Cell Biology and BiophysicsEllenberg Group
Integration of biological data by kernels on graph nodes allows prediction of new genes involved in mitotic chromosome condensation.
Heriche, J.K., Lees, J.G., Morilla, I., Walter, T., Petrova, B., Julia Roberti, M., Hossain, M.J., Adler, P., Fernandez, J.M., Krallinger, M., Haering, C.H., Vilo, J., Valencia, A., Ranea, J.A., Orengo, C. & Ellenberg, J.
Mol Biol Cell. 2014 Jun 18. pii: mbc.E13-04-0221.
The advent of genome-wide RNAi-based screens puts us in the position to identify genes for all functions human cells carry out. However, for many functions, assay complexity and cost make genome-scale knockdown experiments impossible. Methods to predict genes required for cell functions are therefore needed to focus RNAi screens from the whole genome to the most likely candidates. While different bioinformatics tools for gene function prediction exist, they lack experimental validation and are therefore rarely used by experimentalists. To address this, we developed an effective computational gene selection strategy that represents public data about genes as graphs and then analyses these graphs using kernels on graph nodes to predict functional relationships. To demonstrate its performance, we predicted human genes required for a poorly understood cellular function, mitotic chromosome condensation, and experimentally validated the top 100 candidates with a focused RNAi screen by automated microscopy. Quantitative analysis of the images demonstrated that the candidates were indeed strongly enriched in condensation genes including the discovery of several new factors. By combining bioinformatics prediction with experimental validation, our study shows that kernels on graph nodes are a powerful tool to integrate public biological data and predict genes involved in cellular functions of interest.
Mechanisms of HsSAS-6 assembly promoting centriole formation in human cells.
Keller, D., Orpinell, M., Olivier, N., Wachsmuth, M., Mahen, R., Wyss, R., Hachet, V., Ellenberg, J., Manley, S. & Gonczy, P.
J Cell Biol. 2014 Mar 3;204(5):697-712. doi: 10.1083/jcb.201307049.
SAS-6 proteins are thought to impart the ninefold symmetry of centrioles, but the mechanisms by which their assembly occurs within cells remain elusive. In this paper, we provide evidence that the N-terminal, coiled-coil, and C-terminal domains of HsSAS-6 are each required for procentriole formation in human cells. Moreover, the coiled coil is necessary and sufficient to mediate HsSAS-6 centrosomal targeting. High-resolution imaging reveals that GFP-tagged HsSAS-6 variants localize in a torus around the base of the parental centriole before S phase, perhaps indicative of an initial loading platform. Moreover, fluorescence recovery after photobleaching analysis demonstrates that HsSAS-6 is immobilized progressively at centrosomes during cell cycle progression. Using fluorescence correlation spectroscopy and three-dimensional stochastic optical reconstruction microscopy, we uncover that HsSAS-6 is present in the cytoplasm primarily as a homodimer and that its oligomerization into a ninefold symmetrical ring occurs at centrioles. Together, our findings lead us to propose a mechanism whereby HsSAS-6 homodimers are targeted to centrosomes where the local environment and high concentration of HsSAS-6 promote oligomerization, thus initiating procentriole formation.
A quantitative liposome microarray to systematically characterize protein-lipid interactions.
Saliba, A.E., Vonkova, I., Ceschia, S., Findlay, G.M., Maeda, K., Tischer, C., Deghou, S., van Noort, V., Bork, P., Pawson, T., Ellenberg, J. & Gavin, A.C.
Nat Methods. 2014 Jan;11(1):47-50. doi: 10.1038/nmeth.2734. Epub 2013 Nov 24.
Lipids have a role in virtually all biological processes, acting as structural elements, scaffolds and signaling molecules, but they are still largely under-represented in known biological networks. Here we describe a liposome microarray-based assay (LiMA), a method that measures protein recruitment to membranes in a quantitative, automated, multiplexed and high-throughput manner.
Imaging the Assembly, Structure, and Function of the Nuclear Pore Inside Cells.
Otsuka, S., Szymborska, A. & Ellenberg, J.
Methods Cell Biol. 2014;122C:219-238. doi: 10.1016/B978-0-12-417160-2.00010-2.
The nuclear pore complex (NPC) mediates selective transport across the nuclear envelope (NE) and plays crucial roles in several additional cellular functions. In higher eukaryotes, the NPC and the NE disassemble and reassemble during cell division and live-cell imaging has been a powerful tool to analyze these dynamic processes. Here, we present a method for the kinetic analysis of postmitotic NPC assembly and reestablishment of transport competence in intact cells by multicolor 4D imaging and photoswitching. By applying the methods we have established previously using normal rat kidney to HeLa cells, we demonstrate the conservation of NPC assembly in different mammalian cells. We recently showed that the molecular organization of the NPC can be studied by combining stochastic super-resolution microscopy with single-particle averaging and present this method here in detail.
Crowded chromatin is not sufficient for heterochromatin formation and not required for its maintenance.
Walter, A., Chapuis, C., Huet, S. & Ellenberg, J.
J Struct Biol. 2013 Dec;184(3):445-53. doi: 10.1016/j.jsb.2013.10.004. Epub 2013Oct 18.
In contrast to cytoplasmic organelles, which are usually separated from the rest of the cell by phospholipid membranes, nuclear compartments are readily accessible to diffusing proteins and must rely on different mechanisms to maintain their integrity. Specific interactions between scaffolding proteins are known to have important roles for the formation and maintenance of nuclear structures. General physical mechanisms such as molecular crowding, phase separation or colloidal behavior have also been suggested, but their physiological significance remains uncertain. For macromolecular crowding, a role in the maintenance of nucleoli and promyelocytic leukemia (PML) nuclear bodies has been shown. Here, we tested whether a modulation of the compaction state of chromatin, which directly influences the local crowding state, has an impact on the formation and maintenance of densely packed heterochromatin. By osmotic perturbations, we could modify the packing state of chromatin in a controlled manner and show that chromatin compaction, which is associated with increased crowding conditions, is not, per se, sufficient to initiate the formation of new bona fide heterochromatin structures nor is it necessary to maintain already established heterochromatin domains. In consequence, if an increase in crowding induced by chromatin compaction maybe an early step in heterochromatin formation, specific protein-protein interactions are nevertheless required to make heterochromatin long lasting and independent of the crowding state.
Dynamical modelling of phenotypes in a genome-wide RNAi live-cell imaging assay.
Pau, G., Walter, T., Neumann, B., Heriche, J.K., Ellenberg, J. & Huber, W.
BMC Bioinformatics. 2013 Oct 16;14:308. doi: 10.1186/1471-2105-14-308.
BACKGROUND: The combination of time-lapse imaging of live cells with high-throughput perturbation assays is a powerful tool for genetics and cell biology. The Mitocheck project employed this technique to associate thousands of genes with transient biological phenotypes in cell division, cell death and migration. The original analysis of these data proceeded by assigning nuclear morphologies to cells at each time-point using automated image classification, followed by description of population frequencies and temporal distribution of cellular states through event-order maps. One of the choices made by that analysis was not to rely on temporal tracking of the individual cells, due to the relatively low image sampling frequency, and to focus on effects that could be discerned from population-level behaviour. RESULTS: Here, we present a variation of this approach that employs explicit modelling by dynamic differential equations of the cellular state populations. Model fitting to the time course data allowed reliable estimation of the penetrance and time of appearance of four types of disruption of the cell cycle: quiescence, mitotic arrest, polynucleation and cell death. Model parameters yielded estimates of the duration of the interphase and mitosis phases. We identified 2190 siRNAs that induced a disruption of the cell cycle at reproducible times, or increased the durations of the interphase or mitosis phases. CONCLUSIONS: We quantified the dynamic effects of the siRNAs and compiled them as a resource that can be used to characterize the role of their target genes in cell death, mitosis and cell cycle regulation. The described population-based modelling method might be applicable to other large-scale cell-based assays with temporal readout when only population-level measures are available.
Wapl is an essential regulator of chromatin structure and chromosome segregation.
Tedeschi, A., Wutz, G., Huet, S., Jaritz, M., Wuensche, A., Schirghuber, E., Davidson, I.F., Tang, W., Cisneros, D.A., Bhaskara, V., Nishiyama, T., Vaziri, A., Wutz, A., Ellenberg, J. & Peters, J.M.
Nature. 2013 Sep 26;501(7468):564-8. doi: 10.1038/nature12471. Epub 2013 Aug 25.
Mammalian genomes contain several billion base pairs of DNA that are packaged in chromatin fibres. At selected gene loci, cohesin complexes have been proposed to arrange these fibres into higher-order structures, but how important this function is for determining overall chromosome architecture and how the process is regulated are not well understood. Using conditional mutagenesis in the mouse, here we show that depletion of the cohesin-associated protein Wapl stably locks cohesin on DNA, leads to clustering of cohesin in axial structures, and causes chromatin condensation in interphase chromosomes. These findings reveal that the stability of cohesin-DNA interactions is an important determinant of chromatin structure, and indicate that cohesin has an architectural role in interphase chromosome territories. Furthermore, we show that regulation of cohesin-DNA interactions by Wapl is important for embryonic development, expression of genes such as c-myc (also known as Myc), and cell cycle progression. In mitosis, Wapl-mediated release of cohesin from DNA is essential for proper chromosome segregation and protects cohesin from cleavage by the protease separase, thus enabling mitotic exit in the presence of functional cohesin complexes.
Nuclear pore scaffold structure analyzed by super-resolution microscopy and particle averaging.
Szymborska, A., de Marco, A., Daigle, N., Cordes, V.C., Briggs, J.A. & Ellenberg, J.
Science. 2013 Aug 9;341(6146):655-8. doi: 10.1126/science.1240672. Epub 2013 Jul11.
Much of life's essential molecular machinery consists of large protein assemblies that currently pose challenges for structure determination. A prominent example is the nuclear pore complex (NPC), for which the organization of its individual components remains unknown. By combining stochastic super-resolution microscopy, to directly resolve the ringlike structure of the NPC, with single particle averaging, to use information from thousands of pores, we determined the average positions of fluorescent molecular labels in the NPC with a precision well below 1 nanometer. Applying this approach systematically to the largest building block of the NPC, the Nup107-160 subcomplex, we assessed the structure of the NPC scaffold. Thus, light microscopy can be used to study the molecular organization of large protein complexes in situ in whole cells.
EGF-Induced Centrosome Separation Promotes Mitotic Progression and Cell Survival.
Mardin, B.R., Isokane, M., Cosenza, M.R., Kramer, A., Ellenberg, J., Fry, A.M. & Schiebel, E.
Dev Cell. 2013 May 13;25(3):229-40. doi: 10.1016/j.devcel.2013.03.012. Epub 2013May 2.
Timely and accurate assembly of the mitotic spindle is critical for the faithful segregation of chromosomes, and centrosome separation is a key step in this process. The timing of centrosome separation varies dramatically between cell types; however, the mechanisms responsible for these differences and its significance are unclear. Here, we show that activation of epidermal growth factor receptor (EGFR) signaling determines the timing of centrosome separation. Premature separation of centrosomes decreases the requirement for the major mitotic kinesin Eg5 for spindle assembly, accelerates mitosis, and decreases the rate of chromosome missegregation. Importantly, EGF stimulation impacts upon centrosome separation and mitotic progression to different degrees in different cell lines. Cells with high EGFR levels fail to arrest in mitosis upon Eg5 inhibition. This has important implications for cancer therapy because cells with high centrosomal response to EGF are more susceptible to combinatorial inhibition of EGFR and Eg5.
GTSE1 is a microtubule plus-end tracking protein that regulates EB1-dependent cell migration.
Scolz, M., Widlund, P.O., Piazza, S., Bublik, D.R., Reber, S., Peche, L.Y., Ciani, Y., Hubner, N., Isokane, M., Monte, M., Ellenberg, J., Hyman, A.A., Schneider, C. & Bird, A.W.
PLoS One. 2012;7(12):e51259. doi: 10.1371/journal.pone.0051259. Epub 2012 Dec 7.
The regulation of cell migration is a highly complex process that is often compromised when cancer cells become metastatic. The microtubule cytoskeleton is necessary for cell migration, but how microtubules and microtubule-associated proteins regulate multiple pathways promoting cell migration remains unclear. Microtubule plus-end binding proteins (+TIPs) are emerging as important players in many cellular functions, including cell migration. Here we identify a +TIP, GTSE1, that promotes cell migration. GTSE1 accumulates at growing microtubule plus ends through interaction with the EB1+TIP. The EB1-dependent +TIP activity of GTSE1 is required for cell migration, as well as for microtubule-dependent disassembly of focal adhesions. GTSE1 protein levels determine the migratory capacity of both nontransformed and breast cancer cell lines. In breast cancers, increased GTSE1 expression correlates with invasive potential, tumor stage, and time to distant metastasis, suggesting that misregulation of GTSE1 expression could be associated with increased invasive potential.
A fractal model for nuclear organization: current evidence and biological implications.
Bancaud, A., Lavelle, C., Huet, S. & Ellenberg, J.
Nucleic Acids Res. 2012 Oct;40(18):8783-92. doi: 10.1093/nar/gks586. Epub 2012Jul 11.
Chromatin is a multiscale structure on which transcription, replication, recombination and repair of the genome occur. To fully understand any of these processes at the molecular level under physiological conditions, a clear picture of the polymorphic and dynamic organization of chromatin in the eukaryotic nucleus is required. Recent studies indicate that a fractal model of chromatin architecture is consistent with both the reaction-diffusion properties of chromatin interacting proteins and with structural data on chromatin interminglement. In this study, we provide a critical overview of the experimental evidence that support a fractal organization of chromatin. On this basis, we discuss the functional implications of a fractal chromatin model for biological processes and propose future experiments to probe chromatin organization further that should allow to strongly support or invalidate the fractal hypothesis.
Mitotic lamin disassembly is triggered by lipid-mediated signaling.
Mall, M., Walter, T., Gorjanacz, M., Davidson, I.F., Nga Ly-Hartig, T.B., Ellenberg, J., Mattaj, I.W.
J Cell Biol. 2012 Sep 17;198(6):981-90. doi: 10.1083/jcb.201205103.
Disassembly of the nuclear lamina is a key step during open mitosis in higher eukaryotes. The activity of several kinases, including CDK1 (cyclin-dependent kinase 1) and protein kinase C (PKC), has been shown to trigger mitotic lamin disassembly, yet their precise contributions are unclear. In this study, we develop a quantitative imaging assay to study mitotic lamin B1 disassembly in living cells. We find that CDK1 and PKC act in concert to mediate phosphorylation-dependent lamin B1 disassembly during mitosis. Using ribonucleic acid interference (RNAi), we showed that diacylglycerol (DAG)-dependent PKCs triggered rate-limiting steps of lamin disassembly. RNAi-mediated depletion or chemical inhibition of lipins, enzymes that produce DAG, delayed lamin disassembly to a similar extent as does PKC inhibition/depletion. Furthermore, the delay of lamin B1 disassembly after lipin depletion could be rescued by the addition of DAG. These findings suggest that lipins activate a PKC-dependent pathway during mitotic lamin disassembly and provide evidence for a lipid-mediated mitotic signaling event.
The transition from meiotic to mitotic spindle assembly is gradual during early mammalian development.
Courtois, A., Schuh, M., Ellenberg, J. & Hiiragi, T.
J Cell Biol. 2012 Aug 6;198(3):357-70. Epub 2012 Jul 30.
The transition from meiosis to mitosis, classically defined by fertilization, is a fundamental process in development. However, its mechanism remains largely unexplored. In this paper, we report a surprising gradual transition from meiosis to mitosis over the first eight divisions of the mouse embryo. The first cleavages still largely share the mechanism of spindle formation with meiosis, during which the spindle is self-assembled from randomly distributed microtubule-organizing centers (MTOCs) without centrioles, because of the concerted activity of dynein and kinesin-5. During preimplantation development, the number of cellular MTOCs progressively decreased, the spindle pole gradually became more focused, and spindle length progressively scaled down with cell size. The typical mitotic spindle with centrin-, odf2-, kinesin-12-, and CP110-positive centrosomes was established only in the blastocyst. Overall, the transition from meiosis to mitosis progresses gradually throughout the preimplantation stage in the mouse embryo, thus providing a unique system to study the mechanism of centrosome biogenesis in vivo.
Genome-wide RNAi screening identifies human proteins with a regulatory function in the early secretory pathway.
Simpson, J.C., Joggerst, B., Laketa, V., Verissimo, F., Cetin, C., Erfle, H., Bexiga, M.G., Singan, V.R., Heriche, J.K., Neumann, B., Mateos, A., Blake, J., Bechtel, S., Benes, V., Wiemann, S., Ellenberg, J. & Pepperkok, R.
Nat Cell Biol. 2012 Jun 3;14(7):764-74. doi: 10.1038/ncb2510.
The secretory pathway in mammalian cells has evolved to facilitate the transfer of cargo molecules to internal and cell surface membranes. Use of automated microscopy-based genome-wide RNA interference screens in cultured human cells allowed us to identify 554 proteins influencing secretion. Cloning, fluorescent-tagging and subcellular localization analysis of 179 of these proteins revealed that more than two-thirds localize to either the cytoplasm or membranes of the secretory and endocytic pathways. The depletion of 143 of them resulted in perturbations in the organization of the COPII and/or COPI vesicular coat complexes of the early secretory pathway, or the morphology of the Golgi complex. Network analyses revealed a so far unappreciated link between early secretory pathway function, small GTP-binding protein regulation, actin cytoskeleton organization and EGF-receptor-mediated signalling. This work provides an important resource for an integrative understanding of global cellular organization and regulation of the secretory pathway in mammalian cells.
Nucleoporin NUP153 guards genome integrity by promoting nuclear import of 53BP1.
Moudry, P., Lukas, C., Macurek, L., Neumann, B., Heriche, J.K., Pepperkok, R., Ellenberg, J., Hodny, Z., Lukas, J. & Bartek, J.
Cell Death Differ. 2012 May;19(5):798-807. doi: 10.1038/cdd.2011.150. Epub 2011Nov 11.
53BP1 is a mediator of DNA damage response (DDR) and a tumor suppressor whose accumulation on damaged chromatin promotes DNA repair and enhances DDR signaling. Using foci formation of 53BP1 as a readout in two human cell lines, we performed an siRNA-based functional high-content microscopy screen for modulators of cellular response to ionizing radiation (IR). Here, we provide the complete results of this screen as an information resource, and validate and functionally characterize one of the identified 'hits': a nuclear pore component NUP153 as a novel factor specifically required for 53BP1 nuclear import. Using a range of cell and molecular biology approaches including live-cell imaging, we show that knockdown of NUP153 prevents 53BP1, but not several other DDR factors, from entering the nuclei in the newly forming daughter cells. This translates into decreased IR-induced 53BP1 focus formation, delayed DNA repair and impaired cell survival after IR. In addition, NUP153 depletion exacerbates DNA damage caused by replication stress. Finally, we show that the C-terminal part of NUP153 is required for effective 53BP1 nuclear import, and that 53BP1 is imported to the nucleus through the NUP153-importin-beta interplay. Our data define the structure-function relationships within this emerging 53BP1-NUP153/importin-beta pathway and implicate this mechanism in the maintenance of genome integrity.
The quantitative proteome of a human cell line.
Beck, M., Schmidt, A., Malmstroem, J., Claassen, M., Ori, A., Szymborska, A., Herzog, F., Rinner, O., Ellenberg, J. & Aebersold, R.
Mol Syst Biol. 2011 Nov 8;7:549. doi: 10.1038/msb.2011.82.
The generation of mathematical models of biological processes, the simulation of these processes under different conditions, and the comparison and integration of multiple data sets are explicit goals of systems biology that require the knowledge of the absolute quantity of the system's components. To date, systematic estimates of cellular protein concentrations have been exceptionally scarce. Here, we provide a quantitative description of the proteome of a commonly used human cell line in two functional states, interphase and mitosis. We show that these human cultured cells express at least approximately 10 000 proteins and that the quantified proteins span a concentration range of seven orders of magnitude up to 20 000 000 copies per cell. We discuss how protein abundance is linked to function and evolution.
Phenotypic profiling of the human genome reveals gene products involved in plasma membrane targeting of SRC kinases.
Ritzerfeld, J., Remmele, S., Wang, T., Temmerman, K., Brugger, B., Wegehingel, S., Tournaviti, S., Strating, J.R., Wieland, F.T., Neumann, B., Ellenberg, J., Lawerenz, C., Hesser, J., Erfle, H., Pepperkok, R. & Nickel, W.
Genome Res. 2011 Nov;21(11):1955-68. doi: 10.1101/gr.116087.110. Epub 2011 Jul27.
SRC proteins are non-receptor tyrosine kinases that play key roles in regulating signal transduction by a diverse set of cell surface receptors. They contain N-terminal SH4 domains that are modified by fatty acylation and are functioning as membrane anchors. Acylated SH4 domains are both necessary and sufficient to mediate specific targeting of SRC kinases to the inner leaflet of plasma membranes. Intracellular transport of SRC kinases to the plasma membrane depends on microdomains into which SRC kinases partition upon palmitoylation. In the present study, we established a live-cell imaging screening system to identify gene products involved in plasma membrane targeting of SRC kinases. Based on siRNA arrays and a human model cell line expressing two kinds of SH4 reporter molecules, we conducted a genome-wide analysis of SH4-dependent protein targeting using an automated microscopy platform. We identified and validated 54 gene products whose down-regulation causes intracellular retention of SH4 reporter molecules. To detect and quantify this phenotype, we developed a software-based image analysis tool. Among the identified gene products, we found factors involved in lipid metabolism, intracellular transport, and cellular signaling processes. Furthermore, we identified proteins that are either associated with SRC kinases or are related to various known functions of SRC kinases such as other kinases and phosphatases potentially involved in SRC-mediated signal transduction. Finally, we identified gene products whose function is less defined or entirely unknown. Our findings provide a major resource for future studies unraveling the molecular mechanisms that underlie proper targeting of SRC kinases to the inner leaflet of plasma membranes.
Complete kinetochore tracking reveals error-prone homologous chromosome biorientation in mammalian oocytes.
Kitajima, T.S., Ohsugi, M. & Ellenberg, J.
Cell. 2011 Aug 19;146(4):568-81. doi: 10.1016/j.cell.2011.07.031.
Chromosomes must establish stable biorientation prior to anaphase to achieve faithful segregation during cell division. The detailed process by which chromosomes are bioriented and how biorientation is coordinated with spindle assembly and chromosome congression remain unclear. Here, we provide complete 3D kinetochore-tracking datasets throughout cell division by high-resolution imaging of meiosis I in live mouse oocytes. We show that in acentrosomal oocytes, chromosome congression forms an intermediate chromosome configuration, the prometaphase belt, which precedes biorientation. Chromosomes then invade the elongating spindle center to form the metaphase plate and start biorienting. Close to 90% of all chromosomes undergo one or more rounds of error correction of their kinetochore-microtubule attachments before achieving correct biorientation. This process depends on Aurora kinase activity. Our analysis reveals the error-prone nature of homologous chromosome biorientation, providing a possible explanation for the high incidence of aneuploid eggs observed in mammals, including humans.
A system for imaging the regulatory noncoding Xist RNA in living mouse embryonic stem cells.
Ng, K., Daigle, N., Bancaud, A., Ohhata, T., Humphreys, P., Walker, R., Ellenberg, J. & Wutz, A.
Mol Biol Cell. 2011 Jul;22(14):2634-2645. Epub 2011 May 25.
In mammals, silencing of one of the two X chromosomes in female cells compensates for the different number of X chromosomes between the sexes. The noncoding Xist RNA initiates X chromosome inactivation. Xist spreads from its transcription site over the X chromosome territory and triggers the formation of a repressive chromatin domain. To understand localization of Xist over one X chromosome we aimed to develop a system for investigating Xist in living cells. Here we report successful visualization of transgenically expressed MS2-tagged Xist in mouse embryonic stem cells. Imaging of Xist during an entire cell cycle shows that Xist spreads from a single point to a steady state when the chromosome is covered with a constant amount of Xist. Photobleaching experiments of the established Xist cluster indicate that chromosome-bound Xist is dynamic and turns over on the fully Xist covered chromosome. It appears that in interphase the loss of bound Xist and newly produced Xist are in equilibrium. We also show that the turnover of bound Xist requires transcription, and Xist binding becomes stable when transcription is inhibited. Our data reveal a strategy for visualizing Xist and indicate that spreading over the chromosome might involve dynamic binding and displacement.
Automatic quantification of microtubule dynamics enables RNAi-screening of new mitotic spindle regulators.
Sironi L,Solon J,Conrad C,Mayer TU,Brunner D,Ellenberg J
Cytoskeleton (Hoboken). 2011 May;68(5):266-78. doi: 10.1002/cm.20510. Epub 2011Apr 13.
The genetic integrity of every organism depends on the faithful partitioning of its genome between two daughter cells in mitosis. In all eukaryotes, chromosome segregation requires the assembly of the mitotic spindle, a bipolar array of dynamic microtubules. Perturbations in microtubule dynamics affect spindle assembly and maintenance and ultimately result in aberrant cell divisions. To identify new regulators of microtubule dynamics within the hundreds of mitotic hits, reported in RNAi screens performed in C. elegans, Drosophila and mammalian tissue culture cells [Sonnichsen et al., 2005; Goshima et al., 2007; Neumann et al., 2010], we established a fast and quantitative assay to measure microtubule dynamics in living cells. Here we present a fully automated workflow from RNAi transfection, via image acquisition and data processing, to the quantitative characterization of microtubule behaviour. Candidate genes are knocked down by solid-phase reverse transfection with siRNA oligos in HeLa cells stably expressing EB3-EGFP, a microtubule plus end marker. Mitotic cells are selected using an automatic classifier [Conrad et al., 2011] and imaged on a spinning disk confocal microscope at high temporal and spatial resolution. The time-lapse movies are analysed using a multiple particle tracking software, developed in-house, that automatically detects microtubule plus ends, tracks microtubule growth events over consecutive frames and calculates growth speeds, lengths and lifetimes of the tracked microtubules. The entire assay provides a powerful tool to analyse the effect of essential mitotic genes on microtubule dynamics in living cells and to dissect their contribution in spindle assembly and maintenance.
Intracellular transport by an anchored homogeneously contracting F-actin meshwork.
Mori, M., Monnier, N., Daigle, N., Bathe, M., Ellenberg, J. & Lenart, P.
Curr Biol. 2011 Apr 12;21(7):606-11.
Actin-based contractility orchestrates changes in cell shape underlying cellular functions ranging from division to migration and wound healing. Actin also functions in intracellular transport, with the prevailing view that filamentous actin (F-actin) cables serve as tracks for motor-driven transport of cargo. We recently discovered an alternate mode of intracellular transport in starfish oocytes involving a contractile F-actin meshwork that mediates chromosome congression. The mechanisms by which this meshwork contracts and translates its contractile activity into directional transport of chromosomes remained open questions. Here, we use live-cell imaging with quantitative analysis of chromosome trajectories and meshwork velocities to show that the 3D F-actin meshwork contracts homogeneously and isotropically throughout the nuclear space. Centrifugation experiments reveal that this homogeneous contraction is translated into asymmetric, directional transport by mechanical anchoring of the meshwork to the cell cortex. Finally, by injecting inert particles of different sizes, we show that this directional transport activity is size-selective and transduced to chromosomal cargo at least in part by steric trapping or "sieving." Taken together, these results reveal mechanistic design principles of a novel and potentially versatile mode of intracellular transport based on sieving by an anchored homogeneously contracting F-actin meshwork.
A Nup133-dependent NPC-anchored network tethers centrosomes to the nuclear envelope in prophase.
Bolhy, S., Bouhlel, I., Dultz, E., Nayak, T., Zuccolo, M., Gatti, X., Vallee, R., Ellenberg, J. & Doye, V.
J Cell Biol. 2011 Mar 7;192(5):855-71. doi: 10.1083/jcb.201007118.
Centrosomes are closely associated with the nuclear envelope (NE) throughout the cell cycle and this association is maintained in prophase when they separate to establish the future mitotic spindle. At this stage, the kinetochore constituents CENP-F, NudE, NudEL, dynein, and dynactin accumulate at the NE. We demonstrate here that the N-terminal domain of the nuclear pore complex (NPC) protein Nup133, although largely dispensable for NPC assembly, is required for efficient anchoring of the dynein/dynactin complex to the NE in prophase. Nup133 exerts this function through an interaction network via CENP-F and NudE/EL. We show that this molecular chain is critical for maintaining centrosome association with the NE at mitotic entry and contributes to this process without interfering with the previously described RanBP2-BICD2-dependent pathway of centrosome anchoring. Finally, our study reveals that tethering of centrosomes to the nuclear surface at the G2/M transition contributes, along with other cellular mechanisms, to early stages of bipolar spindle assembly.
Micropilot: automation of fluorescence microscopy-based imaging for systems biology.
Conrad, C., Wunsche, A., Tan, T.H., Bulkescher, J., Sieckmann, F., Verissimo, F., Edelstein, A., Walter, T., Liebel, U., Pepperkok, R. & Ellenberg, J.
Nat Methods. 2011 Mar;8(3):246-9. doi: 10.1038/nmeth.1558. Epub 2011 Jan 23.
Quantitative microscopy relies on imaging of large cell numbers but is often hampered by time-consuming manual selection of specific cells. The 'Micropilot' software automatically detects cells of interest and launches complex imaging experiments including three-dimensional multicolor time-lapse or fluorescence recovery after photobleaching in live cells. In three independent experimental setups this allowed us to statistically analyze biological processes in detail and is thus a powerful tool for systems biology.
Fluorescence perturbation techniques to study mobility and molecular dynamics of proteins in live cells: FRAP, photoactivation, photoconversion, and FLIP.
Bancaud, A., Huet, S., Rabut, G. & Ellenberg, J.
Cold Spring Harb Protoc. 2010 Dec 1;2010(12):pdb.top90. doi: 10.1101/pdb.top90.
The technique of fluorescence recovery after photobleaching (FRAP) was introduced in the mid-1970s to study the diffusion of biomolecules in living cells. For several years, it was used mainly by a small number of biophysicists who had developed their own photobleaching systems. Since the mid-1990s, FRAP has gained increasing popularity because of the conjunction of two factors: First, photobleaching techniques are easily implemented on confocal laser-scanning microscopes (CLSMs), and so FRAP has become available to anyone who has access to such equipment. Second, the advent of green fluorescent protein (GFP) has allowed easy fluorescent tagging of proteins and their observation in living cells. Thanks both to the versatility of modern CLSMs, which allow control of laser intensity at any point of the image, and to the development of new fluorescent probes, additional photoperturbation techniques have emerged during the last few years. After the photoperturbation event, one observes and then analyzes how the fluorescence distribution relaxes toward the steady state. Because the photochemical perturbation of suitable fluorophores is essentially irreversible, changes of fluorescence intensity in the perturbed and unperturbed regions are due to the exchange of tagged molecules between those regions. This article first discusses the materials required for performing FRAP experiments on a CLSM and the software for data analysis. It then describes general considerations on how to perform FRAP experiments as well as the necessary controls. Finally, different possible ways to analyze the data are presented.
The protein phosphatase 1 regulator PNUTS is a new component of the DNA damage response.
Landsverk, H.B., Mora-Bermudez, F., Landsverk, O.J., Hasvold, G., Naderi, S., Bakke, O., Ellenberg, J., Collas, P., Syljuasen, R.G. & Kuntziger, T.
EMBO Rep. 2010 Nov;11(11):868-75. Epub 2010 Oct 1.
The function of protein phosphatase 1 nuclear-targeting subunit (PNUTS)--one of the most abundant nuclear-targeting subunits of protein phosphatase 1 (PP1c)--remains largely uncharacterized. We show that PNUTS depletion by small interfering RNA activates a G2 checkpoint in unperturbed cells and prolongs G2 checkpoint and Chk1 activation after ionizing-radiation-induced DNA damage. Overexpression of PNUTS-enhanced green fluorescent protein (EGFP)--which is rapidly and transiently recruited at DNA damage sites--inhibits G2 arrest. Finally, gammaH2AX, p53-binding protein 1, replication protein A and Rad51 foci are present for a prolonged period and clonogenic survival is decreased in PNUTS-depleted cells after ionizing radiation treatment. We identify the PP1c regulatory subunit PNUTS as a new and integral component of the DNA damage response involved in DNA repair.
Live imaging of single nuclear pores reveals unique assembly kinetics and mechanism in interphase.
Dultz, E. & Ellenberg, J.
J Cell Biol. 2010 Oct 4;191(1):15-22. Epub 2010 Sep 27.
In metazoa, new nuclear pore complexes (NPCs) form at two different cell cycle stages: at the end of mitosis concomitant with the reformation of the nuclear envelope and during interphase. However, the mechanisms of these assembly processes may differ. In this study, we apply high resolution live cell microscopy to analyze the dynamics of single NPCs in living mammalian cells during interphase. We show that nuclear growth and NPC assembly are correlated and occur at a constant rate throughout interphase. By analyzing the kinetics of individual NPC assembly events, we demonstrate that they are initiated by slow accumulation of the membrane nucleoporin Pom121 followed by the more rapid association of the soluble NPC subcomplex Nup107-160. This inverse order of recruitment and the overall much slower kinetics compared with postmitotic NPC assembly support the conclusion that the two processes occur by distinct molecular mechanisms.
CellCognition: time-resolved phenotype annotation in high-throughput live cell imaging.
Held, M., Schmitz, M.H., Fischer, B., Walter, T., Neumann, B., Olma, M.H., Peter, M., Ellenberg, J. & Gerlich, D.W.
Nat Methods. 2010 Sep;7(9):747-54. doi: 10.1038/nmeth.1486. Epub 2010 Aug 8.
Fluorescence time-lapse imaging has become a powerful tool to investigate complex dynamic processes such as cell division or intracellular trafficking. Automated microscopes generate time-resolved imaging data at high throughput, yet tools for quantification of large-scale movie data are largely missing. Here we present CellCognition, a computational framework to annotate complex cellular dynamics. We developed a machine-learning method that combines state-of-the-art classification with hidden Markov modeling for annotation of the progression through morphologically distinct biological states. Incorporation of time information into the annotation scheme was essential to suppress classification noise at state transitions and confusion between different functional states with similar morphology. We demonstrate generic applicability in different assays and perturbation conditions, including a candidate-based RNA interference screen for regulators of mitotic exit in human cells. CellCognition is published as open source software, enabling live-cell imaging-based screening with assays that directly score cellular dynamics.
High-throughput microscopy using live mammalian cells.
Terjung, S., Walter, T., Seitz, A., Neumann, B., Pepperkok, R. & Ellenberg, J.
Cold Spring Harb Protoc. 2010 Aug 1;2010(7):pdb.top84. doi: 10.1101/pdb.top84. Europe PMC
Automatic identification and clustering of chromosome phenotypes in a genome wide RNAi screen by time-lapse imaging.
Walter, T., Held, M., Neumann, B., Heriche, J.K., Conrad, C., Pepperkok, R. & Ellenberg, J.
J Struct Biol. 2010 Apr;170(1):1-9. doi: 10.1016/j.jsb.2009.10.004. Epub 2009 Oct23.
High-throughput time-lapse microscopy is an excellent way of studying gene function by collecting time-resolved image data of the cellular responses to gene perturbations. With the increase in both data amount and complexity, computational methods capable of dealing with large image data sets are required. While image processing methods have been successfully applied to endpoint assays in the past, the analysis of complex time-resolved read-outs was so far still too immature to be applied on a large-scale. Here, we present a complete computational processing pipeline for such screens. By automatic image processing and machine learning, a quantitative description of phenotypic dynamics is obtained from the raw bitmaps. In order to visualize the resulting phenotypes in their temporal context, we introduce Event Order Maps allowing a concise representation of the major tendencies of causes and consequences of phenotypic classes. In order to cluster the phenotypic kinetics, we propose a novel technique based on trajectory representation of multidimensional time series. We demonstrate the use of these methods applying them on a genome wide RNAi screen by time-lapse microscopy.
Systematic analysis of human protein complexes identifies chromosome segregation proteins.
Hutchins, J.R., Toyoda, Y., Hegemann, B., Poser, I., Heriche, J.K., Sykora, M.M., Augsburg, M., Hudecz, O., Buschhorn, B.A., Bulkescher, J., Conrad, C., Comartin, D., Schleiffer, A., Sarov, M., Pozniakovsky, A., Slabicki, M.M., Schloissnig, S., Steinmacher, I., Leuschner, M., Ssykor, A., Lawo, S., Pelletier, L., Stark, H., Nasmyth, K., Ellenberg, J., Durbin, R., Buchholz, F., Mechtler, K., Hyman, A.A. & Peters, J.M.
Science. 2010 Apr 30;328(5978):593-9. Epub 2010 Apr 1.
Chromosome segregation and cell division are essential, highly ordered processes that depend on numerous protein complexes. Results from recent RNA interference screens indicate that the identity and composition of these protein complexes is incompletely understood. Using gene tagging on bacterial artificial chromosomes, protein localization, and tandem-affinity purification-mass spectrometry, the MitoCheck consortium has analyzed about 100 human protein complexes, many of which had not or had only incompletely been characterized. This work has led to the discovery of previously unknown, evolutionarily conserved subunits of the anaphase-promoting complex and the gamma-tubulin ring complex--large complexes that are essential for spindle assembly and chromosome segregation. The approaches we describe here are generally applicable to high-throughput follow-up analyses of phenotypic screens in mammalian cells.
Phenotypic profiling of the human genome by time-lapse microscopy reveals cell division genes.
Neumann, B., Walter, T., Heriche, J.K., Bulkescher, J., Erfle, H., Conrad, C., Rogers, P., Poser, I., Held, M., Liebel, U., Cetin, C., Sieckmann, F., Pau, G., Kabbe, R., Wünsche, A., Satagopam, V., Schmitz, M.H., Chapuis, C., Gerlich, D.W., Schneider, R., Eils, R., Huber, W., Peters, J.M., Hyman, A.A., Durbin, R., Pepperkok, R. & Ellenberg, J.
Nature. 2010 Apr 1;464(7289):721-7.
Despite our rapidly growing knowledge about the human genome, we do not know all of the genes required for some of the most basic functions of life. To start to fill this gap we developed a high-throughput phenotypic screening platform combining potent gene silencing by RNA interference, time-lapse microscopy and computational image processing. We carried out a genome-wide phenotypic profiling of each of the approximately 21,000 human protein-coding genes by two-day live imaging of fluorescently labelled chromosomes. Phenotypes were scored quantitatively by computational image processing, which allowed us to identify hundreds of human genes involved in diverse biological functions including cell division, migration and survival. As part of the Mitocheck consortium, this study provides an in-depth analysis of cell division phenotypes and makes the entire high-content data set available as a resource to the community.
Visualization of image data from cells to organisms.
Walter, T., Shattuck, D.W., Baldock, R., Bastin, M.E., Carpenter, A.E., Duce, S., Ellenberg, J., Fraser, A., Hamilton, N., Pieper, S., Ragan, M.A., Schneider, J.E., Tomancak, P. & Heriche, J.K.
Nat Methods. 2010 Mar;7(3 Suppl):S26-41.
Advances in imaging techniques and high-throughput technologies are providing scientists with unprecedented possibilities to visualize internal structures of cells, organs and organisms and to collect systematic image data characterizing genes and proteins on a large scale. To make the best use of these increasingly complex and large image data resources, the scientific community must be provided with methods to query, analyze and crosslink these resources to give an intuitive visual representation of the data. This review gives an overview of existing methods and tools for this purpose and highlights some of their limitations and challenges.
Nuclear import and assembly of influenza A virus RNA polymerase studied in live cells by fluorescence cross-correlation spectroscopy.
Huet, S., Avilov, S.V., Ferbitz, L., Daigle, N., Cusack, S. & Ellenberg, J.
J Virol. 2010 Feb;84(3):1254-64. Epub 2009 Nov 11.
Intracellular transport and assembly of the subunits of the heterotrimeric RNA-dependent RNA polymerase constitute a key component of the replication cycle of influenza virus. Recent results suggest that efficient polymerase assembly is a limiting factor in the viability of reassortant viruses. The mechanism of nuclear import and assembly of the three polymerase subunits, PB1, PB2, and PA, is still controversial, yet it is clearly of great significance in understanding the emergence of new strains with pandemic potential. In this study, we systematically investigated the interactions between the polymerase subunits and their localization in living cells by fluorescence cross-correlation spectroscopy (FCCS) and quantitative confocal microscopy. We could show that PB1 and PA form a dimer in the cytoplasm, which is imported into the nucleus separately from PB2. Once in the nucleus, the PB1/PA dimer associates with PB2 to form the trimeric polymerase. Photon-counting histogram analysis revealed that trimeric polymerase complexes can form higher-order oligomers in the nucleus. We furthermore demonstrate that impairing the nuclear import of PB2 by mutating its nuclear localization signal leads to abnormal formation of the trimeric polymerase in the cytoplasm. Taken together, our results demonstrate which of the previously discussed influenza virus polymerase transport models operates in live cells. Our study sheds light on the interplay between the nuclear import of the subunits and the assembly of the influenza virus polymerase and provides a methodological framework to analyze the effects of different host range mutations in the future.
Molecular crowding affects diffusion and binding of nuclear proteins in heterochromatin and reveals the fractal organization of chromatin.
Bancaud, A., Huet, S., Daigle, N., Mozziconacci, J., Beaudouin, J. & Ellenberg, J.
EMBO J. 2009 Dec 16;28(24):3785-98. Epub .
The nucleus of eukaryotes is organized into functional compartments, the two most prominent being heterochromatin and nucleoli. These structures are highly enriched in DNA, proteins or RNA, and thus thought to be crowded. In vitro, molecular crowding induces volume exclusion, hinders diffusion and enhances association, but whether these effects are relevant in vivo remains unclear. Here, we establish that volume exclusion and diffusive hindrance occur in dense nuclear compartments by probing the diffusive behaviour of inert fluorescent tracers in living cells. We also demonstrate that chromatin-interacting proteins remain transiently trapped in heterochromatin due to crowding induced enhanced affinity. The kinetic signatures of these crowding consequences allow us to derive a fractal model of chromatin organization, which explains why the dynamics of soluble nuclear proteins are affected independently of their size. This model further shows that the fractal architecture differs between heterochromatin and euchromatin, and predicts that chromatin proteins use different target-search strategies in the two compartments. We propose that fractal crowding is a fundamental principle of nuclear organization, particularly of heterochromatin maintenance.
Automatic analysis of dividing cells in live cell movies to detect mitotic delays and correlate phenotypes in time.
Harder, N., Mora-Bermudez, F., Godinez, W.J., Wünsche, A., Eils, R., Ellenberg, J. & Rohr, K.
Genome Res. 2009 Nov;19(11):2113-24. Epub 2009 Oct 1.
Live-cell imaging allows detailed dynamic cellular phenotyping for cell biology and, in combination with small molecule or drug libraries, for high-content screening. Fully automated analysis of live cell movies has been hampered by the lack of computational approaches that allow tracking and recognition of individual cell fates over time in a precise manner. Here, we present a fully automated approach to analyze time-lapse movies of dividing cells. Our method dynamically categorizes cells into seven phases of the cell cycle and five aberrant morphological phenotypes over time. It reliably tracks cells and their progeny and can thus measure the length of mitotic phases and detect cause and effect if mitosis goes awry. We applied our computational scheme to annotate mitotic phenotypes induced by RNAi gene knockdown of CKAP5 (also known as ch-TOG) or by treatment with the drug nocodazole. Our approach can be readily applied to comparable assays aiming at uncovering the dynamic cause of cell division phenotypes.
Formation of the nuclear envelope permeability barrier studied by sequential photoswitching and flux analysis.
Dultz, E., Huet, S. & Ellenberg, J.
Biophys J. 2009 Oct 7;97(7):1891-7.
In higher eukaryotes, the nuclear envelope breaks down during mitosis. It reforms during telophase, and nuclear import is reestablished within <10 min after anaphase onset. It is widely assumed that import functionality simultaneously leads to the exclusion of bulk cytoplasmic proteins. However, nuclear pore complex assembly is not fully completed when import capacity is regained, which raises the question of whether the transport and permeability barrier functions of the nuclear envelope are indeed coupled. In this study, we therefore analyzed the reestablishment of the permeability barrier of the nuclear envelope after mitosis in living cells by monitoring the flux of the reversibly photoswitchable fluorescent protein Dronpa from the cytoplasm into the nucleus after photoactivation. We performed many consecutive flux measurements in the same cell to directly monitor changes in nuclear envelope permeability. Our measurements at different time points after mitosis in individual cells show that contrary to the general view and despite the rapid reestablishment of facilitated nuclear import, the nuclear envelope remains relatively permeable for passive diffusion for the first 2 h after mitosis. Our data demonstrate that reformation of the permeability barrier of nuclear pore complexes occurs only gradually and is uncoupled from regaining active import functionality.
Chromophore-assisted laser inactivation of alpha- and gamma-tubulin SNAP-tag fusion proteins inside living cells.
Keppler, A. & Ellenberg, J.
ACS Chem Biol. 2009 Feb 20;4(2):127-38. doi: 10.1021/cb800298u.
Chromophore-assisted laser inactivation (CALI) can help to unravel localized activities of target proteins at defined times and locations within living cells. Covalent SNAP-tag labeling of fusion proteins with fluorophores such as fluorescein is a fast and highly specific tool to attach the photosensitizer to its target protein in vivo for selective inactivation of the fusion protein. Here, we demonstrate the effectiveness and specificity of SNAP-tag-based CALI by acute inactivation of alpha-tubulin and gamma-tubulin SNAP-tag fusions during live imaging assays of cell division. Singlet oxygen is confirmed as the reactive oxygen species that leads to loss of fusion protein function. The major advantage of SNAP-tag CALI is the ease, reliability, and high flexibility in labeling: the genetically encoded protein tag can be covalently labeled with various dyes matching the experimental requirements. This makes SNAP-tag CALI a very useful tool for rapid inactivation of tagged proteins in living cells.
RNF168 binds and amplifies ubiquitin conjugates on damaged chromosomes to allow accumulation of repair proteins.
Doil, C., Mailand, N., Bekker-Jensen, S., Menard, P., Larsen, D.H., Pepperkok, R., Ellenberg, J., Panier, S., Durocher, D., Bartek, J., Lukas, J. & Lukas, C.
Cell. 2009 Feb 6;136(3):435-46.
DNA double-strand breaks (DSBs) not only interrupt the genetic information, but also disrupt the chromatin structure, and both impairments require repair mechanisms to ensure genome integrity. We showed previously that RNF8-mediated chromatin ubiquitylation protects genome integrity by promoting the accumulation of repair factors at DSBs. Here, we provide evidence that, while RNF8 is necessary to trigger the DSB-associated ubiquitylations, it is not sufficient to sustain conjugated ubiquitin in this compartment. We identified RNF168 as a novel chromatin-associated ubiquitin ligase with an ability to bind ubiquitin. We show that RNF168 interacts with ubiquitylated H2A, assembles at DSBs in an RNF8-dependent manner, and, by targeting H2A and H2AX, amplifies local concentration of lysine 63-linked ubiquitin conjugates to the threshold required for retention of 53BP1 and BRCA1. Thus, RNF168 defines a new pathway involving sequential ubiquitylations on damaged chromosomes and uncovers a functional cooperation between E3 ligases in genome maintenance.
A new model for asymmetric spindle positioning in mouse oocytes.
Schuh, M. & Ellenberg, J.
Curr Biol. 2008 Dec 23;18(24):1986-92. Epub 2008 Dec 8.
An oocyte matures into an egg by extruding half of the chromosomes in a small polar body. This extremely asymmetric division enables the oocyte to retain sufficient storage material for the development of the embryo after fertilization. To divide asymmetrically, mammalian oocytes relocate the spindle from their center to the cortex. In all mammalian species analyzed so far, including human, mouse, cow, pig, and hamster, spindle relocation depends on filamentous actin (F-actin). However, even though spindle relocation is essential for fertility, the involved F-actin structures and the mechanism by which they relocate the spindle are unknown. Here we show in live mouse oocytes that spindle relocation requires a continuously reorganizing cytoplasmic actin network nucleated by Formin-2 (Fmn2). We found that the spindle poles were enriched in activated myosin and pulled on this network. Inhibition of myosin activation by myosin light chain kinase (MLCK) stopped pulling and spindle relocation, indicating that myosin pulling creates the force that drives spindle movement. Based on these results, we propose the first mechanistic model for asymmetric spindle positioning in mammalian oocytes and validate five of its key predictions experimentally.
Sun1 forms immobile macromolecular assemblies at the nuclear envelope.
Lu, W., Gotzmann, J., Sironi, L., Jaeger, V.M., Schneider, M., Luke, Y., Uhlen, M., Szigyarto, C.A., Brachner, A., Ellenberg, J., Foisner, R., Noegel, A.A. & Karakesisoglou, I.
Biochim Biophys Acta. 2008 Dec;1783(12):2415-26. Epub 2008 Sep 19.
SUN-domain proteins form a novel and conserved family of inner nuclear membrane (INM) proteins, which establish physical connections between the nucleoplasm and the cytoskeleton. In the current study, we provide evidence that within the nuclear envelope (NE) Sun1 proteins form highly immobile oligomeric complexes in interphase cells. By performing inverse fluorescence recovery after photobleaching analysis, we demonstrate in vivo that both perinuclear and nucleoplasmic Sun1 segments are essential for maintenance of Sun1 immobility at the NE. Our data in particular underline the self-association properties of the C-terminal coiled-coil Sun1 segment, the ability of which to form dimers and tetramers is demonstrated. Furthermore, the Sun1 tertiary structure involves interchain disulfide bonds that might contribute to higher homo-oligomer formation, although the overall dynamics of the Sun1 C-terminus remains unaffected when the cysteins involved are mutated. While a major Sun1 pool colocalizes with nuclear pore complex proteins, a large fraction of the Sun1 protein assemblies colocalize with immunoreactive foci of Sun2, another SUN-domain paralogue at the NE. We demonstrate that the Sun1 coiled-coil domain permits these heterophilic associations with Sun2. Sun1 therefore provides a non-dynamic platform for the formation of different macromolecular assemblies at the INM. Our data support a model in which SUN-protein-containing multi-variate complexes may provide versatile outer nuclear membrane attachment sites for cytoskeletal filaments.
Work flow for multiplexing siRNA assays by solid-phase reverse transfection in multiwell plates.
Erfle, H., Neumann, B., Rogers, P., Bulkescher, J., Ellenberg, J. & Pepperkok, R.
J Biomol Screen. 2008 Aug;13(7):575-80. Epub 2008 Jul 3.
Solid-phase reverse transfection on cell microarrays is a high-throughput method for the parallel transfection of mammalian cells. However, the cells transfected in this way have been restricted so far to microscopy-based analyses. Analysis methods such as reverse transcriptase-polymerase chain reaction (RT-PCR) and access to higher cell numbers for statistical reasons in microscopy-based assays are not possible with solid-phase reverse transfection on cell microarrays. We have developed a quick and reliable protocol for automated solid-phase reverse transfection of human cells with siRNAs in multiwell plates complementing solid-phase reverse transfection on cell microarrays. The method retains all advantages of solid-phase reverse transfection such as long-term storage capacity after fabrication, reduced cytotoxicity, and reduced cost per screen compared with liquid-phase transfection in multiwell plates. The protocol has been tested for the RNAi-mediated knockdown of several genes in different cell lines including U20S, RPE1, A549, and HeLa cells. We show that even 3 months after production of the "ready to transfect" multiwell plates, there is no reduction in their transfection efficiency as assessed by RT-PCR and nuclear phenotyping by fluorescence microscopy. We conclude that solid-phase reverse transfection in multiwell plates is a cost-efficient and flexible tool for multiplexing cellular assays.
Nuclear pore complex assembly through the cell cycle: regulation and membrane organization.
Antonin, W., Ellenberg, J. & Dultz, E.
FEBS Lett. 2008 Jun 18;582(14):2004-16. Epub 2008 Mar 6.
In eukaryotes, all macromolecules traffic between the nucleus and the cytoplasm through nuclear pore complexes (NPCs), which are among the largest supramolecular assemblies in cells. Although their composition in yeast and metazoa is well characterized, understanding how NPCs are assembled and form the pore through the double membrane of the nuclear envelope and how both processes are controlled still remains a challenge. Here, we summarize what is known about the biogenesis of NPCs throughout the cell cycle with special focus on the membrane reorganization and the regulation that go along with NPC assembly.
EML3 is a nuclear microtubule-binding protein required for the correct alignment of chromosomes in metaphase.
Tegha-Dunghu, J., Neumann, B., Reber, S., Krause, R., Erfle, H., Walter, T., Held, M., Rogers, P., Hupfeld, K., Ruppert, T., Ellenberg, J. & Gruss, O.J.
J Cell Sci. 2008 May 15;121(Pt 10):1718-26. Epub 2008 Apr 29.
Assembly of the mitotic spindle requires a global change in the activity and constitution of the microtubule-binding-protein array at mitotic onset. An important subset of mitotic microtubule-binding proteins localises to the nucleus in interphase and essentially contributes to spindle formation and function after nuclear envelope breakdown. Here, we used a proteomic approach to selectively identify proteins of this category and revealed 50 poorly characterised human gene products, among them the echinoderm microtubule-associated-protein-like gene product, EML3. Indirect immunofluorescence showed that EML3 colocalises with spindle microtubules throughout all mitotic stages. In interphase, EML3 colocalised with cytoplasmic microtubules and accumulated in interphase nuclei. Using YFP-fusion constructs of EML3, we located a nuclear localisation signal and confirmed the microtubule-binding domain of EML3. Functional analysis of EML3 using time-lapse fluorescence microscopy and detailed end-point analysis of phenotypes after siRNA knockdown demonstrates an important role for EML3 in correct metaphase chromosome alignment. Our proteomic identification screen combined with sensitive phenotypic analysis therefore provides a reliable platform for the identification and characterisation of proteins important for correct cell division.
Systematic kinetic analysis of mitotic dis- and reassembly of the nuclear pore in living cells.
Dultz, E., Zanin, E., Wurzenberger, C., Braun, M., Rabut, G., Sironi, L. & Ellenberg, J.
J Cell Biol. 2008 Mar 10;180(5):857-65. Epub 2008 Mar 3.
During mitosis in higher eukaryotes, nuclear pore complexes (NPCs) disassemble in prophase and are rebuilt in anaphase and telophase. NPC formation is hypothesized to occur by the interaction of mitotically stable subcomplexes that form defined structural intermediates. To determine the sequence of events that lead to breakdown and reformation of functional NPCs during mitosis, we present here our quantitative assay based on confocal time-lapse microscopy of single dividing cells. We use this assay to systematically investigate the kinetics of dis- and reassembly for eight nucleoporin subcomplexes relative to nuclear transport in NRK cells, linking the assembly state of the NPC with its function. Our data establish that NPC assembly is an ordered stepwise process that leads to import function already in a partially assembled state. We furthermore find that nucleoporin dissociation does not occur in the reverse order from binding during assembly, which may indicate a distinct mechanism.
Self-organization of MTOCs replaces centrosome function during acentrosomal spindle assembly in live mouse oocytes.
Schuh, M. & Ellenberg, J.
Cell. 2007 Aug 10;130(3):484-98.
Chromosome segregation in mammalian oocytes is driven by a microtubule spindle lacking centrosomes. Here, we analyze centrosome-independent spindle assembly by quantitative high-resolution confocal imaging in live maturing mouse oocytes. We show that spindle assembly proceeds by the self-organization of over 80 microtubule organizing centers (MTOCs) that form de novo from a cytoplasmic microtubule network in prophase and that functionally replace centrosomes. Initially distributed throughout the ooplasm, MTOCs congress at the center of the oocyte, where they contribute to a massive, Ran-dependent increase of the number of microtubules after nuclear envelope breakdown and to the individualization of clustered chromosomes. Through progressive MTOC clustering and activation of kinesin-5, the multipolar MTOC aggregate self-organizes into a bipolar intermediate, which then elongates and thereby establishes chromosome biorientation. Finally, a stable barrel-shaped acentrosomal metaphase spindle with oscillating chromosomes and astral-like microtubules forms that surprisingly exhibits key properties of a centrosomal spindle.
lambda(N)-GFP: an RNA reporter system for live-cell imaging.
Daigle, N. & Ellenberg, J.
Nat Methods. 2007 Aug;4(8):633-6. Epub 2007 Jul 1.
We describe a GFP-based RNA reporter system (lambda(N)-GFP) to visualize RNA molecules in live mammalian cells. It consists of GFP fused to an arginine-rich peptide derived from the phage lambda N protein, lambda(N22), which binds a unique minimal RNA motif and can be used to tag any RNA molecule. lambda(N)-GFP uses a small and easy to engineer RNA tag, reducing the likelihood of perturbing the function of the tagged RNA molecule.
Maximal chromosome compaction occurs by axial shortening in anaphase and depends on Aurora kinase.
Mora-Bermudez, F., Gerlich, D. & Ellenberg, J.
Nat Cell Biol. 2007 Jul;9(7):822-31. Epub 2007 Jun 10.
Eukaryotic cells must first compact their chromosomes before faithfully segregating them during cell division. Failure to do so can lead to segregation defects with pathological consequences, such as aneuploidy and cancer. Duplicated interphase chromosomes are, therefore, reorganized into tight rods before being separated and directed to the newly forming daughter cells. This vital reorganization of chromatin remains poorly understood. To address the dynamics of mitotic condensation of single chromosomes in intact cells, we developed quantitative assays based on confocal time-lapse microscopy of live mammalian cells stably expressing fluorescently tagged core histones. Surprisingly, maximal compaction was not reached in metaphase, but in late anaphase, after sister chromatid segregation. We show that anaphase compaction proceeds by a mechanism of axial shortening of the chromatid arms from telomere to centromere. Chromatid axial shortening was not affected in condensin-depleted cells, but depended instead on dynamic microtubules and Aurora kinase. Acute perturbation of this compaction resulted in failure to rescue segregation defects and in multilobed daughter nuclei, suggesting functions in chromosome segregation and nuclear architecture.
An RNAi screening platform to identify secretion machinery in mammalian cells.
Simpson, J.C., Cetin, C., Erfle, H., Joggerst, B., Liebel, U., Ellenberg, J. & Pepperkok, R.
J Biotechnol. 2007 Apr 30;129(2):352-65. Epub 2007 Jan 14.
Integrative approaches to study protein function in a cellular context are a vital aspect of understanding human disease. Genome sequencing projects provide the basic catalogue of information with which to unravel gene function, but more systematic applications of this resource are now necessary. Here, we describe and test a platform with which it is possible to rapidly use RNA interference in cultured mammalian cells to probe for proteins involved in constitutive protein secretion. Synthetic small interfering RNA molecules are arrayed in chambered slides, then incubated with cells and an assay for secretion performed. Automated microscopy is used to acquire images from the experiments, and automated single-cell analysis rapidly provides reliable quantitative data. In test arrays of 92 siRNA spots targeting 37 prospective membrane traffic proteins, our approach identifies 7 of these as being important for the correct delivery of a secretion marker to the cell surface. Correlating these findings with other screens and bioinformatic information makes these candidates highly likely to be novel membrane traffic machinery components.
Structure and nuclear import function of the C-terminal domain of influenza virus polymerase PB2 subunit.
Tarendeau, F., Boudet, J., Guilligay, D., Mas, P.J., Bougault, C.M., Boulo, S., Baudin, F., Ruigrok, R.W., Daigle, N., Ellenberg, J., Cusack, S., Simorre, J.P. & Hart, D.J.
Nat Struct Mol Biol. 2007 Mar;14(3):229-33. Epub 2007 Feb 25.
The trimeric influenza virus polymerase, comprising subunits PA, PB1 and PB2, is responsible for transcription and replication of the segmented viral RNA genome. Using a novel library-based screening technique called expression of soluble proteins by random incremental truncation (ESPRIT), we identified an independently folded C-terminal domain from PB2 and determined its solution structure by NMR. Using green fluorescent protein fusions, we show that both the domain and the full-length PB2 subunit are efficiently imported into the nucleus dependent on a previously overlooked bipartite nuclear localization sequence (NLS). The crystal structure of the domain complexed with human importin alpha5 shows how the last 20 residues unfold to permit binding to the import factor. The domain contains three surface residues implicated in adaptation from avian to mammalian hosts. One of these tethers the NLS-containing peptide to the core of the domain in the unbound state.
Measuring structural dynamics of chromosomes in living cells by fluorescence microscopy.
Mora-Bermudez, F. & Ellenberg, J.
Methods. 2007 Feb;41(2):158-67.
Mitotic and meiotic chromosomes are the compact packages that faithfully transport the genetic and epigenetic information to the following cell generations. How chromatin dynamically cycles between the decompacted interphase state that supports transcription and replication and the compacted state required for chromosome segregation is not understood. To address this long-standing problem, the structure of chromatin should ideally be studied in the physiological context of intact cells and organisms. We discuss here, the contributions that live-cell imaging can and has made to the study of mitotic chromosome compaction and highlight the power and limitations of this approach. We review methodologies used and suggest that combinatorial approaches and developing new imaging technologies will be key to shedding light on this long-standing question in cell biology.
Reverse transfection on cell arrays for high content screening microscopy.
Erfle, H., Neumann, B., Liebel, U., Rogers, P., Held, M., Walter, T., Ellenberg, J. & Pepperkok, R.
Nat Protoc. 2007;2(2):392-9.
Here, we describe a robust protocol for the reverse transfection of cells on small interfering (siRNA) arrays, which, in combination with multi-channel immunofluorescence or time-lapse microscopy, is suitable for genome-wide RNA interference (RNAi) screens in intact human cells. The automatic production of 48 'transfection ready' siRNA arrays, each containing 384 samples, takes in total 7 h. Pre-fabricated siRNA arrays can be used without loss of transfection efficiency at least up to 15 months after printing. Different human cell lines that have been successfully transfected using the protocol are presented here. The present protocol has been applied to two genome-wide siRNA screens addressing mitosis and constitutive protein secretion.
Minimizing the risk of reporting false positives in large-scale RNAi screens.
Echeverri, C.J., Beachy, P.A., Baum, B., Boutros, M., Buchholz, F., Chanda, S.K., Downward, J., Ellenberg, J., Fraser, A.G., Hacohen, N., Hahn, W.C., Jackson, A.L., Kiger, A., Linsley, P.S., Lum, L., Ma, Y., Mathey-Prevot, B., Root, D.E., Sabatini, D.M., Taipale, J., Perrimon, N. & Bernards, R.
Nat Methods. 2006 Oct;3(10):777-9.
Large-scale RNA interference (RNAi)-based analyses, very much as other 'omic' approaches, have inherent rates of false positives and negatives. The variability in the standards of care applied to validate results from these studies, if left unchecked, could eventually begin to undermine the credibility of RNAi as a powerful functional approach. This Commentary is an invitation to an open discussion started among various users of RNAi to set forth accepted standards that would insure the quality and accuracy of information in the large datasets coming out of genome-scale screens.
High-throughput fluorescence microscopy for systems biology.
Pepperkok, R. & Ellenberg, J.
Nat Rev Mol Cell Biol. 2006 Sep;7(9):690-6. Epub 2006 Jul 19.
In this post-genomic era, we need to define gene function on a genome-wide scale for model organisms and humans. The fundamental unit of biological processes is the cell. Among the most powerful tools to assay such processes in the physiological context of intact living cells are fluorescence microscopy and related imaging techniques. To enable these techniques to be applied to functional genomics experiments, fluorescence microscopy is making the transition to a quantitative and high-throughput technology.
Live-cell imaging reveals a stable cohesin-chromatin interaction after but not before DNA replication.
Gerlich, D., Koch, B., Dupeux, F., Peters, J.M. & Ellenberg, J.
Curr Biol. 2006 Aug 8;16(15):1571-8.
Cohesin is a multisubunit protein complex that links sister chromatids from replication until segregation. The lack of obvious cohesin-targeting-specific sequences on DNA, as well as cohesin's molecular arrangement as a large ring, has led to the working hypothesis that cohesin acts as a direct topological linker. To preserve the identity of sister chromatids, such a linkage would need to stably persist throughout the entire S and G2 phases of the cell cycle. Unexpectedly, cohesin binds chromatin already in telophase, and a large fraction dissociates from chromosomes during prophase in a phosphorylation-dependent manner, whereas initiation of anaphase requires proteolytic cleavage of only a small fraction of cohesin. These observations raised the question of how and when cohesin interacts with chromatin during the cell cycle. Here, we report a cell-cycle dependence in the stability of cohesin binding to chromatin. Using photobleaching and quantitative live-cell imaging, we identified several cohesin pools with different chromatin binding stabilities. Although all chromatin bound cohesin dissociated after a mean residence time of less than 25 min before replication, about one-third of cohesin was bound much more stably after S phase and persisted until metaphase, consistent with long-lived links mediating cohesion between sister chromatids.
Fluorophores for live cell imaging of AGT fusion proteins across the visible spectrum.
Keppler, A., Arrivoli, C., Sironi, L. & Ellenberg, J.
Biotechniques. 2006 Aug;41(2):167-70, 172, 174-5.
O6-alkylguanine-DNA alkyltransferase (AGT) fusion proteins can be specifically and covalently labeled with fluorescent O6-benzylguanine (O6-BG) derivatives for multicolor live cell imaging approaches. Here, we characterize several new BG fluorophores suitable for in vivo AGT labeling that display fluorescence emission maxima covering the visible spectrum from 472 to 673 nm, thereby extending the spectral limits set by fluorescent proteins. We show that the photostability of the cell-permeable dyes BG Rhodamine Green (BG505) and CP tetramethylrhodamine (CP-TMR) is in the range of enhanced green fluorescent protein (EGFP) and monomeric red fluorescent protein (mRFP), and that BG diethylaminomethyl coumarin (BGDEAC), a derivative of coumarin, is even more stable than enhanced cyan fluorescent protein (ECFP). Due to the increasing number of new BG derivatives with interesting fluorescence properties, such as far-red emission, fluorescence labeling of AGT fusion proteins is becoming a versatile alternative to existing live cell imaging approaches.
Resolution of chiasmata in oocytes requires separase-mediated proteolysis.
Kudo, N.R., Wassmann, K., Anger, M., Schuh, M., Wirth, K.G., Xu, H., Helmhart, W., Kudo, H., McKay, M., Maro, B., Ellenberg, J., de Boer, P. & Nasmyth, K.
Cell. 2006 Jul 14;126(1):135-46.
In yeast, resolution of chiasmata in meiosis I requires proteolytic cleavage along chromosome arms of cohesin's Rec8 subunit by separase. Since activation of separase by the anaphase-promoting complex (APC/C) is supposedly not required for meiosis I in Xenopus oocytes, it has been suggested that animal cells might resolve chiasmata by a separase-independent mechanism related to the so-called "prophase pathway" that removes cohesin from chromosome arms during mitosis. By expressing Cre recombinase from a zona pellucida promoter, we have deleted a floxed allele of separase specifically in mouse oocytes. This prevents removal of Rec8 from chromosome arms and resolution of chiasmata. It also hinders extrusion of the first polar body (PBE) and causes female sterility. mRNA encoding wild-type but not catalytically inactive separase restores chiasma resolution. Both types of mRNA restore PBE. Proteolytic activity of separase is therefore essential for Rec8's removal from chromosome arms and for chiasma resolution but not for PBE.
NuSAP, a mitotic RanGTP target that stabilizes and cross-links microtubules.
Ribbeck, K., Groen, A.C., Santarella, R., Bohnsack, M.T., Raemaekers, T., Kocher, T., Gentzel, M., Gorlich, D., Wilm, M., Carmeliet, G., Mitchison, T.J., Ellenberg, J., Hoenger, A. & Mattaj, I.W.
Mol Biol Cell. 2006 Jun;17(6):2646-60. Epub 2006 Mar 29.
Nucleolar and spindle-associated protein (NuSAP) was recently identified as a microtubule- and chromatin-binding protein in vertebrates that is nuclear during interphase. Small interfering RNA-mediated depletion of NuSAP resulted in aberrant spindle formation, missegregation of chromosomes, and ultimately blocked cell proliferation. We show here that NuSAP is enriched on chromatin-proximal microtubules at meiotic spindles in Xenopus oocytes. When added at higher than physiological levels to Xenopus egg extract, NuSAP induces extensive bundling of spindle microtubules and causes bundled microtubules within spindle-like structures to become longer. In vitro reconstitution experiments reveal two direct effects of NuSAP on microtubules: first, it can efficiently stabilize microtubules against depolymerization, and second, it can cross-link large numbers of microtubules into aster-like structures, thick fibers, and networks. With defined components we show that the activity of NuSAP is differentially regulated by Importin (Imp) alpha, Impbeta, and Imp7. While Impalpha and Imp7 appear to block the microtubule-stabilizing activity of NuSAP, Impbeta specifically suppresses aspects of the cross-linking activity of NuSAP. We propose that to achieve full NuSAP functionality at the spindle, all three importins must be dissociated by RanGTP. Once activated, NuSAP may aid to maintain spindle integrity by stabilizing and cross-linking microtubules around chromatin.
High-throughput RNAi screening by time-lapse imaging of live human cells.
Neumann, B., Held, M., Liebel, U., Erfle, H., Rogers, P., Pepperkok, R. & Ellenberg, J.
Nat Methods. 2006 May;3(5):385-90.
RNA interference (RNAi) is a powerful tool to study gene function in cultured cells. Transfected cell microarrays in principle allow high-throughput phenotypic analysis after gene knockdown by microscopy. But bottlenecks in imaging and data analysis have limited such high-content screens to endpoint assays in fixed cells and determination of global parameters such as viability. Here we have overcome these limitations and developed an automated platform for high-content RNAi screening by time-lapse fluorescence microscopy of live HeLa cells expressing histone-GFP to report on chromosome segregation and structure. We automated all steps, including printing transfection-ready small interfering RNA (siRNA) microarrays, fluorescence imaging and computational phenotyping of digital images, in a high-throughput workflow. We validated this method in a pilot screen assaying cell division and delivered a sensitive, time-resolved phenoprint for each of the 49 endogenous genes we suppressed. This modular platform is scalable and makes the power of time-lapse microscopy available for genome-wide RNAi screens.
Nuclear actin: a lack of export allows formation of filaments.
Schuh, M. & Ellenberg, J.
Curr Biol. 2006 May 9;16(9):R321-3.
Actin has been found in nuclei of many cell types, but little is known about its form and function. A recent study has shown that a lack of specific export allows actin to accumulate in the nucleus, where it forms a network of actin filaments that may be required to stabilize the giant nucleus of the Xenopus oocyte.
Dissecting the contribution of diffusion and interactions to the mobility of nuclear proteins.
Beaudouin, J., Mora-Bermudez, F., Klee, T., Daigle, N. & Ellenberg, J.
Biophys J. 2006 Mar 15;90(6):1878-94. Epub 2005 Dec 30.
Quantitative characterization of protein interactions under physiological conditions is vital for systems biology. Fluorescence photobleaching/activation experiments of GFP-tagged proteins are frequently used for this purpose, but robust analysis methods to extract physicochemical parameters from such data are lacking. Here, we implemented a reaction-diffusion model to determine the contributions of protein interaction and diffusion on fluorescence redistribution. The model was validated and applied to five chromatin-interacting proteins probed by photoactivation in living cells. We found that very transient interactions are common for chromatin proteins. Their observed mobility was limited by the amount of free protein available for diffusion but not by the short residence time of the bound proteins. Individual proteins thus locally scan chromatin for binding sites, rather than diffusing globally before rebinding at random nuclear positions. By taking the real cellular geometry and the inhomogeneous distribution of binding sites into account, our model provides a general framework to analyze the mobility of fluorescently tagged factors. Furthermore, it defines the experimental limitations of fluorescence perturbation experiments and highlights the need for complementary methods to measure transient biochemical interactions in living cells.
Condensin I stabilizes chromosomes mechanically through a dynamic interaction in live cells.
Gerlich, D., Hirota, T., Koch, B., Peters, J.M. & Ellenberg, J.
Curr Biol. 2006 Feb 21;16(4):333-44.
BACKGROUND: Restructuring chromatin into morphologically distinct chromosomes is essential for cell division, but the molecular mechanisms underlying this process are poorly understood. Condensin complexes have been proposed as key factors, although controversial conclusions about their contribution to chromosome structure were reached by different experimental approaches in fixed cells or cell extracts. Their function under physiological conditions still needs to be defined. RESULTS: Here, we investigated the specific functions of condensin I and II in live cells by fluorescence microscopy and RNAi depletion. Photobleaching and quantitative time-lapse imaging showed that GFP-tagged condensin II bound stably to chromosomes throughout mitosis. By contrast, the canonical condensin I interacted dynamically with chromatin after completion of prophase compaction, reaching steady-state levels on chromosomes before congression. In condensin I-depleted cells, compaction was normal, but chromosomes were mechanically labile and unable to withstand spindle forces during alignment. However, normal levels of condensin II were not required for chromosome stability. CONCLUSIONS: We conclude that while condensin I seems dispensable for normal chromosome compaction, its dynamic binding after nuclear envelope breakdown locks already condensed chromatin in a rigid state required for mechanically stable spindle attachment.
Monitoring the permeability of the nuclear envelope during the cell cycle.
Lenart, P. & Ellenberg, J.
Methods 2006 Jan;38(1):17-24.
In animal organisms the nuclear envelope (NE) disassembles during cell division resulting in complete intermixing of cytoplasmic and nuclear compartments. This leads to the activation of many mitotic enzymes, which were kept away from their substrates or regulators by nuclear or cytoplasmic sequestration in interphase. Nuclear envelope breakdown (NEBD) is thus an essential step of mitotic entry and commits a cell to M-phase. NEBD begins with the partial disassembly of nuclear pore complexes, leading to a limited permeabilization of the NE for molecules up to approximately 40nm diameter. This is followed by the complete disruption of nuclear pores, which causes local fenestration of the double nuclear membrane and subsequently breakdown of the entire NE structure. Here, we describe the use of different sized inert fluorescent tracer molecules to directly visualize these different steps of NEBD in live cells by fluorescence microscopy.
A contractile nuclear actin network drives chromosome congression in oocytes.
Lenart, P., Bacher, C.P., Daigle, N., Hand, A.R., Eils, R., Terasaki, M. & Ellenberg, J.
Nature 2005 Aug 11;436(7052):812-8. Epub 2005 Jul 13.
Chromosome capture by microtubules is widely accepted as the universal mechanism of spindle assembly in dividing cells. However, the observed length of spindle microtubules and computer simulations of spindle assembly predict that chromosome capture is efficient in small cells, but may fail in cells with large nuclear volumes such as animal oocytes. Here we investigate chromosome congression during the first meiotic division in starfish oocytes. We show that microtubules are not sufficient for capturing chromosomes. Instead, chromosome congression requires actin polymerization. After nuclear envelope breakdown, we observe the formation of a filamentous actin mesh in the nuclear region, and find that contraction of this network delivers chromosomes to the microtubule spindle. We show that this mechanism is essential for preventing chromosome loss and aneuploidy of the egg--a leading cause of pregnancy loss and birth defects in humans.
FRET analyses of the U2AF complex localize the U2AF35/U2AF65 interaction in vivo and reveal a novel self-interaction of U2AF35.
Chusainow, J., Ajuh, P.M., Trinkle-Mulcahy, L., Sleeman, J.E., Ellenberg, J. & Lamond, A.I.
RNA 2005 Aug;11(8):1201-14.
We have analyzed the interaction between the U2AF subunits U2AF35 and U2AF65 in vivo using fluorescence resonance energy transfer (FRET) microscopy. U2 snRNP Auxiliary Factor (U2AF) is an essential pre-mRNA splicing factor complex, comprising 35-kDa (U2AF35) and 65-kDa (U2AF65) subunits. U2AF65 interacts directly with the polypyrimidine tract and promotes binding of U2 snRNP to the pre-mRNA branchpoint, while U2AF35 associates with the conserved AG dinucleotide at the 3' end of the intron and has multiple functions in the splicing process. Using two different approaches for measuring FRET, we have identified and spatially localized sites of direct interaction between U2AF35 and U2AF65 in vivo in live cell nuclei. While U2AF is thought to function as a heterodimeric complex, the FRET data have also revealed a novel U2AF35 self-interaction in vivo, which is confirmed in vitro using biochemical assays. These results suggest that the stoichiometry of the U2AF complex may, at least in part, differ in vivo from the expected heterodimeric complex. The data show that FRET studies offer a valuable approach for probing interactions between pre-mRNA splicing factors in vivo.
Photobleaching techniques to study mobility and molecular dynamics of proteins in live cells: FRAP, iFRAP and FLIP.
Rabut, G. & Ellenberg, J.
In "Live Cell Imaging: A Laboratory Manual Vol. 1", Goldman, R.D. and Spector, D.L., (Eds.), 101-127. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, 2005
Distinct functions of condensin I and II in mitotic chromosome assembly.
Hirota, T., Gerlich, D., Koch, B., Ellenberg, J. & Peters, J.M.
J Cell Sci 2004 Dec 15;117(Pt 26):6435-45. Epub 2004 Nov 30.
Condensin is a protein complex associated with mitotic chromosomes that has been implicated in chromosome condensation. In vertebrates, two types of condensin complexes have recently been identified, called condensin I and II. Here, we show that in mammalian cells condensin II associates with chromatin in prophase, in contrast to condensin I which is cytoplasmic and can thus interact with chromosomes only after nuclear envelope breakdown. RNA interference experiments in conjunction with imaging of live and fixed cells revealed that condensin II is required for chromosome condensation in early prophase, whereas condensin I appears to be dispensable at this stage. By contrast, condensin I is required for the complete dissociation of cohesin from chromosome arms, for chromosome shortening and for normal timing of progression through prometaphase and metaphase, whereas normal condensin II levels are dispensable for these processes. After depletion of both condensin complexes, the onset of chromosome condensation is delayed until the end of prophase, but is then initiated rapidly before nuclear envelope breakdown. These results reveal that condensin II and I associate with chromosomes sequentially and have distinct functions in mitotic chromosome assembly.
LAP2alpha and BAF transiently localize to telomeres and specific regions on chromatin during nuclear assembly.
Dechat, T., Gajewski, A., Korbei, B., Gerlich, D., Daigle, N., Haraguchi, T., Furukawa, K., Ellenberg, J. & Foisner, R.
J Cell Sci 2004 Dec 1;117(Pt 25):6117-28. Epub 2004 Nov 16.
Lamina-associated polypeptide (LAP) 2alpha is a LEM (lamina-associated polypeptide emerin MAN1) family protein associated with nucleoplasmic A-type lamins and chromatin. Using live cell imaging and fluorescence microscopy we demonstrate that LAP2alpha was mostly cytoplasmic in metaphase and associated with telomeres in anaphase. Telomeric LAP2alpha clusters grew in size, formed 'core' structures on chromatin adjacent to the spindle in telophase, and translocated to the nucleoplasm in G1 phase. A subfraction of lamin C and emerin followed LAP2alpha to the core region early on, whereas LAP2beta, lamin B receptor and lamin B initially bound to more peripheral regions of chromatin, before they spread to core structures with different kinetics. Furthermore, the DNA-crosslinking protein barrier-to-autointegration factor (BAF) bound to LAP2alpha in vitro and in mitotic extracts, and subfractions of BAF relocalized to core structures with LAP2alpha. We propose that LAP2alpha and a subfraction of BAF form defined complexes in chromatin core regions and may be involved in chromatin reorganization during early stages of nuclear assembly.
Mapping the dynamic organization of the nuclear pore complex inside single living cells.
Rabut, G., Doye, V. & Ellenberg, J.
Nat Cell Biol 2004 Nov;6(11):1114-21. Epub 2004 Oct 24.
Most cellular activities are executed by multi-protein complexes that form the basic functional modules of their molecular machinery. Proteomic approaches can provide an evermore detailed picture of their composition, but do not reveal how these machines are organized dynamically to accomplish their biological function. Here, we present a method to determine the dissociation rates of protein subunits from complexes that have a traceable localization inside single living cells. As a case study, we systematically analysed the dynamic organization of vertebrate nuclear pore complexes (NPCs), large supramolecular complexes of about 30 different polypeptides. NPC components exhibited a wide range of residence times covering five orders of magnitude from seconds to days. We found the central parts of the NPC to be very stable, consistent with a function as a structural scaffold, whereas more peripheral components exhibited more dynamic behaviour, suggesting adaptor as well as regulatory functions. The presented strategy can be applied to many multi-protein complexes and will help to characterize the dynamic behaviour of complex networks of proteins in live cells.
Automatic real-time three-dimensional cell tracking by fluorescence microscopy.
Rabut, G. & Ellenberg, J.
J Microsc 2004 Nov;216(Pt 2):131-7.
Live cell imaging has become an indispensable technique for cell biologists. However, when grown on coverslip glass used for live cell imaging many cultured cells move even during relatively short observation times and focus can drift as a result of mechanical instabilities and/or temperature fluctuations. Time-lapse imaging therefore requires constant adjustment of the imaging field and focus position to keep the cell of interest centred in the imaged volume. We show here that this limitation can be overcome by tracking cells in a fully automated way using the mass centre of cellular fluorescence. Combined with automated multiple location revisiting, this method dramatically increases the throughput of high-resolution live cell imaging experiments.
Roles of polo-like kinase 1 in the assembly of functional mitotic spindles.
Sumara, I., Gimenez-Abian, J.F., Gerlich, D., Hirota, T., Kraft, C., de la Torre, C., Ellenberg, J. & Peters, J.M.
Curr Biol 2004 Oct 5;14(19):1712-22.
BACKGROUND: The stable association of chromosomes with both poles of the mitotic spindle (biorientation) depends on spindle pulling forces. These forces create tension across sister kinetochores and are thought to stabilize microtubule-kinetochore interactions and to silence the spindle checkpoint. Polo-like kinase 1 (Plk1) has been implicated in regulating centrosome maturation, mitotic entry, sister chromatid cohesion, the anaphase-promoting complex/cyclosome (APC/C), and cytokinesis, but it is unknown if Plk1 controls chromosome biorientation. RESULTS: We have analyzed Plk1 functions in synchronized mammalian cells by RNA interference (RNAi). Plk1-depleted cells enter mitosis after a short delay, accumulate in a preanaphase state, and subsequently often die by apoptosis. Spindles in Plk1-depleted cells lack focused poles and are not associated with centrosomes. Chromosomes attach to these spindles, but the checkpoint proteins Mad2, BubR1, and CENP-E are enriched at many kinetochores. When Plk1-depleted cells are treated with the Aurora B inhibitor Hesperadin, which silences the spindle checkpoint by stabilizing microtubule-kinetochore interactions, cells degrade APC/C substrates and exit mitosis without chromosome segregation and cytokinesis. Experiments with monopolar spindles that are induced by the kinesin inhibitor Monastrol indicate that Plk1 is required for the assembly of spindles that are able to generate poleward pulling forces. CONCLUSIONS: Our results imply that Plk1 is not essential for mitotic entry and APC/C activation but is required for proper spindle assembly and function. In Plk1-depleted cells spindles may not be able to create enough tension across sister kinetochores to stabilize microtubule-kinetochore interactions and to silence the spindle checkpoint.
Hypophosphorylated SR splicing factors transiently localize around active nucleolar organizing regions in telophase daughter nuclei.
Bubulya, P.A., Prasanth, K.V., Deerinck, T.J., Gerlich, D., Beaudouin, J., Ellisman, M.H., Ellenberg, J. & Spector, D.L.
J Cell Biol 2004 Oct 11;167(1):51-63.
Upon completion of mitosis, daughter nuclei assemble all of the organelles necessary for the implementation of nuclear functions. We found that upon entry into daughter nuclei, snRNPs and SR proteins do not immediately colocalize in nuclear speckles. SR proteins accumulated in patches around active nucleolar organizing regions (NORs) that we refer to as NOR-associated patches (NAPs), whereas snRNPs were enriched at other nuclear regions. NAPs formed transiently, persisting for 15-20 min before dissipating as nuclear speckles began to form in G1. In the absence of RNA polymerase II transcription, NAPs increased in size and persisted for at least 2 h, with delayed localization of SR proteins to nuclear speckles. In addition, SR proteins in NAPs are hypophosphorylated, and the SR protein kinase Clk/STY colocalizes with SR proteins in NAPs, suggesting that phosphorylation releases SR proteins from NAPs and their initial target is transcription sites. This work demonstrates a previously unrecognized role of NAPs in splicing factor trafficking and nuclear speckle biogenesis.
Quantitative kinetic analysis of nucleolar breakdown and reassembly during mitosis in live human cells.
Leung, A.K., Gerlich, D., Miller, G., Lyon, C., Lam, Y.W., Lleres, D., Daigle, N., Zomerdijk, J., Ellenberg, J. & Lamond, A.I.
J Cell Biol 2004 Sep 13;166(6):787-800. Epub 2004 Sep 07.
One of the great mysteries of the nucleolus surrounds its disappearance during mitosis and subsequent reassembly at late mitosis. Here, the relative dynamics of nucleolar disassembly and reformation were dissected using quantitative 4D microscopy with fluorescent protein-tagged proteins in human stable cell lines. The data provide a novel insight into the fates of the three distinct nucleolar subcompartments and their associated protein machineries in a single dividing cell. Before the onset of nuclear envelope (NE) breakdown, nucleolar disassembly started with the loss of RNA polymerase I subunits from the fibrillar centers. Dissociation of proteins from the other subcompartments occurred with faster kinetics but commenced later, coincident with the process of NE breakdown. The reformation pathway also follows a reproducible and defined temporal sequence but the order of reassembly is shown not to be dictated by the order in which individual nucleolar components reaccumulate within the nucleus after mitosis.
The entire Nup107-160 complex, including three new members, is targeted as one entity to kinetochores in mitosis.
Loiodice, I., Alves, A., Rabut, G., Van Overbeek, M., Ellenberg, J., Sibarita, J.B. & Doye, V.
Mol Biol Cell. 2004 Jul;15(7):3333-44. Epub 2004 May 14.
In eukaryotes, bidirectional transport of macromolecules between the cytoplasm and the nucleus occurs through elaborate supramolecular structures embedded in the nuclear envelope, the nuclear pore complexes (NPCs). NPCs are composed of multiple copies of approximately 30 different proteins termed nucleoporins, of which several can be biochemically isolated as subcomplexes. One such building block of the NPC, termed the Nup107-160 complex in vertebrates, was so far demonstrated to be composed of six different nucleoporins. Here, we identify three WD (Trp-Asp)-repeat nucleoporins as new members of this complex, two of which, Nup37 and Nup43, are specific to higher eukaryotes. The third new member Seh1 is more loosely associated with the Nup107-160 complex biochemically, but its depletion by RNA interference leads to phenotypes similar to knock down of other constituents of this complex. By combining green fluorescent protein-tagged nucleoporins and specific antibodies, we show that all the constituents of this complex, including Nup37, Nup43, Seh1, and Sec13, are targeted to kinetochores from prophase to anaphase of mitosis. Together, our results indicate that the entire Nup107-160 complex, which comprises nearly one-third of the so-far identified nucleoporins, specifically localizes to kinetochores in mitosis.
Calcium rises locally trigger focal adhesion disassembly and enhance residency of focal adhesion kinase at focal adhesions.
Giannone, G., Ronde, P., Gaire, M., Beaudouin, J., Haiech, J., Ellenberg, J. & Takeda, K.
J Biol Chem 2004 Jul 2;279(27):28715-23. Epub 2004 Apr 21.
Focal adhesion kinase (FAK) activity and Ca(2+) signaling led to a turnover of focal adhesions (FAs) required for cell spreading and migration. We used yellow Cameleon-2 (Ycam), a fluorescent protein-based Ca(2+) sensor fused to FAK or to a FAK-related non-kinase domain, to measure simultaneously local Ca(2+) variations at FA sites and FA dynamics. Discrete subcellular Ca(2+) oscillators initiate both propagating and abortive Ca(2+) waves in migrating U87 astrocytoma cells. Ca(2+)-dependent FA disassembly occurs when the Ca(2+) wave reaches individual FAs, indicating that local but not global Ca(2+) increases trigger FA disassembly. An unexpectedly rapid flux of FAK between cytosolic and FA compartments was revealed by fluorescence recovery after photobleaching studies. The FAK-Ycam recovery half-time (17 s) at FAs was slowed (to 29 s) by Ca(2+) elevation. FAK-related non-kinase domain-Ycam had a faster, Ca(2+)-insensitive recovery half-time (11 s), which is consistent with the effect of Ca(2+) on FAK-Ycam dynamics not being due to a general modification of the dynamics of FA components. Because FAK association at FAs was prolonged by Ca(2+) and FAK autophosphorylation was correlated to intracellular Ca(2+) levels, we propose that local Ca(2+) elevations increase the residency of FAK at FAs, possibly by means of tyrosine phosphorylation of FAK, thereby leading to increased activation of its effectors involved in FA disassembly.
Regulation of sister chromatid cohesion between chromosome arms.
Gimenez-Abian, J.F., Sumara, I., Hirota, T., Hauf, S., Gerlich, D., de la Torre, C., Ellenberg, J. & Peters, J.M.
Curr Biol 2004 Jul 13;14(13):1187-93.
Sister chromatid separation in anaphase depends on the removal of cohesin complexes from chromosomes. In vertebrates, the bulk of cohesin is already removed from chromosome arms during prophase and prometaphase, whereas cohesin remains at centromeres until metaphase, when cohesin is cleaved by the protease separase. In unperturbed mitoses, arm cohesion nevertheless persists throughout metaphase and is principally sufficient to maintain sister chromatid cohesion. How arm cohesion is maintained until metaphase is unknown. Here we show that small amounts of cohesin can be detected in the interchromatid region of metaphase chromosome arms. If prometaphase is prolonged by treatment of cells with microtubule poisons, these cohesin complexes dissociate from chromosome arms, and arm cohesion is dissolved. If cohesin dissociation in prometaphase-arrested cells is prevented by depletion of Plk1 or inhibition of Aurora B, arm cohesion is maintained. These observations imply that, in unperturbed mitoses, small amounts of cohesin maintain arm cohesion until metaphase. When cells lacking Plk1 and Aurora B activity enter anaphase, chromatids lose cohesin. This loss is prevented by proteasome inhibitors, implying that it depends on separase activation. Separase may therefore be able to cleave cohesin at centromeres and on chromosome arms.
Automatic identification of subcellular phenotypes on human cell arrays.
Conrad, C., Erfle, H., Warnat, P., Daigle, N., Lorch, T., Ellenberg, J., Pepperkok, R. & Eils, R.
Genome Res. 2004 Jun;14(6):1130-6.
Light microscopic analysis of cell morphology provides a high-content readout of cell function and protein localization. Cell arrays and microwell transfection assays on cultured cells have made cell phenotype analysis accessible to high-throughput experiments. Both the localization of each protein in the proteome and the effect of RNAi knock-down of individual genes on cell morphology can be assayed by manual inspection of microscopic images. However, the use of morphological readouts for functional genomics requires fast and automatic identification of complex cellular phenotypes. Here, we present a fully automated platform for high-throughput cell phenotype screening combining human live cell arrays, screening microscopy, and machine-learning-based classification methods. Efficiency of this platform is demonstrated by classification of eleven subcellular patterns marked by GFP-tagged proteins. Our classification method can be adapted to virtually any microscopic assay based on cell morphology, opening a wide range of applications including large-scale RNAi screening in human cells.
Dynamics of nuclear pore complex organization through the cell cycle.
Rabut, G., Lenart, P. & Ellenberg, J.
Curr Opin Cell Biol 2004 Jun;16(3):314-21.
In eukaryotic cells, all macromolecules that traffic between the nucleus and the cytoplasm cross the double nuclear membrane through nuclear pore complexes (NPCs). NPCs are elaborate gateways that allow efficient, yet selective, translocation of many different macromolecules. Their protein composition has been elucidated, but how exactly these nucleoporins come together to form the pore is largely unknown. Recent data suggest that NPCs are composed of an extremely stable scaffold on which more dynamic, exchangeable parts are assembled. These could be targets for molecular rearrangements that change nuclear pore transport properties and, ultimately, the state of the cell.
RanBP2/Nup358 Provides a Major Binding Site for NXF1-p15 Dimers at the Nuclear Pore Complex and Functions in Nuclear mRNA Export.
Forler, D., Rabut, G., Ciccarelli, F.D., Herold, A., Kocher, T., Niggeweg, R., Bork, P., Ellenberg, J. & Izaurralde, E.
Mol Cell Biol 2004 Feb;24(3):1155-67.
Metazoan NXF1-p15 heterodimers promote the nuclear export of bulk mRNA across nuclear pore complexes (NPCs). In vitro, NXF1-p15 forms a stable complex with the nucleoporin RanBP2/Nup358, a component of the cytoplasmic filaments of the NPC, suggesting a role for this nucleoporin in mRNA export. We show that depletion of RanBP2 from Drosophila cells inhibits proliferation and mRNA export. Concomitantly, the localization of NXF1 at the NPC is strongly reduced and a significant fraction of this normally nuclear protein is detected in the cytoplasm. Under the same conditions, the steady-state subcellular localization of other nuclear or cytoplasmic proteins and CRM1-mediated protein export are not detectably affected, indicating that the release of NXF1 into the cytoplasm and the inhibition of mRNA export are not due to a general defect in NPC function. The specific role of RanBP2 in the recruitment of NXF1 to the NPC is highlighted by the observation that depletion of CAN/Nup214 also inhibits cell proliferation and mRNA export but does not affect NXF1 localization. Our results indicate that RanBP2 provides a major binding site for NXF1 at the cytoplasmic filaments of the NPC, thereby restricting its diffusion in the cytoplasm after NPC translocation. In RanBP2-depleted cells, NXF1 diffuses freely through the cytoplasm. Consequently, the nuclear levels of the protein decrease and export of bulk mRNA is impaired.
Light microscopy of echinoderm embryos.
Strickland, L., von Dassow, G., Ellenberg, J., Foe, V., Lenart, P. & Burgess, D.
Methods Cell Biol 2004;74:371-409. Europe PMC
Dynamics of chromosome positioning during the cell cycle.
Gerlich, D. & Ellenberg, J.
Curr Opin Cell Biol. 2003 Dec;15(6):664-71.
The arrangement and dynamics of chromosomes inside the nucleus of mammalian cells have been studied intensively over the last two years. Although chromosomes are relatively immobile and occupy non-random positions in interphase, their dynamic movements in mitosis have traditionally been assumed to randomize this arrangement. New methods of live cell imaging now make it possible to follow chromosome movements directly and quantitatively in single cells. Such studies have generated models of chromosome positioning throughout the cell cycle and provide a new basis to address the underlying mechanisms in future experiments.
NuSAP, a novel microtubule-associated protein involved in mitotic spindle organization.
Raemaekers, T., Ribbeck, K., Beaudouin, J., Annaert, W., Van Camp, M., Stockmans, I., Smets, N., Bouillon, R., Ellenberg, J. & Carmeliet, G.
J Cell Biol 2003 Sep 15;162(6):1017-29.
Here, we report on the identification of nucleolar spindle-associated protein (NuSAP), a novel 55-kD vertebrate protein with selective expression in proliferating cells. Its mRNA and protein levels peak at the transition of G2 to mitosis and abruptly decline after cell division. Microscopic analysis of both fixed and live mammalian cells showed that NuSAP is primarily nucleolar in interphase, and localizes prominently to central spindle microtubules during mitosis. Direct interaction of NuSAP with microtubules was demonstrated in vitro. Overexpression of NuSAP caused profound bundling of cytoplasmic microtubules in interphase cells, and this relied on a COOH-terminal microtubule-binding domain. In contrast, depletion of NuSAP by RNA interference resulted in aberrant mitotic spindles, defective chromosome segregation, and cytokinesis. In addition, many NuSAP-depleted interphase cells had deformed nuclei. Both overexpression and knockdown of NuSAP impaired cell proliferation. These results suggest a crucial role for NuSAP in spindle microtubule organization.
4D imaging to assay complex dynamics in live specimens.
Gerlich, D. & Ellenberg, J.
Nat Cell Biol 2003 Sep;Suppl:S14-9.
A full understanding of cellular dynamics is often difficult to obtain from time-lapse microscopy of single optical sections. New microscopes and image-processing software are now making it possible to rapidly record three-dimensional images over time. This four-dimensional imaging allows precise quantitative analysis and enhances visual exploration of data by allowing cellular structures to be interactively displayed from many angles. It has become a key tool for understanding the complex organization of biological processes in live specimens.
Nuclear envelope breakdown in starfish oocytes proceeds by partial NPC disassembly followed by a rapidly spreading fenestration of nuclear membranes.
Lénart, P., Rabut, G., Daigle, N., Hand, A.R., Terasaki, M. & Ellenberg, J.
J Cell Biol 2003 Mar 31;160(7):1055-68.
Breakdown of the nuclear envelope (NE) was analyzed in live starfish oocytes using a size series of fluorescently labeled dextrans, membrane dyes, and GFP-tagged proteins of the nuclear pore complex (NPC) and the nuclear lamina. Permeabilization of the nucleus occurred in two sequential phases. In phase I the NE became increasingly permeable for molecules up to approximately 40 nm in diameter, concurrent with a loss of peripheral nuclear pore components over a time course of 10 min. The NE remained intact on the ultrastructural level during this time. In phase II the NE was completely permeabilized within 35 s. This rapid permeabilization spread as a wave from one epicenter on the animal half across the nuclear surface and allowed free diffusion of particles up to approximately 100 nm in diameter into the nucleus. While the lamina and nuclear membranes appeared intact at the light microscopic level, a fenestration of the NE was clearly visible by electron microscopy in phase II. We conclude that NE breakdown in starfish oocytes is triggered by slow sequential disassembly of the NPCs followed by a rapidly spreading fenestration of the NE caused by the removal of nuclear pores from nuclear membranes still attached to the lamina.
Global chromosome positions are transmitted through mitosis in mammalian cells.
Gerlich, D., Beaudouin, J., Kalbfuss, B., Daigle, N., Eils, R. & Ellenberg, J.
Cell 2003 Mar 21;112(6):751-64.
We investigated positioning of chromosomes during the cell cycle in live mammalian cells with a combined experimental and computational approach. By non-invasive labeling of chromosome subsets and tracking by 4D imaging, we could show that no global rearrangements occurred in interphase. Using the same assay, we also observed a striking order of chromosomes throughout mitosis. By contrast, our computer simulation based on stochastic movements of individual chromosomes predicted randomization of chromosome order in mitosis. In vivo, a quantitative assay for single chromosome positioning during mitosis revealed strong similarities between daughter and mother cells. These results demonstrate that global chromosome positions are heritable through the cell cycle in mammalian cells. Based on tracking of labeled chromosomes and centromeres during chromosome segregation and experimental perturbations of chromosomal order, we propose that chromosome specific timing of sister chromatid separation transmits chromosomal positions from one cell generation to the next.
Cyclic, proteasome-mediated turnover of unliganded and liganded ERalpha on responsive promoters is an integral feature of estrogen signaling.
Reid, G., Hubner, M.R., Metivier, R., Brand, H., Denger, S., Manu, D., Beaudouin, J., Ellenberg, J. & Gannon, F.
Mol Cell 2003 Mar;11(3):695-707.
We present an integrated model of hERalpha-mediated transcription where both unliganded and liganded receptors cycle on estrogen-responsive promoters. Using ChIP, FRAP, and biochemical analysis we evaluate hERalpha at several points in these cycles, establishing the ubiquitination status and subnuclear distribution of hERalpha, its mobility, the kinetics of transcriptional activation, and the cyclic recruitment of E3 ligases and the 19S regulatory component of the proteasome. These experiments, together with an evaluation of the inhibition of transcription and proteasome action, demonstrate that proteasome-mediated degradation and hERalpha-mediated transactivation are inherently linked and act to continuously turn over hERalpha on responsive promoters. Cyclic turnover of hERalpha permits continuous responses to changes in the concentration of estradiol.
Nuclear envelope dynamics in oocytes: from germinal vesicle breakdown to mitosis.
Lenart, P. & Ellenberg, J.
Curr Opin Cell Biol 2003 Feb;15(1):88-95.
We have recently gained new insight into the mechanisms involved in nuclear envelope breakdown, the irreversible step that commits a cell to the M phase. Results from mammalian cell and starfish oocyte studies suggest that mechanical forces of the cytoskeleton, as well as biochemical disassembly of nuclear envelope protein complexes, play important roles in this process.
Ribonucleoprotein-dependent localization of the yeast class V myosin Myo4p.
Kruse, C., Jaedicke, A., Beaudouin, J., Bohl, F., Ferring, D., Guttler, T., Ellenberg, J. & Jansen, R.P.
J Cell Biol 2002 Dec 23;159(6):971-82.
Class V myosins are motor proteins with functions in vesicle transport, organelle segregation, and RNA localization. Although they have been extensively studied, only little is known about the regulation of their spatial distribution. Here we demonstrate that a GFP fusion protein of the budding yeast class V myosin Myo4p accumulates at the bud cortex and is a component of highly dynamic cortical particles. Bud-specific enrichment depends on Myo4p's association with its cargo, a ribonucleoprotein complex containing the RNA-binding protein She2p. Cortical accumulation of Myo4p at the bud tip can be explained by a transient retention mechanism that requires SHE2 and, apparently, localized mRNAs bound to She2p. A mutant She2 protein that is unable to recognize its cognate target mRNA, ASH1, fails to localize Myo4p. Mutant She2p accumulates inside the nucleus, indicating that She2p shuttles between the nucleus and cytoplasm and is exported in an RNA-dependent manner. Consistently, inhibition of nuclear mRNA export results in nuclear accumulation of She2p and cytoplasmic Myo4p mislocalization. Loss of She2p can be complemented by direct targeting of a heterologous lacZ mRNA to a complex of Myo4p and its associated adaptor She3p, suggesting that She2p's function in Myo4p targeting is to link an mRNA to the motor complex.
Chromosomal association of Ran during meiotic and mitotic divisions.
Hinkle, B., Slepchenko, B., Rolls, M.M., Walther, T.C., Stein, P.A., Mehlmann, L.M., Ellenberg, J. & Terasaki, M.
J Cell Sci 2002 Dec 1;115(Pt 23):4685-93.
Recent studies in Xenopus egg extracts indicate that the small G protein Ran has a central role in spindle assembly and nuclear envelope reformation. We determined Ran localization and dynamics in cells during M phase. By immunofluorescence, Ran is accumulated on the chromosomes of meiosis-II-arrested Xenopus eggs. In living cells, fluorescently labeled Ran associated with the chromosomes in Xenopus and remained associated during anaphase when eggs were artificially activated. Fluorescent Ran associated with chromosomes in mouse eggs, during meiotic maturation and early embryonic divisions in starfish, and to a lesser degree during mitosis of a cultured mammalian cell line. Chromosomal Ran undergoes constant flux. From photobleach experiments in immature starfish oocytes, chromosomal Ran has a k(off) of approximately 0.06 second(-1), and binding analysis suggests that there is a single major site. The chromosomal interactions may serve to keep Ran-GTP in the vicinity of the chromosomes for spindle assembly and nuclear envelope reformation.
Remodelling the walls of the nucleus.
Burke, B. & Ellenberg, J.
Nat Rev Mol Cell Biol 2002 Jul;3(7):487-97. Europe PMC
Histone H3 phosphorylation during Xenopus oocyte maturation: regulation by the MAP kinase/p90Rsk pathway and uncoupling from DNA condensation.
Schmitt, A., Gutierrez, G.J., Lenart, P., Ellenberg, J. & Nebreda, A.R.
FEBS Lett 2002 May 8;518(1-3):23-8.
Here we show that during the meiotic maturation of Xenopus oocytes, histone H3 becomes phosphorylated on serine-10 at about the time of maturation promoting factor activation and meiosis I entry. However, overexpression of cAMP-dependent protein kinase that blocks entry into M phase, also leads to massive serine-10 phosphorylation of histone H3 in intact Xenopus oocytes but does not cause chromosome condensation. We also show that the phosphorylation of histone H3 during oocyte maturation requires the activation of the mitogen-activated protein kinase/p90Rsk pathway. Our results indicate that in G2-arrested oocytes, which are about to enter M phase, histone H3 phosphorylation is not sufficient for chromosome condensation.
Nuclear envelope breakdown proceeds by microtubule-induced tearing of the lamina.
Beaudouin, J., Gerlich, D., Daigle, N., Eils, R. & Ellenberg, J.
Cell 2002 Jan 11;108(1):83-96.
The mechanism of nuclear envelope breakdown (NEBD) was investigated in live cells. Early spindle microtubules caused folds and invaginations in the NE up to one hour prior to NEBD, creating mechanical tension in the nuclear lamina. The first gap in the NE appeared before lamin B depolymerization, at the site of maximal tension, by a tearing mechanism. Gap formation relaxed this tension and dramatically accelerated the rate of chromosome condensation. The hole produced in the NE then rapidly expanded over the nuclear surface. NE fragments remaining on chromosomes were removed toward the centrosomes in a microtubule-dependent manner, suggesting a mechanism mediated by a minus-end-directed motor.
Dynamics of nuclear envelope proteins during the cell cycle in mammalian cells.
In "Dynamics of Nuclear Envelope Assembly in Embryos and Somatic Cells, Vol. 23", P. Collas (ed), 18-25. Landes Biosciences, Georgetown, Texas, USA, 2002.
An evolutionarily conserved NPC subcomplex, which redistributes in part to kinetochores in mammalian cells.
Belgareh, N., Rabut, G., Bai, S.W., van Overbeek, M., Beaudouin, J., Daigle, N., Zatsepina, O.V., Pasteau, F., Labas, V., Fromont-Racine, M., Ellenberg, J. & Doye, V.
J Cell Biol 2001 Sep 17;154(6):1147-60.
The nuclear pore complexes (NPCs) are evolutionarily conserved assemblies that allow traffic between the cytoplasm and the nucleus. In this study, we have identified and characterized a novel human nuclear pore protein, hNup133, through its homology with the Saccharomyces cerevisiae nucleoporin scNup133. Two-hybrid screens and immunoprecipitation experiments revealed a direct and evolutionarily conserved interaction between Nup133 and Nup84/Nup107 and indicated that hNup133 and hNup107 are part of a NPC subcomplex that contains two other nucleoporins (the previously characterized hNup96 and a novel nucleoporin designated as hNup120) homologous to constituents of the scNup84 subcomplex. We further demonstrate that hNup133 and hNup107 are localized on both sides of the NPC to which they are stably associated at interphase, remain associated as part of a NPC subcomplex during mitosis, and are targeted at early stages to the reforming nuclear envelope. Throughout mitosis, a fraction of hNup133 and hNup107 localizes to the kinetochores, thus revealing an unexpected connection between structural NPCs constituents and kinetochores. Photobleaching experiments further showed that the mitotic cytoplasm contains kinetochore-binding competent hNup133 molecules and that in contrast to its stable association with the NPCs the interaction of this nucleoporin with kinetochores is dynamic.
Four-dimensional imaging and quantitative reconstruction to analyse complex spatiotemporal processes in live cells.
Gerlich, D., Beaudouin, J., Gebhard, M., Ellenberg, J. & Eils, R.
Nat Cell Biol 2001 Sep;3(9):852-5.
Live-cell imaging technology using fluorescent proteins (green fluorescent protein and its homologues) has revolutionized the study of cellular dynamics. But tools that can quantitatively analyse complex spatiotemporal processes in live cells remain lacking. Here we describe a new technique--fast multi-colour four-dimensional imaging combined with automated and quantitative time-space reconstruction--to fill this gap. As a proof of principle, we apply this method to study the re-formation of the nuclear envelope in live cells. Four-dimensional imaging of three spectrally distinct fluorescent proteins is used to simultaneously visualize three different cellular compartments at high speed and with high spatial resolution. The highly complex data, comprising several thousand images from a single cell, were quantitatively reconstructed in time-space by software developed in-house. This analysis reveals quantitative and qualitative insights into the highly ordered topology of nuclear envelope formation, in correlation with chromatin expansion - results that would have been impossible to achieve by manual inspection alone. Our new technique will greatly facilitate study of the highly ordered dynamic architecture of eukaryotic cells.
Nuclear pore complexes form immobile networks and have a very low turnover in live mammalian cells.
Daigle, N., Beaudouin, J., Hartnell, L., Imreh, G., Hallberg, E., Lippincott-Schwartz, J. & Ellenberg, J.
J Cell Biol 2001 Jul 9;154(1):71-84.
The nuclear pore complex (NPC) and its relationship to the nuclear envelope (NE) was characterized in living cells using POM121-green fluorescent protein (GFP) and GFP-Nup153, and GFP-lamin B1. No independent movement of single pore complexes was found within the plane of the NE in interphase. Only large arrays of NPCs moved slowly and synchronously during global changes in nuclear shape, strongly suggesting mechanical connections which form an NPC network. The nuclear lamina exhibited identical movements. NPC turnover measured by fluorescence recovery after photobleaching of POM121 was less than once per cell cycle. Nup153 association with NPCs was dynamic and turnover of this nucleoporin was three orders of magnitude faster. Overexpression of both nucleoporins induced the formation of annulate lamellae (AL) in the endoplasmic reticulum (ER). Turnover of AL pore complexes was much higher than in the NE (once every 2.5 min). During mitosis, POM121 and Nup153 were completely dispersed and mobile in the ER (POM121) or cytosol (Nup153) in metaphase, and rapidly redistributed to an immobilized pool around chromatin in late anaphase. Assembly and immobilization of both nucleoporins occurred before detectable recruitment of lamin B1, which is thus unlikely to mediate initiation of NPC assembly at the end of mitosis.
Nucleocytoplasmic transport: diffusion channel or phase transition?
Rabut, G. & Ellenberg, J.
Curr Biol 2001 Jul 24;11(14):R551-4.
How exactly large molecules translocate through nuclear pores has been mysterious for a long time. Recent kinetic measurements of transport rates through the pore have led to a novel translocation model that elegantly combines selectivity with very high transport rates.
A new model for nuclear envelope breakdown.
Terasaki, M., Campagnola, P., Rolls, M.M., Stein, P.A., Ellenberg, J., Hinkle, B. & Slepchenko, B.
Mol Biol Cell 2001 Feb;12(2):503-10.
Nuclear envelope breakdown was investigated during meiotic maturation of starfish oocytes. Fluorescent 70-kDa dextran entry, as monitored by confocal microscopy, consists of two phases, a slow uniform increase and then a massive wave. From quantitative analysis of the first phase of dextran entry, and from imaging of green fluorescent protein chimeras, we conclude that nuclear pore disassembly begins several minutes before nuclear envelope breakdown. The best fit for the second phase of entry is with a spreading disruption of the membrane permeability barrier determined by three-dimensional computer simulations of diffusion. We propose a new model for the mechanism of nuclear envelope breakdown in which disassembly of the nuclear pores leads to a fenestration of the nuclear envelope double membrane.
A bromodomain protein, MCAP, associates with mitotic chromosomes and affects G(2)-to-M transition.
Dey, A., Ellenberg, J., Farina, A., Coleman, A.E., Maruyama, T., Sciortino, S., Lippincott-Schwartz, J. & Ozato, K.
Mol Cell Biol 2000 Sep;20(17):6537-49
We describe a novel nuclear factor called mitotic chromosome-associated protein (MCAP), which belongs to the poorly understood BET subgroup of the bromodomain superfamily. Expression of the 200-kDa MCAP was linked to cell division, as it was induced by growth stimulation and repressed by growth inhibition. The most notable feature of MCAP was its association with chromosomes during mitosis, observed at a time when the majority of nuclear regulatory factors were released into the cytoplasm, coinciding with global cessation of transcription. Indicative of its predominant interaction with euchromatin, MCAP localized on mitotic chromosomes with exquisite specificity: (i) MCAP-chromosome association became evident subsequent to the initiation of histone H3 phosphorylation and early chromosomal condensation; and (ii) MCAP was absent from centromeres, the sites of heterochromatin. Supporting a role for MCAP in G(2)/M transition, microinjection of anti-MCAP antibody into HeLa cell nuclei completely inhibited the entry into mitosis, without abrogating the ongoing DNA replication. These results suggest that MCAP plays a role in a process governing chromosomal dynamics during mitosis.
Dynamics and retention of misfolded proteins in native ER membranes
Nehls, S., Snapp, E.L., Cole, N.B., Zaal, K.J., Kenworthy, A.K., Roberts, T.H., Ellenberg, J., Presley, J.F., Siggia, E. & Lippincott-Schwartz, J.
Nat Cell Biol 2000 May;2(5):288-95
When co-translationally inserted into endoplasmic reticulum (ER) membranes, newly synthesized proteins encounter the lumenal environment of the ER, which contains chaperone proteins that facilitate the folding reactions necessary for protein oligomerization, maturation and export from the ER. Here we show, using a temperature-sensitive variant of vesicular stomatitis virus G protein tagged with green fluorescent protein (VSVG-GFP), and fluorescence recovery after photobleaching (FRAP), the dynamics of association of folded and misfolded VSVG complexes with ER chaperones. We also investigate the potential mechanisms underlying protein retention in the ER. Misfolded VSVG-GFP complexes at 40 degrees C are highly mobile in ER membranes and do not reside in post-ER compartments, indicating that they are not retained in the ER by immobilization or retrieval mechanisms. These complexes are immobilized in ATP-depleted or tunicamycin-treated cells, in which VSVG-chaperone interactions are no longer dynamic. These results provide insight into the mechanisms of protein retention in the ER and the dynamics of protein-folding complexes in native ER membranes.
Golgi membranes are absorbed into and reemerge from the ER during mitosis
Zaal, K.J., Smith, C.L., Polishchuk, R.S., Altan, N., Cole, N.B., Ellenberg, J., Hirschberg, K., Presley, J.F., Roberts, T.H., Siggia, E., Phair, R.D. & Lippincott-Schwartz, J.
Cell 1999 Dec 10;99(6):589-601
Quantitative imaging and photobleaching were used to measure ER/Golgi recycling of GFP-tagged Golgi proteins in interphase cells and to monitor the dissolution and reformation of the Golgi during mitosis. In interphase, recycling occurred every 1.5 hr, and blocking ER egress trapped cycling Golgi enzymes in the ER with loss of Golgi structure. In mitosis, when ER export stops, Golgi proteins redistributed into the ER as shown by quantitative imaging in vivo and immuno-EM. Comparison of the mobilities of Golgi proteins and lipids ruled out the persistence of a separate mitotic Golgi vesicle population and supported the idea that all Golgi components are absorbed into the ER. Moreover, reassembly of the Golgi complex after mitosis failed to occur when ER export was blocked. These results demonstrate that in mitosis the Golgi disperses and reforms through the intermediary of the ER, exploiting constitutive recycling pathways. They thus define a novel paradigm for Golgi genesis and inheritance.
Dynamics and mobility of nuclear envelope proteins in interphase and mitotic cells revealed by green fluorescent protein chimeras.
Ellenberg, J. & Lippincott-Schwartz, J.
This is a review article.
Methods 1999 Nov;19(3):362-72
Understanding how membrane proteins are targeted to and retained within the nuclear envelope (NE) and the fate of these proteins during NE disassembly/reassembly in mitosis is central for insight into the function of the NE in nuclear organization and dynamics. To address these issues we have attached green fluorescent protein (GFP) to a well- characterized protein of the inner nuclear membrane, lamin B receptor, believed to be one of the major chromatin docking protein in the NE. We have used this construct in a variety of applications, including dual- color GFP time-lapse imaging, to investigate the mechanisms underlying protein targeting to the NE and NE breakdown and reassembly during mitosis. In this review, we present a summary of the results from such studies and discuss the photobleaching and imaging methodology on which they were derived. Copyright 1999 Academic Press.
Intracellular trafficking of emerin, the Emery-Dreifuss muscular dystrophy protein.
Ostlund, C., Ellenberg, J., Hallberg, E., Lippincott-Schwartz, J. & Worman, H.J.
J Cell Sci 1999 Jun;112 ( Pt 11):1709-19
Emerin is an integral protein of the inner nuclear membrane that is mutated or not expressed in patients with Emery-Dreifuss muscular dystrophy. Confocal immunofluorescence microscopy studies of the intracellular targeting of truncated forms of emerin, some of which are found in patients with Emery-Dreifuss muscular dystrophy, show that the nucleoplasmic, amino-terminal domain is necessary and sufficient for nuclear retention. When this domain is fused to a transmembrane segment of an integral membrane protein of the ER/plasma membrane, the chimeric protein is localized in the inner nuclear membrane. The transmembrane segment of emerin is not targeted to the inner nuclear membrane. Fluorescence photobleaching experiments of emerin fused to green fluorescent protein demonstrate that the diffusional mobility (D) of emerin is decreased in the inner nuclear membrane (D=0.10+/-0.01 microm2/second) compared to the ER membrane (D=0.32+/-0.01 microm2/second). This is in agreement with a model where integral proteins reach the inner nuclear membrane by lateral diffusion and are retained there by association with nucleoplasmic components. Some overexpressed emerin-green fluorescent protein also reaches the plasma membrane of transfected cells, where its diffusion is similar to that in the inner nuclear membrane, suggesting that emerin may also associate with non-nuclear structures.
Dual-colour imaging with GFP variants.
Ellenberg, J., Lippincott-Schwartz, J. & Presley, J.F.
This is a review article.
Trends Cell Biol 1999 Feb;9(2):52-6
Green fluorescent protein (GFP) has become an important tool in cell biology and is widely used as a reporter for imaging intracellular proteins and structures in live cells. Recently, spectral variants of GFP with red- and blue-shifted fluorescence emissions have been characterized, opening the possibility of double labelling with two different-coloured GFP fusion proteins. This article reviews recent advances in this technique, with special emphasis on time-lapse imaging applications in living cells.
Monitoring the dynamics and mobility of membrane proteins tagged with green fluorescent protein.
Lippincott-Schwartz, J., Presley, J.F., Zaal, K.J., Hirschberg, K., Miller, C.D. & Ellenberg, J.
This is a review article.
Methods Cell Biol 1999;58:261-81 Europe PMC
Kinetic analysis of secretory protein traffic and characterization of golgi to plasma membrane transport intermediates in living cells.
Hirschberg, K., Miller, C.M., Ellenberg, J., Presley, J.F., Siggia, E.D., Phair, R.D. & Lippincott-Schwartz, J.
J Cell Biol 1998 Dec 14;143(6):1485-503
Quantitative time-lapse imaging data of single cells expressing the transmembrane protein, vesicular stomatitis virus ts045 G protein fused to green fluorescent protein (VSVG-GFP), were used for kinetic modeling of protein traffic through the various compartments of the secretory pathway. A series of first order rate laws was sufficient to accurately describe VSVG-GFP transport, and provided compartment residence times and rate constants for transport into and out of the Golgi complex and delivery to the plasma membrane. For ER to Golgi transport the mean rate constant (i.e., the fraction of VSVG-GFP moved per unit of time) was 2.8% per min, for Golgi to plasma membrane transport it was 3.0% per min, and for transport from the plasma membrane to a degradative site it was 0.25% per min. Because these rate constants did not change as the concentration of VSVG-GFP in different compartments went from high (early in the experiment) to low (late in the experiment), secretory transport machinery was never saturated during the experiments. The processes of budding, translocation, and fusion of post-Golgi transport intermediates carrying VSVG- GFP to the plasma membrane were also analyzed using quantitative imaging techniques. Large pleiomorphic tubular structures, rather than small vesicles, were found to be the primary vehicles for Golgi to plasma membrane transport of VSVG-GFP. These structures budded as entire domains from the Golgi complex and underwent dynamic shape changes as they moved along microtubule tracks to the cell periphery. They carried up to 10,000 VSVG-GFP molecules and had a mean life time in COS cells of 3.8 min. In addition, they fused with the plasma membrane without intersecting other membrane transport pathways in the cell. These properties suggest that the post-Golgi intermediates represent a unique transport organelle for conveying large quantities of protein cargo from the Golgi complex directly to the plasma membrane.
ZAP-70 association with T cell receptor zeta (TCRzeta): fluorescence imaging of dynamic changes upon cellular stimulation.
Sloan-Lancaster, J., Presley, J., Ellenberg, J., Yamazaki, T., Lippincott-Schwartz, J. & Samelson, L.E.
J Cell Biol 1998 Nov 2;143(3):613-24
The nonreceptor protein tyrosine kinase ZAP-70 is a critical enzyme required for successful T lymphocyte activation. After antigenic stimulation, ZAP-70 rapidly associates with T cell receptor (TCR) subunits. The kinetics of its translocation to the cell surface, the properties of its specific interaction with the TCRzeta chain expressed as a chimeric protein (TTzeta and Tzetazeta), and its mobility in different intracellular compartments were studied in individual live HeLa cells, using ZAP-70 and Tzetazeta fused to green fluorescent protein (ZAP-70 GFP and Tzetazeta-GFP, respectively). Time-lapse imaging using confocal microscopy indicated that the activation-induced redistribution of ZAP-70 to the plasma membrane, after a delayed onset, is of long duration. The presence of the TCRzeta chain is critical for the redistribution, which is enhanced when an active form of the protein tyrosine kinase Lck is coexpressed. Binding specificity to TTzeta was indicated using mutant ZAP-70 GFPs and a truncated zeta chimera. Photobleaching techniques revealed that ZAP-70 GFP has decreased mobility at the plasma membrane, in contrast to its rapid mobility in the cytosol and nucleus. Tzetazeta- GFP is relatively immobile, while peripherally located ZAP-70 in stimulated cells is less mobile than cytosolic ZAP-70 in unstimulated cells, a phenotype confirmed by determining the respective diffusion constants. Examination of the specific molecular association of signaling proteins using these approaches has provided new insights into the TCRzeta-ZAP- 70 interaction and will be a powerful tool for continuing studies of lymphocyte activation.
Two-color green fluorescent protein time-lapse imaging.
Ellenberg, J., Lippincott-Schwartz, J. & Presley, J.F.
Biotechniques 1998 Nov;25(5):838-42, 844-6
The Aequorea victoria green fluorescent protein (GFP) is widely recognized as a powerful tool in cell biology, serving as a vital reporter for monitoring localization and dynamics of intracellular proteins and organelles over time. GFP variants with shifted spectral characteristics have been described and offer enormous potential for double-labeling experiments and protein-protein interaction studies. However, most GFP variant combinations are not suitable for double- label, time-lapse imaging experiments because of either extremely rapid photobleaching of blue-shifted GFP variants or crossover of their excitation and emission spectra, which must then be computer corrected. Here, we describe the successful use of two photostable spectral GFP variants, W7 and 10C, in dual-color, time-lapse imaging of fusion proteins in living cells using either wide-field or confocal microscopy. W7 and 10C were highly photostable during repetitive long- term imaging and were cleanly separated by their different excitation spectra alone with negligible crossover of fluorescence. We present time-lapse image sequences of COS-7 cells co-expressing both a marker of the Golgi complex (galactosyl transferase) fused to W7 and a marker of the nuclear envelope (lamin-B receptor) fused to 10C. To our knowledge, these image sequences provide the first simultaneous visualization of Golgi and nuclear envelope membranes in living cells.
Retrograde transport of Golgi-localized proteins to the ER.
Cole, N.B., Ellenberg, J., Song, J., DiEuliis, D. & Lippincott-Schwartz, J.
J Cell Biol 1998 Jan 12;140(1):1-15
The ER is uniquely enriched in chaperones and folding enzymes that facilitate folding and unfolding reactions and ensure that only correctly folded and assembled proteins leave this compartment. Here we address the extent to which proteins that leave the ER and localize to distal sites in the secretory pathway are able to return to the ER folding environment during their lifetime. Retrieval of proteins back to the ER was studied using an assay based on the capacity of the ER to retain misfolded proteins. The lumenal domain of the temperature- sensitive viral glycoprotein VSVGtsO45 was fused to Golgi or plasma membrane targeting domains. At the nonpermissive temperature, newly synthesized fusion proteins misfolded and were retained in the ER, indicating the VSVGtsO45 ectodomain was sufficient for their retention within the ER. At the permissive temperature, the fusion proteins were correctly delivered to the Golgi complex or plasma membrane, indicating the lumenal epitope of VSVGtsO45 also did not interfere with proper targeting of these molecules. Strikingly, Golgi-localized fusion proteins, but not VSVGtsO45 itself, were found to redistribute back to the ER upon a shift to the nonpermissive temperature, where they misfolded and were retained. This occurred over a time period of 15 min- 2 h depending on the chimera, and did not require new protein synthesis. Significantly, recycling did not appear to be induced by misfolding of the chimeras within the Golgi complex. This suggested these proteins normally cycle between the Golgi and ER, and while passing through the ER at 40 degrees C become misfolded and retained. The attachment of the thermosensitive VSVGtsO45 lumenal domain to proteins promises to be a useful tool for studying the molecular mechanisms and specificity of retrograde traffic to the ER.
Nuclear membrane dynamics and reassembly in living cells: targeting of an inner nuclear membrane protein in interphase and mitosis.
Ellenberg, J., Siggia, E.D., Moreira, J.E., Smith, C.L., Presley, J.F., Worman, H.J. & Lippincott-Schwartz, J.
J Cell Biol 1997 Sep 22;138(6):1193-206
The mechanisms of localization and retention of membrane proteins in the inner nuclear membrane and the fate of this membrane system during mitosis were studied in living cells using the inner nuclear membrane protein, lamin B receptor, fused to green fluorescent protein (LBR- GFP). Photobleaching techniques revealed the majority of LBR-GFP to be completely immobilized in the nuclear envelope (NE) of interphase cells, suggesting a tight binding to heterochromatin and/or lamins. A subpopulation of LBR-GFP within ER membranes, by contrast, was entirely mobile and diffused rapidly and freely (D = 0. 41 +/- 0.1 microm2/s). High resolution confocal time-lapse imaging in mitotic cells revealed LBR-GFP redistributing into the interconnected ER membrane system in prometaphase, exhibiting the same high mobility and diffusion constant as observed in interphase ER membranes. LBR-GFP rapidly diffused across the cell within the membrane network defined by the ER, suggesting the integrity of the ER was maintained in mitosis, with little or no fragmentation and vesiculation. At the end of mitosis, nuclear membrane reformation coincided with immobilization of LBR-GFP in ER elements at contact sites with chromatin. LBR-GFP-containing ER membranes then wrapped around chromatin over the course of 2-3 min, quickly and efficiently compartmentalizing nuclear material. Expansion of the NE followed over the course of 30-80 min. Thus, selective changes in lateral mobility of LBR-GFP within the ER/NE membrane system form the basis for its localization to the inner nuclear membrane during interphase. Such changes, rather than vesiculation mechanisms, also underlie the redistribution of this molecule during NE disassembly and reformation in mitosis.
The transmembrane domain of a carboxyl-terminal anchored protein determines localization to the endoplasmic reticulum.
Yang, M., Ellenberg, J., Bonifacino, J.S. & Weissman, A.M.
J Biol Chem. 1997 Jan 17;272(3):1970-5.
UBC6 is a C-terminal membrane-anchored (type IV) protein, native to Saccharomyces cerevisiae, where it is found in the endoplasmic reticulum. When expressed in mammalian cells, this novel ubiquitin-conjugating enzyme also localizes to the endoplasmic reticulum. UBC6 lacks a lumenal domain and contains no known endoplasmic reticulum retention signals. Analysis of chimeric proteins in which the cytosolic domain of UBC is linked to a heterologous transmembrane domain, or in which the UBC6 transmembrane domain is appended to an unrelated soluble protein, led to the determination that the transmembrane domain of UBC6 plays a dominant role in its compartmental localization. The basis for the transmembrane domain-mediated subcellular targeting of UBC6 was evaluated by lengthening the wild type UBC6 hydrophobic segment from 17 to 21 amino acids, which resulted in re-targeting to the Golgi complex. A further increase in length to 26 amino acids allowed this modified protein to traverse the secretory pathway and gain expression at the plasma membrane. These findings are consistent with models in which, in the absence of dominant cytosolic or lumenal targeting determinants, proteins may be sorted within the secretory pathway based on interactions between their transmembrane domains and the surrounding lipid bilayer.
Fluorescence photobleaching techniques.
Ellenberg, J. & Lippincott-Schwartz, J.
In "Cells: A Laboratory Manual, Vol. 2", Spector, D., Goldman, R. & Leinwand, L. (Eds.), Cold Spring Harbor Laboratory Press, Cold Spring Harbor, 1997, 79.1-79.23