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.
A novel laser nanosurgery approach supports de novo Golgi biogenesis in mammalian cells.
Tangemo, C., Ronchi, P., Colombelli, J., Haselmann, U., Simpson, J.C., Antony, C., Stelzer, E.H., Pepperkok, R. & Reynaud, E.G.
J Cell Sci. 2011 Mar 15;124(Pt 6):978-87. doi: 10.1242/jcs.079640.
The Golgi complex has a central role in the secretory pathway of all higher organisms. To explain the synthesis of its unique stacked structure in mammalian cells, two major models have been proposed. One suggests that it is synthesized de novo from the endoplasmic reticulum. The second model postulates a pre-existing Golgi template that serves as a scaffold for its biogenesis. To test these hypotheses directly, we have developed an approach in which we deplete the Golgi complex from living cells by laser nanosurgery, and subsequently analyze the 'Golgi-depleted' karyoplast using time-lapse and electron microscopy. We show that biosynthetic transport is blocked after Golgi depletion, but is restored 12 hours later. This recovery of secretory transport coincides with an ordered assembly of stacked Golgi structures, and we also observe the appearance of matrix proteins before that of Golgi enzymes. Functional experiments using RNA interference-mediated knockdown of GM130 further demonstrate the importance of the matrix during Golgi biogenesis. By contrast, the centrosome, which can also be removed by laser nanosurgery and is not reformed within the considered time frame, is not required for this process. Altogether, our data provide evidence that de novo Golgi biogenesis can occur in mammalian cells.
Identification of cholesterol-regulating genes by targeted RNAi screening.
Bartz, F., Kern, L., Erz, D., Zhu, M., Gilbert, D., Meinhof, T., Wirkner, U., Erfle, H., Muckenthaler, M., Pepperkok, R. & Runz, H.
Cell Metab. 2009 Jul;10(1):63-75.
Elevated plasma cholesterol levels are considered responsible for excess cardiovascular morbidity and mortality. Cholesterol in plasma is tightly controlled by cholesterol within cells. Here, we developed and applied an integrative functional genomics strategy that allows systematic identification of regulators of cellular cholesterol levels. Candidate genes were identified by genome-wide gene-expression profiling of sterol-depleted cells and systematic literature queries. The role of these genes in cholesterol regulation was then tested by targeted siRNA knockdown experiments quantifying cellular cholesterol levels and the efficiency of low-density lipoprotein (LDL) uptake. With this strategy, 20 genes were identified as functional regulators of cellular cholesterol homeostasis. Of these, we describe TMEM97 as SREBP target gene that under sterol-depleted conditions localizes to endo-/lysosomal compartments and binds to LDL cholesterol transport-regulating protein Niemann-Pick C1 (NPC1). Taken together, TMEM97 and other factors described here are promising to yield further insights into how cells control cholesterol levels.
Secretory cargo regulates the turnover of COPII subunits at single ER exit sites.
Forster, R., Weiss, M., Zimmermann, T., Reynaud, E.G., Verissimo, F., Stephens, D.J. & Pepperkok, R.
Curr Biol. 2006 Jan 24;16(2):173-9.
The COPII coat complex mediates the formation of transport carriers at specialized sites of the endoplasmic reticulum (ERES). It consists of the Sar1p GTPase and the Sec23/24p and the Sec13/31p subcomplexes . Both stimulate the GTPase activity of Sar1p , which itself triggers coat disassembly. This built-in GAP activity makes the COPII complex in principle unstable and raises the question of how sufficient stability required for cargo capture and carrier formation is achieved. To address this, we analyzed COPII turnover at single ERES in living cells. The half times for Sar1p, Sec23p, and Sec24p turnover are 1.1, 3.7, and 3.9 s, respectively. Decreasing the amount of transport-competent cargo in the endoplasmic reticulum accelerates turnover of the Sec23/24p and slows down that of Sar1p. A mathematical model of COPII membrane turnover that reproduces the experimental in vivo FRAP kinetics and is consistent with existing in vitro data predicts that Sec23/24p remains membrane associated even after GTP hydrolysis by Sar1p for a duration that is strongly increased by the presence of cargo. We conclude that secretory cargo retains the COPII complex on membranes, after Sar1p release has occurred, and prevents premature disassembly of COPII during cargo sorting and transport carrier formation.
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