Ellenberg Group
Functional dynamics of nuclear structure during the cell cycle
Figure 1: Mitotic spindle composed of thousands of microscopy images of human cells in which individual genes were silenced. Chromosomes (red) are made of images from genes that affect their segregation, while the mitotic spindle (green) is composed of images from genes affecting its assembly.
Previous and current research
The genome of eukaryotic cells is compartmentalised inside the nucleus, delimited by the nuclear envelope (NE) whose double membrane is continuous with the ER and perforated by nuclear pore complexes (NPCs). In M-phase, most metazoan cells reversibly dismantle the highly ordered structure of the NE. Chromosomes that are surrounded by nuclear membranes in interphase are attached to cytoplasmic spindle microtubules in order to segregate them. After chromosome segregation, the roles are reversed again and the nucleus rapidly reassembles.
The overall aim of our research is to elucidate the mechanisms underlying cell cycle remodelling of the nucleus in live cells. Biogenesis of the nucleus and the formation of M-phase chromosomes are essential but poorly understood processes. To study them, we are using advanced fluorescence microscopy-based methods to understand the dynamics and function of structural and regulatory proteins. Quantitative imaging is coupled with computerised image processing and simulations to extract biophysical parameters and build mechanistic models. As biological systems, we are using somatic mammalian cells for mitosis, as well as mouse oocytes for meiosis, in which we study the asymmetric division they undergo to become a fertilisable egg.
Figure 2: Chromatin (green) in the nucleus of a live somatic mammalian cell leads to volume exclusion of other macromolecules (red, fluorescently labelled dextran) (left panel); and exhibits a fractal organisation (right panel).
In somatic cells, we could show that nuclear membrane formation originates from the endoplasmic reticulum. We found that mitotic nuclear breakdown and reformation is initiated by the ordered dis- and reassembly of NPCs. Interestingly, we found that NPCs are assembled along a different pathway assembly during nuclear growth in interphase. Recently, we have identified many new cell division genes by screening the entire human genome using time-lapse microscopy (figure 1). In animal oocytes, we discovered that asymmetric transport of chromosomes to the cell surface is mediated by a contractile F-actin network rather than by microtubules.
Future projects and goals
The objective of our future work is to gain comprehensive mechanistic insight into cell cycle remodelling of the nucleus. For the interphase nucleus, we are currently focusing on the mechanism of nuclear growth, as well as chromosome architecture and dynamics, where we recently proposed a fractal organisation of chromosomes in interphase nuclei (figure 2). For mitosis, we are aiming to achieve a systems level under- standing and assay all relevant proteins identified in our genome-wide survey of human mitotic genes in living cells. To this end we are continuously automating advanced fluorescence imaging techniques. In oocytes we are pursuing the molecular mechanism of actin-mediated chromosome transport as well as homologous chromosome segregation.