Ellenberg Group

Acentriolar microtubule organising centers (green) form a 3D network around the chromosomes (red) during spindle assembly in a mouse oocyte.

 Previous and current research

The genome of eukaryotic cells is compartmentalised inside the nucleus, delimited by the nuclear envelope (NE) whose double membranes are continuous with the ER and stabilised by the nuclear lamina filament meshwork. The NE is perforated by nuclear pore complexes (NPCs], which allow selective traffic between nucleus and cytoplasm. In M-phase, most metazoan cells reversibly dismantle the highly ordered structure of the NE. Nuclear membranes that surround chromatin in interphase are 'replaced' by cytoplasmic spindle microtubules, which segregate the condensed chromosomes in an 'open' division. After chromosome segregation 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. Breakdown and reassembly of the nucleus and the formation and correct movement of compact mitotic chromosomes are essential but poorly understood processes. To study them, we are assaying fluorescently tagged structural proteins and their regulators using advanced fluorescence microscopy methods coupled with computerised image processing and simulations to extract biophysical parameters and build mechanistic models.

In the past, we could define the ER as the reservoir and means of partitioning for nuclear membrane proteins in mitosis and found that nuclear breakdown is triggered by disassembly of the NPC and then further facilitated by microtubule mediated tearing of the nuclear lamina. During the meiotic division of starfish oocytes, we could show that long-range chromosome motion after nuclear breakdown is driven by actin filaments. In mouse oocytes this occurs only after formation of an acentrosomal spindle, which we could show assembles by self-organisation of cytoplasmic microtubule asters. In mitotic cells, we have analysed chromosome dynamics during their segregation and could show that their overall spatial arrangement is transmitted through mitosis and that their maximal compaction is reached only at the end of anaphase, just before nuclear reformation.

Future projects and goals

Objective of our future work is to gain further mechanistic insight into nuclear remodelling in live cells. In particular, we are focusing on the mechanism of nuclear growth in interphase, nuclear disassembly and reformation as well as chromosome condensation and positioning in somatic cells and microtubule-independent chromosome motion in oocytes. To rapidly obtain quantitative data from intact cells, we aim to automate and standardise advanced fluorescence microscopy assays as much as possible. This enables us to apply them in higher throughput to all relevant proteins and achieve a systems level understanding of the transformations in nuclear structure during cell division. As a first step, we have developed high-throughput live cell imaging in combination with RNAi screening to identify novel genes that function in the above cell division processes.