Aim 2: Differential regulation of abscission between germline stem cells and differentiating daughter cells.
We have found that AuroraB and CyclinB localize at the midbody, and that CycB/Cdk-1 promotes abscission, while AuroraB/Survivin inhibits it. Both complexes have other functions during the cell cycle, which are mainly the control of entry into M phase and the metaphase-anaphase transition. At these two stages, the regulation of AuroraB and Cdk-1 is well-described and a list of known regulators is available 12.
The core machinery that drives the eukaryotic cell cycle is very conserved across evolution, but has only been extensively studied in a few model systems (Morgan, 2007). During animal development, the canonical cell cycle is modulated and adapted in different cell types. For example, the rapid divisions of zebrafish or Drosophila embryos are devoid of Gap phases, while mouse giant trophoblast cells or many larval cells in Drosophila become polyploid by endoreplicating their DNA without any mitosis (M) phase (Budirahardja and Gonczy, 2009). How specific developmental programs alter different steps of the cell cycle remains to be understood in most cases. In this respect, the last stages of cell division, when daughter cells become separated, are probably the most diverse, but also the least explored. In sea urchin embryos, the timing of cytokinesis is shifted, and the completion of cell division only occurs during the S phase of the next cycle (Sanger et al., 1985). Cytokinesis altogether is absent during megakaryocyte differentiation, and can also be arrested at a late stage in spermatocytes of most species (Pepling et al., 1999; Vitrat et al., 1998). Cytokinesis starts by the specification of a cleavage plane and is followed by the ingression of an actomyosin contractile ring. During this transition, the mitotic spindle rearranges at the midzone to form an electron-dense structure known as the midbody, at the center of the intercellular bridge. Finally, by a process called abscission, daughter cells become physically separated. Although abscission failure can lead to tetraploidization, it remains poorly characterized due to several technical challenges (Steigemann and Gerlich, 2009). It is difficult to synchronize cells for the transient abscission process, which hampers most biochemical studies. Proteins involved at this stage may also be required earlier in the cell cycle and genetic mutations in the corresponding genes are thus likely to mask late functions. Finally, the midbody is beyond the resolution limit of most microscopes. Nevertheless, it has become clear that abscission requires remodeling of the membrane and cytoskeleton (Guizetti and Gerlich, 2010). Endosome delivery to the midbody and severing of the microtubule bundle are essential for correct abscission. The upstream regulators that initiate abscission remain unknown however. It is also unclear what regulates the timing of abscission, which can vary from minutes to hours, or can even remain incomplete, as in germ cells (Pepling et al., 1999).