The final step of cell division

Abstract

All multicellular organisms begin life as a single cell called a zygote. A new organism is formed when a sperm cell fertilizes an egg cell. From that point on, the zygote must divide many times through repeated cell division in order to grow and develop. By the time development is complete, the human body consists of trillions of cells. These cells become specialized for different functions and work together to form tissues and organs. Because our bodies are made up of many different specialized cell types, the fertilized egg must do more than simply divide. The fertilized egg must make critical fate decisions that direct cells toward specific lineages. Cells vary in their developmental potential. The zygote itself is totipotent, meaning it can generate every cell type in the embryo as well as the extraembryonic tissues. After several rounds of division, typically by the morula stage, totipotent cells are lost and give rise to pluripotent stem cells. These pluripotent cells can form all three germ layers of the body but can no longer produce extraembryonic tissues. Cell fate changes are often closely linked to cell division. In particular, embryonic stem cells tend to exit pluripotency during the G1 phase of the cell cycle, shortly after division. In this thesis, we investigated the connection between cell division and cell fate decisions in mouse embryonic stem cells (mESCs). In Chapter 2, we examine various contexts in which abscission is delayed or incomplete. We explore the molecular causes of such delays and distinguish between pathological and physiological situations. First, we describe pathways triggered by cellular errors, primarily chromosome segregation defects and nuclear pore complex assembly problems, that lead to delayed abscission in cancer cells. We then highlight physiological examples of delayed or incomplete abscission, focusing on embryo models and stem cells, along with the underlying mechanisms. In Chapter 3, we developed a robust protocol for immunofluorescence staining of mouse embryonic stem cells in both 2D and 3D culture systems. In Chapter 4, we investigated the roles of Aurora B kinase and microtubules in regulating abscission in mESCs. We found that pluripotent mESCs exhibit higher Aurora B activity at the intercellular bridge compared to differentiating cells. They also maintain more stable microtubules in the bridge. Both elevated Aurora B activity and increased microtubule stability contribute to delayed abscission. Notably, reducing Aurora B activity not only accelerates abscission but also decreases microtubule stability at the bridge and promotes faster differentiation of mESCs. Our data indicate that Aurora B regulates abscission timing, at least in part, by phosphorylating the microtubule depolymerase MCAK. In Chapter 5, we explored the upstream regulation of abscission in mESCs. Our results demonstrate that Wnt signaling plays a central role in controlling abscission dynamics. Active Wnt signaling influences abscission speed in a cell-state-dependent manner. We identified two parallel mechanisms: the Wnt/STOP (Stabilization Of Proteins) pathway maintains high levels of Aurora B at the bridge, while downstream Wnt components stabilize microtubules. Together, these effects result in slow abscission specifically in pluripotent mESCs.