| One of the most fundamental biological processes, cell division, requires the precise spatiotemporal coordination of molecular machinery to drive both nuclear and cytoplasmic division. All life is dependent on proper cell division in order to ensure accurate development, maintain tissues and homeostasis. While cell division is a requirement for the propagation of live (and therefore, an ancient process), the actual mechanics and cues are different among the different branches of life. Bacteria have a unique but similar way of dealing with cell division however, the mechanics and mechanisms are just as diverse as the domain they belong to. In eukaryotic cells, the mechanism and machinery are more conserved, but vary slightly amongst the different phyla. For the purposes of my dissertation I will be focusing on animal cell division, using the sea urchin as my model system.;Landmark work by Thomas Schroeder described the structure of the contractile ring during cytokinesis in the sea urchin embryo, providing for the first time the structural basis for how cells pinch in two. Schroeder's observations of the contractile ring, its spatial and temporal overlap with the cleavage furrow indicated that the contractile apparatus is responsible for the physical separation of mother and daughter cells. Additionally, he described for the first time the general organization of the cortical actin cytoskeleton in the early sea urchin embryo. Since these studies, there have been great advancements in our understanding of the biochemistry of the actin cytoskeleton, as well as the actin-associated factors that control actin assembly and organization in sea urchins embryos.;The Rho Family GTPases (Rho, Cdc42 and Rac) are molecular switches that orchestrate cell shape changes in response to extracellular and intracellular cues. However, outside of defining the role of Rho in contractile ring assembly, little progress has been made toward understanding the regulation and dynamics of actin in large spherical cells such as the sea urchin egg.;Using a live cell imaging approach, we describe for the first time the dynamic changes in actin organization that occur during the first cell cycle of the sea urchin embryo, and used these findings as a foundation for two studies. The first study addressed the differential regulation of Rho Gtpases during cytokinesis. While RhoA activity was required for contractile ring assembly, neither Rac nor Cdc42 were required for cell division. However, expression of activated Rac mutants resulted in cytokinesis failure through its promotion of branched actin networks. This provided novel insights into why Rac (and possibly Cdc42) need to be locally inactivated at the cell equator during cell division.;The second study focused on a bright rim of actin localized at the nucleus in the early embryo. This nuclear-associated actin accumulated during interphase, only to disassemble during the course of nuclear envelope breakdown. Surprisingly, this actin network was associated with the cytoplasmic side of the nucleus, in contrast to the intra-nuclear actin meshwork that is assembled within the oocyte germinal vesicle just prior to the first meiotic division. This peri-nuclear actin array may help stabilize the nuclear envelope, but its elaboration appears to be limited to the early stages of development. These results raise interesting questions as to cytoplasmic environment that the nucleus occupies during early development, and how peri-nuclear actin might support its stability and position within the cell Together, these studies provide for the first time, insights into the dynamics and regulation of actin in this important experimental model for cell division. |
