UC Davis

In multicellular organisms, asymmetric cell division generates cell type diversity during both development and tissue homeostasis. For successful asymmetric division to proceed, a cellular axis of polarity, or asymmetry, must be established via any of several molecular mechanisms. This step is followed by two sequential processes requiring precise coordination of the various components of the cytoskeleton. First, the mitotic spindle must be positioned in alignment with the axis of polarity; second, cytokinesis must occur at the right position and time, as instructed by the spindle, to segregate chromosomes and bisect the axis of polarity. In this dissertation I present my contributions to two research projects related to cytoskeletal regulation during asymmetric cell division. Chapter I reviews the state of the field prior to my studies and introduces the two research projects in context. Chapter II is a manuscript in revision for Journal of Cell Science that relates our findings from the first project, which aimed to understand the role of the DEPDC1 homolog LET-99 during cytokinesis. LET-99 was previously characterized as a negative regulator of the dynein-containing force generating complex during spindle positioning, but had also been shown to play a separable role in cytokinesis. With others, I demonstrated that during the first mitotic division LET-99 and the Rac1 homolog CED-10 have antagonistic roles in regulating the balance of branched vs linear actin during cytokinesis to achieve robust and timely furrowing. These roles appear to be global rather than spatially restricted to the contractile ring. However, the findings suggest that CED-10 does not interact with LET-99 to promote spindle positioning in the first mitotic division, further confirming that LET-99’s role as a regulator of the actomyosin cytoskeleton and cytokinesis is separable from its role in spindle positioning. Chapter III is a manuscript in preparation that documents our investigations into the role of CED-10 in spindle orientation of the asymmetrically dividing EMS cell. Two signaling pathways that regulate this spindle orientation were previously identified: a Wnt/Frizzled pathway and a MES-1/SRC-1 pathway. I determined that CED-10 works upstream or at the level of SRC-1 in the MES-1/SRC-1 signaling pathway and furthermore that CED-10 contributes to cortical localization of the same dynein-containing force generating complex involved in spindle positioning at the one-cell stage. I also found that CED-10’s molecular function in this context may be to promote branched actin formation at the EMS/P2 contact, but this evidence is partially contradicted by our findings that loss of the branched-actin regulator GEX-3 does not enhance the rate of EMS spindle rotation defects in Wnt/MOM-2-depleted embryos. An additional curious finding is that CED-10 has a previously unreported role in P1 spindle rotation, as do the branched actin nucleator Arp2/ARX-2 (Arp2) and the Frizzled ortholog MOM-5. Although this research was performed using nematodes as a model organism, the results have broad implications for all animals because most of the molecular components involved are evolutionarily conserved. By understanding in detail the mechanisms of different types of asymmetric divisions, we can understand both normal development and diseases such as cancer, which involves the dysregulation of asymmetric division.