The Arf-GEF Steppke Controls Cell Division In the Early Drosophila Embryo

The early Drosophila embryo forms somatic and germ cells by cell division from a syncytium. This process requires dynamic plasma membrane remodelling shaped by a complex set of interconnected cytoskeletal and membrane trafficking events. To examine how the plasma membrane Arf G proteins contribute to this process, I investigated the role of the Drosophila cytohesin Arf-GEF Steppke, an activator of plasma membrane Arf G proteins, in the early embryo. My first aim was to characterize how Steppke influences cleavage furrow ingression during somatic cell formation. By analyzing steppke loss-of-function and gain-of-function embryos, I discovered that Steppke locally restrains the membrane cytoskeleton to prevent abnormal membrane growth at the base of furrows. Genetic interaction analyses showed that the Steppke/Arf pathway antagonizes Rho1-dependent actomyosin pathways, an antagonism that appears to be partially achieved by stimulating AP-2/clathrin-dependent endocytosis. In my second aim, I examined the role of the Steppke/Arf pathway on restricting primordial germ cell formation. I first showed that it prevents precocious formation of a group of somatic cells at the anterior pole of the embryo. I also examined its effect on primordial germ cell formation at the posterior of the embryo. A series of genetic interaction tests showed that Steppke normally antagonizes germ plasm-induced Rho1 activity, antagonizing the formation of primordial germ cells. Therefore, Steppke seems to set a global inhibitory threshold for cell division. Overall, I defined a critical role of the Steppke/Arf pathway in restraining the actin cytoskeleton during cell division.Ph.D.2018-02-12 00:00:0