The RhoGAPs RGA-8/RICH-1/SH3BP1 and HUM-7/Myo9 regulate actomyosin enrichment during C. elegans embryonic morphogenesis

Organ and tissue formation are highly regulated processes during embryonic development. Defects in this process can lead to birth defects, or premature death. The complexity of vertebrate tissues complicates the study of organ and tissue embryonic development. Epidermal morphogenesis in the nematode Caenorhabditis elegans is an ideal model to study tissue morphogenesis in whole organisms. C. elegans is amenable to genetics and microscopy observation, allowing us to capture live imaging of the migrating tissues. Previous studies from our lab identified a Rac1-dependent branched actin pathway as an important regulator of epidermal cell migration during this process. However, the role of actomyosin contractility during this process was unclear. Actin dynamics and actomyosin contractility are regulated by the Rho GTPases protein family. Rac1 and Cdc42 promote branched actin formation, while Cdc42 and RhoA promote actomyosin contractility. Rho GTPases are molecular switches that become activated by binding to GTP, and deactivated by hydrolyzing GTP to become GDP. The cycle of GTP- and GDP- binding is regulated by GTP exchange factor proteins (GEFs) and GTPase-activating proteins (GAPs), respectively. C. elegans only has seven members of Rho GTPases, but 23 GAPs, suggesting layers of regulation in ii the process of branched actin formation and actomyosin contractility. In this study we performed the first characterization of two RhoGAPs, RGA-8/RICH-1/SH3BP1 and HUM-7/Myo9 and show that they regulate morphogenesis in C. elegans by modulating RHO-1/RhoA and CDC-42. We show that RGA-8/RICH-1/SH3BP1 and HUM-7/Myo9 regulate myosin enrichment during morphogenesis, including at the epidermal pocket cells during ventral enclosure. Previous studies proposed actomyosin contractility is mainly required in underlying neuroblasts to promote epidermal cell migrations. In contrast, the results here show that myosin is polarized, and tightly regulated in the migrating epidermal cells, by RGA-8/RICH- 1/SH3BP1 and HUM-7/Myo9. In addition, we show these proteins contribute to normal morphogenesis by regulating the timing of morphogenetic cell movements. Overall, we place the RhoGAPs HUM-7/Myo9 and RGA-8/RICH-1/SH3BP1 in pathways that regulate both actin and actomyosin contractility through RHO-1 and CDC-42, demonstrating new roles for these GTPases in embryonic epidermal morphogenesis in C. elegans.Ph.D.Includes bibliographical referencesIncludes vitaby Hamidah Raduwa