Characterization of embryonic muscle migration in Caenorhabditis elegans and of the role of unc-54 and mem-1 in inhibiting ectopic muscle membrane extensions

C. elegans body wall muscle is formed after a series of well-orchestrated steps. This thesis describes the plasma membrane dynamics of these migrating embryonic cells and the identification of two genes unc-54/MHB and mem-1 that appear to be involved negatively regulating post-embryonic muscle membrane extensions. During the characterization of embryonic muscle morphogenesis, whereby two rows of cells split and migrate dorsally or ventrally to form the final four muscle quadrants present upon hatching, I observed an anterior migration event, whereby the anterior-most pair of cells in each of the four muscle quadrants extends long processes to the anterior tip of the developing embryo. The anterior-most muscle cells then follow these extensions into their final positions in the developing embryo. Using RNAi and mutant analysis, I have identified laminin as being involved in mediating the dorsal-ventral muscle migrations and that the α-integrin INA-1, the ephrin VAB-2 and its receptor VAB-1 and the Robo receptor SAX-3 indirectly promote the proper extension of the ventral anterior muscle processes by organizing the embryonic neurons so as to provide a clear path for muscle membrane extension. Post embryonically, the loss of either unc-54 (a myosin heavy chain B), or mem-1 (a WD repeat domain protein) results in ectopic membrane extensions from the mature muscle cells. These extensions can be rescued via targeted depletion of actin remodeling or cell adhesion complex components. During the analysis of these mutations I identified a predisposition for generating these ectopic membrane extensions that is conferred by using ectopic expression of PAT-3/β-integrin that is bound to GFP and that using the PAT-3/β-integrin transmembrane domain to localize GFP to the plasma membrane is sufficient to generate this sensitivity.