The focus of our scientific research is the process of myogenesis, with a specific emphasis on the migration, recognition, adhesion and fusion between myoblasts to form multinucleate myotubes. We have chosen embryonic development of the larval body wall muscles in the fruitfly Drosophila melanogaster as a model system, in an effort to utilize the advantages of a combined genetic and molecular approach. Our experimental plan utilizes genes that are associated with serious defects in myoblast fusion as an entry point to identify interacting proteins and signaling pathways that are important to this process. In the embryo, two distinct populations of myoblasts appear to be involved in formation of these fibers. The first, termed muscle founder cells arise in characteristic and reproducible positions in the embryo and contain information that specifies muscle identity, size, position, and attachment. The second and larger group of cells has been termed the fusion-competent myoblasts. Cell-type specific adhesion molecules that are members of the Ig superfamily mediate recognition and fusion between these two distinct populations of myoblasts, and many of the proposed studies focus on the role of these proteins in activating downstream events. SNS, a cell adhesion molecule that is expressed exclusively on the surface of the fusion competent myoblasts, is essential for recognition of Kirre or Rst-expressing founder cells. It appears to activate a signal transduction pathway through interactions that occur with its cytoplasmic domain. Studies within this proposal are aimed at identifying proteins that interact with SNS and are responsible for downstream events. Studies will examine SNS interactions with other transmembrane proteins as well as with cytoplasmic proteins that include SH2- SH3-domain containing adaptor proteins. MBC, the Drosophila ortholog of Dock180, functions as part of an unconventional bipartite guanine nucleotide exchange factor that appears to function downstream of Kirre in founder cells. MBC and its partner Ced-12 are thought to activate the small GTPase Rac1, which has been implicated as a downstream target of multiple pathways in myoblast fusion. We plan to examine this role in more detail by a combination of genetic interaction and live imaging. Lastly, we will use genetic screens that allow us to identify modulators of the above pathways as well as novel genes that function in independent pathways to regulate fusion. Studies have shown that mammalian orthologs of some of these proteins play critical roles in diverse processes in phagocytosis, cell migration and formation of the kidney slit diaphragm. More recent studies have also implicating some of these genes in myoblast fusion in vertebrates.
In a developing embryo, groups of cells of various types must coordinate their movement, interactions and morphology in order to form functional organs. The process of muscle development requires recognition between different myoblasts and fusion of these cells to form muscle fibers. The focus of our research is to examine the mechanisms and essential genes through which cells find each other in the embryo and initiate the process of fusion.
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