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Colleen M. Guerin
La Salle University
Thesis Advisor: Sunita G. Kramer, PhD
Graduate Program: Cell & Developmental Biology
Wednesday, April 8, 2009
Drosophila larval somatic muscle fibers are organized into an intricate, repeating pattern during embryonic development. This pattern depends on individual muscle fibers selecting their appropriate epidermal attachment sites, called tendon cells. Muscles extend filopodia as they migrate and tendon cells release guidance cues to direct muscle fibers to their correct positions. Few molecules have been shown to function in muscle attachment site selection, and identification of novel genes would provide a better understanding of what mechanisms play a role in muscle migration and guidance. We have isolated several Drosophila EMS mutations affecting the ability of somatic muscles to correctly select their proper epidermal attachment sites. The phenotypes include muscles that extend past their appropriate attachment sites, as well as muscles with more general organizational defects. From this screen, we identified RacGAP50C as an essential component of the muscle migratory machinery. RacGAP50C is well studied for its role in cytokinesis, where it coordinates the cytoskeleton for cleavage furrow formation through interactions with Pavarotti, Pebble, and Anillin. We have found a novel role for RacGAP50C and Pavarotti in organizing microtubules (MT) during Drosophila myogenesis. Embryos mutant for either RacGAP50C or pavarotti have similar defects in somatic muscle patterning, whereby muscle fibers are abnormally shaped and display a variety of guidance errors. RacGAP50C localizes to cytoplasmic puncta concentrated at the nuclear periphery in multinucleated myotubes and this localization is dependent on the presence of Pavarotti. The RacGAP50C-containing cytoplasmic puncta co-stain with the MT nucleating protein gamma-tubulin, and gamma-tubulin localization depends on the presence of RacGAP50C, suggesting that the function of RacGAP50C is to promote non-centrosomal MT organization at the nuclear periphery through an interaction with gamma-tubulin. Consistent with this hypothesis, myotubes in RacGAP50C mutants show non-uniform polarity within the MT array. This work provides strong evidence that muscle attachment site selection is MT-based and identifies RacGAP50C and Pavarotti as two molecules crucial for this process.