Identification of a novel splicing factor required for proper myotendenous junction formation and maintenance in Drosophila. Kate M. Rochlin^1,2, Mary Baylies¹. 1) Dept Dev Biol, Sloan-Kettering Inst, New York, NY; 2) Weill Cornell Biomedical University New York, NY.

   The anchoring of the musculoskeletal system links the force-producing muscles to the skeletal system of the organism, allowing stability and movement. In Drosophila, the connection between muscles and the exoskeleton occurs via tendon-like cells that develop in the ectoderm. A number of cross-regulatory interactions are required for targeting muscles to tendons and the subsequent formation of the myotendinous junction (MTJ). Key MTJ pathways rely on rapid changes in protein and isoform expression. However, how this splicing is regulated, either temporally or in a tissue specific manner, is unclear. In a screen to find new genes required for muscle morphogenesis, we uncovered a novel predicted member of the SR family of splicing factors, which are pivotal regulators of all aspects of mRNA metabolism. We named this gene missed connections (mcx) based on its muscle mis-attachment phenotype. Mcx is expressed in the embryonic musculature and localizes to nuclear speckles, consistent with the subcellular localization of other splicing factors. Mutations in mcx fail to form robust MTJs and show changes in localization of crucial attachment site proteins that undergo alternative splicing regulation such as tropomyosin and integrins. The distribution of other proteins known to localize at the MTJ is also affected. We propose that splicing via Mcx is essential to regulate expression and isoform switching of critical proteins required for MTJ formation and maintenance. mcxIdentification of a novel splicing factor required for proper myotendenous junction formation and maintenance in Drosophila is conserved and expressed in mammalian muscle. Since mammalian skeletal muscle also requires alternate splicing and changes in splicing patterns have been linked to muscle disease, we predict our work will define fundamental mechanisms of splicing regulation critical for muscle biology in all organisms.