NFATC2-dependent signaling regulates skeletal muscle growth

Cell fusion is a crucial event in the formation and growth of multinucleated muscle cells as myoblasts fuse during development and regeneration of skeletal muscle. However, few molecules are known to regulate myoblast fusion in mammals. Calcium has a well-known role in myoblast fusion, but the signaling pathways regulated by calcium are not known. The work described in this dissertation identifies a novel signaling pathway activated by calcium that regulates mammalian myoblast fusion and muscle growth involving the calcium sensitive transcription factor, NFATC2. We demonstrate that during the growth of multinucleated muscle cells, myoblasts initially fuse to form myotubes with limited number of nuclei and that subsequent nuclear addition and increases in myotube size are controlled by a molecular pathway regulated by NFATC2. This dissertation further characterizes this pathway by identifying a lipid signaling molecule, prostaglandin $F2a$ as a novel activator of NFATC2. Prostaglandin $F2a$ activates NFATC2 in muscle cells and in doing so, increases myotube size by recruiting the fusion of muscle cells with pre-existing multinucleated cells. Furthermore, the work of this thesis determines that during muscle growth NFATC2 regulates the expression of IL-4, a novel molecular signal that controls myoblast fusion with myotubes. IL-4 is expressed by a subset of muscle cells in fusing muscle cultures and acts through the IL-4 receptor on myoblasts to promote myoblast fusion and muscle growth. We conclude that NFATC2 controls a key step in muscle growth through the novel regulation of myoblast fusion with growing muscle cells. These data implicate the nascent myotube as a heretofore unrecognized component of myogenesis.