[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Skeletal muscle can be isolated into 642 individual muscles and makes up to one third to one half of the mass of the human body. Each of these muscles is specified and patterned prenatally and after birth they will increase in size and take on characteristics suited to each muscle's unique function. To make the muscles functional, each muscle cell must be innervated by a motor neuron, which will also affect the characteristics of the mature muscle. In a healthy adult, muscles will maintain their specialized pattern and function during physiological homeostasis, and will also recapitulate them if the integrity or health of the muscle is disrupted. This repair and regeneration is dependent satellite cells, the skeletal muscle stem cells. In this dissertation, we study a family of receptor tyrosine kinases, Ephs, and their juxtacrine ephrin ligands in the context of skeletal muscle specification and regeneration. First, using a classical ephrin 'stripe' assay to test for contact-mediated repulsion, we found that satellite cells respond to a subset of ephrins with repulsive motility in vitro and that these forward signals through Ephs also promote patterning of differentiating myotubes parallel to ephrin stripes. This pattering can be replicated in a heterologous in vivo system (the hindbrain of the developing quail, where neural crest cells migrate in streams to the branchial arches, and in the forelimb of the developing quail, where presumptive limb myoblasts emigrate from the somite). Second, we present evidence that specific pairwise interactions between Eph receptor tyrosine kinases and ephrin ligands are required to ensure appropriate muscle innervation when it is originally set during postnatal development and when it is recapitulated after muscle or nerve trauma during adulthood. We show expression of a single ephrin, ephrin-A3, exclusively on type I (slow) myofibers shortly after birth, while its receptor EphA8 is only localized to fast motor endplates, suggesting a functional repulsive interaction for motor axon guidance and/or synaptogenesis. Adult EFNA3-/- mutant mice show a significant loss of slow myofibers, while misexpression of ephrin-A3 on fast myofibers results in a switch from a fast fiber type to slow in the context of sciatic nerve injury and regrowth. Third, we show that EphA7 is expressed on satellite cell derived myocytes in vitro, and marks both myocytes and regenerating myofibers in vivo. In the EPHA7 knockout mouse, we find a regeneration defect in a barium chloride injury model starting 3 days post injection in vivo, and that cultured mutant satellite cells are slow to differentiate and divide. Finally, we present other potential Ephs and ephrins that may affect skeletal muscle, such as EphB1 that is expressed on all MyHC-IIb fibers and a subset of MyHC-IIx fibers, and we show a multitude of Ephs and ephrins at the neuromuscular junction that appear to localize on specific myofibers and at different areas of the synapse. We propose that Eph/ephrin signaling, though well studied in development, continues to be important in regulating post natal development, regeneration, and homeostasis of skeletal muscle.
NOTCH inhibition promotes myoblast fusion by releasing HEYL repression on TMEM8C regulatory regions in foetal skeletal muscles
