Adhesion molecule Kirrel3/Neph2 is required for the elongated shape of myocytes during skeletal muscle differentiation.

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Kirrel/Neph proteins are evolutionarily conserved members of the immunoglobulin superfamily of adhesion proteins. Kirrel3 is the mouse orthologue of Dumbfounded (Duf), a family member that regulates myoblast pre-fusion events in Drosophila. Yet, the role of Kirrel3 in mammalian myogenesis has not been demonstrated. Experiments performed here indicate that the mouse Kirrel3 protein regulates morphological changes of myoblasts that are required for their subsequent fusion into multinucleated myotubes. We show that Kirrel3 is transiently expressed at the tips of myocytes during early myoblast differentiation and that its expression is dependent on the myogenic transcription factor, MyoD. Kirrel3 is transported in vesicles into the plasma membrane and its extracellular domain is cleaved in a proteasome-dependent manner. C-terminal deletion mutant lacking most of the intracellular domain accumulates at cellular extensions, does not undergo extracellular cleavage and induces the formation of large cell aggregates. This result suggests that the processing of the extracellular domain is regulated by the receptor's intracellular region. Knock-down of Kirrel3 in primary muscle progenitor cells (MPCs) prevented spindle shape myocyte formation, and significantly reduced their fusion with other myocytes to form multinucleated myotubes. In addition, migration of Kirrel3-deficient MPCs was randomized relative to the directed migration of control MPCs. We conclude that mouse Kirrel3 is a myobast adhesion molecule which promotes the morphological change of rounded MPC to a spindle shaped myocyte that migrates in a directed fashion and participates in the tight interactions between myocytes prior to their fusion.