Myoblasts migration is a key step in myogenesis and regeneration. It allows myoblasts alignment and their fusion into myotubes. The process has been shown to involve m- or m-calpains, two calcium-dependent cysteine proteases. In the present work, we measured calpain activity in cultured cells (fluorometric measurements) and show, for the first time, a peak of activity at the beginning of the differentiation process. We also observed a concomitant and transient increase of the influx of Ca²⁺ and of the expression of TRPC1 protein. Besides, we recently reported that, in adult skeletal muscle fibres, calpains were specifically activated by a store-operated entry of calcium. In the present study, we therefore repressed the expression of TRPC1 in myoblasts and studied the effects on Ca²⁺ fluxes and on differentiation. TRPC1-depleted myoblasts presented a largely reduced store-operated entry of calcium and a significantly diminished transient influx of calcium at the beginning of differentiation. The concomitant peak of calpain activity was abolished.

The fusion of TRPC1-knocked-down myoblasts into myotubes was significantly slowed down, due to a reduced speed of cell migration. Accordingly, migration of control myoblasts was inhibited by 2 to 5 mM GsMTx4 toxin, an inhibitor of TRP channels or by 50 mM Z-Leu-Leu, an inhibitor of calpain. In contrast, stimulation of control myoblasts with IGF-1 increased the basal influx of Ca²⁺, activated calpain and accelerated migration. These effects were not observed in TRPC1 knocked down cells. We therefore suggest that an entry of calcium through TRPC1 channels induces a transient activation of calpain allowing myoblasts migration and fusion.