m-calpain levels increase during fusion of myoblasts in the mutant muscular dysgenesis (mdg) mouse

Previous studies have led to the hypothesis of a possible role for the calc ium-dependent neutral protease m-calpain in myoblast fusion in culture. To evaluate this hypothesis, we chose as our model,the "muscular dysgenesis" m ouse (mdg), which presents in vivo and in vitro characteristics of an eleva ted process of fusion (Yao and Essien, 1975; Dussartre, 1993; Ashby ef al., 1993, Joffroy et al., 1999). The aim of this study was to demonstrate usin g myoblast cell lines and muscle biopsies from this mdg mutant, that the am ount of m-calpain increases significantly as multinucleated myotubes are fo rmed. Using immunoblot analysis, it was shown that the m-calpain concentrat ion in a dysgenic cell line (GLT) increased 3-fold compared to what it was upon the introduction of the differentiation medium. On the other hand, in a normal cell line (NLT), the concentration of m-calpain did not vary signi ficantly. Thus, when the transition from myoblasts to myotubes was slow, an d the absolute level of fusion was reduced, as in the NLT cell line, the le vel of m-calpain was stable. In contrast, when the process of fusion was pr ecocious and fast, and the level of fusion was elevated, such as in the GLT cell line, the concentration of m-calpain increased during fusion. Moreove r, when myoblast fusion was prevented by the addition of calpain inhibitor Il,the process was reduced by approximately 93%. Taking into account these observations, it is clear from our data that the muscular dysgenesis mouse provides a relevant model to study myoblast fusion and that m-calpain is in volved in this process.