DESMIN DEGRADATION AND CA2-DEPENDENT PROTEOLYSIS DURING MYOBLAST FUSION()

It has already been reported that, in vitro, intermediate filaments su ch as desmin and vimentin are very susceptible to proteolysis by calpa ins (Ca2+-activated cysteine proteinases). On the other hand, desmin a nd m-calpain are both present at the onset of myoblast fusion and thro ughout this phenomenon. Based on these observations, the aim of this s tudy was to demonstrate, with cultured rat myoblasts, that the amount of desmin decreased significantly as multinucleated myotubes were form ed. Using immunoblot analysis, it has been shown that the desmin conce ntration decreased 41% as myoblasts fuse. Moreover, under conditions w hich stimulate myoblast fusion, desmin concentration was reduced by 21 % compared to the control culture. Under our experimental conditions, which lead to a reduced desmin level, the amount of m-calpain was incr eased about three-fold. These results suggested that m-calpain could b e involved in myoblast fusion via desmin cleavage. This hypothesis was confirmed by the results obtained after calpeptin treatment. In the p resence of this cell-penetrating inhibitor of calpains, desmin seems n ot-to be degraded. Taking into account the observations obtained after different hydrolysis assays and as compared to those observed-on cult ured cells, it seems conceivable that m-calpain would be able to initi ate desmin cleavage leading to the formation of-proteolytic fragments which should be immediately degraded.