DIFFERENTIAL INTRACELLULAR-DISTRIBUTION AND ACTIVITIES OF MU-CALPAINSAND M-CALPAINS DURING THE DIFFERENTIATION OF HUMAN MYOGENIC CELLS IN CULTURE

Calpains are intracellular calcium-dependent cystein proteases active at neutral pH. There have been found in human adult myogenic cells (i. e. satellite cells) 2 forms of calpains requiring either micromolar Ca 2+:mu-calpain, or millimolar Ca2+:m-calpain. Calpains could be involve d in both intracellular proteolysis and cytoskeleton reorganization re quired for myogenic cell fusion. We showed significant differences in calpains distribution during differentiation of myogenic cells. Using mono- and polyclonal antibodies against both types of calpains, we loc alized mu-calpain and m-calpain in cultured human satellite cells. mu- calpain was detected in the nuclei of myoblasts and in the cytoplasm o f myotubes. m-calpain was only present in the cytoplasm, and was conce ntrated near the nuclear envelope. Biochemical assays for calpain acti vities showed that the amounts of these proteinases were modulated dur ing cell growth and differentiation. m-calpain activity was high at th e proliferation phase (day 4 of culture) and reached a maximum with th e beginning of fusion (day 8) and decreased slightly when the number o f myotubes increased (day 12). This activity profile suggests that m-c alpain could play a role in the initiation of fusion of satellite cell s. The activity of mu-calpain increased regularly with cell growth, th e maximum being reached when the cells differentiate, i.e. when its in tracellular localization shifted fr om the nucleus to the cytoplasm. W e conclude that the activity and the intracellular localization of the 2 forms of calpains differ with the state of differentiation of myoge nic cells.