The role of calpastatin (the specific calpain inhibitor) in myoblast differentiation and fusion.

Using red cells as an experimental model, we previously showed that a limited degradation of certain membrane proteins by calpain (Ca2+-activated thiol protease) was a necessary prerequisite for cell fusion and that fusibility depended on the ratio of calpain to its endogenous inhibitor calpastatin. Here we show that fusion of rat L8 line myoblasts is accompanied by a dramatic change in the calpain/calpastatin ratio. The protein levels of mu-calpain and m-calpain increased only slightly during myoblast differentiation. In contrast, calpastatin diminished by a factor of 10 at the stages of myoblast alignment and start of fusion, allowing calpain activity to become apparent. Calpastatin reappeared at a late stage of myoblast fusion (myotube formation). The results indicate that calpastatin is regulated during myoblast differentiation, and that its diminution is important in determining the activity of the calpain required for myoblast fusion.