Studies on mechanisms of calpain activation in the cell

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Calpain is a "modulator protease" involved in Ca^<2+>-mediated signal transduction processes essential for cellular functions. Physiological functions, although yet unclear, of calpains are so important that a deficient calpain activity causes various diseases including muscular dystrophies, ischemia diseases, cancers, diabetes, and Alzheimer's disease. In these diseases, inhibition of calpain activity is often reported to improve the pathological states of patients, thus indicating the importance of calpain inhibitors and understanding of mechanisms of calpain activation. In this study, we expressed recombinant proteins corresponding to the protease domain of calpain and to EGFP-tagged C105S dominant negative m-calpain, and used to study molecular mechanisms of the regulation of calpain activity. To our surprise, the protease domain of m-calpain showed Ca^<2+>-dependent protease activity, indicating that the protease domain itself binds Ca^<2+>. This is a very important information as to how calpain activity is regulated. Moreover, using C105S-dominanto negative m-calpain, proteolysis of cyteskeletal proteins such as fodrin and talin showed differential susceptibility to m- and μ-calpains. Calpastatin, which is the highly specific and powerful endogenous inhibitor protein for calpain, did not suppress proteolysis of these proteins. These results strongly suggest that for proteolysis of calpain and its regulation, membrane, rather than calpastatin, is very important factor.