Calpain Translocation during Muscle Fiber Necrosis and Regeneration in Dystrophin-Deficient Mice

Previous studies have shown that calpains are autolytically cleaved during the disease process of mdx dystrophy, a mouse model for Duchenne muscular dystrophy, indicating that calpains may be activated and play a role in proteolysis that occurs in muscular dystrophy (J. Biol. Chem.270(18), 10909–10914, 1995). In the present study, we investigated the location of calpain in dystrophic muscle fibers over the course of mdx dystrophy, to relate the protease distribution to its state of activation, and to determine whether calpain translocation was an early event in mdx dystrophy. Immunolabeling of healthy, fully differentiated muscle fibers showed calpain present throughout the cytosol, but more concentrated near the plasma membrane. However, degenerating mdx fibers did not contain higher concentrations of calpain at the plasma membrane and showed only a homogeneous, cytosolic distribution. Calpain distribution was similarly diffuse in young myotubes and regenerating fibers with increased cytosolic concentration in early myotubes. Calpain distribution in adult mdx tissue was similar to that occurring in healthy, fully differentiated fibers, although adult mdx fibers displayed higher concentrations of membrane-associated calpain than those observed in C57 controls. The association of calpain with the plasma membrane was verified by immunoblots of isolated sarcolemmal membrane from adult mdx and control muscle which showed calpain present predominantly in the cytosol along with some membrane association. Thus, changes in calpain distribution coincide with changes in enzymatic cleavage over the course of mdx dystrophy shown previously. Furthermore, the stages of pathology at which calpain cleavage is least coincides with those stages when calpain is most concentrated at the cell membrane, suggesting that calpain is retained in an inactive form at the plasma membrane.