CALPAINS ARE ACTIVATED IN NECROTIC FIBERS FROM MDX DYSTROPHIC MICE

Death of dystrophin-deficient muscle purportedly re suits from increas es in [Ca](in) that cause the activation of calpains. We have tested w hether calpains play a role in this process by assaying for changes in calpain concentration and activation in peak necrotic mdx mice (4 wee ks of age) and in completely regenerated mdx mice (14 weeks of age). B iochemical fractionation and immunoblotting with epitope-specific anti sera allowed measurement of the concentrations of m- and mu-calpains a nd the extent of autoproteolytic modification. Our findings show that total calpain concentration is elevated in both 4-week and 14-week mdx mice. This increase in concentration was shown to result primarily fr om a significant increase in m-calpain concentration at 4 weeks. North ern analysis demonstrated that neither m- nor mu-calpain mRNA concentr ations differed between mdx and controls suggesting that the increased calpain concentration results from post-translational regulation. Imm unoblotting with antibodies directed against amino-terminal peptides r evealed an increase in autoproteolysis of mu-calpain, indicative of in creased activation. The extent of autoproteolysis of mu-calpain return s to control levels during regeneration. This is not a consequence of increased calpastatin mRNA or protein. The findings reported here supp ort a role for calpains in both the degenerative and regenerative aspe cts of mdx dystrophy.