Cell Biology and Metabolism
Calpains Are Activated in Necrotic Fibers from mdx Dystrophic Mice *

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Death of dystrophin-deficient muscle purportedly results from increases in [Ca]in that cause the activation of calpains. We have tested whether calpains play a role in this process by assaying for changes in calpain concentration and activation in peak necrotic mdx mice (4 weeks of age) and in completely regenerated mdx mice (14 weeks of age). Biochemical fractionation and immunoblotting with epitope-specific antisera allowed measurement of the concentrations of m- and μ-calpains and 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 from a significant increase in m-calpain concentration at 4 weeks. Northern analysis demonstrated that neither m- nor μ-calpain mRNA concentrations differed between mdx and controls suggesting that the increased calpain concentration results from post-translational regulation. Immunoblotting with antibodies directed against amino-terminal peptides revealed an increase in autoproteolysis of μ-calpain, indicative of increased activation. The extent of autoproteolysis of μ-calpain returns to control levels during regeneration. This is not a consequence of increased calpastatin mRNA or protein. The findings reported here support a role for calpains in both the degenerative and regenerative aspects of mdx dystrophy.

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This work was supported by Grants AR-40343 from the National Institutes of Health (to J. G. T.), DCB90-96188 and MCB93-19602 from the National Science Foundation (to D. E. C.), a grant from the Muscular Dystrophy Association (to J. G. T.), and the UCLA dissertation year fellowship from the UCLA graduate division (to M. J. S.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.