Chronic
inflammation in tibialis anterior muscles of mdx mice was produced by a single injection of a recombinant adenovirus vector (AV) expressing an immunogenic
beta-galactosidase (beta-gal). In regions of intense beta-gal staining, mononuclear infiltrates abounded, and muscle fibers showed strong extrasynaptic
utrophin immunostaining, restoration of
dystrophin-associated protein complex, and a marked reduction of the prevalence of centronucleation. Immunoblot analysis confirmed an increase of endogenous
utrophin without an increase of the
mRNA of the major muscle
isoform utrA. Significantly better maximal tetanic force values were demonstrated in the inflammatory versus control mdx muscles. The resistance to lengthening contraction- induced damage was also significantly increased in the former. In muscles of mice lacking
TNF-alpha gene, AV vector did not induce
inflammation and extrajunctional
utrophin increase did not occur. In the inflammatory mdx muscles, proteolytic activity of
calcium-activated
calpain was reduced, and in mdx myotubes in vitro, incubation with NO donors also reduced
calpain-mediated
utrophin proteolysis. Since
utrophin was shown to be a natural substrate of
calpain and known inhibitors of
calpain in cultured mdx myotubes increased
utrophin levels, the above results were consistent with the following conclusions: (1) extrasynaptic
utrophin increase is mainly responsible for the antidystrophic effect; (2) extrasynaptic
utrophin increase is a result of posttranscriptional mechanism(s) related to proinflammatory factors; and (3) reduction of endogenous muscle
calpain activity by inflammatory
cytokines has an important role in the stabilization and increase of the extrasynaptic
utrophin.