G.P.85: Truncated dystrophin with exon 45–55 deletion induced muscle atrophy and fiber type change through the hyper-nitrosylation of the ryanodine receptor type-1 and constant release of Ca2+to the cytosol

Abstract

Duchenne muscular dystrophy (DMD), the most common form of muscular dystrophy, is caused by lack of dystrophin. Exon skipping has been developed as a possible treatment of DMD and exon 45–55 skipping strategy could be one of goals of this therapy, partly because patients with exon 45–55 in-frame deletion have shown very mild skeletal muscle symptoms. Moreover, this strategy could theoretically rescue up to 63% of DMD deletion patients, however function of exon 45–55 deleted dystrophin is not well understood. To clarify the functional role of truncated dystrophin, we generated the transgenic (Tg) mice which expressed the exon 45–55 lacking human dystrophin and two Tg lines were established. We, then, mated Tg male mice with <i>mdx</i> female mice to get Tg/<i>mdx</i> male mice and extensively analyzed the phenotypes. These Tg/<i>mdx</i> mice expressed only truncated dystrophin, accompanied by recovery of the dystrophin–glycoprotein complex at the sarcolemma. The Tg/<i>mdx</i> also showed improvement of the function, including serum creatine kinase level, muscle pathology and specific tetanic force, to a level of the wild type. These results suggest that truncated dystrophin with exon 45–55 deletion has a equivalent function to the full length dystrophin. On the other hand, atrophy of type2B fibers and increasing of type2A fibers were observed at 20weeks of Tg/<i>mdx</i> mice. Moreover, the localization of neuronal NOS (nNOS) in Tg/<i>mdx</i> was changed from sarcolemma to cytosol and nNOS itself was activated. This activation induced hyper-nitrosylation of the ryanodine receptor type-1 and Ca²⁺ was constantly released from sarcoplasmic reticulum to cytosol. This Ca²⁺ release might be related to the findings, up-reguletion of Atrogin-1 and MuRF1, which could induce muscle atrophy and PGC1<i>α</i>, which could alter the muscle fiber types. This Tg/<i>mdx</i> mouse could be also a very useful model for analyzing pathogenesis of BMD phenotypes.