Dystrophin deficiency in canine X-linked muscular dystrophy in Japan (CXMD(J)) alters myosin heavy chain expression profiles in the diaphragm more markedly than in the tibialis cranialis muscle

BACKGROUND: Skeletal muscles are composed of heterogeneous collections of muscle fiber types, the arrangement of which contributes to a variety of functional capabilities in many muscle types. Furthermore, skeletal muscles can adapt individual myofibers under various circumstances, such as disease and exercise, by changing fiber types. This study was performed to examine the influence of dystrophin deficiency on fiber type composition of skeletal muscles in canine X-linked muscular dystrophy in Japan (CXMD(J)), a large animal model for Duchenne muscular dystrophy. METHODS: We used tibialis cranialis (TC) muscles and diaphragms of normal dogs and those with CXMD(J )at various ages from 1 month to 3 years old. For classification of fiber types, muscle sections were immunostained with antibodies against fast, slow, or developmental myosin heavy chain (MHC), and the number and size of these fibers were analyzed. In addition, MHC isoforms were detected by gel electrophoresis. RESULTS: In comparison with TC muscles of CXMD(J), the number of fibers expressing slow MHC increased markedly and the number of fibers expressing fast MHC decreased with growth in the affected diaphragm. In populations of muscle fibers expressing fast and/or slow MHC(s) but not developmental MHC of CXMD(J )muscles, slow MHC fibers were predominant in number and showed selective enlargement. Especially, in CXMD(J )diaphragms, the proportions of slow MHC fibers were significantly larger in populations of myofibers with non-expression of developmental MHC. Analyses of MHC isoforms also indicated a marked increase of type I and decrease of type IIA isoforms in the affected diaphragm at ages over 6 months. In addition, expression of developmental (embryonic and/or neonatal) MHC decreased in the CXMD(J )diaphragm in adults, in contrast to continuous high-level expression in affected TC muscle. CONCLUSION: The CXMD(J )diaphragm showed marked changes in fiber type composition unlike TC muscles, suggesting that the affected diaphragm may be effectively adapted toward dystrophic stress by switching to predominantly slow fibers. Furthermore, the MHC expression profile in the CXMD(J )diaphragm was markedly different from that in mdx mice, indicating that the dystrophic dog is a more appropriate model than a murine one, to investigate the mechanisms of respiratory failure in DMD.