Description
Myosin storage myopathy (MSM) is a congenital muscle disorder caused by missense mutations in the _-cardiac/ slow muscle MHC rod and characterized by subsarcolemmal accumulation of MHC that has a hyaline appearance. MSM causing mutations change hydropathy or charge of the residues in the heptad repeat altering interactions within the MHC dimers or between the MHC coiled-coils. We hypothesized that these rod mutations interrupt assembly of coiled-coil rod dimers into thick filaments or alter their stability, resulting in aggregation. We have made a Drosophila model for MSM which can serve as a powerful tool for mechanistic investigations. We introduced the R1845W, L1793P or the E1883K mutation in the Drosophila Mhc gene, and introduced them into the fly genome by germ-line transformation technique. Transgenic fly lines were created that express mutant MHC within the indirect flight muscle (IFM) and jump muscle in the absence of endogenous MHC. Our results indicated a severe compromise in the structure and function of the muscles expressing the mutant MHC. The transgenic flies showed a severe reduction in the flight and jump in both homozygous and heterozygous states with an age-dependent loss of muscle function. Immunofluorescence confocal microscopy of the homozygotes showed alteration of the sarcomeric and myofibrillar structure in young adults. Electron microscopy of IFMs of late-stage pupae of homozygotes showed disrupted myofilament packing in the sarcomeres and cracking in the filament lattice within the myofibrils. Large areas of granular/ filamentous inclusions were identified within the IFMs similar to hyaline bodies found in affected humans. Young heterozygotes also showed altered sarcomeric and myofibrillar structural integrity, with areas of hyaline-like inclusions. In addition, heterozygotes of at least two mutants showed restrictive cardiomyopathy phenotype with arrhythmia that mimics cardiomyopathy reported in human patients. The results of this study demonstrated our success in creating the first animal model for human MSM that mirrors at least some of the muscle phenotypes in humans. In addition, it provides evidence that alteration in the myosin rod region perturbs sarcomeric organization and filament self-assembly, causing myopathic defects. Future studies will be aimed at examining the biophysical properties of the mutant myosin to explore the mechanistic basis of MSM and at identifying potential therapeutic approaches. This will include attempts to clear aggregates and ameliorate induced MSM by over-expressing myosin chaperones or the autophagic response.
