Myosin Storage Myopathy Mutations Disrupt Myofibrillar Assembly/ Stability and Cause Progressive Muscle Degeneration in a Drosophila Model

Abstract

Myosin storage myopathy (MSM) is a congenital disorder caused by dominant missense mutations in the β-cardiac MHC rod and characterized by subsarcolemmal accumulation of β-cardiac myosin that has a hyaline appearance. These mutations map near to or within the assembly competence domain known to be crucial to filament assembly. The mutations disrupt hydropathy or charge of residues in the heptad repeat, altering interactions that stabilize myosin coiled-coil dimers and thick filaments. This potentially disrupts ordered myofibrillar assembly, causing myofibrillar disarray and myosin aggregation. Our Drosophila models for MSM make it possible to examine interactions between wild-type and mutant full-length myosins for pursuing mechanistic investigations. We introduced the R1845W, L1793P or the E1883K mutation into a Drosophila MHC transgene and expressed each in the indirect flight/jump muscles and in the heart. Our studies show a severe reduction in the flight and jump ability of the transgenic flies in both homozygous and heterozygous states, with an age-dependent worsening of muscle function. Electron and confocal microscopy of the indirect flight muscles of transgenic lines show myofibrillar disarray with large areas of granular/ filamentous inclusions similar to hyaline bodies found in affected humans. Semi-automated optical heartbeat analysis of the mutant heterozygotes shows restrictive cardiac physiology and diastolic dysfunction with evidence of worsening cardiac phenotype with age. Lifespans of the MSM mutants are also reduced in comparison to the transgenic control. Future studies will aim at analyzing in vitro filament forming ability of the mutant myosin to determine if defective filament formation and/or instability of the myosin filaments are the basis of MSM. Our model would also potentially help discern if specific chaperones, small molecule chaperone inducers or enhanced autophagy can ameliorate myopathic defects in MSM.