Abstract 1104: Myosin storage myopathy resulting from the loss of Muscle RING Finger 1 and 3
Skeletal muscle myopathy occurs in a wide variety of clinically important settings, such as heart failure, cachexia and prolonged inactivity. A hallmark of myopathies in these disorders is an activation of the ubiquitin-proteasome-system (UPS) and the degradation of contractile proteins of the sarcomere. However, the mechanisms that mediate turnover of these long-lived proteins by the UPS remain poorly defined. The E3 ubiquitin ligases Muscle RING-finger (MuRF) 1 and 3 are restricted to striated muscles and interact with sarcomeric proteins, but their function in maintenance of muscle structure is unknown. We hypothesized that MuRF1 and 3 are involved in the turnover of structural proteins and display, due to their overlapping expression pattern and sequence homology, redundant function. Using co-immunoprecipitation, we identified beta/slow-myosin heavy chain (MHC) and MHCIIa as novel interaction partners of MuRF1 and 3. In addition, we show that MuRF1 and 3 act as E3 ubiquitin ligases that cooperate specifically with the E2 ubiquitin ligases UbcH5a, b and c to mediate the degradation of beta/slow MHC and MHCIIa via the UPS in vivo and in vitro. Ubiquitination assays showed that MuRF1 and 3 ubiquitinate beta/slow MHC and MHCIIa in a RING-finger dependent manner. Accordingly, mice deficient for MuRF1 and 3 develop a skeletal muscle- and cardiomyopathy characterized by a massive subsarcolemmal MHC accumulation, atrophic myofibers and centralized nuclei. Electron microscopy revealed unassociated thick filaments and disoriented sarcomeric fragments and immunoblot analysis showed accumulation of degraded beta/slow MHC within the myopathic muscle. No accumulation of thin filaments was found by immunohistochemistry. The myopathy led to a reduction in maximal force development of the skeletal muscle, cardiac hypertrophy and decreased cardiac function. We conclude that MuRF1 and 3 are key regulators of the UPS-dependent turnover of MHC proteins and that their absence leads to a protein surplus myopathy in striated muscles. This function of MuRF1 and 3 is centrally involved in the maintenance of skeletal muscle and cardiac structure and function and occurs in a redundant manner. These findings reveal a molecular basis for myosin storage myopathies.