Skeletal muscle cells have served as a paradigm for understanding mechanisms leading to cellular differentiation. The proliferation and differentiation of muscle precursor cells require the concerted activity of myogenic regulatory factors including MyoD. In addition, chromatin modifiers mediate dynamic modifications of histone tails that are vital to reprogramming cells toward terminal differentiation. Here, we provide evidence for a unique dimension to epigenetic regulation of skeletal myogenesis. We demonstrate that the lysine methyltransferase G9a is dynamically expressed in myoblasts and impedes differentiation in a methyltransferase activity-dependent manner. In addition to mediating histone H3 lysine-9 di-methylation (H3K9me2) on MyoD target promoters, endogenous G9a interacts with MyoD in precursor cells and directly methylates it at lysine 104 (K104) to constrain its transcriptional activity. Mutation of K104 renders MyoD refractory to inhibition by G9a and enhances its myogenic activity. Interestingly, MyoD methylation is critical for G9a-mediated inhibition of myogenesis. These findings provide evidence of an unanticipated role for methyltransferases in cellular differentiation states by direct posttranslational modification of a transcription factor.

Satellite cells play a critical role in skeletal muscle regeneration in response to injury. Notch signaling is vital for satellite cell activation and myogenic precursor cell expansion but inhibits myogenic differentiation. Thus, precise spatial and temporal regulation of Notch activity is necessary for efficient muscle regeneration. We report that the basic helix-loop-helix transcription factor Stra13 modulates Notch signaling in regenerating muscle. Upon injury, Stra13−/− mice exhibit increased cellular proliferation, elevated Notch signaling, a striking regeneration defect characterized by degenerated myotubes, increased mononuclear cells, and fibrosis. Stra13−/− primary myoblasts also exhibit enhanced Notch activity, increased proliferation, and defective differentiation. Inhibition of Notch signaling ex vivo and in vivo ameliorates the phenotype of Stra13−/− mutants. We demonstrate in vitro that Stra13 antagonizes Notch activity and reverses the Notch-imposed inhibition of myogenesis. Thus, Stra13 plays an important role in postnatal myogenesis by attenuating Notch signaling to reduce myoblast proliferation and promote myogenic differentiation.

In response to a number of genotoxic stimuli that induce DNA damage in cells, the tumour suppressor p53 is activated resulting in cell cycle arrest or apoptosis. In this study, we have identified stimulated with retinoic acid 13 (Stra13), a basic helix-loop-helix transcription factor, as a regulator of ionizing-radiation-induced apoptosis. We show that Stra13 is induced in response to several DNA-damaging agents in a p53-independent manner. Stra13 À/À thymocytes show impaired apoptosis in response to ionizing radiation, and consistently, p53 levels and also expression of its key transcriptional targets Puma and Noxa are reduced in the mutant thymocytes. In vitro, Stra13 regulates p53 levels in a mouse double mutant 2 (Mdm2)-dependent manner by physically interacting with p53 and preventing Mdm2-mediated ubiquitination and nuclear export. Together, our studies provide evidence that Stra13 is involved in DNA-damage-induced apoptosis and indicate its role in tumorigenesis.

Duchenne Muscular Dystrophy (DMD), caused by loss of dystrophin is characterized by progressive muscle cell necrosis. However, the mechanisms leading to muscle degeneration in DMD are poorly understood. Here, we demonstrate that Stra13 protects muscle cells from oxidative damage, and its absence leads to muscle necrosis in response to injury in Stra13-deficient mice. Interestingly, Stra13-/- mutants express elevated levels of TNFalpha, reduced levels of heme-oxygenase-1, and display apparent signs of oxidative stress prior to muscle death. Moreover, Stra13-/- muscle cells exhibit an increased sensitivity to pro-oxidants, and conversely, Stra13 overexpression provides resistance to oxidative damage. Consistently, treatment with anti-oxidant N-acetylcysteine ameliorates muscle necrosis in Stra13-/- mice. We also demonstrate that Stra13 expression is elevated in muscles from dystrophin-deficient (mdx) mice, and mdx/Stra13-/- double mutants exhibit an early onset of muscle degeneration. Our studies underscore the importance of oxidative stress-mediated muscle degeneration in muscular dystrophy, and reveal the contribution of Stra13 in maintenance of muscle integrity.

Sharp-1 is a basic helix-loop-helix (bHLH) transcriptional repressor that is involved in a number of cellular processes. Our previous studies have demonstrated that Sharp-1 is a negative regulator of skeletal myogenesis and it blocks differentiation of muscle precursor cells by modulating the activity of MyoD. In order to understand its role in pre-and post-natal myogenesis, we assessed skeletal muscle development and freeze-injury-induced regeneration in Sharp-1-deficient mice. We show that embryonic skeletal muscle development is not impaired in the absence of Sharp-1; however, post-natally, the regenerative capacity is compromised. Although the initial phases of injury-induced regeneration proceed normally in Sharp-1 2/2 mice, during late stages, the mutant muscle exhibits necrotic fibers, calcium deposits and fibrosis. TGF-b expression, as well as levels of phosphorylated Smad2 and Smad3, are sustained in the mutant tissue and treatment with decorin, which blocks TGF-b signaling, improves the histopathology of Sharp-1 2/2 injured muscles. In vitro, Sharp-1 associates with Smad3, and its overexpression inhibits TGF-b-and Smad3-mediated expression of extracellular matrix genes in myofibroblasts. These results demonstrate that Sharp-1 regulates muscle regenerative capacity, at least in part, by modulation of TGF-b signaling.