Myostatin negatively regulates satellite cell activation and self-renewal

Satellite cells are quiescent muscle stem cells that promote postnatal muscle growth and repair. Here we show that myostatin, a TGF-β member, signals satellite cell quiescence and also negatively regulates satellite cell self-renewal. BrdU labeling in vivo revealed that, among the Myostatin-deficient satellite cells, higher numbers of satellite cells are activated as compared with wild type. In contrast, addition of Myostatin to myofiber explant cultures inhibits satellite cell activation. Cell cycle analysis confirms that Myostatin up-regulated p21, a Cdk inhibitor, and decreased the levels and activity of Cdk2 protein in satellite cells. Hence, Myostatin negatively regulates the G1 to S progression and thus maintains the quiescent status of satellite cells. Immunohistochemical analysis with CD34 antibodies indicates that there is an increased number of satellite cells per unit length of freshly isolated Mstn−/− muscle fibers. Determination of proliferation rate suggests that this elevation in satellite cell number could be due to increased self-renewal and delayed expression of the differentiation gene (myogenin) in Mstn−/− adult myoblasts. Taken together, these results suggest that Myostatin is a potent negative regulator of satellite cell activation and thus signals the quiescence of satellite cells

Irisin treatment improves healing of dystrophic skeletal muscle

Background: Irisin is an exercise induced myokine that is shown to promote browning of adipose tissue and hence, increase energy expenditure. Furthermore, our unpublished results indicate that Irisin improves myogenic differentiation and induces skeletal muscle hypertrophy. Since exercise induced skeletal muscle hypertrophy improves muscle strength, we wanted to investigate if ectopic injection of Irisin peptide improves skeletal muscle function in a mouse model of muscular dystrophy. This utility of Irisin peptide is yet to be studied in animal models. Methods: In order to test this hypothesis, we expressed and purified recombinant murine Irisin peptide from E. coli. Three- to six-week-old male mdx mice were injected IP with either vehicle (dialysis buffer) or Irisin recombinant peptide for two or four weeks, three times-a-week. Results: Irisin injection increased muscle weights and enhanced grip strength in mdx mice. Improved muscle strength can be attributed to the significant hypertrophy observed in the Irisin injected mdx mice. Moreover, Irisin treatment resulted in reduced accumulation of fibrotic tissue and myofiber necrosis in mdx mice. In addition, Irisin improved sarcolemmal stability, which is severely compromised in mdx mice. Conclusion: Irisin injection induced skeletal muscle hypertrophy, improved muscle strength and reduced necrosis and fibrotic tissue in a murine dystrophy model. These results demonstrate the potential therapeutic value of Irisin in muscular dystrophy

Irisin is a pro-myogenic factor that induces skeletal muscle hypertrophy and rescues denervation-induced atrophy

Exercise induces expression of the myokine irisin, which is known to promote browning of white adipose tissue and has been shown to mediate beneficial effects following exercise. Here we show that irisin induces expression of a number of pro-myogenic and exercise response genes in myotubes. Irisin increases myogenic differentiation and myoblast fusion via activation of IL6 signaling. Injection of irisin in mice induces significant hypertrophy and enhances grip strength of uninjured muscle. Following skeletal muscle injury, irisin injection improves regeneration and induces hypertrophy. The effects of irisin on hypertrophy are due to activation of satellite cells and enhanced protein synthesis. In addition, irisin injection rescues loss of skeletal muscle mass following denervation by enhancing satellite cell activation and reducing protein degradation. These data suggest that irisin functions as a pro-myogenic factor in mice

Parkin deficiency exacerbates fasting-induced skeletal muscle wasting in mice

Parkinson's Disease (PD) is a chronic and progressive neurodegenerative disease manifesting itself with tremors, muscle stiffness, bradykinesia, dementia, and depression. Mutations of mitochondrial E3 ligase, PARKIN, have been associated with juvenile PD. Previous studies have characterized muscle atrophy and motor deficits upon loss of functional Parkin in fly and rodent models. However, the mechanisms behind pathophysiology of Parkin deficient muscle remains to be elusive. Here, results suggested that knock down of Parkin significantly increases proteolytic activities in skeletal muscle cell line, the C2C12 myotubes. However, the atrogene levels increase moderately in Parkin deficient cell line. To further investigate the role of Parkin in skeletal muscle atrophy, Parkin knock out (KO) and wild type mice were subjected to 48 h starvation. After 48 h fasting, a greater reduction in skeletal muscle weights was observed in Parkin KO mice as compared to age matched wild type control, suggesting elevated proteolytic activity in the absence of Parkin. Subsequent microarray analyses revealed further enhanced expression of FOXO and ubiquitin pathway in fasted Parkin KO mice. Furthermore, a greater reduction in the expression of cytoskeleton genes was observed in Parkin KO mice following 48 h fasting. Collectively, these results suggest that Parkin deficiency exacerbates fasting-induced skeletal muscle wasting, through upregulating genes involved in catabolic activities in skeletal muscle.Agency for Science, Technology and Research (A*STAR)Nanyang Technological UniversityPublished versionThis work was supported by Nanyang Technological University, National University of Singapore, and Singapore Institute for Clinical Sciences. N.P. was supported with PhD and Postdoctoral fellowships by TEV-SINGA, A*STAR and Nanyang Technological University

Role of myostatin in skeletal muscle growth and cevelopment: implications for sarcopenia

Myostatin is a secreted growth and differentiating factor that belongs to TGF-bsuper-family. Myostatin is expressed in skeletal muscle predominantly. Low levels of myostatin expression are seen in heart, adipose tissue and mammary gland. Naturally occurring mutations in bovine, ovine, canine and human myostatingene or inactivation of the murine myostatingene lead to an increase in muscle mass due to hyperplasia. Molecularly, myostatin has been shown to regulate muscle growth not only by controlling myoblast proliferation and differentiation during fetal myogenesis, but also by regulating satellite cell activation and self-renewal postnatally. Consistent with the molecular genetic studies, injection of several myostatin blockers including Follistatin, myostatin antibodies and the Prodomain of myostatin have all been independently shown to increase muscle regeneration and growth in muscular dystrophy mouse models of muscle wasting. Furthermore, prolonged absence of myostatinin mice has also been shown to reduce sarcopenic muscle loss, due to efficient satellite cell activation and regeneration of skeletal muscle in aged mice. Similarly, treatment of aged mice with Mstn-ant 1 also increased satellite cell activation and enhanced the efficiency of muscles to regen-erate. Given that antagonism of myostatin leads to significant increase in postnatal muscle growth, we propose that myostatin antagonists have tremendous therapeutic value in alleviating sarcopenic muscle loss

Role of myostatin in skeletal muscle growth and cevelopment: implications for sarcopenia

Myostatin is a secreted growth and differentiating factor that belongs to TGF-bsuper-family. Myostatin is expressed in skeletal muscle predominantly. Low levels of myostatin expression are seen in heart, adipose tissue and mammary gland. Naturally occurring mutations in bovine, ovine, canine and human myostatingene or inactivation of the murine myostatingene lead to an increase in muscle mass due to hyperplasia. Molecularly, myostatin has been shown to regulate muscle growth not only by controlling myoblast proliferation and differentiation during fetal myogenesis, but also by regulating satellite cell activation and self-renewal postnatally. Consistent with the molecular genetic studies, injection of several myostatin blockers including Follistatin, myostatin antibodies and the Prodomain of myostatin have all been independently shown to increase muscle regeneration and growth in muscular dystrophy mouse models of muscle wasting. Furthermore, prolonged absence of myostatinin mice has also been shown to reduce sarcopenic muscle loss, due to efficient satellite cell activation and regeneration of skeletal muscle in aged mice. Similarly, treatment of aged mice with Mstn-ant 1 also increased satellite cell activation and enhanced the efficiency of muscles to regen-erate. Given that antagonism of myostatin leads to significant increase in postnatal muscle growth, we propose that myostatin antagonists have tremendous therapeutic value in alleviating sarcopenic muscle loss