BMP signaling regulates satellite cell-dependent postnatal muscle growth.

Postnatal growth of skeletal muscle largely depends on the expansion and differentiation of resident stem cells, the so-called satellite cells. Here, we demonstrate that postnatal satellite cells express components of the bone morphogenetic protein (BMP) signaling machinery. Overexpression of noggin in postnatal mice (to antagonize BMP ligands), satellite cell-specific knockout of Alk3 (the gene encoding the BMP transmembrane receptor) or overexpression of inhibitory SMAD6 decreased satellite cell proliferation and accretion during myofiber growth, and ultimately retarded muscle growth. Moreover, reduced BMP signaling diminished the adult satellite cell pool. Abrogation of BMP signaling in satellite cell-derived primary myoblasts strongly diminished cell proliferation and upregulated the expression of cell cycle inhibitors p21 and p57 In conclusion, these results show that BMP signaling defines postnatal muscle development by regulating satellite cell-dependent myofiber growth and the generation of the adult muscle stem cell pool.

Skeletal muscle cells possess a 'memory' of acute early life TNF-α exposure: role of epigenetic adaptation.

Sufficient quantity and quality of skeletal muscle is required to maintain lifespan and healthspan into older age. The concept of skeletal muscle programming/memory has been suggested to contribute to accelerated muscle decline in the elderly in association with early life stress such as fetal malnutrition. Further, muscle cells in vitro appear to remember the in vivo environments from which they are derived (e.g. cancer, obesity, type II diabetes, physical inactivity and nutrient restriction). Tumour-necrosis factor alpha (TNF-α) is a pleiotropic cytokine that is chronically elevated in sarcopenia and cancer cachexia. Higher TNF-α levels are strongly correlated with muscle loss, reduced strength and therefore morbidity and earlier mortality. We have extensively shown that TNF-α impairs regenerative capacity in mouse and human muscle derived stem cells [Meadows et al. (J Cell Physiol 183(3):330-337, 2000); Foulstone et al. (J Cell Physiol 189(2):207-215, 2001); Foulstone et al. (Exp Cell Res 294(1):223-235, 2004); Stewart et al. (J Cell Physiol 198(2):237-247, 2004); Al-Shanti et al. (Growth factors (Chur, Switzerland) 26(2):61-73, 2008); Saini et al. (Growth factors (Chur, Switzerland) 26(5):239-253, 2008); Sharples et al. (J Cell Physiol 225(1):240-250, 2010)]. We have also recently established an epigenetically mediated mechanism (SIRT1-histone deacetylase) regulating survival of myoblasts in the presence of TNF-α [Saini et al. (Exp Physiol 97(3):400-418, 2012)]. We therefore wished to extend this work in relation to muscle memory of catabolic stimuli and the potential underlying epigenetic modulation of muscle loss. To enable this aim; C2C12 myoblasts were cultured in the absence or presence of early TNF-α (early proliferative lifespan) followed by 30 population doublings in the absence of TNF-α, prior to the induction of differentiation in low serum media (LSM) in the absence or presence of late TNF-α (late proliferative lifespan). The cells that received an early plus late lifespan dose of TNF-α exhibited reduced morphological (myotube number) and biochemical (creatine kinase activity) differentiation vs. control cells that underwent the same number of proliferative divisions but only a later life encounter with TNF-α. This suggested that muscle cells had a morphological memory of the acute early lifespan TNF-α encounter. Importantly, methylation of myoD CpG islands were increased in the early TNF-α cells, 30 population doublings later, suggesting that even after an acute encounter with TNF-α, the cells have the capability of retaining elevated methylation for at least 30 cellular divisions. Despite these fascinating findings, there were no further increases in myoD methylation or changes in its gene expression when these cells were exposed to a later TNF-α dose suggesting that this was not directly responsible for the decline in differentiation observed. In conclusion, data suggest that elevated myoD methylation is retained throughout muscle cells proliferative lifespan as result of early life TNF-α treatment and has implications for the epigenetic control of muscle loss.

A systems-based investigation into vitamin D and skeletal muscle repair, regeneration, and hypertrophy.

Skeletal muscle is a direct target for vitamin D. Observational studies suggest that low 25[OH]D correlates with functional recovery of skeletal muscle following eccentric contractions in humans and crush injury in rats. However, a definitive association is yet to be established. To address this gap in knowledge in relation to damage repair, a randomised, placebo-controlled trial was performed in 20 males with insufficient concentrations of serum 25(OH)D (45 ± 25 nmol/l). Prior to and following 6 wk of supplemental vitamin D3 (4,000 IU/day) or placebo (50 mg of cellulose), participants performed 20 × 10 damaging eccentric contractions of the knee extensors, with peak torque measured over the following 7 days of recovery. Parallel experimentation using isolated human skeletal muscle-derived myoblast cells from biopsies of 14 males with low serum 25(OH)D (37 ± 11 nmol/l) were subjected to mechanical wound injury, which enabled corresponding in vitro studies of muscle repair, regeneration, and hypertrophy in the presence and absence of 10 or 100 nmol 1α,25(OH)2D3. Supplemental vitamin D3 increased serum 25(OH)D and improved recovery of peak torque at 48 h and 7 days postexercise. In vitro, 10 nmol 1α,25(OH)2D3 improved muscle cell migration dynamics and resulted in improved myotube fusion/differentiation at the biochemical, morphological, and molecular level together with increased myotube hypertrophy at 7 and 10 days postdamage. Together, these preliminary data are the first to characterize a role for vitamin D in human skeletal muscle regeneration and suggest that maintaining serum 25(OH)D may be beneficial for enhancing reparative processes and potentially for facilitating subsequent hypertrophy.