Glucose is a major energy source consumed by proliferating mammalian cells. Therefore, in general, proliferating cells have the preference of high glucose contents in extracellular environment. Here, we showed that high glucose concentrations impede the proliferation of satellite cells, which are muscle-specific stem cells, under adherent culture conditions. We found that the proliferation activity of satellite cells was higher in glucose-free DMEM growth medium (low-glucose medium with a glucose concentration of 2 mM) than in standard glucose DMEM (high-glucose medium with a glucose concentration of 19 mM). Satellite cells cultured in the high-glucose medium showed a decreased population of reserve cells, identified by staining for Pax7 expression, suggesting that glucose concentration affects cell fate determination. In conclusion, glucose is a factor that decides the cell fate of skeletal muscle-specific stem cells. Due to this unique feature of satellite cells, hyperglycemia may negatively affect the regenerative capability of skeletal muscle myofibers and thus facilitate sarcopenia.

Chronic antioxidant (piceatannol or enzymatically modified isoquercitrin) supplementation improved metabolic profile and locomotor activity in aging mice.

In this study, we identified a previously uncharacterized skeletal satellite cell‐secreted protein, R3h domain containing‐like (R3hdml). Expression of R3hdml increases during skeletal muscle development and differentiation in mice. Body weight and skeletal muscle mass of R3hdml knockout (KO) mice are lower compared to control mice. Expression levels of cell cycle‐related markers, phosphorylation of Akt, and expression of insulin‐like growth factor within the skeletal muscle are reduced in R3hdml KO mice compared to control mice. Expression of R3hdml increases during muscle regeneration in response to cardiotoxin (CTX)‐induced muscle injury. Recovery of handgrip strength after CTX injection was significantly impaired in R3hdml KO mice, which is rescued by R3hdml. Our results indicate that R3hdml is required for skeletal muscle development, regeneration, and, in particular, satellite cell proliferation and differentiation.

Muscle fibres are broadly categorised into types I and II; the fibre-type ratio determines the contractile and metabolic properties of skeletal muscle tissue. The maintenance of type I fibres is essential for the prevention of obesity and the treatment of muscle atrophy caused by type 2 diabetes or unloading. Some reports suggest that myokines are related to muscle fibre type determination. We thus explored whether a myokine determines whether satellite cells differentiate to type I fibres. By examining the fibre types separately, we identified R-spondin 3 (Rspo3) as a myokine of interest, a secreted protein known as an activator of Wnt signalling pathways. To examine whether Rspo3 induces type I fibres, primary myoblasts prepared from mouse soleus muscles were exposed to a differentiation medium containing the mouse recombinant Rspo3 protein. Expression of myosin heavy chain (MyHC) I, a marker of type I fibre, significantly increased in the differentiated myotubes compared with a control. The Wnt/β-catenin pathway was shown to be the dominant signalling pathway which induces Rspo3-induced MyHC I expression. These results revealed Rspo3 as a myokine that determines whether satellite cells differentiate to type I fibres.

Introduction: Endurance training induces a fiber type shift from fast to slow, mitochondria biogenesis, and increased oxidative capacity, which together are known as the skeletal muscle adaptation. To understand how training induces the skeletal muscle adaptation, the study has generally been done an in vivo model because of the lack of an appropriate cell culture model representing these phenomena. Yet the underlying mechanism remains unknown. Therefore, an in vitro training model with skeletal muscle cells is required to elucidate the molecular mechanism of muscle adaptation induced by training. The purpose of this study is to establish a chronic muscle contraction model of cultured myotubes that mimic the in vivo endurance-training-induced adaptation. Methods: Mouse primary myotubes derived from skeletal muscle satellite cells were used for this study. Mouse extensor digitorum longus was digested in DMEM supplemented with collagenase, and satellite cells were cultured in growth medium including 30% FBS. Satellite-cell-derived myoblasts were differentiated to myotubes by switching the differentiation medium (5% horse serum in DMEM). Three days after differentiation, the myotubes were stimulated with electric pulses at 10 Hz (twitch condition) for different periods (consecutively for 24 h, 72 h, or intermittently for 8 days) or 100 Hz (tetanic condition) for 48 h or 96 h. The myotubes were harvested after stimulation, and the protein expressions of myosin heavy chain (MyHC) I, MyHC II, hexokinase II, glucose transporter 4, myoglobin, and COX IV were quantified by immunoblotting. Results: The contraction of cultured primary myotubes persisted for 8 days under the twitch condition without any visible changes. Contrary to expectations, the protein expressions were not changed by twitch contraction for 24 h, 72 h or 8 days. Under tetanic contraction for 48 h and 96 h, the protein expressions were unchanged. Discussions: We successfully made primary myotubes that endured the twitch condition for 8 days and the tetanic condition for 96 h. However, the protein expressions induced by endurance training were not accompanied by either chronic twitch or tetanic contraction. We suggest two possible reasons that the protein expressions were unchanged after continuous contraction. First, the stimulus conditions adopted in this study were not sufficient to induce a change of protein expression. Second, the protein expression accompanied by endurance training is not only induced by physical contraction but also by other factors such as neurotransmitters and cytokines derived from immune cells.