Lactate, a byproduct of pyruvate in the glycolytic pathway, has been recognized as a signaling molecule and a regulator of gene expression. In skeletal muscles, lactate is dynamically regulated during exercise and influences muscular function, including myogenic differentiation and metabolism. The effects of lactate vary depending on lactate levels, which are influenced by exercise intensity, type, and duration. Furthermore, the effects of lactate on cellular signaling are different during the stages of myogenic differentiation. However, the distribution of lactate signaling in terms of lactate concentration, signaling types, and myogenesis has not been fully elucidated. In this study, we investigated the dual effects of lactate on myogenic differentiation and viability using C2C12 cells and C57BL/6 mice. Low levels of lactate treatment promoted myogenesis in the early stage of C2C12 differentiation, while high lactate concentrations or treatment with 3,5-DHBA, a GPR81 agonist, impaired cell viability during late myogenic differentiation. Transcriptomic analysis and knockdown experiments revealed that lactate promotes myogenesis and muscular metabolic functions through the induction of Ranbp3l and Nfat5 expressions. On the other hand, the detrimental effects of lactate on cell survival are mediated by the GPR81-induced PI3K-Akt/ERK-Atf4 axis. GPR81 signaling also feeds forward the expression of Hcar1 via Akt and ERK. These dual actions of lactate on skeletal muscle were also observed in vivo through lactate or 3,5-DHBA injections and exercise training models. Our study concludes that maintaining a balance in lactate signaling is crucial for regulating skeletal muscle phenotypes in response to exercise and lactate treatments.

Visually emotive stimuli modulate postural control via cognitive pathways, and while colored lenses alter emotional responses, their impact on postural control remains uncertain. Previous studies showed that individually optimized colored glasses enhanced reading performance. However, it remains unclear if these effects extend to postural control. This study aimed to examine the relationship between colors and postural control with colored lenses against a white background, hypothesizing that postural control may be affected by individually different colors. Twelve healthy adults performed a single-leg stance test on a force plate under 26 color conditions, with each condition repeated six times in a non-consecutive manner. Three color conditions with the smallest, 2nd smallest and 3rd smallest environmental area (ENV) of center of pressure (COP) changes in 10 s were individually selected as the participant's best colors. Individual worst colors were selected as those with the largest, 2nd largest and 3rd largest ENV. COP changes in the front hop landing test were examined for the selected 6 color conditions with the transparent condition as a control. The average ENV of the selected best color was significantly smaller than the transparent condition, and that of the selected worst color was significantly larger than the transparent condition in the single-leg stance test. The best or worst colors were individually different, and there were no specific trends in colors or concentrations. The selected color conditions tested in the front hop test revealed that the worst colors had larger ENV than the transparent condition, but the best color conditions had no advantage in postural control. In conclusion, visual color conditions provided by colored lenses have significant impact on postural control both in favorable and unfavorable way, and the color conditions were individually different.

Proteasome-dependent protein degradation and the digestion of peptides by aminopeptidases are essential for myogenesis. Methionine aminopeptidases (MetAPs) are uniquely involved in, both, the proteasomal degradation of proteins and in the regulation of translation (via involvement in post-translational modification). Suppressing MetAP1 and MetAP2 expression inhibits the myogenic differentiation of C2C12 myoblasts. However, the molecular mechanism by which inhibiting MetAPs impairs cellular function remains to be elucidated. Here, we provide evidence for our hypothesis that MetAPs regulate proteostasis and that their inhibition increases ER stress by disrupting the post-translational modification, and thereby compromises cell integrity. Thus, using C2C12 myoblasts, we investigate the effect of inhibiting MetAPs on cell proliferation and the molecular mechanisms underpinning its effects. We found that exposure to bengamide B (a MetAP inhibitor) caused C2C12 myoblasts to lose their proliferative abilities via cell cycle arrest. The underlying mechanism involved the accumulation of abnormal proteins (due to the decrease in the N-terminal methionine removal function) which led to increased endoplasmic reticulum stress, decreased protein synthesis, and a protective activation of the autophagy pathway. To identify the MetAP involved in these effects, we use siRNAs to specifically knockdown MetAP1 and MetAP2 expressions. We found that only MetAP2 knockdown mimicked the effects seen with bengamide B treatment. Thus, we suggest that MetAP2, rather than MetAP1, is involved in maintaining the integrity of C2C12 myoblasts. Our results are useful in understanding muscle regeneration, obesity, and overeating disorders. It will help guide new treatment strategies for these disorders.

Skeletal muscle maintenance depends largely on muscle stem cells (satellite cells) that supply myoblasts required for muscle regeneration and growth. The ubiquitin-proteasome system is the major intracellular protein degradation pathway. We previously reported that proteasome dysfunction in skeletal muscle significantly impairs muscle growth and development. Furthermore, the inhibition of aminopeptidase, a proteolytic enzyme that removes amino acids from the termini of peptides derived from proteasomal proteolysis, impairs the proliferation and differentiation ability of C2C12 myoblasts. However, no evidence has been reported on the role of aminopeptidases with different substrate specificities on myogenesis. In this study, therefore, we investigated whether the knockdown of aminopeptidases in differentiating C2C12 myoblasts affects myogenesis. The knockdown of the X-prolyl aminopeptidase 1, aspartyl aminopeptidase, leucyl-cystinyl aminopeptidase, methionyl aminopeptidase 1, methionyl aminopeptidase 2, puromycine-sensitive aminopeptidase, and arginyl aminopeptidase like 1 gene in C2C12 myoblasts resulted in defective myogenic differentiation. Surprisingly, the knockdown of leucine aminopeptidase 3 (LAP3) in C2C12 myoblasts promoted myogenic differentiation. We also found that suppression of LAP3 expression in C2C12 myoblasts resulted in the inhibition of proteasomal proteolysis, decreased intracellular branched-chain amino acid levels, and enhanced mTORC2-mediated AKT phosphorylation (S473). Furthermore, phosphorylated AKT induced the translocation of TFE3 from the nucleus to the cytoplasm, promoting myogenic differentiation through increased expression of myogenin. Overall, our study highlights the association of aminopeptidases with myogenic differentiation.