Background: Vitamin D is an essential nutrient in musculoskeletal function; however, its relationship to sarcopenia remains ambiguous, and the mechanisms and targets of vitamin D activity have not been elucidated. This study aimed to clarify the role of vitamin D in mature skeletal muscle and its relationship with sarcopenia.Methods: This epidemiological study included 1653 community residents who participated in both the fifth and seventh waves of the National Institute for Longevity Sciences, Longitudinal Study of Aging and had complete background data. Participants were classified into two groups: vitamin D‐deficient (serum 25‐hydroxyvitamin D < 20 ng/mL) and non‐deficient (serum 25‐hydroxyvitamin D ≥ 20 ng/mL); they underwent propensity‐score matching for background factors (age, sex, height, weight, comorbidities, smoker, alcohol intake, energy intake, vitamin D intake, steps, activity, season and sarcopenia). Changes in muscle strength and mass over the 4‐year period were compared. For basic analysis, we generated Myf6^CreERT2 Vitamin D Receptor (VDR)‐floxed (Vdr^mcKO) mice with mature muscle fibre‐specific vitamin D receptor knockout, injected tamoxifen into 8‐week‐old mice and analysed various phenotypes at 16 weeks of age.Results: Grip strength reduction was significantly greater in the deficient group (−1.55 ± 2.47 kg) than in the non‐deficient group (−1.13 ± 2.47 kg; P = 0.019). Appendicular skeletal muscle mass reduction did not differ significantly between deficient (−0.05 ± 0.79 kg) and non‐deficient (−0.01 ± 0.74 kg) groups (P = 0.423). The incidence of new cases of sarcopenia was significantly higher in the deficient group (15 vs. 5 cases; P = 0.039). Skeletal muscle phenotyping of Vdr^mcKO mice showed no significant differences in muscle weight, myofibre percentage or myofibre cross‐sectional area; however, both forelimb and four‐limb muscle strength were significantly lower in Vdr^mcKO mice (males: forelimb, P = 0.048; four‐limb, P = 0.029; females: forelimb, P < 0.001; four‐limb, P < 0.001). Expression profiling revealed a significant decrease in expression of sarcoendoplasmic reticulum Ca²⁺‐ATPase (SERCA) 1 (P = 0.019) and SERCA2a (P = 0.049) genes in the Vdr^mcKO mice. In contrast, expression of non‐muscle SERCA2b and myoregulin genes showed no changes.Conclusions: Vitamin D deficiency affects muscle strength and may contribute to the onset of sarcopenia. Vitamin D‐VDR signalling has minimal influence on the regulation of muscle mass in mature myofibres but has a significant influence on muscle strength.

Microgravity induces skeletal muscle atrophy; however, the underlying mechanism is not clarified. In particular, the influence of microgravity on human skeletal muscle stem/progenitor cells (SMPCs) is not well understood. In this study, we used induced pluripotent stem cell-derived human SMPCs to investigate the effect of microgravity on maintenance of the stem/progenitor cell pool. Human SMPCs were induced by free-floating spherical aggregation culture, and derivatized-SMPC spheres were maintained in a microgravity condition (10-3 G) for 2 weeks using a clinostat rotation system. Microgravity culture deformed the SMPC spheres, with no signs of apoptosis. The most obvious change from microgravity culture was a significant decrease in the expression level of Pax7 in the SMPC spheres, with reduced numbers of myotubes in adhesion culture. Pax7 expression also decreased in the presence of the proteasome inhibitor MG132, indicating that the proteasomal degradation of Pax7 protein is not critical for its reduced expression in microgravity culture. Moreover, microgravity culture decreased the expression level of tumor necrosis factor receptor-associated factor 6 (TRAF6) and phosphorylation of its downstream molecule extracellular-related kinase (ERK) in SMPC spheres. Therefore, microgravity negatively regulates Pax7 expression in human SMPCs possibly through inhibition of the TRAF6/ERK pathway to consequently dysregulate SMPC pool maintenance. Overall, these results suggest that skeletal muscle atrophy is caused by microgravity-induced exhaustion of the stem cell pool.

Background: Recent studies have indicated the importance of muscle quality in addition to muscle quantity in sarcopenia pathophysiology. Intramuscular adipose tissue (IMAT), which originates from mesenchymal progenitors (MPs) in adult skeletal muscle, is a key factor affecting muscle quality in older adults, suggesting that controlling IMAT formation is a promising therapeutic strategy for sarcopenia. However, the molecular mechanism underlying IMAT formation in older adults has not been clarified. We recently found that the vitamin D receptor (VDR) is highly expressed in MPs in comparison to myotubes (P = 0.028, N = 3), indicating a potential role of vitamin D signalling in MPs. In this study, we aimed to clarify the role of vitamin D signalling in MP kinetics, with a focus on adipogenesis.Methods: MPs isolated from mouse skeletal muscles were subjected to adipogenic differentiation conditions with or without vitamin D (1α,25(OH)2D3, 100 nM) for 7 days, and adipogenicity was evaluated based on adipogenic marker expression. For in vivo analysis, tamoxifen‐inducible MP‐specific VDR‐deficient (Vdr^MPcKO) mice were newly developed to investigate whether lack of vitamin D signalling in MPs is involved in IMAT formation. To induce muscle atrophy, Vdr^MPcKO male mice were subjected to tenotomy of the gastrocnemius muscle, and then muscle weight, myofibre cross‐sectional area, adipogenic marker expression, and fatty infiltration into the muscle were evaluated at 3 weeks after operation (N = 3–4). In addition, a vitamin D‐deficient diet was provided to wild‐type male mice (3 and 20 months of age, N = 5) for 3 months to investigate whether vitamin D deficiency causes IMAT formation.Results: Vitamin D treatment nearly completely inhibited adipogenesis of MPs through Runx1‐mediated transcriptional modifications of early adipogenic factors such as PPARγ (P = 0.0031) and C/EBPα (P = 0.0027), whereas VDR‐deficient MPs derived from Vdr^MPcKO mice differentiated into adipocytes even in the presence of vitamin D (P = 0.0044, Oil‐Red O⁺ area). In consistency with in‐vitro findings, Vdr^MPcKO mice and mice fed a vitamin D‐deficient diet exhibited fat deposition in atrophied (P = 0.0311) and aged (P = 0.0216) skeletal muscle, respectively.Conclusions: Vitamin D signalling is important to prevent fate decision of MPs towards the adipogenic lineage. As vitamin D levels decline with age, our data indicate that decreased vitamin D levels may be one of the causes of IMAT formation in older adults, and vitamin D signalling may be a novel therapeutic target for sarcopenia.

Vitamin D, a fat-soluble vitamin, is an important nutrient for tissue homeostasis and is recently gaining attention for its role in sarcopenia. Although several studies have focused on the role of vitamin D in muscle homeostasis, the molecular mechanism underlying its action on skeletal muscle remains unclear. This study investigated the role of vitamin D in myogenesis and muscle fiber maintenance in an immortalized mouse myogenic cell line. A high concentration of active vitamin D, 1α,25(OH)₂D₃, decreased the expression of myogenic regulatory factors (MRFs), myf5 and myogenin in proliferating myoblasts. In addition, high concentration of vitamin D reduced myoblast-to-myoblast and myoblast-to-myotube fusion through the inhibition of Tmem8c (myomaker) and Gm7325 (myomerger), which encode muscle-specific fusion-related micropeptides. A similar inhibitory effect of vitamin D was also observed in immortalized human myogenic cells. A high concentration of vitamin D also induced hypertrophy of multinucleated myotubes by stimulating protein anabolism. The results from this study indicated that vitamin D had both positive and negative effects on muscle homeostasis, such as in muscle regeneration and myofiber maintenance. Elderly individuals face a higher risk of falling and suffering fractures; hence, administration of vitamin D for treating fractures in the elderly could actually promote fusion impairment and, consequently, severe defects in muscle regeneration. Therefore, our results suggest that vitamin D replacement therapy should be used for prevention of age-related muscle loss, rather than for treatment of sarcopenia.

Muscle spindles sense changes in muscle length and transmit them to the central nervous system. Proprioception is essential for gait and postural maintenance, the abnormality of which has been linked to gait disorders and the risk of falling in older adults. However, the effects of aging on the muscle spindle structure remain nebulous. This study investigated age-related structural changes in the muscle spindles (from the equator to the polar) in the soleus and extensor digitorum longus (EDL) muscles of young, middle-aged, and aged mice. The findings indicated that the shape of the annulospiral endings of the sensory neurons began to deteriorate in middle-aged compared with young mice and was further exacerbated in aged mice. These changes were particularly pronounced in the nuclear bag fibers, whereas no significant age-related changes were observed in the intrafusal fibers or capsules. A decline in gait function due to changes in weight-bearing and weight-shifting in aged mice was also observed, suggesting that the deterioration of proprioceptive sensory neurons that innervate the nuclear bag fiber responsible for dynamic sensitivity prevents proper coordinated movement and contributes to movement disorders in aged animals including humans, together with the functional decline of extrafusal fibers.

Sarcopenia is geriatric disease associated with increased mortality and disability. Early diagnosis and intervention are required to prevent it. This study investigated biomarkers for sarcopenia by using combination of comprehensive clinical data and messenger RNA sequencing (RNA-seq) analysis obtained from peripheral blood mononuclear cells. We enrolled total 114 older adults aged 66 – 94 years (52 sarcopenia diagnosed according to the Asian Working Group for Sarcopenia 2019 consensus and 62 normal older people). We used clinical data which were not included diagnosis criteria of sarcopenia, and stride length showed significance by logistic regression analysis (Bonferroni corrected p = .012, odds ratio = 0.06, 95% confidence interval [CI]: 0.05 to 0.40). RNA-seq analysis detected six differential expressed genes (FAR1, GNL2, HERC5, MRPL47, NUBP2, and S100A11). We also performed gene set enrichment analysis and detected two functional modules (i.e., hub genes, MYH9 and FLNA). By using any combination of the nine candidates and basic information (age and sex), risk-prediction models were constructed. The best model by using a combination of stride length, HERC5, S100A11, and FLNA, achieved a high area under the curve (AUC) of 0.91 in a validation cohort (95% CI: 0.78 to 0.95). The quantitative PCR results of the three genes were consistent with the trend observed in the RNA-seq results. When BMI was added, the model achieved a high AUC of 0.95 (95% CI: 0.84 to 0.99). We have discovered potential biomarkers for the diagnosis of sarcopenia. Further refinement may lead to their future practical use in clinical use.