Department of Musculoskeletal Disease, Research Institute, National Center for Geriatrics and Gerontology, Obu, Japan.
Department of Orthopaedic Surgery, Nagoya University Graduate School of Medicine, Nagoya, Japan.
J Cachexia Sarcopenia Muscle. 2024 Jun;15(3):907-918. doi: 10.1002/jcsm.13448. Epub 2024 Mar 27.
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.
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) mice were newly developed to investigate whether lack of vitamin D signalling in MPs is involved in IMAT formation. To induce muscle atrophy, Vdr 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.
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 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 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.
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.
最近的研究表明，在肌少症病理生理学中，除了肌肉量外，肌肉质量也很重要。肌内脂肪组织（IMAT）来源于成年骨骼肌中的间充质祖细胞（MPs），是影响老年人肌肉质量的关键因素，这表明控制 IMAT 的形成是肌少症的一种有前途的治疗策略。然而，老年人中 IMAT 形成的分子机制尚不清楚。我们最近发现，维生素 D 受体（VDR）在 MPs 中的表达水平明显高于肌管（P=0.028，N=3），这表明维生素 D 信号在 MPs 中可能发挥作用。在这项研究中，我们旨在阐明维生素 D 信号在 MPs 动力学中的作用，重点是脂肪生成。
从鼠骨骼肌中分离出 MPs，在有无维生素 D（1α，25（OH）2D3，100 nM）的条件下进行 7 天的成脂分化，根据成脂标志物的表达评估脂肪生成。为了进行体内分析，新开发了可诱导性肌间充质细胞特异性 VDR 缺失（Vdr）的小鼠，以研究 MPs 中缺乏维生素 D 信号是否参与 IMAT 的形成。为了诱导肌肉萎缩，将 Vdr 雄性小鼠的比目鱼肌进行腱切断术，然后在术后 3 周评估肌肉重量、肌纤维横截面积、成脂标志物表达和脂肪浸润到肌肉（N=3-4）。此外，将维生素 D 缺乏饮食给予野生型雄性小鼠（3 和 20 月龄，N=5）3 个月，以研究维生素 D 缺乏是否导致 IMAT 的形成。
维生素 D 处理通过 Runx1 介导的早期脂肪生成因子如 PPARγ（P=0.0031）和 C/EBPα（P=0.0027）的转录修饰，几乎完全抑制了 MPs 的脂肪生成，而源自 Vdr 小鼠的 VDR 缺失的 MPs 即使在存在维生素 D 的情况下也分化为脂肪细胞（P=0.0044，油红 O 面积）。与体外研究结果一致，Vdr 小鼠和维生素 D 缺乏饮食的小鼠在萎缩（P=0.0311）和衰老（P=0.0216）的骨骼肌中表现出脂肪沉积。
维生素 D 信号对于防止 MPs 向脂肪生成谱系的命运决定很重要。随着年龄的增长，维生素 D 水平下降，我们的数据表明，维生素 D 水平的降低可能是老年人中 IMAT 形成的原因之一，而维生素 D 信号可能是肌少症的一种新的治疗靶点。
