CDKN2A-CDK4-E2F3 dependent oxidative skeletal muscle fiber transition in myogenesis, movement, and metabolism

Skeletal muscle function is vital to movement, thermogenesis and metabolism. Muscle fibers differ in contractile ability, mitochondrial content and metabolic properties and muscle fiber transition influences muscle function. However, the molecular mechanisms regulating muscle fiber transition in muscle function are unclear. Here, in over 150 human muscle samples, we observed that markers of oxidative muscle fiber and mitochondria correlate positively with PPARGC1 and CDK4, and, negatively with CDKN2A, a locus significantly associated with type 2 diabetes. Mice expressing an overactive Cdk4 that cannot bind its inhibitor p16INK4a, a product of the CDKN2A locus, are longer, leaner, exhibit increased oxidative myofibers with superior mitochondrial energetics, display enhanced muscle glucose uptake, and are protected from obesity and diabetes. In contrast, Cdk4-deficiency, or skeletal muscle-specific deletion of Cdk4's transcriptional target, E2F3, reduces oxidative myofiber numbers, deteriorates mitochondrial function and exercise capacity, while increasing diabetes susceptibility. E2F3 activates the PPARGC1 promoter and CDK4/E2F3/PPARGC1 levels correlate positively with exercise and fitness, and negatively with adiposity, insulin resistance and lipid accumulation in muscle. These findings provide insight into oxidative muscle fiber transition and function that is of relevance to metabolic and muscular diseases. Expression profiling by RNA-Seq in Quadriceps or Soleus muscle from Wild type, CDK4RR, CDK4KO, MLC-cre or E2F3-mKO mice.骨骼肌功能对于运动、产热及代谢均至关重要。肌纤维在收缩能力、线粒体含量与代谢特性上存在差异，而肌纤维转换过程会影响肌肉功能。然而，调控肌肉功能中肌纤维转换的分子机制目前仍不明确。本研究针对超过150个人类肌肉样本展开分析，结果发现氧化型肌纤维与线粒体的标志物与PPARGC1和CDK4呈正相关，而与CDKN2A呈负相关；其中CDKN2A位点与2型糖尿病存在显著关联。研究中构建了表达功能亢进型Cdk4的小鼠模型，该突变型Cdk4无法结合其抑制剂p16INK4a（由CDKN2A位点编码）；此类小鼠体型更长、体脂更少，拥有更多氧化型肌纤维且线粒体能量代谢更优异，肌肉葡萄糖摄取能力增强，同时可抵抗肥胖与糖尿病。与之相反，Cdk4缺失或骨骼肌特异性敲除Cdk4的转录靶标E2F3，会减少氧化型肌纤维数量，损害线粒体功能与运动能力，并增加糖尿病易感风险。E2F3可激活PPARGC1启动子；CDK4/E2F3/PPARGC1的表达水平与运动能力及健康状态呈正相关，而与肥胖、胰岛素抵抗及肌肉内脂质堆积呈负相关。上述研究结果为氧化型肌纤维转换及其功能的调控机制提供了新见解，相关发现与代谢性疾病及肌肉疾病的研究具有重要关联。本研究通过RNA测序（RNA-Seq）对野生型、CDK4RR、CDK4KO、MLC-cre或E2F3-mKO小鼠的股四头肌或比目鱼肌进行了表达谱分析。