Myf6/MRF4 Mediated Myokine Signaling is Required for the Maintenance of the Adult Skeletal Muscle Stem Cell Pool

In metazoans, skeletal muscle evolved to contract and to produce force. Recent experimental evidence suggests, however, that skeletal muscle has also acquired endocrine functions and produces a vast array of myokines, but how myokine production is regulated and how they affect the resident stem cell population of the skeletal muscle is unknown. Here, we show that in adult skeletal muscle, Myf6/MRF4 is a major regulator of myokine expression. Genetic deletion of Myf6 in skeletal muscles leads to exhaustion of the muscle stem cell (MuSCs) pool in adult mice in a myokine-dependent manner but, surprisingly, does not disrupt muscle differentiation. Using ChIP-Seq and gene expression analyses of myogenic factors, we show that Myf6/MRF4 is essential for the transcriptional activation of many myokines and muscle-secreted proteins, including ligands for canonical signaling pathways such as EGFR, VEGFR and STAT3. Consequently, MuSCs from Myf6 knockout animals show impaired activation of those signaling pathways and exhibit impaired quiescence, defective self-renewal, but nevertheless undergo differentiation. Lastly, we show that induction of myokine expression during aerobic and anaerobic exercise closely correlates with Myf6 expression. Together, these findings indicate that control of myokine signaling by Myf6 is critical to maintain muscle stem cell pool in adult skeletal muscle. ChIP-seq for Myf6 in adult mouse muscle satellite cells, and RNA-seq in WT and Myf6 KO conditions, with or without injury.在后生动物中，骨骼肌的演化核心功能为收缩并产生肌力。然而，近年实验证据表明，骨骼肌还兼具内分泌功能，可分泌大量肌因子（myokine），但目前对于肌因子的表达调控机制，以及其对骨骼肌驻留干细胞群的影响尚不明确。 本研究证实，在成年骨骼肌中，Myf6/MRF4是肌因子表达的核心调控因子。对成年小鼠骨骼肌内的Myf6进行基因敲除，会以肌因子依赖的方式耗竭肌肉干细胞（MuSCs）库，但令人意外的是，该操作并未破坏肌肉分化进程。 通过对肌源性因子开展染色质免疫共沉淀测序（ChIP-Seq）与基因表达分析，本研究发现Myf6/MRF4对众多肌因子及肌肉分泌蛋白的转录激活至关重要，其靶标包括表皮生长因子受体（EGFR）、血管内皮生长因子受体（VEGFR）与信号转导与转录激活因子3（STAT3）等经典信号通路的配体。 由此，来自Myf6敲除（KO）小鼠的肌肉干细胞，其上述信号通路的激活过程受损，同时表现出静息状态异常、自我更新缺陷，但仍可正常完成分化过程。 最后，本研究发现，有氧运动与无氧运动过程中肌因子的表达上调，与Myf6的表达水平呈显著正相关。 综上，上述研究结果表明，Myf6介导的肌因子信号通路调控，对维持成年骨骼肌中的肌肉干细胞库至关重要。本研究附带的测序数据包括：成年小鼠肌肉卫星细胞中Myf6的染色质免疫共沉淀测序（ChIP-Seq）数据，以及野生型（WT）与Myf6敲除（KO）小鼠在有无损伤条件下的RNA测序（RNA-seq）数据。