Myocardin-related Transcription Factors Are Required for Skeletal Muscle Development

Myocardin-related transcription factors (MRTFs) play a central role in the regulation of actin expression and cytoskeletal dynamics. Stimuli that promote actin polymerization allow for shuttling of MRTFs to the nucleus where they activate serum response factor (SRF), a regulator of actin and other cytoskeletal protein genes. SRF is an essential regulator of skeletal muscle differentiation and numerous components of the muscle sarcomere, but the potential involvement of MRTFs in skeletal muscle development has not been examined. We explored the role of MRTFs in muscle development in vivo by generating mutant mice harboring a skeletal muscle-specific deletion of MRTF-B and a global deletion of MRTF-A. These double knockout (dKO) mice were able to form sarcomeres during embryogenesis. However, the sarcomeres were abnormally small and disorganized, causing skeletal muscle hypoplasia and perinatal lethality. Transcriptome analysis demonstrated dramatic dysregulation of actin genes in MRTF dKO mice, highlighting the importance of MRTFs in actin cycling and myofibrillogenesis. MRTFs were also necessary for the survival of skeletal myoblasts and for the efficient formation of intact myotubes. Our findings reveal a central role for MRTFs in sarcomere formation during skeletal muscle development and point to the potential involvement of these transcriptional coactivators in skeletal myopathies. Gene expression profile was generated comparing wild type (WT) and HSA-Cre, MRTF-A/B double knockout mice, by deep sequencing, with three biological replicates, using Illumina HiSeq 2500.心肌素相关转录因子（Myocardin-related transcription factors, MRTFs）在肌动蛋白表达调控与细胞骨架动力学过程中发挥核心作用。能够促进肌动蛋白聚合的刺激信号可介导MRTFs向细胞核穿梭，进而激活血清反应因子（serum response factor, SRF）——SRF是肌动蛋白及其他细胞骨架蛋白基因的关键调控因子。SRF本身是骨骼肌分化及肌肉肌节多种组分的必需调控因子，但目前尚未有研究探讨MRTFs是否参与骨骼肌发育过程。本研究通过构建骨骼肌特异性敲除MRTF-B且全身敲除MRTF-A的突变小鼠模型，在体内探究了MRTFs在肌肉发育中的功能。此类双基因敲除（double knockout, dKO）小鼠在胚胎发育阶段可形成肌节，但所形成的肌节体积异常偏小且排列紊乱，最终引发骨骼肌发育不全与围产期致死。转录组分析结果显示，MRTF双基因敲除小鼠体内的肌动蛋白基因出现显著表达失调，这凸显了MRTFs在肌动蛋白循环与肌原纤维生成过程中的重要作用。此外，MRTFs对于骨骼肌成肌细胞的存活以及完整肌管的高效形成同样不可或缺。我们的研究结果揭示了MRTFs在骨骼肌发育过程中肌节形成的核心作用，并提示这些转录辅激活因子可能参与骨骼肌肌病的发病。本研究通过深度测序技术，设置3次生物学重复，利用Illumina HiSeq 2500测序平台，生成了对比野生型（wild type, WT）与HSA-Cre、MRTF-A/B双基因敲除小鼠的基因表达谱。