The MuSK-BMP pathway maintains myofiber size in slow muscle through regulation of Akt-mTOR signaling

Myofiber size regulation is critical in health, disease, and aging. MuSK (muscle-specific kinase) is a BMP (bone morphogenetic protein) co-receptor that promotes and shapes BMP signaling that is expressed at all neuromuscular junctions and is also present extrasynaptically in the slow soleus muscle. To investigate the role of the MuSK-BMP pathway in vivo we generated mice lacking the BMP-binding MuSK Ig3 domain. These ∆Ig3-MuSK mice are viable and fertile with innervation levels comparable to wild type. In 3-month-old mice myofibers are smaller in the slow soleus, but not in the fast TA. Here we use bulk RNA-seq to perform transcriptomic analysis of slow and fast mouse muscle. RNAseq analysis revealed soleus-selective decreases in RNA metabolism and protein synthesis pathways as well as dysregulation of IGF1 pathway components. Moreover, Akt-mTOR signaling is reduced in soleus but not TA. We propose that the MuSK-BMP pathway acts extrasynaptically to maintain myofiber size in slow muscle by promoting protein synthetic pathways including the IGF1-Akt-mTOR signaling. These results reveal a novel mechanism for regulating myofiber size in slow muscle and introduce the MuSK-BMP pathway as a target for promoting muscle growth and combatting atrophy. 3 month old male mice were used in the study to compare transcriptomic profiles of TA and soleus muscle. 9 WT and 9 ∆Ig3-MuSK tibialis anterior and soleus muscle (total of 36 samples) were used for bulk RNAseq analysis.

肌纤维尺寸调控在健康、疾病及衰老进程中均具有关键意义。肌肉特异性激酶（MuSK, muscle-specific kinase）是骨形态发生蛋白（BMP, bone morphogenetic protein）的共受体，能够促进并塑造BMP信号通路，其表达于所有神经肌肉接头处，同时在慢肌比目鱼肌的突触外区域也有分布。为在活体水平探究MuSK-BMP通路的作用，我们构建了缺失BMP结合性Ig3结构域的MuSK基因敲除小鼠（ΔIg3-MuSK小鼠）。该ΔIg3-MuSK小鼠可正常存活与繁殖，其神经支配水平与野生型（WT, wild type）小鼠相当。在3月龄小鼠中，慢肌比目鱼肌的肌纤维尺寸更小，而快肌胫骨前肌（TA, tibialis anterior）则未出现该变化。本研究通过批量RNA测序（bulk RNA-seq）对小鼠快、慢肌组织开展转录组分析。RNA测序结果显示，比目鱼肌中存在RNA代谢与蛋白质合成通路的选择性下调，同时胰岛素样生长因子1（IGF1, insulin-like growth factor 1）通路组分出现表达失调。此外，比目鱼肌中的Akt-雷帕霉素靶蛋白（Akt-mTOR）信号通路活性降低，而胫骨前肌则无此现象。我们提出，MuSK-BMP通路可通过突触外区域发挥作用，通过促进包括IGF1-Akt-mTOR信号通路在内的蛋白质合成途径，维持慢肌的肌纤维尺寸。本研究揭示了一种调控慢肌肌纤维尺寸的全新机制，并将MuSK-BMP通路确立为促进肌肉生长、对抗肌肉萎缩的潜在靶点。本研究使用3月龄雄性小鼠，对比分析胫骨前肌与比目鱼肌的转录组特征。实验共采集9只野生型小鼠与9只ΔIg3-MuSK小鼠的胫骨前肌及比目鱼肌样本（总计36份样本），用于批量RNA测序分析。