Myogenesis is a highly regulated process ending with the formation of myotubes, the precursors of skeletal muscle fibers. Differentiation of myoblasts into myotubes is controlled by myogenic regulatory factors (MRFs) that act as terminal effectors of signaling cascades involved in the temporal and spatial regulation of muscle development. Such signaling cascades converge and are controlled at the level of intracellular trafficking, but the mechanisms by which myogenesis is regulated by the endosomal machinery and trafficking is largely unexplored. The Endosomal Sorting Complex Required for Transport (ESCRT) machinery composed of four complexes ESCRT-0 to ESCRT-III regulates the biogenesis and trafficking of endosomes as well as the associated signaling and degradation pathways. Here, we investigate its role in regulating myogenesis. We uncovered a new function of the ESCRT-0 hepatocyte growth factor-regulated tyrosine kinase substrate Hrs/Hgs component in the regulation of myogenesis. Hrs depletion strongly impairs the differentiation of murine and human myoblasts. In the C2C12 murine myogenic cell line, inhibition of differentiation was attributed to impaired MRF in the early steps of differentiation. This alteration is associated with an upregulation of the MEK/ERK signaling pathway and a downregulation of the Akt2 signaling both leading to the inhibition of differentiation. The myogenic repressors FOXO1 as well as GSK3β were also found to be both activated when Hrs was absent. Inhibition of the MEK/ERK pathway or of GSK3β by the U0126 or azakenpaullone compounds respectively significantly restores the impaired differentiation observed in Hrs-depleted cells. In addition, functional autophagy that is required for myogenesis was also found to be strongly inhibited. We show for the first time that Hrs/Hgs is a master regulator that modulates myogenesis at different levels through the control of trafficking, signaling, and degradation pathways.

中文翻译：

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ESCRT-0 亚复合成分 Hrs/Hgs 是通过调节信号传导和降解途径来调节肌生成的主要调节因子

肌生成是一个高度调节的过程，以肌管的形成结束，肌管是骨骼肌纤维的前体。成肌细胞向肌管的分化受肌源性调节因子 (MRF) 的控制，这些因子充当参与肌肉发育的时间和空间调节的信号级联的末端效应器。这种信号级联聚合并在细胞内运输水平上受到控制，但肌生成受内体机制和运输调节的机制在很大程度上尚未探索。运输所需的内体分选复合体 (ESCRT) 机制由四个复合体 ESCRT-0 到 ESCRT-III 组成，调节内体的生物发生和运输以及相关的信号传导和降解途径。在这里，我们调查它在调节肌生成中的作用。我们发现了 ESCRT-0 肝细胞生长因子调节的酪氨酸激酶底物 Hrs/Hgs 成分在调节肌生成中的新功能。小时耗竭强烈损害鼠和人成肌细胞的分化。在 C2C12 鼠成肌细胞系中，分化的抑制归因于分化早期步骤中受损的 MRF。这种改变与 MEK/ERK 信号通路的上调和 Akt2 信号通路的下调有关，两者都导致分化的抑制。还发现肌源性抑制因子 FOXO1 和 GSK3β 在 Hrs 不存在时都被激活。U0126 或 azakenpaullon 化合物分别对 MEK/ERK 通路或 GSK3β 的抑制显着恢复了在 Hrs 耗尽细胞中观察到的受损分化。此外，还发现肌生成所需的功能性自噬受到强烈抑制。我们首次表明 Hrs/Hgs 是一种主要调节剂，可通过控制运输、信号传导和降解途径来调节不同水平的肌生成。