Wang Chao, Wang Min, Arrington Justine, Shan Tizhong, Yue Feng, Nie Yaohui, Tao Weiguo Andy, Kuang Shihuan
Department of Animal Science, Purdue University, West Lafayette, IN 47906, USA.
Department of Chemistry, Purdue University, West Lafayette, IN 47906, USA.
Development. 2017 Jan 15;144(2):235-247. doi: 10.1242/dev.138099. Epub 2016 Dec 19.
Myogenic regulatory factors (MRFs), including Myf5, MyoD (Myod1) and Myog, are muscle-specific transcription factors that orchestrate myogenesis. Although MRFs are essential for myogenic commitment and differentiation, timely repression of their activity is necessary for the self-renewal and maintenance of muscle stem cells (satellite cells). Here, we define Ascl2 as a novel inhibitor of MRFs. During mouse development, Ascl2 is transiently detected in a subpopulation of Pax7 MyoD progenitors (myoblasts) that become Pax7 MyoD satellite cells prior to birth, but is not detectable in postnatal satellite cells. Ascl2 knockout in embryonic myoblasts decreases both the number of Pax7 cells and the proportion of Pax7 MyoD cells. Conversely, overexpression of Ascl2 inhibits the proliferation and differentiation of cultured myoblasts and impairs the regeneration of injured muscles. Ascl2 competes with MRFs for binding to E-boxes in the promoters of muscle genes, without activating gene transcription. Ascl2 also forms heterodimers with classical E-proteins to sequester their transcriptional activity on MRF genes. Accordingly, MyoD or Myog expression rescues myogenic differentiation despite Ascl2 overexpression. Ascl2 expression is regulated by Notch signaling, a key governor of satellite cell self-renewal. These data demonstrate that Ascl2 inhibits myogenic differentiation by targeting MRFs and facilitates the generation of postnatal satellite cells.
生肌调节因子（MRFs），包括Myf5、肌分化抗原（MyoD，即Myod1）和肌细胞生成素（Myog），是协调肌生成的肌肉特异性转录因子。尽管MRFs对于肌源性定向分化至关重要，但及时抑制它们的活性对于肌肉干细胞（卫星细胞）的自我更新和维持是必要的。在此，我们将无调性家族蛋白2（Ascl2）定义为一种新型的MRFs抑制剂。在小鼠发育过程中，Ascl2在出生前成为Pax7 MyoD卫星细胞的Pax7 MyoD祖细胞（成肌细胞）亚群中短暂检测到，但在出生后的卫星细胞中无法检测到。胚胎成肌细胞中Ascl2基因敲除会减少Pax7阳性细胞的数量以及Pax7 MyoD细胞的比例。相反，Ascl2的过表达会抑制培养的成肌细胞的增殖和分化，并损害受损肌肉的再生。Ascl2与MRFs竞争结合肌肉基因启动子中的E盒，而不激活基因转录。Ascl2还与经典的E蛋白形成异二聚体，以抑制它们对MRF基因的转录活性。因此，尽管Ascl2过表达，MyoD或Myog的表达仍能挽救肌源性分化。Ascl2的表达受Notch信号通路调节，Notch信号通路是卫星细胞自我更新的关键调控因子。这些数据表明，Ascl2通过靶向MRFs抑制肌源性分化，并促进出生后卫星细胞的产生。
