Ascl2 inhibits myogenesis by antagonizing the transcriptional activity of myogenic regulatory factors

Myogenic regulatory factors (MRFs) including Myf5, MyoD and Myog are muscle-specific transcriptional factors orchestrating myogenesis. Although MRFs are essential for myogenic commitment and differentiation, timely repression of their activity is necessary for self-renewal and maintenance of muscle stem cells (satellite cells). Here we define a novel inhibitor of MRFs: the achaete-scute homologue 2 (Ascl2). During development, Ascl2 is transiently detected in a subpopulation of Pax7+MyoD+ progenitors (myoblasts) that become Pax7+MyoD− satellite cells prior to birth, but not detectable in postnatal satellite cells. Knockout of Ascl2 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 regeneration of injured muscles. At the molecular level, Ascl2 competes with MRFs for binding to E-boxes in the promoters of muscle genes, without activating gene transcription. Ascl2 also forms heterodimer with classical E-proteins to sequester their transcriptional activity on MRFs. Accordingly, MyoD or Myog expression rescues myogenic differentiation despite Ascl2 overexpression. Finally, Ascl2 expression is regulated by Notch signaling, a key governor of satellite cell self-renewal. These data together demonstrate that Ascl2 inhibits myogenic differentiation by targeting MRFs, and facilitates generation of postnatal satellite cells.