Constitutive Notch Activation Upregulates Pax7 and Promotes the Self-Renewal of Skeletal Muscle Satellite Cells

Notch signaling is a conserved cell fate regulator during development and postnatal tissue regeneration. Using skeletal muscle satellite cells as a model and through myogenic cell lineage-specific NICDOE (o vere xpression of constitutively activatedN otch 1i ntrac ellulard omain), here we investigate how Notch signaling regulates the cell fate choice of muscle stem cells. We show that in addition to inhibiting MyoD and myogenic differentiation, NICDOE upregulates Pax7 and promotes the self-renewal of satellite cell-derived primary myoblasts in culture. Using MyoD−/− myoblasts, we further show that NICDOE upregulates Pax7 independently of MyoD inhibition. In striking contrast to previous observations, NICDOE also inhibits S-phase entry and Ki67 expression and thus reduces the proliferation of primary myoblasts. Overexpression of canonical Notch target genes mimics the inhibitory effects of NICDOE onMyoD andKi67 but not the stimulatory effect onPax7 . Instead, NICD regulatesPax7 through interaction with RBP-Jκ, which binds to two consensus sites upstream of thePax7 gene. Importantly, satellite cell-specific NICDOE results in impaired regeneration of skeletal muscles along with increased Pax7+ mononuclear cells. Our results establish a role of Notch signaling in actively promoting the self-renewal of muscle stem cells through direct regulation ofPax7 .