Notch1 and Notch2 signaling from distinct cellular sources exclusively but cooperatively regulate fetal myogenesis

During developmental myogenesis, some continuously proliferating myogenic progenitors (MPs) sustain stem/progenitor states while the rest differentiate into myocytes to form myofibers by fusion. To generate sufficient muscle, fine regulations of cell fate decision of MPs are crucial. Notch signaling has been known to regulate embryonic MPs (eMPs) that play a role in primary myogenesis by promoting cell cycle exit and suppressing premature differentiation. However, the role of Notch signaling in fetal MPs (fMPs) during secondary myogenesis is elusive because the development of fMPs already has been abrogated in Notch manipulation models. In this study, we investigated the role of Notch signaling in fMPs at an individual Notch receptor level. Surprisingly we found that Notch1 and Notch2, most similar among four Notch receptors, distinctly regulate fMPs: Notch1 induces cell cycle exit and Notch2 suppresses premature differentiation, presumably by canonical Notch signaling pathway. Additionally, we suggest that Notch1 and Notch2 signals are regulated by the ligands on myofibers and MP-lineage cells, respectively. This study will lay the groundwork for the in-depth understanding of Notch signaling in muscle formation from embryonic myogenesis to adult muscle regeneration. Total RNA-sequencing analysis was performed using FACS-sorted tdTomato+ myogenic progenitor-lineage cells from limb muscle of Notch1 (Pax7-CreER; Notch1f/f; Rosa-tdTomato), Notch2 (Pax7-CreER; Notch2f/f; Rosa-tdTomato) conditional knock-out and wild-type control (Pax7-CreER; Rosa-tdTomato) E16.5 fetus.