Endothelial-derived Dll4 and PDGF-BB reprogram committed skeletal myoblasts to a pericytes fate without erasing their myogenic memory

Pericytes are vessel associated mural cells that form the smooth muscle layer of vessels. They are able to contribute to skeletal muscle regeneration as previously demonstrated for mesoangioblasts that indeed represent their in vitro counterpart. Pericytes are a heterogeneous population characterized by different marker like Alkaline Phosphatase, Smooth muscle actin, Neuro glial2 (NG2). Endothelial cells recruit pericytes from the surrounding mesoderm progenitor through a PDGF-PDGFrB loop. In this work I have conducted experiments whose results showed that functional pericytes may derive from direct reprogramming of already committed embryonic and, less frequently, fetal skeletal myoblasts. When co-transplanted in vivo with endothelial cells, Pax3 or Myf5 expressing myoblasts, downregulate myogenic genes, with the notable exception of Myf5, upregulate pericyte markers, adopting a perithelial position and morphology in newly formed vessel networks. The activity of endothelial cells can be replaced by exposure to PDGF-BB and Dll4 but not Dll1 or Jagged 1, while inhibition of Notch signaling via a γ-secretase inhibitor completely restores myogenesis, confirming that the skeletal myogenic program is not irreversibly erased. Notch activation in MyoD expressing embryonic cells in vivo abolishes myogenesis but not Myf5 expression that however cannot activate myogenin 7 and trigger myogenesis because Notch, beside suppressing MyoD transcription, activates Id and Twist factors that bind to and inhibit Myf5 transcriptional activity. Myf5 expressing cells activate pericyte genes and adopt a perithelial position, a phenomenon that can be rarely observed also in WT embryos. These data demonstrate that endothelial cells are able to directly reprogram committed skeletal myoblasts to mature pericytes for the formation and stabilization of vessel wall, suggesting that reprogramming occurs as a natural developmental process that leads in a sort of competition between endothelium and muscle.