ALK1 Signaling Inhibits Angiogenesis by Cooperating with the Notch Pathway

Activin receptor-like kinase 1 (ALK1) is an endothelial-specific member of the TGF-β/BMP receptor family that is inactivated in patients with hereditary hemorrhagic telangiectasia (HHT). How ALK1 signaling regulates angiogenesis remains incompletely understood. Here we show that ALK1 inhibits angiogenesis by cooperating with the Notch pathway. Blocking Alk1 signaling during postnatal development in mice leads to retinal hypervascularization and the appearance of arteriovenous malformations (AVMs). Combined blockade of Alk1 and Notch signaling further exacerbates hypervascularization, whereas activation of Alk1 by its high-affinity ligand BMP9 rescues hypersprouting induced by Notch inhibition. Mechanistically, ALK1-dependent SMAD signaling synergizes with activated Notch in stalk cells to induce expression of the Notch targets HEY1 and HEY2, thereby repressing VEGF signaling, tip cell formation, and endothelial sprouting. Taken together, these results uncover a direct link between ALK1 and Notch signaling during vascular morphogenesis that may be relevant to the pathogenesis of HHT vascular lesions. ► Alk1 inhibits retinal neovascularization ► Alk1 signaling promotes a stalk cell phenotype ► Alk1 cooperates with Dll4/Notch via Smad signaling Larrivée et al. find that the BMP9/Alk1 pathway works in parallel with Notch signaling to activate canonical Notch target genes Hey1 and Hey2 in endothelial cells. This convergent regulation helps prevent excessive angiogenesis by suppressing endothelial “tip cell” behavior and provides an intriguing potential explanation for hereditary hemorrhagic telangiectasia.