Identification of BMP9 and BMP10 as functional activators of the orphan activin receptor-like kinase 1 (ALK1) in endothelial cells

ALK1 is an endothelial-specific type I receptor of the TGF beta receptor family whose heterozygous mutations cause hereditary hemorrhagic telangiectasia type 2. Although TGF beta 1 and TGF beta 3 have been shown to bind ALK1 under specific experimental conditions, they may not represent the physiological ligands for this receptor. In the present study, we demonstrate that BMP9 induces the phosphorylation of Smadi/5/8 in microvascular endothelial cells, and this phosphorylation lasts over a period of 24 hours. BMP9 also activates the ID1 promoter-derived BMP response element (BRE) in a dosedependent manner (EC50 = 45 +/- 27 pg/ mL), and this activation is abolished by silencing ALK1 expression or addition of ALK1 extracellular domain. Overexpression of endoglin increases the BMP9 response, whereas silencing of both BMPRII and ActRIIA expressions completely abolishes it. BMP10, which is structurally close to BMP9, is also a potent ALK1 ligand. Finally, we demonstrate that BMP9 and BMP10 potently inhibit endothelial cell migration and growth, and stimulate endothelial expression of a panel of genes that was previously reported to be activated by the constitutively active form of ALKi. Taken together, our results suggest that BMP9 and BMP10 are two specific ALK1 ligands that may physiologically trigger the effects of ALK1 on angiogenesis.