ALK1 signaling inhibits angiogenesis by cooperating with the Notch pathway

Abstract

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.

Figure 1. Alk1 Signaling Regulates Blood Vessel Morphogenesis

(A–C) IsolectinB4 staining of wild-type P4 (A and B) and P6 (C) retinal vessels after treatment with control (A) or ALK1Fc adenovirus (B and C). Scale bar, 500 μm. (D) Quantification of vessel density in the area of retina covered by vessels in control adenovirus compared with ALK1Fc adenovirus at P6 (n = 7 mice/group). (E–L) P12 arterial vascular patterns shown by blue latex dye injected into the left ventricle of control (E–H) and ALK1Fc (I–L) adenovirus-injected pups. (F), (H), (J), and (L) show the corresponding IsolectinB4 staining of retinal vessels. (G), (H), (K), and (L) are higher magnification pictures of boxes shown in (E), (F), (I), and (J). Note abnormal artery formation and hypervascularization in ALK1Fc-treated retinas (I–L). A, artery. Scale bars, 200 μm (E, F, I, and J) and 70 μm (G, H, K, and L). (M) SMA (red) and PECAM-1 (green) staining of P12 ear skin from ALK1Fc adenovirus-treated mice. Note the presence of an AV shunt (arrowhead). Scale bar, 100 μm. (N and O) IsolectinB4 staining of wild-type P12 retinal vessels after treatment with control (N) or Bmp9 (O) adenovirus. Scale bar, 250 μm. (P) Quantification of vessel density in control adenovirus compared with Bmp9 adenovirus at P12 (n = 5 mice/group). All values are mean ± SEM. *p < 0.05; ***p < 0.005; Student's t test.

Figure 2. Effect of Notch and ALK1 Signaling on Endothelial Sprouting In Vitro and In Vivo

(A) Representative images of HUVECs sprouting in a fibrin gel in the presence or absence of ALK1 or Notch agonists and antagonists. Scale bar, 75 μm. (B) Quantification of tube surface area. Graphs represent the average of three to five experiments. (C) Representative images and vascular area quantification of P5 retinas from wild-type or Dll4^+/— pups that received intraperitoneal injections of control or Bmp9 blocking antibody. Scale bar, 120 μm. (D) Quantification of vascular area at P5 (WT plus control antibody, n = 9 retinas; WT plus anti-Bmp9 antibody, n = 4 retinas; ^Dll4+/— plus control antibody, n = 9 retinas; ^Dll4+/— plus anti-Bmp9 antibody, n = 9 retinas). (E) Representative images and quantification of tip cells of P5 retinas from pups injected with vehicle control or DAPT that received intraocular injections of PBS or BMP9. Scale bar, 36 μm. (F) Quantification tip cells at the vascular front 16 hr after intraocular injection (n = 4 retinas for control PBS and BMP9; n = 6 retinas for DAPTPBS and BMP9). All values are mean ± SEM. *p < 0.05; **p < 0.01; ***p < 0.005; Student’s t test.

Figure 3. BMP9 Signals through ALK1 and Regulates Endothelial Gene Expression

(A and B) Western blot analysis of parental HUVECs (A) or control or ALK1 siRNA-transfected HUVECs (B) following stimulation with 10 ng/ml BMP9 for up to 2 hr. (C–E) qPCR analysis of genes involved in SMAD signaling (C), Notch signaling (D), or tip cell specification (E) induced by ALK1 (BMP9) or Notch (sDll4) signaling in HUVECs, alone or in combination after 24 hr stimulation. Graphs represent the average of three to five experiments. All values are mean ± SEM. *p < 0.05; **p < 0.01; ***p < 0.005; Student's t test.

Figure 4. ALK1 Regulates Endothelial Sprouting and Gene Expression through SMAD Signaling and HEY

(A–F) Tube formation of siRNA-transfected HUVECs. (A) Quantification of tube surface area after 3 days of sprouting and 2 days of treatment with or without BMP9 10 ng/ml. (B–F) Representative images of endothelial tubes following siRNA transfection in the absence (top row) or presence (bottom row) of 10 ng/ml BMP9. Scale bar, 75 μm. (G) qPCR analysis of genes induced by ALK1 after transfection with control, SMAD4, HEY1, HEY2, or RBPJ siRNA, with or without BMP9 stimulation. Graphs represent the average of three to five experiments. All values are mean ± SEM. *p < 0.05; **p < 0.01; ***p < 0.005; Student's t test.

Figure 5. ALK1 Signaling through SMAD and HEY Prevents Tip Cell Specification

HUVECs transfected with siRNA were labeled with green or red fluorescent dyes, mixed, coated on microcarrier beads, and embedded in fibrin. (A, B, D, and E) Representative images of sprouts 4 days after siRNA transfection. Scale bar, 65 μm. (C) Quantification of percentage of target siRNA-transfected cells at the tip position of sprouts. Sprouts on 25–30 beads were quantified per experiment. Graphs represent the average of three to four experiments. All values are mean ± SEM. *p < 0.05; Student's t test.

Figure 6. Working Model for ALK1 Signaling in ECs

The data suggest that ALK1 signaling, triggered upon BMP9 binding, activates SMAD1,5,8 signaling, which induces expression of HES1, HEY1, and HEY2 together with Notch signaling. ALK1 also induces expression of UNC5B and VEGFR1, therefore reducing VEGF response and tip cell specification. This modulation of gene expression by Alk1 contributes to the stability and quiescence of the endothelium.