Robo4 is an endothelial cell-specific member of the Roundabout axon guidance receptor family. To identify Robo4 binding partners, we performed a protein-protein interaction screen with the Robo4 extracellular domain. We find that Robo4 specifically binds to UNC5B, a vascular Netrin receptor, revealing unexpected interactions between two endothelial guidance receptors. We show that Robo4 maintains vessel integrity by activating UNC5B, which inhibits signaling downstream of vascular endothelial growth factor (VEGF). Function-blocking monoclonal antibodies against Robo4 and UNC5B increase angiogenesis and disrupt vessel integrity. Soluble Robo4 protein inhibits VEGF-induced vessel permeability and rescues barrier defects in Robo4(-/-) mice, but not in mice treated with anti-UNC5B. Thus, Robo4-UNC5B signaling maintains vascular integrity by counteracting VEGF signaling in endothelial cells, identifying a novel function of guidance receptor interactions in the vasculature.

-Hereditary Hemorrhagic Telangiectasia (HHT) is an inherited vascular disorder that causes arterial-venous malformations (AVMs). Mutations in the genes encoding Endoglin () and Activin-receptor-like kinase 1 ( encoding ALK1) cause HHT type 1 and 2, respectively. Mutations in the gene are present in families with Juvenile Polyposis/HHT syndrome that involves AVMs. SMAD4 is a downstream effector of Transforming growth factor-β (TGFβ)/Bone morphogenetic protein (BMP) family ligands that signal via Activin like kinase receptors (ALKs). Ligand-neutralizing antibodies or inducible, endothelial-specific deletion induce AVMs in mouse models as a result of increased PI3K/AKT signaling. Here we addressed if SMAD4 was required for BMP9-ALK1 effects on PI3K/AKT pathway activation. -We generated a tamoxifen-inducible, postnatal endothelial-specific mutant mice (). -We found that loss of endothelial resulted in AVM formation and lethality. AVMs formed in regions with high blood flow in developing retinas and other tissues. Mechanistically, BMP9 signaling antagonized flow-induced AKT activation in an ALK1 and SMAD4 dependent manner. endothelial cells in AVMs displayed increased PI3K/AKT signaling, and pharmacological PI3K inhibitors or endothelial deletion both rescued AVM formation in mice. BMP9-induced SMAD4 inhibited Casein Kinase 2 () transcription, in turn limiting PTEN phosphorylation and AKT activation. Consequently, CK2 inhibition prevented AVM formation in mice. -Our study reveals SMAD4 as an essential effector of BMP9-10/ALK1 signaling that affects AVM pathogenesis via regulation of expression and PI3K/AKT1 activation.

Ocular neovascular diseases are a leading cause of blindness. Vascular endothelial growth factor (VEGF) blockade improves vision, but not all individuals respond to anti-VEGF treatment, making additional means to prevent neovascularization necessary. Slit-family proteins (Slits) are ligands of Roundabout (Robo) receptors that repel developing axons in the nervous system. Robo1 expression is altered in ocular neovascular diseases, and previous in vitro studies have reported both pro- and anti-angiogenic effects of Slits. However, genetic evidence supporting a role for Slits in ocular neovascularization is lacking. Here we generated conditional knockout mice deficient in various Slit and Robo proteins and found that Slit2 potently and selectively promoted angiogenesis via Robo1 and Robo2 in mouse postnatal retina and in a model of ocular neovascular disease. Mechanistically, Slit2 acting through Robo1 and Robo2 promoted the migration of endothelial cells. These receptors are required for both Slit2- and VEGF-induced Rac1 activation and lamellipodia formation. Thus, Slit2 blockade could potentially be used therapeutically to inhibit angiogenesis in individuals with ocular neovascular disease.