LRG1 promotes angiogenesis by modulating endothelial TGF-β signalling

Aberrant neovascularization contributes to diseases such as cancer, blindness and atherosclerosis, and is the consequence of inappropriate angiogenic signalling. Although many regulators of pathogenic angiogenesis have been identified, our understanding of this process is incomplete. Here we explore the transcriptome of retinal microvessels isolated from mouse models of retinal disease that exhibit vascular pathology, and uncover an upregulated gene, leucine-rich alpha-2-glycoprotein 1 (Lrg1), of previously unknown function. We show that in the presence of transforming growth factor-β1 (TGF-β1), LRG1 is mitogenic to endothelial cells and promotes angiogenesis. Mice lacking Lrg1 develop a mild retinal vascular phenotype but exhibit a significant reduction in pathological ocular angiogenesis. LRG1 binds directly to the TGF-β accessory receptor endoglin, which, in the presence of TGF-β1, results in promotion of the pro-angiogenic Smad1/5/8 signalling pathway. LRG1 antibody blockade inhibits this switch and attenuates angiogenesis. These studies reveal a new regulator of angiogenesis that mediates its effect by modulating TGF-β signalling. LRG1 is identified as a new regulator of TGF-β signalling that promotes angiogenesis via a TβRII–ALK1–ENG–Smad1/5/8 signalling pathway; antibody-mediated inhibition of LRG1 reduces pathogenic neovascularization in a mouse model of retinal injury. Defective angiogenesis is a common feature in many diseases including age-related macular degeneration, atherosclerosis, rheumatoid arthritis and cancer. Here John Greenwood and colleagues identify a novel angiogenic glycoprotein of previously unknown function — leucine-rich-alpha-2-glycoprotein 1 (LRG1) — that exerts its effect through modifying TGF-β signalling. LRG1, upregulated in vitreous samples from humans with proliferative diabetic retinopathy, activates an angiogenic switch by binding to the receptor endoglin and promoting pro-angiogenic TGF-β signalling. Antibody-mediated inhibition of LRG1 reduces pathogenic neovascularization in a mouse model of retinal injury, which suggests that LRG1 is a possible therapeutic target for controlling pathological angiogenesis in ocular disease.