An Angiogenic Switch

A secreted glycoprotein acts as a switch to induce the formation of new blood vessels in disease or injury. Neovascularization, the formation of new blood vessels by angiogenesis, contributes to the pathology of various diseases and is regulated by complex mechanisms. For example, signaling mediated by transforming growth factor–β1 (TGF-β1) in endothelial cells (ECs) can be either angiostatic or pro-angiogenic. Wang et al. found that leucine-rich α-2 glycoprotein 1 (LRG1) mediates a pro-angiogenic switch by interacting with the TGF-β receptor type II (TβRII) complex. Genome-wide transcriptome analysis in mice identified Lrg1 as the most abundantly increased transcript in mouse models of retinal disease or in mice with laser- or hypoxia-induced retinal injury. LRG1 was present in normal retinal vessels in both adult human tissue and wild-type mice but was increased in the disease mouse models and in vitreous samples from patients with proliferative diabetic retinopathy. In human ECs cultured in Matrigel, a three-dimensional matrix, overexpression of LRG1 or treatment with recombinant LRG1 increased proliferation and tube formation, whereas both were inhibited by LRG1 knockdown or antibody treatment. Exogenous LRG1 also increased vessel outgrowth and branching in explanted wild-type fetal mouse metatarsals and aortic rings. Lrg1–/– mice were viable but showed delayed retinal vascular development, and wild-type mice treated with intravitreal injections of LRG1 antibody showed reduced retinal angiogenesis and pathogenic vascular tuft formation after laser injury. In lysates from rat brain ECs, LRG1 immunoprecipitated with the ectodomains of subunits of the TβRII complex. They tested individually and in combination the interaction between LRG1 and endoglin (ENG), activin receptor-like kinase-1 (ALK1), ALK5, and TβRII. Although LRG1 immunoprecipitated with each of the other subunits, LRG1 only immunoprecipitated with ALK1 in the presence of ENG. Depending on which ALK family members are present in the TβRII complex, TGF-β can either promote angiogenesis by activating the transcription factors Smad1 and -5 (Smad1/5) through ALK1 or inhibit EC proliferation by activating Smad2/3 through ALK5. TGF-β1 induced phosphorylation of both the Smad1/5 and Smad2/3 complexes in cultured wild-type brain ECs but only Smad2/3 in Lrg1–/–-derived ECs. Consistent with this regulation of Smad complexes, TGF-β1 induced proliferation in wild-type ECs but inhibited proliferation in Lrg1–/–-derived ECs. Addition of both LRG1 and TGF-β1 induced proliferation and Smad1/5 phosphorylation in ECs from either mouse. Knockdown of ALK1, ENG, or TβRII prevented LRG1-induced tube formation and phosphorylation of Smad1/5 in cultured human ECs, whereas knockdown of ALK5 did not inhibit LRG1-induced tube formation but did inhibit the phosphorylation of Smad2/3. In vivo, conditional knockdown of ENG in mouse lung ECs reduced vessel growth and branching and prevented Smad1/5 phosphorylation. Together, the findings suggest that LRG1, through its interaction with ENG, activates an angiogenic switch in multiple tissues to direct TGF-β1 signaling through the TβRII-ALK1-Smad1/5 pathway. X. Wang, S. Abraham, J. A. G. McKenzie, N. Jeffs, M. Swire, V. B. Tripathi, U. F. O. Luhmann, C. A. K. Lange, Z. Zhai, H. M. Arthur, J. W. B. Bainbridge, S. E. Moss, J. Greenwood, LRG1 promotes angiogenesis by modulating endothelial TGF-β signalling. Nature 499, 306–311 (2013). [Online Journal]