The role of LRG1 in vessel normalization

Purpose In diseases characterized by abnormal neovascularization, the new vessels lack adequate pericyte (PC) coverage. TGFβ is involved in the recruitment of PC through ALK1‐Smad1/5/8 and ALK5‐Smad2/3 signalling. The first signalling pathway inhibits differentiation into mural cells while the latter promotes it. Leucine‐rich α‐2‐glycoprotein 1 (LRG1) is a modulator of TGFβ signalling. By binding to endoglin, LRG1 promotes the ALK1‐Smad1/5/8 signalling pathway. We hypothesized that LRG1 may have an important role in PC recruitment. Methods Retinae from Lrg1−/− and WT mice were trypsin‐digested and the endothelial cell (EC)/PC was calculated. The oxygen‐induced retinopathy mouse model was used to assess PC coverage in the neovascular tufts and the leading edge of the revascularization in Lrg1−/− and WT mice. The retinae were stained for the PC markers NG2 and αSMA and the EC marker CD31. NG2/CD31 and αSMA/CD31 ratios were quantified in the above areas. Moreover, metatarsals from mice fetuses were treated with PBS, LRG1 and angiogenic factors either alone or in combination. Then they were stained for NG2 and CD31 and the fraction of CD31 overlapping with NG2 was quantified. Results Quantification of EC/PC ratio revealed no difference between the control genotypes. In the neovascular tufts, Lrg1 deletion leads to a higher ratio of NG2+ PC whilst at the leading edge of the revascularization of the avascular region, PC coverage was not affected. In the metatarsals the combination of VEGF + LRG1 leads to significantly lower PC coverage compared to the control. Conclusions We demonstrate that under physiological conditions, deficiency of Lrg1 does not alter perivascular coverage. However, during abnormal neovascularization, Lrg1 deficiency seems to lead to increased PC recruitment while exogenous supplementation of LRG1 especially in combination with VEGF decreases it.