Norrin mediates angiogenic properties via the induction of insulin-like growth factor-1

Norrin is an angiogenic signaling molecule that activates canonical Wnt/beta-catenin signaling, and is involved in capillary formation in retina and brain. Moreover, Norrin induces vascular repair following an oxygen-induced retinopathy (OIR), the model of retinopathy of prematurity in mice. Since insulin-like growth factor (IGF)-1 is a very potent angiogenic molecule, we investigated if IGF-1 is a downstream mediator of Norrin's angiogenic properties. In retinae of transgenic mice with an ocular overexpression of Norrin (beta B1-Norrin), we found at postnatal day (P)11 a significant increase of IGF-1 mRNA compared to wild-type littermates. In addition, after treatment of cultured Muller cells or dermal microvascular endothelial cells with Norrin we observed an increase of IGF-1 and its mRNA, an effect that could be blocked with DKK-1, an inhibitor of Wnt/beta-catenin signaling. When OIR was induced, the expression of IGF-1 was significantly suppressed in both transgenic beta B1-Norrin mice and wild-type littermates when compared to wild-type animals that were housed in room air. Furthermore, at P13, one day after the mice had returned to normoxic conditions, IGF-1 levels were significantly higher in transgenic mice compared to wild-type littermates. Finally, after intravitreal injections of inhibitory alpha-IGF-1 antibodies at P12 or at P12 and P14, the Norrin-mediated vascular repair was significantly attenuated. We conclude that Norrin induces the expression of IGF-1 via an activation of the Wnt/beta-catenin signaling pathway, an effect that significantly contributes to the protective effects of Norrin against an OIR. (C) 2015 Elsevier Ltd. All rights reserved.