Astrocyte Hypoxic Response is Essential for Pathological but Not Developmental Angiogenesis of the Retina

Abstract

Purpose: : Astrocytes perform crucial functions in both developmental and pathological angiogenesis of the retina. Vascular endothelial growth factor (VEGF), regulated by the hypoxic response signaling pathway, is essential for these processes. We have analyzed the role of the astrocyte hypoxic response in developmental and pathological retinal angiogenesis using conditional knockout mice and the oxygen-induced retinopathy (OIR) model.

Methods: : Astrocyte-specific (GFAP-cre) conditional knockout mice for VEGF and its upstream regulators von Hippel-Lindau protein (VHL), hypoxia-inducible factor (HIF)-1α, and HIF-2α were created. Retinal vascular morphology in these animals was evaluated by immunohistochemistry at postnatal day 7 (P7) and in adult mice. OIR was induced by hyperoxia treatment from P7 to P12, and retinal vaso-obliteration and pre-retinal neovascularization were quantified at P17. Retinal expression of VEGF, VEGF isoforms, and EPO was assessed by RT-PCR.

Results: : Conditional knockout of VEGF, HIF-1α, or HIF-2α in astrocytes did not affect retinal vascular development and resulted in a normal retinal vasculature in adult animals. In contrast, knockout of VHL caused a grossly abnormal, hypervascular retinal phenotype associated with increased retinal VEGF expression. This phenotype was rescued by additional knockout of HIF-2α or VEGF, but not HIF-1α. Similarly, in the OIR model, knockout of HIF-2α or VEGF, but not HIF-1α, significantly reduced hypoxia-induced pre-retinal neovascularization.

Conclusions: : Our findings demonstrate that (i) astrocyte-derived VEGF is essential for pathological, but not developmental, retinal angiogenesis, and that (ii) the hypoxic response in retinal astrocytes is mediated predominantly by HIF-2α, not HIF-1α.

Keywords: retinal glia • retinal neovascularization • vascular endothelial growth factor