HIF–1 induced VEGF augments, but is not essential for normal retinal vascular development

Abstract

Abstract: : Purpose: It has been hypothesized that the unvascularized developing retina becomes hypoxic, thus inducing HIF–1, which in turn induces VEGF to initiate vasculogenesis. The present study was performed in an attempt to validate this hypothesis. Methods: Vegf^δ/δ mice, which are incapable of upregulating VEGF in hypoxic conditions due to a deletion in the hypoxia response element in the VEGF promoter, were compared to Vegf^+/+ and Vegf^+/δ littermates at various time points during retinal vascular development using sections stained with Griffonia simplicifolia isolectin–B4 (GSA) and retinal flat mounts infused with fluorescein–labelled dextran. Results: Vessels normally begin to penetrate the retina at postnatal day 7 (P7) and the deep capillary bed begins to form by P9 and is completely formed by P10. P9 retinas from Vegf^δ/δ mice were developmentally comparable to P7 retinas from normal mice in that very little penetration of the retina by vessels was apparent. At P10, the deep capillary bed had just begun to form in Vegf^δ/δ mice, but the area of the vascular bed was significantly less than that of Vegf^+/+ and Vegf^+/δ littermates. By P12, the retinas of normal and Vegf^δ/δ mice showed comparable development and the retinal vasculature of adult Vegf^δ/δ mice appeared normal. Conclusions: These results show that in the absence of the HIF–1 binding site on the VEGF promoter, retinal vascular development is delayed, but a normal vasculature ultimately forms. The formation of the deep capillary bed is delayed by about 2 days in Vegf^δ/δ mice, accounting for the main difference and showing that HIF–1 induced VEGF augments the development of the deep capillary bed. It is not clear whether another factor compensates for the absence of HIF–1 induced VEGF or if VEGF eventually is induced through another pathway in order to promote retinal vascular development.

Keywords: growth factors/growth factor receptors • hypoxia • retinal development