Retinal vascular repair and neovascularization are not dependent on CX3CR1 signaling in a model of ischemic retinopathy.

Proliferative retinal neovascularization occurring in response to ischemia is a common mechanism underlying many retinal diseases. In recent studies, retinal microglia have been shown to influence pathological neovascularization, likely through an exchange of cellular signals with associated vascular elements. CX3CR1 is a chemokine receptor located specifically on microglia; its ligand, CX3CL1 (also known as fractalkine or neurotactin) displays pro-angiogenic activity both in in vivo and in vitro. Discovering the regulatory role, if any, that CX3CR1 signaling may have in ischemic retinopathy will shed light on the molecular nature of microglial-vascular interactions and clarify potential targets for future therapy. In this study, we examined this question by inducing and comparing ischemic vascular changes in transgenic mice in which CX3CR1 signaling is either preserved or ablated. Using a well-known oxygen-induced retinopathy (OIR) model, we induced ischemic retinopathy in transgenic mice in which the gene for CX3CR1 has been replaced by green fluorescent protein (GFP) and their wild type controls. CX3CR1(+/+), CX3CR1(+/GFP), and CX3CR1(GFP/GFP) transgenic mice were exposed to 75% oxygen for 5 days starting from postnatal day (P) 7, and then transferred back to room air. At P12 and P17, the extents of vascular repair and neovascularization, and associated changes in retinal microglia distribution, were quantified and compared between mice of different genotypes. Neuronal loss in the retina following ischemia was also evaluated in paraffin sections. Our results show that: (1) CX3CR1 signaling is not required for normal vascular, microglial, and neuronal development in the retina in the first postnatal week, (2) the processes of retinal vascular repair and neovascularization following ischemia occur similarly with and without CX3CR1 signaling, (3) microglia redistribution in the retina and their association with vascular elements occurring concurrently is independent of CX3CR1, and (4) CX3CR1 does not influence the extent of neuronal cell loss in the retina following ischemia. Taken together, our findings indicate that the regulatory signals exchanged between microglia and vascular elements in the ischemic retinopathy animal model are unlikely to involve CX3CR1. These results have implications on therapeutic approaches to, pathological neovascularization involving the modulation of chemokine signaling in general, and the regulation of CX3CR1 signaling specifically.