The Effects of the ApoE Genotype on Murine Retinal Vasculature in Development and Aging

Abstract

Purpose: : ApoE4, the most prevalent genetic risk factor of Alzheimer’s disease, is protective for age related macular degeneration, the most prevalent cause of blindness in the older population in industrialcountries. Our hypothesis suggests that apoE4 affects vascular plastic changes in the retina during development and aging. The objective of this study is to test this hypothesis and to investigatethe mechanisms of this effect.

Methods: : Retinal whole mounts of neonatal, 3 and 8 months old human apoE4 and apoE3 targeted replacement mice were stained for endothelial cells. Confocal images of the vasculature were analyzed. The Oxygen Induced Retinopathy (OIR) model was applied to neonatal apoE4 and apoE3 mice (i.e. exposure to hyperoxia on post-natal days 7 to 12) which were sacrificed at different time points. Paraffin cross-sections of the eyes were analyzed for neovascularization. Quantitative RT-PCR analysis of mRNAs of VEGF ligands and receptors was performed on the OIR retinas.

Results: : 1. Neonatal vascular development. We found a higher vascular density and an increased number of vascular buds in apoE4 mice compared to apoE3 mice (p=0.06 and p=0.002, respectively). These differences were transient and did not show up in adult mice. 2. Neonatal OIR model. This revealed a higher number of neovascular tufts in the apoE4 Vs. the apoE3 mice (p=0.03). The hyperoxia treated mice in both groups had elevated VEGFA, VEGFB, VEGFR1 and VEGFR2 expression levels compared to sham mice, which were however similar in the apoE4 and apoE3 mice. Interestingly, in the non treated control mice the expression of VEGFB in the apoE4 mice was higher than that of the corresponding apoE3 mice . This effect was transient and was maximal on post-natal day 7 (p=0.03). 3. Retinal vasculature in aging. We found an age dependent decrease in retinal vascular density in both groups with a significantly more pronounced decrease in the apoeE4 mice compared to the apoE3 mice (p=0.01).

Conclusions: : 1. The ApoE4 genotype affects retinal vascular development. This is associated with a transient increase in vascular buds, vascular density, and increased expression of VEGFB. 2. The OIR model revealed a similar elevation in VEGF ligands and receptors in both groups of treated mice, with an increased neovascularization in apoE4 mice, suggesting that these effects of apoE4 are not mediated by VEGF. 3. Whereas the density of the retinal vasculature of young adult apoE4 and apoE3 mice is similar, there is a significant reduction in vascular density in aged apoE4 mice compared to corresponding aged apoE3 mice.

Keywords: retinal development • aging • vascular endothelial growth factor