Ferroportin promotes iron import through the retinal vascular endothelium

Abstract

Purpose : Retinal iron accumulation has been implicated in the pathogenesis of age-related macular degeneration (AMD) and other neurodegenerative diseases. The mechanism of iron entry through the retinal and brain vascular endothelial cells (VECs) is not fully understood. Localization of the cellular iron exporter, ferroportin (Fpn), to the abluminal membrane of these cells, leads to the hypothesis that it can import iron across the blood retinal barrier and blood brain barrier.

Methods : A mouse model with deletion of Fpn within the vascular endothelium in both the retina and the brain was developed through tail vein injection of AAV9-Ple261(CLDN5)-cre to both Fpn^F/F, and control Fpn^+/+ mice at P21. Both experimental and control mouse lines contained the CAG-lox-stop-lox-GFP transgene, which was used as a reporter for cre activity. Experimental and control mice were aged to 2, 6 and 9 months. In vivo fundus imaging and quantitative serum iron detection were used for model validation. Eyes and brains were collected for immunofluorescence.

Results : GFP expression within the retinal vasculature of both experimental (Fpn^F/F) and control (Fpn^+/+) mice was observed through fundus imaging. GFP and CD31 co-staining of both retina and brain cryosections revealed a high transduction efficacy of AAV9-Ple261(CLDN5)-cre within VECs. Experimental mice had decreased serum iron concentration and transferrin saturation compared to control mice, and GFP expression was observed in the livers of both groups. Comparing experimental mice to controls, a significant increase in Ferritin-L (Ft-L) expression within GFP positive blood vessels was found in the retinas and brains of the experimentals at several time points (See figure).

Conclusions : Deletion of Fpn from VECs leads to Ft-L accumulation, an indicator of elevated iron levels in the VECs. This occurred despite lower serum iron levels in the experimental mice. This result suggests that Fpn normally transfers iron from VECs into the retina and brain. Consistent with this, retina and brain Ft-L levels were decreased in the experimental mice, which may result from a combination of decreased Fpn-mediated central nervous system iron import and low blood iron levels.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.