Retinal pigment epithelial cells “bulk up” when they “pump iron”: lipid and endolysosome accumulation with mechanistic connection to macular degeneration

Abstract

Purpose : The pathogenesis of age-related macular degeneration (AMD) is poorly defined, but iron may play a role. By generating free radicals through the Fenton reaction, iron can oxidize lipids and create toxic lipid peroxidation products that build up in the retinal pigment epithelium (RPE). This may contribute to the lipid deposits observed in AMD as intracellular lipofuscin and extracellular drusen. Because RPE cells perform daily phagocytosis of photoreceptor outer segments (POS) that are eventually turned over in lysosomes, lipid peroxidation products may also damage the RPE endolysosomal system. We hypothesize that RPE iron overload can cause lipid peroxidation and lysosomal dysfunction, thereby contributing to the pathogenesis of AMD.

Methods : Immunohistochemistry and lipid staining was performed on retinal cryosections from two mouse models with features of AMD: 1) a hepcidin knockout (hepc^KO) model and 2) a ceruloplasmin/hephaestin deficient (CpSla) model. Both mouse models have hypertrophic RPE with elevated iron levels. Eyes were also prepared for transmission electron microscopy.

Results : Filipin and BODIPY 493/503 lipid stains reveal accumulation of unesterified cholesterol in the RPE of hepc^KO mice. These hepc^KO mice also have elevated levels of oxidized LDL/HDL and the lipid peroxidation products 4-HNE and MDA in their RPE. LDL receptor is elevated in the RPE, as is LCAT, an enzyme responsible for cholesterol esterification. LDL receptor, LCAT, and 4-HNE are also elevated in the RPE of CpSla mice while oxidized phospholipids are not. The RPE of both mouse models display increased staining for LAMP1, a marker of late endosomes and lysosomes. Electron microscopy of hepc^KO RPE shows numerous endosome/lysosome-like organelles. Rhodopsin, a component of POS that is normally degraded by RPE lysosomes, accumulates in the RPE of both hepc^KO and CpSla mice.

Conclusions : Cholesterol and lipid peroxidation products accumulate in the RPE of mice with elevated retinal iron. These compounds may cause lysosomal dysfunction and consequent upregulation of lysosomal biogenesis. This suggests that iron may contribute to AMD by catalyzing the formation of lipid peroxidation products and damaging lysosomal proteins so that these compounds cannot be degraded.

This is a 2020 ARVO Annual Meeting abstract.