Retinal iron homeostasis in health and disease

Abstract

Iron is essential for life, but excess iron can be toxic. As a potent free radical creator, iron generates hydroxyl radicals leading to significant oxidative stress. Since iron is not excreted from the body, it accumulates with age in tissues, including the retina, predisposing to age-related oxidative insult. Both hereditary and acquired retinal diseases are associated with increased iron levels. For example, retinal degenerations have been found in hereditary iron overload disorders, like aceruloplasminemia, Friedreich's ataxia, and pantothenate kinase-associated neurodegeneration. Similarly, mice with targeted mutation of the iron exporter ceruloplasmin and its homolog hephaestin showed age-related retinal iron accumulation and retinal degeneration with features resembling human age-related macular degeneration (AMD). Post mortem AMD eyes have increased levels of iron in retina compared to age-matched healthy donors. Iron accumulation in AMD is likely to result, in part, from inflammation, hypoxia, and oxidative stress, all of which can cause iron dysregulation. Fortunately, it has been demonstrated by in vitro and in vivo studies that iron in the retinal pigment epithelium (RPE) and retina is chelatable. Iron chelation protects photoreceptors and retinal pigment epithelial cells (RPE) in a variety of mouse models. This has therapeutic potential for diminishing iron-induced oxidative damage to prevent or treat AMD.

Keywords: age-related macular degeneration (AMD); ceruloplasmin; chelator; ferroportin; hephaestin; iron; oxidative stress; retina.

Figure 1

Adult (6-month-old) Cp^−/−Heph⁻/Y RPE and photoreceptors accumulate iron. (A–C) 6-month-old WT (A), Cp^−/− (B), and Cp^−/−Heph⁻/Y (C) retinas Perls' stained for iron (blue) and counterstained with hematoxylin/eosin. (D) High magnification of Prussian blue Perls' label in 6-month-old Cp^−/−Heph⁻/Y RPE. (E and F) Light photomicrographs of 6-month-old Cp^−/−Heph⁻/Y (E) and WT (F) retinas after DAB enhancement (brown) of Perls' stain. (G and H) Electron micrographs of RPE from 6-month-old Cp^−/−Heph⁻/Y (G) and WT (H) eyes. Only the Cp^−/−Heph⁻/Y RPE (G) contains electrondense vesicles (^*) sometimes fused with melanosomes. Reprinted from Hahn et al., (2004b).

Figure 2

Cp^−/−Heph⁻/Y (9-mon-old) mice have retinal degeneration. (A) Light photomicrograph of WT retina. (B and C) Cp^−/−Heph⁻/Y retina has focal patches of hypertrophic RPE cells in some regions (B) and confluent hypertrophic RPE cells in other areas (C). (D) In an area of RPE hyperplasia (demarcated by arrowheads), Cp^−/−Heph⁻/Y retinas have local photoreceptor degeneration [demarcated by arrows in the outer nuclear layer (ONL)] and subretinal neovascularization (red^*). (E) In an area of hypertrophic, hyperplastic (area demarcated by arrowheads) RPE cells, a necrotic RPE cell also observed by electron microscopy (Left Inset) is present. Within the area of RPE hyperplasia, there is local photoreceptor thinning and subretinal neovascularization (red^*) visible as small vessels containing erythrocytes (Right Inset). The hyperplastic RPE have formed a localized cyst (Cy). (F) Electron micrograph of WT RPE. Br, Bruch's membrane; AM, apical microvilli; OS, photoreceptor outer segments. (G) Electron micrograph of Cp^−/−Heph⁻/Y RPE overloaded with phagosomes and lysosomes containing photoreceptor outer segments at various stages of digestion. Some of these lysosomes (^*) contained multilamellar structures characteristic of outer segment membranes (Inset). (H) Electron micrograph of Cp^−/−Heph⁻/Y deposits between RPE and Bruch's membrane containing wide-spaced collagen (^*). (Scale bars: A–E, 50 μm; F and G, 2 μm; H, 500 nm.) Reprinted from Hahn et al., (2004b).

Figure 3

Schematic diagram showing the retinal layers (A), with enlarged view of the RPE and potential routes of iron traffic (B). Several diseases that disrupt retinal iron transport are indicated in red. Proteins involved in iron transport are illustrated: Cp, ceruloplasmin; DMT1, Divalent metal tranporter-1; Fe, iron; Fe²⁺, ferrous iron; Fe³⁺, ferric iron; Fpn, ferroportin; Frat, Frataxin; HFE, histocompatability leukocyte antigen class I-like protein involved in iron homeostasis; Hp, Hephaestin; Tf, transferrin; TfR, Transferrin receptor.