Abstract
Abstract: :
Purpose: Oxidative stress plays a role in human age–related macular degeneration (AMD). One potential source of oxidative stress is iron, a potent generator of oxidative stress when improperly regulated. Elevated central nervous system iron levels are associated with several neurodegenerative diseases, including Alzheimer’s disease and AMD. Our goal is to better understand the mechanisms of normal retinal iron homeostasis and determine the consequences of iron dysregulation. Methods: Immunohistochemistry and Western analysis were used to localize and quantify the iron handling proteins ferritin and ferroportin in the retinas of normal mice, mice deficient in ferroxidases, ceruloplasmin (Cp) and hephaestin (Heph), and mice deficient in iron regulatory proteins, IRP–1 and IRP–2. Cp/Heph–double knockout retinas were histologically analyzed using the Perls’ Prussian Blue label for iron as well as by light and electron microscopy. Results: The iron storage protein ferritin and the iron transporter ferroportin were present in both the RPE and neural retina. Ferritin was present in particularly high levels at the axon terminals of rod bipolar cells, suggesting a role for ferritin in axonal iron transport. Cp/Heph–double knockout mice had abnormal accumulation of retinal and RPE iron with corresponding increases in ferritin and ferroportin. Irp–1/Irp–2 knockout mice also had elevated retinal ferritin and ferroportin. At 9 months of age, Cp/Heph–double knockout mice developed RPE hypertrophy, hyperplasia and death, subretinal neovascularization, photoreceptor atrophy, and sub–RPE long–spaced collagen deposition. Conclusions: Several iron transport and binding proteins are expressed in the retina, suggesting that the retina tightly regulates iron metabolism. The ferroxidases, Cp and Heph, and iron–transporter, ferroportin, were present in the retina particularly in Muller cell endfeet and in RPE, suggesting a cooperative role in iron export between ferroportin and Cp or Heph. The retinal iron accumulation in the Cp/Heph double knockout mouse most likely resulted from defective retinal iron export. A portion of this increased retinal iron could be bound by IRP–1 and IRP–2, leading to upregulation of ferritin and ferroportin. Iron dysregulation in these Cp/Heph–double knockout mice results in RPE dysfunction and death with subretinal neovascularization, providing a model of some features of AMD.
Keywords: oxidation/oxidative or free radical damage • transgenics/knock–outs • age–related macular degeneration