Iron accumulation in Bruch's membrane and melanosomes of donor eyes with age-related macular degeneration

Abstract

Iron (Fe) accumulation in cytoplasmic storages of the retina and retinal pigment epithelium (RPE) with age has been reported to be a contributing factor to the onset and progression of Age-related Macular Degeneration (AMD). This work investigated whether iron can also be stored in specialized metal-binding melanosomes of the RPE and choroid and in age pigments of the RPE (lipofuscin and melanolipofuscin). As accumulation of debris in Bruch's membrane is an additional hallmark of AMD, the elemental composition of Bruch's membrane was also investigated. Perimacular sections of the retina-choroid complex of six eyes of AMD donors and of seven age-matched healthy controls were investigated using Analytical Electron Microscopy (AEM). The melanosomes of the RPE and choroidal melanocytes of all AMD donors contained about two times higher iron mole fractions (0.06–0.07 at%) compared to the controls, which showed only minor iron mole fractions at or below the detection limit of 0.02 at%. Only melanosomes that contained iron, showed also significant lead peaks (both AMD and control about 0.08 at%). In addition, the electron-dense part of melanolipofuscin granules in the RPE accumulated iron and lead, both for control and AMD donors. Iron in lipofuscin was below the detection limit. The elastic layer of Bruch's membrane of all AMD donors also contained significantly higher iron mole fractions compared to controls (about 0.08 at% Fe), predominantly in areas that were also rich in calcium (Ca) and phosphorus (P), suggesting calcification. Indeed, five of the six AMD donors but only one of the seven controls showed nanocrystalline hydroxyapatite calcifications. Note that such nanocrystalline material can only be detected in EM samples without heavy metal (osmiumtetroxide, uranylacetate) staining. In conclusion, iron accumulation in melanosomal storages and within calcified Bruch's membrane is more pronounced in donors suffering from AMD compared to age-matched controls. This work underlines the common hypothesis that heavy metal homeostasis plays an important role in age-related neuropathy.