Severe lipofuscin accumulation of RPE cells appears to result in a greatly decreased phagocytic capacity. The resulting reduction in ability to cope with the needs of the overlying photoreceptor cells, in order to eliminate the obsolete tips of their POS, may well be of significance in the development of age-related macular degeneration.
Retinal pigment epithelial (RPE) cells are largely postmitotic. They continuously phagocytose the outer tips of the photoreceptor outer segments (POS). Over the life span of an individual, this activity results, although surprisingly slowly, in the intralysomal accumulation of lipofuscin, or age-pigment. Native lipofuscin shows orange-red autofluorescence when exposed to blue light. The loss of energy resulting from the conversion of excitatory blue light into emitted orange-red light may induce photo-oxidative reactions. We exposed neonatal rabbit RPE cells in culture to purified POS from cow eyes. The material were either native or peroxidized by irradiation with UV-light before being added to the RPE cultures. Lipofuscin accumulation was studied by transmission electron microscopy and measured by microfluorometric registration of its autofluorescence. Cells exposed to peroxidized POS accumulated much more lipofuscin than those exposed to native POS, indicating that peroxidized outer segements are not digestable by lysosomal enzymes. Furthermore, lipofuscin-loaded RPE cells were considerable more sensitive to visible blue light than unloaded control cells. The former ones showed lysosomal membrane destabilization with ensuing leakage of lytic enzymes and eventually cell death. We suggest that photo-oxidation of lysosomal membranes surrounding accumulated lipofuscin may be of importance for the development of age-related macular degeneration.
Formation of lipofuscin in cultured retinal pigment epithelial cells exposed to pre-oxidized photoreceptor outer segments.Accumulation of lipofuscin in the retinal pigment epithelium (RPE) with increasing age may affect essential supportive functions for the photoreceptors. Earlier, we described a model system for the study of lipofuscinogenesis in RPE cell cultures and showed that mild oxidative stress enhances lipofuscin formation from phagocytized photoreceptor outer segments (POS). In the present study, bovine POS were photo-oxidized, and turned into a lipofuscin-like material, by irradiation with UV light. Transmission electron microscopy of irradiated POS showed loss of the normal stacks of the disk membranes with conversion into an amorphous osmiophilic electron-dense mass. The formation of thiobarbituric acid reactive substances (TBARS), estimated during the irradiation process, indicated lipid peroxidation. Irradiated POS also showed a strong granular yellow autofluorescence. RPE cell cultures, kept at 21% ambient oxygen, were fed daily for 3, 5 or 7 days with either (i) UV-peroxidized POS, (ii) native POS or (iii) culture medium only. RPE cells fed irradiated POS showed significantly higher levels of lipofuscinspecific autofluorescence compared to cells exposed to native POS after 3 days (p=0.0056), 5 days (p= 0.0037) and 7 days (p=0.0020), and to the non-exposed control cells (3 days: p=0.005.5 days: p=0.0037, 7 days: p=0.0094). The lipofuscin content of cells exposed to irradiated POS increased significantly between days 3 and 7 (p=0.0335). Ultrastructural studies showed much more numerous and larger lipofuscin-like inclusions in RPE cells fed irradiated POS compared to cells exposed to native POS. In the control cells, lipofuscin-like granules were small and sparse. It appears that exposing RPE cells to previously peroxidized POS, thus artificially converted to lipofuscin and obviously not digestible by the lysosomal enzymes, accelerates the formation of severely lipofuscin-loaded cells. The results will be useful for further studies of possible harmful effects of lipofuscin in heavily loaded RPE cells.
This report reviews our experimental work on cultured retinal pigment epithelial (RPE) cells, fed native or UV-irradiated photoreceptor outer segments (POS). We showed that significantly more lipofuscin (LF) was formed in cells cultured in 40% oxygen than in cells cultured in 8% oxygen, indicating an involvement of oxidative mechanisms in LF formation. The antioxidants alpha-tocopherol, lycopene, zeaxanthin and lutein significantly reduced LF formation. RPE cells high in melanin content exhibited significantly less formation of LF than cells low in or devoid of melanin, suggesting that melanin acts as an effective antioxidant. The phagocytic capacity of LF-loaded RPE cells was significantly reduced compared to that of unloaded control cells, indicating that LF-loaded RPE cells may be unable to serve the photoreceptors sufficiently regarding phagocytosis of shed outer segment tips. Blue light irradiation destabilized lysosomal membranes in LF-loaded RPE cells and significantly reduced the viability of such cells compared to unloaded, irradiated control cells. These results may be of significance in relation to the development of age-related macular degeneration (AMD).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.