Proteinopathy in the retinal pigment epithelium (RPE): implications for sight-loss in old age

Abstract

Damage to the retinal pigment epithelium (RPE), which maintains overlying photoreceptors in the retina, is linked with irreversible sight-loss including age-related macular degeneration (AMD), the most common cause of blindness in developed societies. RPE cells internalize and degrade photoreceptor outer segments (POS) as part of the daily photoreceptor renewal, which subjects the RPE to the highest proteolytic burden in the body. However, age and onset of retinopathy correlates with partially degraded POS, which accumulate as lipofuscin and related molecules, constituting a clinically well-documented pathway of RPE death in geographic atrophy AMD that has no effective treatment. To elucidate its molecular mechanisms, we exploited an in-vitro RPE model, which structurally and physiologically recapitulates the native RPE monolayer. Disease conditions linked with AMD including oxidative stress and impaired autophagy were recapitulated using 100m H2O2, oxidatively-modified POS (OxPOS) or 10nM bafilomycin A1, respectively. We also studied effects of the Alzheimer’s-related amyloid beta (A) proteins, which accumulate in aged/AMD retinas. Confocal-immunofluorescence studies combined with ultrastructural imaging revealed the fate of trafficked POS in the phagosome and autophagy-lysosomal pathways, which, depending on the insult, became sequestered in early compartments or were trafficked prematurely to lysosomes. A proportion of POS were also prematurely targeted to autophagy bodies. OxPOS accumulated in late compartments generating increased autofluorescence, which recapitulates aged/damaged RPE in patients. A rapidly accumulated in lysosomes, which, unlike POS cargos, RPE cells were unable to degrade effectively. Our findings revealed contrasting molecular mechanisms underpinning proteinopathy in the RPE that could be manipulated to develop future treatments.

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ORCID for J. Arjuna Ratnayaka: orcid.org/0000-0002-1027-6938

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