Dysfunctional autophagosome and phagosome in age-related macular degeneration (AMD).

Abstract

Purpose: AMD is characterized by accumulation of deposits and dysfunction of the retinal pigment epithelium (RPE) and overlying photoreceptors. Autophagy and phagocytosis are two cellular processes involved in the removal of extracellular and cytosolic components, respectively. In this study, we investigated the role of autophagy and phagocytosis in the pathology of retinal aging and degeneration using histological specimens of human eyes and mice deficient in lysosomal-associated membrane protein 2 (Lamp-2), a major lysosomal membrane protein that has a critical role in the maturation processes of autophagosome as well as phagosome.

Methods: Autophagy was studied in specimens from human eyes with AMD using electron microscopy (EM). Mice with impaired maturation of autophagosomes and phagosomes due to LAMP-2 deficiency were studied over time by fundus photography, fundus autofluorescence (FAF), electroretinograms (ERG), and histology. The mechanism of basolateral accumulation of extracellular material were examined by the use of polarized mouse primary RPE cultures in vitro.

Results: Dilated lysosomes and undigested materials were detectable in human AMD specimens suggesting impaired autophagy and phagocytosis. Mice deficient in Lamp-2 showed age-dependent accumulation of indigested photoreceptor outer segments, lipofuscinosis, and drusenoid deposition under the RPE. We also observed the accumulation of cargo for canonical autophagy and cellular debris that led to necrotic cell death of RPE in Lamp-2 deficient mice. FAF and ERG showed increased autofluorescence and decreased visual function over time, respectively. In vitro mechanistic studies using primary polarized RPE indicated that indigestible contents in autophagosome/autolysosome could be secreted into the extracellular space, particularly in the basolateral side of RPE, suggesting the involvement of impaired autophagy in dysfunctional exocytotic activity of the RPE in drusen formation.

Conclusions: Disruption of autophagy and phagocytosis in a mouse model led to age-dependent dysfunction of RPE and photoreceptor and necrotic cell death of RPE similar to findings in AMD. Modulating of these pathways may be a therapeutic approach for this disease.