Dissecting Regulators of Aging and Age-Related Macular Degeneration in the Retinal Pigment Epithelium

Abstract

Age-related macular degeneration (AMD), the leading cause of blindness in elderly populations, involves the loss of central vision due to progressive dysfunction of the retinal pigment epithelium (RPE) and subsequent loss of light-sensing photoreceptors. While age is a key risk factor, not every aged individual develops AMD. Thus, the critical question is what specific cellular changes tip the balance from healthy aging to disease. To distinguish between changes associated with aging and AMD, we compared the RPE proteome in human eye bank tissue from nondiseased donors during aging ( n = 50 , 29-91 years) and in donors with AMD ( n = 36 ) compared to age-matched donors without disease ( n = 28 ). Proteins from RPE cells were separated on two-dimensional gels, analyzed for content, and identified using mass spectrometry. A total of 58 proteins displayed significantly altered content with either aging or AMD. Proteins involved in metabolism, protein turnover, stress response, and cell death were altered with both aging and AMD. However, the direction of change was predominantly opposite. With aging, we detected an overall decrease in metabolism and reductions in stress-associated proteins, proteases, and chaperones. With AMD, we observed upregulation of metabolic proteins involved in glycolysis, TCA, and fatty acid metabolism, with a concurrent decline in oxidative phosphorylation, suggesting a reprogramming of energy utilization. Additionally, we detected upregulation of proteins involved in the stress response and protein turnover. Predicted upstream regulators also showed divergent results, with inhibition of inflammation and immune response with aging and activation of these processes with AMD. Our results support the idea that AMD is not simply advanced aging but rather the culmination of perturbed protein homeostasis, defective bioenergetics, and increased oxidative stress within the aging RPE, exacerbated by environmental factors and the genetic background of an individual.

Quantitative Proteomics of Human Retinal Pigment Epithelium Reveals Key Regulators for the Pathogenesis of Age-Related Macular Degeneration

“…Three mitochondrial ribosomal proteins (MRPS30, MPRS15, MPRS9) and five proteins involved in mitochondria trafficking and assembly (TSPO, PARL, TMEM126A, RHOT2, TOMM40L) were increased, indicating a potential compensatory response to mitochondrial damage caused by disease conditions. The upregulation of four enzymes of glycolysis (HK1, HK2, PKM, OGDHL) in MGS2 samples is consistent with the increased glycolytic proteins in RPE with AMD [ 22 ]. Interestingly, some enzymes involved in oxidative phosphorylation were also increased at the early stage.…”

“…In our previous studies, we investigated RPE proteomic changes at different stages of AMD using 2-DE coupled to either MALDI-TOF or LC-MS/MS [ 9 , 21 , 22 ]. In the first study, we identified eight spots with significant changes in AMD samples, including three ATP synthase subunits, cytochrome c oxidase VIb, mitofilin, mtHsp70 and mitochondrial translation factor Tu, which are proteins that reside in the mitochondria [ 21 ].…”

“…Many of these altered proteins, with the majority localized to the mitochondria, were associated with both normal aging as well as during AMD progression. However, the direction of change was predominantly opposite, suggesting that aging and AMD involve different biological processes [ 22 ].…”

The Role of Oxidative Stress in the Aging Eye

“…Oxidative stress underlies the pathophysiology of AMD [ 139 ]. Harmful reactive oxygen intermediates are produced at the retina from oxygen consumption, irradiation, oxidation of the fatty acids of the photoreceptor outer segment membrane, photosensitizers at the neurosensory retina and the RPE, and RPE phagocytosis [ 140 , 141 , 142 ].…”

Lipofuscin, Its Origin, Properties, and Contribution to Retinal Fluorescence as a Potential Biomarker of Oxidative Damage to the Retina

“…It has been shown that the AMD retina is under increased oxidative stress in comparison with the age-matched retina and there is a growing body of evidence suggesting that the oxidative stress and products of lipid and bisretinoid oxidation can contribute to the development and progression of AMD by activation of the complement cascade, and other proinflammatory and proangiogenic signalling (reviewed in [193,[380][381][382][383][384][385]). Firstly, RPE affected by early, atrophic, and exudative AMD exhibit about five-fold increased content of total iron and iron chelateable by desferrioxamine in comparison with RPE in age-matched normal retinas [386]. It has been shown in experiments on mice that the retinal iron overload, caused by intravitreal injection of ferric ammonium citrate into the eye, results in i) the modification of proteins by a characteristic product of DHA oxidation forming CEP; ii) the upregulation of interleukin-1β (IL-1β) and cluster of differentiation 68 (CD68) expression; and iii) the photoreceptor and RPE atrophy [93,387].…”