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Marin Gantner, Toshihide Kurihara, Yoshihiko Usui, Peter D Westenskow, Edith Aguilar, Ashley Barnett, Mitchell Prins, Martin Friedlander; Hypoxic stress alters the metabolic capacity and output of RPE cells leading to photoreceptor degeneration. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):39.
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Loss or dysfunction of retinal pigment epithelial (RPE) cells, a monolayer of cells that acts as a conduit between the blood supply and the photoreceptors, is characteristic of age-related macular degeneration (AMD). However, it is unclear what primary events occur in RPE cells that lead to subsequent retinal degeneration. One of the earliest features of AMD is attenuation of the choriocapillaris and reduced blood flow to the overlying tissue. We therefore tested the hypothesis that hypoxic stress alters the metabolic capacity and output of the RPE, reducing the nutrients available for the retina and ultimately leading to photoreceptor degeneration.
To investigate the consequences of hypoxic stress in vitro, we treated primary human RPE cells with the hypoxia-mimetic, DMOG. We used Seahorse Flux Analysis to monitor oxidative capacity of treated cells and we grew cells in transwells to look at the impact of hypoxic stress on secreted metabolites. To determine how hypoxic stress in the RPE cells would impact retinal function in vivo, we examined the retinas of mice lacking Vhl, the negative regulator or HIFs, specifically in RPE cells (VMD-Cre Vhl).<br />
DMOG treatment dramatically reduced the oxidative capacity of RPE cells and increased their glucose uptake and lactate production, as well as reduced glutamine secretion specifically in the apical chamber of transwell cultures. In vivo, stabilization of HIFs in the RPE increased their expression of glycolytic genes, reduced oxidative gene expression and lead to a rapid accumulation of lipid droplets and glycogen. Subsequent to RPE dysfunction, photoreceptor degeneration was observed and ERGs showed functional impairment of both rods and cones.
RPE cells have great metabolic flexibility and can survive despite complete loss of their oxidative capacity when exposed to hypoxic stress. However, their ability to produce and transport energy substrates to the photoreceptors is substantially compromised and as a result photoreceptor dysfunction is observed. Our results suggest that metabolically stressing RPE cells is sufficient to induce dramatic secondary effects on photoreceptors that rely on the metabolic function of the RPE cells.
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