Neuronal death and accumulation of lipid droplets and glycogen granules within retinal pigment epithelium under the influence of mTOR and autophagy.

Abstract

In the course of age-related macular degeneration (AMD) the retinal pigment epithelium undergoes a number of cytopathological alterations that are generated by a dysfunction of specific metabolic pathways. In detail, these include lipid and glycogen accumulation along with dismantling of specific proteins from the plasma membrane. In the present study we analyzed whether 3-methyladenine (3-MA), a classic autophagy inhibitor, may reproduce the pathobiochemical and structural alterations occurring in AMD. Different doses of 3-MA produce a loss of cell viability with lipids and glycogen accumulation, which were quantified by ultrastructural morphometry. This was concomitant with displacement and suppression of autophagy-related proteins along with increased activity of mTOR. A dismantling of phenotype-specific proteins composing tight junctions was observed as well. All these alterations were reverted by the phytochemical autophagy stimulator curcumin which was shown to act as a powerful mTOR inhibitor with an efficacy that was like the classic mTOR inhibitor rapamycin. When these compounds activating autophagy/inhibiting mTOR were administered alone, a beneficial effect was observed even in control cells. The occurrence of 3-MA-induced retinal degeneration was found to be associated with a remarkable aggregation of p62 which is reminiscent of central neurodegenerative disorders, and it was fully prevented by curcumin similarly to rapamycin. These protective effects concern cell viability, altered glycogen and lipid accumulation, and ultrastructural alterations. The present work contributes to understanding degeneration in AMD while extending key biochemical steps to neurodegenerative disorders. The use of natural phytochemicals and light-induced by-products may be used for therapeutic purposes.