Enhancing lysosome function via mTOR/TFEB activation reduces lipofuscin-like granules in early Age-related Macular Degeneration

Abstract

Age-related macular degeneration (AMD) is the most common blinding disease in the western world and is currently incurable. Although the exact causes of AMD are not clear, the primary origin of pathology appears to be in the retinal pigment epithelium (RPE). RPE is responsible for the daily digestion of photoreceptor outer segments (POS), which imposes a heavy continuous burden on the lysosomal network. POS feeding assay in vitro suggested that the accumulation of autofluorescence granules (AFG), similar to lipofuscin in vivo, derives from lysosomal dysfunction. Here we show that synchronous phagocytosis of POS leads to early transient mTOR activation followed by inhibition in late phagosome maturation. One of its substrates, the transcription factor EB (TFEB) increases during phagosome maturation albeit mostly in its inactive phosphorylated form. We questioned whether induction of the mTOR/TFEB axis could improve digestion of POS and hence reduce AFG load. Treatment of POS-fed cells with rapamycin, an mTORC1 inhibitor after the appearance of AFG results in 30% reduction of AFG load. This effect is dependent on active lysosomal enzymes and induction of active dephosphorylated TFEB with consequent activation of GADD34 and lysosomal biogenesis. As a proof of concept, we show that overexpressing a constitutively active form of unphosphorylated TFEB dramatically reduces POS-dependent AFG accumulation. Overall, this study suggests that viral or pharmacological approaches activating the mTOR/TFEB axis in the RPE could be beneficial as cell-protective treatment of early/intermediate cases of AMD, acting to delay progression of the disease.