Enhancing lysosome function via TFEB activation reduces lipofuscin-like granules in the retinal pigment epithelium

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 the aged retinal pigment epithelium (RPE) exhibiting signs of lysosomal dysfunction and oxidative damage. RPE is responsible for the daily digestion of photoreceptor outer segments (POS), imposing a heavy continuous burden on the lysosomal network. A cellular model of RPE lysosomal dysfunction can be achieved by feeding RPE with a single pulse of POS, leading to the accumulation of autofluorescence granules (AFG), similar to lipofuscin in vivo. 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 modulation of the mTOR/TFEB axis could improve POS clearance and hence reduce AFG load. Treatment of POS-fed cells after the appearance of AFGs with rapamycin, an mTORC1 inhibitor 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 TFEB pathway in the RPE could be beneficial as cell-protective treatment of early/intermediate cases of AMD, acting to delay progression of the disease.