Loss of Prom1 impairs autophagy and promotes epithelial‐mesenchymal transition in mouse retinal pigment epithelial cells

Abstract

Mutations in the Prominin‐1 (Prom1) gene disrupt photoreceptor disk morphogenesis, leading to macular dystrophies. We have shown that human retinal pigment epithelial (RPE) homeostasis is under the control of Prom1‐dependent autophagy, demonstrating that Prom1 plays different roles in the photoreceptors and RPE. It is unclear if retinal and macular degeneration caused by the loss of Prom1 function is a cell‐autonomous feature of the RPE or a generalized disease of photoreceptor degeneration. In this study, we investigated whether Prom1 is required for mouse RPE (mRPE) autophagy and phagocytosis, which are cellular processes essential for photoreceptor survival. We found that Prom1‐KO decreases autophagy flux, activates mTORC1, and concomitantly decreases transcription factor EB (TFEB) and Cathepsin‐D activities in mRPE cells. In addition, Prom1‐KO reduces the clearance of bovine photoreceptor outer segments in mRPE cells due to increased mTORC1 and reduced TFEB activities. Dysfunction of Prom1‐dependent autophagy correlates with both a decrease in ZO‐1 and E‐cadherin and a concomitant increase in Vimentin, SNAI1, and ZEB1 levels, consistent with induction of epithelial‐mesenchymal transition (EMT) in Prom1‐KO mRPE cells. Our results demonstrate that Prom1‐mTORC1‐TFEB signaling is a central driver of cell‐autonomous mRPE homeostasis. We show that Prom1‐KO in mRPE leads to RPE defects similar to that seen in atrophic age‐related macular degeneration and opens new avenues of investigation targeting Prom1 in retinal degenerative diseases.

Reduced Retinal Pigment Epithelial Autophagy Due to Loss of Rab12 Prenylation in a Human iPSC-RPE Model of Choroideremia

“…Thus, our findings of increased mTORC1 signaling and reduced autophagy are consistent with these previous reports. Defective RPE autophagy is also a well-established mechanism of disease in age-related macular degeneration (AMD) in numerous published models, as well as Stargardt Disease and PROM1-associated retinal degeneration [ 18 , 19 , 20 , 21 , 22 ]. Activation of autophagy reduced lipofuscin accumulation in RPE cell culture and improved retinal function and reduced Bruch’s membrane thickening in an APOE-deficient mouse model of AMD [ 19 , 20 ].…”

“…In the Abca4 −/− mouse model of Stargardt disease, increased RPE cell endocytosis of complement factor C3a resulted in increased mTOR activation [ 21 ]. Similarly, knockout of PROM1 in both human and mouse RPE cells led to increased mTORC1 activity and reduced autophagy [ 22 , 46 ]. Autophagy is a therapeutic target in pre-clinical studies and in an ongoing clinical trial for Stargardt Disease [NCT04545736].…”

“…Increased mTORC1 signaling and impaired RPE cell autophagy contribute to other forms of retinal degeneration, including age-related macular degeneration (AMD), Stargardt Disease, and PROM1-associated macular dystrophy [ 18 , 19 , 20 , 21 , 22 ]. Autophagy is a cellular catabolic process compensating for nutrient deprivation by lysosomal digest of intracellular damaged proteins and organelles [ 18 ].…”