mTORC1-TFEB pathway in degeneration of the RPE

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The long-term goal of this project is to elucidate the mechanisms by which membrane trafficking is regulated in retinal pigment epithelium (RPE) cells, and to understand the role of defects in this regulation in retinal diseases including age-related macular degeneration (AMD). The RPE is responsible for the removal of daily shed photoreceptor outer segments (POS) by phagocytosis, which activates tightly controlled processes of membrane trafficking and organelle transport. In our published and preliminary studies, we found that phagocytosis activated signaling pathway mediated by the mechanistic target of rapamycin (mTOR). Under physiological conditions, mTOR activation was transient. Aging and degeneration, however, rendered mTOR activation to become prolonged after morning burst. In our newly developed murine model of RPE-specific deletion of mTORC1 upstream suppressor TSC1, constitutively high mTOR activity led to RPE and photoreceptor degeneration, likely caused by deregulated membrane trafficking and delayed POS degradation. We further identified VPS11, a key component of the membrane tethering complexes, was downregulated in TSC1-deficient RPE cells possibly due to inhibition on the transcription factor EB (TFEB). Based on those novel findings, we hypothesize that hyperactivation of mTORC1 can cause RPE and photoreceptor degeneration due to defective membrane trafficking. Enhancing TFEB-mediated expression of RPE trafficking proteins can restore the cellular functions and prevent the degenerative phenotype in retina of TSC1RPE mice. The hypothesis will be tested by three specific aims. Aim 1 is to further characterize the retinal phenotype of mice with conditional knockout of TSC1 in the RPE. Aim 2 is to determine whether overactivation of mTOR inhibits RPE intracellular trafficking and membrane fusion by inhibiting TFEB-mediated VPS11 expression. Aim 3 is to determine whether TFEB gene therapy can prevent or rescue RPE degeneration. Results from the proposed studies are expected to further establish the critical roles of mTOR in controlling the RPE and photoreceptor interaction. Although anti-VEGF therapy has achieved unprecedented success, the majority of AMD patients have the atrophic form of the disease whose etiology is still largely unknown and treatment options are very limited. Identifying novel targets downstream of mTOR in degenerating RPE cells can facilitate the design of therapeutic strategies to prevent or at least delay the disease progression at early stage of AMD.