DARPP32, a target of hyperactive mTORC1 in the retinal pigment epithelium

The mechanistic target of rapamycin (mTOR) is assembled into signaling complexes of mTORC1 or mTORC2, and plays key roles in cell metabolism, stress response, nutrient and growth factor sensing. Accumulating evidence from human and animal model studies has demonstrated a pathogenic role of hyperactive mTORC1 in age-related macular degeneration (AMD). The retinal pigment epithelium (RPE) is a primary injury site in AMD. In mouse models of RPE-specific deletion of Tuberous sclerosis 1 (Tsc1), which encodes an upstream suppressor of mTORC1, the hyperactivated mTORC1 metabolically reprogrammed the RPE and led to the degeneration of the outer retina and choroid (CH). In the current study, we used single-cell RNA sequencing (scRNA-seq) to identify a novel RPE mTORC1 downstream protein, dopamine and cyclic AMP-regulated phosphoprotein of molecular weight 32,000 (DARPP-32). DARPP-32 was not found in healthy RPE but localized to drusen and basal linear deposits in human AMD eyes. In animal models, overexpressing DARPP-32 by adeno-associated virus (AAV) led to abnormal RPE structure and function. The data indicate that DARPP-32 is a previously unidentified signaling protein subjected to mTORC1 regulation and may contribute to RPE degeneration in AMD.雷帕霉素靶蛋白（mechanistic target of rapamycin，mTOR）可组装为mTORC1或mTORC2信号复合体，并在细胞代谢、应激响应、营养与生长因子感知过程中发挥关键作用。来自人体与动物模型研究的大量证据表明，过度激活的mTORC1在年龄相关性黄斑变性（age-related macular degeneration，AMD）的发病过程中发挥致病作用。视网膜色素上皮（retinal pigment epithelium，RPE）是AMD发病的主要损伤部位。在视网膜色素上皮特异性敲除结节性硬化症1（Tuberous sclerosis 1，Tsc1，其编码mTORC1的上游抑制因子）的小鼠模型中，过度激活的mTORC1会使视网膜色素上皮发生代谢重编程，并导致外层视网膜与脉络膜（choroid，CH）发生变性。在本研究中，我们通过单细胞RNA测序（single-cell RNA sequencing，scRNA-seq）筛选得到一种全新的视网膜色素上皮mTORC1下游蛋白——分子量32000的多巴胺与环磷酸腺苷调控磷蛋白（dopamine and cyclic AMP-regulated phosphoprotein of molecular weight 32,000，DARPP-32）。健康视网膜色素上皮中未检测到DARPP-32，但在人类AMD患者眼球的玻璃膜疣与基底线性沉积物中可观测到其定位。在动物模型中，通过腺相关病毒（adeno-associated virus，AAV）过表达DARPP-32会导致视网膜色素上皮的结构与功能异常。上述数据表明，DARPP-32是一种此前未被发现的受mTORC1调控的信号蛋白，或可参与AMD患者视网膜色素上皮的变性过程。