Microsomal Transfer Protein Modulates Lipoprotein Metabolism and Retinal lipid Homeostasis

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Lipid handling is one of the most critical functions of the retinal pigment epithelium (RPE) a single mitotically inactive cell layer that is situated between the neural retina and the Bruch's membrane-Choroid. Visual function depends on the intimate structural, functional and metabolic interactions between the RPE and the neural retina. Within this complex, lipid metabolism is tightly regulated, and its' dysregulation triggers accumulation of excess lipids within the RPE and the adjacent Bruch's membrane and choroidal vasculature. In the human retina the accumulation of neutral lipid deposits is a characteristic of aging and precedes disease associated-lesions. A significant proportion of this neutral lipid is enclosed within apolipoprotein B100 containing lipoproteins (Blps) that resemble cardiomyocyte associated Blps. The prodigious amount of lipid that must be processed by the RPE, through the recycling and metabolism of lipid rich outer segments and ingestion of circulating lipoproteins, predicts that to prevent lipid overload the RPE synthesize and secrete Blp. In this regard, RPE shares metabolic similarity with cardiomyocytes; both utilize fatty acids as an energy source and both secrete unique EC rich Blps. While it has been shown that MTP-mediated secretion of Blps protects cardiomyocytes from lipid accumulation, there is a paucity of studies regarding Blp assembly/secretion by RPE in vivo. In these studies we will use mouse models and cell culture to decipher the pathological consequences associated with dysregulation of Blp assembly and secretion pathway by inhibiting or genetic ablation of the Mttp gene in RPE. Microsomal transfer protein (MTP), the product of the MTTP gene, is an endoplasmic reticulum-resident lipid transfer protein necessary for Blp assembly and secretion. In the first specific aim we will test the hypothesis that MTP-mediated secretion of Blp is a mechanism for protecting against RPE lipid overload. To test the contribution of localized synthesis and secretion of Blp by the RPE to retinal lipid homeostasis and cell function in a metabolically intact system, we generated the RPE-specific MTP deficient (RPEMttp) mouse. In the second specific aim we will test the hypothesis that Blp assembly is modulated by the daily load of ingested OS lipids. We will use RPE differentiated from human induced pluripotent stem cells (iPSCs) to investigate the mechanism by which lipids taken up through outer segment phagocytosis regulate MTP activity and Blp production. Using these cells, we will determine if loss of MTP, either through gene ablation (KD of MTP in iRPE) or pharmacological inhibition, contributes to RPE steatosis. Collectively, these exploratory studies will address the specific role of local Blp assembly in retinal lipid homeostasis. Dysregulation of Blp function can cause non- autonomous changes that negatively affect the entire system and lead to vision loss. These studies maybe foundational to study subsequent steps in the development of age-related changes such as inflammation and degeneration associated with dysregulation of lipoprotein metabolism.