Abstract Triglycerides constitute 90% of the total ingested lipid. Excessive intake of dietary TGs and elevated levels of intestine-derived lipoproteins are key determinants of metabolic disease—a leading cause of mortality in individuals 65 years or older. Increased circulating and tissue TGs strongly correlate with age-related metabolic disease. Despite our understanding of lipid absorption, a fundamental question remains unanswered—how are lipids sensed? mTORC1 (mechanistic target of rapamycin-complex 1) is a PI3K-like kinase that senses amino acids and drives protein synthesis. Our exciting new data show that availability of lipid activates mTORC1 signaling—suggesting that mTOR is a lipid sensor. Experiments with BODIPY-labelled palmitic acid gavaged into mice revealed robust mTORC1 activation—suggesting that mTOR likely senses fatty acid or a fatty acid derivative in vivo. How mTOR senses lipid remains unknown. Our preliminary data in cultured cells show that availability of diacylglycerol triggers the localization of mTOR to lysosomes—suggesting that sensing of diacylglycerols by mTOR occurs at lysosomal membranes. Since mTOR is hyperactivated with age, we propose that increases in lysosomal membrane diacylglycerol with age causes mTORC1 hyperactivation. In addition to the direct cell-autonomous regulation of mTORC1 signaling by lipid, our new data also reveal a novel cell non- autonomous regulation of peripheral mTORC1 signaling. Indeed, acutely suppressing mTORC1 signaling in the mediobasal hypothalamus via targeted stereotaxic delivery of rapamycin, dampens lipid-driven mTORC1 signaling in peripheral tissues. On this basis, we hypothesize that altered lipid sensing by mTOR due to age- related changes in lysosome membrane lipid composition is the mechanism for mTORC1 hyperactivation. We hypothesize further that dysregulation of mTORC1 signaling in hypothalamus contributes to the hyperactivation of mTORC1 signaling in peripheral tissues. To test our hypothesis, we present the following Aims: Aim 1: To determine the mechanism by which mTOR senses cellular diacylglycerols. In Aim 1, we will use biochemical and image-based approaches as well as proteomics and unbiased lipidomics of lysosomal membranes to characterize mTORC1 signaling in response to lipid availability in young and aged mice. We will identify novel regulatory proteins that interact with mTOR when lipid is available. Targeted lipidomics of lysosomal membranes with co-Investigator Dr. Laura Beth McIntire at Columbia University will reveal the specific lipid species that drive mTORC1 hyperactivation in young and old animals. By silencing candidate targets, we will identify novel candidate proteins at lysosomes that mediate the lipid sensing function of mTOR in young and old animals. Aim 2: To dissect systemic lipid sensing by a CNS-to-peripheral mTORC1 axis. In Aim 2, we will use a diverse set of tools including hypothalamic neuron-specific GFP-labeled mice to determine whether li...