Homeostatic Responses to Amino Acid Insufficiency

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PAmino acid nutrition is a key determinant in the development of dysmetabolism and chronic disease. Amino acid insufficiency (AAI) reduces growth, but also acts to increase cellular resistance to stress, improve cardiometabolic fitness and slow metabolic aging. A clearer understanding of the molecular events altering the relationship between amino acid nutrition and resilience of the proteome will advance current dietary and pharmaceutical approaches to promote health span and treat dysmetabolic conditions. The integrated stress response (ISR) is a vital sentinel signaling network in proteostasis maintenance. A central feature of the ISR is the preferential translation of Activating Transcription Factor 4 (ATF4) upon eukaryotic initiation factor 2 phosphorylation (eIF2-P) by the AAI sensor, General Control Nonderepressible 2 (GCN2). GCN2 alters protein synthesis in cooperation with another nutrient-responsive target, the mammalian target of rapamycin complex 1 (mTORC1). Loss of Gcn2 unleashes mTORC1 activity during AAI, creating a mismatch between metabolic state and need and resulting in proteotoxic stress. Aim 1 of this proposal seeks to define the mechanism by which GCN2 suppresses mTORC1 signaling in response to AAI. We will delineate how these signaling networks influence diurnal rhythms in liver proteostasis and metabolism and contribute to the development of liver steatosis. Aim 2 is focused on how combinatorial transcriptional regulation influences physiological outcomes to AAI. To address this, we will first ascertain eIF2-P independent control of ATF4 translation. We will then study two transcription factors that, like ATF4, are preferentially translated in response to eIF2-P. First is CCAAT/enhancer-binding protein Homologous Protein (CHOP) a pro-apoptotic transcription factor and ATF4 binding partner. Second is ATF5, an ATF4 paralog that is uniquely abundant in liver. We will use floxed mouse models to induce liver-specific or whole body deletion of these transcription factors alone or in combination with each other and monitor proteostasis alongside measurements of liver health and metabolism during AAI. Successful completion of the above aims will significantly change how activation of the ISR to AAI is understood and applied biomedically. This is significant because limitations in the canonical pathways currently prevent development of therapeutics properly targeting the proteostasis network and/or the timing of intervention, either to promote resilience or treat disease.