Role of Constitutive Nutrient Signalling in Mammalian Physiology, Metabolism, and Ageing

Resumen

The mechanistic target of rapamycin complex 1 (mTORC1) regulates cell growth and proliferation in response to nutrient and growth factor availability. The impact of hormonal regulation of mTORC1 in the systemic control of metabolism and physiology has been extensively studied by using genetically modified mouse models. In contrast, although nutrient-dependent activation of mTOR is now well defined at the molecular level, its functions in mammalian physiology and metabolism remain minimally understood. Rag family of GTPases act as the molecular link between nutrient sufficiency and mTOR activation. Thereby, to study how nutrient signalling to mTORC1 affects mammalian physiology, we generated RagA-Q66L knock-in mice by replacing a single nucleotide in the coding sequence of RagA, which translates as RagA constitutively active and bound to GTP (RagAGTP). The study of adult physiology of RagAGTP mice was hampered by a lethal neonatal energetic crisis caused by the inability to promote autophagy after the interruption of the trans-placental supply of nutrients. In this thesis, we have genetically overcome the neonatal lethality and studied the impact of constitutive RagA signalling in adult mouse physiology, metabolism, and ageing. We have found several phenotypic alterations including hypopigmentation, craniofacial deformities, decreased length weight and adiposity, and ultimately, a striking reduction in lifespan. By using a combination of metabolomic and proteomic analysis we discovered that constitutive nutrient signalling alters the transition to fasting by impairing PPARα transactivation, affecting mitochondrial and peroxisomal lipid oxidation, ketogenesis and amino acid catabolism. In addition, chronic nutrient signalling also compromises glucose homeostasis by increasing glucose intolerance, fasting glycaemia, and renal gluconeogenesis. Restricted genetic activation of RagA to hepatocytes and not skeletal muscle partially recapitulate these metabolic alterations without leading to severe tissue damage. mTOR activity is now recognised as a major driver of ageing. Ageing is a time-dependent process that produces a gradual decline in physiological functions and increases the risk of developing diseases and death. Genetic and pharmacological inhibition of mTOR has been shown to increase lifespan in a wide variety of organisms from flies to worms and mammals. Interestingly, recent studies are relating the amount, source, and amino acid composition of the proteins in the diet as key factors affecting longevity and thereby suggesting an essential role of the nutrient sensing pathway in ageing. In this thesis, we aim to investigate the impact of ubiquitous, inducible, and tissue-specific activation of RagA in the onset of aged-related pathologies and premature death. We found that systemic activation of RagA reduces lifespan in a process likely independent of ageing. However, when induced by tamoxifen, constitutive RagA signalling leads to liver and kidney damage and increases the incidence of hepatic hemangiomas accelerating the death of the mouse. Interestingly, restricted activation of RagA signalling to the liver does not increases the risk of developing hepatic tumors as occur in other models of hormonal-dependent activation of mTOR, nor produces severe tissue damage. Similar, although RagA activation specifically in skeletal muscle seem to increase fiber damage, does not affect overall survival. Thereby, a systemic activation of RagA has a detrimental effect on health and survival but only causes mild perturbations when is restricted to the liver or to the skeletal muscle.