The Role of the Mitochondrion in the Metabolic Stress Response to Burn Trauma

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PROJECT SUMMARY The efficiency of energy transduction in the body has significant implications for body weight regulation and health. Mitochondrial proton leaks, a process where the proton gradient generated by mitochondria through the oxidation of substrates and electron transfer is transduced to heat rather than being harnessed to generate ATP, is a major contributor to metabolic inefficiency in mammals. Thermoregulation is the principal physiological role ascribed to mitochondria proton leaks, which are responsible for uncoupling around 20-25% of mitochondrial respiration for ATP production. There is significant interest in a putative anti-obesity role for mitochondrial proton leaks. This has been driven by the relatively recent observation that some adult humans possess functional brown adipose tissue (BAT), and that under certain stress states, subcutaneous white adipose tissue (scWAT) can undergo a browning response, developing some BAT characteristics. BAT mitochondria are unique in that they express the inner membrane carrier protein UCP1 (uncoupling protein 1). UCP1-mediated proton leak can almost completely uncouple BAT mitochondria, where fuel oxidation and respiration can proceed uninhibited and in the absence of ATP production. In rodent models of prolonged cold stress, UCP1-mediated proton leak in BAT and scWAT can significantly influence metabolic rate and body mass. However, the role of mitochondrial proton leaks in metabolic regulation beyond models of chronic cold exposure remains poorly understood. We propose a series of innovative studies leveraging severe burn injury as a unique clinical model of hypermetabolism (increased metabolic rate) to probe the role of mitochondrial proton leaks in the regulation of metabolic rate and body mass. Our overarching hypothesis is that UCP1-mediated proton leak in both BAT and scWAT contribute to burn-induced hypermetabolism. In specific aim 1, we will further our understanding of the biochemical basis of hypermetabolism by identifying the tissue specific contributions of mitochondrial proton leaks to burn-induced hypermetabolism, while also identifying the molecular transducers of these proton leaks. In addition, stable isotope studies will be used to determine the role of ATP dependent substrate cycling within scWAT as a novel UCP1-independent mechanism of adipocyte thermogenesis. In specific aim 2, we will investigate the impact of obesity and diminished capacity of BAT activation and scWAT browning on burn- induced hypermetabolism. Specifically, we will determine the key changes in the phospholipid and carrier protein composition of mitochondrial membranes in the liver and skeletal muscle in order to identify key modulators of mitochondrial proton leak beyond UCP1. This new knowledge may hold clinical value in terms of identifying targets that may be modulated either to reduce metabolic rate in the context of hypermetabolism, or to augment mitochondrial inefficiency in the context of nutrient over-supply and obesity.