Hormonal Regulation of Energy Expenditure in Mice

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The goal of our NSERC discovery research program is to understand the regulation of energy metabolism in mammals. Brown adipose tissue (BAT) is an important tissue regulating energy metabolism effects which are mediated through expression of mitochondrial uncoupling protein 1 (UCP1). Supporting a critical role for UCP1 and BAT in the control of energy balance are findings from numerous studies demonstrating that UCP1 promotes energy expenditure. Importantly, recent findings using positron emission tomography (PET) have indicated that BAT activity is also important in humans and may be regulated by obesity, aging, gender and type 2 diabetes. In addition recent studies have demonstrated that there is significant plasticity in white adipose tissue (WAT) to express UCP1 and take on many morphological characteristics analogous to BAT. Despite these recent important observations the endocrine signals regulating BAT activity or the “browning” of WAT are not fully understood and are of fundamental importance to our understanding of factors controlling energy metabolism in mammals. Serotonin is an ancient biogenic amine that is found across phyla and is essential for regulating behaviour, appetite and energy expenditure. What is often under-appreciated is that the vast majority of serotonin in the body is found in the periphery, not the central nervous system. Tryptophan hydroxylase (Tph1) catalyzes the rate-limiting step in the synthesis of serotonin from dietary tryptophan. Importantly, mice lacking Tph1 have reduced serum serotonin, but normal levels of serotonin in the brain, thus allowing us to determine the effects of peripheral serotonin in regulating peripheral energy metabolism. In preliminary studies we have found that circulating serotonin levels are altered by nutritional status including fasting and a diet high in fat and are reduced in response to exercise. Genetic manipulation of peripheral serotonin by studying mice deficient in Tph1 have revealed that low levels of serotonin increase energy expenditure as a result of hyper-activation of BAT and increased browning of white adipocytes. The aims of this NSERC Discovery Grant are to extend on our extensive experience in studying the hormonal regulation of energy metabolism by determining the role of serotonin/Tph1 in controlling adipose tissue energy metabolism. Specifically we will examine: AIM 1: Does chemical inhibition of Tph1 phenocopy the effects of Tph1 genetic deletion on body mass and energy expenditure and is this effect mediated by UCP1? AIM 2: What is the mechanism by which Tph1 and serotonin control white and brown adipose tissue energy metabolism? AIM 3: Is Tph1 required for the browning of white fat induced by physical activity/exercise? Serotonin was discovered more than 60 years ago while searching for vasoconstrictors that cause hypertension, and was aptly named it for its presence in serum (sero) and its vasoactive properties (tonin). We now have substantial preliminary data for a new role of peripheral serotonin in controlling adipose tissue energy metabolism. These findings link an ancient molecule that is essential for regulating energy balance across phyla to the control of adipose tissue energy metabolism in mammals. Our proposed studies will identify how and why serotonin and/or Tph1 controls energy expenditure in mammals and whether UCP1 is essential for these effects. The impact of this discovery-based research may have an important impact on Canadians through improved nutritional policy, improved control over environmental factors and developing new therapeutics, which can only be achieved through an increased understanding of how serotonin controls adipose tissue metabolism and energy balance.