Role of brown adipose tissue (BAT) in energy balance

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

Thermal biology is different between small and large mammals. At typical ambient environmental temperature (eg., 22 C), over one third of energy expenditure in mice is devoted to maintaining core body temperature, largely by brown adipose tissue (BAT). To conserve energy, mice can enter a regulated hypothermia, while humans do not. Since humans expend little or no energy specifically to keep warm, mice studied at 30 C (near thermoneutrality) may be a better model for human energy homeostasis. In mice, dinitrophenol, a protonophore, and CL316243, a beta3-adrenergic agonist, both increased metabolic rate at thermoneutrality but only CL316243 increased it at 22 C. Mice housed at 30 C also may become more obese than mice at 22 C. The effect of environmental temperature must be understood to ensure applicability of mouse experiments to human obesity. BAT is also intimately involved with hypothermia, as induction of hypothermia involves complete inactivation of BAT, while recovery reactivates it. Thus, studies of drugs causing hypothermia should interact in the neural pathways that contribute to the regulation and control of BAT. Progress in FY2022 includes the following: In 2021 we identified preoptic area neurons expressing BRS3 (POA-BRS3) as a population whose activation increased body temperature; inversely, acute inhibition of these neurons reduced body temperature. POA-BRS3 neurons that project to either the paraventricular nucleus of the hypothalamus or the dorsomedial hypothalamus increased body temperature, heart rate, and blood pressure via the sympathetic nervous system. Long-term inactivation of POA-BRS3 neurons caused increased body temperature variability, overshooting both increases and decreases in body temperature set point, with RNA expression profiles suggesting multiple types of POA-BRS3 neurons. Thus, POA-BRS3 neuronal populations regulate body temperature and heart rate, contribute to cold defense, and fine-tune feedback control of body temperature. These findings advance understanding of homeothermy, a defining feature of mammalian biology. An ongoing project is aimed at understanding the role of Otop1 (Otopetrin 1), a proton-selective channel that is highly expressed in brown adipose tissue. The main approach is comparison of wild-type and Otop1 knockout mice.