Brown-fat paucity due to impaired BMP signalling induces compensatory browning of white fat

Maintenance of body temperature is essential for the survival of homeotherms. Brown adipose tissue (BAT) is a specialized fat tissue that is dedicated to thermoregulation(1). Owing to its remarkable capacity to dissipate stored energy and its demonstrated presence in adult humans(2-5), BAT holds great promise for the treatment of obesity and metabolic syndrome(1). Rodent data suggest the existence of two types of brown fat cells: constitutive BAT (cBAT), which is of embryonic origin and anatomically located in the imterscapular region of mice; and recruitable BAT (rBAT), which resides within white adipose tissue (WAT)(6) and skeletal muscle(7), and has alternatively been called beige(8), brite(9) or inducible BAT(10). Bone morphogenetic proteins (BMPs) regulate the formation and thermogenic activity of BAT(10-12). Here we use mouse models to provide evidence for a systemically active regulatory mechanism that controls whole-body BAT activity for thermoregulation and energy homeostasis. Genetic ablation of the type 1A BMP receptor (Bmpr1a) in brown adipogenic progenitor cells leads to a severe paucity of cBAT. This in turn increases sympathetic input to WAT, thereby promoting the formation of rBAT within white fat depots. This previously unknown compensatory mechanism, aimed at restoring total brown-fat-mediated thermogenic capacity in the body, is sufficient to maintain normal temperature homeostasis and resistance to diet-induced obesity. These data suggest an important physiological cross-talk between constitutive and recruitable brown fat cells. This sophisticated regulatory mechanism of body temperature may participate in the control of energy balance and metabolic disease.