Brown Fat Paucity Due to Impaired BMP Signaling Induces Compensatory Browning of White Fat

Maintenance of body temperature is essential for survival of homeotherms. Brown adipose tissue (BAT) is a specialized fat tissue that is dedicated to thermoregulation(1). Due 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: the constitutive BAT (cBAT), which is of embryonic origin and anatomically located in the interscapular region of mice, and the recruitable BAT (rBAT) that resides within white adipose tissue (WAT)(6) and skeletal muscle(7), that 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). We here provide evidence for a systemically active regulatory mechanism that serves to control whole body BAT-activity for thermoregulation and energy homeostasis. Genetic ablation of 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 the constitutive and recruitable brown fat cells. This sophisticated regulatory mechanism of body temperature may participate in the control of energy balance and metabolic disease.