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. Owing to its remarkable capacity to dissipate stored energy and its demonstrated presence in adult humans, BAT holds great promise for the treatment of obesity and metabolic syndrome. 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 interscapular region of mice; and recruitable BAT (rBAT), which resides within white adipose tissue (WAT) and skeletal muscle, and has alternatively been called beige, brite or inducible BAT. Bone morphogenetic proteins (BMPs) regulate the formation and thermogenic activity of BAT. 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.

Bone morphogenetic protein 7 (BMP7) reverses obesity and regulates appetite through a central mTOR pathway.

Body weight is regulated by coordinating energy intake and energy expenditure. Transforming growth factor ß (TGFß)/bone morphogenetic protein (BMP) signaling has been shown to regulate energy balance in lower organisms, but whether a similar pathway exists in mammals is unknown. We have previously demonstrated that BMP7 can regulate brown adipogenesis and energy expenditure. In the current study, we have uncovered a novel role for BMP7 in appetite regulation. Systemic treatment of diet-induced obese mice with BMP7 resulted in increased energy expenditure and decreased food intake, leading to a significant reduction in body weight and improvement of metabolic syndrome. Similar degrees of weight loss with reduced appetite were also observed in BMP7-treated ob/ob mice, suggesting a leptin-independent mechanism utilized by BMP7. Intracerebroventricular administration of BMP7 to mice led to an acute decrease in food intake, which was mediated, at least in part, by a central rapamycin-sensitive mTOR-p70S6 kinase pathway. Together, these results underscore the importance of BMP7 in regulating both food intake and energy expenditure, and suggest new therapeutic approaches for obesity and its comorbidities.

Identification of inducible brown adipocyte progenitors residing in skeletal muscle and white fat.

Brown fat is specialized for energy expenditure and has therefore been proposed to function as a defense against obesity. Despite recent advances in delineating the transcriptional regulation of brown adipocyte differentiation, cellular lineage specification and developmental cues specifying brown-fat cell fate remain poorly understood. In this study, we identify and isolate a subpopulation of adipogenic progenitors (Sca-1(+)/CD45(-)/Mac1(-); referred to as Sca-1(+) progenitor cells, ScaPCs) residing in murine brown fat, white fat, and skeletal muscle. ScaPCs derived from different tissues possess unique molecular expression signatures and adipogenic capacities. Importantly, although the ScaPCs from interscapular brown adipose tissue (BAT) are constitutively committed brown-fat progenitors, Sca-1(+) cells from skeletal muscle and subcutaneous white fat are highly inducible to differentiate into brown-like adipocytes upon stimulation with bone morphogenetic protein 7 (BMP7). Consistent with these findings, human preadipocytes isolated from subcutaneous white fat also exhibit the greatest inducible capacity to become brown adipocytes compared with cells isolated from mesenteric or omental white fat. When muscle-resident ScaPCs are re-engrafted into skeletal muscle of syngeneic mice, BMP7-treated ScaPCs efficiently develop into adipose tissue with brown fat-specific characteristics. Importantly, ScaPCs from obesity-resistant mice exhibit markedly higher thermogenic capacity compared with cells isolated from obesity-prone mice. These data establish the molecular characteristics of tissue-resident adipose progenitors and demonstrate a dynamic interplay between these progenitors and inductive signals that act in concert to specify brown adipocyte development.