Prdm16 is required for the maintenance of brown adipocyte identity and function in adult mice

The role of skeletal-muscle-based thermogenic mechanisms in vertebrate endothermy

Thermogenesis is one of the most important homeostatic mechanisms that evolved during vertebrate evolution. Despite its importance for the survival of the organism, the mechanistic details behind various thermogenic processes remain incompletely understood. Although heat production from muscle has long been recognized as a thermogenic mechanism, whether muscle can produce heat independently of contraction remains controversial. Studies in birds and mammals suggest that skeletal muscle can be an important site of non-shivering thermogenesis (NST) and can be recruited during cold adaptation, although unequivocal evidence is lacking. Much research on thermogenesis during the last two decades has been focused on brown adipose tissue (BAT). These studies clearly implicate BAT as an important site of NST in mammals, in particular in newborns and rodents. However, BAT is either absent, as in birds and pigs, or is only a minor component, as in adult large mammals including humans, bringing into question the BAT-centric view of thermogenesis. This review focuses on the evolution and emergence of various thermogenic mechanisms in vertebrates from fish to man. A careful analysis of the existing data reveals that muscle was the earliest facultative thermogenic organ to emerge in vertebrates, long before the appearance of BAT in eutherian mammals. Additionally, these studies suggest that muscle-based thermogenesis is the dominant mechanism of heat production in many species including birds, marsupials, and certain mammals where BAT-mediated thermogenesis is absent or limited. We discuss the relevance of our recent findings showing that uncoupling of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) by sarcolipin (SLN), resulting in futile cycling and increased heat production, could be the basis for NST in skeletal muscle. The overall goal of this review is to highlight the role of skeletal muscle as a thermogenic organ and provide a balanced view of thermogenesis in vertebrates.

Fighting obesity: When muscle meets fat

The prevalence of obesity has risen to an unprecedented level. According to World Health Organization, over 500 million adults, equivalent to 10%-14% of the world population, were obese with a body mass index (BMI) of 30 kg/m(2) or greater in 2008.(1) This rising prevalence and earlier onset of obesity is believed to be resulted from an interplay of genetic factors, over-nutrition and physical inactivity in modern lifestyles. Obesity also increases the susceptibility to metabolic syndromes, hypertension, cardiovascular diseases, Type 2 diabetes mellitus (T2DM) and cancer.(2-4) The global obesity epidemic has sparked substantial interests in the biology of adipose tissue (fat). In addition, the skeletal muscle and its secretive factors (myokines) have also been shown to play a critical role in controlling body energy balance, adipose homeostasis and inflammation status.(5) Interestingly, skeletal muscle cells share a common developmental origin with brown adipocytes,(6,7) which breaks down lipids to generate heat - thus reducing obesity. Here, we provide a brief overview of the basics and recent progress in muscle-fat crosstalk in the context of body energy metabolism, obesity, and diabetes. We summarize the different types of adipocytes, their developmental origins and implications in body composition. We highlight the role of several novel myokines in regulating fat mass and systemic energy balance, and evaluate the potential of skeletal muscles as a therapeutic target to treat obesity.

The identification and structure of an N-terminal PR domain show that FOG1 is a member of the PRDM family of proteins

FOG1 is a transcriptional regulator that acts in concert with the hematopoietic master regulator GATA1 to coordinate the differentiation of platelets and erythrocytes. Despite considerable effort, however, the mechanisms through which FOG1 regulates gene expression are only partially understood. Here we report the discovery of a previously unrecognized domain in FOG1: a PR (PRD-BF1 and RIZ) domain that is distantly related in sequence to the SET domains that are found in many histone methyltransferases. We have used NMR spectroscopy to determine the solution structure of this domain, revealing that the domain shares close structural similarity with SET domains. Titration with S-adenosyl-L-homocysteine, the cofactor product synonymous with SET domain methyltransferase activity, indicated that the FOG PR domain is not, however, likely to function as a methyltransferase in the same fashion. We also sought to define the function of this domain using both pulldown experiments and gel shift assays. However, neither pulldowns from mammalian nuclear extracts nor yeast two-hybrid assays reproducibly revealed binding partners, and we were unable to detect nucleic-acid-binding activity in this domain using our high-diversity Pentaprobe oligonucleotides. Overall, our data demonstrate that FOG1 is a member of the PRDM (PR domain containing proteins, with zinc fingers) family of transcriptional regulators. The function of many PR domains, however, remains somewhat enigmatic for the time being.

Rictor/mTORC2 loss in the Myf5 lineage reprograms brown fat metabolism and protects mice against obesity and metabolic disease

The in vivo functions of mechanistic target of rapamycin complex 2 (mTORC2) and the signaling mechanisms that control brown adipose tissue (BAT) fuel utilization and activity are not well understood. Here, by conditionally deleting Rictor in the Myf5 lineage, we provide in vivo evidence that mTORC2 is dispensable for skeletal muscle development and regeneration but essential for BAT growth. Furthermore, deleting Rictor in Myf5 precursors shifts BAT metabolism to a more oxidative and less lipogenic state and protects mice from obesity and metabolic disease at thermoneutrality. We additionally find that Rictor is required for brown adipocyte differentiation in vitro and that the mechanism specifically requires AKT1 hydrophobic motif phosphorylation but is independent of pan-AKT signaling and is rescued with BMP7. Our findings provide insights into the signaling circuitry that regulates brown adipocytes and could have important implications for developing therapies aimed at increasing energy expenditure as a means to combat human obesity.