Brown adipose tissue (BAT) is a therapeutic target for metabolic diseases; thus, understanding its metabolic circuitry is clinically important. Many studies of BAT compare rodents mildly cold to those severely cold. Here, we compared BAT remodeling between thermoneutral and mild-cold-adapted mice, conditions more relevant to humans. Although BAT is renowned for catabolic β-oxidative capacity, we find paradoxically that the anabolic de novo lipogenesis (DNL) genes encoding ACLY, ACSS2, ACC, and FASN were among the most upregulated by mild cold and that, in humans, DNL correlates with Ucp1 expression. The regulation and function of adipocyte DNL and its association with thermogenesis are not understood. We provide evidence suggesting that AKT2 drives DNL in adipocytes by stimulating ChREBPβ transcriptional activity and that cold induces the AKT2-ChREBP pathway in BAT to optimize fuel storage and thermogenesis. These data provide insight into adipocyte DNL regulation and function and illustrate the metabolic flexibility of thermogenesis.

Summary Fatty acid synthase (FASN) predominantly generates straight-chain fatty acids using acetyl-CoA as the initiating substrate. However, monomethyl branched-chain fatty acids (mmBCFAs) are also present in mammals but thought to be primarily diet-derived. Here we demonstrate that mmBCFAs are de novo synthesized via mitochondrial BCAA catabolism, exported to the cytosol by adipose-specific expression of carnitine acetyltransferase (CrAT), and elongated by FASN. Brown fat exhibits the highest BCAA catabolic and mmBCFA synthesis fluxes, whereas these lipids are largely absent from liver and brain. mmBCFA synthesis is also sustained in the absence of microbiota. We identify hypoxia as a potent suppressor of BCAA catabolism that decreases mmBCFA synthesis in obese adipose tissue, such that mmBCFAs are significantly decreased in obese animals. These results identify adipose tissue mmBCFA synthesis as a novel link between BCAA metabolism and lipogenesis, highlighting roles for CrAT and FASN promiscuity that influence acyl-chain diversity in the lipidome.

The opsin family of G-protein coupled receptors are employed as light detectors in animals. Opsin 5 (neuropsin, OPN5) is a highly conserved, violet light (380 nm λ max ) sensitive opsin 1 , 2 . In mice, OPN5 is a known photoreceptor in retina 3 and skin 4 but is also expressed in the hypothalamic preoptic area (POA) 5 . Here we describe a light-sensing pathway in which Opn5 expressing POA neurons regulate brown adipose tissue (BAT) thermogenesis. We show Opn5 expression in glutamatergic warm-sensing POA neurons that receive synaptic input from multiple thermoregulatory nuclei. We further show that Opn5 POA neurons project to BAT and decrease its activity under chemogenetic stimulation. Opn5 null mice show overactive BAT, elevated body temperature, and exaggerated thermogenesis when cold challenged. Moreover, violet photostimulation during cold exposure acutely suppresses BAT temperature in wild-type, but not in Opn5 null mice. Direct measurements of intracellular cAMP ex vivo reveal that Opn5 POA neurons increase cAMP when stimulated with violet light. This analysis thus identifies a violet light sensitive deep brain photoreceptor that normally suppresses BAT thermogenesis.

Highlights d Adipocytes express encephalopsin (OPN3), a 480 nm bluelight-sensitive opsin d Mice lacking OPN3 or blue light have diminished thermogenesis during cold exposure d Loss of OPN3 reduces oxygen consumption and energy expenditure d White adipocyte OPN3 promotes lipolysis during cold exposure

SUMMARYHere we present the presence of adiponectin and adiponectin receptors [type 1 (adipoR1) and type 2 (adipoR2)] in rainbow trout (Oncorhynchus mykiss) tissues and cell cultures together with the response to different scenarios. In response to fasting, adiponectin expression was up-regulated in adipose tissue, while the expression of its receptors increased in white and red muscle. Insulin injection decreased adipoR1 expression in white and red muscles. We deduce that the adipoRs in trout muscle show opposite responses to increasing insulin plasma levels, which may maintain sensitivity to insulin in this tissue. Adiponectin expression was inhibited by the inflammatory effect of lipopolysaccharide (LPS) in adipose tissue and red muscle. Moreover, results indicate that LPS may lead to mobilization of fat reserves, increasing adipoR1 expression in adipose tissue. The effects of LPS could be mediated through tumour necrosis factor  (TNF), at least in red muscle. Insulin, growth hormone and TNF all diminished expression of adipoR2 in adipocytes and adipoR1 in myotubes, while insulin increased the expression of adipoR2 in the muscle cells. Adiponectin activates Akt in rainbow trout myotubes, which may lead to an increase in fatty acid uptake and oxidation. Overall, our results show that the adiponectin system responds differently to various physiological challenges and that it is hormonally controlled in vivo and in vitro. To the best of our knowledge, this is the first time this has been demonstrated in teleosts, and it may be a valuable contribution to our understanding of adipokines in fish.

Highlights d Inhibiting brown adipocyte mTORC2 protects against obesity at thermoneutrality d mTORC2 suppresses lipolysis and UCP1 expression d Inhibiting mTORC2 triggers FoxO1 deacetylation by SIRT6 d FoxO1 drives lipid catabolism upon mTORC2 loss

Brown adipose tissue is well known to be a thermoregulatory organ particularly important in small rodents and human infants, but it was only recently that its existence and significance to metabolic fitness in adult humans have been widely realized. The ability of active brown fat to expend high amounts of energy has raised interest in stimulating thermogenesis therapeutically to treat metabolic diseases related to obesity and type 2 diabetes. In parallel, there has been a surge of research aimed at understanding the biology of rodent and human brown fat development, its remarkable metabolic properties, and the phenomenon of white fat browning, in which white adipocytes can be converted into brown like adipocytes with similar thermogenic properties. Here, we review the current understanding of the developmental and metabolic pathways involved in forming thermogenic adipocytes, and highlight some of the many unknown functions of brown fat that make its study a rich and exciting area for future research.

Objective Understanding the signaling mechanisms that control brown adipose tissue (BAT) development is relevant to understanding energy homeostasis and obesity. The AKT kinases are insulin effectors with critical in vivo functions in adipocytes; however, their role in adipocyte development remains poorly understood. The goal of this study was to investigate AKT function in BAT development. Methods We conditionally deleted Akt1 and Akt2 either individually or together with Myf5-Cre, which targets early mesenchymal precursors that give rise to brown adipocytes. Because Myf5-Cre also targets skeletal muscle and some white adipocyte lineages, comparisons were made between AKT function in BAT versus white adipose tissue (WAT) and muscle development. We also deleted both Akt1 and Akt2 in mature brown adipocytes with Ucp1-Cre or Ucp1-CreER to investigate AKT1/2 signaling in BAT maintenance. Results AKT1 and AKT2 are individually dispensable in Myf5-Cre lineages in vivo for establishing brown and white adipocyte precursor cell pools and for their ability to differentiate (i.e. induce PPARγ). AKT1 and AKT2 are also dispensable for skeletal muscle development, and AKT3 does not compensate in either the adipocyte or muscle lineages. In contrast, AKT2 is required for adipocyte lipid filling and efficient downstream AKT substrate phosphorylation. Mice in which both Akt1 and Akt2 are deleted with Myf5-Cre lack BAT but have normal muscle mass, and doubly deleting Akt1 and Akt2 in mature brown adipocytes, either congenitally (with Ucp1-Cre), or inducibly in older mice (with Ucp1-CreER), also ablates BAT. Mechanistically, AKT signaling promotes adipogenesis in part by stimulating ChREBP activity. Conclusions AKT signaling is required in vivo for BAT development but dispensable for skeletal muscle development. AKT1 and AKT2 have both overlapping and distinct functions in BAT development with AKT2 being the most critical individual isoform. AKT1 and AKT2 also have distinct and complementary functions in BAT maintenance.