An improved method for the precise unravelment of non-shivering brown fat thermokinetics

Inhibition of Thyroid Hormone Signaling in the Zona Incerta Alters Basal Metabolic Rate, Behavior, and Serum Glucocorticoids in Male Mice

Background: It has long been known that thyroid disease can lead to changes in energy metabolism, thermoregulation, and anxiety behavior. While these actions have been partially attributed to thyroid hormone (TH) receptor α1 (TRα1) action in the brain, the precise neuroanatomical substrates have remain elusive. Methods: We used PET-CT scans to identify brain regions affected by TH. We then inhibited TRα1 signaling specifically in the most affected region, the zona incerta (ZI), a still mysterious region previously implicated in thermogenesis and anxiety. To this end, we used an adeno-associated virus (AAV) expressing a dominant-negative TRα1R384C in wild-type mice and phenotyped the animals. Finally, we used tyrosine hydroxylase-Cre mice to test specifically the contribution of ZI dopaminergic neurons. Results: Our data showed that AAV-mediated inhibition of TRα1 signaling in the ZI lead to increased energy expenditure at thermoneutrality, while body temperature regulation remained unaffected. Moreover, circulating glucocorticoid levels were increased, and a mild habituation problem was observed in the open field test. No effects were observed when TRα1 signaling was selectively inhibited in dopaminergic neurons. Conclusions: Our findings suggest that altered TH signaling in the ZI is not involved in body temperature regulation but can affect basal metabolism and modulates stress responses.

Prolonged FGF21 treatment increases energy expenditure and induces weight loss in obese mice independently of UCP1 and adrenergic signaling

Fibroblast growth factor 21 (FGF21) reduces body weight, which was attributed to induced energy expenditure (EE). Conflicting data have been published on the role of uncoupling protein 1 (UCP1) in this effect. Therefore, we aimed to revisit the thermoregulatory effects of FGF21 and their implications for body weight regulation. We found that an 8-day treatment with FGF21 lowers body weight to similar extent in both wildtype (WT) and UCP1-deficient (KO) mice fed high-fat diet. In WT mice, this effect is solely due to increased EE, associated with a strong activation of UCP1 and with excess heat dissipated through the tail. This thermogenesis takes place in the interscapular region and can be attenuated by a β-adrenergic inhibitor propranolol. In KO mice, FGF21-induced weight loss correlates with a modest increase in EE, which is independent of adrenergic signaling, and with a reduced energy intake. Interestingly, the gene expression profile of interscapular brown adipose tissue (but not subcutaneous white adipose tissue) of KO mice is massively affected by FGF21, as shown by increased expression of genes encoding triacylglycerol/free fatty acid cycle enzymes. Thus, FGF21 elicits central thermogenic and pyretic effects followed by a concomitant increase in EE and body temperature, respectively. The associated weight loss is strongly dependent on UCP1-based thermogenesis. However, in the absence of UCP1, alternative mechanisms of energy dissipation may contribute, possibly based on futile triacylglycerol/free fatty acid cycling in brown adipose tissue and reduced food intake.

Bisphenol S induces brown adipose tissue whitening and aggravates diet-induced obesity in an estrogen-dependent manner

Bisphenol S (BPS) exposure has been implied epidemiologically to increase obesity risk, but the underlying mechanism is unclear. Here, we propose that BPS exposure at an environmentally relevant dose aggravates diet-induced obesity in female mice by inducing brown adipose tissue (BAT) whitening. We explored the underlying mechanism by which KDM5A-associated demethylation of the trimethylation of lysine 4 on histone H3 (H3K4me3) in thermogenic genes is overactivated in BAT upon BPS exposure, leading to the reduced expression of thermogenic genes. Further studies have suggested that BPS activates KDM5A transcription in BAT by binding to glucocorticoid receptor (GR) in an estrogen-dependent manner. Estrogen-estrogen receptors facilitate the accessibility of the KDM5A gene promoter to BPS-activated GR by recruiting the activator protein 1 (AP-1) complex. These results indicate that BAT is another important target of BPS and that targeting KDM5A-related signals may serve as an approach to counteract the BPS-induced susceptivity to obesity.

Maternal thyroid hormone receptor β activation in mice sparks brown fat thermogenesis in the offspring

It is well established that maternal thyroid hormones play an important role for the developing fetus; however, the consequences of maternal hyperthyroidism for the offspring remain poorly understood. Here we show in mice that maternal 3,3',5-triiodothyronine (T3) treatment during pregnancy leads to improved glucose tolerance in the adult male offspring and hyperactivity of brown adipose tissue (BAT) thermogenesis in both sexes starting early after birth. The activated BAT provides advantages upon cold exposure, reducing the strain on other thermogenic organs like muscle. This maternal BAT programming requires intact maternal thyroid hormone receptor β (TRβ) signaling, as offspring of mothers lacking this receptor display the opposite phenotype. On the molecular level, we identify distinct T3 induced alterations in maternal serum metabolites, including choline, a key metabolite for healthy pregnancy. Taken together, our results connect maternal TRβ activation to the fetal programming of a thermoregulatory phenotype in the offspring.

Rest phase snacking increases energy resorption and weight gain in male mice

OBJECTIVE

Snacking, i.e., the intake of small amounts of palatable food items, is a common behavior in modern societies, promoting overeating and obesity. Shifting food intake into the daily rest phase disrupts circadian rhythms and is also known to stimulate weight gain. We therefore hypothesized that chronic snacking in the inactive phase may promote body weight gain and that this effect is based on disruption of circadian clocks.

METHODS

Male mice were fed a daily chocolate snack either during their rest or their active phase and body weight development and metabolic parameters were investigated. Snacking experiments were repeated in constant darkness and in clock-deficient mutant mice to examine the role of external and internal time cues in mediating the metabolic effects of snacking.

RESULTS

Chronic snacking in the rest phase increased body weight gain and disrupted metabolic circadian rhythms in energy expenditure, body temperature, and locomotor activity. Additionally, these rest phase snacking mice assimilated more energy during the inactive phase. Body weight remained increased in rest phase snacking wildtype mice in constant darkness as well as in clock-deficient mutant mice under a regular light-dark cycle compared to mice snacking in the active phase. Weight gain effects were abolished in clock-deficient mice in constant darkness.

CONCLUSIONS

Our data suggest that mistimed snacking increases energy resorption and promotes body weight gain. This effect requires a functional circadian clock at least under constant darkness conditions.

Temperature modulates systemic and central actions of thyroid hormones on BAT thermogenesis

Thyroid hormones (THs) play a major role regulating energy balance and brown adipose tissue (BAT) thermogenesis, as well as body temperature, as shown in hyperthyroid patients. However, the current landscape of preclinical thyroid hormone models is complex. For example, while rats become catabolic after TH administration, mice gain weight; so, these differences in species need to be analyzed in detail and specially whether temperature could be a factor. Here, we aimed to investigate the effect of environmental temperature on those actions. Rats were subcutaneously treated with L-thyroxine (T4) or stereotaxically within the ventromedial nucleus of the hypothalamus (VMH) with triiodothyronine (T3) and housed at 23°C, 4°C or 30°C; energy balance, BAT thermogenesis and AMP-activated protein kinase (AMPK) in the VMH were analyzed. Our data showed that the effect of both systemic T4 of central T3 on energy balance and BAT thermogenesis was dependent upon environmental temperature. This evidence is of interest in the design of experimental settings highlighting the species-specific metabolic actions of THs, and in understanding its physiological role in the adaptation to temperature.

Uncoupling Protein 1 Does Not Produce Heat without Activation

Mitochondrial uncoupling protein 1 (UCP1) is the crucial mechanistic component of heat production in classical brown fat and the newly identified beige or brite fat. Thermogenesis inevitably comes at a high energetic cost and brown fat, ultimately, is an energy-wasting organ. A constrained strategy that minimizes brown fat activity unless obligate will have been favored during natural selection to safeguard metabolic thriftiness. Accordingly, UCP1 is constitutively inhibited and is inherently not leaky without activation. It follows that increasing brown adipocyte number or UCP1 abundance genetically or pharmacologically does not lead to an automatic increase in thermogenesis or subsequent metabolic consequences in the absence of a plausible route of concomitant activation. Despite its apparent obviousness, this tenet is frequently ignored. Consequently, incorrect conclusions are often drawn from increased BAT or brite/beige depot mass, e.g., predicting or causally linking beneficial metabolic effects. Here, we highlight the inherently inactive nature of UCP1, with a particular emphasis on the molecular brakes and releases of UCP1 activation under physiological conditions. These controls of UCP1 activity represent potential targets of therapeutic interventions to unlock constraints and efficiently harness the energy-expending potential of brown fat to prevent and treat obesity and associated metabolic disorders.

Orally Induced Hyperthyroidism Regulates Hypothalamic AMP-Activated Protein Kinase

Besides their direct effects on peripheral metabolic tissues, thyroid hormones (TH) act on the hypothalamus to modulate energy homeostasis. However, since most of the hypothalamic actions of TH have been addressed in studies with direct central administration, the estimation of the relative contribution of the central vs. peripheral effects in physiologic conditions of peripheral release (or administration) of TH remains unclear. In this study we used two different models of peripherally induced hyperthyroidism (i.e., T4 and T3 oral administration) to assess and compare the serum and hypothalamic TH status and relate them to the metabolic effects of the treatment. Peripheral TH treatment affected feeding behavior, overall growth, core body temperature, body composition, brown adipose tissue (BAT) morphology and uncoupling protein 1 (UCP1) levels and metabolic activity, white adipose tissue (WAT) browning and liver metabolism. This resulted in an increased overall uncoupling capacity and a shift of the lipid metabolism from WAT accumulation to BAT fueling. Both peripheral treatment protocols induced significant changes in TH concentrations within the hypothalamus, with T3 eliciting a downregulation of hypothalamic AMP-activated protein kinase (AMPK), supporting the existence of a central action of peripheral TH. Altogether, these data suggest that peripherally administered TH modulate energy balance by various mechanisms; they also provide a unifying vision of the centrally mediated and the direct local metabolic effect of TH in the context of hyperthyroidism.

BMP8 and activated brown adipose tissue in human newborns

The classical dogma states that brown adipose tissue (BAT) plays a major role in the regulation of temperature in neonates. However, although BAT has been studied in infants for more than a century, the knowledge about its physiological features at this stage of life is rather limited. This has been mainly due to the lack of appropriate investigation methods, ethically suitable for neonates. Here, we have applied non-invasive infrared thermography (IRT) to investigate neonatal BAT activity. Our data show that BAT temperature correlates with body temperature and that mild cold stimulus promotes BAT activation in newborns. Notably, a single short-term cold stimulus during the first day of life improves the body temperature adaption to a subsequent cold event. Finally, we identify that bone morphogenic protein 8B (BMP8B) is associated with the BAT thermogenic response in neonates. Overall, our data uncover key features of the setup of BAT thermogenesis in newborns.

Activity-Based Anorexia Induces Browning of Adipose Tissue Independent of Hypothalamic AMPK

Anorexia nervosa (AN) is an eating disorder leading to malnutrition and, ultimately, to energy wasting and cachexia. Rodents develop activity-based anorexia (ABA) when simultaneously exposed to a restricted feeding schedule and allowed free access to running wheels. These conditions lead to a life-threatening reduction in body weight, resembling AN in human patients. Here, we investigate the effect of ABA on whole body energy homeostasis at different housing temperatures. Our data show that ABA rats develop hyperactivity and hypophagia, which account for a massive body weight loss and muscle cachexia, as well as reduced uncoupling protein 1 (UCP1) expression in brown adipose tissue (BAT), but increased browning of white adipose tissue (WAT). Increased housing temperature reverses not only the hyperactivity and weight loss of animals exposed to the ABA model, but also hypothermia and loss of body and muscle mass. Notably, despite the major metabolic impact of ABA, none of the changes observed are associated to changes in key hypothalamic pathways modulating energy metabolism, such as AMP-activated protein kinase (AMPK) or endoplasmic reticulum (ER) stress. Overall, this evidence indicates that although temperature control may account for an improvement of AN, key hypothalamic pathways regulating thermogenesis, such as AMPK and ER stress, are unlikely involved in later stages of the pathophysiology of this devastating disease.