A recurring problem with the analysis of energy expenditure in genetic models expressing lean and obese phenotypes

Obesity genetics in mouse and human: back and forth, and back again

Obesity is a major public health concern. This condition results from a constant and complex interplay between predisposing genes and environmental stimuli. Current attempts to manage obesity have been moderately effective and a better understanding of the etiology of obesity is required for the development of more successful and personalized prevention and treatment options. To that effect, mouse models have been an essential tool in expanding our understanding of obesity, due to the availability of their complete genome sequence, genetically identified and defined strains, various tools for genetic manipulation and the accessibility of target tissues for obesity that are not easily attainable from humans. Our knowledge of monogenic obesity in humans greatly benefited from the mouse obesity genetics field. Genes underlying highly penetrant forms of monogenic obesity are part of the leptin-melanocortin pathway in the hypothalamus. Recently, hypothesis-generating genome-wide association studies for polygenic obesity traits in humans have led to the identification of 119 common gene variants with modest effect, most of them having an unknown function. These discoveries have led to novel animal models and have illuminated new biologic pathways. Integrated mouse-human genetic approaches have firmly established new obesity candidate genes. Innovative strategies recently developed by scientists are described in this review to accelerate the identification of causal genes and deepen our understanding of obesity etiology. An exhaustive dissection of the molecular roots of obesity may ultimately help to tackle the growing obesity epidemic worldwide.

UCP1 is an essential mediator of the effects of methionine restriction on energy balance but not insulin sensitivity

Dietary methionine restriction (MR) by 80% increases energy expenditure (EE), reduces adiposity, and improves insulin sensitivity. We propose that the MR-induced increase in EE limits fat deposition by increasing sympathetic nervous system-dependent remodeling of white adipose tissue and increasing uncoupling protein 1 (UCP1) expression in both white and brown adipose tissue. In independent assessments of the role of UCP1 as a mediator of MR's effects on EE and insulin sensitivity, EE did not differ between wild-type (WT) and Ucp1(-/-) mice on the control diet, but MR increased EE by 31% and reduced adiposity by 25% in WT mice. In contrast, MR failed to increase EE or reduce adiposity in Ucp1(-/-) mice. However, MR was able to increase overall insulin sensitivity by 2.2-fold in both genotypes. Housing temperatures used to minimize (28°C) or increase (23°C) sympathetic nervous system activity revealed temperature-independent effects of the diet on EE. Metabolomics analysis showed that genotypic and dietary effects on white adipose tissue remodeling resulted in profound increases in fatty acid metabolism within this tissue. These findings establish that UCP1 is required for the MR-induced increase in EE but not insulin sensitivity and suggest that diet-induced improvements in insulin sensitivity are not strictly derived from dietary effects on energy balance.

Triggering receptor expressed on myeloid cells 2 (TREM2) promotes adipogenesis and diet-induced obesity

Triggering receptor expressed on myeloid cells 2 (TREM2) is known to be involved in the anti-inflammatory response and osteoclast development. However, the role of TREM2 in adipogenesis or obesity has not yet been defined. The effect of TREM2 on adipogenesis and obesity was investigated in TREM2 transgenic (TG) mice on a high-fat diet (HFD). To block TREM2 signaling, a neutralizing fusion protein specific for TREM2 (TREM2-Ig) was used. TG mice were much more obese than wild-type mice after feeding with an HFD, independent of the quantity of food intake. These HFD-fed TG mice manifested adipocyte hypertrophy, glucose and insulin resistance, and hepatic steatosis. The expression of adipogenic regulator genes, such as peroxisome proliferator-activated receptor γ and CCAAT/enhancer-binding protein α, was markedly increased in HFD-fed TG mice. Additionally, HFD-fed TG mice exhibited decreased Wnt10b expression and increased GSK-3β (glycogen synthase kinase-3β)-mediated β-catenin phosphorylation. In contrast, the blockade of TREM2 signaling using TREM2-Ig resulted in the inhibition of adipocyte differentiation in vitro and a reduction in body weight in vivo by downregulating the expression of adipogenic regulators. Our data demonstrate that TREM2 promotes adipogenesis and diet-induced obesity by upregulating adipogenic regulators in conjunction with inhibiting the Wnt10b/β-catenin signaling pathway.

The beneficial effects of n-3 polyunsaturated fatty acids on diet induced obesity and impaired glucose control do not require Gpr120

GPR120 (Ffar4) has been postulated to represent an important receptor mediating the improved metabolic profile seen upon ingestion of a diet enriched in polyunsaturated fatty acids (PUFAs). GPR120 is highly expressed in the digestive system, adipose tissue, lung and macrophages and also present in the endocrine pancreas. A new Gpr120 deficient mouse model on pure C57bl/6N background was developed to investigate the importance of the receptor for long-term feeding with a diet enriched with fish oil. Male Gpr120 deficient mice were fed two different high fat diets (HFDs) for 18 weeks. The diets contained lipids that were mainly saturated (SAT) or mainly n-3 polyunsaturated fatty acids (PUFA). Body composition, as well as glucose, lipid and energy metabolism, was studied. As expected, wild type mice fed the PUFA HFD gained less body weight and had lower body fat mass, hepatic lipid levels, plasma cholesterol and insulin levels and better glucose tolerance as compared to those fed the SAT HFD. Gpr120 deficient mice showed a similar improvement on the PUFA HFD as was observed for wild type mice. If anything, the Gpr120 deficient mice responded better to the PUFA HFD as compared to wild type mice with respect to liver fat content, plasma glucose levels and islet morphology. Gpr120 deficient animals were found to have similar energy, glucose and lipid metabolism when fed HFD PUFA compared to wild type mice. Therefore, GPR120 appears to be dispensable for the improved metabolic profile associated with intake of a diet enriched in n-3 PUFA fatty acids.

Chronic oxytocin administration inhibits food intake, increases energy expenditure, and produces weight loss in fructose-fed obese rhesus monkeys

Despite compelling evidence that oxytocin (OT) is effective in reducing body weight (BW) in diet-induced obese (DIO) rodents, studies of the effects of OT in humans and rhesus monkeys have primarily focused on noningestive behaviors. The goal of this study was to translate findings in DIO rodents to a preclinical translational model of DIO. We tested the hypothesis that increased OT signaling would reduce BW in DIO rhesus monkeys by inhibiting food intake and increasing energy expenditure (EE). Male DIO rhesus monkeys from the California National Primate Research Center were adapted to a 12-h fast and maintained on chow and a daily 15% fructose-sweetened beverage. Monkeys received 2× daily subcutaneous vehicle injections over 1 wk. We subsequently identified doses of OT (0.2 and 0.4 mg/kg) that reduced food intake and BW in the absence of nausea or diarrhea. Chronic administration of OT for 4 wk (0.2 mg/kg for 2 wk; 0.4 mg/kg for 2 wk) reduced BW relative to vehicle by 3.3 ± 0.4% (≈0.6 kg; P < 0.05). Moreover, the low dose of OT suppressed 12-h chow intake by 26 ± 7% (P < 0.05). The higher dose of OT reduced 12-h chow intake by 27 ± 5% (P < 0.05) and 8-h fructose-sweetened beverage intake by 18 ± 8% (P < 0.05). OT increased EE during the dark cycle by 14 ± 3% (P < 0.05) and was associated with elevations of free fatty acids and glycerol and reductions in triglycerides suggesting increased lipolysis. Together, these data suggest that OT reduces BW in DIO rhesus monkeys through decreased food intake as well as increased EE and lipolysis.

Specific suppression of insulin sensitivity in growth hormone receptor gene-disrupted (GHR-KO) mice attenuates phenotypic features of slow aging

In addition to their extended lifespans, slow-aging growth hormone receptor/binding protein gene-disrupted (knockout) (GHR-KO) mice are hypoinsulinemic and highly sensitive to the action of insulin. It has been proposed that this insulin sensitivity is important for their longevity and increased healthspan. We tested whether this insulin sensitivity of the GHR-KO mouse is necessary for its retarded aging by abrogating that sensitivity with a transgenic alteration that improves development and secretory function of pancreatic β-cells by expressing Igf-1 under the rat insulin promoter 1 (RIP::IGF-1). The RIP::IGF-1 transgene increased circulating insulin content in GHR-KO mice, and thusly fully normalized their insulin sensitivity, without affecting the proliferation of any non-β-cell cell types. Multiple (nonsurvivorship) longevity-associated physiological and endocrinological characteristics of these mice (namely beneficial blood glucose regulatory control, altered metabolism, and preservation of memory capabilities) were partially or completely normalized, thus supporting the causal role of insulin sensitivity for the decelerated senescence of GHR-KO mice. We conclude that a delayed onset and/or decreased pace of aging can be hormonally regulated.

Interaction of growth hormone receptor/binding protein gene disruption and caloric restriction for insulin sensitivity and attenuated aging

The correlation of physiological sensitivity to insulin ( vis-à-vis glycemic regulation) and longevity is extensively established, creating a justifiable gerontological interest on whether insulin sensitivity is causative, or even predictive, of some or all phenotypes of slowed senescence (including longevity). The growth hormone receptor/ binding protein gene-disrupted (GHR-KO) mouse is the most extensively investigated insulin-sensitive, attenuated aging model. It was reported that, in a manner divergent from similar mutants, GHR-KO mice fail to respond to caloric restriction (CR) by altering their insulin sensitivity. We hypothesized that maximized insulin responsiveness is what causes GHR-KO mice to exhibit a suppressed survivorship response to dietary (including caloric) restriction; and attempted to refute this hypothesis by assessing the effects of CR on GHR-KO mice for varied slow-aging-associated phenotypes. In contrast to previous reports, we found GHR-KO mice on CR to be less responsive than their ad libitum (A.L.) counterparts to the hypoglycemia-inducing effects of insulin. Further, CR had negligible effects on the metabolism or cognition of GHR-KO mice. Therefore, our data suggest that the effects of CR on the insulin sensitivity of GHR-KO mice do not concur with the effects of CR on the aging of GHR-KO mice.

High-resolution mapping of a genetic locus regulating preferential carbohydrate intake, total kilocalories, and food volume on mouse chromosome 17

The specific genes regulating the quantitative variation in macronutrient preference and food intake are virtually unknown. We fine mapped a previously identified mouse chromosome 17 region harboring quantitative trait loci (QTL) with large effects on preferential macronutrient intake-carbohydrate (Mnic1), total kilcalories (Kcal2), and total food volume (Tfv1) using interval-specific strains. These loci were isolated in the [C57BL/6J.CAST/EiJ-17.1-(D17Mit19-D17Mit50); B6.CAST-17.1] strain, possessing a ∼ 40.1 Mb region of CAST DNA on the B6 genome. In a macronutrient selection paradigm, the B6.CAST-17.1 subcongenic mice eat 30% more calories from the carbohydrate-rich diet, ∼ 10% more total calories, and ∼ 9% more total food volume per body weight. In the current study, a cross between carbohydrate-preferring B6.CAST-17.1 and fat-preferring, inbred B6 mice was used to generate a subcongenic-derived F2 mapping population; genotypes were determined using a high-density, custom SNP panel. Genetic linkage analysis substantially reduced the 95% confidence interval for Mnic1 (encompassing Kcal2 and Tfv1) from 40.1 to 29.5 Mb and more precisely established its boundaries. Notably, no genetic linkage for self-selected fat intake was detected, underscoring the carbohydrate-specific effect of this locus. A second key finding was the separation of two energy balance QTLs: Mnic1/Kcal2/Tfv1 for food intake and a newly discovered locus regulating short term body weight gain. The Mnic1/Kcal2/Tfv1 QTL was further de-limited to 19.0 Mb, based on the absence of nutrient intake phenotypes in subcongenic HQ17IIa mice. Analyses of available sequence data and gene ontologies, along with comprehensive expression profiling in the hypothalamus of non-recombinant, cast/cast and b6/b6 F2 controls, focused our attention on candidates within the QTL interval. Zfp811, Zfp870, and Btnl6 showed differential expression and also contain stop codons, but have no known biology related to food intake regulation. The genes Decr2, Ppard and Agapt1 are more appealing candidates because of their involvement in lipid metabolism and down-regulation in carbohydrate-preferring animals.

Oestrogen signalling in white adipose progenitor cells inhibits differentiation into brown adipose and smooth muscle cells

Oestrogen, often via oestrogen receptor alpha (ERα) signalling, regulates metabolic physiology, highlighted by post-menopausal temperature dysregulation (hot flashes), glucose intolerance, increased appetite and reduced metabolic rate. Here we show that ERα signalling has a role in adipose lineage specification in mice. ERα regulates adipose progenitor identity and potency, promoting white adipogenic lineage commitment. White adipose progenitors lacking ERα reprogramme and enter into smooth muscle and brown adipogenic fates. Mechanistic studies highlight a TGFβ programme involved in progenitor reprogramming downstream of ERα signalling. The observed reprogramming has profound metabolic outcomes; both female and male adipose-lineage ERα-mutant mice are lean, have improved glucose sensitivity and are resistant to weight gain on a high-fat diet. Further, they are hypermetabolic, hyperphagic and hyperthermic, all consistent with a brown phenotype. Together, these findings indicate that ERα cell autonomously regulates adipose lineage commitment, brown fat and smooth muscle cell formation, and systemic metabolism, in a manner relevant to prevalent metabolic diseases.

Role of forkhead box protein A3 in age-associated metabolic decline

Aging is associated with increased adiposity and diminished thermogenesis, but the critical transcription factors influencing these metabolic changes late in life are poorly understood. We recently demonstrated that the winged helix factor forkhead box protein A3 (Foxa3) regulates the expansion of visceral adipose tissue in high-fat diet regimens; however, whether Foxa3 also contributes to the increase in adiposity and the decrease in brown fat activity observed during the normal aging process is currently unknown. Here we report that during aging, levels of Foxa3 are significantly and selectively up-regulated in brown and inguinal white fat depots, and that midage Foxa3-null mice have increased white fat browning and thermogenic capacity, decreased adipose tissue expansion, improved insulin sensitivity, and increased longevity. Foxa3 gain-of-function and loss-of-function studies in inguinal adipose depots demonstrated a cell-autonomous function for Foxa3 in white fat tissue browning. Furthermore, our analysis revealed that the mechanisms of Foxa3 modulation of brown fat gene programs involve the suppression of peroxisome proliferator activated receptor γ coactivtor 1 α (PGC1α) levels through interference with cAMP responsive element binding protein 1-mediated transcriptional regulation of the PGC1α promoter. Overall, our data demonstrate a role for Foxa3 in energy expenditure and in age-associated metabolic disorders.

Xbp1s in Pomc neurons connects ER stress with energy balance and glucose homeostasis

The molecular mechanisms underlying neuronal leptin and insulin resistance in obesity and diabetes remain unclear. Here we show that induction of the unfolded protein response transcription factor spliced X-box binding protein 1 (Xbp1s) in pro-opiomelanocortin (Pomc) neurons alone is sufficient to protect against diet-induced obesity as well as improve leptin and insulin sensitivity, even in the presence of strong activators of ER stress. We also demonstrate that constitutive expression of Xbp1s in Pomc neurons contributes to improved hepatic insulin sensitivity and suppression of endogenous glucose production. Notably, elevated Xbp1s levels in Pomc neurons also resulted in activation of the Xbp1s axis in the liver via a cell-nonautonomous mechanism. Together our results identify critical molecular mechanisms linking ER stress in arcuate Pomc neurons to acute leptin and insulin resistance as well as liver metabolism in diet-induced obesity and diabetes.

The browning of white adipose tissue: some burning issues

Igniting thermogenesis within white adipose tissue (i.e., promoting expression and activity of the uncoupling protein UCP1) has attracted much interest. Numerous "browning agents" have now been described (gene ablations, transgenes, food components, drugs, environments, etc.). The implied action of browning agents is that they increase UCP1 through this heat production, leading to slimming. Here, we particularly point to the possibility that cause and effect may on occasion be the reverse: browning agents may disrupt, for example, the fur, leading to increased heat loss, increased thermogenic demand to counteract this heat loss, and thus, through sympathetic nervous system activation, to enhanced UCP1 expression in white (and brown) adipose tissues.

MC4R-expressing glutamatergic neurons in the paraventricular hypothalamus regulate feeding and are synaptically connected to the parabrachial nucleus

Activation of melanocortin-4 receptors (MC4Rs) restrains feeding and prevents obesity; however, the identity, location, and axonal projections of the neurons bearing MC4Rs that control feeding remain unknown. Reexpression of MC4Rs on single-minded 1 (SIM1)(+) neurons in mice otherwise lacking MC4Rs is sufficient to abolish hyperphagia. Thus, MC4Rs on SIM1(+) neurons, possibly in the paraventricular hypothalamus (PVH) and/or amygdala, regulate food intake. It is unknown, however, whether they are also necessary, a distinction required for excluding redundant sites of action. Hence, the location and nature of obesity-preventing MC4R-expressing neurons are unknown. Here, by deleting and reexpressing MC4Rs from cre-expressing neurons, establishing both necessity and sufficiency, we demonstrate that the MC4R-expressing neurons regulating feeding are SIM1(+), located in the PVH, glutamatergic and not GABAergic, and do not express oxytocin, corticotropin-releasing hormone, vasopressin, or prodynorphin. Importantly, these excitatory MC4R-expressing PVH neurons are synaptically connected to neurons in the parabrachial nucleus, which relays visceral information to the forebrain. This suggests a basis for the feeding-regulating effects of MC4Rs.

MicroRNA-378 controls classical brown fat expansion to counteract obesity

Both classical brown adipocytes and brown-like beige adipocytes are considered as promising therapeutic targets for obesity; however, their development, relative importance, and functional coordination are not well understood. Here we show that a modest expression of miR-378/378* in adipose tissue specifically increases classical brown fat (BAT) mass, but not white fat (WAT) mass. Remarkably, BAT expansion, rather than miR-378 per se, suppresses formation of beige adipocytes in subcutaneous WAT. Despite this negative feedback, the expanded BAT depot is sufficient to prevent both genetic and high fat diet-induced obesity. At the molecular level, we find that miR-378 targets phosphodiesterase Pde1b in BAT, but not in WAT. Indeed, miR-378 and Pde1b inversely regulate brown adipogenesis in vitro in the absence of phosphodiesterase inhibitor IBMX. Our work identifies miR-378 as a key regulatory component underlying classical BAT-specific expansion and obesity resistance, and adds novel insights into the physiological cross-talk between BAT and WAT.

Syndecan-1 is required to maintain intradermal fat and prevent cold stress

Homeostatic temperature regulation is fundamental to mammalian physiology and is controlled by acute and chronic responses of local, endocrine and nervous regulators. Here, we report that loss of the heparan sulfate proteoglycan, syndecan-1, causes a profoundly depleted intradermal fat layer, which provides crucial thermogenic insulation for mammals. Mice without syndecan-1 enter torpor upon fasting and show multiple indicators of cold stress, including activation of the stress checkpoint p38α in brown adipose tissue, liver and lung. The metabolic phenotype in mutant mice, including reduced liver glycogen, is rescued by housing at thermoneutrality, suggesting that reduced insulation in cool temperatures underlies the observed phenotypes. We find that syndecan-1, which functions as a facultative lipoprotein uptake receptor, is required for adipocyte differentiation in vitro. Intradermal fat shows highly dynamic differentiation, continuously expanding and involuting in response to hair cycle and ambient temperature. This physiology probably confers a unique role for Sdc1 in this adipocyte sub-type. The PPARγ agonist rosiglitazone rescues Sdc1−/− intradermal adipose tissue, placing PPARγ downstream of Sdc1 in triggering adipocyte differentiation. Our study indicates that disruption of intradermal adipose tissue development results in cold stress and complex metabolic pathology.

All mammals strive to maintain a fixed body temperature, and do so using a remarkable array of different strategies, which vary depending upon the degree of cold challenge. Physiologists many decades ago observed that a fat layer right underneath the epidermis (and above the dermal muscle layer) thickens in response to colder ambient temperatures. This “intradermal fat” provided insulation within days of climate changes. We have found that syndecan-1, which functions as a facultative lipoprotein uptake receptor, is required for intradermal fat expansion in response to cold exposure. This is a highly specific phenotype not shared by other adipocytes. When intradermal fat is absent, mice do not adapt normally to cold stress, and show altered systemic physiologies, including increased brown adipose tissue thermogenesis and hyper-activation of a stress checkpoint (p38α), designed to protect the body against mutagenic and oxidative stressors. The phenotypes associated with loss of Sdc1 function are reversed when mice are housed in warm temperatures, where defense of body temperature is not required. This study is the first to show that intradermal fat can be genetically regulated, with systemic effects on physiology.

Perinatal exposure of mice to the pesticide DDT impairs energy expenditure and metabolism in adult female offspring

Dichlorodiphenyltrichloroethane (DDT) has been used extensively to control malaria, typhus, body lice and bubonic plague worldwide, until countries began restricting its use in the 1970s. Its use in malaria control continues in some countries according to recommendation by the World Health Organization. Individuals exposed to elevated levels of DDT and its metabolite dichlorodiphenyldichloroethylene (DDE) have an increased prevalence of diabetes and insulin resistance. Here we hypothesize that perinatal exposure to DDT disrupts metabolic programming leading to impaired metabolism in adult offspring. To test this, we administered DDT to C57BL/6J mice from gestational day 11.5 to postnatal day 5 and studied their metabolic phenotype at several ages up to nine months. Perinatal DDT exposure reduced core body temperature, impaired cold tolerance, decreased energy expenditure, and produced a transient early-life increase in body fat in female offspring. When challenged with a high fat diet for 12 weeks in adulthood, female offspring perinatally exposed to DDT developed glucose intolerance, hyperinsulinemia, dyslipidemia, and altered bile acid metabolism. Perinatal DDT exposure combined with high fat feeding in adulthood further impaired thermogenesis as evidenced by reductions in core temperature and in the expression of numerous RNA that promote thermogenesis and substrate utilization in the brown adipose tissue of adult female mice. These observations suggest that perinatal DDT exposure impairs thermogenesis and the metabolism of carbohydrates and lipids which may increase susceptibility to the metabolic syndrome in adult female offspring.

Approach to assessing determinants of glucose homeostasis in the conscious mouse

Obesity and type 2 diabetes lessen the quality of life of those afflicted and place considerable burden on the healthcare system. Furthermore, the detrimental impact of these pathologies is expected to persist or even worsen. Diabetes is characterized by impaired insulin action and glucose homeostasis. This has led to a rapid increase in the number of mouse models of metabolic disease being used in the basic sciences to assist in facilitating a greater understanding of the metabolic dysregulation associated with obesity and diabetes, the identification of therapeutic targets, and the discovery of effective treatments. This review briefly describes the most frequently utilized models of metabolic disease. A presentation of standard methods and technologies on the horizon for assessing metabolic phenotypes in mice, with particular emphasis on glucose handling and energy balance, is provided. The article also addresses issues related to study design, selection and execution of metabolic tests of glucose metabolism, the presentation of data, and interpretation of results.

Mathematical model for the contribution of individual organs to non-zero y-intercepts in single and multi-compartment linear models of whole-body energy expenditure

Mathematical models for the dependence of energy expenditure (EE) on body mass and composition are essential tools in metabolic phenotyping. EE scales over broad ranges of body mass as a non-linear allometric function. When considered within restricted ranges of body mass, however, allometric EE curves exhibit ‘local linearity.’ Indeed, modern EE analysis makes extensive use of linear models. Such models typically involve one or two body mass compartments (e.g., fat free mass and fat mass). Importantly, linear EE models typically involve a non-zero (usually positive) y-intercept term of uncertain origin, a recurring theme in discussions of EE analysis and a source of confounding in traditional ratio-based EE normalization. Emerging linear model approaches quantify whole-body resting EE (REE) in terms of individual organ masses (e.g., liver, kidneys, heart, brain). Proponents of individual organ REE modeling hypothesize that multi-organ linear models may eliminate non-zero y-intercepts. This could have advantages in adjusting REE for body mass and composition. Studies reveal that individual organ REE is an allometric function of total body mass. I exploit first-order Taylor linearization of individual organ REEs to model the manner in which individual organs contribute to whole-body REE and to the non-zero y-intercept in linear REE models. The model predicts that REE analysis at the individual organ-tissue level will not eliminate intercept terms. I demonstrate that the parameters of a linear EE equation can be transformed into the parameters of the underlying ‘latent’ allometric equation. This permits estimates of the allometric scaling of EE in a diverse variety of physiological states that are not represented in the allometric EE literature but are well represented by published linear EE analyses.

Oxygen restriction as challenge test reveals early high-fat-diet-induced changes in glucose and lipid metabolism

Challenge tests stress homeostasis and may reveal deviations in health that remain masked under unchallenged conditions. Ideally, challenge tests are non-invasive and applicable in an early phase of an animal experiment. Oxygen restriction (OxR; based on ambient, mild normobaric hypoxia) is a non-invasive challenge test that measures the flexibility to adapt metabolism. Metabolic inflexibility is one of the hallmarks of the metabolic syndrome. To test whether OxR can be used to reveal early diet-induced health effects, we exposed mice to a low-fat (LF) or high-fat (HF) diet for only 5 days. The response to OxR was assessed by calorimetric measurements, followed by analysis of gene expression in liver and epididymal white adipose tissue (eWAT) and serum markers for e.g. protein glycation and oxidation. Although HF feeding increased body weight, HF and LF mice did not differ in indirect calorimetric values under normoxic conditions and in a fasting state. Exposure to OxR; however, increased oxygen consumption and lipid oxidation in HF mice versus LF mice. Furthermore, OxR induced gluconeogenesis and an antioxidant response in the liver of HF mice, whereas it induced de novo lipogenesis and an antioxidant response in eWAT of LF mice, indicating that HF and LF mice differed in their adaptation to OxR. OxR also increased serum markers of protein glycation and oxidation in HF mice, whereas these changes were absent in LF mice. Cumulatively, OxR is a promising new method to test food products on potential beneficial effects for human health.

Central (mainly) actions of GPCRs in energy homeostasis/balance: view from the Chair

To maintain a constant body weight, energy intake must equal energy expenditure; otherwise, there is a risk of overweight and obesity. The hypothalamus is one of the primary brain regions where multiple nutrient-related signals from peripheral and central sources converge and become integrated to regulate both short- and long-term nutritional states. The aim of the afternoon session of the 15th Annual International Symposium of the Laval University Obesity Research Chair held in Quebec City on 9 November 2012 was to present the most recent insights into the complex molecular mechanisms regulating food intake. The aims were to emphasize on the interaction between central and peripheral actions of some of the key players acting not only at the hypothalamic level but also at the periphery. Presentations were focused on melanocortin-3 receptor (MC3R) and melanin-concentrating hormone (MCH) as anorexigenic and orexigenic components of the hypothalamus, on endocannabinoid receptors, initially as a central neuromodulatory signal, and on glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) as peripheral signals. What becomes clear from these four presentations is that the regulation of food intake and energy homeostasis involves several overlapping pathways, and that we have only touched the tip of the iceberg. From the examples presented in this symposium, it could be expected that in the near future, in addition to a low-fat diet and exercise, a combination of appropriate peptides and small molecules is likely to become available to improve/facilitate the objectives of long-term maintenance of energy balance and body weight.

GLP-1(28-36)amide, the Glucagon-like peptide-1 metabolite: friend, foe, or pharmacological folly?

The glucagon-like peptide-1 (GLP-1) axis has emerged as a major therapeutic target for the treatment of type 2 diabetes. GLP-1 mediates its key insulinotropic effects via a G-protein coupled receptor expressed on β-cells and other pancreatic cell types. The insulinotropic activity of GLP-1 is terminated via enzymatic cleavage by dipeptidyl peptidase-4. Until recently, GLP-1-derived metabolites were generally considered metabolically inactive; however, accumulating evidence indicates some have biological activity that may contribute to the pleiotropic effects of GLP-1 independent of the GLP-1 receptor. Recent reports describing the putative effects of one such metabolite, the GLP-1-derived nonapeptide GLP-1(28-36) amide, are the focus of this review. Administration of the nonapeptide elevates cyclic adenosine monophosphate (cAMP) and activates protein kinase A, β-catenin, and cAMP response-element binding protein in pancreatic β-cells and hepatocytes. In stressed cells, the nonapeptide targets the mitochondria and, via poorly defined mechanisms, helps to maintain mitochondrial membrane potential and cellular adenosine triphosphate levels and to reduce cytotoxicity and apoptosis. In mouse models of diet-induced obesity, treatment with the nonapeptide reduces weight gain and ameliorates associated pathophysiology, including hyperglycemia, hyperinsulinemia, and hepatic steatosis. Nonapeptide administration in a streptozotocin-induced model of type 1 diabetes also improves glucose disposal concomitant with elevated insulin levels and increased β-cell mass and proliferation. Collectively, these results suggest some of the beneficial effects of GLP-1 receptor analogs may be mediated by the nonapeptide. However, the concentrations required to elicit some of these effects are in the micromolar range, leading to reservations about potentially related therapeutic benefits. Moreover, although controversial, concerns have been raised about the potential for incretin-based therapies to promote pancreatitis and pancreatic and thyroid cancers. The effects ascribed to the nonapeptide make it a potential contributor to such outcomes, raising additional questions about its therapeutic suitability. Notwithstanding, the nonapeptide, like other GLP-1 metabolites, appears to be biologically active. Increasing understanding of such noncanonical GLP-1 activities should help to improve future incretin-based therapeutics.

Central neural regulation of brown adipose tissue thermogenesis and energy expenditure

Thermogenesis, the production of heat energy, is the specific, neurally regulated, metabolic function of brown adipose tissue (BAT) and contributes to the maintenance of body temperature during cold exposure and to the elevated core temperature during several behavioral states, including wakefulness, the acute phase response (fever), and stress. BAT energy expenditure requires metabolic fuel availability and contributes to energy balance. This review summarizes the functional organization and neurochemical influences within the CNS networks governing the level of BAT sympathetic nerve activity to produce the thermoregulatory and metabolically driven alterations in BAT thermogenesis and energy expenditure that contribute to overall energy homeostasis.

Oxytocin reverses ovariectomy-induced osteopenia and body fat gain

Osteoporosis and overweight/obesity constitute major worldwide public health burdens that are associated with aging. A high proportion of women develop osteoporosis and increased intraabdominal adiposity after menopause. which leads to bone fractures and metabolic disorders. There is no efficient treatment without major side effects for these 2 diseases. We previously showed that the administration of oxytocin (OT) normalizes ovariectomy-induced osteopenia and bone marrow adiposity in mice. Ovariectomized mice, used as an animal model mimicking menopause, were treated with OT or vehicle. Trabecular bone parameters and fat mass were analyzed using micro-computed tomography. Herein, we show that this effect on trabecular bone parameters was mediated through the restoration of osteoblast/osteoclast cross talk via the receptor activator of nuclear factor-κB ligand /osteoprotegerin axis. Moreover, the daily administration of OT normalized body weight and intraabdominal fat depots in ovariectomized mice. Intraabdominal fat mass is more sensitive to OT that sc fat depots, and this inhibitory effect is mediated through inhibition of adipocyte precursor's differentiation with a tendency to lower adipocyte size. OT treatment did not affect food intake, locomotors activity, or energy expenditure, but it did promote a shift in fuel utilization favoring lipid oxidation. In addition, the decrease in fat mass resulted from the inhibition of the adipose precursor's differentiation. Thus, OT constitutes an effective strategy for targeting osteopenia, overweight, and fat mass redistribution without any detrimental effects in a mouse model mimicking the menopause.

Leanness and heightened nonresting energy expenditure: role of skeletal muscle activity thermogenesis

A high-calorie diet accompanied by low levels of physical activity (PA) accounts for the widespread prevalence of obesity today, and yet some people remain lean even in this obesogenic environment. Here, we investigate the cause for this exception. A key trait that predicts high PA in both humans and laboratory rodents is intrinsic aerobic capacity. Rats artificially selected as high-capacity runners (HCR) are lean and consistently more physically active than their low-capacity runner (LCR) counterparts; this applies to both males and females. Here, we demonstrate that HCR show heightened total energy expenditure (TEE) and hypothesize that this is due to higher nonresting energy expenditure (NREE; includes activity EE). After matching for body weight and lean mass, female HCR consistently had heightened nonresting EE, but not resting EE, compared with female LCR. Because of the dominant role of skeletal muscle in nonresting EE, we examined muscle energy use. We found that lean female HCR had higher muscle heat dissipation during activity, explaining their low economy of activity and high activity EE. This may be due to the amplified skeletal muscle expression levels of proteins involved in EE and reduced expression levels of proteins involved in energy conservation in HCR relative to LCR. This is also associated with an increased sympathetic drive to skeletal muscle in HCR compared with LCR. We find little support for the hypothesis that resting metabolic rate is correlated with maximal aerobic capacity if body size and composition are fully considered; rather, the critical factor appears to be activity thermogenesis.

Fatty acid metabolism, energy expenditure and insulin resistance in muscle

Fatty acids (FAs) are essential elements of all cells and have significant roles as energy substrates, components of cellular structure and signalling molecules. The storage of excess energy intake as fat in adipose tissue is an evolutionary advantage aimed at protecting against starvation, but in much of today's world, humans are faced with an unlimited availability of food, and the excessive accumulation of fat is now a major risk for human health, especially the development of type 2 diabetes (T2D). Since the first recognition of the association between fat accumulation, reduced insulin action and increased risk of T2D, several mechanisms have been proposed to link excess FA availability to reduced insulin action, with some of them being competing or contradictory. This review summarises the evidence for these mechanisms in the context of excess dietary FAs generating insulin resistance in muscle, the major tissue involved in insulin-stimulated disposal of blood glucose. It also outlines potential problems with models and measurements that may hinder as well as help improve our understanding of the links between FAs and insulin action.

Ablation of PRDM16 and beige adipose causes metabolic dysfunction and a subcutaneous to visceral fat switch

A clear relationship exists between visceral obesity and type 2 diabetes, whereas subcutaneous obesity is comparatively benign. Here, we show that adipocyte-specific deletion of the coregulatory protein PRDM16 caused minimal effects on classical brown fat but markedly inhibited beige adipocyte function in subcutaneous fat following cold exposure or β3-agonist treatment. These animals developed obesity on a high-fat diet, with severe insulin resistance and hepatic steatosis. They also showed altered fat distribution with markedly increased subcutaneous adiposity. Subcutaneous adipose tissue in mutant mice acquired many key properties of visceral fat, including decreased thermogenic and increased inflammatory gene expression and increased macrophage accumulation. Transplantation of subcutaneous fat into mice with diet-induced obesity showed a loss of metabolic benefit when tissues were derived from PRDM16 mutant animals. These findings indicate that PRDM16 and beige adipocytes are required for the "browning" of white fat and the healthful effects of subcutaneous adipose tissue.

The impact of dietary methionine restriction on biomarkers of metabolic health

Calorie restriction without malnutrition, commonly referred to as dietary restriction (DR), results in a well-documented extension of life span. DR also produces significant, long-lasting improvements in biomarkers of metabolic health that begin to accrue soon after its introduction. The improvements are attributable in part to the effects of DR on energy balance, which limit fat accumulation through reduction in energy intake. Accumulation of excess body fat occurs when energy intake chronically exceeds the energy costs for growth and maintenance of existing tissue. The resulting obesity promotes the development of insulin resistance, disordered lipid metabolism, and increased expression of inflammatory markers in peripheral tissues. The link between the life-extending effects of DR and adiposity is the subject of an ongoing debate, but it is clear that decreased fat accumulation improves insulin sensitivity and produces beneficial effects on overall metabolic health. Over the last 20 years, dietary methionine restriction (MR) has emerged as a promising DR mimetic because it produces a comparable extension in life span, but surprisingly, does not require food restriction. Dietary MR also reduces adiposity but does so through a paradoxical increase in both energy intake and expenditure. The increase in energy expenditure fully compensates for increased energy intake and effectively limits fat deposition. Perhaps more importantly, the diet increases metabolic flexibility and overall insulin sensitivity and improves lipid metabolism while decreasing systemic inflammation. In this chapter, we describe recent advances in our understanding of the mechanisms and effects of dietary MR and discuss the remaining obstacles to implementing MR as a treatment for metabolic disease.

Enhancement of brown fat thermogenesis using chenodeoxycholic acid in mice

OBJECTIVE

Besides their role in lipid absorption, bile acids (BAs) can act as signalling molecules. Cholic acid was shown to counteract obesity and associated metabolic disorders in high-fat-diet (cHF)-fed mice while enhancing energy expenditure through induction of mitochondrial uncoupling protein 1 (UCP1) and activation of non-shivering thermogenesis in brown adipose tissue (BAT). In this study, the effects of another natural BA, chenodeoxycholic acid (CDCA), on dietary obesity, UCP1 in both interscapular BAT and in white adipose tissue (brite cells in WAT), were characterized in dietary-obese mice.

RESEARCH DESIGN

To induce obesity and associated metabolic disorders, male 2-month-old C57BL/6J mice were fed cHF (35% lipid wt wt(-1), mainly corn oil) for 4 months. Mice were then fed either (i) for 8 weeks with cHF or with cHF with two different doses (0.5%, 1%; wt wt(-1)) of CDCA (8-week reversion); or (ii) for 3 weeks with cHF or with cHF with 1% CDCA, or pair-fed (PF) to match calorie intake of the CDCA mice fed ad libitum; mice on standard chow diet were also used (3-week reversion).

RESULTS

In the 8-week reversion, the CDCA intervention resulted in a dose-dependent reduction of obesity, dyslipidaemia and glucose intolerance, which could be largely explained by a transient decrease in food intake. The 3-week reversion revealed mild CDCA-dependent and food intake-independent induction of UCP1-mediated thermogenesis in interscapular BAT, negligible increase of UCP1 in subcutaneous WAT and a shift from carbohydrate to lipid oxidation.

CONCLUSIONS

CDCA could reverse obesity in cHF-fed mice, mainly in response to the reduction in food intake, an effect probably occuring but neglected in previous studies using cholic acid. Nevertheless, CDCA-dependent and food intake-independent induction of UCP1 in BAT (but not in WAT) could contribute to the reduction in adiposity and to the stabilization of the lean phenotype.

Glutamate mediates the function of melanocortin receptor 4 on Sim1 neurons in body weight regulation

The melanocortin receptor 4 (MC4R) is a well-established mediator of body weight homeostasis. However, the neurotransmitter(s) that mediate MC4R function remain largely unknown; as a result, little is known about the second-order neurons of the MC4R neural pathway. Single-minded 1 (Sim1)-expressing brain regions, which include the paraventricular nucleus of hypothalamus (PVH), represent key brain sites that mediate melanocortin action. We conditionally restored MC4R expression in Sim1 neurons in the background of Mc4r-null mice. The restoration dramatically reduced obesity in Mc4r-null mice. The anti-obesity effect was completely reversed by selective disruption of glutamate release from those same Sim1 neurons. The reversal was caused by lower energy expenditure and hyperphagia. Corroboratively, selective disruption of glutamate release from adult PVH neurons led to rapid obesity development via reduced energy expenditure and hyperphagia. Thus, this study establishes glutamate as the primary neurotransmitter that mediates MC4Rs on Sim1 neurons in body weight regulation.

Longitudinal assessment of food intake, fecal energy loss, and energy expenditure after Roux-en-Y gastric bypass surgery in high-fat-fed obese rats

BACKGROUND

The efficacy of Roux-en-Y gastric bypass (RYGB) surgery to produce weight loss has been well-documented, but few studies have measured the key components of energy balance, food intake, and energy expenditure longitudinally.

METHODS

Male Sprague-Dawley rats on a high-fat diet underwent either RYGB, sham operation, or pair feeding and were compared to chow-fed lean controls. Body weight and composition, food intake and preference, energy expenditure, fecal output, and gastric emptying were monitored before and up to 4 months after intervention.

RESULTS

Despite the recovery of initially decreased food intake to levels slightly higher than before surgery and comparable to sham-operated rats after about 1 month, RYGB rats maintained a lower level of body weight and fat mass for 4 months that was not different from chow-fed age-matched controls. Energy expenditure corrected for lean body mass at 1 and 4 months after RYGB was not different from presurgical levels and from all other groups. Fecal energy loss was significantly increased at 6 and 16 weeks after RYGB compared to sham operation, and there was a progressive decrease in fat preference after RYGB.

CONCLUSIONS

In this rat model of RYGB, sustained weight loss is achieved by a combination of initial hypophagia and sustained increases in fecal energy loss, without change in energy expenditure per lean mass. A shift away from high-fat towards low-fat/high-carbohydrate food preference occurring in parallel suggests long-term adaptive mechanisms related to fat absorption.

Mechanisms underlying obesity resistance associated with high spontaneous physical activity

Obesity resistance due to elevated orexin signaling is accompanied by high levels of spontaneous physical activity (SPA). The behavioral and neural mechanisms underlying this observation have not been fully worked out. We determined the contribution of hypothalamic orexin receptors (OXRs) to SPA stimulated by orexin A (OXA), whether OXA-stimulated SPA was secondary to arousal and whether voluntary wheel running led to compensations in 24-h SPA. We further tested whether orexin action on dopamine one receptors (DA1R) in the substantia nigra (SN) plays an important role in the generation of SPA. To test this, SPA response was determined in lean and obese rats with cannulae targeted toward the rostral lateral hypothalamus (rLH) or SN. Sleep/wake states were also measured in rats with rLH cannula and electroencephalogram/electromyogram radiotelemetry transmitters. SPA in lean rats was more sensitive to antagonism of the OX1R and in the early response to the orexin 2 agonist. OXA increased arousal equally in lean and obese rodents, which is discordant from the greater SPA response in lean rats. Obesity-resistant rats ran more and wheel running was directly related to 24-h SPA levels. The OX1R antagonist, SB-334867-A, and the DA1R antagonist, SCH3390, in SN more effectively reduced SPA stimulated by OXA in obesity-resistant rats. These data suggest OXA-stimulated SPA is not secondary to enhanced arousal, propensity for SPA parallels inclination to run and that orexin action on dopaminergic neurons in SN may participate in the mediation of SPA and running wheel activity.

Role of the locus coeruleus in enhanced orexin A-induced spontaneous physical activity in obesity-resistant rats

Orexin/hypocretin terminals innervate noradrenergic locus coeruleus (LC) neurons that project to the prefrontral cortex, which may influence spontaneous physical activity (SPA) and energy balance. Obesity-resistant (OR) rats have higher orexin receptors (OXR) mRNA in the LC and other brain regions, as well as lower adiposity compared with obese rats. These findings led us to hypothesize that orexin activity in the LC is relevant for the OR phenotype. We compared OR rats to Sprague-Dawley rats. We predicted that: 1) brain OXR expression pattern is sufficient to differentiate OR from non-bred Sprague-Dawley rats; 2) nonresting energy expenditure (NREE) and orexin A (OXA)-stimulated SPA after injection in LC would be greater in OR rats; and 3) the effect of OXA on SPA would be greater than its effect on feeding. OXR mRNA from 11 brain sites and the SPA and feeding responses to OXA in the LC were determined. Body composition, basal SPA, and EE were determined. Principal component analysis of the OXR expression pattern differentiates OR and Sprague-Dawley rats and suggests the OXR mRNA in the LC is important in defining the OR phenotype. Compared with Sprague-Dawley rats, OR rats had greater SPA and NREE and lower resting EE and adiposity. SPA responsivity to OXA in the LC was greater in OR rats compared with Sprague-Dawley rats. OXA in the LC did not stimulate feeding in OR or Sprague-Dawley rats. These data suggest that the LC is a prominent site modulating OXA-stimulated SPA, which promotes lower adiposity and higher nonresting EE.

The '39 steps': an algorithm for performing statistical analysis of data on energy intake and expenditure

The epidemics of obesity and diabetes have aroused great interest in the analysis of energy balance, with the use of organisms ranging from nematode worms to humans. Although generating energy-intake or -expenditure data is relatively straightforward, the most appropriate way to analyse the data has been an issue of contention for many decades. In the last few years, a consensus has been reached regarding the best methods for analysing such data. To facilitate using these best-practice methods, we present here an algorithm that provides a step-by-step guide for analysing energy-intake or -expenditure data. The algorithm can be used to analyse data from either humans or experimental animals, such as small mammals or invertebrates. It can be used in combination with any commercial statistics package; however, to assist with analysis, we have included detailed instructions for performing each step for three popular statistics packages (SPSS, MINITAB and R). We also provide interpretations of the results obtained at each step. We hope that this algorithm will assist in the statistically appropriate analysis of such data, a field in which there has been much confusion and some controversy.

Mechanistic comparison between gastric bypass vs. duodenal switch with sleeve gastrectomy in rat models

Background

Both gastric bypass (GB) and duodenal switch with sleeve gastrectomy (DS) have been widely used as bariatric surgeries, and DS appears to be superior to GB. The aim of this study was to better understand the mechanisms leading to body weight loss by comparing these two procedures in experimental models of rats.

Methods

Animals were subjected to GB, DS or laparotomy (controls), and monitored by an open-circuit indirect calorimeter composed of comprehensive laboratory animal monitoring system and adiabatic bomb calorimeter.

Results

Body weight loss was greater after DS than GB. Food intake was reduced after DS but not GB. Energy expenditure was increased after either GB or DS. Fecal energy content was increased after DS but not GB.

Conclusion

GB induced body weight loss by increasing energy expenditure, whereas DS induced greater body weight loss by reducing food intake, increasing energy expenditure and causing malabsorption in rat models.

Hypothalamic malonyl-CoA and the control of food intake

Fatty acid metabolism is implicated in the hypothalamic control of food intake. In this regard, malonyl-CoA, an intermediate in fatty acid synthesis, is emerging as a key player. Malonyl-CoA in the hypothalamus has been proposed as an anorectic mediator in the central control of feeding. A large body of evidence demonstrates that modulating hypothalamic activities of malonyl-CoA metabolic enzymes impacts food intake. Malonyl-CoA action appears to play a significant role in the intracellular signaling pathways underlying leptin anorectic effect in the arcuate nucleus. Ghrelin's hypothalamic effect on feeding may also involve the change in malonyl-CoA metabolism. Hypothalamic malonyl-CoA levels are altered in response to fasting and refeeding, suggesting physiological relevance of the changes in malonyl-CoA level in the controls of feeding and energy balance. Malonyl-CoA inhibits the acyltransferase activity of carnitine palmitoyltransferase-1 (CPT-1), and CPT-1 was considered as a downstream effector in hypothalamic malonyl-CoA effect on feeding. However, recent evidence has not been entirely consistent with this notion. In the arcuate nucleus, the inhibition of CPT-1 acyltransferase activity does not play an important role in the feeding effect of either leptin or cerulenin (a fatty acid synthase inhibitor) that requires the increase in malonyl-CoA level. Alternatively, the brain isoform of CPT-1 (CPT-1c) may act as a downstream target in the malonyl-CoA signaling pathways. CPT-1c does not possess a typical acyltransferase activity, and the exact molecular function of this protein is currently unknown. Recent data indicate it is involved in ceramide metabolism. Of relevance, in the arcuate nucleus, CPT-1c may link malonyl-CoA to ceramide metabolism to affect food intake.

Extended treatment with selective phosphatidylinositol 3-kinase and mTOR inhibitors has effects on metabolism, growth, behaviour and bone strength

The class I phosphatidylinositol 3-kinases (PtdIns3Ks) mediate the effects of many hormones and growth factors on a wide range of cellular processes, and activating mutations or gene amplifications of class I PtdIns3K isoforms are known to contribute to oncogenic processes in a range of tumours. Consequently, a number of small-molecule PtdIns3K inhibitors are under development and in clinical trial. The central signalling role of PtdIns3K in many cellular processes suggests there will be on-target side effects associated with the use of these agents. To gain insights into what these might be we investigated the effect of extended daily dosing of eight small-molecule inhibitors of class Ia PtdIns3Ks. Animals were characterized in metabolic cages to analyse food intake, oxygen consumption and movement. Insulin tolerance and body composition were analysed at the end of the experiment, the latter using EchoMRI. Bone volume and strength was assessed by micro-CT and three-point bending, respectively. Surprisingly, after sustained dosing with pan-PtdIns3K inhibitors and selective inhibitors of the p110α isoform there was a resolution of the impairments in insulin tolerance observed in drug-naïve animals treated with the same drugs. However, pan-PtdIns3K inhibitors and selective inhibitors of the p110α have deleterious effects on animal growth, animal behaviour and bone volume and strength. Together, these findings identify a range of on target effects of PtdIns3K inhibitors and suggest use of these drugs in humans may have important adverse effects on metabolism, body composition, behaviour and skeletal health.

Important role of ventromedial hypothalamic carnitine palmitoyltransferase-1a in the control of food intake

Carnitine palmitoyltransferase-1 (CPT-1) liver isoform, or CPT-1a, is implicated in CNS control of food intake. However, the exact brain nucleus site(s) in mediating this action of CPT-1a has not been identified. In this report, we assess the role of CPT-1a in hypothalamic ventromedial nucleus (VMN). We stereotaxically injected an adenoviral vector containing CPT-1a coding sequence into the VMN of rats to induce overexpression and activation of CPT-1a. The VMN-selective activation of CPT-1a induced an orexigenic effect, suggesting CPT-1a in the VMN is involved in the central control of feeding. Intracerebroventricular administration of etomoxir, a CPT-1 inhibitor, decreases food intake. Importantly, in the animals with VMN overexpression of a CPT-1a mutant that antagonizes the CPT-1 inhibition by etomoxir, the anorectic response to etomoxir was attenuated. This suggests that VMN is involved in mediating the anorectic effect of central inhibition of CPT-1a. In contrast, arcuate nucleus (Arc) overexpression of the mutant did not alter etomoxir-induced inhibition of food intake, suggesting that Arc CPT-1a does not play significant roles in this anorectic action. Furthermore, in the VMN, CPT-1a appears to act downstream of hypothalamic malonyl-CoA action of feeding. Finally, we show that in the VMN CPT-1 activity was altered in concert with fasting and refeeding states, supporting a physiological role of CPT-1a in mediating the control of feeding. All together, CPT-1a in the hypothalamic VMN appears to play an important role in central control of food intake. VMN-selective modulation of CPT-1a activity may therefore be a promising strategy in controlling food intake and maintaining normal body weight.

CTRP9 transgenic mice are protected from diet-induced obesity and metabolic dysfunction

CTRP9 is a secreted multimeric protein of the C1q family and the closest paralog of the insulin-sensitizing adipokine, adiponectin. The metabolic function of this adipose tissue-derived plasma protein remains largely unknown. Here, we show that the circulating levels of CTRP9 are downregulated in diet-induced obese mice and upregulated upon refeeding. Overexpressing CTRP9 resulted in lean mice that dramatically resisted weight gain induced by a high-fat diet, largely through decreased food intake and increased basal metabolism. Enhanced fat oxidation in CTRP9 transgenic mice resulted from increases in skeletal muscle mitochondrial content, expression of enzymes involved in fatty acid oxidation (LCAD and MCAD), and chronic AMPK activation. Hepatic and skeletal muscle triglyceride levels were substantially decreased in transgenic mice. Consequently, CTRP9 transgenic mice had a greatly improved metabolic profile with markedly reduced fasting insulin and glucose levels. The high-fat diet-induced obesity, insulin resistance, and hepatic steatosis observed in wild-type mice were prevented in transgenic mice. Consistent with the in vivo data, recombinant protein significantly enhanced fat oxidation in L6 myotubes via AMPK activation and reduced lipid accumulation in H4IIE hepatocytes. Collectively, these data establish CTRP9 as a novel metabolic regulator and a new component of the metabolic network that links adipose tissue to lipid metabolism in skeletal muscle and liver.

Genetic variation in brown fat activity and body weight regulation in mice: lessons for human studies

The recent characterization of brown fat in humans has generated much excitement on the possibility that increased energy expenditure by heat production by this tissue will be able to reduce obesity. This expectation has largely been stimulated by studies with mice that show strong associations between increased brown fat activity and reductions in obesity and insulin resistance. Research in the mouse has been largely based upon the induction or suppression of brown fat and mitochondrial uncoupling protein by genetic methods. The review of this research literature underscores the idea that reductions in obesity in mice are secondary to the primary role of brown adipose tissue in the regulation of body temperature. Given that the variation in brown fat in humans, as detected by PET imaging, is highly associated with administration of adrenergic agonists and reductions in ambient temperature, the effects on obesity in humans may also be secondary to the regulation of body temperature. Induction of thermogenesis by reduced ambient temperature now becomes like muscle and physical activity, another natural method of increased energy expenditure to combat obesity. Furthermore, there is no evidence to indicate that heat production by adrenergic stimulation via cold exposure or drug treatment or the enriched physical environment is restricted to the thermogenic activity of the brown adipocyte. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Detection of thermogenesis in rodents in response to anti-obesity drugs and genetic modification

Many compounds and genetic manipulations are claimed to confer resistance to obesity in rodents by raising energy expenditure. Examples taken from recent and older literature, demonstrate that such claims are often based on measurements of energy expenditure after body composition has changed, and depend on comparisons of energy expenditure divided by body weight. This is misleading because white adipose tissue has less influence than lean tissue on energy expenditure. Application of this approach to human data would suggest that human obesity is usually due to a low metabolic rate, which is not an accepted view. Increased energy expenditure per animal is a surer way of demonstrating thermogenesis, but even then it is important to know whether this is due to altered body composition (repartitioning), or increased locomotor activity rather than thermogenesis per se. Regression analysis offers other approaches. The thermogenic response to some compounds has a rapid onset and so cannot be due to altered body composition. These compounds usually mimic or activate the sympathetic nervous system. Thermogenesis occurs in, but may not be confined to, brown adipose tissue. It should not be assumed that weight loss in response to these treatments is due to thermogenesis unless there is a sustained increase in 24-h energy expenditure. Thyroid hormones and fibroblast growth factor 21 also raise energy expenditure before they affect body composition. Some treatments and genetic modifications alter the diurnal rhythm of energy expenditure. It is important to establish whether this is due to altered locomotor activity or efficiency of locomotion. There are no good examples of compounds that do not affect short-term energy expenditure but have a delayed effect. How and under what conditions a genetic modification or compound increases energy expenditure influences the decision on whether to seek drugs for the target or take a candidate drug into clinical studies.

Deficiency of the RIIβ subunit of PKA affects locomotor activity and energy homeostasis in distinct neuronal populations

Targeted disruption of RIIβ-protein kinase A (PKA) in mice leads to a lean phenotype, increased nocturnal locomotor activity, and activation of brown adipose tissue. Because RIIβ is abundantly expressed in both white and brown adipose tissue as well as the brain, the contribution of neuronal vs. peripheral PKA to these phenotypes was investigated. We used a Cre-Lox strategy to reexpress RIIβ in a tissue-specific manner in either adipocytes or neurons. Mice with adipocyte-specific RIIβ reexpression remained hyperactive and lean, but pan-neuronal RIIβ reexpression reversed both phenotypes. Selective RIIβ reexpression in all striatal medium spiny neurons with Darpp32-Cre corrected the hyperlocomotor phenotype, but the mice remained lean. Further analysis revealed that RIIβ reexpression in D2 dopamine receptor-expressing medium spiny neurons corrected the hyperlocomotor phenotype, which demonstrated that the lean phenotype in RIIβ-PKA-deficient mice does not develop because of increased locomotor activity. To identify the neurons responsible for the lean phenotype, we used specific Cre-driver mice to reexpress RIIβ in agouti-related peptide (AgRP)-, proopiomelanocortin (POMC)-, single-minded 1 (Sim1)-, or steroidogenic factor 1 (SF1)-expressing neurons in the hypothalamus, but observed no rescue of the lean phenotype. However, when RIIβ was reexpressed in multiple regions of the hypothalamus and striatum driven by Rip2-Cre, or specifically in GABAergic neurons driven by Vgat-ires-Cre, both the hyperactive and lean phenotypes were completely corrected. Bilateral injection of adeno-associated virus1 (AAV1)-Cre directly into the hypothalamus caused reexpression of RIIβ and partially reversed the lean phenotype. These data demonstrate that RIIβ-PKA deficiency in a subset of hypothalamic GABAergic neurons leads to the lean phenotype.

Intestine-specific expression of MOGAT2 partially restores metabolic efficiency in Mogat2-deficient mice

Acyl CoA:monoacylglycerol acyltransferase (MGAT) catalyzes the resynthesis of triacylglycerol, a crucial step in the absorption of dietary fat. Mice lacking the gene Mogat2, which codes for an MGAT highly expressed in the small intestine, are resistant to obesity and other metabolic disorders induced by high-fat feeding. Interestingly, these Mogat2⁻/⁻ mice absorb normal amounts of dietary fat but exhibit a reduced rate of fat absorption, increased energy expenditure, decreased respiratory exchange ratio, and impaired metabolic efficiency. MGAT2 is expressed in tissues besides intestine. To test the hypothesis that intestinal MGAT2 enhances metabolic efficiency and promotes the storage of metabolic fuels, we introduced the human MOGAT2 gene driven by the intestine-specific villin promoter into Mogat2⁻/⁻ mice. We found that the expression of MOGAT2 in the intestine increased intestinal MGAT activity, restored fat absorption rate, partially corrected energy expenditure, and promoted weight gain upon high-fat feeding. However, the changes in respiratory exchange ratio were not reverted, and the recoveries in metabolic efficiency and weight gain were incomplete. These data indicate that MGAT2 in the intestine plays an indispensable role in enhancing metabolic efficiency but also raise the possibility that MGAT2 in other tissues may contribute to the regulation of energy metabolism.

Addition of a low dose of rimonabant to orlistat therapy decreases weight gain and reduces adiposity in dietary obese rats

1. The aim of the present study was to determine whether the addition of a subeffective dose of rimonabant (1 mg/kg) to orlistat would be beneficial in the treatment of diet-induced obesity in rats compared with orlistat monotherapy. 2. Male rats were divided into five groups: (i) rats fed a low-fat diet for 4 months; (ii) rats fed a high-fat diet (HFD) for 4 months and treated daily with vehicle (0.2% Tween-80 solution); (iii) orlistat (10 mg/kg per day)-treated HFD-fed rats; (iv) rimonabant (1 mg/kg per day)-treated HFD-fed rats; and (v) HFD-fed rats treated with a combination of orlistat plus rimonabant. Fasting blood glucose, serum insulin, leptin and adiponectin levels were measured. Liver and adiposity indices were calculated and liver and adipose tissues were processed for histological examination. 3. Over the 4 months of the study, vehicle-treated HFD-fed rats exhibited increased cumulative food intake, bodyweight and liver and adiposity indices. Moreover, vehicle-treated HFD-fed rats exhibited a deterioration in liver function and an abnormal lipid profile. Insulin resistance and serum leptin were increased in this group, whereas serum adiponectin levels were decreased. Orlistat monotherapy or combination therapy with orlistat plus rimonabant improved all these parameters. 4. The addition of the low subeffective dose of rimonabant to orlistat therapy ameliorated HFD-induced obesity to a much greater extent than orlistat monotherapy. This combination showed better weight control and metabolic profile compared with orlistat alone. Therefore, the results of the present study encourage reassessment of the use of a low dose of rimonabant to potentiate the effect of orlistat in the clinical management of obesity if proper clinical safety data are available.

Issues in characterizing resting energy expenditure in obesity and after weight loss

Limitations of current methods: Normalization of resting energy expenditure (REE) for body composition using the 2-compartment model fat mass (FM), and fat-free mass (FFM) has inherent limitations for the interpretation of REE and may lead to erroneous conclusions when comparing people with a wide range of adiposity as well as before and after substantial weight loss.

EXPERIMENTAL OBJECTIVES

We compared different methods of REE normalization: (1) for FFM and FM (2) by the inclusion of %FM as a measure of adiposity and (3) based on organ and tissue masses. RESULTS were compared between healthy subjects with different degrees of adiposity as well as within subject before and after weight loss.

RESULTS

Normalizing REE from an "REE vs. FFM and FM equation" that (1) was derived in obese participants and applied to lean people or (2) was derived before weight loss and applied after weight loss leads to the erroneous conclusion of a lower metabolic rate (i) in lean persons and (ii) after weight loss. This is revealed by the normalization of REE for organ and tissue masses that was not significantly different between lean and obese or between baseline and after weight loss. There is evidence for an increasing specific metabolic rate of FFM with increasing %FM that could be explained by a higher contribution of liver, kidney and heart mass to FFM in obesity. Using "REE vs. FFM and FM equations" specific for different levels of adiposity (%FM) eliminated differences in REE before and after weight loss in women.

CONCLUSION

The most established method for normalization of REE based on FFM and FM may lead to spurious conclusions about metabolic rate in obesity and the phenomenon of weight loss-associated adaptive thermogenesis. Using %FM-specific REE prediction from FFM and FM in kg may improve the normalization of REE when subjects with wide differences in %FM are investigated.

Measuring energy metabolism in the mouse - theoretical, practical, and analytical considerations

The mouse is one of the most important model organisms for understanding human genetic function and disease. This includes characterization of the factors that influence energy expenditure and dysregulation of energy balance leading to obesity and its sequelae. Measuring energy metabolism in the mouse presents a challenge because the animals are small, and in this respect it presents similar challenges to measuring energy demands in many other species of small mammal. This paper considers some theoretical, practical, and analytical considerations to be considered when measuring energy expenditure in mice. Theoretically total daily energy expenditure is comprised of several different components: basal or resting expenditure, physical activity, thermoregulation, and the thermic effect of food. Energy expenditure in mice is normally measured using open flow indirect calorimetry apparatus. Two types of system are available – one of which involves a single small Spartan chamber linked to a single analyzer, which is ideal for measuring the individual components of energy demand. The other type of system involves a large chamber which mimics the home cage environment and is generally configured with several chambers/analyzer. These latter systems are ideal for measuring total daily energy expenditure but at present do not allow accurate decomposition of the total expenditure into its components. The greatest analytical challenge for mouse expenditure data is how to account for body size differences between individuals. This has been a matter of some discussion for at least 120 years. The statistically most appropriate approach is to use analysis of covariance with individual aspects of body composition as independent predictors.

The interaction of amylin with other hormones in the control of eating

Twenty years of research established amylin as an important control of energy homeostasis. Amylin controls nutrient and energy fluxes by reducing energy intake, by modulating nutrient utilization via an inhibition of postprandial glucagon secretion and by increasing energy disposal via a prevention of compensatory decreases of energy expenditure in weight reduced individuals. Like many other gastrointestinal hormones, amylin is secreted in response to meals and it reduces eating by promoting meal-ending satiation. Not surprisingly, amylin interacts with many of these hormones to control eating. These interactions seem to occur at different levels because amylin seems to mediate the eating inhibitory effect of some of these gastrointestinal hormones, and the combination of some of these hormones seems to lead to a stronger reduction in eating than single hormones alone. Amylin's effect on eating is thought to be mediated by a stimulation of specific amylin receptors in the area postrema. Secondary brain sites that were defined to mediate amylin action - and hence potential additional sites of interaction with other hormones - include the nucleus of the solitary tract, the lateral parabrachial nucleus, the lateral hypothalamic area and other hypothalamic nuclei. The focus of this review is to summarize the current knowledge of amylin interactions in the control of eating. In most cases, these interactions have only been studied at a descriptive rather than a mechanistic level and despite the clear knowledge on primary sites of amylin action, the interaction sites between amylin and other hormones are often unknown.

Adaptive thermogenesis with weight loss in humans

Adaptive thermogenesis (AT) with weight loss refers to underfeeding-associated fall in resting and non-resting energy expenditure (REE, non-REE); this is independent of body weight and body composition. In humans, the existence of AT was inconsistently shown and its clinical significance has been questioned.

OBJECTIVES

Discrepant findings are mainly due to different definitions of AT, the use of various and nonstandardized study protocols, and the limits of accuracy of methods to assess energy expenditure. With controlled underfeeding, AT takes more than 2 wk to develop. AT accounts to an average of 0.5 MJ (or 120 kcal) with a considerable between subject variance.

DESIGN AND METHODS

Low-sympathetic nervous system activity, 3,5,3'-tri-iodothyronine (T3) and leptin are likely to add to AT; however, the kinetic changes of their plasma levels with underfeeding differ from the time course of AT and controlled intervention studies substituting and titrating these hormones are rare in humans. AT in response to underfeeding is independent of thermogenesis in response to either diet or cold. Although fat-free mass (FFM) and, thus, liver, and skeletal muscle are considered as major sites of AT, cold-induced nonshivering thermogenesis relates to the metabolism of brown adipose tissue (BAT). In humans, diet-induced thermogenesis is related to postprandial substrate metabolism of FFM with a questionable role of BAT. Obviously, the REE component of AT differs from and its non-REE component with respect to organ contribution as well as mechanisms. Thus, AT cannot be considered as unique.

CONCLUSIONS

AT should be characterized based on individual components of daily energy expenditure, detailed body composition analyses, and mathematical modeling. The biological basis of AT as well as the influences of age, sex, obesity, stress, and inflammation remain to be established in humans.

Moo1 obesity quantitative trait locus in BTBR T+ Itpr3tf/J mice increases food intake

The rising prevalence of obesity is one of the greatest health challenges facing the world today. Discovery of genetic factors affecting obesity risk will provide important insight to its etiology that could suggest new therapeutic approaches. We have previously identified the Modifier of obese 1 (Moo1) quantitative trait locus (QTL) in a cross between leptin-deficient BTBR T(+) Itpr3(tf)/J (BTBR) and C57BL/6J (B6) mice. Understanding the mechanism by which this locus acts will aid in the identification of candidate genes. Here we refined the location of this QTL and sought to determine the mechanism by which Moo1 affects body weight. We found that the effects of Moo1 also alter high fat diet-induced obesity in mice having functional leptin. In detailed metabolic analyses we determined that this locus acts by increasing food intake in BTBR mice, without affecting energy expenditure. The expression levels of the main molecular mediators of food intake in the hypothalamus were not altered, suggesting this locus affects an independent pathway, consistent with its identification in mice lacking functional leptin. Finally, we show that the increased adiposity resulting from Moo1 is sufficient to affect glucose tolerance. These studies show that the Moo1 obesity QTL affects food intake, likely through a novel mechanism, and indicate that modulation of the underlying pathway may not only ameliorate obesity but also its clinical consequences.