Molecular and Physiological Adaptations to Weight Perturbation in Mice

From a medical perspective, obesity may be defined as a degree of relative adiposity sufficient to derange metabolic physiology in a manner that negatively impacts the health of the individual. While population-based cut points based on body mass index (BMI) are frequently used as a means of identifying such individuals, this is an imprecise approach since the critical levels of adiposity in this regard differ substantially among individuals. Our common genetic predisposition to increased adiposity, coupled with an environment conducive to positive energy balance results in an increasing prevalence of human obesity. Weight loss, even when initially successful, is very difficult to maintain due, in part, to a feedback system involving metabolic, behavioral, neuroendocrine and autonomic responses that are initiated to maintain somatic energy stores (fat) at a level considered `ideal' by the central nervous system (CNS). Circulating leptin is an important afferent signal to the CNS relating peripheral energy stores with modulations in key leptin sensing area sensitivity possibly implicated in the functional and molecular basis of defense of body weight. These physiological responses, which include increased metabolic efficiency at lower body weight, may be engaged in individuals at different levels of body fat depending on their genetic makeup, as well as on gestational and post-natal environmental factors that have determined the so-called "set-point". In the work presented in this dissertation the following aspects of the physiology of the defense of body weight were explored: 1) whether levels (thresholds) of defended adiposity can be raised or lowered by environmental manipulation; 2) the physiological and molecular changes that mediate increased metabolic efficiency following weight loss, 3) leptin's role in setting the threshold; 4) the effects of ambient temperature on metabolic phenotypes of weight perturbed to assess whether torpor contributes to metabolic adaptation; and 5) whether changes in gut microbiota accompany changes in diet composition and/or body weight. To assess whether the threshold for defended body weight could be increased or decreased by environmental manipulations (i.e. high fat diet and weight restriction), we identified bioenergetic, behavioral, and CNS structural responses of C57BL/6J in long term diet induced obese (DIO) male mice to weight reduction. We found that maintenance of a body weight 20% below that imposed by a high fat diet results in metabolic adaptation - energy expenditure below that expected for body mass and composition - and structural changes of synapses onto arcuate pro-opiomelanocortin (POMC) cell bodies. These changes are qualitatively and quantitatively similar to those observed in weight-reduced animals that were never obese, suggesting that the previously obese animals are now "defending" a higher body weight. Maintenance of a lower body weight for more than 3 months was not accompanied by remission of the increased metabolic efficiency. Thus, the consequence of long term elevation of body weight suggests an increase in defended body fat that does not abate with time. Mice can enter torpor - a state of decreased metabolic rate and concomitant decrease in body temperature - as a defense mechanism in times of low caloric availability and/or decreased ambient room temperatures. Declines in circulating leptin concentrations and low ambient room temperature have both been implicated in the onset of torpor. To assess the effects of ambient room temperature and leptin concentrations on metabolic adaptation, we characterized C57BL/6J and leptin deficient (Lepob) mice following weight perturbation at both 22°C and 30°C ambients. Weight-reduced C57BL/6J mice show metabolic adaptation at both ambient temperatures and do not enter torpor whereas weight-reduced Lepob animals readily enter torpor at 22°C. This suggests that sufficiently high absolute leptin concentrations may impede the onset of torpor and that torpor itself does not contribute to metabolic adaptation in mice that have an intact leptin axis. To assess whether hyperleptinemia per se was capable of increasing the threshold for defended body weight, leptin was infused by minipumps into C57BL/6J mice for 18 weeks and body weight and metabolic parameters were studied following cessation of leptin infusion. Leptin infused mice did not defend elevated body weights compared to PBS infused mice suggesting that leptin alone may not be capable of setting the threshold for body weight defense implying that other changes accompanying obesity (i.e. increased free fatty acids, endoplasmic reticulum stress and/or inflammation of leptin-sensitive neural areas) are implicated. A caveat and possible confound to this study is the possibility of antibody production against the exogenous leptin that could have drastically decreased the amount of bioavailable leptin in these mice. This experiment did not assess antibody production but subsequent studies should do so. Finally, gut microbiota have been implicated in the regulation of body weight possibly by impacting insulin resistance, inflammation, and adiposity via interactions with epithelial and endocrine cells. We assessed changes in relative abundances of cecal microbiota in mice following sustained changes in body weight and diet composition. In diet-induced obese (DIO) mice, we find that weight reduction resulted in shifts in specific bacteria abundance (Akkermansia and Mucispirillum) and that these changes were correlated with leptin concentrations. Leptin modulates mucin production in the gut possibly altering local microniches for certain bacteria providing a functional link between adiposity and gut-specific changes in bacterial populations. Overall, the major findings of these experiments are that the threshold for body weight defense can be raised but not lowered, that metabolic adaptation observed in weight-reduced mice is not a result of torpor, and that hyperleptinemia (if no anti-bodies were produced) per se isolated from other obesity-related changes does not appear capable of raising the threshold

A Missing Link in Body Weight Homeostasis: The Catabolic Signal of the Overfed State

Mammals regulate fat mass so that increases or reductions in adipose tissue mass activate responses that favor return to one’s previous weight. A reduction in fat mass activates a system that increases food intake and reduces energy expenditure; conversely, overfeeding and rapid adipose tissue expansion reduces food intake and increases energy expenditure. With the identification of leptin nearly two decades ago, the central circuit that defends against reductions in body fat was revealed. However, the systems that defend against rapid expansion of fat mass remain largely unknown. Here we review the physiology of the overfed state and evidence for a distinct regulatory system, which unlike the leptin-mediated system, we propose primarily measures a functional aspect of adipose tissue and not total mass per se

Energy homeostasis in leptin deficient Lep^(ob/ob) mice

Maintenance of reduced body weight is associated both with reduced energy expenditure per unit metabolic mass and increased hunger in mice and humans. Lowered circulating leptin concentration, due to decreased fat mass, provides a primary signal for this response. However, leptin deficient (Lepob/ob) mice (and leptin receptor deficient Zucker rats) reduce energy expenditure following weight reduction by a necessarily non-leptin dependent mechanisms. To identify these mechanisms, Lepob/ob mice were fed ad libitum (AL group; n = 21) or restricted to 3 kilocalories of chow per day (CR group, n = 21). After losing 20% of initial weight (in approximately 2 weeks), the CR mice were stabilized at 80% of initial body weight for two weeks by titrated refeeding, and then released from food restriction. CR mice conserved energy (-17% below predicted based on body mass and composition during the day; -52% at night); and, when released to ad libitum feeding, CR mice regained fat and lean mass (to AL levels) within 5 weeks. CR mice did so while their ad libitum caloric intake was equal to that of the AL animals. While calorically restricted, the CR mice had a significantly lower respiratory exchange ratio (RER = 0.89) compared to AL (0.94); after release to ad libitum feeding, RER was significantly higher (1.03) than in the AL group (0.93), consistent with their anabolic state. These results confirm that, in congenitally leptin deficient animals, leptin is not required for compensatory reduction in energy expenditure accompanying weight loss, but suggest that the hyperphagia of the weight-reduced state is leptin-dependent

A New Symptom of COVID‐19: Loss of Taste and Smell

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Effect of intermittent cold exposure on brown fat activation, obesity, and energy homeostasis in mice.

Homeotherms have specific mechanisms to maintain a constant core body temperature despite changes in thermal environment, food supply, and metabolic demand. Brown adipose tissue, the principal thermogenic organ, quickly and efficiently increases heat production by dissipating the mitochondrial proton motive force. It has been suggested that activation of brown fat, via either environmental (i.e. cold exposure) or pharmacologic means, could be used to increase metabolic rate and thus reduce body weight. Here we assess the effects of intermittent cold exposure (4°C for one to eight hours three times a week) on C57BL/6J mice fed a high fat diet. Cold exposure increased metabolic rate approximately two-fold during the challenge and activated brown fat. In response, food intake increased to compensate fully for the increased energy expenditure; thus, the mice showed no reduction in body weight or adiposity. Despite the unchanged adiposity, the cold-treated mice showed transient improvements in glucose homeostasis. Administration of the cannabinoid receptor-1 inverse agonist AM251 caused weight loss and improvements in glucose homeostasis, but showed no further improvements when combined with cold exposure. These data suggest that intermittent cold exposure causes transient, meaningful improvements in glucose homeostasis, but without synergy when combined with AM251. Since energy expenditure is significantly increased during cold exposure, a drug that dissociates food intake from metabolic demand during cold exposure may achieve weight loss and further metabolic improvements

Weight Categories, Trajectories, Eating Behavior, and Metabolic Consequences during Pregnancy and Postpartum in Women with GDM

Pre-pregnancy overweight and obesity are associated with increased risk for adverse outcomes, such as gestational diabetes mellitus (GDM). This study investigated weight trajectories, eating behaviors, and metabolic consequences in women with GDM during pregnancy and postpartum according to pre-pregnancy BMI. We prospectively included 464 women with GDM. Intuitive eating (Intuitive Eating Scale-2 questionnaire), gestational weight gain (GWG), postpartum weight retention (PPWR) at 6–8 weeks and 1-year postpartum, and glucose intolerance (prediabetes and diabetes) at 1-year were assessed. Women with obesity (WOB) had lower GWG but gained more weight in the postpartum (p p = 0.63), whereas postpartum weight loss was most pronounced in women with normal weight (p p p < 0.001), and the adverse metabolic impact of PPWR was most pronounced in WOB with odds of increased risk of glucose intolerance 8.9 times higher (95% CI 2.956;26.968). These findings suggest an adaptive capacity to relatively rapid weight changes in the perinatal period that is less present with higher BMI

An overfeeding-induced obesity mouse model reveals necessity for Sin3a in postnatal peak ß-Cell mass acquisition

The increase of functional ß-cell mass is paramount to maintaining glucose homeostasis in the setting of systemic insulin resistance and/or augmented metabolic load. Understanding compensatory mechanisms that allow ß-cell mass adaptation may allow for the discovery of therapeutically actionable control nodes. In this study, we report the rapid and robust ß-cell hyperplasic effect in a mouse model of overfeeding-induced obesity (OIO) based on direct gastric caloric infusion. By performing RNA sequencing in islets isolated from OIO mice, we identified Sin3a as a novel transcriptional regulator of ß-cell mass adaptation. ß-Cell¿specific Sin3a knockout animals showed profound diabetes due to defective acquisition of postnatal ß-cell mass. These findings reveal a novel regulatory pathway in ß-cell proliferation and validate OIO as a model for discovery of other mechanistic determinants of ß-cell adaptation.This work was supported by National Institutes of Health grants R01DK103818 and R01DK132661 (U.B.P.), a Russell Berrie Foundation award (A.B.), and American Diabetes Association grant 1-17-PMF-025 (A.B.)

Tissue 2-deoxyglucose uptake.

<p>Mice in the ad lib fed state were administered [¹⁴C]2-deoxyglucose 1 hour prior to euthanasia. Specifically, this was at 22°C in the CON group, at the onset of 4°C in the 1 hour group, and 3 h into the 4°C treatment in the 4 hour group. Data are mean ±SE, N = 8/group. Levels not connected by same letter are significantly different (P<0.05).</p

Transient improvement in glucose tolerance by cold exposure.

<p>In ICE#1 (top) and ICE#2 (bottom), intra-peritoneal glucose tolerance tests (1 g/kg) were performed in the overnight-fasted mice. In ICE#1, the ipGTT was conducted the day after cold exposure, while in ICE#2 it was conducted on the second day following cold exposure. The inset shows the mean area AUC in mg/dl•min ±SE, N = 8/group. Levels not connected by same letter are significantly different (P<0.05).</p

Effects of AM251 and cold exposure.

<p>Mice were administered vehicle or AM251 (3 mg/kg/day) by oral gavage. A: Caloric intake and B: body weight were measured three times per week on days of cold exposure. C: Mice at 30°C were administered CL316243 (100 µg/kg) at time 0 as described in Experimental Procedures (the prior AM251 was given at −24 hours). Inset shows the delta TEE, calculated as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0085876#pone-0085876-g002" target="_blank">Figure 2</a>. 2-way ANOVA showed significant temperature (p<0.01) and drug (p<0.05) effects with no significant interaction (p = 0.62). D: Intra-peritoneal glucose tolerance tests (1 g/kg) were performed in overnight-fasted mice of the indicated treatment group two days after cold exposure. AUC numbers are in mg/dl•minute. E. Insulin tolerance test. Insulin (0.75 U/kg) was injected and blood glucose measured at the indicated times. GTT =  intraperitoneal glucose tolerance test. CL =  CL316243 experiment. ITT =  insulin tolerance test. AM251 was administered 24 h prior to the GTT and ITT. In D & E, a poor-responding outlier in the CON AM group was removed from the analysis. If included in the ipGTT AUC, this group's AUC is 29304 ±3382 mg/dl min and the CON AM significantly different from the ICE AM group. All data are mean ±SE. N = 6/group. Levels not connected by same letter are significantly different (P<0.05).</p