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

Genome-wide meta-analysis uncovers novel loci influencing circulating leptin levels

Leptin is an adipocyte-secreted hormone, the circulating levels of which correlate closely with overall adiposity. Although rare mutations in the leptin (LEP) gene are well known to cause leptin deficiency and severe obesity, no common loci regulating circulating leptin levels have been uncovered. Therefore, we performed a genome-wide association study (GWAS) of circulating leptin levels from 32,161 individuals and followed up loci reaching P <10(-6) in 19,979 additional individuals. We identify five loci robustly associated (P <5 x 10(-8)) with leptin levels in/near LEP, SLC32A1, GCKR, CCNL1 and FTO. Although the association of the FTO obesity locus with leptin levels is abolished by adjustment for BMI, associations of the four other loci are independent of adiposity. The GCKR locus was found associated with multiple metabolic traits in previous GWAS and the CCNL1 locus with birth weight. Knockdown experiments in mouse adipose tissue explants show convincing evidence for adipogenin, a regulator of adipocyte differentiation, as the novel causal gene in the SLC32A1 locus influencing leptin levels. Our findings provide novel insights into the regulation of leptin production by adipose tissue and open new avenues for examining the influence of variation in leptin levels on adiposity and metabolic health.Peer reviewe

Energy expenditure and activity of AL and CR mice.

<p>(A) Energy expenditure during calorie restriction in mice fed <i>ad libitum</i> chow throughout the study (AL) and mice calorically restricted to 80% of initial body weight (CR). Energy expenditure during calorie restriction was measured in the TSE metabolic chambers. Included are the following: TEE–total energy expenditure, REE–resting energy expenditure, NREE–non resting energy expenditure and torpor suppression. (B) Physical activity in AL and CR mice during CR and after release to <i>ad libitum</i> feeding. Activity was measured in the TSE system. Regression of instantaneous TEE as a function of movement (C) during the day and (D) at night in mice fed <i>ad libitum</i> chow throughout the study (AL), mice calorically restricted to 80% of initial body weight (CR) and the CR group after release to <i>ad libitum</i> feeding. P values: *<0.05, ***<0.001.</p

Food intake, plasma glucose and insulin in CR and AL mice.

<p>(A) Mean 24h food intake ±SEM (g) and (B) Cumulative food intake over 8 weeks of body weight re-gain in mice fed <i>ad libitum</i> chow throughout the study (AL) and mice calorically restricted to 80% of initial body weight then released to <i>ad libitum</i> feeding. (C) Mean glucose and (D) insulin ±SEM in <i>ad libitum</i> fed (AL) or calorically restricted (CR) mice measured at 12 weeks of age while CR mice were calorically restricted to maintain 80% of initial body weight. (E) Regression of circulating insulin concentrations against fat mass in the AL and CR groups of mice at 11 weeks of age while CR were weight stable at the reduced body weight. P values: ***<0.001.</p

Body weight and food intake of AL and CR mice in a pilot study.

<p>(A) Mean body weight ±SEM (g) and (B) Mean 24h food intake in mice fed <i>ad libitum</i> throughout the study (AL) and mice calorically restricted to 80% of initial body weight then released to <i>ad libitum</i> feeding. P values: *<0.05, **<0.01.</p

Total energy expenditure and respiratory exchange ratio.

<p>(A) Total energy expenditure after release from calorie restriction in mice fed <i>ad libitum</i> chow throughout the study (AL) and calorically restricted (CR) mice. TEE post-restriction was calculated using the energy balance equation: TEE = FI − (Δ somatic Fat Energy + Δ somatic Fat−Free Energy). (B) average respiratory exchange ratio (RER) measured at each time interval and (C) average 24-hour RER during the day and (D) and at night during and post calorie restriction in mice fed <i>ad libitum</i> chow throughout the study (AL) and mice calorically restricted then released to <i>ad libitum</i> feeding. P values: **<0.01, ***<0.001.</p

Correlations of energy expenditure with body composition in AL and CR mice.

<p>Regression of (A, C) lean mass and (B, D) fat mass against (A, B) total and (C, D) resting energy expenditure in the AL and CR groups of mice during the weight maintenance segment of the CR phase.</p

Body temperature of AL and CR mice.

<p>Body temperature of mice fed <i>ad libitum</i> chow throughout the study (AL) and mice calorically restricted to 80% of initial body weight (CR) measured during the weight maintenance segment of the CR phase.</p

Study design schematic.

<p>Twenty percent weight reduction was achieved by feeding mice 1g of chow daily. During the weight maintenance phase, food intake was increased to 2-3g per day per mouse (the amount of food was adjusted daily when % of initial body weight of a mouse deviated from 80% by more than 2%). Calorically-restricted mice were provided with food twice daily, 1/3 of the total daily calories in the morning (09:00–9:30h) and 2/3 in the evening (18:00–18:30h). Body weight and food intake were monitored daily. During the second week of weight maintenance, mice were placed individually in metabolic cages to assess their energy expenditure (EE; TSE calorimetry system). They were then released to <i>ad libitum</i> feeding and EE was measured for another week. Mice were monitored for eight weeks until the body weight of the previously calorie restricted group reached that of with the never-restricted controls at which point mice were sacrificed.</p