Brent E. Wisse

Despite the suggestion that reduced energy expenditure may be a key contributor to the obesity pandemic, few studies have tested whether acutely reduced energy expenditure is associated with a compensatory reduction in food intake. The homeostatic mechanisms that control food intake and energy expenditure remain controversial and are thought to act over days to weeks. We evaluated food intake in mice using two models of acutely decreased energy expenditure: 1) increasing ambient temperature to thermoneutrality in mice acclimated to standard laboratory temperature or 2) exercise cessation in mice accustomed to wheel running. Increasing ambient temperature (from 21°C to 28°C) rapidly decreased energy expenditure, demonstrating that thermoregulatory energy expenditure contributes to both light cycle (40±1%) and dark cycle energy expenditure (15±3%) at normal ambient temperature (21°C). Reducing thermoregulatory energy expenditure acutely decreased food intake primarily during the light cycle (65±7%), thus conflicting with the delayed compensation model, but did not alter spontaneous activity. Acute exercise cessation decreased energy expenditure only during the dark cycle (14±2% at 21°C; 21±4% at 28°C), while food intake was reduced during the dark cycle (0.9±0.1 g) in mice housed at 28°C, but during the light cycle (0.3±0.1 g) in mice housed at 21°C. Cumulatively, there was a strong correlation between the change in daily energy expenditure and the change in daily food intake (R² = 0.51, p<0.01). We conclude that acutely decreased energy expenditure decreases food intake suggesting that energy intake is regulated by metabolic signals that respond rapidly and accurately to reduced energy expenditure.

Hypothalamic inflammation is a recently described phenomenon that occurs during consumption of high fat diets (HFD) and is hypothesized to promote diet-induced obesity (DIO) by causing resistance to leptin and other adiposity-related inputs. Since evidence supporting this hypothesis was generated in animals with established DIO, it remains unclear whether hypothalamic inflammation occurs early enough to play a causal role in HFD-induced obesity. We report that whereas inflammation in peripheral tissues develops only after months of HFD consumption, low-grade inflammation is evident in both rat and mouse hypothalamus within the first wk of HFD exposure. This increase in inflammatory gene expression is initially transient (~7 d) and coincides temporally with the early onset of both HFD-induced excessive energy intake and hypothalamic leptin resistance. Remarkably, this early phase of the response to HFD in both rats and mice is accompanied by an accumulation of enlarged microglia and reactive astrocytes within the mediobasal hypothalamus (MBH), a region containing key neurocircuits regulating energy homeostasis. The combination of altered astrocyte cell structure and microglial involvement is characteristic of "gliosis," a histopathological hallmark of acute neuronal injury. While the astrocyte component of MBH gliosis resolves over a similar time frame as hypothalamic inflammation, microglia continue to accumulate in the MBH over months of HFD consumption. These results suggest that the initial gliosis response may be protective, limiting HFD-induced neuronal injury and inflammation. Consistent with this hypothesis, microglia isolated from the MBH show reduced inflammatory gene expression in the first wk of HFD feeding, but later appear to perpetuate hypothalamic inflammation. These findings collectively support an "acute neuronal injury" model of DIO, in which the defense of elevated body weight results from fixed injury to neurons responsible for energy homeostasis, and suggest that interventions to avert or reverse neuronal damage have important potential in obesity treatment and prevention. Nothing to Disclose: JPT, C-XY, BHH, XZ, SJG, DAS, GJM, HTN, JDF, DGB, BEW, MHT, MWS

Myostatin deficiency causes dramatically increased skeletal muscle mass and reduced fat mass. Previously, myostatin-deficient mice were reported to have unexpectedly low total energy expenditure (EE) after normalizing to body mass, and thus, a metabolic cause for low fat mass was discounted. To clarify how myostatin deficiency affects the control of body fat mass and energy balance, we compared rates of oxygen consumption, body composition, and food intake in young myostatin-deficient mice relative to wild-type (WT) and heterozygous (HET) controls. We report that after adjusting for total body mass using regression analysis, young myostatin-deficient mice display significantly increased EE relative to both WT (+0.81 ± 0.28 kcal/day, P = 0.004) and HET controls (+0.92 ± 0.31 kcal/day, P = 0.005). Since food intake was not different between groups, increased EE likely accounts for the reduced body fat mass (KO: 8.8 ± 1.1% vs. WT: 14.5 ± 1.3%, P = 0.003) and circulating leptin levels (KO: 0.7 ± 0.2 ng/ml vs. WT: 1.9 ± 0.3 ng/ml, P = 0.008). Interestingly, the observed increase in adjusted EE in myostatin-deficient mice occurred despite dramatically reduced ambulatory activity levels (−50% vs. WT, P < 0.05). The absence of hyperphagia together with increased EE in myostatin-deficient mice suggests that increased leptin sensitivity may contribute to their lean phenotype. Indeed, leptin-induced anorexia (KO: −17 ± 1.2% vs. WT: −5 ± 0.3%) and weight loss (KO: −2.2 ± 0.2 g vs. WT: −1.6 ± 0.1, P < 0.05) were increased in myostatin-deficient mice compared with WT controls. We conclude that increased EE, together with increased leptin sensitivity, contributes to low fat mass in mice lacking myostatin.

Hypothalamic inflammation induced by high-fat feeding causes insulin and leptin resistance and contributes to the pathogenesis of obesity. Since in vitro exposure to saturated fatty acids causes inflammation and insulin resistance in many cultured cell types, we determined how cultured hypothalamic neurons respond to this stimulus. Two murine hypothalamic neuronal cell cultures, N43/5 and GT1–7, were exposed to escalating concentrations of saturated fatty acids for up to 24 h. Harvested cells were evaluated for activation of inflammation by gene expression and protein content. Insulin-treated cells were evaluated for induction of markers of insulin receptor signaling (p-IRS, p-Akt). In both hypothalamic cell lines, inflammation was induced by prototypical inflammatory mediators LPS and TNFα, as judged by induction of IκBα (3- to 5-fold) and IL-6 (3- to 7-fold) mRNA and p-IκBα protein, and TNFα pretreatment reduced insulin-mediated p-Akt activation by 30% (P < 0.05). By comparison, neither mixed saturated fatty acid (100, 250, or 500 μM for ≤6 h) nor palmitate exposure alone (200 μM for ≤24 h) caused inflammatory activation or insulin resistance in cultured hypothalamic neurons, whereas they did in control muscle and endothelial cell lines. Despite the lack of evidence of inflammatory signaling, saturated fatty acid exposure in cultured hypothalamic neurons causes endoplasmic reticulum stress, induces mitogen-activated protein kinase, and causes apoptotic cell death with prolonged exposure. We conclude that saturated fatty acid exposure does not induce inflammatory signaling or insulin resistance in cultured hypothalamic neurons. Therefore, hypothalamic neuronal inflammation in the setting of DIO may involve an indirect mechanism mediated by saturated fatty acids on nonneuronal cells.

Systemic inflammatory stimuli cause anorexia and weight loss by disrupting the physiological regulation of energy balance. Mice lacking MyD88, an intracellular mediator of signal transduction activated by Toll-like receptor 4 or IL-1β receptors, are resistant to anorexia induced by the bacterial endotoxin lipopolysaccharide (LPS), despite a significant circulating cytokine response. Thus, we hypothesized that induction of a peripheral inflammatory response is insufficient to cause LPS-induced anorexia when MyD88 signaling in the central nervous system and other tissues is absent. To test this hypothesis, we used bone marrow transplantation (BMT) to determine if LPS-induced anorexia can be restored to MyD88-deficient mice by reconstituting their bone marrow with wild-type (WT) immune cells. We found that restoring WT circulating immune cells to mice lacking MyD88 conferred only a mild, short-lived anorexia in response to LPS, such that food intake was fully normalized by 20 h post injection (LPS 4.1 ± 0.5 g vs. vehicle 4.3 ± 0.3 g), whereas LPS-induced anorexia was profound and sustained in WT controls after either autologous BMT or sham BMT. Similarly, LPS-mediated induction of hypothalamic mRNA encoding IL-1β and TNFα was robust in both WT control groups but was absent in chimeric MyD88 mice, despite comparable peripheral inflammatory responses across the three groups. We conclude that LPS reduces food intake via a mechanism dependent on MyD88 signaling within brain and/or other tissues and that in the absence of this effect, robust stimulation of circulating immune cells cannot induce sustained anorexia.