Hypothalamic astrocyte NAD+ salvage pathway mediates the coupling of dietary fat overconsumption in a mouse model of obesity.

Nicotinamide adenine dinucleotide (NAD)+ serves as a crucial coenzyme in numerous essential biological reactions, and its cellular availability relies on the activity of the nicotinamide phosphoribosyltransferase (NAMPT)-catalyzed salvage pathway. Here we show that treatment with saturated fatty acids activates the NAD+ salvage pathway in hypothalamic astrocytes. Furthermore, inhibition of this pathway mitigates hypothalamic inflammation and attenuates the development of obesity in male mice fed a high-fat diet (HFD). Mechanistically, CD38 functions downstream of the NAD+ salvage pathway in hypothalamic astrocytes burdened with excess fat. The activation of the astrocytic NAMPT-NAD+-CD38 axis in response to fat overload induces proinflammatory responses in the hypothalamus. It also leads to aberrantly activated basal Ca2+ signals and compromised Ca2+ responses to metabolic hormones such as insulin, leptin, and glucagon-like peptide 1, ultimately resulting in dysfunctional hypothalamic astrocytes. Our findings highlight the significant contribution of the hypothalamic astrocytic NAD+ salvage pathway, along with its downstream CD38, to HFD-induced obesity.

Circulating blood eNAMPT drives the circadian rhythms in locomotor activity and energy expenditure.

Nicotinamide adenine dinucleotide (NAD+) is an essential cofactor of critical enzymes including protein deacetylase sirtuins/SIRTs and its levels in mammalian cells rely on the nicotinamide phosphoribosyltransferase (NAMPT)-mediated salvage pathway. Intracellular NAMPT (iNAMPT) is secreted and found in the blood as extracellular NAMPT (eNAMPT). In the liver, the iNAMPT-NAD+ axis oscillates in a circadian manner and regulates the cellular clockwork. Here we show that the hypothalamic NAD+ levels show a distinct circadian fluctuation with a nocturnal rise in lean mice. This rhythm is in phase with that of plasma eNAMPT levels but not with that of hypothalamic iNAMPT levels. Chemical and genetic blockade of eNAMPT profoundly inhibit the nighttime elevations in hypothalamic NAD+ levels as well as those in locomotor activity (LMA) and energy expenditure (EE). Conversely, elevation of plasma eNAMPT by NAMPT administration increases hypothalamic NAD+ levels and stimulates LMA and EE via the hypothalamic NAD+-SIRT-FOXO1-melanocortin pathway. Notably, obese animals display a markedly blunted circadian oscillation in blood eNAMPT-hypothalamic NAD+-FOXO1 axis as well as LMA and EE. Our findings indicate that the eNAMPT regulation of hypothalamic NAD+ biosynthesis underlies circadian physiology and that this system can be significantly disrupted by obesity.

Mitohormesis in Hypothalamic POMC Neurons Mediates Regular Exercise-Induced High-Turnover Metabolism.

Low-grade mitochondrial stress can promote health and longevity, a phenomenon termed mitohormesis. Here, we demonstrate the opposing metabolic effects of low-level and high-level mitochondrial ribosomal (mitoribosomal) stress in hypothalamic proopiomelanocortin (POMC) neurons. POMC neuron-specific severe mitoribosomal stress due to Crif1 homodeficiency causes obesity in mice. By contrast, mild mitoribosomal stress caused by Crif1 heterodeficiency in POMC neurons leads to high-turnover metabolism and resistance to obesity. These metabolic benefits are mediated by enhanced thermogenesis and mitochondrial unfolded protein responses (UPRmt) in distal adipose tissues. In POMC neurons, partial Crif1 deficiency increases the expression of ß-endorphin (ß-END) and mitochondrial DNA-encoded peptide MOTS-c. Central administration of MOTS-c or ß-END recapitulates the adipose phenotype of Crif1 heterodeficient mice, suggesting these factors as potential mediators. Consistently, regular running exercise at moderate intensity stimulates hypothalamic MOTS-c/ß-END expression and induces adipose tissue UPRmt and thermogenesis. Our findings indicate that POMC neuronal mitohormesis may underlie exercise-induced high-turnover metabolism.

Hypothalamic Macrophage Inducible Nitric Oxide Synthase Mediates Obesity-Associated Hypothalamic Inflammation.

Obesity-associated metabolic alterations are closely linked to low-grade inflammation in peripheral organs, in which macrophages play a central role. Using genetic labeling of myeloid lineage cells, we show that hypothalamic macrophages normally reside in the perivascular area and circumventricular organ median eminence. Chronic consumption of a high-fat diet (HFD) induces expansion of the monocyte-derived macrophage pool in the hypothalamic arcuate nucleus (ARC), which is significantly attributed to enhanced proliferation of macrophages. Notably, inducible nitric oxide synthase (iNOS) is robustly activated in ARC macrophages of HFD-fed obese mice. Hypothalamic macrophage iNOS inhibition completely abrogates macrophage accumulation and activation, proinflammatory cytokine overproduction, reactive astrogliosis, blood-brain-barrier permeability, and lipid accumulation in the ARC of obese mice. Moreover, central iNOS inhibition improves obesity-induced alterations in systemic glucose metabolism without affecting adiposity. Our findings suggest a critical role for hypothalamic macrophage-expressed iNOS in hypothalamic inflammation and abnormal glucose metabolism in cases of overnutrition-induced obesity.

Regulation of energy balance by the hypothalamic lipoprotein lipase regulator Angptl3.

Hypothalamic lipid sensing is important for the maintenance of energy balance. Angiopoietin-like protein 3 (Angptl3) critically regulates the clearance of circulating lipids by inhibiting lipoprotein lipase (LPL). The current study demonstrated that Angptl3 is highly expressed in the neurons of the mediobasal hypothalamus, an important area in brain lipid sensing. Suppression of hypothalamic Angptl3 increased food intake but reduced energy expenditure and fat oxidation, thereby promoting weight gain. Consistently, intracerebroventricular (ICV) administration of Angptl3 caused the opposite metabolic changes, supporting an important role for hypothalamic Angptl3 in the control of energy balance. Notably, ICV Angptl3 significantly stimulated hypothalamic LPL activity. Moreover, coadministration of the LPL inhibitor apolipoprotein C3 antagonized the effects of Angptl3 on energy metabolism, indicating that LPL activation is critical for the central metabolic actions of Angptl3. Increased LPL activity is expected to promote lipid uptake by hypothalamic neurons, leading to enhanced brain lipid sensing. Indeed, ICV injection of Angptl3 increased long-chain fatty acid (LCFA) and LCFA-CoA levels in the hypothalamus. Furthermore, inhibitors of hypothalamic lipid-sensing pathways prevented Angptl3-induced anorexia and weight loss. These findings identify Angptl3 as a novel regulator of the hypothalamic lipid-sensing pathway.