Hepatic Vagal Afferents Convey Clock-Dependent Signals to Regulate Circadian Food Intake.

Circadian desynchrony induced by shiftwork or jetlag is detrimental to metabolic health, but how synchronous/desynchronous signals are transmitted among tissues is unknown. Here we report that liver molecular clock dysfunction is signaled to the brain via the hepatic vagal afferent nerve (HVAN), leading to altered food intake patterns that are corrected by ablation of the HVAN. Hepatic branch vagotomy also prevents food intake disruptions induced by high-fat diet feeding and reduces body weight gain. Our findings reveal a previously unrecognized homeostatic feedback signal that relies on synchrony between the liver and the brain to control circadian food intake patterns. This identifies the hepatic vagus nerve as a therapeutic target for obesity in the setting of chrono-disruption. One Sentence Summary: The hepatic vagal afferent nerve signals internal circadian desynchrony between the brain and liver to induce maladaptive food intake patterns.

Amylin Modulates a Ventral Tegmental Area-to-Medial Prefrontal Cortex Circuit to Suppress Food Intake and Impulsive Food-Directed Behavior.

BACKGROUND: A better understanding of the neural mechanisms regulating impaired satiety to palatable foods is essential to treat hyperphagia linked with obesity. The satiation hormone amylin signals centrally at multiple nuclei including the ventral tegmental area (VTA). VTA-to-medial prefrontal cortex (mPFC) projections encode food reward information to influence behaviors including impulsivity. We hypothesized that modulation of VTA-to-mPFC neurons underlies amylin-mediated decreases in palatable food-motivated behaviors. METHODS: We used a variety of pharmacological, behavioral, genetic, and viral approaches (n = 4-16/experiment) to investigate the anatomical and functional circuitry of amylin-controlled VTA-to-mPFC signaling in rats. RESULTS: To first establish that VTA amylin receptor (calcitonin receptor) activation can modulate mPFC activity, we showed that intra-VTA amylin decreased food-evoked mPFC cFos. VTA amylin delivery also attenuated food-directed impulsive behavior, implicating VTA amylin signaling as a regulator of mPFC functions. Palatable food activates VTA dopamine and mPFC neurons. Accordingly, dopamine receptor agonism in the mPFC blocked the hypophagic effect of intra-VTA amylin, and VTA amylin injection reduced food-evoked phasic dopamine levels in the mPFC, supporting the idea that VTA calcitonin receptor activation decreases dopamine release in the mPFC. Surprisingly, calcitonin receptor expression was not found on VTA-to-mPFC projecting neurons but was instead found on GABAergic (gamma-aminobutyric acidergic) interneurons in the VTA that provide monosynaptic inputs to this pathway. Blocking intra-VTA GABA signaling, through GABA receptor antagonists and DREADD (designer receptor exclusively activated by designer drugs)-mediated GABAergic neuronal silencing, attenuated intra-VTA amylin-induced hypophagia. CONCLUSIONS: These results indicate that VTA amylin signaling stimulates GABA-mediated inhibition of dopaminergic projections to the mPFC to mitigate impulsive consumption of palatable foods.

Metabolic hormone action in the VTA: Reward-directed behavior and mechanistic insights.

Dysfunctional signaling in midbrain reward circuits perpetuates diseases characterized by compulsive overconsumption of rewarding substances such as substance abuse, binge eating disorder, and obesity. Ventral tegmental area (VTA) dopaminergic activity serves as an index for how rewarding stimuli are perceived and triggers behaviors necessary to obtain future rewards. The evolutionary linking of reward with seeking and consuming palatable foods ensured an organism's survival, and hormone systems that regulate appetite concomitantly developed to regulate motivated behaviors. Today, these same mechanisms serve to regulate reward-directed behavior around food, drugs, alcohol, and social interactions. Understanding how hormonal regulation of VTA dopaminergic output alters motivated behaviors is essential to leveraging therapeutics that target these hormone systems to treat addiction and disordered eating. This review will outline our current understanding of the mechanisms underlying VTA action of the metabolic hormones ghrelin, glucagon-like peptide-1, amylin, leptin, and insulin to regulate behavior around food and drugs of abuse, highlighting commonalities and differences in how these five hormones ultimately modulate VTA dopamine signaling.

Tirzepatide suppresses palatable food intake by selectively reducing preference for fat in rodents.

AIM: To investigate the role of glucose-dependent insulinotropic polypeptide receptor (GIPR) agonists alone or combined with glucagon-like peptide-1 receptor (GLP-1R) agonists to regulate palatable food intake and the role of specific macronutrients in these preferences. METHODS: To understand this regulation, we treated mice and rats on several choice diet paradigms of chow and a palatable food option with individual or dual GIPR and GLP-1R agonists. RESULTS: In mice, the dual agonist tirzepatide suppressed total caloric intake, while promoting the intake of chow over a high fat/sucrose diet. Surprisingly, GIPR agonism alone did not alter food choice. The food intake shift observed with tirzepatide in wild-type mice was completely absent in GLP-1R knockout mice, suggesting that GIPR signalling does not regulate food preference. Tirzepatide also selectively suppressed the intake of palatable food but not chow in a rat two-diet choice model. This suppression was specific to lipids, as GLP-1R agonist and dual agonist treatment in rats on a choice paradigm assessing individual palatable macronutrients robustly inhibited the intake of Crisco (lipid) without decreasing the intake of a sucrose (carbohydrate) solution. CONCLUSIONS: Decreasing preference for high-caloric, high-fat foods is a powerful action of GLP-1R and dual GIPR/GLP-1R agonist therapeutics, which may contribute to the weight loss success of these drugs.

REV-ERB nuclear receptors in the suprachiasmatic nucleus control circadian period and restrict diet-induced obesity.

Circadian disruption, as occurs in shift work, is associated with metabolic diseases often attributed to a discordance between internal clocks and environmental timekeepers. REV-ERB nuclear receptors are key components of the molecular clock, but their specific role in the SCN master clock is unknown. We report here that mice lacking circadian REV-ERB nuclear receptors in the SCN maintain free-running locomotor and metabolic rhythms, but these rhythms are notably shortened by 3 hours. When housed under a 24-hour light:dark cycle and fed an obesogenic diet, these mice gained excess weight and accrued more liver fat than controls. These metabolic disturbances were corrected by matching environmental lighting to the shortened endogenous 21-hour clock period, which decreased food consumption. Thus, SCN REV-ERBs are not required for rhythmicity but determine the free-running period length. Moreover, these results support the concept that dissonance between environmental conditions and endogenous time periods causes metabolic disruption.

GIP Receptor Agonism Attenuates GLP-1 Receptor Agonist-Induced Nausea and Emesis in Preclinical Models.

Glucagon-like peptide 1 receptor (GLP-1R) agonists decrease body weight and improve glycemic control in obesity and diabetes. Patient compliance and maximal efficacy of GLP-1 therapeutics are limited by adverse side effects, including nausea and emesis. In three different species (i.e., mice, rats, and musk shrews), we show that glucose-dependent insulinotropic polypeptide receptor (GIPR) signaling blocks emesis and attenuates illness behaviors elicited by GLP-1R activation, while maintaining reduced food intake, body weight loss, and improved glucose tolerance. The area postrema and nucleus tractus solitarius (AP/NTS) of the hindbrain are required for food intake and body weight suppression by GLP-1R ligands and processing of emetic stimuli. Using single-nuclei RNA sequencing, we identified the cellular phenotypes of AP/NTS cells expressing GIPR and GLP-1R on distinct populations of inhibitory and excitatory neurons, with the greatest expression of GIPR in γ-aminobutyric acid-ergic neurons. This work suggests that combinatorial pharmaceutical targeting of GLP-1R and GIPR will increase efficacy in treating obesity and diabetes by reducing nausea and vomiting.

A critical role of hepatic GABA in the metabolic dysfunction and hyperphagia of obesity.

Hepatic lipid accumulation is a hallmark of type II diabetes (T2D) associated with hyperinsulinemia, insulin resistance, and hyperphagia. Hepatic synthesis of GABA, catalyzed by GABA-transaminase (GABA-T), is upregulated in obese mice. To assess the role of hepatic GABA production in obesity-induced metabolic and energy dysregulation, we treated mice with two pharmacologic GABA-T inhibitors and knocked down hepatic GABA-T expression using an antisense oligonucleotide. Hepatic GABA-T inhibition and knockdown decreased basal hyperinsulinemia and hyperglycemia and improved glucose intolerance. GABA-T knockdown improved insulin sensitivity assessed by hyperinsulinemic-euglycemic clamps in obese mice. Hepatic GABA-T knockdown also decreased food intake and induced weight loss without altering energy expenditure in obese mice. Data from people with obesity support the notion that hepatic GABA production and transport are associated with serum insulin, homeostatic model assessment for insulin resistance (HOMA-IR), T2D, and BMI. These results support a key role for hepatocyte GABA production in the dysfunctional glucoregulation and feeding behavior associated with obesity.

Hepatocyte membrane potential regulates serum insulin and insulin sensitivity by altering hepatic GABA release.

Hepatic lipid accumulation in obesity correlates with the severity of hyperinsulinemia and systemic insulin resistance. Obesity-induced hepatocellular lipid accumulation results in hepatocyte depolarization. We have established that hepatocyte depolarization depresses hepatic afferent vagal nerve firing, increases GABA release from liver slices, and causes hyperinsulinemia. Preventing hepatic GABA release or eliminating the ability of the liver to communicate to the hepatic vagal nerve ameliorates the hyperinsulinemia and insulin resistance associated with diet-induced obesity. In people with obesity, hepatic expression of GABA transporters is associated with glucose infusion and disposal rates during a hyperinsulinemic euglycemic clamp. Single-nucleotide polymorphisms in hepatic GABA re-uptake transporters are associated with an increased incidence of type 2 diabetes mellitus. Herein, we identify GABA as a neuro-hepatokine that is dysregulated in obesity and whose release can be manipulated to mute or exacerbate the glucoregulatory dysfunction common to obesity.

The Role of GPR109a Signaling in Niacin Induced Effects on Fed and Fasted Hepatic Metabolism.

Signaling through GPR109a, the putative receptor for the endogenous ligand ß-OH butyrate, inhibits adipose tissue lipolysis. Niacin, an anti-atherosclerotic drug that can induce insulin resistance, activates GPR109a at nM concentrations. GPR109a is not essential for niacin to improve serum lipid profiles. To better understand the involvement of GPR109a signaling in regulating glucose and lipid metabolism, we treated GPR109a wild-type (+/+) and knockout (-/-) mice with repeated overnight injections of saline or niacin in physiological states characterized by low (ad libitum fed) or high (16 h fasted) concentrations of the endogenous ligand, ß-OH butyrate. In the fed state, niacin increased expression of apolipoprotein-A1 mRNA and decreased sterol regulatory element-binding protein 1 mRNA independent of genotype, suggesting a possible GPR109a independent mechanism by which niacin increases high-density lipoprotein (HDL) production and limits transcriptional upregulation of lipogenic genes. Niacin decreased fasting serum non-esterified fatty acid concentrations in both GPR109a +/+ and -/- mice. Independent of GPR109a expression, niacin blunted fast-induced hepatic triglyceride accumulation and peroxisome proliferator-activated receptor α mRNA expression. Although unaffected by niacin treatment, fasting serum HDL concentrations were lower in GPR109a knockout mice. Surprisingly, GPR109a knockout did not affect glucose or lipid homeostasis or hepatic gene expression in either fed or fasted mice. In turn, GPR109a does not appear to be essential for the metabolic response to the fasting ketogenic state or the acute effects of niacin.

Hypothalamic REV-ERB nuclear receptors control diurnal food intake and leptin sensitivity in diet-induced obese mice.

Obesity occurs when energy expenditure is outweighed by energy intake. Tuberal hypothalamic nuclei, including the arcuate nucleus (ARC), ventromedial nucleus (VMH), and dorsomedial nucleus (DMH), control food intake and energy expenditure. Here we report that, in contrast with females, male mice lacking circadian nuclear receptors REV-ERBα and -ß in the tuberal hypothalamus (HDKO mice) gained excessive weight on an obesogenic high-fat diet due to both decreased energy expenditure and increased food intake during the light phase. Moreover, rebound food intake after fasting was markedly increased in HDKO mice. Integrative transcriptomic and cistromic analyses revealed that such disruption in feeding behavior was due to perturbed REV-ERB-dependent leptin signaling in the ARC. Indeed, in vivo leptin sensitivity was impaired in HDKO mice on an obesogenic diet in a diurnal manner. Thus, REV-ERBs play a crucial role in hypothalamic control of food intake and diurnal leptin sensitivity in diet-induced obesity.

Role of ketone signaling in the hepatic response to fasting.

Ketosis is a metabolic adaptation to fasting, nonalcoholic fatty liver disease (NAFLD), and prolonged exercise. ß-OH butyrate acts as a transcriptional regulator and at G protein-coupled receptors to modulate cellular signaling pathways in a hormone-like manner. While physiological ketosis is often adaptive, chronic hyperketonemia may contribute to the metabolic dysfunction of NAFLD. To understand how ß-OH butyrate signaling affects hepatic metabolism, we compared the hepatic fasting response in control and 3-hydroxy-3-methylglutaryl-CoA synthase II (HMGCS2) knockdown mice that are unable to elevate ß-OH butyrate production. To establish that rescue of ketone metabolic/endocrine signaling would restore the normal hepatic fasting response, we gave intraperitoneal injections of ß-OH butyrate (5.7 mmol/kg) to HMGCS2 knockdown and control mice every 2 h for the final 9 h of a 16-h fast. In hypoketonemic, HMGCS2 knockdown mice, fasting more robustly increased mRNA expression of uncoupling protein 2 (UCP2), a protein critical for supporting fatty acid oxidation and ketogenesis. In turn, exogenous ß-OH butyrate administration to HMGCS2 knockdown mice decreased fasting UCP2 mRNA expression to that observed in control mice. Also supporting feedback at the transcriptional level, ß-OH butyrate lowered the fasting-induced expression of HMGCS2 mRNA in control mice. ß-OH butyrate also regulates the glycemic response to fasting. The fast-induced fall in serum glucose was absent in HMGCS2 knockdown mice but was restored by ß-OH butyrate administration. These data propose that endogenous ß-OH butyrate signaling transcriptionally regulates hepatic fatty acid oxidation and ketogenesis, while modulating glucose tolerance. NEW & NOTEWORTHY Ketogenesis regulates whole body glucose metabolism and ß-OH butyrate produced by the liver feeds back to inhibit hepatic ß-oxidation and ketogenesis during fasting.

Non-Mammalian Vertebrates: Distinct Models to Assess the Role of Ion Gradients in Energy Expenditure.

Animals store metabolic energy as electrochemical gradients. At least 50% of mammalian energy is expended to maintain electrochemical gradients across the inner mitochondrial membrane (H+), the sarcoplasmic reticulum (Ca++), and the plasma membrane (Na+/K+). The potential energy of these gradients can be used to perform work (e.g., transport molecules, stimulate contraction, and release hormones) or can be released as heat. Because ectothermic species adapt their body temperature to the environment, they are not constrained by energetic demands that are required to maintain a constant body temperature. In fact, ectothermic species expend seven to eight times less energy than similarly sized homeotherms. Accordingly, ectotherms adopt low metabolic rates to survive cold, hypoxia, and extreme bouts of fasting that would result in energy wasting, lactic acidosis and apoptosis, or starvation in homeotherms, respectively. Ectotherms have also evolved unique applications of ion gradients to allow for localized endothermy. Endothermic avian species, which lack brown adipose tissue, have been integral in assessing the role of H+ and Ca++ cycling in skeletal muscle thermogenesis. Accordingly, the diversity of non-mammalian vertebrate species allows them to serve as unique models to better understand the role of ion gradients in heat production, metabolic flux, and adaptation to stressors, including obesity, starvation, cold, and hypoxia.

Hepatic lipid accumulation: cause and consequence of dysregulated glucoregulatory hormones.

Fatty liver can be diet, endocrine, drug, virus or genetically induced. Independent of cause, hepatic lipid accumulation promotes systemic metabolic dysfunction. By acting as peroxisome proliferator-activated receptor (PPAR) ligands, hepatic non-esterified fatty acids upregulate expression of gluconeogenic, beta-oxidative, lipogenic and ketogenic genes, promoting hyperglycemia, hyperlipidemia and ketosis. The typical hormonal environment in fatty liver disease consists of hyperinsulinemia, hyperglucagonemia, hypercortisolemia, growth hormone deficiency and elevated sympathetic tone. These endocrine and metabolic changes further encourage hepatic steatosis by regulating adipose tissue lipolysis, liver lipid uptake, de novo lipogenesis (DNL), beta-oxidation, ketogenesis and lipid export. Hepatic lipid accumulation may be induced by 4 separate mechanisms: (1) increased hepatic uptake of circulating fatty acids, (2) increased hepatic de novo fatty acid synthesis, (3) decreased hepatic beta-oxidation and (4) decreased hepatic lipid export. This review will discuss the hormonal regulation of each mechanism comparing multiple physiological models of hepatic lipid accumulation. Nonalcoholic fatty liver disease (NAFLD) is typified by increased hepatic lipid uptake, synthesis, oxidation and export. Chronic hepatic lipid signaling through PPARgamma results in gene expression changes that allow concurrent activity of DNL and beta-oxidation. The importance of hepatic steatosis in driving systemic metabolic dysfunction is highlighted by the common endocrine and metabolic disturbances across many conditions that result in fatty liver. Understanding the mechanisms underlying the metabolic dysfunction that develops as a consequence of hepatic lipid accumulation is critical to identifying points of intervention in this increasingly prevalent disease state.