Central NMU signaling in body weight and energy balance regulation: evidence from NMUR2 deletion and chronic central NMU treatment in mice

The neuromedin U system: Pharmacological implications for the treatment of obesity and binge eating behavior

Neuromedin U (NMU) is a bioactive peptide produced in the gut and in the brain, with a role in multiple physiological processes. NMU acts by binding and activating two G protein coupled receptors (GPCR), the NMU receptor 1 (NMU-R1), which is predominantly expressed in the periphery, and the NMU receptor 2 (NMU-R2), mainly expressed in the central nervous system (CNS). In the brain, NMU and NMU-R2 are consistently present in the hypothalamus, commonly recognized as the main "feeding center". Considering its distribution pattern, NMU revealed to be an important neuropeptide involved in the regulation of food intake, with a powerful anorexigenic ability. This has been observed through direct administration of NMU and by studies using genetically modified animals, which revealed an obesity phenotype when the NMU gene is deleted. Thus, the development of NMU analogs or NMU-R2 agonists might represent a promising pharmacological strategy to treat obese individuals. Furthermore, NMU has been demonstrated to influence the non-homeostatic aspect of food intake, playing a potential role in binge eating behavior. This review aims to discuss and summarize the current literature linking the NMU system with obesity and binge eating behavior, focusing on the influence of NMU on food intake and the neuronal mechanisms underlying its anti-obesity properties. Pharmacological strategies to improve the pharmacokinetic profile of NMU will also be reported.

Unanticipated Characteristics of a Selective, Potent Neuromedin-U Receptor 2 Agonist

Neuromedin-U (NMU) mediates several physiological functions via its two cognate receptors, NMUR1 and NMUR2. Disentangling the individual roles of each receptor has largely been undertaken through the use of transgenic mice bearing a deletion in one of the two receptors or by testing native molecules (NMU or its truncated version NMU-8) in a tissue-specific manner, in effect, taking advantage of the distinct receptor expression profiles. These strategies have proved quite useful despite the inherent limitations of overlapping receptor roles and potential compensatory influences of germline gene deletion. With these considerations in mind, the availability of potent, selective NMU compounds with appropriate pharmacokinetic profiles would advance the capabilities of investigators undertaking such efforts. Here, we evaluate a recently reported NMUR2-selective peptide (compound 17) for its in vitro potency (mouse and human), binding affinity, murine pharmacokinetic properties, and in vivo effects. Despite being designed as an NMUR2 agonist, our results show compound 17 unexpectedly binds but does not have functional activity on NMUR1, thereby acting as an R1 antagonist while simultaneously being a potent NMUR2 agonist. Furthermore, evaluation of compound 17 across all known and orphan G-protein-coupled receptors demonstrates multiple receptor partners beyond NMUR2/R1 binding. These properties need to be appreciated for accurate interpretation of results generated using this molecule and may limit the broader ability of this particular entity in disentangling the physiological role of NMU receptor biology.

Sex differences in white adipose tissue expansion: emerging molecular mechanisms

The escalating prevalence of individuals becoming overweight and obese is a rapidly rising global health problem, placing an enormous burden on health and economic systems worldwide. Whilst obesity has well described lifestyle drivers, there is also a significant and poorly understood component that is regulated by genetics. Furthermore, there is clear evidence for sexual dimorphism in obesity, where overall risk, degree, subtype and potential complications arising from obesity all differ between males and females. The molecular mechanisms that dictate these sex differences remain mostly uncharacterised. Many studies have demonstrated that this dimorphism is unable to be solely explained by changes in hormones and their nuclear receptors alone, and instead manifests from coordinated and highly regulated gene networks, both during development and throughout life. As we acquire more knowledge in this area from approaches such as large-scale genomic association studies, the more we appreciate the true complexity and heterogeneity of obesity. Nevertheless, over the past two decades, researchers have made enormous progress in this field, and some consistent and robust mechanisms continue to be established. In this review, we will discuss some of the proposed mechanisms underlying sexual dimorphism in obesity, and discuss some of the key regulators that influence this phenomenon.

Neuromedin U, a Key Molecule in Metabolic Disorders

Obesity is now a public health concern. The leading cause of obesity is an energy imbalance between ingested and expended calories. The mechanisms of feeding behavior and energy metabolism are regulated by a complex of various kinds of molecules, including anorexigenic and orexigenic neuropeptides. One of these neuropeptides, neuromedin U (NMU), was isolated in the 1980s, and its specific receptors, NMUR1 and NMUR2, were defined in 2000. A series of subsequent studies has revealed many of the physiological roles of the NMU system, including in feeding behavior, energy expenditure, stress responses, circadian rhythmicity, and inflammation. Particularly over the past decades, many reports have indicated that the NMU system plays an essential and direct role in regulating body weight, feeding behavior, energy metabolism, and insulin secretion, which are tightly linked to obesity pathophysiology. Furthermore, another ligand of NMU receptors, NMS (neuromedin S), was identified in 2005. NMS has physiological functions similar to those of NMU. This review summarizes recent observations of the NMU system in relation to the pathophysiology of obesity in both the central nervous systems and the peripheral tissues.

Brain region-specific neuromedin U signalling regulates alcohol-related behaviours and food intake in rodents

Albeit neuromedin U (NMU) attenuates alcohol‐mediated behaviours, its mechanisms of action are poorly defined. Providing that the behavioural effects of alcohol are processed within the nucleus accumbens (NAc) shell, anterior ventral tegmental area (aVTA), and laterodorsal tegmental area (LDTg), we assessed the involvement of NMU signalling in the aforementioned areas on alcohol‐mediated behaviours in rodents. We further examined the expression of NMU and NMU receptor 2 (NMUR2) in NAc and the dorsal striatum of high compared with low alcohol‐consuming rats, as this area is of importance in the maintenance of alcohol use disorder (AUD). Finally, we investigated the involvement of NAc shell, aVTA and LDTg in the consumption of chow and palatable peanut butter, to expand the link between NMU and reward‐related behaviours. We demonstrated here, that NMU into the NAc shell, but not aVTA or LDTg, blocked the ability of acute alcohol to cause locomotor stimulation and to induce memory retrieval of alcohol reward, as well as reduced peanut butter in mice. In addition, NMU into NAc shell decreased alcohol intake in rats. On a molecular level, we found increased NMU and decreased NMUR2 expression in the dorsal striatum in high compared with low alcohol‐consuming rats. Both aVTA and LDTg, rather than NAc shell, were identified as novel sites of action for NMU's anorexigenic properties in mice based on NMU's ability to selectively reduce chow intake when injected to these areas. Collectively, these data indicate that NMU signalling in different brain areas selectively modulates different behaviours.

Neuromedin U induces self-grooming in socially-stimulated mice

Emerging evidence suggest that appetite-regulating peptides modulate social behaviors. We here investigate whether the anorexigenic peptide neuromedin U (NMU) modulates sexual behavior in male mice. However, instead of modulating sexual behaviors, NMU administered into the third ventricle increased self-grooming behavior. In addition, NMU-treatment increased self-grooming behavior when exposed to other mice or olfactory social-cues, but not when exposed to non-social environments. As the neuropeptide oxytocin is released during social investigation and exogenous oxytocin induces self-grooming, its role in NMU-induced self-grooming behavior was investigated. In line with our hypothesis, the oxytocin receptor antagonist inhibited NMU-induced self-grooming behavior in mice exposed to olfactory social-cues. Moreover, dopamine in the mesocorticolimbic system is known to be a key regulator of self-grooming behavior. In line with this, we proved that infusion of NMU into nucleus accumbens increased self-grooming behavior in mice confronted with an olfactory social-cue and that this behavior was inhibited by antagonism of dopamine D2, but not D1/D5, receptors. Moreover repeated NMU treatment enhanced ex vivo dopamine levels and decreased the expression of dopamine D2 receptors in nucleus accumbens in socially housed mice. On the other hand, the olfactory stimuli-dependent NMU-induced self-grooming was not affected by a corticotrophin-releasing hormone antagonist, and NMU-treatment did not influence repetitive behaviors in the marble burying test. In conclusion, our results suggest that NMU treatment and, social cues - potentially triggering oxytocin release - together induce excessive grooming behavior in male mice. The mesolimbic dopamine system, including accumbal dopamine D2 receptors, was identified as a crucial downstream mechanism.

Binge-Type Eating in Rats is Facilitated by Neuromedin U Receptor 2 in the Nucleus Accumbens and Ventral Tegmental Area

Binge-eating disorder (BED) is the most common eating disorder, characterized by rapid, recurrent overconsumption of highly palatable food in a short time frame. BED shares an overlapping behavioral phenotype with obesity, which is also linked to the overconsumption of highly palatable foods. The reinforcing properties of highly palatable foods are mediated by the nucleus accumbens (NAc) and the ventral tegmental area (VTA), which have been implicated in the overconsumption behavior observed in BED and obesity. A potential regulator of binge-type eating behavior is the G protein-coupled receptor neuromedin U receptor 2 (NMUR2). Previous research demonstrated that NMUR2 knockdown potentiates binge-type consumption of high-fat food. We correlated binge-type consumption across a spectrum of fat and carbohydrate mixtures with synaptosomal NMUR2 protein expression in the NAc and VTA of rats. Synaptosomal NMUR2 protein in the NAc demonstrated a strong positive correlation with binge intake of a “lower”-fat (higher carbohydrate) mixture, whereas synaptosomal NMUR2 protein in the VTA demonstrated a strong negative correlation with binge intake of an “extreme” high-fat (0% carbohydrate) mixture. Taken together, these data suggest that NMUR2 may differentially regulate binge-type eating within the NAc and the VTA.

Gut-brain axis and addictive disorders: A review with focus on alcohol and drugs of abuse

Due to the limited efficacy of existing medications for addictive disorders including alcohol use disorder (AUD), the need for additional medications is substantial. Potential new medications for addiction can be identified through investigation of the neurochemical substrates mediating the ability of drugs of abuse such as alcohol to activate the mesolimbic dopamine system. Interestingly, recent studies implicate neuropeptides of the gut-brain axis as modulators of reward and addiction processes. The present review therefore summarizes the current studies investigating the ability of the gut-brain peptides ghrelin, glucagon-like peptide-1 (GLP-1), amylin and neuromedin U (NMU) to modulate alcohol- and drug-related behaviors in rodents and humans. Extensive literature demonstrates that ghrelin, the only known orexigenic neuropeptide to date, enhances reward as well as the intake of alcohol, and other drugs of abuse, while ghrelin receptor antagonism has the opposite effects. On the other hand, the anorexigenic peptides GLP-1, amylin and NMU independently inhibits reward from alcohol and drugs of abuse in rodents. Collectively, these rodent and human studies imply that central ghrelin, GLP-1, amylin and NMU signaling may contribute to addiction processes. Therefore, the need for randomized clinical trials investigating the effects of agents targeting these aforementioned systems on drug/alcohol use is substantial.

Small-Molecule Neuromedin U Receptor 2 Agonists Suppress Food Intake and Decrease Visceral Fat in Animal Models

Obesity is a growing public health concern, with 37.5% of the adult population in need of therapeutics that are more efficacious with a better side effect profile. An innovative target in this regard is neuromedin U, a neuropeptide shown to suppress food intake and attenuate weight gain in animal models. These effects of neuromedin U on feeding behavior are thought to be related to agonism at the centrally expressed neuromedin U receptor 2 (NMUR2). As peptides present unique challenges that limit their therapeutic potential, the discovery of small-molecule NMUR2 agonists is needed to validate the targets therapeutic value, but to date, none have been evaluated in any animal model of disease. We therefore assessed two small-molecule NMUR2 agonists for their in vitro signaling and their in vivo efficacy. The NMUR2 agonists were synthesized and both NMUR2 agonists, NY0116 and NY0128, decreased cAMP while stimulating calcium signaling in stably expressing NMUR2 HEK293 cells. When small-molecule NMUR2 agonists were tested in vivo, acute administration significantly decreased high-fat diet consumption. Repeated administration of the compounds decreased body weight and more specifically, decreased the percentage of visceral adipose tissue (VAT) in obese mice. These results have confirmed small-molecule NMUR2 agonists are efficacious in animal models to decrease fat content, food intake, and body weight, suggesting NMUR2 is a promising therapeutic target for metabolic disorders.

Incubation of feeding behavior is regulated by neuromedin U receptor 2 in the paraventricular nucleus of the hypothalamus

A diet of energy-dense food, characterized mainly as a high-fat diet, leads to a persistent excessive consumption defined as overeating. According to the National Institute of Health, more than 2 in 3 adults in the United States are overweight or obese, straining our healthcare system with epidemic proportions. Diets that include abstaining from high-fat foods, ironically, result in an increase in motivation and craving for said high-fat foods, defined as an incubation effect because the behavior aids in developing overeating. Previously, we have shown that modulation of neuromedin U receptor 2 (NMUR2) in the paraventricular nucleus of the hypothalamus (PVN) results in increased food intake and motivation for energy-dense foods. Here, we continue our focus on NMUR2 in the PVN, but in relation to the incubation effect on craving for high-fat food. We employed a model for incubation of craving by having rats abstain from high-fat foods for 30 days before undergoing intake of fatty food on fixed ratio and progressive ratio schedules of reinforcement, and then assess their response to reactivity to cues. Using this model, we compared the feeding behaviors of rats that underwent an mRNA knockdown of the NMUR2 in the PVN to controls after both underwent a 30-day abstinence from high-fat foods. Our results show knockdown of NMUR2 in the PVN blocks the incubation of feeding behavior for food-related cues and high-fat foods.

Cloning and expression patterns of neuromedin U and its receptors in pigs

Neuromedin U (NMU) is a highly conserved neuropeptide that performs a variety of physiological functions in animals via neuromedin U receptor-1 (NMUR1) and neuromedin U receptor-2 (NMUR2). In this study, we cloned the pig NMU, NMUR1 and NMUR2 genes. Bioinformatics analysis demonstrated that the pig NMU cDNA encoded the amino acids Phe-Leu-Phe-Arg-Pro-Arg-Asn-NH2 at the C-terminus and that the NMU receptors, which are G-protein-coupled receptors (GPCRs), contained the seven transmembrane domains typical of GPCRs. Systemic NMU, NMUR1 and NMUR2 mRNA expression was investigated in various pig tissues using real-time RT-PCR. NMU mRNA was expressed both in the central nervous system (CNS) and in peripheral tissues. NMUR1 mRNA was widely expressed in peripheral tissues, whereas NMUR2 mRNA was mainly expressed in the CNS. Immunohistochemistry (IHC) was used to determine the expression patterns of NMU and NMUR1, which were predominantly located in the gastrointestinal tract, genitourinary organs, and immune organs. This study presents molecular and morphological data to aid in additional NMU research in pigs.

Central administration of the anorexigenic peptide neuromedin U decreases alcohol intake and attenuates alcohol-induced reward in rodents

By investigating the neurochemical mechanisms through which alcohol activates the brain reward systems, novel treatment strategies for alcohol use disorder (AUD), a chronic relapsing disease, can be developed. In contrast to the common view of the function of gut-brain peptides, such as neuromedin U (NMU), to regulate food intake and appetite, a novel role in reinforcement mediation has been implied. The anorexigenic effects of NMU are mediated via NMU2 receptors, preferably in the arcuate nucleus and paraventricular nucleus. The expression of NMU2 receptors is also expressed in several reward-related areas in the brain, suggesting a role in reward regulation. The present experiments were therefore set up to investigate the effect of intracerebroventricular administration of NMU on alcohol-mediated behaviors in rodents. We found that central administration of NMU attenuated alcohol-induced locomotor stimulation, accumbal dopamine release and the expression of conditioned place preference in mice. In addition, NMU dose dependently decreased alcohol intake in high, but not in low, alcohol-consuming rats. Central NMU administration did not alter the blood alcohol concentrations nor change the corticosterone levels in rodents. Given that AUD is a major health-care challenge causing an enormous cost to society and novel treatment strategies are warranted, our data suggest that NMU analogues deserve to be evaluated as novel treatment of AUD in humans.

The Anorexigenic Peptide Neuromedin U (NMU) Attenuates Amphetamine-Induced Locomotor Stimulation, Accumbal Dopamine Release and Expression of Conditioned Place Preference in Mice

Amphetamine dependence, besides its substantial economical consequence, is a serious cause of mortality and morbidity. By investigations of the neurochemical correlates through which addictive drugs, such as amphetamine, activate the mesoaccumbal dopamine system unique targets for treatment of drug addiction can be identified. This reward link consists of a dopamine projection from the ventral tegmental area to the nucleus accumbens (NAc) suggesting that these brain areas are important for reward. The physiological function of gut-brain peptides has expanded beyond food intake modulation and involves regulation of drug reinforcement. A novel candidate for reward regulation is the anorexigenic peptide neuromedin U (NMU). We therefore investigated the effects of intracerebroventricular (icv) administration of NMU on amphetamine’s well-documented effects on the mesoaccumbal dopamine system, i.e. locomotor stimulation and accumbal dopamine release in mice. In addition, the effect of accumbal NMU administration on locomotor activity was examined. The effect of NMU, icv or intra-NAc, on the expression of conditioned place preference (CPP) was elucidated. Firstly, we showed that icv administration of NMU attenuate the amphetamine-induced locomotor stimulation, accumbal dopamine release and expression of CPP in mice. Secondly, we found that a lower dose of NMU (icv) reduce the amphetamine-induced locomotor stimulation in mice. Thirdly, we demonstrated that NMU administration into the NAc block the ability of amphetamine to cause a locomotor stimulation in mice. However, accumbal NMU administration did not attenuate the amphetamine-induced expression of CPP in mice. Our novel data suggest that central NMU signalling is involved in development of amphetamine dependence.

A Hamster Model of Diet-Induced Obesity for Preclinical Evaluation of Anti-Obesity, Anti-Diabetic and Lipid Modulating Agents

Aim

Unlike rats and mice, hamsters develop hypercholesterolemia, and hypertriglyceridemia when fed a cholesterol-rich diet. Because hyperlipidemia is a hallmark of human obesity, we aimed to develop and characterize a novel diet-induced obesity (DIO) and hypercholesterolemia Golden Syrian hamster model.

Methods and Results

Hamsters fed a highly palatable fat- and sugar-rich diet (HPFS) for 12 weeks showed significant body weight gain, body fat accumulation and impaired glucose tolerance. Cholesterol supplementation to the diet evoked additional hypercholesterolemia. Chronic treatment with the GLP-1 analogue, liraglutide (0.2 mg/kg, SC, BID, 27 days), normalized body weight and glucose tolerance, and lowered blood lipids in the DIO-hamster. The dipeptidyl peptidase-4 (DPP-4) inhibitor, linagliptin (3.0 mg/kg, PO, QD) also improved glucose tolerance. Treatment with peptide YY_3-36 (PYY_3-36, 1.0 mg/kg/day) or neuromedin U (NMU, 1.5 mg/kg/day), continuously infused via a subcutaneous osmotic minipump for 14 days, reduced body weight and energy intake and changed food preference from HPFS diet towards chow. Co-treatment with liraglutide and PYY_3-36 evoked a pronounced synergistic decrease in body weight and food intake with no lower plateau established. Treatment with the cholesterol uptake inhibitor ezetimibe (10 mg/kg, PO, QD) for 14 days lowered plasma total cholesterol with a more marked reduction of LDL levels, as compared to HDL, indicating additional sensitivity to cholesterol modulating drugs in the hyperlipidemic DIO-hamster. In conclusion, the features of combined obesity, impaired glucose tolerance and hypercholesterolemia in the DIO-hamster make this animal model useful for preclinical evaluation of novel anti-obesity, anti-diabetic and lipid modulating agents.

Neuromedin U receptor 2 knockdown in the paraventricular nucleus modifies behavioral responses to obesogenic high-fat food and leads to increased body weight

Neuromedin U (NMU) is a highly conserved neuropeptide which regulates food intake and body weight. Transgenic mice lacking NMU are hyperphagic and obese, making NMU a novel target for understanding and treating obesity. Neuromedin U receptor 2 (NMUR2) is a high-affinity receptor for NMU found in discrete regions of the central nervous system, in particular the paraventricular nucleus of the hypothalamus (PVN), where it may be responsible for mediating the anorectic effects of NMU. We hypothesized that selective knock down of NMUR2 in the PVN of rats would increase their sensitivity to the reinforcing properties of food resulting in increased intake and preference for high-fat obesogenic food. To this end, we used viral-mediated RNAi to selectively knock down NMUR2 gene expression in the PVN. In rats fed a standard chow, NMUR2 knockdown produced no significant effect on food intake or body weight. However, when the same rats were fed a high-fat diet (45% fat), they consumed significantly more food, gained more body weight, and had increased feed efficiency relative to controls. Furthermore, NMUR2 knockdown rats demonstrated significantly greater binge-type food consumption of the high-fat diet and showed a greater preference for higher-fat food. These results demonstrate that NMUR2 signaling in the PVN regulates consumption and preference for high-fat foods without disrupting feeding behavior associated with non-obesogenic standard chow.

Obesity pharmacotherapy: current perspectives and future directions

The rising tide of obesity and its related disorders is one of the most pressing health concerns worldwide, yet existing medicines to combat the problem are disappointingly limited in number and effectiveness. Recent advances in mechanistic insights into the neuroendocrine regulation of body weight have revealed an expanding list of molecular targets for novel, rationally designed antiobesity pharmaceutical agents. Antiobesity drugs act via any of four mechanisms: 1) decreasing energy intake, 2) increasing energy expenditure or modulating lipid metabolism, 3) modulating fat stores or adipocyte differentiation, and 4) mimicking caloric restriction. Various novel drug candidates and targets directed against obesity are currently being explored. A few of them are also in the later phases of clinical trials. This review discusses the development of novel antiobesity drugs based on current understanding of energy homeostasis

Neuropeptides controlling energy balance: orexins and neuromedins

In this chapter, we review the feeding and energy expenditure effects of orexin (also known as hypocretin) and neuromedin. Orexins are multifunctional neuropeptides that affect energy balance by participating in regulation of appetite, arousal, and spontaneous physical activity. Central orexin signaling for all functions originates in the lateral hypothalamus-perifornical area and is likely functionally differentiated based on site of action and on interacting neural influences. The effect of orexin on feeding is likely related to arousal in some ways but is nonetheless a separate neural process that depends on interactions with other feeding-related neuropeptides. In a pattern distinct from other neuropeptides, orexin stimulates both feeding and energy expenditure. Orexin increases in energy expenditure are mainly by increasing spontaneous physical activity, and this energy expenditure effect is more potent than the effect on feeding. Global orexin manipulations, such as in transgenic models, produce energy balance changes consistent with a dominant energy expenditure effect of orexin. Neuromedins are gut-brain peptides that reduce appetite. There are gut sources of neuromedin, but likely the key appetite-related neuromedin-producing neurons are in the hypothalamus and parallel other key anorectic neuropeptide expression in the arcuate to paraventricular hypothalamic projection. As with other hypothalamic feeding-related peptides, hindbrain sites are likely also important sources and targets of neuromedin anorectic action. Neuromedin increases physical activity in addition to reducing appetite, thus producing a consistent negative energy balance effect. Together with the other various neuropeptides, neurotransmitters, neuromodulators, and neurohormones, neuromedin and orexin act in the appetite network to produce changes in food intake and energy expenditure, which ultimately influences the regulation of body weight.