The melanocortin-3 receptor is a pharmacological target for the regulation of anorexia

Subthreshold activation of the melanocortin system causes generalized sensitization to anorectic agents in mice

The melanocortin-3 receptor (MC3R) regulates GABA release from agouti-related protein (AgRP) nerve terminals and thus tonically suppresses multiple circuits involved in feeding behavior and energy homeostasis. Here, we examined the role of the MC3R and the melanocortin system in regulating the response to various anorexigenic agents. The genetic deletion or pharmacological inhibition of the MC3R, or subthreshold doses of an MC4R agonist, improved the dose responsiveness to glucagon-like peptide 1 (GLP1) agonists, as assayed by inhibition of food intake and weight loss. An enhanced anorectic response to the acute satiety factors peptide YY (PYY3-36) and cholecystokinin (CCK) and the long-term adipostatic factor leptin demonstrated that increased sensitivity to anorectic agents was a generalized result of MC3R antagonism. We observed enhanced neuronal activation in multiple hypothalamic nuclei using Fos IHC following low-dose liraglutide in MC3R-KO mice (Mc3r-/-), supporting the hypothesis that the MC3R is a negative regulator of circuits that control multiple aspects of feeding behavior. The enhanced anorectic response in Mc3r-/- mice after administration of GLP1 analogs was also independent of the incretin effects and malaise induced by GLP1 receptor (GLP1R) analogs, suggesting that MC3R antagonists or MC4R agonists may have value in enhancing the dose-response range of obesity therapeutics.

Hypothalamic hormone deficiency enables physiological anorexia in ground squirrels during hibernation

Mammalian hibernators survive prolonged periods of cold and resource scarcity by temporarily modulating normal physiological functions, but the mechanisms underlying these adaptations are poorly understood. The hibernation cycle of thirteen-lined ground squirrels (Ictidomys tridecemlineatus) lasts for 5-7 months and comprises weeks of hypometabolic, hypothermic torpor interspersed with 24-48-h periods of an active-like interbout arousal (IBA) state. We show that ground squirrels, who endure the entire hibernation season without food, have negligible hunger during IBAs. These squirrels exhibit reversible inhibition of the hypothalamic feeding center, such that hypothalamic arcuate nucleus neurons exhibit reduced sensitivity to the orexigenic and anorexigenic effects of ghrelin and leptin, respectively. However, hypothalamic infusion of thyroid hormone during an IBA is sufficient to rescue hibernation anorexia. Our results reveal that thyroid hormone deficiency underlies hibernation anorexia and demonstrate the functional flexibility of the hypothalamic feeding center.

Neuroanatomical dissection of the MC3R circuitry regulating energy rheostasis

Although mammals resist both acute weight loss and weight gain, the neural circuitry mediating bi-directional defense against weight change is incompletely understood. Global constitutive deletion of the melanocortin-3-receptor (MC3R) impairs the behavioral response to both anorexic and orexigenic stimuli, with MC3R knockout mice demonstrating increased weight gain following anabolic challenges and increased weight loss following anorexic challenges (i.e. impaired energy rheostasis). However, the brain regions mediating this phenotype remain incompletely understood. Here, we utilized MC3R floxed mice and viral injections of Cre-recombinase to selectively delete MC3R from medial hypothalamus (MH) in adult mice. Behavioral assays were performed on these animals to test the role of MC3R in MH in the acute response to orexigenic and anorexic challenges. Complementary chemogenetic approaches were used in MC3R-Cre mice to localize and characterize the specific medial hypothalamic brain regions mediating the role of MC3R in energy homeostasis. Finally, we performed RNAscope in situ hybridization to map changes in the mRNA expression of MC3R, POMC, and AgRP following energy rheostatic challenges. Our results demonstrate that MC3R deletion in MH increased feeding and weight gain following acute high fat diet feeding in males, and enhanced the anorexic effects of semaglutide, in a sexually dimorphic manner. Additionally, activation of DMH MC3R neurons increased energy expenditure and locomotion. Together, these results demonstrate that MC3R mediated effects on energy rheostasis result from the loss of MC3R signaling in the medial hypothalamus of adult animals and suggest an important role for DMH MC3R signaling in energy rheostasis.

Hypothalamic hormone deficiency enables physiological anorexia

Mammalian hibernators survive prolonged periods of cold and resource scarcity by temporarily modulating normal physiological functions, but the mechanisms underlying these adaptations are poorly understood. The hibernation cycle of thirteen-lined ground squirrels (Ictidomys tridecemlineatus) lasts for 5-7 months and comprises weeks of hypometabolic, hypothermic torpor interspersed with 24-48-hour periods of an active-like interbout arousal (IBA) state. We show that ground squirrels, who endure the entire hibernation season without food, have negligible hunger during IBAs. These squirrels exhibit reversible inhibition of the hypothalamic feeding center, such that hypothalamic arcuate nucleus neurons exhibit reduced sensitivity to the orexigenic and anorexigenic effects of ghrelin and leptin, respectively. However, hypothalamic infusion of thyroid hormone during an IBA is sufficient to rescue hibernation anorexia. Our results reveal that thyroid hormone deficiency underlies hibernation anorexia and demonstrate the functional flexibility of the hypothalamic feeding center.

Novel Cocrystal Structures of Peptide Antagonists Bound to the Human Melanocortin Receptor 4 Unveil Unexplored Grounds for Structure-Based Drug Design

Melanocortin 4 receptor (MC4-R) antagonists are actively sought for treating cancer cachexia. We determined the structures of complexes with PG-934 and SBL-MC-31. These peptides differ from SHU9119 by substituting His6 with Pro6 and inserting Gly10 or Arg10. The structures revealed two subpockets at the TM7-TM1-TM2 domains, separated by N2857.36. Two peptide series based on the complexed peptides led to an antagonist activity and selectivity SAR study. Most ligands retained the SHU9119 potency, but several SBL-MC-31-derived peptides significantly enhanced MC4-R selectivity over MC1-R by 60- to 132-fold. We also investigated MC4-R coupling to the K+ channel, Kir7.1. Some peptides activated the channel, whereas others induced channel closure independently of G protein coupling. In cell culture studies, channel activation correlated with increased feeding, while a peptide with Kir7.1 inhibitory activity reduced eating. These results highlight the potential for targeting the MC4-R:Kir7.1 complex for treating positive and restrictive eating disorders.

Chronic Nicotine Consumption and Withdrawal Regulate Melanocortin Receptor, CRF, and CRF Receptor mRNA Levels in the Rat Brain

Alterations in the various neuropeptide systems in the mesocorticolimbic circuitry have been implicated in negative effects associated with drug withdrawal. The corticotropin-releasing factor (CRF) and α-melanocyte-stimulating hormone are two peptides that may be involved. This study investigated the regulatory effects of chronic nicotine exposure and withdrawal on the mRNA levels of melanocortin receptors (MC3R, MC4R), CRF, and CRF receptors (CRFR1 and CRFR2) expressed in the mesocorticolimbic system. Rats were given drinking water with nicotine or without nicotine (control group) for 12 weeks, after which they continued receiving nicotine (chronic exposure) or were withdrawn from nicotine for 24 or 48 h. The animals were decapitated following behavioral testing for withdrawal signs. Quantitative real-time PCR analysis demonstrated that nicotine exposure (with or without withdrawal) increased levels of CRF and CRFR1 mRNA in the amygdala, CRF mRNA in the medial prefrontal cortex, and CRFR1 mRNA in the septum. Nicotine withdrawal also enhanced MC3R and MC4R mRNA levels in different brain regions, while chronic nicotine exposure was associated with increased MC4R mRNA levels in the nucleus accumbens. These results suggest that chronic nicotine exposure and withdrawal regulate CRF and melanocortin signaling in the mesocorticolimbic system, possibly contributing to negative affective state and nicotine addiction.

Inhibition of the melanocortin-3 receptor (MC3R) causes generalized sensitization to anorectic agents

The melanocortin-3 receptor (MC3R) acts presynaptically to regulate GABA release from agouti-related protein (AgRP) nerve terminals and thus may be a negative regulator of multiple circuits involved in feeding behavior and energy homeostasis. Here, we examined the role of MC3R in regulating the response to various anorexigenic agents. Our findings reveal that genetic deletion or pharmacological inhibition of MC3R improves the dose responsiveness to Glucagon-like peptide 1 (GLP1) agonists, as assayed by inhibition of food intake and weight loss. An enhanced anorectic response to other agents, including the acute satiety factors peptide YY (PYY3-36) and cholecystokinin (CCK) and the long-term adipostatic factor, leptin, demonstrated that increased sensitivity to anorectic agents is a generalized result of MC3R antagonism. Enhanced neuronal activation in multiple nuclei, including ARH, VMH, and DMH, was observed using Fos immunohistochemistry following low-dose liraglutide in MC3R knockout mice (Mc3r-/-), supporting the hypothesis that the MC3R is a negative regulator of circuits regulating multiple aspects of feeding behavior. The enhanced anorectic response in Mc3r -/- mice after administration of GLP1 analogs was also independent of the incretin effects and malaise induced by GLP1R analogs, suggesting that MC3R antagonists may have value in enhancing the dose-response range of obesity therapeutics.

Prevalence of Deleterious Variants in MC3R in Patients With Constitutional Delay of Growth and Puberty

CONTEXT

The melanocortin 3 receptor (MC3R) has recently emerged as a critical regulator of pubertal timing, linear growth, and the acquisition of lean mass in humans and mice. In population-based studies, heterozygous carriers of deleterious variants in MC3R report a later onset of puberty than noncarriers. However, the frequency of such variants in patients who present with clinical disorders of pubertal development is currently unknown.

OBJECTIVE

This work aimed to determine whether deleterious MC3R variants are more frequently found in patients clinically presenting with constitutional delay of growth and puberty (CDGP) or normosmic idiopathic hypogonadotropic hypogonadism (nIHH).

METHODS

We examined the sequence of MC3R in 362 adolescents with a clinical diagnosis of CDGP and 657 patients with nIHH, experimentally characterized the signaling properties of all nonsynonymous variants found and compared their frequency to that in 5774 controls from a population-based cohort. Additionally, we established the relative frequency of predicted deleterious variants in individuals with self-reported delayed vs normally timed menarche/voice-breaking in the UK Biobank cohort.

RESULTS

MC3R loss-of-function variants were infrequent but overrepresented in patients with CDGP (8/362 [2.2%]; OR = 4.17; P = .001). There was no strong evidence of overrepresentation in patients with nIHH (4/657 [0.6%]; OR = 1.15; P = .779). In 246 328 women from the UK Biobank, predicted deleterious variants were more frequently found in those self-reporting delayed (aged ≥16 years) vs normal age at menarche (OR = 1.66; P = 3.90E-07).

CONCLUSION

We have found evidence that functionally damaging variants in MC3R are overrepresented in individuals with CDGP but are not a common cause of this phenotype.

Melanocortin-3 receptor expression in AgRP neurons is required for normal activation of the neurons in response to energy deficiency

The melanocortin-3 receptor (MC3R) is a negative regulator of the central melanocortin circuitry via presynaptic expression on agouti-related protein (AgRP) nerve terminals, from where it regulates GABA release onto secondary MC4R-expressing neurons. However, MC3R knockout (KO) mice also exhibit defective behavioral and neuroendocrine responses to fasting. Here, we demonstrate that MC3R KO mice exhibit defective activation of AgRP neurons in response to fasting, cold exposure, or ghrelin while exhibiting normal inhibition of AgRP neurons by sensory detection of food in the ad libitum-fed state. Using a conditional MC3R KO model, we show that the control of AgRP neuron activation by fasting and ghrelin requires the specific presence of MC3R within AgRP neurons. Thus, MC3R is a crucial player in the responsiveness of the AgRP soma to both hormonal and neuronal signals of energy need.

Label-Free Quantitative Thermal Proteome Profiling Reveals Target Transcription Factors with Activities Modulated by MC3R Signaling

Thermal proteome profiling with label-free quantitation using ion-mobility-enhanced LC-MS offers versatile data sets, providing information on protein differential expression, thermal stability, and the activities of transcription factors. We developed a multidimensional data analysis workflow for label-free quantitative thermal proteome profiling (TPP) experiments that incorporates the aspects of gene set enrichment analysis, differential protein expression analysis, and inference of transcription factor activities from LC-MS data. We applied it to study the signaling processes downstream of melanocortin 3 receptor (MC3R) activation by endogenous agonists derived from the proopiomelanocortin prohormone: ACTH, α-MSH, and γ-MSH. The obtained information was used to map signaling pathways downstream of MC3R and to deduce transcription factors responsible for cellular response to ligand treatment. Using our workflow, we identified differentially expressed proteins and investigated their thermal stability. We found in total 298 proteins with altered thermal stability, resulting from MC3R activation. Out of these, several proteins were transcription factors, indicating them as being downstream target regulators that take part in the MC3R signaling cascade. We found transcription factors CCAR2, DDX21, HMGB2, SRSF7, and TET2 to have altered thermal stability. These apparent target transcription factors within the MC3R signaling cascade play important roles in immune responses. Additionally, we inferred the activities of the transcription factors identified in our data set. This was done with Bayesian statistics using the differential expression data we obtained with label-free quantitative LC-MS. The inferred transcription factor activities were validated in our bioinformatic pipeline by the phosphorylated peptide abundances that we observed, highlighting the importance of post-translational modifications in transcription factor regulation. Our multidimensional data analysis workflow allows for a comprehensive characterization of the signaling processes downstream of MC3R activation. It provides insights into protein differential expression, thermal stability, and activities of key transcription factors. All proteomic data generated in this study are publicly available at DOI: 10.6019/PXD039945.

Chemotherapy-induced gastrointestinal toxicity: Pathogenesis and current management

Chemotherapy is the most common treatment for malignant tumors. However, chemotherapy-induced gastrointestinal toxicity (CIGT) has been a major concern for cancer patients, which reduces their quality of life and leads to treatment intolerance and even cessation. Nevertheless, prevention and treatment for CIGT are challenging, due to the prevalence and complexity of the condition. Chemotherapeutic drugs directly damage gastrointestinal mucosa to induce CIGT, including nausea, vomiting, anorexia, gastrointestinal mucositis, and diarrhea, etc. The pathogenesis of CIGT involves multiple factors, such as gut microbiota disorders, inflammatory responses and abnormal neurotransmitter levels, that synergistically contribute to its occurrence and development. In particular, the dysbiosis of gut microbiota is usually linked to abnormal immune responses that increases inflammatory cytokines' expression, which is a common characteristic of many types of CIGT. Chemotherapy-induced intestinal neurotoxicity is also a vital concern in CIGT. Currently, modern medicine is the dominant treatment of CIGT, however, traditional Chinese medicine (TCM) has attracted interest as a complementary and alternative therapy that can greatly alleviate CIGT. Accordingly, this review aimed to comprehensively summarize the pathogenesis and current management of CIGT using PubMed and Google Scholar databases, and proposed that future research for CIGT should focus on the gut microbiota, intestinal neurotoxicity, and promising TCM therapies, which may help to develop more effective interventions and optimize managements of CIGT.

Paraventricular Thalamic MC3R Circuits Link Energy Homeostasis with Anxiety-Related Behavior

The hypothalamic melanocortin system is critically involved in sensing stored energy and communicating this information throughout the brain, including to brain regions controlling motivation and emotion. This system consists of first-order agouti-related peptide (AgRP) and pro-opiomelanocortin (POMC) neurons located in the hypothalamic arcuate nucleus and downstream neurons containing the melanocortin-3 (MC3R) and melanocortin-4 receptor (MC4R). Although extensive work has characterized the function of downstream MC4R neurons, the identity and function of MC3R-containing neurons are poorly understood. Here, we used neuroanatomical and circuit manipulation approaches in mice to identify a novel pathway linking hypothalamic melanocortin neurons to melanocortin-3 receptor neurons located in the paraventricular thalamus (PVT) in male and female mice. MC3R neurons in PVT are innervated by hypothalamic AgRP and POMC neurons and are activated by anorexigenic and aversive stimuli. Consistently, chemogenetic activation of PVT MC3R neurons increases anxiety-related behavior and reduces feeding in hungry mice, whereas inhibition of PVT MC3R neurons reduces anxiety-related behavior. These studies position PVT MC3R neurons as important cellular substrates linking energy status with neural circuitry regulating anxiety-related behavior and represent a promising potential target for diseases at the intersection of metabolism and anxiety-related behavior such as anorexia nervosa.SIGNIFICANCE STATEMENT Animals must constantly adapt their behavior to changing internal and external challenges, and impairments in appropriately responding to these challenges are a hallmark of many neuropsychiatric disorders. Here, we demonstrate that paraventricular thalamic neurons containing the melanocortin-3 receptor respond to energy-state-related information and external challenges to regulate anxiety-related behavior in mice. Thus, these neurons represent a potential target for understanding the neurobiology of disorders at the intersection of metabolism and psychiatry such as anorexia nervosa.

Targeting the central melanocortin system for the treatment of metabolic disorders

A large body of preclinical and clinical data shows that the central melanocortin system is a promising therapeutic target for treating various metabolic disorders such as obesity and cachexia, as well as anorexia nervosa. Setmelanotide, which functions by engaging the central melanocortin circuitry, was approved by the FDA in 2020 for use in certain forms of syndromic obesity. Furthermore, the FDA approvals in 2019 of two peptide drugs targeting melanocortin receptors for the treatment of generalized hypoactive sexual desire disorder (bremelanotide) and erythropoietic protoporphyria-associated phototoxicity (afamelanotide) demonstrate the safety of this class of peptides. These approvals have also renewed excitement in the development of therapeutics targeting the melanocortin system. Here, we review the anatomy and function of the melanocortin system, discuss progress and challenges in developing melanocortin receptor-based therapeutics, and outline potential metabolic and behavioural disorders that could be addressed using pharmacological agents targeting these receptors.

Alpha-melanocyte stimulating hormone (α-MSH): biology, clinical relevance and implication in melanoma

Alpha-melanocyte stimulating hormone (α-MSH) and its receptor, melanocortin 1 receptor (MC1R), have been proposed as potential target for anti-cancer strategies in melanoma research, due to their tissue specific expression and involvement in melanocyte homeostasis. However, their role in prevention and treatment of melanoma is still debated and controversial. Although a large body of evidence supports α-MSH in preventing melanoma development, some preclinical findings suggest that the α-MSH downstream signalling may promote immune escape and cancer resistance to therapy. Additionally, in metastatic melanoma both MC1R and α-MSH have been reported to be overexpressed at levels much higher than normal cells. Furthermore, targeted therapy (e.g. BRAF inhibition in BRAFV600E mutant tumours) has been shown to enhance this phenomenon. Collectively, these data suggest that targeting MC1R could serve as an approach in the treatment of metastatic melanoma. In this review, we explore the molecular biology of α-MSH with particular emphasis into its tumor-related properties, whilst elaborating the experimental evidence currently available regarding the interplay between α-MSH/MC1R axis, melanoma and antitumor strategies.

Cell autonomous regulation of the activation of AgRP neurons by the melanocortin-3 receptor

The melanocortin-3 receptor (MC3R) is a negative regulator of the central melanocortin circuitry via presynaptic expression on AgRP nerve terminals, from where it regulates GABA release onto secondary MC4R-expressing neurons. Hence, animals lacking MC3R (MC3R KO) exhibit hypersensitivity to MC4R agonists. However, MC3R KO mice also exhibit defective behavioral and neuroendocrine responses to fasting. Here, we demonstrate that MC3R KO mice exhibit defective activation of AgRP neurons in response to fasting and cold exposure, while exhibiting normal inhibition of AgRP neurons by sensory detection of food. Further, using an AgRP-specific MC3R knockout model, we show that the control of AgRP neuron activation by MC3R is cell-autonomous. One mechanism underlying this involves the response to ghrelin, which is also blunted in mice with AgRP-specific deletion of the MC3R. Thus, MC3R is a crucial player in the control of energy homeostasis by the central melanocortin system, not only acting presynaptically on AgRP neurons, but via AgRP cell-autonomous regulation of fasting- and cold-induced neuronal activation as well.

The Parallel Structure-Activity Relationship Screening of Three Compounds Identifies the Common Agonist Pharmacophore of Pyrrolidine Bis-Cyclic Guanidine Melanocortin-3 Receptor (MC3R) Small-Molecule Ligands

The melanocortin receptors are involved in numerous physiological pathways, including appetite, skin and hair pigmentation, and steroidogenesis. In particular, the melanocortin-3 receptor (MC3R) is involved in fat storage, food intake, and energy homeostasis. Small-molecule ligands developed for the MC3R may serve as therapeutic lead compounds for treating disease states of energy disequilibrium. Herein, three previously reported pyrrolidine bis-cyclic guanidine compounds with five sites for molecular diversity (R1-R5) were subjected to parallel structure-activity relationship studies to identify the common pharmacophore of this scaffold series required for full agonism at the MC3R. The R2, R3, and R5 positions were required for full MC3R efficacy, while truncation of either the R1 or R4 positions in all three compounds resulted in full MC3R agonists. Two additional fragments, featuring molecular weights below 300 Da, were also identified that possessed full agonist efficacy and micromolar potencies at the mMC5R. These SAR experiments may be useful in generating new small-molecule ligands and chemical probes for the melanocortin receptors to help elucidate their roles in vivo and as therapeutic lead compounds.

KNDy neurons as the GnRH pulse generator: Recent studies in ruminants

This review considers three aspects of recent work on the role of KNDy neurons in GnRH pulse generation in ruminants. First, work on basic mechanisms of pulse generation includes several tests of this hypothesis, all of which support it, and evidence that Kiss1r-containing neurons form a positive feedback circuit with the KNDy neural network that strengthen the activity of this network. The second section on pathways mediating external inputs focuses on the influence of nutrition and photoperiod, and describes the evidence supporting roles for proopiomelanocortin (POMC) and agouti-related peptide (AgRP) afferents to KNDy cells in each of these. Finally, we review studies exploring the potential applications of manipulating signaling by kisspeptin, and the other KNDy peptides, to control reproductive function in domestic animals and conclude that, although these approaches show some promise, they do not have major advantages over current practices at this time.

RISING STARS: Targeting G protein-coupled receptors to regulate energy homeostasis

G protein-coupled receptors (GPCRs) have a critical role in energy homeostasis, contributing to food intake, energy expenditure and glycaemic control. Dysregulation of energy expenditure can lead to metabolic syndrome (abdominal obesity, elevated plasma triglyceride, LDL cholesterol and glucose, and high blood pressure), which is associated with an increased risk of developing obesity, diabetes mellitus, non-alcoholic fatty liver disease and cardiovascular complications. As the prevalence of these chronic diseases continues to rise worldwide, there is an increased need to understand the molecular mechanisms by which energy expenditure is regulated to facilitate the development of effective therapeutic strategies to treat and prevent these conditions. In recent years, drugs targeting GPCRs have been the focus of efforts to improve treatments for type-2 diabetes and obesity, with GLP-1R agonists a particular success. In this review, we focus on nine GPCRs with roles in energy homeostasis that are current and emerging targets to treat obesity and diabetes. We discuss findings from pre-clinical models and clinical trials of drugs targeting these receptors and challenges that must be overcome before these drugs can be routinely used in clinics. We also describe new insights into how these receptors signal, including how accessory proteins, biased signalling, and complex spatial signalling could provide unique opportunities to develop more efficacious therapies with fewer side effects. Finally, we describe how combined therapies, in which multiple GPCRs are targeted, may improve clinical outcomes and reduce off-target effects.

The gut signals to AGRP-expressing cells of the pituitary to control glucose homeostasis

Glucose homeostasis can be improved after bariatric surgery, which alters bile flow and stimulates gut hormone secretion, particularly FGF15/19. FGFR1 expression in AGRP-expressing cells is required for bile acids' ability to improve glucose control. We show that the mouse Agrp gene has 3 promoter/enhancer regions that direct transcription of each of their own AGRP transcripts. One of these Agrp promoters/enhancers, Agrp-B, is regulated by bile acids. We generated an Agrp-B knockin FLP/knockout allele. AGRP-B-expressing cells are found in endocrine cells of the pars tuberalis and coexpress diacylglycerol lipase B - an endocannabinoid biosynthetic enzyme - distinct from pars tuberalis thyrotropes. AGRP-B expression is also found in the folliculostellate cells of the pituitary's anterior lobe. Mice without AGRP-B were protected from glucose intolerance induced by high-fat feeding but not from excess weight gain. Chemogenetic inhibition of AGRP-B cells improved glucose tolerance by enhancing glucose-stimulated insulin secretion. Inhibition of the AGRP-B cells also caused weight loss. The improved glucose tolerance and reduced body weight persisted up to 6 weeks after cessation of the DREADD-mediated inhibition, suggesting the presence of a biological switch for glucose homeostasis that is regulated by long-term stability of food availability.

Melanocortin receptor 3 and 4 mRNA expression in the adult female Syrian hamster brain

Melanocortin 3 receptors (MC3R) and melanocortin 4 receptors (MC4R) are vital in regulating a variety of functions across many species. For example, the dysregulation of these receptors results in obesity and dysfunction in sexual behaviors. Only a handful of studies have mapped the expression of MC3R and MC4R mRNA across the central nervous system, with the primary focus on mice and rats. Because Syrian hamsters are valuable models for functions regulated by melanocortin receptors, our current study maps the distribution of MC3R and MC4R mRNA in the Syrian hamster telencephalon, diencephalon, and midbrain using RNAscope. We found that the expression of MC3R mRNA was lowest in the telencephalon and greatest in the diencephalon, whereas the expression of MC4R mRNA was greatest in the midbrain. A comparison of these findings to previous studies found that MC3R and MC4R expression is similar in some brain regions across species and divergent in others. In addition, our study identifies novel brain regions for the expression of MC3Rs and MC4Rs, and identifies cells that co-express bothMC3 and MC4 receptors within certain brain regions.

Gut microbiome-brain interactions in anorexia nervosa: Potential mechanisms and regulatory strategies

Anorexia nervosa (AN) is a psychiatric disorder characterised by malnutrition, fear of weight gain, and body image disturbances. The aetiology of AN is complex, and may involve environmental factors, genetic factors, and biochemical factors, with the latter meaning that AN may be closely associated with neurons, neurotransmitters, and hormones related to appetite and emotional regulation. In addition, an increasing number of studies have shown there is a link between the intestinal microbiota and psychiatric disorders, such as depression. However, few studies and reviews have focused on AN and gut microbes. Accordingly, in this review, we examine the potential pathogenesis of AN in terms of changes in the gut microbiota and its metabolites, and their effects on AN. The neurobiological function of the nervous system in relation to AN are also been mentioned. Furthermore, we suggest future research directions for this field, and note that probiotics may be developed for use as dietary supplements to help alleviate AN in patients.

Establishment of Restraint Stress-induced Anorexia and Social Isolation-induced Anorexia Mouse Models

Anorexia nervosa (AN) is a devastating neuropsychiatric disease with a prevalence rate of approximately 0.3%-1% among women and morbidity and mortality rates among the highest of all neuropsychiatric disorders. The disease etiology is complex but primarily characterized by reduced food intake and body weight, and intense anxiety and fear associated with gaining weight. Existing rodent models of AN are useful and capture features of the disease, but either require specialized genetic mouse models or are difficult to implement in mice. Here, we describe two simple mouse models of stress-induced anorexia that are easy to implement in basic research labs, and capture core features associated with AN, such as reduced food intake in the context of social/physical stress and increased anxiety-related behavior. These protocols provide reproducible and robust assays for stress-induced anorexia and may be implemented with additional assays to probe the neural circuitry mediating the effects of psychological stress on feeding in mice. Graphical abstract.

A thalamic circuit facilitates stress susceptibility via melanocortin 4 receptor-mediated activation of nucleus accumbens shell

AIMS

Central melanocortin 4 receptor (MC4R) has been reported to induce anhedonia via eliciting dysfunction of excitatory synapses. It is evident that metabolic signals are closely related to chronic stress-induced depression. Here, we investigated that a neural circuit is involved in melanocortin signaling contributing to susceptibility to stress.

METHODS

Chronic social defeat stress (CSDS) was used to develop depressive-like behavior. Electrophysiologic and chemogenetic approaches were performed to evaluate the role of paraventricular thalamus (PVT) glutamatergic to nucleus accumbens shell (NAcsh) circuit in stress susceptibility. Pharmacological and genetic manipulations were applied to investigate the molecular mechanisms of melanocortin signaling in the circuit.

RESULTS

CSDS increases the excitatory neurotransmission in NAcsh through MC4R signaling. The enhanced excitatory synaptic input in NAcsh is projected from PVT glutamatergic neurons. Moreover, chemogenetic manipulation of PVT^Glu -NAcsh projection mediates the susceptibility to stress, which is dependent on MC4R signaling. Overall, these results reveal that the strengthened excitatory neurotransmission in NAcsh originates from PVT glutamatergic neurons, facilitating the susceptibility to stress through melanocortin signaling.

CONCLUSIONS

Our results make a strong case for harnessing a thalamic circuit to reorganize excitatory synaptic transmission in relieving stress susceptibility and provide insights gained on metabolic underpinnings of protection against stress-induced depressive-like behavior.

MC4R Deficiency Causes Dysregulation of Postsynaptic Excitatory Synaptic Transmission as a Crucial Culprit for Obesity

Melanocortin 4 receptor (MC4R) in the paraventricular nucleus of the hypothalamus (PVH) shows bidirectional characterization in modulating food intake and energy homeostasis. We demonstrate that MC4R knockdown (KD) in the PVH can attenuate AMPA receptor (AMPAR)-mediated postsynaptic responses by altering the phosphorylation of AMPAR GluA1 subunit through the protein kinase A (PKA)-dependent signaling cascade and simultaneously lead to rapid body weight gain. Furthermore, PKA KD in the PVH engendered similar electrophysiological and behavioral phenotypes as in MC4R KD mice. Importantly, we observed that the reduction of AMPAR GluA1 expression not only led to attenuated synaptic responses but also caused body weight gain, suggesting that the aberration of synaptic responses may be one of the crucial pathogeny of obesity. Our study provides the synaptic and molecular explanations of how body weight is regulated by MC4R in the PVH.

Organization of neural systems expressing melanocortin-3 receptors in the mouse brain: Evidence for sexual dimorphism

The central melanocortin system is fundamentally important for controlling food intake and energy homeostasis. Melanocortin-3 receptor (MC3R) is one of two major receptors of the melanocortin system found in the brain. In contrast to the well-characterized melanocortin-4 receptor (MC4R), little is known regarding the organization of MC3R-expressing neural circuits. To increase our understanding of the intrinsic organization of MC3R neural circuits, identify specific differences between males and females, and gain a neural systems level perspective of this circuitry, we conducted a brain-wide mapping of neurons labeled for MC3R and characterized the distribution of their projections. Analysis revealed MC3R neuronal and terminal labeling in multiple brain regions that control a diverse range of physiological functions and behavioral processes. Notably, dense labeling was observed in the hypothalamus, as well as areas that share considerable connections with the hypothalamus, including the cortex, amygdala, thalamus, and brainstem. Additionally, MC3R neuronal labeling was sexually dimorphic in several areas, including the anteroventral periventricular area, arcuate nucleus, principal nucleus of the bed nucleus of the stria terminalis, and ventral premammillary region. Altogether, anatomical evidence reported here suggests that MC3R has the potential to influence several different classes of motivated behavior that are essential for survival, including ingestive, reproductive, defensive, and arousal behaviors, and is likely to modulate these behaviors differently in males and females.

Signaling pathways in obesity: mechanisms and therapeutic interventions

Obesity is a complex, chronic disease and global public health challenge. Characterized by excessive fat accumulation in the body, obesity sharply increases the risk of several diseases, such as type 2 diabetes, cardiovascular disease, and nonalcoholic fatty liver disease, and is linked to lower life expectancy. Although lifestyle intervention (diet and exercise) has remarkable effects on weight management, achieving long-term success at weight loss is extremely challenging, and the prevalence of obesity continues to rise worldwide. Over the past decades, the pathophysiology of obesity has been extensively investigated, and an increasing number of signal transduction pathways have been implicated in obesity, making it possible to fight obesity in a more effective and precise way. In this review, we summarize recent advances in the pathogenesis of obesity from both experimental and clinical studies, focusing on signaling pathways and their roles in the regulation of food intake, glucose homeostasis, adipogenesis, thermogenesis, and chronic inflammation. We also discuss the current anti-obesity drugs, as well as weight loss compounds in clinical trials, that target these signals. The evolving knowledge of signaling transduction may shed light on the future direction of obesity research, as we move into a new era of precision medicine.

Incorporation of Indoylated Phenylalanine Yields a Sub-Micromolar Selective Melanocortin-4 Receptor Antagonist Tetrapeptide

The melanocortin family is involved in many physiological functions, including pigmentation, steroidogenesis, and appetite. The centrally expressed melanocortin-3 and melanocortin-4 receptors (MC3R and MC4R) possess overlapping but distinct roles in energy homeostasis. Herein, the third and fourth positions of a tetrapeptide lead compound [Ac-Arg-Arg-(pI)DPhe-Tic-NH₂], previously reported to possess MC3R agonist and MC4R antagonist activities, were substituted with indoylated phenylalanine (Wsf/Wrf) residues in an attempt to generate receptor subtype selective compounds. At the third position, d-amino acids were required for melanocortin agonist activity, while both l- and d-residues resulted in MC4R antagonist activity. These results indicate that l-indoylated phenylalanine residues at the third position of the scaffold can generate MC4R over MC3R selective antagonist ligands, resulting in a substitution pattern that may be exploited for novel MC4R ligands that can be used to probe the in vivo activity of the MC4R without involvement of the MC3R.

Metabolic Profiling of Mice with Deletion of the Orphan G Protein-Coupled Receptor, GPR37L1

Understanding the neurogenic causes of obesity may reveal novel drug targets to counter the obesity crisis and associated sequelae. Here, we investigate whether the deletion of GPR37L1, an astrocyte-specific orphan G protein-coupled receptor, affects whole-body energy homeostasis in mice. We subjected male Gpr37l1−/− mice and littermate wildtype (Gpr37l1+/+, C57BL/6J background) controls to either 12 weeks of high-fat diet (HFD) or chow feeding, or to 1 year of chow diet, with body composition quantified by EchoMRI, glucose handling by glucose tolerance test and metabolic rate by indirect calorimetry. Following an HFD, Gpr37l1−/− mice had similar glucose handling, body weight and fat mass compared with wildtype controls. Interestingly, we observed a significantly elevated respiratory exchange ratio in HFD- and chow-fed Gpr37l1−/− mice during daylight hours. After 1 year of chow feeding, we again saw no differences in glucose and insulin tolerance or body weight between genotypes, nor in energy expenditure or respiratory exchange ratio. However, there was significantly lower fat mass accumulation, and higher ambulatory activity in the Gpr37l1−/− mice during night hours. Overall, these results indicate that while GPR37L1 may play a minor role in whole-body metabolism, it is not a viable clinical target for the treatment of obesity.

The impact of undernutrition on KNDy (kisspeptin/neurokinin B/dynorphin) neurons in female lambs

Undernutrition limits reproduction through inhibition of gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) secretion. Because KNDy neurons coexpress neuropeptides that play stimulatory (kisspeptin and neurokinin B [NKB]) and inhibitory (dynorphin) roles in pulsatile GnRH/LH release, we hypothesized that undernutrition would inhibit kisspeptin and NKB expression at the same time as increasing dynorphin expression. Fifteen ovariectomized lambs were either fed to maintain pre-study body weight (controls) or feed-restricted to lose 20% of pre-study body weight (FR) over 13 weeks. Blood samples were collected and plasma from weeks 0 and 13 were assessed for LH by radioimmunoassay. At week 13, animals were killed, and brain tissue was processed for assessment of KNDy peptide mRNA or protein expression. Mean LH and LH pulse amplitude were lower in FR lambs compared to controls. We observed lower mRNA abundance for kisspeptin within KNDy neurons of FR lambs compared to controls with no significant change in mRNA for NKB or dynorphin. We also observed that FR lambs had fewer numbers of arcuate nucleus kisspeptin and NKB perikarya compared to controls. These findings support the idea that KNDy neurons are important for regulating reproduction during undernutrition in female sheep.

Translational advances of melanocortin drugs: Integrating biology, chemistry and genetics

Melanocortin receptors have emerged as important targets with a very unusual versatility, as their widespread distribution on multiple tissues (e.g. skin, adrenal glands, brain, immune cells, exocrine glands) together with the variety of physiological processes they control (pigmentation, cortisol release, satiety mechanism, inflammation, secretions), place this family of receptors as genuine therapeutic targets for many disorders. This review focuses in the journey of the development of melanocortin receptors as therapeutic targets from the discovery of their existence in the early 1990 s to the approval of the first few drugs of this class. Two major areas of development characterise the current state of melanocortin drug development: their role in obesity, recently culminated with the approval of setmelanotide, and their potential for the treatment of chronic inflammatory and autoimmune diseases like rheumatoid arthritis, multiple sclerosis or fibrosis. The pro-resolving nature of these drugs offers the advantage of acting by mimicking the way our body naturally resolves inflammation, expecting fewer side effects and a more balanced (i.e. non-immunosuppressive) response from them. Here we also review the approaches followed for the design and development of novel compounds, the importance of the GPCR nature of these receptors in the process of drug development, therapeutic value, current challenges and successes, and the potential for the implementation of precision medicine approaches through the incorporation of genetics advances.

The Role of Leptin in the Development of Energy Homeostatic Systems and the Maintenance of Body Weight

LEP is a pleiotropic gene and the actions of leptin extend well beyond simply acting as the signal of the size of adipose tissue stores originally proposed. This is a discussion of the multi-system interactions of leptin with the development of the neural systems regulating energy stores, and the subsequent maintenance of energy stores throughout the lifespan. The prenatal, perinatal, and later postnatal effects of leptin on systems regulating body energy stores and on the energy stores themselves are heavily influenced by the nutritional environment which leptin exposure occurs. This review discusses the prenatal and perinatal roles of leptin in establishing the neuronal circuitry and other systems relevant to the adiposity set-point (or “threshold”) and the role of leptin in maintaining weight homeostasis in adulthood. Therapeutic manipulation of the intrauterine environment, use of leptin sensitizing agents, and identification of specific cohorts who may be more responsive to leptin or other means of activating the leptin signaling pathway are ripe areas for future research.

MC3R links nutritional state to childhood growth and the timing of puberty

The state of somatic energy stores in metazoans is communicated to the brain, which regulates key aspects of behaviour, growth, nutrient partitioning and development1. The central melanocortin system acts through melanocortin 4 receptor (MC4R) to control appetite, food intake and energy expenditure2. Here we present evidence that MC3R regulates the timing of sexual maturation, the rate of linear growth and the accrual of lean mass, which are all energy-sensitive processes. We found that humans who carry loss-of-function mutations in MC3R, including a rare homozygote individual, have a later onset of puberty. Consistent with previous findings in mice, they also had reduced linear growth, lean mass and circulating levels of IGF1. Mice lacking Mc3r had delayed sexual maturation and an insensitivity of reproductive cycle length to nutritional perturbation. The expression of Mc3r is enriched in hypothalamic neurons that control reproduction and growth, and expression increases during postnatal development in a manner that is consistent with a role in the regulation of sexual maturation. These findings suggest a bifurcating model of nutrient sensing by the central melanocortin pathway with signalling through MC4R controlling the acquisition and retention of calories, whereas signalling through MC3R primarily regulates the disposition of calories into growth, lean mass and the timing of sexual maturation.

Miniature microscopes for manipulating and recording in vivo brain activity

Here we describe the development and application of miniature integrated microscopes (miniscopes) paired with microendoscopes that allow for the visualization and manipulation of neural circuits in superficial and subcortical brain regions in freely behaving animals. Over the past decade the miniscope platform has expanded to include simultaneous optogenetic capabilities, electrically-tunable lenses that enable multi-plane imaging, color-corrected optics, and an integrated data acquisition platform that streamlines multimodal experiments. Miniscopes have given researchers an unprecedented ability to monitor hundreds to thousands of genetically-defined neurons from weeks to months in both healthy and diseased animal brains. Sophisticated algorithms that take advantage of constrained matrix factorization allow for background estimation and reliable cell identification, greatly improving the reliability and scalability of source extraction for large imaging datasets. Data generated from miniscopes have empowered researchers to investigate the neural circuit underpinnings of a wide array of behaviors that cannot be studied under head-fixed conditions, such as sleep, reward seeking, learning and memory, social behaviors, and feeding. Importantly, the miniscope has broadened our understanding of how neural circuits can go awry in animal models of progressive neurological disorders, such as Parkinson's disease. Continued miniscope development, including the ability to record from multiple populations of cells simultaneously, along with continued multimodal integration of techniques such as electrophysiology, will allow for deeper understanding into the neural circuits that underlie complex and naturalistic behavior.