Current Mechanistic and Pharmacodynamic Understanding of Melanocortin-4 Receptor Activation

Predisposition of the Common MC4R rs17782313 Female Carriers to Elevated Obesity and Interaction with Eating Habits

The global rise in obesity is attributed to genetic predisposition interaction with an obesogenic environment. Melanocortin 4 receptor (MC4R) rs17782313 polymorphism has been linked to common obesity with varying influence across different populations. MC4R is a crucial player in the leptin proopiomelanocortin pathway that regulates weight hemostasis. We aimed to study MC4R rs17782313 and its interaction with eating behaviors on obesity predisposition in the Israeli population. Adults' (n = 5785, >18 y) genotype and anthropometric and demographic data were analyzed using logistic regression models adjusting for age, sex, T1DM, and T2DM. MC4R rs17782313 significantly predisposes to elevated obesity risk under the recessive and additive models (OR = 1.38, 95% CI: 1.1-1.72, p = 0.005 and OR = 1.1, 95% CI: 1.01-1.2, p = 0.03, respectively) adjusted for confounders (age, sex, T1DM, and T2DM). Stratification by sex demonstrated that carrying the common MC4R rs17782313 is significantly associated with an elevated predisposition to obesity under the recessive model among females only (OR = 1.41, 95% CI: 1.09-1.82, p = 0.01), with an average of 0.85 BMI increment compared with wild type and one risk allele carriers. MC4R rs17782313 significantly interacted with several eating behaviors to enhance the risk of obesity. Our findings demonstrate that MC4R rs17782313 homozygous female carriers are significantly predisposed to obesity amplified by eating behaviors.

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.

Obesity medication lorcaserin activates brainstem GLP-1 neurons to reduce food intake and augments GLP-1 receptor agonist induced appetite suppression

OBJECTIVE

Overweight and obesity are endemic in developed countries, with a substantial negative impact on human health. Medications developed to treat obesity include agonists for the G-protein coupled receptors glucagon-like peptide-1 (GLP-1R; e.g. liraglutide), serotonin 2C (5-HT2CR; e.g, lorcaserin), and melanocortin4 (MC4R) which reduce body weight primarily by suppressing food intake. However, the mechanisms underlying the therapeutic food intake suppressive effects are still being defined and were investigated here.

METHODS

We profiled PPG neurons in the nucleus of the solitary tract (PPGNTS) using single nucleus RNA sequencing (Nuc-Seq) and histochemistry. We next examined the requirement of PPGNTS neurons for obesity medication effects on food intake by virally ablating PPGNTS neurons. Finally, we assessed the effects on food intake of the combination of liraglutide and lorcaserin.

RESULTS

We found that 5-HT2CRs, but not GLP-1Rs or MC4Rs, were widespread in PPGNTS clusters and that lorcaserin significantly activated PPGNTS neurons. Accordingly, ablation of PPGNTS neurons prevented the reduction of food intake by lorcaserin but not MC4R agonist melanotan-II, demonstrating the functional significance of PPGNTS 5-HT2CR expression. Finally, the combination of lorcaserin with GLP-1R agonists liraglutide or exendin-4 produced greater food intake reduction as compared to either monotherapy.

CONCLUSIONS

These findings identify a necessary mechanism through which obesity medication lorcaserin produces its therapeutic benefit, namely brainstem PPGNTS neurons. Moreover, these data reveal a strategy to augment the therapeutic profile of the current frontline treatment for obesity, GLP-1R agonists, via coadministration with 5-HT2CR agonists.

CRTC1 deficiency, specifically in melanocortin-4 receptor-expressing cells, induces hyperphagia, obesity, and insulin resistance

Melanocortin-4 receptor (MC4R) is a critical regulator of appetite and energy expenditure in rodents and humans. MC4R deficiency causes hyperphagia, reduced energy expenditure, and impaired glucose metabolism. Ligand binding to MC4R activates adenylyl cyclase, resulting in increased levels of intracellular cyclic adenosine monophosphate (cAMP), a secondary messenger that regulates several cellular processes. Cyclic adenosine monophosphate responsive element-binding protein-1-regulated transcription coactivator-1 (CRTC1) is a cytoplasmic coactivator that translocates to the nucleus in response to cAMP and is reportedly involved in obesity. However, the precise mechanism through which CRTC1 regulates energy metabolism remains unknown. Additionally, there are no reports linking CRTC1 and MC4R, although both CRTC1 and MC4R are known to be involved in obesity. Here, we demonstrate that mice lacking CRTC1, specifically in MC4R cells, are sensitive to high-fat diet (HFD)-induced obesity and exhibit hyperphagia and increased body weight gain. Moreover, the loss of CRTC1 in MC4R cells impairs glucose metabolism. MC4R-expressing cell-specific CRTC1 knockout mice did not show changes in body weight gain, food intake, or glucose metabolism when fed a normal-chow diet. Thus, CRTC1 expression in MC4R cells is required for metabolic adaptation to HFD with respect to appetite regulation. Our results revealed an important protective role of CRTC1 in MC4R cells against dietary adaptation.

The metabolic conditioning of obesity: A review of the pathogenesis of obesity and the epigenetic pathways that "program" obesity from conception

Understanding the developmental origins of health and disease is integral to overcome the global tide of obesity and its metabolic consequences, including atherosclerotic cardiovascular disease, type 2 diabetes, hyperlipidemia, and nonalcoholic fatty liver disease. The rising prevalence of obesity has been attributed, in part, to environmental factors including the globalization of the western diet and unhealthy lifestyle choices. In this review we argue that how and when such exposures come into play from conception significantly impact overall risk of obesity and later health outcomes. While the laws of thermodynamics dictate that obesity is caused by an imbalance between caloric intake and energy expenditure, the drivers of each of these may be laid down before the manifestation of the phenotype. We present evidence over the last half-century that suggests that the temporospatial evolution of obesity from intrauterine life and beyond is, in part, due to the conditioning of physiological processes at critical developmental periods that results in maladaptive responses to obesogenic exposures later in life. We begin the review by introducing studies that describe an association between perinatal factors and later risk of obesity. After a brief discussion of the pathogenesis of obesity, including the systemic regulation of appetite, adiposity, and basal metabolic rate, we delve into the mechanics of how intrauterine, postnatal and early childhood metabolic environments may contribute to adult obesity risk through the process of metabolic conditioning. Finally, we detail the specific epigenetic pathways identified both in preclinical and clinical studies that synergistically "program" obesity.

Ligands for Melanocortin Receptors: Beyond Melanocyte-Stimulating Hormones and Adrenocorticotropin

The discovery of melanocortins in 1916 has resulted in more than 100 years of research focused on these peptides. Extensive studies have elucidated well-established functions of melanocortins mediated by cell surface receptors, including MSHR (melanocyte-stimulating hormone receptor) and ACTHR (adrenocorticotropin receptor). Subsequently, three additional melanocortin receptors (MCRs) were identified. Among these five MCRs, MC3R and MC4R are expressed primarily in the central nervous system, and are therefore referred to as the neural MCRs. Since the central melanocortin system plays important roles in regulating energy homeostasis, targeting neural MCRs is emerging as a therapeutic approach for treating metabolic conditions such as obesity and cachexia. Early efforts modifying endogenous ligands resulted in the development of many potent and selective ligands. This review focuses on the ligands for neural MCRs, including classical ligands (MSH and agouti-related peptide), nonclassical ligands (lipocalin 2, β-defensin, small molecules, and pharmacoperones), and clinically approved ligands (ACTH, setmelanotide, bremelanotide, and several repurposed drugs).

Pharmacologic Weight Management in the Era of Adolescent Obesity

CONTEXT

Pediatric obesity is a serious health problem in the United States. While lifestyle modification therapy with dietary changes and increased physical activity are integral for the prevention and treatment of mild to moderate obesity in youth, only a modest effect on sustained weight reduction is observed in children and young adults with severe obesity. This underscores the need for additional evidence-based interventions for children and adolescents with severe obesity, including pharmacotherapy, before considering invasive procedures such as bariatric surgery.

EVIDENCE ACQUISITION

This publication focuses on recent advances in pharmacotherapy of obesity with an emphasis on medications approved for common and rarer monogenic forms of pediatric obesity.

EVIDENCE SYNTHESIS

We review medications currently available in the United States, both those approved for weight reduction in children and "off-label" medications that have a broad safety margin.

CONCLUSION

It is intended that this review will provide guidance for practicing clinicians and will encourage future exploration for successful pharmacotherapy and other interventions for obesity in youth.

Pharmacotherapy in Childhood Obesity

BACKGROUND

The increasing number of obese children and adolescence is a major problem in health-care systems. Currently, the gold standard for the treatment of these patients with obesity is a multicomponent lifestyle intervention. Unfortunately, this strategy is not leading to a substantial and long-lasting weight loss in the majority of patients. This is the reason why there is an urgent need to establish new treatment strategies for children and adolescents with obesity to reduce the risk for the development of any comorbidities like cardiovascular diseases or diabetes mellitus type 2.

SUMMARY

In this review, we outline available pharmacological therapeutic options for children and compare the available study data with the outcome of conservative treatment approaches.

KEY MESSAGES

We discussed, in detail, how knowledge about underlying molecular mechanisms might support the identification of effective antiobesity drugs in the future and in which way this might modulate current treatment strategies to support children and adolescence with obesity to lose body weight.

Targeting appetite and satiety in diabetes and obesity, via G protein-coupled receptors

Type 2 diabetes and obesity have reached pandemic proportions throughout the world, so much so that the World Health Organisation coined the term "Globesity" to help encapsulate the magnitude of the problem. G protein-coupled receptors (GPCRs) are highly tractable drug targets due to their wide involvement in all aspects of physiology and pathophysiology, indeed, GPCRs are the targets of approximately 30% of the currently approved drugs. GPCRs are also broadly involved in key physiologies that underlie type 2 diabetes and obesity including feeding reward, appetite and satiety, regulation of blood glucose levels, energy homeostasis and adipose function. Despite this, only two GPCRs are the target of approved pharmaceuticals for treatment of type 2 diabetes and obesity. In this review we discuss the role of these, and select other candidate GPCRs, involved in various facets of type 2 diabetic or obese pathophysiology, how they might be targeted and the potential reasons why pharmaceuticals against these targets have not progressed to clinical use. Finally, we provide a perspective on the current development pipeline of anti-obesity drugs that target GPCRs.

Mutations in melanocortin-4 receptor: From fish to men

Melanocortin-4 receptor (MC4R), expressed abundantly in the hypothalamus, is a critical regulator of energy homeostasis, including both food intake and energy expenditure. Shortly after the publication in 1997 of the Mc4r knockout phenotypes in mice, including increased food intake and severe obesity, the first mutations in MC4R were reported in humans in 1998. Studies in the subsequent two decades have established MC4R mutation as the most common monogenic form of obesity, especially in early-onset severe obesity. Studies in animals, from fish to mammals, have established the conserved physiological roles of MC4R in all vertebrates in regulating energy balance. Drug targeting MC4R has been recently approved for treating morbid genetic obesity. How the MC4R can be exploited for animal production is highly worthy of active investigation.

Stimulation of Gs signaling in MC4R cells by DREADD increases energy expenditure, suppresses food intake, and increases locomotor activity in mice

The melanocortin 4 receptor (MC4R) plays an important role in the regulation of appetite and energy expenditure in humans and rodents. Impairment of MC4R signaling causes severe obesity. MC4R mainly couples to the G-protein G_s. Ligand binding to MC4R activates adenylyl cyclase resulting in increased intracellular cAMP levels. cAMP acts as a secondary messenger, regulating various cellular processes. MC4R can also couple with G_q and other signaling pathways. Therefore, the contribution of MC4R/G_s signaling to energy metabolism and appetite remains unclear. To study the effect of G_s signaling activation in MC4R cells on whole body energy metabolism and appetite, we generated a novel mouse strain that expresses a G_s-coupled designer receptors exclusively activated by designer drugs [Gs-DREADD (GsD)] selectively in MC4R-expressing cells (GsD-MC4R mice). Chemogenetic activation of the GsD by a designer drug [deschloroclozapine (DCZ); 0.01∼0.1 mg/kg body wt] in MC4R-expressing cells significantly increased oxygen consumption and locomotor activity. In addition, GsD activation significantly reduced the respiratory exchange ratio, promoting fatty acid oxidation, but did not affect core (rectal) temperature. A low dose of DCZ (0.01 mg/kg body wt) did not suppress food intake, but a high dose of DCZ (0.1 mg/kg body wt) suppressed food intake in MC4R-GsD mice, although either DCZ dose (0.01 or 0.1 mg/kg body wt) did not affect food intake in the control mice. In conclusion, the current study demonstrated that the stimulation of G_s signaling in MC4R-expressing cells increases energy expenditure and locomotor activity and suppresses appetite.NEW & NOTEWORTHY We report that G_s signaling in melanocortin 4 receptor (MC4R)-expressing cells regulates energy expenditure, appetite, and locomotor activity. These findings shed light on the mechanism underlying the regulation of energy metabolism and locomotor activity by MC4R/cAMP signaling.

The rs483145 polymorphism of MC4R gene is not associated with obesity in the Chilean population: Results of GENADIO study

INTRODUCTION

The melanocortin receptor 4 (MC4R) participates in the control of appetite at the level of the central nervous system, through the leptin-melanocortin pathway. An association between different polymorphisms of the MC4R gene and obesity has been reported. However, there are few studies of the rs483145 single nucleotide polymorphism (SNP) of this gene.

OBJECTIVE

To investigate its prevalence and association with adiposity markers in Chilean adults.

METHODS

The prevalence of SNP rs483145, of the MC4R gene, was determined in 259 participants of the GENADIO study (genes, environment, diabetes and obesity) by means of real-time polymerase chain reaction (PCR). The association between the risk allele of MC4R (A) and adiposity markers (body weight, body mass index, fat mass percentage, hip circumference, waist circumference, waist-to-hip ratio) was performed by linear regression analysis and adjusted for confusion variables (socio-demographic and physic activity) using three statistical models.

RESULTS

It was determined that the prevalence of the risk allele (A) of the SNP rs483145 of the MC4R gene is 24.5% in the Chilean adult population included in this study, without finding an association with any of the adiposity markers studied, both in adjusted and unadjusted models.

CONCLUSION

The presence of the risk allele of SNP rs483145 of the MC4R gene is not associated with adiposity markers in the Chilean adult population studied. New studies with a bigger sample size will be necessary to confirm these results.

Anti-obesity drug discovery: advances and challenges

Enormous progress has been made in the last half-century in the management of diseases closely integrated with excess body weight, such as hypertension, adult-onset diabetes and elevated cholesterol. However, the treatment of obesity itself has proven largely resistant to therapy, with anti-obesity medications (AOMs) often delivering insufficient efficacy and dubious safety. Here, we provide an overview of the history of AOM development, focusing on lessons learned and ongoing obstacles. Recent advances, including increased understanding of the molecular gut-brain communication, are inspiring the pursuit of next-generation AOMs that appear capable of safely achieving sizeable and sustained body weight loss.

Naturally occurring mutations in G protein-coupled receptors associated with obesity and type 2 diabetes mellitus

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors involved in the regulation of almost all known physiological processes. Dysfunctions of GPCR-mediated signaling have been shown to cause various diseases. The prevalence of obesity and type 2 diabetes mellitus (T2DM), two strongly associated disorders, is increasing worldwide, with tremendous economical and health burden. New safer and more efficacious drugs are required for successful weight reduction and T2DM treatment. Multiple GPCRs are involved in the regulation of energy and glucose homeostasis. Mutations in these GPCRs contribute to the development and progression of obesity and T2DM. Therefore, these receptors can be therapeutic targets for obesity and T2DM. Indeed some of these receptors, such as melanocortin-4 receptor and glucagon-like peptide 1 receptor, have provided important new drugs for treating obesity and T2DM. This review will focus on the naturally occurring mutations of several GPCRs associated with obesity and T2DM, especially incorporating recent large genomic data and insights from structure-function studies, providing leads for future investigations.

Drugs for Treating Obesity

Older medications approved for chronic weight management (orlistat, naltrexone/bupropion, liraglutide 3 mg and, in the USA, phentermine/topiramate) have not been widely adopted by health care providers. Those medications produce only modest additional weight loss when used to augment lifestyle intervention. However, semaglutide 2.4 mg weekly has recently emerged and produces much more weight loss - on average 15% weight loss at 1 year. Semaglutide's enhanced efficacy and that its class (GLP-1 receptor analogs) is well-known may result in more clinicians adopting pharmacotherapy. Furthermore, the first dedicated cardiovascular outcome trial powered for superiority testing an anti-obesity medication (SELECT) is underway with semaglutide 2.4 mg. A positive outcome will further promote the concept that weight management should be a primary target for cardiometabolic disease control. In phase 3, tirzepatide and cagrilintide/semaglutide combination are showing promise for even greater weight loss efficacy. Another recently approved medication takes a personalized medicine approach; setmelanotide is approved as a therapy for those with some of the ultra-rare genetic diseases characterized by severe, early onset obesity. This chapter reviews the currently available and anticipated medications for chronic weight management as well as those approved for the genetic and syndromic obesities.

Next Generation Antiobesity Medications: Setmelanotide, Semaglutide, Tirzepatide and Bimagrumab: What do They Mean for Clinical Practice?

There is a new generation of antiobesity drugs in development or just arriving on the scene. First, setmelanotide has been approved for three of the ultrarare genetic conditions that cause obesity–pro-opiomelanocortin deficiency, proprotein convertase subtilisin and kexin type 1 (an important enzyme in the melanocortin pathway) and leptin receptor deficiency. Setmelanotide marks the first in a personalized medicine approach to obesity. Second, semaglutide 2.4 mg once weekly has been submitted to regulators in the United States and the European Union for approval for patients with obesity (body mass index [BMI] ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) and at least one weight related comorbidity. This drug has been studied in five phase 3 clinical trials, four discussed herein: semaglutide produces roughly twice as much weight loss as we have seen in older antiobesity medications. Semaglutide is already in use for treatment of diabetes and, as a glucagon-like peptide 1 (GLP-1) receptor analog, is part of a class of drugs used widely in diabetes. Tirzepatide, a glucose-insulin peptide and GLP-1 dual agonist is in phase 3 study for obesity management, and bimagrumab is a new agent in phase 2 with a unique mechanism of action; they are generating much interest. The purpose of this narrative review is lay the groundwork for a discussion of the clinical impact of these new medications on the clinical practice of obesity. Further, these developments shall be used to launch a speculation of what is likely to be their impact on the future of obesity pharmacotherapy.

High-Throughput Screening of Mouse Gene Knockouts Identifies Established and Novel High Body Fat Phenotypes

PURPOSE

Obesity is a major public health problem. Understanding which genes contribute to obesity may better predict individual risk and allow development of new therapies. Because obesity of a mouse gene knockout (KO) line predicts an association of the orthologous human gene with obesity, we reviewed data from the Lexicon Genome5000TM high throughput phenotypic screen (HTS) of mouse gene KOs to identify KO lines with high body fat.

MATERIALS AND METHODS

KO lines were generated using homologous recombination or gene trapping technologies. HTS body composition analyses were performed on adult wild-type and homozygous KO littermate mice from 3758 druggable mouse genes having a human ortholog. Body composition was measured by either DXA or QMR on chow-fed cohorts from all 3758 KO lines and was measured by QMR on independent high fat diet-fed cohorts from 2488 of these KO lines. Where possible, comparisons were made to HTS data from the International Mouse Phenotyping Consortium (IMPC).

RESULTS

Body fat data are presented for 75 KO lines. Of 46 KO lines where independent external published and/or IMPC KO lines are reported as obese, 43 had increased body fat. For the remaining 29 novel high body fat KO lines, Ksr2 and G2e3 are supported by data from additional independent KO cohorts, 6 (Asnsd1, Srpk2, Dpp8, Cxxc4, Tenm3 and Kiss1) are supported by data from additional internal cohorts, and the remaining 21 including Tle4, Ak5, Ntm, Tusc3, Ankk1, Mfap3l, Prok2 and Prokr2 were studied with HTS cohorts only.

CONCLUSION

These data support the finding of high body fat in 43 independent external published and/or IMPC KO lines. A novel obese phenotype was identified in 29 additional KO lines, with 27 still lacking the external confirmation now provided for Ksr2 and G2e3 KO mice. Undoubtedly, many mammalian obesity genes remain to be identified and characterized.

The rs483145 polymorphism of MC4R gene is not associated with obesity in the Chilean population: Results of GENADIO study

INTRODUCTION

The melanocortin receptor 4 (MC4R) participates in the control of appetite at the level of the central nervous system, through the leptin-melanocortin pathway. An association between different polymorphisms of the MC4R gene and obesity has been reported. However, there are few studies of the rs483145 single nucleotide polymorphism (SNP) of this gene.

OBJECTIVE

To investigate its prevalence and association with adiposity markers in Chilean adults.

METHODS

The prevalence of SNP rs483145, of the MC4R gene, was determined in 259 participants of the GENADIO study (genes, environment, diabetes and obesity) by means of real-time polymerase chain reaction (PCR). The association between the risk allele of MC4R (A) and adiposity markers (body weight, body mass index, fat mass percentage, hip circumference, waist circumference, waist-to-hip ratio) was performed by linear regression analysis and adjusted for confusion variables (socio-demographic and physic activity) using three statistical models.

RESULTS

It was determined that the prevalence of the risk allele (A) of the SNP rs483145 of the MC4R gene is 24.5% in the Chilean adult population included in this study, without finding an association with any of the adiposity markers studied, both in adjusted and unadjusted models.

CONCLUSION

The presence of the risk allele of SNP rs483145 of the MC4R gene is not associated with adiposity markers in the Chilean adult population studied. New studies with a bigger sample size will be necessary to confirm these results.

Human MC4R variants affect endocytosis, trafficking and dimerization revealing multiple cellular mechanisms involved in weight regulation

The Melanocortin-4 Receptor (MC4R) plays a pivotal role in energy homeostasis. We used human MC4R mutations associated with an increased or decreased risk of obesity to dissect mechanisms that regulate MC4R function. Most obesity-associated mutations impair trafficking to the plasma membrane (PM), whereas obesity-protecting mutations either accelerate recycling to the PM or decrease internalization, resulting in enhanced signaling. MC4R mutations that do not affect canonical Gα_s protein-mediated signaling, previously considered to be non-pathogenic, nonetheless disrupt agonist-induced internalization, β-arrestin recruitment, and/or coupling to Gα_s, establishing their causal role in severe obesity. Structural mapping reveals ligand-accessible sites by which MC4R couples to effectors and residues involved in the homodimerization of MC4R, which is disrupted by multiple obesity-associated mutations. Human genetic studies reveal that endocytosis, intracellular trafficking, and homodimerization regulate MC4R function to a level that is physiologically relevant, supporting the development of chaperones, agonists, and allosteric modulators of MC4R for weight loss therapy.

Whole-brain activation signatures of weight-lowering drugs

OBJECTIVE

The development of effective anti-obesity therapeutics relies heavily on the ability to target specific brain homeostatic and hedonic mechanisms controlling body weight. To obtain further insight into neurocircuits recruited by anti-obesity drug treatment, the present study aimed to determine whole-brain activation signatures of six different weight-lowering drug classes.

METHODS

Chow-fed C57BL/6J mice (n = 8 per group) received acute treatment with lorcaserin (7 mg/kg; i.p.), rimonabant (10 mg/kg; i.p.), bromocriptine (10 mg/kg; i.p.), sibutramine (10 mg/kg; p.o.), semaglutide (0.04 mg/kg; s.c.) or setmelanotide (4 mg/kg; s.c.). Brains were sampled two hours post-dosing and whole-brain neuronal activation patterns were analysed at single-cell resolution using c-Fos immunohistochemistry and automated quantitative three-dimensional (3D) imaging.

RESULTS

The whole-brain analysis comprised 308 atlas-defined mouse brain areas. To enable fast and efficient data mining, a web-based 3D imaging data viewer was developed. All weight-lowering drugs demonstrated brain-wide responses with notable similarities in c-Fos expression signatures. Overlapping c-Fos responses were detected in discrete homeostatic and non-homeostatic feeding centres located in the dorsal vagal complex and hypothalamus with concurrent activation of several limbic structures as well as the dopaminergic system.

CONCLUSIONS

Whole-brain c-Fos expression signatures of various weight-lowering drug classes point to a discrete set of brain regions and neurocircuits which could represent key neuroanatomical targets for future anti-obesity therapeutics.

The Role of Melanocortin Plasticity in Pain-Related Outcomes After Alcohol Exposure

The global COVID-19 pandemic has shone a light on the rates and dangers of alcohol misuse in adults and adolescents in the US and globally. Alcohol exposure during adolescence causes persistent molecular, cellular, and behavioral changes that increase the risk of alcohol use disorder (AUD) into adulthood. It is established that alcohol abuse in adulthood increases the likelihood of pain hypersensitivity and the genesis of chronic pain, and humans report drinking alcohol to relieve pain symptoms. However, the longitudinal effects of alcohol exposure on pain and the underlying CNS signaling that mediates it are understudied. Specific brain regions mediate pain effects, alcohol effects, and pain-alcohol interactions, and neural signaling in those brain regions is modulated by neuropeptides. The CNS melanocortin system is sensitive to alcohol and modulates pain sensitivity, but this system is understudied in the context of pain-alcohol interactions. In this review, we focus on the role of melanocortin signaling in brain regions sensitive to alcohol and pain, in particular the amygdala. We also discuss interactions of melanocortins with other peptide systems, including the opioid system, as potential mediators of pain-alcohol interactions. Therapeutic strategies that target the melanocortin system may mitigate the negative consequences of alcohol misuse during adolescence and/or adulthood, including effects on pain-related outcomes.

The central melanocortin system and human obesity

The prevalence of obesity and the associated comorbidities highlight the importance of understanding the regulation of energy homeostasis. The central melanocortin system plays a critical role in controlling body weight balance. Melanocortin neurons sense and integrate the neuronal and hormonal signals, and then send regulatory projections, releasing anorexigenic or orexigenic melanocortin neuropeptides, to downstream neurons to regulate the food intake and energy expenditure. This review summarizes the latest progress in our understanding of the role of the melanocortin pathway in energy homeostasis. We also review the advances in the identification of human genetic variants that cause obesity via mechanisms that affect the central melanocortin system, which have provided rational targets for treatment of genetically susceptible patients.

Melanocortin pathways: suppressed and stimulated melanocortin-4 receptor (MC4R)

Leptin-melanocortin pathway plays an essential role in the body weight regulation. Enhanced melanocortin signaling in the hypothalamus results in both decreased food intake and increased energy expenditure. The discovery of monogenic obesities with dysfunction of melanocortin-4 receptor (MC4R) greatly contributed to understanding of energy balance regulation. This review presents phenotypical characterization and prevalence of the MC4R gene mutations. Genome-wide association studies revealed that MC4R gene is significantly related not only to monogenic obesities but also to common obesity. An interaction of variants in the MC4R gene with fat mass and obesity associated (FTO) gene significantly increases the risk for obesity, particularly in adolescence. On the other hand, about 15 % of the MC4R gene variants result in a gain of function that protects against obesity and is associated with favorable metabolic profile. Long-term attempts to activate the MC4R have recently been finalized by a discovery of setmelanotide, a novel specific MC4R agonist that is devoid of untoward cardiovascular side-effects. The employment of specific MC4R agonists may open new horizons not only in the treatment of rare monogenic obesities but also in some common obesities where stimulation of MC4R could be achieved.

Structural Complexity and Plasticity of Signaling Regulation at the Melanocortin-4 Receptor

The melanocortin-4 receptor (MC4R) is a class A G protein-coupled receptor (GPCR), essential for regulation of appetite and metabolism. Pathogenic inactivating MC4R mutations are the most frequent cause of monogenic obesity, a growing medical and socioeconomic problem worldwide. The MC4R mediates either ligand-independent or ligand-dependent signaling. Agonists such as α-melanocyte-stimulating hormone (α-MSH) induce anorexigenic effects, in contrast to the endogenous inverse agonist agouti-related peptide (AgRP), which causes orexigenic effects by suppressing high basal signaling activity. Agonist action triggers the binding of different subtypes of G proteins and arrestins, leading to concomitant induction of diverse intracellular signaling cascades. An increasing number of experimental studies have unraveled molecular properties and mechanisms of MC4R signal transduction related to physiological and pathophysiological aspects. In addition, the MC4R crystal structure was recently determined at 2.75 Å resolution in an inactive state bound with a peptide antagonist. Underpinned by structural homology models of MC4R complexes simulating a presumably active-state conformation compared to the structure of the inactive state, we here briefly summarize the current understanding and key players involved in the MC4R switching process between different activity states. Finally, these perspectives highlight the complexity and plasticity in MC4R signaling regulation and identify gaps in our current knowledge.

Association between MC4R rs17782313 genotype and obesity: A meta-analysis

BACKGROUND

Obesity is a huge burden of the world. It is commonly recognized that dietary structure and physical inactivity is essential in the progress of obesity. However, some individuals still face the trouble of obese even though they live a healthy life. Except for the combination of diseases, the operation of both lifestyle and genetic features contributes to obesity. Melanocortin-4-receptor (MC4R) gene is one of the known hereditary factors of obesity. rs17782313, a single nucleotide variant in MC4R gene, has been reported unclear results in whether it plays a role in obesity. This meta-analysis is to estimate the association between MC4R rs17782313 genotype and obesity.

METHOD

A systematic literature retrieval was conducted in four databases: PubMed, Embase, Web of Science and Cochrane Library with specific search strategy. Select qualified studies to identify relevant studies. Odds ratios (ORs) with 95% confidence intervals (CI), P value and I² value were used to assess the strength of the association in meta-analysis and adjusted with ethnicity, quality and single nucleotide polymorphism (SNP) testing method.

RESULT

6 eligible studies involving 3133 obese cases and 3123 normal-weight participants were selected from 378 articles. Allele B of MC4R rs17782313 present a statistically significant association with obesity under allele contrast model (OR = 1.325, 95%CI: 1.219-1.439), dominant model (OR = 1.320, 95%CI: 1.184-1.472), recessive model (OR = 1.690, 95%CI: 1.420-2.011) and homozygous type of co-dominant model (OR = 1.925, 95%CI: 1.590-2.330), respectively, and P < 0.05.

CONCLUSION

Mutated MC4R rs17782313 is associated with higher risk of obesity. People with homozygous mutant genotype of MC4R rs17782313 would be more likely to suffer from obesity, while heterozygous mutant genotype needs further studies to clarify.

Signal Transduction and Pathogenic Modifications at the Melanocortin-4 Receptor: A Structural Perspective

The melanocortin-4 receptor (MC4R) can be endogenously activated by binding of melanocyte-stimulating hormones (MSH), which mediates anorexigenic effects. In contrast, the agouti-related peptide (AgRP) acts as an endogenous inverse agonist and suppresses ligand-independent basal signaling activity (orexigenic effects). Binding of ligands to MC4R leads to the activation of different G-protein subtypes or arrestin and concomitant signaling pathways. This receptor is a key protein in the hypothalamic regulation of food intake and energy expenditure and naturally-occurring inactivating MC4R variants are the most frequent cause of monogenic obesity. In general, obesity is a growing problem on a global scale and is of social, medical, and economic relevance. A significant goal is to develop optimized pharmacological tools targeting MC4R without adverse effects. To date, this has not been achieved because of inter alia non-selective ligands across the five functionally different MCR subtypes (MC1-5R). This motivates further investigation of (i) the three-dimensional MC4R structure, (ii) binding mechanisms of various ligands, and (iii) the molecular transfer process of signal transduction, with the aim of understanding how structural features are linked with functional-physiological aspects. Unfortunately, experimentally elucidated structural information is not yet available for the MC receptors, a group of class A G-protein coupled receptors (GPCRs). We, therefore, generated MC4R homology models and complexes with interacting partners to describe approximate structural properties associated with signaling mechanisms. In addition, molecular insights from pathogenic mutations were incorporated to discriminate more precisely their individual malfunction of the signal transfer mechanism.