A role for the melanocortin 4 receptor in sexual function

Harnessing the melanocortin system in the control of food intake and glucose homeostasis

The central and peripheral melanocortin system, comprising of five receptors and their endogenous ligands, is responsible for a wide array of physiological functions such as skin pigmentation, sexual function and development, and inflammation. A growing body of both clinical and pre-clinical research is demonstrating the relevance of this system in metabolic health. Disruption of hypothalamic melanocortin signalling is the most common cause of monogenic obesity in humans. Setmelanotide, an FDA-approved analogue of alpha-melanocyte stimulating hormone (α-MSH) that functions by restoring central melanocortin signalling, has proven to be a potent pharmacological tool in the treatment of syndromic obesity. As the first effective therapy targeting the melanocortin system to treat metabolic disorders, its approval has sparked research to further harness the links between these melanocortin receptors and metabolic processes. Here, we outline the structure of the central and peripheral melanocortin system, discuss its critical role in the regulation of food intake, and review promising targets that may hold potential to treat metabolic disorders in humans.

Incorporation of Three Extracyclic Arginine Residues into a Melanocortin Macrocyclic Agonist (c[Pro-His-DPhe-Arg-Trp-Dap-Lys(Arg-Arg-Arg-Ac)-DPro]) Decreases Food Intake When Administered Intrathecally or Subcutaneously Compared to a Macrocyclic Ligand Lacking Extracyclic Arginine Residues (c[Pro-His-DPhe-Arg-Trp-Dap-Ala-DPro)]

Of the three Food and Drug Administration-approved melanocortin peptide drugs, two possess a cyclic scaffold, demonstrating that cyclized melanocortin peptides have therapeutic relevance. An extracyclic Arg residue, critical for pharmacological activity in the approved melanocortin cyclic drug setmelanotide, has also been demonstrated to increase the signal when fluorescently labeled cell-penetrating cyclic peptides are incubated with HeLa cells, with the maximal signal observed with three extracyclic Arg amino acids. Herein, a branching Lys residue was substituted into two macrocyclic melanocortin peptide agonists to incorporate 0-3 extracyclic Arg amino acids. Incorporation of the Arg residues resulted in equipotent or increased agonist potency at the mouse melanocortin receptors in vitro, suggesting that these substitutions were tolerated in the macrocyclic scaffolds. Further in vivo evaluation of one parent ligand (c[Pro-His-DPhe-Arg-Trp-Dap-Ala-Pro]) and the three Arg derivative (c[Pro-His-DPhe-Arg-Trp-Dap-Lys(Ac-Arg-Arg-Arg)-Pro)] demonstrated that the three Arg derivative further decreased food intake compared to the parent macrocycle when the compounds were administered either via intrathecal injection or subcutaneous dosing. This suggests that three extracyclic Arg amino acids may be beneficial in the design of cyclic melanocortin ligands and that in vitro pharmacological profiling may not predict the in vivo efficacy of melanocortin ligands.

Cloning, distribution, and effects of growth regulation of MC3R and MC4R in red crucian carp (Carassius auratus red var.)

Background

Melanocortin-3 and -4 receptors (MC3R and MC4R), G protein-coupled receptors, play vital roles in the regulation of energy homeostasis. To understand the functions of mc3r and mc4r in the energy homeostasis of red crucian carp (Carassius auratus red var., RCC), we cloned mc3r and mc4r, analyzed the tissue expression and localization of the genes, and investigated the effects of knockout of mc3r (mc3r +/-) and mc4r (mc4r +/-) in RCC.

Results

The full-length cDNAs of RCC mc3r and mc4r were 1459 base pairs (bp) and 1894 bp, respectively. qRT-PCR indicated that mc3r and mc4r were profusely expressed in the brain, but lower expressed in the periphery tissues. ISH revealed that mc3r and mc4r were located in NPP, NPO, NAPv, NSC, NAT, NRL, NLTl, and NLTp of the brain, suggesting that mc3r and mc4r might regulate many physiological and behavioral aspects in RCC. To further verify the roles of mc3r and mc4r in energy homeostasis, the mc3r+/- and mc4r+/- fish were obtained by the CRISPR/Cas9 system. The average body weights, total lengths, body depths, and food intake of mc4r+/- fish were significantly higher than those of mc3r+/- and the normal wild-type (WT) fish, but there was no difference between the mc3r+/- and WT fish, indicating that the RCC phenotype and food intake were mainly influenced by mc4r but not mc3r. Interestingly, mc4r+/- fish displayed more visceral fat mass than mc3r+/- and WT fish, and mc3r+/- fish also exhibited slightly more visceral fat mass compared to WT. RNA-seq of the liver and muscle revealed that a large number of differentially expressed genes (DEGs) differed in WT vs. mc3r+/-, WT vs. mc4r+/-, and mc3r+/- vs. mc4r+/-, mainly related to lipid, glucose, and energy metabolism. The KEGG enrichment analysis revealed that DEGs were mainly enriched in pathways such as steroid biosynthesis, fatty acid metabolism, fatty acid biosynthesis, glycolysis/gluconeogenesis, wnt signaling pathway, PPAR signaling pathway, and MAPK signaling pathway, thereby affecting lipid accumulation and growth.

Conclusion

In conclusion, these results will assist in the further investigation of the molecular mechanisms in which MC3R and MC4R were involved in the regulation of energy homeostasis in fish.

Development of Melanocortin 4 Receptor Agonists by Exploiting Animal-Derived Macrocyclic, Disulfide-Rich Peptide Scaffolds

G protein-coupled receptors are among the most widely studied classes of drug targets. A major challenge in this field is to develop ligands that will selectively modulate a single receptor subtype to overcome the disadvantages of undesired "off target" effects caused by lack of target and thus signaling specificity. In the current study, we explored ligand design for the melanocortin 4 receptor (MC4R) since it is an attractive target for developing antiobesity drugs. Endogenously, the receptor is activated by peptide ligands, i.e., three melanocyte-stimulating hormones (α-MSH, β-MSH, and γ-MSH) and by adrenocorticotropic hormone. Therefore, we utilized a peptide drug design approach, utilizing "molecular grafting" of pharmacophore peptide sequence motifs onto a stable nature-derived peptide scaffold. Specifically, protegrin-4-like-peptide-1 (Pr4LP1) and arenicin-1-like-peptide-1 (Ar3LP1) fully activated MC4R in a functional cAMP assay with potencies of 3.7 and 1.0 nM, respectively. In a nanoluciferase complementation assay with less signal amplification, the designed peptides fully recruited mini-Gs with subnanomolar and nanomolar potencies. Interestingly, these novel peptide MC4R ligands recruited β-arrestin-2 with ∼2-fold greater efficacies and ∼20-fold increased potencies as compared to the endogenous α-MSH. The peptides were inactive at related MC1R and MC3R in a cAMP accumulation assay. These findings highlight the applicability of animal-derived disulfide-rich scaffolds to design pathway and subtype selective MC4R pharmacological probes. In the future, this approach could be exploited to develop functionally selective ligands that could offer safer and more effective obesity drugs.

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.

MC4R in Central and Peripheral Systems

Obesity has emerged as a critical and urgent health burden during the current global pandemic. Among multiple genetic causes, melanocortin receptor-4 (MC4R), involved in food intake and energy metabolism regulation through various signaling pathways, has been reported to be the lead genetic factor in severe and early onset obesity and hyperphagia disorders. Most previous studies have illustrated the roles of MC4R signaling in energy intake versus expenditure in the central system, while some evidence indicates that MC4R is also expressed in peripheral systems, such as the gut and endocrine organs. However, its physiopathological function remains poorly defined. This review aims to depict the central and peripheral roles of MC4R in energy metabolism and endocrine hormone homeostasis, the diversity of phenotypes, biased downstream signaling caused by distinct MC4R mutations, and current drug development targeting the receptor.

MC1R Peptide Agonist Self-Assembles into a Hydrogel That Promotes Skin Pigmentation for Treating Vitiligo

Vitiligo, a common skin disease that seriously affects 0.5-2.0% of the worldwide population, lacks approved therapeutics due to a wide range of adverse side effects. As a key regulator of skin pigmentation, MC1R may be an effective therapeutic target for vitiligo. Herein, we report an MC1R peptide agonist that directly self-assembles into nanofibrils that form a hydrogel matrix under normal physiological conditions. This hydrogel exhibits higher stability than free peptides, sustained release, rapid recovery from shear-thinning, and resistance to enzymatic proteolysis. Furthermore, this peptidal MC1R agonist upregulates tyrosinase, tyrosinase-related protein-1 (TYRP-1), and tyrosinase-related protein-2 (TYRP-2) to stimulate melanin synthesis. More importantly, MC1R agonist hydrogel promotes skin pigmentation in mice more potently than free MC1R agonist. This study supports the development of this MC1R agonist hydrogel as a promising pharmacological intervention for vitiligo.

A review on pharmacological options for the treatment of erectile dysfunction: state of the art and new strategies

INTRODUCTION

Erectile dysfunction (ED) affects between 12.9% and 28.1% of men worldwide, presenting a strong aged-correlated prevalence. Several pharmacological treatments are currently available for ED, which can be classified into oral, injection, and topical/intraurethral therapies.

AREAS COVERED

Extensive research on PubMed/MEDLINE until February 2023 was performed. For each of the aforementioned drug classes, available molecules, and formulations, their efficacy and most common adverse events as well as general guidelines on prescription were investigated and extensively described. A glimpse into future directions regarding ED pharmacotherapy is also present.

EXPERT OPINION

In recent years, there have been significant developments in pharmacological treatments for ED. It is essential for physicians to identify the best treatment option for patients based on their preferences and sexual habits. The treatment approach for ED has shifted from a sequential to a parallel paradigm, where all treatment options are available as first-line therapies. While there are promising regenerative therapies for ED, such as shockwaves and platelet-rich plasma injections, pharmacological treatment is still the most effective option for most patients.

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.

Flexibility of neural circuits regulating mating behaviors in mice and flies

Mating is essential for the reproduction of animal species. As mating behaviors are high-risk and energy-consuming processes, it is critical for animals to make adaptive mating decisions. This includes not only finding a suitable mate, but also adapting mating behaviors to the animal's needs and environmental conditions. Internal needs include physical states (e.g., hunger) and emotional states (e.g., fear), while external conditions include both social cues (e.g., the existence of predators or rivals) and non-social factors (e.g., food availability). With recent advances in behavioral neuroscience, we are now beginning to understand the neural basis of mating behaviors, particularly in genetic model organisms such as mice and flies. However, how internal and external factors are integrated by the nervous system to enable adaptive mating-related decision-making in a state- and context-dependent manner is less well understood. In this article, we review recent knowledge regarding the neural basis of flexible mating behaviors from studies of flies and mice. By contrasting the knowledge derived from these two evolutionarily distant model organisms, we discuss potential conserved and divergent neural mechanisms involved in the control of flexible mating behaviors in invertebrate and vertebrate brains.

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).

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.

Genes Predisposing to Erectile Dysfunction and Management: A Review

Background: The burden of erectile dysfunction (ED) is rising worldwide due to unresponsiveness of some affected individuals to existing drugs and treatment strategies. Fortunately, improvement in biological techniques has led to the understanding that some cases of the disorder may have a genetic etiology, which, when fully understood, may lead to improved treatment.

Objective: This review articulated established ED candidate genes and pathophysiology to assist researchers and medical practitioners to formulate effective drugs and treatment procedures.

Methods: The Google search engine was used to retrieve relevant information on the topic from reputable academic databases, including PubMed, Medline, Google Scholar, Scopus, and SpringerLink.

Results: The search discovered 10 ED candidate genes, which are SIM1, SLC6A4, 5-HTTLPR, TGFB1, DAT1, MC4R, NOS3, GNB3, AR, and MTHFR. Polymorphisms or mutations in these genes may disrupt erectile activities of the hypothalamus, neurotransmitters such as dopamine, serotonin, and nitric oxide as well as relaxation of penile tissues. Clinical presentations of ED include loss of erection, weak vaginal penetration, premature ejaculation, and anejaculation. Each gene has a distinct mechanism, which, if targeted in the affected may reverse the disorder or reduce the effects.

Conclusion: Some cases of ED are genetic, which, when fully understood, may give an insight into new treatment procedures or improve on the current ones. Medical practitioners are advised to formulate treatment procedures that target the affected gene (s) in individuals.

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.

Persistent sexual dysfunction after SSRI withdrawal: a scoping review and presentation of 86 cases from the Netherlands

BACKGROUND

Sexual dysfunction is highly prevalent worldwide. A specific form is persistent sexual dysfunction after SSRI withdrawal. We conducted a systematic literature review in order to characterize factors related to post SSRI sexual dysfunction (PSSD) and analyzed spontaneous reports of persistent sexual dysfunction reported to the Netherlands Pharmacovigilance Center Lareb.

RESEARCH DESIGN AND METHODS

A systematic literature review was conducted following the PRISMA-ScR guidelines. In addition, reports of PSSD submitted to the Netherlands Pharmacovigilance Center Lareb between 1992 and 2021 were analyzed.

RESULTS

A total of 237 articles were retrieved through the search and 33 articles were selected for inclusion in this review, in accordance with the inclusion criteria. Information regarding the characteristics of the condition, its clinical management, patient characteristics, and impact of PSSD is presented. A total of 86 reports of persistent sexual dysfunction were analyzed. The longest case being a patient with PSSD for 23 years. The main symptoms were: loss or decreased libido (n = 53), erectile dysfunction (n = 23) and anorgasmia (n = 5).

CONCLUSIONS

PSSD impact includes sexual, psychological, and social consequences. Little is known about the mechanisms underlying PSSD and no effective treatment exists. It is necessary to increase recognition of PSSD among prescribers and improve its management at the clinical level.

Pharmacokinetics of the Melanocortin Type 1 Receptor Agonist PL8177 After Subcutaneous Administration

Background and Objective

PL8177 is a selective melanocortin 1 receptor agonist in development for the treatment of various immunologic and inflammatory conditions. Here we describe the pharmacokinetics of PL8177 after subcutaneous (sc) delivery in animals and humans.

Methods

Mice, rats, and dogs were administered sc PL8177 at single doses of 1.0 and 3.0 mg/kg (mice); 1.0, 5.0, and 25.0 mg/kg/day (rats); or 1.5, 8.0, and 40.0 mg/day (dogs). Blood was collected over 24 h (mice) or 28 days (rats and dogs). Safety and pharmacokinetics of single and multiple sc doses were also examined in human volunteers. Two dose levels were tested in two dosing cohorts of 1.0 and 3.0 mg/day for 7 days. Blood samples were collected through Day 1 and on Days 2 to 6 at peak and trough times based on analysis of the first two single-dose cohorts.

Results

In mice, 3 mg/kg PL8177 resulted in an area under the plasma concentration–time curve from 0 to infinity (AUC_∞) of 1727 ng·h/mL, a maximum plasma concentration (C_max) of 2440 ng/mL, an elimination half-life (t_½) of 0.5 h, and a time to maximum concentration (t_max) of 0.25 h. Results for the 1-mg/kg dose were generally proportional. In rats, mean t_max values were independent of dose and ranged from 0.25 to 1.0 h for single and multiple dosing. C_max values ranged from 516 to 695 ng/mL (1-mg/kg dose) and from 666 to 1180 ng/mL (25-mg/kg dose). In dogs, mean t_max values ranged from 0.4 to 1.3 h for single and multiple dosing. Values for t_max decreased with increasing dose and mean plasma C_max increased less than dose proportionally (96–129 ng·h/mL [1.5 mg], 275–615 ng·h/mL [8.0 mg], and 633–1280 ng·h/mL [40.0 mg]). In humans, PL8177 was observed in the plasma within 15 min after a single dose and persisted for up to 48 h at higher doses. The t_max was 30–45 min (single dose) and 15–45 min (multiple doses). In multiple-dose studies, maximum steady-state plasma concentration (C_max,ss) and AUC_∞ increased with dose. Geometric mean C_max,ss values were 20.1 ng/mL (1.0 mg) and 57.2 ng/mL (3.0 mg). AUC_∞ values were 54.3 ng·h/mL (1.0 mg) and 199 ng·h/mL (3.0 mg). Unchanged PL8177 excreted in the urine was ≤ 1%, and accumulation was minimal.

Conclusion

PL8177 administration resulted in a consistent pharmacokinetic profile. The measured exposure levels resulted in pharmacologically active PL8177 concentrations at the targeted MC1R. Rapid absorption was seen in healthy volunteers, and multiple-dose administration over 7 days resulted in pharmacokinetic characteristics similar to those observed after single-dose administration. Results support the continued development of PL8177 to treat immunologic and inflammatory conditions.

The melanocortin pathway and energy homeostasis: From discovery to obesity therapy

BACKGROUND

Over the past 20 years, insights from human and mouse genetics have illuminated the central role of the brain leptin-melanocortin pathway in controlling mammalian food intake, with genetic disruption resulting in extreme obesity, and more subtle polymorphic variations influencing the population distribution of body weight. At the end of 2020, the U.S. Food and Drug Administration (FDA) approved setmelanotide, a melanocortin 4 receptor agonist, for use in individuals with severe obesity due to either pro-opiomelanocortin (POMC), proprotein convertase subtilisin/kexin type 1 (PCSK1), or leptin receptor (LEPR) deficiency.

SCOPE OF REVIEW

Herein, we chart the melanocortin pathway's history, explore its pharmacology, genetics, and physiology, and describe how a neuropeptidergic circuit became an important druggable obesity target.

MAJOR CONCLUSIONS

Unravelling the genetics of the subset of severe obesity has revealed the importance of the melanocortin pathway in appetitive control; coupling this with studying the molecular pharmacology of compounds that bind melanocortin receptors has brought a new obesity drug to the market. This process provides a drug discovery template for complex disorders, which for setmelanotide took 25 years to transform from a single gene into an approved drug.

Structure reveals the activation mechanism of the MC4 receptor to initiate satiation signaling

Obesity is a global epidemic that causes morbidity and impaired quality of life. The melanocortin receptor 4 (MC4R) is at the crux of appetite, energy homeostasis, and body-weight control in the central nervous system and is a prime target for anti-obesity drugs. Here, we present the cryo-electron microscopy (cryo-EM) structure of the human MC4R-Gs signaling complex bound to the agonist setmelanotide, a cyclic peptide recently approved for the treatment of obesity. The work reveals the mechanism of MC4R activation, highlighting a molecular switch that initiates satiation signaling. In addition, our findings indicate that calcium (Ca2+) is required for agonist, but not antagonist, efficacy. These results fill a gap in the understanding of MC4R activation and could guide the design of future weight-management drugs.

Discovery of Nanomolar Melanocortin-3 Receptor (MC3R)-Selective Small Molecule Pyrrolidine Bis-Cyclic Guanidine Agonist Compounds Via a High-Throughput "Unbiased" Screening Campaign

The central melanocortin-3 and melanocortin-4 receptors (MC3R, MC4R) are key regulators of body weight and energy homeostasis. Herein, the discovery and characterization of first-in-class small molecule melanocortin agonists with selectivity for the melanocortin-3 receptor over the melanocortin-4 receptor are reported. Identified via "unbiased" mixture-based high-throughput screening approaches, pharmacological evaluation of these pyrrolidine bis-cyclic guanidines resulted in nanomolar agonist activity at the melanocortin-3 receptor. The pharmacological profiles at the remaining melanocortin receptor subtypes tested indicated similar agonist potencies at both the melanocortin-1 and melanocortin-5 receptors and antagonist or micromolar agonist activities at the melanocortin-4 receptor. This group of small molecules represents a new area of chemical space for the melanocortin receptors with mixed receptor pharmacology profiles that may serve as novel lead compounds to modulate states of dysregulated energy balance.

MRAP2 Interaction with Melanocortin-4 Receptor in SnakeHead (Channa argus)

The melanocortin-4 receptor (MC4R) plays an important role in the regulation of food intake and energy expenditure. Melanocortin-2 receptor accessory protein 2 (MRAP2) modulates trafficking, ligand binding, and signaling of MC4R. The Northern snakehead (Channa argus) is an economically important freshwater fish native to East Asia. To explore potential interaction between snakehead MC4R and MRAP2, herein we cloned snakehead mc4r and mrap2. The snakehead mc4r consisted of a 984 bp open reading frame encoding a protein of 327 amino acids, while snakehead mrap2 contained a 693 bp open reading frame encoding a protein of 230 amino acids. Synteny analysis indicated that mc4r was highly conserved with similar gene arrangement, while mrap2 contained two isoforms in teleost with different gene orders. Snakehead mc4r was primarily expressed in the brain, whereas mrap2 was expressed in the brain and intestine. Snakehead mc4r and mrap2 expression was modulated by fasting and refeeding. Further pharmacological experiments showed that the cloned snakehead MC4R was functional, capable of binding to peptide agonists and increasing intracellular cAMP production in a dose-dependent manner. Snakehead MC4R exhibited high constitutive activity. MRAP2 significantly decreased basal and agonist-stimulated cAMP signaling. These findings suggest that snakehead MC4R might be involved in energy balance regulation by interacting with MRAP2. Further studies are needed to elucidate MC4R in regulating diverse physiological processes in snakehead.

Multiresidue Tetrapeptide Substitutions Yield a 140-fold Selective Melanocortin-3 over Melanocortin-4 Receptor Agonist

The five melanocortin receptors regulate numerous physiological functions. Although many ligands have been developed for the melanocortin-4 receptor (MC4R), the melanocortin-3 receptor (MC3R) has been less-well characterized, in part due to the lack of potent, selective tool compounds. Previously an Ac-His-Arg-(pI)DPhe-Tic-NH₂ scaffold, inverting the Phe-Arg motif of the native melanocortin signal sequence, was identified to possess mMC3R over mMC4R selective agonist activity. In this study, a library of 12 compounds derived from this scaffold was synthesized and assayed at the mouse melanocortin receptors (MCRs), utilizing substitutions previously shown to increase mMC3R agonist potency and/or selectivity. One compound (8, Ac-Val-Gln-DBip-DTic-NH₂) was identified as greater than 140-fold selective for the mMC3R over the mMC4R, possessed 70 nM potency at the mMC3R, and partially stimulated the mMC4R at 100 μM concentrations without antagonist activity. This pharmacological profile may be useful in developing new tool and therapeutic ligands that selective signal through the MC3R.

Phosphodiesterase Type 5 Inhibitors and Risk of Skin Cancers in Men: A Meta-Analysis and Trial Sequential Analysis Involving 7,479,852 Subjects

Purpose

We conducted a systematic review and meta-analysis to quantify the association between phosphodiesterase type 5 inhibitors (PDE5Is) use and skin cancers and we also examined whether down-expression of the PDE5A gene was related to worse prognosis for malignant melanoma (MM) patients.

Materials and Methods

The PubMed, Cochrane Library, Web of Science, EMBASE, and ClinicalTrails.gov databases were searched. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to evaluate the association between PDE5Is use and risk of skin cancers. Cumulative meta-analysis and trial sequential analysis (TSA) were also conducted. Survival outcomes were analyzed online.

Results

After pooling all 8 eligible studies comprising 7,479,852 subjects, we found that PDE5Is use was significantly associated with slightly increased risk of developing MM (OR: 1.13, 95% CI: 1.05 to 1.21, I²=67.1%), basal cell carcinoma (OR: 1.16, 95% CI: 1.13 to 1.19, I²=49.6%), and squamous cell carcinoma (OR: 1.07, 95% CI: 1.01 to 1.13, I²=0.0%). Totally, PDE5Is increased the risk of developing skin cancers (OR: 1.13, 95% CI: 1.09 to 1.17, I²=70.8%). TSA results showed that the sample size was enough to reach a positive conclusion.

Conclusions

The use of PDE5Is may be slightly associated with increased risk of developing skin cancers. There should be a balance between drug benefits and potential safety issues. However, the pooled results should be considered tentative until confounding factors such as sun exposure and lifestyle are well-controlled in further studies.

Comparative Intracerebroventricular and Intrathecal Administration of a Nanomolar Macrocyclic Melanocortin Receptor Agonist MDE6-5-2c (c[Pro-His-DPhe-Arg-Trp-Dap-Ala-DPro]) Decreases Food Intake in Mice

There is a critical need to find safe therapeutics to treat an increasingly obese population and diseases associated with an imbalance in energy homeostasis. The melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R) ligands have long been the focus to help scientists understand energy homeostasis and the regulation of feeding behavior. Herein, we use a nanomolar macrocyclic melanocortin receptor agonist ligand MDE6-5-2c (c[Pro-His-DPhe-Arg-Trp-Dap-Ala-DPro) to examine metabolic and energy hemostasis profiles upon intrathecal (IT) administration directly into the spinal cord as compared to intracerebroventricular (ICV) administration directly into the brain. Overall, central ICV administration of MDE6-5-2c resulted in decreased food intake, in a dose-dependent manner, and decreased respiratory exchange ratio (RER). Comparison of IT versus ICV routes of MDE6-5-2c administration resulted in MDE6-5-2c possessing a longer duration of action on both feeding behavior and RER via IT. The C-peptide, ghrelin, GIP, leptin, IL-6, and resistin plasma hormones and biomarkers were compared using IT versus ICV MDE6-5-2c routes of administration. Plasma resistin levels were decreased upon ICV treatment of MDE6-5-2c, as compared to ICV vehicle control treatment. Intrathecal treatment resulted in significantly decreased inflammatory cytokine interleukin-6 (IL-6) levels compared to ICV administration. Investigation of the nonselective MC3R and MC4R macrocyclic agonist MDE6-5-2c molecule revealed differences in food intake, RER, and plasma biomarker profiles based upon ICV or IT routes of administration and characterize this novel molecular chemotype as a molecular probe to study the melanocortin system in vivo.

Selective Cyclized α-Melanocyte-Stimulating Hormone Derivative with Multiple N-Methylations for Melanoma Imaging with Positron Emission Tomography

In this study, we designed and evaluated a novel α-melanocyte-stimulating hormone derivative with four N-methylations for melanocortin 1 receptor-targeted melanoma imaging with positron emission tomography (PET). The resulting peptide, DOTA-Pip-Nle⁴-Cyclo[Asp⁵-N-Me-His⁶-d-Phe⁷-N-Me-Arg⁸-N-Me-Trp⁹-N-Me-Lys¹⁰]αMSH_4-10-NH₂ (CCZ01099), showed high receptor selectivity, greatly improved stability, and rapid internalization. [⁶⁸Ga]Ga-CCZ01099 showed clear tumor visualization and excellent tumor-to-normal tissue contrast with PET imaging in a preclinical melanoma model. Therefore, CCZ01099 is a promising compound for imaging and potentially radioligand therapy for melanoma.

Novel insights into the metabolic action of Kiss1 neurons

Kiss1 neurons are essential regulators of the hypothalamic-pituitary-gonadal (HPG) axis by regulating gonadotropin-releasing hormone (GnRH) release. Compelling evidence suggests that Kiss1 neurons of the arcuate nucleus (Kiss1ARC), recently identified as the hypothalamic GnRH pulse generator driving fertility, also participate in the regulation of metabolism through kisspeptinergic and glutamatergic interactions with, at least, proopiomelanocortin (POMC) and agouti-related peptide (AgRP)/neuropeptide Y (NPY) neurons, located in close apposition with Kiss1ARC. This review offers a comprehensive overview of the recent developments, mainly derived from animal models, on the role of Kiss1 neurons in the regulation of energy balance, including food intake, energy expenditure and the influence of circadian rhythms on this role. Furthermore, the possible neuroendocrine pathways underlying this effect, and the existing controversies related to the anorexigenic action of kisspeptin in the different experimental models, are also discussed.

The Bio-Psycho-Social Dimension in Women's Sexual Desire: 'Argumentum ad novitatem'

Sexual desire includes complex motivation and drive. In the context of biological and cognitive- emotive state art of science, it is often a neglected field in medicine. In regard to the treatment, study on women's sexual function received less attention compared to the men's sexuality. In the past, this endeavor was relatively not well disseminated in the scientific community. Recently, there was a revolutionized surge of drug targets available to treat women with low sexual desire. It is timely to review the relevant biological approach, especially in the context of pharmacotherapy to understand this interesting clinical entity which was modulated by numerous interactive psychosocial inter-play and factors. The complex inter-play between numerous dimensional factors lends insights to understand the neural mechanism, i.e. the rewards centre pathway and its interaction with external psychosocialstimulus, e.g. relationship or other meaningful life events. The function of hormones, e.g. oxytocin or testosterone regulation was described. The role of neurotransmitters as reflected by the introduction of a molecule of flibenserin, a full agonist of the 5-HT1A and partial agonist of the D4 to treat premenopausal women with low sexual desire was deliberated. Based on this fundamental scientific core knowledge, we suggest an outline on know-how of introduction for sex therapy (i.e. "inner-self" and "outer-self") where the role of partner is narrated. Then, we also highlighted on the use of pharmacological agent as an adjunct scope of therapy, i.e. phosphodiasterase-5 (PDE-5) inhibitors and hormonal treatment in helping the patient with low sexual desire.

Melanocortin-4 receptor regulation of reproductive function in black rockfish (Sebastes schlegelii)

Melanocortin-4 receptor (MC4R) is a G protein-coupled receptor with multiple functions in mammals. However, the functions of MC4R in fish have not been investigated extensively. The purpose of this study was to determine potential regulation of reproduction by the MC4R. We cloned the black rockfish MC4R and analyzed its tissue distribution and function. The results showed that black rockfish mc4r cDNA consisted of 981 nucleotides encoding a protein of 326 amino acids. The quantitative PCR data showed that mc4r mRNA was primarily expressed in the brain, gonad, stomach and intestine. In the brain, mc4r was found to be primarily located in the hypothalamus. Both α-MSH and β-MSH increased gnih expression and decreased sgnrh and cgnrh expression (P < 0.05). α-MSH and β-MSH had opposite effects on kisspeptin expression. In contrast, α-MSH and β-MSH increased the expression of cyp11, cyp19, 3β-hsd and star. In summary, our study shows that MC4R in black rockfish might regulate reproductive function and that the effects of α-MSH and β-MSH might differ.

Combined Computational and Structural Approach into Understanding the Role of Peptide Binding and Activation of Melanocortin Receptor 4

Melanocortin receptor 4 (MC4R) is expressed predominantly in the central nervous system and regulates food intake and sexual function and is also thought to be responsible for effects on mood and cognition. It belongs to the melanocortin receptor subfamily of G protein-coupled receptors (GPCRs). Here, we have synthesized and structurally characterized three peptides that bind to MC4R, producing different signaling events. AgRP is a naturally occurring antagonist, HLWNRS is the minimal sequence of the N-terminal with partial agonist activity, and aMSH is a full agonistic peptide. By implementing molecular dynamics simulations on the different peptide-receptor complexes, we propose their molecular basis of binding to investigate their differential molecular properties regarding the activation states of the receptor. Our analysis shows that the agonist and partial agonist may induce rotation in transmembrane helix 3, which is known to be involved in the key events occurring during GPCR activation, and this movement is impacted by certain aromatic residues and their positioning in the orthosteric binding site of the receptor.

Diet-induced DNA methylation within the hypothalamic arcuate nucleus and dysregulated leptin and insulin signaling in the pathophysiology of obesity

Obesity rates continue to rise in an unprecedented manner in what could be the most rapid population‐scale shift in human phenotype ever to occur. Increased consumption of unhealthy, calorie‐dense foods, coupled with sedentary lifestyles, is the main factor contributing to a positive energy balance and the development of obesity. Leptin and insulin are key hormones implicated in pathogenesis of this disorder and are crucial for controlling whole‐body energy homeostasis. Their respective function is mediated by the counterbalance of anorexigenic and orexigenic neurons located within the hypothalamic arcuate nucleus. Dysregulation of leptin and insulin signaling pathways within this brain region may contribute not only to the development of obesity, but also systemically affect the peripheral organs, thereby manifesting as metabolic diseases. Although the exact mechanisms detailing how these hypothalamic nuclei contribute to disease pathology are still unclear, increasing evidence suggests that altered DNA methylation may be involved. This review evaluates animal studies that have demonstrated diet‐induced DNA methylation changes in genes that regulate energy homeostasis within the arcuate nucleus, and elucidates possible mechanisms causing hypothalamic leptin and insulin resistance leading to the development of obesity and metabolic diseases.

Oxytocin Neurons Enable Melanocortin Regulation of Male Sexual Function in Mice

The melanocortin pathway has been implicated in both metabolism and sexual function. When the melanocortin 4 receptor (MC4R) is knocked out globally, male mice display obesity, low sexual desire, and copulatory difficulties; however, it is unclear whether these phenotypes are interdependent. To elucidate the neuronal circuitry involved in sexual dysfunction in MC4R knockouts, we re-expressed the MC4R in these mice exclusively on Sim1 neurons (tbMC4RSim1 mice) or on a subset of Sim1 neurons, namely oxytocin neurons (tbMC4Roxt mice). The groups were matched at young ages to control for the effects of obesity. Interestingly, young MC4R null mice had no deficits in sexual motivation or erectile function. However, MC4R null mice were found to have an increased latency to reach ejaculation compared to control mice, which was restored in both tbMC4RSim1 and tbMC4Roxt mice. These results indicate that melanocortin signaling via the MC4R on oxytocin neurons is important for normal ejaculation independent of the male's metabolic health.

Enkephalins and ACTH in the mammalian nervous system

The pentapeptides methionine-enkephalin and leucine-enkephalin belong to the opioid family of peptides, and the non-opiate peptide adrenocorticotropin hormone (ACTH) to the melanocortin peptide family. Enkephalins/ACTH are derived from pro-enkephalin, pro-dynorphin or pro-opiomelanocortin precursors and, via opioid and melanocortin receptors, are responsible for many biological activities. Enkephalins exhibit the highest affinity for the δ receptor, followed by the μ and κ receptors, whereas ACTH binds to the five subtypes of melanocortin receptor, and is the only member of the melanocortin family of peptides that binds to the melanocortin-receptor 2 (ACTH receptor). Enkephalins/ACTH and their receptors exhibit a widespread anatomical distribution. Enkephalins are involved in analgesia, angiogenesis, blood pressure, embryonic development, emotional behavior, feeding, hypoxia, limbic system modulation, neuroprotection, peristalsis, and wound repair; as well as in hepatoprotective, motor, neuroendocrine and respiratory mechanisms. ACTH plays a role in acetylcholine release, aggressive behavior, blood pressure, bone maintenance, hyperalgesia, feeding, fever, grooming, learning, lipolysis, memory, nerve injury repair, neuroprotection, sexual behavior, sleep, social behavior, tissue growth and stimulates the synthesis and secretion of glucocorticoids. Enkephalins/ACTH are also involved in many pathologies. Enkephalins are implicated in alcoholism, cancer, colitis, depression, heart failure, Huntington's disease, influenza A virus infection, ischemia, multiple sclerosis, and stress. ACTH plays a role in Addison's disease, alcoholism, cancer, Cushing's disease, dermatitis, encephalitis, epilepsy, Graves' disease, Guillain-Barré syndrome, multiple sclerosis, podocytopathies, and stress. In this review, we provide an updated description of the enkephalinergic and ACTH systems.

Current Mechanistic and Pharmacodynamic Understanding of Melanocortin-4 Receptor Activation

In this work we summarize our understanding of melanocortin 4 receptor (MC4R) pathway activation, aiming to define a safe and effective therapeutic targeting strategy for the MC4R. Delineation of cellular MC4R pathways has provided evidence for distinct MC4R signaling events characterized by unique receptor activation kinetics. While these studies remain narrow in scope, and have largely been explored with peptidic agonists, the results provide a possible correlation between distinct ligand groups and differential MC4R activation kinetics. In addition, when a set of small-molecule and peptide MC4R agonists are compared, evidence of biased signaling has been reported. The results of such mechanistic studies are discussed.

Structure⁻Activity Relationships of the Tetrapeptide Ac-His-Arg-(pI)DPhe-Tic-NH2 at the Mouse Melanocortin Receptors: Modification at the (pI)DPhe Position Leads to mMC3R Versus mMC4R Selective Ligands

The five melanocortin receptors (MC1R–MC5R) are involved in numerous biological pathways, including steroidogenesis, pigmentation, and food intake. In particular, MC3R and MC4R knockout mice suggest that the MC3R and MC4R regulate energy homeostasis in a non-redundant manner. While MC4R-selective agonists have been utilized as appetite modulating agents, the lack of MC3R-selective agonists has impeded progress in modulating this receptor in vivo. In this study, the (pI)DPhe position of the tetrapeptide Ac-His-Arg-(pI)DPhe-Tic-NH₂ (an MC3R agonist/MC4R antagonist ligand) was investigated with a library of 12 compounds. The compounds in this library were found to have higher agonist efficacy and potency at the mouse (m) MC3R compared to the MC4R, indicating that the Arg-DPhe motif preferentially activates the mMC3R over the mMC4R. This observation may be used in the design of new MC3R-selective ligands, leading to novel probe and therapeutic lead compounds that will be useful for treating metabolic disorders.

The melanocortin pathway and control of appetite-progress and therapeutic implications

The initial discovery that ob/ob mice become obese because of a recessive mutation of the leptin gene has been crucial to discover the melanocortin pathway to control appetite. In the melanocortin pathway, the fed state is signaled by abundance of circulating hormones such as leptin and insulin, which bind to receptors expressed at the surface of pro-opiomelanocortin (POMC) neurons to promote processing of POMC to the mature hormone α-melanocyte-stimulating hormone (α-MSH). The α-MSH released by POMC neurons then signals to decrease energy intake by binding to melanocortin-4 receptor (MC4R) expressed by MC4R neurons to the paraventricular nucleus (PVN). Conversely, in the 'starved state' activity of agouti-related neuropeptide (AgRP) and of neuropeptide Y (NPY)-expressing neurons is increased by decreased levels of circulating leptin and insulin and by the orexigenic hormone ghrelin to promote food intake. This initial understanding of the melanocortin pathway has recently been implemented by the description of the complex neuronal circuit that controls the activity of POMC, AgRP/NPY and MC4R neurons and downstream signaling by these neurons. This review summarizes the progress done on the melanocortin pathway and describes how obesity alters this pathway to disrupt energy homeostasis. We also describe progress on how leptin and insulin receptors signal in POMC neurons, how MC4R signals and how altered expression and traffic of MC4R change the acute signaling and desensitization properties of the receptor. We also describe how the discovery of the melanocortin pathway has led to the use of melanocortin agonists to treat obesity derived from genetic disorders.

Neuroanatomical Framework of the Metabolic Control of Reproduction

A minimum amount of energy is required for basic physiological processes, such as protein biosynthesis, thermoregulation, locomotion, cardiovascular function, and digestion. However, for reproductive function and survival of the species, extra energy stores are necessary. Production of sex hormones and gametes, pubertal development, pregnancy, lactation, and parental care all require energy reserves. Thus the physiological systems that control energy homeostasis and reproductive function coevolved in mammals to support both individual health and species subsistence. In this review, we aim to gather scientific knowledge produced by laboratories around the world on the role of the brain in integrating metabolism and reproduction. We describe essential neuronal networks, highlighting key nodes and potential downstream targets. Novel animal models and genetic tools have produced substantial advances, but critical gaps remain. In times of soaring worldwide obesity and metabolic dysfunction, understanding the mechanisms by which metabolic stress alters reproductive physiology has become crucial for human health.

Sex and Depression

Sexual dysfunction is one of the more common features of depressive disorders, presenting with dysfunction across sexual response cycle. Variety of factors play a role in causing sexual dysfunction in these patients, such as psychological, biological, social and interpersonal factors. Another cause of sexual dysfunction in these patients can be the side effect of antidepressants making it difficult to conclude if the dysfunction is the result of the depression or the treatment of depression. Clinicians need to be aware about the sexual dysfunction as it can have big impact on the overall quality of life of an individual.

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.

Discovery of Melanocortin Ligands via a Double Simultaneous Substitution Strategy Based on the Ac-His-dPhe-Arg-Trp-NH2 Template

The melanocortin system regulates an array of diverse physiological functions including pigmentation, feeding behavior, energy homeostasis, cardiovascular regulation, sexual function, and steroidogenesis. Endogenous melanocortin agonist ligands all possess the minimal messaging tetrapeptide sequence His-Phe-Arg-Trp. Based on this endogenous sequence, the Ac-His¹-dPhe²-Arg³-Trp⁴-NH₂ tetrapeptide has previously been shown to be a useful scaffold when utilizing traditional positional scanning approaches to modify activity at the various melanocortin receptors (MC1-5R). The study reported herein was undertaken to evaluate a double simultaneous substitution strategy as an approach to further diversify the Ac-His¹-dPhe²-Arg³-Trp⁴-NH₂ tetrapeptide with concurrent introduction of natural and unnatural amino acids at positions 1, 2, or 4, as well as an octanoyl residue at the N-terminus. The designed library includes the following combinations: (A) double simultaneous substitution at capping group position (Ac) together with position 1, 2, or 4, (B) double simultaneous substitution at positions 1 and 2, (C) double simultaneous substitution at positions 1 and 4, and (D) double simultaneous substitution at positions 2 and 4. Several lead ligands with unique pharmacologies were discovered in the current study including antagonists targeting the neuronal mMC3R with minimal agonist activity and ligands with selective profiles for the various melanocortin subtypes. The results suggest that the double simultaneous substitution strategy is a suitable approach in altering melanocortin receptor potency or selectivity or converting agonists into antagonists and vice versa.

Genetic variation in the SIM1 locus is associated with erectile dysfunction

Erectile dysfunction is a common condition of men in middle and older ages. Twin studies suggest that about one-third of the risk is due to genetic factors, independent of other known erectile dysfunction risk factors. However, studies that have searched for specific genetic contributors have been limited due to small sample sizes, candidate gene approaches, and weak phenotyping. As a result, there are no confirmed genetic risk factors for erectile dysfunction. This study finds a specific genetic cause for erectile dysfunction.

Erectile dysfunction affects millions of men worldwide. Twin studies support the role of genetic risk factors underlying erectile dysfunction, but no specific genetic variants have been identified. We conducted a large-scale genome-wide association study of erectile dysfunction in 36,649 men in the multiethnic Kaiser Permanente Northern California Genetic Epidemiology Research in Adult Health and Aging cohort. We also undertook replication analyses in 222,358 men from the UK Biobank. In the discovery cohort, we identified a single locus (rs17185536-T) on chromosome 6 near the single-minded family basic helix-loop-helix transcription factor 1 (SIM1) gene that was significantly associated with the risk of erectile dysfunction (odds ratio = 1.26, P = 3.4 × 10⁻²⁵). The association replicated in the UK Biobank sample (odds ratio = 1.25, P = 6.8 × 10⁻¹⁴), and the effect is independent of known erectile dysfunction risk factors, including body mass index (BMI). The risk locus resides on the same topologically associating domain as SIM1 and interacts with the SIM1 promoter, and the rs17185536-T risk allele showed differential enhancer activity. SIM1 is part of the leptin–melanocortin system, which has an established role in body weight homeostasis and sexual function. Because the variants associated with erectile dysfunction are not associated with differences in BMI, our findings suggest a mechanism that is specific to sexual function.

Developing a Biased Unmatched Bivalent Ligand (BUmBL) Design Strategy to Target the GPCR Homodimer Allosteric Signaling (cAMP over β-Arrestin 2 Recruitment) Within the Melanocortin Receptors

Understanding the functional relevance of G protein-coupled receptor (GPCR) homodimerization has been limited by the insufficient tools to assess asymmetric signaling occurring within dimers comprised of the same receptor type. We present unmatched bivalent ligands (UmBLs) to study the asymmetric function of melanocortin homodimers. UmBLs contain one agonist and one antagonist pharmacophore designed to target a melanocortin homodimer such that one receptor is occupied by an agonist and the other receptor by an antagonist pharmacophore. First-in-class biased UmBLs (BUmBLs) targeting the human melanocortin-4 receptor (hMC4R) were discovered. The BUmBLs displayed biased agonism by potently stimulating cAMP signaling (EC₅₀ ∼ 2-6 nM) but minimally activating the β-arrestin recruitment pathway (≤55% maximum signal at 10 μM). To our knowledge, we report the first single-compound strategy to pharmacologically target melanocortin receptor allosteric signaling that occurs between homodimers that can be applied straightforwardly in vitro and in vivo to other GPCR systems.

MC4R Agonists: Structural Overview on Antiobesity Therapeutics

The melanocortin-4 receptor (MC4R) regulates adipose tissue formation and energy homeostasis, and is believed to be a monogenic target for novel antiobesity therapeutics. Several research efforts targeting this receptor have identified potent and selective agonists. While viable agonists have been characterized in vitro, undesirable side effects frequently appeared during clinical trials. The most promising candidates have diverse structures, including linear peptides, cyclic peptides, and small molecules. Herein, we present a compilation of potent MC4R agonists and discuss the pivotal structural differences within those molecules that resulted in good selectivity for MC4R over other melanocortins. We provide insight on recent progress in the field and reflect on directions for development of new agonists.

Connecting metabolism and gonadal function: Novel central neuropeptide pathways involved in the metabolic control of puberty and fertility

Albeit essential for perpetuation of species, reproduction is an energy-demanding function that can be adjusted to body metabolic status. Reproductive maturation and function can be suppressed in conditions of energy deficit, but can be altered also in situations of persistent energy excess, e.g., morbid obesity. This metabolic-reproductive integration, of considerable pathophysiological relevance to explain different forms of perturbed puberty and sub/infertility, is implemented by the concerted action of numerous central and peripheral regulators, which impinge at different levels of the hypothalamic-pituitary-gonadal (HPG) axis, permitting a tight fit between nutritional/energy status and gonadal function. We summarize here the major physiological mechanisms whereby nutritional and metabolic cues modulate the maturation and function of the HPG axis. We will focus on recent progress on the major central neuropeptide pathways, including kisspeptins, neurokinin B and the products of POMC and NPY neurons, which convey metabolic information to GnRH neurons, as major hierarchical hub of our reproductive brain.

Sim1 Neurons Are Sufficient for MC4R-Mediated Sexual Function in Male Mice

Sexual dysfunction is a poorly understood condition that affects up to one-third of men around the world. Existing treatments that target the periphery do not work for all men. Previous studies have shown that central melanocortins, which are released by pro-opiomelanocortin neurons in the arcuate nucleus of the hypothalamus, can lead to male erection and increased libido. Several studies specifically implicate the melanocortin 4 receptor (MC4R) in the central control of sexual function, but the specific neural circuitry involved is unknown. We hypothesized that single-minded homolog 1 (Sim1) neurons play an important role in the melanocortin-mediated regulation of male sexual behavior. To test this hypothesis, we examined the sexual behavior of mice expressing MC4R only on Sim1-positive neurons (tbMC4Rsim1 mice) in comparison with tbMC4R null mice and wild-type controls. In tbMC4Rsim1 mice, MC4R reexpression was found in the medial amygdala and paraventricular nucleus of the hypothalamus. These mice were paired with sexually experienced females, and their sexual function and behavior was scored based on mounting, intromission, and ejaculation. tbMC4R null mice showed a longer latency to mount, a reduced intromission efficiency, and an inability to reach ejaculation. Expression of MC4R only on Sim1 neurons reversed the sexual deficits seen in tbMC4R null mice. This study implicates melanocortin signaling via the MC4R on Sim1 neurons in the central control of male sexual behavior.