Structure, function and regulation of the melanocortin receptors

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.

Cryo-electron microscopy for GPCR research and drug discovery in endocrinology and metabolism

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors, with many GPCRs having crucial roles in endocrinology and metabolism. Cryogenic electron microscopy (cryo-EM) has revolutionized the field of structural biology, particularly regarding GPCRs, over the past decade. Since the first pair of GPCR structures resolved by cryo-EM were published in 2017, the number of GPCR structures resolved by cryo-EM has surpassed the number resolved by X-ray crystallography by 30%, reaching >650, and the number has doubled every ~0.63 years for the past 6 years. At this pace, it is predicted that the structure of 90% of all human GPCRs will be completed within the next 5-7 years. This Review highlights the general structural features and principles that guide GPCR ligand recognition, receptor activation, G protein coupling, arrestin recruitment and regulation by GPCR kinases. The Review also highlights the diversity of GPCR allosteric binding sites and how allosteric ligands could dictate biased signalling that is selective for a G protein pathway or an arrestin pathway. Finally, the authors use the examples of glycoprotein hormone receptors and glucagon-like peptide 1 receptor to illustrate the effect of cryo-EM on understanding GPCR biology in endocrinology and metabolism, as well as on GPCR-related endocrine diseases and drug discovery.

Discovery of CRN04894: A Novel Potent Selective MC2R Antagonist

A novel class of nonpeptide melanocortin type 2 receptor (MC2R) antagonists was discovered through modification of known nonpeptide MC4R ligands. Structure-activity relationship (SAR) studies led to the discovery of 17h (CRN04894), a highly potent and subtype-selective first-in-class MC2R antagonist, which demonstrated remarkable efficacy in a rat model of adrenocorticotrophic hormone (ACTH)-stimulated corticosterone secretion. Oral administration of 17h suppressed ACTH-stimulated corticosterone secretion in a dose-dependent manner at doses ≥3 mg/kg. With its satisfactory pharmaceutical properties, 17h was advanced to Phase 1 human clinical trials in healthy volunteers with the goal of moving into patient trials to evaluate CRN04894 for the treatment of ACTH-dependent diseases, including congenital adrenal hyperplasia (CAH) and Cushing's disease (CD).

α-Melanocyte-stimulating hormone alleviates pathological cardiac remodeling via melanocortin 5 receptor

α-Melanocyte-stimulating hormone (α-MSH) regulates diverse physiological functions by activating melanocortin receptors (MC-R). However, the role of α-MSH and its possible target receptors in the heart remain completely unknown. Here we investigate whether α-MSH could be involved in pathological cardiac remodeling. We found that α-MSH was highly expressed in the mouse heart with reduced ventricular levels after transverse aortic constriction (TAC). Administration of a stable α-MSH analog protected mice against TAC-induced cardiac hypertrophy and systolic dysfunction. In vitro experiments revealed that MC5-R in cardiomyocytes mediates the anti-hypertrophic signaling of α-MSH. Silencing of MC5-R in cardiomyocytes induced hypertrophy and fibrosis markers in vitro and aggravated TAC-induced cardiac hypertrophy and fibrosis in vivo. Conversely, pharmacological activation of MC5-R improved systolic function and reduced cardiac fibrosis in TAC-operated mice. In conclusion, α-MSH is expressed in the heart and protects against pathological cardiac remodeling by activating MC5-R in cardiomyocytes. These results suggest that analogs of naturally occurring α-MSH, that have been recently approved for clinical use and have agonistic activity at MC5-R, may be of benefit in treating heart failure.

The Neuropeptide α-Melanocyte-Stimulating Hormone Prevents Persistent Corneal Edema following Injury

Corneal endothelial cells (CEnCs) regulate corneal hydration and maintain tissue transparency through their barrier and pump function. However, these cells exhibit limited regenerative capacity following injury. Currently, corneal transplantation is the only established therapy for restoring endothelial function, and there are no pharmacologic interventions available for restoring endothelial function. This study investigated the efficacy of the neuropeptide α-melanocyte-stimulating hormone (α-MSH) in promoting endothelial regeneration during the critical window between ocular injury and the onset of endothelial decompensation using an established murine model of injury using transcorneal freezing. Local administration of α-MSH following injury prevented corneal edema and opacity, reduced leukocyte infiltration, and limited CEnC apoptosis while promoting their proliferation. These results suggest that α-MSH has a proregenerative and cytoprotective function on CEnCs and shows promise as a therapy for the prevention and management of corneal endothelial dysfunction.

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.

The melanocortin receptor signaling system and its role in neuroprotection against neurodegeneration: Therapeutic insights

The melanocortin signaling system consists of the melanocortin peptides, their distinctive receptors, accessory proteins, and endogenous antagonists. Melanocortin peptides are small peptide hormones that have been studied in a variety of physiological and pathological conditions. There are five types of melanocortin receptors, and they are distributed within the central nervous system and in several tissues of the periphery. The G protein-coupled melanocortin receptors typically signal through adenylyl cyclase and other downstream signaling pathways. Depending on the ligand, surface expression of melanocortin receptor, receptor occupancy period, related proteins, the type of cell, and other parameters, the signaling pathways are complicated and pleiotropic. While it is known that all five melanocortin receptors are coupled to Gs, they can also occasionally couple to Gq or Gi. Both direct and indirect neuroprotection are induced by the melanocortin signaling system. Targeting several of the components of the melanocortin signaling system (ligands, receptors, accessory proteins, signaling effectors, and regulators) may provide therapeutic opportunities. Activation of the melanocortin system improves different functional traits in neurodegenerative diseases. There is a potential for additional melanocortin system interventions by interfering with dimerization or dissociation. This review aims to discuss the melanocortin receptor signaling system and its role in neuroprotection, as well as its therapeutic potential.

Effects of the Combination of the C1473G Mutation in the Tph2 Gene and Lethal Yellow Mutations in the Raly-Agouti Locus on Behavior, Brain 5-HT and Melanocortin Systems in Mice

Tryptophan hydroxylase 2 (TPH2) is the key and rate-limited enzyme of serotonin (5-HT) synthesis in the brain. The C1473G mutation in the Tph2 gene results in a two-fold decrease in enzyme activity in the mouse brain. The lethal yellow (A^Y) mutation in the Raly-Agouti locus results in the overexpression of the Agouti gene in the brain and causes obesity and depressive-like behavior in mice. Herein, the possible influences of these mutations and their combination on body mass, behavior, brain 5-HT and melanocortin systems in mice of the B6-1473CC/aa. B6-1473CC/A^Ya, B6-1473GG/aa are investigated. B6-1473GG/A^Ya genotypes were studied. The 1473G and A^Y alleles increase the activity of TPH2 and the expression of the Agouti gene, respectively, but they do not alter 5-HT and 5-HIAA levels or the expression of the genes Tph2, Maoa, Slc6a4, Htr1a, Htr2a, Mc3r and Mc4r in the brain. The 1473G allele attenuates weight gain and depressive-like immobility in the forced swim test, while the A^Y allele increases body weight gain and depressive-like immobility. The combination of these alleles results in hind limb dystonia in the B6-1473GG/A^Ya mice. This is the first evidence for the interaction between the C1473G and A^Y mutations.

The Melanocortin System: A Promising Target for the Development of New Antidepressant Drugs

Major depression is one of the most prevalent mental disorders, causing significant human suffering and socioeconomic loss. Since conventional antidepressants are not sufficiently effective, there is an urgent need to develop new antidepressant medications. Despite marked advances in the neurobiology of depression, the etiology and pathophysiology of this disease remain poorly understood. Classical and newer hypotheses of depression suggest that an imbalance of brain monoamines, dysregulation of the hypothalamic-pituitary-adrenal axis (HPAA) and immune system, or impaired hippocampal neurogenesis and neurotrophic factors pathways are cause of depression. It is assumed that conventional antidepressants improve these closely related disturbances. The purpose of this review was to discuss the possibility of affecting these disturbances by targeting the melanocortin system, which includes adrenocorticotropic hormone-activated receptors and their peptide ligands (melanocortins). The melanocortin system is involved in the regulation of various processes in the brain and periphery. Melanocortins, including peripherally administered non-corticotropic agonists, regulate HPAA activity, exhibit anti-inflammatory effects, stimulate the levels of neurotrophic factors, and enhance hippocampal neurogenesis and neurotransmission. Therefore, endogenous melanocortins and their analogs are able to complexly affect the functioning of those body’s systems that are closely related to depression and the effects of antidepressants, thereby demonstrating a promising antidepressant potential.

Evaluating the association between the Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diet, mental health, and cardio-metabolic risk factors among individuals with obesity

BACKGROUND

Several previous investigations have examined the brain-protective role of the Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diet. However, more knowledge is needed about the MIND diet's other favorable impacts. The purpose of this study was to examine the relationship between the MIND diet, mental health, and metabolic markers in individuals with obesity.

METHODS

In this cross-sectional study, we included 339 individuals with obesity (BMI ≥ 30 kg/m²) aged 20-50 years. We utilized a semi-quantitative Food Frequency Questionnaire (FFQ), we assessed dietary intake, including 168 food items, and calculated the value of MIND. Metabolic syndrome (MetS) was defined according to the National Cholesterol Education Program Adult Treatment Panel III (NCEP-ATP III) guidelines. We assessed biochemical parameters using Enzymatic methods. Blood pressure and body composition were also determined.

RESULTS

Higher tertiles of the MIND diet score were associated with significantly higher energy intake, macronutrients, and brain-healthy food intakes (P < 0.001). Among the brain-unhealthy foods, only the intake of sweets and pastries was significantly lower in the highest versus lowest MIND tertiles. We also observed lower odds of stress (P < 0.05) and higher insulin sensitivity (P < 0.05) in the highest versus lowest MIND diet tertiles. We witnessed no significant changes in other parameters.

CONCLUSION

Lower stress levels and higher insulin sensitivity independent of some confounders like age, BMI, sex, and physical activity were associated with the highest tertile of MIND diet score.

Melanocortin receptor agonist NDP-α-MSH improves cognitive deficits and microgliosis but not amyloidosis in advanced stages of AD progression in 5XFAD and 3xTg mice

Introduction

Alzheimer's disease (AD) is the most frequent cause of dementia and still lacks effective therapy. Clinical signs of AD include low levels of endogenous melanocortins (MCs) and previous studies have shown that treatment with MC analogs induces neuroprotection in the early stages of AD.

Methods

We investigated the neuroprotective role of MCs in two transgenic mouse models of severe AD using 5 and 7 month-old (mo) 5XFAD mice and 9 and 12 mo 3xTg mice. These mice were subjected to a chronic stimulation of MC receptors (MCRs) with MC analogue Nle4-D-Phe7-α-melanocyte stimulating hormone (NDP-α-MSH, 340 μg/kg, i.p.). Mouse behavior and ex-vivo histological and biochemical analyses were performed after 50 days of treatment.

Results

Our analysis demonstrated an improvement in cognitive abilities of AD mice at late stage of AD progression. We also showed that these protective effects are associated with decreased levels of hyperphosphorylated Tau but not with Aβ burden, that was unaffected in the hippocampus and in the cortex of AD mice. In addition, an age-dependent NDP effect on glial reactivity was observed only in 3xTg mice whereas a global downregulation of p38 mitogen-activated protein kinase was selectively observed in 7 mo 5XFAD and 14 mo 3xTg mice.

Conclusion

Our results suggest that MCR stimulation by NDP-α-MSH could represent a promising therapeutic strategy in managing cognitive decline also at late stage of AD, whereas the effects on neuroinflammation may be restricted to specific stages of AD progression.

Lethal Yellow Mutation Causes Anxiety, Obsessive-compulsive Behavior and Affects the Brain Melanocortin System in Males and Females of Mice

BACKGROUND

The brain melanocortin system regulates numerous physiological functions and kinds of behavior. The agouti protein inhibits melanocortin receptors in melanocytes. The lethal yellow (AY) mutation puts the Agouti gene under the control of the Raly gene promotor and causes the agouti protein expression in the brain. In the present article, we investigated the effects of the AY mutation on brain mRNA levels of Agouti, Raly, and melanocortin-related genes such as Agrp, Pomc, Mc3r, Mc4r, and their relationship to behavior.

METHODS

The experiment was performed on 6-month-old males and females of AY/a and a/a (control) mice. Anxiety and obsessive-compulsive behavior were studied in elevated plus-maze and marble- burying tests. The mRNA levels were quantified by qPCR.

RESULTS

AY mutation caused anxiety in males and obsessive-compulsive behavior in females. Positive correlation between Agouti and Raly genes mRNA levels were shown in the hypothalamus, hippocampus, and frontal cortex in AY/a mice. Reduced RNA concentrations of Mc3r and Mc4r genes were found respectively in the hypothalamus and frontal cortex in AY/a males. The Raly gene expression positively correlates with mRNA concentrations of the Mc3r gene in the hypothalamus and the Mc4r gene in the hypothalamus and frontal cortex.

CONCLUSION

Possible association of obsessive-compulsive behavior with reduced Raly, Mc3r, or Mc4r gene expression is suggested.

α-MSH is partially involved in the immunomodulation of Nile tilapia (Oreochromis niloticus) antibacterial immunity

α-Melanocyte-stimulating hormone (α-MSH) is a well-studied neuropeptide controlling skin and hair color. Besides, numerous immunomodulation roles of α-MSH were recorded in humans and mice. However, the regulatory effects of α-MSH in teleost immunity haven't been well elucidated. In this study, several precursor molecules of α-MSH (POMCs) and its receptors (MCRs) in Nile tilapia (Oreochromis niloticus) were characterized, and their expression characteristics and specific functions on antibacterial immunity were determined. Overall, POMCs and MCRs were principally detected in the brain, skin, and liver, and were remarkably promoted post Streptococcus agalactiae infection. However, tiny POMCs and MCRs were observed in tilapia immune organs (head kidney and spleen) or lymphocytes, and no evident immunomodulation effect was detected in vitro. Moreover, the in vivo challenge experiments revealed that α-MSH protects tilapia from bacterial infection by regulating responses in the brain and intestine. This study lays theoretical data for a deeper comprehension of the immunomodulation mechanisms of teleost α-MSH and the evolutional process of the vertebrate melanocortin system.

Late-Stage Functionalization with Cysteine Staples Generates Potent and Selective Melanocortin Receptor-1 Agonists

In this work, cysteine staples were used as a late-stage functionalization strategy to diversify peptides and build conjugates targeting the melanocortin G-protein-coupled receptors [melanocortin receptor-1 (MC1R) and MC3R-MC5R]. Monocyclic and bicyclic agonists based on sunflower trypsin inhibitor-1 were used to generate a selection of stapled peptides that were evaluated for binding (pK_i) and functional activation (pEC₅₀) of the melanocortin receptor subtypes. Stapled peptides generally had improved activity, with aromatic stapled peptides yielding selective MC1R agonists, including a xylene-stapled peptide (2) with an EC₅₀ of 1.9 nM for MC1R and >150-fold selectivity for MC3R and MC4R. Selected stapled peptides were further functionalized with linkers and payloads, generating a series of conjugated peptides with potent MC1R activity, including one pyridazine-functionalized peptide (21) with picomolar activity at MC1R (K_i 58 pM; EC₅₀ < 9 pM). This work demonstrates that staples can be used as modular synthetic tools to tune potency and selectivity in peptide-based drug design.

Immunomodulatory Role of Neuropeptides in the Cornea

The transparency of the cornea along with its dense sensory innervation and resident leukocyte populations make it an ideal tissue to study interactions between the nervous and immune systems. The cornea is the most densely innervated tissue of the body and possesses both immune and vascular privilege, in part due to its unique repertoire of resident immune cells. Corneal nerves produce various neuropeptides that have a wide range of functions on immune cells. As research in this area expands, further insights are made into the role of neuropeptides and their immunomodulatory functions in the healthy and diseased cornea. Much remains to be known regarding the details of neuropeptide signaling and how it contributes to pathophysiology, which is likely due to complex interactions among neuropeptides, receptor isoform-specific signaling events, and the inflammatory microenvironment in disease. However, progress in this area has led to an increase in studies that have begun modulating neuropeptide activity for the treatment of corneal diseases with promising results, necessitating the need for a comprehensive review of the literature. This review focuses on the role of neuropeptides in maintaining the homeostasis of the ocular surface, alterations in disease settings, and the possible therapeutic potential of targeting these systems.

The Multifaceted Melanocortin Receptors

The 5 known melanocortin receptors (MCs) have established physiological roles. With the exception of MC2, these receptors can behave unpredictably, and since they are more widely expressed than their established roles would suggest, it is likely that they have other poorly characterized functions. The aim of this review is to discuss some of the less well-explored aspects of the 4 enigmatic members of this receptor family (MC1,3-5) and describe how these are multifaceted G protein-coupled receptors (GPCRs). These receptors appear to be promiscuous in that they bind several endogenous agonists (products of the proopiomelanocortin [POMC] gene) and antagonists but with inconsistent relative affinities and effects. We propose that this is a result of posttranslational modifications that determine receptor localization within nanodomains. Within each nanodomain there will be a variety of proteins, including ion channels, modifying proteins, and other GPCRs, that can interact with the MCs to alter the availability of receptor at the cell surface as well as the intracellular signaling resulting from receptor activation. Different combinations of interacting proteins and MCs may therefore give rise to the complex and inconsistent functional profiles reported for the MCs. For further progress in understanding this family, improved characterization of tissue-specific functions is required. Current evidence for interactions of these receptors with a range of partners, resulting in modulation of cell signaling, suggests that each should be studied within the full context of their interacting partners. The role of physiological status in determining this context also remains to be characterized.

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.

Functional characterization of cAMP signaling of variant porcine MC1R alleles in PK15 cells

The melanocortin 1 receptor (MC1R), encoded by the classical extension (E) coat color locus, is expressed on the surface of melanocytes and plays a critical role in switching melanin synthesis from pheomelanin (red/yellow) to eumelanin (black/brown). Different MC1R alleles associated with various coat color patterns in pigs have been identified over the past decades. However, functional analysis of variant porcine MC1R alleles has not yet been performed. Therefore, in this study, we examined the subcellular localization and cyclic adenosine monophosphate (cAMP) signaling capability of MC1R variants in porcine kidney epithelial cells (PK15) overexpressing different MC1R alleles. Transcriptional slippage may partially restore the reading frame of the E^P allele, possibly accounting for the observed spot phenotype. The A243T substitution in the e allele severely disrupted the membrane localization of the MC1R receptor, resulting in a severely impaired cAMP signaling capability. Both the V95M and L102P substitutions in the E^D1 allele may contribute to the constitutively active function of MC1R, thus accounting for the dominant black phenotype. The D124N substitution in the E^D2 allele severely attenuated the cAMP signaling capability of MC1R; however, whether this mutation contributes to the distinct phenotype of Hampshire pigs requires further investigation. Thus, our results provide new insights into the functional characteristics of MC1R variants and their roles in porcine coat color formation.

Pharmacological Evaluation of Melanocortin 2 Receptor Accessory Protein 2 on Axolotl Neural Melanocortin Signaling

The Melanocortin-3 receptor (MC3R) and Melanocortin-4 receptor (MC4R), two members of the key hypothalamic neuropeptide signaling, function as complex mediators to control the central appetitive and energy homeostasis. The melanocortin 2 receptor accessory protein 2 (MRAP2) is well-known for its modulation on the trafficking and signaling of MC3R and MC4R in mammals. In this study, we cloned and elucidated the pharmacological profiles of MRAP2 on the regulation of central melanocortin signaling in a relatively primitive poikilotherm amphibian species, the Mexican axolotl (Ambystoma mexicanum). Our results showed the higher conservation of axolotl mc3r and mc4r across species than mrap2, especially the transmembrane regions in these proteins. Phylogenetic analysis indicated that the axolotl MC3R/MC4R clustered closer to their counterparts in the clawed frog, whereas MRAP2 fell in between the reptile and amphibian clade. We also identified a clear co-expression of mc3r, mc4r, and mrap2 along with pomc and agrp in the axolotl brain tissue. In the presence of MRAP2, the pharmacological stimulation of MC3R by α-MSH or ACTH significantly decreased. MRAP2 significantly decreased the cell surface expression of MC4R in a dose dependent manner. The co-localization and formation of the functional complex of axolotl MC3R/MC4R and MRAP2 on the plasma membrane were further confirmed in vitro. Dramatic changes of the expression levels of mc3r, mrap2, pomc, and agrp in the fasting axolotl hypothalamus indicated their critical roles in the metabolic regulation of feeding behavior and energy homeostasis in the poikilotherm aquatic amphibian.

Intracerebroventricular administration of α-melanocyte-stimulating hormone (α-MSH) enhances thigmotaxis and induces anxiety-like behavior in the goldfish Carassius auratus

α-Melanocyte-stimulating hormone (α-MSH) is a body pigmentation-regulating hormone secreted from the intermediate lobe of the pituitary in vertebrates. It is also produced in the brain, and acts as an anorexigenic neuropeptide involved in feeding regulation. In rodents, intracerebroventricular (ICV) administration of α-MSH has been shown to affect not only feeding behavior, but also psychomotor activity. However, there is still no information regarding the psychophysiological effects of α-MSH on behavior in fish. Therefore, we examined the effect of synthetic α-MSH on psychomotor activity in goldfish. Since this species prefers the edge to the central area of a tank, we used this as a preference test for assessing psychomotor activity. When α-MSH was administered ICV at 1 and 10 pmol g^-1 body weight (BW), the time spent in the edge area of a tank was prolonged at 10 pmol g^-1 BW. However, α-MSH at these doses did not affect locomotor activity. The action of α-MSH mimicked those of FG-7142 (a central-type benzodiazepine receptor (CBR) inverse agonist with an anxiogenic effect) at 10 pmol g^-1 BW and melanotan II (a melanocortin 4 receptor (MC4R) agonist) at 50 pmol g^-1 BW, whereas ICV administration of tofisopam (a CBR agonist with an anxiolytic effect) at 10 pmol g^-1 BW prolonged the time spent in the central area. The anxiogenic-like effect of α-MSH was abolished by treatment with the MC4R antagonist HS024 at 50 pmol g^-1 BW. These data indicate that α-MSH affects psychomotor activity in goldfish, and exerts an anxiogenic-like effect via the MC4R-signaling pathway.

Investigation of Hemodynamic Receptors of the Internal Carotid Artery Segments

Objectives

Internal carotid artery (ICA), the main artery of the brain, passes through the cavernous sinus (CS) which forms one of these venous pools. During this transition, while there is arterial blood in the lumen of ICA, its outer surface is in contact with venous blood from the brain. Herein, we aimed to detect the receptor differences of ICA in this highly specialized anatomical region of the skull base.

Methods

We performed the study on 10 human cadavers and searched CGRPR, TRP12, ASIC3 and ACTHR receptors via immunostaining using laser scanning confocal microscopy.

Results

We determined TRP12 receptor positive in the tunica media and tunica adventitia layers of the cavernous segment of ICA. We did not detect similar positivity in the cervical part of the ICA. In the receptor scan we made in terms of CGRPR, while we detected positivity in the tunica media layer of the cavernous segment, we found positivity in the tunica intima layer of the cervicalis segment of the ICA. We did not detect any positivity for ASIC3 and ACTHR receptors in both parts of the ICA.

Conclusions

As a result, we observed various differences in receptors between ICA segments. While the outer surface of the ICA in the cervical region did not show any receptor positivity, we detected TRP12 receptor positivity along the tissue contour of vessel in the CS. We assume that it may provide a new perspective on pathologies of the CS/ICA and preservation of brain hemodynamics for clinicians.

Structural insights into ligand recognition and activation of the melanocortin-4 receptor

Melanocortin-4 receptor (MC4R) plays a central role in the regulation of energy homeostasis. Its high sequence similarity to other MC receptor family members, low agonist selectivity and the lack of structural information concerning MC4R-specific activation have hampered the development of MC4R-seletive therapeutics to treat obesity. Here, we report four high-resolution structures of full-length MC4R in complex with the heterotrimeric G_s protein stimulated by the endogenous peptide ligand α-MSH, FDA-approved drugs afamelanotide (Scenesse™) and bremelanotide (Vyleesi™), and a selective small-molecule ligand THIQ, respectively. Together with pharmacological studies, our results reveal the conserved binding mode of peptidic agonists, the distinctive molecular details of small-molecule agonist recognition underlying receptor subtype selectivity, and a distinct activation mechanism for MC4R, thereby offering new insights into G protein coupling. Our work may facilitate the discovery of selective therapeutic agents targeting MC4R.

Novel bifunctional hybrid compounds designed to enhance the effects of opioids and antagonize the pronociceptive effects of nonopioid peptides as potent analgesics in a rat model of neuropathic pain

ABSTRACT

The purpose of our work was to determine the role of nonopioid peptides derived from opioid prohormones in sensory hypersensitivity characteristics of neuropathic pain and to propose a pharmacological approach to restore the balance of these endogenous opioid systems. Nonopioid peptides may have a pronociceptive effect and therefore contribute to less effective opioid analgesia in neuropathic pain. In our study, we used unilateral chronic constriction injury (CCI) of the sciatic nerve as a neuropathic pain model in rats. We demonstrated the pronociceptive effects of proopiomelanocortin- and proenkephalin-derived nonopioid peptides assessed by von Frey and cold plate tests, 7 to 14 days after injury. The concentration of proenkephalin-derived pronociceptive peptides was increased more robustly than that of Met-enkephalin in the ipsilateral lumbar spinal cord of CCI-exposed rats, as shown by mass spectrometry, and the pronociceptive effect of one of these peptides was blocked by an antagonist of the melanocortin 4 (MC4) receptor. The above results confirm our hypothesis regarding the possibility of creating an analgesic drug for neuropathic pain based on enhancing opioid activity and blocking the pronociceptive effect of nonopioid peptides. We designed and synthesized bifunctional hybrids composed of opioid (OP) receptor agonist and MC4 receptor antagonist (OP-linker-MC4). Moreover, we demonstrated that they have potent and long-lasting antinociceptive effects after a single administration and a delayed development of tolerance compared with morphine after repeated intrathecal administration to rats subjected to CCI. We conclude that the bifunctional hybrids OP-linker-MC4 we propose are important prototypes of drugs for use in neuropathic pain.

Beyond pigmentation: signs of liver protection during afamelanotide treatment in Swiss patients with erythropoietic protoporphyria, an observational study

Erythropoietic protoporphyria (EPP) is an ultra-rare inherited disorder with overproduction of protoporphyrin in maturating erythroblasts. This excess protoporphyrin leads to incapacitating phototoxic burns in sunlight exposed skin. Its biliary elimination causes cholestatic liver injury in 20% and terminal liver failure in 4% of EPP patients. Thereby, the risk of liver injury increases with increasing erythrocyte protoporphyrin concentrations. Afamelanotide, an α-melanocyte-stimulating hormone (MSH) analog inducing skin pigmentation, was shown to improve sunlight tolerance in EPP. Beyond this well-known effect on pigmentation, the MSHs have liver-protective effects and improve survival of maturating erythroblasts, effects described in animal or in vitro models to date only. We investigated whether afamelanotide treatment in EPP has effects on erythropoiesis, protoporphyrin concentrations, and liver injury by analyzing retrospectively our long-term safety data.

Methods

From the 47 Swiss EPP-patients treated at our center since 2006, we included those 38 patients in the current analysis who received at least one afamelanotide dose between 2016 and 2018 and underwent regular laboratory testing before and during the treatment. We compared the means of pretreatment measurements with those during the treatment.

Results

Protoporphyrin concentrations dropped from 21.39 ± 11.12 (mean ± SD) before afamelanotide to 16.83 ± 8.24 µmol/L (p < .0001) during treatment. Aspartate aminotransferase decreased from 26.67 ± 13.16 to 22.9 ± 7.76 IU/L (p = .0146). For both entities, patients with higher values showed a more progressive decrease, indicating a risk reduction of EPP-related liver disease. The pre-existing hypochromia and broad mean red-cell distribution width were further augmented under afamelanotide. This was more likely due to an influence of afamelanotide on maturating erythroblasts than due to an exacerbated iron deficiency, as mean zinc-protoporphyrin decreased significantly and ferritin remained unchanged. No serious afamelanotide-related adverse events were observed for a total of 240 treatment years.

Conclusion

Our findings point to a protective effect of afamelanotide on erythroblast maturation and protoporphyrin-induced liver injury.

Plain Language summary

Afamelanotide, a skin tanning hormone, may protect patients with erythropoietic protoporphyria not only from skin burns, but also from liver injury associated with the disease. Patients with erythropoietic protoporphyria (EPP), an inherited metabolic disease, suffer from light-induced skin burns and liver injury elicited by the accumulated light sensitizer protoporphyrin. The excess protoporphyrin is produced in red cell precursors in the bone marrow, and it is eliminated from the body via the liver and bile. A high protoporphyrin excretion burden damages the liver cells, the risk for this increases with higher protoporphyrin concentrations. About 20% of EPP patients show some sign of liver injury and 4% develop life-threatening liver dysfunction.Afamelanotide, closely related to natural α-melanocyte stimulating hormone (MSH), induces skin tanning. This effect protects EPP patients from light-induced skin burns as shown in previous studies. We have treated Swiss EPP patients with afamelanotide since 2006, and we regularly perform safety tests of this treatment.Recent in vitro and animal studies demonstrated α-MSH effects other than skin tanning, including an improved synthesis of red blood cell precursors in the bone-marrow and protection of the liver from experimentally induced damage. Until now, it is unknown whether afamelanotide has similar effects in the human organism.To study this question, we analyzed retrospectively the safety laboratory data of 38 Swiss patients, who received at least one dose of afamelanotide from 2016 to 2019. We found that both, the average protoporphyrin concentrations and aspartate aminotransferase, a test for liver function, improved during afamelanotide treatment as compared to before.We concluded that afamelanotide applied to EPP patients to protect them from light-induced skin burns also may reduce their risk of liver injury.

Harnessing cyclotides to design and develop novel peptide GPCR ligands

Cyclotides are plant-derived cyclic, disulfide-rich peptides with a unique cyclic cystine knot topology that confers them with remarkable structural stability and resistance to proteolytic degradation. Recently, cyclotides have emerged as promising scaffold molecules for designing peptide-based therapeutics. Here, we provide examples of how engineering cyclotides using molecular grafting may lead to the development of novel peptide ligands of G protein-coupled receptors (GPCRs), today's most exploited drug targets. Integrating bioactive epitopes into stable cyclotide scaffolds can lead to improved pharmacokinetics and oral activity as well as selectivity and high enzymatic stability. We also discuss and highlight the importance of engineered cyclotides as novel tools to study GPCR signaling.

The Fight Against Obesity Escalates: New Drugs on the Horizon and Metabolic Implications

PURPOSE OF REVIEW

There is currently a steep rise in the global prevalence of obesity. Pharmaceutical therapy is a valuable component of conservative obesity therapy. Herein, medications currently in the phase of preclinical or clinical testing are reviewed, along with an overview of the mechanisms that regulate energy intake and expenditure. In addition, the current and potential future directions of obesity drug therapy are discussed.

RECENT FINDINGS

Although the current arsenal of obesity pharmacotherapy is limited, a considerable number of agents that exert their actions through a variety of pharmacodynamic targets and mechanisms are in the pipeline. This expansion shapes a potential near future of obesity conservative management, characterized by tailored combined therapeutic regimens, targeting not only weight loss but also improved overall health outcomes. The progress regarding the elucidation of the mechanisms which regulate the bodily energy equilibrium has led to medications which mimic hormonal adaptations that follow bariatric surgery, in the quest for a "Medical bypass." These, combined with agents which could increase energy expenditure, point to a brilliant future in the conservative treatment of obesity.

Anti-inflammatory effects of α-MSH through p-CREB expression in sarcoidosis like granuloma model

Lung inflammation due to sarcoidosis is characterized by a complex cascade of immunopathologic events, including leukocyte recruitment and granuloma formation. α-melanocyte stimulating hormone (α-MSH) is a melanocortin signaling peptide with anti-inflammatory properties. We aimed to evaluate the effects of α-MSH in a novel in vitro sarcoidosis model. An in vitro sarcoidosis-like granuloma model was developed by challenging peripheral blood mononuclear cells (PBMCs) derived from patients with confirmed treatment-naïve sarcoidosis with microparticles generated from Mycobacterium abscessus cell walls. Unchallenged PBMCsand developed granulomas were treated daily with 10 μM α-MSH or saline as control. Cytokine concentrations in supernatants of culture and in cell extracts were measured using Illumina multiplex Elisa and western blot, respectively. Gene expression was analyzed using RNA-Seq and RT-PCR. Protein secretion and gene expression of IL-7, IL-7R, IFN-γ, MC1R, NF-κB, phosphorylated NF-κB (p-NF-κB), MARCO, and p-CREB were measured with western blot and RNAseq. A significant increase in IL-7, IL-7R, and IFN-γ protein expression was found in developed granulomas comparing to microparticle unchallenged PBMCs. IL-7, IL-7R, and IFN-γ protein expression was significantly reduced in developed granulomas after exposure to α-MSH compared with saline treated granulomas. Compared with microparticle unchallenged PBMCs, total NF-κB and p-NF-κB were significantly increased in developed granulomas, while expression of p-CREB was not changed. Treatment with α-MSH promoted a significantly higher concentration of p-CREB in granulomas. The anti-inflammatory effects of α-MSH were blocked by specific p-CREB inhibition. α-MSH has anti-inflammatory properties in this in vitro granuloma model, which is an effect mediated by induction of phosphorylation of CREB.

Expression of melanocortin 1 receptor before and after narrowband UVB phototherapy treatment in patients with stable vitiligo: A prospective study

Vitiligo is a disease characterized by skin depigmentation caused by the selective destruction of melanocytes. The melanocortin system participates as a regulator of melanogenesis and skin pigmentation. Narrowband UVB phototherapy (nb-UVB) is currently considered to be the gold standard and first choice treatment method for vitiligo vulgaris. The aim of the present study was to analyze the clinical and biochemical parameters of vitiligo, as well as to determine the expression of proopiomelanocortin (POMC), melanocortin 1 receptor (MC1R) and melanocortin 4 receptor (MC4R) genes in the skin of patients with stable vitiligo receiving nb-UVB phototherapy. Patient clinical and biochemical parameters, and the skin biopsies of 22 patients with stable vitiligo were analyzed. These biopsies were obtained before and after nb-UVB phototherapy. The genetic expression analysis of POMC, MC1R and MC4R genes was performed via RNA-Sequence analysis. A statistical evaluation of the clinical and biochemical parameters, the degree of response to treatment and the expression profiles of the melanocortin system genes were performed to identify their association with treatment response. A two-sided P≤0.05 value was considered to indicate a statistically significant difference. Alterations were observed in the expression profiles of MC1R following nb-UVB phototherapy (P≤0.05). In addition, elevated levels of triiodothyronine were associated with a poor response to nb-UVB phototherapy. In conclusion the current study revealed that nb-UVB phototherapy altered the expression profile of the MC1R gene.

Neuroendocrinology and neurobiology of sebaceous glands

The nervous system communicates with peripheral tissues through nerve fibres and the systemic release of hypothalamic and pituitary neurohormones. Communication between the nervous system and the largest human organ, skin, has traditionally received little attention. In particular, the neuro-regulation of sebaceous glands (SGs), a major skin appendage, is rarely considered. Yet, it is clear that the SG is under stringent pituitary control, and forms a fascinating, clinically relevant peripheral target organ in which to study the neuroendocrine and neural regulation of epithelia. Sebum, the major secretory product of the SG, is composed of a complex mixture of lipids resulting from the holocrine secretion of specialised epithelial cells (sebocytes). It is indicative of a role of the neuroendocrine system in SG function that excess circulating levels of growth hormone, thyroxine or prolactin result in increased sebum production (seborrhoea). Conversely, growth hormone deficiency, hypothyroidism, and adrenal insufficiency result in reduced sebum production and dry skin. Furthermore, the androgen sensitivity of SGs appears to be under neuroendocrine control, as hypophysectomy (removal of the pituitary) renders SGs largely insensitive to stimulation by testosterone, which is crucial for maintaining SG homeostasis. However, several neurohormones, such as adrenocorticotropic hormone and α-melanocyte-stimulating hormone, can stimulate sebum production independently of either the testes or the adrenal glands, further underscoring the importance of neuroendocrine control in SG biology. Moreover, sebocytes synthesise several neurohormones and express their receptors, suggestive of the presence of neuro-autocrine mechanisms of sebocyte modulation. Aside from the neuroendocrine system, it is conceivable that secretion of neuropeptides and neurotransmitters from cutaneous nerve endings may also act on sebocytes or their progenitors, given that the skin is richly innervated. However, to date, the neural controls of SG development and function remain poorly investigated and incompletely understood. Botulinum toxin-mediated or facial paresis-associated reduction of human sebum secretion suggests that cutaneous nerve-derived substances modulate lipid and inflammatory cytokine synthesis by sebocytes, possibly implicating the nervous system in acne pathogenesis. Additionally, evidence suggests that cutaneous denervation in mice alters the expression of key regulators of SG homeostasis. In this review, we examine the current evidence regarding neuroendocrine and neurobiological regulation of human SG function in physiology and pathology. We further call attention to this line of research as an instructive model for probing and therapeutically manipulating the mechanistic links between the nervous system and mammalian skin.

Relationships Between Neurodegeneration and Vascular Damage in Diabetic Retinopathy

Diabetic retinopathy (DR) is a common complication of diabetes and constitutes a major cause of vision impairment and blindness in the world. DR has long been described exclusively as a microvascular disease of the eye. However, in recent years, a growing interest has been focused on the contribution of neuroretinal degeneration to the pathogenesis of the disease, and there are observations suggesting that neuronal death in the early phases of DR may favor the development of microvascular abnormalities, followed by the full manifestation of the disease. However, the mediators that are involved in the crosslink between neurodegeneration and vascular changes have not yet been identified. According to our hypothesis, vascular endothelial growth factor (VEGF) could probably be the most important connecting link between the death of retinal neurons and the occurrence of microvascular lesions. Indeed, VEGF is known to play important neuroprotective actions; therefore, in the early phases of DR, it may be released in response to neuronal suffering, and it would act as a double-edged weapon inducing both neuroprotective and vasoactive effects. If this hypothesis is correct, then any retinal stress causing neuronal damage should be accompanied by VEGF upregulation and by vascular changes. Similarly, any compound with neuroprotective properties should also induce VEGF downregulation and amelioration of the vascular lesions. In this review, we searched for a correlation between neurodegeneration and vasculopathy in animal models of retinal diseases, examining the effects of different neuroprotective substances, ranging from nutraceuticals to antioxidants to neuropeptides and others and showing that reducing neuronal suffering also prevents overexpression of VEGF and vascular complications. Taken together, the reviewed evidence highlights the crucial role played by mediators such as VEGF in the relationship between retinal neuronal damage and vascular alterations and suggests that the use of neuroprotective substances could be an efficient strategy to prevent the onset or to retard the development of DR.

Molecular and pharmacological characterization of poultry (Gallus gallus, Anas platyrhynchos, Anser cygnoides domesticus) and pig (Sus scrofa domestica) melanocortin-5 receptors and their mutants

The melanocortin-5 receptor (MC5R) is a member of the G protein-coupled receptor superfamily that plays a critical role in lipid production, skeletal muscle fatty acid oxidation, and adipocyte lipolysis. Although multiple functions and important value of MC5R in human beings have been fully demonstrated, however, the potential molecular cloning, pharmacological characteristics and key amino acids in poultry and pig were still not fully understood. Herein, we successfully cloned MC5R genes from chicken (Gallus gallus, cMC5R), duck (Anas platyrhynchos, dMC5R), goose (Anser cygnoides domesticus, gMC5R) and pig (Sus scrofa domestica, pMC5R), and compared their genetic and protein difference with hMC5R through phylogenetic analysis and homology models. Besides, we constructed three alanine-substitution mutants for each of MC5Rs through homologous reorganization, including c/d/gMC5R-D119A/F254A/H257A and pMC5R-D204A/F339A/H342A. Subsequently, we focused our investigation on the pharmacological characterization of four wide-type MC5Rs and their mutants in HEK293T cells, including the intracellular cAMP generation and phosphorylation level of ERK1/2. The results showed that these mutants had decreased cAMP levels under the stimulation of ligands, in spite of enhanced basal activity for c/d/gF254A and pH342A, indicating their important roles in the location and activation of receptors. Notably, these MC5Rs and mutants displayed significant species-specific phenotypes in the activation of pERK1/2 with ligands, which was not completely consistent with hMC5R. These findings demonstrated that presence of interspecies differences for MC5Rs, particularly for the pERK1/2 pathway. Taken together, our study expands current knowledge about the molecular and pharmacological characterization of c/d/g/pMC5Rs, providing preliminary data for MC5R-targeted drug screening or genetic breeding of economic animals in the future.

Analyzing the signaling properties of gar (Lepisosteus oculatus) melanocortin receptors: Evaluating interactions with MRAP1 and MRAP2

RT-PCR analysis of gar pituitary and brain indicated that different combinations of gar melanocortin receptor mRNAs are present in the same tissues with mRNAs for gar mrap1 and gar mrap2. Against this background, an objective of this study was to determine whether the ligand sensitivity for either ACTH or α-MSH was affected when gar (g) melanocortin receptors (Mcrs) were co-expressed with either of the accessory proteins gMrap1 or gMrap2 in Chinese Hamster Ovary cells. The results indicated that gMc2r has an obligatory requirement for co-expression with gMrap1 in order for the receptor to be activated by hACTH(1-24). In addition, activation of gMc2r did not occur when the receptor was expressed alone or co-expressed with gMrap2. Furthermore, co-expression of gMc2r with gMrap1 followed by stimulation with NDP-MSH resulted in a low level of activation (only at 10-7 M and 10-6 M). However, gMc1r, gMc3r, gMc4r, and gMc5r responded to stimulation by NDP-MSH in a more robust manner. Co-expression of gMc1r, gMc3r, gMc4r, and gMc5r with gMRAP1 had no effect on sensitivity to stimulation by NDP-MSH or hACTH(1-24). Co-expression with gMRAP2 had no negative or positive effect on ligand sensitivity for gMc1r, gMc3r, and gMc5r, however this treatment did increase the activation of CHO cells transfected with gMc4r following stimulation with both hACTH(1-24) (p < 0.001), and NDP-MSH (p < 0.001). Co-expression of gMC5R with either gMRAP1 or gMRAP2 increased trafficking of gMC5R to the plasma membrane. These pharmacological observations are compared to the response of melanocortin receptors from other neopterygian fishes, cartilaginous fishes, and tetrapods to stimulation by ACTH(1-24) and forms of α-MSH.

A Novel Pale-Yellow Coat Color of Rabbits Generated via MC1R Mutation With CRISPR/Cas9 System

Coat color is of great importance in animal breed characteristics; it is not only a significant productive trait but also an indispensable economic trait, especially in the rabbit industry. In the present study, the relationship between melanocortin 1 receptor (MC1R) genotypes and coat color phenotypes was observed in five rabbit breeds with popular coat colors that are present in China. These breeds comprised the Lianshan black rabbit (BR), Fujian yellow rabbit (YR), New Zealand white rabbit (WR), Gray Giant rabbit (GR), and Checkered Giant rabbit (CR), which were firstly determined, and the results showed that GR had an E allele; WR, CR, and BR had a 6-bp in-frame deletion (c.281_286del6, E^D allele); and YR had a 30-bp deletion (c.304_333del30 E allele). To explore the feasibility of obtaining a novel rabbit coat color through the mutation of MC1R with the CRISPR/Cas9 system, two single-guide RNAs (sgRNAs) were designed for the MC1R gene, and the editing efficiency was confirmed by injection of rabbits' zygotes. Unlike the donor rabbits whose coat color was originally black, two novel pale-yellow-coated rabbits were generated in the founders. A total of six novel MC1R gene deletions were identified in the two founder rabbits, in which the longest deletion was more than 700 bp. The histological hematoxylin-and-eosin (H&E) staining results indicated that eumelanin amounts were absent in hair follicles of MC1R-knockout (KO) rabbits, when compared with that of donor BR. In addition, the messenger RNA (mRNA) levels of some key downstream genes in the MC1R pathway were all downregulated in MC1R-KO rabbits compared with BR and YR. These results further indicate that loss-of-function MC1R contributed to blocking the synthesis of eumelanin and created a novel pale-yellow coat color in the MC1R-KO rabbits, and gene editing technology may be a useful tool to generate novel phenotypes in rabbit breeding.

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.

Neuroprotective Peptides in Retinal Disease

In the pathogenesis of many disorders, neuronal death plays a key role. It is now assumed that neurodegeneration is caused by multiple and somewhat converging/overlapping death mechanisms, and that neurons are sensitive to unique death styles. In this respect, major advances in the knowledge of different types, mechanisms, and roles of neurodegeneration are crucial to restore the neuronal functions involved in neuroprotection. Several novel concepts have emerged recently, suggesting that the modulation of the neuropeptide system may provide an entirely new set of pharmacological approaches. Neuropeptides and their receptors are expressed widely in mammalian retinas, where they exert neuromodulatory functions including the processing of visual information. In multiple models of retinal diseases, different peptidergic substances play neuroprotective actions. Herein, we describe the novel advances on the protective roles of neuropeptides in the retina. In particular, we focus on the mechanisms by which peptides affect neuronal death/survival and the vascular lesions commonly associated with retinal neurodegenerative pathologies. The goal is to highlight the therapeutic potential of neuropeptide systems as neuroprotectants in retinal diseases.

The Importance of Melanocortin Receptors and Their Agonists in Pulmonary Disease

Melanocortin agonists are ancient neuropeptides that have steroidogenesis and anti-inflammatory properties. They activate melanocortin receptors (MCR), a family of five seven-transmembrane G-protein coupled receptors. MC1R and MC3R are mainly involved in immunomodulatory effects. Adrenocorticotropin hormone (ACTH) and alpha-Melanocortin stimulating hormone (α-MSH) reduce pro-inflammatory cytokines in several pulmonary inflammatory disorders including asthma, sarcoidosis, and the acute respiratory distress syndrome. They have also been shown to reduce fibrogenesis in animal models with pulmonary fibrosis. By understanding the functions of MCR in macrophages, T-helper cell type 1, and T-helper cell type 17, we may uncover the mechanism of action of melanocortin agonists in sarcoidosis. Further translational and clinical research is needed to define the role of ACTH and α-MSH in pulmonary diseases.

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.

Biomedical applications of radioiodinated peptides

The overexpression of peptide receptors in certain tumors as compared to endogeneous expression levels represents the molecular basis for the design of peptide-based tools for targeted nuclear imaging and therapy. Receptor targeting with radiolabelled peptides became a very important imaging and/or therapeutic approach in nuclear medicine and oncology. A great variety of peptides has been radiolabelled with clinical relevant radionuclides, such as radiometals and radiohalogens. However, to the best of our knowledge concise and updated reviews providing information about the biomedical application of radioiodinated peptides are still missing. This review outlines the synthetic efforts in the preparation of radioiodinated peptides highlighting the importance of radioiodine in nuclear medicine, giving an overview of the most relevant radioiodination strategies that have been employed and describes relevant examples of their use in the biomedical field.

Insights into the Allosteric Mechanism of Setmelanotide (RM-493) as a Potent and First-in-Class Melanocortin-4 Receptor (MC4R) Agonist To Treat Rare Genetic Disorders of Obesity through an in Silico Approach

Human melanocortin-4 receptor (hMC4R) mutations have been implicated as the cause for about 6-8% of all severe obesity cases. Drug-like molecules that are able to rescue the functional activity of mutated receptors are highly desirable to combat genetic obesity among this population of patients. One such molecule is the selective MC4R agonist RM-493 (setmelanotide). While this molecule has been shown to activate mutated receptors with 20-fold higher potency over the endogenous agonist, little is known about its binding mode and how it effectively interacts with hMC4R despite the presence of mutations. In this study, a MC4R homology model was constructed based on the X-ray crystal structure of the adenosine A2A receptor in the active state. Four MC4R mutations commonly found in genetically obese patients and known to effect ligand binding in vitro were introduced into the constructed model. RM-493 was then docked into the wild-type and mutated models in order to better elucidate the possible binding modes for this promising drug candidate and assess how it may be interacting with MC4R to effectively activate receptor polymorphisms. The results reflected the orthosteric interactions of both the endogenous and synthetic ligands with the MC4R, which is supported by the site-directed mutagenesis studies. Meanwhile it helped explain the decremental affinity and potency of these ligands with the receptor polymorphisms. More significantly, our findings indicated that the structural characteristics of RM-493 may allow for enhanced receptor-ligand interactions, particularly through those with the putative allosteric binding sites, which facilitated the ligand to stabilize the active state of native and mutant MC4Rs to maintain reasonably high affinity and potency.

Melanocortin 1 Receptor-Targeted α-Particle Therapy for Metastatic Uveal Melanoma

New effective therapies are greatly needed for metastatic uveal melanoma, which has a very poor prognosis with a median survival of less than 1 y. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanoma metastases, and a MC1R-specific ligand (MC1RL) with high affinity and selectivity for MC1R was previously developed. Methods: The ²²⁵Ac-DOTA-MC1RL conjugate was synthesized in high radiochemical yield and purity and was tested in vitro for biostability and for MC1R-specific cytotoxicity in uveal melanoma cells, and the lanthanum-DOTA-MC1RL analog was tested for binding affinity. Non-tumor-bearing BALB/c mice were tested for maximum tolerated dose and biodistribution. Severe combined immunodeficient mice bearing uveal melanoma tumors or engineered MC1R-positive and -negative tumors were studied for biodistribution and efficacy. Radiation dosimetry was calculated using mouse biodistribution data and blood clearance kinetics from Sprague-Dawley rat data. Results: High biostability, MC1R-specific cytotoxicity, and high binding affinity were observed. Limiting toxicities were not observed at even the highest administered activities. Pharmacokinetics and biodistribution studies revealed rapid blood clearance (<15 min), renal and hepatobillary excretion, MC1R-specific tumor uptake, and minimal retention in other normal tissues. Radiation dosimetry calculations determined pharmacokinetics parameters and absorbed α-emission dosages from ²²⁵Ac and its daughters. Efficacy studies demonstrated significantly prolonged survival and decreased metastasis burden after a single administration of ²²⁵Ac-DOTA-MC1RL in treated mice relative to controls. Conclusion: These results suggest significant potential for the clinical translation of ²²⁵Ac-DOTA-MC1RL as a novel therapy for metastatic uveal melanoma.

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

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.

Molecular Imaging and Radionuclide Therapy of Melanoma Targeting the Melanocortin 1 Receptor

Melanoma is a deadly disease at late metastatic stage, and early diagnosis and accurate staging remain the key aspects for managing melanoma. The melanocortin 1 receptor (MC1 R) is overexpressed in primary and metastatic melanomas, and its endogenous ligand, the α-melanocyte-stimulating hormone (αMSH), has been extensively studied for the development of MC1 R-targeted molecular imaging and therapy of melanoma. Natural αMSH is not well suited for this purpose due to low stability in vivo. Unnatural amino acid substitutions substantially stabilized the peptide, while cyclization via lactam bridge and metal coordination further improved binding affinity and stability. In this study, we summarized the development and the in vitro and in vivo characteristics of the radiolabeled αMSH analogues, including ^99mTc-, ¹¹¹In-, ⁶⁷ Ga-, or ¹²⁵I-labeled αMSH analogues for imaging with single-photon emission computed tomography; ⁶⁸Ga-, ⁶⁴Cu-, or ¹⁸F-labeled αMSH analogues for imaging with positron emission tomography; and ¹⁸⁸Re-, ¹⁷⁷Lu-, ⁹⁰Y-, or ²¹²Pb-labeled αMSH analogues for radionuclide therapy. These radiolabeled αMSH analogues showed promising results with high tumor uptake and rapid normal tissue activity clearance in the preclinical model of B16F1 and B16F10 mouse melanomas. These results highlight the potential of using radiolabeled αMSH analogues in clinical applications for molecular imaging and radionuclide therapy of melanoma.

Molecular characterization of feline melanocortin 4 receptor and melanocortin 2 receptor accessory protein 2

Melanocortin 4 receptor (MC4R), which is a member of the G protein-coupled receptor (GPCR) family, mediates regulation of energy homeostasis upon the binding of α-melanocyte-stimulating hormone (α-MSH) in the central nervous system (CNS). Melanocortin 2 receptor accessory protein 2 (MRAP2) modulates the function of MC4R. We performed cDNA cloning of cat MC4R and MRAP2 and characterized their amino acid sequences, mRNA expression patterns in cat tissues, protein-protein interactions, and functions. We found high sequence homology (>88%) with other mammalian MC4R and MRAP2 encoding 332 and 206 amino acid residues, respectively. Reverse transcription-polymerase chain reaction analysis revealed that cat MC4R and MRAP2 mRNA were expressed highly in the CNS. In CHO-K1 cells transfected with cat MC4R, stimulation with α-MSH increased intracellular cyclic adenosine monophosphate (cAMP) concentration in a dose-dependent manner. Furthermore, the presence of MRAP2 enhanced the cat MC4R-mediated cAMP production. These results suggested that cat MC4R acts as a neuronal mediator in the CNS and that its function is modulated by MRAP2. In addition, our NanoBiT study showed the dynamics of their interactions in living cells; stimulation with α-MSH slightly affected the interaction between MC4R and MRAP2, and did not affect MC4R homodimerization, suggesting that they interact in the basal state and that structural change of MC4R by activation may affect the interaction between MC4R and MRAP2.

Click-Chemistry-Mediated Synthesis of Selective Melanocortin Receptor 4 Agonists

The melanocortin receptor 4 (MC4R) subtype of the melanocortin receptor family is a target for therapeutics to ameliorate metabolic dysfunction. Endogenous MC4R agonists possess a critical pharmacophore (HFRW), and cyclization of peptide agonists often enhances potency. Thus, 17 cyclized peptides were synthesized by solid phase click chemistry to develop novel, potent, selective MC4R agonists. Using cAMP measurements and a transcriptional reporter assay, we observed that several constrained agonists generated by a cycloaddition reaction displayed high selectivity (223- to 467-fold) toward MC4R over MC3R and MC5R receptor subtypes without compromising agonist potency. Significant variation was also observed between the EC₅₀ values for the two assays, with robust levels of reporter expression measured at lower concentrations than those effecting appreciable increases in cAMP levels for the majority of the compounds tested. Collectively, we characterized significant elements that modulate the activity of the core pharmacophore for MC4R and provide a rationale for careful assay selection for agonist screening.

Key amino acid residue in Melanocortin-1 receptor (melanocyte α-MSH receptor) for ligand selectivity

The melanocortin-1 receptor (MC1R) is a subtype of the melanocortin receptor family and NDP-α-MSH is a non-selective agonist for MC1R. The core sequence of NDP-α-MSH, His-Phe-Arg-Trp, is important for ligand binding and biological activities at the melanocortin receptor subtypes (MCRs). A recent study indicates that Trp⁹ in NDP-α-MSH plays an important role in ligand selectivity. Deletion of Trp⁹ in NDP-α-MSH (des-Trp⁹-NDP-α-MSH) resulted in loss of agonist activity at MC4R, although remains agonist activity at MC1R. The molecular basis for this receptor ligand selectivity is unknown. In this study we examined what region of the MC1R is responsible for des-NDP-α-MSH selectivity. Our results indicate that (1) substitution of TM3 of MC4R with the corresponding region of MC1R switches des-Trp⁹-NDP-α-MSH from no activity to agonist; (2) des-Trp⁹-NDP-α-MSH exhibits agonistic activity at the L133M mutation of the MC4R; and (3) substitution of non-conserved amino acid residue M128 in TM3 of MC1R significantly reduced des-Trp⁹-NDP-α-MSH agonist activity. Our results demonstrate that amino acid residue 128 in TM3 of MC1R, or amino acid residue L133 in TM3 of the MC4R, play crucial roles in ligand des-Trp⁹-NDP-α-MSH selectivity at MC1R or MC4R.

Adjunctive Steroid Therapy for Treatment of Pediatric Septic Shock

Septic shock remains the major cause of childhood morbidity and mortality worldwide. Although early sepsis recognition, fluid resuscitation, timely administration of antimicrobials, and vasoactive-inotropic drug infusions are all key to achieving good sepsis outcomes, therapy using various steroid drug classes remains an attractive adjunctive intervention to minimize the duration of septic shock and transition to multiple organ dysfunction syndrome. All steroid drug classes possess biological plausibility to affect a beneficial clinical effect among children with septic shock, but none has undergone rigorous, prospective assessment in a large, high-quality pediatric interventional trial.