A role for the melanocortin 4 receptor in sexual function

The role of proopiomelanocortin (POMC) neurones in feeding behaviour

The precursor protein, proopiomelanocortin (POMC), produces many biologically active peptides via a series of enzymatic steps in a tissue-specific manner, yielding the melanocyte-stimulating hormones (MSHs), corticotrophin (ACTH) and β-endorphin. The MSHs and ACTH bind to the extracellular G-protein coupled melanocortin receptors (MCRs) of which there are five subtypes. The MC3R and MC4R show widespread expression in the central nervous system (CNS), whilst there is low level expression of MC1R and MC5R. In the CNS, cell bodies for POMC are mainly located in the arcuate nucleus of the hypothalamus and the nucleus tractus solitarius of the brainstem. Both of these areas have well defined functions relating to appetite and food intake. Mouse knockouts (ko) for pomc, mc4r and mc3r all show an obese phenotype, as do humans expressing mutations of POMC and MC4R. Recently, human subjects with specific mutations in β-MSH have been found to be obese too, as have mice with engineered β-endorphin deficiency. The CNS POMC system has other functions, including regulation of sexual behaviour, lactation, the reproductive cycle and possibly central cardiovascular control. However, this review will focus on feeding behaviour and link it in with the neuroanatomy of the POMC neurones in the hypothalamus and brainstem.

Structure-activity relationships of melanocortin agonists containing the benzimidazole scaffold

The melanocortin system has been implicated in regulating various physiological processes including pigmentation, energy homeostasis, obesity, steroidogenesis cardiovascular, and exocrine gland function. The five melanocortin receptors that belong to the super family of G protein-coupled receptors are stimulated by naturally occurring agonists. The aim of this research was focused on the design, synthesis, and pharmacological characterization of melanocortin ligands that contain the 1,2,5-trisubstituted benzimidazole scaffold. A series of benzimidazole analogues, with three points of diversity at positions 1, 2, and 5, were designed, synthesized, pharmacologically assayed at the mouse melanocortin receptors MC1R, MC3R, MC4R, and MC5R and resulted in ligands possessing a range of agonist activity from nm to no stimulation at up to 100 microM concentrations. This study demonstrates that the benzimidazole structure template can be appended with key melanocortin agonist amino acids for the design melanocortin receptor agonist ligands.

Automated Pharmacophore Query Optimization with Genetic AlgorithmsA Case Study Using the MC4R System

Due to the recent availability of high quality small molecule databases, such as ZINC and PubChem,1,2 virtual screening is playing an even more important role in identifying biologically relevant molecules in drug discovery campaigns. The success of pharmacophore-based virtual screening (PBVS) relies largely on the accuracy and specificity of the pharmacophore query employed. Deriving a pharmacophore query from a single structure inevitably introduces uncertainty, and the derived query is unlikely to be optimal against every collection of input compounds, especially when it is desired to discriminate among compounds with similar chemical structures. In this study, we present an optimization approach empowered by genetic algorithms (GA) to enhance the accuracy and specificity of a primary pharmacophore query. The example utilized is the human melanocortin type 4 receptor (hMC4R), for which the pharmacophore query was built on the basis of the structure of a rigid cyclic peptide agonist.(3) The optimized query is shown to be capable of identifying 37 positive hMC4R agonists with no false positives from a training set containing 55 agonists and 51 nonagonists. This represents a significant improvement from the initial query which exhibited a 37/32 hit rate. The final, optimized query is challenged with a testing set comprising of 55 hMC4R agonists and 50 nonagonists and achieves a hit rate of 33/8, that improved from 40/31. The impact of GA controlling parameters, including mutation rate, crossover rate, fitness function, population size, and convergence criterion, on performance of optimization are examined and discussed.

Structure–activity relationships of novel piperazines as antagonists for the melanocortin-4 receptor

During the investigation of antagonists for the MC4 receptor, we found that 10ab having a naphthyl group showed almost the same binding affinity for the MC4 receptor as that of the lead compound 1 with a benzoyl group. We also developed a new type of compounds, namely, bis-piperazines, and found that the bis-piperazines 10 exhibited a high affinity for the MC4 receptor. In particular, (-)-10bg exhibited the highest affinity for the MC4 receptor with an IC50 value of 8.13nM. In this paper, we present the design, synthesis, and structure-activity relationships of the novel bis-piperazines as MC4 receptor antagonists.

Novel piperazines: Potent melanocortin-4 receptor antagonists with anxiolytic-like activity

In the present study, we found that a novel piperazine compound, 11a, showed a moderate affinity (IC(50)=333nM) for the MC4 receptor. We developed the new type of piperazine compounds and found that mono-piperazine 11b exhibited a high-affinity (IC(50)=40.3nM) for the MC4 receptor. We also found that a series of biphenyl analogues exhibited a high-affinity for the receptor, and in particular, compound 11j exhibited the highest affinity for the MC4 receptor with an IC(50) value of 14.5nM. Furthermore, some of these compounds, when administered orally, significantly reversed the stress-induced anxiety-like behavior in rats. In this paper, we report the synthesis, structure-activity relationships, and oral activity of the novel mono-piperazines as MC4 receptor antagonists.

Hypothalamic proopiomelanocortin (POMC) neurons have a cholinergic phenotype

Neuronal networks originating in the hypothalamic arcuate nucleus play fundamental roles in the control of energy balance. Neuropeptide Y (NPY)-producing neurons in the arcuate nucleus stimulate food intake, whereas arcuate nucleus neurons that release the proopiomelanocortin (POMC)-derived peptide alpha-melanocyte-stimulating hormone (alpha-MSH) potently reduce food intake. Relatively little attention has been focused on classical neurotransmitters in regulation of food intake. Here, we have investigated the potential presence of acetylcholine (ACh) in NPY- and POMC-containing neuronal populations of the arcuate nucleus. Antisera to proteins required for cholinergic neurotransmission, including choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter (VAChT), were employed in double-labeling immunohistochemical experiments. In colchicine-treated rats, ChAT- and VAChT-immunopositive cell bodies were located in the ventral aspect of the arcuate nucleus. ChAT and VAChT immunoreactivities were demonstrated in alpha-MSH- and cocaine- and amphetamine-regulated transcript (CART)-containing cell bodies of the arcuate nucleus, whereas cell bodies containing NPY or agouti-related peptide (AGRP) were distinct from VAChT-immunoreactive neuronal perikarya. VAChT immunoreactivity was also present in a large number of alpha-MSH-containing nerve fiber varicosities throughout the central nervous system. In the commissural part of the nucleus tractus solitarius, no alpha-MSH-containing cell bodies were found to have ChAT or VAChT immunoreactivity. The presence of markers for cholinergic neurotransmission in a subpopulation of hypothalamic POMC/CART neurons suggests co-release of ACh with peptides derived from the POMC precursor and CART. The results indicate a role for ACh in control of energy balance, mediating the effects of peripheral hormones such as leptin and insulin.

Characterization of Mahogunin Ring Finger-1 expression in mice

Mutations in mouse Mahogunin Ring Finger-1 (Mgrn1) were first recognized for their effect on agouti-mediated pigment-type switching. Mgrn1 null mutants are completely black and develop spongiform degeneration of the brain. Mgrn1 hypomorphs have dark fur but do not develop neurodegeneration. We characterized a new Mgrn1 hypomorphic allele caused by a gene-trap insertion. Mice homozygous for this mutation are slightly darker than non-mutant animals. They show reduced overall expression of Mgrn1 and two of the four normal Mgrn1 isoforms are replaced by beta-GEO fusion proteins that differ from the normal proteins at their carboxy termini. To investigate the role of different Mgrn1 isoforms in pigment-type switching, we used quantitative relative reverse transcriptase polymerase chain reaction to examine their expression in the skin of Mgrn1 mutant and control mice. Most Mgrn1 mutants produce little or no normal Mgrn1 in the skin. Mgrn1 null mutant mice overexpressing isoform I or III, which are normally absent or weakly expressed in adult skin, had normal agouti-banded hairs. Our results indicate that reduced levels of MGRN1 cause the pigmentation phenotypes of Mgrn1 mutant mice and that there are no significant differences in the function of the four MGRN1 isoforms in pigment-type switching.

Melanocortin receptors, melanotropic peptides and penile erection

Penile erection is a complex physiologic event resulting from the interactions of the nervous system on a highly specialized vascular organ. Activation of central nervous system melanocortinergic (MC) receptors with either endogenous or synthetic melanotropic ligands may initiate and/or facilitate spontaneous penile erection. While the CNS contains principally the MC3 and MC4 receptor subtypes, there is conflicting data as to which receptor mediates erection. Although the MC4R is emerging as the principle effector of MC induced erection, the role of the MC3R is poorly understood. Manipulation of each receptor subtype with newly synthesized receptor specific agonists and antagonists, as well as knockout mice, has elucidated their individual contributions. Novel data from our laboratories suggests that antagonism of forebrain MC3R may enhance melanocortin-induced erections. Furthermore, melanocortin agents may interact with better-studied systems such as oxytocinergic pathways at the hypothalamic, brainstem or spinal level. Current therapies for erectile dysfunction target end organ vascular tissue. Manipulation of MC receptors may provide an alternative, centrally mediated therapeutic approach for erectile and other sexual dysfunctions. The non-specific "superpotent" MC agonist, PT-141, which is the carboxylate derivative of MT-II, has reached phase II human trials. Through their centrally mediated activity, melanocortin agonists have potential to treat erectile dysfunction as well as possible applications to the unmet medical needs of decreased sexual motivation and loss of libido.

Synthesis of Diphenylmethyl Analogues and Their Affinity for the Melanocortin-4 Receptor and the Serotonin Transporter

While examining antagonists of the melanocortin-4 receptor (MC4 receptor), we found that compound 12b, containing a diphenylmethyl moiety, had a relatively high affinity for the MC4 receptor. When diphenylmethyl analogues were further examined, compounds 12c and 18 were also found to exhibit a high affinity for the MC4 receptor (IC(50)=46.7 nM and 33.2 nM, respectively). Furthermore, compound 12c was also found to show a high affinity for the serotonin transporter (IC(50)=10.7 nM). Here, we describe the synthesis and biological evaluation of various diphenylmethyl analogues in relation to their actions on the MC4 receptor and the serotonin transporter.

Design and synthesis of (ant)-agonists that alter appetite and adiposity

Over the past decade, hypothalamic circuits have been described that impact energy homeostasis in rodents and humans. Our drug development efforts for the treatment of obesity and the metabolic syndrome have largely focused on selected genetic and/or pharmacologically validated pathways. The translation of these pathways into therapeutics for the treatment of obesity will find its first clinical successes over the coming decade. Initial efforts have focused on gaining a better understanding of the relevance of rodent pharmacological and genetic observations for the development of therapeutics for the treatment of human obesity. We pursue pathways defined by the expression of the ghrelin receptor, melanin-concentrating hormone receptors, melanocortin receptors, cannabinoid receptors and neuropeptide Y1 and Y5 receptors. In this review, we will discuss drug development efforts for the treatment of obesity, focused on selective melanocortin 4 receptor agonists and neuropeptide Y1 and Y5 receptor antagonists. These drug development efforts required an in-depth understanding of cell-based observations which drive the development of compound structure-activity relationships. These include understanding of receptor function in selected cell-based backgrounds and early evaluation and validation of ex vivo observations in appropriate in vivo models. In order to develop selective and safe anti-obesity drugs, diverse approaches are needed to increase the likelihood of clinical success, including: (i) developing a detailed understanding of the predictive value of rodent pathways for treatment of human disease; (ii) knowledge of the exact location of targeted receptor subtypes for the clinical indication under study in order to derive a suitable compound profile; (iii) predictive measures of in vivo and/or ex vivo receptor occupancy required to bring about a desired physiological effect; (iv) predictive parameters that outline that the drug-derived effects are safe and mechanism-based; and (v) the refinement of selected compound classes, aimed at their clinical use.

Role of the melanocortin system in inflammation

In recent years, scientific interest in melanocortins (MC) has progressively increased due to their wide range of effects and expression on various tissues. Primarily discovered as mediators of skin pigmentation, recent research has shown their important roles in various body functions, such as energy homeostasis, sexual function, and inflammation. The anti-inflammatory and immunomodulatory properties discovered so far have led to the hypothesis that alpha-melanocyte-stimulating hormone (MSH) and its cognate receptors might present potential anti-inflammatory treatment options.

Identification of differential melanocortin 4 receptor agonist profiles at natively expressed receptors in rat cortical astrocytes and recombinantly expressed receptors in human embryonic kidney cells

Using cAMP accumulation as a functional readout, we pharmacologically characterized the response of native melanocortin receptors in cultured rat astrocytes, and found this response to be mediated by the melanocortin 4 receptor (MC4R). Melancortin agonists stimulate cAMP in a concentration-dependent manner in both astrocytes and human embryonic kidney cells recombinantly expressing rat MC4R (HEK-rMC4R), however, the relative potency and intrinsic activity of both small molecule and peptide agonists are reduced in the native system. As such, the small molecules THIQ, NBI-702 and MB243 display 43, 30 and 18% of the maximal response elicited by alpha-MSH in astrocytes. Likewise, the peptides MTII and ACTH display 55 and 72% of the maximal response elicited by alpha-MSH in these cells. In contrast, all of these compounds elicit full agonist responses with similar intrinsic activity to alpha-MSH in HEK-rMC4R cells. MC4R mRNA was detected in astrocytes, however radioligand binding experiments failed to detect measurable MC4R in astrocyte membranes, in contrast to membranes from HEK-rMC4R cells that display a binding site density of 18.1+/-1.5 fmol/mg. We propose that the divergent observations in functional activity between the cell types reflect differences in receptor expression and that caution should be exercised when interpreting agonist activity in over-expression systems for the purposes of drug discovery.

Melanocortin-4 receptor-mediated inhibition of apoptosis in immortalized hypothalamic neurons via mitogen-activated protein kinase

The melanocortin-4 receptor (MC4R) is a seven transmembrane member of the melanocortin receptor family. The GT1-1 cell line exhibits endogenous expression of MC4R. In this study, GT1-1 cells were used to study MC4R signaling pathways and to examine the effects of melanocortin receptor agonist NDP-MSH on apoptosis. MC4R mRNA expression was demonstrated by RT-PCR. Functional melanocortin receptor expression was implied by specific binding of NDP-MSH and cAMP production. NDP-MSH-stimulated GnRH release in a dose-dependent manner. Serum deprivation-induced apoptosis in GT1-1 cells, and the NDP-MSH inhibited this effect. The melanocortin receptor antagonist SHU9119 blocked the antiapoptotic actions of NDP-MSH, and the MAP kinase inhibitor PD98059 significantly attenuated the antiapoptotic effect. NDP-MSH-stimulated ERK1/2 phosphorylation in a dose-dependent manner. ERK1/2 phosphorylation could be abolished by SHU9119. In GT1-1 cells, melanocortin receptor activation causes ERK1/2 phosphorylation. In these cells, MC4R activation is also associated with antiapoptotic effects.

An effect on the subjective sexual response in premenopausal women with sexual arousal disorder by bremelanotide (PT-141), a melanocortin receptor agonist

INTRODUCTION

Melanocortins affect multiple physiological responses, including sexual behaviors. Bremelanotide is a synthetic peptide melanocortin analog of alpha-melanocyte-stimulating hormone that is an agonist at melanocortin receptors MC3R and MC4R.

AIM

To evaluate a single intranasal dose of bremelanotide for potential effects on physiological and subjective measurements of sexual arousal and desire in premenopausal women with sexual arousal disorder.

MAIN OUTCOME MEASURES

Change in vaginal pulse amplitude during neutral and erotic videos after treatment with bremelanotide or placebo and subjects' perceptions of physiological and sexual response within 24 hours of treatment with bremelanotide or placebo.

METHODS

Eighteen premenopausal women with a primary diagnosis of female sexual arousal disorder were randomly assigned to receive a single intranasal dose of 20 mg bremelanotide or matching placebo in a double-blind manner during the first in-clinic treatment session, and the alternate medication during the second in-clinic treatment session. During each session, subjects viewed a 20-minute neutral video followed by a 20-minute sexually explicit video. Vaginal photoplethysmography was used to monitor vaginal vasocongestion and questionnaires were used to evaluate perceptions of sexual response within the following 24-hour period.

RESULTS

More women reported moderate or high sexual desire following bremelanotide treatment vs. placebo (P = 0.0114), and a trend toward more positive responses regarding feelings of genital arousal occurred after bremelanotide compared with placebo (P = 0.0833). Among women who attempted sexual intercourse within 24 hours after treatment, significantly more were satisfied with their level of sexual arousal following bremelanotide, compared with placebo (P = 0.0256). Vaginal vasocongestion did not change significantly while viewing erotic videos following bremelanotide administration compared with placebo.

CONCLUSION

This preliminary evaluation suggests the potential for bremelanotide to positively affect desire and arousal in women with female sexual arousal disorder and indicates that bremelanotide is a promising candidate for further evaluation in an at-home study.

The MC4 receptor and control of appetite

Mutations in the human melanocortin (MC)4 receptor have been associated with obesity, which underscores the relevance of this receptor as a drug target to treat obesity. Infusion of MC4R agonists decreases food intake, whereas inhibition of MC receptor activity by infusion of an MC receptor antagonist or with the inverse agonist AgRP results in increased food intake. This review addresses the role of the MC system in different aspects of feeding behaviour. MC4R activity affects meal size and meal choice, but not meal frequency, and the type of diet affects the efficacy of MC4R agonists to reduce food intake. The central sites involved in the different aspects of feeding behaviour that are affected by MC4R signalling are being unravelled. The paraventricular nucleus plays an important role in food intake per se, whereas MC signalling in the lateral hypothalamus is associated with the response to a high fat diet. MC4R signalling in the brainstem has been shown to affect meal size. Further genetic, behavioural and brain-region specific studies need to clarify how the MC4R agonists affect feeding behaviour in order to determine which obese individuals would benefit most from treatment with these drugs. Application of MCR agonists in humans has already revealed side effects, such as penile erections, which may complicate introduction of these drugs in the treatment of obesity.

Cell signaling and trafficking of human melanocortin receptors in real time using two-photon fluorescence and confocal laser microscopy: differentiation of agonists and antagonists

Melanocortin hormones and neurotransmitters regulate a vast array of physiologic processes by interacting with five G-protein-coupled melanocortin receptor types. In the present study, we have systematically studied the regulation of individual human melanocortin receptor wild subtypes using a synthetic rhodamine-labeled human melanotropin agonist and antagonist, arrestins fused to green fluorescent protein in conjunction with two-photon fluorescence laser scanning microscopy and confocal microscopy. Stimulation of the melanocortin receptors by its cognate agonist triggered rapid arrestin recruitment and receptor internalization for all four human melanocortin receptors examined. Antagonists-bound melanocortin receptors, on the other hand, did not recruit beta-arrestins, and remained in the cell membrane even after long-term (30 min) treatment. Agonist-mediated internalization of all melanocortin receptor subtypes was sensitive to inhibitors of clathrin-dependent endocytosis, but not to caveolae inhibitors. In summary, agonist-mediated internalization of all subtypes of melanocortin receptors are dependent upon beta-arrestin-mediated clathrin-coated pits, whereas, beta-arrestin-2 conjugated green fluorescence protein (beta-arrestin-2-GFP) recruitment is not dependent on protein kinase A activation. Real time two-photon fluorescence laser scanning microscopy is a most powerful tool to study the dynamic processes in living cells and tissues, without inflicting significant and often lethal damage to the specimen.

Effects of alpha-melanocyte-stimulating hormone on magnocellular oxytocin neurones and their activation at intromission in male rats

The peptides alpha-melanocyte-stimulating hormone (alpha-MSH) and oxytocin have very similar effects on several behaviours, including male sexual behaviour. Both induce penile erection and enhance copulatory behaviour when given centrally, suggesting that their central actions are not independent. Here, we used intromission as a physiological stimulus to investigate whether some central effects of alpha-MSH during male sexual behaviour are mediated by oxytocin neurones. We used the expression of the immediate-early gene product Fos to investigate oxytocin neurone activation at intromission and after intracerebroventricular (i.c.v.) administration of alpha-MSH (1 microg/5 microl) and studied the effects of i.c.v. administration of a MC4 receptor antagonist on Fos expression and on the latency of male rats to exhibit sexual behaviour in the presence of a receptive female. In rats that showed intromission, Fos was expressed in magnocellular oxytocin neurones in both the paraventricular nucleus (PVN) and the supraoptic nucleus (SON), but there was no significant activation of parvocellular oxytocin neurones of the PVN. Similarly, alpha-MSH increased Fos expression in magnocellular oxytocin neurones but had little or no effect in parvocellular oxytocin neurones. In male rats that achieved intromission, central injection of a MC4 receptor antagonist significantly attenuated the increase in Fos expression in magnocellular oxytocin neurones in both the PVN and the SON and increased mount and intromission latencies compared to vehicle-injected controls. Together, the results indicate that magnocellular oxytocin neurones are involved in the central regulation of male sexual behaviour, and that some of the central effects of alpha-MSH are likely to be mediated by magnocellular oxytocin neurones.

alpha-Melanocyte-stimulating hormone and oxytocin: a peptide signalling cascade in the hypothalamus

alpha-Melanocyte-stimulating hormone (alpha-MSH) and oxytocin share remarkable similarities of effects on behaviour in rats; in particular, they both inhibit feeding behaviour and stimulate sexual behaviour. Recently, we showed that alpha-MSH interacts with the magnocellular oxytocin system in the supraoptic nucleus; alpha-MSH induces the release of oxytocin from the dendrites of magnocellular neurones but it inhibits the secretion of oxytocin from their nerve terminals in the posterior pituitary. This effect of alpha-MSH on supraoptic nucleus oxytocin neurones is remarkable for two reasons. First, it illustrates the capacity of magnocellular neurones to differentially regulate peptide release from dendrites and axons and, second, it emphasises the putative role of magnocellular neurones as a major source of central oxytocin release, and as a likely substrate of some oxytocin-mediated behaviours. The ability of peptides to differentially control secretion from different compartments of their targets indicates one way by which peptide signals might have a particularly significant effect on neuronal circuitry. This suggests a possible explanation for the striking way in which some peptides can influence specific, complex behaviours.

Effects of macrocycle size and rigidity on melanocortin receptor-1 and -5 selectivity in cyclic lactam alpha-melanocyte-stimulating hormone analogs

The effects of the linker arm rigidity and size on melanocortin receptor selectivity were explored in a series of compounds using cyclic lactam alpha-melanocyte-stimulating hormone template. A variety of dicarboxylic acid linkers introduced between the alpha-amino group of His(6) and the epsilon-amino group of Lys(10) lead to high-affinity, selective human melanocortin receptor-1 and -5 (hMC1R and hMC5R) antagonists. The incorporation of hydrophilic functions into the linker arm was found to be unfavorable for both binding potency and receptor selectivity. Analogs 8 and 9 containing highly conformationally constrained hydrophobic linkers (m- and p-phthalic acids) were found to be selective nanomolar range hMC1R antagonists (IC(50) = 7 and 4 nm, respectively), whereas the employment of a small conformationally constrained linker (maleic acid) resulted in a high-affinity (IC(50) = 19 nm) and selective hMC5R antagonist (analog 12). These newly developed melanotropins will serve as critical biochemical tools for elucidating the full spectrum of functions performed by the physiologically important melanocortin-1 and -5 receptors.

A novel cell-based assay for G-protein-coupled receptor-mediated cyclic adenosine monophosphate response element binding protein phosphorylation

Currently, the most popular means of assessing functional activity of Gs/olf-coupled receptors is via the measurement of intracellular cyclic adenosine monophosphate (cAMP) accumulation. An additional readout is the downstream phosphorylation of cAMP response element binding protein (CREB), which gives an indication of gene transcription, the ultimate response of many G-protein-coupled receptor (GPCR) signals. Current methods of quantifying CREB phosphorylation are low throughput, and so we have designed a novel higher throughput method using the Odyssey infrared imaging system. Functional potencies of both agonists and antagonists correlate well with radioligand binding affinities determined using examples of both an endogenous (adenosine(2A) receptor in PC-12 cells) and a heterologous (human melanocortin 4 receptor in HEK-293 cells) expression system. For example, the antagonist ZM241385 demonstrates 0.23+/-0.03 nM affinity for the A(2A) receptor and has a functional potency of 0.26+/-0.04 nM determined using cAMP and 0.15+/-0.06 nM using CREB phosphorylation. These data demonstrate that this novel approach for the measurement of CREB phosphorylation is a useful tool for the assessment of GPCR activity in whole cells and is more amenable to the throughput required for the purposes of drug discovery.

D-aspartate regulates melanocortin formation and function: behavioral alterations in D-aspartate oxidase-deficient mice

D-aspartate, an abundant D-amino acid enriched in neuroendocrine tissues, can be degraded by D-aspartate oxidase (Ddo). To elucidate the function of D-aspartate, we generated mice with targeted deletion of Ddo (Ddo(-/-)) and observe massive but selective augmentations of D-aspartate in various tissues. The pituitary intermediate lobe, normally devoid of D-aspartate from endogenous Ddo expression, manifests pronounced increases of immunoreactive D-aspartate in Ddo(-/-) mice. Ddo(-/-) mice show markedly diminished synthesis and levels of pituitary proopiomelanocortin/alpha-MSH, associated with decreased melanocortin-dependent behaviors. Therefore, Ddo is the endogenous enzyme that degrades D-aspartate, and Ddo-enriched organs, low in D-aspartate, may represent areas of high turnover where D-aspartate may be physiologically important.

Melanocortin peptide therapeutics: historical milestones, clinical studies and commercialization

The melanocortins (MCs) are a family of multifunctional peptidergic hormones. Several superpotent, prolonged acting, enzymatically resistant, MC analogs have been designed and synthesized to help clarify the nature and role of MCs and their receptors (MCRs) in physiological functions. Two of these analogs, a linear peptide, melanotan I, and a cyclic truncated peptide, melanotan II (MTI and MTII, respectively) have been patented and tested clinically for studies on tanning of the skin (MTI) and for diagnosis and treatment of male erectile dysfunction (MTII). A new MTII analog, PT-141 (Palatin Technologies) has initial phase I/II trials and is scheduled to enter pivotal stage III clinical trials leading to commercialization. MTI may provide a therapeutic tan with the potential to lower the risk of skin cancer. PT-141 may improve sexual functionality in both males and females.

Development of cyclic gamma-MSH analogues with selective hMC3R agonist and hMC3R/hMC5R antagonist activities

A series of cyclic lactam analogues of gamma-MSH (H-Tyr1-Val2-Met3-Gly4-His5-Phe6-Arg7-Trp8-Asp9-Arg10-Phe11-Gly12-OH) with a bulky hydrophobic residue in the direct proximity to the pharmacophore (Xaa-D-Phe/D-Nal(2')-Arg-Trp) were designed and synthesized by solid-phase methods. A variety of amino acids with a broad range of hydrophobic/hydrophilic properties was introduced in position 5 to further explore their complementary role in receptor selectivity. Biological evaluation of these peptides revealed several analogues with potent hMC3R agonist and hMC3R/hMC5R antagonist activities, and good receptor selectivity. Analogue 4, c[Nle-Arg-D-Phe-Arg-Trp-Glu]-NH2, was found to be a very potent and selective hMC3R agonist (EC50=1.2 nM, 112% act). In addition, analogue 13, c[Nle-Val-D-Nal(2')-Arg-Trp-Glu]-NH2, was identified as an hMC3R/hMC5R antagonist with the best selectivity against the hMC4R in this series (pA2(hMC3R)=8.4; pA2(hMC5R)=8.7). These results indicate the significance of steric factors in melanocortin receptor selectivity and suggest that introduction of bulky residues in the direct proximity to the melanocortin pharmacophore is an effective approach to design of novel hMC3R and hMC5R selective ligands.

Targeting melanocortin receptors as potential novel therapeutics

Adrenocorticotrophic hormone (ACTH(1-39)) and the melanocortins (alpha, beta and gamma-melanocyte-stimulating hormone [MSH]) are derived from a larger precursor molecule known as the pro-opiomelanocortin (POMC) protein. They exert their numerous biological effects by activating 7 transmembrane G-protein coupled receptors (GPCR), leading to adenylyl cyclase activation and subsequent cAMP accumulation within the target cell. To date, 5 melanocortin receptors (MCR) have been identified and termed MC1R to MC5R, they have been shown to have a wide and varied distribution throughout the body, being found in the central nervous system (CNS), periphery and immune cells. Melanocortins have a multitude of actions including: (i) modulating disease pathologies including arthritis, asthma, obesity; (ii) affecting functions, for example erectile dysfunction, skin tanning; and (iii) organ systems, for example cardiovascular system. Recently a mechanistic approach has been identified with alpha-MSH preventing NF-kappaB activation via the preservation and expression of IkappaBalphaprotein. This leads to a reduction of pro-inflammatory mediators including cytokines and inhibition of adhesion molecule expression, with subsequent reduction in leukocyte emigration. Development of selective ligands with an appropriate pharmacokinetic profile will enable a pharmacological evaluation of the potential beneficial effects of the melanocortins. In this review I have discussed the potential mechanistic action for the melanocortins and some of the disease pathologies shown to be modulated. This review proposes targeting the MCR with the ultimate aim of controlling many of the diseases that we face today.

MC4R oligomerizes independently of extracellular cysteine residues

The melanocortin 4 receptor (MC4R) plays an essential role in weight regulation. Recently we could show that the MC4R is able to form receptor dimers. In the present study we investigated the role of extracellular cysteine residues and the structure of the third extracellular loop for receptor dimerization. None of the four extracellular cysteine residues nor the structure of the third extracellular loop play a role for MC4R-MC4R interaction as all investigated mutants display the same dimerization pattern as the wild-type receptor. Therefore for MC4R dimerization structures of the transmembrane-spanning helices are more likely to be involved.

Dimerization of the melanocortin 4 receptor: a study using bioluminescence resonance energy transfer

The melanocortin 4 receptor is important in the regulation of satiety. In this study we have investigated the propensity of the MC4 receptor to homodimerize. MC4 receptors with either a modified green fluorescent protein (GFP(2)) or Renilla luciferase (RLuc) at their C-terminus were constructed. These receptors showed equivalent binding and functional properties to the wild-type MC4 receptor. Bioluminescence resonance energy transfer readings indicated that the MC4 receptor exists as a constitutive homodimer, which was not regulated by peptide interaction. The efficiency of MC4 receptor to form homodimers was greatly enhanced compared to its ability to heterodimerize with the kappa opioid receptor.

Obesity-associated mutations in the human melanocortin-4 receptor gene

Mutations in the human melanocortin-4 receptor (MC4R) gene have been associated with severe obesity. Many of the mutations result in partial or complete loss-of-function based on the nature of the mutation or the function of mutated receptors when tested in heterologous expression systems. This review discusses the role of MC4R in the central regulation of body weight, the pathogenic mechanisms of the mutations, and the validity of MC4R as an anti-obesity drug target.

Melanotan-II: Investigation of the inducer and facilitator effects on penile erection in anaesthetized rat

The effects of melanotan-II, a non-specific agonist of melanocortin receptors, on erection and its possible sites of action were investigated in anesthetized rats. Delivered i.v. (0.1, 0.3 and 1 mg/kg) or within the paraventricular nucleus of the hypothalamus (0.1 and 1 microg), melanotan-II exerted a dose-dependent inducer activity on erection by eliciting erectile events and shortening latency of the first erectile event to occur. Erectile events were of higher amplitude in rats treated with melanotan-II i.t. (0.2 microg) delivered at the L6-S1 level than in animals treated with the vehicle i.t. delivered. Erectile responses elicited by cavernous nerve stimulation were increased after i.v. melanotan-II (1 mg/kg), thereby exerting facilitator effect on erection. In contrast, melanotan-II injected within the corpus cavernosum (1 microg) did not display any facilitator activity. To investigate the neural pathways involved in the facilitator effect of melanotan-II, we performed acute spinalization (T8 level) and differential selective nerve transections. Neither spinalization nor bilateral transection of pelvic nerves or dorsal penile nerves impaired facilitator activity of i.v. melanotan-II (1 mg/kg). Conversely, the facilitator effect of melanotan-II was abolished after acute removal of the lumbar paravertebral sympathetic chain. These results lead to the conclusion that central and peripheral melanocortin pathways are recruited by melanotan-II, depending on its route of delivery, to exert both inducer and facilitator activities on erection.

Melanocortin-4 receptor mRNA is expressed in sympathetic nervous system outflow neurons to white adipose tissue

Energy balance results from the coordination of multiple pathways affecting energy expenditure and food intake. Candidate neuropeptides involved in energy balance are the melanocortins. Several species, including Siberian hamsters studied here, decrease and increase food intake in response to stimulation and blockade of the melanocortin 4-receptor (MC4-R). In addition, central application of the MC3/4-R agonist melanotan-II decreases body fat (increases lipolysis) beyond that accounted for by its ability to decrease food intake. Because an increase in the sympathetic nervous system drive to white adipose tissue (WAT) is the principal initiator of lipolysis, we tested whether the sympathetic outflow circuitry from brain to WAT contained MC4-R mRNA expressing cells. This was accomplished by labeling the sympathetic outflow to inguinal WAT using the pseudorabies virus (PRV), a transneuronal retrograde viral tract tracer, and then processing the brain for colocalization of PRV immunoreactivity with MC4-R mRNA, the latter assessed by in situ hybridization. MC4-R mRNA was impressively colocalized in PRV-labeled cells (approximately greater than 60%) in many brain areas across the neuroaxis, including those typically implicated in lipid mobilization (e.g., hypothalamic paraventricular, suprachiasmatic, arcuate and dorsomedial nuclei, lateral hypothalamic area), as well as those not traditionally identified with lipolysis (e.g., preoptic area, subzona incerta of the lateral hypothalamus, periaqueductal gray, solitary nucleus). These data provide compelling neuroanatomical evidence that could underlie a direct central modulation of the sympathetic outflow to WAT by the melanocortins through the MC4-Rs resulting in changes in lipid mobilization and adiposity.

Central Nervous System Agents in the Treatment of Erectile Dysfunction

In the last two decades, a better understanding of the mechanisms governing erectile function and the pathophysiologies underlying erectile dysfunction (ED) have led re-searchers to investigate novel treatment concepts. Selective type-5 phosphodiesterase inhibitors are recommended as first-line therapy because of their high efficacy, but 30% to 40% of patients who have ED do not respond adequately to these agents and require alternative methods. The central nervous system plays a fundamental role in sexual behavior. Animal models have advanced our understanding of the neuroanatomic and neuropharmacologic basis of centrally induced penile erections. Clinical research with apomorphine has demonstrated efficacy in men who have a range of ED. Recent interest has focused on other centrally acting agents for ED treatment, including the melanocortin receptor agonists.

Interactions of human melanocortin 4 receptor with nonpeptide and peptide agonists

Specific interactions of human melanocortin-4 receptor (hMC4R) with its nonpeptide and peptide agonists were studied using alanine-scanning mutagenesis. The binding affinities and potencies of two synthetic, small-molecule agonists (THIQ, MB243) were strongly affected by substitutions in transmembrane alpha-helices (TM) 2, 3, 6, and 7 (residues Glu(100), Asp(122), Asp(126), Phe(261), His(264), Leu(265), and Leu(288)). In addition, a I129A mutation primarily affected the binding and potency of THIQ, while F262A, W258A, Y268A mutations impaired interactions with MB243. By contrast, binding affinity and potency of the linear peptide agonist NDP-MSH were substantially reduced only in D126A and H264A mutants. Three-dimensional models of receptor-ligand complexes with their agonists were generated by distance-geometry using the experimental, homology-based, and other structural constraints, including interhelical H-bonds and two disulfide bridges (Cys(40)-Cys(279), Cys(271)-Cys(277)) of hMC4R. In the models, all pharmacophore elements of small-molecule agonists are spatially overlapped with the corresponding key residues (His(6), d-Phe(7), Arg(8), and Trp(9)) of the linear peptide: their charged amine groups interact with acidic residues from TM2 and TM3, similar to His(6) and Arg(6) of NDP-MSH; their substituted piperidines mimic Trp(9) of the peptide and interact with TM5 and TM6, while the d-Phe aromatic rings of all three agonists contact with Leu(133), Trp(258), and Phe(261) residues.

Involvement of melanocortin-4 receptor in anxiety and depression

The melanocortins, which are derived from proopiomelanocortin, have a variety of physiological functions mediated membrane surface receptors. To date, five subtypes have been cloned. With the cloning of melanocortin receptors, studies with genetic models, and development of selective compounds, the physiological roles of the five melanocortin receptors have begun to be understood. The melanocortin-4 receptor (MC4R), which is predominantly expressed in the central nervous system, has in particular become the focus of much attention in recent years because of the critical roles it plays in a wide range of functions, including feeding, sexual behavior, and stress. Recent development of selective antagonists for the MC4R has provided pharmacological evidence that blockade of MC4R could be a useful way of alleviating numerous conditions such as anxiety/depression, pain, and addiction to drugs of abuse.

Gene polymorphisms and their effects in the melanocortin system

In addition to its role in human pigmentation, components of the melanocortin system regulate appetite, energy homeostasis and hormone production. Recent studies have suggested possible roles of this system in immunity, transmission of pain signals, and reproductive potential. A number of polymorphisms have been identified in genes of the melanocortin system and are associated with pigmentation in humans, as well as being causative of disorders of adrenal hormone production and obesity. This review gives an outline of these polymorphisms, their functional significance and possible application to or impact on diagnosis and pharmacotherapy based on melanocortin pathways.

Melanocortinergic control of penile erection

Melanocortin receptors in the forebrain and spinal cord can be activated by endogenous or synthetic ligands to induce penile erection in rats and human subjects. To better understand how melanocortin circuits play a role in sex behavior, we review the contribution of melanocortin receptors and/or neurons in the hypothalamus, hindbrain, spinal cord and peripheral nerves to erectile function. New information regarding neuropeptides that mediate penile erection has extended our understanding of the central control of sex behavior, and melanocortin agonists may provide alternatives to existing treatment for highly prevalent problems including erectile dysfunction.

A review of melanocortin receptor small molecule ligands

The melanocortin system (MC) is implicated in the regulation of a variety of physiological pathways including pigmentation, steroid function, energy homeostasis, food intake, obesity, cardiovascular, sexual function, and normal gland regulation. The melanocortin system consists of five receptors identified to date (MC1-5R), melanocortin agonists derived from the pro-opiomelanocortin prohormone (POMC) and two naturally existing antagonists. Melanocortin receptor ligand structure-activity studies have been performed since the 1960s, primarily focused on the pigmentation aspect of physiology. During the 1990s, the melanocortin-4 receptor was identified to play a significant physiological role in the regulation of both food intake and obesity. Subsequently, a concerted drug design effort has focused on the design and discovery of melanocortin receptor small molecules. Herein, we present an overview of melanocortin receptor heterocyclic small molecules.

Discovery and activity of (1R,4S,6R)-N-[(1R)-2-[4-cyclohexyl-4-[[(1,1-dimethylethyl)amino]carbonyl]-1-piperidinyl]-1-[(4-fluorophenyl)methyl]-2-oxoethyl]-2-methyl-2-azabicyclo[2.2.2]octane-6-carboxamide (3, RY764), a potent and selective melanocortin subtype-4 receptor agonist

A novel isoquinuclidine containing selective melanocortin subtype-4 receptor small molecule agonist, 3 (RY764), is reported. Its in vivo characterization revealed mechanism-based food intake reduction and erectile activity augmentation in rodents.

1-Amino-1,2,3,4-tetrahydronaphthalene-2-carboxylic acid as a Tic mimetic: Application in the synthesis of potent human melanocortin-4 receptor selective agonists

The discovery of 1-amino-1,2,3,4-tetrahydronaphthalene-2-carboxylic acid analogs as potent human melanocortin-4 selective agonists is described.

The Use of Telemetry Technology to Test the Proerectile Effect of Melanotan-II (MT-II) in Conscious Rats

OBJECTIVE

The aim of this study is to demonstrate that monitoring, by means of telemetry technology, the increases in intracavernosal pressure (ICP) in freely moving rats using melanotan-II (MT-II) as a proerectile inducer compound is a relevant experimental model to investigate the effects of pharmacological agents on erection.

METHODS

Adult rats were implanted in the corpus cavernosum with a pressure sensor which permitted telemetric monitoring of ICP in freely moving animals following MT-II (0.1, 0.3 and 1 mg/kg) or saline i.v. injections. ICP was also measured after MT-II (0.1, 0.3 or 1 mg/kg) or saline i.v. delivery in anesthetized rats.

RESULTS

In conscious rats, MT-II (1 mg/kg) significantly increased overall erectile activity compared to saline. In anesthetized rats, MT-II-induced increase in overall erectile activity was not statistically significant but displayed a similar pattern.

CONCLUSIONS

The use of telemetry technology allowed to collect quantifiable and reliable data regarding the proerectile activity of MT-II in physiological conditions. The telemetry model appears suitable for investigating the potential inducer proerectile properties of pharmacological agents.

N-terminal fatty acylated His-dPhe-Arg-Trp-NH(2) tetrapeptides: influence of fatty acid chain length on potency and selectivity at the mouse melanocortin receptors and human melanocytes

The melanocortin system is involved in the regulation of a diverse number of physiologically important pathways including pigmentation, feeding behavior, weight and energy homeostasis, inflammation, and sexual function. All the endogenous melanocortin agonist ligands possess the conserved His-Phe-Arg-Trp tetrapeptide sequence that is postulated to be important for melanocortin receptor molecular recognition and stimulation. Previous studies by our laboratory resulted in the discovery that increasing alkyl chain length at the N-terminal "capping" region of the His-dPhe-Arg-Trp-NH(2) tetrapeptide resulted in a 100-fold increased melanocortin receptor agonist potency. This study was undertaken to systematically evaluate the pharmacological effects of increasing N-capping alkyl chain length of the CH(3)(CH(2))(n)CO-His-dPhe-Arg-Trp-NH(2) (n = 6-16) tetrapeptide template. Twelve analogues were synthesized and pharmacologically characterized at the mouse melanocortin receptors MC1R and MC3R-MC5R and human melanocytes known to express the MC1R. These peptides demonstrated melanocortin receptor selectivity profiles different from those of previously published tetrapeptides. The most notable results of enhanced ligand potency (20- to 200-fold) and receptor selectivity were observed at the MC1R. Tetrapeptides that possessed greater than nine alkyl groups were superior to alpha-MSH in terms of the stimulation of human melanocyte tyrosinase activity. Additionally, the n-pentadecanoyl derivative had a residual effect on tyrosinase activity that existed for at least 4 days after the peptide was removed from the human melanocyte culture medium. These data demonstrate the utility, potency, and residual effect of melanocortin tetrapeptides by adding N-terminal fatty acid moieties.

Structure-activity relationships of the unique and potent agouti-related protein (AGRP)-melanocortin chimeric Tyr-c[beta-Asp-His-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH2 peptide template

The melanocortin receptor system consists of endogenous agonists, antagonists, G-protein coupled receptors, and auxiliary proteins that are involved in the regulation of complex physiological functions such as energy and weight homeostasis, feeding behavior, inflammation, sexual function, pigmentation, and exocrine gland function. Herein, we report the structure-activity relationship (SAR) of a new chimeric hAGRP-melanocortin agonist peptide template Tyr-c[beta-Asp-His-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) that was characterized using amino acids previously reported in other melanocortin agonist templates. Twenty peptides were examined in this study, and six peptides were selected for (1)H NMR and computer-assisted molecular modeling structural analysis. The most notable results include the identification that modification of the chimeric template at the His position with Pro and Phe resulted in ligands that were nM mouse melanocortin-3 receptor (mMC3R) antagonists and nM mouse melanocortin-4 receptor (mMC4R) agonists. The peptides Tyr-c[beta-Asp-His-DPhe-Ala-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) and Tyr-c[beta-Asp-His-DNal(1')-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) resulted in 730- and 560-fold, respectively, mMC4R versus mMC3R selective agonists that also possessed nM agonist potency at the mMC1R and mMC5R. Structural studies identified a reverse turn occurring in the His-DPhe-Arg-Trp domain, with subtle differences observed that may account for the differences in melanocortin receptor pharmacology. Specifically, a gamma-turn secondary structure involving the DPhe(4) in the central position of the Tyr-c[beta-Asp-Phe-DPhe-Arg-Trp-Asn-Ala-Phe-Dpr]-Tyr-NH(2) peptide may differentiate the mixed mMC3R antagonist and mMC4R agonist pharmacology.

HLDF-6 peptide affects behavioral reactions and organism functions dependent on androgen hormones in normal and castrated male mice

The hexapeptide Thr-Gly-Glu-Asn-His-Arg (HLDF-6), which was first identified as an active fragment of the human leukemia differentiation factor (HLDF) molecule, displays differentiation-inducing, neuroprotective and anti-drug abuse activities. Most of its in vivo effects were revealed only on male animals. We have studied HLDF-6 effects on a variety of organism functions and behavioral reactions, which are known to be dependent on androgen steroid hormones, both on castrated and normal (sham-operated) animals. Male NMRI mice were castrated or sham-operated at the age of 55 days (after puberty). After that, HLDF-6 peptide was injected daily during 3 weeks, followed by behavioral, morphological and biochemical testing. HLDF-6 increased testosterone level (1.5- to 2-fold) both in sham-operated and castrated animals. Sexual activity and pain sensitivity, which are strongly reduced in castrates, were completely or partially recovered by HLDF-6. At the same time, the peptide caused some effects similar to castration in sham-operated animals: aggression and locomotor activity were decreased; oral grooming was prolonged. Morphological studies of accessory sex glands showed that HLDF-6 partially normalizes the morphology and functional activity of seminal vesicles in castrates, but it does not prevent castration-induced apoptosis of prostate epithelial cells. Based on these observations, we can assume that HLDF-6 peptide displays at least two effects on androgen hormones metabolism in males: it stimulates testosterone biosynthesis by both testes and adrenals and simultaneously inhibits its conversion to dihydrotestosterone (DHT), most probably by diminution of 5alpha-reductase isoform 1 mRNA expression.

Functional selectivity of melanocortin 4 receptor peptide and nonpeptide agonists: evidence for ligand-specific conformational states

Agonists of the melanocortin 4 (MC4) receptor have potential pharmaceutical benefit in the treatment of obesity and sexual dysfunction. In this study, we have compared the ability of a number of peptide and nonpeptide agonists to activate a FLAG-tagged human MC4 (FMC4) receptor, as measured by both cAMP accumulation and calcium mobilization using a fluorometric imaging plate reader (FLIPR). In addition, we have analyzed the ability of these agonists to cause receptor internalization, as measured by fluorescence-activated cell sorting analysis. The endogenous agonist alpha-melanocortin-stimulating hormone (alpha-MSH) increased cAMP accumulation, calcium mobilization, and receptor internalization in a dose-dependent manner in human embryonic kidney 293 cells expressing the FMC4 receptor. The activity of the other agonists varied considerably in these assays, and overall, the potency and intrinsic activity of the agonists in the cAMP accumulation assays did not correlate with their potency or intrinsic activity in either the FLIPR or receptor internalization assays. Agonists could be clearly separated into two functional classes based on their structure. Peptide agonists beta-MSH, des-acetyl-alpha-MSH, and [Nle(4), D-Phe(7)]-alpha-melanocortin-stimulating hormone exhibited 80 to 112% of the maximal alpha-MSH response in cAMP accumulation and 62 to 96% in FLIPR assays and were able to cause 75 to 118% of receptor internalization induced by alpha-MSH. Conversely, although the nonpeptide agonists exhibited 73 to 149% of the alpha-MSH response in the cAMP accumulation assays, they were significantly impaired in the FLIPR (7-40%) and receptor internalization (-5-38%) assays. These findings demonstrate an important difference in activation and internalization of the MC4 receptor by nonpeptide versus peptide agonists and provides evidence of agonist-specific conformational states.

Melanocortin subtype-4 receptor agonists containing a piperazine core with substituted aryl sulfonamides

The biological activity for a set of melanocortin-4 receptor (MC4R) agonists containing a piperazine core with an ortho-substituted aryl sulfonamide is described. Compounds from this set had binding and functional activities at MC4R less than 30 nM. The most selective compound in this series was >25,000-fold more potent at MC4R than MC3R, and 490-fold more potent at MC4R than MC5R. This compound also reduced food intake after oral dosing at 25, 50, and 100 mg kg(-1) in fasted mice.

Stereochemical studies of the monocyclic agouti-related protein (103-122) Arg-Phe-Phe residues: conversion of a melanocortin-4 receptor antagonist into an agonist and results in the discovery of a potent and selective melanocortin-1 agonist

The agouti-related protein (AGRP) is an endogenous antagonist of the centrally expressed melanocortin receptors. The melanocortin-4 receptor (MC4R) is involved in energy homeostasis, food intake, sexual function, and obesity. The endogenous hAGRP protein is 132 amino acids in length, possesses five disulfide bridges at the C-terminus of the molecule, and is expressed in the hypothalamus of the brain. We have previously reported that a monocyclic hAGRP(103-122) peptide is an antagonist at the melanocortin receptors expressed in the brain. Stereochemical inversion from the endogenous l- to d-isomers of single or multiple amino acid modifications in this monocyclic truncated hAGRP sequence resulted in molecules that are converted from melanocortin receptor antagonists into melanocortin receptor agonists. The Asp-Pro-Ala-Ala-Thr-Ala-Tyr-cyclo[Cys-Arg-DPhe-DPhe-Asn-Ala-Phe-Cys]-Tyr-Ala-Arg-Lys-Leu peptide resulted in a 60 nM melanocortin-1 receptor agonist that is 100-fold selective versus the mMC4R, 1000-fold selective versus the mMC3R, and ca. 180-fold selective versus the mMC5R. In attempts to identify putative ligand-receptor interactions that may be participating in the agonist induced stimulation of the MC4R, selected ligands were docked into a homology molecular model of the mMC4R. These modeling studies have putatively identified hAGRP ligand DArg111-mMC4RAsn115 (TM3) and the hAGRP DPhe113-mMC4RPhe176 (TM4) interactions as important for agonist activity.

Voluntary exercise delays monogenetic obesity and overcomes reproductive dysfunction of the melanocortin-4 receptor knockout mouse

The melanocortin system is involved in hypothalamic regulation of energy homeostasis. The melanocortin-4 receptor (MC4R) has been linked to both obesity and reproductive dysfunction. Deletion of the MC4R from the mouse genome has resulted in phenotypes including adult onset obesity, hyperphagia, and difficulty in reproducing when homozygote parents are bred. Additionally, polymorphisms of the human MC4R have been identified in morbidly obese children and adults. Herein, we have identified that voluntary exercise, provided via the presence of a running wheel, impedes the monogenetic obesity (at 20 weeks of age running wheel housed body weight=31+/-1.8 g versus conventionally housed body weight=41+/-2.3 g, a 25% decrease in body weight p<0.01), hyperphagia (average cumulative food intake is not statistically different than wild type mice housed in running wheel cages), and reproductive dysfunction phenotypes associated with the MC4R knockout mice housed by conventional means. These data demonstrate the novel finding that voluntary exercise at a young age may hinder genetically induced obesity.

Discovery of (2S)-N-[(1R)-2-[4-cyclohexyl-4-[[(1,1-dimethylethyl)amino]carbonyl]-1-piperidinyl]-1-[(4-fluorophenyl)methyl]-2-oxoethyl]-4-methyl-2-piperazinecarboxamide (MB243), a potent and selective melanocortin subtype-4 receptor agonist

We report the discovery and optimization of substituted 2-piperazinecarboxamides as potent and selective agonists of the melanocortin subtype-4 receptor. Further in vivo development of lead agonist, MB243, is disclosed.