Binding and cAMP studies of melanotropin peptides with the cloned human peripheral melanocortin receptor, hMC1R

Recommended Tool Compounds for the Melanocortin Receptor (MCR) G Protein-Coupled Receptors (GPCRs)

The melanocortin receptors are a centrally and peripherally expressed family of Class A GPCRs with physiological roles, including pigmentation, steroidogenesis, energy homeostasis, and others yet to be fully characterized. There are five melanocortin receptor subtypes that, apart from the melanocortin-2 receptor (MC2R), are stimulated by a shared set of endogenous agonists. Until 2020, X-ray crystallographic and cryo-electron microscopic (cryo-EM) structures of these receptors were unavailable, and the investigation of their mechanisms of action and putative ligand-receptor interactions was driven by site-directed mutagenesis studies of the receptors and targeted structure-activity relationship (SAR) studies of the endogenous and derivative synthetic ligands. Synthetic derivatives of the endogenous agonist ligand α-MSH have evolved into a suite of powerful ligands such as NDP-MSH (melanotan I), melanotan II (MTII), and SHU9119. This suite of tool compounds now enables the study of the melanocortin receptors and serves as scaffolds for FDA-approved drugs, means of validating stably expressing melanocortin receptor cell lines, core ligands in assessing cryo-EM structures of active and inactive receptor complexes, and essential references for high-throughput discovery and mechanism of action studies. Herein, we review the history and significance of a finite set of these essential tool compounds and discuss how they are being utilized to further the field's understanding of melanocortin receptor physiology and greater druggability.

Divergence of cAMP signalling pathways mediating augmented nucleotide excision repair and pigment induction in melanocytes

Loss-of-function melanocortin 1 receptor (MC1R) polymorphisms are common in UV-sensitive fair-skinned individuals and are associated with blunted cAMP second messenger signalling and higher lifetime risk of melanoma because of diminished ability of melanocytes to cope with UV damage. cAMP signalling positions melanocytes to resist UV injury by upregulating synthesis of UV-blocking eumelanin pigment and by enhancing the repair of UV-induced DNA damage. cAMP enhances melanocyte nucleotide excision repair (NER), the genome maintenance pathway responsible for the removal of mutagenic UV photolesions, through cAMP-activated protein kinase (protein kinase A)-mediated phosphorylation of the ataxia telangiectasia-mutated and Rad3-related (ATR) protein on the S435 residue. We investigated the interdependence of cAMP-mediated melanin upregulation and cAMP-enhanced DNA repair in primary human melanocytes and a melanoma cell line. We observed that the ATR-dependent molecular pathway linking cAMP signalling to the NER pathway is independent of MITF activation. Similarly, cAMP-mediated upregulation of pigment synthesis is independent of ATR, suggesting that the key molecular events driving MC1R-mediated enhancement of genome maintenance (eg PKA-mediated phosphorylation of ATR) and MC1R-induced pigment induction (eg MITF activation) are distinct.

Afamelanotide: A Review in Erythropoietic Protoporphyria

Afamelanotide (SCENESSE(®)) is a synthetic α-melanocyte stimulating hormone analogue and first-in-class melanocortin-1 receptor agonist that is approved in the EU for the prevention of phototoxicity in adults with erythropoietic protoporphyria (EPP). It is administered subcutaneously as a biodegradable, controlled-release implant containing 16 mg of afamelanotide. This article reviews the clinical efficacy and tolerability of afamelanotide in EPP and summarizes its pharmacological properties. In the phase III trial, CUV039, afamelanotide treatment improved light tolerance in patients with EPP. Compared with placebo, afamelanotide treatment enabled patients to spend more time in direct sunlight without pain and increased the time to the appearance of the first symptoms of phototoxicity provoked by a standardized light source. Afamelanotide was generally well tolerated in this trial, with no drug-related serious adverse events reported. Commonly occurring adverse reactions included headache and implant-site reactions. Efficacy and safety data from earlier phase III trials are consistent with those from the CUV039 trial. Afamelanotide, approved in the EU for the prevention of EPP phototoxicity, represents a useful addition to the management of the disorder.

Cyclic lactam hybrid α-MSH/Agouti-related protein (AGRP) analogues with nanomolar range binding affinities at the human melanocortin receptors

A novel hybrid melanocortin pharmacophore was designed based on the topographical similarities between the pharmacophores of Agouti related protein (AGRP) an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. When employed in two different 23-membered macrocyclic lactam peptide templates, the designed hybrid AGRP/MSH pharmacophore yielded non-competitive ligands with nanomolar range binding affinities. The topography-based pharmacophore hybridization strategy will prove useful in development of unique non-competitive melanocortin receptor modulators.

Solid-phase peptide head-to-side chain cyclodimerization: discovery of C(2)-symmetric cyclic lactam hybrid α-melanocyte-stimulating hormone (MSH)/agouti-signaling protein (ASIP) analogues with potent activities at the human melanocortin receptors

A novel hybrid melanocortin pharmacophore was designed based on the pharmacophores of the agouti-signaling protein (ASIP), an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. The designed hybrid ASIP/MSH pharmacophore was explored in monomeric cyclic, and cyclodimeric templates. The monomeric cyclic disulfide series yielded peptides with hMC3R-selective non-competitive binding affinities. The direct on-resin peptide lactam cyclodimerization yielded nanomolar range (25-120 nM) hMC1R-selective full and partial agonists in the cyclodimeric lactam series which demonstrates an improvement over the previous attempts at hybridization of MSH and agouti protein sequences. The secondary structure-oriented pharmacophore hybridization strategy will prove useful in development of unique allosteric and orthosteric melanocortin receptor modulators. This report also illustrates the utility of peptide cyclodimerization for the development of novel GPCR peptide ligands.

Multiple N-methylation of MT-II backbone amide bonds leads to melanocortin receptor subtype hMC1R selectivity: pharmacological and conformational studies

Multiple N-methylation is a novel technology to improve bioavailability of peptides and increase receptor subtype selectivity. This technique has been applied here to the superpotent but nonselective cyclic peptide MT-II. A library of all possible 31 backbone N-methylated derivatives has been synthesized and tested for binding and activation at melanocortin receptor subtypes 1, 3, 4, and 5. It turned out that selectivity is improved with every introduced N-methyl group, resulting in several N-methylated selective and potent agonists for the hMC1R. The most potent of these derivatives is N-methylated on four out of five amide bonds in the cyclic structure. Its solution structure indicates a strongly preferred backbone conformation that resembles other alpha-MSH analogs but possesses much less flexibility and in addition distinct differences in the spatial arrangement of individual amino acid side chains.

The peptide NDP-MSH induces phenotype changes in the heart that resemble ischemic preconditioning

alpha-Melanocyte-stimulating hormone (alpha-MSH) is a pro-opiomelanocortin (POMC)-derived peptide that exerts multiple protective effects on host cells. Previous investigations showed that treatment with alpha-MSH or synthetic melanocortin agonists reduces heart damage in reperfusion injury and transplantation. The aim of this preclinical research was to determine whether melanocortin treatment induces preconditioning-like cardioprotection. In particular, the plan was to assess whether melanocortin administration causes phenotype changes similar to those induced by repetitive ischemic events. The idea was conceived because both ischemic preconditioning and melanocortin signaling largely depend on cAMP response element binding protein (CREB) phosphorylation. Rats received single i.v. injections of 750microg/kg of the alpha-MSH analogue Nle(4),DPhe(7)-alpha-MSH (NDP-MSH) or saline and were sacrificed at 0.5, 1, 3, or 5h. Western blot analysis showed that rat hearts expressed melanocortin 1 receptor (MC1R) protein. Treatment with NDP-MSH was associated with early and marked increase in interleukin 6 (IL-6) mRNA. This was followed by signal transducer and activator of transcription 3 (STAT3) phosphorylation and induction of suppressor of cytokine signaling 3 (SOCS3). There were no changes in expression of other cytokines of the IL-6 family. Expression of IL-10, IL-1beta, and TNF-alpha was likewise unaltered. In hearts of rats treated with NDP-MSH there was increased expression of the orphan nuclear receptor Nur77. The data indicate that NDP-MSH induces phenotype changes that closely resemble ischemic preconditioning and likely contribute to its established protection against reperfusion injury. In addition, the increased expression of Nur77 and SOCS3 could be part of a broader anti-inflammatory effect.

Organic chemistry and biology: chemical biology through the eyes of collaboration

From a scientific perspective, efforts to understand biology including what constitutes health and disease has become a chemical problem. However, chemists and biologists "see" the problems of understanding biology from different perspectives, and this has retarded progress in solving the problems especially as they relate to health and disease. This suggests that close collaboration between chemists and biologists is not only necessary but essential for progress in both the biology and chemistry that will provide solutions to the global questions of biology. This perspective has directed my scientific efforts for the past 45 years, and in this overview I provide my perspective of how the applications of synthetic chemistry, structural design, and numerous other chemical principles have intersected in my collaborations with biologists to provide new tools, new science, and new insights that were only made possible and fruitful by these collaborations.

Substitution of arginine with proline and proline derivatives in melanocyte-stimulating hormones leads to selectivity for human melanocortin 4 receptor

A new series of melanotropin analogues with His or Arg residues in the core pharmacophores of MTII, SHU9119, and Ac-NDP-gamma-MSH-NH(2) replaced by Pro or trans-/cis-4-guanidinyl-Pro derivatives were designed and synthesized to introduce selectivity toward the human melanocortin 4 receptor (hMC4R). Analogues 1, 2, 3, 6, 7, 8 were found to be hMC4R selective. Second messenger studies have demonstrated that analogues 1 and 2 are insurmountable inhibitors of MTII agonist activity at the hMC4R. Molecular modeling studies suggest that the hMC4R selectivity is due to a beta-turn shift induced by the Pro ring that makes the global minimum structures of these analogues resemble the NMR solution structure of the hASIP melanocortin receptor binding motif. Substitution of His in MTII also provided functional selectivity for the hMC3R or the hMC4R. These findings are important for a better understanding of the selectivity mechanism at the hMC3R/hMC4R and the development of therapeutic ligands selectively targeting the hMC4R.

Structure-activity relationships of cyclic lactam analogues of alpha-melanocyte-stimulating hormone (alpha-MSH) targeting the human melanocortin-3 receptor

A variety of dicarboxylic acid linkers introduced between the alpha-amino group of Pro(6) and the -amino group of Lys(10) of the cyclic lactam alpha-melanocyte-stimulating hormone (alpha-MSH)-derived Pro(6)-D-Phe(7)/D-Nal(2')(7)-Arg(8)-Trp(9)-Lys(10)-NH2 pentapeptide template lead to nanomolar range and selective hMC3R agonists and antagonists. Replacement of the Pro(6) residue and the dicarboxylic acid linker with 2,3-pyrazine-dicarboxylic acid furnished a highly selective nanomolar range hMC3R partial agonist (analogue 12, c[CO-2,3-pyrazine-CO-D-Phe-Arg-Trp-Lys]-NH2, EC50 = 27 nM, 70% max cAMP) and an hMC3R antagonist (analogue 13, c[CO-2,3-pyrazine-CO-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 23 nM). Modeling experiments suggest that 2,3-pyrazinedicarboxylic acid stabilizes a beta-turn-like structure with the D-Phe/D-Nal(2') residues, which explains the high potency of the corresponding peptides. Placement of a Nle residue in position 6 produced a hMC3R/hMC5R antagonist (analogue 15, c[CO-(CH 2)2-CO-Nle-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 12 and 17 nM, respectively), similarly to the previously described cyclic gamma-melanocyte-stimulating hormone (gamma-MSH)-derived hMC3R/hMC5R antagonists. These newly developed melanotropins will serve as critical biochemical tools for elucidating the full spectrum of functions performed by the physiologically important melanocortin-3 receptor.

Synthesis and biological activity of novel peptide mimetics as melanocortin receptor agonists

A series of novel peptidomimetic analogs was prepared containing cyclohexyl, phenyl, or heterocyclic groups to ostensibly orient the guanidine or mimic of an arginine in a putative melanocortin receptor ligand pharmacophore. Some binding affinity at the melanocortin receptors MC(3) and MC(4) was noted. In silico docking also indicated that the relative positions of the hydrogen-bonding sites and hydrophobic regions of the compounds are reasonably well matched to the receptor-binding site. This may present a lead entry into a selective series of MC(4)R agonists.

Brain–adipose tissue cross talk

While investigating the reversible seasonal obesity of Siberian hamsters, direct sympathetic nervous system (SNS) postganglionic innervation of white adipose tissue (WAT) has been demonstrated using anterograde and retrograde tract tracers. The primary function of this innervation is lipid mobilization. The brain SNS outflow to WAT has been defined using the pseudorabies virus (PRV), a retrograde transneuronal tract tracer. These PRV-labelled SNS outflow neurons are extensively co-localized with melanocortin-4 receptor mRNA, which, combined with functional data, suggests their involvement in lipolysis. The SNS innervation of WAT also regulates fat cell number, as noradrenaline inhibits and WAT denervation stimulates fat cell proliferationin vitroandin vivorespectively. The sensory innervation of WAT has been demonstrated by retrograde tract tracing, electrophysiological recording and labelling of the sensory-associated neuropeptide calcitonin gene-related peptide in WAT. Local injections of the sensory nerve neurotoxin capsaicin into WAT selectively destroy this innervation. Just as surgical removal of WAT pads triggers compensatory increases in lipid accretion by non-excised WAT depots, capsaicin-induced sensory denervation triggers increases in lipid accretion of non-capsaicin-injected WAT depots, suggesting that these nerves convey information about body fat levels to the brain. Finally, parasympathetic nervous system innervation of WAT has been suggested, but the recent finding of no WAT immunoreactivity for the possible parasympathetic marker vesicular acetylcholine transporter (VAChT) argues against this claim. Collectively, these data suggest several roles for efferent and afferent neural innervation of WAT in body fat regulation.

Novel selective human melanocortin-3 receptor ligands: use of the 4-amino-1,2,4,5-tetrahydro-2-benzazepin-3-one (Aba) scaffold

In search of new selective antagonists and/or agonists for the human melanocortin receptor subtypes hMC1R to hMC5R to elucidate the specific biological roles of each GPCR, we modified the structures of the superagonist MT-II (Ac-Nle-c[Asp-His-D-Phe-Arg-Trp-Lys]-NH(2)) and the hMC3R/hMC4R antagonist SHU9119 (Ac-Nle-c[Asp-His-D-Nal(2')-Arg-Trp-Lys]-NH(2)) by replacing the His-d-Phe and His-d-Nal(2') fragments in MT-II and SHU9119, respectively, with Aba-Xxx (4-amino-1,2,4,5-tetrahydro-2-benzazepin-3-one-Xxx) dipeptidomimetics (Xxx=D-Phe/pCl-D-Phe/D-Nal(2')). Employment of the Aba mimetic yielded novel selective high affinity hMC3R and hMC3R/hMC5R antagonists.

Design, synthesis, and biological evaluation of a new class of small molecule peptide mimetics targeting the melanocortin receptors

A new bicyclic template has been developed for the synthesis of peptide mimetics. Straightforward synthetic steps, starting from amino acids, allow the facile construction of a wide range of analogs. This system was designed to target the melanocortin receptors (MCRs), with functional group selection based on a known pharmacophore and guidance from molecular modeling to rationally identify positional and stereochemical isomers likely to be active. The functions of hMCRs are critical to myriad biological activities, including pigmentation, steroidogenesis, energy homeostasis, erectile activity, and inflammation. These G-protein-coupled receptors (GPCRs) are targets for drug discovery in a number of areas, including cancer, pain, and obesity therapeutics. All compounds from this series tested to date are antagonists which bind with high affinity. Importantly, many are highly selective for a particular MCR subtype, including some of the first completely hMC5R-selective antagonists reported.

The melanocortin circuit in obese and lean strains of chicks

Agonists of membranal melanocortin 3 and 4 receptors (MC3/4Rs) are known to take part in the complex control mechanism of energy balance. In this study, we compared the physiological response to an exogenous MC3/4R agonist and the hypothalamic expression of proopic melanocortin (POMC) gene, encoding few MC3/4R ligands, between broiler and layer chicken strains. These strains, representing the two most prominent commercial strains of chickens grown for meat (broilers) and egg production (layers), differ in their food intake, fat accumulation, and reproductive performance and, therefore, form a good model of obese and lean phenotypes, respectively. A single i.v. injection of the synthetic peptide melanotan-II (MT-II; 1 mg/kg body weight) into the wing vein of feed-restricted birds led to attenuation of food intake upon exposure to feeding ad libitum in both broiler and layer chickens. A study of the POMC mRNA encoding the two prominent natural MC3/4R agonists, alpha-MSH and ACTH, also revealed a general similarity between the strains. Under feeding conditions ad libitum, POMC mRNA levels were highly similar in chicks of both strains and this level was significantly reduced upon feed restriction. However, POMC mRNA down-regulation upon feed restriction was more pronounced in layers than in broilers. These results suggest: (i) a role for MC3/4R agonists in the control of appetite; (ii) that the physiological differences between broilers and layers are not related to unresponsiveness of broiler chickens to the satiety signal of MC3/4R ligands. Therefore, these findings suggest that artificial activation of this circuit in broiler chicks could help to accommodate with their agricultural shortcomings of overeating, fattening, and impaired reproduction.

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.

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.

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.

Design of cyclic and other templates for potent and selective peptide alpha-MSH analogues

For over three decades, the design of linear peptide ligands often has incorporated cyclic constraints to improve potency, receptor selectivity, proteolytic stability and biodistribution. Its importance has been so well established that modern day schemes for ligand-based drug design often start with cyclization of linear peptides to rigidify peptide structure, to limit its conformational possibilities, and to find key pharmacophore elements in three-dimensional space. In the past several years, cyclic constraints have been used to develop ligands with improved efficacy, binding affinity, biostability and receptor selectivity for alpha-melanocyte-stimulating hormone (alpha-MSH). Furthermore, potent cyclic alpha-MSH analogues, such as MT-II and SHU-9119, have made structure-activity relationship studies and molecular modeling more useful for creating new three-dimensional, topographical pharmacophore templates.

Synthesis of a novel potent cyclic peptide MC4-ligand by ring-closing metathesis

The synthesis of a novel potent cyclic peptide MC4-ligand by ring-closing metathesis (RCM) is described. Based on the Ac-Nle-Gly-Lys-D-Phe-Arg-Trp-Gly-NH2-MC4 ligand, Ac-Nle-Alg-Lys-D-Phe-Arg-Trp-Alg-NH2 was designed and synthesized followed by cyclization using RCM. Both compounds are high affinity and selective MC4-R-agonists. The cyclic RCM-peptide was more potent in a rat-grooming assay.

Biological and conformational study of beta-substituted prolines in MT-II template: steric effects leading to human MC5 receptor selectivity

To investigate the molecular basis for the interaction of the chi-constrained conformation of melanotropin peptide with the human melanocortin receptors, a series of beta-substituted proline analogs were synthesized and incorporated into the Ac-Nle-C[Asp-His-D-Phe-Arg-Trp-Lys]-NH2 (MT-II) template at the His6 and D-Phe7 positions. It was found that the binding affinities generally diminished as the steric bulk of the p-substituents of the 3-phenylproline residues increased. From (2S, 3R)-3-phenyl-Pro6 to (2S, 3R)-3-(p-methoxyphenyl)-Pro6 analogs the binding affinity decreased 23-fold at the human melanocortin-3 receptor (hMC3R), 17-fold at the hMC4R, and eight-fold at the hMC5R, but selectivity for the hMC5R increased. In addition, the substitution of the D-Phe7 residue with a (2R, 3S)-3-phenyl-Pro resulted in greatly reduced binding affinity (10(3)-10(5)) at these melanocortin receptors. Macromodel's Large Scale Low Mode (LLMOD) with OPLS-AA force field simulations revealed that both MT-II and SHU-9119 share a similar backbone conformation and topography with the exception of the orientation of the side chains of D-Phe7/D-Nal (2')7 in chi space. Introduction of the dihedrally constrained phenylproline analogs into the His6 position (analogs 2-6) caused topographical changes that might be responsible for the lower binding affinities. Our findings indicate that hMC3 and hMC4 receptors are more sensitive to steric effects and conformational constraints than the hMC5 receptor. This is the first example for melanocortin receptor selectivity where the propensity of steric interactions in chi space of beta-modified Pro6 analogs of MT-II has been shown to play a critical role for binding as well as bioefficacy of melanotropins at hMC3 and hMC4 receptors, but not at the hMC5 receptor.

A novel 3-step enantioselective synthesis of pyrenylalanine with subsequent incorporation into opioid, CCK, and melanotropin ligands

Pyrene possesses unique spectroscopic properties such as a high quantum yield, a long half-life in the excited state, and the ability to form excimers when in proximity to each other in the excited state. These properties allow pyrenylalanine, which is a pyrene moiety incorporated into an amino acid, to be used as a fluorescent probe in peptides and proteins. The common route for the synthesis of pyrenylalanine involves 5 steps, with subsequent separation of the two isomers by recrystallization. This paper reports a novel 3-step asymmetric synthesis of pyrenylalanine with high enantioselectivity, good yields, and facile isomer purification. After synthesis, pyrenylalanine was incorporated into a series of opioid, CCK, and melanotropin peptide ligands in order to study the effects of aromaticity, lipophilicity, and steric properties on their potency and efficacy at their corresponding biological receptors. The change in binding and efficacy of the labeled ligands as compared to the unlabeled ligands demonstrates the possible role of lipophilicity/aromaticity in the binding and signal transduction of the ligand-receptor interaction.

Melanocortin receptor signaling through mitogen-activated protein kinase in vitro and in rat hypothalamus

The central melanocortin system has emerged as a potential regulator of food intake. This action of melanocortins appears to occur through intrahypothalamic, melanocortin-containing projections, including those from the arcuate to the paraventricular nucleus (PVN). Although the complexity of feeding behavior and the long duration of the effects of melanocortins on food intake suggest changes in gene expression, the mechanism by which such changes occur has been elusive. In the present report, we describe experiments using in vitro and in vivo approaches to demonstrate melanocortin-induced phosphorylation (activation) of members of the mitogen-activated protein kinase (MAPK) family of transcription factors. First, application of the melanocortin agonist MTII to COS-1 cells resulted in an increase in phosphorylated MAPK after the cells were transfected with the melanocortin type 4 receptor (MC4-R), but not the type 3 receptor. Formation of cAMP, however, was observed when either receptor subtype was transfected. Subsequent experiments revealed that the effect of MTII on MAPK activation in MC4-R-transfected cells was dose-dependent and was maximal after 10 min of MTII exposure. Second, central injections of MTII increased the number of phospho-MAPK-immunoreactive cells in the rat PVN compared to vehicle-injected animals. When coupled with immunohistochemical identification of PVN neurons containing oxytocin, a clear segregation was apparent, allowing for a precise anatomical description of the pattern of activated MAPK within the PVN. These data are the first to suggest a differential coupling of MC4-R and may describe a mechanism through which the long-term and persistent behavioral actions of melanocortins are mediated.

Stereoselective synthesis of novel dipeptide beta-turn mimetics targeting melanocortin peptide receptors

[reaction: see text] The novel dipeptide beta-turn mimetic, 4,8-disubstituted azabicyclo[4.3.0]nonane amino acid ester (15), has been synthesized to serve as a peptide mimetic of the dipeptide Phe-Arg, which contains two important pharmacophore elements in melanotropin peptides. Introduction of side-chain functionality at C-8 was achieved by using beta-functionalized pyroglutamate (8) as a synthetic precursor. The side chain at C-4 was introduced by bromination of dehydroamino acid intermediate (10) followed by Suzuki cross-coupling.

The Seiji memorial lecture: the melanosome: an ideal model to study cellular differentiation

Melanosomes provide an intriguing model for study at many levels. In part this is due to their unique structure and function, but also in part to their involvement in pigmentary diseases and as a model to study basic cellular mechanisms of organelle biogenesis. Recent studies have elucidated the full proteome of the melanosome and the metabolic and molecular lesions involved in a number of pigmentary diseases have been resolved. This paper summarizes recent advances in the field in these areas.

Design of peptides, proteins, and peptidomimetics in chi space

Peptide and protein biological activities depend on their three dimensionals structures in the free state and when interacting with their receptors/acceptors. The backbone conformations such as alpha-helix, beta-sheet, beta-turn, and so forth provide critical templates for the three-dimensional structure, but the overall shape and intrinsic stereoelectronic properties of the peptide or protein important for molecular recognition, signal transduction, enzymatic specificity, immunomodulation, and other biological effects depend on arrangement of the side chain groups in three-dimensional chi space (their chi 1, chi 2, etc. torsional angles). In this paper we explore approaches to the de novo design of polypeptides and peptidomimetics with biased or specific conformational/topographical properties in chi space. We consider computational and experimental methods that can be used to examine the effects of specific structural modifications in constraining side chain groups of amino acid residues and their similarities in chi space to the natural amino acids to evaluate what sort of mimetics are likely to mimic normal amino acids. We then examine some of the asymmetric synthetic methods that are being developed to obtain the amino acid mimetics. Finally, we consider selected examples in the literature where these specialized amino acids have been incorporated in biologically active peptides and the specific insights they have provided regarding the topographical requirements for bioactive peptide potency, selectivity, and other biochemical and pharmacological properties. Constraints in chi space show great promise as useful tools in peptide, protein, and peptidomimetic de novo design of structures and pharmacophores with specific stereostructural, biochemical and biological properties.

Structure and dynamics of alpha-MSH using DRISM integral equation theory and stochastic dynamics

The structural and dynamical features of the hormone alpha-MSH in solution have been examined over a 100 ns time scale by using free energy molecular mechanics models at room temperature. The free energy surface has been modeled using methods from integral equation theory and the dynamics by the Langevin equation. A modification of the accessible surface area friction drag model was used to calculate the atomic friction coefficients. The molecule shows a stable beta-turn conformation in the message region and a close interaction between the side chains of His6, Phe7, and Trp9. A salt bridge between Glu5 and Arg8 was found not to be a preferred interaction, whereas a Glu5 and Lys11 salt bridge was not sampled, presumably due to relatively high free energy barriers. The message region was more conformationally rigid than the N-terminal region. Several structural features observed here agree well with experimental results. The conformational features suggest a receptor-hormone interaction model where the hydrophobic side chains of Phe7 and Trp9 interact with the transmembrane portion of the MC1 receptor. Also, the positively charged side chain of Arg8 and the imidazole side chain of His6 may interact with the negatively charged portions of the receptor which may even be on the receptor's extracellular loops.

Induction of autocrine factor inhibiting cell motility from murine B16-BL6 melanoma cells by alpha-melanocyte stimulating hormone

We have previously reported that neuropeptide alpha-melanocyte stimulating hormone (alpha-MSH) successfully inhibited Matrigel invasion and haptotactic migration of B16-BL6 melanoma cells towards both fibronectin and laminin without affecting their growth. In the present study, we investigated the inhibitory mechanism of tumor cell motility by alpha-MSH. Alpha-MSH significantly blocked the autocrine motility factor (AMF)-enhanced cell motility. However, alpha-MSH did neither prevent the secretion of AMF from B16-BL6 cells nor alter the expression level of AMF receptor (gp78). On the other hand, alpha-MSH induced the secretion of the motility inhibitory factor(s) from B16-BL6 cells in a concentration- and time-dependent manner. The induction of the motility inhibitor(s) was proportional to increasing levels of intracellular cAMP induced by alpha-MSH as well as forskolin, and the activity was abolished by an adenylate cyclase inhibitor, 2',5'-dideoxyadenosine (DDA). The motility-inhibiting activity in conditioned medium (CM) from alpha-MSH-treated B16-BL6 cells was found to have a m.w. below 3 kDa after fractionation. This activity was abolished by boiling but insensitive to trypsin. The treatment of tumor cells with cycloheximide reduced the activity in alpha-MSH-stimulated CM. Our results suggest that alpha-MSH inhibited the motility of B16-BL6 cells through induction of autocrine factor(s).

Molecular basis for the interaction of [Nle4,D-Phe7]melanocyte stimulating hormone with the human melanocortin-1 receptor

The melanocortin-1 receptor (MC1R) is a seven-transmembrane (TM) G-protein-coupled receptor whose natural ligands are the melanocortin peptides, adrenocorticotropic hormone, and alpha-, beta-, and gamma- melanocyte stimulating hormone (MSH). To test a previously constructed three-dimensional model of the molecular interaction between the long-acting, superpotent alpha-MSH analog [Nle4,D-Phe7]MSH (NDP-MSH) and the human MC1R we examined the effects of site-directed receptor mutagenesis on the binding affinity and potency of NDP-MSH. In addition, we also examined the effects of these same mutations on the binding affinity and potency of the structurally related agonists alpha-MSH, gamma-MSH, and Ac-Nle4-cyclic-[Asp5,His6,D-Phe7,Arg8,Trp9,Lys10]NH2 (MT-II). Mutagenesis of acidic receptor residues Glu94 in TM2 and Asp117 or Asp121 in TM3 significantly altered the binding affinity and potency of all four agonists suggesting that these receptor residues are important to the ligand-receptor interactions of all. A disproportionate change in agonist potency versus affinity observed with simultaneous mutation of these acidic residues (mutant constructs D117A/D121A or E94A/D117A/D121A) or introduction of a single positive charge (mutant construct D121K) also implicates these residues in receptor activation. In addition, results from the individual mutation of aromatic receptor residues Phe175, Phe196, and Phe257, and simultaneous mutation of multiple TM4, -5, and -6 tyrosine and phenylalanine residues suggests that aromatic-aromatic ligand-receptor interactions also participate in binding these melanocortins to the MC1R. These experiments appear to have identified some of the critical receptor residues involved in the ligand-receptor interactions between these melanocortins and the hMC1R.

Phenotypic characterization and functional correlation of alpha-MSH binding to pituitary cells

It is clear that alpha-melanocyte-stimulating hormone (alpha-MSH), released by the hypophysial neurointermediate lobe, is a mediator of suckling-induced prolactin release, but several questions surrounding its role remain unresolved. Accordingly, the objectives of the present study were 1) to establish whether alpha-MSH could bind in a reversible manner to a specific secretory type cell within the adenohypophysis (AP), 2) to resolve the issue of whether the peptide could compete with dopamine for the same receptor binding site, and 3) to seek a functional signaling correlate for alpha-MSH binding. In pursuit of these objectives, we subjected pituitary cells from lactating rats to alpha-MSH receptor autoradiography, AP hormone immunocytochemistry, or digital imaging fluorescence microscopy with fura 2 as a calcium-sensitive probe. Our results show that alpha-MSH binding is restricted to mammotropes and that a specific subpopulation of these express functional alpha-MSH receptors that are coupled to a Ca2+ signaling pathway. Moreover, alpha-MSH does not compete with dopamine antagonists/agonists for the same binding site.

Truncation studies of alpha-melanotropin peptides identify tripeptide analogues exhibiting prolonged agonist bioactivity

Truncation studies of alpha-melanotropin peptides identify tripeptide analogues exhibiting prolonged agonist bioactivity: PEPTIDES 17(6) 995-1002, 1996.-Systematic analysis of fragment derivatives of the superpotent alpha-MSH analogue. Ac-Ser.Tyr-Ser-Nle4-Glu- His-DPhe7-Arg-Trp-Gly-Lys-Pro-Val-NH2(NDP-MSH), led to the discovery of tripeptide agonists possessing prolonged bioactivity in the frog skin assay. Of particular significance to this discovery was Ac-DPhe-Arg-DTrp-NH2, which was the most potent tripeptide in this series exhibiting sustained melanotropic activity. Different pharmacophore models appear to exist that are dependent on the substructure and stereochemistry of the MSH(6-9) "active site." The tripeptides Ac-DPhe-Arg-Trp-NH2, Ac-DPhe-Arg-DTrp-NH2, and Ac-DPhe-DArg-Trp-NH2 stereo-chemical combinations require only Phe7-Xaa8-Trp9, whereas Ac-DPhe-DArg-DTrp-NH2, Ac-Phe-Arg-DTrp-NH2, and Ac-Phe-Arg-Trp-NH2 additionally require His4 for minimal biological activity. Ac-DPhe-Arg-DTrp-NH2 represents a novel prototype lead for the development of MSH-based peptidomimetic agonists.