Chimeric melanocortin MC1 and MC3 receptors: identification of domains participating in binding of melanocyte-stimulating hormone peptides

Radiochemical and radiopharmacological characterization of a 64 Cu-labeled α-MSH analog conjugated with different chelators

Radiolabeled α-melanocyte-stimulating hormone (α-MSH) derivatives have a high potential for diagnosis and treatment of melanoma, because of high specificity and binding affinity to the melanocortin-1 receptor (MC1R). Hence, the α-MSH-derived peptide NAP-NS1 with a β-Ala linker (ε-Ahx-β-Ala-Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH₂ ) was conjugated to different chelators: either to NOTA (p-SCN-Bn-1,4,7-triazacyclononane-1,4,7-triacetic acid), to a hexadentate bispidine carbonate derivative (dimethyl-9-(((4-nitrophenoxy)carbonyl)oxy)-2,4-di(pyridin-2-yl)-3,7-bis(pyridin-2-ylmethyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylate), or to DMPTACN (p-SCN-Ph-bis(2-pyridyl-methyl)-1,4,7-triaza-cyclononane), labeled with ⁶⁴ Cu, and investigated in terms of radiochemical and radiopharmacological properties. For the three ⁶⁴ Cu-labeled conjugates negligible transchelation, suitable buffer and serum stability, as well as appropriate water solubility, was determined. The three conjugates exhibited high binding affinity (low nanomolar range) in murine B16F10, human MeWo, and human TXM13 cells. The B_max values of [⁶⁴ Cu]Cu-bispidine-NAP-NS1 ([⁶⁴ Cu]Cu-2) and [⁶⁴ Cu]Cu-DMPTACN-NAP-NS1 ([⁶⁴ Cu]Cu-3) were higher than those of [⁶⁴ Cu]Cu-NOTA-NAP-NS1 ([⁶⁴ Cu]Cu-1), implying that different charged chelate units might have an impact on binding capacity. Preliminary in vivo biodistribution studies suggested the main excretion pathway of [⁶⁴ Cu]Cu-1 and [⁶⁴ Cu]Cu-3 to be renal, while that of [⁶⁴ Cu]Cu-2 seemed to be both renal and hepatobiliary. An initial moderate uptake in the kidney decreased clearly after 60 minutes. All three ⁶⁴ Cu-labeled conjugates should be considered for further in vivo investigations using a suitable xenograft mouse model.

Synthesis, Characterization, and Initial Biological Evaluation of [99m Tc]Tc-Tricarbonyl-labeled DPA-α-MSH Peptide Derivatives for Potential Melanoma Imaging

α-Melanocyte stimulating hormone (α-MSH) derivatives target the melanocortin-1 receptor (MC1R) specifically and selectively. In this study, the α-MSH-derived peptide NAP-NS1 (Nle-Asp-His-d-Phe-Arg-Trp-Gly-NH₂ ) with and without linkers was conjugated with 5-(bis(pyridin-2-ylmethyl)amino)pentanoic acid (DPA-COOH) and labeled with [^99m Tc]Tc-tricarbonyl by two methods. With the one-pot method the labeling was faster than with the two-pot method, while obtaining similarly high yields. Negligible trans-chelation and high stability in physiological solutions was determined for the [^99m Tc]Tc-tricarbonyl-peptide conjugates. Coupling an ethylene glycol (EG)-based linker increased the hydrophilicity. The peptide derivatives displayed high binding affinity in murine B16F10 melanoma cells as well as in human MeWo and TXM13 melanoma cell homogenates. Preliminary in vivo studies with one of the [^99m Tc]Tc-tricarbonyl-peptide conjugates showed good stability in blood and both renal and hepatobiliary excretion. Biodistribution was performed on healthy rats to gain initial insight into the potential relevance of the ^99m Tc-labeled peptides for in vivo imaging.

ACTH Receptor (MC2R) Specificity: What Do We Know About Underlying Molecular Mechanisms?

Coincidentally, the release of this Research Topic in Frontiers in Endocrinology takes place 25 years after the discovery of the adrenocorticotropic hormone receptor (ACTHR) by Mountjoy and colleagues. In subsequent years, following the discovery of other types of mammalian melanocortin receptors (MCRs), ACTHR also became known as melanocortin type 2 receptor (MC2R). At present, five types of MCRs have been reported, all of which share significant sequence similarity at the amino acid level, and all of which specifically bind melanocortins (MCs)-a group of biologically active peptides generated by proteolysis of the proopiomelanocortin precursor. All MCs share an identical -H-F-R-W- pharmacophore sequence. α-Melanocyte-stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH) are the most extensively studied MCs and are derived from the same region. Essentially, α-MSH is formed from the first 13 amino acid residues of ACTH. ACTHR is unique among MCRs because it binds one sole ligand-ACTH, which makes it a very attractive research object for molecular pharmacologists. However, much research has failed, and functional studies of this receptor are lagging behind other MCRs. The reason for these difficulties has already been outlined by Mountjoy and colleagues in their publication on ACTHR coding sequence discovery where the Cloudman S91 melanoma cell line was used for receptor expression because it was a "more sensitive assay system." Subsequent work showed that ACTHR could be successfully expressed only in endogenous MCR-expressing cell lines, since in other cell lines it is retained within the endoplasmic reticulum. The resolution of this methodological problem came in 2005 with the discovery of melanocortin receptor accessory protein, which is required for the formation of functionally active ACTHR. The decade that followed this discovery was filled with exciting research that provided insight into the molecular mechanisms underlying the action of ACTHR. The purpose of this review is to summarize the advances in this fascinating research field.

Radiopharmacological characterization of ⁶⁴Cu-labeled α-MSH analogs for potential use in imaging of malignant melanoma

The melanocortin-1 receptor (MC1R) plays an important role in melanoma growth, angiogenesis and metastasis, and is overexpressed in melanoma cells. α-Melanocyte stimulating hormone (α-MSH) and derivatives are known to bind with high affinity at this receptor that provides the potential for selective targeting of melanoma. In this study, one linear α-MSH-derived peptide Nle-Asp-His-D-Phe-Arg-Trp-Gly-NH2 (NAP-NS1) without linker and with εAhx-β-Ala linker, and a cyclic α-MSH derivative, [Lys-Glu-His-D-Phe-Arg-Trp-Glu]-Arg-Pro-Val-NH2 (NAP-NS2) with εAhx-β-Ala linker were conjugated with p-SCN-Bn-NOTA and labeled with (64)Cu. Radiochemical and radiopharmacological investigations were performed with regard to transchelation, stability, lipophilicity and in vitro binding assays as well as biodistribution in healthy rats. No transchelation reactions, but high metabolic stability and water solubility were demonstrated. The linear derivatives showed higher affinity than the cyclic one. [(64)Cu]Cu-NOTA-εAhx-β-Ala-NAP-NS1 ([(64)Cu]Cu-2) displayed rapid cellular association and dissociation in murine B16F10 cell homogenate. All [(64)Cu]Cu-labeled conjugates exhibited affinities in the low nanomolar range in B16F10. [(64)Cu]Cu-2 showed also high affinity in human MeWo and TXM13 cell homogenate. In vivo studies suggested that [(64)Cu]Cu-2 was stable, with about 85 % of intact peptide in rat plasma at 2 h p.i. Biodistribution confirmed the renal pathway as the major elimination route. The uptake of [(64)Cu]Cu-2 in the kidney was 5.9 % ID/g at 5 min p.i. and decreased to 2.0 % ID/g at 60 min p.i. Due to the prospective radiochemical and radiopharmacological properties of the linear α-MSH derivative [(64)Cu]Cu-2, this conjugate is a promising candidate for tracer development in human melanoma imaging.

Selected melanocortin 1 receptor single-nucleotide polymorphisms differentially alter multiple signaling pathways

The melanocortin 1 receptor (MC1R) is a highly polymorphic G protein-coupled receptor, which is known to modulate pigmentation and inflammation. In the current study, we investigated the pharmacological effects of select single-nucleotide polymorphisms (SNPs) (V60L, R163Q, and F196L). After transient expression of MC1Rs in human embryonic kidney 293 cells, basal and ligand-induced cAMP signaling and mitogen-activated protein kinase (MAPK) activation were assessed by using luciferase reporter gene assays and Western blot analysis, respectively. All receptor variants showed decreased basal cAMP activity. With the V60L and F196L variants, the decrease in constitutive activity was attributable, at least in part, to a reduction in surface expression. The F196L variant also displayed a significant reduction in potency for both the peptide agonist α-melanocyte-stimulating hormone (α-MSH) and the small-molecule agonist 1-[1-(3-methyl-L-histidyl-O-methyl-D-tyrosyl)-4-phenyl-4-piperidinyl]-1-butanone (BMS-470539). In MAPK signaling assays, the F196L variant showed decreased phospho-extracellular signal-regulated kinase levels after stimulation with either α-MSH or BMS-470539. In contrast, the R163Q variant displayed a selective loss of α-MSH-induced MAPK activation; whereas responsiveness to the small-molecule agonist BMS-470539 was preserved. Further assessment of MC1R variants in A549 cells, an in vitro model of inflammation, revealed an enhanced inflammatory response resulting from expression of the F196L variant (versus the wild-type MC1R). This alteration in function was restored by treatment with BMS-470539. Overall, these studies illustrate novel signaling profiles linked to distinct MC1R SNPs. Furthermore, our investigations highlight the potential for small-molecule drugs to rescue the function of MC1R variants that show reduced basal and/or α-MSH stimulated activity.

Melanocortin 4 receptor polymorphism is associated with carcass fat in beef cattle

McLean, K. L. and Schmutz, S. M. 2011. Melanocortin 4 receptor polymorphism is associated with carcass fat in beef cattle. Can. J. Anim. Sci. 91: 75–79. Melanocortin 4 receptor (MC4R) binds α-melanocyte stimulating hormone (α-MSH) reducing feed and energy intake in several species of animals. One variant in swine has been reported to increase daily gain, backfat deposition and feed intake. MC4R sequence was obtained from 20 random crossbred steers where a novel Ser330Asn polymorphism was detected. Three hundred and eighty-two crossbred Canadian steers and 985 crossbred American steers were genotyped for this polymorphism. The Ser330Asn polymorphism had a minor allele frequency of 0.01 in the Canadian and 0.02 in the American steer populations. The Canadian steers with the heterozygous genotype had increased grade fat (P=0.036) and decreased lean meat yield (P=0.032). The American heterozygous steers had increased backfat (P=0.031) and less desirable yield grades (P=0.022), but also lower longissimus dorsi measurements (P=0.031). The association of the Asn330 allele was validated in two typical crossbred steer populations in two countries, suggesting it has effects of commercial significance.

Identification of three novel genetic variants in the melanocortin-3 receptor of obese children

The melanocortin-3 receptor (MC3R), a G-protein-coupled receptor expressed in the hypothalamus, is a key component of the leptin-melanocortin pathway that regulates energy homeostasis. It is suggested that an MC3R defect leads to an increased feed efficiency, by which nutrients are partitioned preferentially into fat. In this study, we hypothesized that early-onset obesity could be induced by mutations in MC3R. To investigate this hypothesis, we screened the entire coding region of the MC3R gene for mutations in obese subjects. A total of 404 overweight and obese children and adolescents, 86 severely obese adults (BMI ≥40 kg/m²), and 150 normal-weight control adults were included. Besides three synonymous coding variations in the MC3R gene (S69S, L95L, I226I), we were able to identify three novel heterozygous, nonsynonymous, coding mutations (N128S, V211I, L299V) in three unrelated obese children. None of these mutations were found in any of the control subjects. Functional studies assessing localization and signaling properties of the mutant receptors provided proof for impaired function of the L299V mutated receptor, whereas no conclusive evidence for functional impairment of the N128S and V211I mutated receptors could be established. First, these results provide supporting evidence for a role of the MC3R gene in the pathogenesis of obesity in a small subset of patients. Second, they show that caution is called for the interpretation of newly discovered mutations in MC3R.

Identification of domains responsible for specific membrane transport and ligand specificity of the ACTH receptor (MC2R)

The adrenocorticotropic hormone (ACTH) receptor has highly specific membrane expression that is limited to adrenal cells; in other cell types the polypeptide fails to be transported to the cell surface. Unlike other evolutionarily related members of the melanocortin receptor family (MC1R-MC5R) that recognize different melanocortin peptides, ACTHR (MC2R) binds only ACTH. We used a mutagenesis approach involving systematic construction of chimeric ACTHR/MC4R receptors to identify the domains determining the selectivity of ACTHR membrane transport and ACTH binding. In total 15 chimeric receptors were created by replacement of selected domains of human ACTHR with the corresponding regions of human MC4R. We developed an analytical method to accurately quantify cell-membrane localization of recombinant receptors fused with enhanced green fluorescent protein by confocal fluorescence microscopy. The chimeric receptors were also tested for their ability to bind ACTH (1-24) and the melanocyte-stimulating hormone (MSH) analog, Nle4, DPhe7-alpha-MSH, and to induce a cAMP response. Our results indicate that substitution of the MC4R N-terminal segment with the homologous segment of ACTHR significantly decreased membrane transport. We also identified another signal localized in the third and fourth transmembrane regions as the main determinant of ACTHR intracellular retention. In addition, we found that the fourth and fifth transmembrane domains of the ACTHR are involved in ACTH binding selectivity. We discuss the mechanisms involved in bypassing these arrest signals via an interaction with melanocortin 2 receptor accessory protein (MRAP) and the possible mechanisms that determine the high ligand-binding specificity of ACTHR.

The melanocortin-1 receptor gene polymorphism and association with human skin cancer

The melanocortin-1 receptor (MC1R) is a key gene involved in the regulation of melanin synthesis and encodes a G-protein coupled receptor expressed on the surface of the melanocyte in the skin and hair follicles. MC1R activation after ultraviolet radiation exposure results in the production of the dark eumelanin pigment and the tanning process in humans, providing physical protection against DNA damage. The MC1R gene is highly polymorphic in Caucasian populations with a number of MC1R variant alleles associated with red hair, fair skin, freckling, poor tanning, and increased risk of melanoma and nonmelanoma skin cancer. Variant receptors have shown alterations in biochemical function, largely due to intracellular retention or impaired G-protein coupling, but retain some signaling ability. The association of MC1R variant alleles with skin cancer risk remains after correction for pigmentation phenotype, indicating regulation of nonpigmentary pathways. Notably, MC1R activation has been linked to DNA repair and may also contribute to the regulation of immune responses.

Rough set-based proteochemometrics modeling of G-protein-coupled receptor-ligand interactions

G-Protein-coupled receptors (GPCRs) are among the most important drug targets. Because of a shortage of 3D crystal structures, most of the drug design for GPCRs has been ligand-based. We propose a novel, rough set-based proteochemometric approach to the study of receptor and ligand recognition. The approach is validated on three datasets containing GPCRs. In proteochemometrics, properties of receptors and ligands are used in conjunction and modeled to predict binding affinity. The rough set (RS) rule-based models presented herein consist of minimal decision rules that associate properties of receptors and ligands with high or low binding affinity. The information provided by the rules is then used to develop a mechanistic interpretation of interactions between the ligands and receptors included in the datasets. The first two datasets contained descriptors of melanocortin receptors and peptide ligands. The third set contained descriptors of adrenergic receptors and ligands. All the rule models induced from these datasets have a high predictive quality. An example of a decision rule is "If R1_ligand(Ethyl) and TM helix 2 position 27(Methionine) then Binding(High)." The easily interpretable rule sets are able to identify determinative receptor and ligand parts. For instance, all three models suggest that transmembrane helix 2 is determinative for high and low binding affinity. RS models show that it is possible to use rule-based models to predict ligand-binding affinities. The models may be used to gain a deeper biological understanding of the combinatorial nature of receptor-ligand interactions.

Worldwide polymorphism at the MC1R locus and normal pigmentation variation in humans

While there have been many advances in our understanding of the genetics of pathological skin pigmentation in humans, our knowledge about what determines variation in normal skin color is still incomplete. Variation in one gene, melanocortin 1 receptor (MC1R), has been associated with red hair and fair skin in Europeans. However, this gene might also play an important role in shaping pigmentation of other human populations, where it experiences different selective pressures. Below we review what is currently known about polymorphism and selection at the MC1R coding and promoter regions in human populations, the pattern of MC1R evolution in nonhuman primates, and the interaction of MC1R with other genes.

Evolutionary conservation of the structural, pharmacological, and genomic characteristics of the melanocortin receptor subtypes

We have cloned melanocortin receptors (MCRs) from several species of fish. The MC4R and MC5R subtypes arose early in vertebrate evolution and their primary structure is remarkably conserved. Expression and pharmacological characterization of the MCRs in fish has revealed that they bind and respond to melanocortin peptides with high potency. Detailed characterization of the binding properties of the different subtypes suggests that MCRs in early vertebrates had preference for adrenocorticotropic hormone (ACTH) peptides, while the high sensitivity for the shorter proopiomelanocortin (POMC) products, such as the alpha-, beta-, and gamma-melanocyte-stimulating hormone (MSH), has appeared later, perhaps as the MCR subtypes gained more specialized functions. The MCR repertoire shows in general high similarities in their primary structures, while they are however not similar in terms of functional roles. The MCRs serve therefore as an interesting model family to understand the molecular mechanisms of how functions of the genes can diverge during evolution. In this review, we provide an overview of our recent studies on the cloning, expression, pharmacology, 3D modeling, and genomic studies of the MCRs in non-mammalian species.

Co-occurrence of two partially inactivating polymorphisms of MC3R is associated with pediatric-onset obesity

Both human linkage studies and MC3R knockout mouse models suggest that the MC3R may play an important role in energy homeostasis. Here we show that among 355 overweight and nonoverweight children, 8.2% were double homozygous for a pair of missense MC3R sequence variants (Thr6Lys and Val81Ile). Such children were significantly heavier (BMI and BMI SD score: P < 0.0001), had more body fat (body fat mass and percentage fat mass: P < 0.001), and had greater plasma leptin (P < 0.0001) and insulin concentrations (P < 0.001) and greater insulin resistance (P < 0.008) than wild-type or heterozygous children. Both sequence variants were more common in African-American than Caucasian children. In vitro expression studies found the double mutant MC3R was partially inactive, with significantly fewer receptor binding sites, decreased signal transduction, and less protein expression. We conclude that diminished MC3R expression in this double MC3R variant may be a predisposing factor for excessive body weight gain in children.

Analogs of sub-nanomolar hMC1R agonist LK-184 [Ph(CH2)3CO-His-d-Phe-Arg-Trp-NH2]. An additional binding site within the human melanocortin receptor 1?

Twenty nine analogs of a superpotent MC1R agonist LK-184 (1) were tested at human melanocortin receptors (hMC1, hMC3, and hMC4Rs). All derivatives with the spacer between the N-terminus and the aromatic ring longer or shorter than C(3) were much less potent at hMC1R than 1. Only LK-312 PhCO(CH(2))(3)CO-His-d-Phe-Arg-Trp-NH(2) (3), partially mimicking the pi-system of 1, had an EC(50) of 0.05 nM at hMC1R, which confirms the localization of the pi-binding zone of the receptor. Truncation of 1 to Ph(CH(2))(3)CO-His-d-Phe-Arg-NH(2) gave a full MC1 agonist, LK-394 (30), with an EC(50) of 5 nM and a weak partial agonism at MC3/4Rs. This suggests the existence of an additional binding site within hMC1R next to that for the core sequence His-d-Phe-Arg-Trp-NH(2).

Stable expression of human melanocortin 3 receptor fused to EGFP in the HEK293 cells

Among the melanocortins alpha-MSH is known to be involved in feeding behavior. These hormones mediate their effects through G protein-coupled receptors by stimulating adenylate cyclase. In this study, we have developed an in vitro expression model for human melanocortin 3 receptor (hMC3R) tagged at its C terminus with EGFP. The corresponding chimeric cDNA was stably expressed in HEK293 cells. The selected clones expressing the hMC3R-EGFP exhibited cell surface fluorescence and responded to NDP-MSH stimulation by producing cAMP in a dose-dependent manner (EC(50): 0.3 nM). Binding studies revealed a single class of binding sites with a K(D) of 2.24 nM. Moreover, Agouti-related protein was also demonstrated to be an antagonist of the hMC3R-EGFP. Thus, the hMC3R tagged with EGFP stably expressed in HEK293 cells, exhibiting the same characteristics than the wild-type hMC3R, is the only model of expression of this receptor allowing its direct localization inside living cells.

Association of feather colour with constitutively active melanocortin 1 receptors in chicken

Seven alleles of the chicken melanocortin (MC) 1 receptor were cloned into expression vectors, expressed in mammalian cells and pharmacologically characterized. Four of the clones e(+R), e(+B&D), e(wh)/e(y), E(Rfayoumi) gave receptors to which melanocortin stimulating hormone (alpha-MSH) and NDP-MSH bound with similar IC50 values and responded to alpha-MSH by increasing intracellular cAMP levels in a dose-dependent manner. Three of the cMC1 receptors; e(b), E and E(R), did not show any specific binding to the radioligand, but were found to be constitutively active in the cAMP assay. The E and E(R) alleles are associated with black feather colour in chicken while the eb allele gives rise to brownish pigmentation. The three constitutively active receptors share a mutation of Glu to Lys in position 92. This mutation was previously found in darkly pigmented sombre mice, but constitutively active MC receptors have not previously been shown in any nonmammalian species. We also inserted the Glu to Lys mutation in the human MC1 and MC4 receptors. In contrast with the chicken clones, the hMC1-E94K receptor bound to the ligand, but was still constitutively active independently of ligand concentration. The hMC4-E100K receptor did not bind to the MSH ligand and was not constitutively active. The results indicate that the structural requirements that allow the receptor to adapt an active conformation without binding to a ligand, as a consequence of this E/K mutation, are not conserved within the MC receptors. The results are discussed in relationship to feather colour in chicken, molecular receptor structures and evolution. We suggest that properties for the 'E92K switch' mechanism may have evolved in an ancestor common to chicken and mammals and were maintained over long time periods through evolutionary pressure, probably on closely linked structural features.

Presence of melanocortin (MC4) receptor in spiny dogfish suggests an ancient vertebrate origin of central melanocortin system

We report the cloning, expression, pharmacological characterization and tissue distribution of a melanocortin (MC) receptor gene in a shark, the spiny dogfish (Squalus acanthias) (Sac). Phylogenetic analysis showed that this receptor is an ortholog of the MC4 subtype, sharing 71% overall amino acid identity with the human (Hsa) MC4 receptor. When expressed and characterized by radioligand binding assay for the natural MSH (melanocyte-stimulating hormone) peptides alpha-, beta-, and gamma-MSH, the SacMC4 receptor showed pharmacological properties very similar to the HsaMC4 receptor. Stimulation of SacMC4 receptor transfected cells with alpha-MSH caused a dose-dependent increase in intracellular cAMP levels. The SacMC4 receptor has Ala in position 59 where all other cloned MC receptors have Glu. We confirmed that this was not due to individual polymorphism and subsequently mutated the residue 'back' to Glu but the mutation did not affect the pharmacological properties of the receptor. SacMC4 receptor mRNA was detected by RT-PCR in the optic tectum, hypothalamus, brain stem, telencephalon and olfactory bulb but not in cerebellum or in peripheral tissues. This study describes the first characterization of an MC receptor in a cartilaginous fish, the most distant MC receptor gene cloned to date. Conservation of gene structure, pharmacological properties and tissue distribution suggests that this receptor may have similar roles in sharks as in mammals and that these were established more than 450 million years ago.

Melanocortin-3 receptor gene variants in a Maori kindred with obesity and early onset type 2 diabetes

Genetic studies suggest a diabetes susceptibility locus on human chromosome 20, near the melanocortin receptor-3 (MC3-R) gene. We examined the MC3-R as a candidate gene for type 2 diabetes in 12 members of a large Maori kindred with multiple affected members. The coding region of the MC3-R gene was sequenced for both diabetic and non-diabetic individuals. Two separate single base pair substitutions were found in the MC3-R coding sequence and these resulted in amino acid changes, Lysine6Threonine and Isoleucine81Valine. Neither of these MC3-R variants tracked with the presence of diabetes. Furthermore, the variant and wild type MC3-R showed similar functional coupling to adenylyl cyclase. A polymorphic marker (D20S32e) close to the human MC3-R (hMC3-R) coding sequence was investigated in a 60-member pedigree for association with diabetes and other metabolic parameters. There was an association between D20S32e genotype and fasting insulin (P=0.0085) and the insulin resistance index, HOMA-R (P=0.0042). An association was also found between genotype and HDL levels during oral glucose tolerance testing (P=0.0037). These associations were independent of BMI, age, gender and diabetes. Our data do not support a role for variations in the coding region of the hMC3-R in the development of type 2 diabetes in this Maori kindred, but suggest that a locus on chromosome 20 q, close to D20S32e, may contribute to both insulin secretion and action in the Maori kindred.

One melanocortin 4 and two melanocortin 5 receptors from zebrafish show remarkable conservation in structure and pharmacology

We report the cloning, genome mapping, functional expression, pharmacology and anatomical distribution of three melanocortin (MC) receptors from zebrafish (z). Phylogenetic analysis showed with high bootstrap support that these genes represent one MC4 receptor and two MC5 receptors. Chromosomal mapping showed conserved synteny between regions containing zMC4 and human (h) MC4 receptors, whereas the two zMC5 receptor genes map on chromosome segments in which the zebrafish has several genes with two orthologues of a single mammalian gene. It is likely that the two copies of zMC5 receptors arose through a separate duplication in the teleost lineage. The zMC4, zMC5a, and zMC5b receptors share 70-71% overall amino acid identity with the respective human orthologues and over 90% in three TM regions believed to be most important for ligand binding. All three zebrafish receptors also show pharmacological properties remarkably similar to their human orthologues, with similar affinities and the same potency order, when expressed and characterized in radioligand binding assay for the natural MSH) peptides alpha-, beta-, and gamma-MSH. Stimulation of transfected mammalian cells with alpha-MSH caused a dose-dependent increase in intracellular cAMP levels for all three zebrafish receptors. All three genes were expressed in the brain, eye, ovaries and gastrointestinal tract, whereas the zMC5b receptor was also found in the heart, as determined by RT-PCR. Our studies, which represent the first characterization of MC receptors in a nonamniote species, indicate that the MC receptor subtypes arose very early in vertebrate evolution. Important pharmacological and functional properties, as well as gene structure and syntenic relationships have been highly conserved over a period of more than 400 million years implying that these receptors participate in vital physiological functions.

Expression, candidate gene, and population studies of the melanocortin 5 receptor

In mouse the melanocortin 5 receptor is known to regulate sebaceous gland function. To clarify its role in man, we have studied melanocortin 5 receptor expression in skin, and allelic variation at the melanocortin 5 receptor locus in diverse human populations and candidate disease groups. Melanocortin 5 receptor protein and mRNA expression were studied by immunohistochemistry and reverse transcriptase polymerase chain reaction. Melanocortin 5 receptor mRNA was detected in normal skin and cultured keratinocytes but not in cultured fibroblasts or melanocytes. Immunohistochemistry revealed melanocortin 5 receptor immunoreactivity in the epithelium and appendages, including the sebaceous gland, eccrine glands, and apocrine glands, as well as low level expression in the interfollciular epidermis. In order to screen for genetic diversity in the melanocortin 5 receptor that might be useful for allelic association studies we sequenced the entire melanocortin 5 receptor coding region in a range of human populations. One nonsynonymous change (Phe209Leu) and four synonymous changes (Ala81Ala, Asp108Asp, Ser125Ser, and Thr248Thr) were identified. Similar results were found in each of the populations except for the Inuit in which only the Asp108Asp variant was seen. The apparent "global distribution" of melanocortin 5 receptor variants may indicate that they are old in evolutionary terms. Variation of melanocortin 5 receptor was examined in patients with acne (n = 21), hidradenitis supprativa (n = 4), and sebaceous gland lesions comprising sebaceous nevi, adenomas, and hyperplasia (n = 13). No additional mutations were found. In order to determine the functional status of the Phe209Leu change, increase in cAMP in response to stimulation with alpha-melanocyte-stimulating hormone was measured in HEK-293 cells transfected with either wild-type or the Phe209Leu variant. The variant melanocortin 5 receptor was shown to act in a concentration-dependent manner, which did not differ from that of wild type. We have therefore found no evidence of a causative role for melanocortin 5 receptor in sebaceous gland dysfunction, and in the absence of any association between variation at the locus and disease group, the pathophysiologic role of the melanocortin 5 receptor in man requires further study.

Detection of regions in the MC1 receptor of importance for the selectivity of the MC1 receptor super-selective MS04/MS05 peptides

We have investigated the ability of our earlier identified MS04-MS05 MSH-peptide analogues to bind to chimeric MC1-MC3 receptors. While the MS04 and MS05 peptides bind with nanomolar and sub-nanomolar affinities to the wild type MC1 receptor, they bind only with micromolar affinities for the wild type MC3 receptor, thus being the hitherto most MC1 receptor selective ligands. Upon exchanging portions involving transmembrane regions TM1, TM2-3, and TM6-7 of the MC1 receptor with corresponding portions of the MC3 receptor both of these peptides showed major losses of affinities. By contrast exchanges involving TM4-5 did not appreciably affect the affinity of either MS04 or MS05. Our data suggest that the binding pocket for the MS04-MS05 MSH-peptides is located between TM1-3 and TM6-7 of the melanocortin receptors.

PLS modeling of chimeric MS04/MSH-peptide and MC1/MC3-receptor interactions reveals a novel method for the analysis of ligand-receptor interactions

A novel method has been developed for the analysis of ligand-receptor interactions. The method utilizes binding data generated from the analysis of chimeric proteins with chimeric peptides. To each chimeric part of the peptide and receptor are assigned descriptors, thus creating a matrix of X descriptors. These descriptors are then correlated with the experimentally determined interaction binding affinities for each chimeric receptor/peptide pair by use of partial least-squares projection to latent structures (PLS). The method was applied to analyze the interactions of chimeric MSH-peptides with wild-type MC1 and MC3 receptors, and MC1/MC3 receptor chimeras (in total 40 peptide-receptor combinations). Two types of PLS models could be created, one that revealed the relationships between receptor and peptide structure and peptide binding pK(i) values (i.e., affinity) (R2 and Q2 being 0.71 and 0.62, respectively), and another that revealed the relationships between peptide and receptor structure and peptide-receptor selectivity (R2 and Q2 being 0.64 and 0.57, respectively). After addition of cross-terms these models improved significantly; the R2 and Q2 being 0.93 and 0.75 for affinity, and 0.92 and 0.72 for selectivity, respectively. The analysis shows that the high affinity of the MSH-peptides is primarily achieved by interactions of the peptides' C-terminal amino acids with TM2 and TM3 of the receptor, and, to a lesser extent, by the interaction of the N-terminus with TM1, TM2 and TM3 of the receptor. However, in contrast, the MC1 receptor selectivity is primarily determined by an interaction of the peptides' N-termini with TM2/3 of the receptor. Moreover, the cross-terms of the PLS model revealed the existence of a strong interaction between TM6/7 and TM2/3 of the receptors.

Loss of function mutations of the human melanocortin 1 receptor are common and are associated with red hair

The melanocortin 1 receptor is a G-protein-coupled receptor that acts as a control point for control of the eumelanin/phaeomelanin ratio in mouse hair. MC1 receptor loss of function mutations lead to an increase in the ratio of phaeomelanin/eumelanin in many mammals resulting in yellow or red coat colours. We have previously shown that several common point mutations in the human MC1 receptor are overrepresented in North European redheads and in individuals with pale skin. In order to determine the functional significance of these changes we have carried out transfection and binding studies. Expression of the Val60Leu, Arg142His, Arg151Cys, Arg160Trp, and Asp294His receptors in COS 1 cells revealed that these receptors were unable to stimulate cAMP production as strongly as the wild type receptor in response to alpha-melanocyte-stimulating hormone stimulation. None of the mutant receptors displayed complete loss of alphaMSH binding, with only the Arg142His and Asp294His displaying a slight reduction in binding affinity.

Thyrotropin releasing hormone (TRH) selectively binds and activates the melanocortin 1 receptor

We tested the endogenous tripeptide TRH (thyrotropin releasing hormone) ability to bind to MC (melanocortin) receptor subtypes. We discovered that TRH binds to the human MCI receptor expressed in COS cells and to murine B16 melanoma cells with 5790+/-1010 nM and 6370+/-1260 nM Ki's, respectively. TRH did not bind to the human MC3, MC4 or MC5 receptor subtypes. Moreover, TRH also stimulated cAMP production in murine B16 melanoma cells reaching the same maximum level of cAMP as found for alpha-MSH. However, several analogues of TRH, including TRH-OH, TRH-Gly-NH2 and other analogues, where each of the three amino acid residues in TRH had been exchanged by a related residue, did not bind to any of the MC receptors tested in this study. C(alpha) atoms of molecular models of TRH and the core of a MSH peptide were aligned with r.m.s. of 0.01 A. Moreover, TRH could be docked into a binding pocket of a molecular model of the MC1 receptor at only a little higher energy than a short cyclic MSH peptide. The data indicate a similarity in the mode of TRH and MSH activation of the MCI receptor.