Cloned Melanocortin Receptors Research Articles

Single Nucleotide Polymorphisms in the Melanocortin His-Phe-Arg-Trp Sequences Decrease Tetrapeptide Potency and Efficacy.

The melanocortin receptors are stimulated by agonists (α-MSH, β-MSH, γ-MSH, and ACTH) processed from the proopiomelanocortin (POMC) gene transcript and possess a common His-Phe-Arg-Trp tetrapeptide sequence critical for receptor activation. Deficiency in POMC signaling in humans is associated with adrenal insufficiency, altered pigmentation, and rapid, early onset weight gain. Herein, 12 single nucleotide polymorphisms (SNPs) deposited into the Variation Viewer database within the His-Phe-Arg-Trp sequences of ACTH/α-MSH, β-MSH, and γ-MSH were substituted into tetrapeptide scaffolds to examine the in vitro signaling effects of these polymorphisms at the cloned melanocortin receptors. Every polymorphism decreased agonist potency and/or efficacy at the melanocortin receptors assayed, indicating that polymorphisms within the signaling sequence of POMC-derived agonists negatively impacts receptor activation. Future work to incorporate these substitutions into the full-length POMC agonists would confirm these findings, identifying new patient populations that might benefit from therapeutic regiments to treat POMC-deficient signaling.

Melanocortin receptors and erectile function.

Review the historical and current evidence that suggests that activation of melanocortin receptors modulates erectile activity. The available literature was reviewed. Melanocortin peptides derived from the pro-opiomelanocortin (POMC) precursor protein exert a host of diverse physiological effects in the periphery and in the CNS through interactions with one or more of the five cloned melanocortin receptors. Natural and synthetic melanocortin peptide agonists influence erectile and sexual function in a range of preclinical species. Emerging clinical evidence now suggests that the proerectile effects observed in preclinical species are evident in man as well. Preclinical and clinical results support the involvement of melanocortins in the modulation of erectile and sexual function. Current evidence indicates that the melanocortin 4 receptor subtype contributes to the proerectile effects observed with pan-receptor agonists. However, the putative receptor subtypes, pathways and mechanisms implicated in mediating the proerectile effects of melanocortins remain to be fully elucidated.

The Role of Melanocortin Peptides and Receptors in Regulation of Energy Balance

Energy balance is a highly regulated, complex process which is modulated by central and peripheral systems. Dysregulation of energy homeostasis can result in metabolic disorders, such as obesity and type II diabetes. Obesity and type II diabetes are two of the most prevalent and challenging clinical conditions in society today. A growing body of evidence has implicated the melanocortin system as an important component in the maintenance of energy balance. alpha-MSH, a 13 amino acid peptide secreted as a product of the pro-opiomelanocortin (POMC) gene in the pituitary is a potent agonist of 4 of the 5 cloned melanocortin receptors (MCR). MC receptors are members of a G-protein-coupled receptor (GPCR) family, which signal through cAMP. Agouti and agouti-related protein (AGRP) are natural antagonists of melanocortin receptors and participate in regulation of skin/fur pigmentation, body weight, and adiposity. Stimulation of MC receptors has pleiotropic effects, which impact the nervous system as well as endocrine and immune functions. One of the most prominent effects of MC receptor stimulation is a dramatic suppression of food intake and body weight, which has led to the hypothesis that the MC receptor system plays a primary role in the maintenance of energy balance. This idea is supported by a large body of pharmacological, molecular and human genetic evidence. The following review summarizes the role of melanocortin receptors in the regulation of food intake and energy homeostasis and highlights the opportunities for MC receptors as drug development targets in treating eating disorders and diabetes.

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.

Evidence that α -MSH induced grooming is not primarily mediated by any of the cloned melanocortin receptors

It is well established that melanocortic peptides, such as melanocyte-stimulating hormone (MSH) and adrenocorticotropin, induce grooming behavior. The MC3 and MC4 receptors are the MC receptors which are most abundantly expressed in the brain. γ-MSH, a peptide with preference to the MC3 receptor, however, does not induce grooming. Recent studies have shown that MC4 receptor antagonists are very effective in inhibiting α-MSH induced grooming. These data have indicated that grooming behavior in rodents may be mediated by the MC4 receptor. In this study we investigated if the recently developed MC1 receptor selective agonist MS05 was able to induce grooming in comparison with α-MSH. The results show that MS05 is effective in inducing grooming after either intracerebroventricular or ventral tegmental area administration in rats. Central administration of either MS05 or α-MSH besides grooming also induced stretching, yawning, rearing and locomotion. The results indicate that the earlier hypothesis that the MC4 receptor is the main mediator of grooming behavior has to be modified. Moreover, as this behaviour does not pharmacologically correlate to the profile of any of the five cloned MC receptors, we suggest that α-MSH induced grooming may not primarily be mediated by any of these receptors.

Characterization of Agouti-related protein binding to melanocortin receptors.

Agouti-related protein (AGRP) is a naturally occurring antagonist of melanocortin action that is thought to play an important role in the hypothalamic control of feeding behavior. The exact mechanism of AGRP and Agouti protein action has been difficult to examine, in part because of difficulties in producing homogeneous forms of these molecules that can be used for direct binding assays. In this report we describe the application of chemical protein synthesis to the construction of two novel AGRP variants. Examination of the biological activity of the AGRP variants demonstrates that a truncated variant, human AGRP(87-132), a 46-amino acid variant based on the carboxyl-terminal cysteine-rich domain of AGRP, is equipotent to an 111-amino acid variant, mouse [Leu127Pro]AGRP (mature AGRP minus its signal sequence), in its ability to dose dependently inhibit alpha-MSH-generated cAMP generation at the cloned melanocortin receptors. Furthermore, deletion of the amino-terminal portion of the full-length variant did not alter the MCR subtype specificity of AGRP(87-132). Finally, iodination of human AGRP(87-132) provided a useful reagent with which the binding properties of AGRP could be analyzed. In both conventional and photoemulsion binding studies [125I]AGRP(87-132) was observed only to bind to cells expressing melanocortin receptors MC3R, MC4R, and MC5R. These results demonstrate that the residues critical for receptor binding, alpha-MSH inhibition, and melanocortin receptor subtype specificity are all located in the carboxyl terminus of the molecule. Because [Nle4, D-Phe7] (NDP)-MSH displaces the binding of [125I]AGRP(87-132) to MCRs and AGRP(87-132) displaces the binding of [125I]NDP-MSH, we conclude that these molecules bind in a competitive fashion to melanocortin receptors.

Different cardiovascular profiles of three melanocortins in conscious rats; evidence for antagonism between gamma 2-MSH and ACTH-(1-24).

1. We investigated the effects of [Nle4,D-Phe7]alpha-melanocyte-stimulating hormone (NDP-MSH), adrenocorticotropin-(1-24) (ACTH-(1-24)) and gamma 2-MSH, three melanocortins with different agonist selectivity for the five cloned melanocortin receptors, on blood pressure and heart rate in conscious, freely moving rats following intravenous administration. 2. As was previously found by other investigators as well as by us gamma 2-MSH, a peptide suggested to be an agonist with selectivity for the melanocortin MC3 receptor, caused a dose-dependent, short lasting pressor response in combination with a tachycardia. Despite the fact that NDP-MSH is a potent agonist of various melanocortin receptor subtypes, among which the melanocortin MC1 receptor, it did not affect blood pressure or heart rate, when administered i.v. in doses of up to 1000 nmol kg-1. 3. ACTH-(1-24) caused a dose-dependent decrease in blood pressure in combination with a dose-dependent increase in heart rate in a dose-range from 15 to 500 nmol kg-1. The cardiovascular effects of ACTH-(1-24) were independent of the presence of the adrenals. 4. Pretreatment with ACTH-(1-24) caused a pronounced, dose-dependent parallel shift to the right of the dose-response curve for the pressor and tachycardiac effects of gamma 2-MSH. The antagonistic effect of ACTH-(1-24) was already apparent following a dose of this peptide as low as 10 nmol kg-1, which when given alone had no intrinsic hypotensive activity. 5. These results form further support for the notion that it is not via activation of one of the as yet cloned melanocortin receptors that gamma-MSH-like peptides increase blood pressure and heart rate. The cardiovascular effects of ACTH-(1-24) seem not to be mediated by the adrenal melanocortin MC3 receptors, for which ACTH-(1-24) is a selective agonist, or by adrenal catecholamines. 6. There appears to be a functional antagonism between ACTH-(1-24) and gamma 2-MSH, two melanocortins derived from a common precursor, with respect to their effect on blood pressure and heart rate. Whether this antagonism plays a (patho)physiological role remains to be shown.