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Perspective

New Insights into G-Protein-Coupled Receptor Signaling from the Melanocortin Receptor System

Departments of Psychiatry and Pharmacology, University of California, San Diego, La Jolla, California

Received April 15, 2003; accepted April 15, 2003

For decades, geneticists, as well as breeders of "fancy" pets, have been interested in the interaction of the melanocortin 1 receptor locus (Mc1r; also known as melanocyte-stimulating hormone receptor, Mshr) with the Agouti locus because of the array of coat colors that alterations at these loci generate. In the simplest case, a mouse with two wild-type Mc1r alleles and two recessive Agouti alleles (Fig. 1b, Mc1r⁺/Mc1r⁺, a/a) is expectedly darker than the wild-type mouse (Fig. 1a, Mc1r⁺/Mc1r⁺, A/A) of "agouti" coloring, and a mouse with a pair of recessive alleles at both the Mc1r and Agouti loci is yellow (Fig. 1c, Mc1r^-/Mc1r^-, a/a). Geneticists have taken advantage of this unique system in model organisms to serve as readily visible markers for such experiments as gene targeting (Simpson et al., 1997). However, it is a century-old enigmatic observation that a mouse with wild-type Mc1r alleles and one of several dominant Agouti alleles (Fig. 1d, Mc1r⁺/Mc1r⁺, A^Y/a) is not only yellow, but obese. Furthermore, a pair of recessive alleles at yet another locus, mahogany (also known as Attractin, Atrn), ablates the affects of the dominant Agouti allele (Fig. 1e). These observations have led more recently to a series of investigations identifying new receptors and mediators of the melanocortin receptor (MCR) pathway making this signal transduction pathway an important model in the study of G-protein coupled receptor (GPCR) pathways in complex disease and for pharmacological insights as well.

View larger version (17K): [in this window] [in a new window]   Fig. 1. Phenotype of (a) wild-type "agouti" mouse with a yellow subapical band pattern in hair caused by transient expression of Agouti during hair development; (b) mouse with genotype consisting of two wild-type Mc1r alleles and two recessive Agouti alleles (Mc1r+/Mc1r+, a/a) producing hair without subapical yellow band; (c) mouse with a pair of recessive alleles at both the Mc1r and agouti loci (Mc1r-/Mc1r-, a/a) is yellow; (d) mouse with genotype consisting of two wild-type Mc1r alleles and one dominant Agouti allele (Mc1r+/Mc1r+, AY/a) is yellow and obese; (e) mouse with two recessive mahogany alleles on a dominant Agouti background is similar to wild-type mouse in pigmentation and body mass.

The MCR pathway includes five known differentially expressed GPCRs: MC1R, corticotropin receptor (ACTHR), MC3R, MC4R, and MC5R (Mountjoy et al., 1992). Although all five receptors are known to be Gs-coupled, MC3R has also been shown to function through phospholipase C-mediated hydrolysis of phosphoinositides (Konda et al., 1994). MC1R is predominantly expressed in melanocytes, where it is known for its classic role in skin and hair pigmentation in many species. MC1R is also expressed in other tissues and cells, such as the pituitary and leukocytes (Chhajlani 1996), indicating putative physiological roles yet to be unveiled. MC3R and MC4R are found in the central nervous system but are notably absent from the melanocytes (Gantz et al., 1993); both are highly expressed in the hypothalamus, where they are involved in energy homeostasis. ACTHR is expressed in the adrenal cortex and MC5R in peripheral cells such as adipocytes. Melanocortins are the endogenous agonists to which the MCRs have differential affinities. These are small peptides derived from proopiomelanocortin: ACTH, -MSH, -MSH, and -MSH. -MSH is the predominant melanocortin of action in the hypothalamus and skin and potently activates all MCRs except ACTHR. The effects of -MSH in vivo are modulated by two endogenous paracrine peptides, agouti (or agouti signaling protein) and agouti-related protein (AGRP). Specifically, AGRP potently antagonizes MC3R, MC4R, and MC5R, whereas agouti potently antagonizes MC1R, ACTHR, and MC4R (Yang et al., 1997). Agouti and AGRP are normally expressed in the skin and brain, respectively.

In mice, agouti is a 131-amino acid peptide synthesized in the skin that causes the mouse melanocytes to produce yellow pigment instead of the brown/black pigment by competitively inhibiting the action of -MSH on MC1R (Lu et al., 1994). The "agouti" coloring of wild-type animals is produced by a brown/black pigmented hair with a subapical yellow pigmented band resulting from transient expression of Agouti during hair development (Fig. 1a). Dominant mutations of Mc1r in which the receptor is constitutively active or has enhanced affinity for -MSH result in dark, nonagouti mice, even in the presence of agouti, implicating agouti's role upstream of the receptor; recessive mutations of murine Mc1r rendering a nonfunctional MC1R produces yellow mice (Robbins et al., 1993). The role of agouti in human pigmentation or other physiological roles is not understood; however, agouti is found in human tissue and has been shown to antagonize human MCRs in vitro (Yang et al., 1997). Unlike agouti's elusive role in human physiology, AGRP is thought to regulate energy homeostasis. The exact mechanism of action on the MCRs by agouti and AGRP is still not well understood. Several potential explanations for AGRP action exist, including competitive antagonism, inverse agonism, or action on an effector other than adenylyl cyclase (Siegrist et al., 1997, Yang et al., 1999). In this issue of Molecular Pharmacology, Yang et al. (2003) use a series of MC1R/MC4R chimeric constructs to study AGRP's role in competitive inhibition of -MSH to elucidate the role of MCR antagonism in energy homeostasis as well as to assess the significance of specific GPCR domains and residues in receptor binding and antagonism.

Naturally occurring variants with associated phenotypes of the MCR signal transduction (Table 1) have contributed to our understanding of the role of components of the MCR pathway, particularly the physiological role of MC4R and AGRP. The pleiotropic effects of altered expression of Agouti in four inherited dominant mouse Agouti mutations (A^y, A^iy, A^sy, A^vy) revealed an important physiological role for MC4R. As summarized above, mice that are wild-type for Mc1r have an expected yellow coat color with the expression of a dominant Agouti allele. However, some dominant Agouti polymorphisms are associated with obesity as well as increased tumor susceptibility and insulin resistance. These mutations facilitated the cloning of the Agouti gene, whose gene product provides a model for in vivo competitive inhibition of GPCRs. A^y results from a large deletion that fuses the promoter of the Raly gene with the Agouti gene. The Raly gene is constitutively expressed in all somatic cells, and its promoter over-rides the regulation of the agouti gene, causing ectopic over-expression of Agouti (Bultman et al., 1992). A^iy and A^vy ectopic overexpression results from the insertion of a retrotransposon and A^sy from a novel DNA sequence that ultimately deregulates the Agouti promoter by altering Agouti transcripts via molecular mechanisms such as altered splicing. (Duhl et al., 1994). It was after the cloning of AGRP that the obesity phenotype associated with ubiquitous expression of the Agouti gene was understood. In these dominant Agouti mutant mice, agouti mimics AGRP's competitive inhibition of -MSH action on MC4R (Yang et al., 1997) in the central nervous system, thereby affecting energy homeostasis analogous to its action on MC1R in skin and pigmentation (Fig. 2).

View larger version (31K): [in this window] [in a new window]   Fig. 2. Mechanism of agouti and AGRP inhibition of -MSH action on MCRs in pigmentation and energy homeostasis. (a) -MSH stimulates MC1R in skin increasing brown/black pigment. (b) Agouti inhibits the action of -MSH on MC1R, producing yellow/red pigments. (c) -MSH stimulates MC4R in the hypothalamus decreasing food intake. (d) AGRP and ectopically expressed agouti inhibits the action of -MSH on MC4R increasing food intake. Attractin, the product of the mahogany locus may act as a low affinity receptor for agouti, increasing the local cell surface concentration of agouti.

AGRP was cloned through sequence similarity with Agouti and found to be expressed primarily in the hypothalamus, where it serves as a selective antagonist for MC3R and MC4R; in contrast to the ubiquitously expressing Agouti mouse model that implicated agouti as an antagonist for MC4R as confirmed by in vitro studies, ubiquitous expression of AGRP in transgenic mice did not affect pigmentation (Ollmann et al., 1997). This indicated that AGRP lacks affinity for MC1R, as later confirmed by in vitro studies (Yang et al., 1999). In their article in this issue of Molecular Pharmacology, Yang et al. (2003) have used this AGRP binding selectivity for MC4R over MC1R in their chimeric receptors to begin to unravel the GPCR domains involved in differential binding properties of agonists and antagonists. For example, Yang et al. (2003) show that although the third and fourth transmembrane domains of human MC4R play a significant role in C-terminal AGRP binding in vitro, the fourth transmembrane domain does not affect agonist [(Nle⁴, D-Phe⁷)-MSH, an analog of -MSH] binding. These and further such pharmacological studies of the domains of MC1R, MC4R, agouti, and AGRP should help determine the differential agonist and antagonist binding properties of GPCRs.

Through these studies, a model is emerging on the action of AGRP and MC4R on feeding and obesity phenotype in mouse (Fig. 2). Neurons producing -MSH and AGRP respond to energy balance changes to regulate food intake via the MC3 and MC4 receptors of downstream neurons. AGRP antagonist increases food intake by inhibiting the action of -MSH on MC4R, as agouti antagonizes the effect of -MSH on MC1R to regulate pigmentation (reviewed in Barsh and Schwartz, 2002).

The MCR pathway has also introduced new regulators of the G-protein-coupled receptor pathway. The pleiotropic effects of the dominant Agouti mutations described above were not seen in mice carrying two recessive alleles at the mahogany locus. The product of the murine mahogany locus was cloned and found to be expressed in many cells and tissues, including melanocytes and the hypothalamus (Gunn et al., 1999). It is orthologous to a transmembrane domain-coding splice variant of the human attractin molecule (Tang et al., 2000). The mechanism of action of this membrane-bound protein is also not fully understood; however, He et al. (2001) provided biochemical evidence of a mechanism in which the mahogany product, attractin, functions as a low-affinity receptor for agouti, but not for AGRP, that increases its local cell-surface concentration, enabling agouti to antagonize the action of -MSH on MC1R and, when ectopically expressed, on MC4R (Fig. 2, b and d).

As described here, the plethora of naturally occurring polymorphisms and mutations of the MCR signal transduction pathway in human and mouse, coupled with their visibly altered phenotypes, provide for a unique model for understanding the mechanisms by which components of the GPCR signal transduction pathway interact and contribute to complex disease and traits. This system introduces new components that modify activation of the GPCR signal transduction pathway, provides numerous phenotype-associated human and mouse single nucleotide polymorphisms in GPCRs for "natural" site-directed mutagenesis studies into the roles of specific amino acid residues in GPCR structure-function, and provides naturally occurring animal variant models to study the action of endogenous antagonists on GPCRs and their modifiers. In addition to their traditional roles in pigmentation and adrenal function and the now established role in energy homeostasis, melanocortins have been observed to have numerous other actions such as on anti-inflammatory, analgesic, learning and memory, and sexual function (reviewed in Wikberg et al., 2000; Gantz and Fong, 2003). Further pharmacological studies on the interactions of these small peptides with their MCR pathway components have the potential to lead to new therapeutic approaches in disease involving GPCRs.

	Footnotes

 
ABBREVIATIONS: MCR, melanocortin receptor; GPCR, G protein-coupled receptor; ACTHR, corticotropin receptor; MSH, melanocyte-stimulating hormone; AGRP, agouti-related protein.

Address correspondence to: Dr. Brinda K. Rana, Department of Psychiatry, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0603. E-mail: bkrana{at}ucsd.edu

	References

Top References

 
Argyropoulos G, Rankinen T, Neufeld DR, Rice T, Province MA, Leon AS, Skinner JS, Wilmore JH, Rao DC, and Bouchard C (2002) A polymorphism in the human agouti-related protein is associated with late-onset obesity. J Clin Endocrinol Metab 87: 4198-4202.[Abstract/Free Full Text]

Barsh GS and Schwartz MW (2002) Genetic approaches to studying energy balance: perception and integration. Nat Rev Genet 3: 589-600.[CrossRef][Medline]

Bultman SJ, Michaud EJ, and Woychik RP (1992) Molecular characterization of the mouse agouti locus. Cell 71: 1195-1204.[CrossRef][Medline]

Chen AS, Marsh DJ, Trumbauer ME, Frazier EG, Guan X-M, Yu H, Rosenblum CI, Vongs A, Feng Y, Cao L, et al. (2000) Inactivation of the mouse melanocortin-3 receptor results in increased fat mass and reduced lean body mass. Nat Genet 26: 97-102.[CrossRef][Medline]

Chhajlani V (1996) Distribution of cDNA for melanocortin receptor subtypes in human tissues. Biochem Mol Biol Int 38: 73-80.[Medline]

Clark AJ, McLoughlin L, and Grossman A (1993) Familial glucocorticoid deficiency associated with point mutation in the adrenocorticotropin receptor. Lancet 34: 461-462.

Flanagan N, Healy E, Ray A, Philips S, Todd C, Jackson IJ, Birch-Machin MA, and Rees JL (2000) Pleiotropic effects of the melanocortin 1 receptor (MC1R) gene on human pigmentation. Hum Mol Genet 9: 2531-2537.[Abstract/Free Full Text]

Gantz I, Miwa H, Konda Y, Shimoto Y, Tashiro T, Watson SJ, DelValle J, and Yamada T (1993) Molecular cloning, expression, and gene localization of a fourth melanocortin receptor. J Biol Chem 268: 15174-15179.[Abstract/Free Full Text]

Gantz I and Fong TM (2003) The melanocortin system. Am J Physiol 284: E468-E474.

Duhl DM, Vrieling H, Miller KA, Wolff GL, and Barsh GS (1994) Neomorphic agouti mutations in obese yellow mice. Nat Genet 8: 59-65.[CrossRef][Medline]

Gunn TM, Miller KA, He L, Hyman RW, Davis RW, Azarani A, Schlossman SF, Duke-Cohan JS, and Barsh GS (1999) The mouse mahogany locus encodes a transmembrane form of human attractin. Nature (Lond) 398: 152-156.[CrossRef][Medline]

He L, Gunn TM, Bouley DM, Lu XY, Watson SJ, Schlossman SF, Duke-Cohan JS, and Barsh GS (2001) A biochemical function for attractin in agouti-induced pigmentation and obesity. Nat Genet 27: 40-47.[Medline]

Konda Y, Gantz I, DelValle J, Shimoto Y, Miwa H, and Yamada T (1994) Interaction of dual intracellular signaling pathways activated by the melanocortin-3 receptor. J Biol Chem 269: 13162-13166.[Abstract/Free Full Text]

Krude H, Biebermann H, Luck W, Horn R, Brabant G, and Gruters A (1998) Severe early-onset obesity, adrenal insufficiency and red hair pigmentation caused by POMC mutations in humans. Nat Genet 19: 155-157.[CrossRef][Medline]

Lu D, Willard D, Patel IR, Kadwell S, Overton L, Kost T, Luther M, Chen W, Woychik RP, Wilkison WO, et al. (1994) Agouti protein is an antagonist of the melanocyte-stimulating-hormone receptor. Nature (Lond) 371: 799-802.[CrossRef][Medline]

Mogil JS, Wilson SG, Chesler EJ, Rankin AL, Nemmani KV, Lariviere WR, Groce MK, Wallace MR, Kaplan L, Staud R, et al. (2003) The melanocortin-1 receptor gene mediates female-specific mechanisms of analgesia in mice and humans. Proc Natl Acad Sci USA 100: 4867-4872.[Abstract/Free Full Text]

Mountjoy KG, Robbins LS, Mortrud MT, and Cone RD (1992) The cloning of a family of genes that encode the melanocortin receptors. Science (Wash DC) 257: 1248-1251.[Abstract/Free Full Text]

Ollmann MM, Wilson BD, Yang Y-K, Kerns JA, Chen Y, Gantz I, and Barsh GS (1997) Agouti-related protein is an endogenous antagonist of the melanocortin-4 receptor in vitro and in vivo. Science (Wash DC) 278: 135-138.[Abstract/Free Full Text]

Palmer JS, Duffy DL, Box NF, Aitken JF, O'Gorman LE, Green AC, Hayward NK, Martin NG, and Sturm RA (2000) Melanocortin-1 receptor polymorphisms and risk of melanoma: is the association explained solely by pigmentation phenotype? Am J Hum Genet 66: 176-186.[CrossRef][Medline]

Robbins LS, Nadeau JH, Johnson KR, Kelly MA, Roselli-Rehfuss L, Baack E, Mountjoy KG, and Cone RD (1993) Pigmentation phenotypes of variant extension locus alleles result from point mutations that alter MSH receptor function. Cell 72: 827-834.[CrossRef][Medline]

Siegrist W, Drozdz R, Cotti R, Willard DH, Wilkison WO, and Eberle AN (1997) Interactions of alpha-melanotropin and agouti on B16 melanoma cells: evidence for inverse agonism of agouti. J Recept Signal Transduct Res 17: 75-98.[Medline]

Simpson EM, Linder CC, Sargent EE, Davisson MT, Mobraaten LE, and Sharp J (1997) Genetic variation among 129 substrains and its importance for "targeted mutagenesis" in mice. Nat Genet 16: 19-27.[CrossRef][Medline]

Tang W, Gunn TM, McLaughlin DF, Barsh GS, Schlossman SF, and Duke-Cohan JS (2000) Secreted and membrane attractin result from alternative splicing of the human ATRN gene. Proc Natl Acad Sci USA 97: 6025-6030.[Abstract/Free Full Text]

Tsigos C, Arai K, Hung W, and Chrousos GP (1993) Hereditary isolated glucocorticoid deficiency is associated with abnormalities of the adrenocorticotropin receptor gene. J Clin Investig 92: 2458-2461.

Vaisse C, Clement K, Guy-Grand B, and Froguel P (1998) A frameshift mutation in human MC4R is associated with a dominant form of obesity. Nat Genet 20: 113-114.[CrossRef][Medline]

Wikberg JE, Muceniece R, Mandrika I, Prusis P, Lindblom J, Post C, and Skottner A (2000) New aspects on the melanocortins and their receptors. Pharmacol Res 42: 393-420.[CrossRef][Medline]

Yang Y, Chen M, Lai Y, Gantz I, Yagmurlu A, Georgeson KE, and Harmon CM (2003) Molecular determination of agouti-related protein binding to human melanocortin-4 receptor. Mol Pharmacol 64: 94-103[Abstract/Free Full Text]

Yang YK, Dickinson C, Kent SB, and Gantz I (1999) Characterization of AGRP binding to the melanocortin receptors. Mol Endorinology 13: 148-155.

Yang Y-K, Ollmann MM, Wilson BD, Dickinson C, Yamada T, Barsh GS, and Gantz I (1997) Effects of recombinant ASIP on melanocortin action. Mol Endocrinol 11: 274-280.[Abstract/Free Full Text]

Yeo GSH, Farooqi IS, Aminian S, Halsall DJ, Stanhope RG, and O'Rahilly SA (1998) frameshift mutation in MC4R associated with dominantly inherited human obesity. Nat Genet 20: 111-112.[CrossRef][Medline]

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