New insights into G-protein-coupled receptor signaling from the melanocortin receptor system.

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/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. 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 antag-