The agouti gene: turned on to yellow

A genome-wide epistatic network underlies the molecular architecture of continuous color variation of body extremities

Deciphering the molecular architecture of coat coloration for a better understanding of the biological mechanisms underlying pigmentation still remains a challenge. We took advantage of a rabbit French experimental population in which both a pattern and a gradient of coloration from white to brown segregated within the himalayan phenotype. The whole experimental design was genotyped using the high density Affymetrix® AxiomOrcun™ SNP Array and phenotyped into 6 different groups ordered from the lighter to the darker. Genome-wide association analyses pinpointed an oligogenic determinism, under recessive and additive inheritance, involving genes already known in melanogenesis (ASIP, KIT, MC1R, TYR), and likely processed pseudogenes linked to ribosomal function, RPS20 and RPS14. We also identified (i) gene-gene interactions through ASIP:MC1R affecting light cream/beige phenotypes while KIT:RPS responsible of dark chocolate/brown colors and (ii) a genome-wide epistatic network involving several others coloration genes such as POT1 or HPS5. Finally, we determined the recessive inheritance of the English spotting phenotype likely involving a copy number variation affecting at least the end of the coding sequence of the KIT gene. Our analyses of coloration as a continuous trait allowed us to go beyond much of the established knowledge through the detection of additional genes and gene-gene interactions that may contribute to the molecular architecture of the coloration phenotype.

Molecular Pathways and Pigments Underlying the Colors of the Pearl Oyster Pinctada margaritifera var. cumingii (Linnaeus 1758)

The shell color of the Mollusca has attracted naturalists and collectors for hundreds of years, while the molecular pathways regulating pigment production and the pigments themselves remain poorly described. In this study, our aim was to identify the main pigments and their molecular pathways in the pearl oyster Pinctada margaritifera—the species displaying the broadest range of colors. Three inner shell colors were investigated—red, yellow, and green. To maximize phenotypic homogeneity, a controlled population approach combined with common garden conditioning was used. Comparative analysis of transcriptomes (RNA-seq) of P. margaritifera with different shell colors revealed the central role of the heme pathway, which is involved in the production of red (uroporphyrin and derivates), yellow (bilirubin), and green (biliverdin and cobalamin forms) pigments. In addition, the Raper–Mason, and purine metabolism pathways were shown to produce yellow pigments (pheomelanin and xanthine) and the black pigment eumelanin. The presence of these pigments in pigmented shell was validated by Raman spectroscopy. This method also highlighted that all the identified pathways and pigments are expressed ubiquitously and that the dominant color of the shell is due to the preferential expression of one pathway compared with another. These pathways could likely be extrapolated to many other organisms presenting broad chromatic variation.

Genetic deciphering of the antagonistic activities of the melanin-concentrating hormone and melanocortin pathways in skin pigmentation

The genetic origin of human skin pigmentation remains an open question in biology. Several skin disorders and diseases originate from mutations in conserved pigmentation genes, including albinism, vitiligo, and melanoma. Teleosts possess the capacity to modify their pigmentation to adapt to their environmental background to avoid predators. This background adaptation occurs through melanosome aggregation (white background) or dispersion (black background) in melanocytes. These mechanisms are largely regulated by melanin-concentrating hormone (MCH) and α-melanocyte–stimulating hormone (α-MSH), two hypothalamic neuropeptides also involved in mammalian skin pigmentation. Despite evidence that the exogenous application of MCH peptides induces melanosome aggregation, it is not known if the MCH system is physiologically responsible for background adaptation. In zebrafish, we identify that MCH neurons target the pituitary gland-blood vessel portal and that endogenous MCH peptide expression regulates melanin concentration for background adaptation. We demonstrate that this effect is mediated by MCH receptor 2 (Mchr2) but not Mchr1a/b. mchr2 knock-out fish cannot adapt to a white background, providing the first genetic demonstration that MCH signaling is physiologically required to control skin pigmentation. mchr2 phenotype can be rescued in adult fish by knocking-out pomc, the gene coding for the precursor of α-MSH, demonstrating the relevance of the antagonistic activity between MCH and α-MSH in the control of melanosome organization. Interestingly, MCH receptor is also expressed in human melanocytes, thus a similar antagonistic activity regulating skin pigmentation may be conserved during evolution, and the dysregulation of these pathways is significant to our understanding of human skin disorders and cancers.

Melanocytes produce melanin, a natural skin pigment, for body coloration which helps to protect and camouflage an organism and to attract mates. Melanocytes are ubiquitous pigment cells in vertebrates and the genes underlying their development are well conserved, making fishes that possess the ability to modify their pigmentation, biologically relevant and successful models for human skin disorders. Many human skin diseases including albinism, vitiligo, and melanoma are derived from mutations in conserved pigmentation genes. However, much of the conserved molecular mechanisms behind these diseases and human pigmentation remain unknown. For instance, melanin concentrating hormone (MCH) was originally identified as a peptide that when injected, could make fish paler by promoting melanin aggregation but no mutants demonstrating an endogenous function for MCH in pigmentation have been reported. Here, we use zebrafish mutants of MCH and the MCH receptor to determine their specific genetic function in pigmentation. Additionally, we demonstrate that MCH has an antagonistic pigmentation function to the melanocortin system, where MCH expression promotes lighter pigmentation and melanocortin activity promotes darkening. Thus, we find that the balance between the MCH and melanocortin system activities are likely required for skin pigmentation and dysregulation of these pathways could underlie adverse human skin conditions.

Melanocortins contribute to sequential differentiation and enucleation of human erythroblasts via melanocortin receptors 1, 2 and 5

In this study, we showed that adrenocorticotropic hormone (ACTH) promoted erythroblast differentiation and increased the enucleation ratio of erythroblasts. Because ACTH was contained in hematopoietic medium as contamination, the ratio decreased by the addition of anti-ACTH antibody (Ab). Addition of neutralizing Abs (nAbs) for melanocortin receptors (MCRs) caused erythroblast accumulation at specific stages, i.e., the addition of anti-MC2R nAb led to erythroblast accumulation at the basophilic stage (baso-E), the addition of anti-MC1R nAb caused accumulation at the polychromatic stage (poly-E), and the addition of anti-MC5R nAb caused accumulation at the orthochromatic stage (ortho-E). During erythroblast differentiation, ERK, STAT5, and AKT were consecutively phosphorylated by erythropoietin (EPO). ERK, STAT5, and AKT phosphorylation was inhibited by blocking MC2R, MC1R, and MC5R, respectively. Finally, the phosphorylation of myosin light chain 2, which is essential for the formation of contractile actomyosin rings, was inhibited by anti-MC5R nAb. Taken together, our study suggests that MC2R and MC1R signals are consecutively required for the regulation of EPO signal transduction in erythroblast differentiation, and that MC5R signal transduction is required to induce enucleation. Thus, melanocortin induces proliferation and differentiation at baso-E, and polarization and formation of an actomyosin contractile ring at ortho-E are required for enucleation.

Agouti revisited: transcript quantification of the ASIP gene in bovine tissues related to protein expression and localization

Beside its role in melanogenesis, the agouti signaling protein (ASIP) has been related to obesity. The potentially crucial role in adipocyte development makes it a tempting candidate for economic relevant, fat related traits in farm animals. The objective of our study was to characterize the mRNA expression of different ASIP transcripts and of putative targets in different bovine tissues, as well as to study consequences on protein abundance and localization. ASIP mRNA abundance was determined by RT-qPCR in adipose and further tissues of cattle representing different breeds and crosses. ASIP mRNA was up-regulated more than 9-fold in intramuscular fat of Japanese Black cattle compared to Holstein (p<0.001). Further analyses revealed that a transposon-derived transcript was solely responsible for the increased ASIP mRNA abundance. This transcript was observed in single individuals of different breeds indicating a wide spread occurrence of this insertion at the ASIP locus in cattle. The protein was detected in different adipose tissues, skin, lung and liver, but not in skeletal muscle by Western blot with a bovine-specific ASIP antibody. However, the protein abundance was not related to the observed ASIP mRNA over-expression. Immuno-histochemical analyses revealed a putative nuclear localization of ASIP additionally to the expected cytosolic signal in different cell types. The expression of melanocortin receptors (MCR) 1 to 5 as potential targets for ASIP was analyzed by RT-PCR in subcutaneous fat. Only MC1R and MC4R were detected indicating a similar receptor expression like in human adipose tissue. Our results provide evidence for a widespread expression of ASIP in bovine tissues at mRNA and, for the first time, at protein level. ASIP protein is detectable in adipocytes as well as in further cells of adipose tissue. We generated a basis for a more detailed investigation of ASIP function in peripheral tissues of various mammalian species.

The effect of dose rate on the frequency of specific-locus mutations induced in mouse spermatogonia is restricted to larger lesions; a retrospective analysis of historical data

A series of 19 large-scale germ-cell mutagenesis experiments conducted several decades ago led to the conclusion that low-LET radiation delivered to mouse spermatogonia at dose rates of 0.8 R/min and below induced only about one-third as many specific-locus mutations as did single, acute exposures at 24 R/min and above. A two-hit origin of the mutations was deemed unlikely in view of the then prevailing evidence for the small size of genetic lesions in spermatogonia. Instead, the dose-rate effect was hypothesized to be the result of a repair system that exists in spermatogonia, but not in more mature male reproductive cells. More recent genetic and molecular studies on the marker genes have identified the phenotypes associated with specific states of the mutant chromosomes, and it is now possible retrospectively to classify individual past mutations as "large lesions" or "other lesions". The mutation-frequency difference between high and low dose rates is restricted to the large lesion mutations, for which the dose-curve slopes differ by a factor exceeding 3.4. For other lesion mutations, there is essentially no difference between the slopes for protracted and acute irradiations; induced other lesions frequencies per unit dose remain similar for dose rates ranging over more than 7 orders of magnitude. For large lesions, these values rise sharply at dose rates >0.8 R/min, though they remain similar within the whole range of protracted doses, failing to provide evidence for a threshold dose rate. The downward bend at high doses that had been noted for X-ray-induced specific-locus mutations as a whole and ascribed to a positive correlation between spermatogonial death and mutation load is now found to be restricted to large lesion mutations. There is a marked difference between the mutation spectra (distributions among the seven loci) for large lesions and other lesions. Within each class, however, the spectra are similar for acute and protracted irradiation.

The developmental role of Agouti in color pattern evolution

Animal color patterns can affect fitness in the wild; however, little is known about the mechanisms that control their formation and subsequent evolution. We took advantage of two locally camouflaged populations of Peromyscus mice to show that the negative regulator of adult pigmentation, Agouti, also plays a key developmental role in color pattern evolution. Genetic and functional analyses showed that ventral-specific embryonic expression of Agouti establishes a prepattern by delaying the terminal differentiation of ventral melanocytes. Moreover, a skin-specific increase in both the level and spatial domain of Agouti expression prevents melanocyte maturation in a regionalized manner, resulting in a novel and adaptive color pattern. Thus, natural selection favors late-acting, tissue-specific changes in embryonic Agouti expression to produce large changes in adult color pattern.

Measuring natural selection on genotypes and phenotypes in the wild

A complete understanding of the role of natural selection in driving evolutionary change requires accurate estimates of the strength of selection acting in the wild. Accordingly, several approaches using a variety of data-including patterns of DNA variability, spatial and temporal changes in allele frequencies, and fitness estimates-have been developed to identify and quantify selection on both genotypes and phenotypes. Here, we review these approaches, drawing on both recent and classic examples to illustrate their utility and limitations. We then argue that by combining estimates of selection at multiple levels-from individual mutations to phenotypes-and at multiple timescales-from ecological to evolutionary-with experiments that demonstrate why traits are under selection, we can gain a much more complete picture of the adaptive process.

Volume-sensitive outwardly rectifying chloride channel in white adipocytes from normal and diabetic mice

The volume-sensitive outwardly rectifying (VSOR) chloride channel is ubiquitously expressed and involved in cell volume regulation after osmotic swelling, called regulatory volume decrease (RVD), in various cell types. In adipocytes, the expression of the VSOR channel has not been explored to date. Here, by employing the whole-cell patch-clamp technique, we examined whether or not the VSOR channel is expressed in white adipocytes freshly isolated from epididymal fat pads of normal (C57BL/6 or KK) and diabetic (KKA(y)) mice. Whole cell voltage-clamp recordings revealed that Cl(-) currents were gradually activated upon cell swelling induced by application of a hypotonic solution, both in normal and diabetic adipocytes. Although both the mean cell size (or cell capacitance) and the current magnitude in KKA(y) adipocytes were larger than those in C57BL/6 cells, the current density was significantly lower in KKA(y) adipocytes (23.32 +/- 1.94 pA in C57BL/6 adipocytes vs. 13.04 +/- 2.41 pA in KKA(y) adipocytes at +100 mV). Similarly, the current density in diabetic KKA(y) adipocytes was lower than that in adipocytes from KK mice (a parental strain of KKA(y) mice), which do not present diabetes until an older age. The current was inhibited by Cl(-) channel blockers, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and glibenclamide, or hypertonic solution, and showed outward rectification and inactivation kinetics at large positive potentials. These electrophysiological and pharmacological properties are consistent with those of the VSOR channel in other cell types. Moreover, adipocytes showed RVD, which was inhibited by NPPB. In KKA(y) adipocytes, RVD was significantly slower (tau; 8.42 min in C57BL/6 adipocytes vs. 11.97 min in KKA(y) adipocytes) and incomplete during the recording period (25 min). It is concluded that the VSOR channel is functionally expressed and involved in volume regulation in white adipocytes. RVD is largely impaired in adipocytes from diabetic mice, presumably as a consequence of the lower density of the functional VSOR channel in the plasma membrane.

Spiny mice modulate eumelanin to pheomelanin ratio to achieve cryptic coloration in "evolution canyon," Israel

BACKGROUND

Coat coloration in mammals is an explicit adaptation through natural selection. Camouflaging with the environment is the foremost evolutionary drive in explaining overall coloration. Decades of enquiries on this topic have been limited to repetitive coat color measurements to correlate the morphs with background/habitat blending. This led to an overwhelming endorsement of concealing coloration as a local phenotypic adaptation in animals, primarily rodents to evade predators. However, most such studies overlooked how rodents actually achieve such cryptic coloration. Cryptic coloration could be attained only through optimization between the yellow- to brown-colored "pheomelanin" and gray to black-colored "eumelanin" in the hairs. However, no study has explored this conjecture yet. "Evolution Canyon" (EC) in Israel is a natural microscale laboratory where the relationship between organism and environment can be explored. EC is comprised of an "African" slope (AS), which exhibits a yellow-brownish background habitat, and a "European" slope (ES), exhibiting a dark grayish habitat; both slopes harbor spiny mice (Acomys cahirinus). Here, we examine how hair melanin content of spiny mice living in the opposing slopes of EC evolves toward blending with their respective background habitat.

METHODOLOGY/PRINCIPAL FINDINGS

We measured hair-melanin (both eumelanin and pheomelanin) contents of 30 spiny mice from the EC using high-performance liquid chromatography (HPLC) that detects specific degradation products of eumelanin and pheomelanin. The melanin pattern of A. cahirinus approximates the background color of the slope on which they dwell. Pheomelanin is slightly (insignificantly) higher in individuals found on the AS to match the brownish background, whereas individuals of the ES had significantly greater eumelanin content to mimic the dark grayish background. This is further substantiated by a significantly higher eumelanin and pheomelanin ratio on the ES than on the AS.

CONCLUSION/SIGNIFICANCE

It appears that rodents adaptively modulate eumelanin and pheomelanin contents to achieve cryptic coloration in contrasting habitats even at a microscale.

New ligands for melanocortin receptors

Named originally for their effects on peripheral end organs, the melanocortin system controls a diverse set of physiological processes through a series of five G-protein-coupled receptors and several sets of small peptide ligands. The central melanocortin system plays an essential role in homeostatic regulation of body weight, in which two alternative ligands, alpha-melanocyte-stimulating hormone and agouti-related protein, stimulate and inhibit receptor signaling in several key brain regions that ultimately affect food intake and energy expenditure. Much of what we know about the relationship between central melanocortin signaling and body weight regulation stems from genetic studies. Comparative genomic studies indicate that melanocortin receptors used for controlling pigmentation and body weight regulation existed more than 500 million years ago in primitive vertebrates, but that fine-grained control of melanocortin receptors through neuropeptides and endogenous antagonists developed more recently. Recent studies based on dog coat-color genetics revealed a new class of melanocortin ligands, the beta-defensins, which reveal the potential for cross talk between the melanocortin and the immune systems.

Genetics of Sex-linked yellow in the Syrian hamster

Alternating patches of black and yellow pigment are a ubiquitous feature of mammalian color variation that contributes to camouflage, species recognition, and morphologic diversity. X-linked determinants of this pattern--recognized by variegation in females but not in males--have been described in the domestic cat as Orange, and in the Syrian hamster as Sex-linked yellow (Sly), but are curiously absent from other vertebrate species. Using a comparative genomic approach, we develop molecular markers and a linkage map for the euchromatic region of the Syrian hamster X chromosome that places Sly in a region homologous to the centromere-proximal region of human Xp. Comparison to analogous work carried out for Orange in domestic cats indicates, surprisingly, that the cat and hamster mutations lie in nonhomologous regions of the X chromosome. We also identify the molecular cause of recessively inherited black coat color in hamsters (historically referred to as nonagouti) as a Cys115Tyr mutation in the Agouti gene. Animals doubly mutant for Sly and nonagouti exhibit a Sly phenotype. Our results indicate that Sly represents a melanocortin pathway component that acts similarly to, but is genetically distinct from, Mc1r and that has implications for understanding both the evolutionary history and the mutational mechanisms of pigment-type switching.

Regulation of eumelanin/pheomelanin synthesis and visible pigmentation in melanocytes by ligands of the melanocortin 1 receptor

The production of melanin in the hair and skin is tightly regulated by the melanocortin 1 receptor (MC1R) whose activation is controlled by two secreted ligands, alpha-melanocyte stimulating hormone (alphaMSH) and agouti signal protein (ASP). As melanin is extremely stable, lasting years in biological tissues, the mechanism underlying the relatively rapid decrease in visible pigmentation elicited by ASP is of obvious interest. In this study, the effects of ASP and alphaMSH on the regulation of melanin synthesis and on visible pigmentation were assessed in normal murine melanocytes and were compared with the quick depigmenting effect of the tyrosinase inhibitor, phenylthiourea (PTU). alphaMSH increased pheomelanin levels prior to increasing eumelanin content over 4 days of treatment. Conversely, ASP switched off the pigment synthesis pathway, reducing eu- and pheo-melanin synthesis within 1 day of treatment that was proportional to the decrease in tyrosinase protein level and activity. These results demonstrate that the visible depigmentation of melanocytes induced by ASP does not require the degradation of existing melanin but rather is due to the dilution of existing melanin by melanocyte turnover, which emphasizes the importance of pigment distribution to visible color.

Characterization of Japanese quail yellow as a genomic deletion upstream of the avian homolog of the mammalian ASIP (agouti) gene

ASIP is an important pigmentation gene responsible for dorsoventral and hair-cycle-specific melanin-based color patterning in mammals. We report some of the first evidence that the avian ASIP gene has a role in pigmentation. We have characterized the genetic basis of the homozygous lethal Japanese quail yellow mutation as a >90-kb deletion upstream of ASIP. This deletion encompasses almost the entire coding sequence of two upstream loci, RALY and EIF2B, and places ASIP expression under control of the RALY promoter, leading to the presence of a novel transcript. ASIP mRNA expression was upregulated in many tissues in yellow compared to wild type but was not universal, and consistent differences were not observed among skins of yellow and wild-type quail. In a microarray analysis on developing feather buds, the locus with the largest downregulation in yellow quail was SLC24A5, implying that it is regulated by ASIP. Finally, we document the presence of ventral skin-specific isoforms of ASIP mRNA in both wild-type quails and chickens. Overall, there are remarkable similarities between yellow in quail and lethal yellow in mouse, which involve a deletion in a similar genomic position. The presence of ventral-specific ASIP expression in birds shows that this feature is conserved across vertebrates.

Coloration and the genetics of adaptation

Coat color is often used as camouflage and so has evolutionary benefit. How is coat color determined?

From genesis to gene sequencing: historical progress in the understanding of skin color

Since at least biblical times, humans have pondered on why there might be variation in skin color and what might constitute the nature of that difference. In this article, two historical trails are followed, one beginning with the Ancient Greeks, the other with the Ancient Chinese. These two paths converge to provide us with some historical evidence to back recent scientific discoveries in the dynamic regulation of skin pigmentation, focusing on melanocyte-stimulating hormone and its natural antagonist agouti-signaling protein.

Investigation of the role of the agouti signaling protein gene (ASIP) in coat color evolution in primates

We investigated variation in the gene encoding the agouti signaling protein (ASIP) in relation to coat color evolution in primates. We found little evidence that mutations in the coding region of ASIP have been involved in color changes among closely related primate species. Among many closely related species with differing coat color, the coding region of ASIP was identical. In two cases (Sulawesi macaque and black lion tamarin) where species with almost completely black coat color had derived point mutations in exon 4 of the ASIP coding sequence, the same mutations did not alter coloration in other mammals and so probably do not affect ASIP function. Evolutionary reconstructions of two key phenotypes that are typically related to ASIP function--transverse phaeomelanin bands on hairs and pale ventral coloration--showed that these usually evolved concurrently, suggesting that loci acting downstream of ASIP may be involved. Analysis of dN/dS ratios revealed a likely change in functional constraint on ASIP following loss of agouti-banded hairs + pale ventral coloration, particularly in catarrhine primates (humans, apes, and Old World monkeys). Together with previous results on a lack of association of coat color with MC1R variation, these results suggest that other loci probably have an important role in primate coat color evolution.

Melanocortin-1 receptor structure and functional regulation

The melanogenic actions of the melanocortins are mediated by the melanocortin-1 receptor (MC1R). MC1R is a member of the G-protein-coupled receptors (GPCR) superfamily expressed in cutaneous and hair follicle melanocytes. Activation of MC1R by adrenocorticotrophin or alpha-melanocyte stimulating hormone is positively coupled to the cAMP signaling pathway and leads to a stimulation of melanogenesis and a switch from the synthesis of pheomelanins to the production of eumelanic pigments. The functional behavior of the MC1R agrees with emerging concepts in GPCR signaling including dimerization, coupling to more than one signaling pathway and a high agonist-independent constitutive activity accounting for inverse agonism phenomena. In addition, MC1R displays unique properties such as an unusually high number of natural variants often associated with clearly visible phenotypes and the occurrence of endogenous peptide antagonists. Therefore MC1R is an ideal model to study GPCR function. Here we review our current knowledge of MC1R structure and function, with emphasis on information gathered from the analysis of natural variants. We also discuss recent data on the regulation of MC1R function by paracrine and endocrine factors and by external stimuli such as ultraviolet light.

The mutation causing the black-and-tan pigmentation phenotype of Mangalitza pigs maps to the porcine ASIP locus but does not affect its coding sequence

The gene for agouti signaling protein (ASIP) is centrally involved in the expression of coat color traits in animals. The Mangalitza pig breed is characterized by a black-and-tan phenotype with black dorsal pigmentation and yellow or white ventral pigmentation. We investigated a Mangalitza x Piétrain cross and observed a coat color segregation pattern in the F2 generation that can be explained by virtue of two alleles at the MC1R locus and two alleles at the ASIP locus. Complete linkage of the black-and-tan phenotype to microsatellite alleles at the ASIP locus on SSC 17q21 was observed. Corroborated by the knowledge of similar mouse coat color mutants, it seems therefore conceivable that the black-and-tan pigmentation of Mangalitza pigs is caused by an ASIP allele a(t), which is recessive to the wild-type allele A. Toward positional cloning of the a(t) mutation, a 200-kb genomic BAC/PAC contig of this chromosomal region has been constructed and subsequently sequenced. Full-length ASIP cDNAs obtained by RACE differed in their 5' untranslated regions, whereas they shared a common open reading frame. Comparative sequencing of all ASIP exons and ASIP cDNAs between Mangalitza and Piétrain pigs did not reveal any differences associated with the coat color phenotype. Relative qRT-PCR analyses showed different dorsoventral skin expression intensities of the five ASIP transcripts in black-and-tan Mangalitza. The a(t) mutation is therefore probably a regulatory ASIP mutation that alters its dorsoventral expression pattern.

Antidiabetic effect of Nitobegiku, the herb Tithonia diversifolia, in KK-Ay diabetic mice

Nitobegiku (the herb of Tithonia diversifolia (HEMSL) A. GRAY) has been used as a medicinal plant for diabetes. The antidiabetic effect of an 80% ethanol extract of Nitobegiku (Td) was investigated in KK-Ay-mice, an animal model of type 2 diabetes. Td (500 mg/kg body weight) reduced the blood glucose of KK-Ay mice 7 h after a single oral dose. No change in blood glucose in Td-treated normal mice (ddY) was seen. Td (500 mg/kg) reduced blood glucose in KK-Ay mice 3 weeks after a single oral dose and also significantly lowered plasma insulin in KK-Ay mice under similar conditions. Td-treated KK-Ay mouse blood glucose was significantly decreased in an insulin tolerance test. These results support the hypothesis that Td improves glucose metabolism by reducing insulin resistance. Therefore, Nitobegiku may be useful for the treatment of type 2 diabetes.

Two cysteine substitutions in the MC1R generate the blue variant of the Arctic fox (Alopex lagopus) and prevent expression of the white winter coat

We have characterized two mutations in the MC1R gene of the blue variant of the arctic fox (Alopex lagopus) that both incorporate a novel cysteine residue into the receptor. A family study in farmed arctic foxes verified that the dominant expression of the blue color phenotype cosegregates completely with the allele harboring these two mutations. Additionally to the altered pigment synthesis, the blue fox allele suppresses the seasonal change in coat color found in the native arctic fox. Consequently, these findings suggest that the MC1R/agouti regulatory system is involved in the seasonal changes of coat color found in arctic fox.

Population Differences in the Frequency of the Agouti Signaling Protein g.8818A>G Polymorphism

The role of agouti signaling protein (ASIP) in human pigmentation pathways is not definitively understood although its murine homologue regulates, in part, pheomelanogenesis. We have reported an association of a polymorphism in the 3'-untranslated region of ASIP (g.8818A>G) with dark hair and eye color among a group of European-Americans (Am J Hum Genet 2002 March;70:770). Among 147 healthy control subjects, the frequency of the G-allele was 0.12. We hypothesized that this polymorphism would occur at different frequencies among different population groups. Using PCR-RFLP, we genotyped 25 East Asian, 86 African-American, and 207 West African individuals for the ASIP g.8818A>G polymorphism. The g.8818G-allele was present in the West African sample at a frequency of 0.80, in the African-American sample at a frequency of 0.62, and in the East Asian sample at 0.28. The difference in allele frequency among population groups was statistically significant (P < 0.0001). Although the effect of the g.8818A>G polymorphism upon ASIP function is unknown, the large difference in allele frequency between our West African and European-American sample populations lends support to the notion that this gene may be important in human pigmentation.

Recent developments in our understanding of melanocortin system in the regulation of food intake

Obesity has become one of the most significant public health problems facing the world today. However, the pathogenesis of obesity is multifactorial and involves the interaction of genetic and environmental factors. There is a pressing need to better understand the biochemical pathways that control energy intake and expenditure. In the last few years, a number of important signalling molecules have been identified that play important roles in obesity. One family of these molecules is the melanocortin system, which consists of several components: (1) melanocortin peptides; (2) the five seven-transmembrane G-protein coupled melanocortin receptors (MCRs); (3) the endogenous MCR antagonists, agouti and agouti-related protein; (4) the endogenous melanocortin mediators, mahogany, and syndecan. This system plays a key role in the central nervous system control of feeding behaviour and energy expenditure. This article will provide an overview of the anatomy, physiology, and molecular biology of the melanocortin system, and recent developments in our understanding of this system in obesity.

PPARgamma coactivator 1beta/ERR ligand 1 is an ERR protein ligand, whose expression induces a high-energy expenditure and antagonizes obesity

A well balanced body energy budget controlled by limitation of calorie uptake and/or increment of energy expenditure, which is typically achieved by proper physical exercise, is most effective against obesity and diabetes mellitus. Recently, peroxisome proliferator-activated receptor (PPAR) gamma, a member of the nuclear receptor, and its cofactors have been shown to be involved in lipid metabolism and in the control of energy expenditure. Here we show that PPARgamma coactivator 1 (PGC-1) beta functions as ERRL1 (for ERR ligand 1), which can bind and activate orphan ERRs (estrogen receptor-related receptors) in vitro. Consistently, PGC-1beta/ERRL1 transgenic mice exhibit increased expression of the medium-chain acyl CoA dehydrogenase, a known ERR target and a pivotal enzyme of mitochondrial beta-oxidation in skeletal muscle. As a result, the PGC-1beta/ERRL1 mice show a state similar to an athlete; namely, the mice are hyperphagic and of elevated energy expenditure and are resistant to obesity induced by a high-fat diet or by a genetic abnormality. These results demonstrate that PGC-1beta/ERRL1 can function as a protein ligand of ERR, and that its level contributes to the control of energy balance in vivo, and provide a strategy for developing novel antiobesity drugs.

Pigmentary switches in domestic animal species

Although homogeneous pigmentation usually is observed in wild animals, most domestic animal species display a wide variety of coat colors. In fur animals, the coat color is an important production trait, and in other species such as cattle and sheep, the coat color is a major breed characteristic. Variability in coat color is seen both within and between breeds, and makes domesticated species unique for studying gene function and gene regulation of loci affecting pigmentation. In several species, mutations in the MC1-R gene have been shown to cause the dominant expression of black pigment. In fox, alleles of both the agouti and the MC1-R gene could cause eumelanin synthesis. In addition, a nonepistatic interaction between MC1-R and agouti has been observed, resulting in several different coat color phenotypes expressing a mixture of red and black pigmentation. Also in cattle and sheep, amino acid substitutions within the MC1-R explain the dominant inheritance of black pigmentation. Unlike the constitutively activated MC1-R found in the Alaska silver fox, dominant variants of the MC1-R found in cattle and sheep seem to be completely dominant with no antagonizing effect of agouti. MC1-R variants with premature stop codons are widespread in several cattle populations, indicating that this well-conserved gene has no other fundamental function beside pigmentation. Other well-established breed characteristics include distinct coat color patterns in which the distribution of melanocytes, partly regulated by the c-kit gene, seems to be involved.

Inverse agonism gains weight

Inverse agonism is emerging as a new endogenous principle for receptor regulation. Agouti-related protein (AgRP), following its release in the brain, stimulates food intake. AgRP binds to brain melanocortin receptors, which are involved in the regulation of body weight. In addition to antagonizing the effects of the melanocortin receptor agonist alpha-melanocyte-stimulating hormone (alpha-MSH), AgRP suppresses the constitutive activity of melanocortin MC(3) and MC(4) receptors, which characterizes AgRP as an inverse agonist rather than a neutral antagonist. The balance between the activity of AgRP-containing neurons and alpha-MSH-containing neurons determines the extent of activation of melanocortin receptors in neurons onto which they project. The identification of AgRP as an endogenous inverse agonist provides physiological relevance to inverse agonism in the control of body weight.

Accessory proteins for melanocortin signaling: attractin and mahogunin

Switching from eumelanin to pheomelanin synthesis during hair growth is accomplished by transient synthesis of Agouti protein, an inverse agonist for the melanocortin-1 receptor (Mc1r). The coat color mutations mahogany and mahoganoid prevent hair follicle melanocytes from responding to Agouti protein. The gene mutated in mahogany, which is also known as Attractin (Atrn), encodes a type I transmembrane protein that functions as an accessory receptor for Agouti protein. We have recently determined that the gene mutated in mahoganoid, which is also known as Mahogunin (Mgrn1), encodes an E3 ubiquitin ligase. Like Attractin, Mahogunin is conserved in invertebrate genomes, and its absence causes a pleiotropic phenotype that includes spongiform neurodegeneration.

The genetic basis of adaptive melanism in pocket mice

Identifying the genes underlying adaptation is a major challenge in evolutionary biology. Here, we describe the molecular changes underlying adaptive coat color variation in a natural population of rock pocket mice, Chaetodipus intermedius. Rock pocket mice are generally light-colored and live on light-colored rocks. However, populations of dark (melanic) mice are found on dark lava, and this concealing coloration provides protection from avian and mammalian predators. We conducted association studies by using markers in candidate pigmentation genes and discovered four mutations in the melanocortin-1-receptor gene, Mc1r, that seem to be responsible for adaptive melanism in one population of lava-dwelling pocket mice. Interestingly, another melanic population of these mice on a different lava flow shows no association with Mc1r mutations, indicating that adaptive dark color has evolved independently in this species through changes at different genes.

Antidiabetic effect of nitobegiku in KK-Ay diabetic mice

In the past, nitobegiku (the herb of Tithonia diversifolia (Hemsl) A. Gray) has been used as a medicinal plant for diabetes. Antidiabetic effect of the water extract of Nitobegiku (NG) was investigated in KK-Ay-mice--one of the animal models of type 2 diabetes. NG (1,500 mg/kg body weight) reduced the blood glucose of KK-Ay mice from 509 +/- 22 mg/dl to 340 +/- 14 mg/dl (p < 0.001) and also lowered the plasma insulin (p < 0.05) 7 hours after single oral administration. No change in blood glucose of NG-treated normal mice (ddY) was seen. These results support that NG improve glucose metabolism by reducing insulin resistance. Therefore, NG may be useful for treatment of type 2 diabetes.

The regulatory basis of melanogenic switching

Melanin produced in follicular melanocytes is the major basis for pigmentation of hair and wool in mammals. Two major types of melanin may be synthesized, the black/brown eumelanin and the reddish/yellow pheomelanin. Based on available cell biological evidence and reasonable assumptions, a mathematical model is developed to improve our understanding of melanogenic switching, i.e. the switching between eumelanin and pheomelanin production depending on the extracellular signalling context. In 1993, Ito proposed that melanogenic switching is due to the covalent binding of the intermediate DOPAquinone to the enzyme glutathione reductase. We were only able to obtain a good fit to available experimental data on the relation between pheomelanin levels and the activity of the key enzyme tyrosinase by taking Ito's hypothesis into account. Thus, our results support Ito's hypothesis, and suggest that melanogenic switching may be due to a jump between two stable production pattern states when the tyrosinase activity varies between two bifurcation levels. This implies that small changes in the levels of external regulatory factors may cause an accentuated change in the proportion of the produced colour pigments and may explain the fact that mammalian coat patterns often exhibit sharply delimited patches of either black or reddish colour.

Induction and characterization of mutations at the b locus of the medaka, Oryzias latipes

The b locus is one of the most familiar pigmentation loci in the medaka, but its biochemical function is still unknown. Here we report induction of new mutations at the b locus by radiation and ENU. We also characterized all these mutations and previously isolated spontaneous ones on the phenotypic basis. Unexpectively, all the 18 induced mutations reduced melanin contents in both eyes and skin correlatively, although degree of reduction was varied from mutations to mutations. Moreover, presumed null mutants (bs8, bg8, bc2, bd3, bd6, bg13, bg19, bg24) had slightly melanized (dark red) eyes. These results suggest that the b-locus product plays an important but not a critical role in melanogenesis. The spontaneous mutants were divided into two types: one (bdl2, bdl3, and bp) had similarities with the induced mutants in that they had slightly colored eyes and skin, the other (bv, B', bd, bdl1, and b) exhibited normally black eyes but lightly colored skin. The present study supports our recent results (Fukamachi et al., 2001) that mutational changes were found in the coding region of the b gene in some of the mutants which reduced both eyes and skin melanogenesis, while the mutational change for the b allele could not be found there. We speculate that the bv, B', bd, bdl1, and b alleles might arise by the mutations in the regulatory region for skin melanogenesis.

A polymorphism in the agouti signaling protein gene is associated with human pigmentation

In mice and humans, binding of alpha-melanocyte--stimulating hormone to the melanocyte-stimulating--hormone receptor (MSHR), the protein product of melanocortin-1 receptor (MC1R) gene, leads to the synthesis of eumelanin. In the mouse, ligation of MSHR by agouti signaling protein (ASP) results in the production of pheomelanin. The role of ASP in humans is unclear. We sought to characterize the agouti signaling protein gene (ASIP) in a group of white subjects, to assess whether ASIP was a determinant of human pigmentation and whether this gene may be associated with increased melanoma risk. We found no evidence of coding-region sequence variation in ASIP, but detected a g.8818A-->G polymorphism in the 3' untranslated region. We genotyped 746 participants in a study of melanoma susceptibility for g.8818A-->G, by means of polymerase chain reaction and restriction fragment--length polymorphism analysis. Among the 147 healthy controls, the frequency of the G allele was.12. Carriage of the G allele was significantly associated with dark hair (odds ratio 1.8; 95% confidence interval [CI] 1.2--2.8) and brown eyes (odds ratio 1.9; 95% CI 1.3--2.8) after adjusting for age, gender, and disease status. ASIP g.8818A-->G was not associated independently with disease status. This is the first report of an association of ASIP with specific human pigmentation characteristics. It remains to be investigated whether the interaction of MC1R and ASIP can enhance prediction of human pigmentation and melanoma risk.

Monogenic forms of obesity and diabetes mellitus

Great progress has been made in identifying several genes and in understanding the molecular pathogenesis of inherited syndromes of obesity and diabetes mellitus (DM). In humans, mutations in leptin, leptin receptor, proopiomelanocortin (POMC), melanocortin-4 receptor (MC4R) and prohormone convertase 1 (PC1) have been described in patients with severe obesity. Most of these obesity disorders, with the exception of the MC4R mutations, exhibit recessive inheritance and a distinct phenotype with varying degrees of hypothalamic dysfunction, and they unravel the critical role of the central leptin and melanocortin pathways in human appetite control and energy homeostasis. Maturity onset diabetes of the young (MODY) is a genetically and clinically heterogeneous subtype of type 2 DM with early onset autosomal dominant inheritance and a primary defect in insulin secretion. To date, six MODY genes have been identified, the glucokinase gene and five beta cell-specific transcription factor genes, hepatocyte nuclear factor-1alpha (HNF-1alpha), HNF-1beta, HNF-4alpha, insulin promoter factor-1 (IPF-1) and NeuroD1/BETA2. Mitochondrial DNA mutations cause another form of DM with an insulin secretory defect that is commonly associated with neurosensory hearing impairment, and has strict maternal inheritance. At the other end of the spectrum are the inherited syndromes of insulin resistance that are caused by mutations in the insulin receptor gene and in the adipocyte-specific transcription factor PPARgamma. The advances in our knowledge of the phenotypic manifestations and underlying molecular mechanisms of genetic syndromes of obesity and DM raise expectations for molecular diagnosis, as well as for more etiological therapies and better prevention of the continuously increasing prevalence of obesity and DM in our modern societies.

Agouti: from mouse to man, from skin to fat

The agouti protein regulates pigmentation in the mouse hair follicle producing a black hair with a subapical yellow band. Its effect on pigmentation is achieved by antagonizing the binding of alpha-melanocyte stimulating hormone (alpha-MSH) to melanocortin 1 receptor (Mc1r), switching melanin synthesis from eumelanin (black/brown) to phaeomelanin (red/yellow). Dominant mutations in the non-coding region of mouse agouti cause yellow coat colour and ectopic expression also results in obesity, type 11 diabetes, increased somatic growth and tumourigenesis. At least some of these pleiotropic effects can be explained by antagonism of other members of the melanocortin receptor family by agouti protein. The yellow coat colour is the result of agouti chronically antagonizing the binding of alpha-MSH to Mc1r and the obese phenotype results from agouti protein antagonizing the binding of alpha-MSH to Mc3r and/or Mc4r. Despite the existence of a highly homologous agouti protein in humans, agouti signal protein (ASIP), its role has yet to be defined. However it is known that human ASIP is expressed at highest levels in adipose tissue where it may antagonize one of the melanocortin receptors. The conserved nature of the agouti protein combined with the diverse phenotypic effects of agouti mutations in mouse and the different expression patterns of human and mouse agouti, suggest ASIP may play a role in human energy homeostasis and possibly human pigmentation.

Genetic and biochemical studies of the Agouti-attractin system

Pleiotropic effects of melanocortin signaling were first described nearly 100 years ago when mice carrying the lethal yellow (A(y)) allele of the Agouti coat color gene were recognized to develop increased growth and adiposity. Work from our laboratory and others over the last several years has demonstrated that the non-pigmentary effects of A(y) are caused by ectopic expression of Agouti protein, a paracrine signaling molecule whose normal function is to inhibit signaling through the melanocortin 1 receptor (Mc1r), but which can mimic the effects of Agouti-related protein (Agrp), a homologous neuropeptide produced in the medial portion of the arcuate nucleus that acts as a potent antagonist of the Mc3r and Mc4r. Recently we have used the genetics of pigmentation as an in vivo screening system to analyze other mutations in the Agouti-melanocortin pathway, leading to the identification of Attractin (Atrn), a widely expressed type I transmembrane protein that serves as an accessory receptor for Agouti protein. Surprisingly, homologs of Atrn are found in fruitflies and nematodes, even though Agouti and/or Agouti-related protein are found only in vertebrates. Insight into this apparent paradox now comes from studies of different Atrn alleles, in which we find hyperactivity, abnormal myelination, and widespread CNS vacuolation. We suggest that the neurodegenerative phenotype reflects the ancestral function of Atrn to facilitate and/or maintain cell-cell interactions in the nervous system. Expression in neurectodermal cells during vertebrate evolution may have allowed Atrn to be recruited by the Agouti-melanocortin system to control coat color.

Hypoleptinemia, but not hypoinsulinemia, induces hyperphagia in streptozotocin-induced diabetic rats

To assess the dominance between hypoinsulinemia and hypoleptinemia as factors in the development of hyperphagia in streptozotocin (STZ)-induced diabetes mellitus (STZ-DM) rodents with respect to hormone-neuropeptide interactions, changes in gene expression of agouti gene-related protein (AGRP) in the arcuate nucleus of the hypothalamus were investigated using STZ-DM rats, fasting Zucker fa/fa rats and STZ-DM agouti (STZ-DM A(y)/a) mice. AGRP mRNA and neuropeptide Y mRNA were both significantly up-regulated in STZ-DM rats, which are associated with body weight loss, hyperglycemia, hypoinsulinemia and hypoleptinemia. We proceeded to analyze whether insulin or leptin played the greater role in the regulation of AGRP using Zucker fa/fa rats. The AGRP mRNA did not differ significantly between fasted fa/fa rats, which have both leptin-insensitivity and hypoinsulinemia, and fed Zuckers, which have leptin-insensitivity and hyperinsulinemia. We further found that up-regulation of AGRP expression was normalized by infusion of leptin into the third cerebroventricle (i3vt), but not by i3vt infusion of insulin, although up-regulation of AGRP was partially corrected by systemic insulin infusion. The latter finding supports hypoleptinemia as a key-modulator of STZ-DM-induced hyperphagia because systemic insulin infusion, at least partially, restored hypoleptinemia through its acceleration of fat deposition, as demonstrated by the partial recovery of lost body weight. After STZ-DM induction, A(y)/a mice whose melanocortin-4 receptor (MC4-R) was blocked by ectopic expression of agouti protein additionally accelerated hyperphagia and up-regulated AGRP mRNA, implying that the mechanism is triggered by a leptin deficit rather than by the main action of the message through MC4-R. Hypoleptinemia, but not hypoinsulinemia per se, thus develops hyperphagia in STZ-DM rodents. These results are very much in line with evidence that hypothalamic neuropeptides are potently regulated by leptin as downstream targets of its actions.

Involvement of ITF2 in the transcriptional regulation of melanogenic genes

In response to agouti signal protein, melanocytes switch from producing eumelanin to pheomelanin concomitant with the down-regulation of melanogenic gene transcription. We previously reported that a ubiquitous basic helix-loop-helix transcription factor, known as ITF2, is up-regulated during this switch, and we now report that treatment of melanocytes with melanocyte-stimulating hormone down-regulates expression of ITF2. To more fully characterize the involvement of ITF2 in regulating melanogenic gene transcription, ITF2 sense or antisense constructs were introduced into melan-a melanocytes. Gene and protein expression analyses and luciferase reporter assays using promoters from melanogenic genes showed that up-regulation of ITF2 suppressed melanogenic gene expression as well as the expression of Mitf, a melanocyte-specific transcription factor. In addition, stable ITF2 sense transfectants had significant reductions in pigmentation and a less dendritic phenotype compared with mock transfectants. In contrast, ITF2 antisense-transfected melanocytes were more pigmented and more dendritic. These results demonstrate that up-regulation of ITF2 during the pheomelanin switch is functionally significant and reveal that differential expression of a ubiquitous basic helix-loop-helix transcription factor can modulate expression of melanogenic genes and the differentiation of melanocytes.

Antagonist and agonist activities of the mouse agouti protein fragment (91-131) at the melanocortin-1 receptor

Antagonist and agonist activities of chemically synthetized mouse agouti protein fragment (91-131) (AP91-131) at the melanocortin type-1 receptor (MC1-R) were assessed using B 16-F1 mouse melanoma cells in vitro and the following assay systems: (i) receptor binding, (ii) adenylate cyclase, (iii) tyrosinase, (iv) melanin production, and (v) cell proliferation. In competition binding studies AP91-131 was about 3-fold less potent than the natural agonist alpha-melanocyte-stimulating hormone (alpha-MSH) in displacing the radioligand [125I]-[Nle4, D-Phe7]-alpha-MSH (Ki 6.5 +/- 0.8 nmol/l). Alpha-MSH-induced tyrosinase activation and melanin production were completely inhibited by a 100-fold higher concentration of AP9 l -131; the IC50 values for AP91-131 in thetwo assay systems were 91 +/- 22 nM and 95 +/- 15 nM respectively. Basal melanin production and adenylate cyclase activity in the absence of agonist were decreased by AP91-131 with IC50 values of 9.6+/-1.8 nM and 5.0+/-2.4 nM, respectively. This indicates inverse agonist activity of AP91-131 similar to that of native AP. The presence of 10 nM melanin-concentrating hormone (MCH) slightly potentiated the inhibitory activity of AP91-131 in the adenylate cyclase and melanin assays. On the other hand, AP91-131 inhibited cell growth similar to alpha-MSH (IC50 11.0 +/- 2.1 nM; maximal inhibition 1.8-fold higher than that of alpha-MSH). Furthermore, MC1-R was down-regulated by AP91-131 with about the same potency and time-course as with alpha-MSH. These results demonstrate that AP91-131 displays both agonist and antagonist activities at the MC1-R and hence that it is the cysteine-rich region of agouti protein which inhibits and mimics the different alpha-MSH functions, most likely by simultaneous modulation of different intracellular signalling pathways.

Transgenic complementation of leptin-receptor deficiency. I. Rescue of the obesity/diabetes phenotype of LEPR-null mice expressing a LEPR-B transgene

Mice homozygous for the Leprdb3J (db3J) mutation are null for all known isoforms of the leptin receptor (LEPR). These animals are obese, hyperphagic, cold intolerant, insulin resistant, and infertile. Mice homozygous for the Leprdb (db) mutation (lacking the B isoform only) have the same phenotype as db3J animals. To better understand the function(s) of the LEPR isoforms in vivo, we generated db3J/db3J and db/db mice bearing a transgene (neuron-specific enolase [NSE]-Rb) expressing the B isoform of LEPR, the isoform capable of activating the signal transducer and activator of transcription (STAT) pathway, under the control of the neuron-specific enolase enhancer/promoter. The NSE-Rb transgene was expressed in the brain, with low levels of expression in adrenals, testis, and white adipose tissue. LEPR-B transgene expression in NSE-Rb db3J/db3J mice partially corrected the increased fat mass, hyperphagia, and glucose intolerance while restoring fertility in males and rescuing the cold intolerance in both sexes. The body weights of NSE-Rb transgenic mice that possessed the full complement of short LEPR isoforms (NSE-Rb db/db mice) were similar to those of NSE-Rb db3J/db3J mice, suggesting that the short LEPR isoforms play little role in body weight regulation. Based on quantitative analysis of hypothalamic neuropeptide gene expression in the transgenic animals, we infer full restoration of leptin sensitivity to proopiomelanocortin (POMC) neurons, partial correction of leptin sensitivity in agouti gene-related protein (AGRP)/neuropeptide Y (NPY) neurons, and a lack of effect on leptin sensitivity of melanin concentrating hormone neurons. Thus, hypothalamic POMC and AGRP/NPY neurons are primary candidates as the mediators of the effects of the NSE-Rb transgene on energy homeostasis, ingestive behavior, the neuroendocrine system, and glucose metabolism.

Mouse genetics/genomics: an effective approach for drug target discovery and validation

The mouse has become the premier mammalian system for the identification of the genetic basis of both mono- and oligogenic disorders, as well as the understanding of complex diseases with gene-gene and gene-environment interactions. The similarity between human and mouse genetic disease is sometimes striking, while in other cases the phenotypes are less similar. The ability to genetically map and then clone single gene disorders rapidly, and the emerging technologies that will allow the economical identification of the polygenes controlling quantitative traits further demonstrate the utility of the mouse as a model for gene discovery. Additionally, the ability to genetically manipulate the mouse through transgenesis and gene targeting allows for the testing of hypotheses regarding specific gene function and their role in disease. The utility of the mouse extends beyond being just a gene discovery tool to provide prevalidated targets. It can also be used for the development of animal models, and the testing of compounds in specifically constructed transgenic and knockout strains to further define the target and pathway of a therapeutic compound.

Genetics of body-weight regulation

The role of genetics in obesity is twofold. Studying rare mutations in humans and model organisms provides fundamental insight into a complex physiological process, and complements population-based studies that seek to reveal primary causes. Remarkable progress has been made on both fronts, and the pace of advance is likely to accelerate as functional genomics and the human genome project expand and mature. Approaches based on mendelian and quantitative genetics may well converge, and lead ultimately to more rational and selective therapies.

In situ localization of agouti signal protein in murine skin using immunohistochemistry with an ASP-specific antibody

Switching between production of eumelanin or pheomelanin in follicular melanocytes is responsible for hair color in mammals; in mice, this switch is controlled by the agouti locus, which encodes agouti signal protein (ASP) through the action of melanocortin receptor 1. To study expression and processing patterns of ASP in the skin and its regulation of pigment production in hair follicles, we have generated a rabbit antibody (termed alphaPEP16) against a synthetic peptide that corresponds to the carboxyl terminus of ASP. The specificity of that antibody was measured by ELISA and was confirmed by Western blot analysis. Using immunohistochemistry, we characterized the expression of ASP in the skin of newborn mice at 3, 6, and 9 days postnatally. Expression in nonagouti (a/a) black mouse skin was negative at all times examined, as expected, and high expression of ASP was observed in 6 day newborn agouti (A/+) and in 6 and 9 day newborn lethal yellow (A(y)/a) mouse skin. In lethal yellow (pheomelanogenic) mice, ASP expression increased day by day as the hair color became more yellow. These expression patterns suggest that ASP is delivered quickly and efficiently to melanocytes and to hair matrix cells in the hair bulbs where it regulates melanin production.

Implications of proopiomelanocortin (POMC) mutations in humans: the POMC deficiency syndrome

The recent discovery of the contribution of proopiomelanocortin (POMC)-derived peptides to the regulation of energy homeostasis and exocrine gland secretion in mice aroused new interest in the complex function of the endocrine POMC network. In addition, the first mutations in the gene encoding POMC have been identified in two patients affected by adrenal insufficiency, early onset severe obesity and red hair pigmentation. Therefore, the focus of this brief review will be the detailed discussion of the implications of these new findings in the physiology of the human POMC ligand-receptor system.