黑素皮质素受体1——哺乳动物黑色素形成中的关键基因MC1R——the Key Gene in Mammalian Melanin Synthesis

黑色素的形成与产生在动物的生长发育过程中受到许多基因的调控。本文综述了近年来被广泛研究的哺乳动物黑色素形成调控中的一个关键基因——黑素皮质素受体1（melanocortin-1-receptor, MC1R）基因的作用机制、DNA序列变异与黑色素性状之间的关系,并且对另一重要的脊椎动物类群——鸟类中MC1R基因的确定与突变情况作以概述,此外对乌骨鸡富含黑色素的原因和鸟类的黑色素形成机制也进行了探讨。Abstract: The study of the molecular regulation mechanism of melanin synthesis during animal development has become a new focus recently .The synthesis and production of melanin during animal development are regulated by many genes. This paper summarized the molecular function mechanism of melanocortin-1-receptor (MC1R) gene and the relationship between the consequences of polymorphic variation of the gene and melanin traits, in addition to summarized the identification and mutation of MC1R gene in birds. Furthermore, the reason of abundant melanin in silkies and melanin synthesis mechanism in birds are all also discussed here.     

Melanocortin－1 receptor gene variants in four Chinese ethnic populations

INTRODUCTIONThe variation in human hall and skin color in~ geographic regions of the world is the result Of differences in two Principal forms Of melanin,the red-yellow phaeomelalilns and the bldebrowneUmelanins, which are present in the epidermallayer of hUman skin and hair[1, 2]. The type ofmelanin Produced is under the control of two genes,identified initially by the mouse mutation, extension and agouti. The eXtension gene is expressedin melanocytes, Producillg the melanocyte stimul…     

犬MC1R基因T105A基因座多态性及 与毛色性状相关性的研究The

为了检测犬MC1R基因T105A基因座的多态性,并分析该多态性与犬毛色表型的相关性,抽取111只外科手术学实验用杂种犬血液并提取DNA,记录毛色表型。采用PCR-RFLP技术,对MC1R基因T105A基因座进行基因多态性分析,并对该基因座DNA进行克隆测序;用二元变量相关分析的统计学方法分析基因座多态性与毛色性状之间的相关性。经PCR-RFLP分析结果表明,T105A基因座序列具有多态性,表现为A、B二个等位基因和AA、AB及BB 3种基因型。A、B等位基因频率分别为72.97%和27.03%,基因杂合度（H）为0.39。基因型AA频率为55.86%,BB为9.91%,AB为34.23%。对T105A多态性片段DNA克隆测序后发现,MC1R基因在编码第105位氨基酸的密码子第一个碱基存在由G到A的单碱基突变,该突变导致第105位氨基酸发生由丙氨酸向苏氨酸的改变。统计分析结果表明MC1R基因T105A基因座的多态性与毛色性状不存在显著的相关性,这可能是由于外科手术学实验用犬是杂种犬,其遗传背景不同所致,尚须在纯种犬群体中进一步研究MC1R基因对毛色的影响。 Abstract: In order to detect the polymorphism of T105A in MC1R gene in dogs and to analyze the relationship between the genetic polymorphisms and phenotypes of dog coat color, the blood samples of 111 cross-breed dogs were taken and their genomic DNAs were extracted. The phenotypes of dog coat color were recorded. The T105A locus of MC1R gene in the canine was detected through the technology of PCR-RFLP. Furthermore, the polymorphic fragments at T105A were sequenced. The relationships between the polymorphism of T105A and coat color trait were analyzed by the statistical methods of bivarate correlation analysis. By the method of PCR-RFLP, the T105A polymorphism was found with two alleles A and B and three genotypes AA, AB and BB. The frequencies of two alleles were 72.97% and 27.03%, respectively. The heterozygosity of T105A locus was 0.39. The frequencies of three genotypes were 55.86%, 34.23% and 9.91%, respectively. According to the results of sequencing, one base change from G to A at the position 105 was found at T105A locus and it altered amino acid at the position 105 from alanine to threonine. According to the statistical analysis, no significant association between the polymorphism of MC1R gene and the coat color was found and the result may be due to the differences of genetic background. Further research on MC1R gene should be done in pure breed dogs.     

Comparative Skin Histology of Frogs Reveals High-elevation Adaptation of the Tibetan Nanorana parkeri

Adaptations to extreme environmental conditions are intriguing. Animal skin, which directly interacts with external environment, plays diverse and important roles in adaptive evolution. The thin and bare skin of amphibians is sensitive to external environmental conditions and, thus, it facilitates investigations into adaptations for living in extreme environments. Herein, we compare the structures of skin in four anuran species living at elevations ranging from 100 m to 4500 m to assess phenotypic innovations in the skin of Nanorana parkeri, which lives at extremely high elevations. Analyses reveal similar basic skin structures, but N. parkeri differs from the other species by having more epidermal capillaries and granular glands, which correlate highly with responses to hypoxia and/or ultraviolet(UV) radiation. Further intraspecific comparisons from frogs taken at ～4500 m and ～2900 m reveal that all of the changes are fixed. Changes occurring only in the higher elevation population, such as possessing more skin pigments, may represent local adaptations to coldness and/or UV radiation. These results provide a morphological basis for understanding further the molecular adaptations of these frogs.     

In vivo skin penetration and metabolic path of quantum dots

The skin is the largest organ of the body and is a potential route of exposure to sunscreens and cosmetics containing nanoparticles;however,the permeability of the skin to these nanoparticles is currently unknown.In this paper,we studied the transdermal delivery capacity through mouse skin of water-soluble CdSeS quantum dots(QDs) and the deposition of these QDs in the body.QD solution was coated onto the dorsal hairless skin of male ICR mice.Fluorescence microscopy and transmission electron microscopy(TEM) were used to observe the distribution of QDs in the skin and organs,and inductively coupled plasma-mass spectrometry(ICP-MS) was used to measure the 111Cd content to indicate the concentration of QDs in plasma and organs.Experimental results indicate that QDs can penetrate into the dermal layer and are limited to the uppermost stratum corneum layers and the hair follicles.Through blood circulation,QDs deposit mostly in liver and kidney and are difficult to clear.111Cd concentration was greater than 14 ng g-1 in kidney after 120 h after 0.32 nmol QDs was applied to a mouse.These results suggest that QDs have in vivo transdermal delivery capacity through mouse skin and are harmful to the liver and kidney.     

Changes in the cellularity of the cortex of human hairs as an indicator of radiation exposure

  Growing hair follicles with their rapid cell proliferation would be expected to be sensitive organs to cytotoxic agents such as radiation. Various abnormalities in the hair and hair follicles have been reported in the past. Changes in the number of cells in the newly forming hair cortex have been shown in the mouse to be one of the more sensitive assays for radiation effects, and this approach could provide a basis for a biological dosimeter. Here we show for the first time using hair cortex cell counts some preliminary data indicating that the number of cell nuclei in a unit of length (140 μm) of the cortex of human hairs from the chest and scalp of patients undergoing fractionated radiotherapy falls significantly (P = 0.005) by 5%–10% 3 days after the first dose in a fractionated sequence of irradiations. The first dose was delivered on a Friday, and no further exposures were delivered until after the hair sample was taken on the 3rd day (Monday). No significant effect of radiation dose could be detected over the available, limited range of doses studied (5 – 6.5 Gy with one exit dose sample at 2.6 Gy). Also, the width varies from hair to hair. If the width of the hair is taken into account and the cortical nuclei counts are normalised to the width of each hair, the effects seen at day 3 become slightly more significant (P = 0.002), and those at day 5 also become significant (P = 0.012). Samples taken on the 5th day after the first (Friday) exposure were also 2 days after the second exposure and 1 day after the third exposure. However, little expression of damage attributable to the 2nd and 3rd exposures was anticipated since their effects would take some time to be expressed in the cortical region examined, which is some distance from the proliferative region of the follicle. Received: 28 September 1995 / Accepted in revised form: 23 February 1996     

Alteration of β-secretase traffic by the receptor tyrosine kinase signaling pathway- a new mechanism for regulating Alzheimer's β-amyloid production

The receptor tyrosine kinases (RTKs) are a family of cellsurface proteins with diverse functions in proliferation, dif-ferentiation or cell-cell communication. When a specific li-gand binds to its cognate receptor, a conformational changeof this receptor due to the ligand-receptor interaction willlead to activation of the intrinsic tyrosine kinase residing inthe intracellular domain of the receptor. The activation ofthis tyrosine kinase is essential for transducing the signals toa cascade of its downstream molecules that eventually causerelated physiological responses [1]. For example, binding ofnerve growth factor (NGF) to its receptor TrkA is essentialfor the proper development, patterning, and maintenanceof the mammalian nervous system. This ligand and recep-tor interaction will lead to the formation of a crab-shapedhomodimeric TrkA structure [2], and the subsequent activa-tion of its intrinsic RTK will cause auto-phosphorylationof its own intracellular tyrosine residues. PhosphorylatedTrkA receptors recruit and increase the phosphorylationof PLC-γ and Shc, which leads to activation of either thePI3K/Akt pathway or Ras/raf/ERK pathway. In the brainOf Alzheimer's disease (AD) patients, alterations of nervegrowth factor (NGF) and its receptor TrkA have beenreported to associate with AD pathogenesis [3]. However,the underlying mechanisms remain elusive.     

High polymorphism at the human melanocortin 1 receptor locus

Variation in human skin/hair pigmentation is due to varied amounts of eumelanin (brown/black melanins) and phaeomelanin (red/yellow melanins) produced by the melanocytes. The melanocortin 1 receptor (MC1R) is a regulator of eu- and phaeomelanin production in the melanocytes, and MC1R mutations causing coat color changes are known in many mammals. We have sequenced the MC1R gene in 121 individuals sampled from world populations with an emphasis on Asian populations. We found variation at five nonsynonymous sites (resulting in the variants Arg67Gln, Asp84Glu, Val92Met, Arg151Cys, and Arg163Gln), but at only one synonymous site (A942G). Interestingly, the human consensus protein sequence is observed in all 25 African individuals studied, but at lower frequencies in the other populations examined, especially in East and Southeast Asians. The Arg163Gln variant is absent in the Africans studied, almost absent in Europeans, and at a low frequency (7%) in Indians, but is at an exceptionally high frequency (70%) in East and Southeast Asians. The MC1R gene in common and pygmy chimpanzees, gorilla, orangutan, and baboon was sequenced to study the evolution of MC1R. The ancestral human MC1R sequence is identical to the human consensus protein sequence, while MC1R varies considerably among higher primates. A comparison of the rates of substitution in genes in the melanocortin receptor family indicates that MC1R has evolved the fastest. In addition, the nucleotide diversity at the MC1R locus is shown to be several times higher than the average nucleotide diversity in human populations, possibly due to diversifying selection.     

Agonist‐Independent,High Constitutive Activity of the Human Melanocortin 1 Receptor

The melanocortins (α‐melanocyte‐stimulating hormone and adrenocorticotropin) act on epidermal melanocytes to increase melanogenesis, the eumelanin/pheomelanin ratio and dendricity. These actions are mediated by the heptahelical melanocortin 1 receptor (MC1R), positively coupled to adenylyl cyclase. Gain‐of‐function mouse Mc1r alleles are associated with a dark, eumelanic coat. Conversely, loss‐of‐function variants, or overexpression of agouti, a natural melanocortin antagonist, yield yellow, pheomelanic furs. In humans, loss‐of‐function MC1R variants are associated with fair skin, poor tanning, propensity to freckle and increased skin cancer risk. Therefore, MC1R is a key regulator of mammalian pigmentation. Several observations such as induction of constitutive pigmentation in amelanotic mouse melanoma cells following expression of MC1R indicate that the receptor might display agonist‐independent activity. We report a systematic and comparative study of MC1R and Mc1r constitutive activity. We show that expression of MC1R in heterologous systems leads to an agonist‐independent increase in cyclic adenosine monophophate (cAMP). Basal signalling is a function of receptor expression and is two to fourfold higher for MC1R than for Mc1r. Moreover, it is observed in human melanoma cells over‐expressing the MC1R. Constitutive signalling is abolished or reduced by point mutations of MC1R impairing the response to agonists, and is only doubled by the Lys94Glu mutation, mimicking the constitutively active mouse E^so‐3J allele. Stable or transient expression of wild‐type MC1R, but not of loss‐of‐function mutants, potently stimulates forskolin activation of adenylyl cyclase, a common feature of constitutively active Gs‐coupled receptors. Therefore, human MC1R displays a strong agonist‐independent constitutive activity.     

Lack of somatic alterations of MC1R in primary melanoma

Germline variation of the melanocortin 1 receptor gene (MC1R) is a risk factor for cutaneous melanoma. Recent studies have indicated that the risk is significantly higher for melanomas with somatic BRAF mutations, suggesting that MC1R variants may have a more specific role than their demonstrated effects on skin and hair pigmentation. To address the possibility that MC1R may act like a tumor suppressor gene by creating a permissive condition for melanocytes with specific somatic mutations to proliferate or survive, we analyzed 103 primary melanomas for somatic MC1R mutations and copy number alterations. This cohort included melanomas from skin with and without chronic sun-induced damage, mucosal membranes, and acral skin (palms, soles, and subungual). We did not find somatic mutations or frequent DNA copy number alterations at the MC1R locus, nor any skewed pattern of copy number alterations that would favor one allele type over the other. In conclusion, our findings indicate that MC1R is not a frequent target of somatic alterations in melanoma.     

Agonist-independent, high constitutive activity of the human melanocortin 1 receptor

The melanocortins (alpha-melanocyte-stimulating hormone and adrenocorticotropin) act on epidermal melanocytes to increase melanogenesis, the eumelanin/pheomelanin ratio and dendricity. These actions are mediated by the heptahelical melanocortin 1 receptor (MC1R), positively coupled to adenylyl cyclase. Gain-of-function mouse Mc1r alleles are associated with a dark, eumelanic coat. Conversely, loss-of-function variants, or overexpression of agouti, a natural melanocortin antagonist, yield yellow, pheomelanic furs. In humans, loss-of-function MC1R variants are associated with fair skin, poor tanning, propensity to freckle and increased skin cancer risk. Therefore, MC1R is a key regulator of mammalian pigmentation. Several observations such as induction of constitutive pigmentation in amelanotic mouse melanoma cells following expression of MC1R indicate that the receptor might display agonist-independent activity. We report a systematic and comparative study of MC1R and Mc1r constitutive activity. We show that expression of MC1R in heterologous systems leads to an agonist-independent increase in cyclic adenosine monophophate (cAMP). Basal signalling is a function of receptor expression and is two to fourfold higher for MC1R than for Mc1r. Moreover, it is observed in human melanoma cells over-expressing the MC1R. Constitutive signalling is abolished or reduced by point mutations of MC1R impairing the response to agonists, and is only doubled by the Lys94Glu mutation, mimicking the constitutively active mouse E(so-3J) allele. Stable or transient expression of wild-type MC1R, but not of loss-of-function mutants, potently stimulates forskolin activation of adenylyl cyclase, a common feature of constitutively active Gs-coupled receptors. Therefore, human MC1R displays a strong agonist-independent constitutive activity.     

黑素皮质激素受体1（MC1R）基因系统发育树与犬的毛色

犬的驯养迄今约有1万多年,由于不同环境和不同目的人工选择形成了犬品种间或品种内极丰富的毛色多样性,经证实,这些犬的很多毛色类型与MC1R相关 ,MC1R在一些物种中有同源基因 本文阐述了犬MC1R多态性研究进展,并选择其它9个有代表性的哺乳动物物种与犬MC1R同源基因进行了比较,以此建立系统发育树。结果显示,10个物种的MC1R基因的分子进化关系与物种的经典分类学地位基本相符。      

Spatial and Temporal Localization of the Melanocortin 1 Receptor and Its Ligand ���CMelanocyte-Stimulating Hormone during Cutaneous Wound Repair

Growing evidence indicates that the melanocortin 1 receptor (MC1R) and its ligand α–melanocyte-stimulating hormone (α-MSH) have other functions in the skin in addition to pigment production. Activation of the MC1R/α-MSH signaling pathway has been implicated in the regulation of both inflammation and extracellular matrix homeostasis. However, little is known about the role of MC1R/α-MSH signaling in the regulation of inflammatory and fibroproliferative responses to cutaneous injury. Although MC1R and α-MSH localization has been described in uninjured skin, their spatial and temporal expression during cutaneous wound repair has not been investigated. In this study, the authors report the localization of MC1R and α-MSH in murine cutaneous wounds, human acute burns, and hypertrophic scars. During murine wound repair, MC1R and α-MSH were detected in inflammatory cells and suprabasal keratinocytes at the leading edge of the migrating epithelial tongue. MC1R and α-MSH protein levels were upregulated in human burn wounds and hypertrophic scars compared to uninjured human skin, where receptor and ligand were absent. In burn wounds and hypertrophic scars, MC1R and α-MSH localized to epidermal keratinocytes and dermal fibroblasts. This spatiotemporal localization of MC1R and α-MSH in cutaneous wounds warrants future investigation into the role of MC1R/α-MSH signaling in the inflammatory and fibroproliferative responses to cutaneous injury. This article contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.     

Skin phototype: a new perspective

Cutaneous phototype is considered mainly related to cutaneous pigmentation and to the eumelanin/pheomelanin ratio, which is mostly genetically determined by the melanocortin 1 receptor (MC1R) polymorphisms. However, data in literature indicate that, in addition to stimulation of eumelanin synthesis, the MC1R signalling activates antioxidant, DNA repair and survival pathways. New emerging aspects regarding photoprotection and skin phototypes are going beyond those features connected to the melanin content in the skin. Important new findings link the MC1R to nuclear receptors activation, shedding light on new extra‐melanogenic effects dependent on the α‐melanocyte‐stimulating hormone (α‐MSH) activity and new ways through which such functions are modulated. These evidences indicate that several factors including melanin play a part in defining the basis for individual sun sensitivity, suggesting that the cutaneous phototype represents a ‘biochemical fingerprint’.     

Variation of the melanocortin 1 receptor gene in the macaques

Melanocortin 1 receptor (MC1R), a G-coupled seven-transmembrane receptor protein, plays a key role in the regulation of melanin synthesis in mammals. Sequence variation of the MC1R gene (MC1R) has been associated with pigmentation phenotypes in humans and in several animal species. The macaques (genus Macaca) are known to show a marked inter-specific variation in coat color although the causative genetic variation remains unclear. We investigated nucleotide sequences of the MC1R in 67 individuals of 18 macaque species with different coat color phenotypes including black and agouti. Twenty-eight amino acid replacements were identified in the macaques, but none of these amino acid replacements could explain the black coat color of Macaca silenus and the Sulawesi macaque species. Our molecular evolutionary analysis has revealed that nonsynonymous substitution/synonymous substitution (dN/dS) ratio of the MC1R has not been uniform in the macaque groups and, moreover, their coat color and dN/dS ratio were not related. These results suggest that the MC1R is unlikely to be responsible for the coat color variation of the macaques and functions of MC1R other than pigmentation might be associated with the different selective pressures on the MC1R in macaques.     

MC1R and the response of melanocytes to ultraviolet radiation

The constitutive color of our skin plays a dramatic role in our photoprotection from solar ultraviolet radiation (UVR) that reaches the Earth and in minimizing DNA damage that gives rise to skin cancer. More than 120 genes have been identified and shown to regulate pigmentation, one of the key genes being melanocortin 1 receptor (MC1R) that encodes the melanocortin 1 receptor (MC1R), a seven-transmembrane G protein-coupled receptor expressed on the surface of melanocytes. Modulation of MC1R function regulates melanin synthesis by melanocytes qualitatively and quantitatively. The MC1R is regulated by the physiological agonists alpha-melanocyte-stimulating hormone (alphaMSH) and adrenocorticotropic hormone (ACTH), and antagonist agouti signaling protein (ASP). Activation of the MC1R by binding of an agonist stimulates the synthesis of eumelanin primarily via activation of adenylate cyclase. The significance of cutaneous pigmentation lies in the photoprotective effect of melanin, particularly eumelanin, against sun-induced carcinogenesis. Epidermal melanocytes and keratinocytes respond to UVR by increasing their expression of alphaMSH and ACTH, which up-regulate the expression of MC1R, and consequently enhance the response of melanocytes to melanocortins. Constitutive skin pigmentation dramatically affects the incidence of skin cancer. The pigmentary phenotype characterized by red hair, fair complexion, inability to tan and tendency to freckle is an independent risk factor for all skin cancers, including melanoma. The MC1R gene is highly polymorphic in human populations, and allelic variation at this locus accounts, to a large extent, for the variation in pigmentary phenotypes and skin phototypes (SPT) in humans. Several allelic variants of the MC1R gene are associated with the red hair and fair skin (RHC) phenotype, and carrying one of these variants is thought to diminish the ability of the epidermis to respond to DNA damage elicited by UVR. The MC1R gene is considered a melanoma susceptibility gene, and its significance in determining the risk for skin cancer is of tremendous interest.     

The melanocortin 1 receptor (MC1R): more than just red hair

The melanocortin 1 receptor, a seven pass transmembrane G protein coupled receptor, is a key control point in melanogenesis. Loss-of-function mutations at the MC1R are associated with a switch from eumelanin to phaeomelanin production, resulting in a red or yellow coat colour. Activating mutations, in animals at least, lead to enhanced eumelanin synthesis. In man, a number of loss-of-function mutations in the MC1R have been described. The majority of red-heads (red-haired persons) are compound heterozygotes or homozygotes for up to five frequent loss-of-function mutations. A minority of redheads are, however, only heterozygote. The MC1R is, therefore, a major determinant of sun sensitivity and a genetic risk factor for melanoma and non-melanoma skin cancer. Recent work suggests that the MC1R also shows a clear heterozygote effect on skin type, with up to 30% of the population harbouring loss-of-function mutations. Activating mutations of the MC1R in man have not been described. The MC1R is particularly informative and a tractable gene for studies of human evolution and migration. In particular, study of the MC1R may provide insights into the lightening of skin colour observed in most European populations. The world wide pattern of MC1R diversity is compatible with functional constraint operating in Africa, whereas the greater allelic diversity seen in non-African populations is consistent with neutral predictions rather than selection. Whether this conclusion is as a result of weakness in the statistical testing procedures applied, or whether it will be seen in other pigment genes will be of great interest for studies of human skin colour evolution.