The interaction of agouti signal protein and melanocyte stimulating hormone to regulate melanin formation in mammals

Siberian cats help in solving part of the mystery surrounding golden cats

Golden cats have been appreciated since the beginning of the cat fancy. Golden is a modification of the tabby coat. In the Siberian breed, a specific golden phenotype, named sunshine, has been described. Sunshine tabby cats exhibit a warm tone of tabby, a pink nose lacking the black lining and a large light cream area around the nose. Pedigree analyses revealed an autosomal recessive inheritance pattern. A single candidate region was identified by genome-wide association study (GWAS) and homozygosity mapping. Within that region, we identified CORIN (Corin, serine peptidase) as a strong candidate gene, since CORIN variants have been identified in mice and tigers with a golden phenotype and CORIN has been described as a modifier of the ASIP (Agouti Signaling Protein) pathway. A homozygous CORIN:c.2383C>T missense variant was identified in sunshine tabby cats. Segregation of the variant was consistent with recessive inheritance. The variant was also found in three Kurilian bobtail cats and in two ToyBob cats from the 99 Lives dataset but genotyping of 106 cats from 13 breeds failed to identify carriers in cats from other breeds. The CORIN:c.2383C>T variant was predicted to change an arginine to a cysteine at position 795 in the protein: CORIN:p.(Arg795Cys). Finally, hair observation in Siberian cats was consistent with elongated ASIP signaling as golden hair showed a large yellow band instead of the short subapical one usually observed in agouti hair. These results support an association of the Siberian sunshine modification with the CORIN:c.2383C>T variant. The Siberian cat has helped us to decipher one of the golden phenotypes observed in cats and we propose that the CORIN:c.2383C>T variant represents the wb^SIB (Siberian recessive wideband) allele in the domestic cat.

Examining the molecular basis of coat color in a nocturnal primate family (Lorisidae)

Organisms use color for camouflage, sexual signaling, or as a warning sign of danger. Primates are one of the most vibrantly colored Orders of mammals. However, the genetics underlying their coat color are poorly known, limiting our ability to study molecular aspects of its evolution. The role of the melanocortin 1 receptor (MC1R) in color evolution has been implicated in studies on rocket pocket mice (Chaetodipus intermediusi), toucans (Ramphastidae), and many domesticated animals. From these studies, we know that changes in MC1R result in a yellow/red or a brown/black morphology. Here, we investigate the evolution of MC1R in Lorisidae, a monophyletic nocturnal primate family, with some genera displaying high contrast variation in color patterns and other genera being monochromatic. Even more unique, the Lorisidae family has the only venomous primate: the slow loris (Nycticebus). Research has suggested that the contrasting coat patterns of slow lorises are aposematic signals for their venom. If so, we predict the MC1R in slow lorises will be under positive selection. In our study, we found that Lorisidae MC1R is under purifying selection (ω = 0.0912). In Lorisidae MC1R, there were a total of 75 variable nucleotides, 18 of which were nonsynonymous. Six of these nonsynonymous substitutions were found on the Perodicticus branch, which our reconstructions found to be the only member of Lorisidae that has predominantly lighter coat color; no substitutions were associated with Nycticebus. Our findings generate new insight into the genetics of pelage color and evolution among a unique group of nocturnal mammals and suggest putative underpinnings of monochromatic color evolution in the Perodicticus lineage.

RNA-seq profiling of skin in temperate and tropical cattle

Skin is a major thermoregulatory organ in the body controlling homeothermy, a critical function for climate adaptation. We compared genes expressed between tropical- and temperate-adapted cattle to better understand genes involved in climate adaptation and hence thermoregulation. We profiled the skin of representative tropical and temperate cattle using RNA-seq. A total of 214,754,759 reads were generated and assembled into 72,993,478 reads and were mapped to unique regions in the bovine genome. Gene coverage of unique regions of the reference genome showed that of 24,616 genes, only 13,130 genes (53.34%) displayed more than one count per million reads for at least two libraries and were considered suitable for downstream analyses. Our results revealed that of 255 genes expressed differentially, 98 genes were upregulated in tropically-adapted White Fulani (WF; Bos indicus) and 157 genes were down regulated in WF compared to Angus, AG (Bos taurus). Fifteen pathways were identified from the differential gene sets through gene ontology and pathway analyses. These include the significantly enriched melanin metabolic process, proteinaceous extracellular matrix, inflammatory response, defense response, calcium ion binding and response to wounding. Quantitative PCR was used to validate six representative genes which are associated with skin thermoregulation and epithelia dysfunction (mean correlation 0.92; p < 0.001). Our results contribute to identifying genes and understanding molecular mechanisms of skin thermoregulation that may influence strategic genomic selection in cattle to withstand climate adaptation, microbial invasion and mechanical damage.

Population structure, genetic diversity and selection signatures within seven indigenous Pakistani goat populations

Goat farming in Pakistan depends on indigenous breeds that have adapted to specific agro-ecological conditions. Pakistan has a rich resource of goat breeds, and the genetic diversity of these goat breeds is largely unknown. In this study, genetic diversity and population structure were characterized from seven indigenous goat breeds using the goat 50K SNP chip. The genetic diversity analysis showed that Bugi toori goats have the highest inbreeding level, consistent with the highest linkage disequilibrium, lowest diversity and long run of heterozygosity segments. This indicates that this breed should be prioritized in future conservation activities. The population structure analysis revealed four fairly distinct clusters (including Bugi toori, Bari, Black Tapri and some Kamori) and three other breeds that are seemingly the results of admixture between these or related groups (some Kamori, Pateri, Tapri and White Tapri). The selection signatures were evaluated in each breed. A total of 2508 putative selection signals were reported. The 26 significant windows were identified in more than four breeds, and selection signatures spanned several genes that directly or indirectly influence traits included coat colour variation (KIT), reproduction (BMPR1B, GNRHR, INSL6, JAK2 and EGR4), body size (SOCS2), ear size (MSRB3) and milk composition (ABCG2, SPP1, CSN1S2, CSN2, CSN3 and PROLACTIN).

Characterizing neutral genomic diversity and selection signatures in indigenous populations of Moroccan goats (Capra hircus) using WGS data

Since the time of their domestication, goats (Capra hircus) have evolved in a large variety of locally adapted populations in response to different human and environmental pressures. In the present era, many indigenous populations are threatened with extinction due to their substitution by cosmopolitan breeds, while they might represent highly valuable genomic resources. It is thus crucial to characterize the neutral and adaptive genetic diversity of indigenous populations. A fine characterization of whole genome variation in farm animals is now possible by using new sequencing technologies. We sequenced the complete genome at 12× coverage of 44 goats geographically representative of the three phenotypically distinct indigenous populations in Morocco. The study of mitochondrial genomes showed a high diversity exclusively restricted to the haplogroup A. The 44 nuclear genomes showed a very high diversity (24 million variants) associated with low linkage disequilibrium. The overall genetic diversity was weakly structured according to geography and phenotypes. When looking for signals of positive selection in each population we identified many candidate genes, several of which gave insights into the metabolic pathways or biological processes involved in the adaptation to local conditions (e.g., panting in warm/desert conditions). This study highlights the interest of WGS data to characterize livestock genomic diversity. It illustrates the valuable genetic richness present in indigenous populations that have to be sustainably managed and may represent valuable genetic resources for the long-term preservation of the species.

Modulation of social behavior by the agouti pigmentation gene

Agouti is a secreted neuropeptide that acts as an endogenous antagonist of melanocortin receptors. Mice and rats lacking agouti (called non-agouti) have dark fur due to a disinhibition of melanocortin signaling and pigment deposition in the hair follicle. Non-agouti animals have also been reported to exhibit altered behavior, despite no evidence for the expression of agouti outside the skin. Here we confirm that non-agouti mice show altered social behavior and uncover expression of agouti in the preputial gland, a sebaceous organ in the urinary tract that secretes molecules involved in social behavior. Non-agouti mice had enlarged preputial glands and altered levels of putative preputial pheromones and surgical removal of the gland reversed the behavioral phenotype. These findings demonstrate the existence of an autologous, out-of-skin pathway for the modulation of social behavior.

Susceptibility to Cutaneous Squamous Cell Carcinoma in Renal Transplant Recipients Associates with Genes Regulating Melanogenesis Independent of their Role in Pigmentation

The highly polymorphic melanocortin 1 receptor (MC1R) gene plays a crucial role in pigmentation. Variants of the gene have been implicated in risk of cutaneous squamous cell carcinoma (SCC) in the general population. In renal transplant (RT) recipients these cancers are more aggressive and very common. To evaluate the risk of SCC relative to MC1R and the pigmentation-associated genes ASIP, TYR, and TYRP1, a group of 217 RT recipients with and without SCC was genotyped. Associations with SCC risk were indicated in carriers of the red hair color associated MC1R variant p.Arg151Cys (OR = 1.99; 1.05-3.75), and in carriers of two of any of the MC1R variants disclosed (OR = 2.36; 1.08-5.15). These associations appeared independent of traditionally protective phenotypes, also supported by the stratifications from skin phototype and hair color. A tendency towards an increased SCC risk was observed for a specific ASIP haplotype (OR = 1.87; 0.91-3.83), while no such associations were observed for the TYR and TYRP1 variants. Thus, the risk of developing SCC in RT patients is modulated by MC1R variation irrespective of phenotypes considered to be protective. Heterozygous combinations of MC1R variants appear to be more relevant in assessing SCC risk than the effects of variants individually.

Mechanism of Mitf inhibition and morphological differentiation effects of hirsein A on B16 melanoma cells revealed by DNA microarray

BACKGROUND

We have previously reported that hirsein A inhibits melanogenesis in B16 melanoma cells by downregulating the Mitf gene expression.

OBJECTIVE

In this study, microarray was employed to determine the transcriptional response of B16 cells to hirsein A (HA) treatment and to find out the mechanism underlying Mitf downregulation.

METHODS

DNA microarray, spotted with 265 genes for melanogenesis and signal transduction, was performed using the total RNA isolated from B16 cells treated with HA. Validation of the results was done using real-time PCR. In addition, real-time PCR using primers for Mda-7 gene and F-actin staining were performed. Transfection experiments were performed to knockdown the expression of the Mc1r gene to evaluate its role in the cell morphological change observed.

RESULTS

As expected, the expressions of the Mitf-regulated melanosome transport genes and the Mc1r gene were downregulated. Furthermore, the expressions of the MAPK pathway intermediates were either up- or downregulated. Genes associated with cell differentiation, such as Gadd45b, were upregulated and prompted us to determine the expression of the Il-24 (Mda-7) gene using real-time PCR. There was an increase in the Mda-7 mRNA expression in B16 and HMV-II melanoma cells, and in human melanocytes. To better visualize the cell morphology, F-actin staining was performed and the results showed an increase in the dendrite outgrowth in HA-treated cells. Silencing the Mc1r gene did not cause a change in the B16 cell morphology observed in cells treated with HA.

CONCLUSION

This study demonstrated that HA downregulates Mitf gene expression by regulating the expressions of the MAPK signaling pathway intermediates. In addition, the inhibited Mc1r gene expression also contributed to the overall Mitf downregulation but does not play a role in the observed change in B16 cell morphology. HA surprisingly can regulate genes associated with differentiating cells (Mda-7) suggesting a role for HA in the melanoma cell differentiation induction. While the exact molecular mechanism by which HA promotes cell differentiation remain to be determined, it is clear that HA can downregulate Mitf expression and promote cell differentiation and has the potential to be used in the development of therapy for melanoma.

Epigenetic programming by maternal nutrition: shaping future generations

Within the Western world's aging and increasingly overweight population, we are seeing an increasing prevalence of adult-onset, lifestyle-related disease such as diabetes, hypertension and atherosclerosis. There is significant evidence that suboptimal nutrition in pregnancy can lead to an increased risk of these diseases developing in offspring, and that this increased risk can be heritable. Thus, poor in utero nutrition may be a major contributor to the current cycle of obesity. While the molecular basis of this phenomenon is unknown, available evidence suggests that it can be mediated by epigenetic changes to gene expression. Here, we discuss epigenetics as a mediator of disease risk in response to nutritional cues. The potential for maternal nutrition to heritably alter epigenetic states may have implications for population health and adaptive evolution.

Skin-lightening products revisited

Skin colour typology depends on the amount and location of its chromophores. Among them, eumelanins derived from 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and 5,6-dihydroxyindole (DHI), and phaeomelanins are of utmost importance. These biomolecules result from the multi-step enzymatic and non-enzymatic conversion of tyrosine into melanins. Pigmentation disorders are multiple and depend on alterations in the density in active melanocytes, and on specific abnormalities of any of the complex melanogenesis mechanisms. This review presents some of the main skin-lightening agents with respect to their mechanisms of action and side-effects. Some of the novel compounds may lead to new perspectives in the fields of dermatology and cosmetology. The methods commonly used to assess efficacy of skin-lightening products rely on in vitro models including cell-free enzymatic assays, melanocyte cultures and reconstructed epidermis bioassays. Animal models have little relevance. By contrast, human testing with the support of instrumental evaluations is the most informative.

Genetic variants in pigmentation genes, pigmentary phenotypes, and risk of skin cancer in Caucasians

Human pigmentation is a polygenic quantitative trait with high heritability. Although a large number of single nucleotide polymorphisms (SNPs) have been identified in pigmentation genes, very few SNPs have been examined in relation to human pigmentary phenotypes and skin cancer risk. We evaluated the associations between 15 SNPs in 8 candidate pigmentation genes (TYR, TYRP1, OCA2, SLC24A5, SLC45A2, POMC, ASIP and ATRN) and both pigmentary phenotypes (hair color, skin color and tanning ability) and skin cancer risk in a nested case-control study of Caucasians within the Nurses' Health Study (NHS) among 218 melanoma cases, 285 squamous cell carcinoma (SCC) cases, 300 basal cell carcinoma (BCC) cases and 870 common controls. We found that the TYR Arg402Gln variant was significantly associated with skin color (p-value = 7.7 x 10(-4)) and tanning ability (p-value = 7.3 x 10(-4)); the SLC45A2 Phe374Leu variant was significantly associated with hair color (black to blonde) (p-value = 2.4 x 10(-7)), skin color (p-value = 1.1 x 10(-7)) and tanning ability (p-value = 2.5 x 10(-4)). These associations remained significant after controlling for MC1R variants. No significant associations were found between these polymorphisms and the risk of skin cancer. We observed that the TYRP1 rs1408799 and SLC45A2 1721 C>G were associated with melanoma risk (OR, 0.77; 95% CI, 0.60-0.98 and OR, 0.75; 95% CI, 0.60-0.95, respectively). The TYR Ser192Tyr was associated with SCC risk (OR, 1.23; 95% CI, 1.00-1.50). The TYR haplotype carrying only the Arg402Gln variant allele was significantly associated with SCC risk (OR, 1.35; 95% CI, 1.04-1.74). The OCA2 Arg419Gln and ASIP g.8818 A>G were associated with BCC risk (OR, 1.50; 95% CI, 1.06-2.13 and OR, 0.73; 95% CI, 0.53-1.00, respectively). The haplotype near ASIP (rs4911414[T] and rs1015362[G]) was significantly associated with fair skin color (OR, 2.28; 95% CI, 1.46-3.57) as well as the risks of melanoma (OR, 1.68; 95% CI, 1.18-2.39) and SCC (OR, 1.54; 95% CI, 1.08-2.19). These associations remained similar after adjusting for pigmentary phenotypes and MC1R variants. The statistical power of our study was modest and additional studies are warranted to confirm the associations observed in the present study. Our study provides evidence for the contribution of pigmentation genetic variants, in addition to the MC1R variants, to variation in human pigmentary phenotypes and possibly the development of skin cancer.

Characterisation of the melanocortin-1 receptor gene in alpaca and identification of possible markers associated with phenotypic variations in colour

The aim of this study was to determine if any correlation exists between melanocortin-1 receptor (MC1R) polymorphisms and skin and fibre colour in alpacas. Primers capable of amplifying the entire alpaca MC1R gene were designed from a comparative alignment of Bos taurus and Mus musculus MC1R gene sequences. The complete MC1R gene of 41 alpacas exhibiting a range of fibre colours, and which were sourced from farms across Australia, was sequenced from PCR products. Twenty-one single nucleotide polymorphisms were identified within MC1R. Two of these polymorphisms (A82G and C901T) have the potential to reduce eumelanin production by disrupting the activity of MC1R. No agreement was observed between fibre colour alone and MC1R genotype in the 41 animals in this study. However, when the animals were assigned to groups based on the presence or absence of eumelanin in their fibre and skin, only animals that had at least one allele with the A82/C901 combination expressed eumelanin. We propose that A82/C901 is the wild-type dominant ‘E’ MC1R allele, while alpacas with either G82/T901 or G82/Y901 are homozygous for the recessive ‘e’ MC1R allele and are therefore unable to produce eumelanin.

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.

Involvement of dynein and spectrin with early melanosome transport and melanosomal protein trafficking

Melanosomes are unique membrane-bound organelles specialized for the synthesis and distribution of melanin. Mechanisms involved in the trafficking of proteins to melanosomes and in the transport of mature pigmented melanosomes to the dendrites of melanocytic cells are being characterized, but details about those processes during early stages of melanosome maturation are not well understood. Early melanosomes must remain in the perinuclear area until critical components are assembled. In this study, we characterized the processing of two distinct melanosomal proteins, tyrosinase (TYR) and Pmel17, to elucidate protein processing in early or late steps of the secretory pathway, respectively, and to determine mechanisms underlying the subcellular localization and transport of early melanosomes. We used immunological, biochemical, and molecular approaches to demonstrate that the movement of early melanosomes in the perinuclear area depends primarily on microtubules but not on actin filaments. In contrast, the trafficking of TYR and Pmel17 depends on cytoplasmic dynein and its interaction with the spectrin/ankyrin system, which is involved with the sorting of cargo from the plasma membrane. These results provide important clues toward understanding the processes involved with early events in melanosome formation and transport.

Melanocyte biology and skin pigmentation

Melanocytes are phenotypically prominent but histologically inconspicuous skin cells. They are responsible for the pigmentation of skin and hair, and thereby contribute to the appearance of skin and provide protection from damage by ultraviolet radiation. Pigmentation mutants in various species are highly informative about basic genetic and developmental pathways, and provide important clues to the processes of photoprotection, cancer predisposition and even human evolution. Skin is the most common site of cancer in humans. Continued understanding of melanocyte contributions to skin biology will hopefully provide new opportunities for the prevention and treatment of skin diseases.

The insertion of a full-length Bos taurus LINE element is responsible for a transcriptional deregulation of the Normande Agouti gene

Mammalian pigmentation is controlled by the concerted action of Tyr, Tyrp1 and Dct producing eumelanin and/or pheomelanin in melanocytes. The ratio of these two pigments is determined by the agonist alpha-melanocyte stimulating hormone and the antagonist Agouti protein acting on the Mc1r. Here we show that the Agouti gene is over-expressed in Normande breed compared with Prim'Holstein breed. The Normande cattle have a characteristic coat color phenotype with a variable presence of black (eumelanin) hair over a red/brown background. We have found a previously undescribed full-length L1-BT element inserted in the 5'-genomic sequence of the Agouti gene in Normande cattle which promotes the over-expression of alternative transcripts. The variable expression of the alternative transcript directed by the long interspersed nuclear element promoter may be the origin of the brindle coat color pattern of the Normande breed. This new bovine Agouti allele isolated in Normande breed has been named Abr. Finally, as ectopic over-expression of Agouti in Ay mice is responsible for the obesity syndrome, we discuss the possible consequences of Abr for meat and milk production in cattle.

[Nle4-D-Phe7]-alpha-melanocyte-stimulating hormone significantly increased pigmentation and decreased UV damage in fair-skinned Caucasian volunteers

Epidermal melanin reduces some effects of UV radiation, the major cause of skin cancer. To examine whether induced melanin can provide protection from sunburn injury, 65 subjects completed a trial with the potent synthetic melanotropin, [Nle4-D-Phe7]-alpha-melanocyte-stimulating hormone ([Nle4-D-Phe7]-alpha-MSH) delivered by subcutaneous injection into the abdomen at 0.16 mg/kg for three 10-day cycles over 3 months. Melanin density, measured by reflectance spectroscopy, increased significantly in all [Nle4-D-Phe7]-alpha-MSH-treated subjects. The highest increases were in volunteers with lowest baseline skin melanin levels. In subjects with low minimal erythemal dose (MED) skin type, melanin increased by an average of 41% (from 2.55 to 3.59, P < 0.0001 vs placebo) over eight separate skin sites compared with only 12% (from 4.18 to 4.70, P < 0.0001 vs placebo) in subjects with a high-MED skin type. Epidermal sunburn cells resulting from exposure to 3 MED of UV radiation were reduced by more than 50% after [Nle4-D-Phe7]-alpha-MSH treatment in the volunteers with low baseline MED. Thymine dimer formation was also shown to be reduced by 59% (P = 0.002) in the epidermal basal layer. This study has shown for the first time the potential ability of a synthetic hormone that augments melanin production to provide photoprotection to people who normally burn in direct sunlight.

Seasonal pelage color change: news based on a South American Rodent

Mammalian seasonalmolting and color change are known to be influencedby photoperiod changes. Calomys laucha, a South American rodent, exhibits seasonal pelage color change; however, unlike Northern hemisphere rodents, which present a gray or brown color during summer and a whitish color during winter, C. laucha pelage changes from an orange color during summer to a dark gray color during winter. Animals maintained for over a year in stationary photoperiod (LD 12:12h, 22ºC) presented orange pelage color during the summer corresponding month (January), and gray color during the winter corresponding month (July). Same age animals were evaluated during summer or winter months, and also showed different colors. Animals exposed for 12 weeks to summer or winter artificial conditions displayed color change, not according to the environmental conditions, as expected, but similar to that of animals maintained in stationary photoperiod. These results suggest that pelage color change in C. laucha is controlled by an endogenous circannual rhythm. The adaptive function of C. laucha color change is discussed.

Biogenesis of pigment granules: a sensitive way to regulate melanocyte function

Pigmentation not only provides a wide range of cosmetic coloration to the skin, hair and eyes, but also provides the underlying tissue significant protection from ultraviolet (UV) damage, which can lead to photoaging and photocarcinogenesis. The melanin pigment is synthesized and deposited within a unique, membrane-bound organelle termed the melanosome. Recent advances in molecular biology and biochemistry have allowed a greater appreciation of how melanocytes generate this organelle and how its biogenesis, structure and function is regulated by the environment. Melanosomes serve as ideal models for the study of organelle biogenesis, protein trafficking, organelle movement and cell-cell interactions that occur during the transfer of melanosomes to keratinocytes. Our understanding of the mechanisms behind a wide range of human pigmentary diseases have grown remarkably as melanosomes have been unraveled.

Dorsoventral patterning of the mouse coat by Tbx15

Many members of the animal kingdom display coat or skin color differences along their dorsoventral axis. To determine the mechanisms that control regional differences in pigmentation, we have studied how a classical mouse mutation, droopy ear (de^H), affects dorsoventral skin characteristics, especially those under control of the Agouti gene. Mice carrying the Agouti allele black-and-tan (a^t) normally have a sharp boundary between dorsal black hair and yellow ventral hair; the de^H mutation raises the pigmentation boundary, producing an apparent dorsal-to-ventral transformation. We identify a 216 kb deletion in de^H that removes all but the first exon of the Tbx15 gene, whose embryonic expression in developing mesenchyme correlates with pigmentary and skeletal malformations observed in de^H/de^H animals. Construction of a targeted allele of Tbx15 confirmed that the de^H phenotype was caused by Tbx15 loss of function. Early embryonic expression of Tbx15 in dorsal mesenchyme is complementary to Agouti expression in ventral mesenchyme; in the absence of Tbx15, expression of Agouti in both embryos and postnatal animals is displaced dorsally. Transplantation experiments demonstrate that positional identity of the skin with regard to dorsoventral pigmentation differences is acquired by E12.5, which is shortly after early embryonic expression of Tbx15. Fate-mapping studies show that the dorsoventral pigmentation boundary is not in register with a previously identified dermal cell lineage boundary, but rather with the limb dorsoventral boundary. Embryonic expression of Tbx15 in dorsolateral mesenchyme provides an instructional cue required to establish the future positional identity of dorsal dermis. These findings represent a novel role for T-box gene action in embryonic development, identify a previously unappreciated aspect of dorsoventral patterning that is widely represented in furred mammals, and provide insight into the mechanisms that underlie region-specific differences in body morphology.

Greg Barsh and colleagues show that a member of the well-known family of T-box genes helps to form an important pigmentation boundary in mice

The Seiji memorial lecture: the melanosome: an ideal model to study cellular differentiation

Melanosomes provide an intriguing model for study at many levels. In part this is due to their unique structure and function, but also in part to their involvement in pigmentary diseases and as a model to study basic cellular mechanisms of organelle biogenesis. Recent studies have elucidated the full proteome of the melanosome and the metabolic and molecular lesions involved in a number of pigmentary diseases have been resolved. This paper summarizes recent advances in the field in these areas.

The biology of melanocytes

In veterinary medicine, our understanding of the biology and regulation of melanocytic function is mostly based on information realized from human and murine studies. Improved understanding of the biology of melanocytes is needed to develop more effective treatment regimens for malignant melanoma and other melanocytic disorders. In vertebrates, melanocytes are well known for their role in skin pigmentation, hair and feather coloration, and for their ability to produce and distribute melanin to surrounding keratinocytes. Enzymes involved in melanin synthesis are present exclusively in melanosomes. The type of melanin synthesized by melanocytes in mammals is regulated at a genetic, biochemical and environmental level. These regulatory factors affect not only the phenotypic appearance, but also the photoprotective properties of melanin. This review addresses the biology of melanocytes, melanin synthesis and the photoprotective properties of melanin.

The mouse ocular albinism 1 gene product is an endolysosomal protein

To gain insight into the role of Oa1, the mouse homolog of the human X-linked ocular albinism 1 protein, its properties and subcellular localization were investigated. Antiserum raised against an expressed segment of the Oa1 protein recognized a band of approximately 48 kDa in immunoblots of extracts of cultured mouse melan-a melanocytes, but not of cells of non-melanocyte origin. When melanocyte extracts were treated with glycopeptidase F, a approximately 44 kDa band appeared. Like the melanogenic enzyme tyrosinase, expression of Oa1 was stimulated by alpha-melanocyte stimulating hormone and inhibited by agouti signal protein. Upon density gradient centrifugation of organelles of melan-a cells, Oa1 protein colocalized with the late endosomal/lysosomal marker Lamp1, but only partial overlap was observed with melanosomal proteins in the high density region of the gradient. Immunofluorescence staining revealed that neither endogenous Oa1 nor an Oa1-green fluorescent protein fusion product colocalized with the melanosomal protein tyrosinase related protein-1 in the cell periphery. In contrast, colocalization of Oa1 and Oa1-green fluorescent protein fusion product with Lamp1 was extensive throughout the cell. These results indicate that Oa1 is a melanocyte-specific integral membrane glycoprotein localized to late endosomes/lysosomes but not mature melanosomes. Considering the microscopic findings in patients with X-linked ocular albinism 1, we speculate that Oa1 may play a role in the trafficking of vesicles to developing melanosomes.

AgRP(83-132) acts as an inverse agonist on the human-melanocortin-4 receptor

The central melanocortin (MC) system has been demonstrated to act downstream of leptin in the regulation of body weight. The system comprises alpha-MSH, which acts as agonist, and agouti-related protein (AgRP), which acts as antagonist at the MC3 and MC4 receptors (MC3R and MC4R). This property suggests that MCR activity is tightly regulated and that opposing signals are integrated at the receptor level. We here propose another level of regulation within the melanocortin system by showing that the human (h) MC4R displays constitutive activity in vitro as assayed by adenylyl cyclase (AC) activity. Furthermore, human AgRP(83-132) acts as an inverse agonist for the hMC4R since it was able to suppress constitutive activity of the hMC4R both in intact B16/G4F melanoma cells and membrane preparations. The effect of AgRP(83-132) on the hMC4R was blocked by the MC4R ligand SHU9119. Also the hMC3R and the mouse(m)MC5R were shown to be constitutively active. AgRP(83-132) acted as an inverse agonist on the hMC3R but not on the mMC5R. Thus, AgRP is able to regulate MCR activity independently of alpha-MSH. These findings form a basis to further investigate the relevance of constitutive activity of the MC4R and of inverse agonism of AgRP for the regulation of body weight.

Bioactive motifs of agouti signal protein

The switch between the synthesis of eu- and pheomelanins is modulated by the interaction of two paracrine signaling molecules, alpha-melanocyte stimulating hormone (MSH) and agouti signal protein (ASP), which interact with melanocytes via the MSH receptor (MC1R). Comparison of the primary sequence of ASP with the known MSH pharmacophore provides no suggestion about the putative bioactive domain(s) of ASP. To identify such bioactive motif(s), we synthesized 15-mer peptides that spanned the primary sequence of ASP and determined their effects on the melanogenic activities of murine melanocytes. Northern and Western blotting were used, together with chemical analysis of melanins and enzymatic assays, to identify three distinct bioactive regions of ASP that down-regulate eumelanogenesis. The decrease in eumelanin production was mediated by down-regulation of mRNA levels for tyrosinase and other melanogenic enzymes, as occurs in vivo, and these effects were comparable to those elicited by intact recombinant ASP. Shorter peptides in those motifs were synthesized and their effects on melanogenesis were further investigated. The amino acid arginine, which is present in the MSH peptide pharmacophore (HFRW), is also in the most active domain of ASP (KVARP). Our data suggest that lysines and an arginine (in motifs such as KxxxxKxxR or KxxRxxxxK) are important for the bioactivity of ASP. Identification of the specific ASP epitope that interacts with the MC1R has potential pharmacological applications in treating dysfunctions of skin pigmentation.

Activation of a cAMP pathway and induction of melanogenesis correlate with association of p16(INK4) and p27(KIP1) to CDKs, loss of E2F-binding activity, and premature senescence of human melanocytes

There is strong evidence that the senescent phenotype, whether induced by telomere shortening, oxidative damage, or oncogenic stimuli, is an important tumor suppressive mechanism. The melanocyte is a cell of neural crest origin that produces the pigment melanin and can develop into malignant melanomas. To understand how malignant cells escape senescence, it is first crucial to define what genes control senescence in the normal cell. Prolonged exposure to high levels of cAMP results in accumulation of melanin and terminal differentiation of human melanocytes. Here we present evidence that activation of a cAMP pathway correlates with multiple cellular changes in these cells: (1) increased expression of the transcription factor microphthalmia; (2) increased melanogenesis; (3) increased association of the cyclin-dependent kinase inhibitors (CDK-Is) p27(KIP1) and p16(INK4) with CDK2 and CDK4, respectively; (4) failure to phosphorylate the retinoblastoma protein (pRB); (5) decreased expression of E2F1, E2F2, and E2F4 proteins; (6) loss of E2F DNA-binding activity; and (7) phenotypic changes characteristic of senescent cells. Senescent melanocytes have potent E2F inhibitory activity, because extracts from these cells completely abolished E2F DNA-binding activity that was present in extracts from the early proliferative phase. We propose that increased activity of the CDK-Is p27 and p16 and loss of E2F activity in human melanocytes characterize a senescence program activated by the cAMP pathway. Disruption of cAMP-mediated and melanogenesis-induced senescence may cause immortalization of human melanocytes, an early step in the development of melanomas.

POMC-derived peptides and their biological action

It has long been known that a large number of POMC-related peptides are found in skin. In this introduction I describe the formation of POMC-derived peptides in various tissues to indicate that processing is largely tissue-dependent. I focus on the peptides from the N-terminal fragment, such as gamma-MSH, ACTH and alpha-MSH, and beta-lipopropin as well as beta-endorphin. I touch on the factors that control the synthesis of the various peptides, which are now numerous and varied, and again are tissue specific. The biologic activity of the peptides generated from POMC are described in relation to their possible action in skin. In addition, I describe a new class of peptides induced in skin following injury and which are of great interest.

Agouti-related maturation and tissue distribution of alpha-Melanocyte Stimulating Hormone in wild-type (AwJ/AwJ) and mutant (Ay/a,a/a) mice

Because of ectopic overproduction of agouti protein, yellow alleles (A(y) and A(vy)) of the murine agouti gene may secondarily modulate the synthesis, maturation (i.e., acetylation), and/or tissue deployment of alpha-Melanocyte Stimulating Hormone (MSH). We used HPLC to test the hypothesis that A(y)/a mice exhibit altered concentrations of desacetyl-, monoacetyl-, and diacetyl-alpha-MSH in pituitaries, sera, and telogen hair bulbs when compared to black (a/a) mice. We also used RIA to measure total MSH in those same tissues of A(y)a,a/a, and white-bellied agouti (A(wJ)/A(wJ)) mice (Strain C57BL/6J). We found no evidence that A(y)/a mice possessed an imbalance of des-, mono-, and diacetylated alpha-MSH species. However, radioimmunoassay (RIA) analyses of total MSH suggest that wild-type agouti mice (A(wJ)/A(wJ)) exhibited significantly decreased (P < 0.05) tissue levels of total alpha-MSH in pituitaries, sera, and regenerating hair bulbs when compared to those of mutant A(y)/a and a/a mice.

What controls melanogenesis?

The pigments eumelanin and pheomelanin are the visually most striking products of specialized neural crest-derived cells (melanocytes), and provide color to both epidermis and hair shafts. While the intriguing and controversial biological functions of these multifaceted heteropolymers will be discussed in a later feature, here it is explored how their generation (melanogenesis) is controlled. For decades, this has been the object of much controversy, the salient features of which are delineated in the following contributions.

[Oculocutaneous albinism: clinical, historical and anthropological aspects]

Albinism represents a group of inherited abnormalities of the melanin pigment system in which the synthesis of melanin is absent or reduced, generalized (oculocutaneous albinism) or localised (ocular albinism). Recent molecular studies provide insight into the pathophysiological processes of pigmentation regulation and help our understanding of the genetic heterogeneity of human albinism. It rarely affects Europeans, frequently Africans, only a minority of Amerindians, who nevertheless, when an ethnic group is concerned, presents one of the highest incidence in the world. Historically, the African albinos were used as an alibi by the European theologians to support Adam's descent of humanity and by naturalists to affirm the alleged superiority of the white men. Anthropological data are mainly issued from Amerindians with contradictories attitudes towards albinos: both acceptance and rejection. Only the Kuna of Panama have given albinos a major place in their mythology, although in reality they frequently reject them.

Tyrosine transport into melanosomes is increased following stimulation of melanocyte differentiation

A variety of physiological factors can stimulate differentiation of melanocytes to increase pigmentation, and critical to this process is the transport of the melanogenic substrate (tyrosine) into melanosomes. In this study, we examined whether stimulation of melanogenesis affects melanosomal tyrosine transport. Tyrosine uptake increased almost 2-fold in melanosomes derived from melanocytes treated with melanocyte-stimulating hormone (MSH), which acts to increase intracellular cAMP levels, resulting in the up-regulation of many genes involved in melanogenesis. Stimulation of melanoma cells with dibutyryl cAMP increased melanosomal tyrosine transport 2- to 3-fold after 24 to 48 hrs, with peak levels occurring after 3 to 5 days of treatment, suggesting that de novo gene expression may be required. The cAMP-induced increase in melanosomal tyrosine transport could be effectively competed with phenylalanine or tryptophan, but not with dopamine or proline, suggesting either that a pool of transporters with greater tyrosine transporting ability pre-exists, or that a greater number of tyrosine transporters reside within the melanosomal membrane. These results illustrate a rare example of hormonal plasma membrane stimulation which transduces a signal for increased vesicular transport of an amino acid.

Characterization of genes modulated during pheomelanogenesis using differential display

Molecular and biochemical mechanisms that modulate the production of eumelanin or pheomelanin pigments involve the opposing effects of two intercellular signaling molecules, alpha-melanocyte stimulating hormone (MSH) and agouti signal protein (ASP). ASP is an antagonist of MSH signaling through the melanocyte-specific MSH receptor, although its mechanism(s) of action is controversial. We previously have reported significant down-regulation of all known melanogenic genes during the eumelanin to pheomelanin switch in murine hair follicle melanocytes and in cultured melanocytes treated with recombinant ASP. To identify factors that might be involved in the switch to pheomelanogenesis, we screened ASP-treated melanocytes by using differential display and identified three up-regulated genes: a DNA replication control protein, a basic helix-loop-helix transcription factor, and a novel gene. We have simultaneously identified six down-regulated genes in ASP-treated melanocytes; two of those encode tyrosinase and TRP2, melanogenic genes known to be down-regulated during pheomelanogenesis, which provide good internal controls for this approach. These results suggest that there are complex mechanisms involved in the switch to pheomelanin production, and that these modulated genes might be involved in the pleiotropic changes seen in yellow mice, including the change in coat color.

The site and time of expression of MIF in frog development

A ventrally localized melanization-inhibiting factor (MIF) has been suggested to play a role in the expression of dorsal-ventral pigment patterns in amphibia. Here we investigate the onset and localization of MIF appearance in frog development. The expression of MIF was analyzed in the wild-type and gray-eyed mutant (g/g) of Rana japonica by immunoblotting and immunohistochemistry using an anti-MIF neutralizing monoclonal antibody. Western blot analysis revealed that the anti-MIF antibody recognized approximately 51 kDa and approximately 58 kDa bands. The 51 kDa band first appeared at the external gill stage, while 58 kDa band was additionally detected at the hindlimb bud stage. With the use of immunohistochemistry, it was found that the anti-MIF antibody stained the whole epidermis of the embryos at the external gill stage; however, the staining was stronger in lateral and ventral epidermis than in dorsal. Staining with the anti-MIF antibody was observed only in the outer epidermis of the ventral skin, but not in the dorsal skin during and after metamorphosis. The spatial expression of MIF in the wild-type was the same as that in the gray-eyed mutant. The same immunohistochemical result was obtained in the adults of R. nigromaculata. These results suggest that MIF is involved in the formation of the dorsal-ventral pigment pattern.

The role of alpha-MSH, its agonists, and C-AMP in in vitro avian melanocytes

Little is known about the effect of alpha-MSH and other melanogenic stimulators on avian melanocytes. Tissue cultures of Barred Plymouth Rock regenerating feather melanocytes were established and the culture medium contained selected concentrations of alpha-MSH and other melanogenic stimulators in Ham's F-10 medium supplemented with antibiotics and 10% new born calf serum. Cultures were maintained at 37 degrees C in 95% air/5% CO2. No increase in melanogenesis over control levels due to the addition of 10(-5) M Forskolin, 10(-4) M IBMX, 10(-3) M c-GMP, and 10(-3) M db-c-AMP was observed in the cultures on days 5 and 7. However, 2.5 (optimum), 5, and 10 micrograms/ml alpha-MSH and 10(-3) M 8-bromo-c-AMP significantly increased melanogenesis over control levels on days 5 and 7. The stimulation of melanogenesis was detectable by a significantly increased number of melanocytes containing numerous stage IV melanosomes. No increase in melanocyte cell number was observed in any of the experimental cultures. The addition of 1, 2 (optimum), or 3 mM calcium did enhance the increased pigmentation effect of 2.5 micrograms/ml alpha-MSH. Two very convincing experiments showed that c-AMP was the second messenger for alpha-MSH in these birds. First, the c-AMP inhibitor, 10(-3) M Rp-c-AMPS, completely inhibited the stimulatory effect of alpha-MSH in these in vitro melanocytes. Second, direct measurements of c-AMP levels in feather tissue showed a significant increase in c-AMP levels 10.min after alpha-MSH treatment. Controls received no alpha-MSH. The results showed that these avian melanocytes have alpha-MSH receptors and were able to respond to the hormone. C-AMP was the second messenger in this system. Apparently db-c-AMP was not able to enter these mature, highly-differentiated cells and c-AMP agonists, Forskolin and IBMX, were also either unable to enter these older cells or, if they did enter the cells, were unable to stimulate c-AMP production. Evidently the more lipophilic 8-bromo-c-AMP was able to enter these cells and stimulate melanogenesis.

Insights into pigmentary phenomena provided by grafting and chimera formation in the axolotl

The expression of pigmentation patterns in axolotl pigmentary mutants was observed following three types of experimental manipulations including chimera formation, reciprocal neural crest grafts, of gonadal primordia. Three pigmentary genes were utilized including the wild type (D), white (d), and albino (a). In chimeras between white and albino embryos, melanoblasts from the white half crossed the graft interface to differentiate in albino skin. Neural crest grafts from white embryos to albinos provided melanophores of white origin that were capable of differentiation in albino skin. Grafts of gonadal primordia from albino to white embryos provided albino germ cells that formed unpigmented ovocytes together with dark ovocytes: white ovocytes from the albino grafted ovary, and dark ovocytes from the host ovary. The donor albino white ectoderm included in the graft was able to support the differentiation of melanophores, iridophores, and xanthophores that invaded the graft ectoderm from the neural crest of the white host. It was concluded that manifestation of the white or wild phenotypes may be related to the possible presence or absence of inhibiting or stimulating pigmentary factors in the skin. This possibility was discussed in the light of recent discoveries of such factors as Agouti Signaling Protein (ASP) from mammalian skin.