Isolation, chromosomal mapping, and expression of the mouse tyrosinase gene

Genetic connection of carotid atherosclerosis with coat color and body weight in an intercross between hyperlipidemic mouse strains

Atherosclerosis in the carotid artery is a major cause of ischemic stroke and has a strong genetic component. The aim of this study was to identify genetic factors contributing to carotid atherosclerosis. 154 female F2mice were generated from an intercross between LP/J and BALB/cJ Apoe-null (Apoe^-/-) mice and fed 12 weeks of Western diet. Atherosclerotic lesions, body weight, and coat color were measured and genotyping was performed using miniMUGA arrays. A significant QTL on chromosome (Chr) 7, named Cath20, and 5 suggestive QTL on Chr 6, 12, 13, 15, and X were identified for carotid lesions. Three significant QTL, Bwfq2, Bw1n, Bwtq6, on Chr 2, 7, and 15 were identified for body weight. Two significant QTL, Chop2 and Albc2, on Chr 4 and 7 were identified for coat color, with Tyr, encoding tyrosinase, being the causal gene of Albc2. Cath20 coincided precisely with QTL Bw1n and Albc2 on Chr7. Carotid lesion sizes were significantly correlated with body weight and graded coat color in F2 mice. Cath20 on Chr7 disappeared after adjustment for coat color but remained after adjustment for body weight. These results demonstrate genetic connections of carotid atherosclerosis with body weight and coat color in hyperlipidemic mice and suggest a potential role for Tyr in carotid atherosclerosis.

Genetic Connection between Hyperglycemia and Carotid Atherosclerosis in Hyperlipidemic Mice

Type 2 diabetes (T2D) is a major risk for atherosclerosis and its complications. Apoe-null (Apoe^−/−) mouse strains exhibit a wide range of variations in susceptibility to T2D and carotid atherosclerosis, with the latter being a major cause of ischemic stroke. To identify genetic connections between T2D and carotid atherosclerosis, 145 male F2 mice were generated from LP/J and BALB/cJ Apoe^−/− mice and fed 12 weeks of a Western diet. Atherosclerotic lesions in the carotid arteries, fasting, and non-fasting plasma glucose levels were measured, and genotyping was performed using miniMUGA arrays. Two significant QTL (quantitative trait loci) on chromosomes (Chr) 6 and 15 were identified for carotid lesions. The Chr15 QTL coincided precisely with QTL Bglu20 for fasting and non-fasting glucose levels. Carotid lesion sizes showed a trend toward correlation with fasting and non-fasting glucose levels in F2 mice. The Chr15 QTL for carotid lesions was suppressed after excluding the influence from fasting or non-fasting glucose. Likely candidate genes for the causal association were Tnfrsf11b, Deptor, and Gsdmc2. These results demonstrate a causative role for hyperglycemia in the development of carotid atherosclerosis in hyperlipidemic mice.

Coloration in Mammals

Mammalian colors and color patterns are some of the most diverse and conspicuous traits found in nature and have been widely studied from genetic/developmental and evolutionary perspectives. In this review we first discuss the proximate causes underlying variation in pigment type (i.e., color) and pigment distribution (i.e., color pattern) and highlight both processes as having a distinct developmental basis. Then, using multiple examples, we discuss ultimate factors that have driven the evolution of coloration differences in mammals, which include background matching, intra- and interspecific signaling, and physiological influences. Throughout, we outline bridges between developmental and functional investigatory approaches that help broaden knowledge of mammals' memorable external appearances, and we point out areas for future interdisciplinary research.

Inhibition of melanin production by anthracenone dimer glycosides isolated from Cassia auriculata seeds

The methanol extract of Cassia auriculata seeds was found to inhibit melanogenesis in B16 melanoma 4A5 cells under conditions of theophylline stimulation. Two new phlegmacin-type anthracenone dimer glycosides, auriculataosides A and B, were isolated from the active methanol fraction, and their inhibitory effects were observed in the concentration range of 0.03 to 0.3 μM. Inhibition of microphthalmia-associated transcription factor, tyrosinase, tyrosinase-related protein (TRP)-1, and TRP-2 protein expression was observed, suggesting that the inhibition of these factors is part of the mechanism of action underlying melanogenesis inhibition.

Regional expression of tyrosinase in central catecholaminergic systems of colored mice

A relationship between coat color and behavioral characteristics has been reported for numerous species. We previously indicated that particular behavioral traits contributing to the genotype at the agouti locus manifest only when possessing a wild-type allele at the albino (i.e., tyrosinase: Tyr) locus. The present study was performed to investigate tyrosinase expression with marked activity in central nervous systems. The whole brain of male B10 and B10-c mice, a B10 congenic strain of the albino locus from BALB/c, at 8 to 9 weeks of age was removed, and obtained several regions of brain, especially catecholaminergic. Comparatively large amounts of Tyr mRNA and its translation products of approximately 68 kDa were found in the regions obtained, and definitely possessed the enzyme activity for the oxidation of L-tyrosine. The present results indicate the possibility that the amount of catecholamines produced in albino mice is higher than that of colored mice due to the deficit in tyrosinase heritably.

Tyrosinase Depletion Prevents the Maturation of Melanosomes in the Mouse Hair Follicle

The mechanisms that lead to variation in human skin and hair color are not fully understood. To better understand the molecular control of skin and hair color variation, we modulated the expression of Tyrosinase (Tyr), which controls the rate-limiting step of melanogenesis, by expressing a single-copy, tetracycline-inducible shRNA against Tyr in mice. Moderate depletion of TYR was sufficient to alter the appearance of the mouse coat in black, agouti, and yellow coat color backgrounds, even though TYR depletion did not significantly inhibit accumulation of melanin within the mouse hair. Ultra-structural studies revealed that the reduction of Tyr inhibited the accumulation of terminal melanosomes, and inhibited the expression of genes that regulate melanogenesis. These results indicate that color in skin and hair is determined not only by the total amount of melanin within the hair, but also by the relative accumulation of mature melanosomes.

C57BL/6N albino/agouti mutant mice as embryo donors for efficient germline transmission of C57BL/6 ES cells

We generated C57BL/6NTac mice carrying a tyrosinase loss-of function mutation and a reversion of the nonagouti locus to agouti. This strain has a high superovulation response, allows visual detection of chimeric coat color contribution of C57BL/6 ES-cells and provides a simplified breeding format that generates black G1 offspring of pure inbred C57BL/6 background in one step, providing the ideal host for genetically manipulated C57BL/6 ES cells.

Albinism-causing mutations in recombinant human tyrosinase alter intrinsic enzymatic activity

Background

Tyrosinase (TYR) catalyzes the rate-limiting, first step in melanin production and its gene (TYR) is mutated in many cases of oculocutaneous albinism (OCA1), an autosomal recessive cause of childhood blindness. Patients with reduced TYR activity are classified as OCA1B; some OCA1B mutations are temperature-sensitive. Therapeutic research for OCA1 has been hampered, in part, by the absence of purified, active, recombinant wild-type and mutant human enzymes.

Methodology/Principal Findings

The intra-melanosomal domain of human tyrosinase (residues 19–469) and two OCA1B related temperature-sensitive mutants, R422Q and R422W were expressed in insect cells and produced in T. ni larvae. The short trans-membrane fragment was deleted to avoid potential protein insolubility, while preserving all other functional features of the enzymes. Purified tyrosinase was obtained with a yield of >1 mg per 10 g of larval biomass. The protein was a monomeric glycoenzyme with maximum enzyme activity at 37°C and neutral pH. The two purified mutants when compared to the wild-type protein were less active and temperature sensitive. These differences are associated with conformational perturbations in secondary structure.

Conclusions/Significance

The intramelanosomal domains of recombinant wild-type and mutant human tyrosinases are soluble monomeric glycoproteins with activities which mirror their in vivo function. This advance allows for the structure – function analyses of different mutant TYR proteins and correlation with their corresponding human phenotypes; it also provides an important tool to discover drugs that may improve tyrosinase activity and treat OCA1.

Oculocutaneous albinism in Suncus murinus: establishment of a strain and identification of its responsible gene

The house musk shrew Suncus murinus (Insectivora, Soricidae) is referred to as suncus in a laboratory context. Although the capture of albino-like shrews (wild suncus) has been reported previously, albino-like strains have never been established, and the molecular basis of the character has remained elusive. We have established an OCAO mutant strain (oculocutaneous albinism Okinawa), from a wild suncus with a white coat and red eyes, which was captured in 2002. During the course of establishing the strain, it was revealed that the albino-like phenotype was inherited in an autosomal recessive manner. To elucidate the molecular basis of this phenotype, we cloned the suncus cDNAs for tyrosinase (Tyr), pink-eyed dilution (p), and solute carrier family 45, member 2 (Slc45a2), since these genes are involved in oculocutaneous albinism in various species, including humans. Several polymorphisms were identified in these genes; however, linkage analysis excluded the involvement of Tyr and p. On the other hand, two amino acid substitutions (V240A and G366E) were identified in Slc45a2 that cosegregated with the phenotype in the OCAO mutant strain. While V240A was also present in colored suncus collected from Okinawa, G366E was unique to the albino-like suncus and heterozygous carriers. Thus, we conclude that a mutation in Slc45a2 (G366E) is responsible for an albino-like phenotype in Suncus murinus.

Distinct distal regulatory elements control tyrosinase expression in melanocytes and the retinal pigment epithelium

Pigment cells of mammals are characterized by two different developmental origins: cells of the retinal pigment epithelium (RPE) originate from the optic cup of the developing forebrain, whereas melanocytes arise from the neural crest. The pigmentation gene tyrosinase is expressed in all pigment cells but differentially regulated in melanocytes and RPE. The tyrosinase promoter does not confer strong expression in pigment cells in vivo, while inclusion of a distal regulatory element at position -15 kb is necessary and sufficient to provide strong expression in melanocytes. Nevertheless, the regulatory elements responsible for correct spatial and temporal tyrosinase expression in the RPE remained unidentified so far. In this report, we show that a 186 kb BAC containing the tyrosinase gene provides transgene expression in both RPE and melanocytes indicating the presence of regulatory sequences required for expression in the RPE. A deletion analysis of the BAC was performed demonstrating that a RPE-regulatory element resides between -17 and -75 kb. Using multi-species comparative genomic analysis we identified three conserved sequences within this region. When tested in transgenic mice one of these sequences located at -47 kb targeted expression to the RPE. In addition, deletion of this regulatory element within a tyrosinase::lacZ BAC provided evidence that this sequence is not only sufficient but also required for correct spatial and temporal expression in the RPE. The identification of this novel element demonstrates that tyrosinase gene expression is controlled by separate distal regulatory sequences in melanocytes and RPE.

Genetics, development and evolution of adaptive pigmentation in vertebrates

The study of pigmentation has played an important role in the intersection of evolution, genetics, and developmental biology. Pigmentation's utility as a visible phenotypic marker has resulted in over 100 years of intense study of coat color mutations in laboratory mice, thereby creating an impressive list of candidate genes and an understanding of the developmental mechanisms responsible for the phenotypic effects. Variation in color and pigment patterning has also served as the focus of many classic studies of naturally occurring phenotypic variation in a wide variety of vertebrates, providing some of the most compelling cases for parallel and convergent evolution. Thus, the pigmentation model system holds much promise for understanding the nature of adaptation by linking genetic changes to variation in fitness-related traits. Here, I first discuss the historical role of pigmentation in genetics, development and evolutionary biology. I then discuss recent empirically based studies in vertebrates, which rely on these historical foundations to make connections between genotype and phenotype for ecologically important pigmentation traits. These studies provide insight into the evolutionary process by uncovering the genetic basis of adaptive traits and addressing such long-standing questions in evolutionary biology as (1) are adaptive changes predominantly caused by mutations in regulatory regions or coding regions? (2) is adaptation driven by the fixation of dominant mutations? and (3) to what extent are parallel phenotypic changes caused by similar genetic changes? It is clear that coloration has much to teach us about the molecular basis of organismal diversity, adaptation and the evolutionary process.

A polymorphism in the agouti signalling protein (ASIP) is associated with decreased levels of mRNA

To date, a role for agouti signalling protein (ASIP) in human pigmentation has not been well characterized. It is known that agouti plays a pivotal role in the pigment switch from the dark eumelanin to the light pheomelanin in the mouse. However, because humans do not have an agouti banded hair pattern, its role in human pigmentation has been questioned. We previously identified a single polymorphism in the 3'-untranslated region (UTR) of ASIP that was found at a higher frequency in African-Americans compared with other population groups. To compare allele frequencies between European-Australians and indigenous Australians, the g.8818A --> G polymorphism was genotyped. Significant differences were seen in allele frequencies between these groups (P < 0.0001) with carriage of the G allele highest in Australian Aborigines. In the Caucasian sample set a strong association was observed between the G allele and dark hair colour (P = 0.004) (odds ratio 4.6; 95% CI 1.4-15.27). The functional consequences of this polymorphism are not known but it was postulated that it might result in message instability and premature degradation of the transcript. To test this hypothesis, ASIP mRNA levels were quantified in melanocytes carrying the variant and non-variant alleles. Using quantitative real-time polymerase chain reaction the mean ASIP mRNA ratio of the AA genotype to the AG genotype was 12 (P < 0.05). This study suggests that the 3'-UTR polymorphism results in decreased levels of ASIP and therefore less pheomelanin production.

An inducible mouse model of melanoma expressing a defined tumor antigen

Cancer immunotherapy based on vaccination with defined tumor antigens has not yet shown strong clinical efficacy, despite promising results in preclinical models. This discrepancy might result from the fact that available preclinical models rely on transplantable tumors, which do not recapitulate the long-term host-tumor interplay that occurs in patients during progressive tumor development and results in tumor tolerance. To create a faithful preclinical model for cancer immunotherapy, we generated a transgenic mouse strain developing autologous melanomas expressing a defined tumor antigen recognized by T cells. We chose the antigen encoded by P1A, a well-characterized murine cancer germ line gene. To transform melanocytes, we aimed at simultaneously activating the Ras pathway and inactivating tumor suppressor Ink4a/Arf, thereby reproducing two genetic events frequently observed in human melanoma. The melanomas are induced by s.c. injection of 4-OH-tamoxifen (OHT). By activating a CreER recombinase expressed from a melanocyte-specific promoter, this treatment induces the loss of the conditional Ink4a/Arf gene in melanocytes. Because the CreER gene itself is also flanked by loxP sites, the activation of CreER also induces the deletion of its own coding sequence and thereby allows melanocyte-specific expression of genes H-ras and P1A, which are located downstream on the same transgene. All melanomas induced in those mice with OHT show activation of the Ras pathway and deletion of gene Ink4a/Arf. In addition, these melanomas express P1A and are recognized by P1A-specific T lymphocytes. This model will allow to characterize the interactions between the immune system and naturally occurring tumors and thereby to optimize immunotherapy approaches targeting a defined tumor antigen.

The Tyr (albino) locus of the laboratory mouse

The albino mouse was already known in ancient times and was apparently selectively bred in Egypt, China, and Japan. Thus, it is not surprising that the c or albino locus (now the Tyr locus) was among the first used to demonstrate Mendelian inheritance in mammals at the dawn of the past century. This locus is now known to encode tyrosinase, the rate-limiting enzyme in the production of melanin pigment, and the molecular basis of the albino ( Tyr(c)) mutation is known. Here we describe the congenic series of Tyr-locus alleles, from wild type to null ( albino). We compare eye and skin pigmentation phenotypes and the genetic lesions that cause each. We suggest that this panel of congenic mutants contains rich, untapped resources for the study of many questions of basic cell biological interest.

Mouse alopecia areata models: an array of data on mechanisms and genetics

Laboratory mice have become the premier animal model for most human and domestic animal diseases, and they have long been the model of choice for studying mammalian genetics, especially since the advent of genetic engineering. Many remarkable discoveries have been made through intense study of these wonderful small mammals, and undoubtably many more will be made. It is no surprise that one mouse model for alopecia areata (AA) has been found possibly many more will be, some of which exhibit rare phenotypes found in subpopulations of humans with the disease, such as nail deformities, thyroid disease, inflammatory bowel disease, and autoimmune polyendocrine syndrome type 1. Intense investigation by many groups into the first model, the adult onset form of AA (using the C3H/HeJ inbred strain), found similarities as well as differences with commonly held ideas concerning human AA. Regardless of some of the controversies, much insight has been gained from studying these and other rodent and domestic animal models which has opened up new ideas and discussions of AA and its treatment.

Coexpression of wild-type tyrosinase enhances maturation of temperature-sensitive tyrosinase mutants

Tyrosinase is a type I membrane glycoprotein whose activity is essential for melanin synthesis. Loss of function mutations in tyrosinase is the cause of oculocutaneous albinism 1. In the milder oculocutaneous albinism 1B form in which mutant proteins retain residual activity, the severity of albinism depends on the type of mutations expressed in the melanocyte. In this study, we show that coexpression of wild-type protein with temperature-sensitive tyrosinase mutants corrects the mutant conformation defect in an activity-dependent manner. Exit from the endoplasmic reticulum and complex carbohydrate processing in the Golgi was promoted when temperature-sensitive tyrosinase mutants were ectopically expressed in host melanocytes carrying wild-type protein even at the nonpermissive temperature. Incubation of transfected melanocytes with DOPA (the cofactor and substrate for tyrosinase), or tyrosine (the substrate), further enhanced processing of ectopic mutant proteins. The analysis of glycosylation-deficient mutants revealed regions in tyrosinase with high, low, and intermediate dependency on glycans for maturation. We concluded that the presence of tyrosinase activity enhances the maturation of temperature-sensitive and glycosylation-deficient forms of tyrosinase. The results may explain the variation in pigmentation and the development of pigment later in life in patients carrying different mutant alleles of oculocutaneous albinism 1B.

Quantitative trait locus mapping in laboratory mice derived from a replicated selection experiment for open-field activity

Bidirectional selection in rodents has been used to derive animal models of human behavior. An important question is whether selection for behavior operates on a limited number of QTL or whether the number and individual contribution of QTL varies between selection experiments. To address this question, we mapped QTL in two large F2 intercrosses (N = 815 and 821) from the four lines derived from a replicated selection experiment for open-field activity, an animal model for susceptibility to anxiety. Our analyses indicate that selection operated on the same relatively small number of loci in both crosses. Haplotype information and the direction of effect of each QTL allele were used to confirm that the QTL mapped in the two crosses lie in the same chromosomal regions, although we were unable to determine whether QTL in the two crosses represent the same genes. We conclude that the genetic architecture of the selected strains is similar and relatively simple.

Endoplasmic reticulum retention is a common defect associated with tyrosinase-negative albinism

Tyrosinase is a melanocyte-specific enzyme critical for the synthesis of melanin, a process normally restricted to a post-Golgi compartment termed the melanosome. Loss-of-function mutations in tyrosinase are the cause of oculocutaneous albinism, demonstrating the importance of the enzyme in pigmentation. In the present study, we explored the possibility that trafficking of albino tyrosinase from the endoplasmic reticulum (ER) to the Golgi apparatus and beyond is disrupted. Toward this end, we analyzed the common albino mouse mutation Tyr(C85S), the frequent human albino substitution TYR(T373K), and the temperature-sensitive tyrosinase TYR(R402Q)/Tyr(H402A) found in humans and mice, respectively. Intracellular localization was monitored in albino melanocytes carrying the native mutation, as well as in melanocytes ectopically expressing green fluorescent protein-tagged tyrosinase. Enzymatic characterization of complex glycans and immunofluorescence colocalization with organelle-specific resident proteins established that all four mutations produced defective proteins that were retained in the ER. TYR(R402Q)/Tyr(H402A) Golgi processing and transport to melanosomes were promoted at the permissive temperature of 32 degrees C, but not at the nonpermissive 37 degrees C temperature. Furthermore, evidence of protein misfolding was demonstrated by the prolonged association of tyrosinase mutants with calnexin and calreticulin, known ER chaperones that play a key role in the quality-control processes of the secretory pathway. From these results we concluded that albinism, at least in part, is an ER retention disease.

Stimulation of melanogenesis in a human melanoma cell line by bistratene A

The polyether toxin, bistratene A, induced morphological and functional differentiation of a human melanoma cell line (MM96E). The cells became blocked at the G2/M transition and elaborated a number of processes. Tyrosinase activity and melanin content were substantially increased. Northern blot analysis showed up-regulation of mRNA for several genes known to be involved in melanin biosynthesis (pmel17, pmel34, and tyrosinase related proteins, TRP-1 and TRP-2). Bistratene A induced the phosphorylation of several proteins as assessed by 2D gel electrophoresis and one of these was identified as stathmin (oncoprotein 18), a cell-cycle regulated phosphoprotein. Bistratene A specifically induced the translocation of protein kinase Cdelta (PKCdelta) from a soluble to a particulate fraction without affecting other isoforms. These results implicate a role for protein kinase Cdelta in the induction of differentiation of this human melanoma cell line.

Regulation of melanogenesis in B16 mouse melanoma cells by protein kinase C

Melanogenesis is regulated by a variety of environmental and hormonal factors. In this study, we showed that protein kinase C (PKC) plays a major role in regulating melanogenesis in B16 mouse melanoma cells. Chronic treatment of B16 cells with phorbol dibutyrate resulted in a concentration-dependent loss of density-dependent induction of tyrosinase activity, which correlated positively with a concentration-dependent loss of PKC enzyme activity. In contrast, B16 clones overexpressing PKC alpha had increased tyrosinase activity. Different phorbol derivatives inhibited tyrosinase activity and depleted cellular PKC alpha in a manner that reflected their reported tumor-promoting activity. Western blotting analysis showed that phorbol dibutyrate decreased the amount of the brown locus gene product (TRP-1) by 50% and lowered the amount of the albino locus gene product (tyrosinase) to undetectable levels. None of the phorbol derivatives affected the level of the slaty locus protein (TRP-2). The decrease in tyrosinase and TRP-1 protein levels was found to be due to a decrease in the mRNA encoded by these genes. In addition to inhibiting the density-dependent increase in tyrosinase activity, phorbol dibutyrate inhibited some, but not all, of the 8-bromocyclic AMP-induced increase in tyrosinase activity. This was accompanied by a decrease in the amount of tyrosinase protein induced by 8-bromocyclic AMP. Although 8-bromocyclic AMP did not change the level of TRP-1, it did reverse the decrease in the amount of this protein induced by phorbol dibutyrate. The amount of TRP-2 was not altered by any of these agents. These data suggest that PKC regulates melanogenesis primarily by controlling the constitutive expression of tyrosinase and, to a lesser extent, TRP-1.

DNA damage enhances melanogenesis

Although the ability of UV irradiation to induce pigmentation in vivo and in vitro is well documented, the intracellular signals that trigger this response are poorly understood. We have recently shown that increasing DNA repair after irradiation enhances UV-induced melanization. Moreover, addition of small DNA fragments, particularly thymine dinucleotides (pTpT), selected to mimic sequences excised during the repair of UV-induced DNA photoproducts, to unirradiated pigment cells in vitro or to guinea pig skin in vivo induces a pigment response indistinguishable from UV-induced tanning. Here we present further evidence that DNA damage and/or the repair of this damage increases melanization. (i) Treatment with the restriction enzyme Pvu II or the DNA-damaging chemical agents methyl methanesulfonate (MMS) or 4-nitroquinoline 1-oxide (4-NQO) produces a 4- to 10-fold increase in melanin content in Cloudman S91 murine melanoma cells and an up to 70% increase in normal human melanocytes, (ii) UV irradiation, MMS, and pTpT all upregulate the mRNA level for tyrosinase, the rate-limiting enzyme in melanin biosynthesis. (iii) Treatment with pTpT or MMS increases the response of S91 cells to melanocyte-stimulating hormone (MSH) and increases the binding of MSH to its cell surface receptor, as has been reported for UV irradiation. Together, these data suggest that UV-induced DNA damage and/or the repair of this damage is an important signal in the pigmentation response to UV irradiation. Because Pvu II acts exclusively on DNA and because MMS and 4-NQO, at the concentrations used, primarily interact with DNA, such a stimulus alone appears sufficient to induce melanogenesis. Of possible practical importance, the dinucleotide pTpT mimics most, if not all, of the effects of UV irradiation on pigmentation, tyrosinase mRNA regulation, and response to MSH without the requirement for antecedent DNA damage.

Misrouting of tyrosinase with a truncated cytoplasmic tail as a result of the murine platinum (cp) mutation

Mice homozygous for the platinum (cp) allele at the albino locus manifest severe oculocutaneous albinism despite the presence in vitro of tyrosinase activity at 25% wild-type levels. We demonstrate that the cp allele results from an A-->T substitution, changing a lysine residue at position 489 to a termination codon, with truncation of tyrosinase's cytoplasmic tail. In choroidal melanocytes of neonatal mutant mice, tyrosinase activity could be detected in the trans Golgi network, but was absent from melanosomes. Instead, it was detected in vesicles in the cell periphery and dendrites, and on the extracellular surface. In the retinal pigment epithelium, activity was present on the extracellular apical and basolateral surfaces. Our results demonstrate misrouting of a mutant tyrosinase lacking its cytoplasmic tail, providing an explanation for the severe effect of this mutation on ocular and cutaneous pigmentation.

Analysis of the tyrosinase gene of the Japanese pond frog, Rana nigromaculata: cloning and nucleotide sequence of the genomic DNA containing the tyrosinase gene and its flanking regions

Three genomic DNA fragments containing the tyrosinase-encoding gene (TYR) of the Japanese pond frog, Rana nigromaculata, were cloned. The first, clone I, was isolated from a genomic library of sperm DNA using the mouse TYR cDNA as the probe and contained a DNA segment similar to exon 4 of the mouse TYR gene. Subsequently, the TYR cDNA was isolated by screening a frog embryo cDNA library using clone I as the probe. Two clones that contain genomic DNA of the TYR gene were isolated also from a blood cell DNA library using the frog TYR cDNA as the probe. Comparison of the nucleotide (nt) sequences of the genomic clone II DNA and the cDNA revealed that clone II contained a 3,140-bp DNA fragment consisting of the 5'-flanking region, the first exon, and a part of the first intron. The region upstream of the coding region contained the characteristic sequences for regulatory elements, including TATA- and CAAT-motifs, and also a pigment cell-specific promoter element, which is shared by the promoter regions of the vertebrate TYR genes. A 764-bp segment containing an upstream 748-bp non-coding region and 16-bp coding region was functional for expression of the promoter-less cat gene on a plasmid in the transiently transformed albino frog melanophore. The genomic clone III contained the 3'-untranslated region of the mRNA and its 3'-flanking region. Thus, the cDNA plus genomic DNA fragments isolated here cover the entire TYR gene and its flanking regions.

High-molecular-weight forms of tyrosinase and the tyrosinase-related proteins: evidence for a melanogenic complex

Tyrosinase, tyrosinase-related protein-1 (TRP-1), and tyrosinase-related protein-2, (TRP-2, dopachrome tautomerase) were shown by immunoblotting and enzyme assays to copurify from extracts of Cloudman S91 melanoma cells. Antibodies to TRP-1 and TRP-2 immunoprecipitated tyrosinase activity, suggesting a stable interaction (complex) among these proteins. The tyrosine hydroxylase activity of tyrosinase was reduced in the complexed form; treatment with Triton X-100 dissociated the complex and activated the tyrosinase present within it. To further study this complex, we employed sucrose gradient density centrifugation of extracts from cultured murine melanocytes. Tyrosinase, TRP-1, and TRP-2 all existed in high molecular weight "multimers" of approximately 200 to > 700 kilodaltons. Extraction of cells with buffers containing the detergent CHAPS preserved the high molecular weight multimers; Triton X-100 caused their dissociation into monomers. Low pH, low ionic strength, and millimolar concentrations of calcium ions favored the maintenance of multimers. The results of this study demonstrate that the participation of tyrosinase, TRP-1, and TRP-2 in a multimeric complex could have important physiologic consequences, and raise the possibility that some of the well-known interactions between coat color genes may be explained by intermolecular interactions between the gene products.

Structural organization of the human tyrosinase gene and sequence analysis and characterization of its promoter region

Tyrosinase is the principal enzyme in the biosynthesis of melanin. The expression of tyrosinase is tissue-specific and appears to be regulated by various hormonal and environmental factors. Elucidation of the genomic structure and molecular basis of control of tyrosinase gene expression will greatly enhance our understanding of the regulation of human pigmentation. To this end, we have isolated and performed restriction mapping of recombinant cosmid and lambda phage clones containing the human tyrosinase gene, sequenced a 2.2-kilobase (kb) region of its promoter, and determined the potential regions regulating the tyrosinase gene expression in transient-expression system. The human tyrosinase gene is comprised of five exons and four introns. Based on our restriction mapping studies, the gene spans a distance of over 65-kb on chromosome 11 (q14-->q21). We constructed a series of plasmids (pHTY-CAT) that contain 5' sequential deletions of the human tyrosinase 5' flanking sequence fused to the reporter gene, chloramphenicol acetyltransferase (CAT). The plasmids were used to locate promoter regions that are potential regulators of tyrosinase gene expression in a transient expression system using melanoma cell lines. In human melanoma cells, the plasmid construct with a -2020 base pair (bp) promoter yielded the highest CAT activity. When the deletions reached -1739 bp, the CAT activity was dramatically reduced, indicating that important enhancer elements for transcription control are present between -1739 and -2020 bp. Further deletions up to -550 bp also resulted in dramatic decreases of CAT activity. However, when the deletion included -550 bp of the 5' flanking sequence, there was 26 percent of the CAT activity compared to that of the -2020 bp promoter. Deletions beyond -550 bp also showed markedly decreased CAT activity. Based on our data, we suggest that human tyrosinase gene expression is governed by both tissue-specific and multiple regulatory elements.

Pathogenesis of the platinum (cp) mutation, a model for oculocutaneous albinism

The platinum mutation at the C (albino) locus in the mouse is a potential model for oculocutaneous albinism in humans other than type IA (tyrosinase-negative) albinism. Although tissues from mice homozygous for the mutation display substantial tyrosinase activity, cutaneous and ocular pigmentation is severely restricted in affected animals. Using specific antipeptide antisera, we demonstrate that ocular extracts from wild-type mice contain two isoforms of tyrosinase bearing either the amino-terminal PEP5 epitope or the carboxy-terminal PEP7 epitope. The latter isoform participates in a high-molecular-weight complex detectable on sucrose density gradients. In platinum mice, antiserum to the PEP7 epitope fails to recognize any protein species, and the high-molecular-weight form of tyrosinase is not detectable. Our results support a key role for this high-molecular-weight complex in melanogenesis, and suggest that mutations that interfere with the ability of tyrosinase to participate in a multimeric complex may be a cause of oculocutaneous albinism in people.

From gene to protein: determination of melanin synthesis

Melanin production in mammals is regulated at a variety of levels (tissue, cellular, and subcellular), and many gene loci are involved in the determination of color patterns directed by the melanocyte. Many of the genes involved in these complex processes have now been cloned, and even the simplest mutation can lead to dramatic changes in the phenotype of the individual. Many, if not all, of the pigment related genes have pleiotropic effects on the development and differentiation of the organism, and perhaps because of this, the melanocyte is evolving as an important model for the study of gene regulation and action at the functional level. In view of the importance of pigmentation as a photoprotective barrier and as a cosmetic factor affecting appearance and social acceptance, the importance of these studies seems destined to increase significantly in the future.

Fibroblasts expressing mouse c locus tyrosinase produce an authentic enzyme and synthesize phaeomelanin

Recent advances in the study of the molecular biology of mouse pigmentation have led to the discovery of a family of proteins involved in the control of melanin synthesis. It has been confirmed that the product of the mouse c (albino) locus is the key melanogenic enzyme tyrosinase, but study of its function and regulation have been hampered by the presence of closely related proteins within melanin-synthesising cells. To overcome these problems, we have established lines of mouse fibroblasts expressing the c locus mouse tyrosinase. Here we describe characterisation of the tyrosinase synthesised by these cells and demonstrate considerable similarity between the expressed tyrosinase and the native enzyme. The expressed tyrosinase is proteolytically cleaved to produce membrane-bound and soluble forms of the expected molecular mass and is rich in N-linked carbohydrate, suggesting that melanocytic differentiation is not a prerequisite for post-translational modification of the protein. The expressed enzyme has tyrosinase activity, but not catalase or dopachrome tautomerase activity, confirming that it is an authentic tyrosinase. Transfected fibroblasts expressing tyrosinase are shown to share several physiological characteristics with melanoma cell lines, including increased pigmentation and tyrosinase activity in response to increased cell density. Since tyrosinase is expressed under a heterologous promoter, these shared characteristics probably reflect translational or post-translational controls that operate in both non-melanocytic and melanocytic cell types. We demonstrate that pigmented fibroblasts contain the melanin synthesis intermediates 5-S-cysteinyldopa and 5-S-glutathionyl-dopa, and produce a phaeomelanin-like pigment, but do not contain detectable eumelanin. Expression of tyrosine is therefore sufficient for the synthesis of a form of melanin pigment in fibroblasts.

A murine monoclonal antibody, MoAb HMSA-5, against a melanosomal component highly expressed in early stages, and common to normal and neoplastic melanocytes

The melanosome is a secretory organelle unique to the melanocyte and its neoplastic counterpart, malignant melanoma. The synthesis and assembly of these intracytoplasmic organelles is not yet fully understood. We have developed a murine monoclonal antibody (MoAb) against melanosomes isolated from human melanocytes (newborn foreskin) cultured in the presence of 12-O tetradecanoyl phorbol-13-acetate (TPA). This MoAb, designated HMSA-5 (Human Melanosome-Specific Antigen-5) (IgG1), recognised a cytoplasmic antigen in both normal human melanocytes and neoplastic cells, such as common and dysplastic melanocytic nevi, and malignant melanoma. None of the carcinoma or sarcoma specimens tested showed positive reactivity with MoAb HMSA-5. Under immunoelectron microscopy, immuno-gold deposition was seen on microvesicles associated with melanosomes, and a portion of the ER-Golgi complexes. Radioimmunoprecipitation analysis showed that the HMSA-5 reactive antigen was a glycoprotein of M(r) 69 to 73 kDa. A pulse-chase time course study showed that the amount of antigen detected by MoAb HMSA-5 decreased over a 24 h period without significant expression on the cell surface, or corresponding appearance of the antigen in the culture supernatant. This glycoprotein appears to play a role in the early stages of melanosomal development, and the HMSA-5 reactive epitope may be lost during subsequent maturation processes. Importantly, HMSA-5 can be identified in all forms of human melanocytes, hence it can be considered a new common melanocytic marker even on routine paraffin sections.

A cis-acting element involved in mouse tyrosinase gene expression and partial purification of its binding protein

The tyrosinase gene is specifically expressed in melanocytes. Understanding the molecular basis of tissue-specific expression of the tyrosinase gene will greatly explain the mechanisms controlling pigmentation. We report a nucleotide sequence, TGATGTATTC, located -236 base pairs upstream of the transcription start site, that enhances tyrosinase gene expression in mouse melanoma cells. The sequence is referred to as the tyrosinase element-1 (TE-1). TE-1 was protected from DNase I cleavage by pigment cell nuclear extracts but was not protected by non-pigment cell nuclear extract. Partial purification of TE-1 binding protein (TEBP-1) was performed from the B16 mouse melanoma cell nuclear extract using biotin-cellulose affinity chromatography. The affinity-purified fraction exhibited binding to the DNA fragment containing TE-1, and to a synthetic oligomer representing TE-1. UV-cross-linking indicated that the size of TEBP-1 is approximately 49 kD. TE-1 also directed enhanced CAT activity in the B16 melanoma cells but not in non-pigment cells. These data indicate that TE-1 may be an enhancer element that is responsible for pigment cell specific expression of the tyrosinase gene.

The tyrosinase-related protein-1 gene has a structure and promoter sequence very different from tyrosinase

We have determined the exon structure of the mouse tyrosinase-related protein-1 (TRP-1) gene. The gene is only 15kb in length, but contains seven introns, in contrast to the tyrosinase gene which is almost 100kb long with only four introns. Only two introns are located in homologous positions in both genes. Intron I of TRP-1 has three alternative 5' splice sites clustered within 21bp, which all splice to the same 3' site. Intron V has a very unusual 5' splice site, which has the dinucleotide GC rather than the conventional GT. We show that as little as 370bp of 5'-flanking DNA is sufficient to direct cell-specific expression of the chloramphenicol acetyl transferase gene. The flanking DNA of TRP-1, unlike tyrosinase, does not contain a TATA box or a CCAAT box. Both mouse genes, however, share an 11bp sequence, also found in human tyrosinase, which we suggest may be a melanocyte-specific promoter element.

Mapping the human CAS2 gene, the homologue of the mouse brown (b) locus, to human chromosome 9p22-pter

Melanin biosynthesis is a multistep process with the first step being the conversion of L-tyrosine to L-Dopa catalyzed by the enzyme tyrosinase. The enzymes which catalyze the other steps of melanogenesis are not known. One murine pigmentation gene, the brown (b) locus, when mutated, leads to a brown or hypopigmented coat. The b-locus protein has been shown to display catalase activity. The human b-locus, therefore, is designated as CAS2. We used the mouse b-locus cDNA to isolate the human homologue, which in turn, was used to map the CAS2 locus to a human chromosome. The potential CAS2 protein codes for 527 amino acids containing a putative signal sequence and transmembrane domain. The CAS2 protein has primary and probably secondary structures similar to human tyrosinase. The CAS2 was mapped to human Chromosome 9 by somatic cell hybridization and, more specifically, to 9p22-pter by in situ hybridization. The assignment of CAS2 on the human Chromosome 9 extends this region of known homology on mouse Chromosome 4.

Conserved cysteine to serine mutation in tyrosinase is responsible for the classical albino mutation in laboratory mice

Albinism, due to a lack of melanin pigment, is one of the oldest known mutations in mice. Tyrosinase (monophenol oxygenase, EC 1.14.18.1) is the first enzyme in the pathway for melanin synthesis, and the gene encoding this enzyme has been mapped to the mouse albino (c) locus. We have used mouse tyrosinase cDNA clones and genomic sequencing to study the albino mutation in laboratory mice. Within the tyrosinase gene coding sequences, a G to C transversion at nucleotide 308, causing a cysteine to serine mutation at amino acid 103, is sufficient to abrogate pigment production in transgenic mice. This same base pair change is fully conserved in classical albino strains of laboratory mice. These results indicate that a conserved mutation in the tyrosinase coding sequences is responsible for the classical albino mutation in laboratory mice, and also that most albino laboratory mouse strains have been derived from a common ancestor.

Identification of the albino mutation of mouse tyrosinase by analysis of an in vitro revertant

From within an albino melanocyte line grown in vitro we identified and cloned cells that apparently had reverted to wild type. We sequenced a part of the tyrosinase gene, encompassing a candidate mutation, from wild-type, albino, and revertant cell DNAs. The revertant cells contain, on one chromosome, a perfect base reversion to the wild-type sequence of this candidate mutation, proving that this is the sole defect in the tyrosinase gene of albino mutant mice. The revertant cells readily regain the albino phenotype after freezing and thawing. Taking advantage of a Dde I restriction site created by the albino mutation, we demonstrated that the regained phenotype is due to allele loss involving the wild-type chromosome. The Dde I site also allowed us to show that all inbred albino mice carry the same mutation and so must be derived from the same progenitor.

Murine and human b locus pigmentation genes encode a glycoprotein (gp75) with catalase activity

Melanogenesis is regulated in large part by tyrosinase (monophenol monooxygenase; monophenol, L-dopa:oxygen oxidoreductase, EC 1.14.18.1), and defective tyrosinase leads to albinism. The mechanisms for other pigmentation determinants (e.g., those operative in tyrosinase-positive albinism and in murine coat-color mutants) are not yet known. One murine pigmentation gene, the brown (b) locus, when mutated leads to a brown (b/b) or hypopigmented (Blt/Blt) coat versus the wild-type black (B/B). We show that the b locus codes for a glycoprotein with the activity of a catalase (hydrogen-peroxide:hydrogen-peroxide oxidoreductase, EC 1.11.1.6) (catalase B). Only the c locus protein is a tyrosinase. Because peroxides may be by-products of melanogenic activity and hydrogen peroxide in particular is known to destroy melanin precursors and melanin, we conclude that pigmentation is controlled not only by tyrosinase but also by a hydroperoxidase. Our studies indicate that catalase B is identical with gp75, a known human melanosomal glycoprotein; that the b mutation is in a heme-associated domain; and that the Blt mutation renders the protein susceptible to rapid proteolytic degradation.

A point mutation in the tyrosinase gene of BALB/c albino mouse causing the cysteine----serine substitution at position 85

Murine albinism is characterized by complete lack of melanin pigments in skin and retina. In order to study the molecular basis of albinism, we have cloned and characterized the tyrosinase gene of BALB/c mice (c/c). Sequence analysis of this gene reveals a point mutation at nucleotide residue 387 (G----C transversion) causing a Cys----Ser substitution at position 85 in one of the cysteine-rich domains of the tyrosinase molecule. Since this G----C transversion creates an additional DdeI site, we were able to confirm that this mutation is actually present in BALB/c genomic DNA using DNA amplification techniques. In contrast, both C57BL/6 (C/C) and DBA/2 (C/C) mouse strains carry the G residue at the same position, suggesting that this point mutation is specific for the albino mutation at the c locus. Moreover, we were able to show that the tyrosinase containing Ser-85 is not functional in transient expression of its cDNA. We therefore suggest that a G----C transversion at nucleotide residue 387 of the tyrosinase gene could lead to the albino phenotype of BALB/c mouse.