Pink-eyed dilution protein controls the processing of tyrosinase

Modulating OCA2 Expression as a Promising Approach to Enhance Skin Brightness and Reduce Dark Spots

The oculocutaneous albinism II (OCA2) gene encodes a melanosomal transmembrane protein involved in melanogenesis. Recent genome-wide association studies have identified several single nucleotide polymorphisms within OCA2 genes that are involved in skin pigmentation. Nevertheless, there have been no attempts to modulate this gene to improve skin discoloration. Accordingly, our aim was to identify compounds that can reduce OCA2 expression and to develop a formula that can improve skin brightness and reduce hyperpigmented spots. In this study, we investigated the effects of OCA2 expression reduction on melanin levels, melanosome pH, and autophagy induction through siRNA knockdown. Additionally, we identified several bioactives that effectively reduce OCA2 expression. Ultimately, in a clinical trial, we demonstrated that topical application of those compounds significantly improved skin tone and dark spots compared to vitamin C, a typical brightening agent. These findings demonstrate that OCA2 is a promising target for the development of efficacious cosmetics and therapeutics designed to treat hyperpigmentation.

Whole-transcriptome sequencing reveals a melanin-related ceRNA regulatory network in the breast muscle of Xichuan black-bone chicken

The economic losses incurred due to reduced muscle pigmentation highlight the crucial role of melanin-based coloration in the meat of black-bone chickens. Melanogenesis in the breast muscle of black-bone chickens is currently poorly understood in terms of molecular mechanisms. This study employed whole-transcriptome sequencing to analyze black and white breast muscle samples from black-bone chickens, leading to the identification of 367 differentially expressed (DE) mRNAs, 48 DElncRNAs, 104 DEcircRNAs, and 112 DEmiRNAs involved in melanin deposition. Based on these findings, a competitive endogenous RNA (ceRNA) network was developed to better understand the complex mechanisms of melanin deposition. Furthermore, our analysis revealed key DEmRNAs (TYR, DCT, EDNRB, MLPH and OCA2) regulated by DEmiRNAs (gga-miR-140-5p, gga-miR-1682, gga-miR-3529, gga-miR-499-3p, novel-m0012-3p, gga-miR-200b-5p, gga-miR-203a, gga-miR-6651-5p, gga-miR-7455-3p, gga-miR-31-5p, miR-140-x, miR-455-x, novel-m0065-3p, gga-miR-29b-1-5p, miR-455-y, novel-m0085-3p, and gga-miR-196-1-3p). These DEmiRNAs competitively interacted with DElncRNAs including MSTRG.2609.2, MSTRG.4185.1, LOC112530666, LOC112533366, LOC771030, LOC107054724, LOC121107411, LOC100859072, LOC101750037, LOC121108550, LOC121109224, LOC121110876, and LOC101749016, as well as DEcircRNAs, such as novel_circ_000158, novel_circ_000623, novel_001518, and novel_circ_003596. The findings from this study provide insight into the mechanisms that regulate lncRNA, circRNA, miRNA, and mRNA expression in chicken melanin deposition.

Natural tyrosinase enzyme inhibitors: A path from melanin to melanoma and its reported pharmacological activities

The skin containing melanin pigment acts as a protective barrier and counteracts the UVR and other environmental stressors to maintain or restore disrupted cutaneous homeostasis. The production of melanin pigment is dependent on tyrosine levels. L-tyrosine and L-dihydroxyphenylalanine (L-DOPA) can serve both as a substrates and intermediates of melanin synthetic pathway and as inducers and positive regulators of melanogenesis. The biosynthesis of melanin is stimulated upon exposure to UVR, which can also stimulate local production of hormonal factors, which can stimulate melanoma development by altering the chemical properties of eu- and pheomelanin. The process of melanogenesis can be altered by several pathways. One involves activation of POMC, with the production of POMC peptides including MSH and ACTH, which increase intracellular cAMP levels, which activates the MITF, and helps to stimulate tyrosinase (TYR) expression and activity. Defects in OCA1 to 4 affects melanogenic activity via posttranslational modifications resulting in proteasomal degradation and reducing pigmentation. Further, altering, the MITF factor, helps to regulate the expression of MRGE in melanoma, and helps to increase the TYR glycosylation in ER. CRH stimulates POMC peptides that regulate melanogenesis and also by itself can stimulate melanogenesis. The POMC, P53, ACTH, MSH, MC1R, MITF, and 6-BH4 are found to be important regulators for pigmentation. Melanogenesis can affect melanoma behaviour and inhibit immune responses. Therefore, we reviewed natural products that would alter melanin production. Our special focus was on targeting melanin synthesis and TYR enzyme activity to inhibit melanogenesis as an adjuvant therapy of melanotic melanoma. Furthermore, this review also outlines the current updated pharmacological studies targeting the TYR enzyme from natural sources and its consequential effects on melanin production.

A Comprehensive Review of Mammalian Pigmentation: Paving the Way for Innovative Hair Colour-Changing Cosmetics

The natural colour of hair shafts is formed at the bulb of hair follicles, and it is coupled to the hair growth cycle. Three critical processes must happen for efficient pigmentation: (1) melanosome biogenesis in neural crest-derived melanocytes, (2) the biochemical synthesis of melanins (melanogenesis) inside melanosomes, and (3) the transfer of melanin granules to surrounding pre-cortical keratinocytes for their incorporation into nascent hair fibres. All these steps are under complex genetic control. The array of natural hair colour shades are ascribed to polymorphisms in several pigmentary genes. A myriad of factors acting via autocrine, paracrine, and endocrine mechanisms also contributes for hair colour diversity. Given the enormous social and cosmetic importance attributed to hair colour, hair dyeing is today a common practice. Nonetheless, the adverse effects of the long-term usage of such cosmetic procedures demand the development of new methods for colour change. In this context, case reports of hair lightening, darkening and repigmentation as a side-effect of the therapeutic usage of many drugs substantiate the possibility to tune hair colour by interfering with the biology of follicular pigmentary units. By scrutinizing mammalian pigmentation, this review pinpoints key targetable processes for the development of innovative cosmetics that can safely change the hair colour from the inside out.

Versatility of Hydrogels: From Synthetic Strategies, Classification, and Properties to Biomedical Applications

Hydrogels are three-dimensional, cross-linked, and supramolecular networks that can absorb significant volumes of water. Hydrogels are one of the most promising biomaterials in the biological and biomedical fields, thanks to their hydrophilic properties, biocompatibility, and wide therapeutic potential. Owing to their nontoxic nature and safe use, they are widely accepted for various biomedical applications such as wound dressing, controlled drug delivery, bone regeneration, tissue engineering, biosensors, and artificial contact lenses. Herein, this review comprises different synthetic strategies for hydrogels and their chemical/physical characteristics, and various analytical, optical, and spectroscopic tools for their characterization are discussed. A range of synthetic approaches is also covered for the synthesis and design of hydrogels. It will also cover biomedical applications such as bone regeneration, tissue engineering, and drug delivery. This review addressed the fundamental, general, and applied features of hydrogels in order to facilitate undergraduates, graduates, biomedical students, and researchers in a variety of domains.

Melanosome Biogenesis in the Pigmentation of Mammalian Skin

Melanins, the main pigments of the skin and hair in mammals, are synthesized within membrane-bound organelles of melanocytes called melanosomes. Melanosome structure and function are determined by a cohort of resident transmembrane proteins, many of which are expressed only in pigment cells and localize specifically to melanosomes. Defects in the genes that encode melanosome-specific proteins or components of the machinery required for their transport in and out of melanosomes underlie various forms of ocular or oculocutaneous albinism, characterized by hypopigmentation of the hair, skin, and eyes and by visual impairment. We review major components of melanosomes, including the enzymes that catalyze steps in melanin synthesis from tyrosine precursors, solute transporters that allow these enzymes to function, and structural proteins that underlie melanosome shape and melanin deposition. We then review the molecular mechanisms by which these components are biosynthetically delivered to newly forming melanosomes-many of which are shared by other cell types that generate cell type-specific lysosome-related organelles. We also highlight unanswered questions that need to be addressed by future investigation.

Biophysical Compatibility of a Heterotrimeric Tyrosinase-TYRP1-TYRP2 Metalloenzyme Complex

Tyrosinase (TYR) is a copper-containing monooxygenase central to the function of melanocytes. Alterations in its expression or activity contribute to variations in skin, hair and eye color, and underlie a variety of pathogenic pigmentary phenotypes, including several forms of oculocutaneous albinism (OCA). Many of these phenotypes are linked to individual missense mutations causing single nucleotide variants and polymorphisms (SNVs) in TYR. We previously showed that two TYR homologues, TYRP1 and TYRP2, modulate TYR activity and stabilize the TYR protein. Accordingly, to investigate whether TYR, TYRP1, and TYRP2 are biophysically compatible with various heterocomplexes, we computationally docked a high-quality 3D model of TYR to the crystal structure of TYRP1 and to a high-quality 3D model of TYRP2. Remarkably, the resulting TYR-TYRP1 heterodimer was complementary in structure and energy with the TYR-TYRP2 heterodimer, with TYRP1 and TYRP2 docking to different adjacent surfaces on TYR that apposed a third realistic protein interface between TYRP1-TYRP2. Hence, the 3D models are compatible with a heterotrimeric TYR-TYRP1-TYRP2 complex. In addition, this heterotrimeric TYR-TYRP1-TYRP2 positioned the C-terminus of each folded enzymatic domain in an ideal position to allow their C-terminal transmembrane helices to form a putative membrane embedded three-helix bundle. Finally, pathogenic TYR mutations causing OCA1A, which also destabilize TYR biochemically, cluster on an unoccupied protein interface at the periphery of the heterotrimeric complex, suggesting that this may be a docking site for OCA2, an anion channel. Pathogenic OCA2 mutations result in similar phenotypes to those produced by OCA1A TYR mutations. While this complex may be difficult to detect in vitro, due to the complex environment of the vertebrate cellular membranous system, our results support the existence of a heterotrimeric complex in melanogenesis.

Chemical and biochemical control of skin pigmentation with special emphasis on mixed melanogenesis

Melanins are widely distributed in animals and plants; in vertebrates, most melanins are present on the body surface. The diversity of pigmentation in vertebrates is mainly attributed to the quantity and ratio of eumelanin and pheomelanin synthesis. Most natural melanin pigments in animals consist of both eumelanin and pheomelanin in varying ratios, and thus, their combined synthesis is called "mixed melanogenesis." Gene expression is an established mechanism for controlling melanin synthesis; however, there are multiple factors that affect melanin synthesis besides gene expression. Due to the differential sensitivity of the eumelanin and pheomelanin synthetic pathways to pH, melanosomal pH likely plays a major role in mixed melanogenesis. Here, we focused on various factors affecting mixed melanogenesis including (1) chemical regulation of melanin synthesis, (2) melanosomal pH regulation during normal melanogenesis and effect on mixed melanogenesis, and (3) mechanisms of melanosomal pH control (proton pumps, channels, transporters, and signaling pathways).

Current landscape of Oculocutaneous Albinism in Japan

Oculocutaneous albinism (OCA), which is roughly divided into non-syndromic and syndromic OCA, is a group of autosomal recessive disorders caused by mutations in genes associated with pigmentation. Patients with OCA have hypopigmentation and ocular manifestations such as photophobia, amblyopia, and nystagmus. Hermansky-Pudlak syndrome (HPS), the most common syndromic OCA, is characterized by the additional features of a bleeding tendency and other critical systemic comorbidities such as pulmonary fibrosis and immunodeficiency. NGS-based gene analyses have identified several new causative genes for OCA and have detected rare subtypes of OCA with high accuracy including Japanese patients. In our survey of 190 Japanese OCA patients/families, OCA4 is the most common subtype (25.3%) followed by OCA1 (20.0%), HPS1 (14.7%), and OCA2 (8.4%). Similar to the A481T variant in OCA2, which is associated with a mild form of OCA2 and skin color variation, the c.-492_489delAATG variant located in the promoter region of SLC45A2 has been uniquely identified in Japanese patients with a mild form of OCA4. Further, rare OCA subtypes, including OCA3, HPS2, HPS3, HPS4, HPS5, HPS6, and HPS9, have also been identified in Japanese patients. The clinical characteristics and underlying molecular mechanisms of each subtype of OCA are concisely summarized in this review.

Membrane transport proteins in melanosomes: Regulation of ions for pigmentation

Melanosomes are unique organelles in melanocytes that produce melanin, the pigment for skin, hair, and eye color. Tyrosinase is the essential and rate-limiting enzyme for melanin production, that strictly requires neutral pH for activity. pH maintenance is a result of the combinational function of multiple ion transport proteins. Thus, ion homeostasis in melanosomes is crucial for melanin synthesis. Defect of the ion transport system causes various pigmentation phenotypes, from mild effect to severe disorders such as albinism. In this review, we summarize the up-to-date knowledge of the ion transport system, such as transport function, structure, and the physiological roles and mechanisms of the ion transport proteins in melanosomes. In addition, we propose a model of melanosomal ion transport system-how the functional coupling of multiple transport proteins modulates and maintains ion homeostasis. We discuss melanin synthesis in terms of the ion transport system.

Nonhuman Primate Model of Oculocutaneous Albinism with TYR and OCA2 Mutations

Human visual acuity is anatomically determined by the retinal fovea. The ontogenetic development of the fovea can be seriously hindered by oculocutaneous albinism (OCA), which is characterized by a disorder of melanin synthesis. Although people of all ethnic backgrounds can be affected, no efficient treatments for OCA have been developed thus far, due partly to the lack of effective animal models. Rhesus macaques are genetically homologous to humans and, most importantly, exhibit structures of the macula and fovea that are similar to those of humans; thus, rhesus macaques present special advantages in the modeling and study of human macular and foveal diseases. In this study, we identified rhesus macaque models with clinical characteristics consistent with those of OCA patients according to observations of ocular behavior, fundus examination, and optical coherence tomography. Genomic sequencing revealed a biallelic p.L312I mutation in TYR and a homozygous p.S788L mutation in OCA2, both of which were further confirmed to affect melanin biosynthesis via in vitro assays. These rhesus macaque models of OCA will be useful animal resources for studying foveal development and for preclinical trials of new therapies for OCA.

Novel compound heterozygous mutations in OCA2 gene associated with non-syndromic oculocutaneous albinism in a Chinese Han patient: a case report

Background

Oculocutaneous albinism (OCA) is a group of rare genetically heterogeneous disorders. The present study aimed to identify the genetic cause of a Chinese Han family with non-syndromic oculocutaneous albinism (OCA).

Case presentation

Here, we report an 11-month-old male proband from a Chinese Han non-consanguineous family, who presented with milky skin, yellow white hair, nystagmus, astigmatism, and hypermetropia. We performed the targeted next-generation sequencing (NGS) on the proband and identified two novel compound heterozygous variants (c.1865 T > C (p.Leu622Pro) and exons 17–21 deletion) in OCA2 gene associated with OCA type 2 (OCA2, OMIM 203200). Meanwhile, a previously reported heterozygous mutation (c.4805G > A) in MYO7 gene related with Usher syndrome type 1B was found. The online tools SIFT, PolyPhen-2, and Mutation Taster predicted variant c.1865 T > C was probably damaging. The residue p.Leu622 was in a highly conserved region among species by CLUSTALW. Three-dimensional homology model with I-TASSER indicated that p.Leu622Pro variant disturbed the formation of the α-helix, resulting in a random coil structure. The gross deletion (exons 17–21) in OCA2 gene has was not been reported previously. These two novel variants in OCA2 gene were inherited from each parent respectively, after verification by Sanger sequencing and quantitative PCR (qPCR) in the family.

Conclusions

This study indicates the two novel compound heterozygous mutations in OCA2 gene may be responsible for clinical manifestations of OCA2. It expands the mutation spectrum of OCA2 gene and is helpful to screen for large deletions with targeted NGS protocol in monogenic disease. It also assists the genetic counselling, carrier screening and personalized healthcare of the disease.

Electronic supplementary material

The online version of this article (10.1186/s12881-019-0850-7) contains supplementary material, which is available to authorized users.

Molecular outcomes, clinical consequences, and genetic diagnosis of Oculocutaneous Albinism in Pakistani population

Nonsyndromic oculocutaneous Albinism (nsOCA) is clinically characterized by the loss of pigmentation in the skin, hair, and iris. OCA is amongst the most common causes of vision impairment in children. To date, pathogenic variants in six genes have been identified in individuals with nsOCA. Here, we determined the identities, frequencies, and clinical consequences of OCA alleles in 94 previously unreported Pakistani families. Combination of Sanger and Exome sequencing revealed 38 alleles, including 22 novel variants, segregating with nsOCA phenotype in 80 families. Variants of TYR and OCA2 genes were the most common cause of nsOCA, occurring in 43 and 30 families, respectively. Twenty-two novel variants include nine missense, four splice site, two non-sense, one insertion and six gross deletions. In vitro studies revealed retention of OCA proteins harboring novel missense alleles in the endoplasmic reticulum (ER) of transfected cells. Exon-trapping assays with constructs containing splice site alleles revealed errors in splicing. As eight alleles account for approximately 56% (95% CI: 46.52–65.24%) of nsOCA cases, primarily enrolled from Punjab province of Pakistan, hierarchical strategies for variant detection would be feasible and cost-efficient genetic tests for OCA in families with similar origin. Thus, we developed Tetra-primer ARMS assays for rapid, reliable, reproducible and economical screening of most of these common alleles.

Variation in Hsp70-1A Expression Contributes to Skin Color Diversity

The wide range in human skin color results from varying levels of the pigment melanin. Genetic mechanisms underlying coloration differences have been explored, but identified genes do not account for all variation seen in the skin color spectrum. Post-transcriptional and post-translational regulation of factors that determine skin color, including melanin synthesis in epidermal melanocytes, melanosome transfer to keratinocytes, and melanosome degradation, is also critical for pigmentation. We therefore investigated proteins that are differentially expressed in melanocytes derived from either white or African American skin. Two-dimensional gel electrophoresis and mass spectrometry demonstrated that heat shock protein 70-1A (Hsp70-1A) protein levels were significantly higher in African American melanocytes compared with white melanocytes. Hsp70-1A expression significantly correlated with levels of tyrosinase, the rate-limiting melanogenic enzyme, consistent with a proposed role for Hsp70 family members in tyrosinase post-translational modification. In addition, pharmacologic inhibition and small interfering RNA-mediated downregulation of Hsp70-1A correlated with pigmentation changes in cultured melanocytes, modified human skin substitutes, and ex vivo skin. Furthermore, Hsp70-1A inhibition led to increased autophagy-mediated melanosome degradation in keratinocytes. Our data thus reveal that epidermal Hsp70-1A contributes to the diversity of skin color by regulating the amount of melanin synthesized in melanocytes and modulating autophagic melanosome degradation in keratinocytes.

A Genetic Mechanism for Convergent Skin Lightening during Recent Human Evolution

Skin lightening among Eurasians is thought to have been a convergence occurring independently in Europe and East Asia as an adaptation to high latitude environments. Among Europeans, several genes responsible for such lightening have been found, but the information available for East Asians is much more limited. Here, a genome-wide comparison between dark-skinned Africans and Austro-Asiatic speaking aborigines and light-skinned northern Han Chinese identified the pigmentation gene OCA2, showing unusually deep allelic divergence between these groups. An amino acid substitution (His615Arg) of OCA2 prevalent in most East Asian populations—but absent in Africans and Europeans—was significantly associated with skin lightening among northern Han Chinese. Further transgenic and targeted gene modification analyses of zebrafish and mouse both exhibited the phenotypic effect of the OCA2 variant manifesting decreased melanin production. These results indicate that OCA2 plays an important role in the convergent skin lightening of East Asians during recent human evolution.

l-tyrosine induces melanocyte differentiation in novel pink-eyed dilution castaneus mouse mutant showing age-related pigmentation

BACKGROUND

The mouse pink-eyed dilution (oculocutaneous albinism II; p/Oca2(p)) locus is known to control tyrosinase activity, melanin content, and melanosome development in melanocytes. Pink-eyed dilution castaneus (p(cas)/Oca2(p-cas)) is a novel mutant allele on mouse chromosome 7 that arose spontaneously in Indonesian wild mice, Mus musculus castaneus. Mice homozygous for Oca2(p-cas) usually exhibit pink eyes and beige-colored coat on nonagouti C57BL/6 (B6) background. Recently, a novel spontaneous mutation occurred in the progeny between this mutant and B6 mice. The eyes of this novel mutant progressively become black from pink and the coat becomes dark gray from beige with aging.

OBJECTIVE

The aim of this study is to clarify whatever differences exist in melanocyte proliferation and differentiation between the ordinary (pink-eyed) and novel (black-eyed) mutant using serum-free primary culture system.

METHODS

The characteristics of melanocyte proliferation and differentiation were investigated by serum-free primary culture system using melanocyte-proliferation medium (MDMD).

RESULTS

The proliferation of melanoblasts in MDMD did not differ between the two mice. However, when the epidermal cell suspensions were cultured with MDMD supplemented with l-tyrosine (Tyr), the differentiation of black-eyed melanocytes was greatly induced in a concentration-dependent manner compared with pink-eyed melanocytes. Immunocytochemistry demonstrated that the expression of tyrosinase and tyrosinase-related protein-1 (Tyrp1) was greatly induced or stimulated both in pink-eyed and black-eyed melanocytes, whereas the expression of microphthalmia-associated transcription factor (Mitf) was stimulated only in black-eyed melanocytes.

CONCLUSION

These results suggest that the age-related coat darkening in black-eyed mutant may be caused by the increased ability of melanocyte differentiation dependent on l-Tyr through the upregulation of tyrosinase, Tyrp1, and Mitf. This mutant mouse may be useful for animal model to clarify the mechanisms of age-related pigmentation in human skin, such as melasma and solar lentigines.

Homology modelling and virtual screening of P-protein in a quest for novel antimelanogenic agent and in vitro assessments

An adequate knowledge on molecular mechanism of melanogenesis provides an opportunity to find the novel molecular targets for the discovery and development of new cosmetics. Among various genes, the OCA2 is being essential for proper melanin synthesis, and mutation or deletion of this gene leads to oculocutaneous albinism type 2. Thus, for this study, the product of this gene, that is P-protein, was targeted in quest for novel inhibitors as antimelanogenic agents. Based on pattern search of amino acid sequence and homology analysis, the protein structure was modelled. The role of this protein has been predicted as a tyrosine transporter of melanosomes. Thus, the molecular library was generated on the basis of tyrosine transporter inhibitor. Based on the dock score, 20 molecules have been considered as putative inhibitors for P-protein. Among these compounds, five molecules (compound #1, #4, #8, #13 and #17) were found to be quite effective as antimelanogenic without having any toxicity. Further investigations to establish the mechanism of action, the indirect methods such as tyrosinase assay, analysis for eumelanin and pheomelanins and investigation of mRNA levels were being carried out. The results from the studies offered a new lead in antimelanogenic therapy and may be very useful for further optimization work in developing them as novel depigmenting agents.

A nonsense nucleotide substitution in the oculocutaneous albinism II gene underlies the original pink-eyed dilution allele (Oca2(p)) in mice

The original pink-eyed dilution (p) on chromosome 7 is a very old spontaneous mutation in mice. The oculocutaneous albinism II (Oca2) gene has previously been identified as the p gene. Oca2 transcripts have been shown to be absent in the skin of SJL/J mice with the original p mutant allele (Oca2^p); however, the molecular genetic lesion underlying the original Oca2^p allele has never been reported. The NCT mouse (commonly known as Nakano cataract mouse) has a pink-eyed dilution phenotype, which prompted us to undertake a molecular genetic analysis of the Oca2 gene of this strain. Our genetic linkage analysis suggests that the locus for the pink-eyed dilution phenotype of NCT is tightly linked to the Oca2 locus. PCR cloning and nucleotide sequence analysis indicates that the NCT mouse has a nonsense nucleotide substitution at exon 7 of the Oca2 gene. Examination of three mouse strains (NZW/NSlc, SJL/J, and 129X1/SvJJmsSlc) with the original Oca2^p allele revealed the presence of a nonsense nucleotide substitution identical to that in the NCT strain. RT-PCR analysis revealed that the Oca2 transcripts were absent in the skin of NCT mice, suggesting intervention of the nonsense-mediated mRNA decay pathway. Collectively, the data in this study indicate that the nonsense nucleotide substitution in the Oca2 gene underlies the Oca2^p allele. Our data also indicate that the NCT mouse can be used not only as a cataract model, but also as a model for human type II oculocutaneous albinism.

An intracellular anion channel critical for pigmentation

Intracellular ion channels are essential regulators of organellar and cellular function, yet the molecular identity and physiological role of many of these channels remains elusive. In particular, no ion channel has been characterized in melanosomes, organelles that produce and store the major mammalian pigment melanin. Defects in melanosome function cause albinism, characterized by vision and pigmentation deficits, impaired retinal development, and increased susceptibility to skin and eye cancers. The most common form of albinism is caused by mutations in oculocutaneous albinism II (OCA2), a melanosome-specific transmembrane protein with unknown function. Here we used direct patch-clamp of skin and eye melanosomes to identify a novel chloride-selective anion conductance mediated by OCA2 and required for melanin production. Expression of OCA2 increases organelle pH, suggesting that the chloride channel might regulate melanin synthesis by modulating melanosome pH. Thus, a melanosomal anion channel that requires OCA2 is essential for skin and eye pigmentation.

Intravital imaging of SRF and Notch signalling identifies a key role for EZH2 in invasive melanoma cells

The acquisition of cell motility is an early step in melanoma metastasis. Here we use intravital imaging of signalling reporter cell-lines combined with genome-wide transcriptional analysis to define signalling pathways and genes associated with melanoma metastasis. Intravital imaging revealed heterogeneous cell behaviour in vivo: <10% of cells were motile and both singly moving cells and streams of cells were observed. Motile melanoma cells had increased Notch- and SRF-dependent transcription. Subsequent genome-wide analysis identified an overlapping set of genes associated with high Notch and SRF activity. We identified EZH2, a histone methyltransferase in the Polycomb repressive complex 2, as a regulator of these genes. Heterogeneity of EZH2 levels is observed in melanoma models, and co-ordinated upregulation of genes positively regulated by EZH2 is associated with melanoma metastasis. EZH2 was also identified as regulating the amelanotic phenotype of motile cells in vivo by suppressing expression of the P-glycoprotein Oca2. Analysis of patient samples confirmed an inverse relationship between EZH2 levels and pigment. EZH2 targeting with siRNA and chemical inhibition reduced invasion in mouse and human melanoma cell lines. The EZH2-regulated genes KIF2C and KIF22 are required for melanoma cell invasion and important for lung colonization. We propose that heterogeneity in EZH2 levels leads to heterogeneous expression of a cohort of genes associated with motile behaviour including KIF2C and KIF22. EZH2-dependent increased expression of these genes promotes melanoma cell motility and early steps in metastasis.

Genetic variation in regulatory DNA elements: the case of OCA2 transcriptional regulation

Mutations within the OCA2 gene or the complete absence of the OCA2 protein leads to oculocutaneous albinism type 2. The OCA2 protein plays a central role in melanosome biogenesis, and it is a strong determinant of the eumelanin content in melanocytes. Transcript levels of the OCA2 gene are strongly correlated with pigmentation intensities. Recent studies demonstrated that the transcriptional level of OCA2 is to a large extent determined by the noncoding SNP rs12913832 located 21.5 kb upstream of the OCA2 gene promoter. In this review, we discuss current hypotheses and the available data on the mechanism of OCA2 transcriptional regulation and how this is influenced by genetic variation. Finally, we will explore how future epigenetic studies can be used to advance our insight into the functional biology that connects genetic variation to human pigmentation.

Basis for enhanced barrier function of pigmented skin

Humans with darkly pigmented skin display superior permeability barrier function in comparison with humans with lightly pigmented skin. The reduced pH of the stratum corneum (SC) of darkly pigmented skin could account for enhanced function, because acidifying lightly pigmented human SC resets barrier function to darkly pigmented levels. In SKH1 (nonpigmented) versus SKH2/J (pigmented) hairless mice, we evaluated how a pigment-dependent reduction in pH could influence epidermal barrier function. Permeability barrier homeostasis is enhanced in SKH2/J versus SKH1 mice, correlating with a reduced pH in the lower SC that colocalizes with the extrusion of melanin granules. Darkly pigmented human epidermis also shows substantial melanin extrusion in the outer epidermis. Both acute barrier disruption and topical basic pH challenges accelerate reacidification of SKH2/J (but not SKH1) SC, while inducing melanin extrusion. SKH2/J mice also display enhanced expression of the SC acidifying enzyme, secretory phospholipase A2f (sPLA2f). Enhanced barrier function of SKH2/J mice could be attributed to enhanced activity of two acidic pH-dependent, ceramide-generating enzymes, β-glucocerebrosidase and acidic sphingomyelinase, leading to accelerated maturation of SC lamellar bilayers. Finally, organotypic cultures of darkly pigmented human keratinocytes display enhanced barrier function in comparison with lightly pigmented cultures. Together, these results suggest that the superior barrier function of pigmented epidermis can be largely attributed to the pH-lowering impact of melanin persistence/extrusion and enhanced sPLA2f expression.

Ion transport in pigmentation

Skin melanocytes and ocular pigment cells contain specialized organelles called melanosomes, which are responsible for the synthesis of melanin, the major pigment in mammals. Defects in the complex mechanisms involved in melanin synthesis and regulation result in vision and pigmentation deficits, impaired development of the visual system, and increased susceptibility to skin and eye cancers. Ion transport across cellular membranes is critical for many biological processes, including pigmentation, but the molecular mechanisms by which it regulates melanin synthesis, storage, and transfer are not understood. In this review we first discuss ion channels and transporters that function at the plasma membrane of melanocytes; in the second part we consider ion transport across the membrane of intracellular organelles, with emphasis on melanosomes. We discuss recently characterized lysosomal and endosomal ion channels and transporters associated with pigmentation phenotypes. We then review the evidence for melanosomal channels and transporters critical for pigmentation, discussing potential molecular mechanisms mediating their function. The studies investigating ion transport in pigmentation physiology open new avenues for future research and could reveal novel molecular mechanisms underlying melanogenesis.

Functional characterization of two novel splicing mutations in the OCA2 gene associated with oculocutaneous albinism type II

Oculocutaneous albinism (OCA) is characterized by hypopigmentation of the skin, hair and eye, and by ophthalmologic abnormalities caused by a deficiency in melanin biosynthesis. OCA type II (OCA2) is one of the four commonly-recognized forms of albinism, and is determined by mutation in the OCA2 gene. In the present study, we investigated the molecular basis of OCA2 in two siblings and one unrelated patient. The mutational screening of the OCA2 gene identified two hitherto-unknown putative splicing mutations. The first one (c.1503+5G>A), identified in an Italian proband and her affected sibling, lies in the consensus sequence of the donor splice site of OCA2 intron 14 (IVS14+5G>A), in compound heterozygosity with a frameshift mutation, c.1450_1451insCTGCCCTGACA, which is predicted to determine the premature termination of the polypeptide chain (p.I484Tfs*19). In-silico prediction of the effect of the IVS14+5G>A mutation on splicing showed a score reduction for the mutant splice site and indicated the possible activation of a newly-created deep-intronic acceptor splice site. The second mutation is a synonymous transition (c.2139G>A, p.K713K) involving the last nucleotide of exon 20. This mutation was found in a young African albino patient in compound heterozygosity with a previously-reported OCA2 missense mutation (p.T404M). In-silico analysis predicted that the mutant c.2139G>A allele would result in the abolition of the splice donor site. The effects on splicing of these two novel mutations were investigated using an in-vitro hybrid-minigene approach that led to the demonstration of the causal role of the two mutations and to the identification of aberrant transcript variants.

Loss of Oca2 disrupts the unfolded protein response and increases resistance to endoplasmic reticulum stress in melanocytes

Accumulation of proteins in the endoplasmic reticulum (ER) typically induces stress and initiates the unfolded protein response (UPR) to facilitate recovery. If homeostasis is not restored, apoptosis is induced. However, adaptation to chronic UPR activation can increase resistance to subsequent acute ER stress. We therefore investigated adaptive mechanisms in Oculocutaneous albinism type 2 (Oca2)-null melanocytes where UPR signaling is arrested despite continued tyrosinase accumulation leading to resistance to the chemical ER stressor thapsigargin. Although thapsigargin triggers UPR activation, instead of Perk-mediated phosphorylation of eIF2α, in Oca2-null melanocytes, eIF2α was rapidly dephosphorylated upon treatment. Dephosphorylation was mediated by the Gadd34-PP1α phosphatase complex. Gadd34-complex inhibition blocked eIF2α dephosphorylation and significantly increased Oca2-null melanocyte sensitivity to thapsigargin. Thus, Oca2-null melanocytes adapt to acute ER stress by disruption of pro-apoptotic Perk signaling, which promotes cell survival. This is the first study to demonstrate rapid eIF2α dephosphorylation as an adaptive mechanism to ER stress.

Reduced glutathione disrupts the intracellular trafficking of tyrosinase and tyrosinase-related protein-1 but not dopachrome tautomerase and Pmel17 to melanosomes, which results in the attenuation of melanization

We previously reported that treatment of B16 melanotic melanoma cells with reduced glutathione (GSH) converts them to amelanotic cells without any significant down-regulation of tyrosinase activity. To characterize the cellular mechanism(s) involved, we determined the intracellular distribution of melanocyte-specific proteins, especially in melanin synthesis-specific organelles, termed melanosomes by subcellular fractionation followed by Western blotting and confocal laser microscopy (CFLM). In the melanosome-rich large granule fraction and in highly purified melanosome fractions, while GSH-induced amelanotic B16 cells have significantly diminished levels of protein/activity of tyrosinase and tyrosinase-related protein-1 compared with control melanized B16 cells, there was substantially no difference in the distribution and levels of dopachrome tautomerase and the processed isoform of Pmel17 (HMB45) between control melanized and GSH-induced amelanotic B16 cells. Analysis of merged images obtained by CFLM revealed that whereas tyrosinase, Pmel17 and dopachrome tautomerase colocalize with each other in the control melanized B16 cells, tyrosinase does not colocalize with Pmel17 or its processed isoform and with dopachrome tautomerase in GSH-induced amelanotic B16 cells. The sum of these findings suggests that reduced glutathione selectively disrupts the intracellular trafficking of tyrosinase and tyrosinase-related protein-1 but not dopachrome tautomerase and Pmel17 to melanosomes, which results in the attenuation of melanization, probably serving as a putative model for oculocutaneous albinism type 4.

Report of a novel OCA2 gene mutation and an investigation of OCA2 variants on melanoma risk in a familial melanoma pedigree

BACKGROUND

Oculocutaneous albinism type 2 (OCA2) is caused by mutations of the OCA2 gene. Individuals affected by OCA2 as well as other types of albinism are at a significantly increased risk for sun-induced skin-cancers, including malignant melanoma (MM).

OBJECTIVE

To identify the molecular etiology of oculocutaneous albinism in a previously uncharacterized melanoma pedigree and to investigate the relationship between two OCA2 variants and melanoma predisposition in this pedigree.

METHODS

DNA and RNA were isolated from the peripheral blood of seven patients in a familial melanoma pedigree. Electron microscopy was performed on the individual with clinical oculocutaneous albinism. OCA2, TYRP1, MC1R, CDKN2A/p16, CDKN2A/p19ARF, and CDK4 genes were sequenced in affected individuals. The relationship between OCA2 variants and melanoma was assessed using a pedigree likelihood-based method.

RESULTS

The proband was determined to be an OCA2 compound heterozygous mutation carrier with a previously reported conservative missense mutation (V443I) and a novel non-conservative missense mutation (L734R). The pedigree contained individuals diagnosed with both cutaneous and iris melanoma. Based on co-segregation analysis, the odds of these OCA2 variants being high penetrance loci for melanoma was: 1.3-to-1 if we include the iris melanoma as affected and 6.5-to-1 if we only consider cutaneous melanoma as affected.

CONCLUSION

The discovery of this novel OCA2 variant adds to the body of evidence on the detrimental effects of OCA2 gene mutations on pigmentation, supports existing GWAS data on the relevance of the OCA2 gene in melanoma predisposition, and may ultimately assist in the development of targeted molecular therapies in the treatment of OCA and melanoma.

Pigmented-MDCK (P-MDCK) cell line with tunable melanin expression: an in vitro model for the outer blood-retinal barrier

Retinal pigment epithelium, which forms the outer blood-retinal barrier, is a critical barrier for transport of drugs to the retina. The purpose of this study was to develop a pigmented MDCK (P-MDCK) cell line as a rapidly established in vitro model for the outer blood-retinal barrier to assess the influence of melanin pigment on solute permeability. A melanin synthesizing P-MDCK cell line was developed by lentiviral transduction of human tyrosinase and p-protein genes in MDCK (NBL-2) cells. Melanin content, tyrosinase activity (conversion of L-dopa to dopachrome), and transepithelial electrical resistance (TEER) were measured. Expression of tyrosinase protein and p-protein in P-MDCK cells was confirmed by confocal microscopy. Effect of l-tyrosine (0 to 2 mM) in culture medium on melanin synthesis in P-MDCK cells was evaluated. Cell uptake and transepithelial transport of pigment-binding chloroquine (Log D = 1.59) and a negative control salicylic acid (Log D = -1.14) were investigated. P-MDCK cells expressed tyrosinase and p-protein. Tyrosinase activity was 4.5-fold higher in P-MDCK cells compared to wild type MDCK cells. The transepithelial electrical resistance stabilized by day 4 in both cell types, with the TEER being 958 ± 33 and 964 ± 58 Ω·cm(2) for P-MDCK and wild type cells, respectively. Melanin content in P-MDCK cells depended on the concentration of l-tyrosine in culture medium, and increased from 3 to 54 μg/mg protein with an increase in l-tyrosine content from 0 to 2 mM. When the cells were grown in 2 mM l-tyrosine, uptake of chloroquine was 2.3-fold higher and the transepithelial transport was 2.2-fold lower in P-MDCK cells when compared to wild type MDCK cells. No significant difference was observed for both cell uptake and transport of salicylic acid. We developed a P-MDCK cell line with tunable melanin synthesis as a rapidly developing surrogate for retinal pigment epithelium.

Mechanisms of protein delivery to melanosomes in pigment cells

Vertebrate pigment cells in the eye and skin are useful models for cell types that use specialized endosomal trafficking pathways to partition cargo proteins to unique lysosome-related organelles such as melanosomes. This review describes current models of protein trafficking required for melanosome biogenesis in mammalian melanocytes.

Molecular genetic studies and delineation of the oculocutaneous albinism phenotype in the Pakistani population

Background

Oculocutaneous albinism (OCA) is caused by a group of genetically heterogeneous inherited defects that result in the loss of pigmentation in the eyes, skin and hair. Mutations in the TYR, OCA2, TYRP1 and SLC45A2 genes have been shown to cause isolated OCA. No comprehensive analysis has been conducted to study the spectrum of OCA alleles prevailing in Pakistani albino populations.

Methods

We enrolled 40 large Pakistani families and screened them for OCA genes and a candidate gene, SLC24A5. Protein function effects were evaluated using in silico prediction algorithms and ex vivo studies in human melanocytes. The effects of splice-site mutations were determined using an exon-trapping assay.

Results

Screening of the TYR gene revealed four known (p.Arg299His, p.Pro406Leu, p.Gly419Arg, p.Arg278*) and three novel mutations (p.Pro21Leu, p.Cys35Arg, p.Tyr411His) in ten families. Ex vivo studies revealed the retention of an EGFP-tagged mutant (p.Pro21Leu, p.Cys35Arg or p.Tyr411His) tyrosinase in the endoplasmic reticulum (ER) at 37°C, but a significant fraction of p.Cys35Arg and p.Tyr411His left the ER in cells grown at a permissive temperature (31°C). Three novel (p.Asp486Tyr, p.Leu527Arg, c.1045-15 T > G) and two known mutations (p.Pro743Leu, p.Ala787Thr) of OCA2 were found in fourteen families. Exon-trapping assays with a construct containing a novel c.1045-15 T > G mutation revealed an error in splicing. No mutation in TYRP1, SLC45A2, and SLC24A5 was found in the remaining 16 families. Clinical evaluation of the families segregating either TYR or OCA2 mutations showed nystagmus, photophobia, and loss of pigmentation in the skin or hair follicles. Most of the affected individuals had grayish-blue colored eyes.

Conclusions

Our results show that ten and fourteen families harbored mutations in the TYR and OCA2 genes, respectively. Our findings, along with the results of previous studies, indicate that the p.Cys35Arg, p.Arg278* and p.Gly419Arg alleles of TYR and the p.Asp486Tyr and c.1045-15 T > G alleles of OCA2 are the most common causes of OCA in Pakistani families. To the best of our knowledge, this study represents the first documentation of OCA2 alleles in the Pakistani population. A significant proportion of our cohort did not have mutations in known OCA genes. Overall, our study contributes to the development of genetic testing protocols and genetic counseling for OCA in Pakistani families.

Gabra5-gene haplotype block associated with behavioral properties of the full agonist benzodiazepine chlordiazepoxide

The gabra5 gene is associated with pharmacological properties (myorelaxant, amnesic, anxiolytic) of benzodiazepines. It is tightly located (0.5 cM) close to the pink-eyed dilution (p) locus which encodes for fur color on mouse chromosome 7. We tested the putative role of the gabra5 gene in pharmacological properties of the full non specific agonist chlordiazepoxide (CDP), using behavioral and molecular approaches in mutated p/p mice and wild type F2 from crosses between two multiple markers inbred strain ABP/Le and C57BL/6By strain. From our results, using rotarod, light-dark box, elevated maze and radial arm maze tests, we demonstrate that p/p mice are more sensitive than WT to the sensory motor, anxiolytic and amnesic effect of CDP. This is associated with the presence of a haplotypic block on the murine chromosome 7 and with an up regulation of gabra5 mRNAs in hippocampi of p/p F2 mice.

Informed reasoning: repositioning of nitisinone to treat oculocutaneous albinism

Oculocutaneous albinism (OCA) is a group of genetic disorders characterized by hypopigmentation of the skin, hair, and eyes. Affected individuals experience reduced visual acuity and substantially increased skin cancer risk. There are four major types of OCA (OCA1-OCA4) that result from disruption in production of melanin from tyrosine. Current treatment options for individuals with OCA are limited to attempts to correct visual problems and counseling to promote use of sun protective measures. However, Onojafe et al., reporting in this issue of the JCI, provide hope for a new treatment approach for OCA, as they demonstrate that treating mice that model OCA-1b with nitisinone, which is FDA approved for treating hereditary tyrosinemia type 1, elevates plasma tyrosine levels, and increases eye and hair pigmentation.

How are proliferation and differentiation of melanocytes regulated?

Coat colors are determined by melanin (eumelanin and pheomelanin). Melanin is synthesized in melanocytes and accumulates in special organelles, melanosomes, which upon maturation are transferred to keratinocytes. Melanocytes differentiate from undifferentiated precursors, called melanoblasts, which are derived from neural crest cells. Melanoblast/melanocyte proliferation and differentiation are regulated by the tissue environment, especially by keratinocytes, which synthesize endothelins, steel factor, hepatocyte growth factor, leukemia inhibitory factor and granulocyte-macrophage colony-stimulating factor. Melanocyte differentiation is also stimulated by alpha-melanocyte stimulating hormone; in the mouse, however, this hormone is likely carried through the bloodstream and not produced locally in the skin. Melanoblast migration, proliferation and differentiation are also regulated by many coat color genes otherwise known for their ability to regulate melanosome formation and maturation, pigment type switching and melanosome distribution and transfer. Thus, melanocyte proliferation and differentiation are not only regulated by genes encoding typical growth factors and their receptors but also by genes classically known for their role in pigment formation.

Functional interactions between OCA2 and the protein complexes BLOC-1, BLOC-2, and AP-3 inferred from epistatic analyses of mouse coat pigmentation

The biogenesis of melanosomes is a multistage process that requires the function of cell-type-specific and ubiquitously expressed proteins. OCA2, the product of the gene defective in oculocutaneous albinism type 2, is a melanosomal membrane protein with restricted expression pattern and a potential role in the trafficking of other proteins to melanosomes. The ubiquitous protein complexes AP-3, BLOC-1, and BLOC-2, which contain as subunits the products of genes defective in various types of Hermansky-Pudlak syndrome, have been likewise implicated in trafficking to melanosomes. We have tested for genetic interactions between mutant alleles causing deficiency in OCA2 (pink-eyed dilution unstable), AP-3 (pearl), BLOC-1 (pallid), and BLOC-2 (cocoa) in C57BL/6J mice. The pallid allele was epistatic to pink-eyed dilution, and the latter behaved as a semi-dominant phenotypic enhancer of cocoa and, to a lesser extent, of pearl. These observations suggest functional links between OCA2 and these three protein complexes involved in melanosome biogenesis.

The mouse pink-eyed dilution allele of the P-gene greatly inhibits eumelanin but not pheomelanin synthesis

The mouse pink-eyed dilution (p) locus is known to control eumelanin synthesis, melanosome morphology, and tyrosinase activity in melanocytes. However, it has not been fully determined whether the mutant allele, p affects pheomelanin synthesis. Effects of the p allele on eumelanin and phemelanin synthesis were investigated by chemical analysis of dorsal hairs of 5-week-old mice obtained from the F(2) generations (black, pink-eyed black, recessive yellow, pink-eyed recessive yellow, agouti, and pink-eyed agouti) between C57BL/10JHir (B10)-congenic pink-eyed black mice (B10-p/p) and recessive yellow (B10-Mc1r(e)/Mc1r(e)) or agouti (B10-A/A) mice. The eumelanin content was dramatically (>20-fold) decreased in pink-eyed black and pink-eyed agouti mice, whereas the pheomelanin content did not decrease in pink-eyed black, pink-eyed recessive yellow, or pink-eyed agouti mice compared to the corresponding P/- mice. These results suggest that the pink-eyed dilution allele greatly inhibits eumelanin synthesis, but not pheomelanin synthesis.

Spectrum of candidate gene mutations associated with Indian familial oculocutaneous and ocular albinism

PURPOSE

Albinism is a group of genetic disorders, showing a broad spectrum of different phenotypes. The purpose of this study was to screen known candidate genes for oculocutaneous albinism (OCA) and ocular albinism (OA) mutations in Indian patients.

METHODS

Blood samples were collected from 23 probands and 13 affected family members from 23 genetically unrelated Indian families (22 diagnosed as OCA and 1 diagnosed as OA) and analyzed by bidirectional DNA sequencing of the classic OCA genes--tyrosinase (TYR, or oculocutaneous albinism IA), pink eyed dilution (P; or oculocutaneous albinism II (OCA2]), tyrosinase-related protein 1 (TYRP1), solute carrier family 45, member 2 (SLC45A2; or membrane-associated transporter protein [MATP])--and the OA1 gene, G protein-coupled receptor 143 (GPR143).

RESULTS

Three missense mutations, c. 715 C>T (R239W), c. 896 G>A (R299H), c.1255 G>A (G419R), and one termination c. 832 C>T (R278X), were identified in TYR, as well as one novel mutation, c.1453 G>A (G485R) in P. One novel single nucleotide polymorphism (SNP) was identified in both TYR and P; few reported SNPs were identified. The G>A base substitution caused relatively conservative amino acid changes, which altered glycine to arginine residues within the topological domain. The novel OCA2 mutation was not present in 100 control samples. This study identified two probands carrying mutations alone, 16 probands carrying SNPs alone, 4 probands carrying both mutations and SNPs and only one proband carrying neither mutations nor SNPs.

CONCLUSIONS

Although sequence analysis was performed with all five candidate genes, only four (17.39%) of the 23 probands showed mutations in TYR and 2 probands (8.69%) showed an unreported novel mutation in P. Genetic counseling for phenotypical diagnosis and genetic mutation screening of these genes will help to minimize the incidence of OCA and OA in future generations.

Comprehensive analysis of the molecular basis of oculocutaneous albinism in Indian patients lacking a mutation in the tyrosinase gene

BACKGROUND

Oculocutaneous albinism (OCA) refers to a group of inherited disorders where the patients have little or no pigment in the eyes, skin and hair. Mutations in genes regulating multi-step melanin biosynthesis are the basis of four 'classical' OCA types with overlapping clinical features. There are a few reports on defects in TYR and a single report on SLC45A2 in Indians affected with OCA but no report on OCA2 (a major locus related to the disease) and TYRP1.

OBJECTIVES

To assess and describe a comprehensive picture of the molecular genetic basis of OCA among Indians with no apparent mutations in TYR.

METHODS

Twenty-four affected pedigrees from 14 different ethnicities were analysed for mutations in OCA2, TYRP1, SLC45A2 and SLC24A5 using the polymerase chain reaction-sequencing approach.

RESULTS

Two splice-site and four missense mutations were detected in OCA2 in seven unrelated pedigrees, including four novel mutations. Haplotype analysis revealed a founder mutation (Ala787Thr) in two unrelated families of the same ethnicity. A patient homozygous for a novel SLC45A2 mutation also harboured a novel OCA2 defect. No mutation was detected in TYRP1 or SLC24A5.

CONCLUSIONS

Our results suggest that an OCA2 gene defect is the second most prevalent type of OCA in India after TYR. The presence of homozygous mutations in the affected pedigrees underscores the lack of intermixing between the affected ethnicities. Direct detection of the genetic lesions prevalent in specific ethnic groups could be used for carrier detection and genetic counselling to contain the disease.

The unfolded protein response in melanocytes: activation in response to chemical stressors of the endoplasmic reticulum and tyrosinase misfolding

Accumulation of proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response (UPR), comprising three signaling pathways initiated by Ire1, Perk and Atf6 respectively. Unfolded protein response activation was compared in chemically stressed murine wildtype melanocytes and mutant melanocytes that retain tyrosinase in the ER. Thapsigargin, an ER stressor, activated all pathways in wildtype melanocytes, triggering Caspase 12-mediated apoptosis at toxic doses. Albino melanocytes expressing mutant tyrosinase showed evidence of ER stress with increased Ire1 expression, but the downstream effector, Xbp1, was not activated even following thapsigargin treatment. Attenuation of Ire1 signaling was recapitulated in wildtype melanocytes treated with thapsigargin for 8 days, with diminished Xbp1 activation observed after 4 days. Atf6 was also activated in albino melanocytes, with no response to thapsigargin, while the Perk pathway was not activated and thapsigargin treatment elicited robust expression of the downstream effector CCAAT-enhancer-binding protein homologous protein. Thus, melanocytes adapt to ER stress by attenuating two UPR pathways.

Management of melanoma families

In this review we have aimed to focus on the clinical management of familial melanoma patients and their relatives. Along this line three major topics will be discussed: (1) management/screening of familial melanoma families: what is advised and what is the evidence thereof; (2) variability of families worldwide with regard to clinical phenotype, including cancer spectrum and likelihood of finding germline mutations and (3) background information for clinicians on the molecular biology of familial melanoma and recent developments in this field.

Inheritance of a novel mutated allele of the OCA2 gene associated with high incidence of oculocutaneous albinism in a Polynesian community

Oculocutaneous albinism type 2 (OCA2) is a human autosomal-recessive hypopigmentation disorder associated with pathological mutations of the OCA2 gene. In this study, we investigated a form of OCA in a Polynesian population with an observed phenotype characterized by fair skin, some brown nevi present in the sun-exposed areas and green or blue eyes. Hair presented with a unique red coloration since birth, with tones ranging across individuals from Yellow-Red to Brown-Red, or Auburn. We genetically screened for mutations in the OCA2 and MC1R genes as their products have previously been shown to be associated with red hair/fair skin and OCA2. The SLC45A2 gene was also screened to identify any possible relation to skin color variation. We have identified a novel missense substitution in the OCA2 gene (Gly775Asp) responsible for OCA2 in individuals of Polynesian heritage from Tuvalu. The estimated incidence of this form of OCA2 in the primary study community is believed to occur at one of the highest recorded rates of albinism at approximately 1 per 669 individuals. In addition, we have analyzed four unrelated individuals with albinism who have Polynesian heritage from three other separate communities and found they carry the same OCA2 mutation. We also analyzed an out-group comprising three unrelated individuals with albinism of Melanesian ancestries from two separate communities, one Australian Aboriginal and three Australian Caucasians, and did not detect this mutation. We hypothesize that this mutation may be Polynesian specific and that it originated from a common founder.

New insights into the active site structure and catalytic mechanism of tyrosinase and its related proteins

Tyrosinases are widely distributed in nature. They are copper-containing oxidases belonging to the type 3 copper protein family, together with catechol oxidases and haemocyanins. Tyrosinases are essential enzymes in melanin biosynthesis and therefore responsible for pigmentation of skin and hair in mammals, where two more enzymes, the tyrosinase-related proteins (Tyrps), participate in the pathway. The structure and catalytic mechanism of mammalian tyrosinases have been extensively studied but they are not completely understood because of the lack of information on the tertiary structure. The availability of crystallographic data of one plant catechol oxidase and one bacterial tyrosinase has improved the model of the three-dimensional structure of the active site of the enzyme. Furthermore, sequence comparison of tyrosinase and the Tyrps reveals that the three orthologue proteins share many key structural features, because of their common origin from an ancestral gene, although the specific residues responsible for their different catalytic capabilities have not been identified yet. This review summarizes our current knowledge of tyrosinase and Tyrps structure and function and describes the catalytic mechanism of tyrosinase and Dct/Tyrp2, which are better characterized.

Involvement of ABC transporters in melanogenesis and the development of multidrug resistance of melanoma

Because melanomas are intrinsically resistant to conventional radiotherapy and chemotherapy, many alternative treatment approaches have been developed such as biochemotherapy and immunotherapy. The most common cause of multidrug resistance (MDR) in human cancers is the expression and function of one or more ATP-binding cassette (ABC) transporters that efflux anticancer drugs from cells. Melanoma cells express a group of ABC transporters (such as ABCA9, ABCB1, ABCB5, ABCB8, ABCC1, ABCC2, and ABCD1) that may be associated with the resistance of melanoma cells to a broad range of anticancer drugs and/or of melanocytes to toxic melanin intermediates and metabolites. In this review, we propose a model (termed the ABC-M model) in which the intrinsic MDR of melanoma cells is at least in part because of the transporter systems that may also play a critical role in reducing the cytotoxicity of the melanogenic pathway in melanocytes. The ABC-M model suggests molecular strategies to reverse MDR function in the context of the melanogenic pathway, which could open therapeutic avenues towards the ultimate goal of circumventing clinical MDR in patients with melanoma.

Influence of melanosome dynamics on melanoma drug sensitivity

BACKGROUND

Malignant melanomas are intrinsically resistant to many conventional treatments, such as radiation and chemotherapy, for reasons that are poorly understood. Here we propose and test a model that explains drug resistance or sensitivity in terms of melanosome dynamics.

METHODS

The growth and sensitivity to cisplatin of MNT-1 cells, which are melanotic and enriched with mature stage III and IV melanosomes, and SK-MEL-28 cells, which have only immature stage I and II melanosomes, were compared using clonogenic assays. Differences in pigmentation, melanosome stages, melanosome number, and cellular structures in different cell lines in response to various treatments were examined by electron microscopy. The relative numbers of melanosomes of different stages were compared after treatment with 1-phenyl-2-thiourea. The relationship between drug transporter function and endogenous melanogenic toxicity was assessed by treating cells with the cyclosporin analog PSC-833 and by assessing vacuole formation and cell growth inhibition. All statistical tests were two-sided.

RESULTS

Endogenous melanogenic cytotoxicity, produced by damaged melanosomes, resulted in pronounced cell growth inhibition in MNT-1 cells compared with amelanotic SK-MEL-28 cells. The sensitivity to CDDP of MNT-1 cells was 3.8-fold higher than that of SK-MEL-28 cells (mean IC(50) for SK-MEL-28 and MNT-1 = 2.13 microM and 0.56 microM, respectively; difference = 1.57 microM, 95% confidence interval = 1.45 to 1.69; P = .0017). After treatment with 6.7 microM CDDP for 72 hours, the number of stage II-III melanosomes in surviving MNT-1 cells was 6.8-fold that of untreated cells. Modulation of MNT-1 cells to earlier-stage (II, II-III, III) melanosomes by treatment with the tyrosinase inhibitor 1-phenyl-2-thiourea dramatically increased CDDP resistance. Furthermore, PSC-833 principally suppressed MNT-1 melanotic cell growth via an elevation of autophagosome-like vacuolar structures, possibly by inhibiting melanosome membrane transporters.

CONCLUSIONS

Melanosome dynamics (including their biogenesis, density, status, and structural integrity) regulate the drug resistance of melanoma cells. Manipulation of melanosome functions may be an effective way to enhance the therapeutic activity of anticancer drugs against melanoma.

Localization to mature melanosomes by virtue of cytoplasmic dileucine motifs is required for human OCA2 function

Oculocutaneous albinism type 2 is caused by defects in the gene OCA2, encoding a pigment cell-specific, 12-transmembrane domain protein with homology to ion permeases. The function of the OCA2 protein remains unknown, and its subcellular localization is under debate. Here, we show that endogenous OCA2 in melanocytic cells rapidly exits the endoplasmic reticulum (ER) and thus does not behave as a resident ER protein. Consistently, exogenously expressed OCA2 localizes within melanocytes to melanosomes, and, like other melanosomal proteins, localizes to lysosomes when expressed in nonpigment cells. Mutagenized OCA2 transgenes stimulate melanin synthesis in OCA2-deficient cells when localized to melanosomes but not when specifically retained in the ER, contradicting a proposed primary function for OCA2 in the ER. Steady-state melanosomal localization requires a conserved consensus acidic dileucine-based sorting motif within the cytoplasmic N-terminal region of OCA2. A second dileucine signal within this region confers steady-state lysosomal localization in melanocytes, suggesting that OCA2 might traverse multiple sequential or parallel trafficking routes. The two dileucine signals physically interact in a differential manner with cytoplasmic adaptors known to function in trafficking other proteins to melanosomes. We conclude that OCA2 is targeted to and functions within melanosomes but that residence within melanosomes may be regulated by secondary or alternative targeting to lysosomes.