The microphthalmia transcription factor (Mitf) controls expression of the ocular albinism type 1 gene: link between melanin synthesis and melanosome biogenesis

Melanocytes in regenerative medicine applications and disease modeling

Melanocytes are dendritic cells localized in skin, eyes, hair follicles, ears, heart and central nervous system. They are characterized by the presence of melanosomes enriched in melanin which are responsible for skin, eye and hair pigmentation. They also have different functions in photoprotection, immunity and sound perception. Melanocyte dysfunction can cause pigmentary disorders, hearing and vision impairments or increased cancer susceptibility. This review focuses on the role of melanocytes in homeostasis and disease, before discussing their potential in regenerative medicine applications, such as for disease modeling, drug testing or therapy development using stem cell technologies, tissue engineering and extracellular vesicles.

mitfa deficiency promotes immune vigor and potentiates antitumor effects in zebrafish

The mitfa gene is a well-known transcription factor associated with microphthalmia and is essential for early melanophore development. However, little is known about how mitfa affects the immune system. Here, we generated a novel mitfa knock-out zebrafish line using the CRISPR/Cas9 system. The mitfa-/- zebrafish exhibited reduced melanin levels compared to the nacre mutant. We investigated the impact on the immune system after exposure to Edwardsiella tarda and bifenazate in zebrafish larvae, and observed that the macrophage numbers were reduced in both treated groups. Remarkably, the expression levels of immune-related genes exhibited significant increases after bacterial challenge or bifenazate exposure in the mitfa-/- zebrafish, except for tlr4 and rela. Furthermore, we conducted xenograft experiments using mouse B16 melanoma cells. Notably, the cancer cells didn't show a high cell migration ratio, implying that the immune system was highly activated after the loss of mifta. Taken together, our findings suggest that mitfa-/- zebrafish serve as a valuable model for investigating the relationship between the immune system and melanocytes, providing new insights into the role of mitfa in immune responses.

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.

A Network of MicroRNAs and mRNAs Involved in Melanosome Maturation and Trafficking Defines the Lower Response of Pigmentable Melanoma Cells to Targeted Therapy

BACKGROUND

The ability to increase their degree of pigmentation is an adaptive response that confers pigmentable melanoma cells higher resistance to BRAF inhibitors (BRAFi) compared to non-pigmentable melanoma cells.

METHODS

Here, we compared the miRNome and the transcriptome profile of pigmentable 501Mel and SK-Mel-5 melanoma cells vs. non-pigmentable A375 melanoma cells, following treatment with the BRAFi vemurafenib (vem). In depth bioinformatic analyses (clusterProfiler, WGCNA and SWIMmeR) allowed us to identify the miRNAs, mRNAs and biological processes (BPs) that specifically characterize the response of pigmentable melanoma cells to the drug. Such BPs were studied using appropriate assays in vitro and in vivo (xenograft in zebrafish embryos).

RESULTS

Upon vem treatment, miR-192-5p, miR-211-5p, miR-374a-5p, miR-486-5p, miR-582-5p, miR-1260a and miR-7977, as well as GPR143, OCA2, RAB27A, RAB32 and TYRP1 mRNAs, are differentially expressed only in pigmentable cells. These miRNAs and mRNAs belong to BPs related to pigmentation, specifically melanosome maturation and trafficking. In fact, an increase in the number of intracellular melanosomes-due to increased maturation and/or trafficking-confers resistance to vem.

CONCLUSION

We demonstrated that the ability of pigmentable cells to increase the number of intracellular melanosomes fully accounts for their higher resistance to vem compared to non-pigmentable cells. In addition, we identified a network of miRNAs and mRNAs that are involved in melanosome maturation and/or trafficking. Finally, we provide the rationale for testing BRAFi in combination with inhibitors of these biological processes, so that pigmentable melanoma cells can be turned into more sensitive non-pigmentable cells.

The retinal pigmentation pathway in human albinism: Not so black and white

Albinism is a pigment disorder affecting eye, skin and/or hair. Patients usually have decreased melanin in affected tissues and suffer from severe visual abnormalities, including foveal hypoplasia and chiasmal misrouting. Combining our data with those of the literature, we propose a single functional genetic retinal signalling pathway that includes all 22 currently known human albinism disease genes. We hypothesise that defects affecting the genesis or function of different intra-cellular organelles, including melanosomes, cause syndromic forms of albinism (Hermansky-Pudlak (HPS) and Chediak-Higashi syndrome (CHS)). We put forward that specific melanosome impairments cause different forms of oculocutaneous albinism (OCA1-8). Further, we incorporate GPR143 that has been implicated in ocular albinism (OA1), characterised by a phenotype limited to the eye. Finally, we include the SLC38A8-associated disorder FHONDA that causes an even more restricted "albinism-related" ocular phenotype with foveal hypoplasia and chiasmal misrouting but without pigmentation defects. We propose the following retinal pigmentation pathway, with increasingly specific genetic and cellular defects causing an increasingly specific ocular phenotype: (HPS1-11/CHS: syndromic forms of albinism)-(OCA1-8: OCA)-(GPR143: OA1)-(SLC38A8: FHONDA). Beyond disease genes involvement, we also evaluate a range of (candidate) regulatory and signalling mechanisms affecting the activity of the pathway in retinal development, retinal pigmentation and albinism. We further suggest that the proposed pigmentation pathway is also involved in other retinal disorders, such as age-related macular degeneration. The hypotheses put forward in this report provide a framework for further systematic studies in albinism and melanin pigmentation disorders.

MITF in Normal Melanocytes, Cutaneous and Uveal Melanoma: A Delicate Balance

Microphthalmia-associated transcription factor (MITF) is an important regulator of melanogenesis and melanocyte development. Although it has been studied extensively in cutaneous melanoma, the role of MITF in uveal melanoma (UM) has not been explored in much detail. We review the literature about the role of MITF in normal melanocytes, in cutaneous melanoma, and in UM. In normal melanocytes, MITF regulates melanocyte development, melanin synthesis, and melanocyte survival. The expression profile and the behaviour of MITF-expressing cells suggest that MITF promotes local proliferation and inhibits invasion, inflammation, and epithelial-to-mesenchymal (EMT) transition. Loss of MITF expression leads to increased invasion and inflammation and is more prevalent in malignant cells. Cutaneous melanoma cells switch between MITF-high and MITF-low states in different phases of tumour development. In UM, MITF loss is associated with loss of BAP1 protein expression, which is a marker of poor prognosis. These data indicate a dual role for MITF in benign and malignant melanocytic cells.

The Many Faces of G Protein-Coupled Receptor 143, an Atypical Intracellular Receptor

GPCRs transform extracellular stimuli into a physiological response by activating an intracellular signaling cascade initiated via binding to G proteins. Orphan G protein-coupled receptors (GPCRs) hold the potential to pave the way for development of new, innovative therapeutic strategies. In this review we will introduce G protein-coupled receptor 143 (GPR143), an enigmatic receptor in terms of classification within the GPCR superfamily and localization. GPR143 has not been assigned to any of the GPCR families due to the lack of common structural motifs. Hence we will describe the most important motifs of classes A and B and compare them to the protein sequence of GPR143. While a precise function for the receptor has yet to be determined, the protein is expressed abundantly in pigment producing cells. Many GPR143 mutations cause X-linked Ocular Albinism Type 1 (OA1, Nettleship-Falls OA), which results in hypopigmentation of the eyes and loss of visual acuity due to disrupted visual system development and function. In pigment cells of the skin, loss of functional GPR143 results in abnormally large melanosomes (organelles in which pigment is produced). Studies have shown that the receptor is localized internally, including at the melanosomal membrane, where it may function to regulate melanosome size and/or facilitate protein trafficking to the melanosome through the endolysosomal system. Numerous additional roles have been proposed for GPR143 in determining cancer predisposition, regulation of blood pressure, development of macular degeneration and signaling in the brain, which we will briefly describe as well as potential ligands that have been identified. Furthermore, GPR143 is a promiscuous receptor that has been shown to interact with multiple other melanosomal proteins and GPCRs, which strongly suggests that this orphan receptor is likely involved in many different physiological actions.

Identification and characterization of two novel noncoding tyrosinase (TYR) gene variants leading to oculocutaneous albinism type 1

Oculocutaneous albinism type 1 (OCA1), resulting from pathogenic variants in the tyrosinase (TYR) gene, refers to a group of phenotypically heterogeneous autosomal recessive disorders characterized by a partial or a complete absence of pigment in the skin/hair and is also associated with common developmental eye defects. In this study, we identified two novel compound heterozygous TYR variants from a Chinese hypopigmentary patient by whole-exome sequencing. Specifically, the two variants were c.-89T>G, located at the core of the initiator E-box (Inr E-box) of the TYR promoter, and p.S16Y (c.47C>A), located within the signal sequence. We performed both in silico analysis and experimental validation and verified these mutations as OCA1 variants that caused either impaired or complete loss of function of TYR. Mechanistically, the Inr E-box variant dampened TYR binding to microphthalmia-associated transcription factor, a master transcriptional regulator of the melanocyte development, whereas the S16Y variant contributed to endoplasmic reticulum retention, a common and principal cause of impaired TYR activity. Interestingly, we found that the Inr E-box variant creates novel protospacer adjacent motif sites, recognized by nucleases SpCas9 and SaCas9-KKH, respectively, without compromising the functional TYR coding sequence. We further used allele-specific genomic editing by CRISPR activation to specifically target the variant promoter and successfully activated its downstream gene expression, which could lead to potential therapeutic benefits. In conclusion, this study expands the spectrum of TYR variants, especially those within the promoter and noncoding regions, which can facilitate genetic counseling and clinical diagnosis of OCA1.

Distinct skin morphological and transcriptomic profiles between wild and albino Oscar Astronotus ocellatus

Oscar Astronotus ocellatus is an important ornamental fish, including albino and wild varieties. Albino individuals attract aquarium hobbyists due to their unique body color, but studies on the species' albinism mechanism are currently scarce. Here, we investigated the morphological and transcriptomic profiles of the skin of albino and wild Oscar. The results showed that the albino type had fewer oval-shaped melanophores and immature melanosomes but that the wild type contained more stellate-shaped melanophores and mature melanosomes. Albino Oscar had a degenerative pigment layer without obvious melanin deposition and content, while the wild type contained more concentrated melanin within the pigment layer. A total of 272,392 unigenes were detected, 109 of which were identified as differentially expressed genes (DEGs) between albino and wild Oscar. Pathways of DEGs, including those involved in complement and coagulation cascades, novobiocin biosynthesis, Th1 and Th2 cell differentiation, and tropane, piperidine and pyridine alkaloid biosynthesis, were significantly enriched. DEGs, including upregulated Sfrp5 and Tat, and downregulated Wnt-10a, Ppp3c, Notch1 and Trim27 involved in the Wnt signaling pathway, Notch signaling pathway, tyrosine metabolism, MAPK signaling pathway and melanogenesis, might be associated with the albinism of Oscar. This study characterized the difference in melanophore morphology between wild and albino Oscar and identified some albinism-related candidate genes and signaling pathways, helping to understand the genetic mechanism of fish albinism.

Evidence for Protein-Protein Interaction between Dopamine Receptors and the G Protein-Coupled Receptor 143

Protein-protein interactions between G protein-coupled receptors (GPCRs) can augment their functionality and increase the repertoire of signaling pathways they regulate. New therapeutics designed to modulate such interactions may allow for targeting of a specific GPCR activity, thus reducing potential for side effects. Dopamine receptor (DR) heteromers are promising candidates for targeted therapy of neurological conditions such as Parkinson's disease since current treatments can have severe side effects. To facilitate development of such therapies, it is necessary to identify the various DR binding partners. We report here a new interaction partner for DRD₂ and DRD₃, the orphan receptor G protein-coupled receptor 143 (GPR143), an atypical GPCR that plays multiple roles in pigment cells and is expressed in several regions of the brain. We previously demonstrated that the DRD₂/ DRD₃ antagonist pimozide also modulates GPR143 activity. Using confocal microscopy and two FRET methods, we observed that the DRs and GPR143 colocalize and interact at intracellular membranes. Furthermore, co-expression of wildtype GPR143 resulted in a 57% and 67% decrease in DRD₂ and DRD₃ activity, respectively, as determined by β-Arrestin recruitment assay. GPR143-DR dimerization may negatively modulate DR activity by changing affinity for dopamine or delaying delivery of the DRs to the plasma membrane.

Role of Amine Neurotransmitters and Their Receptors in Skin Pigmentation: Therapeutic Implication

Skin pigmentation can occur due to increased melanin, including melanocyte proliferation, melanin biosynthesis, or melanocyte migration. There are many factors that influence the melanin production process, but the role of neurotransmitters in this process is still unclear. We found that histamine and serotonin influence the different stages of melanogenesis and melanogenesis, which increase melanogenesis. Since then, several related papers have been published, and from these papers, it has been recognised that the role of neurotransmitters in skin-pigment-related diseases needs to be summarised. By introducing the role of neurotransmitters in the regulation of various pigment disorders, including vitiligo and melasma, through this review, many researchers can be expected to try to apply neurotransmitter-related agonists and antagonists as treatments for skin pigment disorders.

Comparative transcriptome and histological analyses provide insights into the skin pigmentation in Minxian black fur sheep (Ovis aries)

Background

Minxian black fur (MBF) sheep are found in the northwestern parts of China. These sheep have developed several special traits. Skin color is a phenotype subject to strong natural selection and diverse skin colors are likely a consequence of differences in gene regulation.

Methods

Skin structure, color differences, and gene expression (determined by RNA sequencing) were evaluated the Minxian black fur and Small-tail Han sheep (n = 3 each group), which are both native Chinese sheep breeds.

Results

Small-tail Han sheep have a thicker skin and dermis than the Minxian black fur sheep (P < 0.01); however, the quantity of melanin granules is greater (P < 0.01) in Minxian black fur sheep with a more extensive distribution in skin tissue and hair follicles. One hundred thirty-three differentially expressed genes were significantly associated with 37 ontological terms and two critical KEGG pathways for pigmentation (“tyrosine metabolism” and “melanogenesis” pathways). Important genes from those pathways with known involvement in pigmentation included OCA2 melanosomal transmembrane protein (OCA2), dopachrome tautomerase (DCT), tyrosinase (TYR) and tyrosinase related protein (TYRP1), melanocortin 1 receptor (MC1R), and premelanosome protein (PMEL). The results from our histological and transcriptome analyses will form a foundation for additional investigation into the genetic basis and regulation of pigmentation in these sheep breeds.

Oregano Phytocomplex Induces Programmed Cell Death in Melanoma Lines via Mitochondria and DNA Damage

Plant secondary metabolites possess chemopreventive and antineoplastic properties, but the lack of information about their exact mechanism of action in mammalian cells hinders the translation of these compounds in suitable therapies. In light of this, firstly, Origanum vulgare L. hydroalcoholic extract was chemically characterized by spectrophotometric and chromatographic analyses; then, the molecular bases underlying its antitumor activity on B16-F10 and A375 melanoma cells were investigated. Oregano extract induced oxidative stress and inhibited melanogenesis and tumor cell proliferation, triggering programmed cell death pathways (both apoptosis and necroptosis) through mitochondria and DNA damage. By contrast, oregano extract was safe on healthy tissues, revealing no cytotoxicity and mutagenicity on C2C12 myoblasts, considered as non-tumor proliferating cell model system, and on Salmonella strains, by the Ames test. All these data provide scientific evidence about the potential application of this food plant as an anticancer agent in in vivo studies and clinical trials.

Detection and prognostic role of heterogeneous populations of melanoma circulating tumour cells

BACKGROUND

Circulating tumour cells (CTCs) can be assessed through a minimally invasive blood sample with potential utility as a predictive, prognostic and pharmacodynamic biomarker. The large heterogeneity of melanoma CTCs has hindered their detection and clinical application.

METHODS

Here we compared two microfluidic devices for the recovery of circulating melanoma cells. The presence of CTCs in 43 blood samples from patients with metastatic melanoma was evaluated using a combination of immunocytochemistry and transcript analyses of five genes by RT-PCR and 19 genes by droplet digital PCR (ddPCR), whereby a CTC score was calculated. Circulating tumour DNA (ctDNA) from the same patient blood sample, was assessed by ddPCR targeting tumour-specific mutations.

RESULTS

Our analysis revealed an extraordinary heterogeneity amongst melanoma CTCs, with multiple non-overlapping subpopulations. CTC detection using our multimarker approach was associated with shorter overall and progression-free survival. Finally, we found that CTC scores correlated with plasma ctDNA concentrations and had similar pharmacodynamic changes upon treatment initiation.

CONCLUSIONS

Despite the high phenotypic and molecular heterogeneity of melanoma CTCs, multimarker derived CTC scores could serve as viable tools for prognostication and treatment response monitoring in patients with metastatic melanoma.

Cell cycle regulatory markers in melanoma: New strategies in diagnosis and treatment

Background: Melanoma has been known as an aggressive type of skin cancer in recent years. Reports have distributed the spread rate of melanoma among white skin populations. Also, many studies have mentioned several causes of melanoma. Ultraviolet radiation was represented to be the most important reason for occurrence of melanoma. However, recent studies have found that a combination of factors, such as environmental and genetic factors, can contribute to occurrence of various cancers, specifically melanoma.

Methods: Different studies have been conducted on the efficacy of genetic disorders in melanoma. These surveys marked the key role of specific biomarkers in molecular and cellular processes, and investigations have found the expression of several genes in these processes. In addition, aberrant expression of these genes due to mutation and methylation can affect the whole process.

Results: The expression process of these genes is regulated by microRNAs. These new biomolecules have been considered as negative regulators because of managing molecular and cellular processes. MicroRNAs are small conserved regulators attached to their targets leading to rearrangement of gene expression. Adherence of these noncoding RNAs can cause mRNA degradation or inhibit its translation.

Conclusion: Recently, the application of specific genes in melanoma has been studied. In this review, the way melanoma is regulated because of these biomarkers and their demand through cell cycle in diagnosis, prognosis, and therapeutic periods was considered. Keywords: Melanoma, Biomarkers, Cell cycle, Biomolecules

Intrinsic and extrinsic regulation of human skin melanogenesis and pigmentation

In human skin, melanogenesis is a tightly regulated process. Indeed, several extracellular signals are transduced via dedicated signalling pathways and mostly converge to MITF, a transcription factor integrating upstream signalling and regulating downstream genes involved in the various inherent mechanisms modulating melanogenesis. The synthesis of melanin pigments occurs in melanocytes inside melanosomes where melanogenic enzymes (tyrosinase and related proteins) are addressed with the help of specific protein complexes. The melanosomes loaded with melanin are then transferred to keratinocytes. A more elaborate level of melanogenesis regulation comes into play via the action of non-coding RNAs (microRNAs, lncRNAs). Besides this canonical regulation, melanogenesis can also be modulated by other non-specific intrinsic pathways (hormonal environment, inflammation) and by extrinsic factors (solar irradiation such as ultraviolet irradiation, environmental pollution). We developed a bioinformatic interaction network gathering the multiple aspects of melanogenesis and skin pigmentation as a resource to better understand and study skin pigmentation biology.

MITF-M regulates melanogenesis in mouse melanocytes

BACKGROUND

Although the impact of the microphthalamia-associated transcription factor (Mitf) signaling pathway on melanocytes progression has been extensively studied, the specific molecular mechanisms behind MITF-M-enhanced melanin production in melanocytes still need to be clarified.

METHODS

In this study, we analyzed the levels of Mitf-M in skin tissues of different coat mice in order to further reveal the relationship between Mitf-M and skin pigmentation. To address the function of Mitf-M on melanogenesis, we have used an overexpression system and combined morphological and biochemical methods to investigate its localization in different coat color mice and pigmentation-related genes' expression in mouse melanocytes.

RESULTS

The qRT-PCR assay and Western blotting analysis revealed that Mitf-M mRNA and protein were synthesized in all tested mice skin samples, with the highest expression level in brown skin, a moderate expression level in grey skin and the lowest expression level in black skin. Simultaneously, immunofluorescence staining revealed that MITF-M was mainly expressed in the hair follicle matrix and inner and outer root sheath in the skin tissues with different coat colors. Furthermore, overexpression of MITF-M led to increased melanin content and variable pigmentation-related gene expression.

CONCLUSION

These results directly demonstrate that MITF-M not only influences melanogenesis, but also determines the progression of melanosomal protein in mouse melanocytes.

Administration of a polyphenol-enriched feed to farmed sea bass (Dicentrarchus labrax L.): Kidney melanomacrophages response

The reinforcement of the defense mechanism of fish, through the administration of immunostimulants, is considered as a promising alternative to vaccines. Natural immunostimulants such as polyphenols, flavanoids, pigments and essential oils can modulate the innate immune response. In lower vertebrates, melano-macrophage centres, i.e. clusters of pigment-containing cells forming the extracutaneous pigment system, are wide-spread in the stroma of the haemopoietic tissue, mainly in kidney and spleen. In fishes, melano-macrophage centres play an important role in the immune response against antigenic stimulants and pathogens. In the present study, we evaluated the effect of a polyphenol-enriched diet on the health status of European sea bass (Dicentrarchus labrax L.). Farmed sea bass were administered a feed containing a phytocomplex, rich in catechins and epigallocatechins, which was obtained from the seeds of Canosina Nero di Troia Vitis vinifera and mixed with conventional feed at two different concentrations. The effects of such a diet were investigated in juvenile and commercial size samples, i.e. undergoing a short- and long-term period of diet, respectively, focusing on their extracutaneous pigmentary system and, in more detail, on the enzymatic activities leading to melanin biosynthesis. Our results show that prolonged dietary treatments with higher concentration of polyphenols might modulate tyrosinase activity and gene expression in commercial size fishes. An increase of melano-macrophage activity is correlated to a stimulation of cytoprotective functions against antigenic stimulants and pathogens, as an expression of a robust and protective adaptive immune response.

Identification of Novel G Protein-Coupled Receptor 143 Ligands as Pharmacologic Tools for Investigating X-Linked Ocular Albinism

Purpose

GPR143 regulates melanosome biogenesis and organelle size in pigment cells. The mechanisms underlying receptor function remain unclear. G protein–coupled receptors (GPCRs) are excellent pharmacologic targets; thus, we developed and applied a screening approach to identify potential GPR143 ligands and chemical modulators.

Methods

GPR143 interacts with β-arrestin; we therefore established a β-arrestin recruitment assay to screen for compounds that modulate activity. Because GPR143 is localized intracellularly, screening with the wild-type receptor would be restricted to agents absorbed by the cell. For the screen we used a mutant receptor, which shows similar basal activity as the wild type but traffics to the plasma membrane. We tested two compound libraries and investigated validated hits for their effects on melanocyte pigmentation.

Results

GPR143, which showed high constitutive activity in the β-arrestin assay, was inhibited by several compounds. The three validated inhibitors (pimozide, niclosamide, and ethacridine lactate) were assessed for impact on melanocytes. Pigmentation and expression of tyrosinase, a key melanogenic enzyme, were reduced by all compounds. Because GPR143 appears to be constitutively active, these compounds may turn off its activity.

Conclusions

X-linked ocular albinism type I, characterized by developmental eye defects, results from GPR143 mutations. Identifying pharmacologic agents that modulate GPR143 activity will contribute significantly to our understanding of its function and provide novel tools with which to study GPCRs in melanocytes and retinal pigment epithelium. Pimozide, one of three GPR143 inhibitors identified in this study, maybe be a good lead structure for development of more potent compounds and provide a platform for design of novel therapeutic agents.

FGF21 regulates melanogenesis in alpaca melanocytes via ERK1/2-mediated MITF downregulation

Fibroblast growth factor 21 (FGF21) is known as a metabolic regulator to regulate the metabolism of glucose and lipids. However, the underlying mechanism of FGF21 on melanin synthesis remains unknown. Therefore, the current study investigates the effect of FGF21 on melanogenesis in alpaca melanocytes. We transfected the FGF21 into alpaca melanocytes, then detected the melanin contents, protein and mRNA levels of pigmentation-related genes in order to determine the melanogenesis-regulating pathway of FGF21. The results showed that FGF21 overexpression suppressed melanogenesis and decreased the expression of the major target genes termed microphthalmia-associated transcription factor (MITF) and its downstream genes, including tyrosinase (TYR) and tyrosinase-related protein 2 (TRP2). However FGF21 increased the expression of phospho-extracellular signal-regulated kinase (p-Erk1/2). In contrast, FGF21-siRNA, a small interference RNA mediating FGF21 silencing, abolished the inhibition of melanogenesis. Altogether, FGF21 may decrease melanogenesis in alpaca melanocytes via ERK activation and subsequent MITF downregulation, which is then followed by the suppression of melanogenic enzymes and melanin production.

Interaction between G Protein-Coupled Receptor 143 and Tyrosinase: Implications for Understanding Ocular Albinism Type 1

Developmental eye defects in X-linked ocular albinism type 1 are caused by G-protein coupled receptor 143 (GPR143) mutations. Mutations result in dysfunctional melanosome biogenesis and macromelanosome formation in pigment cells, including melanocytes and retinal pigment epithelium. GPR143, primarily expressed in pigment cells, localizes exclusively to endolysosomal and melanosomal membranes unlike most G protein-coupled receptors, which localize to the plasma membrane. There is some debate regarding GPR143 function and elucidating the role of this receptor may be instrumental for understanding neurogenesis during eye development and for devising therapies for ocular albinism type I. Many G protein-coupled receptors require association with other proteins to function. These G protein-coupled receptor-interacting proteins also facilitate fine-tuning of receptor activity and tissue specificity. We therefore investigated potential GPR143 interaction partners, with a focus on the melanogenic enzyme tyrosinase. GPR143 coimmunoprecipitated with tyrosinase, while confocal microscopy demonstrated colocalization of the proteins. Furthermore, tyrosinase localized to the plasma membrane when coexpressed with a GPR143 trafficking mutant. The physical interaction between the proteins was confirmed using fluorescence resonance energy transfer. This interaction may be required in order for GPR143 to function as a monitor of melanosome maturation. Identifying tyrosinase as a potential GPR143 binding protein opens new avenues for investigating the mechanisms that regulate pigmentation and neurogenesis.

Ocular Albinism Type 1 Regulates Melanogenesis in Mouse Melanocytes

To investigate whether ocular albinism type 1 (OA1) is differentially expressed in the skin of mice with different coat colors and to determine its correlation with coat color establishment in mouse, the expression patterns and tissue distribution characterization of OA1 in the skin of mice with different coat colors were qualitatively and quantitatively analyzed by real-time quantitative PCR (qRT-PCR), immunofluorescence staining and Western blot. The qRT-PCR analysis revealed that OA1 mRNA was expressed in all mice skin samples tested, with the highest expression level in brown skin, a moderate expression level in black skin and the lowest expression level in gray skin. Positive OA1 protein bands were also detected in all skin samples by Western blot analysis. The relative expression levels of OA1 protein in both black and brown skin were significantly higher than that in gray skin, but there was no significant difference between black and brown mice. Immunofluorescence assays revealed that OA1 was mainly expressed in the hair follicle matrix, the inner and outer root sheath in the skin tissues with different coat colors. To get further insight into the important role of OA1 in the melanocytes’ pigmentation, we transfected the OA1 into mouse melanocytes and then detected the relative expression levels of pigmentation-related gene. Simultaneously, we tested the melanin content of melanocytes. As a result, the overexpression of OA1 significantly increased the expression levels of microphthalmia-associated transcription factor (MITF), tyrosinase (TYR), tyrosinase-related protein 1 (TRP1) and premelanosome protein (PMEL). However, the tyrosinase-related protein 2 (TRP2) level was attenuated. By contrast, the level of glycoprotein non-metastatic melanoma protein b (GPNMB) was unaffected by OA1 overexpression. Furthermore, we observed a significant increase in melanin content in mouse melanocyte transfected OA1. Therefore, we propose that OA1 may participate in the formation of coat color by regulating the level of MITF and the number, size, motility and maturation of melanosome.

Nanodiamonds coupled with 5,7-dimethoxycoumarin, a plant bioactive metabolite, interfere with the mitotic process in B16F10 cells altering the actin organization

For the first time, we coupled reduced detonation nanodiamonds (NDs) with a plant secondary metabolite, citropten (5,7-dimethoxycoumarin), and demonstrated how this complex was able to reduce B16F10 tumor cell growth more effectively than treatment with the pure molecule. These results encouraged us to find out the specific mechanism underlying this phenomenon. Internalization kinetics and quantification of citropten in cells after treatment with its pure or ND-conjugated form were measured, and it was revealed that the coupling between NDs and citropten was essential for the biological properties of the complex. We showed that the adduct was not able to induce apoptosis, senescence, or differentiation, but it determined cell cycle arrest, morphological changes, and alteration of mRNA levels of the cytoskeletal-related genes. The identification of metaphasic nuclei and irregular disposition of β-actin in the cell cytoplasm supported the hypothesis that citropten conjugated with NDs showed antimitotic properties in B16F10 cells. This work can be considered a pioneering piece of research that could promote and support the biomedical use of plant drug-functionalized NDs in cancer therapy.

The newt (Cynops pyrrhogaster) RPE65 promoter: molecular cloning, characterization and functional analysis

The adult newt has the ability to regenerate the neural retina following injury, a process achieved primarily by the retinal pigment epithelium (RPE). To deliver exogenous genes to the RPE for genetic manipulation of regenerative events, we isolated the newt RPE65 promoter region by genome walking. First, we cloned the 2.8 kb RPE65 promoter from the newt, Cynops pyrrhogaster. Sequence analysis revealed several conserved regulatory elements described previously in mouse and human RPE65 promoters. Second, having previously established an I-SceI-mediated transgenic protocol for the newt, we used it here to examine the -657 bp proximal promoter of RPE65. The promoter assay used with F0 transgenic newts confirmed transgene expression of mCherry fluorescent protein in the RPE. Using bioinformatic tools and the TRANSFAC database, we identified a 340 bp CpG island located between -635 and -296 bp in the promoter; this region contains response elements for the microphthalmia-associated transcription factor known as MITF (CACGTG, CATGTG), and E-boxes (CANNTG). Sex-determining region box 9 (or SOX9) response element previously reported in the regulation of RPE genes (including RPE65) was also identified in the newt RPE65 promoter. Third, we identified DNA motif boxes in the newt RPE65 promoter that are conserved among other vertebrates. The newt RPE65 promoter is an invaluable tool for site-specific delivery of exogenous genes or genetic manipulation systems for the study of retinal regeneration in this animal.

A molecular systems approach to modelling human skin pigmentation: identifying underlying pathways and critical components

BACKGROUND

Ultraviolet radiations (UV) serve as an environmental stress for human skin, and result in melanogenesis, with the pigment melanin having protective effects against UV induced damage. This involves a dynamic and complex regulation of various biological processes that results in the expression of melanin in the outer most layers of the epidermis, where it can exert its protective effect. A comprehensive understanding of the underlying cross talk among different signalling molecules and cell types is only possible through a systems perspective. Increasing incidences of both melanoma and non-melanoma skin cancers necessitate the need to better comprehend UV mediated effects on skin pigmentation at a systems level, so as to ultimately evolve knowledge-based strategies for efficient protection and prevention of skin diseases.

METHODS

A network model for UV-mediated skin pigmentation in the epidermis was constructed and subjected to shortest path analysis. Virtual knock-outs were carried out to identify essential signalling components.

RESULTS

We describe a network model for UV-mediated skin pigmentation in the epidermis. The model consists of 265 components (nodes) and 429 directed interactions among them, capturing the manner in which one component influences the other and channels information. Through shortest path analysis, we identify novel signalling pathways relevant to pigmentation. Virtual knock-outs or perturbations of specific nodes in the network have led to the identification of alternate modes of signalling as well as enabled determining essential nodes in the process.

CONCLUSIONS

The model presented provides a comprehensive picture of UV mediated signalling manifesting in human skin pigmentation. A systems perspective helps provide a holistic purview of interconnections and complexity in the processes leading to pigmentation. The model described here is extensive yet amenable to expansion as new data is gathered. Through this study, we provide a list of important proteins essential for pigmentation which can be further explored to better understand normal pigmentation as well as its pathologies including vitiligo and melanoma, and enable therapeutic intervention.

PAX6 regulates melanogenesis in the retinal pigmented epithelium through feed-forward regulatory interactions with MITF

During organogenesis, PAX6 is required for establishment of various progenitor subtypes within the central nervous system, eye and pancreas. PAX6 expression is maintained in a variety of cell types within each organ, although its role in each lineage and how it acquires cell-specific activity remain elusive. Herein, we aimed to determine the roles and the hierarchical organization of the PAX6-dependent gene regulatory network during the differentiation of the retinal pigmented epithelium (RPE). Somatic mutagenesis of Pax6 in the differentiating RPE revealed that PAX6 functions in a feed-forward regulatory loop with MITF during onset of melanogenesis. PAX6 both controls the expression of an RPE isoform of Mitf and synergizes with MITF to activate expression of genes involved in pigment biogenesis. This study exemplifies how one kernel gene pivotal in organ formation accomplishes a lineage-specific role during terminal differentiation of a single lineage.

It is currently poorly understood how a single developmental transcription regulator controls early specification as well as a broad range of highly specialized differentiation schemes. PAX6 is one of the most extensively investigated factors in central nervous system development, yet its role in execution of lineage-specific programs remains mostly elusive. Here, we directly investigated the involvement of PAX6 in the differentiation of one lineage, the retinal pigmented epithelium (RPE), a neuroectodermal-derived tissue that is essential for retinal development and function. We revealed that PAX6 accomplishes its role through a unique regulatory interaction with the transcription factor MITF, a master regulator of the pigmentation program. During the differentiation of the RPE, PAX6 regulates the expression of an RPE-specific isoform of Mitf and importantly, at the same time, PAX6 functions together with MITF to directly activate the expression of downstream genes required for pigment biogenesis. These findings provide comprehensive insight into the gene hierarchy that controls RPE development: from a kernel gene (a term referring to the upper-most gene in the gene regulatory network) that is broadly expressed during CNS development through a lineage-specific transcription factor that together with the kernel gene creates cis-regulatory input that contributes to transcriptionally activate a battery of terminal differentiation genes.

Melanoma: from melanocyte to genetic alterations and clinical options

Metastatic melanoma remained for decades without any effective treatment and was thus considered as a paradigm of cancer resistance. Recent progress with understanding of the molecular mechanisms underlying melanoma initiation and progression revealed that melanomas are genetically and phenotypically heterogeneous tumors. This recent progress has allowed for the development of treatment able to improve for the first time the overall disease-free survival of metastatic melanoma patients. However, clinical responses are still either too transient or limited to restricted patient subsets. The complete cure of metastatic melanoma therefore remains a challenge in the clinic. This review aims to present the recent knowledge and discoveries of the molecular mechanisms involved in melanoma pathogenesis and their exploitation into clinic that have recently facilitated bench to bedside advances.

A dual role for SOX10 in the maintenance of the postnatal melanocyte lineage and the differentiation of melanocyte stem cell progenitors

During embryogenesis, the transcription factor, Sox10, drives the survival and differentiation of the melanocyte lineage. However, the role that Sox10 plays in postnatal melanocytes is not established. We show in vivo that melanocyte stem cells (McSCs) and more differentiated melanocytes express SOX10 but that McSCs remain undifferentiated. Sox10 knockout (Sox10(fl); Tg(Tyr::CreER)) results in loss of both McSCs and differentiated melanocytes, while overexpression of Sox10 (Tg(DctSox10)) causes premature differentiation and loss of McSCs, leading to hair graying. This suggests that levels of SOX10 are key to normal McSC function and Sox10 must be downregulated for McSC establishment and maintenance. We examined whether the mechanism of Tg(DctSox10) hair graying is through increased expression of Mitf, a target of SOX10, by asking if haploinsufficiency for Mitf (Mitf(vga9) ) can rescue hair graying in Tg(DctSox10) animals. Surprisingly, Mitf(vga9) does not mitigate but exacerbates Tg(DctSox10) hair graying suggesting that MITF participates in the negative regulation of Sox10 in McSCs. These observations demonstrate that while SOX10 is necessary to maintain the postnatal melanocyte lineage it is simultaneously prevented from driving differentiation in the McSCs. This data illustrates how tissue-specific stem cells can arise from lineage-specified precursors through the regulation of the very transcription factors important in defining that lineage.

Melanoma: from mutations to medicine

Melanoma is often considered one of the most aggressive and treatment-resistant human cancers. It is a disease that, due to the presence of melanin pigment, was accurately diagnosed earlier than most other malignancies and that has been subjected to countless therapeutic strategies. Aside from early surgical resection, no therapeutic modality has been found to afford a high likelihood of curative outcome. However, discoveries reported in recent years have revealed a near avalanche of breakthroughs in the melanoma field-breakthroughs that span fundamental understanding of the molecular basis of the disease all the way to new therapeutic strategies that produce unquestionable clinical benefit. These discoveries have been born from the successful fruits of numerous researchers working in many-sometimes-related, although also distinct-biomedical disciplines. Discoveries of frequent mutations involving BRAF(V600E), developmental and oncogenic roles for the microphthalmia-associated transcription factor (MITF) pathway, clinical efficacy of BRAF-targeted small molecules, and emerging mechanisms underlying resistance to targeted therapeutics represent just a sample of the findings that have created a striking inflection in the quest for clinically meaningful progress in the melanoma field.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Study of a US cohort supports the role of ZNF644 and high-grade myopia susceptibility

PURPOSE

Myopia, or nearsightedness, is highly prevalent in Asian countries and is considered a serious public health issue globally. High-grade myopia can predispose individuals to myopic maculopathy, premature cataracts, retinal detachment, and glaucoma. A recent study implicated zinc finger protein 644 isoform 1 (ZNF644) variants with non-syndromic high-grade myopia in a Chinese-Asian population. Herein we focused on investigating the role for ZNF644 variants in high-grade myopia in a United States (US) cohort.

METHODS

DNA from a case cohort of 131 subject participants diagnosed with high-grade myopia was screened for ZNF644 variants. Spherical refractive error of -≤-6.00 diopters (D) in at least one eye was defined as affected. All coding, intron/exon boundaries were screened using Sanger sequencing. Single nucleotide allele frequencies were determined by screening 672 ethnically matched controls.

RESULTS

Sequencing analysis did not detect previously reported mutations. However, our analysis identified 2 novel single nucleotide variants (c.725C>T, c.821A>T) in 2 high-grade myopia individuals- one Caucasian and one African American, respectively. These variants were not found in normal controls. A rare variant - dbsSNP132 (rs12117237→c.2119A>G) - with a minor allele frequency of 0.2% was present in 6 additional cases, but was also present in 5 controls.

CONCLUSIONS

Our study has identified two novel variants in ZNF644 associated with high-grade myopia in a US cohort. Our results suggest that ZNF644 may play a role in myopia development.

A small molecule inhibitor of Mitf-E-box DNA binding and its depigmenting effect in melan-a cells

BACKGROUND

Microphthalmia associated transcription factor (Mitf) is a key regulatory transcriptional factor of pigmentation-related genes including tyrosinase. Inhibition of tyrosinase transcription by blocking the binding of Mitf with its promoter E-box DNA can control the pigmentation. However, no such chemicals were reported so far.

OBJECTIVE

To discover and evaluate the small molecule inhibitors of Mitf-E-box DNA.

METHODS

Candidate chemicals were screened by virtual screening from pharmacophore data followed by Mitf E-box DNA protein chip. After selecting the chemical, its inhibitory activity on binding interaction between Mitf and E-box DNA, electrophoretic mobility shift assay (EMSA) was performed. To evaluate the depigmenting activity of Compound #17, cellular melanin assa, and Western blot were performed in melan-a cells.

RESULTS

Among 27 chemicals selected from a pharmacophore data by virtual screening, Compound #17 was screened, which showed the most potent inhibitory activity against Mitf-E-box DNA binding in protein chip. EMSA results confirmed the specific inhibition of Compound #17 on Mitf-E-box DNA binding. In melan-a cells, Compound #17 reduced tyrosinase expression and melanin synthesis (62.5% at 25 μM).

CONCLUSIONS

  The results show that Compound #17 is the first small molecule inhibitor of Mitf-E-box DNA binding with depigmenting activity.

MicroRNA-restricted transgene expression in the retina

Background

Gene transfer using adeno-associated viral (AAV) vectors has been successfully applied in the retina for the treatment of inherited retinal dystrophies. Recently, microRNAs have been exploited to fine-tune transgene expression improving therapeutic outcomes. Here we evaluated the ability of retinal-expressed microRNAs to restrict AAV-mediated transgene expression to specific retinal cell types that represent the main targets of common inherited blinding conditions.

Methodology/Principal Findings

To this end, we generated AAV2/5 vectors expressing EGFP and containing four tandem copies of miR-124 or miR-204 complementary sequences in the 3′UTR of the transgene expression cassette. These vectors were administered subretinally to adult C57BL/6 mice and Large White pigs. Our results demonstrate that miR-124 and miR-204 target sequences can efficiently restrict AAV2/5-mediated transgene expression to retinal pigment epithelium and photoreceptors, respectively, in mice and pigs. Interestingly, transgene restriction was observed at low vector doses relevant to therapy.

Conclusions

We conclude that microRNA-mediated regulation of transgene expression can be applied in the retina to either restrict to a specific cell type the robust expression obtained using ubiquitous promoters or to provide an additional layer of gene expression regulation when using cell-specific promoters.

An integrated approach to uncover drivers of cancer

Systematic characterization of cancer genomes has revealed a staggering number of diverse aberrations that differ among individuals, such that the functional importance and physiological impact of most tumor genetic alterations remain poorly defined. We developed a computational framework that integrates chromosomal copy number and gene expression data for detecting aberrations that promote cancer progression. We demonstrate the utility of this framework using a melanoma data set. Our analysis correctly identified known drivers of melanoma and predicted multiple tumor dependencies. Two dependencies, TBC1D16 and RAB27A, confirmed empirically, suggest that abnormal regulation of protein trafficking contributes to proliferation in melanoma. Together, these results demonstrate the ability of integrative Bayesian approaches to identify candidate drivers with biological, and possibly therapeutic, importance in cancer.

The ethyl acetate extract of PGP (Phellinus linteus grown on Panax ginseng) suppresses B16F10 melanoma cell proliferation through inducing cellular differentiation and apoptosis

ETHNOPHARMACOLOGICAL RELEVANCE

Phellinus linteus and Panax ginseng have been widely used as traditional herbal medicines to treat various diseases including cancer in East Asia.

AIM OF THE STUDY

The present study sought to investigate the possible mechanism in anti-proliferative effect of Phellinus linteus that was grown on Panax ginseng (PGP) on B16F10 melanoma cells.

MATERIALS AND METHODS

The anti-proliferative effect of PGP on B16F10 was evaluated by CCK-8 assays. Apoptotic cells were detected by flow cytometry analysis. The proteins involved in apoptosis and cellular differentiation were assessed by immunoblot analysis. Ginsenosides contents of PG or PGP were analyzed using HPLC.

RESULTS

The ethyl acetate fraction (EtOAc) of PGP exhibited the strongest anti-proliferative activity among PGP fractions (butanol or water) on B16F10 cells. PGP EtOAc extract showed stronger inhibitory effect than Panax ginseng (PG) or Phellinus linteus (PL) EtOAc extract on B16F10 melanoma cell proliferation. PGP EtOAc extract induced the dendrite-like structures and the melanin production in B16F10 cells. PGP EtOAc extract increased a sub-G1 cell population through inducing p53/p21 and activated caspase-8 protein expression in B16F10 cells. Notably, PGP EtOAc extract contained ginsenosides Rd, Rg3, Rb2, Rg1 and Rb1 more than PG EtOAc extract. Rd and Rg3 significantly inhibited B16F10 cell proliferation.

CONCLUSION

Our data suggest that PGP EtOAc extract inhibits B16F10 cell proliferation through inducing apoptosis and cellular differentiation.

Signaling pathways in melanosome biogenesis and pathology

Melanosomes are the specialized intracellular organelles of pigment cells devoted to the synthesis, storage and transport of melanin pigments, which are responsible for most visible pigmentation in mammals and other vertebrates. As a direct consequence, any genetic mutation resulting in alteration of melanosomal function, either because affecting pigment cell survival, migration and differentiation, or because interfering with melanosome biogenesis, transport and transfer to keratinocytes, is immediately translated into color variations of skin, fur, hair or eyes. Thus, over 100 genes and proteins have been identified as pigmentary determinants in mammals, providing us with a deep understanding of this biological system, which functions by using mechanisms and processes that have parallels in other tissues and organs. In particular, many genes implicated in melanosome biogenesis have been characterized, so that melanosomes represent an incredible source of information and a model for organelles belonging to the secretory pathway. Furthermore, the function of melanosomes can be associated with common physiological phenotypes, such as variation of pigmentation among individuals, and with rare pathological conditions, such as albinism, characterized by severe visual defects. Among the most relevant mechanisms operating in melanosome biogenesis are the signal transduction pathways mediated by two peculiar G protein-coupled receptors: the melanocortin-1 receptor (MC1R), involved in the fair skin/red hair phenotype and skin cancer; and OA1 (GPR143), whose loss-of-function results in X-linked ocular albinism. This review will focus on the most recent novelties regarding the functioning of these two receptors, by highlighting emerging signaling mechanisms and general implications for cell biology and pathology.

{alpha}MSH and Cyclic AMP elevating agents control melanosome pH through a protein kinase A-independent mechanism

Melanins are synthesized in melanocytes within specialized organelles called melanosomes. Numerous studies have shown that the pH of melanosome plays a key role in the regulation of melanin synthesis. However, until now, acute regulation of melanosome pH by a physiological stimulus has never been demonstrated. In the present study, we show that the activation of the cAMP pathway by alphaMSH or forskolin leads to an alkalinization of melanosomes and a concomitant regulation of vacuolar ATPases and ion transporters of the solute carrier family. The solute carrier family members include SLC45A2, which is mutated in oculocutaneous albinism type IV, SLC24A4 and SLC24A5, proteins implicated in the control of eye, hair, and skin pigmentation, and the P protein, encoded by the oculocutaneous albinism type II locus. Interestingly, H89, a pharmacological inhibitor of protein kinase A (PKA), prevents the cAMP-induced pigmentation and induces acidification of melanosomes. The drastic depigmenting effect of H89 is not due to an inhibition of tyrosinase expression. Indeed, H89 blocks the induction of melanogenesis induced by LY294002, a potent inhibitor of the PI 3-kinase pathway, without any effect on tyrosinase expression. Furthermore, PKA is not involved in the inhibition of pigmentation promoted by H89 because LY294002 induces pigmentation independently of PKA. Also, other PKA inhibitors do not affect pigmentation. Taken together, our results strengthen the support for a key role of melanosome pH in the regulation of melanin synthesis and, for the first time, demonstrate that melanosome pH is regulated by cAMP and alphaMSH. Notably, these are both mediators of the response to solar UV radiation, the main physiological stimulus of skin pigmentation.

Novel MITF targets identified using a two-step DNA microarray strategy

Malignant melanoma is a chemotherapy-resistant cancer with high mortality. Recent advances in our understanding of the disease at the molecular level have indicated that it shares many characteristics with developmental precursors to melanocytes, the mature pigment-producing cells of the skin and hair follicles. The development of melanocytes absolutely depends on the action of the microphthalmia-associated transcription factor (MITF). MITF has been shown to regulate a broad variety of genes, whose functions range from pigment production to cell-cycle regulation, migration and survival. However, the existing list of targets is not sufficient to explain the role of MITF in melanocyte development and melanoma progression. DNA microarray analysis of gene expression offers a straightforward approach to identify new target genes, but standard analytical procedures are susceptible to the generation of false positives and require additional experimental steps for validation. Here, we introduce a new strategy where two DNA microarray-based approaches for identifying transcription factor targets are combined in a cross-validation protocol designed to help control false-positive generation. We use this two-step approach to successfully re-identify thirteen previously recorded targets of MITF-mediated upregulation, as well as 71 novel targets. Many of these new targets have known relevance to pigmentation and melanoma biology, and further emphasize the critical role of MITF in these processes.

BEST1 expression in the retinal pigment epithelium is modulated by OTX family members

A number of genes preferentially expressed in the retinal pigment epithelium (RPE) are associated with retinal degenerative disease. One of these, BEST1, encodes bestrophin-1, a protein that when mutated causes Best macular dystrophy. As a model for RPE gene regulation, we have been studying the mechanisms that control BEST1 expression, and recently demonstrated that members of the MITF-TFE family modulate BEST1 transcription. The human BEST1 upstream region from -154 to +38 bp is sufficient to direct expression in the RPE, and positive-regulatory elements exist between -154 and -104 bp. Here, we show that the -154 to -104 bp region is necessary for RPE expression in transgenic mice and contains a predicted OTX-binding site (Site 1). Since another non-canonical OTX site (Site 2) is located nearby, we tested the function of these sites using BEST1 promoter/luciferase constructs by in vivo electroporation and found that mutation of both sites reduces promoter activity. Three OTX family proteins - OTX1, OTX2 and CRX - bound to both Sites 1 and 2 in vitro, and all of them increased BEST1 promoter activity. Surprisingly, we found that human and bovine RPE expressed not only OTX2 but also CRX, the CRX genomic region in bovine RPE was hypersensitive to DNase I, consistent with active transcription, and that both OTX2 and CRX bound to the BEST1 proximal promoter in vivo. These results demonstrate for the first time CRX expression in the RPE, and suggest that OTX2 and CRX may act as positive modulators of the BEST1 promoter in the RPE.

[Phenotype of the visual system in oculocutaneous and ocular albinism]

In spite of albinism being one of the visual impairments which has been known for over a century, it has only been known for a few decades that albinism is correlated to severe cerebral morphological developmental alterations. The increasing knowledge about the role of melanin in the development and orientation of cerebral neurons not only renders more insight into albinism, but also a greater insight in the physiological neuronal and cerebral development in man. Concerning the morphological and visual phenotype there are new clinical findings which enlarge the known spectrum of albinism. In a representative group of 506 persons with oculocutaneous and ocular albinism who are in care at the Department of Ophthalmology at the University of Saarland (UKS), we present a staging of morphological findings of the iris, retinal pigment epithelium and macula, and of the optic nerve head which has been in use for 10 years. Albinism may present with a remarkably mild ocular phenotype and a near to normal functional phenotype. We present correlations between molecular genetic types of albinism, ocular phenotype and visual function. Of great importance concerning later visual acuity is the dysplasia of the optic nerve head (ONH), which is a frequent finding in albinism. The appearance of the ONH should always be included in any clinical description of an albinism patient. It is highly possible that due to a moderate phenotype there are still many patients who have not been diagnosed yet. Visual acuity of 30/20 to 20/20 and no nystagmus do not rule out albinism. In addition, when performing albino VEPs in phenotypically normal children with infantile strabismus, small ONHs, but normal visual acuity and no nystagmus, the classical atypical chiasmal crossing is sometimes found. Therefore, the number of persons having undiagnosed albinism is probably quite high, perhaps there even is a very broad transition zone from normal to albinotic.

Complex inhibition of tyrosinase by thiol-composed Cu2+ chelators: a clue for designing whitening agents

The inhibition of tyrosinase has attracted considerable attention for potential medicinal and cosmetic applications, as well as in agriculture. This study investigated the inhibition effects of thiol-associated Cu(2+) chelators and deduced a strategy for designing and/or selecting tyrosinase inhibitors. Among the several compounds tested, dithioglycerine (DTGC) was selected for further experiments on the inhibition kinetics on tyrosinase. Different types of tyrosinases derived from mushroom and from the transient overexpression in HEK293 cells were tested individually. The results showed that DTGC significantly inhibited human tyrosinase in a complex manner (slope-parabolic mixed-type inhibition), which was comparable to mushroom tyrosinase. The affinity of DTGC affinity to human tyrosinase was evaluated by setting up a K(i slope) equation. The results suggest that a Cu(2+) chelator modified with thiol groups has potential as a whitening agent. In addition, a strategy for designing and/or selecting tyrosinase inhibitors that target the active enzyme site was also suggested.

VMD2 promoter requires two proximal E-box sites for its activity in vivo and is regulated by the MITF-TFE family

The retinal pigment epithelium (RPE) is crucial for the function and survival of retinal photoreceptors. VMD2 encodes bestrophin, an oligomeric chloride channel that is preferentially expressed in the RPE and, when mutated, causes Best macular dystrophy. Previously, we defined the VMD2 upstream region from -253 to +38 bp as being sufficient to direct RPE-specific expression in the eye, and we suggested microphthalmia-associated transcription factor (MITF) as a possible positive regulator. Here we show that in transgenic mice the -154 to +38 bp region is sufficient for RPE expression, and mutation of two E-boxes, 1 and 2, within this region leads to loss of promoter activity. A yeast one-hybrid screen using bait containing E-box 1 identified clones encoding MITF, TFE3, and TFEB, and chromatin immunoprecipitation with antibodies against these proteins enriched the VMD2 proximal promoter. Analysis using in vivo electroporation with constructs containing mutation of each E-box indicated that expression in native RPE requires both E-boxes, yet in vitro DNA binding studies suggested that MITF binds well to E-box 1 but only minimally to E-box 2. MITF knockdown by small interfering RNA (siRNA) in cell culture revealed a strong correlation between MITF and VMD2 mRNA levels. Sequential transfection of a luciferase construct with expression vectors following MITF siRNA revealed that TFE3 and TFEB can also transactivate the VMD2 promoter. Taken together, we suggest that VMD2 is regulated by the MITF-TFE family through two E-boxes, with E-box 1 required for a direct interaction of MITF-TFE factors and E-box 2 for binding of the as yet unidentified factor(s).

Malignant melanoma: genetics and therapeutics in the genomic era

Cell for cell, probably no human cancer is as aggressive as melanoma. It is among a handful of cancers whose dimensions are reported in millimeters. Tumor thickness approaching 4 mm presents a high risk of metastasis, and a diagnosis of metastatic melanoma carries with it an abysmal median survival of 6-9 mo. What features of this malignancy account for such aggressive behavior? Is it the migratory history of its cell of origin or the programmed adaptation of its differentiated progeny to environmental stress, particularly ultraviolet radiation? While the answers to these questions are far from complete, major strides have been made in our understanding of the cellular, molecular, and genetic underpinnings of melanoma. More importantly, these discoveries carry profound implications for the development of therapies focused directly at the molecular engines driving melanoma, suggesting that we may have reached the brink of an unprecedented opportunity to translate basic science into clinical advances. In this review, we attempt to summarize our current understanding of the genetics and biology of this disease, drawing from expanding genomic information and lessons from development and genetically engineered mouse models. In addition, we look forward toward how these new insights will impact on therapeutic options for metastatic melanoma in the near future.

Metastatic potential of melanomas defined by specific gene expression profiles with no BRAF signature

The molecular biology of metastatic potential in melanoma has been studied many times previously and changes in the expression of many genes have been linked to metastatic behaviour. What is lacking is a systematic characterization of the regulatory relationships between genes whose expression is related to metastatic potential. Such a characterization would produce a molecular taxonomy for melanoma which could feasibly be used to identify epigenetic mechanisms behind changes in metastatic behaviour. To achieve this we carried out three separate DNA microarray analyses on a total of 86 cultures of melanoma. Significantly, multiple testing correction revealed that previous reports describing correlations of gene expression with activating mutations in BRAF or NRAS were incorrect and that no gene expression patterns correlate with the mutation status of these MAPK pathway components. Instead, we identified three different sample cohorts (A, B and C) and found that these cohorts represent melanoma groups of differing metastatic potential. Cohorts A and B were susceptible to transforming growth factor-beta (TGFbeta)-mediated inhibition of proliferation and had low motility. Cohort C was resistant to TGFbeta and demonstrated high motility. Meta-analysis of the data against previous studies linking gene expression and phenotype confirmed that cohorts A and C represent transcription signatures of weakly and strongly metastatic melanomas, respectively. Gene expression co-regulation suggested that signalling via TGFbeta-type and Wnt/beta-catenin pathways underwent considerable change between cohorts. These results suggest a model for the transition from weakly to strongly metastatic melanomas in which TGFbeta-type signalling upregulates genes expressing vasculogenic/extracellular matrix remodelling factors and Wnt signal inhibitors, coinciding with a downregulation of genes downstream of Wnt signalling.

The ocular albinism type 1 (OA1) protein and the evidence for an intracellular signal transduction system involved in melanosome biogenesis

Ocular albinism type 1 is an X-linked disorder characterized by severe reduction of visual acuity, retinal hypopigmentation, foveal hypoplasia, optic misrouting and the presence of giant melanosomes (macromelanosomes) in skin melanocytes and retinal pigment epithelium. The protein product of the OA1 gene is a pigment cell specific membrane glycoprotein, displaying structural and functional features of G protein-coupled receptors (GPCRs). However, in contrast to all other previously characterized GPCRs, OA1 is not localized to the plasma membrane, but is targeted to intracellular organelles, namely late endosomes/lysosomes and melanosomes. These unique characteristics suggest that OA1 represents the first example described so far of an exclusively intracellular GPCR and regulates melanosome biogenesis by transducing signals from the organelle lumen to the cytosol. These findings support previous hypotheses that GPCR-mediated signaling might also operate at the internal membranes in mammalian cells.