Transcriptional and signaling regulation in neural crest stem cell-derived melanocyte development: do all roads lead to Mitf?

BMP4 is required for the initial expression of MITF in melanocyte precursor differentiation from embryonic stem cells

Although the differentiation of melanoblasts to melanocytes is known to depend on many distinct factors, it is still poorly understood which factors lead to the induction of melanoblasts. To determine which factors might induce melanoblasts, we examined a set of candidate factors for their ability to induce expression of MITF, a master regulator of melanoblast development, in an ES cell-based melanocyte differentiation system. It appears that BMP4 is capable of inducing MITF expression in stem cells. In contrast, a number of other factors normally implicated in the development of the melanocyte lineage, including WNT1, WNT3a, SCF, EDN3, IGF1, PDGF, and RA, cannot induce MITF expression. Nevertheless, BMP4 alone does not allow MITF-expressing precursors to become differentiated melanocytes, but the addition of EDN3 further promotes differentiation of the precursors into mature melanocytes. Our results support a model in which BMP4 induces MITF expression in pluripotent stem cells and EDN3 subsequently promotes differentiation of these MITF expressing cells along the melanocyte lineage.

SYT14L, especially its C2 domain, is involved in regulating melanocyte differentiation

BACKGROUND

The formation of dendrites by melanocytes is highly analogous to that process in neural cells. We previously reported that a C2 domain-containing protein, copine-1, is involved in the extension of dendrites by neural cells. However, the effect of C2 domain-containing proteins in dendrite formation by melanocytes has not yet been elucidated.

OBJECTIVE

The aim of this study was to screen novel C2 domain-containing proteins related to dendrite outgrowth in melanocytes and to investigate their precise roles in melanocyte dendrite formation during differentiation.

METHODS

We transduced mouse melan-a melanocytes with a recombinant adenovirus expressing a C2 domain library. Dendrite elongation, melanin content, tyrosinase activity and Western blot analyses were conducted to elucidate the possible underlying mechanisms of action in melanocytes.

RESULTS

Sixteen sets of C2 domain-containing proteins were identified whose over-expression resulted in dendrite lengthening. Among those, we focused on the C2 domain of SYT14L (truncated mutant of SYT14L) in this study. Forced expression of full length SYT14L or the C2 domain of SYT14L induced a significant elongation of dendrite length accompanied by the induction of melanocyte differentiation-related markers, including melanin synthesis, tyrosinase catalytic activity and the expression of tyrosinase (TYR), tyrosinase related protein-1 (TRP-1) and TRP-2. In addition, over-expression of either the C2 domain or the full length form of SYT14L significantly increased the phosphorylation of ERK and CREB.

CONCLUSION

These results suggest that SYT14L, especially its C2 domain, may play an important role in regulating melanocyte differentiation through the modulation of ERK and (or) CREB signaling.

MITF, the Janus transcription factor of melanoma

Current models of melanoma propose that transition from the proliferative to the invasive stages of tumor development involves a dynamic and reversible switch in cell phenotype. The almost mutually exclusive proliferative and invasive phenotypes are defined by distinct gene expression signatures, which are themselves controlled by the level of functional MITF protein present in the cell. Recently, new signaling pathways and transcription factors that regulate MITF expression have been defined, and high throughput genomics have identified novel MITF target genes. MITF acts both as a transcription activator to promote expression of genes involved in cell cycle, but also as a transcriptional repressor of genes involved in invasion. A novel human germline mutation in MITF has been identified that blocks its sumoylation, thereby altering its transcriptional properties and conferring an increased risk of melanoma. These new studies depict an ever more complex function for MITF in melanoma.

The genetics of melanoma: recent advances

Cutaneous malignant melanoma results from the interplay of genetic, host, and environmental factors. Genetic factors implicated in melanoma etiology include inherited high-, intermediate-, and low-risk susceptibility genes as well as numerous somatic mutations in melanoma tumors. CDKN2A is the major high-risk melanoma susceptibility gene identified to date. Recent identification of low-risk loci has been accomplished predominantly through genome-wide association studies. Whole-exome and whole-genome studies have identified numerous genes somatically altered in melanoma tumors and highlighted a higher mutation load in melanoma tumors compared with those in other cancers. This higher load is believed to be attributable to the preponderance of cytosine-to-thymine nucleotide substitutions as a result of UV radiation exposure. Technological advances, particularly next-generation sequencing, have increased the opportunities for germline and somatic gene discovery in melanoma and are opening up new avenues for understanding melanoma pathogenesis as well as leading to new opportunities for treatment.

A conditional zebrafish MITF mutation reveals MITF levels are critical for melanoma promotion vs. regression in vivo

The microphthalmia-associated transcription factor (MITF) is the “master melanocyte transcription factor” with a complex role in melanoma. MITF protein levels vary between and within clinical specimens, and amplifications and gain- and loss-of-function mutations have been identified in melanoma. How MITF functions in melanoma development and the effects of targeting MITF in vivo are unknown because MITF levels have not been directly tested in a genetic animal model. Here, we use a temperature-sensitive mitf zebrafish mutant to conditionally control endogenous MITF activity. We show that low levels of endogenous MITF activity are oncogenic with BRAF^V600E to promote melanoma that reflects the pathology of the human disease. Remarkably, abrogating MITF activity in BRAF^V600Emitf melanoma leads to dramatic tumor regression marked by melanophage infiltration and increased apoptosis. These studies are significant because they show that targeting MITF activity is a potent antitumor mechanism, but also show that caution is required because low levels of wild-type MITF activity are oncogenic.

Geranylgeranylacetone inhibits melanin synthesis via ERK activation in Mel-Ab cells

AIMS

Geranylgeranylacetone (GGA) has shown cytoprotective activity through induction of a 70-kDa heat shock protein (HSP70). Although HSP70 is reported to regulate melanogenesis, the effects of GGA on melanin synthesis in melanocytes have not been previously studied. Therefore, this study investigated the effects of GGA on melanogenesis and the related signaling pathways.

MAIN METHODS

Melanin content and tyrosinase activities were measured in Mel-Ab cells. GGA-induced signal transduction pathways were investigated by western blot analysis.

KEY FINDINGS

Our results showed that GGA significantly decreased melanin content in a concentration-dependent manner. Similarly, GGA reduced tyrosinase activity dose-dependently, but it did not directly inhibit tyrosinase. Western blot analysis indicated that GGA downregulated microphthalmia-associated transcription factor (MITF) and tyrosinase protein expression, whereas it increased the phosphorylation of extracellular signal-regulated kinase (ERK) and mammalian target of rapamycin (mTOR). Furthermore, a specific ERK pathway inhibitor, PD98059, blocked GGA-induced melanin reduction and then prevented downregulation of MITF and tyrosinase by GGA. However, a specific mTOR inhibitor, rapamycin, only slightly restored inhibition of melanin production by GGA, indicating that mTOR signaling is not a key mechanism regulating the inhibition of melanin production.

SIGNIFICANCE

These findings suggest that activation of ERK by GGA reduces melanin synthesis in Mel-Ab cells through downregulation of MITF and tyrosinase expression.

Differential evolution of duplicated medakafish mitf genes

Gene duplication is a major force of evolution. One whole genome duplication (WGD) event in the fish ancestor generated genome-wide duplicates in all modern species. Coloration and patterning on the animal body surface exhibit enormous diversity, representing a mysterious and ideal system for understanding gene evolution. Surface colors and patterns are determined primarily by pigment cells in the skin and eye. Thus, microphthalmia-associated transcription factor (Mitf) as a master regulator of melanocyte development is excellent for studying the evolution of WGD-derived gene duplicates. Here we report the evolution of mitf duplicate, mitf1 and mitf2, in the fish medaka (Oryzias latipes), which encode medaka co-homologs Mitf1 and Mitf2 of the mouse Mitf. Compared to mitf1, mitf2 exhibits an accelerated sequence divergence and loses melanocytic expression in embryos at critical developmental stages. Compared to a Xiphophorus counterpart, the medaka Mitf2 displayed a reduced activity in activating melanogenic gene expression by reporter assays and RT-PCR analyses. We show that the medaka Mitf2 has the ability to induce melanocyte differentiation in medaka embryonic stem cells but at a remarkably reduced efficiency compared to the Xiphophorus counterpart. Our data suggest differential evolution of the medaka mitf duplicate, with mitf1 adopting conservation and mitf2 employing degeneration, which is different from the duplication-degeneration-complementation proposed as the mechanism to preserve many gene duplicates in zebrafish. Our finding reveals species-specific variations for mitf duplicate evolution, in agreement with enormous diversity of body coloration and patterning.

Mutations in c10orf11, a melanocyte-differentiation gene, cause autosomal-recessive albinism

Autosomal-recessive albinism is a hypopigmentation disorder with a broad phenotypic range. A substantial fraction of individuals with albinism remain genetically unresolved, and it has been hypothesized that more genes are to be identified. By using homozygosity mapping of an inbred Faroese family, we identified a 3.5 Mb homozygous region (10q22.2-q22.3) on chromosome 10. The region contains five protein-coding genes, and sequencing of one of these, C10orf11, revealed a nonsense mutation that segregated with the disease and showed a recessive inheritance pattern. Investigation of additional albinism-affected individuals from the Faroe Islands revealed that five out of eight unrelated affected persons had the nonsense mutation in C10orf11. Screening of a cohort of autosomal-recessive-albinism-affected individuals residing in Denmark showed a homozygous 1 bp duplication in C10orf11 in an individual originating from Lithuania. Immunohistochemistry showed localization of C10orf11 in melanoblasts and melanocytes in human fetal tissue, but no localization was seen in retinal pigment epithelial cells. Knockdown of the zebrafish (Danio rerio) homolog with the use of morpholinos resulted in substantially decreased pigmentation and a reduction of the apparent number of pigmented melanocytes. The morphant phenotype was rescued by wild-type C10orf11, but not by mutant C10orf11. In conclusion, we have identified a melanocyte-differentiation gene, C10orf11, which when mutated causes autosomal-recessive albinism in humans.

Modeling melanoblast development

Melanoblasts are a particular type of cell that displays extensive cellular proliferation during development to contribute to the skin. There are only a few melanoblast founders, initially located just dorsal to the neural tube, and they sequentially colonize the dermis, epidermis, and hair follicles. In each compartment, melanoblasts are exposed to a wide variety of developmental cues that regulate their expansion. The colonization of the dermis and epidermis by melanoblasts involves substantial proliferation to generate thousands of cells or more from a few founders within a week of development. This review addresses the cellular and molecular events occurring during melanoblast development. We focus on intrinsic and extrinsic factors that control melanoblast proliferation. We also present a robust mathematical model for estimating the doubling-time of dermal and epidermal melanoblasts for all coat color phenotypes from black to white.

Melanocyte stem cells: biology and current aspects

Epidermal stem cells have become an object of intensive research. The epidermis constitutes one of the main sources of stem cells and is a tissue of choice for use in exploring their biology.

Stratified squamous epithelium (epidermis) possesses the capacity for self-renewal and repair due to the presence of epidermal stem cells (ESC). They have been identified within basal layer of the interfollicular epidermis (IFE), in the “bulge” of the hair follicles of rodents, and also in the human follicular bulge. Melanocyte stem cells (MSC) from hair follicles (precisely from the bulge region, which also contains epidermal stem cells) provide an attractive model for the study of stem cells and their regulation at the niche.

This review summarizes the rapidly developing field of epidermal stem cell research and their application in regenerative medicine, paying particular attention to melanocyte stem cells, their biology and some of the processes that occur during hair graying and regeneration of the pigmentary system, as well as discussing how aged-associated changes in the melanocyte stem cells compartment impact hair graying. This review also includes differentiation of human skin stem cells into functional epidermal melanocytes.

Functional analysis of MITF gene mutations associated with Waardenburg syndrome type 2

MITF mutations results in an abnormal melanocyte development and lead to Waardenburg syndrome type 2 (WS2). Here, we analyzed the in vitro activities of two recently identified WS2-associated MITF mutations (p.R217I and p.T192fsX18). The R217I MITF retained partial activity, normal DNA-binding ability and nuclear distribution, whereas the T192fsX18 MITF failed to activate TYR promoter and showed aberrant subcellular localization which may be caused by deletion of nuclear localization signal (NLS) at aa 213-218 (ERRRRF). These results suggest that haploinsufficiency may be the underlying mechanism for the mild phenotypes of WS2 caused by these two mutations.

Integration of ChIP-seq and machine learning reveals enhancers and a predictive regulatory sequence vocabulary in melanocytes

We take a comprehensive approach to the study of regulatory control of gene expression in melanocytes that proceeds from large-scale enhancer discovery facilitated by ChIP-seq; to rigorous validation in silico, in vitro, and in vivo; and finally to the use of machine learning to elucidate a regulatory vocabulary with genome-wide predictive power. We identify 2489 putative melanocyte enhancer loci in the mouse genome by ChIP-seq for EP300 and H3K4me1. We demonstrate that these putative enhancers are evolutionarily constrained, enriched for sequence motifs predicted to bind key melanocyte transcription factors, located near genes relevant to melanocyte biology, and capable of driving reporter gene expression in melanocytes in culture (86%; 43/50) and in transgenic zebrafish (70%; 7/10). Next, using the sequences of these putative enhancers as a training set for a supervised machine learning algorithm, we develop a vocabulary of 6-mers predictive of melanocyte enhancer function. Lastly, we demonstrate that this vocabulary has genome-wide predictive power in both the mouse and human genomes. This study provides deep insight into the regulation of gene expression in melanocytes and demonstrates a powerful approach to the investigation of regulatory sequences that can be applied to other cell types.

Constitutive gray hair in mice induced by melanocyte-specific deletion of c-Myc

c-Myc is involved in the control of diverse cellular processes and implicated in the maintenance of different tissues including the neural crest. Here, we report that c-Myc is particularly important for pigment cell development and homeostasis. Targeting c-Myc specifically in the melanocyte lineage using the floxed allele of c-Myc and Tyr::Cre transgenic mice results in a congenital gray hair phenotype. The gray coat color is associated with a reduced number of functional melanocytes in the hair bulb and melanocyte stem cells in the hair bulge. Importantly, the gray phenotype does not progress with time, suggesting that maintenance of the melanocyte through the hair cycle does not involve c-Myc function. In embryos, at E13.5, c-Myc-deficient melanocyte precursors are affected in proliferation in concordance with a reduction in numbers, showing that c-Myc is required for the proper melanocyte development. Interestingly, melanocytes from c-Myc-deficient mice display elevated levels of the c-Myc paralog N-Myc. Double deletion of c-Myc and N-Myc results in nearly complete loss of the residual pigmentation, indicating that N-Myc is capable of compensating for c-Myc loss of function in melanocytes.

Screening of MITF and SOX10 regulatory regions in Waardenburg syndrome type 2

Waardenburg syndrome (WS) is a rare auditory-pigmentary disorder that exhibits varying combinations of sensorineural hearing loss and pigmentation defects. Four subtypes are clinically defined based on the presence or absence of additional symptoms. WS type 2 (WS2) can result from mutations within the MITF or SOX10 genes; however, 70% of WS2 cases remain unexplained at the molecular level, suggesting that other genes might be involved and/or that mutations within the known genes escaped previous screenings. The recent identification of a deletion encompassing three of the SOX10 regulatory elements in a patient presenting with another WS subtype, WS4, defined by its association with Hirschsprung disease, led us to search for deletions and point mutations within the MITF and SOX10 regulatory elements in 28 yet unexplained WS2 cases. Two nucleotide variations were identified: one in close proximity to the MITF distal enhancer (MDE) and one within the U1 SOX10 enhancer. Functional analyses argued against a pathogenic effect of these variations, suggesting that mutations within regulatory elements of WS genes are not a major cause of this neurocristopathy.

The three M's: melanoma, microphthalmia-associated transcription factor and microRNA

Studies examining intratumor heterogeneity have indicated that several cancer types, including melanoma, can display phenotypic plasticity, corresponding to their capacity to undergo transient reversible cellular changes. Conceptual models constructed to explain the process of cancer propagation differ in their treatment of intratumor heterogeneity. Recent observations of reversible phenotypic heterogeneity in melanoma have led to the proposal of a novel 'phenotypic plasticity' model of cancer propagation. Microphthalmia-associated transcription factor (MITF), the melanocyte 'lineage-specific' transcription factor, has emerged as one of the central players in melanoma phenotypic plasticity. Here we discuss the conceptual models suggested to explain the relations between MITF and melanoma plasticity, in addition to the complex regulatory roles that MITF plays in melanocytes and melanoma development. Finally, we provide an in-depth literature survey of microRNAs (miRNAs) involved in MITF activity, melanoma propagation and metastasis, in addition to their potential use as agents of personalized therapy.

The effects of NB-UVB on the hair follicle-derived neural crest stem cells differentiating into melanocyte lineage in vitro

BACKGROUND

Narrow-band UVB (NB-UVB) is an effective therapeutic option in the treatment of vitiligo. Despite the apparent clinical efficacy, the underlying mechanism of how topical NB-UVB induces repigmentation in vitiligo has not been clearly elucidated.

OBJECTIVES

To investigate the effects of NB-UVB on the maturation of melanocyte lineage differentiated from hair follicle-derived neural crest stem cells (HF-NCSCs) in vitro.

METHODS

HF-NCSCs were isolated from mouse whisker follicles. The isolated cells were multipotent and expressed embryonic NCSC biomarkers. The effects of NB-UVB on development and differentiation of HF-NCSCs were evaluated. We assessed cell viability, melanogenesis and migration of melanocytes derived from HF-NCSCs after NB-UVB radiation. Tyrosinase, Tyrp1, Dct, Kit, Mc1R, Fzd4, NT3R, Ednra, EP1, TGFβR, Sox10, Mitf, Lef1 and Pax3 gene expression was measured by quantitative RT-PCR, while Tyrosinase, Sox10 and Mitf protein expression were measured by Western blot analysis. Cell migration was measured by Boyden chamber transwell assay.

RESULTS

NB-UVB increased the expression of tyrosinase during melanocytic differentiation from mouse HF-NCSCs, however, NB-UVB inhibited proliferation of melanocytes derived from HF-NCSCs. Mechanistically, increased melanocyte maturation after NB-UVB treatment was resulted from increased expression of several key melanogenic factors, including Sox10, Kit and Mc1R, which play a critical role to promote tyrosinase expression. Furthermore, the migration of the HF-NCSCs-derived melanocytes was downregulated as NB-UVB doses increased. However, the migration of HF-NCSCs was upregulated under 0.4J NB-UVB radiation.

CONCLUSIONS

Those data provide in vitro evidence demonstrating some direct effects of NB-UVB on pigmentation of melanocyte lineage differentiated from HF-NCSCs, and may provide a possible mechanism for the effect of NB-UVB in vitiligo.

Don't it make my blue eyes brown: heterochromia and other abnormalities of the iris

Eye colour is one of the most important characteristics in determining facial appearance. In this paper I shall discuss the anatomy and genetics of normal eye colour, together with a wide and diverse range of conditions that may produce an alteration in normal iris pigmentation or form.

Microphthalmia-associated transcription factor acts through PEDF to regulate RPE cell migration

Cells of the retinal pigment epithelium (RPE) play major roles in metabolic functions, maintenance of photoreceptor function, and photoreceptor survival in the retina. They normally form a stable monolayer, but migrate during disease states. Although growth factors produced by the RPE cells primarily control these cellular events, how these factors are regulated in RPE cells remain largely unknown. Here we show that the basic-helix-loop-helix-leucine zipper microphthalmia-associated transcription factor (MITF), which plays central roles in the development and function of a variety of cell types including RPE cells, upregulates the expression of a multifunctional factor PEDF in RPE cells. Consequently, the upregulation of PEDF impairs microtubule assembly and thus inhibits RPE cell migration. Conversely, specific knockdown of PEDF partially rescues the impairment of microtubule assembly and cell migration proceeds in MITF overexpressing stable cells. We conclude that MITF acts through PEDF to inhibit RPE cell migration and to play a significant role in regulating RPE cellular function. We suggest that MITF has a novel and important role in maintaining RPE cells as a stable monolayer and the down-regulation of PEDF that may contribute to retinal degenerative diseases.

Biomarkers as key contributors in treating malignant melanoma metastases

Melanoma is a human neurocristopathy associated with developmental defects in the neural crest-derived epidermal melanocytes. At the present time, at least three hypotheses were identified that may explain melanoma aetiology, as follows: (1) a model of linear progression from differentiated melanocytes to metastatic cancer cells (2) a model involving the appearance of melanoma stem-like cells, and (3) an epigenetic progenitor model of cancer. Treating metastatic melanoma is one of the most serious challenges in the 21st century. This is justified because of a subpopulation of cells presenting a remarkable molecular heterogeneity, which is able to explain the drug resistance and the growing mortality rates worldwide. Fortunately, there are now evidences sustaining the importance of genetic, epigenetic, and metabolomic alterations as biomarkers for classification, staging, and better management of melanoma patients. To illustrate some fascinating insights in this field, the genes BRAF^V600E and CTLA4 have been recognized as bona fide targets to benefit melanoma patients. Our research attempts to carefully evaluate data from the literature in order to highlight the link between a molecular disease model and the key contribution of biomarkers in treating malignant melanoma metastases.

C-KIT signaling depends on microphthalmia-associated transcription factor for effects on cell proliferation

The development of melanocytes is regulated by the tyrosine kinase receptor c-KIT and the basic-helix-loop-helix-leucine zipper transcription factor Mitf. These essential melanocyte survival regulators are also well known oncogenic factors in malignant melanoma. Despite their importance, not much is known about the regulatory mechanisms and signaling pathways involved. In this study, we therefore sought to identify the signaling pathways and mechanisms involved in c-KIT mediated regulation of Mitf. We report that c-KIT stimulation leads to the activation of Mitf specifically through the c-KIT phosphorylation sites Y721 (PI3 kinase binding site), Y568 and Y570 (Src binding site). Our study not only confirms the involvement of Ras-Erk signaling pathway in the activation of Mitf, but also establishes that Src kinase binding to Y568 and Y570 of c-KIT is required. Using specific inhibitors we observe and verify that c-KIT induced activation of Mitf is dependent on PI3-, Akt-, Src-, p38- or Mek kinases. Moreover, the proliferative effect of c-KIT is dependent on Mitf in HEK293T cells. In contrast, c-KIT Y568F and Y721F mutants are less effective in driving cell proliferation, compared to wild type c-KIT. Our results reveal novel mechanisms by which c-KIT signaling regulates Mitf, with implications for understanding both melanocyte development and melanoma.

Interpretation of complex phenotypes: lessons from the Mitf gene

SUMMARY

Mutations provide important structure–function relationships by allowing the correlation of phenotypes to the underlying genotypes. Knockout mutations that lead to loss-of-function are important and informative in this respect. However, spontaneous and induced mutations sometimes provide surprising phenotypes, which lead to unexpected functional insights and novel biochemical pathways, especially when multiple mutations(alleles) exist at a locus. An excellent example is provided by the microphthalmia (Mitf) locus in the mouse.The multiple Mitf alleles have their own phenotypic properties, most of which have been explained by the underlying mutation. However, one allele, the Mitf (Mi-White) (Mitf (Mi-Wh)) mutation, exhibits phenotypes that have not yet been fully explained. Here, the molecular, genetic, and phenotypic properties of this mutation are reviewed and an attempt made to explain the underlying biochemical reason for its observed effects.

SOX10 directly modulates ERBB3 transcription via an intronic neural crest enhancer

Background

The ERBB3 gene is essential for the proper development of the neural crest (NC) and its derivative populations such as Schwann cells. As with all cell fate decisions, transcriptional regulatory control plays a significant role in the progressive restriction and specification of NC derived lineages during development. However, little is known about the sequences mediating transcriptional regulation of ERBB3 or the factors that bind them.

Results

In this study we identified three transcriptional enhancers at the ERBB3 locus and evaluated their regulatory potential in vitro in NC-derived cell types and in vivo in transgenic zebrafish. One enhancer, termed ERBB3_MCS6, which lies within the first intron of ERBB3, directs the highest reporter expression in vitro and also demonstrates epigenetic marks consistent with enhancer activity. We identify a consensus SOX10 binding site within ERBB3_MCS6 and demonstrate, in vitro, its necessity and sufficiency for the activity of this enhancer. Additionally, we demonstrate that transcription from the endogenous Erbb3 locus is dependent on Sox10. Further we demonstrate in vitro that Sox10 physically interacts with that ERBB3_MCS6. Consistent with its in vitro activity, we also show that ERBB3_MCS6 drives reporter expression in NC cells and a subset of its derivative lineages in vivo in zebrafish in a manner consistent with erbb3b expression. We also demonstrate, using morpholino analysis, that Sox10 is necessary for ERBB3_MCS6 expression in vivo in zebrafish.

Conclusions

Taken collectively, our data suggest that ERBB3 may be directly regulated by SOX10, and that this control may in part be facilitated by ERBB3_MCS6.

Post-embryonic nerve-associated precursors to adult pigment cells: genetic requirements and dynamics of morphogenesis and differentiation

The pigment cells of vertebrates serve a variety of functions and generate a stunning variety of patterns. These cells are also implicated in human pathologies including melanoma. Whereas the events of pigment cell development have been studied extensively in the embryo, much less is known about morphogenesis and differentiation of these cells during post-embryonic stages. Previous studies of zebrafish revealed genetically distinct populations of embryonic and adult melanophores, the ectotherm homologue of amniote melanocytes. Here, we use molecular markers, vital labeling, time-lapse imaging, mutational analyses, and transgenesis to identify peripheral nerves as a niche for precursors to adult melanophores that subsequently migrate to the skin to form the adult pigment pattern. We further identify genetic requirements for establishing, maintaining, and recruiting precursors to the adult melanophore lineage and demonstrate novel compensatory behaviors during pattern regulation in mutant backgrounds. Finally, we show that distinct populations of latent precursors having differential regenerative capabilities persist into the adult. These findings provide a foundation for future studies of post-embryonic pigment cell precursors in development, evolution, and neoplasia.

Understanding the biology of post-embryonic stem and progenitor cells is of both basic and translational importance. To identify mechanisms by which stem and progenitor cells are established, maintained, and recruited to particular fates, we are using the zebrafish adult pigment pattern. Previous work showed that embryonic and adult pigment cells have different genetic requirements, but little is known about the molecular or proliferative phenotypes of precursors to adult pigment cells or where these precursors reside during post-embryonic development. We show here that post-embryonic pigment cell precursors are associated with peripheral nerves and that these cells migrate to the skin during the larval-to-adult transformation when the adult pigment pattern forms. We also define morphogenetic and differentiative roles for several genes in promoting these events. Finally, we demonstrate that latent precursor pools persist into the adult and that different pools have different capacities for supplying new pigment cells in the context of pattern regeneration. Our study sets the stage for future analyses to identify additional common and essential features of pigment stem cell biology.

The Dark brown plumage color in chickens is caused by an 8.3-kb deletion upstream of SOX10

The Dark brown (DB) mutation in chickens reduces expression of black eumelanin and enhances expression of red pheomelanin, but only in certain parts of the plumage. Here, we present genetic evidence that an 8.3-kb deletion upstream of the SOX10 transcription start site is the causal mutation underlying the DB phenotype. The SOX10 transcription factor has a well-established role in melanocyte biology and is essential for melanocyte migration and survival. Previous studies have demonstrated that the mouse homolog of a highly conserved element within the deleted region is a SOX10 enhancer. The mechanism of action of this mutation remains to be established, but one possible scenario is that the deletion leads to reduced SOX10 expression which in turn down-regulates expression of key enzymes in pigment synthesis such as tyrosinase. Lower tyrosinase activity leads to a shift toward a more pheomelanistic (reddish) plumage color, which is the characteristic feature of the DB phenotype.

Essential role of microphthalmia transcription factor for DNA replication, mitosis and genomic stability in melanoma

Malignant melanoma is an aggressive cancer known for its notorious resistance to most current therapies. The basic helix-loop-helix microphthalmia transcription factor (MITF) is the master regulator determining the identity and properties of the melanocyte lineage, and is regarded as a lineage-specific 'oncogene' that has a critical role in the pathogenesis of melanoma. MITF promotes melanoma cell proliferation, whereas sustained supression of MITF expression leads to senescence. By combining chromatin immunoprecipitation coupled to high throughput sequencing (ChIP-seq) and RNA sequencing analyses, we show that MITF directly regulates a set of genes required for DNA replication, repair and mitosis. Our results reveal how loss of MITF regulates mitotic fidelity, and through defective replication and repair induces DNA damage, ultimately ending in cellular senescence. These findings reveal a lineage-specific control of DNA replication and mitosis by MITF, providing new avenues for therapeutic intervention in melanoma. The identification of MITF-binding sites and gene-regulatory networks establish a framework for understanding oncogenic basic helix-loop-helix factors such as N-myc or TFE3 in other cancers.

A curated online resource for SOX10 and pigment cell molecular genetic pathways

We describe the creation of a specialized web-accessible database named the Pigment Cell Gene Resource, which contains information on the genetic pathways that regulate pigment cell development and function. This manually curated database is comprised of two sections, an annotated literature section and an interactive transcriptional network diagram. Initially, this database focuses on the transcription factor SOX10, which has essential roles in pigment cell development and function, but the database has been designed with the capacity to expand in the future, allowing inclusion of many more pigmentation genes. Database URL: http://research.nhgri.nih.gov/pigment_cell/

Differentiation of zebrafish melanophores depends on transcription factors AP2 alpha and AP2 epsilon

A model of the gene-regulatory-network (GRN), governing growth, survival, and differentiation of melanocytes, has emerged from studies of mouse coat color mutants and melanoma cell lines. In this model, Transcription Factor Activator Protein 2 alpha (TFAP2A) contributes to melanocyte development by activating expression of the gene encoding the receptor tyrosine kinase Kit. Next, ligand-bound Kit stimulates a pathway activating transcription factor Microphthalmia (Mitf), which promotes differentiation and survival of melanocytes by activating expression of Tyrosinase family members, Bcl2, and other genes. The model predicts that in both Tfap2a and Kit null mutants there will be a phenotype of reduced melanocytes and that, because Tfap2a acts upstream of Kit, this phenotype will be more severe, or at least as severe as, in Tfap2a null mutants in comparison to Kit null mutants. Unexpectedly, this is not the case in zebrafish or mouse. Because many Tfap2 family members have identical DNA–binding specificity, we reasoned that another Tfap2 family member may work redundantly with Tfap2a in promoting Kit expression. We report that tfap2e is expressed in melanoblasts and melanophores in zebrafish embryos and that its orthologue, TFAP2E, is expressed in human melanocytes. We provide evidence that Tfap2e functions redundantly with Tfap2a to maintain kita expression in zebrafish embryonic melanophores. Further, we show that, in contrast to in kita mutants where embryonic melanophores appear to differentiate normally, in tfap2a/e doubly-deficient embryonic melanophores are small and under-melanized, although they retain expression of mitfa. Interestingly, forcing expression of mitfa in tfap2a/e doubly-deficient embryos partially restores melanophore differentiation. These findings reveal that Tfap2 activity, mediated redundantly by Tfap2a and Tfap2e, promotes melanophore differentiation in parallel with Mitf by an effector other than Kit. This work illustrates how analysis of single-gene mutants may fail to identify steps in a GRN that are affected by the redundant activity of related proteins.

Neural crest-derived pigment cells, known as melanocytes, are important to an organism's survival because they protect skin cells from ultraviolet radiation, camouflage the organism from predators, and contribute to sexual selection. Networks of regulatory proteins control the steps of melanocyte development, including lineage specification, migration, survival, and differentiation. Gaps in our understanding of these networks hamper progress in effective prevention and treatment of diseases of melanocytes, including metastatic melanoma and vitiligo. Studies conducted in tissue-culture cells and mouse embryos implicate regulatory proteins including the transcription factor TFAP2A, the growth factor receptor KIT, and the transcription factor MITF as being important for multiple steps in melanocyte development. Abnormalities in TFAP2A, KIT, and MITF expression in melanoma highlight the importance of this pathway in human disease. Here we show that a gene closely related to TFAP2A, tfap2e, is expressed in zebrafish embryonic melanocytes and human melanocytes. We provide evidence that Tfap2e cooperates with Tfap2a to promote expression of zebrafish kita in embryonic melanocytes. Further we show that an effector of Tfap2a/e activity other than Kita is required for melanocyte differentiation and that this effector acts upstream or in parallel with Mitfa activity. These findings reveal unexpected complexity to the gene-regulatory network governing melanocyte differentiation.

Signatures of microRNAs and selected microRNA target genes in human melanoma

Small noncoding microRNAs (miRNA) regulate the expression of target mRNAs by repressing their translation or orchestrating their sequence-specific degradation. In this study, we investigated miRNA and miRNA target gene expression patterns in melanoma to identify candidate biomarkers for early and progressive disease. Because data presently available on miRNA expression in melanoma are inconsistent thus far, we applied several different miRNA detection and profiling techniques on a panel of 10 cell lines and 20 patient samples representing nevi and primary or metastatic melanoma. Expression of selected miRNAs was inconsistent when comparing cell line-derived and patient-derived data. Moreover, as expected, some discrepancies were also detected when miRNA microarray data were correlated with qPCR-measured expression levels. Nevertheless, we identified miRNA-200c to be consistently downregulated in melanocytes, melanoma cell lines, and patient samples, whereas miRNA-205 and miRNA-23b were markedly reduced only in patient samples. In contrast, miR-146a and miR-155 were upregulated in all analyzed patients but none of the cell lines. Whole-genome microarrays were performed for analysis of selected melanoma cell lines to identify potential transcriptionally regulated miRNA target genes. Using Ingenuity pathway analysis, we identified a deregulated gene network centered around microphthalmia-associated transcription factor, a transcription factor known to play a key role in melanoma development. Our findings define miRNAs and miRNA target genes that offer candidate biomarkers in human melanoma.

Allele-specific genetic interactions between Mitf and Kit affect melanocyte development

The tyrosine kinase receptor KIT and the transcription factor MITF, each required for melanocyte development, have been shown to interact functionally both in vitro and in vivo. In vitro, KIT signaling leads to MITF phosphorylation, affecting MITF activity and stability. In vivo, the presence of the Mitf (Mi-wh) allele exacerbates the spotting phenotype associated with heterozygosity for Kit mutations. Here, we show that among a series of other Mitf alleles, only the recessive Mitf (mi-bws) mimics the effect of Mitf (Mi-wh) on Kit. Intriguingly, Mitf (mi-bws) is characterized by a splice defect that leads to a reduction of RNAs containing MITF exon 2B which encodes serine-73, a serine phosphorylated upon KIT signaling. Nevertheless, other Mitf alleles that generally affect Mitf RNA levels, or carry a serine-73-to-alanine mutation that specifically reduces exon 2B-containing RNAs, do not show similar interactions with Kit in vivo. We conclude that the recessive Mitf (mi-bws) is a complex allele that can display a semi-dominant effect when present in a Kit-sensitized background. We suggest that human disease variability may equally be due to complex, allele-specific interactions between different genes.

The melanocortin-1 receptor gene polymorphism and association with human skin cancer

The melanocortin-1 receptor (MC1R) is a key gene involved in the regulation of melanin synthesis and encodes a G-protein coupled receptor expressed on the surface of the melanocyte in the skin and hair follicles. MC1R activation after ultraviolet radiation exposure results in the production of the dark eumelanin pigment and the tanning process in humans, providing physical protection against DNA damage. The MC1R gene is highly polymorphic in Caucasian populations with a number of MC1R variant alleles associated with red hair, fair skin, freckling, poor tanning, and increased risk of melanoma and nonmelanoma skin cancer. Variant receptors have shown alterations in biochemical function, largely due to intracellular retention or impaired G-protein coupling, but retain some signaling ability. The association of MC1R variant alleles with skin cancer risk remains after correction for pigmentation phenotype, indicating regulation of nonpigmentary pathways. Notably, MC1R activation has been linked to DNA repair and may also contribute to the regulation of immune responses.

Comparison of melanoblast expression patterns identifies distinct classes of genes

A full understanding of transcriptional regulation requires integration of information obtained from multiple experimental datasets. These include datasets annotating gene expression within the context of an entire organism under normal and genetically perturbed conditions. Here we describe an expression dataset annotating pigment cell-expressed genes of the developing melanocyte and retinal pigmented epithelium lineages. Expression images are annotated and available at http://research.nhgri.nih.gov/manuscripts/Loftus/March2009/. Data are also summarized in a standardized manner using a universal melanoblast scoring scale that accounts for the embryonic location of cells and regional cell density. This approach allowed us to classify 14 pigment genes into four groupings classified by cell lineage expression, temporal-spatial context, and differential alteration in response to altered MITF and SOX10 status. Significant differences in regional populations were also observed across inbred strain backgrounds, highlighting the value of this approach to identify modifier allele influences on melanoblast number and distributions. This analysis revealed novel features of in vivo expression patterns that are not measurable by in vitro-based assays, providing data that in combination with genomic analyses will allow modeling of pigment cell gene expression in development and disease.

NRG1 / ERBB3 signaling in melanocyte development and melanoma: inhibition of differentiation and promotion of proliferation

Neuregulin (NRG) signaling through the receptor tyrosine kinase, ERBB3, is required for embryonic development, and dysregulated signaling has been associated with cancer progression. Here, we show that NRG1/ERBB3 signaling inhibits melanocyte (MC) maturation and promotes undifferentiated, migratory and proliferative cellular characteristics. Embryonic analyses demonstrated that initial MC specification and distribution were not dependent on ERBB3 signaling. However NRG1/ERBB3 signaling was both necessary and sufficient to inhibit differentiation of later stages of MC development in culture. Analysis of tissue arrays of human melanoma samples suggests that ERBB3 signaling may also contribute to metastatic progression of melanoma as ERBB3 was phosphorylated in primary tumors compared with nevi or metastatic lesions. Neuregulin 1-treated MCs demonstrated increased proliferation and invasion and altered morphology concomitant with decreased levels of differentiation genes, increased levels of proliferation genes and altered levels of melanoma progression and metastases genes. ERBB3 activation in primary melanomas suggests that NRG1/ERBB3 signaling may contribute to the progression of melanoma from benign nevi to malignancies. We propose that targeting ERBB3 activation and downstream genes identified in this study may provide novel therapeutic interventions for malignant melanoma.

The role of MITF phosphorylation sites during coat color and eye development in mice analyzed by bacterial artificial chromosome transgene rescue

The microphthalmia-associated transcription factor (Mitf) has emerged as an important model for gene regulation in eukaryotic organisms. In vertebrates, it regulates the development of several cell types including melanocytes and has also been shown to play an important role in melanoma. In vitro, the activity of MITF is regulated by multiple signaling pathways, including the KITL/KIT/B-Raf pathway, which results in phosphorylation of MITF on serine residues 73 and 409. However, the precise role of signaling to MITF in vivo remains largely unknown. Here, we use a BAC transgene rescue approach to introduce specific mutations in MITF to study the importance of specific phospho-acceptor sites and protein domains. We show that mice that carry a BAC transgene where single-amino-acid substitutions have been made in the Mitf gene rescue the phenotype of the loss-of-function mutations in Mitf. This may indicate that signaling from KIT to MITF affects other phospho-acceptor sites in MITF or that alternative sites can be phosphorylated when Ser73 and Ser409 have been mutated. Our results have implications for understanding signaling to transcription factors. Furthermore, as MITF and signaling mechanisms have been shown to play an important role in melanomas, our findings may lead to novel insights into this resilient disease.

Melanocyte pigmentation stiffens murine cardiac tricuspid valve leaflet

Pigmentation of murine cardiac tricuspid valve leaflet is associated with melanocyte concentration, which affects its stiffness. Owing to its biological and viscoelastic nature, estimation of the in situ stiffness measurement becomes a challenging task. Therefore, quasi-static and nanodynamic mechanical analysis of the leaflets of the mouse tricuspid valve is performed in the current work. The mechanical properties along the leaflet vary with the degree of pigmentation. Pigmented regions of the valve leaflet that contain melanocytes displayed higher storage modulus (7-10 GPa) than non-pigmented areas (2.5-4 GPa). These results suggest that the presence of melanocytes affects the viscoelastic properties of the mouse atrioventricular valves and are important for their proper functioning in the organism.

Frequent mutations in the MITF pathway in melanoma

Microphthalmia-associated transcription factor (MITF) is involved in melanocyte cell development, pigmentation and neoplasia. To determine whether MITF is somatically mutated in melanoma, we compared the sequence of MITF from primary and metastatic lesions to patient-matched normal DNA. In the 50 metastatic melanoma tumor lines analysed, we discovered four samples that had genomic amplifications of MITF and four that had MITF mutations in the regions encoding the transactivation, DNA binding or basic, helix-loop-helix domains. Sequence analysis for SOX10, a transcription factor, which both acts upstream of MITF and synergizes with MITF, identified an additional three samples with frameshift or nonsense mutations. Microphthalmia-associated transcription factor and SOX10 were found to be mutated in a mutually exclusive fashion, possibly suggesting disruption in a common genetic pathway. Taken together we found that over 20% of the metastatic melanoma cases had alterations in the MITF pathway. We show that the MITF pathway is also altered in primary melanomas: 2/26 demonstrated mutations in MITF and 6/55 demonstrated mutations in SOX10. Our findings suggest that altered MITF function during melanomagenesis can be achieved by MITF amplification, MITF single base substitutions or by mutation of its regulator SOX10.