Melanocytes in development, regeneration, and cancer

Stem Cells in Cancer: From Mechanisms to Therapeutic Strategies

Stem cells have emerged as a pivotal area of research in the field of oncology, offering new insights into the mechanisms of cancer initiation, progression, and resistance to therapy. This review provides a comprehensive overview of the role of stem cells in cancer, focusing on cancer stem cells (CSCs), their characteristics, and their implications for cancer therapy. We discuss the origin and identification of CSCs, their role in tumorigenesis, metastasis, and drug resistance, and the potential therapeutic strategies targeting CSCs. Additionally, we explore the use of normal stem cells in cancer therapy, focusing on their role in tissue regeneration and their use as delivery vehicles for anticancer agents. Finally, we highlight the challenges and future directions in stem cell research in cancer.

Somatic mutations distinguish melanocyte subpopulations in human skin

To better understand the homeostatic mechanisms governing melanocytes, we performed deep phenotyping of clonal expansions of single melanocytes from human skin. In total, we interrogated the mutational landscapes, gene expression profiles, and morphological features of 297 melanocytes from 31 donors. To our surprise, a population of melanocytes with low mutation burden was maintained in sun damaged skin. These melanocytes were more stem-like, smaller, less dendritic and displayed distinct gene expression profiles compared to their counterparts with high mutation burdens. We used single-cell spatial transcriptomics (10X Xenium) to reveal the spatial distribution of melanocytes inferred to have low and high mutation burdens (LowMut and HighMut cells), based on their gene expression profiles. LowMut melanocytes were found in hair follicles as well as in the interfollicular epidermis, whereas HighMut melanocytes resided almost exclusively in the interfollicular epidermis. We propose that melanocytes in the hair follicle occupy a privileged niche, protected from UV radiation, but periodically migrate out of the hair follicle to replenish the interfollicular epidermis after waves of photodamage. More broadly, our study illustrates the advantages of a cell atlas that includes mutational information, as cells can change their cellular states and positional coordinates over time, but mutations are like scars, providing a historical record of the homeostatic processes that were operative on each cell.

Specific SOX10 enhancer elements modulate phenotype plasticity and drug resistance in melanoma

Recent studies indicate that the development of drug resistance and increased invasiveness in melanoma is largely driven by transcriptional plasticity rather than canonical coding mutations. Understanding the mechanisms behind cell identity shifts in oncogenic transformation and cancer progression is crucial for advancing our understanding of melanoma and other aggressive cancers. While distinct melanoma phenotypic states have been well characterized, the processes and transcriptional controls that enable cells to shift between these states remain largely unknown. In this study, we initially leverage the well-established zebrafish melanoma model as a high-throughput system to dissect and analyze transcriptional control elements that are hijacked by melanoma. We identify key characteristics of these elements, making them translatable to human enhancer identification despite the lack of direct sequence conservation. Building on our identification of a zebrafish sox10 enhancer necessary for melanoma initiation, we extend these findings to human melanoma, identifying two human upstream enhancer elements that are critical for full SOX10 expression. Stable biallelic deletion of these enhancers using CRISPR-Cas9 induces a distinct phenotype shift across multiple human melanoma cell lines from a melanocytic phenotype towards an undifferentiated phenotype and is also characterized by an increase in drug resistance that mirrors clinical data including an upregulation of NTRK1, a tyrosine kinase, and potential therapeutic target. These results provide new insights into the transcriptional regulation of SOX10 in human melanoma and underscore the role of individual enhancer elements and potentially NTRK1 in driving melanoma phenotype plasticity and drug resistance. Our work lays the groundwork for future gene-based and combination kinase-inhibitor therapies targeting SOX10 regulation and NTRK1 as a potential avenue for enhancing the efficacy of current melanoma treatments.

Role of hydrogen sulfide in dermatological diseases

Hydrogen sulfide (H2S), together with carbon monoxide (CO) and nitric oxide (NO), is recognized as a vital gasotransmitter. H2S is biosynthesized by enzymatic pathways in the skin and exerts significant physiological effects on a variety of biological processes, such as apoptosis, modulation of inflammation, cellular proliferation, and regulation of vasodilation. As a major health problem, dermatological diseases affect a large proportion of the population every day. It is urgent to design and develop effective drugs to deal with dermatological diseases. Dermatological diseases can arise from a multitude of etiologies, including neoplastic growth, infectious agents, and inflammatory processes. The abnormal metabolism of H2S is associated with many dermatological diseases, such as melanoma, fibrotic diseases, and psoriasis, suggesting its therapeutic potential in the treatment of these diseases. In addition, therapies based on H2S donors are being developed to treat some of these conditions. In the review, we discuss recent advances in the function of H2S in normal skin, the role of altering H2S metabolism in dermatological diseases, and the therapeutic potential of diverse H2S donors for the treatment of dermatological diseases.

CD133-Dependent Activation of Phosphoinositide 3-Kinase /AKT/Mammalian Target of Rapamycin Signaling in Melanoma Progression and Drug Resistance

Melanoma frequently harbors genetic alterations in key molecules leading to the aberrant activation of PI3K and its downstream pathways. Although the role of PI3K/AKT/mTOR in melanoma progression and drug resistance is well documented, targeting the PI3K/AKT/mTOR pathway showed less efficiency in clinical trials than might have been expected, since the suppression of the PI3K/mTOR signaling pathway-induced feedback loops is mostly associated with the activation of compensatory pathways such as MAPK/MEK/ERK. Consequently, the development of intrinsic and acquired resistance can occur. As a solid tumor, melanoma is notorious for its heterogeneity. This can be expressed in the form of genetically divergent subpopulations including a small fraction of cancer stem-like cells (CSCs) and non-cancer stem cells (non-CSCs) that make the most of the tumor mass. Like other CSCs, melanoma stem-like cells (MSCs) are characterized by their unique cell surface proteins/stemness markers and aberrant signaling pathways. In addition to its function as a robust marker for stemness properties, CD133 is crucial for the maintenance of stemness properties and drug resistance. Herein, the role of CD133-dependent activation of PI3K/mTOR in the regulation of melanoma progression, drug resistance, and recurrence is reviewed.

Canonical Wnt and TGF-β/BMP signaling enhance melanocyte regeneration but suppress invasiveness, migration, and proliferation of melanoma cells

Melanoma is the deadliest form of skin cancer and develops from the melanocytes that are responsible for the pigmentation of the skin. The skin is also a highly regenerative organ, harboring a pool of undifferentiated melanocyte stem cells that proliferate and differentiate into mature melanocytes during regenerative processes in the adult. Melanoma and melanocyte regeneration share remarkable cellular features, including activation of cell proliferation and migration. Yet, melanoma considerably differs from the regenerating melanocytes with respect to abnormal proliferation, invasive growth, and metastasis. Thus, it is likely that at the cellular level, melanoma resembles early stages of melanocyte regeneration with increased proliferation but separates from the later melanocyte regeneration stages due to reduced proliferation and enhanced differentiation. Here, by exploiting the zebrafish melanocytes that can efficiently regenerate and be induced to undergo malignant melanoma, we unravel the transcriptome profiles of the regenerating melanocytes during early and late regeneration and the melanocytic nevi and malignant melanoma. Our global comparison of the gene expression profiles of melanocyte regeneration and nevi/melanoma uncovers the opposite regulation of a substantial number of genes related to Wnt signaling and transforming growth factor beta (TGF-β)/(bone morphogenetic protein) BMP signaling pathways between regeneration and cancer. Functional activation of canonical Wnt or TGF-β/BMP pathways during melanocyte regeneration promoted melanocyte regeneration but potently suppressed the invasiveness, migration, and proliferation of human melanoma cells in vitro and in vivo. Therefore, the opposite regulation of signaling mechanisms between melanocyte regeneration and melanoma can be exploited to stop tumor growth and develop new anti-cancer therapies.

2,2',4,4'-tetrabromodiphenyl ether causes depigmentation in zebrafish larvae via a light-mediated pathway

Polybrominated diphenyl ethers (PBDEs) are organic pollutants widely detected in various environmental media due to their high persistence and bioaccumulation. PBDE-induced visual impairment and neurotoxicity were previously demonstrated using zebrafish (Danio rerio) models, and recent research reported the phenotypic depigmentation effect of PBDEs at high concentrations on zebrafish, but whether those effects are still present at environment-relevant levels is still unclear. Herein, we performed both phenotypic examination and mechanism investigation in zebrafish embryos (48 hpf) and larvae (5 dpf) about their pigmentation status when exposing to PBDE congener BDE-47 (2,2',4,4'-tetrabrominated diphenyl ether) at levels from 0.25 to 25 μg/L. Results showed that low-level BDE-47 can restrain the relative melanin abundance of zebrafish larvae to 70.47% (p < 0.05) and 61.54% (p < 0.01) respectively under 2.5 and 25 μg/L BDE-47 compared with control, and the thickness of retinal pigment epithelium (RPE) remarkably reduced from 571.4 nm to 350.3 nm (p < 0.001) under 25 μg/L BDE-47 exposure. We also observed disrupted expressions of melanin synthesis genes and disorganized mitfa differentiation patterns based on Tg(mifta:EGFP), as well as visual impairment resulting from thinner RPE. Considering both processes of visual development and melanin synthesis are highly sensitive to ambient light conditions, we prolonged the light regime of maintaining zebrafish larvae from 14 hours light versus 10 hours dark (14L:10D) to 18 hours light versus 6 hours dark (18L:6D). Lengthening photoperiod successfully rescued the fluorescent level of mitfa in zebrafish epidermis and most gene expressions associated with melanin synthesis under 25 μg/L BDE-47 exposure to the normal level. In conclusion, our work reported the effects of low-level PBDEs on melanin production using zebrafish embryos and larvae, and identified the potential role of a light-mediated pathway in the neurotoxic mechanism of PBDEs.

Functional and long-lived melanocytes from human pluripotent stem cells with transient ectopic expression of JMJD3

BACKGROUND

Melanocytes are an essential part of the epidermis, and their regeneration has received much attention because propagation of human adult melanocytes in vitro is too slow for clinical use. Differentiation from human pluripotent stem cells to melanocytes has been reported, but the protocols to produce them require multiple and complex differentiation steps.

METHOD

We differentiated human embryonic stem cells (hESCs) that transiently express JMJD3 to pigmented cells. We investigated whether the pigmented cells have melanocytic characteristics and functions by qRT-PCR, immunocytochemical analysis and flow cytometry. We also investigated their biocompatibility by injecting the cells into immunodeficient mice for clinical use.

RESULT

We successfully differentiated and established a pure culture of melanocytes. The melanocytes maintained their growth rate for a long time, approximately 200 days, and were functional. They exhibited melanogenesis and transfer of melanin to peripheral keratinocytes. Moreover, melanocytes simulated the developmental processes from melanoblasts to melanocytes. The melanocytes had high engraftability and biocompatibility in the immunodeficient mice.

CONCLUSION

The robust generation of functional and long-lived melanocytes are key to developing clinical applications for the treatment of pigmentary skin disorders.

Noncoding RNA circuitry in melanoma onset, plasticity, and therapeutic response

Melanoma, the cancer of the melanocyte, is the deadliest form of skin cancer with an aggressive nature, propensity to metastasize and tendency to resist therapeutic intervention. Studies have identified that the re-emergence of developmental pathways in melanoma contributes to melanoma onset, plasticity, and therapeutic response. Notably, it is well known that noncoding RNAs play a critical role in the development and stress response of tissues. In this review, we focus on the noncoding RNAs, including microRNAs, long non-coding RNAs, circular RNAs, and other small RNAs, for their functions in developmental mechanisms and plasticity, which drive onset, progression, therapeutic response and resistance in melanoma. Going forward, elucidation of noncoding RNA-mediated mechanisms may provide insights that accelerate development of novel melanoma therapies.

A single-cell level comparison of human inner ear organoids with the human cochlea and vestibular organs

Inner ear disorders are among the most common congenital abnormalities; however, current tissue culture models lack the cell type diversity to study these disorders and normal otic development. Here, we demonstrate the robustness of human pluripotent stem cell-derived inner ear organoids (IEOs) and evaluate cell type heterogeneity by single-cell transcriptomics. To validate our findings, we construct a single-cell atlas of human fetal and adult inner ear tissue. Our study identifies various cell types in the IEOs including periotic mesenchyme, type I and type II vestibular hair cells, and developing vestibular and cochlear epithelium. Many genes linked to congenital inner ear dysfunction are confirmed to be expressed in these cell types. Additional cell-cell communication analysis within IEOs and fetal tissue highlights the role of endothelial cells on the developing sensory epithelium. These findings provide insights into this organoid model and its potential applications in studying inner ear development and disorders.

FZD6 Promotes Melanoma Cell Invasion but Not Proliferation by Regulating Canonical Wnt Signaling and Epithelial‒Mesenchymal Transition

FZD6 is a key gene that controls tissue polarity during development. Increasing evidence suggests that it also plays active roles in various cancers. In this study, we show that FZD6 is overexpressed in multiple melanoma cell lines and human samples. Knockdown or knockout of FZD6 does not affect cell proliferation but significantly reduces the invasive ability of melanoma cells. In addition, we have found that knockout of Fzd6 dramatically reduces lung metastasis in the Pten/BRaf mouse model of melanoma. Mechanistic studies in vitro and in vivo reveal a surprising involvement of canonical Wnt signaling and epithelial‒mesenchymal pathway in the FZD6-mediated invasive phenotype. Together, our study supports a promoter role of FZD6 in melanoma progression.

Generating Functional and Highly Proliferative Melanocytes Derived from Human Pluripotent Stem Cells: A Promising Tool for Biotherapeutic Approaches to Treat Skin Pigmentation Disorders

Melanocytes are essential for skin homeostasis and protection, and their loss or misfunction leads to a wide spectrum of diseases. Cell therapy utilizing autologous melanocytes has been used for years as an adjunct treatment for hypopigmentary disorders such as vitiligo. However, these approaches are hindered by the poor proliferative capacity of melanocytes obtained from skin biopsies. Recent advances in the field of human pluripotent stem cells have fueled the prospect of generating melanocytes. Here, we have developed a well-characterized method to produce a pure and homogenous population of functional and proliferative melanocytes. The genetic stability and potential transformation of melanocytes from pluripotent stem cells have been evaluated over time during the in vitro culture process. Thanks to transcriptomic analysis, the molecular signatures all along the differentiation protocol have been characterized, providing a solid basis for standardizing the protocol. Altogether, our results promise meaningful, broadly applicable, and longer-lasting advances for pigmentation disorders and open perspectives for innovative biotherapies for pigment disorders.

Surgical procedures and innovative approaches for vitiligo regenerative treatment and melanocytorrhagy

Surgical treatments for vitiligo are a safe and effective treatment modality for select patients with vitiligo. Many techniques of vitiligo surgery exist, each with unique advantages and disadvantages. We reviewed the various surgical therapies and innovative approaches for vitiligo regenerative treatment reported in the literature. Surgical therapies can be subdivided into tissue grafting methods and cellular grafting methods. Tissue grafting methods mainly include mini punch grafts, suction blister roof grafts, and hair follicle grafts. Cellular grafting methods include cultured and non-cultured treatments. The efficacy needs to be improved largely due to the poor proliferation and quality of the autologous melanocytes. Rho-associated protein kinase inhibitor enhances primary melanocyte culture proliferation from vitiligo patients to prevent apoptosis. Innovative approaches using stem cell methods could prove invaluable in developing a novel cell therapy for patients suffering from vitiligo. We succeeded in inducing melanin pigmentation in mice skin in vivo using our human induced pluripotent stem cell-derived melanocytes. In addition, we reviewed melanocytorrhagy, detachment and transepidermal loss of melanocytes, and melanocyte-related adhesion molecules. These adhesion molecules include epithelial cadherin, discoidin domain receptor tyrosine kinase 1, glycoprotein non-metastatic melanoma protein B, macrophage migration inhibiting factor, 17β-hydroxysteroid dehydrogenase 1, and E26 transformation-specific 1.

Brain Regeneration Resembles Brain Cancer at Its Early Wound Healing Stage and Diverges From Cancer Later at Its Proliferation and Differentiation Stages

Gliomas are the most frequent type of brain cancers and characterized by continuous proliferation, inflammation, angiogenesis, invasion and dedifferentiation, which are also among the initiator and sustaining factors of brain regeneration during restoration of tissue integrity and function. Thus, brain regeneration and brain cancer should share more molecular mechanisms at early stages of regeneration where cell proliferation dominates. However, the mechanisms could diverge later when the regenerative response terminates, while cancer cells sustain proliferation. To test this hypothesis, we exploited the adult zebrafish that, in contrast to the mammals, can efficiently regenerate the brain in response to injury. By comparing transcriptome profiles of the regenerating zebrafish telencephalon at its three different stages, i.e., 1 day post-lesion (dpl)-early wound healing stage, 3 dpl-early proliferative stage and 14 dpl-differentiation stage, to those of two brain cancers, i.e., low-grade glioma (LGG) and glioblastoma (GBM), we reveal the common and distinct molecular mechanisms of brain regeneration and brain cancer. While the transcriptomes of 1 dpl and 3 dpl harbor unique gene modules and gene expression profiles that are more divergent from the control, the transcriptome of 14 dpl converges to that of the control. Next, by functional analysis of the transcriptomes of brain regeneration stages to LGG and GBM, we reveal the common and distinct molecular pathways in regeneration and cancer. 1 dpl and LGG and GBM resemble with regard to signaling pathways related to metabolism and neurogenesis, while 3 dpl and LGG and GBM share pathways that control cell proliferation and differentiation. On the other hand, 14 dpl and LGG and GBM converge with respect to developmental and morphogenetic processes. Finally, our global comparison of gene expression profiles of three brain regeneration stages, LGG and GBM exhibit that 1 dpl is the most similar stage to LGG and GBM while 14 dpl is the most distant stage to both brain cancers. Therefore, early convergence and later divergence of brain regeneration and brain cancer constitutes a key starting point in comparative understanding of cellular and molecular events between the two phenomena and development of relevant targeted therapies for brain cancers.

Acute multidrug delivery via a wearable bioreactor facilitates long-term limb regeneration and functional recovery in adult Xenopus laevis

Limb regeneration is a frontier in biomedical science. Identifying triggers of innate morphogenetic responses in vivo to induce the growth of healthy patterned tissue would address the needs of millions of patients, from diabetics to victims of trauma. Organisms such as Xenopus laevis-whose limited regenerative capacities in adulthood mirror those of humans-are important models with which to test interventions that can restore form and function. Here, we demonstrate long-term (18 months) regrowth, marked tissue repatterning, and functional restoration of an amputated X. laevis hindlimb following a 24-hour exposure to a multidrug, pro-regenerative treatment delivered by a wearable bioreactor. Regenerated tissues composed of skin, bone, vasculature, and nerves significantly exceeded the complexity and sensorimotor capacities of untreated and control animals' hypomorphic spikes. RNA sequencing of early tissue buds revealed activation of developmental pathways such as Wnt/β-catenin, TGF-β, hedgehog, and Notch. These data demonstrate the successful "kickstarting" of endogenous regenerative pathways in a vertebrate model.

Neural Crest-Like Stem Cell Transcriptome Analysis Identifies LPAR1 in Melanoma Progression and Therapy Resistance

Metastatic melanoma is challenging to clinically address. Although standard-of-care targeted therapy has high response rates in patients with BRAF-mutant melanoma, therapy relapse occurs in most cases. Intrinsically resistant melanoma cells drive therapy resistance and display molecular and biologic properties akin to neural crest-like stem cells (NCLSC) including high invasiveness, plasticity, and self-renewal capacity. The shared transcriptional programs and vulnerabilities between NCLSCs and cancer cells remains poorly understood. Here, we identify a developmental LPAR1-axis critical for NCLSC viability and melanoma cell survival. LPAR1 activity increased during progression and following acquisition of therapeutic resistance. Notably, genetic inhibition of LPAR1 potentiated BRAFi ± MEKi efficacy and ablated melanoma migration and invasion. Our data define LPAR1 as a new therapeutic target in melanoma and highlights the promise of dissecting stem cell-like pathways hijacked by tumor cells. SIGNIFICANCE: This study identifies an LPAR1-axis critical for melanoma invasion and intrinsic/acquired therapy resistance.

An Optimized Zebrafish Nursery Feeding Regimen Improves Growth Rates and Labor Costs

Setting nutritional standards for larval zebrafish (Danio rerio) that maximize growth, survival, and reproductive success is challenging. We evaluated the effects of different feeding regimens on larval zebrafish by comparing Gemma Micro 75 pelleted diet and live-type L rotifers (Brachionus plicatilis) in 3 feeding regimens starting at 9 days postfertilization (dpf): bolus feeding of live diet (BL), continuous feeding of live diet (CL), and pelleted diet (PD). Animals in the PD and CL groups were longer than the BL group at 4-5 weeks postfertilization. The PD group was also greater in body depth than both live diet groups. There was no significant difference in weight between the groups. There were also no significant differences in fecundity or sex ratios indicating that all feeding methods successfully promote growth of a useful breeding stock of fish. In addition, we quantified the equipment, consumable, and labor costs associated with these methods, and found that the PD regimen was superior to both live diet regimens. These data suggest that providing a high nutrient-density pelleted diet to larval and juvenile zebrafish is an effective means to increase early growth and to decrease cost and labor associated with nursery care.

Nucleotide stress responses in neural crest cell fate and melanoma

Melanoma is the deadliest form of skin cancer. While clinical developments have significantly improved patient prognosis, effective treatment is often obstructed by limited response rates, intrinsic or acquired resistance to therapy, and adverse events. Melanoma initiation and progression are associated with transcriptional reprogramming of melanocytes to a cell state that resembles the lineage from which the cells are specified during development, that is the neural crest. Convergence to a neural crest cell (NCC)-like state revealed the therapeutic potential of targeting developmental pathways for the treatment of melanoma. Neural crest cells have a unique sensitivity to metabolic dysregulation, especially nucleotide depletion. Mutations in the pyrimidine biosynthesis enzyme dihydroorotate dehydrogenase (DHODH) particularly affect neural crest-derived tissues and cause Miller syndrome, a genetic disorder characterized by craniofacial malformations in patients. The developmental susceptibility of the neural crest to nucleotide deficiency is conserved in melanoma and provides a metabolic vulnerability that can be exploited for therapeutic purposes. We review the current knowledge on nucleotide stress responses in neural crest and melanoma and discuss how the recent scientific advances that have improved our understanding of transcriptional regulation during nucleotide depletion can impact melanoma treatment.

Rhabdoid melanoma in a harpy eagle (Harpia harpyja)

A 28-year-old male harpy eagle (Harpia harpyja) with a history of anorexia, hyporexia, lethargy, and progressive weight loss was found dead and submitted for post-mortem examination. Gross findings include dark brown discolouration of testes and lungs; the testes were bilaterally enlarged, glistening brown-grey to blackish in appearance, firm, smooth, and multilobulated. The lungs contained a mass with similar features to the testicles, irregularly shaped with multiple nodules. Histology of testis showed round, polygonal and pleomorphic cells, containing melanin pigments and a typical eosinophilic vacuole in their cytoplasm and with severe pleomorphism. An immunohistochemistry panel with Melan-A, vimentin, CK AE1/AE/3, MUM-1 and CD-68 were performed, yielding a positive reaction for Melan-A and vimentin. The morphology of the tumour cells, the presence of melanin pigment and the immunoreactivity for Melan-A and vimentin by the cells led to a diagnosis of rhabdoid melanoma. This is the first case of this pathology in the testis with lung metastasis in a harpy eagle.

NRAS mutant melanoma: Towards better therapies

Genetic alterations affecting RAS proteins are commonly found in human cancers. Roughly a fourth of melanoma patients carry activating NRAS mutations, rendering this malignancy particularly challenging to treat. Although the development of targeted as well as immunotherapies led to a substantial improvement in the overall survival of non-NRAS^mut melanoma patients (e.g. BRAF^mut), patients with NRAS^mut melanomas have an overall poorer prognosis due to the high aggressiveness of RAS^mut tumors, lack of efficient targeted therapies or rapidly emerging resistance to existing treatments. Understanding how NRAS-driven melanomas develop therapy resistance by maintaining cell cycle progression and survival is crucial to develop more effective and specific treatments for this group of melanoma patients. In this review, we provide an updated summary of currently available therapeutic options for NRAS^mut melanoma patients with a focus on combined inhibition of MAPK signaling and CDK4/6-driven cell cycle progression and mechanisms of the inevitably developing resistance to these treatments. We conclude with an outlook on the most promising novel therapeutic approaches for melanoma patients with constitutively active NRAS. STATEMENT OF SIGNIFICANCE: An estimated 75000 patients are affected by NRAS^mut melanoma each year and these patients still have a shorter progression-free survival than BRAF^mut melanomas. Both intrinsic and acquired resistance occur in NRAS-driven melanomas once treated with single or combined targeted therapies involving MAPK and CDK4/6 inhibitors and/or checkpoint inhibiting immunotherapy. Oncolytic viruses, mRNA-based vaccinations, as well as targeted triple-agent therapy are promising alternatives, which could soon contribute to improved progression-free survival of the NRAS^mut melanoma patient group.

Hydrogen Sulfide in Skin Diseases: A Novel Mediator and Therapeutic Target

Together with nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H₂S) is now recognized as a vital gaseous transmitter. The ubiquitous distributions of H₂S-producing enzymes and potent chemical reactivities of H₂S in biological systems make H₂S unique in its ability to regulate cellular and organ functions in both health and disease. Acting as an antioxidant, H₂S can combat oxidative species such as reactive oxygen species (ROS) and reactive nitrogen species (RNS) and protect the skin from oxidative stress. The aberrant metabolism of H₂S is involved in the pathogenesis of several skin diseases, such as vascular disorders, psoriasis, ulcers, pigment disorders, and melanoma. Furthermore, H₂S donors and some H₂S hybrids have been evaluated in many experimental models of human disease and have shown promising therapeutic results. In this review, we discuss recent advances in understanding H₂S and its antioxidant effects on skin pathology, the roles of altered H₂S metabolism in skin disorders, and the potential value of H₂S as a therapeutic intervention in skin diseases.

Human and mouse melanoma cells recapitulate an EMT-like program in response to mesenchymal stromal cells secretome

The mechanisms underlying the propensity of melanomas to metastasize are not completely understood. We hypothesized that melanoma cells are capable of promptly activating an epithelial-to-mesenchymal transition (EMT)-like profile in response to stroma-derived factors. Thus, we investigated the role of mesenchymal stromal cells (MSCs), a cell population considered as a precursor of tumor stroma, on the activation of an EMT-like profile and acquisition of metastatic traits in melanoma cells. After subcutaneous co-injection with mouse B16 melanoma cells, MSCs occupied perivascular sites within tumors and enhanced B16 metastasis to the lungs. In vitro, MSCs' secretome activated an EMT-like profile in B16 cells, reducing their avidity to fibronectin, and increasing their motility and invasiveness. These effects were abrogated upon blocking of MET phosphorylation in B16 cells using small molecule inhibitors. MSCs also activated an EMT-like profile in human melanoma cells from different stages of progression. Activation of EMT in human cells was associated with increased levels of p-STAT1 and p-STAT3. In conclusion, both mouse and human melanoma cells are equipped to activate an EMT-like program and acquire metastatic traits through the activation of distinct pathways by MSCs' secretome.

β-Galactosylceramidase Promotes Melanoma Growth via Modulation of Ceramide Metabolism

Disturbance of sphingolipid metabolism may represent a novel therapeutic target in metastatic melanoma, the most lethal form of skin cancer. β-Galactosylceramidase (GALC) removes β-galactose from galactosylceramide and other sphingolipids. In this study, we show that downregulation of galcb, a zebrafish ortholog of human GALC, affects melanoblast and melanocyte differentiation in zebrafish embryos, suggesting a possible role for GALC in melanoma. On this basis, the impact of GALC expression in murine B16-F10 and human A2058 melanoma cells was investigated following its silencing or upregulation. Galc knockdown hampered growth, motility, and invasive capacity of B16-F10 cells and their tumorigenic and metastatic activity when grafted in syngeneic mice or zebrafish embryos. Galc-silenced cells displayed altered sphingolipid metabolism and increased intracellular levels of ceramide, paralleled by a nonredundant upregulation of Smpd3, which encodes for the ceramide-generating enzyme neutral sphingomyelinase 2. Accordingly, GALC downregulation caused SMPD3 upregulation, increased ceramide levels, and inhibited the tumorigenic activity of human melanoma A2058 cells, whereas GALC upregulation exerted opposite effects. In concordance with information from melanoma database mining, RNAscope analysis demonstrated a progressive increase of GALC expression from common nevi to stage IV human melanoma samples that was paralleled by increases in microphthalmia transcription factor and tyrosinase immunoreactivity inversely related to SMPD3 and ceramide levels. Overall, these findings indicate that GALC may play an oncogenic role in melanoma by modulating the levels of intracellular ceramide, thus providing novel opportunities for melanoma therapy. SIGNIFICANCE: Data from zebrafish embryos, murine and human cell melanoma lines, and patient-derived tumor specimens indicate that β-galactosylceramidase plays an oncogenic role in melanoma and may serve as a therapeutic target.

Melanocytes derived from mouse hair follicles: A novel study model to assess pigmentation disorders

Melanocytes are the major cells responsible for skin and fair pigmentation in vertebrates. They localize to hair follicles(HFs) and the epidermis during embryonic development. A reduced number or dysfunction of melanocytes results in pigmentation disorders.Thus, methods for isolation, culture, and identification of melanocytes in mouse hair follicles provide an experimental basis for thestudy of of pigmentation disorders. In the current work, we harvested the melanocytes from the anagen phase dorsal skin of C57BL/6 mice.After its separation from the skin, the dermis was digested, and the HFs were released. HFs were then also digested, and the cells released from HFs were cultured in melanocyte growth medium. Immunofluorescence and immunohistochemistry staining were used to localize the distribution of melanocytes in HFs . Reverse transcription polymerase chain reaction was performed to detect the expression of specific melanocyte marker genes. Immunofluorescence, immunohistochemistry, flow cytometry, and western blot were carried out to detect the expression of marker proteins in cells. 3,4-Dihydroxy-L-phenylalanine (L-DOPA) staining was used to detect the pigmentation functionality of melaonocytes. Based on our results, we conclude that mature and functional melanocytes can be successfully obtained from theHFs, providing a cell model to study pigmentation disorders. The current findings provide novel insights for the treatment of pigmentation disorders by autologous cell transplantation. Further, we believe that issues related to skin damage, insufficient numbers of autologous cells, and autoimmune problems can be resolved in future though the use of functional melanocytes.

Loss of prdm1a accelerates melanoma onset and progression

Melanoma is an aggressive, deadly skin cancer derived from melanocytes, a neural crest cell derivative. Melanoma cells mirror the developmental program of neural crest cells in that they exhibit the same gene expression patterns and utilize similar cellular mechanisms, including increased cell proliferation, epithelial-mesenchymal transition, and migration. Here we studied the role of neural crest regulator PRDM1 in melanoma onset and progression. In development, Prdm1a functions to promote neural crest progenitor fate, and in melanoma, we found that PRDM1 has reduced copy number and is recurrently deleted in both zebrafish and humans. When examining expression of neural crest and melanocyte development genes, we show that sox10 progenitor expression is high in prdm1a-/- mutants, while more differentiated melanocyte markers are reduced, suggesting that normally Prdm1a is required for differentiation. Data mining of human melanoma datasets indicates that high PRDM1 expression in human melanoma is correlated with better patient survival and decreased PRDM1 expression is common in metastatic tumors. When one copy of prdm1a is lost in the zebrafish melanoma model Tg(mitfa:BRAFV600E );p53-/- ;prdm1a+/- , melanoma onset occurs more quickly, and the tumors that form have a larger area with increased expression of sox10. These data demonstrate a novel role for PRDM1 as a tumor suppressor in melanoma.

Decoding the Role of CD271 in Melanoma

The evolution of melanoma, the most aggressive type of skin cancer, is triggered by driver mutations that are acquired in the coding regions of particularly BRAF (rat fibrosarcoma serine/threonine kinase, isoform B) or NRAS (neuroblastoma-type ras sarcoma virus) in melanocytes. Although driver mutations strongly determine tumor progression, additional factors are likely required and prerequisite for melanoma formation. Melanocytes are formed during vertebrate development in a well-controlled differentiation process of multipotent neural crest stem cells (NCSCs). However, mechanisms determining the properties of melanocytes and melanoma cells are still not well understood. The nerve growth factor receptor CD271 is likewise expressed in melanocytes, melanoma cells and NCSCs and programs the maintenance of a stem-like and migratory phenotype via a comprehensive network of associated genes. Moreover, CD271 regulates phenotype switching, a process that enables the rapid and reversible conversion of proliferative into invasive or non-stem-like states into stem-like states by yet largely unknown mechanisms. Here, we summarize current findings about CD271-associated mechanisms in melanoma cells and illustrate the role of CD271 for melanoma cell migration and metastasis, phenotype-switching, resistance to therapeutic interventions, and the maintenance of an NCSC-like state.

Long-Term Inhibition of Notch in A-375 Melanoma Cells Enhances Tumor Growth Through the Enhancement of AXIN1, CSNK2A3, and CEBPA2 as Intermediate Genes in Wnt and Notch Pathways

Notch suppression by gamma-secretase inhibitors is a valid approach against melanoma. However, most of studies have evaluated the short-term effect of DAPT on tumor cells or even cancer stem cells. In the present study, we surveyed the short-term and long-term effects of DAPT on the stem cell properties of A375 and NA8 as melanoma cell lines. The effects of DAPT were tested both in vitro and in vivo using xenograft models. In A375 with B-raf mutation, DAPT decreased the level of NOTCH1, NOTH2, and HES1 as downstream genes of the Notch pathway. This was accompanied by enhanced apoptosis after 24 h treatment, arrest in the G2-M phase, and impaired ability of colony and melanosphere formation at the short term. Moreover, tumor growth also reduced during 13 days of treatment. However, long-term treatment of DAPT promoted tumor growth in the xenograft model and enhanced the number and size of colonies and spheroids in vitro. The gene expression studies confirmed the up-regulation of Wnt and Notch downstream genes as well as AXIN1, CSNK2A3, and CEBPA2 following the removal of Notch inhibitor in vitro and in the xenograft model. Moreover, the Gompertz-based mathematical model determined a new drug resistance term in the present study. Our data supported that the long-term and not short-term inhibition of Notch by DAPT may enhance tumor growth and motility through up-regulation of AXIN1, CSNK2A3, and CEBPA2 genes in B-raf mutated A375 cells.

Hydrogen Sulfide Promotes Cell Proliferation and Melanin Synthesis in Primary Human Epidermal Melanocytes

BACKGROUND/AIM

Hydrogen sulfide (H2S) has been found to act as a physiological intercellular messenger to regulate cell survival. In this study, we evaluated whether H2S could promote cell proliferation and melanin synthesis in human epidermal melanocytes (HEMs).

METHODS

Primary HEMs were cocultured with sodium hydrosulfide (NaHS, the most widely used H2S donor) or endogenously overexpressed with cystathionine-γ-lyase (CSE) gene, which is the most predominant H2S-producing enzyme. Then, cell viability, intracellular melanin content, tyrosinase (TYR) activity, and expression of microphthalmia-associated transcription factor (MITF), TYR, together with TYR-related protein 1 (TRP-1) in both transcript and protein levels, were detected.

RESULTS

We first confirmed that NaHS (10-100 μm) increased cell viability, intracellular melanin content, and TYR activity in a dose-dependent manner. Then, we found that endogenous H2S production also promoted cell proliferation, intracellular melanin content, and TYR activity. In addition, we observed the mRNA and protein expression of MITF, TYR, and TRP-1 was significantly up-regulated after NaHS treatment and CSE gene transfection.

CONCLUSIONS

This study demonstrates that H2S promotes cell proliferation and melanin synthesis in HEMs, which indicates pharmacologic regulation of H2S may be potential treatment for skin disorders caused by loss of melanocytes or dysfunction of melanogenesis.

Histone variant dictates fate biasing of neural crest cells to melanocyte lineage

In the neural crest lineage, progressive fate restriction and stem cell assignment are crucial for both development and regeneration. Whereas fate commitment events have distinct transcriptional footprints, fate biasing is often transitory and metastable, and is thought to be moulded by epigenetic programmes. Therefore, the molecular basis of specification is difficult to define. In this study, we established a role for a histone variant, H2a.z.2, in specification of the melanocyte lineage from multipotent neural crest cells. H2a.z.2 silencing reduces the number of melanocyte precursors in developing zebrafish embryos and from mouse embryonic stem cells in vitro We demonstrate that this histone variant occupies nucleosomes in the promoter of the key melanocyte determinant mitf, and enhances its induction. CRISPR/Cas9-based targeted mutagenesis of this gene in zebrafish drastically reduces adult melanocytes, as well as their regeneration. Thereby, our study establishes the role of a histone variant upstream of the core gene regulatory network in the neural crest lineage. This epigenetic mark is a key determinant of cell fate and facilitates gene activation by external instructive signals, thereby establishing melanocyte fate identity.

Opsin3 Downregulation Induces Apoptosis of Human Epidermal Melanocytes via Mitochondrial Pathway

G protein‐coupled receptors (GPCRs) are core switches connecting excellular survival or death signals with cellular signaling pathways in a context‐dependent manner. Opsin 3 (OPN3) belongs to the GPCR superfamily. However, whether OPN3 can control the survival or death of human melanocytes is not known. Here, we try to investigate the inherent function of OPN3 on the survival of melanocytes. Our results demonstrate that OPN3 knockdown by RNAi‐OPN3 in human epidermal melanocytes leads to cell apoptosis. The downregulation of OPN3 markedly reduces intracellular calcium levels and decreases phosphorylation of BAD. Attenuated BAD phosphorylation and elevated BAD protein level alter mitochondria membrane permeability, which trigger activation of BAX and inhibition of BCL‐2 and raf‐1. Activated BAX results in the release of cytochrome c and the loss of mitochondrial membrane potential. Cytochrome c complexes associate with caspase 9, forming a postmitochondrial apoptosome that activate effector caspases including caspase 3 and caspase 7. The release of apoptotic molecules eventually promotes the occurrence of apoptosis. In conclusion, we hereby are the first to prove that OPN3 is a key signal responsible for cell survival through a calcium‐dependent G protein‐coupled signaling and mitochondrial pathway.

Dnmt3a is required for the tumor stemness of B16 melanoma cells

The relationship of carcinogenesis and DNA methyltransferases has attracted extensive attention in tumor research. We reported previously that inhibition of de novo DNA methyltransferase 3a (Dnmt3a) in murine B16 melanoma cells significantly suppressed tumor growth and metastasis in xenografted mouse model. Here, we further demonstrated that knockdown of Dnmt3a enhanced the proliferation in anchor-independent conditions of B16 cells, but severely disrupted its multipotent differentiation capacity in vitro. Furthermore, transforming growth factor β1, a key trigger in stem cell differentiation and tumor cell epithelial-mesenchymal transition (EMT), mainly induced apoptosis, but not EMT in Dnmt3a-deficient B16 cells. These data suggested that Dnmt3a is required for maintaining the tumor stemness of B16 cells and it assists B16 cells to escape from death during cell differentiation. Thus it is hypothesized that not only extraordinary self-renewal ability, but also the capacity of multipotent differentiation is necessary for the melanoma tumorigenesis. Inhibition of multipotent differentiation of tumor cells may shed light on the tumor treatment.

CD271 is a molecular switch with divergent roles in melanoma and melanocyte development

Dysregulation of signaling networks controlling self-renewal and migration of developmental cell lineages is closely linked to the proliferative and invasive properties of tumors. Identification of such signaling pathways and their critical regulators is vital for successful design of effective targeted therapies against neoplastic tissue growth. The neurotrophin receptor (CD271/NGFR/p75NTR) is a key regulator of the melanocytic cell lineage through its ability to mediate cell growth, survival, and differentiation. Using clinical melanoma samples, normal melanocytes and global gene expression profiling we have investigated the role of CD271 in rewiring signal transduction networks of melanoma cells during neoplastic transformation. Our analysis demonstrates that depending on the cell fate of tumor initiation vs normal development, elevated levels of CD271 can serve as a switch between proliferation/survival and differentiation/cell death. Two divergent arms of neurotrophin signaling hold the balance between positive regulators of tumor growth controlled by E2F, MYC, SREBP1 and AKT3 pathways on the one hand, and differentiation, senescence, and apoptosis controlled by TRAF6/IRAK-dependent activation of AP1 and TP53 mediated processes on the other hand. A molecular network map revealed in this study uncovers CD271 as a context-specific molecular switch between normal development and malignant transformation.

Down-regulation of FZD3 receptor suppresses growth and metastasis of human melanoma independently of canonical WNT signaling

Frizzled 3 receptor (FZD3) plays an important role in the homeostasis of the neural crest and its derivatives, which give rise to pigment-synthesizing cells, melanocytes. While the role for FZD3 in specification of the melanocytic lineage from neural crest is well established, its significance in the formation of melanoma, its associated malignancy, is less understood. In this study we identified FZD3 as a critical regulator of human melanoma tumorigenesis. Down-regulation of FZD3 abrogated growth, colony-forming potential, and invasive capacity of patient-derived melanoma cells. Xenotransplantation of tumor cells with down-regulated FZD3 levels originating from melanomas carrying the BRAF(V600) mutation uniformly suppressed their capacity for tumor and metastasis formation. FZD3 knockdown leads to the down-regulation of the core cell cycle protein components (cyclins D1, E2, B1, and CDKs 1, 2, and 4) in melanomas with a hyperactive BRAF oncogene, indicating a dominant role of this receptor during melanoma pathogenesis. Enriched pathway analysis revealed that FZD3 inhibits transcriptional networks controlled by CREB5, FOXD1, and ATF3, which suppress the activity of MAPK-mediated signaling. Thus, FZD3 establishes a positive-feedback mechanism that activates MAPK signal transduction network, critical to melanoma carcinogenesis. Importantly, high levels of FZD3 mRNA were found to be correlated with melanoma advancement to metastatic stages and limited patient survival. Changes in gene-expression patterns mediated by FZD3 activity occur in the absence of nuclear β-catenin function, thus representing an important therapeutic target for the melanoma patients whose disease progresses independent of canonical WNT signaling.

The Power of Fish Models to Elucidate Skin Cancer Pathogenesis and Impact the Discovery of New Therapeutic Opportunities

Animal models play important roles in investigating the pathobiology of cancer, identifying relevant pathways, and developing novel therapeutic tools. Despite rapid progress in the understanding of disease mechanisms and technological advancement in drug discovery, negative trial outcomes are the most frequent incidences during a Phase III trial. Skin cancer is a potential life-threatening disease in humans and might be medically futile when tumors metastasize. This explains the low success rate of melanoma therapy amongst other malignancies. In the past decades, a number of skin cancer models in fish that showed a parallel development to the disease in humans have provided important insights into the fundamental biology of skin cancer and future treatment methods. With the diversity and breadth of advanced molecular genetic tools available in fish biology, fish skin cancer models will continue to be refined and expanded to keep pace with the rapid development of skin cancer research. This review begins with a brief introduction of molecular characteristics of skin cancers, followed by an overview of teleost models that have been used in the last decades in melanoma research. Next, we will detail the importance of the zebrafish (Danio rerio) animal model and other emerging fish models including platyfish (Xiphophorus sp.), and medaka (Oryzias latipes) in future cutaneous malignancy studies. The last part of this review provides the recent development and genome editing applications of skin cancer models in zebrafish and the progress in small molecule screening.

Identification of Novel Melanin Synthesis Inhibitors From Crataegus pycnoloba Using an in Vivo Zebrafish Phenotypic Assay

Zebrafish has emerged as a powerful model organism for high throughput drug screening. Several morphological criteria, transgenic lines and in situ expression screens have been developed to identify novel bioactive compounds and their mechanism of action. Here, we used the inhibition of melanogenesis during early zebrafish embryo development to identify natural compounds that block melanogenesis. We identified an extract from the Greek hawthorn Crataegus pycnoloba as a potent inhibitor of melanin synthesis and used activity based subfractionation to identify active subfractions and eventually three single compounds of the same family (dibenzofurans). These compounds show reversible inhibition of melanin synthesis and do not act via inhibition of tyrosinase. We also showed that they do not interfere with neural crest differentiation or migration. We identified via in silico modeling that the compounds can bind to the aryl hydrocarbon receptor (AHR) and verified activation of the Ahr signaling pathway showing the induction of the expression of target genes.

Integration of zebrafish fin regeneration genes with expression data of human tumors in silico uncovers potential novel melanoma markers

Tissue regeneration requires expression of a large, unknown number of genes to initiate and maintain cellular processes such as proliferation, extracellular matrix synthesis, differentiation and migration. A unique model to simulate this process in a controlled manner is the re-growth of the caudal fin of zebrafish after amputation. Within this tissue stem cells differentiate into fibroblasts, epithelial and endothelial cells as well as melanocytes. Many genes implicated in the regeneration process are deregulated in cancer. We therefore undertook a systematic gene expression study to identify genes upregulated during the re-growth of caudal fin tissue. By applying a high stringency cut-off value of 4-fold change, we identified 54 annotated genes significantly overexpressed in regenerating blastema. Further bioinformatics data mining studies showed that 22 out of the 54 regeneration genes where overexpressed in melanoma compared to normal skin or other cancers. Whereas the role of TNC (tenascin C) and FN1 (fibronectin 1) in melanoma development is well documented, implication of MARCKS, RCN3, BAMBI, PEA3/ETV4 and the FK506 family members FKBP7, FKBP10 and FKBP11 in melanoma progression is unclear. Corresponding proteins were detected in melanoma tissue but not in normal skin. High expression of FKBP7, DPYSL5 and MDK was significantly associated with poor survival. We discuss a potential role of these novel melanoma genes, which have promising potential as new therapeutic targets or diagnostic markers.

Artificial Pigmented Human Skin Created by Muse Cells

The skin composes physiological and chemical barrier and renews skin component cells throughout the human life. Melanocytes locate in the basal layer of the epidermis and produce melanin to protect the skin from ultraviolet. Melanin plays key roles in determining human skin and hair color. Melanocyte dysfunction observed in albinism and vitiligo not only causes cosmetic problems but also increases risk of skin cancer. As rejuvenate therapy, embryonic stem (ES) cells and induced pluripotent stem (iPS) cells have been reported to generate melanocytes. Other than ES and iPS cells, human skin tissues maintain pluripotent stem cells, named multilineage-differentiating stress-enduring (Muse) cells. We employ Muse cells isolated from human fibroblasts and adipose tissue to differentiate into melanocytes (Muse-MC). Muse-MC express melanocyte-related molecules, such as tyrosinase and DCT, and show tyrosinase activity. We also succeeded to differentiate Muse cells into fibroblasts and keratinocytes and created three-dimensional (3D) reconstituted skin with Muse cell-derived melanocytes, fibroblasts, and keratinocytes. The 3D reconstituted skin of Muse cell-derived cells coordinately showed epidermis layers and Muse-MC localized in the basal layer of the epidermis. Thus Muse cells in the human skin can be a source of rejuvenation medicine for the skin reconstruction.

SOX10-MITF pathway activity in melanoma cells

Melanoma is one of the most dangerous and lethal skin cancers, with a considerable metastatic potential and drug resistance. It involves a malignant transformation of melanocytes. The exact course of events in which melanocytes become melanoma cells remains unclear. Nevertheless, this process is said to be dependent on the occurrence of cells with the phenotype of progenitor cells - cells characterized by expression of proteins such as nestin, CD-133 or CD-271. The development of these cells and their survival were found to be potentially dependent on the neural crest stem cell transcription factor SOX10. This is just one of the possible roles of SOX10, which contributes to melanomagenesis by regulating the SOX10-MITF pathway, but also to melanoma cell survival, proliferation and metastasis formation. The aim of this review is to describe the broad influence of the SOX10-MITF pathway on melanoma cells.

A Review of the Aetiopathogenesis and Clinical and Histopathological Features of Oral Mucosal Melanoma

Oral mucosal melanoma is an uncommon, usually heavily melanin-pigmented, but occasionally amelanotic aggressive tumour with a poor prognosis. Despite radical surgery, radiotherapy, or chemotherapy, local recurrence and distant metastasis are frequent. Microscopical examination is essential for diagnosis, and routine histological staining must be supplemented by immunohistochemical studies. The aetiology is unknown, the pathogenesis is poorly understood, and the 5-year survival rate rarely exceeds 30%. In most cases, oral mucosal melanoma arises from epithelial melanocytes in the basal layer of the epithelium and less frequently from immature melanocytes arrested in the lamina propria. In both cases the melanocytes undergo malignant transformation, invade deeper tissues, and metastasize to regional lymph nodes and to distant sites. Very rarely metastasis from skin melanoma may give rise to oral mucosal melanoma that may be mistaken for primary oral mucosal melanoma. The pathogenesis of oral mucosal melanoma is complex involving multiple interactions between cytogenetic factors including dysregulation of the cKit signalling pathways, cell cycle, apoptosis, and cell-to-cell interactions on the one hand and melanin itself, melanin intermediates, and local microenvironmental agents regulating melanogenesis on the other hand. The detailed mechanisms that initiate the malignant transformation of oral melanocytes and thereafter sustain and promote the process of melanomagenesis are unknown.

The rise of photoresponsive protein technologies applications in vivo: a spotlight on zebrafish developmental and cell biology

The zebrafish ( Danio rerio) is a powerful vertebrate model to study cellular and developmental processes in vivo. The optical clarity and their amenability to genetic manipulation make zebrafish a model of choice when it comes to applying optical techniques involving genetically encoded photoresponsive protein technologies. In recent years, a number of fluorescent protein and optogenetic technologies have emerged that allow new ways to visualize, quantify, and perturb developmental dynamics. Here, we explain the principles of these new tools and describe some of their representative applications in zebrafish.

Identification of a small molecule that downregulates MITF expression and mediates antimelanoma activity in vitro

Melanoma is a type of cancer arising from the melanocytes, which are the cells that make up the pigment melanin and are derived from the neural crest. There is no particularly effective therapy once the disease is metastatic, highlighting the need for discovery of novel potent agents. In this investigation, we adopted a zebrafish embryonic pigmentation model to identify antimelanoma agents by screening an in-house small molecule library. With this assay, we found that a small molecule compound, SKLB226, blocked zebrafish pigmentation and pigment cell migration. Mechanism of action studies showed that SKLB226 downregulated MITF mRNA level in both zebrafish embryos and mammalian melanoma cells. Further studies showed that it could efficiently suppress the viability and migration of mammalian melanoma cells. In summary, SKLB226 can be used as a chemical tool to study melanocyte development as well as an antimelanoma lead compound that should be subjected to further structural optimization.

Wnt/β-catenin signaling pathway activates melanocyte stem cells in vitro and in vivo

BACKGROUND

Melanocyte stem cells (McSCs) are the origin of melanocytes that are periodically refreshed in skin and hair follicle. Previously, we reported that Wnt3a could promote melanogenesis, but the mechanism of McSCs activation remains unclear.

OBJECTIVE

We aimed to illustrate the roles of Wnt/ß-catenin signaling pathway during McSC activation.

METHODS

Adenovirus-mediated overexpression of Wnt3a and Wnt10b were used. In vitro experiments were performed on the immortalized melanocyte progenitor cell line iMC23, wheres in vivo experiments were performed in Dct-LacZ mice. Immunofluorescence and western blot were used to determine the protein expression.

RESULTS

Wnt3a promotes the differentiation and melanogenesis of iMC23, by activating Wnt/β-catenin signaling pathway. Wnt3a induces hair follicle regeneration and McSC activation. Detailed analysis indicats that Wnt3a activated Wnt/β-catenin signaling pathway, thus promoting the differentiation of McSCs during this process. Wnt10b, another canonical Wnt signaling ligand, induces hair follicle regeneration and McSC activation as well.

CONCLUSION

Wnt/β-catenin signaling pathway activates McSCs both in vitro and in vivo.

Feeder-free Derivation of Melanocytes from Human Pluripotent Stem Cells

Human pluripotent stem cells (hPSCs) represent a platform to study human development in vitro under both normal and disease conditions. Researchers can direct the differentiation of hPSCs into the cell type of interest by manipulating the culture conditions to recapitulate signals seen during development. One such cell type is the melanocyte, a pigment-producing cell of neural crest (NC) origin responsible for protecting the skin against UV irradiation. This protocol presents an extension of a currently available in vitro Neural Crest differentiation protocol from hPSCs to further differentiate NC into fully pigmented melanocytes. Melanocyte precursors can be enriched from the Neural Crest protocol via a timed exposure to activators of WNT, BMP, and EDN3 signaling under dual-SMAD-inhibition conditions. The resultant melanocyte precursors are then purified and matured into fully pigmented melanocytes by culture in a selective medium. The resultant melanocytes are fully pigmented and stain appropriately for proteins characteristic of mature melanocytes.

Cross-species models of human melanoma

Although transformation of melanocytes to melanoma is rare, the rapid growth, systemic spread, as well as the chemoresistance of melanoma present significant challenges for patient care. Here we review animal models of melanoma, including murine, canine, equine, and zebrafish models, and detail the immense contribution these models have made to our knowledge of human melanoma development, and to melanocyte biology. We also highlight the opportunities for cross-species comparative genomic studies of melanoma to identify the key molecular events that drive this complex disease.

Chromatin-Remodelling Complex NURF Is Essential for Differentiation of Adult Melanocyte Stem Cells

MIcrophthalmia-associated Transcription Factor (MITF) regulates melanocyte and melanoma physiology. We show that MITF associates the NURF chromatin-remodelling factor in melanoma cells. ShRNA-mediated silencing of the NURF subunit BPTF revealed its essential role in several melanoma cell lines and in untransformed melanocytes in vitro. Comparative RNA-seq shows that MITF and BPTF co-regulate overlapping gene expression programs in cell lines in vitro. Somatic and specific inactivation of Bptf in developing murine melanoblasts in vivo shows that Bptf regulates their proliferation, migration and morphology. Once born, Bptf-mutant mice display premature greying where the second post-natal coat is white. This second coat is normally pigmented by differentiated melanocytes derived from the adult melanocyte stem cell (MSC) population that is stimulated to proliferate and differentiate at anagen. An MSC population is established and maintained throughout the life of the Bptf-mutant mice, but these MSCs are abnormal and at anagen, give rise to reduced numbers of transient amplifying cells (TACs) that do not express melanocyte markers and fail to differentiate into mature melanin producing melanocytes. MSCs display a transcriptionally repressed chromatin state and Bptf is essential for reactivation of the melanocyte gene expression program at anagen, the subsequent normal proliferation of TACs and their differentiation into mature melanocytes.

The melanocytes pigmenting the coat of adult mice derive from the melanocyte stem cell population residing in the permanent bulge area of the hair follicle. At each angen phase, melanocyte stem cells are stimulated to generate proliferative transient amplifying cells that migrate to the bulb of the follicle where they differentiate into mature melanin producing melanocytes, a processes involving MIcrophthalmia-associated Transcription Factor (MITF) the master regulator of the melanocyte lineage. We show that MITF associates with the NURF chromatin-remodelling factor in melanoma cells. NURF acts downstream of MITF in melanocytes and melanoma cells co-regulating gene expression in vitro. In vivo, mice lacking the NURF subunit Bptf in the melanocyte lineage show premature greying as they are unable to generate mature melanocytes from the adult stem cell population. We find that the melanocyte stem cells from these animals are abnormal and that once they are stimulated at anagen, Bptf is required to ensure the expression of melanocyte markers and their differentiation into mature adult melanocytes. Chromatin remodelling by NURF therefore appears to be essential for the transition of the transcriptionally quiescent stem cell to the differentiated state.

Cell injury, retrodifferentiation and the cancer treatment paradox

This “opinion article” is an attempt to take an overview of some significant changes that have happened in our understanding of cancer status during the last half century and its evolution under the progressive influence of molecular biology. As an active worker in cancer research and developmental biology during most of this period, I would like to comment briefly on these changes and to give my critical appreciation of their outcome as it affects our knowledge of cancer development as well as the current treatment of the disease. A recall of my own contribution to the subject is also included. Two subjects are particularly developed: cell injury and cell-killing therapies. Cell injury, whatever its origin, has acquired the status of a pivotal event for the initiation of cancer emergence. It is postulated that cell injury, a potential case of cellular death, may also be the origin of a process of stepwise cell reversion (retrodifferentiation or retroprogrammation) leading, by division, mature or stem cells to progressive immaturity. The genetic instability and mutational changes that accompanies this process of cell injury and rejuvenation put normal cells in a status favourable to neoplastic transformation or may evolve cancer cells toward clones with higher malignant potentiality. Thus, cell injury suggests lifestyle as the major upstream initiator of cancer development although this not exclude randomness as an unavoidable contributor to the disease. Cell-killing agents (mainly cytotoxic drugs and radiotherapy) are currently used to treat cancer. At the same time, it is agreed that agents with high cell injury potential (ultraviolet light, ionising radiations, tobacco, environmental pollutants, etc.) contribute to the emergence of malignant tumours. This represents a real paradox. In spite of the progress accomplished in cancer survival, one is tempted to suggest that we have very few chances of really cure cancer as long as we continue to treat malignancies with cell-killing therapies. Indeed, the absence of alternatives to such treatments justifies the pursuit of current procedures of cancer care. But, this should be, precisely, an urgent stimulus to explore other therapeutic approaches. Tumour reversion, immunotherapy, stem cell management and genomic analysis of embryo-foetal development could be, among others, appropriated candidates for future active research.

Conserved lamin A protein expression in differentiated cells in the earthworm Eudrilus eugeniae

Lamin A is an intermediate filament protein found in most of the differentiated vertebrate cells but absent in stem cells. It shapes the skeletal frame structure beneath the inner nuclear membrane of the cell nucleus. As there are few studies of the expression of lamin A in invertebrates, in the present work, we have analyzed the sequence, immunochemical conservation and expression pattern of lamin A protein in the earthworm Eudrilus eugeniae, a model organism for tissue regeneration. The expression of lamin A has been confirmed in E. eugeniae by immunoblot. Its localization in the nuclear membrane has been observed by immunohistochemistry using two different rabbit anti-sera raised against human lamin A peptides, which are located at the C-terminus of the lamin A protein. These two antibodies detected 70 kDa lamin A protein in mice and a single 65 kDa protein in the earthworm. The Oct-4 positive undifferentiated blastemal tissues of regenerating earthworm do not express lamin A, while the Oct-4 negative differentiated cells express lamin A. This pattern was also confirmed in the earthworm prostate gland. The present study is the first evidence for the immunochemical identification of lamin A and Oct-4 in the earthworm. Along with the partial sequence obtained from the earthworm genome, the present results suggest that lamin A protein and its expression pattern is conserved from the earthworm to humans.

The melanocyte lineage in development and disease

Melanocyte development provides an excellent model for studying more complex developmental processes. Melanocytes have an apparently simple aetiology, differentiating from the neural crest and migrating through the developing embryo to specific locations within the skin and hair follicles, and to other sites in the body. The study of pigmentation mutations in the mouse provided the initial key to identifying the genes and proteins involved in melanocyte development. In addition, work on chicken has provided important embryological and molecular insights, whereas studies in zebrafish have allowed live imaging as well as genetic and transgenic approaches. This cross-species approach is powerful and, as we review here, has resulted in a detailed understanding of melanocyte development and differentiation, melanocyte stem cells and the role of the melanocyte lineage in diseases such as melanoma.

Summary: This Review discusses melanocyte development and differentiation, melanocyte stem cells, and the role of the melanocyte lineage in diseases such as melanoma.