Clonal analyses reveal roles of organ founding stem cells, melanocyte stem cells and melanoblasts in establishment, growth and regeneration of the adult zebrafish fin

Analysis of yellow mutant rainbow trout transcriptomes at different developmental stages reveals dynamic regulation of skin pigmentation genes

Yellow mutant rainbow trout (YR), an economically important aquaculture species, is popular among consumers due to its excellent meat quality and attractive appearance. Skin color is a key economic trait for YR, but little is known about the molecular mechanism of skin color development. In this study, YR skin transcriptomes were analyzed to explore temporal expression patterns of pigmentation-related genes in three different stages of skin color development. In total, 16,590, 16,682, and 5619 genes were differentially expressed between fish at 1 day post-hatching (YR1d) and YR45d, YR1d and YR90d, and YR45d and YR90d. Numerous differentially expressed genes (DEGs) associated with pigmentation were identified, and almost all of them involved in pteridine and carotenoid synthesis were significantly upregulated in YR45d and YR90d compared to YR1d, including GCH1, PTS, QDPR, CSFIR1, SLC2A11, SCARB1, DGAT2, PNPLA2, APOD, and BCO2. Interestingly, many DEGs enriched in melanin synthesis pathways were also significantly upregulated, including melanogenesis (MITF, MC1R, SLC45A2, OCA2, and GPR143), tyrosine metabolism (TYR, TYRP1, and DCT), and MAPK signaling (KITA) pathways. Using short time-series expression miner, we identified eight differential gene expression pattern profiles, and DEGs in profile 7 were associated with skin pigmentation. Protein–protein interaction network analysis showed that two modules were related to xanthophores and melanophores. In addition, 1,812,329 simple sequence repeats and 2,011,334 single-nucleotide polymorphisms were discovered. The results enhance our understanding of the molecular mechanism underlying skin pigmentation in YR, and could accelerate the molecular breeding of fish species with valuable skin color traits and will likely be highly informative for developing new therapeutic approaches to treat pigmentation disorders and melanoma.

Integrative mRNA-miRNA interaction analysis reveals the molecular mechanism of skin color variation between wild-type and yellow mutant rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss) is an important economic fish in China. Skin color affects the economic value of trout. However, the molecular mechanism of the skin color variation between wild-type (WR) and yellow mutant rainbow trout (YR) is unclear. We sequenced mRNAs and miRNAs of dorsal skin to identify key color variation-associated mRNAs and miRNAs between WR and YR. Overall, 2060 out of 3625 differentially expressed genes were upregulated in YR, and 196 out of 275 differentially expressed miRNAs were downregulated in WR. We identified three key YR-upregulated genes related to the formation of xanthophores (GCH1, SLC2A11, and SOX10). Interestingly, several genes related to melanogenesis (TYR, TYRP1, TYRP2, MC1R, MITF, PMEL, SLC45A2, and OCA2) were downregulated in WR. Integrated analysis identified five miRNAs that target at least two skin color-related genes (miR-495-y, miR-543-y, miR-665-z, miR-433-y, and miR-382-x). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses of target genes identified noncoding RNA metabolic process as the most significantly enriched GO term, and several metabolic pathways associated with skin color were enriched significantly, such as tyrosine metabolism, histidine metabolism, and vitamin B6 metabolism. Quantitative real-time PCR of selected mRNAs and miRNAs validated the reliability of the integrated analysis. This study provides in-depth insights into the molecular mechanism of skin color variation between WR and YR, which will accelerate the genetic selection and breeding of rainbow trout with consumer-favored traits.

Development and genetics of red coloration in the zebrafish relative Danio albolineatus

Animal pigment patterns play important roles in behavior and, in many species, red coloration serves as an honest signal of individual quality in mate choice. Among Danio fishes, some species develop erythrophores, pigment cells that contain red ketocarotenoids, whereas other species, like zebrafish (D. rerio) only have yellow xanthophores. Here, we use pearl danio (D. albolineatus) to assess the developmental origin of erythrophores and their mechanisms of differentiation. We show that erythrophores in the fin of D. albolineatus share a common progenitor with xanthophores and maintain plasticity in cell fate even after differentiation. We further identify the predominant ketocarotenoids that confer red coloration to erythrophores and use reverse genetics to pinpoint genes required for the differentiation and maintenance of these cells. Our analyses are a first step toward defining the mechanisms underlying the development of erythrophore-mediated red coloration in Danio and reveal striking parallels with the mechanism of red coloration in birds.

Evolution of pigment cells and patterns: recent insights from teleost fishes

Skin pigment patterns of vertebrates are stunningly diverse, and nowhere more so than in teleost fishes. Several species, including relatives of zebrafish, recently evolved cichlid fishes of East Africa, clownfishes, deep sea fishes, and others are providing insights into pigment pattern evolution. This overview describes recent advances in understanding periodic patterns, like stripes and spots, the loss of patterns, and the role of cell-type diversification in generating pigmentation phenotypes. Advances in this area are being facilitated by the application of modern methods of gene editing, genomics, computational analysis, and other approaches to non-traditional model organisms having interesting pigmentary phenotypes. Several topics worthy of future attention are outlined as well.

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.

Modeling Stripe Formation on Growing Zebrafish Tailfins

As zebrafish develop, black and gold stripes form across their skin due to the interactions of brightly colored pigment cells. These characteristic patterns emerge on the growing fish body, as well as on the anal and caudal fins. While wild-type stripes form parallel to a horizontal marker on the body, patterns on the tailfin gradually extend distally outward. Interestingly, several mutations lead to altered body patterns without affecting fin stripes. Through an exploratory modeling approach, our goal is to help better understand these differences between body and fin patterns. By adapting a prior agent-based model of cell interactions on the fish body, we present an in silico study of stripe development on tailfins. Our main result is a demonstration that two cell types can produce stripes on the caudal fin. We highlight several ways that bone rays, growth, and the body-fin interface may be involved in patterning, and we raise questions for future work related to pattern robustness.

Maintenance of Melanocyte Stem Cell Quiescence by GABA-A Signaling in Larval Zebrafish

In larval zebrafish, melanocyte stem cells (MSCs) are quiescent, but can be recruited to regenerate the larval pigment pattern following melanocyte ablation. Through pharmacological experiments, we found that inhibition of γ-aminobutyric acid (GABA)-A receptor function, specifically the GABA-A ρ subtype, induces excessive melanocyte production in larval zebrafish. Conversely, pharmacological activation of GABA-A inhibited melanocyte regeneration. We used clustered regularly interspaced short palindromic repeats/Cas9 to generate two mutant alleles of gabrr1, a subtype of GABA-A receptors. Both alleles exhibited robust melanocyte overproduction, while conditional overexpression of gabrr1 inhibited larval melanocyte regeneration. Our data suggest that gabrr1 signaling is necessary to maintain MSC quiescence and sufficient to reduce, but not eliminate, melanocyte regeneration in larval zebrafish.

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.

Evolution of Endothelin signaling and diversification of adult pigment pattern in Danio fishes

Fishes of the genus Danio exhibit diverse pigment patterns that serve as useful models for understanding the genes and cell behaviors underlying the evolution of adult form. Among these species, zebrafish D. rerio exhibit several dark stripes of melanophores with sparse iridophores that alternate with light interstripes of dense iridophores and xanthophores. By contrast, the closely related species D. nigrofasciatus has an attenuated pattern with fewer melanophores, stripes and interstripes. Here we demonstrate species differences in iridophore development that presage the fully formed patterns. Using genetic and transgenic approaches we identify the secreted peptide Endothelin-3 (Edn3)—a known melanogenic factor of tetrapods—as contributing to reduced iridophore proliferation and fewer stripes and interstripes in D. nigrofasciatus. We further show the locus encoding this factor is expressed at lower levels in D. nigrofasciatus owing to cis-regulatory differences between species. Finally, we show that functions of two paralogous loci encoding Edn3 have been partitioned between skin and non-skin iridophores. Our findings reveal genetic and cellular mechanisms contributing to pattern differences between these species and suggest a model for evolutionary changes in Edn3 requirements for pigment patterning and its diversification across vertebrates.

Neural crest derived pigment cells generate the spectacular variation in skin pigment patterns among vertebrates. Mammals and birds have just a single skin pigment cell, the melanocyte, whereas ectothermic vertebrates have several pigment cells including melanophores, iridophores and xanthophores, that together organize into a diverse array of patterns. In the teleost zebrafish, Danio rerio, an adult pattern of stripes depends on interactions between pigment cell classes and between pigment cells and their tissue environment. The close relative D. nigrofasciatus has fewer stripes and prior analyses suggested a difference between these species that lies extrinsic to the pigment cells themselves. A candidate for mediating this difference is Endothelin-3 (Edn3), essential for melanocyte development in warm-blooded animals, and required by all three classes of pigment cells in an amphibian. We show that Edn3 specifically promotes iridophore development in Danio, and that differences in Edn3 expression contribute to differences in iridophore complements, and striping, between D. rerio and D. nigrofasciatus. Our study reveals a novel function for Edn3 and provides new insights into how changes in gene expression yield morphogenetic outcomes to effect diversification of adult form.

The developmental and genetic trajectory of coloration in the guppy (Poecilia reticulata)

Examining the association between trait variation and development is crucial for understanding the evolution of phenotypic differences. Male guppy ornamental caudal fin coloration is one trait that shows a striking degree of variation within and between guppy populations. Males initially have no caudal fin coloration, then gradually develop it as they reach sexual maturity. For males, there is a trade-off between female preference for caudal fin coloration and increased visibility to predators. This trade-off may reach unique endpoints in males from different predation regimes. Caudal fin coloration includes black melanin, orange/yellow pteridines or carotenoids, and shimmering iridescence. This study examined the phenotypic trajectory and genetics associated with color development. We found that black coloration always developed first, followed by orange/yellow, then iridescence. The ordering and timing of color appearance was the same regardless of predation regime. The increased expression of melanin synthesis genes correlated well with the visual appearance of black coloration, but there was no correlation between carotenoids or pteridine synthesis gene expression and the appearance of orange/yellow. The lack of orange/yellow coloration in earlier male caudal fin developmental stages may be due to reduced expression of genes underlying the development of orange/yellow xanthophores.

Three-Dimensional Printed Fish Graders: A Tool to Rapidly and Reliably Size Select Zebrafish

Research into adult zebrafish often requires fish to be of a specific size. Currently, fish must be individually measured to achieve this goal. Here, we design and utilize fish graders to quickly sort fish by width. We characterize graders individually for the length of fish they discriminate between and we also analyze graders in pairs to define the range of lengths for a retained population of fish. We note that a 1 mm increase of fish width increases fish length by 6.2-7.2 mm. We provide the schematics to print a series of eight retention widths, and note that graders of any desired retention width can easily be printed by slightly modifying our design files.

Zebrafish adult pigment stem cells are multipotent and form pigment cells by a progressive fate restriction process: Clonal analysis identifies shared origin of all pigment cell types

Skin pigment pattern formation is a paradigmatic example of pattern formation. In zebrafish, the adult body stripes are generated by coordinated rearrangement of three distinct pigment cell-types, black melanocytes, shiny iridophores and yellow xanthophores. A stem cell origin of melanocytes and iridophores has been proposed although the potency of those stem cells has remained unclear. Xanthophores, however, seemed to originate predominantly from proliferation of embryonic xanthophores. Now, data from Singh et al. shows that all three cell-types derive from shared stem cells, and that these cells generate peripheral neural cell-types too. Furthermore, clonal compositions are best explained by a progressive fate restriction model generating the individual cell-types. The numbers of adult pigment stem cells associated with the dorsal root ganglia remain low, but progenitor numbers increase significantly during larval development up to metamorphosis, likely via production of partially restricted progenitors on the spinal nerves.

A light-colored region of caudal fin: a niche of melanocyte progenitors in crucian carp (Cyprinus carpio L.)

Melanocyte stem cells are a population of immature cells which sustain the self-renewal and replenish the differentiated melanocytes. In this research, a light-colored region (LCR) is observed at the heel of caudal fin in juvenile crucian carp. By cutting off the caudal fin, the operated caudal fin can regenerate in accordance with the original pigment pattern from the retained LCR. As markers of stem cells, Oct4 and Sox2 have been found to be highly expressed in the LCR as well as Mitfa, a label of the melanoblasts. In vitro, Mitfa⁺ melanoblasts are observed in the cells which are derived from the LCR and transfected with Mitfa-EGFP reporter by using Tol2 transposon system. Furthermore, by real-time qPCR, it is shown that the level of sox2 mRNA is gradually decreased from the LCR to proximal and distal caudal fin, and that of mitfa mRNA in the proximal caudal fin (PCF) is higher than that in the LCR, while it is the lowest in the distal caudal fin. Hence, we propose that the LCR is a pigment progenitor niche, sending melanocytes to the distal of caudal fin, which gradually emerges as caudal fin grow. We reveal that the LCR of caudal fin might be a niche of pigment progenitors, and contribute to pigment-producing stem cells in crucian carp.

Pax7 is required for establishment of the xanthophore lineage in zebrafish embryos

A conceptually novel role for Pax7 is found in zebrafish pigment formation. Absence of Pax7 leads to an expansion of the embryonic and larval melanophore lineage and a depletion of xanthophores, suggesting a model in which Pax7 is involved in early chromatophore specification processes.

The pigment pattern of many animal species is a result of the arrangement of different types of pigment-producing chromatophores. The zebrafish has three different types of chromatophores: black melanophores, yellow xanthophores, and shimmering iridophores arranged in a characteristic pattern of golden and blue horizontal stripes. In the zebrafish embryo, chromatophores derive from the neural crest cells. Using pax7a and pax7b zebrafish mutants, we identified a previously unknown requirement for Pax7 in xanthophore lineage formation. The absence of Pax7 results in a severe reduction of xanthophore precursor cells and a complete depletion of differentiated xanthophores in embryos as well as in adult zebrafish. In contrast, the melanophore lineage is increased in pax7a/pax7b double-mutant embryos and larvae, whereas juvenile and adult pax7a/pax7b double-mutant zebrafish display a severe decrease in melanophores and a pigment pattern disorganization indicative of a xanthophore- deficient phenotype. In summary, we propose a novel role for Pax7 in the early specification of chromatophore precursor cells.

Zebrafish stripes as a model for vertebrate colour pattern formation

Colour patterns are prominent features of many animals and have important functions in communication, such as camouflage, kin recognition and mate choice. As targets for natural as well as sexual selection, they are of high evolutionary significance. The molecular mechanisms underlying colour pattern formation in vertebrates are not well understood. Progress in transgenic tools, in vivo imaging and the availability of a large collection of mutants make the zebrafish (Danio rerio) an attractive model to study vertebrate colouration. Zebrafish display golden and blue horizontal stripes that form during metamorphosis as mosaics of yellow xanthophores, silvery or blue iridophores and black melanophores in the hypodermis. Lineage tracing revealed the origin of the adult pigment cells and their individual cellular behaviours during the formation of the striped pattern. Mutant analysis indicated that interactions between all three pigment cell types are required for the formation of the pattern, and a number of cell surface molecules and signalling systems have been identified as mediators of these interactions. The understanding of the mechanisms that underlie colour pattern formation is an important step towards deciphering the genetic basis of variation in evolution.

Poised Regeneration of Zebrafish Melanocytes Involves Direct Differentiation and Concurrent Replenishment of Tissue-Resident Progenitor Cells

Efficient regeneration following injury is critical for maintaining tissue function and enabling organismal survival. Cells reconstituting damaged tissue are often generated from resident stem or progenitor cells or from cells that have dedifferentiated and become proliferative. While lineage-tracing studies have defined cellular sources of regeneration in many tissues, the process by which these cells execute the regenerative process is largely obscure. Here, we have identified tissue-resident progenitor cells that mediate regeneration of zebrafish stripe melanocytes and defined how these cells reconstitute pigmentation. Nearly all regeneration melanocytes arise through direct differentiation of progenitor cells. Wnt signaling is activated prior to differentiation, and inhibition of Wnt signaling impairs regeneration. Additional progenitors divide symmetrically to sustain the pool of progenitor cells. Combining direct differentiation with symmetric progenitor divisions may serve as a means to rapidly repair injured tissue while preserving the capacity to regenerate.

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.

Origins of adult pigmentation: diversity in pigment stem cell lineages and implications for pattern evolution

Teleosts comprise about half of all vertebrate species and exhibit an extraordinary diversity of adult pigment patterns that function in shoaling, camouflage, and mate choice and have played important roles in speciation. Here, we review studies that have identified several distinct neural crest lineages, with distinct genetic requirements, that give rise to adult pigment cells in fishes. These lineages include post-embryonic, peripheral nerve-associated stem cells that generate black melanophores and iridescent iridophores, cells derived directly from embryonic neural crest cells that generate yellow-orange xanthophores, and bipotent stem cells that generate both melanophores and xanthophores. This complexity in adult chromatophore lineages has implications for our understanding of adult traits, melanoma, and the evolutionary diversification of pigment cell lineages and patterns.

Thyroid hormone-dependent adult pigment cell lineage and pattern in zebrafish

Pigment patterns are useful for elucidating fundamental mechanisms of pattern formation and how these mechanisms evolve. In zebrafish, several pigment cell classes interact to generate stripes, yet the developmental requirements and origins of these cells remain poorly understood. Using zebrafish and a related species, we identified roles for thyroid hormone (TH) in pigment cell development and patterning, and in postembryonic development more generally. We show that adult pigment cells arise from distinct lineages having distinct requirements for TH and that differential TH dependence can evolve within lineages. Our findings demonstrate critical functions for TH in determining pigment pattern phenotype and highlight the potential for evolutionary diversification at the intersection of developmental and endocrine mechanisms.

Clonal analysis of kit ligand a functional expression reveals lineage-specific competence to promote melanocyte rescue in the mutant regenerating caudal fin

The study of regeneration in an in vivo vertebrate system has the potential to reveal targetable genes and pathways that could improve our ability to heal and repair damaged tissue. We have developed a system for clonal labeling of discrete cell lineages and independently inducing gene expression under control of the heat shock promoter in the zebrafish caudal fin. Consequently we are able to test the affects of overexpressing a single gene in the context of regeneration within each of the nine different cell lineage classes that comprise the caudal fin. This can test which lineage is necessary or sufficient to provide gene function. As a first example to demonstrate this approach, we explored which lineages were competent to functionally express the kit ligand a protein as assessed by the local complementation of the mutation in the sparse-like (kitlga^tc244b) background. We show that dermal fibroblast expression of kit ligand a robustly supports the rescue of melanocytes in the regenerating caudal fin. kit ligand a expression from skin and osteoblasts results in more modest and variable rescue of melanocytes, while lateral line expression was unable to complement the mutation.

Autophagy is required for zebrafish caudal fin regeneration

Regeneration is the ability of multicellular organisms to replace damaged tissues and regrow lost body parts. This process relies on cell fate transformation that involves changes in gene expression as well as in the composition of the cytoplasmic compartment, and exhibits a characteristic age-related decline. Here, we present evidence that genetic and pharmacological inhibition of autophagy - a lysosome-mediated self-degradation process of eukaryotic cells, which has been implicated in extensive cellular remodelling and aging - impairs the regeneration of amputated caudal fins in the zebrafish (Danio rerio). Thus, autophagy is required for injury-induced tissue renewal. We further show that upregulation of autophagy in the regeneration zone occurs downstream of mitogen-activated protein kinase/extracellular signal-regulated kinase signalling to protect cells from undergoing apoptosis and enable cytosolic restructuring underlying terminal cell fate determination. This novel cellular function of the autophagic process in regeneration implies that the role of cellular self-digestion in differentiation and tissue patterning is more fundamental than previously thought.

Melanocytes, melanocyte stem cells, and melanoma stem cells

Melanocyte stem cells differ greatly from melanoma stem cells; the former provide pigmented cells during normal tissue homeostasis and repair, and the latter play an active role in a lethal form of cancer. These 2 cell types share several features and can be studied by similar methods. Aspects held in common by both melanocyte stem cells and melanoma stem cells include their expression of shared biochemical markers, a system of similar molecular signals necessary for their maintenance, and a requirement for an ideal niche microenvironment for providing these factors. This review provides a perspective of both these cell types and discusses potential models of stem cell growth and propagation. Recent findings provide a strong foundation for the development of new therapeutics directed at isolating and manipulating melanocyte stem cells for tissue engineering or at targeting and eradicating melanoma specifically, while sparing nontumor cells.

Kit signaling is involved in melanocyte stem cell fate decisions in zebrafish embryos

Adult stem cells are crucial for growth, homeostasis and repair of adult animals. The melanocyte stem cell (MSC) and melanocyte regeneration is an attractive model for studying regulation of adult stem cells. The process of melanocyte regeneration can be divided into establishment of the MSC, recruitment of the MSC to produce committed daughter cells, and the proliferation, differentiation and survival of these daughter cells. Reduction of Kit signaling results in dose-dependent reduction of melanocytes during larval regeneration. Here, we use clonal analysis techniques to develop assays to distinguish roles for these processes during zebrafish larval melanocyte regeneration. We use these clonal assays to investigate which processes are affected by the reduction in Kit signaling. We show that the regeneration defect in kita mutants is not due to defects in MSC recruitment or in the proliferation, differentiation or survival of the daughter cells, but is instead due to a defect in stem cell establishment. Our analysis suggests that the kit MSC establishment defect results from inappropriate differentiation of the MSC lineage.

Does melanoma begin in a melanocyte stem cell?

What is the cellular origin of melanoma? What role do melanocyte stem cells (MSC) and other melanocyte precursors play in the development of melanoma? Are MSCs and other latent melanocyte precursors more susceptible to solar radiation? These and many other questions can be very effectively addressed using the zebrafish model. Zebrafish have a robust regenerative capability, permitting the study of how MSCs are regulated and recruited at specific times and places to generate the pigment pattern following fin amputation or melanocyte ablation. They can be used to determine the effects of environmental radiation on the proliferation, survival, repair, and differentiation of MSCs. Our lab is using zebrafish to investigate how UVA- (320-400 nm) and UVB- (290-320 nm) induced damage to MSCs may contribute to the development of melanoma. A review is given of MSCs in zebrafish as well as experimental techniques and drugs for manipulating MSC populations. These techniques can be used to design experiments to help answer many questions regarding the role of MSCs or melanocyte precursors in the formation of melanoma stem cells and tumors following exposure to UVA/UVB radiation.

The cell biology of regeneration

Regeneration of complex structures after injury requires dramatic changes in cellular behavior. Regenerating tissues initiate a program that includes diverse processes such as wound healing, cell death, dedifferentiation, and stem (or progenitor) cell proliferation; furthermore, newly regenerated tissues must integrate polarity and positional identity cues with preexisting body structures. Gene knockdown approaches and transgenesis-based lineage and functional analyses have been instrumental in deciphering various aspects of regenerative processes in diverse animal models for studying regeneration.

Extrafollicular dermal melanocyte stem cells and melanoma

Recent studies suggest that extrafollicular dermal melanocyte stem cells (MSCs) persist after birth in the superficial nerve sheath of peripheral nerves and give rise to migratory melanocyte precursors when replacements for epidermal melanocytes are needed on the basal epidermal layer of the skin. If a damaged MSC or melanocyte precursor can be shown to be the primary origin of melanoma, targeted identification and eradication of it by antibody-based therapies will be the best method to treat melanoma and a very effective way to prevent its recurrence. Transcription factors and signaling pathways involved in MSC self-renewal, expansion and differentiation are reviewed. A model is presented to show how the detrimental effects of long-term UVA/UVB radiation on DNA and repair mechanisms in MSCs convert them to melanoma stem cells. Zebrafish have many advantages for investigating the role of MSCs in the development of melanoma. The signaling pathways regulating the development of MSCs in zebrafish are very similar to those found in humans and mice. The ability to easily manipulate the MSC population makes zebrafish an excellent model for studying how damage to MSCs may lead to melanoma.

Clonal and lineage analysis of melanocyte stem cells and their progeny in the zebrafish

The study of melanocyte biology in the zebrafish presents a highly tractable system for understanding fundamental principles of developmental biology. Melanocytes are visible in the transparent embryo and in the mature fish following metamorphosis, a physical transformation from the larval to adult form. While early developing larval melanocytes are direct derivatives of the neural crest, the remainder of melanocytes develop from unpigmented precursors, or melanocyte stem cells (MSCs). The Tol2 transposable element has facilitated the construction of stable transgenic lines that label melanocytes. In another application, integration of Tol2 constructs makes possible clonal analysis of melanocyte and MSC lineages. Drugs that block melanin synthesis, ablate melanocytes, and block establishment of MSC populations allow the interrogation of this model system for mechanisms of adult stem cell development and regulation.

Diverse mechanisms for assembly of branchiomeric nerves

The formation of branchiomeric nerves (cranial nerves V, VII, IX and X) from their sensory, motor and glial components is poorly understood. The current model for cranial nerve formation is based on the Vth nerve, in which sensory afferents are formed first and must enter the hindbrain in order for the motor efferents to exit. Using transgenic zebrafish lines to discriminate between motor neurons, sensory neurons and peripheral glia, we show that this model does not apply to the remaining three branchiomeric nerves. For these nerves, the motor efferents form prior to the sensory afferents, and their pathfinding show no dependence on sensory axons, as ablation of cranial sensory neurons by ngn1 knockdown had no effect. In contrast, the sensory limbs of the IXth and Xth nerves (but not the Vth or VIIth) were misrouted in gli1 mutants, which lack hindbrain bmn, suggesting that the motor efferents are crucial for appropriate sensory axon projection in some branchiomeric nerves. For all four nerves, peripheral glia were the intermediate component added and had a critical role in nerve integrity but not in axon guidance, as foxd3 null mutants lacking peripheral glia exhibited defasciculation of gVII, gIX, and gX axons. The bmn efferents were unaffected in these mutants. These data demonstrate that multiple mechanisms underlie formation of the four branchiomeric nerves. For the Vth, sensory axons initiate nerve formation, for the VIIth the sensory and motor limbs are independent, and for the IXth/Xth the motor axons initiate formation. In all cases the glia are patterned by the initiating set of axons and are needed to maintain axon fasciculation. These results reveal that coordinated interactions between the three neural cell types in branchiomeric nerves differ according to their axial position.

Fate restriction in the growing and regenerating zebrafish fin

We use transposon-based clonal analysis to identify the lineage classes that make the adult zebrafish caudal fin. We identify nine distinct lineage classes, including epidermis, melanocyte/xanthophore, iridophore, intraray glia, lateral line, osteoblast, dermal fibroblast, vascular endothelium, and resident blood. These lineage classes argue for distinct progenitors, or organ founding stem cells (FSCs), for each lineage, which retain fate restriction throughout growth of the fin. Thus, distinct FSCs exist for the four neuroectoderm lineages, and dermal fibroblasts are not progenitors for fin ray osteoblasts; however, artery and vein cells derive from a shared lineage in the fin. Transdifferentiation of cells or lineages in the regeneration blastema is often postulated. However, our studies of single progenitors or FSCs reveal no transfating or transdifferentiation between these lineages in the regenerating fin. This result shows that, the same as in growth, lineages retain fate restriction when passed through the regeneration blastema.

Lineage relationship of direct-developing melanocytes and melanocyte stem cells in the zebrafish

Previous research in zebrafish has demonstrated that embryonic and larval regeneration melanocytes are derived from separate lineages. The embryonic melanocytes that establish the larval pigment pattern do not require regulative melanocyte stem cell (MSC) precursors, and are termed direct-developing melanocytes. In contrast, the larval regeneration melanocytes that restore the pigment pattern after ablation develop from MSC precursors. Here, we explore whether embryonic melanocytes and MSCs share bipotent progenitors. Furthermore, we explore when fate segregation of embryonic melanocytes and MSCs occurs in zebrafish development. In order to achieve this, we develop and apply a novel lineage tracing method. We first demonstrate that Tol2-mediated genomic integration of reporter constructs from plasmids injected at the 1-2 cell stage occurs most frequently after the midblastula transition but prior to shield stage, between 3 and 6 hours post-fertilization. This previously uncharacterized timing of Tol2-mediated genomic integration establishes Tol2-mediated transposition as a means for conducting lineage tracing in zebrafish. Combining the Tol2-mediated lineage tracing strategy with a melanocyte regeneration assay previously developed in our lab, we find that embryonic melanocytes and larval regeneration melanocytes are derived from progenitors that contribute to both lineages. We estimate 50-60 such bipotent melanogenic progenitors to be present in the shield-stage embryo. Furthermore, our examination of direct-developing and MSC-restricted lineages suggests that these are segregated from bipotent precursors after the shield stage, but prior to the end of convergence and extension. Following this early fate segregation, we estimate approximately 100 embryonic melanocyte and 90 MSC-restricted lineages are generated to establish or regenerate the zebrafish larval pigment pattern, respectively. Thus, the dual strategies of direct-development and MSC-derived development are established in the early gastrula, via fate segregation of the two lineages.

Time-lapse imaging of neural development: zebrafish lead the way into the fourth dimension

Time-lapse imaging is often the only way to appreciate fully the many dynamic cell movements critical to neural development. Zebrafish possess many advantages that make them the best vertebrate model organism for live imaging of dynamic development events. This review will discuss technical considerations of time-lapse imaging experiments in zebrafish, describe selected examples of imaging studies in zebrafish that revealed new features or principles of neural development, and consider the promise and challenges of future time-lapse studies of neural development in zebrafish embryos and adults.

mitfa is required at multiple stages of melanocyte differentiation but not to establish the melanocyte stem cell

The mitfa gene encodes a zebrafish ortholog of the microphthalmia-associated transcription factor (Mitf) which, like its counterparts in other species, is absolutely required for development of neural crest melanocytes. In order to evaluate mitfa's role in different stages of melanocyte development, we have identified hypomorphic alleles of mitfa, including two alleles that are temperature-sensitive for melanocyte development. Molecular analysis revealed that the mitf(fh53)ts results from a single base pair change producing an asparagine to tyrosine amino acid substitution in the DNA-binding domain, and the mitfa(vc7)ts allele is a mutation in a splice donor site that reduces the level of correctly-spliced transcripts. Splicing in the mitfa(vc7) allele does not itself appear to be temperature-dependent. A third, hypomorphic allele, mitfa(z25) results in an isoleucine to phenylalanine substitution in the first helix domain of the protein. Temperature upshift experiments with mitfa(fh53)ts show that mitfa is required at several stages of melanocyte differentiation, including for expression of the early melanoblast marker dct, again for progression from dct expression to differentiation, and again for maintenance of dendritic form following differentiation. mitfa(fh53)ts mutants recover melanocytes within 2-3days when downshifted at all stages of larval development. However, when melanocyte stem cells (MSCs) are ablated by early treatment with the erbB3 inhibitor AG1478, melanocyte recovery is lost by 48 h. This result indicates first that the MSC is established at the restrictive temperature, and that melanoblasts die or lose the ability to recover after being held at the restrictive temperature for approximately one day.