Gene-regulatory interactions in neural crest evolution and development

Genomically integrated transgenes are stably and conditionally expressed in neural crest cell-specific lineages

Neural crest cells (NCCs) are a transient embryonic structure that gives rise to a variety of cells including peripheral nervous system, melanocytes, and Schwann cells. To understand the molecular mechanisms underlying NCC development, a gene manipulation of NCCs by in ovo electroporation technique is a powerful tool, particularly in chicken embryos, the model animal that has long been used for the NCC research. However, since expression of introduced genes by the conventional electroporation method is transient, the mechanisms of late development of NCCs remain unexplored. We here report novel methods by which late-developing NCCs are successfully manipulated with electroporated genes. Introduced genes can be stably and/or conditionally expressed in a NCC-specific manner by combining 4 different techniques: Tol2 transposon-mediated genomic integration (Sato et al., 2007), a NCC-specific enhancer of the Sox10 gene (identified in this study), Cre/loxP system, and tet-on inducible expression (Watanabe et al., 2007). This is the first demonstration that late-developing NCCs in chickens are gene-manipulated specifically and conditionally. These methods have further allowed us to obtain ex vivo live-images of individual Schwann cells that are associated in axon bundles in peripheral tissues. Cellular activity and morphology dynamically change as development proceeds. This study has opened a new way to understand at the molecular and cellular levels how late NCCs develop in association with other tissues during embryogenesis.

Signals controlling neural crest contributions to the heart

Cardiac neural crest cells represent a unique subpopulation of cranial neural crest cells that are specified, delaminate and migrate from the developing neural tube to the caudal pharynx where they support aortic arch artery development. From the caudal pharynx, a subset of these cells migrates into the cardiac outflow tract where they are needed for outflow septation. Many signaling factors are known to be involved in specifying and triggering the migration of neural crest cells. These factors have not been specifically studied in cardiac crest but are assumed to be the same as for the other regions of crest. Signaling factors like Ephs and Semaphorins guide the cells into the caudal pharynx. Support of the cells in the pharynx is from endothelin, PDGF and the TGFbeta/BMP signaling pathways. Mutants in the TGFbeta/BMP pathway show abnormal migration or survival in the pharynx, whereas the migration of the neural crest cells into the outflow tract is orchestrated by Semaphorin/Plexin signaling. Although TGFbeta family members have been well studied and show defective neural crest function in outflow septation, their mechanism of action remains unclear.

Maternal intake of folic acid and neural crest stem cells

Maternal folic acid (FA) intake has beneficial effects in preventing neural tube defects and may also play a role in the prevention of adult onset diseases such as Alzheimer's disease, dementia, neuropsychiatric disorders, cardiovascular diseases, and cerebral ischemia. This review will focus on the effects of maternal FA intake on neural crest stem cell proliferation and differentiation. Although FA is generally considered beneficial, it has the potential of promoting cell proliferation at the expense of differentiation. In some situations, this may lead to miscarriage or postnatal developmental abnormalities. Therefore, a blind approach such as "FA for everyone" is not necessarily the best course of action. Ultimately, the best approach for FA supplementation, and potentially other nutritional supplements, will include customized patient genomic profiles for determining dose and duration.

The neural crest-enriched microRNA miR-452 regulates epithelial-mesenchymal signaling in the first pharyngeal arch

Neural crest cells (NCCs) are a subset of multipotent, migratory stem cells that populate a large number of tissues during development and are important for craniofacial and cardiac morphogenesis. Although microRNAs (miRNAs) have emerged as important regulators of development and disease, little is known about their role in NCC development. Here, we show that loss of miRNA biogenesis by NCC-specific disruption of murine Dicer results in embryos lacking craniofacial cartilaginous structures, cardiac outflow tract septation and thymic and dorsal root ganglia development. Dicer mutant embryos had reduced expression of Dlx2, a transcriptional regulator of pharyngeal arch development, in the first pharyngeal arch (PA1). miR-452 was enriched in NCCs, was sufficient to rescue Dlx2 expression in Dicer mutant pharyngeal arches, and regulated non-cell-autonomous signaling involving Wnt5a, Shh and Fgf8 that converged on Dlx2 regulation in PA1. Correspondingly, knockdown of miR-452 in vivo decreased Dlx2 expression in the mandibular component of PA1, leading to craniofacial defects. These results suggest that post-transcriptional regulation by miRNAs is required for differentiation of NCC-derived tissues and that miR-452 is involved in epithelial-mesenchymal signaling in the pharyngeal arch.

Reiterative AP2a activity controls sequential steps in the neural crest gene regulatory network

The neural crest (NC) emerges from combinatorial inductive events occurring within its progenitor domain, the neural border (NB). Several transcription factors act early at the NB, but the initiating molecular events remain elusive. Recent data from basal vertebrates suggest that ap2 might have been critical for NC emergence; however, the role of AP2 factors at the NB remains unclear. We show here that AP2a initiates NB patterning and is sufficient to elicit a NB-like pattern in neuralized ectoderm. In contrast, the other early regulators do not participate in ap2a initiation at the NB, but cooperate to further establish a robust NB pattern. The NC regulatory network uses a multistep cascade of secreted inducers and transcription factors, first at the NB and then within the NC progenitors. Here we report that AP2a acts at two distinct steps of this cascade. As the earliest known NB specifier, AP2a mediates Wnt signals to initiate the NB and activate pax3; as a NC specifier, AP2a regulates further NC development independent of and downstream of NB patterning. Our findings reconcile conflicting observations from various vertebrate organisms. AP2a provides a paradigm for the reiterated use of multifunctional molecules, thereby facilitating emergence of the NC in vertebrates.

Prohibitin1 acts as a neural crest specifier in Xenopus development by repressing the transcription factor E2F1

Prohibitin 1 (phb1), which was initially described as an inhibitor of cell proliferation, is a highly conserved protein found in multiple cellular compartments. In the nucleus it interacts with the transcriptional regulators Rb and E2F1 and controls cell proliferation and apoptosis. Here we unravel an unexpected novel function for phb1 in Xenopus cranial neural crest (CNC) development. Xphb1 is maternally expressed; zygotically expressed neurula stage transcripts accumulate in the CNC and the neural tube. Knockdown of Xphb1 by antisense morpholino injection results in the loss of foxD3, snail2 and twist expression, whereas expression of c-myc, AP-2 and snail1 remains unaffected. Xphb2, its closest relative, cannot substitute for Xphb1, underlining the specificity of Xphb1 function. Epistatic analyses place Xphb1 downstream of c-myc and upstream of foxD3, snail2 and twist. To elucidate which subdomain in Xphb1 is required for neural crest gene regulation we generated deletion mutants and tested their rescue ability in Xphb1 morphants. The E2F1-binding domain was found to be necessary for Xphb1 function in neural crest development. Gain- and loss-of-function experiments reveal that Xphb1 represses E2F1 activity; suppression of E2F1 through Xphb1 is required for twist, snail2 and foxD3 expression in the CNC. With the Xphb1 dependency of a subset of CNC specifiers downstream of c-myc, we have identified a new branching point in the neural crest gene regulatory network.

Early acquisition of neural crest competence during hESCs neuralization

Background

Neural crest stem cells (NCSCs) are a transient multipotent embryonic cell population that represents a defining characteristic of vertebrates. The neural crest (NC) gives rise to many derivatives including the neurons and glia of the sensory and autonomic ganglia of the peripheral nervous system, enteric neurons and glia, melanocytes, and the cartilaginous, bony and connective tissue of the craniofacial skeleton, cephalic neuroendocrine organs, and some heart vessels.

Methodology/Principal Findings

We present evidence that neural crest (NC) competence can be acquired very early when human embryonic stem cells (hESCs) are selectively neuralized towards dorsal neuroepithelium in the absence of feeder cells in fully defined conditions. When hESC-derived neurospheres are plated on fibronectin, some cells emigrate onto the substrate. These early migratory Neural Crest Stem Cells (emNCSCs) uniformly upregulate Sox10 and vimentin, downregulate N-cadherin, and remodel F-actin, consistent with a transition from neuroepithelium to a mesenchymal NC cell. Over 13% of emNCSCs upregulate CD73, a marker of mesenchymal lineage characteristic of cephalic NC and connexin 43, found on early migratory NC cells. We demonstrated that emNCSCs give rise in vitro to all NC lineages, are multipotent on clonal level, and appropriately respond to developmental factors. We suggest that human emNCSC resemble cephalic NC described in model organisms. Ex vivo emNCSCs can differentiate into neurons in Ret.k^- mouse embryonic gut tissue cultures and transplanted emNCSCs incorporate into NC-derived structures but not CNS tissues in chick embryos.

Conclusions/Significance

These findings will provide a framework for further studying early human NC development including the epithelial to mesenchymal transition during NC delamination.

prdm1a Regulates sox10 and islet1 in the development of neural crest and Rohon-Beard sensory neurons

The PR domain containing 1a, with ZNF domain factor, gene (prdm1a) plays an integral role in the development of a number of different cell types during vertebrate embryogenesis, including neural crest cells, Rohon-Beard (RB) sensory neurons and the cranial neural crest-derived craniofacial skeletal elements. To better understand how Prdm1a regulates the development of various cell types in zebrafish, we performed a microarray analysis comparing wild type and prdm1a mutant embryos and identified a number of genes with altered expression in the absence of prdm1a. Rescue analysis determined that two of these, sox10 and islet1, lie downstream of Prdm1a in the development of neural crest cells and RB neurons, respectively. In addition, we identified a number of other novel downstream targets of Prdm1a that may be important for the development of diverse tissues during zebrafish embryogenesis.

Assembling neural crest regulatory circuits into a gene regulatory network

The neural crest is a multipotent stem cell–like population that gives rise to a wide range of derivatives in the vertebrate embryo including elements of the craniofacial skeleton and peripheral nervous system as well as melanocytes. The neural crest forms in a series of regulatory steps that include induction and specification of the prospective neural crest territory–neural plate border, specification of bona fide neural crest progenitors, and differentiation into diverse derivatives. These individual processes during neural crest ontogeny are controlled by regulatory circuits that can be assembled into a hierarchical gene regulatory network (GRN). Here we present an overview of the GRN that orchestrates the formation of cranial neural crest cells. Formulation of this network relies on information largely inferred from gene perturbation studies performed in several vertebrate model organisms. Our representation of the cranial neural crest GRN also includes information about direct regulatory interactions obtained from the cis-regulatory analyses performed to date, which increases the resolution of the architectural circuitry within the network.

Mechanisms driving neural crest induction and migration in the zebrafish and Xenopus laevis

The neural crest is an evolutionary adaptation, with roots in the formation of mesoderm. Modification of neural crest behavior has been is critical for the evolutionary diversification of the vertebrates and defects in neural crest underlie a range of human birth defects. There has been a tremendous increase in our knowledge of the molecular, cellular, and inductive interactions that converge on defining the neural crest and determining its behavior. While there is a temptation to look for simple models to explain neural crest behavior, the reality is that the system is complex in its circuitry. In this review, our goal is to identify the broad features of neural crest origins (developmentally) and migration (cellularly) using data from the zebrafish (teleost) and Xenopus laevis (tetrapod amphibian) in order to illuminate where general mechanisms appear to be in play, and equally importantly, where disparities in experimental results suggest areas of profitable study.

Cadherin 6B induces BMP signaling and de-epithelialization during the epithelial mesenchymal transition of the neural crest

The development of neural crest cells involves an epithelial-mesenchymal transition (EMT) associated with the restriction of cadherin 6B expression to the pre-migratory neural crest cells (PMNCCs), as well as a loss of N-cadherin expression. We find that cadherin 6B, which is highly expressed in PMNCCs, persists in early migrating neural crest cells and is required for their emigration from the neural tube. Cadherin 6B-expressing PMNCCs exhibit a general loss of epithelial junctional polarity and acquire motile properties before their delamination from the neuroepithelium. Cadherin 6B selectively induces the de-epithelialization of PMNCCs, which is mediated by stimulation of BMP signaling, whereas N-cadherin inhibits de-epithelialization and BMP signaling. As BMP signaling also induces cadherin 6B expression and represses N-cadherin, cadherin-regulated BMP signaling may create two opposing feedback loops. Thus, the overall EMT of neural crest cells occurs via two distinct steps: a cadherin 6B and BMP signaling-mediated de-epithelialization, and a subsequent delamination through the basement membrane.

PAX3 across the spectrum: from melanoblast to melanoma

The PAX3 transcription factor is critical for the proper development of neural crest lineages including melanocytes. These cells show continued PAX3 expression from formation to differentiation. While many expression, misexpression and mutation studies clarify the importance of PAX3 in melanocyte development, less well understood, and more perplexing, is the continued PAX3 expression in the adult skin. In this article we explore the multiple roles of PAX3 in melanocyte genesis, and draw on evidence from expression in developing melanoblasts, adult melanocytes and melanocyte stem cells. From this, we present a more encompassing theory that PAX3 is a key regulator of the myriad steps in melanocytic cell determination. These roles may be accomplished by differential association with cofactors, via alternate transcripts or posttranslational protein modification(s). In light of the plethora of information gleaned from development we then consider its roles in melanoma and provide here a comprehensive consideration of the significance of PAX3 expression in melanoma. PAX3 and Pax3 indicate human and mouse transcription factors respectively.

Conserved expression of mouse Six1 in the pre-placodal region (PPR) and identification of an enhancer for the rostral PPR

All cranial sensory organs and sensory neurons of vertebrates develop from cranial placodes. In chick, amphibians and zebrafish, all placodes originate from a common precursor domain, the pre-placodal region (PPR), marked by the expression of Six1/4 and Eya1/2. However, the PPR has never been described in mammals and the mechanism involved in the formation of PPR is poorly defined. Here, we report the expression of Six1 in the horseshoe-shaped mouse ectoderm surrounding the anterior neural plate in a pattern broadly similar to that of non-mammalian vertebrates. To elucidate the identity of Six1-positive mouse ectoderm, we searched for enhancers responsible for Six1 expression by in vivo enhancer assays. One conserved non-coding sequence, Six1-14, showed specific enhancer activity in the rostral PPR of chick and Xenopus and in the mouse ectoderm. These results strongly suggest the presence of PPR in mouse and that it is conserved in vertebrates. Moreover, we show the importance of the homeodomain protein-binding sites of Six1-14, the Six1 rostral PPR enhancer, for enhancer activity, and that Dlx5, Msx1 and Pax7 are candidate binding factors that regulate the level and area of Six1 expression, and thereby the location of the PPR. Our findings provide critical information and tools to elucidate the molecular mechanism of early sensory development and have implications for the development of sensory precursor/stem cells.

Analysis of early human neural crest development

The outstanding migration and differentiation capacities of neural crest cells (NCCs) have fascinated scientists since Wilhelm His described this cell population in 1868. Today, after intense research using vertebrate model organisms, we have gained considerable knowledge regarding the origin, migration and differentiation of NCCs. However, our understanding of NCC development in human embryos remains largely uncharacterized, despite the role the neural crest plays in several human pathologies. Here, we report for the first time the expression of a battery of molecular markers before, during, or following NCC migration in human embryos from Carnegie Stages (CS) 12 to 18. Our work demonstrates the expression of Sox9, Sox10 and Pax3 transcription factors in premigratory NCCs, while actively migrating NCCs display the additional transcription factors Pax7 and AP-2alpha. Importantly, while HNK-1 labels few migrating NCCs, p75(NTR) labels a large proportion of this population. However, the broad expression of p75(NTR) - and other markers - beyond the neural crest stresses the need for the identification of additional markers to improve our capacity to investigate human NCC development, and to enable the generation of better diagnostic and therapeutic tools.

Origin and genetic evolution of the vertebrate skeleton

The current understanding of the origin and evolution of the genetic cassette for the vertebrate skeletal system is reviewed. Molecular phylogenetic analyses of fibrillar collagen genes, which encode the main component of both cartilage and mineralized bone, suggest that genome duplications in vertebrate ancestors were essential for producing distinct collagen fibers for cartilage and mineralized bone. Several data Indicate co-expression of the ancestral copy of fibrillar collagen with the SoxE and Runx transcription factors. Therefore, the genetic cassette may have already existed in protochordate ancestors, and may operate in the development of the pharyngeal gill skeleton. Accompanied by genome duplications in vertebrate ancestors, this genetic cassette may have also been duplicated and co-opted for cartilage and bone. Subsequently, the genetic cassette for cartilage recruited novel genetic material via domain shuffling. Aggrecan, acquired by means of domain shuffling, performs an essential role in cartilage as a shock absorber. In contrast, the cassette for bone recruited new genetic material produced by tandem duplication of the SPARC/osteonectin genes. Some of the duplicated copies of SPARC/osteonectin became secretory Cabinding phosphoproteins (SCPPs) performing a central role in mineralization by regulating the calcium phosphate concentration. Comparative genome analysis revealed similar molecular evolutionary histories for the genetic cassettes for cartilage and bone, namely duplication of the ancestral genetic cassette and recruitment of novel genetic material.

PAX3 expression in normal skin melanocytes and melanocytic lesions (naevi and melanomas)

BACKGROUND

Cutaneous Malignant Melanoma is an aggressive form of skin cancer, arising in cutaneous melanocytes. The transcription factor PAX3 regulates melanocyte specification from neural crest cells during development but expression in differentiated melanocytes is uncertain. By contrast it is frequently found in melanomas and naevi and is a marker for melanoma staging and detection. In this study we analysed the expression of PAX3 across the spectrum of melanocytic cells, from normal melanocytes to cells of benign and malignant lesions to better assess its function in these various tissues. Pax3 and PAX3 (italicized) refer to the mouse and human gene, respectively; whereas Pax3 and PAX3 (non-italicized) refer to the corresponding mouse and human protein.

METHODOLOGY AND PRINCIPAL FINDINGS

PAX3 expression was analysed by immunohistochemistry and qRT-PCR. Immunofluorescence was used for co-expression with differentiation, migration and survival markers. As expected PAX3 expression was observed in naevi and melanoma cells. It was also found in melanocytes of normal skin where it co-expressed with melanocyte markers, MITF and MLANA. Co-expression with its downstream target, antiapoptotic factor BCL2L1 confirms PAX3 as a cell survival regulator. PAX3 was also co-expressed with melanoma cell migration marker MCAM in dermal naevi and melanoma cell nests, but this downstream target of PAX3 was not present in normal epidermal melanocytes, suggesting differential roles for PAX3 in normal epidermal melanocytes and melanoma cells. Most interestingly, a proportion of PAX3-positive epidermal melanocytes in normal skin show HES1 and Ki67 co-expression, indicating their less differentiated proliferative phenotype.

CONCLUSIONS AND SIGNIFICANCE

Our results suggest that a previously identified role for PAX3, that of regulator of an undifferentiated plastic state, may operate in melanocytes of normal skin. This role, possibly required for cellular response to environmental stimuli, may contribute to formation and development of melanocytic lesions in which PAX3 expression is prominent.

Isolation and propagation of enteric neural crest progenitor cells from mouse embryonic stem cells and embryos

Neural crest is a source of diverse cell types, including the peripheral nervous system. The transcription factor Sox10 is expressed throughout early neural crest. We exploited Sox10 reporter and selection markers created by homologous recombination to investigate the generation, maintenance and expansion of neural crest progenitors. Sox10-GFP-positive cells are produced transiently from mouse embryonic stem (ES) cells by treatment with retinoic acid in combination with Fgf8b and the cytokine leukaemia inhibitory factor (Lif). We found that expression of Sox10 can be maintained using noggin, Wnt3a, Lif and endothelin (NWLE). ES cell-derived Sox10-GFP-positive cells cultured in NWLE exhibit molecular markers of neural crest progenitors. They differentiate into peripheral neurons in vitro and are able to colonise the enteric network in organotypic gut cultures. Neural crest cells purified from embryos using the Sox10 reporter also survive in NWLE, but progressively succumb to differentiation. We therefore applied selection to eliminate differentiating cells. Sox10-selected cells could be clonally expanded, cryopreserved, and multiplied for over 50 days in adherent culture. They remained neurogenic in vitro and in foetal gut grafts. Generation of neural crest from mouse ES cells opens a new route to the identification and validation of determination factors. Furthermore, the ability to propagate undifferentiated progenitors creates an opportunity for experimental dissection of the stimuli and molecular circu that govern neural crest lineage progression. Finally, the demonstration of robust enteric neurogenesis provides a system for investigating and modelling cell therapeutic approaches to neurocristopathies such as Hirschsprung's disease.

Involvement of Neptune in induction of the hatching gland and neural crest in the Xenopus embryo

Neptune, a Krüppel-like transcription factor, is expressed in various regions of the developing Xenopus embryo and it has multiple functions in the process of development in various organs. In situ hybridization analysis showed that Neptune is expressed in the boundary region between neural and non-neural tissues at the neurula stage, but little is known about the function of Neptune in this region. Here, we examined the expression and function of Neptune in the neural plate border (NPB) in the Xenopus embryo. Depletion of Neptune protein in developing embryos by using antisense MO caused loss of the hatching gland and otic vesicle as well as malformation of neural crest-derived cranial cartilages and melanocytes. Neptune MO also suppressed the expression of hatching gland and neural crest markers such as he, snail2, sox9 and msx1 at the neurula stage. Subsequent experiments showed that Neptune is necessary and sufficient for the differentiation of hatching gland cells and that it is located downstream of pax3 in the signal regulating the differentiation of these cells. Thus, Neptune is a new member of hatching gland specifier and plays a physiological role in determination and specification of multiple lineages derived from the NPB region.

Genomic code for Sox10 activation reveals a key regulatory enhancer for cranial neural crest

The neural crest is a multipotent, stem cell-like population that migrates extensively in the embryo and forms a wide array of derivatives, ranging from neurons to melanocytes and cartilage. Analyses of the gene regulatory network driving neural crest development revealed Sox10 as one of the earliest neural crest-specifying genes, cell-autonomously driving delamination and directly regulating numerous downstream effectors and differentiation gene batteries. In search of direct inputs to the neural crest specifier module, we dissected the chick Sox10 genomic region and isolated two downstream regulatory regions with distinct spatiotemporal activity. A unique element, Sox10E2 represents the earliest-acting neural crest cis-regulatory element, critical for initiating Sox10 expression in newly formed cranial, but not vagal and trunk neural crest. A second element, Sox10E1, acts in later migrating vagal and trunk crest cells. Deep characterization of Sox10E2 reveals Sox9, Ets1, and cMyb as direct inputs mediating enhancer activity. ChIP, DNA-pull down, and gel-shift assays demonstrate their direct binding to the Sox10E2 enhancer in vivo, whereas mutation of their corresponding binding sites, or inactivation of the three upstream regulators, abolishes both reporter and endogenous Sox10 expression. Using cis-regulatory analysis as a tool, our study makes critical connections within the neural crest gene regulatory network, thus being unique in establishing a direct link of upstream effectors to a key neural crest specifier.

The Pax3 and Pax7 paralogs cooperate in neural and neural crest patterning using distinct molecular mechanisms, in Xenopus laevis embryos

Pax3 and Pax7 paralogous genes have functionally diverged in vertebrate evolution, creating opportunity for a new distribution of roles between the two genes and the evolution of novel functions. Here we focus on the regulation and function of Pax7 in the brain and neural crest of amphibian embryos, which display a different pax7 expression pattern, compared to the other vertebrates already described. Pax7 expression is restricted to the midbrain, hindbrain and anterior spinal cord, and Pax7 activity is important for maintaining the fates of these regions, by restricting otx2 expression anteriorly. In contrast, pax3 displays broader expression along the entire neuraxis and Pax3 function is important for posterior brain patterning without acting on otx2 expression. Moreover, while both genes are essential for neural crest patterning, we show that they do so using two distinct mechanisms: Pax3 acts within the ectoderm which will be induced into neural crest, while Pax7 is essential for the inducing activity of the paraxial mesoderm towards the prospective neural crest.

The F-box protein Cdc4/Fbxw7 is a novel regulator of neural crest development in Xenopus laevis

Background

The neural crest is a unique population of cells that arise in the vertebrate ectoderm at the neural plate border after which they migrate extensively throughout the embryo, giving rise to a wide range of derivatives. A number of proteins involved in neural crest development have dynamic expression patterns, and it is becoming clear that ubiquitin-mediated protein degradation is partly responsible for this.

Results

Here we demonstrate a novel role for the F-box protein Cdc4/Fbxw7 in neural crest development. Two isoforms of Xenopus laevis Cdc4 were identified, and designated xCdc4α and xCdc4β. These are highly conserved with vertebrate Cdc4 orthologs, and the Xenopus proteins are functionally equivalent in terms of their ability to degrade Cyclin E, an established vertebrate Cdc4 target. Blocking xCdc4 function specifically inhibited neural crest development at an early stage, prior to expression of c-Myc, Snail2 and Snail.

Conclusions

We demonstrate that Cdc4, an ubiquitin E3 ligase subunit previously identified as targeting primarily cell cycle regulators for proteolysis, has additional roles in control of formation of the neural crest. Hence, we identify Cdc4 as a protein with separable but complementary functions in control of cell proliferation and differentiation.

Melanocytes in development and cancer

Melanocytes are pigment-producing cells in the skin of humans and other vertebrates. A number of genes involved in melanocyte development and vertebrate pigmentation have been characterized, largely through studies of a diversity of pigment mutations in a variety of species. Embryonic development of the melanocyte initiates with cell fate specification in the neural crest, which is then followed by cell migration and niche localization. Many genes involved in melanocyte development have also been implicated in the development of melanoma, an aggressive and fatal form of skin cancer that originates in the melanocyte. Although early stage melanomas that have not spread to the lymph nodes can be excised with little risk of recurrence, patients diagnosed with metastatic melanoma have a high mortality rate due to the resistance of most tumors to radiotherapy and chemotherapy. Transformed melanocytes that develop into melanomas proliferate abnormally and often begin to grow radially in the skin. Vertical growth can then follow this radial growth, leading to an invasion through the basement membrane into the underlying dermis and subsequent metastasis. It is still unclear, however, how a normal melanocyte becomes a melanoma cell, and how melanoma utilizes the properties of the normal melanocyte and its progenitors in its progression. The goal of this mini-review is to highlight the role of melanocyte developmental pathways in melanoma, and to discuss recent studies and tools being used to illuminate this connection.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Isolation and characterization of neural crest stem cells derived from in vitro-differentiated human embryonic stem cells

The neural crest is a transient structure of vertebrate embryos that initially generates neural crest stem cells (NCSCs) which then migrate throughout the body to produce a diverse array of mature tissue types. Due to the rarity of adult NCSCs as well as ethical and technical issues surrounding isolation of early embryonic tissues, biologic studies of human NCSCs are extremely challenging. Thus, much of what is known about human neural crest development has been inferred from model organisms. In this study, we report that functional NCSCs can be rapidly generated and isolated from in vitro-differentiated human embryonic stem cells (hESCs). Using the stromal-derived inducing activity (SDIA) of PA6 fibroblast co-culture we have induced hESCs to differentiate into neural crest. Within 1 week, migrating cells that express the early neural crest markers p75 and HNK1 as well as numerous other genes associated with neural crest induction such as SNAIL, SLUG, and SOX10 are detectable. Fluorescence-activated cell sorting (FACS)-based isolation of the p75-positive population enriches for cells with genetic, phenotypic, and functional characteristics of NCSCs. These p75-enriched cells readily form neurospheres in suspension culture, self-renew to form secondary spheres, and give rise under differentiation conditions to multiple neural crest lineages including peripheral nerves, glial, and myofibroblastic cells. Importantly, these cells differentiate into neural crest derivatives when transplanted into developing chick embryos in vivo. Thus, this SDIA protocol can be used to successfully and efficiently isolate early human NCSCs from hESCs in vitro. This renewable source of NCSCs provides an invaluable source of cells for studies of both normal and disordered human neural crest development.

The posteriorizing gene Gbx2 is a direct target of Wnt signalling and the earliest factor in neural crest induction

Wnt signalling is required for neural crest (NC) induction; however, the direct targets of the Wnt pathway during NC induction remain unknown. We show here that the homeobox gene Gbx2 is essential in this process and is directly activated by Wnt/beta-catenin signalling. By ChIP and transgenesis analysis we show that the Gbx2 regulatory elements that drive expression in the NC respond directly to Wnt/beta-catenin signalling. Gbx2 has previously been implicated in posteriorization of the neural plate. Here we unveil a new role for this gene in neural fold patterning. Loss-of-function experiments using antisense morpholinos against Gbx2 inhibit NC and expand the preplacodal domain, whereas Gbx2 overexpression leads to transformation of the preplacodal domain into NC cells. We show that the NC specifier activity of Gbx2 is dependent on the interaction with Zic1 and the inhibition of preplacodal genes such as Six1. In addition, we demonstrate that Gbx2 is upstream of the neural fold specifiers Pax3 and Msx1. Our results place Gbx2 as the earliest factor in the NC genetic cascade being directly regulated by the inductive molecules, and support the notion that posteriorization of the neural folds is an essential step in NC specification. We propose a new genetic cascade that operates in the distinction between anterior placodal and NC territories.

Chordate roots of the vertebrate nervous system: expanding the molecular toolkit

The vertebrate brain is highly complex with millions to billions of neurons. During development, the neural plate border region gives rise to the neural crest, cranial placodes and, in anamniotes, to Rohon-Beard sensory neurons, whereas the boundary region of the midbrain and hindbrain develops organizer properties. Comparisons of developmental gene expression and neuroanatomy between vertebrates and the basal chordate amphioxus, which has only thousands of neurons and lacks a neural crest, most placodes and a midbrain-hindbrain organizer, indicate that these vertebrate features were built on a foundation already present in the ancestral chordate. Recent advances in genomics have provided insights into the elaboration of the molecular toolkit at the invertebrate-vertebrate transition that may have facilitated the evolution of these vertebrate characteristics.

Cephalochordates (amphioxus or lancelets): a model for understanding the evolution of chordate characters

Cephalochordates, commonly called amphioxus or lancelets, are marine invertebrate chordates. They were traditionally held as the closest living relatives of vertebrates and have long occupied a key phylogenetic position in our discussions on the origin of vertebrates. However, recent molecular phylogenetic analyses have placed tunicates as the sister taxon of vertebrates and cephalochordates as the most basal chordate group. Therefore, studies on cephalochordates can also provide important insights for understanding the conserved patterning mechanisms for all chordates. Cephalochordates and vertebrates share several key chordate features, namely, the dorsal nerve cord, notochord, segmented somites, and pharyngeal gill slits. However, vertebrates have ~25% more genes than amphioxus and several features not present in cephalochordates, including a more elaborate head and forebrain, migratory neural crest, and neurogenic placodes. During the last two decades, expression patterns of important developmental genes have been used extensively to infer homologies between cephalochordate and vertebrate embryos. These studies have answered some long-standing questions concerning the evolution of vertebrates from their invertebrate ancestors and have also generated interesting hypotheses for further investigations. With the completion of a cephalochordate genome project in 2008, comparison between cephalochordate and vertebrate genomes has allowed us to postulate a picture for the genome of the last common ancestor of all chordates. Such comparisons are also providing important biological insights into the functional biology of cephalochordates and the evolutionary origin of developmental mechanisms that led to the emergence of the vertebrate body plan.

Gene regulatory networks in embryonic stem cells and brain development

Embryonic stem cells (ESCs) are endowed with the ability to generate multiple cell lineages and carry great therapeutic potentials in regenerative medicine. Future application of ESCs in human health and diseases will embark on the delineation of molecular mechanisms that define the biology of ESCs. Here, we discuss how the finite ESC components mediate the intriguing task of brain development and exhibit biomedical potentials to cure diverse neurological disorders.

Epithelial-mesenchymal transitions: the importance of changing cell state in development and disease

The events that convert adherent epithelial cells into individual migratory cells that can invade the extracellular matrix are known collectively as epithelial-mesenchymal transition (EMT). Throughout evolution, the capacity of cells to switch between these two cellular states has been fundamental in the generation of complex body patterns. Here, we review the EMT events that build the embryo and further discuss two prototypical processes governed by EMT in amniotes: gastrulation and neural crest formation. Cells undergo EMT to migrate and colonize distant territories. Not surprisingly, this is also the mechanism used by cancer cells to disperse throughout the body.

Epithelial-mesenchymal transitions: the importance of changing cell state in development and disease

The events that convert adherent epithelial cells into individual migratory cells that can invade the extracellular matrix are known collectively as epithelial-mesenchymal transition (EMT). Throughout evolution, the capacity of cells to switch between these two cellular states has been fundamental in the generation of complex body patterns. Here, we review the EMT events that build the embryo and further discuss two prototypical processes governed by EMT in amniotes: gastrulation and neural crest formation. Cells undergo EMT to migrate and colonize distant territories. Not surprisingly, this is also the mechanism used by cancer cells to disperse throughout the body.

Comprehensive spatiotemporal analysis of early chick neural crest network genes

Specification of neural crest progenitors begins during gastrulation at the neural plate border, long before migration or differentiation. Neural crest cell fate is acquired by progressive activation of discrete groups of transcription factors that appear to be highly conserved in vertebrates; however, comprehensive analysis of their expression has been lacking in chick, an important model system for neural crest development. To address this, we analyzed expression of 10 transcription factors that are known specifiers of neural plate border and neural crest fate and compared them across developmental stages from gastrulation to neural crest migration. Surprisingly, we find that most neural crest specifiers are expressed during gastrulation in chick, concomitant with and in similar domains as neural plate border specifiers. This finding suggests that interactions between these molecules may occur much earlier than previously thought, an important consideration for interpretation of functional studies.

Melanoma biology and the promise of zebrafish

Advantageous organismal and technical attributes of the zebrafish are being increasingly applied to study cancer biology. Along with other tumor models, zebrafish that develop melanomas have been generated. In both genetics and phenotype, zebrafish melanomas are strikingly similar to their human counterparts. For this reason, studies in the zebrafish are poised to make significant contributions to melanoma biology. In this review, we summarize important features of human melanoma and discuss how the zebrafish can be used to address many questions that remain unanswered about this devastating disease.

The endothelin system: evolution of vertebrate-specific ligand-receptor interactions by three rounds of genome duplication

Morphological innovations like the acquisition of the neural crest as well as gene family expansions by genome duplication are considered as major leaps in the evolution of the vertebrate lineage. Using comparative genomic analyses, we have reconstructed the evolutionary history of the endothelin system, a signaling pathway consisting of endothelin ligands and their G protein-coupled receptors. The endothelin system plays a key role in cardiovascular regulation as well as in the development of diverse neural crest derivatives like pigment cells and craniofacial bone structures, which are hot spots of diversity in vertebrates. However, little is known about the origin and evolution of the endothelin system in the vertebrate lineage. We show that the endothelin core system, that is, endothelin ligands (Edn) and their receptors (Ednr), is a vertebrate-specific innovation. The components of the endothelin core system in modern vertebrate genomes date back to single genes that have been duplicated during whole-genome duplication events. After two rounds of genome duplication during early vertebrate evolution, the endothelin system of an ancestral gnathostome consisted of four ligand and four receptor genes. The previously unknown fourth endothelin ligand Edn4 has been kept in teleost fish but lost in tetrapods. Bony vertebrates generally possess three receptor genes, EdnrA, EdnrB1, and EdnrB2. EdnrB2 has been lost secondarily in the mammalian lineage from a chromosome that gave rise to the sex chromosomes in therians (marsupials and placentals). The endothelin system of fishes was further expanded by a fish-specific genome duplication and duplicated edn2, edn3, ednrA, and ednrB1 genes have been retained in teleost fishes. Functional divergence analyses suppose that following each round of genome duplication, coevolution of ligands and their binding regions in the receptors has occurred, adjusting the endothelin signaling system to the increase of possible ligand-receptor interactions. Furthermore, duplications of genes involved in the endothelin system are associated with functional specialization for the development of particular neural crest derivatives. Our results support an important role for newly emerging ligands and receptors as components of signaling pathways and their expansion through genome duplications in the evolution of the vertebrate neural crest.

Evolution of the neural crest viewed from a gene regulatory perspective

Neural crest cells are a vertebrate innovation and form a wide variety of embryonic cell types as diverse as peripheral neurons and facial skeleton. They undergo complex migration and differentiation processes from their site of origin in the developing central nervous system to their final destinations in the periphery. In this review, we summarize recent data on the current formulation of a gene regulatory network underlying neural crest formation and its roots at the base of the vertebrate lineage. Analyzing neural crest formation from a gene regulatory viewpoint provides insights into both the developmental mechanisms and evolutionary origins of this vertebrate-specific cell type.

Genomics and the evolutionary origins of nervous system complexity

Advances in genomics are leading to increased understanding of the evolution of complexity, especially by beginning to bridge genotype and phenotype. Here, using examples from nervous system evolution, we define general patterns of increased complexity seen across levels of biological organization. We also explore specific evolutionary mechanisms that increase complexity, namely those that increase the number of biological units (parts) in a system. We provide specific neurobiological examples of increased complexity in genes, gene networks, cell types, and tissues/organs. These examples illustrate that while a variety of different mechanisms increase biological complexity, they can be understood in a generalized comparative framework.

Cell communication with the neural plate is required for induction of neural markers by BMP inhibition: evidence for homeogenetic induction and implications for Xenopus animal cap and chick explant assays

In Xenopus, the animal cap is very sensitive to BMP antagonists, which result in neuralization. In chick, however, only cells at the border of the neural plate can be neuralized by BMP inhibition. Here we compare the two systems. BMP antagonists can induce neural plate border markers in both ventral Xenopus epidermis and non-neural chick epiblast. However, BMP antagonism can only neuralize ectodermal cells when the BMP-inhibited cells form a continuous trail connecting them to the neural plate or its border, suggesting that homeogenetic neuralizing factors can only travel between BMP-inhibited cells. Xenopus animal cap explants contain cells fated to contribute to the neural plate border and even to the anterior neural plate, explaining why they are so easily neuralized by BMP-inhibition. Furthermore, chick explants isolated from embryonic epiblast behave like Xenopus animal caps and express border markers. We propose that the animal cap assay in Xenopus and explant assays in the chick are unsuitable for studying instructive signals in neural induction.

Dissecting early regulatory relationships in the lamprey neural crest gene network

The neural crest, a multipotent embryonic cell type, originates at the border between neural and nonneural ectoderm. After neural tube closure, these cells undergo an epithelial-mesenchymal transition, migrate to precise, often distant locations, and differentiate into diverse derivatives. Analyses of expression and function of signaling and transcription factors in higher vertebrates has led to the proposal that a neural crest gene regulatory network (NC-GRN) orchestrates neural crest formation. Here, we interrogate the NC-GRN in the lamprey, taking advantage of its slow development and basal phylogenetic position to resolve early inductive events, 1 regulatory step at the time. To establish regulatory relationships at the neural plate border, we assess relative expression of 6 neural crest network genes and effects of individually perturbing each on the remaining 5. The results refine an upstream portion of the NC-GRN and reveal unexpected order and linkages therein; e.g., lamprey AP-2 appears to function early as a neural plate border rather than a neural crest specifier and in a pathway linked to MsxA but independent of ZicA. These findings provide an ancestral framework for performing comparative tests in higher vertebrates in which network linkages may be more difficult to resolve because of their rapid development.

A double-negative feedback loop between ZEB1-SIP1 and the microRNA-200 family regulates epithelial-mesenchymal transition

Epithelial to mesenchymal transition occurs during embryologic development to allow tissue remodeling and is proposed to be a key step in the metastasis of epithelial-derived tumors. The miR-200 family of microRNAs plays a major role in specifying the epithelial phenotype by preventing expression of the transcription repressors, ZEB1/deltaEF1 and SIP1/ZEB2. We show here that miR-200a, miR-200b, and the related miR-429 are all encoded on a 7.5-kb polycistronic primary miRNA (pri-miR) transcript. We show that the promoter for the pri-miR is located within a 300-bp segment located 4 kb upstream of miR-200b. This promoter region is sufficient to confer expression in epithelial cells and is repressed in mesenchymal cells by ZEB1 and SIP1 through their binding to a conserved pair of ZEB-type E-box elements located proximal to the transcription start site. These findings establish a double-negative feedback loop controlling ZEB1-SIP1 and miR-200 family expression that regulates cellular phenotype and has direct relevance to the role of these factors in tumor progression.

dlx3b/4b are required for the formation of the preplacodal region and otic placode through local modulation of BMP activity

The vertebrate inner ear arises from the otic placode, a transient thickening of ectodermal epithelium adjacent to neural crest domains in the presumptive head. During late gastrulation, cells fated to comprise the inner ear are part of a domain in cranial ectoderm that contain precursors of all sensory placodes, termed the preplacodal region (PPR). The combination of low levels of BMP activity coupled with high levels of FGF signaling are required to establish the PPR through induction of members of the six/eya/dach, iro, and dlx families of transcription factors. The zebrafish dlx3b/4b transcription factors are expressed at the neural plate border where they play partially redundant roles in the specification of the PPR, otic and olfactory placodes. We demonstrate that dlx3b/4b assist in establishing the PPR through the transcriptional regulation of the BMP antagonist cv2. Morpholino-mediated knockdown of Dlx3b/4b results in loss of cv2 expression in the PPR and a transient increase in Bmp4 activity that lasts throughout early somitogenesis. Through the cv2-mediated inhibition of BMP activity, dlx3b/4b create an environment where FGF activity is favorable for PPR and otic marker expression. Our results provide insight into the mechanisms of PPR specification as well as the role of dlx3b/4b function in PPR and otic placode induction.

An evolutionarily conserved intronic region controls the spatiotemporal expression of the transcription factor Sox10

Background

A major challenge lies in understanding the complexities of gene regulation. Mutation of the transcription factor SOX10 is associated with several human diseases. The disease phenotypes reflect the function of SOX10 in diverse tissues including the neural crest, central nervous system and otic vesicle. As expected, the SOX10 expression pattern is complex and highly dynamic, but little is known of the underlying mechanisms regulating its spatiotemporal pattern. SOX10 expression is highly conserved between all vertebrates characterised.

Results

We have combined in vivo testing of DNA fragments in zebrafish and computational comparative genomics to identify the first regulatory regions of the zebrafish sox10 gene. Both approaches converged on the 3' end of the conserved 1^st intron as being critical for spatial patterning of sox10 in the embryo. Importantly, we have defined a minimal region crucial for this function. We show that this region contains numerous binding sites for transcription factors known to be essential in early neural crest induction, including Tcf/Lef, Sox and FoxD3. We show that the identity and relative position of these binding sites are conserved between zebrafish and mammals. A further region, partially required for oligodendrocyte expression, lies in the 5' region of the same intron and contains a putative CSL binding site, consistent with a role for Notch signalling in sox10 regulation. Furthermore, we show that β-catenin, Notch signalling and Sox9 can induce ectopic sox10 expression in early embryos, consistent with regulatory roles predicted from our transgenic and computational results.

Conclusion

We have thus identified two major sites of sox10 regulation in vertebrates and provided evidence supporting a role for at least three factors in driving sox10 expression in neural crest, otic epithelium and oligodendrocyte domains.

Trunk lateral cells are neural crest-like cells in the ascidian Ciona intestinalis: insights into the ancestry and evolution of the neural crest

Neural crest-like cells (NCLC) that express the HNK-1 antigen and form body pigment cells were previously identified in diverse ascidian species. Here we investigate the embryonic origin, migratory activity, and neural crest related gene expression patterns of NCLC in the ascidian Ciona intestinalis. HNK-1 expression first appeared at about the time of larval hatching in dorsal cells of the posterior trunk. In swimming tadpoles, HNK-1 positive cells began to migrate, and after metamorphosis they were localized in the oral and atrial siphons, branchial gill slits, endostyle, and gut. Cleavage arrest experiments showed that NCLC are derived from the A7.6 cells, the precursors of trunk lateral cells (TLC), one of the three types of migratory mesenchymal cells in ascidian embryos. In cleavage arrested embryos, HNK-1 positive TLC were present on the lateral margins of the neural plate and later became localized adjacent to the posterior sensory vesicle, a staging zone for their migration after larval hatching. The Ciona orthologues of seven of sixteen genes that function in the vertebrate neural crest gene regulatory network are expressed in the A7.6/TLC lineage. The vertebrate counterparts of these genes function downstream of neural plate border specification in the regulatory network leading to neural crest development. The results suggest that NCLC and neural crest cells may be homologous cell types originating in the common ancestor of tunicates and vertebrates and support the possibility that a putative regulatory network governing NCLC development was co-opted to produce neural crest cells during vertebrate evolution.