Induction of the neural crest: a multigene process

The activity of Pax3 and Zic1 regulates three distinct cell fates at the neural plate border

In Xenopus, the neural plate border gives rise to at least three cell populations: the neural crest, the preplacodal ectoderm, and the hatching gland. To understand the molecular mechanisms that regulate the formation of these lineages, we have analyzed the role of two transcription factors, Pax3 and Zic1, which are among the earliest genes activated in response to neural plate border-inducing signals. At the end of gastrulation, Pax3 and Zic1 are coexpressed in the neural crest forming region. In addition, Pax3 is expressed in progenitors of the hatching gland, and Zic1 is detected in the preplacodal ectoderm. Using gain of function and knockdown approaches in whole embryos and animal explants, we demonstrate that Pax3 and Zic1 are necessary and sufficient to promote hatching gland and preplacodal fates, respectively, whereas their combined activity is essential to specify the neural crest. Moreover, we show that by manipulating the levels of Pax3 and Zic1 it is possible to shift fates among these cells. These findings provide novel information on the mechanisms regulating cell fate decisions at the neural plate border.

The transcription factor MEF2C is required for craniofacial development

MEF2 transcription factors are well-established regulators of muscle development. We have discovered an unanticipated role for MEF2C in the neural crest, where tissue-specific inactivation results in neonatal lethality due to severe craniofacial defects. We show that MEF2C is required for expression of the Dlx5, Dlx6, and Hand2 transcription factor genes in the branchial arches, and we identify a branchial arch-specific enhancer in the Dlx5/6 locus, which is activated synergistically by MEF2C and Dlx5, demonstrating that these factors interact to induce transcription. Mef2c and Dlx5/6 also interact genetically. Mice heterozygous for either Dlx5/6 or Mef2c are normal at birth and survive to weaning. By contrast, heterozygosity for both Mef2c and Dlx5/6 results in defective palate development and neonatal lethality. Taken together, the studies presented here define a feed-forward transcriptional circuit between the MADS-box transcription factor MEF2C and the homeodomain transcription factors Dlx5 and Dlx6 in craniofacial development.

The mother superior mutation ablates foxd3 activity in neural crest progenitor cells and depletes neural crest derivatives in zebrafish

The zebrafish mutation mother superior (mosm188) leads to a depletion of neural crest (NC) derivatives including the craniofacial cartilage skeleton, the peripheral nervous system (sympathetic neurons, dorsal root ganglia, enteric neurons), and pigment cells. The loss of derivatives is preceded by a reduction in NC-expressed transcription factors, snail1b, sox9b, sox10, and a specific loss of foxd3 expression in NC progenitor cells. We employed genetic linkage analysis and physical mapping to place the mosm188 mutation on zebrafish chromosome 6 in the vicinity of the foxd3 gene. Furthermore, we found that mosm188 does not complement the sym1/foxd3 mutation, indicating that mosm188 resides within the foxd3 locus. Injection of PAC clones containing the foxd3 gene into mosm188 embryos restored foxd3 expression in NC progenitors and suppressed the mosm188 phenotype. However, sequencing the foxd3 transcribed area in mosm188 embryos did not reveal nucleotide changes segregating with the mosm188 phenotype, implying that the mutation most likely resides outside the foxd3-coding region. Based on these findings, we propose that the mosm188 mutation perturbs a NC-specific foxd3 regulatory element. Further analysis of mosm188 mutants and foxd3 morphants revealed that NC cells are initially formed, suggesting that foxd3 function is required to maintain the pool of NC progenitors.

Non-malignant migration of B16 mouse melanoma cells in the neural crest and invasive growth in the eye cup of the chick embryo

Melanocytes originate from the neural crest. In a previous study, we observed that human SK-Mel 28 human melanoma cells resumed neural crest cell migration after transplantation into the chick embryo neural tube. Here, we used transgenic mouse B16-F1 melanoma cells transfected with green fluorescent protein-vasodilator-stimulated phosphoprotein construct to extend these observations. After the injection of a cell suspension into the trunk neural tube of E2 chick embryos, the migration of melanoma cells was followed by live fluorescence microscopy. Within 12 h, the melanoma cells formed clusters in the neural tube at the levels of the intersegmental clefts between somites. After 24 h, a segmental pattern of emigration was visible. Emigrated melanoma cells were identified in serial paraffin sections by immunohistochemistry with ab732 as a marker for melanoma cells and by in-situ hybridization of mouse-specific repetitive genomic sequence mL1. After 24 h, melanoma cells were found along the medial neural crest pathway and in the sympathetic trunk ganglia and, after 48 h, also in the lateral melanocytic pathway. During migration along the neural crest pathways, mouse melanoma cells underwent apoptosis, which was assessed by anti-caspase 3 and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick-end labeling staining. To prove the ablation of malignant behavior after back-transplantation into the original embryonic neural crest environment, we injected the same cell suspension into the eye cup of the E3 embryo. In this location, invasive melanomas formed.

Human fetal neuroblast and neuroblastoma transcriptome analysis confirms neuroblast origin and highlights neuroblastoma candidate genes

Transcriptome profiling of neuroblasts and neuroblastoma tumor cells provides strong support for a neuroblast origin of neuroblastoma and highlights new candidate neuroblastoma genes

Background

Neuroblastoma tumor cells are assumed to originate from primitive neuroblasts giving rise to the sympathetic nervous system. Because these precursor cells are not detectable in postnatal life, their transcription profile has remained inaccessible for comparative data mining strategies in neuroblastoma. This study provides the first genome-wide mRNA expression profile of these human fetal sympathetic neuroblasts. To this purpose, small islets of normal neuroblasts were isolated by laser microdissection from human fetal adrenal glands.

Results

Expression of catecholamine metabolism genes, and neuronal and neuroendocrine markers in the neuroblasts indicated that the proper cells were microdissected. The similarities in expression profile between normal neuroblasts and malignant neuroblastomas provided strong evidence for the neuroblast origin hypothesis of neuroblastoma. Next, supervised feature selection was used to identify the genes that are differentially expressed in normal neuroblasts versus neuroblastoma tumors. This approach efficiently sifted out genes previously reported in neuroblastoma expression profiling studies; most importantly, it also highlighted a series of genes and pathways previously not mentioned in neuroblastoma biology but that were assumed to be involved in neuroblastoma pathogenesis.

Conclusion

This unique dataset adds power to ongoing and future gene expression studies in neuroblastoma and will facilitate the identification of molecular targets for novel therapies. In addition, this neuroblast transcriptome resource could prove useful for the further study of human sympathoadrenal biogenesis.

Bone morphogenetic protein-mediated modulation of lineage diversification during neural differentiation of embryonic stem cells

Embryonic stem cells (ES cells) can give rise to a broad spectrum of neural cell types. The biomedical application of ES cells will require detailed knowledge on the role of individual factors modulating fate specification during in vitro differentiation. Bone morphogenetic proteins (BMPs) are known to exert a multitude of diverse differentiation effects during embryonic development. Here, we show that exposure to BMP2 at distinct stages of neural ES cell differentiation can be used to promote specific cell lineages. During early ES cell differentiation, BMP2-mediated inhibition of neuroectodermal differentiation is associated with an increase in mesoderm and smooth muscle differentiation. In fibroblast growth factor 2-expanded ES cell-derived neural precursors, BMP2 supports the generation of neural crest phenotypes, and, within the neuronal lineage, promotes distinct subtypes of peripheral neurons, including cholinergic and autonomic phenotypes. BMP2 also exerts a density-dependent promotion of astrocyte differentiation at the expense of oligodendrocyte formation. Experiments involving inhibition of the serine threonine kinase FRAP support the notion that these effects are mediated via the JAK/STAT pathway. The preservation of diverse developmental BMP2 effects in differentiating ES cell cultures provides interesting prospects for the enrichment of distinct neural phenotypes in vitro.

Migratory patterns and developmental potential of trunk neural crest cells in the axolotl embryo

Using cell markers and grafting, we examined the timing of migration and developmental potential of trunk neural crest cells in axolotl. No obvious differences in pathway choice were noted for DiI-labeling at different lateral or medial positions of the trunk neural folds in neurulae, which contributed not only to neural crest but also to Rohon-Beard neurons. Labeling wild-type dorsal trunks at pre- and early-migratory stages revealed that individual neural crest cells migrate away from the neural tube along two main routes: first, dorsolaterally between the epidermis and somites and, later, ventromedially between the somites and neural tube/notochord. Dorsolaterally migrating crest primarily forms pigment cells, with those from anterior (but not mid or posterior) trunk neural folds also contributing glia and neurons to the lateral line. White mutants have impaired dorsolateral but normal ventromedial migration. At late migratory stages, most labeled cells move along the ventromedial pathway or into the dorsal fin. Contrasting with other anamniotes, axolotl has a minor neural crest contribution to the dorsal fin, most of which arises from the dermomyotome. Taken together, the results reveal stereotypic migration and differentiation of neural crest cells in axolotl that differ from other vertebrates in timing of entry onto the dorsolateral pathway and extent of contribution to some derivatives.

Lens induction in vertebrates: Variations on a conserved theme of signaling events

This review provides an overview of our current understanding of signaling mechanisms involved in lens induction, which are presented in context of the major stages of lens induction (competence, bias, inhibition and specification). Although the process of lens induction is generally well conserved, we highlight aspects of induction that vary among species. Finally, this review identifies future challenges in forming an integrated network of signaling pathways involved in lens induction.

Endogenous bone morphogenetic protein antagonists regulate mammalian neural crest generation and survival

We demonstrate here that Chordin and Noggin function as bone morphogenetic protein (BMP) antagonists in vivo to promote mammalian neural crest development. Using Chrd and Nog single and compound mutants, we find that Noggin has a major role in promoting neural crest formation, in which Chordin is partially redundant. BMP signaling is increased in dorsal tissues lacking Noggin and is further increased when Chordin is also absent. The early neural crest domain is expanded with decreased BMP antagonism in vivo. Noggin and Chordin also regulate subsequent neural crest cell emigration from the neural tube. However, reduced levels of these BMP antagonists ultimately result in perturbation of neural crest cell derived peripheral nervous system and craniofacial skeletal elements. Such defects reflect, at least in part, a function to limit apoptosis in neural crest cells. Noggin and Chordin, therefore, function together to regulate both the generation and survival of neural crest cells in mammalian development.

Functional analysis of Sox8 during neural crest development in Xenopus

Among the families of transcription factors expressed at the neural plate border, Sox proteins have been shown to regulate multiple aspects of neural crest development. Sox8, Sox9 and Sox10, exhibit overlapping expression domains in neural crest progenitors, and studies in mouse suggest that Sox8 functions redundantly with Sox9 and Sox10 during neural crest development. Here, we show that in Xenopus, Sox8 accumulates at the lateral edges of the neural plate at the mid-gastrula stage; in contrast to its mouse and chick orthologs, Sox8 expression precedes that of Sox9 and Sox10 in neural crest progenitors. Later in development, Sox8 expression persists in migrating cranial crest cells as they populate the pharyngeal arches and in trunk neural crest cells, in a pattern that recapitulates both Sox9 and Sox10 expression domains. Although morpholino-mediated knockdown of Sox8 protein did not prevent the formation of neural crest progenitors, the timing of their induction was severely affected. This delay in neural crest specification had dramatic consequences on the development of multiple lineages of the neural crest. We demonstrate that these defects are due to the inability of neural crest cells to migrate into the periphery, rather than to a deficiency in neural crest progenitors specification and survival. These results indicate that the control of Sox8 expression at the neural plate border is a key process in initiating neural crest formation in Xenopus, and highlight species-specific differences in the relative importance of SoxE proteins during neural crest development.

N-cadherin is required for neural crest remodeling of the cardiac outflow tract

Cardiac neural crest cells undergo extensive cell rearrangements during the formation of the aorticopulmonary septum in the outflow tract. However, the morphogenetic mechanisms involved in this fundamental process remain poorly understood. To determine the function of the Ca2+-dependent cell adhesion molecule, N-cadherin, in murine neural crest, we applied the Cre/loxP system and created mouse embryos genetically mosaic for N-cadherin. Specifically, deletion of N-cadherin in neural crest cells led to embryonic lethality with distinct cardiovascular defects. Neural crest cell migration and homing to the cardiac outflow tract niche were unaffected by loss of N-cadherin. However, N-cadherin-deficient neural crest cells were unable to undergo the normal morphogenetic changes associated with outflow tract remodeling, resulting in persistent truncus arteriosus in the majority of mutant embryos. Other mutant embryos initiated aorticopulmonary septum formation; however, the neural crest cells were unable to elongate and align properly along the midline and remained rounded with limited contact with their neighbors. Interestingly, rotation of the outflow tract was incomplete in these mutants suggesting that alignment of the channels is dependent on N-cadherin-generated cytoskeletal forces. A second cardiac phenotype was observed where loss of N-cadherin in the epicardium led to disruption of heterotypic cell interactions between the epicardium and myocardium resulting in a thinned ventricular myocardium. Thus, we conclude that in addition to its role in myocardial cell adhesion, N-cadherin is required for neural crest cell rearrangements critical for patterning of the cardiac outflow tract and in the maintenance of epicardial-myocardial cell interactions.

Absence of Nodal signaling promotes precocious neural differentiation in the mouse embryo

After implantation, mouse embryos deficient for the activity of the transforming growth factor-beta member Nodal fail to form both the mesoderm and the definitive endoderm. They also fail to specify the anterior visceral endoderm, a specialized signaling center which has been shown to be required for the establishment of anterior identity in the epiblast. Our study reveals that Nodal-/- epiblast cells nevertheless express prematurely and ectopically molecular markers specific of anterior fate. Our analysis shows that neural specification occurs and regional identities characteristic of the forebrain are established precociously in the Nodal-/- mutant with a sequential progression equivalent to that of wild-type embryo. When explanted and cultured in vitro, Nodal-/- epiblast cells readily differentiate into neurons. Genes normally transcribed in organizer-derived tissues, such as Gsc and Foxa2, are also expressed in Nodal-/- epiblast. The analysis of Nodal-/-;Gsc-/- compound mutant embryos shows that Gsc activity plays no critical role in the acquisition of forebrain characters by Nodal-deficient cells. This study suggests that the initial steps of neural specification and forebrain development may take place well before gastrulation in the mouse and highlights a possible role for Nodal, at pregastrula stages, in the inhibition of anterior and neural fate determination.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

Zebrafish foxd3 is selectively required for neural crest specification, migration and survival

The vertebrate neural crest is a pluripotent cell population that generates a large variety of cell types, including peripheral neurons, cartilage and pigment cells. Mechanisms that control the patterning of the neural crest toward specific cell fates remain only partially understood. Zebrafish homozygous for the sympathetic mutation 1 (sym1) have defects in a subset of neural crest derivatives, such as peripheral neurons, glia and cartilage, but retain normal numbers of melanocytes. The sym1 mutation is a nucleotide deletion that disrupts the forkhead DNA-binding domain of the foxd3 gene, which encodes a conserved winged-helix transcription factor. We show that sym1 mutants have normal numbers of premigratory neural crest cells, but these cells express reduced levels of snai1b and sox10, implicating foxd3 as an essential regulator of these transcription factors in the premigratory neural crest. The onset of neural crest migration is also delayed in sym1 mutants, and there is a reduction in the number of migratory trunk neural crest cells, particularly along the medial migration pathway. TUNEL analysis revealed aberrant apoptosis localized to the hindbrain neural crest at the 15-somite stage, indicating a critical role for foxd3 in the survival of a subpopulation of neural crest cells. These results show that foxd3 selectively specifies premigratory neural crest cells for a neuronal, glial or cartilage fate, by inducing the expression of lineage-associated transcription factors in these cells and regulating their subsequent migration.

c-Met expression is regulated by Mitf in the melanocyte lineage

Hepatocyte growth factor (HGF)/c-Met signaling is thought to be a key pathway in both melanocyte development and melanoma metastasis. Here, HGF stimulation of melanocytes was seen to up-regulate c-Met expression. In an effort to decipher the mechanism by which HGF up-regulates its receptor, we found that c-Met is a direct transcriptional target of Mitf. This was confirmed with chromatin immunoprecipitation experiments of the human c-Met promoter, as well as by the ability of adenovirally expressed Mitf to modulate endogenous c-Met protein levels in melanocytes. Disruption of Mitf blocked HGF-dependent increases in endogenous c-Met message and protein levels, indicating that HGF regulates its own receptor levels via Mitf. Finally, dominant-negative inhibition of Mitf resulted in profound resistance of melanocytes and melanoma cells to HGF-dependent matrix invasion, suggesting a physiologic role for this pathway in melanocytic development and melanoma.

The contribution of the neural crest to the vertebrate body

As a transitory structure providing adult tissues of the vertebrates with very diverse cell types, the neural crest (NC) has attracted for long the interest of developmental biologists and is still the subject of ongoing research in a variety of animal models. Here we review a number of data from in vivo cell tracing and in vitro single cell culture experiments, which gained new insights on the mechanisms of cell migration, proliferation and differentiation during NC ontogeny. We put emphasis on the role of Hox genes, morphogens and interactions with neighbouring tissues in specifying and patterning the skeletogenic NC cells in the head. We also include advances made towards characterizing multipotent stem cells in the early NC as well as in various NC derivatives in embryos and even in adult.

BMP2 and FGF2 cooperate to induce neural-crest-like fates from fetal and adult CNS stem cells

CNS stem cells are best characterized by their ability to self-renew and to generate multiple differentiated derivatives, but the effect of mitogenic signals, such as fibroblast growth factor 2 (FGF2), on the positional identity of these cells is not well understood. Here, we report that bone morphogenetic protein 2 (BMP2) induces telencephalic CNS stem cells to fates characteristic of neural crest and choroid plexus mesenchyme, a cell type of undetermined lineage in rodents. This induction occurs both in dissociated cell culture and cortical explants of embryonic day 14.5 (E14.5) embryos, but only when cells have been exposed to FGF2. Neither EGF nor IGF1 can substitute for FGF2. An early step in this response is activation of β-catenin, a mediator of Wnt activity. The CNS stem cells first undergo an epithelial-to-mesenchymal transition and subsequently differentiate to smooth-muscle and non-CNS glia cells. Similar responses are seen with stem cells from E14.5 cortex, E18.5 cortex and adult subventricular zone, but with a progressive shift toward gliogenesis that is characteristic of normal development. These data indicate that FGF2 confers competence for dorsalization independently of its mitogenic action. This rapid and efficient induction of dorsal fates may allow identification of positional identity effectors that are co-regulated by FGF2 and BMP2.

Abnormalities in neural crest cell migration in laminin alpha5 mutant mice

Although numerous in vitro experiments suggest that extracellular matrix molecules like laminin can influence neural crest migration, little is known about their function in the embryo. Here, we show that laminin alpha5, a gene up-regulated during neural crest induction, is localized in regions of newly formed cranial and trunk neural folds and adjacent neural crest migratory pathways in a manner largely conserved between chick and mouse. In laminin alpha5 mutant mice, neural crest migratory streams appear expanded in width compared to wild type. Conversely, neural folds exposed to laminin alpha5 in vitro show a reduction by half in the number of migratory neural crest cells. During gangliogenesis, laminin alpha5 mutants exhibit defects in condensing cranial sensory and trunk sympathetic ganglia. However, ganglia apparently recover at later stages. These data suggest that the laminin alpha5 subunit functions as a cue that restricts neural crest cells, focusing their migratory pathways and condensation into ganglia. Thus, it is required for proper migration and timely differentiation of some neural crest populations.

Tissues and signals involved in the induction of placodal Six1 expression in Xenopus laevis

Ectodermal placodes, from which many cranial sense organs and ganglia develop, arise from a common placodal primordium defined by Six1 expression. Here, we analyse placodal Six1 induction in Xenopus using microinjections and tissue grafts. We show that placodal Six1 induction occurs during neural plate and neural fold stages. Grafts of anterior neural plate but not grafts of cranial dorsolateral endomesoderm induce Six1 ectopically in belly ectoderm, suggesting that only the neural plate is sufficient for inducing Six1 in ectoderm. However, extirpation of either anterior neural plate or of cranial dorsolateral endomesoderm abolishes placodal Six1 expression indicating that both tissues are required for its induction. Elevating BMP-levels blocks placodal Six1 induction, whereas ectopic sources of BMP inhibitors expand placodal Six1 expression without inducing Six1 ectopically. This suggests that BMP inhibition is necessary but needs to cooperate with additional factors for Six1 induction. We show that FGF8, which is expressed in the anterior neural plate, can strongly induce ectopic Six1 in ventral ectoderm when combined with BMP inhibitors. In contrast, FGF8 knockdown abolishes placodal Six1 expression. This suggests that FGF8 is necessary and together with BMP inhibitors sufficient to induce placodal Six1 expression in cranial ectoderm, implicating FGF8 as a central component in generic placode induction.

msh/Msx gene family in neural development

The involvement of Msx homeobox genes in skull and tooth formation has received a great deal of attention. Recent studies also indicate a role for the msh/Msx gene family in development of the nervous system. In this article, we discuss the functions of these transcription factors in neural-tissue organogenesis. We will deal mainly with the interactions of the Drosophila muscle segment homeobox (msh) gene with other homeobox genes and the repressive cascade that leads to neuroectoderm patterning; the role of Msx genes in neural-crest induction, focusing especially on the differences between lower and higher vertebrates; their implication in patterning of the vertebrate neural tube, particularly in diencephalon midline formation. Finally, we will examine the distinct activities of Msx1, Msx2 and Msx3 genes during neurogenesis, taking into account their relationships with signalling molecules such as BMP.

Role of morphogens in neural crest cell determination

The neural crest is a transient, migratory cell population found in all vertebrate embryos that generate a diverse range of cell and tissue derivatives including, but not limited, to the neurons and glia of the peripheral nervous system, smooth muscle, connective tissue, melanocytes, craniofacial cartilage, and bone. Over the past few years, many studies have provided tremendous insights into understanding the mechanisms regulating the induction and migration of neural crest cell development. This review highlights the surprising and perhaps unexpected roles for morphogens in these distinct processes. A comparison of studies performed in several different vertebrates emphasizes the requirement for coordination between multiple signaling pathways in the induction and migration of neural crest cells in the developing embryo.

Combined deficiencies of Msx1 and Msx2 cause impaired patterning and survival of the cranial neural crest

The neural crest is a multipotent, migratory cell population that contributes to a variety of tissues and organs during vertebrate embryogenesis. Here, we focus on the function of Msx1 and Msx2, homeobox genes implicated in several disorders affecting craniofacial development in humans. We show that Msx1/2 mutants exhibit profound deficiencies in the development of structures derived from the cranial and cardiac neural crest. These include hypoplastic and mispatterned cranial ganglia, dysmorphogenesis of pharyngeal arch derivatives and abnormal organization of conotruncal structures in the developing heart. The expression of the neural crest markers Ap-2alpha, Sox10 and cadherin 6 (cdh6) in Msx1/2 mutants revealed an apparent retardation in the migration of subpopulations of preotic and postotic neural crest cells, and a disorganization of neural crest cells paralleling patterning defects in cranial nerves. In addition, normally distinct subpopulations of migrating crest underwent mixing. The expression of the hindbrain markers Krox20 and Epha4 was altered in Msx1/2 mutants, suggesting that defects in neural crest populations may result, in part, from defects in rhombomere identity. Msx1/2 mutants also exhibited increased Bmp4 expression in migratory cranial neural crest and pharyngeal arches. Finally, proliferation of neural crest-derived mesenchyme was unchanged, but the number of apoptotic cells was increased substantially in neural crest-derived cells that contribute to the cranial ganglia and the first pharyngeal arch. This increase in apoptosis may contribute to the mispatterning of the cranial ganglia and the hypoplasia of the first arch.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).

A balance of FGF, BMP and WNT signalling positions the future placode territory in the head

The sensory nervous system in the vertebrate head arises from two different cell populations: neural crest and placodal cells. By contrast, in the trunk it originates from neural crest only. How do placode precursors become restricted exclusively to the head and how do multipotent ectodermal cells make the decision to become placodes or neural crest? At neural plate stages,future placode cells are confined to a narrow band in the head ectoderm, the pre-placodal region (PPR). Here, we identify the head mesoderm as the source of PPR inducing signals, reinforced by factors from the neural plate. We show that several independent signals are needed: attenuation of BMP and WNT is required for PPR formation. Together with activation of the FGF pathway, BMP and WNT antagonists can induce the PPR in naïve ectoderm. We also show that WNT signalling plays a crucial role in restricting placode formation to the head. Finally, we demonstrate that the decision of multipotent cells to become placode or neural crest precursors is mediated by WNT proteins:activation of the WNT pathway promotes the generation of neural crest at the expense of placodes. This mechanism explains how the placode territory becomes confined to the head, and how neural crest and placode fates diversify.

Dynamic alterations in gene expression after Wnt-mediated induction of avian neural crest

The Wnt signaling pathway is important in the formation of neural crest cells in many vertebrates, but the downstream targets of neural crest induction by Wnt are largely unknown. Here, we examined quantitative changes in gene expression regulated by Wnt-mediated neural crest induction using quantitative PCR (QPCR). Induction was recapitulated in vitro by adding soluble Wnt to intermediate neural plate tissue cultured in collagen, and induced versus control tissue were assayed using gene-specific primers at times corresponding to premigratory (18 and 24 h) or early (36 h) stages of crest migration. The results show that Wnt signaling up-regulates in a distinct temporal pattern the expression of several genes normally expressed in the dorsal neural tube (slug, Pax3, Msx1, FoxD3, cadherin 6B) at "premigratory" stages. While slug is maintained in early migrating crest cells, Pax3, FoxD3, Msx1 and cadherin 6B all are down-regulated by the start of migration. These results differ from the temporal profile of these genes in response to the addition of recombinant BMP4, where gene expression seems to be maintained. Interestingly, expression of rhoB is unchanged or even decreased in response to Wnt-mediated induction at all times examined, though it is up-regulated by BMP signals. The temporal QPCR profiles in our culture paradigm approximate in vivo expression patterns of these genes before neural crest migration, and are consistent with Wnt being an initial neural crest inducer with additional signals like BMP and other factors maintaining expression of these genes in vivo. Our results are the first to quantitatively describe changes in gene expression in response to a Wnt or BMP signal during transformation of a neural tube cell into a migratory neural crest cell.

Neural and eye-specific defects associated with loss of the imitation switch (ISWI) chromatin remodeler in Xenopus laevis

Imitation Switch (ISWI) is a member of the SWI2/SNF2 superfamily of ATP-dependent chromatin remodelers, which regulate transcription and maintain chromatin structure by mobilizing nucleosomes using the energy of ATP. Four distinct ISWI complexes have been identified in Xenopus oocytes. The developmental role of Xenopus ISWI, however, has not previously been investigated in vivo. Here we report the tissue specificity, developmental expression, and requirement of ISWI for development of Xenopus embryos. Whole mount in situ hybridization shows ISWI localized in the lateral sides of the neural plate, brain, eye, and in later stages, the spinal cord. Injection of antisense ISWI RNA, morpholino oligonucleotides or dominant-negative ISWI mutant mRNA into fertilized eggs inhibits gastrulation and neural fold closure. Genes involved in neural patterning and development, such as BMP4 and Sonic hedgehog (Shh), are misregulated in the absence of functional ISWI, and ISWI binds to the BMP4 gene in vivo. Developmental and transcriptional defects caused by dominant-negative ISWI are rescued by co-injection of wild-type ISWI mRNA. Inhibition of ISWI function results in aberrant eye development and the formation of cataracts. These data suggest a critical role for ISWI chromatin remodeling complexes in neural development, including eye differentiation, in the Xenopus laevis embryo.

Induction and specification of the vertebrate ectodermal placodes: precursors of the cranial sensory organs

The sensory organs of the vertebrate head derive from two embryological structures, the neural crest and the ectodermal placodes. Although quite a lot is known about the secreted and transcription factors that regulate neural crest development, until recently little was known about the molecular pathways that regulate placode development. Herein we review recent findings on the induction and specification of the pre-placodal ectoderm, and the transcription factors that are involved in regulating placode fate and initial differentiation.

Dedifferentiation precedes invasion in the progression from Barrett's metaplasia to esophageal adenocarcinoma

PURPOSE

Adenocarcinoma arises in Barrett's esophagus by progression from metaplasia to cancer through grades of dysplasia. Our aim in this exploratory study was to characterize the broad changes in gene expression that underlie this histologic progression to cancer and assess the potential for using these gene expression changes as a marker predictive of malignant progression in Barrett's epithelium.

EXPERIMENTAL DESIGN

Microarray analysis was used to obtain individual gene expression profiles from endoscopic biopsies of nine esophageal adenocarcinomas and the Barrett's epithelia from which three of the cancers had arisen. Pooled samples from the Barrett's epithelia of six patients without cancer or dysplasia served as a reference.

RESULTS

Barrett's epithelia from which cancer had arisen differed from the reference Barrett's epithelia primarily by underexpression of genes, many of which function in governing cell differentiation. These changes in gene expression were found even in those specimens of Barrett's epithelia from which cancer had arisen that lacked dysplasia. Each cancer differed from the Barrett's epithelium from which it had arisen primarily by an overexpression of genes, many of which were associated with tissue remodeling and invasiveness. Cancers without identifiable Barrett's epithelium differed from cancers that had arisen from a Barrett's epithelium by having an even greater number of these overexpressed genes.

CONCLUSIONS

Histologic progression from Barrett's epithelium to cancer is associated with a gradient of increasing changes in gene expression characterized by an early loss of gene function governing differentiation that begins before histologic change; gain in function of genes related to remodeling and invasiveness follows later. This correlation of histologic progression with increasing changes in gene expression suggests that gene expression changes in biopsies taken from Barrett's epithelium potentially could serve as a marker for neoplastic progression that could be used to predict risk for developing cancer.

Hereditary hormone excess: genes, molecular pathways, and syndromes

Hereditary origin of a tumor helps toward early discovery of its mutated gene; for example, it supports the compilation of a DNA panel from index cases to identify that gene by finding mutations in it. The gene for a hereditary tumor may contribute also to common tumors. For some syndromes, such as hereditary paraganglioma, several genes can cause a similar syndrome. For other syndromes, such as multiple endocrine neoplasia 2, one gene supports variants of a syndrome. Onset usually begins earlier and in more locations with hereditary than sporadic tumors. Mono- or oligoclonal ("clonal") tumor usually implies a postnatal delay, albeit less delay than for sporadic tumor, to onset and potential for cancer. Hormone excess from a polyclonal tissue shows onset at birth and no benefit from subtotal ablation of the secreting organ. Genes can cause neoplasms through stepwise loss of function, gain of function, or combinations of these. Polyclonal hormonal excess reflects abnormal gene dosage or effect, such as activation or haploinsufficiency. Polyclonal hyperplasia can cause the main endpoint of clinical expression in some syndromes or can be a precursor to clonal progression in others. Gene discovery is usually the first step toward clarifying the molecule and pathway mutated in a syndrome. Most mutated pathways in hormone excess states are only partly understood. The bases for tissue specificity of hormone excess syndromes are usually uncertain. In a few syndromes, tissue selectivity arises from mutation in the open reading frame of a regulatory gene (CASR, TSHR) with selective expression driven by its promoter. Polyclonal excess of a hormone is usually from a defect in the sensor system for an extracellular ligand (e.g., calcium, glucose, TSH). The final connections of any of these polyclonal or clonal pathways to hormone secretion have not been identified. In many cases, monoclonal proliferation causes hormone excess, probably as a secondary consequence of accumulation of cells with coincidental hormone-secretory ability.

Sox proteins and neural crest development

Among the families of transcription factors expressed at the neural plate border in response to neural crest-inducing signals, Sox proteins have emerged as important players in regulating multiple aspects of neural crest development. Here, we summarize the expression of six Sox genes, namely Sox8, Sox9, Sox10, LSox5, Sox4 and Sox11, in neural crest progenitors and their derivatives, and review some aspects of their function pertaining to neural crest development in several species.

Computational model for cell migration in three-dimensional matrices

Although computational models for cell migration on two-dimensional (2D) substrata have described how various molecular and cellular properties and physiochemical processes are integrated to accomplish cell locomotion, the same issues, along with certain new ones, might contribute differently to a model for migration within three-dimensional (3D) matrices. To address this more complicated situation, we have developed a computational model for cell migration in 3D matrices using a force-based dynamics approach. This model determines an overall locomotion velocity vector, comprising speed and direction, for individual cells based on internally generated forces transmitted into external traction forces and considering a timescale during which multiple attachment and detachment events are integrated. Key parameters characterize cell and matrix properties, including cell/matrix adhesion and mechanical and steric properties of the matrix; critical underlying molecular properties are incorporated explicitly or implicitly. Model predictions agree well with experimental results for the limiting case of migration on 2D substrata as well as with recent experiments in 3D natural tissues and synthetic gels. Certain predicted features such as biphasic behavior of speed with density of matrix ligands for 3D migration are qualitatively similar to their 2D counterparts, but new effects generally absent in 2D systems, such as effects due to matrix sterics and mechanics, are now predicted to arise in many 3D situations. As one particular sample manifestation of these effects, the optimal levels of cell receptor expression and matrix ligand density yielding maximal migration are dependent on matrix mechanical compliance.

Essential role of non-canonical Wnt signalling in neural crest migration

Migration of neural crest cells is an elaborate process that requires the delamination of cells from an epithelium and cell movement into an extracellular matrix. In this work, it is shown for the first time that the non-canonical Wnt signalling [planar cell polarity (PCP) or Wnt-Ca2+] pathway controls migration of neural crest cells. By using specific Dsh mutants, we show that the canonical Wnt signalling pathway is needed for neural crest induction, while the non-canonical Wnt pathway is required for neural crest migration. Grafts of neural crest tissue expressing non-canonical Dsh mutants, as well as neural crest cultured in vitro, indicate that the PCP pathway works in a cell-autonomous manner to control neural crest migration. Expression analysis of non-canonical Wnt ligands and their putative receptors show that Wnt11 is expressed in tissue adjacent to neural crest cells expressing the Wnt receptor Frizzled7 (Fz7). Furthermore, loss- and gain-of-function experiments reveal that Wnt11 plays an essential role in neural crest migration. Inhibition of neural crest migration by blocking Wnt11 activity can be rescued by intracellular activation of the non-canonical Wnt pathway. When Wnt11 is expressed opposite its normal site of expression, neural crest migration is blocked. Finally, time-lapse analysis of cell movement and cell protrusion in neural crest cultured in vitro shows that the PCP or Wnt-Ca2+ pathway directs the formation of lamellipodia and filopodia in the neural crest cells that are required for their delamination and/or migration.

A developmental switch in the response of DRG neurons to ETS transcription factor signaling

Two ETS transcription factors of the Pea3 subfamily are induced in subpopulations of dorsal root ganglion (DRG) sensory and spinal motor neurons by target-derived factors. Their expression controls late aspects of neuronal differentiation such as target invasion and branching. Here, we show that the late onset of ETS gene expression is an essential requirement for normal sensory neuron differentiation. We provide genetic evidence in the mouse that precocious ETS expression in DRG sensory neurons perturbs axonal projections, the acquisition of terminal differentiation markers, and their dependence on neurotrophic support. Together, our findings indicate that DRG sensory neurons exhibit a temporal developmental switch that can be revealed by distinct responses to ETS transcription factor signaling at sequential steps of neuronal maturation.

By expressing ETS transcription factors at different developmental stages of dorsal root ganglion development, the authors show that late onset of ETS expression is essential for normal sensory neuron differentiation

Neural crest determination by co-activation of Pax3 and Zic1 genes in Xenopus ectoderm

A number of regulatory genes have been implicated in neural crest development. However, the molecular mechanism of how neural crest determination is initiated in the exact ectodermal location still remains elusive. Here, we show that the cooperative function of Pax3 and Zic1 determines the neural crest fate in the amphibian ectoderm. Pax3 and Zic1 are expressed in an overlapping manner in the presumptive neural crest area of the Xenopus gastrula, even prior to the onset of the expression of the early bona fide neural crest marker genes Foxd3 and Slug. Misexpression of both Pax3 and Zic1 together efficiently induces ectopic neural crest differentiation in the ventral ectoderm, whereas overexpression of either one of them only expands the expression of neural crest markers within the dorsolateral ectoderm. The induction of neural crest differentiation by Pax3 and Zic1 requires Wnt signaling. Loss-of-function studies in vivo and in the animal cap show that co-presence of Pax3 and Zic1 is essential for the initiation of neural crest differentiation. Thus, co-activation of Pax3 and Zic1, in concert with Wnt, plays a decisive role for early neural crest determination in the correct place of the Xenopus ectoderm.

To proliferate or to die: role of Id3 in cell cycle progression and survival of neural crest progenitors

The neural crest is a unique population of mitotically active, multipotent progenitors that arise at the vertebrate neural plate border. Here, we show that the helix-loop-helix transcriptional regulator Id3 has a novel role in cell cycle progression and survival of neural crest progenitors in Xenopus. Id3 is localized at the neural plate border during gastrulation and neurulation, overlapping the domain of neural crest induction. Morpholino oligonucleotide-mediated depletion of Id3 results in the absence of neural crest precursors and a resultant loss of neural crest derivatives. This appears to be mediated by cell cycle inhibition followed by cell death of the neural crest progenitor pool, rather than a cell fate switch. Conversely, overexpression of Id3 increases cell proliferation and results in expansion of the neural crest domain. Our data suggest that Id3 functions by a novel mechanism, independent of cell fate determination, to mediate the decision of neural crest precursors to proliferate or die.

Neural crest induction by the canonical Wnt pathway can be dissociated from anterior-posterior neural patterning in Xenopus

While Wnt signaling is known to be involved in early steps of neural crest development, the mechanism remains unclear. Because Wnt signaling is able to posteriorize anterior neural tissues, neural crest induction by Wnts has been proposed to be an indirect consequence of posteriorization of neural tissues rather than a direct effect of Wnt signaling. To address the relationship between posteriorization and neural crest induction by Wnt signaling, we have used gain of function and loss of function approaches in Xenopus to modulate the level of Wnt signaling at multiple points in the pathway. We find that modulating the level of Wnt signaling allows separation of neural crest induction from the effects of Wnts on anterior-posterior neural patterning. We also find that activation of Wnt signaling induces ectopic neural crest in the anterior region without posteriorizing anterior neural tissues. In addition, Wnt signaling induces neural crest when its posteriorizing activity is blocked by inhibition of FGF signaling in neuralized explants. Finally, depletion of beta-catenin confirms that the canonical Wnt pathway is required for initial neural crest induction. While these observations do not exclude a role for posteriorizing signals in neural crest induction, our data, together with previous observations, strongly suggest that canonical Wnt signaling plays an essential and direct role in neural crest induction.

The transcriptional control of trunk neural crest induction, survival, and delamination

Trunk neural crest cells are generated at the border between the neural plate and nonneural ectoderm, where they initiate a distinct program of gene expression, undergo an epithelial-mesenchymal transition (EMT), and delaminate from the neuroepithelium. Here, we provide evidence that members of three families of transcription induce these properties in premigratory neural crest cells. Sox9 acts to provide the competence for neural crest cells to undergo an EMT and is required for trunk neural crest survival. In the absence of Sox9, cells apoptose prior to or shortly after delamination. Slug/Snail, in the presence of Sox9, is sufficient to induce an EMT in neural epithelial cells, while FoxD3 regulates the expression of cell-cell adhesion molecules required for neural crest migration. Together, the data suggest a model in which a combination of transcription factors regulates the acquisition of the diverse properties of neural crest cells.

Msx1 and Pax3 cooperate to mediate FGF8 and WNT signals during Xenopus neural crest induction

FGF, WNT, and BMP signaling promote neural crest formation at the neural plate boundary in vertebrate embryos. To understand how these signals are integrated, we have analyzed the role of the transcription factors Msx1 and Pax3. Using a combination of overexpression and morpholino-mediated knockdown strategies in Xenopus, we show that Msx1 and Pax3 are both required for neural crest formation, display overlapping but nonidentical activities, and that Pax3 acts downstream of Msx1. In neuralized ectoderm, Msx1 is sufficient to induce multiple early neural crest genes. Msx1 induces Pax3 and ZicR1 cell autonomously, in turn, Pax3 combined with ZicR1 activates Slug in a WNT-dependent manner. Upstream of this, WNTs initiate Slug induction through Pax3 activity, whereas FGF8 induces neural crest through both Msx1 and Pax3 activities. Thus, WNT and FGF8 signals act in parallel at the neural border and converge on Pax3 activity during neural crest induction.

Xenopus Id3 is required downstream of Myc for the formation of multipotent neural crest progenitor cells

Neural crest cells, a population of proliferative, migratory, tissue-invasive stem cells, are a defining feature of vertebrate embryos. These cells arise at the neural plate border during a time in development when precursors of the central nervous system and the epidermis are responding to the extracellular signals that will ultimately dictate their fates. Neural crest progenitors, by contrast, must be maintained in a multipotent state until after neural tube closure. Although the molecular mechanisms governing this process have yet to be fully elucidated, recent work has suggested that Myc functions to prevent premature cell fate decisions in neural crest forming regions of the early ectoderm. Here, we show that the small HLH protein Id3 is a Myc target that plays an essential role in the formation and maintenance of neural crest stem cells. A morpholino-mediated 'knockdown' of Id3 protein results in embryos that lack neural crest. Moreover, forced expression of Id3 maintains the expression of markers of the neural crest progenitor state beyond the time when they would normally be downregulated and blocks the differentiation of neural crest derivatives. These results shed new light on the mechanisms governing the formation and maintenance of a developmentally and clinically important cell population.

The actin depolymerizing factor n-cofilin is essential for neural tube morphogenesis and neural crest cell migration

Cofilin/ADF proteins are a ubiquitously expressed family of F-actin depolymerizing factors found in eukaryotic cells including plants. In vitro, cofilin/ADF activity has been shown to be essential for actin driven motility, by accelerating actin filament turnover. Three actin depolymerizing factors (n-cofilin, m-cofilin, ADF) can be found in mouse and human. Here we show that in mouse the non-muscle-specific gene-n-cofilin-is essential for migration of neural crest cells as well as other cell types in the paraxial mesoderm. The main defects observed in n-cofilin mutant embryos are an impaired delamination and migration of neural crest cells, affecting the development of neural crest derived tissues. Neural crest cells lacking n-cofilin do not polarize, and F-actin bundles or fibers are not detectable. In addition, n-cofilin is required for neuronal precursor cell proliferation and scattering. These defects result in a complete lack of neural tube closure in n-cofilin mutant embryos. Although ADF is overexpressed in mutant embryos, this cannot compensate the lack of n-cofilin, suggesting that they might have a different function in embryonic development. Our data suggest that in mammalian development, regulation of the actin cytoskeleton by the F-actin depolymerizing factor n-cofilin is critical for epithelial-mesenchymal type of cell shape changes as well as cell proliferation.

Zebrafish narrowminded disrupts the transcription factor prdm1 and is required for neural crest and sensory neuron specification

Specification of both neural crest cells and Rohon-Beard (RB) sensory neurons involves a complex series of interactions between the neural and non-neural ectoderm. The molecular mechanisms directing this process are not well understood. The zebrafish narrowminded (nrd) mutation is unique, since it is one of two mutations in which defects are observed in both cell populations: it leads to a complete absence of RB neurons and a reduction in neural crest cells and their derivatives. Here, we show that nrd is a mutation in prdm1, a SET/zinc-finger domain transcription factor. A Morpholino-mediated depletion of prdm1 phenocopies the nrd mutation, and conversely overexpression of prdm1 mRNA rescues the nrd RB sensory neuron and neural crest phenotype. prdm1 is expressed at the border of the neural plate within the domain where neural crest cells and RB sensory neurons form. Analysis of prdm1 function by overexpression indicates that prdm1 functions to promote the cell fate specification of both neural crest cells and RB sensory neurons, most likely as a downstream effector of the BMP signaling pathway.

Transcriptional program of bone morphogenetic protein-2-induced epithelial and smooth muscle differentiation of pluripotent human embryonal carcinoma cells

Pluripotent human embryonal carcinoma NTera2/cloneD1 (NT2/D1) cells respond to multiple vertebrate patterning factors and offer a unique model system to investigate the signaling events associated with lineage determination and cell differentiation. Here, we define the temporal changes in global gene expression patterns in NT2/D1 cells upon treatment with bone morphogenetic protein-2 (BMP-2). Exposure to BMP-2 rapidly induced the expression of several transcription factors involved in establishing non-neural ectodermal fate followed by the appearance of epithelial-specific markers. Subsequent loss of stem cell markers was coupled to gene expression changes associated with decreased proliferative activity. Temporal clustering of gene expression patterns revealed a concurrent down-regulation of multiple transcripts involved in neurogenesis, neurite outgrowth, and axonal guidance, suggesting that the BMP-mediated differentiation process involves pro-epithelial as well as anti-neurogenic mechanisms. In addition, increased expression of smooth muscle markers both by gene expression and immunohistochemistry was detected. Several neural crest markers were induced preceding such a differentiation, compatible with a neural crest origin of NT2/D1-derived smooth muscle cells. Comparison of changes in transcript expression between BMP-2-induced epithelial versus all-trans-retinoic acid (ATRA)-induced neural differentiation revealed potential candidates for regulation of BMP-2 signaling and suppression of neural fate by BMP-2. This study suggests that BMP-2-induced differentiation of NT2/D1 cells provides a powerful assay to study early human epithelial and smooth muscle development.

Blimp-1 specifies neural crest and sensory neuron progenitors in the zebrafish embryo

Developmental origins of the neural crest (NC), a quintessential and pluripotent vertebrate cell type, has historically been a topic of extensive investigation but continues to remain poorly understood. In the zebrafish embryo, NC and primary sensory neurons are thought to segregate from a common population of progenitor cells in response to lateral inhibition. Here, we show that the zebrafish homolog of the B-lymphocyte-induced maturation protein (Blimp-1) gene, u-boot (ubo), is induced by BMP signaling in cells at the boundary of the neural plate and nonneural ectoderm. Loss of Ubo activity not only inhibits specification of the NC but also impairs development of the primary sensory neurons. Conversely, misexpression of ubo results in the generation of supernumerary primary sensory neurons consistent with this cell type representing the default fate within the progenitor equivalence group. These results establish a link between the activity of the transcriptional regulator Blimp-1 and the inductive effects of BMP signaling in the inception of NC progenitor fate.

Analysis of forkhead and snail expression reveals epithelial-mesenchymal transitions during embryonic and larval development of Nematostella vectensis

The winged helix transcription factor Forkhead and the zinc finger transcription factor Snail are crucially involved in germ layer formation in Bilateria. Here, we isolated and characterized a homolog of forkhead/HNF3 (FoxA/group 1) and of snail from a diploblast, the sea anemone Nematostella vectensis. We show that Nematostella forkhead expression starts during late Blastula stage in a ring of cells that demarcate the blastopore margin during early gastrulation, thereby marking the boundary between ectodermal and endodermal tissue. snail, by contrast, is expressed in a complementary pattern in the center of forkhead-expressing cells marking the presumptive endodermal cells fated to ingress during gastrulation. In a significant portion of early gastrulating embryos, forkhead is expressed asymmetrically around the blastopore. While snail-expressing cells form the endodermal cell mass, forkhead marks the pharynx anlage throughout embryonic and larval development. In the primary polyp, forkhead remains expressed in the pharynx. The detailed analysis of forkhead and snail expression during Nematostella embryonic and larval development further suggests that endoderm formation results from epithelial invagination, mesenchymal immigration, and reorganization of the endodermal epithelial layer, that is, by epithelial-mesenchymal transitions (EMT) in combination with extensive morphogenetic movements. snail also governs EMT at different processes during embryonic development in Bilateria. Our data indicate that the function of snail in Diploblasts is to regulate motility and cell adhesion, supporting that the triggering of changes in cell behavior is the ancestral role of snail in Metazoa.

Early development of the cranial sensory nervous system: from a common field to individual placodes

Sensory placodes are unique columnar epithelia with neurogenic potential that develop in the vertebrate head ectoderm next to the neural tube. They contribute to the paired sensory organs and the cranial sensory ganglia generating a wide variety of cell types ranging from lens fibres to sensory receptor cells and neurons. Although progress has been made in recent years to identify the molecular players that mediate placode specification, induction and patterning, the processes that initiate placode development are not well understood. One hypothesis suggests that all placode precursors arise from a common territory, the pre-placodal region, which is then subdivided to generate placodes of specific character. This model implies that their induction begins through molecular and cellular mechanisms common to all placodes. Embryological and molecular evidence suggests that placode induction is a multi-step process and that the molecular networks establishing the pre-placodal domain as well as the acquisition of placodal identity are surprisingly similar to those used in Drosophila to specify sensory structures.

A slug, a fox, a pair of sox: transcriptional responses to neural crest inducing signals

The neural crest, a cell type found only in vertebrate embryos, gives rise to the structures of the skull and face and most of the peripheral nervous system, as well as other cell types characteristic of vertebrates. These cells are of great clinical significance and a wide variety of congenital defects are due to aberrant neural crest development. Increasing numbers of studies are contributing to our understanding of how this group of cells form and differentiate during normal development. Wnt, FGF, BMP, and Notch-mediated signals all have essential roles in this process, and several of these signals appear to play multiple temporally distinct roles. Changes in the response of neural crest cells to the same signal over time may be mediated, in part, by an ever-changing cocktail of transcription factors expressed within these cells. Neural crest development is thus a complex multistep process, and elucidating the molecular mechanisms that mediate distinct aspects of this process will require that we determine the role of each of these factors alone and in combination. Here, we review some recent advances in our understanding of the signals and downstream transcription factors involved in neural crest cell formation.

Neural crest contribution to mammalian tooth formation

The cranial neural crest cells, which are specialized cells of neural origin, are central to the process of mammalian tooth development. They are the only source of mesenchyme able to sustain tooth development, and give rise not only to most of the dental tissues, but also to the periodontium, the surrounding tissues that hold teeth in position. Tooth organogenesis is regulated by a series of interactions between cranial neural crest cells and the oral epithelium. In the development of a tooth, the epithelium covering the inside of the developing oral cavity provides the first instructive signals. Signaling molecules secreted by the oral epithelium 1) establish large cellular fields competent to form a specific tooth shape (mono- or multicuspid) along a proximodistal axis; 2) define an oral (capable of forming teeth) and non-oral mesenchyme along a rostrocaudal axis; and 3) position the sites of future tooth development. The critical information to model tooth shape resides later in the neural crest-derived mesenchyme. Cranial neural crest cells ultimately differentiate into highly specialized cell types to produce mature dental organs. Some cranial neural crest cells located in the dental pulp, however, maintain plasticity in their developmental potential up to postnatal life, offering new prospects for regeneration of dental tissues.

Recycling signals in the neural crest

Vertebrate neural crest cells are multipotent and differentiate into structures that include cartilage and the bones of the face, as well as much of the peripheral nervous system. Understanding how different model vertebrates utilize signaling pathways reiteratively during various stages of neural crest formation and differentiation lends insight into human disorders associated with the neural crest.