Requirement ofFoxD3-class signaling for neural crest determination inXenopus

Gene expression patterns of the developing human face at single cell resolution reveal cell type contributions to normal facial variation and disease risk

Craniofacial development gives rise to the complex structures of the face and involves the interplay of diverse cell types. Despite its importance, our understanding of human-specific craniofacial developmental mechanisms and their genetic underpinnings remains limited. Here, we present a comprehensive single-nucleus RNA sequencing (snRNA-seq) atlas of human craniofacial development from craniofacial tissues of 24 embryos that span six key time points during the embryonic period (4-8 post-conception weeks). This resource resolves the transcriptional dynamics of seven major cell types and uncovers distinct major cell types, including muscle progenitors and cranial neural crest cells (CNCCs), as well as dozens of subtypes of ectoderm and mesenchyme. Comparative analyses reveal substantial conservation of major cell types, alongside human biased differences in gene expression programs. CNCCs, which play a crucial role in craniofacial morphogenesis, exhibit the lowest marker gene conservation, underscoring their evolutionary plasticity. Spatial transcriptomics further localizes cell populations, providing a detailed view of their developmental roles and anatomical context. We also link these developmental processes to genetic variation, identifying cell type-specific enrichments for common variants associated with facial morphology and rare variants linked to orofacial clefts. Intriguingly, Neanderthal-introgressed sequences are enriched near genes with biased expression in cartilage and specialized ectodermal subtypes, suggesting their contribution to modern human craniofacial features. This atlas offers unprecedented insights into the cellular and genetic mechanisms shaping the human face, highlighting conserved and distinctly human aspects of craniofacial biology. Our findings illuminate the developmental origins of craniofacial disorders, the genetic basis of facial variation, and the evolutionary legacy of ancient hominins. This work provides a foundational resource for exploring craniofacial biology, with implications for developmental genetics, evolutionary biology, and clinical research into congenital anomalies.

Xenopus as a model system for studying pigmentation and pigmentary disorders

Human pigmentary disorders encompass a broad spectrum of phenotypic changes arising from disruptions in various stages of melanocyte formation, the melanogenesis process, or the transfer of pigment from melanocytes to keratinocytes. A large number of pigmentation genes associated with pigmentary disorders have been identified, many of them awaiting in vivo confirmation. A more comprehensive understanding of the molecular basis of pigmentary disorders requires a vertebrate animal model where changes in pigmentation are easily observable in vivo and can be combined to genomic modifications and gain/loss-of-function tools. Here we present the amphibian Xenopus with its unique features that fulfill these requirements. Changes in pigmentation are particularly easy to score in Xenopus embryos, allowing whole-organism based phenotypic screening. The development and behavior of Xenopus melanocytes closely mimic those observed in mammals. Interestingly, both Xenopus and mammalian skins exhibit comparable reactions to ultraviolet radiation. This review highlights how Xenopus constitutes an alternative and complementary model to the more commonly used mouse and zebrafish, contributing to the advancement of knowledge in melanocyte cell biology and related diseases.

SoxB1 transcription factors are essential for initiating and maintaining neural plate border gene expression

SoxB1 transcription factors (Sox2/3) are well known for their role in early neural fate specification in the embryo, but little is known about functional roles for SoxB1 factors in non-neural ectodermal cell types, such as the neural plate border (NPB). Using Xenopus laevis, we set out to determine whether SoxB1 transcription factors have a regulatory function in NPB formation. Here, we show that SoxB1 factors are necessary for NPB formation, and that prolonged SoxB1 factor activity blocks the transition from a NPB to a neural crest state. Using ChIP-seq, we demonstrate that Sox3 is enriched upstream of NPB genes in early NPB cells and in blastula stem cells. Depletion of SoxB1 factors in blastula stem cells results in downregulation of NPB genes. Finally, we identify Pou5f3 factors as potential Sox3 partners in regulating the formation of the NPB and show that their combined activity is needed for normal NPB gene expression. Together, these data identify a role for SoxB1 factors in the establishment and maintenance of the NPB, in part through partnership with Pou5f3 factors.

The Emerging Roles of the Cephalic Neural Crest in Brain Development and Developmental Encephalopathies

The neural crest, a unique cell population originating from the primitive neural field, has a multi-systemic and structural contribution to vertebrate development. At the cephalic level, the neural crest generates most of the skeletal tissues encasing the developing forebrain and provides the prosencephalon with functional vasculature and meninges. Over the last decade, we have demonstrated that the cephalic neural crest (CNC) exerts an autonomous and prominent control on the development of the forebrain and sense organs. The present paper reviews the primary mechanisms by which CNC can orchestrate vertebrate encephalization. Demonstrating the role of the CNC as an exogenous source of patterning for the forebrain provides a novel conceptual framework with profound implications for understanding neurodevelopment. From a biomedical standpoint, these data suggest that the spectrum of neurocristopathies is broader than expected and that some neurological disorders may stem from CNC dysfunctions.

Repressive Interactions Between Transcription Factors Separate Different Embryonic Ectodermal Domains

The embryonic ectoderm is composed of four domains: neural plate, neural crest, pre-placodal region (PPR) and epidermis. Their formation is initiated during early gastrulation by dorsal-ventral and anterior-posterior gradients of signaling factors that first divide the embryonic ectoderm into neural and non-neural domains. Next, the neural crest and PPR domains arise, either via differential competence of the neural and non-neural ectoderm (binary competence model) or via interactions between the neural and non-neural ectoderm tissues to produce an intermediate neural border zone (NB) (border state model) that subsequently separates into neural crest and PPR. Many previous gain- and loss-of-function experiments demonstrate that numerous TFs are expressed in initially overlapping zones that gradually resolve into patterns that by late neurula stages are characteristic of each of the four domains. Several of these studies suggested that this is accomplished by a combination of repressive TF interactions and competence to respond to local signals. In this study, we ectopically expressed TFs that at neural plate stages are characteristic of one domain in a different domain to test whether they act cell autonomously as repressors. We found that almost all tested TFs caused reduced expression of the other TFs. At gastrulation these effects were strictly within the lineage-labeled cells, indicating that the effects were cell autonomous, i.e., due to TF interactions within individual cells. Analysis of previously published single cell RNAseq datasets showed that at the end of gastrulation, and continuing to neural tube closure stages, many ectodermal cells express TFs characteristic of more than one neural plate stage domain, indicating that different TFs have the opportunity to interact within the same cell. At neurula stages repression was observed both in the lineage-labeled cells and in adjacent cells not bearing detectable lineage label, suggesting that cell-to-cell signaling has begun to contribute to the separation of the domains. Together, these observations directly demonstrate previous suggestions in the literature that the segregation of embryonic ectodermal domains initially involves cell autonomous, repressive TF interactions within an individual cell followed by the subsequent advent of non-cell autonomous signaling to neighbors.

Function of chromatin modifier Hmgn1 during neural crest and craniofacial development

The neural crest is a dynamic embryonic structure that plays a major role in the formation of the vertebrate craniofacial skeleton. Neural crest formation is regulated by a complex sequence of events directed by a network of transcription factors working in concert with chromatin modifiers. The high mobility group nucleosome binding protein 1 (Hmgn1) is a nonhistone chromatin architectural protein, associated with transcriptionally active chromatin. Here we report the expression and function of Hmgn1 during Xenopus neural crest and craniofacial development. Hmgn1 is broadly expressed at the gastrula and neurula stages, and is enriched in the head region at the tailbud stage, especially in the eyes and the pharyngeal arches. Hmgn1 knockdown affected the expression of several neural crest specifiers, including sox8, sox10, foxd3, and twist1, while other genes (sox9 and snai2) were only marginally affected. The specificity of this phenotype was confirmed by rescue, where injection of Hmgn1 mRNA was able to restore sox10 expression in morphant embryos. The reduction in neural crest gene expression at the neurula stage in Hmgn1 morphant embryos correlated with a decreased number of sox10- and twist1-positive cells in the pharyngeal arches at the tailbud stage, and hypoplastic craniofacial cartilages at the tadpole stage. These results point to a novel role for Hmgn1 in the control of gene expression essential for neural crest and craniofacial development. Future work will investigate the precise mode of action of Hmgn1 in this context.

Lineage-specific control of convergent differentiation by a Forkhead repressor

During convergent differentiation, multiple developmental lineages produce a highly similar or identical cell type. However, few molecular players that drive convergent differentiation are known. Here, we show that the C. elegans Forkhead transcription factor UNC-130 is required in only one of three convergent lineages that produce the same glial cell type. UNC-130 acts transiently as a repressor in progenitors and newly-born terminal cells to allow the proper specification of cells related by lineage rather than by cell type or function. Specification defects correlate with UNC-130:DNA binding, and UNC-130 can be functionally replaced by its human homolog, the neural crest lineage determinant FoxD3. We propose that, in contrast to terminal selectors that activate cell type-specific transcriptional programs in terminally differentiating cells, UNC-130 acts early and specifically in one convergent lineage to produce a cell type that also arises from molecularly distinct progenitors in other lineages.

BMP signaling is enhanced intracellularly by FHL3 controlling WNT-dependent spatiotemporal emergence of the neural crest

The spatiotemporal coordination of multiple morphogens is essential for embryonic patterning yet poorly understood. During neural crest (NC) formation, dynamic bone morphogenetic protein (BMP), fibroblast growth factor (FGF), and WNT signals cooperate by acting on mesoderm and ectoderm. Here, we show that Fhl3, a scaffold LIM domain protein, modulates BMP gradient interpretation during NC induction. During gastrulation, low BMP signaling neuralizes the neural border (NB) ectoderm, while Fhl3 enhances Smad1 intracellular response in underlying paraxial mesoderm, triggering the high WNT8 signals needed to pattern the NB. During neurulation, fhl3 activation in NC ectoderm promotes simultaneous high BMP and BMP-dependent WNT activity required for specification. Mechanistically, Fhl3 interacts with Smad1 and promotes Smad1 binding to wnt8 promoter in a BMP-dependent manner. Consequently, differential Fhl3 expression in adjacent cells ensures a finely tuned coordination of BMP and WNT signaling at several stages of NC development, starting by positioning the NC-inducing mesoderm center under competent NB ectoderm.

Kindlin2 regulates neural crest specification via integrin-independent regulation of the FGF signaling pathway

The focal adhesion protein Kindlin2 is essential for integrin activation, a process that is fundamental to cell-extracellular matrix adhesion. Kindlin 2 (Fermt2) is widely expressed in mouse embryos, and its absence causes lethality at the peri-implantation stage due to the failure to trigger integrin activation. The function of kindlin2 during embryogenesis has not yet been fully elucidated as a result of this early embryonic lethality. Here, we showed that kindlin2 is essential for neural crest (NC) formation in Xenopus embryos. Loss-of-function assays performed with kindlin2-specific morpholino antisense oligos (MOs) or with CRISPR/Cas9 techniques in Xenopus embryos severely inhibit the specification of the NC. Moreover, integrin-binding-deficient mutants of Kindlin2 rescued the phenotype caused by loss of kindlin2, suggesting that the function of kindlin2 during NC specification is independent of integrins. Mechanistically, we found that Kindlin2 regulates the fibroblast growth factor (FGF) pathway, and promotes the stability of FGF receptor 1. Our study reveals a novel function of Kindlin2 in regulating the FGF signaling pathway and provides mechanistic insights into the function of Kindlin2 during NC specification.

Mitochondrial dysfunction interferes with neural crest specification through the FoxD3 transcription factor

The neural crest is an important group of cells with pluripotency and migratory ability that is crucially involved in tissue and cell specification during development. Craniofacial shaping, sensory neurons, body asymmetry, and pigmentation are linked to neural crest functionality. Despite its prominent role in embryogenesis, neural crest specification as well as the possible part mitochondria play in such a process remains unclarified. Mitochondria are important organelles not only for respiration, but also for regulation of cell proliferation, differentiation and death. Modulation of mitochondrial fitness and depletion of mitochondrial ATP synthesis has been shown to down-regulate Wnt signaling, both in vitro and in vivo. Since Wnt signaling is one of the crucial players during neural crest induction/specification, we hypothesized a signaling cascade connecting mitochondria to embryonic development and neural crest migration and differentiation. Here, by using pharmacological and genetic modulators of mitochondrial function, we provide evidence that a crosstalk between mitochondrial energy homeostasis and Wnt signaling is important in the development of neural crest-derived tissues. Furthermore, our results highlight the possibility to modulate neural crest cell specification by tuning mitochondrial metabolism via FoxD3, an important transcription factor that is regulated by Wnt. FoxD3 ensures the correct embryonic development and contributes to the maintenance of cell stemness and to the induction of epithelial-to-mesenchymal transition. In summary, our work offers new insights into the molecular mechanism of action of FoxD3 and demonstrates that mitochondrial fitness is linked to the regulation of this important transcription factor via Wnt signaling in the context of neural crest specification.

On the road again: Establishment and maintenance of stemness in the neural crest from embryo to adulthood

Unique to vertebrates, the neural crest (NC) is an embryonic stem cell population that contributes to a greatly expanding list of derivatives ranging from neurons and glia of the peripheral nervous system, facial cartilage and bone, pigment cells of the skin to secretory cells of the endocrine system. Here, we focus on what is specifically known about establishment and maintenance of NC stemness and ultimate fate commitment mechanisms, which could help explain its exceptionally high stem cell potential that exceeds the "rules set during gastrulation." In fact, recent discoveries have shed light on the existence of NC cells that coexpress commonly accepted pluripotency factors like Nanog, Oct4/PouV, and Klf4. The coexpression of pluripotency factors together with the exceptional array of diverse NC derivatives encouraged us to propose a new term "pleistopotent" (Greek for abundant, a substantial amount) to be used to reflect the uniqueness of the NC as compared to other post-gastrulation stem cell populations in the vertebrate body, and to differentiate them from multipotent lineage restricted stem cells. We also discuss studies related to the maintenance of NC stemness within the challenging context of being a transient and thus a constantly changing population of stem cells without a permanent niche. The discovery of the stem cell potential of Schwann cell precursors as well as multiple adult NC-derived stem cell reservoirs during the past decade has greatly increased our understanding of how NC cells contribute to tissues formed after its initial migration stage in young embryos.

The transcription factor Hypermethylated in Cancer 1 (Hic1) regulates neural crest migration via interaction with Wnt signaling

The transcription factor Hypermethylated in Cancer 1 (HIC1) is associated with both tumorigenesis and the complex human developmental disorder Miller-Dieker Syndrome. While many studies have characterized HIC1 as a tumor suppressor, HIC1 function in development is less understood. Loss-of-function mouse alleles show embryonic lethality accompanied with developmental defects, including craniofacial abnormalities that are reminiscent of human Miller-Dieker Syndrome patients. However, the tissue origin of the defects has not been reported. In this study, we use the power of the Xenopus laevis model system to explore Hic1 function in early development. We show that hic1 mRNA is expressed throughout early Xenopus development and has a spatial distribution within the neural plate border and in migrating neural crest cells in branchial arches. Targeted manipulation of hic1 levels in the dorsal ectoderm that gives rise to neural and neural crest tissues reveals that both overexpression and knockdown of hic1 result in craniofacial defects with malformations of the craniofacial cartilages. Neural crest specification is not affected by altered hic1 levels, but migration of the cranial neural crest is impaired both in vivo and in tissue explants. Mechanistically, we find that Hic1 regulates cadherin expression profiles and canonical Wnt signaling. Taken together, these results identify Hic1 as a novel regulator of the canonical Wnt pathway during neural crest migration.

The vertebrate-specific VENTX/NANOG gene empowers neural crest with ectomesenchyme potential

During Cambrian, unipotent progenitors located at the neural (plate) border (NB) of an Olfactoria chordate embryo acquired the competence to form ectomesenchyme, pigment cells and neurons, initiating the rise of the multipotent neural crest cells (NC) specific to vertebrates. Surprisingly, the known vertebrate NB/NC transcriptional circuitry is a constrained feature also found in invertebrates. Therefore, evidence for vertebrate-specific innovations endowing vertebrate NC with multipotency is still missing. Here, we identified VENTX/NANOG and POU5/OCT4 as vertebrate-specific innovations. When VENTX was depleted in vivo and in directly-induced NC, the NC lost its early multipotent state and its skeletogenic potential, but kept sensory neuron and pigment identity, thus reminiscent of invertebrate NB precursors. In vivo, VENTX gain-of-function enabled NB specifiers to reprogram embryonic non-neural ectoderm towards early NC identity. We propose that skeletogenic NC evolved by acquiring VENTX/NANOG activity, promoting a novel multipotent progenitor regulatory state into the pre-existing sensory neuron/pigment NB program.

Blastula stage specification of avian neural crest

Cell fate specification defines the earliest steps towards a distinct cell lineage. Neural crest, a multipotent stem cell population, is thought to be specified from the ectoderm, but its varied contributions defy canons of segregation potential and challenges its embryonic origin. Aiming to resolve this conflict, we have assayed the earliest specification of neural crest using blastula stage chick embryos. Specification assays on isolated chick epiblast explants identify an intermediate region specified towards the neural crest cell fate. Furthermore, low density culture suggests that the specification of intermediate cells towards the neural crest lineage is independent of contact mediated induction and Wnt-ligand induced signaling, but is, however, dependent on transcriptional activity of β-catenin. Finally, we have validated the regional identity of the intermediate region towards the neural crest cell fate using fate map studies. Our results suggest a model of neural crest specification within a restricted epiblast region in blastula stage chick embryos.

PRDM1 controls the sequential activation of neural, neural crest and sensory progenitor determinants

During early embryogenesis, the ectoderm is rapidly subdivided into neural, neural crest and sensory progenitors. How the onset of lineage determinants and the loss of pluripotency markers are temporally and spatially coordinated in vivo is still debated. Here, we identify a crucial role for the transcription factor PRDM1 in the orderly transition from epiblast to defined neural lineages in chick. PRDM1 is initially expressed broadly in the entire epiblast, but becomes gradually restricted as cell fates are specified. We find that PRDM1 is required for the loss of some pluripotency markers and the onset of neural, neural crest and sensory progenitor specifier genes. PRDM1 directly activates their expression by binding to their promoter regions and recruiting the histone demethylase Kdm4a to remove repressive histone marks. However, once neural lineage determinants become expressed, they in turn repress PRDM1, whereas prolonged PRDM1 expression inhibits neural, neural crest and sensory progenitor genes, suggesting that its downregulation is necessary for cells to maintain their identity. Therefore, PRDM1 plays multiple roles during ectodermal cell fate allocation.

Specification and formation of the neural crest: Perspectives on lineage segregation

The neural crest is a fascinating embryonic population unique to vertebrates that is endowed with remarkable differentiation capacity. Thought to originate from ectodermal tissue, neural crest cells generate neurons and glia of the peripheral nervous system, and melanocytes throughout the body. However, the neural crest also generates many ectomesenchymal derivatives in the cranial region, including cell types considered to be of mesodermal origin such as cartilage, bone, and adipose tissue. These ectomesenchymal derivatives play a critical role in the formation of the vertebrate head, and are thought to be a key attribute at the center of vertebrate evolution and diversity. Further, aberrant neural crest cell development and differentiation is the root cause of many human pathologies, including cancers, rare syndromes, and birth malformations. In this review, we discuss the current findings of neural crest cell ontogeny, and consider tissue, cell, and molecular contributions toward neural crest formation. We further provide current perspectives into the molecular network involved during the segregation of the neural crest lineage.

Specifying neural crest cells: From chromatin to morphogens and factors in between

Neural crest (NC) cells are a stem-like multipotent population of progenitor cells that are present in vertebrate embryos, traveling to various regions in the developing organism. Known as the "fourth germ layer," these cells originate in the ectoderm between the neural plate (NP), which will become the brain and spinal cord, and nonneural tissues that will become the skin and the sensory organs. NC cells can differentiate into more than 30 different derivatives in response to the appropriate signals including, but not limited to, craniofacial bone and cartilage, sensory nerves and ganglia, pigment cells, and connective tissue. The molecular and cellular mechanisms that control the induction and specification of NC cells include epigenetic control, multiple interactive and redundant transcriptional pathways, secreted signaling molecules, and adhesion molecules. NC cells are important not only because they transform into a wide variety of tissue types, but also because their ability to detach from their epithelial neighbors and migrate throughout developing embryos utilizes mechanisms similar to those used by metastatic cancer cells. In this review, we discuss the mechanisms required for the induction and specification of NC cells in various vertebrate species, focusing on the roles of early morphogenesis, cell adhesion, signaling from adjacent tissues, and the massive transcriptional network that controls the formation of these amazing cells. This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics Signaling Pathways > Cell Fate Signaling.

The b-HLH transcription factor Hes3 participates in neural plate border formation by interfering with Wnt/β-catenin signaling

Hes3 belongs to the Hes basic helix-loop-helix family of transcriptional repressors that play central roles in maintaining progenitor cells and regulating binary cell fate decisions in the embryo. During Xenopus laevis development, hes3 is expressed in the embryonic ectoderm in a horseshoe shape domain at the edge of the developing neural pate. Hes3 mis-expression at early neurula stage blocks neural crest (snai2, sox8, sox9 and sox10) and cranial placode (six1 and dmrta1) gene expression, and promotes neural plate (sox2 and sox3) fate. At tailbud stage, these embryos exhibited a massive up-regulation of both sox8 and sox10 expression, associated with an increase in genes important for melanocytes differentiation (mitf and dct). Using a hormone inducible construct we show that Hes3 does not induce a pigment cell differentiation program de novo, rather it maintains progenitor cells in an undifferentiated state, and as Hes3 expression subsides overtime these cells adopt a pigment cell fate. We demonstrate that mechanistically Hes3 mediates its activity through inhibition of Wnt/β-catenin signaling, a molecular pathway critical for neural crest specification and pigment cell lineage differentiation. We propose that Hes3 at the edge of the neural plate spatially restricts the response to mesoderm-derived Wnt ligands, thereby contributing to the establishment of sharp boundaries of gene expression at the neural plate border.

A gene regulatory network underlying the formation of pre-placodal ectoderm in Xenopus laevis

BACKGROUND

The neural plate border ectoderm gives rise to key developmental structures during embryogenesis, including the neural crest and the preplacodal ectoderm. Many sensory organs and ganglia of vertebrates develop from cranial placodes, which themselves arise from preplacodal ectoderm, defined by expression of transcription factor Six1 and its coactivator Eya1. Here we elucidate the gene regulatory network underlying the specification of the preplacodal ectoderm in Xenopus, and the functional interactions among transcription factors that give rise to this structure.

RESULTS

To elucidate the gene regulatory network upstream of preplacodal ectoderm formation, we use gain- and loss-of-function studies to explore the role of early ectodermal transcription factors for establishing the preplacodal ectoderm and adjacent ectodermal territories, and the role of Six1 and Eya1 in feedback regulation of these transcription factors. Our findings suggest that transcription factors with expression restricted to ventral (non-neural) ectoderm (AP2, Msx1, FoxI1, Vent2, Dlx3, GATA2) and those restricted to dorsal (neural) ectoderm (Pax3, Hairy2b, Zic1) are required for specification of both preplacodal ectoderm and neural crest in a context-dependent fashion and are cross-regulated by Eya1 and Six1.

CONCLUSION

These findings allow us to elucidate a detailed gene regulatory network at the neural plate border upstream of preplacodal ectoderm formation based on functional interactions between ectodermal transcription factors. We propose a new model to explain the formation of immediately juxtaposed preplacodal ectoderm and neural crest territories at the neural plate border, uniting previous models.

The neural border: Induction, specification and maturation of the territory that generates neural crest cells

The neural crest is induced at the edge between the neural plate and the nonneural ectoderm, in an area called the neural (plate) border, during gastrulation and neurulation. In recent years, many studies have explored how this domain is patterned, and how the neural crest is induced within this territory, that also participates to the prospective dorsal neural tube, the dorsalmost nonneural ectoderm, as well as placode derivatives in the anterior area. This review highlights the tissue interactions, the cell-cell signaling and the molecular mechanisms involved in this dynamic spatiotemporal patterning, resulting in the induction of the premigratory neural crest. Collectively, these studies allow building a complex neural border and early neural crest gene regulatory network, mostly composed by transcriptional regulations but also, more recently, including novel signaling interactions.

Control of neural crest induction by MarvelD3-mediated attenuation of JNK signalling

Tight junctions are required for the formation of tissue barriers and function as suppressors of signalling mechanisms that control gene expression and cell behaviour; however, little is known about the physiological and developmental importance of such signalling functions. Here, we demonstrate that depletion of MarvelD3, a transmembrane protein of tight junctions, disrupts neural crest formation and, consequently, development of neural crest-derived tissues during Xenopus embryogenesis. Using embryos and explant cultures combined with a small molecule inhibitor or mutant mRNAs, we show that MarvelD3 is required to attenuate JNK signalling during neural crest induction and that inhibition of JNK pathway activation is sufficient to rescue the phenotype induced by MarvelD3 depletion. Direct JNK stimulation disrupts neural crest development, supporting the importance of negative regulation of JNK. Our data identify the junctional protein MarvelD3 as an essential regulator of early vertebrate development and neural crest induction and, thereby, link tight junctions to the control and timing of JNK signalling during early development.

ERK-mediated phosphorylation regulates SOX10 sumoylation and targets expression in mutant BRAF melanoma

In human mutant BRAF melanoma cells, the stemness transcription factor FOXD3 is rapidly induced by inhibition of ERK1/2 signaling and mediates adaptive resistance to RAF inhibitors. However, the mechanism underlying ERK signaling control of FOXD3 expression remains unknown. Here we show that SOX10 is both necessary and sufficient for RAF inhibitor-induced expression of FOXD3 in mutant BRAF melanoma cells. SOX10 activates the transcription of FOXD3 by binding to a regulatory element in FOXD3 promoter. Phosphorylation of SOX10 by ERK inhibits its transcription activity toward multiple target genes by interfering with the sumoylation of SOX10 at K55, which is essential for its transcription activity. Finally, depletion of SOX10 sensitizes mutant BRAF melanoma cells to RAF inhibitors in vitro and in vivo. Thus, our work discovers a novel phosphorylation-dependent regulatory mechanism of SOX10 transcription activity and completes an ERK1/2/SOX10/FOXD3/ERBB3 axis that mediates adaptive resistance to RAF inhibitors in mutant BRAF melanoma cells.

Alteration of the Retinoid Acid-CBP Signaling Pathway in Neural Crest Induction Contributes to Enteric Nervous System Disorder

Hirschsprung Disease (HSCR) and/or hypoganglionosis are common pediatric disorders that arise from developmental deficiencies of enteric neural crest cells (ENCCs). Retinoid acid (RA) signaling has been shown to affect neural crest (NC) development. However, the mechanisms underlying RA deficiency-induced HSCR or hypoganglionosis are not well-defined. In this report, we found that in HSCR patient bowels, the RA nuclear receptor RARα and its interacting coregulator CREB-binding protein (CBP) were expressed in enteric neural plexuses in the normal ganglionic segment. However, the expression of these two genes was significantly inhibited in the pathological aganglionic segment. In a Xenopus laevis animal model, endogenous RARα interacted with CBP and was expressed in NC territory. Morpholino-mediated knockdown of RARα blocked expression of the NC marker genes Sox10 and FoxD3 and inhibited NC induction. The morphant embryos exhibited reduced nervous cells in the gastrointestinal anlage, a typical enteric nervous deficiency-associated phenotype. Injection of CBP mRNA rescued NC induction and reduced enteric nervous deficiency-associated phenotypes. Our work demonstrates that RARα regulates Sox10 expression via CBP during NC induction, and alteration of the RA-CBP signaling pathway may contribute to the development of enteric nervous system disorders.

Trunk neural crest origin of dermal denticles in a cartilaginous fish

Cartilaginous fishes (e.g., sharks and skates) possess a postcranial dermal skeleton consisting of tooth-like "denticles" embedded within their skin. As with teeth, the principal skeletal tissue of dermal denticles is dentine. In the head, cranial neural crest cells give rise to the dentine-producing cells (odontoblasts) of teeth. However, trunk neural crest cells are generally regarded as nonskeletogenic, and so the embryonic origin of trunk denticle odontoblasts remains unresolved. Here, we use expression of FoxD3 to pinpoint the specification and emigration of trunk neural crest cells in embryos of a cartilaginous fish, the little skate (Leucoraja erinacea). Using cell lineage tracing, we further demonstrate that trunk neural crest cells do, in fact, give rise to odontoblasts of trunk dermal denticles. These findings expand the repertoire of vertebrate trunk neural crest cell fates during normal development, highlight the likely primitive skeletogenic potential of this cell population, and point to a neural crest origin of dentine throughout the ancestral vertebrate dermal skeleton.

Implication of thyroid hormone signaling in neural crest cells migration: Evidence from thyroid hormone receptor beta knockdown and NH3 antagonist studies

Thyroid hormones (TH) have been mainly associated with post-embryonic development and adult homeostasis but few studies report direct experimental evidence for TH function at very early phases of embryogenesis. We assessed the outcome of altered TH signaling on early embryogenesis using the amphibian Xenopus as a model system. Precocious exposure to the TH antagonist NH-3 or impaired thyroid receptor beta function led to severe malformations related to neurocristopathies. These include pathologies with a broad spectrum of organ dysplasias arising from defects in embryonic neural crest cell (NCC) development. We identified a specific temporal window of sensitivity that encompasses the emergence of NCCs. Although the initial steps in NCC ontogenesis appeared unaffected, their migration properties were severely compromised both in vivo and in vitro. Our data describe a role for TH signaling in NCCs migration ability and suggest severe consequences of altered TH signaling during early phases of embryonic development.

Neural crest stem cells and their potential therapeutic applications

The neural crest (NC) is a remarkable transient structure generated during early vertebrate development. The neural crest progenitors have extensive migratory capacity and multipotency, harboring stem cell-like characteristics such as self-renewal. They can differentiate into a variety of cell types from craniofacial skeletal tissues to the trunk peripheral nervous system (PNS). Multiple regulators such as signaling factors, transcription factors, and migration machinery components are expressed at different stages of NC development. Gain- and loss-of-function studies in various vertebrate species revealed epistatic relationships of these molecules that could be assembled into a gene regulatory network defining the processes of NC induction, specification, migration, and differentiation. These basic developmental studies led to the subsequent establishment and molecular validation of neural crest stem cells (NCSCs) derived by various strategies. We provide here an overview of the isolation and characterization of NCSCs from embryonic, fetal, and adult tissues; the experimental strategies for the derivation of NCSCs from embryonic stem cells, induced pluripotent stem cells, and skin fibroblasts; and recent developments in the use of patient-derived NCSCs for modeling and treating neurocristopathies. We discuss future research on further refinement of the culture conditions required for the differentiation of pluripotent stem cells into axial-specific NC progenitors and their derivatives, developing non-viral approaches for the generation of induced NC cells (NCCs), and using a genomic editing approach to correct genetic mutations in patient-derived NCSCs for transplantation therapy. These future endeavors should facilitate the therapeutic applications of NCSCs in the clinical setting.

Ror2 signaling is required for local upregulation of GDF6 and activation of BMP signaling at the neural plate border

The receptor tyrosine kinase Ror2 is a major Wnt receptor that activates β-catenin-independent signaling and plays a conserved role in the regulation of convergent extension movements and planar cell polarity in vertebrates. Mutations in the ROR2 gene cause recessive Robinow syndrome in humans, a short-limbed dwarfism associated with craniofacial malformations. Here, we show that Ror2 is required for local upregulation of gdf6 at the neural plate border in Xenopus embryos. Ror2 morphant embryos fail to upregulate neural plate border genes and show defects in the induction of neural crest cell fate. These embryos lack the spatially restricted activation of BMP signaling at the neural plate border at early neurula stages, which is required for neural crest induction. Ror2-dependent planar cell polarity signaling is required in the dorsolateral marginal zone during gastrulation indirectly to upregulate the BMP ligand Gdf6 at the neural plate border and Gdf6 is sufficient to rescue neural plate border specification in Ror2 morphant embryos. Thereby, Ror2 links Wnt/planar cell polarity signaling to BMP signaling in neural plate border specification and neural crest induction.

Musculocontractural Ehlers-Danlos syndrome and neurocristopathies: dermatan sulfate is required for Xenopus neural crest cells to migrate and adhere to fibronectin

Of all live births with congenital anomalies, approximately one-third exhibit deformities of the head and face. Most craniofacial disorders are associated with defects in a migratory stem and progenitor cell population, which is designated the neural crest (NC). Musculocontractural Ehlers-Danlos syndrome (MCEDS) is a heritable connective tissue disorder with distinct craniofacial features; this syndrome comprises multiple congenital malformations that are caused by dysfunction of dermatan sulfate (DS) biosynthetic enzymes, including DS epimerase-1 (DS-epi1; also known as DSE). Studies in mice have extended our understanding of DS-epi1 in connective tissue maintenance; however, its role in fetal development is not understood. We demonstrate that DS-epi1 is important for the generation of isolated iduronic acid residues in chondroitin sulfate (CS)/DS proteoglycans in early Xenopus embryos. The knockdown of DS-epi1 does not affect the formation of early NC progenitors; however, it impairs the correct activation of transcription factors involved in the epithelial-mesenchymal transition (EMT) and reduces the extent of NC cell migration, which leads to a decrease in NC-derived craniofacial skeleton, melanocytes and dorsal fin structures. Transplantation experiments demonstrate a tissue-autonomous role for DS-epi1 in cranial NC cell migration in vivo Cranial NC explant and single-cell cultures indicate a requirement of DS-epi1 in cell adhesion, spreading and extension of polarized cell processes on fibronectin. Thus, our work indicates a functional link between DS and NC cell migration. We conclude that NC defects in the EMT and cell migration might account for the craniofacial anomalies and other congenital malformations in MCEDS, which might facilitate the diagnosis and development of therapies for this distressing condition. Moreover, the presented correlations between human DS-epi1 expression and gene sets of mesenchymal character, invasion and metastasis in neuroblastoma and malignant melanoma suggest an association between DS and NC-derived cancers.

Neural crest specification by inhibition of the ROCK/Myosin II pathway

Neural crest is a population of multipotent progenitor cells that form at the border of neural and non-neural ectoderm in vertebrate embryos, and undergo epithelial-mesenchymal transition and migration. According to the traditional view, the neural crest is specified in early embryos by signaling molecules including BMP, FGF, and Wnt proteins. Here, we identify a novel signaling pathway leading to neural crest specification, which involves Rho-associated kinase (ROCK) and its downstream target nonmuscle Myosin II. We show that ROCK inhibitors promote differentiation of human embryonic stem cells (hESCs) into neural crest-like progenitors (NCPs) that are characterized by specific molecular markers and ability to differentiate into multiple cell types, including neurons, chondrocytes, osteocytes, and smooth muscle cells. Moreover, inhibition of Myosin II was sufficient for generating NCPs at high efficiency. Whereas Myosin II has been previously implicated in the self-renewal and survival of hESCs, we demonstrate its role in neural crest development during ESC differentiation. Inhibition of this pathway in Xenopus embryos expanded neural crest in vivo, further indicating that neural crest specification is controlled by ROCK-dependent Myosin II activity. We propose that changes in cell morphology in response to ROCK and Myosin II inhibition initiate mechanical signaling leading to neural crest fates.

Growth-Arrest-Specific 7 Gene Regulates Neural Crest Formation and Craniofacial Development in Zebrafish

Growth-arrest-specific 7 (Gas7) is preferentially expressed in the nervous system and plays an important role during neuritogenesis in vertebrates. We recently demonstrated that gas7 is highly expressed in zebrafish neurons, where it regulates neural development. The possibility that gas7 may also regulate the development of other tissues remains to be examined. In this study, we investigate the role of Gas7 in the development of craniofacial tissues. Knockdown of gas7 using morpholino oligomers produced abnormal phenotypes in neural crest (NC) cells and their derivatives. NC-derived cartilage maturation was altered in Gas7 morphants as revealed by aberrant sox9b and dlx2 expression, a phenotype that could be rescued by coinjection of gas7 mRNA. While rhombomere morphology remained normal in Gas7 morphants, we observed reduced expression of the prechondrogenic genes sox9b and dlx2 in cells populating the posterior pharyngeal arches, but the fundamental structure of pharyngeal arches was preserved. In addition, NC cell sublineages that migrate to form neurons, glial cells, and melanocytes were altered in Gas7 morphants as revealed by aberrant expression of neurod, foxd3, and mitfa, respectively. Development of NC progenitors was also examined in Gas7 morphants at 12 hpf, and we observed that the reduction of cell precursors in Gas7 morphants was due to increased apoptosis level. These results indicate that the formation of NC progenitors and derivatives depends on Gas7 expression. Our observations also suggest that Gas7 regulates the formation of NC derivatives constituting the internal tissues of pharyngeal arches, without affecting the fundamental structure of mesodermal-derived pharyngeal arches.

Neural crest specification by Prohibitin1 depends on transcriptional regulation of prl3 and vangl1

A complex network of transcription factors regulates specification of neural crest cells at early neurula stage by stabilizing neural crest identity and activating neural crest effector genes so that distinct subpopulations evolve. In this network, c-myc acts on top of the gene hierarchy controlling snail2, AP2 and prohibitin1 (phb1) expression. While snail2 and AP2 are well studied neural crest specifier genes little is known about the role of phb1 in this process. To identify phb1 regulated genes we analyzed the transcriptome of neural crest explants of phb1 morphant Xenopus embryos. Among 147 phb1 regulated genes we identified the membrane-associated protein-tyrosine phosphatase PRP4A3 (prl3) and the atypical cadherin and Wnt-PCP component van gogh like1 (vangl1). Gain of function, loss of function and epistasis experiments allowed us to allocate both genes in the neural crest specification network between phb1 and twist. Interestingly, both, vangl1 and prl3 regulate only a small subset of neural crest marker genes. The identification of two membrane-associated proteins as novel neural crest specifiers indicates that in addition to gene regulation by combinatory effects of transcription factors also post-translational modifications (prl3) and cell-cell adhesion and/or regulation of cell-polarity (vangl1) specify the identity of neural crest cell populations. genesis 53:627-639, 2015. © 2015 Wiley Periodicals, Inc.

PAX3 and FOXD3 Promote CXCR4 Expression in Melanoma

Metastatic melanoma is an aggressive and deadly disease. The chemokine receptor CXCR4 is active in melanoma metastasis, although the mechanism for the promotion and maintenance of CXCR4 expression in these cells is mostly unknown. Here, we find melanoma cells express two CXCR4 isoforms, the common version and a variant that is normally restricted to cells during development or to mature blood cells. CXCR4 expression is driven through a highly conserved intronic enhancer element by the transcription factors PAX3 and FOXD3. Inhibition of these transcription factors slows melanoma cell growth, migration, and motility, as well as reduces CXCR4 expression. Overexpression of these transcription factors drives the production of increased CXCR4 levels. Loss of PAX3 and FOXD3 transcription factor activity results in a reduction in cell motility, migration, and chemotaxis, all of which are rescued by CXCR4 overexpression. Here, we discover a molecular pathway wherein PAX3 and FOXD3 promote CXCR4 gene expression in melanoma.

Animal models for studying neural crest development: is the mouse different?

The neural crest is a uniquely vertebrate cell type and has been well studied in a number of model systems. Zebrafish, Xenopus and chick embryos largely show consistent requirements for specific genes in early steps of neural crest development. By contrast, knockouts of homologous genes in the mouse often do not exhibit comparable early neural crest phenotypes. In this Spotlight article, we discuss these species-specific differences, suggest possible explanations for the divergent phenotypes in mouse and urge the community to consider these issues and the need for further research in complementary systems.

The interactions of genes, age, and environment in glaucoma pathogenesis

Glaucoma, a progressive degenerative condition that results in the death of retinal ganglion cells, is one of the leading causes of blindness, affecting millions worldwide. The mechanisms underlying glaucoma are not well understood, although years of studies have shown that the largest risk factors are elevated intraocular pressure, age, and genetics. Eleven genes and multiple loci have been identified as contributing factors. These genes act by a number of mechanisms, including mechanical stress, ischemic/oxidative stress, and neurodegeneration. We summarize the recent advances in the understanding of glaucoma and propose a unified hypothesis for glaucoma pathogenesis. Glaucoma does not result from a single pathological mechanism, but rather a combination of pathways that are influenced by genes, age, and environment. In particular, we hypothesize that, in the presence of genetic risk factors, exposure to environment stresses results in an earlier age of onset for glaucoma. This hypothesis is based upon the overlap of the molecular pathways in which glaucoma genes are involved. Because of the interactions between these processes, it is likely that there are common therapies that may be effective for different subtypes of glaucoma.

Xenopus Nkx6.3 is a neural plate border specifier required for neural crest development

In vertebrates, the neural plate border (NPB) is established by a group of transcription factors including Dlx3, Msx1 and Zic1. The crosstalk between these NPB specifiers governs the separation of the NPB region into placode and neural crest (NC) territories and also their further differentiation. Understanding the mechanisms of NPB formation and NC development is critical for our knowledge of related human diseases. Here we identified Nkx6.3, a transcription factor of the Nkx family, as a new NPB specifier required for neural crest development in Xenopus embryos. XNkx6.3 is expressed in the ectoderm of the neural plate border region at neurula stages, covering the epidermis, placode and neural crest territories, but not the neural plate. Inhibition of Nkx6.3 by dominant negative construct or specific morpholino leads to neural crest defects, while overexpression of Nkx6.3 induces ectopic neural crest in the anterior neural fold. In animal caps, Nkx6.3 alone is able to initiate the whole neural crest regulatory network and induces neural crest fate robustly. We showed that overexpression of Nkx6.3 affects multiple signaling pathways, creating a high-Wnt, low-BMP environment required for neural crest development. Gain- and loss-of-function of Nkx6.3 have compound effects on the expression of known NPB genes, which is largely opposite to that of Dlx3. Overexpression of Dlx3 blocks the NC inducing activity of Nkx6.3. The crosstalk between Nkx6.3, Dlx3 and Msx1 is likely crucial for proper NPB formation and neural crest development in Xenopus.

The neural crest, a multifaceted structure of the vertebrates

In this review, several features of the cells originating from the lateral borders of the primitive neural anlagen, the neural crest (NC) are considered. Among them, their multipotentiality, which together with their migratory properties, leads them to colonize the developing body and to participate in the development of many tissues and organs. The in vitro analysis of the developmental capacities of single NC cells (NCC) showed that they present several analogies with the hematopoietic cells whose differentiation involves the activity of stem cells endowed with different arrays of developmental potentialities. The permanence of such NC stem cells in the adult organism raises the problem of their role at that stage of life. The NC has appeared during evolution in the vertebrate phylum and is absent in their Protocordates ancestors. The major role of the NCC in the development of the vertebrate head points to a critical role for this structure in the remarkable diversification and radiation of this group of animals.

In vivo collective cell migration requires an LPAR2-dependent increase in tissue fluidity

Collective cell migration (CCM) and epithelial-mesenchymal transition (EMT) are common to cancer and morphogenesis, and are often considered to be mutually exclusive in spite of the fact that many cancer and embryonic cells that have gone through EMT still cooperate to migrate collectively. Here we use neural crest (NC) cells to address the question of how cells that have down-regulated cell-cell adhesions can migrate collectively. NC cell dissociation relies on a qualitative and quantitative change of the cadherin repertoire. We found that the level of cell-cell adhesion is precisely regulated by internalization of N-cadherin downstream of lysophosphatidic acid (LPA) receptor 2. Rather than promoting the generation of single, fully mesenchymal cells, this reduction of membrane N-cadherin only triggers a partial mesenchymal phenotype. This intermediate phenotype is characterized by an increase in tissue fluidity akin to a solid-like-to-fluid-like transition. This change of plasticity allows cells to migrate under physical constraints without abolishing cell cooperation required for collectiveness.

The Fox/Forkhead transcription factor family of the hemichordate Saccoglossus kowalevskii

Background

The Fox gene family is a large family of transcription factors that arose early in organismal evolution dating back to at least the common ancestor of metazoans and fungi. They are key components of many gene regulatory networks essential for embryonic development. Although much is known about the role of Fox genes during vertebrate development, comprehensive comparative studies outside vertebrates are sparse. We have characterized the Fox transcription factor gene family from the genome of the enteropneust hemichordate Saccoglossus kowalevskii, including phylogenetic analysis, genomic organization, and expression analysis during early development. Hemichordates are a sister group to echinoderms, closely related to chordates and are a key group for tracing the evolution of gene regulatory mechanisms likely to have been important in the diversification of the deuterostome phyla.

Results

Of the 22 Fox gene families that were likely present in the last common ancestor of all deuterostomes, S. kowalevskii has a single ortholog of each group except FoxH, which we were unable to detect, and FoxQ2, which has three paralogs. A phylogenetic analysis of the FoxQ2 family identified an ancestral duplication in the FoxQ2 lineage at the base of the bilaterians. The expression analyses of all 23 Fox genes of S. kowalevskii provide insights into the evolution of components of the regulatory networks for the development of pharyngeal gill slits (foxC, foxL1, and foxI), mesoderm patterning (foxD, foxF, foxG), hindgut development (foxD, foxI), cilia formation (foxJ1), and patterning of the embryonic apical territory (foxQ2).

Conclusions

Comparisons of our results with data from echinoderms, chordates, and other bilaterians help to develop hypotheses about the developmental roles of Fox genes that likely characterized ancestral deuterostomes and bilaterians, and more recent clade-specific innovations.

FoxD3 regulates cranial neural crest EMT via downregulation of tetraspanin18 independent of its functions during neural crest formation

The scaffolding protein tetraspanin18 (Tspan18) maintains epithelial cadherin-6B (Cad6B) to antagonize chick cranial neural crest epithelial-to-mesenchymal transition (EMT). For migration to take place, Tspan18 must be downregulated. Here, we characterize the role of the winged-helix transcription factor FoxD3 in the control of Tspan18 expression. Although we previously found that Tspan18 mRNA persists several hours past the stage it would normally be downregulated in FoxD3-deficient neural folds, we now show that Tspan18 expression eventually declines. This indicates that while FoxD3 is crucial for initial downregulation of Tspan18, other factors subsequently impact Tspan18 expression. Remarkably, the classical EMT transcription factor Snail2 is not one of these factors. As in other vertebrates, FoxD3 is required for chick cranial neural crest specification and migration, however, FoxD3 has surprisingly little impact on chick cranial neural crest cell survival. Strikingly, Tspan18 knockdown rescues FoxD3-dependent neural crest migration defects, although neural crest specification is still deficient. This indicates that FoxD3 promotes cranial neural crest EMT by eliciting Tspan18 downregulation separable from its Tspan18-independent activity during neural crest specification and survival.

Stem cell-dependent formation of a functional anterior regeneration pole in planarians requires Zic and Forkhead transcription factors

Planarians can regenerate their head within days. This process depends on the direction of adult stem cells to wound sites and the orchestration of their progenitors to commit to appropriate lineages and to arrange into patterned tissues. We identified a zinc finger transcription factor, Smed-ZicA, as a downstream target of Smed-FoxD, a Forkhead transcription factor required for head regeneration. Smed-zicA and Smed-FoxD are co-expressed with the Wnt inhibitor notum and the Activin inhibitor follistatin in a cluster of cells at the anterior-most tip of the regenerating head - the anterior regeneration pole - and in surrounding stem cell progeny. Depletion of Smed-zicA and Smed-FoxD by RNAi abolishes notum and follistatin expression at the pole and inhibits head formation downstream of initial polarity decisions. We suggest a model in which ZicA and FoxD transcription factors synergize to control the formation of Notum- and Follistatin-producing anterior pole cells. Pole formation might constitute an early step in regeneration, resulting in a signaling center that orchestrates cellular events in the growing tissue.

Establishing the pre-placodal region and breaking it into placodes with distinct identities

Specialized sensory organs in the vertebrate head originate from thickenings in the embryonic ectoderm called cranial sensory placodes. These placodes, as well as the neural crest, arise from a zone of ectoderm that borders the neural plate. This zone separates into a precursor field for the neural crest that lies adjacent to the neural plate, and a precursor field for the placodes, called the pre-placodal region (PPR), that lies lateral to the neural crest. The neural crest domain and the PPR are established in response to signaling events mediated by BMPs, FGFs and Wnts, which differentially activate transcription factors in these territories. In the PPR, members of the Six and Eya families, act in part to repress neural crest specific transcription factors, thus solidifying a placode developmental program. Subsequently, in response to environmental cues the PPR is further subdivided into placodal territories with distinct characteristics, each expressing a specific repertoire of transcription factors that provide the necessary information for their progression to mature sensory organs. In this review we summarize recent advances in the characterization of the signaling molecules and transcriptional effectors that regulate PPR specification and its subdivision into placodal domains with distinct identities.

Setting appropriate boundaries: fate, patterning and competence at the neural plate border

The neural crest and craniofacial placodes are two distinct progenitor populations that arise at the border of the vertebrate neural plate. This border region develops through a series of inductive interactions that begins before gastrulation and progressively divide embryonic ectoderm into neural and non-neural regions, followed by the emergence of neural crest and placodal progenitors. In this review, we describe how a limited repertoire of inductive signals-principally FGFs, Wnts and BMPs-set up domains of transcription factors in the border region which establish these progenitor territories by both cross-inhibitory and cross-autoregulatory interactions. The gradual assembly of different cohorts of transcription factors that results from these interactions is one mechanism to provide the competence to respond to inductive signals in different ways, ultimately generating the neural crest and cranial placodes.

A novel N-terminal motif is responsible for the evolution of neural crest-specific gene-regulatory activity in vertebrate FoxD3

The neural crest is unique to vertebrates and has allowed the evolution of their complicated craniofacial structures. During vertebrate evolution, the acquisition of the neural crest must have been accompanied by the emergence of a new gene regulatory network (GRN). Here, to investigate the role of protein evolution in the emergence of the neural crest GRN, we examined the neural crest cell (NCC) differentiation-inducing activity of chordate FoxD genes. Amphioxus and vertebrate (Xenopus) FoxD proteins both exhibited transcriptional repressor activity in Gal4 transactivation assays and bound to similar DNA sequences in vitro. However, whereas vertebrate FoxD3 genes induced the differentiation of ectopic NCCs when overexpressed in chick neural tube, neither amphioxus FoxD nor any other vertebrate FoxD paralogs exhibited this activity. Experiments using chimeric proteins showed that the N-terminal portion of the vertebrate FoxD3 protein is critical to its NCC differentiation-inducing activity. Furthermore, replacement of the N-terminus of amphioxus FoxD with a 39-amino-acid segment from zebrafish FoxD3 conferred neural crest-inducing activity on amphioxus FoxD or zebrafish FoxD1. Therefore, fixation of this N-terminal amino acid sequence may have been crucial in the evolutionary recruitment of FoxD3 to the vertebrate neural crest GRN.

Distinct functional and temporal requirements for zebrafish Hdac1 during neural crest-derived craniofacial and peripheral neuron development

The regulation of gene expression is accomplished by both genetic and epigenetic means and is required for the precise control of the development of the neural crest. In hdac1(b382) mutants, craniofacial cartilage development is defective in two distinct ways. First, fewer hoxb3a, dlx2 and dlx3-expressing posterior branchial arch precursors are specified and many of those that are consequently undergo apoptosis. Second, in contrast, normal numbers of progenitors are present in the anterior mandibular and hyoid arches, but chondrocyte precursors fail to terminally differentiate. In the peripheral nervous system, there is a disruption of enteric, DRG and sympathetic neuron differentiation in hdac1(b382) mutants compared to wildtype embryos. Specifically, enteric and DRG-precursors differentiate into neurons in the anterior gut and trunk respectively, while enteric and DRG neurons are rarely present in the posterior gut and tail. Sympathetic neuron precursors are specified in hdac1(b382) mutants and they undergo generic neuronal differentiation but fail to undergo noradrenergic differentiation. Using the HDAC inhibitor TSA, we isolated enzyme activity and temporal requirements for HDAC function that reproduce hdac1(b382) defects in craniofacial and sympathetic neuron development. Our study reveals distinct functional and temporal requirements for zebrafish hdac1 during neural crest-derived craniofacial and peripheral neuron development.

A dynamic code of dorsal neural tube genes regulates the segregation between neurogenic and melanogenic neural crest cells

Understanding when and how multipotent progenitors segregate into diverse fates is a key question during embryonic development. The neural crest (NC) is an exemplary model system with which to investigate the dynamics of progenitor cell specification, as it generates a multitude of derivatives. Based on 'in ovo' lineage analysis, we previously suggested an early fate restriction of premigratory trunk NC to generate neural versus melanogenic fates, yet the timing of fate segregation and the underlying mechanisms remained unknown. Analysis of progenitors expressing a Foxd3 reporter reveals that prospective melanoblasts downregulate Foxd3 and have already segregated from neural lineages before emigration. When this downregulation is prevented, late-emigrating avian precursors fail to upregulate the melanogenic markers Mitf and MC/1 and the guidance receptor Ednrb2, generating instead glial cells that express P0 and Fabp. In this context, Foxd3 lies downstream of Snail2 and Sox9, constituting a minimal network upstream of Mitf and Ednrb2 to link melanogenic specification with migration. Consistent with the gain-of-function data in avians, loss of Foxd3 function in mouse NC results in ectopic melanogenesis in the dorsal tube and sensory ganglia. Altogether, Foxd3 is part of a dynamically expressed gene network that is necessary and sufficient to regulate fate decisions in premigratory NC. Their timely downregulation in the dorsal neural tube is thus necessary for the switch between neural and melanocytic phases of NC development.

Conserved structural domains in FoxD4L1, a neural forkhead box transcription factor, are required to repress or activate target genes

FoxD4L1 is a forkhead transcription factor that expands the neural ectoderm by down-regulating genes that promote the onset of neural differentiation and up-regulating genes that maintain proliferative neural precursors in an immature state. We previously demonstrated that binding of Grg4 to an Eh-1 motif enhances the ability of FoxD4L1 to down-regulate target neural genes but does not account for all of its repressive activity. Herein we analyzed the protein sequence for additional interaction motifs and secondary structure. Eight conserved motifs were identified in the C-terminal region of fish and frog proteins. Extending the analysis to mammals identified a high scoring motif downstream of the Eh-1 domain that contains a tryptophan residue implicated in protein-protein interactions. In addition, secondary structure prediction programs predicted an α-helical structure overlapping with amphibian-specific Motif 6 in Xenopus, and similarly located α-helical structures in other vertebrate FoxD proteins. We tested functionality of this site by inducing a glutamine-to-proline substitution expected to break the predicted α-helical structure; this significantly reduced FoxD4L1’s ability to repress zic3 and irx1. Because this mutation does not interfere with Grg4 binding, these results demonstrate that at least two regions, the Eh-1 motif and a more C-terminal predicted α-helical/Motif 6 site, additively contribute to repression. In the N-terminal region we previously identified a 14 amino acid motif that is required for the up-regulation of target genes. Secondary structure prediction programs predicted a short β-strand separating two acidic domains. Mutant constructs show that the β-strand itself is not required for transcriptional activation. Instead, activation depends upon a glycine residue that is predicted to provide sufficient flexibility to bring the two acidic domains into close proximity. These results identify conserved predicted motifs with secondary structures that enable FoxD4L1 to carry out its essential functions as both a transcriptional repressor and activator of neural genes.

Tetraspanin18 is a FoxD3-responsive antagonist of cranial neural crest epithelial-to-mesenchymal transition that maintains cadherin-6B protein

During epithelial-to-mesenchymal transition (EMT), tightly associated, polarized epithelial cells become individual mesenchymal cells capable of migrating. Here, we investigate the role of the transmembrane protein tetraspanin18 (Tspan18) in chick cranial neural crest EMT. Tspan18 mRNA is expressed in premigratory cranial neural crest cells, but is absent from actively migrating neural crest cells. Tspan18 knockdown leads to a concomitant loss of cadherin-6B (Cad6B) protein, whereas Cad6B protein persists when Tspan18 expression is extended. The temporal profile of Cad6B mRNA downregulation is unaffected in these embryos, which indicates that Tspan18 maintains Cad6B protein levels and reveals that Cad6B is regulated by post-translational mechanisms. Although downregulation of Tspan18 is necessary, it is not sufficient for neural crest migration: the timing of neural crest emigration, basal lamina breakdown and Cad7 upregulation proceed normally in Tspan18-deficient cells. This emphasizes the need for coordinated transcriptional and post-translational regulation of Cad6B during EMT and illustrates that Tspan18-antagonized remodeling of cell-cell adhesions is only one step in preparation for cranial neural crest migration. Unlike Cad6B, which is transcriptionally repressed by Snail2, Tspan18 expression is downstream of the winged-helix transcription factor FoxD3, providing a new transcriptional input into cranial neural crest EMT. Together, our data reveal post-translational regulation of Cad6B protein levels by Tspan18 that must be relieved by a FoxD3-dependent mechanism in order for cranial neural crest cells to migrate. These results offer new insight into the molecular mechanisms of cranial neural crest EMT and expand our understanding of tetraspanin function relevant to metastasis.