Neural crest specification by noncanonical Wnt signaling and PAR-1

Neural crest cells development and neuroblastoma progression: Role of Wnt signaling

Neuroblastoma (NB) is one of the most common heterogeneous extracranial cancers in infancy that arises from neural crest (NC) cells of the sympathetic nervous system. The Wnt signaling pathway, both canonical and noncanonical pathway, is a highly conserved signaling pathway that regulates the development and differentiation of the NC cells during embryogenesis. Reports suggest that aberrant activation of Wnt ligands/receptors in Wnt signaling pathways promote progression and relapse of NB. Wnt signaling pathways regulate NC induction and migration in a similar manner; it regulates proliferation and metastasis of NB. Inhibiting the Wnt signaling pathway or its ligands/receptors induces apoptosis and abrogates proliferation and tumorigenicity in all major types of NB cells. Here, we comprehensively discuss the Wnt signaling pathway and its mechanisms in regulating the development of NC and NB pathogenesis. This review highlights the implications of aberrant Wnt signaling in the context of etiology, progression, and relapse of NB. We have also described emerging strategies for Wnt-based therapies against the progression of NB that will provide new insights into the development of Wnt-based therapeutic strategies for NB.

The homeodomain transcription factor Ventx2 regulates respiratory progenitor cell number and differentiation timing during Xenopus lung development

Ventx2 is an Antennapedia superfamily/NK-like subclass homeodomain transcription factor best known for its roles in the regulation of early dorsoventral patterning during Xenopus gastrulation and in the maintenance of neural crest multipotency. In this work we characterize the spatiotemporal expression pattern of ventx2 in progenitor cells of the Xenopus respiratory system epithelium. We find that ventx2 is directly induced by BMP signaling in the ventral foregut prior to nkx2-1, the earliest epithelial marker of the respiratory lineage. Functional studies demonstrate that Ventx2 regulates the number of Nkx2-1/Sox9+ respiratory progenitor cells induced during foregut development, the timing and level of surfactant protein gene expression, and proper tracheal-esophageal separation. Our data suggest that Ventx2 regulates the balance of respiratory progenitor cell expansion and differentiation. While the ventx gene family has been lost from the mouse genome during evolution, humans have retained a ventx2-like gene (VENTX). Finally, we discuss how our findings might suggest a possible function of VENTX in human respiratory progenitor cells.

Expression atlas of avian neural crest proteins: Neurulation to migration

Neural crest (NC) cells are a dynamic population of embryonic stem cells that create various adult tissues in vertebrate species including craniofacial bone and cartilage and the peripheral and enteric nervous systems. NC development is thought to be a conserved and complex process that is controlled by a tightly-regulated gene regulatory network (GRN) of morphogens, transcription factors, and cell adhesion proteins. While multiple studies have characterized the expression of several GRN factors in single species, a comprehensive protein analysis that directly compares expression across development is lacking. To address this lack in information, we used three closely related avian models, Gallus gallus (chicken), Coturnix japonica (Japanese quail), and Pavo cristatus (Indian peafowl), to compare the localization and timing of four GRN transcription factors, PAX7, SNAI2, SOX9, and SOX10, from the onset of neurulation to migration. While the spatial expression of these factors is largely conserved, we find that quail NC cells express SNAI2, SOX9, and SOX10 proteins at the equivalent of earlier developmental stages than chick and peafowl. In addition, quail NC cells migrate farther and more rapidly than the larger organisms. These data suggest that despite a conservation of NC GRN players, differences in the timing of NC development between species remain a significant frontier to be explored with functional studies.

WNT11-FZD7-DAAM1 signalling supports tumour initiating abilities and melanoma amoeboid invasion

Melanoma is a highly aggressive tumour that can metastasize very early in disease progression. Notably, melanoma can disseminate using amoeboid invasive strategies. We show here that high Myosin II activity, high levels of ki-67 and high tumour-initiating abilities are characteristic of invasive amoeboid melanoma cells. Mechanistically, we find that WNT11-FZD7-DAAM1 activates Rho-ROCK1/2-Myosin II and plays a crucial role in regulating tumour-initiating potential, local invasion and distant metastasis formation. Importantly, amoeboid melanoma cells express both proliferative and invasive gene signatures. As such, invasive fronts of human and mouse melanomas are enriched in amoeboid cells that are also ki-67 positive. This pattern is further enhanced in metastatic lesions. We propose eradication of amoeboid melanoma cells after surgical removal as a therapeutic strategy.

Pinhead signaling regulates mesoderm heterogeneity via the FGF receptor-dependent pathway

Among the three embryonic germ layers, the mesoderm plays a central role in the establishment of the vertebrate body plan. The mesoderm is specified by secreted signaling proteins from the FGF, Nodal, BMP and Wnt families. No new classes of extracellular mesoderm-inducing factors have been identified in more than two decades. Here, we show that the pinhead (pnhd) gene encodes a secreted protein that is essential for the activation of a subset of mesodermal markers in the Xenopus embryo. RNA sequencing revealed that many transcriptional targets of Pnhd are shared with those of the FGF pathway. Pnhd activity was accompanied by Erk phosphorylation and required FGF and Nodal but not Wnt signaling. We propose that during gastrulation Pnhd acts in the marginal zone to contribute to mesoderm heterogeneity via an FGF receptor-dependent positive feedback mechanism.

Wnt Signaling in Neural Crest Ontogenesis and Oncogenesis

Neural crest (NC) cells are a temporary population of multipotent stem cells that generate a diverse array of cell types, including craniofacial bone and cartilage, smooth muscle cells, melanocytes, and peripheral neurons and glia during embryonic development. Defective neural crest development can cause severe and common structural birth defects, such as craniofacial anomalies and congenital heart disease. In the early vertebrate embryos, NC cells emerge from the dorsal edge of the neural tube during neurulation and then migrate extensively throughout the anterior-posterior body axis to generate numerous derivatives. Wnt signaling plays essential roles in embryonic development and cancer. This review summarizes current understanding of Wnt signaling in NC cell induction, delamination, migration, multipotency, and fate determination, as well as in NC-derived cancers.

Williams Syndrome, Human Self-Domestication, and Language Evolution

Language evolution resulted from changes in our biology, behavior, and culture. One source of these changes might be human self-domestication. Williams syndrome (WS) is a clinical condition with a clearly defined genetic basis which results in a distinctive behavioral and cognitive profile, including enhanced sociability. In this paper we show evidence that the WS phenotype can be satisfactorily construed as a hyper-domesticated human phenotype, plausibly resulting from the effect of the WS hemideletion on selected candidates for domestication and neural crest (NC) function. Specifically, we show that genes involved in animal domestication and NC development and function are significantly dysregulated in the blood of subjects with WS. We also discuss the consequences of this link between domestication and WS for our current understanding of language evolution.

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 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.

Regulation of neural crest development by the formin family protein Daam1

The neural crest (NC) multipotent progenitor cells form at the neural plate border and migrate to diverse locations in the embryo to differentiate into many cell types. NC is specified by several embryonic pathways, however the role of noncanonical Wnt signaling in this process remains poorly defined. Daam1 is a formin family protein that is present in embryonic ectoderm at the time of NC formation and can mediate noncanonical Wnt signaling. Our interference experiments indicated that Daam1 is required for NC gene activation. To further study the function of Daam1 in NC development we used a transgenic reporter Xenopus line, in which GFP transcription is driven by sox10 upstream regulatory sequences. The activation of the sox10:GFP reporter in a subset of NC cells was suppressed after Daam1 depletion and in embryos expressing N-Daam1, a dominant interfering construct. Moreover, N-Daam1 blocked reporter activation in neuralized ectodermal explants in response to Wnt11, but not Wnt8 or Wnt3a, confirming that the downstream pathways are different. In complementary experiments, a constitutively active Daam1 fragment expanded the NC territory, but this gain-of-function activity was eliminated in a construct with a point mutation in the FH2 domain that is critical for actin polymerization. These observations suggest a new role of Daam1 and actin remodeling in NC specification.

Neural crest development in Xenopus requires Protocadherin 7 at the lateral neural crest border

In vertebrates, the neural crest is a unique population of pluripotent cells whose development is dependent on signaling from neighboring tissues. Cadherin family members, including protocadherins, are emerging as major players in neural crest development, largely through their roles in cell adhesion and sorting in embryonic tissues. Here, we show that Protocadherin 7 (Pcdh7), previously shown to function in sensorial layer integrity and neural tube closure in Xenopus, is also involved in neural crest specification and survival. Pcdh7 expression partly overlaps the neural crest domain at the lateral neural crest border. Pcdh7 knockdown in embryos does not alter neural crest induction; however, neural crest specification markers, including Snail2 and Sox9, are lost, due to apoptosis of the neural crest starting after stage 13. Pcdh7 knockdown also results in downregulation of Wnt11b; both of which are co-expressed in the sensorial layer lateral to the neural crest, suggestive of a role for Wnt11b in the neural crest apoptosis. Confirming this role, apoptosis, Snail2 expression and the developmental fate of the neural crest can be partially rescued by ectopic expression of Wnt11b. These results indicate that Pcdh7 plays an important role in maintaining the sensorial layer at the lateral neural crest border, which is necessary for the secretion of survival factors, including Wnt11b.

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.

Signaling pathways and tissue interactions in neural plate border formation

The neural crest is a transient cell population that gives rise to various cell types of multiple tissues and organs in the vertebrate embryo. Neural crest cells arise from the neural plate border, a region localized at the lateral borders of the prospective neural plate. Temporally and spatially coordinated interaction with the adjacent tissues, the non-neural ectoderm, the neural plate and the prospective dorsolateral mesoderm, is required for neural plate border specification. Signaling molecules, namely BMP, Wnt and FGF ligands and corresponding antagonists are derived from these tissues and interact to induce the expression of neural plate border specific genes. The present mini-review focuses on the current understanding of how the NPB territory is formed and accentuates the need for coordinated interaction of BMP and Wnt signaling pathways and precise tissue communication that are required for the definition of the prospective NC in the competent ectoderm.

ROR-Family Receptor Tyrosine Kinases

ROR-family receptor tyrosine kinases form a small subfamily of receptor tyrosine kinases (RTKs), characterized by a conserved, unique domain architecture. ROR RTKs are evolutionary conserved throughout the animal kingdom and act as alternative receptors and coreceptors of WNT ligands. The intracellular signaling cascades activated downstream of ROR receptors are diverse, including but not limited to ROR-Frizzled-mediated activation of planar cell polarity signaling, RTK-like signaling, and antagonistic regulation of WNT/β-Catenin signaling. In line with their diverse repertoire of signaling functions, ROR receptors are involved in the regulation of multiple processes in embryonic development such as development of the axial and paraxial mesoderm, the nervous system and the neural crest, the axial and appendicular skeleton, and the kidney. In humans, mutations in the ROR2 gene cause two distinct developmental syndromes, recessive Robinow syndrome (RRS; MIM 268310) and dominant brachydactyly type B1 (BDB1; MIM 113000). In Robinow syndrome patients and animal models, the development of multiple organs is affected, whereas BDB1 results only in shortening of the distal phalanges of fingers and toes, reflecting the diversity of functions and signaling activities of ROR-family RTKs. In this chapter, we give an overview on ROR receptor structure and function. We discuss their signaling functions and role in vertebrate embryonic development with a focus on those developmental processes that are affected by mutations in the ROR2 gene in human patients.

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.

A PTK7/Ror2 Co-Receptor Complex Affects Xenopus Neural Crest Migration

Neural crest cells are a highly migratory pluripotent cell population that generates a wide array of different cell types and failure in their migration can result in severe birth defects and malformation syndromes. Neural crest migration is controlled by various means including chemotaxis, repellent guidance cues and cell-cell interaction. Non-canonical Wnt PCP (planar cell polarity) signaling has previously been shown to control cell-contact mediated neural crest cell guidance. PTK7 (protein tyrosine kinase 7) is a transmembrane pseudokinase and a known regulator of Wnt/PCP signaling, which is expressed in Xenopus neural crest cells and required for their migration. PTK7 functions as a Wnt co-receptor; however, it remains unclear by which means PTK7 affects neural crest migration. Expressing fluorescently labeled proteins in Xenopus neural crest cells we find that PTK7 co-localizes with the Ror2 Wnt-receptor. Further, co-immunoprecipitation experiments demonstrate that PTK7 interacts with Ror2. The PTK7/Ror2 interaction is likely relevant for neural crest migration, because Ror2 expression can rescue the PTK7 loss of function migration defect. Live cell imaging of explanted neural crest cells shows that PTK7 loss of function affects the formation of cell protrusions as well as cell motility. Co-expression of Ror2 can rescue these defects. In vivo analysis demonstrates that a kinase dead Ror2 mutant cannot rescue PTK7 loss of function. Thus, our data suggest that Ror2 can substitute for PTK7 and that the signaling function of its kinase domain is required for this effect.

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.

Ror2 receptor mediates Wnt11 ligand signaling and affects convergence and extension movements in zebrafish

The receptor-tyrosine kinase Ror2 acts as an alternative receptor or co-receptor for Wnt5a and mediates Wnt5a-induced convergent extension movements during embryogenesis in mice and Xenopus as well as the polarity and migration of several cell types during development. However, little is known about whether Ror2 function is conserved in other vertebrates or is involved in other non-canonical Wnt ligands in vivo. In this study we demonstrated that overexpression of dominant-negative ror2 (ror2-TM) mRNA in zebrafish embryos resulted in convergence and extension defects and incompletely separated eyes, which is consistent with observations from slb/wnt11 mutants or wnt11 knockdown morphants. Moreover, the co-injection of ror2-TM mRNA and a wnt11 morpholino or the coexpression of ror2 and wnt11 in zebrafish embryos synergetically induced more severe convergence and extension defects. Transplantation studies further demonstrated that the Ror2 receptor responded to the Wnt11 ligand and regulated cell migration and cell morphology during gastrulation. DnRor2 inhibited the action of Wnt11, which was revealed by a decreased percentage of Wnt11-induced convergence and extension defects. Ror2 physically interacts with Wnt11. Theintracellular Tyr-647andSer-863 sites ofRor2are essential for mediating the action of Wnt11. Dishevelled and RhoA act downstream of Wnt11-Ror2 to regulate convergence and extension movements. Overall, our data suggest an important role of Ror2 in mediating Wnt11 signaling and in regulating convergence and extension movements in zebrafish.

Vangl2 cooperates with Rab11 and Myosin V to regulate apical constriction during vertebrate gastrulation

Core planar cell polarity (PCP) proteins are well known to regulate polarity in Drosophila and vertebrate epithelia; however, their functions in vertebrate morphogenesis remain poorly understood. In this study, we describe a role for PCP signaling in the process of apical constriction during Xenopus gastrulation. The core PCP protein Vangl2 is detected at the apical surfaces of cells at the blastopore lip, and it functions during blastopore formation and closure. Further experiments show that Vangl2, as well as Daam1 and Rho-associated kinase (Rock), regulate apical constriction of bottle cells at the blastopore and ectopic constriction of ectoderm cells triggered by the actin-binding protein Shroom3. At the blastopore lip, Vangl2 is required for the apical accumulation of the recycling endosome marker Rab11. We also show that Rab11 and the associated motor protein Myosin V play essential roles in both endogenous and ectopic apical constriction, and might be involved in Vangl2 trafficking to the cell surface. Overexpression of Rab11 RNA was sufficient to partly restore normal blastopore formation in Vangl2-deficient embryos. These observations suggest that Vangl2 affects Rab11 to regulate apical constriction during blastopore formation.

FGF signaling transforms non-neural ectoderm into neural crest

The neural crest arises at the border between the neural plate and the adjacent non-neural ectoderm. It has been suggested that both neural and non-neural ectoderm can contribute to the neural crest. Several studies have examined the molecular mechanisms that regulate neural crest induction in neuralized tissues or the neural plate border. Here, using the chick as a model system, we address the molecular mechanisms by which non-neural ectoderm generates neural crest. We report that in response to FGF the non-neural ectoderm can ectopically express several early neural crest markers (Pax7, Msx1, Dlx5, Sox9, FoxD3, Snail2, and Sox10). Importantly this response to FGF signaling can occur without inducing ectopic mesodermal tissues. Furthermore, the non-neural ectoderm responds to FGF by expressing the prospective neural marker Sox3, but it does not express definitive markers of neural or anterior neural (Sox2 and Otx2) tissues. These results suggest that the non-neural ectoderm can launch the neural crest program in the absence of mesoderm, without acquiring definitive neural character. Finally, we report that prior to the upregulation of these neural crest markers, the non-neural ectoderm upregulates both BMP and Wnt molecules in response to FGF. Our results provide the first effort to understand the molecular events leading to neural crest development via the non-neural ectoderm in amniotes and present a distinct response to FGF signaling.

Paladin is an antiphosphatase that regulates neural crest cell formation and migration

Although a network of transcription factors that specifies neural crest identity in the ectoderm has been defined, expression of neural crest transcription factors does not guarantee eventual migration as a neural crest cell. While much work has gone into determining regulatory relationships within the transcription factor network, the ability of protein modifications like phosphorylation to modulate the function of neural crest regulatory factors and determine when and where they are active also has crucial implications. Paladin, which was previously classified as a phosphatase based on sequence similarity, is expressed in chick neural crest precursors and is maintained throughout their epithelial to mesenchymal transition and migration. Loss of Paladin delays the expression of transcription factors Snail2 and Sox10 in premigratory neural crest cells, but does not affect accumulation of FoxD3, Cad6B or RhoB, indicating that Paladin differentially modulates the expression of genes previously thought to be coregulated within the neural crest gene regulatory network. Both gain and loss of Paladin function result in disrupted neural crest migration, reinforcing the importance of precisely regulated phosphorylation for neural crest migration. Mutation of critical, catalytic cysteine residues within Paladin's predicted phosphatase active site motifs did not abolish the function of Paladin in the neural crest. Collectively, these data indicate that Paladin is an antiphosphatase that modulates the activity of specific neural crest regulatory factors during neural crest development. Our work identifies a novel regulator of phosphorylation status that provides an additional layer of regulation in the neural crest.

Signaling and transcriptional regulation in neural crest specification and migration: lessons from xenopus embryos

The neural crest is a population of highly migratory and multipotent cells, which arises from the border of the neural plate in vertebrate embryos. In the last few years, the molecular actors of neural crest early development have been intensively studied, notably by using the frog embryo, as a prime model for the analysis of the earliest embryonic inductions. In addition, tremendous progress has been made in understanding the molecular and cellular basis of Xenopus cranial neural crest migration, by combining in vitro and in vivo analysis. In this review, we examine how the action of previously known neural crest-inducing signals [bone morphogenetic protein (BMP), wingless-int (Wnt), fibroblast growth factor (FGF)] is controlled by newly discovered modulators during early neural plate border patterning and neural crest specification. This regulation controls the induction of key transcription factors that cooperate to pattern the premigratory neural crest progenitors. These data are discussed in the perspective of the gene regulatory network that controls neural and neural crest patterning. We then address recent findings on noncanonical Wnt signaling regulation, cell polarization, and collective cell migration which highlight how cranial neural crest cells populate their target tissue, the branchial arches, in vivo. More than ever, the neural crest stands as a powerful and attractive model to decipher complex vertebrate regulatory circuits in vivo.

Current perspectives of the signaling pathways directing neural crest induction

The neural crest is a migratory population of embryonic cells with a tremendous potential to differentiate and contribute to nearly every organ system in the adult body. Over the past two decades, an incredible amount of research has given us a reasonable understanding of how these cells are generated. Neural crest induction involves the combinatorial input of multiple signaling pathways and transcription factors, and is thought to occur in two phases from gastrulation to neurulation. In the first phase, FGF and Wnt signaling induce NC progenitors at the border of the neural plate, activating the expression of members of the Msx, Pax, and Zic families, among others. In the second phase, BMP, Wnt, and Notch signaling maintain these progenitors and bring about the expression of definitive NC markers including Snail2, FoxD3, and Sox9/10. In recent years, additional signaling molecules and modulators of these pathways have been uncovered, creating an increasingly complex regulatory network. In this work, we provide a comprehensive review of the major signaling pathways that participate in neural crest induction, with a focus on recent developments and current perspectives. We provide a simplified model of early neural crest development and stress similarities and differences between four major model organisms: Xenopus, chick, zebrafish, and mouse.