Molecular mechanisms of neural crest induction

From neural tube to spinal cord: The dynamic journey of the dorsal neuroepithelium

In a developing embryo, formation of tissues and organs is remarkably precise in both time and space. Through cell-cell interactions, neighboring progenitors coordinate their activities, sequentially generating distinct types of cells. At present, we only have limited knowledge, rather than a systematic understanding, of the underlying logic and mechanisms responsible for cell fate transitions. The formation of the dorsal aspect of the spinal cord is an outstanding model to tackle these dynamics, as it first generates the peripheral nervous system and is later responsible for transmitting sensory information from the periphery to the brain and for coordinating local reflexes. This is reflected first by the ontogeny of neural crest cells, progenitors of the peripheral nervous system, followed by formation of the definitive roof plate of the central nervous system and specification of adjacent interneurons, then a transformation of roof plate into dorsal radial glia and ependyma lining the forming central canal. How do these peripheral and central neural branches segregate from common progenitors? How are dorsal radial glia established concomitant with transformation of the neural tube lumen into a central canal? How do the dorsal radial glia influence neighboring cells? This is only a partial list of questions whose clarification requires the implementation of experimental paradigms in which precise control of timing is crucial. Here, we outline some available answers and still open issues, while highlighting the contributions of avian models and their potential to address mechanisms of neural patterning and function.

Intermediate Cells of Dual Embryonic Origin Follow a Basal to Apical Gradient of Ingression Into the Lateral Wall of the Cochlea

Intermediate cells of the stria vascularis are neural crest derived melanocytes. They are essential for the establishment of the endocochlear potential in the inner ear, which allows mechanosensory hair cells to transduce sound into nerve impulses. Despite their importance for normal hearing, how these cells develop and migrate to their position in the lateral wall of the cochlea has not been studied. We find that as early as E10.5 some Schwann cell precursors in the VIIIth ganglion begin to express melanocyte specific markers while neural crest derived melanoblasts migrate into the otic vesicle. Intermediate cells of both melanoblast and Schwann cell precursor origin ingress into the lateral wall of the cochlea starting at around E15.5 following a basal to apical gradient during embryonic development, and continue to proliferate postnatally.

Seq Your Destiny: Neural Crest Fate Determination in the Genomic Era

Neural crest stem/progenitor cells arise early during vertebrate embryogenesis at the border of the forming central nervous system. They subsequently migrate throughout the body, eventually differentiating into diverse cell types ranging from neurons and glia of the peripheral nervous system to bones of the face, portions of the heart, and pigmentation of the skin. Along the body axis, the neural crest is heterogeneous, with different subpopulations arising in the head, neck, trunk, and tail regions, each characterized by distinct migratory patterns and developmental potential. Modern genomic approaches like single-cell RNA- and ATAC-sequencing (seq) have greatly enhanced our understanding of cell lineage trajectories and gene regulatory circuitry underlying the developmental progression of neural crest cells. Here, we discuss how genomic approaches have provided new insights into old questions in neural crest biology by elucidating transcriptional and posttranscriptional mechanisms that govern neural crest formation and the establishment of axial level identity. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

From Neural Crest to Definitive Roof Plate: The Dynamic Behavior of the Dorsal Neural Tube

Research on the development of the dorsal neural tube is particularly challenging. In this highly dynamic domain, a temporal transition occurs between early neural crest progenitors that undergo an epithelial-to-mesenchymal transition and exit the neural primordium, and the subsequent roof plate, a resident epithelial group of cells that constitutes the dorsal midline of the central nervous system. Among other functions, the roof plate behaves as an organizing center for the generation of dorsal interneurons. Despite extensive knowledge of the formation, emigration and migration of neural crest progenitors, little is known about the mechanisms leading to the end of neural crest production and the transition into a roof plate stage. Are these two mutually dependent or autonomously regulated processes? Is the generation of roof plate and dorsal interneurons induced by neural tube-derived factors throughout both crest and roof plate stages, respectively, or are there differences in signaling properties and responsiveness as a function of time? In this review, we discuss distinctive characteristics of each population and possible mechanisms leading to the shift between the above cell types.

Renal Manifestations of Tuberous Sclerosis Complex

Tuberous sclerosis complex (TSC) is a genetic condition caused by a mutation in either the TSC1 or TSC2 gene. Disruption of either of these genes leads to impaired production of hamartin or tuberin proteins, leading to the manifestation of skin lesions, tumors, and seizures. TSC can manifest in multiple organ systems with the cutaneous and renal systems being the most commonly affected. These manifestations can secondarily lead to the development of hypertension, chronic kidney disease, and neurocognitive declines. The renal pathologies most commonly seen in TSC are angiomyolipoma, renal cysts, and less commonly, oncocytomas. In this review, we highlight the current understanding on the renal manifestations of TSC along with current diagnosis and treatment guidelines.

Extracellular Matrix in Secondary Palate Development

The secondary palate arises from outgrowths of epithelia-covered embryonic mesenchyme that grow from the maxillary prominence, remodel to meet over the tongue, and fuse at the midline. These events require the coordination of cell proliferation, migration, and gene expression, all of which take place in the context of the extracellular matrix (ECM). Palatal cells generate their ECM, and then stiffen, degrade, or otherwise modify its properties to achieve the required cell movement and organization during palatogenesis. The ECM, in turn, acts on the cells through their matrix receptors to change their gene expression and thus their phenotype. The number of ECM-related gene mutations that cause cleft palate in mice and humans is a testament to the crucial role the matrix plays in palate development and a reminder that understanding that role is vital to our progress in treating palate deformities. This article will review the known ECM constituents at each stage of palatogenesis, the mechanisms of tissue reorganization and cell migration through the palatal ECM, the reciprocal relationship between the ECM and gene expression, and human syndromes with cleft palate that arise from mutations of ECM proteins and their regulators. Anat Rec, 2019. © 2019 American Association for Anatomy.

Wnt Receptor Frizzled-4 as a Marker for Isolation of Enteric Neural Progenitors in Human Children

Identification and isolation of neural progenitor cells from the human enteric nervous system (ENS) is currently hampered by the lack of reliable, specific markers. Here, we define the Wnt-receptor frizzled-4 as a marker for the isolation of enteric neural progenitor cells derived from paediatric gut samples. We show that the Wnt-receptor frizzled-4 is expressed in the human colon and in Tunica muscularis-derived enterospheres. To obtain a purified culture, we carried out fluorescence-activated cell sorting (FACS) using PE-conjugated frizzled-4 antibodies. Frizzled-4^positive cells gave rise to neurosphere-like bodies and ultimately differentiated into neurons as revealed by BrdU-proliferation assays and immunocytochemistry, whereas in frizzled-4^negative cultures we did not detect any neuronal and glial cells. By using a patch-clamp approach, we also demonstrated the expression of functional sodium and potassium channels in frizzled-4^positive cell cultures after differentiation in vitro.

Impact of prenatal arsenate exposure on gene expression in a pure population of migratory cranial neural crest cells

Prenatal exposure to arsenic, a naturally occurring toxic element, causes neural tube defects (NTDs) and, in animal models, orofacial anomalies. Since aberrant development or migration of cranial neural crest cells (CNCCs) can also cause similar anomalies within developing embryos, we examined the effects of in utero exposure to sodium arsenate on gene expression patterns in pure populations of CNCCs, isolated by fluorescence activated cell sorting (FACS), from Cre/LoxP reporter mice. Changes in gene expression were analyzed using Affymetrix GeneChip^® microarrays and expression of selected genes was verified by TaqMan quantitative real-time PCR. We report, for the first time, arsenate-induced alterations in the expression of a number of novel candidate genes and canonical cascades that may contribute to the pathogenesis of orofacial defects. Ingenuity Pathway and NIH-DAVID analyses revealed cellular response pathways, biological themes, and potential upstream regulators, that may underlie altered fetal programming of arsenate exposed CNCCs.

Generating retinoic acid gradients by local degradation during craniofacial development: One cell's cue is another cell's poison

Retinoic acid (RA) is a vital morphogen for early patterning and organogenesis in the developing embryo. RA is a diffusible, lipophilic molecule that signals via nuclear RA receptor heterodimeric units that regulate gene expression by interacting with RA response elements in promoters of a significant number of genes. For precise RA signaling, a robust gradient of the morphogen is required. The developing embryo contains regions that produce RA, and specific intracellular concentrations of RA are created through local degradation mediated by Cyp26 enzymes. In order to elucidate the mechanisms by which RA executes precise developmental programs, the kinetics of RA metabolism must be clearly understood. Recent advances in techniques for endogenous RA detection and quantification have paved the way for mechanistic studies to shed light on downstream gene expression regulation coordinated by RA. It is increasingly coming to light that RA signaling operates not only at precise concentrations but also employs mechanisms of degradation and feedback inhibition to self-regulate its levels. A global gradient of RA throughout the embryo is often found concurrently with several local gradients, created by juxtaposed domains of RA synthesis and degradation. The existence of such local gradients has been found especially critical for the proper development of craniofacial structures that arise from the neural crest and the cranial placode populations. In this review, we summarize the current understanding of how local gradients of RA are established in the embryo and their impact on craniofacial development.

Should I stay or should I go? Cadherin function and regulation in the neural crest

Our increasing comprehension of neural crest cell development has reciprocally advanced our understanding of cadherin expression, regulation, and function. As a transient population of multipotent stem cells that significantly contribute to the vertebrate body plan, neural crest cells undergo a variety of transformative processes and exhibit many cellular behaviors, including epithelial-to-mesenchymal transition (EMT), motility, collective cell migration, and differentiation. Multiple studies have elucidated regulatory and mechanistic details of specific cadherins during neural crest cell development in a highly contextual manner. Collectively, these results reveal that gradual changes within neural crest cells are accompanied by often times subtle, yet important, alterations in cadherin expression and function. The primary focus of this review is to coalesce recent data on cadherins in neural crest cells, from their specification to their emergence as motile cells soon after EMT, and to highlight the complexities of cadherin expression beyond our current perceptions, including the hypothesis that the neural crest EMT is a transition involving a predominantly singular cadherin switch. Further advancements in genetic approaches and molecular techniques will provide greater opportunities to integrate data from various model systems in order to distinguish unique or overlapping functions of cadherins expressed at any point throughout the ontogeny of the neural crest.

Role of Sp5 as an essential early regulator of neural crest specification in xenopus

BACKGROUND

The neural crest (NC) is a multipotent embryonic cell population, which is induced by an integration of secreted signals including BMP, Wnt, and FGF and, subsequently, NC cell fates are specified by a regulatory network of specific transcription factors. This study was undertaken to identify a role of Sp5 transcription factor in vertebrates.

RESULTS

Xenopus Sp5 is expressed in the prospective neural crest regions from gastrulation through the tadpole stages in early development. Knockdown of Sp5 caused severe defects in craniofacial cartilage, pigmentation, and dorsal fin. Gain- and loss-of-function of Sp5 led to up- and down-regulation of the expression of NC markers in the neural fold, respectively. In contrast, Sp5 had no effect on neural induction and patterning. Sp5 regulated the expression of neural plate border (NPB) specifiers, Msx1 and Pax3, and these regulatory factors recovered the expression of NC marker in the Sp5-deficient embryos. Depletion of Sp5 impaired NC induction by Wnt/β-catenin or FGF signal, whereas its co-expression rescued NC markers in embryos in which either signal was blocked.

CONCLUSIONS

These results suggest that Sp5 functions as a critical early factor in the genetic cascade to regulate NC induction downstream of Wnt and FGF pathways.

Loss of Xenopus cadherin-11 leads to increased Wnt/β-catenin signaling and up-regulation of target genes c-myc and cyclin D1 in neural crest

Xenopus cadherin-11 (Xcadherin-11) is an exceptional cadherin family member, which is predominantly expressed in cranial neural crest cells (NCCs). Apart from mediating cell-cell adhesion it promotes cranial NCC migration by initiating filopodia and lamellipodia formation. Here, we demonstrate an unexpected function of Xcadherin-11 in NCC specification by interfering with canonical Wnt/β-catenin signaling. Loss-of-function experiments, using a specific antisense morpholino oligonucleotide against Xcadherin-11, display a nuclear β-catenin localization in cranial NCCs and a broader expression domain of the proto-oncogene cyclin D1 which proceeds c-myc up-regulation. Additionally, we observe an enhanced NCC proliferation and an expansion of specific NCC genes like AP2 and Sox10. Thereby, we could allocate NCC proliferation and specification to different gene functions. To clarify which domain in Xcadherin-11 is required for early NCC development we tested different deletion mutants for their rescue ability in Xcadherin-11 morphants. We identified the cytoplasmic tail, specifically the β-catenin binding domain, to be necessary for proper NCC development. We propose that Xcadherin-11 is necessary for controlled NCC proliferation and early NCC specification in tuning the expression of the canonical Wnt/β-catenin target genes cyclin D1 and c-myc by regulating the concentration of the nuclear pool of β-catenin.

Cell differentiation of pluripotent tissue sheets immobilized on supported membranes displaying cadherin-11

Investigating cohesive tissue sheets in controlled cultures still poses a challenge since the complex intercellular interactions are difficult to mimic in in vitro models. We used supported lipid membranes functionalized by the adhesive part of the extracellular domain of the cell adhesion molecule cadherin-11 for the immobilization of pluripotent tissue sheets, the animal cap isolated from Xenopus laevis blastula stage embryos. Cadherin-11 was bound via histidine tag to lipid membranes with chelator head groups. In the first step, quantitative functionalization of the membranes with cadherin-11 was confirmed by quartz crystal microbalance and high energy specular X-ray reflectivity. In the next step, animal cap tissue sheets induced to neural crest cell fate were cultured on the membranes functionalized with cadherin-11. The adhesion of cells within the cohesive tissue was significantly dependent on changes in lateral densities of cadherin-11. The formation of filopodia and lamellipodia in the cohesive tissue verified the viability and sustainability of the culture over several hours. The expression of the transcription factor slug in externally induced tissue demonstrated the applicability of lipid membranes displaying adhesive molecules for controlled differentiation of cohesive pluripotent tissue sheets.

Pax7 lineage contributions to the mammalian neural crest

Background

Neural crest cells are vertebrate-specific multipotent cells that contribute to a variety of tissues including the peripheral nervous system, melanocytes, and craniofacial bones and cartilage. Abnormal development of the neural crest is associated with several human maladies including cleft/lip palate, aggressive cancers such as melanoma and neuroblastoma, and rare syndromes, like Waardenburg syndrome, a complex disorder involving hearing loss and pigment defects. We previously identified the transcription factor Pax7 as an early marker, and required component for neural crest development in chick embryos. In mammals, Pax7 is also thought to play a role in neural crest development, yet the precise contribution of Pax7 progenitors to the neural crest lineage has not been determined.

Methodology/Principal Findings

Here we use Cre/loxP technology in double transgenic mice to fate map the Pax7 lineage in neural crest derivates. We find that Pax7 descendants contribute to multiple tissues including the cranial, cardiac and trunk neural crest, which in the cranial cartilage form a distinct regional pattern. The Pax7 lineage, like the Pax3 lineage, is additionally detected in some non-neural crest tissues, including a subset of the epithelial cells in specific organs.

Conclusions/Significance

These results demonstrate a previously unappreciated widespread distribution of Pax7 descendants within and beyond the neural crest. They shed light regarding the regionally distinct phenotypes observed in Pax3 and Pax7 mutants, and provide a unique perspective into the potential roles of Pax7 during disease and development.

Normalized shape and location of perturbed craniofacial structures in the Xenopus tadpole reveal an innate ability to achieve correct morphology

BACKGROUND

Embryonic development can often adjust its morphogenetic processes to counteract external perturbation. The existence of self-monitoring responses during pattern formation is of considerable importance to the biomedicine of birth defects, but has not been quantitatively addressed. To understand the computational capabilities of biological tissues in a molecularly tractable model system, we induced craniofacial defects in Xenopus embryos, then tracked tadpoles with craniofacial deformities and used geometric morphometric techniques to characterize changes in the shape and position of the craniofacial structures.

RESULTS

Canonical variate analysis revealed that the shapes and relative positions of perturbed jaws and branchial arches were corrected during the first few months of tadpole development. Analysis of the relative movements of the anterior-most structures indicates that misplaced structures move along the anterior-posterior and left-right axes in ways that are significantly different from their normal movements.

CONCLUSIONS

Our data suggest a model in which craniofacial structures use a measuring mechanism to assess and adjust their location relative to other local organs. Understanding the correction mechanisms at work in this system could lead to the better understanding of the adaptive decision-making capabilities of living tissues and suggest new approaches to correct birth defects in humans.

Indian hedgehog signaling is required for proper formation, maintenance and migration of Xenopus neural crest

Neural crest induction is the result of the combined action at the neural plate border of FGF, BMP, and Wnt signals from the neural plate, mesoderm and nonneural ectoderm. In this work we show that the expression of Indian hedgehog (Ihh, formerly named Banded hedgehog) and members of the Hedgehog pathway occurs at the prospective neural fold, in the premigratory and migratory neural crest. We performed a functional analysis that revealed the requirement of Ihh signaling in neural crest development. During the early steps of neural crest induction loss of function experiments with antisense morpholino or locally grafted cyclopamine-loaded beads suppressed the expression of early neural crest markers concomitant with the increase in neural and epidermal markers. We showed that changes in Ihh activity produced no alterations in either cell proliferation or apoptosis, suggesting that this signal involves cell fate decisions. A temporal analysis showed that Hedgehog is continuously required not only in the early and late specification but also during the migration of the neural crest. We also established that the mesodermal source of Ihh is important to maintain specification and also to support the migratory process. By a combination of embryological and molecular approaches our results demonstrated that Ihh signaling drives in the migration of neural crest cells by autocrine or paracrine mechanisms. Finally, the abrogation of Ihh signaling strongly affected only the formation of cartilages derived from the neural crest, while no effects were observed on melanocytes. Taken together, our results provide insights into the role of the Ihh cell signaling pathway during the early steps of neural crest development.

Neural crest specification: tissues, signals, and transcription factors

The neural crest is a transient population of multipotent and migratory cells unique to vertebrate embryos. Initially derived from the borders of the neural plate, these cells undergo an epithelial to mesenchymal transition to leave the central nervous system, migrate extensively in the periphery, and differentiate into numerous diverse derivatives. These include but are not limited to craniofacial cartilage, pigment cells, and peripheral neurons and glia. Attractive for their similarities to stem cells and metastatic cancer cells, neural crest cells are a popular model system for studying cell/tissue interactions and signaling factors that influence cell fate decisions and lineage transitions. In this review, we discuss the mechanisms required for neural crest formation in various vertebrate species, focusing on the importance of signaling factors from adjacent tissues and conserved gene regulatory interactions, which are required for induction and specification of the ectodermal tissue that will become neural crest.

SOX10 directly modulates ERBB3 transcription via an intronic neural crest enhancer

Background

The ERBB3 gene is essential for the proper development of the neural crest (NC) and its derivative populations such as Schwann cells. As with all cell fate decisions, transcriptional regulatory control plays a significant role in the progressive restriction and specification of NC derived lineages during development. However, little is known about the sequences mediating transcriptional regulation of ERBB3 or the factors that bind them.

Results

In this study we identified three transcriptional enhancers at the ERBB3 locus and evaluated their regulatory potential in vitro in NC-derived cell types and in vivo in transgenic zebrafish. One enhancer, termed ERBB3_MCS6, which lies within the first intron of ERBB3, directs the highest reporter expression in vitro and also demonstrates epigenetic marks consistent with enhancer activity. We identify a consensus SOX10 binding site within ERBB3_MCS6 and demonstrate, in vitro, its necessity and sufficiency for the activity of this enhancer. Additionally, we demonstrate that transcription from the endogenous Erbb3 locus is dependent on Sox10. Further we demonstrate in vitro that Sox10 physically interacts with that ERBB3_MCS6. Consistent with its in vitro activity, we also show that ERBB3_MCS6 drives reporter expression in NC cells and a subset of its derivative lineages in vivo in zebrafish in a manner consistent with erbb3b expression. We also demonstrate, using morpholino analysis, that Sox10 is necessary for ERBB3_MCS6 expression in vivo in zebrafish.

Conclusions

Taken collectively, our data suggest that ERBB3 may be directly regulated by SOX10, and that this control may in part be facilitated by ERBB3_MCS6.

Evidence for a dynamic spatiotemporal fate map and early fate restrictions of premigratory avian neural crest

Colonization of trunk neural crest derivatives in avians follows a ventral to dorsal order beginning with sympathetic ganglia, Schwann cells, sensory ganglia and finally melanocytes. Continuous crest emigration underlies this process, which is accounted for by a progressive ventral to dorsal relocation of neural tube progenitors prior to departure. This causes a gradual narrowing of FoxD3, Sox9 and Snail2 expression domains in the dorsal tube that characterize the neural progenitors of the crest and these genes are no longer transcribed by the time melanoblasts begin emigrating. Consistently, the final localization of crest cells can be predicted from their relative ventrodorsal position within the premigratory domain or by their time of delamination. Thus, a dynamic spatiotemporal fate map of crest derivatives exists in the dorsal tube at flank levels of the axis with its midline region acting as a sink for the ordered ingression and departure of progenitors. Furthermore, discrete lineage analysis of the dorsal midline at progressive stages generated progeny in single rather than multiple derivatives, revealing early fate restrictions. Compatible with this notion, when early emigrating `neural' progenitors were diverted into the lateral `melanocytic' pathway, they still adopted neural traits, suggesting that initial fate acquisition is independent of the migratory environment and that the potential of crest cells prior to emigration is limited.

Mitochondrial C1-tetrahydrofolate synthase (MTHFD1L) supports the flow of mitochondrial one-carbon units into the methyl cycle in embryos

Mitochondrial folate-dependent one-carbon (1-C) metabolism converts 1-C donors such as serine and glycine to formate, which is exported and incorporated into the cytoplasmic tetrahydrofolate (THF) 1-C pool. Developing embryos depend on this mitochondrial pathway to provide 1-C units for cytoplasmic process such as de novo purine biosynthesis and the methyl cycle. This pathway is composed of sequential methylene-THF dehydrogenase, methenyl-THF cyclohydrolase, and 10-formyl-THF synthetase activities. In embryonic mitochondria, the bifunctional MTHFD2 enzyme catalyzes the dehydrogenase and cyclohydrolase reactions, but the enzyme responsible for the mitochondrial synthetase reaction has not been identified in embryos. A monofunctional 10-formyl-THF synthetase (MTHFD1L gene product) functions in adult mitochondria and is a likely candidate for the embryonic activity. Here we show that the MTHFD1L enzyme is present in mitochondria from normal embryonic tissues and embryonic fibroblast cell lines, and embryonic mitochondria possess the ability to synthesize formate from glycine. The MTHFD1L transcript was detected at all stages of mouse embryogenesis examined. In situ hybridizations showed that MTHFD1L was expressed ubiquitously throughout the embryo but with localized regions of higher expression. The spatial pattern of MTHFD1L expression was virtually indistinguishable from that of MTHFD2 and MTHFD1 (cytoplasmic C(1)-THF synthase) in embryonic day 9.5 mouse embryos, suggesting coordinated regulation. Finally, we show using stable isotope labeling that in an embryonic mouse cell line, greater than 75% of 1-C units entering the cytoplasmic methyl cycle are mitochondrially derived. Thus, a complete pathway of enzymes for supplying 1-C units from the mitochondria to the methyl cycle in embryonic tissues is established.

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

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

Fate map and morphogenesis of presumptive neural crest and dorsal neural tube

In contrast to the classical assumption that neural crest cells are induced in chick as the neural folds elevate, recent data suggest that they are already specified during gastrulation. This prompted us to map the origin of the neural crest and dorsal neural tube in the early avian embryo. Using a combination of focal dye injections and time-lapse imaging, we find that neural crest and dorsal neural tube precursors are present in a broad, crescent-shaped region of the gastrula. Surprisingly, static fate maps together with dynamic confocal imaging reveal that the neural plate border is considerably broader and extends more caudally than expected. Interestingly, we find that the position of the presumptive neural crest broadly correlates with the BMP4 expression domain from gastrula to neurula stages. Some degree of rostrocaudal patterning, albeit incomplete, is already evident in the gastrula. Time-lapse imaging studies show that the neural crest and dorsal neural tube precursors undergo choreographed movements that follow a spatiotemporal progression and include convergence and extension, reorientation, cell intermixing, and motility deep within the embryo. Through these rearrangement and reorganization movements, the neural crest and dorsal neural tube precursors become regionally segregated, coming to occupy predictable rostrocaudal positions along the embryonic axis. This regionalization occurs progressively and appears to be complete in the neurula by stage 7 at levels rostral to Hensen's node.

Differential requirements of BMP and Wnt signalling during gastrulation and neurulation define two steps in neural crest induction

The neural crest is induced by a combination of secreted signals. Although previous models of neural crest induction have proposed a step-wise activation of these signals, the actual spatial and temporal requirement has not been analysed. Through analysing the role of the mesoderm we show for the first time that specification of neural crest requires two temporally and chemically different steps: first, an induction at the gastrula stage dependent on signals arising from the dorsolateral mesoderm; and second, a maintenance step at the neurula stage dependent on signals from tissues adjacent to the neural crest. By performing tissue recombination experiments and using specific inhibitors of different inductive signals, we show that the first inductive step requires Wnt activation and BMP inhibition, whereas the later maintenance step requires activation of both pathways. This change in BMP necessity from BMP inhibition at gastrula to BMP activation at neurula stages is further supported by the dynamic expression of BMP4 and its antagonists, and is confirmed by direct measurements of BMP activity in the neural crest cells. The differential requirements of BMP activity allow us to propose an explanation for apparently discrepant results between chick and frog experiments. The demonstration that Wnt signals are required for neural crest induction by mesoderm solves an additional long-standing controversy. Finally, our results emphasise the importance of considering the order of exposure to signals during an inductive event.

A negative modulatory role for rho and rho-associated kinase signaling in delamination of neural crest cells

Background

Neural crest progenitors arise as epithelial cells and then undergo a process of epithelial to mesenchymal transition that precedes the generation of cellular motility and subsequent migration. We aim at understanding the underlying molecular network. Along this line, possible roles of Rho GTPases that act as molecular switches to control a variety of signal transduction pathways remain virtually unexplored, as are putative interactions between Rho proteins and additional known components of this cascade.

Results

We investigated the role of Rho/Rock signaling in neural crest delamination. Active RhoA and RhoB are expressed in the membrane of epithelial progenitors and are downregulated upon delamination. In vivo loss-of-function of RhoA or RhoB or of overall Rho signaling by C3 transferase enhanced and/or triggered premature crest delamination yet had no effect on cell specification. Consistently, treatment of explanted neural primordia with membrane-permeable C3 or with the Rock inhibitor Y27632 both accelerated and enhanced crest emigration without affecting cell proliferation. These treatments altered neural crest morphology by reducing stress fibers, focal adhesions and downregulating membrane-bound N-cadherin. Reciprocally, activation of endogenous Rho by lysophosphatidic acid inhibited emigration while enhancing the above. Since delamination is triggered by BMP and requires G1/S transition, we examined their relationship with Rho. Blocking Rho/Rock function rescued crest emigration upon treatment with noggin or with the G1/S inhibitor mimosine. In the latter condition, cells emigrated while arrested at G1. Conversely, BMP4 was unable to rescue cell emigration when endogenous Rho activity was enhanced by lysophosphatidic acid.

Conclusion

Rho-GTPases, through Rock, act downstream of BMP and of G1/S transition to negatively regulate crest delamination by modifying cytoskeleton assembly and intercellular adhesion.

WITHDRAWN: The Neural Cell Adhesion Molecule NCAM2/OCAM/RNCAM, a Close Relative to NCAM

Cell adhesion molecules (CAMs) constitute a large class of plasma membrane-anchored proteins that mediate attachment between neighboring cells and between cells and the surrounding extracellular matrix (ECM). However, CAMs are more than simple mediators of cell adhesion. The neural cell adhesion molecule (NCAM) is a well characterized, ubiquitously expressed CAM that is highly expressed in the nervous system. In addition to mediating cell adhesion, NCAM participates in a multitude of cellular events, including survival, migration, and differentiation of cells, outgrowth of neurites, and formation and plasticity of synapses. NCAM shares an overall sequence identity of approximately 44% with the neural cell adhesion molecule 2 (NCAM2), a protein also known as olfactory cell adhesion molecule (OCAM) and Rb-8 neural cell adhesion molecule (RNCAM), and the region-for-region sequence homology between the two proteins suggests that they are transcribed from paralogous genes. However, very little is known about the function of NCAM2, although it originally was described more than 20 years ago. In this review we summarize the known properties and functions of NCAM2 and describe some of the differences and similarities between NCAM and NCAM2.

Emerging roles for zic genes in early development

Members of the Zic family of zinc finger transcription factors play critical roles in a variety of developmental processes. They are involved in development of neural tissues and the neural crest, in left-right axis patterning, in somite development, and in formation of the cerebellum. In addition to their roles in cell-fate specification, zic genes also promote cell proliferation. Further, they are expressed in postmitotic cells of the cerebellum and in retinal ganglion cells. Efforts to determine the role of individual zic genes within an array of developmental and cellular processes are complicated by overlapping patterns of zic gene expression and strong sequence conservation within this gene family. Nevertheless, substantial progress has been made. This review summarizes our knowledge of the molecular events that govern the activities of zic family members, including emerging relationships between upstream signaling pathways and zic genes. In addition, advancements in our understanding of the molecular events downstream of Zic transcription factors are reviewed. Despite significant progress, however, much remains to be learned regarding the mechanisms through which zic genes exert their function in a variety of different contexts.

Neural crests are actively precluded from the anterior neural fold by a novel inhibitory mechanism dependent on Dickkopf1 secreted by the prechordal mesoderm

It is known the interactions between the neural plate and epidermis generate neural crest (NC), but it is unknown why the NC develops only at the lateral border of the neural plate and not in the anterior fold. Using grafting experiments we show that there is a previously unidentified mechanism that precludes NC from the anterior region. We identify prechordal mesoderm as the tissue that inhibits NC in the anterior territory and show that the Wnt/beta-catenin antagonist Dkk1, secreted by this tissue, is sufficient to mimic this NC inhibition. We show that Dkk1 is required for preventing the formation of NC in the anterior neural folds as loss-of-function experiments using a Dkk1 blocking antibody in Xenopus as well as the analysis of Dkk1-null mouse embryos transform the anterior neural fold into NC. This can be mimicked by Wnt/beta-catenin signaling activation without affecting the anterior posterior patterning of the neural plate, or placodal specification. Finally, we show that the NC cells induced at the anterior neural fold are able to migrate and differentiate as normal NC. These results demonstrate that anterior regions of the embryo lack NC because of a mechanism, conserved from fish to mammals, that suppresses Wnt/beta-catenin signaling via Dkk1.

Embryogenetics: gene control of the embryogenesis of the eye

In recent years there have been significant advances in our knowledge and understanding of the genetic control of embryonic development. The aim of this paper is to review the current state of knowledge regarding genetic control of embryogenesis, the first stage of ocular development. Products of numerous genes participate in signaling, induction and control during the formation of the neural plate, groove and tube, key events leading to the development of the future eye.

Palatal fusion - where do the midline cells go? A review on cleft palate, a major human birth defect

Formation of the palate, the organ that separates the oral cavity from the nasal cavity, is a developmental process characteristic to embryos of higher vertebrates. Failure in this process results in palatal cleft. During the final steps of palatogenesis, two palatal shelves outgrowing from the sides of the embryonic oronasal cavity elevate above the tongue, meet in the midline, and rapidly fuse together. Over the decades, multiple mechanisms have been proposed to explain how the superficial mucous membranes disappear from the contact line, thus allowing for normal midline mesenchymal confluence. A substantial body of experimental evidence exists for cell death, cell migration, epithelial-to-mesenchymal transdifferentiation (EMT), replacement through new tissue intercalation, and other mechanisms. However, the most recent use of gene recombination techniques in cell fate tracking disfavors the EMT concept, and suggests that apoptosis is the major fate of the midline cells during physiological palatal fusion. This article summarizes the benefits and drawbacks of histochemical and molecular tools used to determine the fates of cells within the palatal midline. Mechanisms of normal disintegration of the midline epithelial seam are reviewed together with pathologic processes that prevent this disintegration, thus causing cleft palate.

Analysis of connexin expression during mouse Schwann cell development identifies connexin29 as a novel marker for the transition of neural crest to precursor cells

Connexins are transmembrane proteins forming gap junction channels for direct intercellular and, for example in myelinating glia cells, intracellular communication. In mature myelin-forming Schwann cells, expression of multiple connexins, i.e. connexin (Cx) 43, Cx29, Cx32, and Cx46 (after nerve injury) has been detected. However, little is known about connexin protein expression during Schwann cell development. Here we use histochemical methods on wildtype and Cx29lacZ transgenic mice to investigate the developmental expression of connexins in the Schwann cell lineage. Our data demonstrate that in the mouse Cx43, Cx29, and Cx32 protein expression is activated in a developmental sequence that is clearly correlated with major developmental steps in the lineage. Only Cx43 was expressed from neural crest cells onwards. Cx29 protein expression was absent from neural crest cells but appeared as neural crest cells generated precursors (embryonic day 12) both in vivo and in vitro. This identifies Cx29 as a novel marker for cells of the defined Schwann cell lineage. The only exception to this were dorsal roots, where the expression of Cx29 was delayed four days relative to ventral roots and spinal nerves. Expression of Cx32 commenced postnatally, coinciding with the onset of myelination. Thus, the coordinated expression of connexin proteins in cells of the embryonic and postnatal Schwann cell lineage might point to a potential role in peripheral nerve development and maturation.

Antagonistic roles of full-length N-cadherin and its soluble BMP cleavage product in neural crest delamination

During neural crest ontogeny, an epithelial to mesenchymal transition is necessary for cell emigration from the dorsal neural tube. This process is likely to involve a network of gene activities, which remain largely unexplored. We demonstrate that N-cadherin inhibits the onset of crest delamination both by a cell adhesion-dependent mechanism and by repressing canonical Wnt signaling previously found to be necessary for crest delamination by acting downstream of BMP4. Furthermore, N-cadherin protein, but not mRNA, is normally downregulated along the dorsal tube in association with the onset of crest delamination, and we find that this process is triggered by BMP4. BMP4 stimulates cleavage of N-cadherin into a soluble cytoplasmic fragment via an ADAM10-dependent mechanism. Intriguingly, when overexpressed, the cytoplasmic N-cadherin fragment translocates into the nucleus, stimulates cyclin D1 transcription and crest delamination, while enhancing transcription of beta-catenin. CTF2 also rescues the mesenchymal phenotype of crest cells in ADAM10-inhibited neural primordia. Hence, by promoting its cleavage, BMP4 converts N-cadherin inhibition into an activity that is likely to participate, along with canonical Wnt signaling, in the stimulation of neural crest emigration.

Development of the upper lip: morphogenetic and molecular mechanisms

The vertebrate upper lip forms from initially freely projecting maxillary, medial nasal, and lateral nasal prominences at the rostral and lateral boundaries of the primitive oral cavity. These facial prominences arise during early embryogenesis from ventrally migrating neural crest cells in combination with the head ectoderm and mesoderm and undergo directed growth and expansion around the nasal pits to actively fuse with each other. Initial fusion is between lateral and medial nasal processes and is followed by fusion between maxillary and medial nasal processes. Fusion between these prominences involves active epithelial filopodial and adhering interactions as well as programmed cell death. Slight defects in growth and patterning of the facial mesenchyme or epithelial fusion result in cleft lip with or without cleft palate, the most common and disfiguring craniofacial birth defect. Recent studies of craniofacial development in animal models have identified components of several major signaling pathways, including Bmp, Fgf, Shh, and Wnt signaling, that are critical for proper midfacial morphogenesis and/or lip fusion. There is also accumulating evidence that these signaling pathways cross-regulate genetically as well as crosstalk intracellularly to control cell proliferation and tissue patterning. This review will summarize the current understanding of the basic morphogenetic processes and molecular mechanisms underlying upper lip development and discuss the complex interactions of the various signaling pathways and challenges for understanding cleft lip pathogenesis.

Concise Review: Bone Morphogenetic Protein Pleiotropism in Neural Stem Cells and Their Derivatives-Alternative Pathways, Convergent Signals

Bone morphogenetic proteins (BMPs) are a class of morphogens that are critical regulators of the central nervous system (CNS), peripheral nervous system, and craniofacial development. Modulation of BMP signaling also appears to be an important component of the postnatal stem cell niche. However, describing a comprehensive model of BMP actions is complicated by their paradoxical effects in precursor cells, which include dorsal specification, promoting proliferation or mitotic arrest, cell survival or death, and neuronal or glial fate. In addition, in postmitotic neurons BMPs can promote dendritic growth, act as axonal chemorepellants, and stabilize synapses. Although many of these responses depend on interactions with other incoming signals, some reflect the recruitment of distinct BMP signal transduction pathways. In this review, we classify the diverse effects of BMPs on neural cells, focus on the known mechanisms that specify distinct responses, and discuss the remaining challenges in identifying the cellular basis of BMP pleiotropism. Addressing these issues may have importance for stem cell mobilization, differentiation, and cell integration/survival in reparative therapies.

Frizzled7 mediates canonical Wnt signaling in neural crest induction

The neural crest is a multipotent cell population that migrates from the dorsal edge of the neural tube to various parts of the embryo where it differentiates into a remarkable variety of different cell types. Initial induction of neural crest is mediated by a combination of BMP, Wnt, FGF, Retinoic acid and Notch/Delta signaling. The two-signal model for neural crest induction suggests that BMP signaling induces the competence to become neural crest. The second signal involves Wnt acting through the canonical pathway and leads to expression of neural crest markers such as slug. Wnt signals from the neural plate, non-neural ectoderm and paraxial mesoderm have all been suggested to play a role in neural crest induction. We show that Xenopus frizzled7 (Xfz7) is expressed in the dorsal ectoderm including early neural crest progenitors and is a key mediator of the Wnt inductive signal. We demonstrate that Xfz7 expression is induced in response to a BMP antagonist, noggin, and that Xfz7 can induce neural crest specific genes in noggin-treated ectodermal explants (animal caps). Morpholino-mediated or dominant negative inhibition of Xfz7 inhibits Wnt induced Xslug expression in the animal cap assay and in the whole embryo leading to a loss of neural crest derived pigment cells. Full-length Xfz7 rescues the morpholino-induced phenotype, as does activated beta-catenin, suggesting that Xfz7 is signaling through the canonical pathway. We therefore demonstrate that Xfz7 is regulated by BMP antagonism and is required for neural crest induction by Wnt in the developing vertebrate embryo.

Regulation of murine TGFbeta2 by Pax3 during early embryonic development

Previously our laboratory identified TGFbeta2 as a potential downstream target of Pax3 by utilizing microarray analysis and promoter data base mining (Mayanil, C. S. K., George, D., Freilich, L., Miljan, E. J., Mania-Farnell, B. J., McLone, D. G., and Bremer, E. G. (2001) J. Biol. Chem. 276, 49299-49309). Here we report that Pax3 directly regulates TGFbeta2 transcription by binding to cis-regulatory elements within its promoter. Chromatin immunoprecipitation revealed that Pax3 bound to the cis-regulatory elements on the TGFbeta2 promoter (GenBanktrade mark accession number AF118263). Both TGFbeta2 promoter-luciferase activity measurements in transient cotransfection experiments and electromobility shift assays supported the idea that Pax3 regulates TGFbeta2 by directly binding to its cis-regulatory regions. Additionally, by using a combination of co-immunoprecipitation and chromatin immunoprecipitation, we show that the TGFbeta2 cis-regulatory elements between bp 741-940 and bp 1012-1212 bind acetylated Pax3 and are associated with p300/CBP and histone deacetylases. The cis-regulatory elements between bp 741 and 940 in addition to associating with acetylated Pax3 and HDAC1 also associated with SIRT1. Whole mount in situ hybridization and quantitative real time reverse transcription-PCR showed diminished levels of TGFbeta2 transcripts in Pax3(-/-) mouse embryos (whose phenotype is characterized by neural tube defects) as compared with Pax3(+/+) littermates (embryonic day 10.0; 30 somite stage), suggesting that Pax3 regulation of TGFbeta2 may play a pivotal role during early embryonic development.

Specification of the neural crest occurs during gastrulation and requires Pax7

The neural crest is a stem population critical for development of the vertebrate craniofacial skeleton and peripheral ganglia. Neural crest cells originate along the border between the neural plate and epidermis, migrate extensively and generate numerous derivatives, including neurons and glia of the peripheral nervous system, melanocytes, bone and cartilage of the head skeleton. Impaired neural crest development is associated with human defects, including cleft palate. Classically, the neural crest has been thought to form by interactions at the border between neural and non-neural ectoderm or mesoderm, and defined factors such as bone morphogenetic proteins (BMPs) and Wnt proteins have been postulated as neural crest-inducers. Although competence to induce crest cells declines after stage 10 (ref. 14), little is known about when neural crest induction begins in vivo. Here we report that neural crest induction is underway during gastrulation and well before proper neural plate appearance. We show that a restricted region of chick epiblast (stage 3-4) is specified to generate neural crest cells when explanted under non-inducing conditions. This region expresses the transcription factor Pax7 by stage 4 + and later contributes to neural folds and migrating neural crest. In chicken embryos, Pax7 is required for neural crest formation in vivo, because blocking its translation inhibits expression of the neural crest markers Slug, Sox9, Sox10 and HNK-1. Our results indicate that neural crest specification initiates earlier than previously assumed, independently of mesodermal and neural tissues, and that Pax7 has a crucial function during neural crest development.

Etiology, pathogenesis and prevention of neural tube defects

Spina bifida, anencephaly, and encephalocele are commonly grouped together and termed neural tube defects (NTD). Failure of closure of the neural tube during development results in anencephaly or spina bifida aperta but encephaloceles are possibly post-closure defects. NTD are associated with a number of other central nervous system (CNS) and non-neural malformations. Racial, geographic and seasonal variations seem to affect their incidence. Etiology of NTD is unknown. Most of the non-syndromic NTD are of multifactorial origin. Recent in vitro and in vivo studies have highlighted the molecular mechanisms of neurulation in vertebrates but the morphologic development of human neural tube is poorly understood. A multisite closure theory, extrapolated directly from mouse experiments highlighted the clinical relevance of closure mechanisms to human NTD. Animal models, such as circle tail, curly tail, loop tail, shrm and numerous knockouts provide some insight into the mechanisms of NTD. Also available in the literature are a plethora of chemically induced preclosure and a few post-closure models of NTD, which highlight the fact that CNS malformations are of hetergeneitic nature. No Mendelian pattern of inheritance has been reported. Association with single gene defects, enhanced recurrence risk among siblings, and a higher frequency in twins than in singletons indicate the presence of a strong genetic contribution to the etiology of NTD. Non-availability of families with a significant number of NTD cases makes research into genetic causation of NTD difficult. Case reports and epidemiologic studies have implicated a number of chemicals, widely differing therapeutic drugs, environmental contaminants, pollutants, infectious agents, and solvents. Maternal hyperthermia, use of valproate by epileptic women during pregnancy, deficiency and excess of certain nutrients and chronic maternal diseases (e.g. diabetes mellitus) are reported to cause a manifold increase in the incidence of NTD. A host of suspected teratogens are also available in the literature. The UK and Hungarian studies showed that periconceptional supplementation of women with folate (FA) reduces significantly both the first occurrence and recurrence of NTD in the offspring. This led to mandatory periconceptional FA supplementation in a number of countries. Encouraged by the results of clinical studies, numerous laboratory investigations focused on the genes involved in the FA, vitamin B12 and homocysteine metabolism during neural tube development. As of today no clinical or experimental study has provided unequivocal evidence for a definitive role for any of these genes in the causation of NTD suggesting that a multitude of genes, growth factors and receptors interact in controlling neural tube development by yet unknown mechanisms. Future studies must address issues of gene-gene, gene-nutrient and gene-environment interactions in the pathogenesis of NTD.

PCNS: a novel protocadherin required for cranial neural crest migration and somite morphogenesis in Xenopus

Protocadherins (Pcdhs), a major subfamily of cadherins, play an important role in specific intercellular interactions in development. These molecules are characterized by their unique extracellular domain (EC) with more than 5 cadherin-like repeats, a transmembrane domain (TM) and a variable cytoplasmic domain. PCNS (Protocadherin in Neural crest and Somites), a novel Pcdh in Xenopus, is initially expressed in the mesoderm during gastrulation, followed by expression in the cranial neural crest (CNC) and somites. PCNS has 65% amino acid identity to Xenopus paraxial protocadherin (PAPC) and 42-49% amino acid identity to Pcdh 8 in human, mouse, and zebrafish genomes. Overexpression of PCNS resulted in gastrulation failure but conferred little if any specific adhesion on ectodermal cells. Loss of function accomplished independently with two non-overlapping antisense morpholino oligonucleotides resulted in failure of CNC migration, leading to severe defects in the craniofacial skeleton. Somites and axial muscles also failed to undergo normal morphogenesis in these embryos. Thus, PCNS has essential functions in these two important developmental processes in Xenopus.

Relations and interactions between cranial mesoderm and neural crest populations

The embryonic head is populated by two robust mesenchymal populations, paraxial mesoderm and neural crest cells. Although the developmental histories of each are distinct and separate, they quickly establish intimate relations that are variably important for the histogenesis and morphogenesis of musculoskeletal components of the calvaria, midface and branchial regions. This review will focus first on the genesis and organization within nascent mesodermal and crest populations, emphasizing interactions that probably initiate or augment the establishment of lineages within each. The principal goal is an analysis of the interactions between crest and mesoderm populations, from their first contacts through their concerted movements into peripheral domains, particularly the branchial arches, and continuing to stages at which both the differentiation and the integrated three-dimensional assembly of vascular, connective and muscular tissues is evident. Current views on unresolved or contentious issues, including the relevance of head somitomeres, the processes by which crest cells change locations and constancy of cell-cell relations at the crest-mesoderm interface, are addressed.

Vitamin A improves Pax3 expression that is decreased in the heart of rats with experimental diaphragmatic hernia

BACKGROUND/AIMS

Rats with nitrofen-induced congenital diaphragmatic hernia (CDH) have hypoplasia and malformations of the heart. The mechanism of action of nitrofen involves changes in neural crest signaling. Pax3 function is required for cardiac neural crest cells to complete their migration to the developing heart. Vitamin A improves heart hypoplasia. The aims of this study were to examine whether Pax3 expression is decreased in the heart of E13 E15 and E21 rats exposed to nitrofen and if vitamin A reverts this effect.

MATERIAL AND METHODS

Pregnant rats received either 100 mg nitrofen or olive oil on E9.5. Each group was divided into 2 subgroups according to the subsequent treatment with intragastric vitamin A (15000 IU) or vehicle on E10.5 to E11.5. The pups were recovered on E13, E15 and E21 and the hearts were dissected out. Pax3 mRNA expression was determined by quantitative real time PCR. Comparisons among groups were made with ANOVA and Bonferroni post hoc tests with a threshold of significance of P < .05.

RESULTS

Pax3 mRNA expression was significantly decreased on E13 and E15 in the hearts of nitrofen-treated embryos and it remained decreased although not significantly on E21. Vitamin A recovered this expression on E13, partially on E15 and above normal levels on E21.

CONCLUSIONS

Pax3 is underexpressed in the hearts of nitrofen exposed embryonal rats on days 13th and 15th of gestation and tends to be lower than normal near term. Vitamin A up-regulates this expression on the 3 end points. The mechanism of action of Pax3 should be further investigated because it could be one of the targets for future prenatal transplacental intervention.

Expression of TFAP2beta and TFAP2gamma genes in Xenopus laevis

The embryonic expression patterns of two additional members of the transcription factor TFAP2 family in Xenopus laevis, TFAP2beta and TFAP2gamma, are described. Both genes share overlapping expression domains with the previously characterized TFAP2alpha in this species, although differences exist. All three genes are expressed in the neural crest (NC) region at late gastrula to early neurula stages. TFAP2alpha and TFAP2gamma are also expressed in outer, epidermal cells, while TFAP2beta is essentially NC-specific. All three are induced by Wnt/beta-catenin -- BMP signals and all bind to a consensus TFAP2 recognition site from an epidermal keratin gene.

Recent advances in craniofacial morphogenesis

Craniofacial malformations are involved in three fourths of all congenital birth defects in humans, affecting the development of head, face, or neck. Tremendous progress in the study of craniofacial development has been made that places this field at the forefront of biomedical research. A concerted effort among evolutionary and developmental biologists, human geneticists, and tissue engineers has revealed important information on the molecular mechanisms that are crucial for the patterning and formation of craniofacial structures. Here, we highlight recent advances in our understanding of evo-devo as it relates to craniofacial morphogenesis, fate determination of cranial neural crest cells, and specific signaling pathways in regulating tissue-tissue interactions during patterning of craniofacial apparatus and the morphogenesis of tooth, mandible, and palate. Together, these findings will be beneficial for the understanding, treatment, and prevention of human congenital malformations and establish the foundation for craniofacial tissue regeneration.

Abnormalities in neural crest cell migration in laminin alpha5 mutant mice

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

msh/Msx gene family in neural development

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

Regulatory targets for transcription factor AP2 in Xenopus embryos

The transcription factor AP2 (TFAP2) has an important role in regulating gene expression in both epidermis and neural crest cells. In order to further characterize these functions we have used a hormone inducible TFAP2alpha fusion protein in a Xenopus animal cap assay to identify downstream targets of this factor. The most common pattern comprised genes predominantly expressed in the epidermis. A second group was expressed at high levels in the neural crest, but all of these were also expressed in the epidermis as well as in other tissues in which TFAP2alpha has not been detected, suggesting modular control involving both TFAP2-dependent and TFAP2-independent components. In addition, a few strongly induced genes did not overlap at all in expression pattern with that of TFAP2alpha in the early embryo, and were also activated precociously in the experimentally manipulated ectoderm, and thus likely represent inappropriate regulatory interactions. A final group was identified that were repressed by TFAP2alpha and were expressed in the neural plate. These results provide further support for the importance of TFAP2alpha in ectoderm development, and also highlight the molecular linkage between the epidermis and neural crest in the Xenopus embryo.

Regulatory mechanisms for neural crest formation

In early vertebrate development, the neural crest is specified in the embryonic ectoderm at the boundary of the neural plate and the nonneural ectoderm. After the induction, the neural crest cells undergo epithelial-mesenchymal transition, delaminating from the epithelium, and migrate extensively in the embryonic environment to give rise to a wide variety of tissues and cell types. In this review, we try to summarize the recent progress in understanding the molecular nature of the inductive signals and transcription factors involved in neural crest formation and following steps of the neural crest development. We also point out the underlying problems that need to be solved to understand the process further.

New insights into craniofacial morphogenesis

No region of our anatomy more powerfully conveys our emotions nor elicits more profound reactions when disease or genetic disorders disfigure it than the face. Recent progress has been made towards defining the tissue interactions and molecular mechanisms that control craniofacial morphogenesis. Some insights have come from genetic manipulations and others from tissue recombinations and biochemical approaches, which have revealed the molecular underpinnings of facial morphogenesis. Changes in craniofacial architecture also lie at the heart of evolutionary adaptation, as new studies in fish and fowl attest. Together, these findings reveal much about molecular and tissue interactions behind craniofacial development.

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

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