Xenopus Nbx, a novel NK-1 related gene essential for neural crest formation

Craniofacial Development Is Fine-Tuned by Sox2

The precise control of neural crest stem cell delamination, migration and differentiation ensures proper craniofacial and head development. Sox2 shapes the ontogeny of the cranial neural crest to ensure precision of the cell flow in the developing head. Here, we review how Sox2 orchestrates signals that control these complex developmental processes.

Sox2: To crest or not to crest?

Precise control of neural progenitor transformation into neural crest stem cells ensures proper craniofacial and head development. In the neural progenitor pool, SoxB factors play an essential role as cell fate determinants of neural development, whereas during neural crest stem cell formation, Sox2 plays a predominant role as a guardian of the developmental clock that ensures precision of cell flow in the developing head.

Integration of signals along orthogonal axes of the vertebrate neural tube controls progenitor competence and increases cell diversity

FGF gates competence to generate Floor Plate and Neural Crest in response to Shh and BMP signals by controlling expression of the transcription factor Nkx1.2.

A relatively small number of signals are responsible for the variety and pattern of cell types generated in developing embryos. In part this is achieved by exploiting differences in the concentration or duration of signaling to increase cellular diversity. In addition, however, changes in cellular competence—temporal shifts in the response of cells to a signal—contribute to the array of cell types generated. Here we investigate how these two mechanisms are combined in the vertebrate neural tube to increase the range of cell types and deliver spatial control over their location. We provide evidence that FGF signaling emanating from the posterior of the embryo controls a change in competence of neural progenitors to Shh and BMP, the two morphogens that are responsible for patterning the ventral and dorsal regions of the neural tube, respectively. Newly generated neural progenitors are exposed to FGF signaling, and this maintains the expression of the Nk1-class transcription factor Nkx1.2. Ventrally, this acts in combination with the Shh-induced transcription factor FoxA2 to specify floor plate cells and dorsally in combination with BMP signaling to induce neural crest cells. As development progresses, the intersection of FGF with BMP and Shh signals is interrupted by axis elongation, resulting in the loss of Nkx1.2 expression and allowing the induction of ventral and dorsal interneuron progenitors by Shh and BMP signaling to supervene. Hence a similar mechanism increases cell type diversity at both dorsal and ventral poles of the neural tube. Together these data reveal that tissue morphogenesis produces changes in the coincidence of signals acting along orthogonal axes of the neural tube and this is used to define spatial and temporal transitions in the competence of cells to interpret morphogen signaling.

During embryonic development different cell types arise at different times and places. This diversity is produced by a relatively small number of signals and depends, at least in part, on changes in the way cells respond to each signal. One example of this so-called change in “competence” is found in the vertebrate spinal cord where a signal, Sonic Hedgehog (Shh), induces a glial cell type known as floor plate (FP) at early developmental times, while the same signal later induces specific types of neurons. Here, we dissected the molecular mechanism underlying the change in competence, and found that another signal, FGF, is involved through its control of the transcription factor Nkx1.2. In embryos, Shh and FGF are produced perpendicular to one another and FP is induced where the two signals intersect. The position of this intersection changes as the embryo elongates and this determines the place and time FP is produced. A similar strategy also appears to apply to another cell type, neural crest. In this case, the intersection of FGF with BMP signal is crucial. Together the data provide new insight into the spatiotemporal control of cell type specification during development of the vertebrate spinal cord.

Involvement of XZFP36L1, an RNA-binding protein, in Xenopus neural development

Xenopus ZFP36L1 (zinc finger protein 36, C3H type-like 1) belongs to the ZFP36 family of RNA-binding proteins, which contains two characteristic tandem CCCH-type zinc-finger domains. The ZFP36 proteins can bind AU-rich elements in 3' untranslated regions of target mRNAs and promote their turnover. However, the expression and role of ZFP36 genes during neural development in Xenopus embryos remains largely unknown. The present study showed that Xenopus ZFP36L1 was expressed at the dorsal part of the forebrain, forebrain-midbrain boundary, and midbrain-hindbrain boundary from late neurula stages to tadpole stages of embryonic development. Overexpression of XZFP36L1 in Xenopus embryos inhibited neural induction and differentiation, leading to severe neural tube defects. The function of XZP36L1 requires both its zinc finger and C terminal domains, which also affect its subcellular localization. These results suggest that XZFP36L1 is likely involved in neural development in Xenopus and might play an important role in post-transcriptional regulation.

Maternal intake of folic acid and neural crest stem cells

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

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.

Conservation and Phylogeny of a Novel Family of Non-Hox Genes of the Antp Class in Demospongiae (Porifera)

A survey across the most basal animal phylum, the Porifera, for the presence of homeobox-containing genes led to the isolation of 24 partial or complete homeobox sequences from 21 sponge species distributed in 15 families and 6 orders of Demospongiae. All the new sequences shared a high identity/similarity with EmH-3 (Ephydatia muelleri), a non-Hox gene from the Antp class. The Demox sequences, EmH-3, and related homeodomains formed a well-supported clade with no true affinity with any known bilaterian family, including the Tlx/Hox11 family, suggesting that the EmH-3 family of genes, comprising 31 members, represents a novel family of non-Hox genes, called the Demox family, widespread among Demospongiae. The presence of the Tlx/Hox11 specific signature in the Demox family and common regulatory elements suggested that the Demox and Tlx/Hox11 families are closely related. In the phylogenetic analyses, freshwater Haplosclerida appeared as monophyletic, and Haplosclerida and Halichondrida as polyphyletic, with a clade comprising Agelas species and Axinella corrugata. As for their expression, high levels of Demox transcripts were found in adult tissues. Our data add to the number of published poriferan homeobox sequences and provide independent confirmation of the current Demospongiae phylogenies.

Transcriptional control of early tract formation in the embryonic chick midbrain

The earliest step in establishing the complex neuronal networks in the vertebrate brain is the formation of a scaffold of axon tracts. How the formation of the early axon scaffold is controlled at the molecular level is unclear. Forming part of the scaffold, neurons located at the ventral midbrain-forebrain border (MFB) give rise to the medial longitudinal fascicle (mlf) and the posterior commissure (pc). We demonstrate that the homeobox genes Sax1, Six3, Emx2 and Pax6 are expressed in distinct domains in this area, suggesting that the specification of mlf and pc neurons might be controlled by the combinatorial activity of these transcription factors. We have tested this hypothesis by analysing the function of Sax1 in the embryonic chick brain. Gain-of-function experiments with Sax1 result in alterations to the early axon scaffold, most prominently an enlargement of the mlf at the expense of the pc. Ectopic expression of Sax1 also affects the expression of other ventral homeobox genes, particularly Six3 and Emx2. Our results indicate that the specification of neurons forming the early axon scaffold is governed by a homeobox code, thus resembling the mechanism of neuronal specification in the spinal cord.

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

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

Induction of the neural crest and the opportunities of life on the edge

The neural crest is a multipotent population of migratory cells unique to the vertebrate embryo. Neural crest arises at the lateral edge of the neural plate and migrates throughout the embryo to give rise to a wide variety of cell types including peripheral and enteric neurons and glia, craniofacial cartilage and bone, smooth muscle, and pigment cells. Here we review recent studies that have addressed the role of several signaling pathways in the induction of the neural crest. Work in the mouse, chick, Xenopus, and zebrafish have shown that a complex network of genes is activated at the neural plate border in response to neural crest-inducing signals. We also summarize some of these findings and discuss how the differential activation of these genes may contribute to the establishment of neural crest diversity.

Early requirement of the transcriptional activator Sox9 for neural crest specification in Xenopus

The neural crest is a multipotent population of cells that arises at the neural plate border in the vertebrate embryo. We have previously shown that a member of the Sox family of transcription factors, Sox9, is a regulator of neural crest formation in Xenopus, as Sox9-depleted embryos failed to form neural crest progenitors. Here, we describe experiments that further investigate Sox9 function during neural crest development. Induction of neural crest progenitors in Xenopus is regulated by Wnt signaling. We show that this process is largely dependent on Sox9 function as Wnt-mediated neural crest induction is inhibited in the context of Sox9-depleted embryos. Moreover, we demonstrate that Sox9 functions as a transcriptional activator during neural crest formation. Expression of a construct in which Sox9 DNA-binding domain (HMG box) is fused to the repressor domain of Drosophila engrailed blocked neural crest formation, thereby mimicking the phenotype of Sox9-depleted embryos. Finally, using a hormone-inducible inhibitory mutant of Sox9, lacking the transactivation domain, we show that Sox9 function is required for neural crest specification but not for its subsequent migration.

Rho guanine nucleotide exchange factor xNET1 implicated in gastrulation movements during Xenopus development

During Xenopus development, embryonic cells dramatically change their shape and position. Rho family small GTPases, such as RhoA, Rac, and Cdc42, play important roles in this process. These GTPases are generally activated by guanine nucleotide exchange factors (GEFs); however, the roles of RhoGEFs in Xenopus development have not yet been elucidated. We therefore searched for RhoGEF genes in our Xenopus EST database, and we identified several genes expressed during embryogenesis. Among them, we focused on one gene, designated xNET1. It is similar to mammalian NET1, a RhoA-specific GEF. An in vitro binding assay revealed that xNET1 bound to RhoA, but not to Rac or Cdc42. In addition, transient expression of xNET1 activated endogenous RhoA. These results indicated that xNET1 is a GEF for RhoA. Epitope-tagged xNET1 was localized mainly to the nucleus, and the localization was regulated by nuclear localization signals in the N-terminal region of xNET1. Overexpression of either wild-type or a mutant form of xNET1 severely inhibited gastrulation movements. We demonstrated that xNET1 was co-immunoprecipitated with the Dishevelled protein, which is an essential signaling component in the non-canonical Wnt pathway. This pathway has been shown to activate RhoA and regulate gastrulation movements. We propose that xNET1 or a similar RhoGEF may mediate Dishevelled signaling to RhoA in the Wnt pathway.

Postnatal lethality in mice lacking the Sax2 homeobox gene homologous to Drosophila S59/slouch: evidence for positive and negative autoregulation

Homeobox gene transcription factors direct multiple functions during development. They are involved in early patterning of the embryo as well as cell specification, cell differentiation, and organogenesis. Here we describe a previously uncharacterized murine homeobox gene, Sax2, that shows high similarity to the Drosophila S59/slouch and murine Sax1 genes. We show that Sax2 gene expression occurs early during embryogenesis in the midbrain, the midbrain-hindbrain boundary, the ventral neural tube, the developing eye, and the apical ectodermal ridge of the limb. To determine the role of Sax2 during development, we generated a knockout mouse line by replacing part of the Sax2 coding sequences with the lacZ gene. The Sax2 null allele mutants exhibit a strong phenotype indicated by growth retardation starting immediately after birth and leading to premature death within the first 3 weeks postnatal. Intriguingly, our studies also demonstrated a striking autoregulation of the Sax2 gene in both positive- and negative-feedback mechanisms depending on the specific cell type expressing Sax2.

PKC delta is essential for Dishevelled function in a noncanonical Wnt pathway that regulates Xenopus convergent extension movements

Protein kinase C (PKC) has been implicated in the Wnt signaling pathway; however, its molecular role is poorly understood. We identified novel genes encoding delta-type PKC in the Xenopus EST databases. Loss of PKC delta function revealed that it was essential for convergent extension during gastrulation. We then examined the relationship between PKC delta and the Wnt pathway. PKC delta was translocated to the plasma membrane in response to Frizzled signaling. In addition, loss of PKC delta function inhibited the translocation of Dishevelled and the activation of c-Jun N-terminal kinase (JNK) by Frizzled. Furthermore, PKC delta formed a complex with Dishevelled, and the activation of PKC delta by phorbol ester was sufficient for Dishevelled translocation and JNK activation. Thus, PKC delta plays an essential role in the Wnt/JNK pathway by regulating the localization and activity of Dishevelled.