MaternalXenopus Zic2negatively regulatesNodal-relatedgene expression during anteroposterior patterning

Evolution of the gene regulatory network of body axis by enhancer hijacking in amphioxus

A central goal of evolutionary developmental biology is to decipher the evolutionary pattern of gene regulatory networks (GRNs) that control embryonic development, and the mechanism underlying GRNs evolution. The Nodal signaling that governs the body axes of deuterostomes exhibits a conserved GRN orchestrated principally by Nodal, Gdf1/3, and Lefty. Here we show that this GRN has been rewired in cephalochordate amphioxus. We found that while the amphioxus Gdf1/3 ortholog exhibited nearly no embryonic expression, its duplicate Gdf1/3-like, linked to Lefty, was zygotically expressed in a similar pattern as Lefty. Consistent with this, while Gdf1/3-like mutants showed defects in axial development, Gdf1/3 mutants did not. Further transgenic analyses showed that the intergenic region between Gdf1/3-like and Lefty could drive reporter gene expression as that of the two genes. These results indicated that Gdf1/3-like has taken over the axial development role of Gdf1/3 in amphioxus, possibly through hijacking Lefty enhancers. We finally demonstrated that, to compensate for the loss of maternal Gdf1/3 expression, Nodal has become an indispensable maternal factor in amphioxus and its maternal mutants caused axial defects as Gdf1/3-like mutants. We therefore demonstrated a case that the evolution of GRNs could be triggered by enhancer hijacking events. This pivotal event has allowed the emergence of a new GRN in extant amphioxus, presumably through a stepwise process. In addition, the co-expression of Gdf1/3-like and Lefty achieved by a shared regulatory region may have provided robustness during body axis formation, which provides a selection-based hypothesis for the phenomena called developmental system drift.

ZIC2 promotes colorectal cancer growth and metastasis through the TGF-β signaling pathway

ZIC2 is involved in the tumor progression of many types of cancers. The role of ZIC2 in the metastasis of colorectal cancer and its mechanism are not yet clear. In this study, we found that high ZIC2 expression was not only associated with poor prognosis, relapse-free survival and advanced metastasis but was also an independent prognostic factor in colorectal cancer patients. Moreover, ZIC2 knockdown inhibited cell proliferation, migration and invasion, while the upregulation of ZIC2 had the opposite effect in vitro. ZIC2 overexpression induced TGF-β1 expression and increased Smad3 phosphorylation. The carcinogenic effects of elevated ZIC2 expression can be eliminated by interfering with the TGF-β1 receptor with inhibitors. This further verified the promoting effect of ZIC2 on the TGF-β signaling pathway. In vivo experiments have also confirmed that ZIC2 can promote liver metastases of colorectal cancer. The results suggest that ZIC2 is associated with poor prognosis and relapse-free survival in colorectal cancer patients. Moreover, ZIC2 promoted colorectal cancer progression and metastasis by activating the TGF-β signaling pathway. Hence, ZIC2 is expected to be a new therapeutic and prognostic target for colorectal cancer in the future.

The crucial role of model systems in understanding the complexity of cell signaling in human neurocristopathies

Animal models are useful to study the molecular, cellular, and morphogenetic mechanisms underlying normal and pathological development. Cell-based study models have emerged as an alternative approach to study many aspects of human embryonic development and disease. The neural crest (NC) is a transient, multipotent, and migratory embryonic cell population that generates a diverse group of cell types that arises during vertebrate development. The abnormal formation or development of the NC results in neurocristopathies (NCPs), which are characterized by a broad spectrum of functional and morphological alterations. The impaired molecular mechanisms that give rise to these multiphenotypic diseases are not entirely clear yet. This fact, added to the high incidence of these disorders in the newborn population, has led to the development of systematic approaches for their understanding. In this article, we have systematically reviewed the ways in which experimentation with different animal and cell model systems has improved our knowledge of NCPs, and how these advances might contribute to the development of better diagnostic and therapeutic tools for the treatment of these pathologies. This article is categorized under: Congenital Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Stem Cells and Development Congenital Diseases > Molecular and Cellular Physiology Neurological Diseases > Genetics/Genomics/Epigenetics.

Role of dipeptidyl peptidase-4 as a potentiator of activin/nodal signaling pathway

DPP4 (dipeptidyl peptidase-4), a highly conserved transmembrane glycoprotein with an exo-peptidase activity, has been shown to contribute to glucose metabolism, immune regulation, signal transduction, and cell differentiation. Here, we show that DPP4 is involved in control of activin/nodal signaling in Xenopus early development. In support of this, gain of function of DPP4 augmented Smad2 phosphorylation as well as expression of target genes induced by activin or nodal signal. In addition, Dpp4 and Xnr1 showed synergistic effect on induction of ectopic dorsal body axis, when co-injected at suboptimal doses in early embryos. Conversely, saxagliptin, a DPP4 inhibitor repressed activin induction of Smad2 phosphorylation. Notably, overexpression of Dpp4 disrupted specification of dorsal body axis of embryo, leading to malformed phenotypes such as spina bifida and a shortened and dorsally bent axis. Together, these results suggest that DPP4 functions as a potentiator of activin/nodal signaling pathway.

Role of maternal Xenopus syntabulin in germ plasm aggregation and primordial germ cell specification

The localization and organization of mitochondria- and ribonucleoprotein granule-rich germ plasm is essential for many aspects of germ cell development. In Xenopus, germ plasm is maternally inherited and is required for the specification of primordial germ cells (PGCs). Germ plasm is aggregated into larger patches during egg activation and cleavage and is ultimately translocated perinuclearly during gastrulation. Although microtubule dynamics and a kinesin (Kif4a) have been implicated in Xenopus germ plasm localization, little is known about how germ plasm distribution is regulated. Here, we identify a role for maternal Xenopus Syntabulin in the aggregation of germ plasm following fertilization. We show that depletion of sybu mRNA using antisense oligonucleotides injected into oocytes results in defects in the aggregation and perinuclear transport of germ plasm and subsequently in reduced PGC numbers. Using live imaging analysis, we also characterize a novel role for Sybu in the collection of germ plasm in vegetal cleavage furrows by surface contraction waves. Additionally, we show that a localized kinesin-like protein, Kif3b, is also required for germ plasm aggregation and that Sybu functionally interacts with Kif3b and Kif4a in germ plasm aggregation. Overall, these data suggest multiple coordinate roles for kinesins and adaptor proteins in controlling the localization and distribution of a cytoplasmic determinant in early development.

Neural tube closure: cellular, molecular and biomechanical mechanisms

Neural tube closure has been studied for many decades, across a range of vertebrates, as a paradigm of embryonic morphogenesis. Neurulation is of particular interest in view of the severe congenital malformations - 'neural tube defects' - that result when closure fails. The process of neural tube closure is complex and involves cellular events such as convergent extension, apical constriction and interkinetic nuclear migration, as well as precise molecular control via the non-canonical Wnt/planar cell polarity pathway, Shh/BMP signalling, and the transcription factors Grhl2/3, Pax3, Cdx2 and Zic2. More recently, biomechanical inputs into neural tube morphogenesis have also been identified. Here, we review these cellular, molecular and biomechanical mechanisms involved in neural tube closure, based on studies of various vertebrate species, focusing on the most recent advances in the field.

Neural transcription factors bias cleavage stage blastomeres to give rise to neural ectoderm

The decision by embryonic ectoderm to give rise to epidermal versus neural derivatives is the result of signaling events during blastula and gastrula stages. However, there also is evidence in Xenopus that cleavage stage blastomeres contain maternally derived molecules that bias them toward a neural fate. We used a blastomere explant culture assay to test whether maternally deposited transcription factors bias 16-cell blastomere precursors of epidermal or neural ectoderm to express early zygotic neural genes in the absence of gastrulation interactions or exogenously supplied signaling factors. We found that Foxd4l1, Zic2, Gmnn, and Sox11 each induced explants made from ventral, epidermis-producing blastomeres to express early neural genes, and that at least some of the Foxd4l1 and Zic2 activities are required at cleavage stages. Similarly, providing extra Foxd4l1 or Zic2 to explants made from dorsal, neural plate-producing blastomeres significantly increased the expression of early neural genes, whereas knocking down either significantly reduced them. These results show that maternally delivered transcription factors bias cleavage stage blastomeres to a neural fate. We demonstrate that mouse and human homologs of Foxd4l1 have similar functional domains compared to the frog protein, as well as conserved transcriptional activities when expressed in Xenopus embryos and blastomere explants. genesis 54:334-349, 2016. © 2016 Wiley Periodicals, Inc.

Myocyte enhancer factor 2D regulates ectoderm specification and adhesion properties of animal cap cells in the early Xenopus embryo

In Xenopus, animal cap (AC) cells give rise to ectoderm and its derivatives: epidermis and the central nervous system. Ectoderm has long been considered a default pathway of embryonic development, with cells that are not under the influence of vegetal Nodal signaling adopting an ectodermal program of gene expression. In the present study, we describe the involvement of the animally-localized maternal transcription factor myocyte enhancer factor (Mef) 2D in regulating the identity of AC cells. We find that Mef2D is required for the formation of both ectodermal lineages: neural and epidermis. Gain and loss of function experiments indicate that Mef2D regulates early gastrula expression of key ectodermal/epidermal genes in the animal region. Mef2D controls the activity of zygotic bone morphogenetic protein (BMP) signaling known to dictate the epidermal differentiation program. Exogenous expression of Mef2D in vegetal blastomeres was sufficient to induce ectopic expression of ectoderm/epidermal genes in the vegetal half of the embryo, when Nodal signaling was inhibited. Depletion of Mef2D caused a loss of AC cell adhesion that was rescued by the expression of E-cadherin or bone morphogenetic protein 4. In addition, expression of Mef2D in the prospective endoderm caused unusual aggregation of vegetal cells with animal cells in vitro and inappropriate segregation to other germ layers in vivo. Mef2D cooperates with another animally-expressed transcription factor, FoxI1e. Together, they regulate the expression of genes encoding signaling proteins and the transcription factors that control the regional identity of animal cells. Therefore, we describe a new role for the animally-localized Mef2D protein in early ectoderm specification, which is similar to that of the vegetally-localized VegT in endoderm and mesoderm formation.

Early neural ectodermal genes are activated by Siamois and Twin during blastula stages

BMP signaling distinguishes between neural and non-neural fates by activating epidermis-specific transcription and repressing neural-specific transcription. The neural ectoderm forms after the Organizer secrets antagonists that prevent these BMP-mediated activities. However, it is not known whether neural genes also are transcriptionally activated. Therefore, we tested the ability of nine Organizer transcription factors to ectopically induce the expression of four neural ectodermal genes in epidermal precursors. We found evidence for two pathways: Foxd4 and Sox11 were only induced by Sia and Twn, whereas Gmnn and Zic2 were induced by Sia, Twn, as well as seven other Organizer transcription factors. The induction of Foxd4, Gmnn and Zic2 by Sia/Twn was both non-cell autonomous (requiring an intermediate protein) and cell autonomous (direct), whereas the induction of Sox11 required Foxd4 activity. Because direct induction by Sia/Twn could occur endogenously in the dorsal-equatorial blastula cells that give rise to both the Organizer mesoderm and the neural ectoderm, we knocked down Sia/Twn in those cells. This prevented the blastula expression of Foxd4 and Sox11, demonstrating that Sia/Twn directly activate some neural genes before the separation of the Organizer mesoderm and neural ectoderm lineages.

Siah ubiquitin ligases modulate nodal signaling during zebrafish embryonic development

Siah2 is a zebrafish homologue of mammalian Siah family. Siah acts as an E3 ubiquitin ligase that binds proteins destined for degradation. Extensive homology between siah and Drosophila Siah homologue (sina) suggests their important physiological roles during embryonic development. However, detailed functional studies of Siah in vertebrate development have not been carried out. Here we report that Siah2 specifically augments nodal related gene expression in marginal blastomeres at late blastula through early gastrula stages of zebrafish embryos. Siah2 dependent Nodal signaling augmentation is confirmed by cell-based reporter gene assays using 293T cells and 3TPluciferase reporter plasmid. We also established a molecular hierarchy of Siah as a upstream regulator of FoxH1/Fast1 transcriptional factor in Nodal signaling. Elevated expression of nodal related genes by overexpression of Siah2 was enough to override the inhibitory effects of atv and lft2 on the Nodal signaling. In particular, E3 ubiquitin ligase activity of Siah2 is critical to limit the duration and/or magnitude of Nodal signaling. Additionally, since the embryos injected with Siah morpholinos mimicked the atv overexpression phenotype at least in part, our data support a model in which Siah is involved in mesendoderm patterning via modulating Nodal signaling.

Novel animal pole-enriched maternal mRNAs are preferentially expressed in neural ectoderm

BACKGROUND

Many animals utilize maternal mRNAs to pre-pattern the embryo before the onset of zygotic transcription. In Xenopus laevis, vegetal factors specify the germ line, endoderm, and dorsal axis, but there are few studies demonstrating roles for animal-enriched maternal mRNAs. Therefore, we carried out a microarray analysis to identify novel maternal transcripts enriched in 8-cell-stage animal blastomeres.

RESULTS

We identified 39 mRNAs isolated from 8-cell animal blastomeres that are >4-fold enriched compared to vegetal pole mRNAs. We characterized 14 of these that are of unknown function. We validated the microarray results for 8/14 genes by qRT-PCR and for 14/14 genes by in situ hybridization assays. Because no developmental functions are reported yet, we provide the expression patterns for each of the 14 genes. Each is expressed in the animal hemisphere of unfertilized eggs, 8-cell animal blastomeres, and diffusely in blastula animal cap ectoderm, gastrula ectoderm and neural ectoderm, neural crest (and derivatives) and cranial placodes (and derivatives). They have varying later expression in some mesodermal and endodermal tissues in tail bud through larval stages.

CONCLUSIONS

Novel animal-enriched maternal mRNAs are preferentially expressed in ectodermal derivatives, particularly neural ectoderm. However, they are later expressed in derivatives of other germ layers.

Regulation of neurogenesis by Fgf8a requires Cdc42 signaling and a novel Cdc42 effector protein

Fibroblast growth factor (FGF) signaling is required for numerous aspects of neural development, including neural induction, CNS patterning and neurogenesis. The ability of FGFs to activate Ras/MAPK signaling is thought to be critical for these functions. However, it is unlikely that MAPK signaling can fully explain the diversity of responses to FGFs. We have characterized a Cdc42-dependent signaling pathway operating downstream of the Fgf8a splice isoform. We show that a Cdc42 effector 4-like protein (Cdc42ep4-l or Cep4l) has robust neuronal-inducing activity in Xenopus embryos. Furthermore, we find that Cep4l and Cdc42 itself are necessary and sufficient for sensory neurogenesis in vivo. Furthermore, both proteins are involved in Fgf8a-induced neuronal induction, and Cdc42/Cep4l association is promoted specifically by the Fgf8a isoform of Fgf8, but not by Fgf8b, which lacks neuronal inducing activity. Overall, these data suggest a novel role for Cdc42 in an Fgf8a-specific signaling pathway essential for vertebrate neuronal development.

The ZIC gene family encodes multi-functional proteins essential for patterning and morphogenesis

The zinc finger of the cerebellum gene (ZIC) discovered in Drosophila melanogaster (odd-paired) has five homologs in Xenopus, chicken, mice, and humans, and seven in zebrafish. This pattern of gene copy expansion is accompanied by a divergence in gene and protein structure, suggesting that Zic family members share some, but not all, functions. ZIC genes are implicated in neuroectodermal development and neural crest cell induction. All share conserved regions encoding zinc finger domains, however their heterogeneity and specification remain unexplained. In this review, the evolution, structure, and expression patterns of the ZIC homologs are described; specific functions attributable to individual family members are supported. A review of data from functional studies in Xenopus and murine models suggest that ZIC genes encode multifunctional proteins operating in a context-specific manner to drive critical events during embryogenesis. The identification of ZIC mutations in congenital syndromes highlights the relevance of these genes in human development.

Differential role of Axin RGS domain function in Wnt signaling during anteroposterior patterning and maternal axis formation

Axin is a critical component of the β-catenin destruction complex and is also necessary for Wnt signaling initiation at the level of co-receptor activation. Axin contains an RGS domain, which is similar to that of proteins that accelerate the GTPase activity of heterotrimeric Gα/Gna proteins and thereby limit the duration of active G-protein signaling. Although G-proteins are increasingly recognized as essential components of Wnt signaling, it has been unclear whether this domain of Axin might function in G-protein regulation. This study was performed to test the hypothesis that Axin RGS-Gna interactions would be required to attenuate Wnt signaling. We tested these ideas using an axin1 genetic mutant (masterblind) and antisense oligo knockdowns in developing zebrafish and Xenopus embryos. We generated a point mutation that is predicted to reduce Axin-Gna interaction and tested for the ability of the mutant forms to rescue Axin loss-of-function function. This Axin point mutation was deficient in binding to Gna proteins in vitro, and was unable to relocalize to the plasma membrane upon Gna overexpression. We found that the Axin point mutant construct failed to rescue normal anteroposterior neural patterning in masterblind mutant zebrafish, suggesting a requirement for G-protein interactions in this context. We also found that the same mutant was able to rescue deficiencies in maternal axin1 loss-of-function in Xenopus. These data suggest that maternal and zygotic Wnt signaling may differ in the extent of Axin regulation of G-protein signaling. We further report that expression of a membrane-localized Axin construct is sufficient to inhibit Wnt/β-catenin signaling and to promote Axin protein turnover.

Transcription factor Zic2 inhibits Wnt/β-catenin protein signaling

The Zic transcription factors play critical roles during embryonic development. Mutations in the ZIC2 gene are associated with human holoprosencephaly, but the etiology is still unclear. Here, we report a novel function for ZIC2 as a regulator of β-catenin·TCF4-mediated transcription. We show that ZIC2 can bind directly to the DNA-binding high mobility group box of TCF4 via its zinc finger domain and inhibit the transcriptional activity of the β-catenin·TCF4 complex. However, the binding of TCF4 to DNA was not affected by ZIC2. Zic2 RNA injection completely inhibited β-catenin-induced axis duplication in Xenopus embryos and strongly blocked the ability of β-catenin to induce expression of known Wnt targets in animal caps. Moreover, Zic2 knockdown in transgenic Xenopus Wnt reporter embryos led to ectopic Wnt signaling activity mainly at the midbrain-hindbrain boundary. Together, our results demonstrate a previously unknown role for ZIC2 as a transcriptional regulator of the β-catenin·TCF4 complex.

Identification of germ plasm-associated transcripts by microarray analysis of Xenopus vegetal cortex RNA

RNA localization is a common mechanism for regulating cell structure and function. Localized RNAs in Xenopus oocytes are critical for early development, including germline specification by the germ plasm. Despite the importance of these localized RNAs, only approximately 25 have been identified and fewer are functionally characterized. Using microarrays, we identified a large set of localized RNAs from the vegetal cortex. Overall, our results indicate a minimum of 275 localized RNAs in oocytes, or 2-3% of maternal transcripts, which are in general agreement with previous findings. We further validated vegetal localization for 24 candidates and further characterized three genes expressed in the germ plasm. We identified novel germ plasm expression for reticulon 3.1, exd2 (a novel exonuclease-domain encoding gene), and a putative noncoding RNA. Further analysis of these and other localized RNAs will likely identify new functions of germ plasm and facilitate the identification of cis-acting RNA localization elements.

Vegetally localized Xenopus trim36 regulates cortical rotation and dorsal axis formation

Specification of the dorsoventral axis in Xenopus depends on rearrangements of the egg vegetal cortex following fertilization, concomitant with activation of Wnt/beta-catenin signaling. How these processes are tied together is not clear, but RNAs localized to the vegetal cortex during oogenesis are known to be essential. Despite their importance, few vegetally localized RNAs have been examined in detail. In this study, we describe the identification of a novel localized mRNA, trim36, and characterize its function through maternal loss-of-function experiments. We find that trim36 is expressed in the germ plasm and encodes a ubiquitin ligase of the Tripartite motif-containing (Trim) family. Depletion of maternal trim36 using antisense oligonucleotides results in ventralized embryos and reduced organizer gene expression. We show that injection of wnt11 mRNA rescues this effect, suggesting that Trim36 functions upstream of Wnt/beta-catenin activation. We further find that vegetal microtubule polymerization and cortical rotation are disrupted in trim36-depleted embryos, in a manner dependent on Trim36 ubiquitin ligase activity. Additionally, these embryos can be rescued by tipping the eggs 90 degrees relative to the animal-vegetal axis. Taken together, our results suggest a role for Trim36 in controlling the stability of proteins regulating microtubule polymerization during cortical rotation, and subsequently axis formation.

Zic-associated holoprosencephaly: zebrafish Zic1 controls midline formation and forebrain patterning by regulating Nodal, Hedgehog, and retinoic acid signaling

Holoprosencephaly (HPE) is the most frequently observed human embryonic forebrain defect. Recent evidence indicates that the two major forms of HPE, classic HPE and midline interhemispheric (MIH) HPE, are elicited by two different mechanisms. The only gene known to be associated with both forms of HPE is Zic2. We used the zebrafish Danio rerio as a model system to study Zic knockdown during midline formation by looking at the close homolog Zic1, which is expressed in an overlapping fashion with Zic2. Zic1 knockdown in zebrafish leads to a strong midline defect including partial cyclopia due to attenuated Nodal and Hedgehog signaling in the anterior ventral diencephalon. Strikingly, we were able to show that Zic1 is also required for maintaining early forebrain expression of the retinoic acid (RA)-degrading enzyme cyp26a1. Zic1 LOF leads to increased RA levels in the forebrain, subsequent ventralization of the optic vesicle and down-regulation of genes involved in dorsal BMP signaling. Repression of BMP signaling in dorsal forebrain has been implicated in causing MIH HPE. This work provides a mechanistical explanation at the molecular level of why Zic factors are associated with both major forms of HPE.

Pathogenesis of holoprosencephaly

Holoprosencephaly (HPE), the most common human forebrain malformation, occurs in 1 in 250 fetuses and 1 in 16,000 live births. HPE is etiologically heterogeneous, and its pathology is variable. Several mouse models of HPE have been generated, and some of the molecular causes of different forms of HPE and the mechanisms underlying its variable pathology have been revealed by these models. Herein, we summarize the current knowledge on the genetic alterations that cause HPE and discuss some important questions about this disease that remain to be answered.

Maternal Interferon Regulatory Factor 6 is required for the differentiation of primary superficial epithelia in Danio and Xenopus embryos

Early in the development of animal embryos, superficial cells of the blastula form a distinct lineage and adopt an epithelial morphology. In different animals, the fate of these primary superficial epithelial (PSE) cells varies, and it is unclear whether pathways governing segregation of blastomeres into the PSE lineage are conserved. Mutations in the gene encoding Interferon Regulatory Factor 6 (IRF6) are associated with syndromic and non-syndromic forms of cleft lip and palate, consistent with a role for Irf6 in development of oral epithelia, and mouse Irf6 targeted null mutant embryos display abnormal differentiation of oral epithelia and skin. In Danio rerio (zebrafish) and Xenopus laevis (African clawed frog) embryos, zygotic irf6 transcripts are present in many epithelial tissues including the presumptive PSE cells and maternal irf6 transcripts are present throughout all cells at the blastula stage. Injection of antisense oligonucleotides with ability to disrupt translation of irf6 transcripts caused little or no effect on development. By contrast, injection of RNA encoding a putative dominant negative Irf6 caused epiboly arrest, loss of gene expression characteristic of the EVL, and rupture of the embryo at late gastrula stage. The dominant negative Irf6 disrupted EVL gene expression in a cell autonomous fashion. These results suggest that Irf6 translated in the oocyte or unfertilized egg suffices for early development. Supporting the importance of maternal Irf6, we show that depletion of maternal irf6 transcripts in X. laevis embryos leads to gastrulation defects and rupture of the superficial epithelium. These experiments reveal a conserved role for maternally-encoded Irf6 in differentiation of a simple epithelium in X. laevis and D. rerio. This epithelium constitutes a novel model tissue in which to explore the Irf6 regulatory pathway.

Maternal Tgif1 regulates nodal gene expression in Xenopus

In Xenopus, the maternal transcription factor VegT is necessary and sufficient to initiate the expression of nodal-related genes, which are central to many aspects of early development. However, little is known about regulation of VegT activity. Using maternal loss-of-function experiments, we show that the maternal homeoprotein, Tgif1, antagonizes VegT and plays a central role in anteroposterior patterning by negatively regulating a subset of nodal-related genes. Depletion of Tgif1 causes the anteriorization of embryos and the up-regulation of nodal paralogues nr5 and nr6. Furthermore, Tgif1 inhibits activation of nr5 by VegT in a manner that requires a C-terminal Sin3 corepressor-interacting domain. Tgif1 has been implicated in the transcriptional corepression of transforming growth factor-beta (TGFbeta) and retinoid signaling. However, we show that Tgif1 does not inhibit these pathways in early development. These results identify an essential role for Tgif1 in the control of nodal expression and provide insight into Tgif1 function and mechanisms controlling VegT activity.

A combinatorial code of maternal GATA, Ets and beta-catenin-TCF transcription factors specifies and patterns the early ascidian ectoderm

Our understanding of the maternal factors that initiate early chordate development, and of their direct zygotic targets, is still fragmentary. A molecular cascade is emerging for the mesendoderm, but less is known about the ectoderm, giving rise to epidermis and nervous tissue. Our cis-regulatory analysis surprisingly places the maternal transcription factor Ci-GATAa (GATA4/5/6) at the top of the ectodermal regulatory network in ascidians. Initially distributed throughout the embryo, Ci-GATAa activity is progressively repressed in vegetal territories by accumulating maternal beta-catenin. Once restricted to the animal hemisphere, Ci-GATAa directly activates two types of zygotic ectodermal genes. First, Ci-fog is activated from the 8-cell stage throughout the ectoderm, then Ci-otx is turned on from the 32-cell stage in neural precursors only. Whereas the enhancers of both genes contain critical and interchangeable GATA sites, their distinct patterns of activation stem from the additional presence of two Ets sites in the Ci-otx enhancer. Initially characterized as activating elements in the neural lineages, these Ets sites additionally act as repressors in non-neural lineages, and restrict GATA-mediated activation of Ci-otx. We thus identify a precise combinatorial code of maternal factors responsible for zygotic onset of a chordate ectodermal genetic program.

How the mother can help: studying maternal Wnt signaling by anti-sense-mediated depletion of maternal mRNAs and the host transfer technique

Early development in Xenopus laevis is controlled by maternal gene products synthesized during oogenesis. The dorsal/ventral and anterior/posterior axes are established as a result of canonical Wnt signaling activity. The functions of maternal genes in embryonic development are most effectively studied by introducing anti-sense, oligos complementary to their mRNAs into oocytes and culturing the oocytes long enough to allow for the breakdown of the target RNAs and the turnover of existing cognate proteins before fertilization. This method has been used to establish the role of Wnt signaling in Xenopus axis formation. Here we describe the methodology for targeting of maternal mRNAs and for successful fertilization of mRNA-depleted oocytes.

Calcium fluxes in dorsal forerunner cells antagonize beta-catenin and alter left-right patterning

Establishment of the left-right axis is essential for normal organ morphogenesis and function. Ca(2+) signaling and cilia function in the zebrafish Kuppfer's Vesicle (KV) have been implicated in laterality. Here we describe an endogenous Ca(2+) release event in the region of the KV precursors (dorsal forerunner cells, DFCs), prior to KV and cilia formation. Manipulation of Ca(2+) release to disrupt this early flux does not impact early DFC specification, but results in altered DFC migration or cohesion in the tailbud at somite stages. This leads to disruption of KV formation followed by bilateral expression of asymmetrical genes, and randomized organ laterality. We identify beta-catenin inhibition as a Ca(2+)-signaling target and demonstrate that localized loss of Ca(2+) within the DFC region or DFC-specific activation of beta-catenin is sufficient to alter laterality in zebrafish. We identify a previously unknown DFC-like cell population in Xenopus and demonstrate a similar Ca(2+)-sensitive stage. As in zebrafish, manipulation of Ca(2+) release results in ectopic nuclear beta-catenin and altered laterality. Overall, our data support a conserved early Ca(2+) requirement in DFC-like cell function in zebrafish and Xenopus.

Nodal signaling is required for closure of the anterior neural tube in zebrafish

Background

Nodals are secreted signaling proteins with many roles in vertebrate development. Here, we identify a new role for Nodal signaling in regulating closure of the rostral neural tube of zebrafish.

Results

We find that the neural tube in the presumptive forebrain fails to close in zebrafish Nodal signaling mutants. For instance, the cells that will give rise to the pineal organ fail to move from the lateral edges of the neural plate to the midline of the diencephalon. The open neural tube in Nodal signaling mutants may be due in part to reduced function of N-cadherin, a cell adhesion molecule expressed in the neural tube and required for neural tube closure. N-cadherin expression and localization to the membrane are reduced in fish that lack Nodal signaling. Further, N-cadherin mutants and morphants have a pineal phenotype similar to that of mutants with deficiencies in the Nodal pathway. Overexpression of an activated form of the TGFβ Type I receptor Taram-A (Taram-A*) cell autonomously rescues mesendoderm formation in fish with a severe decrease in Nodal signaling. We find that overexpression of Taram-A* also corrects their open neural tube defect. This suggests that, as in mammals, the mesoderm and endoderm have an important role in regulating closure of the anterior neural tube of zebrafish.

Conclusion

This work helps establish a role for Nodal signals in neurulation, and suggests that defects in Nodal signaling could underlie human neural tube defects such as exencephaly, a fatal condition characterized by an open neural tube in the anterior brain.

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.

Negative regulation of Activin/Nodal signaling by SRF during Xenopus gastrulation

Activin/Nodal signaling is essential for germ-layer formation and axial patterning during embryogenesis. Recent evidence has demonstrated that the intra- or extracellular inhibition of this signaling is crucial for ectoderm specification and correct positioning of mesoderm and endoderm. Here, we analyzed the function of Xenopus serum response factor (XSRF) in establishing germ layers during early development. XSRF transcripts are restricted to the animal pole ectoderm in Xenopus early embryos. Ectopic expression of XSRF RNA suppresses mesoderm induction, both in the marginal zone in vivo and caused by Activin/Nodal signals in animal caps. Dominant-negative mutant or antisense morpholino oligonucleotide-mediated inhibition of XSRF function expands the expression of mesendodermal genes toward the ectodermal territory and enhances the inducing activity of the Activin signal. SRF interacts with Smad2 and FAST-1, and inhibits the formation of the Smad2-FAST-1 complex induced by Activin. These results suggest that XSRF might act to ensure proper mesoderm induction in the appropriate region by inhibiting the mesoderm-inducing signals during early embryogenesis.

FoxD3 regulation of Nodal in the Spemann organizer is essential for Xenopus dorsal mesoderm development

Induction and patterning of the mesodermal germ layer is a key early step of vertebrate embryogenesis. We report that FoxD3 function in the Xenopus gastrula is essential for dorsal mesodermal development and for Nodal expression in the Spemann organizer. In embryos and explants, FoxD3 induced mesodermal genes, convergent extension movements and differentiation of axial tissues. Engrailed-FoxD3, but not VP16-FoxD3, was identical to native FoxD3 in mesoderm-inducing activity, indicating that FoxD3 functions as a transcriptional repressor to induce mesoderm. Antagonism of FoxD3 with VP16-FoxD3 or morpholino-knockdown of FoxD3 protein resulted in a complete block to axis formation, a loss of mesodermal gene expression, and an absence of axial mesoderm, indicating that transcriptional repression by FoxD3 is required for mesodermal development. FoxD3 induced mesoderm in a non-cell-autonomous manner, indicating a role for secreted inducing factors in the response to FoxD3. Consistent with this mechanism, FoxD3 was necessary and sufficient for the expression of multiple Nodal-related genes, and inhibitors of Nodal signaling blocked mesoderm induction by FoxD3. Therefore, FoxD3 is required for Nodal expression in the Spemann organizer and this function is essential for dorsal mesoderm formation.

Mesoderm induction: from caps to chips

Vertebrate mesoderm induction is one of the classical problems in developmental biology. Various developmental biology approaches, particularly in Xenopus and zebrafish, have identified many of the key factors that are involved in this process and have provided major insights into how these factors interact as part of a signalling and transcription-factor network. These data are beginning to be refined by high-throughput approaches such as microarray assays. Future challenges include understanding how the prospective mesodermal cells integrate the various signals they receive and how they resolve this information to regulate their morphogenetic behaviours and cell-fate decisions.

Patterning the early Xenopus embryo

Developmental biology teachers use the example of the frog embryo to introduce young scientists to the wonders of vertebrate development, and to pose the crucial question, 'How does a ball of cells become an exquisitely patterned embryo?'. Classical embryologists also recognized the power of the amphibian model and used extirpation and explant studies to explore early embryo polarity and to define signaling centers in blastula and gastrula stage embryos. This review revisits these early stages of Xenopus development and summarizes the recent explosion of information on the intrinsic and extrinsic factors that are responsible for the first phases of embryonic patterning.