Animal and vegetal pole cells of early Xenopus embryos respond differently to maternal dorsal determinants: implications for the patterning of the organiser

Maternal Wnt11b regulates cortical rotation during Xenopus axis formation: analysis of maternal-effect wnt11b mutants

Asymmetric signalling centres in the early embryo are essential for axis formation in vertebrates. These regions (e.g. amphibian dorsal morula, mammalian anterior visceral endoderm) require stabilised nuclear β-catenin, but the role of localised Wnt ligand signalling activity in their establishment remains unclear. In Xenopus, dorsal β-catenin is initiated by vegetal microtubule-mediated symmetry breaking in the fertilised egg, known as 'cortical rotation'. Localised wnt11b mRNA and ligand-independent activators of β-catenin have been implicated in dorsal β-catenin activation, but the extent to which each contributes to axis formation in this paradigm remains unclear. Here, we describe a CRISPR-mediated maternal-effect mutation in Xenopus laevis wnt11b.L. We find that wnt11b is maternally required for robust dorsal axis formation and for timely gastrulation, and zygotically for left-right asymmetry. Importantly, we show that vegetal microtubule assembly and cortical rotation are reduced in wnt11b mutant eggs. In addition, we show that activated Wnt coreceptor Lrp6 and Dishevelled lack behaviour consistent with roles in early β-catenin stabilisation, and that neither is regulated by Wnt11b. This work thus implicates Wnt11b in the distribution of putative dorsal determinants rather than in comprising the determinants themselves. This article has an associated 'The people behind the papers' interview.

Asymmetric Localization and Distribution of Factors Determining Cell Fate During Early Development of Xenopus laevis

Asymmetric division is a property of eukaryotic cells that is fundamental to the formation of higher life forms. Despite its importance, the mechanism behind it remains elusive. Asymmetry in the cell is induced by polarization of cell fate determinants that become unevenly distributed among progeny cells. So far dozens of determinants have been identified. Xenopus laevis is an ideal system to study asymmetric cell division during early development, because of the huge size of its oocytes and early-stage blastomeres. Here, we present the current knowledge about localization and distribution of cell fate determinants along the three body axes: animal-vegetal, dorsal-ventral, and left-right. Uneven distribution of cell fate determinants during early development specifies the formation of the embryonic body plan.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

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.

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.

Single blastomere expression profiling of Xenopus laevis embryos of 8 to 32-cells reveals developmental asymmetry

We have measured the expression of 41 maternal mRNAs in individual blastomeres collected from the 8 to 32-cell Xenopus laevis embryos to determine when and how asymmetry in the body plan is introduced. We demonstrate that the asymmetry along the animal-vegetal axis in the oocyte is transferred to the daughter cells during early cell divisions. All studied mRNAs are distributed evenly among the set of animal as well as vegetal blastomeres. We find no asymmetry in mRNA levels that might be ascribed to the dorso-ventral specification or the left-right axis formation. We hypothesize that while the animal-vegetal asymmetry is a consequence of mRNA gradients, the dorso-ventral and left-right axes specifications are induced by asymmetric distribution of other biomolecules, probably proteins.

Maternal Dead-End1 is required for vegetal cortical microtubule assembly during Xenopus axis specification

Vertebrate axis specification is an evolutionarily conserved developmental process that relies on asymmetric activation of Wnt signaling and subsequent organizer formation on the future dorsal side of the embryo. Although roles of Wnt signaling during organizer formation have been studied extensively, it is unclear how the Wnt pathway is asymmetrically activated. In Xenopus and zebrafish, the Wnt pathway is triggered by dorsal determinants, which are translocated from the vegetal pole to the future dorsal side of the embryo shortly after fertilization. The transport of dorsal determinants requires a unique microtubule network formed in the vegetal cortex shortly after fertilization. However, molecular mechanisms governing the formation of vegetal cortical microtubule arrays are not fully understood. Here we report that Dead-End 1 (Dnd1), an RNA-binding protein required for primordial germ cell development during later stages of embryogenesis, is essential for Xenopus axis specification. We show that knockdown of maternal Dnd1 specifically interferes with the formation of vegetal cortical microtubules. This, in turn, impairs translocation of dorsal determinants, the initiation of Wnt signaling, organizer formation, and ultimately results in ventralized embryos. Furthermore, we found that Dnd1 binds to a uridine-rich sequence in the 3'-UTR of trim36, a vegetally localized maternal RNA essential for vegetal cortical microtubule assembly. Dnd1 anchors trim36 to the vegetal cortex in the egg, promoting high concentrations of Trim36 protein there. Our work thus demonstrates a novel and surprising function for Dnd1 during early development and provides an important link between Dnd1, mRNA localization, the microtubule cytoskeleton and axis specification.

Xenopus Nanos1 is required to prevent endoderm gene expression and apoptosis in primordial germ cells

Nanos is expressed in multipotent cells, stem cells and primordial germ cells (PGCs) of organisms as diverse as jellyfish and humans. It functions together with Pumilio to translationally repress targeted mRNAs. Here we show by loss-of-function experiments that Xenopus Nanos1 is required to preserve PGC fate. Morpholino knockdown of maternal Nanos1 resulted in a striking decrease in PGCs and a loss of germ cells from the gonads. Lineage tracing and TUNEL staining reveal that Nanos1-deficient PGCs fail to migrate out of the endoderm. They appear to undergo apoptosis rather than convert to normal endoderm. Whereas normal PGCs do not become transcriptionally active until neurula, Nanos1-depleted PGCs prematurely exhibit a hyperphosphorylated RNA polymerase II C-terminal domain at the midblastula transition. Furthermore, they inappropriately express somatic genes characteristic of endoderm regulated by maternal VegT, including Xsox17α, Bix4, Mixer, GATA4 and Edd. We further demonstrate that Pumilio specifically binds VegT RNA in vitro and represses, along with Nanos1, VegT translation within PGCs. Repressed VegT RNA in wild-type PGCs is significantly less stable than VegT in Nanos1-depleted PGCs. Our data indicate that maternal VegT RNA is an authentic target of Nanos1/Pumilio translational repression. We propose that Nanos1 functions to translationally repress RNAs that normally specify endoderm and promote apoptosis, thus preserving the germline.

Cortical rotation and messenger RNA localization in Xenopus axis formation

In Xenopus eggs, fertilization initiates a rotational movement of the cortex relative to the cytoplasm, resulting in the transport of critical determinants to the future dorsal side of the embryo. Cortical rotation is mediated by microtubules, resulting in activation of the Wnt/β-catenin signaling pathway and expression of organizer genes on the dorsal side of the blastula. Similar cytoplasmic localizations resulting in β-catenin activation occur in many chordate embryos, suggesting a deeply conserved mechanism for patterning early embryos. This review summarizes the experimental evidence for the molecular basis of this model, focusing on recent maternal loss-of-function studies that shed light on two main unanswered questions: (1) what regulates microtubule assembly during cortical rotation and (2) how is Wnt/β-catenin signaling activated dorsally? In addition, as these processes depend on vegetally localized molecules in the oocyte, the mechanisms of RNA localization and novel roles for localized RNAs in axis formation are discussed. The work reviewed here provides a beginning framework for understanding the coupling of asymmetry in oogenesis with the establishment of asymmetry in the embryo.

Production of hepatocyte-like cells from human amnion

Cells isolated from the placenta have been the subject of intense investigation because many of the cells express characteristics of multipotent or even pluripotent stem cells. Cells from the placental tissues such as amnion and chorion have been reported to display multilineage differentiation and surface marker and gene expression patterns consistent with embryonic stem (ES) and mesenchymal stem cells, respectively. We have reported that epithelial cells isolated from term placenta contain cells that express surface markers such as the stage-specific embryonic antigens (SSEA) and a gene expression profile that is similar to ES cells. When subjected to specific differentiation protocols, amniotic epithelial cells display markers of differentiation to cardiomyocytes, neurons, pancreatic cells and hepatocytes. If specific and efficient methods could be developed to induce differentiation of these cells to hepatocytes, the amnion may become a useful source of cells for hepatocyte transplants. Cells isolated from amnion also have some unique properties as compared to some other stem cell sources in that they are isolated from a tissue that is normally discarded following birth, they are quite plentiful and easily isolated and they do not produce tumors when transplanted. Cells isolated from the amnion may be a uniquely useful and noncontroversial stem cell source.

Control of gastrula cell motility by the Goosecoid/Mix.1/ Siamois network: basic patterns and paradoxical effects

In the vegetal half of the Xenopus gastrula, cell populations differ with respect to migration on fibronectin substratum. We show that the paired-class homeodomain transcription factors Goosecoid (Gsc), Mix.1, and Siamois (Sia) are involved in the modulation of migration velocity and cell polarity. Mix.1 is expressed in the whole vegetal half and serves as a competence factor that is necessary, but not sufficient, for rapid cell migration and polarization. In the head mesoderm, Gsc and Sia are coexpressed with Mix.1, promoting rapid cell migration and polarization. Ectopic expression of Gsc and Sia in both vegetal and ventral regions often generates paradoxical effects; if a factor activates a certain motility trait in one region, it inhibits it in the other. Migration velocity and cell polarity are regulated independently. Fast and efficiently migrating multipolar cells and slow-moving polarized cells can be obtained by ectopic expression of these transcription factors in different combinations.

Expression of Siamois and Twin in the blastula Chordin/Noggin signaling center is required for brain formation in Xenopus laevis embryos

The blastula Chordin- and Noggin-expressing (BCNE) center located in the dorsal animal region of the Xenopus blastula embryo contains both prospective anterior neuroectoderm and Spemann organizer precursor cells. Here we show that, contrary to previous reports, the canonical Wnt target homeobox genes, Double knockdown of these genes using antisense morpholinos in Xenopus laevis blocked head formation, reduced the expression of the other BCNE center genes, upregulated Bmp4 expression, and nullified hyperdorsalization by lithium chloride. Moreover, gain- and loss-of-function experiments showed that Siamois and Twin expression is repressed by the vegetal transcription factor VegT. We propose that VegT expression causes maternal beta-Catenin signals to restrict Siamois and Twin expression to the BCNE region. A two-step inhibition of BMP signals by Siamois and Twin-- first by transcriptional repression of Bmp4 and then by activation of the expression of the BMP inhibitors Chordin and Noggin--in the BCNE center is required for head formation.

The Xenopus Nieuwkoop center and Spemann-Mangold organizer share molecular components and a requirement for maternal Wnt activity

In Xenopus embryos, the dorso-ventral and antero-posterior axes are established by the Spemann-Mangold organizer. According to the prevalent model of early development, the organizer is induced by the dorsalizing Nieuwkoop signal, which is secreted by the Nieuwkoop center. Formation of the center requires the maternal Wnt pathway, which is active on the dorsal side of embryos. Nevertheless, the molecular nature of the Nieuwkoop signal remains unclear. Since the Nieuwkoop center and the organizer both produce dorsalizing signals in vitro, we asked if they might share molecular components. We find that vegetal explants, the source of Nieuwkoop signal in recombination assays, express a number of organizer genes. The product of one of these genes, chordin, is required for signaling, suggesting that the organizer and the center share at least some molecular components. Furthermore, experiments with whole embryos show that maternal Wnt activity is required in the organizer just as it is needed in the Nieuwkoop center in vitro. We conclude that the maternal Wnt pathway generates the Nieuwkoop center in vitro and the organizer in vivo by activating a common set of genes, without the need of an intermediary signaling step.

Tsukushi modulates Xnr2, FGF and BMP signaling: regulation of Xenopus germ layer formation

Background

Cell-cell communication is essential in tissue patterning. In early amphibian development, mesoderm is formed in the blastula-stage embryo through inductive interactions in which vegetal cells act on overlying equatorial cells. Members of the TGF-β family such as activin B, Vg1, derrière and Xenopus nodal-related proteins (Xnrs) are candidate mesoderm inducing factors, with further activity to induce endoderm of the vegetal region. TGF-β-like ligands, including BMP, are also responsible for patterning of germ layers. In addition, FGF signaling is essential for mesoderm formation whereas FGF signal inhibition has been implicated in endoderm induction. Clearly, several signaling pathways are coordinated to produce an appropriate developmental output; although intracellular crosstalk is known to integrate multiple pathways, relatively little is known about extracellular coordination.

Methodology/Principal Findings

Here, we show that Xenopus Tsukushi (X-TSK), a member of the secreted small leucine rich repeat proteoglycan (SLRP) family, is expressed in ectoderm, endoderm, and the organizer during early development. We have previously reported that X-TSK binds to and inhibits BMP signaling in cooperation with chordin. We now demonstrate two novel interactions: X-TSK binds to and inhibits signaling by FGF8b, in addition to binding to and enhancement of Xnr2 signaling. This signal integration by X-TSK at the extracellular level has an important role in germ layer formation and patterning. Vegetally localized X-TSK potentiates endoderm formation through coordination of BMP, FGF and Xnr2 signaling. In contrast, X-TSK inhibition of FGF-MAPK signaling blocks ventrolateral mesoderm formation, while BMP inhibition enhances organizer formation. These actions of X-TSK are reliant upon its expression in endoderm and dorsal mesoderm, with relative exclusion from ventrolateral mesoderm, in a pattern shaped by FGF signals.

Conclusions/Significance

Based on our observations, we propose a novel mechanism by which X-TSK refines the field of positional information by integration of multiple pathways in the extracellular space.

R-spondin1 is a high affinity ligand for LRP6 and induces LRP6 phosphorylation and beta-catenin signaling

R-spondin proteins are newly identified secreted molecules that activate beta-catenin signaling. However, the mechanism of R-spondin action and its relationship with Wnt signaling remain unclear. Here we show that human R-spondin1 (hRspo1) is a high affinity ligand for the Wnt co-receptor LRP6 (K(d) = 1.2 nm). hRspo1 induces glycogen synthase kinase 3-dependent phosphorylation and activation of LRP6. DKK1, an LRP6 antagonist, inhibits hRspo1-induced LRP6 phosphorylation. We further demonstrate that hRspo1 synergizes with Frizzled5 in Xenopus axis induction assays and induces the phosphorylation of Dishevelled, a cytoplasmic component downstream of Frizzled function. Our study reveals interesting similarity and distinction between Wnt and R-spondin signaling.

XIC is required for Siamois activity and dorsoanterior development

Siamois is the transcriptional mediator of the dorsal Wnt signaling pathway and is necessary for formation of the Spemann organizer and dorsoanterior development in Xenopus. We have determined that XIC, a Xenopus I-mfa domain protein that regulates Tcf3 binding, is required for dorsoaxial development and specifically for Siamois activity in establishing the dorsal organizer. In loss-of-function studies, we found that embryos injected with a morpholino to XIC mRNA (XIC morphpolino) are missing head structures, neural tube, notochord, and paraxial mesoderm as well as NCAM and XMyoD expression. Although Siamois, Twin, and Xnr3 expression is normal in morpholino-injected embryos, levels of downstream organizer factors, including goosecoid, Xnot, Cerberus, and chordin, are severely reduced. Ectopic axis formation induced by Siamois is repressed by injection of the XIC morpholino and further repressed by coinjection of beta-catenin or a constitutively active Tcf3/HMG/G4A fusion. Activation of reporters driven by the Siamois-responsive proximal element of the goosecoid promoter is inhibited in the presence of the morpholino and can be rescued by murine I-mfa and by a dominant-negative Tcf3. The data indicate a role for XIC in limiting Tcf3-dependent repression of Siamois activities that are required for goosecoid transcription and for dorsal organizer formation.

Neural Tissue in Ascidian Embryos Is Induced by FGF9/16/20, Acting via a Combination of Maternal GATA and Ets Transcription Factors

In chordates, formation of neural tissue from ectodermal cells requires an induction. The molecular nature of the inducer remains controversial in vertebrates. Here, using the early neural marker Otx as an entry point, we dissected the neural induction pathway in the simple embryos of Ciona intestinalis. We first isolated the regulatory element driving Otx expression in the prospective neural tissue, showed that this element directly responds to FGF signaling and that FGF9/16/20 acts as an endogenous neural inducer. Binding site analysis and gene loss of function established that FGF9/16/20 induces neural tissue in the ectoderm via a synergy between two maternal response factors. Ets1/2 mediates general FGF responsiveness, while the restricted activity of GATAa targets the neural program to the ectoderm. Thus, our study identifies an endogenous FGF neural inducer and its early downstream gene cascade. It also reveals a role for GATA factors in FGF signaling.

Evolution of Brachyury proteins: identification of a novel regulatory domain conserved within Bilateria

Orthologues of Brachyury, a subfamily of T-box transcription factors, specify distinct cell types in different metazoan phyla, suggesting that the function of these genes has changed through the course of evolution. To investigate this evolutionary process, we have compared the activities of Brachyury orthologues from all major phyla in a single cellular context, the pluripotent Xenopus laevis animal cap. In this assay, an ancestral function is revealed: most orthologues, including the Hydra protein, mimic the action of endogenous Xenopus Brachyury, in that they induce mesoderm but not endoderm. Orthologues from Drosophila and ascidians, however, display an additional derived property, represented in our assay by the induction of endoderm. Misexpression of chimeric versions of Brachyury reveals that the C-terminal half of the protein is important for the strength of the induced response but not for its specificity. In contrast, amino acids located within the T-domain and in a short N-terminal peptide are involved in restricting the activity of Brachyury proteins to induction of mesoderm and not endoderm. Possession of this N-terminal motif is correlated with early circumblastoporal expression of Brachyury orthologues. We propose that restriction of Brachyury activity by this motif plays a conserved role in the control of Bilaterian gastrulation.

Conserved requirement of Lim1 function for cell movements during gastrulation

To investigate Lim1 function during gastrulation, we used transcript depletion through DEED antisense oligonucleotides in Xenopus and cell transplantation in mice. Xenopus embryos depleted of Lim1 lack anterior head structures and fail to form a proper axis as a result of a failure of gastrulation movements, even though mesodermal cell identities are specified. Similar disruption of cell movements in the mesoderm is also observed in Lim1(-/-) mice. Paraxial protocadherin (PAPC) expression is lost in the nascent mesoderm of Lim1(-/-) mouse embryos and in the organizer of Lim1-depleted Xenopus embryos; the latter can be rescued to a considerable extent by supplying PAPC exogenously. We conclude that a primary function of Lim1 in the early embryo is to enable proper cell movements during gastrulation.

A role for biliverdin IXalpha in dorsal axis development of Xenopus laevis embryos

The determinants of Xenopus laevis embryos that act before their first cell division are mandatory for the formation of mRNas required to establish the dorsal axis. Although their chemical identities are unknown, a number of their properties have long been recognized. One of the determinants is present in the cytoplasm and is sensitive to UV light. Thus, exposing stage 1 embryos to either standard 254-nm or, as shown here, to 366-nm UV light during the 0.3-0.4 time fraction of their first cycle inactivates the cytoplasmic determinant. As a consequence, both types of irradiated embryos fail to express dorsal markers, e.g., goosecoid and chordin, without affecting formation of ventral markers, e.g., Vent-1. The developmental outcome is dorsal axis-deficient morphology. We report here that biliverdin IXalpha, a normal constituent of cytoplasmic yolk platelets, is photo-transformed by irradiation with either 254- or 366-nm UV light and that the transformation triggers the dorsal axis deficiency. When the 254- or 366-nm UV-irradiated embryos, fated to dorsal axis deficiency, are incubated solely with microM amounts of biliverdin, they recover and form the axis. In contrast, incubation with either in vitro photo-transformed biliverdin or biliverdin IXalpha dimethyl ester does not induce recovery. The results define an approach to produce dorsal axis-deficient embryos by photo-transforming its biliverdin by irradiation with 366-nm UV light and identify an unsuspected role for biliverdin IXalpha in X. laevis embryogenesis.

Siamois functions in the early blastula to induce Spemann's organiser

In Xenopus, the Spemann organiser is defined as a dorsal territory in the early gastrula that initiates development of the embryonic axis. It has been shown that the early zygotic transcription factor Siamois is essential for Spemann's organiser formation. By the onset of gastrulation, the organiser is patterned into a vegetal head organiser, which induces anterior structures upon transplantation, and a more animal trunk organiser, which induces a posterior neuraxis. However, it is unclear when these distinct organiser domains are initially specified. To shed light on this question, we analysed the temporal activity of Siamois, as this factor induces both head and trunk development, when ectopically expressed via mRNA injection. In this study, we expressed Siamois ectopically at different time points and analysed the extent of axial development. Using a hormone-inducible version of Siamois, we found evidence for a tight window of competence during which ventral cells can respond to Siamois by commencing both the head and the trunk genetic programmes. The competence to form Spemann's organiser was lost 2 h before gastrulation, although partial axis formation could still occur following delayed activation of Siamois. We demonstrate that this late response to Siamois involves a new role for this gene, which can indirectly repress ventral gene expression, in the absence of known organiser genes.

Mesoderm formation in Eleutherodactylus coqui: body patterning in a frog with a large egg

The direct developing frog, Eleutherodactylus coqui, develops from a large egg (diameter 3.5 mm). To investigate the effect of egg size on germ-layer formation, we studied mesoderm formation in E. coqui and compared it to that of Xenopus laevis (diameter 1.3 mm). First, we identified the position of prospective mesoderm in the 16-cell E. coqui embryo by cell-lineage tracing. Although the animal blastomeres are small, they form most of the blastocoel roof and make extensive contributions to some mesodermal tissues. Second, we performed recombinant analysis with X. laevis animal caps to define the distribution of mesoderm-inducing activity. Mesoderm-inducing activity in E. coqui was restricted around the marginal zone with strong activity in the superficial cells. Neither the vegetal pole nor the blastocoel floor had activity, although these same regions from X. laevis induced mesoderm. Third, we cloned Ecbra, a homologue of Xbra, an early mesoderm marker in X. laevis. Ecbra was expressed in the marginal ring close to the surface, similar to X. laevis, but E. coqui had weaker expression on the dorsal side. Our results suggest that mesoderm formation is shifted more animally and superficially in E. coqui compared to X. laevis.

The casein kinase I family in Wnt signaling

The canonical Wnt-signaling pathway is critical for many aspects of development, and mutations in components of the Wnt pathway are carcinogenic. Recently, sufficiency tests identified casein kinase Iepsilon (CKIepsilon) as a positive component of the canonical Wnt/beta-catenin pathway, and necessity tests showed that CKIepsilon is required in vertebrates to transduce Wnt signals. In addition to CKIepsilon, the CKI family includes several other isoforms (alpha, beta, gamma, and delta) and their role in Wnt sufficiency tests had not yet been clarified. However, in Caenorhabditis elegans studies, loss-of-function of a CKI isoform most similar to alpha produced the mom phenotype, indicative of loss-of-Wnt signaling. In this report, we examine the ability of the various CKI isoforms to activate Wnt signaling and find that all the wild-type CKI isoforms do so. Dishevelled (Dsh), another positive component of the Wnt pathway, becomes phosphorylated in response to Wnt signals. All the CKI isoforms, with the exception of gamma, increase the phosphorylation of Dsh in vivo. In addition, CKI directly phosphorylates Dsh in vitro. Finally, we find that CKI is required in vivo for the Wnt-dependent phosphorylation of Dsh. These studies advance our understanding of the mechanism of Wnt action and suggest that more than one CKI isoform is capable of transducing Wnt signals in vivo.

Axis Induction by Wnt Signaling: Target Promoter Responsiveness Regulates Competence

The modulation of inductive competence is a major theme in embryonic development, but, in most cases, the underlying mechanisms are not well understood. In principle, the capacity of extracellular signals to elicit particular responses could be regulated by changes in cell surface receptors, in intracellular signaling pathways, or in the responsiveness of individual target gene promoters. As an example of regulated competence, we have examined dorsal axis induction in Xenopus embryos by Wnt signaling. Competence of Wnt proteins such as Xwnt-8 to induce an ectopic axis or the dorsal early response genes siamois and Xnr3 is lost by the onset of gastrulation, when these same ligands now produce a distinct set of "late" effects, including anterior truncation and induction of the midbrain/hindbrain marker engrailed-2. Although other Wnts apparently make use of alternative signaling mechanisms, we demonstrate that late-expressed Xwnt-8 continues to employ the canonical Wnt signaling pathway used earlier in dorsal axis induction, stabilizing cytosolic beta-catenin, and activating gene expression through Tcf/Lef transcription factors. Moreover, an activated, hormone-inducible version of XTcf-3 (TVGR) that can reproduce both early and late Wnt responses when activated at appropriate stages becomes unable to induce siamois and secondary axes at the same time as Wnt ligands themselves. Finally, we show that TVGR also loses the ability to induce expression of a reporter construct containing a small fragment of the siamois promoter, implying that this fragment contains sequences governing the loss of Wnt responsiveness before gastrulation. Together, these results argue that the competence of Wnts to induce a dorsal axis is lost in the nucleus, as a result of changes in the responsiveness of target promoters.

Patterning and lineage specification in the amphibian embryo

Xenopus has been widely used to study early embryogenesis because the embryos allow for efficient functional assays of gene products by the overexpression of RNA. The first asymmetry of the embryo is initiated during oogenesis and is manifested by the darkly pigmented animal hemisphere and lightly pigmented vegetal hemisphere. Upon fertilization a second asymmetry, the dorsal-ventral asymmetry, is established, with the sperm entry site defining the prospective ventral region. During the cleavage stage, a vegetal cortical cytoplasm (VCC)/beta-catenin signaling pathway is differentially activated on the prospective dorsal side of the embryo. The overlapping of the VCC/beta-catenin and transforming growth factor beta (TGF-beta) pathways in the dorsal vegetal quadrant specifies dorsal-vental axis formation by regulating formation of the Spemann organizer, including the anterior endomesoderm. The organizer initiates gastrulation to form a triploblastic embryo in which the mesoderm layer is located between the ectoderm layer and the endoderm layer. The interplay between maternal and zygotic TGF-beta s and the T-box transcription factors in the vegetal hemisphere initiates the specification of germ-layer lineages. TGF-beta signaling originating from the vegetal region induces mesoderm in the equatorial region, and initiates endoderm differentiation directly in the vegetal region. The ectoderm develops from the animal region, which does not come into contact with the vegetal TGF-beta signals. A large number of the downstream components and transcriptional targets of early developmental pathways have been identified and characterized. This review gives an overview of recent advances in the understanding of the functional roles and interactions of the molecular players important for axis determination and germ-layer specification during early Xenopus embryogenesis.

Initiation and early patterning of the endoderm

We review the early stages of endoderm formation in the major animal models. In Amphibia maternal molecules are important in initiating endoderm formation. This is followed by successive signaling events that establish and then pattern the endoderm. In other organisms there are differences in endodermal development, particularly in the initial, prephylotypic stages. Later many of the same key families of transcription factors and signaling cassettes are used in all animals, but more work will be needed to establish exact evolutionary homologies.

Maternal VegT is the initiator of a molecular network specifying endoderm in Xenopus laevis

During cleavage stages, maternal VegT mRNA and protein are localized to the Xenopus embryo's vegetal region from which the endoderm will arise and where several zygotic gene transcripts will be localized. Previous loss-of-function experiments on this T-box transcription factor suggested a role for VegT in Xenopus endoderm formation. Here, we test whether VegT is required to initiate endoderm formation using a loss of function approach. We find that the endodermal genes, Bix1, Bix3, Bix4, Milk (Bix2), Mix.1, Mix.2, Mixer, Xsox17 alpha, Gata4, Gata5, Gata6 and endodermin, as well as the anterior endodermal genes Xhex and cerberus, and the organizer specific gene, Xlim1, are downstream of maternal VegT. We also find that the TGF beta s, Xnr1, Xnr2, Xnr4 and derriere rescue expression of these genes, supporting the idea that cell interactions are critical for proper endoderm formation. Additionally, inhibitory forms of Xnr2 and Derriere blocked the ability of VegT mRNA injection to rescue VegT-depleted embryos. Furthermore, a subset of endodermal genes was rescued in VegT-depleted vegetal masses by induction from an uninjected vegetal mass. Finally, we begin to establish a gene hierarchy downstream of VegT by testing the ability of Mixer and Gata5 to rescue the expression of other endodermal genes. These results identify VegT as the maternal regulator of endoderm initiation and illustrate the complexity of zygotic pathways activated by VegT in the embryo's vegetal region.

Phosphatidylinositol-3 kinase acts in parallel to the ERK MAP kinase in the FGF pathway during Xenopus mesoderm induction

Phosphoinositide 3-kinases (PI3Ks) are lipid kinases that can phosphorylate phosphaditylinositides leading to the cell type-specific regulation of intracellular protein kinases. PI3Ks are involved in a wide variety of cellular events including mitogenic signalling, regulation of growth and survival, vesicular trafficking, and control of the cytoskeleton. Some of these enzymes also act downstream of receptor tyrosine kinases or G-protein-coupled receptors. Using two strategies to inhibit PI3K signalling in embryos, we have analysed the role of PI3Ks during early Xenopus development. We find that a class 1A PI3K catalytic activity is required for the definition of trunk mesoderm during the blastula stages, but is less important for endoderm and prechordal plate mesoderm induction or for organiser formation. It is required in the FGF signalling pathway downstream of Ras and in parallel to the extracellular signal-regulated kinase (ERK) MAP kinases. In addition, our results show that ERKs and PI3Ks can synergise to convert ectoderm into mesoderm. These data provide the first evidence that class 1 PI3Ks are required for a specific set of patterning events in vertebrate embryos. Furthermore, they bring new insight into the FGF signalling cascade in Xenopus.

The establishment of Spemann's organizer and patterning of the vertebrate embryo

Molecular studies have begun to unravel the sequential cell-cell signalling events that establish the dorsal-ventral, or 'back-to-belly', axis of vertebrate animals. In Xenopus and zebrafish, these events start with the movement of membrane vesicles associated with dorsal determinants. This mediates the induction of mesoderm by generating gradients of growth factors. Dorsal mesoderm then becomes a signalling centre, the Spemann's organizer, which secretes several antagonists of growth-factor signalling. Recent studies have led to new models for the regulation of cell-cell signalling during development, which may also apply to the homeostasis of adult tissues.

A beta-catenin/engrailed chimera selectively suppresses Wnt signaling

beta-catenin plays an integral role in cell-cell adhesion by linking the cadherin complex of the adherens junction to the underlying actin cytoskeleton. In addition, beta-catenin transduces intracellular signals within the Wnt developmental pathway that are crucial to the proper establishment of embryonic axes and pattern formation of early mesoderm and ectoderm. For example, in the context of a defined dorsal ‘organizer’ region of early Xenopus embryos, beta-catenin enters the nucleus and associates with transcription factors of the HMG (High Mobility Group) Lef/Tcf protein family. Consequently, genes such as siamois, a homeobox gene contributing to the specification of the dorsoanterior axis, are activated. To further examine the role that beta-catenin plays in Wnt signaling, we generated a chimeric protein, beta-Engrailed (beta-Eng), in which the C-terminal trans-activation domain of beta-catenin is replaced with the transcriptional repression domain of Drosophila Engrailed. Dorsal overexpression of this mRNA in early Xenopus embryos leads to suppression of organizer-specific molecular markers such as siamois, Xnr-3 and goosecoid, corresponding with the dramatic morphological ventralization of embryos. Ventralized embryos further exhibit reduced activity of the Wnt pathway, as indicated by the loss of the notochord/organizer marker, chordin. Importantly, beta-Eng associates and functions normally with the known components of the cadherin complex, providing the experimental opportunity to repress beta-catenin's signaling function apart from its role in cadherin-mediated cell-cell adhesion.

The putative wnt receptor Xenopus frizzled-7 functions upstream of beta-catenin in vertebrate dorsoventral mesoderm patterning

We have isolated one member of the frizzled family of wnt receptors from Xenopus (Xfz7) to study the role of cell-cell communication in the establishment of the vertebrate axis. We demonstrate that this maternally encoded protein specifically synergizes with wnt proteins in ectopic axis induction. Embryos derived from oocytes depleted of maternal Xfz7 RNA by antisense oligonucleotide injection are deficient in dorsoanterior structures. Xfz7-depleted embryos are deficient in dorsal but not ventral mesoderm due to the reduced expression of the wnt target genes siamois, Xnr3 and goosecoid. These signaling defects can be restored by the addition of beta-catenin but not Xwnt8b. Xfz7 thus functions upstream of the known GSK-3/axin/beta-catenin intracellular signaling complex in vertebrate dorsoventral mesoderm specification.

Dorsal induction from dorsal vegetal cells in Xenopus occurs after mid-blastula transition

We performed some experiments to investigate the temporal and spatial details of the dorsal induction exerted by dorsal vegetal cells in Xenopus embryo. Two dorsal vegetal cells (D1 cells) were transplanted into the ventral vegetal region of a recipient at the 32-cell stage. At various times after transplantation, the ventral animal-equatorial part was explanted and cultured. The explants isolated 5.5 h after transplantation (time 5.5) elongated and formed somites. In RT-PCR analysis, the expression of dorsal gene, chordin was activated in the explants isolated after time 4.0 (about the 4000-cell stage which corresponds to the mid blastula transition (MBT)) at control stage 10. In another series of experiments, ventral animal-equatorial and dorsal vegetal parts were isolated from the 4000-cell stage embryos and they were combined for 2.0-2.5 h. These ventral animal-equatorial explants elongated and formed somites. The chordin expression was also observed in the explants. But the 32- and 256-cell stage dorsal vegetal cells failed to exert the dorsalizing activity within the 2.0-2.5 h of the conjugation. These results suggest that 2 h contact after MBT is necessary and sufficient for the dorsal induction from the dorsal vegetal cells and it occurs as a result of the zygotic gene expression. Consistent with this idea, the zygotic dorsal genes, siamois and chordin were expressed on the upper regions of the transplanted D1 descendants at stage 10. Furthermore, this region began to gastrulate when the D1 cell was transplanted with upside-down orientation. Our data indicate that the upper region of the D1 descendants by itself act as the Spemann organizer rather than the Nieuwkoop center.

HNF1(beta) is required for mesoderm induction in the Xenopus embryo

XHNF1(β) is a homeobox-containing gene initially expressed at the blastula stage in the vegetal part of the Xenopus embryo. We investigated its early role by functional ablation, through mRNA injection of an XHNF1(beta)/engrailed repressor fusion construct (XHNF1(beta)/EngR). Dorsal injections of XHNF1(beta)/EngR mRNA abolish dorsal mesoderm formation, leading to axial deficiencies; ventral injections disrupt ventral mesoderm formation without affecting axial development. XHNF1(beta)/EngR phenotypic effects specifically depend on the DNA-binding activity of its homeodomain and are fully rescued by coinjection of XHNF1(beta) mRNA. Vegetal injection of XHNF1(beta)/EngR mRNA blocks the mesoderm-inducing ability of vegetal explants. Both B-Vg1 and VegT maternal determinants trigger XHNF1(beta) expression in animal caps. XHNF1(beta)/EngR mRNA blocks B-Vg1-mediated, but not by eFGF-mediated, mesoderm induction in animals caps. However, wild-type XHNF1(beta) mRNA does not trigger Xbra expression in animal caps. We conclude that XHNF1(beta) function is essential, though not sufficient, for mesoderm induction in the Xenopus embryo.

Regulation of the early expression of the Xenopus nodal-related 1 gene, Xnr1

The Xenopus nodal related-1 (Xnr1) gene has a complex expression pattern in embryos, with two temporal phases. In the first phase, transcripts are first detected in perinuclear sites in the vegetal region of the blastula. During gastrulation, this expression disappears and transcripts become localised to the dorsal marginal zone. Expression stops and then restarts in a second phase at neurula and tailbud stages, firstly in two symmetric patches near the posterior end of the notochord, and then asymmetrically in a large domain in the left lateral plate mesoderm. In this study, we have investigated the regulation of the early phase of expression of Xnr1. We show that the T-box transcription factor VegT can induce Xnr1. It had previously been shown that Xnr1 can induce VegT in ectoderm cells and we show that the early expression of Xnr1 is regulated by an autoregulatory loop. By inspection of the Xnr1 promoter sequence, we have identified two non-palindromic T-box-binding sites, which are 10 bp apart. Using mutational analysis, we have shown that these elements are required for the VegT induction of Xnr1. The Xnr1 promoter shows striking homologies with the Xnr3 promoter. In particular, two elements that are required for Wnt signaling are conserved between these two promoters, but the two T-box sites are not conserved, and Xnr3 is not induced by VegT. A region of the promoter containing the T-box sites and the Wnt sites is sufficient to drive expression of a reporter gene in a dorsal domain in transgenic Xenopus at the gastrula stage. We show that this pattern of expression of the transgene in gastrulae is not dependent on the T-box sites.

Interaction among GSK-3, GBP, axin, and APC in Xenopus axis specification

Glycogen synthase kinase 3 (GSK-3) is a constitutively active kinase that negatively regulates its substrates, one of which is beta-catenin, a downstream effector of the Wnt signaling pathway that is required for dorsal-ventral axis specification in the Xenopus embryo. GSK-3 activity is regulated through the opposing activities of multiple proteins. Axin, GSK-3, and beta-catenin form a complex that promotes the GSK-3-mediated phosphorylation and subsequent degradation of beta-catenin. Adenomatous polyposis coli (APC) joins the complex and downregulates beta-catenin in mammalian cells, but its role in Xenopus is less clear. In contrast, GBP, which is required for axis formation in Xenopus, binds and inhibits GSK-3. We show here that GSK-3 binding protein (GBP) inhibits GSK-3, in part, by preventing Axin from binding GSK-3. Similarly, we present evidence that a dominant-negative GSK-3 mutant, which causes the same effects as GBP, keeps endogenous GSK-3 from binding to Axin. We show that GBP also functions by preventing the GSK-3-mediated phosphorylation of a protein substrate without eliminating its catalytic activity. Finally, we show that the previously demonstrated axis-inducing property of overexpressed APC is attributable to its ability to stabilize cytoplasmic beta-catenin levels, demonstrating that APC is impinging upon the canonical Wnt pathway in this model system. These results contribute to our growing understanding of how GSK-3 regulation in the early embryo leads to regional differences in beta-catenin levels and establishment of the dorsal axis.

Relationship of vegetal cortical dorsal factors in the Xenopus egg with the Wnt/beta-catenin signaling pathway

In Xenopus, the dorsal factor in the vegetal cortical cytoplasm (VCC) of the egg is responsible for axis formation of the embryo. Previous studies have shown that VCC dorsal factor has properties similar to activators of the Wnt/beta-catenin-signaling pathway. In this study, we examined the relationship of the VCC dorsal factor with components of the pathway. First, we tested whether beta-catenin protein, which is known to be localized on the dorsal side of early embryos, accounts for the dorsal axis activity of VCC. Reduction of beta-catenin mRNA and protein in oocytes did not diminish the activity of VCC to induce a secondary axis in recipient embryos. The amount of beta-catenin protein was not enriched in VCC compared to animal cortical cytoplasm, which has no dorsal axis activity. These results indicate that beta-catenin is unlikely to be the VCC dorsal axis factor. Secondly, we examined the effects of four Wnt-pathway-interfering constructs (dominant-negative Xdsh, XGSK3, Axin, and dominant-negative XTcf3) on the ability of VCC to induce expression of the early Wnt target genes, Siamois and Xnr3. The activity of VCC was inhibited by Axin and dominant negative XTcf3 but not by dominant negative Xdsh or XGSK3. We also showed that VCC decreased neither the amount nor the activity of exogenous XGSK3, suggesting that the VCC dorsal factor does not act by affecting XGSK3 directly. Finally, we tested six Wnt-pathway activating constructs (Xwnt8, Xdsh, dominant negative XGSK3, dominant negative Axin, XAPC and beta-catenin) for their responses to the four Wnt-pathway-interfering constructs. We found that only XAPC exhibited the same responses as VCC; it was inhibited by Axin and dominant negative XTcf3 but not by dominant negative Xdsh or XGSK3. Although the connection between XAPC and the VCC dorsal factor is not yet clear, the fact that APC binds Axin suggests that the VCC dorsal factor could act on Axin rather than XGSK3.

A cell-free assay system for beta-catenin signaling that recapitulates direct inductive events in the early xenopus laevis embryo

In vertebrate embryos, signaling via the beta-catenin protein is known to play an essential role in the induction of the dorsal axis. In its signaling capacity, beta-catenin acts directly to affect target gene transcription, in concert with transcription factors of the TCF/LEF family. We have developed a cell-free in vitro assay for beta-catenin signaling activity that utilizes transcriptionally active nuclei and cytoplasm from cleavage-blocked Xenopus laevis embryos. Under these assay conditions, we demonstrate that either addition of beta-catenin protein or upstream activation of the beta-catenin signaling pathway can induce the expression of developmentally relevant target genes. Addition of exogenous beta-catenin protein induced expression of Siamois, XTwin, Xnr3, and Cerberus mRNAs in a protein synthesis independent manner, whereas a panel of other Spemann organizer-specific genes did not respond to beta-catenin. Lithium induction of the beta-catenin signaling pathway, which is thought to cause beta-catenin accumulation by inhibiting its proteasome-dependent degradation, caused increased expression of Siamois in a protein synthesis independent fashion. This result suggests that beta-catenin derived from a preexisting pool can be activated to signal, and that accumulation of this activated form does not require ongoing synthesis. Furthermore, activation of the signaling pathway with lithium did not detectably alter cytoplasmic beta-catenin levels and was insensitive to inhibition of the proteasome- dependent degradation pathway. Taken together, these results suggest that activation of beta-catenin signaling by lithium in this system may occur through a distinct activation mechanism that does not require modulation of levels through regulation of proteasomal degradation.

Transcriptional regulation in Xenopus: a bright and froggy future

Xenopus has played a key role in defining the general mechanisms that underlie early vertebrate development. Recent studies reveal how the transcriptional regulation of signaling and transcription factors is used to pattern the early dorsal-ventral axis. With the development of new methods for producing transgenic frogs, Xenopus will become a very attractive system for studying transcriptional regulation at all stages of embryogenesis.

Gastrulation in zebrafish: what mutants teach us

A major approach to the study of development is to compare the phenotypes of normal and mutant individuals for a given genetic locus. Understanding the development of a complex metazoan therefore requires examination of many mutants. Relatively few organisms are being studied this way, and zebrafish is currently the best example of a vertebrate for which large-scale mutagenesis screens have successfully been carried out. The number of genes mutated in zebrafish that have been cloned expands rapidly, bringing new insights into a number of developmental pathways operating in vertebrates. Here, we discuss work on zebrafish mutants affecting gastrulation and patterning of the early embryo. Gastrulation is orchestrated by the dorsal organizer, which forms in a region where maternally derived beta-catenin signaling is active. Mutation in the zygotic homeobox gene bozozok disrupts the organizer genetic program and leads to severe axial deficiencies, indicating that this gene is a functional target of beta-catenin signaling. Once established, the organizer releases inhibitors of ventralizing signals, such as BMPs, and promotes dorsoanterior fates within all germ layers. In zebrafish, several mutations affecting dorsal-ventral (D/V) patterning inactivate genes functioning in the BMP pathway, stressing the central role of this pathway in the gastrula embryo. Cells derived from the organizer differentiate into several axial structures, such as notochord and prechordal mesoderm, which are thought to induce various fates in adjacent tissues, such as the floor plate, after the completion of gastrulation. Studies with mutants in nodal-related genes, in one-eyed pinhead, which is required for nodal signaling, and in the Notch pathway reveal that midline cell fate specification is, in fact, initiated during gastrulation. Furthermore, the organizer coordinates morphogenetic movements, and zebrafish mutants in T-box mesoderm-specific genes help clarify the mechanism of convergence movements required for the formation of axial and paraxial mesoderm.

An anterior signalling centre in Xenopus revealed by the homeobox gene XHex

BACKGROUND

Signals from anterior endodermal cells that express the homeobox gene Hex initiate development of the most rostral tissues of the mouse embryo. The dorsal/anterior endoderm of the Xenopus gastrula, which expresses Hex and the putative head-inducing gene cerberus, is proposed to be equivalent to the mouse anterior endoderm. Here, we report the origin and signalling properties of this population of cells in the early Xenopus embryo.

RESULTS

Xenopus anterior endoderm was found to derive in part from cells at the centre of the blastocoel floor that express XHex, the Xenopus cognate of Hex. Like their counterparts in the mouse embryo, these Hex-expressing blastomeres moved to the dorsal side of the Xenopus embryo as gastrulation commenced, and populated deep endodermal adjacent to Spemann's organiser. Experiments involving the induction of secondary axes confirmed that XHex expression was associated with anterior development. Ventral misexpression of XHex induced ectopic cerberus expression and conferred anterior signalling properties to the endoderm. Unlike the effect of misexpressing cerberus, these signals could not neuralise overlying ectoderm.

CONCLUSIONS

XHex expression reveals the unexpected origin of an anterior signalling centre in Xenopus, which arises in part from the centre of the blastula and localises to the deep endoderm adjacent to Spemann's organiser. Signals originating from these endodermal cells impart an anterior identity to the overlying ectoderm, but are insufficient for neural induction. The anterior movement of Hex-expressing cells in both Xenopus and mouse embryos suggests that this process is a conserved feature of vertebrate development.

A two-step model for the fate determination of presumptive endodermal blastomeres in Xenopus embryos

BACKGROUND

In Xenopus, the endoderm germ layer is derived from the vegetal blastomeres of cleavage-stage embryos. Cell transplantation experiments have revealed that the endodermal fate becomes gradually fixed during the late blastula stages. Sox17alpha, Mix.1, Mixer and GATA-4 encode vegetal zygotic transcription factors with endoderm-inducing activity. The accumulation of their transcripts during the late blastula stages may cause determination of the endodermal fate. VegT, a T-box transcription factor, the maternal transcripts of which are vegetally localised, is also required for endoderm formation.

RESULTS

We analysed the events leading to the progressive accumulation of the transcripts for Sox17alpha, Mix.1, Mixer and GATA-4. Two phases could be distinguished in the endodermal programme. In phase 1, Sox17alpha, Mix.1, and the genes encoding transforming growth factor beta-related signalling molecules Xnr1, Xnr2 and Derrière were activated cell-autonomously at around the mid-blastula transition (MBT) by maternal determinants. In phase 2, TGFbeta signalling, possibly involving Xnr1, Xnr2 and Derrière, led to the activation of Mixer and GATA-4 in late blastula stages and to the reinforcement of the expression of Sox17alpha and Mix.1. Overexpression of VegT in animal caps triggered a developmental programme qualitatively similar to that observed in vegetal blastomeres, except that Xnr1 and GATA-4 were not activated by the early gastrula stage.

CONCLUSIONS

Our results support a two-step model for endoderm determination between fertilisation and the onset of gastrulation. The initial cell-autonomous activation of early endodermal genes by maternal determinants including, but not limited to, VegT is relayed by the action of zygotic TGFbetas such as Xnr1, Xnr2 and Derrière.

Wnt signaling and dorso-ventral axis specification in vertebrates

The dorso-ventral axis is specified in vertebrates through the formation of a dorsal signaling center known as the Spemann organizer. This process depends on signal transduction by beta-catenin that can be regulated by secreted Wnt proteins. Recent discoveries of new players in this signaling pathway have narrowed down the search for the initial cues for axis specification in vertebrate embryos.

Spatially distinct head and heart inducers within the Xenopus organizer region

BACKGROUND

The mouse anterior visceral endoderm, an extraembryonic tissue, expresses several genes essential for normal development of structures rostral to the anterior limit of the notochord and has been termed the head organizer. This tissue also has heart-inducing activity and expresses mCer1 which, like its Xenopus homolog cerberus, can induce markers of cardiac specification and anterior neural tissue when ectopically expressed. We investigated the relationship between head and heart induction in Xenopus embryos, which lack extraembryonic tissues.

RESULTS

We found three regions of gene expression in the Xenopus organizer: deep endoderm, which expressed cerberus; prechordal mesoderm, which showed overlapping but non-identical expression of genes characteristic of the murine head organizer, such as XHex and XANF-1; and leading-edge dorsoanterior endoderm, which expressed both cerberus and a subset of the genes expressed by the prechordal mesoderm. Microsurgical ablation of the cerberus-expressing endoderm decreased the incidence of heart, but not head, formation. Removal of prechordal mesoderm, in contrast, caused deficits of anterior head structures. Finally, although misexpression of cerberus induced ectopic heads, it was unable to induce genes thought to participate in head induction.

CONCLUSIONS

In Xenopus, the cerberus-expressing endoderm is required for heart, but not head, inducing activity. Therefore, this tissue is not the topological equivalent of the murine anterior visceral endoderm. We propose that, in Xenopus, cerberus is redundant to other bone morphogenetic protein (BMP) and Wnt antagonists located in prechordal mesoderm for head induction, but may be necessary for heart induction.

Regulation of dorsal gene expression in Xenopus by the ventralizing homeodomain gene Vox

Patterning in the vertebrate embryo is controlled by an interplay between signals from the dorsal organizer and the ventrally expressed BMPs. Here we examine the function of Vox, a homeodomain-containing gene that is activated by the ventralizing signal BMP-4. Inhibition of BMP signaling using a dominant negative BMP receptor (DeltaBMPR) leads to the ectopic activation of dorsal genes in the ventral marginal zone, and this activation is prevented by co-injection of Vox. chordin is the most strongly activated of those genes that are up-regulated by DeltaBMPR and is the gene most strongly inhibited by Vox expression. We demonstrate that Vox acts as a transcriptional repressor, showing that the activity of native Vox is mimicked by a Vox-repressor fusion (VoxEnR) and that a Vox-activator fusion (VoxG4A) acts as an antimorph, causing the formation of a partial secondary axis when expressed on the ventral side of the embryo. Although Vox can ectopically activate BMP-4 expression in whole embryos, we see no activation of BMP-4 by VoxG4A, demonstrating that this activation is indirect. Using a hormone-inducible version of VoxG4A, we find that a critical time window for Vox function is during the late blastula period. Using this construct, we demonstrate that only a subset of dorsal genes is directly repressed by Vox, revealing that there are different modes of regulation for organizer genes. Since the major direct target for Vox repression is chordin, we propose that Vox acts in establishing a BMP-4 morphogen gradient by restricting the expression domain of chordin.

Organisation of Xenopus oocyte and egg cortices

The division of the Xenopus oocyte cortex into structurally and functionally distinct "animal" and "vegetal" regions during oogenesis provides the basis of the organisation of the early embryo. The vegetal region of the cortex accumulates specific maternal mRNAs that specify the development of the endoderm and mesoderm, as well as functionally-defined "determinants" of dorso-anterior development, and recognisable "germ plasm" determinants that segregate into primary germ cells. These localised elements on the vegetal cortex underlie both the primary animal-vegetal polarity of the egg and the organisation of the developing embryo. The animal cortex meanwhile becomes specialised for the events associated with fertilisation: sperm entry, calcium release into the cytoplasm, cortical granule exocytosis, and polarised cortical contraction. Cortical and subcortical reorganisations associated with meiotic maturation, fertilisation, cortical rotation, and the first mitotic cleavage divisions redistribute the vegetal cortical determinants, contributing to the specification of dorso-anterior axis and segregation of the germ line. In this article we consider what is known about the changing organisation of the oocyte and egg cortex in relation to the mechanisms of determinant localisation, anchorage, and redistribution, and show novel ultrastructural views of cortices isolated at different stages and processed by the rapid-freeze deep-etch method. Cortical organisation involves interactions between the different cytoskeletal filament systems and internal membranes. Associated proteins and cytoplasmic signals probably modulate these interactions in stage-specific ways, leaving much to be understood.

Spemann organizer activity of Smad10

The Spemann organizer induces neural tissue, dorsalizes mesoderm and generates a second dorsal axis. We report the isolation and characterization of Smad10, which has all three of these Spemann activities. Smad10 is expressed at the appropriate time to transduce Spemann signals endogenously. Like the organizer, Smad10 generates anterior and posterior neural tissues. Smad10 appears to function downstream of the Spemann organizer, consistent with a role in mediating organizer-derived signals. Interestingly, Smad10, unlike previously characterized mediators of Spemann activity, does not appear to block BMP signals. This finding, coupled with the functional activity and expression profile, suggests that Smad10 mediates Spemann action in a novel manner.

Embryonic induction: is the Nieuwkoop centre a useful concept?

Regionalisation of the amphibian embryo is classically thought to involve induction by the Spemann organiser, itself induced by the Nieuwkoop centre. This model has now been extended to teleosts, with the identification of a gene that appears to define the zebrafish equivalent of the Nieuwkoop centre.