Dorsal determinants in the Xenopus egg are firmly associated with the vegetal cortex and behave like activators of the Wnt pathway

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.

Maternal syntabulin is required for dorsal axis formation and is a germ plasm component in Xenopus

In amphibians and teleosts, early embryonic axial development is driven by maternally deposited mRNAs and proteins, called dorsal determinants, which migrate to the presumptive dorsal side of the embryo in a microtubule-dependent manner after fertilization. Syntabulin is an adapter protein that binds to kinesin KIF5B and to the transmembrane protein Syntaxin1. In zebrafish, a mutation in Syntabulin causes complete embryo ventralization. It is unknown whether Syntabulin plays an analogous role during early development of other species, a question addressed here in Xenopus laevis. in situ hybridization of syntabulin mRNA was carried out at different stages of Xenopus development. In oocytes, syntabulin transcripts were localized to the vegetal cortex of large oocytes and the mitochondrial cloud of very young oocytes. We extended the zebrafish data by finding that during cleavage Xenopus syntabulin mRNA localized to the germ plasm and was later expressed in primordial germ cells (PGCs). This new finding suggested a role for Syntabulin during germ cell differentiation. The functional role of maternal syntabulin mRNA was investigated by knock-down with phosphorothioate DNA antisense oligos followed by oocyte transfer. The results showed that syntabulin mRNA depletion caused the complete loss of dorso-anterior axis formation in frog embryos. Consistent with the ventralized phenotype, syntabulin-depleted embryos displayed severe reduction of dorsal markers and ubiquitous transcription of the ventral marker sizzled. Syntabulin was required for the maternal Wnt/β-Catenin signal, since ventralization could be completely rescued by injection of β-catenin (or syntabulin) mRNA. The data suggest an evolutionarily conserved role for Syntabulin, a protein that bridges microtubule motors and membrane vesicles, during dorso-ventral axis formation in the vertebrates.

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.

Asymmetric Localization of CK2α During Xenopus Oogenesis

The establishment of the dorso-ventral axis is a fundamental process that occurs after fertilization. Dorsal axis specification in frogs starts immediately after fertilization, and depends upon activation of Wnt/β-catenin signaling. The protein kinase CK2α can modulate Wnt/β-catenin signaling and is necessary for dorsal axis specification in Xenopus laevis. Our previous experiments show that CK2α transcripts and protein are animally localized in embryos, overlapping the region where Wnt/β-catenin signaling is activated. Here we determined whether the animal localization of CK2α in the embryo is preceded by its localization in the oocyte. We found that CK2α transcripts were detected from stage I, their levels increased during oogenesis, and were animally localized as early as stage III. CK2α transcripts were translated during oogenesis and CK2α protein was localized to the animal hemisphere of stage VI oocytes. We cloned the CK2α 3’UTR and showed that the 2.8 kb CK2α transcript containing the 3’UTR was enriched during oogenesis. By injecting ectopic mRNAs, we demonstrated that both the coding and 3’UTR regions were necessary for proper CK2α transcript localization. This is the first report showing the involvement of coding and 3’UTR regions in animal transcript localization. Our findings demonstrate the pre-localization of CK2α transcript and thus, CK2α protein, in the oocyte. This may help restrict CK2α expression in preparation for dorsal axis specification.

A co-dependent requirement of xBcl9 and Pygopus for embryonic body axis development in Xenopus

The Wnt/beta-catenin transcriptional activation complex requires the adapter protein Pygopus (Pygo), which links the basal transcription machinery to beta-catenin, by its association with legless (Lgs)/ B-cell lymphoma-9 (Bcl9). Pygo was shown to be required for development in vertebrates, but the role of Lgs/Bcl9 is unknown. We identified an amphibian orthologue of Lgs/Bcl9, XBcl9, which interacted biochemically with Xbeta-catenin and XPygo2. The body axis promoting ability of Xbeta-catenin was diminished when residues required for its interaction with XBcl9 were mutated. In blastula embryos, XBcl9 was transiently preferentially expressed in nuclei of dorsoanterior cells and ectopically expressed XBcl9 required XPygo2 to localize to nuclei. Furthermore, while neither XBcl9 nor XPygo2 alone affected development when ectopically expressed, both were required to induce supernumerary axis and dorsal gene activation. Like XPygo2, depletion of maternal XBcl9 alone caused dorsal defects. These results indicated an essential role of the Pygo-Bcl9 duet in vertebrate body axis formation.

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.

Hex acts with beta-catenin to regulate anteroposterior patterning via a Groucho-related co-repressor and Nodal

In Xenopus, the establishment of the anteroposterior axis involves two key signalling pathways, canonical Wnt and Nodal-related TGFbeta. There are also a number of transcription factors that feedback upon these pathways. The homeodomain protein Hex, an early marker of anterior positional information, acts as a transcriptional repressor, suppressing induction and propagation of the Spemman organiser while specifying anterior identity. We show that Hex promotes anterior identity by amplifying the activity of canonical Wnt signalling. Hex exerts this activity by inhibiting the expression of Tle4, a member of the Groucho family of transcriptional co-repressors that we identified as a Hex target in embryonic stem (ES) cells and Xenopus embryos. This Hex-mediated enhancement of Wnt signalling results in the upregulation of the Nieuwkoop centre genes Siamois and Xnr3, and the subsequent increased expression of the anterior endodermal marker Cerberus and other mesendodermal genes downstream of Wnt signalling. We also identified Nodal as a Hex target in ES cells. We demonstrate that in Xenopus, the Nodal-related genes Xnr1 and Xnr2, but not Xnr5 and Xnr6, are regulated directly by Hex. The identification of Nodal-related genes as Hex targets explains the ability of Hex to suppress induction and propagation of the organiser. Together, these results support a model in which Hex acts early in development to reinforce a Wnt-mediated, Nieuwkoop-like signal to induce anterior endoderm, and later in this tissue to block further propagation of Nodal-related signals. The ability of Hex to regulate the same targets in both Xenopus and mouse implies this model is conserved.

Potential structural role of non-coding and coding RNAs in the organization of the cytoskeleton at the vegetal cortex of Xenopus oocytes

The localization of RNA within a cell or embryo is crucial for proper cellular function or development. There is evidence that the cytoskeleton and RNA may function in the anchoring of localized RNAs at the vegetal cortex of Xenopus laevis oocytes. We found that the organization of the cytokeratin filaments but not the actin cytoskeleton depends on the presence of intact VegT mRNA and a noncoding RNA, Xlsirts. Destruction of either of these transcripts results in disruption of the cytokeratin cytoskeleton in a transcript-specific manner and interferes with proper formation of the germinal granules and subsequent development of the germline. Analysis of the distribution of endogenous VegT and Xlsirts in live oocytes using molecular beacons showed that these RNAs are integrated into the cytokeratin cytoskeleton. These results demonstrate a novel structural role of coding and noncoding RNAs in the organization of the vegetal cortex of Xenopus oocytes.

The ARID domain protein dril1 is necessary for TGF(beta) signaling in Xenopus embryos

ARID domain proteins are members of a highly conserved family involved in chromatin remodeling and cell-fate determination. Dril1 is the founding member of the ARID family and is involved in developmental processes in both Drosophila and Caenorhabditis elegans. We describe the first embryological characterization of this gene in chordates. Dril1 mRNA expression is spatiotemporally regulated and is detected in the involuting mesoderm during gastrulation. Inhibition of dril1 by either a morpholino or an engrailed repressor-dril1 DNA binding domain fusion construct inhibits gastrulation and perturbs induction of the zygotic mesodermal marker Xbra and the organizer markers chordin, noggin, and Xlim1. Xenopus tropicalis dril1 morphants also exhibit impaired gastrulation and axial deficiencies, which can be rescued by coinjection of Xenopus laevis dril1 mRNA. Loss of dril1 inhibits the response of animal caps to activin and secondary axis induction by smad2. Dril1 depletion in animal caps prevents both the smad2-mediated induction of dorsal mesodermal and endodermal markers and the induction of ventral mesoderm by smad1. Mesoderm induction by eFGF is uninhibited in dril1 morphant caps, reflecting pathway specificity for dril1. These experiments identify dril1 as a novel regulator of TGF(beta) signaling and a vital component of mesodermal patterning and embryonic morphogenesis.

Genetic evidence for involvement of maternally derived Wnt canonical signaling in dorsal determination in zebrafish

In zebrafish, the program for dorsal specification begins soon after fertilization. Dorsal determinants are localized initially to the vegetal pole, then transported to the blastoderm, where they are thought to activate the canonical Wnt pathway, which induces the expression of dorsal-specific genes. We identified a novel maternal-effect recessive mutation, tokkaebi (tkk), that affects formation of the dorsal axis. Severely ventralized phenotypes, including a lack of dorso-anterior structures, were seen in 5-100% of the embryos obtained from tkk homozygous transmitting females. tkk embryos displayed defects in the nuclear accumulation of beta-catenin on the dorsal side, and reduced or absent expression of dorsal-specific genes. Mesoderm and endoderm formation outside the dorsal axis was not significantly affected. Injection of RNAs for activated beta-catenin, dominant-negative forms of Axin1 and GSK3beta, and wild-type Dvl3, into the tkk embryos suppressed the ventralized phenotypes and/or dorsalized the embryos, and restored or induced an ectopic and expanded expression of bozozok/dharma and goosecoid. However, dorsalization by wnt RNAs was affected in the tkk embryos. Inhibition of cytoplasmic calcium release elicited an ectopic and expanded expression of chordin in the wild-type, but did not restore chordin expression efficiently in the tkk embryos. These data indicate that the tkk gene product functions upstream of or parallel to the beta-catenin-degradation machinery to control the stability of beta-catenin. The tkk locus was mapped to chromosome 16. These data provide genetic evidence that the maternally derived canonical Wnt pathway upstream of beta-catenin is involved in dorsal axis formation in zebrafish.

Protein kinase CK2 is required for dorsal axis formation in Xenopus embryos

Dorsal axis formation in Xenopus embryos is dependent upon asymmetrical localization of beta-catenin, a transducer of the canonical Wnt signaling pathway. Recent biochemical experiments have implicated protein kinase CK2 as a regulator of members of the Wnt pathway including beta-catenin. Here, we have examined the role of CK2 in dorsal axis formation. CK2 was present in the developing embryo at an appropriate time and place to participate in dorsal axis formation. Overexpression of mRNA encoding CK2 in ventral blastomeres was sufficient to induce a complete ectopic axis, mimicking Wnt signaling. A kinase-inactive mutant of CK2alpha was able to block ectopic axis formation induced by XWnt8 and beta-catenin and was capable of suppressing endogenous axis formation when overexpressed dorsally. Taken together, these studies demonstrate that CK2 is a bona fide member of the Wnt pathway and has a critical role in the establishment of the dorsal embryonic axis.

Parallel microtubules and other conserved elements of dorsal axial specification in the direct developing frog, Eleutherodactylus coqui

Specification of the dorsal axis in commonly studied frogs, such as Xenopus laevis and Rana pipiens, depends on a microtubule-mediated movement of cytoplasm in the fertilized egg. The Puerto Rican tree frog, Eleutherodactylus coqui, has an egg that is twenty times the volume of that of X. laevis, raising the question as to whether the mechanism of dorsal axial specification is conserved in these large eggs. Fertilized eggs of E. coqui develop a transient array of parallel microtubules, similar to other frogs, but proportionately larger. The array persists after first cleavage, longer than in other frogs, and is gone by the third cleavage. Correlated with the longer life of the parallel microtubules, both 2- and 8-cell E. coqui embryos remain sensitive to gravity-mediated axial specification, a sensitivity lost in X. laevis before the 2-cell stage. Activation of the Wnt/beta-catenin pathway by injected Xwnt8 RNA causes axial formation as in X. laevis. The results indicate that elements of dorsal axial specification are conserved in E. coqui, but they occur later compared to in X. laevis.

β-catenin, MAPK and Smad signaling during earlyXenopusdevelopment

Knowledge of when and where signaling pathways are activated is crucial for understanding embryonic development. In this study, we have systematically analyzed and compared the signaling pattern of four major pathways by localization of the activated key components β-catenin (Wnt proteins), MAPK (tyrosine kinase receptors/FGF), Smad1 (BMP proteins) and Smad2 (Nodal/activin/Vg1). We have determined semi-quantitatively the distribution of these components at 18 consecutive stages in Xenopus development, from early blastula to tailbud stages, by immunofluorescence on serial cryosections. The image obtained is that of very dynamic and widespread activities, with very few inactive regions. Signaling fields can vary from large gradients to restricted areas with sharp borders. They do not respect tissue boundaries. This direct visualization of active signaling verifies several predictions inferred from previous functional data. It also reveals unexpected signal patterns, pointing to some poorly understood aspects of early development. In several instances, the patterns strikingly overlap, suggesting extensive interplay between the various pathways. To test this possibility, we have manipulated maternal β-catenin signaling and determined the effect on the other pathways in the blastula embryo. We found that the patterns of P-MAPK, P-Smad1 and P-Smad2 are indeed strongly dependent on β-catenin at this stage. supplementary material: Supplementary Information

Development in frogs with large eggs and the origin of amniotes

The origin of the amniote egg is one of the most significant events in the evolution of terrestrial vertebrates. This innovation was probably driven by increased egg size, and to find potential parallels, we can examine the derived development of extant amphibians with large eggs. The embryo of the Puerto Rican tree frog, Eleutherodactylus coqui, exhibits an alteration of its fate map and a secondary coverage of its yolky cells, reflecting the large 3.5 mm egg. Comparable changes may have occurred with the derivation of an amniote pattern of development. Future investigations should focus on the molecular organization of the egg. In the model amphibian for development, Xenopus laevis, information for embryonic germ layers, the dorsal axis, and germ cells is stored mainly as localized RNAs at the vegetal pole of the egg. These localizations would likely be changed with increased egg size. A review of the orthologues of the key X. laevis genes raises the possibility that their activities are not conserved in other vertebrates.

RNA anchoring in the vegetal cortex of the Xenopus oocyte

The body plan of the embryo is established by a polarized source of developmental information in the oocyte. The Xenopus laevis oocyte creates polarity by anchoring mRNAs in the vegetal cortex, including Vg1 and Xwnt-11, which might function in body plan specification, and Xcat-2, which might function in germ cell development. To identify components of the RNA anchoring mechanism, we used the manually isolated vegetal cortex (IVC) to assay loss or change in spatial arrangement of mRNAs caused by disruption of cortical elements. The role of cytoskeleton in mRNA anchoring was tested by treating oocytes with inhibitors that selectively disrupted actin microfilaments and cytokeratin filaments. Treatment of oocytes with cytochalasin B caused clumping of Vg1 and Xwnt-11 as revealed by in situ hybridization of the IVC, but did not cause their release, as confirmed by RT-PCR analysis. These mRNA clumps did not match the distribution of actin microfilament clumps, but were distributed similarly to the remnant cytokeratin filaments. Treatment of oocytes with monoclonal anti-cytokeratin antibody C11 released these mRNAs from the cortex. C11 altered the texture of the cytokeratin network, but did not affect the actin meshwork. These results show that Vg1 and Xwnt-11 are retained by a cytokeratin filament-dependent mechanism, and that organization of the cytokeratin network depend on an intact actin meshwork. Colcemid did not disrupt Vg1 and Xwnt-11 retention in the IVC, so anchoring of these mRNAs are independent of microtubules. Membrane disruption in the IVC by Triton X-100 decreased Vg1 and Xwnt-11. Loss of these mRNAs was due mainly to ribonuclease activity released from membrane components. However, when ribonuclease activity was suppressed under cold temperature, a higher amount of Vg1 and Xwnt-11 was recovered in the supernatant. This result suggested that a fraction of these mRNAs required membranes to be retained in the cortex. By contrast, Xcat-2 mRNA was neither released nor degraded following treatments with cytochalasin B, C11, colcemid and Triton X-100 under cold temperature, so no cortical element could be implicated in its anchoring.

Local inhibition of cortical rotation in Xenopus eggs by an anti-KRP antibody

The dorsal-ventral axis of amphibian embryos is specified by the "cortical rotation," a translocation of the egg cortex relative to the vegetal yolk mass. The mechanism of cortical rotation is not understood but is thought to involve an array of aligned, commonly oriented microtubules. We have demonstrated an essential requirement for kinesin-related proteins (KRPs) in the cortical rotation by microinjection beneath the vegetal cortex of an antipeptide antibody recognising multiple Xenopus egg KRPs. Time-lapse videomicroscopy revealed a striking local inhibition of the cortical rotation around the injection site, indicating that KRP-mediated translocation of the cortex is generated by forces acting across the vegetal subcortical region. Anti-tubulin immunofluorescence showed that the antibody disrupted both formation and maintenance of the aligned microtubule array. Direct examination of rhodamine-labelled microtubules by confocal microscopy showed that the anti-KRP antibody provoked striking three-dimensional flailing movement of the subcortical microtubules. In contrast, microtubules in antibody-free regions undulated only within the plane of the cortex, a significant population exhibiting little or no net movement. These findings suggest that KRPs have a critical role during cortical rotation in tethering microtubules to the cortex and that they may not contribute significantly to the translocation force as previously thought.

Beta-catenin signaling activity dissected in the early Xenopus embryo: a novel antisense approach

Xenopus embryos develop dorsal/ventral and anterior/posterior axes as a result of the activity of a maternal Xwnt pathway, in which beta-catenin is an essential component, acting as a transactivator of transcription of zygotic genes. However, the questions of where and when beta-catenin is required in early embryogenesis have not been addressed directly, because no loss-of-function method has been available. Here we report the use of a novel antisense approach that allows us to target depletion of protein to individual blastomeres. When a "morpholino" oligo complementary to beta-catenin mRNA is injected into early embryos, it depletes beta-catenin protein effectively through the neurula stage. By targeting the oligo to different cleavage blastomeres, we block beta-catenin activity in different areas and at different times. Dorsal vegetal injection at the 2- and 4-cell stages blocks dorsal axis formation and at the 8-cell stage blocks head formation, while A-tier injection at the 32-cell stage causes abnormal cement gland formation. This approach shows the complex involvement of Xwnt pathways in embryonic patterning and offers a rapid method for the functional analysis of both maternal and early zygotic gene products in Xenopus.

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.

LiCl disrupts axial development in mouse but does not act through the beta-catenin/Lef-1 pathway

Chimera and cell marking studies suggest that axial determination in mouse embryos occurs at postimplantation stages. In contrast, Xenopus laevis axes are determined early due to the asymmetric distribution of maternally derived factors in the one-cell zygote. In our earlier study we used lithium chloride (LiCl) to perturb development of mouse axes. Here we investigate whether the lithium induced axial defects in mouse are being mediated by the beta-catenin/Lef-1 pathway as in Xenopus laevis. In lithium treated embryos we did not observe any changes in the amount or localization of beta-catenin protein. Furthermore, the lack of Lef-1 mRNA in treated and untreated embryos indicates the LiCl induced axial defects in the mouse are not mediated by the beta-catenin/Lef-1 pathway.

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.

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.

Gravity influences the position of the dorsoventral axis in medaka fish embryos (Oryzias latipes)

To determine whether gravity influences the plane of bilateral symmetry in medaka embryos, zygotes were placed with their animal-vegetal axis orientated vertically and with their vegetal pole elevated. Then, at regular intervals during the first cell cycle, the zygotes were tilted 90 degrees for about 10 min and subsequently returned to their original orientation. In embryos tilted during the first half of the first cell cycle, the embryonic shield formed on the side that had been lowermost when the zygote was tilted. In embryos that were tilted twice, first in one direction and then in the opposite direction, the embryonic shield formed on the side that was lowermost the first time. When zygotes were centrifuged at 5 g, the embryonic shield formed on the outwardly radial (centrifugal) side of the embryo. The orientation of the array of parallel microtubules in the vegetal pole region was also influenced by tilting or centrifuging zygotes. No correlation was found between the positions of the polar body and the micropyle and the plane of bilateral symmetry. It was concluded that gravity influences both the plane of bilateral symmetry and the orientation of microtubules in the vegetal pole region of medaka embryos.

beta-TrCP is a negative regulator of Wnt/beta-catenin signaling pathway and dorsal axis formation in Xenopus embryos

The Wnt/beta-catenin signaling pathway is responsible for the establishment of dorsoventral axis of Xenopus embryos. The recent finding of the F-box/WD40-repeat protein slimb in Drosophila, whose loss-of-function mutation causes ectopic activation of wingless signaling (Jiang, J., Struhl, G., 1998. Nature 391, 493-496), led us to examine the role of its vertebrate homolog betaTrCp in the Wnt/beta-catenin signaling and dorsal axis formation in Xenopus embryos. Co-injection of betaTrCp mRNA diminished Xwnt8 mRNA-induced axis formation and expression of Siamois and Xnr3, suggesting that betaTrCP is a negative regulator of the Wnt/beta-catenin signaling pathway. An mRNA for a betaTrCp mutant construct (DeltaF), which lacked the F-box domain, induced an ectopic axis and expression of Siamois and Xnr3. Because this activity of DeltaF was suppressed by co-injection of DeltaF TrCP mRNA, DeltaF likely acts in a dominant negative fashion. The activity of DeltaF was diminished by C-cadherin, glycogen synthase kinase 3 and Axin, but not by a dominant negative dishevelled. These results suggest that betaTrCp can act as a negative regulator of dorsal axis formation in Xenopus embryos.

beta-Catenin is essential for patterning the maternally specified animal-vegetal axis in the sea urchin embryo

In sea urchin embryos, the animal-vegetal axis is specified during oogenesis. After fertilization, this axis is patterned to produce five distinct territories by the 60-cell stage. Territorial specification is thought to occur by a signal transduction cascade that is initiated by the large micromeres located at the vegetal pole. The molecular mechanisms that mediate the specification events along the animal-vegetal axis in sea urchin embryos are largely unknown. Nuclear beta-catenin is seen in vegetal cells of the early embryo, suggesting that this protein plays a role in specifying vegetal cell fates. Here, we test this hypothesis and show that beta-catenin is necessary for vegetal plate specification and is also sufficient for endoderm formation. In addition, we show that beta-catenin has pronounced effects on animal blastomeres and is critical for specification of aboral ectoderm and for ectoderm patterning, presumably via a noncell-autonomous mechanism. These results support a model in which a Wnt-like signal released by vegetal cells patterns the early embryo along the animal-vegetal axis. Our results also reveal similarities between the sea urchin animal-vegetal axis and the vertebrate dorsal-ventral axis, suggesting that these axes share a common evolutionary origin.

Propagation and localization of Wnt signaling

Cellular mechanisms for the transport and localization of Wnt signaling components are important for the propagation, distribution, and polarization of Wnt signals in embryonic tissues. Wnt signals are distributed through tissues by vesicular transport of Wnt proteins, localized in embryos by directed transport of cytoplasmic Wnt-signaling components, and propagated asymmetrically during cell division.

From cortical rotation to organizer gene expression: toward a molecular explanation of axis specification in Xenopus

After fertilization of Xenopus eggs, the cortex rotates relative to the cytoplasm, resulting in the formation of a cytoplasmic and transplantable dorsal-determining activity opposite the sperm entry point. This activity induces the dorsal expression of regulatory genes, which in turn establishes the Spemann organizer at the start of gastrulation. There has been considerable debate as to whether Vg1, or components of the Wnt-1 signaling pathway, normally function as this early dorsal determinant. Experiments now support the hypothesis that beta-catenin, a component of the Wnt pathway, provides the initial dorsoventral polarity to the embryo, and that Vg1 functions at a subsequent step in development. Specifically, beta-catenin is required for formation of the endogenous axes, and it is expressed at greater levels in dorsal cells during the early cleavage stages. Moreover, on the dorsal side of the embryo, complexes of beta-catenin and Tcf-3 directly bind the promoter of the dorsal regulatory genes siamois and twin and facilitate their expression, thereby contributing to the subsequent formation of the Spemann organizer. On the ventral side of the embryo, Tcf-3 likely works in the absence of beta-catenin as a transcriptional repressor of siamois. These and other data are considered in the context of how the initial polarization of the fertilized egg by the localized accumulation of beta-catenin establishes a range of subsequent dorsoventral asymmetries in the embryo.