Mesoderm formation in Xenopus ectodermal explants overexpressing Xwnt8: evidence for a cooperating signal reaching the animal pole by gastrulation

Pinhead signaling regulates mesoderm heterogeneity via the FGF receptor-dependent pathway

Among the three embryonic germ layers, the mesoderm plays a central role in the establishment of the vertebrate body plan. The mesoderm is specified by secreted signaling proteins from the FGF, Nodal, BMP and Wnt families. No new classes of extracellular mesoderm-inducing factors have been identified in more than two decades. Here, we show that the pinhead (pnhd) gene encodes a secreted protein that is essential for the activation of a subset of mesodermal markers in the Xenopus embryo. RNA sequencing revealed that many transcriptional targets of Pnhd are shared with those of the FGF pathway. Pnhd activity was accompanied by Erk phosphorylation and required FGF and Nodal but not Wnt signaling. We propose that during gastrulation Pnhd acts in the marginal zone to contribute to mesoderm heterogeneity via an FGF receptor-dependent positive feedback mechanism.

Maintaining embryonic stem cell pluripotency with Wnt signaling

Wnt signaling pathways control lineage specification in vertebrate embryos and regulate pluripotency in embryonic stem (ES) cells, but how the balance between progenitor self-renewal and differentiation is achieved during axis specification and tissue patterning remains highly controversial. The context- and stage-specific effects of the different Wnt pathways produce complex and sometimes opposite outcomes that help to generate embryonic cell diversity. Although the results of recent studies of the Wnt/β-catenin pathway in ES cells appear to be surprising and controversial, they converge on the same conserved mechanism that leads to the inactivation of TCF3-mediated repression.

VegT, eFGF and Xbra cause overall posteriorization while Xwnt8 causes eye-level restricted posteriorization in synergy with chordin in early Xenopus development

We examined several candidate posterior/mesodermal inducing molecules using permanent blastula-type embryos (PBEs) as an assay system. Candidate molecules were injected individually or in combination with the organizer factor chordin mRNA. Injection of chordin alone resulted in a white hemispherical neural tissue surrounded by a large circular cement gland, together with anterior neural gene expression and thus the development of the anterior-most parts of the embryo, without mesodermal tissues. When VegT, eFGF or Xbra mRNAs were injected into a different blastomere of the chordin-injected PBEs, the embryos elongated and formed eye, muscle and pigment cells, and expressed mesodermal and posterior neural genes. These embryos formed the full spectrum of the anteroposterior embryonic axis. In contrast, injection of CSKA-Xwnt8 DNA into PBEs injected with chordin resulted in eye formation and expression of En2, a midbrain/hindbrain marker, and Xnot, a notochord marker, but neither elongation, muscle formation nor more posterior gene expression. Injection of chordin and posteriorizing molecules into the same cell did not result in elongation of the embryo. Thus, by using PBEs as the host test system we show that (i) overall anteroposterior neural development, mesoderm (muscle) formation, together with embryo elongation can occur through the synergistic effect(s) of the organizer molecule chordin, and each of the 'verall posteriorizing molecules'eFGF, VegT and Xbra; (ii) Xwnt8-mediated posteriorization is restricted to the eye level and is independent of mesoderm formation; and (iii) proper anteroposterior patterning requires a separation of the dorsalizing and posteriorizing gene expression domains.

A Vertebrate Homolog of the Cell Cycle Regulator Dbf4 Is an Inhibitor of Wnt Signaling Required for Heart Development

Early stages of vertebrate heart development have been linked to Wnt signaling. Here we show in both gain- and loss-of-function experiments that XDbf4, a known regulator of Cdc7 kinase, is an inhibitor of the canonical Wnt signaling pathway. Depletion of endogenous XDbf4 protein did not disturb gastrulation movements or early organizer genes but resulted in embryos with morphologically defective heart and eyes and suppressed cardiac markers. These markers were restored by overexpressed XDbf4, or an XDbf4 mutant that inhibits Wnt signaling but lacks the ability to regulate Cdc7. This indicates that the function of XDbf4 in heart development is independent of its role in the cell cycle. Moreover, our data suggest that XDbf4 acts through the physical and functional interaction with Frodo, a context-dependent regulator of Wnt signaling. These findings establish an unexpected function for a vertebrate Dbf4 homolog and demonstrate the requirement for Wnt inhibition in early cardiac specification.

Zygotic Wnt Activity Is Required for Brachyury Expression in the Early Xenopus laevis Embryo

The canonical, beta-catenin-dependent Wnt pathway is a crucial player in the early events of Xenopus development. Dorsal axis formation and mesoderm patterning are accepted effects of this pathway, but the regulation of expression of genes involved in mesoderm specification is not. This conclusion is based largely on the inability of the Wnt pathway to induce mesoderm in animal cap explants. Using injections of inhibitors of canonical Wnt signaling, we demonstrate that expression of the general mesodermal marker Brachyury (Xbra) requires a zygotic, ligand-dependent Wnt activity throughout the marginal zone. Analysis of the Xbra promoter reveals that putative TCF-binding sites mediate Wnt activation, the first sites in this well-studied promoter to which an activation role can be ascribed. However, established mesoderm inducers like eFGF and activin can bypass the Wnt requirement for Xbra expression. Another mesoderm promoting factor, VegT, activates Xbra in a Wnt-dependent manner. We also show that the activin/nodal signaling is necessary for ectopic Xbra induction by the Wnt pathway, but not by VegT. Our data significantly change the understanding of Brachyury regulation in Xenopus, implying the existence of an unknown zygotic Wnt ligand in Spemann's organizer. Since Brachyury is considered to have a major role in mesoderm formation, it is possible that Wnts might play a role in mesoderm specification, in addition to patterning.

Non-canonical Wnt signaling in Xenopus: regulation of axis formation and gastrulation

Wnt proteins form a family of secreted glycoproteins that are involved in different developmental processes such as differentiation, proliferation, cell migration and cell polarity. To exert its function, Wnt proteins activate different intracellular signaling cascades. Whereas the canonical, Wnt/beta-catenin pathway is well characterized, less is known about the function of non-canonical Wnt pathways in vertebrates. I here summarize recent findings implicating important roles for Wnt/Ca(2+) and Wnt/JNK signaling during different aspects of early Xenopus laevis development, namely axis formation and gastrulation movements.

The Wnt/beta-catenin pathway posteriorizes neural tissue in Xenopus by an indirect mechanism requiring FGF signalling

In order to identify factors involved in posteriorization of the central nervous system, we undertook a functional screen in Xenopus animal cap explants which involved coinjecting noggin RNA together with pools of RNA from a chick somite cDNA library. In the course of this screen, we isolated a clone encoding a truncated form of beta-catenin, which induced posterior neural and dorsal mesodermal markers when coinjected with noggin in animal caps. Similar results were obtained with Xwnt-8 and Xwnt-3a, suggesting that these effects are a consequence of activating the canonical Wnt signalling pathway. To investigate whether the activation of posterior neural markers requires mesoderm induction, we performed experiments using a chimeric inducible form of beta-catenin. Activation of this protein during blastula stages resulted in the induction of both posterior neural and mesodermal markers, while activation during gastrula stages induced only posterior neural markers. We show that this posteriorizing activity occurs by an indirect and noncell-autonomous mechanism requiring FGF signalling.

Antagonistic regulation of convergent extension movements in Xenopus by Wnt/beta-catenin and Wnt/Ca2+ signaling

Convergent extension movements are the main driving force of Xenopus gastrulation. A fine-tuned regulation of cadherin-mediated cell-cell adhesion is thought to be required for this process. Members of the Wnt family of extracellular glycoproteins have been shown to modulate cadherin-mediated cell-cell adhesion, convergent extension movements, and cell differentiation. Here we show that endogenous Wnt/beta-catenin signaling activity is essential for convergent extension movements due to its effect on gene expression rather than on cadherins. Our data also suggest that XLEF-1 rather than XTCF-3 is required for convergent extension movements and that XLEF-1 functions in this context in the Wnt/beta-catenin pathway to regulate Xnr-3. In contrast, activation of the Wnt/Ca2+ pathway blocks convergent extension movements, with potential regulation of the Wnt/beta-catenin pathway at two different levels. PKC, activated by the Wnt/Ca2+ pathway, blocks the Wnt/beta-catenin pathway upstream of beta-catenin and phosphorylates Dishevelled. CamKII, also activated by the Wnt/Ca2+ pathway, inhibits the Wnt/beta-catenin signaling cascade downstream of beta-catenin. Thus, an opposing cross-talk of two distinct Wnt signaling cascades regulates convergent extension movements in Xenopus.

Activated mutants of SHP-2 preferentially induce elongation of Xenopus animal caps

In Xenopus ectodermal explants (animal caps), fibroblast growth factor (FGF) evokes two major events: induction of ventrolateral mesodermal tissues and elongation. The Xenopus FGF receptor (XFGFR) and certain downstream components of the XFGFR signal transduction pathway (e.g., members of the Ras/Raf/MEK/mitogen-activated protein kinase [MAPK] cascade) are required for both of these processes. Likewise, activated versions of these signaling components induce mesoderm and promote animal cap elongation. Previously, using a dominant negative mutant approach, we showed that the protein-tyrosine phosphatase SHP-2 is necessary for FGF-induced MAPK activation, mesoderm induction, and elongation of animal caps. Taking advantage of recent structural information, we now have generated novel, activated mutants of SHP-2. Here, we show that expression of these mutants induces animal cap elongation to an extent comparable to that evoked by FGF. Surprisingly, however, activated mutant-induced elongation can occur without mesodermal cytodifferentiation and is accompanied by minimal activation of the MAPK pathway and mesodermal marker expression. Our results implicate SHP-2 in a pathway(s) directing cell movements in vivo and identify potential downstream components of this pathway. Our activated mutants also may be useful for determining the specific functions of SHP-2 in other signaling systems.

Molecular mechanisms of neural crest formation

The neural crest is a transient population of multipotent precursor cells named for its site of origin at the crest of the closing neural folds in vertebrate embryos. Following neural tube closure, these cells become migratory and populate diverse regions throughout the embryo where they give rise to most of the neurons and support cells of the peripheral nervous system (PNS), pigment cells, smooth muscle, craniofacial cartilage, and bone. Because of its remarkable ability to generate such diverse derivatives, the neural crest has fascinated developmental biologists for over one hundred years. A great deal has been learned about the migratory pathways neural crest cells follow and the signals that may trigger their differentiation, but until recently comparatively little was known about earlier steps in neural crest development. In the past few years progress has been made in understanding these earlier events, including how the precursors of these multipotent cells are specified in the early embryo and the mechanisms by which they become migratory. In this review, we first examine the mechanisms underlying neural crest induction, paying particular attention to a number of growth factor and transcription factor families that have been implicated in this process. We also discuss when and how the fate of neural crest precursors may diverge from those of nearby neural and epidermal populations. Finally, we review recent advances in our understanding of how neural crest cells become migratory and address the process of neural crest diversification.

Establishment of substratum polarity in the blastocoel roof of the Xenopus embryo

The fibronectin fibril matrix on the blastocoel roof of the Xenopus gastrula contains guidance cues that determine the direction of mesoderm cell migration. The underlying guidance-related polarity of the blastocoel roof is established in the late blastula under the influence of an instructive signal from the vegetal half of the embryo, in particular from the mesoderm. Formation of an oriented substratum depends on functional activin and FGF signaling pathways in the blastocoel roof. Besides being involved in tissue polarization, activin and FGF also affect fibronectin matrix assembly. Activin treatment of the blastocoel roof inhibits fibril formation, whereas FGF modulates the structure of the fibril network. The presence of intact fibronectin fibrils is permissive for directional mesoderm migration on the blastocoel roof extracellular matrix.

Functional and biochemical interactions of Wnts with FrzA, a secreted Wnt antagonist

Wnts are highly conserved developmental regulators that mediate inductive signaling between neighboring cells and participate in the determination of embryonic axes. Frizzled proteins constitute a large family of putative transmembrane receptors for Wnt signals. FrzA is a novel protein that shares sequence similarity with the extracellular domain of Frizzled. The Xenopus homologue of FrzA is dynamically regulated during early development. At the neurula stages, XfrzA mRNA is abundant in the somitic mesoderm, but later becomes strongly expressed in developing heart, neural crest derivatives, endoderm, otic vesicle and other sites of organogenesis. To evaluate possible biological functions of FrzA, we analyzed its effect on early Xenopus development. Microinjection of bovine or Xenopus FrzA mRNA into dorsal blastomeres resulted in a shortened body axis, suggesting a block of convergent extension movements. Consistent with this possibility, FrzA blocked elongation of ectodermal explants in response to activin, a potent mesoderm-inducing factor. FrzA inhibited induction of secondary axes by Xwnt8 and human Wnt2, but not by Xdsh, supporting the idea that FrzA interferes with Wnt signaling. Furthermore, FrzA suppressed Wnt-dependent activation of the early response genes in ectodermal explants and in the marginal zone. Finally, immunoprecipitation experiments demonstrate that FrzA binds to the soluble Wingless protein in cell culture supernatants in vitro. Our results indicate that FrzA is a naturally occurring secreted antagonist of Wnt signaling.

Xenopus cadherin-11 (Xcadherin-11) expression requires the Wg/Wnt signal

In this study we describe the isolation of Xcadherin-11, the Xenopus homologue to the mesenchymal cadherin-11. Similar to epithelial and neural cadherins, overexpression of Xcadherin-11 led to posteriorised phenotypes due to inhibition of convergent extension movement. Because zygotic expression of Xcadherin-11 starts with gastrulation, we analysed the ability of different growth factors involved in mesoderm differentiation to induce the expression of Xcadherin-11. Using the animal cap assay, we demonstrated that Xcadherin-11 is activated by Xwnt-8 or beta-catenin, but repressed by BMP-4. Activin did not induce Xcadherin-11 but its synergistic function was required for the Xwnt-8/beta-catenin-mediated activation of Xcadherin-11. Because Xcadherin-11 and Xenopus E- and N-cadherin are differentially regulated by growth factors in the Xenopus animal cap, our results also reveal that this assay provides a helpful model system to elucidate the molecular control mechanism of epithelial-mesenchymal conversion.

The Xenopus homeobox gene twin mediates Wnt induction of goosecoid in establishment of Spemann's organizer

We describe the isolation of the Xenopus homeobox gene twin (Xtwn), which was identified in an expression cloning screen for molecules with dorsalizing activities. Injection of synthetic Xtwn mRNA restores a complete dorsal axis in embryos lacking dorsal structures and induces a complete secondary dorsal axis when ectopically expressed in normal embryos. The sequence homology, expression pattern and gain-of-function phenotype of Xtwn is most similar to the previously isolated Xenopus homeobox gene siamois (Xsia) suggesting that Xtwn and Xsia comprise a new subclass of homeobox genes important in dorsal axis specification. We find that Xtwn is able to activate the Spemann organizer-specific gene goosecoid (gsc) via direct binding to a region of the gsc promoter previously shown to mediate Wnt induction. Since Xtwn expression is strongly induced in ectodermal (animal cap) cells in response to overexpression of a dorsalizing Wnt molecule, we examined the possibility that Xtwn might be a direct target of a Wnt signal transduction cascade. First, we demonstrate that purified LEF1 protein can interact, in vitro, with consensus LEF1/TCF3-binding sites found within the Xtwn promoter. Second, these binding sites were shown to be required for Wnt-mediated induction of a Xtwn reporter gene containing these sites. As LEF1/TCF3 family transcription factors have previously been shown to directly mediate Wnt signaling, these results suggest that Xtwn induction by Wnt may be direct. Finally, in UV-hyperventralized embryos, expression of endogenous Xtwn is confined to the vegetal pole and a Xtwn reporter gene is hyperinduced vegetally in a LEF1/TCF3-binding-site-dependent manner. These results suggest that cortical rotation distributes Wnt-like dorsal determinants to the dorsal side of the embryo, including the dorsal marginal zone, and that these determinants may directly establish Spemann's organizer in this region.

Graded amounts of Xenopus dishevelled specify discrete anteroposterior cell fates in prospective ectoderm

Signals emitted from the prospective dorsal marginal zone (the organizer) are thought to specify neuroectodermal cell fates along the anteroposterior (AP) axis, but the mechanisms underlying this signaling event remain to be elucidated. To assess the effect of Xenopus Dishevelled (Xdsh), a proposed component of the Wnt, Notch and Frizzled signal transduction pathways, on AP axis determination, it was supplied in varying doses to presumptive ectodermal cells. Two-fold increments in levels of microinjected Xdsh mRNA revealed a gradual shift in cell fates along the AP axis. Lower doses of Xdsh mRNA activated anterior neuroectodermal markers, XAG1 and Xotx2, whereas the higher doses induced more posterior neural tissue markers such as En2, Krox20 and HoxB9. At the highest dose of Xdsh mRNA, explants contained maximal amount of HoxB9 transcripts and developed notochord and somites. When compared with Xdsh, Xwnt8 mRNA also activated anterior neuroectodermal markers, but failed to elicit mesoderm formation. Analysis of explants overexpressing Xdsh at the gastrula stage revealed activation of several organizer-specific genes which have been implicated in determination of neural tissue (Xotx2, noggin, chordin and follistatin). Whereas Goosecoid, Xlim1 and Xwnt8 were not induced in these explants, another early marginal zone marker, Xbra, was activated at the highest level of Xdsh mRNA. These observations suggest that the effects of Xdsh on AP axis specification may be mediated by combinatorial action of several early patterning genes. Increasing levels of Xdsh mRNA activate posterior markers, whereas increasing amounts of the organizer stimulate the extent of anterior development (Stewart, R.M. and Gerhart, J.C. (1990) Development 109, 363-372). These findings argue against induction of the entire organizer by Xdsh in ectodermal cells and implicate signal transduction pathways involving Xdsh in AP axis determination. Thus, different levels of a single molecule, Xdsh, can specify distinct cell states along the AP axis.

Lithium inhibits glycogen synthase kinase-3 activity and mimics wingless signalling in intact cells

BACKGROUND

Exposing eukaryotic cells to lithium ions (Li+) during development has marked effects on cell fate and organization. The phenotypic consequences of Li+ treatment on Xenopus embryos and sporulating Dictyostelium are similar to the effects of inhibition or disruption, respectively, of a highly conserved protein serine/threonine kinase, glycogen synthase kinase-3 (GSK-3). In Drosophila, the GSK-3 homologue is encoded by zw3sgg, a segment-polarity gene involved in embryogenesis that acts downstream of wg. In higher eukaryotes, GSK-3 has been implicated in signal transduction pathways downstream of phosphoinositide 3-kinase and mitogen-activated protein kinases.

RESULTS

We investigated the effect of Li+ on the activity of the GSK-3 family. At physiological doses, Li+ inhibits the activity of human GSK-3 beta and Drosophila Zw3Sgg, but has no effect on other protein kinases. The effect of Li+ on GSK-3 is reversible in vitro. Treatment of cells with Li+ inhibits GSK-3-dependent phosphorylation of the microtubule-associated protein Tau. Li+ treatment of Drosophila S2 cells and rat PC12 cells induces accumulation of cytoplasmic Armadillo/beta-catenin, demonstrating that Li+ can mimic Wingless signalling in intact cells, consistent with its inhibition of GSK-3.

CONCLUSIONS

Li+ acts as a specific inhibitor of the GSK-3 family of protein kinases in vitro and in intact cells, and mimics Wingless signalling. This reveals a possible molecular mechanism of Li+ action on development and differentiation.

Analysis of Dishevelled signalling pathways during Xenopus development

BACKGROUND

Recent studies have demonstrated that the Wnt, Frizzled and Notch proteins are involved in a variety of developmental processes in fly, worm, frog and mouse embryos. The Dishevelled (Dsh) protein is required for Drosophila cells to respond to Wingless, Notch and Frizzled signals, but the molecular mechanisms of its action are not well understood. Using the ability of a mutant form of the Xenopus homologue of Dsh (Xdsh) to block Wnt and Dsh signalling in a model system, this work attempts to clarify the role of the endogenous Xdsh during the early stages of vertebrate development.

RESULTS

A mutant Xdsh (Xdd1) with an internal deletion of the conserved PDZ/DHR domain was constructed. Overexpression of Xdd1 mRNA in ventral blastomeres of Xenopus embryos strongly inhibited induction of secondary axes by the wild-type Xdsh and Xwnt8 mRNAs, but did not affect the axis-inducing ability of beta-catenin mRNA. These observations suggest that Xdd1 acts as a dominant-negative mutant. Dorsal expression of Xdd1 caused severe posterior truncations in the injected embryos, whereas wild-type Xdsh suppressed this phenotype. Xdd1 blocked convergent extension movements in ectodermal explants stimulated with mesoderm-inducing factors and in dorsal marginal zone explants, but did not affect mesoderm induction and differentiation.

CONCLUSIONS

A vertebrate homologue of Dsh is a necessary component of Wnt signal transduction and functions upstream of beta-catenin. These findings also establish a requirement for the PDZ domain in signal transduction by Xdsh, and suggest that endogenous Xdsh controls morphogenetic movements in the embryo.

Maternal beta-catenin establishes a ‘dorsal signal’ in early Xenopus embryos

In previous work, we demonstrated that maternally encoded beta-catenin, the vertebrate homolog of armadillo, is required for formation of dorsal axial structures in early Xenopus embryos (Heasman, J., Crawford, A., Goldstone, K., Garner-Hamrick, P., Gumbiner, B., Kintner, C., Yoshida-Noro, C. and Wylie, C. (1994). Cell 79, 791–803). Here we investigated, firstly, the role(s) of beta-catenin in spatial terms, in different regions of the embryo, by injecting beta-catenin mRNA into individual blastomeres of beta-catenin-depleted embryos at the 32 cell stage. The results indicate that beta-catenin can rescue the dorsal axial structures in a non-cell-autonomous way and without changing the fates of the injected cells. This suggests that cells overexpressing beta-catenin send a ‘dorsal signal’ to other cells. This was confirmed by showing that beta-catenin overexpressing animal caps did not cause wild-type caps to form mesoderm, but did cause isolated beta-catenin-deficient marginal zones to form dorsal mesoderm. Furthermore beta-catenin-deficient vegetal masses treated with overexpressing caps regained their ability to act as Nieuwkoop Centers. Secondly, we studied the temporal activity of beta-catenin. We showed that zygotic transcription of beta-catenin starts after the midblastula transition (MBT), but does not rescue dorsal axial structures. We further demonstrated that the vegetal mass does not release a dorsal signal until after the onset of transcription, at the midblastula stage, suggesting that maternal beta-catenin protein is required at or before this time. Thirdly we investigated where, in relationship to other gene products known to be active in axis formation, beta-catenin is placed. We find that BVg1, bFGF, tBR (the truncated form of BMP2/4R), siamois and noggin activities are all downstream of beta-catenin, as shown by the fact that injection of their mRNAs rescues the effect of depleting maternally encoded beta-catenin. Interference with the action of glycogen synthase kinase (GSK), a vertebrate homolog of the Drosophila gene product, zeste white 3 kinase, does not rescue the effect, suggesting that it is upstream.

Activin-like signal activates dorsal-specific maternal RNA between 8- and 16-cell stages of Xenopus

In many animals the dorsal-ventral axis forms by an initial localization of maternal molecules, which then regulate the spatial location of signals that directly influence the expression of axis-specific fates. Several recent studies have demonstrated that dorsal-animal blastomeres of the Xenopus morula (8-32 cells) are biased toward dorsal fates prior to mesoderm inductive signaling. In this study we ask whether the dorsal bias is the result of autonomous expression of maternal molecules specifically localized within dorsal cells or of early activating signals. It was found that although 16-cell dorsal-animal blastomeres (D1.1) can differentiate into dorsal tissues when cultured alone, the 8-cell mothers (D1) can not. Likewise, although RNA extracted from D1.1 can induce an extra dorsal axis when injected into vegetal blastomeres, RNA extracted from D1 can not. However, D1 does express dorsal tissues if co-cultured with dorsal-vegetal cells or with culture medium containing a mixture of activins (PIF-medium). Furthermore, short-term culture of D1 in PIF-medium enables the D1 RNA to induce an ectopic dorsal axis. Ventral-animal blastomeres also can express dorsal axial tissues when co-cultured with dorsal-vegetal blastomeres or in PIF-medium, but the RNA from the activin-treated ventral cells cannot induce ectopic dorsal axes. These studies demonstrate that there are maternal RNAs that, shortly after fertilization, are present only in the dorsal-animal region. They do not act cell autonomously, but require an activin-like signal. These RNAs may function by increasing the responsiveness of dorsal-animal blastomeres to the mesoderm inductive signals present in both the morula and the blastula.

Properties of the dorsal activity found in the vegetal cortical cytoplasm of Xenopus eggs

The Xenopus egg contains a maternal dorsal determinant that is specifically localized to the vegetal cortex. We have previously shown that vegetal cortical cytoplasm can generate a full dorsal axis when it is injected into ventral vegetal blastomeres of a cleavage-stage embryo. In this study, we have defined further the properties of the dorsal activity. The cortical dorsal activity arises during oocyte maturation after germinal vesicle breakdown. When injected into the four extreme animal pole blastomeres of ultraviolet-ventralized 32-cell embryos, vegetal cortical cytoplasm partially rescued dorsal axial structures. As revealed by lineage tracing, these axial structures formed ectopically from the progeny of the cells that were injected. Injection of animal cortical cytoplasm had no effect. When mid-blastula (stage 8) animal caps from these injected embryos were isolated and cultured, both vegetal cortex-enriched and animal cortex-enriched animal caps produced only epidermis. Therefore vegetal cortex, on its own, is not a mesoderm inducer. Between stage 8 (blastula) and stage 10 (gastrula), a ventral mesoderm-inducing signal spreads from vegetal cells towards the animal pole. We tested whether this natural mesoderm-inducing factor interacts with the activity found in the vegetal cortex. Injection of vegetal cortex enhanced the formation of neural tissue and cement gland when animal caps were isolated at stage 10. When cultured from stage 8 in the presence of the ventral mesoderm-inducing fibroblast growth factor, animal caps enriched in vegetal cortex developed significantly more neural tissue and cement gland than ones enriched in animal cortex. These results indicate that the dorsal activity localized to the egg vegetal cortex alters the response of cells to mesoderm inducers.

bFGF as a possible morphogen for the anteroposterior axis of the central nervous system in Xenopus

Vertebrate neural development is initiated during gastrulation by the inductive action of the dorsal mesoderm (Spemann's organizer in amphibians) on neighbouring ectoderm, which eventually gives rise to the central nervous system from forebrain to spinal cord. Here we present evidence that bFGF can mimic the organizer action by inducing Xenopus ectoderm cells in culture to express four position-specific neural markers (XeNK-2, En-2, XIHbox1 and XIHbox6) along the anteroposterior axis. bFGF also induced the expression of a general neural marker NCAM but not the expression of immediate-early mesoderm markers (goosecoid, noggin, Xbra and Xwnt-8), suggesting that bFGF directly neuralized ectoderm cells without forming mesodermal cells. The bFGF dose required to induce the position-specific markers was correlated with the anteroposterior location of their expression in vivo, with lower doses eliciting more anterior markers and higher doses more posterior markers. These data indicate that bFGF or its homologue is a promising candidate for a neural morphogen for anteroposterior patterning in Xenopus. Further, we showed that the ability of ectoderm cells to express the anterior markers in response to bFGF was lost by mid-gastrula, before the organizer mesoderm completely underlies the anterior dorsal ectoderm. Thus, an endogenous FGF-like molecule released from the involuting organizer may initiate the formation of the anteroposterior axis of the central nervous system during the early stages of gastrulation by forming a concentration gradient within the plane of dorsal ectoderm.

Dorsalizing and neuralizing properties of Xdsh, a maternally expressed Xenopus homolog of dishevelled

Signaling factors of the Wnt proto-oncogene family are implicated in dorsal axis formation during vertebrate development, but the molecular mechanism of this process is not known. Studies in Drosophila have indicated that the dishevelled gene product is required for wingless (Wnt1 homolog) signal transduction. We demonstrate that injection of mRNA encoding a Xenopus homolog of dishevelled (Xdsh) into prospective ventral mesodermal cells triggers a complete dorsal axis formation in Xenopus embryos. Lineage tracing experiments show that cells derived from the injected blastomere contribute to anterior and dorsal structures of the induced axis. In contrast to its effect on mesoderm, overexpression of Xdsh mRNA in prospective ectodermal cells triggers anterior neural tissue differentiation. These studies suggest that Wnt signal transduction pathway is conserved between Drosophila and vertebrates and point to a role for maternal Xdsh product in dorsal axis formation and in neural induction.

Effect of activin and lithium on isolated Xenopus animal blastomeres and response alteration at the midblastula transition

Dorsoventral mesoderm patterning in the amphibian embryo involves a series of interactions mediated by several peptide growth factors. Animal blastomeres isolated at the 8-cell stage are useful for studying mesoderm patterning, since they contain the prospective (uninduced) mesoderm region and allow examination of the default state of animal cells. When activin is applied to these dorsal and ventral animal half explants, a competence prepattern for responding to activin is observed. In order to investigate the characteristics of prepatterning, we treated animal blastomeres with the embryo dorsalizing agent LiCl. Treatment with lithium alone did not induce normal trunk mesoderm in either blastomere. Lithium did, however, alter the competence of animal blastomeres to activin. Dorsal mesoderm was formed in the ventral blastomeres, as well as in the dorsal blastomeres. This result reveals that the early dorsoventral polarity in the animal hemisphere is not fixed. Using goosecoid(gsc) and Xwnt-8 genes as dorsal and ventral mesoderm markers, it was verified that lithium modifies the competence to activin. Unexpectedly, lithium treatment on its own resulted in gsc expression in the animal half explants. This suggests that embryo goosecoid expression may be induced by the effect of dorsal determination activity, but not by mesoderm induction. However, lithium induced also the expression of brachyury (Xbra) gene at very low levels. This would indicate the formation of dorsal-anterior mesoderm, which was not identified by the tissue observations. Expression of Xwnt-8, a ventral mesoderm marker usually induced in blastula animal caps by activin, was hardly induced in the blastomere explants. We isolated whole animal half explants at the 8-cell stage and exposed to activin at different stages. It was found that the same concentration of activin induces gsc before the midblastula stage, and induces Xwnt-8 at later stages. This suggests that the response of animal blastomeres alters depending on the stage of activin signaling.

Zebrafish wnt8 and wnt8b share a common activity but are involved in distinct developmental pathways

The specification of the vertebrate body plan is dependent on numerous signaling molecules, including members of the Wnt family. We have identified two zebrafish wnt8 paralogs related to Xwnt-8B and Xwnt-8, respectively. A RT-PCR assay demonstrated that wnt8 is expressed maternally, with transcripts detected throughout embryogenesis, whereas wnt8b transcripts were first detected during late gastrulation. The wnt8 transcripts at 50% epiboly are spatially restricted to those cells at the blastoderm margin, overlying gsc-expressing cells in the axial hypoblast. During late gastrulation, wnt8 was no longer detected in the marginal cells at the dorsal midline and by mid-segmentation, transcripts were found in the presumptive tail bud. In contrast, wnt8b expression is spatially restricted to prospective neuroepithelium, and later to neural-specific structures. Overexpression of both wnts results in two major phenotypes: radialized embryos and embryos with anterior defects. These phenotypes were preceded by significant changes in the spatial expression patterns of gsc and ntl transcripts, reminiscent of activities of Xwnt-8 in Xenopus, and consistent with a role for wnt8 in the specification or patterning of mesoderm.

Heparan sulfate proteoglycans are required for mesoderm formation in Xenopus embryos

Mesoderm forms in the vertebrate embryo as a result of inductive interactions involving secreted growth factors and cell surface molecules. Proteoglycans have recently been implicated in the control of cell adhesion, migration and growth factor responsiveness. We have found that removal of glycosaminoglycan chains of proteoglycans from Xenopus ectodermal explants by heparinase, but not by chondroitinase, results in inhibition of elongation and mesodermal differentiation in response to signaling factors: activin, FGF and Wnt. Heparinase treatment differentially affected expression of early general and region-specific mesodermal markers, suggesting that mesodermal cell fates become specified in the early embryo via at least two signaling pathways which differ in their requirements for heparan sulfate proteoglycans. Addition of soluble heparan sulfate restored activin-mediated induction of muscle-specific actin gene in heparinase-treated explants. Finally, heparinase inhibited autonomous morphogenetic movements and mesodermal, but not neural, differentiation in dorsal marginal zone explants, which normally give rise to mesoderm in the embryo. These results directly demonstrate that heparan sulfate proteoglycans participate in gastrulation and mesoderm formation in the early embryo.

The cleavage stage origin of Spemann's Organizer: analysis of the movements of blastomere clones before and during gastrulation in Xenopus

Recent investigations into the roles of early regulatory genes, especially those resulting from mesoderm induction or first expressed in the gastrula, reveal a need to elucidate the developmental history of the cells in which their transcripts are expressed. Although fates both of the early blastomeres and of regions of the gastrula have been mapped, the relationship between the two sets of fate maps is not clear and the clonal origin of the regions of the stage 10 embryo are not known. We mapped the positions of each blastomere clone during several late blastula and early gastrula stages to show where and when these clones move. We found that the dorsal animal clone (A1) begins to move away from the animal pole at stage 8, and the dorsal animal marginal clone (B1) leaves the animal cap by stage 9. The ventral animal clones (A4 and B4) spread into the dorsal animal cap region as the dorsal clones recede. At stage 10, the ventral animal clones extend across the entire dorsal animal cap. These changes in the blastomere constituents of the animal cap during epiboly may contribute to the changing capacity of the cap to respond to inductive growth factors. Pregastrulation movements of clones also result in the B1 clone occupying the vegetal marginal zone to become the primary progenitor of the dorsal lip of the blastopore (Spemann's Organizer). This report provides the fundamental descriptions of clone locations during the important periods of axis formation, mesoderm induction and neural induction. These will be useful for the correct targeting of genetic manipulations of early regulatory events.

Neural induction by the secreted polypeptide noggin

The Spermann organizer induces neural tissue from dorsal ectoderm and dorsalizes lateral and ventral mesoderm in Xenopus. The secreted factor noggin, which is expressed in the organizer, can mimic the dorsalizing signal of the organizer. Data are presented showing that noggin directly induces neural tissue, that it induces neural tissue in the absence of dorsal mesoderm, and that it acts at the appropriate stage to be an endogenous neural inducing signal. Noggin induces cement glands and anterior brain markers, but not hindbrain or spinal cord markers. Thus, noggin has the expression pattern and activity expected of an endogenous neural inducer.