Hensen's node induces neural tissue in Xenopus ectoderm. Implications for the action of the organizer in neural induction.

The development of the vertebrate nervous system is initiated in amphibia by inductive interactions between ectoderm and a region of the embryo called the organizer. The organizer tissue in the dorsal lip of the blastopore of Xenopus and Hensen's node in chick embryos have similar neural inducing properties when transplanted into ectopic sites in their respective embryos. To begin to determine the nature of the inducing signals of the organizer and whether they are conserved across species we have examined the ability of Hensen's node to induce neural tissue in Xenopus ectoderm. We show that Hensen's node induces large amounts of neural tissue in Xenopus ectoderm. Neural induction proceeds in the absence of mesodermal differentiation and is accompanied by tissue movements which may reflect notoplate induction. The competence of the ectoderm to respond to Hensen's node extends much later in development than that to activin-A or to induction by vegetal cells, and parallels the extended competence to neural induction by axial mesoderm. The actions of activin-A and Hensen's node are further distinguished by their effects on lithium-treated ectoderm. These results suggest that neural induction can occur efficiently in response to inducing signals from organizer tissue arrested at a stage prior to gastrulation, and that such early interactions in the blastula may be an important component of neural induction in vertebrate embryos.     

Directionality of heart looping: effects of Pitx2c misexpression on flectin asymmetry and midline structures

The insulin-like growth factors (IGFs) are well known mitogens, both in vivo and in vitro, while functions in cellular differentiation have also been indicated. Here, we demonstrate a new role for the IGF pathway in regulating head formation in Xenopus embryos. Both IGF-1 and IGF-2, along with their receptor IGF-1R, are expressed early during embryogenesis, and the IGF-1R is present particularly in anterior and dorsal structures. Overexpression of IGF-1 leads to anterior expansion of head neural tissue as well as formation of ectopic eyes and cement gland, while IGF-1 receptor depletion using antisense morpholino oligonucleotides drastically reduces head structures. Furthermore, we demonstrate that IGF signaling exerts this effect by antagonizing the activity of the Wnt signal transduction pathway in the early embryo, at the level of beta-catenin. Thus, the IGF pathway is required for head formation during embryogenesis.     

A novel member of the Xenopus Zic family, Zic5, mediates neural crest development

We characterized Xenopus Zic5 which belongs to a novel class of the Zic family. Zic5 is more specifically expressed in the prospective neural crest than other Zic genes. Overexpression of Zic5 in embryos led to ectopic expression of the early neural crest markers, Xsna and Xslu, with the loss of epidermal marker expression. In Zic5-overexpressing animal cap explants, there was marked induction of neural crest markers, without mesodermal and anterior neural markers. This was in contrast to other Xenopus Zic genes, which induce both anterior and the neural crest markers in the same assay. Injection of a dominant-negative form of Zic5 can block neural crest formation in vivo. These results indicate that Zic5 expression converts cells from an epidermal fate to a neural crest cell fate. This is the first evidence for neural crest tissue inductive activity separate from anterior neural tissue inductive activity in a Zic family gene.     

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.     

Inducible gene expression in transgenic Xenopus embryos

The amphibian Xenopus laevis has been successfully used for many years as a model system for studying vertebrate development. Because of technical limitations, however, molecular investigations have mainly concentrated on early stages. We have developed a straightforward method for stage-specific induction of gene expression in transgenic Xenopus embryos [1] [2]. This method is based on the Xenopus heat shock protein 70 (Xhsp70 [3]) promoter driving the expression of desired gene products. We found that ubiquitous expression of the transgene is induced upon relatively mild heat treatment. Green fluorescent protein (GFP) was used as a marker to monitor successful induction of gene expression in transgenic embryos. We used this method to study the stage specificity of Wnt signalling function. Transient ectopic Wnt-8 expression during early neurulation was sufficient to repress anterior head development and this capacity was restricted to early stages of neurulation. By transient over-expression at different stages of development, we show that frizzled-7 disrupted morphogenesis sequentially from anterior to posterior along the dorsal axis as development proceeds. These results demonstrate that this method for inducible gene expression in transgenic Xenopus embryos will be a very powerful tool for temporal analysis of gene function and for studying molecular mechanisms of vertebrate organogenesis.     

Homoiogenetic Neural Induction in Xenopus Chimeric Explants

We previously raised monoclonal antibodies specific for epidermis (7) and neural tissue (8) of Xenopus for use as markers of tissue differentiation in induction experiments (8). Here we have used these monoclonal antibodies to examine homoiogenetic neural induction, by which cells induced to differentiate to neural tissues can in turn induce competent ectoderm to do the same. Presumptive anterior neural plate excised from late gastrulae of Xenopus laevis was conjugated with competent ectoderm from the initial gastrula of Xenopus borealis , either side by side or with their inner surfaces together. The chimeric explants enabled us to distinguish induced neural tissues from inducing neural tissues. In both types of explant, neural tissues identified by the neural tissue-specific antibody, NEU-1, were induced in the competent ectoderm by the presumptive anterior neural plate. The results suggest that homoiogenetic neural induction does occur in Xenopus embryos.     

Neural crest induction by the canonical Wnt pathway can be dissociated from anterior-posterior neural patterning in Xenopus

While Wnt signaling is known to be involved in early steps of neural crest development, the mechanism remains unclear. Because Wnt signaling is able to posteriorize anterior neural tissues, neural crest induction by Wnts has been proposed to be an indirect consequence of posteriorization of neural tissues rather than a direct effect of Wnt signaling. To address the relationship between posteriorization and neural crest induction by Wnt signaling, we have used gain of function and loss of function approaches in Xenopus to modulate the level of Wnt signaling at multiple points in the pathway. We find that modulating the level of Wnt signaling allows separation of neural crest induction from the effects of Wnts on anterior-posterior neural patterning. We also find that activation of Wnt signaling induces ectopic neural crest in the anterior region without posteriorizing anterior neural tissues. In addition, Wnt signaling induces neural crest when its posteriorizing activity is blocked by inhibition of FGF signaling in neuralized explants. Finally, depletion of beta-catenin confirms that the canonical Wnt pathway is required for initial neural crest induction. While these observations do not exclude a role for posteriorizing signals in neural crest induction, our data, together with previous observations, strongly suggest that canonical Wnt signaling plays an essential and direct role in neural crest induction.     

Conditional BMP inhibition in Xenopus reveals stage-specific roles for BMPs in neural and neural crest induction

Bone morphogenetic protein (BMP) inhibition has been proposed as the primary determinant of neural cell fate in the developing Xenopus ectoderm. The evidence supporting this hypothesis comes from experiments in explanted "animal cap" ectoderm and in intact embryos using BMP antagonists that are unregulated and active well before gastrulation. While informative, these experiments cannot answer questions regarding the timing of signals and the behavior of cells in the more complex environment of the embryo. To examine the effects of BMP antagonism at defined times in intact embryos, we have generated a novel, two-component system for conditional BMP inhibition. We find that while blocking BMP signals induces ectopic neural tissue both in animal caps and in vivo, in intact embryos, it can only do so prior to late blastula stage (stage 9), well before the onset of gastrulation. Later inhibition does not induce neural identity, but does induce ectopic neural crest, suggesting that BMP antagonists play temporally distinct roles in establishing neural and neural crest identity. By combining BMP inhibition with fibroblast growth factor (FGF) activation, the neural inductive response in whole embryos is greatly enhanced and is no longer limited to pre-gastrula ectoderm. Thus, BMP inhibition during gastrulation is insufficient for neural induction in intact embryos, arguing against a BMP gradient as the sole determinant of ectodermal cell fate in the frog.     

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.     

A restrictive role for Hedgehog signalling during otic specification in Xenopus

Vertebrate inner ear development is initiated by the specification of the otic placode, an ectodermal structure induced by signals from neighboring tissue. Although several signaling molecules have been identified as candidate otic inducers, many details of the process of inner ear induction remain elusive. Here, we report that otic induction is responsive to the level of Hedgehog (Hh) signaling activity in Xenopus, making use of both gain- and loss-of-function approaches. Ectopic activation of Hedgehog signaling resulted in the development of ectopic vesicular structures expressing the otic marker genes XPax-2, Xdll-3, and Xwnt-3A, thus revealing otic identity. Induction of ectopic otic vesicles was also achieved by misexpression of two different inhibitors of Hh signaling: the putative Hh antagonist mHIP and XPtc1deltaLoop2, a dominant-negative form of the Hh receptor Patched. In addition, misexpression of XPtc1deltaLoop2 as well as treatment of Xenopus embryos with the specific Hh signaling antagonist cyclopamine resulted in the formation of enlarged otic vesicles. In summary, our observations suggest that a defined level of Hh signaling provides a restrictive environment for otic fate in Xenopus embryos.     

Active signals,gradient formation and regional specificity in neural induction

The question of how the vertebrate embryo gives rise to a nervous system is of paramount interest in developmental biology. Neural induction constitutes the earliest step in this process and is tightly connected with development of the embryonic body axes. In the Xenopus embryo, perpendicular gradients of BMP and Wnt signals pattern the dorsoventral and anteroposterior body axes. Both pathways need to be inhibited to allow anterior neural induction to occur. FGF8 and IGF are active neural inducers that together with BMP and Wnt signals are integrated at the level of Smad 1/5/8 phosphorylation. Hedgehog (Hh) also contributes to anterior neural induction. Suppressor-of-fused plays an important role in intertwining the Hh and Wnt pathways. Distinct mechanisms are discussed that establish morphogen gradients and integrate retinoic acid and FGF signals during posterior development. These findings not only improve our understanding of regional specification in neural induction, but have profound implications for mammalian stem cell research and regenerative medicine.     

The effects of N-cadherin misexpression on morphogenesis in Xenopus embryos

N-cadherin is a calcium-dependent, cell adhesion molecule that has been proposed to play a role in morphogenesis in vertebrate embryos. Throughout early neural development, N-cadherin is expressed during the morphogenetic changes that occur when ectoderm, in response to neural induction, forms a neural plate and tube. To study the role of N-cadherin in these processes, cDNA clones encoding Xenopus laevis N-cadherin were isolated and used to study the expression of N-cadherin in frog embryos. These studies showed that N-cadherin RNA is not expressed at detectable levels in early cleavage embryos or in isolated ectoderm in the absence of neural induction. However, N-cadherin RNA rapidly appeared in ectoderm exposed to a heterologous neural inducer, indicating that N-cadherin expression, as an early response to induction, precedes the morphogenetic events associated with early neural development. The role of N-cadherin in these morphogenetic events was studied by ectopically expressing N-cadherin in the ectoderm of embryos prior to induction. The ectopic expression of this protein in ectoderm led to the formation of cell boundaries and to severe morphological defects. These results are consistent with the hypothesis that the morphogenetic changes associated with early neural development are controlled, in part, by the induced expression of N-cadherin in the neural plate.     

FGF-8 stimulates neuronal differentiation through FGFR-4a and interferes with mesoderm induction in Xenopus embryos

The role of fibroblast growth factors (FGFs) in neural induction is controversial [1,2]. Although FGF signalling has been implicated in early neural induction [3-5], a late role for FGFs in neural development is not well established. Indeed, it is thought that FGFs induce a precursor cell fate but are not able to induce neuronal differentiation or late neural markers [6-8]. It is also not known whether the same or distinct FGFs and FGF receptors (FGFRs) mediate the effects on mesoderm and neural development. We report that Xenopus embryos expressing ectopic FGF-8 develop an abundance of ectopic neurons that extend to the ventral, non-neural, ectoderm, but show no ectopic or enhanced notochord or somitic markers. FGF-8 inhibited the expression of an early mesoderm marker, Xbra, in contrast to eFGF, which induced ectopic Xbra robustly and neuronal differentiation weakly. The effect of FGF-8 on neurogenesis was blocked by dominant-negative FGFR-4a (DeltaXFGFR-4a). Endogenous neurogenesis was also blocked by DeltaXFGFR-4a and less efficiently by dominant-negative FGFR-1 (XFD), suggesting that it depends preferentially on signalling through FGFR-4a. The results suggest that FGF-8 and FGFR-4a signalling promotes neurogenesis and, unlike other FGFs, FGF-8 interferes with mesoderm induction. Thus, different FGFs show specificity for mesoderm induction versus neurogenesis and this may be mediated, at least in part, by the use of distinct receptors.     

Kermit 2/XGIPC, an IGF1 receptor interacting protein, is required for IGF signaling in Xenopus eye development

GIPC is a PDZ-domain-containing protein identified in vertebrate and invertebrate organisms through its interaction with a variety of binding partners including many membrane proteins. Despite the multiple reports identifying GIPC, its endogenous function and the physiological significance of these interactions are much less studied. We have previously identified the Xenopus GIPC homolog kermit as a frizzled 3 interacting protein that is required for frizzled 3 induction of neural crest in ectodermal explants. We identified a second Xenopus GIPC homolog, named kermit 2 (also recently described as an IGF receptor interacting protein and named XGIPC). Despite its high amino acid similarity with kermit, kermit 2/XGIPC has a distinct function in Xenopus embryos. Loss-of-function analysis indicates that kermit 2/XGIPC is specifically required for Xenopus eye development. Kermit 2/XGIPC functions downstream of IGF in eye formation and is required for maintaining IGF-induced AKT activation. A constitutively active PI3 kinase partially rescues the Kermit 2/XGIPC loss-of-function phenotype. Our results provide the first in vivo loss of function analysis of GIPC in embryonic development and also indicate that kermit 2/XGIPC is a novel component of the IGF pathway, potentially functioning through modulation of the IGF1 receptor.     

Induction of otic structures by canonical Wnt signalling in medaka

The Wnt family of signalling proteins is known to participate in multiple developmental decisions during embryogenesis. We misexpressed Wnt1 in medaka embryos and observed anterior truncations, similar to those described for ectopic activation of canonical Wnt signalling in other species. Interestingly, when we induced a heat-shock Wnt1 transgenic line exactly at 30% epiboly, we observed multiple ectopic otic vesicles in the truncated embryos. The vesicles then fused, forming a single large ear structure. These “cyclopic ears” filled the complete anterior region of the embryos. The ectopic induction of otic development can be explained by the juxtaposition of hindbrain tissue with anterior ectoderm. Fibroblast growth factor (Fgf) ligands are thought to mediate the otic-inducing properties of the hindbrain. However, signals different from Fgf3 and Fgf8 are necessary to explain the formation of the ectopic ear structures, suggesting that Wnt signalling is involved in the otic induction process in medaka.     

Anterior patterning by synergistic activity of the early gastrula organizer and the anterior germ layer tissues of the mouse embryo.

Fragments of the germ layer tissues isolated from the early-primitive-streak (early-streak) stage mouse embryos were tested for axis induction activity by transplantation to late-gastrula (late-streak to early-bud) stage host embryos. The posterior epiblast fragment that contains the early gastrula organizer was able to recruit the host tissues to form an ectopic axis. However, the most anterior neural gene that was expressed in the ectopic axis was Krox20 that marks parts of the hindbrain, but markers of the mid- and forebrain (Otx2 and En1) were not expressed. Anterior visceral endoderm or the anterior epiblast alone did not induce any ectopic neural tissue. However, when these two anterior germ layer tissues were transplanted together, they can induce the formation of ectopic host-derived neural tissues but these tissues rarely expressed anterior neural genes and did not show any organization of an ectopic axis. Therefore, although the anterior endoderm and epiblast together may display some inductive activity, they do not act like a classical organizer. Induction of the anterior neural genes in the ectopic axis was achieved only when a combination of the posterior epiblast fragment, anterior visceral endoderm and the anterior epiblast was transplanted to the host embryo. The formation of anterior neural structures therefore requires the synergistic interaction of the early gastrula organizer and anterior germ layer tissues.     

The cerberus-related gene, Cerr1, is not essential for mouse head formation

The Xenopus cerberus gene encodes a secreted factor expressed in the Spemann organizer that can cause ectopic head formation when its mRNA is injected into Xenopus embryos. In mouse, the cerberus-related gene, Cerr1, is expressed in the anterior mesendoderm that underlies the presumptive anterior neural plate and its expression is downregulated in Lim1 headless embryos. To determine whether Cerr1 is required for head formation we generated a null mutation in Cerr1 by gene targeting in mouse embryonic stem cells. We found that head formation is normal in Cerr1(-/-) embryos and we detected no obvious phenotypic defects in adult Cerr1(-/-) mice. However, in embryonic tissue layer recombination assays, Cerr1(-/-) presomitic/somitic mesoderm, unlike Cerr1-expressing wild-type presomitic/somitic mesoderm, was unable to maintain expression of the anterior neural marker gene Otx2 in ectoderm explants. These findings suggest that establishment of anterior identity in the mouse may involve the action of multiple functionally redundant factors.     

Conversion of ectoderm into a neural fate by ATH-3, a vertebrate basic helix-loop-helix gene homologous to Drosophila proneural gene atonal.

We have isolated a novel basic helix-loop-helix (bHLH) gene homologous to the Drosophila proneural gene atonal, termed ATH-3, from Xenopus and mouse. ATH-3 is expressed in the developing nervous system, with high levels of expression in the brain, retina and cranial ganglions. Injection of ATH-3 RNA into Xenopus embryos dramatically expands the neural tube and induces ectopic neural tissues in the epidermis but inhibits non-neural development. This ATH-3-induced neural hyperplasia does not require cell division, indicating that surrounding cells which are normally non-neural types adopt a neural fate. In a Xenopus animal cap assay, ATH-3 is able to convert ectodermal cells into neurons expressing anterior markers without inducing mesoderm. Interestingly, a single amino acid change from Ser to Asp in the basic region, which mimics phosphorylation of Ser, severely impairs the anterior marker-inducing ability without affecting general neurogenic activities. These results provide evidence that ATH-3 can directly convert non-neural or undetermined cells into a neural fate, and suggest that the Ser residue in the basic region may be critical for the regulation of ATH-3 activity by phosphorylation.