Cerberus is a head-inducing secreted factor expressed in the anterior endoderm of Spemann's organizer

BMP signaling and skeletogenesis

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor (TGF)-beta superfamily of signal molecules that mediate many diverse biological processes ranging from early embryonic tissue patterning to postnatal tissue homeostasis. BMPs trigger cell responses mainly through the canonical signaling pathway where intracellular Smads play central roles in delivering the extracellular signals to the nucleus. While the same Smads are used by BMPs in all types of cells, different transcription factors account in part for the functional diversity of BMPs. These transcription factors are recruited by Smads to regulate the expression of specific subsets of target genes depending on the cell types. Among the transcription factors are Hox proteins. Experimental gain and loss-of-function studies as well as naturally occurring mutations in Hox genes demonstrate their central roles in embryonic skeletal patterning. In addition to the interactions with Smads observed for several Hox proteins, there is also evidence that the expression of a number of Hox genes is regulated by BMPs. It is suggested that Hox proteins play an important role in the BMP pathway.

SOX7 and SOX18 are essential for cardiogenesis in Xenopus

Early in vertebrate development, endodermal signals act on mesoderm to induce cardiogenesis. The F-type SOXs SOX7 and SOX18beta are expressed in the cardiogenic region of the early Xenopus embryo. Injection of RNAs encoding SOX7 or SOX18beta, but not the related F-type SOX, SOX17, leads to the nodal-dependent expression of markers of cardiogenesis in animal cap explants. Injection of morpholinos directed against either SOX7 or SOX18mRNAs lead to a partial inhibition of cardiogenesis in vivo, while co-injection of SOX7 and SOX18 morpholinos strongly inhibited cardiogenesis. SOX7 RNA rescued the effects of the SOX18 morpholino and visa versa, indicating that the proteins have redundant functions. In animal cap explants, it appears that SOX7 and SOX18 act indirectly through Xnr2 to induce mesodermal (Eomesodermin, Snail, Wnt11), organizer (Cerberus) and endodermal (endodermin, Hex) tissues, which then interact to initiate cardiogenesis. Versions of SOX7 and SOX18 with their C-terminal, beta-catenin interaction domains replaced by a transcriptional activator domain failed to antagonize beta-catenin activation of Siamois, but still induced cardiogenesis. These observations identify SOX7 and SOX18 as important, and previously unsuspected, regulators of cardiogenesis in Xenopus.

Absence of Nodal signaling promotes precocious neural differentiation in the mouse embryo

After implantation, mouse embryos deficient for the activity of the transforming growth factor-beta member Nodal fail to form both the mesoderm and the definitive endoderm. They also fail to specify the anterior visceral endoderm, a specialized signaling center which has been shown to be required for the establishment of anterior identity in the epiblast. Our study reveals that Nodal-/- epiblast cells nevertheless express prematurely and ectopically molecular markers specific of anterior fate. Our analysis shows that neural specification occurs and regional identities characteristic of the forebrain are established precociously in the Nodal-/- mutant with a sequential progression equivalent to that of wild-type embryo. When explanted and cultured in vitro, Nodal-/- epiblast cells readily differentiate into neurons. Genes normally transcribed in organizer-derived tissues, such as Gsc and Foxa2, are also expressed in Nodal-/- epiblast. The analysis of Nodal-/-;Gsc-/- compound mutant embryos shows that Gsc activity plays no critical role in the acquisition of forebrain characters by Nodal-deficient cells. This study suggests that the initial steps of neural specification and forebrain development may take place well before gastrulation in the mouse and highlights a possible role for Nodal, at pregastrula stages, in the inhibition of anterior and neural fate determination.

Asymmetric expression of the BMP antagonists chordin and gremlin in the sea anemone Nematostella vectensis: implications for the evolution of axial patterning

The evolutionary origin of the anterior-posterior and the dorsoventral body axes of Bilateria is a long-standing question. It is unclear how the main body axis of Cnidaria, the sister group to the Bilateria, is related to the two body axes of Bilateria. The conserved antagonism between two secreted factors, BMP2/4 (Dpp in Drosophila) and its antagonist Chordin (Short gastrulation in Drosophila) is a crucial component in the establishment of the dorsoventral body axis of Bilateria and could therefore provide important insight into the evolutionary origin of bilaterian axes. Here, we cloned and characterized two BMP ligands, dpp and GDF5-like as well as two secreted antagonists, chordin and gremlin, from the basal cnidarian Nematostella vectensis. Injection experiments in zebrafish show that the ventralizing activity of NvDpp mRNA is counteracted by NvGremlin and NvChordin, suggesting that Gremlin and Chordin proteins can function as endogenous antagonists of NvDpp. Expression analysis during embryonic and larval development of Nematostella reveals asymmetric expression of all four genes along both the oral-aboral body axis and along an axis perpendicular to this one, the directive axis. Unexpectedly, NvDpp and NvChordin show complex and overlapping expression on the same side of the embryo, whereas NvGDF5-like and NvGremlin are both expressed on the opposite side. Yet, the two pairs of ligands and antagonists only partially overlap, suggesting complex gradients of BMP activity along the directive axis but also along the oral-aboral axis. We conclude that a molecular interaction between BMP-like molecules and their secreted antagonists was already employed in the common ancestor of Cnidaria and Bilateria to create axial asymmetries, but that there is no simple relationship between the oral-aboral body axis of Nematostella and one particular body axis of Bilateria.

Genomic analysis of Xenopus organizer function

Background

Studies of the Xenopus organizer have laid the foundation for our understanding of the conserved signaling pathways that pattern vertebrate embryos during gastrulation. The two primary activities of the organizer, BMP and Wnt inhibition, can regulate a spectrum of genes that pattern essentially all aspects of the embryo during gastrulation. As our knowledge of organizer signaling grows, it is imperative that we begin knitting together our gene-level knowledge into genome-level signaling models. The goal of this paper was to identify complete lists of genes regulated by different aspects of organizer signaling, thereby providing a deeper understanding of the genomic mechanisms that underlie these complex and fundamental signaling events.

Results

To this end, we ectopically overexpress Noggin and Dkk-1, inhibitors of the BMP and Wnt pathways, respectively, within ventral tissues. After isolating embryonic ventral halves at early and late gastrulation, we analyze the transcriptional response to these molecules within the generated ectopic organizers using oligonucleotide microarrays. An efficient statistical analysis scheme, combined with a new Gene Ontology biological process annotation of the Xenopus genome, allows reliable and faithful clustering of molecules based upon their roles during gastrulation. From this data, we identify new organizer-related expression patterns for 19 genes. Moreover, our data sub-divides organizer genes into separate head and trunk organizing groups, which each show distinct responses to Noggin and Dkk-1 activity during gastrulation.

Conclusion

Our data provides a genomic view of the cohorts of genes that respond to Noggin and Dkk-1 activity, allowing us to separate the role of each in organizer function. These patterns demonstrate a model where BMP inhibition plays a largely inductive role during early developmental stages, thereby initiating the suites of genes needed to pattern dorsal tissues. Meanwhile, Wnt inhibition acts later during gastrulation, and is essential for maintenance of organizer gene expression throughout gastrulation, a role which may depend on its ability to block the expression of a host of ventral, posterior, and lateral fate-specifying factors.

A Serpin family gene, protease nexin-1 has an activity distinct from protease inhibition in early Xenopus embryos

Protease nexin-1 (PN-1)/glia-derived nexin (GDN) is a member of the Serpin (serine proteinase inhibitor) family, and can inhibit thrombin, plasmin, and plasminogen activators. PN-1 has been shown to be a neuroprotective factor in a number of assay systems, and this activity has been assumed to be a function of its protease inhibitory function. Here, we report cloning and characterization of a Xenopus orthologue of PN-1 (xPN-1). xPN-1 was isolated in a functional screen of an egg cDNA library for factors that modify early axial patterning. xPN-1 is expressed maternally through late tadpole stages, and is expressed preferentially in the notochord, the pharyngeal endoderm, the otic vesicle, and the ventral region of the brain in tailbud embryos. Over-expression of xPN-1 causes defective gastrulation, inhibits convergent extension movements in activin induced animal caps, and inhibits expression of a distinct subset of activin induced mesendodermal markers. Interestingly, expression of point or deletion mutation of the Reactive Center Loop of xPN1,which is essential for the protease inhibitory activity of all serpins, had effects on Xenopus development indistinguishable from those of wild type xPN-1. These observations suggest the possibility that xPN-1 has a novel activity in addition to its established function as an inhibitor of serine proteases.

Induction and specification of cranial placodes

Cranial placodes are specialized regions of the ectoderm, which give rise to various sensory ganglia and contribute to the pituitary gland and sensory organs of the vertebrate head. They include the adenohypophyseal, olfactory, lens, trigeminal, and profundal placodes, a series of epibranchial placodes, an otic placode, and a series of lateral line placodes. After a long period of neglect, recent years have seen a resurgence of interest in placode induction and specification. There is increasing evidence that all placodes despite their different developmental fates originate from a common panplacodal primordium around the neural plate. This common primordium is defined by the expression of transcription factors of the Six1/2, Six4/5, and Eya families, which later continue to be expressed in all placodes and appear to promote generic placodal properties such as proliferation, the capacity for morphogenetic movements, and neuronal differentiation. A large number of other transcription factors are expressed in subdomains of the panplacodal primordium and appear to contribute to the specification of particular subsets of placodes. This review first provides a brief overview of different cranial placodes and then synthesizes evidence for the common origin of all placodes from a panplacodal primordium. The role of various transcription factors for the development of the different placodes is addressed next, and it is discussed how individual placodes may be specified and compartmentalized within the panplacodal primordium. Finally, tissues and signals involved in placode induction are summarized with a special focus on induction of the panplacodal primordium itself (generic placode induction) and its relation to neural induction and neural crest induction. Integrating current data, new models of generic placode induction and of combinatorial placode specification are presented.

Zebrafish endoderm formation is regulated by combinatorial Nodal, FGF and BMP signalling

In the zebrafish embryo, the mesoderm and endoderm originate from common precursors and segregate during gastrulation by mechanisms that are largely unknown. Understanding how the signalling pathways that regulate endoderm and mesoderm formation interact is crucial to understanding how the germ layers are established. Here, we have analysed how the FGF and BMP pathways interact with Nodal signalling during the process of endoderm formation. We found that activation of the FGF/ERK pathway disrupts endoderm formation in the embryo and antagonizes the ability of an activated form of Tar/Acvr1b to induce endoderm at the animal pole. By contrast, inhibition of FGF signalling increases the number of endodermal precursors and potentiates the ability of Tar*/Acvr1b to induce endoderm at the animal pole. Using a pharmacological inhibitor of the FGF receptor, we show that reducing FGF signalling partially rescues the deficit of endoderm precursors in bon mutant embryos. Furthermore, we found that overexpression of BMPs compromises endoderm formation, suggesting that formation of endoderm precursors is negatively regulated by BMPs on the ventral side. We show that simultaneous inhibition of the FGF/Ras and BMP pathways results in a dramatic increase in the number of endoderm precursors. Taken together, these data strongly suggest that BMP and FGF-ERK pathways cooperate to restrict the number of endodermal progenitors induced in response to Nodal signalling. Finally, we investigated the molecular basis for the FGF-MAPK-dependent repression of endoderm formation. We found that FGF/ERK signalling causes phosphorylation of Casanova/Sox32, an important regulator of endoderm determination, and provide evidence that this phosphorylation attenuates its ability to induce sox17. These results identify a molecular mechanism whereby FGF attenuates Nodal-induced endodermal transcription factors and highlight a potential mechanism whereby mesoderm and endoderm fates could segregate from each other.

Tes regulates neural crest migration and axial elongation in Xenopus

Tes is a member of an emerging family of proteins sharing a set of protein motifs referred to as PET-LIM domains. PET-LIM proteins such as Prickle regulate cell behavior during gastrulation in Xenopus and zebrafish, and to ask whether Tes is also involved in controlling cell behavior, we isolated its Xenopus orthologue. Xtes is expressed as a maternal transcript that is maintained at low levels until neurula stages when expression is elevated in the head and axial structures. Depletion of Xtes leads to a foreshortened head and severe defects in axis elongation. The anterior defect is due in part to the inhibition of cranial neural crest migration while the defects in elongation may be due to perturbation of expression of XFGF8, Xdelta-1 and Xcad-3 and thereby to disruption of posterior somitogenesis. Finally, we note that simultaneous depletion of Xtes and Xenopus Prickle results in axial defects that are more severe than those resulting from depletion of Xtes alone, suggesting that the two proteins act together to control axial elongation.

Spemann's organizer and self-regulation in amphibian embryos

In 1924, Spemann and Mangold demonstrated the induction of Siamese twins in transplantation experiments with salamander eggs. Recent work in amphibian embryos has followed their lead and uncovered that cells in signalling centres that are located at the dorsal and ventral poles of the gastrula embryo communicate with each other through a network of secreted growth-factor antagonists, a protease that degrades them, a protease inhibitor and bone-morphogenic-protein signals.

Early steps in neural development

We studied early neurulation events in vitro by transplanting quail Hensen's node, central prenodal regions (before the nodus as such develops), or upper layer parts of it on the not yet definitively committed upper layer of chicken anti-sickle regions (of unincubated blastoderms), eventually associated with central blastoderm fragments. We could demonstrate by this quail-chicken chimera technique that after the appearance of a pronounced thickening of the chicken upper layer by the early inductive effect of neighboring endophyll, a floor plate forms by insertion of Hensen's node-derived quail cells into the median part of the groove. This favors, at an early stage, the floor plate "allocation" model that postulates a common origin for notochord and median floor plate cells from the vertebrate's secondary major organizer (Hensen's node in this case). A comparison is made with results obtained after transplantation of similar Hensen's nodes in isolated chicken endophyll walls or with previously obtained results after the use of the grafting procedure in the endophyll walls of whole chicken blastoderms.

Genomic profiling of mixer and Sox17beta targets during Xenopus endoderm development

The transcription factors Mixer and Sox17beta have well-characterized roles in endoderm specification during Xenopus embryogenesis. In order to more thoroughly understand the mechanisms by which these endodermal regulators act, we expressed Mixer and Sox17beta in naïve ectodermal tissue and, using oligonucleotide-based microarrays, compared their genomic transcriptional profile to that of unaffected tissue. Using this approach, we identified 71 transcripts that are upregulated by Mixer or Sox17beta, 63 of which have previously uncharacterized roles in endoderm development. Furthermore, an in situ hybridization screen using antisense probes for several of these clones identified six targets of Mixer and/or Sox17beta that are expressed in the endoderm during gastrula stages, providing new and regional markers of the endoderm. Our results contribute further insight into the functions of Mixer and Sox17beta and bring us closer to understanding at the molecular level the pathways that regulate endoderm development.

Maternal determinants of embryonic cell fate

How important is the contribution of mRNAs and proteins stored in the oocyte for determining the body plan of the Xenopus embryo? Here we review the current understanding of the roles of maternally supplied transcription factors, signaling molecules, and signaling regulators in establishing the ectoderm, mesoderm, and endoderm germ layers and the embryonic axes. Key essential asymmetries of VegT, Wnt11, and Ectodermin are described, as well as the complexity of maternal transcription factors that are involved in the initial expression of early zygotic genes.

An obligatory caravanserai stop on the silk road to neural induction: Inhibition of BMP/GDF signaling

Work in Xenopus laevis produced the first molecular explanation for neural specification, the default model, where inactivation of the BMP pathway in ectodermal cells changes fates from epidermal to neural. This review covers the present status of our understanding of neural specification, with emphasis on Xenopus, but including relevant facts in other model systems. While recent experiments have increased the complexity of the molecular picture, they have also provided additional support for the default model and the central position of the BMP pathway. We conclude that synergy between accumulated knowledge and technical progress will maintain Xenopus at the forefront of research in neural development.

Cooperative non-cell and cell autonomous regulation of Nodal gene expression and signaling by Lefty/Antivin and Brachyury in Xenopus

Dynamic spatiotemporal expression of the nodal gene and its orthologs is involved in the dose-dependent induction and patterning of mesendoderm during early vertebrate embryogenesis. We report loss-of-function studies that define a high degree of synergistic negative regulation on the Xenopus nodal-related genes (Xnrs) by extracellular Xenopus antivin/lefty (Xatv/Xlefty)-mediated functional antagonism and Brachyury-mediated transcriptional suppression. A strong knockdown of Xlefty/Xatv function was achieved by mixing translation- and splicing-blocking morpholino oligonucleotides that target both the A and B alloalleles of Xatv. Secreted and cell-autonomous inhibitors of Xnr signaling were used to provide evidence that Xnr-mediated induction was inherently long-range in this situation in the large amphibian embryo, essentially being capable of spreading over the entire animal hemisphere. There was a greater expansion of the Organizer and mesendoderm tissues associated with dorsal specification than noted in previous Xatv knockdown experiments in Xenopus, with consequent exogastrulation and long-term maintenance of expanded axial tissues. Xatv deficiency caused a modest animal-ward expansion of the marginal zone expression territory of the Xnr1 and Xnr2 genes. In contrast, introducing inhibitory Xbra-En(R) fusion constructs into Xatv-deficient embryos caused a much larger increase in the level and spatial extent of Xnr expression. However, in both cases (Xatv/Xlefty-deficiency alone, or combined with Xbra interference), Xnr2 expression was constrained to the superficial cell layer, suggesting a fundamental tissue-specific competence in the ability to express Xnrs, an observation with direct implications regarding the induction of endodermal vs. mesodermal fates. Our experiments reveal a two-level suppressive mechanism for restricting the level, range, and duration of Xnr signaling via extracellular inhibition by Xatv/Xlefty coupled with potent indirect transcriptional repression by Xbra.

Extracellular modulation of BMP activity in patterning the dorsoventral axis

Signaling via bone morphogenetic proteins (BMPs) regulates a vast array of diverse biological processes in the developing embryo and in postembryonic life. Many insights into BMP signaling derive from studies of the BMP signaling gradients that pattern cell fates along the embryonic dorsal-ventral (DV) axis of both vertebrates and invertebrates. This review examines recent developments in the field of DV patterning by BMP signaling, focusing on extracellular modulation as a key mechanism in the formation of BMP signaling gradients in Drosophila, Xenopus, and zebrafish.

Primary body axes of vertebrates: Generation of a near-Cartesian coordinate system and the role of Spemann-type organizer

A rationale for the complex-appearing generation of the primary body axes in vertebrates can be obtained if this process is divided into two parts. First, an ancestral system is responsible for the anteroposterior (AP) patterning of the brain and the positioning of the heart. The blastopore (marginal zone) acts as a source region that generates primary AP-positional information for the brain, a process that is largely independent of the organizer. This evolutionary old system was once organizing the single axis of radial-symmetric ancestors. Second, the trunk is assumed to be an evolutionary later addition. The AP organization of the trunk depends on a time-controlled posterior transformation in which an oscillation plays a crucial role. This oscillation also leads to the repetitive nature of the trunk pattern as seen in somites or segments. The function of the Spemann-type organizer is not to specify the dorsoventral (DV) positional information directly but to initiate the formation of a stripe-shaped midline organizer, realized with different structures in the brain and in the trunk (prechordal plate vs. notochord). The distance of the cells to this midline (rather than to the organizer) is crucial for the DV specification. The basically different modes of axes formation in vertebrates and insects is proposed to have their origin in the initial positioning of the mesoderm. Only in vertebrates the mesoderm is initiated in a ring at a posterior position. Thus, only in vertebrates complex tissue movements are required to transform the ring-shaped posterior mesoderm into the rod-shaped axial structures.

Nodal-related geneXnr5 is amplified in theXenopus genome

In Xenopus, six nodal-related genes (Xnrs) have been identified to date. We found numerous tandem duplications of Xnr5 in the Xenopus laevis and Xenopus tropicalis genomes that involve highly conserved copies of coding and regulatory regions. The duplicated versions of Xnr5 were expressed in both the superficial and deep layer of dorsal endoderm and in the deep layer of ventral endoderm, where the initial inducers of mesendoderm formation would be expected to be localized. Overexpression of secreted inhibitors of Xnrs led to a substantially enhanced transcription of the duplicated Xnr5 genes and Xnr6 in embryos. Therefore, Xnr5 and Xnr6 have a novel feedback loop to inhibit transcription of Xnr5 and Xnr6. These results suggest that the initialization of a strong Xnr5 and Xnr6 signal is enabled by the rapid transcription from multiple genes. The novel feedback loop may negatively regulate transcription of Xnr5s and Xnr6 to limit overproduction of these potent inducers, with the Xnr5/Xnr6 signal then activating positive (Xnrs) and negative (Xlefty) loops, which regulate the range of mesodermal tissues produced.

FGF signaling is necessary for establishing gut tube domains along the anterior-posterior axis in vivo

At the end of gastrulation in avians and mammals, the endoderm germ layer is an undetermined sheet of cells. Over the next 24-48 h, endoderm forms a primitive tube and becomes regionally specified along the anterior-posterior axis. Fgf4 is expressed in gastrulation and somite stage embryos in the vicinity of posterior endoderm that gives rise to the posterior gut. Moreover, the posterior endoderm adjacent to Fgf4-expressing mesoderm expresses the FGF-target genes Sprouty1 and 2 suggesting that endoderm respond to an FGF signal in vivo. Here, we report the first evidence suggesting that FGF4-mediated signaling is required for establishing gut tube domains along the A-P axis in vivo. At the gastrula stage, exposing endoderm to recombinant FGF4 protein results in an anterior shift in the Pdx1 and CdxB expression domains. These expression domains remain sensitive to FGF4 levels throughout early somite stages. Additionally, FGF4 represses the anterior endoderm markers Hex1 and Nkx2.1 and disrupts foregut morphogenesis. FGF signaling directly patterns endoderm and not via a secondary induction from another germ layer, as shown by expression of dominant-active FGFR1 specifically in endoderm, which results in ectopic anterior expression of Pdx1. Loss-of-function studies using the FGF receptor antagonist SU5402 demonstrate that FGF signaling is necessary for establishing midgut gene expression and for maintaining gene expression boundaries between the midgut and hindgut from gastrulation through somitogenesis. Moreover, FGF signaling in the primitive streak is necessary to restrict Hex1 expression to anterior endoderm. These data show that FGF signaling is critical for patterning the gut tube by promoting posterior and inhibiting anterior endoderm cell fate.

Xnr2 and Xnr5 unprocessed proteins inhibit Wnt signaling upstream of dishevelled

Nodal and Nodal-related proteins activate the Activin-like signal pathway and play a key role in the formation of mesoderm and endoderm in vertebrate development. Recent studies have shown additional activities of Nodal-related proteins apart from the canonical Activin-like signal pathway. Here we report a novel function of Nodal-related proteins using cleavage mutants of Xenopus nodal-related genes (cmXnr2 and cmXnr5), which are known to be dominant-negative inhibitors of nodal family signaling. cmXnr2 and cmXnr5 inhibited both BMP signaling and Wnt signaling without activating the Activin-like signal in animal cap assays. Pro region construct of Xnr2 and Xnr5 did not inhibit Xwnt8, and pro/mature region chimera mutant cmActivin-Xnr2 and cmActivin-Xnr5 also did not inhibit Xwnt8 activity. These results indicate that the pro domains of Xnr2 and Xnr5 are necessary, but not sufficient, for Wnt inhibition, by Xnr family proteins. In addition, Western blot analysis and immunohistochemistry analysis revealed that the unprocessed Xnr5 protein is stably produced and secreted as effectively as mature Xnr5 protein, and that the unprocessed Xnr5 protein diffused in the extracellular space. These results suggest that unprocessed Xnr2 and Xnr5 proteins may be involved in inhibiting both BMP and Wnt signaling and are able to be secreted to act on somewhat distant target cells, if these are highly produced.

Vg 1 is an essential signaling molecule in Xenopus development

Xenopus Vg 1, a transforming growth factor beta (Tgfbeta) family member, was one of the first maternally localized mRNAs identified in vertebrates. Its restriction to the vegetal pole of the egg made it the ideal candidate to be the mesoderm-inducing signal released by vegetal cells, but its function in vivo has never been resolved. We show that Vg 1 is essential for Xenopus embryonic development, and is required for mesoderm induction and for the expression of several key Bmp antagonists. Although the original Vg 1 transcript does not rescue Vg 1-depleted embryos, we report that a second allele is effective. This work resolves the mystery of Vg 1 function, and shows it to be an essential maternal regulator of embryonic patterning.

Tissues and signals involved in the induction of placodal Six1 expression in Xenopus laevis

Ectodermal placodes, from which many cranial sense organs and ganglia develop, arise from a common placodal primordium defined by Six1 expression. Here, we analyse placodal Six1 induction in Xenopus using microinjections and tissue grafts. We show that placodal Six1 induction occurs during neural plate and neural fold stages. Grafts of anterior neural plate but not grafts of cranial dorsolateral endomesoderm induce Six1 ectopically in belly ectoderm, suggesting that only the neural plate is sufficient for inducing Six1 in ectoderm. However, extirpation of either anterior neural plate or of cranial dorsolateral endomesoderm abolishes placodal Six1 expression indicating that both tissues are required for its induction. Elevating BMP-levels blocks placodal Six1 induction, whereas ectopic sources of BMP inhibitors expand placodal Six1 expression without inducing Six1 ectopically. This suggests that BMP inhibition is necessary but needs to cooperate with additional factors for Six1 induction. We show that FGF8, which is expressed in the anterior neural plate, can strongly induce ectopic Six1 in ventral ectoderm when combined with BMP inhibitors. In contrast, FGF8 knockdown abolishes placodal Six1 expression. This suggests that FGF8 is necessary and together with BMP inhibitors sufficient to induce placodal Six1 expression in cranial ectoderm, implicating FGF8 as a central component in generic placode induction.

Identification and expression of XRTN2 and XRTN3 during Xenopus development

The reticulon (RTN) family of proteins has been described as a new eukaryotic protein family. We have isolated Xenopus cDNA homologues of RTN2 and RTN3 and examined their expression patterns during Xenopus development. XRTN2 has two transcripts, XRTN2-B and XRTN2-C, which encode 321 and 191 amino acids, respectively. XRTN3 has only one transcript that encodes 214 amino acids. We detected the XRTN2-B transcript in the neural tissues and brain from the early neurula stage. XRTN2-C is strongly expressed in the myotome, future skeletal muscle. The XRTN3 mRNA is localized in the animal hemisphere of the egg and blastula stage embryos and then subsequently restricted, mainly in the neural tissues. At the subcellular level, The XRTN proteins are expressed in the endoplasmic reticulum network structure of the animal cap cells as well as COS-7 cells. Our results suggest the potential roles of XRTN2s and XRTN3 during Xenopus embryogenesis.

Mesodermal Differentiation: Signal Integration During Development

This Teaching Resource provides a summary and slides derived from a lecture on the signaling cascades that mediate early embryogenesis and is part of the course "Cell Signaling Systems: A Course for Graduate Students." The lecture focuses on the mechanisms underlying germ layer formation, patterning, and suppression in the vertebrate embryo, with particular emphasis on the regulation of mesodermal development in the model organism Xenopus laevis.

XIC is required for Siamois activity and dorsoanterior development

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

Neural induction: old problem, new findings, yet more questions

During neural induction, the embryonic neural plate is specified and set aside from other parts of the ectoderm. A popular molecular explanation is the 'default model' of neural induction, which proposes that ectodermal cells give rise to neural plate if they receive no signals at all, while BMP activity directs them to become epidermis. However, neural induction now appears to be more complex than once thought, and can no longer be fully explained by the default model alone. This review summarizes neural induction events in different species and highlights some unanswered questions about this important developmental process.

BMP antagonists: Their roles in development and involvement in pathophysiology

Bone morphogenetic proteins (BMPs) are phylogenetically conserved signaling molecules that belong to the transforming growth factor (TGF)-beta superfamily, and are involved in the cascades of body patterning and morphogenesis. The activities of BMPs are precisely regulated by certain classes of molecules that are recently recognized as BMP antagonists. BMP antagonists function through direct association with BMPs, thus prohibiting BMPs from binding their cognate receptors. In this review, the classification and functions of BMP antagonists will be discussed, especially focusing on the new family of tissue-specific BMP antagonists composed of uterine sensitization-associated gene 1 (USAG-1) and sclerostin.

Choice of either beta-catenin or Groucho/TLE as a co-factor for Xtcf-3 determines dorsal-ventral cell fate of diencephalon during Xenopus development

Co-repressor Groucho/Transducin-Like Enhancer of split (TLE) interacts with transcription factors that are expressed in the central nervous system (CNS), and regulates transcriptional activities. In this study, we examined the contribution of Groucho/TLE to CNS development in Xenopus. The functional inhibition of Groucho/TLE using the WRPW motif as a competitor resulted in the conversion of the ventral cell into the dorsal fate in the prospective diencephalon. We also found that the neural plate was expanded laterally without inhibiting neural crest development. In tailbud, the disturbance of trigeminal ganglion development was observed. These observations allow us to conclude that Groucho/TLE plays important roles in the induction and patterning of distinct CNS territories. We found that Xtcf-3 is involved in some of the patterning in these territories. We generated the variant of Xtcf-3, Xtcf-3BDN-, which is suspected to interfere with the interaction between endogenous Groucho/TLE and Xtcf-3. The transcriptional activation of the Xtcf-3-target genes in response to endogenous Wnt/beta-catenin signaling by the overexpression of Xtcf-3BDN- led to a reduction of the ventral diencephalon. This result indicates that transcriptional repression by the Groucho/TLE-Xtcf-3 complex is important for ventral diencephalon patterning. This idea is supported by the finding that the overexpression of the dominant-negative form of Xtcf-3 or axil causes the expansion of the ventral diencephalon. Based on these data, we propose that the localized activation of Wnt/beta-catenin signaling, which converts Tcf from a repressor to an activator, is required for the establishment of dorsal-ventral patterning in the prospective diencephalon.

WNTs in the vertebrate nervous system: from patterning to neuronal connectivity

WNT signalling has a key role in early embryonic patterning through the regulation of cell fate decisions, tissue polarity and cell movements. In the nervous system, WNT signalling also regulates neuronal connectivity by controlling axon pathfinding, axon remodelling, dendrite morphogenesis and synapse formation. Studies, from invertebrates to mammals, have led to a considerable understanding of WNT signal transduction pathways. This knowledge provides a framework for the study of the mechanisms by which WNTs regulate diverse neuronal functions. Manipulation of the WNT pathways could provide new strategies for nerve regeneration and neuronal circuit modulation.

Xema, a foxi-class gene expressed in the gastrula stage Xenopus ectoderm, is required for the suppression of mesendoderm

The molecular basis of vertebrate germ layer formation has been the focus of intense scrutiny for decades, and the inductive interactions underlying this process are well defined. Only recently, however, have studies demonstrated that the regulated inhibition of ectopic germ layer formation is also crucial for patterning the early vertebrate embryo. We report here the characterization of Xema (Xenopus Ectodermally-expressed Mesendoderm Antagonist), a novel member of the Foxi-subclass of winged-helix transcription factors that is involved in the suppression of ectopic germ layer formation in the frog, Xenopus laevis. Xema transcripts are restricted to the animal pole ectoderm during early Xenopus development. Ectopic expression of Xema RNA inhibits mesoderm induction, both by growth factors and in the marginal zone, in vivo. Conversely, introduction of antisense morpholino oligonucleotides directed against the Xema transcript stimulates the expression of a broad range of mesodermal and endodermal marker genes in the animal pole. Our studies demonstrate that Xema is both necessary and sufficient for the inhibition of ectopic mesendoderm in the cells of the presumptive ectoderm, and support a model in which Fox proteins function in part to restrict inappropriate germ layer development throughout the vertebrate embryo.

Expression of FoxE4 and Rx Visualizes the Timing and Dynamics of Critical Processes Taking Place during Initial Stages of Vertebrate Eye Development

Several transcription factors have a critical function during initial stages of vertebrate eye formation. In this paper, we discuss the role of the Rx subfamily of homeobox-containing genes in retinal development, and the role of the Foxe3 and FoxE4 subfamily of forkhead box-containing genes in lens development. Rx genes are expressed in the initial stages of retinal development and they play a critical role in eye formation. Elimination of Rx function in mice results in lack of eye formation. Abnormal eye development observed in the mouse mutation eyeless (ey1), the medakatemperature-sensitive mutation eyeless (el), and the zebrafish mutation chokh are caused by abnormal regulation or function of Rx genes. In humans, a mutation in Rx leads to anophthalmia. In contrast, Foxe3 and FoxE4 genes are expressed in the lens and they play an essential role in its formation. Mutations in the Foxe3 gene are the cause of the mouse mutation dysgenetic lens (dyl) and in humans, mutation in FOXE3 leads to anterior segment dysgenesis and cataracts. Since Rx and FoxE4 are expressed in the earliest stages of retina and lens development, their expression visualizes the timing and dynamics of the crucial processes that comprise eye formation. In this paper we present a model of eye development based on the expression pattern of these two genes.

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.

Identification and characterization of Xenopus OMP25

This study describes the isolation of mitochondrial outer membrane protein 25 (OMP25) from Xenopus laevis and an analysis of its role in early development. X. laevis OMP25 (xOMP25) is a transmembrane protein of the mitochondrial outer membrane with a PDZ domain in the cytoplasmic tail, and an approximate molecular size of 25 kDa. We isolated xOMP25 from a cDNA library of X. laevis tailbud embryos. Amino acid sequence analysis of xOMP25 showed 57% identity to mouse OMP25, with 73% identity in the PDZ domains. XOMP25 mRNA is expressed maternally, and at a constant level throughout early development. The transcript is localized to eye, otic vesicle, branchial arch and neural tube. Mitochondrial targeting of an EGFP-fusion protein of xOMP25 was visualized using a mitochondria-specific fluorescent dye. Overexpression of xOMP25 in embryos caused curved axes, small eyes and disorganized head structures. Knockdown of xOMP25 protein using antisense morpholino oligonucleotides resulted in slightly shortened axes and decreased neural tissue. Although the mechanism remains unclear, our results implicate xOMP25 protein in the formation of the intact neural tube.

Heart induction by Wnt antagonists depends on the homeodomain transcription factor Hex

Inhibition of canonical Wnt/beta-catenin signaling by Dickkopf-1 (Dkk-1) or Crescent initiates cardiogenesis in vertebrate embryos. However, nearly nothing is known about the downstream effectors of these secreted Wnt antagonists or the mechanism by which they activate heart formation. Here we show that Wnt antagonists in Xenopus stimulate cardiogenesis non-cell-autonomously, up to several cells away from those in which canonical Wnt/beta-catenin signaling is blocked, indicative of an indirect role in heart induction. A screen for downstream mediators revealed that Dkk-1 and other inhibitors of the canonical Wnt pathway induce the homeodomain transcription factor Hex, which is normally expressed in endoderm underlying the presumptive cardiac mesoderm in amphibian, bird, and mammalian embryos. Loss of Hex function blocks both endogenous heart development and ectopic heart induction by Dkk-1. As with the canonical Wnt pathway antagonists, ectopic Hex induces expression of cardiac markers non-cell-autonomously. Thus, to initiate cardiogenesis, Wnt antagonists act on endoderm to up-regulate Hex, which, in turn, controls production of a diffusible heart-inducing factor. This novel function for Hex suggests an etiology for the cardiac malformations in Hex mutant mice and will make possible the isolation of factors that induce heart directly in the mesoderm.

Identification of novel genes affecting mesoderm formation and morphogenesis through an enhanced large scale functional screen in Xenopus

The formation of mesoderm is an important developmental process of vertebrate embryos, which can be broken down into several steps; mesoderm induction, patterning, morphogenesis and differentiation. Although mesoderm formation in Xenopus has been intensively studied, much remains to be learned about the molecular events responsible for each of these steps. Furthermore, the interplay between mesoderm induction, patterning and morphogenesis remains obscure. Here, we describe an enhanced functional screen in Xenopus designed for large-scale identification of genes controlling mesoderm formation. In order to improve the efficiency of the screen, we used a Xenopus tropicalis unique set of cDNAs, highly enriched in full-length clones. The screening strategy incorporates two mesodermal markers, Xbra and Xmyf-5, to assay for cell fate specification and patterning, respectively. In addition we looked for phenotypes that would suggest effects in morphogenesis, such as gastrulation defects and shortened anterior-posterior axis. Out of 1728 full-length clones we isolated 82 for their ability to alter the phenotype of tadpoles and/or the expression of Xbra and Xmyf-5. Many of the clones gave rise to similar misexpression phenotypes (synphenotypes) and many of the genes within each synphenotype group appeared to be involved in similar pathways. We determined the expression pattern of the 82 genes and found that most of the genes were regionalized and expressed in mesoderm. We expect that many of the genes identified in this screen will be important in mesoderm formation.

Myocardin is sufficient and necessary for cardiac gene expression in Xenopus

Myocardin is a cardiac- and smooth muscle-specific cofactor for the ubiquitous transcription factor serum response factor (SRF). Using gain-of-function approaches in the Xenopus embryo, we show that myocardin is sufficient to activate transcription of a wide range of cardiac and smooth muscle differentiation markers in non-muscle cell types. We also demonstrate that, for the myosin light chain 2 gene (MLC2), myocardin cooperates with the zinc-finger transcription factor Gata4 to activate expression. Inhibition of myocardin activity in Xenopus embryos using morpholino knockdown methods results in inhibition of cardiac development and the absence of expression of cardiac differentiation markers and severe disruption of cardiac morphological processes. We conclude that myocardin is an essential component of the regulatory pathway for myocardial differentiation.

Neural induction requires BMP inhibition only as a late step, and involves signals other than FGF and Wnt antagonists

A dominant molecular explanation for neural induction is the 'default model', which proposes that the ectoderm is pre-programmed towards a neural fate, but is normally inhibited by endogenous BMPs. Although there is strong evidence favouring this in Xenopus, data from other organisms suggest more complexity, including an involvement of FGF and modulation of Wnt. However, it is generally believed that these additional signals also act by inhibiting BMPs. We have investigated whether BMP inhibition is necessary and/or sufficient for neural induction. In the chick, misexpression of BMP4 in the prospective neural plate inhibits the expression of definitive neural markers (Sox2 and late Sox3), but does not affect the early expression of Sox3, suggesting that BMP inhibition is required only as a late step during neural induction. Inhibition of BMP signalling by the potent antagonist Smad6, either alone or together with a dominant-negative BMP receptor, Chordin and/or Noggin in competent epiblast is not sufficient to induce expression of Sox2 directly, even in combination with FGF2, FGF3, FGF4 or FGF8 and/or antagonists of Wnt signalling. These results strongly suggest that BMP inhibition is not sufficient for neural induction in the chick embryo. To test this in Xenopus, Smad6 mRNA was injected into the A4 blastomere (which reliably contributes to epidermis but not to neural plate or its border) at the 32-cell stage: expression of neural markers (Sox3 and NCAM) is not induced. We propose that neural induction involves additional signalling events that remain to be identified.

Molecular mechanisms of neural crest induction

The neural crest is an embryonic cell population that originates at the border between the neural plate and the prospective epidermis. Around the time of neural tube closure, neural crest cells emigrate from the neural tube, migrate along defined paths in the embryo and differentiate into a wealth of derivatives. Most of the craniofacial skeleton, the peripheral nervous system, and the pigment cells of the body originate from neural crest cells. This cell type has important clinical relevance, since many of the most common craniofacial birth defects are a consequence of abnormal neural crest development. Whereas the migration and differentiation of the neural crest have been extensively studied, we are just beginning to understand how this tissue originates. The formation of the neural crest has been described as a classic example of embryonic induction, in which specific tissue interactions and the concerted action of signaling pathways converge to induce a multipotent population of neural crest precursor cells. In this review, we summarize the current status of knowledge on neural crest induction. We place particular emphasis on the signaling molecules and tissue interactions involved, and the relationship between neural crest induction, the formation of the neural plate and neural plate border, and the genes that are upregulated as a consequence of the inductive events.

Shisa Promotes Head Formation through the Inhibition of Receptor Protein Maturation for the Caudalizing Factors, Wnt and FGF

Head formation requires simultaneous inhibition of multiple caudalizing signals during early vertebrate embryogenesis. We identified a novel antagonist against Wnt and FGF signaling for head formation, Shisa, which functions cell autonomously in the endoplasmic reticulum (ER). Shisa is specifically expressed in the prospective head ectoderm and the Spemann organizer of Xenopus gastrulae. Overexpression of Shisa inhibited both Wnt and FGF signaling in Xenopus embryos and in a cell line. Loss of Shisa function sensitized the neuroectoderm to Wnt signaling and suppressed head formation during gastrulation. Shisa physically interacted with immature forms of the Wnt receptor Frizzled and the FGF receptor within the ER and inhibited their posttranslational maturation and trafficking to the cell surface. Taken together, these findings indicate that Shisa is a novel molecule that controls head formation by regulating the establishment of the receptors for caudalizing factors.

Identification of neural genes usingXenopus DNA microarrays

To isolate novel genes regulating neural induction, we used a DNA microarray approach. As neural induction is thought to occur by means of the inhibition of bone morphogenetic protein (BMP) signaling, BMP signaling was inhibited in ectodermal cells by overexpression of a dominant-negative receptor. RNAs were isolated from control animal cap explants and from dominant-negative BMP receptor expressing animal caps and subjected to a microarray experiment using newly generated high-density Xenopus DNA microarray chips representing over 17,000 unigenes. We have identified 77 genes that are induced in animal caps after inhibition of BMP signaling, and all of these genes were subjected to whole-mount in situ hybridization analysis. Thirty-two genes showed specific expression in neural tissues. Of the 32, 14 genes have never been linked to neural induction. Two genes that are highly induced by BMP inhibition are inhibitors of Wnt signaling, suggesting that a key step in neural induction is to produce Wnt antagonists to promote anterior neural plate development. Our current analysis also proves that a microarray approach is useful in identifying novel candidate factors involved in neural induction and patterning.

Origin, fate, and function of the components of the avian germ disc region and early blastoderm: Role of ooplasmic determinants

In the avian oocytal germ disc region, at the end of oogenesis, we discerned four ooplasms (alpha, beta, gamma, delta) presenting an onion-peel distribution (from peripheral and superficial to central and deep. Their fate was followed during early embryonic development. The most superficial and peripheral alpha ooplasm plays a fundamental role during cleavage. The beta ooplasm, originally localized in the peripheral region of the blastodisc, becomes mainly concentrated in the primitive streak. At the moment of bilateral symmetrization, a spatially oblique, sickle-shaped uptake of gamma and delta ooplasms occurs so that gamma and delta ooplasms become incorporated into the deeper part of the avian blastoderm. These ooplasms seem to contain ooplasmic determinants that initiate either early neurulation or gastrulation events. The early neural plate-inducing structure that forms a deep part of the blastoderm is the delta ooplasm-containing endophyll (primary hypoblast). Together with the primordial germ cells, it is derived from the superficial centrocaudal part of the nucleus of Pander, which also contains delta ooplasm. The other structure (gamma ooplasm) that is incorporated into the caudolateral deep part of the blastoderm forms Rauber's sickle. It induces gastrulation in the concavity of Rauber's sickle and blood island formation exterior to Rauber's sickle. Rauber's sickle develops by ingrowth of blastodermal cells into the gamma ooplasm, which surrounds the nucleus of Pander. Rauber's sickle constitutes the primary major organizer of the avian blastoderm and generates only extraembryonic tissues (junctional and sickle endoblast). By imparting positional information, it organizes and dominates the whole blastoderm (controlling gastrulation, neurulation, and coelom and cardiovascular system formation). Fragments of the horns of Rauber's sickle extend far cranially into the lateral quadrants of the unincubated blastoderm, so that often Rauber's sickle material forms three quarters of a circle. This finding explains the regulative capacities of isolated blastoderm parts, with the exception of the anti-sickle region and central blastoderm region, where no Rauber's sickle material is present. In avian blastoderms, there exists a competitive inhibition by Rauber's sickle on the primitive streak and neural plate-inducing effects of sickle endoblast. Avian primordial germ cells contain delta ooplasm derived from the superficial part of the nucleus of Pander. Their original deep and central ooplasmic localization has been confirmed by the use of a chicken vasa homologue. We conclude that the unincubated blastoderm consists of three elementary tissues: upper layer mainly containing beta ooplasm, endophyll containing delta ooplasm, and Rauber's sickle containing gamma ooplasm). These elementary tissues form before the three classic germ layers have developed.

Neural induction in Xenopus requires early FGF signalling in addition to BMP inhibition

Neural induction constitutes the first step in the generation of the vertebrate nervous system from embryonic ectoderm. Work with Xenopus ectodermal explants has suggested that epidermis is induced by BMP signals, whereas neural fates arise by default following BMP inhibition. In amniotes and ascidians, however, BMP inhibition does not appear to be sufficient for neural fate acquisition, which is initiated by FGF signalling. We decided to re-evaluate in the context of the whole embryo the roles of the BMP and FGF pathways during neural induction in Xenopus. We find that ectopic BMP activity converts the neural plate into epidermis, confirming that this pathway must be inhibited during neural induction in vivo. Conversely, inhibition of BMP, or of its intracellular effector SMAD1 in the non-neural ectoderm leads to epidermis suppression. In no instances, however, is BMP/SMAD1 inhibition sufficient to elicit neural induction in ventral ectoderm. By contrast, we find that neural specification occurs when weak eFGF or low ras signalling are combined with BMP inhibition. Using all available antimorphic FGF receptors (FGFR), as well as the pharmacological FGFR inhibitor SU5402, we demonstrate that pre-gastrula FGF signalling is required in the ectoderm for the emergence of neural fates. Finally, we show that although the FGF pathway contributes to BMP inhibition, as in other model systems, it is also essential for neural induction in vivo and in animal caps in a manner that cannot be accounted for by simple BMP inhibition. Taken together, our results reveal that in contrast to predictions from the default model, BMP inhibition is required but not sufficient for neural induction in vivo. This work contributes to the emergence of a model whereby FGF functions as a conserved initiator of neural specification among chordates.

LDL receptor-related protein LRP6 regulates proliferation and survival through the Wnt cascade in vascular smooth muscle cells

Initial studies have established expression of low-density lipoprotein (LDL) receptor-related protein 6 (LRP6) in vascular smooth muscle cells (VSMCs). We hypothesized that LRP6 is a critical mediator governing the regulation of the canonical Wnt/β-catenin/T cell factor 4 (Tcf-4) cascade in the vasculature. This hypothesis was based on our previous work demonstrating a role for the β-catenin/Tcf-4 pathway in vascular remodeling as well as work in other cell systems establishing a role for LRP family members in the Wnt cascade. In line with our hypothesis, LRP6 upregulation significantly increased Wnt-1-induced Tcf activation. Moreover, a dominant interfering LRP6 mutant lacking the carboxyl intracellular domain (LRP6ΔC) abolished Tcf activity. LRP6-induced stimulation of Tcf was blocked in VSMCs harboring constitutive expression of a dominant negative Tcf-4 transgene lacking the β-catenin binding domain, suggesting that LRP6-induced activation of Tcf was mediated through a β-catenin-dependent signal. Expression of the dominant interfering LRP6ΔC transgene was sufficient to abolish the Wnt-induced survival as well as cyclin D1 activity and cell cycle progression. In conclusion, these findings provide the first evidence of a role for an LDL receptor-related protein in the regulation of VSMC proliferation and survival through the evolutionary conserved Wnt signaling cascade.

Hypoblast controls mesoderm generation and axial patterning in the gastrulating rabbit embryo

Gastrulation in higher vertebrate species classically commences with the generation of mesoderm cells in the primitive streak by epithelio-mesenchymal transformation of epiblast cells. However, the primitive streak also marks, with its longitudinal orientation in the posterior part of the conceptus, the anterior-posterior (or head-tail) axis of the embryo. Results obtained in chick and mouse suggest that signals secreted by the hypoblast (or visceral endoderm), the extraembryonic tissue covering the epiblast ventrally, antagonise the mesoderm induction cascade in the anterior part of the epiblast and thereby restrict streak development to the posterior pole (and possibly initiate head development anteriorly). In this paper we took advantage of the disc-shape morphology of the rabbit gastrula for defining the expression compartments of the signalling molecules Cerberus and Dickkopf at pre-gastrulation and early gastrulation stages in a mammal other than the mouse. The two molecules are expressed in novel expression compartments in a complementary fashion both in the hypoblast and in the emerging primitive streak. In loss-of-function experiments, carried out in a New-type culturing system, hypoblast was removed prior to culture at defined stages before and at the beginning of gastrulation. The epiblast shows a stage-dependent and topographically restricted susceptibility to express Brachyury, a T-box gene pivotal for mesoderm formation, and to transform into (histologically proven) mesoderm. These results confirm for the mammalian embryo that the anterior-posterior axis of the conceptus is formed first as a molecular prepattern in the hypoblast and then irrevocably fixed, under the control of signals secreted from the hypoblast, by epithelio-mesenchymal transformation (primitive streak formation) in the epiblast.

Stroke and TGF-beta proteins: glial cell line-derived neurotrophic factor and bone morphogenetic protein

Recent studies have indicated that proteins in the transforming growth factor-beta superfamily alter damage induced by various neuronal injuries. Of these proteins, glial cell line-derived neurotrophic factor (GDNF) and bone morphogenetic protein-7 (BMP-7) have unique protective and regenerative effects in stroke animals. Delivery of GDNF or BMP-7 to brain tissue reduced cerebral infarction and improved motor functions in stroke animals. Pretreatment with these factors reduced caspase-3 activity and DNA fragmentation in the ischemic brain region, suggesting that antiapoptotic effects are involved. Beside the protective effects, BMP-7 given after stroke improves locomotor function. These regenerative effects of BMP-7 may involve the enhancement of dendritic growth and remodeling. In this review, we illustrate the neuroprotective and neuroregenerative properties of GDNF and BMP-7 and emphasize their therapeutic potential for stroke.

The activity of the Nodal antagonist Cerl-2 in the mouse node is required for correct L/R body axis

Correct establishment of the left/right (L/R) body asymmetry in the mouse embryo requires asymmetric activation of the evolutionarily conserved Nodal signaling cascade in the left lateral plate mesoderm (L-LPM). Furthermore, the presence of Nodal in the node is essential for its own expression in the L-LPM. Here, we have characterized the function of cerl-2, a novel Nodal antagonist, which displays a unique asymmetric expression on the right side of the mouse node. cerl-2 knockout mice display multiple laterality defects including randomization of the L/R axis. These defects can be partially rescued by removing one nodal allele. Our results demonstrate that Cerl-2 plays a key role in restricting the Nodal signaling pathway toward the left side of the mouse embryo by preventing its activity in the right side.

Six1 promotes a placodal fate within the lateral neurogenic ectoderm by functioning as both a transcriptional activator and repressor

Cranial placodes, which give rise to sensory organs in the vertebrate head, are important embryonic structures whose development has not been well studied because of their transient nature and paucity of molecular markers. We have used markers of pre-placodal ectoderm (PPE) (six1, eya1) to determine that gradients of both neural inducers and anteroposterior signals are necessary to induce and appropriately position the PPE. Overexpression of six1 expands the PPE at the expense of neural crest and epidermis, whereas knock-down of Six1 results in reduction of the PPE domain and expansion of the neural plate, neural crest and epidermis. Using expression of activator and repressor constructs of six1 or co-expression of wild-type six1 with activating or repressing co-factors (eya1 and groucho, respectively), we demonstrate that Six1 inhibits neural crest and epidermal genes via transcriptional repression and enhances PPE genes via transcriptional activation. Ectopic expression of neural plate, neural crest and epidermal genes in the PPE demonstrates that these factors mutually influence each other to establish the appropriate boundaries between these ectodermal domains.