Nodal and Fgf pathways interact through a positive regulatory loop and synergize to maintain mesodermal cell populations

REREa/Atrophin-2 interacts with histone deacetylase and Fgf8 signaling to regulate multiple processes of zebrafish development

The transcriptional regulator RERE/Atrophin-2 (RERE) is required for the normal patterning of the early vertebrate embryo, including the central nervous system, pharyngeal arches, and limbs. Consistent with a role as a transcriptional corepressor, RERE binds histone deacetylase 1 and 2 (HDAC1/2), and orphan nuclear receptors such as Tlx. Here, we identify the zebrafish babyface (bab) as a mutant in rerea and show that it interacts genetically with fibroblast growth factor 8 (fgf8). We suggest that this finding is largely due to its interactions with HDAC, because genetic or pharmacological disruptions of HDAC phenocopy many features of the bab mutant. Furthermore, removing the functions of either REREa or HDAC synergizes with loss of Fgf8 function to disrupt posterior mesoderm formation during somitogenesis, midbrain-hindbrain boundary maintenance, and pharyngeal cartilage development. Together, these results reveal novel in vivo roles for REREa in HDAC-mediated regulation of Fgf signaling. We present a model for RERE-dependent patterning in which tissue-specific transcriptional repression, by means of an REREa-HDAC complex, modulates growth factor signaling during embryogenesis.

Chordin expression, mediated by Nodal and FGF signaling, is restricted by redundant function of two beta-catenins in the zebrafish embryo

Using embryos transgenic for the TOP-GFP reporter, we show that the two zebrafish beta-catenins have different roles in the organizer and germ-ring regions of the embryo. beta-Catenin-activated transcription in the prospective organizer region specifically requires beta-catenin-2, whereas the ventrolateral domain of activated transcription is abolished only when both beta-catenins are inhibited. chordin expression during zebrafish gastrulation has been previously shown in both axial and paraxial domains, but is excluded from ventrolateral domains. We show that this gene is expressed in paraxial territories adjacent to the domain of ventrolateral beta-catenin-activated transcription, with only slight overlap, consistent with the now well-known inhibitory effects of Wnt8 on dorsal gene expression. Eliminating both Wnt8/beta-catenin signaling and organizer activity by inhibition of expression of the two beta-catenins results in massive ectopic circumferential expression of chordin and later, by formation of a distinctive embryonic phenotype ('ciuffo') that expresses trunk and anterior neural markers with correct relative anteroposterior patterning. We show that chordin expression is required for this neural gene expression. The Nodal gene squint has been shown to be necessary for optimal expression of chordin and is sufficient in some contexts for its expression. However, chordin is not normally expressed in the ventrolateral germ-ring despite robust expression of squint in this domain. We show the ectopic circumferential expression of chordin and other dorsal genes to be completely dependent on Nodal and FGF signaling, and to be independent of a functional organizer. We propose that whereas the axial domain of chordin expression is formed by cells that are derived from the organizer, the paraxial domain is the result of axial-derived anti-Wnt signals, which relieve the repression that otherwise is set by the Wnt8/beta-catenin/vox,vent pathway on latent germ-ring Nodal/FGF-activated expression.

The genetics and embryology of zebrafish metamerism

Somites are the most obvious metameric structures in the vertebrate embryo. They are mesodermal segments that form in bilateral pairs flanking the notochord and are created sequentially in an anterior to posterior sequence concomitant with the posterior growth of the trunk and tail. Zebrafish somitogenesis is regulated by a clock that causes cells in the presomitic mesoderm (PSM) to undergo cyclical activation and repression of several notch pathway genes. Coordinated oscillation among neighboring cells manifests as stripes of gene expression that pass through the cells of the PSM in a posterior to anterior direction. As axial growth continually adds new cells to the posterior tail bud, cells of the PSM become relatively less posterior. This gradual assumption of a more anterior position occurs over developmental time and constitutes part of a maturation process that governs morphological segmentation in conjunction with the clock. Segment morphogenesis involves a mesenchymal to epithelial transition as prospective border cells at the anterior end of the mesenchymal PSM adopt a polarized, columnar morphology and surround a mesenchymal core of cells. The segmental pattern influences the development of the somite derivatives such as the myotome, and the myotome reciprocates to affect the formation of segment boundaries. While somites appear to be serially homologous, there may be variation in the segmentation mechanism along the body axis. Moreover, whereas the genetic architecture of the zebrafish, mouse, and chick segmentation clocks shares many common elements, there is evidence that the gene networks have undergone independent modification during evolution.

Nodal signaling activates differentiation genes during zebrafish gastrulation

Nodal signals induce mesodermal and endodermal progenitors during vertebrate development. To determine the role of Nodal signaling at a genomic level, we isolated Nodal-regulated genes by expression profiling using macroarrays and gene expression databases. Putative Nodal-regulated genes were validated by in situ hybridization screening in wild type and Nodal signaling mutants. 46 genes were identified, raising the currently known number of Nodal-regulated genes to 72. Based on their expression patterns along the dorsoventral axis, most of these genes can be classified into two groups. One group is expressed in the dorsal margin, whereas the other group is expressed throughout the margin. In addition to transcription factors and signaling components, the screens identified several new functional classes of Nodal-regulated genes, including cytoskeletal components and molecules involved in protein secretion or endoplasmic reticulum stress. We found that x-box binding protein-1 (xbp1) is a direct target of Nodal signaling and required for the terminal differentiation of the hatching gland, a specialized secretory organ whose specification is also dependent on Nodal signaling. These results indicate that Nodal signaling regulates not only specification genes but also differentiation genes.

FGF signaling is required for {beta}-catenin-mediated induction of the zebrafish organizer

We have used the maternal effect mutant ichabod, which is deficient in maternal beta-catenin signaling, to test for the epistatic relationship between beta-catenin activation, FGF signaling and bozozok, squint and chordin expression. Injection of beta-catenin RNA into ichabod embryos can completely rescue normal development. By contrast, when FGF signaling is inhibited, beta-catenin did not induce goosecoid and chordin, repress bmp4 expression or induce a dorsal axis. These results demonstrate that FGF signaling is necessary for beta-catenin induction of the zebrafish organizer. We show that FGFs function downstream of squint and bozozok to turn on chordin expression. Full rescue of ichabod by Squint is dependent on FGF signaling, and partial rescue by FGFs is completely dependent on chordin. By contrast, Bozozok can rescue the complete anteroposterior axis, but not notochord, in embryos blocked in FGF signaling. Surprisingly, accumulation of bozozok transcript in beta-catenin RNA-injected ichabod embryos is also dependent on FGF signaling, indicating a role of FGFs in maintenance of bozozok RNA. These experiments show that FGF-dependent organizer function operates through both bozozok RNA accumulation and a pathway consisting of beta-catenin-->Squint-->FGF-->Chordin, in which each component is sufficient for expression of the downstream factors of the pathway, and in which Nodal signaling is required for FGF gene expression and FGF signaling is required for Squint induction of chordin.

Fgf signaling negatively regulates Nodal-dependent endoderm induction in zebrafish

In zebrafish development, Nodal signaling is critical for the induction of endoderm and mesoderm. Three transcription factors downstream of Nodal, Bonnie and Clyde (Bon), Faust (Fau)/Gata5 and Casanova (Cas), are required for endoderm induction. However, it is not yet fully understood how the Nodal signaling pathway regulates the decision process of endoderm and mesoderm induction. In this study, we focused on Fgf signaling, downstream of Nodal signaling, during endoderm induction. We found that activation of Fgf signaling decreases the number of cas-expressing endodermal cells. Conversely, inhibition of this signaling increases the number of endodermal cells without affecting the expression of Nodal, Nodal antagonists, bon or fau/gata5. Inhibition of Fgf signaling in endoderm mutants suggests that this signaling negatively regulates cas expression by a pathway parallel to Bon and Fau/Gata5 in the molecular cascade leading to endoderm. Furthermore, activation of Fgf signaling can overcome Cas-mediated abrogation of mesodermal gene expression. Altogether, these results suggest that Fgf signaling negatively regulates endoderm induction, possibly through repression of cas expression and down-regulation of Cas function.

Genomic organization, alternative splicing, and multiple regulatory regions of the zebrafish fgf8 gene

Fgf8 is among the members of the fibroblast growth factor (FGF) family that play pivotal roles in vertebrate development. In the present study, the genomic DNA of the zebrafish fgf8 gene was cloned to elucidate the regulatory mechanism behind the temporally and spatially restricted expression of the gene in vertebrate embryos. Structural analysis revealed that the exon-intron organization of fgf8 is highly conserved during vertebrate evolution, from teleosts to mammals. Close inspection of the genomic sequence and reverse transcription-polymerase chain reaction analysis revealed that zebrafish fgf8 encodes two splicing variants, corresponding to Fgf8a and Fgf8b, among the four to seven splicing variants known in mammals. Misexpression of the two variants in zebrafish embryos following mRNA injection showed that both variants have dorsalizing activities on zebrafish embryos, with Fgf8b being more potent. Reporter gene analysis of the transcriptional regulation of zebrafish fgf8 suggested that its complicated expression pattern, which is considered essential for its multiple roles in development, is mediated by combinations of different regulatory regions in the upstream and downstream regions of the gene. Furthermore, comparison of the genomic sequence of fgf8 among different vertebrate species suggests that this regulatory mechanism is conserved during vertebrate evolution.

The regulation of mesodermal progenitor cell commitment to somitogenesis subdivides the zebrafish body musculature into distinct domains

The vertebrate musculature is produced from a visually uniform population of mesodermal progenitor cells (MPCs) that progressively bud off somites populating the trunk and tail. How the MPCs are regulated to continuously release cells into the presomitic mesoderm throughout somitogenesis is not understood. Using a genetic approach to study the MPCs, we show that a subset of MPCs are set aside very early in zebrafish development, and programmed to cell-autonomously enter the tail domain beginning with the 16th somite. Moreover, we show that the trunk is subdivided into two domains, and that entry into the anterior trunk, posterior trunk, and tail is regulated by interactions between the Nodal and bone morphogenetic protein (Bmp) pathways. Finally, we show that the tail MPCs are held in a state we previously called the Maturation Zone as they wait for the signal to begin entering somitogenesis.

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.

Molecular genetics of axis formation in zebrafish

The basic vertebrate body plan of the zebrafish embryo is established in the first 10 hours of development. This period is characterized by the formation of the anterior-posterior and dorsal-ventral axes, the development of the three germ layers, the specification of organ progenitors, and the complex morphogenetic movements of cells. During the past 10 years a combination of genetic, embryological, and molecular analyses has provided detailed insights into the mechanisms underlying this process. Maternal determinants control the expression of transcription factors and the location of signaling centers that pattern the blastula and gastrula. Bmp, Nodal, FGF, canonical Wnt, and retinoic acid signals generate positional information that leads to the restricted expression of transcription factors that control cell type specification. Noncanonical Wnt signaling is required for the morphogenetic movements during gastrulation. We review how the coordinated interplay of these molecules determines the fate and movement of embryonic cells.

Activin/Nodal and FGF pathways cooperate to maintain pluripotency of human embryonic stem cells

Maintenance of pluripotency is crucial to the mammalian embryo's ability to generate the extra-embryonic and embryonic tissues that are needed for intrauterine survival and foetal development. The recent establishment of embryonic stem cells from human blastocysts (hESCs) provides an opportunity to identify the factors supporting pluripotency at early stages of human development. Using this in vitro model, we have recently shown that Nodal can block neuronal differentiation, suggesting that TGFbeta family members are involved in cell fate decisions of hESCs, including preservation of their pluripotency. Here, we report that Activin/Nodal signalling through Smad2/3 activation is necessary to maintain the pluripotent status of hESCs. Inhibition of Activin/Nodal signalling by follistatin and by overexpression of Lefty or Cerberus-Short, or by the Activin receptor inhibitor SB431542, precipitates hESC differentiation. Nevertheless, neither Nodal nor Activin is sufficient to sustain long-term hESC growth in a chemically defined medium without serum. Recent studies have shown that FGF2 can also maintain long-term expression of pluripotency markers, and we find that inhibition of the FGF signalling pathway by the tyrosine kinase inhibitor SU5402 causes hESC differentiation. However, this effect of FGF on hESC pluripotency depends on Activin/Nodal signalling, because it is blocked by SB431542. Finally, long-term maintenance of in-vitro pluripotency can be achieved with a combination of Activin or Nodal plus FGF2 in the absence of feeder-cell layers, conditioned medium or Serum Replacer. These findings suggest that the Activin/Nodal pathway maintains pluripotency through mechanism(s) in which FGF acts as a competence factor and therefore provide further evidence of distinct mechanisms for preservation of pluripotency in mouse and human ESCs.

Overexpression of Nodal promotes differentiation of mouse embryonic stem cells into mesoderm and endoderm at the expense of neuroectoderm formation

Understanding how to direct the fate of embryonic stem (ES) cells upon differentiation is critical to their eventual use in therapeutic applications. Clues for controlling ES cell differentiation may be found in the early embryo because mouse ES cells form derivatives of all three embryonic germ layers upon injection into blastocysts. One promising candidate for influencing the differentiation of ES cells into the embryonic germ layers is the transforming growth factor-beta (TGF-beta) growth factor, Nodal. Nodal null mouse mutants lack mesoderm, and injection of Nodal mRNA into nonmammalian embryos induces mesodermal and endodermal tissues. We find that overexpression of Nodal in mouse ES cells leads not only to up-regulation of mesodermal and endodermal cell markers but also to downregulation of neuroectodermal markers. These findings demonstrate the importance of Nodal's influence on the differentiation of pluripotent cells to all three of the primary germ layers. Accordingly, altering expression of factors responsible for cell differentiation in the intact embryo provides an approach for directing ES cell fates in vitro toward therapeutically useful cell types.

Sinup, a novel Siaz-interacting nuclear protein, modulates neural plate formation in the zebrafish embryos

Siah, the vertebrate homologue of the Drosophila seven in absentia (sina) gene, is well conserved from Drosophila to mammal and involved in ubiquitination and proteasome-dependent degradation of various target proteins. To identify cellular proteins interacting with Siah, we screened a zebrafish cDNA library with zebrafish Siah (Siaz) as bait in a yeast two-hybrid assay. We identified a cDNA encoding a novel protein composed of 145 amino acids and termed it as Sinup (Siaz-interacting-nuclear-protein). Sinup is a novel nuclear protein that binds to the highly conserved C-terminal protein-interacting domain of Siaz both in vivo and in vitro. During development, sinup transcripts are abundant from the one-cell stage to the early blastula and then markedly diminished, suggesting sinup largely exists as maternal transcripts. sinup overexpression induced lateral expansion of the neural plate and in consequence caused ectopic expression of otx-2 and hoxb1b during the late gastrula stage. In addition, the lateral/paraxial expression of wnt8 at the onset of gastrulation is suppressed by the forced expression of sinup while the expression levels of various dorso-ventral markers are unaffected. In contrast, interfering with sinup functions using sinup morpholino oligonucleotides gradually diminished the anterior neuroectoderm from the posterior region, and resulted in compete loss of hindbrain at the 3-somites stage. Our report suggests that sinup expression should be tightly regulated during early embryonic development for the proper neural plate formation.

Expression and potential role of fibroblast growth factor 2 and its receptors in human embryonic stem cells

Although the detection of several components of the fibroblast growth factor (FGF) signaling pathway in human embryonic stem cells (hESCs) has been reported, the functionality of that pathway and effects on cell fate decisions are yet to be established. In this study we characterized expression of FGF-2, the prototypic member of the FGF family, and its receptors (FGFRs) in undifferentiated and differentiating hESCs; subsequently, we analyzed the effects of FGF-2 on hESCs, acting as both exogenous and endogenous factors. We have determined that undifferentiated hESCs are abundant in several molecular-mass isoforms of FGF-2 and that expression pattern of these isoforms remains unchanged under conditions that induce hESC differentiation. Significantly, FGF-2 is released by hESCs into the medium, suggesting an autocrine activity. Expression of FGFRs in undifferentiated hESCs follows a specific pattern, with FGFR1 being the most abundant species and other receptors showing lower expression in the following order: FGFR1 --> FGFR3 --> FGFR4 --> FGFR2. Initiation of differentiation is accompanied by profound changes in FGFR expression, particularly the upregulation of FGFR1. When hESCs are exposed to exogenous FGF-2, extracellular signal-regulated kinases are phosphorylated and thereby activated. However, the presence or absence of exogenous FGF-2 does not significantly affect the proliferation of hESCs. Instead, increased concentration of exogenous FGF-2 leads to reduced outgrowth of hESC colonies with time in culture. Finally, the inhibitor of FGFRs, SU5402, was used to ascertain whether FGF-2 that is released by hESCs exerts its activities via autocrine pathways. Strikingly, the resultant inhibition of FGFR suppresses activation of downstream protein kinases and causes rapid cell differentiation, suggesting an involvement of autocrine FGF signals in the maintenance of proliferating hESCs in the undifferentiated state. In conclusion from our data, we propose that this endogenous FGF signaling pathway can be implicated in self-renewal or differentiation of hESCs.

FGF-induced vesicular release of Sonic hedgehog and retinoic acid in leftward nodal flow is critical for left-right determination

The precise specification of left-right asymmetry is an essential process for patterning internal organs in vertebrates. In mouse embryonic development, the symmetry-breaking process in left-right determination is initiated by a leftward extraembryonic fluid flow on the surface of the ventral node. However, it is not known whether the signal transduction mechanism of this flow is chemical or mechanical. Here we show that fibroblast growth factor (FGF) signalling triggers secretion of membrane-sheathed objects 0.3-5 microm in diameter termed 'nodal vesicular parcels' (NVPs) that carry Sonic hedgehog and retinoic acid. These NVPs are transported leftward by the fluid flow and eventually fragment close to the left wall of the ventral node. The silencing effects of the FGF-receptor inhibitor SU5402 on NVP secretion and on a downstream rise in Ca2+ were sufficiently reversed by exogenous Sonic hedgehog peptide or retinoic acid, suggesting that FGF-triggered surface accumulation of cargo morphogens may be essential for launching NVPs. Thus, we propose that NVP flow is a new mode of extracellular transport that forms a left-right gradient of morphogens.

Wnt/beta-catenin regulation of the Sp1-related transcription factor sp5l promotes tail development in zebrafish

Tail formation in vertebrates involves the specification of a population of multipotent precursors, the tailbud, which will give rise to all of the posterior structures of the embryo. Wnts are signaling proteins that are candidates for promoting tail outgrowth in zebrafish, although which Wnts are involved, what genes they regulate, and whether Wnts are required for initiation or maintenance steps in tail formation has not been resolved. We show here that both wnt3a and wnt8 are expressed in the zebrafish tailbud and that simultaneous inhibition of both wnt3a and wnt8 using morpholino oligonucleotides can completely block tail formation. In embryos injected with wnt3a and wnt8 morpholinos, expression of genes in undifferentiated presomitic mesoderm is initiated, but not maintained. To identify genes that might function downstream of Wnts in tail formation, a DNA microarray screen was conducted, revealing that sp5l, a member of the Sp1 family of zinc-finger transcription factors, is activated by Wnt signaling. Moreover, we show that sp5l expression in the developing tail is dependent on both wnt3a and wnt8 function. Supporting a role for sp5l in tail formation, we find that inhibition of sp5l strongly enhances the effects of wnt3a inhibition, and overexpression of sp5l RNA is able to completely restore normal tail development in wnt3a morphants. These data place sp5l downstream of wnt3a and wnt8 in a Wnt/beta-catenin signaling pathway that controls tail development in zebrafish.

Chordin, FGF signaling, and mesodermal factors cooperate in zebrafish neural induction

The ectoderm gives rise to both neural tissue and epidermis. In vertebrates, specification of the neural plate requires repression of bone morphogenetic protein (BMP) signaling in the dorsal ectoderm. The extracellular BMP antagonist Chordin and other signals from the dorsal mesoderm play important roles in this process. We utilized zebrafish mutant combinations that disrupt Chordin and mesoderm formation to reveal additional signals that contribute to the establishment of the neural domain. We demonstrate that fibroblast growth factor (FGF) signaling accounts for the additional activity in neural specification. Impeding FGF signaling results in a shift of ectodermal markers from neural to epidermal. However, following inhibition of FGF signaling, expression of anterior neural markers recovers in a Nodal-dependent fashion. Simultaneously blocking, Chordin, mesoderm formation, and FGF signaling eliminates neural marker expression during gastrula stages. We observed that FGF signaling is required for chordin expression but that it also acts via other mechanisms to repress BMP transcription during late blastula stages. Activation of FGF signaling was also able to repress BMP transcription in the absence of protein synthesis. Our results support a model in which specification of anterior neural tissue requires early FGF-mediated repression of BMP transcript levels and later activities of Chordin and mesodermal factors.