Evidence for a frizzled-mediated wnt pathway required for zebrafish dorsal mesoderm formation

Combinatorial Wnt signaling landscape during brachiopod anteroposterior patterning

BACKGROUND

Wnt signaling pathways play crucial roles in animal development. They establish embryonic axes, specify cell fates, and regulate tissue morphogenesis from the early embryo to organogenesis. It is becoming increasingly recognized that these distinct developmental outcomes depend upon dynamic interactions between multiple ligands, receptors, antagonists, and other pathway modulators, consolidating the view that a combinatorial "code" controls the output of Wnt signaling. However, due to the lack of comprehensive analyses of Wnt components in several animal groups, it remains unclear if specific combinations always give rise to specific outcomes, and if these combinatorial patterns are conserved throughout evolution.

RESULTS

In this work, we investigate the combinatorial expression of Wnt signaling components during the axial patterning of the brachiopod Terebratalia transversa. We find that T. transversa has a conserved repertoire of ligands, receptors, and antagonists. These genes are expressed throughout embryogenesis but undergo significant upregulation during axial elongation. At this stage, Frizzled domains occupy broad regions across the body while Wnt domains are narrower and distributed in partially overlapping patches; antagonists are mostly restricted to the anterior end. Based on their combinatorial expression, we identify a series of unique transcriptional subregions along the anteroposterior axis that coincide with the different morphological subdivisions of the brachiopod larval body. When comparing these data across the animal phylogeny, we find that the expression of Frizzled genes is relatively conserved, whereas the expression of Wnt genes is more variable.

CONCLUSIONS

Our results suggest that the differential activation of Wnt signaling pathways may play a role in regionalizing the anteroposterior axis of brachiopod larvae. More generally, our analyses suggest that changes in the receptor context of Wnt ligands may act as a mechanism for the evolution and diversification of the metazoan body axis.

Zebrafish Wtx is a negative regulator of Wnt signaling but is dispensable for embryonic development and organ homeostasis

BACKGROUND

The X-chromosomally linked gene WTX is a human disease gene and a member of the AMER family. Mutations in WTX are found in Wilms tumor, a form of pediatric kidney cancer and in patients suffering from OSCS (Osteopathia striata with cranial sclerosis), a sclerosing bone disorder. Functional data suggest WTX to be an inhibitor of the Wnt/β-catenin signaling pathway. Deletion of Wtx in mouse leads to perinatal death, impeding the analysis of its physiological role.

RESULTS

To gain insights into the function of Wtx in development and homeostasis we have used zebrafish as a model and performed both knockdown and knockout studies using morpholinos and transcription activator-like effector nucleases (TALENs), respectively. Wtx knockdown led to increased Wnt activity and embryonic dorsalization. Also, wtx mutants showed a transient upregulation of Wnt target genes in the context of caudal fin regeneration. Surprisingly, however, wtx as well as wtx/amer2/amer3 triple mutants developed normally, were fertile and did not show any anomalies in organ maintenance.

CONCLUSIONS

Our data show that members of the zebrafish wtx/amer gene family, while sharing a partially overlapping expression pattern do not compensate for each other. This observation demonstrates a remarkable robustness during development and regeneration in zebrafish.

A mutation in FRIZZLED2 impairs Wnt signaling and causes autosomal dominant omodysplasia

Autosomal dominant omodysplasia is a rare skeletal dysplasia characterized by short humeri, radial head dislocation, short first metacarpals, facial dysmorphism and genitourinary anomalies. We performed next-generation whole-exome sequencing and comparative analysis of a proband with omodysplasia, her unaffected parents and her affected daughter. We identified a de novo mutation in FRIZZLED2 (FZD2) in the proband and her daughter that was not found in unaffected family members. The FZD2 mutation (c.1644G>A) changes a tryptophan residue at amino acid 548 to a premature stop (p.Trp548*). This altered protein is still produced in vitro, but we show reduced ability of this mutant form of FZD2 to interact with its downstream target DISHEVELLED. Furthermore, expressing the mutant form of FZD2 in vitro is not able to facilitate the cellular response to canonical Wnt signaling like wild-type FZD2. We therefore conclude that the FRIZZLED2 mutation is a de novo, novel cause for autosomal dominant omodysplasia.

ZNRF3 promotes Wnt receptor turnover in an R-spondin-sensitive manner

R-spondin proteins strongly potentiate Wnt signalling and function as stem-cell growth factors. Despite the biological and therapeutic significance, the molecular mechanism of R-spondin action remains unclear. Here we show that the cell-surface transmembrane E3 ubiquitin ligase zinc and ring finger 3 (ZNRF3) and its homologue ring finger 43 (RNF43) are negative feedback regulators of Wnt signalling. ZNRF3 is associated with the Wnt receptor complex, and inhibits Wnt signalling by promoting the turnover of frizzled and LRP6. Inhibition of ZNRF3 enhances Wnt/β-catenin signalling and disrupts Wnt/planar cell polarity signalling in vivo. Notably, R-spondin mimics ZNRF3 inhibition by increasing the membrane level of Wnt receptors. Mechanistically, R-spondin interacts with the extracellular domain of ZNRF3 and induces the association between ZNRF3 and LGR4, which results in membrane clearance of ZNRF3. These data suggest that R-spondin enhances Wnt signalling by inhibiting ZNRF3. Our study provides new mechanistic insights into the regulation of Wnt receptor turnover, and reveals ZNRF3 as a tractable target for therapeutic exploration.

Wnt6 activates endoderm in the sea urchin gene regulatory network

In the sea urchin, entry of β-catenin into the nuclei of the vegetal cells at 4th and 5th cleavages is necessary for activation of the endomesoderm gene regulatory network. Beyond that, little is known about how the embryo uses maternal information to initiate specification. Here, experiments establish that of the three maternal Wnts in the egg, Wnt6 is necessary for activation of endodermal genes in the endomesoderm GRN. A small region of the vegetal cortex is shown to be necessary for activation of the endomesoderm GRN. If that cortical region of the egg is removed, addition of Wnt6 rescues endoderm. At a molecular level, the vegetal cortex region contains a localized concentration of Dishevelled (Dsh) protein, a transducer of the canonical Wnt pathway; however, Wnt6 mRNA is not similarly localized. Ectopic activation of the Wnt pathway, through the expression of an activated form of β-catenin, of a dominant-negative variant of GSK-3β or of Dsh itself, rescues endomesoderm specification in eggs depleted of the vegetal cortex. Knockdown experiments in whole embryos show that absence of Wnt6 produces embryos that lack endoderm, but those embryos continue to express a number of mesoderm markers. Thus, maternal Wnt6 plus a localized vegetal cortical molecule, possibly Dsh, is necessary for endoderm specification; this has been verified in two species of sea urchin. The data also show that Wnt6 is only one of what are likely to be multiple components that are necessary for activation of the entire endomesoderm gene regulatory network.

Znrg, a novel gene expressed mainly in the developing notochord of zebrafish

The notochord, a defining characteristic of the chordate embryo is a critical midline structure required for axial skeletal formation in vertebrates, and acts as a signaling center throughout embryonic development. We utilized the digital differential display program of the National Center for Biotechnology Information, and identified a contig of expressed sequence tags (no. Dr. 83747) from the zebrafish ovary library in Genbank. Full-length cDNA of the identified gene was cloned by 5'- and 3'- RACE, and the resulting sequence was confirmed by polymerase chain reaction and sequencing. The cDNA clone contains 2,505 base pairs and encodes a novel protein of 707 amino acids that shares no significant homology with any known proteins. This gene was expressed in mature oocytes and at the one-cell stage, and persisted until the 5th day of development, as determined by RT-PCR. Transcripts were detected by whole-mount RNA in situ hybridization from the two-cell stage to 72 h of embryonic development. This gene was uniformly distributed from the cleavage stage up to the blastula stage. During early gastrulation, it was present in the dorsal region, and became restricted to the notochord and pectoral fin at 48 and 72 h of embryonic development. Based on its abundance in the notochord, we hypothesized that the novel gene may play an important role in notochord development in zebrafish; we named this gene, zebrafish notochord-related gene, or znrg.

Developmental gene regulatory networks in the zebrafish embryo

The genomic developmental program operates mainly through the regulated expression of genes encoding transcription factors and signaling pathways. Complex networks of regulatory genetic interactions control developmental cell specification and fates. Development in the zebrafish, Danio rerio, has been studied extensively and large amounts of experimental data, including information on spatial and temporal gene expression patterns, are available. A wide variety of maternal and zygotic regulatory factors and signaling pathways have been discovered in zebrafish, and these provide a useful starting point for reconstructing the gene regulatory networks (GRNs) underlying development. In this review, we describe in detail the genetic regulatory subcircuits responsible for dorsoanterior-ventroposterior patterning and endoderm formation. We describe a number of regulatory motifs, which appear to act as the functional building blocks of the GRNs. Different positive feedback loops drive the ventral and dorsal specification processes. Mutual exclusivity in dorsal-ventral polarity in zebrafish is governed by intra-cellular cross-inhibiting GRN motifs, including vent/dharma and tll1/chordin. The dorsal-ventral axis seems to be determined by competition between two maternally driven positive-feedback loops (one operating on Dharma, the other on Bmp). This is the first systematic approach aimed at developing an integrated model of the GRNs underlying zebrafish development. Comparison of GRNs' organizational motifs between different species will provide insights into developmental specification and its evolution. The online version of the zebrafish GRNs can be found at http://www.zebrafishGRNs.org.

Wnt signaling mediates diverse developmental processes in zebrafish

A combination of forward and reverse genetic approaches in zebrafish has revealed novel roles for canonical Wnt and Wnt/PCP signaling during vertebrate development. Forward genetics in zebrafish provides an exceptionally powerful tool to assign roles in vertebrate developmental processes to novel genes, as well as elucidating novel roles played by known genes. This has indeed turned out to be the case for components of the canonical Wnt signaling pathway. Non-canonical Wnt signaling in the zebrafish is also currently a topic of great interest, due to the identified roles of this pathway in processes requiring the integration of cell polarity and cell movement, such as the directed migration movements that drive the narrowing and lengthening (convergence and extension) of the embryo during early development.

Manipulation of gene expression during zebrafish embryonic development using transient approaches

The rapid embryonic development and high fecundity of zebrafish contribute to the great advantages of this model for the study of developmental genetics. Transient disruption of the normal function of a gene during development can be achieved by microinjecting mRNA, DNA or short chemically stabilized anti-sense oligomers, called morpholinos (MOs), into early zebrafish embryos. The ensuing develop ment of the microinjected embryos is observed over the following hours and days to analyze the impact of the microinjected products on embryogenesis. Compared to stable reverse genetic approaches (sta ble transgenesis, targeted mutants recovered by TILLING), these transient reverse genetic approaches are vastly quicker, relatively affordable, and require little animal facility space. Common applications of these methodologies allow analysis of gain-of-function (gene overexpression or dominant active), loss-of-function (gene knock down or dominant negative), mosaic analysis, lineage-restricted studies and cell tracing experiments. The use of these transient approaches for the manipulation of gene expression has improved our understanding of many key developmental pathways including both the Wnt/beta-catenin and Wnt/PCP pathways, as covered in some detail in Chapter 17 of this book. This chapter describes the most common and versatile approaches: gain of function and loss of function using DNA and mRNA injections and loss of function using MOs.

Analysis of dishevelled localization and function in the early sea urchin embryo

Dishevelled (Dsh) is a key signaling molecule in the canonical Wnt pathway. Although the mechanism by which Dsh transduces a Wnt signal remains elusive, the subcellular localization of Dsh may be critical for its function. In the early sea urchin embryo, Dsh is concentrated in punctate structures within the cytoplasm of vegetal blastomeres. In these cells, Dsh stabilizes beta-catenin and causes it to accumulate in nuclei, resulting in the activation of transcriptional gene regulatory networks that drive mesoderm and endoderm formation. Here, we present a systematic mutational analysis of Lytechinus variegatus Dsh (LvDsh) that identifies motifs required for its vegetal cortical localization (VCL). In addition to a previously identified lipid-binding motif near the N-terminus of Dsh (Weitzel, H.E., Illies, M.R., Byrum, C.A., Xu, R., Wikramanayake, A.H., Ettensohn, C.A., 2004. Differential stability of beta-catenin along the animal-vegetal axis of the sea urchin embryo mediated by dishevelled. Development 131, 2947-56), we identify a short (21 amino acid) motif between the PDZ and DEP domains that is required for VCL. Phosphorylation of threonine residues in this region regulates both the targeting and stability of LvDsh. We also identify functional nuclear import and export signals within LvDsh. We provide additional evidence that LvDsh is active locally in the vegetal region of the embryo but is inactive in animal blastomeres and show that the inability of LvDsh to function in animal cells is not a consequence of impaired nuclear import. The DIX domain of LvDsh functions as a potent dominant negative when overexpressed (Weitzel, H.E., Illies, M.R., Byrum, C.A., Xu, R., Wikramanayake, A.H., Ettensohn, C.A., 2004. Differential stability of beta-catenin along the animal-vegetal axis of the sea urchin embryo mediated by dishevelled. Development 131, 2947-56). Here, we show that the dominant negative effect of DIX is dependent on a highly conserved, lipid-binding motif that includes residues K57 and E58. The dominant negative effect of DIX is not a consequence of blocking VCL or the nuclear import of LvDsh. We provide evidence that isolated DIX domains interact with full-length LvDsh in vivo. In addition, we show that the K57/E58 lipid-binding motif of DIX is essential for this interaction. We propose that binding of the isolated DIX domain to full-length Dsh may be facilitated by interactions with lipids, and that this interaction may inhibit signaling by a) preventing endogenous Dsh from interacting with Axin, or b) blocking the ability of Dsh to recruit other proteins, such as GBP/Frat1, to the beta-catenin degradation complex.

Zebrafish Naked1 and Naked2 antagonize both canonical and non-canonical Wnt signaling

Wnt signaling controls a wide range of developmental processes and its aberrant regulation can lead to disease. To better understand the regulation of this pathway, we identified zebrafish homologues of Naked Cuticle (Nkd), Nkd1 and Nkd2, which have previously been shown to inhibit canonical Wnt/beta-catenin signaling. Zebrafish nkd1 expression increases substantially after the mid-blastula transition in a pattern mirroring that of activated canonical Wnt/beta-catenin signaling, being expressed in both the ventrolateral blastoderm margin and also in the axial mesendoderm. In contrast, zebrafish nkd2 is maternally and ubiquitously expressed. Overexpression of Nkd1 or Nkd2a suppressed canonical Wnt/beta-catenin signaling at multiple stages of early zebrafish development and also exacerbated the cyclopia and axial mesendoderm convergence and extension (C&E) defect in the non-canonical Wnt/PCP mutant silberblick (slb/wnt11). Thus, Nkds are sufficient to antagonize both canonical and non-canonical Wnt signaling. Reducing Nkd function using antisense morpholino oligonucleotides resulted in increased expression of canonical Wnt/beta-catenin target genes. Finally, reducing Nkd1 function in slb mutants suppressed the axial mesendoderm C&E defect. These data indicate that zebrafish Nkd1 and Nkd2 function to limit both canonical and non-canonical Wnt signaling.

hecate, a zebrafish maternal effect gene, affects dorsal organizer induction and intracellular calcium transient frequency

A zebrafish maternal effect mutation, in the gene hecate, results in embryos that have defects in the formation of dorsoanterior structures and altered calcium release. hecate mutant embryos lack nuclear accumulation of beta-catenin and have reduced expression of genes specific to the dorsal organizer. We found that hecate mutant embryos exhibit a nearly 10-fold increase in the frequency of intracellular Ca2+ transients normally present in the enveloping layer during the blastula stages. Inhibition of Ca2+ release leads to ectopic expression of dorsal genes in mutant embryos suggesting that Ca2+ transients are important in mediating dorsal gene expression. Inhibition of Ca2+ release also results in the expression of dorsal-specific genes in the enveloping layer in a beta-catenin-independent manner, which suggests an additional function for the Ca2+ transients in this cellular layer. The mutant phenotype can be reversed by the expression of factors that activate Wnt/beta-catenin signaling, suggesting that the Wnt/beta-catenin pathway, at least as activated by an exogenous Wnt ligand, is intact in hec mutant embryos. Our results are consistent with a role for the hecate gene in the regulation of Ca2+ release during the cleavage stages, which in turn influences dorsal gene expression in both marginal cells along the dorsoventral axis and in the enveloping layer.

Glycogen synthase kinase 3 has a proapoptotic function in Hydra gametogenesis

In an approach to study the evolutionary conservation of molecules of the Wnt signal transduction pathway, we analyzed the function of the major negative regulator of this pathway, GSK3 (glycogen synthase kinase 3), in the basal metazoan Hydra. Microinjection assays reveal that HyGSK3 inhibits beta-catenin in zebrafish embryos, indicating that the function of GSK3 in the canonical Wnt signaling pathway is evolutionarily conserved. In Hydra, HyGSK3 transcripts are strongly upregulated during gametogenesis. Treatment of female polyps with the GSK3 inhibitors lithium and alsterpaullone prevents the differentiation of nurse cells and subsequent oocyte formation. Our data indicate that GSK3 is required for the early induction of apoptosis in germline cells, which has been shown to be an initial step in Hydra gametogenesis. Our experiments show that main functions of GSK3 are evolutionarily conserved and unique to multicellular animals, a conclusion which is additionally supported by the presence of specific regulatory domains in the HyGSK3 protein.

Maternal Wnt11 Activates the Canonical Wnt Signaling Pathway Required for Axis Formation in Xenopus Embryos

Wnt signaling pathways play essential roles in patterning and proliferation of embryonic and adult tissues. In many organisms, this signaling pathway directs axis formation. Although the importance of intracellular components of the pathway, including beta-catenin and Tcf3, has been established, the mechanism of their activation is uncertain. In Xenopus, the initiating signal that localizes beta-catenin to dorsal nuclei has been suggested to be intracellular and Wnt independent. Here, we provide three lines of evidence that the pathway specifying the dorsal axis is activated extracellularly in Xenopus embryos. First, we identify Wnt11 as the initiating signal. Second, we show that activation requires the glycosyl transferase X.EXT1. Third, we find that the EGF-CFC protein, FRL1, is also essential and interacts with Wnt11 to activate canonical Wnt signaling.

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

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

Maternal factors in zebrafish development

All processes that occur before the activation of the zygotic genome at the midblastula transition are driven by maternal products, which are produced during oogenesis and stored in the mature oocyte. Upon egg activation and fertilization, these maternal factors initiate developmental cascades that carry out the embryonic developmental program. Even after the initiation of zygotic gene expression, perduring maternal products continue performing essential functions, either together with other maternal factors or through interactions with newly expressed zygotic products. Advances in zebrafish research have placed this organism in a unique position to contribute to a detailed understanding of the role of maternal factors in early vertebrate development. This review summarizes our knowledge on the processes involved in the production and redistribution of maternal factors during zebrafish oogenesis and early development, as well as our understanding of the function of these factors in axis formation, germ layer and germ cell specification, and other early embryonic processes.

Conservation of structure and functional divergence of duplicated Wnt8s in pufferfish

The zebrafish wnt8 locus differs from its tetrapod counterparts in that it produces two functionally overlapping but distinct Wnt8 proteins. Studies of zebrafish wnt8 have suggested that the two major Wnt8 proteins produced are functionally similar yet may behave differently depending on the assay context. To determine whether the bicistronic wnt8 and its accompanying unique protein activities found in zebrafish are more widespread (and perhaps universal) among teleosts, we have extended our studies to the pufferfish Takifugu rubripes. We have found that Takifugu wnt8 is also bicistronic, indicating that the wnt8 duplication occurred before the divergence of these teleosts approximately 150 million years ago. Furthermore, overexpression assays in zebrafish embryos show that functional differences between the zebrafish Wnt8.1 and Wnt8.2 proteins are conserved in their Takifugu orthologs. Thus, despite the fact that Wnt8.1 and Wnt8.2 proteins are as similar to each other as each is to Xenopus Xwnt-8, Wnt8 family members can behave quite differently in the context of zebrafish embryos. This finding suggests that zebrafish (and possibly teleost in general) Wnt8 receptors are able to discriminate between highly related ligands.

The ankyrin repeat protein Diversin recruits Casein kinase Iepsilon to the beta-catenin degradation complex and acts in both canonical Wnt and Wnt/JNK signaling

Wnt signals control decisive steps in development and can induce the formation of tumors. Canonical Wnt signals control the formation of the embryonic axis, and are mediated by stabilization and interaction of beta-catenin with Lef/Tcf transcription factors. An alternative branch of the Wnt pathway uses JNK to establish planar cell polarity in Drosophila and gastrulation movements in vertebrates. We describe here the vertebrate protein Diversin that interacts with two components of the canonical Wnt pathway, Casein kinase Iepsilon (CKIepsilon) and Axin/Conductin. Diversin recruits CKIepsilon to the beta-catenin degradation complex that consists of Axin/Conductin and GSK3beta and allows efficient phosphorylation of beta-catenin, thereby inhibiting beta-catenin/Tcf signals. Morpholino-based gene ablation in zebrafish shows that Diversin is crucial for axis formation, which depends on beta-catenin signaling. Diversin is also involved in JNK activation and gastrulation movements in zebrafish. Diversin is distantly related to Diego of Drosophila, which functions only in the pathway that controls planar cell polarity. Our data show that Diversin is an essential component of the Wnt-signaling pathway and acts as a molecular switch, which suppresses Wnt signals mediated by the canonical beta-catenin pathway and stimulates signaling via JNK.

Lateral line, nervous system, and maternal expression of Frizzled 7a during zebrafish embryogenesis

We have isolated and mapped a new wnt receptor frizzled family member, zebrafish frizzled 7a. Fz7a and a previously reported zebrafish fz7 (El-Messaoudi and Renucci, 2001) make an orthologous gene pair, however, they display distinct expression patterns. Fz7a shows strong maternal as well as zygotic expression. Fz7a transcript is enriched dorsally starting with the shield stage. At the end of gastrulation, Fz7a is abundantly expressed within anterior neuroectoderm and expressed more weakly within lateral mesoderm. Fz7a is detected during somitogenesis within the central nervous system, somatic and posterior lateral mesoderm. At 24hpf, fz7a is expressed in migrating lateral line primordium. At 48hpf, fz7a is detected in the ear, pectoral fin bud, and within neuromasts, which had originated from the lateral line primordium. Radiation hybrid mapping using panel LN54 (Hukriede et al., 1999) places fz7a on linkage group 6, linked to the marker fi11h08 (distance 0.00cR, LOD score 14.1). To prove that fz7 and fz7a are indeed different genes we mapped fz7 as well using the same LN54 panel. Fz7 mapped to linkage group 9 with a LOD of 12.5, 27.31 cR from wnt 10a in between markers IBD2759 and fb50e04.

Differential distribution of the G protein gamma3 subunit in the developing zebrafish nervous system

G proteins play an essential role in the transduction and propagation of extracellular signals across the plasma membrane. It was once thought that the G protein alpha subunit was the sole regulator of intracellular molecules. The G protein betagamma complex is now recognized as participating in many signaling events. While screening a zebrafish cDNA library to identify members of the protein 4.1 superfamily (Kelly, G.M., Reversade, B., Biochem. Cell Biol. 75 (1997), 623), we fortuitously identified a clone that encodes a zebrafish G protein gamma subunit. The 666 nucleotides of the zebrafish G protein gamma subunit cDNA encodes a polypeptide of 75 amino acids with high degree of homology to human, bovine, rat and mouse gamma subunits. BLAST search analysis of GenBank revealed that the zebrafish gamma subunit is 93% identical and 97% similar to the mammalian gamma3 subunit. The gamma3 gene was mapped to the zebrafish linkage group 21, approximately 10.76 cRays from bf, a gene with sequence homology to the human properdin factor gene. RT-PCR and in situ hybridization analyses first detected gamma3 mRNA during late somitogenesis, where it was expressed preferentially in the Vth cranial nerve, the forebrain and in ventrolateral regions of the mid- and hindbrain including the spinal cord. The ability of the zebrafish gamma3 subunit to form a signaling heterodimeric complex with a beta subunit was tested using a human beta2 subunit. The gamma3 formed a heterodimer with beta2 and the complex was capable of binding calmodulin in a calcium-dependent manner. Overexpression of the beta2gamma3 complex in zebrafish embryos lead to the loss of dorsoanterior structures and heart defects, possibly owing to an up-regulation of mitogen-activated protein kinase activity and/or decline in protein kinase A signaling. Together, these data imply that a betagamma heterodimer plays a role in signal transduction events involving G protein coupled receptors and that these events occur in specific regions in the nervous system of the developing zebrafish.

New thoughts on the role of the beta-gamma subunit in G-protein signal transduction

Heterotrimeric G proteins are involved in numerous biological processes, where they mediate signal transduction from agonist-bound G-protein-coupled receptors to a variety of intracellular effector molecules and ion channels. G proteins consist of two signaling moieties: a GTP-bound alpha subunit and a beta-gamma heterodimer. The beta-gamma dimer, recently credited as a significant modulator of G-protein-mediated cellular responses, is postulated to be a major determinant of signaling fidelity between G-protein-coupled receptors and downstream effectors. In this review we have focused on the role of beta-gamma signaling and have included examples to demonstrate the heterogeneity in the heterodimer composition and its implications in signaling fidelity. We also present an overview of some of the effectors regulated by beta-gamma and draw attention to the fact that, although G proteins and their associated receptors play an instrumental role in development, there is rather limited information on beta-gamma signaling in embryogenesis.

Expression pattern of the frizzled 7 gene during zebrafish embryonic development

We have identified a novel frizzled gene in zebrafish, (Danio rerio): frizzled 7, highly related to mouse, chick and Xenopus fz7. No maternal expression was detected. Zygotic transcription starts at the end of gastrulation anteriorly in the presumptive neurectoderm and in the presomitic mesoderm. During somitogenesis expression is detected in developing central nervous system, including forebrain, midbrain, hindbrain and spinal cord, the lateral mesoderm and the anterior part of the forming somites. The strong sequence conservation of fz7 to the mouse, chick and Xenopus counterparts is also true for their expression pattern.

Maternally controlled (beta)-catenin-mediated signaling is required for organizer formation in the zebrafish

We have identified and characterized a zebrafish recessive maternal effect mutant, ichabod, that results in severe anterior and dorsal defects during early development. The ichabod mutation is almost completely penetrant, but exhibits variable expressivity. All mutant embryos fail to form a normal embryonic shield; most fail to form a head and notochord and have excessive development of ventral tail fin tissue and blood. Abnormal dorsal patterning can first be observed at 3.5 hpf by the lack of nuclear accumulation of (beta)-catenin in the dorsal yolk syncytial layer, which also fails to express bozozok/dharma/nieuwkoid and znr2/ndr1/squint. At the onset of gastrulation, deficiencies in expression of dorsal markers and expansion of expression of markers of ventral tissues indicate a dramatic alteration of dorsoventral identity. Injection of (beta)-catenin RNA markedly dorsalized ichabod embryos and often completely rescued the phenotype, but no measurable dorsalization was obtained with RNAs encoding upstream Wnt pathway components. In contrast, dorsalization was obtained when RNAs encoding either Bozozok/Dharma/Nieuwkoid or Znr2/Ndr1/Squint were injected. Moreover, injection of (beta)-catenin RNA into ichabod embryos resulted in activation of expression of these two genes, which could also activate each other. RNA injection experiments strongly suggest that the component affected by the ichabod mutation acts on a step affecting (beta)-catenin nuclear localization that is independent of regulation of (beta)-catenin stability. This work demonstrates that a maternal gene controlling localization of (beta)-catenin in dorsal nuclei is necessary for dorsal yolk syncytial layer gene activity and formation of the organizer in the zebrafish.

The maternal Xenopus beta-catenin signaling pathway, activated by frizzled homologs, induces goosecoid in a cell non-autonomous manner

In spite of abundant evidence that Wnts play essential roles in embryonic induction and patterning, little is known about the expression or activities of Wnt receptors during embryogenesis. The isolation and expression of two maternal Xenopus frizzled genes, Xfrizzled-1 and Xfrizzled-7, is described. It is also demonstrated that both can activate the Wnt/beta-catenin signaling pathway as monitored by the induction of specific target genes. Activation of the beta-Catenin pathway has previously been shown to be necessary and sufficient for specifying the dorsal axis of Xenopus. beta-Catenin is thought to work through the cell-autonomous induction of the homeobox genes siamois and twin, that in turn bind to and activate the promoter of another homeobox gene, goosecoid. However, it was found that the beta-catenin pathway regulated the expression of both endogenous goosecoid, and a goosecoid promoter construct, in a cell non-autonomous manner. These data demonstrate that maternal Frizzleds can activate the Wnt/beta-catenin pathway in Xenopus embryos, and that induction of a known downstream gene can occur in a cell non-autonomous manner.

Dual roles of Wnt signaling during chondrogenesis in the chicken limb

Long bones of the appendicular skeleton are formed from a cartilage template in a process known as endochondral bone development. Chondrocytes within this template undergo a progressive program of differentiation from proliferating to postmitotic prehypertrophic to hypertrophic chondrocytes, while mesenchymal cells immediately surrounding the early cartilage template form the perichondrium. Recently, members of the Wnt family of secreted signaling molecules have been implicated in regulating chondrocyte differentiation. We find that Wnt-5a, Wnt-5b and Wnt-4 genes are expressed in chondrogenic regions of the chicken limb: Wnt-5a is expressed in the perichondrium, Wnt-5b is expressed in a subpopulation of prehypertrophic chondrocytes and in the outermost cell layer of the perichondrium, and Wnt-4 is expressed in cells of the joint region. Misexpression experiments demonstrate that two of these Wnt molecules, Wnt-5a and Wnt-4, have opposing effects on the differentiation of chondrocytes and that these effects are mediated through divergent signaling pathways. Specifically, Wnt-5a misexpression delays the maturation of chondrocytes and the onset of bone collar formation, while Wnt-4 misexpression accelerates these two processes. Misexpression of a stabilized form of beta-catenin also results in accelerated chondrogenesis, suggesting that a beta-catenin/TCF-LEF complex is involved in mediating the positive regulatory effect of Wnt-4. A number of the genes involved in Wnt signal tranduction, including two members of the Frizzled gene family, which are believed to encode Wnt-receptors, show very dynamic and distinct expression patterns in cartilaginous elements of developing chicken limbs. Misexpression of putative dominant-negative forms of the two Frizzled proteins results in severe shortening of the infected cartilage elements due to a delay in chondrocyte maturation, indicating that an endogenous Wnt signal does indeed function to promote chondrogenic differentiation.

Requirement for glycogen synthase kinase-3beta in cell survival and NF-kappaB activation

Glycogen synthase kinase-3 (GSK-3)-alpha and -beta are closely related protein-serine kinases, which act as inhibitory components of Wnt signalling during embryonic development and cell proliferation in adult tissues. Insight into the physiological function of GSK-3 has emerged from genetic analysis in Drosophila, Dictyostelium and yeast. Here we show that disruption of the murine GSK-3beta gene results in embryonic lethality caused by severe liver degeneration during mid-gestation, a phenotype consistent with excessive tumour necrosis factor (TNF) toxicity, as observed in mice lacking genes involved in the activation of the transcription factor activation NF-kappaB. GSK-3beta-deficient embryos were rescued by inhibition of TNF using an anti-TNF-alpha antibody. Fibroblasts from GSK-3beta-deficient embryos were hypersensitive to TNF-alpha and showed reduced NF-kappaB function. Lithium treatment (which inhibits GSK-3; refs 8, 9) sensitized wild-type fibroblasts to TNF and inhibited transactivation of NF-kappaB. The early steps leading to NF-kappaB activation (degradation of I-kappaB and translocation of NF-kappaB to the nucleus) were unaffected by the loss of GSK-3beta, indicating that NF-kappaB is regulated by GSK-3beta at the level of the transcriptional complex. Thus, GSK-3beta facilitates NF-kappaB function.

The putative wnt receptor Xenopus frizzled-7 functions upstream of beta-catenin in vertebrate dorsoventral mesoderm patterning

We have isolated one member of the frizzled family of wnt receptors from Xenopus (Xfz7) to study the role of cell-cell communication in the establishment of the vertebrate axis. We demonstrate that this maternally encoded protein specifically synergizes with wnt proteins in ectopic axis induction. Embryos derived from oocytes depleted of maternal Xfz7 RNA by antisense oligonucleotide injection are deficient in dorsoanterior structures. Xfz7-depleted embryos are deficient in dorsal but not ventral mesoderm due to the reduced expression of the wnt target genes siamois, Xnr3 and goosecoid. These signaling defects can be restored by the addition of beta-catenin but not Xwnt8b. Xfz7 thus functions upstream of the known GSK-3/axin/beta-catenin intracellular signaling complex in vertebrate dorsoventral mesoderm specification.

Xenopus frizzled 7 can act in canonical and non-canonical Wnt signaling pathways: implications on early patterning and morphogenesis

Here we report the cloning of a Xenopus frizzled transmembrane receptor, Xfz7, and describe its expression pattern during early embryogenesis. Xfz7 mRNA is provided maternally and zygotic transcription peaks in gastrula stages. At that time, transcripts are preferentially localized to the marginal zone and become restricted to distinct regions of the tadpoles in tailbud stages. Overexpression of Xfz7 in embryos perturbs the morphogenesis of trunk and tail, blocks convergence-extension movements in animal caps induced with activin and dorsal lip explants and decreases cadherin-mediated cell adhesion. Xfz7 can interact specifically with Xwnt-8b and signal in the canonical, dorsalizing Wnt pathway. Overexpression of Xfz7 does not trigger the Wnt-1-type pathway but acts in a non-canonical Wnt or morphogenetic-effector pathway involving the activation of protein kinase C (PKC). Xfz7 seems to be involved in different aspects of Wnt signaling during the course of embryogenesis.

Sequence, expression, and location of zebrafish frizzled 10

Members of the frizzled gene family encode seven-pass transmembrane proteins that function in the interpretation and reception of Wnt-mediated cell-cell communication events. To investigate frizzled function in early zebrafish development, we isolated the maternally contributed frizzled 10 (fz10) gene and localized it to linkage group 8 using radiation hybrid mapping. The cloned zebrafish fz10 is closely related to the fz10 group from other organisms. Zygotic expression of fz10 is observed in the posterior tail mesenchyme, dorsal neural tube, and different parts of the brain.

Dishevelled phosphorylation, subcellular localization and multimerization regulate its role in early embryogenesis

Dishevelled (Dsh) induces a secondary axis and can translocate to the membrane when activated by Frizzleds; however, dominant-negative approaches have not supported a role for Dsh in primary axis formation. We demonstrate that the Dsh protein is post-translationally modified at the dorsal side of the embryo: timing and position of this regulation suggests a role of Dsh in dorsal-ventral patterning in Xenopus. To create functional links between these properties of Dsh we analyzed the influence of endogenous Frizzleds and the Dsh domain dependency for these characteristics. Xenopus Frizzleds phosphorylate and translocate Xdsh to the membrane irrespective of their differential ectopic axes inducing abilities, showing that translocation is insufficient for axis induction. Dsh deletion analysis revealed that axis inducing abilities did not segregate with Xdsh membrane association. The DIX region and a short stretch at the N-terminus of the DEP domain are necessary for axis induction while the DEP region is required for Dsh membrane association and its phosphorylation. In addition, Dsh forms homomeric complexes in embryos suggesting that multimerization is important for its proper function.

Requirement for beta-catenin in anterior-posterior axis formation in mice

The anterior-posterior axis of the mouse embryo is defined before formation of the primitive streak, and axis specification and subsequent anterior development involves signaling from both embryonic ectoderm and visceral endoderm. Tauhe Wnt signaling pathway is essential for various developmental processes, but a role in anterior-posterior axis formation in the mouse has not been previously established. Beta-catenin is a central player in the Wnt pathway and in cadherin-mediated cell adhesion. We generated beta-catenin-deficient mouse embryos and observed a defect in anterior-posterior axis formation at embryonic day 5.5, as visualized by the absence of Hex and Hesx1 and the mislocation of cerberus-like and Lim1 expression. Subsequently, no mesoderm and head structures are generated. Intercellular adhesion is maintained since plakoglobin substitutes for beta-catenin. Our data demonstrate that beta-catenin function is essential in anterior-posterior axis formation in the mouse, and experiments with chimeric embryos show that this function is required in the embryonic ectoderm.

Zebrafish Dkk1 functions in forebrain specification and axial mesendoderm formation

We identified a zebrafish homologue of Dickkopf-1 (Dkk1), which was previously identified in Xenopus as a Wnt inhibitor with potent head-inducing activity. Zebrafish dkk1 is expressed in the dorsal marginal blastoderm and also in the dorsal yolk syncytial layer after mid-blastula transition. At later blastula stages, the expression expands to the entire blastoderm margin. During gastrulation, dkk1-expressing cells are confined to the embryonic shield and later to the anterior axial mesendoderm, prospective prechordal plate. Embryos, in which dkk1 was ectopically expressed, exhibited enlarged forebrain, eyes, and axial mesendoderm such as prechordal plate and notochord. dkk1 expression in the dorso-anterior mesendoderm during gastrulation was prominently reduced in zebrafish mutants bozozok (boz), squint (sqt), and one-eyed pinhead (oep), which all display abnormalities in the formation and function of the Spemann organizer and axial mesendoderm. dkk1 expression was normal in these embryos during the blastula period, indicating that zygotic functions of these genes are required for maintenance but not establishment of dkk1 expression. Overexpression of dkk1 suppressed defects in the development of forebrain, eyes, and notochord in boz mutants. Overexpression of dkk1 promoted anterior neuroectoderm development in the embryos injected with antivin RNA, which lack most of the mesoderm and endoderm, suggesting that Dkk1 can affect regionalization of neuroectoderm independently of dorso-anterior mesendoderm. These data indicate that Dkk1, expressed in dorsal mesendoderm, functions in the formation of both the anterior nervous system and the axial mesendoderm in zebrafish.

Wnt signalling: pathway or network?

Members of the Wnt family of secreted glycoproteins participate in many signalling events during development. Recent findings suggest that Wnt signals can sometimes play a permissive role during cell-fate assignment. Wnt proteins have been shown to interact with a number of extracellular and cell-surface proteins, whereas many intracellular components of the Wnt-signalling pathway are also involved in other cellular functions. The consequences of Wnt signalling can be affected by members of the MAP kinase family. These observations suggest that the future understanding of Wnt signalling may require models that are based on a signalling network rather than a single linear pathway.

Wnt signaling and dorso-ventral axis specification in vertebrates

The dorso-ventral axis is specified in vertebrates through the formation of a dorsal signaling center known as the Spemann organizer. This process depends on signal transduction by beta-catenin that can be regulated by secreted Wnt proteins. Recent discoveries of new players in this signaling pathway have narrowed down the search for the initial cues for axis specification in vertebrate embryos.