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

Zebrafish foxo3b negatively regulates canonical Wnt signaling to affect early embryogenesis

FOXO genes are involved in many aspects of development and vascular homeostasis by regulating cell apoptosis, proliferation, and the control of oxidative stress. In addition, FOXO genes have been showed to inhibit Wnt/β-catenin signaling by competing with T cell factor to bind to β-catenin. However, how important of this inhibition in vivo, particularly in embryogenesis is still unknown. To demonstrate the roles of FOXO genes in embryogenesis will help us to further understand their relevant physiological functions. Zebrafish foxo3b gene, an orthologue of mammalian FOXO3, was expressed maternally and distributed ubiquitously during early embryogenesis and later restricted to brain. After morpholino-mediated knockdown of foxo3b, the zebrafish embryos exhibited defects in axis and neuroectoderm formation, suggesting its critical role in early embryogenesis. The embryo-developmental marker gene staining at different stages, phenotype analysis and rescue assays revealed that foxo3b acted its role through negatively regulating both maternal and zygotic Wnt/β-catenin signaling. Moreover, we found that foxo3b could interact with zebrafish β-catenin1 and β-catenin2 to suppress their transactivation in vitro and in vivo, further confirming its role relevant to the inhibition of Wnt/β-catenin signaling. Taken together, we revealed that foxo3b played a very important role in embryogenesis and negatively regulated maternal and zygotic Wnt/β-catenin signaling by directly interacting with both β-catenin1 and β-catenin2. Our studies provide an in vivo model for illustrating function of FOXO transcription factors in embryogenesis.

Naked1 antagonizes Wnt signaling by preventing nuclear accumulation of β-catenin

Cyto-nuclear shuttling of β-catenin is at the epicenter of the canonical Wnt pathway and mutations in genes that result in excessive nuclear accumulation of β-catenin are the driving force behind the initiation of many cancers. Recently, Naked Cuticle homolog 1 (Nkd1) has been identified as a Wnt-induced intracellular negative regulator of canonical Wnt signaling. The current model suggests that Nkd1 acts between Disheveled (Dvl) and β-catenin. Here, we employ the zebrafish embryo to characterize the cellular and biochemical role of Nkd1 in vivo. We demonstrate that Nkd1 binds to β-catenin and prevents its nuclear accumulation. We also show that this interaction is conserved in mammalian cultured cells. Further, we demonstrate that Nkd1 function is dependent on its interaction with the cell membrane. Given the conserved nature of Nkd1, our results shed light on the negative feedback regulation of Wnt signaling through the Nkd1-mediated negative control of nuclear accumulation of β-catenin.

Kzp controls canonical Wnt8 signaling to modulate dorsoventral patterning during zebrafish gastrulation

During vertebrate embryonic development, the body axis formation requires the action of Wnt signals and their antagonists. Zygotic canonical wnt8 expression appears exclusively at the ventrolateral margin and mediates Wnt/β-catenin activities to promote posterior and ventral cell fate. However, the mechanisms involved in the initiation of zygotic wnt8 signals are poorly understood. Here, we identify a novel, maternally derived transcription factor, Kzp (Kaiso zinc finger-containing protein), as an important determinant for the initiation of zygotic Wnt signals in zebrafish. Kzp is a DNA-binding transcription factor that recognizes specific consensus DNA sequences, 5'-(t/a/g)t(a/t/g)nctgcca-3', through zinc fingers and controls the initiation of zygotic wnt8 expression by directly binding to the wnt8 promoter during zebrafish embryonic development. Depletion of Kzp strongly dorsalized embryos, which was characterized by the expansion of dorsal gene expression. Overexpression of Kzp caused posteriorization. These phenotypes were highly similar to ones induced by wnt8 depletion or overexpression and were rescued by alteration of wnt8 activity. Thus, our results provide the first insight into the mechanism involved in the initiation of zygotic canonical Wnt signals by a maternally derived transcription factor.

Retinoic acid-dependent establishment of positional information in the hindbrain was conserved during vertebrate evolution

Zebrafish hoxb1b is expressed during epiboly in the posterior neural plate, with its anterior boundary at the prospective r4 region providing a positional cue for hindbrain formation. A similar function and expression is known for Hoxa1 in mice, suggesting a shared regulatory mechanism for hindbrain patterning in vertebrate embryos. To understand the evolution of the regulatory mechanisms of key genes in patterning of the central nervous system, we examined how hoxb1b transcription is regulated in zebrafish embryos and compared the regulatory mechanisms between mammals and teleosts that have undergone an additional genome duplication. By promoter analysis, we found that the expression of the reporter gene recapitulated hoxb1b expression when driven in transgenic embryos by a combination of the upstream 8.0-kb DNA and downstream 4.6-kb DNA. Furthermore, reporter expression expanded anteriorly when transgenic embryos were exposed to retinoic acid (RA) or LiCl, or injected with fgf3/8 mRNA, implicating the flanking DNA examined here in the responsiveness of hoxb1b to posteriorizing signals. We further identified at least two functional RA responsive elements in the downstream DNA that were shown to be major regulators of early hoxb1b expression during gastrulation, while the upstream DNA, which harbors repetitive sequences with apparent similarity to the autoregulatory sequence of mouse Hoxb1, contributed only to later hoxb1b expression, during somitogenesis. Possible implications in vertebrate evolution are discussed based on these findings.

Myristoylated Naked2 antagonizes Wnt-beta-catenin activity by degrading Dishevelled-1 at the plasma membrane

In Drosophila, naked cuticle is an inducible antagonist of the Wnt-beta-catenin pathway, likely acting at the level of Dishevelled (Dsh/Dvl), an essential component of this pathway. The mechanism by which naked cuticle and its two vertebrate orthologs, Naked1 (NKD1) and Naked2 (NKD2), inhibit Dvl function is unknown. NKD2 is myristoylated, a co-translational modification that leads to its plasma membrane localization. In contrast, myristoylation-deficient G2A NKD2 is cytoplasmic. Herein we show that the ability of Nkd2/NKD2 to antagonize Wnt-beta-catenin activity during zebrafish embryonic development and in mammalian HEK293 cells is myristoylation-dependent. NKD2 and Dvl-1 interact and co-localize at the lateral membrane of polarized epithelial cells. In reciprocal overexpression and siRNA knockdown experiments, NKD2 and Dvl-1 destabilize each other via enhanced polyubiquitylation; this effect is also dependent upon Naked2 myristoylation. Cell fractionation and ubiquitylation assays indicate that endogenous NKD2 interacts with a slower migrating, ubiquitylated form of Dvl-1 in plasma membrane fractions. These results provide a mechanism by which NKD2 antagonizes Wnt signaling: myristoylated NKD2 interacts with Dvl-1 at the plasma membrane, and this interaction leads to their mutual ubiquitin-mediated proteasomal degradation.

Prostaglandin Gbetagamma signaling stimulates gastrulation movements by limiting cell adhesion through Snai1a stabilization

Gastrulation movements form the germ layers and shape them into the vertebrate body. Gastrulation entails a variety of cell behaviors, including directed cell migration and cell delamination, which are also involved in other physiological and pathological processes, such as cancer metastasis. Decreased Prostaglandin E(2) (PGE(2)) synthesis due to interference with the Cyclooxygenase (Cox) and Prostaglandin E synthase (Ptges) enzymes halts gastrulation and limits cancer cell invasiveness, but how PGE(2) regulates cell motility remains unclear. Here we show that PGE(2)-deficient zebrafish embryos, impaired in the epiboly, internalization, convergence and extension gastrulation movements, exhibit markedly increased cell-cell adhesion, which contributes to defective cell movements in the gastrula. Our analyses reveal that PGE(2) promotes cell protrusive activity and limits cell adhesion by modulating E-cadherin transcript and protein, in part through stabilization of the Snai1a (also known as Snail1) transcriptional repressor, an evolutionarily conserved regulator of cell delamination and directed migration. We delineate a pathway whereby PGE(2) potentiates interaction between the receptor-coupled G protein betagamma subunits and Gsk3beta to inhibit proteasomal degradation of Snai1a. However, overexpression of beta-catenin cannot stabilize Snai1a in PGE(2)-deficient gastrulae. Thus, the Gsk3beta-mediated and beta-catenin-independent inhibition of cell adhesion by Prostaglandins provides an additional mechanism for the functional interactions between the PGE(2) and Wnt signaling pathways during development and disease. We propose that ubiquitously expressed PGE(2) synthesizing enzymes, by promoting the stability of Snai1a, enable the precise and rapid regulation of cell adhesion that is required for the dynamic cell behaviors that drive various gastrulation movements.

Mesodermal Wnt signaling organizes the neural plate via Meis3

In vertebrates, canonical Wnt signaling controls posterior neural cell lineage specification. Although Wnt signaling to the neural plate is sufficient for posterior identity, the source and timing of this activity remain uncertain. Furthermore, crucial molecular targets of this activity have not been defined. Here, we identify the endogenous Wnt activity and its role in controlling an essential downstream transcription factor, Meis3. Wnt3a is expressed in a specialized mesodermal domain, the paraxial dorsolateral mesoderm, which signals to overlying neuroectoderm. Loss of zygotic Wnt3a in this region does not alter mesoderm cell fates, but blocks Meis3 expression in the neuroectoderm, triggering the loss of posterior neural fates. Ectopic Meis3 protein expression is sufficient to rescue this phenotype. Moreover, Wnt3a induction of the posterior nervous system requires functional Meis3 in the neural plate. Using ChIP and promoter analysis, we show that Meis3 is a direct target of Wnt/beta-catenin signaling. This suggests a new model for neural anteroposterior patterning, in which Wnt3a from the paraxial mesoderm induces posterior cell fates via direct activation of a crucial transcription factor in the overlying neural plate.

A novel Wilms tumor 1 (WT1) target gene negatively regulates the WNT signaling pathway

Mammalian kidney development requires the functions of the Wilms tumor gene WT1 and the WNT/beta-catenin signaling pathway. Recent studies have shown that WT1 negatively regulates WNT/beta-catenin signaling, but the molecular mechanisms by which WT1 inhibits WNT/beta-catenin signaling are not completely understood. In this study, we identified a gene, CXXC5, which we have renamed WID (WT1-induced Inhibitor of Dishevelled), as a novel WT1 transcriptional target that negatively regulates WNT/beta-catenin signaling. WT1 activates WID transcription through the upstream enhancer region. In the developing kidney, Wid and Wt1 are coexpressed in podocytes of maturing nephrons. Structure-function analysis demonstrated that WID interacts with Dishevelled via its C-terminal CXXC zinc finger and Dishevelled binding domains and potently inhibits WNT/beta-catenin signaling in vitro and in vivo. WID is evolutionarily conserved, and ablation of wid in zebrafish embryos with antisense morpholino oligonucleotides perturbs embryonic kidney development. Taken together, our results demonstrate that the WT1 negatively regulates WNT/beta-catenin pathway via its target gene WID and further suggest a role for WID in nephrogenesis.

Wnt signaling and the polarity of the primary body axis

How animals establish and pattern the primary body axis is one of the most fundamental problems in biology. Data from diverse deuterostomes (frog, fish, mouse, and amphioxus) and from planarians (protostomes) suggest that Wnt signaling through beta-catenin controls posterior identity during body plan formation in most bilaterally symmetric animals. Wnt signaling also influences primary axis polarity of pre-bilaterian animals, indicating that an axial patterning role for Wnt signaling predates the evolution of bilaterally symmetric animals. The use of posterior Wnt signaling and anterior Wnt inhibition might be a unifying principle of body plan development in most animals.

Emx3 is required for the differentiation of dorsal telencephalic neurons

emx3 is first expressed in prospective telencephalic cells at the anterior border of the zebrafish neural plate. Knockdown of Emx3 function by morpholino reduces the expression of markers specific to dorsal telencephalon, and impairs axon tract formation. Rescue of both early and late markers requires low-level expression of emx3 at the one- or two-somite stage. Higher emx3 expression levels cause dorsal telencephalic markers to expand ventrally, which points to a possible role of emx3 in specifying dorsal telencephalon and a potential new function for Wnt/beta-catenin pathway activation. In contrast to mice, where Emx2 plays a major role in dorsal telencephalic development, knockdown of zebrafish Emx2 apparently does not affect telencephalic development. Similarly, Emx1 knockdown has little effect. Previously, emx3 was thought to be fish-specific. However, we found all three emx orthologs in Xenopus tropicalis and opossum (Monodelphis domestica) genomes, indicating that emx3 was present in an ancestral tetrapod genome.

Her6 regulates the neurogenetic gradient and neuronal identity in the thalamus

During vertebrate brain development, the onset of neuronal differentiation is under strict temporal control. In the mammalian thalamus and other brain regions, neurogenesis is regulated also in a spatially progressive manner referred to as a neurogenetic gradient, the underlying mechanism of which is unknown. Here we describe the existence of a neurogenetic gradient in the zebrafish thalamus and show that the progression of neurogenesis is controlled by dynamic expression of the bHLH repressor her6. Members of the Hes/Her family are known to regulate proneural genes, such as Neurogenin and Ascl. Here we find that Her6 determines not only the onset of neurogenesis but also the identity of thalamic neurons, marked by proneural and neurotransmitter gene expression: loss of Her6 leads to premature Neurogenin1-mediated genesis of glutamatergic (excitatory) neurons, whereas maintenance of Her6 leads to Ascl1-mediated production of GABAergic (inhibitory) neurons. Thus, the presence or absence of a single upstream regulator of proneural gene expression, Her6, leads to the establishment of discrete neuronal domains in the thalamus.

Endogenous Wnt signaling maintains neural progenitor cell potency

Wnt signaling regulates neural stem cell (NSC) function and development throughout an individual's lifetime. Intriguingly, adult hippocampal progenitors (AHPs) produce several Wnts, and the intracellular machinery necessary to respond to them, creating the potential for an active autocrine-signaling loop within this stem cell niche. However, the standard luciferase-based Wnt assay failed to detect this signaling loop. This assay is inherently less temporally sensitive to activity among a population of unsynchronized proliferating cells because it relies on the rapidly degrading reporter luciferase. We circumvented this limitation using a promoter assay that employs green fluorescent protein (GFP), as a relatively long-lived reporter of canonical Wnt activity. We found that at baseline, AHPs secreted functional Wnt that self-stimulates low-level canonical Wnt signaling. Elimination baseline Wnt activity, via application of an extracellular Wnt antagonist promoted neurogenesis, based on a combination of unbiased gene expression analysis and cell-fate analysis. A detailed clonal analysis of progenitors transduced with specific intracellular antagonists of canonical signaling, either Axin or truncated cadherin (beta-catenin sequestering), revealed that loss of baseline signaling depletes the population of multipotent precursors, thereby driving an increasing fraction to assume a committed cell fate (i.e., unipotent progenitors). Similarly, baseline Wnt signaling repressed differentiation of human NSCs. Although the specific Wnts produced by neural precursors vary with age and between species, their effects remain remarkably consistent. In sum, this study establishes that autonomous Wnt signaling is a conserved feature of the neurogenic niche that preserves the delicate balance between NSC maintenance and differentiation.

Apc1 is required for maintenance of local brain organizers and dorsal midbrain survival

The tumor suppressor Apc1 is an intracellular antagonist of the Wnt/beta-catenin pathway, which is vital for induction and patterning of the early vertebrate brain. However, its role in later brain development is less clear. Here, we examined the mechanisms underlying effects of an Apc1 zygotic-effect mutation on late brain development in zebrafish. Apc1 is required for maintenance of established brain subdivisions and control of local organizers such as the isthmic organizer (IsO). Caudal expansion of Fgf8 from IsO into the cerebellum is accompanied by hyperproliferation and abnormal cerebellar morphogenesis. Loss of apc1 results in reduced proliferation and apoptosis in the dorsal midbrain. Mosaic analysis shows that Apc is required cell-autonomously for maintenance of dorsal midbrain cell fate. The tectal phenotype occurs independently of Fgf8-mediated IsO function and is predominantly caused by stabilization of beta-catenin and subsequent hyperactivation of Wnt/beta-catenin signalling, which is mainly mediated through LEF1 activity. Chemical activation of the Wnt/beta-catenin in wild-type embryos during late brain maintenance stages phenocopies the IsO and tectal phenotypes of the apc mutants. These data demonstrate that Apc1-mediated restriction of Wnt/beta-catenin signalling is required for maintenance of local organizers and tectal integrity.

Zebrafish gbx1 refines the midbrain-hindbrain boundary border and mediates the Wnt8 posteriorization signal

Background

Studies in mouse, Xenopus and chicken have shown that Otx2 and Gbx2 expression domains are fundamental for positioning the midbrain-hindbrain boundary (MHB) organizer. Of the two zebrafish gbx genes, gbx1 is a likely candidate to participate in this event because its early expression is similar to that reported for Gbx2 in other species. Zebrafish gbx2, on the other hand, acts relatively late at the MHB. To investigate the function of zebrafish gbx1 within the early neural plate, we used a combination of gain- and loss-of-function experiments.

Results

We found that ectopic gbx1 expression in the anterior neural plate reduces forebrain and midbrain, represses otx2 expression and repositions the MHB to a more anterior position at the new gbx1/otx2 border. In the case of gbx1 loss-of-function, the initially robust otx2 domain shifts slightly posterior at a given stage (70% epiboly), as does MHB marker expression. We further found that ectopic juxtaposition of otx2 and gbx1 leads to ectopic activation of MHB markers fgf8, pax2.1 and eng2. This indicates that, in zebrafish, an interaction between otx2 and gbx1 determines the site of MHB development. Our work also highlights a novel requirement for gbx1 in hindbrain development. Using cell-tracing experiments, gbx1 was found to cell-autonomously transform anterior neural tissue into posterior. Previous studies have shown that gbx1 is a target of Wnt8 graded activity in the early neural plate. Consistent with this, we show that gbx1 can partially restore hindbrain patterning in cases of Wnt8 loss-of-function. We propose that in addition to its role at the MHB, gbx1 acts at the transcriptional level to mediate Wnt8 posteriorizing signals that pattern the developing hindbrain.

Conclusion

Our results provide evidence that zebrafish gbx1 is involved in positioning the MHB in the early neural plate by refining the otx2 expression domain. In addition to its role in MHB formation, we have shown that gbx1 is a novel mediator of Wnt8 signaling during hindbrain patterning.

Current perspectives in zebrafish reverse genetics: moving forward

Use of the zebrafish as a model of vertebrate development and disease has expanded dramatically over the past decade. While many articles have discussed the strengths of zebrafish forward genetics (the phenotype-driven approach), there has been less emphasis on equally important and frequently used reverse genetics (the candidate gene-driven approach). Here we review both current and prospective reverse genetic techniques that are applicable to the zebrafish model. We include discussion of pharmacological approaches, popular gain-of-function and knockdown approaches, and gene targeting strategies. We consider the need for temporal and spatial control over gain/loss of gene function, and discuss available and developing techniques to achieve this end. Our goal is both to reveal the current technical advantages of the zebrafish and to highlight those areas where work is still required to allow this system to be exploited to full advantage.

The Netrin-related domain of Sfrp1 interacts with Wnt ligands and antagonizes their activity in the anterior neural plate

BACKGROUND

Secreted frizzled related proteins (SFRPs) are multifunctional modulators of Wnt and BMP (Bone Morphogenetic Protein) signalling necessary for the development of most organs and the homeostasis of different adult tissues. SFRPs fold in two independent domains: the cysteine rich domain (SfrpCRD) related to the extracellular portion of Frizzled (Fz, Wnt receptors) and the Netrin module (SfrpNTR) defined by homologies with molecules such as Netrin-1, inhibitors of metalloproteinases and complement proteins. Due to its structural relationship with Fz, it is believed that SfrpCRD interferes with Wnt signalling by binding and sequestering the ligand. In contrast, the functional relevance of the SfrpNTR has been barely addressed.

RESULTS

Here, we combine biochemical studies, mutational analysis and functional assays in cell culture and medaka-fish embryos to show that the Sfrp1NTR mimics the function of the entire molecule, binds to Wnt8 and antagonizes Wnt canonical signalling. This activity requires intact tertiary structure and is shared by the distantly related Netrin-1NTR. In contrast, the Sfrp1CRD cannot mirror the function of the entire molecule in vivo but interacts with Fz receptors and antagonizes Wnt8-mediated beta-catenin transcriptional activity.

CONCLUSION

On the basis of these results, we propose that SFRP modulation of Wnt signalling may involve multiple and differential interactions among Wnt, Fz and SFRPs.

Epithelial-mesenchymal transition: at the crossroads of development and tumor metastasis

The epithelial-mesenchymal transition is a highly conserved cellular program that allows polarized, immotile epithelial cells to convert to motile mesenchymal cells. This important process was initially recognized during several critical stages of embryonic development and has more recently been implicated in promoting carcinoma invasion and metastasis. In this review, we summarize and compare major signaling pathways that regulate the epithelial-mesenchymal transitions during both development and tumor metastasis. Studies in both fields are critical for our molecular understanding of cell migration and morphogenesis.

Regulation of canonical Wnt signaling by Brachyury is essential for posterior mesoderm formation

The T box transcription factor Brachyury is essential for the formation of the posterior body in all vertebrates, although its critical transcriptional targets have remained elusive. Loss-of-function studies of mouse Brachyury and the zebrafish Brachyury ortholog Ntl indicated that Brachyury plays a more significant role in higher vertebrates than lower vertebrates. We have identified a second zebrafish Brachyury ortholog (Bra), and show that a combined loss of Ntl and Bra recapitulates the mouse phenotype, demonstrating an ancient role for Brachyury in patterning all but the most anterior somites. Using cell transplantation, we show that the only essential role for Brachyury during somite formation is non-cell autonomous, and demonstrate that Ntl and Bra are required for and can induce expression of the canonical Wnts wnt8 and wnt3a. We propose that a positive autoregulatory loop between Ntl/Bra and canonical Wnt signaling maintains the mesodermal progenitors to facilitate posterior somite development in chordates.

Xenopus Tbx6 mediates posterior patterning via activation of Wnt and FGF signalling

In vertebrates, the patterning of anterior-posterior (AP) axis is a fundamental process during embryogenesis. Wnt and FGF signalling pathways play important roles in regulating the patterning of embryo AP axis. Mouse Tbx6 encodes a transcription factor that has been demonstrated to be involved in the specification of the posterior tissue in mouse embryonic body. Here, we prove that morpholino-induced knockdown of XTbx6 impairs posterior development, indicating the requirement of XTbx6 in this process. Meanwhile, gain of XTbx6 function is sufficient to induce ectopic posterior structures in Xenopus embryos. Furthermore, XTbx6 activates the expression of Xwnt8 and FGF8, which are two mediators of posterior development, suggesting a mechanism by which XTbx6 modulates posterior patterning via Wnt and FGF signalling pathway activation.

A role for N-cadherin in mesodermal morphogenesis during gastrulation

Cell adhesion molecules mediate numerous developmental processes necessary for the segregation and organization of tissues. Here we show that the zebrafish biber (bib) mutant encodes a dominant allele at the N-cadherin locus. When knocked down with antisense oligonucleotides, bib mutants phenocopy parachute (pac) null alleles, demonstrating that bib is a gain-of-function mutation. The mutant phenotype disrupts normal cell-cell contacts throughout the mesoderm as well as the ectoderm. During gastrulation stages, cells of the mesodermal germ layer converge slowly; during segmentation stages, the borders between paraxial and axial tissues are irregular and somite borders do not form; later, myotomes are fused. During neurulation, the neural tube is disorganized. Although weaker, all traits present in bib mutants were found in pac mutants. When the distribution of N-cadherin mRNA was analyzed to distinguish mesodermal from neuroectodermal expression, we found that N-cadherin is strongly expressed in the yolk cell and hypoblast in the early gastrula, just preceding the appearance of the bib mesodermal defects. Only later is N-cadherin expressed in the anlage of the CNS, where it is found as a radial gradient in the forming neural plate. Hence, besides a well-established role in neural and somite morphogenesis, N-cadherin is essential for morphogenesis of the mesodermal germ layer during gastrulation.

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.

Zebrafish colgate/hdac1 functions in the non-canonical Wnt pathway during axial extension and in Wnt-independent branchiomotor neuron migration

Vertebrate gastrulation involves the coordinated movements of populations of cells. These movements include cellular rearrangements in which cells polarize along their medio-lateral axes leading to cell intercalations that result in elongation of the body axis. Molecular analysis of this process has implicated the non-canonical Wnt/Frizzled signaling pathway that is similar to the planar cell polarity pathway (PCP) in Drosophila. Here we describe a zebrafish mutant, colgate (col), which displays defects in the extension of the body axis and the migration of branchiomotor neurons. Activation of the non-canonical Wnt/PCP pathway in these mutant embryos by overexpressing DeltaNdishevelled, rho kinase2 and van gogh-like protein 2 (vangl2) rescues the extension defects suggesting that col acts as a positive regulator of the non-canonical Wnt/PCP pathway. Further, we show that col normally regulates the caudal migration of nVII facial hindbrain branchiomotor neurons and that the mutant phenotype can be rescued by misexpression of vangl2 independent of the Wnt/PCP pathway. We cloned the col locus and found that it encodes histone deacetylase1 (hdac1). Our previous results and studies by others have implicated hdac1 in repressing the canonical Wnt pathway. Here, we demonstrate novel roles for zebrafish hdac1 in activating non-canonical Wnt/PCP signaling underlying axial extension and in promoting Wnt-independent caudal migration of a subset of hindbrain branchiomotor neurons.

Calcium signaling in vertebrate embryonic patterning and morphogenesis

Signaling pathways that rely on the controlled release and/or accumulation of calcium ions are important in a variety of developmental events in the vertebrate embryo, affecting cell fate specification and morphogenesis. One such major developmentally important pathway is the Wnt/calcium signaling pathway, which, through its antagonism of Wnt/beta-catenin signaling, appears to regulate the formation of the early embryonic organizer. In addition, the Wnt/calcium pathway shares components with another non-canonical Wnt pathway involved in planar cell polarity, suggesting that these two pathways form a loose network involved in polarized cell migratory movements that fashion the vertebrate body plan. Furthermore, left-right axis determination, neural induction and somite formation also display dynamic calcium release, which may be critical in these patterning events. Finally, we summarize recent evidence that propose a role for calcium signaling in stem cell biology and human developmental disorders.

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.

Evolution of axis specification mechanisms in jawed vertebrates: insights from a chondrichthyan

The genetic mechanisms that control the establishment of early polarities and their link with embryonic axis specification and patterning seem to substantially diverge across vertebrates. In amphibians and teleosts, the establishment of an early dorso-ventral polarity determines both the site of axis formation and its rostro-caudal orientation. In contrast, amniotes retain a considerable plasticity for their site of axis formation until blastula stages and rely on signals secreted by extraembryonic tissues, which have no clear equivalents in the former, for the establishment of their rostro-caudal pattern. The rationale for these differences remains unknown. Through detailed expression analyses of key development genes in a chondrichthyan, the dogfish Scyliorhinus canicula, we have reconstructed the ancestral pattern of axis specification in jawed vertebrates. We show that the dogfish displays compelling similarities with amniotes at blastula and early gastrula stages, including the presence of clear homologs of the hypoblast and extraembryonic ectoderm. In the ancestral state, these territories are specified at opposite poles of an early axis of bilateral symmetry, homologous to the dorso-ventral axis of amphibians or teleosts, and aligned with the later forming embryonic axis, from head to tail. Comparisons with amniotes suggest that a dorsal expansion of extraembryonic ectoderm, resulting in an apparently radial symmetry at late blastula stages, has taken place in their lineage. The synthesis of these results with those of functional analyses in model organisms supports an evolutionary link between the dorso-ventral polarity of amphibians and teleosts and the embryonic-extraembryonic organisation of amniotes. It leads to a general model of axis specification in gnathostomes, which provides a comparative framework for a reassessment of conservations both among vertebrates and with more distant metazoans.

Depletion of Med10 enhances Wnt and suppresses Nodal signaling during zebrafish embryogenesis

The transcriptional Mediator (MED) is a multiprotein complex that transmits information from transcription factors to RNA polymerase II (PolII) to regulate transcription. At present, the role of distinct MED subunits in general transcription versus transcription stimulated by specific signaling pathways is unclear. By means of positional cloning, we reveal that the zebrafish mutant tennismatch is a hypomorphic allele of Med10, a conserved MED middle domain subunit. Using morpholino antisense oligonucleotides, we further demonstrate that reduction of Med10 levels led to an enhancement of the Wnt signaling pathway, while also suggesting a role for Med10 in mediating the Nodal signaling pathway. In contrast to the dual roles of Med10, reduction of Med12 and Med13 levels, two MED subunits in the regulatory domain, led to an enhancement of the Wnt signaling pathway but not the Nodal pathway, while reduction of Med15 levels, a MED subunit in the tail domain, suppressed the Nodal signaling pathway but not the Wnt signaling pathway. Thus, Med10 appears to be a unique MED subunit that differentially transduces information from distinct signaling pathways during zebrafish embryogenesis.

Cloning and embryonic expression of five distinct sfrp genes in the zebrafish Danio rerio

Recently, a new member of the secreted frizzled-related protein (sFRP) family, named tlc, has been identified as expressed by the anterior neural border (ANB) cells in the zebrafish Danio rerio. Tlc plays an important role in telencephalic induction and patterning. In absence of Tlc, formation of the telencephalon is severely delayed, but not abolished. This prompted us to clone the other zebrafish sfrp family members and analyse their expression patterns, in search of a family member that may partly functionally overlap with Tlc. Except sizzled, expression profile of sfrp genes in zebrafish has not been reported so far. Here, we describe the cloning of full-length cDNA for sfrp1a, sfrp1b, sfrp2, sfrp3 and sfrp5 gene transcripts and we examine their expression at different embryonic stages. Only sfrp1a is expressed in the anterior neural plate including the ANB cells where and when tlc is expressed. Interestingly, compared to both tlc and sfrp1a, wnt genes are complementary expressed more posteriorly in the neural plate. Later, both sfrp1a and sfrp5 expression profiles are overlapping, in particular at pharyngula stage these genes are expressed in the ventral part of the forebrain, midbrain and hindbrain. sfrp1b, sfrp2 and sfrp3 are mainly expressed in mesodermal and endodermal embryonic tissues. Expression profiles of these different genes in zebrafish gave interesting clues on the possible function and evolution of sFRPs in zebrafish and other organisms.

Mesoderm induction: from caps to chips

Vertebrate mesoderm induction is one of the classical problems in developmental biology. Various developmental biology approaches, particularly in Xenopus and zebrafish, have identified many of the key factors that are involved in this process and have provided major insights into how these factors interact as part of a signalling and transcription-factor network. These data are beginning to be refined by high-throughput approaches such as microarray assays. Future challenges include understanding how the prospective mesodermal cells integrate the various signals they receive and how they resolve this information to regulate their morphogenetic behaviours and cell-fate decisions.

The KLHL12-Cullin-3 ubiquitin ligase negatively regulates the Wnt-beta-catenin pathway by targeting Dishevelled for degradation

Dishevelled is a conserved protein that interprets signals received by Frizzled receptors. Using a tandem-affinity purification strategy and mass spectrometry we have identified proteins associated with Dishevelled, including a Cullin-3 ubiquitin ligase complex containing the Broad Complex, Tramtrack and Bric à Brac (BTB) protein Kelch-like 12 (KLHL12). This E3 ubiquitin ligase complex is recruited to Dishevelled in a Wnt-dependent manner that promotes its poly-ubiquitination and degradation. Functional analyses demonstrate that regulation of Dishevelled by this ubiquitin ligase antagonizes the Wnt-beta-catenin pathway in cultured cells, as well as in Xenopus and zebrafish embryos. Considered with evidence that the distinct Cullin-1 based SCF(beta-TrCP)complex regulates beta-catenin stability, our data on the stability of Dishevelled demonstrates that two distinct ubiquitin ligase complexes regulate the Wnt-beta-catenin pathway.

Essential and opposing roles of zebrafish beta-catenins in the formation of dorsal axial structures and neurectoderm

In Xenopus, Wnt signals and their transcriptional effector beta-catenin are required for the development of dorsal axial structures. In zebrafish, previous loss-of-function studies have not identified an essential role for beta-catenin in dorsal axis formation, but the maternal-effect mutation ichabod disrupts beta-catenin accumulation in dorsal nuclei and leads to a reduction of dorsoanterior derivatives. We have identified and characterized a second zebrafish beta-catenin gene, beta-catenin-2, located on a different linkage group from the previously studied beta-catenin-1, but situated close to the ichabod mutation on LG19. Although the ichabod mutation does not functionally alter the beta-catenin-2 reading frame, the level of maternal beta-catenin-2, but not beta-catenin-1, transcript is substantially lower in ichabod, compared with wild-type, embryos. Reduction of beta-catenin-2 function in wild-type embryos by injection of morpholino antisense oligonucleotides (MOs) specific for this gene (MO2) results in the same ventralized phenotypes as seen in ichabod embryos, and administration of MO2 to ichabod embryos increases the extent of ventralization. MOs directed against beta-catenin-1 (MO1), by contrast, had no ventralizing effect on wild-type embryos. beta-catenin-2 is thus specifically required for organizer formation and this function is apparently required maternally, because the ichabod mutation causes a reduction in maternal transcription of the gene and a reduced level of beta-catenin-2 protein in the early embryo. A redundant role of beta-catenins in suppressing formation of neurectoderm is revealed when both beta-catenin genes are inhibited. Using a combination of MO1 and MO2 in wild-type embryos, or by injecting solely MO1 in ichabod embryos, we obtain expression of a wide spectrum of neural markers in apparently appropriate anteroposterior pattern. We propose that the early, dorsal-promoting function of beta-catenin-2 is essential to counteract a later, dorsal- and neurectoderm-repressing function that is shared by both beta-catenin genes.

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.

FoxA3 and goosecoid promote anterior neural fate through inhibition of Wnt8a activity before the onset of gastrulation

Formation of the nervous system initially requires the acquisition of neural identity, which is achieved through the inhibition of epidermalizing factors. A regional patterning then takes place within the neural plate through the activity of caudalizing factors. These two processes are tightly regulated early in development by the dorsal organizer. Here, we show that, in zebrafish embryos, two transcription factors, FoxA3 and Goosecoid, coexpressed at the dorsal blastula margin, are required for the definition of anterior neural fate. Their inactivation results in deletions of anterior head structures associated with an increase of Wnt8 activity at the dorsal blastula margin. These phenotypes can be fully rescued by overexpression of Wnt inhibitors or by inactivation of wnt8a. Altogether, foxA3 and goosecoid cooperate to promote formation of anterior neural tissue by protecting, as early as blastula stage, presumptive anterior neural cells from an irreversible caudalization by the posteriorizing factor Wnt8a.

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.

Rhombomere boundaries are Wnt signaling centers that regulate metameric patterning in the zebrafish hindbrain

The vertebrate hindbrain develops from a series of segments (rhombomeres) distributed along the anteroposterior axis. We are studying the roles of Wnt and Delta-Notch signaling in maintaining rhombomere boundaries as organizing centers in the zebrafish hindbrain. Several wnt genes (wnt1, wnt3a, wnt8b, and wnt10b) show elevated expression at rhombomere boundaries, whereas several delta genes (dlA, dlB, and dlD) are expressed in transverse stripes flanking rhombomere boundaries. Partial disruption of Wnt signaling by knockdown of multiple wnt genes, or the Wnt mediator tcf3b, ablates boundaries and associated cell types. Expression of dlA is chaotic, and cell types associated with rhombomere centers are disorganized. Similar patterning defects are observed in segmentation mutants spiel-ohne-grenzen (spg) and valentino (val), which fail to form rhombomere boundaries due to faulty interactions between adjacent rhombomeres. Stripes of wnt expression are variably disrupted, with corresponding disturbances in metameric patterning. Mutations in dlA or mind bomb (mib) disrupt Delta-Notch signaling and cause a wide range of patterning defects in the hindbrain. Stripes of wnt1 are initially normal but subsequently dissipate, and metameric patterning becomes increasingly disorganized. Driving wnt1 expression using a heat-shock construct partially rescues metameric patterning in mib mutants. Thus, rhombomere boundaries act as Wnt signaling centers required for precise metameric patterning, and Delta signals from flanking cells provide feedback to maintain wnt expression at boundaries. Similar feedback mechanisms operate in the Drosophila wing disc and vertebrate limb bud, suggesting coaptation of a conserved signaling module that spatially organizes cells in complex organ systems.

D-glucuronyl C5-epimerase acts in dorso-ventral axis formation in zebrafish

BACKGROUND

Heparan sulfate (HS) is an ubiquitous component of the extracellular matrix that binds and modulates the activity of growth factors, cytokines and proteases. Animals with defective HS biosynthesis display major developmental abnormalities however the processes that are affected remain to be defined. D-glucuronyl-C5-epimerase (Glce) is a key HS chain modifying enzyme that catalyses the conversion of glucuronic acid into iduronic acid, a biosynthetic step that enhances HS biological activity. In this study the role of Glce during early zebrafish development has been investigated.

RESULTS

Two Glce-like proteins (Glce-A and -B) are expressed in zebrafish at all times. They are the products of two distinct genes that, based on chromosomal mapping, are both orthologues of the same single human gene. Transcripts for both proteins were detected in fertilized zebrafish embryos prior to the onset of zygotic transcription indicating their maternal origin. At later developmental stages the epimerases are expressed widely throughout gastrulation and then become restricted to the hindbrain at 24 h post-fertilization. By monitoring the expression of well characterized marker genes during gastrulation, we have found that misexpression of Glce causes a dose-dependent expansion of the ventral structures, whereas protein knockdown using targeted antisense morpholino oligonucleotides promotes axis dorsalization. The ventralizing activity of Bmp2b is enhanced by Glce overexpression whereas Glce knockdown impairs Bmp2b activity.

CONCLUSION

Glce activity is an important determinant of of dorso-ventral axis formation and patterning in zebrafish. In particular Glce acts during gastrulation by affecting Bmp-mediated cell specification. The results obtained further corroborate the concept that HS encodes information that affect morphogenesis during early vertebrate development.

The Sp1-related transcription factors sp5 and sp5-like act downstream of Wnt/beta-catenin signaling in mesoderm and neuroectoderm patterning

BACKGROUND

Wnt/beta-catenin signaling regulates many processes during vertebrate development, including patterning of the mesoderm along the dorso-ventral axis and patterning of the neuroectoderm along the anterior-posterior axis during gastrulation. However, relatively little is known about Wnt target genes mediating these effects.

RESULTS

Using zebrafish DNA microarrays, we have identified several new targets of Wnt/beta-catenin signaling, including sp5-like (sp5l, previously called spr2), a zinc-finger transcription factor of the Sp1 family. sp5-like is a direct target of Wnt/beta-catenin signaling and acts together with its paralog sp5 (previously called bts1) downstream of wnt8 in patterning of the mesoderm and neuroectoderm because (1) overexpression of sp5-like, like overexpression of wnt8, posteriorizes the neuroectoderm, (2) sp5-like morpholino-mediated knockdown, like wnt8 knockdown, causes anteriorization of the hindbrain, (3) combined knockdown of sp5 and sp5-like, like loss of wnt8, causes expansion of dorsal mesoderm, (4) sp5-like knockdown reduces the defects in mesoderm and neuroectoderm patterning caused by wnt8 overexpression, and (5) inhibition of sp5-like enhances the effects of hypomorphic loss of wnt8. Importantly, (6) overexpression of sp5-like is able to partially restore normal hindbrain patterning in wnt8 morphants.

CONCLUSIONS

sp5-like is a direct target of Wnt/beta-catenin signaling during gastrulation and, together with sp5, acts as a required mediator of the activities of wnt8 in patterning the mesoderm and neuroectoderm. We conclude that sp5 transcription factors mediate the downstream responses to Wnt/beta-catenin signaling in several developmental processes in zebrafish.

Functional genomic analysis of the Wnt-wingless signaling pathway

The Wnt-Wingless (Wg) pathway is one of a core set of evolutionarily conserved signaling pathways that regulates many aspects of metazoan development. Aberrant Wnt signaling has been linked to human disease. In the present study, we used a genomewide RNA interference (RNAi) screen in Drosophila cells to screen for regulators of the Wnt pathway. We identified 238 potential regulators, which include known pathway components, genes with functions not previously linked to this pathway, and genes with no previously assigned functions. Reciprocal-Best-Blast analyses reveal that 50% of the genes identified in the screen have human orthologs, of which approximately 18% are associated with human disease. Functional assays of selected genes from the cell-based screen in Drosophila, mammalian cells, and zebrafish embryos demonstrated that these genes have evolutionarily conserved functions in Wnt signaling. High-throughput RNAi screens in cultured cells, followed by functional analyses in model organisms, prove to be a rapid means of identifying regulators of signaling pathways implicated in development and disease.

Regulation of hhex expression in the yolk syncytial layer, the potential Nieuwkoop center homolog in zebrafish

The Nieuwkoop center is the earliest signaling center during dorsal-ventral pattern formation in amphibian embryos and has been implied to function in induction of the Spemann-Mangold organizer. In zebrafish, Nieuwkoop-center-like activity resides in the dorsal yolk syncytial layer (YSL) at the interface of the vegetal yolk cell and the blastoderm. hex homologs are expressed in the anterior endomesoderm in frogs (Xhex), the anterior visceral endoderm in mice, and the dorsal YSL in zebrafish (hhex). Here, we investigate the control of hhex expression in the YSL. We demonstrate that bozozok (boz) is absolutely required for early hhex expression, while overexpression of boz causes ectopic hhex expression. Activation of Wnt/beta-catenin signaling by LiCl induces hhex expression in wild-type YSL but not in boz mutant embryos, revealing that boz activity is required downstream of Wnt/beta-catenin signaling for hhex expression. Further, we show that the boz-mediated induction of hhex is independent of the Boz-mediated repression of bmp2b. Our data reveal that repressive effects of both Vega1 and Vega2 may be responsible for the exclusion of hhex expression from the ventral and lateral parts of the YSL. In summary, zebrafish hhex appears to be activated by Wnt/beta-catenin in the dorsal YSL, where Boz acts in a permissive way to limit repression of hhex by Vega1 and Vega2.

Interaction of Wnt and caudal-related genes in zebrafish posterior body formation

Although Wnt signaling plays an important role in body patterning during early vertebrate embryogenesis, the mechanisms by which Wnts control the individual processes of body patterning are largely unknown. In zebrafish, wnt3a and wnt8 are expressed in overlapping domains in the blastoderm margin and later in the tailbud. The combined inhibition of Wnt3a and Wnt8 by antisense morpholino oligonucleotides led to anteriorization of the neuroectoderm, expansion of the dorsal organizer, and loss of the posterior body structure-a more severe phenotype than with inhibition of each Wnt alone-indicating a redundant role for Wnt3a and Wnt8. The ventrally expressed homeobox genes vox, vent, and ved mediated Wnt3a/Wnt8 signaling to restrict the organizer domain. Of posterior body-formation genes, expression of the caudal-related cdx1a and cdx4/kugelig, but not bmps or cyclops, was strongly reduced in the wnt3a/wnt8 morphant embryos. Like the wnt3a/wnt8 morphant embryos, cdx1a/cdx4 morphant embryos displayed complete loss of the tail structure, suggesting that Cdx1a and Cdx4 mediate Wnt-dependent posterior body formation. We also found that cdx1a and cdx4 expression is dependent on Fgf signaling. hoxa9a and hoxb7a expression was down-regulated in the wnt3a/wnt8 and cdx1a/cdx4 morphant embryos, and in embryos with defects in Fgf signaling. Fgf signaling was required for Cdx-mediated hoxa9a expression. Both the wnt3a/wnt8 and cdx1a/cdx4 morphant embryos failed to promote somitogenesis during mid-segmentation. These data indicate that the cdx genes mediate Wnt signaling and play essential roles in the morphogenesis of the posterior body in zebrafish.

Noncanonical Wnt signaling regulates midline convergence of organ primordia during zebrafish development

Several components of noncanonical Wnt signaling pathways are involved in the control of convergence and extension (CE) movements during zebrafish and Xenopus gastrulation. However, the complexity of these pathways and the wide patterns of expression and activity displayed by some of their components immediately suggest additional morphogenetic roles beyond the control of CE. Here we show that the key modular intracellular mediator Dishevelled, through a specific activation of RhoA GTPase, controls the process of convergence of endoderm and organ precursors toward the embryonic midline in the zebrafish embryo. We also show that three Wnt noncanonical ligands wnt4a, silberblick/wnt11, and wnt11-related regulate this process by acting in a largely redundant way. The same ligands are also required, nonredundantly, to control specific aspects of CE that involve interaction of Dishevelled with mediators different from that of RhoA GTPase. Overall, our results uncover a late, previously unexpected role of noncanonical Wnt signaling in the control of midline assembly of organ precursors during vertebrate embryo development.

Signal transduction during amyloid-β-peptide neurotoxicity: role in Alzheimer disease

Alzheimer's disease (AD) is a neurodegenerative disorder with progressive dementia accompanied by two main structural changes in the brain: intracellular protein deposits termed neurofibrillary tangles (NFT) and extracellular amyloid protein deposits surrounded by dystrophic neurites that constitutes the senile plaques. Currently, it is widely accepted that amyloid beta-peptide (A beta) metabolism disbalance is crucial for AD progression. A beta deposition may be enhanced by molecular chaperones, including metals like copper and proteins like acetylcholinesterase (AChE). At the neuronal level, several AD-related proteins interact with transducers of the Wnt/beta-catenin signaling pathway, including beta-catenin and glycogen synthase kinase 3 beta (GSK-3 beta) and both in vitro and in vivo studies suggest that Wnt/beta-catenin signaling is a target for A beta toxicity. Accordingly, activation of this signaling by lithium or Wnt ligands in AD-experimental animal models or in primary hippocampal neurons attenuate A beta neurotoxicity by recovering beta-catenin levels and Wnt-target gene expression of survival genes such as bcl-2. On the other hand, peroxisomal proliferator-activated receptor gamma (PPAR gamma) and muscarinic acetylcholine receptor (mAChR) agonists also activate Wnt/beta-catenin signaling and they have neuroprotective effects on hippocampal neurons. Our studies are consistent with the idea that a sustained loss of function of Wnt signaling components would trigger a series of events, determining the onset and development of AD and that modulation of this pathway through the activation of cross-talking signaling cascades should be considered as a possible therapeutic strategy for AD treatment.

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.

Sequential antagonism of early and late Wnt-signaling by zebrafish colgate promotes dorsal and anterior fates

The establishment of the vertebrate body plan involves patterning of the ectoderm, mesoderm, and endoderm along the dorsoventral and antero-posterior axes. Interactions among numerous signaling molecules from several multigene families, including Wnts, have been implicated in regulating these processes. Here we provide evidence that the zebrafish colgate(b382) (col) mutation results in increased Wnt signaling that leads to defects in dorsal and anterior development. col mutants display early defects in dorsoventral patterning manifested by a decrease in the expression of dorsal shield-specific markers and ectopic expression of ventrolaterally expressed genes during gastrulation. In addition to these early patterning defects, col mutants display a striking regional posteriorization within the neuroectoderm, resulting in a reduction in anterior fates and an expansion of posterior fates within the forebrain and midbrain-hindbrain regions. We are able to correlate these phenotypes to the overactivation of Wnt signaling in col mutants. The early dorsal and anterior patterning phenotypes of the col mutant embryos are selectively rescued by inactivation of Wnt8 function by morpholino translational interference. In contrast, the regionalized neuroectoderm posterioriorization phenotype is selectively rescued by morpholino-mediated inactivation of Wnt8b. These results suggest that col-mediated antagonism of early and late Wnt-signaling activity during gastrulation is normally required sequentially for both early dorsoventral patterning and the specification and patterning of regional fates within the anterior neuroectoderm.

PPAR gamma signaling exacerbates mammary gland tumor development

Breast cancer cell lines that express the nuclear peroxisome proliferator-activated receptor gamma (PPAR gamma) can be prompted to undergo growth arrest and differentiation when treated with synthetic PPAR gamma ligands. To evaluate the therapeutic potential of increased PPAR gamma signaling in vivo, we generated transgenic mice that express a constitutively active form of PPAR gamma in mammary gland. These mice are indistinguishable from their wild-type littermates. However, when bred to a transgenic strain prone to mammary gland cancer, bigenic animals develop tumors with greatly accelerated kinetics. Surprisingly, in spite of their more malignant nature, bigenic tumors are more secretory and differentiated. The molecular basis of this tumor-promoting effect may be an increase in Wnt signaling, as ligand activation of PPAR gamma potentiates Wnt function in an in vivo model of this pathway. These results suggest that once an initiating event has taken place, increased PPAR gamma signaling serves as a tumor promoter in the mammary gland.

Combinatorial Wnt control of zebrafish midbrain–hindbrain boundary formation

Wnt signaling is known to be required for the normal development of the vertebrate midbrain and hindbrain, but genetic loss of function analyses in the mouse and zebrafish yield differing results regarding the relative importance of specific Wnt loci. In the zebrafish, Wnt1 and Wnt10b functionally overlap in their control of gene expression in the ventral midbrain-hindbrain boundary (MHB), but they are not required for the formation of the MHB constriction. Whether other wnt loci are involved in zebrafish MHB development is unclear, although the expression of at least two wnts, wnt3a and wnt8b, is maintained in wnt1/wnt10b mutants. In order to address the role of wnt3a in zebrafish, we have isolated a full length cDNA and examined its expression and function via knockdown by morpholino antisense oligonucleotide (MO)-mediated knockdown. The expression pattern of wnt3a appears to be evolutionarily conserved between zebrafish and mouse, and MO knockdown shows that Wnt3a, while not uniquely required for MHB development, is required in the absence of Wnt1 and Wnt10b for the formation of the MHB constriction. In zebrafish embryos lacking Wnt3a, Wnt1 and Wnt10b, the expression of engrailed orthologs, pax2a and fgf8 is not maintained after mid-somitogenesis. In contrast to acerebellar and no isthmus mutants, in which midbrain and hindbrain cells acquire new fates but cell number is not significantly affected until late in embryogenesis, zebrafish embryos lacking Wnt3a, Wnt1 and Wnt10b undergo extensive apoptosis in the midbrain and cerebellum anlagen beginning in mid-somitogenesis, which results in the absence of a significant portion of the midbrain and cerebellum. Thus, the requirement for Wnt signaling in forming the MHB constriction is evolutionarily conserved in vertebrates and it is possible in zebrafish to dissect the relative impact of multiple Wnt loci in midbrain and hindbrain development.