Induction of a secondary embryonic axis in zebrafish occurs following the overexpression of beta-catenin

Symmetrical parapagus diprosopus: A comparative, computed tomographic, and pathoanatomical study of a new case in domestic pig

BACKGROUND

Parapagus diprosopus are conjoined twins characterized by craniofacial duplication and only one body, representing one of the rarest types of these twins. Their occurrence has been recorded in different species of vertebrates, including humans, but few cases have been studied in domestic pigs.

CASE

A pair of conjoined twin pigs was studied using x-rays, computed tomography, and necropsy. The abnormalities found were compared with those of the rare swine cases presented in the literature as well as with other species, and the different etiopathogenetic possibilities were addressed. The degree of duplication of the head bones decreased caudally, as did that of the structures of the central nervous system. In the two oral cavities, there was a complete cleft palate. All the cervical vertebrae and thoracic vertebrae up to T3 were partially duplicated. The heart and great vessels were normal, as were the other thoracic and abdominal organs.

CONCLUSIONS

The conjoined twin pigs of this study are a case of parapagus diprosopus tetraophthalmus triotus, presenting the same pattern of abnormalities of human diprosopus and that of other species. The scarcity of detailed studies on craniofacial duplication in pigs and the lack of a definitive explanation on the etiology and pathogenesis of conjoined twins shows the need for further research and the publication of more cases.

Altered ovarian transcriptome is linked to early mortality and abnormalities in zebrafish embryos after maternal exposure to gamma irradiation

Recent laboratory studies focusing on multigenerational approach demonstrated drastic phenotypic effects after chronic fish irradiation exposure. No irradiation effect at phenotypic scale was observed for F0 (reproductive performances) while early mortality and malformations were observed in F1 offspring whether they were irradiated or not. The objective was to study molecular mechanisms likely to be involved in these phenotypic effects induced by parental irradiation. Thus, F0 adult zebrafish were irradiated for ten days until reproduction and maternal involvement in offspring development was assessed. Levels of maternal provided cortisol and vitellogenin, needed for embryo development, were not impacted by irradiation. However, maternal transcriptome highlighted irradiation effect on processes involved in oocyte development, as well as on essential maternal factors needed for offspring development. Therefore, this study highlighted the importance of parental exposure on offspring fate and of the importance of multigenerational exposure in risk assessment.

Cis-regulatory code for determining the action of Foxd as both an activator and a repressor in ascidian embryos

In early embryos of Ciona, an invertebrate chordate, the animal-vegetal axis is established by the combinatorial actions of maternal factors. One target of these maternal factors, Foxd, is specifically expressed in the vegetal hemisphere and stabilizes the animal-vegetal axis by activating vegetal hemisphere-specific genes and repressing animal hemisphere-specific genes. This dual functionality is essential for the embryogenesis of early ascidian embryos; however, the mechanism by which Foxd can act as both a repressor and an activator is unknown. Here, we identify a Foxd binding site upstream of Lhx3/4, which is activated by Foxd, and compare it with a repressive Foxd binding site upstream of Dmrt.a. We found that activating sites bind Foxd with low affinity while repressive sites bind Foxd with high affinity. Reporter assays confirm that this qualitative difference between activating and repressive Foxd binding sites is sufficient to change Foxd functionality. We therefore conclude that the outcome of Foxd transcriptional regulation is encoded in cis-regulatory elements.

Inhibitory effects on melanogenesis by thymoquinone are mediated through the β‑catenin pathway in B16F10 mouse melanoma cells

Thymoquinone (TQ) is a component found in the seeds of Nigella sativa, an annual plant growing on the Mediterranean coast, and is known for its anticancer and anti‑inflammatory effects. However, to date, at least to the best of our knowledge, limited studies are available examining the molecular mechanisms through which TQ inhibits melanogenesis. Accordingly, this study aimed to treat B16F10 mouse melanoma cells with TQ to investigate its apparent effects and its molecular regulatory mechanisms. Treatment of the B16F10 cells with 10, 15 and 20 µM of TQ for 48 h resulted in a dose‑dependent decrease in the expression of microphthalmia‑associated transcription factor (MITF), tyrosinase expression and tyrosinase activity, and these treatments simultaneously led to a decrease in the protein expression and transcription of β‑catenin, a Wnt signaling pathway protein. Pre‑treatment of the cells with the proteasome inhibitor, MG132, to confirm the inhibition of melanogenesis through the β‑catenin pathway by TQ treatment resulted in an increase in the expression of β‑catenin that was initially reduced by TQ, and the expression and activity of MITF and tyrosinase also increased. Pre‑treatment with LiCl, which is known to inactivate glycogen synthase kinase 3β (GSK3β) by inducing the phosphorylation of the Ser‑9 site, resulted in an increased phospho‑GSK3β expression accompanied by β‑catenin that was initially reduced by TQ, and the recovery of the expression and activity of tyrosinase was also confirmed. The transfection of S37A cDNA into B16F10 cells that overexpress β‑catenin resulted in the recovery of β‑catenin expression that was initially reduced by TQ, and this treatment also recovered the expression and activity of tyrosinase. When zebrafish eggs were treated with 1, 2.5 and 5 µM of TQ at 10 h following fertilization, their melanin content decreased in a dose‑dependent manner. On the whole, these findings demonstrated that the inhibition of melanogenesis in B16F10 mouse melanoma cells by TQ treatment resulted from the inhibition of the β‑catenin pathway and confirmed that TQ treatment inhibited melanogenesis in zebrafish.

TSPAN12 Is a Norrin Co-receptor that Amplifies Frizzled4 Ligand Selectivity and Signaling

Accessory proteins in Frizzled (FZD) receptor complexes are thought to determine ligand selectivity and signaling amplitude. Genetic evidence indicates that specific combinations of accessory proteins and ligands mediate vascular β-catenin signaling in different CNS structures. In the retina, the tetraspanin TSPAN12 and the ligand norrin (NDP) mediate angiogenesis, and both genes are linked to familial exudative vitreoretinopathy (FEVR), yet the molecular function of TSPAN12 remains poorly understood. Here, we report that TSPAN12 is an essential component of the NDP receptor complex and interacts with FZD4 and NDP via its extracellular loops, consistent with an action as co-receptor that enhances FZD4 ligand selectivity for NDP. FEVR-linked mutations in TSPAN12 prevent the incorporation of TSPAN12 into the NDP receptor complex. In vitro and in Xenopus embryos, TSPAN12 alleviates defects of FZD4 M105V, a mutation that destabilizes the NDP/FZD4 interaction. This study sheds light on the poorly understood function of accessory proteins in FZD signaling.

Transgenic Expression of A Venous Malformation Related Mutation, TIE2-R849W, Significantly Induces Multiple Malformations of Zebrafish

A TIE2 mutation causing arginine-to-tryptophan substitution at residue 849 (TIE2-R849W) is commonly identified in heredofamilial venous malformation. However, there is no in vivo model to confirm the pathogenic role of TIE2-R849W. Humanized TIE2-R849W plasmid was constructed via PCR-mediated site-directed mutagenesis. After transcription and micro-injection, TIE2-R849W significantly induces multiple malformations in zebrafish: caudal vein plexus (CVP) defect, eye abnormalities, forebrain formation perturbations, and mandibular malformation. Histologically, these phenotypes accompany aphakia, confused retina plexiform layer, abnormal mandibular cartilage, ectopic myelencephalon proliferation and aberrant location of neurogliocytes. According to qRT-PCR, except for high expression of egfl7, the other CVP-related genes cd146, nr2f1a, and s1pr1 are not significantly different from control. TIE2-R849W also induced upregulation of the wnt signaling pathway. Gene array in vitro shows that under the effect of TIE2-R849W, consistent with high expression of pik3 and foxo1, high levels of egfl7, wnt9a, lrp5 and dkk1 were partly confirmed. This in vivo model directly identifies the venous-related pathogenic role of TIE2-R849W. Under up-regulation of TIE2-R849W, egfl7 could be considered a potential reason for venous defects. Moreover, the wnt pathway may perform an important role as a key trigger for head multi-malformations.

Imaging early embryonic calcium activity with GCaMP6s transgenic zebrafish

Intracellular Ca²⁺ signaling regulates cellular activities during embryogenesis and in adult organisms. We generated stable Tg[βactin2:GCaMP6s]^stl351 and Tg[ubi:GCaMP6s]^stl352 transgenic lines that combine the ubiquitously-expressed Ca²⁺ indicator GCaMP6s with the transparent characteristics of zebrafish embryos to achieve superior in vivo Ca²⁺ imaging. Using the Tg[βactin2:GCaMP6s]^stl351 line featuring strong GCaMP6s expression from cleavage through gastrula stages, we detected higher frequency of Ca²⁺ transients in the superficial blastomeres during the blastula stages preceding the midblastula transition. Additionally, GCaMP6s also revealed that dorsal-biased Ca²⁺ signaling that follows the midblastula transition persisted longer during gastrulation, compared with earlier studies. We observed that dorsal-biased Ca²⁺ signaling is diminished in ventralized ichabod/β-catenin2 mutant embryos and ectopically induced in embryos dorsalized by excess β-catenin. During gastrulation, we directly visualized Ca²⁺ signaling in the dorsal forerunner cells, which form in a Nodal signaling dependent manner and later give rise to the laterality organ. We found that excess Nodal increases the number and the duration of Ca²⁺ transients specifically in the dorsal forerunner cells. The GCaMP6s transgenic lines described here enable unprecedented visualization of dynamic Ca²⁺ events from embryogenesis through adulthood, augmenting the zebrafish toolbox.

Chordin and dickkopf-1b are essential for the formation of head structures through activation of the FGF signaling pathway in zebrafish

The ability of the Spemann organizer to induce dorsal axis formation is dependent on downstream factors of the maternal Wnt/β-catenin signaling pathway. The fibroblast growth factor (FGF) signaling pathway has been identified as one of the downstream components of the maternal Wnt/β-catenin signaling pathway. The ability of the FGF signaling pathway to induce the formation of a dorsal axis with a complete head structure requires chordin (chd) expression; however, the molecular mechanisms involved in this developmental process, due to activation of FGF signaling, remain unclear. In this study, we showed that activation of the FGF signaling pathway induced the formation of complete head structures through the expression of chd and dickkopf-1b (dkk1b). Using the organizer-deficient maternal mutant, ichabod, we identified dkk1b as a novel downstream factor in the FGF signaling pathway. We also demonstrate that dkk1b expression is necessary, after activation of the FGF signaling pathway, to induce neuroectoderm patterning along the anteroposterior (AP) axis and for formation of complete head structures. Co-injection of chd and dkk1b mRNA resulted in the formation of a dorsal axis with a complete head structure in ichabod embryos, confirming the role of these factors in this developmental process. Unexpectedly, we found that chd induced dkk1b expression in ichabod embryos at the shield stage. However, chd failed to maintain dkk1b expression levels in cells of the shield and, subsequently, in the cells of the prechordal plate after mid-gastrula stage. In contrast, activation of the FGF signaling pathway maintained the dkk1b expression from the beginning of gastrulation to early somitogenesis. In conclusion, activation of the FGF signaling pathway induces the formation of a dorsal axis with a complete head structure through the expression of chd and subsequent maintenance of dkk1b expression levels.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

The increase in maternal expression of axin1 and axin2 contribute to the zebrafish mutant ichabod ventralized phenotype

β-Catenin is a central effector of the Wnt pathway and one of the players in Ca(+)-dependent cell-cell adhesion. While many wnts are present and expressed in vertebrates, only one β-catenin exists in the majority of the organisms. One intriguing exception is zebrafish that carries two genes for β-catenin. The maternal recessive mutation ichabod presents very low levels of β-catenin2 that in turn affects dorsal axis formation, suggesting that β-catenin1 is incapable to compensate for β-catenin2 loss and raising the question of whether these two β-catenins may have differential roles during early axis specification. Here we identify a specific antibody that can discriminate selectively for β-catenin1. By confocal co-immunofluorescent analysis and low concentration gain-of-function experiments, we show that β-catenin1 and 2 behave in similar modes in dorsal axis induction and cellular localization. Surprisingly, we also found that in the ich embryo the mRNAs of the components of β-catenin regulatory pathway, including β-catenin1, are more abundant than in the Wt embryo. Increased levels of β-catenin1 are found at the membrane level but not in the nuclei till high stage. Finally, we present evidence that β-catenin1 cannot revert the ich phenotype because it may be under the control of a GSK3β-independent mechanism that required Axin's RGS domain function.

Mutations in DDX3X Are a Common Cause of Unexplained Intellectual Disability with Gender-Specific Effects on Wnt Signaling

Intellectual disability (ID) affects approximately 1%-3% of humans with a gender bias toward males. Previous studies have identified mutations in more than 100 genes on the X chromosome in males with ID, but there is less evidence for de novo mutations on the X chromosome causing ID in females. In this study we present 35 unique deleterious de novo mutations in DDX3X identified by whole exome sequencing in 38 females with ID and various other features including hypotonia, movement disorders, behavior problems, corpus callosum hypoplasia, and epilepsy. Based on our findings, mutations in DDX3X are one of the more common causes of ID, accounting for 1%-3% of unexplained ID in females. Although no de novo DDX3X mutations were identified in males, we present three families with segregating missense mutations in DDX3X, suggestive of an X-linked recessive inheritance pattern. In these families, all males with the DDX3X variant had ID, whereas carrier females were unaffected. To explore the pathogenic mechanisms accounting for the differences in disease transmission and phenotype between affected females and affected males with DDX3X missense variants, we used canonical Wnt defects in zebrafish as a surrogate measure of DDX3X function in vivo. We demonstrate a consistent loss-of-function effect of all tested de novo mutations on the Wnt pathway, and we further show a differential effect by gender. The differential activity possibly reflects a dose-dependent effect of DDX3X expression in the context of functional mosaic females versus one-copy males, which reflects the complex biological nature of DDX3X mutations.

microRNA regulation of Wnt signaling pathways in development and disease

Wnt signaling pathways and microRNAs (miRNAs) are critical regulators of development. Aberrant Wnt signaling pathways and miRNA levels lead to developmental defects and diverse human pathologies including but not limited to cancer. Wnt signaling pathways regulate a plethora of cellular processes during embryonic development and maintain homeostasis of adult tissues. A majority of Wnt signaling components are regulated by miRNAs which are small noncoding RNAs that are expressed in both animals and plants. In animal cells, miRNAs fine tune gene expression by pairing primarily to the 3'untranslated region of protein coding mRNAs to repress target mRNA translation and/or induce target degradation. miRNA-mediated regulation of signaling transduction pathways is important in modulating dose-sensitive response of cells to signaling molecules. This review discusses components of the Wnt signaling pathways that are regulated by miRNAs in the context of development and diseases. A fundamental understanding of miRNA functions in Wnt signaling transduction pathways may yield new insight into crosstalks of regulatory mechanisms essential for development and disease pathophysiology leading to novel therapeutics.

Centrosome-Associated Degradation Limits β-Catenin Inheritance by Daughter Cells after Asymmetric Division

Caenorhabditis elegans embryos rapidly diversify cell fate using a modified Wnt/β-catenin signaling strategy to carry out serial asymmetric cell divisions (ACDs). Wnt-dependent ACDs rely on nuclear asymmetry of the transcriptional coactivator SYS-1/β-catenin between daughter cells to differentially activate Wnt-responsive target genes. Here, we investigate how dynamic localization of SYS-1 to mitotic centrosomes influences SYS-1 inheritance in daughter cells and cell-fate outcomes after ACD. Through yeast two-hybrid screening, we identify the centrosomal protein RSA-2 as a SYS-1 binding partner and show that localization of SYS-1 to mitotic centrosomes is dependent on RSA-2. Uncoupling SYS-1 from the centrosome by RSA-2 depletion increases SYS-1 inheritance after ACD and promotes Wnt-dependent cell fate. Photobleaching experiments reveal that centrosome-bound SYS-1 turns over rapidly. Interestingly, disruption of the proteasome leads to an increased accumulation of SYS-1 at the centrosome but disrupts its dynamic turnover. We conclude that centrosomal targeting of SYS-1 promotes its degradation during asymmetric cell division. We propose a model whereby centrosome-associated SYS-1 degradation couples negative regulation with cell-division timing to facilitate SYS-1 clearance from the mother cell at the time of asymmetric division. Based on our observations of centrosomal SYS-1 dynamics, we discuss the possibility that the centrosome may coordinate various cell-cycle-dependent processes by synchronizing mitosis and protein regulation.

Transgenic fish systems and their application in ecotoxicology

The use of transgenics in fish is a relatively recent development for advancing understanding of genetic mechanisms and developmental processes, improving aquaculture, and for pharmaceutical discovery. Transgenic fish have also been applied in ecotoxicology where they have the potential to provide more advanced and integrated systems for assessing health impacts of chemicals. The zebrafish (Daniorerio) is the most popular fish for transgenic models, for reasons including their high fecundity, transparency of their embryos, rapid organogenesis and availability of extensive genetic resources. The most commonly used technique for producing transgenic zebrafish is via microinjection of transgenes into fertilized eggs. Transposon and meganuclease have become the most reliable methods for insertion of the genetic construct in the production of stable transgenic fish lines. The GAL4-UAS system, where GAL4 is placed under the control of a desired promoter and UAS is fused with a fluorescent marker, has greatly enhanced model development for studies in ecotoxicology. Transgenic fish have been developed to study for the effects of heavy metal toxicity (via heat-shock protein genes), oxidative stress (via an electrophile-responsive element), for various organic chemicals acting through the aryl hydrocarbon receptor, thyroid and glucocorticoid response pathways, and estrogenicity. These models vary in their sensitivity with only very few able to detect responses for environmentally relevant exposures. Nevertheless, the potential of these systems for analyses of chemical effects in real time and across multiple targets in intact organisms is considerable. Here we illustrate the techniques used for generating transgenic zebrafish and assess progress in the development and application of transgenic fish (principally zebrafish) for studies in environmental toxicology. We further provide a viewpoint on future development opportunities.

Simplet/Fam53b is required for Wnt signal transduction by regulating β-catenin nuclear localization

Canonical β-catenin-dependent Wnt signal transduction is important for several biological phenomena, such as cell fate determination, cell proliferation, stem cell maintenance and anterior-posterior axis formation. The hallmark of canonical Wnt signaling is the translocation of β-catenin into the nucleus where it activates gene transcription. However, the mechanisms regulating β-catenin nuclear localization are poorly understood. We show that Simplet/Fam53B (Smp) is required for Wnt signaling by positively regulating β-catenin nuclear localization. In the zebrafish embryo, the loss of smp blocks the activity of two β-catenin-dependent reporters and the expression of Wnt target genes, and prevents nuclear accumulation of β-catenin. Conversely, overexpression of smp increases β-catenin nuclear localization and transcriptional activity in vitro and in vivo. Expression of mutant Smp proteins lacking either the nuclear localization signal or the β-catenin interaction domain reveal that the translocation of Smp into the nucleus is essential for β-catenin nuclear localization and Wnt signaling in vivo. We also provide evidence that mammalian Smp is involved in regulating β-catenin nuclear localization: the protein colocalizes with β-catenin-dependent gene expression in mouse intestinal crypts; siRNA knockdown of Smp reduces β-catenin nuclear localization and transcriptional activity; human SMP mediates β-catenin transcriptional activity in a dose-dependent manner; and the human SMP protein interacts with human β-catenin primarily in the nucleus. Thus, our findings identify the evolutionary conserved SMP protein as a regulator of β-catenin-dependent Wnt signal transduction.

Calcium signaling in extraembryonic domains during early teleost development

It is becoming recognized that the extraembryonic domains of developing vertebrates, that is, those that make no cellular contribution to the embryo proper, act as important signaling centers that induce and pattern the germ layers and help establish the key embryonic axes. In the embryos of teleost fish, in particular, significant progress has been made in understanding how signaling activity in extraembryonic domains, such as the enveloping layer, the yolk syncytial layer, and the yolk cell, might help regulate development via a combination of inductive interactions, cellular dynamics, and localized gene expression. Ca(2+) signaling in a variety of forms that include propagating waves and standing gradients is a feature found in all three teleostean extraembryonic domains. This leads us to propose that in addition to their other well-characterized signaling activities, extraembryonic domains are well suited (due to their relative stability and continuity) to act as Ca(2+) signaling centers and conduits.

GRG5/AES interacts with T-cell factor 4 (TCF4) and downregulates Wnt signaling in human cells and zebrafish embryos

Transcriptional control by TCF/LEF proteins is crucial in key developmental processes such as embryo polarity, tissue architecture and cell fate determination. TCFs associate with β-catenin to activate transcription in the presence of Wnt signaling, but in its absence act as repressors together with Groucho-family proteins (GRGs). TCF4 is critical in vertebrate intestinal epithelium, where TCF4-β-catenin complexes are necessary for the maintenance of a proliferative compartment, and their abnormal formation initiates tumorigenesis. However, the extent of TCF4-GRG complexes' roles in development and the mechanisms by which they repress transcription are not completely understood. Here we characterize the interaction between TCF4 and GRG5/AES, a Groucho family member whose functional relationship with TCFs has been controversial. We map the core GRG interaction region in TCF4 to a 111-amino acid fragment and show that, in contrast to other GRGs, GRG5/AES-binding specifically depends on a 4-amino acid motif (LVPQ) present only in TCF3 and some TCF4 isoforms. We further demonstrate that GRG5/AES represses Wnt-mediated transcription both in human cells and zebrafish embryos. Importantly, we provide the first evidence of an inherent repressive function of GRG5/AES in dorsal-ventral patterning during early zebrafish embryogenesis. These results improve our understanding of TCF-GRG interactions, have significant implications for models of transcriptional repression by TCF-GRG complexes, and lay the groundwork for in depth direct assessment of the potential role of Groucho-family proteins in both normal and abnormal development.

Dimerized glycosaminoglycan chains increase FGF signaling during zebrafish development

Proteoglycans (PGs) modulate numerous signaling pathways during development through binding of their glycosaminoglycan (GAG) side chains to various signaling molecules, including fibroblast growth factors (FGFs). A majority of PGs possess two or more GAG side chains, suggesting that GAG multivalency is imperative for biological functions in vivo. However, only a few studies have examined the biological significance of GAG multivalency. In this report, we utilized a library of bis- and tris-xylosides that produce two and three GAG chains on the same scaffold, respectively, thus mimicking PGs, to examine the importance of GAG valency and chain type in regulating FGF/FGFR interactions in vivo in zebrafish. A number of bis- and tris-xylosides, but not mono-xylosides, caused an elongation phenotype upon their injection into embryos. In situ hybridization showed that elongated embryos have elevated expression of the FGF target gene mkp3 but unchanged expression of reporters for other pathways, indicating that FGF/FGFR signaling was specifically hyperactivated. In support of this observation, elongation can be reversed by the tyrosine kinase inhibitor SU5402, mRNA for the FGFR antagonist sprouty4, or FGF8 morpholino. Endogenous GAGs seem to be unaffected after xyloside treatment, suggesting that this is a gain-of-function phenotype. Furthermore, expression of a multivalent but not a monovalent GAG containing syndecan-1 proteoglycan recapitulates the elongation phenotype observed with the bivalent xylosides. On the basis of these in vivo findings, we propose a new model for GAG/FGF/FGFR interactions in which dimerized GAG chains can activate FGF-mediated signal transduction pathways.

Medaka embryonic stem cells are capable of generating entire organs and embryo-like miniatures

Embryonic stem (ES) cells have the potency to produce many cell types of the embryo and adult body. Upon transplantation into early host embryos, ES cells are able to differentiate into various specialized cells and contribute to host tissues and organs of all germ layers. Here we present data in the fish medaka (Oryzias latipes) that ES cells have a novel ability to form extra organs and even embryo-like miniatures. Upon transplantation as individual cells according to the standard procedure, ES cells distributed widely to various organ systems of 3 germ layers. Upon transplantation as aggregates, ES cells were able to form extra organs, including the hematopoietic organ and contracting heart. We show that localized ES cell transplantation often led to the formation of extra axes that comprised essentially of either host cells or donor ES cells. These extra axes were associated with the head region of the embryo proper or formed at ectopic sites on the yolk sac. Surprisingly, certain ectopic axes were even capable of forming embryo-like miniatures. We conclude that ES cells have the ability to form entire organs and even embryo-like miniatures under proper environmental conditions. This finding points to a new possibility to generate ES cell-derived axes and organs.

Mesogenin causes embryonic mesoderm progenitors to differentiate during development of zebrafish tail somites

The molecular mechanism underlying somite development differs along the embryonic antero-posterior axis. In zebrafish, cell lineage tracing and genetic analysis have revealed a difference in somite development between the trunk and tail. For instance, spadetail/tbx16 (spt) mutant embryos lack trunk somites but not tail ones. Trunk and tail somites are developed from mesodermal progenitor cells (MPCs) located in the tailbud. While the undifferentiated state of MPCs is maintained by mutual activation between Wnt and Brachyury/Ntl, the mechanism by which the MPCs differentiate into presomitic mesoderm (PSM) cells remains largely unclear. Especially, the molecules that promote PSM differentiation during tail development should be clarified. Here, we show that zebrafish embryos defective in mesogenin1 (msgn1) and spt failed to differentiate into PSM cells in tail development and show increased expression of wnt8 and ntl. Msgn1 acted in a cell-autonomous manner and as a transcriptional activator in PSM differentiation. The expression of msgn1 initially overlapped with that of ntl in the ventral tailbud, as previously reported; and its mis-expression caused ectopic expression of tbx24, a PSM marker gene, only in the tailbud and posterior notochord, both of which expressed ntl in zebrafish embryos. Furthermore, the PSM-inducing activity of misexpressed msgn1 was enhanced by co-expression with ntl. Thus, Msgn1 exercised its PSM-inducing activity in cells expressing ntl. Based on these results, we speculate that msgn1 expression in association with that of ntl may allow the differentiation of progenitor cells to proceed during development of somites in the tail.

2-O-sulfotransferase regulates Wnt signaling, cell adhesion and cell cycle during zebrafish epiboly

O-sulfotransferases modify heparan sulfate proteoglycans (HSPGs) by catalyzing the transfer of a sulfate to a specific position on heparan sulfate glycosaminoglycan (GAG) chains. Although the roles of specific HSPG modifications have been described in cell culture and invertebrates, little is known about their functions or abilities to modulate specific cell signaling pathways in vertebrate development. Here, we report that 2-O-sulfotransferase (2-OST) is an essential component of canonical Wnt signaling in zebrafish development. 2-OST-deficient embryos have reduced GAG chain sulfation and are refractory to exogenous Wnt8 overexpression. Embryos in which maternally encoded 2-OST is knocked down have normal activation of several zygotic mesoderm, endoderm and ectoderm patterning genes, but have decreased deep cell adhesion and fail to initiate epiboly, which can be rescued by re-expression of 2-OST protein. Reduced cell adhesion and altered cell cycle regulation in 2-OST-deficient embryos are associated with decreased β-catenin and E-cadherin protein levels at cell junctions, and these defects can be rescued by reactivation of the intracellular Wnt pathway, utilizing stabilized β-catenin or dominant-negative Gsk3, but not by overexpression of Wnt8 ligand. Together, these results indicate that 2-OST functions within the Wnt pathway, downstream of Wnt ligand signaling and upstream of Gsk3β and β-catenin intracellular localization and function.

The transcriptional mediator component Med12 is required for hindbrain boundary formation

Background

Rhombomere boundaries form during hindbrain segmentation and are critical for maintaining segmental integrity and regulating migration in the hindbrain. Some genetic models affecting hindbrain boundary formation have been described, but involvement of components of the transcriptional mediator complex in boundary formation has not reported so far.

Principal Findings

The kto/med12 mutant zebrafish, which affects the Mediator component Med12, causes specific loss of rhombomere boundary cells even though segmentation of the hindbrain takes place at least in part. In kto mutant embryos, cells forming rhombomere boundaries were largely absent as indicated by the use of several marker genes. While no obvious increase in cell death was observed, we found a notable reduction of cell proliferation in the hindbrain of kto mutant zebrafish.

Conclusions

The kto/med12 mutation results in specific defects of boundary cell formation in the zebrafish hindbrain.

Sonic hedgehog (Shh)-Gli signaling controls neural progenitor cell division in the developing tectum in zebrafish

Despite considerable progress, the mechanisms that control neural progenitor differentiation and behavior, as well as their functional integration into adult neural circuitry, are far from being understood. Given the complexity of the mammalian brain, non-mammalian models provide an excellent model to study neurogenesis, including both the cellular composition of the neurogenic microenvironment, and the factors required for precursor growth and maintenance. In particular, we chose to address the question of the control of progenitor proliferation by Sonic hedgehog (Shh) using the zebrafish dorsal mesencephalon, known as the optic tectum (OT), as a model system. Here we show that either inhibiting pharmacologically or eliminating hedgehog (Hh) signaling by using mutants that lack essential components of the Hh pathway reduces neural progenitor cell proliferation affecting neurogenesis in the OT. On the contrary, pharmacological gain-of-function experiments result in significant increase in proliferation. Importantly, Shh-dependent function controls neural progenitor cell behavior as sox2-positive cell populations were lost in the OT in the absence of Hh signaling, as evidenced in slow-muscle-omitted (smu) mutants and with timed cyclopamine inhibition. Expressions of essential components of the Hh pathway reveal for the first time a late dorsal expression in the embryonic OT. Our observations argue strongly for a role of Shh in neural progenitor biology in the OT and provide comparative data to our current understanding of progenitor/stem cell mechanisms that place Shh as a key niche factor in the dorsal brain.

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.

Induction of otic structures by canonical Wnt signalling in medaka

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

Noncanonical Wnt signaling in tumor progression and metastasis

For almost 15 years, the concept that noncanonical (beta-catenin-independent) Wnt signaling pathways play key roles in embryonic development has grown steadily in the scientific literature. Significant progress has been made toward understanding how these pathways regulate morphogenetic processes as diverse as gastrulation cell movements and the formation of cilia. More recently, however, data have implicated components of noncanonical Wnt/Ca(2+) and Wnt/planar cell polarity signaling in directly promoting the invasiveness and malignant progression of diverse forms of human cancer. Here I review this emerging field of cancer research using data from developmental model systems to provide a framework for addressing future questions.

A Wnt survival guide: from flies to human disease

It has been two decades since investigators discovered the link between the Drosophila wingless (Wg) gene and the vertebrate oncogene int-1, thus establishing the family of signaling proteins known as Wnts. Since the inception of the Wnt signaling field, there have been 19 Wnt isoforms identified in humans. These secreted glycoproteins can activate at least two distinct signaling pathways in vertebrate cells, leading to cellular changes that regulate a vast array of biological processes, including embryonic development, cell fate, cell proliferation, cell migration, stem cell maintenance, tumor suppression, and oncogenesis. In certain contexts, one subset of Wnt isoforms activates the canonical Wnt/beta-catenin pathway that is characterized by the activation of certain beta-catenin-responsive target genes in response to the binding of Wnt ligand to its cognate receptors. Similarly, a second subset of Wnt isoforms activates beta-catenin-independent pathways, including the Wnt/calcium (Wnt/Ca) pathway and the Wnt/planar cell polarity (Wnt/PCP) pathway, in certain cellular contexts. In addition, research has identified several secreted proteins known to regulate Wnt signaling, including the Dickkopf (DKK) family, secreted Frizzled-related proteins (sFRPs), and Wnt inhibitory factor-1 (WIF-1). The advent of technologies that can provide genome-wide expression data continues to implicate Wnts and proteins that regulate Wnt signaling pathways in a growing number of disease processes. The aim of this review is to provide a context on the Wnt field that will facilitate the interpretation and study of Wnt signaling in the context of human disease.

Characterization and comparison of intercellular adherent junctions expressed by human corneal endothelial cells in vivo and in vitro

PURPOSE

Human corneal endothelial cell (HCEC) proliferation is controlled by HCEC junctions, but the mechanism of proliferation remains unknown. The authors sought to characterize adherent junction components of in vivo HCECs and to compare their gene expression and their proliferative potential with those of in vitro counterparts.

METHODS

Stripped human Descemet membranes were digested with collagenase A, and the resultant HCEC aggregates were cultured for 7, 14, and 21 days in supplemented hormonal epithelial medium (SHEM). The growth of HCEC monolayers was monitored by BrdU labeling performed 24 hours before termination. In vivo and in vitro HCECs were subjected to immunostaining to FITC-phalloidin and antibodies to different junction components and BrdU. Their mRNA expressions were determined by RT-PCR.

RESULTS

In vivo HCECs expressed transcripts of N-, VE-, E-, and P-cadherins, alpha-, beta-, gamma-, and p120-catenins, and p190. In vitro HCEC counterparts also expressed all these mRNAs except P-cadherin. In vivo HCECs displayed continuous circular F-actin, N-cadherin, beta- and p120-catenins, and p190, discontinuous circular VE-cadherin bands at or close to cell junctions, and E-cadherin in the cytoplasm. Such an in vivo pattern was gradually achieved by in vitro HCECs at day 21 and was correlated with a progressive decline of BrdU labeling.

CONCLUSIONS

In vivo and in vitro HCECs displayed distinct protein cytolocalization of N-, VE-, and E-cadherins, beta- and p120-catenins, and p190. Progressive maturation of adherent junctions was associated with a decline of the proliferative potential. This information allows us to devise new strategies to engineer in vitro HCECs by targeting these components.

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.

Cystic kidney gene seahorse regulates cilia-mediated processes and Wnt pathways

Recently the cilium has emerged as an important sensory organelle for a wide range of cell types in vertebrates. However, the signaling cascade that links ciliary signals to cellular events remains poorly understood. Here, we show that the zebrafish cystic kidney gene seahorse is closely associated with ciliary functions: seahorse is required for establishing left-right asymmetry and for preventing kidney cyst formation; seahorse transcript is highly enriched in heavily ciliated tissues; and seahorse genetically interacts with the ciliary gene inversin. Yet seahorse is dispensable for cilia assembly or motility and the Seahorse protein is cytoplasmic. We provide evidence that Seahorse associates with Dishevelled. Finally, we show that seahorse constrains the canonical Wnt pathway and promotes the noncanonical Wnt pathway during gastrulation. Together, these data suggest that Seahorse may provide a link between ciliary signals and Wnt pathways.

Novel genes involved in canonical Wnt/beta-catenin signaling pathway in early Ciona intestinalis embryos

We report here characterization of five genes for novel components of the canonical Wnt/beta-catenin signaling pathway. These genes were identified in the ascidian Ciona intestinalis through a loss-of-function screening for genes required for embryogenesis with morpholinos, and four of them have counterparts in vertebrates. The five genes we studied are as follows: Ci-PGAP1, a Ciona orthologue of human PGAP1, which encodes GPI (glycosylphosphatidylinositol) inositol-deacylase, Ci-ZF278, a gene encoding a C2H2 zinc-finger protein, Ci-C10orf11, a Ciona orthologue of human C10orf11 that encodes a protein with leucine-rich repeats, Ci-Spatial/C4orf17, a single counterpart for two human genes Spatial and C4orf17, and Ci-FLJ10634, a Ciona orthologue of human FLJ10634 that encodes a member of the J-protein family. Knockdown of each of the genes mimicked beta-catenin knockdown and resulted in suppression of the expression of beta-catenin downstream genes (Ci-FoxD, Ci-Lhx3, Ci-Otx and Ci-Fgf9/16/20) and subsequent endoderm formation. For every gene, defects in knockdown embryos were rescued by overexpression of a constitutively active form, but not wild-type, of Ci-beta-catenin. Dosage-sensitive interactions were found between Ci-beta-catenin and each of the genes. These results suggest that these five genes act upstream of or parallel to Ci-beta-catenin in the Wnt/beta-catenin signaling pathway in early Ciona embryos.

Beta-catenin is required for the establishment of vegetal embryonic fates in the nemertean, Cerebratulus lacteus

Downstream components of the canonical Wnt signaling pathway that result in the nuclear localization of beta-catenin are involved in diverse developmental processes including the formation of the mesendoderm, the regulation of axial properties and asymmetric cell divisions in a wide array of metazoans. The nemertean worm, Cerebratulus lacteus, represents a member of the understudied lophotrochozoan clade that exhibits a highly stereotyped spiral cleavage program in which ectodermal, endodermal, and mesodermal origins are known from intracellular fate mapping studies. Here, the embryonic distribution of beta-catenin protein was studied using injection of synthetic mRNA, encoding GFP-tagged beta-catenin, into fertilized eggs. During the early cleavage stages beta-catenin was destabilized/degraded in animal hemisphere blastomeres and became localized to the nuclei of the four vegetal-most cells at the 64-cell stage, which give rise to definitive larval and adult endoderm. Functional assays indicate that beta-catenin plays a key role in the development of the endoderm. Morpholino knockdown of endogenous beta-catenin, as confirmed by Western analysis, resulted in the failure to gastrulate, absence of the gut and an animalized phenotype in the resulting larvae, including the formation of ectopic (anterior) apical organ tissue with elongated apical tuft cilia and no indications of dorsoventral polarity. Similarly, over-expression of the cytoplasmic domain of cadherin or a beta-catenin-engrailed repressor fusion construct prevented endoderm formation and generated the same animalized phenotype. Injections of mRNA encoding either a stabilized, constitutively activated form of beta-catenin or a dominant negative form of GSK3-beta converted all or nearly all cells into endodermal fates expressing gut-specific esterase. Thus, beta-catenin appears to be both necessary and sufficient to promote endoderm formation in C. lacteus, consistent with its role in endoderm and endomesoderm formation in anthozoan cnidarians, ascidians, and echinoderms. Consistent with the results of other studies, beta-catenin may be viewed as playing a role in the development of posterior/vegetal larval fates (i.e., endoderm) in C. lacteus. However, unlike the case found in polychaete annelid and soil nematode embryos, there is no evidence for a role of beta-catenin in regulating cell fates and asymmetric cell divisions along the entire anterior-posterior axis.

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.

The role of maternal Activin-like signals in zebrafish embryos

Maternal Activin-like proteins, a subgroup of the TGF-beta superfamily, play a key role in establishing the body axes in many vertebrates, but their role in teleosts is unclear. At least two maternal Activin-like proteins are expressed in zebrafish, including the Vg1 orthologue, zDVR-1, and the nodal-related gene, Squint. Our analysis of embryos lacking both maternal and zygotic squint function revealed that maternal squint is required in some genetic backgrounds for the formation of dorsal and anterior tissues. Conditional inactivation of the ALK4, 5 and 7 receptors by SB-505124 treatment during the cleavage stages ruled out a role for maternal Squint, zDVR-1, or other Activin-like ligands before the mid-blastula transition, when the dorsal axis is established. Furthermore, we show that maternal Squint and zDVR-1 are not required during the cleavage stages to induce zygotic nodal-related gene expression. nodal-related gene expression decreases when receptor inhibition continues past the mid-blastula transition, resulting in a progressive loss of mesoderm and endoderm. We conclude that maternally expressed Activin-like signals do not act before the mid-blastula transition in zebrafish, but do have a variably penetrant role in the later stages of axis formation. This contrasts with the early role for these signals during Xenopus development.

Association of Dishevelled with the clathrin AP-2 adaptor is required for Frizzled endocytosis and planar cell polarity signaling

Upon activation by Wnt, the Frizzled receptor is internalized in a process that requires the recruitment of Dishevelled. We describe a novel interaction between Dishevelled2 (Dvl2) and micro2-adaptin, a subunit of the clathrin adaptor AP-2; this interaction is required to engage activated Frizzled4 with the endocytic machinery and for its internalization. The interaction of Dvl2 with AP-2 requires simultaneous association of the DEP domain and a peptide YHEL motif within Dvl2 with the C terminus of micro2. Dvl2 mutants in the YHEL motif fail to associate with micro2 and AP-2, and prevent Frizzled4 internalization. Corresponding Xenopus Dishevelled mutants show compromised ability to interfere with gastrulation mediated by the planar cell polarity (PCP) pathway. Conversely, a Dvl2 mutant in its DEP domain impaired in PCP signaling exhibits defective AP-2 interaction and prevents the internalization of Frizzled4. We suggest that the direct interaction of Dvl2 with AP-2 is important for Frizzled internalization and Frizzled/PCP signaling.

Specification of primordial germ cells in medaka (Oryzias latipes)

BACKGROUND

Primordial germ cells (PGCs) give rise to gametes that are responsible for the development of a new organism in the next generation. Two modes of germ line specification have been described: the inheritance of asymmetrically-localized maternally provided cytoplasmic determinants and the induction of the PGC fate by other cell types. PGCs specification in zebrafish appears to depend on inheritance of germ plasm in which several RNA molecules such as vasa and nanos reside. Whether the specification mode of PGCs found in zebrafish is general for other fish species was brought into question upon analysis of olvas expression--the vasa homologue in another teleost, medaka (Oryzias latipes). Here, in contrast to the findings in zebrafish, the PGCs are found in a predictable position relative to a somatic structure, the embryonic shield. This finding, coupled with the fact that vasa mRNA, which is localized to the germ plasm of zebrafish but does not label a similar structure in medaka opened the possibility of fundamentally different mechanisms governing PGC specification in these two fish species.

RESULTS

In this study we addressed the question concerning the mode of PGC specification in medaka using embryological experiments, analysis of RNA stability in the PGCs and electron microscopy observations. Dramatic alterations in the somatic environment, i.e. induction of a secondary axis or mesoderm formation alteration, did not affect the PGC number. Furthermore, the PGCs of medaka are capable of protecting specific RNA molecules from degradation and could therefore exhibit a specific mRNA expression pattern controlled by posttrancriptional mechanisms. Subsequent analysis of 4-cell stage medaka embryos using electron microscopy revealed germ plasm-like structures located at a region corresponding to that of zebrafish germ plasm.

CONCLUSION

Taken together, these results are consistent with the idea that in medaka the inheritance of maternally provided asymmetrically-localized cytoplasmic determinants directs cells to assume the germ line fate similar to zebrafish PGCs.

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.

The Emergence of Pattern in Embryogenesis: Regulation of beta-Catenin Localization During Early Sea Urchin Development

The accumulation of beta-catenin in the nuclei of blastomeres at one pole of the early embryo is a highly conserved and essential feature of animal development. In the sea urchin, beta-catenin accumulates in the nuclei of vegetal blastomeres during early cleavage and activates gene regulatory networks that drive mesoderm and endoderm formation. Measurements of beta-catenin half-life in vivo have demonstrated a gradient in stability along the animal-vegetal axis. Dishevelled (Dsh), a protein that regulates beta-catenin turnover, is localized in the vegetal cortex, where it has an essential role in stabilizing beta-catenin and activating endomesodermal gene networks. Two motifs of Dsh are required for targeting to the vegetal cortex. Overexpression of Dsh in animal blastomeres does not alter their fate, which suggests that a localized activator of Dsh may be missing in these cells. Wnt signaling may be localized in the early sea urchin embryo, as it is in Xenopus, but findings point to possible differences in the initial polarizing signal in amphibians and echinoderms. Further studies will be required to determine the extent to which mechanisms that control beta-catenin nuclearization in early embryogenesis have been conserved during animal evolution.

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.

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.

Beta-catenin signaling marks the prospective site of primitive streak formation in the mouse embryo

Beta-catenin signaling has been shown to be involved in triggering axis formation in several organisms, including Xenopus and zebrafish. Genetic analysis has demonstrated that the Wnt/beta-catenin signaling pathway is also involved in axis formation in the mouse, since a targeted deletion of beta-catenin results in embryos that have a block in anterior-posterior axis formation, fail to initiate gastrulation, and do not form mesoderm. However, because beta-catenin is ubiquitously expressed, the precise time and cell types in which this signaling pathway is active during early embryonic development remain unknown. Thus, to better understand the role of the Wnt/beta-catenin signaling pathway in axis formation and mesoderm specification, we have examined both the distribution and signaling activity of beta-catenin during early embryonic development in the mouse. We show that the N-terminally nonphosphorylated form of beta-catenin as well as beta-catenin signaling is first detectable in the extraembryonic visceral endoderm in day 5.5 embryos. Before the initiation of gastrulation at day 6.0, beta-catenin signaling is asymmetrically distributed within the epiblast and is localized to a small group of cells adjacent to the embryonic--extraembryonic junction. At day 6.5 and onward, beta-catenin signaling was detected in the primitive streak and mature node. Thus, beta-catenin signaling precedes primitive streak formation and is present in epiblast cells that will go on to form the primitive streak. These results support a critical role for the Wnt/beta-catenin pathway in specifying cells to form the primitive streak and node in the mammalian embryo as well as identify a novel domain of Wnt/beta-catenin signaling activity during early embryogenesis.

Inhibition of the Beta-Catenin Signaling Pathway in Blastocyst and Uterus During the Window of Implantation in Mice1

Beta-catenin, the mammalian homolog of Drosophila armadillo protein, was first identified as a cadherin-associated protein at cell-cell junctions. Another function of beta-catenin is the transduction of cytosolic signals to the nucleus in a variety of cellular contexts, which usually are elicited by the active form of beta-catenin. The aim of the present study was to examine the potential role of active beta-catenin in the mouse embryo and uterus during embryo implantation. Active beta-catenin was detected differentially in mouse embryos and uteri during the peri-implantation period. Aberrant activation of beta-catenin by LiCl, a well-known glycogen synthase kinase-3 inhibitor, significantly inhibited blastocyst hatching and subsequent adhesion and outgrowth on fibronectin. Results obtained from pseudopregnant and implantation-delayed mice imply an important role for implanting blastocysts in the temporal and spatial changes of active beta-catenin in the uterus during the window of implantation. Collectively, these results suggest that the beta-catenin signaling pathway is inhibited in both blastocyst and uterus during the window of implantation, which may represent a new mechanism to synchronize the development of preimplantation embryos and differentiation of the uterus during this process.

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.