Xwnt-11: a maternally expressed Xenopus wnt gene

Dorsal lip maturation and initial archenteron extension depend on Wnt11 family ligands

Wnt11 family proteins are ligands that activate a type of Dishevelled-mediated, non-canonical Wnt signaling pathway. Loss of function causes defects in gastrulation and/or anterior-posterior axis extension in all vertebrates. Non-mammalian vertebrate genomes encode two Wnt11 family proteins whose distinct functions have been unclear. We knocked down Wnt11b and Wnt11, separately and together, in Xenopus laevis. Single morphants exhibited very similar phenotypes of delayed blastopore closure, but they had different phenotypes during the tailbud period. In response to their very similar gastrulation phenotypes, we chose to characterize dual morphants. Using dark field illuminated time-lapse imaging and kymograph analysis, we identified a failure of dorsal blastopore lip maturation that correlated with slower blastopore closure and failure to internalize the endoderm at the dorsal blastopore lip. We connected these externally visible phenotypes to cellular events in the internal tissues by imaging intact fixed embryos stained for anillin and microtubules. We found that the initial extension of the archenteron is correlated with blastopore lip maturation, and archenteron extension is dramatically disrupted by decreased Wnt11 family signaling. We were aided in our interpretation of the immunofluorescence by the novel, membrane proximal location of the cleavage furrow protein anillin in the epithelium of the blastopore lip and early archenteron.

Evolutionary conservation of maternal RNA localization in fishes and amphibians revealed by TOMO-Seq

Asymmetrical localization of biomolecules inside the egg, results in uneven cell division and establishment of many biological processes, cell types and the body plan. However, our knowledge about evolutionary conservation of localized transcripts is still limited to a few models. Our goal was to compare localization profiles along the animal-vegetal axis of mature eggs from four vertebrate models, two amphibians (Xenopus laevis, Ambystoma mexicanum) and two fishes (Acipenser ruthenus, Danio rerio) using the spatial expression method called TOMO-Seq. We revealed that RNAs of many known important transcripts such as germ layer determinants, germ plasm factors and members of key signalling pathways, are localized in completely different profiles among the models. It was also observed that there was a poor correlation between the vegetally localized transcripts but a relatively good correlation between the animally localized transcripts. These findings indicate that the regulation of embryonic development within the animal kingdom is highly diverse and cannot be deduced based on a single model.

Alignment of the cell long axis by unidirectional tension acts cooperatively with Wnt signalling to establish planar cell polarity

Planar cell polarity (PCP) is the aligned cell polarity within a tissue plane. Mechanical signals are known to act as a global cue for PCP, yet their exact role is still unclear. In this study, we focused on PCP in the posterior neuroectoderm of Xenopus laevis and investigated how mechanical signals regulate polarity. We reveal that the neuroectoderm is under a greater tension in the anterior-posterior direction and that perturbation of this tension causes PCP disappearance. We show that application of uniaxial stretch to explant tissues can control the orientation of PCP and that cells sense the tissue stretch indirectly through a change in their shape, rather than directly through detection of anisotropic tension. Furthermore, we reveal that PCP is most strongly established when the orientation of tissue stretch coincides with that of diffusion of locally expressed Wnt ligands, suggesting a cooperative relationship between these two PCP regulators.

Putting in the Erk: Growth factor signaling and mesoderm morphogenesis

It has long been known that FGF signaling contributes to mesoderm formation, a germ layer found in triploblasts that is composed of highly migratory cells that give rise to muscles and to the skeletal structures of vertebrates. FGF signaling activates several pathways in the developing mesoderm, including transient activation of the Erk pathway, which triggers mesodermal fate specification through the induction of the gene brachyury and activates morphogenetic programs that allow mesodermal cells to position themselves in the embryo. In this review, we discuss what is known about the generation and interpretation of transient Erk signaling in mesodermal tissues across species. We focus specifically on mechanisms that translate the level and duration of Erk signaling into cell fate and cell movement instructions and discuss strategies for further interrogating the role that Erk signaling dynamics play in mesodermal gastrulation and morphogenesis.

Massive Gene Loss and Function Shuffling in Appendicularians Stretch the Boundaries of Chordate Wnt Family Evolution

Gene loss is a pervasive source of genetic variation that influences species evolvability, biodiversity and the innovation of evolutionary adaptations. To better understand the evolutionary patterns and impact of gene loss, here we investigate as a case study the evolution of the wingless (Wnt) family in the appendicularian tunicate Oikopleura dioica, an emergent EvoDevo model characterized by its proneness to lose genes among chordates. Genome survey and phylogenetic analyses reveal that only four of the thirteen Wnt subfamilies have survived in O. dioica—Wnt5, Wnt10, Wnt11, and Wnt16,—representing the minimal Wnt repertoire described in chordates. While the loss of Wnt4 and Wnt8 likely occurred in the last common ancestor of tunicates, representing therefore a synapomorphy of this subphylum, the rest of losses occurred during the evolution of appendicularians. This work provides the first complete Wnt developmental expression atlas in a tunicate and the first insights into the evolution of Wnt developmental functions in appendicularians. Our work highlights three main evolutionary patterns of gene loss: (1) conservation of ancestral Wnt expression domains not affected by gene losses; (2) function shuffling among Wnt paralogs accompanied by gene losses; and (3) extinction of Wnt expression in certain embryonic directly correlated with gene losses. Overall our work reveals that in contrast to “conservative” pattern of evolution of cephalochordates and vertebrates, O. dioica shows an even more radical “liberal” evolutionary pattern than that described ascidian tunicates, stretching the boundaries of the malleability of Wnt family evolution in chordates.

Spatio-temporal mRNA tracking in the early zebrafish embryo

Early stages of embryogenesis depend on subcellular localization and transport of maternal mRNA. However, systematic analysis of these processes is hindered by a lack of spatio-temporal information in single-cell RNA sequencing. Here, we combine spatially-resolved transcriptomics and single-cell RNA labeling to perform a spatio-temporal analysis of the transcriptome during early zebrafish development. We measure spatial localization of mRNA molecules within the one-cell stage embryo, which allows us to identify a class of mRNAs that are specifically localized at an extraembryonic position, the vegetal pole. Furthermore, we establish a method for high-throughput single-cell RNA labeling in early zebrafish embryos, which enables us to follow the fate of individual maternal transcripts until gastrulation. This approach reveals that many localized transcripts are specifically transported to the primordial germ cells. Finally, we acquire spatial transcriptomes of two xenopus species and compare evolutionary conservation of localized genes as well as enriched sequence motifs.

Inhibition in growth and cardiotoxicity of tris (2-butoxyethyl) phosphate through down-regulating Wnt signaling pathway in early developmental stage of zebrafish (Danio rerio)

As a common organophosphorus flame retardant, tris (2-butoxyethyl) phosphate (TBOEP) is detected in water environment and aquatic animals extensively. Despite previous researches have reported the developmental toxicity of TBOEP in zebrafish (Danio rerio) larvae, few research focused on its underlying mechanisms. In this study, zebrafish embryos were exposed to 0, 20, 200, 1000 and 2000 µg/L TBOEP from 2 until 120 h post-fertilization (hpf) to determine potential mechanisms of developmental toxicity of this compound. Early developmental stage parameters such as body length, survival rate, hatching rate and heart rate were decreased, while malformation rate was ascended. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) assay was carried out at 12, 24, 72 and 120 hpf to demonstrate alterations in expression of genes of Wnt signaling pathway. The results indicated that axin1 was significantly up-regulated, while β-catenin, pkc and wnt11 were down-regulated. Correlation analysis indicated that expression of these genes was significantly correlated with body length. Furthermore, apoptosis was detected in heart region by acridine orange (AO) staining and terminal deoxynucleotide transferase-mediated deoxy-UTP nick end labeling (TUNEL) assay. In addition, at 120 hpf, occurrence of oxidative stress was observed in zebrafish larvae. Moreover, 6-Bromoindirubin-3'-oxime (BIO)， an activator of Wnt pathway, was found to alleviate the inhibiting effects of TBOEP on zebrafish growth. The overall outcomes offered novel viewpoints in toxic effects of TBOEP, and down-regulating Wnt signaling pathway were able to reveal some potential mechanisms of developmental toxicity of TBOEP in zebrafish larvae.

Early Xenopus gene regulatory programs, chromatin states, and the role of maternal transcription factors

For decades, the early development of the Xenopus embryo has been an essential model system to study the gene regulatory mechanisms that govern cellular specification. At the top of the hierarchy of gene regulatory networks, maternally deposited transcription factors initiate this process and regulate the expression of zygotic genes that give rise to three distinctive germ layer cell types (ectoderm, mesoderm, and endoderm), and subsequent generation of organ precursors. The onset of germ layer specification is also closely coupled with changes associated with chromatin modifications. This review will examine the timing of maternal transcription factors initiating the zygotic genome activation, the epigenetic landscape of embryonic chromatin, and the network structure that governs the process.

Regulation of Translationally Repressed mRNAs in Zebrafish and Mouse Oocytes

From the beginning of oogenesis, oocytes accumulate tens of thousands of mRNAs for promoting oocyte growth and development. A large number of these mRNAs are translationally repressed and localized within the oocyte cytoplasm. Translational activation of these dormant mRNAs at specific sites and timings plays central roles in driving progression of the meiotic cell cycle, axis formation, mitotic cleavages, transcriptional initiation, and morphogenesis. Regulation of the localization and temporal translation of these mRNAs has been shown to rely on cis-acting elements in the mRNAs and trans-acting factors recognizing and binding to the elements. Recently, using model vertebrate zebrafish, localization itself and formation of physiological structures such as RNA granules have been shown to coordinate the accurate timings of translational activation of dormant mRNAs. This subcellular regulation of mRNAs is also utilized in other animals including mouse. In this chapter, we review fundamental roles of temporal regulation of mRNA translation in oogenesis and early development and then focus on the mechanisms of mRNA regulation in the oocyte cytoplasm by which the activation of dormant mRNAs at specific timings is achieved.

A gene regulatory program controlling early Xenopus mesendoderm formation: Network conservation and motifs

Germ layer formation is among the earliest differentiation events in metazoan embryos. In triploblasts, three germ layers are formed, among which the endoderm gives rise to the epithelial lining of the gut tube and associated organs including the liver, pancreas and lungs. In frogs (Xenopus), where early germ layer formation has been studied extensively, the process of endoderm specification involves the interplay of dozens of transcription factors. Here, we review the interactions between these factors, summarized in a transcriptional gene regulatory network (GRN). We highlight regulatory connections conserved between frog, fish, mouse, and human endodermal lineages. Especially prominent is the conserved role and regulatory targets of the Nodal signaling pathway and the T-box transcription factors, Vegt and Eomes. Additionally, we highlight network topologies and motifs, and speculate on their possible roles in development.

Mechanisms of Vertebrate Germ Cell Determination

Two unique characteristics of the germ line are the ability to persist from generation to generation and to retain full developmental potential while differentiating into gametes. How the germ line is specified that allows it to retain these characteristics within the context of a developing embryo remains unknown and is one focus of current research. Germ cell specification proceeds through one of two basic mechanisms: cell autonomous or inductive. Here, we discuss how germ plasm driven germ cell specification (cell autonomous) occurs in both zebrafish and the frog Xenopus. We describe the segregation of germ cells during embryonic development of solitary and colonial ascidians to provide an evolutionary context to both mechanisms. We conclude with a discussion of the inductive mechanism as exemplified by both the mouse and axolotl model systems. Regardless of mechanism, several general themes can be recognized including the essential role of repression and posttranscriptional regulation of gene expression.

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.

Maternal messages to live by: a personal historical perspective

In the 1980s, the study of localized maternal mRNAs was just emerging as a new research area. Classic embryological studies had linked the inheritance of cytoplasmic domains with specific cell lineages, but the underlying molecular nature of these putative determinants remained a mystery. The model system Xenopus would play a pivotal role in the progress of this new field. In fact, the first localized maternal mRNA to be identified and cloned from any organism was Xenopus vg1, a TGF-beta family member. This seminal finding opened the door to many subsequent studies focused on how RNAs are localized and what functions they had in development. As the field moves into the future, Xenopus remains the system of choice for studies identifying RNA/protein transport particles and maternal RNAs through RNA-sequencing.

Wnt proteins can direct planar cell polarity in vertebrate ectoderm

The coordinated orientation of cells across the tissue plane, known as planar cell polarity (PCP), is manifested by the segregation of core PCP proteins to different sides of the cell. Secreted Wnt ligands are involved in many PCP-dependent processes, yet whether they act as polarity cues has been controversial. We show that in Xenopus early ectoderm, the Prickle3/Vangl2 complex was polarized to anterior cell edges and this polarity was disrupted by several Wnt antagonists. In midgastrula embryos, Wnt5a, Wnt11, and Wnt11b, but not Wnt3a, acted across many cell diameters to orient Prickle3/Vangl2 complexes away from their sources regardless of their positions relative to the body axis. The planar polarity of endogenous Vangl2 in the neuroectoderm was similarly redirected by an ectopic Wnt source and disrupted after depletion of Wnt11b in the presumptive posterior region of the embryo. These observations provide evidence for the instructive role of Wnt ligands in vertebrate PCP.

DOI:http://dx.doi.org/10.7554/eLife.16463.001

Molecular specification of germ layers in vertebrate embryos

In order to generate the tissues and organs of a multicellular organism, different cell types have to be generated during embryonic development. The first step in this process of cellular diversification is the formation of the three germ layers: ectoderm, endoderm and mesoderm. The ectoderm gives rise to the nervous system, epidermis and various neural crest-derived tissues, the endoderm goes on to form the gastrointestinal, respiratory and urinary systems as well as many endocrine glands, and the mesoderm will form the notochord, axial skeleton, cartilage, connective tissue, trunk muscles, kidneys and blood. Classic experiments in amphibian embryos revealed the tissue interactions involved in germ layer formation and provided the groundwork for the identification of secreted and intracellular factors involved in this process. We will begin this review by summarising the key findings of those studies. We will then evaluate them in the light of more recent genetic studies that helped clarify which of the previously identified factors are required for germ layer formation in vivo, and to what extent the mechanisms identified in amphibians are conserved across other vertebrate species. Collectively, these studies have started to reveal the gene regulatory network (GRN) underlying vertebrate germ layer specification and we will conclude our review by providing examples how our understanding of this GRN can be employed to differentiate stem cells in a targeted fashion for therapeutic purposes.

Sulf1 has ligand-dependent effects on canonical and non-canonical Wnt signalling

Wnt signalling plays essential roles during embryonic development and is known to be mis-regulated in human disease. There are many molecular mechanisms that ensure tight regulation of Wnt activity. One such regulator is the heparan-sulfate-specific 6-O-endosulfatase Sulf1. Sulf1 acts extracellularly to modify the structure of heparan sulfate chains to affect the bio-availability of Wnt ligands. Sulf1 could, therefore, influence the formation of Wnt signalling complexes to modulate the activation of both canonical and non-canonical pathways. In this study, we use well-established assays in Xenopus to investigate the ability of Sulf1 to modify canonical and non-canonical Wnt signalling. In addition, we model the ability of Sulf1 to influence morphogen gradients using fluorescently tagged Wnt ligands in ectodermal explants. We show that Sulf1 overexpression has ligand-specific effects on Wnt signalling: it affects membrane accumulation and extracellular levels of tagged Wnt8a and Wnt11b ligands differently, and inhibits the activity of canonical Wnt8a but enhances the activity of non-canonical Wnt11b.

The alternative splicing regulator Tra2b is required for somitogenesis and regulates splicing of an inhibitory Wnt11b isoform

Alternative splicing is pervasive in vertebrates, yet little is known about most isoforms or their regulation. transformer-2b (tra2b) encodes a splicing regulator whose endogenous function is poorly understood. Tra2b knockdown in Xenopus results in embryos with multiple defects, including defective somitogenesis. Using RNA sequencing, we identify 142 splice changes (mostly intron retention and exon skipping), 89% of which are not in current annotations. A previously undescribed isoform of wnt11b retains the last intron, resulting in a truncated ligand (Wnt11b-short). We show that this isoform acts as a dominant-negative ligand in cardiac gene induction and pronephric tubule formation. To determine the contribution of Wnt11b-short to the tra2b phenotype, we induce retention of intron 4 in wnt11b, which recapitulates the failure to form somites but not other tra2b morphant defects. This alternative splicing of a Wnt ligand adds intricacy to a complex signaling pathway and highlights intron retention as a regulatory mechanism.

Symmetry breakage in the vertebrate embryo: when does it happen and how does it work?

Asymmetric development of the vertebrate embryo has fascinated embryologists for over a century. Much has been learned since the asymmetric Nodal signaling cascade in the left lateral plate mesoderm was detected, and began to be unraveled over the past decade or two. When and how symmetry is initially broken, however, has remained a matter of debate. Two essentially mutually exclusive models prevail. Cilia-driven leftward flow of extracellular fluids occurs in mammalian, fish and amphibian embryos. A great deal of experimental evidence indicates that this flow is indeed required for symmetry breaking. An alternative model has argued, however, that flow simply acts as an amplification step for early asymmetric cues generated by ion flux during the first cleavage divisions. In this review we critically evaluate the experimental basis of both models. Although a number of open questions persist, the available evidence is best compatible with flow-based symmetry breakage as the archetypical mode of symmetry breakage.

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.

In vivo T-box transcription factor profiling reveals joint regulation of embryonic neuromesodermal bipotency

The design of effective cell replacement therapies requires detailed knowledge of how embryonic stem cells form primary tissues, such as mesoderm or neurectoderm that later become skeletal muscle or nervous system. Members of the T-box transcription factor family are key in the formation of these primary tissues, but their underlying molecular activities are poorly understood. Here, we define in vivo genome-wide regulatory inputs of the T-box proteins Brachyury, Eomesodermin, and VegT, which together maintain neuromesodermal stem cells and determine their bipotential fates in frog embryos. These T-box proteins are all recruited to the same genomic recognition sites, from where they activate genes involved in stem cell maintenance and mesoderm formation while repressing neurogenic genes. Consequently, their loss causes embryos to form an oversized neural tube with no mesodermal derivatives. This collaboration between T-box family members thus ensures the continuous formation of correctly proportioned neural and mesodermal tissues in vertebrate embryos during axial elongation.

Wnt signaling through the Ror receptor in the nervous system

The receptor tyrosine kinase-like orphan receptor (Ror) proteins are conserved tyrosine kinase receptors that play roles in a variety of cellular processes that pattern tissues and organs during vertebrate and invertebrate development. Ror signaling is required for skeleton and neuronal development and modulates cell migration, cell polarity, and convergent extension. Ror has also been implicated in two human skeletal disorders, brachydactyly type B and Robinow syndrome. Rors are widely expressed during metazoan development including domains in the nervous system. Here, we review recent progress in understanding the roles of the Ror receptors in neuronal migration, axonal pruning, axon guidance, and synaptic plasticity. The processes by which Ror signaling execute these diverse roles are still largely unknown, but they likely converge on cytoskeletal remodeling. In multiple species, Rors have been shown to act as Wnt receptors signaling via novel non-canonical Wnt pathways mediated in some tissues by the adapter protein disheveled and the non-receptor tyrosine kinase Src. Rors can either activate or repress Wnt target expression depending on the cellular context and can also modulate signal transduction by sequestering Wnt ligands away from their signaling receptors. Future challenges include the identification of signaling components of the Ror pathways and bettering our understanding of the roles of these pleiotropic receptors in patterning the nervous system.

Sfrp5 modulates both Wnt and BMP signaling and regulates gastrointestinal organogenesis [corrected] in the zebrafish, Danio rerio

Sfrp5 belongs to the family of secreted frizzled related proteins (Sfrp), secreted inhibitors of Wingless-MMTV Integration Site (Wnt) signaling, which play an important role in cancer and development. We selected sfrp5 because of its compelling expression profile in the developing endoderm in zebrafish, Danio rerio. In this study, overexpression of sfrp5 in embryos results in defects in both convergent extension (CE) by inhibition of non-canonical Wnt signaling and defects in dorsoventral patterning by inhibition of Tolloid-mediated proteolysis of the BMP inhibitor Chordin. From 25 hours post fertilization (hpf) to 3 days post fertilization (dpf), both overexpression and knockdown of Sfrp5 decrease the size of the endoderm, significantly reducing liver cell number. At 3 dpf, insulin-positive endodermal cells fail to coalesce into a single pancreatic islet. We show that Sfrp5 inhibits both canonical and non-canonical Wnt signaling during embryonic and endodermal development, resulting in endodermal abnormalities.

Localisation of RNAs into the germ plasm of vitellogenic Xenopus oocytes

We have studied the localisation of mRNAs in full-grown Xenopus laevis oocytes by injecting fluorescent RNAs, followed by confocal microscopy of the oocyte cortex. Concentrating on RNA encoding the Xenopus Nanos homologue, nanos1 (formerly Xcat2), we find that it consistently localised into aggregated germ plasm ribonucleoprotein (RNP) particles, independently of cytoskeletal integrity. This implies that a diffusion/entrapment-mediated mechanism is active, as previously reported for previtellogenic oocytes. Sometimes this was accompanied by localisation into scattered particles of the “late”, Vg1/VegT pathway; occasionally only late pathway localisation was seen. The Xpat RNA behaved in an identical fashion and for neither RNA was the localisation changed by any culture conditions tested. The identity of the labelled RNP aggregates as definitive germ plasm was confirmed by their inclusion of abundant mitochondria and co-localisation with the germ plasm protein Hermes. Further, the nanos1/Hermes RNP particles are interspersed with those containing the germ plasm protein Xpat. These aggregates may be followed into the germ plasm of unfertilized eggs, but with a notable reduction in its quantity, both in terms of injected molecules and endogenous structures. Our results conflict with previous reports that there is no RNA localisation in large oocytes, and that during mid-oogenesis even germ plasm RNAs localise exclusively by the late pathway. We find that in mid oogenesis nanos1 RNA also localises to germ plasm but also by the late pathway. Late pathway RNAs, Vg1 and VegT, also may localise into germ plasm. Our results support the view that mechanistically the two modes of localisation are extremely similar, and that in an injection experiment RNAs might utilise either pathway, the distinction in fates being very subtle and subject to variation. We discuss these results in relation to their biological significance and the results of others.

Regulation of cell polarity and RNA localization in vertebrate oocytes

It has long been appreciated that the inheritance of maternal cytoplasmic determinants from different regions of the egg can lead to differential specification of blastomeres during cleavage. Localized RNAs are important determinants of cell fate in eggs and embryos but are also recognized as fundamental regulators of cell structure and function. This chapter summarizes recent molecular and genetic experiments regarding: (1) mechanisms that regulate polarity during different stages of vertebrate oogenesis, (2) pathways that localize presumptive protein and RNA determinants within the polarized oocyte and egg, and (3) how these determinants act in the embryo to determine the ultimate cell fates. Emphasis is placed on studies done in Xenopus, where extensive work has been done in these areas, and comparisons are drawn with fish and mammals. The prospects for future work using in vivo genome manipulation and other postgenomic approaches are also discussed.

Characterization of rainbow trout gonad, brain and gill deep cDNA repertoires using a Roche 454-Titanium sequencing approach

Rainbow trout, Oncorhynchus mykiss, is an important aquaculture species worldwide and, in addition to being of commercial interest, it is also a research model organism of considerable scientific importance. Because of the lack of a whole genome sequence in that species, transcriptomic analyses of this species have often been hindered. Using next-generation sequencing (NGS) technologies, we sought to fill these informational gaps. Here, using Roche 454-Titanium technology, we provide new tissue-specific cDNA repertoires from several rainbow trout tissues. Non-normalized cDNA libraries were constructed from testis, ovary, brain and gill rainbow trout tissue samples, and these different libraries were sequenced in 10 separate half-runs of 454-Titanium. Overall, we produced a total of 3million quality sequences with an average size of 328bp, representing more than 1Gb of expressed sequence information. These sequences have been combined with all publicly available rainbow trout sequences, resulting in a total of 242,187 clusters of putative transcript groups and 22,373 singletons. To identify the predominantly expressed genes in different tissues of interest, we developed a Digital Differential Display (DDD) approach. This approach allowed us to characterize the genes that are predominantly expressed within each tissue of interest. Of these genes, some were already known to be tissue-specific, thereby validating our approach. Many others, however, were novel candidates, demonstrating the usefulness of our strategy and of such tissue-specific resources. This new sequence information, acquired using NGS 454-Titanium technology, deeply enriched our current knowledge of the expressed genes in rainbow trout through the identification of an increased number of tissue-specific sequences. This identification allowed a precise cDNA tissue repertoire to be characterized in several important rainbow trout tissues. The rainbow trout contig browser can be accessed at the following publicly available web site (http://www.sigenae.org/).

Maternal xNorrin, a canonical Wnt signaling agonist and TGF-β antagonist, controls early neuroectoderm specification in Xenopus

Xenopus maternal Norrin, which activates Wnt signaling but inhibits TGF-β family molecules, is essential for neuroectoderm formation. Loss of TGF-β inhibition in Norrin may contribute to the development of Norrie disease.

Dorsal–ventral specification in the amphibian embryo is controlled by β-catenin, whose activation in all dorsal cells is dependent on maternal Wnt11. However, it remains unknown whether other maternally secreted factors contribute to β-catenin activation in the dorsal ectoderm. Here, we show that maternal Xenopus Norrin (xNorrin) promotes anterior neural tissue formation in ventralized embryos. Conversely, when xNorrin function is inhibited, early canonical Wnt signaling in the dorsal ectoderm and the early expression of the zygotic neural inducers Chordin, Noggin, and Xnr3 are severely suppressed, causing the loss of anterior structures. In addition, xNorrin potently inhibits BMP- and Nodal/Activin-related functions through direct binding to the ligands. Moreover, a subset of Norrin mutants identified in humans with Norrie disease retain Wnt activation but show defective inhibition of Nodal/Activin-related signaling in mesoderm induction, suggesting that this disinhibition causes Norrie disease. Thus, xNorrin is an unusual molecule that acts on two major signaling pathways, Wnt and TGF-β, in opposite ways and is essential for early neuroectoderm specification.

A key step during early embryogenesis is the generation of neural precursors, which later form the central nervous system. In vertebrates, this process requires proper dorsal–ventral axis specification, and we know that the canonical Wnt and BMP signaling pathways help pattern the dorsal ectoderm. In this study, we examine other factors that are involved in neuroectoderm development in the frog species Xenopus laevis. We find that maternal Xenopus Norrin (xNorrin) is required for canonical Wnt signaling in the dorsal ectoderm, functions upstream of neural inducers, and is required for neural formation. We also find that xNorrin not only activates Wnt signaling, but also inhibits BMP/Nodal-related signaling. In humans, mutations in Norrin cause Norrie disease. Using Norrin mutants identified in patients with Norrie disease, we find that some Norrin mutants fail to inhibit BMP/Nodal-related signaling (specifically, TGF-β) but retain the ability to activate the Wnt pathway, suggesting that loss of TGF-β inhibition may contribute to Norrie disease development.

Nerve growth factor regulates axial rotation during early stages of chick embryo development

Nerve growth factor (NGF) was discovered because of its neurotrophic actions on sympathetic and sensory neurons in the developing chicken embryo. NGF was subsequently found to influence and regulate the function of many neuronal and non neuronal cells in adult organisms. Little is known, however, about the possible actions of NGF during early embryonic stages. However, mRNAs encoding for NGF and its receptors TrkA and p75(NTR) are expressed at very early stages of avian embryo development, before the nervous system is formed. The question, therefore, arises as to what might be the functions of NGF in early chicken embryo development, before its well-established actions on the developing sympathetic and sensory neurons. To investigate possible roles of NGF in the earliest stages of development, stage HH 11-12 chicken embryos were injected with an anti-NGF antibody (mAb αD11) that binds mature NGF with high affinity. Treatment with anti-NGF, but not with a control antibody, led to a dose-dependent inversion of the direction of axial rotation. This effect of altered rotation after anti NGF injection was associated with an increased cell death in somites. Concurrently, a microarray mRNA expression analysis revealed that NGF neutralization affects the expression of genes linked to the regulation of development or cell proliferation. These results reveal a role for NGF in early chicken embryo development and, in particular, in the regulation of somite survival and axial rotation, a crucial developmental process linked to left-right asymmetry specification.

Cortical rotation and messenger RNA localization in Xenopus axis formation

In Xenopus eggs, fertilization initiates a rotational movement of the cortex relative to the cytoplasm, resulting in the transport of critical determinants to the future dorsal side of the embryo. Cortical rotation is mediated by microtubules, resulting in activation of the Wnt/β-catenin signaling pathway and expression of organizer genes on the dorsal side of the blastula. Similar cytoplasmic localizations resulting in β-catenin activation occur in many chordate embryos, suggesting a deeply conserved mechanism for patterning early embryos. This review summarizes the experimental evidence for the molecular basis of this model, focusing on recent maternal loss-of-function studies that shed light on two main unanswered questions: (1) what regulates microtubule assembly during cortical rotation and (2) how is Wnt/β-catenin signaling activated dorsally? In addition, as these processes depend on vegetally localized molecules in the oocyte, the mechanisms of RNA localization and novel roles for localized RNAs in axis formation are discussed. The work reviewed here provides a beginning framework for understanding the coupling of asymmetry in oogenesis with the establishment of asymmetry in the embryo.

Putting RNAs in the right place at the right time: RNA localization in the frog oocyte

Localization of maternal mRNAs in many developing organisms provides the basis for both initial polarity during oogenesis and patterning during embryogenesis. Prominent examples of this phenomenon are found in Xenopus laevis, where localized maternal mRNAs generate developmental polarity along the animal/vegetal axis. Targeting of mRNA molecules to specific subcellular regions is a fundamental mechanism for spatial regulation of gene expression, and considerable progress has been made in defining the underlying molecular pathways.

Wnt11 in 2011 - the regulation and function of a non-canonical Wnt

Genetic studies of Wnt11 have revealed many insights into the roles and regulation of Wnt11, particularly during development. New tools to study Wnt11 have recently become available, making it timely to review the literature regarding this unique Wnt family member. In this study, we focus on mammalian Wnt11, describing its main sites of expression during development, and how the Wnt11 gene is regulated. We highlight an emerging theme in which canonical Wnt signals regulate Wnt11 expression through transcription factors in addition to, or other than, Tcf/LEF family members. We also discuss the frizzled family and other receptors that bind to Wnt11, the intracellular kinases and small GTPases that act downstream of Wnt11, and the effects of Wnt11 on Wnt/β-catenin signalling. Finally, we elaborate on the relevance of Wnt11 to human cancer, where it appears to be important both for proliferation and/or survival during normal differentiation and for migration/invasion.

New insights into the regulation of RNP granule assembly in oocytes

In a variety of cell types in plants, animals, and fungi, ribonucleoprotein (RNP) complexes play critical roles in regulating RNA metabolism. These RNP granules include processing bodies and stress granules that are found broadly across cell types, as well as RNP granules unique to the germline, such as P granules, polar granules, sponge bodies, and germinal granules. This review focuses on RNP granules localized in oocytes of the major model systems, Caenorhabditis elegans, Drosophila, Xenopus, mouse, and zebrafish. The signature families of proteins within oocyte RNPs include Vasa and other RNA-binding proteins, decapping activators and enzymes, Argonaute family proteins, and translation initiation complex proteins. This review describes the many recent insights into the dynamics and functions of RNP granules, including their roles in mRNA degradation, mRNA localization, translational regulation, and fertility. The roles of the cytoskeleton and cell organelles in regulating RNP granule assembly are also discussed.

Planar cell polarity signaling pathway in congenital heart diseases

Congenital heart disease (CHD) is a common cardiac disorder in humans. Despite many advances in the understanding of CHD and the identification of many associated genes, the fundamental etiology for the majority of cases remains unclear. The planar cell polarity (PCP) signaling pathway, responsible for tissue polarity in Drosophila and gastrulation movements and cardiogenesis in vertebrates, has been shown to play multiple roles during cardiac differentiation and development. The disrupted function of PCP signaling is connected to some CHDs. Here, we summarize our current understanding of how PCP factors affect the pathogenesis of CHD.

Maternal and zygotic control of zebrafish dorsoventral axial patterning

Vertebrate development begins with precise molecular, cellular, and morphogenetic controls to establish the basic body plan of the embryo. In zebrafish, these tightly regulated processes begin during oogenesis and proceed through gastrulation to establish and pattern the axes of the embryo. During oogenesis a maternal factor is localized to the vegetal pole of the oocyte that is a determinant of dorsal tissues. Following fertilization this vegetally localized dorsal determinant is asymmetrically translocated in the egg and initiates formation of the dorsoventral axis. Dorsoventral axis formation and patterning is then mediated by maternal and zygotic factors acting through Wnt, BMP (bone morphogenetic protein), Nodal, and FGF (fibroblast growth factor) signaling pathways, each of which is required to establish and/or pattern the dorsoventral axis. This review addresses recent advances in our understanding of the molecular factors and mechanisms that establish and pattern the dorsoventral axis of the zebrafish embryo, including establishment of the animal-vegetal axis as it relates to formation of the dorsoventral axis.

Identification and mechanism of regulation of the zebrafish dorsal determinant

In vertebrates, the animal-vegetal axis is determined during oogenesis and at ovulation, the egg is radially symmetric. In anamniotes, following fertilization, a microtubule-dependent movement leads to the displacement of maternal dorsal determinants from the vegetal pole to the future dorsal side of the embryo, providing the initial breaking of radial symmetry [Weaver C, Kimelman D (2004) Development 131:3491-3499]. These dorsal determinants induce β-catenin nuclear translocation in dorsal cells of the blastula. Previous work in amphibians has shown that secreted Wnt11/5a complexes, regulated by the Wnt antagonist Dkk-1, are required for the initiation of embryonic axis formation [Cha et al. (2009) Curr Biol 29:1573-1580]. In the current study, we determined that the vegetal maternal dorsal determinant in fish is not the Wnt11/5a complex but the canonical Wnt, Wnt8a. Translation of this mRNA and secretion of the Wnt8a protein result in a dorsal-to-ventral gradient of Wnt stimulation, extending across the entire embryo. This gradient is counterbalanced by two Wnt inhibitors, Sfrp1a and Frzb. These proteins are essential to restrict the activation of the canonical Wnt pathway to the dorsal marginal blastomeres by defining the domain where the Wnt8a activity gradient is above the threshold value necessary for triggering the canonical β-catenin pathway. In summary, this study establishes that the zebrafish maternal dorsal determinant, Wnt8a, is required to localize the primary dorsal center, and that the extent of this domain is defined by the activity of two maternally provided Wnt antagonists, Sfrp1a and Frzb.

An ATF2-based luciferase reporter to monitor non-canonical Wnt signaling in Xenopus embryos

Non-canonical/planar cell polarity (PCP) Wnt signaling plays important roles in embryonic development and tissue homeostasis, and is implicated in human disease. Monitoring Wnt/PCP signaling relies mostly on semi-quantitative bioassays or biochemical analysis. Here we describe a luciferase reporter assay based on an ATF2 response element, which faithfully monitors non-canonical Wnt signaling in Xenopus embryos. The assay is simple, quantitative, and robust. It can be used to detect non-canonical Wnt signaling changes following gain and loss of function of pathway components, including Wnt, Frizzled, Ror2, Disheveled, Rac1, MKK7, and JNK. Wnt/PCP signaling has recently been implicated in left-right asymmetry and our reporter assay suggests that in gastrula embryos there is a right-ward bias in Wnt/PCP signaling. We also mapped Wnt/PCP signaling in the early Xenopus embryo and find that it peaks in the dorso-vegetal region, paralleling Wnt/β-catenin signaling.

Expression of XNOA 36 in the mitochondrial cloud of Xenopus laevis oocytes

In Xenopus laevis oocytes a mitochondrial cloud (MC) is found between the nucleus and the plasma membrane at stages I–II of oogenesis. The MC contains RNAs that are transported to the future vegetal pole at stage II of oogenesis. In particular, germinal plasm mRNAs are found in the Message Transport Organiser (METRO) region, the MC region opposite to the nucleus. At stages II–III, a second pathway transports Vg1 and VegT mRNAs to the area where the MC content merges with the vegetal cortex. Microtubules become polarized at the sites of migration of Vg1 and VegT mRNAs through an unknown signalling mechanism. In early meiotic stages, the centrioles are almost completely lost with their remnants being dispersed into the cytoplasm and the MC, which may contain a MTOC to be used in the later localization pathway of the mRNAs. In mammals, XNOA 36 encodes a member of a highly conserved protein family and localises to the nucleolus or in the centromeres. In the Xenopus late stage I oocyte, XNOA 36 mRNA is transiently segregated in one half of the oocyte, anchored by a cytoskeletal network that contains spectrin. Here we found that XNOA 36 transcript also localises to the nucleoli and in the METRO region. XNOA 36 protein immunolocalization, using an antibody employed for the library immunoscreening that depicted XNOA 36 expression colonies, labels the migrating MC, the cytoplasm of stage I oocytes and in particular the vegetal cortex facing the MC. The possible role of XNOA 36 in mRNA anchoring to the vegetal cortex or in participating in early microtubule reorganization is discussed.

Shared molecular mechanisms regulate multiple catenin proteins: canonical Wnt signals and components modulate p120-catenin isoform-1 and additional p120 subfamily members

Wnt signaling pathways have fundamental roles in animal development and tumor progression. Here, employing Xenopus embryos and mammalian cell lines, we report that the degradation machinery of the canonical Wnt pathway modulates p120-catenin protein stability through mechanisms shared with those regulating β-catenin. For example, in common with β-catenin, exogenous expression of destruction complex components, such as GSK3β and axin, promotes degradation of p120-catenin. Again in parallel with β-catenin, reduction of canonical Wnt signals upon depletion of LRP5 and LRP6 results in p120-catenin degradation. At the primary sequence level, we resolved conserved GSK3β phosphorylation sites in the amino-terminal region of p120-catenin present exclusively in isoform-1. Point-mutagenesis of these residues inhibited the association of destruction complex components, such as those involved in ubiquitylation, resulting in stabilization of p120-catenin. Functionally, in line with predictions, p120 stabilization increased its signaling activity in the context of the p120-Kaiso pathway. Importantly, we found that two additional p120-catenin family members, ARVCF-catenin and δ-catenin, associate with axin and are degraded in its presence. Thus, as supported using gain- and loss-of-function approaches in embryo and cell line systems, canonical Wnt signals appear poised to have an impact upon a breadth of catenin biology in vertebrate development and, possibly, human cancers.

Negative regulation of wnt11 expression by Jnk signaling during zebrafish gastrulation

Stress-induced Sapk/Jnk signaling is involved in cell survival and apoptosis. Recent studies have increased our understanding of the physiological roles of Jnk signaling in embryonic development. However, still unclear is the precise function of Jnk signaling during gastrulation, a critical step in the establishment of the vertebrate body plan. Here we use morpholino-mediated knockdown of the zebrafish orthologs of the Jnk activators Mkk4 and Mkk7 to examine the effect of Jnk signaling abrogation on early vertebrate embryogenesis. Depletion of zebrafish Mkk4b led to abnormal convergent extension (CE) during gastrulation, whereas Mkk7 morphants exhibited defective somitogenesis. Surprisingly, Mkk4b morphants displayed marked upregulation of wnt11, which is the triggering ligand of CE and stimulates Jnk activation via the non-canonical Wnt pathway. Conversely, ectopic activation of Jnk signaling by overexpression of an active form of Mkk4b led to wnt11 downregulation. Mosaic lineage tracing studies revealed that Mkk4b-Jnk signaling suppressed wnt11 expression in a non-cell-autonomous manner. These findings provide the first evidence that wnt11 itself is a downstream target of the Jnk cascade in the non-canonical Wnt pathway. Our work demonstrates that Jnk activation is indispensable for multiple steps during vertebrate body plan formation. Furthermore, non-canonical Wnt signaling may coordinate vertebrate CE movements by triggering Jnk activation that represses the expression of the CE-triggering ligand wnt11.

Identification of germ plasm-associated transcripts by microarray analysis of Xenopus vegetal cortex RNA

RNA localization is a common mechanism for regulating cell structure and function. Localized RNAs in Xenopus oocytes are critical for early development, including germline specification by the germ plasm. Despite the importance of these localized RNAs, only approximately 25 have been identified and fewer are functionally characterized. Using microarrays, we identified a large set of localized RNAs from the vegetal cortex. Overall, our results indicate a minimum of 275 localized RNAs in oocytes, or 2-3% of maternal transcripts, which are in general agreement with previous findings. We further validated vegetal localization for 24 candidates and further characterized three genes expressed in the germ plasm. We identified novel germ plasm expression for reticulon 3.1, exd2 (a novel exonuclease-domain encoding gene), and a putative noncoding RNA. Further analysis of these and other localized RNAs will likely identify new functions of germ plasm and facilitate the identification of cis-acting RNA localization elements.

The multiple phases and faces of wnt signaling during cardiac differentiation and development

Understanding heart development on a molecular level is a prerequisite for uncovering the causes of congenital heart diseases. Therapeutic approaches that try to enhance cardiac regeneration or that involve the differentiation of resident cardiac progenitor cells or patient-specific induced pluripotent stem cells will also benefit tremendously from this knowledge. Wnt proteins have been shown to play multiple roles during cardiac differentiation and development. They are extracellular growth factors that activate different intracellular signaling branches. Here, we summarize our current understanding of how these factors affect different aspects of cardiogenesis, starting from early specification of cardiac progenitors and continuing on to later developmental steps, such as morphogenetic processes, valve formation, and establishment of the conduction system.

Xenopus Wnt11b is identified as a potential pronephric inducer

In this study, we aimed to establish if known wnt signaling molecules could be responsible for inducing early pronephros specification, using a novel and effective in vitro bioassay in Xenopus embryos. Anterior somites have the unique biological activity to signal to unspecified intermediate mesoderm to induce pronephros formation in Xenopus embryos. We have used a molecular candidate gene approach to analyze both canonical and noncanonical wnt expression in isolated anterior and posterior somites and dissected presumptive pronephros, pronephric anlagen, and pronephros from stage 12.5-35 embryos. We have identified potential candidate wnt genes expressed in the right time and place to specify pronephric development. These candidates were then directly tested in an in vitro pronephrogenesis assay based on Holtfreter sandwich cultures. Results revealed that noncanonical wnt11b and wnt11 can induce pronephros formation in vitro. Loss-of-function experiments confirmed that these genes are necessary for normal pronephros development.

Syntabulin, a motor protein linker, controls dorsal determination

In amphibian and teleost embryos, the dorsal determinants (DDs) are believed to be initially localized to the vegetal pole and then transported to the prospective dorsal side of the embryo along a microtubule array. The DDs are known to activate the canonical Wnt pathway and thereby promote the expression of genes that induce the dorsal organizer. Here, by identifying the locus of the maternal-effect ventralized mutant tokkaebi, we show that Syntabulin, a linker of the kinesin I motor protein, is essential for dorsal determination in zebrafish. We found that syntabulin mRNA is transported to the vegetal pole during oogenesis through the Bucky ball (Buc)-mediated Balbiani body-dependent pathway, which is necessary for establishment of animal-vegetal (AV) oocyte polarity. We demonstrate that Syntabulin is translocated from the vegetal pole in a microtubule-dependent manner. Our findings suggest that Syntabulin regulates the microtubule-dependent transport of the DDs, and provide evidence for the link between AV and dorsoventral axis formation.

Noncanonical Wnt11 signaling and cardiomyogenic differentiation

Although the molecular details remain unclear, Wnt signaling via both canonical and noncanonical pathways is integral to cardiac specification and morphogenesis. A growing body of evidence also suggests substantial overlap between these supposedly discrete pathways in cell- and context-dependent manners. The ability of Wnt11 to induce cardiomyogenesis both during embryonic development and in adult cells makes it an attractive candidate for preprogramming cells for cardiac repair. This review primarily discusses various aspects of noncanonical Wnt signaling in cardiogenesis with particular emphasis on Wnt11.

Xenopus Wntless and the retromer complex cooperate to regulate XWnt4 secretion

Wnt signaling is implicated in a variety of developmental and pathological processes. The molecular mechanisms governing the secretion of Wnt ligands remain to be elucidated. Wntless, an evolutionarily conserved multipass transmembrane protein, is a dedicated secretion factor of Wnt proteins that participates in Drosophila melanogaster embryogenesis. In this study, we show that Xenopus laevis Wntless (XWntless) regulates the secretion of a specific Wnt ligand, XWnt4, and that this regulation is specifically required for eye development in Xenopus. Moreover, the Retromer complex is required for XWntless recycling to regulate the XWnt4-mediated eye development. Inhibition of Retromer function by Vps35 morpholino (MO) resulted in various Wnt deficiency phenotypes, affecting mesoderm induction, gastrulation cell movements, neural induction, neural tube closure, and eye development. Overexpression of XWntless led to the rescue of Vps35 MO-mediated eye defects but not other deficiencies. These results collectively suggest that XWntless and the Retromer complex are required for the efficient secretion of XWnt4, facilitating its role in Xenopus eye development.

Identification of asymmetrically localized transcripts along the animal-vegetal axis of the Xenopus egg

In many organisms, proper embryo development depends on the asymmetrical distribution of mRNA in the cytoplasm of the egg. Here we report comprehensive screening of RNA localized in the animal or vegetal hemisphere of the Xenopus egg. Macroarrays including over 40,000 independent embryonic cDNA clones, representing at least 17,000 unigenes, were differentially hybridized with labeled probes synthesized from the mRNA of animal or vegetal blastomeres. After two rounds of screening, we identified 33 clones of transcripts that may be preferentially distributed in the vegetal region of the early stage embryo, but transcripts localized in the animal region were not found. To assess the array results, we performed northern blot and quantitative real-time reverse transcription-polymerase chain reaction analysis. As a result, 21 transcripts of the 33 were confirmed to be localized in the vegetal region of the early stage embryo. Whole-mount in situ hybridization analysis revealed that 11 transcripts, including 7 previously reported genes, were localized in the vegetal hemisphere of the egg. These 11 transcripts were categorized into three groups according to their expression patterns in the egg. The first group, which contained four transcripts, showed uniform expression in the vegetal hemisphere, similar to VegT. The second group, which contained three transcripts, showed gradual expression from the vegetal pole to the equator, similar to Vg1. The last group, which contained three transcripts, was expressed at the germ plasm, similar to Xdazl. One transcript, Xwnt11, showed both the second and the third expression patterns.