No tail co-operates with non-canonical Wnt signaling to regulate posterior body morphogenesis in zebrafish

Dolutegravir and Folic Acid Interaction during Neural System Development in Zebrafish Embryos

Dolutegravir (DTG) is one of the most prescribed antiretroviral drugs for treating people with HIV infection, including women of child-bearing potential or pregnant. Nonetheless, neuropsychiatric symptoms are frequently reported. Early reports suggested that, probably in relation to folic acid (FA) shortage, DTG may induce neural tube defects in infants born to women taking the drug during pregnancy. Subsequent reports did not definitively confirm these findings. Recent studies in animal models have highlighted the association between DTG exposure in utero and congenital anomalies, and an increased risk of neurologic abnormalities in children exposed during in utero life has been reported. Underlying mechanisms for DTG-related neurologic symptoms and congenital anomalies are not fully understood. We aimed to deepen our knowledge on the neurodevelopmental effects of DTG exposure and further explore the protective role of FA by the use of zebrafish embryos. We treated embryos at 4 and up to 144 h post fertilization (hpf) with a subtherapeutic DTG concentration (1 μM) and observed the disruption of the anterior-posterior axis and several morphological malformations in the developing brain that were both prevented by pre-exposure (2 hpf) and rescued by post-exposure (10 hpf) with FA. By whole-mount in situ hybridization with riboprobes for genes that are crucial during the early phases of neurodevelopment (ntl, pax2a, ngn1, neurod1) and by in vivo visualization of the transgenic Tg(ngn1:EGFP) zebrafish line, we found that DTG induced severe neurodevelopmental defects over time in most regions of the nervous system (notochord, midbrain-hindbrain boundary, eye, forebrain, midbrain, hindbrain, spinal cord) that were mostly but not completely rescued by FA supplementation. Of note, we observed the disruption of ngn1 expression in the dopaminergic regions of the developing forebrain, spinal cord neurons and spinal motor neuron projections, with the depletion of the tyrosine hydroxylase (TH)+ dopaminergic neurons of the dorsal diencephalon and the strong reduction in larvae locomotion. Our study further supports previous evidence that DTG can interfere with FA pathways in the developing brain but also provides new insights regarding the mechanisms involved in the increased risk of DTG-associated fetal neurodevelopmental defects and adverse neurologic outcomes in in utero exposed children, suggesting the impairment of dopaminergic pathways.

Zebrafish Insights into Nanomaterial Toxicity: A Focused Exploration on Metallic, Metal Oxide, Semiconductor, and Mixed-Metal Nanoparticles

Nanomaterials are widely used in various fields, and ongoing research is focused on developing safe and sustainable nanomaterials. Using zebrafish as a model organism for studying the potentially toxic effects of nanomaterials highlights the importance of developing safe and sustainable nanomaterials. Studies conducted on nanomaterials and their toxicity and potential risks to human and environmental health are vital in biomedical sciences. In the present review, we discuss the potential toxicity of nanomaterials (inorganic and organic) and exposure risks based on size, shape, and concentration. The review further explores various types of nanomaterials and their impacts on zebrafish at different levels, indicating that exposure to nanomaterials can lead to developmental defects, changes in gene expressions, and various toxicities. The review also covers the importance of considering natural organic matter and chorion membranes in standardized nanotoxicity testing. While some nanomaterials are biologically compatible, metal and semiconductor nanomaterials that enter the water environment can increase toxicity to aquatic creatures and can potentially accumulate in the human body. Further investigations are necessary to assess the safety of nanomaterials and their impacts on the environment and human health.

wnt11f2 Zebrafish, an Animal Model for Development and New Insights in Bone Formation

Wnt signaling is a key regulator of osteoblast differentiation and mineralization in humans and animals, mediated by the canonical Wnt/β-catenin and noncanonical signaling pathways. Both pathways are crucial in regulating osteoblastogenesis and bone formation. The zebrafish silberblick (slb) carries a mutation in wnt11f2, a gene that contributes to embryonic morphogenesis; however, its role in bone morphology is unknown. wnt11f2 was originally known as wnt11; it was recently reclassified to avoid confusion in comparative genetics and disease modeling. The goal of this review is to summarize the characterization of the wnt11f2 zebrafish mutant and to deliver some new insights concerning its role in skeletal development. In addition to the previously described defects in early development in this mutant as well as craniofacial dysmorphia, we show an increase in tissue mineral density in the heterozygous mutant that points to a possible role of wnt11f2 in high bone mass phenotypes.

Drug discovery efforts toward inhibitors of canonical Wnt/β-catenin signaling pathway in the treatment of cancer: A composition-of-matter review (2010-2020)

The Wnt/β-catenin pathway has a crucial role in the proliferation and differentiation of normal cells as well as the self-renewal and pluripotency of stem cells, including cancer stem cells (CSCs). Targeting this pathway with small-molecule chemotherapeutics, discovered via conventional efforts, has proved difficult. Recently, computer-aided drug discovery efforts have produced promising chemotherapeutics. A concerted effort to develop inhibitors of this pathway through more efficient and cost-effective drug discovery methods could lead to a significant increase in clinically relevant therapeutics. Herein, patents from 2010 to 2020 are reviewed to identify those that have disclosed composition of matter for small-molecule inhibitors of the Wnt/ β-catenin pathway for cancer. We believe that such efforts will provide insights for future therapeutic candidate discovery and development in this field.

Toxicity of polymer-modified CuS nanoclusters on zebrafish embryo development

Despite the vast amount of research on the toxicity of copper-based nanoparticles, the toxicity of CuS nanoparticles is still largely unknown. Due to the application of CuS-based nanomaterials in biomedical engineering, it is necessary to study their potential toxicity and biological effects. In this study, we evaluated the toxicity of polymer-modified CuS nanoclusters (PATA3-C4@CuS) on embryo development through exposing zebrafish embryos to 1, 2.5, 5, 7.5, and 10 mg/L PATA3-C4@CuS at 0.75-h post-fertilization. The morphological results demonstrated that PATA3-C4@CuS at concentrations greater than 1 mg/L PATA3-C4@CuS induced abnormal phenotypes including smaller heads and eyes, pericardial edema, and epiboly retardation and it increased mortality, lowered the hatching rate, and inhibited swim bladder inflation. In situ hybridization and quantitative reverse transcription polymerase chain reaction showed that PATA3-C4@CuS could alter the expression patterns of tbxta, dlx3, and cstlb and increase the expression levels of wnt5 and wnt11, which suggested that PATA3-C4@CuS disrupts cell migration by increasing the levels of wnt5 and wnt11 during gastrulation. It was also discovered that PATA3-C4@CuS exposure caused a slow heart rate and smaller ventricles in zebrafish larvae. Immunofluorescence and behavioral analyses showed that PATA3-C4@CuS could damage the ventral projection of the primary motor neurons CaP, which was in accordance with the reduction in locomotion ability. Together, our data demonstrated that functional PATA3-C4@CuS could disrupt cell migration during gastrulation, affect cardiac development and function, and decrease locomotive activity.

Mechanical control of tissue shape and morphogenetic flows during vertebrate body axis elongation

Shaping embryonic tissues into their functional morphologies requires cells to control the physical state of the tissue in space and time. While regional variations in cellular forces or cell proliferation have been typically assumed to be the main physical factors controlling tissue morphogenesis, recent experiments have revealed that spatial variations in the tissue physical (fluid/solid) state play a key role in shaping embryonic tissues. Here we theoretically study how the regional control of fluid and solid tissue states guides morphogenetic flows to shape the extending vertebrate body axis. Our results show that both the existence of a fluid-to-solid tissue transition along the anteroposterior axis and the tissue surface tension determine the shape of the tissue and its ability to elongate unidirectionally, with large tissue tensions preventing unidirectional elongation and promoting blob-like tissue expansions. We predict both the tissue morphogenetic flows and stresses that enable unidirectional axis elongation. Our results show the existence of a sharp transition in the structure of morphogenetic flows, from a flow with no vortices to a flow with two counter-rotating vortices, caused by a transition in the number and location of topological defects in the flow field. Finally, comparing the theoretical predictions to quantitative measurements of both tissue flows and shape during zebrafish body axis elongation, we show that the observed morphogenetic events can be explained by the existence of a fluid-to-solid tissue transition along the anteroposterior axis. These results highlight the role of spatiotemporally-controlled fluid-to-solid transitions in the tissue state as a physical mechanism of embryonic morphogenesis.

Brachyury in the gastrula of basal vertebrates

The Brachyury gene encodes a transcription factor that is conserved across all animals. In non-chordate metazoans, brachyury is primarily expressed in ectoderm regions that are added to the endodermal gut during development, and often form a ring around the site of endoderm internalization in the gastrula, the blastopore. In chordates, this brachyury ring is conserved, but the gene has taken on a new role in the formation of the mesoderm. In this phylum, a novel type of mesoderm that develops into notochord and somites has been added to the ancestral lateral plate mesoderm. Brachyury contributes to a shift in cell fate from neural ectoderm to posterior notochord and somites during a major lineage segregation event that in Xenopus and in the zebrafish takes place in the early gastrula. In the absence of this brachyury function, impaired formation of posterior mesoderm indirectly affects the gastrulation movements of peak involution and convergent extension. These movements are confined to specific regions and stages, leaving open the question why brachyury expression in an extensive, coherent ring, before, during and after gastrulation, is conserved in the two species whose gastrulation modes differ considerably, and also in many other metazoan gastrulae of diverse structure.

Mechanics of Anteroposterior Axis Formation in Vertebrates

The vertebrate anteroposterior axis forms through elongation of multiple tissues during embryogenesis. This process is based on tissue-autonomous mechanisms of force generation and intertissue mechanical coupling whose failure leads to severe developmental anomalies such as body truncation and spina bifida. Similar to other morphogenetic modules, anteroposterior body extension requires both the rearrangement of existing materials-such as cells and extracellular matrix-and the local addition of new materials, i.e., anisotropic growth, through cell proliferation, cell growth, and matrix deposition. Numerous signaling pathways coordinate body axis formation via regulation of cell behavior during tissue rearrangements and/or volumetric growth. From a physical perspective, morphogenesis depends on both cell-generated forces and tissue material properties. As the spatiotemporal variation of these mechanical parameters has recently been explored in the context of vertebrate body elongation, the study of this process is likely to shed light on the cross talk between signaling and mechanics during morphogenesis.

The effects of five types of tea solutions on epiboly process, neural and cardiovascular development, and locomotor capacity of zebrafish

The effects of teas on embryonic development are still known little. The objective of this study was to compare and analyze developmental effects of green tea, delicate flavor oolong tea, strong flavor oolong tea, black tea, and pu'er tea using zebrafish embryos. Embryos were exposed in tea solutions from one-cell stage; the morphology, locomotor capacity, and gene expression of embryos or larvae were analyzed. The results showed that either tea could decrease the length of body and the size of head and eyes. The effect of green tea had the most significant effects on morphology. Only green tea disturbed cell movement, epiboly, and nervous system development. All five tea solutions caused heart structure alternations and lowered heart rates, and effects caused by green tea were severe. Green tea inhibited the formation of dorsal aorta and segmental arteries and decreased the velocity and total movement distance of larvae. In conclusion, the toxicity of green tea to epiboly, neural and cardiovascular development, and locomotor capacity is more severe than that of other teas. Our study played a warning role for safety consumption of teas and provided references for further study of tea's physiological and pharmacological effects and biological activity.

Basal constriction during midbrain-hindbrain boundary morphogenesis is mediated by Wnt5b and focal adhesion kinase

Basal constriction occurs at the zebrafish midbrain–hindbrain boundary constriction (MHBC) and is likely a widespread morphogenetic mechanism. 3D reconstruction demonstrates that MHBC cells are wedge-shaped, and initially constrict basally, with subsequent apical expansion. wnt5b is expressed in the MHB and is required for basal constriction. Consistent with a requirement for this pathway, expression of dominant negative Gsk3β overcomes wnt5b knockdown. Immunostaining identifies focal adhesion kinase (Fak) as active in the MHB region, and knockdown demonstrates Fak is a regulator of basal constriction. Tissue specific knockdown further indicates that Fak functions cell autonomously within the MHBC. Fak acts downstream of wnt5b, suggesting that Wnt5b signals locally as an early step in basal constriction and acts together with more widespread Fak activation. This study delineates signaling pathways that regulate basal constriction during brain morphogenesis.

Summary: Focal adhesion kinase acts downstream of Wnt5b to mediate basal constriction of neuroepithelial cells during the formation of the midbrain–hindbrain boundary.

Embryonic vascular disruption adverse outcomes: Linking high throughput signaling signatures with functional consequences

Embryonic vascular disruption is an important adverse outcome pathway (AOP) as chemical disruption of cardiovascular development induces broad prenatal defects. High throughput screening (HTS) assays aid AOP development although linking in vitro data to in vivo apical endpoints remains challenging. This study evaluated two anti-angiogenic agents, 5HPP-33 and TNP-470, across the ToxCastDB HTS assay platform and anchored the results to complex in vitro functional assays: the rat aortic explant assay (AEA), rat whole embryo culture (WEC), and the zebrafish embryotoxicity (ZET) assay. Both were identified as putative vascular disruptive compounds (pVDCs) in ToxCastDB and disrupted angiogenesis and embryogenesis in the functional assays. Differences were observed in potency and adverse effects: 5HPP-33 was embryolethal (WEC and ZET); TNP-470 produced caudal defects at lower concentrations. This study demonstrates how a tiered approach using HTS signatures and complex functional in vitro assays might be used to prioritize further in vivo developmental toxicity testing.

A role for planar cell polarity during early endoderm morphogenesis

The zebrafish endoderm begins to develop at gastrulation stages as a monolayer of cells. The behaviour of the endoderm during gastrulation stages is well understood. However, knowledge of the morphogenic movements of the endoderm during somitogenesis stages, as it forms a mesenchymal rod, is lacking. Here we characterise endodermal development during somitogenesis stages, and describe the morphogenic movements as the endoderm transitions from a monolayer of cells into a mesenchymal endodermal rod. We demonstrate that, unlike the overlying mesoderm, endodermal cells are not polarised during their migration to the midline at early somitogenesis stages. Specifically, we describe the stage at which endodermal cells begin to leave the monolayer, a process we have termed 'midline aggregation'. The planar cell polarity (PCP) signalling pathway is known to regulate mesodermal and ectodermal cell convergence towards the dorsal midline. However, a role for PCP signalling in endoderm migration to the midline during somitogenesis stages has not been established. In this report, we investigate the role for PCP signalling in multiple phases of endoderm development during somitogenesis stages. Our data exclude involvement of PCP signalling in endodermal cells as they leave the monolayer.

Embryonic transplantation experiments: Past, present, and future

The process of taking a piece of tissue and transplanting it into a novel location has been of paramount importance for life sciences. The technique of transplantation has served an important role in providing a basic understanding of all facets of biology ranging from cancer and evolutionary biology to developmental biology. First employed by early embryologists, transplantation has played a particularly critical role in elucidating virtually every aspect of embryonic development including cell specification, commitment, cell fate determination, embryonic induction, and plasticity. This review will detail the essential role cell transplantation experiments have played in uncovering fundamental developmental and cell biological processes as well as their valuable contribution to contemporary developmental biology. Finally, it will suggest fruitful directions that this technique, in conjunction with current molecular and sequencing technologies, could play in future work.

Species-specific contribution of volumetric growth and tissue convergence to posterior body elongation in vertebrates

Posterior body elongation is a widespread mechanism propelling the generation of the metazoan body plan. The posterior growth model predicts that a posterior growth zone generates sufficient tissue volume to elongate the posterior body. However, there are energy supply-related differences between vertebrates in the degree to which growth occurs concomitantly with embryogenesis. By applying a multi-scalar morphometric analysis in zebrafish embryos, we show that posterior body elongation is generated by an influx of cells from lateral regions, by convergence-extension of cells as they exit the tailbud, and finally by a late volumetric growth in the spinal cord and notochord. Importantly, the unsegmented region does not generate additional tissue volume. Fibroblast growth factor inhibition blocks tissue convergence rather than volumetric growth, showing that a conserved molecular mechanism can control convergent morphogenesis through different cell behaviours. Finally, via a comparative morphometric analysis in lamprey, dogfish, zebrafish and mouse, we propose that elongation via posterior volumetric growth is linked to increased energy supply and is associated with an overall increase in volumetric growth and elongation.

Distinct requirements for Wntless in habenular development

Secreted Wnt proteins play pivotal roles in development, including regulation of cell proliferation, differentiation, progenitor maintenance and tissue patterning. The transmembrane protein Wntless (Wls) is necessary for secretion of most Wnts and essential for effective Wnt signaling. During a mutagenesis screen to identify genes important for development of the habenular nuclei in the dorsal forebrain, we isolated a mutation in the sole wls gene of zebrafish and confirmed its identity with a second, independent allele. Early embryonic development appears normal in homozygous wls mutants, but they later lack the ventral habenular nuclei, form smaller dorsal habenulae and otic vesicles, have truncated jaw and fin cartilages and lack swim bladders. Activation of a reporter for β-catenin-dependent transcription is decreased in wls mutants, indicative of impaired signaling by the canonical Wnt pathway, and expression of Wnt-responsive genes is reduced in the dorsal diencephalon. Wnt signaling was previously implicated in patterning of the zebrafish brain and in the generation of left-right (L-R) differences between the bilaterally paired dorsal habenular nuclei. Outside of the epithalamic region, development of the brain is largely normal in wls mutants and, despite their reduced size, the dorsal habenulae retain L-R asymmetry. We find that homozygous wls mutants show a reduction in two cell populations that contribute to the presumptive dorsal habenulae. The results support distinct temporal requirements for Wls in habenular development and reveal a new role for Wnt signaling in the regulation of dorsal habenular progenitors.

Exploring the Effects of Different Types of Surfactants on Zebrafish Embryos and Larvae

Currently, surfactants are widely distributed in the environment. As organic pollutants, their toxicities have drawn extensive attention. In this study, the effects of anionic [sodium dodecyl sulphate (SDS) ], cationic [dodecyl dimethyl benzyl ammonium chloride (1227)] and non-ionic [fatty alcohol polyoxyethylene ether (AEO) ] surfactants on zebrafish larval behaviour were evaluated. Five behavioural parameters were recorded using a larval rest/wake assay, including rest total, number of rest bouts, rest bouts length, total activity and waking activity. The results revealed that 1227 and AEO at 1 μg/mL were toxic to larval locomotor activity and that SDS had no significant effects. Moreover, we tested the toxicities of the three surfactants in developing zebrafish embryos. AEO exposure resulted in smaller head size, smaller eye size and shorter body length relative to SDS and 1227. All three surfactants incurred concentration-dependent responses. Furthermore, in situ hybridisation indicated that smaller head size may be associated with a decreased expression of krox20. The altered expression of ntl demonstrated that the developmental retardation stemmed from inhibited cell migration and growth. These findings provide references for ecotoxicological assessments of different types of surfactants, and play a warning role in the application of surfactants.

How to make an intestine

With the high prevalence of gastrointestinal disorders, there is great interest in establishing in vitro models of human intestinal disease and in developing drug-screening platforms that more accurately represent the complex physiology of the intestine. We will review how recent advances in developmental and stem cell biology have made it possible to generate complex, three-dimensional, human intestinal tissues in vitro through directed differentiation of human pluripotent stem cells. These are currently being used to study human development, genetic forms of disease, intestinal pathogens, metabolic disease and cancer.

Zebrafish Tbx16 regulates intermediate mesoderm cell fate by attenuating Fgf activity

Progenitors of the zebrafish pronephros, red blood and trunk endothelium all originate from the ventral mesoderm and often share lineage with one another, suggesting that their initial patterning is linked. Previous studies have shown that spadetail (spt) mutant embryos, defective in tbx16 gene function, fail to produce red blood cells, but retain the normal number of endothelial and pronephric cells. We report here that spt mutants are deficient in all the types of early blood, have fewer endothelial cells as well as far more pronephric cells compared to wildtype. In vivo cell tracing experiments reveal that blood and endothelium originate in spt mutants almost exclusive from the dorsal mesoderm whereas, pronephros and tail originate from both dorsal and ventral mesoderm. Together these findings suggest possible defects in posterior patterning. In accord with this, gene expression analysis shows that mesodermal derivatives within the trunk and tail of spt mutants have acquired more posterior identity. Secreted signaling molecules belonging to the Fgf, Wnt and Bmp families have been implicated as patterning factors of the posterior mesoderm. Further investigation demonstrates that Fgf and Wnt signaling are elevated throughout the nonaxial region of the spt gastrula. By manipulating Fgf signaling we show that Fgfs both promote pronephric fate and repress blood and endothelial fate. We conclude that Tbx16 plays an important role in regulating the balance of intermediate mesoderm fates by attenuating Fgf activity.

Regulated tissue fluidity steers zebrafish body elongation

The tailbud is the posterior leading edge of the growing vertebrate embryo and consists of motile progenitors of the axial skeleton, musculature and spinal cord. We measure the 3D cell flow field of the zebrafish tailbud and identify changes in tissue fluidity revealed by reductions in the coherence of cell motion without alteration of cell velocities. We find a directed posterior flow wherein the polarization between individual cell motion is high, reflecting ordered collective migration. At the posterior tip of the tailbud, this flow makes sharp bilateral turns facilitated by extensive cell mixing due to increased directional variability of individual cell motions. Inhibition of Wnt or Fgf signaling or cadherin 2 function reduces the coherence of the flow but has different consequences for trunk and tail extension. Modeling and additional data analyses suggest that the balance between the coherence and rate of cell flow determines whether body elongation is linear or whether congestion forms within the flow and the body axis becomes contorted.

Development of the human tail bud and splanchnic mesenchyme

The purpose of this paper was to shed some light on anorectal development from a viewpoint of the tail bud and splanchnic mesenchyme for better understanding of the morphogenesis of the human anorectum. Human embryos ranging from Carnegie stage 11 to 23 (CS 11 to 23) were adopted in this study. Seventeen embryos preserved at the Congenital Anomaly Research Center of Kyoto University Graduate School of Medicine were histologically examined. The cloaca, extending caudally to the hindgut, was dramatically enlarged, particularly both its dorsal portion and membrane, that is, the cloacal membrane resulting from the development of the tailgut derived from the tail bud. The splanchnic mesenchyme surrounding the hindgut was spread out in the direction of the urorectal septum ventrally, suggesting that it participated in the formation of the septum. No fusion of the urorectal septum and the cloacal membrane was found. The splanchnic mesenchyme proliferated and developed into smooth muscle (circular and longitudinal) layers from cranial to caudal along the hindgut. The tail bud seems to cause both the adequate dilation of the dorsal cloaca and the elongation of the cloacal membrane; its dorsal portion in particular will be necessary for normal anorectal development. The splanchnic mesenchyme developed and descended toward the pectinate line and formed the internal sphincter muscle at the terminal bowel.

The differentiation and movement of presomitic mesoderm progenitor cells are controlled by Mesogenin 1

Somites are formed from the presomitic mesoderm (PSM) and give rise to the axial skeleton and skeletal muscles. The PSM is dynamic; somites are generated at the anterior end, while the posterior end is continually renewed with new cells entering from the tailbud progenitor region. Which genes control the conversion of tailbud progenitors into PSM and how is this process coordinated with cell movement? Using loss- and gain-of-function experiments and heat-shock transgenics we show in zebrafish that the transcription factor Mesogenin 1 (Msgn1), acting with Spadetail (Spt), has a central role. Msgn1 allows progression of the PSM differentiation program by switching off the progenitor maintenance genes ntl, wnt3a, wnt8 and fgf8 in the future PSM cells as they exit from the tailbud, and subsequently induces expression of PSM markers such as tbx24. msgn1 is itself positively regulated by Ntl/Wnt/Fgf, creating a negative-feedback loop that might be crucial to regulate homeostasis of the progenitor population until somitogenesis ends. Msgn1 drives not only the changes in gene expression in the nascent PSM cells but also the movements by which they stream out of the tailbud into the PSM. Loss of Msgn1 reduces the flux of cells out of the tailbud, producing smaller somites and an enlarged tailbud, and, by delaying exhaustion of the progenitor population, results in supernumerary tail somites. Through its combined effects on gene expression and cell movement, Msgn1 (with Spt) plays a key role both in genesis of the paraxial mesoderm and in maintenance of the progenitor population from which it derives.

Exploring the toxicity of a bismuth-asparagine coordination polymer on the early development of zebrafish embryos

Nanoparticles are widely used in nanomedicine, raising concerns about their toxicity. In this study, the toxicity of bismuth-asparagine coordination polymer spheres (BACP-2) was assessed in zebrafish embryos. Injection of 1-4 cell stage embryos with BACP-2 resulted in smaller head size (particularly smaller eye size), shorter body length, and pericardial edemas. The severity and occurrence of the resulting phenotype were concentration-dependent. The expression of genes such as krox20, orthodenticle homeobox 2 (otx2), and cardiac myosin light chain-2 (cmlc2) indicates that the effects of BACP-2 on the head and heart were related to changes in gene expression patterns. A delay in epiboly was observed, and the expression levels of the no tail (ntl) gene indicated that the delay in epiboly resulted both from the effect of BACP-2 on cell migration during epiboly and from slow growth. These findings indicate that BACP-2 exhibits concentration-dependent developmental toxicity, providing insight into the nanotoxicity of bismuth derivatives, which must be rigorously evaluated with respect to toxicity before their application in nanomedicine.

Crosstalk between Fgf and Wnt signaling in the zebrafish tailbud

Fibroblast growth factor (Fgf) and Wnt signaling are necessary for the intertwined processes of tail elongation, mesodermal development and somitogenesis. Here, we use pharmacological modifiers and time-resolved quantitative analysis of both nascent transcription and protein phosphorylation in the tailbud, to distinguish early effects of signal perturbation from later consequences related to cell fate changes. We demonstrate that Fgf activity elevates Wnt signaling by inhibiting transcription of the Wnt antagonists dkk1 and notum1a. PI3 kinase signaling also increases Wnt signaling via phosphorylation of Gsk3β. Conversely, Wnt can increase signaling within the Mapk branch of the Fgf pathway as Gsk3β phosphorylation elevates phosphorylation levels of Erk. Despite the reciprocal positive regulation between Fgf and Wnt, the two pathways generally have opposing effects on the transcription of co-regulated genes. This opposing regulation of target genes may represent a rudimentary relationship that manifests as out-of-phase oscillation of Fgf and Wnt target genes in the mouse and chick tailbud. In summary, these data suggest that Fgf and Wnt signaling are tightly integrated to maintain proportional levels of activity in the zebrafish tailbud, and this balance is important for axis elongation, cell fate specification and somitogenesis.

Diverse chemical scaffolds support direct inhibition of the membrane-bound O-acyltransferase porcupine

Secreted Wnt proteins constitute one of the largest families of intercellular signaling molecules in vertebrates with essential roles in embryonic development and adult tissue homeostasis. The functional redundancy of Wnt genes and the many forms of cellular responses they elicit, including some utilizing the transcriptional co-activator β-catenin, has limited the ability of classical genetic strategies to uncover their roles in vivo. We had previously identified a chemical compound class termed Inhibitor of Wnt Production (or IWP) that targets Porcupine (Porcn), an acyltransferase catalyzing the addition of fatty acid adducts onto Wnt proteins. Here we demonstrate that diverse chemical structures are able to inhibit Porcn by targeting its putative active site. When deployed in concert with small molecules that modulate the activity of Tankyrase enzymes and glycogen synthase kinase 3 β (GSK3β), additional transducers of Wnt/β-catenin signaling, the IWP compounds reveal an essential role for Wnt protein fatty acylation in eliciting β-catenin-dependent and -independent forms of Wnt signaling during zebrafish development. This collection of small molecules facilitates rapid dissection of Wnt gene function in vivo by limiting the influence of redundant Wnt gene functions on phenotypic outcomes and enables temporal manipulation of Wnt-mediated signaling in vertebrates.

BMP and non-canonical Wnt signaling are required for inhibition of secondary tail formation in zebrafish

The role of bone morphogenetic protein (BMP) signaling in specifying cell fate in the zebrafish tailbud has been well established. In addition to a loss of ventral tissues, such as ventral tailfin and cloaca, some embryos with compromised BMP signaling produce an additional phenotype: a ventrally located secondary tail containing both somitic muscle and notochord. This phenotype has been proposed to reflect a fate-patterning defect due to a change in a hypothesized BMP activity gradient. Here, we show that a defect in morphogenetic movements, not fate patterning, underlies the formation of secondary tails in BMP-inhibited embryos. Our data indicate that BMP signaling is activated in the ventroposterior tailbud to promote cell migration during tailbud protrusion, and that defective migration of these cells in BMP mutants ultimately leads to bifurcation of the caudal notochord. Additionally, we show that non-canonical Wnt signaling is also required for proper tail morphogenesis, possibly by maintaining cohesion of notochord progenitors by regulation of cadherin localization. We propose a model in which BMP and the non-canonical Wnt pathway regulate tail morphogenesis by controlling cell migration and cell adhesion within the tailbud.

Essential genes for astroglial development and axon pathfinding during zebrafish embryogenesis

The formation of the central nervous system depends on the coordinated development of neural and glial cell types that arise from a common precursor. Using an existing group of zebrafish mutants generated by viral insertion, we performed a "shelf-screen" to identify genes necessary for astroglial development and axon scaffold formation. We screened 274 of 315 viral insertion lines using antibodies that label axons (anti-Acetylated Tubulin) and astroglia (anti-Gfap) and identified 25 mutants with defects in gliogenesis, glial patterning, neurogenesis, and axon guidance. We also identified a novel class of mutants affecting radial glial cell numbers. Defects in astroglial patterning were always associated with axon defects, supporting an important role for axon-glial interactions during axon scaffold development. The genes disrupted in these viral lines have all been identified, providing a powerful new resource for the study of axon guidance, glio- and neurogenesis, and neuron-glial interactions during development of the vertebrate CNS.

Genome-wide RNAi screen reveals disease-associated genes that are common to Hedgehog and Wnt signaling

The Hedgehog (Hh) and Wnt signal transduction pathways are master regulators of embryogenesis and tissue renewal and represent anticancer therapeutic targets. Using genome-wide RNA interference screening in murine cultured cells, we established previously unknown associations between these signaling pathways and genes linked to developmental malformations, diseases of premature tissue degeneration, and cancer. We identified functions in both pathways for the multitasking kinase Stk11 (also known as Lkb1), a tumor suppressor implicated in lung and cervical cancers. We found that Stk11 loss resulted in disassembly of the primary cilium, a cellular organizing center for Hh pathway components, thus dampening Hh signaling. Loss of Stk11 also induced aberrant signaling through the Wnt pathway. Chemicals that targeted the Wnt acyltransferase Porcupine or that restored primary cilia length by inhibiting the tubulin deacetylase HDAC6 (histone deacetylase 6) countered deviant pathway activities driven by Stk11 loss. Our study demonstrates that Stk11 is a critical mediator in both the Hh and the Wnt pathways, and our approach provides a platform to support the development of targeted therapeutic strategies.

Structural components and morphogenetic mechanics of the zebrafish yolk extension, a developmental module

The yolk extension (YE) appears to be a novel developmental module that has been inserted into the phylotypic period of teleostean development, specifically in the order Cypriniformes. The zebrafish YE informs the study of morphogenetic movements reshaping ventral tissues because (1) this trait is easily visible, so disruptions are easy to score; (2) its ontogenesis occurs quickly; and (3) the yolk cell isolates the tissues elongating the ventrum from the rest of the embryo, serving as a three-dimensional in vivo "tissue culture." We determined that three histological compartments comprise the structural components of the YE: (1) the internal yolk cell; (2) the mesendodermal mantle external to the yolk cell; and (3) the external embryonic integument, consisting of an embryonic epidermis plus enveloping layer cells. These structural components interact with one another in a hierarchical manner, resulting in the morphogenesis of the elongated and tubular embryonic zebrafish ventrum as the cylindrical YE forms. Time-lapse videomicroscopy and experimental manipulation show that the yolk mass is a cohesive, viscoelastic foam, which resists compression. Moreover, as the mesodermal mantle participates in tubulation of the posterior trunk, Kupffer's Vesicle, the organ of laterality in teleosts, separates from the posterior pole of the yolk syncytial layer. Additionally, the embryonic integument becomes contractile over the posterior yolk cell, constricting the yolk mass to form the YE. These findings constitute an initial assessment of the morphogenetic mechanics underlying formation of the YE developmental module in zebrafish.

no tail integrates two modes of mesoderm induction

During early zebrafish development the nodal signalling pathway patterns the embryo into three germ layers, in part by inducing the expression of no tail (ntl), which is essential for correct mesoderm formation. When nodal signalling is inhibited ntl fails to be expressed in the dorsal margin, but ventral ntl expression is unaffected. These observations indicate that ntl transcription is under both nodal-dependent and nodal-independent regulation. Consistent with these observations and with a role for ntl in mesoderm formation, some somites form within the tail region of embryos lacking nodal signalling. In an effort to understand how ntl is regulated and thus how mesoderm forms, we have mapped the elements responsible for nodal-dependent and nodal-independent expression of ntl in the margin of the embryo. Our work demonstrates that expression of ntl in the margin is the consequence of two separate enhancers, which act to mediate different mechanisms of mesoderm formation. One of these enhancers responds to nodal signalling, and the other to Wnt and BMP signalling. We demonstrate that the nodal-independent regulation of ntl is essential for tail formation. Misexpression of Wnt and BMP ligands can induce the formation of an ectopic tail, which contains somites, in embryos devoid of nodal signalling, and this tail formation is dependent on ntl function. Similarly, nodal-independent tail somite formation requires ntl. At later stages in development ntl is required for notochord formation, and our analysis has also led to the identification of the enhancer required for ntl expression in the developing notochord.

A gene regulatory network directed by zebrafish No tail accounts for its roles in mesoderm formation

Using chromatin immunoprecipitation combined with genomic microarrays we have identified targets of No tail (Ntl), a zebrafish Brachyury ortholog that plays a central role in mesoderm formation. We show that Ntl regulates a downstream network of other transcription factors and identify an in vivo Ntl binding site that resembles the consensus T-box binding site (TBS) previously identified by in vitro studies. We show that the notochord-expressed gene floating head (flh) is a direct transcriptional target of Ntl and that a combination of TBSs in the flh upstream region are required for Ntl-directed expression. Using our genome-scale data we have assembled a preliminary gene regulatory network that begins to describe mesoderm formation and patterning in the early zebrafish embryo.

Identification of common and unique modifiers of zebrafish midline bifurcation and cyclopia

Loss of the zebrafish Nodal-related protein Squint causes a spectrum of phenotypes including cyclopia and midline bifurcations (MB). Here we examine MBs and their relation to cyclopia in maternal-zygotic squint (MZsqt) mutants. There is a concordance of MB with cyclopia in MZsqt embryos. Heat treatment and depletion of Hsp90a are "common" risk factors, each of which increases the incidence of both phenotypes. Midline identity is specified on both sides of MBs, and deep-layer cells are initially lacking within bifurcations, whereas enveloping layer cells are intact. Bifurcations do not appear until the completion of gastrulation and are preceded by gaps in the expression of wnt5b, an essential regulator of dorsal convergence. The incidence of early MBs and wnt5b expression defects in heated MZsqt embryos is high, but there is also substantial recovery. Wnt5b depletion increases the incidence of MB, but not cyclopia, and as such Wnt5b is a "unique" risk factor for MB. Reciprocally, depletion of Wnt11 or Hsp90b increases cyclopia only. In summary, we find that MB arises after gastrulation in regions that fail to express wnt5b, and we show that two complex dysmorphologies - MB and cyclopia - can be promoted by either common or unique risk factors.

Localised axial progenitor cell populations in the avian tail bud are not committed to a posterior Hox identity

The outgrowth of the vertebrate tail is thought to involve the proliferation of regionalised stem/progenitor cell populations formed during gastrulation. To follow these populations over extended periods, we used cells from GFP-positive transgenic chick embryos as a source for donor tissue in grafting experiments. We determined that resident progenitor cell populations are localised in the chicken tail bud. One population, which is located in the chordoneural hinge (CNH), contributes descendants to the paraxial mesoderm, notochord and neural tube, and is serially transplantable between embryos. A second population of mesodermal progenitor cells is located in a separate dorsoposterior region of the tail bud, and a corresponding population is present in the mouse tail bud. Using heterotopic transplantations, we show that the fate of CNH cells depends on their environment within the tail bud. Furthermore, we show that the anteroposterior identity of tail bud progenitor cells can be reset by heterochronic transplantation to the node region of gastrula-stage chicken embryos.

The migration of paraxial and lateral plate mesoderm cells emerging from the late primitive streak is controlled by different Wnt signals

BACKGROUND

Co-ordinated cell movement is a fundamental feature of developing embryos. Massive cell movements occur during vertebrate gastrulation and during the subsequent extension of the embryonic body axis. These are controlled by cell-cell signalling and a number of pathways have been implicated. Here we use long-term video microscopy in chicken embryos to visualize the migration routes and movement behaviour of mesoderm progenitor cells as they emerge from the primitive streak (PS) between HH stages 7 and 10.

RESULTS

We observed distinct cell movement behaviours along the length of the streak and determined that this is position dependent with cells responding to environmental cues. The behaviour of cells was altered by exposing embryos or primitive streak explants to cell pellets expressing Wnt3a and Wnt5a, without affecting cell fates, thus implicating these ligands in the regulation of cell movement behaviour. Interestingly younger embryos were not responsive, suggesting that Wnt3a and Wnt5a are specifically involved in the generation of posterior mesoderm, consistent with existing mouse and zebrafish mutants. To investigate which downstream components are involved mutant forms of dishevelled (dsh) and prickle1 (pk1) were electroporated into the primitive streak. These had differential effects on the behaviour of mesoderm progenitors emerging from anterior or posterior regions of the streak, suggesting that multiple Wnt pathways are involved in controlling cell migration during extension of the body axis in amniote embryos.

CONCLUSION

We suggest that the distinct behaviours of paraxial and lateral mesoderm precursors are regulated by the opposing actions of Wnt5a and Wnt3a as they leave the primitive streak in neurula stage embryos. Our data suggests that Wnt5a acts via prickle to cause migration of cells from the posterior streak. In the anterior streak, this is antagonised by Wnt3a to generate non-migratory medial mesoderm.

Regionally autonomous segmentation within zebrafish presomitic mesoderm

Similar to chick, mouse, and Xenopus, a number of intercellular signaling pathways are required for zebrafish segmentation. However, the spatial scales over which these signaling pathways operate in zebrafish remain largely unknown. During zebrafish segmentation, waves of her1 transcription (a) initiate within the tailbud region, (b) propagate anteriorly through presomitic mesoderm (PSM), and (c) terminate in the anlage of newly-forming somites. These observations raise the question of whether the tailbud region serves as a "pacemaker" or "organizing center" for the initiation of propagating her1 expression waves. Microsurgical manipulations reveal that the anteriorly waves of her1 transcription are not perturbed by removal of the zebrafish tailbud. Furthermore, expression patterns of deltaD, paraxial protocadherin C (papc), and myoD within recently formed somites also appear to be relatively unperturbed by either removal of the tailbud, or by removal of lateral plate mesoderm. Although dynamic gene networks rapidly specify mesenchymal or epithelial cellular identities within forming somites, this specification is plastic. Time-lapse analysis has shown that the cellular progeny of mitotically-active epithelial border cells within newly formed somites can adopt different cellular identities than their precursor cells. Overall, these results indicate that sustained long-range intercellular communication with the tailbud, anterior somites, or lateral plate mesoderm is not necessary for segmentation or somitogenesis to proceed within the PSM. The basic segmentation and somitogenesis processes in zebrafish presomitic mesoderm appear to be largely regionally autonomous and governed by local morphogenetic cell behaviors.

Brachyury-downstream gene sets in a chordate, Ciona intestinalis: integrating notochord specification, morphogenesis and chordate evolution

In vertebrates, Brachyury, a T-box transcription factor gene, seems to have a dual role in the differentiation of axial midline mesoderm cells into notochord and gastrulation cell movements regulated by non-canonical Wnt/planar cell polarity (Wnt/PCP) signaling. To understand the function of Brachyury-downstream genes in chordate embryos, from a series of our survey on differential expression, including subtractive hybridization, dot-blot assays, EST sequences and the expression patterns in whole-mount in situ hybridization at embryonic stages, we developed a knowledge database called "CINOBI: CionaNotochord and Brachyury-downstream gene Index" to create comprehensive catalogues of Brachyury-downstream gene sets in Ciona intestinalis. Combining genome and large-scale cDNA data, we were able to characterize 450 non-redundant Brachyury-downstream genes: Twenty-four genes were newly annotated as notochord-expressed genes. Several genes are components of signaling pathways such as Wnt/PCP, Nf kappaB and TGF-beta signaling. We propose that Brachyury is linked to these pathways regulating the expression of each component, and such a regulatory mechanism might be conserved among chordates.

Apoptosis in zebrafish embryos: removing cells from inappropriate locations

The purpose of this study was to analyze by transmission electron microscopy the morphology and dynamics of the excess cells remaining in the truncated tail of the zebrafish compound mutant, knypek;no tail (kny;ntl). We mainly focused on two periods during tail extension: first, the stage when the primordia of both notochord and spinal cord should be already formed; second, the stage when morphogenesis of the tail is almost complete. Not only double mutants but also wild-type (wt) embryos were examined to compare with. A massive wave of morphologically normal apoptotic cellular death was detected by 22 h of development in the kny;ntl compound mutants. However, apoptosis was a rare event in the tail of wt embryos. We described the ultrastructural features and possible evolution of the apoptotic cells in the tail region, and also assayed the distribution of apoptotic cells in the different tissues conforming the tail: the covering epithelium or epidermis, the bulk of mesenchymal tissue, and the primordia of the spinal cord and the notochord. Huge amounts of apoptotic bodies were found in the mesenchymal tissue of the double-mutant embryos, just the location where excess cells are erroneously accumulated inside the truncated tail. This fact, together with striking findings regarding both some morphological aspects and the distribution of apoptosis, is discussed and suggested to be an indirect consequence of the genetic defects in kny;ntl mutants.

Ciliary function and Wnt signal modulation

With the increase in complexity of morphogenetic signaling cascades over the course of evolution and the emergence of broadly ciliated organisms, the cilium seems to have acquired a role as regulator of paracrine signal transduction. Recently, several lines of evidence have provided a link between basal body and ciliary proteins and Wnt signaling. In this chapter, we will evaluate the evidence linking the basal body and cilium with the regulation of beta-catenin-dependent (canonical) and beta-catenin-independent (noncanonical) signaling processes as well as which role(s) Wnt signaling might play in ciliogenesis. In addition, we will discuss aberrant Wnt signaling could contribute to phenotypes common to most ciliopathies and why these phenotypes might be driven by loss of noncanonical rather than gain of noncanonical Wnt signaling.

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.

Cadherin-mediated adhesion regulates posterior body formation

BACKGROUND

The anterior-posterior axis of the vertebrate embryo undergoes a dramatic elongation during early development. Convergence and extension of the mesoderm, occurring during gastrulation, initiates the narrowing and lengthening of the embryo. However the lengthening of the axis continues during post-gastrula stages in the tailbud region, and is thought to involve convergent extension movements as well as other cell behaviors specific to posterior regions.

RESULTS

We demonstrate here, using a semi-dominant N-cadherin allele, that members of the classical cadherin subfamily of cell-cell adhesion molecules are required for tailbud elongation in the zebrafish. In vivo imaging of cell behaviors suggests that the extension of posterior axial mesodermal cells is impaired in embryos that carry the semi-dominant N-cadherin allele. This defect most likely results from a general loss of cell-cell adhesion in the tailbud region. Consistent with these observations, N-cadherin is expressed throughout the tailbud during post-gastrulation stages. In addition, we show that N-cadherin interacts synergistically with vang-like 2, a member of the non-canonical Wnt signaling/planar cell polarity pathway, to mediate tail morphogenesis.

CONCLUSION

We provide the first evidence here that N-cadherin and other members of the classical cadherin subfamily function in parallel with the planar cell polarity pathway to shape the posterior axis during post-gastrulation stages. These findings further highlight the central role that adhesion molecules play in the cellular rearrangements that drive morphogenesis in vertebrates and identify classical cadherins as major contributors to tail development.

Disruption of the basal body compromises proteasomal function and perturbs intracellular Wnt response

Primary cilia and basal bodies are evolutionarily conserved organelles that mediate communication between the intracellular and extracellular environments. Here we show that bbs1, bbs4 and mkks (also known as bbs6), which encode basal body proteins, are required for convergence and extension in zebrafish and interact with wnt11 and wnt5b. Suppression of bbs1, bbs4 and mkks transcripts results in stabilization of beta-catenin with concomitant upregulation of T-cell factor (TCF)-dependent transcription in both zebrafish embryos and mammalian ciliated cells, a defect phenocopied by the silencing of the axonemal kinesin subunit KIF3A but not by chemical disruption of the cytoplasmic microtubule network. These observations are attributable partly to defective degradation by the proteasome; suppression of BBS4 leads to perturbed proteasomal targeting and concomitant accumulation of cytoplasmic beta-catenin. Cumulatively, our data indicate that the basal body is an important regulator of Wnt signal interpretation through selective proteolysis and suggest that defects in this system may contribute to phenotypes pathognomonic of human ciliopathies.

Convergence and extension movements mediate the specification and fate maintenance of zebrafish slow muscle precursors

During vertebrate gastrulation, concurrent inductive events and cell movements fashion the body plan. Convergence and extension (C&E) gastrulation movements narrow the vertebrate embryonic body mediolaterally while elongating it rostrocaudally. Segmented somites are shaped and positioned by C&E alongside the notochord and differentiate into skeleton, fast, and slow muscles during somitogenesis. In zebrafish, simultaneous inactivation of non-canonical Wnt signaling components Knypek and Trilobite strongly impairs C&E gastrulation movements. Here we show that knypek;trilobite double mutants exhibit a severe deficit in slow muscles and their precursor, adaxial cells, revealing essential roles of C&E movements in adaxial cell development. Adaxial cells become distinguishable in the presomitic mesoderm during late gastrulation by their expression of myogenic factors and axial-adjacent position. Using cell tracing analyses and genetic manipulations, we demonstrate that C&E movements regulate the number of prospective adaxial cells specified during gastrulation by determining the size of the interface between the inductive axial and target presomitic tissues. During segmentation, when the range of Hedgehog signaling from the axial tissue declines, tight apposition of prospective adaxial cells to the notochord, which is achieved by convergence movements, is necessary for their continuous Hedgehog reception and fate maintenance. We provide direct evidence to show that the deficiency of adaxial cells in knypek;trilobite double mutants is due to impaired C&E movements, rather than an alteration in Hedgehog signal and its reception, or a cell-autonomous requirement for Knypek and Trilobite in adaxial cell development. Our results underscore the significance of precise coordination between cell movements and inductive tissue interactions during cell fate specification.

Ledgerline, a Novel Xenopus laevis Gene, Regulates Differentiation of Presomitic Mesoderm During Somitogenesis

Segmentation of the vertebrate body via the sequential formation of somites is an important process in embryogenesis. This sequential process is governed by the activation and regulation of Notch-related molecular oscillators by fibroblast growth factor and retinoic acid (RA) signaling. In this study, we identified ledgerline, a novel gene of Xenopus laevis expressed specifically in the presomitic mesoderm. Knockdown of ledgerline using antisense morpholino oligonucleotides shifted the developing somite front and altered the expression of genes that regulate molecular oscillation, including Delta2, ESR5, Hairy2a, and Thylacine1. Knockdown of ledgerline also downregulated RALDH-2 expression. Injection of RARalpha-CA, a constitutively active mutant of the RA receptor RARalpha, subsequently reduced the altered Thylacine1 expression. These results strongly suggest that ledgerline is essential for mesodermal RA activity and differentiation of the presomitic mesoderm during Xenopus somitogenesis.

RhoA acts downstream of Wnt5 and Wnt11 to regulate convergence and extension movements by involving effectors Rho Kinase and Diaphanous: Use of zebrafish as an in vivo model for GTPase signaling

Gastrulation shapes the early embryos by forming three germ layers, ectoderm, mesoderm and endoderm. In vertebrates, this process requires massive cell rearrangement including convergence and extension (CE) movements that involve narrowing and lengthening of embryonic tissues as well as cell elongation. Such polarization and movements require precise reorganization and regulation of the cytoskeleton network and cell adhesion. Rho small GTPases are key regulators for dynamic actin cytoskeleton. However, the signaling mechanisms underlying their functions in CE remain to be further elucidated. We have cloned the zebrafish Danio rerio rhoA and by capitalizing on the specific functional knockdown using morpholinos against rhoA and the availability of CE mutants defective in Wnt signaling, we showed that rhoA morphants were reminiscent to noncanonical wnt mutants with serious disruption in CE movements. Injection of rhoA mRNA effectively rescued such defects in wnt5 and wnt11 mutants. Furthermore, CE defects in rhoA knockdown or wnt mutants can be suppressed through functional bypass after ectopic expression of the two mammalian Rho effectors, the Rho kinase and Diaphanous (mDia). These results provide the first evidence that the RhoA in vivo acts downstream of Wnt5 and Wnt11 to effect, without affecting cell fates, on the CE movements in zebrafish embryos. Significantly, it elicits such effect via both effectors, Rho kinase and Dia. These findings also support the versatility of the zebrafish as a model to further investigate the roles of various classes of small GTPases in regulating cell dynamics in vivo.

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.

In vivo cell biology: following the zebrafish trend

A deeper understanding of the mechanisms of cell behavior is essential if we want to comprehend how an organism develops and functions. Changes in cellular processes, including the orientation of cell divisions, cell shape, polarity, differentiation and migration, account for tissue rearrangements during development and homeostasis. The in vivo relevance of in vitro findings is being constantly debated and the need for in vivo systems becoming more pressing. The zebrafish (Danio rerio) might become the vertebrate system of choice for a wide spectrum of biological questions that need to be investigated in vivo at cellular and subcellular resolutions. Here, we discuss some recent studies in which the zebrafish was used to gain insight into cell-biological mechanisms. Although this model system has been predominantly appreciated for its amenability to forward genetics, current advances in imaging technology and an increasing number of transgenic lines are bringing it closer to its full potential.

Efficient RNA interference in zebrafish embryos using siRNA synthesized with SP6 RNA polymerase

Double-stranded RNA (dsRNA) has been shown to be a useful tool for silencing genes in zebrafish (Danio rerio), while the blocking specificity of dsRNA is still of major concern for application. It was reported that siRNA (small interfering RNA) prepared by endoribonuclease digestion (esiRNA) could efficiently silence endogenous gene expression in mammalian embryos. To test whether esiRNA could work in zebrafish, we utilized Escherichia coli RNaseIII to digest dsRNA of zebrafish no tail (ntl), a mesoderm determinant in zebrafish and found that esi-ntl could lead to developmental defects, however, the effective dose was so close to the toxic dose that esi-ntl often led to non-specific developmental defects. Consequently, we utilized SP6 RNA polymerase to produce si-ntl, siRNA designed against ntl, by in vitro transcription. By injecting in vitro synthesized si-ntl into zebrafish zygotes, we obtained specific phenocopies of reported mutants of ntl. We achieved up to a 59%no tail phenotype when the injection concentration was as high as 4 microg/microL. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization analysis showed that si-ntl could largely and specifically reduce mRNA levels of the ntl gene. As a result, our data indicate that esiRNA is unable to cause specific developmental defects in zebrafish, while siRNA should be an alternative for downregulation of specific gene expression in zebrafish in cases where RNAi techniques are applied to zebrafish reverse genetics.

Wnt signalling mediated by Tbx2b regulates cell migration during formation of the neural plate

During gastrulation, optimal adhesion and receptivity to signalling cues are essential for cells to acquire new positions and identities via coordinated cell movements. T-box transcription factors and the Wnt signalling pathways are known to play important roles in these processes. Zebrafish tbx2b, a member of the TBX2 family, has previously been shown to be required for the specification of midline mesoderm. We show here that tbx2b transcripts are present during mid-gastrula before its expression is detected by whole-mount in situ hybridization. Isolated ectodermal cells deficient in Tbx2b have altered cell surface properties and the level of cadherins in these cells is lower. In chimaeric embryos generated by cell transplantation and single blastomere injections, Tbx2b-deficient cells are defective in cell movement in a cell-autonomous manner, resulting in their exclusion from the developing neural plate. Using this ;exclusion' phenotype as a screen, we show that Tbx2b acts within the context of Fz7 signalling. The exclusion of cells lacking T-box proteins in chimeras during development was demonstrated with other T-box genes and may indicate a general functional mechanism for T-box proteins.

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

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