Xwnt11 is a target of Xenopus Brachyury: regulation of gastrulation movements via Dishevelled, but not through the canonical Wnt pathway

Highly conserved functions of the Brachyury gene on morphogenetic movements: insight from the early-diverging phylum Ctenophora

Brachyury, a member of the T-box transcription family identified in a diverse array of metazoans, was initially recognized for its function in mesoderm formation and notochord differentiation in vertebrates; however, its ancestral role has been suggested to be in control of morphogenetic movements. Here, we show that morpholino oligonucleotide knockdown of Brachyury (MlBra) in embryos of a ctenophore, one of the most ancient groups of animals, prevents the invagination of MlBra expressing stomodeal cells and is rescued with corresponding RNA injections. Injection of RNA encoding a dominant-interfering construct of MlBra causes identical phenotypes to that of RNA encoding a dominant-interfering form of Xenopus Brachyury (Xbra) in Xenopus embryos. Both injected embryos down-regulate Xbra downstream genes, Xbra itself and Xwnt11 but not axial mesodermal markers, resulting in failure to complete gastrulation due to loss of convergent extension movements. Moreover, animal cap assay reveals that MlBra induces Xwnt11 like Xbra. Overall results using Xenopus embryos show that these two genes are functionally interchangeable. These functional experiments demonstrate for the first time in a basal metazoan that the primitive role of Brachyury is to regulate morphogenetic movements, rather than to specify endomesodermal fates, and the role is conserved between non-bilaterian metazoans and vertebrates.

Line up and listen: Planar cell polarity regulation in the mammalian inner ear

The inner ear sensory organs possess extraordinary structural features necessary to conduct mechanosensory transduction for hearing and balance. Their structural beauty has fascinated scientists since the dawn of modern science and ensured a rigorous pursuit of the understanding of mechanotransduction. Sensory cells of the inner ear display unique structural features that underlie their mechanosensitivity and resolution, and represent perhaps the most distinctive form of a type of cellular polarity, known as planar cell polarity (PCP). Until recently, however, it was not known how the precise PCP of the inner ear sensory organs was achieved during development. Here, we review the PCP of the inner ear and recent advances in the quest for an understanding of its formation.

The involvement of lethal giant larvae and Wnt signaling in bottle cell formation in Xenopus embryos

Lethal giant larvae (Lgl) plays a critical role in establishment of cell polarity in epithelial cells. While Frizzled/Dsh signaling has been implicated in the regulation of the localization and activity of Lgl, it remains unclear whether specific Wnt ligands are involved. Here we show that Wnt5a triggers the release of Lgl from the cell cortex into the cytoplasm with the concomitant decrease in Lgl stability. The observed changes in Lgl localization were independent of atypical PKC (aPKC), which is known to influence Lgl distribution. In ectodermal cells, both Wnt5a and Lgl triggered morphological and molecular changes characteristic of apical constriction, whereas depletion of their functions prevented endogenous and ectopic bottle cell formation. Furthermore, Lgl RNA partially rescued bottle cell formation in embryos injected with a dominant negative Wnt5a construct. These results suggest a molecular link between Wnt5a and Lgl that is essential for apical constriction during vertebrate gastrulation.

Regulation of convergence and extension movements during vertebrate gastrulation by the Wnt/PCP pathway

Vertebrate gastrulation entails massive cell movements that establish and shape the germ layers. During gastrulation, the individual cell behaviors are strictly coordinated in time and space by various signaling pathways. These pathways instruct the cells about proliferation, shape, fate and migration into proper location. Convergence and extension (C&E) movements during vertebrate gastrulation play a major role in the shaping of the embryonic body. In vertebrates, the Wnt/Planar Cell Polarity (Wnt/PCP) pathway is a key regulator of C&E movements, essential for several polarized cell behaviors, including directed cell migration, and mediolateral and radial cell intercalation. However, the molecular mechanisms underlying the acquisition of Planar Cell Polarity by highly dynamic mesenchymal cells engaged in C&E are still not well understood. Here we review new evidence implicating the Wnt/PCP pathway in specific cell behaviors required for C&E during zebrafish gastrulation, in comparison to other vertebrates. We also discuss findings on the molecular regulation and the interaction of the Wnt/PCP pathway with other signaling pathways during gastrulation movements.

Complex and dynamic patterns of Wnt pathway gene expression in the developing chick forebrain

Background

Wnt signalling regulates multiple aspects of brain development in vertebrate embryos. A large number of Wnts are expressed in the embryonic forebrain; however, it is poorly understood which specific Wnt performs which function and how they interact. Wnts are able to activate different intracellular pathways, but which of these pathways become activated in different brain subdivisions also remains enigmatic.

Results

We have compiled the first comprehensive spatiotemporal atlas of Wnt pathway gene expression at critical stages of forebrain regionalisation in the chick embryo and found that most of these genes are expressed in strikingly dynamic and complex patterns. Several expression domains do not respect proposed compartment boundaries in the developing forebrain, suggesting that areal identities are more dynamic than previously thought. Using an in ovo electroporation approach, we show that Wnt4 expression in the thalamus is negatively regulated by Sonic hedgehog (Shh) signalling from the zona limitans intrathalamica (ZLI), a known organising centre of forebrain development.

Conclusion

The forebrain is exposed to a multitude of Wnts and Wnt inhibitors that are expressed in a highly dynamic and complex fashion, precluding simple correlative conclusions about their respective functions or signalling mechanisms. In various biological systems, Wnts are antagonised by Shh signalling. By demonstrating that Wnt4 expression in the thalamus is repressed by Shh from the ZLI we reveal an additional level of interaction between these two pathways and provide an example for the cross-regulation between patterning centres during forebrain regionalisation.

Wnt5 is required for notochord cell intercalation in the ascidian Halocynthia roretzi

BACKGROUND INFORMATION

In the embryos of various animals, the body elongates after gastrulation by morphogenetic movements involving convergent extension. The Wnt/PCP (planar cell polarity) pathway plays roles in this process, particularly mediolateral polarization and intercalation of the embryonic cells. In ascidians, several factors in this pathway, including Wnt5, have been identified and found to be involved in the intercalation process of notochord cells.

RESULTS

In the present study, the role of the Wnt5 genes, Hr-Wnt5alpha (Halocynthia roretzi Wnt5alpha) and Hr-Wnt5beta, in convergent extension was investigated in the ascidian H. roretzi by injecting antisense oligonucleotides and mRNAs into single precursor blastomeres of various tissues, including notochord, at the 64-cell stage. Hr-Wnt5alpha is expressed in developing notochord and was essential for notochord morphogenesis. Precise quantitative control of its expression level was crucial for proper cell intercalation. Overexpression of Wnt5 proteins in notochord and other tissues that surround the notochord indicated that Wnt5alpha plays a role within the notochord, and is unlikely to be the source of polarizing cues arising outside the notochord. Detailed mosaic analysis of the behaviour of individual notochord cells overexpressing Wnt5alpha indicated that a Wnt5alpha-manipulated cell does not affect the behaviour of neighbouring notochord cells, suggesting that Wnt5alpha works in a cell-autonomous manner. This is further supported by comparison of the results of Wnt5alpha and Dsh (Dishevelled) knockdown experiments. In addition, our results suggest that the Wnt/PCP pathway is also involved in mediolateral intercalation of cells of the ventral row of the nerve cord (floor plate) and the endodermal strand.

CONCLUSION

The present study highlights the role of the Wnt5alpha signal in notochord convergent extension movements in ascidian embryos. Our results raise the novel possibility that Wnt5alpha functions in a cell-autonomous manner in activation of the Wnt/PCP pathway to polarize the protrusive activity that drives convergent extension.

Dickkopf-1: a suitable target for the management of myeloma bone disease

Bone disease remains a major problem in the management of patients with multiple myeloma (MM) and is characterized by the presence of lytic lesions due to increased osteoclastic activity and reduced osteoblast function. Wingless-type and integrase 1 (Wnt)/beta-catenin signaling is a central pathway for bone development and homeostasis. Dickkopf-1 (Dkk-1) is a soluble inhibitor of Wnt, which disrupts osteoblast differentiation and action. Dkk-1 is produced by myeloma cells and overexpressed in myeloma microenvironment of patients with extensive bone disease. In addition to its direct inhibitory effect of Dkk-1 on osteoblasts, Dkk-1 disrupts the Wnt3a-regulated osteoprotegerin and receptor activator of NF-kappaB ligand (RANKL) expression in osteoblasts and thus it indirectly enhances osteoclast function in MM. Dkk-1 serum and bone marrow plasma levels are increased in MM patients and correlated with advanced International Staging System stage and presence of osteolytic lesions. Preclinical studies in mouse myeloma models showed that targeting Dkk-1 with neutralizing anti-Dkk-1 antibodies resulted in increased numbers of osteoblasts, reduced numbers of multinucleated osteoclasts and increased bone volume. The bone anabolic effect of anti-Dkk-1 may also be associated with reduced myeloma burden. These data show that Dkk-1 has a pivotal role in bone health and disease and is a novel target for the management of myeloma patients with lytic bone disease.

Progress and challenges in understanding planar cell polarity signaling

During development, epithelial cells in some tissues acquire a polarity orthogonal to their apical-basal axis. This polarity, referred to as planar cell polarity (PCP), or tissue polarity, is essential for the normal physiological function of many epithelia. Early studies of PCP focused on insect epithelia (Lawrence, 1966 [1]), and the earliest genetic analyses were carried out in Drosophila (Held et al., 1986; Gubb and Garcia-Bellido, 1982 [2,3]). Indeed, most of our mechanistic understanding of PCP derives from the ongoing use of Drosophila as a model system. However, a range of medically important developmental defects and physiological processes are under the control of PCP mechanisms that appear to be at least partially conserved, driving considerable interest in studying PCP both in Drosophila and in vertebrate model systems. Here, I present a model of the PCP signaling mechanism based on studies in Drosophila. I highlight two areas in which our understanding is deficient, and which lead to current confusion in the literature. Future studies that shed light on these areas will substantially enhance our understanding of the fascinating yet challenging problem of understanding the mechanisms that generate PCP.

A microarray screen for direct targets of Zic1 identifies an aquaporin gene, aqp-3b, expressed in the neural folds

The Zic1 transcription factor plays multiple roles during early development, for example, in patterning the early neural plate and formation of the neural crest, somites, and cerebellum. To identify direct downstream target genes of Zic1, a microarray screen was conducted in Xenopus laevis that identified 85 genes upregulated twofold or more. These include transcription factors, receptors, enzymes, proteins involved in retinoic acid signaling, and an aquaglyceroporin (aqp-3b), but surprisingly no genes known to be involved in cell proliferation. We show that both aqp-3 and aqp-3b were expressed in adult tissues, while during early embryonic development, only aqp-3b was transcribed. During neurula stages, aqp-3b was expressed specifically in the neural folds. This pattern of aqp-3b expression closely resembled that of NF-protocadherin (NFPC), which is involved in cell adhesion and neural tube closure. Aqp-3b may also be involved in neural tube closure, since mammalian Aqp-3 promotes cell migration and proliferation.

Active cell movements coupled to positional induction are involved in lineage segregation in the mouse blastocyst

In the mouse blastocyst, some cells of the inner cell mass (ICM) develop into primitive endoderm (PE) at the surface, while deeper cells form the epiblast. It remained unclear whether the position of cells determines their fate, such that gene expression is adjusted to cell position, or if cells are pre-specified at random positions and then sort. We have tracked and characterised dynamics of all ICM cells from the early to late blastocyst stage. Time-lapse microscopy in H2B-EGFP embryos shows that a large proportion of ICM cells change position between the surface and deeper compartments. Most of this cell movement depends on actin and is associated with cell protrusions. We also find that while most cells are precursors for only one lineage, some give rise to both, indicating that lineage segregation is not complete in the early ICM. Finally, changing the expression levels of the PE marker Gata6 reveals that it is required in surface cells but not sufficient for the re-positioning of deeper cells. We provide evidence that Wnt9A, known to be expressed in the surface ICM, facilitates re-positioning of Gata6-expressing cells. Combining these experimental results with computer modelling suggests that PE formation involves both cell sorting movements and position-dependent induction.

Proximal events in Wnt signal transduction

The Wnt family of secreted ligands act through many receptors to stimulate distinct intracellular signalling pathways in embryonic development, in adults and in disease processes. Binding of Wnt to the Frizzled family of receptors and to low density lipoprotein receptor-related protein 5 (LRP5) or LRP6 co-receptors stimulates the intracellular Wnt-beta-catenin signalling pathway, which regulates beta-cateninstability and context-dependent transcription. This signalling pathway controls many processes, such as cell fate determination, cell proliferation and self-renewal of stem and progenitor cells. Intriguingly, the transmembrane receptor Tyr kinases Ror2 and Ryk, as well as Frizzledreceptors that act independently of LRP5 or LRP6, function as receptors for Wnt and activate beta-catenin-independent pathways. This leads to changes in cell movement and polarity and to the antagonism of the beta-catenin pathway.

Wnt9b signaling regulates planar cell polarity and kidney tubule morphogenesis

Although many vertebrate organs, such as kidneys, lungs and liver, are composed of epithelial tubules, little is known of the mechanisms that establish the length or diameter of these tubules. In the kidney, defects in the establishment or maintenance of tubule diameter are associated with one of the most common inherited human disorders, polycystic kidney disease. Here we show that attenuation of Wnt9b signaling during kidney morphogenesis affects the planar cell polarity of the epithelium and leads to tubules with significantly increased diameter. Although previous studies showed that polarized cell divisions maintain the diameter of postnatal kidney tubules, we find that cell divisions are randomly oriented during embryonic development. Our data suggest that diameter is established during early morphogenetic stages by convergent extension processes and maintained by polarized cell divisions. Wnt9b, signaling through the non-canonical Rho/Jnk branch of the Wnt pathway, is necessary for both of these processes.

Molecular basis of morphogenesis during vertebrate gastrulation

Gastrulation is a crucial step in early embryogenesis. During gastrulation, a set of morphogenetic processes takes place leading to the establishment of the basic body plan and formation of primary germ layers. A rich body of knowledge about these morphogenetic processes has been accumulated over decades. The understanding of the molecular mechanism that controls the complex cell movement and inductive processes during gastrulation remains a challenge. Substantial progress has been made recently to identify and characterize pathways and molecules implicated in the modulation of morphogenesis during vertebrate gastrulation. Here, we summarize recent findings in the analysis of signaling pathways implicated in gastrulation movements, with the aim to generalize the basic molecular principles of vertebrate morphogenesis.

Blocking Dishevelled signaling in the noncanonical Wnt pathway in sea urchins disrupts endoderm formation and spiculogenesis, but not secondary mesoderm formation

Dishevelled (Dsh) is a phosphoprotein key to beta-catenin dependent (canonical) and beta-catenin independent (noncanonical) Wnt signaling. Whereas canonical Wnt signaling has been intensively studied in sea urchin development, little is known about other Wnt pathways. To examine roles of these beta-catenin independent pathways in embryogenesis, we used Dsh-DEP, a deletion construct blocking planar cell polarity (PCP) and Wnt/Ca(2+) signaling. Embryos overexpressing Dsh-DEP failed to gastrulate or undergo skeletogenesis, but produced pigment cells. Although early mesodermal gene expression was largely unperturbed, embryos exhibited reduced expression of genes regulating endoderm specification and differentiation. Overexpressing activated beta-catenin failed to rescue Dsh-DEP embryos, indicating that Dsh-DEP blocks endoderm formation downstream of initial canonical Wnt signaling. Because Dsh-DEP-like constructs block PCP signaling in other metazoans, and disrupting RhoA or Fz 5/8 in echinoids blocks subsets of the Dsh-DEP phenotypes, our data suggest that noncanonical Wnt signaling is crucial for sea urchin endoderm formation and skeletogenesis.

A quest for the mechanism regulating global planar cell polarity of tissues

Most epithelial cells, besides their ubiquitous apical-basal polarity, are polarized within the plane of the epithelium, which is called planar cell polarity (PCP). Using Drosophila as a model, meaningful progress has been made in the identification of key PCP factors and the dissection of their intracellular molecular interactions. The long-range, global aspects of coordinated polarization and the overlying regulatory mechanisms that create the initial polarity direction have, however, remained elusive. Several recent publications have outlined potential mechanisms of how the global regulation of PCP might be controlled and how the distinct core factor groups might interact via frizzled, Van Gogh or flamingo. This review focuses on these exciting features and attempts to provide an integrated picture of these recent and novel insights.

Conformational stability and DNA binding specificity of the cardiac T-box transcription factor Tbx20

The transcription factor Tbx20 acts within a hierarchy of T-box factors in lineage specification and morphogenesis in the mammalian heart and is mutated in congenital heart disease. T-box family members share a approximately 20-kDa DNA-binding domain termed the T-box. The question of how highly homologous T-box proteins achieve differential transcriptional control in heart development, while apparently binding to the same DNA sequence, remains unresolved. Here we show that the optimal DNA recognition sequence for the T-box of Tbx20 corresponds to a T-half-site. Furthermore, we demonstrate using purified recombinant domains that distinct T-boxes show significant differences in the affinity and kinetics of binding and in conformational stability, with the T-box of Tbx20 displaying molten globule character. Our data highlight unique features of Tbx20 and suggest mechanistic ways in which cardiac T-box factors might interact synergistically and/or competitively within the cardiac regulatory network.

Cystic kidney diseases and planar cell polarity signaling

Renal cystic diseases are a major clinical concern as they are the most common genetic cause of end-stage renal disease. While many of the genes causing cystic disease have been identified in recent years, knowing the molecular nature of the mutations has not clarified the mechanisms underlying cyst formation. Recent research in model organisms has suggested that cyst formation may be because of defective planar cell polarity (PCP) and/or ciliary defects. In this review, we first outline the clinical features of renal cystic diseases and then discuss current research linking our understanding of cystic kidney disease to PCP and cilia.

Tightening the connections between cadherins and fibronectin matrix

Fibronectin extracellular matrix is assembled and remodeled throughout embryogenesis and plays key roles in early vertebrate development. In this issue of Developmental Cell, Dzamba et al. reveal, through their studies of Xenopus embryos, a novel mechanism for regulating fibronectin matrix assembly through Wnt signaling and cadherin-mediated cell-cell adhesion.

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.

Essential role of beta-1,4-galactosyltransferase 2 during medaka (Oryzias latipes) gastrulation

Glycans are known to play important roles in vertebrate development; however, it is difficult to analyze in mammals because it takes place in utero. Therefore, we used medaka (Oryzias latipes) to clarify the roles of glycans during vertebrate development. beta-1,4-Galactosyltransferase is one of the key enzymes in the biosynthesis of the lactosamine structures that are commonly found on glycoproteins and glycolipids. Here, we show the essential role of beta4GalT2 during medaka development. Depletion of beta4GalT2 by morpholino antisense oligonucleotide injection resulted in significant morphological defects, such as shortening of the anterior-posterior axis, cyclopia, impaired somite segmentation, and head hypoplasia. In situ hybridization analyses revealed that the loss of beta4GalT2 led to defective anterior-posterior axis elongation during gastrulation without affecting organizer formation. Furthermore, a cell tracing experiment demonstrated that beta4GalT2 knockdown mainly affects mediolateral cell intercalation, which contributes to anterior-posterior axis elongation. A cell transplantation experiment indicated that glycans are produced by beta4GalT2 cell-autonomously during gastrulation. beta4GalT2 depletion also led to enhanced apoptosis; however, this does not account for the phenotypic abnormalities, as blockade of apoptosis failed to compensate for the beta4GalT2 depletion. Our data suggest that beta4GalT2 activity is cell-autonomously required in cells undergoing mediolateral cell intercalation, which drives extension movements during medaka gastrulation.

Role of CCN, a vertebrate specific gene family, in development

The CCN family of genes constitutes six members of small secreted cysteine rich proteins, which exists only in vertebrates. The major members of CCN are CCN1 (Cyr61), CCN2 (CTGF), and CCN3 (Nov). CCN4, CCN5, and CCN6 were formerly reported to be in the Wisp family, but they are now integrated into CCN due to the resemblance of their four principal modules: insulin like growth factor binding protein, von Willebrand factor type C, thrombospondin type 1, and carboxy-terminal domain. CCNs show a wide and highly variable expression pattern in adult and in embryonic tissues, but most studies have focused on their principal role in osteo/chondrogenesis and vasculo/angiogenesis from the aspect of migration, growth, and differentiation of mesenchymal cells. CCN proteins simultaneously integrate and modulate the signals of integrins, bone morphogenetic protein, vascular endothelial growth factor, Wnt, and Notch by direct binding. However, the priority in the use of the signals is different depending on the cell status. Even the equivalent counterparts show a difference in signal usage among species. It may be that the evolution of the CCN family continues to keep pace with vertebrate evolution itself.

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.

Noncanonical Wnt/PCP signaling during vertebrate gastrulation

The branch of the Wnt pathway, related to planar cell polarity signaling in Drosophila, is fundamental not only to the establishment of tissue polarity but also to a variety of morphogenetic processes in vertebrates. The genetic pathway has been noted for its similarity as well as divergence of between vertebrates and Drosophila. This review focuses on issues related to the complexity of the output of the planar cell polarity pathway during gastrulation in zebrafish and Xenopus and, to a lesser extent, during gastrulation/neurulation in mice.

Cadherin adhesion, tissue tension, and noncanonical Wnt signaling regulate fibronectin matrix organization

In this study we demonstrate that planar cell polarity signaling regulates morphogenesis in Xenopus embryos in part through the assembly of the fibronectin (FN) matrix. We outline a regulatory pathway that includes cadherin adhesion and signaling through Rac and Pak, culminating in actin reorganization, myosin contractility, and tissue tension, which, in turn, directs the correct spatiotemporal localization of FN into a fibrillar matrix. Increased mechanical tension promotes FN fibril assembly in the blastocoel roof (BCR), while reduced BCR tension inhibits matrix assembly. These data support a model for matrix assembly in tissues where cell-cell adhesions play an analogous role to the focal adhesions of cultured cells by transferring to integrins the tension required to direct FN fibril formation at cell surfaces.

Wnt activity guides facial branchiomotor neuron migration, and involves the PCP pathway and JNK and ROCK kinases

BACKGROUND

Wnt proteins play roles in many biological processes, including axon guidance and cell migration. In the mammalian hindbrain, facial branchiomotor (FBM) neurons undergo a striking rostral to caudal migration, yet little is known of the underlying molecular mechanisms. In this study, we investigated a possible role of Wnts and the planar cell polarity (PCP) pathway in this process.

RESULTS

Here we demonstrate a novel role for Wnt proteins in guiding FBM neurons during their rostral to caudal migration in the hindbrain. We found that Wnt5a is expressed in a caudal high to rostral low gradient in the hindbrain. Wnt-coated beads chemoattracted FBM neurons to ectopic positions in an explant migration assay. The rostrocaudal FBM migration was moderately perturbed in Wnt5a mutant embryos and severely disrupted in Frizzled3 mutant mouse embryos, and was aberrant following inhibition of Wnt function by secreted Frizzled-related proteins. We also show the involvement of the Wnt/PCP pathway in mammalian FBM neuron migration. Thus, mutations in two PCP genes, Vangl2 and Scribble, caused severe defects in FBM migration. Inhibition of JNK and ROCK kinases strongly and specifically reduced the FBM migration, as well as blocked the chemoattractant effects of ectopic Wnt proteins.

CONCLUSION

These results provide in vivo evidence that Wnts chemoattract mammalian FBM neurons and that Wnt5a is a candidate to mediate this process. Molecules of the PCP pathway and the JNK and ROCK kinases also play a role in the FBM migration and are likely mediators of Wnt signalling.

Modulation of the beta-catenin signaling pathway by the dishevelled-associated protein Hipk1

Background

Wnts are evolutionarily conserved ligands that signal through β-catenin-dependent and β-catenin–independent pathways to regulate cell fate, proliferation, polarity, and movements during vertebrate development. Dishevelled (Dsh/Dvl) is a multi-domain scaffold protein required for virtually all known Wnt signaling activities, raising interest in the identification and functions of Dsh-associated proteins.

Methodology

We conducted a yeast-2-hybrid screen using an N-terminal fragment of Dsh, resulting in isolation of the Xenopus laevis ortholog of Hipk1. Interaction between the Dsh and Hipk1 proteins was confirmed by co-immunoprecipitation assays and mass spectrometry, and further experiments suggest that Hipk1 also complexes with the transcription factor Tcf3. Supporting a nuclear function during X. laevis development, Myc-tagged Hipk1 localizes primarily to the nucleus in animal cap explants, and the endogenous transcript is strongly expressed during gastrula and neurula stages. Experimental manipulations of Hipk1 levels indicate that Hipk1 can repress Wnt/β-catenin target gene activation, as demonstrated by β-catenin reporter assays in human embryonic kidney cells and by indicators of dorsal specification in X. laevis embryos at the late blastula stage. In addition, a subset of Wnt-responsive genes subsequently requires Hipk1 for activation in the involuting mesoderm during gastrulation. Moreover, either over-expression or knock-down of Hipk1 leads to perturbed convergent extension cell movements involved in both gastrulation and neural tube closure.

Conclusions

These results suggest that Hipk1 contributes in a complex fashion to Dsh-dependent signaling activities during early vertebrate development. This includes regulating the transcription of Wnt/β-catenin target genes in the nucleus, possibly in both repressive and activating ways under changing developmental contexts. This regulation is required to modulate gene expression and cell movements that are essential for gastrulation.

The tumor suppressor PRDM5 regulates Wnt signaling at early stages of zebrafish development

PRDM genes are a family of transcriptional regulators that modulate cellular processes such as differentiation, cell growth and apoptosis. Some family members are involved in tissue or organ maturation, and are differentially expressed in specific phases of embryonic development. PRDM5 is a recently identified family member that functions as a transcriptional repressor and behaves as a putative tumor suppressor in different types of cancer. Using gene expression profiling, we found that transcriptional targets of PRDM5 in human U2OS cells include critical genes involved in developmental processes, and specifically in regulating wnt signaling. We therefore assessed PRDM5 function in vivo by performing loss-of-function and gain-of-function experiments in zebrafish embryos. Depletion of prdm5 resulted in impairment of morphogenetic movements during gastrulation and increased the occurrence of the masterblind phenotype in axin+/- embryos, characterized by the loss of eyes and telencephalon. Overexpression of PRDM5 mRNA had opposite effects on the development of anterior neural structures, and resulted in embryos with a shorter body axis due to posterior truncation, a bigger head and abnormal somites. In situ hybridization experiments aimed at analyzing the integrity of wnt pathways during gastrulation at the level of the prechordal plate revealed inhibition of non canonical PCP wnt signaling in embryos overexpressing PRDM5, and over-activation of wnt/beta-catenin signaling in embryos lacking Prdm5. Our data demonstrate that PRDM5 regulates the expression of components of both canonical and non canonical wnt pathways and negatively modulates wnt signaling in vivo.

Kaiso is a bimodal modulator for Wnt/beta-catenin signaling

The Wnt family of secreted ligands plays critical roles during embryonic development and tumorigenesis. Here we show that Kaiso, a dual specific DNA-binding protein, functions as a bimodal regulator of canonical Wnt signaling. Loss-of-function analysis of Kaiso abrogated Wnt-mediated reporter activity and axis duplication, whereas gain-of-function analysis of Kaiso dose-dependently resulted in synergistic and suppressive effects. Our analyses further suggest Kaiso can regulate TCF/LEF1-activity for these effects via modulating HDAC1 and beta-catenin-complex formation. Our studies together provide insights into why Kaiso null mice display resistance to intestinal tumors when crossed onto an Apc(Min/+) background.

Regulation of convergent extension by non-canonical Wnt signaling in the Xenopus embryo

Non-canonical Wnt signaling is an important regulator of gastrulation in Xenopus. In particular, it has been implicated in the control of convergent extension movements. Convergent extension in the gastrula occurs primarily in the dorsal tissue of the marginal zone, and explants of this tissue will continue to undergo these movements in isolation. This observation has led to an assay to examine convergent extension movements that is unique to the Xenopus system, and is described herein.

Contact inhibition of locomotion in vivo controls neural crest directional migration

Contact inhibition of locomotion was discovered by Abercrombie more than 50 years ago and describes the behaviour of fibroblast cells confronting each other in vitro, where they retract their protrusions and change direction on contact. Its failure was suggested to contribute to malignant invasion. However, the molecular basis of contact inhibition of locomotion and whether it also occurs in vivo are still unknown. Here we show that neural crest cells, a highly migratory and multipotent embryonic cell population, whose behaviour has been likened to malignant invasion, demonstrate contact inhibition of locomotion both in vivo and in vitro, and that this accounts for their directional migration. When two migrating neural crest cells meet, they stop, collapse their protrusions and change direction. In contrast, when a neural crest cell meets another cell type, it fails to display contact inhibition of locomotion; instead, it invades the other tissue, in the same manner as metastatic cancer cells. We show that inhibition of non-canonical Wnt signalling abolishes both contact inhibition of locomotion and the directionality of neural crest migration. Wnt-signalling members localize at the site of cell contact, leading to activation of RhoA in this region. These results provide the first example of contact inhibition of locomotion in vivo, provide an explanation for coherent directional migration of groups of cells and establish a previously unknown role for non-canonical Wnt signalling.

Involvement of AP-2rep in morphogenesis of the axial mesoderm in Xenopus embryo

We have previously isolated a cDNA clone coding for Xenopus AP-2rep (activator protein-2 repressor), a member of the Krüppel-like factor family, and reported its expression pattern in developing Xenopus embryos. In the present study, the physiological function of AP-2rep in the morphogenetic movements of the dorsal mesoderm and ectoderm was investigated. Embryos injected with either AP-2rep or VP16repC (a dominant-negative mutant) into the dorsal marginal zone at the 4-cell stage exhibited abnormal morphology in dorsal structures. Both AP-2rep and VP16repC also inhibited the elongation of animal cap explants treated with activin without affecting the expression of differentiation markers. Whole-mount in situ hybridization analysis revealed that expression of brachyury and Wnt11 was greatly suppressed by injection of VP16repC or AP-2rep morpholino, but expression was restored by the simultaneous injection of wild-type AP-2rep RNA. Furthermore, the morphogenetic abnormality induced by injection of VP16repC or AP-2rep morpholino was restored by simultaneous injection of brachyury or Wnt11 mRNA. These results show that AP-2rep is involved in the morphogenesis of the mesoderm at the gastrula stage, via the brachyury and/or Wnt pathways.

WNT11 acts as a directional cue to organize the elongation of early muscle fibres

The early vertebrate skeletal muscle is a well-organized tissue in which the primitive muscle fibres, the myocytes, are all parallel and aligned along the antero-posterior axis of the embryo. How myofibres acquire their orientation during development is unknown. Here we show that during early chick myogenesis WNT11 has an essential role in the oriented elongation of the myocytes. We find that the neural tube, known to drive WNT11 expression in the medial border of somites, is necessary and sufficient to orient myocyte elongation. We then show that the specific inhibition of WNT11 function in somites leads to the disorganization of myocytes. We establish that WNT11 mediates this effect through the evolutionary conserved planar cell polarity (PCP) pathway, downstream of the WNT/beta-catenin-dependent pathway, required to initiate the myogenic program of myocytes and WNT11 expression. Finally, we demonstrate that a localized ectopic source of WNT11 can markedly change the orientation of myocytes, indicating that WNT11 acts as a directional cue in this process. All together, these data show that the sequential action of the WNT/PCP and the WNT/beta-catenin pathways is necessary for the formation of fully functional embryonic muscle fibres. This study also provides evidence that WNTs can act as instructive cues to regulate the PCP pathway in vertebrates.

Conservation of notochord gene expression across chordates: insights from the Leprecan gene family

The notochord is a defining character of the chordates, and the T-box transcription factor Brachyury has been shown to be required for notochord development in all chordates examined. In the ascidian Ciona intestinalis, at least 44 notochord genes have been identified as bona fide transcriptional targets of Brachyury. We examined the embryonic expression of a subset of murine orthologs of Ciona Brachyury target genes in the notochord to assess its conservation throughout chordate evolution. We focused on analyzing the Leprecan gene family, which in mouse is composed of three genes, as opposed to the single-copy Ciona gene. We found that all three mouse Leprecan genes are expressed in the notochord. Additionally, while Leprecan expression in C. intestinalis is confined to the notochord, expression of its mouse orthologs includes dorsal root ganglia, limb buds, branchial arches, and developing kidneys. These results have interesting implications for the evolution and development of chordates.

Sfrp5 coordinates foregut specification and morphogenesis by antagonizing both canonical and noncanonical Wnt11 signaling

Cell identity and tissue morphogenesis are tightly orchestrated during organogenesis, but the mechanisms regulating this are poorly understood. We show that interactions between Wnt11 and the secreted Wnt antagonist secreted frizzled-related protein 5 (Sfrp5) coordinate cell fate and morphogenesis during Xenopus foregut development. sfrp5 is expressed in the surface cells of the foregut epithelium, whereas wnt11 is expressed in the underlying deep endoderm. Depletion of Sfrp5 results in reduced foregut gene expression and hypoplastic liver and ventral pancreatic buds. In addition, the ventral foregut cells lose adhesion and fail to form a polarized epithelium. We show that the cell fate and epithelial defects are due to inappropriate Wnt/beta-catenin and Wnt/PCP signaling, respectively, both mediated by Wnt11. We provide evidence that Sfrp5 locally inhibits Wnt11 to maintain early foregut identity and to allow an epithelium to form over a mass of tissue undergoing Wnt-mediated cell movements. This novel mechanism coordinating canonical and noncanonical Wnt signaling may have broad implications for organogenesis and cancer.

Planar polarization in embryonic epidermis orchestrates global asymmetric morphogenesis of hair follicles

Mammalian body hairs align along the anterior-posterior (A-P) axis and offer a striking but poorly understood example of global cell polarization, a phenomenon known as planar cell polarity (PCP). We have discovered that during embryogenesis, marked changes in cell shape and cytoskeletal polarization occur as nascent hair follicles become anteriorly angled, morphologically polarized and molecularly compartmentalized along the A-P axis. Hair follicle initiation coincides with asymmetric redistribution of Vangl2, Celsr1 and Fzd6 within the embryonic epidermal basal layer. Moreover, loss-of-function mutations in Vangl2 and Celsr1 show that they have an essential role in hair follicle polarization and orientation, which develop in part through non-autonomous mechanisms. Vangl2 and Celsr1 are both required for their planar localization in vivo, and physically associate in a complex in vitro. Finally, we provide in vitro evidence that homotypic intracellular interactions of Celsr1 are required to recruit Vangl2 and Fzd6 to sites of cell-cell contact.

Convergent extension movements and ciliary function are mediated by ofd1, a zebrafish orthologue of the human oral-facial-digital type 1 syndrome gene

In humans, OFD1 is mutated in oral-facial-digital type I syndrome leading to prenatal death in hemizygous males and dysmorphic faces and brain malformations, with polycystic kidneys presenting later in life in heterozygous females. To elucidate the function of Ofd1, we have studied its function during zebrafish embryonic development. In wild-type embryos, ofd1 mRNA is widely expressed and Ofd1-green fluorescent protein (GFP) fusion localizes to the centrosome/basal body. Disrupting Ofd1 using antisense morpholinos (MOs) led to bent body axes, hydrocephalus and oedema. Laterality was randomized in the brain, heart and viscera, likely a consequence of shorter cilia with disrupted axonemes and perturbed intravesicular fluid flow in Kupffer's vesicle. Embryos injected with ofd1 MOs also displayed convergent extension (CE) defects, which were enhanced by loss of Slb/Wnt11 or Tri/Vangl2, two proteins functioning in a non-canonical Wnt/Planar Cell Polarity (PCP) pathway. Pronephric glomerular midline fusion was compromised in vangl2 and ofd1 loss of function embryos and we suggest this anomaly may be a novel CE defect. Thus, Ofd1 is required for ciliary motility and function in zebrafish, supporting data showing that Ofd1 is essential for primary cilia function in mice. In addition, our data show that Ofd1 is important for CE during gastrulation, consistent with data linking primary cilia and non-canonical Wnt/PCP signalling.

The plane facts of PCP in the CNS

Planar cell polarity (PCP) pathways have been defined by their ability to direct the development of obviously polarized cellular architectures. Recent studies indicate that PCP pathways also regulate aspects of cell morphology that are not restricted to the plane of the epithelium. In the developing nervous system, PCP-mediated changes in the cytoskeleton are fundamental to neuronal migration, neuronal polarity, axon guidance, and dendritic arborization, highlighting the importance of "planar polarity" genes for defining the shape of a neuron in all dimensions.

GATA transcription factors integrate Wnt signalling during heart development

Cardiogenesis is inhibited by canonical Wnt/beta-catenin signalling and stimulated by non-canonical Wnt11/JNK signalling, but how these two signalling pathways crosstalk is currently unknown. Here, we show that Wnt/beta-catenin signalling restricts cardiogenesis via inhibition of GATA gene expression, as experimentally reinstating GATA function overrides beta-catenin-mediated inhibition and restores cardiogenesis. Furthermore, we show that GATA transcription factors in turn directly regulate Wnt11 gene expression, and that Wnt11 is required to a significant degree for mediating the cardiogenesis-promoting function of GATA transcription factors. These results demonstrate that GATA factors occupy a central position between canonical and non-canonical Wnt signalling in regulating heart muscle 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.

A role for planar cell polarity signaling in angiogenesis

The planar cell polarity (PCP) pathway is a highly conserved signaling cascade that coordinates both epithelial and axonal morphogenic movements during development. Angiogenesis also involves the growth and migration of polarized cells, although the mechanisms underlying their intercellular communication are poorly understood. Here, using cell culture assays, we demonstrate that inhibition of PCP signaling disrupts endothelial cell growth, polarity, and migration, all of which can be rescued through downstream activation of this pathway by expression of either Daam-1, Diversin or Inversin. Silencing of either Dvl2 or Prickle suppressed endothelial cell proliferation. Moreover, loss of p53 rescues endothelial cell growth arrest but not the migration inhibition caused by PCP disruption. In addition, we show that the zebrafish Wnt5 mutant (pipetail (ppt)), which has impaired PCP signaling, displays vascular developmental defects. These findings reveal a potential role for PCP signaling in the coordinated assembly of endothelial cells into vascular structures and have important implications for vascular remodeling in development and disease.

Highly efficient differentiation of hESCs to functional hepatic endoderm requires ActivinA and Wnt3a signaling

Human embryonic stem cells (hESCs) are a valuable source of pluripotential primary cells. To date, however, their homogeneous cellular differentiation to specific cell types in vitro has proven difficult. Wnt signaling has been shown to play important roles in coordinating development, and we demonstrate that Wnt3a is differentially expressed at critical stages of human liver development in vivo. The essential role of Wnt3a in hepatocyte differentiation from hESCs is paralleled by our in vitro model, demonstrating the importance of a physiologic approach to cellular differentiation. Our studies provide compelling evidence that Wnt3a signaling is important for coordinated hepatocellular function in vitro and in vivo. In addition, we demonstrate that Wnt3a facilitates clonal plating of hESCs exhibiting functional hepatic differentiation. These studies represent an important step toward the use of hESC-derived hepatocytes in high-throughput metabolic analysis of human liver function.

Non-canonical Wnt signaling through Wnt5a/b and a novel Wnt11 gene, Wnt11b, regulates cell migration during avian gastrulation

Knowledge of the molecular mechanisms regulating cell ingression, epithelial-mesenchymal transition and migration movements during amniote gastrulation is steadily improving. In the frog and fish embryo, Wnt5 and Wnt11 ligands are expressed around the blastopore and play an important role in regulating cell movements associated with gastrulation. In the chicken embryo, although Wnt5a and Wnt5b are expressed in the primitive streak, the known Wnt11 gene is expressed in paraxial and intermediate mesoderm, and in differentiated myocardial cells, but not in the streak. Here, we identify a previously uncharacterized chicken Wnt11 gene, Wnt11b, that is orthologous to the frog Wnt11 and zebrafish Wnt11 (silberblick) genes. Chicken Wnt11b is expressed in the primitive streak in a pattern similar to chicken Wnt5a and Wnt5b. When non-canonical Wnt signaling is blocked using a Dishevelled dominant-negative protein, gastrulation movements are inhibited and cells accumulate in the primitive streak. Furthermore, disruption of non-canonical Wnt signaling by overexpression of full-length or dominant-negative Wnt11b or Wnt5a constructions abrogates normal cell migration through the primitive streak. We conclude that non-canonical Wnt signaling, mediated in part by Wnt11b, is important for regulation of gastrulation cell movements in the avian embryo.

Nucleoredoxin regulates the Wnt/planar cell polarity pathway in Xenopus

The Wnt signaling pathway is conserved across species, and is essential for early development. We previously identified nucleoredoxin (NRX) as a protein that interacts with dishevelled (Dvl) in vivo to negatively regulate the Wnt/beta-catenin pathway. However, whether NRX affects another branch of the Wnt pathway, the Wnt/planar cell polarity (PCP) pathway, remains unclear. Here we show that NRX regulates the Wnt/PCP pathway. In Xenopus laevis, over-expression or depletion of NRX by injection of NRX mRNA or antisense morpholino oligonucleotide, respectively, yields the bent-axis phenotype that is typically observed in embryos with abnormal PCP pathway activity. In co-injection experiments of Dvl and NRX mRNA, NRX suppresses the Dvl-induced bent-axis phenotype. Over-expression or depletion of NRX also suppresses the convergent extension movements that are believed to underlie normal gastrulation. We also found that NRX can inhibit Dvl-induced up-regulation of c-Jun phosphorylation. These results indicate that NRX plays crucial roles in the Wnt/PCP pathway through Dvl and regulates Xenopus gastrulation movements.

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

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

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.