Role of frizzled 7 in the regulation of convergent extension movements during gastrulation in Xenopus laevis

Calpain2 protease: A new member of the Wnt/Ca(2+) pathway modulating convergent extension movements in Xenopus

Calpains are a family of calcium-dependent intracellular cysteine proteases that regulate several physiological processes by limited cleavage of different substrates. The role of Calpain2 in embryogenesis is not clear with conflicting evidence from a number of mouse knockouts. Here we report the temporal and spatial expression of Calpain2 in Xenopus laevis embryos and address its role in Xenopus development. We show that Calpain2 is expressed maternally with elevated expression in neural tissues and that Calpain2 activity is spatially and temporally regulated. Using a Calpain inhibitor, a dominant negative and a morpholino oligonoucleotide we demonstrate that impaired Calpain2 activity results in defective convergent extension both in mesodermal and neural tissues. Specifically, Calpain2 downregulation results in loss of tissue polarity and blockage of mediolateral intercalation in Keller explants without affecting adherens junction turnover. We further show that Calpain2 is activated in response to Wnt5a and that the inhibitory effect of Wnt5a expression on animal cap elongation can be rescued by blocking Calpain2 function. This suggests that Calpain2 activity needs to be tightly regulated during convergent extension. Finally we show that expression of Xdd1 blocks the membrane translocation of Calpain2 suggesting that Calpain2 activation is downstream of Dishevelled. Overall our data show that Calpain2 activation through the Wnt/Ca(2+) pathway and Dishevelled can modulate convergent extension movements.

Wnt signaling through the Ror receptor in the nervous system

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

The distribution of Dishevelled in convergently extending mesoderm

Convergent extension (CE) is a conserved morphogenetic movement that drives axial lengthening of the primary body axis and depends on the planar cell polarity (PCP) pathway. In Drosophila epithelia, a polarised subcellular accumulation of PCP core components, such as Dishevelled (Dvl) protein, is associated with PCP function. Dvl has long been thought to accumulate in the mediolateral protrusions in Xenopus chordamesoderm cells undergoing CE. Here we present a quantitative analysis of Dvl intracellular localisation in Xenopus chordamesoderm cells. We find that, surprisingly, accumulations previously observed at mediolateral protrusions of chordamesodermal cells are not protrusion-specific but reflect yolk-free cytoplasm and are quantitatively matched by the distribution of the cytoplasm-filling lineage marker dextran. However, separating cell cortex-associated from bulk Dvl signal reveals a statistical enrichment of Dvl in notochord-somite boundary-(NSB)-directed protrusions, which is dependent upon NSB proximity. Dvl puncta were also observed, but only upon elevated overexpression. These puncta showed no statistically significant spatial bias, in contrast to the strongly posteriorly-enriched GFP-Dvl puncta previously reported in zebrafish. We propose that Dvl distribution is more subtle and dynamic than previously appreciated and that in vertebrate mesoderm it reflects processes other than protrusion as such.

Requirement for frzb and fzd7a in cranial neural crest convergence and extension mechanisms during zebrafish palate and jaw morphogenesis

Regulation of convergence and extension by wnt-frizzled signaling is a common theme in embryogenesis. This study examines the functional requirements of frzb and fzd7a in convergence and extension mechanisms during craniofacial development. Using a morpholino knockdown approach, we found that frzb and fzd7a are dispensable for directed migration of the bilateral trabeculae, but necessary for the convergence and extension of the palatal elements, where the extension process is mediated by chondrocyte proliferation, morphologic change and intercalation. In contrast, frzb and fzd7a are required for convergence of the mandibular prominences, where knockdown of either frzb or fzd7a resulted in complete loss of lower jaw structures. Further, we found that bapx1 was specifically downregulated in the wnt9a/frzb/fzd7a morphants, while general neural crest markers were unaffected. In addition, expression of wnt9a and frzb was also absent in the edn-/- mutant. Notably, over-expression of bapx1 was sufficient to partially rescue mandibular elements in the wnt9a/frzb/fzd7a morphants, demonstrating genetic epistasis of bapx1 acting downstream of edn1 and wnt9a/frzb/fzd7a in lower jaw development. This study underscores the important role of wnt-frizzled signaling in convergence and extension in palate and craniofacial morphogenesis, distinct regulation of upper vs. lower jaw structures, and integration of wnt-frizzled with endothelin signaling to coordinate shaping of the facial form.

Different thresholds of Wnt-Frizzled 7 signaling coordinate proliferation, morphogenesis and fate of endoderm progenitor cells

Wnt signaling has multiple dynamic roles during development of the gastrointestinal and respiratory systems. Differential Wnt signaling is thought to be a critical step in Xenopus endoderm patterning such that during late gastrula and early somite stages of embryogenesis, Wnt activity must be suppressed in the anterior to allow the specification of foregut progenitors. However, the foregut endoderm also expresses the Wnt-receptor Frizzled 7 (Fzd7) as well as several Wnt ligands suggesting that the current model may be too simple. In this study, we show that Fzd7 is required to transduce a low level of Wnt signaling that is essential to maintain foregut progenitors. Foregut-specific Fzd7-depletion from the Xenopus foregut resulted in liver and pancreas agenesis. Fzd7-depleted embryos failed to maintain the foregut progenitor marker hhex and exhibited decreased proliferation; in addition the foregut cells were enlarged with a randomized orientation. We show that in the foregut Fzd7 signals via both the Wnt/β-catenin and Wnt/JNK pathways and that different thresholds of Wnt-Fzd7 activity coordinate progenitor cell fate, proliferation and morphogenesis.

Characterization of sFRP2-like in amphioxus: insights into the evolutionary conservation of Wnt antagonizing function

Wnt signaling plays a key role in embryonic patterning and morphogenetic movements. The secreted Frizzled-related proteins (sFRPs) antagonize Wnt signaling, but their roles in development are poorly understood. To determine whether function of sFRPs is conserved between amphioxus and vertebrates, we characterized sFRP2-like function in the amphioxus, Branchiostoma belcheri tsingtauense (B. belcheri). As in other species of Branchiostome, in B. belcheri, expression of sFRP2-like is restricted to the mesendoderm during gastrulation and to the anterior mesoderm and endoderm during neurulation. Functional analyses in frog (Xenopus laevis) indicate that amphioxus sFRP2-like potently inhibits both canonical and non-canonical Wnts. Thus, sFRP-2 probably functions in amphioxus embryos to inhibit Wnt signaling anteriorly. Moreover, dorsal overexpression of amphioxus sFRP2-like in Xenopus embryos, like inhibition of Wnt11, blocks gastrulation movements. This implies that sFRP2-like may also modulate Wnt signaling during gastrulation movements in amphioxus.

A systematic survey of expression and function of zebrafish frizzled genes

Wnt signaling is crucial for the regulation of numerous processes in development. Consistent with this, the gene families for both the ligands (Wnts) and receptors (Frizzleds) are very large. Surprisingly, while we have a reasonable understanding of the Wnt ligands likely to mediate specific Wnt-dependent processes, the corresponding receptors usually remain to be elucidated. Taking advantage of the zebrafish model's excellent genomic and genetic properties, we undertook a comprehensive analysis of the expression patterns of frizzled (fzd) genes in zebrafish. To explore their functions, we focused on testing their requirement in several developmental events known to be regulated by Wnt signaling, convergent extension movements of gastrulation, neural crest induction, and melanocyte specification. We found fourteen distinct fzd genes in the zebrafish genome. Systematic analysis of their expression patterns between 1-somite and 30 hours post-fertilization revealed complex, dynamic and overlapping expression patterns. This analysis demonstrated that only fzd3a, fzd9b, and fzd10 are expressed in the dorsal neural tube at stages corresponding to the timing of melanocyte specification. Surprisingly, however, morpholino knockdown of these, alone or in combination, gave no indication of reduction of melanocytes, suggesting the important involvement of untested fzds or another type of Wnt receptor in this process. Likewise, we found only fzd7b and fzd10 expressed at the border of the neural plate at stages appropriate for neural crest induction. However, neural crest markers were not reduced by knockdown of these receptors. Instead, these morpholino knockdown studies showed that fzd7a and fzd7b work co-operatively to regulate convergent extension movement during gastrulation. Furthermore, we show that the two fzd7 genes function together with fzd10 to regulate epiboly movements and mesoderm differentiation.

WNT signalling pathways as therapeutic targets in cancer

Since the initial discovery of the oncogenic activity of WNT1 in mouse mammary glands, our appreciation for the complex roles for WNT signalling pathways in cancer has increased dramatically. WNTs and their downstream effectors regulate various processes that are important for cancer progression, including tumour initiation, tumour growth, cell senescence, cell death, differentiation and metastasis. Although WNT signalling pathways have been difficult to target, improved drug-discovery platforms and new technologies have facilitated the discovery of agents that can alter WNT signalling in preclinical models, thus setting the stage for clinical trials in humans.

Whole-genome microRNA screening identifies let-7 and mir-18 as regulators of germ layer formation during early embryogenesis

Tight control over the segregation of endoderm, mesoderm, and ectoderm is essential for normal embryonic development of all species, yet how neighboring embryonic blastomeres can contribute to different germ layers has never been fully explained. We postulated that microRNAs, which fine-tune many biological processes, might modulate the response of embryonic blastomeres to growth factors and other signals that govern germ layer fate. A systematic screen of a whole-genome microRNA library revealed that the let-7 and miR-18 families increase mesoderm at the expense of endoderm in mouse embryonic stem cells. Both families are expressed in ectoderm and mesoderm, but not endoderm, as these tissues become distinct during mouse and frog embryogenesis. Blocking let-7 function in vivo dramatically affected cell fate, diverting presumptive mesoderm and ectoderm into endoderm. siRNA knockdown of computationally predicted targets followed by mutational analyses revealed that let-7 and miR-18 down-regulate Acvr1b and Smad2, respectively, to attenuate Nodal responsiveness and bias blastomeres to ectoderm and mesoderm fates. These findings suggest a crucial role for the let-7 and miR-18 families in germ layer specification and reveal a remarkable conservation of function from amphibians to mammals.

The planar cell polarity protein VANGL2 coordinates remodeling of the extracellular matrix

Understanding how planar cell polarity (PCP) is established, maintained, and coordinated in migrating cell populations is an important area of research with implications for both embryonic morphogenesis and tumor cell invasion. We recently reported that the PCP protein Vang-like 2 (VANGL2) regulates the endocytosis and cell surface level of membrane type-1 matrix metalloproteinase (MMP14 or MT1-MMP). Here, we further discuss these findings in terms of extracellular matrix (ECM) remodeling, cell migration, and zebrafish gastrulation. We also demonstrate that VANGL2 function impacts the focal degradation of ECM by human cancer cells including the formation or stability of invadopodia. Together, our findings implicate MMP14 as a downstream effector of VANGL2 signaling and suggest a model whereby the regulation of pericellular proteolysis is a fundamental aspect of PCP in migrating cells.

Collective mesendoderm migration relies on an intrinsic directionality signal transmitted through cell contacts

Collective cell migration is key to morphogenesis, wound healing, or cancer cell migration. However, its cellular bases are just starting to be unraveled. During vertebrate gastrulation, axial mesendoderm migrates in a group, the prechordal plate, from the embryonic organizer to the animal pole. How this collective migration is achieved remains unclear. Previous work has suggested that cells migrate as individuals, with collective movement resulting from the addition of similar individual cell behavior. Through extensive analyses of cell trajectories, morphologies, and polarization in zebrafish embryos, we reveal that all prechordal plate cells show the same behavior and rely on the same signaling pathway to migrate, as expected if they do so individually. However, by using cell transplants, we demonstrate that prechordal plate migration is a true collective process, as isolated cells do not migrate toward the animal pole. They are still polarized and motile but lose directionality. Directionality is restored upon contact with the endogenous prechordal plate. This contact dependent orientation relies on E-cadherin, Wnt-PCP signaling, and Rac1. Importantly, groups of cells also need contact with the endogenous plate to orient correctly, showing an instructive role of the plate in establishing directionality. Overall, our results lead to an original model of collective migration in which directional information is contained within the moving group rather than provided by extrinsic cues, and constantly maintained in cells by contacts with their neighbors. This self-organizing model could account for collective invasion of new territories, as observed in cancer strands, without requirement for any attractant in the colonized tissue.

Planar cell polarity and the developmental control of cell behavior in vertebrate embryos

Planar cell polarity (PCP), the orientation and alignment of cells within a sheet, is a ubiquitous cellular property that is commonly governed by the conserved set of proteins encoded by so-called PCP genes. The PCP proteins coordinate developmental signaling cues with individual cell behaviors in a wildly diverse array of tissues. Consequently, disruptions of PCP protein functions are linked to defects in axis elongation, inner ear patterning, neural tube closure, directed ciliary beating, and left/right patterning, to name only a few. This review attempts to synthesize what is known about PCP and the PCP proteins in vertebrate animals, with a particular focus on the mechanisms by which individual cells respond to PCP cues in order to execute specific cellular behaviors.

Activation of Wnt11 by transforming growth factor-β drives mesenchymal gene expression through non-canonical Wnt protein signaling in renal epithelial cells

Transforming growth factor β1 (TGF-β) promotes renal interstitial fibrosis in vivo and the expression of mesenchymal genes in vitro; however, most of its direct targets in epithelial cells are still elusive. In a screen for genes directly activated by TGF-β, we found that components of the Wnt signaling pathway, especially Wnt11, were targets of activation by TGF-β and Smad3 in primary renal epithelial cells. In gain and loss of function experiments, Wnt11 mediated the actions of TGF-β through enhanced activation of mesenchymal marker genes, such as Zeb1, Snail1, Pai1, and αSMA, without affecting Smad3 phosphorylation. Inhibition of Wnt11 by receptor knockdown or treatment with Wnt inhibitors limited the effects of TGF-β on gene expression. We found no evidence that Wnt11 activated the canonical Wnt signaling pathway in renal epithelial cells; rather, the function of Wnt11 was mediated by the c-Jun N-terminal kinase (JNK) pathway. Consistent with the in vitro results, all the TGF-β, Wnt11, and JNK targets were activated in a unilateral ureteral obstruction (UUO) model of renal fibrosis in vivo. Our findings demonstrated cooperativity among the TGF-β, Wnt11, and JNK signaling pathways and suggest new targets for anti-fibrotic therapy in renal tissue.

The tangled web of non-canonical Wnt signalling in neural migration

In all multicellular animals, successful embryogenesis is dependent on the ability of cells to detect the status of the local environment and respond appropriately. The nature of the extracellular environment is communicated to the intracellular compartment by ligand/receptor interactions at the cell surface. The Wnt canonical and non-canonical signalling pathways are found in the most primitive metazoans, and they play an essential role in the most fundamental developmental processes in all multicellular organisms. Vertebrates have expanded the number of Wnts and Frizzled receptors and have additionally evolved novel Wnt receptor families (Ryk, Ror). The multiplicity of potential interactions between Wnts, their receptors and downstream effectors has exponentially increased the complexity of the signal transduction network. Signalling through each of the Wnt pathways, as well as crosstalk between them, plays a critical role in the establishment of the complex architecture of the vertebrate central nervous system. In this review, we explore the signalling networks triggered by non-canonical Wnt/receptor interactions, focussing on the emerging roles of the non-conventional Wnt receptors Ryk and Ror. We describe the role of these pathways in neural tube formation and axon guidance where Wnt signalling controls tissue polarity, coordinated cell migration and axon guidance via remodelling of the cytoskeleton.

New insights into the mechanism of Wnt signaling pathway activation

Wnts compromise a large family of secreted, hydrophobic glycoproteins that control a variety of developmental and adult processes in all metazoan organisms. Recent advances in the Wnt-signal studies have revealed that distinct Wnts activate multiple intracellular cascades that regulate cellular proliferation, differentiation, migration, and polarity. Although the mechanism by which Wnts regulate different pathways selectively remains to be clarified, evidence has accumulated that in addition to the formation of ligand-receptor pairs, phosphorylation of receptors, receptor-mediated endocytosis, acidification, and the presence of cofactors, such as heparan sulfate proteoglycans, are also involved in the activation of specific Wnt pathways. Here, we review the mechanism of activation in Wnt signaling initiated on the cell-surface membrane. In addition, the mechanisms for fine-tuning by cross talk between Wnt and other signaling are also discussed.

VANGL2 regulates membrane trafficking of MMP14 to control cell polarity and migration

Planar cell polarity (PCP) describes the polarized orientation of cells within the plane of a tissue. Unlike epithelial PCP, the mechanisms underlying PCP signaling in migrating cells remain undefined. Here, the establishment of PCP must be coordinated with dynamic changes in cell adhesion and extracellular matrix (ECM) organization. During gastrulation, the membrane type-1 matrix metalloproteinase (MT1-MMP or MMP14) is required for PCP and convergence and extension cell movements. We report that the PCP protein Vang-like 2 (VANGL2) regulates the endocytosis and cell-surface availability of MMP14 in manner that is dependent on focal adhesion kinase. We demonstrate that zebrafish trilobite/vangl2 mutant embryos exhibit increased Mmp14 activity and decreased ECM. Furthermore, in vivo knockdown of Mmp14 partially rescues the Vangl2 loss-of-function convergence and extension phenotype. This study identifies a mechanism linking VANGL2 with MMP14 trafficking and suggests that establishment of PCP in migrating gastrula cells requires regulated proteolytic degradation or remodeling of the ECM. Our findings implicate matrix metalloproteinases as downstream effectors of PCP and suggest a broadly applicable mechanism whereby VANGL2 affects diverse morphogenetic processes.

Analysis of cell shape and polarity during zebrafish gastrulation

Gastrulation is a complex set of cellular rearrangements that establish the overall shape of the body plan during development. In addition to being an essential and fascinating aspect of development, the cells of the gastrulating zebrafish embryo also provide an ideal in vivo system to study the interplay of cell polarity and movement in a native 3D environment. During gastrulation, zebrafish mesodermal cells undergo a series of conversions from initial non-polarized amoeboid cell movements to more mesenchymal and finally highly polarized and intercalative cell behaviors. Many of the cellular behavior changes of these cells are under the control of the RhoA pathway, which in turn is regulated by many signals, including non-canonical Wnts. The goal of this chapter is to provide researchers with the necessary protocols to examine changes in cell polarity and movement in the developing zebrafish embryo.

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

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

Planar cell polarity in coordinated and directed movements

Planar cell polarity is a fundamental concept to understanding the coordination of cell movements in the plane of a tissue. Since the planar cell polarity pathway was discovered in mesenchymal tissues involving cell interaction during vertebrate gastrulation, there is an emerging evidence that a variety of mesenchymal and epithelial cells utilize this genetic pathway to mediate the coordination of cells in directed movements. In this review, we focus on how the planar cell polarity pathway is mediated by migrating cells to communicate with one another in different developmental processes.

The translational repressor 4E-BP mediates the hypoxia-induced defects in myotome cells

Cell growth, proliferation, differentiation, and survival are influenced by the availability of oxygen. The effect of hypoxia on embryonic cells and the underlying molecular mechanisms to maintain cellular viability are still poorly understood. In this study, we show that hypoxia during Xenopus embryogenesis rapidly leads to a significant developmental delay and to cell apoptosis after prolonged exposure. We provide strong evidence that hypoxia does not affect somitogenesis but affects the number of mitotic cells and muscle-specific protein accumulation in somites, without interfering with the expression of MyoD and MRF4 transcription factors. We also demonstrate that hypoxia reversibly decreases Akt phosphorylation and increases the total amount of the translational repressor 4E-BP, in combination with an increase of the 4E-BP associated with eIF4E. Interestingly, the inhibition of PI3-Kinase or mTOR, with LY29002 or rapamycin respectively, triggers the 4E-BP accumulation in Xenopus embryos. Finally, the overexpression of the non-phosphorylatable 4E-BP protein induces, similar to hypoxia, a decrease in mitotic cells and a decrease in muscle-specific protein accumulation in somites. Taken together, our studies suggest that 4E-BP plays a central role under hypoxia in promoting the cap-independent translation at the expense of cap-dependent translation and triggers specific defects in muscle development.

Possible involvement of Wnt11 in colorectal cancer progression

Our previous report revealed that the expression of Frizzled-7 (FZD7) in colorectal cancer (CRC) and its possible role in CRC progression. In this study we measured the expression levels of candidate FZD7 ligands, Wnt3 and Wnt11 in colon cancer cell lines (n = 7) and primary CRC tissues (n = 133) by quantitative RT-PCR. We also examined the functional effects of Wnt11 with the use of Wnt11 transfectants of colon cancer HCT-116 cells. Wnt11 transfectants showed the increased proliferation and migration/invasion activities compared to mock cells. Western blot analysis of transfectants revealed that phosphorylation of JNK and c-jun was increased after Wnt11 transfection. Wnt11 mRNA expression was significantly higher in the stage I, II, III, or IV tumor tissues than in non-tumor tissues (overall P < 0.003), while there was no significant difference in Wnt3 mRNA expression between tumor and non-tumor tissues. In addition, Wnt11 mRNA expression was significantly higher in patients with recurrence or death after surgery than in those with no recurrence (disease free) after surgery (P = 0.018). We also compared the expression levels of Wnt11 mRNA with those of FZD7 mRNA in the same CRC samples. Wnt11 mRNA expression was significantly higher in patients with higher FZD7 mRNA levels than in those with lower FZD7 mRNA levels (P = 0.0005). The expression levels of Wnt11 mRNA were correlated with those of FZD7 mRNA (P < 0.0001). These data suggest that Wnt11 may play an important role in CRC progression.

Mink1 regulates β-catenin-independent Wnt signaling via Prickle phosphorylation

β-Catenin-independent Wnt signaling pathways have been implicated in the regulation of planar cell polarity (PCP) and convergent extension (CE) cell movements. Prickle, one of the core proteins of these pathways, is known to asymmetrically localize proximally at the adherens junction of Drosophila melanogaster wing cells and to locally accumulate within plasma membrane subdomains in cells undergoing CE movements during vertebrate development. Using mass spectrometry, we have identified the Ste20 kinase Mink1 as a Prickle-associated protein and found that they genetically interact during the establishment of PCP in the Drosophila eye and CE in Xenopus laevis embryos. We show that Mink1 phosphorylates Prickle on a conserved threonine residue and regulates its Rab5-dependent endosomal trafficking, a process required for the localized plasma membrane accumulation and function of Prickle. Mink1 also was found to be important for the clustering of Vangl within plasma membrane puncta. Our results provide a link between Mink and the Vangl-Prickle complex and highlight the importance of Prickle phosphorylation and endosomal trafficking for its function during Wnt-PCP signaling.

Roles of planar cell polarity pathways in the development of neural [correction of neutral] tube defects

Neural tube defects (NTDs) are the second most common birth defect in humans. Despite many advances in the understanding of NTDs and the identification of many genes related to NTDs, the fundamental etiology for the majority of cases of NTDs remains unclear. Planar cell polarity (PCP) signaling pathway, which is important for polarized cell movement (such as cell migration) and organ morphogenesis through the activation of cytoskeletal pathways, has been shown to play multiple roles during neural tube closure. The disrupted function of PCP pathway is connected with some NTDs. Here, we summarize our current understanding of how PCP factors affect the pathogenesis of NTDs.

Dissecting the PCP pathway: one or more pathways?: Does a separate Wnt-Fz-Rho pathway drive morphogenesis?

Planar cell polarity (PCP), the alignment of cells within 2D tissue planes, involves a set of core molecular regulators highly conserved between animals and cell types. These include the transmembrane proteins Frizzled (Fz) and VanGogh and the cytoplasmic regulators Dishevelled (Dsh) and Prickle. It is widely accepted that this core forms part of a 'PCP pathway' for signal transduction, which can affect cell morphology through activation of an evolutionary ancient regulatory module involving Rho family GTPases and Myosin II, and/or the JNK kinase cascade. We have re-examined the evidence for interactions between the proposed PCP pathway components, and question the placing of the cell morphology regulators in the same pathway as the PCP core. While Fz and Dsh are clearly involved in both PCP and Rho-based cell morphology regulation, available evidence cannot currently discriminate whether these processes are linked mechanistically by a shared Fz/Dsh population, or pass by two distinct pathways.

Planar cell polarity in kidney development and disease

Planar cell polarity (PCP) describes the coordinated polarization of tissue cells in a direction that is orthogonal to their apical/basal axis. In the last several years, studies in flies and vertebrates have defined evolutionarily conserved pathways that establish and maintain PCP in various cellular contexts. Defective responses to the polarizing signal(s) have deleterious effects on the development and repair of a wide variety of organs/tissues. In this review, we cover the known and hypothesized roles for PCP in the metanephric kidney. We highlight the similarities and differences in PCP establishment in this organ compared with flies, especially the role of Wnt signaling in this process. Finally, we present a model whereby the signal(s) that organizes PCP in the kidney epithelium, at least in part, comes from the adjacent stromal fibroblasts.

Planar cell polarity in Drosophila

In all multicellular organisms, epithelial cells are not only polarized along the apical-basal axis, but also within the epithelial plane, giving cells a sense of direction. Planar cell polarity (PCP) signaling regulates establishment of polarity within the plane of an epithelium. The outcomes of PCP signaling are diverse and include the determination of cell fates, the generation of asymmetric but highly aligned structures, such as the stereocilia in the human inner ear or the hairs on a fly wing, or the directional migration of cells during convergence and extension during vertebrate gastrulation. In humans, aberrant PCP signaling can result in severe developmental defects, such as open neural tubes (spina bifida), and can cause cystic kidneys. In this review, we discuss the basic mechanism and more recent findings of PCP signaling focusing on Drosophila melanogaster, the model organism in which most key PCP components were initially identified.

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

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

Principles of planar polarity in animal development

Planar polarity describes the coordinated polarisation of cells or structures in the plane of a tissue. The patterning mechanisms that underlie planar polarity are well characterised in Drosophila, where many events are regulated by two pathways: the 'core' planar polarity complex and the Fat/Dachsous system. Components of both pathways also function in vertebrates and are implicated in diverse morphogenetic processes, some of which self-evidently involve planar polarisation and some of which do not. Here, we review the molecular mechanisms and cellular consequences of planar polarisation in diverse contexts, seeking to identify the common principles across the animal kingdom.

Punctuated actin contractions during convergent extension and their permissive regulation by the non-canonical Wnt-signaling pathway

Actomyosin networks linked to the micro-environment through the plasma membrane are thought to be key players in regulating cell behaviors within multicellular tissues, such as converging and extending mesoderm. Here, we observe the dynamics of actin contractions called 'punctuated actin contractions' in the mid-cell body of embryonic mesenchymal cells in the mesoderm. These contraction dynamics are a common feature of Xenopus embryonic tissues and are important for cell shape changes during morphogenesis. Quantitative morphological analysis of these F-actin dynamics indicates that frequent and aligned movements of multiple actin contractions accompany mesoderm cells as they intercalate and elongate. Using inhibitors combined with fluorescence recovery after photobleaching (FRAP) analysis, we find that the dynamics of actin contractions are regulated by both myosin contractility and F-actin polymerization. Furthermore, we find that the non-canonical Wnt-signaling pathway permissively regulates levels of punctuated actin contractions. Overexpression of Xfz7 (Fzd7) can induce early maturation of actin contractions in mesoderm and produce mesoderm-like actin contractions in ectoderm cells. By contrast, expression of the dominant-negative Xenopus disheveled construct Xdd1 blocks the progression of actin contractions into their late mesoderm dynamics but has no effect in ectoderm. Our study reveals punctuated actin contractions within converging and extending mesoderm and uncovers a permissive role for non-canonical Wnt-signaling, myosin contractility and F-actin polymerization in regulating these dynamics.

Stereotypical cell division orientation controls neural rod midline formation in zebrafish

The development of multicellular organisms is dependent on the tight coordination between tissue growth and morphogenesis. The stereotypical orientation of cell divisions has been proposed to be a fundamental mechanism by which proliferating and growing tissues take shape. However, the actual contribution of stereotypical division orientation (SDO) to tissue morphogenesis is unclear. In zebrafish, cell divisions with stereotypical orientation have been implicated in both body-axis elongation and neural rod formation, although there is little direct evidence for a critical function of SDO in either of these processes. Here we show that SDO is required for formation of the neural rod midline during neurulation but dispensable for elongation of the body axis during gastrulation. Our data indicate that SDO during both gastrulation and neurulation is dependent on the noncanonical Wnt receptor Frizzled 7 (Fz7) and that interfering with cell division orientation leads to severe defects in neural rod midline formation but not body-axis elongation. These findings suggest a novel function for Fz7-controlled cell division orientation in neural rod midline formation during neurulation.

The Drosophila GIPC homologue can modulate myosin based processes and planar cell polarity but is not essential for development

Epithelia often show, in addition to the ubiquitous apico-basal (A/B) axis, a polarization within the plane of the epithelium, perpendicular to the A/B axis. Such planar cell polarity (PCP) is for example evident in the regular arrangement of the stereocilia in the cochlea of the mammalian inner ear or in (almost) all Drosophila adult external structures. GIPCs (GAIP interacting protein, C terminus) were first identified in mammals and bind to the Galphai GTPase activating protein RGS-GAIP. They have been proposed to act in a G-protein coupled complex controlling vesicular trafficking. Although GIPCs have been found to bind to numerous proteins including Frizzled receptors, which participate in PCP establishment, there is little in vivo evidence for the functional role(s) of GIPCs. We show here that overexpressed Drosophila dGIPC alters PCP generation in the wing. We were however unable to find any binding between dGIPC and the Drosophila receptors Fz1 and Fz2. The effect of overexpressed dGIPC is likely due to an effect on the actin cytoskeleton via myosins, since it is almost entirely suppressed by removing a genomic copy of the Myosin VI/jaguar gene. Surprisingly, although dGIPC can interfere with PCP generation and myosin based processes, the complete loss-of-function of dGIPC gives viable adults with no PCP or other detectable defects arguing for a non-essential role of dGIPC in viability and normal Drosophila development.

Ror2/Frizzled complex mediates Wnt5a-induced AP-1 activation by regulating Dishevelled polymerization

The receptor tyrosine kinase Ror2 acts as a receptor or coreceptor for Wnt5a to mediate Wnt5a-induced activation of the Wnt/JNK pathway and inhibition of the beta-catenin-dependent canonical Wnt pathway. However, little is known about how Ror2 cooperates with another receptor component(s) to mediate Wnt5a signaling. We show here that Ror2 regulates Wnt5a-induced polymerization of Dishevelled (Dvl) and that this Ror2-mediated regulation of Dvl is independent of the cytoplasmic region of Ror2. Ror2 can associate with Frizzled7 (Fz7) via its extracellular cysteine-rich domain to form a receptor complex that is required for the regulation of Dvl and activation of the AP-1 promoter after Wnt5a stimulation. Suppressed expression of Fz7 indeed results in the inhibition of Wnt5a-induced polymerization of Dvl and AP-1 activation. Interestingly, both the DIX and the DEP domains of Dvl are indispensable for Dvl polymerization and subsequent AP-1 activation after Wnt5a stimulation. We further show that polymerized Dvl is colocalized with Rac1 and that suppressed expression of Rac1 inhibits Wnt5a-induced AP-1 activation. Collectively, our results indicate that Ror2/Fz receptor complex plays an important role in the Wnt5a/Rac1/AP-1 pathway by regulating the polymerization of Dvl.

The RNA-binding protein Seb4/RBM24 is a direct target of MyoD and is required for myogenesis during Xenopus early development

RNA-binding proteins play an important role to post-transcriptionally regulate gene expression. During early development they exhibit temporally and spatially regulated expression pattern. The expression of Xenopus laevis Seb4 gene, also known as RBM24 in other vertebrates, is restricted to the lateral and ventral mesoderm during gastrulation and then localized to the somitic mesoderm, in a similar pattern as XMyoD gene. Using a hormone-inducible form of MyoD to identify potential direct MyoD target genes, we find that Seb4 expression is directly regulated by MyoD at the gastrula stage. We further show that a 0.65kb X. tropicalis RBM24 regulatory region contains multiple E boxes (CANNTG), which are potential binding sites for MyoD and other bHLH proteins. By injecting a RBM24 reporter construct into the animal pole of X. laevis embryos, we find that this reporter gene is indeed specifically activated by MyoD and repressed by a dominant negative MyoD mutant. Knockdown of Seb4 produces similar effects as those obtained by the dominant negative MyoD mutant, indicating that it is required for the expression of myogenic genes and myogenesis in the embryo. In cultured ectodermal explants, although overexpression of Seb4 has no obvious effect on myogenesis, knockdown of Seb4 inhibits the expression of myogenic genes and myogenesis induced by MyoD. These results reveal that Seb4 is a target of MyoD during myogenesis and is required for myogenic gene expression.

Distinct Xenopus Nodal ligands sequentially induce mesendoderm and control gastrulation movements in parallel to the Wnt/PCP pathway

The vertebrate body plan is established in two major steps. First, mesendoderm induction singles out prospective endoderm, mesoderm and ectoderm progenitors. Second, these progenitors are spatially rearranged during gastrulation through numerous and complex movements to give rise to an embryo comprising three concentric germ layers, polarised along dorsoventral, anteroposterior and left-right axes. Although much is known about the molecular mechanisms of mesendoderm induction, signals controlling gastrulation movements are only starting to be revealed. In vertebrates, Nodal signalling is required to induce the mesendoderm, which has precluded an analysis of its potential role during the later process of gastrulation. Using time-dependent inhibition, we show that in Xenopus, Nodal signalling plays sequential roles in mesendoderm induction and gastrulation movements. Nodal activity is necessary for convergent extension in axial mesoderm and for head mesoderm migration. Using morpholino-mediated knockdown, we found that the Nodal ligands Xnr5 and Xnr6 are together required for mesendoderm induction, whereas Xnr1 and Xnr2 act later to control gastrulation movements. This control is operated via the direct regulation of key movement-effector genes, such as papc, has2 and pdgfralpha. Interestingly, however, Nodal does not appear to mobilise the Wnt/PCP pathway, which is known to control cell and tissue polarity. This study opens the way to the analysis of the genetic programme and cell behaviours that are controlled by Nodal signalling during vertebrate gastrulation. It also provides a good example of the sub-functionalisation that results from the expansion of gene families in evolution.

Wnt to build a tube: contributions of Wnt signaling to epithelial tubulogenesis

Epithelial tubes are crucial to the function of organ systems including the cardiovascular system, pulmonary system, gastrointestinal tract, reproductive organ systems, excretory system, and auditory system. Using a variety of animal model systems, recent studies have substantiated the role of Wnt signaling via the canonical/beta-catenin-mediated trajectory, the non-canonical Wnt trajectories, or both, in forming epithelial tubular tissues. This review focuses on the involvement of the Wnt pathways in the induction, specification, proliferation, and morphogenesis involved in tubulogenesis within tissues including the lungs, kidneys, ears, mammary glands, gut, and heart. The ultimate goal is to describe the developmental processes forming the various tubulogenic organ systems to determine the relationships between these processes.

Expression of five frizzleds during zebrafish craniofacial development

Wnt/Planar Cell Polarity (PCP) signaling is critical for proper animal development. While initially identified in Drosophila, this pathway is also essential for the proper development of vertebrates. Zebrafish mutants, defective in the Wnt/PCP pathway, frequently display defects in convergence and extension gastrulation movements and additional later abnormalities including problems with craniofacial cartilage morphogenesis. Although multiple Frizzled (Fzd) homologues, Wnt receptors, were identified in zebrafish, it is unknown which Fzd plays a role in shaping the early larvae head skeleton. In an effort to determine which Frizzleds are involved in this process, we analyzed the expression of five zebrafish frizzled homologues fzd2, 6, 7a, 7b, and 8a from 2-4days post-fertilization (dpf). During the analyzed developmental time points fzd2 and fzd6 are broadly expressed throughout the head, while the expression of fzd7a, 7b and 8a is much more restricted. Closer examination revealed that fzd7b is expressed in the neural crest and the mesodermal core of the pharyngeal arches and in the chondrocytes of newly stacked craniofacial cartilage elements. However, fzd7a is only expressed in the neural crest of the pharyngeal arches and fzd8a is expressed in the pharyngeal endoderm.

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.

In vivo analyzes of dystroglycan function during somitogenesis in Xenopus laevis

Dystroglycan (Dg) is a cell adhesion receptor for laminin that has been reported to play a role in skeletal muscle cell stability, cytoskeletal organization, cell polarity, and signaling. Here we show that Dg is expressed at both the notochord/somite and the intersomitic boundaries, where laminin and fibronectin are accumulated during somitogenesis. Inhibition of Dg function with morpholino antisense oligonucleotides or a dominant negative mutant results in the normal segmentation of the presomitic mesoderm but affects the number, the size, and the integrity of somites. Depletion of Dg disrupts proliferation and alignment of myoblasts without affecting XMyoD and XMRF4 expression. It also leads to defects in laminin deposition at the intersomitic junctions, whereas expression of integrin beta1 subunits and fibronectin assembly occur normally. Our results show that Dg is critical for both proliferation and elongation of somitic cells and that the Dg-cytoplasmic domain is required for the laminin assembly at the intersomitic boundaries. Developmental Dynamics 238:1332-1345, 2009. (c) 2008 Wiley-Liss, Inc.

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.

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.

XsFRP5 modulates endodermal organogenesis in Xenopus laevis

Canonical Wnt signalling is known to be involved in the regulation of differentiation and proliferation in the context of endodermal organogenesis. Wnt mediated beta-catenin activation is understood to be modulated by secreted Frizzled-related proteins, such as XsFRP5, which is dynamically expressed in the prospective liver/ventral pancreatic precursor cells during late neurula stages, becoming liver specific at tailbud stages and shifting to the posterior stomach/anterior duodenum territory during tadpole stages of Xenopus embryogenesis. These expression characteristics prompted us to analyse the function of XsFRP5 in the context of endodermal organogenesis. We demonstrate that XsFRP5 can form a complex with and inhibit a multitude of different Wnt ligands, including both canonical and non-canonical ones. Knockdown of XsFRP5 results in transient pancreatic hypoplasia as well as in an enlargement of the stomach. In VegT-injected animal cap explants, XsFRP5 can induce expression of exocrine but not endocrine pancreatic marker genes. Both, its expression characteristics as well as its interactions with XsFRP5, define Wnt2b as a putative target for XsFRP5 in vivo. Knockdown of Wnt2b results in a hypoplastic stomach as well as in hypoplasia of the pancreas. On the basis of these findings we propose that XsFRP5 exerts an early regulatory function in the specification of the ventral pancreas, as well as a late function in controlling stomach size via inhibition of Wnt signalling.

Noncanonical Wnt signaling orchestrates early developmental events toward hematopoietic cell fate from human embryonic stem cells

During human development, signals that govern lineage specification versus expansion of cells committed to a cell fate are poorly understood. We demonstrate that activation of canonical Wnt signaling by Wnt3a promotes proliferation of human embryonic stem cells (hESCs)--precursors already committed to the hematopoietic lineage. In contrast, noncanonical Wnt signals, activated by Wnt11, control exit from the pluripotent state and entry toward mesoderm specification. Unique to embryoid body (EB) formation of hESCs, Wnt11 induces development and arrangement of cells expressing Brachyury that coexpress E-cadherin and Frizzled-7 (Fzd7). Knockdown of Fzd7 expression blocks Wnt11-dependent specification. Our study reveals an unappreciated role for noncanonical Wnt signaling in hESC specification that involves development of unique mesoderm precursors via morphogenic organization within human EBs.

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.

Wnt signaling pathways meet Rho GTPases

Wnt ligands and their receptors orchestrate many essential cellular and physiological processes. During development they control differentiation, proliferation, migration, and patterning, while in the adult, they regulate tissue homeostasis, primarily through their effects on stem cell proliferation and differentiation. Underpinning these diverse biological activities is a complex set of intracellular signaling pathways that are still poorly understood. Rho GTPases have emerged as key mediators of Wnt signals, most notably in the noncanonical pathways that involve polarized cell shape changes and migrations, but also more recently in the canonical pathway leading to beta-catenin-dependent transcription. It appears that Rho GTPases integrate Wnt-induced signals spatially and temporally to promote morphological and transcriptional changes affecting cell behavior.

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.