Xwnt11 and the regulation of gastrulation in Xenopus

Senescent cells perturb intestinal stem cell differentiation through Ptk7 induced noncanonical Wnt and YAP signaling

Cellular senescence and the senescence-associated secretory phenotype (SASP) are implicated in aging and age-related disease, and SASP-related inflammation is thought to contribute to tissue dysfunction in aging and diseased animals. However, whether and how SASP factors influence the regenerative capacity of tissues remains unclear. Here, using intestinal organoids as a model of tissue regeneration, we show that SASP factors released by senescent fibroblasts deregulate stem cell activity and differentiation and ultimately impair crypt formation. We identify the secreted N-terminal domain of Ptk7 as a key component of the SASP that activates non-canonical Wnt / Ca2+ signaling through FZD7 in intestinal stem cells (ISCs). Changes in cytosolic [Ca2+] elicited by Ptk7 promote nuclear translocation of YAP and induce expression of YAP/TEAD target genes, impairing symmetry breaking and stem cell differentiation. Our study discovers secreted Ptk7 as a factor released by senescent cells and provides insight into the mechanism by which cellular senescence contributes to tissue dysfunction in aging and disease.

Brachyury in the gastrula of basal vertebrates

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

A deficiency in SUMOylation activity disrupts multiple pathways leading to neural tube and heart defects in Xenopus embryos

Background

Adenovirus protein, Gam1, triggers the proteolytic destruction of the E1 SUMO-activating enzyme. Microinjection of an empirically determined amount of Gam1 mRNA into one-cell Xenopus embryos can reduce SUMOylation activity to undetectable, but nonlethal, levels, enabling an examination of the role of this post-translational modification during early vertebrate development.

Results

We find that SUMOylation-deficient embryos consistently exhibit defects in neural tube and heart development. We have measured differences in gene expression between control and embryos injected with Gam1 mRNA at three developmental stages: early gastrula (immediately following the initiation of zygotic transcription), late gastrula (completion of the formation of the three primary germ layers), and early neurula (appearance of the neural plate). Although changes in gene expression are widespread and can be linked to many biological processes, three pathways, non-canonical Wnt/PCP, snail/twist, and Ets-1, are especially sensitive to the loss of SUMOylation activity and can largely account for the predominant phenotypes of Gam1 embryos. SUMOylation appears to generate different pools of a given transcription factor having different specificities with this post-translational modification involved in the regulation of more complex, as opposed to housekeeping, processes.

Conclusions

We have identified changes in gene expression that underlie the neural tube and heart phenotypes resulting from depressed SUMOylation activity. Notably, these developmental defects correspond to the two most frequently occurring congenital birth defects in humans, strongly suggesting that perturbation of SUMOylation, either globally or of a specific protein, may frequently be the origin of these pathologies.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5773-3) contains supplementary material, which is available to authorized users.

[Wnt/β-catenin signaling is involved in the Icariin induced proliferation of bone marrow mesenchymal stem cells]

OBJECTIVE

To investigate the effect of icariin on proliferation of bone marrow mesenchymal stem cells (BMSCs) in Sprague-Dawley (SD) rats.

METHODS

BMSCs were obtained from SD rat bone marrow with differential time adherent method. Its characteristic was identified through differentiation cell surface antigens and the multi-lineage (osteo/adipo/chondo) differentiation potential. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method and 5-Bromo-2-Deoxyuridine (BrdU) incorporation were applied to detect the effect of icariin on BMSCs proliferation. Flow cytometry was used to detect proliferation index of BMSCs. The RNA level and the distribution of β-catenin were evaluated by Real-time Polymerase Chain Reaction (PCR) and Immunofluorescent staining respectively. Western blot was used to detect protein expression levels of β-catenin, glycogen synthase kinase-3 beta (GSK-3β), phospho-glycogen synthase kinase-3 beta (pGSK-3β) and cyclinD1.

RESULTS

Icariin promoted BMSCs proliferation at the concentration of 0.05-2.0 mg/L. The percentage of BrdU positive cells of BMSCs was increased from 40.98% to 70.42%, and the proliferation index value was increased from 8.9% to 17.5% with the treatment of 0.05 mg/L icariin, which significance values were both less than 0.05. Compared with the control group, total and nuclear β-catenin proteins, as well as β-catenin mRNA expression, were all increased with icariin treatment. Meanwhile, the phosphorylation level of GSK-3β and cyclinD1 protein expressions were also increased in BMSCs with icariin treatment.

CONCLUSION

The findings of the present study demonstrated that low dosage of icariin could promote BMSCs proliferation. The activation of Wnt/β-catenin pathways was involved in this process.

Mammary Development and Breast Cancer: A Wnt Perspective

The Wnt pathway has emerged as a key signaling cascade participating in mammary organogenesis and breast oncogenesis. In this review, we will summarize the current knowledge of how the pathway regulates stem cells and normal development of the mammary gland, and discuss how its various components contribute to breast carcinoma pathology.

Ror2 receptor mediates Wnt11 ligand signaling and affects convergence and extension movements in zebrafish

The receptor-tyrosine kinase Ror2 acts as an alternative receptor or co-receptor for Wnt5a and mediates Wnt5a-induced convergent extension movements during embryogenesis in mice and Xenopus as well as the polarity and migration of several cell types during development. However, little is known about whether Ror2 function is conserved in other vertebrates or is involved in other non-canonical Wnt ligands in vivo. In this study we demonstrated that overexpression of dominant-negative ror2 (ror2-TM) mRNA in zebrafish embryos resulted in convergence and extension defects and incompletely separated eyes, which is consistent with observations from slb/wnt11 mutants or wnt11 knockdown morphants. Moreover, the co-injection of ror2-TM mRNA and a wnt11 morpholino or the coexpression of ror2 and wnt11 in zebrafish embryos synergetically induced more severe convergence and extension defects. Transplantation studies further demonstrated that the Ror2 receptor responded to the Wnt11 ligand and regulated cell migration and cell morphology during gastrulation. DnRor2 inhibited the action of Wnt11, which was revealed by a decreased percentage of Wnt11-induced convergence and extension defects. Ror2 physically interacts with Wnt11. Theintracellular Tyr-647andSer-863 sites ofRor2are essential for mediating the action of Wnt11. Dishevelled and RhoA act downstream of Wnt11-Ror2 to regulate convergence and extension movements. Overall, our data suggest an important role of Ror2 in mediating Wnt11 signaling and in regulating convergence and extension movements in zebrafish.

Wnt11b is involved in cilia-mediated symmetry breakage during Xenopus left-right development

Breakage of bilateral symmetry in amphibian embryos depends on the development of a ciliated epithelium at the gastrocoel roof during early neurulation. Motile cilia at the gastrocoel roof plate (GRP) give rise to leftward flow of extracellular fluids. Flow is required for asymmetric gene expression and organ morphogenesis. Wnt signaling has previously been involved in two steps, Wnt/ß-catenin mediated induction of Foxj1, a regulator of motile cilia, and Wnt/planar cell polarity (PCP) dependent cilia polarization to the posterior pole of cells. We have studied Wnt11b in the context of laterality determination, as this ligand was reported to activate canonical and non-canonical Wnt signaling. Wnt11b was found to be expressed in the so-called superficial mesoderm (SM), from which the GRP derives. Surprisingly, Foxj1 was only marginally affected in loss-of-function experiments, indicating that another ligand acts in this early step of laterality specification. Wnt11b was required, however, for polarization of GRP cilia and GRP morphogenesis, in line with the known function of Wnt/PCP in cilia-driven leftward flow. In addition Xnr1 and Coco expression in the lateral-most GRP cells, which sense flow and generate the first asymmetric signal, was attenuated in morphants, involving Wnt signaling in yet another process related to symmetry breakage in Xenopus.

Xenopus laevis nucleotide binding protein 1 (xNubp1) is important for convergent extension movements and controls ciliogenesis via regulation of the actin cytoskeleton

Nucleotide binding protein 1 (Nubp1) is a highly conserved phosphate loop (P-loop) ATPase involved in diverse processes including iron-sulfur protein assembly, centrosome duplication and lung development. Here, we report the cloning, expression and functional characterization of Xenopus laevis Nubp1. We show that xNubp1 is expressed maternally, displays elevated expression in neural tissues and is required for convergent extension movements and neural tube closure. In addition, xNubp1knockdown leads to defective ciliogenesis of the multi-ciliated cells of the epidermis as well as the monociliated cells of the gastrocoel roof plate. Specifically, xNubp1 is required for basal body migration, spacing and docking in multi-ciliated cells and basal body positioning and axoneme elongation in monociliated gastrocoel roof plate cells. Live imaging of the different pools of actin and basal body migration during the process of ciliated cell intercalation revealed that two independent pools of actin are present from the onset of cell intercalation; an internal network surrounding the basal bodies, anchoring them to the cell cortex and an apical pool of punctate actin which eventually matures into the characteristic apical actin network. We show that xNubp1 colocalizes with the apical actin network of multiciliated cells and that problems in basal body transport in xNubp1 morphants are associated with defects of the internal network of actin, while spacing and polarity issues are due to a failure of the apical and sub-apical actin pools to mature into a network. Effects of xNubp1 knockdown on the actin cytoskeleton are independent of RhoA localization and activation, suggesting that xNubp1 may have a direct role in the regulation of the actin cytoskeleton.

Integration of canonical and noncanonical Wnt signaling pathways patterns the neuroectoderm along the anterior-posterior axis of sea urchin embryos

Three different Wnt signaling pathways function to restrict the anterior neuroectoderm state to the anterior end of the sea urchin embryo, a mechanism of anterior fate restriction that could be conserved among deuterostomes.

Patterning the neuroectoderm along the anterior–posterior (AP) axis is a critical event in the early development of deuterostome embryos. However, the mechanisms that regulate the specification and patterning of the neuroectoderm are incompletely understood. Remarkably, the anterior neuroectoderm (ANE) of the deuterostome sea urchin embryo expresses many of the same transcription factors and secreted modulators of Wnt signaling, as does the early vertebrate ANE (forebrain/eye field). Moreover, as is the case in vertebrate embryos, confining the ANE to the anterior end of the embryo requires a Wnt/β-catenin-dependent signaling mechanism. Here we use morpholino- or dominant negative-mediated interference to demonstrate that the early sea urchin embryo integrates information not only from Wnt/β-catenin but also from Wnt/Fzl5/8-JNK and Fzl1/2/7-PKC pathways to provide precise spatiotemporal control of neuroectoderm patterning along its AP axis. Together, through the Wnt1 and Wnt8 ligands, they orchestrate a progressive posterior-to-anterior wave of re-specification that restricts the initial, ubiquitous, maternally specified, ANE regulatory state to the most anterior blastomeres. There, the Wnt receptor antagonist, Dkk1, protects this state through a negative feedback mechanism. Because these different Wnt pathways converge on the same cell fate specification process, our data suggest they may function as integrated components of an interactive Wnt signaling network. Our findings provide strong support for the idea that the sea urchin ANE regulatory state and the mechanisms that position and define its borders represent an ancient regulatory patterning system that was present in the common echinoderm/vertebrate ancestor.

The initial regulatory state of most cells in many deuterostome embryos, including those of vertebrates and sea urchins, supports anterior neural fate specification. It is important to restrict this neurogenic potential to the anterior end of the embryo during early embryogenesis, but the molecular mechanisms by which this re-specification of posterior fate occurs are incompletely understood in any embryo. The sea urchin embryo is ideally suited to study this process because, in contrast to vertebrates, anterior–posterior neuroectoderm patterning occurs independently of dorsal-ventral axis patterning and takes place before the complex cell movements of gastrulation. In this study, we show that a linked, three-step process involving at least three different Wnt signaling pathways provides precise spatiotemporal restriction of the anterior neuroectoderm regulatory state to the anterior end of the sea urchin embryo. Because these three pathways impinge on the same developmental process, they could be functioning as an integrated Wnt signaling network. Moreover, striking parallels among gene expression patterns and functional studies suggest that this mechanism of anterior fate restriction could be highly conserved among deuterostomes.

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

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

Wnt/β-catenin signaling pathway is a direct enhancer of thyroid transcription factor-1 in human papillary thyroid carcinoma cells

The Wnt/β-catenin signaling pathway is involved in the normal development of thyroid gland, but its disregulation provokes the appearance of several types of cancers, including papillary thyroid carcinomas (PTC) which are the most common thyroid tumours. The follow-up of PTC patients is based on the monitoring of serum thyroglobulin levels which is regulated by the thyroid transcription factor 1 (TTF-1): a tissue-specific transcription factor essential for the differentiation of the thyroid. We investigated whether the Wnt/β-catenin pathway might regulate TTF-1 expression in a human PTC model and examined the molecular mechanisms underlying this regulation. Immunofluorescence analysis, real time RT-PCR and Western blot studies revealed that TTF-1 as well as the major Wnt pathway components are co-expressed in TPC-1 cells and human PTC tumours. Knocking-down the Wnt/β-catenin components by siRNAs inhibited both TTF-1 transcript and protein expression, while mimicking the activation of Wnt signaling by lithium chloride induced TTF-1 gene and protein expression. Functional promoter studies and ChIP analysis showed that the Wnt/β-catenin pathway exerts its effect by means of the binding of β-catenin to TCF/LEF transcription factors on the level of an active TCF/LEF response element at [−798, −792 bp] in TTF-1 promoter. In conclusion, we demonstrated that the Wnt/β-catenin pathway is a direct and forward driver of the TTF-1 expression. The localization of TCF-4 and TTF-1 in the same area of PTC tissues might be of clinical relevance, and justifies further examination of these factors in the papillary thyroid cancers follow-up.

Xenopus Wnt11b is identified as a potential pronephric inducer

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

Towards an integrated view of Wnt signaling in development

Wnt signaling is crucial for embryonic development in all animal species studied to date. The interaction between Wnt proteins and cell surface receptors can result in a variety of intracellular responses. A key remaining question is how these specific responses take shape in the context of a complex, multicellular organism. Recent studies suggest that we have to revise some of our most basic ideas about Wnt signal transduction. Rather than thinking about Wnt signaling in terms of distinct, linear, cellular signaling pathways, we propose a novel view that considers the integration of multiple, often simultaneous, inputs at the level of both Wnt-receptor binding and the downstream, intracellular response.

Down syndrome critical region protein 5 regulates membrane localization of Wnt receptors, Dishevelled stability and convergent extension in vertebrate embryos

The Glypican family of heparan sulfate proteoglycans regulates Wnt signaling and convergent extension (CE) in vertebrate embryos. They are predicted to be glycosylphosphatidylinositol (GPI)-tethered membrane-bound proteins, but there is no functional evidence of their regulation by the GPI synthesis complex. Down syndrome critical region protein 5 (Dscr5, also known as Pigp) is a component of the GPI-N-acetylglucosaminyltransferase (GPI-GnT) complex, and is associated with specific features of Down syndrome. Here we report that Dscr5 regulates CE movements through the non-canonical Wnt pathway. Both dscr5 overexpression and knockdown impaired convergence and extension movements. Dscr5 functionally interacted with Knypek/Glypican 4 and was required for its localization at the cell surface. Knockdown of dscr5 disrupted Knypek membrane localization and caused an enhanced Frizzled 7 receptor endocytosis in a Caveolin-dependent manner. Furthermore, dscr5 knockdown promoted specific Dishevelled degradation by the ubiquitin-proteosome pathway. These results reveal a functional link between Knypek/Glypican 4 and the GPI synthesis complex in the non-canonical Wnt pathway, and provide the new mechanistic insight that Dscr5 regulates CE in vertebrate embryos by anchoring different Wnt receptors at the cell surface and maintaining Dishevelled stability.

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.

PTK7 is essential for polarized cell motility and convergent extension during mouse gastrulation

Despite being implicated as a mechanism driving gastrulation and body axis elongation in mouse embryos, the cellular mechanisms underlying mammalian convergent extension (CE) are unknown. Here we show, with high-resolution time-lapse imaging of living mouse embryos, that mesodermal CE occurs by mediolateral cell intercalation, driven by mediolaterally polarized cell behavior. The initial events in the onset of CE are mediolateral elongation, alignment and orientation of mesoderm cells as they exit the primitive streak. This cell shape change occurs prior to, and is required for, the subsequent onset of mediolaterally polarized protrusive activity. In embryos mutant for PTK7, a novel cell polarity protein, the normal cell elongation and alignment upon leaving the primitive streak, the subsequent polarized protrusive activity, and CE and axial elongation all failed. The mesoderm normally thickens and extends, but on failure of convergence movements in Ptk7 mutants, the mesoderm underwent radial intercalation and excessive thinning, which suggests that a cryptic radial cell intercalation behavior resists excessive convergence-driven mesodermal thickening in normal embryos. When unimpeded by convergence forces in Ptk7 mutants, this unopposed radial intercalation resulted in excessive thinning of the mesoderm. These results show for the first time the polarized cell behaviors underlying CE in the mouse, demonstrate unique aspects of these behaviors compared with those of other vertebrates, and clearly define specific roles for planar polarity and for the novel planar cell polarity gene, Ptk7, as essential regulators of mediolateral cell intercalation during mammalian CE.

Non-canonical Wnt signaling and N-cadherin related beta-catenin signaling play a role in mechanically induced osteogenic cell fate

Background

Understanding how the mechanical microenvironment influences cell fate, and more importantly, by what molecular mechanisms, will enhance not only the knowledge of mesenchymal stem cell biology but also the field of regenerative medicine. Mechanical stimuli, specifically loading induced oscillatory fluid flow, plays a vital role in promoting healthy bone development, homeostasis and morphology. Recent studies suggest that such loading induced fluid flow has the potential to regulate osteogenic differentiation via the upregulation of multiple osteogenic genes; however, the molecular mechanisms involved in the transduction of a physical signal into altered cell fate have yet to be determined.

Methods and Principal Findings

Using immuno-staining, western blot analysis and luciferase assays, we demonstrate the oscillatory fluid flow regulates β-catenin nuclear translocation and gene transcription. Additionally, real time RT-PCR analysis suggests that flow induces Wnt5a and Ror2 upregulation, both of which are essential for activating the small GTPase, RhoA, upon flow exposure. Furthermore, although β-catenin phosphorylation is not altered by flow, its association with N-cadherin is, indicating that flow-induced β-catenin signaling is initiated by adherens junction signaling.

Conclusion

We propose that the mechanical microenvironment of bone has the potential to regulate osteogenic differentiation by initiating multiple key molecular pathways that are essential for such lineage commitment. Specifically, non-canonical Wnt5a signaling involving Ror2 and RhoA as well as N-cadherin mediated β-catenin signaling are necessary for mechanically induced osteogenic differentiation.

The physical state of fibronectin matrix differentially regulates morphogenetic movements in vivo

This study demonstrates that proper spatiotemporal expression and the physical assembly state of fibronectin (FN) matrix play key roles in the regulation of morphogenetic cell movements in vivo. We examine the progressive assembly and 3D fibrillar organization of FN and its role in regulating cell and tissue movements in Xenopus embryos. Expression of the 70 kD N-terminal fragment of FN blocks FN fibril assembly at gastrulation but not initial FN binding to integrins at the cell surface. We find that fibrillar FN is necessary to maintain cell polarity through oriented cell division and to promote epiboly, possibly through maintenance of tissue-surface tension. In contrast, FN fibrils are dispensable for convergence and extension movements required for axis elongation. Closure of the migratory mesendodermal mantle was accelerated in the absence of a fibrillar matrix. Thus, the macromolecular assembly of FN matrices may constitute a general regulatory mechanism for coordination of distinct morphogenetic movements.

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.

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

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

Primary cilia in planar cell polarity regulation of the inner ear

Primary cilia are essential components of diverse cellular processes. Many of the requirements can be linked to the apparent signaling function of primary cilia. Recent studies have also uncovered a role for primary cilia in planar cell polarity (PCP) signaling. PCP refers to the coordinated orientation of cells along an axis parallel to the plane of the cell sheet. In vertebrates, the inner ear sensory organs display distinctive forms of PCP. One of the inner ear PCP characteristics is the coordinated positioning of a primary cilium eccentrically in every sensory hair cell within each organ. The inner ear, therefore, provides an opportunity to explore the cellular role of primary cilia in PCP signaling. In this chapter, we will introduce the PCP of the inner ear sensory organs, describe the conserved mechanism underlying the establishment of the planar polarity axis in invertebrates and vertebrates, and highlight a unique requirement for primary cilia in PCP regulation in vertebrates. Additionally, we will discuss a potentially ubiquitous role for cilia in cellular polarization in general.

Wnt signalling regulates paxillin ubiquitination essential for mesodermal cell motility

Gastrulation movements are critical for establishing the three germ layers and the architecture of vertebrate embryos. During Xenopus laevis gastrulation, mesodermal tissue migrates on the blastocoel roof and elongates along the antero-posterior axis. During this process, cells in the dorsal mesoderm are polarized and intercalate with each other, which is defined as convergent extension and is known to be regulated by the non-canonical Wnt pathway. Here, we show that paxillin plays an essential role in this process. Paxillin is a focal-adhesion associated protein implicated in the regulation of actin cytoskeletal organization and cell motility, but its role in Xenopus embryogenesis has not yet been clarified. We demonstrate that the Wnt pathway controls the ubiquitination and stability of paxillin, and that this regulatory mechanism is essential for convergent extension movements. We identified a RING finger protein XRNF185, which physically binds to paxillin and the proteasome. XRNF185 destabilizes paxillin at focal adhesions and promotes mesodermal cell migration during convergent extension. We propose a mechanism to regulate gastrulation movements that involves paxillin ubiquitination and stability controlled by Wnt signalling.

Wnt5a functions in planar cell polarity regulation in mice

Planar cell polarity (PCP) refers to the polarization of cells within the plane of a cell sheet. A distinctive epithelial PCP in vertebrates is the uniform orientation of stereociliary bundles of the sensory hair cells in the mammalian cochlea. In addition to establishing epithelial PCP, planar polarization is also required for convergent extension (CE); a polarized cellular movement that occurs during neural tube closure and cochlear extension. Studies in Drosophila and vertebrates have revealed a conserved PCP pathway, including Frizzled (Fz) receptors. Here we use the cochlea as a model system to explore the involvement of known ligands of Fz, Wnt morphogens, in PCP regulation. We show that Wnt5a forms a reciprocal expression pattern with a Wnt antagonist, the secreted frizzled-related protein 3 (Sfrp3 or Frzb), along the axis of planar polarization in the cochlear epithelium. We further demonstrate that Wnt5a antagonizes Frzb in regulating cochlear extension and stereociliary bundle orientation in vitro, and that Wnt5a(-/-) animals have a shortened and widened cochlea. Finally, we show that Wnt5a is required for proper subcellular distribution of a PCP protein, Ltap/Vangl2, and that Wnt5a interacts genetically with Ltap/Vangl2 for uniform orientation of stereocilia, cochlear extension, and neural tube closure. Together, these findings demonstrate that Wnt5a functions in PCP regulation in mice.

Planar cell polarity signaling in vertebrates

Planar cell polarity (PCP) refers to the polarization of a field of cells within the plane of a cell sheet. This form of polarization is required for diverse cellular processes in vertebrates, including convergent extension (CE), the establishment of PCP in epithelial tissues and ciliogenesis. Perhaps the most distinct example of vertebrate PCP is the uniform orientation of stereociliary bundles at the apices of sensory hair cells in the mammalian auditory sensory organ. The establishment of PCP in the mammalian cochlea occurs concurrently with CE in this ciliated epithelium, therefore linking three cellular processes regulated by the vertebrate PCP pathway in the same tissue and emerging as a model system for dissecting PCP signaling. This review summarizes the morphogenesis of this model system to assist the interpretation of the emerging data and proposes molecular mechanisms underlying PCP signaling in vertebrates.

Shaping the mammalian auditory sensory organ by the planar cell polarity pathway

The human ear is capable of processing sound with a remarkable resolution over a wide range of intensity and frequency. This ability depends largely on the extraordinary feats of the hearing organ, the organ of Corti and its sensory hair cells. The organ of Corti consists of precisely patterned rows of sensory hair cells and supporting cells along the length of the snail-shaped cochlear duct. On the apical surface of each hair cell, several rows of actin-containing protrusions, known as stereocilia, form a "V"-shaped staircase. The vertices of all the "V"-shaped stereocilia point away from the center of the cochlea. The uniform orientation of stereocilia in the organ of Corti manifests a distinctive form of polarity known as planar cell polarity (PCP). Functionally, the direction of stereociliary bundle deflection controls the mechanical channels located in the stereocilia for auditory transduction. In addition, hair cells are tonotopically organized along the length of the cochlea. Thus, the uniform orientation of stereociliary bundles along the length of the cochlea is critical for effective mechanotransduction and for frequency selection. Here we summarize the morphological and molecular events that bestow the structural characteristics of the mammalian hearing organ, the growth of the snail-shaped cochlear duct and the establishment of PCP in the organ of Corti. The PCP of the sensory organs in the vestibule of the inner ear will also be described briefly.

FGF8, Wnt8 and Myf5 are target genes of Tbx6 during anteroposterior specification in Xenopus embryo

The T-box transcription factor Tbx6 is required for somite formation and loss-of-function or reduced activity of Tbx6 result in absence of posterior paraxial mesoderm or disorganized somites, but how it is involved in a regulatory hierarchy during Xenopus early development is not clear. We show here that Tbx6 is expressed in the lateral and ventral mesoderm of early gastrula, and it is necessary and sufficient to directly and indirectly regulate the expression of a subset of early mesodermal and endodermal genes. Ectopic expression of Tbx6 inhibits early neuroectodermal gene expression by strongly inducing the expression of posterior mesodermal genes, and expands the mesoderm territory at the expense of neuroectoderm. Conversely, overexpression of a dominant negative Tbx6 mutant in the ventral mesoderm inhibits the expression of several mesodermal genes and results in neural induction in a dose-dependent manner. Using a hormone-inducible form of Tbx6, we have identified FGF8, Xwnt8 and XMyf5 as immediate early responsive genes of Tbx6, and the induction of these genes by Tbx6 is independent of Xbra and VegT. These target genes act downstream and mediate the function of Tbx6 in anteroposterior specification. Our results therefore identify a regulatory cascade governed by Tbx6 in the specification of posterior mesoderm during Xenopus early development.

Kaiso/p120-catenin and TCF/beta-catenin complexes coordinately regulate canonical Wnt gene targets

Beta-catenin-dependent or canonical Wnt signals are fundamental in animal development and tumor progression. Using Xenopus laevis, we report that the BTB/POZ zinc finger family member Kaiso directly represses canonical Wnt gene targets (Siamois, c-Fos, Cyclin-D1, and c-Myc) in conjunction with TCF/LEF (TCF). Analogous to beta-catenin relief of TCF repressive activity, we show that p120-catenin relieves Kaiso-mediated repression of Siamois. Furthermore, Kaiso and TCF coassociate, and combined Kaiso and TCF derepression results in pronounced Siamois expression and increased beta-catenin coprecipitation with the Siamois promoter. The functional interdependency is underlined by Kaiso suppression of beta-catenin-induced axis duplication and by TCF-3 rescue of Kaiso depletion phenotypes. These studies point to convergence of parallel p120-catenin/Kaiso and beta-catenin/TCF signaling pathways to regulate gene expression in vertebrate development and possibly carcinogenesis.

Antagonistic interaction between IGF and Wnt/JNK signaling in convergent extension in Xenopus embryo

The homeobox gene Otx2 is expressed during gastrulation in the anterior domain of the vertebrate embryo and is involved in neural and head induction during Xenopus early development. It also prevents convergent extension movements in trunk and posterior mesoderm. Insulin-like growth factors (IGFs) were shown to have similar function. However, whether they interact and the mechanism by which they affect convergent extension remain unclear. We show that IGF pathway specifically induces the expression of Otx2 in the early gastrula and blocks convergent extension of neuroectoderm and mesoderm through the transcriptional activation of Otx2 gene. Otx2 represses the expression of Xbra and Xwnt-11, and the effects of IGF on gastrulation movements can be partially rescued by antisense Otx2 morpholino oligonucleotide. These indicate that IGF pathway interacts with Otx2 to restrict Xbra and Xwnt-11 expression in the trunk and posterior regions. Consistent with this, we show that inhibition of IGF signaling or Otx2 function induces Xbra and Xwnt11 expression and convergent extension in ectodermal cells. Furthermore, the blockade of convergent extension by IGF-I and Otx2 can be rescued by coexpression of Xwnt-11 or a constitutively active Jun N-terminal kinase (JNK). Because Xbra and Xwnt-11 are required for convergent extension movements and Xwnt-11 activates the non-canonical Wnt-11/JNK pathway, our results reveal a mutually exclusive function between IGF and Wnt-11/JNK pathways in regulating cell behaviours during vertebrate head and trunk development.

Essential role of non-canonical Wnt signalling in neural crest migration

Migration of neural crest cells is an elaborate process that requires the delamination of cells from an epithelium and cell movement into an extracellular matrix. In this work, it is shown for the first time that the non-canonical Wnt signalling [planar cell polarity (PCP) or Wnt-Ca2+] pathway controls migration of neural crest cells. By using specific Dsh mutants, we show that the canonical Wnt signalling pathway is needed for neural crest induction, while the non-canonical Wnt pathway is required for neural crest migration. Grafts of neural crest tissue expressing non-canonical Dsh mutants, as well as neural crest cultured in vitro, indicate that the PCP pathway works in a cell-autonomous manner to control neural crest migration. Expression analysis of non-canonical Wnt ligands and their putative receptors show that Wnt11 is expressed in tissue adjacent to neural crest cells expressing the Wnt receptor Frizzled7 (Fz7). Furthermore, loss- and gain-of-function experiments reveal that Wnt11 plays an essential role in neural crest migration. Inhibition of neural crest migration by blocking Wnt11 activity can be rescued by intracellular activation of the non-canonical Wnt pathway. When Wnt11 is expressed opposite its normal site of expression, neural crest migration is blocked. Finally, time-lapse analysis of cell movement and cell protrusion in neural crest cultured in vitro shows that the PCP or Wnt-Ca2+ pathway directs the formation of lamellipodia and filopodia in the neural crest cells that are required for their delamination and/or migration.

Non-canonical Wnt signals are modulated by the Kaiso transcriptional repressor and p120-catenin

Gastrulation movements are critical for establishing the three principal germ layers and the basic architecture of vertebrate embryos. Although the individual molecules and pathways involved are not clearly understood, non-canonical Wnt signals are known to participate in developmental processes, including planar cell polarity and directed cell rearrangements. Here we demonstrate that the dual-specificity transcriptional repressor Kaiso, first identified in association with p120-catenin, is required for Xenopus gastrulation movements. In addition, depletion of xKaiso results in increased expression of the non-canonical xWnt11, which contributes to the xKaiso knockdown phenotype as it is significantly rescued by dominant-negative Wnt11. We further demonstrate that xWnt11 is a direct gene target of xKaiso and that p120-catenin association relieves xKaiso repression in vivo. Our results indicate that p120-catenin and Kaiso are essential components of a new developmental gene regulatory pathway that controls vertebrate morphogenesis.

Move it or lose it: axis specification in Xenopus

A long-standing question in developmental biology is how amphibians establish a dorsoventral axis. The prevailing view has been that cortical rotation is used to move a dorsalizing activity from the bottom of the egg towards the future dorsal side. We review recent evidence that kinesin-dependent movement of particles containing components of the Wnt intracellular pathway contributes to the formation of the dorsal organizer, and suggest that cortical rotation functions to align and orient microtubules, thereby establishing the direction of particle transport. We propose a new model in which active particle transport and cortical rotation cooperate to generate a robust movement of dorsal determinants towards the future dorsal side of the embryo.

Wnt signaling in axon guidance

Recent studies have identified Wnt proteins as conserved axon guidance molecules in vertebrates and invertebrates. Wnt proteins are a large family of diffusible factors that play several important roles, both in embryonic development and in adult function. The signaling mechanisms of Wnt proteins are complex and, because Wnts are newly discovered as axon guidance cues, little is known about how Wnt signaling controls the direction of growth cone navigation - a process that is crucial in development of the nervous system. This review summarizes recent work on the role of Wnts in axon guidance and discusses the possible signaling mechanisms involved in growth cone guidance. Understanding how Wnts regulate axon wiring will not only help us to understand how the nervous system is connected but also provide possible tools for axon regeneration.

Role of the TAK1-NLK-STAT3 pathway in TGF-beta-mediated mesoderm induction

Transforming growth factor (TGF)-beta-activated kinase 1 (TAK1) and Nemo-like kinase (NLK) function in Xenopus, Drosophila, and Caenorhabditis elegans development. Here we report that serine phosphorylation of STAT3 induced by TAK1-NLK cascade is essential fo TGF-beta-mediated mesoderm induction in Xenopus embryo. Depletion of TAK1, NLK, or STAT3 blocks TGF-beta-mediated mesoderm induction. Coexpression of NLK and STAT3 induces mesoderm by a mechanism that requires serine phosphorylation of STAT3. Activin activates NLK, which in turn directly phosphorylates STAT3. Moreover, depletion of either TAK1 or NLK inhibits endogenous serine phosphorylation of STAT3. These results provide the first evidence that TAK1-NLK-STAT3 cascade participates in TGF-beta-mediated mesoderm induction.

T-box genes in early embryogenesis

The T-box gene family, encoding related DNA-binding transcriptional regulators, plays an essential role in controlling many aspects of embryogenesis in a wide variety of organisms. The T-box genes exhibit diverse patterns of spatial and temporal expression in the developing embryo, and both genetic and molecular embryological studies have demonstrated their importance in regulating cell fate decisions that establish the early body plan, and in later processes underlying organogenesis. Despite these studies, little is known of either the regulation of the T-box genes or the identities of their transcriptional targets. The aim of this review is to examine the diverse yet conserved roles of several T-box genes in regulating early patterning in chordates and to discuss possible mechanisms through which this functional diversity might arise. Developmental Dynamics 229:201-218, 2004.

Dishevelled activates Ca2+ flux, PKC, and CamKII in vertebrate embryos

Wnt ligands and Frizzled (Fz) receptors have been shown to activate multiple intracellular signaling pathways. Activation of the Wnt-beta-catenin pathway has been described in greatest detail, but it has been reported that Wnts and Fzs also activate vertebrate planar cell polarity (PCP) and Wnt-Ca2+ pathways. Although the intracellular protein Dishevelled (Dsh) plays a dual role in both the Wnt-beta-catenin and the PCP pathways, its potential involvement in the Wnt-Ca2+ pathway has not been investigated. Here we show that a Dsh deletion construct, XDshDeltaDIX, which is sufficient for activation of the PCP pathway, is also sufficient for activation of three effectors of the Wnt-Ca2+ pathway: Ca2+ flux, PKC, and calcium/calmodulin-dependent protein kinase II (CamKII). Furthermore, we find that interfering with endogenous Dsh function reduces the activation of PKC by Xfz7 and interferes with normal heart development. These data suggest that the Wnt-Ca2+ pathway utilizes Dsh, thereby implicating Dsh as a component of all reported Fz signaling pathways.

Shaping the zebrafish notochord

Promptly after the notochord domain is specified in the vertebrate dorsal mesoderm, it undergoes dramatic morphogenesis. Beginning during gastrulation, convergence and extension movements change a squat cellular array into a narrow, elongated one that defines the primary axis of the embryo. Convergence and extension might be coupled by a highly organized cellular intermixing known as mediolateral intercalation behavior (MIB). To learn whether MIB drives early morphogenesis of the zebrafish notochord, we made 4D recordings and quantitatively analyzed both local cellular interactions and global changes in the shape of the dorsal mesodermal field. We show that MIB appears to mediate convergence and can account for extension throughout the dorsal mesoderm. Comparing the notochord and adjacent somitic mesoderm reveals that extension can be regulated separately from convergence. Moreover, mutational analysis shows that extension does not require convergence. Hence, a cellular machine separate from MIB that can drive dorsal mesodermal extension exists in the zebrafish gastrula. The likely redundant control of morphogenesis may provide for plasticity at this critical stage of early development.

brachyury Target genes in the early sea urchin embryo isolated by differential macroarray screening

Brachyury is a transcription factor that functions in gastrulation and endoderm development throughout the Bilateria. Here, we identify genes that are expressed downstream of brachyury during gastrulation of the sea urchin embryo. Screens with two different complex probes generated by subtractive hybridization were carried out on high-density arrays of embryonic cDNA libraries. An mRNA sequence population from embryos expressing brachyury at its peak stage of expression was subtracted with message sequence from embryos in which Brachyury function had been "knocked-out" by injection of a morpholine-substituted antisense oligonucleotide to generate a differential probe for brachyury target genes. Another probe was made by using an mRNA population from embryos that mis-express brachyury at a stage just prior to the normal onset of expression, subtracted with message sequence taken from normal embryos at this stage. Screens carried out with these probes target overlapping but distinct sets of downstream genes. After partial sequence characterization, promising genes were independently analyzed by quantitative real-time PCR and by in situ hybridization. Two major classes of genes emerge in this study: genes expressed in the subset of the secondary mesenchyme cells (SMC) that will become pigment cells, and genes that are expressed in portions of the endoderm coincident with brachyury expression. The latter genes are candidates for direct transcriptional targets of Brachyury. Some of the endodermal genes that respond to Brachyury are cytoskeletal modulators that may play a role in gut morphogenesis. This finding is consistent with the block in gastrulation induced by interfering with Brachyury function in sea urchins, and with known or suggested Brachyury function in other species. Other endodermal target genes are expressed in the archenteron and might be terminal differentiation enzymes of the gut. Brachyury expression patterns for Strongylocentrotus purpuratus reported in this paper are entirely consistent with data from other echinoderm species. Brachyury expression in the vegetal plate is confined to the presumptive endodermal cells. Therefore, the SMC genes are likely to be indirect targets of Brachyury-induced signaling from the surrounding endoderm to the central mesoderm, or the effects on these genes may be indirect consequences of gross disruption of the vegetal plate. These results and other data suggest that the brachyury gene transduces information about the state of endodermal specification to genes that modulate morphogenesis and genes that perform terminal functions in the gut.

The T-box transcription factor Brachyury mediates cartilage development in mesenchymal stem cell line C3H10T1/2

The BMP2-dependent onset of osteo/chondrogenic differentiation in the acknowledged pluripotent murine mesenchymal stem cell line (C3H10T1/2) is accompanied by the immediate upregulation of Fibroblast Growth Factor Receptor 3 (FGFR3) and a delayed response by FGFR2. Forced expression of FGFR3 in C3H10T1/2 is sufficient for chondrogenic differentiation, indicating an important role for FGF-signaling during the manifestation of the chondrogenic lineage in this cell line. Screening for transcription factors exhibiting a chondrogenic capacity in C3H10T1/2 indentified that the T-box containing transcription factor Brachyury is upregulated by FGFR3-mediated signaling. Forced expression of Brachyury in C3H10T1/2 was sufficient for differentiation into the chondrogenic lineage in vitro and in vivo after transplantation into muscle. A dominant-negative variant of Brachyury, consisting of its DNA-binding domain (T-box), interferes with BMP2-mediated cartilage formation. These studies indicate that BMP-initiated FGF-signaling induces a novel type of transcription factor for the onset of chondrogenesis in a mesenchymal stem cell line. A potential role for this T-box factor in skeletogenesis is further delineated from its expression profile in various skeletal elements such as intervertebral disks and the limb bud at late stages (18.5 d.p.c.) of murine embryonic development.

Heads or tails? Amphioxus and the evolution of anterior-posterior patterning in deuterostomes

In Xenopus, the canonical Wnt-signaling pathway acting through beta-catenin functions both in establishing the dorso-ventral axis and in patterning the anterior-posterior axis. This pathway also acts in patterning the animal-vegetal axis in sea urchins. However, because sea urchin development is typically indirect, and adult sea urchins have pentamerous symmetry and lack a longitudinal nerve cord, it has not been clear how the roles of the canonical Wnt-signaling pathway in axial patterning in sea urchins and vertebrates are evolutionarily related. The developmental expression patterns of Notch, brachyury, caudal, and eight Wnt genes have now been determined for the invertebrate chordate Amphioxus, which, like sea urchins, has an early embryo that gastrulates by invagination, but like vertebrates, has a later embryo with a dorsal hollow nerve cord that elongates posteriorly from a tail bud. Comparisons of Amphioxus with other deuterostomes suggest that patterning of the ancestral deuterostome embryo along its anterior-posterior axis during the late blastula and subsequent stages involved a posterior signaling center including Wnts, Notch, and transcription factors such as brachyury and caudal. In tunicate embryos, in which cell numbers are reduced and cell fates largely determined during cleavage stages, only vestiges of this signaling center are still apparent; these include localization of Wnt-5 mRNA to the posterior cytoplasm shortly after fertilization and localization of beta-catenin to vegetal nuclei during cleavage stages. Neither in tunicates nor in Amphioxus is there any evidence that the canonical Wnt-signaling pathway functions in establishment of the dorso-ventral axis. Thus, roles for Wnt-signaling in dorso-ventral patterning of embryos may be a vertebrate innovation that arose in connection with the evolution of yolky eggs and gastrulation by extensive involution.