Differences in boundary behavior in the 3D vertex and Voronoi models

An important open question in the modeling of biological tissues is how to identify the right scale for coarse-graining, or equivalently, the right number of degrees of freedom. For confluent biological tissues, both vertex and Voronoi models, which differ only in their representation of the degrees of freedom, have effectively been used to predict behavior, including fluid-solid transitions and cell tissue compartmentalization, which are important for biological function. However, recent work in 2D has hinted that there may be differences between the two models in systems with heterotypic interfaces between two tissue types, and there is a burgeoning interest in 3D tissue models. Therefore, we compare the geometric structure and dynamic sorting behavior in mixtures of two cell types in both 3D vertex and Voronoi models. We find that while the cell shape indices exhibit similar trends in both models, the registration between cell centers and cell orientation at the boundary are significantly different between the two models. We demonstrate that these macroscopic differences are caused by changes to the cusp-like restoring forces introduced by the different representations of the degrees of freedom at the boundary, and that the Voronoi model is more strongly constrained by forces that are an artifact of the way the degrees of freedom are represented. This suggests that vertex models may be more appropriate for 3D simulations of tissues with heterotypic contacts.

Getting reshaped by the building of your substrate: Extracellular matrix assembly joins the morphogenesis toolkit

In this issue of Developmental Cell, Serna-Morales et al. show evidence that autonomous assembly of the extracellular matrix drives remodeling of the Drosophila embryonic nervous system. This finding is consistent with the notion that self-assembly of the matrix generates stress that can be exploited by morphogenetic processes.

EpCAM cellular functions in adhesion and migration, and potential impact on invasion: A critical review

EpCAM has long been known as a cell surface protein highly expressed in carcinomas. It has since become one of the key cancer biomarkers. Despite its high fame, its actual role in cancer development is still controversial. Beyond a flurry of correlative studies, which point either to a positive or a negative link with tumour progression, there has been surprisingly few studies on the actual cellular mechanisms of EpCAM and on their functional consequences. Clearly, EpCAM plays multiple important roles, in cell proliferation as well as in cell adhesion and migration. The two latter functions, directly relevant for metastasis, are the focus of this review. We attempt here to bring together the available experimental data to build a global coherent view of EpCAM functions. We also include in this overview EpCAM2/Trop2, the close relative of EpCAM. At the core of EpCAM (and EpCAM2/Trop2) function stands the ability to repress contractility of the actomyosin cell cortex. This activity appears to involve direct inhibition by EpCAM of members of the novel PKC family and of a specific downstream PKD-Erk cascade. We will discuss how this activity can result in a variety of adhesive and migratory phenotypes, thus potentially explaining at least part of the apparent inconsistencies between different studies. The picture remains fragmented, and we will highlight some of the conflicting evidence and the many unsolved issues, starting with the controversy around its original description as a cell-cell adhesion molecule.

EpCAM as Modulator of Tissue Plasticity

The Epithelial Cell Adhesion Molecule or EpCAM is a well-known marker highly expressed in carcinomas and showing a strong correlation with poor cancer prognosis. While its name relates to its proposed function as a cell adhesion molecule, EpCAM has been shown to have various signalling functions. In particular, it has been identified as an important positive regulator of cell adhesion and migration, playing an essential role in embryonic morphogenesis as well as intestinal homeostasis. This activity is not due to its putative adhesive function, but rather to its ability to repress myosin contractility by impinging on a PKC signalling cascade. This mechanism confers EpCAM the unique property of favouring tissue plasticity. I review here the currently available data, comment on possible connections with other properties of EpCAM, and discuss the potential significance in the context of cancer invasion.

Sorting at embryonic boundaries requires high heterotypic interfacial tension

The establishment of sharp boundaries is essential for segregation of embryonic tissues during development, but the underlying mechanism of cell sorting has remained unclear. Opposing hypotheses have been proposed, either based on global tissue adhesive or contractile properties or on local signalling through cell contact cues. Here we use ectoderm-mesoderm separation in Xenopus to directly evaluate the role of these various parameters. We find that ephrin-Eph-based repulsion is very effective at inducing and maintaining separation, whereas differences in adhesion or contractility have surprisingly little impact. Computer simulations support and generalise our experimental results, showing that a high heterotypic interfacial tension between tissues is key to their segregation. We propose a unifying model, in which conditions of sorting previously considered as driven by differential adhesion/tension should be viewed as suboptimal cases of heterotypic interfacial tension.The mechanisms that cause different cells to segregate into distinct tissues are unclear. Here the authors show in Xenopus that formation of a boundary between two tissues is driven by local tension along the interface rather than by global differences in adhesion or cortical contractility.

Nuclear β-Catenin under Control

During Wnt stimulation, β-catenin accumulates in the nucleus, where it regulates gene transcription. In this issue of Developmental Cell, Lu et al. (2015) report a mechanism that specifically targets this nuclear pool for degradation, using lysine demethylation as ubiquitination signal.

The cellular basis of tissue separation

The subdivision of the embryo into physically distinct regions is one of the most fundamental processes in development. General hypotheses for tissue separation based on differential adhesion or tension have been proposed in the past, but with little experimental support. During the last decade, the field has experienced a strong revival, largely driven by renewed interest in biophysical modeling of development. Here, I will discuss the various models of boundary formation and summarize recent studies that have shifted our understanding of the process from the simple juxtaposition of global tissue properties to the characterization of local cellular reactions. Current evidence favors a model whereby separation is controlled by cell surface cues, which, upon cell-cell contact, generate acute changes in cytoskeletal and adhesive properties to inhibit cell mixing, and whereby the integration of multiple local cues may dictate both the global morphogenetic properties of a tissue and its separation from adjacent cell populations.

Regulation of the phosphorylation and nuclear import and export of β-catenin by APC and its cancer-related truncated form

We report the first direct analysis of the endogenous β-catenin phosphorylation activity in colon cancer SW480 cells. By comparing parental SW480 cells that harbor a typical truncated adenomatous polyposis coli (APC) form, cells expressing full-length APC and APC-depleted cells, we provide the formal demonstration that APC is necessary for β-catenin phosphorylation, both for priming of the protein at residue serine 45 and for the subsequent phosphorylation of residues 33, 37 and 41. Truncated APC still sustains a surprisingly high phosphorylation activity, which requires the protein to bind to β-catenin through the APC 20-amino-acid (20AA) repeats, thus providing a biochemical explanation for the precise truncations found in cancer cells. We also show that most of the β-catenin phosphorylation activity is associated with a dense insoluble fraction. We finally examine the impact of full-length and truncated APC on β-catenin nuclear transport. We observe that β-catenin is transported much faster than previously thought. Although this fast translocation is largely insensitive to the presence of wild-type or truncated APC, the two forms appear to limit the pool of β-catenin that is available for transport, which could have an impact on β-catenin nuclear activities in normal and cancer cells.

EpCAM controls actomyosin contractility and cell adhesion by direct inhibition of PKC

Epithelial cell adhesion molecule (EpCAM) is a cell-surface protein highly expressed in embryonic tissues and in malignant carcinomas. We report that EpCAM acts as a potent inhibitor of novel protein kinase C (nPKC) in both embryos and cancer cells. We observed dramatic effects of loss of EpCAM on amphibian embryonic tissues, which include sequentially strong overstimulation of PKC activity and of the Erk pathway, leading to exacerbated myosin contractility, loss of cadherin-mediated adhesion, tissue dissociation, and, ultimately, cell death. We show that PKC inhibition is caused by a short segment of the EpCAM cytoplasmic tail. This motif resembles the pseudosubstrate inhibitory domains of PKCs and binds nPKCs with high affinity. A bioinformatics search reveals the existence of similar motifs in other plasma membrane proteins, most of which are cell-cell adhesion molecules. Thus, direct inhibition of PKC by EpCAM represents a general mode of regulation of signal transduction by cell-surface proteins.

A molecular base for cell sorting at embryonic boundaries: contact inhibition of cadherin adhesion by ephrin/ Eph-dependent contractility

The mechanism responsible for subdividing the embryo into individual tissues is a fundamental, yet still poorly understood, question in developmental biology. Various general hypotheses have been proposed, involving differences in cell adhesion, contractility, or contact-mediated repulsion. However, the key parameter in tissue separation, i.e., the regulation of cadherin-based adhesion at the boundary, has not yet been investigated. We show that cadherin clustering is specifically inhibited at the vertebrate notochord-presomitic mesoderm boundary, preventing formation of adhesive bonds between cells of the two different types. This local regulation depends on differentially expressed ephrins and Eph receptors, which increase cell contractility and generate a membrane blebbing-like behavior along the boundary. Inhibiting myosin activity is sufficient to induce cadherin clustering and formation of stable contacts across the boundary, causing notochord and presomitic tissues to fuse. Local inhibition of cadherin adhesion explains how sharp separation can be achieved in response to cell-cell contact signals.

Proteomic analysis of differences in ectoderm and mesoderm membranes by DiGE

Ectoderm and mesoderm can be considered as prototypes for epithelial and mesenchymal cell types. These two embryonic tissues display clear differences in adhesive and motility properties, which are phenomenologically well characterized but remain largely unexplored at the molecular level. Because the key downstream regulations must occur at the plasma membrane and in the underlying actin cortical structures, we have set out to compare the protein content of membrane fractions from Xenopus ectoderm and mesoderm tissues using 2-dimensional difference gel electrophoresis (DiGE). We have thus identified several proteins that are enriched in one or the other tissues, including regulators of the cytoskeleton and of cell signaling. This study represents to our knowledge the first attempt to use proteomics specifically targeted to the membrane-cortex compartment of embryonic tissues. The identified components should help unraveling a variety of tissue-specific functions in the embryo.

Maternal Wnt/β-catenin signaling coactivates transcription through NF-κB binding sites during Xenopus axis formation

Maternal Wnt/β-Catenin signaling establishes a program of dorsal-specific gene expression required for axial patterning in Xenopus. We previously reported that a subset of dorsally expressed genes depends not only on Wnt/β-Catenin stimulation, but also on a MyD88-dependent Toll-like receptor/IL1-receptor (TLR/IL1-R) signaling pathway. Here we show that these two signal transduction cascades converge in the nucleus to coactivate gene transcription in blastulae through a direct interaction between β-Catenin and NF-κB proteins. A transdominant inhibitor of NF-κB, ΔNIκBα, phenocopies loss of MyD88 protein function, implicating Rel/NF-κB proteins as selective activators of dorsal-specific gene expression. Sensitive axis formation assays in the embryo demonstrate that dorsalization by Wnt/β-Catenin requires NF-κB protein activity, and vice versa. Xenopus nodal-related 3 (Xnr3) is one of the genes with dual β-Catenin/NF-κB input, and a proximal NF-κB consensus site contributes to the regional activity of its promoter. We demonstrate in vitro binding of Xenopus β-Catenin to several XRel proteins. This interaction is observed in vivo upon Wnt-stimulation. Finally, we show that a synthetic luciferase reporter gene responds to both endogenous and exogenous β-Catenin levels in an NF-κB motif dependent manner. These results suggest that β-Catenin acts as a transcriptional co-activator of NF-κB-dependent transcription in frog primary embryonic cells.

A method to separate nuclear, cytosolic, and membrane-associated signaling molecules in cultured cells

Direct comparison of protein distribution between the nucleus, the cytoplasm, and the plasma membrane is important for understanding cellular processes and, in particular, signal transduction, where cascades generated at the cell surface regulate functions in other cellular compartments, such as the nucleus. Yet, many commonly used methods fail to effectively separate the plasma membrane and the cytoskeleton from the nucleus, and the cytosol from the nucleosol. This problem has led to confounding results in the study of signaling pathways due to incorrect assignment of cellular localization to signaling molecules and presents challenges in the biochemical study of soluble proteins that shuttle between the cytoplasm and the nucleus. We present a simple method, based on partial membrane permeabilization with detergent followed by density gradient centrifugation, that provides rapid separation of cytosolic, nucleosolic, nuclear insoluble, and membrane components in various mammalian cell lines.

EphrinB/EphB signaling controls embryonic germ layer separation by contact-induced cell detachment

BACKGROUND

The primordial organization of the metazoan body is achieved during gastrulation by the establishment of the germ layers. Adhesion differences between ectoderm, mesoderm, and endoderm cells could in principle be sufficient to maintain germ layer integrity and prevent intermixing. However, in organisms as diverse as fly, fish, or amphibian, the ectoderm-mesoderm boundary not only keeps these germ layers separated, but the ectoderm also serves as substratum for mesoderm migration, and the boundary must be compatible with repeated cell attachment and detachment.

PRINCIPAL FINDINGS

We show that localized detachment resulting from contact-induced signals at the boundary is at the core of ectoderm-mesoderm segregation. Cells alternate between adhesion and detachment, and detachment requires ephrinB/EphB signaling. Multiple ephrinB ligands and EphB receptors are expressed on each side of the boundary, and tissue separation depends on forward signaling across the boundary in both directions, involving partially redundant ligands and receptors and activation of Rac and RhoA.

CONCLUSION

This mechanism differs from a simple differential adhesion process of germ layer formation. Instead, it involves localized responses to signals exchanged at the tissue boundary and an attachment/detachment cycle which allows for cell migration across a cellular substratum.

The tumor-associated EpCAM regulates morphogenetic movements through intracellular signaling

Epithelial cell adhesion molecule (EpCAM) is best known as a tumor-associated protein highly expressed in carcinomas. The function of this cell surface protein during embryonic development and its potential role in cancer are still poorly understood. We identified EpCAM in a gain-of-function screen for inducers of abnormal tissue mixing during gastrulation. Elevated EpCAM levels in either the ectoderm or the mesoderm confer "invasive" properties to cells in both populations. We found that this phenotype represents an "overstimulation" of an essential activity of EpCAM in controlling cell movements during embryonic development. Surprisingly, this property is independent of the putative adhesive function of EpCAM, and rather relies on a novel signaling function that operates through down-regulation of PKC activity. We show that inhibition of novel PKCs accounts entirely for the invasive phenotype induced by abnormally high levels of EpCAM as well as for its normal function in regulating cell rearrangement during early development.

Inhibition of cell adhesion by xARVCF indicates a regulatory function at the plasma membrane

The cytoplasmic tail of cadherins is thought to regulate the strength and dynamics of cell-cell adhesion. Part of its regulatory activity has been attributed to a membrane-proximal region, the juxtamembrane domain (JMD), and its interaction with members of the p120 catenin subfamily. We show that titration of xARVCF, a member of this family, to the plasma membrane disrupts adhesion in the early embryo. Adhesion can be restored by coexpression of constitutively active Rac, suggesting that intracellular signaling is the primary cause in the loss of adhesion phenotype. Our observations suggest that the recruitment of p120 type catenins to the plasma membrane by the cadherin cytoplasmic tail may create protein complexes, which actively modulate the adhesion "status" of embryonic cells.

beta-Catenin controls cell sorting at the notochord-somite boundary independently of cadherin-mediated adhesion

In Xenopus laevis, patterning of the trunk mesoderm into the dorsal notochord and lateral somites depends on differential regulation of Wnt-beta-catenin signaling. To study the cellular requirements for the physical separation of these tissues, we manipulated beta-catenin activity in individual cells that were scattered within the trunk mesoderm. We found that high activity led to efficient cell sorting from the notochord to the somites, whereas reduced activity led to sorting in the opposite direction. Analysis of individual cells overexpressing beta-catenin revealed that these cells were unable to establish stable contacts with notochord cells but could freely cross the boundary to integrate within the somitic tissue. Interference with cadherin-mediated adhesion disrupted tissue architecture, but it did not affect sorting and boundary formation. Based on these results, we propose that the boundary itself is the result of cell-autonomous changes in contact behavior that do not rely on differences in absolute levels of adhesion.

Nucleo-cytoplasmic shuttling of Axin, a negative regulator of the Wnt-beta-catenin Pathway

Axin is a negative regulator of the Wnt pathway essential for down-regulation of beta-catenin. Axin has been considered so far as a cytoplasmic protein. Here we show that, although cytoplasmic at steady state, Axin shuttles in fact in and out of the nucleus; Axin accumulates in the nucleus of cells treated with leptomycin B, a specific inhibitor of the CRM1-mediated nuclear export pathway and is efficiently exported from Xenopus oocyte nuclei in a RanGTP- and CRM1-dependent manner. We have characterized the sequence requirement for export and identified two export domains, which do not contain classical nuclear export consensus sequences, and we show that Axin binds directly to the export factor CRM1 in the presence of RanGTP.

A role for maternal beta-catenin in early mesoderm induction in Xenopus

Mesoderm formation results from an inducing process that requires maternal and zygotic FGF/MAPK and TGFbeta activities, while maternal activation of the Wnt/beta-catenin pathway determines the anterior-dorsal axis. Here, we show a new role of Wnt/beta-catenin signaling in mesoderm induction. We find that maternal beta-catenin signaling is not only active dorsally but also all around the equatorial region, coinciding with the prospective mesoderm. Maternal beta-catenin function is required both for expression of dorsal genes and for activation of MAPK and the mesodermal markers Xbra and eomesodermin. beta-catenin acts in a non- cell-autonomous manner upstream of zygotic FGF and nodal signals. The Wnt/beta-catenin activity in the equatorial region of the early embryo is the first example of a maternally provided mesoderm inducer restricted to the prospective mesoderm.

β-catenin, MAPK and Smad signaling during earlyXenopusdevelopment

Knowledge of when and where signaling pathways are activated is crucial for understanding embryonic development. In this study, we have systematically analyzed and compared the signaling pattern of four major pathways by localization of the activated key components β-catenin (Wnt proteins), MAPK (tyrosine kinase receptors/FGF), Smad1 (BMP proteins) and Smad2 (Nodal/activin/Vg1). We have determined semi-quantitatively the distribution of these components at 18 consecutive stages in Xenopus development, from early blastula to tailbud stages, by immunofluorescence on serial cryosections. The image obtained is that of very dynamic and widespread activities, with very few inactive regions. Signaling fields can vary from large gradients to restricted areas with sharp borders. They do not respect tissue boundaries. This direct visualization of active signaling verifies several predictions inferred from previous functional data. It also reveals unexpected signal patterns, pointing to some poorly understood aspects of early development. In several instances, the patterns strikingly overlap, suggesting extensive interplay between the various pathways. To test this possibility, we have manipulated maternal β-catenin signaling and determined the effect on the other pathways in the blastula embryo. We found that the patterns of P-MAPK, P-Smad1 and P-Smad2 are indeed strongly dependent on β-catenin at this stage. supplementary material: Supplementary Information

CRM1- and Ran-independent nuclear export of beta-catenin

BACKGROUND

Activation of the Wnt pathway induces beta-catenin to localize inside the nucleus, where it interacts with transcription factors such as TCF/LEF-1. Regulation of the pathway occurs through a beta-catenin-degrading complex based on Axin and the tumor suppressor APC. We have previously found that beta-catenin import occurs independently of nuclear import factors but is similar to the import of the transport factors themselves do. APC, which can shuttle in and out of the nucleus, has been proposed to be responsible for reexport of beta-catenin in a CRM1-dependent manner.

RESULTS

We have studied beta-catenin export in vivo and in semipermeabilized cells. beta-catenin contains three export sequences. Export is insensitive to leptomycin B, a specific inhibitor of the CRM1-mediated pathway. It does not require nuclear RanGTP, and it can be reconstituted in the absence of additional soluble factors; this is consistent with nondirectional translocation of beta-catenin. Further observations suggest that beta-catenin subcellular distribution in vivo may depend primarily on retention through interaction with other cellular components. Finally, we show evidence that reexport is required for degradation of nuclear beta-catenin and that nuclei lack Axin, an essential component of the degradation machinery.

CONCLUSIONS

beta-catenin is exported independently of the CRM1 pathway. We propose a model of free, nondirectional nuclear translocation for beta-catenin, its localization being regulated by retention in the nucleus and degradation in the cytoplasm.

Domains of axin and disheveled required for interaction and function in wnt signaling

Disheveled blocks the degradation of beta-catenin in response to Wnt signal by interacting with the scaffolding protein, Axin. To define this interaction in detail we undertook a mutational and binding analysis of the murine Axin and Disheveled proteins. The DIX domain of Axin was found to be important for association with Disheveled and two other regions of Axin (between residues 1-168 and 600-810) were identified that can promote the association of Axin and Disheveled. We found that the DIX domain of Disheveled is critical for association with Axin in vivo and for Disheveled activity. The Disheveled DIX domain controlled the ability of Disheveled to induce the accumulation of cytosolic beta-catenin whereas the PDZ domain was not essential to this function.

Domains of axin involved in protein-protein interactions, Wnt pathway inhibition, and intracellular localization

Axin was identified as a regulator of embryonic axis induction in vertebrates that inhibits the Wnt signal transduction pathway. Epistasis experiments in frog embryos indicated that Axin functioned downstream of glycogen synthase kinase 3beta (GSK3beta) and upstream of beta-catenin, and subsequent studies showed that Axin is part of a complex including these two proteins and adenomatous polyposis coli (APC). Here, we examine the role of different Axin domains in the effects on axis formation and beta-catenin levels. We find that the regulators of G-protein signaling domain (major APC-binding site) and GSK3beta-binding site are required, whereas the COOH-terminal sequences, including a protein phosphatase 2A binding site and the DIX domain, are not essential. Some forms of Axin lacking the beta-catenin binding site can still interact indirectly with beta-catenin and regulate beta-catenin levels and axis formation. Thus in normal embryonic cells, interaction with APC and GSK3beta is critical for the ability of Axin to regulate signaling via beta-catenin. Myc-tagged Axin is localized in a characteristic pattern of intracellular spots as well as at the plasma membrane. NH2-terminal sequences were required for targeting to either of these sites, whereas COOH-terminal sequences increased localization at the spots. Coexpression of hemagglutinin-tagged Dishevelled (Dsh) revealed strong colocalization with Axin, suggesting that Dsh can interact with the Axin/APC/GSK3/beta-catenin complex, and may thus modulate its activity.

Nuclear localization signal-independent and importin/karyopherin-independent nuclear import of beta-catenin

BACKGROUND

Control of the nuclear localization of specific proteins is an important mechanism for regulating many signal transduction pathways. Upon activation of the Wnt signaling pathway, beta-catenin localizes into the nucleus and interacts with TCF/LEF-1 (T-cell factor/lymphocyte enhancer factor-1) transcription factors, triggering activation of downstream genes. The role of regulated nuclear localization in beta-catenin signaling is still unclear. Beta-catenin has no nuclear localization sequence (NLS). Although it has been reported that beta-catenin can piggyback into the nucleus by binding to TCF/LEF-1, there is evidence that its import is independent of TCF/LEF-1 in vivo. Therefore, the mechanism for beta-catenin nuclear localization remains to be established.

RESULTS

We have analyzed beta-catenin nuclear import in an in vitro assay using permeabilized cells. Beta-catenin docks specifically onto the nuclear envelope in the absence of other cytosolic factors. Docking is not inhibited by an NLS peptide and does not require importins/karyopherins, the receptors for classical NLS substrates. Rather, docking is specifically competed by importin-beta/beta-karyopherin, indicating that beta-catenin and importin-beta/beta-karyopherin both interact with common nuclear pore components. Nuclear translocation of beta-catenin is energy dependent and is inhibited by nonhydrolyzable GTP analogs and by a dominant-negative mutant form of the Ran GTPase. Cytosol preparations contain inhibitory activities for beta-catenin import that are distinct from the competition by importin-beta/beta-karyopherin and may be involved in the physiological regulation of the pathway.

CONCLUSIONS

Beta-catenin is imported into the nucleus by binding directly to the nuclear pore machinery, similar to importin-beta/beta-karyopherin or other importin-beta-like import factors, such as transportin. These findings provide an explanation for how beta-catenin localizes to the nucleus without an NLS and independently of its interaction with TCF/LEF-1. This is a new and unusual mechanism for the nuclear import of a signal transduction protein. The lack of beta-catenin import activity in the presence of normal cytosol suggests that its import may be regulated by upstream events in the Wnt signaling pathway.

The mouse Fused locus encodes Axin, an inhibitor of the Wnt signaling pathway that regulates embryonic axis formation

Mutations at the mouse Fused locus have pleiotropic developmental effects, including the formation of axial duplications in homozygous embryos. The product of the Fused locus, Axin, displays similarities to RGS (Regulators of G-Protein Signaling) and Dishevelled proteins. Mutant Fused alleles that cause axial duplications disrupt the major mRNA, suggesting that Axin negatively regulates the response to an axis-inducing signal. Injection of Axin mRNA into Xenopus embryos inhibits dorsal axis formation by interfering with signaling through the Wnt pathway. Furthermore, ventral injection of an Axin mRNA lacking the RGS domain induces an ectopic axis, apparently through a dominant-negative mechanism. Thus, Axin is a novel inhibitor of Wnt signaling and regulates an early step in embryonic axis formation in mammals and amphibians.

Induction of the primary dorsalizing center in Xenopus by the Wnt/GSK/beta-catenin signaling pathway, but not by Vg1, Activin or Noggin

The molecular nature of the primary dorsalizing inducing event in Xenopus is controversial and several secreted factors have been proposed as potential candidates: Wnts, Vg1, Activin and Noggin. Recent studies, however, have provided new insight into the activity of the dorsalizing region, called the Nieuwkoop Center. (1) The activity of this dorsalizing center involves an entire signal transduction pathway that requires maternal beta-catenin (Heasman, J., Crawford, A., Goldstone, K., Garner-Hamrick, P., Gumbiner, B., McCrea, P., Kintner, C., Noro, C. Y. and Wylie, C. (1994) Cell 79, 791–803). (2) A transcription factor with potent dorsalizing activity, Siamois, is expressed within the Nieuwkoop Center (Lemaire, P., Garrett, N. and Gurdon, J. B. (1995) Cell 81, 85–94). We have used these two properties of the Nieuwkoop Center to evaluate the dorsalizing activity of the four secreted factors Wnt8, Vg1, Activin and Noggin. The requirement for beta-catenin was tested by coexpressing a cadherin, which sequesters beta-catenin at the cell membrane and specifically blocks its intracellular signaling activity (Fagotto, F., Funayama, N., Gluck, U. and Gumbiner, B. M. (1996) J. Cell Biol. 132, 1105–1114). Induction of Siamois expression was detected by RT-PCR. Of the four growth factors, only Wnt was sensitive to inhibition of beta-catenin activity and only Wnt could induce Siamois expression. Therefore, Wnt is able to induce a bonafide Nieuwkoop Center, while Vg1, Activin and Noggin probably induce dorsal structures by a different mechanism. To order the steps in the Nieuwkoop Center signaling cascade, we have tested the relationship between beta-catenin and GSK, a serine-threonine kinase that has been implicated in axis formation in a step downstream of Wnt. We found that GSK acts upstream of beta-catenin, similar to the order of these components in the Wingless pathway in Drosophila. We have also examined the relationship between the Wnt/beta-catenin pathway and Siamois. We show that beta-catenin induces expression of Siamois and that the free signaling pool of beta-catenin is required for normal expression of endogenous Siamois. We conclude that the sequence of steps in the signaling pathway is Wnt-->GSK-->beta-catenin-->Siamois.

Binding to cadherins antagonizes the signaling activity of beta-catenin during axis formation in Xenopus

beta-Catenin, a cytoplasmic protein known for its association with cadherin cell adhesion molecules, is also part of a signaling cascade involved in embryonic patterning processes such as the determination of the dorsoventral axis in Xenopus and determination of segment polarity in Drosophila. Previous studies suggest that increased cytoplasmic levels of beta-catenin correlate with signaling, raising questions about the need for in- teraction with cadherins in this process. We have tested the role of the beta-catenin-cadherin interaction in axis formation. Using beta-catenin deletion mutants, we demonstrate that significant binding to cadherins can be eliminated without affecting the signaling activity. Also, depletion of the soluble, cytosolic pool of beta-catenin by binding to overexpressed C-cadherin completely inhibited beta-catenin-inducing activity. We conclude that binding to cadherins is not required for beta-catenin signaling, and therefore the signaling function of beta-catenin is independent of its role in cell adhesion. Moreover, because beta-catenin signaling is antagonized by binding to cadherins, we suggest that cadherins can act as regulators of the intracellular beta-catenin signaling pathway.

Embryonic axis induction by the armadillo repeat domain of beta-catenin: evidence for intracellular signaling

beta-catenin was identified as a cytoplasmic cadherin-associated protein required for cadherin adhesive function (Nagafuchi, A., and M. Takeichi. 1989. Cell Regul. 1:37-44; Ozawa, M., H. Baribault, and R. Kemler. 1989. EMBO [Eur. Mol. Biol. Organ.] J. 8:1711-1717). Subsequently, it was found to be the vertebrate homologue of the Drosophila segment polarity gene product Armadillo (McCrea, P. D., C. W. Turck, and B. Gumbiner. 1991. Science [Wash. DC]. 254:1359-1361; Peifer, M., and E. Wieschaus. 1990. Cell. 63:1167-1178). Also, antibody perturbation experiments implicated beta-catenin in axial patterning of the early Xenopus embryo (McCrea, P. D., W. M. Brieher, and B. M. Gumbiner. 1993. J. Cell Biol. 123:477-484). Here we report that overexpression of beta-catenin in the ventral side of the early Xenopus embryo, by injection of synthetic beta-catenin mRNA, induces the formation of a complete secondary body axis. Furthermore, an analysis of beta-catenin deletion constructs demonstrates that the internal armadillo repeat region is both necessary and sufficient to induce axis duplication. This region interacts with C-cadherin and with the APC tumor suppressor protein, but not with alpha-catenin, that requires the amino-terminal region of beta-catenin to bind to the complex. Since alpha-catenin is required for cadherin-mediated adhesion, the armadillo repeat region alone probably cannot promote cell adhesion, making it unlikely that beta-catenin induces axis duplication by increasing cell adhesion. We propose, rather, that beta-catenin acts in this circumstance as an intracellular signaling molecule. Subcellular fractionation demonstrated that all of the beta-catenin constructs that contain the armadillo repeat domain were present in both the soluble cytosolic and the membrane fraction. Immunofluorescence staining confirmed the plasma membrane and cytoplasmic localization of the constructs containing the armadillo repeat region, but revealed that they also accumulate in the nucleus, especially the construct containing only the armadillo repeat domain. These findings and the beta-catenin protein interaction data offer several intriguing possibilities for the site of action or the protein targets of beta-catenin signaling activity.

Changes in yolk platelet pH during Xenopus laevis development correlate with yolk utilization. A quantitative confocal microscopy study

The variations of the pH in Xenopus yolk platelets have been estimated by fluorescence confocal microscopy and computer image processing. For pH measurements in vitellogenic oocytes, the pH-sensitive fluorescent dye, DM-NERF, was coupled to vitellogenin, and the DM-NERF-vitellogenin was taken up by oocytes via receptor-mediated endocytosis. Dual emission ratio measurements of internalized DM-NERF-vitellogenin indicated that the mature yolk platelets are mildly acidic (pH 5.6). Their precursors, the primordial yolk platelets, have a similar pH. This pH is probably sufficiently low for the partial cleavage of vitellogenin to yolk proteins, but not for yolk degradation. The yolk platelet pH at various developmental stages was estimated by measuring the accumulation of Acridine Orange, both in isolated yolk platelets and in disaggregated embryonic cells. During oogenesis, the yolk platelets accumulated a constant amount of Acridine Orange, corresponding to a pH of around 5.7. During embryogenesis, however, yolk platelets became progressively much more acidic (pH < 5). Acidification correlated with yolk degradation in the various tissues examined, and yolk utilization was blocked when acidification was inhibited with bafilomycin, an inhibitor of vacuolar H+-ATPase. Bafilomycin also inhibited differentiation of cells isolated from stage 13–15 embryos. These data show that the yolk platelet pH is developmentally regulated and is involved in triggering yolk degradation. Also, yolk acidification and degradation appeared to be associated with cell differentiation and with the formation of the endosomal/lysosomal compartment, typical of adult cells, but absent in early embryos.

Beta-catenin localization during Xenopus embryogenesis: accumulation at tissue and somite boundaries

beta-catenin is a cytoplasmic protein associated with cadherin adhesion molecules and has been implicated in axis formation in Xenopus (McCrea, P. D., Brieher, W. M. and Gumbiner, B. M. (1993) J. Cell Biol. 127, 477–484). We have studied its distribution in Xenopus embryos by immunofluorescence on frozen sections. Consistent with its function in cell-cell adhesion, beta-catenin is present in every cell. However, high levels are expressed in certain regions and different tissues of the embryo. No simple correlation appears to exist between the levels of beta-catenin with the expected strength of adhesion. High levels of beta-catenin were found in regions undergoing active morphogenetic movements, such as the marginal zone of blastulae and gastrulae. This suggests that high expression of beta-catenin could be involved in dynamic adhesion events. Surprisingly, beta-catenin also accumulates on plasma membranes that probably do not establish direct or strong contacts with other cells. In particular, high amounts of beta-catenin are found transiently at boundaries between tissue anlagen and at the intersomitic boundaries. This unexpected pattern of beta-catenin expression raises the possibility that this molecule participates in developmental processes, perhaps independently of its classical role in cell-cell adhesion.

Yolk platelets in Xenopus oocytes maintain an acidic internal pH which may be essential for sodium accumulation

Yolk platelets constitute an embryonic endocytic compartment that stores maternally synthesized nutrients. The pH of Xenopus yolk platelets, measured by photometry on whole oocytes which had endocytosed FITC-vitellogenin, was found to be acidic (around pH 5.6). Experiments on digitonin-permeabilized oocytes showed that acidification was due to the activity of an NEM- and bafilomycin A1-sensitive vacuolar proton-ATPase. Proton pumping required chloride, but was not influenced by potassium or sodium. Passive proton leakage was slow, probably due to the buffer capacity of the yolk, and was dependent on the presence of cytoplasmic monovalent cations. In particular, sodium could drive proton efflux through an amiloride-sensitive Na+/H+ exchanger. 8-Bromo-cyclic-AMP was found to increase acidification, suggesting that pH can be regulated by intracellular second messengers. The moderately acidic pH does not promote degradation of the yolk platelets, which in oocytes are stable for weeks, but it is likely to be required to maintain the integrity of these organelles. Furthermore, the pH gradient created by the proton pump, when coupled with the Na+/H+ exchanger, is probably responsible for the accumulation and storage of sodium into the yolk platelets during oogenesis.