The roles of maternal alpha-catenin and plakoglobin in the early Xenopus embryo

Proteomic Analysis of Domestic Cat Blastocysts and Their Secretome Produced in an In Vitro Culture System without the Presence of the Zona Pellucida

Domestic cat blastocysts cultured without the zona pellucida exhibit reduced implantation capacity. However, the protein expression profile has not been evaluated in these embryos. The objective of this study was to evaluate the protein expression profile of domestic cat blastocysts cultured without the zona pellucida. Two experimental groups were generated: (1) domestic cat embryos generated by IVF and cultured in vitro (zona intact, (ZI)) and (2) domestic cat embryos cultured in vitro without the zona pellucida (zona-free (ZF group)). The cleavage, morula, and blastocyst rates were estimated at days 2, 5 and 7, respectively. Day 7 blastocysts and their culture media were subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS). The UniProt Felis catus database was used to identify the standard proteome. No significant differences were found in the cleavage, morula, or blastocyst rates between the ZI and ZF groups (p > 0.05). Proteomic analysis revealed 22 upregulated and 20 downregulated proteins in the ZF blastocysts. Furthermore, 14 proteins involved in embryo development and implantation were present exclusively in the culture medium of the ZI blastocysts. In conclusion, embryo culture without the zona pellucida did not affect in vitro development, but altered the protein expression profile and release of domestic cat blastocysts.

The α-Catenin mechanosensing M region is required for cell adhesion during tissue morphogenesis

α-Catenin couples the cadherin-catenin complex to the actin cytoskeleton. The mechanosensitive α-Catenin M region undergoes conformational changes upon application of force to recruit interaction partners. Here, we took advantage of the tension landscape in the Drosophila embryo to define three different states of α-Catenin mechanosensing in support of cell adhesion. Low-, medium-, and high-tension contacts showed a corresponding recruitment of Vinculin and Ajuba, which was dependent on the α-Catenin M region. In contrast, the Afadin homolog Canoe acts in parallel to α-Catenin at bicellular low- and medium-tension junctions but requires an interaction with α-Catenin for its tension-sensitive enrichment at high-tension tricellular junctions. Individual M region domains make complex contributions to cell adhesion through their impact on interaction partner recruitment, and redundancies with the function of Canoe. Our data argue that α-Catenin and its interaction partners are part of a cooperative and partially redundant mechanoresponsive network that supports AJs remodeling during morphogenesis.

Desmoplakin is required for epidermal integrity and morphogenesis in the Xenopus laevis embryo

Desmoplakin (Dsp) is a unique and critical desmosomal protein, that is integral to epidermal development. However, it is unclear whether this protein is required specifically for epidermal morphogenesis. Using morpholinos or Crispr/Cas9 mutagenesis we decreased the function of Dsp in frog embryos to better understand its role during epidermal development. Dsp morphant and mutant embryos had developmental defects such as epidermal fragility that mimicked what has been reported in mammals. Most importantly, we also uncovered a novel function for Dsp in the morphogenesis of the epidermis in X. laevis. In particular, Dsp is required during the process of radial intercalation where basally located cells move into the outer epidermal layer. Once inserted these newly intercalated cells expand their apical surface and then they differentiate into specific epidermal cell types. Decreased levels of Dsp resulted in the failure of the radially intercalating cells to expand their apical surface, thereby reducing the number of differentiated multiciliated and secretory cells. Such defects correlate with changes in E-cadherin levels and actin and microtubule localization which could explain the defects in apical expansion. A mutated form of Dsp that maintains cell-cell adhesion but eliminates the connections to the cytoskeleton results in the same epidermal morphogenesis defect. These results suggest a specific role for Dsp in the apical expansion of cells during radial intercalation. We have developed a novel system, in the frog, to demonstrate for the first time that desmosomes not only protect against mechanical stress but are also critical for epidermal morphogenesis.

zGrad is a nanobody-based degron system that inactivates proteins in zebrafish

The analysis of protein function is essential to modern biology. While protein function has mostly been studied through gene or RNA interference, more recent approaches to degrade proteins directly have been developed. Here, we adapted the anti-GFP nanobody-based system deGradFP from flies to zebrafish. We named this system zGrad and show that zGrad efficiently degrades transmembrane, cytosolic and nuclear GFP-tagged proteins in zebrafish in an inducible and reversible manner. Using tissue-specific and inducible promoters in combination with functional GFP-fusion proteins, we demonstrate that zGrad can inactivate transmembrane and cytosolic proteins globally, locally and temporally with different consequences. Global protein depletion results in phenotypes similar to loss of gene activity, while local and temporal protein inactivation yields more restricted and novel phenotypes. Thus, zGrad is a versatile tool to study the spatial and temporal requirement of proteins in zebrafish.

Force-dependent allostery of the α-catenin actin-binding domain controls adherens junction dynamics and functions

α-catenin is a key mechanosensor that forms force-dependent interactions with F-actin, thereby coupling the cadherin-catenin complex to the actin cytoskeleton at adherens junctions (AJs). However, the molecular mechanisms by which α-catenin engages F-actin under tension remained elusive. Here we show that the α1-helix of the α-catenin actin-binding domain (αcat-ABD) is a mechanosensing motif that regulates tension-dependent F-actin binding and bundling. αcat-ABD containing an α1-helix-unfolding mutation (H1) shows enhanced binding to F-actin in vitro. Although full-length α-catenin-H1 can generate epithelial monolayers that resist mechanical disruption, it fails to support normal AJ regulation in vivo. Structural and simulation analyses suggest that α1-helix allosterically controls the actin-binding residue V796 dynamics. Crystal structures of αcat-ABD-H1 homodimer suggest that α-catenin can facilitate actin bundling while it remains bound to E-cadherin. We propose that force-dependent allosteric regulation of αcat-ABD promotes dynamic interactions with F-actin involved in actin bundling, cadherin clustering, and AJ remodeling during tissue morphogenesis.

Patterns in space: coordinating adhesion and actomyosin contractility at E-cadherin junctions

Cadherin adhesion receptors are fundamental determinants of tissue organization in health and disease. Increasingly, we have come to appreciate that classical cadherins exert their biological actions through active cooperation with the contractile actin cytoskeleton. Rather than being passive resistors of detachment forces, cadherins can regulate the assembly and mechanics of the contractile apparatus itself. Moreover, coordinate spatial patterning of adhesion and contractility is emerging as a determinant of morphogenesis. Here we review recent developments in cadherins and actin cytoskeleton cooperativity, by focusing on E-cadherin adhesive patterning in the epithelia. Next, we discuss the underlying principles of cellular rearrangement during Drosophila germband extension and epithelial cell extrusion, as models of how planar and apical-lateral patterns of contractility organize tissue architecture.

The maternal-to-zygotic transition targets actin to promote robustness during morphogenesis

Robustness is a property built into biological systems to ensure stereotypical outcomes despite fluctuating inputs from gene dosage, biochemical noise, and the environment. During development, robustness safeguards embryos against structural and functional defects. Yet, our understanding of how robustness is achieved in embryos is limited. While much attention has been paid to the role of gene and signaling networks in promoting robust cell fate determination, little has been done to rigorously assay how mechanical processes like morphogenesis are designed to buffer against variable conditions. Here we show that the cell shape changes that drive morphogenesis can be made robust by mechanisms targeting the actin cytoskeleton. We identified two novel members of the Vinculin/α-Catenin Superfamily that work together to promote robustness during Drosophila cellularization, the dramatic tissue-building event that generates the primary epithelium of the embryo. We find that zygotically-expressed Serendipity-α (Sry-α) and maternally-loaded Spitting Image (Spt) share a redundant, actin-regulating activity during cellularization. Spt alone is sufficient for cellularization at an optimal temperature, but both Spt plus Sry-α are required at high temperature and when actin assembly is compromised by genetic perturbation. Our results offer a clear example of how the maternal and zygotic genomes interact to promote the robustness of early developmental events. Specifically, the Spt and Sry-α collaboration is informative when it comes to genes that show both a maternal and zygotic requirement during a given morphogenetic process. For the cellularization of Drosophilids, Sry-α and its expression profile may represent a genetic adaptive trait with the sole purpose of making this extreme event more reliable. Since all morphogenesis depends on cytoskeletal remodeling, both in embryos and adults, we suggest that robustness-promoting mechanisms aimed at actin could be effective at all life stages.

Every embryo develops under its own unique set of circumstances, with variable inputs coming from mother, father, and the environment. To then ensure a reliable outcome, mechanisms are built into development to buffer against challenges like genetic deficiency, maternal fever, alcohol exposure, etc. This buffering, called “robustness”, can be overwhelmed, ending in miscarriage, pre-mature birth, and structural and functional birth defects. Thus, we need to understand how robustness arises in order to define an embryo's susceptibilities to genetic background and environment; and to ultimately promote healthy reproduction. In this work we provide new insight into how morphogenesis, the process of tissue building in embryos, is made more robust. First, we show that early gene expression by the embryo, or zygote, supplements the stockpile of proteins already supplied by the mother to ensure the robustness of early morphogenesis. Specifically, our data suggests that a specific gene, sry-α, and its expression by the embryo at the maternal-to-zygotic transition, is a genetic adaptation with the sole function of making the first tissue building event in the fruit fly more robust. In addition, we show that the robustness of this morphogenetic event is promoted by mechanisms regulating the actin cytoskeleton.

Principles of E-cadherin supramolecular organization in vivo

BACKGROUND

E-cadherin plays a pivotal role in tissue morphogenesis by forming clusters that support intercellular adhesion and transmit tension. What controls E-cadherin mesoscopic organization in clusters is unclear.

RESULTS

We use 3D superresolution quantitative microscopy in Drosophila embryos to characterize the size distribution of E-cadherin nanometric clusters. The cluster size follows power-law distributions over three orders of magnitude with exponential decay at large cluster sizes. By exploring the predictions of a general theoretical framework including cluster fusion and fission events and recycling of E-cadherin, we identify two distinct active mechanisms setting the cluster-size distribution. Dynamin-dependent endocytosis targets large clusters only, thereby imposing a cutoff size. Moreover, interactions between E-cadherin clusters and actin filaments control the fission in a size-dependent manner.

CONCLUSIONS

E-cadherin clustering depends on key cortical regulators, which provide tunable and local control over E-cadherin organization. Our data provide the foundation for a quantitative understanding of how E-cadherin distribution affects adhesion and might regulate force transmission in vivo.

Stability and dynamics of cell-cell junctions

Adherens junctions display dual properties of robustness and plasticity. In multicellular organisms, they support both strong cell-cell adhesion and rapid cell-cell contact remodeling during development and wound healing. The core components of adherens junctions are clusters of cadherin molecules, which mediate cell-cell adhesion through homophilic interactions in trans. Interactions of cadherins with the actin cytoskeleton are essential for providing both stability and plasticity to adherens junctions. Cadherins regulate the turnover of actin by regulating its polymerization and anchor tensile actomyosin networks at the cell cortex. In turn, actin regulates cadherin turnover by regulating its endocytosis and actomyosin networks exert forces driving remodeling of cell-cell contacts. The interplay between adherens junctions and contractile actomyosin networks has striking outcomes during epithelial morphogenesis. Their integrated dynamics result in different morphogenetic patterns shaping tissues and organs.

The evolutionary origin of epithelial cell-cell adhesion mechanisms

A simple epithelium forms a barrier between the outside and the inside of an organism, and is the first organized multicellular tissue found in evolution. We examine the relationship between the evolution of epithelia and specialized cell-cell adhesion proteins comprising the classical cadherin/β-catenin/α-catenin complex (CCC). A review of the divergent functional properties of the CCC in metazoans and non-metazoans, and an updated phylogenetic coverage of the CCC using recent genomic data reveal: (1) The core CCC likely originated before the last common ancestor of unikonts and their closest bikont sister taxa. (2) Formation of the CCC may have constrained sequence evolution of the classical cadherin cytoplasmic domain and β-catenin in metazoa. (3) The α-catenin-binding domain in β-catenin appears to be the favored mutation site for disrupting β-catenin function in the CCC. (4) The ancestral function of the α/β-catenin heterodimer appears to be an actin-binding module. In some metazoan groups, more complex functions of α-catenin were gained by sequence divergence in the non-actin-binding (N-, M-) domains. (5) Allosteric regulation of α-catenin may have evolved for more complex regulation of the actin cytoskeleton.

What my mother told me: Examining the roles of maternal gene products in a vertebrate

In Xenopus, mRNAs synthesized during oocyte differentiation are inherited by the egg and direct all protein synthesis until the late-blastula stage. This provides an opportunity to study the roles of maternally expressed genes in embryonic development of a vertebrate. Oocytes can be depleted of specific mRNAs by the injection of antisense deoxyoligonucleotides and then fertilized to assay for developmental abnormalities. The ease of experimental manipulation of early Xenopus embryos in culture gives considerable opportunity for the analysis of the abnormalities seen.

Regulation of classical cadherin membrane expression and F-actin assembly by alpha-catenins, during Xenopus embryogenesis

Alpha (α)-E-catenin is a component of the cadherin complex, and has long been thought to provide a link between cell surface cadherins and the actin skeleton. More recently, it has also been implicated in mechano-sensing, and in the control of tissue size. Here we use the early Xenopus embryos to explore functional differences between two α-catenin family members, α-E- and α-N-catenin, and their interactions with the different classical cadherins that appear as tissues of the embryo become segregated from each other. We show that they play both cadherin-specific and context-specific roles in the emerging tissues of the embryo. α-E-catenin interacts with both C- and E-cadherin. It is specifically required for junctional localization of C-cadherin, but not of E-cadherin or N-cadherin at the neurula stage. α-N-cadherin interacts only with, and is specifically required for junctional localization of, N-cadherin. In addition, α -E-catenin is essential for normal tissue size control in the non-neural ectoderm, but not in the neural ectoderm or the blastula. We also show context specificity in cadherin/ α-catenin interactions. E-cadherin requires α-E-catenin for junctional localization in some tissues, but not in others, during early development. These specific functional cadherin/alpha-catenin interactions may explain the basis of cadherin specificity of actin assembly and morphogenetic movements seen previously in the neural and non-neural ectoderm.

CRIM1 complexes with ß-catenin and cadherins, stabilizes cell-cell junctions and is critical for neural morphogenesis

In multicellular organisms, morphogenesis is a highly coordinated process that requires dynamically regulated adhesion between cells. An excellent example of cellular morphogenesis is the formation of the neural tube from the flattened epithelium of the neural plate. Cysteine-rich motor neuron protein 1 (CRIM1) is a single-pass (type 1) transmembrane protein that is expressed in neural structures beginning at the neural plate stage. In the frog Xenopus laevis, loss of function studies using CRIM1 antisense morpholino oligonucleotides resulted in a failure of neural development. The CRIM1 knockdown phenotype was, in some cases, mild and resulted in perturbed neural fold morphogenesis. In severely affected embryos there was a dramatic failure of cell adhesion in the neural plate and complete absence of neural structures subsequently. Investigation of the mechanism of CRIM1 function revealed that it can form complexes with ß-catenin and cadherins, albeit indirectly, via the cytosolic domain. Consistent with this, CRIM1 knockdown resulted in diminished levels of cadherins and ß-catenin in junctional complexes in the neural plate. We conclude that CRIM1 is critical for cell-cell adhesion during neural development because it is required for the function of cadherin-dependent junctions.

Mutational analysis supports a core role for Drosophila α-catenin in adherens junction function

α-catenin associates the cadherin-catenin complex with the actin cytoskeleton. α-catenin binds to β-catenin, which links it to the cadherin cytoplasmic tail, and F-actin, but also to a multitude of actin-associated proteins. These interactions suggest a highly complex cadherin-actin interface. Moreover, mammalian αE-catenin has been implicated in a cadherin-independent cytoplasmic function in Arp2/3-dependent actin regulation, and in cell signaling. The function and regulation of individual molecular interactions of α-catenin, in particular during development, are not well understood. We have generated mutations in Drosophila α-Catenin (α-Cat) to investigate α-Catenin function in this model, and to establish a setup for testing α-Catenin-related constructs in α-Cat-null mutant cells in vivo. Our analysis of α-Cat mutants in embryogenesis, imaginal discs and oogenesis reveals defects consistent with a loss of cadherin function. Compromising components of the Arp2/3 complex or its regulator SCAR ameliorate the α-Cat loss-of-function phenotype in embryos but not in ovaries, suggesting negative regulatory interactions between α-Catenin and the Arp2/3 complex in some tissues. We also show that the α-Cat mutant phenotype can be rescued by the expression of a DE-cadherin::α-Catenin fusion protein, which argues against an essential cytosolic, cadherin-independent role of Drosophila α-Catenin.

Cadherin function during Xenopus gastrulation

Xenopus gastrulation consists of the orderly deformation of a single, multilayered cell sheet that resembles a multilayered epithelium, and flexible cell-cell adhesion has to provide tissue cohesion while allowing for cell rearrangements that drive gastrulation. A few classic cadherins are expressed in the Xenopus early embryo. The prominent C-cadherin is essential for the cohesion of the animal part of the gastrula including ectoderm and chordamesoderm, and it contributes to the adhesion of endoderm and anterior mesoderm in the vegetal moiety. The cadherin/catenin complex is expressed in a graded pattern which is stable during early development. Regional differences in cell adhesion conform to the graded cadherin/catenin expression pattern. However, although the cadherin/catenin pattern seems to be actively maintained, and cadherin function is modulated to reinforce differential adhesiveness, it is not clear how regional differences in tissue cohesion affect gastrulation. Manipulating cadherin expression or function does not induce cell sorting or boundary formation in the embryo. Moreover, known boundary formation mechanisms in the gastrula are based on active cell repulsion. Cell rearrangement is also compatible with variable tissue cohesion. Thus, identifying roles for differential adhesion in the Xenopus gastrula remains a challenge.

αE-catenin regulates cell-cell adhesion and membrane blebbing during zebrafish epiboly

αE-catenin is an actin-binding protein associated with the E-cadherin-based adherens junction that regulates cell-cell adhesion. Recent studies identified additional E-cadherin-independent roles of αE-catenin in regulating plasma membrane dynamics and cell migration. However, little is known about the roles of αE-catenin in these different cellular processes in vivo during early vertebrate development. Here, we examined the functions of αE-catenin in cell-cell adhesion, cell migration and plasma membrane dynamics during morphogenetic processes that drive epiboly in early Danio rerio (zebrafish) development. We show that depletion of αE-catenin caused a defect in radial intercalation that was associated with decreased cell-cell adhesion, in a similar manner to E-cadherin depletion. Depletion of αE-catenin also caused deep cells to have protracted plasma membrane blebbing, and a defect in plasma membrane recruitment of ERM proteins that are involved in controlling membrane-to-cortex attachment and membrane blebbing. Significantly, depletion of both E-cadherin and αE-catenin suppressed plasma membrane blebbing. We suggest that during radial intercalation the activities of E-cadherin and αE-catenin in the maintenance of membrane-to-cortex attachment are balanced, resulting in stabilization of cell-cell adhesion and suppression of membrane blebbing, thereby enabling proper radial intercalation.

Force generation, transmission, and integration during cell and tissue morphogenesis

Cell shape changes underlie a large set of biological processes ranging from cell division to cell motility. Stereotyped patterns of cell shape changes also determine tissue remodeling events such as extension or invagination. In vitro and cell culture systems have been essential to understanding the fundamental physical principles of subcellular mechanics. These are now complemented by studies in developing organisms that emphasize how cell and tissue morphogenesis emerge from the interplay between force-generating machines, such as actomyosin networks, and adhesive clusters that transmit tensile forces at the cell cortex and stabilize cell-cell and cell-substrate interfaces. Both force production and transmission are self-organizing phenomena whose adaptive features are essential during tissue morphogenesis. A new era is opening that emphasizes the similarities of and allows comparisons between distant dynamic biological phenomena because they rely on core machineries that control universal features of cytomechanics.

Differential requirement for the dual functions of β-catenin in embryonic stem cell self-renewal and germ layer formation

Canonical Wnt-signalling has been implicated in mouse and human embryonic stem cell (ESC) maintenance, however its requirement is controversial. β-catenin is the key component in this highly conserved Wnt pathway, acting as a transcriptional transactivator. Yet, β-catenin has additional roles at the plasma membrane regulating cell-cell adhesion, complicating the analyses of cells/tissues lacking β-catenin. We report here the generation of a β-catenin deficient mouse ESC (mESC) line and show that self-renewal is maintained in absence of β-catenin. Cell-adhesion is partially rescued by plakoglobin up-regulation, but fails to be maintained during differentiation. When differentiated as aggregates, wild-type mESCs form descendents of all three germ layers, while mesendodermal germ layer formation and neuronal differentiation are defective in β-catenin deficient mESCs. A Tcf/Lef-signalling defective β-catenin variant, which re-establishes cadherin-mediated cell-adhesion, rescues definitive endoderm and neuroepithelial formation, suggesting that β-catenin cell-adhesion function is more important than its signalling function for these processes.

Novel insight into the function and regulation of alphaN-catenin by Snail2 during chick neural crest cell migration

The neural crest is a transient population of migratory cells that differentiates to form a variety of cell types in the vertebrate embryo, including melanocytes, the craniofacial skeleton, and portions of the peripheral nervous system. These cells initially exist as adherent epithelial cells in the dorsal aspect of the neural tube and only later become migratory after an epithelial-to-mesenchymal transition (EMT). Snail2 plays a critical role in mediating chick neural crest cell EMT and migration due to its expression by both premigratory and migratory cranial neural crest cells and its ability to down-regulate intercellular junctions components. In an attempt to delineate the role of cellular junction components in the neural crest, we have identified the adherens junction molecule neural alpha-catenin (alphaN-catenin) as a Snail2 target gene whose repression is critical for chick neural crest cell migration. Knock-down and overexpression of alphaN-catenin enhances and inhibits neural crest cell migration, respectively. Furthermore, our results reveal that alphaN-catenin regulates the appropriate movement of neural crest cells away from the neural tube into the embryo. Collectively, our data point to a novel function of an adherens junction protein in facilitating the proper migration of neural crest cells during the development of the vertebrate embryo.

The Drosophila afadin homologue Canoe regulates linkage of the actin cytoskeleton to adherens junctions during apical constriction

Cadherin-based adherens junctions (AJs) mediate cell adhesion and regulate cell shape change. The nectin–afadin complex also localizes to AJs and links to the cytoskeleton. Mammalian afadin has been suggested to be essential for adhesion and polarity establishment, but its mechanism of action is unclear. In contrast, Drosophila melanogaster’s afadin homologue Canoe (Cno) has suggested roles in signal transduction during morphogenesis. We completely removed Cno from embryos, testing these hypotheses. Surprisingly, Cno is not essential for AJ assembly or for AJ maintenance in many tissues. However, morphogenesis is impaired from the start. Apical constriction of mesodermal cells initiates but is not completed. The actomyosin cytoskeleton disconnects from AJs, uncoupling actomyosin constriction and cell shape change. Cno has multiple direct interactions with AJ proteins, but is not a core part of the cadherin–catenin complex. Instead, Cno localizes to AJs by a Rap1- and actin-dependent mechanism. These data suggest that Cno regulates linkage between AJs and the actin cytoskeleton during morphogenesis.

Cell adhesion in amphibian gastrulation

The amphibian gastrula can be regarded as a single coherent tissue which folds and distorts itself in a reproducible pattern to establish the embryonic germ layers. It is held together by cadherins which provide the flexible adhesion required for the massive cell rearrangements that accompany gastrulation. Cadherin expression and adhesiveness increase as one goes from the vegetal cell mass through the anterior mesendoderm to the chordamesoderm, and then decrease again slightly in the ectoderm. Together with a basic random component of cell motility, this flexible, differentially expressed adhesiveness generates surface and interfacial tension effects which, in principle, can exert strong forces. However, conclusive evidence for an in vivo role of differential adhesion-related effects in gastrula morphogenesis is still lacking. The most important morphogenetic process in the amphibian gastrula seems to be intercellular migration, where cells crawl actively across each other's surface. The crucial aspect of this process is that cell motility is globally oriented, leading for example to mediolateral intercalation of bipolar cells during convergent extension of the chordamesoderm or to the directional migration of unipolar cells during translocation of the anterior mesendoderm on the ectodermal blastocoel roof. During these movements, the boundary between ectoderm and mesoderm is maintained by a tissue separation process.

Investigating the correspondence between transcriptomic and proteomic expression profiles using coupled cluster models

MOTIVATION

Modern transcriptomics and proteomics enable us to survey the expression of RNAs and proteins at large scales. While these data are usually generated and analyzed separately, there is an increasing interest in comparing and co-analyzing transcriptome and proteome expression data. A major open question is whether transcriptome and proteome expression is linked and how it is coordinated.

RESULTS

Here we have developed a probabilistic clustering model that permits analysis of the links between transcriptomic and proteomic profiles in a sensible and flexible manner. Our coupled mixture model defines a prior probability distribution over the component to which a protein profile should be assigned conditioned on which component the associated mRNA profile belongs to. We apply this approach to a large dataset of quantitative transcriptomic and proteomic expression data obtained from a human breast epithelial cell line (HMEC). The results reveal a complex relationship between transcriptome and proteome with most mRNA clusters linked to at least two protein clusters, and vice versa. A more detailed analysis incorporating information on gene function from the Gene Ontology database shows that a high correlation of mRNA and protein expression is limited to the components of some molecular machines, such as the ribosome, cell adhesion complexes and the TCP-1 chaperonin involved in protein folding.

AVAILABILITY

Matlab code is available from the authors on request.

A two-tiered mechanism for stabilization and immobilization of E-cadherin

Epithelial tissues maintain a robust architecture which is important for their barrier function, but they are also remodelled through the reorganization of cell-cell contacts. Tissue stability requires intercellular adhesion mediated by E-cadherin, in particular its trans-association in homophilic complexes supported by actin filaments through beta- and alpha-catenin. How alpha-catenin dynamic interactions between E-cadherin/beta-catenin and cortical actin control both stability and remodelling of adhesion is unclear. Here we focus on Drosophila homophilic E-cadherin complexes rather than total E-cadherin, including diffusing 'free' E-cadherin, because these complexes are a better proxy for adhesion. We find that E-cadherin complexes partition in very stable microdomains (that is, bona fide adhesive foci which are more stable than remodelling contacts). Furthermore, we find that stability and mobility of these microdomains depend on two actin populations: small, stable actin patches concentrate at homophilic E-cadherin clusters, whereas a rapidly turning over, contractile network constrains their lateral movement by a tethering mechanism. alpha-Catenin controls epithelial architecture mainly through regulation of the mobility of homophilic clusters and it is largely dispensable for their stability. Uncoupling stability and mobility of E-cadherin complexes suggests that stable epithelia may remodel through the regulated mobility of very stable adhesive foci.

Embryonic cells depleted of beta-catenin remain competent to differentiate into dorsal mesodermal derivatives

Disruption of axis specification leads to defects in dorsal tissue patterning and cell movements. Here, we examine how beta-catenin coordinately affects gastrulation movements and dorsal mesoderm differentiation. The reduction of beta-catenin protein levels by morpholino oligonucleotides complementary to beta-catenin mRNA causes a disruption in gastrulation movements. Time-lapse imaging of beta-catenin morphants during gastrulation reveals that involution occurs simultaneously around the blastopore in the absence of convergent extension cell movements. Transplantation experiments show that morphant cells grafted from the marginal zone into wild-type hosts differentiate into notochord and muscle. However, wild-type mesoderm cells grafted to the marginal zone of beta-catenin morphants do not form dorsal tissues. These data argue that beta-catenin is not required for the initial establishment of dorsal mesoderm cell competency, but it is required for the maintenance of that competency. We propose that tissue interactions that occur during convergent extension movements are necessary for maintaining dorsal tissue competency.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

The role of Ret receptor tyrosine kinase in dopaminergic neuron development

Glial cell line-derived neurotrophic factor (GDNF) is one of the most potent trophic factors identified for promoting survival and function of dopaminergic (DA) neurons in the midbrain. Ret, a member of the receptor tyrosine kinase (RTK) superfamily transduces GDNF signaling. The role of Ret in the development of DA neurons is not clear however. Here we demonstrate the involvement of Ret in the DA neuron development both in vitro and in vivo. The dopamine transporter (DAT) gene was clearly induced in rat embryonic neural precursors that had been transfected with Ret. Temporary blockade of Ret expression in embryos using Ret antisense oligonucleotides (Ret-AS-ODN) in vivo led to reduced striatal DA content and a decrease of tyrosine hydroxylase (TH) positive fibers in the striatum. Additionally, some DA neurons in the substantia nigra (SN) underwent apoptotic cell death following the Ret-AS-ODN treatment. Taken together, the data suggest that normal function of Ret is required in vivo for the maturation of DA neurons, in particular for cell survival and fiber innervation. We further demonstrated Ret-induced expression of DAT in vitro.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

VegT induces endoderm by a self-limiting mechanism and by changing the competence of cells to respond to TGF-beta signals

The maternal determinant VegT is required for both endoderm and mesoderm formation by the Xenopus embryo. An important downstream mediator of VegT action is Xsox17, which has been proposed to be induced in cell-autonomous, then signal-dependent phases. We show that Xsox17 is a direct VegT target, but that direct induction of Xsox17 by VegT is rapidly inhibited. This inhibition is relieved by TGF- beta signalling, to which the future endoderm cell is sensitised by VegT, resulting in the observed dependence on cell contact for maintained Xsox17 expression. We propose that this change in regulation is a consequence of a VegT-induced repressor, inhibiting direct induction of early endoderm markers by VegT, and contributing to the formation of the boundary of the endodermal domain.

Zygotic Wnt Activity Is Required for Brachyury Expression in the Early Xenopus laevis Embryo

The canonical, beta-catenin-dependent Wnt pathway is a crucial player in the early events of Xenopus development. Dorsal axis formation and mesoderm patterning are accepted effects of this pathway, but the regulation of expression of genes involved in mesoderm specification is not. This conclusion is based largely on the inability of the Wnt pathway to induce mesoderm in animal cap explants. Using injections of inhibitors of canonical Wnt signaling, we demonstrate that expression of the general mesodermal marker Brachyury (Xbra) requires a zygotic, ligand-dependent Wnt activity throughout the marginal zone. Analysis of the Xbra promoter reveals that putative TCF-binding sites mediate Wnt activation, the first sites in this well-studied promoter to which an activation role can be ascribed. However, established mesoderm inducers like eFGF and activin can bypass the Wnt requirement for Xbra expression. Another mesoderm promoting factor, VegT, activates Xbra in a Wnt-dependent manner. We also show that the activin/nodal signaling is necessary for ectopic Xbra induction by the Wnt pathway, but not by VegT. Our data significantly change the understanding of Brachyury regulation in Xenopus, implying the existence of an unknown zygotic Wnt ligand in Spemann's organizer. Since Brachyury is considered to have a major role in mesoderm formation, it is possible that Wnts might play a role in mesoderm specification, in addition to patterning.

Plakoglobin is required for maintenance of the cortical actin skeleton in early Xenopus embryos and for cdc42-mediated wound healing

Early Xenopus embryos are large, and during the egg to gastrula stages, when there is little extracellular matrix, the cytoskeletons of the individual blastomeres are thought to maintain their spherical architecture and provide scaffolding for the cellular movements of gastrulation. We showed previously that depletion of plakoglobin protein during the egg to gastrula stages caused collapse of embryonic architecture. Here, we show that this is due to loss of the cortical actin skeleton after depletion of plakoglobin, whereas the microtubule and cytokeratin skeletons are still present. As a functional assay for the actin skeleton, we show that wound healing, an actin-based behavior in embryos, is also abrogated by plakoglobin depletion. Both wound healing and the amount of cortical actin are enhanced by overexpression of plakoglobin. To begin to identify links between plakoglobin and the cortical actin polymerization machinery, we show here that the Rho family GTPase cdc42, is required for wound healing in the Xenopus blastula. Myc-tagged cdc42 colocalizes with actin in purse-strings surrounding wounds. Overexpression of cdc42 dramatically enhances wound healing, whereas depletion of maternal cdc42 mRNA blocks it. In combinatorial experiments we show that cdc42 cannot rescue the effects of plakoglobin depletion, showing that plakoglobin is required for cdc42-mediated cortical actin assembly during wound healing. However, plakoglobin does rescue the effect of cdc42 depletion, suggesting that cdc42 somehow mediates the distribution or function of plakoglobin. Depletion of alpha-catenin does not remove the cortical actin skeleton, showing that plakoglobin does not mediate its effect by its known linkage through alpha-catenin to the actin skeleton. We conclude that in Xenopus, the actin skeleton is a major determinant of cell shape and overall architecture in the early embryo, and that plakoglobin plays an essential role in the assembly, maintenance, or organization of this cortical actin.

The role of maternal axin in patterning the Xenopus embryo

Regulation of the stability of beta catenin protein is a critical role of Wnt signaling cascades. In early Xenopus development, dorsal axis specification depends on regulation of beta catenin by both cytoplasmic and nuclear mechanisms. While the cytoplasmic protein axin is known as a key component of the cytoplasmic beta catenin degradation complex, loss-of-function studies are needed to establish whether it is required for dorso-ventral patterning in the embryo, and to test where in the embryo it carries out its function. Here, we show that embryos lacking maternal axin protein have increased levels of soluble beta catenin protein and increased nuclear localization of beta catenin in ventral nuclei at the blastula stage. These embryos gastrulate abnormally and develop with excessive notochord and head structures, and reduced tail and ventral components. They show increased expression of dorsal markers, including siamois, Xnr3, chordin, gsc, Xhex, and Otx2, decreased expression of Xwnt 8 and Xbra, and little alteration of BMP4 and Xvent1 and -2 mRNA levels. The ventral halves of axin-depleted embryos at the gastrula stage have dramatically increased levels of chordin expression, and severely decreased levels of Xwnt 8 mRNA expression, while BMP4 transcript levels are only slightly reduced. This dorso-anterior phenotype is rescued by axin mRNA injected into the vegetal pole of axin-depleted oocytes before fertilization. Interestingly, the phenotype was rescued by ventral but not dorsal injection of axin mRNA, at the 4-cell stage, although dorsal injection into wild-type embryos does cause ventralization. These results show directly that the localized ventral activity of maternal axin is critical for the correct patterning of the early Xenopus embryo.

beta-Catenin controls hair follicle morphogenesis and stem cell differentiation in the skin

beta-Catenin is an essential molecule in Wnt/wingless signaling, which controls decisive steps in embryogenesis. To study the role of beta-catenin in skin development, we introduced a conditional mutation of the gene in the epidermis and hair follicles using Cre/loxP technology. When beta-catenin is mutated during embryogenesis, formation of placodes that generate hair follicles is blocked. We show that beta-catenin is required genetically downstream of tabby/downless and upstream of bmp and shh in placode formation. If beta-catenin is deleted after hair follicles have formed, hair is completely lost after the first hair cycle. Further analysis demonstrates that beta-catenin is essential for fate decisions of skin stem cells: in the absence of beta-catenin, stem cells fail to differentiate into follicular keratinocytes, but instead adopt an epidermal fate.

Cadherins and catenins, Wnts and SOXs: embryonic patterning in Xenopus

Wnt signaling plays a critical role in a wide range of developmental and oncogenic processes. Altered gene regulation by the canonical Wnt signaling pathway involves the cytoplasmic stabilization of beta-catenin, a protein critical to the assembly of cadherin-based cell-cell adherence junctions. In addition to binding to cadherins, beta-catenin also interacts with transcription factors of the TCF-subfamily of HMG box proteins and regulates their activity. The Xenopus embryo has proven to be a particularly powerful experimental system in which to study the role of Wnt signaling components in development and differentiation. We review this literature, focusing on the role of Wnt signaling and interacting components in establishing patterns within the early embryo.

Hyperproliferation and defects in epithelial polarity upon conditional ablation of alpha-catenin in skin

When surface epithelium was conditionally targeted for ablation of alpha-catenin, hair follicle development was blocked and epidermal morphogenesis was dramatically affected, with defects in adherens junction formation, intercellular adhesion, and epithelial polarity. Differentiation occurred, but epidermis displayed hyperproliferation, suprabasal mitoses, and multinucleated cells. In vitro, alpha-catenin null keratinocytes were poorly contact inhibited and grew rapidly. These differences were not dependent upon intercellular adhesion and were in marked contrast to keratinocytes conditionally null for another essential intercellular adhesion protein, desmoplakin (DP). KO keratinocytes exhibited sustained activation of the Ras-MAPK cascade due to aberrations in growth factor responses. Thus, remarkably, features of precancerous lesions often attributed to defects in cell cycle regulatory genes can be generated by compromising the function of alpha-catenin.

Plakoglobin and beta-catenin: protein interactions, regulation and biological roles

Beta-catenin can play different roles in the cell, including one as a structural protein at cell-cell adherens junctions and another as a transcriptional activator mediating Wnt signal transduction. Plakoglobin (gamma)-catenin), a close homolog of beta-catenin, shares with beta-catenin common protein partners and can fulfill some of the same functions. The complexing of catenins with various protein partners is regulated by phosphorylation and by intramolecular interactions. The competition between different catenin partners for binding to catenins mediates the cross-talk between cadherin-based adhesion, catenin-dependent transcription and Wnt signaling. Although plakoglobin differs from beta-catenin in its functions and is unable to compensate for defects in Wnt signaling resulting from lack of beta-catenin, recent evidence suggests that plakoglobin plays a unique role in Wnt signaling that is different from that of beta-catenin. The functional difference between catenins is reflected in their differential involvement in embryonic development and cancer progression.

Assembly of tight junctions during early vertebrate development

Tight junction formation during development is critical for embryonic patterning and organization. We consider mechanisms of junction biogenesis in cleaving mouse and Xenopus eggs. Junction assembly follows the establishment of cell polarity at 8-cell (mouse) or 2-cell (Xenopus) stages, characterized by sequential membrane delivery of constituents, coordinated by embryonic (mouse) or maternal (Xenopus) expression programmes. Cadherin adhesion is permissive for tight junction construction only in the mouse. Occludin post-translational modification and membrane delivery, mediated by delayed ZO-1 alpha(+)isoform expression in the mouse, provides a mechanism for completion of tight junction biogenesis and sealing, regulating the timing of blastocoel cavitation.

Differential mechanisms of LEF/TCF family-dependent transcriptional activation by beta-catenin and plakoglobin

beta-Catenin and plakoglobin are highly homologous components of cell-cell adherens junctions linking cadherin receptors to the actin cytoskeleton. beta-Catenin, in addition, activates transcription by forming a complex with LEF/TCF family transcription factors in the nucleus. Plakoglobin can also bind to LEF-1 and, when overexpressed in mammalian cells, enhances LEF-1-directed transcription. Plakoglobin overexpression, however, results in the elevation and nuclear translocation of endogenous beta-catenin. We show here, by DNA mobility shift analysis, that the formation of a plakoglobin-LEF/TCF-DNA complex in vitro is very inefficient compared to a complex containing beta-catenin-LEF-DNA. Moreover, in plakoglobin-transfected cells plakoglobin-LEF/TCF-DNA complexes were not formed; rather, the endogenous beta-catenin, whose level is elevated by plakoglobin transfection, formed a beta-catenin-LEF-DNA complex. Removal of the N- and C-terminal domains of both beta-catenin and plakoglobin (leaving the armadillo repeat domain intact) induced plakoglobin-LEF-DNA complex formation and also enhanced beta-catenin-LEF-DNA complexing, both with in vitro-translated components and in transfected cells. Transfection with these truncated catenins increased endogenous beta-catenin levels, but the truncated catenins acted as dominant-negative inhibitors of beta-catenin-driven transcription by forming transcriptionally inactive complexes with LEF-1. When these catenin mutants were prevented from entering the nucleus, by their fusion to the connexin transmembrane domain, they indirectly activated transcription by increasing endogenous beta-catenin levels. These results suggest that overexpression of plakoglobin does not directly activate transcription and that formation of catenin-LEF-DNA complexes is negatively regulated by the catenin N- and C-terminal domains.

Differential interaction of plakoglobin and beta-catenin with the ubiquitin-proteasome system

Beta-catenin and plakoglobin are closely related armadillo family proteins with shared and distinct properties; Both are associated with cadherins in actin-containing adherens junctions. Plakoglobin is also found in desmosomes where it anchors intermediate filaments to the desmosomal plaques. Beta-catenin, on the other hand, is a component of the Wnt signaling pathway, which is involved in embryonic morphogenesis and tumorigenesis. A key step in the regulation of this pathway involves modulation of beta-catenin stability. A multiprotein complex, regulated by Wnt, directs the phosphorylation of beta-catenin and its degradation by the ubiquitin-proteasome system. Plakoglobin can also associate with members of this complex, but inhibition of proteasomal degradation has little effect on its levels while dramatically increasing the levels of beta-catenin. Beta-TrCP, an F-box protein of the SCF E3 ubiquitin ligase complex, was recently shown to play a role in the turnover of beta-catenin. To elucidate the basis for the apparent differences in the turnover of beta-catenin and plakoglobin we compared the handling of these two proteins by the ubiquitin-proteasome system. We show here that a deletion mutant of beta-TrCP, lacking the F-box, can stabilize the endogenous beta-catenin leading to its nuclear translocation and induction of beta-catenin/LEF-1-directed transcription, without affecting the levels of plakoglobin. However, when plakoglobin was overexpressed, it readily associated with beta-TrCP, efficiently competed with beta-catenin for binding to beta-TrCP and became polyubiquitinated. Fractionation studies revealed that about 85% of plakoglobin in 293 cells, is Triton X-100-insoluble compared to 50% of beta-catenin. These results suggest that while both plakoglobin and beta-catenin can comparably interact with beta-TrCP and the ubiquitination system, the sequestration of plakoglobin by the membrane-cytoskeleton system renders it inaccessible to the proteolytic machinery and stabilizes it.

Requirement for beta-catenin in anterior-posterior axis formation in mice

The anterior-posterior axis of the mouse embryo is defined before formation of the primitive streak, and axis specification and subsequent anterior development involves signaling from both embryonic ectoderm and visceral endoderm. Tauhe Wnt signaling pathway is essential for various developmental processes, but a role in anterior-posterior axis formation in the mouse has not been previously established. Beta-catenin is a central player in the Wnt pathway and in cadherin-mediated cell adhesion. We generated beta-catenin-deficient mouse embryos and observed a defect in anterior-posterior axis formation at embryonic day 5.5, as visualized by the absence of Hex and Hesx1 and the mislocation of cerberus-like and Lim1 expression. Subsequently, no mesoderm and head structures are generated. Intercellular adhesion is maintained since plakoglobin substitutes for beta-catenin. Our data demonstrate that beta-catenin function is essential in anterior-posterior axis formation in the mouse, and experiments with chimeric embryos show that this function is required in the embryonic ectoderm.

Nonchordate classic cadherins have a structurally and functionally unique domain that is absent from chordate classic cadherins

Classic cadherins, which are adhesion molecules in cell-cell adherens junctions, have a large contribution to the construction of the animal body. Their molecular structures show clear differences between chordate and nonchordate metazoans. Although nonchordate classic cadherins have cadherin superfamily-specific extracellular repeats (CRs) and a highly conserved cytoplasmic domain (CP), these cadherins have a unique extracellular domain that is absent from vertebrate and ascidian classic cadherins. We called this the primitive classic cadherin domain (PCCD). To understand the roles of the PCCD, we constructed and characterized a series of mutant forms of the Drosophila classic cadherin DE-cadherin. Biochemical analyses indicated that the last two CRs and PCCD form a special structure with proteolytic cleavage. Mutations in the PCCD did not eliminate the cell-cell-binding function of DE-cadherin in cultured cells, but prevented the cadherin from efficiently translocating to the plasma membrane in epithelial cells of the developing embryo. In addition, genetic rescue assays suggested that although CP-mediated control plays a central role in tracheal fusion, the role of the PCCD in efficient recruitment of DE-cadherin to apical areas of the plasma membranes is also important for dynamic epithelial morphogenesis. We propose that there is a fundamental difference in the mode of classic cadherin-mediated cell-cell adhesion between chordate and nonchordate metazoans.

Cadherin-catenin complexes during zebrafish oogenesis: heterotypic junctions between oocytes and follicle cells

During vertebrate oogenesis, the germ cells and associated somatic cells remain connected by a variety of adhering junctional complexes. However, the molecular composition of these cellular structures is largely unknown. To identify the proteins forming the heterotypic adherens junctions between oocytes and follicle cells in the zebrafish (Danio rerio), the cDNAs encoding alphaE-catenin and plakoglobin were isolated. Using these cDNAs, in combination with the previously isolated beta-catenin cDNA, and antibodies specific for alpha- and beta-catenin, plakoglobin, and N- and E-cadherin, we found differences in catenin and plakoglobin gene expression during oogenesis. The immunolocalization of these plaque proteins, as well as of cadherins, in the ovarian follicle indicated an enrichment of alpha- and beta-catenin and of E-cadherin-like protein(s) in the oocyte cortex, notably at sites of oocyte-follicle cell contacts, suggesting the presence of hitherto unknown heterotypic adherens junctions between these cells. By contrast, plakoglobin and N-cadherin localization was restricted to cell-cell contacts in the follicle cell layer. During oocyte maturation, mRNAs for alphaE- and beta-catenin and plakoglobin accumulated, and all three plaque-forming proteins were stored in unfertilized eggs, either in complexed forms with cadherins or as free cytoplasmic pools. These findings suggest possible roles of these junctional proteins during early embryogenesis.

Functional characterization of multiple transactivating elements in beta-catenin, some of which interact with the TATA-binding protein in vitro

beta-Catenin, a member of the family of Armadillo repeat proteins, plays a dual role in cadherin-mediated cell adhesion and in signaling by Wnt growth factors. Upon Wnt stimulation beta-catenin undergoes nuclear translocation and serves as transcriptional coactivator of T cell factor DNA-binding proteins. Previously the transactivation potential of different portions of beta-catenin has been demonstrated, but the precise location of transactivating elements has not been established. Also, the mechanism of transactivation by beta-catenin and the molecular basis for functional differences between beta-catenin and the closely related proteins Armadillo and Plakoglobin are poorly understood. Here we have used a yeast system for the detailed characterization of the transactivation properties of beta-catenin. We show that its transactivation domains possess a modular structure, consist of multiple subelements that cover broad regions at its N and C termini, and extend considerably into the Armadillo repeat region. Compared with beta-catenin the N termini of Plakoglobin and Armadillo have different transactivation capacities that may explain their distinct signaling properties. Furthermore, transactivating elements of beta-catenin interact specifically and directly with the TATA-binding protein in vitro providing further evidence that a major function of beta-catenin during Wnt signaling is to recruit the basal transcription machinery to promoter regions of Wnt target genes.

Immunocytochemical studies of the interactions of cadherins and catenins in the early Xenopus embryo

Linkage of cadherins to the cytoskeleton is crucial for their adhesive function. Since alpha- and beta-catenin play a key role in this linkage, these proteins are possible targets for processes that control cell-cell adhesion. To achieve a better understanding of the regulation of cell-cell adhesion in embryonic morphogenesis, we used immunohistology to investigate how in Xenopus blastomeres catenins respond to disturbances in the expression of maternal cadherins. Overexpression of myc-tagged maternal cadherin leads to a proportionate increase of the level of beta-catenin. The two proteins colocalize in the endoplasmic reticulum, in cytoplasmic vesicles, and along the cell membrane, indicating that the beta-catenin binds to overexpressed cadherin early in its passage to the plasma membrane. Expression of cadherin is essential for the stable presence of beta-catenin, as depletion from maternal cadherin mRNA leads to a complete loss of beta-catenin from the blastomeres. alpha-Catenin behaves differently. Overexpression of cadherin leaves the amount and localization of alpha-catenin largely unaffected, and additional cadherin inserts itself into the membrane without a proportionate rise in the level of membrane-bound alpha-catenin. However, cadherin mRNA depletion leads to a redistribution of alpha-catenin from the membrane to the cytoplasm. Thus, cadherin is required to localize alpha-catenin to the membrane, but the amount of alpha-catenin along the membrane seems to be restricted to a certain level which cannot be exceeded. The relevance of these observations for the regulation of cadherin-mediated cell adhesion in the Xenopus embryo is discussed. Additionally, we demonstrate that plakoglobin, like beta-catenin an armadillo repeat protein, shows neither accumulation after overexpression nor colocalization with the overexpressed cadherin.

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.

Misexpression of the catenin p120(ctn)1A perturbs Xenopus gastrulation but does not elicit Wnt-directed axis specification

Modulators of cadherin function are of great interest given that the cadherin complex actively contributes to the morphogenesis of virtually all tissues. The catenin p120(ctn) (formerly p120cas) was first identified as a src- and receptor-protein tyrosine kinase substrate and later shown to interact directly with cadherins. In common with beta-catenin and plakoglobin (gamma-catenin), p120(ctn) contains a central Armadillo repeat region by which it binds cadherin cytoplasmic domains. However, little is known about the function of p120(ctn) within the cadherin complex. We examined the role of p120(ctn)1A in early vertebrate development via its exogenous expression in Xenopus. Ventral overexpression of p120(ctn)1A, in contrast to beta-catenin, did not induce the formation of duplicate axial structures resulting from the activation of the Wnt signaling pathway, nor did p120(ctn) affect mesoderm induction. Rather, dorsal misexpression of p120(ctn) specifically perturbed gastrulation. Lineage tracing of cells expressing exogenous p120(ctn) indicated that cell movements were disrupted, while in vitro studies suggested that this may have been a consequence of reduced adhesion between blastomeres. Thus, while cadherin-binding proteins beta-catenin, plakoglobin, and p120(ctn) are members of the Armadillo protein family, it is clear that these proteins have distinct biological functions in early vertebrate development. This work indicates that p120(ctn) has a role in cadherin function and that heightened expression of p120(ctn) interferes with appropriate cell-cell interactions necessary for morphogenesis.

Identification of four CCCH zinc finger proteins in Xenopus, including a novel vertebrate protein with four zinc fingers and severely restricted expression

Tristetraprolin (TTP), the prototype of a class of CCCH zinc finger proteins, is a phosphoprotein that is rapidly and transiently induced by growth factors and serum in fibroblasts. Recent evidence suggests that a physiological function of TTP is to inhibit tumor necrosis factor alpha secretion from macrophages by binding to and destabilizing its mRNA (Carballo, E., Lai, W.S., Blackshear, P.J., 1998. Science, 281, 1001-1005). To investigate possible functions of CCCH proteins in early development of Xenopus, we isolated four Xenopus cDNAs encoding members of this class. Based on 49% overall amino acid identity and 84% amino acid identity within the double zinc finger domain, one of the Xenopus proteins (XC3H-1) appears to be the homologue of TTP. By similar analyses, XC3H-2 and XC3H-3 are homologues of ERF-1 (cMG1, TIS11B) and ERF-2 (TIS11D). A fourth protein, XC3H-4, is a previously unidentified member of the CCCH class of vertebrate zinc finger proteins; it contains four Cx8Cx5Cx3H repeats, two of which are YKTEL Cx8Cx5Cx3H repeats that are closely related to sequences found in the other CCCH proteins. Whereas XC3H-1, XC3H-2, and XC3H-3 were widely expressed in adult tissues, XC3H-4 mRNA was not detected in any of the adult tissues studied except for the ovary. Its expression appeared to be limited to the ovary, oocyte, egg and the early embryonic stages leading up to the mid-blastula transition. Its mRNA was highly expressed in oocytes of all ages, and was enriched in the animal pole cytosol of mature oocytes. Maternal expression was also seen with the other three messages, suggesting the possibility that these proteins are involved in regulating mRNA stability in oocyte maturation and/or early embryogenesis.

The junction-associated protein AF-6 interacts and clusters with specific Eph receptor tyrosine kinases at specialized sites of cell-cell contact in the brain

The AF-6/afadin protein, which contains a single PDZ domain, forms a peripheral component of cell membranes at specialized sites of cell-cell junctions. To identify potential receptor-binding targets of AF-6 we screened the PDZ domain of AF-6 against a range of COOH-terminal peptides selected from receptors having potential PDZ domain-binding termini. The PDZ domain of AF-6 interacts with a subset of members of the Eph subfamily of RTKs via its COOH terminus both in vitro and in vivo. Cotransfection of a green fluorescent protein-tagged AF-6 fusion protein with full-length Eph receptors into heterologous cells induces a clustering of the Eph receptors and AF-6 at sites of cell-cell contact. Immunohistochemical analysis in the adult rat brain reveals coclustering of AF-6 with Eph receptors at postsynaptic membrane sites of excitatory synapses in the hippocampus. Furthermore, AF-6 is a substrate for a subgroup of Eph receptors and phosphorylation of AF-6 is dependent on a functional kinase domain of the receptor. The physical interaction of endogenous AF-6 with Eph receptors is demonstrated by coimmunoprecipitation from whole rat brain lysates. AF-6 is a candidate for mediating the clustering of Eph receptors at postsynaptic specializations in the adult rat brain.

Desmoplakin is required early in development for assembly of desmosomes and cytoskeletal linkage

Desmosomes first assemble in the E3.5 mouse trophectoderm, concomitant with establishment of epithelial polarity and appearance of a blastocoel cavity. Throughout development, they increase in size and number and are especially abundant in epidermis and heart muscle. Desmosomes mediate cell-cell adhesion through desmosomal cadherins, which differ from classical cadherins in their attachments to intermediate filaments (IFs), rather than actin filaments. Of the proteins implicated in making this IF connection, only desmoplakin (DP) is both exclusive to and ubiquitous among desmosomes. To explore its function and importance to tissue integrity, we ablated the desmoplakin gene. Homozygous -/- mutant embryos proceeded through implantation, but did not survive beyond E6.5. Surprisingly, analysis of these embryos revealed a critical role for desmoplakin not only in anchoring IFs to desmosomes, but also in desmosome assembly and/or stabilization. This finding not only unveiled a new function for desmoplakin, but also provided the first opportunity to explore desmosome function during embryogenesis. While a blastocoel cavity formed and epithelial cell polarity was at least partially established in the DP (-/-) embryos, the paucity of desmosomal cell-cell junctions severely affected the modeling of tissue architecture and shaping of the early embryo.