Expressed recombinant cadherins mediate cell sorting in model systems

Cadherin-dependent mechanotransduction depends on ligand identity but not affinity

This study investigates the relationship between classical cadherin binding affinities and mechanotransduction through cadherin-mediated adhesions. The mechanical properties of cadherin-dependent intercellular junctions are generally attributed to differences in the binding affinities of classical cadherin subtypes that contribute to cohesive energies between cells. However, cell mechanics and mechanotransduction may also regulate intercellular contacts. We used micropipette measurements to quantify the two-dimensional affinities of cadherins at the cell surface, and two complementary mechanical measurements to assess ligand-dependent mechanotransduction through cadherin adhesions. At the cell surface, the classical cadherins investigated in this study form both homophilic and heterophilic bonds with two-dimensional affinities that differ by less than threefold. In contrast, mechanotransduction through cadherin adhesions is strongly ligand dependent such that homophilic, but not heterophilic ligation mediates mechanotransduction, independent of the cadherin binding affinity. These findings suggest that ligand-selective mechanotransduction may supersede differences in cadherin binding affinities in regulating intercellular contacts.

Cross talk between engulfment receptors stabilin-2 and integrin αvβ5 orchestrates engulfment of phosphatidylserine-exposed erythrocytes

Efficient cell corpse clearance is critical for health in organisms. Apoptotic cells displaying phosphatidylserine (PS) are recognized by engulfment receptors and ingested through two conserved pathways. In one pathway, engulfment receptor brain-specific angiogenesis inhibitor 1 (BAI-1) or integrin functions upstream of ELMO/DOCK180 and activate the small GTPase Rac1. In the other pathway, engulfment receptor CED-1 or stabilin-2 acts in concert with the adaptor protein GULP to activate Rac1. Stabilin-2, a PS receptor, facilitates phagocytosis of apoptotic cells and mediates the production of anti-inflammatory cytokines. Here, we propose that the stabilin-2 extracellular domain consisting of integrin-binding fasciclin 1 (FAS1) domains coordinates the activities of the two phagocytic pathways via direct interactions with integrin. Interactions between stabilin-2 and integrin were determined using biochemical assays, including coimmunoprecipitation and fluorescence resonance energy transfer (FRET). These interactions appear to have functional relevance, since knockdown of endogenous αvβ5 expression or treatment with a function-blocking αvβ5 antibody significantly decreased stabilin-2-mediated phagocytosis in the absence of soluble factors. Our data collectively suggest that the engulfment receptors of the two phagocytic pathways communicate with each other to orchestrate engulfment of damaged erythrocytes. Coordinated phagocytic signaling would be advantageous for physiological and pathological circumstances that require rapid clearance of abnormal (apoptotic or aged) cells.

Plakoglobin: role in tumorigenesis and metastasis

Plakoglobin (γ-catenin) is a member of the Armadillo family of proteins and a homolog of β-catenin. As a component of both the adherens junctions and desmosomes, plakoglobin plays a pivotal role in the regulation of cell-cell adhesion. Furthermore, similar to β-catenin, plakoglobin is capable of participating in cell signaling. However, unlike β-catenin that has well-documented oncogenic potential through its involvement in the Wnt signaling pathway, plakoglobin generally acts as a tumor/metastasis suppressor. The exact roles that plakoglobin plays during tumorigenesis and metastasis are not clear; however, recent evidence suggests that it may regulate gene expression, cell proliferation, apoptosis, invasion, and migration. In this paper, we describe plakoglobin, its discovery and characterization, its role in regulating cell-cell adhesion, and its signaling capabilities in regulation of tumorigenesis and metastasis.

Molecular mechanisms of cell segregation and boundary formation in development and tumorigenesis

The establishment and maintenance of precisely organized tissues requires the formation of sharp borders between distinct cell populations. The maintenance of segregated cell populations is also required for tissue homeostasis in the adult, and deficiencies in segregation underlie the metastatic spreading of tumor cells. Three classes of mechanisms that underlie cell segregation and border formation have been uncovered. The first involves differences in cadherin-mediated cell-cell adhesion that establishes interfacial tension at the border between distinct cell populations. A second mechanism involves the induction of actomyosin-mediated contraction by intercellular signaling, such that cortical tension is generated at the border. Third, activation of Eph receptors and ephrins can lead to both decreased adhesion by triggering cleavage of E-cadherin, and to repulsion of cells by regulation of the actin cytoskeleton, thus preventing intermingling between cell populations. These mechanisms play crucial roles at distinct boundaries during development, and alterations in cadherin or Eph/ephrin expression have been implicated in tumor metastasis.

Biophysics of cadherin adhesion

Since the identification of cadherins and the publication of the first crystal structures, the mechanism of cadherin adhesion, and the underlying structural basis have been studied with a number of different experimental techniques, different classical cadherin subtypes, and cadherin fragments. Earlier studies based on biophysical measurements and structure determinations resulted in seemingly contradictory findings regarding cadherin adhesion. However, recent experimental data increasingly reveal parallels between structures, solution binding data, and adhesion-based biophysical measurements that are beginning to both reconcile apparent differences and generate a more comprehensive model of cadherin-mediated cell adhesion. This chapter summarizes the functional, structural, and biophysical findings relevant to cadherin junction assembly and adhesion. We emphasize emerging parallels between findings obtained with different experimental approaches. Although none of the current models accounts for all of the available experimental and structural data, this chapter discusses possible origins of apparent discrepancies, highlights remaining gaps in current knowledge, and proposes challenges for further study.

Evolution of the cadherin-catenin complex

Adherens junctions are the most common junction type found in animal epithelia. Their core components are classical cadherins and catenins, which form membrane-spanning complexes that mediate intercellular binding on the extracellular side and associate with the actin cytoskeleton on the intracellular side. Junctional cadherin-catenin complexes are key elements involved in driving animal morphogenesis. Despite their ubiquity and importance, comparative studies of classical cadherins, catenins and their related molecules suggest that the cadherin/catenin-based adherens junctions have undergone structural and compositional transitions during the diversification of animal lineages. This chapter describes the molecular diversities related to the cadherin-catenin complex, based on accumulated molecular and genomic information. Understanding when and how the junctional cadherin-catenin complex originated, and its subsequent diversification in animals, promotes a comprehensive understanding of the mechanisms of animal morphological diversification.

Synapse adhesion: a dynamic equilibrium conferring stability and flexibility

Cell adhesion molecules (CAMs) linked to cytoskeleton generate stable cell-cell junctions. Cadherins provide a canonical example, but paradoxically, they participate in a multitude of transient and regulatable interactions. Their extracellular binding generates weak adhesion that is modified by clustering; interactions with F-actin are regulated, can be transient, and can alter F-actin dynamics. Additionally, cadherin recycling from the cell surface can modify the size and location of junctions and strength of adhesion. In epithelial cells, this ongoing dynamic behavior is important for maintaining stable junctions. Recent work supports that cadherins act similarly at synapses where their actions are likely to be shared by integrins and other actin-linked CAMs. Together the collaborative activities of such CAMs provide a stable, but flexible structure that can promote and support changes in synapse shape and size while maintaining stable junctions to permit information flow.

Engineering artificial signaling centers to polarize embryoid body differentiation

Embryonic stem (ES) cells differentiating as aggregates self-organize dependent on Wnt signaling that is initially localized to discrete sites in the aggregate. As differentiation proceeds, Wnt signaling expands to most of the aggregates, thus resulting in widespread differentiation of mesendodermal progenitors. This process resembles primitive streak formation, but the lack of organized positional information makes the differentiating aggregates develop in a disorganized fashion. Here, we report that exogenous, cellular signaling sources can control the site where differentiation initiates in ES cell aggregates. Fibroblasts engineered to express cadherins are assembled with ES cells to form composite aggregates where the fibroblasts are positioned as a discrete pole. When engineered to express secreted Wnt agonists or antagonists, this pole functions to localize signaling in a way that polarizes the differentiating aggregates. The use of cell adhesion molecules to control morphology of developing stem cell aggregates should be widely applicable in tissue engineering.

Embryo developmental disruption during organogenesis produced by CF-1 murine periconceptional alcohol consumption

The aim was to study the control females (CF)-1 mouse embryo differentiation, growth, morphology on embryonic E- and N-cadherin expression at midgestation after periconceptional moderate alcohol ingestion. Adult female mice were exposed to 10% ethanol in drinking water for 17 days previous to and up to day 10 of gestation (ethanol-exposed females, EF) and were compared with nonexposed CF. EF presented reduced quantities of E10 to E10.5 embryos, greater percentage of embryos at stages less than E7.5, reduced implantation site numbers/female, and increased resorptions compared with CF. EF-embryo growth was significantly affected as evidenced by reduced cephalic and body sizes of E10 and E10.5 embryos (scanning electron microscopy) and decreased protein content of E10.5 embryos vs. CF embryos. A significantly higher percentage of EF-E10-10.5 embryos presented abnormal neural tube (NT) closure vs. the percentage of CF. E10 embryos from EF presented elevated tissue disorganization, pyknosis and nuclear condensation in somites, mesenchymal and neuroepithelial tissue. Immunohistochemical E- and N-cadherin distribution patterns were similar in organic structures of E10 embryos between groups. However, western blot revealed that E- and N-cadherin expression levels were significantly increased in EF-derived embryos vs. controls. Perigestational ethanol consumption by CF-1 mice induced significant damage in the organogenic embryogenesis by producing delayed differentiation, growth deficiencies, and increasing the frequency of NT defects. Ethanol exposure may disrupt cell-cell adhesion leading to upregulation of E- and N-cadherin expression suggesting that deregulation of cell adhesion molecules could be involved in the disruption of embryo development at organogenesis in CF-1 mouse.

A variable cytoplasmic domain segment is necessary for γ-protocadherin trafficking and tubulation in the endosome/lysosome pathway

The variable portion of the γ-protocadherin (Pcdh-γ) cytoplasmic domain (VCD) controls Pcdh-γ trafficking and organelle tubulation in the endolysosome system. Active VCD segments are conserved in Pcdh-γA and Pcdh-γB subfamilies.

Clustered protocadherins (Pcdhs) are arranged in gene clusters (α, β, and γ) with variable and constant exons. Variable exons encode cadherin and transmembrane domains and ∼90 cytoplasmic residues. The 14 Pcdh-αs and 22 Pcdh-γs are spliced to constant exons, which, for Pcdh-γs, encode ∼120 residues of an identical cytoplasmic moiety. Pcdh-γs participate in cell–cell interactions but are prominently intracellular in vivo, and mice with disrupted Pcdh-γ genes exhibit increased neuronal cell death, suggesting nonconventional roles. Most attention in terms of Pcdh-γ intracellular interactions has focused on the constant domain. We show that the variable cytoplasmic domain (VCD) is required for trafficking and organelle tubulation in the endolysosome system. Deletion of the constant cytoplasmic domain preserved the late endosomal/lysosomal trafficking and organelle tubulation observed for the intact molecule, whereas deletion or excision of the VCD or replacement of the Pcdh-γA3 cytoplasmic domain with that from Pcdh-α1 or N-cadherin dramatically altered trafficking. Truncations or internal deletions within the VCD defined a 26–amino acid segment required for trafficking and tubulation in the endolysosomal pathway. This active VCD segment contains residues that are conserved in Pcdh-γA and Pcdh-γB subfamilies. Thus the VCDs of Pcdh-γs mediate interactions critical for Pcdh-γ trafficking.

Cleavage of E-cadherin by ADAM10 mediates epithelial cell sorting downstream of EphB signalling

The formation and maintenance of complex organs requires segregation of distinct cell populations into defined territories (that is, cell sorting) and the establishment of boundaries between them. Here we have investigated the mechanism by which Eph/ephrin signalling controls the compartmentalization of cells in epithelial tissues. We show that EphB/ephrin-B signalling in epithelial cells regulates the formation of E-cadherin-based adhesions. EphB receptors interact with E-cadherin and with the metalloproteinase ADAM10 at sites of adhesion and their activation induces shedding of E-cadherin by ADAM10 at interfaces with ephrin-B1-expressing cells. This process results in asymmetric localization of E-cadherin and, as a consequence, in differences in cell affinity between EphB-positive and ephrin-B-positive cells. Furthermore, genetic inhibition of ADAM10 activity in the intestine of mice results in a lack of compartmentalization of Paneth cells within the crypt stem cell niche, a defect that phenocopies that of EphB3-null mice. These results provide important insights into the regulation of cell migration in the intestinal epithelium and may help in the understanding of the nature of the cell sorting process in other epithelial tissues where Eph-ephrin interactions play a central role.

Prolines in βA-sheet of neural cadherin act as a switch to control the dynamics of the equilibrium between monomer and dimer

Neural cadherins dimerize through the formation of calcium-dependent strand-crossover structures. Dimerization of cadherins leads to cell-cell adhesion in multicellular organisms. Strand-crossover dimer forms exclusively between the first N-terminal extracellular modules (EC1) of the adhesive partners via swapping of their βA-sheets and docking of tryptophan-2 in the hydrophobic pocket. In the apo-state wild-type cadherin is predominantly monomer, which indicates that the dimerization is energetically unfavorable in the absence of calcium. Addition of calcium favors dimer formation by creating strain in the monomer and lowering the energetic barrier between monomer and dimer. Dynamics of the monomer-dimer equilibrium is vital for plasticity of synapses. Prolines recurrently occur in proteins that form strand-crossover dimer and are believed to be the source of the strain in the monomer. N-cadherins have two proline residues in the βA-sheet. We focused our studies on the role of these two prolines in calcium-dependent dimerization. Spectroscopic, electrophoretic, and chromatopgraphic studies showed that mutations of both prolines to alanines increased the dimerization affinity by ~20-fold and relieved the requirement of calcium in dimerization. The P5A and P6A mutant formed very stable dimers that required denaturation of protein to disassemble in the apo conditions. In summary, the proline residues act as a switch to control the dynamics of the equilibrium between monomer and dimer which is crucial for the plasticity of synapses.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Epigenetic regulation of cellular adhesion in cancer

Epigenetics describes the development and maintenance of stable heritable gene expression patterns, which allow cells to show different phenotypes despite of a commonly shared genetic code. The increasing knowledge in this field during the last decades reveals its importance for many physiological processes like differentiation, embryogenesis and parental imprinting, but also for some diseases such as cancer. Recent data have shown that the complexity of carcinogenesis can no longer be explained solely on the basis of genetic changes, but epigenomic alterations such as changes of the DNA methylation pattern and/or post-translational histone modifications and changes of microRNA expression need to be equally considered. Such epigenetic alterations may cause permanent changes in gene expression patterns and may therefore essentially contribute to some of the known phenotypic characteristics of cancer cells like the loss of growth control, altered intercellular communication and enhanced motility. The two latter may essentially be associated with the downregulation of cellular adhesion molecules, which may therefore be relevant in the context of cancer invasiveness and prognosis. The targeted modification of the epigenome may therefore open new horizons within the increasingly important field of epigenetic therapeutics-particularly in view of the regulation of cellular adhesion with particular attention to tumor cell invasion and metastasis.

Cell-extracellular matrix versus cell-cell interactions during the development of the cochlear-vestibular ganglion

Cells destined to become the neurones of the cochlear-vestibular ganglion (CVG) originate within the otic epithelium. Early in development they detach from their neighbors and migrate out of the epithelium, where they coalesce to form the CVG. To accomplish this process, the neuroblasts must modify their interactions with other cells within the epithelium and with proteins in the extracellular matrix to allow for repositioning. The aim of this study was to investigate the roles of the major families of adhesion molecules that mediate cellular interactions with the extracellular matrix, the integrins, and with other cells, the cadherins, in neuroblast segregation from the otic epithelium. The expression of classical cadherins increased in migrating neuroblasts compared with the otic epithelium. Quantitative RT-PCR revealed that this was concomitant with down-regulation of E-cadherin and up-regulation of N-cadherin in the migrating cells. In contrast, the level of β1 integrin expression by the epithelium was maintained in migrating neuroblasts. However, although multiple integrin ligands were expressed within the otic basement membrane at this stage of development, only fibronectin (FN) supported neuroblast migration along the substrate in vitro. Inhibition of β1 integrins resulted in significantly reduced linear migration on FN. Importantly, neuroblasts retained the ability to segregate from the epithelium but remained compacted immediately adjacent to the originating tissue, suggesting dominance of cell-cell over cell-matrix interactions. These data suggest that the balance between cell-cell and cell-substratum interactions directs otic neuroblast migration and gangliogenesis.

Sequential binding of calcium leads to dimerization in neural cadherin

Neural cadherin (N-cadherin) is a calcium-dependent homophilic cell-adhesive molecule and critical for synaptogenesis and synapse maintenance. The extracellular region plays an important role in cadherin-mediated cell adhesion and has five tandemly repeated ectodomains (EC1-EC5) with three calcium-binding sites situated between each of these domains. Adhesive dimer formation is significantly dependent on binding of calcium such that mutations in the calcium-binding sites adversely affect cell adhesion. To investigate the relative significance of the calcium-binding sites at the EC1-EC2 interface in calcium-induced dimerization, we mutated three important amino acids, D134, D136, and D103, in NCAD12, a construct containing EC1 and EC2. Spectroscopic and chromatographic experiments showed that all three mutations affected calcium binding and dimerization. Mutation of D134, a bidentate chelator in site 3, severely impaired the binding of calcium to all three sites. These findings confirm that binding to site 3 is required for binding to occur at site 2 and site 1. Interestingly, while the D103A mutation diminished only the affinity for calcium, it completely eliminated dimerization. Equilibrium dialysis experiments showed a stoichiometry of 3 at 2 mM calcium for D103A, but no dimerization was apparent even at 10 mM calcium. These results indicate that calcium binding alone is not sufficient for dimerization but requires cooperativity between calcium-binding sites. In summary, our findings confirm that the calcium-binding sites are occupied sequentially in the order of site 3, then site 2 and site 1, and that cooperativity between site 2 and site 1 is essential for formation of adhesive dimers by N-cadherin.

Neurogenin3 initiates stepwise delamination of differentiating endocrine cells during pancreas development

During development, pancreatic endocrine cells are specified within the pancreatic epithelium. They subsequently delaminate out of the epithelium and cluster in the mesenchyme to form the islets of Langerhans. Neurogenin3 (Ngn3) is a transcription factor required for the differentiation of all endocrine cells and we investigated its role in their delamination. We observed in the mouse pancreas that most Ngn3-positive cells have lost contact with the lumen of the epithelium, showing that the delamination from the progenitor layer is initiated in endocrine progenitors. Subsequently, in both mouse and chick newly born endocrine cells at the periphery of the epithelium strongly decrease E-cadherin, break-down the basal lamina and cluster into islets of Langerhans. Repression of E-cadherin is sufficient to promote delamination from the epithelium. We further demonstrate that Ngn3 indirectly controls Snail2 protein expression post-transcriptionally to repress E-cadherin. In the chick embryo, Ngn3 independently controls epithelium delamination and differentiation programs.

Loss of the p53/p63 regulated desmosomal protein Perp promotes tumorigenesis

Dysregulated cell–cell adhesion plays a critical role in epithelial cancer development. Studies of human and mouse cancers have indicated that loss of adhesion complexes known as adherens junctions contributes to tumor progression and metastasis. In contrast, little is known regarding the role of the related cell–cell adhesion junction, the desmosome, during cancer development. Studies analyzing expression of desmosome components during human cancer progression have yielded conflicting results, and therefore genetic studies using knockout mice to examine the functional consequence of desmosome inactivation for tumorigenesis are essential for elucidating the role of desmosomes in cancer development. Here, we investigate the consequences of desmosome loss for carcinogenesis by analyzing conditional knockout mice lacking Perp, a p53/p63 regulated gene that encodes an important component of desmosomes. Analysis of Perp-deficient mice in a UVB-induced squamous cell skin carcinoma model reveals that Perp ablation promotes both tumor initiation and progression. Tumor development is associated with inactivation of both of Perp's known functions, in apoptosis and cell–cell adhesion. Interestingly, Perp-deficient tumors exhibit widespread downregulation of desmosomal constituents while adherens junctions remain intact, suggesting that desmosome loss is a specific event important for tumorigenesis rather than a reflection of a general change in differentiation status. Similarly, human squamous cell carcinomas display loss of PERP expression with retention of adherens junctions components, indicating that this is a relevant stage of human cancer development. Using gene expression profiling, we show further that Perp loss induces a set of inflammation-related genes that could stimulate tumorigenesis. Together, these studies suggest that Perp-deficiency promotes cancer by enhancing cell survival, desmosome loss, and inflammation, and they highlight a fundamental role for Perp and desmosomes in tumor suppression. An understanding of the factors affecting cancer progression is important for ultimately improving the diagnosis, prognostication, and treatment of cancer.

Changes in tissue architecture, such as loss of adhesion between cells, have been shown to facilitate cancer development, especially metastasis where cells can detach from a tumor and spread throughout the body. While various studies have demonstrated that inactivation of an adhesion complex known as the adherens junction promotes cancer development and metastasis, little is known about the role of the desmosome—a related cell–cell adhesion complex—in tumorigenesis. Here we examine the consequence of desmosome-deficiency for tumor development by studying mice lacking a key component of desmosomes in the skin, a protein known as Perp. Using a mouse model for human skin cancer, in which ultraviolet light promotes skin cancer development, we demonstrate that Perp-deficiency indeed leads to accelerated skin tumorigenesis. We similarly observe that PERP is lost during human skin cancer development, suggesting that PERP is also important as a tumor suppressor in humans. These findings demonstrate that desmosome-deficiency achieved by Perp inactivation can promote cancer and suggest the potential utility of monitoring PERP status for staging, prognostication, or treatment of human cancers.

γ-protocadherins are enriched and transported in specialized vesicles associated with the secretory pathway in neurons

Gamma protocadherins (Pcdh-γs) resemble classical cadherins and have the potential to engage in cell-cell interactions with homophilic properties. Emerging evidence suggests non-conventional roles for some protocadherins in neural development. We sought to determine whether Pcdh-γ trafficking in neurons is consistent with an intracellular role for these molecules. Here we show that, in contrast to the largely surface localization of classical cadherins, endogenous Pcdh-γs are primarily intracellular in rat neurons in vivo and are equally distributed within organelles of subsynaptic dendritic and axonal compartments. A strikingly higher proportion of Pcdh-γ-containing organelles in synaptic compartments was observed at postnatal day 16. To determine the origin of Pcdh-γ-trafficking organelles, we isolated organelles with Pcdh-γ antibody-coupled magnetic beads from brain organelle suspensions. Vesicles with high levels of COPII and endoplasmic reticulum-Golgi intermediate compartment (ERGIC) components were isolated with the Pcdh-γ antibody but not with the classical cadherin antibody. In cultured hippocampal neurons, Pcdh-γ immunolabeling partially overlapped with calnexin- and COPII-positive puncta in dendrites. Mobile Pcdh-γ-GFP profiles dynamically codistributed with a DsRed construct coupled to ER retention signals by live imaging. Pcdh-γ expression correlated with accumulations of tubulovesicular and ER-like organelles in dendrites. Our results are consistent with the possibility that Pcdh-γs could have a unique function within the secretory pathway in addition to their documented surface roles.

E-cadherin gene-engineered feeder systems for supporting undifferentiated growth of mouse embryonic stem cells

Conventionally, embryonic stem (ES) cells are cultured on a cell layer of mouse embryonic fibroblasts (MEFs) as feeder cells to support undifferentiated growth of ES cells. In this study, cell-cell interactions between mouse ES and feeder cells were artificially engineered via an epithelial cell adhesion molecule, E-cadherin, whose expression is considerable in ES cells. Mouse mesenchymal STO and NIH3T3 cells that were genetically engineered to express E-cadherin were used in ES cell cultures as feeder cells. ES cells cultured on the E-cadherin-expressing feeder cells maintained the expression of stem cell markers, alkaline phosphatase (AP), Oct3/4, Nanog and Sox2, and the efficiency of AP-positive colony formation was comparable to MEFs, and much better than parental STO and NIH3T3 cells. Furthermore, ES cells maintained on the E-cadherin-expressing feeder cells possessed the ability to differentiate into the three germ layers both in vitro and in vivo. The results indicated that E-cadherin expression in feeder cells could improve the performance of feeder cells, which may be further applicable to create new artificial feeder cell lines.

Compartment boundaries: sorting cells with tension

The subdivision of proliferating tissues into groups of non-intermingling sets of cells, termed compartments, is a common process of animal development. Signaling between adjacent compartments induces the local expression of morphogens that pattern the surrounding tissue. Sharp and straight boundaries between compartments stabilize the source of such morphogens during tissue growth and, thus, are of crucial importance for pattern formation. Signaling pathways required to maintain compartment boundaries have been identified, yet the physical mechanisms that maintain compartment boundaries remained elusive. Recent data now show that a local increase in actomyosin-based mechanical tension on cell bonds is vital for maintaining compartment boundaries in Drosophila.

The polarized distribution of Na+,K+-ATPase: role of the interaction between {beta} subunits

Na⁺,K⁺-ATPase polarity depends on the interaction between the β subunits of Na⁺,K⁺-ATPases located on neighboring cells. In the present work, we use energy transfer methods (FRET), in vivo to demonstrate that these β subunits interact directly at the intercellular space of epithelial cells.

The very existence of higher metazoans depends on the vectorial transport of substances across epithelia. A crucial element of this transport is the membrane enzyme Na⁺,K⁺-ATPase. Not only is this enzyme distributed in a polarized manner in a restricted domain of the plasma membrane but also it creates the ionic gradients that drive the net movement of glucose, amino acids, and ions across the entire epithelium. In a previous work, we have shown that Na⁺,K⁺-ATPase polarity depends on interactions between the β subunits of Na⁺,K⁺-ATPases located on neighboring cells and that these interactions anchor the entire enzyme at the borders of the intercellular space. In the present study, we used fluorescence resonance energy transfer and coprecipitation methods to demonstrate that these β subunits have sufficient proximity and affinity to permit a direct interaction, without requiring any additional extracellular molecules to span the distance.

Two-step adhesive binding by classical cadherins

Crystal structures of classical cadherins have revealed two dimeric configurations. In the first, N-terminal beta-strands of EC1 domains 'swap' between partner molecules. The second configuration (the 'X dimer'), also observed for T-cadherin, is mediated by residues near the EC1-EC2 calcium binding sites, and N-terminal beta-strands of partner EC1 domains, though held adjacent, do not swap. Here we show that strand-swapping mutants of type I and II classical cadherins form X dimers. Mutant cadherins impaired for X-dimer formation show no binding in short-time frame surface plasmon resonance assays, but in long-time frame experiments, they have homophilic binding affinities close to that of wild type. Further experiments show that exchange between monomers and dimers is slowed in these mutants. These results reconcile apparently disparate results from prior structural studies and suggest that X dimers are binding intermediates that facilitate the formation of strand-swapped dimers.

Reelin promotes neuronal orientation and dendritogenesis during preplate splitting

The secreted ligand Reelin is thought to regulate the translocation and positioning of prospective layer 6 (L6) neurons into the preplate, a plexus of neurons overlying the ventricular zone. We examined wild type and Reelin-deficient cortices and found that L6 neurons were equivalently positioned beneath the pia during the period of preplate splitting and initial cortical plate (CP) formation. The absence of detectable L6 ectopia in "reeler" cortices at this developmental time point indicates that Reelin-signaling might not regulate L6 neuron migration or gross positioning during preplate splitting. To explore the acute response of L6 neurons to Reelin, subpial injections of Reelin were made into Reelin-deficient explants. Reelin injection caused L6 neurons to orient their nuclei and polarize their Golgi toward the pia while initiating exuberant dendritic (MAP2+) outgrowth within 4 h. This rapid Reelin-dependent neuronal orientation and alignment created CP-like histology without any significant change in the mean position of the population of L6 neurons. Conversely, subplate cells and chondroitin sulfate proteoglycan immunoreactivity were found at significantly deeper positions from the pial surface after injection, suggesting that Reelin partially rescues preplate splitting within 4 h. Thus, Reelin has a direct role in promoting rapid morphological differentation and orientation of L6 neurons during preplate splitting.

Beyond structure, to survival: activation of Stat3 by cadherin engagement

Cells in normal tissues or in tumors have extensive opportunities for adhesion to their neighbors and the importance of cell to cell contact in the study of fundamental cellular processes is beginning to emerge. In this review, we discuss recent evidence of dramatic changes in the activity of an important signal transducer found to be profoundly affected by cell to cell adhesion, the signal transducer and activator of transcription-3 (Stat3). Direct cadherin engagement, growth of cells to postconfluence, or formation of multicellular aggregates were found to induce a striking increase in the levels of Stat3 activity, Rac1/Cdc42, and members of the IL6 receptor family in different settings. This activation was specific to Stat3, in that the levels of the extracellular signal regulated kinase (Erk1/2), a signal transducer often coordinately activated with Stat3 by a number of growth factors or oncogenes, remained unaffected by cell density. Density-dependent Stat3 activation may play a key role in survival, and could contribute to the establishment of cell polarity. It is clear that at any given time the total Stat3 activity levels in a cell are the sum of the effects of cell to cell adhesion plus the conventional Stat3 activating factors present.

Cathepsin G, a neutrophil protease, induces compact cell-cell adhesion in MCF-7 human breast cancer cells

Cathepsin G is a serine protease secreted by activated neutrophils that play a role in the inflammatory response. Because neutrophils are known to be invading leukocytes in various tumors, their products may influence the characteristics of tumor cells such as the growth state, motility, and the adhesiveness between cells or the extracellular matrix. Here, we demonstrate that cathepsin G induces cell-cell adhesion of MCF-7 human breast cancer cells resulting from the contact inhibition of cell movement on fibronectin but not on type IV collagen. Cathepsin G subsequently induced cell condensation, a very compact cell colony, resulting due to the increased strength of E-cadherin-mediated cell-cell adhesion. Cathepsin G action is protease activity-dependent and was inhibited by the presence of serine protease inhibitors. Cathepsin G promotes E-cadherin/catenin complex formation and Rap1 activation in MCF-7 cells, which reportedly regulates E-cadherin-based cell-cell junctions. Cathepsin G also promotes E-cadherin/protein kinase D1 (PKD1) complex formation, and Go6976, the selective PKD1 inhibitor, suppressed the cathepsin G-induced cell condensation. Our findings provide the first evidence that cathepsin G regulates E-cadherin function, suggesting that cathepsin G has a novel modulatory role against tumor cell-cell adhesion.

Adherens junction: molecular architecture and regulation

The adherens junction (AJ) is an element of the cell-cell junction in which cadherin receptors bridge the neighboring plasma membranes via their homophilic interactions. Cadherins associate with cytoplasmic proteins, called catenins, which in turn bind to cytoskeletal components, such as actin filaments and microtubules. These molecular complexes further interact with other proteins, including signaling molecules, rendering the AJs into highly dynamic and regulatable structures. The AJs of such nature contribute to the physical linking of cells, as well as to the regulation of cell-cell contacts, which is essential for morphogenesis and remodeling of tissues and organs. Thus, elucidating the molecular architecture of the AJs and their regulatory mechanisms are crucial for understanding how the multicellular system is organized.

Effects of N-cadherin overexpression on the adhesion properties of embryonic stem cells

Constitutive overexpression of N-cadherin in mouse embryonic stem cells led to marked changes in the phenotype and adhesion properties of these cells. The changes included the formation of smaller embryonic bodies, elevated mRNA and total protein levels of N-cadherin, and increased amounts of p120 catenin and connexin-43. N-cadherin cells exhibited decreased attachment to non-cell surfaces, while their adhesiveness to each other and to rat neonatal cardiomyocytes was significantly elevated. The findings suggest that N-cadherin overexpression can facilitate electromechanical integration of stem cells into excitable tissues with endogenously high levels of N-cadherin, such as the heart and brain.

Rap1 maintains adhesion between cells to affect Egfr signaling and planar cell polarity in Drosophila

The small GTPase Rap1 affects cell adhesion and cell motility in numerous developmental contexts. Loss of Rap1 in the Drosophila wing epithelium disrupts adherens junction localization, causing mutant cells to disperse, and dramatically alters epithelial cell shape. While the adhesive consequences of Rap1 inactivation have been well described in this system, the effects on cell signaling, cell fate specification, and tissue differentiation are not known. Here we demonstrate that Egfr-dependent cell types are lost from Rap1 mutant tissue as an indirect consequence of DE-cadherin mislocalization. Cells lacking Rap1 in the developing wing and eye are capable of responding to an Egfr signal, indicating that Rap1 is not required for Egfr/Ras/MAPK signal transduction. Instead, Rap1 regulates adhesive contacts necessary for maintenance of Egfr signaling between cells, and differentiation of wing veins and photoreceptors. Rap1 is also necessary for planar cell polarity in these tissues. Wing hair alignment and ommatidial rotation, functional readouts of planar cell polarity in the wing and eye respectively, are both affected in Rap1 mutant tissue. Finally, we show that Rap1 acts through the effector Canoe to regulate these developmental processes.

An adhesion-independent, aPKC-dependent function for cadherins in morphogenetic movements

Cadherin shedding affects migration and occurs in development and cancer progression. By examining the in vivo biological function of the extracellular cadherin domain (CEC1-5) independently of the shedding process itself, we identified a novel function for cadherins in convergent extension (CE) movements in Xenopus. CEC1-5 interfered with CE movements during gastrulation. Unexpectedly, CEC1-5 did not alter cell aggregation or adhesion to cadherin substrates. Instead, gastrulation defects were rescued by a membrane-anchored cadherin cytoplasmic domain, the polarity protein atypical PKC (aPKC) or constitutive active Rac, indicating that CEC1-5 modulates a cadherin-dependent signalling pathway. We found that the cadherin interacts with aPKC and, more importantly, that the extracellular domain alters this association as well as the phosphorylation status of aPKC. This suggests that CE movements require a dynamic regulation of cadherin-aPKC interaction. Our results show that cadherins play a dual role in CE movements: a previously identified adhesive activity and an adhesion-independent function that requires aPKC and Rac, thereby directly connecting cadherins with polarity. Our results also suggest that increased cadherin shedding, often observed in cancer progression, can regulate migration and invasion by modulating polarity protein activity.

Role of E-cadherin in membrane-cortex interaction probed by nanotube extrusion

This study aims to define the role of E-cadherin (Ecad) engagement in cell-cell contact during membrane-cortex interaction. As a tool, we used a hydrodynamic membrane tube extrusion technique to characterize the mechanical interaction between the plasma membrane and the underlying cortical cytoskeleton. Cells were anchored on 4.5 microm beads coated with polylysine (PL) to obtain nonspecific cell adhesion or with an antibody against Ecad to mimic specific Ecad-mediated cell adhesion. We investigated tube length dynamics L(t) over time and through successive extrusions applied to the cell at regular time intervals. A constant slow velocity was observed for the first extrusion, for PL-attached cells. Subsequent extrusions had two phases: an initial high-velocity regime followed by a low-velocity regime. Successive extrusions gradually weakened the binding of the membrane around the tube neck to the underlying cortical cytoskeleton. Cells specifically attached via Ecad first exhibited a very low extrusion velocity regime followed by a faster extrusion regime similar to nonspecific extrusion. This indicates that Ecad strengthens the membrane-cortical cytoskeleton interaction, but only in a restricted area corresponding to the site of contact between the cell and the bead. Occasional giant "cortex" tubes were extruded with specifically anchored cells, demonstrating that the cortex remained tightly bound to the membrane through Ecad-mediated adhesion at the contact site.

Homophilic and heterophilic polycystin 1 interactions regulate E-cadherin recruitment and junction assembly in MDCK cells

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited human renal disease and is caused by mutations in two genes, PKD1 (85%) and PKD2 (15%). Cyst epithelial cells are characterised by a complex cellular phenotype including changes in proliferation, apoptosis, basement membrane composition and apicobasal polarity. Since polycystin 1 (PC1), the PKD1 protein, has been located in the basolateral membrane of kidney epithelial cells, we hypothesised that it might have a key role in mediating or stabilising cell-cell interactions. In non-ciliated L929 cells, stable or transient surface expression of the PC1 extracellular domain was sufficient to confer an adhesive phenotype and stimulate junction formation. In MDCK cells, we found that PC1 was recruited to the lateral membranes coincident with E-cadherin within 30 minutes after a ;calcium switch'. Recruitment of both proteins was significantly delayed when cells were treated with a PC1 blocking antibody raised to the PKD domains. Finally, PC1 and E-cadherin could be coimmunoprecipitated together from MDCK cells. We conclude that PC1 has a key role in initiating junction formation via initial homophilic interactions and facilitates junction assembly and the establishment of apicobasal polarity by E-cadherin recruitment.

Stabilin-2 mediates homophilic cell-cell interactions via its FAS1 domains

Stabilin-2 was recently shown to mediate a heterophilic interaction with integrin alpha M beta 2 via its FAS1 domain. Here, we demonstrate that stabilin-2 also mediates homophilic cell-cell interactions. L cells expressing stabilin-2 mediate a significant level of cell aggregation, and this aggregation is significantly inhibited by anti-stabilin-2 antibody. Stabilin-2-mediated aggregation is mediated by homophilic interactions and enhanced in the presence of Ca(2+) and Mg(2+). Interestingly, exogenous addition of FAS1 domains but not EGF-like domains enhances stabilin-2-mediated cell aggregation, suggesting that exogenous FAS1 domains may form polymeric structure with FAS1 domains of stabilin-2. Together, these data show the participation of stabilin-2 in homophilic cell adhesion and role of FAS1 domains.

Regulatory roles of the cadherin superfamily

Charged with the task of providing a molecular link between adjacent cells, the cadherin superfamily consists of over 100 members and populates the genomes of organisms ranging from vertebrates to cniderians. This breadth hints at what decades of research has confirmed: that cadherin-based adhesion and signaling events regulate diverse cellular processes including cell-sorting, differentiation, cell survival, proliferation, cell polarity, and cytoskeletal organization.

E-cadherin controls beta-catenin and NF-kappaB transcriptional activity in mesenchymal gene expression

E-cadherin and its transcriptional repressor Snail1 (Snai1) are two factors that control epithelial phenotype. Expression of Snail1 promotes the conversion of epithelial cells to mesenchymal cells, and occurs concomitantly with the downregulation of E-cadherin and the upregulation of expression of mesenchymal genes such as those encoding fibronectin and LEF1. We studied the molecular mechanism controlling the expression of these genes in mesenchymal cells. Forced expression of E-cadherin strongly downregulated fibronectin and LEF1 RNA levels, indicating that E-cadherin-sensitive factors are involved in the transcription of these genes. E-cadherin overexpression decreased the transcriptional activity of the fibronectin promoter and reduced the interaction of beta-catenin and NF-kappaB with this promoter. Similar to beta-catenin, NF-kappaB was found, by co-immunoprecipitation and pull-down assays, to be associated with E-cadherin and other cell-adhesion components. Interaction of the NF-kappaB p65 subunit with E-cadherin or beta-catenin was reduced when adherens junctions were disrupted by K-ras overexpression or by E-cadherin depletion using siRNA. These conditions did not affect the association of p65 with the NF-kappaB inhibitor IkappaBalpha. The functional significance of these results was stressed by the stimulation of NF-kappaB transcriptional activity, both basal and TNF-alpha-stimulated, induced by an E-cadherin siRNA. Therefore, these results demonstrate that E-cadherin not only controls the transcriptional activity of beta-catenin but also that of NF-kappaB. They indicate too that binding of this latter factor to the adherens junctional complex prevents the transcription of mesenchymal genes.

Co-operative roles for E-cadherin and N-cadherin during lens vesicle separation and lens epithelial cell survival

The classical cadherins are known to have both adhesive and signaling functions. It has also been proposed that localized regulation of cadherin activity may be important in cell assortment during development. In the context of eye development, it has been suggested that cadherins are important for separation of the invaginated lens vesicle from the surface ectoderm. To test this hypothesis, we conditionally deleted N-cadherin or E-cadherin from the presumptive lens ectoderm of the mouse. Conditional deletion of either cadherin alone did not produce a lens vesicle separation defect. However, these conditional mutants did exhibit common structural deficits, including microphthalmia, severe iris hyperplasia, persistent vacuolization within the fibre cell region, and eventual lens epithelial cell deterioration. To assess the co-operative roles of E-cadherin and N-cadherin within the developing lens, double conditional knockout embryos were generated. These mice displayed distinct defects in lens vesicle separation and persistent expression of another classical cadherin, P-cadherin, within the cells of the persistent lens stalk. Double mutant lenses also exhibited severe defects in lens epithelial cell adhesion and survival. Finally, the severity of the lens phenotype was shown to be sensitive to the number of wild-type E- and N-cadherin alleles. These data suggest that the co-operative expression of both E- and N-cadherin during lens development is essential for normal cell sorting and subsequent lens vesicle separation.

Immunohistochemical observation of co-expression of E- and N-cadherins in rat organogenesis

Cadherins are a family of transmembrane glycoproteins that mediate cell-to-cell adhesion. Isoforms, including E- and N-cadherin, have been identified and shown to regulate morphogenesis through homophilic binding. In the ontogeny, the expressions of E- and N-cadherin change spatiotemporally, and the changes in cadherin isoforms, called cadherin switching, impact the mechanical adhesion of cells. Furthermore, cadherin functions as a receptor that transfers information from outside to inside cells, and in terms of switching, it affects cell phenotypes. To observe the expression patterns of E- and N-cadherins during embryogenesis and to identify cells that transiently coexpress both cadherins, we employed a recently developed immunohistochemical double staining technique in rat fetuses. At embryonic day 9, embryonic ectodermal cells more dominantly expressed E-cadherin, while mesodermal cells more dominantly expressed N-cadherin. At embryonic day 10, the expression pattern of E-cadherin in the surface ectoderm and endoderm and that of N-cadherin in the neuroectoderm were established. After embryonic day 10, unique co-expression of E- and N-cadherin was observed in primordia, such as the bulbus cordis, otic pit, notochord, and Rathke’s pouch. In the present study, it was possible to visualize the expression patterns of E- and N-cadherin during early fetal development, which enabled us to morphologically clarify cadherin switching.

Biophysical properties of cadherin bonds do not predict cell sorting

Differential binding between cadherin subtypes is widely believed to mediate cell sorting during embryogenesis. However, a fundamental unanswered question is whether cell sorting is dictated by the biophysical properties of cadherin bonds, or by broader, cadherin-dependent differences in intercellular adhesion or membrane tension. This report describes atomic force microscope measurements of the strengths and dissociation rates of homophilic and heterophilic cadherin (CAD) bonds. Measurements conducted with chicken N-CAD, canine E-CAD, and Xenopus C-CAD demonstrated that all three cadherins cross-react and form multiple, intermolecular bonds. The mechanical and kinetic properties of the heterophilic bonds are similar to the homophilic interactions. The thus quantified bond parameters, together with previously reported adhesion energies were further compared with in vitro cell aggregation and sorting assays, which are thought to mimic in vivo cell sorting. Trends in quantified biophysical properties of the different cadherin bonds do not correlate with sorting outcomes. These results suggest that cell sorting in vivo and in vitro is not governed solely by biophysical differences between cadherin subtypes.

New insights into cadherin function in epidermal sheet formation and maintenance of tissue integrity

Co-expression and gene linkage have hampered elucidating the physiological relevance of cadherins in mammalian tissues. Here, we combine conditional gene ablation and transgenic RNA interference to uncover new roles for E- and P-cadherins in epidermal sheet formation in vitro and maintenance of epidermal integrity in vivo. By devising skin-specific RNAi technology, we demonstrate that cadherin inhibition in vivo impairs junction formation and intercellular adhesion and increases apoptosis. These defects compromise epidermal barrier function and tissue integrity. In vitro, with only E-cadherin missing, epidermal sheet formation is delayed, but when both cadherins are suppressed, defects extend to adherens junctions, desmosomes, tight junctions and cortical actin dynamics. Using different rescue strategies, we show that cadherin level rather than subtype is critical. Finally, by comparing conditional loss-of-function studies of epidermal catenins and cadherins, we dissect cadherin-dependent and independent roles of adherens junction components in tissue physiology.

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.

The cadherin superfamily in Anopheles gambiae: a comparative study with Drosophila melanogaster

The cadherin superfamily is a diverse and multifunctional group of proteins with extensive representation across genomes of phylogenetically distant species that is involved in cell–cell communication and adhesion. The mosquito Anopheles gambiae is an emerging model organism for the study of innate immunity and host–pathogen interactions, where the malaria parasite induces a profound rearrangement of the actin cytoskeleton at critical stages of infection. We have used bioinformatics tools to retrieve present sequence knowledge about the complete repertoire of cadherins in A. gambiae and compared it to that of the fruit fly, Drosophila melanogaster. In A. gambiae, we have identified 43 genes coding for cadherin extracellular domains that were re-annotated to 38 genes and represent an expansion of this gene family in comparison to other invertebrate organisms. The majority of Drosophila cadherins show a 1 : 1 Anopheles orthologue, but we have observed a remarkable expansion in some groups in A. gambiae, such as N-cadherins, that were recently shown to have a role in the olfactory system of the fruit fly. In vivo dsRNA silencing of overrepresented genes in A. gambiae and other genes showing expression at critical tissues for parasite infection will likely advance our understanding of the problems of host preference and host–pathogen interactions in this mosquito species.