occurs during normal developmental processes to allow cell types to segregate from one another. Tumor cells often recapitulate this activity and the result is an aggressive tumor cell that gains the ability to leave the site of the tumor and metastasize. At present, we understand some of the mechanisms that promote cadherin switching and some of the pathways downstream of this process that influence cell behavior. Specific cadherin family members influence growthfactor-receptor signaling and Rho GTPases to promote cell motility and invasion. In addition, p120-catenin probably plays multiple roles in cadherin switching, regulating Rho GTPases and stabilizing cadherins. Journal of Cell Science 728cadherin expression has been shown to promote motility and invasion (Hazan et al., 2000; Islam et al., 1996;Nieman et al., 1999). This loss of E-cadherin expression and gain of N-cadherin expression is reminiscent of the cadherin switching that is seen during normal embryonic development and probably underpins many of the phenotypic changes that occur in the participating cells (reviewed in Cavallaro et al., 2002; Christofori, 2003; Gerhart et al., 2004).The term cadherin switching usually refers to a switch from expression of E-cadherin to expression of N-cadherin, but also includes situations in which E-cadherin expression levels do not change significantly but the cells turn on (or increase) expression of N-cadherin. It also includes examples in which other cadherins replace or are co-expressed with E-cadherin, including R-cadherin, cadherin 11, T-cadherin and even P-cadherin, and the expression of the 'inappropriate cadherin' might alter the behavior of the tumor cells (Derycke and Bracke, 2004;Nakajima et al., 2004;Paredes et al., 2005;Patel, I. et al., 2003;Riou et al., 2006;Stefansson et al., 2004;Taniuchi et al., 2005;Tomita et al., 2000). It has even been reported that E-cadherin can influence tumorigenesis in tissues that do not normally express this cadherin. For example, ovarian surface epithelium normally expresses N-cadherin. However, during progression to the neoplastic state, the cells show decreased N-cadherin expression and increased E-cadherin and Pcadherin expression; the E-cadherin might play a role in the initiation of the aberrant differentiation that characterizes ovarian carcinogenesis (Patel, I. et al., 2003;Wong et al., 1999;Wu et al., 2007). Table 1 presents examples of cadherin switching that have been reported during normal developmental processes and during tumorigenesis.One role of cadherin switching is to allow a select population of cells to separate from their neighbors -for example, during processes such as gastrulation, epiblast cell ingression through the primitive streak and neural crest emigration from the neural tube (Edelman et al., 1983; Hatta and Takeichi, 1986;Takeichi, 1988;Takeichi et al., 2000). It is well known that cells expressing different cadherins segregate from one another in in vitro aggregation assays (Nose et al., 1988;Steinberg and Takeichi, 1994) and it is easy...

E-cadherin is a transmembrane glycoprotein that mediates calcium-dependent, homotypic cell–cell adhesion and plays a role in maintaining the normal phenotype of epithelial cells. Decreased expression of E-cadherin has been correlated with increased invasiveness of breast cancer. In other systems, inappropriate expression of a nonepithelial cadherin, such as N-cadherin, by an epithelial cell has been shown to downregulate E-cadherin expression and to contribute to a scattered phenotype. In this study, we explored the possibility that expression of nonepithelial cadherins may be correlated with increased motility and invasion in breast cancer cells. We show that N-cadherin promotes motility and invasion; that decreased expression of E-cadherin does not necessarily correlate with motility or invasion; that N-cadherin expression correlates both with invasion and motility, and likely plays a direct role in promoting motility; that forced expression of E-cadherin in invasive, N-cadherin–positive cells does not reduce their motility or invasive capacity; that forced expression of N-cadherin in noninvasive, E-cadherin–positive cells produces an invasive cell, even though these cells continue to express high levels of E-cadherin; that N-cadherin–dependent motility may be mediated by FGF receptor signaling; and that cadherin-11 promotes epithelial cell motility in a manner similar to N-cadherin.

p120ctn is a catenin whose direct binding to the juxtamembrane domain of classical cadherins suggests a role in regulating cell–cell adhesion. The juxtamembrane domain has been implicated in a variety of roles including cadherin clustering, cell motility, and neuronal outgrowth, raising the possibility that p120 mediates these activities. We have generated minimal mutations in this region that uncouple the E-cadherin–p120 interaction, but do not affect interactions with other catenins. By stable transfection into E-cadherin–deficient cell lines, we show that cadherins are both necessary and sufficient for recruitment of p120 to junctions. Detergent-free subcellular fractionation studies indicated that, in contrast to previous reports, the stoichiometry of the interaction is extremely high. Unlike α- and β-catenins, p120 was metabolically stable in cadherin-deficient cells, and was present at high levels in the cytoplasm. Analysis of cells expressing E-cadherin mutant constructs indicated that p120 is required for the E-cadherin–mediated transition from weak to strong adhesion. In aggregation assays, cells expressing p120-uncoupled E-cadherin formed only weak cell aggregates, which immediately dispersed into single cells upon pipetting. As an apparent consequence, the actin cytoskeleton failed to insert properly into peripheral E-cadherin plaques, resulting in the inability to form a continuous circumferential ring around cell colonies. Our data suggest that p120 directly or indirectly regulates the E-cadherin–mediated transition to tight cell–cell adhesion, possibly blocking subsequent events necessary for reorganization of the actin cytoskeleton and compaction.

Cadherins are transmembrane glycoproteins that mediate calcium-dependent cell-cell adhesion. The cadherin family is large and diverse, and proteins are considered to be members of this family if they have one or more cadherin repeats in their extracellular domain. Cadherin family members are the transmembrane components of a number of cellular junctions, including adherens junctions, desmosomes, cardiac junctions, endothelial junctions, and synaptic junctions. Cadherin function is critical in normal development, and alterations in cadherin function have been implicated in tumorigenesis. The strength of cadherin interactions can be regulated by a number of proteins, including the catenins, which serve to link the cadherin to the cytoskeleton. Cadherins have been implicated in a number of signaling pathways that regulate cellular behavior, and it is becoming increasingly clear that integration of information received from cell-cell signaling, cell-matrix signaling, and growth factor signaling determines ultimate cellular phenotype and behavior.

Abstract. Cadherins are CaZ+-dependent, cell surface glycoproteins involved in cell--cell adhesion. Extracellularly, transmembrane cadherins such as E-, P-, and N-cadherin self-associate, while intracellularly they interact indirectly with the actin-based cytoskeleton. Several intraceUular proteins termed catenins, including a-catenin, B-catenin, and plakoglobin, are tightly associated with these cadherins and serve to link them to the cytoskeleton. Here, we present evidence that in fibroblasts a-actinin, but not vinculin, colocalizes extensively with the N-cadherin/catenin complex. This is in contrast to epithelial cells where both cytoskeletal proteins colocalize extensively with E-cadherin and catenins. We further show that a-actinin, but not vinculin, co-immunoprecipitates specifically with a-and ~3-catenin from N-and E-cadherin--expressing cells, but only if a-catenin is present. Moreover, we show that a-actinin coimmunoprecipitates with the N-cadherin/catenin complex in an actin-independent manner. We therefore propose that cadherin/catenin complexes are linked to the actin cytoskeleton via a direct association between a-actinin and a-catenin.

Abstract. Molecular mechanisms linking pre-and postsynaptic membranes at the interneuronal synapses are little known. We tested the cadherin adhesion system for its localization in synapses of mouse and chick brains. We found that two classes of cadherin-associated proteins, aN-and 13-catenin, are broadly distributed in adult brains, colocalizing with a synaptic marker, synaptophysin. At the ultrastructural level, these proteins were localized in synaptic junctions of various types, forming a symmetrical adhesion structure. These structures sharply bordered the transmitter release sites associated with synaptic vesicles, although their segregation was less clear in certain types of synapses. N-cadherin was also localized at a similar site of synaptic junctions but in restricted brain nuclei. In developing synapses, the catenin-bearing contacts dominated their junctional structures. These findings demonstrate that interneuronal synaptic junctions comprise two subdomains, transmitter release zone and cateninbased adherens junction. The catenins localized in these junctions are likely associated with certain cadherin molecules including N-cadherin, and the cadherin/catenin complex may play a critical role in the formation or maintenance of synaptic junctions.T HE synapse is a site where the axon terminal of a neuron comes into functional contact with a target cell. To generate the synapses, each neuron selectively contacts and communicates with other particular neurons. Despite the importance of understanding how specific neuronal connections are established, little is known about what kinds of adhesion molecules are essential for the formation and maintenance of interneuronal synaptic junctions. This lack of information is contrasted with a great deal of knowledge about other synaptic structures and functions, including synaptic vesicle traffic, channel/ receptor function, and signal transduction in synapses (Jessell and Kandel, 1993; Stidhof, 1995).Various electron microscopic studies have revealed the ultrastructural features of interneuronal synaptic junctions. Electron-dense cytoplasmic materials accumulate under both pre-and postsynaptic plasma membranes; and the former is, in general, associated with a cluster of synaptic vesicles. These junctional structures are considered to be the sites where synaptic transmission takes place. The interneuronal synaptic junctions exhibit some variations in morphological characteristics of the cytoplasmic density. Gray (1959) and Colonnier (1968) showed that there are two types of synapses, asymmetric (Gray's type I) and symmetric (Gray's type II). The intercellular space beAddress all correspondence to Masatoshi Takeichi, Department of Biophysics, Faculty of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-01, Japan. Tel.: (81) 75 751 2111. Fax: (81) 75 753 4197. e-maih takeichi@take.biophys.kyoto-u.ac.jp. tween the membrane thickenings, called the synaptic cleft, is occupied by filamentous materials.Molecular analysis of the synaptic junctions has thus far b...

Epithelium-to-mesenchyme transitions (EMTs) are characterized by morphological and behavioral changes in cells. During an EMT, E-cadherin is downregulated while N-cadherin is upregulated. The goal of this study was to understand the role cadherin switching plays in EMT using a classical model system: transforming growth factor β1 (TGF-β1)-mediated EMT in mammary epithelial cells. We showed that stress fibers and focal adhesions are increased, and cell-cell junctions are decreased in response to TGF-β1. Moreover, these changes were reversible upon removal of TGF-β1. Downregulation of E-cadherin and upregulation of N-cadherin were both transcriptional. Neither experimental knockdown nor experimental overexpression of N-cadherin interfered with the morphological changes. In addition, the morphological changes associated with EMT preceded the downregulation of E-cadherin. Interestingly, TGF-β1-induced motility in N-cadherin-knockdown cells was significantly reduced. Together, these data suggest that cadherin switching is necessary for increased motility but is not required for the morphological changes that accompany EMT.

pl20"S is a tyrosine kinase substrate implicated in ligand-induced receptor signaling through the epidermal growth factor, platelet-derived growth factor, and colony-stimulating factor receptors and in cell transformation by Src. Here we report that p120 associates with a complex containing E-cadherin, a-catenin, 0-catenin, and plakoglobin. Furthermore, p120 precisely colocalizes with E-cadherin and catenins in vivo in both normal and Src-transformed MDCK cells. Unlike IB-catenin and plakoglobin, p120 has at least four isoforms which are differentially expressed in a variety of cell types, suggesting novel means of modulating cadherin activities in cells. In Src-transformed MDCK cells, p120, IB-catenin, and plakoglobin were heavily phosphorylated on tyrosine, but the physical associations between these proteins were not disrupted. Association of p120 with the cadherin machinery indicates that both Src and receptor tyrosine kinases cross talk with proteins important for cadherin-mediated cell adhesion. These results also strongly suggest a role for p120 in cell adhesion. p120 is a major Src substrate whose phosphorylation on tyrosine correlates with cell transformation (41). Unlike most Src substrates, p120 is not phosphorylated by nonmyristylated Src mutants which cannot phosphorylate critical membraneassociated substrates necessary for cell transformation. In addition, p120 is tyrosine phosphorylated in response to ligand-induced stimulation of several receptor tyrosine kinases, including those for epidermal growth factor, platelet-derived growth factor, and colony-stimulating factor 1 (6, 17). Although these observations suggest that p120 plays a role in cell transformation and ligand-induced signaling, the predicted amino acid sequence of p120 lacks motifs (e.g., SH2 and SH3 domains) that might suggest its participation in mitogenic signaling.One striking feature of the p120 sequence is the presence of an Arm domain (36, 40) comprising 11 copies of a 42-aminoacid motif originally described for the Drosophila segment polarity gene product, armadillo (38,42). The function of the Arm domain is unknown, but armadillo's vertebrate homologs, 0-catenin and plakoglobin, bind to the cytoplasmic tail of E-cadherin, an interaction essential for cadherin-mediated cell-cell adhesion (29,33). The cadherins are a family of transmembrane glycoproteins that connect cells by Ca2+-dependent homophilic interactions between their extracellular domains (for a review, see reference 51). Their intracellular cytoplasmic segments are thought to anchor cadherins to the actin cytoskeleton through a-catenin, 0-catenin, and -y-catenin (plakoglobin). a-Catenin probably mediates the interaction with actin, because it is partially homologous to the actinbinding protein vinculin (12,30), and its presence in complexes with cadherin correlates with the ability of these complexes to bind to DNase I actin-binding columns (33). Critical roles for P-catenin and plakoglobin in adhesion are largely inferred,