A short core region of E-cadherin is essential for catenin binding and is highly phosphorylated

Germline mutations in E-cadherin do not explain association of hereditary prostate cancer, gastric cancer and breast cancer

Somatic mutations in the E-cadherin (CDH1) gene have frequently been reported in cases with diffuse gastric and lobular breast cancers. Recently, germline mutations have been identified in families with diffuse gastric cancers. In families with hereditary prostate cancer (HPC), a significant association of prostate cancer, gastric and/or breast cancer has been observed in epidemiological studies. The aim of this study was to investigate if germline mutations in CDH1 could explain the risk for cancer in HPC families with an excess of gastric and breast cancer. In total, 17 members from 13 HPC families and 3 members from 3 families with hereditary gastric cancer (HGC) were screened for germline CDH1 sequence alterations using PCR/Denaturing HPLC for initial screening of nucleotide variants followed by confirmatory direct sequencing analysis. The frequency of identified novel germline mutations were tested for in 136 cases with hereditary prostate cancer and 215 cases of sporadic prostate cancer with 422 age matched controls in an allelic discrimination assay. In total, 8 sequence variants were detected in 20 samples tested. In the HPC families, we found 2 missense mutations, A592T in exon 12 and a novel D777N in exon 15 and a mutation in intron 5, 687+92T>A. A previously known polymorphism in exon 13 and 3 sequence variations in introns and untranslated regions were also found, of which the significance is unknown. In HGC-023 with early onset diffuse gastric cancer a truncating mutation, R335X, was identified in exon 7. None of the missense mutations or 687+92T>A were found in the extended HPC material or in the sporadic prostate cancer cases with age-matched controls in the allelic discrimination assay. We found several germline mutations of unknown clinical significance in the CDH1 gene that probably do not explain the association of prostate, gastric and/or breast cancers in the HPC-families. Two missense mutations and a mutation in intron 5 were identified that do not influence the risk of hereditary or sporadic prostate cancer in general and are considered to be pedigree specific. In a family with hereditary gastric cancer of the diffuse type, we identified the first truncating germline mutation in a Scandinavian family.

Axial protocadherin is a mediator of prenotochord cell sorting in Xenopus

Prenotochord cell sorting is regarded as one of the first cell sorting events in early chordate development. We recently demonstrated that this sorting event occurs in vitro, although the mediator of this activity remains unidentified. Herein, we report the isolation of a full-length cDNA clone of Axial protocadherin (AXPC), the homologue of human protocadherin-1 (PCD1). AXPC encodes a transmembrane protein (AXPC) that is expressed exclusively in the notochord at the neurula stage and in the pronephros, somites, heart, optic vesicle, otic vesicle, and distinct parts of the brain at the tailbud stage. Cell dissociation and reaggregation assays and in vivo microinjection experiments demonstrated that cells overexpressing a membrane-tethered form of AXPC (MT-AXPC) acquired the same adhesive properties as prenotochord cells. Moreover, microinjection of either mRNA encoding the dominant negative form of AXPC (DN-AXPC) or morpholino oligonucleotides interferes with the sorting activity of prenotochord cells and normal axis formation. This study suggests that AXPC is necessary and sufficient for prenotochord cell sorting in the gastrulating embryo, and may also mediate sorting events later in development.

The cytoplasmic face of cell contact sites

The cytoplasmic face of cell contact sites comprises large macromolecular assemblies that link transmembrane cell adhesion molecules to the cytoskeleton. These assemblies are dynamic structures that are the targets of regulatory signals that control cell adhesiveness. Recent studies of the biochemistry and structure of the cadherin-catenin complex, vinculin and proteins of the ezrin/radixin/moesin family have begun to reveal the architecture of these assemblies and the mechanisms that are involved in their regulation.

Cell Junction Dynamics in the Testis: Sertoli-Germ Cell Interactions and Male Contraceptive Development

Spermatogenesis is an intriguing but complicated biological process. However, many studies since the 1960s have focused either on the hormonal events of the hypothalamus-pituitary-testicular axis or morphological events that take place in the seminiferous epithelium. Recent advances in biochemistry, cell biology, and molecular biology have shifted attention to understanding some of the key events that regulate spermatogenesis, such as germ cell apoptosis, cell cycle regulation, Sertoli-germ cell communication, and junction dynamics. In this review, we discuss the physiology and biology of junction dynamics in the testis, in particular how these events affect interactions of Sertoli and germ cells in the seminiferous epithelium behind the blood-testis barrier. We also discuss how these events regulate the opening and closing of the blood-testis barrier to permit the timely passage of preleptotene and leptotene spermatocytes across the blood-testis barrier. This is physiologically important since developing germ cells must translocate across the blood-testis barrier as well as traverse the seminiferous epithelium during their development. We also discuss several available in vitro and in vivo models that can be used to study Sertoli-germ cell anchoring junctions and Sertoli-Sertoli tight junctions. An in-depth survey in this subject has also identified several potential targets to be tackled to perturb spermatogenesis, which will likely lead to the development of novel male contraceptives.

Distinct regions of the cadherin cytoplasmic domain are essential for functional interaction with Galpha 12 and beta-catenin

Heterotrimeric G proteins of the G(12) subfamily mediate cellular signals leading to events such as cytoskeletal rearrangements, cell proliferation, and oncogenic transformation. Several recent studies have revealed direct effector proteins through which G(12) subfamily members may transmit signals leading to various cellular responses. Our laboratory recently demonstrated that Galpha(12) and Galpha(13) specifically interact with the cytoplasmic domains of several members of the cadherin family of cell adhesion molecules (Meigs, T. E., Fields, T. A., McKee, D. D., and Casey, P. J. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 519-524). This interaction causes beta-catenin to release from cadherin and relocalize to the cytoplasm and nucleus, where it participates in transcriptional activation. Here we report that two distinct regions of the epithelial cadherin (E-cadherin) tail are required for interaction with beta-catenin and Galpha(12), respectively. Deletion of an acidic, 19-amino acid region of E-cadherin abolishes its ability to bind beta-catenin in vitro, to inhibit beta-catenin-mediated transactivation, or to stabilize beta-catenin; causes subcellular mislocalization of beta-catenin; and disrupts cadherin-mediated cell adhesion. On the other hand, deletion of a distinct 11-amino acid region of E-cadherin dramatically attenuates interaction with Galpha(12); furthermore, Galpha(12) is ineffective in stimulating beta-catenin release from an E-cadherin cytoplasmic domain lacking this putative Galpha(12)-binding region. These findings indicate that Galpha(12) and beta-catenin do not compete for the same binding site on cadherin and provide molecular targets for selectively disrupting the interaction of these proteins with cadherin.

Adhesion signaling: how beta-catenin interacts with its partners

The multi-functional protein beta-catenin plays essential roles in cell-cell adhesion and nuclear signaling. Elucidation of the structures of beta-catenin complexes is beginning to clarify how beta-catenin uses the same surface to bind its various partners, and provides insights into how these interactions might be regulated.

Expression of alpha-catenin in alpha-catenin-deficient cells increases resistance to sphingosine-induced apoptosis

Alpha-catenin, an intracellular protein, associates with the COOH-terminal region of cadherin cell adhesion molecules through interactions with either beta-catenin or gamma-catenin (plakoglobin). The full activity of cadherins requires a linkage to the actin cytoskeleton mediated by catenins. We transfected alpha-catenin-deficient colon carcinoma cells with a series of alpha-catenin constructs to determine that alpha-catenin expression increases the resistance to apoptosis induced by sphingosine. Two groups of constructs, containing deletions in either the middle segment of the molecule or the COOH terminus, induced morphological changes, cell compaction, and decreases in cell death. In alpha-catenin-expressing cells, inhibition of cadherin cell adhesion by treatment with anti-E-cadherin antibodies did not decrease the cells viability. alpha-Catenin expression partially suppressed the downregulation of Bcl-xL and the activation of caspase 3. Expression of p27kip1 protein, an inhibitor of cyclin-dependent kinases, was increased by alpha-catenin expression in low density cell cultures. The increased levels of p27kip1 correlated with both increased resistance to cell death and morphological changes in transfectants containing deletion mutants. Transfection-mediated upregulation of p27kip1 decreases sphingosine-induced cell death in alpha-catenin-deficient cells. We postulate that alpha-catenin mediates transduction of signals from the cadherin-catenin complex to regulate the apoptotic cascade via p27kip1.

Identification of Src phosphorylation sites in the catenin p120ctn

p120-catenin (p120(ctn)) interacts with the cytoplasmic tail of cadherins and is thought to regulate cadherin clustering during formation of adherens junctions. Several observations suggest that p120 can both positively and negatively regulate cadherin adhesiveness depending on signals that so far remain unidentified. Although p120 tyrosine phosphorylation is a leading candidate, the role of this modification in normal and Src-transformed cells remains unknown. Here, as a first step toward pinpointing this role, we have employed two-dimensional tryptic mapping to directly identify the major sites of Src-induced p120 phosphorylation. Eight sites were identified by direct mutation of candidate tyrosines to phenylalanine and elimination of the accompanying spots on the two-dimensional maps. Identical sites were observed in vitro and in vivo, strongly suggesting that the physiologically important sites have been correctly identified. Changing all of these sites to phenylalanine resulted in a p120 mutant, p120-8F, that could not be efficiently phosphorylated by Src and failed to interact with SHP-1, a tyrosine phosphatase shown previously to interact selectively with tyrosine-phosphorylated p120 in cells stimulated with epidermal growth factor. Using selected tyrosine to phenylalanine p120 mutants as dominant negative reagents, it may now be possible to selectively block events postulated to be dependent on p120 tyrosine phosphorylation.

A dileucine motif targets E-cadherin to the basolateral cell surface in Madin-Darby canine kidney and LLC-PK1 epithelial cells

E-cadherin is a major adherens junction protein of epithelial cells, with a central role in cell-cell adhesion and cell polarity. Newly synthesized E-cadherin is targeted to the basolateral cell surface. We analyzed targeting information in the cytoplasmic tail of E-cadherin by utilizing chimeras of E-cadherin fused to the ectodomain of the interleukin-2alpha (IL-2alpha) receptor expressed in Madin-Darby canine kidney and LLC-PK(1) epithelial cells. Chimeras containing the full-length or membrane-proximal half of the E-cadherin cytoplasmic tail were correctly targeted to the basolateral domain. Sequence analysis of the membrane-proximal tail region revealed the presence of a highly conserved dileucine motif, which was analyzed as a putative targeting signal by mutagenesis. Elimination of this motif resulted in the loss of Tac/E-cadherin basolateral localization, pinpointing this dileucine signal as being both necessary and sufficient for basolateral targeting of E-cadherin. Truncation mutants unable to bind beta-catenin were correctly targeted, showing, contrary to current understanding, that beta-catenin is not required for basolateral trafficking. Our results also provide evidence that dileucine-mediated targeting is maintained in LLC-PK(1) cells despite the altered polarity of basolateral proteins with tyrosine-based signals in this cell line. These results provide the first direct insights into how E-cadherin is targeted to the basolateral membrane.

The structure of the beta-catenin/E-cadherin complex and the molecular basis of diverse ligand recognition by beta-catenin

As a component of adherens junctions and the Wnt signaling pathway, beta-catenin binds cadherins, Tcf family transcription factors, and the tumor suppressor APC. We have determined the crystal structures of both unphosphorylated and phosphorylated E-cadherin cytoplasmic domain complexed with the arm repeat region of beta-catenin. The interaction spans all 12 arm repeats, and features quasi-independent binding regions that include helices which interact with both ends of the arm repeat domain and an extended stretch of 14 residues which closely resembles a portion of XTcf-3. Phosphorylation of E-cadherin results in interactions with a hydrophobic patch of beta-catenin that mimics the binding of an amphipathic XTcf-3 helix. APC contains sequences homologous to the phosphorylated region of cadherin, and is likely to bind similarly.

E-cadherin suppresses cellular transformation by inhibiting beta-catenin signaling in an adhesion-independent manner

E-cadherin is a tumor suppressor protein with a well-established role in cell-cell adhesion. Adhesion could contribute to tumor suppression either by physically joining cells or by facilitating other juxtacrine signaling events. Alternatively, E-cadherin tumor suppressor activity could result from binding and antagonizing the nuclear signaling function of beta-catenin, a known proto-oncogene. To distinguish between an adhesion- versus a beta-catenin signaling-dependent mechanism, chimeric cadherin constructs were expressed in the SW480 colorectal tumor cell line. Expression of wild-type E-cadherin significantly inhibits the growth of this cell line. Growth inhibitory activity is retained by all constructs that have the beta-catenin binding region of the cytoplasmic domain but not by E-cadherin constructs that exhibit adhesive activity, but lack the beta-catenin binding region. This growth suppression correlates with a reduction in beta-catenin/T cell factor (TCF) reporter gene activity. Importantly, direct inhibition of beta-catenin/TCF signaling inhibits the growth of SW480 cells, and the growth inhibitory activity of E-cadherin is rescued by constitutively activated forms of TCF. Thus, the growth suppressor activity of E-cadherin is adhesion independent and results from an inhibition of the beta-catenin/TCF signaling pathway, suggesting that loss of E-cadherin expression can contribute to upregulation of this pathway in human cancers. E-cadherin-mediated growth suppression was not accompanied by overall depletion of beta-catenin from the cytosol and nucleus. This appears to be due to the existence of a large pool of cytosolic beta-catenin in SW480 cells that is refractory to both cadherin binding and TCF binding. Thus, a small pool of beta-catenin that can bind TCF (i.e., the transcriptionally active pool) can be selectively depleted by E-cadherin expression. The existence of functionally distinct pools of cytosolic beta-catenin suggests that there are mechanisms to regulate beta-catenin signaling in addition to controlling its level of accumulation.

Cadherin sequences that inhibit beta-catenin signaling: a study in yeast and mammalian cells

Drosophila Armadillo and its mammalian homologue beta-catenin are scaffolding proteins involved in the assembly of multiprotein complexes with diverse biological roles. They mediate adherens junction assembly, thus determining tissue architecture, and also transduce Wnt/Wingless intercellular signals, which regulate embryonic cell fates and, if inappropriately activated, contribute to tumorigenesis. To learn more about Armadillo/beta-catenin's scaffolding function, we examined in detail its interaction with one of its protein targets, cadherin. We utilized two assay systems: the yeast two-hybrid system to study cadherin binding in the absence of Armadillo/beta-catenin's other protein partners, and mammalian cells where interactions were assessed in their presence. We found that segments of the cadherin cytoplasmic tail as small as 23 amino acids bind Armadillo or beta-catenin in yeast, whereas a slightly longer region is required for binding in mammalian cells. We used mutagenesis to identify critical amino acids required for cadherin interaction with Armadillo/beta-catenin. Expression of such short cadherin sequences in mammalian cells did not affect adherens junctions but effectively inhibited beta-catenin-mediated signaling. This suggests that the interaction between beta-catenin and T cell factor family transcription factors is a sensitive target for disruption, making the use of analogues of these cadherin derivatives a potentially useful means to suppress tumor progression.

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.

Essential tyrosine residues for interaction of the non-receptor protein-tyrosine phosphatase PTP1B with N-cadherin

Expression of a dominant-negative, catalytically inactive form of the nonreceptor protein-tyrosine phosphatase PTP1B in L-cells constitutively expressing N-cadherin results in loss of N-cadherin-mediated cell-cell adhesion. PTP1B interacts directly with the cytoplasmic domain of N-cadherin, and this association is regulated by phosphorylation of tyrosine residues in PTP1B. The following three tyrosine residues in PTP1B are potential substrates for tyrosine kinases: Tyr-66, Tyr-152, and Tyr-153. To determine the tyrosine residue(s) that are crucial for the cadherin-PTP1B interaction we used site-directed mutagenesis to create catalytically inactive PTP1B constructs bearing additional single, double, or triple mutations in which tyrosine was substituted by phenylalanine. Mutation Y152F eliminates binding to N-cadherin in vitro, whereas mutations Y66F and Y153F do not. Overexpression of the catalytically inactive PTP1B with the Y152F mutation in L-cells constitutively expressing N-cadherin has no effect on N-cadherin-mediated adhesion, and immunoprecipitation reveals that the mutant Y152F PTP1B does not associate with N-cadherin in situ. Furthermore, among cells overexpressing the Y152F mutant endogenous PTP1B associates with N-cadherin and is tyrosine-phosphorylated.

Intestinal cell adhesion molecules. Liver-intestine cadherin

The cadherin superfamily comprises a large number of cell adhesion molecules, several of which are expressed in the gastrointestinal tract. LI-cadherin represents a novel type of cadherin within the cadherin superfamily distinguished from other cadherins by structural and functional features described in this review. In the mouse and human, LI-cadherin is selectively expressed on the basolateral surface of enterocytes and goblet cells in the small and large intestine, whereas in the rat this cadherin is additionally detectable in hepatocytes. LI-cadherin is capable of mediating Ca(2+)-dependent homophilic cell-cell adhesion independent of interactions with the cytoskeleton, indicating that the adhesive function of this novel cadherin is complementary to that of E-cadherin and desmosomal cadherins co expressed in the intestinal mucosa.

p120 catenin affects cell motility via modulation of activity of Rho-family GTPases: a link between cell-cell contact formation and regulation of cell locomotion

The molecular basis for contact inhibition of cell locomotion is still largely unknown. Cadherins, the major receptors mediating cell-cell adhesion, associate in the cytoplasm with armadillo family proteins, including beta- and gamma-catenin and p120 catenin (p120ctn). E-cadherin-mediated contact formation was shown to inhibit cellular motility. We examine whether p120ctn may have a role in this regulation. We show here that overexpression of p120ctn in fibroblasts and epithelial cells induces pronounced changes in cell shape, motility and adhesion to the extracellular matrix. p120ctn-transfected cells display increased filopodial/lamellipodial activity, decreased contractility and focal adhesion formation, and augmented migratory ability. These effects of p120ctn are mediated by small GTPases of the Rho family. Direct assessment of the activity of these GTPases in cells expressing a 5-fold higher level of p120ctn as compared to non-transfected control cells revealed significant augmentation of Cdc42 and Rac activity. Moreover, co-transfection of p120ctn with dominant-negative Cdc42 and Rac, or constitutively active Rho suppressed morphological effects of p120ctn. Confocal immunofluorescence visualization of the distribution of endogenous p120ctn in dense cultures showed that formation of cadherin-mediated cell-cell contacts is accompanied by sequestering of p120ctn to the junction regions. In sparse cultures p120ctn is distributed over the cytoplasm. Co-transfection with an excess of E-cadherin leads to sequestration of exogenous p120ctn to cell-cell junctions or to small cadherin-containing vesicles, and abolishes p120ctn effects on cell morphology. Thus, p120ctn may couple the formation and disruption of cadherin-mediated contacts with regulation of cell motility by triggering pathway(s) affecting Rho family GTPases.

N-Cadherin extracellular repeat 4 mediates epithelial to mesenchymal transition and increased motility

E- and N-cadherin are members of the classical cadherin family of proteins. E-cadherin plays an important role in maintaining the normal phenotype of epithelial cells. Previous studies from our laboratory and other laboratories have shown that inappropriate expression of N-cadherin by tumor cells derived from epithelial tissue results in conversion of the cell to a more fibroblast-like cell, with increased motility and invasion. Our present study was designed to determine which domains of N-cadherin make it different from E-cadherin, with respect to altering cellular behavior, such as which domains are responsible for the epithelial to mesenchymal transition and increased cell motility and invasion. To address this question, we constructed chimeric cadherins comprised of selected domains of E- and N-cadherin. The chimeras were transfected into epithelial cells to determine their effect on cell morphology and cellular behavior. We found that a 69-amino acid portion of EC-4 of N-cadherin was necessary and sufficient to promote both an epithelial to mesenchymal transition in squamous epithelial cells and increased cell motility. Here, we show that different cadherin family members promote different cellular behaviors. In addition, we identify a novel activity that can be ascribed to the extracellular domain of N-cadherin.

Crystal structure of a beta-catenin/Tcf complex

The Wnt signaling pathway plays critical roles in embryonic development and tumorigenesis. Stimulation of the Wnt pathway results in the accumulation of a nuclear beta-catenin/Tcf complex, activating Wnt target genes. A crystal structure of beta-catenin bound to the beta-catenin binding domain of Tcf3 (Tcf3-CBD) has been determined. The Tcf3-CBD forms an elongated structure with three binding modules that runs antiparallel to beta-catenin along the positively charged groove formed by the armadillo repeats. Structure-based mutagenesis defines three sites in beta-catenin that are critical for binding the Tcf3-CBD and are differentially involved in binding APC, cadherin, and Axin. The structural and mutagenesis data reveal a potential target for molecular drug design studies.

Vezatin, a novel transmembrane protein, bridges myosin VIIA to the cadherin-catenins complex

Defects in myosin VIIA are responsible for deafness in the human and mouse. The role of this unconventional myosin in the sensory hair cells of the inner ear is not yet understood. Here we show that the C-terminal FERM domain of myosin VIIA binds to a novel transmembrane protein, vezatin, which we identified by a yeast two-hybrid screen. Vezatin is a ubiquitous protein of adherens cell-cell junctions, where it interacts with both myosin VIIA and the cadherin-catenins complex. Its recruitment to adherens junctions implicates the C-terminal region of alpha-catenin. Taken together, these data suggest that myosin VIIA, anchored by vezatin to the cadherin-catenins complex, creates a tension force between adherens junctions and the actin cytoskeleton that is expected to strengthen cell-cell adhesion. In the inner ear sensory hair cells vezatin is, in addition, concentrated at another membrane-membrane interaction site, namely at the fibrillar links interconnecting the bases of adjacent stereocilia. In myosin VIIA-defective mutants, inactivity of the vezatin-myosin VIIA complex at both sites could account for splaying out of the hair cell stereocilia.

Characterization of three novel human cadherin genes (CDH7, CDH19, and CDH20) clustered on chromosome 18q22-q23 and with high homology to chicken cadherin-7

Full-length coding sequences of two novel human cadherin cDNAs were obtained by sequence analysis of several EST clones and 5' and 3' rapid amplification of cDNA ends (RACE) products. Exons for a third cDNA sequence were identified in a public-domain human genomic sequence, and the coding sequence was completed by 3' RACE. One of the sequences (CDH7L1, HGMW-approved gene symbol CDH7) is so similar to chicken cadherin-7 gene that we consider it to be the human orthologue. In contrast, the published partial sequence of human cadherin-7 is identical to our second cadherin sequence (CDH7L2), for which we propose CDH19 as the new name. The third sequence (CDH7L3, HGMW-approved gene symbol CDH20) is almost identical to the mouse "cadherin-7" cDNA. According to phylogenetic analysis, this mouse cadherin-7 and its here presented human homologue are most likely the orthologues of Xenopus F-cadherin. These novel human genes, CDH7, CDH19, and CDH20, are localized on chromosome 18q22-q23, distal of both the gene CDH2 (18q11) encoding N-cadherin and the locus of the six desmosomal cadherin genes (18q12). Based on genetic linkage maps, this genomic region is close to the region to which Paget's disease was linked. Interestingly, the expression patterns of these three closely related cadherins are strikingly different.

Structural basis of the membrane-targeting and unmasking mechanisms of the radixin FERM domain

Radixin is a member of the ezrin/radixin/moesin (ERM) family of proteins, which play a role in the formation of the membrane-associated cytoskeleton by linking actin filaments and adhesion proteins. This cross-linking activity is regulated by phosphoinositides such as phosphatidylinositol 4,5-bisphosphate (PIP2) in the downstream of the small G protein Rho. The X-ray crystal structures of the radixin FERM domain, which is responsible for membrane binding, and its complex with inositol-(1,4, 5)-trisphosphate (IP3) have been determined. The domain consists of three subdomains featuring a ubiquitin-like fold, a four-helix bundle and a phosphotyrosine-binding-like domain, respectively. These subdomains are organized by intimate interdomain interactions to form characteristic grooves and clefts. One such groove is negatively charged and so is thought to interact with basic juxta-membrane regions of adhesion proteins. IP3 binds a basic cleft that is distinct from those of pleckstrin homology domains and is located on a positively charged flat molecular surface, suggesting an electrostatic mechanism of plasma membrane targeting. Based on the structural changes associated with IP3 binding, a possible unmasking mechanism of ERM proteins by PIP2 is proposed.

A role for alpha-and beta-catenins in bacterial uptake

Interaction of internalin with E-cadherin promotes entry of Listeria monocytogenes into human epithelial cells. This process requires actin cytoskeleton rearrangements. Here we show, by using a series of stably transfected cell lines expressing E-cadherin variants, that the ectodomain of E-cadherin is sufficient for bacterial adherence and that the intracytoplasmic domain is required for entry. The critical cytoplasmic region was further mapped to the beta-catenin binding domain. Because beta-catenin is known to interact with alpha-catenin, which binds to actin, we generated a fusion molecule consisting of the ectodomain of E-cadherin and the actin binding site of alpha-catenin. Cells expressing this chimera were as permissive as E-cadherin-expressing cells. In agreement with these data, alpha- and beta-catenins as well as E-cadherin clustered and colocalized at the entry site, where F-actin then accumulated. Taken together, these results reveal that E-cadherin, via beta- and alpha-catenins, can trigger dynamic events of actin polymerization and membrane extensions culminating in bacterial uptake.

Evidence that tyrosine phosphorylation regulates N-cadherin turnover during retinal development

N-cadherin, a member of the cadherin family of calcium-dependent cell adhesion molecules, mediates adhesive and signaling interactions between cells during development. N-Cadherin undergoes dynamic spatiotemporal changes in expression which correlate with morphogenetic movements of cells during organogenesis and histogenesis. We have previously shown that N-cadherin expression during development is regulated by several mechanisms, including mRNA expression, cytokine modulation, and proteolytically mediated turnover, yielding the NCAD90 protein. The present study was directed at determining the extent to which N-cadherin in primary embryonic cells is the target of endogenous kinases and phosphatases, as well as the effects of modulation of these enzymes on NCAD90 expression. The results of phosphoamino acid analyses, peptide mapping, and measurements of N-cadherin and NCAD90 expression in embryonic tissues indicate that N-cadherin is indeed the target of endogenous kinase and phosphatase action, and that modulation of different classes of these enzymes can result in either stimulation or inhibition of NCAD90 production. These results provide a mechanistic explanation for observations that cadherin function is downregulated following expression of exogenously introduced viral tyrosine kinases and provide a function for the tyrosine phosphatases recently found in association with cadherins. The results indicate that N-cadherin expression during retinal development is possibly regulated in part by modulation of its phosphorylation state, the balance of which may determine whether N-cadherin remains stably expressed or is targeted for proteolytically mediated turnover to produce NCAD90.

The disruption of adherens junctions is associated with a decrease of E-cadherin phosphorylation by protein kinase CK2

The down-regulation of E-cadherin is a common event in carcinogenesis. Phosphorylation/dephosphorylation is one posttranscriptional process which may regulate intercellular junctions. Here we show that in okadaic acid-treated keratinocytes, E-cadherin expression is shifted from the membrane to the cytoplasm, preventing cells from forming aggregates. These changes of E-cadherin localization and function are associated with a decrease in its phosphorylation state. The decrease in E-cadherin phosphorylation was essentially detected in okadaic acid-treated cell lysates isolated from 0.5% Triton-soluble fraction and not in the Triton-insoluble fraction linked to the cytoskeleton, suggesting a role of E-cadherin phosphorylation in cell-cell interactions. E-cadherin was markedly phosphorylated by CK2, either the purified recombinant enzyme or the endogenous enzyme. Using specific CK2 inhibitors such as heparin and 5, 6-dichloro-1-beta-d-ribofuranosylbenzimidazole, endogenous CK2 was confirmed as the main enzyme phosphorylating E-cadherin. The decrease in E-cadherin phosphorylation by endogenous CK2 was not restored by the addition of purified CK2, confirming that it is not due to a defect in CK2 expression or to its reduced activity, but rather to the incapacity of CK2 to phosphorylate E-cadherin. The co-immunoprecipitation and colocalization of E-cadherin and CK2 suggests that CK2 may play a critical role in the maintenance of epidermis cohesion.

The nonreceptor tyrosine kinase fer mediates cross-talk between N-cadherin and beta1-integrins

Cadherins and integrins must function in a coordinated manner to effectively mediate the cellular interactions essential for development. We hypothesized that exchange of proteins associated with their cytoplasmic domains may play a role in coordinating function. To test this idea, we used Trojan peptides to introduce into cells and tissues peptide sequences designed to compete for the interaction of specific effectors with the cytoplasmic domain of N-cadherin, and assayed their effect on cadherin- and integrin-mediated adhesion and neurite outgrowth. We show that a peptide mimicking the juxtamembrane (JMP) region of the cytoplasmic domain of N-cadherin results in inhibition of N-cadherin and beta1-integrin function. The effect of JMP on beta1-integrin function depends on the expression of N-cadherin and is independent of transcription or translation. Treatment of cells with JMP results in the release of the nonreceptor tyrosine kinase Fer from the cadherin complex and its accumulation in the integrin complex. A peptide that mimics the first coiled-coil domain of Fer prevents Fer accumulation in the integrin complex and reverses the inhibitory effect of JMP. These findings suggest a new mechanism through which N-cadherin and beta1-integrins are coordinately regulated: loss of an effector from the cytoplasmic domain of N-cadherin and gain of that effector by the beta1-integrin complex.

Phylogenetic analysis of the cadherin superfamily allows identification of six major subfamilies besides several solitary members

Cadherins play an important role in specific cell-cell adhesion events. Their expression appears to be tightly regulated during development and each tissue or cell type shows a characteristic pattern of cadherin molecules. Inappropriate regulation of their expression levels or functionality has been observed in human malignancies, in many cases leading to aggravated cancer cell invasion and metastasis. The cadherins form a superfamily with at least six subfamilies, which can be distinguished on the basis of protein domain composition, genomic structure, and phylogenetic analysis of the protein sequences. These subfamilies comprise classical or type-I cadherins, atypical or type-II cadherins, desmocollins, desmogleins, protocadherins and Flamingo cadherins. In addition, several cadherins clearly occupy isolated positions in the cadherin superfamily (cadherin-13, -15, -16, -17, Dachsous, RET, FAT, MEGF1 and most invertebrate cadherins). We suggest a different evolutionary origin of the protocadherin and Flamingo cadherin genes versus the genes encoding desmogleins, desmocollins, classical cadherins, and atypical cadherins. The present phylogenetic analysis may accelerate the functional investigation of the whole cadherin superfamily by allowing focused research of prototype cadherins within each subfamily.

The p120 catenin family: complex roles in adhesion, signaling and cancer

p120 catenin (p120) is the prototypic member of a growing subfamily of Armadillo-domain proteins found at cell-cell junctions and in nuclei. In contrast to the functions of the classical catenins (alpha-catenin, beta-catenin, and gamma-catenin/plakoglobin), which have been studied extensively, the first clues to p120's biological function have only recently emerged, and its role remains controversial. Nonetheless, it is now clear that p120 affects cell-cell adhesion through its interaction with the highly conserved juxtamembrane domain of classical cadherins, and is likely to have additional roles in the nucleus. Here, we summarize the data on the potential involvement of p120 both in promotion of and in prevension of adhesion, and propose models that attempt to reconcile some of the disparities in the literature. We also discuss the structural relationships and functions of several known p120 family members, as well as the potential roles of p120 in signaling and cancer.

Casein kinase II phosphorylation of E-cadherin increases E-cadherin/beta-catenin interaction and strengthens cell-cell adhesion

Beta-catenin, a member of the Armadillo repeat protein family, binds directly to the cytoplasmic domain of E-cadherin, linking it via alpha-catenin to the actin cytoskeleton. A 30-amino acid region within the cytoplasmic domain of E-cadherin, conserved among all classical cadherins, has been shown to be essential for beta-catenin binding. This region harbors several putative casein kinase II (CKII) and glycogen synthase kinase-3beta (GSK-3beta) phosphorylation sites and is highly phosphorylated. Here we report that in vitro this region is indeed phosphorylated by CKII and GSK-3beta, which results in an increased binding of beta-catenin to E-cadherin. Additionally, in mouse NIH3T3 fibroblasts expression of E-cadherin with mutations in putative CKII sites resulted in reduced cell-cell contacts. Thus, phosphorylation of the E-cadherin cytoplasmic domain by CKII and GSK-3beta appears to modulate the affinity between beta-catenin and E-cadherin, ultimately modifying the strength of cell-cell adhesion.

Truncation of the beta-catenin binding domain of E-cadherin precedes epithelial apoptosis during prostate and mammary involution

A potential target of hormone action during prostate and mammary involution is the intercellular junction of adjacent secretory epithelium. This is supported by the long-standing observation that one of the first visible stages of prostate and mammary involution is the disruption of interepithelial adhesion prior to the onset of apoptosis. In a previous study addressing this aspect of involution, we acquired compelling evidence indicating that the disruption of E-cadherin-dependent adhesion initiates apoptotic programs during prostate and mammary involution. In cultured prostate and mammary epithelial cells, inhibition of E-cadherin-dependent aggregation resulted in cell death following apoptotic stimuli. Loss of cell-cell adhesion in the nonaggregated population appeared to result from the rapid truncation within the cytosolic domain of the mature, 120-kDa species of E-cadherin (E-cad(120)). Immunoprecipitations from cell culture and involuting mammary gland demonstrated that this truncation removed the beta-catenin binding domain from the cytoplasmic tail of E-cadherin, resulting in a non-beta-catenin binding, membrane-bound 97-kDa species (E-cad(97)) and a free cytoplasmic 35-kDa form (E-cad(35)) that is bound to beta-catenin. Examination of E-cadherin expression and cellular distribution during prostate and mammary involution revealed a dramatic reduction in junctional membrane staining that correlated with a similar reduction in E-cad(120) and accumulation of E-cad(97) and E-cad(35). The observation that E-cadherin was truncated during involution suggested that hormone depletion activated the same apoptotic pathway in vivo as observed in vitro. Based on these findings, we hypothesize that truncation of E-cadherin results in the loss of beta-catenin binding and cellular dissociation that may signal epithelial apoptosis during prostate and mammary involution. Thus, E-cadherin may be central to homeostatic regulation in these tissues by coordinating adhesion-dependent survival and dissociation-induced apoptosis.

Selective uncoupling of p120(ctn) from E-cadherin disrupts strong adhesion

p120(ctn) 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 alpha- and beta-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.

Regulation of E-cadherin/Catenin association by tyrosine phosphorylation

Alteration of cadherin-mediated cell-cell adhesion is frequently associated to tyrosine phosphorylation of p120- and beta-catenins. We have examined the role of this modification in these proteins in the control of beta-catenin/E-cadherin binding using in vitro assays with recombinant proteins. Recombinant pp60(c-src) efficiently phosphorylated both catenins in vitro, with stoichiometries of 1.5 and 2.0 mol of phosphate/mol of protein for beta-catenin and p120-catenin, respectively. pp60(c-src) phosphorylation had opposing effects on the affinities of beta-catenin and p120 for the cytosolic domain of E-cadherin; it decreased (in the case of beta-catenin) or increased (for p120) catenin/E-cadherin binding. However, a role for p120-catenin in the modulation of beta-catenin/E-cadherin binding was not observed, since addition of phosphorylated p120-catenin did not modify the affinity of phosphorylated (or unphosphorylated) beta-catenin for E-cadherin. The phosphorylated Tyr residues were identified as Tyr-86 and Tyr-654. Experiments using point mutants in these two residues indicated that, although Tyr-86 was a better substrate for pp60(c-src), only modification of Tyr-654 was relevant for the interaction with E-cadherin. Transient transfections of different mutants demonstrated that Tyr-654 is phosphorylated in conditions in which adherens junctions are disrupted and evidenced that binding of beta-catenin to E-cadherin in vivo is controlled by phosphorylation of beta-catenin Tyr-654.

The juxtamembrane domain of cadherin regulates integrin-mediated adhesion and neurite outgrowth

Axons are guided along their trajectories during development by many different systems of adhesion, attraction, and repulsion. Thus, many distinct, and potentially competing, receptor systems respond to environmental cues, and the information must be coordinated inside the growth cone to ensure that extension follows the appropriate path. In this brief review we bring together two studies, each of which has defined different aspects of a pathway through which N-cadherin regulates beta1-integrin function allowing for coordinated responses to environmental cues during neurite extension. First we review progress in defining the binding to cells and the subsequent effects on adhesion and neurite outgrowth of the chondroitin sulfate proteoglycan, neurocan. Neurocan binds to a cell surface glycosyltransferase associated with N-cadherin (but not integrin), initiating a signal which results in loss of cadherin and integrin-function-suggesting that these two adhesion receptor systems engage in cross-talk, allowing coordinate regulation. Second, we review the use of "Trojan" peptides, peptides which mimic specific sequences in the cytoplasmic domain of N-cadherin attached to a cell permeation sequence, to reveal protein-protein interactions critical to cadherin-integrin cross-talk. One peptide mimicking a 20 amino acid sequence in the juxtamembrane region of N-cadherin has the same effect as neurocan, blocking both cadherin- and integrin-mediated adhesion and neurite outgrowth. Both neurocan and the peptide cause the release of the non-receptor tyrosine kinase Fer from the cadherin complex and its binding to the integrin complex. These data define an epigenetic pathway through which environmental cues are capable of coordinately regulating the activity of two developmentally important adhesion systems.

Cell adhesion and the actin cytoskeleton of the enveloping layer in the zebrafish embryo during epiboly

As the zebrafish embryo undergoes gastrulation and epiboly, the cells of the enveloping layer (EVL) expand, covering the entire yolk cell. During the epiboly process, the EVL cells move as a coherent layer, remaining tightly attached to each other and to the underlying yolk syncytial layer (YSL). In view of the central role of the actin cytoskeleton, in both cell motility and cell cell adhesion, we have labeled these cells in situ with fluorescent phalloidin and anti-actin antibodies. We show that, throughout their migration, the EVL cells retain a conspicuous cortical actin cytoskeletal belt coinciding with cell surface cadherins. At the margins approaching the YSL, the EVL cells extend, from their apicolateral domains, actin-rich filopodial protrusions devoid of detectable cadherin. We have studied the role of the actin cytoskeleton in the maintenance of EVL cohesion during epiboly. Cytochalasin treatment of embryos induces EVL dissociation accompanied by general detachment of the rest of the embryonic cells. In the dissociating EVL cells, the cortical actin belt undergoes fragmentation with the formation of actin aggregates; cadherins, on the other hand, remain evenly distributed at the junctional cell surface. Removal of Ca2+ by ethyleneglycolbis (amino-ethyl-ether)-tetraacetic acid (EGTA) treatment also induces cell dissociation without visible disruption of the cortical actin belt. The protein kinase inhibitor (1-isoquinolinylsulfonyl)-2-methyl-piperazine dihydrochloride (H-7), which blocks acto-myosin contractility and disrupts actin cables in cultured cells, also potentiates cytochalasin-induced dissociation and promotes the projection of numerous actin-rich lamellipodial extensions. The fact that EVL cells produce microspike-like structures towards the YSL and are capable of lamellipodial activity lend further support to the suggestion (R.W. Keller and J.P. Trinkaus. 1987. Dev. Biol. 120: 12-24) that the EVL cells are not passively mobilized on the expanding YSL but actively participate in epiboly.Key words: actin, adhesion, cadherin, cytochalasin, embryo, zebrafish.

N-cadherin promotes motility in human breast cancer cells regardless of their E-cadherin expression

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.

Mechanism of cell-cell adhesion complex assembly

Cell-cell adhesion complexes play an important role in the organization and behavior of cells in tissues. An important step in the formation of such complexes is the clustering of the adhesion receptors; this is critical for proper adhesion, for anchorage of the cytoskeleton to the plasma membrane, and for generation of different intracellular signals. Recent advances reveal that several interconnected mechanisms are responsible for clustering of the different adhesion receptors.

Serotonin synchronises convergent extension of ectoderm with morphogenetic gastrulation movements in Drosophila

During Drosophila gastrulation, convergent extension of the ectoderm is required for germband extension. Adhesive heterogeneity within ectodermal cells has been proposed to trigger the intercalation of cells responsible for this movement. Segmentation genes would impose this heterogeneity by establishing a pair-rule pattern of cell adhesion properties. We previously reported that the serotonin receptor (5-ht(2Dro)) is expressed in the presumptive ectoderm with a pair-rule pattern. Here, we show that the peaks of 5-ht(2Dro) expression and serotonin synthesis coincide precisely with the onset of convergent extension of the ectoderm. Gastrulae genetically depleted of serotonin or the 5-ht(2Dro) receptor do not extend their germband properly, and the ectodermal movements becomes asynchronous with the morphogenetic movements in the endoderm and mesoderm. Associated with the beginning of this desynchronisation, is an altered subcellular localisation of adherens junctions within the ectoderm. Combined, these data highlight the role of the ectoderm in Drosophila gastrulation and support the notion that serotonin signalling through the 5-HT(2Dro) receptor triggers changes in cell adhesiveness that are necessary for cell intercalation.

Regulation of E-cadherin: does hypoxia initiate the metastatic cascade?

The ability of tumours to metastasis is regarded as one of the hallmarks of malignancy. The process through which tumours evolve to achieve this has been termed the metastatic cascade. This cascade has been the subject of much investigation over many years. One of the vital events identified by these investigations is the reduction of adhesion between tumour cells facilitating invasion of the surrounding tissues and vascular channels, ultimately leading to the development of a distant metastasis. E-cadherin and its associated catenin complex have been identified as key molecules in cell adhesion. This review looks at the structure and interaction of the E-cadherin-catenin complex and the factors that appear to regulate E-cadherin expression and thus cell adhesion. From the data gathered, it has become possible to propose the hypothesis that the development of tumour hypoxia is the initiating factor that sets the tumour on the road to metastasis.

The role of the E-cadherin/catenin adhesion complex in the development and progression of cancer

The E-cadherin/catenin protein complex regulates the functional integrity of epithelia by mediating specific intercellular adhesion, Defects in the transmembrane E-cadherin protein play an important role in several human cancer types. E-cadherin-inactivating mutations were mainly found in sporadic lobular breast carcinoma and in both familial and sporadic diffuse gastric carcinoma. Armadillo proteins such as beta-catenin and p120ctn are complexed to the cytoplasmic tail of E-cadherin, whereas the vinculin-related alphaE-catenin protein forms a link to the actin cytoskeleton. The latter shows inactivating deletions in various tumor cell lines. Apparently, both E-cadherin and alphaE-catenin serve as tumor suppressor and invasion suppressor molecules. On the other hand, protein-stabilizing oncogenic mutations of beta-catenin were found at high frequency in particular human tumor types. Mutated beta-catenin protein is imported into the nucleus, and its binding to LEF/TCF transcription factors modulates transcription of intriguing target genes. Also p120ctn was recently found to arrive in the nucleus and to interact with a transcription factor. Furthermore, a wide variety of mechanisms have been described to regulate in a reversible way E-cadherin/catenin-mediated cell adhesion and differentiation. These phenomena appear to be crucial in human cancer development and progression.

Mechanism of extracellular domain-deleted dominant negative cadherins

The cadherin/catenin complex mediates Ca2+-dependent cell-cell interactions that are essential for normal developmental processes. It has been proposed that sorting of cells during embryonic development is due, at least in part, to expression of different cadherin family members or to expression of differing levels of a single family member. Expression of dominant-negative cadherins has been used experimentally to decrease cell-cell interactions in whole organisms and in cultured cells. In this study, we elucidated the mechanism of action of extracellular domain-deleted dominant-negative cadherin, showing that it is not cadherin isotype-specific, and that it must be membrane-associated but the orientation within the membrane does not matter. In addition, membrane-targeted cytoplasmic domain cadherin with the catenin-binding domain deleted does not function as a dominant-negative cadherin. Expression of extracellular domain-deleted dominant-negative cadherin results in down-regulation of endogenous cadherins which presumably contributes to the non-adhesive phenotype.

Coupling assembly of the E-cadherin/beta-catenin complex to efficient endoplasmic reticulum exit and basal-lateral membrane targeting of E-cadherin in polarized MDCK cells

The E-cadherin/catenin complex regulates Ca++-dependent cell-cell adhesion and is localized to the basal-lateral membrane of polarized epithelial cells. Little is known about mechanisms of complex assembly or intracellular trafficking, or how these processes might ultimately regulate adhesion functions of the complex at the cell surface. The cytoplasmic domain of E-cadherin contains two putative basal-lateral sorting motifs, which are homologous to sorting signals in the low density lipoprotein receptor, but an alanine scan across tyrosine residues in these motifs did not affect the fidelity of newly synthesized E-cadherin delivery to the basal-lateral membrane of MDCK cells. Nevertheless, sorting signals are located in the cytoplasmic domain since a chimeric protein (GP2CAD1), comprising the extracellular domain of GP2 (an apical membrane protein) and the transmembrane and cytoplasmic domains of E-cadherin, was efficiently and specifically delivered to the basal-lateral membrane. Systematic deletion and recombination of specific regions of the cytoplasmic domain of GP2CAD1 resulted in delivery of <10% of these newly synthesized proteins to both apical and basal-lateral membrane domains. Significantly, >90% of each mutant protein was retained in the ER. None of these mutants formed a strong interaction with beta-catenin, which normally occurs shortly after E-cadherin synthesis. In addition, a simple deletion mutation of E-cadherin that lacks beta-catenin binding is also localized intracellularly. Thus, beta-catenin binding to the whole cytoplasmic domain of E-cadherin correlates with efficient and targeted delivery of E-cadherin to the lateral plasma membrane. In this capacity, we suggest that beta-catenin acts as a chauffeur, to facilitate transport of E-cadherin out of the ER and the plasma membrane.

Cadherin function in junctional complex rearrangement and posttranslational control of cadherin expression

The role of E-cadherin, a calcium-dependent adhesion protein, in organizing and maintaining epithelial junctions was examined in detail by expressing a fusion protein (GP2-Cad1) composed of the extracellular domain of a nonadherent glycoprotein (GP2) and the transmembrane and cytoplasmic domains of E-cadherin. All studies shown were also replicated using an analogous cell line that expresses a mutant cadherin construct (T151) under the control of tet repressor. Mutant cadherin was expressed at approximately 10% of the endogenous E-cadherin level and had no apparent effect on tight junction function or on distributions of adherens junction, tight junction, or desmosomal marker proteins in established Madin-Darby canine kidney cell monolayers. However, GP2-Cad1 accelerated the disassembly of epithelial junctional complexes and delayed their reassembly in calcium switch experiments. Inducing expression of GP2-Cad1 to levels approximately threefold greater than endogenous E-cadherin expression levels in control cells resulted in a decrease in endogenous E-cadherin levels. This was due in part to increased protein turnover, indicating a cellular mechanism for sensing and controlling E-cadherin levels. Cadherin association with catenins is necessary for strong cadherin-mediated cell-cell adhesion. In cells expressing low levels of GP2-Cad1, protein levels and stoichiometry of the endogenous cadherin-catenin complex were unaffected. Thus effects of GP2-Cad1 on epithelial junctional complex assembly and stability were not due to competition with endogenous E-cadherin for catenin binding. Rather, we suggest that GP2-Cad1 interferes with the packing of endogenous cadherin-catenin complexes into higher-order structures in junctional complexes that results in junction destabilization.

Desmosomes: intercellular adhesive junctions specialized for attachment of intermediate filaments

Cell-cell adhesion is thought to play important roles in development, in tissue morphogenesis, and in the regulation of cell migration and proliferation. Desmosomes are adhesive intercellular junctions that anchor the intermediate filament network to the plasma membrane. By functioning both as an adhesive complex and as a cell-surface attachment site for intermediate filaments, desmosomes integrate the intermediate filament cytoskeleton between cells and play an important role in maintaining tissue integrity. Recent observations indicate that tissue integrity is severely compromised in autoimmune and genetic diseases in which the function of desmosomal molecules is impaired. In addition, the structure and function of many of the desmosomal molecules have been determined, and a number of the molecular interactions between desmosomal proteins have now been elucidated. Finally, the molecular constituents of desmosomes and other adhesive complexes are now known to function not only in cell adhesion, but also in the transduction of intracellular signals that regulate cell behavior.

The membrane-proximal region of the E-cadherin cytoplasmic domain prevents dimerization and negatively regulates adhesion activity

Cadherins are transmembrane glycoproteins involved in Ca2+-dependent cell-cell adhesion. Deletion of the COOH-terminal residues of the E-cadherin cytoplasmic domain has been shown to abolish its cell adhesive activity, which has been ascribed to the failure of the deletion mutants to associate with catenins. Based on our present results, this concept needs revision. As was reported previously, leukemia cells (K562) expressing E-cadherin with COOH-terminal deletion of 37 or 71 amino acid residues showed almost no aggregation. Cells expressing E-cadherin with further deletion of 144 or 151 amino acid residues, which eliminates the membrane-proximal region of the cytoplasmic domain, showed E-cadherin-dependent aggregation. Thus, deletion of the membrane-proximal region results in activation of the nonfunctional E-cadherin polypeptides. However, these cells did not show compaction. Chemical cross-linking revealed that the activated E-cadherin polypeptides can be cross-linked to a dimer on the surface of cells, whereas the inactive polypeptides, as well as the wild-type E-cadherin polypeptide containing the membrane-proximal region, can not. Therefore, the membrane-proximal region participates in regulation of the adhesive activity by preventing lateral dimerization of the extracellular domain.

Molecular organization of the desmoglein-plakoglobin complex

Different epithelial intercellular junctions contain distinct complexes incorporating plakoglobin. In adherens junctions, plakoglobin interacts with two molecules, the transmembrane adhesion protein of the cadherin family (e.g. E-cadherin) and alpha-catenin. The latter is thought to anchor the cadherin-plakoglobin complex to the cortical actin cytoskeleton. In desmosomes, plakoglobin forms a complex with desmosomal cadherins, either desmoglein (Dsg) or desmocollin (Dsc), but not with alpha-catenin. To further understand the structure and assembly of the plakoglobin-cadherin complexes we analyzed amino acid residues involved in plakoglobin-Dsg interactions using alanine scanning mutagenesis. Previously, we have shown that plakoglobin interacts with a 72 amino acid-long cytoplasmic domain (C-domain) that is conserved among desmosomal and classic cadherins. In this paper, we show that a row of the large hydrophobic residues located at the C-terminal portion of the Dsg C-domain is indispensable for interaction with plakoglobin. To study a reciprocal site we expressed plakoglobin (MPg) or its mutants tagged by 6 myc epitope in epithelial A-431 cells. Using sucrose gradient centrifugation and subsequent co-immunoprecipitation, MPg was found to be efficiently incorporated into the same type of complexes as endogenous plakoglobin. A major pool of Dsg-plakoglobin complexes sedimented at 8S and exhibited a 1:1 stoichiometry. Using alanine scanning mutagenesis and the co-immunoprecipitation assay we identified nine hydrophobic amino acids within the arm repeats 1–3 of plakoglobin, that are required for binding to Dsg and Dsc. Eight of these amino acids also participate in the interaction with alpha-catenin. No mutations were found to reduce the affinity of plakoglobin binding to E-cadherin. These data provide direct evidence that the same hydrophobic plakoglobin surface is essential for mutually exclusive interaction with distinct proteins such as alpha-catenin and desmosomal cadherins.

E-Cadherin-dependent growth suppression is mediated by the cyclin-dependent kinase inhibitor p27(KIP1)

Recent studies have demonstrated the importance of E-cadherin, a homophilic cell-cell adhesion molecule, in contact inhibition of growth of normal epithelial cells. Many tumor cells also maintain strong intercellular adhesion, and are growth-inhibited by cell- cell contact, especially when grown in three-dimensional culture. To determine if E-cadherin could mediate contact-dependent growth inhibition of nonadherent EMT/6 mouse mammary carcinoma cells that lack E-cadherin, we transfected these cells with an exogenous E-cadherin expression vector. E-cadherin expression in EMT/6 cells resulted in tighter adhesion of multicellular spheroids and a reduced proliferative fraction in three-dimensional culture. In addition to increased cell-cell adhesion, E-cadherin expression also resulted in dephosphorylation of the retinoblastoma protein, an increase in the level of the cyclin-dependent kinase inhibitor p27(kip1) and a late reduction in cyclin D1 protein. Tightly adherent spheroids also showed increased levels of p27 bound to the cyclin E-cdk2 complex, and a reduction in cyclin E-cdk2 activity. Exposure to E-cadherin-neutralizing antibodies in three-dimensional culture simultaneously prevented adhesion and stimulated proliferation of E-cadherin transfectants as well as a panel of human colon, breast, and lung carcinoma cell lines that express functional E-cadherin. To test the importance of p27 in E-cadherin-dependent growth inhibition, we engineered E-cadherin-positive cells to express inducible p27. By forcing expression of p27 levels similar to those observed in aggregated cells, the stimulatory effect of E-cadherin-neutralizing antibodies on proliferation could be inhibited. This study demonstrates that E-cadherin, classically described as an invasion suppressor, is also a major growth suppressor, and its ability to inhibit proliferation involves upregulation of the cyclin-dependent kinase inhibitor p27.

Differential nuclear translocation and transactivation potential of beta-catenin and plakoglobin

beta-Catenin and plakoglobin are homologous proteins that function in cell adhesion by linking cadherins to the cytoskeleton and in signaling by transactivation together with lymphoid-enhancing binding/T cell (LEF/TCF) transcription factors. Here we compared the nuclear translocation and transactivation abilities of beta-catenin and plakoglobin in mammalian cells. Overexpression of each of the two proteins in MDCK cells resulted in nuclear translocation and formation of nuclear aggregates. The beta-catenin-containing nuclear structures also contained LEF-1 and vinculin, while plakoglobin was inefficient in recruiting these molecules, suggesting that its interaction with LEF-1 and vinculin is significantly weaker. Moreover, transfection of LEF-1 translocated endogenous beta-catenin, but not plakoglobin to the nucleus. Chimeras consisting of Gal4 DNA-binding domain and the transactivation domains of either plakoglobin or beta-catenin were equally potent in transactivating a Gal4-responsive reporter, whereas activation of LEF-1- responsive transcription was significantly higher with beta-catenin. Overexpression of wild-type plakoglobin or mutant beta-catenin lacking the transactivation domain induced accumulation of the endogenous beta-catenin in the nucleus and LEF-1-responsive transactivation. It is further shown that the constitutive beta-catenin-dependent transactivation in SW480 colon carcinoma cells and its nuclear localization can be inhibited by overexpressing N-cadherin or alpha-catenin. The results indicate that (a) plakoglobin and beta-catenin differ in their nuclear translocation and complexing with LEF-1 and vinculin; (b) LEF-1-dependent transactivation is preferentially driven by beta-catenin; and (c) the cytoplasmic partners of beta-catenin, cadherin and alpha-catenin, can sequester it to the cytoplasm and inhibit its transcriptional activity.

Tyrosine phosphorylation and src family kinases control keratinocyte cell-cell adhesion

In their progression from the basal to upper differentiated layers of the epidermis, keratinocytes undergo significant structural changes, including establishment of close intercellular contacts. An important but so far unexplored question is how these early structural events are related to the biochemical pathways that trigger differentiation. We show here that beta-catenin, gamma-catenin/plakoglobin, and p120-Cas are all significantly tyrosine phosphorylated in primary mouse keratinocytes induced to differentiate by calcium, with a time course similar to that of cell junction formation. Together with these changes, there is an increased association of alpha-catenin and p120-Cas with E-cadherin, which is prevented by tyrosine kinase inhibition. Treatment of E-cadherin complexes with tyrosine-specific phosphatase reveals that the strength of alpha-catenin association is directly dependent on tyrosine phosphorylation. In parallel with the biochemical effects, tyrosine kinase inhibition suppresses formation of cell adhesive structures, and causes a significant reduction in adhesive strength of differentiating keratinocytes. The Fyn tyrosine kinase colocalizes with E-cadherin at the cell membrane in calcium-treated keratinocytes. Consistent with an involvement of this kinase, fyn-deficient keratinocytes have strongly decreased tyrosine phosphorylation levels of beta- and gamma-catenins and p120-Cas, and structural and functional abnormalities in cell adhesion similar to those caused by tyrosine kinase inhibitors. Whereas skin of fyn-/- mice appears normal, skin of mice with a disruption in both the fyn and src genes shows intrinsically reduced tyrosine phosphorylation of beta-catenin, strongly decreased p120-Cas levels, and important structural changes consistent with impaired keratinocyte cell adhesion. Thus, unlike what has been proposed for oncogene-transformed or mitogenically stimulated cells, in differentiating keratinocytes tyrosine phosphorylation plays a positive role in control of cell adhesion, and this regulatory function appears to be important both in vitro and in vivo.

The juxtamembrane region of the cadherin cytoplasmic tail supports lateral clustering, adhesive strengthening, and interaction with p120ctn

Cadherin cell-cell adhesion molecules form membrane-spanning molecular complexes that couple homophilic binding by the cadherin ectodomain to the actin cytoskeleton. A fundamental issue in cadherin biology is how this complex converts the weak intrinsic binding activity of the ectodomain into strong adhesion. Recently we demonstrated that cellular cadherins cluster in a ligand-dependent fashion when cells attached to substrata coated with the adhesive ectodomain of Xenopus C-cadherin (CEC1-5). Moreover, forced clustering of the ectodomain alone significantly strengthened adhesiveness (Yap, A.S., W.M. Brieher, M. Pruschy, and B.M. Gumbiner. Curr. Biol. 7:308-315). In this study we sought to identify the determinants of the cadherin cytoplasmic tail responsible for clustering activity. A deletion mutant of C-cadherin (CT669) that retained the juxtamembrane 94-amino acid region of the cytoplasmic tail, but not the beta-catenin-binding domain, clustered upon attachment to substrata coated with CEC1-5. Like wild-type C-cadherin, this clustering was ligand dependent. In contrast, mutant molecules lacking either the complete cytoplasmic tail or just the juxtamembrane region did not cluster. The juxtamembrane region was itself sufficient to induce clustering when fused to a heterologous membrane-anchored protein, albeit in a ligand-independent fashion. The CT669 cadherin mutant also displayed significant adhesive activity when tested in laminar flow detachment assays and aggregation assays. Purification of proteins binding to the juxtamembrane region revealed that the major associated protein is p120(ctn). These findings identify the juxtamembrane region of the cadherin cytoplasmic tail as a functionally active region supporting cadherin clustering and adhesive strength and raise the possibility that p120(ctn) is involved in clustering and cell adhesion.

Calcium-induced intercellular adhesion of keratinocytes does not involve accumulation of beta 1 integrins at cell-cell contacts and does not involve changes in the levels or phosphorylation of catenins

On initiation of terminal differentiation human epidermal keratinocytes detach from the underlying basement membrane as a result of inactivation and subsequent loss of integrins from the cell surface. Assembly of keratinocytes into multilayered sheets requires functional E- and P-cadherin and when stratification is inhibited in low calcium medium differentiating keratinocytes continue to express functional integrins. Using immunofluorescence microscopy, we found that on addition of calcium ions to keratinocyte monolayers there was colocalisation of the beta 1 integrins and E-cadherin along the lateral membranes except for a zone close to the substratum which exclusively contained integrins. Quantitative immunoelectron microscopy showed that on induction of stable cell-cell contacts the density of beta 1 integrins was the same on the apical and lateral membranes, suggesting that the accumulation of integrins on the lateral membranes observed by immunofluorescence microscopy is due to the increased area of contact between adjacent cells and not to an increase in receptor density. There were no changes in the levels of catenins and their degree of phosphorylation after induction of cell-cell contacts. These observations provide new sights into the mechanism of calcium-dependent intercellular adhesion of keratinocytes.