The adhesive binding site of cadherins revisited

Mechanotransduction by PCDH15 Relies on a Novel cis-Dimeric Architecture

The tip link, a filament formed by protocadherin 15 (PCDH15) and cadherin 23, conveys mechanical force from sound waves and head movement to open hair-cell mechanotransduction channels. Tip-link cadherins are thought to have acquired structural features critical for their role in mechanotransduction. Here, we biophysically and structurally characterize the unusual cis-homodimeric architecture of PCDH15. We show that PCDH15 molecules form double-helical assemblies through cis-dimerization interfaces in the extracellular cadherin EC2-EC3 domain region and in a unique membrane-proximal domain. Electron microscopy studies visualize the cis-dimeric PCDH15 assembly and reveal the PCDH15 extracellular domain as a parallel double helix with cis cross-bridges at the two locations we defined. The helical configuration suggests the potential for elasticity through helix winding and unwinding. Functional studies in hair cells show that mutations that perturb PCDH15 dimerization contacts affect mechanotransduction. Together, these data reveal the cis-dimeric architecture of PCDH15 and show that dimerization is critical for sensing mechanical stimuli.

Early events in the assembly of E-cadherin adhesions

E-cadherin is a calcium dependent cell adhesion molecule that is key to the organization of cells in the epithelial tissue. It is a multidomain, trans-membrane protein in which the extracellular domain forms the homotypic, adhesive interaction while the intracellular domain interacts with the actin cytoskeleton through the catenin family of adaptor proteins. A number of recent studies have provided novel insights into the mechanism of adhesion formation by this class of adhesion proteins. Here, we describe an updated view of the process of E-cadherin adhesion formation with an emphasis on the role of molecular mobility, clustering, and active cellular processes.

Protocadherins and hypothalamic development: do they play an unappreciated role?

Normal brain development requires coordinated cell movements at precise times. It has long been established that cell-cell adhesion proteins of the cadherin superfamily are involved in the adhesion and sorting of cells during tissue morphogenesis. In the present review, we focus on protocadherins, which form the largest subfamily of the cadherin superfamily and mediate homophilic cell-cell adhesion in the developing brain. These molecules are highly expressed during neural development and the exact roles that they play are still emerging. Although, historically, protocadherins were considered to provide mechanical and chemical connections between adjacent cells, recent research suggests that they may also serve as molecular identity markers of neurones to help guide cell recognition and sorting, cell migration, outgrowth of neuronal processes, and synapse formation. This phenomenon of single cell diversity stems, in part, from the vast variation in protein structure, genomic organisation and molecular function of the protocadherins. Although expression profiles and genetic manipulations have provided evidence for the role of protocadherins in the developing brain, we have only begun to construct a complete understanding of protocadherin function. We examine our current understanding of how protocadherins influence brain development and discuss the possible roles for this large superfamily within the hypothalamus. We conclude that further research into these underappreciated but vitally important genes will shed insight into hypothalamic development and perhaps the underlying aetiology of neuroendocrine disorders.

Structural and energetic determinants of adhesive binding specificity in type I cadherins

Type I cadherin cell-adhesion proteins are similar in sequence and structure and yet are different enough to mediate highly specific cell-cell recognition phenomena. It has previously been shown that small differences in the homophilic and heterophilic binding affinities of different type I family members can account for the differential cell-sorting behavior. Here we use a combination of X-ray crystallography, analytical ultracentrifugation, surface plasmon resonance and double electron-electron resonance (DEER) electron paramagnetic resonance spectroscopy to identify the molecular determinants of type I cadherin dimerization affinities. Small changes in sequence are found to produce subtle structural and dynamical changes that impact relative affinities, in part via electrostatic and hydrophobic interactions, and in part through entropic effects because of increased conformational heterogeneity in the bound states as revealed by DEER distance mapping in the dimers. These findings highlight the remarkable ability of evolution to exploit a wide range of molecular properties to produce closely related members of the same protein family that have affinity differences finely tuned to mediate their biological roles.

Role of cadherin-11 in synovial joint formation and rheumatoid arthritis pathology

Cadherin-11 is a classic cadherin adhesion molecule that mediates homophilic cell-to-cell adhesion. Cadherin-11 is involved in the function of embryonic development, tissue morphogenesis, tumor invasion and metastasis, and signal transduction. This review summarizes the function of cadherin-11 in synovial joint formation and rheumatoid arthritis (RA), including its relative function with bone and cartilage development and growth plate, synovial, and tendon formation. The role of cadherin-11 in RA is also discussed, both in fibroblasts inflammation and fibroblast-like synoviocyte (FLSs) migration and invasion. The potential of anti-cadherin-11 therapy for RA is introduced in comparison with the other current RA therapies.

Remarkable disparity in mechanical response among the extracellular domains of type I and II cadherins

Cadherins, a large family of calcium-dependent adhesion molecules, are critical for intercellular adhesion. While crystallographic structures for several cadherins show clear structural similarities, their relevant adhesive strengths vary and their mechanisms of adhesion between types I and II cadherin subfamilies are still unclear. Here, stretching of cadherins was explored experimentally by atomic force microscopy and computationally by steered molecular dynamics (SMD) simulations, where partial unfolding of the E-cadherin ectodomains was observed. The SMD simulations on strand-swapping cadherin dimers displayed similarity in binding strength, suggesting contributions of other mechanisms to explain the strength differences of cell adhesion in vivo. Systematic simulations on the unfolding of the extracellular domains of type I and II cadherins revealed diverse pathways. However, at the earliest stage, a remarkable similarity in unfolding was observed for the various type I cadherins that was distinct from that for type II cadherins. This likely correlates positively with their distinct adhesive properties, suggesting that the initial forced deformation in type I cadherins may be involved in cadherin-mediated adhesion. An animated Interactive 3D Complement (I3DC) is available in Proteopedia at http://proteopedia.org/w/Journal:JBSD:25.

Cadherins in brain morphogenesis and wiring

Cadherins are Ca(2+)-dependent cell-cell adhesion molecules that play critical roles in animal morphogenesis. Various cadherin-related molecules have also been identified, which show diverse functions, not only for the regulation of cell adhesion but also for that of cell proliferation and planar cell polarity. During the past decade, understanding of the roles of these molecules in the nervous system has significantly progressed. They are important not only for the development of the nervous system but also for its functions and, in turn, for neural disorders. In this review, we discuss the roles of cadherins and related molecules in neural development and function in the vertebrate brain.

Thinking outside the cell: how cadherins drive adhesion

Cadherins are a superfamily of cell surface glycoproteins whose ectodomains contain multiple repeats of β-sandwich extracellular cadherin (EC) domains that adopt a similar fold to immunoglobulin domains. The best characterized cadherins are the vertebrate 'classical' cadherins, which mediate adhesion via trans homodimerization between their membrane-distal EC1 domains that extend from apposed cells, and assemble intercellular adherens junctions through cis clustering. To form mature trans adhesive dimers, cadherin domains from apposed cells dimerize in a 'strand-swapped' conformation. This occurs in a two-step binding process involving a fast-binding intermediate called the 'X-dimer'. Trans dimers are less flexible than cadherin monomers, a factor that drives junction assembly following cell-cell contact by reducing the entropic cost associated with the formation of lateral cis oligomers. Cadherins outside the classical subfamily appear to have evolved distinct adhesive mechanisms that are only now beginning to be understood.

Cross talk between focal adhesion kinase and cadherins: role in regulating endothelial barrier function

A layer of endothelial cells attached to their underlying matrices by complex transmembrane structures termed focal adhesion (FA) proteins maintains the barrier property of microvascular endothelium. FAs sense the physical properties of the extracellular matrix (ECM) and organize the cytoskeleton accordingly. The close association of adherens junction (AJ) protein, cadherin, with the cytoskeleton is known to be essential in coordinating the appropriate mechanical properties to cell-cell contacts. Recently, it has become clear that a crosstalk exists between focal adhesion kinase (FAK) and cadherin that regulates signaling at intercellular endothelial junctions. This review discusses recent advances in our understanding of the dynamic regulation of the molecular connections between FAK and the cadherin complex and cadherin-catenin-actin interaction-dependent changes as well as the role of small GTPases in endothelial barrier regulation. This review also discusses how a signaling network regulates a range of cellular processes important for barrier function and diseases.

Molecular design principles underlying β-strand swapping in the adhesive dimerization of cadherins

Cell adhesion by classical cadherins is mediated by dimerization of their EC1 domains through the “swapping” of N-terminal β-strands. We use molecular simulations, measurements of binding affinities, and x-ray crystallography to provide a detailed picture of the structural and energetic factors that control the adhesive dimerization of cadherins. We show that strand swapping in EC1 is driven by conformational strain in cadherin monomers which arises from the anchoring of their short N-terminal strand at one end by the conserved Trp2 and at the other by ligation to Ca²⁺ ions. We also demonstrate that a conserved pro-pro motif functions to avoid the formation of an overly tight interface where affinity differences between different cadherins, crucial at the cellular level, are lost. We use these findings to design site-directed mutations which transform a monomeric EC2-EC3 domain cadherin construct, into a strand-swapped dimer.

Structure and biochemistry of cadherins and catenins

Classical cadherins mediate specific adhesion at intercellular adherens junctions. Interactions between cadherin ectodomains from apposed cells mediate cell-cell contact, whereas the intracellular region functionally links cadherins to the underlying cytoskeleton. Structural, biophysical, and biochemical studies have provided important insights into the mechanism and specificity of cell-cell adhesion by classical cadherins and their interplay with the cytoskeleton. Adhesive binding arises through exchange of beta strands between the first extracellular cadherin domains (EC1) of partner cadherins from adjacent cells. This "strand-swap" binding mode is common to classical and desmosomal cadherins, but sequence alignments suggest that other cadherins will bind differently. The intracellular region of classical cadherins binds to p120 and beta-catenin, and beta-catenin binds to the F-actin binding protein alpha-catenin. Rather than stably bridging beta-catenin to actin, it appears that alpha-catenin actively regulates the actin cytoskeleton at cadherin-based cell-cell contacts.

Combinatorial homophilic interaction between gamma-protocadherin multimers greatly expands the molecular diversity of cell adhesion

The specificity of interactions between neurons is believed to be mediated by diverse cell adhesion molecules, including members of the cadherin superfamily. Whereas mechanisms of classical cadherin adhesion have been studied extensively, much less is known about the related protocadherins (Pcdhs), which together make up the majority of the superfamily. Here we use quantitative cell aggregation assays and biochemical analyses to characterize cis and trans interactions among the 22-member gamma-Pcdh family, which have been shown to be critical for the control of synaptogenesis and neuronal survival. We show that gamma-Pcdh isoforms engage in trans interactions that are strictly homophilic. In contrast to classical cadherins, gamma-Pcdh interactions are only partially Ca(2+)-dependent, and their specificity is mediated through the second and third extracellular cadherin (EC) domains (EC2 and EC3), rather than through EC1. The gamma-Pcdhs also interact both covalently and noncovalently in the cis-orientation to form multimers both in vitro and in vivo. In contrast to gamma-Pcdh trans interactions, cis interactions are highly promiscuous, with no isoform specificity. We present data supporting a model in which gamma-Pcdh cis-tetramers represent the unit of their adhesive trans interactions. Unrestricted tetramerization in cis, coupled with strictly homophilic interactions in trans, predicts that the 22 gamma-Pcdhs could form 234,256 distinct adhesive interfaces. Given the demonstrated role of the gamma-Pcdhs in synaptogenesis, our data have important implications for the molecular control of neuronal specificity.

Structural basis for mechanical force regulation of the adhesin FimH via finger trap-like beta sheet twisting

The Escherichia coli fimbrial adhesive protein, FimH, mediates shear-dependent binding to mannosylated surfaces via force-enhanced allosteric catch bonds, but the underlying structural mechanism was previously unknown. Here we present the crystal structure of FimH incorporated into the multiprotein fimbrial tip, where the anchoring (pilin) domain of FimH interacts with the mannose-binding (lectin) domain and causes a twist in the beta sandwich fold of the latter. This loosens the mannose-binding pocket on the opposite end of the lectin domain, resulting in an inactive low-affinity state of the adhesin. The autoinhibition effect of the pilin domain is removed by application of tensile force across the bond, which separates the domains and causes the lectin domain to untwist and clamp tightly around the ligand like a finger-trap toy. Thus, beta sandwich domains, which are common in multidomain proteins exposed to tensile force in vivo, can undergo drastic allosteric changes and be subjected to mechanical regulation.

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.

Regulation of homotypic cell-cell adhesion by branched N-glycosylation of N-cadherin extracellular EC2 and EC3 domains

The effects of altering N-cadherin N-glycosylation on several cadherin-mediated cellular behaviors were investigated using small interfering RNA and site-directed mutagenesis. In HT1080 fibrosarcoma cells, small interfering RNA-directed knockdown of N-acetylglucosaminyltransferase V (GnT-V), a glycosyltransferase up-regulated by oncogene signaling, caused decreased expression of N-linked beta(1,6)-branched glycans expressed on N-cadherin, resulting in enhanced N-cadherin-mediated cell-cell adhesion, but had no effect on N-cadherin expression on the cell surface. This effect on adhesion was accompanied by decreased cell migration and invasion, opposite of the effects observed when GnT-V was overexpressed in these cells (Guo, H. B., Lee, I., Kamar, M., and Pierce, M. (2003) J. Biol. Chem. 278, 52412-52424). A detailed study using site-directed mutagenesis demonstrated that three of the eight putative N-glycosylation sites in the N-cadherin sequence showed N-glycan expression. Moreover, all three of these sites, located in the extracellular domains EC2 and EC3, were shown by leucoagglutinating phytohemagglutinin binding to express at least some beta(1,6)-branched glycans, products of GnT-V activity. Deletion of these sites had no effect on cadherin levels on the cell surface but led to increased stabilization of cell-cell contacts, cell-cell adhesion- mediated intracellular signaling, and reduced cell migration. We show for the first time that these deletions had little effect on formation of the N-cadherin-catenin complex but instead resulted in increased N-cadherin cis-dimerization. Branched N-glycan expression at three sites in the EC2 and -3 domains regulates N-cadherin-mediated cell-cell contact formation, outside-in signaling, and cell migration and is probably a significant contributor to the increase in the migratory/invasive phenotype of cancer cells that results when GnT-V activity is up-regulated by oncogene signaling.

Linking molecular affinity and cellular specificity in cadherin-mediated adhesion

Many cell-cell adhesive events are mediated by the dimerization of cadherin proteins presented on apposing cell surfaces. Cadherin-mediated processes play a central role in the sorting of cells into separate tissues in vivo, but in vitro assays aimed at mimicking this behavior have yielded inconclusive results. In some cases, cells that express different cadherins exhibit homotypic cell sorting, forming separate cell aggregates, whereas in other cases, intermixed aggregates are formed. A third pattern is observed for mixtures of cells expressing either N- or E-cadherin, which form distinct homotypic aggregates that adhere to one another through a heterotypic interface. The molecular basis of cadherin-mediated cell patterning phenomena is poorly understood, in part because the relationship between cellular adhesive specificity and intermolecular binding free energies has not been established. To clarify this issue, we have measured the dimerization affinities of N-cadherin and E-cadherin. These proteins are similar in sequence and structure, yet are able to mediate homotypic cell patterning behavior in a variety of tissues. N-cadherin is found to form homodimers with higher affinity than does E-cadherin and, unexpectedly, the N/E-cadherin heterophilic binding affinity is intermediate in strength between the 2 homophilic affinities. We can account for observed cell aggregation behaviors by using a theoretical framework that establishes a connection between molecular affinities and cell-cell adhesive specificity. Our results illustrate how graded differences between different homophilic and heterophilic cadherin dimerizaton affinities can result in homotypic cell patterning and, more generally, show how proteins that are closely related can, nevertheless, be responsible for highly specific cellular adhesive behavior.

Pharmacology of cell adhesion molecules of the nervous system

Cell adhesion molecules (CAMs) play a pivotal role in the development and maintenance of the nervous system under normal conditions. They also are involved in numerous pathological processes such as inflammation, degenerative disorders, and cancer, making them attractive targets for drug development. The majority of CAMs are signal transducing receptors. CAM-induced intracellular signalling is triggered via homophilic (CAM-CAM) and heterophilic (CAM - other counter-receptors) interactions, which both can be targeted pharmacologically. We here describe the progress in the CAM pharmacology focusing on cadherins and CAMs of the immunoglobulin (Ig) superfamily, such as NCAM and L1. Structural basis of CAM-mediated cell adhesion and CAM-induced signalling are outlined. Different pharmacological approaches to study functions of CAMs are presented including the use of specific antibodies, recombinant proteins, and synthetic peptides. We also discuss how unravelling of the 3D structure of CAMs provides novel pharmacological tools for dissection of CAM-induced signalling pathways and offers therapeutic opportunities for a range of neurological disorders.

A soluble truncated cadherin induces breast cancer cell apoptosis and growth inhibition

PURPOSE

To identify the characteristics and function of the truncated cadherin cDNA which encodes a soluble molecule containing the sequence of VE-cadherin extracellular domain repeats from repeat 1 to 4 (designated as CED1-4) and a secreting signal peptide at N terminal.

METHODS

A pMSCV/CED1-4 vector was constructed. Recombinant retrovirus ReCED1-4 and ReEmpty were produced by 293 package cells and transfected into MDA-MB435 human breast cancer cells. The expression of CED1-4 in transfectants and their supernatant was analyzed by RT-PCR and Western blot, respectively. MDA-MB435 cell proliferation assays were performed in vitro and in vivo. CED-14-induced apoptosis was demonstrated using Annexin V binding, TUNEL and caspase 3 assays. The expression of integrin beta1 and c-fos mRNA was detected by RT-PCR.

RESULTS

The constructed soluble CED1-4 encoded 484 amino acids and a secreting signal peptide (27 amino acids). CED1-4 was expressed by MDA-MB435/CED1-4 cells, and detected in the supernatant of CED1-4 tranfectants. CED1-4 transfection significantly inhibited the growth of MDA-MB435 cells in vitro and in vivo. About 22-fold increase in the early apoptotic cells in MDA-MB435/CED1-4 cells was observed as compared with MDA-MB435/empty cells. Increased activity of caspase 3 in MDA-MB435/CED1-4 cells was more than two times as compared with that of the control cells. Interestingly, integrin beta1 transcriptional level in MDA-MB435/CED1-4 cells was down-regulated as compared with control cells. The resistance of fibronectin to CED1-4 apoptotic inducibility was confirmed by detection of caspase 3. The blockage of c-fos transcriptional expression was detected in MDA-MB435/CED1-4 cells.

CONCLUSIONS

The soluble truncated cadherin may be considered an apoptotic inducer and growth inhibitor in the MDA-MB435 breast carcinoma cell line. Down-regulation of integrin beta1 and blockage of c-fos expression may be related to CED1-4-induced apoptosis and growth inhibition.

Type II cadherin ectodomain structures: implications for classical cadherin specificity

Type I and II classical cadherins help to determine the adhesive specificities of animal cells. Crystal-structure determination of ectodomain regions from three type II cadherins reveals adhesive dimers formed by exchange of N-terminal beta strands between partner extracellular cadherin-1 (EC1) domains. These interfaces have two conserved tryptophan side chains that anchor each swapped strand, compared with one in type I cadherins, and include large hydrophobic regions unique to type II interfaces. The EC1 domains of type I and type II cadherins appear to encode cell adhesive specificity in vitro. Moreover, perturbation of motor neuron segregation with chimeric cadherins depends on EC1 domain identity, suggesting that this region, which includes the structurally defined adhesive interface, encodes type II cadherin functional specificity in vivo.

Effect of linker segments on the stability of epithelial cadherin domain 2

Epithelial cadherin is a transmembrane protein that is essential in calcium-dependent cell-cell recognition and adhesion. It contains five independently folded globular domains in its extracellular region. Each domain has a seven-strand beta-sheet immunoglobulin fold. Short seven-residue peptide segments connect the globular domains and provide oxygens to chelate calcium ions at the interface between the domains (Nagar et al., Nature 1995;380:360-364). Recently, stability studies of ECAD2 (Prasad et al., Biochemistry 2004;43:8055-8066) were undertaken with the motivation that Domain 2 is a representative domain for this family of proteins. The definition of a domain boundary is somewhat arbitrary; hence, it was important to examine the effect of the adjoining linker regions that connect Domain 2 to the adjacent domains. Present studies employ temperature-denaturation and proteolytic susceptibility to provide insight into the impact of these linkers on Domain 2. The significant findings of our present study are threefold. First, the linker segments destabilize the core domain in the absence of calcium. Second, the destabilization due to addition of the linker segments can be partially reversed by the addition of calcium. Third, sodium chloride stabilizes all constructs. This result implies that electrostatic repulsion is a contributor to destabilization of the core domain by addition of the linkers. Thus, the context of Domain 2 within the whole molecule affects its thermodynamic characteristics.

Structure-based models of cadherin-mediated cell adhesion: the evolution continues

Cadherins are glycoproteins that are responsible for homophilic, Ca2+-dependent cell-cell adhesion and play crucial roles in many cellular adhesion processes ranging from embryogenesis to the formation of neuronal circuits in the central nervous system. Many different experimental approaches have been used to unravel the molecular basis for cadherin-mediated adhesion. In particular, several high-resolution structures have provided models for cadherin-cadherin interactions that are illuminative in many respects yet contradictory in others. This review gives an overview of the structural studies of cadherins over the past decade while focusing on recent developments that reconcile some of the earlier findings.

A dimeric version of the short N-cadherin binding motif HAVDI promotes neuronal cell survival by activating an N-cadherin/fibroblast growth factor receptor signalling cascade

The HAVDI and INPISGQ sequences have been identified as functional binding motifs in extracellular domain 1 (ECD1) of N-cadherin. Cyclic peptides containing a tandem repeat of the individual motifs function as N-cadherin agonists and stimulate neurite outgrowth. We now show that the cyclic peptide N-Ac-CHAVDINGHAVDIC-NH2 (SW4) containing the HAVDI sequence in tandem is efficacious also in promoting the in vitro survival of several populations of central nervous system neurons in paradigms where fibroblast growth factor-2 (FGF-2) is active. SW4 supported the survival of rat postnatal cerebellar granule neurons plated in serum-free medium and limited the death of differentiated granule neurons induced to die by switch to low K+ medium. In addition, SW4 rescued embryonic hippocampal and cortical neurons from injury caused by glutamic acid excitotoxicity. The neuroprotective effects of SW4 displayed a concentration dependence similar to those inducing neuritogenesis, were inhibited by a monomeric version of the same motif and by a specific FGF receptor antagonist (PD173074), and were not mimicked by the linear peptide. Inhibitors of the phosphatidylinositol 3-kinase (PI 3-kinase), MAP kinase, and p38 kinase signalling pathways did not interfere with SW4 function. These data suggest that SW4 functions by binding to and clustering N-cadherin in neurons and thereby activating and N-cadherin/FGF receptor signalling cascade, and propose that such agonists may represent a starting point for the development of therapeutic agents promoting neuronal cell survival and regeneration.

N-Acetylglucosaminyltransferase V Expression Levels Regulate Cadherin-associated Homotypic Cell-Cell Adhesion and Intracellular Signaling Pathways

A common glycan alteration in transformed cells and human tumors is the highly elevated levels of N-linked beta(1,6)glycans caused by increased transcription of N-acetylglucosaminyltransferase V (GnT-V). Here, we define the involvement of GnT-V in modulation of homotypic cell-cell adhesion in human fibrosarcoma HT1080 and mouse NIH3T3 cells. Increased GnT-V expression resulted in a significant decrease in the rates of calcium-dependent cell-cell adhesion. Reduced cell-cell adhesion was blocked by function-blocking antibody against N-cadherin and abrogated by pre-treatment of cells with swainsonine, demonstrating the involvement of N-cadherin in the cell-cell adhesion and that changes in N-linked beta(1,6)glycan expression are responsible for the reduction in rates of adhesion, although this reduction could be mediated by the altered N-linked glycosylation of glycoproteins other than N-cadherin. Overexpression of GnT-V had no effect on the levels of cell surface expression of N-cadherin; however, it did cause a marked enhancement of both beta(1,6) branching and poly-N-acetyllactosamine expression on N-cadherin. GnT-V overexpression resulted in decreased N-cadherin clustering on the cell surface induced by anti-N-cadherin antibody and affected the outside-in signal transduction pathway of ERK mediated by N-cadherin. Overexpression of GnT-V sensitized stimulation of tyrosine phosphorylation of catenins by growth factors and expression of v-src, which is consistent with its reduction of cell-cell adhesion. In vitro, GnT-V-overexpressing cells showed increased motility concomitant with increased phosphorylation of catenins. Moreover, GnT-V-deficient embryo fibroblasts from GnT-V homozygous null mice (GnT-V(-/-)) express N-cadherin and showed significantly increased levels of N-cadherin-based cell-cell adhesion compared with those from GnT-V(+/-) mice. These levels of adhesion were inhibited significantly by transient expression of GnT-V, confirming the hypothesis that levels of GnT-V can regulate cadherin-associated homotypic cell-cell adhesion. Aberrant N-linked beta(1,6) branching that occurs during oncogenesis can, therefore, lessen cell-cell adhesion, contributing to increased cellular motility and invasiveness.

Developmental patterns of cadherin expression and localization in relation to compartmentalized thalamocortical terminations in rat barrel cortex

The wiring of synaptic circuitry during development is remarkably precise, but the molecular interactions that enable such precision remain largely to be defined. Cadherins are cell adhesion molecules hypothesized to play roles in axon growth and synaptic targeting during development. We previously showed that N-cadherin localizes to ventrobasal (VB) thalamocortical synapses in rat somatosensory (barrel) cortex during formation of the whisker-map in layer IV (Huntley and Benson [1999] J. Comp. Neurol. 407:453-471). Such specific association of N-cadherin with one identified afferent pathway raises the prediction that other cadherins are expressed in barrel cortex and that these are, in some combination, also differentially associated with distinct inputs. Here, we first show that N-cadherin and three other classic cadherins (cadherin-6, -8, and -10) are expressed contemporaneously in barrel cortex with relative levels of postnatal expression that are highest during the first 2 weeks, when afferent and intrinsic circuitries are forming and synaptogenesis is maximal. Each displayed distinct, but partly overlapping laminar patterns of expression that changed over time. Cadherin-8 probe hybridization formed a particularly striking pattern of intermittent, columnar patches extending from layer V through layer III, which was first detectable at approximately postnatal day 3. The patches were centered precisely over regions of dysgranular layer IV and, in the whisker barrel field, over barrel septa. This pattern is similar to that formed by the terminal distribution of thalamocortical afferents arising from the posterior nucleus (POm), suggesting cadherin-8 association with the POm thalamocortical synaptic circuit. Consistent with this, cadherin-8 mRNAs were enriched in the POm nucleus, and cadherin-8 immunolabeling in layer IV was enriched in barrel septa and codistributed with labeled POm thalamocortical synaptic-like puncta. The striking molecular parcellation of at least two different cadherins to the two, converging thalamic pathways that terminated in non-overlapping barrel center and septal compartments in layer IV strongly suggested that cadherins provide requisite molecular recognition and targeting that enable precise construction of thalamocortical and other synaptic circuitry.

Fast dissociation kinetics between individual E-cadherin fragments revealed by flow chamber analysis

E-cadherin is the predominant adhesion molecule of epithelia. The interaction between extracellular segments of E-cadherin in the membrane of opposing cells is homophilic and calcium dependent. Whereas it is widely accepted that the specificity of the adhesive interaction is localized to the N-terminal domain, the kinetics of the recognition process are unknown. We report the first quantitative data describing the dissociation kinetics of individual E-cadherin interactions. Aggregation assays indicate that the two outermost domains of E-cadherin (E/EC1-2) retain biological activity when chemically immobilized on glass beads. Cadherin fragment trans-interaction was analysed using a flow chamber technique. Transient tethers had first-order kinetics, suggesting a unimolecular interaction. The unstressed lifetime of individual E-cadherin interactions was as brief as 2 s. A fast off rate and the low tensile strength of the E-cadherin bond may be necessary to support the high selectivity and plasticity of epithelial cell interactions.

Differential displacement of classical cadherins by VE-cadherin

VE-cadherin is an endothelial cell-specific, type II classical cadherin that plays an important role in permeability, vasculogenesis, and vascular remodeling. Endothelial cells express equal levels of VE- and N-cadherin; VE-cadherin is present injunctions while N-cadherin is diffusely expressed over the surface of the cell. The present study was designed first to determine if the ability of VE-cadherin to displace N-cadherin from junctions was endothelial-cell specific, and second to determine if VE-cadherin could displace other classical cadherins from cell junctions. Our data suggest that VE-cadherin specifically influences the cellular localization of N-cadherin, independent of cell type, and does not effect the localization of other classical cadherins.

Dimeric versions of two short N-cadherin binding motifs (HAVDI and INPISG) function as N-cadherin agonists

N-cadherin is a member of the classical cadherin family of homophilic binding molecules. Peptide competition studies have identified the HAVDI and INPISGQ sequences as functional binding motifs in extracellular domain 1 (ECD1) of N-cadherin. Whereas monomeric versions of these motifs function as specific N-cadherin antagonists, we now show that cyclic peptides containing a tandem repeat of the individual motifs function as N-cadherin agonists. In this context, when presented to neurons as soluble molecules, the dimeric versions of the motifs stimulate neurite outgrowth in a similar manner to native N-cadherin. The response to the dimeric agonist peptides was inhibited by monomeric versions of the same motif and also by recombinant N-cadherin ECD1 protein. The responses were also inhibited by antibodies to a fibroblast growth factor receptor (FGFR) binding motif in ECD4 of N-cadherin and by a specific FGFR antagonist (PD17304). These data suggest that the peptides function by binding to and clustering N-cadherin in neurons and thereby activating an N-cadherin/FGFR signaling cascade. The novel agonists will be invaluable for dissecting out those cadherin functions that rely on signaling as opposed to adhesion and clearly have the potential to be developed as therapeutic agents for the promotion of cell survival and axonal regeneration.

Identification of an N-cadherin motif that can interact with the fibroblast growth factor receptor and is required for axonal growth

In this study, we show that the neurite outgrowth response stimulated by N-cadherin is inhibited by a recently developed and highly specific fibroblast growth factor receptor (FGFR) antagonist. To test whether the N-cadherin response also requires FGF function, we developed peptide mimetics of the receptor binding sites on FGFs. Most mimetics inhibit the neurite outgrowth response stimulated by FGF in the absence of any effect on the N-cadherin response. The exceptions to this result were two mimetics of a short FGF1 sequence, which has been shown to interact with the region of the FGFR containing the histidine-alanine-valine motif. These peptides inhibited FGF and N-cadherin responses with similar efficacy. The histidine-alanine-valine region of the FGFR has previously been implicated in the N-cadherin response, and a candidate interaction site has been identified in extracellular domain 4 of N-cadherin. We now show that antibodies directed to this site on N-cadherin inhibit the neurite outgrowth response stimulated by N-cadherin, and peptide mimetics of the site inhibit N-cadherin and FGF responses. Thus, we can conclude that N-cadherin contains a novel motility motif in extracellular domain 4, and that peptide mimetics of this motif can interact with the FGFR.

Multiple cadherin extracellular repeats mediate homophilic binding and adhesion

The extracellular homophilic-binding domain of the cadherins consists of 5 cadherin repeats (EC1-EC5). Studies on cadherin specificity have implicated the NH(2)-terminal EC1 domain in the homophilic binding interaction, but the roles of the other extracellular cadherin (EC) domains have not been evaluated. We have undertaken a systematic analysis of the binding properties of the entire cadherin extracellular domain and the contributions of the other EC domains to homophilic binding. Lateral (cis) dimerization of the extracellular domain is thought to be required for adhesive function. Sedimentation analysis of the soluble extracellular segment of C-cadherin revealed that it exists in a monomer-dimer equilibrium with an affinity constant of approximately 64 microm. No higher order oligomers were detected, indicating that homophilic binding between cis-dimers is of significantly lower affinity. The homophilic binding properties of a series of deletion constructs, lacking successive or individual EC domains fused at the COOH terminus to an Fc domain, were analyzed using a bead aggregation assay and a cell attachment-based adhesion assay. A protein with only the first two NH(2)-terminal EC domains (CEC1-2Fc) exhibited very low activity compared with the entire extracellular domain (CEC1-5Fc), demonstrating that EC1 alone is not sufficient for effective homophilic binding. CEC1-3Fc exhibited high activity, but not as much as CEC1-4Fc or CEC1-5Fc. EC3 is not required for homophilic binding, however, since CEC1-2-4Fc and CEC1-2-4-5Fc exhibited high activity in both assays. These and experiments using additional EC combinations show that many, if not all, the EC domains contribute to the formation of the cadherin homophilic bond, and specific one-to-one interaction between particular EC domains may not be required. These conclusions are consistent with a previous study on direct molecular force measurements between cadherin ectodomains demonstrating multiple adhesive interactions (Sivasankar, S., W. Brieher, N. Lavrik, B. Gumbiner, and D. Leckband. 1999. PROC: Natl. Acad. Sci. USA. 96:11820-11824; Sivasankar, S., B. Gumbiner, and D. Leckband. 2001. Biophys J. 80:1758-68). We propose new models for how the cadherin extracellular repeats may contribute to adhesive specificity and function.