Cross talk between adhesion molecules: control of N-cadherin activity by intracellular signals elicited by beta1 and beta3 integrins in migrating neural crest cells

Molecular adhesion development in a neural cell monolayer forming in an ultrasound trap

A 2-dimensional aggregate of C6 neural cells was formed rapidly (within 30 s) in suspension in a recently developed 1.5 MHz ultrasound standing wave trap. A typical 1 mm diameter aggregate contained about 3,500 cells. Spreading of membrane occurred between the aggregated cells. The rate of spreading of the tangentially developing intercellular contact area was 0.19 microm/min. The form of the suspended aggregate changed from one of a hexagonal arrangement of cells to one of a cell-monolayer-like continuous sheet of mostly quadrilateral and pentagonal cells as in a cell monolayer on a solid substratum. A range of fluorescent indicators showed that the >99% viability of the cells did not change during 1 h exposures; therefore cell viability was not compromised during the monolayer development. The average integral intensities from stained actin filaments at the spreading cell-cell interfaces after 1, 8 and 30 min were 14, 25 and 46 microm(2) respectively. The cells in this work progressed from physical aggregation, through molecular adhesion, to displaying the intracellular consequences of receptor interactions. The ability to form mechanically strong confluent monolayer structures that can be monitored in situ or harvested from the trap provides a technique with general potential for monitoring the synchronous development of cell responses to receptor-triggered adhesion.

Roles of cell adhesion molecules nectin and nectin-like molecule-5 in the regulation of cell movement and proliferation

In response to chemoattractants, migrating cells form protrusions, such as lamellipodia and filopodia, and structures, such as ruffles over lamellipodia, focal complexes and focal adhesions at leading edges. The formation of these leading edge structures is essential for directional cell movement. Nectin-like molecule-5 (Necl-5) interacts in cis with PDGF receptor and integrin alpha(v)beta(3), and enhances the activation of signalling molecules associated with these transmembrane proteins, which results in the formation of leading edge structures and enhancement of directional cell movement. When migrating cells come into contact with each other, cell-cell adhesion is initiated, resulting in reduced cell velocity. Necl-5 first interacts in trans with nectin-3. This interaction is transient and induces down-regulation of Necl-5 expression at the cell surface, resulting in reduced cell movement. Cell proliferation is also suppressed by the down-regulation of Necl-5, because the inhibitory effect of Necl-5 on Sprouty2, a negative regulator of the Ras signalling, is diminished. PDGF receptor and integrin alpha(v)beta(3), which have interacted with Necl-5, then form a complex with nectin, which initiates cell-cell adhesion and recruits cadherin to the nectin-based cell-cell adhesion sites to form stable adherens junctions. The formation of adherens junctions stops cell movement, in part through inactivation of integrin alpha(v)beta(3) caused by the trans-interaction of nectin. Thus, nectin and Necl-5 play key roles in the regulation of cell movement and proliferation.

Nectins and nectin-like molecules: roles in contact inhibition of cell movement and proliferation

Nectins and nectin-like molecules (Necls) are immunoglobulin-like transmembrane cell adhesion molecules that are expressed in various cell types. Homophilic and heterophilic engagements between family members provide cells with molecular tools for intercellular communications. Nectins primarily regulate cell-cell adhesions, whereas Necls are involved in a greater variety of cellular functions. Recent studies have revealed that nectins and NECL-5, in cooperation with integrin alphavbeta3 and platelet-derived growth factor receptor, are crucial for the mechanisms that underlie contact inhibition of cell movement and proliferation; this has important implications for the development and tissue regeneration of multicellular organisms and the phenotypes of cancer cells.

Connexons and cell adhesion: a romantic phase

Recent evidence indicates, that gap junction forming proteins do not only contribute to intercellular communication (Kanno and Saffitz in Cardiovasc Pathol 10:169–177, 2001; Saez et al. in Physiol Rev 83:1359–1400, 2003), ion homeostasis and volume control (Goldberg et al. in J Biol Chem 277:36725–36730, 2002; Saez et al. in Physiol Rev 83:1359–1400, 2003). They also serve biological functions in a mechanical sense, supporting adherent connections between neighbouring cells of epithelial and non-epithelial tissues (Clair et al. in Exp Cell Res 314:1250–1265, 2008; Shaw et al. in Cell 128:547–560, 2007), where they stabilize migratory pathways in the developing central nervous system (Elias et al. in Nature 448:901–907, 2007; Malatesta et al. in Development 127:5253–5263, 2000; Noctor et al. in Nature 409:714–720, 2001; Rakic in Brain Res 33:471–476, 1971; J Comp Neurol 145:61–83 1972; Science 241:170–176, 1988), or mediate polarized movements and directionality of neural crest cells during organogenesis (Kirby and Waldo in Circ Res 77:211–215, 1995; Xu et al. in Development 133:3629–3639, 2006). Since, most data describing adhesive properties of gap junctions delt with connexin 43 (Cx43) (Beardslee et al. in Circ Res 83:629–635, 1998), we will focus our brief review on this isoform.

Cross-talk among integrin, cadherin, and growth factor receptor: roles of nectin and nectin-like molecule

Integrin, cadherin, and growth factor receptor are key molecules for fundamental cellular functions including cell movement, proliferation, differentiation, adhesion, and survival. These cell surface molecules cross-talk with each other in the regulation of such cellular functions. Nectin and nectin-like molecule (Necl) have been identified as cell adhesion molecules that belong to the immunoglobulin superfamily. Nectin and Necl play important roles in the integration of integrin, cadherin, and growth factor receptor at the cell-cell adhesion sites of contacting cells and at the leading edges of moving cells, and thus are also involved in the fundamental cellular functions together with integrin, cadherin, and growth factor receptor. This chapter describes how newly identified cell adhesion molecules, nectin and Necl, modulate the cross-talk among integrin, cadherin, and growth factor receptor and how these integrated molecules act in the regulation of fundamental cellular functions.

Wettability of substrata controls cell–substrate and cell–cell adhesions

The maintenance of endothelial cell (EC) monolayer architecture requires stable adhesions not only between neighboring cells but also between cells and the extracellular matrix. While the influence of biomaterials surface wettability on cell-substratum adhesion is rather well studied, its impact on cell-cell cohesion has not been extensively investigated. In the present study a model system consisting of hydrophilic and hydrophobic glass pre-coated with fibronectin and fibrinogen was used to study the influence of surface wettability on both types of cell adhesions. It was demonstrated that the substrate wettability controls the adhesion and cytoskeletal organization of endothelial cells, which has an impact on the subsequent ability of cells to establish stable cell-cell cohesions. These effects were related to the accessibility of specific domains of the adsorbed proteins. While the hydrophobic substratum promoted cell-cell cohesion, on hydrophilic substrata cell-substrate adhesion was dominant. In addition, evidence for an influence of surface wettability on the cross talk between integrins and cadherins was found.

Cellular prion protein interaction with vitronectin supports axonal growth and is compensated by integrins

The physiological functions of the cellular prion protein, PrP(C), as a cell surface pleiotropic receptor are under debate. We report that PrP(C) interacts with vitronectin but not with fibronectin or collagen. The binding sites mediating this PrP(C)-vitronectin interaction were mapped to residues 105-119 of PrP(C) and the residues 307-320 of vitronectin. The two proteins were co-localized in embryonic dorsal root ganglia from wild-type mice. Vitronectin addition to cultured dorsal root ganglia induced axonal growth, which could be mimicked by vitronectin peptide 307-320 and abrogated by anti-PrP(C) antibodies. Full-length vitronectin, but not the vitronectin peptide 307-320, induced axonal growth of dorsal root neurons from two strains of PrP(C)-null mice. Functional assays demonstrated that relative to wild-type cells, PrP(C)-null dorsal root neurons were more responsive to the Arg-Gly-Asp peptide (an integrin-binding site), and exhibited greater alphavbeta3 activity. Our findings indicate that PrP(C) plays an important role in axonal growth, and this function may be rescued in PrP(C)-knockout animals by integrin compensatory mechanisms.

Involvement of integrin-induced activation of protein kinase C in the formation of adherens junctions

In epithelial cells, tight junctions (TJs) and adherens junctions (AJs) form junctional complexes. At AJs, cadherins and nectins are the major cell-cell adhesion molecules. Nectins first form cell-cell adhesions and then recruit cadherins to the nectin-based cell-cell adhesion sites to form AJs in coordination with the activation of integrin alpha(v)beta(3), followed by the formation of TJs. We previously demonstrated that when MDCK cells precultured at a low Ca(2+) concentration were treated with the protein kinase C (PKC) activator 12-O-tetradecanoyl-phorbol-13-acetate (TPA), incomplete AJs and a TJ-like structure were achieved. However, it remains unknown how PKC is activated and how it regulates the formation of cell-cell junctions. When MDCK cells precultured at a low Ca(2+) concentration were treated with TPA, incomplete AJs were formed without the activation of integrin alpha(v)beta(3). Treatment of cells with TPA also enhanced the phosphorylation of FAK, which transmits the outside-in signal of integrin and plays a role in the nectin-induced formation of AJs. In addition, inhibition of PKC suppressed the formation of AJs. These results indicate that the activation of PKC functions downstream of integrin alpha(v)beta(3) and upstream of FAK, and is important for the nectin-induced formation of AJs.

N-cadherin as a therapeutic target in cancer

During tumor progression, cancer cells undergo dramatic changes in the expression profile of adhesion molecules resulting in detachment from original tissue and acquisition of a highly motile and invasive phenotype. A hallmark of this change, also referred to as the epithelial-mesenchymal transition, is the loss of E- (epithelial) cadherin and the de novo expression of N- (neural) cadherin adhesion molecules. N-cadherin promotes tumor cell survival, migration and invasion, and a high level of its expression is often associated with poor prognosis. N-cadherin is also expressed in endothelial cells and plays an essential role in the maturation and stabilization of normal vessels and tumor-associated angiogenic vessels. Increasing experimental evidence suggests that N-cadherin is a potential therapeutic target in cancer. A peptidic N-cadherin antagonist (ADH-1) has been developed and has entered clinical testing. In this review, the authors discuss the biochemical and functional features of N-cadherin, its potential role in cancer and angiogenesis, and summarize the preclinical and clinical results achieved with ADH-1.

Connexin 43-mediated modulation of polarized cell movement and the directional migration of cardiac neural crest cells

Connexin 43 knockout (Cx43α1KO) mice have conotruncal heart defects that are associated with a reduction in the abundance of cardiac neural crest cells (CNCs) targeted to the heart. In this study, we show CNCs can respond to changing fibronectin matrix density by adjusting their migratory behavior,with directionality increasing and speed decreasing with increasing fibronectin density. However, compared with wild-type CNCs, Cx43α1KO CNCs show reduced directionality and speed, while CNCs overexpressing Cx43α1 from the CMV43 transgenic mice show increased directionality and speed. Altered integrin signaling was indicated by changes in the distribution of vinculin containing focal contacts, and altered temporal response of Cx43α1KO and CMV43 CNCs to β1 integrin function blocking antibody treatment. High resolution motion analysis showed Cx43α1KO CNCs have increased cell protrusive activity accompanied by the loss of polarized cell movement. They exhibited an unusual polygonal arrangement of actin stress fibers that indicated a profound change in cytoskeletal organization. Semaphorin 3A, a chemorepellent known to inhibit integrin activation, was found to inhibit CNC motility, but in the Cx43α1KO and CMV43 CNCs, cell processes failed to retract with semaphorin 3A treatment. Immunohistochemical and biochemical analyses suggested close interactions between Cx43α1,vinculin and other actin-binding proteins. However, dye coupling analysis showed no correlation between gap junction communication level and fibronectin plating density. Overall, these findings indicate Cx43α1 may have a novel function in mediating crosstalk with cell signaling pathways that regulate polarized cell movement essential for the directional migration of CNCs.

N-cadherin is required for neural crest remodeling of the cardiac outflow tract

Cardiac neural crest cells undergo extensive cell rearrangements during the formation of the aorticopulmonary septum in the outflow tract. However, the morphogenetic mechanisms involved in this fundamental process remain poorly understood. To determine the function of the Ca2+-dependent cell adhesion molecule, N-cadherin, in murine neural crest, we applied the Cre/loxP system and created mouse embryos genetically mosaic for N-cadherin. Specifically, deletion of N-cadherin in neural crest cells led to embryonic lethality with distinct cardiovascular defects. Neural crest cell migration and homing to the cardiac outflow tract niche were unaffected by loss of N-cadherin. However, N-cadherin-deficient neural crest cells were unable to undergo the normal morphogenetic changes associated with outflow tract remodeling, resulting in persistent truncus arteriosus in the majority of mutant embryos. Other mutant embryos initiated aorticopulmonary septum formation; however, the neural crest cells were unable to elongate and align properly along the midline and remained rounded with limited contact with their neighbors. Interestingly, rotation of the outflow tract was incomplete in these mutants suggesting that alignment of the channels is dependent on N-cadherin-generated cytoskeletal forces. A second cardiac phenotype was observed where loss of N-cadherin in the epicardium led to disruption of heterotypic cell interactions between the epicardium and myocardium resulting in a thinned ventricular myocardium. Thus, we conclude that in addition to its role in myocardial cell adhesion, N-cadherin is required for neural crest cell rearrangements critical for patterning of the cardiac outflow tract and in the maintenance of epicardial-myocardial cell interactions.

Nectins and nectin-like molecules: roles in cell adhesion, polarization, movement, and proliferation

Nectins and nectin-like molecules (Necls) are immunoglobulin-like cell adhesion molecules that constitute families containing four and five members, respectively. All members, except for Necl-5, trans-interact homophilically. Furthermore, all members, including Necl-5, trans-interact heterophilically with their respective specific partners among the members. Necl-5 regulates cell movement and proliferation cooperatively with integrin alphavbeta3 and growth factor receptors. Nectins function as cell-cell adhesion molecules at a variety of cell-cell junctions, including adherens junctions, and regulate the initial step of cell-cell junction formation. Nectins and integrin alphavbeta3 are further involved in the cross-talk between cell-matrix and cell-cell junctions. Thus, both nectin and Necl family members play important roles in fundamental cellular functions, including cell adhesion, polarization, movement, and proliferation.

α1β1-integrin engagement to distinct laminin-1 domains orchestrates spreading, migration and survival of neural crest cells through independent signaling pathways

Integrin engagement regulates cell adhesion, shape, migration, growth, and differentiation, but molecular mechanisms coordinating these functions in cells remain unclear. Because of their migratory and differentiation potential, neural crest cells constitute a powerful paradigm to address this question. Here, we describe that laminin-1, a major component of their migration routes, promotes crest cell spreading, migration and survival through two distinct integrin-binding domains that are situated on both sides of its α1 subunit and can be separated in the LN-1 elastase proteolytic fragments E1' and E8. Interaction with either domain was mediated by the same integrin α1β1 but produced distinct, complementary responses through specific signaling cascades. FAK activation upon E8 binding induced spreading, formation of actin bundles and focal adhesions, stimulated oriented migration, but failed to support survival. Conversely, Erk activation upon E1' binding promoted long-term survival and random migration without actin reorganization. Consistent with this, interaction with laminin-5 or laminin-10/11, which do not harbor integrin-binding domains in the N-terminal side of their α chains, failed to support survival. Thus, the signaling activity and function of integrins might depend on binding domains in their ligands, thereby revealing ligand control of integrin function as a possible mechanism for the modulation and coordination of cell response to adhesive signals.

Interaction of integrin alpha(v)beta3 with nectin. Implication in cross-talk between cell-matrix and cell-cell junctions

Cell-matrix and cell-cell junctions cross-talk together, and these two junctions cooperatively regulate cell movement, proliferation, adhesion, and polarization. However, the mechanism of this cross-talk remains unknown. An immunoglobulin-like cell-cell adhesion molecule nectin first trans-interacts with each other to form cell-cell adhesion and induces activation of Rap1, Cdc42, and Rac small G proteins through c-Src. Trans-interacting nectin then recruits another cell-cell adhesion molecule cadherin to the nectin-based cell-cell adhesion sites and forms adherens junctions (AJs). Here, we show that integrin alpha(v)beta3 functionally and physically associates with nectin. Integrin alpha(v)beta3 colocalized with nectin at the nectin-based cell-cell adhesion sites. The association of integrin alpha(v)beta3 with nectin was direct and was mediated through their extracellular regions. This interaction was necessary for the nectin-induced signaling. Focal adhesion kinase, which relays the integrin-initiated outside-in signals to the intracellular signaling molecules, was also involved in the nectin-induced signaling. During the formation of AJs, the high affinity form of integrin alpha(v)beta3 co-localized with nectin at the primordial cell-cell contact sites, and then after the establishment of AJs, this high affinity form of integrin alpha(v)beta3 was converted to the low affinity form, which continued to co-localize with nectin. Thus, integrin alpha(v)beta3 and nectin play pivotal roles in the cross-talk between cell-matrix and cell-cell junctions and the formation of cadherin-based AJs.

Phosphoinositide-3 kinase-Rac1-c-Jun NH2-terminal kinase signaling mediates collagen I-induced cell scattering and up-regulation of N-cadherin expression in mouse mammary epithelial cells

During epithelial-to-mesenchymal transitions (EMTs), cells must change their interactions with one another and with their extracellular matrix in a synchronized manner. To characterize signaling pathways cells use to coordinate these changes, we used NMuMG mammary epithelial cells. We showed that these cells become fibroblastic and scattered, with increased N-cadherin expression when cultured on collagen I. Rac1 and c-Jun NH2-terminal kinase (JNK) were activated when cells were plated on collagen I, and dominant inhibitory Rac1 (RacN17) or inhibition of JNK signaling prevented collagen I-induced morphological changes and N-cadherin up-regulation. Furthermore, inhibiting phosphoinositide-3 kinase (PI3K) activity prevented Rac1 and JNK activation as well as collagen I-induced N-cadherin up-regulation. These data implicate PI3K-Rac1-JNK signaling in collagen I-induced changes in NMuMG cells. To establish a role for N-cadherin in collagen I-induced cell scattering, we generated N-cadherin overexpressing and knockdown NMuMG cells and showed that knocking down N-cadherin expression prevented collagen I-induced morphological changes. Motility assays showed that cells overexpressing N-cadherin were significantly more motile than mock-transfected cells and that N-cadherin-mediated motility was collagen I dependent. In addition, we showed that cord formation and branching in three-dimensional culture (EMT-dependent events) required N-cadherin expression and PI3K-Rac1-JNK signaling.

Lack of beta1 integrins in enteric neural crest cells leads to a Hirschsprung-like phenotype

The enteric nervous system arises mainly from vagal and sacral neural crest cells that colonise the gut between 9.5 and 14 days of development in mice. Using the Cre-LoxP system, we removed beta1 integrins in the neural crest cells when they emerge from the neural tube. beta1-null enteric neural crest cells fail to colonise the gut completely, leading to an aganglionosis of the descending colon, which resembles the human Hirschsprung's disease. Moreover, beta1-null enteric neural crest cells form abnormal aggregates in the gut wall, leading to a severe alteration of the ganglia network organisation. Organotypic cultures of gut explants reveal that beta1-null enteric neural crest cells show impaired adhesion on extracellular matrix and enhanced intercellular adhesion properties. They display migration defects in collagen gels and gut tissue environments. We also provide evidence that beta1 integrins are required for the villi innervation in the small intestine. Our findings highlight the crucial roles played by beta1 integrins at various steps of enteric nervous system development.

Neural crest delamination and migration: integrating regulations of cell interactions, locomotion, survival and fate

During the entire process of neural crest development from specification till final differentiation, delamination and migration are critical steps where nascent crest cells face multiple challenges: within a relatively short period of time that does not exceed several hours, they have to change drastically their cell- and substrate-adhesion properties, lose cell polarity and activate the locomotory machinery, while keeping proliferating, surviving and maintaining a pool of precursors in the neural epithelium. Then, as soon as they are released from the neural tube, neural crest cells have to adapt to a new, rapidly-changing environment and become able to interpret multiple cues which guide them to appropriate target sites and prevent them from distributing in aberrant locations. It appears from recent studies that, behind an apparent linearity and unity, neural crest development is subdivided into several independent steps, each being governed by a multiplicity of rules and referees. Here resides probably one of the main reasons of the success of neural crest cells to accomplish their task.

Laminin-5-integrin interaction signals through PI 3-kinase and Rac1b to promote assembly of adherens junctions in HT-29 cells

Human intestinal cell differentiation is mediated by signaling pathways that remain largely undefined. We and others have shown that cell migration and differentiation along the crypt-villus axis is associated with temporal and spatial modulations of the repertoire, as well as with the function of integrins and E-cadherins and their substrates. Cross-talk between integrin and cadherin signaling was previously described and seems to coordinate this differentiation process. Here, we report that engagement of alpha6 and, to a lesser extent, alpha3 integrin subunits after HT-29 cell adhesion on laminin 5 increases the expression of E-cadherin, which then organizes into nascent adherens junctions. We further identify that phosphoinositide 3-kinase (PI 3-kinase) activation plays a key role in this cross-talk. Indeed, integrin-dependent adhesion on laminin 5 stimulates PI 3-kinase activity. Immunofluorescence and immunoprecipitation experiments revealed that activated PI 3-kinase is recruited at cell-cell contacts. Using LY294002, an inhibitor of PI 3-kinase activity, we found that this activation is essential for E-cadherin connection with the cytoskeleton and for biogenesis of adherens junctions. Finally, we demonstrated that PI 3-kinase could signal through Rac1b activation to control adherens junction assembly. Our results provide a mechanistic insight into integrin-cadherin cross-talk and identify a novel role for PI 3-kinase in the establishment of adherens junctions.

Rap1: a turnabout for the crosstalk between cadherins and integrins

The coordinate modulation of the cellular functions of cadherins and integrins plays an essential role in fundamental physiological and pathological processes, including morphogenesis, tissue differentiation and renewal, wound healing, immune surveillance, inflammatory response, tumour progression, and metastasis. However, the molecular mechanisms underlying the fine-balanced relationship between cadherin and integrin functions are still elusive. This review focuses on recent findings on the involvement of the small GTPase Rap1 in the regulation of cadherin- and integrin-dependent cell adhesion and signal transduction. In particular, it highlights some of the novel results recently obtained that raise the possibility of a pivotal role for Rap1 in the functional crosstalk between cadherins and integrins, suggesting interesting new regulatory mechanisms.

Role of the beta1-integrin subunit in the adhesion, extravasation and migration of T24 human bladder carcinoma cells

The abilities of tumor cells to extravasate from the blood vessel system and to migrate through the connective tissue are prerequisites in metastasis formation. Both processes are chiefly mediated by integrins, which mediate both cell-cell and cell-matrix interactions. We investigated the role of integrin subunits in the adhesion, extravasation and migration of the highly invasive human bladder carcinoma cell line T24. Here we show that inhibition of the beta(1)-integrin subunit using the specific beta(1)-integrin blocking antibody 4B4 significantly reduces the adhesion to HUVEC and transmigratory activity of T24 cells. The blockade of the beta(1)-integrin subunit also resulted in a significantly reduced locomotory activity of T24 cells. A detailed cell migration analysis on a single cell level revealed that blockade of the beta(1)-integrin subunit leads to an altered migration pattern of single cells but does not influence migration per se. Migration parameters such as time active, velocity and distance migrated were significantly reduced as compared to untreated control cells. Our observations strongly suggest a central role for the beta(1)-integrin subunit in forming the cell-cell and cell-matrix bonds necessary for adhesion, extravasation and migration.

Phosphorylation of N-cadherin-associated cortactin by Fer kinase regulates N-cadherin mobility and intercellular adhesion strength

Cortactin regulates the strength of nascent N-cadherin-mediated intercellular adhesions through a tyrosine phosphorylation-dependent mechanism. Currently, the functional significance of cortactin phosphorylation and the kinases responsible for the regulation of adhesion strength are not defined. We show that the nonreceptor tyrosine kinase Fer phosphorylates cadherin-associated cortactin and that this process is involved in mediating intercellular adhesion strength. In wild-type fibroblasts N-cadherin ligation-induced transient phosphorylation of Fer, indicating that junction formation activates Fer kinase. Tyrosine phosphorylation of cortactin after N-cadherin ligation was strongly reduced in fibroblasts expressing only catalytically inactive Fer (D743R), compared with wild-type cells. In wild-type cells, N-cadherin-coated bead pull-off assays induced fourfold greater endogenous N-cadherin association than in D743R cells. Fluorescence recovery after photobleaching showed that GFP-N-cadherin mobility at nascent contacts was 50% faster in wild-type than D743R cells. In shear wash-off assays, nascent intercellular adhesion strength was twofold higher in wild-type than D743R cells. Cortactin recruitment to adhesions was independent of Fer kinase activity, but was impacted by N-cadherin ligation-provoked Rac activation. We conclude that N-cadherin ligation induces Rac-dependent cortactin recruitment and Fer-dependent cortactin phosphorylation, which in turn promotes enhanced mobilization and interaction of surface expressed N-cadherin in contacting cells.

Force measurements in E-cadherin-mediated cell doublets reveal rapid adhesion strengthened by actin cytoskeleton remodeling through Rac and Cdc42

We have used a modified, dual pipette assay to quantify the strength of cadherin-dependent cell–cell adhesion. The force required to separate E-cadherin–expressing paired cells in suspension was measured as an index of intercellular adhesion. Separation force depended on the homophilic interaction of functional cadherins at the cell surface, increasing with the duration of contact and with cadherin levels. Severing the link between cadherin and the actin cytoskeleton or disrupting actin polymerization did not affect initiation of cadherin-mediated adhesion, but prevented it from developing and becoming stronger over time. Rac and Cdc42, the Rho-like small GTPases, were activated when E-cadherin–expressing cells formed aggregates in suspension. Overproduction of the dominant negative form of Rac or Cdc42 permitted initial E-cadherin–based adhesion but affected its later development; the dominant active forms prevented cell adhesion outright. Our findings highlight the crucial roles played by Rac, Cdc42, and actin cytoskeleton dynamics in the development and regulation of strong cell adhesion, defined in terms of mechanical forces.

Comparison of molecular mechanisms mediating cell contact phenomena in model developmental systems: an exploration of universality

Are there universal molecular mechanisms associated with cell contact phenomena during metazoan ontogenesis? Comparison of adhesion systems in disparate model systems indicates the existence of unifying principles. Requirements for multicellularity are (a) the construction of three-dimensional structures involving a crucial balance between adhesiveness and motility; and (b) the establishment of integration at molecular, cellular, tissue, and organismal levels of organization. Mechanisms for (i) cell-cell and cell-substrate adhesion, (ii) cell movement, (iii) cell-cell communication, (iv) cellular responses, (v) regulation of these processes, and (vi) their integration with patterning, growth, and other developmental processes are all crucial to metazoan development, and must have been present for the emergence and radiation of Metazoa. The principal unifying themes of this review are the dynamics and regulation of cell contact phenomena. Our knowledge of the dynamic molecular mechanisms underlying cell contact phenomena remains fragmentary. Here we examine the molecular bases of cell contact phenomena using extant model developmental systems (representing a wide range of phyla) including the simplest i.e. sponges, and the eukaryotic protist Dictyostelium discoideum, the more complex Drosophila melanogaster, and vertebrate systems. We discuss cell contact phenomena in a broad developmental context. The molecular language of cell contact phenomena is complex; it involves a plethora of structurally and functionally diverse molecules, and diverse modes of intermolecular interactions mediated by protein and/or carbohydrate moieties. Reasons for this are presumably the necessity for a high degree of specificity of intermolecular interactions, the requirement for a multitude of different signals, and the apparent requirement for an increasingly large repertoire of cell contact molecules in more complex developmental systems, such as the developing vertebrate nervous system. However, comparison of molecular models for dynamic adhesion in sponges and in vertebrates indicates that, in spite of significant differences in the details of the way specific cell-cell adhesion is mediated, similar principles are involved in the mechanisms employed by members of disparate phyla. Universal requirements are likely to include (a) rapidly reversible intermolecular interactions; (b) low-affinity intermolecular interactions with fast on-off rates; (c) the compounding of multiple intermolecular interactions; (d) associated regulatory signalling systems. The apparent widespread employment of molecular mechanisms involving cadherin-like cell adhesion molecules suggests the fundamental importance of cadherin function during development, particularly in epithelial morphogenesis, cell sorting, and segregation of cells.

Cortactin associates with N-cadherin adhesions and mediates intercellular adhesion strengthening in fibroblasts

The regulation of N-cadherin-mediated intercellular adhesion strength in fibroblasts is poorly characterized; this is due, in part, to a lack of available quantitative models. We used a recombinant N-cadherin chimeric protein and a Rat 2 fibroblast, donor-acceptor cell model, to study the importance of cortical actin filaments and cortactin in the strengthening of N-cadherin adhesions. In wash-off assays, cytochalasin D (1 microM) reduced intercellular adhesion by threefold, confirming the importance of cortical actin filaments in strengthening of N-cadherin-mediated adhesions. Cortactin, an actin filament binding protein, spatially colocalized to, and directly associated with, nascent N-cadherin adhesion complexes. Transfection of Rat-2 cells with cortactin-specific, RNAi oligonucleotides reduced cortactin protein by 85% and intercellular adhesion by twofold compared with controls (P<0.005) using the donor-acceptor model. Cells with reduced cortactin exhibited threefold less N-cadherin-mediated intercellular adhesion strength compared with controls in wash-off assays using N-cadherin-coated beads. Immunoprecipitation and immunoblotting showed that N-cadherin-associated cortactin was phosphorylated on tyrosine residue 421 after intercellular adhesion. While tyrosine phosphorylation of cortactin was not required for recruitment to N-cadherin adhesions it was necessary for cadherin-mediated intercellular adhesion strength. Thus cortactin, and phosphorylation of its tyrosine residues, are important for N-cadherin-mediated intercellular adhesion strength.

Induction of apoptosis in cultured endothelial cells by a cadherin antagonist peptide: involvement of fibroblast growth factor receptor-mediated signalling

Cadherins are a family of transmembrane glycoproteins mediating calcium-dependent, homophilic cell-cell adhesion. In addition, these molecules are involved in signaling events, regulating such processes as cell motility, proliferation, and apoptosis. Members of the cadherin subfamily, called either classical or type I cadherins, contain a highly conserved sequence at their homophilic binding site consisting of the three amino acids--histidine-alanine-valine (HAV). Previous studies have shown that peptides containing the HAV motif inhibit cadherin-dependent events such as cell aggregation, compaction, and neurite outgrowth. We report here that a cyclic peptide, N-Ac-CHAVC-NH2 can perturb cadherin-mediated endothelial cell interactions, resulting in a progressive apoptotic cell death. This effect depends on cell density, as it is only observed when dense cultures are treated with the peptide. Adherens junction (AJ)-associated cadherin and catenins are differentially affected by the N-Ac-CHAVC-NH2 treatment, as judged by double immunofluorescence labeling followed by immunofluorescence-ratio imaging. However, cell-cell adhesions are largely retained during the first few hours after addition of the peptide. It was also observed that following treatment, actin filaments partially lose their plasma membrane anchorage at AJs and translocate towards the cell center. Interestingly, addition of basic fibroblast growth factor to confluent, peptide-treated, endothelial cell cultures, completely blocks apoptosis and the inhibitory peptide reduce the phosphorylation of the FGF receptor target protein FRS2, suggesting that the peptide exerts its effect by inhibiting cadherin-mediated activation of fibroblast growth factor receptor signaling. We propose that cadherin-mediated signaling is essential for maintaining viability of confluent endothelial cells, and that its perturbation by N-Ac-CHAVC-NH2 drives these cells to apoptosis.

p120 catenin associates with kinesin and facilitates the transport of cadherin-catenin complexes to intercellular junctions

p120 catenin (p120) is a component of adherens junctions and has been implicated in regulating cadherin-based cell adhesion as well as the activity of Rho small GTPases, but its exact roles in cell–cell adhesion are unclear. Using time-lapse imaging, we show that p120-GFP associates with vesicles and exhibits unidirectional movements along microtubules. Furthermore, p120 forms a complex with kinesin heavy chain through the p120 NH₂-terminal head domain. Overexpression of p120, but not an NH₂-terminal deletion mutant deficient in kinesin binding, recruits endogenous kinesin to N-cadherin. Disruption of the interaction between N-cadherin and p120, or the interaction between p120 and kinesin, leads to a delayed accumulation of N-cadherin at cell–cell contacts during calcium-initiated junction reassembly. Our analyses identify a novel role of p120 in promoting cell surface trafficking of cadherins via association and recruitment of kinesin.

Cell surface molecules and truncal neural crest ontogeny: A perspective

The neural crest cell is synonymous with vertebrates and can be viewed as a transitory, mobile vector that conveys neuroepithelial stem cells to a diverse number of remote locations in the embryo. Neural crest cells have been studied intensively over the past 30 years, and it is increasingly apparent that their fate is, at least in part, directed extrinsically by the environment to which they are exposed in vivo. The interface between the cell surface and the opposing environment is clearly an important compartment for the correct deployment of the neural crest. Here, we review some of the molecules present in this location and how they influence the fate of the neural crest and generate disease.

Cellular localization and signaling activity of ?-catenin in migrating neural crest cells

In the vertebrate embryo, development of the neural crest is accompanied by sequential changes in cellular adhesiveness, allowing cells to delaminate from the neural epithelium, to undergo migration through extracellular matrix material, and to coalesce into ganglia of the peripheral nervous system. Because of its dual role in cell adhesion, as a link between cadherins and the actin cytoskeleton, and in cell signaling, as a key mediator of the Wnt-signaling pathway, beta-catenin is a good candidate to play a central role in the control of neural crest cell development. In the present study, we analyzed, by using an in vitro culture system, whether the cellular localization and the signaling activity of beta-catenin are regulated in conjunction with cell migration during ontogeny of trunk neural crest cells in the avian embryo. beta-Catenin molecules were found primarily in association with N-cadherin in the regions of intercellular contacts in most migrating neural crest cells, and only early-migrating cells situated in proximity with the dorsal side of the neural tube showed detectable beta-catenin in their nuclei. This finding indicates that beta-catenin may be recruited for signaling in neural crest cells only transiently at the onset of migration and that sustained beta-catenin signals are not necessary for the progression of migration. The nuclear distribution of beta-catenin within crest cells was not affected upon modification of the N-cadherin-mediated cell-cell contacts, revealing that recruitment of beta-catenin for signaling is not driven by changes in intercellular cohesion during migration. Overstimulation of beta-catenin signals in neural crest cells at the time of their migration, using LiCl treatment or coculture with Wnt-1-producing cells, induced nuclear translocation of beta-catenin and Lef-1 up-regulation in neural crest cells and provoked a marked inhibition of cell delamination and migration. The effect of LiCl and exogenous Wnt-1 on neural crest cells could be essentially attributed to a dramatic decrease in integrin-mediated cell-matrix adhesion as well as a massive reduction of cell proliferation. In addition, although it apparently did not affect expression of neural crest markers, Wnt-1 exposure dramatically affected signaling events involving Notch-Delta, presumably also accounting for the strong reduction in cell delamination. In conclusion, our data indicate that beta-catenin functions primarily in cell adhesion events during migration and may be recruited transiently for signaling during delamination possibly to regulate the balance between cell proliferation and cell differentiation.

A dual role for Sonic hedgehog in regulating adhesion and differentiation of neuroepithelial cells

In vertebrates, the nervous system arises from a flat sheet of epithelial cells, the neural plate, that gradually transforms into a hollow neural tube. This process, called neurulation, involves sequential changes in cellular interactions that are precisely coordinated both spatially and temporally by the combined actions of morphogens. To gain further insight into the molecular events regulating cell adhesion during neurulation, we investigated whether the adhesive and migratory capacities of neuroepithelial cells might be modulated by Sonic hedgehog (Shh), a signaling molecule involved in the control of cell differentiation in the ventral neural tube. When deposited onto extracellular matrix components in vitro, neural plates explanted from avian embryos at early neurulation readily dispersed into monolayers of spread cells, thereby revealing their intrinsic ability to migrate. In the presence of Shh added in solution to the culture medium, the explants still exhibited the same propensity to disperse. In contrast, when Shh was immobilized to the substrate or produced by neuroepithelial cells themselves after transfection, neural plate explants failed to disperse and instead formed compact structures. Changes in the adhesive capacities of neuroepithelial cells caused by Shh could be accounted for by inactivation of surface beta1-integrins combined with an increase in N-cadherin-mediated cell adhesion. Furthermore, immobilized Shh promoted differentiation of neuroepithelial cells into motor neurons and floor plate cells with the same potency as soluble Shh. However, the effect of Shh on the neuroepithelial cell adhesion was discernible and apparently independent from its differentiation effect and was not mediated by the signaling cascade elicited by the Patched-Smoothened receptor and involving the Gli transcription factors. Thus, our experiments indicate that Shh is able to control sequentially adhesion and differentiation of neuroepithelial cells through different mechanisms, leading to a coordinated regulation of the various cell interactions essential for neural tube morphogenesis.

Cadherin-mediated adhesion is essential for myofibril continuity across the plasma membrane but not for assembly of the contractile apparatus

The strong coordinated contraction of heart muscle is dependent on the correct alignment and connection of the myofibrils across the plasma membrane. Previous studies indicate that N-cadherin is involved in cardiac myocyte adhesion and myofibrillogenesis. To investigate whether N-cadherin is specifically required for normal myocyte structure and function, we cultured myocytes from wild-type, N-cadherin-null and mutant embryos expressing the epithelial cadherin E-cadherin. In contrast to previous studies in chicken using N-cadherin-perturbing antibodies, our in vitro studies with mouse cells demonstrate that N-cadherin is not required for myofibrillogenesis, but is critical for myofibril organization. That is, N-cadherin-deficient myocytes beat and myofibrils were well formed; however, alignment of the myofibrils through regions of cell-cell contact was lost, resulting in their random orientation. Gap junctions were perturbed in the N-cadherin-null myocytes. By contrast, focal contacts appeared normal in the mutant cells. Furthermore, E-cadherin restored normal cell morphology and behavior to the N-cadherin-deficient myocytes, including proper alignment of the myofibrils. We conclude that a different adhesive system, most probably integrin, is responsible for myofibrillogenesis in the N-cadherin-null myocytes.

Expression of Ep-CAM shifts the state of cadherin-mediated adhesions from strong to weak

Various adhesion molecules play an important role in defining cell fate and maintaining tissue integrity. Therefore, cross-signaling between adhesion receptors should be a common phenomenon to support the orchestrated changes of cells' connections to the substrate and to the neighboring cells during tissue remodeling. Recently, we have demonstrated that the epithelial cell adhesion molecule Ep-CAM negatively modulates cadherin-mediated adhesions in direct relation to its expression levels. Here, we used E-cadherin/alpha-catenin chimera constructs to define the site of Ep-CAM's negative effect on cadherin-mediated adhesions. Murine L-cells transfected with either E-cadherin/alpha-catenin fusion protein, or E-cadherin fused to the carboxy-terminal half of alpha-catenin, were subsequently supertransfected with an inducible Ep-CAM construct. Introduction of Ep-CAM altered the cell's morphology, weakened the strength of cell-cell interactions, and decreased the cytoskeleton-bound fraction of the cadherin/catenin chimeras in both cell models. Furthermore, expression of Ep-CAM induced restructuring of F-actin, with changes in thickness and orientation of the actin filaments. The results showed that Ep-CAM affects E-cadherin-mediated adhesions without involvement of beta-catenin by disrupting the link between alpha-catenin and F-actin. The latter is likely achieved through remodeling of the actin cytoskeleton by Ep-CAM, possibly through pp120.

The thrombospondin type 1 repeat (TSR) and neuronal differentiation: roles of SCO-spondin oligopeptides on neuronal cell types and cell lines

SCO-spondin is a large glycoprotein secreted by ependymal cells of the subcommissural organ. It shares functional domains called thrombospondin type 1 repeats (TSRs) with a number of developmental proteins expressed in the central nervous system, and involved in axonal pathfinding. Also, SCO-spondin is highly conserved in the chordate phylum and its multiple domain organization is probably a chordate innovation. The putative involvement of SCO-spondin in neuron/glia interaction in the course of development is assessed in various cell culture systems. SCO-spondin interferes with several developmental processes, including neuronal survival, neurite extension, neuronal aggregation, and fasciculation. The TSR motifs, and especially the WSGWSSCSVSCG sequence, are most important in these neuronal responses. Integrins and growth factor receptors may cooperate as integrative signals. We discuss the putative involvement of the subcommissural organ/Reissner's fiber complex in developmental events, as a particular extracellular signaling system.

The fibronectin-binding integrins alpha5beta1 and alphavbeta3 differentially modulate RhoA-GTP loading, organization of cell matrix adhesions, and fibronectin fibrillogenesis

We have studied the formation of different types of cell matrix adhesions in cells that bind to fibronectin via either alpha5beta1 or alphavbeta3. In both cases, cell adhesion to fibronectin leads to a rapid decrease in RhoA activity. However, alpha5beta1 but not alphavbeta3 supports high levels of RhoA activity at later stages of cell spreading, which are associated with a translocation of focal contacts to peripheral cell protrusions, recruitment of tensin into fibrillar adhesions, and fibronectin fibrillogenesis. Expression of an activated mutant of RhoA stimulates alphavbeta3-mediated fibrillogenesis. Despite the fact that alpha5beta1-mediated adhesion to the central cell-binding domain of fibronectin supports activation of RhoA, other regions of fibronectin are required for the development of alpha5beta1-mediated but not alphavbeta3-mediated focal contacts. Using chimeras of beta1 and beta3 subunits, we find that the extracellular domain of beta1 controls RhoA activity. By expressing both beta1 and beta3 at high levels, we show that beta1-mediated control of the levels of beta3 is important for the distribution of focal contacts. Our findings demonstrate that the pattern of fibronectin receptors expressed on a cell dictates the ability of fibronectin to stimulate RhoA-mediated organization of cell matrix adhesions.

Neural crest cell motility in valproic acid

Neural crest cells (NCCs) exit the dorsal neural tube and migrate to sites where they form diverse tissues. Valproic acid (VPA) is an anticonvulsant drug that induces neural tube and related defects. Altered NCC migration and proliferation have been proposed as mechanisms of teratogenicity. We cultured neural tube segments from chick embryos in 0.75-3.0mM VPA. We used image analysis, proliferation assays, and fluorescence localization to investigate NCCs during VPA exposure. VPA inhibited attachment of explants and the number that produced migrating cells. VPA markedly decreased the proportion of cells migrating individually, promoting migration as epithelial sheets. VPA at 3mM decreased cellular spreading. Area and perimeter change per minute were reduced, but migration velocity was not. VPA at 2mM reduced proliferation 11% and 3mM arrested proliferation. Immunostaining of VPA-exposed explants revealed N-cadherin-positive cell boundaries within sheets, but independent NCCs did not stain. F-actin staining was reduced in independent NCCs. The data support a VPA mechanism involving interference with epithelial-mesenchymal transition.

Role of N-cadherin in bone formation

Cell-cell adhesion mediated by cadherins is essential for the function of bone forming cells during osteogenesis. Here, the evidence that N-cadherin is an important regulator of osteoblast differentiation and osteogenesis is reviewed. Osteoblasts express a limited number of cadherins, including the classic N-cadherin. The expression profile of N-cadherin in osteoblasts during bone formation in vivo and in vitro suggests a role of this molecule in osteogenesis. Functional studies using neutralizing antibodies or antisense oligonucleotides indicate that N-cadherin is involved in the control the expression of osteoblast marker gene expression and differentiation. Cleavage of N-cadherin during osteoblast apoptosis also suggests a role of N-cadherin-mediated-cell-cell adhesion in osteoblast survival. Hormonal and local factors that regulate osteoblast function also regulate N-cadherin expression and subsequent cell-cell adhesion associated with osteoblast differentiation or survival. Signaling mechanisms involved in N-cadherin-mediated cell-cell adhesion and osteoblast gene expression have also been identified. Alterations of N-cadherin expression are associated with abnormal osteoblast differentiation and osteogenesis in pathological conditions. These findings indicate that N-cadherin plays a role in normal and pathological bone formation and provide some insight into the process involved in N-cadherin-mediated cell-cell adhesion and differentiation in osteoblasts.

Biogenesis of N-cadherin-dependent cell-cell contacts in living fibroblasts is a microtubule-dependent kinesin-driven mechanism

Cadherin-mediated cell-cell adhesion is a dynamic process that is regulated during embryonic development, cell migration, and differentiation. Different cadherins are expressed in specific tissues consistent with their roles in cell type recognition. In this study, we examine the formation of N-cadherin-dependent cell-cell contacts in fibroblasts and myoblasts. In contrast to E-cadherin, both endogenous and ectopically expressed N-cadherin shuttles between an intracellular and a plasma membrane pool. Initial formation of N-cadherin-dependent cell-cell contacts results from the recruitment of the intracellular pool of N-cadherin to the plasma membrane. N-cadherin also localizes to the Golgi apparatus and both secretory and endocytotic vesicles. We demonstrate that the intracellular pool of N-cadherin is tightly associated with the microtubule (MT) network and that junction formation requires MTs. In addition, localization of N-cadherin to the cortex is dependent on an intact F-actin cytoskeleton. We show that N-cadherin transport requires the MT network as well as the activity of the MT-associated motor kinesin. In conclusion, we propose that N-cadherin distribution is a regulated process promoted by cell-cell contact formation, which controls the biogenesis and turnover of the junctions through the MT network.

Cadherins in neural crest cell development and transformation

Cadherins constitute a superfamily of cell adhesion molecules involved in cell-cell interaction, histogenesis and cellular transformation. They have been implicated in the development of various lineages, including derivatives of the neural crest. Neural crest cells (NCC) emerge from the dorsal part of the neural tube after an epithelio-mesenchymal transition (EMT) and migrate through the embryo. After homing and differentiation, NCC give rise to many cell types, such as neurons, Schwann cells and melanocytes. During these steps, the pattern of expression of the various cadherins studied is very dynamic. Cadherins also display plasticity of expression during the transformation of neural crest cell derivatives. Here, we review the pattern of expression and the role of the main cadherins involved in the development and transformation of neural crest cell derivatives.

Structure and control of a cell-cell adhesion complex associated with spermiation in rat seminiferous epithelium

Spermiation, the release of late spermatids from the Sertoli cell, is disrupted by a number of toxicants. Control of the spermiation process, and the proteins that interact to adhere mature spermatids to Sertoli cells, is poorly understood. In these studies we used immunohistochemistry, coimmunoprecipitation/Western blotting, and mass spectrometry to refine an earlier model of sperm adhesion proposed by our laboratory. We have identified specific proteins linked together as part of a multiprotein complex, as well as several additional proteins (cortactin, ERK1/2, and 14-3-3 zeta) that may be functioning in both structural and signal transduction roles. The current and prior data suggest that protein phosphorylation is central to the control of spermiation. We also present and characterize an in vitro tubule culture system that allowed functional testing of the spermiation model by pharmacologic manipulation, and yielded data consistent with the importance of protein phosphorylation in spermiation.

Leaving the neighborhood: molecular mechanisms involved during epithelial-mesenchymal transition

Several molecular mechanisms contribute directly and mechanically to the loss of epithelial phenotype. During epithelial-mesenchymal transition (EMT), adherens junctions and desmosomes are at least partially dissociated. At the same time, a massive cytoskeleton reorganization takes place, involving the rho family and the remodeling of the actin microfilament mesh. Numerous pathways have been described in vitro that control phenotype transition in specific cell models. In vivo developmental studies suggest that transcriptional control, activated by a specific pathway involving Ras, Src and potentially the Wnt pathway, is an essential step. Recent functional and localization experiments indicate that the slug/snail family of transcription factors functions overall as an epithelial phenotype repressor and could represent a key EMT contributor.

Smad proteins and hepatocyte growth factor control parallel regulatory pathways that converge on beta1-integrin to promote normal liver development

Smads serve as intracellular mediators of transforming growth factor beta (TGF-beta) signaling. After phosphorylation by activated type I TGF-beta receptors, Smad proteins translocate to the nucleus, where they serve as transcription factors and increase or decrease expression of TGF-beta target genes. Mice lacking one copy each of Smad2 and Smad3 suffered midgestation lethality due to liver hypoplasia and anemia, suggesting essential dosage requirements of TGF-beta signal components. This is likely due to abnormal adhesive properties of the mutant hepatocytes, which may result from a decrease in the level of the beta1-integrin and abnormal processing and localization of E-cadherin. Culture of mutant livers in vitro revealed the existence of a parallel developmental pathway mediated by hepatocyte growth factor (HGF), which could rescue the mutant phenotype independent of Smad activation. These pathways merge at the beta1-integrin, the level of which was increased by HGF in the cultured mutant livers. HGF treatment reversed the defects in cell proliferation and hepatic architecture in the Smad2(+/-); Smad3(+/-) livers.

Loss of alpha3beta1 integrin function results in an altered differentiation program in the mouse submandibular gland

Mammalian submandibular gland (SMG) development leads to the establishment of highly organized secretory acinar and nonsecretory ductal epithelial cells. The ability of maturing salivary epithelial cells to attain their differentiated state has been shown to depend, in part, on interactions between extracellular matrix (ECM) proteins and their integrin receptors. In a search for key regulators of salivary cell lineage, we have studied alpha3beta1 integrin, a receptor for the basement membrane protein laminin, by characterizing embryonic day 18 (E18) SMGs isolated from mice carrying a targeted mutation in the alpha3 integrin gene. Transmission electron microscopy studies showed that the mutant SMGs exhibited an aberrant differentiation phenotype with defects in the apical-basal polarity axis and in the basement membrane. Based on immunohistochemistry and Western blot analyses, the alpha3beta1-deficient SMGs had altered expression and/or localization of several ECM and adhesive molecules, including laminin beta1, fibronectin, alpha5 integrin, and E-cadherin. These changes correlated with alterations in the activation state of Ras-extracellular signal-regulated kinase (ERK), as well as the expression and/or localization of Cdc42 and RhoA, two Rho GTPases that regulate the organization of the actin cytoskeleton. We conclude that alpha3beta1 is required for normal salivary cell differentiation and that its absence affects multiple components of adhesive complexes and their associated signalling pathways.

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.

Integrin regulation of cell-cell adhesion during epithelial tubule formation

The extracellular matrix plays an important role in regulation of epithelial development and organization. To determine more precisely the function of extracellular matrix in this process, the initial steps in collagen-mediated formation of epithelial tubules were studied using a model cell culture system. Previous studies have demonstrated that incubation of Madin-Darby canine kidney (MDCK) epithelial cells with a collagen gel overlay induces (beta)1 integrin-regulated epithelial remodeling accompanied by extensive cell rearrangements and formation of epithelial tubules. During epithelial remodeling there was extensive disruption of the epithelial junctional complex. Progressive opening of tight junctions was observed over 8 hours using transepithelial resistance measurements and immunofluorescence microscopy demonstrated that tight and adherens junction proteins were dispersed throughout the apical and basolateral membranes. Junction complex disruption allowed the formation of apical cell extensions and subsequent migration of selected cell sheets from the epithelial monolayer. Confocal microscopy demonstrated the presence of adherens junction (E-cadherin, (alpha)-catenin, (beta)-catenin, plakoglobin) and desmosomal (desmoplakin-1/2, plakoglobin) proteins on, and within, cell extensions demonstrating that cell junctions had undergone considerable disassembly. However, groups of cell extensions appeared to be associated by E-cadherin/catenin-mediated interactions. Association of E-cadherin/catenin complexes with the epithelial cytoskeleton was analyzed by differential detergent extraction. SDS-PAGE and immunoblot analysis demonstrated that adherens junction proteins were primarily cytoskeleton-associated in control cells. During integrin-regulated remodeling, there was a progressive reduction in the interaction of adherens junction proteins with the cytoskeleton suggesting that they play an important role in the maintenance of epithelial integrity. Since loss of transepithelial electrical resistance and disruption of junctional complexes were inhibited by an antifunctional integrin antibody, we propose that activation of integrin signaling pathways regulate junctional complex stability, cell-cell interactions and cell migration. These observations provide evidence that integrin-regulated MDCK epithelial tubule formation can serve as a model system for studying rearrangements of epithelial sheets which occur during development.

Integrin alpha3beta1 engagement disrupts intercellular adhesion

During tissue morphogenesis and tumor invasion, epithelial cells must undergo intercellular rearrangement in which cells are repositioned with respect to one another and the surrounding mesenchymal extracellular matrix. Using three-dimensional aggregates of squamous epithelial cells, we show that such intercellular rearrangements can be triggered by activation of beta1 integrins after their ligation with extracellular matrices. On nonadherent substrates, multicellular aggregates (MCAs) formed rapidly via E-cadherin junctional complexes and over time became compacted spheroids exhibiting a more epithelial phenotype. After MCAs were replated on culture substrates, the spheroids collapsed to yield tightly arranged cell monolayers. Cell-cell contact induced rapid elevation in E-cadherin levels, which was due to an increase in the metabolic stability of junctional receptors. During MCA remodeling of cell-cell adhesions, and monolayer formation, their E-cadherin levels fell rapidly. Similar behavior was obtained regardless of which ECM ligand-collagen type I, fibronectin, or laminin 1-MCAs were seeded on. In contrast, when seeded onto a matrix elaborated by squamous epithelial cells, cells in the MCA attached, spread, lost cell-cell junctions, and dispersed. Analysis identified laminin 5 as the active ECM ligand in this matrix, and MCA dispersion required functional beta1 integrin and specifically alpha3beta1. Furthermore, substrate-immobilized anti-integrin antibody effectively reproduced the epithelial-mesenchymal-like transition induced by the laminin 5 matrix. During the early stages of aggregate rearrangement and collapse, cells on laminin 5 substrates, but not those on collagen I substrates, exhibited intense cortical arrays of F-actin, microspikes, and fascin accumulation at their peripheral surfaces. These results suggest that engagement of specific integrin-ligand pairs regulates cadherin junctional adhesions during events common to epithelial morphogenesis and tumor invasion.

Olfactory ensheathing cells and Schwann cells differ in their in vitro interactions with astrocytes

Transplanted olfactory ensheathing cells (OECs) are able to remyelinate demyelinated axons and support regrowth of transected axons after transplantation into the adult CNS. Transplanted Schwann cells (SCs) share these repair properties but have limitations imposed on their behavior by the presence of astrocytes (ACs). Because OECs exist alongside astrocytes in the olfactory bulb, we have hypothesized that they have advantages over SCs in transplant-mediated CNS repair due to an increased ability to integrate and migrate within an astrocytic environment. In this study, we have tested this hypothesis by comparing the interactions between astrocytes and either SCs or OECs, using a range of in vitro assays. We have shown that (1) astrocytes and SCs segregate into defined non-overlapping domains in co-culture, whereas astrocytes and OECs freely intermingle; (2) both SCs and OECs will migrate across astrocyte monolayers, but only OECs will migrate into an area containing astrocytes; (3) SCs spend less time in contact with astrocytes than do OECs; and (4) astrocytes undergo hypertrophy when in contact with SCs, but not with OECs. Expression of N-cadherin has been implicated as a key mediator of the failure of SCs to integrate with astrocytes. However, we found no differences in the intensity of N-cadherin immunoreactivity between SCs and OECs, suggesting that it is not the adhesion molecule that accounts for the observed differences. In addition, the number of astrocytes expressing chondroitin sulfate proteoglycans (CSPG) is increased when astrocytes are co-cultured with Schwann cells compared with the number when astrocytes are grown alone or with OECs. Taken together, these data support the hypothesis that OECs will integrate more extensively than Schwann cells in astrocytic environments and are therefore better candidates for transplant-mediated repair of the damaged CNS.

The integrins alpha3beta1 and alpha6beta1 physically and functionally associate with CD36 in human melanoma cells. Requirement for the extracellular domain OF CD36

Lateral association between different transmembrane glycoproteins can serve to modulate integrin function. Here we characterize a physical association between the integrins alpha(3)beta(1) and alpha(6)beta(1) and CD36 on the surface of melanoma cells and show that ectopic expression of CD36 by CD36-negative MV3 melanoma cells increases their haptotactic migration on extracellular matrix components. The association was demonstrated by co-immunoprecipitation, reimmunoprecipitation, and immunoblotting of surface-labeled cells lysed in Brij 96 detergent. Confocal microscopy illustrated the co-association of alpha(3) and CD36 in cell membrane projections and ruffles. A requirement for the extracellular domain of CD36 in this association was shown by co-immunoprecipitation experiments using surface-labeled MV3 melanoma or COS-7 cells that had been transiently transfected with chimeric constructs between CD36 and intercellular adhesion molecule 1 (ICAM-1) or with a truncation mutant of CD36. CD36 is known to engage in signal transduction and to localize to membrane microdomains or rafts in several cell types. Toward a mechanistic explanation for the functional effects of CD36 expression, we demonstrate that in fractionated Triton X-100 lysates of the MV3 cells stably transfected with CD36, CD36 was greatly enriched with the detergent-insoluble fractions that represent plasma membrane rafts. Significantly, when these fractionated lysates were reprobed for endogenous beta(1) integrin, it was found that a 4-fold increase in the proportion of the mature protein was contained within the detergent-insoluble fractions when extracted from the CD36-transfected cells compared with MV3 cells transfected with vector only. These results suggest that in melanoma cells CD36 expression may induce the sequestration of certain integrins into membrane microdomains and promote cell migration.

Matricellular proteins as modulators of cell-matrix interactions: adhesive defect in thrombospondin 2-null fibroblasts is a consequence of increased levels of matrix metalloproteinase-2

Thrombospondin 2 (TSP2)-null mice, generated by disruption of the Thbs2 gene, display a variety of connective tissue abnormalities, including fragile skin and the presence of abnormally large collagen fibrils with irregular contours in skin and tendon. In this study we demonstrate that TSP2-null skin fibroblasts show a defect in attachment to a number of matrix proteins, and a reduction in cell spreading. To investigate the molecular mechanisms responsible for these abnormal cell-matrix interactions, we compared the levels of matrix metalloproteinases (MMPs) in wild-type and mutant fibroblasts. Isolation and analysis of gelatinases from conditioned media by gelatin-agarose affinity chromatography and gelatinolytic assays demonstrated that TSP2-null fibroblasts produce a 2-fold increase in gelatinase A (MMP2) compared with wild-type cells. The adhesive defect was corrected by treatment of TSP2-null fibroblasts with soluble TSP2, with the MMP inhibitors BB94 and tissue inhibitor of metalloproteinase-2, and with a neutralizing antibody to MMP2. Moreover, stable transfection of TSP2-null fibroblasts with mouse TSP2 cDNA corrected both the adhesive defect and the altered expression of MMP2. Finally, MMP2 was shown to interact with TSP2 in a direct-binding plate assay. We conclude that TSP2 plays an important role in cell-matrix interactions, and that a deficiency in the protein results in increased levels of MMP2 that contribute to the adhesive defect in TSP2-null fibroblasts and could play a role in the complex phenotype of TSP2-null mice.

Analysis of cadherin/catenin complexes in transformed thyroid epithelial cells: modulation by beta 1 integrin subunit

We have analysed the expression of cadherin/catenin complex molecules in PC C13 rat thyroid cells transformed in vitro with different oncogenes. No significant downregulation of either E-cadherin, alpha-, beta- and gamma-catenin was detected following the introduction of activated forms of myc, adenovirus E1A, ras, raf, myc + ras, E1A + raf. However, ras- and raf-transformed PC C13 cells showed altered adherens junctions. An altered distribution of cadherin/catenin complexes characterized by radially oriented membrane spikes perpendicular to cell edges was the most prominent feature evidenced by immunofluorescence. No beta1 integrin localization was observed in areas where this altered pattern of E-cadherin expression was detected. However, beta1 integrin subunit expression was detected at areas of cell-cell contact where E-cadherin showed a normal pattern of expression. Furthermore, ras- and raf-transformed PC C13 cells showed the ability to migrate in collagen gels, in contrast to their normal untransformed counterpart. Overexpression of beta1 integrin was found to restore normal E-cadherin localization at cell-cell contacts and to partially inhibit the ability to migrate in collagen gels. Finally, two cell lines obtained by ras transformation in vivo, and derived from a rat primary thyroid carcinoma (TK6) and its lung metastasis (MPTK6), were found to have lost gamma-catenin expression. TK6 lost also E-cadherin expression and membrane localization of alpha-catenin. These results suggest that: i) in vitro thyroid cell transformation is associated to a change in cadherin/catenin complexes distribution rather than to a decrease in expression; ii) in vivo transformation is associated to the loss of expression of some of these molecules likely due to tumor progression; iii) alterations in beta1 integrin subunit expression can result in changes in cadherin/catenin function thus implying that an integrin-cadherin synergy may exist in thyroid cells.