Quantitative analysis of intercellular adhesive specificity in freshly explanted and cultured cells

Nectin-3, a new member of immunoglobulin-like cell adhesion molecules that shows homophilic and heterophilic cell-cell adhesion activities

We have isolated a novel cell-cell adhesion system localized at cadherin-based adherens junctions (AJs). This system consists of at least nectin, a Ca(2+)-independent immunoglobulin-like adhesion molecule, and afadin, an actin filament-binding protein, that connects nectin to the actin cytoskeleton. Nectin constitutes a family consisting of two members, nectin-1 and -2. We have isolated here a third member of the nectin family and named it nectin-3. Nectin-3 has three splicing variants, nectin-3alpha (biggest), -3beta (middle), and -3gamma (smallest). Like nectin-1 and -2, nectin-3alpha consists of three extracellular immunoglobulin-like domains, a transmembrane segment, and a cytoplasmic region with the C-terminal consensus motif for binding to the PDZ domain. Nectin-3alpha formed a cis-homo-dimer and showed Ca(2+)-independent trans-homo-interaction to cause homophilic cell-cell adhesion. Nectin-3alpha furthermore showed trans-hetero-interaction with nectin-1 or -2 but did not form a cis-hetero-dimer with nectin-1 or -2. Nectin-1 did not show trans-hetero-interaction with nectin-2. The affinity of trans-hetero-interaction of nectin-3alpha with nectin-1 or -2 was higher than that of trans-homo-interaction of nectin-1, -2, or -3alpha. Nectin-2 and -3 were ubiquitously expressed, whereas nectin-1 was abundantly expressed in brain. Nectin-3alpha was colocalized with nectin-2 at cadherin-based AJs and interacted with afadin. These results indicate that the nectin family consists of at least three members, nectin-1, -2, and -3, all of which show homophilic and heterophilic cell-cell adhesion activities and are localized at cadherin-based AJs.

Interaction of nectin with afadin is necessary for its clustering at cell-cell contact sites but not for its cis dimerization or trans interaction

We have recently found a novel functional unit of cell-cell adhesion at cadherin-based adherens junctions, consisting of at least nectin, a homophilic cell adhesion molecule, and afadin, an actin filament-binding protein, which connects nectin to the actin cytoskeleton. Here we studied a mechanism of cell-cell adhesion of the nectin-afadin system by use of a cadherin-deficient L cell line stably expressing the intact form of mouse nectin-2alpha, a truncated form of nectin-2alpha incapable of interacting with afadin (nectin-2alpha-DeltaC), or a point-mutated form of nectin-2alpha capable of interacting with afadin and a cadherin-expressing EL cell line, which transiently expressed the point-mutated form of nectin-2alpha. We found that the interaction of nectin-2alpha with afadin was necessary for their clustering at cell-cell contact sites. However, nectin-2alpha-DeltaC showed cis dimerization and trans interaction, both of which did not require the interaction of nectin-2alpha with afadin. We have previously shown in EL cells that the interaction of nectin-1 with afadin is necessary for its recruitment to adherens junctions. We found that the trans interaction of nectin-2alpha was furthermore necessary for this recruitment. On the basis of these observations, we propose a model for the mechanism of cell-cell adhesion of nectin and roles of afadin in this mechanism.

Tyrosine residue in exon 14 of the cytoplasmic domain of platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) regulates ligand binding specificity

Platelet/endothelial cell adhesion molecule (PECAM-1) is a cell adhesion molecule of the immunoglobulin superfamily that plays a role in a number of vascular processes including leukocyte transmigration through endothelium. The presence of a specific 19- amino acid exon within the cytoplasmic domain of PECAM-1 regulates the binding specificity of the molecule; specifically, isoforms containing exon 14 mediate heterophilic cell-cell aggregation while those variants missing exon 14 mediate homophilic cell-cell aggregation. To more precisely identify the region of exon 14 responsible for ligand specificity, a series of deletion mutants were created in which smaller regions of exon 14 were removed. After transfection into L cells, they were tested for their ability to mediate aggregation. For heterophilic aggregation to occur, a conserved 5-amino acid region (VYSEI in the murine sequence or VYSEV in the human sequence) in the mid-portion of the exon was required. A final construct, in which this tyrosine was mutated into a phenylalanine, aggregated in a homophilic manner when transfected into L cells. Inhibition of phosphatase activity by exposure of cells expressing wild type or mutant forms of PECAM-1 to sodium orthovanadate resulted in high levels of cytoplasmic tyrosine phosphorylation and led to a switch from heterophilic to homophilic aggregation. Our data thus indicate either loss of this tyrosine from exon 14 or its phosphorylation results in a change in ligand specificity from heterophilic to homophilic binding. Vascular cells could thus determine whether PECAM-1 functions as a heterophilic or homophilic adhesion molecule by processes such as alternative splicing or by regulation of the balance between tyrosine phosphorylation or dephosphorylation. Defining the conditions under which these changes occur will be important in understanding the biology of PECAM-1 in transmigration, angiogenesis, development, and other processes in which this molecule plays a role.

Platelet endothelial cell adhesion molecule-1 (PECAM-1) homophilic adhesion is mediated by immunoglobulin-like domains 1 and 2 and depends on the cytoplasmic domain and the level of surface expression

PECAM-1/CD31 is vascular cell adhesion and signaling molecule of the Ig superfamily that plays a role in neutrophil recruitment at inflammatory sites and may be involved the release of leukocytes from the bone marrow and in cardiovascular development. The interactions of PECAM-1 with its ligands are complex in that it is able to bind both with itself (homophilic adhesion) or with non-PECAM-1 ligands (heterophilic adhesion). Although the factors that regulate ligand binding are not fully understood, these interactions are regulated in part by its large cytoplasmic domain, a region of 118 amino acids encoded by 8 exons of its gene (exons 9-16). The purpose of this work was to better define the mechanisms of PECAM-1-dependent homophilic adhesion by analyzing the binding interactions of L-cells expressing full-length and selectively mutated forms of human, murine, and human/murine chimeric PECAM-1 molecules in an established aggregation assay. These studies demonstrate that 1) the minimal length of the cytoplasmic domain required for cellular aggregation is represented within the sequences encoded by exons 9 and 10, 2) removal or addition of the sequences encoded by exon 14 from the cytoplasmic domain can determine whether the mechanism of aggregation is a heterophilic calcium-dependent process or a homophilic calcium-independent process, 3) high levels of surface expression of PECAM-1 on the cell surface change the mechanism of aggregation from heterophilic to homophilic, and 4) PECAM-1-dependent homophilic binding appears to involve the direct interaction of only the first two extracellular Ig-like domains. These data suggest that PECAM-1-ligand interactions can be regulated through multiple pathways including alterations of the cytoplasmic domain and the level of surface expression.

Alternative splicing of a specific cytoplasmic exon alters the binding characteristics of murine platelet/endothelial cell adhesion molecule-1 (PECAM-1)

Platelet/endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a membrane glycoprotein expressed on endothelial cells, platelets, and leukocytes. Analysis of PECAM-1 expression in the developing mouse embryo has revealed the presence of multiple isoforms of murine PECAM-1 (muPECAM-1) that appeared to result from the alternative splicing of exons encoding cytoplasmic domain sequences (exons 10-16) (Baldwin, H. S., Shen, H. M., Yan, H., DeLisser, H. M., Chung, A., Mickanin, C., Trask, T., Kirschbaum, N. E. Newman, P. J., Albelda, S., and Buck, C. A. (1994) Development 120, 2539-2553). To investigate the functional consequences of alternatively spliced muPECAM-1 cytoplasmic domains, L-cells were transfected with cDNA for each variant and their ability to promote cell aggregation was compared. In this assay, full-length muPECAM-1 and all three isoforms containing exon 14 behaved like human PECAM-1 in that they mediated calcium- and heparin-dependent heterophilic aggregation. In contrast, three muPECAM-1 variants, all missing exon 14, mediated calcium- and heparin-independent homophilic aggregation. Exon 14 thus appears to modulate the ligand and adhesive interactions of the extracellular domain of PECAM-1. These findings suggest that alternative splicing may represent a mode of regulating the adhesive function of PECAM-1 in vivo and provides direct evidence that alternative splicing involving the cytoplasmic domain affects the ligand specificity and binding properties of a cell adhesion receptor.

Functional properties of human vascular endothelial cadherin (7B4/cadherin-5), an endothelium-specific cadherin

Human vascular endothelial cadherin (VE-cadherin, 7B4/cadherin-5) is an endothelial-specific cadherin localized at the intercellular junctions. To directly investigate the functional role of this molecule we cloned the full-length cDNA from human endothelial cells and transfected its coding region into Chinese hamster ovary cells. The product of the transfected cDNA had the same molecular weight as the natural VE-cadherin in human endothelial cells, and reacted with several VE-cadherin mouse monoclonal antibodies. Furthermore, it selectively concentrated at intercellular junctions, where it codistributed with alpha-catenin. VE-cadherin conferred adhesive properties to transfected cells. It mediated homophilic, calcium-dependent aggregation and cell-to-cell adhesion. In addition, it decreased intercellular permeability to high-molecular weight molecules and reduced cell migration rate across a wounded area. Thus, VE-cadherin may exert a relevant role in endothelial cell biology through control of the cohesion and organization of the intercellular junctions.

The receptor tyrosine kinase ARK mediates cell aggregation by homophilic binding

The ARK (AXL, UFO) receptor is a member of a new family of receptor tyrosine kinases whose extracellular domain contains a combination of fibronectin type III and immunoglobulin motifs similar to those found in many cell adhesion molecules. ARK mRNA is expressed at high levels in the mouse brain, prevalently in the hippocampus and cerebellum, and this pattern of expression resembles that of adhesion molecules that are capable of promoting cell aggregation through homophilic or heterophilic binding. We report here the ability of the murine ARK receptor to mediate homophilic binding. Expression of the ARK protein in Drosophila S2 cells induces formation of cell aggregates consisting of ARK-expressing cells, and aggregation leads to receptor activation, with an increase in receptor phosphorylation. Homophilic binding does not require ARK tyrosine kinase activity, since S2 cells expressing a receptor in which the intracellular domain was deleted were able to undergo aggregation as well as cells expressing the wild-type ARK receptor. Similar results were obtained with NIH 3T3 and CHO cells expressing high levels of ARK, although in this case ARK expression appeared to be accompanied by constitutive activation. The purified recombinant extracellular domain of ARK can induce homotypic aggregation of coated fluorescent beads (Covaspheres), and this protein can also function as a substrate for adhesion by S2 and NIH 3T3 cells expressing ARK. These results suggest that ARK represents a new cell adhesion molecule that through its homophilic interaction may regulate cellular functions during cell recognition.

Deletions in the cytoplasmic domain of platelet-endothelial cell adhesion molecule-1 (PECAM-1, CD31) result in changes in ligand binding properties

Platelet/endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a member of the immunoglobulin superfamily present on platelets, endothelial cells, and leukocytes that may function as a vascular cell adhesion molecule. The purpose of this study was to examine the role of the cytoplasmic domain in PECAM-1 function. To accomplish this, wild-type and mutated forms of PECAM-1 cDNA were transfected into murine fibroblasts and the functional characteristics of the cells analyzed. Wild-type PECAM-1 localized to the cell-cell borders of adjacently transfected cells and mediated heterophilic, calcium-dependent L-cell aggregation that was inhibitable by a polyclonal and two monoclonal anti-PECAM-1 antibodies. A mutant protein lacking the entire cytoplasmic domain did not support aggregation or move to cell-cell borders. In contrast, both forms of PECAM-1 with partially truncated cytoplasmic domains (missing either the COOH-terminal third or two thirds of the cytoplasmic domain) localized to cell-cell borders in 3T3 cells in a manner analogous to the distribution seen in cultured endothelial cells. L-cells expressing these mutants demonstrated homophilic, calcium-independent aggregation that was blocked by the polyclonal anti-PECAM-1 antibody, but not by the two bioactive monoclonal antibodies. Although changes in the cytoplasmic domain of other receptors have been shown to alter ligand-binding affinity, to our knowledge, PECAM-1 is the first example of a cell adhesion molecule where changes in the cytoplasmic domain result in a switch in the basic mechanism of adhesion leading to different ligand-binding specificity. Variations in the cytoplasmic domain could thus be a potential mechanism for regulating PECAM-1 activity in vivo.

Homophilic adhesion of the myelin P0 protein requires glycosylation of both molecules in the homophilic pair

The myelin P0 protein is glycosylated at a single site, asparagine 93, within its only immunoglobulin (Ig)-like domain. We have previously shown that P0 behaves like a homophilic adhesion molecule (Filbin, M. T., F. S. Walsh, B. D. Trapp, J. A. Pizzey, and G. I. Tennekoon. 1990. Nature (Lond.). 344:871-872). To determine if the sugar residues of this molecule contribute to its adhesiveness, the glycosylation site was eliminated by replacing asparagine 93 with an alanine, through site-directed mutagenesis of the P0 cDNA. The mutated P0 cDNA was transfected into CHO cells and surface expression of the mutated P0 was assessed by immunofluorescence, limited trypsinization and an ELISA. A cell line was chosen which expressed approximately equivalent amounts of the unglcosylated P0 (UNGP0) at the cell surface as did a cell line expressing the fully glycosylated P0 (GPo); the adhesive properties of these two cell lines were compared. It was found that when a single cell suspension of the UNGPo cells were incubated, by 60 min, unlike the GP0 cells, they had not formed large aggregates; they were indistinguishable from the control transfected cells. This suggests that the UNGP0 protein does not behave like an adhesion molecule. To establish if only one molecule in the P0:P0 homophilic pair must be glycosylated for adhesion to occur, the ability of UNGP0 cells to adhere to GP0 cells was assessed both qualitatively and quantitatively. The results of both types of assay imply that, indeed, both P0 molecules in the homophilic pair must be glycosylated for adhesion to take place.

A heterophilic adhesion mechanism for platelet/endothelial cell adhesion molecule 1 (CD31)

The molecular nature of cell adhesion mediated by platelet/endothelial cell adhesion molecule 1 (PECAM-1; CD31) was examined using stably transfected L cells in a PECAM-dependent aggregation assay. This adhesion was temperature sensitive and divalent cation dependent, with Mg2+ supporting aggregation to a greater degree than Ca2+. PECAM-dependent aggregation was heterophilic: PECAM-1 transfectants bound as readily to control-transfected L cells as to other PECAM-1 transfectants, demonstrating that a molecule endogenously expressed on the L cells serves as the ligand for PECAM in this system and presumably substitutes for the natural human ligand.

Role of myelin P0 protein as a homophilic adhesion molecule

Peripheral nervous system myelin is an extension of the Schwann cell's plasma membrane that tightly enwraps axons in many layers and permits nerve impulses to be rapidly conducted. It is not known how these multiple membrane layers are held together in this compact form. Here we present evidence supporting the hypothesis that the extracellular leaflets of myelin are held together by the most abundant protein of myelin of the peripheral nervous system, P0, by homophilic interaction of its extracellular domains. Transfected Chinese hamster ovary cells expressing P0 protein adhere to each other in suspension, to form large aggregates, whereas cells that are identical but which do not express P0 do not. We also show that this aggregation is mediated by homophilic binding between P0-expressing cells and that the apposing plasma membranes of these cells specifically form desmosomes, whereas control transfected cells do not. As the only difference between the two cell populations is the expression of P0, this protein is apparently responsible for the changes in morphology and adhesion in the cells that express it. The idea that P0 is a homophilic adhesion molecule is supported by its inclusion in the immunoglobulin supergene family, all members of which are involved in recognition and/or adhesion.

Intercellular adhesion mediated by human muscle neural cell adhesion molecule: effects of alternative exon use

Mouse 3T3 fibroblasts were permanently transfected with cDNAs encoding isoforms of the neural cell adhesion molecule (N-CAM) present in human skeletal muscle and brain. Parental and transfected cells were then used in a range of adhesion assays. In the absence of external shear forces, transfection with cDNAs encoding either transmembrane or glycosylphosphatidylinositol (GPI)-linked N-CAM species significantly increased the intercellular adhesiveness of 3T3 cells in suspension. Transfection of a cDNA encoding a secreted N-CAM isoform was without effect on adhesion. Cells transfected with cDNAs containing or lacking the muscle-specific domain 1 sequence, a four-exon group spliced into the muscle but not the brain GPI-linked N-CAM species, were equally adhesive in the assays used. We also demonstrate that N-CAM-mediated intercellular adhesiveness is inhibited by 0.2 mg/ml heparin; but, at higher concentrations, reduced adhesion of parental cells was also seen. Coaggregation of fluorescently labeled and unlabeled cell populations was performed and measured by comparing their distribution within aggregates with distributions that assume nonspecific (random) aggregation. These studies demonstrate that random aggregation occurs between transfected cells expressing the transmembrane and GPI-linked N-CAM species and between parental cells and those expressing the secreted N-CAM isoform. Other combinations of these populations tested exhibited partial adhesive specificity, indicating homophilic binding between surface-bound N-CAM. Thus, the approach exploited here allows for a full analysis of the requirements, characteristics, and specificities of the adhesive behavior of individual N-CAM isoforms.

Requirements for the Ca2+-independent component in the initial intercellular adhesion of C2 myoblasts

Using a sensitive and quantitative adhesion assay, we have studied the initial stages of the intercellular adhesion of the C2 mouse myoblast line. After dissociation in low levels of trypsin in EDTA, C2 cells can rapidly reaggregate by Ca2+-independent mechanisms to form large multicellular aggregates. If cells are allowed to recover from dissociation by incubation in defined media, this adhesive system is augmented by a Ca2+-dependent mechanism with maximum recovery seen after 4 h incubation. The Ca2+-independent adhesion system is inhibited by preincubation of cell monolayers with cycloheximide before dissociation. Aggregation is also reduced after exposure to monensin, implicating a role for surface-translocated glycoproteins in this mechanism of adhesion. In coaggregation experiments using C2 myoblasts and 3T3 fibroblasts in which the Ca2+-dependent adhesion system was inactivated, no adhesive specificity between the two cell types was seen. Although synthetic peptides containing the RGD sequence are known to inhibit cell-substratum adhesion in various cell types, incubation of C2 myoblasts with the integrin-binding tetrapeptide, RGDS, greatly stimulated the Ca2+-independent aggregation of these cells while control analogs had no effect. These results show that a Ca2+-independent mechanism alone is sufficient to allow for the rapid formation of multicellular aggregates in a mouse myoblast line, and that many of the requirements and perturbants of the Ca2+-independent system of intercellular myoblast adhesion are similar to those of the Ca2+-dependent adhesion mechanisms.

Dual adhesion systems of chick myoblasts

Cultured chick myoblasts (Mb) were resuspended by incubation with 100 micrograms/ml trypsin/2.5 mM CaCl2 (to yield TC-Mb), or with 5 micrograms/ml trypsin/2.5 mM EDTA (to yield LTE-Mb). As measured in a particle counter, TC-Mb aggregation was Ca2+ dependent, whereas LTE-Mb aggregated equally well in the presence of CaCl2 or EDTA. Cells subjected to the same treatments in sequence, like cells dissociated directly with 100 micrograms/ml trypsin/2.5 mM EDTA, did not aggregate significantly in the presence or absence of Ca2+. Adhesive specificity was assessed by mixing unlabeled cells with cells labeled with a fluorescent dye and then analyzing the distribution of fluorescent and nonfluorescent cells in aggregates. No adhesive specificity was seen in controls (i.e., TC-Mb aggregated randomly with TC-Mb, or LTE-Mb with LTE-Mb), but TC-Mb and LTE-Mb did not cross-adhere. These results indicate the existence of two independent, noncomplementing, adhesion systems, and suggest that the differential treatments preserve or activate one system while destroying the other. Myoblasts dissociated with 2.5 mM EDTA in the absence of exogenous trypsin (E-Mb) have both adhesion systems active on their surfaces, as do Mb grown in Ca2+-free medium and then dissociated with 0.7 mM EDTA (Knudsen, K. A., and Horwitz, A. F., Dev. Biol. 58, 328-338, 1977). Although aggregation of E-Mb is largely Ca2+ independent and that of Knudsen/Horwitz-Mb is largely Ca2+ dependent, they adhere well to each other and to LTE-Mb while segregating from TC-Mb. Fibroblasts also have dual adhesion systems, one Ca2+ dependent and the other Ca2+ independent, but TC-Fb do not cross-adhere to TC-Mb (nor E-Fb to E-Mb). Cell type-specific adhesive selectivity may thus contribute to the selectivity of myocyte fusion.

Adhesive interactions between normal and dystrophic human skin fibroblasts

The adhesive properties of skin fibroblasts from patients with Duchenne muscular dystrophy (DMD) were studied by analysing cell aggregate formation in suspensions consisting of normal and DMD fibroblasts. By the use of aggregation kinetics and fluorescent labelling, the genotypic composition of aggregates in mixed-cell suspensions could be visualised. The distribution of normal and DMD cells within these aggregates could then be compared to theoretical binomial distributions which assume no difference in the specific adhesiveness between the two genotypes. Analysis of the 3- and 4-cell aggregates which were produced by co-aggregating normal and DMD cells demonstrate that there is no qualitative (specific) difference in the adhesiveness between normal and DMD fibroblasts. However, quantitative changes in the cell-cell adhesion of DMD fibroblasts may be present, and this is supported by the relatively small proportion of intermediate size heterotypic aggregates which were formed in mixed-genotype cell suspensions. In such mixtures, fewer aggregates consisting of 5 or more cells were formed compared to fibroblast suspensions derived from pairs of normal individuals. Furthermore, cell suspensions from pairs of DMD patients produced even less greater than or equal to 5-cell aggregates than were found in the mixed-genotype experiments. These findings are considered in relation to previous reports of abnormal cell adhesiveness and other adhesion-related mechanisms in DMD cells.

The measurement of specific cell: cell interactions by dual-parameter flow cytometry

The Fc receptor-mediated aggregation of antibody-coated spleen cells with cells from the P388D1 mouse macrophage line was followed using a novel flow cytometric technique. P388D1 and spleen cells were directly labeled with green-emitting (fluorescein isothiocyanate) and red-emitting (substituted rhodamine isothiocyanate) fluorophores, respectively. They were mixed, incubated in suspension at 4 degrees C, and analyzed for aggregation with a dual laser flow cytometer. Unconjugated cells appeared as particles which were either red or green, while conjugates were detected as particles which were both red and green. Using this assay procedure, 5 X 10(4) cells were analyzed in 2-3 min for the percentages of conjugates, free spleen cells, and free P388D1 cells. Intercellular aggregation required both antibody on the spleen cells and free Fc receptors on the P388D1 cells; nonspecific aggregates accounted for 1% or less of the total particles analyzed. Measurements of the fluorescence distributions within conjugates indicated that the majority of conjugates contained a single P388D1 cell bound to 1-3 spleen cells, and that only heterophilic aggregation occurred. The flow cytometric technique described here should be applicable for the measurement of the initial events of intercellular aggregation in other systems as well.