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.     

Triple effect of mimetic peptides interfering with neural cell adhesion molecule homophilic cis interactions

The neural cell adhesion molecule (NCAM) is pivotal in neural development, regeneration, and learning. Here we characterize two peptides, termed P1-B and P2, derived from the homophilic binding sites in the first two N-terminal immunoglobulin (Ig) modules of NCAM, with regard to their effects on neurite extension and adhesion. To evaluate how interference of these mimetic peptides with NCAM homophilic interactions in cis influences NCAM binding in trans, we employed a coculture system in which PC12-E2 cells were grown on monolayers of fibroblasts with or without NCAM expression and the rate of neurite outgrowth subsequently was analyzed. P2, but not P1-B, induced neurite outgrowth in the absence of NCAM binding in trans. When PC12-E2 cells were grown on monolayers of NCAM-expressing fibroblasts, the effect of both P1-B and P2 on neurite outgrowth was dependent on peptide concentrations. P1-B and P2 acted as conventional antagonists, agonists, and reverse agonists of NCAM at low, intermediate, and high peptide concentrations, respectively. The demonstrated in vitro triple pharmacological effect of mimetic peptides interfering with the NCAM homophilic cis binding will be valuable for the understanding of the actions of these mimetics in vivo.     

Glycosyl phosphatidylinositol--anchored T-cadherin mediates calcium-dependent, homophilic cell adhesion.

Cadherins are a family of cell adhesion molecules that exhibit calcium-dependent, homophilic binding. Their function depends on both an HisAlaVal sequence in the first extracellular domain, EC1, and the interaction of a conserved cytoplasmic region with intracellular proteins. T-cadherin is an unusual member of the cadherin family that lacks the HisAlaVal motif and is anchored to the membrane through a glycosyl phosphatidylinositol moiety (Ranscht, B., and M. T. Dours-Zimmermann. 1991. Neuron. 7:391-402). To assay the function of T-cadherin in cell adhesion, we have transfected T-cadherin cDNA into CHO cells. Two proteins, mature T-cadherin and the uncleaved T-cadherin precursor, were produced from T-cadherin cDNA. The T-cadherin proteins differed from classical cadherins in several aspects. First, the uncleaved T-cadherin precursor was expressed, together with mature T-cadherin, on the surface of the transfected cells. Second, in the absence of calcium, T-cadherin was more resistant to proteolytic cleavage than other cadherins. Lastly, in contrast to classical cadherins, T-cadherin was not concentrated into cell-cell contacts between transfected cells in monolayer cultures. In cellular aggregation assays, T-cadherin induced calcium-dependent, homophilic adhesion which was abolished by treatment of T-cadherin-transfected cells with phosphatidylinositol-specific phospholipase C. These results demonstrate that T-cadherin is a functional cadherin that differs in several properties from classical cadherins. The function of T-cadherin in homophilic cell recognition implies that the mechanism of T-cadherin-induced adhesion is distinct from that of classical cadherins.     

Single molecule adhesion measurements reveal two homophilic neural cell adhesion molecule bonds with mechanically distinct properties

Neural cell adhesion molecule (NCAM) is a cell surface adhesion glycoprotein that plays an important role in the development and stability of nervous tissue. The homophilic binding mechanism of NCAM is still a subject of debate on account of findings that appear to support different mechanisms. This paper describes single molecule force measurements with both full-length NCAM and NCAM mutants that lack different immunoglobulin (Ig) domains. By systematically applying an external, time-dependent force to the bond, we obtained parameters that describe the energy landscape of NCAM-NCAM bonds. Histograms of the rupture forces between the full-length NCAM extracellular domains revealed two binding events, one rupturing at higher forces than the other. These bond rupture data show that the two bonds have the same dissociation rates. Despite the energetic and kinetic similarities, the bond strengths differ significantly, and are mechanically distinct. Measurements with NCAM domain deletion mutants mapped the weaker bond to the Ig1-2 segment, and the stronger bond to the Ig3 domain. Finally, the quantitative agreement between the fragment adhesion and the strengths of both NCAM bonds shows that the domain deletions considered in this study do not alter the intrinsic strengths of either of the two bonds.     

Characterization of the kinetics of neural cell adhesion molecule homophilic binding

A solid-phase assay has been developed for the investigation of the kinetics of neural cell adhesion molecule (NCAM) binding. Using this assay we can show that NCAM binds to itself in a time-dependent and saturable manner. Binding constants (KB values) of 6.9 x 10(-8) M and 1.23 x 10(-6) M, respectively, were obtained for adult and newborn rat NCAM homophilic binding. Binding is specifically inhibited by Fab' fragments of polyclonal anti-NCAM antibodies but is unaffected by heparin or chondroitin sulphate. This indicates that the NCAM homophilic binding site is separate from and independent of the heparin-binding site and that a developmental modification, probably polysialation, gives rise to marked differences in the adhesive properties of NCAM.     

Multiple cadherin extracellular repeats mediate homophilic binding and adhesion.

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

Cloning and functional characterization of DSCAML1, a novel DSCAM-like cell adhesion molecule that mediates homophilic intercellular adhesion.

DSCAM, a conserved gene involved in neuronal differentiation, is a member of the Ig superfamily of cell adhesion molecules. Herein, we report the functional characterization of a human DSCAM (Down syndrome cell adhesion molecule) paralogue, DSCAML1, located on chromosome 11q23. The deduced DSCAML1 protein contains 10 Ig domains, six fibronectin-III domains, and an intracellular domain, all of which are structurally identical to DSCAM. When compared to DSCAM, DSCAML1 protein showed 64% identity to the extracellular domain and 45% identity to the cytoplasmic domain. In the mouse brain, DSCAML1 is predominantly expressed in Purkinje cells of the cerebellum, granule cells of the dentate gyrus, and in neurons of the cerebral cortex and olfactory bulb. Biochemical and immunofluorescence analyses indicated that DSCAML1 is a cell surface molecule that targets axonal features in differentiated PC12 cells. DSCAML1 exhibits homophilic binding activity that does not require divalent cations. Based on its structural and functional properties and similarities to DSCAM, we suggest that DSCAML1 may be involved in formation and maintenance of neural networks. The chromosomal locus for DSCAML1 makes it an ideal candidate for neuronal disorders (such as Gilles de la Tourette and Jacobsen syndromes) that have been mapped on 11q23.     

The role of complex carbohydrates in adhesion of the myelin protein, P0.

The most abundant protein of peripheral nerve myelin, a glycoprotein termed P0, is believed to be involved in the compaction of the myelin sheath and is postulated to be the closest relative to the ancestral gene for the immunoglobulin superfamily. Recently, P0 has indeed been shown to behave like a homophilic adhesion molecule via interactions of its extracellular domains. Here we demonstrate the importance of the oligosaccharide moieties of P0 in its functioning as a homophilic adhesion molecule. Expression of the complex form of P0 glycoprotein in transfected Chinese hamster ovary cells greatly increased the adhesiveness of those cells, whereas expression of the high-mannose form of P0 glycoprotein did not. This is the first step in the dissection of P0-P0 interaction at the molecular level.     

The homophilic binding of junctional adhesion molecule-C mediates tumor cell-endothelial cell interactions

The junctional adhesion molecule C (JAM-C) was recently shown to undergo a heterophilic interaction with the leukocyte beta2 integrin Mac-1, thereby mediating interactions between vascular cells in inflammatory cell recruitment. Here, the homophilic interaction of JAM-C is presented and functionally characterized to mediate tumor cell-endothelial cell interactions. Recombinant soluble JAM-C in fluid phase bound to immobilized JAM-C as assessed in a purified system; moreover, JAM-C-transfected Chinese hamster ovary (CHO) cells adhered to immobilized JAM-C. The homophilic interaction of JAM-C was mediated by the isolated amino-terminal Ig domain (D1), but not the carboxyl-terminal Ig domain (D2), of the molecule. Dimerization of JAM-A is dependent on the sequence RVE in the amino-terminal Ig domain. This motif is conserved in JAM-C (Arg64-Ile65-Glu66), and a single amino acid mutation in this motif (E66R) abolished the homophilic interaction of JAM-C. The lung carcinoma cell line NCI-H522 was found to express JAM-C. NCI-H522 cells adhered to immobilized JAM-C, as well as to JAM-C-transfected CHO cells, but not to mock-transfected CHO cells or to CHO cells transfected with the JAM-C mutant (E66R). Adhesion of NCI-H522 cells to JAM-C protein or JAM-C-transfected CHO cells was abolished in the presence of soluble JAM-C or the isolated D1. Furthermore, the adhesion of NCI-H522 cells to endothelial cells was significantly blocked by soluble JAM-C or the isolated D1. Thus, JAM-C undergoes a homophilic interaction via the Arg64-Ile65-Glu66 motif on the membrane-distal Ig domain of the molecule. The homophilic interaction of JAM-C can mediate tumor cell-endothelial cell interactions and may thereby be involved in the process of tumor cell metastasis.     

Pathological missense mutations of neural cell adhesion molecule L1 affect homophilic and heterophilic binding activities.

Mutations in the gene for neural cell adhesion molecule L1 (L1CAM) result in a debilitating X-linked congenital disorder of brain development. At the neuronal cell surface L1 may interact with a variety of different molecules including itself and two other CAMs of the immunoglobulin superfamily, axonin-1 and F11. However, whether all of these interactions are relevant to normal or abnormal development has not been determined. Over one-third of patient mutations are single amino acid changes distributed across 10 extracellular L1 domains. We have studied the effects of 12 missense mutations on binding to L1, axonin-1 and F11 and shown for the first time that whereas many mutations affect all three interactions, others affect homophilic or heterophilic binding alone. Patient pathology is therefore due to different types of L1 malfunction. The nature and functional consequence of mutation is also reflected in the severity of the resultant phenotype with structural mutations likely to affect more than one binding activity and result in early mortality. Moreover, the data indicate that several extracellular domains of L1 are required for homophilic and heterophilic interactions.     

Single-molecule force spectroscopy of the aplysia cell adhesion molecule reveals two homophilic bonds

Aplysia californica neurons comprise a powerful model system for quantitative analysis of cellular and biophysical properties that are essential for neuronal development and function. The Aplysia cell adhesion molecule (apCAM), a member of the immunoglobulin superfamily of cell adhesion molecules, is present in the growth cone plasma membrane and involved in neurite growth, synapse formation, and synaptic plasticity. apCAM has been considered to be the Aplysia homolog of the vertebrate neural cell adhesion molecule (NCAM); however, whether apCAM exhibits similar binding properties and neuronal functions has not been fully established because of the lack of detailed binding data for the extracellular portion of apCAM. In this work, we used the atomic force microscope to perform single-molecule force spectroscopy of the extracellular region of apCAM and show for the first time (to our knowledge) that apCAM, like NCAM, is indeed a homophilic cell adhesion molecule. Furthermore, like NCAM, apCAM exhibits two distinct bonds in the trans configuration, although the kinetic and structural parameters of the apCAM bonds are quite different from those of NCAM. In summary, these single-molecule analyses further indicate that apCAM and NCAM are species homologs likely performing similar functions.     

Ethanol inhibits L1 cell adhesion molecule activation of mitogen-activated protein kinases

Inhibition of the functions of L1 cell adhesion molecule (L1) by ethanol has been implicated in the pathogenesis of the neurodevelopmental aspects of the fetal alcohol syndrome (FAS). Ethanol at pharmacological concentrations has been shown to inhibit L1-mediated neurite outgrowth of rat post-natal day 6 cerebellar granule cells (CGN). Extracellular signal-related kinases (ERK) 1/2 activation occurs following L1 clustering. Reduction in phosphoERK1/2 by inhibition of mitogen-activated protein kinase kinase (MEK) reduces neurite outgrowth of cerebellar neurons. Here, we examine the effects of ethanol on L1 activation of ERK1/2, and whether this activation occurs via activation of fibroblast growth factor receptor 1 (FGFR1). Ethanol at 25 mm markedly inhibited ERK1/2 activation by both clustering L1 with cross-linked monoclonal antibodies, or by L1-Fc chimeric proteins. Clustering L1 with subsequent ERK1/2 activation did not result in tyrosine phosphorylation of the FGFR1. In addition, inhibition of FGFR1 tyrosine kinase blocked basic fibroblast growth factor (bFGF) activation of ERK1/2, but did not affect activation of ERK1/2 by clustered L1. We conclude that ethanol disrupts the signaling pathway between L1 clustering and ERK1/2 activation, and that this occurs independently of the FGFR1 pathway in cerebellar granule 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.     

Low-expression of E-cadherin in leukaemia cells causes loss of homophilic adhesion and promotes cell growth

E-cadherin (epithelial cadherin) belongs to the calcium-dependent adhesion molecule superfamily and is implicated in the interactions of haematopoietic progenitors and bone marrow stromal cells. Adhesion capacity to bone marrow stroma was impaired for leukaemia cells, suggesting that a breakdown of adhesive mechanisms governed by an adhesion molecule may exist in leukaemic microenvironment. We previously found that E-cadherin was low expressed in primary acute leukaemia cells compared with normal bone marrow mononuclear cells. In this study, we investigate the functional importance of low E-cadherin expression in leukaemia cell behaviours and investigate its effects in the abnormal interaction of leukaemic cells with stromal cells. After expression of E-cadherin was restored by a demethylating agent in leukaemia cells, E-cadherin-specific adhesion was enhanced. Additionally, siRNA (small interfering RNA)-mediated silencing of E-cadherin in Raji cells resulted in a reduction of cell homophilic adhesion and enhancement of cell proliferation and colony formation. These results suggest that low expression of E-cadherin contributes to the vigorous growth and transforming ability of leukaemic cells.     

Functional complementation of yeast ribosomal P0 protein with Plasmodium falciparum P0

A complex of three phosphoproteins (P0, P1 and P2) constitutes the stalk region at the GTPase center of the eukaryotic large ribosomal subunit, amongst which the protein P0 plays the most crucial role. Earlier studies have shown the functional complementation of the conditional P0-null mutant of Saccharomyces cerevisiae (W303dGP0) with orthologous P0 genes from fungal and mammalian organisms, but not the protozoan parasite Leishmania infantum. In this paper we show that the PfP0 gene from the protozoan malaria parasite Plasmodium falciparum can functionally complement the conditional P0-null W303dGP0 mutant of S. cerevisiae. Unlike the above orthologous genes, PfP0 gene could also rescue the D67dGP0 strain, which in addition to being a conditional null for ScP0 gene, is a null-mutant for both ScP1alpha and beta genes. However, under stress conditions such as high temperature, salt and osmolarity, PfP0 gene could not rescue D67dGP0 strain. Ribosomes purified from W303dGP0 carrying PfP0 gene did not contain ScP1 protein, indicating a lack of binding of ScP1 to PfP0 protein. Yeast 2-hybrid analysis further confirmed the lack of binding of ScP1 to PfP0 protein. The polymerizing activities of ribosomes with ScP0 or PfP0 protein, in the absence of ScP1 protein, were found to be about 40-45% that of ribosomes with all the yeast P-proteins. In its sensitivity to the inhibitor sordarin, PfP0 was similar to the P0 protein from the fungus Aspergillus fumigatus. These results indicate a closer functional relationship of P. falciparum P0 gene to fungal P0 genes.     

Disruption of the GDNF binding site in NCAM dissociates ligand binding and homophilic cell adhesion

Most plasma membrane proteins are capable of sensing multiple cell-cell and cell-ligand interactions, but the extent to which this functional versatility is founded on their modular design is less clear. We have identified the third immunoglobulin domain of the Neural Cell Adhesion Molecule (NCAM) as the necessary and sufficient determinant for its interaction with Glial Cell Line-derived Neurotrophic Factor (GDNF). Four charged contacts were identified by molecular modeling as the main contributors to binding energy. Their mutation abolished GDNF binding to NCAM but left intact the ability of NCAM to mediate cell adhesion, indicating that the two functions are genetically separable. The GDNF-NCAM interface allows complex formation with the GDNF family receptor alpha1, shedding light on the molecular architecture of a multicomponent GDNF receptor.     

The differential effect of normal cells and their neoplastic transformants on embryonic cell adhesion

The adhesion of 7 day old embryonic chick neural retina cells was reduced by adult calf serum whereas foetal calf serum diminished adhesion to a lesser extent. Conditioned media from normal cells either increased the adhesion of neural retina cells or had no effect. Media from malignant cells reduced the adhesion of the retina cells. The results are discussed in relation to malignant invasion.