Visualization of neural cell adhesion molecule by electron microscopy

Role of CD56 in Normal Kidney Development and Wilms Tumorigenesis

The cell-surface glycoprotein CD56 has three major isoforms that play important roles in cell adhesion and signaling, which may promote cell proliferation, differentiation, survival, or migration. It is an important molecule in normal kidney development and acts as a key marker in Wilms tumor stem and progenitor cells. Here, we review the structural and genetic features of the CD56 glycoprotein, and summarize its roles in the normal versus diseased metanephric blastema. We discuss areas of CD56-related research that may complement or improve existing Wilms tumor treatment strategies, including the antibody-drug conjugate lorvotuzumab mertansine that binds to CD56.

NCAM1 Polysialylation: The Prion Protein's Elusive Reason for Being?

Much confusion surrounds the physiological function of the cellular prion protein (PrP^C). It is, however, anticipated that knowledge of its function will shed light on its contribution to neurodegenerative diseases and suggest ways to interfere with the cellular toxicity central to them. Consequently, efforts to elucidate its function have been all but exhaustive. Building on earlier work that uncovered the evolutionary descent of the prion founder gene from an ancestral ZIP zinc transporter, we recently investigated a possible role of PrP^C in a morphogenetic program referred to as epithelial-to-mesenchymal transition (EMT). By capitalizing on PrP^C knockout cell clones in a mammalian cell model of EMT and using a comparative proteomics discovery strategy, neural cell adhesion molecule-1 emerged as a protein whose upregulation during EMT was perturbed in PrP^C knockout cells. Follow-up work led us to observe that PrP^C regulates the polysialylation of the neural cell adhesion molecule NCAM1 in cells undergoing morphogenetic reprogramming. In addition to governing cellular migration, polysialylation modulates several other cellular plasticity programs PrP^C has been phenotypically linked to. These include neurogenesis in the subventricular zone, controlled mossy fiber sprouting and trimming in the hippocampal formation, hematopoietic stem cell renewal, myelin repair and maintenance, integrity of the circadian rhythm, and glutamatergic signaling. This review revisits this body of literature and attempts to present it in light of this novel contextual framework. When approached in this manner, a coherent model of PrP^C acting as a regulator of polysialylation during specific cell and tissue morphogenesis events comes into focus.

Polysialic acid: biosynthesis, novel functions and applications

As an anti-adhesive, a reservoir for key biological molecules, and a modulator of signaling, polysialic acid (polySia) is critical for nervous system development and maintenance, promotes cancer metastasis, tissue regeneration and repair, and is implicated in psychiatric diseases. In this review, we focus on the biosynthesis and functions of mammalian polySia, and the use of polySia in therapeutic applications. PolySia modifies a small subset of mammalian glycoproteins, with the neural cell adhesion molecule, NCAM, serving as its major carrier. Studies show that mammalian polysialyltransferases employ a unique recognition mechanism to limit the addition of polySia to a select group of proteins. PolySia has long been considered an anti-adhesive molecule, and its impact on cell adhesion and signaling attributed directly to this property. However, recent studies have shown that polySia specifically binds neurotrophins, growth factors, and neurotransmitters and that this binding depends on chain length. This work highlights the importance of considering polySia quality and quantity, and not simply its presence or absence, as its various roles are explored. The capsular polySia of neuroinvasive bacteria allows these organisms to evade the host immune response. While this "stealth" characteristic has made meningitis vaccine development difficult, it has also made polySia a worthy replacement for polyetheylene glycol in the generation of therapeutic proteins with low immunogenicity and improved circulating half-lives. Bacterial polysialyltransferases are more promiscuous than the protein-specific mammalian enzymes, and new studies suggest that these enzymes have tremendous therapeutic potential, especially for strategies aimed at neural regeneration and tissue repair.

Structural model and trans-interaction of the entire ectodomain of the olfactory cell adhesion molecule

The ectodomain of olfactory cell adhesion molecule (OCAM/NCAM2/RNCAM) consists of five immunoglobulin (Ig) domains (IgI-V), followed by two fibronectin-type 3 (Fn3) domains (Fn3I-II). A complete structural model of the entire ectodomain of human OCAM has been assembled from crystal structures of six recombinant proteins corresponding to different regions of the ectodomain. The model is the longest experimentally based composite structural model of an entire IgCAM ectodomain. It displays an essentially linear arrangement of IgI-V, followed by bends between IgV and Fn3I and between Fn3I and Fn3II. Proteins containing IgI-IgII domains formed stable homodimers in solution and in crystals. Dimerization could be disrupted in vitro by mutations in the dimer interface region. In conjunction with the bent ectodomain conformation, which can position IgI-V parallel with the cell surface, the IgI-IgII dimerization enables OCAM-mediated trans-interactions with an intercellular distance of about 20 nm, which is consistent with that observed in synapses.

Sequences at the interface of the fifth immunoglobulin domain and first fibronectin type III repeat of the neural cell adhesion molecule are critical for its polysialylation

Polysialic acid is an anti-adhesive glycan that modifies a select group of mammalian proteins. The primary substrate of the polysialyltransferases (polySTs) is the neural cell adhesion molecule (NCAM). Polysialic acid negatively regulates cell adhesion, is required for proper brain development, and is expressed in specific areas of the adult brain where it promotes on-going cell migration and synaptic plasticity. The first fibronectin type III repeat (FN1) of NCAM is required for polysialylation of the N-glycans on the adjacent immunoglobulin-like domain (Ig5), and acidic residues on the surface of FN1 play a role in polyST recognition. Recent work demonstrated that the FN1 domain from the unpolysialylated olfactory cell adhesion molecule (OCAM) was able to partially replace NCAM FN1 (Foley, D. A., Swartzentruber, K. G., Thompson, M. G., Mendiratta, S. S., and Colley, K. J. (2010) J. Biol. Chem. 285, 35056-35067). Here we demonstrate that individually replacing three identical regions shared by NCAM and OCAM FN1, (500)PSSP(503) (PSSP), (526)GGVPI(530) (GGVPI), and (580)NGKG(583) (NGKG), dramatically reduces NCAM polysialylation. In addition, we show that the polyST, ST8SiaIV/PST, specifically binds NCAM and that this binding requires the FN1 domain. Replacing the FN1 PSSP sequences and the acidic patch residues decreases NCAM-polyST binding, whereas replacing the GGVPI and NGKG sequences has no effect. The location of GGVPI and NGKG in loops that flank the Ig5-FN1 linker and the proximity of PSSP to this linker suggest that GGVPI and NGKG sequences may be critical for stabilizing the Ig5-FN1 linker, whereas PSSP may play a dual role maintaining the Ig5-FN1 interface and a polyST recognition site.

Structure and mutagenesis of neural cell adhesion molecule domains: evidence for flexibility in the placement of polysialic acid attachment sites

The addition of alpha2,8-polysialic acid to the N-glycans of the neural cell adhesion molecule, NCAM, is critical for brain development and plays roles in synaptic plasticity, learning and memory, neuronal regeneration, and the growth and invasiveness of cancer cells. Our previous work indicates that the polysialylation of two N-glycans located on the fifth immunoglobulin domain (Ig5) of NCAM requires the presence of specific sequences in the adjacent fibronectin type III repeat (FN1). To understand the relationship of these two domains, we have solved the crystal structure of the NCAM Ig5-FN1 tandem. Unexpectedly, the structure reveals that the sites of Ig5 polysialylation are on the opposite face from the FN1 residues previously found to be critical for N-glycan polysialylation, suggesting that the Ig5-FN1 domain relationship may be flexible and/or that there is flexibility in the placement of Ig5 glycosylation sites for polysialylation. To test the latter possibility, new Ig5 glycosylation sites were engineered and their polysialylation tested. We observed some flexibility in glycosylation site location for polysialylation and demonstrate that the lack of polysialylation of a glycan attached to Asn-423 may be in part related to a lack of terminal processing. The data also suggest that, although the polysialyltransferases do not require the Ig5 domain for NCAM recognition, their ability to engage with this domain is necessary for polysialylation to occur on Ig5 N-glycans.

Structure of the tandem fibronectin type 3 domains of neural cell adhesion molecule

Activation of the fibroblast growth factor receptor (FGFR) by neural cell adhesion molecule (NCAM) is essential for NCAM-mediated neurite outgrowth. Previous peptide studies have identified two regions in the fibronectin type 3 (FN3)-like domains of NCAM as being important for these activities. Here we report the crystal structure of the NCAM FN3 domain tandem, which reveals an acutely bent domain arrangement. Mutation of a non-conserved surface residue (M610R) led to a second crystal form showing a substantially different conformation. Thus, the FN3 domain linker is highly flexible, suggesting that it corresponds to the hinge seen in electron micrographs of NCAM. The two putative FGFR1-binding segments, one in each NCAM FN3 domain, are situated close to the domain interface. They form a contiguous patch in the more severely bent conformation but become separated upon straightening of the FN3 tandem, suggesting that conformational changes within NCAM may modulate FGFR1 activation. Surface plasmon resonance experiments demonstrated only a very weak interaction between the NCAM FN3 tandem and soluble FGFR1 proteins expressed in mammalian cells (dissociation constant >100 muM). Thus, the NCAM-FGFR1 interaction at the cell surface is likely to depend upon avidity effects due to receptor clustering.

Cell adhesion molecule DM-GRASP presented as nanopatterns to neurons regulates attachment and neurite growth

Adhesion and neurite formation of neurons and neuroblastoma cells critically depends on the lateral spacing of the cell adhesion molecule DM-GRASP offered as nanostructured substrate.

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.

Specific Amino Acids in the First Fibronectin Type III Repeat of the Neural Cell Adhesion Molecule Play a Role in Its Recognition and Polysialylation by the Polysialyltransferase ST8Sia IV/PST

Polysialic acid is an anti-adhesive protein modification that promotes cell migration and the plasticity of cell interactions. Because so few proteins carry polysialic acid, we hypothesized that polysialylation is a protein-specific event and that a specific polysialyltransferase-substrate interaction is the basis of this specificity. The major substrate for the polysialyltransferases is the neural cell adhesion molecule, NCAM. Previous work demonstrates that the first fibronectin type III repeat of NCAM (FN1) was necessary for the polysialylation of the N-glycans on the adjacent immunoglobulin domain (Ig5) (Close, B. E., Mendiratta, S. S., Geiger, K. M., Broom, L. J., Ho, L. L., and Colley, K. J. (2003) J. Biol. Chem. 278, 30796-30805). This suggested that FN1 may be a recognition site for the polysialyltransferases. In this study, we showed that the second fibronectin type III repeat (FN2) of NCAM cannot replace FN1. Arg substitution of three unique acidic amino acids on the surface of FN1 eliminated polysialylation not only of a minimal Ig5-FN1 substrate but also of full-length NCAM. Ala substitution of these residues eliminated Ig5-FN1 polysialylation but not that of full-length NCAM, suggesting that the two proteins are interacting differently with the enzymes and that multiple residues are involved in the enzyme-NCAM interaction. By using another truncated protein, Ig5-FN1-FN2, we confirmed the importance of enzyme-substrate positioning for optimal recognition and polysialylation. In sum, we have found that acidic residues on the surface of FN1 are part of a larger protein interaction region that is critical for NCAM recognition and polysialylation by the polysialyltransferases.

Nanomechanics of adhesion proteins

Recent advances in molecular force measurements have resulted in the quantification of the nanomechanical properties of single molecular bonds, and elucidated novel relationships between molecular architecture and biomolecular adhesion. The measured forces to rupture single intermolecular bonds revealed novel and unexpected ways that proteins respond to mechanical force. Measurement of the magnitude of interprotein forces and the distances over which they act further determined how protein architecture may contribute to both the stability and structural organization of adhesive junctions.

Neural cell adhesion molecule (N-CAM) homophilic binding mediated by the two N-terminal Ig domains is influenced by intramolecular domain-domain interactions

The mechanism by which the neural cell adhesion molecule, N-CAM, mediates homophilic interactions between cells has been variously attributed to an isologous interaction of the third immunoglobulin (Ig) domain, to reciprocal binding of the two N-terminal Ig domains, or to reciprocal interactions of all five Ig domains. Here, we have used a panel of recombinant proteins in a bead binding assay, as well as transfected and primary cells, to clarify the molecular mechanism of N-CAM homophilic binding. The entire extracellular region of N-CAM mediated bead aggregation in a concentration- and temperature-dependent manner. Interactions of the N-terminal Ig domains, Ig1 and Ig2, were essential for bead binding, based on deletion and mutation experiments and on antibody inhibition studies. These findings were largely in accord with aggregation experiments using transfected L cells or primary chick brain cells. Additionally, maximal binding was dependent on the integrity of the intramolecular domain-domain interactions throughout the extracellular region. We propose that these interactions maintain the relative orientation of each domain in an optimal configuration for binding. Our results suggest that the role of Ig3 in homophilic binding is largely structural. Several Ig3-specific reagents failed to affect N-CAM binding on beads or on cells, while an inhibitory effect of an Ig3-specific monoclonal antibody is probably due to perturbations at the Ig2-Ig3 boundary. Thus, it appears that reciprocal interactions between Ig1 and Ig2 are necessary and sufficient for N-CAM homophilic binding, but that maximal binding requires the quaternary structure of the extracellular region defined by intramolecular domain-domain interactions.

Structure and interactions of NCAM Ig1-2-3 suggest a novel zipper mechanism for homophilic adhesion

The neural cell adhesion molecule, NCAM, mediates Ca(2+)-independent cell-cell and cell-substratum adhesion via homophilic (NCAM-NCAM) and heterophilic (NCAM-non-NCAM molecules) binding. NCAM plays a key role in neural development, regeneration, and synaptic plasticity, including learning and memory consolidation. The crystal structure of a fragment comprising the three N-terminal Ig modules of rat NCAM has been determined to 2.0 A resolution. Based on crystallographic data and biological experiments we present a novel model for NCAM homophilic binding. The Ig1 and Ig2 modules mediate dimerization of NCAM molecules situated on the same cell surface (cis interactions), whereas the Ig3 module mediates interactions between NCAM molecules expressed on the surface of opposing cells (trans interactions) through simultaneous binding to the Ig1 and Ig2 modules. This arrangement results in two perpendicular zippers forming a double zipper-like NCAM adhesion complex.

Mechanism of homophilic adhesion by the neural cell adhesion molecule: use of multiple domains and flexibility

The extracellular regions of adhesion proteins of the Ig superfamily comprise multiple, tandemly arranged domains. We used directforce measurements to investigate how this modular architecture contributes to the adhesive interactions of the neural cell adhesion molecule (NCAM), a representative of this protein class. The extracellular region of NCAM comprises five immunoglobulin and two fibronectin domains. Previous investigations generated different models for the mechanism of homophilic adhesion that each use different domains. We use force measurements to demonstrate that NCAM binds in two spatially distinct configurations. Igdomain deletion mutants identified the domains responsible for each of the adhesive bonds. The measurements also confirmed the existence of a flexible hinge that alters the orientation of the adhesive complexes and the intermembrane distance. These results suggest that a combination of multiple bound states and internal molecular flexibility allows for sequentially synergistic bond formation and the ability to accommodate differences in intercellular space.

Ultrastructure of the zonula adherens revealed by rapid-freeze deep-etching

The zonula adherens (ZA) in adult chicken retinal pigment epithelium was examined with cryo-electron microscopic methods. Deep-etching of the cross-fractured ZA showed globules in the intercellular space. These globules apparently correspond to the electron-dense structure seen in thin sections. Deep-etching of obliquely fractured ZA further revealed rod-like structures extending from the extracellular surface into the intercellular space. These rods (mean approximately 9 nm thick, approximately 20 nm long) were straight and sometimes divided into two or three segments. The rods typically canted at approximately 60 degrees with respect to the plasma membrane, and they were often connected to the intercellular globules at their distal ends. When the rods are compared with the isolated cadherins reported previously, it is suggested that a combination of a rod and a globule may represent an extracellular part of cadherin. Membrane particles were observed on the P-face of the ZA plasma membrane, and their distribution density was approximately seven times that of the rods. The freeze-etching also revealed a characteristic particle complex on the ZA cytoplasmic surface, which may represent the cytosolic proteins linking cadherins to actin bundles.

Computer simulation approach to the quantification of immunogold labelling on plasma membrane of cultured neurons

Cell culture is a convenient model system to study the expression of plasma membrane-bound proteins in nerve cells. Analysing it with an ultrastructural detail researchers often apply transmission electron microscopy together with immunogold labelling. Plasma membrane profiles are one-dimensional (1D) and provide little information about the topography of membrane-bound proteins. In order to convert 1D estimates of spatial arrangement for preembedding immunogold labelled proteins into two-dimensional (2D) quantities, namely the 2D pattern and density of labelling, this paper presents a simple computer simulation technique. This technique is based on a mathematical model permitting a simulated immunogold labelled membrane to be sampled in a way similar to microtome sectioning. An interlabel distance (ILD) estimate is used to define the position of immunogold particles in membrane profiles. In order to interpret experimental ILD measurements the simulated distribution best fit to the experimental data is selected and the corresponding 2D density and pattern of particle scattering are considered to explain the real situation. Various parameters including a cell section thickness, immunogold particle size etc can be adjusted to suit the demands of a particular experiment. The technique was applied to quantify the NCAM preembedding immunogold labelling in the plasma membrane of cultured rat hippocampal neurons.

Structural analysis of the murine cell adhesion molecule L1 by electron microscopy and computer-assisted modelling

In the present study we have analysed the morphology of two fragments with apparent molecular weights of 180 and 140 kDA (L1-180 and L1-140) derived from the extracellular region of the murine neural cell adhesion molecule L1. The fragment L1-180 consists of almost the entire extracellular part of the molecule, and is built up of six immunoglobulin-like and five fibronectin type III-like domains. Fragment L1-140 lacks one-half of the third, the fourth and the fifth fibronectin type III-like domains. By electron microscopic analysis of rotary-shadowed molecules, L1-140 and L1-180 revealed fibrillar structures 31-43 nm long and 7-12 nm wide with one pronounced globular terminal domain. As determined by complex formation with an L1 antibody, this terminal part of the molecule is formed by the fibronectin type III-like domains. The individual structures showed variation and complexity, and four distinct aspects were identified. These different forms probably represent two-dimensional projections of the same three-dimensional helical structure. Computer-assisted modelling of the L1 molecule, i.e. the protein backbone, showed no strong intramolecular interaction between the different fibronectin type III- or Ig-like domains, suggesting that the formation of the globular part of the molecule is probably achieved by protein-carbohydrate and/or carbohydrate-carbohydrates rather than protein-protein interactions. In addition, our model proposes that interactions occur within the interfaces between the different domains. The highly conserved amino acid residues in these regions point to the necessity of maintaining the orientation between the different domains.

Homophilic adhesion mediated by the neural cell adhesion molecule involves multiple immunoglobulin domains

The neural cell adhesion molecule (N-CAM) mediates homophilic binding between a variety of cell types including neurons, neurons and glia, and neurons and muscle cells. The mechanism by which N-CAM on one cell interacts with N-CAM on another, however, is unknown. Attempts to identify which of the five immunoglobulin-like domains (Ig I-V) and the two fibronectin type III repeats (FnIII 1-2) in the extracellular region of N-CAM are involved in this process have led to ambiguous results. We have generated soluble recombinant proteins corresponding to each of the individual immunoglobulin domains and the combined FnIII 1-2 and prepared polyclonal antibodies specific for each. The purified proteins and antibodies were used in aggregation experiments with fluorescent microspheres and chicken embryo brain cells to determine possible contributions of each domain to homophilic adhesion. The recombinant domains were tested for their ability to bind to purified native N-CAM, to bind to each other, and to inhibit the aggregation of N-CAM on microspheres and the aggregation of neuronal cells. Each of the immunoglobulin domains bound to N-CAM, and in solution all of the immunoglobulin domains inhibited the aggregation of N-CAM-coated microspheres. Soluble Ig II, Ig III, and Ig IV inhibited neuronal aggregation; antibodies against whole N-CAM, the Ig III domain, and the Ig I domain all strongly inhibited neuronal aggregation, as well as the aggregation of N-CAM-coated microspheres. Of all the domains, the third immunoglobulin domain alone demonstrated the ability to self-aggregate, whereas Ig I bound to Ig V and Ig II bound to Ig IV. The combined FnIII 1-2 exhibited a slight ability to self-aggregate but did not bind to any of the immunoglobulin-like domains. These results suggest that N-CAM-N-CAM binding involves all five immunoglobulin domains and prompt the hypothesis that in homophilic cell-cell binding mediated by N-CAM these domains may interact pairwise in an antiparallel orientation.

An anti-invasive concentration of the alkyl-lysophospholipid ET-18-OCH3 enhances the motility of embryonal chick heart cells cultured on solid substrate

Pretreatment of embryonal chick heart fragments with ET-18-OCH3 is known to induce resistance to invasion by several malignant cell lines. Embryonal chick heart fragments or cell suspensions prepared from such fragments were explanted on solid substrate and treated in medium with 10 micrograms/ml ET-18-OCH3 or with drug-free medium (control) for 48 h. This medium was washed away and replaced by drug-free fresh medium. Twenty-four to 48 h later the fast plasma membrane movements (involved in ruffling, blebbing, fast shape change and fast translocation) were quantified using a simple method based on subtracting two video images taken with an interval of 28 s. The ET-18-OCH3-treated cells showed a higher intensity of fast plasma membrane movements than control cells. Cells around a treated explant did not show the same radial alignment as in controls, suggesting loss of contact inhibition of movement. Cells from a cell suspension derived from a treated fragment showed faster translocation on solid substrate and faster shape change. We speculate that increased motility of host cells may be involved in resistance to invasion.

Axonal glycoproteins with immunoglobulin- and fibronectin type III-related domains in vertebrates: structural features, binding activities, and signal transduction

The L1- and F11-like axonal glycoproteins, implicated in neurite outgrowth and fasciculation, are members of the Ig superfamily comprising multiple fibronectin type III-like domains. Their Ig-like and fibronectin type III-related domains are likely to be composed of seven beta-strands arranged in two opposing beta-sheets of highly similar topology. Whereas the F11-like molecules lack a transmembrane sequence and are anchored in the plasma membrane by a glycosylphosphatidylinositol, the L1-like molecules comprise cytoplasmic domains with highly conserved sequence motifs. Most of the latter proteins occur in different isoforms generated by alternative pre-mRNA splicing, which has not been documented for molecules of the F11 subgroup. L1-like proteins undergo heterophilic as well as homophilic interactions, whereas only the former mode of binding was observed for F11-like proteins. Evidence is accumulating that these Ig superfamily molecules with fibronectin type III-like domains are interacting in a complex manner with each other and molecules of the extracellular matrix. Investigations assigning structure to function reveal that their individual extracellular domains serve distinct binding activities. Recent studies also suggest that L1 and NCAM are implicated in the transduction of transmembrane signals.

Functional topography of the myelin-associated glycoprotein. I. Mapping of domains by electron microscopy

The functional topography of the myelin-associated glycoprotein (MAG) was investigated by electron microscopic analysis of rotary-shadowed molecules of a MAG fragment (MAG 90) comprising the five immunoglobulin-like domains of the extracellular part of the molecule. MAG 90 molecules appeared as rod-like structures (18.5 +/- 1.2 nm long and 4.0 +/- 0.8 nm wide) with a globular domain at one end. Antibodies directed against the amino- and carboxy-terminus of MAG 90 interacted with the non-globular terminal region, indicating that the molecule is bent in the globular region with the amino- and carboxy-terminal arms in close apposition to each other. An antibody which interferes with the binding of MAG to neurons interacted predominantly with the globular domain of MAG 90. The fibril-forming collagen types I, III and V bound mainly to the non-globular terminal region of MAG 90, whereas the majority of heparin molecules interacted with the globular region of the molecule. The L2/HNK-1 carbohydrate structure was localized at the non-globular region in the protein fragment comprising the fourth and fifth immunoglobulin-like domains.

Structural properties of the myelin-associated glycoprotein ectodomain

Myelin-associated glycoprotein (MAG) has been proposed to mediate adhesive interactions during myelin development. We have used the baculovirus expression system to produce a truncated form of this molecule [soluble extracellular domain of MAG (sMAG)] consisting of the complete extracellular ectodomain. Spectroscopic studies indicate a high beta-sheet content, consistent with the prediction of Ig-like structure. Hydrodynamic studies indicate an asymmetric monomer, with a Stokes radius of 4.1-4.6 nm, a sedimentation coefficient of 3.6S, and a frictional ratio of approximately 1.6. We postulate that the outer two Ig-like domains form a unit that folds back over the rest of the molecule. Fluorescence quenching studies indicate that sMAG interacts with divalent cations and may have a functional lectin domain.

Ankyrin-binding proteins related to nervous system cell adhesion molecules: candidates to provide transmembrane and intercellular connections in adult brain

A major class of ankyrin-binding glycoproteins have been identified in adult rat brain of 186, 155, and 140 kD that are alternatively spliced products of the same pre-mRNA. Characterization of cDNAs demonstrated that ankyrin-binding glycoproteins (ABGPs) share 72% amino acid sequence identity with chicken neurofascin, a membrane-spanning neural cell adhesion molecule in the Ig super-family expressed in embryonic brain. ABGP polypeptides have the following features consistent with a role as ankyrin-binding proteins in vitro and in vivo: (a) ABGPs and ankyrin associate as pure proteins in a 1:1 molar stoichiometry; (b) the ankyrin-binding site is located in the COOH-terminal 21 kD of ABGP186 which contains the predicted cytoplasmic domain; (c) ABGP186 is expressed at approximately the same levels as ankyrin (15 pmoles/milligram of membrane protein); and (d) ABGP polypeptides are co-expressed with the adult form of ankyrinB late in postnatal development and are colocalized with ankyrinB by immunofluorescence. Similarity in amino acid sequence and conservation of sites of alternative splicing indicate that genes encoding ABGPs and neurofascin share a common ancestor. However, the major differences in developmental expression reported for neurofascin in embryos versus the late postnatal expression of ABGPs suggest that ABGPs and neurofascin represent products of gene duplication events that have subsequently evolved in parallel with distinct roles. The predicted cytoplasmic domains of rat ABGPs and chicken neurofascin are nearly identical to each other and closely related to a group of nervous system cell adhesion molecules with variable extracellular domains, which includes L1, Nr-CAM, and Ng-CAM of vertebrates, and neuroglian of Drosophila. The ankyrin-binding site of rat ABGPs is localized to the C-terminal 200 residues which encompass the cytoplasmic domain, suggesting the hypothesis that ability to associate with ankyrin may be a shared feature of neurofascin and related nervous system cell adhesion molecules.

Distinct NCAM splicing events are differentially regulated during rat brain development

Primary transcripts for the neural cell adhesion molecule NCAM are highly alternatively spliced potentially giving rise to over 100 different mRNA forms. These mRNAs encode three major polypeptide isoforms of 120, 140, and 180 kDa each of which is thought to be composed of a mixture of polypeptides that differ by the variable presence of small exons at two locations. These NCAM 'microsplicing' patterns were examined within mRNA populations encoding each of the major isoforms to identify exactly which NCAM forms are present during brain development. The proportion of NCAM mRNAs containing at the exon 7/8 junction the alternatively microspliced 30 bp VASE exon increased similarly during brain development in mRNAs encoding all three major size classes. Perinatal brain, cultures of neurons from embryonic rats, and of glia from newborn rats all had low VASE levels while about 50% of the mRNAs in adult brain expressed VASE. In contrast, microsplicing at the exon 12/exon 13 junction was differentially regulated among NCAM major size classes. mRNAs containing microspliced exons totaling 3.15, or 18 base pairs (bp) represented greater than 50% of the total mRNAs encoding the 120 kDa forms at all ages. However, these exons were present in less than 15% of the 140 and 180 kDa encoding mRNAs in rats older than embryonic day 15. Similar results at the exon 12/13 junction were observed with mRNAs from neuronal cultures while glial cultures had greater levels of a 3 bp pair exon at this junction.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of soluble neural cell adhesion molecule in rat brain, CSF, and plasma

The polypeptide composition and glycosylation of soluble isoforms of neural cell adhesion molecule (NCAM) in developing rat brain, CSF, and plasma were characterized. Soluble NCAM in rat brain consisted of several glycosylated isoforms. The degree of glycosylation was developmentally regulated. After desialylation, four polypeptides of M(r) values of approximately 190,000 (s1), 135,000 (s2), 115,000 (s3), and 110,000 (s4) were observed. Polypeptides s1, s2, and s3 were also present in CSF, whereas only s3 and s4 were observed in plasma. Treatment of soluble brain NCAM with N-glycosidase F, which removes N-linked carbohydrates, produced polypeptides of M(r) values of approximately 190,000, 125,000, and 108,000-97,000. The monoclonal antibody OB11, which recognizes an epitope on the cytoplasmic part of transmembrane forms of NCAM, did not react with any of the soluble isoforms. Purified soluble NCAM, consisting mainly of s3, contained an N-terminal sequence identical to that of membrane-associated NCAM. Gel filtration of s3 indicated that it was present as a dimer under the chosen conditions. NCAM-expressing glioma cells adhered specifically to immobilized soluble NCAM. This implies that functionally significant soluble forms of NCAM are present in the extracellular fluid.

Identification of a peptide sequence involved in homophilic binding in the neural cell adhesion molecule NCAM

The neural cell adhesion molecule NCAM is capable of mediating cell-cell adhesion via homophilic interactions. In this study, three strategies have been combined to identify regions of NCAM that participate directly in NCAM-NCAM binding: analysis of domain deletion mutations, mapping of epitopes of monoclonal antibodies, and use of synthetic peptides to inhibit NCAM activity. Studies on L cells transfected with NCAM mutant cDNAs using cell aggregation and NCAM-covasphere binding assays indicate that the third immunoglobulin-like domain is involved in homophilic binding. The epitopes of four monoclonal antibodies that have been previously shown to affect cell-cell adhesion mediated by NCAM were also mapped to domain 3. Overlapping hexapeptides were synthesized on plastic pins and assayed for binding with these monoclonal antibodies. One of them (PP) reacted specifically with the sequence KYSFNY. Synthetic oligopeptides containing the PP epitope were potent and specific inhibitors of NCAM binding activity. A substratum containing immobilized peptide conjugates also exhibited adhesiveness for neural retinal cells. Cell attachment was specifically inhibited by peptides that contained the PP-epitope and by anti-NCAM univalent antibodies. The shortest active peptide has the sequence KYSFNYDGSE, suggesting that this site is directly involved in NCAM homophilic interaction.

Alternative splicing of neural-cell-adhesion molecule mRNA in human small-cell lung-cancer cell line H69

The neural-cell-adhesion molecule (NCAM) is expressed in all small-cell lung cancers (SCLC) and in approximately 20% of non-small-cell lung tumors (non-SCLC). These NCAM-positive lung tumors have a poor prognosis compared with NCAM-negative tumors. Multiple NCAM protein isoforms are expressed from a single-copy gene as a result of alternative splicing and/or post-translational modifications. Therefore, we studied the NCAM isoforms expressed in a human small-cell lung-cancer cell line, H69. NCAM mRNA transcripts of 7.2, 6.7, 4.3 and 4.0 were detected in these cells on Northern blots. Since the various NCAM isoforms may have different biological properties, we performed a more precise examination of NCAM mRNAs, using polymerase chain reactions (PCR) with primers flanking the various NCAM exon boundaries. The shortest alternatively spliced sequence that we found was the trinucleotide AAG located between exon 12 and 13 in the so-called hinge region of the NCAM protein. This AAG trinucleotide was present in the majority of the NCAM mRNAs. A second alternatively spliced 30 nt-exon VASE (immunoglobulin-variable domain-like alternatively spliced exon) was present in all NCAM transcript isoforms at the exon 7/exon 8 junction. VASE resulted in the insertion of 10 amino acids into the 4th immunoglobulin-like loop of the NCAM protein. Within the limits of the PCR methodology, no evidence for the presence of mRNA containing exon 15, encoding the glycosyl-phosphatidylinositol-linked (GPI-linked) NCAM isoform in H69 cells was obtained. Considering that H69 cells express 2 major NCAM protein classes (NCAM-180 and NCAM-140), and that the VASE and AAG alternative mRNA splice variants result in minor differences in protein sizes, at least 8 polypeptide isoforms of NCAM might be expressed in H69 cells that contribute to the binding interactions of NCAM.

Intercellular space is affected by the polysialic acid content of NCAM

We have previously proposed that polysialic acid (PSA), which is attached to NCAM on the cell surface, can serve to regulate a variety of cell-cell interactions. The present study provides evidence that hydrated PSA influences a sufficiently large volume at the cell surface to exert broad steric effects, and that the removal of PSA in fact causes a detectable change in intercellular space. Using F11 neuron/neuroblastoma hybrid cells as a model system, the measured density and size of PSA suggests that a substantial fraction of the space between two apposed cell surface membranes could be sterically influenced by the presence of PSA. Specific enzymatic removal of PSA, which is similar in magnitude to changes that occur in many tissues during normal development, caused about a 25% decrease in the distance between two apposed cells. By contrast, removal of both heparan sulfate and chondroitin sulfate from the cells had no effect on this parameter. It is proposed that such changes in membrane-membrane distance could serve to alter selectively the efficiency of encounter between complementary receptors on apposing cells, and explain at least in part the broad biological influences of PSA.

NCAM polysialic acid can regulate both cell-cell and cell-substrate interactions

We have proposed previously that the polysialic acid (PSA) moiety of NCAM can influence membrane-membrane apposition, and thereby serve as a selective regulator of a variety of contact-dependent cell interactions. In this study, cell and tissue culture models are used to obtain direct evidence that the presence of PSA on the surface membrane can affect both cell-cell and cell-substrate interactions. Using a neuroblastoma/sensory neuron cell hybrid, it was found that removal of PSA with a specific neuraminidase (endo-N) augments cell-cell aggregation mediated by the L1 cell adhesion molecule as well as cell attachment to a variety of tissue culture substrates. In studies of embryonic spinal cord axon bundling, which involves both cell-cell and cell-substrate interactions, the pronounced defasciculation produced by removal of PSA is most easily explained by an increase in cell-substrate interaction. The fact that in both studies NCAM's intrinsic adhesion function was found not to be an important variable further illustrates that regulation of the cell surface by PSA can extend beyond binding mediated by the NCAM polypeptide.

Characterization of pemphigus foliaceus antigen from human epidermis

Pemphigus foliaceus (PF) and its endemic form, Fogo Selvagem (FS), are characterized by subcorneal vesicles and pathogenic IgG autoantibodies directed against keratinocyte surface antigens. A major pool of FS antigen(s) remains bound to the insoluble epidermal envelope fraction. In this paper we demonstrate that this antigen(s) can be released from the envelope fraction by sonication. By immune precipitation four components can be detected, having molecular weights (MW) of 260, 80, 62, and 45 kD. The 260-kD component is lost by boiling or extraction with glycine HCl at pH 2.8. The major components appear to be the 80- and 62-kD poly-peptides. They chromatograph as a unit by gel filtration in 0.1% SDS, in the MW range of 115-120 kD. The FS antigen(s) appears to be cationic, forming insoluble complexes at low pH with SDS, and is labile to ammonium sulfate and freezing and thawing. It is unaffected by positive pressure concentration, 50% acetone precipitation, and reduction/alkylation. The FS antigen(s) is precipitated by all FS and nonendemic PF sera except those in complete clinical and serologic remission. The FS antigen(s) is also precipitated by 50% of pemphigus vulgaris but none of the bullous pemphigoid sera tested. All FS antigenic components are immunoprecipitated by IgG4 autoantibodies, but the IgG1 subclass from the same patients appear to immunoprecipitate only the 62-kD polypeptide. The FS antigen(s) is able to adsorb human autoantibodies against human desmoglein 1 (DG1), but not rabbit antisera against bovine DG1 or 2. This paper shows that physical stress, i.e., sonication, may be able to solubilize sufficient FS antigen(s) from the epidermal envelope fractions for further chemical characterization. The relationship of these FS antigen(s) to other reported FS antigens is presently unknown.

Binding properties of detergent-solubilized NCAM

An assay has been designed for the identification of NCAM-binding proteins present in an NP-40 detergent extract of brain membranes. This method, which is capable of analyzing both heterophilic and homophilic interactions, uses species-specific antibodies against NCAM in combination with radioiodination, so that after unlabeled chicken and iodinated frog brain membrane proteins were allowed to interact, the chicken NCAM could be specifically isolated by immunoaffinity adsorption. The radiolabeled frog proteins coisolated with chicken NCAM were then characterized by one- and two-dimensional gel electrophoresis in combination with immunoblotting. The only detectable NCAM-binding proteins were identified as the 140- and 180-kD forms of NCAM. The presence and absence of polysialic acid on NCAM did not change the amount or nature of the frog proteins immunopurified under these conditions. As an alternative for detecting heterophilic ligands, a simplified immunoprecipitation method was employed using either iodine or sulfate radiolabels. Again under these conditions only NCAM was detected. These results are consistent with the hypothesis that the major binding protein for NCAM is NCAM itself, and suggest that differences in polysialic acid content do not directly alter the properties of NCAM's homophilic binding site.

The neural cell adhesion molecule N-CAM enhances L1-dependent cell-cell interactions

On neural cells, the cell adhesion molecule L1 is generally found coexpressed with N-CAM. The two molecules have been suggested, but not directly shown, to affect each other's function. To investigate the possible functional relationship between the two molecules, we have characterized the adhesive interactions between the purified molecules and between cultured cells expressing them. Latex beads were coated with purified L1 and found to aggregate slowly. N-CAM-coated beads did not aggregate, but did so after addition of heparin. Beads coated with both L1 and N-CAM aggregated better than L1-coated beads. Strongest aggregation was achieved when L1-coated beads were incubated together with beads carrying both L1 and N-CAM. In a binding assay, the complex of L1 and N-CAM bound strongly to immobilized L1, but not to the cell adhesion molecules J1 or myelin-associated glycoprotein. N-CAM alone did not bind to these glycoproteins. Cerebellar neurones adhered to and sent out processes on L1 immobilized on nitrocellulose. N-CAM was less effective as substrate. Neurones interacted most efficiently with the immobilized complex of L1 and N-CAM. They adhered to this complex even when its concentration was at least 10 times lower than the lowest concentration of L1 found to promote adhesion. The complex became adhesive for cells only when the two glycoproteins were preincubated together for approximately 30 min before their immobilization on nitrocellulose. The adhesive properties between cells that express L1 only or both L1 and N-CAM were also studied. ESb-MP cells, which are L1-positive, but N-CAM negative, aggregated slowly under low Ca2+. Their aggregation could be completely inhibited by antibodies to L1 and enhanced by addition of soluble N-CAM to the cells before aggregation. N2A cells, which are L1 and N-CAM positive aggregated well under low Ca2+. Their aggregation was partially inhibited by either L1 or N-CAM antibodies and almost completely by the combination of both antibodies. N2A and ESb-MP cells coaggregated rapidly and their interaction was similarly inhibited by L1 and N-CAM antibodies. These results indicate that L1 is involved in two types of binding mechanisms. In one type, L1 serves as its own receptor with slow binding kinetics. In the other, L1 is modulated in the presence of N-CAM on one cell (cis-binding) to form a more potent receptor complex for L1 on another cell (trans-binding).

Lead-impaired neurodevelopment. Mechanisms and threshold values in the rodent

Mechanisms underlying the cognitive deficits arising from chronic low level lead exposure are viewed as a consequence of impaired neurodevelopmental events. These events are exemplified within the cerebellum which is formed completely after birth and, therefore, encompasses all major epochs of development. Further, the unique pharmacokinetic distribution of juvenile blood lead levels is developmentally regulated and can be correlated to the periods of impaired structuring and threshold values indicating no-effect levels established. Here, the endpoint value is related to lead-induced dysfunctioning of a morphoregulator--the neural cell adhesion molecule (NCAM). During final synaptic structuring its normal developmental sequence is perturbed when blood lead levels exceed 20 micrograms/dl. These events are similar, and compared to, those seen in the cerebellum of the staggerer mouse mutant (sg/sg) where the selection of circuits to be preserved from those transiently overproduced during development is impaired.

The mouse neuronal cell surface protein F3: a phosphatidylinositol-anchored member of the immunoglobulin superfamily related to chicken contactin

Several members of the Ig superfamily are expressed on neural cells where they participate in surface interactions between cell bodies and processes. Their Ig domains are more closely related to each other than to Ig variable and constant domains and have been grouped into the C2 set. Here, we report the cloning and characterization of another member of this group, the mouse neuronal cell surface antigen F3. The F3 cDNA sequence contains an open reading frame that could encode a 1,020-amino acid protein consisting of a signal sequence, six Ig-like domains of the C2 type, a long premembrane region containing two segments that exhibit sequence similarity to fibronectin type III repeats and a moderately hydrophobic COOH-terminal sequence. The protein does not contain a typical transmembrane segment but appears to be attached to the membrane by a phosphatidylinositol anchor. Antibodies against the F3 protein recognize a prominent 135-kD protein in mouse brain. In fetal brain cultures, they stain the neuronal cell surface and, in cultures maintained in chemically defined medium, most prominently neurites and neurite bundles. The mouse f3 gene maps to band F of chromosome 15. The gene transcripts detected in the brain by F3 cDNA probes are developmentally regulated, the highest amounts being expressed between 1 and 2 wk after birth. The F3 nucleotide and deduced amino acid sequence show striking similarity to the recently published sequence of the chicken neuronal cell surface protein contactin. However, there are important differences between the two molecules. In contrast to F3, contactin has a transmembrane and a cytoplasmic domain. Whereas contactin is insoluble in nonionic detergent and is tightly associated with the cytoskeleton, about equal amounts of F3 distribute between buffer-soluble, nonionic detergent-soluble, and detergent-insoluble fractions. Among other neural cell surface proteins, F3 most resembles the neuronal cell adhesion protein L1, with 25% amino acid identity between their extracellular domains. Based on its structural similarity with known cell adhesion proteins of nervous tissue and with L1 in particular, we propose that F3 mediates cell surface interactions during nervous system development.

Characterization of a heparan sulfate proteoglycan that copurifies with the neural cell adhesion molecule

We have demonstrated previously that the neural cell adhesion molecule (NCAM) interacts with a neuronal heparan sulfate proteoglycan. The binding of this proteoglycan(s) by NCAM appears to be required for NCAM-mediated cell adhesion, although the mechanism is unclear. In the present study we show that a heparan sulfate proteoglycan copurifies with NCAM, and provide an initial biochemical characterization of the proteoglycan. The copurification of a heparan sulfate proteoglycan with NCAM was demonstrated following immunopurification of NCAM from a detergent extract of cell membranes derived from Na2(35)SO4-labeled neural retinal cells. A large-molecular-weight, 35SO4-labeled molecule copurified with NCAM isolated from these neural cell cultures, and was resistant to chondroitinase ABC treatment, but degraded completely by nitrous acid treatment. These results indicate that the molecule is a heparan sulfate proteoglycan. Although this proteoglycan copurifies with NCAM, it is not detected when the neuron-glia cell adhesion molecule (NgCAM) is immunopurified using the 8D9 monoclonal antibody. The heparan sulfate proteoglycan may also be a membrane-associated proteoglycan since it interacts with phenyl-Sepharose. Molecular weight characterization of the proteoglycan by gel filtration chromatography indicates a molecular weight of 400-520 kDa. The heparan sulfate glycosaminoglycan chains were shown to have an average molecular weight of approximately 40 kDa, and the polypeptide backbone was estimated to be 120 kDa by polyacrylamide gel electrophoresis. These data therefore demonstrate that a neuronal heparan sulfate proteoglycan copurifies with NCAM.

Alternative splicing of the neural cell adhesion molecule gene generates variant extracellular domain structure in skeletal muscle and brain

Myotube mRNA isoforms of the neural cell adhesion molecule (N-CAM) contain a novel sequence block termed muscle-specific domain 1 (MSD1), which is inserted within the extracellular coding region. Here, we report a characterization of the genomic organization of MSD1 and its pattern of expression within cellular N-CAM RNA and polypeptide species. S1 nuclease protection analyses and sequence analysis of an N-CAM human genomic clone containing MSD1 sequences indicated that MSD1 is comprised of three discrete exons of 15, 48, and 42 bp, designated MSD1a, MSD1b, and MSD1c, respectively. Although the MSD1a exon was present in a small proportion of mRNAs from both brain and muscle cells, the entire MSD1 sequence occurred predominantly in mRNAs from differentiated myotube cells. In addition, antiserum raised to a synthetic, MSD1b-encoded peptide sequence was found to stain the cell surface of human skeletal myotubes in culture, whereas myoblasts, fibroblasts, and neural cells were negative. MSD1a, MSD1b, and MSD1c sequences thus arise collectively in N-CAM mRNA and polypeptide isoforms as a result of muscle tissue-specific and developmentally regulated alternative mRNA splicing events. In addition, the occurrence of brain and muscle mRNAs containing only MSD1a indicate that alternative splicing may occur within the MSD region itself to generate further diversity.

Topology of cell adhesion molecules

The neural cell adhesion molecule (N-CAM) exists in two major forms [ld (large cytoplasmic domain) peptide and sd (small cytoplasmic domain) peptide] that contain transmembrane segments and different cytoplasmic domains and in a third form [ssd (small surface domain) peptide] that lacks transmembrane and cytoplasmic regions. All forms have the same extracellular region of more than 600 amino acid residues, a region also found in a fragment (Fr2) that can be released from cells by proteolysis. The liver cell adhesion molecule (L-CAM) is expressed as a single species that is distinct from N-CAM, but its extracellular region can also be obtained as a proteolytic fragment (Ft1). Examination of the various forms of N-CAM and the Ft1 fragment of L-CAM by electron microscopy of rotary shadowed molecules indicated that they all have rod-shaped structures that contain a hinge region which is apparently flexible. Both the ssd chain and the Fr2 fragment of N-CAM are single rods bent into arms approximately 18 and 10 nm long. The ld and sd chains are longer bent rods that form rosettes comprising two to six branches; detergent treatment disrupts these rosettes into single rods. Specific antibodies that block homophilic N-CAM binding labeled the distal ends of the branches of the ld/sd rosettes and the ends of the longer arm of both the ssd chain and the Fr2 fragment. Antibodies that bind to the sialic acid-rich region of N-CAM bound near the hinge. These data indicate that the N-CAM rosettes are formed by interaction between their transmembrane or cytoplasmic domains and not by interactions involving their homophilic binding sites. The L-CAM Ft1 fragment is also a bent rod with an apparently flexible hinge; like the ssd chain and the Fr2 fragment of N-CAM, it does not form aggregates. The similarities between L-CAM and N-CAM, despite their differences in amino acid sequence, suggest that their general configuration and the presence of a flexible hinge are important elements in assuring effective and specific cell-cell adhesion.

Identification of a heparin binding domain of the neural cell adhesion molecule N-CAM using synthetic peptides

The neural cell adhesion molecule (N-CAM) plays an integral role in cell interactions during neural development, with the binding of heparan sulfate proteoglycan to the amino-terminal region of N-CAM being required for N-CAM function. In the present study we have used synthetic peptides (HBD-1 and HBD-2), derived from the primary amino acid sequence of rat N-CAM, to identify the region of N-CAM that binds heparan sulfate. The 28 amino acid HBD-1 synthetic peptide was shown to bind both [3H]heparin and dissociated retinal cells. Retinal cells also attach to a substratum of HBD-2 peptide, but fail to bind to a control peptide containing a scrambled amino acid sequence of HBD-2. The HBD-2 peptide also inhibits retinal cell adhesion to N-CAM, demonstrating the physiological importance of the amino acid sequence encoded by the HBD peptide. These data therefore permit the localization of a heparin binding domain to a 17 amino acid region of immunoglobulin-like loop 2.

Polypeptide variation in an N-CAM extracellular immunoglobulin-like fold is developmentally regulated through alternative splicing

The alternative splicing of a previously undiscovered 30 base exon confers a new level of polypeptide diversity on the N-CAM family of cell-surface glycoproteins. It results in the insertion of 10 amino acids into the fourth of five extracellular immunoglobulin-like folds. Each major size class of rat brain N-CAM mRNAs consists of members that contain or lack the exon. Furthermore, this splicing event is developmentally controlled: RNAs containing the inserted exon are expressed at extremely low levels (less than 3%) in embryonic brain but increase postnatally to 40%-45% of all N-CAM mRNAs in adult brain. Antibodies that recognize the alternative 10 amino acid segment react with a subset of N-CAM-expressing neurons in cultures of embryonic rat cells.

Molecular modelling of the immunoglobulin-like domains of the neural cell adhesion molecule (NCAM): implications for the positioning of functionally important sugar side chains

The neural cell adhesion molecule (NCAM) is thought to mediate cell-cell adhesion by a homophilic mechanism involving binding sites located in the N-terminal region of the protein. This region of the molecule consists of five domains that are homologous to each other and share conserved residues with immunoglobulin domains. We report here secondary structure predictions for the five NCAM domains and three-dimensional models for two of them. The results are entirely consistent with an immunoglobulin-like folding of the NCAM domains into seven strands forming two beta-sheets. NCAM-NCAM binding may thus be analogous to the pairwise associations of immunoglobulin constant domains, which are involved in dimer formation. Insertions and deletions are located mostly in beta-turn regions. Two alpha-helical regions in the third and fourth domain are predicted with high probability. NCAM bears two kinds of functionally important sugar side chains, sialic acid polymers in the fifth domain, which modulate NCAM binding, and the L2 moiety, which is involved in cell adhesion and can be assigned to the third domain. Three-dimensional modelling of the corresponding domains indicates that two of the three sites for N-linked glycosylation in the fifth and the single site in the third domain are located on the face of the domain, which in immunoglobulin constant regions engages in intermolecular interactions.

The reaction-limited kinetics of membrane-to-surface adhesion and detachment

Biological adhesion is frequently mediated by specific membrane proteins (adhesion molecules). Starting with the notion of adhesion molecules, we present a simple model of the physics of membrane-to-surface attachment and detachment. This model consists of coupling the equations for deformation of an elastic membrane with equations for the chemical kinetics of the adhesion molecules. We propose a set of constitutive laws relating bond stress to bond strain and also relating the chemical rate constants of the adhesion molecules to bond strain. We derive an exact formula for the critical tension. We also describe a fast and accurate finite difference algorithm for generating numerical solutions of our model. Using this algorithm, we are able to compute the transient behaviour during the initial phases of adhesion and detachment as well as the steady-state geometry of adhesion and the velocity of the contact. An unexpected consequence of our model is the predicted occurrence of states in which adhesion cannot be reversed by application of tension. Such states occur only if the adhesion molecules have certain constitutive properties (catch-bonds). We discuss the rational for such catch-bonds and their possible biological significance. Finally, by analysis of numerical solutions, we derive an accurate and general expression for the steady-state velocity of attachment and detachment. As applications of the theory, we discuss data on the rolling velocity of granulocytes in post-capillary venules and data on lectin-mediated adhesion of red cells.

Neural protein 1B236/myelin-associated glycoprotein (MAG) defines a subgroup of the immunoglobulin superfamily

We have reviewed the structure and properties of the neural protein 1B236/MAG. This molecule consists largely of five Ig-like domains separated from its carboxyl terminal tail by a single membrane-spanning region. Two forms of the protein differ in the length and sequence of the carboxyl terminus: these are encoded by alternatively spliced mRNAs that are differentially expressed during postnatal neural development. The Ig-like domains of 1B236/MAG are unusual in having structural similarities to Ig V domains but with short Cys-Cys distances characteristic of C domains. Several other Ig-like molecules exhibit this structural feature, including the cell adhesion molecule N-CAM, which is most closely related in sequence to 1B236/MAG. We have proposed 1B236/MAG as the prototype for this subgroup of the Ig family and offer a model for this type of Ig domain structure. 1B236/MAG probably acts as a cell adhesion molecule to mediate interactions between cells in a fashion similar to that proposed for N-CAM. In particular, 1B236/MAG may be involved in interactions between myelinating oligodendrocytes or Schwann cells and axons or between adjacent layers of myelin membrane during the process of myelin compaction. It is most likely that the homophilic or heterophilic interactions of 1B236/MAG occur through binding to the Ig-like domains. The structure of 1B236/MAG is therefore quite consistent with its proposed function and may serve as the model for this class of cell-cell interaction molecules. One would predict, for example, that the neuron-glia cell adhesion molecule Ng-CAM, also known as NILE or L1 (Bock et al. 1985, Friedlander et al. 1985), which mediates interactions between neurons and glial cells, would have a very similar structure to those of N-CAM and 1B236/MAG. In addition, the carboxyl terminal tails of the 1B236/MAG proteins may also be involved in interactions with cytoskeletal components, during membrane vesicle transport through the glial cytoplasm during myelination or through neuronal axoplasm or cytoplasm. The availability of full-length cDNA clones of 1B236/MAG mRNAs with the ability to express these products in vitro will enable the structure and interactions of 1B236/MAG to be tested in detail.