Mapping of three carbohydrate attachment sites in embryonic and adult forms of the neural cell adhesion molecule

Host-biomaterial interactions in mesh complications after pelvic floor reconstructive surgery

Polypropylene (PPL) mesh is widely used in pelvic floor reconstructive surgery for prolapse and stress urinary incontinence. However, some women, particularly those treated using transvaginal PPL mesh placement for prolapse, experience intractable pain and mesh exposure or extrusion. Explanted tissue from patients with complications following transvaginal implantation of mesh is typified by a dense fibrous capsule with an immune cell-rich infiltrate, suggesting that the host immune response has a role in transvaginal PPL mesh complications through the separate contributions of the host (patient), the biological niche within which the material is implanted and biomaterial properties of the mesh. This immune response might be strongly influenced by both the baseline inflammatory status of the patient, surgical technique and experience, and the unique hormonal, immune and microbial tissue niche of the vagina. Mesh porosity, surface area and stiffness also might have an effect on the immune and tissue response to transvaginal mesh placement. Thus, a regulatory pathway is needed for mesh development that recognizes the roles of host and biological factors in driving the immune response to mesh, as well as mandatory mesh registries and the longitudinal surveillance of patients.

Capsular polysaccharide vaccine for Group B Neisseria meningitidis, Escherichia coli K1, and Pasteurella haemolytica A2

We reviewed the literature that is the basis for our proposal that (2→8)-α-Neu5Ac conjugates will be safe and effective vaccines for Group B meningococci (GBMs), Escherichia coli K1, and Pasteurella haemolytica A2. Although (2→8)-α-Neu5Ac is a virulence factor and a protective antigen of these three pathogens, it is also a component of normal tissues (neural cell adhesion molecule). Natural, anti-(2→8)-α-Neu5Ac present in most adults, vaccine-induced antibodies, and even high levels of spontaneously appearing monoclonal anti-(2→8)-α-Neu5Ac did not cause autoimmunity. Although it is not possible to prove a null hypothesis, there are no epidemiologic, serologic, immunologic, or clinical data to indicate that (2→8)-α-Neu5Ac antibodies will induce pathology or an autoimmune disease. No increased pathology caused by these antibodies was found, even in neonates and infants of mothers recovered from GBM meningitis. The lack of pathology mediated by anti-(2→8)-α-Neu5Ac may be explained by different presentations of (2→8)-α-Neu5Ac on bacterial and mammalian cells and by the unusual physicochemical properties of anti-(2→8)-α-Neu5Ac. Based on clinical and experimental data collected over 30 y and because (2→8)-α-Neu5Ac is an essential virulence factor and a protective antigen for GBM, E. coli K1, and P. haemolytica A2, protein conjugates of it are easy to prepare using inexpensive and plentiful ingredients, and they would be compatible with routinely administered infant vaccines, clinical studies of these conjugates should proceed.

A short segment within the cytoplasmic domain of the neural cell adhesion molecule (N-CAM) is essential for N-CAM-induced NF-kappa B activity in astrocytes

The neural cell adhesion molecule (N-CAM) is expressed on the surface of astrocytes, where its homophilic binding leads to the activation of the transcription factor NF-kappaB. Transfection of astrocytes with a construct encompassing the transmembrane region and the cytoplasmic domain of N-CAM (designated Tm-Cyto, amino acids 685-839 in the full-length molecule) inhibited this activation up to 40%, and inhibited N-CAM-induced translocation of NF-kappaB to the nucleus. N-CAM also activated NF-kappaB in astrocytes from N-CAM knockout mice, presumably through binding to a heterophile. This activation, however, was not blocked by Tm-Cyto expression, indicating that the inhibitory effect of the Tm-Cyto construct is specific for cell surface N-CAM. Deletions and point mutations of the cytoplasmic portion of the Tm-Cyto construct indicated that the region between amino acids 780 and 800 were essential for inhibitory activity. This region contains four threonines (788, 793, 794, and 797). Mutation to alanine of T788, T794, or T797, but not T793, abolished inhibitory activity, as did mutation of T788 or T797 to aspartic acid. A Tm-Cyto construct with T794 mutated to aspartic acid retained inhibitory activity but did not itself induce a constitutive NF-kappaB response. This result suggests that phosphorylation of T794 may be necessary but is not the triggering event. Overall, these findings define a short segment of the N-CAM cytoplasmic domain that is critical for N-CAM-induced activation of NF-kappaB and may be important in other N-CAM-mediated signaling.

The N-glycan acceptor specificity of a glucuronyltransferase, GlcAT-P, associated with biosynthesis of the HNK-1 epitope

The acceptor specificity of a rat brain glucuronyltransferase, GlcAT-P, associated with biosynthesis of the HNK-1 epitope on glycoproteins, was investigated using asialoorosomucoid as a model acceptor substrate. Structural analysis of N-linked oligosaccharides, to which glucuronic acid was transferred by GlcAT-P, by means of two-dimensional mapping of pyridylamino-oligosaccharides and MS spectrometry, demonstrated that the enzyme transferred glucuronic acid to bi-, tri-, and tetra-antennary complex type sugar chains, with almost equal efficiency, indicating that the enzyme has no preference as to the number of acceptor sugar branches. Next, we studied the branch specificity of this enzyme by means of the selective branch scission method involving two step exoglycosidase digestion using authentic pyridylamino-oligosaccharides. The GlcAT-P is highly specific for the terminal N-acetyllactosamine structure and no glucuronic acid was incorporated into a Galbeta1-3GlcNAc moiety. The GlcAT-P transferred glucuronic acid to the galactose residues in the N-acetyllactosamine branches of bi-, tri-, and tetra-antennary oligosaccharide chains, with different efficiencies and most preferentially to those in the Galbeta1-4GlcNAcbeta1-4Manalpha1-3 branch.

Palmitoylation of the cytoplasmic domain of the neural cell adhesion molecule N-CAM serves as an anchor to cellular membranes

The neural cell adhesion molecule N-CAM is expressed at key sites during embryonic development and mediates homophilic adhesion between cells both in the embryo and in the adult. N-CAM is expressed in multiple forms and two of the major isoforms differ in their cytoplasmic domains, one (ld form) having an insert of 261 amino acids that is missing in the other (sd form). N-CAM has been previously shown to be palmitoylated, but the sites of acylation have not been localized. We show here that the cytoplasmic domain of the N-CAM became palmitoylated after transfection of a cDNA encoding N-CAM into COS-7 cells, and that this acylation occurs on the four closely spaced cysteines in the cytoplasmic domain of N-CAM. Moreover, when a cDNA encoding only the cytoplasmic domain was transfected into cells, the protein was palmitoylated and associated with membranes even though it lacked a membrane spanning segment. Site directed mutagenesis of the four cysteine residues to serines at positions 5, 11, 16, and 22 in the cytoplasmic domain (723, 729, 734, and 740 in the native protein) eliminated both the palmitoylation and association with the membrane fraction. Mutagenesis of the cysteines individually, in pairs, and in groups of three indicated that C5 is not acylated with either palmitate or oleate, but the other three cysteines are acylated to different extents. Cytoplasmic domains with single cysteine mutations localized primarily in the membrane fraction, while those with three mutations were found primarily in the cytoplasm. Proteins containing two mutated cysteines were found in both the cytoplasm and the membrane fraction with C11 and C16 having the most influence on the distribution in accord with their higher level of acylation. Mutation of the cysteines did not affect the ability of full-length N-CAM to promote aggregation when transfected into COS-7 cells. Based on these results we suggest that the primary role of palmitoylation is to provide a second anchor in the plasma membrane to direct the protein to discrete membrane microdomains or to organize the cytoplasmic region for interaction with factors that affect signaling events resulting from N-CAM mediated adhesion.

Polysialic acid at the cell surface: biophysics in service of cell interactions and tissue plasticity

Polysialic acid (PSA) is a long polymer of negatively-charged sialic acid associated with the neural cell adhesion molecule. PSA serves as a potent negative regulator of cell interactions via its unusual biophysical properties. During development the abundant and regulated expression of this carbohydrate is closely correlated with axon pathfinding and targeting, and with certain aspects of muscle formation. Its level can also be modulated by synaptic activity. PSA expression is more restricted in the neonatal and adult brain, being primarily associated with regions capable of morphological or physiological changes. Studies on the function of PSA studies suggest that its primary role is to promote developmentally-controlled and activity-dependent plasticity in cell interactions and thereby facilitate changes in the structure and function of the nervous system. The presence of PSA on a variety of metastatic tumor lines has also attracted the attention of oncologists, and its late appearance in evolution raises interesting questions about the phylogeny of complex tissue formation.

Developmental expression of two rat sialyltransferases that modify the neural cell adhesion molecule, N-CAM

Polysialylation of the neural cell adhesion molecule (N-CAM) reduces the efficacy of N-CAM-mediated homophilic binding and is regulated both during development and in regions undergoing neurogenesis or remodeling in the adult. Hamster PST-1 (PST) and rat STX are two related sialytransferases that catalyze the polysialylation of N-CAM. We have isolated a cDNA clone for the rat homologue of PST and compared its amino acid and nucleotide sequence to that of rat STX. This analysis revealed regions of high sequence similarity corresponding to the enzymatic domains of the two molecules. Other regions of lower similarity were used to generate specific probes for in situ hybridization. The distribution of PST and STX mRNAs, polysialic acid, and N-CAM were analyzed at three developmental stages. PST and STX mRNAs were expressed abundantly throughout the nervous system at embryonic day 15 and postnatal day 4 and were coexpressed in most tissues examined. In the adult brain, STX expression was reduced relative to PST and expression of both mRNAs was restricted to subsets of cells in areas undergoing constant synaptic rearrangement including hippocampus and olfactory system. The results suggest that both PST and STX participate in the polysialylation of N-CAM in vivo and that their expression levels are dynamically controlled during development and regeneration.

Neural cell adhesion molecules in activity-dependent development and synaptic plasticity

Cell adhesion molecules (CAMs) have a vital role in forming connections between neurons during embryonic development. Increasing evidence suggests that CAMs also participate in activity-dependent plasticity during development and synaptic plasticity in adults. Neural impulses of appropriate patterns can regulate expression of specific CAMs in mouse neurons from dorsal-root ganglia, alter cell-cell adhesion and produce structural reorganization of axon terminals in culture. Synaptic plasticity in Aplysia, learning in chick and long-term potentiation in rat hippocampus are accompanied by changes in CAM expression. Long-term potentiation can be blocked by disrupting CAM function in rat hippocampus, and learning deficits result from antibody blockade of CAMs in chicks and in transgenic mice lacking specific CAMs. Cell adhesion molecules might produce these effects by controlling several cellular processes, including cell adhesion, cytoskeletal structure and intracellular signaling.

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.

Migration of luteinizing hormone-releasing hormone (LHRH) neurons in early human embryos

Luteinizing hormone-releasing hormone (LHRH) neurons originate in the epithelium of the medial olfactory pit and migrate from the nose into the forebrain along nerve fibers rich in neural cell adhesion molecule (N-CAM). The present study examined the ontogenesis of LHRH neurons in early human embryos and found a similar pattern of development of these cells. Luteinizing hormone-releasing hormone immunoreactivity was detected in the epithelium of the medial olfactory pit and in cells associated with the terminal-vomeronasal nerves at 42 (but not 28-32) days of gestation. The migration route of these cells was examined with antibodies to N-CAM and antibodies to polysialic acid (PSA-N-CAM), which is present on N-CAM at certain stages of development. Neural cell adhesion molecule immunoreactivity was present in a population of cells in the olfactory placode of the earliest embryos examined (28-32 days) and later (42 and 46 days) throughout the migration route. The PSA-N-CAM immunoreactivity was not detected until 42 days and was present in a more limited distribution in nerve fibers streaming from the olfactory placode and along the caudal part of the migration route below the forebrain. Previous studies have indicated that the highly sialated form of N-CAM is less adhesive. The PSA-N-CAM may therefore facilitate the migration of these cells by lessening the adhesion between the fascicles that make up the migration route, expediting the passage of cords of LHRH cells between the nerve fibers as these cells move toward the brain.

Protein determinants for specific polysialylation of the neural cell adhesion molecule

Expression of polysialic acid (PSA) involves its specific attachment to the neural cell adhesion molecule (NCAM). Here we identify the amino acid residues within NCAM that are polysialylated and structural domains of the NCAM polypeptide that are required for addition of PSA in cells. Chicken NCAM cDNAs containing amino acid mutations, domain deletions, and domain substitutions were expressed in the F11 rat/mouse hybrid cell line, which can produce polysialylated NCAM. Polysialylation of the chicken NCAM was evaluated by immunopurification and electrophoresis. Mutation of all three potential N-glycosylation sites within the fifth immunoglobulin domain (Ig5) abrogated polysialylation. Analysis of paired mutations revealed that Asn-459 is heavily polysialylated, Asn-430 has a lower level of substitution, and Asn-404 receives little or no PSA. Analysis of domain deletions established that the intracellular domain, Ig domains 1-3, and the COOH-terminal fibronectin-type III (FNIII) repeat are not required for polysialylation, but that deletion of either the adjacent Ig4 or FNIII-type domain prevented addition of PSA. Accordingly, a minimal polypeptide for polysialylation was found to contain Ig domains 4 and 5, the adjacent FNIII repeat, plus a membrane attachment. These results suggest that although all PSA is located within Ig5, regions outside Ig5 also play a role in PSA addition to NCAM. Furthermore, molecular modeling indicates spatial proximity of Asn-430 and Asn-459 and a tight-locking arrangement between Ig4, Ig5, and FNIII#1 that would be consistent with their formation of a spatially discrete enzyme recognition site for polysialylation.

Polysialic acids

1. Polysialic acids are linear homopolymers of N-acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc) and deaminated neuraminic acid (KDN) residues joined by alpha 2,8, alpha 2-9 or alpha 2,8/alpha 2,9 ketosidic linkages. 2. They occur in glycoproteins of embryonic neural membranes (playing a role of neural cell adhesion molecules), in non-neural tissues (postnatal kidney), tumours, (neuroectodermal tumours), fish eggs and in the capsule of certain bacteria such as Neisseria meningitidis group B. 3. These polymers are synthesized through reactions which involve (a) the synthesis of sialic acid; (b) its activation to a cytidine monophosphate sugar nucleotide and (c) the polymerization of the different residues by a polysialyl-transferase complex. 4. Polysialic acids are involved in organogenesis and in cell growth. In several tissues they act as oncodevelopmental antigens, and in bacteria are also virulent determinants.

Expression of the metabotropic glutamate receptor mGluR1 alpha and the ionotropic glutamate receptor GluR1 in the brain during the postnatal development of normal mouse and in the cerebellum from mutant mice

Expression of the metabotropic glutamate receptor type 1 alpha (mGluR1 alpha) and the non-N-methyl-D-aspartate (NMDA) ionotropic glutamate receptor type 1 (GluR1) in mouse brain was investigated using the antibodies raised against the synthetic peptides corresponding to their C-terminal amino acid sequences. Both receptor proteins are glycosylated predominantly in an asparagine-linked manner, and are abundant in post-synaptic membranes. We showed that mGluR1 alpha and GluR1 expression within the first 3 postnatal weeks undergoes dramatic changes in time and space, i.e., in the hippocampus and cerebellum. These spatio-temporal expression patterns appear to be correlated with the postnatal ontogenesis and establishment of the glutamatergic neurotransmission system in the hippocampus and cerebellum, cell migration, dendritic and axonal growth, spine formation, and synaptogenesis. In the adult cerebellum, mGluR1 alpha is intensely expressed in Purkinje neurons and GluR1 in Bergmann glial cells. Both receptors are expressed to a fair degree in weaver mutant cerebellum despite granule cell degeneration. However, the intrinsic expression levels of both mGluR1 alpha and GluR1 are markedly reduced in the cerebellum of the Purkinje cell-deficient and underdeveloped mutant mice, Purkinje-cell-degeneration, Lurcher, and staggerer, suggesting that GluR1 expression in Bergmann glia cells may be correlated with the sustained interaction with adjacent Purkinje neurons.

Transient expression of adheron molecules during chick retinal development

Neuritogenesis and synapse formation are transient phenomena mediated in part by filopodial attachments (Tsui, Lankford, and Klein, Proc. Natl. Acad. Sci. 82:8256-8260 1985). The attachments can be labeled by antisera against adherons, adhesive microparticles isolated from cell culture media (Tsui, Schubert, and Klein, J. Cell Biol. 106:2095-2108 1988). Here, two monoclonal antibodies raised against adherons have been found to recognize transiently expressed membrane antigens of developing avian retina. Early in development, monoclonal antibody (mAb) AD1 stained antigens that spanned the entire tissue. With time, immunoreactivity became restricted to optic fiber, ganglion cell, and inner plexiform layers. Immunoblots of embryonic day (E) 13 retina showed a broad band at 66-72 kD for particulate fractions and a fine band at 70 kD for soluble fractions. The particulate forms disappeared as retinas matured, but the soluble form did not. mAb AD2 initially labeled retina antigens of optic fiber, ganglion cell, and inner plexiform layers (IPL). Labeling in the plexiform layer showed discrete lamina. Immunoreactivity first appeared at E9, peaked at E15, and then disappeared shortly after hatching. In isolated cells, AD2 labeled small cell surface aggregates. Cytoarchitectural studies, using whole-mount transmission electron microscopy, showed AD2 antigen in cell surface microfilaments, including some that joined filopodia together. The adheron antigens recognized by mAbs AD1 and AD2 thus were (1) topographically restricted; (2) associated with cell surfaces; and (3) developmentally down-regulated. This pattern suggests a role in developmentally transient cell surface phenomena, such as neurite extension or junction biogenesis.

Different extracellular domains of the neural cell adhesion molecule (N-CAM) are involved in different functions

The neural cell adhesion molecule (N-CAM) engages in diverse functional roles in neural cell interactions. Its extracellular part consists of five Ig-like domains and two fibronectin type III homologous (type III) repeats. To investigate the functional properties of the different structural domains of the molecule in cell interactions and signal transduction to the cell interior, we have synthesized, in a bacterial expression system, the individual domains and tandem sets of individual domains as protein fragments. These protein fragments were tested for their capacity to influence adhesion and spreading of neuronal cell bodies, promote neurite outgrowth, and influence cellular migration patterns from cerebellar microexplants in vitro. Ig-like domains I and II and the combined type III repeats I-II were most efficient for adhesion of neuronal cell bodies, when coated as substrates. Neurite outgrowth was best on the substrate-coated combined type III repeats I-II, followed by the combined Ig-like domains I-V and Ig-like domain I. Spreading of neuronal cell bodies was best on substrate-coated combined type III repeats I-II, followed by Ig-like domain I and the combined Ig-like domains I-V. The cellular migration pattern from cerebellar microexplant cultures plated on a mixture of laminin and poly-L-lysine was modified by Ig-like domains I, III, and IV, while Ig-like domains II and V and the combined type III repeats I-II did not show significant modifications, when added as soluble fragments. Outgrowth of astrocytic processes from the explant core was influenced only by Ig-like domain I. Metabolism of inositol phosphates was strongly increased by Ig-like domain I and less by the Ig-like domains II, III, IV, and V, and not influenced by the combined type III repeats I-II. Intracellular concentrations of Ca2+ and pH values were increased only by the Ig-like domains I and II. Intracellular levels of cAMP and GMP were not influenced by any protein fragment. These experiments indicate that different domains of N-CAM subserve different functional roles in cell recognition and signal transduction, and are functionally competent without nervous system-derived carbohydrate structures.

Structural variants of the neural cell adhesion molecule (N-CAM) in developing feathers

The neural cell adhesion molecule (N-CAM) is expressed in a specific spatiotemporal pattern during feather development, suggesting that adhesion mediated by this molecule is involved in feather morphogenesis. To begin to investigate N-CAM's function in developing feathers, we determined what forms of N-CAM polypeptide are present and the distribution of polysialic acid (PSA), a carbohydrate moiety that decreases N-CAM-mediated cellular adhesion. N-CAM in skin appears as a Mr 145-kDa polypeptide compared to the 140-kDa brain N-CAM polypeptide, and is encoded by a 6.4-kb mRNA, compared to the 6.1-kb mRNA in brain. Polymerase chain reaction analysis of the exon splicing pattern of skin N-CAM shows that the 6.4-kb mRNA band represents two transcripts, with and without a 93-bp insert between exons 12 and 13. Thus, two N-CAM polypeptides are expressed in skin, but the 93-bp insert does not account for the larger size of the skin mRNAs and polypeptides. We show that the size difference of the polypeptides is instead due to N-linked oligosaccharides attached to the skin N-CAM proteins. The larger size of the skin mRNAs may be due to use of a different transcriptional start site. Staining of skin sections and wholemounts confirms previous descriptions of N-CAM in developing feathers, but reveals that N-CAM is also present at low levels on epidermal cells as early as stage 29 (E6). We find that PSA is expressed only on a subset of the cells that express N-CAM, in particular on dermal cells in the feather rudiments from stage 35-36 (E9-10) and on smooth muscle cells at the base of the filaments from stage 37 (E11) until the latest stage examined (stage 44, E18). The known effects on cell-cell adhesion of amount of N-CAM and PSA suggest that the variations we observe in skin may regulate cell-cell interactions that are important in feather development.

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.

Cell adhesion molecules and the regulation of development

Cell adhesion molecules, in conjunction with the other morphoregulatory molecules, substrate adhesion molecules and cell junctional molecules, are dynamically expressed in coordinate patterns throughout development. Their activities are linked to a variety of cellular processes, and their ability to influence mechanochemical processes allows them to influence a variety of other fundamental developmental events. The clinical significance of these molecules remains to be determined, but they are clearly involved in a number of pathologic conditions and could become the focus of a wide range of diagnostic techniques and eventually even therapeutic designs.

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.

An immunoblot assay for the simultaneous quantification of several antigens

A radioimmunologic assay method that allows for the simultaneous quantification of several antigens in one sample is described. Polypeptide antigens are resolved electrophoretically and electroblotted to nitrocellulose. The nitrocellulose is then reacted with a mixture of several antisera simultaneously, and antibody-binding proteins are visualized by incubation with 125I-protein A and by autoradiography. Antigens are identified according to their molecular weights and quantified by counting the bound radioactivity. The sensitivity of the assay is in the low nanogram range and can be adjusted individually for each antigen by appropriately diluting the first antiserum. The procedure is presently applied to the detection of three neural antigens, neural cell adhesion molecule, neuron-specific enolase, and synaptophysin, in adult brain tissue and to the assessment of expression of the latter two during development of brain cells in primary culture. The method is fast, comparatively cheap, and associated with a low radiation exposure. It should prove especially useful when only scarce amounts of sample are available.

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.

Purification and characterization of P400 protein, a glycoprotein characteristic of Purkinje cell, from mouse cerebellum

P400 protein is a concanavalin A (Con A)-binding, 250-kilodalton glycoprotein characteristic of cerebellum. Extraction conditions for P400 protein were investigated, and complete solubilization of P400 protein from a submicrosomal fraction (P31 fraction) of mouse cerebellum was attained by the combination of 4% Zwittergent 3-14 and 4 M guanidinium chloride. The solubilized P400 protein was purified using Sepharose CL-4B and Con A-Sepharose chromatography. A monoclonal antibody (18A10) was prepared against P400 protein. Endo-beta-N-acetylglucosaminidase F digestion of P400 protein revealed that P400 protein has a small number of asparagine-linked oligosaccharide chains and that the epitope that is recognized by 18A10 monoclonal antibody is not on the asparagine-linked oligosaccharide portion. Tissue distribution of P400 protein was investigated by immunoblot analysis using 18A10 monoclonal antibody. P400 protein was abundant in the cerebellum, but a very small amount of P400 protein or related antigen was also detected in other parts of the nervous system and in nonneural tissues. Immunohistochemical studies indicated that P400 protein was distributed abundantly in the soma, the dendritic arborization, and the axon of the Purkinje cell. No immunoreaction was observed in the other types of 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.

Differentiation-dependent sialylation of individual neural cell adhesion molecule polypeptides during postnatal development

The postnatal sialylation of individual neural cell adhesion molecule (N-CAM) polypeptides by a developmentally regulated sialyltransferase in Golgi-enriched fractions isolated from rat brain is described. The 120-kilodalton polypeptide of N-CAM was found to be sialylated at each developmental age examined. This was in contrast to the 140- and 180-kilodalton N-CAM polypeptides which were only sialylated until postnatal day 10 and from postnatal day 12, respectively. Immunoblotting procedures demonstrated that all N-CAM polypeptides were expressed in the Golgi fractions at each developmental stage examined. The heavily sialylated "embryonic" form of N-CAM was found to be reexpressed at postnatal days 10 and 12, a time coincident with extensive fibre outgrowth. The "embryonic" form of N-CAM incorporated similar amounts of [14C]sialic acid into its constituent polypeptides reflecting the difference in sialic acid to protein ratio, as this form of N-CAM was virtually undetectable in the immunoblots of postnatal material.

Biosynthesis, membrane association, and release of N-CAM-120, a phosphatidylinositol-linked form of the neural cell adhesion molecule

The neural cell adhesion molecule (N-CAM) of rodents comprises three distinct proteins of Mr 180,000, 140,000, and 120,000 (designated N-CAM-180, -140, and -120). They are expressed in different proportions by different tissues and cell types. but the individual contribution of each form to cell adhesion is presently unknown. Previous studies have shown that the two N-CAM species of higher relative molecular mass span the membrane whereas N-CAM-120 lacks a transmembrane domain and can be released from the cell surface by phosphatidylinositol-specific phospholipase C. In this report, we provided evidence that N-CAM-120 contained covalently bound phosphatidylinositol and studied N-CAM-120 from its biosynthesis to its membrane insertion and finally to its release from the cell surface. Evidence was presented showing that the lipid tail of N-CAM-120 contained ethanolamine as is the case for other lipid-linked molecules. The phospholipid anchor was attached to the protein during the first minutes after completion of the polypeptide chain. This process took place in the endoplasmic reticulum as judged from endoglycosidase H digestion experiments. Immediately after a 2-min pulse with [35S]methionine, we detected also a short-lived precursor that had not yet acquired the lipid tail. Pulse-chase studies established that N-CAM-120 was transported to the cell surface from which it was slowly released into the extracellular milieu. The molecules recovered in the incubation medium appeared to have lost all of their bound fatty acid but only around half of the ethanolamine. Upon fractionation of brain tissue, approximately 75% of N-CAM-120 was recovered with a membrane fraction and approximately 25% in a membrane-free supernatant. A small proportion (approximately 6%) was found to be resistant to extraction by non-ionic detergent. A major posttranslational modification of N-CAM is polysialylation. Our results showed that also N-CAM-120 was polysialylated in the young postnatal brain and released in this form from cultured cerebellar cells. The presence of N-CAM in a form that can be released from the cell surface and accumulates in the extracellular fluid suggests a novel mechanism by which N-CAM-mediated adhesion may be modulated.

Identification of a cDNA clone that contains the complete coding sequence for a 140-kD rat NCAM polypeptide

Neural cell adhesion molecules (NCAMs) are cell surface glycoproteins that appear to mediate cell-cell adhesion. In vertebrates NCAMs exist in at least three different polypeptide forms of apparent molecular masses 180, 140, and 120 kD. The 180- and 140-kD forms span the plasma membrane whereas the 120-kD form lacks a transmembrane region. In this study, we report the isolation of NCAM clones from an adult rat brain cDNA library. Sequence analysis indicated that the longest isolate, pR18, contains a 2,574 nucleotide open reading frame flanked by 208 bases of 5' and 409 bases of 3' untranslated sequence. The predicted polypeptide encoded by clone pR18 contains a single membrane-spanning region and a small cytoplasmic domain (120 amino acids), suggesting that it codes for a full-length 140-kD NCAM form. In Northern analysis, probes derived from 5' sequences of pR18, which presumably code for extracellular portions of the molecule hybridized to five discrete mRNA size classes (7.4, 6.7, 5.2, 4.3, and 2.9 kb) in adult rat brain but not to liver or muscle RNA. However, the 5.2- and 2.9-kb mRNA size classes did not hybridize to either a large restriction fragment or three oligonucleotides derived from the putative transmembrane coding region and regions that lie 3' to it. The 3' probes did hybridize to the 7.4-, 6.7-, and 4.3-kb message size classes. These combined results indicate that clone pR18 is derived from either the 7.4-, 6.7-, or 4.3-kb adult rat brain RNA size class. Comparison with chicken and mouse NCAM cDNA sequences suggests that pR18 represents the amino acid coding region of the 6.7- or 4.3-kb mRNA. The isolation of pR18, the first cDNA that contains the complete coding sequence of an NCAM polypeptide, unambiguously demonstrates the predicted linear amino acid sequence of this probable rat 140-kD polypeptide. This cDNA also contains a 30-base pair segment not found in NCAM cDNAs isolated from other species. The significance of this segment and other structural features of the 140-kD form of NCAM can now be studied.

Characterization of neural cell adhesion molecules (NCAM) expressed by Ewing and neuroblastoma cell lines

The status of the neural cell adhesion molecule NCAM gene which is mapped to human chromosome 11q23-24 has been investigated in Ewing-tumor-derived cell lines which present the t(11;22)(q23-24;q12) translocation characteristic of this malignancy. No rearrangement was detected when 2 different non-overlapping probes to mouse NCAM were used. The expression of the NCAM gene was analysed at both the protein and messenger levels in material extracted from Ewing cell lines, human neuroblastoma cell line and fetal mouse brain. Immune blot and immunoprecipitation studies showed that the neuroblastoma cell line contained more NCAM material than the Ewing lines. In neuroblastoma but not in Ewing, the NCAM material had the electrophoretic characteristics of molecules with long polysialic acid chains. After treatment with endosialidase, the diffusely migrating neuroblastoma material was resolved into 3 discrete bands of 120, 140 and 180 kDa. In Ewing extract, high-molecular-weight NCAM species were also detected with a 3-band pattern more reminiscent of mature brain. Endoglycosidase F treatment of Ewing NCAM indicated that all 3 species were largely N-glycosylated. Northern blot analysis confirmed that NCAM was expressed more abundantly in neuroblastoma than in Ewing cell lines. Among the 4 NCAM messengers (7.0, 6.5, 4.3 and 4.1 kb) detected in the neuroblastoma, the 6.5 kb species was largely predominant. The Ewing messenger RNA pattern was clearly different as the largest 7.0-kb species was virtually absent and the other bands were of similar intensities.

Visualization of neural cell adhesion molecule by electron microscopy

The 130- and 160-kD polypeptide forms of the neural cell adhesion molecule (NCAM) were analyzed by electron microscopy after low angle rotary shadowing and freeze replication. Individual NCAM molecules appeared as uniformly thick rods, with a distinct bend or hinge region near their middle. Aggregates were also present, containing two to six rods in a pinwheel-like configuration without measurable overlap between rods. The 130- and 160-kD NCAM forms had lengths of 38 and 51 nm, respectively, with a difference in arm length distal to the bend, but not toward the center of the pinwheel. Although enzymatic removal of the polysialic acid moiety on NCAM did not alter the appearance of individual molecules, it did increase the average number of arms per aggregate. Monoclonal antibodies that recognize defined regions of the NCAM polypeptide were used to provide landmarks on the observed molecular figures. Two antibodies specific for cytoplasmic epitopes near the COOH terminus were clustered at the distal tip of aggregated arms. Two other antibodies that react with epitopes near the NH2 terminus and the middle of the molecule bound to sites more centrally located on the pinwheel structure. Together, these results suggest that the observed aggregates represent an association of molecules near their NH2-terminal homophilic binding site, and have led to several predictions about the nature of an NCAM-mediated cell-cell bond.

Neural cell adhesion molecule: structure, immunoglobulin-like domains, cell surface modulation, and alternative RNA splicing

The neural cell adhesion molecule, N-CAM, appears on early embryonic cells and is important in the formation of cell collectives and their boundaries at sites of morphogenesis. Later in development it is found on various differentiated tissues and is a major CAM mediating adhesion among neurons and between neurons and muscle. To provide a molecular basis for understanding N-CAM function, the complete amino acid sequences of the three major polypeptides of N-CAM and most of the noncoding sequences of their messenger RNA's were determined from the analysis of complementary DNA clones and were verified by amino acid sequences of selected CNBr fragments and proteolytic fragments. The extracellular region of each N-CAM polypeptide includes five contiguous segments that are homologous in sequence to each other and to members of the immunoglobulin superfamily, suggesting that interactions among immunoglobulin-like domains form the basis for N-CAM homophilic binding. Although different in their membrane-associated and cytoplasmic domains, the amino acid sequences of the three polypeptides appear to be identical throughout this extracellular region (682 amino acids) where the binding site is located. Variations in N-CAM activity thus do not occur by changes in the amino acid sequence that alter the specificity of binding. Instead, regulation is achieved by cell surface modulation events that alter N-CAM affinity, prevalence, mobility, and distribution on the surface. A major mechanism for modulation is alternative RNA splicing resulting in N-CAM's with different cytoplasmic domains that differentially interact with the cell membrane. Such regulatory mechanisms may link N-CAM binding function with other primary cellular processes during the embryonic development of pattern.

Developmental change in the amount of polysialosyl glycopeptides isolated from the rat brain

Polysialosyl glycopeptides were coisolated with glycosaminoglycans by Pronase digestion of the whole brains of perinatal rats and could be separated from known glycosaminoglycans by two-dimensional electrophoresis on cellulose acetate film. The polysialosyl glycopeptides could not be obtained from fetal rat brain on day 13 of gestation, but began to be detected on day 14. The amount of polysialosyl glycopeptides was estimated from the dye concentration of the Alcian blue-stained spot in the electrophoretogram. The glycopeptide content increased almost linearly, on the basis of brain DNA, up to 10 days after birth. Thereafter, the content decreased rapidly, and hardly any polysialosyl glycopeptides could be isolated from the brain at approximately 30 days. This developmental change may be involved in morphogenesis and maturation of the brain. The polysialosyl glycopeptides could be isolated from the cerebellum, from the cerebrum, or from the brainstem of the neonatal rat. However, each region of the brain had a postnatal developmental change in glycopeptide content different from those of the other regions.

Isolation and nucleotide sequence of mouse NCAM cDNA that codes for a Mr 79,000 polypeptide without a membrane-spanning region

The neural cell adhesion molecule (NCAM) exists in several isoforms which are selectively expressed by different cell types and at different stages of development. In the mouse, three proteins with apparent Mr's of 180,000, 140,000 and 120,000 have been distinguished that are encoded by 4-5 different mRNAs. Here we report the full amino acid sequence of a NCAM protein inferred from the sequences of overlapping cDNA clones. The 706-residue polypeptide contains, towards its N-terminus, 5 domains that share structural homology with members of the immunoglobulin supergene family. The sequence does not encode a typical membrane-spanning segment, but ends with 24 uncharged amino acids followed by two stop codons. This fact, together with size considerations, make it highly likely that our sequence represents NCAM-120, which lacks transmembrane or cytoplasmic domains and is attached to the membrane by phospholipid. Probes from the 5' region detect all four NCAM gene transcripts present in mouse brain consistent with the notion that the extracellular domains are common to most NCAM forms. However, a 3' probe corresponding to the hydrophobic tail and non-coding region hybridizes specifically with the smallest mRNA species. S1 nuclease protection experiments indicate that this region is encoded by exon(s) spliced out from the other mRNAs. Furthermore, our clones that are highly homologous to a published chicken NCAM sequence which codes for putative transmembrane and cytoplasmic domains elsewhere, diverge from it at the presumptive splice junction. It appears thus that alternate use of exons determines whether NCAM proteins with membrane-spanning domains are synthesized.(ABSTRACT TRUNCATED AT 250 WORDS)

cDNA clones of the neural cell adhesion molecule (N-CAM) lacking a membrane-spanning region consistent with evidence for membrane attachment via a phosphatidylinositol intermediate

In embryonic chicken brains, the neural cell adhesion molecule N-CAM is expressed mainly as two polypeptides, the large intracellular-domain polypeptide (ld) (Mr = 160,000) and the small intracellular-domain polypeptide (sd) (Mr = 130,000) chains, that differ in their cytoplasmic domains and that arise by alternative splicing of RNA transcribed from a single gene. There is evidence for a minor N-CAM polypeptide of Mr = 120,000 that is similar to the ld and sd chains for most of its amino-terminal sequence, but which lacks a cytoplasmic domain. We report here the isolation and characterization of a cDNA clone, lambda N151, that appears to encode this third N-CAM polypeptide, which we designate the ssd (small surface-domain) polypeptide chain. The cDNA insert of lambda N151 consists of 2437 base pairs (bp). DNA hybridization and sequencing indicate that the first 1721 bp are nearly identical to the corresponding sequences of clone lambda N208, which encodes the ld chain. Following in the same reading frame, lambda N151 encodes 25 amino acids not present in lambda N208. The rest of lambda N151 consists of a 637-bp noncoding region containing an AATACA polyadenylylation sequence and a 55-bp poly(A) tract. Messenger RNAs complementary to lambda N151 appear later in development than those complementary to the ld and sd chains, and their appearance is correlated with the appearance of the ssd polypeptide. Although the polypeptide encoded by lambda N151 lacks a membrane region that would define a cytoplasmic domain, it does contain at its carboxyl end a relatively hydrophobic stretch of amino acids similar to those seen in precursors of membrane proteins that are attached to membranes via the lipid phosphatidylinositol. We show here that the ssd chain of chicken N-CAM can be released from brain vesicles by treatment with phospholipase C, suggesting that it too may have a phosphatidylinositol anchor. These results define two additional modes by which N-CAM expression can be modulated: by RNA splicing at a new site and by differential membrane attachment of the resulting polypeptide through a lipid intermediate.

Topography of N-CAM structural and functional determinants. I. Classification of monoclonal antibody epitopes

12 distinct neural cell adhesion molecule (N-CAM) epitopes, each recognized by a different monoclonal antibody (mAb), have been characterized in terms of the major structural and functional features of the molecule. Seven antibodies, each recognizing the amino-terminal region of the molecule, altered the rate of N-CAM-mediated adhesion. Four of these were inhibitors, two of which also recognized a heparin-binding N-CAM fragment. The other three antibodies specifically enhanced the rate of N-CAM-mediated adhesion. Three epitopes, one polypeptide- and two carbohydrate-dependent, were associated with the sialic acid-rich central portion of the molecule. The remaining two antibodies were found to react with intracellular determinants, and are specific for the largest of the three major N-CAM polypeptide forms. Studies on the ability of one antibody to hinder recognition of native N-CAM by another antibody suggested that the epitopes associated with N-CAM binding functions are in close proximity compared with the other determinants. The classification of these mAb epitopes has allowed the topographical placement of key N-CAM features, as described in the following paper, and provides valuable probes for analysis of both the structure and function of N-CAM.

Topography of N-CAM structural and functional determinants. II. Placement of monoclonal antibody epitopes

The accompanying report (Watanabe, M., A. L. Frelinger III, and U. Rutishauser, 1986, J. Cell Biol., 103:1721-1727) describes a set of monoclonal antibodies (mAbs) directed against N-CAM epitopes representing the known major structural and functional domains of the molecule. In this study, we have generated and separated a variety of peptide fragments from N-CAM, and then used their size and reactivity with each antibody to position the antigenic sites along the peptide chain. This epitope map, together with the biological properties of the antibodies and previous studies on N-CAM, have been used to construct a topographical model for the molecule in the cell membrane.

Differential contributions of Ng-CAM and N-CAM to cell adhesion in different neural regions

Individual neurons can express both the neural cell adhesion molecule (N-CAM) and the neuron-glia cell adhesion molecule (Ng-CAM) at their cell surfaces. To determine how the functions of the two molecules may be differentially controlled, we have used specific antibodies to each cell adhesion molecule (CAM) to perturb its function, first in brain membrane vesicle aggregation and then in tissue culture assays testing the fasciculation of neurite outgrowths from cultured dorsal root ganglia, the migration of granule cells in cerebellar explants, and the formation of histological layers in the developing retina. Our strategy was initially to delineate further the binding mechanisms for each CAM. Antibodies to Ng-CAM and N-CAM each inhibited brain membrane vesicle aggregation but the binding mechanisms of the two CAMs differed. As expected from the known homophilic binding mechanism of N-CAM, anti-N-CAM-coated vesicles did not co-aggregate with uncoated vesicles. Anti-Ng-CAM-coated vesicles readily co-aggregated with uncoated vesicles in accord with a postulated heterophilic binding mechanism. It was also shown that N-CAM was not a ligand for Ng-CAM. In contrast to assays with brain membrane vesicles, cellular systems can reveal functional differences for each CAM reflecting its relative amount (prevalence modulation) and location (polarity modulation). Consistent with this, each of the three cellular processes examined in vitro was preferentially inhibited only by anti-N-CAM or by anti-Ng-CAM antibodies. Both neurite fasciculation and the migration of cerebellar granule cells were preferentially inhibited by anti-Ng-CAM antibodies. Anti-N-CAM antibodies inhibited the formation of histological layers in the retina. The data on perturbation by antibodies were correlated with the relative levels of expression of Ng-CAM and N-CAM in each of these different neural regions. Quantitative immunoblotting experiments indicated that the relative Ng-CAM/N-CAM ratios in comparable extracts of brain, dorsal root ganglia, and retina were respectively 0.32, 0.81, and 0.04. During culture of dorsal root ganglia in the presence of nerve growth factor, the Ng-CAM/N-CAM ratio rose to 4.95 in neurite outgrowths and 1.99 in the ganglion proper, reflecting both polarity and prevalence modulation. These results suggest that the relative ability of anti-Ng-CAM and anti-N-CAM antibodies to inhibit cell-cell interactions in different neural tissues is strongly correlated with the local Ng-CAM/N-CAM ratio.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell agglutination by a novel cell surface sialoglycopeptide inhibitor and the relationship between its protease and biological activities

A bovine sialoglycopeptide, purified to homogeneity and capable of inhibiting cellular protein synthesis and proliferation, was shown to agglutinate a wide variety of nontransformed and transformed cells. The cell agglutination activity was shown to be independent of the biological inhibitory action and most likely related to a protease activity that could not be physically separated during purification of the sialoglycopeptide. Samples that were completely biologically inactivated retained full protease activity and their ability to agglutinate target cells. Balb/c 3T3 cells were not agglutinated by the sialoglycopeptide and they elicited a protein that interfered with the agglutination reaction and even redispursed cells that already had been aggregated by the inhibitor.

Sequence of a cDNA clone encoding the polysialic acid-rich and cytoplasmic domains of the neural cell adhesion molecule N-CAM

Purified fractions of the neural cell-adhesion molecule N-CAM from embryonic chicken brain contain two similar polypeptides (Mr, 160,000 and 130,000), each containing an amino-terminal external binding region, a carbohydrate-rich central region, and a carboxyl-terminal region that is associated with the cell. Previous studies indicate that the two polypeptides arise by alternative splicing of mRNAs transcribed from a single gene. We report here the 3556-nucleotide sequence of a cDNA clone (pEC208) that encodes 964 amino acids from the carbohydrate and cell-associated domains of the larger N-CAM polypeptide followed by 664 nucleotides of 3' untranslated sequence. The predicted protein sequence contains attachment sites for polysialic acid-containing oligosaccharides, four tandem homologous regions of polypeptide resembling those seen in the immunoglobulin superfamily, and a single hydrophobic sequence that appears to be the membrane-spanning segment. The cytoplasmic domain carboxyl terminal to this segment includes a block of approximately equal to 250 amino acids present in the larger but not in the smaller N-CAM polypeptide. We designate these the ld (large domain) polypeptide and the sd (small domain) polypeptide. The intracellular domains of the ld and sd polypeptides are likely to be critical for cell-surface modulation of N-CAM by interacting in a differential fashion with other intrinsic proteins or with the cytoskeleton.

Specific alteration of NCAM-mediated cell adhesion by an endoneuraminidase

A phage endoneuraminidase that specifically cleaves alpha-2, 8-linked polysialic acid has been found to be a useful probe for examining the biological role of this sugar moiety on the neural cell adhesion molecule (NCAM). The enzyme caused a 3.3-fold increase in the rate of NCAM-dependent aggregation of membrane vesicles from chicken embryonic brain, without the nonspecific effects previously encountered with the use of exoneuraminidases. The enhancement of aggregation was closely correlated with removal of sialic acid as assessed by electrophoretic mobility. Extension of this analysis to cultures of spinal ganglia indicated that removal of sialic acid by the endoneuraminidase results in an increase in the thickness of neurite bundles. This enhancement of fasciculation was reversed by addition of anti-NCAM Fab, suggesting that the enzyme treatment was not toxic and did not produce nonspecific effects on adhesion. Injection of the enzyme into the eyes of 3.5-d chicken embryos consistently produced a striking array of abnormalities in those parts of the neural retina that contained the highest concentrations of NCAM at the time of injection. These perturbations included a dramatic thickening of the neural epithelium in the posterior eye, a failure of cells in this region to elongate radially, formation of an ectopic optic fiber layer, and an incomplete association of the presumptive pigmented epithelium with the neural retina. These results provide the first direct evidence that the polysialic acid on NCAM has a regulatory effect on adhesion between living cells, and that the amount of this carbohydrate is critical for the normal morphogenesis of nerve tissue.