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

Characterization of NCAM diversity in cultured neurons

A single transcript of the NCAM gene undergoes differential processing resulting in a multiplicity of mRNAs and their translation products. In this study, the diversity of NCAM in rat primary neuronal cultures was investigated utilizing immuno- and Northern blot analyses. NCAM polypeptides of 190 kDa (NCAM-A) and 135 kDa (NCAM-B) were shown to be associated with the neuronal phenotype. These data were confirmed by Northern blotting, which in both neocortical neurons and cerebellar granule neurons revealed mRNA classes of 7.4 kb and 6.7 kb encoding for NCAM-A and -B, respectively. However, oligonucleotide probes, specific for selected exons or exon combinations, revealed special features of cerebellar granule neurons as compared to neocortical neurons: expression of 4.3 kb NCAM mRNA, a relatively low amount of VASE-containing variants, and an apparent lack of mRNA species containing exons alpha and an AAG insert between exons 12 and 13. Distinct patterns of NCAM mRNA may putatively be related to the regional origin and functional specificity of the investigated neurons.

Axon growth is enhanced by NCAM lacking the VASE exon when expressed in either the growth substrate or the growing axon

The neural cell adhesion molecule NCAM exists as several related peptides formed by alternative splicing of the single NCAM gene. Here the ability of NCAM containing and lacking the alternatively spliced VASE exon to act as a permissive growth substrate was tested by examining retinal axon outgrowth on normal L cell fibroblasts and L cells expressing stably transfected 140 kD NCAM +/- VASE. L cells expressing either NCAM form were a more permissive substrate than control L cells. At higher substrate cell densities, greater axon outgrowth occurred on substrate cells expressing NCAM - VASE than on those expressing NCAM + VASE. Similar experiments tested retinal axon growth on neuronal substrates by utilizing clonal B35 cells, C3 cells that are NCAM lacking variants of B35, and C3 cells into which 140 kD NCAM +/- VASE has been restored by transfection. Axon growth on C3 cells transfected with NCAM - VASE was greater than that on all other substrates including cells transfected with NCAM + VASE. In these experiments C3 cells and transfected C3 expressing NCAM + VASE cell promoted similar outgrowth. The influence on neurite growth of the NCAM isoform of the neurite itself was tested by examining neurite formation using combinations of C3 cells and C3 NCAM transfectants both in the growth monolayer and as responding cells. C3 cells were able to extend neurites, indicating NCAM is not required for neurite growth. However, C3 derivatives transfected with NCAM +/- VASE had greater neurite outgrowth. The most extensive neurite growth was found when NCAM - VASE was expressed by both substrate cells and the responding neurite growing cells. Thus NCAM enhances axon or neurite outgrowth when present either in the growth substrate or on the growing axon. NCAM - VASE has a significantly greater growth promoting capability than NCAM + VASE. The expression of NCAM + VASE by more mature neural cells could thus be a significant factor in the reduced axonation capabilities of mature neurons.

NILE/L1 and NCAM-polysialic acid expression on growing axons of isolated neurons

The neuron adhesion molecules NILE/L1 and NCAM may be involved in axonal guidance and cell recognition. To investigate all exposed membrane domains of single neurons, something which has not previously been done for any adhesion molecule, we used digitally processed scanning electron microscopy with a high-energy backscatter electron detector. This allowed a quantitative analysis of immunogold staining densities on all surfaces of isolated rat hippocampal neurons in culture to study NILE/L1 and NCAM expression independent of potentially inductive innervation. During early stages of neuritic extension, all growth cones showed similar NILE/L1 expression, but as soon as a single process extended farther than the others (by 20 hours), this putative axon and its growth cone generally showed a stronger level of NILE/L1 immunogold labeling than the other neurites. This is the earliest evidence of plasma membrane differentiation between axons and dendrites. With further neuritic growth, the relative NILE/L1 expression on axons and their growth cones continued to increase. In contrast to some earlier reports, NILE/L1 was expressed on axonal growth cones growing on both polylysine-coated glass and astrocyte substrates. Strong immunostaining for NCAM-related polysialic acid (PSA) was found on axonal growth cones and filopodia, suggesting that the homophilic adhesive action of NCAM may be reduced during axonal growth. PSA showed greater labeling on distal axons than on other areas of the neuron, indicating a variable NCAM-mediated adhesion on different regions of the same cell. Neither NILE/L1, NCAM, nor PSA appeared to show regional differences in axons fasciculating or defasciculating on themselves. A strong intercellular heterogeneity of NILE/L1, NCAM, and PSA expression levels on neurons in the same culture dish was found, suggesting that subsets of cells from the hippocampus may express biologically relevant differences in adhesion molecules compared to neighboring neurons. In light of the growing body of evidence pointing to the multifaceted array of homophilic and heterophilic binding interactions that NILE/L1 and NCAM may exhibit, and the functional importance of molecular densities, the quantitative data here support the hypothesis that sufficient cellular and subcellular heterogeneity exists for these molecules to be involved in some aspects of axonal guidance.

Two neural-cell adhesion molecule (NCAM)-encoding genes in Xenopus laevis are expressed during development and in adult tissues

Expression of genes encoding neural-cell adhesion molecules (NCAM) is regulated to a large extent at the level of alternative splicing. Xenopus laevis contains two functional copies of the NCAM gene and comparison of the amino acid sequences of NCAM proteins derived from both genes shows that they differ at approximately 8% of positions. Differential expression of these proteins during development or in adult tissues could provide an additional level of regulation above that obtained by alternative splicing. PCR analysis however, shows that both NCAM genes are expressed at similar levels throughout embryonic development and in all adult tissues examined.

The upregulation of a glial cell surface antigen at the astrocytic scar in the rat

The upregulation of a 106-kDa glial protein was examined using indirect immunohistochemical methods. The protein is recognized by the monoclonal antibody AMP1. Previous studies demonstrated that the AMP1 antigen is found on the external surface of cultured astrocytes and is involved in stabilizing adhesive interactions between these cells. In sections of injured adult rat brain, the spatial distribution of the AMP1 antigen correlated with the region of reactive gliosis. The relative intensity of immunofluorescence indicates that the AMP1 antigen is dramatically upregulated at the gliotic scar. The correlation between AMP1 antigen upregulation and reactive gliosis suggests that this molecule is critical to the process of CNS scar formation.

Age-related changes in expression of the neural cell adhesion molecule in skeletal muscle: a comparative study of newborn, adult and aged rats

Neural cell adhesion molecule (NCAM) is expressed by muscle and involved in muscle-neuron and muscle-muscle cell interactions. The expression in muscle is regulated during myogenesis and by the state of innervation. In aged muscle, both neurogenic and myogenic degenerative processes occur. We here report quantitative and qualitative changes in NCAM protein and mRNA forms during aging in normal rat skeletal muscle. Determination of the amount of NCAM by e.l.i.s.a. showed that the level decreased from perinatal to adult age, followed by a considerable increase in 24-month-old rat muscle. Thus NCAM concentration in aged muscle was sixfold higher than in young adult muscle. In contrast with previous reports, NCAM polypeptides of 200, 145, 125 and 120 kDa were observed by immunoblotting throughout postnatal development and aging, the relative proportions of the individual NCAM polypeptides remaining virtually unchanged at all ages examined. However, changes in the extent of sialylation of NCAM were demonstrated. Even though the relative amounts of the various NCAM polypeptides were unchanged during aging, distinct changes in NCAM mRNA classes were observed. Three NCAM mRNA classes of 6.7, 5.2 and 2.9 kb were present in perinatal and young adult skeletal muscle, whereas only the 5.2 and 2.9 kb mRNA classes could be demonstrated in aged muscle. This indicates that metabolism of the various NCAM polypeptides is individually regulated during aging. Alternative splicing of NCAM mRNA in skeletal muscle was studied by Northern blotting using DNA oligonucleotide probes specifically hybridizing to selected exons or exon combinations. Exon VASE, which has previously been shown to be present in both brain and heart NCAM mRNA, was virtually absent from skeletal muscle at all ages studied. In contrast, the majority of NCAM mRNA in postnatal skeletal muscle was shown to contain extra exons inserted between exons 12 and 13. Of the various possible exon combinations at this splice site, the combinations 12-a-AAG-13 and 12-a-b seemed to be prevalent in postnatal skeletal muscle. No significant change in the relative proportion of these two exon combinations occurred during aging. The observed upregulation of NCAM protein in aged muscle supports the assumption that an increasing proportion of muscle fibres are denervated in aged muscle. Selective upregulation of the 5.2 and 2.9 kb mRNA forms have previously been demonstrated in muscle cell lines and in primary cultures of muscle cells during formation of myotubes in vitro, and this switch in NCAM mRNA classes has been suggested to correlate with myogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Human biliary glycoprotein gene: characterization of a family of novel alternatively spliced RNAs and their expressed proteins

Eight different human biliary glycoprotein (BGP) isoantigens, structurally related members of the carcinoembryonic antigen family, CD66/67 family, and immunoglobulin superfamily, are derived by alternative splicing from a single genomic transcription unit. Novel BGP isoforms have been identified by polymerase chain reaction amplification and by DNA sequencing of amplified cDNA segments. In addition to verifying previously documented BGPs, we describe four new forms, two of which have unusual nonimmunoglobulin exons contributed by inverted Alu repeats. Determination of the genomic DNA sequence encompassing most of the known extracellular and intracellular domains demonstrates that the translatable Alu-like sequences are encoded in bona fide exons. The third novel BGP isoform contains none of the extracellular disulfide-linked immunoglobulin-like domains typical of these molecules but retains N-terminal and intracellular domains, suggesting distinct functions for N-terminal versus other disulfide-linked domains. cDNAs coding for each identified isoform have been transfected into COS7 monkey cells, and the resulting polypeptides are heavily N glycosylated but can be deglycosylated to their expected primary sizes. Many of these deglycosylated forms can be correlated with unique patterns of BGP expression in different cell lines, while in granulocytes, some previously undescribed or alternatively modified forms may predominate. The BGP family represents a potentially large but unknown source of functional diversity among cells of epithelial and hematopoietic origin. The availability of a defined set of expressed of BGP cDNAs should permit critical definition of their function.

Human biliary glycoprotein gene: characterization of a family of novel alternatively spliced RNAs and their expressed proteins

Eight different human biliary glycoprotein (BGP) isoantigens, structurally related members of the carcinoembryonic antigen family, CD66/67 family, and immunoglobulin superfamily, are derived by alternative splicing from a single genomic transcription unit. Novel BGP isoforms have been identified by polymerase chain reaction amplification and by DNA sequencing of amplified cDNA segments. In addition to verifying previously documented BGPs, we describe four new forms, two of which have unusual nonimmunoglobulin exons contributed by inverted Alu repeats. Determination of the genomic DNA sequence encompassing most of the known extracellular and intracellular domains demonstrates that the translatable Alu-like sequences are encoded in bona fide exons. The third novel BGP isoform contains none of the extracellular disulfide-linked immunoglobulin-like domains typical of these molecules but retains N-terminal and intracellular domains, suggesting distinct functions for N-terminal versus other disulfide-linked domains. cDNAs coding for each identified isoform have been transfected into COS7 monkey cells, and the resulting polypeptides are heavily N glycosylated but can be deglycosylated to their expected primary sizes. Many of these deglycosylated forms can be correlated with unique patterns of BGP expression in different cell lines, while in granulocytes, some previously undescribed or alternatively modified forms may predominate. The BGP family represents a potentially large but unknown source of functional diversity among cells of epithelial and hematopoietic origin. The availability of a defined set of expressed of BGP cDNAs should permit critical definition of their function.

Expression of NCAM mRNA and polypeptides in aging rat brain

In aging brain, degenerative as well as compensatory regenerative processes are believed to occur. The neural cell adhesion molecule NCAM is involved in developmental and regenerative processes in the brain. However, the role of NCAM in aging brain has not been characterized. In this study, the expression of NCAM mRNAs and polypeptides was investigated in aging rat brain. The 7.4 and 6.7 kb NCAM mRNAs were selectively downregulated during postnatal development, and the 5.2 and 2.9 kb NCAM mRNAs were upregulated. However, from postnatal day 40 to old age no change in NCAM mRNA classes was observed. The fraction of NCAM mRNA containing the VASE exon increased postnatally but remained stable during adult life. VASE, which is believed to modulate the binding capacity, seemed to be relatively more abundant in the 7.4 and 6.7 kb NCAM mRNAs, encoding transmembrane NCAM forms, than in the 5.2 and 2.9 kb NCAM mRNAs, coding for glycosyl phosphatidylinositol (GPI) linked NCAM. Conversely, insertion of exons a and AAG between exons 12 and 13, a region containing two fibronectin type III repeats, seemed to be more pronounced in 5.2 and 2.9 kb NCAM mRNAs than in the 7.4 and 6.7 kb mRNAs. During postnatal development an increase in the fraction of 6.7 kb NCAM mRNA containing the exons a and AAG was observed. However, during aging the fraction of NCAM mRNAs containing this exon combination seemed constant. At the protein level, NCAM-A was downregulated both during development and aging. No changes were observed during aging in the composition of soluble NCAM forms in the brain, cerebrospinal fluid or blood plasma. The amount of NCAM in rat brain decreased during postnatal development, but remained at a constant level from postnatal day 40 to old age. To conclude, several changes in NCAM expression occur during early postnatal development emphasizing the important role of this molecule in the morphogenetic processes. During aging, a significant selective downregulation of NCAM-A was observed indicating that in general only minor regenerative processes occur in the brain.

Cell adhesion molecules, second messengers and axonal growth

Recent studies on NCAM-related molecules suggest that individual cell adhesion molecules might function to both promote axonal growth during development and maintain synaptic structure in the adult. Evidence that differential alternative splicing contributes to this apparent bifunctionality of cell adhesion molecules is discussed.

Molecular cloning and expression of a new rat liver cell-CAM105 isoform. Differential phosphorylation of isoforms

An hepatocyte cell-adhesion molecule (cell-CAM105) was recently shown to be identical with the liver plasma-membrane ecto-ATPase. This protein has structural features of the immunoglobulin superfamily and is homologous with carcinoembryonic antigen proteins. We have cloned a cDNA encoding a new form of the cell-CAM105 which is a variant of the previously isolated clone. In addition to having a shorter cytoplasmic domain, the new isoform also has substitutions clustered in the first 130 amino acids of the extracellular domain. Both of these isoforms are expressed on the surface of hepatocytes with the shorter variant being the predominant form. The previously isolated cell-CAM105 (long form) has more potential phosphorylation sites than does the new isoform (short form). Both isoforms are found to be phosphorylated after incubation with [32P]phosphate in vitro, with the long form being phosphorylated to a significantly higher extent. This observed differential phosphorylation could be one of the mechanisms for the regulation of isoform functions. Using antipeptide antibodies specific for the long form and antibodies that are reactive with both isoforms, we have shown that both isoforms are localized in the canalicular domain of hepatocytes. The sequence differences between these two isoforms suggest that they are probably derived from different genes rather than from alternative splicing.

Alternative splicing of the cytoplasmic domain of neural cell adhesion molecule alters its ability to act as a substrate for neurite outgrowth

A full-length cDNA encoding 180-kDa neural cell adhesion molecule (NCAM 180) has been transfected into mouse NIH-3T3 fibroblasts, and stable clones expressing the transgene have been isolated and characterised. Transfection was associated with the expression of a major protein band of 180 kDa and a minor related band of 140 kDa. Antibodies reactive exclusively with human NCAM immunoprecipitated both proteins but failed to coprecipitate any other proteins. The ability of transfected NCAM to stimulate neurite outgrowth was determined by culturing rat cerebellar neurons on top of confluent monolayers of parental 3T3 cells or clones of transfected 3T3 cells expressing either NCAM 140 or NCAM 180. The results show that NCAM 180 is less able to act as a substrate for neurite outgrowth than NCAM 140.

NCAM: structural diversity, function and regulation of expression

NCAM is a large family of structurally closely related proteins with cell-cell adhesive properties and a temporo-spatially regulated expression throughout development. This review covers recent work on NCAM with an emphasis on the still open questions of the full extent of structural diversity and the mechanism whereby it arises, the chemistry and functional consequences of the binding event and the intricacies of the developmental regulation of NCAM, all of which have ramifications in its likely role as an effector of morphogenesis.

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.

The VASE exon downregulates the neurite growth-promoting activity of NCAM 140

Axonal growth, guidance and synapse formation are controlled by receptors on neuronal growth cones that can recognize positive and inhibitory cues in the local microenvironment. Four well characterized receptor systems are known that recognize the growth-promoting activities associated with the extracellular matrix and the membranes of cells such as astrocytes, muscle cells and Schwann cells; these are the integrins and the homophilically binding cell adhesion molecules neural-cell adhesion molecule (NCAM), N-cadherin and L1 (refs 5-12). Alternative splicing generates 20-30 isoforms of NCAM and these can also be differentially glycosylated. There are two sites where alternative splicing changes the extracellular structure of membrane-bound NCAM and one of these (the MSD1 region) does not obviously affect function. Here we report that the variable alternatively spliced exon (VASE) in immunoglobulin domain 4 downregulates the neurite outgrowth-promoting activity of NCAM. The high level of VASE expression in the adult central as compared with peripheral nervous system could contribute to the poor regenerative capacity of the former.

Expression of the neural cell adhesion molecule (NCAM) on the haemopoietic cell line Nalm-16

The Nalm-16 cell line was originally described as being of the null acute lymphoblastic leukaemia (ALL) phenotype. Using phenotypic and genotypic markers, we have demonstrated the line carries markers associated with cells of the B lineage. In addition, Nalm-16 binds a series of monoclonal antibodies characterized as predominantly recognising neuroectodermal tissues. Amongst these antibodies, UJ13A, 5.1.H11 and ERIC-1 have been shown to recognise the neural cell adhesion molecule (NCAM). Expression of the NCAM molecule is highly complex, with several isoforms of the protein resulting from the differential splicing of the NCAM mRNA transcript. Western-blot analysis of Nalm-16 cell extracts indicates that cells express the heavily polysialylated form of the molecule found on many embryonic tumours. Neuraminidase digestion indicates that the 140 kD isoform is predominantly expressed on Nalm-16, although the 120 kD isoform is present to a lesser degree. These findings have been confirmed using Northern-blot analysis.

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.

Splicing of the VASE exon of neural cell adhesion molecule (NCAM) in human small-cell lung carcinoma (SCLC)

Expression of the neural cell adhesion molecule (NCAM) on small-cell lung carcinoma (SCLC) cell lines and tumour tissue has been investigated. Cell lines were found to express highly sialylated NCAM. Neuraminidase treatment revealed the presence of the 140- and 120-kDa isoforms with differential expression of a 95-kDa protein. Similar data were obtained with SCLC tumour tissues. These results were corroborated by Northern blotting where mRNA of 6.7 and 5.5 kb coding for the 140- and 120-kDa isoforms, respectively, were identified. In a few tumours, a weaker band of 7.4-kb mRNA coding for the 180-kDa NCAM was also identified. This result could not be confirmed biochemically due to shortage of material. Finally, a 5-kb transcript was identified in all SCLC samples examined. The NCAM isoform coded by this mRNA remains unknown. Using the polymerase chain reaction (PCR), we have demonstrated the presence of the VASE mini-exon in some isoforms of SCLC NCAM. The VASE mini-exon sequence in human SCLC differs from the published murine sequence by only one base change. This substitution does not result in altered amino-acid sequence.

Developmental regulation of alternative splicing in the mRNA encoding Xenopus laevis neural cell adhesion molecule (NCAM)

The neural cell adhesion molecule (NCAM) is thought to play a role in the formation of the vertebrate nervous system. In mammals and chicken, it is known that more than 100 different forms of the NCAM protein can be generated by alternative splicing of one primary transcript and it is possible that these different forms have distinct biological functions. A large part of the diversity is generated by alternative mRNA splicing in two regions, called the pi and the muscle specific domain (MSD), that encode portions of the extracellular domain of the NCAM protein. In this report, we describe the tissue and developmental expression of the pi and MSD sequences in the amphibian, Xenopus laevis. Our experiments show that NCAM transcripts are present in all tissues examined including muscle, heart, liver, kidney, and brain. We have identified a 30-base exon, similar to the pi domain observed in mammals, that is not present in maternal NCAM RNA but appears in a subset of the NCAM mRNA population shortly after neural induction. At the predicted location of the MSD we have detected only two alternatively spliced exons, 3 bases and 15 bases in length. In no X. laevis tissue examined did we detect the two additional alternatively spliced exons which are present in the MSD region of mammalian and chicken NCAM RNAs. Finally, the analysis has revealed a dynamic and complex pattern of expression of alternatively spliced NCAM mRNAs during embryogenesis. High levels of expression of specific forms of NCAM RNA correlate with major morphogenic events such as neural tube formation and metamorphosis.

A novel cell adhesion molecule, G-CAM, found on cultured rat glia

Using a monoclonal antibody (mAb), designated AMP1, a novel cell adhesion molecule was identified on rat astrocytes and oligodendrocytes. When confluent monolayers of cultured rat astrocytes were labeled with AMP1, the antigen was discretely localized to the cell surface in regions of cell-cell contacts. The antibody did not label embryonic rat cortical neurons plated on monolayers of neonatal astrocytes, indicating that the antigen is neither present on cultured neurons nor does it reorganize on the glial surface under the neurons. On immunoblots of astrocytic or brain proteins, mAb AMP1 recognized a 106 kDa protein. In the present paper, data are presented demonstrating that the AMP1 antigen is a cell adhesion molecule and is distinct from all the known cell adhesion molecules present on astrocytes: N-CAM, N-cadherin, or members of the beta 1 integrin family. We have tentatively termed this molecule 'glial cell adhesion molecule' (G-CAM).

A carbohydrate epitope defined by monoclonal antibody VC1.1 is found on N-CAM and other cell adhesion molecules

VC1.1 is a monoclonal antibody that stains the surfaces of neuronal subsets in the brain. Previous work showed that VC1.1 recognizes 3 polypeptide bands with molecular weights of 95-105 kDa, 140 kDa and 170 kDa and two high molecular weight proteoglycans with weights of approximately 680 and 650-700 kDa. The heterogeneity and molecular weight range of these bands suggested that VC1.1 might recognize a carbohydrate moiety associated with a family of cell surface molecules. It had been previously demonstrated that a separate monoclonal antibody, HNK-1 also recognized a cell surface associated epitope characterized as a sulfate- and glucuronic acid-containing N-linked carbohydrate. This epitope has been shown to be present on members of the N-CAM adhesion molecule family. In this report, we demonstrate that VC1.1 recognizes an N-linked carbohydrate group that is attached to myelin-associated glycoprotein and N-CAM. Immunocytochemical and biochemical comparisons of VC1.1 and HNK-1 staining in rat and cat brain indicate that these two antibodies probably recognize overlapping, or identical carbohydrate epitopes.

Expression of alternative isoforms of the neural cell adhesion molecule (NCAM) on normal brain and a variety of brain tumours

A panel of monoclonal antibodies, including a reagent designated ERIC-1, have been characterized as binding to the human neural cell adhesion molecule (NCAM). These monoclonal antibodies bind in a relatively uniform manner to a variety of normal and neoplastic tissues arising from the neuroectoderm. However, multiple forms of the protein are known to arise from the differential splicing of exons within the NCAM gene located on chromosome 11 at q23. On human adult brain, four isoforms of 180, 170, 145 and 120 kDa have been identified. Here, we report the identification of another NCAM isoform of 95 kDa that is apparent on tissues following either N-glycanase or neuraminidase treatment to remove carbohydrate and sialic acid residues from the molecule respectively. NCAM expression is further complicated by differential post-translational modification of the molecule which is developmentally regulated. In general, fetal NCAM is more heavily polysialylated than the adult forms of the molecule. Human fetal brain has been shown to express the heavily sialylated embryonic form of NCAM, but following neuraminidase digestion, a similar pattern of NCAM expression is seen to that in adult brain. A variety of human brain tumours examined also show different patterns of NCAM expression, despite their uniform staining with monoclonal antibodies. The significance of these observations for designing new molecular and immunological approaches to the diagnosis of a variety of primary tumours is reviewed.

A new variant of glycoprotein CD44 confers metastatic potential to rat carcinoma cells

Using a monoclonal antibody (MAb1.1ASML) raised against a surface glycoprotein of the metastasizing rat pancreatic carcinoma cell line BSp73ASML, cDNA clones have been isolated that encode glycoproteins with partial homology to CD44, a presumed adhesion molecule. In one of the clones, pMeta-1, the epitope marks an additional extracellular domain of 162 amino acids inserted into the rat CD44 protein between amino acid positions 223 and 247 (by analogy to human and murine CD44). The new variants are expressed only in the metastasizing cell lines of two rat tumors, the pancreatic carcinoma BSp73 and the mammary adenocarcinoma 13762NF; they are not expressed in the non-metastasizing tumor cell lines nor in most normal rat tissues. Overexpression of pMeta-1 in the nonmetastasizing BSp73AS cells suffices to establish full metastatic behavior.

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.

At least 27 alternatively spliced forms of the neural cell adhesion molecule mRNA are expressed during rat heart development

The major membrane-associated or transmembrane isoforms of the neural cell adhesion molecule (NCAM) are generated by alternative splicing at the 3' end of the mRNA. Further diversity in NCAM structure is observed in the extracellular region of the polypeptide, where the insertion of additional amino acid residues can result from alternative splicing events occurring at the exon 7-exon 8 and exon 12-exon 13 junctions. Here we report the characterization of tissue-specific patterns of alternative splicing at the exon 12-exon 13 junction by using the polymerase chain reaction. Nine alternatively spliced sequences in rat heart between exon 12 and exon 13 were identified. Each sequence consisted of different combinations of the three small exons (15, 48, and 42 bp in length) and the AAG triplet that make up MSD1, the 108-bp muscle-specific sequence found in human skeletal muscle NCAM (G. Dickson, H.J. Gower, C. H. Barton, H. M. Prentice, V. L. Elsom, S. E. Moore, R. D. Cox, C. Quinn, W. Putt, and F. S. Walsh, Cell 50:1119-1130, 1987). Although the rat equivalent of MSD1 (designated 15+ 48+ 42+ 3+) was detected in all ages of heart examined, it was only one of four or five major splice combinations at any given age. The only alternatively spliced sequence found in the exon 7-exon 8 junction of heart NCAM mRNA was the 30-bp variable alternatively spliced exon previously identified in rat brain. Twenty-seven NCAM forms with distinct sequences were found by analysis of individual NCAM transcripts from postnatal day 1 heart tissue for alternative splicing at the exon 7-exon 8 junction, the exon 12-exon 13 junction and the 3' end. Several combinations of splicing patterns in these three different regions of the gene appeared to be preferentially expressed. The observation that the expression of alternatively spliced forms of NCAM is developmentally regulated suggests a role for NCAM diversity in cardiac development.

At least 27 alternatively spliced forms of the neural cell adhesion molecule mRNA are expressed during rat heart development

The major membrane-associated or transmembrane isoforms of the neural cell adhesion molecule (NCAM) are generated by alternative splicing at the 3' end of the mRNA. Further diversity in NCAM structure is observed in the extracellular region of the polypeptide, where the insertion of additional amino acid residues can result from alternative splicing events occurring at the exon 7-exon 8 and exon 12-exon 13 junctions. Here we report the characterization of tissue-specific patterns of alternative splicing at the exon 12-exon 13 junction by using the polymerase chain reaction. Nine alternatively spliced sequences in rat heart between exon 12 and exon 13 were identified. Each sequence consisted of different combinations of the three small exons (15, 48, and 42 bp in length) and the AAG triplet that make up MSD1, the 108-bp muscle-specific sequence found in human skeletal muscle NCAM (G. Dickson, H.J. Gower, C. H. Barton, H. M. Prentice, V. L. Elsom, S. E. Moore, R. D. Cox, C. Quinn, W. Putt, and F. S. Walsh, Cell 50:1119-1130, 1987). Although the rat equivalent of MSD1 (designated 15+ 48+ 42+ 3+) was detected in all ages of heart examined, it was only one of four or five major splice combinations at any given age. The only alternatively spliced sequence found in the exon 7-exon 8 junction of heart NCAM mRNA was the 30-bp variable alternatively spliced exon previously identified in rat brain. Twenty-seven NCAM forms with distinct sequences were found by analysis of individual NCAM transcripts from postnatal day 1 heart tissue for alternative splicing at the exon 7-exon 8 junction, the exon 12-exon 13 junction and the 3' end. Several combinations of splicing patterns in these three different regions of the gene appeared to be preferentially expressed. The observation that the expression of alternatively spliced forms of NCAM is developmentally regulated suggests a role for NCAM diversity in cardiac development.

Expression of the unique NCAM VASE exon is independently regulated in distinct tissues during development

During development of the rat central nervous system, neural cell adhesion molecule (NCAM) mRNAs containing in the extracellular domain a 30-bp alternative exon, here named VASE, replace RNAs that lack this exon. The presence of this alternative exon between previously described exons 7 and 8 changes the predicted loop structure of the derived polypeptide from one resembling an immunoglobulin constant region domain to one resembling an immunoglobulin variable domain. This change could have significant effects on NCAM polypeptide function and cell-cell interaction. In this report we test multiple rat tissues for the presence of additional alternative exons at this position and also examine the regulation of splicing of the previously described exon. To sensitively examine alternative splicing, polymerase chain reactions (PCRs) with primers flanking the exon 7/exon 8 alternative splicing site were performed. Four categories of RNA samples were tested for new exons: whole brain from embryonic day 11 to adult, specific brain regions dissected from adult brain, clonal lines of neural cells in vitro, and muscle cells and tissues cultured in vitro and obtained by dissection. Within the limits of the PCR methodology, no evidence for any alternative exon other than the previously identified VASE was obtained. The regulation of expression of this exon was found to be complex and tissue specific. Expression of the 30-bp exon in the heart and nervous system was found to be regulated independently; a significant proportion of embryonic day 15 heart NCAM mRNAs contain VASE while only a very small amount of day 15 nervous system mRNAs contain VASE. Some adult central nervous system regions, notably the olfactory bulb and the peripheral nervous system structures adrenal gland and dorsal root ganglia, express NCAM which contains very little VASE. VASE is undetectable in NCAM PCR products from the olfactory epithelium. Other nervous system regions express significant quantities of NCAM both with and without VASE. Clonal cell lines in culture generally expressed very little VASE. These results indicate that a single alternative exon, VASE, is found in NCAM immunoglobulin-like loop 4 and that distinct tissues and nervous system regions regulate expression of VASE independently both during development and in adult animals.

The large isoform of myelin-associated glycoprotein is scarcely expressed in the quaking mouse brain

Two polypeptide isoforms of myelin-associated glycoprotein (MAG) with molecular masses of 72 and 67 kDa are produced by alternative splicing of the exon 12 portion. Our previous work has demonstrated that in the quaking mouse brain this alternative splicing is lacking and that the mRNA coding the large MAG isoform (L-MAG) is scarcely expressed, whereas that of small MAG isoform (S-MAG) is overexpressed. In the present study, we prepared antisera specific to the S-MAG and L-MAG amino acid residues, respectively. Immunoblots showed that the L-MAG band was scarcely detectable in the quaking mouse brain, whereas the S-MAG band had an apparently higher molecular mass than in the normal control. Our immunohistochemical study also showed that L-MAG was scarcely stained in the quaking mouse brain. These results seemed to reflect a reduction in content of L-MAG mRNA and abnormal glycosylation in the quaking mouse brain.

Neurite outgrowth in response to transfected N-CAM changes during development and is modulated by polysialic acid

We have used monolayers of control 3T3 cells and 3T3 cells transfected with a cDNA encoding human N-CAM as a culture substrate for embryonic chick retinal ganglion cells (RGCs). At embryonic day 6 (E6), but not at E11, RGCs extended longer neurites on monolayers of N-CAM-transfected cells. This loss of RGC responsiveness was not associated with substantial changes in the level of N-CAM expression on RGC growth cones. The neurite outgrowth response from E6 RGCs could be inhibited by removal of N-CAM from the monolayer, by removal of alpha 2-8-linked polysialic acid from neuronal N-CAM, or by antibodies that bind exclusively to chick (neuronal) N-CAM. In contrast, the response was not dependent on neuronal beta 1 integrin function. These data provide substantive evidence for a homophilic binding mechanism directly mediating N-CAM-dependent neurite outgrowth, and suggest that changes in polysialic acid expression on neuronal N-CAM may modulate N-CAM-dependent axonal growth during development.

Highly sialylated N-CAM is expressed in adult mouse optic nerve and retina

The localization of the neural cell adhesion molecule (N-CAM) and its highly sialylated form, which is prevalent in young tissues and has therefore been called embryonic neural cell adhesion molecule, was studied in the developing and adult mouse optic nerve and retina immunohistologically and immunochemically. At embryonic and early postnatal ages, neuroblasts and young postmitotic neurons, Müller cells and astrocytes in the retina, and retinal ganglion cell axons and all glial cells in the optic nerve express highly sialylated neural cell adhesion molecule. Beginning with the third postnatal week, highly sialylated neural cell adhesion molecule disappears from retinal ganglion cell axons in the optic nerve and from neuronal cell bodies and processes in the retina. In addition, it is not detectable on oligodendrocytes in 3-week-old animals. However, highly sialylated neural cell adhesion molecule continues to be expressed in the adult optic nerve and retina by astrocytes and Müller cells. On these cells it is only absent from cell membranes contacting basal lamina. Weakly sialylated neural cell adhesion molecule, in contrast, is expressed by all cell types of retinal and optic nerve during development and in the adult. The loss of highly sialylated neural cell adhesion molecule from neurons and oligodendrocytes must therefore be considered as a cell type-specific conversion of the so-called embryonic to the adult form of neural cell adhesion molecule and does not simply reflect the disappearance of neural cell adhesion molecule from these cells. Weakly sialylated neural cell adhesion molecule, however, is absent from outer segments of photoreceptor cells and, as is the case for the highly sialylated form, from glial cell surfaces contacting basal lamina. Thus, the expression of highly sialylated neural cell adhesion molecule by pre- and postmitotic neurons and by oligodendrocytes is restricted mainly to the period of histogenetic events in retina and optic nerve, i.e. cell division, cell migration, dendritic and axonal growth and synaptogenesis. In addition to the observation that this form of neural cell adhesion molecule is less adhesive than the weakly sialylated, adult form, it is likely that highly sialylated neural cell adhesion molecule plays an important role during dynamic morphogenetic events. Furthermore, the expression of highly sialylated neural cell adhesion molecule by astrocytes and Müller cells in adult optic nerves and retinae suggests some histogenetically plastic functions for these cells in the adult mouse visual system.

Transcription initiation sites and structural organization of the extreme 5' region of the rat neural cell adhesion molecule gene

Through analysis of rat genomic cosmid clones, the 5'-most exon of the rat neural cell adhesion molecule (NCAM) gene was identified. This exon, here named exon 0, contained the entire 5' untranslated region and the N-terminal signal sequence of the polypeptide. Exon 0 was isolated from a 1.6-kilobase (kb) EcoRI-HindIII fragment of rat genomic cosmid clone 9 which was 35 kb in length. This fragment was sequenced and found to contain approximately 940 base pairs (bp) of 5'-flanking sequence, exon 0, which was approximately 245 bp in length, and approximately 400 bp of the following intron 0. By using information derived from this fragment and the pR18 rat NCAM cDNA, the transcription initiation sites were determined with two assays. Both primer extensions and nuclease S1 protection assays of postnatal day 7 rat brain RNA identified seven initiation sites within a single 10-bp region at positions -195 to -186 relative to the translation start site. An additional minor site was found at position -329. In the immediate 5' region, no consensus TATA or CCAAT sequences were found. Potential regulatory elements within this region include Sp1 consensus binding sites and also a 178-bp homopurine-homopyrimidine sequence containing several mirror repeats. NCAM has multiple transcripts which are regulated in a developmental and tissue-specific fashion. To determine whether these transcripts are initiated at the same sites, transcription initiation sites were analyzed in postnatal day 7 and adult rat brain and also in cultured cell lines of neuronal, glial, and muscle phenotypes. These tissues and cells exhibited distinct NCAM transcript populations in Northern (RNA) dot blot analysis. In all cases similar transcription start sites were found, suggesting that all major NCAM transcripts have similar or identical initiation sites. These results provide essential information to begin analysis of NCAM regulation in different tissues and during development.

Transcription initiation sites and structural organization of the extreme 5' region of the rat neural cell adhesion molecule gene

Through analysis of rat genomic cosmid clones, the 5'-most exon of the rat neural cell adhesion molecule (NCAM) gene was identified. This exon, here named exon 0, contained the entire 5' untranslated region and the N-terminal signal sequence of the polypeptide. Exon 0 was isolated from a 1.6-kilobase (kb) EcoRI-HindIII fragment of rat genomic cosmid clone 9 which was 35 kb in length. This fragment was sequenced and found to contain approximately 940 base pairs (bp) of 5'-flanking sequence, exon 0, which was approximately 245 bp in length, and approximately 400 bp of the following intron 0. By using information derived from this fragment and the pR18 rat NCAM cDNA, the transcription initiation sites were determined with two assays. Both primer extensions and nuclease S1 protection assays of postnatal day 7 rat brain RNA identified seven initiation sites within a single 10-bp region at positions -195 to -186 relative to the translation start site. An additional minor site was found at position -329. In the immediate 5' region, no consensus TATA or CCAAT sequences were found. Potential regulatory elements within this region include Sp1 consensus binding sites and also a 178-bp homopurine-homopyrimidine sequence containing several mirror repeats. NCAM has multiple transcripts which are regulated in a developmental and tissue-specific fashion. To determine whether these transcripts are initiated at the same sites, transcription initiation sites were analyzed in postnatal day 7 and adult rat brain and also in cultured cell lines of neuronal, glial, and muscle phenotypes. These tissues and cells exhibited distinct NCAM transcript populations in Northern (RNA) dot blot analysis. In all cases similar transcription start sites were found, suggesting that all major NCAM transcripts have similar or identical initiation sites. These results provide essential information to begin analysis of NCAM regulation in different tissues and during development.

Characterization of cDNA clones defining variant forms of human neural cell adhesion molecule N-CAM

The neural cell adhesion molecule N-CAM has been identified in a number of species and comprises at least three major cell surface polypeptides of different molecular structures and tissue distributions. We report here the isolation and characterization of cDNA clones encoding two of the three major forms of N-CAM from a human neuroblastoma cDNA library. One of the clones, NII-6, provides the first complete sequence of a small cytoplasmic domain (140 kDa) form of the molecule in humans and differs in a number of respects from cDNA clones derived from human muscle. These differences include the presence of a 30-bp insert in the fourth immunoglobulin-like domain of N-CAM, a 3-bp insert in the extracellular portion of the molecule, and an additional 6 bp in the middle of the membrane-spanning segment. Based on the analysis of a genomic DNA clone spanning these regions of N-CAM, the first two differences arise by alternate splicing of RNA and occur in some, but not all clones; the additional 6 bp may reflect a genetic polymorphism. A second cDNA clone, NI-10, encodes the complete sequence of a segment that is specific to the large cytoplasmic domain (180 kDa) polypeptide of human N-CAM and is very similar to corresponding segments of mouse, chicken, and rat N-CAM. This sequence also arises by alternative splicing of RNA. In addition, we have identified a genomic DNA segment encoding sequences specific to the third, small surface domain (120 kDa) polypeptide of N-CAM. The data presented here and previously define the DNA sequences of the membrane-bound forms and known variants of human N-CAM. From these sequences, a wide variety of probes can be generated for investigating the expression of particular N-CAM polypeptides in normal and pathological tissues.

N-myc down regulates neural cell adhesion molecule expression in rat neuroblastoma

In human neuroblastoma, amplification of the N-myc oncogene is correlated with increased metastatic ability. We recently showed that transfection of the rat neuroblastoma cell line B104 with an N-myc expression vector resulted in an increase in metastatic ability and a significant reduction in the expression of major histocompatibility complex class I antigens. We examined whether N-myc causes additional phenotypic changes in these cells. We showed that expression of N-myc leads to a dramatic reduction in the levels of neural cell adhesion molecule (NCAM) polypeptides and mRNAs. Spontaneous revertants of the high N-myc phenotype were found to have regained significant levels of NCAM expression, indicating that the continued expression of N-myc is required to maintain the low NCAM phenotype. NCAM was not reduced in B104 cells transfected with the neomycin resistance vector alone, and other neuronal markers were not specifically reduced in N-myc-transfected B104 cells. As NCAM functions in cell-cell adhesion, decreased NCAM expression could contribute significantly to the increased metastatic potential of N-myc-amplified neuroblastomas.

Characterization of the human N-CAM promoter

In contrast with the complex series of splicing choices that generate the various membrane-associated isoforms of the neural cell-adhesion molecule alternative splicing of 5' exons does not contribute to additional molecular diversity. A single regulatory unit in genomic DNA, mapping to a 5 kb restriction-endonuclease-HindIII fragment, controls the expression of all major RNA size classes. DNA sequence analysis of a 2 kb fragment spanning the two major identified transcriptional initiation sites (194 and 188 bp from the ATG codon) and translation start codon indicates that the regulatory unit does not possess classical TATA or CCAAT motifs. The region of the putative promoter exhibits a GC-rich content and a high frequency of the dinucleotide CpG, both characteristics of a HTF(HpaII tiny fragments)-island. Introduction of deletion-mutant chimaeric-gene constructs into human and rodent N-CAM-expressing cell lines defines an active promoter region of 467 bp (-144 to -611 bp from the ATG codon). This region of genomic DNA contains consensus sites for the interaction of known transcriptional factors.

Differential splicing generates a nervous system-specific form of Drosophila neuroglian

We recently described the characterization and cloning of Drosophila neuroglian, a member of the immunoglobulin superfamily. Neuroglian contains six immunoglobulin-like domains and five fibronectin type III domains and shows strong sequence homology to the mouse neural cell adhesion molecule L1. Here we show that the neuroglian gene generates at least two different protein products by tissue-specific alternative splicing. The two protein forms differ in their cytoplasmic domains. The long form is restricted to the surface of neurons in the CNS and neurons and some support cells in the PNS; in contrast, the short form is expressed on a wide range of other cells and tissues. Thus, whereas the mouse L1 gene appears to encode only one protein that functions largely as a neural cell adhesion molecule, its Drosophila homolog, the neuroglian gene, encodes at least two protein forms that may play two different roles, one as a neural cell adhesion molecule and the other as a more general cell adhesion molecule involved in other tissues and imaginal disc morphogenesis.

N-myc down regulates neural cell adhesion molecule expression in rat neuroblastoma

In human neuroblastoma, amplification of the N-myc oncogene is correlated with increased metastatic ability. We recently showed that transfection of the rat neuroblastoma cell line B104 with an N-myc expression vector resulted in an increase in metastatic ability and a significant reduction in the expression of major histocompatibility complex class I antigens. We examined whether N-myc causes additional phenotypic changes in these cells. We showed that expression of N-myc leads to a dramatic reduction in the levels of neural cell adhesion molecule (NCAM) polypeptides and mRNAs. Spontaneous revertants of the high N-myc phenotype were found to have regained significant levels of NCAM expression, indicating that the continued expression of N-myc is required to maintain the low NCAM phenotype. NCAM was not reduced in B104 cells transfected with the neomycin resistance vector alone, and other neuronal markers were not specifically reduced in N-myc-transfected B104 cells. As NCAM functions in cell-cell adhesion, decreased NCAM expression could contribute significantly to the increased metastatic potential of N-myc-amplified neuroblastomas.

Olfactory neurons express a unique glycosylated form of the neural cell adhesion molecule (N-CAM)

mAb-based approaches were used to identify cell surface components involved in the development and function of the frog olfactory system. We describe here a 205-kD cell surface glycoprotein on olfactory receptor neurons that was detected with three mAbs: 9-OE, 5-OE, and 13-OE. mAb 9-OE immunoreactivity, unlike mAbs 5-OE and 13-OE, was restricted to only the axons and terminations of the primary sensory olfactory neurons in the frog nervous system. The 9-OE polypeptide(s) were immunoprecipitated and tested for cross-reactivity with known neural cell surface components including HNK-1, the cell adhesion molecule L1, and the neural cell adhesion molecule (N-CAM). These experiments revealed that 9-OE-reactive molecules were not L1 related but were a subset of the 200-kD isoforms of N-CAM. mAb 9-OE recognized epitopes associated with N-linked carbohydrate residues that were distinct from the polysialic acid chains present on the embryonic form of N-CAM. Moreover, 9-OE N-CAM was a heterogeneous population consisting of subsets both with and without the HNK-1 epitope. Thus, combined immunohistochemical and immunoprecipitation experiments have revealed a new glycosylated form of N-CAM unique to the olfactory system. The restricted spatial expression pattern of this N-CAM glycoform suggests a possible role in the unusual regenerative properties of this sensory system.

Characterization of rat brain NCAM mRNA using DNA oligonucleotide probes

A number of different isoforms of the neural cell adhesion molecule (NCAM) have been identified. The difference between these is due to alternative splicing of a single NCAM gene. In rat brain NCAM mRNAs with sizes of 7.4, 6.7, 5.2, 4.3 and 2.9 kb have been reported. We have synthesized six DNA oligonucleotides, that hybridize to different exons in the NCAM gene. Furthermore we have constructed three oligonucleotides, that exclusively hybridize to mRNAs lacking certain exons, by letting them consist of sequences adjacent to both sides of the splice sites. By means of these probes we have characterized the five NCAM mRNAs in rat brain.

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

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

Generation of multiple N-CAM polypeptides from a single gene

The neural cell adhesion molecule (N-CAM) is believed to be a key regulator of adhesive events in the nervous system and skeletal muscle. The recent isolation of N-CAM cDNAs from different tissues has identified a high degree of diversity in primary amino acid sequence between different isoforms. In this article, we review these recent studies and discuss methods for unravelling the functional consequences of the generation of multiple N-CAM polypeptides using gene transfection approaches.

Understanding the molecules of neural cell contacts: emerging patterns of structure and function

Neural cells make and break many contacts during their lifetime. The processes of neuroblast migration, axon elongation and guidance, synaptogenesis, myelination and synaptic rearrangement all require the selective formation and elimination of cell-cell and cell-substratum associations.