Clusters of neural cell adhesion molecule at sites of cell-cell contact

The neural cell adhesion molecule NCAM regulates neuritogenesis by multiple mechanisms of interaction

The neural cell adhesion molecule NCAM and its glycosylation with polysialic acid (polySia) are crucially involved in proliferation, migration and differentiation of neural progenitors. Modification with polySia, homophilic and heterophilic interactions set the function of NCAM, but little is known on their interplay. We have shown recently that removal of polySia induces neuronal differentiation via heterophilic NCAM interactions at cell contacts between SH-SY5Y neuroblastoma cells. Here we analyze the additional impact of NCAM-positive fibroblasts as a ligand-presenting cellular environment, a model often used to demonstrate the neuritogenic effect of homophilic NCAM interactions. Native SH-SY5Y cells did not respond to interactions with fibroblast NCAM. However, after induction of neuronal differentiation by retinoic acid the previously ineffective NCAM signals activated extracellular signal-regulated kinase (ERK) and promoted neuritogenesis. Removal of polySia increased neuritogenesis in retinoic acid-treated cells additive to the NCAM substrate effect. The change in responsiveness to substrate NCAM was associated with a rearrangement of polysialylated NCAM away from its enrichment at homotypic cell-cell contacts and with the appearance of non-polysialylated NCAM, i.e. changes facilitating NCAM interactions with the substrate. Thus, heterophilic and homophilic NCAM interactions are integrated into the cell's response yet they have the capacity to independently trigger neuritogenesis. The actual occurrence of each of these interactions, however, depends on the cellular context, targeted cell surface presentation of NCAM and the dynamic regulation of its modification by polysialic acid. In summary, this study reveals how the complex interplay of NCAM interactions and polysialylation provides an elaborate system to regulate neuritogenesis.

Regulation of HLA-DR and co-stimulatory molecule expression on natural killer T cells by granulocyte-macrophage colony-stimulating factor

A subset of mononuclear cells present in most tissues coexpresses receptors of both natural killer (NK) and T cells. Although linked to antiviral immunity, the function of these putative NKT cells is uncertain. We present evidence that human CD56+ DR- NKT cells exhibit hybrid adaptive and innate immune functions. These cells spontaneously lysed tumour cell targets and upon engagement of T-cell antigen receptors secreted the cytokines interferon-gamma and granulocyte-macrophage colony-stimulating factor (GM-CSF). Conversely, GM-CSF treatment transformed the NKT cells into dendritic cells, inducing rapid expression of HLA-DR and the co-stimulatory molecules CD80 and CD86. The ability to stimulate tetanus toxoid-specific responses from naïve T cells was acquired within 3 days of activating CD56+ NKT cells with GM-CSF. These results suggest a potential role for NKT cells in the initiation and control of primary immunity during the acute phase of infection.

Effects of osmoprotectant compounds on NCAM polysialylation under hyperosmotic stress and elevated pCO(2)

Elevated osmolality and pCO(2) have been shown to alter sialylation in a protein-specific manner. In Chinese hamster ovary (CHO)MT2-l-8 cells, tPA sialylation changed only slightly from 40 to 250 mm Hg pCO(2), whereas neural cell adhesion molecule polysialic acid (NCAM PSA) content decreased by up to 70% at 250 mm Hg pCO(2), pH 7.2. NCAM PSA content also decreased with increasing NaCl or NH(4)Cl concentration. This suggests that PSA content is a sensitive indicator of conditions that may alter glycosylation. Amino acids and their derivatives have been used to protect hybridoma and CHO cell growth under hyperosmotic stress. We examined the impact of osmoprotectants on NCAM PSA content in CHO MT2-1-8 cells under hyperosmolality (up to 545 mOsm/kg) and at 195 and 250 mm Hg pCO(2). NCAM PSA content at 545 mOsm/kg was at least two-fold greater in the presence of glycine betaine or L-proline compared to that without osmoprotectant. Surprisingly, in the presence of 20 mM glycine betaine, PSA levels were 50-60% of the control level for osmolalities ranging from 320 to 545 mOsm/kg. Thus, glycine betaine inhibits NCAM polysialylation at osmolalities below 435 mOsm/kg and is beneficial at higher osmolalities. In contrast to glycine betaine, L-proline increased PSA content by 25-120% relative to the unprotected culture at < or =545 mOsm/kg. The decrease in NCAM PSA levels of CHO MT2-1-8 cells cultured at 195 mm Hg pCO(2)-435 mOsm/kg was not mitigated by the presence of 25 mM glycine betaine, glycine, or L-threonine, even though all of these compounds enhanced cell growth. At 250 mm Hg pCO(2), all osmoprotectants tested (20 mM L-threonine, L-proline, glycine, or glycine betaine) increased NCAM polysialylation, with 20 mM glycine betaine restoring NCAM PSA to near control levels. Thus, osmoprotectants may (partially) offset changes in glycosylation, as well as the inhibition of growth, in cells under environmental stress. Supernatant beta-galactosidase levels, which increase upon alkalization of acidic organelles, did not differ significantly under elevated pCO(2) and hyperosmolality from that at control conditions.

Point mutations identified in Lec8 Chinese hamster ovary glycosylation mutants that inactivate both the UDP-galactose and CMP-sialic acid transporters

Nucleotide-sugar transporters (NSTs) are critical components of glycosylation pathways in eukaryotes. The identification of structural elements that are involved in NST functions provides an important task. Chinese hamster ovary glycosylation mutants defective in nucleotide-sugar transport provide access to inactive transporters that can define such structure/function relationships. In this study, we have cloned the hamster UDP-galactose transporter (UGT) and identified defects in UGT gene transcripts from nine independent Chinese hamster ovary mutants that belong to the Lec8 complementation group. Reverse transcription polymerase chain reaction with primers that span the UGT open reading frame showed that three Lec8 mutants express a full-length open reading frame, while six Lec8 mutants predominantly express truncated UGT gene transcripts. Sequencing identified different single or triplet nucleotide changes in full-length UGT transcripts from three of the mutants. These mutations translate into three different amino acid changes at positions that are highly conserved in all the known mammalian NSTs. Transfection of a cDNA encoding either of the mutations Delta serine 213 or G281D failed to correct the UDP-galactose transport defect in Lec8 transfectants. Most importantly, introducing these same mutations into the homologous region of the murine CMP-sialic acid transporter caused inactivation of this transporter. Thus, identifying point mutations that inactivate UGT in Lec8 mutants resulted in the discovery of amino acids that are critical to the activity of both UGT and CST, the two most divergent mammalian NSTs.

Regulation of neural cell adhesion molecule polysialylation state by cell-cell contact and protein kinase C delta

Post-translational modification of neural cell adhesion molecule (NCAM) with alpha2,8-linked polysialic acid, which regulates homophilic adhesion and/or signal transduction events, is crucial to synaptic plasticity in the developing and adult brain. Evidence from in vitro models has implicated polysialylation in the regulation of cell growth, migration, and differentiation. Here, using two in vitro models, we demonstrate that polysialylation is downregulated by cell-cell contact and correlated with a state of neuronal differentiation. Furthermore, we report a role for protein kinase C delta (PKCdelta) in the regulation of NCAM polysialylation. Pharmacological studies using the PKC activator, phorbol myristate acetate, and inhibitors, calphostin-C, and staurosporine, demonstrated PKC activity to be inversely related to NCAM polysialylation in the mouse neuro-2A cell line. Isoform-specific immunoblot studies indicated this effect to be mediated by the calcium-independent PKCdelta isozyme, as its expression was inversely related to NCAM polysialylation state in both neuro-2A and rat PC-12 cell lines. Isoform specificity was further confirmed using the PKCdelta-selective inhibitor rottlerin, which produced a marked increase in PSA expression (36.9+/-5.25 a.u. vs. 24.7+/-0.80 arbitrary units control) coupled with a neuritogenic response. Likewise, decreased expression of PKCdelta was seen in nerve growth factor (NGF)-differentiated PC-12 cells. These findings suggest that the neuronal differentiation process may involve inhibition of PKCdelta, resulting in enhanced morphological plasticity, as evidenced by activation of NCAM polysialylation.

Role of NCAM, cadherins, and microfilaments in cell-cell contact formation in TM4 immature mouse sertoli cells

To determine events that lead to the formation of intercellular contacts, we examined the spatial and temporal distribution of NCAM, cadherins, and F-actin in TM4 cells by immunofluorescence and laser scanning confocal microscopy. TM4 cells exhibited epithelioid characteristics and formed large overlapping lamella-like cell-cell contacts that contained a high concentration of NCAM. NCAM-rich lamellae formed from smaller NCAM patches at the ends of filopodia-like contacts between adjacent cells. Cadherins, as visualized by a pan-cadherin antibody, were present in a pattern distinctly different from that of NCAM. Although in filopodia-like contacts, both cadherins and NCAM were often concentrated at filopodial tips, in the larger lamella-like contacts that developed later, cadherins were located in an irregular punctate pattern only at the distal and more apical margins of the slanted NCAM-rich contact regions. Patterns of NCAM and microfilament (MF) bundle distribution were distinctly different, suggesting that the ends of these MF bundles were not physically linked to NCAM. By contrast, cadherins were concentrated at the ends of MF bundles at all stages of contact formation examined. Interestingly, this association of cadherins with MF bundles was mostly seen at the edge of the overlapping processes. In the lower cell process, MF bundles at the contact site were often arranged in random fashion, indicating an asymmetric distribution of MF in the junctional region. However, N-cadherin was enriched only at sites where MF bundles from both the upper and lower cell processes were aligned and terminated at the junctional membrane. Thus the organization of the actin cytoskeleton at cell-cell contact sites is influenced by the differential localization of different cadherins. These data also suggest that different mechanisms are involved in the accumulation of NCAM and cadherins in cell-cell contact regions.

Fluorescence tracing of intracellular proteins

Developments in fluorescence microscopy and the availability of fluorescently labeled antibodies and probes for localization of molecules and organelles have made the microscope an indispensable tool with which one can map specific molecules to subcellular loci allowing deep insight into cell and organelle biology. Furthermore, confocal microscopy permits analysis of the three dimensional architecture of cells that could not be accomplished by conventional light microscopy. The goal of fluorescence protein tracing by microscopy is to visualize cellular constituents and general cytoarchitecture as close to native organization as possible. To achieve this, and to preserve cellular structure in the best possible manner, the specimen is usually fixed chemically. Here I review several standard fixation, permeabilization and labeling schemes followed by examples of several standard imaging techniques.

Bicarbonate concentration and osmolality are key determinants in the inhibition of CHO cell polysialylation under elevated pCO(2) or pH

Accumulation of CO(2) in animal cell cultures can be a significant problem during scale-up and production of recombinant glycoprotein biopharmaceuticals. By examining the cell-surface polysialic acid (PSA) content, we show that elevated CO(2) partial pressure (pCO(2)) can alter protein glycosylation. PSA is a high-molecular-weight polymer attached to several complex N-linked oligosaccharides on the neural cell adhesion molecule (NCAM), so that small changes in either core glycosylation or in polysialylation are amplified and easily measured. Flow-cytometric analysis revealed that PSA levels on Chinese hamster ovary (CHO) cells decrease with increasing pCO(2) in a dose-dependent manner, independent of any change in NCAM content. The results are highly pH-dependent, with a greater decrease in PSA at higher pH. By manipulating medium pH and pCO(2), we showed that decreases in PSA correlate well with bicarbonate concentration ([HCO(3)(-)]). In fact, it was possible to offset a 60% decrease in PSA content at 120 mm Hg pCO(2) by decreasing the pH from 7.3 to 6.9, such that [HCO(3)(-)] was lowered to that of control (38 mm Hg pCO(2)). When the increase in osmolality associated with elevated [HCO(3)(-)] was offset by decreasing the basal medium [NaCl], elevated [HCO(3)(-)] still caused a decrease in PSA, although less extensive than without osmolality control. By increasing [NaCl], we show that hyperosmolality alone decreases PSA content, but to a lesser extent than for the same osmolality increase due to elevated [NaHCO(3)]. In conclusion, we demonstrate the importance of pH and pCO(2) interactions, and show that [HCO(3)(-)] and osmolality can account for the observed changes in PSA content over a wide range of pH and pCO(2) values.

Expression of NCAM recapitulates tubulogenic development in kidneys recovering from acute ischemia

Recovery of the kidney from acute renal failure relies on a sequence of events including epithelial cell dedifferentiation and proliferation followed by differentiation and restoration of the functional integrity of the nephron. The factors responsible for, and the significance of, reversion to a less differentiated cell phenotype and its relationship to the proliferative response after ischemia are poorly understood. In an attempt to identify adhesion molecules that may be influential in the recovery process, the expression of neural cell adhesion molecule (NCAM) and markers of epithelial differentiation and proliferation were analyzed at various times after an ischemic insult. In maturing nephrons, NCAM is detectable by immunohistochemistry in renal vesicles, S-shaped bodies, and early tubules. There is minimal cellular NCAM expression in normal tubules of the adult kidney. In contrast, in postischemic kidneys, NCAM expression is abundant in S3 proximal tubule cells 5 days after reperfusion. As in developing tubules, NCAM is concentrated in basal and lateral aspects of cells that have no apical gp330 or dipeptidyl peptidase IV detectable on their brush border. The expression of NCAM is preceded by disassembly of the brush border and proliferation of surviving S3 cells, which is most prominent at 2 days postischemia. NCAM expression persists in some flattened and dedifferentiated cells for up to 7 wk after ischemia. Thus proximal tubule epithelial cells of the postischemic kidney express NCAM in a pattern that recapitulates the expression of NCAM in the developing kidney. Such reversion of phenotype extends at least back to the early stages of renal vesicle formation, and this reversion may represent a critical step in the reestablishment of a normal tubule. NCAM-matrix interactions may mediate the motogenic and mitogenic responses of the dedifferentiated epithelium that are critical to reestablishment of a functional proximal tubule.

Sequence-Specific Impairment of Memory Formation by NCAM Antisense Oligonucleotides

The functional role of NCAM gene expression in memory formation was studied in the one-trial passive avoidance task in day-old chicks by pretraining injections of one of three different 18-mer end-protected oligonucleotides corresponding to positions 190-, 207-, and 332- of the NCAM Ig1 domain. Twenty-four-hour-old chicks were trained by pecking at a bitter-tasting bead and tested for avoidance 30 min, 3, 8, or 24 hr later. Memory retention was significantly reduced only in the group of animals injected with the NCAM antisense corresponding to position 207- (AS-ODN-207), and only if given twice, both immediately after hatching and 12 hr before training. This antisense was without effect on the general behavior of the chicks, training or acquisition, and did not produce observable neurotoxic damage. Under such conditions amnesia was evident by 3 hr after training and lasted until at least 24 hr after training. The two other tested oligonucleotides were without behavioral effect. To control for nonsequence-specific effects of AS-ODN-207, brains from injected and trained animals were processed for Western blotting and probed using anti-NCAM, anti-L1, and anti-actin antibodies. NCAM antisense corresponding to position 207- significantly reduced the level of NCAM, whereas the level of L1 and actin remained unchanged. These results confirm our earlier conclusion that NCAM is necessary for longer term memory retention.

Sequence-specific impairment of memory formation by NCAM antisense oligonucleotides

The functional role of NCAM gene expression in memory formation was studied in the one-trial passive avoidance task in day-old chicks by pretraining injections of one of three different 18-mer end-protected oligonucleotides corresponding to positions 190-, 207-, and 332- of the NCAM Ig1 domain. Twenty-four-hour-old chicks were trained by pecking at a bitter-tasting bead and tested for avoidance 30 min, 3, 8, or 24 hr later. Memory retention was significantly reduced only in the group of animals injected with the NCAM antisense corresponding to position 207- (AS-ODN-207), and only if given twice, both immediately after hatching and 12 hr before training. This antisense was without effect on the general behavior of the chicks, training or acquisition, and did not produce observable neurotoxic damage. Under such conditions amnesia was evident by 3 hr after training and lasted until at least 24 hr after training. The two other tested oligonucleotides were without behavioral effect. To control for nonsequence-specific effects of AS-ODN-207, brains from injected and trained animals were processed for Western blotting and probed using anti-NCAM, anti-L1, and anti-actin antibodies. NCAM antisense corresponding to position 207- significantly reduced the level of NCAM, whereas the level of L1 and actin remained unchanged. These results confirm our earlier conclusion that NCAM is necessary for longer term memory retention.

Ammonia inhibits neural cell adhesion molecule polysialylation in Chinese hamster ovary and small cell lung cancer cells

Ammonia is a major concern in biotechnology because it often limits recombinant protein production by animal cells. Conditions, such as ammonia accumulation, in large-scale production systems can parallel those that develop within fast-growing solid tumors such as small cell lung cancer (SCLC). Ammonia's specific inhibition of the sialylation of secreted glycoproteins is well documented, but it is not known how ammonia affects membrane-bound proteins, nor what role it may have on important glycosylation determinants in cancer. We therefore examined the effects of NH4Cl on polysialic acid (PolySia) in the neural cell adhesion molecule (NCAM). By using flow cytometry combined with two NCAM antibodies, one specific for the peptide backbone and another that recognizes PolySia chains, we show that ammonia causes rapid, dose-dependent, and reversible inhibition of NCAM polysialylation in Chinese hamster ovary (CHO) and SCLC NCI-N417 cells. The decrease in PolySia was accompanied by a small increase in NCAM, suggesting that the changes were specific to the oligosaccharide. Inhibition by ammonia was greater for CHO cells, with PolySia cell surface content decreasing to 10% of control after a 4-day culture with 10 mM NH4Cl, while N417 cell PolySia was reduced by only 35%. Ammonia caused a 60% decrease in the CHO cell yield from glucose, while N417 cells were barely affected, suggesting that increased resistance to ammonia by N41 7 cells is a global rather than glycosylation-specific phenomenon. The data presented show that the tumor microenvironment may be an important factor in the regulation of PolySia expression.

Studies on the teratogen pharmacophore of valproic acid analogues: evidence of interactions at a hydrophobic centre

Propyl-4-yn-valproic acid (2-propyl-4-pentynoic acid), an analogue of valproic acid with a triple bond in one alkyl side chain, potently induces exencephaly in mice. Given that propyl-4-yn-valproic acid is a branched chain carboxylic acid, we synthesized a series of analogues with n-alkyl side chains of increasing length and correlated their potential to induce neural tube defects and to inhibit proliferation and induce differentiation in cells of neural origin, the latter being crucial to the orderly structuring of the embryo. All analogues significantly increased the incidence of neural tube defects in the embryos of dams exposed to a single dose of 1.25 mmol/kg on day 8 of gestation. This effect occurred in a dose-dependent manner and the rate of exencephaly increased with the progressive increase in n-alkyl side chain length. Moreover, increasing chain length resulted in a dose-dependent inhibition of C6 glioma proliferation rate over a concentration range of 0-3 mM and this was independent of the cell type employed and mode of estimating proliferative rate. The antiproliferative action of these analogues was associated with profound shape change in neuro-2A neuroblastoma involving extensive neuritogenesis and an associated increase in neural cell adhesion molecule (NCAM) prevalence at points of cell-cell contact, the latter exhibiting a dose-dependent increase when the n-alkyl chain was extended to five carbon units. These results suggest an interaction with a specific site in which the n-alkyl side is proposed to serve as an 'anchor' within a hydrophobic pocket to facilitate the ionic and/or H-bonding of the carboxylic acid and high electron density of the carbon-carbon triple bond.

Monoclonal antibody interaction with the third immunoglobulin-like domain of N-CAM is sufficient to cause cell migration

Cellular adhesion molecules can influence a variety of biological mechanisms in the nervous system. These range from the processes of normal development and maintenance to neural plasticity and recovery following injury. The elucidation of the intricate contributions of these molecules will require the correlation of functional assays with specific molecules and the specific binding domains of such molecules with multiple signaling pathways. The data presented in this paper show that the monoclonal antibody anti-NCAM16, directed against the third immunoglobulin-like domain of the neural cell adhesion molecule N-CAM, is capable of stimulating the complex biological process of cell migration in primary embryonic motor neurons and human neuronal cell lines.

Expression cloning of the Golgi CMP-sialic acid transporter

Translocation of nucleotide sugars across the membrane of the Golgi apparatus is a prerequisite for the synthesis of complex carbohydrate structures. While specific transport systems for different nucleotide sugars have been identified biochemically in isolated microsomes and Golgi vesicles, none of these transport proteins has been characterized at the molecular level. Chinese hamster ovary (CHO) mutants of the complementation group Lec2 exhibit a strong reduction in sialylation of glycoproteins and glycolipids due to a defect in the CMP-sialic acid transport system. By complementation cloning in the mutant 6B2, belonging to the Lec2 complementation group, we were able to isolate a cDNA encoding the putative murine Golgi CMP-sialic acid transporter. The cloned cDNA encodes a highly hydrophobic, multiple membrane spanning protein of 36.4 kDa, with structural similarity to the recently cloned ammonium transporters. Transfection of a hemagglutinin-tagged fusion protein into the mutant 6B2 led to Golgi localization of the hemagglutinin epitope. Our results, together with the observation that the cloned gene shares structural similarities to other recently cloned transporter proteins, strongly suggest that the isolated cDNA encodes the CMP-sialic acid transporter.

Integrity of the homophilic binding site is required for the preferential localization of NCAM in intercellular contacts

The neural cell adhesion molecule NCAM is a member of the immunoglobulin (Ig) superfamily. NCAM can undergo homophilic binding and heterophilic interactions with cell surface components and is often concentrated at sites of intercellular contact. To investigate the molecular basis of this biased surface distribution, we examined L cell transfectants expressing wild-type or mutant forms of chick NCAM-140 by laser scanning confocal microscopy. Mutant NCAMs that lacked Ig-like domains 1, 2, 4, or 5 were preferentially localized in contact regions. However, the relative concentration of these mutant NCAMs in contact sites was substantially reduced compared with wild-type NCAM. In contrast, NCAM redistribution to intercellular contacts was abolished in cells expressing mutant NCAMs that either lacked Ig-like domain 3 or contained mutations in the homophilic binding site in this domain. In heterotypic contacts between PC12 cells and L cell transfectants, colocalization of rat NCAM and chick NCAM was again dependent on the integrity of the homophilic binding site of the NCAM expressed on L cells. These results provide evidence that homophilic binding is the main mechanism by which NCAM becomes redistributed to intercellular contacts. They also implicate a role for other Ig-like domains in the accumulation of NCAM at cell-cell contacts.

Catenin-dependent and -independent functions of vascular endothelial cadherin

Vascular endothelial cadherin (VE-cadherin, cadherin-5, or 7B4) is an endothelial specific cadherin that regulates cell to cell junction organization in this cell type. Cadherin linkage to intracellular catenins was found to be required for their adhesive properties and for localization at cell to cell junctions. We constructed a mutant form of VE-cadherin lacking the last 82 amino acids of the cytoplasmic domain. Surprisingly, despite any detectable association of this truncated VE-cadherin to catenin-cytoskeletal complex, the molecule was able to cluster at cell-cell contacts in a manner similar to wild type VE-cadherin. Truncated VE-cadherin was also able to promote calcium-dependent cell to cell aggregation and to partially inhibit cell detachment and migration from a confluent monolayer. In contrast, intercellular junction permeability to high molecular weight molecules was severely impaired by truncation of VE-cadherin cytoplasmic domain. These results suggest that the VE-cadherin extracellular domain is enough for early steps of cell adhesion and recognition. However, interaction of VE-cadherin with the cytoskeleton is necessary to provide strength and cohesion to the junction. The data also suggest that cadherin functional regulation might not be identical among the members of the family.

Molecular characterization of eukaryotic polysialyltransferase-1

Polysialic acid (PSA) is a dynamically regulated product of post-translational modification of the neural cell adhesion molecule, NCAM. Presence of the large anionic carbohydrate modulates NCAM binding properties and, by increasing the intercellular space, influences interactions between other cell surface molecules. PSA expression underlies cell type- and developmental-specific alterations and correlates with stages of cellular motility. In the adult, PSA becomes restricted to regions of permanent neural plasticity and regenerating neural and muscle tissues. Recent data implicate its important function in spatial learning and memory, and in tumour biology. Here we describe the molecular characterization of polysialyltransferase-1, the key enzyme of eukaryotic PSA synthesis. In reconstitution experiments, the newly cloned enzyme induces PSA synthesis in all NCAM-expressing cell lines. Our data therefore represent convincing evidence that the polycondensation of alpha-2,8-linked sialic acids in mammals is the result of a single enzymatic activity and provide a new basis for studying the functional role of PSA in neuro- and tumour biology.

Characterization of neural cell adhesion molecule (NCAM) expression in thyroid follicular cells: induction by cytokines and over-expression in autoimmune glands

NCAM (CD56) is a cell surface glycoprotein of the immunoglobulin superfamily expressed on neuroendocrine and natural killer (NK) cells which has considerable molecular heterogeneity due to differential splicing and post-translational modifications. NCAM has been detected in the thyroid follicular cells (thyrocytes) immunohistologically. We report here the molecular form, the modulation by cytokines and the levels of expression in thyroid pathology. By using a panel of MoAbs to NCAM on Western blots from thyrocyte extract we have determined that these cells express the 140- and 180-kD forms of NCAM. Exposure of primary cultures of thyrocytes to interferon-gamma (IFN-gamma), and even more, to the combination of IFN-gamma plus tumour necrosis factor-alpha (TNF-alpha) induced a clear increase in the expression of NCAM as assessed by FACS analysis. NCAM expression in thyrocytes was assessed by immunofluorescence in 59 surgical specimens of thyroid glands, and was found increased in 11/17 (64%) of Graves', in 5/25 (20%) of multinodular goitre (MNG) and in occasional adenoma glands. No correlation was found with the expression of HLA class I, class II or the degree of lymphocytic infiltration scored in adjacent sections, but it was often seen in areas infiltrated by macrophages. In conclusion, NCAM is an adhesion molecule whose expression is clearly increased in thyrocytes in autoimmune glands, probably as a consequence of exposure to cytokines locally released. Since one of the forms of NCAM expressed by thyrocytes has the capability to generate intracellular signal it may play a role in normal thyroid function. In addition, NCAM may facilitate the recognition of thyrocytes by lymphocytes, particularly by NK CD56+ lymphocytes.

Augmentation of adherens junction formation in mesenchymal cells by co-expression of N-CAM or short-term stimulation of tyrosine-phosphorylation

Adherens-type junctions (AJ) are specialized intercellular contacts, mediated by cadherins and characterized by the association with actin filaments through a vinculin- and catenin-rich submembrane plaque. We describe here two mechanisms which potentiate AJ formation in mesenchymal cells. These include the augmentation of AJ by the co-expression of another adhesion molecule, namely NCAM, and the stimulation of tyrosine phosphorylation. These effects were obtained in NIH-3T3 cells, which, under normal conditions, have poor cadherin- and vinculin-containing intercellular junctions. The transfection of these cells with cDNA encoding the 140kD NCAM resulted in the extensive formation of cadherin- and vinculin-rich AJ, demonstrating a cooperativity between the two junctional systems. AJ could also be induced in 3T3, and in CEF and COS cells, upon a brief exposure to H2O2/vanadate, which elevates cellular levels of phosphotyrosine due to inhibition of tyrosine-specific phosphatases. This induction was, however, transient since prolonged exposure to H2O2/vanadate resulted in an overall destruction of AJ and detachment of cells from each other and from the extracellular matrix. AJ formation appears, therefore, to be modulated by a variety of factors including the level of expression of its intrinsic components, the cooperative effect of other adhesion molecules, and by tyrosine-phosphorylation.

Reduced receptor aggregation and altered cytoskeleton in cultured myocytes after space-flight

We carried out parallel experiments first on the slow clinostat and then in space-flight to examine the effects of altered gravity on the aggregation of the nicotinic acetylcholine receptors and the structure of the cytoskeleton in cultured Xenopus embryonic muscle cells. By examining the concordance between results from space flight and the clinostat, we tested whether the slow clinostat is a relevant simulation paradigm. Space-flown cells showed marked changes in the distribution and organization of actin filaments and had a reduced incidence of acetylcholine receptor aggregates at the site of contact with polystyrene beads. Similar effects were found after clinostat rotation. The sensitivity of synaptic receptor aggregation and cytoskeletal morphology suggests that in the microgravity of space cell behavior may be importantly altered.

Co-localization of 1,4-dihydropyridine receptor alpha 2/delta subunit and N-CAM during early myogenesis in vitro

The surface distribution of the alpha 2/delta subunit of the 1,4-dihydropyridine receptor and its topographical relationship with the neural cell adhesion molecule (N-CAM) were investigated during early myogenesis in vitro, by double immunocytochemical labeling with the monoclonal antibody 3007 and an anti-N-CAM polyclonal antiserum. The monoclonal antibody 3007 has been previously shown to immunoprecipitate dihydropyridine receptor from skeletal muscle T-tubules. In further immunoprecipitation experiments on such preparations and muscle cell cultures, it was demonstrated here that the monoclonal antibody 3007 exclusively recognizes the alpha 2/delta subunit of the 1,4-dihydropyridine receptor. In rabbit muscle cell cultures, the labeling for both alpha 2/delta and N-CAM was first detected on myoblasts, in the form of spots on the membrane and perinuclear patches. Spots of various sizes organized in aggregates were then found on the membrane of myotubes. At fusion (T0), aggregates of N-CAM spots alone were found at the junction between fusing cells. At T6 and later stages, all alpha 2/delta aggregates present on myotubes co-localized with N-CAM, while less than 3% of N-CAM aggregates did not co-localize with alpha 2/delta. A uniform N-CAM staining also made its appearance. At T12, when myotubes showed prominent contractility, alpha 2/delta-N-CAM aggregates diminished in size. Dispersed alpha 2/delta spots of a small regular size spread over the whole surface of the myotubes and alignments of these spots became visible. Corresponding N-CAM spots were now occasionally seen, and uniform N-CAM staining was prominent. These results show that alpha 2/delta and N-CAM are co-localized and that their distributions undergo concomitant changes during early myogenesis until the T-tubule network starts to be organized. This suggest that these two proteins might jointly participate in morphogenetic events preceding the formation of T-tubules.

Restricted localization of L1 and N-CAM at sites of contact between Schwann cells and neurites in culture

Schwann cell-axon contacts in developing and regenerating peripheral nerve in situ contain high levels of the recognition molecules L1 and N-CAM, while the molecules are not detectable at the ab-axonal cell surface of Schwann cells. To investigate whether Schwann cells, axons, or both contribute to the localization of the molecules at Schwann cell-axon contacts, a heterologous cell culture system consisting of Schwann cells from mice and neurons from chicken was investigated by immunoelectron microscopy using species-specific L1 and N-CAM antibodies. We showed that Schwann cells expressed both molecules only at sites of contact between Schwann cells and neurites and other Schwann cells. Schwann cells not in contact with other cells expressed both molecules on their entire cell surface. In contrast, neurites expressed G4, an L1-related molecule in chicken, on their entire cell surface independently of whether they were in contact with other cells or not. Thus, cultured Schwann cells localize L1 and N-CAM selectively at cell contact sites and may thereby stabilize their attachment to the neighboring cellular partners.

Differential effects of over-expressed neural cell adhesion molecule isoforms on myoblast fusion

We have used a transfection based approach to analyze the role of neural cell adhesion molecule (NCAM) in myogenesis at the stage of myoblast fusion to form multinucleate myotubes. Stable cell lines of myogenic C2 cells were isolated that express the transmembrane 140- or 180-kD NCAM isoforms or the glycosylphosphatidylinositol (GPI) linked isoforms of 120 or 125 kD. We found that expression of the 140-kD transmembrane isoform led to a potent enhancement of myoblast fusion. The 125-kD GPI-linked NCAM also enhanced the rate of fusion but less so when a direct comparison of cell surface levels of the 140-kD transmembrane form was carried out. While the 180-kD transmembrane NCAM isoform was effective in promoting C2 cell fusion similar to the 140-kD isoform, the 120-kD isoform did not have an effect on fusion parameters. It is possible that these alterations in cell fusion are associated with cis NCAM interactions in the plane of the membrane. While all of the transfected human NCAMs (the transmembrane 140- and 180-kD isoforms and the 125- and 120-kD GPI isoforms) could be clustered in the plane of the plasma membrane by species-specific antibodies there was a concomitant clustering of the endogenous mouse NCAM protein in all cases except with the 120-kD human isoform. These studies show that different isoforms of NCAM can undergo specific interactions in the plasma membrane which are likely to be important in fusion. While the transmembrane and the 125-kD GPI-anchored NCAMs are capable of enhancing fusion the 120-kD GPI NCAM is not. Thus it is likely that interactions associated with NCAM intracellular domains and also the muscle specific domain (MSD) region in the extracellular domain of the GPI-linked 125-kD NCAM are important. In particular this is the first role ascribed to the O-linked carbohydrate containing MSD region which is specifically expressed in skeletal muscle.

Apical polarization of N-CAM in retinal pigment epithelium is dependent on contact with the neural retina

The retinal pigment epithelium (RPE) is unique among epithelia in that its apical surface does not face a lumen, but, instead, is specialized for interaction with the neural retina. The molecules involved in the interaction of the RPE with the neural retina are not known. We show here that the neural cell adhesion molecule (N-CAM) is found both on the apical surface of RPE in situ and on the outer segments of photoreceptors, fulfilling an important requisite for an adhesion role between both structures. Strikingly, culture of RPE results in rapid redistribution of N-CAM to the basolateral surface. This is not due to an isoform shift, since the N-CAM expressed by cultured cells (140 kD) is the same as that expressed by RPE in vivo. Rather, the reversed polarity of N-CAM appears to result from the disruption of the contact between the RPE and the photoreceptors of the neural retina. We suggest that N-CAM in RPE and photoreceptors participate in these interactions.

Adhesive capacity of human long-term bone marrow cultures from normals and patients with acute myeloid leukaemia: the influence of adhesion molecules

In order to study the adhesive interactions of the human bone marrow microenvironment and acute myeloid leukaemic cells, we investigated the binding capacity of KG-1 cells upon human long-term bone marrow cultures derived from 17 healthy volunteers and 12 patients with acute myeloid leukemia. Adhesion was measured using a 51-chromium labelling assay. Adhesion of KG-1 cells upon 'normal' stromal layers: 33% +/- 4.0, n = 17 (mean +/- SEM) was higher as compared to the binding to 'leukaemic' stromas: 24% +/- 3.7, n = 12 (p < 0.05). Blocking monoclonal antibodies against adhesion molecules reduced the binding of KG-1 cells upon 'normal' stroma, when anti-VLA4 (p < 0.03), anti-Mac1 (p < 0.03) and anti-p150/95 (p < 0.04) were used. Binding of KG-1 cells on 'leukaemic' stromas was partly inhibited by anti-VCAM1 (p < 0.03). Blocking achieved by single or combined antibodies was never complete, suggesting that the adhesion is a multifactorial process, including a variety of adhesion molecules and/or adhesion mechanisms.

Early encounters of the repetitive kind: a prelude to cell adhesion in conjugating Tetrahymena thermophila

The relationship between direct cell contacts and subsequent cell-cell adhesion was studied in the ciliated protozoan, Tetrahymena thermophila. During sexual reproduction, adhesion into pairs begins at approximately 1 hr after mixing starved complementary mating types. However, direct contacts between cells prior to pairing are known to be required for the development of adhesion-readiness. We find here that the initial contact interactions are necessary but not sufficient to drive the cells to adhesion-readiness. Secondary interactions are needed. Two distinct experimental strategies were used. First, we examined the effects of a mutant that is unable to pair but which can stimulate two different wild-type mating type cells to pair when mixed. We showed that stimulation by the mutant is only partial; in response to mutant cells, wild-type cells ceased forming food vacuoles but did not undergo tip transformation or concanavalin A (Con A)-receptor tipping. Further, kinetic analysis shows that when mixed together, pair-formation among partially stimulated wild-type cells is slightly delayed, allowing time for these pre-pairing processes to occur. This indicates that, beyond the initial contact interaction, mutant-stimulated wild-type cells require a subsequent interaction which cannot be fulfilled by the mutants. Secondly, we found that by blocking contact interactions between wild-type mating types at various time intervals after they were mixed, additional increase in tip transformation and Con A receptor tipping was prevented. Further, both processes underwent a regression. This indicates that multiple contact interactions are required to drive the cells to adhesion readiness and to prevent developmental slip-back.

Induction of the neural cell adhesion molecule and neuronal aggregation by osteogenic protein 1

The neural cell adhesion molecule (N-CAM) plays a fundamental role in nervous system development and regeneration, yet the regulation of the expression of N-CAM in different brain regions has remained poorly understood. Osteogenic protein 1 (OP-1) is a member of the transforming growth factor beta superfamily that is expressed in the nervous system. Treatment of the neuroblastoma-glioma hybrid cell line NG108-15 for 1-4 days with recombinant human OP-1 (hOP-1) induced alterations in cell shape, formation of epithelioid sheets, and aggregation of cells into multilayered clusters. Immunofluorescence studies and Western blots demonstrated a striking differential induction of the three N-CAM isoforms in hOP-1-treated cells. hOP-1 caused a 6-fold up-regulation of the 140-kDa N-CAM, the isoform showing the highest constitutive expression, and a 29-fold up-regulation of the 180-kDa isoform. The 120-kDa isoform was not detected in control NG108-15 cells but was readily identified in hOP-1-treated cells. Incubation of NG108-15 cells with an antisense N-CAM oligonucleotide reduced the induction of N-CAM by hOP-1 and decreased the formation of multilayered cell aggregates. Anti-N-CAM monoclonal antibodies also diminished the formation of multilayered cell aggregates by hOP-1 and decreased cell-cell adhesion when hOP-1-treated NG108-15 cells were dispersed and replated. Thus, hOP-1 produces morphologic changes in NG108-15 cells, at least in part, by inducing N-CAM. These observations suggest that OP-1 or a homologue may participate in the regulation of N-CAM during nervous system development and regeneration.