Cellular site of synthesis and dynamics of cell surface re-expression of polysialic acid of the neural cell adhesion molecule

Neural glycomics: the sweet side of nervous system functions

The success of investigations on the structure and function of the genome (genomics) has been paralleled by an equally awesome progress in the analysis of protein structure and function (proteomics). We propose that the investigation of carbohydrate structures that go beyond a cell's metabolism is a rapidly developing frontier in our expanding knowledge on the structure and function of carbohydrates (glycomics). No other functional system appears to be suited as well as the nervous system to study the functions of glycans, which had been originally characterized outside the nervous system. In this review, we describe the multiple studies on the functions of LewisX, the human natural killer cell antigen-1 (HNK-1), as well as oligomannosidic and sialic (neuraminic) acids. We attempt to show the sophistication of these structures in ontogenetic development, synaptic function and plasticity, and recovery from trauma, with a view on neurodegeneration and possibilities to ameliorate deterioration. In view of clinical applications, we emphasize the need for glycomimetic small organic compounds which surpass the usefulness of natural glycans in that they are metabolically more stable, more parsimonious to synthesize or isolate, and more advantageous for therapy, since many of them pass the blood brain barrier and are drug-approved for treatments other than those in the nervous system, thus allowing a more ready access for application in neurological diseases. We describe the isolation of such mimetic compounds using not only Western NIH, but also traditional Chinese medical libraries. With this review, we hope to deepen the interests in this exciting field.

Expression of polysialic acid in primary laryngeal squamous cell carcinoma

AIMS

Expression of polySia is associated with metastatic dissemination and progression of various malignant diseases. In particular, it may contribute to tumorigenesis by a negative modulatory effect on cellular signaling cascades responsible for cellular migration, differentiation and proliferation. In this study, we investigated the expression of polySia in primary metastatic and non-metastatic laryngeal squamous cell carcinoma (LSCC) tumor tissues and its potential impact on the LSCC progression.

MAIN METHODS

The expression of polySia in metastatic and non-metastatic primary laryngeal squamous cell carcinoma (LSCC) tumor biopsy specimens was investigated by immunohistochemistry, while the expression of polysialyltransferase IV (ST8SiaIV)(), fibroblast growth factor receptor 1 (FGFR1), extracellular signal regulated kinases 1 and 2 (Erk 1/2) and c-Raf was tested in metastatic and non-metastatic primary tumor tissues (including the corresponding non-tumor control tissues) by Western blot analysis.

KEY FINDINGS

The expression of polySia was detected in LSCC biopsies specimens with generally stronger immunoreactivity in non-metastatic tumor LSCC sections and in histologically undifferentiated tumors. Also, increased polySia expression was observed in adjacent histologically unaltered laryngeal tumor-associated tissue of the metastatic sections. In addition, we provide an evidence of increased polysialyltransferase IV (ST8SiaIV) expression, involved in polySia synthesis in both metastatic and non-metastatic primary tumors which is accompanied by decreased levels of FGFR1, Erk 1/2 and c-Raf.

SIGNIFICANCE

We present for the first time the evidence for the polySia expression in LSCC biopsies specimens which suggests its potential impact on initial steps of LSCC malignant transformation.

MALE GAMETOPHYTE DEFECTIVE 2, encoding a sialyltransferase-like protein, is required for normal pollen germination and pollen tube growth in Arabidopsis

Sialyltransferases (SiaTs) exist widely in vertebrates and play important roles in a variety of biological processes. In plants, several genes have also been identified to encode the proteins that share homology with the vertebrate SiaTs. However, very little is known about their functions in plants. Here we report the identification and characterization of a novel Arabidopsis gene, MALE GAMETOPHYTE DEFECTIVE 2 (MGP2) that encodes a sialyltransferase-like protein. MGP2 was expressed in all tissues including pollen grains and pollen tubes. The MGP2 protein was targeted to Golgi apparatus. Knockout of MGP2 significantly inhibited the pollen germination and retarded pollen tube growth in vitro and in vivo, but did not affect female gametophytic functions. These results suggest that the sialyltransferase-like protein MGP2 is important for normal pollen germination and pollen tube growth, giving a novel insight into the biological roles of the sialyltransferase-like proteins in plants.

In vivo activation of channelrhodopsin-2 reveals that normal patterns of spontaneous activity are required for motoneuron guidance and maintenance of guidance molecules

Spontaneous, highly rhythmic episodes of propagating bursting activity are present early during the development of chick and mouse spinal cords. Acetylcholine, and GABA and glycine, which are both excitatory at this stage, provide the excitatory drive. It was previously shown that a moderate decrease in the frequency of bursting activity, caused by in ovo application of the GABA(A) receptor blocker, picrotoxin, resulted in motoneurons making dorsal-ventral (D-V) pathfinding errors in the limb and in the altered expression of guidance molecules associated with this decision. To distinguish whether the pathfinding errors were caused by perturbation of the normal frequency of bursting activity or interference with GABA(A) receptor signaling, chick embryos were chronically treated in ovo with picrotoxin to block GABA(A) receptors, while light activation by channelrhodopsin-2 was used to restore bursting activity to the control frequency. The restoration of normal patterns of neural activity in the presence of picrotoxin prevented the D-V pathfinding errors in the limb and maintained the normal expression levels of EphA4, EphB1, and polysialic acid on neural cell adhesion molecule, three molecules previously shown to be necessary for this pathfinding choice. These observations demonstrate that developing spinal motor circuits are highly sensitive to the precise frequency and pattern of spontaneous activity, and that any drugs that alter this activity could result in developmental defects.

Transcriptional regulation of PSA-NCAM expression by NMDA receptor activation in RA-differentiated C6 glioma cultures

N-Methyl-d-aspartate (NMDA) receptors exhibit a dichotomy of signaling with both toxic and plastic responses. Recent reports have shown that exposure to subtoxic concentration of NMDA results in a neuroprotective state that was measured when these neurons were subsequently challenged with toxic doses of glutamate or kainate. Control of polysialylated neural cell adhesion molecule (PSA-NCAM) expression by NMDA receptor activation has been described in several systems, suggesting a functional link between these two proteins. The perception of glial role in CNS function has changed dramatically over the past few years from simple trophic functions to that of cells with important roles in development and maintenance of CNS in cooperation with neurons. We report here the transcriptional regulation of PSA-NCAM expression by subtoxic dose of NMDA in retinoic acid differentiated C6 glioma cell cultures. C6 glioma cell cultures differentiated with retinoic acid (10microM) were exposed to NMDA (100microM) or to antagonist MK-801 (200nM) prior to treatment with NMDA and cells were harvested after 24h of treatment to study the expression of total NCAM, PSA-NCAM, nuclear factor kappaB (NF-kappaB) and activator protein-1 (AP-1) by Western blotting and dual immunocytofluorescence and expression of PST mRNA by fluorescent in situ hybridization (FISH). Significant increase in the levels of PSA-NCAM, NF-kappaB, AP-1 and PST mRNA was observed in NMDA treated cultures. Treatment of cultures with MK-801, a non-competitive NMDA receptor antagonist, prior to NMDA exposure prevented the NMDA-mediated changes indicating the involvement of NMDA receptor activation. The results elucidate the possible cellular and molecular mechanisms of regulation of PSA-NCAM expression in astroglial cultures by extracellular signals.

Polysialic acid and activity-dependent synapse remodeling

Polysialic acid (PSA) is a large carbohydrate added post-translationally to the extracellular domain of the Neural Cell Adhesion Molecule (NCAM) that influences its adhesive and other functional properties. PSA-NCAM is widely distributed in the developing nervous system where it promotes dynamic cell interactions, like those responsible for axonal growth, terminal sprouting and target innervation. Its expression becomes restricted in the adult nervous system where it is thought to contribute to various forms of neuronal and glial plasticity. We here review evidence, obtained mainly from hypothalamic neuroendocrine centers and the olfactory system, that it intervenes in structural synaptic plasticity and accompanying neuronal-glial transformations, making possible the formation and elimination of synapses that occur under particular physiological conditions. While the mechanism of action of this complex sugar is unknown, it is now clear that it is a necessary molecular component of various cell transformations, including those responsible for activity-dependent synaptic remodeling.

The polysialylated neural cell adhesion molecule reaches cell surfaces of hypothalamic neurons and astrocytes via the constitutive pathway

Understanding how neurons and glia sort and deliver cell adhesion molecules to their cell surface should provide important clues as to how such molecules participate in dynamic neuronal functions in the developing and adult brain. The present study examines translocation of polysialylated neural cell adhesion molecule (PSA-NCAM), a negative regulator of cell adhesion, in cells of the rat hypothalamo-neurohypophysial system in which it is expressed throughout life and which undergo morphological remodelling in response to stimulation. PSA-NCAM expression in this system does not vary markedly in relation to different conditions of regulated neurosecretion, suggesting that the glycoprotein reaches cell surfaces via the constitutive pathway. To study this more directly, we here used immunofluorescence for PSA on NCAM in live, unpermeabilized cells to monitor PSA-NCAM surface expression in organotypic slice cultures from postnatal rat hypothalami. Subsequent immunolabelling for oxytocin confirmed that the cultures included magnocellular oxytocinergic neurons displaying many properties of adult neurosecretory neurons in situ. In the cultures, immunoreaction for PSA-NCAM was visible on the surface of oxytocinergic and non-oxytocinergic axons. This reaction disappeared after exposure of the cultures to endoneuraminidase, an enzyme which specifically cleaves alpha-2-8-linked PSA from NCAM. PSA-NCAM reappeared on axonal surfaces 4h after enzyme washout. Such reexpression was visibly not affected by neuronal activity inhibition (blockade of Ca(2+) channels with Mn(2+), of Na(+) channels with tetrodotoxin, or of glutamate receptors with 6-cyano-7-nitroquinoxaline-2,3-dione or D-2-amino-5-phosphonopentanoic acid) or facilitation (K(+) depolarization or GABA-A receptor blockade with bicuculline). In contrast, PSA-NCAM surface translocation was inhibited reversibly by cooling the cultures at 20 degrees C, a procedure which blocks constitutive secretion and which resulted in accumulation of PSA-NCAM in the cytoplasm of oxytocinergic and non-oxytocinergic neurons. This treatment also revealed PSA-NCAM in the cytoplasm of underlying astrocytes. Our observations provide direct evidence that PSA-NCAM reaches the cell surface of hypothalamic neurons and astrocytes via the constitutive pathway, independently of Ca(2+) entry and enhanced neuronal activity. Thus, PSA-NCAM in the hypothalamo-neurohypophysial system would be continuously available to permit its cells to undergo remodelling whenever the proper stimulus intervenes.

Protein kinase C delta regulates neural cell adhesion molecule polysialylation state in the rat brain

Polysialylation of neural cell adhesion molecule (NCAM PSA) modulates cell-cell homophilic binding and signalling during brain development and the remodelling of discrete brain regions in the adult. Following learning, a transient increase in the frequency of polysialylated neurones occurs in the dentate gyrus of the hippocampal formation, and this has been correlated with the selective retention and/or elimination of synapses that are transiently overproduced during memory consolidation. We now demonstrate that protein kinase C delta (PKCdelta) negatively regulates polysialyltransferase activity in the rat brain during development and also in the hippocampus during memory consolidation, where its down-regulation in the Golgi membrane fraction coincides with the transient increase in NCAM PSA expression. Decreased expression of PKCdelta was also observed in the hippocampus of rats reared in a complex environment and this directly contrasted the significant increase in frequency of hippocampal polysialylated neurones observed in these animals. These effects were isoform-specific as no change in total PKC enzyme activity was detected during memory consolidation and complex environment rearing had no effect on the hippocampal expression of PKCalpha, beta, gamma or epsilon. By sequential immunoprecipitation and immunoblot analysis, phosphorylation of polysialyltransferase protein(s) was (were) demonstrated to occur on both serine and tyrosine residues and this was associated with decreased enzyme activity. Moreover, a similar experimental approach revealed the degree of PKCdelta co-precipitation with polysialyltransferase protein(s) to be inversely correlated with polysialyltransferase activity. These findings support in vitro evidence indicating PKCdelta to regulate polysialyltransferase activity and NCAM polysialylation state.

Retinoic acid-induced changes in polysialyltransferase mRNA expression and NCAM polysialylation in human neuroblastoma cells

Polysialic acid (PSA) is a dynamically regulated carbohydrate modification of the neural cell adhesion molecule NCAM, which is implicated in neural differentiation and cellular plasticity. The cloning and characterization of two polysialyltransferases, termed ST8SiaII (STX) and ST8SiaIV (PST), opened up new perspectives in the search for factors that control this unique cell surface glycosylation. In vitro and transfection approaches revealed that ST8SiaII and ST8SiaIV are independently capable of synthesizing PSA on NCAM with slightly different specificities towards the major NCAM isoforms and glycosylation sites. Their overlapping but distinct expression patterns during brain development point towards an independent transcriptional regulation. However, the factors driving their joint or distinct expression, as well as the significance of divergent expression patterns in vivo, are not yet understood. In the present study, the mRNA expression of ST8SiaII and ST8SiaIV was comparatively analyzed in neuronal differentiation of PSA-positive human neuroblastoma cell lines induced by retinoic acid (RA), phorbolester, or growth factors. Using a semiquantitative RT-PCR strategy, we demonstrated a general decrease in the mRNA level of ST8SiaII upon differentiation of SH-SY5Y and LAN-5 cells. In contrast, a drastic increase of ST8SiaIV was specifically induced by RA-treatment of SH-SY5Y cells. To explore the significance of these changes, the cellular capacity to perform PSA synthesis and the degree of NCAM polysialylation were analyzed. Our data indicate that the increased expression of ST8SiaIV enables an accelerated polysialylation of NCAM, which, however, is not converted into higher amounts of PSA.

Control of NCAM polysialylation by the differential expression of polysialyltransferases ST8SiaII and ST8SiaIV

Polysialic acid (PSA) is a developmentally regulated carbohydrate consisting of alpha-2,8-linked sialic acid residues attached to the neural cell adhesion molecule NCAM. PSA promotes plasticity of cell-cell interactions in the nervous system and appears linked to the malignant potential of several tumors. Two enzymes, the polysialyltransferases ST8SiaII (STX) and ST8SiaIV (PST) have been identified and shown to be independently able to synthesize PSA. However, in vivo studies have demonstrated that in the majority of PSA-positive tissues the two polysialyltransferases are expressed simultaneously. Therefore, this study was undertaken to elucidate in which way the individual enzymes contribute to PSA expression under in vivo conditions. Using a semiquantitative RT-PCR strategy PSA-positive human tumor cell lines were screened for expression of ST8SiaII and ST8SiaIV at the mRNA level. Divergent patterns observed in some cell lines suggest that polysialyltransferases are independently regulated at the transcriptional level. In subsequent analyses the different mRNA levels of ST8SiaII and ST8SiaIV in these tumor cells were correlated with the degree of PSA expression and the cellular capacity to rapidly synthesize PSA. Our data indicate that ST8SiaIV is the major regulator of NCAM polysialylation in vivo.

Alcohol exposure alters the expression pattern of neural cell adhesion molecules during brain development

Neural cell adhesion molecules (NCAMs) play critical roles during development of the nervous system. The aim of this study is to investigate the possible effect of ethanol exposure on the pattern of expression and sialylation of NCAM isoforms during postnatal rat brain development because alterations in NCAM content and distribution have been associated with defects in cell migration, synapse formation, and memory consolidation, and deficits in these processes have been observed after in utero alcohol exposure. The expression of NCAM isoforms in the developing cerebral cortex of pups from control and alcohol-fed mothers was assessed by western blotting, ribonuclease protection assay, and immunocytochemistry. The highly sialylated form of NCAM [polysialic acid (PSA)-NCAM] is mainly expressed during the neonatal period and then is down-regulated in parallel with the appearance of NCAM 180 and NCAM 140. Ethanol exposure increases PSA-NCAM levels during the neonatal period, delays the loss of PSA-NCAM, decreases the amount of NCAM 180 and NCAM 140 isoforms, and reduces sialyltransferase activity during postnatal brain development. Neuraminidase treatment of ethanol-exposed neonatal brains leads to more intense band degradation products, suggesting a higher content of NCAM polypeptides carrying PSA in these samples. However, NCAM mRNA levels are not changed by ethanol. Immunocytochemical analysis demonstrates that ethanol triggers an increase in PSA-NCAM immunolabeling in the cytoplasm of astroglial cells, accompanied by a decrease in immunogold particles over the plasma membrane. These findings indicate that ethanol exposure during brain development alters the pattern of NCAM expression and suggest that modification of NCAM could affect neuronal-glial interactions that might contribute to the brain defects observed after in utero alcohol exposure.

Differential expression of two adhesion molecules of the immunoglobulin superfamily, F3 and polysialylated NCAM, in hypothalamic magnocellular neurones capable of plasticity

The adult hypothalamo-neurohypophysial system undergoes activity-dependent, reversible morphological changes which result in reduced astrocytic coverage of its neurones and an increase in their synaptic contacts. Our recent observations show that neurones and glia of the hypothalamo-neurohypophysial system continue to express 'embryonic' molecular features which may underlie their capacity to undergo such plasticity. These include expression of cell surface molecules like the glycosyl phosphatidyl inositol (GPI)-linked glycoprotein F3, which intervenes in axonal outgrowth, and the polysialylated isoform of the neural cell adhesion molecule (PSA-NCAM), which reduces cell adhesion and promotes dynamic cell interactions. F3 is colocalised with vasopressin and oxytocin hormones in neurosecretory granules and follows an activity-dependent, regulated pathway for surface expression on neurohypophysial axons. In contrast, PSA-NCAM appears to follow a constitutive pathway, independent of the activity of the hypothalamo-neurohypophysial system, for expression on axonal and glial surfaces, in the hypothalamic magnocellular nuclei and in the neurohypophysis. The role of F3 remains to be determined but in view of its presumptive functions during development, we propose that it promotes remodelling of neurosecretory terminals. On the other hand, we provide direct evidence that surface expression of PSA on NCAM is essential to morphological plasticity since its specific enzymatic degradation in vivo inhibited the neuronal-glial and synaptic changes normally induced by stimulation of secretion from the hypothalamo-neurohypophysial system.

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.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

Cell biology of polysialic acid

The unusual carbohydrate polysialic acid (PSA), attached uniquely to neural cell adhesion molecule (NCAM) through a developmentally regulated process, modulates neural cell interactions. Major advances in the past two years have increased our understanding of PSA biosynthesis and regulation. Of particular interest is the cloning of the genes encoding polysialyltransferases (PSTs) and the finding that a single enzyme is able to confer polysialylation to NCAM. The electrical activity of neurons and transmembrane signalling are probably major players in controlling both PSA biosynthesis and its expression at the cell surface. A direct causal relationship between PSA expression and activity-induced synaptic plasticity has been reported.

Polysialic acid and the regulation of cell interactions

Polysialic acid, a unique glycosylation of the neural cell adhesion molecule, is highly regulated in its expression. Its function is manifested in the modulation of cell interactions, probably through its unusual physical properties. Recent advances have clarified the enzymatic mechanism of polysialic acid biosynthesis, expanded its role in cell migration and axon guidance, and suggested that it promotes plasticity in the adult nervous system.

Protein glycosylation in the endoplasmic reticulum and the Golgi apparatus and cell type-specificity of cell surface glycoconjugate expression: analysis by the protein A-gold and lectin-gold techniques

High resolution immunolabeling applying the protein A-gold technique and carbohydrate cytochemistry using lectin-gold labeling on Lowicryl K4M and thawed-frozen thin sections are most useful approaches for the detection of protein antigens and lectin binding sites in intracellular organelles and the plasma membrane. They provided the basis for modern electron microscopic studies on protein glycosylation reactions and the identification of their subcellular localization as reviewed here. These studies have demonstrated organelle subcompartments and the cell type-specific compartmentation of endoplasmic reticulum and Golgi apparatus-associated glycosylation reactions. The other subject reviewed in this paper is cell surface glycoconjugates, as they are expressed in relation to specific cell types present in various organs and during cellular differentiation processes.