Cleavage of the polysialosyl units of brain glycoproteins by a bacteriophage endosialidase. Involvement of a long oligosaccharide segment in molecular interactions of polysialic acid

Polysialylation controls immune function of myeloid cells in murine model of pneumococcal pneumonia

Polysialic acid (polySia) is a post-translational modification of a select group of cell-surface proteins that guides cellular interactions. As the overall impact of changes in expression of this glycan on leukocytes during infection is not known, we evaluate the immune response of polySia-deficient ST8SiaIV-/- mice infected with Streptococcus pneumoniae (Spn). Compared with wild-type (WT) mice, ST8SiaIV-/- mice are less susceptible to infection and clear Spn from airways faster, with alveolar macrophages demonstrating greater viability and phagocytic activity. Leukocyte pulmonary recruitment, paradoxically, is diminished in infected ST8SiaIV-/- mice, corroborated by adoptive cell transfer, microfluidic migration experiments, and intravital microscopy, and possibly explained by dysregulated ERK1/2 signaling. PolySia is progressively lost from neutrophils and monocytes migrating from bone marrow to alveoli in Spn-infected WT mice, consistent with changing cellular functions. These data highlight multidimensional effects of polySia on leukocytes during an immune response and suggest therapeutic interventions for optimizing immunity.

Improved methods to characterize the length and quantity of highly unstable PolySialic acids subject category: (Carbohydrates, chromatographic techniques)

Polysialic acid (polySia) is a linear homopolymer of α2-8-linked sialic acids that is highly expressed during early stages of mammalian brain development and modulates a multitude of cellular functions. While degree of polymerization (DP) can affect such functions, currently available methods do not accurately characterize this parameter, because of the instability of the polymer. We developed two improved methods to characterize the DP and total polySia content in biological samples. PolySia chains with exposed reducing termini can be derivatized with DMB for subsequent HPLC analysis. However, application to biological samples of polySia-glycoproteins requires release of polySia chains from the underlying glycan, which is difficult to achieve without concurrent partial hydrolysis of the α2-8-linkages of the polySia chain, affecting its accurate characterization. We report an approach to protect internal α2-8sia linkages of long polySia chains, using previously known esterification conditions that generate stable polylactone structures. Such polylactonized molecules are more stable during acid hydrolysis release and acidic DMB derivatization. Additionally, we used the highly specific Endoneuraminidase-NF enzyme to discriminate polysialic acid and other sialic acid and developed an approach to precisely measure the total content of polySia in a biological sample. These two methods provide improved quantification and characterization of polySia.

Characterization of the Polysialylation Status in Ovaries of the Salmonid Fish Coregonus maraena and the Percid Fish Sander lucioperca

In vertebrates, the carbohydrate polymer polysialic acid (polySia) is especially well known for its essential role during neuronal development, regulating the migration and proliferation of neural precursor cells, for instance. Nevertheless, sialic acid polymers seem to be regulatory elements in other physiological systems, such as the reproductive tract. Interestingly, trout fish eggs have polySia, but we know little of its cellular distribution and role during oogenesis. Therefore, we localized α2,8-linked N-acetylneuraminic acid polymers in the ovaries of Coregonus maraena by immunohistochemistry and found that prevalent clusters of oogonia showed polySia signals on their surfaces. Remarkably, the genome of this salmonid fish contains two st8sia2 genes and one st8sia4 gene, that is, three polysialyltransferases. The expression analysis revealed that for st8sia2-r2, 60 times more mRNA was present than st8sia2-r1 and st8sia4. To compare polysialylation status regarding various polySiaT configurations, we performed a comparable analysis in Sander lucioperca. The genome of this perciform fish contains only one st8sia2 and no st8sia4 gene. Here, too, clusters of oogonia showed polysialylated cell surfaces, and we detected high mRNA values for st8sia2. These results suggest that in teleosts, polySia is involved in the cellular processes of oogonia during oogenesis.

Recent advances in the analysis of polysialic acid from complex biological systems

Polysialic acid (polySia) is a unique, well-characterised carbohydrate polymer highly-expressed on the cell surface of neurons in the early stages of mammalian brain development. Post-embryogenesis, it is also re-expressed in a number of tumours of neuroendocrine origin. It plays important roles in modulating cell-cell, and cell-matrix adhesion and migration, tumour invasion and metastasis. Techniques for structural and quantitative characterisation of polySia from tumours and cancer cells are thus essential in exploring the relationship between polySia expression levels and structural and functional changes associated with cancer progression and metastasis. A variety of techniques have been developed to structurally and quantitatively analyse polySia in clinical tissues and other biological samples. In this review, analytical approaches used for the determination of polySia in biological matrices in the past 20 years are discussed, with a particular focus on chemical approaches, and quantitative analysis.

Recent approaches for directly profiling cell surface sialoform

Sialic acids (SAs) are nine-carbon monosaccharides existing at the terminal location of glycan structures on the cell surface and secreted glycoconjugates. The expression levels and linkages of SAs on cells and tissues, collectively known as sialoform, present the hallmark of the cells and tissues of different systems and conditions. Accordingly, detecting or profiling cell surface sialoforms is very critical for understanding the function of cell surface glycans and glycoconjugates and even the molecular mechanisms of their underlying biological processes. Further, it may provide therapeutic and diagnostic applications for different diseases. In the past decades, several kinds of SA-specific binding molecules have been developed for detecting and profiling specific sialoforms of cells and tissues; the experimental materials have expanded from frozen tissue to living cells; and the analytical technologies have advanced from histochemistry to fluorescent imaging, flow cytometry and microarrays. This review summarizes the recent bioaffinity approaches for directly detecting and profiling specific SAs or sialylglycans, and their modifications of different cells and tissues.

Enzymatic Synthesis of 6'-Sialyllactose, a Dominant Sialylated Human Milk Oligosaccharide, by a Novel exo-α-Sialidase from Bacteroides fragilis NCTC9343

Gut bacteria provide a rich source of glycosidases that can recognize and/or hydrolyze glycans for nutrition. Interestingly, some glycosidases have also been found to catalyze transglycosylation reactions in vitro and thus can be used for oligosaccharide synthesis. In this work, six putative and one known exo-α-sialidase genes-three from Bacteroides fragilis NCTC9343, three from Clostridium perfringens ATCC 13124, and one known from Bifidobacterium bifidum JCM1254-were subjected to gene cloning and heterogeneous expression in Escherichia coli The recombinant enzymes were purified, characterized for substrate specificity, and screened for transglycosylation activity. A sialidase, named BfGH33C, from B. fragilis NCTC9343 was found to possess excellent transglycosylation activity for the synthesis of sialylated human milk oligosaccharide. The native BfGH33C was a homodimer with a molecular weight of 113.6 kDa. The K_m and k_cat values for 4-methylumbelliferyl N-acetyl-α-d-neuraminic acid and sialic acid dimer were determined to be 0.06 mM and 283.2 s^-1, and 0.75 mM and 329.6 s^-1, respectively. The enzyme was able to transfer sialyl from sialic acid dimer or oligomer to lactose with high efficiency and strict α2-6 regioselectivity. The influences of the initial substrate concentration, pH, temperature, and reaction time on transglycosylation were investigated in detail. Using 40 mM sialic acid dimer (or 40 mg/ml oligomer) and 1 M lactose (pH 6.5) at 50°C for 10 min, BfGH33C could specifically produce 6'-sialyllactose, a dominant sialylated human milk oligosaccharide, at a maximal conversion ratio above 20%. It provides a promising alternative to the current chemical and enzymatic methods for obtaining sialylated oligosaccharides.IMPORTANCE Sialylated human milk oligosaccharides are significantly beneficial to the neonate, as they play important roles in supporting resistance to pathogens, gut maturation, immune function, and brain and cognitive development. Therefore, access to the sialylated oligosaccharides has attracted increasing attention both for the study of saccharide functions and for the development of infant formulas that could mimic the nutritional value of human milk. Nevertheless, nine-carbon sialic acids are rather complicated for the traditional chemical modifications, which require multiple protection and deprotection steps to achieve a specific glycosidic bond. Here, the exo-α-sialidase BfGH33C synthesized 6'-sialyllactose in a simple step with high transglycosylation activity and strict regioselectivity. Additionally, it could utilize oligosialic acid, which was newly prepared in an easy, economical way to reduce the substrate cost, as a glycosyl donor. All the studies laid a foundation for the practical use of BfGH33C in large-scale synthesis of sialylated oligosaccharides in the future.

Internalization of a polysialic acid-binding Escherichia coli bacteriophage into eukaryotic neuroblastoma cells

Eukaryotic organisms are continuously exposed to bacteriophages, which are efficient gene transfer agents in bacteria. However, bacteriophages are considered not to pass the eukaryotic cell membrane and enter nonphagocytic cells. Here we report the binding and penetration of Escherichia coli PK1A2 bacteriophage into live eukaryotic neuroblastoma cells in vitro. The phage interacts with cell surface polysialic acid, which shares structural similarity with the bacterial phage receptor. Using fluorescence and electron microscopy, we show that phages are internalized via the endolysosomal route and persist inside the human cells up to one day without affecting cell viability. Phage capsid integrity is lost in lysosomes, and the phage DNA is eventually degraded. We did not detect the entry of phage DNA into the nucleus; however, we speculate that this might occur as a rare event, and propose that this potential mechanism could explain prokaryote–eukaryote gene flow.

Eukaryotic organisms are continuously exposed to bacteriophages, but these are not thought to enter non-phagocytic cells. Here, Lehti et al. show that a bacteriophage can bind to a specific receptor on the surface of human neuroblastoma cells in vitro, and be internalized via the endolysosomal route.

Intraventricular administration of endoneuraminidase-N facilitates ectopic migration of subventricular zone-derived neural progenitor cells into 6-OHDA lesioned striatum of mice

Polysialic acid (PSA), a carbohydrate polymer associated with the neural cell adhesion molecule (NCAM), plays an important role in the migration, differentiation and maturation of neuroblasts. Endoneuraminidase-N (Endo-N) can specifically cleave PSA from NCAM. The objective of the present study was to examine: the effect of Endo-N on characteristics of subventricular zone (SVZ)-derived neural progenitor cells (NPCs) in vitro; whether intraventricular administration of Endo-N could increase ectopic migration of SVZ-derived NPCs into 6-hydroxydopamine (6-OHDA)-lesioned striatum, and whether migrated NPCs could differentiate into neuronal and glial cells. In in vitro study, Endo-N was found to inhibit the migration of NPCs, and to enhance the differentiation of NPCs. In in vivo study, mice sequentially received injections of 6-OHDA into the right striatum, Endo-N into the right lateral ventricle, and bromodeoxyuridine (BrdU) intraperitoneally. The data showed that intraventricular injections of Endo-N disorganized the normal structure of the rostral migratory stream (RMS), and drastically increased the number of BrdU-immunoreactive (IR) cells in 6-OHDA-lesioned striatum. In addition, a number of BrdU-IR cells were double labeled for doublecortin (DCX), NeuN or glial fibrillary acidic protein (GFAP). The results suggest that interruption of neuroblast chain pathway with Endo-N facilitates ectopic migration of SVZ-derived NPCs into the lesioned striatum, and migrated NPCs can differentiate into neurons and astrocytes.

The effect of Young's modulus on the neuronal differentiation of mouse embryonic stem cells

There is substantial evidence that cells produce a diverse response to changes in ECM stiffness depending on their identity. Our aim was to understand how stiffness impacts neuronal differentiation of embryonic stem cells (ESC's), and how this varies at three specific stages of the differentiation process. In this investigation, three effects of stiffness on cells were considered; attachment, expansion and phenotypic changes during differentiation. Stiffness was varied from 2 kPa to 18 kPa to finally 35 kPa. Attachment was found to decrease with increasing stiffness for both ESC's (with a 95% decrease on 35 kPa compared to 2 kPa) and neural precursors (with a 83% decrease on 35 kPa). The attachment of immature neurons was unaffected by stiffness. Expansion was independent of stiffness for all cell types, implying that the proliferation of cells during this differentiation process was independent of Young's modulus. Stiffness had no effect upon phenotypic changes during differentiation for mESC's and neural precursors. 2 kPa increased the proportion of cells that differentiated from immature into mature neurons. Taken together our findings imply that the impact of Young's modulus on attachment diminishes as neuronal cells become more mature. Conversely, the impact of Young's modulus on changes in phenotype increased as cells became more mature.

Chemical biology approaches to designing defined carbohydrate vaccines

Carbohydrate antigens have shown promise as important targets for developing effective vaccines and pathogen detection strategies. Modifying purified microbial glycans through synthetic routes or completely synthesizing antigenic motifs are attractive options to advance carbohydrate vaccine development. However, limited knowledge on structure-property correlates hampers the discovery of immunoprotective carbohydrate epitopes. Recent advancements in tools for glycan modification, high-throughput screening of biological samples, and 3D structural analysis may facilitate antigen discovery process. This review focuses on advances that accelerate carbohydrate-based vaccine development and various technologies that are driving these efforts. Herein we provide a critical overview of approaches and resources available for rational design of better carbohydrate antigens. Structurally defined and fully synthetic oligosaccharides, designed based on molecular understanding of antigen-antibody interactions, offer a promising alternative for developing future carbohydrate vaccines.

Polysialic acid: biosynthesis, novel functions and applications

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

Sialic acids in the brain: gangliosides and polysialic acid in nervous system development, stability, disease, and regeneration

Every cell in nature carries a rich surface coat of glycans, its glycocalyx, which constitutes the cell's interface with its environment. In eukaryotes, the glycocalyx is composed of glycolipids, glycoproteins, and proteoglycans, the compositions of which vary among different tissues and cell types. Many of the linear and branched glycans on cell surface glycoproteins and glycolipids of vertebrates are terminated with sialic acids, nine-carbon sugars with a carboxylic acid, a glycerol side-chain, and an N-acyl group that, along with their display at the outmost end of cell surface glycans, provide for varied molecular interactions. Among their functions, sialic acids regulate cell-cell interactions, modulate the activities of their glycoprotein and glycolipid scaffolds as well as other cell surface molecules, and are receptors for pathogens and toxins. In the brain, two families of sialoglycans are of particular interest: gangliosides and polysialic acid. Gangliosides, sialylated glycosphingolipids, are the most abundant sialoglycans of nerve cells. Mouse genetic studies and human disorders of ganglioside metabolism implicate gangliosides in axon-myelin interactions, axon stability, axon regeneration, and the modulation of nerve cell excitability. Polysialic acid is a unique homopolymer that reaches >90 sialic acid residues attached to select glycoproteins, especially the neural cell adhesion molecule in the brain. Molecular, cellular, and genetic studies implicate polysialic acid in the control of cell-cell and cell-matrix interactions, intermolecular interactions at cell surfaces, and interactions with other molecules in the cellular environment. Polysialic acid is essential for appropriate brain development, and polymorphisms in the human genes responsible for polysialic acid biosynthesis are associated with psychiatric disorders including schizophrenia, autism, and bipolar disorder. Polysialic acid also appears to play a role in adult brain plasticity, including regeneration. Together, vertebrate brain sialoglycans are key regulatory components that contribute to proper development, maintenance, and health of the nervous system.

Changes in polysialic acid expression on myeloid cells during differentiation and recruitment to sites of inflammation: role in phagocytosis

Polysialic acid (polySia) is a unique linear homopolymer of α2,8-linked sialic acid that has been studied extensively as a posttranslational modification of neural cell adhesion molecule in the central nervous system. Only two proteins are known to be polysialylated in cells of the immune system: CD56 on human natural killer cells and murine bone marrow (BM) leukocytes, and neuropilin-2 (NRP-2) on dendritic cells (DCs). We tested the hypothesis that polySia expression is regulated during maturation and migration of leukocytes and plays a role in functional activity. Using wild-type and NCAM(-/-) mice, we show that BM neutrophils express only polysialylated CD56, whereas a subset of BM monocytes expresses polysialylated CD56 and/or another polysialylated protein(s). We demonstrate that polysialylated CD56 expression is progressively down-regulated in wild-type monocytes and monocyte-derived cells during migration from BM through peripheral blood to pulmonary and peritoneal sites of inflammation. Freshly isolated monocyte-derived peritoneal macrophages are devoid of polySia yet re-express polySia on NRP-2 and an additional protein(s) after maintenance in culture. Removal of polySia from these cells enhances phagocytosis of Klebsiella pneumoniae, suggesting that down-regulation of polySia on macrophages facilitates bacterial clearance. Using wild-type and NRP-2(-/-) mice, we demonstrate that NRP-2 and an additional protein(s) are polysialylated by ST8 SiaIV in BM-derived DCs. We conclude that polySia expression in monocyte-derived cells is dynamically regulated by ST8 SiaIV activity and by expression of carrier proteins during recruitment to sites of inflammation and influences cellular interactions with microbes, contributing to innate and adaptive immune responses.

Enzymatic removal of polysialic acid from neural cell adhesion molecule interrupts gonadotropin releasing hormone (GnRH) neuron-glial remodeling

There is abundant evidence to prove that the astrocytes are highly dynamic cell type in CNS and under physiological conditions such as reproduction, these cells display a remarkable structural plasticity especially at the level of their distal processes ensheathing the gonadotropin releasing hormone (GnRH) axon terminals. The morphology of GnRH axon terminals and astrocytes in the median eminence region of hypothalamus show activity dependent structural plasticity during different phases of estrous cycle. In the current study, we have assessed the functional contribution of ∞-2,8-linked polysialic acid (PSA) on neural cell adhesion molecule (PSA-NCAM) in this neuronal-glial plasticity using both in vitro and in vivo model systems. In vivo experiments were carried out after stereotaxic injection of endoneuraminidase enzyme (endo-N) near median eminence region of hypothalamus to specifically remove PSA residues on NCAM followed by localization of GnRH, PSA-NCAM and glial fibrillary acidic protein (GFAP) by immunostaining. Using in vitro model, structural remodeling of GnV-3 cells, (a conditionally immortalized GnRH cell line) co-cultured with primary astrocytes was studied after treating the cells with endo-N. Marked morphological changes were observed in GnRH axon terminals in proestrous phase rats and control GnV-3 cells as compared to endo-N treatment i.e. after removal of PSA. The specificity of endo-N treatment was also confirmed by studying the expression of PSA-NCAM by Western blotting in cultures treated with and without endo-N. Removal of PSA from surfaces with endo-N prevented stimulation associated remodeling of GnRH axon terminals as well as their associated glial cells under both in vivo and in vitro conditions. The current data confirms the permissive role of PSA to promote dynamic remodeling of GnRH axon terminals and their associated glia during reproductive cycle in rats.

Endosialidases: Versatile Tools for the Study of Polysialic Acid

Polysialic acid is an α2,8-linked N-acetylneuraminic acid polymer found on the surface of both bacterial and eukaryotic cells. Endosialidases are bacteriophage-borne glycosyl hydrolases that specifically cleave polysialic acid. The crystal structure of an endosialidase reveals a trimeric mushroom-shaped molecule which, in addition to the active site, harbors two additional polysialic acid binding sites. Folding of the protein crucially depends on an intramolecular C-terminal chaperone domain that is proteolytically released in an intramolecular reaction. Based on structural data and previous considerations, an updated catalytic mechanism is discussed. Endosialidases degrade polysialic acid in a processive mode of action, and a model for its mechanism is suggested. The review summarizes the structural and biochemical elucidations of the last decade and the importance of endosialidases in biochemical and medical applications. Active endosialidases are important tools in studies on the biological roles of polysialic acid, such as the pathogenesis of septicemia and meningitis by polysialic acid-encapsulated bacteria, or its role as a modulator of the adhesion and interactions of neural and other cells. Endosialidase mutants that have lost their polysialic acid cleaving activity while retaining their polysialic acid binding capability have been fused to green fluorescent protein to provide an efficient tool for the specific detection of polysialic acid.

Relationship between polysialylated neural cell adhesion molecule and beta-endorphin- or gonadotropin releasing hormone-containing neurons during activation of the gonadotrope axis in short daylength in the ewe

Morphological plasticity has been demonstrated between breeding and anestrous seasons in the ewe hypothalamus, particularly for the gonadotropin-releasing hormone (GnRH) system. We sought to determine the impact of a photoperiodic transition, from long days (LD, 16 h light/24 h) to short days (SD; 8 h light/24 h), on the association between a marker of cerebral plasticity, the polysialylated form of neural cell adhesion molecule (PSA-NCAM), and two diencephalic populations: the GnRH and beta-endorphin (beta-END) neurons, the latter being potent inhibitors of GnRH neuronal activity. We also estimated the number of contacts on GnRH neurons after the passage to SD, using synaptophysin as a marker for synaptic buttons. Those parameters were evaluated in ovariectomized estradiol-replaced ewes using double immunocytochemistry and confocal microscopy at different times after the transition to SD: day 0 (D0), D30, D45, D60 and D112. Luteinizing hormone (LH) secretion was recorded throughout the experiment. High LH levels were observed only at D112. Significantly more PSA-NCAM was found in the GnRH neuron perimeters in the D112 group than in the other groups. This increase was not associated with any change in the number of synaptophysin-immunoreactive contacts on GnRH neurons. The beta-END peri-neuronal space was affected negatively by the transition to SD: the percentage of PSA-NCAM on beta-END neurons decreased between D45 and D112 in the posterior two thirds of the arcuate nucleus (ARC). These results suggest that photoperiod may reorganize cell interactions in different hypothalamic areas, ultimately reactivating GnRH neurons, in our model of ovariectomized-estradiol replaced ewes.

Polysialic acid immobilized on silanized glass surfaces: a test case for its use as a biomaterial for nerve regeneration

The immobilization of polysialic acid (polySia) on glass substrates has been investigated with regard to the applicability of this polysaccharide as a novel, biocompatible and bioresorbable material for tissue engineering, especially with regard to its use in nerve regeneration. PolySia, a homopolymer of alpha-2,8-linked sialic acid, is involved in post-translational modification of the neural cell adhesion molecule (NCAM). The degradation of polySia can be controlled which makes it an interesting material for coating and for scaffold construction in tissue engineering. Here, we describe the immobilization of polySia on glass surfaces via an epoxysilane linker. Whereas glass surfaces will not actually be used in nerve regeneration scaffolds, they provide a simple and efficient means for testing various methods for the investigation of immobilized polySia. The modified surfaces were investigated with contact angle measurements and the quantity of immobilized polySia was examined by the thiobarbituric acid assay and a specific polySia-ELISA. The interactions between the polySia-modified surface and immortalized Schwann cells were evaluated via cell adhesion and cell viability assays. The results show that polySia can be immobilized on glass surfaces via the epoxysilane linker and that surface-bound polySia has no toxic effects on Schwann cells. Therefore, as a key substance in the development of vertebrates and as a favourable substrate for the cultivation of Schwann cells, it offers interesting features for the use in nerve guidance tubes for treatment of peripheral nerve injuries.

A new sialidase mechanism: bacteriophage K1F endo-sialidase is an inverting glycosidase

Bacteriophages specific for Escherichia coli K1 express a tailspike protein that degrades the polysialic acid coat of E. coli K1 that is essential for bacteriophage infection. This enzyme is specific for polysialic acid and is a member of a family of endo-sialidases. This family is unusual because all other previously reported sialidases outside of this family are exo- or trans-sialidases. The recently determined structure of an endo-sialidase derived from bacteriophage K1F (endoNF) revealed an active site that lacks a number of the residues that are conserved in other sialidases, implying a new, endo-sialidase-specific catalytic mechanism. Using synthetic trifluoromethylumbelliferyl oligosialoside substrates, kinetic parameters for hydrolysis at a single cleavage site were determined. Measurement of kcat/Km at a series of pH values revealed a dependence on a single protonated group of pKa 5. Mutation of a putative active site acidic residue, E581A, resulted in complete loss of sialidase activity. Direct 1H NMR analysis of the hydrolysis of trifluoromethylumbelliferyl sialotrioside revealed that endoNF is an inverting sialidase. All other wild type sialidases previously reported are retaining glycosidases, implying a new mechanism of sialidase action specific to this family of endo-sialidases.

Polysialylated Neuropilin-2 Is Expressed on the Surface of Human Dendritic Cells and Modulates Dendritic Cell-T Lymphocyte Interactions

Polysialic acid (PSA) is a unique linear homopolymer of alpha2,8-linked sialic acid that has been identified as a posttranslational modification on only five mammalian proteins. Studied predominantly on neural cell adhesion molecule (NCAM) during development of the vertebrate nervous system, PSA modulates cell interactions mediated by NCAM and other adhesion molecules. An isoform of NCAM (CD56) on natural killer (NK) cells is the only protein known to be polysialylated in cells of the immune system, yet the function of PSA in NK cells remains unclear. We show here that neuropilin-2 (NRP-2), a receptor for the semaphorin and vascular endothelial growth factor families in neurons and endothelial cells, respectively, is expressed on the surface of human dendritic cells and is polysialylated. Expression of NRP-2 is up-regulated during dendritic cell maturation, coincident with increased expression of ST8Sia IV, one of the key enzymes of PSA biosynthesis, and with the appearance of PSA on the cell surface. PSA on NRP-2 is resistant to digestion with peptide N-glycosidase F but is sensitive to release under alkaline conditions, suggesting that PSA chains are added to O-linked glycans of NRP-2. Removal of polysialic acid from the surface of dendritic cells or binding of NRP-2 with specific IgG promoted dendritic cell-induced activation and proliferation of T lymphocytes. Thus, this newly recognized polysialylated protein on the surface of dendritic cells influences dendritic cell-T lymphocyte interactions through one or more of its distinct extracellular domains.

Structural and biological properties of the carbohydrate units of nervous tissue glycoproteins

We have identified structures in nervous tissue glycoproteins that are novel for glycoproteins in general or enriched in nervous tissue or cells of neural origin. These include: (alpha 2-8)-linked polysialic acid units, the linear form of poly-N-acetyllactosamine glycans, the sialylated X antigen determinant NeuAc(alpha 2-3)-Gal(beta 1-4) [Fuc(alpha 1-3)]GlcNAc, a series of Man-O-Ser(Thr)-linked glycans, and the O-glycosidically linked disaccharide unit Gal(alpha 1-3)GalNAc. The polysialic and poly-N-acetyllactosamine glycans are also developmentally regulated. The polysialic acid units in the cell adhesion molecule N-CAM. The poly-N-acetyllactosamine units occur in the adhesion molecule NILE (which is immunologically similar to Ng-CAM and L1) and in some other components revealed by a cell surface-labelling method specific for these glycans. The mannose-linked glycans occur in a chondroitin sulphate proteoglycan involved in neuron-glia interactions. Other biological interactions of the carbohydrates include their serving as bacterial receptors in meningitis, their serving as models for molecular mimicry by the capsules of meningitis-causing bacteria, and the role of some structures as antigens in autoimmune conditions. At the molecular level, two types of mechanisms are suggested for the glycans in molecular interactions: they may function either as mediators of interactions by serving as specific recognition ligands, or as modulators of the interactions determined by polypeptides or other molecules.

Evolution of bacteriophages infecting encapsulated bacteria: lessons from Escherichia coli K1-specific phages

Bacterial capsules are not only important virulence factors, but also provide attachment sites for bacteriophages that possess capsule degrading enzymes as tailspike proteins. To gain insight into the evolution of these specialized viruses, we studied a panel of tailed phages specific for Escherichia coli K1, a neuroinvasive pathogen with a polysialic acid capsule. Genome sequencing of two lytic K1-phages and comparative analyses including a K1-prophage revealed that K1-phages did not evolve from a common ancestor. By contrast, each phage is related to a different progenitor type, namely T7-, SP6-, and P22-like phages, and gained new host specificity by horizontal uptake of an endosialidase gene. The new tailspikes emerged by combining endosialidase domains with the capsid binding module of the respective ancestor. For SP6-like phages, we identified a degenerated tailspike protein which now acts as versatile adaptor protein interconnecting tail and newly acquired tailspikes and demonstrate that this adapter utilizes an N-terminal undecapeptide interface to bind otherwise unrelated tailspikes. Combining biochemical and sequence analyses with available structural data, we provide new molecular insight into basic mechanisms that allow changes in host specificity while a conserved head and tail architecture is maintained. Thereby, the present study contributes not only to an improved understanding of phage evolution and host-range extension but may also facilitate the on purpose design of therapeutic phages based on well-characterized template phages.

Site-directed antibodies to low-voltage-activated calcium channel CaV3.3 (alpha1I) subunit also target neural cell adhesion molecule-180

Synthetic peptides of defined amino acid sequence are commonly used as unique antigens for production of antibodies to more complex target proteins. We previously showed that an affinity-purified, site-directed polyclonal antibody (CW90) raised against a peptide antigen (CNGRMPNIAKDVFTKM) anticipated to be specific to a T-type voltage-dependent Ca(2+) channel subunit identified recombinant rat alpha1I/Ca(V)3.3 and two endogenous mouse proteins distinct in their developmental expression and apparent molecular mass (neonatal form 260 kDa, mature form 190 kDa) [Yunker AM, Sharp AH, Sundarraj S, Ranganathan V, Copeland TD, McEnery MW (2003) Immunological characterization of T-type voltage-dependent calcium channel Ca(V)3.1 (alpha 1G) and Ca(V)3.3 (alpha 1I) isoforms reveal differences in their localization, expression, and neural development. Neuroscience 117:321-335]. In the present study, we further characterize the biochemical properties of the CW90 antigens. We show for the first time that recombinant alpha1I/Ca(V)3.3 is modified by N-glycosylation. Using peptide:N-glycosidase F (PNGase F), an enzyme that removes polysaccharides attached at Asn residues, and endoneuraminidase-N (Endo-N), which specifically removes polysialic acid modifications, we reveal that differential glycosylation fully accounts for the large difference in apparent molecular mass between neonatal and adult CW90 antigens and that the neonatal form is polysialylated. As very few proteins are substrates for Endo-N, we carried out extensive analyses and herein present evidence that CW90 reacts with recombinant alpha1I/Ca(V)3.3 as well as endogenous neural cell adhesion molecule-180 (NCAM-180). We demonstrate the basis for CW90 cross-reactivity is a five amino acid epitope (AKDVF) present in both alpha1I/Ca(V)3.3 and NCAM-180. To extend these findings, we introduce a novel polyclonal anti-peptide antibody (CW678) that uniquely recognizes NCAM-180 and a new antibody (CW109) against alpha1I/Ca(V)3.3. Western blot analyses obtained with CW678, CW109 and CW90 on a variety of samples confirm that the endogenous CW90 signals are fully attributed to the two developmental forms of NCAM-180. Using CW678, we present novel data on differentiation-dependent NCAM-180 expression in human neuroblastoma IMR32 cells. These results strongly suggest the need for careful analyses to validate anti-peptide antibodies when targeting membrane proteins of low abundance.

PSA-NCAM in mammalian structural plasticity and neurogenesis

Polysialic acid (PSA) is a linear homopolymer of alpha2-8-N acetylneuraminic acid whose major carrier in vertebrates is the neural cell adhesion molecule (NCAM). PSA serves as a potent negative regulator of cell interactions via its unusual biophysical properties. PSA on NCAM is developmentally regulated thus playing a prominent role in different forms of neural plasticity spanning from embryonic to adult nervous system, including axonal growth, outgrowth and fasciculation, cell migration, synaptic plasticity, activity-induced plasticity, neuronal-glial plasticity, embryonic and adult neurogenesis. The cellular distribution, developmental changes and possible function(s) of PSA-NCAM in the central nervous system of mammals here are reviewed, along with recent findings and theories about the relationships between NCAM protein and PSA as well as the role of different polysialyltransferases. Particular attention is focused on postnatal/adult neurogenesis, an issue which has been deeply investigated in the last decade as an example of persisting structural plasticity with potential implications for brain repair strategies. Adult neurogenic sites, although harbouring all subsequent steps of cell differentiation, from stem cell division to cell replacement, do not faithfully recapitulate development. After birth, they undergo morphological and molecular modifications allowing structural plasticity to adapt to the non-permissive environment of the mature nervous tissue, that are paralled by changes in the expression of PSA-NCAM. The use of PSA-NCAM as a marker for exploring differences in structural plasticity and neurogenesis among mammalian species is also discussed.

Direct Evidence That Neural Cell Adhesion Molecule (NCAM) Polysialylation Increases Intermembrane Repulsion and Abrogates Adhesion

Molecular force measurements quantified the impact of polysialylation on the adhesive properties both of membrane-bound neural cell adhesion molecule (NCAM) and of other proteins on the same membrane. These results show quantitatively that NCAM polysialylation increases the range and magnitude of intermembrane repulsion. The repulsion is sufficient to overwhelm both homophilic NCAM and cadherin attraction at physiological ionic strength, and it abrogates the protein-mediated intermembrane adhesion. The steric repulsion is ionic strength dependent and decreases substantially at high monovalent salt concentrations with a concomitant increase in the intermembrane attraction. The magnitude of the repulsion also depends on the amount of polysialic acid (PSA) on the membranes, and the PSA-dependent attenuation of cadherin adhesion increases with increasing PSA-NCAM:cadherin ratios. These findings agree qualitatively with independent reports based on cell adhesion studies and reveal the likely molecular mechanism by which NCAM polysialylation regulates cell adhesion and intermembrane space.

First Synthetic Probe for the Detection and Quantification of a Protein with a Potential α,(2→8)Sialyltransferase Activity

There is considerable interest in monitoring alpha,(2-->8)sialyltransferase (ST8) levels; however, there are few specific and sensitive methods to directly detect and quantitate the protein. This paper reports the development of a synthetic probe composed of oxidized colominic acid coupled to biotinyl-L-lysine hydrazide to detect and quantify ST8 with putative "initiase" activity and its use in three solid-phase applications. The detection limit observed for ST8 purified from K562 cells was approximately 2 pg by dot-blot analysis. In Western blots the probe bound and specifically recognized a protein band corresponding to ST8. In ELISA a linear dose response was obtained for pure protein in the range of 50-200 pg. Analysis of 3'-azido-3'deoxythymidine-treated cells by all three methods showed a reduction in ST8 compared to control cells; treated cells had 73% of control levels by ELISA. This probe will be useful for studies on the expression ST8 and its role in glycoconjugate biosynthesis.

Polysialic acid and mucin type o-glycans on the neural cell adhesion molecule differentially regulate myoblast fusion

Polysialic acid attached to the neural cell adhesion molecule (NCAM) is thought to play a critical role in development. NCAM in muscle tissue contains a muscle-specific domain (MSD) to which mucin type O-glycans are attached. In the present study, using the C2C12 myoblast system, we show that NCAM containing MSD is increasingly expressed on the cell surface as myotubes form. Polysialic acid is primarily attached to N-glycans of NCAM, and polysialylated NCAM is expressed on the outer surface of myotube bundles. By transfecting cDNAs encoding wild type and mutant forms of NCAM, we found that NCAM containing MSD facilitates myoblast fusion, and this effect is diminished by mutating O-glycosylation sites at MSD. By contrast, forced expression of polysialic acid in early differentiation stages reduces myotube formation and delays the expression of NCAM containing the MSD domain. Strikingly, inhibition of polysialic acid synthesis by antisense DNA approach induced differentiation in both human rhabdomyosarcoma cells, which overexpress polysialic acid, and C2C12 cells. These results indicate that polysialic acid and mucin type O-glycans on NCAM differentially regulate myoblast fusion, playing critical roles in muscle development.

Conformations of terminal sialyloligosaccharide fragments--a molecular dynamics study

Molecular dynamics simulations have been performed to understand the conformational features of the terminal sialyloligosaccharide fragments NeuNAc alpha(2-3)Gal, NeuNAc alpha(2-6)Gal, NeuNAc alpha(2-8)NeuNAc and NeuNAc alpha(2-9)NeuNAc. The conformational regions A(i), B(i) and C(i) were identified in the Ramachandran plot. Analysis of the 1000 ps trajectories collected through simulation (2000 ps in the case of NeuNAc alpha (2-9)NeuNAc) revealed that these molecules have conformational propensity in region B(i). The occurrence of these molecules in the common conformational space leads to a structural similarity between them. This structural similarity may be an essential requirement for the neuraminidase activity towards sialyloligosaccharides. The local change in the conformation of the active site residues of neuraminidases may contribute for the specificity differences between different linkages of sialyloligosaccharides. A highly conserved water-mediated hydrogen bond observed in these structures between the sugar residues, acts as an additional stabilizing force.

High affinity binding of long-chain polysialic acid to antibody, and modulation by divalent cations and polyamines

Long-chain polysialic acid (PSA) is expressed on the vertebrate neural cell adhesion molecule (NCAM) during neuronal plasticity. Its structural similarity to the capsular PSAs of some pathogenic bacteria has hampered the development of polysaccharide vaccines against meningitis. The antibodies formed during immunization require a long epitope for binding, and cross-react with host tissue PSA. The nature of the epitope and possible external effectors involved are still unclear. We have evaluated the interaction of PSA with its antibody mAb735 by surface plasmon resonance. The influences of PSA chain length, pH, temperature, ionic environment, and polyamines were also determined. The antibody binding affinity was found to dramatically increase with PSA chain length. A sub-nanomolar dissociation constant (K(D)=8.5 x 10(-10)M) was obtained for the binding of very long chain native MenB polysaccharides (approximately 200 Neu5Ac-residues). Colominic acid from Escherichia coli K1 (approximately 100 residues) and shorter polymers exhibited progressively weaker affinities. The antibody also bound tightly (K(D) approximately 5 x 10(-9)M) to polysialylated glycopeptides from human embryonal brain. The effects of pH and ionic strength suggested that the interaction is largely electrostatic. Ca2+ and Mn2+ ions promoted the observed surface plasmon resonance response in a concentration dependent fashion. Spermine increased the response in a similar way. Our results suggest that divalent cations and polyamines may play significant role in the regulation of the PSA epitope presentation in vivo.

Serological and conformational properties ofE. coliK92 capsular polysaccharide and itsN-propionylated derivative both illustrate that induced antibody does not recognize extended epitopes of polysialic acid: Implications for a comprehensive conjugate vaccine against groups B and CN. meningitidis

The capsular polysaccharide of E. coli K92 (K92P) contains elements in common with the capsular polysaccharides of both groups B and C N. meningitidis, and may therefore form the basis of a bivalent vaccine. In an attempt to augment the cross-protective immune response to group B meningococci, the N-acetyl groups of the K92P were replaced by N-propionyl groups (NPrK92P) and conjugated to protein. This strategy had previously been applied with success to the poorly immunogenic capsular polysaccharide of group B meningococcus (GBMP), and the bactericidal epitope was found to be exclusively mimicked by extended helical segments of the NPrGBMP. The NPrK92P-conjugate, in relation to a K92P-conjugate, failed to enhance the response to GBMP but did generate a measurable response to NPrGBMP, but only at the expense of a greatly reduced GCMP response. Despite the presence of an immune response to NPrGBMP, the anti-NPrK92 serum was not bactericidal. Competitive inhibition studies with NPrGBMP oligosaccharides suggested the NPrK92 antibodies could not cross-react with the protective epitope on group B meningococci, as defined by extended helical segments of the NPrGBMP, but only recognized short non-bactericidal NPrGBMP epitopes. This hypothesis was supported from the conformational and molecular dynamics studies of the K92P, which demonstrated a lack of extended conformations that resemble the GBMP extended epitope. Indeed, the conformational properties of the K92P more closely resembled those of the GCMP, thereby explaining the observed moderate cross-protection of the K92P antiserum towards group C meningococci. Thus, on the basis of these results, it can be concluded that K92P, regardless of N-propionyl modification, will not serve as an effective single vaccine component against both groups B and C meningococci.Key words: conjugate vaccine, Neisseria meningitidis, polysialic acid, NMR, molecular dynamics.

Activity and cross-reactivity of antibodies induced in mice by immunization with a group B meningococcal conjugate

The capsular polysaccharide of group B Neisseria meningitidis is composed of a linear homopolymer of alpha(2-8) N-acetyl neuraminic acid or polysialic acid (PSA) that is also carried by isoforms of the mammalian neural cell adhesion molecule (NCAM), which is especially expressed on brain cells during development. Here we analyzed the ability of antibodies induced by the candidate vaccine N-propionyl polysaccharide tetanus toxoid conjugate to recognize PSA-NCAM. We hyperimmunized mice to produce a pool of antisera and a series of immunoglobulin G monoclonal antibodies and evaluated their self-reactivity profile by using a battery of tests (immunoprecipitation, immunoblotting, and immunofluorescence detection on live cells and human tissue sections) chosen for their sensitivity and specificity to detect PSA-NCAM in various environments. We also searched for the effects of the vaccine-induced antibodies in two functional assays involving cell lysis or cell migration. Although they were highly bactericidal, all the antibodies tested showed very low or no recognition of PSA-NCAM, in contrast to PSA-specific monoclonal antibodies used as controls. Different patterns of cross-reactions were revealed by the tests used, likely due to affinity and specificity differences among the populations of induced antibodies. Furthermore, neither cell lysis nor perturbation of migration was observed in the presence of the tested antibodies. Importantly, we showed that whereas enzymatic removal of PSA groups from the surfaces of live cells perturbed their migration, blocking them with PSA-specific antibodies was not functionally detrimental. Taken together, our data indicated that this candidate vaccine induced antibodies that could not demonstrate an immunopathologic effect.

ST8Sia IV mRNA corresponds with the biosynthesis of alpha2,8sialyl polymers but not oligomers in rat oligodendrocytes

As oligodendrocytes mature they progress through a series of distinct differentiation steps characterized by the expression of specific markers. One such marker, polysialic acid found on the neural cell adhesion molecule (NCAM), is detected by antibodies and is present on progenitor oligodendrocytes, but is not detected to the same extent on mature oligodendrocytes. Two closely related polysialyltransferases, ST8Sia II (STX) and ST8Sia IV (PST) have been cloned previously and shown to synthesize polysialic acid on NCAM and other glycoproteins. To determine whether or not polyalpha2,8sialyltransferases are downregulated during the differentiation of oligodendrocytes, the enzyme activity and expression of ST8Sia II and ST8Sia IV mRNA at two stages of maturation in JS12/1 and JS3/16 oligodendrocytes were examined. Differentiation in both oligodendroglial cell lines was accompanied by more than a 50% reduction in the biosynthesis of polymers of alpha2,8sialic acid when fetuin was used as substrate. Most interestingly, extracts of JS12/1 mature cells synthesized 60% more short oligomers of alpha2,8sialic acid than the progenitor cells, whereas JS3/16 mature cells synthesized barely detectable amounts of the short oligomers. Transcripts for ST8Sia IV mRNA were present in both JS12/1 and JS3/16 and were reduced when the biosynthesis was markedly reduced. In contrast ST8Sia II mRNA was barely detectable in JS3/16 cells and although detectable in JS12/1 cells, there was no clear modulation with maturation. These results were supported by the examination of the brains of rats from embryonic to Day 21 ages. The enzyme activity and mRNA experiments show that polyalpha2,8sialyltransferase itself is down regulated to cause the reduction in sialyl polymers on mature oligodendrocytes. Moreover, ST8Sia IV is responsible for the polysialylation of NCAM in oligodendrocytes.

The impact of N-glycosylation on the functions of polysialyltransferases

Poly-alpha-2,8-sialic acid (polysialic acid) is a post-translational modification of the neural cell adhesion molecule (NCAM) and an important regulator of neuronal cell-cell interactions. The synthesis of polysialic acid depends on the two polysialyltransferases ST8SiaII and ST8SiaIV. Understanding the catalytic mechanisms of the polysialyltransferases is critical toward the aim of influencing physiological and pathophysiological functions mediated by polysialic acid. We recently demonstrated that polysialyltransferases are bifunctional enzymes exhibiting auto- and NCAM polysialylation activity. Autopolysialylation occurs on N-glycans of the enzymes, and glycosylation variants lacking sialic acid and galactose were found to be inactive for both auto- and NCAM polysialylation. In the present study, we have analyzed the number and functional importance of N-linked oligosaccharides present on polysialyltransferases. We demonstrate that autopolysialylation depends on specific N-glycans attached to Asn(74) in ST8SiaIV and Asn(89) and Asn(219) in ST8SiaII. Deletion of polysialic acid acceptor sites by site-directed mutagenesis rendered the polysialyltransferases inactive in vitro and in vivo. The inactivity of autopolysialylation-negative polysialyltransferases in vivo was not caused by the absence or default targeting of the enzymes. The data presented in this study clearly show that active polysialyltransferases are competent to perform autopolysialylation and provide strong evidence for a tight functional link between the two catalytic functions.

Prospects offered by genome studies for combating meningococcal disease by vaccination

Meningococcal disease was first recognised and Neisseria meningitidis isolated as the causative agent over 100 years ago, but despite more than a century of research, attempts to eliminate this distressing illness have so far been thwarted. The main problem lies in the fact that N. meningitidis usually exists as a harmless commensal inhabitant of the human nasopharynx, the pathogenic state being the exception rather than the norm. As man is its only host, the meningococcus is uniquely adapted to this ecological niche and has evolved an array of mechanisms for evading clearance by the human immune response. Progress has been made in combating the disease by developing vaccines that target specific pathogenic serogroups of meningococci. However, a fully comprehensive vaccine that protects against all pathogenic strains is still just beyond reach. The publication of the genome sequences of two meningococcal strains, one each from serogroups A and B and the imminent completion of a third illustrates the extent of the problems to be overcome, namely the vast array of genetic mechanisms for the generation of meningococcal diversity. Fortunately, genome studies also provide new hope for solutions to these problems in the potential for a greater understanding of meningococcal pathogenesis and possibilities for the identification of new vaccine candidates. This review describes some of the approaches that are currently being used to exploit the information from meningococcal genome sequences and seeks to identify future prospects for combating meningococcal disease.

Roles, regulation, and mechanism of polysialic acid function during neural development

The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) appeared during the evolution of vertebrates as a new mechanism for regulation of cell interactions. This large and abundant glycoprotein can exert steric effects at the cell surface that lead to the attenuation of cell-cell bonds mediated not only by NCAM but also a variety of other adhesion receptors. PSA-NCAM expression changes both as a result of developmental programs and physiological inputs. This global modulation of cell-cell attachment has been shown to facilitate cell migration, axon pathfinding and targeting, and plastic changes in the embryonic and adult nervous system.

A challenge to the ultrasensitive chemical method for the analysis of oligo- and polysialic acids at a nanogram level of colominic acid and a milligram level of brain tissues

Polysialic acid (polySia) is a functional epitope and is known: 1) to regulate normal fertilization of lower vertebrates and invertebrates; 2) to be expressed on neural cell adhesion molecule (NCAM) when the formation or re-arrangement of nervous tissues takes place during embryonic stages as well as in adults of higher vertebrates; and 3) to be re-expressed in several human tumors. Thus, polySia serves as oncodevelopmental antigen. To date sensitive biochemical diagnostic probes (antibodies and endo-N-acylneuraminidase) to detect polySia are known. However, these reagents are not commercially available yet and they are only reactive to specific types of polySia structure. Moreover, precise information not only on diversity but also on the length or degree of polymerization (DP) of extended polySia chains is considered important in understanding the molecular mechanism of biosynthesis of polySia chains and fine-tuning of NCAM-NCAM adhesive interaction by polySia chain but cannot be obtained with these biochemical probes. We have been continuously making efforts to develop and improve the sensitivity of chemical methods for polySia analysis toward these challenging problems. This article presents our most recently developed chemical method for polySia analysis and its use in obtaining new information on DP of colominic acid samples and polySia chains present in rat brain tissues with the highest sensitivity that has ever been attained.

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.

The highly sialylated isoform of the neural cell adhesion molecule is required for estradiol-induced morphological synaptic plasticity in the adult arcuate nucleus

The large quantities of polysialic acid (PSA) characterizing highly sialylated isoform of the neural cell adhesion molecule (PSA-NCAM), greatly reduce cell adhesion and render this particular cell surface adhesion molecule a likely candidate to intervene in dynamic neuronal phenomena, such as synaptic plasticity. The hypothalamic arcuate nucleus expresses high levels of PSA-NCAM and maintains a high capacity for neuroplastic changes in the adult. Thus, in the arcuate nucleus of female rats, varying circulating levels of estrogen give rise to a reversible reduction in the number of axo-somatic GABA synapses, together with a changing ensheathing of neuronal somata by astrocytes. To examine the role of PSA in such changes, we perturbed its expression, either by blockade with antibodies raised against this carbohydrate moiety (delivered intracerebroventricularly), or by its enzymatic cleavage after microinjection of endoneuraminidase N over the arcuate nucleus. Either procedure was performed in ovariectomized adult rats that received concurrent treatment with 17 beta-estradiol. Morphological synaptic plasticity was analysed using the unbiased disector method to assess synaptic densities in ultrathin sections of the arcuate nucleus immunogold-labelled for GABA. As expected, 17 beta-estradiol induced a significant reduction in the number of GABAergic axo-somatic synapses, a reduction which did not occur after infusion of anti-PSA antibodies or in vivo enzymatic removal of PSA from NCAM. Taken together, our results provide strong evidence that the presence of large quantities of the PSA moiety on NCAM is a necessary prerequisite for estrogen-induced phasic remodelling of synapses in the adult female arcuate nucleus.

Structural requirements for the adherence of Plasmodium falciparum-infected erythrocytes to chondroitin sulfate proteoglycans of human placenta

Plasmodium falciparum infection during pregnancy results in the accumulation of infected red blood cells (IRBCs) in the placenta, leading to poor pregnancy outcome. In the preceding paper (Achur, R. N., Valiyaveettil, M., Alkhalil, A., Ockenhouse, C. F., and Gowda, D. C. (2000) J. Biol. Chem. 275, 40344-40356), we reported that unusually low sulfated chondroitin sulfate proteoglycans (CSPGs) in the intervillous spaces of the placenta mediate the IRBC adherence. In this study, we report the structural requirements for the adherence and the minimum chondroitin 4-sulfate (C4S) structural motif that supports IRBC adherence. Partially sulfated C4Ss with varying sulfate contents were prepared by solvolytic desulfation of a fully sulfated C4S. These and other nonmodified C4Ss, with different proportions of 4-, 6-, and nonsulfated disaccharide repeats, were analyzed for inhibition of IRBC adherence to the placental CSPG. C4Ss containing 30-50% 4-sulfated and 50-70% nonsulfated disaccharide repeats efficiently inhibited IRBC adherence; C6S had no inhibitory activity. Oligosaccharides of varying sizes were prepared by the partial depolymerization of C4Ss containing varying levels of 4-sulfation, and their ability to inhibit the IRBC adherence was studied. Oligosaccharides with six or more disaccharide repeats inhibited IRBC adherence to the same level as that of the intact C4Ss, indicating that a dodecasaccharide is the minimum structural motif required for optimal IRBC adherence. Of the C4S dodecasaccharides, only those with two or three sulfate groups per molecule showed maximum IRBC inhibition. These data define the structural requirements for the IRBC adherence to placental CSPGs with implications for the development of therapeutics for maternal malaria.

Biosynthetic incorporation of unnatural sialic acids into polysialic acid on neural cells

In this study we demonstrate that polysialyltransferases are capable of accepting unnatural substrates in terminally differentiated human neurons. Polysialyltransferases catalyze the glycosylation of the neural cell adhesion molecule (NCAM) with polysialic acid (PSA). The unnatural sialic acid analog, N-levulinoyl sialic acid (SiaLev), was incorporated into cell surface glycoconjugates including PSA by the incubation of cultured neurons with the metabolic precursor N-levulinoylmannosamine (ManLev). The ketone group within the levulinoyl side chain of SiaLev was then used as a chemical handle for detection using a biotin probe. The incorporation of SiaLev residues into PSA was demonstrated by protection from sialidases that can cleave natural sialic acids but not those bearing unnatural N-acyl groups. The presence of SiaLev groups on the neuronal cell surface did not impede neurite outgrowth or significantly affect the distribution of PSA on neuronal compartments. Since PSA is important in neural plasticity and development, this mechanism for modulating PSA structure might be useful for functional studies.

Capillary electrophoresis of N-acetylneuraminic acid polymers and hyaluronic acid: correlation between migration order reversal and biological functions

High-resolution analysis of polymers of N-acetylneuraminic acid and hyaluronic acid was performed by capillary electrophoresis in a buffer containing a neutral polymer. Both polysaccharides having more than 100 monosaccharide residues were well separated into their molecular species by capillary electrophoresis using a combination of a chemically modified capillary and a buffer containing poly(ethylene glycol) as an additive. During optimization of the separation conditions, small oligomers of both polysaccharides were observed to migrate in the reverse order of their molecular masses on the electropherograms. However, oligomers larger than pentamer and decamer for N-acetylneuraminic acid polymers and hyaluronic acid, respectively, migrated in the order of their molecular masses. We propose that these unusual migration patterns are closely related to the stereochemical structures and the oligomer migrating the fastest is the minimum unit that forms the regular three-dimensional structure required for the biological function.

The polysialic acid units of the neural cell adhesion molecule N-CAM form filament bundle networks

Polysialic acid is a developmentally regulated component in the neural cell adhesion molecule N-CAM which also occurs as the capsular polysaccharide of bacteria causing meningitis. Polysialic acid has been considered as a repulsive element that regulates intermolecular and intercellular adhesion. Using atomic force microscopy we unexpectedly find that oligomers of polysialic acid assemble with each other into filament bundle networks. Filaments were formed from oligomers containing 12 or more N-acetylneuraminic acid residues, and they were sensitive to sialidase digestion. The networks were also formed by the polysialic acid-containing carbohydrate units of N-CAM. The formation of filament bundles is a novel and unexpected property of polysialic acid and of short carbohydrate oligomers in general and represents a previously unrecognized molecular interaction mechanism which impacts both eukaryotic and prokaryotic cell-cell adhesions.

Polysialic acid: three-dimensional structure, biosynthesis and function

Polysialic acid is a unique cell surface polysaccharide found in the capsule of neuroinvasive bacteria and as a highly regulated post-translational modification of the neural cell adhesion molecule. Recent progress has been achieved in research on both the physicochemical properties of polysialic acid and the biosynthetic pathways leading to polysialic acid expression in bacteria and mammals.

Polysialic acid, a unique glycan that is developmentally regulated by two polysialyltransferases, PST and STX, in the central nervous system: from biosynthesis to function

Polysialic acid is a developmentally regulated carbohydrate composed of a linear homopolymer of alpha-2,8-linked sialic acid residues. This unique glycan is mainly attached to the neural cell adhesion molecule (N-CAM) and implicated in many morphogenic events of the neural cells by modulating the adhesive property of N-CAM. Recently, the cDNA that encodes polysialyltransferase, which is responsible for the polysialylation of N-CAM, was successfully cloned from three mammalian species. This review focuses on the molecular cloning of human polysialyltransferase, designated PST. It then describes the number of enzymes actually required for the polysialylation of N-CAM using an in vitro polysialyltransferase assay. Comparisons between PST and another polysialyltransferase, sialyltransferase X (STX), are made and it is demonstrated that both enzymes can independently form polysialic acid in vitro, but that during neural development they coordinately but distinctly synthesize polysialic acid on N-CAM. The role of polysialic acid in the central nervous system is also discussed. Finally, evidence that the two polysialyltransferases, PST and STX, apparently have distinct roles in the development of neural cells is provided by using a neurite outgrowth assay.

The significance of hypersialylation of dipeptidyl peptidase IV (CD26) in the inhibition of its activity by Tat and other cationic peptides. CD26: a subverted adhesion molecule for HIV peptide binding

The functionality of DPP-IV, purified from human placenta and isolated from CD4+/CD26+ T cells of noninfected and HIV-1-infected individuals, was investigated as to its ability to bind certain specific peptides. Using isoelectric focusing and the specificity of substrate-impregnated overlay membranes, we found that DPP-IV from term placenta and from T cells of HIV-infected individuals was significantly more sialylated compared with enzyme isozyme patterns of other tissues. We report here that (1) the number of isoforms of DPP-IV and extent of sialylation are critical to function and peptide binding; (2) the number of sialylated isoforms isolated from PBMCs increases significantly with age greater than 40 years; (3) hypersialylation by extreme anionic isoforms is highly associated with HIV infection and pathognomonic to remaining CD4+ cells in overt AIDS; and (4) highly sialylated DPP-IV is more significantly inhibited by Tat and cationic peptides. We conclude that hypersialylation of DPP-IV modifies surface charge of the CD26 antigen, promoting binding of HIV peptides through their cationic domains to the sialic acid residues of DPP-IV, and that certain HIV moieties are likely to engage this phenomenon as an auxiliary adhesion mechanism to fuse with cells. Furthermore, as a consequence of this occurrence, DPP-IV enzymatic activity can be significantly reduced, competitively.