Regulation of cell adhesion by polysialic acid. Effects on cadherin, immunoglobulin cell adhesion molecule, and integrin function and independence from neural cell adhesion molecule binding or signaling activity.

The polysialylation of neural cell adhesion molecule (NCAM) evolved in vertebrates to carry out biological functions related to changes in cell position and morphology. Many of these effects involve the attenuation of cell interactions that are not mediated through NCAM's own adhesion properties. A proposed mechanism for this global effect on cell interaction is the steric inhibition of membrane-membrane apposition based solely on polysialic acid (PSA) biophysical properties. However, it remains possible that the intrinsic binding or signaling properties of the NCAM polypeptide are also involved. To help resolve this issue, this study uses a quantitative cell detachment assay together with cells engineered to display different adhesion receptors together with a variety of polysialylated NCAM polypeptide isoforms and functional domain deletion mutations. The results obtained indicate that regulation by PSA occurs with adhesion receptors as diverse as an IgCAM, a cadherin and an integrin, and does not require NCAM functional domains other than those minimally required for polysialylation. These findings are most consistent with the cell apposition mechanism for PSA action, as this model predicts that the inhibitory effects of PSA-NCAM on cell adhesion should be independent of the nature of the adhesion system and of any intrinsic binding or signaling properties of the NCAM polypeptide itself.     

Polysialylation increases lateral diffusion of neural cell adhesion molecule in the cell membrane

Polysialic acid (PSA) is a polymer of N-acetylneuraminic acid residues added post-translationally to the membrane-bound neural cell adhesion molecule (NCAM). The large excluded volume created by PSA polymer is thought to facilitate cell migration by decreasing cell adhesion. Here we used live cell imaging (spot fluorescence recovery after photobleaching and fluorescence correlation spectroscopy) combined with biochemical approaches in an attempt to uncover a link between cell motility and the impact of polysialylation on NCAM dynamics. We show that PSA regulates specifically NCAM lateral diffusion and this is dependent on the integrity of the cytoskeleton. However, whereas the glial-derivative neurotrophic factor chemotactic effect is dependent on PSA, the molecular dynamics of PSA-NCAM is not directly affected by glial-derivative neurotrophic factor. These findings reveal a new intrinsic mechanism by which polysialylation regulates NCAM dynamics and thereby a biological function like cell migration.     

Polysialyltransferase-1 autopolysialylation is not requisite for polysialylation of neural cell adhesion molecule

Polysialyltransferase-1 (PST; ST8Sia IV) is one of the alpha2, 8-polysialyltransferases responsible for the polysialylation of the neural cell adhesion molecule (NCAM). The presence of polysialic acid on NCAM has been shown to modulate cell-cell and cell-matrix interactions. We previously reported that the PST enzyme itself is modified by alpha2,8-linked polysialic acid chains in vivo. To understand the role of autopolysialylation in PST enzymatic activity, we employed a mutagenesis approach. We found that PST is modified by five Asn-linked oligosaccharides and that the vast majority of the polysialic acid is found on the oligosaccharide modifying Asn-74. In addition, the presence of the oligosaccharide on Asn-119 appeared to be required for folding of PST into an active enzyme. Co-expression of the PST Asn mutants with NCAM demonstrated that autopolysialylation is not required for PST polysialyltransferase activity. Notably, catalytically active, non-autopolysialylated PST does not polysialylate any endogenous COS-1 cell proteins, highlighting the protein specificity of polysialylation. Immunoblot analyses of NCAM polysialylation by polysialylated and non-autopolysialylated PST suggests that the NCAM is polysialylated to a higher degree by autopolysialylated PST. We conclude that autopolysialylation of PST is not required for, but does enhance, NCAM polysialylation.     

The effect of multivalent cations on adhesion time for cellular adhesion to solid surfaces

The dynamic behavior of the adhesion of a charge-regulated cell to a solid surface of constant potential is investigated. In particular, the effect of the presence of multivalent cations in the suspension medium on adhesion time is discussed. By neglecting the effect of hydrodynamic retardation and assuming that the bulk liquid phase is stagnant, we show that the presence of multivalent cations has the effect of retarding cell adhesion. At a fixed level of ionic strength, the adhesion time increases with the increase of the concentration of multivalent cations in the suspension medium, and decreases with the increase in magnitude of the Hamaker constant. For a fixed concentration of cations, the adhesion time decreases with the increase of ionic strength. The effect of the magnitude of Hamaker constant on adhesion time is appreciable if both the ionic strength and the concentration of cations are high.     

Polysialytation as a regulator of neural plasticity in rodent learning and aging

Although generally accepted to play an important role in development, the precise functional significance of NCAM remains to be elucidated. Correlative and interventive studies suggest a role for polysialylated NCAM in neurite elaboration. In the adult NCAM polysialylation continues to be expressed in regions of the central nervous system which retain neuroplastic potential. During memory formation modulation of polysialylation on the synapse-enriched isoform of NCAM occurs in the hippocampus. The polysialylated neurons of this structure have been located at the border of the granule cell layer and hilar region of the dentate and their number increases dramatically during memory consolidation. The converse is also true for a profound decline in the basal number of polysialylated neurons occurs with ageing when neural plasticity becomes attenuated. In conclusion, it is suggested that NCAM polysialylation regulates ultrastructural plasticity associated with synaptic elaboration.Abbreviations PSA polysialic acid - NCAM neural cell adhesion molecule - SGL sub-granular cell layer - MF mossy fibers Special issue dedicated to Dr. Robert Balazs.     

Molecular dissection of the ST8Sia IV polysialyltransferase. Distinct domains are required for neural cell adhesion molecule recognition and polysialylation

Polysialic acid, a homopolymer of alpha2,8-linked sialic acid expressed on the neural cell adhesion molecule (NCAM), is thought to play critical roles in neural development. Two highly homologous polysialyltransferases, ST8Sia II and ST8Sia IV, which belong to the sialyltransferase gene family, synthesize polysialic acid on NCAM. By contrast, ST8Sia III, which is moderately homologous to ST8Sia II and ST8Sia IV, adds oligosialic acid to itself but very inefficiently to NCAM. Here, we report domains of polysialyltransferases required for NCAM recognition and polysialylation by generating chimeric enzymes between ST8Sia IV and ST8Sia III or ST8Sia II. We first determined the catalytic domain of ST8Sia IV by deletion mutants. To identify domains responsible for NCAM polysialylation, different segments of the ST8Sia IV catalytic domain, identified by the deletion experiments, were replaced with corresponding segments of ST8Sia II and ST8Sia III. We found that larger polysialic acid was formed on the enzymes themselves (autopolysialylation) when chimeric enzymes contained the carboxyl-terminal region of ST8Sia IV. However, chimeric enzymes that contain only the carboxyl-terminal segment of ST8Sia IV and the amino-terminal segment of ST8Sia III showed very weak activity toward NCAM, even though they had strong activity in polysialylating themselves. In fact, chimeric enzymes containing the amino-terminal portion of ST8Sia IV fused to downstream sequences of ST8Sia III inhibited NCAM polysialylation in vitro, although they did not polysialylate NCAM. These results suggest that in polysialyltransferases the NCAM recognition domain is distinct from the polysialylation domain and that some chimeric enzymes may act as a dominant negative enzyme for NCAM polysialylation.     

The Polysialyltransferases Interact with Sequences in Two Domains of the Neural Cell Adhesion Molecule to Allow Its Polysialylation

The neural cell adhesion molecule (NCAM) is the major substrate for the polysialyltransferases (polySTs), ST8SiaII/STX and ST8SiaIV/PST. The polysialylation of NCAM N-glycans decreases cell adhesion and alters signaling. Previous work demonstrated that the first fibronectin type III repeat (FN1) of NCAM is required for polyST recognition and the polysialylation of the N-glycans on the adjacent Ig5 domain. In this work, we highlight the importance of an FN1 acidic patch in polyST recognition and also reveal that the polySTs are required to interact with sequences in the Ig5 domain for polysialylation to occur. We find that features of the Ig5 domain of the olfactory cell adhesion molecule (OCAM) are responsible for its lack of polysialylation. Specifically, two basic OCAM Ig5 residues (Lys and Arg) found near asparagines equivalent to those carrying the polysialylated N-glycans in NCAM substantially decrease or eliminate polysialylation when used to replace the smaller and more neutral residues (Ser and Asn) in analogous positions in NCAM Ig5. This decrease in polysialylation does not reflect altered glycosylation but instead is correlated with a decrease in polyST-NCAM binding. In addition, inserting non-conserved OCAM sequences into NCAM Ig5, including an “extra” N-glycosylation site, decreases or completely blocks NCAM polysialylation. Taken together, these results indicate that the polySTs not only recognize an acidic patch in the FN1 domain of NCAM but also must contact sequences in the Ig5 domain for polysialylation of Ig5 N-glycans to occur.     

A novel alpha-helix in the first fibronectin type III repeat of the neural cell adhesion molecule is critical for N-glycan polysialylation

Polysialic acid is a developmentally regulated, anti-adhesive glycan that is added to the neural cell adhesion molecule, NCAM. Polysialylated NCAM is critical for brain development and plays roles in synaptic plasticity, axon guidance, and cell migration. The first fibronectin type III repeat of NCAM, FN1, is necessary for the polysialylation of N-glycans on the adjacent immunoglobulin domain. This repeat cannot be replaced by other fibronectin type III repeats. We solved the crystal structure of human NCAM FN1 and found that, in addition to a unique acidic surface patch, it possesses a novel alpha-helix that links strands 4 and 5 of its beta-sandwich structure. Replacement of the alpha-helix did not eliminate polysialyltransferase recognition, but shifted the addition of polysialic acid from the N-glycans modifying the adjacent immunoglobulin domain to O-glycans modifying FN1. Other experiments demonstrated that replacement of residues in the acidic surface patch alter the polysialylation of both N- and O-glycans in the same way, while the alpha-helix is only required for the polysialylation of N-glycans. Our data are consistent with a model in which the FN1 alpha-helix is involved in an Ig5-FN1 interaction that is critical for the correct positioning of Ig5 N-glycans for polysialylation.     

Repetitive and Transient Increases in Hippocampal Neural Cell Adhesion Molecule Polysialylation State Following Multitrial Spatial Training

Abstract: Polysialylated neurons, located at the inner border of the dentate granule cell layer, have been demonstrated to exhibit time-dependent change in their frequency at 10–12 h following training in the Morris water maze, a spatial learning paradigm. Such a change was not observed in animals required to locate a visible platform or in those rendered amnesic with scopolamine. This frequency response was capable of rapid reactivation following further training stimuli in a manner that was independent of circadian influence. These learning-associated modulations in neural cell adhesion molecule (NCAM) polysialylation state did not increase in magnitude despite improved performance, suggesting their activation is required for processing information rather than contributing to previously stored, task-associated memory. An increase in NCAM polysialylation appears to be a universal learning response to both spatial and nonspatial paradigms as similar time-dependent changes occurred following training in a one-trial, step-through, passive avoidance response subsequent to water maze training.     

The minimal structural domains required for neural cell adhesion molecule polysialylation by PST/ST8Sia IV and STX/ST8Sia II

A limited number of mammalian proteins are modified by polysialic acid, with the neural cell adhesion molecule (NCAM) being the most abundant of these. We hypothesize that polysialylation is a protein-specific glycosylation event and that an initial protein-protein interaction between polysialyltransferases and glycoprotein substrates mediates this specificity. To evaluate the regions of NCAM required for recognition and polysialylation by PST/ST8Sia IV and STX/ST8Sia II, a series of domain deletion proteins were generated, co-expressed with each enzyme, and their polysialylation analyzed. A protein consisting of the fifth immunoglobulin-like domain (Ig5), which contains the reported sites of polysialylation, and the first fibronectin type III repeat (FN1) was polysialylated by both enzymes, whereas a protein consisting of Ig5 alone was not polysialylated by either enzyme. This demonstrates that the Ig5 domain of NCAM and FN1 are sufficient for polysialylation, and suggests that the FN1 may constitute an enzyme recognition and docking site. Two other NCAM mutants, NCAM-6 (Ig1-5) and NCAM-7 (FN1-FN2), were weakly polysialylated by PST/ST8Sia IV, suggesting that a weaker enzyme recognition site may exist within the Ig domains, and that glycans in the FN region are polysialylated. Further analysis indicated that O-linked oligosaccharides in NCAM-7, and O-linked and N-linked glycans in full-length NCAM, are polysialylated when these proteins are co-expressed with the polysialyltransferases in COS-1 cells. Our data support a model in which the polysialyltransferases bind to the FN1 of NCAM to polymerize polysialic acid chains on appropriately presented glycans in adjacent regions.     

Experimental approaches to interfere with the polysialylation of the neural cell adhesion molecule in vitro and in vivo

Sialic acid (Sia) is expressed as terminal sugar in many glycoconjugates and plays an important role during development and regeneration. Addition of homopolymers of Sia (polysialic acid; polySia/PSA) is a unique and highly regulated post-translational modification of the neural cell adhesion molecule (NCAM). The presence of polySia affects NCAM-dependent cell adhesion and plays an important role during brain development, neural regeneration, and plastic processes including learning and memory. PolySia-NCAM is expressed on several neuroendocrine tumors of high malignancy and correlates with poor prognosis. Two closely related enzymes, the polysialyltransferases ST8SiaII and ST8SiaIV, catalyze the biosynthesis of polySia. This review summarizes recent knowledge on Sia biosynthesis and the correlation between Sia biosynthesis and polysialylation of NCAM and report on approaches to modify the degree of polySia on NCAM in vitro and in vivo. First, we describe the inhibition of polysialylation of NCAM in ST8SiaII-expressing cells using synthetic Sia precursors. Second, we demonstrate that the key enzyme of the Sia biosynthesis (UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase) regulates and limits the synthesis of polySia by controlling the cellular Sia concentration.     

Specific amino acids in the first fibronectin type III repeat of the neural cell adhesion molecule play a role in its recognition and polysialylation by the polysialyltransferase ST8Sia IV/PST

Polysialic acid is an anti-adhesive protein modification that promotes cell migration and the plasticity of cell interactions. Because so few proteins carry polysialic acid, we hypothesized that polysialylation is a protein-specific event and that a specific polysialyltransferase-substrate interaction is the basis of this specificity. The major substrate for the polysialyltransferases is the neural cell adhesion molecule, NCAM. Previous work demonstrates that the first fibronectin type III repeat of NCAM (FN1) was necessary for the polysialylation of the N-glycans on the adjacent immunoglobulin domain (Ig5) (Close, B. E., Mendiratta, S. S., Geiger, K. M., Broom, L. J., Ho, L. L., and Colley, K. J. (2003) J. Biol. Chem. 278, 30796-30805). This suggested that FN1 may be a recognition site for the polysialyltransferases. In this study, we showed that the second fibronectin type III repeat (FN2) of NCAM cannot replace FN1. Arg substitution of three unique acidic amino acids on the surface of FN1 eliminated polysialylation not only of a minimal Ig5-FN1 substrate but also of full-length NCAM. Ala substitution of these residues eliminated Ig5-FN1 polysialylation but not that of full-length NCAM, suggesting that the two proteins are interacting differently with the enzymes and that multiple residues are involved in the enzyme-NCAM interaction. By using another truncated protein, Ig5-FN1-FN2, we confirmed the importance of enzyme-substrate positioning for optimal recognition and polysialylation. In sum, we have found that acidic residues on the surface of FN1 are part of a larger protein interaction region that is critical for NCAM recognition and polysialylation by the polysialyltransferases.     

Enzyme-dependent variations in the polysialylation of the neural cell adhesion molecule (NCAM) in vivo

Polysialic acid (polySia), an alpha2,8-linked polymer of N-acetylneuraminic acid, represents an essential regulator of neural cell adhesion molecule (NCAM) functions. Two polysialyltransferases, ST8SiaII and ST8SiaIV, account for polySia synthesis, but their individual roles in vivo are still not fully understood. Previous in vitro studies defined differences between the two enzymes in their usage of the two NCAM N-glycosylation sites affected and suggested a synergistic effect. Using mutant mice, lacking either enzyme, we now assessed in vivo the contribution of ST8SiaII and ST8SiaIV to polysialylation of NCAM. PolySia-NCAM was isolated from mouse brains and trypsinized, and polysialylated glycopeptides as well as glycans were analyzed in detail. Our results revealed an identical glycosylation and almost complete polysialylation of N-glycosylation sites 5 and 6 in polySia-NCAM irrespective of the enzyme present. The same sets of glycans were substituted by identical numbers of polySia chains in vivo, the length distribution of which, however, differed with the enzyme setting. Expression of ST8SiaIV alone led to higher amounts of short polySia chains and gradual decrease with length, whereas exclusive action of ST8SiaII evoked a slight reduction in long polySia chains only. These variations were most pronounced at N-glycosylation site 5, whereas the polysialylation pattern at N-glycosylation site 6 did not differ between NCAM from wild-type and ST8SiaII- or ST8SiaIV-deficient mice. Thus, our fine structure analyses suggest a comparable quality of polysialylation by ST8SiaII and ST8SiaIV and a distinct synergistic action of the two enzymes in the synthesis of long polySia chains at N-glycosylation site 5 in vivo.     

The polysialyltransferase ST8Sia II/STX: posttranslational processing and role of autopolysialylation in the polysialylation of neural cell adhesion molecule.

The presence of alpha2,8-linked polysialic acid on the neural cell adhesion molecule (NCAM) is known to modulate cell interactions during development and oncogenesis. Two enzymes, the alpha2,8-polysialyltransferases ST8Sia IV()/PST and ST8Sia II()/STX are responsible for the polysialylation of NCAM. We previously reported that both ST8Sia IV/PST and ST8Sia II/STX enzymes are themselves modified by alpha2,8-linked polysialic acid chains, a process called autopolysialylation. In the case of ST8Sia IV/PST, autopolysialylation is not required for enzymatic activity. However, whether the autopolysialylation of ST8Sia II/STX is required for its ability to polysialylate NCAM is unknown. To understand how autopolysialylation impacts ST8Sia II/STX enzymatic activity, we employed a mutagenesis approach. We found that ST8Sia II/STX is modified by six Asn-linked oligosaccharides and that polysialic acid is distributed among the oligosaccharides modifying Asn 89, 219, and 234. Coexpression of a nonautopolysialylated ST8Sia II/STX mutant with NCAM demonstrated that autopolysialylation is not required for ST8Sia II/STX polysialyltransferase activity. In addition, catalytically active, nonautopolysialylated ST8Sia II/STX does not polysialylate any endogenous COS-1 cell proteins, highlighting the protein specificity of polysialylation. Furthermore, immunoblot analysis of NCAM polysialylation by autopolysialylated and nonautopolysialylated ST8Sia II/STX suggests that the NCAM is polysialylated to a higher degree by autopolysialylated ST8Sia II/STX. Therefore, we conclude that autopolysialylation of ST8Sia II/STX, like that of ST8Sia IV/PST, is not required for, but does enhance, NCAM polysialylation.     

Structural studies of the neural-cell-adhesion molecule by X-ray and neutron reflectivity

The structures of adhesion proteins play an important role in the formation of intercellular junctions and the control of intermembrane spacing. This paper describes the combination of neutron and X-ray specular reflectivity measurements to investigate the structure of the ectodomain of the neural-cell-adhesion molecule (NCAM). The measurements with unmodified NCAM suggest the presence of a bend in the extracellular region. Measurements with the polysialic-acid-modified form of NCAM reveal that, at physiological ionic strength, the carbohydrate chains extend beyond the range of the unmodified protein. The excluded volume of the polymer is also ionic-strength-dependent, as expected for a polyelectrolyte. The structural characteristics obtained from these independent analyses of X-ray and neutron reflectivity data agree with each other, with prior reflectivity studies, and with molecular dimensions obtained from direct-force measurements. These results provide structural insights into the configuration of the NCAM ectodomain and the regulation of NCAM adhesion by post-translational modification.     

Polysialylation promotes neural cell adhesion molecule-mediated cell migration in a fibroblast growth factor receptor-dependent manner, but independent of adhesion capability

Polysialic acid (PSA), a carbohydrate polymer mainly present in the neural cell adhesion molecule (NCAM), promotes neural plasticity; however, its mode of action in tumor malignancy remains largely unknown. In this study, we investigated the influence of polysialylation on cell migration. PSA consistently promoted cell migration on different extracellular matrices (ECMs) but differentially affected cell adhesion. All of these actions were reversed by endo-N-acetylneuraminidase treatment, and PSA-driven migration was inhibited by the specific fibroblast growth factor receptor (FGFR) inhibitor Su5402. Consistent with this latter observation, PSA-stimulated migration on different ECMs was paralleled by activation of the FGFR and its downstream signaling components, PLC-γ, focal adhesion kinase and extracellular signal-regulated kinase 1/2. In contrast, the pattern of p59(fyn) activation correlated with differential adhesion to different ECMs. Collectively, these results indicate that PSA-conjugated NCAM potentiates signal transduction by the FGFR pathway and thereby enhances cell migration independent of adhesion capability, providing additional insights into the role of PSA in cancer development.     

Polysialyltransferase ST8Sia II (STX) polysialylates all of the major isoforms of NCAM and facilitates neurite outgrowth

The neural cell adhesion molecule (NCAM) has different isoforms due to different sizes in its polypeptide and plays a significant role in neural development. In neural development, the function of NCAM is modified by polysialylation catalyzed by two polysialyltransferases, ST8Sia II and ST8Sia IV. Previously, it was reported by others that ST8Sia II polysialylates only transmembrane isoforms of the NCAM, such as NCAM-140 and NCAM-180, but not NCAM-120 and NCAM-125 anchored by a glycosylphosphotidylinositol. In the present study, we first discovered that ST8Sia II polysialylates all isoforms of the NCAM examined, and we demonstrated that polysialylation of NCAM expressed on 3T3 cells facilitates neurite outgrowth regardless of isoforms of NCAM, where polysialic acid is attached. We then show that neurite outgrowth is significantly facilitated only when polysialylated NCAM is present in cell membranes. Moreover, the soluble NCAM coated on plates did not have an effect on neurite outgrowth exerted by soluble L1 adhesion molecule coated on plates. These results, taken together, indicate that ST8Sia II plays critical roles in modulating the function of all major isoforms of NCAM. The results also support previous studies showing that a signal cascade initiated by NCAM differs from that initiated by L1 molecule.     

Differential biosynthesis of polysialic acid on neural cell adhesion molecule (NCAM) and oligosaccharide acceptors by three distinct alpha 2,8-sialyltransferases, ST8Sia IV (PST), ST8Sia II (STX), and ST8Sia III

Polysialylated neural cell adhesion molecule (NCAM) is thought to play a critical role in neural development. Polysialylation of NCAM was shown to be achieved by two alpha2,8-polysialyltransferases, ST8Sia IV (PST) and ST8Sia II (STX), which are moderately related to another alpha2,8-sialyltransferase, ST8Sia III. Here we describe that all three alpha2,8-sialyltransferases can utilize oligosaccharides as acceptors but differ in the efficiency of adding polysialic acid on NCAM. First, we found that ST8Sia III can form polysialic acid on the enzyme itself (autopolysialylation) but not on NCAM. These discoveries prompted us to determine if ST8Sia IV and ST8Sia II share the property of ST8Sia III in utilizing low molecular weight oligosaccharides as acceptors. By using a newly established method, we found that ST8Sia IV, ST8Sia II, and ST8Sia III all add oligosialic and polysialic acid on various sialylated N-acetyllactosaminyl oligosaccharides, including NCAM N-glycans, fetuin N-glycans, synthetic sialylated N-acetyllactosamines, and on alpha(2)-HS-glycoprotein. Our results also showed that monosialyl and disialyl N-acetyllactosamines can serve equally as an acceptor, suggesting that no initial addition of alpha2,8-sialic acid is necessary for the action of polysialyltransferases. Polysialylation of NCAM by ST8Sia IV and ST8Sia II is much more efficient than polysialylation of N-glycans isolated from NCAM. Moreover, ST8Sia IV and ST8Sia II catalyze polysialylation of NCAM much more efficiently than ST8Sia III. These results suggest that no specific acceptor recognition is involved in polysialylation of low molecular weight sialylated oligosaccharides, whereas the enzymes exhibit pronounced acceptor specificities if glycoproteins are used as acceptors.     

Polysialylation of the neural cell adhesion molecule: interfering with polysialylation and migration in neuroblastoma cells

Polysialic acid represents a unique posttranslational modification of the neural cell adhesion molecule (NCAM). It is built as a homopolymer of up to 150 molecules of alpha 2-8-linked sialic acids on N-glycans of the fifth immunoglobulin-like domain of NCAM. Besides its role in cell migration and axonal growth during development, polysialic acids are closely related to tumor malignancy as they are linked to the malignant potential of several tumors, such as undifferentiated neuroblastoma. Polysialic acid expression is significantly more frequent in high-grade tumors than in low-grade tumors. It is synthesized in the Golgi apparatus by the activity of two closely related enzymes, the polysialyltransferases ST8SiaII and ST8SiaIV. Interestingly, polysialylation of tumors is not equally synthesized by both polysialyltransferases. It has been shown that especially the ST8SiaII gene is not expressed in some normal tissue, but is strongly expressed in tumor tissue. Here we summarize some knowledge on the role of polysialic acid in cell migration and tumor progression and present novel evidence that interfering with polysialylation using unnatural sialic acid precursors decreases the migration of neuroblastoma cells.