Analysis of O-acetylated sialic acids by 3-nitrophenylhydrazine derivatization combined with LC-MS/MS.

Sialic acids are a family of monosaccharides that share a nine-carbon backbone and a carboxyl group. A recent derivatization method based on 3-nitrophenylhydrazine (3-NPH) provides a mild chemical labeling technique for biomolecules containing carbonyl or carboxyl groups. In this study, we utilized 3-NPH to label sialic acids via a two-step derivatization process. The derivatized species can produce a common reporter ion corresponding to C1-C3 with two labels, and a fragment differentiating between Neu5Ac, Neu5Gc, and KDN. This method is compatible with O-acetylated sialic acids and provides high sensitivity to Neu5Gc and KDN, and since the utilization of dual labeling significantly enhances the hydrophobicity of derivatives, it can effectively mitigate matrix effects when combined with parallel reaction monitoring technology. Negative-ion tandem mass spectrometry (MS/MS) analysis reveals a distinctive fragmentation profile for the 4-O-acetylated species, while the other sialic acids yield similar MS/MS spectra with a high abundance of reporter ions. Using the reporter ion as a transition, this analytical strategy is effective for analyzing complex biological samples. For example, it was successfully employed to quantify sialic acids in the intestinal tissues of several carp species, demonstrating its potential in sialylation research.

Quantitation of bioactive components in infant formulas: Milk oligosaccharides, sialic acids and corticosteroids.

Human milk is considered the optimal food for infants with abundant nutrients and bioactive components, which play key roles in infant health and development. Infant formulas represent appropriate substitutes for human milk. There are many brands of infant formula with different ingredient sources and functions on the market. The present study aims to quantify important bioactive components, i.e., milk oligosaccharides (MOS), sialic acids (Sia) and corticosteroids, in different infant formulas and compare these to human milk. In total, 12 different infant formulas available on the Dutch market were analyzed in this study. The concentrations of MOS and Sia were characterized by UHPLC-FLD and LC-MS, while corticosteroids were determined using established UHPLC-MS/MS methods. Among infant formulas, 15 structures of oligosaccharides were identified, of which 2'-Fucosyllactose (2'FL), 3'-Galactosyllactose (3'GL) and 6'-Galactosyllactose (6́'GL) were found in all infant formulas. The oligosaccharide concentrations differed between milk source and brands and were 3-5 times lower than in human milk. All infant formulas contained Sia, N-acetylneuraminic acid (Neu5Ac) was dominant in bovine milk-based formulas, while N-glycolylneuraminic acid (Neu5Gc) was major in goat milk-based formula. All infant formulas contained corticosteroids, yet, at lower concentrations than human milk. Insight in concentrations of bioactive components in infant formula compared to human milk may give direction to dietary advices and/or novel formula design.

Predicting Sialic Acid Content of N-Linked Glycans Using the Isotopic Pattern of Chlorine.

Sialic acids play several roles in both physiological and pathological processes; however, due to their labile nature, they are difficult to analyze using mass spectrometry. Previous work has shown that infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) is able to detect intact sialylated N-linked glycans without the use of chemical derivatization. In this work, we describe a new rule that can predict the number of sialic acids on a glycan. Formalin-fixed paraffin-embedded human kidney tissue was prepared using previously established methods and analyzed using IR-MALDESI in negative-ion mode mass spectrometry. Using the experimental isotopic distribution of a detected glycan, we can predict the number of sialic acids on the glycan; #sialic acids is equal to the charge state minus the number of chlorine adducts, or z - #Cl-. This new rule grants confident glycan annotations and compositions beyond accurate mass measurements, thereby further improving the capability of IR-MALDESI to study sialylated N-linked glycans within biological tissues.

Acarbose Mitigates Age-Dependent Alterations in Erythrocyte Membrane Transporters During Aging in Rats.

Acarbose (ACA), a well-studied and effective inhibitor of α-amylase and α-glucosidase, is a postprandial-acting antidiabetic medicine. The membrane of the erythrocyte is an excellent tool for analyzing different physiological and biochemical activities since it experiences a range of metabolic alterations throughout aging. It is uncertain if ACA modulates erythrocyte membrane activities in an age-dependent manner. As a result, the current study was conducted to explore the influence of ACA on age-dependent deteriorated functions of transporters/exchangers, disrupted levels of various biomarkers such as lipid hydroperoxides (LHs), protein carbonyl (PCO), sialic acid (SA), total thiol (-SH), and erythrocyte membrane osmotic fragility. In addition to a concurrent increase in Na+/H+ exchanger activity and concentration of LH, PCO, and osmotic fragility, we also detected a considerable decrease in membrane-linked activities of Ca2+-ATPase (PMCA) and Na+/K+-ATPase (NKA), as well as concentrations of SA and -SH in old-aged rats. The aging-induced impairment of the activities of membrane-bound ATPases and the changed levels of redox biomarkers were shown to be effectively restored by ACA treatment.

Analysis of the intramolecular 1,7-lactone of N-acetylneuraminic acid using HPLC-MS: relationship between detection and stability.

A subclass of the sialic acid family consists of intramolecular lactones that may function as key indicators of physiological and pathological states. However, the existence of these compounds in free form is highly improbable, since they are unlikely to exist in an aqueous solution due to their lability. Current analytical method used to detect them in biological fluids has not recognized their reactivity in solution and is prone to misidentification. However, recent advances in synthetic methods for 1,7-lactones have allowed the preparation of these sialic acid derivatives as authentic reference standards. We report here the development of a new HPLC-MS method for the simultaneous detection of the 1,7-lactone of N-acetylneuraminic acid, its γ-lactone derivative, and N-acetylneuraminic acid that overcomes the limitations of the previous analytical procedure for their identification.

Determination of Sialic Acid Isomers from Released N-Glycans Using Ion Mobility Spectrometry.

Complex carbohydrates are ubiquitous in nature and represent one of the major classes of biopolymers. They can exhibit highly diverse structures with multiple branched sites as well as a complex regio- and stereochemistry. A common way to analytically address this complexity is liquid chromatography (LC) in combination with mass spectrometry (MS). However, MS-based detection often does not provide sufficient information to distinguish glycan isomers. Ion mobility-mass spectrometry (IM-MS)─a technique that separates ions based on their size, charge, and shape─has recently shown great potential to solve this problem by identifying characteristic isomeric glycan features such as the sialylation and fucosylation pattern. However, while both LC-MS and IM-MS have clearly proven their individual capabilities for glycan analysis, attempts to combine both methods into a consistent workflow are lacking. Here, we close this gap and combine hydrophilic interaction liquid chromatography (HILIC) with IM-MS to analyze the glycan structures released from human alpha-1-acid glycoprotein (hAGP). HILIC separates the crude mixture of highly sialylated multi-antennary glycans, MS provides information on glycan composition, and IMS is used to distinguish and quantify α2,6- and α2,3-linked sialic acid isomers based on characteristic fragments. Further, the technique can support the assignment of antenna fucosylation. This feature mapping can confidently assign glycan isomers with multiple sialic acids within one LC-IM-MS run and is fully compatible with existing workflows for N-glycan analysis.

Fast Discrimination of Sialylated N-Glycan Linkage Isomers with One-Step Derivatization by Microfluidic Capillary Electrophoresis-Mass Spectrometry.

Linkage isomers (α-2,3- or α-2,6-linkage) of sialylated N-glycans are involved in the emergence and progression of some diseases, so they are of great significance for diagnosing and monitoring diseases. However, the qualitative and quantitative analysis of sialylated N-glycan linkage isomers remains challenging due to their low abundance and limited isomeric separation techniques. Herein, we developed a novel strategy integrating one-step sialic acid derivatization, positive charge-sensitive separation and highly sensitive detection based on microfluidic capillary electrophoresis-mass spectrometry (MCE-MS) for fast and specific analysis of α-2,3- and α-2,6-linked sialylated N-glycan isomers. A kind of easily charged long-chain amino compound was screened first for one-step sialic acid derivatization so that only α-2,3- and α-2,6-linked isomers can be quickly and efficiently separated within 10 min by MCE due to the difference in structural conformation, whose separation mechanism was further theoretically supported by molecular dynamic simulation. In addition, different sialylated N-glycans were separated in order according to the number of sialic acids, so that a migration time-based prediction of the number of sialic acids was achieved. Finally, the sialylated N-glycome of human serum was profiled within 10 min and 6 of the 52 detected sialylated N-glycans could be potential diagnostic biomarkers of cervical cancer (CC), whose α-2,3- and α-2,6-linked isomers were distinguished by α-2,3Neuraminidase S.

DMTMM-mediated methylamidation for MALDI mass spectrometry analysis of N-glycans with structurally conserved sialic acid residues in biological fluids "via direttissima".

Characterization of serum glycoprotein N-glycans with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) in positive-ion mode needs a derivatization step to stabilize and neutralize the negative charge on sialic acids. The acidic sugars are attached to the end of glycoproteins, glycolipids or gangliosides. Here, we present a method for sialic acid stabilization via modification based on derivatization of carboxylic acid group activated with 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM) with methylamine. DMTMM substitutes in many processes N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide (EDC/NHS) as activation reagent due to its better performance and higher stability in water. Glycosylated proteins are used as solid phase support for glycan derivatization and purification from excess of derivatization reagents. We evaluated our glycan analysis method in murine sera and intestinal lavages. The stabilization of sialic acid enables a complete conservation of the glycan structures, in contrast to other methods where sialic acids are partially lost. In BALB/c mouse sera, we detected predominantly mono- and di-sialylated N-glycans with mostly N-Glycolylneuraminic acid (Neu5Gc) and only trace amounts of N-Acetyl neuraminic acid (Neu5Ac). BALB/c mouse intestinal lavages glycoproteins contained asialo N-glycans. DMTMM-mediated methylamidation of N-glycans for MALDI mass spectrometry analysis is a fast and cheap method for structurally conserved glycan derivatization.

Sialic acids in gynecological cancer development and progression: Impact on diagnosis and treatment.

Gynecological cancers are in the top 10 of most common cancers in women. Survival and outcome are strongly related to the stage at diagnosis. Therefore, early diagnosis is essential in reducing morbidity and mortality. The high mortality rate of gynecological cancers can mainly be attributed to ovarian cancer (OC). OC is commonly diagnosed at an advanced stage due to a lack of proper screening tools allowing early detection. Endometrial cancer (EC) on the contrary, is mostly diagnosed at an early stage and has, in general, better outcomes. The incidence of nonendometrioid EC has increased in the last decade, displaying a shared tumor biology with OC and consequently significantly worse outcome. New approaches allowing detection of gynecological cancers in an early stage are therefore desired. Recent studies on cancer biology have shown the relevance of altered glycosylation in the occurrence and progression of cancer. The aberrant expression of sialic acid, a specific carbohydrate terminating glycoproteins and glycolipids on the cell-surface, is frequently correlated with malignancy. We aimed to determine the current understanding of sialic acid function in different gynecological cancers to identify the gaps in knowledge and its potential use for new diagnostic and therapeutic avenues. Therefore we performed a review on current literature focusing on studies where sialylation was linked to gynecological cancers. The identified studies showed elevated levels of sialic acid in serum, tissue and sialylated antigens in most patients with gynecological cancers, underlining its potential for diagnosis.

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.

Metabolic Labeling of Legionaminic Acid in Flagellin Glycosylation of Campylobacter jejuni Identifies Maf4 as a Putative Legionaminyl Transferase.

Campylobacter jejuni is the major human food-borne pathogen. Its bipolar flagella are heavily O-glycosylated with microbial sialic acids and essential for its motility and pathogenicity. However, both the glycosylation of flagella and the exact contribution of legionaminic acid (Leg) to flagellar activity is poorly understood. Herein, we report the development of a metabolic labeling method for Leg glycosylation on bacterial flagella with probes based on azide-modified Leg precursors. The hereby azido-Leg labeled flagellin could be detected by Western blot analysis and imaged on intact bacteria. Using the probes on C. jejuni and its isogenic maf4 mutant we also further substantiated the identification of Maf4 as a putative Leg glycosyltransferase. Further evidence was provided by UPLC-MS detection of labeled CMP-Leg and an in silico model of Maf4. This method and the developed probes will facilitate the study of Leg glycosylation and the functional role of this modification in C. jejuni motility and invasiveness.

Chemical Synthesis of Sialyl N-Glycans and Analysis of Their Recognition by Neuraminidase.

The chemical synthesis of a fully sialylated tetraantennary N-glycan has been achieved for the first time by using the diacetyl strategy, in which NHAc is protected as NAc2 to improve reactivity by preventing intermolecular hydrogen bonds. Another key was the glycosylation to the branched mannose in an ether solvent, which promoted the desired glycosylation by stabilizing the oxocarbenium ion intermediate. Furthermore, high α-selectivity of these glycosylation reactions was realized by utilizing remote participation. Two asymmetrically deuterium labeled sialyl N-glycans were also synthesized by the same strategy. The synthesized N-glycans were used to probe the molecular basis of H1N1 neuraminidase recognition. The asymmetrically deuterated N-glycans revealed a difference in the recognition of sialic acid on each branch. Meanwhile, the tetraantennary N-glycan was used to evaluate the effects of multivalency and steric hinderance by forming branching structures.

Detection of cell-surface sialic acids and photodynamic eradication of cancer cells using dye-modified polydopamine-coated gold nanobipyramids.

A nanoprobe based on polydopamine-coated gold nanobipyramids surface modified with molecules of a phenylboronic acid-substituted distyryl boron dipyrromethene has been fabricated and characterised using various physical and spectroscopic methods. It serves as an ultrasensitive sensor for sialic acids on the surface of cancer cells based on its dual surface-enhanced Raman scattering and fluorescence response. This biomarker can also trigger the photodynamic activity of these nanobipyramids, effectively eradicating the cancer cells mainly through apoptosis as shown by various bioassays.

Reversible O-Acetyl Migration within the Sialic Acid Side Chain and Its Influence on Protein Recognition.

O-Acetylation is a common naturally occurring modification of carbohydrates and is especially widespread in sialic acids, a family of nine-carbon acidic monosaccharides. O-Acetyl migration within the exocyclic glycerol-like side chain of mono-O-acetylated sialic acid reported previously was from the C7- to C9-hydroxyl group with or without an 8-O-acetyl intermediate, which resulted in an equilibrium that favors the formation of the 9-O-acetyl sialic acid. Herein, we provide direct experimental evidence demonstrating that O-acetyl migration is bidirectional, and the rate of equilibration is influenced predominantly by the pH of the sample. While the O-acetyl group on sialic acids and sialoglycans is stable under mildly acidic conditions (pH < 5, the rate of O-acetyl migration is extremely low), reversible O-acetyl migration is observed readily at neutral pH and becomes more significant when the pH increases to slightly basic. Sialoglycan microarray studies showed that esterase-inactivated porcine torovirus hemagglutinin-esterase bound strongly to sialoglycans containing a more stable 9-N-acetylated sialic acid analog, but these compounds were less resistant to periodate oxidation treatment compared to their 9-O-acetyl counterparts. Together with prior studies, the results support the possible influence of sialic acid O-acetylation and O-acetyl migration to host-microbe interactions and potential application of the more stable synthetic N-acetyl mimics.

An assay for quantitative analysis of polysialic acid expression in cancer cells.

Polysialic acid (polySia) is a linear polysaccharide comprised of N-acetylneuraminic acid residues and its over-expression in cancer cells has been correlated with poor clinical prognosis. An assay has been developed for quantitative analysis of cellular polySia expression. This was achieved by extracting and purifying released polySia from glycoproteins by mild acid hydrolysis and optimised organic extraction. The polySia was further hydrolysed into Sia monomers, followed by fluorescent labelling and quantitative analysis. The assay was qualified utilising endoneuraminidase-NF to remove polySia from the surface of C6-ST8SiaII cancer cells (EC50 = 2.13 ng/mL). The result was comparable to that obtained in a polySia-specific cellular ELISA assay. Furthermore, the assay proved suitable for evaluation of changes in polySia expression following treatment with a small molecule inhibitor of polysialylation. Given the importance of polySia in multiple disease states, notably cancer, this is a potentially vital tool with applications in the fields of drug discovery and glycobiology.

Polysialic acid in the rat brainstem and thoracolumbar spinal cord: Distribution, cellular location, and comparison with mouse.

Polysialic acid (polySia), a homopolymer of α2,8-linked glycans, is a posttranslational modification on a few glycoproteins, most commonly in the brain, on the neural cell adhesion molecule. Most research in the adult central nervous system has focused on its expression in higher brain regions, where its distribution coincides with regions known to exhibit high levels of synaptic plasticity. In contrast, scant attention has been paid to the expression of polySia in the hindbrain. The main aims of the study were to examine the distribution of polySia immunoreactivity in the brainstem and thoracolumbar spinal cord, to compare the distribution of polySia revealed by two commercial antibodies commonly used for its investigation, and to compare labeling in the rat and mouse. We present a comprehensive atlas of polySia immunoreactivity: we report that polySia labeling is particularly dense in the dorsal tegmentum, medial vestibular nuclei and lateral parabrachial nucleus, and in brainstem regions associated with autonomic function, including the dorsal vagal complex, A5, rostral ventral medulla, A1, and midline raphe, as well as sympathetic preganglionic neurons in the spinal cord and central targets of primary sensory afferents (nucleus of the solitary tract, spinal trigeminal nucleus, and dorsal horn [DH]). Ultrastructural examination showed labeling was present predominantly on the plasma membrane/within the extracellular space/in or on astrocytes. Labeling throughout the brainstem and spinal cord were very similar for the two antibodies and was eliminated by the polySia-specific sialidase, Endo-NF. Similar patterns of distribution were found in rat and mouse brainstem with differences evident in DH.

Assays for the identification and quantification of sialic acids: Challenges, opportunities and future perspectives.

N-Acetyl neuraminic acid (sialic acid) is a monosaccharide generally found as the terminating unit on glycans, which in turn are found on the surface of cells and glycoproteins. These glycans aid in a variety of biological functions such as cell interactions and immune response. Sialic acid has been identified as a biomarker for cardiovascular disease, diabetes and a range of other inflammatory and degenerative conditions. It has also been identified as a marker for different types of cancer. Sialic acid levels vary depending on the level of inflammation present during the course of an inflammatory disease and it is overexpressed by tumours as a shield against the immune system. Since the discovery of sialic acid, numerous assays have been developed for the identification and quantification of different sialic acid derivative monosaccharides and these assays fall into four main groups: colorimetric, fluorometric, enzymatic and chromatographic/mass spectrometric, with much overlap between these. Given the importance of sialic acids in biological pathways, this review article critically appraises assays that are used to detect and quantify sialic acid and its derivatives. Thus it details the method, sensitivity, specificity and wider scope of a range of assays, and concludes by suggesting some future directions for assay development and application. In this way, insight is provided into assays that allow for the accurate quantitation of sialic acid in biological samples, which may facilitate identification of the roles of sialic acid in healthy and disease pathways.

Blends of Human Milk Oligosaccharides Confer Intestinal Epithelial Barrier Protection in Vitro.

Breastfeeding is integral in the proper maturation of the intestinal barrier and protection against inflammatory diseases. When human milk (HM) is not available, supplementation with HM bioactives like Human Milk Oligosaccharides (HMOs) may help in providing breastfeeding barrier-protective benefits. An increasing HMO variety is becoming industrially available, enabling approaching the HMO complexity in HM. We aimed at assessing the impact of blends of available HMOs on epithelial barrier function in vitro. The capacity of individual [2'-Fucosyllactose (2'FL), Difucosyllactose, Lacto-N-tetraose, Lacto-N-neotetraose, 3'-Siallylactose and 6'-Siallylactose] or varying combinations of 3, 5 and 6 HMOs to modulate fluorescein-isothiocyanate (FITC)-labelled Dextran 4 KDa (FD4) translocation and/or transepithelial resistance (TEER) was characterized in Caco-2: HT29- methotrexate (MTX) cell line monolayers before and after an inflammatory challenge with TNF-α and IFN-γ. The six HMO blend (HMO6) dose-dependently limited the cytokine-induced FD4 translocation and TEER decrease and increased TEER values before challenge. Similarly, 3 and 5 HMO blends conferred a significant protection against the challenge, with 2'FL, one of the most abundant but most variable oligosaccharides in HM, being a key contributor. Overall, our results suggest differential ability of specific HMOs in modulating the intestinal barrier and support the potential of supplementation with combinations of available HMOs to promote gut health and protect against intestinal inflammatory disorders.

Erythrocytic membrane anionic charge, sialic acid content, and their correlations with urinary glycosaminoglycans in preeclampsia and eclampsia.

Compared to healthy pregnant women, changes in erythrocytic membrane anionic charge (EAC) and urinary glycosaminoglycans (UGAGS) have been reported in African women with preeclampsia. A single previous study showed a decrease in erythrocytic membrane sialic acid (EMSA) in preeclampsia compared to healthy pregnancy; however, EMSA was not significantly different between women with preeclampsia and non-pregnant women. No study has focused on the relationships between EAC, EMSA, and UGAGS in preeclampsia and eclampsia compared to healthy pregnant and non-pregnant women of reproductive age. Moreover, the erythrocyte membrane contains sialoglycoproteins and proteoglycans involved in creating the negatively charged cell surface, disruption of which leads to erythrocyte aggregation seen in preeclampsia/eclampsia. However, the etiopathogenesis of preeclampsia and eclampsia remains unclear. Therefore, we evaluated the relationship between EAC, UGAGS, and EMSA in preeclampsia and eclampsia. Three groups of 30 women each were enrolled: Group A (non-pregnant women), Group B (healthy pregnant women without complications), and Group C (women with preeclampsia/eclampsia). EMSA was diminished under oxidative stress prevalent in eclampsia and preeclampsia which might have caused a decreased EAC. EAC was negatively correlated with UGAGS and positively correlated with EMSA (p < .001). EMSA was negatively correlated with UGAGS (p < .001). In conclusion, a loss of sialic acid from the erythrocyte membrane causes a significant decrease in the EAC which mirrors the decrease in the negative charge of the renal glomerular basement membrane and might lead to proteinuria and increased UGAGS excretion in preeclampsia and eclampsia.

Glycoproteomic measurement of site-specific polysialylation.

Polysialylation is the enzymatic addition of a highly negatively charged sialic acid polymer to the non-reducing termini of glycans. Polysialylation plays an important role in development, and is involved in neurological diseases, neural tissue regeneration, and cancer. Polysialic acid (PSA) is also a biodegradable and non-immunogenic conjugate to therapeutic drugs to improve their pharmacokinetics. PSA chains vary in length, composition, and linkages, while the specific sites of polysialylation are important determinants of protein function. However, PSA is difficult to analyse by mass spectrometry (MS) due to its high negative charge and size. Most analytical approaches for analysis of PSA measure its degree of polymerization and monosaccharide composition, but do not address the key questions of site specificity and occupancy. Here, we developed a high-throughput LC-ESI-MS/MS glycoproteomics method to measure site-specific polysialylation of glycoproteins. This method measures site-specific PSA modification by using mild acid hydrolysis to eliminate PSA and sialic acids while leaving the glycan backbone intact, together with protease digestion followed by LC-ESI-MS/MS glycopeptide detection. PSA-modified glycopeptides are not detectable by LC-ESI-MS/MS, but become detectable after desialylation, allowing measurement of site-specific PSA occupancy. This method is an efficient analytical workflow for the study of glycoprotein polysialylation in biological and therapeutic settings.