Characterization of oligo- and polysialic acids by MALDI-TOF-MS

N-glycolylneuraminic acid in red meat and processed meat is a health concern: A review on the formation, health risk, and reduction

One of the most consistent epidemiological associations between diet and human disease risk is the impact of consuming red meat and processed meat products. In recent years, the health concerns surrounding red meat and processed meat have gained worldwide attention. The fact that humans have lost the ability to synthesize N-glycolylneuraminic acid (Neu5Gc) makes red meat and processed meat products the most important source of exogenous Neu5Gc for humans. As our research of Neu5Gc has increased, it has been discovered that Neu5Gc in red meat and processed meat is a key factor in many major diseases. Given the objective evidence of the harmful risk caused by Neu5Gc in red meat and processed meat to human health, there is a need for heightened attention in the field of food. This updated review has several Neu5Gc aspects given including biosynthetic pathway of Neu5Gc and its accumulation in the human body, the distribution of Neu5Gc in food, the methods for detecting Neu5Gc, and most importantly, a systematic review of the existing methods for reducing the content of Neu5Gc in red meat and processed meat. It also provides some insights into the current status and future directions in this area.

Advances in high-resolution mass spectrometry techniques for analysis of high mass-to-charge ions

A major challenge in modern mass spectrometry (MS) is achieving high mass resolving power and accuracy for precision analyses in high mass-to-charge (m/z) regions. To advance the capability of MS for increasingly demanding applications, understanding limitations of state-of-the-art techniques and their status in applied sciences is essential. This review summarizes important instruments in high-resolution mass spectrometry (HRMS) and related advances to extend their working range to high m/z regions. It starts with an overview of HRMS techniques that provide adequate performance for macromolecular analysis, including Fourier-transform, time-of-flight (TOF), quadrupole-TOF, and related data-processing techniques. Methodologies and applications of HRMS for characterizing macromolecules in biochemistry and material sciences are summarized, such as top-down proteomics, native MS, drug discovery, structural virology, and polymer analyses.

Photothermal Therapy of Neuroblastoma via Polysialic Acid-Targeting Nanomissiles

Exploring new targets and developing novel targeted therapies are urgently needed for neuroblastoma therapy. Polysialic acid (polySia), a linear homopolymer of sialic acid units that correlates well with tumor progression and poor prognosis, has emerged as a potential target for neuroblastoma. However, the lack of polySia-specific binding reagents has severely limited the development of polySia-targeting therapeutics for neuroblastoma. Herein, the construction of polySia-targeting nanomissiles via molecular imprinting for the photothermal therapy of neuroblastoma is reported. Oligosialic acid (oligoSia) containing 3-4 units is considered as a characteristic structure for the recognition of polySia, while oligoSia containing 4-7 units digested from polySia is employed as the template. Via boronate-affinity controllable oriented surface imprinting, oligoSia-imprinted nanoparticles (oSia-MIP) are prepared. The oSia-MIP allows for specifically recognizing polySia and targeting polySia overexpressed neuroblastoma cells in vitro and in vivo. oSia-MIP loaded with indocyanine green is prepared and experimentally demonstrated to be a potent targeted photothermal therapeutic for neuroblastoma. Equipping the core substrate with functional entities, the developed polySia targeting nanoplatform can be accommodated to various therapeutic modalities, holding great promise for neuroblastoma targeted therapy.

Polysialic Acid in the Immune System

Polysialic acid (polySia) is a highly regulated polymer of sialic acid (Sia) with such potent biophysical characteristics that when expressed drastically influences the interaction properties of cells. Although much of what is known of polySia in mammals has been elucidated from the study of its role in the central nervous system (CNS), polySia is also expressed in other tissues, including the immune system where it presents dynamic changes during differentiation, maturation, and activation of different types of immune cells of the innate and adaptive response, being involved in key regulatory mechanisms. At least six polySia protein carriers (CCR7, ESL-1, NCAM, NRP2, ST8Sia 2, and ST8Sia 4) are expressed in different types of immune cells, but there is still much to be explored in regard not only to the regulatory mechanisms that determine their expression and the structure of polySia chains but also to the identification of the cis- and trans- ligands of polySia that establish signaling networks. This review summarizes the current knowledge on polySia in the immune system, addressing its biosynthesis, its tools for identification and structural characterization, and its functional roles and therapeutic implications.

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.

Long-term intrathecal administration of morphine vs. baclofen: Differences in CSF glycoconjugate profiles using multiglycomics

Opioid use for treatment of persistent pain has increased dramatically over the past two decades, but it has not resulted in improved pain management outcomes. To understand the molecular mechanisms of opioids, molecular signatures that arise from opioid exposure are often sought after, using various analytical methods. In this study, we performed proteomics, and multiglycomics via sequential analysis of polysialic acids, glycosaminoglycans, N-glycans and O-glycans, using the same cerebral spinal fluid (CSF) sample from patients that had long-term (>2 years), intrathecal morphine or baclofen administered via an indwelling pump. Proteomics and N-glycomics signatures between the two treatment groups were highly conserved, while significant differences were observed in polysialic acid, heparan sulfate glycosaminoglycan and O-glycan profiles between the two treatment groups. This represents the first study to investigate the potential relationships between diverse CSF conjugated glycans and long-term intrathecal drug exposure. The unique changes, observed by a sequential analytical workflow, reflect previously undescribed molecular effects of opioid administration and pain management.

The Loss of Polysialic Acid Impairs the Contractile Phenotype of Peritubular Smooth Muscle Cells in the Postnatal Testis

In the testis, the germinal epithelium of seminiferous tubules is surrounded by contractile peritubular cells, which are involved in sperm transport. Interestingly, in postnatal testis, polysialic acid (polySia), which is also an essential player for the development of the brain, was observed around the tubules. Western blotting revealed a massive decrease of polySia from postnatal day 1 towards puberty, together with a fundamental reduction of the net-like intertubular polySia. Using polysialyltransferase knockout mice, we investigated the consequences of the loss of polySia in the postnatal testis. Compared to postnatal wild-type animals, polySia knockouts showed slightly reduced smooth muscle actin (SMA) immunostaining of peritubular smooth muscle cells (SMCs), while calponin, marking more differentiated SMCs, dramatically decreased. In contrast, testicular SMA and calponin immunostaining remained unchanged in vascular SMCs in all genotypes. In addition, the cGMP-dependent protein kinase PKG I, a key enzyme of SMC relaxation, was nearly undetectable in the peritubular SMCs. Cell proliferation in the peritubular layer increased significantly in the knockouts, as shown by proliferating cell nuclear anti (PCNA) staining. Taken together, in postnatal testis, the absence of polySia resulted in an impaired differentiation of peritubular, but not vascular, SMCs to a more synthetic phenotype. Thus, polySia might influence the maintenance of a differentiated phenotype of non-vascular SMCs.

The structure and role of lactone intermediates in linkage-specific sialic acid derivatization reactions

Sialic acids occur ubiquitously throughout vertebrate glycomes and often endcap glycans in either α2,3- or α2,6-linkage with diverse biological roles. Linkage-specific sialic acid characterization is increasingly performed by mass spectrometry, aided by differential sialic acid derivatization to discriminate between linkage isomers. Typically, during the first step of such derivatization reactions, in the presence of a carboxyl group activator and a catalyst, α2,3-linked sialic acids condense with the subterminal monosaccharides to form lactones, while α2,6-linked sialic acids form amide or ester derivatives. In a second step, the lactones are converted into amide derivatives. Notably, the structure and role of the lactone intermediates in the reported reactions remained ambiguous, leaving it unclear to which extent the amidation of α2,3-linked sialic acids depended on direct aminolysis of the lactone, rather than lactone hydrolysis and subsequent amidation. In this report, we used mass spectrometry to unravel the role of the lactone intermediate in the amidation of α2,3-linked sialic acids by applying controlled reaction conditions on simple and complex glycan standards. The results unambiguously show that in common sialic acid derivatization protocols prior lactone formation is a prerequisite for the efficient, linkage-specific amidation of α2,3-linked sialic acids, which proceeds predominantly via direct aminolysis. Furthermore, nuclear magnetic resonance spectroscopy confirmed that exclusively the C2 lactone intermediate is formed on a sialyllactose standard. These insights allow a more rationalized method development for linkage-specific sialic derivatization in the future.

Glycomics studies using sialic acid derivatization and mass spectrometry

Proteins can undergo glycosylation during and/or after translation to afford glycoconjugates, which are often secreted by a cell or populate cell surfaces. Changes in the glycan portion can have a strong influence on a glycoconjugate and are associated with a multitude of human pathologies. Of particular interest are sialylated glycoconjugates, which exist as constitutional isomers that differ in their linkages (α2,3, α2,6, α2,8 or α2,9) between sialic acids and their neighbouring monosaccharides. In general, mass spectrometry enables the rapid and sensitive characterization of glycosylation, but there are challenges specific to identifying and (relatively) quantifying sialic acid isomers. These challenges can be addressed using linkage-specific methodologies for sialic acid derivatization, after which mass spectrometry can enable product identification. This Review is concerned with the new and important derivatization approaches reported in the past decade, which have been implemented in various mass-spectrometry-glycomics workflows and have found clinical glycomics applications. The convenience and wide applicability of the approaches make them attractive for studies of sialylation in different types of glycoconjugate.

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.

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Exploring and Exploiting Acceptor Preferences of the Human Polysialyltransferases as a Basis for an Inhibitor Screen

α2,8-Linked polysialic acid (polySia) is an oncofoetal antigen with high abundance during embryonic development. It reappears in malignant tumours of neuroendocrine origin. Two polysialyltransferases (polySTs) ST8SiaII and IV are responsible for polySia biosynthesis. During development, both enzymes are essential to control polySia expression. However, in tumours ST8SiaII is the prevalent enzyme. Consequently, ST8SiaII is an attractive target for novel cancer therapeutics. A major challenge is the high structural and functional conservation of ST8SiaII and -IV. An assay system that enables differential testing of ST8SiaII and -IV would be of high value to search for specific inhibitors. Here we exploited the different modes of acceptor recognition and elongation for this purpose. With DMB-DP3 and DMB-DP12 (fluorescently labelled sialic acid oligomers with a degree of polymerisation of 3 and 12, respectively) we identified stark differences between the two enzymes. The new acceptors enabled the simple comparative testing of the polyST initial transfer rate for a series of CMP-activated and N-substituted sialic acid derivatives. Of these derivatives, the non-transferable CMP-Neu5Cyclo was found to be a new, competitive ST8SiaII inhibitor.

In vitro generation of polysialylated cervical mucins by bacterial polysialyltransferases to counteract cytotoxicity of extracellular histones

Neutrophil extracellular traps (NET) are formed against pathogens. However, various diseases are directly linked to this meshwork of DNA. The cytotoxic properties of extracellular histones especially seem to be an important trigger during these diseases. Furthermore, NET accumulation on implants is discussed to result in an impaired efficiency or failure, depending on the category of implant. Interestingly, mucins have been investigated as surface coatings potentially capable of reducing neutrophil adhesion. Similarly, polysialic acid was shown to inactivate the cytotoxic properties of extracellular histones. We wanted to combine the probability to decrease the adhesion of neutrophils using mucins with the capability of sialic acid polymers to counteract histone-mediated cytotoxicity. To this end, we elongate cervical mucins using bacterial polysialyltransferases. Subsequent cell-based experiments demonstrated the activity of elongated mucins against histone-mediated cytotoxicity. Thus, polysialylated mucins may represent a novel component to coat implants or to combat diseases with exaggerated NET formation.

Mass Spectrometric Analysis of Oligo- and Polysialic Acids

Oligo- and polysialic acids (oligo/polySia) are involved in numerous biological processes depending on the chain length, the comprised type of sialic acids, as well as the glycosidic linkages. Here, we describe the determination of the composition, the sequence, as well as the linkages between the sialic acid residues of lactonized oligo/polySia using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS)(/MS) and electrospray-ionization (ESI)-MS((n)).

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.

High sensitivity combined with extended structural coverage of labile compounds via nanoelectrospray ionization at subambient pressures

Subambient pressure ionization with nanoelectrospray (SPIN) has proven to be effective in producing ions with high efficiency and transmitting them to low pressures for increased sensitivity in mass spectrometry (MS) analysis. Here we present evidence that the SPIN source not only improves MS sensitivity but also facilitates the detection of more labile compounds. The gentleness of conventional heated capillary electrospray ionization (ESI) and the SPIN designs was compared in conjunction with the liquid chromatography mass spectrometry (LC–MS) analysis of colominic acid and N-glycans containing sialic acid. Prior experiments conducted with the SPIN interface demonstrated the ability to detect labile glycans such as heavily sialylated and polysialic acid N-glycans, which are difficult to detect with a conventional ESI-MS interface. Colominic acid is a mixture of sialic acid polymers of different lengths containing labile glycosidic linkages between monomer units necessitating a gentle ion source. These labile covalent bonds may display similar behavior to sialic acid chains in N-glycans during MS analysis. By coupling the SPIN source with high-resolution mass spectrometry and using advanced data processing tools, we demonstrate much extended coverage of sialic acid polymer chains as compared to conventional ESI-MS and the ability to detect sialic acid containing N-glycans without the need of sample derivatization. In addition, we show that SPIN-LC–MS is effective in elucidating polymer features with high efficiency and high sensitivity previously unattainable by the conventional ESI-LC–MS methods.

Engineering the product profile of a polysialyltransferase

Oligo- and polysaccharides have myriad applications as therapeutic reagents from glycoconjugate vaccines to matrices for tissue engineering. Polysaccharide length may vary over several orders of magnitude and is a critical determinant of both their physical properties and biological activities. Therefore, the tailored synthesis of oligo- and polysaccharides of defined size is a major goal for glycoengineering. By mutagenesis and screening of a bacterial polysialyltransferase (polyST), we identified a single-residue switch that controls the size distribution of polymeric products. Specific substitutions at this site yielded distributive enzymes that synthesize polysaccharides with narrow size distribution ideal for glycoengineering applications. Mechanistic investigation revealed that the wild-type enzyme has an extended binding site that accommodates at least 20 residues of the growing polymer; changes in affinity along this binding site allow fine-tuning of the enzyme's product distribution.

Laser microdissection of paraffin embedded tissue as a tool to estimate the sialylation status of selected cell populations

In vertebrates, sialic acids occur at the terminal end of glycans mediating numerous biological processes like cell differentiation or tumor metastasis. Consequently, the cellular sialylation status under healthy and pathological conditions is of high interest. Existing analytical strategies to determine sialylation patterns are mostly applied to tissue samples consisting of a mixture of different cell types. Alterations in the sialylation status in a distinct area of tissues or in a specific cell population may, therefore, be easily overlooked. Likewise, estimated variations in sialylation in tissue homogenates might be simply the result of a changed cell composition. To overcome these limitations, we employed laser microdissection to isolate defined cell types or functional subunits and cell populations of paraffin embedded specimens which represent the most abundant supply of human tissue associated with clinical records. For qualitative and quantitative estimation of the sialylation status, sialic acids were released, fluorescently labeled, and analyzed by an online high-performance liquid chromatography-electrospray ionization-mass spectrometry (HPLC-ESI-MS) system. As a proof of principle, this strategy was successfully applied to characterize the sialylation of the apical region of epididymal epithelial cells. Furthermore, it was possible to detect an impaired sialylation during kidney maturation in a transgenic mouse model, which was restricted to glomeruli, whereas no differences in sialylation were observed when whole kidney homogenates were used. Thus, starting from paraffin embedded tissue samples, the outlined approach offers a sensitive method to detect and quantify sialic acids on defined cell populations, which may be useful to explore novel sialic acid dependent roles during physiological and pathological processes.

Glycomic strategy for efficient linkage analysis of di-, oligo- and polysialic acids

Sialic acid polymers of glycoproteins and glycolipids are characterized by a high diversity in nature and are involved in distinct biological processes depending inter alia on the glycosidic linkages between the present sialic acid residues. Though suitable protocols are available for chain length and sialic acid determination, sensitive methods for linkage analysis of di-, oligo-, and polysialic acids (di/oligo/polySia) are still pending. In this study, we have established a highly sensitive glycomic strategy for this purpose which is based on permethylation of di/oligo/polySia after tagging their reducing ends with the fluorescent dye 1,2-diamino-4,5-methylenedioxybenzene (DMB). Using DMB-labeled sialic acid di/oligo/polymers glycosidic linkages could be efficiently determined and, optionally, the established working procedure can be combined with HPLC for in depth characterization of distinct di/oligo/polySia chains. Moreover, the outlined approach can be directly applied to mammalian tissue samples and linkage analysis of sialic acid polymers present in biopsy samples of neuroblastoma tissue demonstrating the usefulness of the outlined work flow to screen, for example, cancer tissue for the presence of distinct variants of di/oligo/polySia as potentially novel biomarkers. Hence, the described strategy offers a highly sensitive and efficient strategy for identification of glycosidic linkages in sialic acid di/oligo/polymers of glycoproteins and glycolipids.

Recent advances in sialic acid-focused glycomics

Recent emergences of glycobiology, glycotechnology and glycomics have been clarifying enormous roles of carbohydrates in biological recognition systems. For example, cell surface carbohydrates existing as glycoconjugates (glycolipids, glycoproteins and proteoglycans) play crucial roles in cell-cell communication, cell proliferation and differentiation, tumor metastasis, inflammatory response or viral infection. In particular, sialic acids (SAs) existing as terminal residues in carbohydrate chains on cell surface are involved in signal recognition and adhesion to ligands, antibodies, enzymes and microbes. In addition, plasma free SAs and sialoglycans have shown great potential for disease biomarker discovery. Therefore, the development of efficient analytical methods for structural and functional studies of SAs and sialylglycans are very important and highly demanded. The problems of SAs and sialylglycans analysis are vanishingly small sample amount, complicated and unstable structures, and complex mixtures. Nevertheless, in the past decade, mass spectrometry in combination with chemical derivatization and modern separation methodologies has become a powerful and versatile technique for structural analysis of SAs and sialylglycans. This review summarizes these recent advances in glycomic studies on SAs and sialylglycans. Specially, derivatization and capturing of SAs and sialylglycans combined with mass spectrometry analysis are highlighted.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Tagging saccharides for signal enhancement in mass spectrometric analysis

MALDI-MS provides a rapid and sensitive analysis of large biomolecules such as proteins and nucleic acids. However, oligo- and polysaccharides are less sensitive in MS analysis partly due to their neutral and hydrophilic nature to cause low ionization efficiency. In this study, four types of oligosaccharides including aldoses, aminoaldoses, alduronic acids and α-keto acids were modified by appropriate tags at the reducing termini to improve their ionization efficiency. Bradykinin (BK), a vasoactive nonapeptide (RPPGFSPFR), containing two arginine and two phenylalanine residues turned out to be an excellent MS signal enhancer for maltoheptaose, GlcNAc oligomers and oligogalacturonic acids. In the MALDI-TOF-MS analysis using 2,5-dihydroxybenzoic acid (2,5-DHB) as the matrix, the GalA4-BK and GalA5-BK conjugates prepared by reductive amination showed the detection limit at 0.1 fmol, i.e. ∼800-fold enhancement over the unmodified pentagalacturonic acids. The remarkable MS enhancement was attributable to the synergistic effect of the basic arginine residues for high proton affinity and the hydrophobic property phenylalanine residues for facile ionization. A tetrapeptide GFGR(OMe) and an arginine linked phenylenediamine (H(2) N)(2) Ph-R(OMe) were thus designed to act as potent tags of oligosaccharides in MS analysis. Interestingly, concurrent condensation and lactonization of α2,8-linked tetrasialic acid (SA4) was carried out with (H(2) N)(2) Ph-R(OMe) to obtain a quinoxalinone derivative, which showed > 200-fold enhancement over unmodified SA4 in the MALDI-TOF-MS analysis.

Quantification of nucleotide-activated sialic acids by a combination of reduction and fluorescent labeling

Sialic acids usually represent the terminal monosaccharide of glycoconjugates and are directly involved in many biological processes. The cellular concentration of their nucleotide-activated form is one pacemaker for the highly variable sialylation of glycoconjugates. Hence, the determination of CMP-sialic acid levels is an important factor to understand the complex glycosylation machinery of cells and to standardize the production of glycotherapeutics. We have established a highly sensitive strategy to quantify the concentration of nucleotide-activated sialic acid by a combination of reduction and fluorescent labeling using the fluorophore 1,2-diamino-4,5-methylenedioxybenzene (DMB). The labeling with DMB requires free keto as well as carboxyl groups of the sialic acid molecule. Reduction of the keto group prior to the labeling process precludes the labeling of nonactivated sialic acids. Since the keto group is protected against reduction by the CMP-substitution, labeling of nucleotide-activated sialic acids is still feasible after reduction. Subsequent combination of the DMB-high-performance liquid chromatography (HPLC) application with mass spectrometric approaches, such as matrix-assisted laser desorption/ionization time-of-flight-mass spectrometry (MALDI-TOF-MS) and electrospray-ionization (ESI)-MS, allows the unambiguous identification of both natural and modified CMP-sialic acids and localization of potential substituents. Thus, the described strategy offers a sensitive detection, identification, and quantification of nucleotide-activated sialic acid derivatives in the femtomole range without the need for nucleotide-activated standards.

Synaptic cell adhesion molecule SynCAM 1 is a target for polysialylation in postnatal mouse brain

Among the large set of cell surface glycan structures, the carbohydrate polymer polysialic acid (polySia) plays an important role in vertebrate brain development and synaptic plasticity. The main carrier of polySia in the nervous system is the neural cell adhesion molecule NCAM. As polySia with chain lengths of more than 40 sialic acid residues was still observed in brain of newborn Ncam(-/-) mice, we performed a glycoproteomics approach to identify the underlying protein scaffolds. Affinity purification of polysialylated molecules from Ncam(-/-) brain followed by peptide mass fingerprinting led to the identification of the synaptic cell adhesion molecule SynCAM 1 as a so far unknown polySia carrier. SynCAM 1 belongs to the Ig superfamily and is a powerful inducer of synapse formation. Importantly, the appearance of polysialylated SynCAM 1 was not restricted to the Ncam(-/-) background but was found to the same extent in perinatal brain of WT mice. PolySia was located on N-glycans of the first Ig domain, which is known to be involved in homo- and heterophilic SynCAM 1 interactions. Both polysialyltransferases, ST8SiaII and ST8SiaIV, were able to polysialylate SynCAM 1 in vitro, and polysialylation of SynCAM 1 completely abolished homophilic binding. Analysis of serial sections of perinatal Ncam(-/-) brain revealed that polySia-SynCAM 1 is expressed exclusively by NG2 cells, a multifunctional glia population that can receive glutamatergic input via unique neuron-NG2 cell synapses. Our findings sug-gest that polySia may act as a dynamic modulator of SynCAM 1 functions during integration of NG2 cells into neural networks.

Mass spectrometry in the analysis of N-linked and O-linked glycans

Mass spectrometry (MS) continues to play a vital role in defining the structures of N-glycans and O-glycans in glycoproteins via glycomic and glycoproteomic methodologies. The former seeks to define the total N-glycan and/or O-glycan repertoire in a biological sample whilst the latter is concerned with the analysis of glycopeptides. Recent technical developments have included improvements in tandem mass spectrometry (MS/MS and MS(n)) sequencing methodologies, more sensitive methods for analysing sulfated and polysialylated glycans and better procedures for defining the sites of O-glycosylation. New tools have been introduced to assist data handling and publicly accessible databases are being populated with glycomics data. Progress is exemplified by recent research in the fields of glycoimmunology, reproductive glycobiology, stem cells, bacterial glycosylation and non-mucin O-glycosylation.

Brain development needs sugar: the role of polysialic acid in controlling NCAM functions

Polysialic acid (polySia) is a major regulator of cell-cell interactions in the developing nervous system and a key factor in maintaining neural plasticity. As a polyanionic molecule with high water binding capacity, polySia increases the intercellular space and creates conditions that are permissive for cellular plasticity. While the prevailing model highlights polySia as a non-specific regulator of cell-cell contacts, this review concentrates on recent studies in knockout mice indicating that a crucial function of polySia resides in controlling interactions mediated by its predominant protein carrier, the neural cell adhesion molecule NCAM.