Mass Spectrometric Fragmentation Analysis of Oligosialic and Polysialic Acids

Identification of a buried β-strand as a novel disease-related motif in the human polysialyltransferases

The polysialyltransferases ST8SIA2 and ST8SIA4 and their product, polysialic acid (polySia), are known to be related to cancers and mental disorders. ST8SIA2 and ST8SIA4 have conserved amino acid (AA) sequence motifs essential for the synthesis of the polySia structures on the neural cell adhesion molecule. To search for a new motif in the polysialyltransferases, we adopted the in silico Individual Meta Random Forest program that can predict disease-related AA substitutions. The Individual Meta Random Forest program predicted a new eight-amino-acids sequence motif consisting of highly pathogenic AA residues, thus designated as the pathogenic (P) motif. A series of alanine point mutation experiments in the pathogenic motif (P motif) showed that most P motif mutants lost the polysialylation activity without changing the proper enzyme expression levels or localization in the Golgi. In addition, we evaluated the enzyme stability of the P motif mutants using newly established calculations of mutation energy, demonstrating that the subtle change of the conformational energy regulates the activity. In the AlphaFold2 model, we found that the P motif was a buried β-strand underneath the known surface motifs unique to ST8SIA2 and ST8SIA4. Taken together, the P motif is a novel buried β-strand that regulates the full activity of polysialyltransferases from the inside of the molecule.

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.

Comparative Studies of Polysialic Acids Derived from Five Different Vertebrate Brains

Polysialic acid (polySia/PSA) is a linear homopolymer of sialic acid (Sia) that primarily modifies the neural cell adhesion molecule (NCAM) in mammalian brains. PolySia-NCAM not only displays an anti-adhesive function due to the hydration effect, but also possesses a molecule-retaining function via a direct binding to neurologically active molecules. The quality and quantity of polySia determine the function of polySia-NCAM and are considered to be profoundly related to the maintenance of normal brain functions. In this study, to compare the structures of polySia-NCAM in brains of five different vertebrates (mammals, birds, reptiles, amphibians, and fish), we adopted newly developed combinational methods for the analyses. The results revealed that the structural features of polySia considerably varied among different species. Interestingly, mice, as a mammal, possess eminently distinct types of polySia, in both quality and quantity, compared with those possessed by other animals. Thus, the mouse polySia is of larger quantities, of longer and more diverse chain lengths, and of a larger molecular size with higher negative charge, compared with polySia of other species. These properties might enable more advanced brain function. Additionally, it is suggested that the polySia/Sia ratio, which likely reflects the complexity of brain function, can be used as a new promising index to evaluate the intelligence of different vertebrate brains.

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.

Nanoparticles Equipped with α2,8-Linked Sialic Acid Chains Inhibit the Release of Neutrophil Extracellular Traps

Neutrophils can combat the invasion of pathogens by the formation of neutrophil extracellular traps (NETs). The NET mechanism is not only an effective tool for combating pathogens, but is also associated with diseases. Therefore, NETs are a potential target for combating pathologies, such as cystic fibrosis and thrombosis. We investigated the potential of nanoparticles, which were modified with α2,8-linked sialic acid chains, to modulate NET release during phorbol myristate acetate stimulation. Interestingly, when these nanoparticles were applied, the formation of reactive oxygen species was partly inhibited and the release of NET was counteracted. However, although the release of NET fibers was prevented, the nuclei still lost their characteristic segmented structure and became swollen, indicating that only the release, and not complete activation was suppressed. Intriguingly, coincubation of α2,8-sialylated particles with free sialic acid chains prevented the outlined inhibitory effects. Thus, the sialic acid chains must be attached to a linker molecule to generate an active bioconjugate that is able to inhibit the release of NET.

Effective analysis of degree of polymerization of polysialic acids in mass spectrometry by combining novel sample preparation and dynamic instrument optimization methods

This work demonstrates a mass spectrometry technique to improve data reliability when analyzing degree of polymerization (DP) of high-mass polysialic acids (PSAs). Matrix-assisted laser-desorption/ionization (MALDI) time-of-flight mass spectrometry is the technique of choice for analyzing large molecules due to its wide mass working range; however, the observed DP of PSAs using such an instrument is unreliable owing to sensitivity bias towards low-mass ions. A multi-layer MALDI sample preparation protocol is demonstrated in the current study to improve PSA sensitivity, and a dynamic instrument optimization method (DIOM) is employed to minimize detector saturation over a wide mass range. The DP information obtained in the DIOM combines a series of mass spectral data obtained with individually optimized instrument parameters to minimize the problem of sensitivity bias in respective mass ranges. The resultant mass spectra facilitate unambiguous determination of DP in the high-mass range due to significantly improved spectral quality. The main instrument parameters involved in the optimization process include extraction delay in MALDI ion source as well as the cutoff mass of the ion detector. In comparison to conventional methods, the DIOM doubles the maximum DP that can be unambiguously identified by mass spectrometry.

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.

Polysialic Acid in Human Plasma Can Compensate the Cytotoxicity of Histones

The innate immune system has numerous mechanisms to fight against pathogens, including the formation of neutrophil extracellular traps (NETs). By spreading out chromatin, antimicrobial peptides and enzymes, neutrophils efficiently trap pathogens like bacteria and facilitate their elimination. During this process, high concentrations of extracellular histones can be reached. Several researchers have demonstrated that the cytotoxic characteristics of these histones can trigger diseases like sepsis. Interestingly, the carbohydrate polysialic acid (polySia) can bind histones and reduce histone-mediated cytotoxicity in a chain length-dependent manner. In the present study, we examined the chain length of polySia in plasma and tested its ability to decrease the cytotoxic characteristics of extracellular histones. Remarkably, we detected polySia not only in the soluble fraction of plasma, but also on enriched extracellular vesicles (EVs). Chain length analysis revealed that polySia chains originating from human plasma can consists of more than 40 sialic acid residues and show a cytoprotective effect against extracellular histones. Intriguingly, polySia is not only present in human plasma but also in fish and other branches of vertebrates. Thus, polySia is a physiological element in plasma and may represent a natural buffer for extracellular histones.

Artificial Polysialic Acid Chains as Sialidase-Resistant Molecular-Anchors to Accumulate Particles on Neutrophil Extracellular Traps

Neutrophils are involved in numerous immunological events. One mechanism of neutrophils to combat pathogens is the formation of neutrophil extracellular traps (NETs). Thereby, neutrophils use DNA fibers to form a meshwork of DNA and histones as well as several antimicrobial components to trap and kill invaders. However, the formation of NETs can lead to pathological conditions triggering among other things (e.g., sepsis or acute lung failure), which is mainly a consequence of the cytotoxic characteristics of accumulated extracellular histones. Interestingly, the carbohydrate polysialic acid represents a naturally occurring antagonist of the cytotoxic properties of extracellular histones. Inspired by polysialylated vesicles, we developed polysialylated nanoparticles. Since sialidases are frequently present in areas of NET formation, we protected the sensitive non-reducing end of these homopolymers. To this end, the terminal sialic acid residue of the non-reducing end was oxidized and directly coupled to nanoparticles. The covalently linked sialidase-resistant polysialic acid chains are still able to neutralize histone-mediated cytotoxicity and to initiate binding of these polysialylated particles to NET filaments. Furthermore, polysialylated fluorescent microspheres can be used as a bioanalytical tool to stain NET fibers. Thus, polySia chains might not only be a useful agent to reduce histone-mediated cytotoxicity but also an anchor to accumulate nanoparticles loaded with active substances in areas of NET formation.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article 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 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

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.

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.

High-throughput mass finger printing and Lewis blood group assignment of human milk oligosaccharides

The structural diversity of human milk oligosaccharides (HMOs) strongly depends on the Lewis (Le) blood group status of the donor which allows a classification of these glycans into three different groups. Starting from 50 μL of human milk, a new high-throughput, standardized, and widely automated mass spectrometric approach has been established which can be used for correlation of HMO structures with the respective Lewis blood groups on the basis of mass profiles of the entire mixture of glycans together with selected fragment ion spectra. For this purpose, the relative abundance of diagnostically relevant compositional species, such as Hex(2)Fuc(2) and Hex(3)HexNAc(1)Fuc(2), as well as the relative intensities of characteristic fragment ions obtained thereof are of key importance. For each Lewis blood group, i.e., Le(a-b+), Le(a+b-), and Le(a-b-), specific mass profile and fragment ion patterns could be thus verified. The described statistically proven classification of the derived glycan patterns may be a valuable tool for analysis and comparison of large sets of milk samples in metabolic studies. Furthermore, the outlined protocol may be used for rapid screening in clinical studies and quality control of milk samples donated to milk banks.