Frequent occurrence of pre-existing alpha 2-->8-linked disialic and oligosialic acids with chain lengths up to 7 Sia residues in mammalian brain glycoproteins. Prevalence revealed by highly sensitive chemical methods and anti-di-, oligo-, and poly-Sia antibodies specific for defined chain lengths

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.

Comprehensive Analysis of Oligo/Polysialylglycoconjugates in Cancer Cell Lines

In cancer cells, cell-surface sialylation is altered, including a change in oligo/polysialic acid (oligo/polySia) structures. Since they are unique and rarely expressed in normal cells, oligo/polySia structures may serve as promising novel biomarkers and targets for therapies. For the diagnosis and treatment of the disease, a precise understanding of the oligo/polySia structures in cancer cells is necessary. In this study, flow cytometric analysis and gene expression datasets were obtained from sixteen different cancer cell lines. These datasets demonstrated the ability to predict glycan structures and their sialylation status. Our results also revealed that sialylation patterns are unique to each cancer cell line. Thus, we can suggest promising combinations of antibody and cancer cell for glycan prediction. However, the precise prediction of minor glycans need to be further explored.

Polysialylation in a DISC1 Mutant Mouse

Schizophrenia is a serious psychiatric disorder that affects the social life of patients. Psychiatric disorders are caused by a complex combination of genetic (G) and environmental (E) factors. Polysialylation represents a unique posttranslational modification of a protein, and such changes in neural cell adhesion molecules (NCAMs) have been reported in postmortem brains from patients with psychiatric disorders. To understand the G × E effect on polysialylated NCAM expression, in this study, we performed precise measurements of polySia and NCAM using a disrupted-in-schizophrenia 1 (DISC1)-mutant mouse (G), a mouse model of schizophrenia, under acute stress conditions (E). This is the first study to reveal a lower number and smaller length of polySia in the suprachiasmatic nucleus of DISC1 mutants relative to those in wild-type (WT) mice. In addition, an analysis of polySia and NCAM responses to acute stress in five brain regions (olfactory bulb, prefrontal cortex, suprachiasmatic nucleus, amygdala, and hippocampus) revealed that the pattern of changes in these responses in WT mice and DISC1 mutants differed by region. These differences could indicate the vulnerability of DISC1 mutants to stress.

The α2,8-sialyltransferase 6 (St8sia6) localizes in the ER and enhances the anchorage-independent cell growth in cancer

Sialylation, the final stage of post-translational modification of proteins, is achieved in the Golgi apparatus and is related to the malignant phenotype of cancer. Disialylation of ganglioside (GD3) by St8sia1 and polysialylation by St8sia2 and 4 have been shown to be related to malignant phenotypes; however, di/oligosialylation by St8sia6 is still unknown. In this study, we analyzed the malignant phenotype of St8sia6 and found that upregulation of St8sia6 in melanoma B16 cells increased anchorage-independent cell growth, which was not due to sialic acid cleavage by a sialidase. Moreover, unlike other sialyltransferases, St8sia6 localized to the endoplasmic reticulum (ER). We found that the localization to the Golgi apparatus could be regulated by swapping experiments using St8sia2; however, the malignant phenotype did not change. These data demonstrate that the enhancement of anchorage-independent cell growth by St8sia6 is not due to its localization of ER, but is due to the expression of the protein itself.

The dynamic brain N-glycome

The attachment of carbohydrates to other macromolecules, such as proteins or lipids, is an important regulatory mechanism termed glycosylation. One subtype of protein glycosylation is asparagine-linked glycosylation (N-glycosylation) which plays a key role in the development and normal functioning of the vertebrate brain. To better understand the role of N-glycans in neurobiology, it's imperative we analyse not only the functional roles of individual structures, but also the collective impact of large-scale changes in the brain N-glycome. The systematic study of the brain N-glycome is still in its infancy and data are relatively scarce. Nevertheless, the prevailing view has been that the neuroglycome is inherently restricted with limited capacity for variation. The development of improved methods for N-glycomics analysis of brain tissue has facilitated comprehensive characterisation of the complete brain N-glycome under various experimental conditions on a larger scale. Consequently, accumulating data suggest that it's more dynamic than previously recognised and that, within a general framework, it has a given capacity to change in response to both intrinsic and extrinsic stimuli. Here, we provide an overview of the many factors that can alter the brain N-glycome, including neurodevelopment, ageing, diet, stress, neuroinflammation, injury, and disease. Given this emerging evidence, we propose that the neuroglycome has a hitherto underappreciated plasticity and we discuss the therapeutic implications of this regarding the possible reversal of pathological changes via interventions. We also briefly review the merits and limitations of N-glycomics as an analytical method before reflecting on some of the outstanding questions in the field.

Bioinformatics Analyses Identify the Therapeutic Potential of ST8SIA6 for Colon Cancer

Sialylation of glycoproteins is modified by distinct sialyltransferases such as ST3Gal, ST6Gal, ST6GalNAc, or ST8SIA with α2,3-, α2,6-, or α2,8-linkages. Alteration of these sialyltransferases causing aberrant sialylation is associated with the progression of colon cancer. However, among the ST8- sialyltransferases, the role of ST8SIA6 in colon cancer remains poorly understood. In this study, we explored the involvement of ST8SIA6 in colon cancer using multiple gene databases. The relationship between ST8SIA6 expression and tumor stages/grades was investigated by UALCAN analysis, and Kaplan–Meier Plotter analysis was used to analyze the expression of ST8SIA6 on the survival outcome of colon cancer patients. Moreover, the biological functions of ST8SIA6 in colon cancer were explored using LinkedOmics and cancer cell metabolism gene DB. Finally, TIMER and TISMO analyses were used to delineate ST8SIA6 levels in tumor immunity and immunotherapy responses, respectively. ST8SIA6 downregulation was associated with an advanced stage and poorly differentiated grade; however, ST8SIA6 expression did not affect the survival outcomes in patients with colon cancer. Gene ontology analysis suggested that ST8SIA6 participates in cell surface adhesion, angiogenesis, and membrane vesicle trafficking. In addition, ST8SIA6 levels affected immunocyte infiltration and immunotherapy responses in colon cancer. Collectively, these results suggest that ST8SIA6 may serve as a novel therapeutic target towards personalized medicine for colon cancer.

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.

Analysis of biochemical features of ST8 α-N-acetyl-neuraminide α2,8-sialyltransferase (St8sia) 5 isoforms

Gangliosides are important components of the membrane and are involved in many biological activities. St8sia5 is an α2,8-sialyltransferase involved in ganglioside synthesis, and has three isoforms. In this study, we analyzed the features of three isoforms, St8sia5-S, -M, and -L that had not been analyzed, and found that only St8sia5-L was localized in the Golgi, while the majority of St8sia5-M and -S were localized in the ER. The localization of Golgi of St8sia5 depended on the stem region. In addition, the incorporation of exogenous GD3 was upregulated only in St8sia5-L expressing cells. Taken together, the localization of St8sia5 is important for the activity of the enzyme.

Characterization of Thrombospondin Type 1 Repeat in Haliotis diversicolor and Its Possible Role in Osteoinduction

Thrombospondin repeats (TSR) are important peptide domains present in the sequences of many extracellular and transmembrane proteins with which a variety of ligands interact. In this study, we characterized HdTSR domains in the ADAMTS3 protein of Thai abalone, Haliotis diversicolor, based on the transcriptomic analysis of its mantle tissues. PCR amplification and localization studies demonstrated the existence of HdTSR transcript and protein in H. diversicolor tissues, particularly in both the inner and outer mantle epithelial folds. We, therefore, generated a short recombinant protein, termed HdTSR1/2, based on the existence of the WxxWxxW or WxxxxW motif (which binds to TGF-β, a known signaling in bone formation/repair) in HdTSR1 and HdTSR2 sequences and used it to test the osteoinduction function in the pre-osteoblastic cell line, MC3T3-E1. This recombinant protein demonstrated the ability to induce the differentiation of MC3T3-E1 cells by the concentration- and time-dependent upregulation of many known osteogenic markers, including RUNX2, COL1A1, OCN, and OPN. We also demonstrated the upregulation of the SMAD2 gene after cell treatment with HdTSR1/2 proteinindicating its possible interaction through TGF-β, which thus activates its downstream signaling cascade and triggers the biomineralization process in the differentiated osteoblastic cells. Together, HdTSR domains existed in an extracellular ADAMTS3 protein in the mantle epithelium of H. diversicolor and played a role in osteoinduction as similar to the other nacreous proteins, opening up its possibility to be developed as an inducing agent of bone repair.

N-Glycosylation in isolated rat nerve terminals

N-linked glycosylation is a ubiquitous protein modification that is capable of modulating protein structure, function and interactions. Many proteins in the brain associated with the synapse and important for synaptic transmission are highly glycosylated and their glycosylation could be important for learning and memory related molecular processes and synaptic plasticity. In the present study, we extend the knowledge of the synaptic glycome and glycoproteome by performing glycan- and intact glycopeptide-focused analyses of isolated rat nerve terminals (synaptosomes) by LC-MS/MS. Overall, glycomics identified a total of 41 N-glycans in isolated synaptosomes. Sialylated N-glycans represented only 7% of the total abundance of the rat synaptosome N-glycome with oligomannose, neutral hybrid and complex type N-glycans being the most abundant structures. Using detergent extraction of the active zone proteins from the synaptosomes revealed a change in the active zone glycan abundance in comparison with the rest of the synaptosome glycan content. Characterization of intact sialylated N-linked glycopeptides enriched by titanium dioxide chromatography revealed more than 85% selectivity of sialylated species and the presence of NeuGc on active zone proteins. In addition, both disialic and trisialic acid modified glycans were present on synaptic glycoproteins, although oxonium ion profiling revealed that trisialic units were only present on glycoproteins in the detergent soluble fraction. However, correct identification of intact sialylated N-linked glycopeptides using the Byonic program failed, most likely due to the lack of peptide backbone fragmentation during tandem mass spectrometry.

Identification and functional characterization of a Siglec-7 counter-receptor on K562 cells

Sialic acid (Sia)-binding immunoglobulin-like lectin 7 (Siglec-7) is an inhibitory receptor primarily expressed on natural killer (NK) cells and monocytes. Siglec-7 is known to negatively regulate the innate immune system through Sia binding to distinguish self and nonself; however, a counter-receptor bearing its natural ligand remains largely unclear. Here, we identified a counter-receptor of Siglec-7 using K562 hematopoietic carcinoma cells presenting cell surface ligands for Siglec-7. We affinity-purified the ligands using Fc-ligated recombinant Siglec-7 and diSia-dextran polymer, a strong inhibitor for Siglec-7. We then confirmed the counter-receptor for Siglec-7 as leukosialin (CD43) through mass spectrometry, immunoprecipitation, and proximity labeling. Additionally, we demonstrated that the cytotoxicity of NK cells toward K562 cells was suppressed by overexpression of leukosialin in a Siglec-7-dependent manner. Taken together, our data suggest that leukosialin on K562 is a counter-receptor for Siglec-7 on NK cells and that a cluster of the Sia-containing glycan epitope on leukosialin is key as trans-ligand for unmasking the cis-ligand.

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.

Depolarization-dependent Induction of Site-specific Changes in Sialylation on N-linked Glycoproteins in Rat Nerve Terminals

Synaptic transmission leading to release of neurotransmitters in the nervous system is a fast and highly dynamic process. Previously, protein interaction and phosphorylation have been thought to be the main regulators of synaptic transmission. Here we show that sialylation of N-linked glycosylation is a novel potential modulator of neurotransmitter release mechanisms by investigating depolarization-dependent changes of formerly sialylated N-linked glycopeptides. We suggest that negatively charged sialic acids can be modulated, similarly to phosphorylation, by the action of sialyltransferases and sialidases thereby changing local structure and function of membrane glycoproteins. We characterized site-specific alteration in sialylation on N-linked glycoproteins in isolated rat nerve terminals after brief depolarization using quantitative sialiomics. We identified 1965 formerly sialylated N-linked glycosites in synaptic proteins and found that the abundances of 430 glycosites changed after 5 s depolarization. We observed changes on essential synaptic proteins such as synaptic vesicle proteins, ion channels and transporters, neurotransmitter receptors and cell adhesion molecules. This study is to our knowledge the first to describe ultra-fast site-specific modulation of the sialiome after brief stimulation of a biological system.

Combinational Analyses with Multiple Methods Reveal the Existence of Several Forms of Polysialylated Neural Cell Adhesion Molecule in Mouse Developing Brains

Polysialic acid (polySia/PSA) is an anionic glycan polymer of sialic acid, and it mostly modifies the neural cell adhesion molecule (NCAM) in mammalian brains. Quality and quantity of the polySia of the polySia-NCAM is spatio-temporally regulated in normal brain development and functions, and their impairments are reported to be related to diseases, such as psychiatric disorders and cancers. Therefore, precise understanding of the state of polySia-NCAM structure would lead to the diagnosis of diseases for which their suitable evaluation methods are necessary. In this study, to develop these evaluation methods, structures of polySia-NCAM from mouse brains at six different developmental stages were analyzed by several conventional and newly developed methods. Integrated results of these experiments clearly demonstrated the existence of different types of polySia-NCAMs in developing brains. In addition, combinational analyses were shown to be useful for precise understanding of the quantity and quality of polySia, which can provide criteria for the diagnosis of diseases.

Acute stress-induced change in polysialic acid levels mediated by sialidase in mouse brain

Stress is an important environmental factor influencing human behaviour and causing several mental disorders. Alterations in the structure of polysialic acid (polySia/PSA) due to genetic alterations in ST8SIA2, which encodes a polySia-synthesizing enzyme, are related to certain mental disorders. However, whether stress as an environmental factor leads to changes in polySia structure is unknown. Here we studied the effects of acute stress on polySia expression and found reductions in both the quantity and quality of polySia in the olfactory bulb and prefrontal cortex, even with short-term exposure to acute stress. The use of inhibitors for sialidase, microglia and astrocytes revealed that these declines were due to a transient action of sialidase from microglia and astrocytes in the olfactory bulb and prefrontal cortex, respectively. These data suggest that sialidase dynamically regulates polySia expression in a brain region-specific manner.

N-linked mannose glycoconjugates on shrimp thrombospondin, pmTSP-II, and their involvement in the sperm acrosome reaction

Glycoconjugates in egg extracellular matrices are known to serve several functions in reproductive processes. Here, the presence of N-linked mannose (Man) glycoconjugates on shrimp thrombospondin ( pmTSP-II) and their physiological functions were investigated in the black tiger shrimp Penaeus monodon. A molecular analysis of pmTSP-II demonstrated anchorage sites for N-linked glycans in both the chitin-binding and TSP3 domains. The presence of Man residues was verified by concanavalin A lectin histochemistry on the purified fraction of pmTSP-II (250 kDa with protease inhibitor). The function of the Man glycoconjugates was evident by the Con A interference with the pmTSP-II-induced acrosome reaction (AR) as well as by the ability to recover the induction of the AR by the inclusion of Mans in the treatment mixture. In addition, the recombinant proteins of the three signature pmTSP-II domains expressed in E. coli (lacking glycosylation) and mannosidase-treated pmTSP-II showed a minimal ability to initiate the AR response. Together, these results provide evidence of the pivotal role that Man-linked pmTSP-II plays in modulating the shrimp sperm AR, a novel role for a TSP family protein in shrimp reproductive biology.

Novel Zebrafish Mono-α2,8-sialyltransferase (ST8Sia VIII): An Evolutionary Perspective of α2,8-Sialylation

The mammalian mono-α2,8-sialyltransferase ST8Sia VI has been shown to catalyze the transfer of a unique sialic acid residues onto core 1 O-glycans leading to the formation of di-sialylated O-glycosylproteins and to a lesser extent to diSia motifs onto glycolipids like GD1a. Previous studies also reported the identification of an orthologue of the ST8SIA6 gene in the zebrafish genome. Trying to get insights into the biosynthesis and function of the oligo-sialylated glycoproteins during zebrafish development, we cloned and studied this fish α2,8-sialyltransferase homologue. In situ hybridization experiments demonstrate that expression of this gene is always detectable during zebrafish development both in the central nervous system and in non-neuronal tissues. Intriguingly, using biochemical approaches and the newly developed in vitro MicroPlate Sialyltransferase Assay (MPSA), we found that the zebrafish recombinant enzyme does not synthetize diSia motifs on glycoproteins or glycolipids as the human homologue does. Using comparative genomics and molecular phylogeny approaches, we show in this work that the human ST8Sia VI orthologue has disappeared in the ray-finned fish and that the homologue described in fish correspond to a new subfamily of α2,8-sialyltransferase named ST8Sia VIII that was not maintained in Chondrichtyes and Sarcopterygii.

Sialic Acids in Neurology

Sialic acid (Sia) is involved in many biological activities and commonly occurs as a monosialyl residue at the nonreducing terminal end of glycoconjugates. The loss of activity of UDP-GlcNAc2-epimerase/ManNAc kinase, which is a key enzyme in Sia biosynthesis, is lethal to the embryo, which clearly indicates the importance of Sia in embryogenesis. Occasionally, oligo/polymeric Sia structures such as disialic acid (diSia), oligosialic acid (oligoSia), and polysialic acid (polySia) occur in glycoconjugates. In particular, polySia, a well-known epitope that commonly occurs in neuroinvasive bacteria and vertebrate brains, is one of the most well-known and biologically/neurologically important glycotopes in vertebrates. The biological effects of polySia, especially on neural cell-adhesion molecules, have been well studied, and in-depth knowledge regarding polySia has been accumulated. In addition, the importance of diSia and oligoSia epitopes has been reported. In this chapter, the recent advances in the study of diSia, oligoSia, and polySia residues in glycoproteins in neurology, and their history, definition, occurrence, analytical methods, biosynthesis, and biological functions evaluated by phenotypes of gene-targeted mice, biochemical features, and related diseases are described.

Different properties of polysialic acids synthesized by the polysialyltransferases ST8SIA2 and ST8SIA4

Polysialic acid (polySia) is mainly found as a modification of neural cell adhesion molecule (NCAM) in whole embryonic brains, as well as restricted areas of adult vertebrate brains, including the hippocampus. PolySia shows not only repulsive effects on NCAM-involved cell-cell interactions due to its bulky and hydrated properties, but also attractive effects on the interaction with neurologically active molecules, which exerts a reservoir function. Two different polysialyltransferases, ST8SIA2 and ST8SIA4, are involved in the synthesis of polySia chains; however, to date, the differences of the properties between polySia chains synthesized by these two enzymes remain unknown. In this study, to clarify this point, we first prepared polySia-NCAMs from HEK293 cells stably expressing ST8SIA4 and ST8SIA2, or ST8SIA2 (SNP-7), a mutant ST8SIA2 derived from a schizophrenia patient. The conventional sensitive chemical and immunological characterizations showed that the quantity and quality (structural features) of polySia are not so much different between ST8SIA4- and ST8SIA2-synthesized ones, apart from those of ST8SIA2 (SNP-7). Then, we assessed the homophilic and heterophilic interactions mediated by polySia-NCAM by adopting a surface plasmon resonance measurement as an in vitro analytical method. Our novel findings are as follows: (i) the ST8SIA2- and ST8SIA4-synthesized polySia-NCAMs exhibited different attractive and repulsive effects than each other; (ii) both polySia- and oligoSia-NCAMs synthesized by ST8SIA2 were able to bind polySia-NCAMs; (iii) the polySia-NCAM synthesized by a ST8SIA2 (SNP-7) showed markedly altered attractive and repulsive properties. Collectively, polySia-NCAM is suggested to simultaneously possess both attractive and repulsive properties that are highly regulated by the two polysialyltransferases.

Diverse subcellular localizations of the insect CMP-sialic acid synthetases

The occurrence and biological importance of sialic acid (Sia) and its metabolic enzymes in insects have been studied using Drosophila melanogaster. The most prominent feature of D. melanogaster CMP-Sia synthetase (DmCSS) is its Golgi-localization, contrasted with nuclear localization of vertebrate CSSs. However, it remains unclear if the Golgi-localization is common to other insect CSSs and why it happens. To answer these questions, Aedes aegypti (mosquito) CSS (AaCSS) and Tribolium castaneum (beetle) CSS (TcCSS) were cloned and characterized for their activity and subcellular localization. Our new findings show: (1) AaCSS and TcCSS share a common overall structure with DmCSS in terms of evolutionarily conserved motifs and the absence of the C-terminal domain typical to vertebrate CSSs; (2) when expressed in mammalian and insect cells, AaCSS and TcCSS showed in vivo and in vitro CSS activities, similar to DmCSS. In contrast, when expressed in bacteria, they lacked CSS activity because the N-terminal hydrophobic region appeared to induce protein aggregation; (3) when expressed in Drosophila S2 cells, AaCSS and TcCSS were predominantly localized in the ER, but not in the Golgi. Surprisingly, DmCSS was mainly secreted into the culture medium, although partially detected in Golgi. Consistent with these results, the N-terminal hydrophobic regions of AaCSS and TcCSS functioned as a signal peptide to render them soluble in the ER, while the N-terminus of DmCSS functioned as a membrane-spanning region of type II transmembrane proteins whose cytosolic KLK sequence functioned as an ER export signal. Accordingly, the differential subcellular localization of insect CSSs are distinctively more diverse than previously recognized.

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.

Cellular O-Glycome Reporter/Amplification to explore O-glycans of living cells

Protein O-glycosylation plays key roles in many biological processes, but the repertoire of O-glycans synthesized by cells is difficult to determine. Here we describe a new approach termed Cellular O-Glycome Reporter/Amplification (CORA), a sensitive method to amplify and profile mucin-type O-glycans synthesized by living cells. Cells incubated with peracetylated benzyl-α-N-acetylgalactosamine (GalNAc-α-Benzyl) convert it to a large variety of modified O-glycan derivatives that are secreted from cells, allowing easy purification for analysis by HPLC and mass spectrometry (MS). CORA results in ~100–1000-fold increase in sensitivity over conventional O-glycan analyses and identifies a more complex repertoire of O-glycans in more than a dozen cell types from Homo sapiens and Mus musculus. Furthermore, CORA coupled with computational modeling allows predictions on the diversity of the human O-glycome and offers new opportunities to identify novel glycan biomarkers for human diseases.

Molecular characterization and expression analyses of ST8Sia II and IV in piglets during postnatal development: lack of correlation between transcription and posttranslational levels

The two mammalian α2,8-polysialyltransferases (polyST's), ST8Sia II (STX) and ST8Sia IV (PST), catalyze synthesis of the α2-8-linked polysialic acid (polySia) glycans on neural cell adhesion molecules (NCAMs). The objective of this study was to clone the coding sequence of the piglet ST8Sia II and determine the mRNA expression levels of ST8Sia II, ST8Sia IV, NCAM and neuropilin-2 (NRP-2), also a carrier protein of polySia, during postnatal development. The amino acid sequence deduced from the coding sequence of ST8Sia II was compared with seven other mammalian species. Piglet ST8Sia II was highly conserved and shared 67.8% sequence identity with ST8Sia IV. Genes coding for ST8Sia II and IV were differentially expressed and distinctly different in neural and non-neural tissues at postnatal days 3 and 38. Unexpectedly, the cellular levels of mRNA coding for ST8Sia II and IV showed no correlation with the posttranslational level of polySia glycans in different tissues. In contrast, mRNA abundance coding for NCAM and neuropilin-2 correlated with expression of ST8Sia II and IV. These findings show that the cellular abundance of ST8Sia II and IV in postnatal piglets is regulated at the level of translation/posttranslation, and not at the level of transcription, a finding that has not been previously reported. These studies further highlight differences in the molecular mechanisms controlling polysialylation in adult rodents and neonatal piglets.

LC-MS/MS glycomic analyses of free and conjugated forms of the sialic acids, Neu5Ac, Neu5Gc and KDN in human throat cancers

An elevated level of the free deaminated sialic acid, 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN), was first discovered in human ovarian cancers (OCs), suggesting that KDN may be an oncodevelopmental antigen (Inoue S, Lin SL, Chang T, Wu SH, Yao CW, Chu TY, Troy FA II, Inoue Y. 1998. J Biol Chem. 273(42):27199-27204). To determine if this unexpected finding was unique to OC, we developed an LC-MS/MS glycomic approach to quantitatively determine the level of free and conjugated forms of KDN, Neu5Ac and Neu5Gc in head and neck cancers of the throat, and in a subpopulation of matched lymph nodes. These findings were correlated with tumor (T), nodal (N), metastatic (M) involvement and the differentiation status of the tumors. The following new findings are reported: (i) The level of free KDN in 49 throat cancers and a subpopulation of 10 regional lymph nodes accounted for 94.5 and 93.3%, respectively, of the total level of KDN (∼2 µg/g); (ii) in marked contrast, the level of free Neu5Ac in throat cancer and lymph nodes accounted for only 6.5 and 5.1% of the total level of Neu5Ac (85 µg/g); (3) The level of Neu5Gc (0.03 µg/g) in throat cancers was 0.30% of the level of Neu5Ac, two-thirds were conjugated and one-third was free. The central importance of these new findings is that the elevated level of free KDN relative to free Neu5Ac and Neu5GC in throat cancers showing no lymphatic metastasis, and which are poorly to moderately differentiated, suggests that free KDN may be useful as a biomarker for detecting some early-stage cancers at biopsy, and be of possible prognostic value in determining the potential degree of malignancy.

Cell- and Protein-Directed Glycosylation of Native Cleaved HIV-1 Envelope

The gp120/gp41 HIV-1 envelope glycoprotein (Env) is highly glycosylated, with up to 50% of its mass consisting of N-linked glycans. This dense carbohydrate coat has emerged as a promising vaccine target, with its glycans acting as epitopes for a number of potent and broadly neutralizing antibodies (bnAbs). Characterizing the glycan structures present on native HIV-1 Env is thus a critical goal for the design of Env immunogens. In this study, we used a complementary, multistep approach involving ion mobility mass spectrometry and high-performance liquid chromatography to comprehensively characterize the glycan structures present on HIV-1 gp120 produced in peripheral blood mononuclear cells (PBMCs). The capacity of different expression systems, including pseudoviral particles and recombinant cell surface trimers, to reproduce native-like glycosylation was then assessed. A population of oligomannose glycans on gp120 was reproduced across all expression systems, supporting this as an intrinsic property of Env that can be targeted for vaccine design. In contrast, Env produced in HEK 293T cells failed to accurately reproduce the highly processed complex-type glycan structures observed on PBMC-derived gp120, and in particular the precise linkage of sialic acid residues that cap these glycans. Finally, we show that unlike for gp120, the glycans decorating gp41 are mostly complex-type sugars, consistent with the glycan specificity of bnAbs that target this region. These findings provide insights into the glycosylation of native and recombinant HIV-1 Env and can be used to inform strategies for immunogen design and preparation.

IMPORTANCE

Development of an HIV vaccine is desperately needed to control new infections, and elicitation of HIV bnAbs will likely be an important component of an effective vaccine. Increasingly, HIV bnAbs are being identified that bind to the N-linked glycans coating the HIV envelope glycoproteins gp120 and gp41, highlighting them as important targets for vaccine design. It is therefore important to characterize the glycan structures present on native, virion-associated gp120 and gp41 for development of vaccines that accurately mimic native-Env glycosylation. In this study, we used a number of analytical techniques to precisely study the structures of both the oligomannose and complex-type glycans present on native Env to provide a reference for determining the ability of potential HIV immunogens to accurately replicate the glycosylation pattern on these native structures.

Rapid Trimming of Cell Surface Polysialic Acid (PolySia) by Exovesicular Sialidase Triggers Release of Preexisting Surface Neurotrophin

As acidic glycocalyx on primary mouse microglial cells and a mouse microglial cell line Ra2, expression of polysialic acid (polySia/PSA), a polymer of the sialic acid Neu5Ac (N-acetylneuraminic acid), was demonstrated. PolySia is known to modulate cell adhesion, migration, and localization of neurotrophins mainly on neural cells. PolySia on Ra2 cells disappeared very rapidly after an inflammatory stimulus. Results of knockdown and inhibitor studies indicated that rapid surface clearance of polySia was achieved by secretion of endogenous sialidase Neu1 as an exovesicular component. Neu1-mediated polySia turnover was accompanied by the release of brain-derived neurotrophic factor normally retained by polySia molecules. Introduction of a single oxygen atom change into polySia by exogenous feeding of the non-neural sialic acid Neu5Gc (N-glycolylneuraminic acid) caused resistance to Neu1-induced polySia turnover and also inhibited the associated release of brain-derived neurotrophic factor. These results indicate the importance of rapid turnover of the polySia glycocalyx by exovesicular sialidases in neurotrophin regulation.

Novel strategy for herbal species classification based on UPLC-HRMS oligosaccharide profiling

Oligosaccharides, which exist widely in herbs, present diverse important pharmacological activities. However, the complexity of oligosaccharides seriously challenges their profiling, quality control, and elucidation of activity. In this paper, a novel oligosaccharide analytical method based on a new derivatization pretreatment and ultra-performance liquid chromatography coupled with high resolution tandem mass spectrometry (UPLC-HRMS) procedure was developed to rapidly profile and identify the oligosaccharides of Epimedium. Oligosaccharides are easily derivatized by 2,4-bis(diethylamino)-6-hydrazino-1,3,5-triazine under convenient and mild conditions. Without any further purification steps, oligosaccharides were analyzed by an established UPLC-HRMS/MS method with high sensitivity, good separation efficiency and speed. Benefitting from the derivatization, the oligosaccharides generated a response in the MS two orders of magnitude higher than that of the free oligosaccharide. Oligosaccharides of 52 Epimedium samples were profiled and identified based on the high-resolution mass spectral data. A total of 66 oligosaccharide compounds detected in 52 Epimedium herbs were relatively quantified and statistically processed by principal component analysis (PCA). The 52 Epimedium herbs could be classified into different species based on their oligosaccharide composition and content. Thirteen oligosaccharide compounds demonstrated potential as markers for Epimedium species classification, and their structures were preliminarily identified using MS/MS spectra.

Why Is N-Glycolylneuraminic Acid Rare in the Vertebrate Brain?

The sialic acids N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) differ by a single oxygen atom and are widely found at the terminal position of glycans on vertebrate cell surfaces. In animals capable of synthesizing Neu5Gc, most tissues and cell types express both sialic acids, in proportions that vary between species. However, it has long been noted that Neu5Gc is consistently expressed at trace to absent levels in the brains of all vertebrates studied to date. Although several reports have claimed to find low levels of Neu5Gc-containing glycans in neural tissue, no study definitively excludes the possibility of contamination with glycans from non-neural cell types. This distribution of a molecule - prominently but variably expressed in extraneural tissues but very low or absent in the brain - is, to our knowledge, unique. The evolutionarily conserved brain-specific suppression of Neu5Gc may indicate that its presence is toxic to this organ; however, no studies to date have directly addressed this very interesting question. Here we provide a historical background to this issue and discuss potential mechanisms causing the suppression of Neu5Gc expression in brain tissue, as well as mechanisms by which Neu5Gc may exert the presumed toxicity. Finally, we discuss future approaches towards understanding the mechanisms and implications of this unusual finding.

Advanced Technologies in Sialic Acid and Sialoglycoconjugate Analysis

Although the structural diversity of sialic acid (Sia) is rapidly expanding, understanding of its biological significance has lagged behind. Advanced technologies to detect and probe diverse structures of Sia are absolutely necessary not only to understand further biological significance but also to pursue medicinal and industrial applications. Here we describe analytical methods for detection of Sia that have recently been developed or improved, with a special focus on 9-O-acetylated N-acetylneuraminic acid (Neu5,9Ac), N-glycolylneuraminic acid (Neu5Gc), deaminoneuraminic acid (Kdn), O-sulfated Sia (SiaS), and di-, oligo-, and polysialic acid (diSia/oligoSia/polySia) in glycoproteins and glycolipids. Much more attention has been paid to these Sia and sialoglycoconjugates during the last decade, in terms of regulation of the immune system, neural development and function, tumorigenesis, and aging.

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.

Parvovirus glycan interactions

Members of the Parvoviridae utilize glycan receptors for cellular attachment and subsequent interactions determine transduction efficiency or pathogenic outcome. This review focuses on the identity of the glycan receptors utilized, their capsid binding footprints, and a discussion of the overlap of these sites with tropism, transduction, and pathogenicity determinants. Despite high sequence diversity between the different genera, most parvoviruses bind to negatively charged glycans, such as sialic acid and heparan sulfate, abundant on cell surface membranes. The capsid structure of these viruses exhibit high structural homology enabling common regions to be utilized for glycan binding. At the same time the sequence diversity at the common footprints allows for binding of different glycans or differential binding of the same glycan.

Characterization of Escherichia coli K1 colominic acid-specific murine antibodies that are cross-protective against Neisseria meningitidis groups B, C, and Y

The capsular polysaccharide (PS) of Neisseria meningitidis serogroup B (NMGB) is α(2-8)-linked N-acetylneuraminic acid (Neu5Ac), which is almost identical to the O-acetylated colominic acid (CA) of Escherichia coli K1 Although E. coli K1 has long been known to elicit cross-protective antibodies against NMGB, limited information on these highly cross-reactive antibodies is available. In the present study, six new monoclonal antibodies (mAbs) specific to both E. coli K1 CA and NMGB PS were produced by immunizing Balb/c mice with E. coli K1, and their serological and molecular properties were characterized, together with 12 previously reported hybridoma mAbs. Among the bactericidal mAbs against NMGB, both HmenB5 and HmenB18, which are genetically identical though of different mouse origins, were able to kill serogroup C and Y meningococci. Based on SPR sensograms, the binding affinity of HmenB18 for PS was suggested to be associated with at least two different binding forces: the polyanionicity of Neu5Ac and an interaction with the O-acetyl groups of Neu5Ac. Molecular analysis showed that similar to most mAbs presenting a few restricted V region germline genes, the V region genes of HmenB18 were 979% and 986% identical to the closest IGHV1-1401 and IGLV15-10301 germline gene alleles, respectively, and V-D-J editing in this mAb generated an unusually long VH-CDR3 sequence (17 amino acid residues), containing one basic arginine, two hydrophobic isoleucine residues and a 'YAMDY' motif. Models of the mAb combining sites demonstrate that most of the mAbs exhibited a wide, shallow groove with a high overall positive charge, as seen in mAb735, which is specific for a polyanionic helical epitope. In contrast, the combining site of HmenB18 was shown to be wide but to present a relatively weak positive charge, consistent with the extensive recognition by HmenB18 of the various structural epitopes formed with the Neu5Ac residue and its O-acetylation.

Profiling of glycan receptors for minute virus of mice in permissive cell lines towards understanding the mechanism of cell recognition

The recognition of sialic acids by two strains of minute virus of mice (MVM), MVMp (prototype) and MVMi (immunosuppressive), is an essential requirement for successful infection. To understand the potential for recognition of different modifications of sialic acid by MVM, three types of capsids, virus-like particles, wild type empty (no DNA) capsids, and DNA packaged virions, were screened on a sialylated glycan microarray (SGM). Both viruses demonstrated a preference for binding to 9-O-methylated sialic acid derivatives, while MVMp showed additional binding to 9-O-acetylated and 9-O-lactoylated sialic acid derivatives, indicating recognition differences. The glycans recognized contained a type-2 Galβ1-4GlcNAc motif (Neu5Acα2-3Galβ1-4GlcNAc or 3′SIA-LN) and were biantennary complex-type N-glycans with the exception of one. To correlate the recognition of the 3′SIA-LN glycan motif as well as the biantennary structures to their natural expression in cell lines permissive for MVMp, MVMi, or both strains, the N- and O-glycans, and polar glycolipids present in three cell lines used for in vitro studies, A9 fibroblasts, EL4 T lymphocytes, and the SV40 transformed NB324K cells, were analyzed by MALDI-TOF/TOF mass spectrometry. The cells showed an abundance of the sialylated glycan motifs recognized by the viruses in the SGM and previous glycan microarrays supporting their role in cellular recognition by MVM. Significantly, the NB324K showed fucosylation at the non-reducing end of their biantennary glycans, suggesting that recognition of these cells is possibly mediated by the Lewis X motif as in 3′SIA-Le^X identified in a previous glycan microarray screen.

Crystal structure of anti-polysialic acid antibody single chain Fv fragment complexed with octasialic acid: insight into the binding preference for polysialic acid

Polysialic acid is a linear homopolymer of α2-8-linked sialic acids attached mainly onto glycoproteins. Cell surface polysialic acid plays roles in cell adhesion and differentiation events in a manner that is often dependent on the degree of polymerization (DP). Anti-oligo/polysialic acid antibodies have DP-dependent antigenic specificity, and such antibodies are widely utilized in biological studies for detecting and distinguishing between different oligo/polysialic acids. A murine monoclonal antibody mAb735 has a unique preference for longer polymers of polysialic acid (DP >10), yet the mechanism of recognition at the atomic level remains unclear. Here, we report the crystal structure of mAb735 single chain variable fragment (scFv735) in complex with octasialic acid at 1.8 Å resolution. In the asymmetric unit, two scFv735 molecules associate with one octasialic acid. In both complexes of the unit, all the complementarity-determining regions except for L3 interact with three consecutive sialic acid residues out of the eight. A striking feature of the complex is that 11 ordered water molecules bridge the gap between antibody and ligand, whereas the direct antibody-ligand interaction is less extensive. The dihedral angles of the trisialic acid unit directly interacting with scFv735 are not uniform, indicating that mAb735 does not strictly favor the previously proposed helical conformation. Importantly, both reducing and nonreducing ends of the bound ligand are completely exposed to solvent. We suggest that mAb735 gains its apparent high affinity for a longer polysialic acid chain by recognizing every three sialic acid units in a paired manner.

Inhibition of brain ST8SiaIII sialyltransferase leads to impairment of procedural memory in mice

Several glycoproteins in mammalian brains contain α2,8-linked disialic acid residues. We previously showed a constant expression of disialic acid (DiSia) in the hippocampus, olfactory bulb and cortex, and a gradual decrease of expression in the cerebellum from neonatal to senile mice. Previous publications indicate that neurite extension of neuroblastoma-derived Neuro2A cells is inhibited in the presence of DiSia antibody. Based on this, we treated Neuro2A cell cultures with RNA interference for ST8SiaIII mRNA, the enzyme responsible for DiSia formation. We observed that neurite extension was inhibited by this treatment. Taking this evidence into consideration and the relationship of the cerebellum with learning and memory, we studied the role of DiSia expression in a learning task. Through delivery of pST8SiaIII into the brains of C57BL/6 neonatal mice, we inhibited the expression of ST8SiaIII. ST8SiaIII mRNA and protein expressions were analyzed by real-time PCR and western blot, respectively. In this work, we showed that pST8SiaIII-treated mice presented a significantly reduced level of ST8SiaIII mRNA in the cerebellum (p<0.01) in comparison to control mice at 8 days after treatment. It is also noted that these levels returned to baseline values in the adulthood. Then, we evaluated behavioural performance in the T-Maze, a learning task that estimates procedural memory. At all ages, pST8SiaIII-treated mice showed a lower performance in the test session, being most evident at older ages (p<0.001). Taken all together, we conclude that gene expression of ST8SiaIII is necessary for some cognitive tasks at early postnatal ages, since reduced levels impaired procedural memory in adult mice.

Advanced mass spectrometry and chemical analyses reveal the presence of terminal disialyl motif on mouse B-cell glycoproteins

The occurrence of a terminal disialyl motif on mammalian O-glycans is increasingly being identified through recent mass spectrometry (MS)-based glycomic profiling. In most cases, it is carried on simple core 1 structures in which both the galactose and N-acetyl galactosamine can be disialylated. In contrast, a disialyl motif on N-glycans is less readily revealed by MS mapping, since additional MS/MS analysis is required to determine the distribution of the various sialic acids on typically multisialylated complex type N-glycans. In our MS-based glycomic screening, we found that a mouse B lymphoma cell line, BCL1, ranks among those that have the highest amount of disialyl motif on its O-glycans, including those carried on CD45. More intriguingly, detailed chemical and MS/MS analyses unambiguously showed that the Neu5Gcα2-8Neu5Gc disialyl motif is also present on the N-glycans and that it can be carried on the termini of polylactosaminoglycan chains, which can be further sulfated on the proximal GlcNAc, occurring alongside other monosialylated sulfated LacNAc termini. Upon silencing the expression of mouse α2,8-sialyltransferase VI (ST8Sia VI), the overall disialyl content decreases significantly, but more so for that on the N-glycans than the O-glycans. ST8Sia VI was further shown to be the most significantly upregulated ST8Sia during plasma cell differentiation, which coincides with increasing content of the disialyl motif. Increasing terminal disialylation without leading to polysialylation may thus have important biological consequences awaiting further investigation. Likewise, the expression of mono- and disialylated sulfated LacNAc may constitute novel recognition codes modulating B-cell activation and differentiation.

Pig sperm membrane microdomains contain a highly glycosylated 15-25-kDa wheat germ agglutinin-binding protein

A highly glycosylated protein, which has unique, novel features in localization, structure, and potential function, is found in pig sperm, and named WGA-gp due to its high binding property with wheat germ agglutinin (WGA). WGA-gp is localized mainly in flagella and enriched in membrane microdomains or lipid rafts. It is not detected by ordinary protein staining methods due to a high content of both N- and O-glycans consisting of neutral monosaccharides. Interestingly, WGA-gp may be involved in intracellular Ca(2+) regulation. Treatment of sperm with anti-WGA-gp antibody enhances the amplitude of Ca(2+) oscillation without changing the basal intracellular Ca(2+) concentrations. All these features of WGA-gp, except for different carbohydrate structures occupying most part of the molecules, are similar to those of flagellasialin in sea urchin sperm, which regulates the intracellular Ca(2+) concentration. Presence of carbohydrate-enriched flagellar proteins involved in intracellular Ca(2+) regulation may be a common feature among animal sperm.

Metabolism of vertebrate amino sugars with N-glycolyl groups: resistance of α2-8-linked N-glycolylneuraminic acid to enzymatic cleavage

The sialic acid (Sia) N-acetylneuraminic acid (Neu5Ac) and its hydroxylated derivative N-glycolylneuraminic acid (Neu5Gc) differ by one oxygen atom. CMP-Neu5Gc is synthesized from CMP-Neu5Ac, with Neu5Gc representing a highly variable fraction of total Sias in various tissues and among different species. The exception may be the brain, where Neu5Ac is abundant and Neu5Gc is reported to be rare. Here, we confirm this unusual pattern and its evolutionary conservation in additional samples from various species, concluding that brain Neu5Gc expression has been maintained at extremely low levels over hundreds of millions of years of vertebrate evolution. Most explanations for this pattern do not require maintaining neural Neu5Gc at such low levels. We hypothesized that resistance of α2-8-linked Neu5Gc to vertebrate sialidases is the detrimental effect requiring the relative absence of Neu5Gc from brain. This linkage is prominent in polysialic acid (polySia), a molecule with critical roles in vertebrate neural development. We show that Neu5Gc is incorporated into neural polySia and does not cause in vitro toxicity. Synthetic polymers of Neu5Ac and Neu5Gc showed that mammalian and bacterial sialidases are much less able to hydrolyze α2-8-linked Neu5Gc at the nonreducing terminus. Notably, this difference was not seen with acid-catalyzed hydrolysis of polySias. Molecular dynamics modeling indicates that differences in the three-dimensional conformation of terminal saccharides may partly explain reduced enzymatic activity. In keeping with this, polymers of N-propionylneuraminic acid are sensitive to sialidases. Resistance of Neu5Gc-containing polySia to sialidases provides a potential explanation for the rarity of Neu5Gc in the vertebrate brain.

Sialidase NEU4 hydrolyzes polysialic acids of neural cell adhesion molecules and negatively regulates neurite formation by hippocampal neurons

Modulation of levels of polysialic acid (polySia), a sialic acid polymer, predominantly associated with the neural cell adhesion molecule (NCAM), influences neural functions, including synaptic plasticity, neurite growth, and cell migration. Biosynthesis of polySia depends on two polysialyltransferases ST8SiaII and ST8SiaIV in vertebrate. However, the enzyme involved in degradation of polySia in its physiological turnover remains uncertain. In the present study, we identified and characterized a murine sialidase NEU4 that catalytically degrades polySia. Murine NEU4, dominantly expressed in the brain, was found to efficiently hydrolyze oligoSia and polySia chains as substrates in sialidase in vitro assays, and also NCAM-Fc chimera as well as endogenous NCAM in tissue homogenates of postnatal mouse brain as assessed by immunoblotting with anti-polySia antibodies. Degradation of polySia by NEU4 was also evident in neuroblastoma Neuro2a cells that were co-transfected with Neu4 and ST8SiaIV genes. Furthermore, in mouse embryonic hippocampal primary neurons, the endogenously expressed NEU4 was found to decrease during the neuronal differentiation. Interestingly, GFP- or FLAG-tagged NEU4 was partially co-localized with polySia in neurites and significantly suppressed their outgrowth, whereas silencing of NEU4 showed the acceleration together with an increase in polySia expression. These results suggest that NEU4 is involved in regulation of neuronal function by polySia degradation in mammals.

Evolution of siglec-11 and siglec-16 genes in hominins

We previously reported a human-specific gene conversion of SIGLEC11 by an adjacent paralogous pseudogene (SIGLEC16P), generating a uniquely human form of the Siglec-11 protein, which is expressed in the human brain. Here, we show that Siglec-11 is expressed exclusively in microglia in all human brains studied-a finding of potential relevance to brain evolution, as microglia modulate neuronal survival, and Siglec-11 recruits SHP-1, a tyrosine phosphatase that modulates microglial biology. Following the recent finding of a functional SIGLEC16 allele in human populations, further analysis of the human SIGLEC11 and SIGLEC16/P sequences revealed an unusual series of gene conversion events between two loci. Two tandem and likely simultaneous gene conversions occurred from SIGLEC16P to SIGLEC11 with a potentially deleterious intervening short segment happening to be excluded. One of the conversion events also changed the 5' untranslated sequence, altering predicted transcription factor binding sites. Both of the gene conversions have been dated to ~1-1.2 Ma, after the emergence of the genus Homo, but prior to the emergence of the common ancestor of Denisovans and modern humans about 800,000 years ago, thus suggesting involvement in later stages of hominin brain evolution. In keeping with this, recombinant soluble Siglec-11 binds ligands in the human brain. We also address a second-round more recent gene conversion from SIGLEC11 to SIGLEC16, with the latter showing an allele frequency of ~0.1-0.3 in a worldwide population study. Initial pseudogenization of SIGLEC16 was estimated to occur at least 3 Ma, which thus preceded the gene conversion of SIGLEC11 by SIGLEC16P. As gene conversion usually disrupts the converted gene, the fact that ORFs of hSIGLEC11 and hSIGLEC16 have been maintained after an unusual series of very complex gene conversion events suggests that these events may have been subject to hominin-specific selection forces.

Parvoviruses: structure and infection

Parvoviruses package a ssDNA genome. Both nonpathogenic and pathogenic members exist, including those that cause fetal infections, encompassing the entire spectrum of virus phenotypes. Their small genomes and simple coding strategy has enabled functional annotation of many steps in the infectious life cycle. They assemble a multifunctional capsid responsible for cell recognition and the transport of the packaged genome to the nucleus for replication and progeny virus production. It is also the target of the host immune response. Understanding how the capsid structure relates to the function of parvoviruses provides a platform for recombinant engineering of viral gene delivery vectors for the treatment of clinical diseases, and is fundamental for dissecting the viral determinants of pathogenicity. This review focuses on our current understanding of parvovirus capsid structure and function with respect to the infectious life cycle.