Advanced Technologies in Sialic Acid and Sialoglycoconjugate Analysis

Forced expression of α2,3-sialyltransferase IV rescues impaired heart development in α2,6-sialyltransferase I-deficient medaka

Sialic acids (Sias) are often linked to galactose (Gal) residues by α2,6- and α2,3-linkages in glycans of glycoproteins. Sias are indispensable for vertebrate development, because organisms deficient in some enzymes in the Sia synthetic pathway are lethal during the development. However, it remains unknown if the difference of Siaα2,6Gal or α2,3Gal linkage has a critical meaning. To find a clue to understand significance of the linkage difference at the organism level, medaka was used as a vertebrate model. In embryos, Siaα2,6Gal epitopes recognized by Sambucus nigra lectin (SNA) and Siaα2,3Gal epitopes recognized by Maackia amurensis lectin (MAA) were enriched in the blastodisc and the yolk sphere, respectively. When these lectins were injected in the perivitelline space, SNA, but not MAA, impaired embryo body formation at 1 day post-fertilization (dpf). Most Siaα2,6Gal epitopes occurred on N-glycans owing to their sensitivity to peptide:N-glycanase. Of knockout-medaka (KO) for either of two β-galactoside:α2,6-sialyltransferase genes, ST6Gal I and ST6Gal II, only ST6Gal I-KO showed severe cardiac abnormalities at 7-16 dpf, leading to lethality at 14-18 dpf. Interestingly, however, these cardiac abnormalities of ST6Gal I-KO were rescued not only by forced expression of ST6Gal I, but also by that of ST6Gal II and the β-galactoside:α2,3-sialyltransferase IV gene (ST3Gal IV). Taken together, the Siaα2,6Gal linkage synthesized by ST6Gal I are critical in heart development; however, it can be replaced by the linkages synthesized by ST6Gal II and ST3Gal IV. These data suggest that sialylation itself is more important than its particular linkage for the heart development.

Identification and characterization of a deaminoneuraminic acid (Kdn)-specific aldolase from Sphingobacterium species

Sialic acid (Sia) is a group of acidic sugars with a 9-carbon backbone, and classified into 3 species based on the substituent group at C5 position: N-acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc), and deaminoneuraminic acid (Kdn). In Escherichia coli, the sialate aldolase or N-acetylneuraminate aldolase (NanA) is known to catabolize these Sia species into pyruvate and the corresponding 6-carbon mannose derivatives. However, in bacteria, very little is known about the catabolism of Kdn, compared with Neu5Ac. In this study, we found a novel Kdn-specific aldolase (Kdn-aldolase), which can exclusively degrade Kdn, but not Neu5Ac or Neu5Gc, from Sphingobacterium sp., which was previously isolated from a Kdn-assimilating bacterium. Kdn-aldolase had the optimal pH and temperature at 7.0-8.0 and 50 °C, respectively. It also had the synthetic activity of Kdn from pyruvate and mannose. Site-specific mutagenesis revealed that N50 residue was important for the Kdn-specific reaction. Existence of the Kdn-aldolase suggests that Kdn-specific metabolism may play a specialized role in some bacteria.

Genome-Wide CRISPR/Cas9 Screen Reveals a Role for SLC35A1 in the Adsorption of Porcine Deltacoronavirus

Porcine deltacoronavirus (PDCoV), an emerging enteropathogenic coronavirus, not only causes diarrhea in piglets but also possesses the potential to infect humans. To better understand host-virus genetic dependencies and find potential therapeutic targets for PDCoV, we used a porcine single-guide RNA (sgRNA) lentivirus library to screen host factors related to PDCoV infection in LLC-PK1 cells. The solute carrier family 35 member A1 (SLC35A1), a key molecule in the sialic acid (SA) synthesis pathway, was identified as a host factor required for PDCoV infection. A knockout of SLC35A1 caused decreases in the amounts of cell surface sialic acid (SA) and viral adsorption; meanwhile, trypsin promoted the use of SA in PDCoV infection. By constructing and assessing a series of recombinant PDCoV strains with the deletion or mutation of possible critical domain or amino acid residues for SA binding in the S1 N-terminal domain, we found that S T182 might be a PDCoV SA-binding site. However, the double knockout of SLC35A1 and amino peptidase N (APN) could not block PDCoV infection completely. Additionally, we found that different swine enteric coronaviruses, including transmissible gastroenteritis coronavirus, porcine epidemic diarrhea virus, and swine acute diarrhea syndrome coronavirus, are differentially dependent on SA. Overall, our study uncovered a collection of host factors that can be exploited as drug targets against PDCoV infection and deepened our understanding of the relationship between PDCoV and SA. IMPORTANCE Identifying the host factors required for replication will be helpful to uncover the pathogenesis mechanisms and develop antivirals against the emerging coronavirus porcine deltacoronavirus (PDCoV). Herein, we performed a genome-wide clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 knockout screen, the results of which revealed that the solute carrier family 35 member A1 (SLC35A1) is a host factor required for PDCoV infection that acts by regulating cell surface sialic acid (SA). We also identified the T182 site in the N-terminal domain of PDCoV S1 subunit as being associated with the SA-binding site and found that trypsin promotes the use of cell surface SA by PDCoV. Furthermore, different swine enteric coronaviruses use SLC35A1 differently for infection. This is the first study to screen host factors required for PDCoV replication using a genome-wide CRISPR-Cas9 functional knockout, thereby providing clues for developing antiviral drugs against PDCoV infection.

Sulfation of sialic acid is ubiquitous and essential for vertebrate development

Glycosylation of proteins and lipids occurs in vertebrates, usually terminating with sialylation, which regulates the physicochemical and biological properties of these glycoconjugates. Although less commonly known, sialic acid residues also undergo various modifications, such as acetylation, methylation, and sulfation. However, except for acetylation, the enzymes or functions of the other modification processes are unknown. To the best of our knowledge, this study is the first to demonstrate the ubiquitous occurrence of sulfated sialic acids and two genes encoding the sialate: O-sulfotransferases 1 and 2 in vertebrates. These two enzymes showed about 50% amino acid sequence identity, and appeared to be complementary to each other in acceptor substrate preferences. Gene targeting experiments showed that the deficiency of these genes was lethal for medaka fish during young fry development and accompanied by different phenotypes. Thus, the sulfation of sialic acids is essential for the vertebrate development.

Implication of N-glycolylneuraminic acid in regulation of cell adhesiveness of C2C12 myoblast cells during differentiation into myotube cells

A transition of sialic acid (Sia) species on GM3 ganglioside from N-acetylneuraminic acid (Neu5Ac) to N-glycolylneuraminic acid (Neu5Gc) takes place in mouse C2C12 myoblast cells during their differentiation into myotube cells. However, the meaning of this Sia transition remains unclear. This study thus aims to gain a functional insight into this phenomenon. The following lines of evidence show that the increased de novo synthesis of Neu5Gc residues in differentiating myoblast cells promotes adhesiveness of the cells, which is beneficial for promotion of differentiation. First, the Sia transition occurred even in the C2C12 cells cultured in serum-free medium, indicating that it happens through de novo synthesis of Neu5Gc. Second, GM3(Neu5Gc) was localized in myoblast cells, but not in myotube cells, and related to expression of the CMP-Neu5Ac hydroxylase (CMAH) gene. Notably, expression of CMAH precedes myotube formation not only in differentiating C2C12 cells, but also in mouse developing embryos. Since the myoblast cells were attached on the dish surface more strongly than the myotube cells, expression of GM3(Neu5Gc) may be related to the surface attachment of the myoblast cells. Third, exogenous Neu5Gc, but not Neu5Ac, promoted differentiation of C2C12 cells, thus increasing the number of cells committed to fuse with each other. Fourth, the CMAH-transfected C2C12 cells were attached on the gelatin-coated surface much more rapidly than the mock-cells, suggesting that the expression of CMAH promotes cell adhesiveness through the expression of Neu5Gc.

N-glycolylneuraminic acid serum biomarker levels are elevated in breast cancer patients at all stages of disease

Background

Normal human tissues do not express glycans terminating with the sialic acid N-glycolylneuraminic acid (Neu5Gc), yet Neu5Gc-containing glycans have been consistently found in human tumor tissues, cells and secretions and have been proposed as a cancer biomarker. We engineered a Neu5Gc-specific lectin called SubB2M, and previously reported elevated Neu5Gc biomarkers in serum from ovarian cancer patients using a Surface Plasmon Resonance (SPR)-based assay. Here we report an optimized SubB2M SPR-based assay and use this new assay to analyse sera from breast cancer patients for Neu5Gc levels.

Methods

To enhance specificity of our SPR-based assay, we included a non-sialic acid binding version of SubB, SubB_A12, to control for any non-specific binding to SubB2M, which improved discrimination of cancer-free controls from early-stage ovarian cancer. We analysed 96 serum samples from breast cancer patients at all stages of disease compared to 22 cancer-free controls using our optimized SubB2M-_A12-SPR assay. We also analysed a collection of serum samples collected at 6 monthly intervals from breast cancer patients at high risk for disease recurrence or spread.

Results

Analysis of sera from breast cancer cases revealed significantly elevated levels of Neu5Gc biomarkers at all stages of breast cancer. We show that Neu5Gc serum biomarker levels can discriminate breast cancer patients from cancer-free individuals with 98.96% sensitivity and 100% specificity. Analysis of serum collected prospectively, post-diagnosis, from breast cancer patients at high risk for disease recurrence showed a trend for a decrease in Neu5Gc levels immediately following treatment for those in remission.

Conclusions

Neu5Gc serum biomarkers are a promising new tool for early detection and disease monitoring for breast cancer that may complement current imaging- and biopsy-based approaches.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09428-0.

Cytidine Monophosphate N-Acetylneuraminic Acid Synthetase and Solute Carrier Family 35 Member A1 Are Required for Reovirus Binding and Infection

Engagement of cell surface receptors by viruses is a critical determinant of viral tropism and disease. The reovirus attachment protein σ1 binds sialylated glycans and proteinaceous receptors to mediate infection, but the specific requirements for different cell types are not entirely known. To identify host factors required for reovirus-induced cell death, we conducted a CRISPR-knockout screen targeting over 20,000 genes in murine microglial BV2 cells. Candidate genes required for reovirus to cause cell death were highly enriched for sialic acid synthesis and transport. Two of the top candidates identified, CMP N-acetylneuraminic acid synthetase (Cmas) and solute carrier family 35 member A1 (Slc35a1), promote sialic acid expression on the cell surface. Two reovirus strains that differ in the capacity to bind sialic acid, T3SA⁺ and T3SA^-, were used to evaluate Cmas and Slc35a1 as potential host genes required for reovirus infection. Following CRISPR-Cas9 disruption of either gene, cell surface expression of sialic acid was diminished. These results correlated with decreased binding of strain T3SA⁺, which is capable of engaging sialic acid. Disruption of either gene did not alter the low-level binding of T3SA^-, which does not engage sialic acid. Furthermore, infectivity of T3SA⁺ was diminished to levels similar to those of T3SA^- in cells lacking Cmas and Slc35a1 by CRISPR ablation. However, exogenous expression of Cmas and Slc35a1 into the respective null cells restored sialic acid expression and T3SA⁺ binding and infectivity. These results demonstrate that Cmas and Slc35a1, which mediate cell surface expression of sialic acid, are required in murine microglial cells for efficient reovirus binding and infection.IMPORTANCE Attachment factors and receptors are important determinants of dissemination and tropism during reovirus-induced disease. In a CRISPR cell survival screen, we discovered two genes, Cmas and Slc35a1, which encode proteins required for sialic acid expression on the cell surface and mediate reovirus infection of microglial cells. This work elucidates host genes that render microglial cells susceptible to reovirus infection and expands current understanding of the receptors on microglial cells that are engaged by reovirus. Such knowledge may lead to new strategies to selectively target microglial cells for oncolytic applications.

Sialic acid sulfation is induced by the antibiotic treatment in mammalian cells

Sialic acids (Sias) are an outermost-situated sugar of glycoproteins and glycolipids to play important roles in various biological phenomena. They are often modified by additional substituents, such as O-acetyl group, to display more than 50 different structures in nature. Of those modified Sia, nothing is known about the occurrence and biological functions of sulfated Sias (SiaSs) in mammals. To elucidate the significance of sialic acid sulfation, we investigated various mammalian-cultured cell lines for the expression of SiaS using the specific antibody 3G9. First, SiaS is expressed in a cell line-dependent and a cell density-dependent manner. Second, in CHO cells, the expression of SiaS is reversibly induced by treatment with the antibiotic G418. Taken together, the expression of SiaS is changed by intrinsic and extrinsic factors in mammalian cells. This is the first demonstration of regulated expression of SiaS.

Novel Molecular Mechanisms of Gangliosides in the Nervous System Elucidated by Genetic Engineering

Acidic glycosphingolipids, i.e., gangliosides, are predominantly and consistently expressed in nervous tissues of vertebrates at high levels. Therefore, they are considered to be involved in the development and function of nervous systems. Recent studies involving genetic engineering of glycosyltransferase genes have revealed novel aspects of the roles of gangliosides in the regulation of nervous tissues. In this review, novel findings regarding ganglioside functions and their modes of action elucidated mainly by studies of gene knockout mice are summarized. In particular, the roles of gangliosides in the regulation of lipid rafts to maintain the integrity of nervous systems are reported with a focus on the roles in the regulation of neuro-inflammation and neurodegeneration via complement systems. In addition, recent advances in studies of congenital neurological disorders due to genetic mutations of ganglioside synthase genes and also in the techniques for the analysis of ganglioside functions are introduced.

Identification and characterization of a novel, versatile sialidase from a Sphingobacterium that can hydrolyze the glycosides of any sialic acid species at neutral pH

Bacterial sialidases are widely used to remove sialic acid (Sia) residues from glycans. Most of them cleave the glycosides of N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) under acidic pHs; however, currently available bacterial sialidases had no activity to the glycosides of deaminoneuraminic acid (Kdn). In this study, we found a novel sialidase from Sphingobacterium sp. strain HMA12 that could cleave any of the glycosides of Neu5Ac, Neu5Gc, and Kdn. It also had a broad linkage specificity, i.e., α2,3-, α2,6-, α2,8-, and α2,9-linkages, and the optimal pH at neutral ranges, pH 6.5-7.0. These properties are particularly important when sialidases are applied for in vivo digestion of the cell surface sialosides under physiological conditions. Interestingly, 2,3-didehydro-2-deoxy-N-acetylneuraminic acid (Neu5Ac2en), which is a transition state analog-based inhibitor, competitively inhibited the enzyme-catalyzed reaction for Kdn as well as for Neu5Ac, suggesting that the active site is common to the Neu5Ac and Kdn residues. Taken together, this sialidase is versatile and useful for the in vivo research on sialo-glycoconjugates.

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.

Detection of N-glycolylneuraminic acid biomarkers in sera from patients with ovarian cancer using an engineered N-glycolylneuraminic acid-specific lectin SubB2M

N-glycolylneuraminic acid (Neu5Gc)-containing glycans are a prominent form of aberrant glycosylation found in human tumor cells and have been proposed as cancer biomarkers. The B subunit of the subtilase cytotoxin (SubB) produced by Shiga toxigenic Escherichia coli recognises Neu5Gc containing glycans. We have previously engineered this lectin, SubB2M, for greater specificity and enhanced recognition of Neu5Gc-containing glycans. Here we further explore the utility of SubB2M to detect Neu5Gc tumor biomarkers in sera from patients with ovarian cancer. Using surface plasmon resonance (SPR) we show that SubB2M can detect the established ovarian cancer biomarker, CA125, in a highly sensitive and specific fashion in the context of human serum. These studies established conditions for screening serum samples from patients with ovarian cancer for Neu5Gc glycans. We found that serum from patients with all stages of ovarian cancer had significantly elevated mean levels of Neu5Gc glycans compared to normal controls. Serum from patients with late stage disease (stages IIIC, IV) had uniformly elevated levels of Neu5Gc glycans. Detection of Neu5Gc-glycans using SubB2M has the potential to be used as a diagnostic ovarian cancer biomarker, as well as a tool for monitoring treatment and disease progression in late stage disease.

Sialylation of N-glycans: mechanism, cellular compartmentalization and function

Sialylated N-glycans play essential roles in the immune system, pathogen recognition and cancer. This review approaches the sialylation of N-glycans from three perspectives. The first section focuses on the sialyltransferases that add sialic acid to N-glycans. Included in the discussion is a description of these enzymes' glycan acceptors, conserved domain organization and sequences, molecular structure and catalytic mechanism. In addition, we discuss the protein interactions underlying the polysialylation of a select group of adhesion and signaling molecules. In the second section, the biosynthesis of sialic acid, CMP-sialic acid and sialylated N-glycans is discussed, with a special emphasis on the compartmentalization of these processes in the mammalian cell. The sequences and mechanisms maintaining the sialyltransferases and other glycosylation enzymes in the Golgi are also reviewed. In the final section, we have chosen to discuss processes in which sialylated glycans, both N- and O-linked, play a role. The first part of this section focuses on sialic acid-binding proteins including viral hemagglutinins, Siglecs and selectins. In the second half of this section, we comment on the role of sialylated N-glycans in cancer, including the roles of β1-integrin and Fas receptor N-glycan sialylation in cancer cell survival and drug resistance, and the role of these sialylated proteins and polysialic acid in cancer metastasis.