Glycan-based interactions involving vertebrate sialic-acid-recognizing proteins

Recent approaches for directly profiling cell surface sialoform

Sialic acids (SAs) are nine-carbon monosaccharides existing at the terminal location of glycan structures on the cell surface and secreted glycoconjugates. The expression levels and linkages of SAs on cells and tissues, collectively known as sialoform, present the hallmark of the cells and tissues of different systems and conditions. Accordingly, detecting or profiling cell surface sialoforms is very critical for understanding the function of cell surface glycans and glycoconjugates and even the molecular mechanisms of their underlying biological processes. Further, it may provide therapeutic and diagnostic applications for different diseases. In the past decades, several kinds of SA-specific binding molecules have been developed for detecting and profiling specific sialoforms of cells and tissues; the experimental materials have expanded from frozen tissue to living cells; and the analytical technologies have advanced from histochemistry to fluorescent imaging, flow cytometry and microarrays. This review summarizes the recent bioaffinity approaches for directly detecting and profiling specific SAs or sialylglycans, and their modifications of different cells and tissues.

Endogenous pore-forming protein complex targets acidic glycosphingolipids in lipid rafts to initiate endolysosome regulation

Bacterial pore-forming toxin aerolysin-like proteins (ALPs) are widely distributed in animals and plants. However, functional studies on these ALPs remain in their infancy. βγ-CAT is the first example of a secreted pore-forming protein that functions to modulate the endolysosome pathway via endocytosis and pore formation on endolysosomes. However, the specific cell surface molecules mediating the action of βγ-CAT remain elusive. Here, the actions of βγ-CAT were largely attenuated by either addition or elimination of acidic glycosphingolipids (AGSLs). Further study revealed that the ALP and trefoil factor (TFF) subunits of βγ-CAT bind to gangliosides and sulfatides, respectively. Additionally, disruption of lipid rafts largely impaired the actions of βγ-CAT. Finally, the ability of βγ-CAT to clear pathogens was attenuated in AGSL-eliminated frogs. These findings revealed a previously unknown double binding pattern of an animal-secreted ALP in complex with TFF that initiates ALP-induced endolysosomal pathway regulation, ultimately leading to effective antimicrobial responses.

Engineering of CHO cells for the production of vertebrate recombinant sialyltransferases

Background

Sialyltransferases (SIATs) are a family of enzymes that transfer sialic acid (Sia) to glycan chains on glycoproteins, glycolipids, and oligosaccharides. They play key roles in determining cell–cell and cell-matrix interactions and are important in neuronal development, immune regulation, protein stability and clearance. Most fully characterized SIATs are of mammalian origin and these have been used for in vitro and in vivo modification of glycans. Additional versatility could be achieved by the use of animal SIATs from other species that live in much more variable environments. Our aim was to generate a panel of stable CHO cell lines expressing a range of vertebrate SIATs with different physicochemical and functional properties.

Methods

The soluble forms of various animal ST6Gal and ST3Gal enzymes were stably expressed from a Gateway-modified secretion vector in CHO cells. The secreted proteins were IMAC-purified from serum-free media. Functionality of the protein was initially assessed by lectin binding to the host CHO cells. Activity of purified proteins was determined by a number of approaches that included a phosphate-linked sialyltransferase assay, HILIC-HPLC identification of sialyllactose products and enzyme-linked lectin assay (ELLA).

Results

A range of sialyltransferase from mammals, birds and fish were stably expressed in CHO Flp-In cells. The stable cell lines expressing ST6Gal1 modify the glycans on the surface of the CHO cells as detected by fluorescently labelled lectin microscopy. The catalytic domains, as isolated by Ni Sepharose from culture media, have enzymatic activities comparable to commercial enzymes. Sialyllactoses were identified by HILIC-HPLC on incubation of the enzymes from lactose or whey permeate. The enzymes also increased SNA-I labelling of asialofetuin when incubated in a plate format.

Conclusion

Stable cell lines are available that may provide options for the in vivo sialylation of glycoproteins. Proteins are active and should display a variety of biological and physicochemical properties based on the animal source of the enzyme.

A water-soluble AIE-active polyvalent glycocluster: design, synthesis and studies on carbohydrate–lectin interactions for visualization of Siglec distributions in living cell membranes

An AIE-active tetraphenylethene-decorated pseudo-trisialic acidTPE3Swas synthesized and utilized for visualization of Siglecs expressed on the surface of cells.

Gold(i)-promoted α-selective sialylation of glycosylortho-hexynylbenzoates for the latent-active synthesis of oligosialic acids

A gold(i)-promoted α-selective glycosylation approach with sialylortho-hexynylbenzoates as donors is developed for the latent-active synthesis of α-(2 → 9)-linked oligosialic acids.

Accumulation of α-2,6-sialyoglycoproteins in the Muscle Sarcoplasm Due to Trichinella Sp. Invasion

The sialylation of the glycoproteins in skeletal muscle tissue is not well investigated, even though the essential role of the sialic acids for the proper muscular function has been proven by many researchers. The invasion of the parasitic nematode Trichinella spiralis in the muscles with subsequent formation of Nurse cell-parasite complex initiates increased accumulation of sialylated glycoproteins within the affected area of the muscle fiber. The aim of this study is to describe some details of the α-2,6-sialylation in invaded muscle cells. Asynchronous invasion with infectious T. spiralis larvae was experimentally induced in mice. The areas of the occupied sarcoplasm were reactive towards α-2,6-sialic acid specific Sambucus nigra agglutinin during the whole process of transformation to a Nurse cell.The cytoplasm of the developing Nurse cell reacted with Helix pomatia agglutinin, Arachis hypogea agglutinin and Vicia villosa lectin-B4 after neuraminidase pretreatment.Up-regulation of the enzyme ST6GalNAc1 and down-regulation of the enzyme ST6GalNAc3 were detected throughout the course of this study. The results from our study assumed accumulation of sialyl-Tn-Ag, 6`-sialyl lactosamine, SiA-α-2,6-Gal-β-1,3-GalNAc-α-Ser/Thr and Gal-β-1,3-GalNAc(SiA-α-2,6-)-α-1-Ser/Thr oligosaccharide structures into the occupied sarcoplasm. Further investigations in this domain will develop the understanding about the amazing adaptive capabilities of skeletal muscle tissue.

Down-regulation of siglec-2 (CD22) predicts worse overall survival from HBV-related early-stage hepatocellular carcinoma: a preliminary analysis from Gene Expression Omnibus

Sialic-acid-binding immunoglobulin-like lectin (siglec) regulates cell death, anti-proliferative effects and mediates a variety of cellular activities. Little was known about the relationship between siglecs and hepatocellular carcinoma (HCC) prognosis. Siglec gene expression between tumor and non-tumor tissues were compared and correlated with overall survival (OS) from HCC patients in GSE14520 microarray expression profile. Siglec-1 to siglec-9 were all down-regulated in tumor tissues compared with those in non-tumor tissues in HCC patients (all P < 0.05). Univariate and multivariate Cox regression analysis revealed that siglec-2 overexpression could predict better OS (HR = 0.883, 95%CI = 0.806-0.966, P = 0.007). Patients with higher siglec-2 levels achieved longer OS months than those with lower siglec-2 levels in the Kaplan-Meier event analysis both in training and validation sets (P < 0.05). Alpha-fetoprotein (AFP) levels in siglec-2 low expression group were significantly higher than those in siglec-2 high expression group using Chi-square analysis (P = 0.043). In addition, both logistic regression analysis and ROC curve method showed that siglec-2 down-regulation in tumor tissues was significantly associated with AFP elevation over 300 ng/ml (P < 0.05). In conclusion, up-regulation of siglec-2 in tumor tissues could predict better OS in HCC patients. Mechanisms of siglec-2 in HCC development need further research.

Selective fragmentation of the N-glycan moiety and protein backbone of ribonuclease B on an Orbitrap Fusion Lumos Tribrid mass spectrometer

RATIONALE

The functional study and application of an intact glycoprotein require the structural characterization of both the protein backbone and the glycan moiety; the former has been successfully demonstrated with selective fragmentation of the protein backbone in CID and ExD; whether the latter can be achieved with selective fragmentation of the glycan moiety remains to be explored.

METHODS

RNase B solution was electrosprayed and its intact glycoforms of GlcNAc₂ Man_n (n = 5-9) with the highest abundance (charge state z = 16) were isolated individually and fragmented using CID, ETD, HCD, ETciD, and EThcD on the Orbitrap Fusion Lumos Tribrid mass spectrometer; the dissociation parameters were optimized for selective fragmentation of the N-glycan moiety and protein backbone as well as high sequence coverage. The obtained spectra were interpreted using the protein and N-glycan database search engines ProteinGoggle and GlySeeker, respectively.

RESULTS

With exploration of different dissociation parameters for all the five methods, selective fragmentation of the N-glycan moiety (the protein backbone staying intact) was observed in both HCD and EThcD at low collisional energies, but only a few matched product ions were observed; more comprehensive fragmentation was observed at high collisional energies (the protein backbone lost). Selective protein backbone fragmentation was observed in all the five dissociation methods.

CONCLUSIONS

For comprehensive structural characterization of intact N-glycoproteins using tandem mass spectrometry, the composition and topology of the N-glycan moiety can be identified using HCD and EThcD complementarily at low and high energies; while the amino acid sequence and glycosite can be identified using CID, ETD, HCD, ETciD, and EThcD with their optimal dissociation parameters.

Sialic acid is a critical fetal defense against maternal complement attack

The negatively charged sugar sialic acid (Sia) occupies the outermost position in the bulk of cell surface glycans. Lack of sialylated glycans due to genetic ablation of the Sia-activating enzyme CMP-sialic acid synthase (CMAS) resulted in embryonic lethality around day 9.5 post coitum (E9.5) in mice. Developmental failure was caused by complement activation on trophoblasts in Cmas-/- implants and was accompanied by infiltration of maternal neutrophils at the fetal-maternal interface, intrauterine growth restriction, impaired placental development, and a thickened Reichert's membrane. This phenotype, which shared features with complement receptor 1-related protein Y (Crry) depletion, was rescued in E8.5 Cmas-/- mice upon injection of cobra venom factor, resulting in exhaustion of the maternal complement component C3. Here we show that Sia is dispensable for early development of the embryo proper but pivotal for fetal-maternal immune homeostasis during pregnancy, i.e., for protecting the allograft implant against attack by the maternal innate immune system. Finally, embryos devoid of cell surface sialylation suffered from malnutrition due to inadequate placentation as a secondary effect.

Modulation of Immune Tolerance via Siglec-Sialic Acid Interactions

One of the key features of the immune system is its extraordinary capacity to discriminate between self and non-self and to respond accordingly. Several molecular interactions allow the induction of acquired immune responses when a foreign antigen is recognized, while others regulate the resolution of inflammation, or the induction of tolerance to self-antigens. Post-translational signatures, such as glycans that are part of proteins (glycoproteins) and lipids (glycolipids) of host cells or pathogens, are increasingly appreciated as key molecules in regulating immunity vs. tolerance. Glycans are sensed by glycan binding receptors expressed on immune cells, such as C-type lectin receptors (CLRs) and Sialic acid binding immunoglobulin type lectins (Siglecs), that respond to specific glycan signatures by triggering tolerogenic or immunogenic signaling pathways. Glycan signatures present on healthy tissue, inflamed and malignant tissue or pathogens provide signals for “self” or “non-self” recognition. In this review we will focus on sialic acids that serve as “self” molecular pattern ligands for Siglecs. We will emphasize on the function of Siglec-expressing mononuclear phagocytes as sensors for sialic acids in tissue homeostasis and describe how the sialic acid-Siglec axis is exploited by tumors and pathogens for the induction of immune tolerance. Furthermore, we highlight how the sialic acid-Siglec axis can be utilized for clinical applications to induce or inhibit immune tolerance.

The use of the novel glycosyl acceptor and supramer analysis in the synthesis of sialyl-α(2-3)-galactose building block

A new glycosyl acceptor to be used in sialylation was designed as a 3-hydroxy derivative of 4-methoxyphenyl β-d-galactopyranoside with 2-O-acetyl group and O-4 and O-6 protected as benzylidene acetal. Two alternative syntheses of this compound were compared. Sialylation of 3-OH group of the glycosyl acceptor with O-chloroacetylated N-trifluoroacetylneuraminic acid phenyl thioglycoside (NIS, TfOH, MeCN, MS 3 Å, -40 °C) was studied in a wide concentration range (5-150 mmol L^-1). The outcome of sialylation generally followed the predictions of supramer analysis of solutions of sialyl donor in MeCN, which was performed by polarimetry and static light scattering and revealed two concentration ranges differing in solution structure and the structures of supramers of glycosyl donor. The optimized conditions of sialylation (C = 50 mmol L^-1) were used to synthesize protected Neu-α(2-3)-Gal disaccharide (78%, α:β = 13:1), which was then converted to sialyl-α(2-3)-galactose imidate building block useful for the synthesis of complex sialo-oligosaccharides.

Advances in mass spectrometry-based glycomics

The diversification of the chemical properties and biological functions of proteins is attained through posttranslational modifications, such as glycosylation. Glycans, which are covalently attached to proteins, play a vital role in cell activities. The microheterogeneity and complexity of glycan structures associated with proteins make comprehensive glycomic analysis challenging. However, recent advancements in mass spectrometry (MS), separation techniques, and sample preparation methods have primarily facilitated structural elucidation and quantitation of glycans. This review focuses on describing recent advances in MS-based techniques used for glycomic analysis (2012-2018), including ionization, tandem MS, and separation techniques coupled with MS. Progress in glycomics workflow involving glycan release, purification, derivatization, and separation will also be highlighted here. Additionally, the recent development of quantitative glycomics through comparative and multiplex approaches will also be described.

Recent Chemical Biology Approaches for Profiling Cell Surface Sialylation Status

Sialic acids (SAs) often exist as the terminal sugars of glycans of either glycoproteins or glycolipids on the cell surface and thus are directly involved in biological processes, such as cell-cell, cell-ligand, and cell-pathogen interactions. Cell surface SA expression levels and their linkages are collectively termed cell surface sialylation status, which represent varying cellular states and contribute to the overall functionality of a cell. Accordingly, systemic and specific profiling of the cell surface sialyation status is critical in deciphering the structures and functions of cell surface glycoconjugates and the molecular mechanisms of their underlying biological processes. In recent decades, several advanced chemical biology approaches have been developed to profile the cell surface sialyation status of both in vitro and in vivo samples, including metabolic labeling, direct chemical modification, and boronic acid coupling approaches. Various investigative technologies have also been explored for their unique competence, including fluorescent imaging, flow cytometry, Raman imaging, magnetic resonance imaging (MRI), and matrix-assisted laser desorption ionization imaging mass spectrometry. In particular, the sialylation status of a specific glycoprotein on the cell surface has been investigated. This review highlights the recent advancements in chemical biology approaches for profiling cell surface sialyation status. It is expected that this review will provide researchers different choices for both biological and biomedical research and applications.

Mass spectrometry for protein sialoglycosylation

Sialic acids are a family of structurally unique and negatively charged nine-carbon sugars, normally found at the terminal positions of glycan chains on glycoproteins and glycolipids. The glycosylation of proteins is a universal post-translational modification in eukaryotic species and regulates essential biological functions, in which the most common sialic acid is N-acetyl-neuraminic acid (2-keto-5-acetamido-3,5-dideoxy-D-glycero-D-galactononulopyranos-1-onic acid) (Neu5NAc). Because of the properties of sialic acids under general mass spectrometry (MS) conditions, such as instability, ionization discrimination, and mixed adducts, the use of MS in the analysis of protein sialoglycosylation is still challenging. The present review is focused on the application of MS related methodologies to the study of both N- and O-linked sialoglycans. We reviewed MS-based strategies for characterizing sialylation by analyzing intact glycoproteins, proteolytic digested glycopeptides, and released glycans. The review concludes with future perspectives in the field.

Siglecs: A journey through the evolution of sialic acid-binding immunoglobulin-type lectins

Siglecs (sialic acid-binding immunoglobulin-type lectins) are a family of immune regulatory receptors predominantly found on the cells of the hematopoietic system. A V-set Ig-like domain mediates the recognition of different sialylated glycoconjugates, which can lead to the activation or inhibition of the immune response, depending on the involved Siglecs. Siglecs are categorized into two subgroups: one including all CD33-related Siglecs and the other consisting of Siglec-1 (Sialoadhesin), Siglec-2 (CD22), Siglec-4 (myelin-associated glycoprotein, MAG) and Siglec-15. In contrast to the members of the CD33-related Siglecs, which share ∼50-99% sequence identity, Siglecs of the other subgroup show quite low homology (approximately 25-30% sequence identity). Based on the published sequences and functions of Siglecs, we performed phylogenetic analyses and sequence alignments to reveal the conservation of Siglecs throughout evolution. Therefore, we focused on the presence of Siglecs in different classes of vertebrates (fishes, amphibians, birds, reptiles and mammals), offering a bridge between the presence of different Siglecs and the biological situations of the selected animals.

Identification and characterization of a novel β-D-galactosidase that releases pyruvylated galactose

Pyruvyl modification of oligosaccharides is widely seen in both prokaryotes and eukaryotes. Although the biosynthetic mechanisms of pyruvylation have been investigated, enzymes that metabolize and degrade pyruvylated oligosaccharides are not well known. Here, we searched for a pyruvylated galactose (PvGal)-releasing enzyme by screening soil samples. We identified a Bacillus strain, as confirmed by the 16S ribosomal RNA gene analysis, that exhibited PvGal-ase activity toward p-nitrophenyl-β-D-pyruvylated galactopyranose (pNP-β-D-PvGal). Draft genome sequencing of this strain, named HMA207, identified three candidate genes encoding potential PvGal-ases, among which only the recombinant protein encoded by ORF1119 exhibited PvGal-ase activity. Although ORF1119 protein displayed broad substrate specificity for pNP sugars, pNP-β-D-PvGal was the most favorable substrate. The optimum pH for the ORF1119 PvGal-ase was determined as 7.5. A BLAST search suggested that ORF1119 homologs exist widely in bacteria. Among two homologs tested, BglC from Clostridium but not BglH from Bacillus showed PvGal-ase activity. Crystal structural analysis together with point mutation analysis revealed crucial amino acids for PvGal-ase activity. Moreover, ORF1119 protein catalyzed the hydrolysis of PvGal from galactomannan of Schizosaccharomyces pombe, suggesting that natural polysaccharides might be substrates of the PvGal-ase. This novel PvGal-catalyzing enzyme might be useful for glycoengineering projects to produce new oligosaccharide structures.

Quantification of Multivalency in Protein-Oligomer-Coated Nanoparticles Targeting Dynamic Membrane Glycan Receptors

Multivalent binding of proteins to glycan receptors on the host cell quantitatively controls the initial adhesion of most viruses. However, quantifying such multivalency in terms of binding valency has always been a challenge because of the hierarchy of multivalency involving multiple protein oligomers on the virus, limiting our understanding of virus adhesion and virulence. To address this challenge, we mimicked virus adhesion to cell surfaces by attaching protein-oligomer-coated nanoparticles (NPs) to fluidic glycolipid membranes with surface glycan density varying over 4 orders of magnitude. Using total internal reflection fluorescence microscopy to track single attached NPs, we show that the binding isotherms exhibit two regions, attributed to monovalent and multivalent protein/glycan interactions at low and high glycan densities, respectively. The bimodal binding curve allows the quantification of the different valency and binding constants of monovalent and multivalent interactions. In addition, the competitive inhibition of multivalency by the glycopolymer presenting multiple glycan moieties is quantitatively appreciated. This work is essential to mapping and understanding the complex binding specificities of glycan-binding proteins and inhibitory drug designs and applications.

Crystal structures and kinetic analyses of N-acetylmannosamine-6-phosphate 2-epimerases from Fusobacterium nucleatum and Vibrio cholerae

Sialic acids are nine-carbon sugars that are found abundantly on the cell surfaces of mammals as glycoprotein or glycolipid complexes. Several Gram-negative and Gram-positive bacteria have the ability to scavenge and catabolize sialic acids to use as a carbon source. This gives them an advantage in colonizing sialic acid-rich environments. The genes of the sialic acid catabolic pathway are generally present as the operon nanAKE. The third gene in the operon encodes the enzyme N-acetylmannosamine-6-phosphate 2-epimerase (NanE), which catalyzes the conversion of N-acetylmannosamine 6-phosphate to N-acetylglucosamine 6-phosphate, thus committing it to enter glycolysis. The NanE enzyme belongs to the isomerase class of enzymes possessing the triose phosphate isomerase (TIM) barrel fold. Here, comparative structural and functional characterizations of the NanE epimerases from two pathogenic Gram-negative bacteria, Fusobacterium nucleatum (Fn) and Vibrio cholerae (Vc), have been carried out. Structures of NanE from Vc (VcNanE) with and without ligand bound have been determined to 1.7 and 2.7 Å resolution, respectively. The structure of NanE from Fn (FnNanE) has been determined to 2.2 Å resolution. The enzymes show kinetic parameters that are consistent with those of Clostridium perfringens NanE. These studies allowed an evaluation of whether NanE may be a good drug target against these pathogenic bacteria.

Evolution of sialic acids: Implications in xenotransplant biology

All living cells are covered with a dense “sugar-coat” of carbohydrate chains (glycans) conjugated to proteins and lipids. The cell surface glycome is determined by a non-template driven process related to the collection of enzymes that assemble glycans in a sequential manner. In mammals, many of these glycans are topped with sialic acids (Sia), a large family of acidic sugars. The “Sialome” is highly diverse owing to various Sia types, linkage to underlying glycans, range of carriers, and complex spatial organization. Presented at the front of cells, Sia play a major role in immunity and recognition of “self” versus “non-self,” largely mediated by the siglecs family of Sia-binding host receptors. Albeit many mammalian pathogens have evolved to hijack this recognition system to avoid host immune attack, presenting a fascinating host-pathogen evolutionary arms race. Similarly, cancer cells exploit Sia for their own survival and propagation. As part of this ongoing fitness, humans lost the ability to synthesize the Sia type N-glycolylneuraminic acid (Neu5Gc), in contrast to other mammals. While this loss had provided an advantage against certain pathogens, humans are continuously exposed to Neu5Gc through mammalian-derived diet (eg, red meat), consequently generating a complex immune response against it. Circulating anti-Neu5Gc antibodies together with Neu5Gc on some human tissues mediate chronic inflammation “xenosialitis” that exacerbate various human diseases (eg, cancer and atherosclerosis). Similarly, Neu5Gc-containing xenografts are exposed to human anti-Neu5Gc antibodies with implications to sustainability. This review aimed to provide a glimpse into the evolution of Sia and their implications to xenotransplantation.

Human evolutionary loss of epithelial Neu5Gc expression and species-specific susceptibility to cholera

While infectious agents have typical host preferences, the noninvasive enteric bacterium Vibrio cholerae is remarkable for its ability to survive in many environments, yet cause diarrheal disease (cholera) only in humans. One key V. cholerae virulence factor is its neuraminidase (VcN), which releases host intestinal epithelial sialic acids as a nutrition source and simultaneously remodels intestinal polysialylated gangliosides into monosialoganglioside GM1. GM1 is the optimal binding target for the B subunit of a second virulence factor, the AB5 cholera toxin (Ctx). This coordinated process delivers the CtxA subunit into host epithelia, triggering fluid loss via cAMP-mediated activation of anion secretion and inhibition of electroneutral NaCl absorption. We hypothesized that human-specific and human-universal evolutionary loss of the sialic acid N-glycolylneuraminic acid (Neu5Gc) and the consequent excess of N-acetylneuraminic acid (Neu5Ac) contributes to specificity at one or more steps in pathogenesis. Indeed, VcN was less efficient in releasing Neu5Gc than Neu5Ac. We show enhanced binding of Ctx to sections of small intestine and isolated polysialogangliosides from human-like Neu5Gc-deficient Cmah-/- mice compared to wild-type, suggesting that Neu5Gc impeded generation of the GM1 target. Human epithelial cells artificially expressing Neu5Gc were also less susceptible to Ctx binding and CtxA intoxication following VcN treatment. Finally, we found increased fluid secretion into loops of Cmah-/- mouse small intestine injected with Ctx, indicating an additional direct effect on ion transport. Thus, V. cholerae evolved into a human-specific pathogen partly by adapting to the human evolutionary loss of Neu5Gc, optimizing multiple steps in cholera pathogenesis.

Main Strategies of Plant Expression System Glycoengineering for Producing Humanized Recombinant Pharmaceutical Proteins

Most the pharmaceutical proteins are derived not from their natural sources, rather their recombinant analogs are synthesized in various expression systems. Plant expression systems, unlike mammalian cell cultures, combine simplicity and low cost of procaryotic systems and the ability for posttranslational modifications inherent in eucaryotes. More than 50% of all human proteins and more than 40% of the currently used pharmaceutical proteins are glycosylated, that is, they are glycoproteins, and their biological activity, pharmacodynamics, and immunogenicity depend on the correct glycosylation pattern. This review examines in detail the similarities and differences between N- and O-glycosylation in plant and mammalian cells, as well as the effect of plant glycans on the activity, pharmacokinetics, immunity, and intensity of biosynthesis of pharmaceutical proteins. The main current strategies of glycoengineering of plant expression systems aimed at obtaining fully humanized proteins for pharmaceutical application are summarized.

Mosquito Saliva Reshapes Alphavirus Infection and Immunopathogenesis

Alphaviruses are transmitted to humans via bites of infected mosquitoes. Although alphaviruses have caused a wide range of outbreaks and crippling disease, the availability of licensed vaccines or antiviral therapies remains limited. Mosquito vectors such as Aedes and Culex are the main culprits in the transmission of alphaviruses. This review explores how mosquito saliva may promote alphavirus infection. Identifying the roles of mosquito-derived factors in alphavirus pathogenesis will generate novel tools to circumvent and control mosquito-borne alphavirus infections in humans.

Metabolic glycan labeling-assisted discovery of cell-surface markers for primary neural stem and progenitor cells

A chemical approach was developed for identifying cell-surface markers for primary neural stem cells (NSCs). Using an in vitro coculture system of primary NSCs combined with metabolic labeling of sialoglycans with bioorthogonal functional groups, we selectively enriched and identified a list of cell-surface sialoglycoproteins that were more abundantly expressed in neural stem and progenitor cells.

Substrate-bound outward-open structure of a Na+-coupled sialic acid symporter reveals a new Na+ site

Many pathogenic bacteria utilise sialic acids as an energy source or use them as an external coating to evade immune detection. As such, bacteria that colonise sialylated environments deploy specific transporters to mediate import of scavenged sialic acids. Here, we report a substrate-bound 1.95 Å resolution structure and subsequent characterisation of SiaT, a sialic acid transporter from Proteus mirabilis. SiaT is a secondary active transporter of the sodium solute symporter (SSS) family, which use Na⁺ gradients to drive the uptake of extracellular substrates. SiaT adopts the LeuT-fold and is in an outward-open conformation in complex with the sialic acid N-acetylneuraminic acid and two Na⁺ ions. One Na⁺ binds to the conserved Na2 site, while the second Na⁺ binds to a new position, termed Na3, which is conserved in many SSS family members. Functional and molecular dynamics studies validate the substrate-binding site and demonstrate that both Na⁺ sites regulate N-acetylneuraminic acid transport.

Examination of Signatures of Recent Positive Selection on Genes Involved in Human Sialic Acid Biology

Sialic acids are nine carbon sugars ubiquitously found on the surfaces of vertebrate cells and are involved in various immune response-related processes. In humans, at least 58 genes spanning diverse functions, from biosynthesis and activation to recycling and degradation, are involved in sialic acid biology. Because of their role in immunity, sialic acid biology genes have been hypothesized to exhibit elevated rates of evolutionary change. Consistent with this hypothesis, several genes involved in sialic acid biology have experienced higher rates of non-synonymous substitutions in the human lineage than their counterparts in other great apes, perhaps in response to ancient pathogens that infected hominins millions of years ago (paleopathogens). To test whether sialic acid biology genes have also experienced more recent positive selection during the evolution of the modern human lineage, reflecting adaptation to contemporary cosmopolitan or geographically-restricted pathogens, we examined whether their protein-coding regions showed evidence of recent hard and soft selective sweeps. This examination involved the calculation of four measures that quantify changes in allele frequency spectra, extent of population differentiation, and haplotype homozygosity caused by recent hard and soft selective sweeps for 55 sialic acid biology genes using publicly available whole genome sequencing data from 1,668 humans from three ethnic groups. To disentangle evidence for selection from confounding demographic effects, we compared the observed patterns in sialic acid biology genes to simulated sequences of the same length under a model of neutral evolution that takes into account human demographic history. We found that the patterns of genetic variation of most sialic acid biology genes did not significantly deviate from neutral expectations and were not significantly different among genes belonging to different functional categories. Those few sialic acid biology genes that significantly deviated from neutrality either experienced soft sweeps or population-specific hard sweeps. Interestingly, while most hard sweeps occurred on genes involved in sialic acid recognition, most soft sweeps involved genes associated with recycling, degradation and activation, transport, and transfer functions. We propose that the lack of signatures of recent positive selection for the majority of the sialic acid biology genes is consistent with the view that these genes regulate immune responses against ancient rather than contemporary cosmopolitan or geographically restricted pathogens.

Boronic acid functionalized g-C3N4 nanosheets for ultrasensitive and selective sensing of glycoprotein in the physiological environment

As important biomarkers, glycoprotein sensing is frequently facilitated by boronic acid binding with its cis-diols. However, boronic acid based sensors suffer from drawbacks of alkali restriction and/or sensitivity limitation. Herein, we report boronic acid decorated g-C₃N₄ nanosheets (B-g-CN) with a Wulff-type boronic acid feature, which selectively bind glycoprotein under physiological conditions. Meanwhile, the binding causes significant enhancement of the B-g-CN nanosheet fluorescence, providing the basis for glycoprotein sensing. With IgG as a model, a detection limit (LOD) of 2.2 nM (3σ/s, n = 11) was obtained within a linear range of 6.7-67 nM. The LOD was further improved to 52 pM subject to enrichment of the nanosheets, which well enables IgG assay in human urine samples. Moreover, it was successful in imaging endogenous and exogenous glycoproteins in living cells.

Siglec genes confer resistance to systemic lupus erythematosus in humans and mice

A recent meta-analysis revealed the contribution of the SIGLEC6 locus to the risk of developing systemic lupus erythematosus (SLE). However, no specific Siglec (sialic acid-binding immunoglobulin-like lectin) genes (Siglecs) have been implicated in the pathogenesis of SLE. Here, we performed in silico analysis of the function of three major protective alleles in the locus and found that these alleles were expression quantitative trait loci that enhanced expression of the adjacent SIGLEC12 gene. These data suggest that SIGLEC12 may protect against the development of SLE in Asian populations. Consistent with human genetic data, we identified two missense mutations in lupus-prone B6.NZM^{Sle1/Sle2/Sle3} (Sle1-3) mice in Siglece, which is the murine Siglec with the greatest homology to human SIGLEC12. Since the mutations resulted in reduced binding of Siglec E to splenic cells, we evaluated whether Siglece^-/- mice had SLE phenotypes. We found that Siglece^-/- mice showed increased autoantibody production, glomerular immune complex deposition and severe renal pathology reminiscent of human SLE nephropathy. Our data demonstrate that the Siglec genes confer resistance to SLE in mice and humans.

Polydiacetylene liposomes with phenylboronic acid tags: a fluorescence turn-on sensor for sialic acid detection and cell-surface glycan imaging

Sialic acid (SA) located at the terminal end of glycans on cell membranes has been shown to play an important yet distinctive role in various biological and pathological processes. Effective methods for the facile, sensitive and in situ analysis of SA on living cell surfaces are of great significance in terms of clinical diagnostics and therapeutics. Here, a new polydiacetylene (PDA) liposome-based sensor system bearing phenylboronic acid (PBA) and 1,8-naphthalimide derived fluorophore moieties was developed as a fluorescence turn-on sensor for the detection of free SA in aqueous solution and the in situ imaging of SA-terminated glycans on living cell surfaces. In the sensor system, three diacetylene monomers, PCDA-pBA, PCDA-Nap and PCDA-EA, were designed and synthesized to construct the composite PDA liposome sensor. The monomer PCDA-pBA modified with PBA molecules was employed as a receptor for SA recognition, while the monomer PCDA-Nap containing a 1,8-naphthalimide derivative fluorophore was used for fluorescence signaling. When the composite PDA liposomes were formed, the energy transfer between the fluorophore and the conjugated backbone could directly quench the fluorescence of the fluorophore. In the presence of additional SA or SA abundant cells, the strong binding of SA with PBA moieties disturbed the pendent side chain conformation, resulting in the fluorescence restoration of the fluorophore. The proposed methods realized the fluorescence turn-on detection of free SA in aqueous solution and the in situ imaging of SA on living MCF-7 cell surfaces. This work provides a new potential tool for simple and selective analysis of SA on living cell membranes.

Automation aided optimization of cloning, expression and purification of enzymes of the bacterial sialic acid catabolic and sialylation pathways enzymes for structural studies

The process of obtaining a well-expressing, soluble and correctly folded constructs can be made easier and quicker by automating the optimization of cloning, expression and purification. While there are many semiautomated pipelines available for cloning, expression and purification, there is hardly any pipeline that involves complete automation. Here, we achieve complete automation of all the steps involved in cloning and in vivo expression screening. This is demonstrated using 18 genes involved in sialic acid catabolism and the surface sialylation pathway. Our main objective was to clone these genes into a His-tagged Gateway vector, followed by their small-scale expression optimization in vivo. The constructs that showed best soluble expression were then selected for purification studies and scaled up for crystallization studies. Our technique allowed us to quickly find conditions for producing significant quantities of soluble proteins in Escherichia coli, their large-scale purification and successful crystallization of a number of these proteins. The method can be implemented in other cases where one needs to screen a large number of constructs, clones and expression vectors for successful recombinant production of functional proteins.

Nucleophile Promiscuity of Engineered Class II Pyruvate Aldolase YfaU from E. Coli

Pyruvate-dependent aldolases exhibit a stringent selectivity for pyruvate, limiting application of their synthetic potential, which is a drawback shared with other existing aldolases. Structure-guided rational protein engineering rendered a 2-keto-3-deoxy-l-rhamnonate aldolase variant, fused with a maltose-binding protein (MBP-YfaU W23V/L216A), capable of efficiently converting larger pyruvate analogues, for example, those with linear and branched aliphatic chains, in aldol addition reactions. Combination of these nucleophiles with N-Cbz-alaninal (Cbz=benzyloxycarbonyl) and N-Cbz-prolinal electrophiles gave access to chiral building blocks, for example, derivatives of (2S,3S,4R)-4-amino-3-hydroxy-2-methylpentanoic acid (68 %, d.r. 90:10) and the enantiomer of dolaproine (33 %, d.r. 94:6) as well as a collection of unprecedented α-amino acid derivatives of the proline and pyrrolizidine type. Conversions varied between 6-93 % and diastereomeric ratios from 50:50 to 95:5 depending on the nucleophilic and electrophilic components.

Sialic Acid-Binding Immunoglobulin-Like Lectin1 as a Novel Predictive Biomarker for Relapse in Graves' Disease: A Multicenter Study

BACKGROUND

There are currently no reliable biomarkers to predict relapse in Graves' disease (GD). In the present study, we investigated novel diagnostic biomarkers to predict the long-term remission of or relapse in GD.

METHODS

A DNA microarray analysis was performed to examine gene expression in the peripheral leukocytes of a frequently relapsing patient with GD and a patient in long-term remission after the discontinuation of antithyroid drugs (ATDs). Based on the DNA microarray analysis, we focused on Sialic acid-binding immunoglobulin-like lectin1 (SIGLEC1) as a candidate novel biomarker to predict GD relapse. Three hundred and fifty-eight patients with GD in the thyroid clinics of four different hospitals in Japan were included in a cross-sectional study to establish whether SIGLEC1 mRNA levels distinguish GD relapse experience from long-term remission. An additional 55 patients with GD were enrolled in a prospective study to clarify whether SIGLEC1 mRNA levels at ATD discontinuation predict GD relapse.

RESULTS

SIGLEC1 mRNA levels were significantly higher in patients with GD relapse experience than in those in long-term remission. Based on the receiver operating characteristic analysis, we found that high SIGLEC1 mRNA levels (≥258.9 copies) significantly distinguished GD relapse experience from long-term remission (p < 0.0001; sensitivity 66.7%, specificity 70.1%). In the prospective study, when the optimal cutoff value from the receiver operating characteristic curve analysis was applied to SIGLEC1 mRNA positivity at ATD discontinuation, SIGLEC1-positive patients (≥258.9 copies) showed a significantly higher cumulative risk of relapse than SIGLEC1-negative patients (<258.9 copies) (p = 0.022, the log-rank test).

CONCLUSIONS

SIGLEC1 mRNA levels have potential as a novel predictive biomarker for GD relapse.

Impact of host cell line choice on glycan profile

Protein glycosylation is post-translational modification (PTM) which is important for pharmacokinetics and immunogenicity of recombinant glycoprotein therapeutics. As a result of variations in monosaccharide composition, glycosidic linkages and glycan branching, glycosylation introduces considerable complexity and heterogeneity to therapeutics. The host cell line used to produce the glycoprotein has a strong influence on the glycosylation because different host systems may express varying repertoire of glycosylation enzymes and transporters that contributes to specificity and heterogeneity in glycosylation profiles. In this review, we discuss the types of host cell lines currently used for recombinant therapeutic production, their glycosylation potential and the resultant impact on glycoprotein properties. In addition, we compare the reported glycosylation profiles of four recombinant glycoproteins: immunoglobulin G (IgG), coagulation factor VII (FVII), erythropoietin (EPO) and alpha-1 antitrypsin (A1AT) produced in different mammalian cells to establish the influence of mammalian host cell lines on glycosylation.

Functional Metabolomics Characterizes a Key Role for N-Acetylneuraminic Acid in Coronary Artery Diseases

BACKGROUND

As new biomarkers of coronary artery diseases (CAD) emerge via metabolomics, the underlying functional mechanisms remain to be elucidated. Functional metabolomics aims to translate metabolomics-derived biomarkers to disease mechanisms.

METHODS

A cohort of 2324 patients who underwent coronary angiography from 4 independent centers was studied. A combination of ultra-performance liquid chromatography and quadrupole time-of-flight mass spectrometry in the negative ion mode was used for untargeted analysis of metabolites in plasma. Significant differential metabolites were identified by cross-comparisons with and within CAD types, including normal coronary artery, nonobstructvie coronary atherosclerosis, stable angina, unstable angina, and acute myocardial infarction. A tandem liquid chromatography-mass spectrometry-based approach using isotope-labeled standard addition was subsequently performed for targeted analysis of the metabolic marker N-acetylneuraminic acid (Neu5Ac). A functional metabolomics strategy was proposed to investigate the role of Neu5Ac in the progression of CAD by using in vitro and in vivo models.

RESULTS

We identified a total of 36 differential metabolites, 35 of which were confirmed with reference compounds. Elevation of Neu5Ac was observed in plasma during CAD progression in center 1 (P=4.0e-64, n=2019) and replicated in 3 independent centers (n=305). The increased level of Neu5Ac in plasma was confirmed by accurate targeted quantification. Mechanistically, Neu5Ac was able to trigger myocardial injury in vitro and in vivo by activation of the Rho/Rho-associated coiled-coil containing protein kinase signaling pathway through binding to RhoA and Cdc42, but not Rac1. Silencing neuraminidase-1, the enzyme that regulates Neu5Ac generation, ameliorated oxygen-glucose deprivation-induced injury in cardiomyocytes and ligation/isoprenaline-induced myocardial ischemia injury in rats. Pharmacological inhibition of neuraminidase by anti-influenza drugs, oseltamivir and zanamivir, also protected cardiomyocytes and the heart from myocardial injury.

CONCLUSIONS

Functional metabolomics identified a key role for Neu5Ac in acute myocardial infarction, and targeting neuraminidase-1 may represent an unrecognized therapeutic intervention for CAD.

Comprehensive metabolic profiling of chronic low-grade inflammation among generally healthy individuals

BACKGROUND

Inflammation occurs as an immediate protective response of the immune system to a harmful stimulus, whether locally confined or systemic. In contrast, a persisting, i.e., chronic, inflammatory state, even at a low-grade, is a well-known risk factor in the development of common diseases like diabetes or atherosclerosis. In clinical practice, laboratory markers like high-sensitivity C-reactive protein (hsCRP), white blood cell count (WBC), and fibrinogen, are used to reveal inflammatory processes. In order to gain a deeper insight regarding inflammation-related changes in metabolism, the present study assessed the metabolic patterns associated with alterations in inflammatory markers.

METHODS

Based on mass spectrometry and nuclear magnetic resonance spectroscopy we determined a comprehensive panel of 613 plasma and 587 urine metabolites among 925 apparently healthy individuals. Associations between inflammatory markers, namely hsCRP, WBC, and fibrinogen, and metabolite levels were tested by linear regression analyses controlling for common confounders. Additionally, we tested for a discriminative signature of an advanced inflammatory state using random forest analysis.

RESULTS

HsCRP, WBC, and fibrinogen were significantly associated with 71, 20, and 19 plasma and 22, 3, and 16 urine metabolites, respectively. Identified metabolites were related to the bradykinin system, involved in oxidative stress (e.g., glutamine or pipecolate) or linked to the urea cycle (e.g., ornithine or citrulline). In particular, urine 3'-sialyllactose was found as a novel metabolite related to inflammation. Prediction of an advanced inflammatory state based solely on 10 metabolites was well feasible (median AUC: 0.83).

CONCLUSIONS

Comprehensive metabolic profiling confirmed the far-reaching impact of inflammatory processes on human metabolism. The identified metabolites included not only those already described as immune-modulatory but also completely novel patterns. Moreover, the observed alterations provide molecular links to inflammation-associated diseases like diabetes or cardiovascular disorders.

Membrane-enclosed multienzyme (MEME) synthesis of 2,7-anhydro-sialic acid derivatives

Naturally occurring 2,7-anhydro-alpha-N-acetylneuraminic acid (2,7-anhydro-Neu5Ac) is a transglycosylation product of bacterial intramolecular trans-sialidases (IT-sialidases). A facile one-pot two-enzyme approach has been established for the synthesis of 2,7-anhydro-sialic acid derivatives including those containing different sialic acid forms such as Neu5Ac and N-glycolylneuraminic acid (Neu5Gc). The approach is based on the use of Ruminoccocus gnavus IT-sialidase for the release of 2,7-anhydro-sialic acid from glycoproteins, and the conversion of free sialic acid by a sialic acid aldolase. This synthetic method, which is based on a membrane-enclosed enzymatic synthesis, can be performed on a preparative scale. Using fetuin as a substrate, high-yield and cost-effective production of 2,7-anhydro-Neu5Ac was obtained to high-purity. This method was also applied to the synthesis of 2,7-anhydro-Neu5Gc. The membrane-enclosed multienzyme (MEME) strategy reported here provides an efficient approach to produce a variety of sialic acid derivatives.

A polyacrylamide-based silica stationary phase for the separation of carbohydrates using alcohols as the weak eluent in hydrophilic interaction liquid chromatography

A hydrophilic interaction liquid chromatography (HILIC) stationary phase was prepared by a two-step synthesis method, immobilizing polyacrylamide on silica sphere particles. The stationary phase (named PA, 5μm dia) was evaluated using a mixture of carbohydrates in HILIC mode and the column efficiency reached 121,000Nm^-1. The retention behavior of carbohydrates on PA stationary phase was investigated with three different organic solvents (acetonitrile, ethanol and methanol) employed as the weak eluent. The strongest hydrophilicity of PA stationary phase was observed in both acetonitrile and methanol as the weak eluent, when compared with another two amide stationary phases. Attributing to its high hydrophilicity, three oligosaccharides (xylooligosaccharide, fructooligosaccharide and chitooligosaccharides) presented good retention on PA stationary phase using alcohols/water as mobile phase. Finally, PA stationary phase was successfully applied for the purification of galactooligosaccharides and saponins of Paris polyphylla. It is feasible to use safer and cheaper alcohols to replace acetonitrile as the weak eluent for green analysis and purification of polar compounds on PA stationary phase.

Stereoselective sialylation with O-trifluoroacetylated thiosialosides: hydrogen bonding involved?

A series of novel sialyl donors containing O-trifluoroacetyl (TFA) groups at various positions was synthesized. The choice of protecting groups in sialyl donors was based on hypothesis that variations in ability of different acyl groups to act as hydrogen bond acceptors would influence the supramolecular structure of reaction mixture (solution structure), hence the outcome of sialylation. These glycosyl donors were examined in the model glycosylation of the primary hydroxyl group of 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose in comparison with sialyl donors without O-TFA groups. The presence of O-TFA groups in a sialyl donor strongly affected the outcome of sialylation. Several sialyl donors studied showed promising results: yields of disaccharides can be as high as 86% as can be the stereoselectivities (α/β up to 15:1). The results obtained suggest that varying acyl O-protecting groups in sialyl donor may result in dramatic changes in the outcome of sialylation although further studies are required to dissect the influence of intermolecular hydrogen bonding and intramolecular substituent effects related to variations of electron-withdrawing properties of different acyl groups.

Solid-Phase Chemical Modification for Sialic Acid Linkage Analysis: Application to Glycoproteins of Host Cells Used in Influenza Virus Propagation

Differentiation between the sialyl linkages is often critical to understanding biological consequence. Here we present a facile method for determining these linkages in glycans. Analysis of sialic acids is challenging due to their labile nature during sample preparation and ionization. Derivatization is often required via chemical reaction. Amidation derivatizes all sialic acids regardless of linkage, while esterification enables differentiation between α2,3-linked and α2,6-linked sialic acids. Reactions have been primarily performed on free glycans in solution but have been recently adapted to solid-phase providing unique advantages such as simplified sample preparation, improved yield, and high throughput applications. Here, we immobilized glycoproteins on resin via reductive amination, modified α2,6-linked sialic acids through ethyl esterification, and α2,3-linked sialic acids via amidation. N-glycans and O-glycans were released via enzyme and chemical reactions. The method was applied for analysis of three different MDCK cell lines used for influenza propagation and where distributions of α2,3 and α2,6 sialic acids are critical for cell performance. Linkage specific distribution of these sialic acids was quantitatively determined and unique for each cell line. Our study demonstrates that protein sialylation can be reliably and quantitatively characterized in terms of sialic acid linkage of each glycan using the solid-phase esterification/amidation strategy.

Cellular glycosylation senses metabolic changes and modulates cell plasticity during epithelial to mesenchymal transition

Epithelial to mesenchymal transition (EMT) is a developmental program reactivated by tumor cells that leads to the switch from epithelial to mesenchymal phenotype. During EMT, cells are transcriptionally regulated to decrease E-cadherin expression while expressing mesenchymal markers such as vimentin, fibronectin, and N-cadherin. Growing body of evidences suggest that cells engaged in EMT undergo a metabolic reprograming process, redirecting glucose flux toward hexosamine biosynthesis pathway (HBP), which fuels aberrant glycosylation patterns that are extensively observed in cancer cells. HBP depends on nutrient availability to produce its end product UDP-GlcNAc, and for this reason is considered a metabolic sensor pathway. UDP-GlcNAc is the substrate used for the synthesis of major types of glycosylation, including O-GlcNAc and cell surface glycans. In general, the rate limiting enzyme of HBP, GFAT, is overexpressed in many cancer types that present EMT features as well as aberrant glycosylation. Moreover, altered levels of O-GlcNAcylation can modulate cell morphology and favor EMT. In this review, we summarize some of the current knowledge that correlates glucose metabolism, aberrant glycosylation and hyper O-GlcNAcylation supported by HBP that leads to EMT activation. Developmental Dynamics 247:481-491, 2018. © 2017 Wiley Periodicals, Inc.