Notable Aspects of Glycan-Protein Interactions

Quantitative Glycan-Protein Cross-Linking Mass Spectrometry Using Enrichable Linkers Reveals Extensive Glycan-Mediated Protein Interaction Networks

Protein-protein interactions in the cell membrane are typically mediated by glycans, with terminal sialic acid often involved in these interactions. To probe the nature of the interactions, we developed quantitative cross-linking methods involving the glycans of the glycoproteins and the polypeptide moieties of proteins. We designed and synthesized biotinylated enrichable cross-linkers that were click-tagged to metabolically incorporate azido-sialic acid on cell surface glycans to allow cross-linking of the azido-glycans with lysine residues on proximal polypeptides. The glycopeptide-peptide cross-links (GPx) were enriched using biotin groups through affinity purification with streptavidin resin beads. Workflows using two linkers, one photocleavable and the other disulfide, were developed and applied to reveal the sialic acid-mediated cell-surface protein networks of PNT2 (prostate) cells. Glycopeptide-peptide pairs were identified, with 12000 GPx linked by sialylated glycoforms revealing interactions between source glycoproteins and nearly 700 target proteins. Protein-protein interactions were characterized by as many as 40 peptide pairs, and the extent of the interactions between proteins was prioritized by the number of GPx. Quantitation was performed by counting the number of GPx that identify the protein pairs. Abundant membrane proteins such as ITGB1 yielded an interactome consisting of around 400 other proteins, which were ranked from the most extensive interaction, having the largest number of GPx, to at least one. The interactome was further confirmed separately by published databases. This tool will enhance our understanding of glycosylation on protein-protein interactions and provide new potential targets for therapeutics.

Glycopolymers Prepared by Alternating Ring-Opening Metathesis Polymerization Provide Access to Distinct, Multivalent Structures for the Probing of Biological Activity

A myriad of biological processes are facilitated by ligand-receptor interactions. The low affinities of these interactions are typically enhanced by multivalent engagements to promote binding. However, each biological interaction requires a unique display and orientation of ligands. Therefore, the availability and diversity of synthetic multivalent probes are invaluable to the investigation of ligand-receptor binding interactions. Here, we report glycopolymers prepared from bicyclo[4.2.0]oct-6-ene-7-carboxamide and 4,7-dihydro-1,3-dioxepin or cyclohexene. These glycopolymers, synthesized by alternating ring-opening metathesis polymerization, display precise ligand spacing as well as the option of a hydrophobic or acetal-functionalized polymer backbone. Small-angle X-ray scattering (SAXS) data analysis revealed that these [4.2.0] glycopolymers adopted distinct conformations in solution. In aqueous media, [4.2.0]-dioxepin glycopolymers formed swollen polymer chains with rod-like, flexible structures while [4.2.0]-cyclohexene glycopolymers assumed compact, globular structures. To illustrate how these glycopolymers could aid in the exploration of ligand-receptor interactions, we incorporated the [4.2.0] glycopolymers into a biological assay to assess their potential as activators of acrosomal exocytosis (AE) in mouse sperm. The results of the biological assay confirmed that the differing structures of the [4.2.0] glycopolymers would evoke distinct biological responses; [4.2.0]-cyclohexene glycopolymers induced AE in mouse sperm while [4.2.0]-dioxepin glycopolymers did not. Herein, we provide two options for glycopolymers with low to moderate molecular weight dispersities and low cytotoxicity that can be implemented into biological assays based on the desired hydrophobicity, rigidity, and structural conformation of the polymer probe.

The role of galectins in mediating the adhesion of circulating cells to vascular endothelium

Vascular cell adhesion is a complex orchestration of events that commonly feature lectin-ligand interactions between circulating cells, such as immune, stem, and tumor cells, and endothelial cells (ECs) lining post-capillary venules. Characteristically, circulating cell adherence to the vasculature endothelium is initiated through interactions between surface sialo-fucosylated glycoprotein ligands and lectins, specifically platelet (P)- or endothelial (E)-selectin on ECs or between leukocyte (L)-selectin on circulating leukocytes and L-selectin ligands on ECs, culminating in circulating cell extravasation. This lectin-ligand interplay enables the migration of immune cells into specific tissue sites to help maintain effective immunosurveillance and inflammation control, the homing of stem cells to bone marrow or tissues in need of repair, and, unfortunately, in some cases, the dissemination of circulating tumor cells (CTCs) to distant metastatic sites. Interestingly, there is a growing body of evidence showing that the family of β-galactoside-binding lectins, known as galectins, can also play pivotal roles in the adhesion of circulating cells to the vascular endothelium. In this review, we present contemporary knowledge on the significant roles of host- and/or tumor-derived galectin (Gal)-3, -8, and -9 in facilitating the adhesion of circulating cells to the vascular endothelium either directly by acting as bridging molecules or indirectly by triggering signaling pathways to express adhesion molecules on ECs. We also explore strategies for interfering with galectin-mediated adhesion to attenuate inflammation or hinder the metastatic seeding of CTCs, which are often rich in galectins and/or their glycan ligands.

Glycan-modified cellular nanosponges for enhanced neutralization of botulinum toxin

Botulinum toxin (BoNT) is a potent neurotoxin that poses a significant threat as a biowarfare weapon and a potential bioterrorist tool. Currently, there is a lack of effective countermeasures to combat BoNT intoxication in the event of a biological attack. Here, we report on a novel solution by combining cell metabolic engineering with cell membrane coating nanotechnology, resulting in the development of glycan-modified cellular nanosponges that serve as a biomimetic and broad-spectrum BoNT detoxification strategy. Specifically, we increase the expression levels of gangliosides on THP-1 cells through metabolic engineering, and then collect the modified THP-1 cell membrane and coat it onto synthetic polymeric cores, creating cellular nanosponges that closely mimic host cells. Our findings demonstrate that higher levels of gangliosides on the cellular nanosponges result in greater binding capacities with BoNT. The glycan-modified cellular nanosponges exhibit superior efficacy in neutralizing BoNT cytotoxicity in vitro when compared to their unmodified counterparts. In a mouse model of BoNT intoxication, the glycan-modified cellular nanosponges show more pronounced survival benefits when administered both as a treatment and a preventative regimen. These results highlight the potential of cellular nanosponges, especially when modified with glycans, as a promising countermeasure platform against BoNT and related clostridial toxins.

Transforms of Cell Surface Glycoproteins Charge Influences Tumor Cell Metastasis via Atypically Inhibiting Epithelial-Mesenchymal Transition Including Matrix Metalloproteinases and Cell Junctions

Cell communication and signal transduction rely heavily on the charge on the cell surface. The cell surface is negatively charged, with glycoproteins on the cell membrane providing a large percentage of the charge. Sialic acid is found on the outermost side of glycan chains and contributes to glycoprotein's negative charge. Sialic acid is highly expressed in tumor cells and plays an important role in tumor metastasis and immune escape by interacting with extracellular ligands. However, the specific effect of negative charge changes on glycoproteins is still poorly understood. In this study, we used 9-azido sialic acid (9Az-Sia) to create artificial epitopes on glycoproteins via metabolic glycan labeling, and we attached charged groups such as amino and carboxyl to 9Az-Sia via a click reaction with dibenzocyclooctyne (DBCO). The charge of glycoproteins was changed by metabolic glycan labeling and click modification. The results showed that the migration and invasion ability of the MDA-MB-231 cell labeled with 9Az-Sia was significantly reduced after the modification with amino groups rather than carboxyl groups. Epithelial-mesenchymal transition (EMT) is the biological process of metastatic tumor cells, with an increasing ability of tumor cells to migrate and invade. In particular, the expression of adhesion molecules increased in the amine-linked group, whereas the expression of matrix metalloproteinases (MMPs) increased significantly, which is not identical to EMT characteristics. In vivo experiments have demonstrated that the loss of negative charge on glycoproteins has an inhibitory effect on tumors. In conclusion, modifying the positive charge on the surface of glycoproteins can inhibit tumor cell metastasis and has great potential for tumor therapy.

Sialylation: fate decision of mammalian sperm development, fertilization, and male fertility†

Sperm development, maturation, and successful fertilization within the female reproductive tract are intricate and orderly processes that involve protein translation and post-translational modifications. Among these modifications, sialylation plays a crucial role. Any disruptions occurring throughout the sperm's life cycle can result in male infertility, yet our current understanding of this process remains limited. Conventional semen analysis often fails to diagnose some infertility cases associated with sperm sialylation, emphasizing the need to comprehend and investigate the characteristics of sperm sialylation. This review reanalyzes the significance of sialylation in sperm development and fertilization and evaluates the impact of sialylation damage on male fertility under pathological conditions. Sialylation serves a vital role in the life journey of sperm, providing a negatively charged glycocalyx and enriching the molecular structure of the sperm surface, which is beneficial to sperm reversible recognition and immune interaction. These characteristics are particularly crucial during sperm maturation and fertilization within the female reproductive tract. Moreover, enhancing the understanding of the mechanism underlying sperm sialylation can promote the development of relevant clinical indicators for infertility detection and treatment.

Galectin-3 and Epithelial MUC1 Mucin-Interactions Supporting Cancer Development

Aberrant glycosylation of cell surface proteins is a very common feature of many cancers. One of the glycoproteins, which undergoes specific alterations in the glycosylation of tumor cells is epithelial MUC1 mucin, which is highly overexpressed in the malignant state. Such changes lead to the appearance of tumor associated carbohydrate antigens (TACAs) on MUC1, which are rarely seen in healthy cells. One of these structures is the Thomsen-Friedenreich disaccharide Galβ1-3GalNAc (T or TF antigen), which is typical for about 90% of cancers. It was revealed that increased expression of the T antigen has a big impact on promoting cancer progression and metastasis, among others, due to the interaction of this antigen with the β-galactose binding protein galectin-3 (Gal-3). In this review, we summarize current information about the interactions between the T antigen on MUC1 mucin and Gal-3, and their impact on cancer progression and metastasis.

Chemoproteomic mapping of human milk oligosaccharide (HMO) interactions in cells

Human milk oligosaccharides (HMOs) are a family of unconjugated soluble glycans found in human breast milk that exhibit a myriad of biological activity. While recent studies have uncovered numerous biological functions for HMOs (antimicrobial, anti-inflammatory & probiotic properties), the receptors and protein binding partners involved in these processes are not well characterized. This can be attributed largely in part to the low affinity and transient nature of soluble glycan-protein interactions, precluding the use of traditional characterization techniques to survey binding partners in live cells. Here, we present the use of synthetic photoactivatable HMO probes to capture, enrich and identify HMO protein targets in live cells using mass spectrometry-based chemoproteomics. Following initial validation studies using purified lectins, we profiled the targets of HMO probes in live mouse macrophages. Using this strategy, we mapped hundreds of HMO binding partners across multiple cellular compartments, including many known glycan-binding proteins as well as numerous proteins previously not known to bind glycans. We expect our findings to inform future investigations of the diverse roles of how HMOs may regulate protein function.

Glycoconjugates: Synthesis, Functional Studies, and Therapeutic Developments

Glycoconjugates are major constituents of mammalian cells that are formed via covalent conjugation of carbohydrates to other biomolecules like proteins and lipids and often expressed on the cell surfaces. Among the three major classes of glycoconjugates, proteoglycans and glycoproteins contain glycans linked to the protein backbone via amino acid residues such as Asn for N-linked glycans and Ser/Thr for O-linked glycans. In glycolipids, glycans are linked to a lipid component such as glycerol, polyisoprenyl pyrophosphate, fatty acid ester, or sphingolipid. Recently, glycoconjugates have become better structurally defined and biosynthetically understood, especially those associated with human diseases, and are accessible to new drug, diagnostic, and therapeutic developments. This review describes the status and new advances in the biological study and therapeutic applications of natural and synthetic glycoconjugates, including proteoglycans, glycoproteins, and glycolipids. The scope, limitations, and novel methodologies in the synthesis and clinical development of glycoconjugates including vaccines, glyco-remodeled antibodies, glycan-based adjuvants, glycan-specific receptor-mediated drug delivery platforms, etc., and their future prospectus are discussed.

Mass Spectrometry-Based Shotgun Glycomics Using Labeled Glycan Libraries

Mass spectrometry-based shotgun glycomics (MS-SG) is a rapid, sensitive, label-, and immobilization-free approach for the discovery of natural ligands of glycan-binding proteins (GBPs). To perform MS-SG, natural libraries of glycans derived from glycoconjugates in cells or tissues are screened against a target GBP using catch-and-release electrospray ionization mass spectrometry (CaR-ESI-MS). Because glycan concentrations are challenging to determine, ligand affinities cannot be directly measured. In principle, relative affinities can be ranked by combining CaR-ESI-MS data with relative concentrations established by hydrophilic interaction liquid chromatography (HILIC) performed on the fluorophore-labeled glycan library. To validate this approach, as well as the feasibility of performing CaR-ESI-MS directly on labeled glycans, libraries of labeled N-glycans extracted from the human monocytic U937 cells or intestinal tissues were labeled with 2-aminobenzamide (2-AB), 2-aminobenzoic acid (2-AA), or procainamide (proA). The libraries were screened against plant and human GBPs with known specificities for α2-3- and α2-6-linked sialosides and quantified by HILIC. Dramatic differences, in some cases, were found for affinity rankings obtained with libraries labeled with different fluorophores, as well as those produced using the combined unlabeled/labeled library approach. The origin of these differences could be explained by differential glycan labeling efficiencies, the impact of specific labels on glycan affinities for the GBPs, and the relative efficiency of release of ligands from GBPs in CaR-ESI-MS. Overall, the results of this study suggest that the 2-AB(CaR-ESI-MS)/2-AB(HILIC) combination provides the most reliable description of the binding specificities of GBPs for N-glycans and is recommended for MS-SG applications.

Current Methods for Extraction and Concentration of Foodborne Bacteria with Glycan-Coated Magnetic Nanoparticles: A Review

Rapid and accurate food pathogen detection is an essential step to preventing foodborne illnesses. Before detection, removal of bacteria from the food matrix and concentration to detectable levels are often essential steps. Although many reviews discuss rapid concentration methods for foodborne pathogens, the use of glycan-coated magnetic nanoparticles (MNPs) is often omitted. This review seeks to analyze the potential of this technique as a rapid and cost-effective solution for concentration of bacteria directly from foods. The primary focus is the mechanism of glycan-coated MNP binding, as well as its current applications in concentration of foodborne pathogens. First, a background on the synthesis, properties, and applications of MNPs is provided. Second, synthesis of glycan-coated particles and their theorized mechanism for bacterial adhesion is described. Existing research into extraction of bacteria directly from food matrices is also analyzed. Finally, glycan-coated MNPs are compared to the magnetic separation technique of immunomagnetic separation (IMS) in terms of cost, time, and other factors. At its current state, glycan-coated MNPs require more research to fully identify the mechanism, potential for optimization, and extraction capabilities directly in food matrices. However, current research indicates glycan-coated MNPs are an incredibly cost-effective method for rapid food pathogen extraction and concentration.

96-well plate format in conjunction with ultra-high-performance liquid chromatography coupled to orbitrap mass spectrometry for high-throughput screening protein binders from ginseng

Conventional strategies for screening of protein binders cannot be used for complicated samples such as ligand libraries created by combinatorial chemistry or from natural product extracts. In the current study, we developed a novel method in a competitive binding configuration for screening protein binders from complicated samples by a combination of streptavidin-coated 96-well plate format in conjunction with ultra-high-performance liquid chromatography coupled with Orbitrap mass spectrometry (UHPLC-Orbitrap-MS). The concanavalin A (Con A) modified 96-well plate and lysozyme modified 96-well plate (as control) were incubated with oligosaccharide standards respectively, and the compounds with the decreased peak areas in experimental group compared to those in the control group were detected as binders by UHPLC-ESI-MS. The factors such as incubation time, incubation temperature, and buffer, which might affect the binding affinity and reproducibility were optimized. The potential of the approach is examined using the extracts of Radix ginseng cruda and American ginseng. The relative binding degrees (RBDs) of the detected disaccharides were relatively high in the extracts of Radix ginseng cruda, and those of the trisaccharides were similar in the extracts of the two kinds of ginseng. To our knowledge, it's the first time to reveal the differences and analogies in lectin peanut agglutinin (PNA)-binding capabilities of oligosaccharides between the extracts of radix ginseng cruda and American ginseng, indicating the efficiency of the method for analysis of complicated samples.

The N-linked glycosylations of TIGIT Asn32 and Asn101 facilitate PVR/TIGIT interaction

Although the PVR/TIGIT immune checkpoint axis has been suggested as a promising target for cancer immunotherapy and multiple TIGIT-targeting therapies are undergoing clinical trials, the underlying regulatory mechanisms of PVR/TIGIT interaction remain inconclusive. Here we show that TIGIT N-glycosylations are critical for maintaining the interaction between TIGIT and PVR. TIGIT has two N-glycosylation residues, N32 and N101. N-glycosylation on N101 of TIGIT and, to less extent, on N32, play potent roles in PVR binding. Taken together, these findings suggest that the N-glycosylation sites on TIGIT, especially residue N101, may be potential targets for PVR/TIGIT immune checkpoint blockade.

Glycan-protein cross-linking mass spectrometry reveals sialic acid-mediated protein networks on cell surfaces

A cross-linking method is developed to elucidate glycan-mediated interactions between membrane proteins through sialic acids. The method provides information on previously unknown extensive glycomic interactions on cell membranes. The vast majority of membrane proteins are glycosylated with complicated glycan structures attached to the polypeptide backbone. Glycan-protein interactions are fundamental elements in many cellular events. Although significant advances have been made to identify protein-protein interactions in living cells, only modest advances have been made on glycan-protein interactions. Mechanistic elucidation of glycan-protein interactions has thus far remained elusive. Therefore, we developed a cross-linking mass spectrometry (XL-MS) workflow to directly identify glycan-protein interactions on the cell membrane using liquid chromatography-mass spectrometry (LC-MS). This method involved incorporating azido groups on cell surface glycans through biosynthetic pathways, followed by treatment of cell cultures with a synthesized reagent, N-hydroxysuccinimide (NHS)-cyclooctyne, which allowed the cross-linking of the sialic acid azides on glycans with primary amines on polypeptide backbones. The coupled peptide-glycan-peptide pairs after cross-linking were identified using the latest techniques in glycoproteomic and glycomic analyses and bioinformatics software. With this approach, information on the site of glycosylation, the glycoform, the source protein, and the target protein of the cross-linked pair were obtained. Glycoprotein-protein interactions involving unique glycoforms on the PNT2 cell surface were identified using the optimized and validated method. We built the GPX network of the PNT2 cell line and further investigated the biological roles of different glycan structures within protein complexes. Furthermore, we were able to build glycoprotein-protein complex models for previously unexplored interactions. The method will advance our future understanding of the roles of glycans in protein complexes on the cell surface.

HIV-1 Envelope Glycosylation and the Signal Peptide

The RV144 trial represents the only vaccine trial to demonstrate any protective effect against HIV-1 infection. While the reason(s) for this protection are still being evaluated, it serves as justification for widespread efforts aimed at developing new, more effective HIV-1 vaccines. Advances in our knowledge of HIV-1 immunogens and host antibody responses to these immunogens are crucial to informing vaccine design. While the envelope (Env) protein is the only viral protein present on the surface of virions, it exists in a complex trimeric conformation and is decorated with an array of variable N-linked glycans, making it an important but difficult target for vaccine design. Thus far, efforts to elicit a protective humoral immune response using structural mimics of native Env trimers have been unsuccessful. Notably, the aforementioned N-linked glycans serve as a component of many of the epitopes crucial for the induction of potentially protective broadly neutralizing antibodies (bnAbs). Thus, a greater understanding of Env structural determinants, most critically Env glycosylation, will no doubt be of importance in generating effective immunogens. Recent studies have identified the HIV-1 Env signal peptide (SP) as an important contributor to Env glycosylation. Further investigation into the mechanisms by which the SP directs glycosylation will be important, both in the context of understanding HIV-1 biology and in order to inform HIV-1 vaccine design.

The influence of ABO blood groups on COVID-19 susceptibility and severity: A molecular hypothesis based on carbohydrate-carbohydrate interactions

The world is experiencing one of the most difficult moments in history with the COVID-19 pandemic, a disease caused by SARS-CoV-2, a new type of coronavirus. Virus infectivity is mediated by the binding of Spike transmembrane glycoprotein to specific protein receptors present on cell host surface. Spike is a homotrimer that emerges from the virion, each monomer containing two subunits named S1 and S2, which are related to cell recognition and membrane fusion, respectively. S1 is subdivided in domains S1A (or NTD) and S1B (or RBD), with experimental and in silico studies suggesting that the former binds to sialic acid-containing glycoproteins, such as CD147, whereas the latter binds to ACE2 receptor. Recent findings indicate that the ABO blood system modulates susceptibility and progression of infection, with type-A individuals being more susceptible to infection and/or manifestation of a severe condition. Seeking to understand the molecular mechanisms underlying this susceptibility, we carried out an extensive bibliographic survey on the subject. Based on this survey, we hypothesize that the correlation between the ABO blood system and susceptibility to SARS-CoV-2 infection can be presumably explained by the modulation of sialic acid-containing receptors distribution on host cell surface induced by ABO antigens through carbohydrate-carbohydrate interactions, which could maximize or minimize the virus Spike protein binding to the host cell. This model could explain previous sparse observations on the molecular mechanism of infection and can direct future research to better understand of COVID-19 pathophysiology.

Mass Spectrometry-Based Shotgun Glycomics for Discovery of Natural Ligands of Glycan-Binding Proteins

Glycans attached to lipids and membrane-bound and secreted proteins and peptides mediate many important physiological and pathophysiological processes through interactions with glycan-binding proteins (GBPs). However, uncovering functional glycan ligands is challenging due to the large number of naturally occurring glycan structures, the limited availability of glycans in their purified form, the low affinities of GBP-glycan interactions, and limitations in existing binding assays. This work explores the application of catch-and-release electrospray ionization mass spectrometry (CaR-ESI-MS) for screening libraries of N-glycans derived from natural sources. The assay was tested by screening a small-defined library of complex N-glycans at equimolar concentrations against plant and human GBPs with known specificities for either α2-3- or α2-6-linked sialosides, with affinities in the millimolar to micromolar range. Validation experiments, performed in negative ion mode, revealed that bound N-glycan ligands are readily released, as intact deprotonated ions, from GBPs in the gas phase using collision-induced dissociation. Moreover, the relative abundances of the released ligands closely match their solution affinities. The results obtained for a natural N-glycan library produced from cultured immune cells serve to highlight the ease with which CaR-ESI-MS can screen complex mixtures of N-glycans for interactions. Additionally, scaling the relative abundances of released glycan ligands according to their relative abundances in solution, as determined by hydrophilic interaction-ultrahigh-performance liquid chromatography of the fluorescently labeled library, allows the relative affinities of glycan ligands to be ranked.

Glycan-dependent cell adhesion mechanism of Tc toxins

Toxin complex (Tc) toxins are virulence factors of pathogenic bacteria. Tcs are composed of three subunits: TcA, TcB and TcC. TcA facilitates receptor-toxin interaction and membrane permeation, TcB and TcC form a toxin-encapsulating cocoon. While the mechanisms of holotoxin assembly and pore formation have been described, little is known about receptor binding of TcAs. Here, we identify heparins/heparan sulfates and Lewis antigens as receptors for different TcAs from insect and human pathogens. Glycan array screening reveals that all tested TcAs bind negatively charged heparins. Cryo-EM structures of Morganella morganii TcdA4 and Xenorhabdus nematophila XptA1 reveal that heparins/heparan sulfates unexpectedly bind to different regions of the shell domain, including receptor-binding domains. In addition, Photorhabdus luminescens TcdA1 binds to Lewis antigens with micromolar affinity. Here, the glycan interacts with the receptor-binding domain D of the toxin. Our results suggest a glycan dependent association mechanism of Tc toxins on the host cell surface.

Targeting the N332-supersite of the HIV-1 envelope for vaccine design

Introduction: Broadly neutralizing antibodies (bNAbs) that are able to target diverse global viruses are widely believed to be crucial for an HIV-1 vaccine. Several conserved targets recognized by these antibodies have been identified on the HIV-1 envelope glycoprotein. One such target that shows particular promise for vaccination is the N332-supersite.Areas covered: This review describes the potential of the N332-supersite epitope as an immunogen design platform. We discuss the structure of the epitope and the bNAbs that target it, emphasizing their diverse modes of binding. Furthermore, the successes and limitations of recent N332-supersite immunization studies are discussed.Expert opinion: During HIV-1 infection, some of the broadest and most potent bNAbs target the N332-supersite. Furthermore, some of these antibodies require less affinity maturation than the high levels typical of many bNAbs, making these potentially more achievable vaccine targets. In addition, bNAbs bind this epitope with multiple angles of approach and glycan dependencies, perhaps increasing the probability of eliciting such responses by vaccination. Animal studies have shown that N332-supersite bNAb precursors can be activated by novel immunogens. While follow-up studies must establish whether boosting strategies can drive the maturation of bNAbs from these precursors, the development of targeted N332-supersite immunogens expands our arsenal of potential HIV-1 vaccine candidates.

Influenza A Virus Hemagglutinin-Neuraminidase-Receptor Balance: Preserving Virus Motility

Influenza A viruses (IAVs) occasionally cross the species barrier and adapt to novel host species. This requires readjustment of the functional balance of the sialic acid receptor-binding hemagglutinin (HA) and the receptor-destroying neuraminidase (NA) to the sialoglycan-receptor repertoire of the new host. Novel techniques have revealed mechanistic details of this HA-NA-receptor balance, emphasizing a previously underappreciated crucial role for NA in driving the motility of receptor-associated IAV particles. Motility enables virion penetration of the sialylated mucus layer as well as attachment to, and uptake into, underlying epithelial cells. As IAVs are essentially irreversibly bound in the absence of NA activity, the fine-tuning of the HA-NA-receptor balance rather than the binding avidity of IAV particles per se is an important factor in determining host species tropism.

Deciphering Isomers with a Multiple Reaction Monitoring Method for the Complete Detectable O-Glycan Repertoire of the Candidate Therapeutic, Lubricin

Glycosylation is a fundamental post-translational modification, occurring on half of all proteins. Despite its significance, our understanding is limited, in part due to the inherent difficulty in studying these branched, multi-isomer structures. Accessible, detailed, and quantifiable methods for studying glycans, particularly O-glycans, are needed. Here we take a multiple reaction monitoring (MRM) approach to differentiate and relatively quantify all detectable glycans, including isomers, on the heavily O-glycosylated protein lubricin. Lubricin (proteoglycan 4) is essential for lubrication of the joint and eye. Given the therapeutic potential of lubricin, it is essential to understand its O-glycan repertoire in biological and recombinantly produced samples. O-Glycans were released by reductive β-elimination and defined, showing a range of 26 neutral, sulfated, sialylated, and both sulfated and sialylated core 1 (Galβ1-3GalNAcα1-) and core 2 (Galβ1-3(GlcNAcβ1-6)GalNAcα1-) structures. Isomer-specific MRM transitions allowed effective differentiation of neutral glycan isomers as well as sulfated isomeric structures, where the sulfate was retained on the fragment ions. This strategy was not as effective with labile sialylated structures; instead, it was observed that the optimal collision energy for the m/z 290.1 sialic acid B-fragment differed consistently between sialic acid isomers, allowing differentiation between isomers when fragmentation spectra were insufficient. This approach was also effective for purchased Neu5Acα2-3Galβ1-4Glc and Neu5Acα2-6Galβ1-4Glc and for Neu5Acα2-3Galβ1-4GlcNAc and Neu5Acα2-6Galβ1-4GlcNAc linkage isomers with the Neu5Acα2-6 consistently requiring more energy for optimal generation of the m/z 290.1 fragment. Overall, this method provides an effective and easily accessible approach for the quantification and annotation of complex released O-glycan samples.

Protein and Glycan Mimicry in HIV Vaccine Design

Antigenic mimicry is a fundamental tenet of structure-based vaccinology. Vaccine strategies for the human immunodeficiency virus type 1 (HIV-1) focus on the mimicry of its envelope spike (Env) due to its exposed location on the viral membrane and role in mediating infection. However, the virus has evolved to minimize the immunogenicity of conserved epitopes on the envelope spike. This principle is starkly illustrated by the presence of an extensive array of host-derived glycans, which act to shield the underlying protein from antibody recognition. Despite these hurdles, a subset of HIV-infected individuals eventually develop broadly neutralizing antibodies that recognize these virally presented glycans. Effective HIV-1 immunogens are therefore likely to involve some degree of mimicry of both the protein and glycan components of Env. As such, considerable efforts have been made to characterize the structure of the envelope spike and its glycan shield. This review summarizes the recent progress made in this field, with an emphasis on our growing understanding of the factors shaping the glycan shield of Env derived from both virus and soluble immunogens. We argue that recombinant mimics of the envelope spike are currently capable of capturing many features of the native viral glycan shield. Finally, we explore strategies through which the immunogenicity of Env glycans may be enhanced in the development of future immunogens.

Assembly of Divalent Ligands and Their Effect on Divalent Binding to Pseudomonas aeruginosa Lectin LecA

Divalent ligands were prepared as inhibitors for the adhesion protein of the problematic Pseudomonas aeruginosa pathogen. Bridging two binding sites enables simultaneous binding of two galactose moieties, which strongly enhances binding. An alternating motif of glucose and triazole and aryl groups was shown to have the right mix of rigidity, solubility, and ease of synthesis. Spacers were varied with respect to the core unit as well as the aglycon portions in an attempt to optimize dynamics and enhance interactions with the protein. Affinities of the divalent ligands were measured by ITC, and K_d’s as low as 12 nM were determined, notably for a compounds with either a rigid (phenyl) or flexible (butyl) unit at the core. Introducing a phenyl aglycon moiety next to the galactoside ligands on both termini did indeed lead to a higher enthalpy of binding, which was more than compensated by entropic costs. The results are discussed in terms of thermodynamics and theoretical calculations of the expected and observed multivalency effects.

A bite so sweet: the glycobiology interface of tick-host-pathogen interactions

Vector-borne diseases constitute 17% of all infectious diseases in the world; among the blood-feeding arthropods, ticks transmit the highest number of pathogens. Understanding the interactions between the tick vector, the mammalian host and the pathogens circulating between them is the basis for the successful development of vaccines against ticks or the tick-transmitted pathogens as well as for the development of specific treatments against tick-borne infections. A lot of effort has been put into transcriptomic and proteomic analyses; however, the protein-carbohydrate interactions and the overall glycobiology of ticks and tick-borne pathogens has not been given the importance or priority deserved. Novel (bio)analytical techniques and their availability have immensely increased the possibilities in glycobiology research and thus novel information in the glycobiology of ticks and tick-borne pathogens is being generated at a faster pace each year. This review brings a comprehensive summary of the knowledge on both the glycosylated proteins and the glycan-binding proteins of the ticks as well as the tick-transmitted pathogens, with emphasis on the interactions allowing the infection of both the ticks and the hosts by various bacteria and tick-borne encephalitis virus.

Kinetic analysis of the influenza A virus HA/NA balance reveals contribution of NA to virus-receptor binding and NA-dependent rolling on receptor-containing surfaces

Interactions of influenza A virus (IAV) with sialic acid (SIA) receptors determine viral fitness and host tropism. Binding to mucus decoy receptors and receptors on epithelial host cells is determined by a receptor-binding hemagglutinin (HA), a receptor-destroying neuraminidase (NA) and a complex in vivo receptor-repertoire. The crucial but poorly understood dynamics of these multivalent virus-receptor interactions cannot be properly analyzed using equilibrium binding models and endpoint binding assays. In this study, the use of biolayer interferometric analysis revealed the virtually irreversible nature of IAV binding to surfaces coated with synthetic sialosides or engineered sialoglycoproteins in the absence of NA activity. In addition to HA, NA was shown to be able to contribute to the initial binding rate while catalytically active. Virus-receptor binding in turn contributed to receptor cleavage by NA. Multiple low-affinity HA-SIA interactions resulted in overall extremely high avidity but also permitted a dynamic binding mode, in which NA activity was driving rolling of virus particles over the receptor-surface. Virus dissociation only took place after receptor density of the complete receptor-surface was sufficiently decreased due to NA activity of rolling IAV particles. The results indicate that in vivo IAV particles, after landing on the mucus layer, reside continuously in a receptor-bound state while rolling through the mucus layer and over epithelial cell surfaces driven by the HA-NA-receptor balance. Quantitative BLI analysis enabled functional examination of this balance which governs this dynamic and motile interaction that is expected to be crucial for penetration of the mucus layer and subsequent infection of cells by IAV but likely also by other enveloped viruses carrying a receptor-destroying enzyme in addition to a receptor-binding protein.

Influenza A virus (IAV) tropism is largely determined by the interaction of virus particles with the sialic acid receptor repertoire of the host. IAVs encounter a diverse range of sialic acid receptors that can function as decoys (e.g. in the mucus that covers epithelial cells) or as entry receptors. We studied the dynamics of IAV-receptor interactions in real-time using biolayer interferometry (BLI) in combination with synthetic glycans and recombinant sialoglycoproteins mimicking in vivo receptors. Thereby we could show that IAVs do not continuously associate and dissociate with receptor-coated surfaces but actually were rolling over the surface with which they remained permanently associated until the receptors were sufficiently cleared. This required the concerted action of the receptor-binding hemagglutinin (HA) and the receptor-destroying neuraminidase (NA) on the receptor surface. We could quantify the precise HA-NA-receptor balance that determined the speed of rolling and eventual elution from the surface by BLI and propose a model in which IAV is permanently, but dynamically, associated with receptors on mucus or host cells in vivo.

Chemoenzymatic Synthesis of Asymmetrical Multi-Antennary N-Glycans to Dissect Glycan-Mediated Interactions between Human Sperm and Oocytes

Complex N-glycans of glycoproteins of the zona pellucida (ZP) of human oocytes have been implicated in the binding of spermatozoa. The termini of these unusual bi-, tri-, and tetra-antennary N-glycans consist of the tetrasaccharide sialyl-Lewis^x (SLe^x ), which was previously identified as the minimal epitope for sperm binding. We describe here the chemoenzymatic synthesis of highly complex triantennary N-glycans derived from ZP carrying SLe^x moieties at the C-2 and C-2' arm and a sialyl-Lewis^x -Lewis^x (SLe^x -Le^x ) residue at the C-6 antenna and two closely related analogues. The compounds were examined for their ability to inhibit the interaction of human sperm to ZP. It was found that the SLe^x -Le^x moiety is critical for inhibitory activity, whereas the other SLe^x moieties exerted minimal effect. Further studies with SLe^x -Le^x and SLe^x showed that the extended structure is the more potent inhibitor. In addition, trivalent SLe^x -Le^x and SLe^x were prepared which showed greater inhibitory activity compared to their monovalent counterparts. Our studies show that although SLe^x can inhibit the binding of spermatozoa, presenting this epitope in the context of a complex N-glycan results in a loss of inhibitory potential, and in this context only SLe^x -Le^x can make productive interactions. It is not the multivalent display of SLe^x on a multi-antennary glycan but the presentation of multiple SLe^x -Le^x on the various glycosylation sites of ZP that accounts for high avidity binding.

The Role of the Carbohydrates in Plasmatic Membrane

In the following paper, authors describe glycans present on cell membranes as they affect the folding, the spatial arrangement, the behavior and the interaction with the substrate of some membrane proteins. Authors describe the synthesis and assembly of a glycan on a protein, the formation of N-glycans, the maturation of an N-glycan in different cellular compartments, the structure of the glycocalyx and how it interacts with any pathogens. The study of the E-cadherin and the potassium channel to demonstrate how glycans affect the spatial arrangement, the stability and activity of the glycoproteins on the membranes. Subsequently, authors analyze the correlation between disorder glycosylation and human health. Authors define glycosylation disorders as a genetic defect that alter the structure or biosynthesis of glycans (sugar chains) in one or more biosynthetic pathways. Human glycosylation disorders reflect the disruption of early steps in the pathways of glycan biosynthesis. More in details, authors analyze the role of glycoprotein in tumor cell adhesion, in particular, in cells MCF-7 and MDA-MB-231 on zeolite scaffold. In the same time, the role of metalloproteinase is described in the mobilization of cancer cells and in metastasis.

Paucimannose-Rich N-glycosylation of Spatiotemporally Regulated Human Neutrophil Elastase Modulates Its Immune Functions

Human neutrophil elastase (HNE) is an important N-glycosylated serine protease in the innate immune system, but the structure and immune-modulating functions of HNE N-glycosylation remain undescribed. Herein, LC-MS/MS-based glycan, glycopeptide and glycoprotein profiling were utilized to first determine the heterogeneous N-glycosylation of HNE purified from neutrophil lysates and then from isolated neutrophil granules of healthy individuals. The spatiotemporal expression of HNE during neutrophil activation and the biological importance of its N-glycosylation were also investigated using immunoblotting, cell surface capture, native MS, receptor interaction, protease inhibition, and bacteria growth assays. Site-specific HNE glycoprofiling demonstrated that unusual paucimannosidic N-glycans, particularly Manα1,6Manβ1,4GlcNAcβ1,4(Fucα1,6)GlcNAcβ, predominantly occupied Asn124 and Asn173. The equally unusual core fucosylated monoantenna complex-type N-sialoglycans also decorated these two fully occupied sites. In contrast, the mostly unoccupied Asn88 carried nonfucosylated paucimannosidic N-glycans probably resulting from low glycosylation site solvent accessibility. Asn185 was not glycosylated. Subcellular- and site-specific glycoprofiling showed highly uniform N-glycosylation of HNE residing in distinct neutrophil compartments. Stimulation-induced cell surface mobilization demonstrated a spatiotemporal regulation, but not cell surface-specific glycosylation signatures, of HNE in activated human neutrophils. The three glycosylation sites of HNE were located distal to the active site indicating glycan functions other than interference with HNE enzyme activity. Functionally, the paucimannosidic HNE glycoforms displayed preferential binding to human mannose binding lectin compared with the HNE sialoglycoforms, suggesting a glycoform-dependent involvement of HNE in complement activation. The heavily N-glycosylated HNE protease inhibitor, α1-antitrypsin, displayed concentration-dependent complex formation and preferred glycoform-glycoform interactions with HNE. Finally, both enzymatically active HNE and isolated HNE N-glycans demonstrated low micromolar concentration-dependent growth inhibition of clinically-relevant Pseudomonas aeruginosa, suggesting some bacteriostatic activity is conferred by the HNE N-glycans. Taken together, these observations support that the unusual HNE N-glycosylation, here reported for the first time, is involved in modulating multiple immune functions central to inflammation and infection.

Ubiquitous Importance of Protein Glycosylation

More than half of all proteins are glycosylated. The attached glycans provide proteins with important structural and functional properties and glycan parts of glycoproteins have essential roles in many key biological processes. This chapter describes the effect of glycosylation on the structure and function of proteins, with emphasis on regulation of protein half-life and modulation of protein function by alternative glycosylation. In addition, this chapter highlights the importance of glycan-lectin interactions, the ability of glycans to block phosphorylation of proteins, and the importance of glycans in disease.

A Viral Immunity Chromosome in the Marine Picoeukaryote, Ostreococcus tauri

Micro-algae of the genus Ostreococcus and related species of the order Mamiellales are globally distributed in the photic zone of world's oceans where they contribute to fixation of atmospheric carbon and production of oxygen, besides providing a primary source of nutrition in the food web. Their tiny size, simple cells, ease of culture, compact genomes and susceptibility to the most abundant large DNA viruses in the sea render them attractive as models for integrative marine biology. In culture, spontaneous resistance to viruses occurs frequently. Here, we show that virus-producing resistant cell lines arise in many independent cell lines during lytic infections, but over two years, more and more of these lines stop producing viruses. We observed sweeping over-expression of all genes in more than half of chromosome 19 in resistant lines, and karyotypic analyses showed physical rearrangements of this chromosome. Chromosome 19 has an unusual genetic structure whose equivalent is found in all of the sequenced genomes in this ecologically important group of green algae.

We propose that chromosome 19 of O. tauri is specialized in defence against viral attack, a constant threat for all planktonic life, and that the most likely cause of resistance is the over-expression of numerous predicted glycosyltransferase genes. O. tauri thus provides an amenable model for molecular analysis of genome evolution under environmental stress and for investigating glycan-mediated host-virus interactions, such as those seen in herpes, influenza, HIV, PBCV and mimivirus.

Ganglioside-Lipid and Ganglioside-Protein Interactions Revealed by Coarse-Grained and Atomistic Molecular Dynamics Simulations

Gangliosides are glycolipids in which an oligosaccharide headgroup containing one or more sialic acids is connected to a ceramide. Gangliosides reside in the outer leaflet of the plasma membrane and play a crucial role in various physiological processes such as cell signal transduction and neuronal differentiation by modulating structures and functions of membrane proteins. Because the detailed behavior of gangliosides and protein-ganglioside interactions are poorly known, we investigated the interactions between the gangliosides GM1 and GM3 and the proteins aquaporin (AQP1) and WALP23 using equilibrium molecular dynamics simulations and potential of mean force calculations at both coarse-grained (CG) and atomistic levels. In atomistic simulations, on the basis of the GROMOS force field, ganglioside aggregation appears to be a result of the balance between hydrogen bond interactions and steric hindrance of the headgroups. GM3 clusters are slightly larger and more ordered than GM1 clusters due to the smaller headgroup of GM3. The different structures of GM1 and GM3 clusters from atomistic simulations are not observed at the CG level based on the Martini model, implying a difference in driving forces for ganglioside interactions in atomistic and CG simulations. For protein-ganglioside interactions, in the atomistic simulations, GM1 lipids bind to specific sites on the AQP1 surface, whereas they are depleted from WALP23. In the CG simulations, the ganglioside binding sites on the AQP1 surface are similar, but ganglioside aggregation and protein-ganglioside interactions are more prevalent than in the atomistic simulations. Using the polarizable Martini water model, results were closer to the atomistic simulations. Although experimental data for validation is lacking, we proposed modified Martini parameters for gangliosides to more closely mimic the sizes and structures of ganglioside clusters observed at the atomistic level.

Influence of Core β-1,2-Xylosylation on Glycoprotein Recognition by Murine C-type Lectin Receptors and Its Impact on Dendritic Cell Targeting

Targeting antigens to dendritic cell subsets is a promising strategy to enhance the efficacy of vaccines. C-type lectin receptors (CLRs) expressed by dendritic cells are particularly attractive candidates since CLR engagement may promote cell uptake and may further stimulate antigen presentation and subsequent T cell activation. While most previous approaches have involved antibody-mediated CLR-targeting, glycan-based CLR targeting has become more and more attractive in recent years. In the present study, we show that small structural glycan modifications may markedly influence CLR recognition, dendritic cell targeting, and subsequent T cell activation. A biantennary N-glycan (G0) and its analogous O-2 core xylosylated N-glycan (XG0) were synthesized, covalently conjugated to the model antigen ovalbumin, and analyzed for binding to a set of murine CLR-Fc fusion proteins using lectin microarray. To evaluate whether the differential binding of G0 and XG0 to CLRs impacted dendritic cell targeting, uptake studies using murine dendritic cells were performed. Finally, effects of the ovalbumin glycoconjugates on T cell activation were measured in a dendritic cell/T cell cocultivation assay. Our results highlight the utility of glycan-based dendritic cell targeting and demonstrate that small structural differences may have a major impact on dendritic cell targeting efficacy.

Emerging roles of protein mannosylation in inflammation and infection

Proteins are frequently modified by complex carbohydrates (glycans) that play central roles in maintaining the structural and functional integrity of cells and tissues in humans and lower organisms. Mannose forms an essential building block of protein glycosylation, and its functional involvement as components of larger and diverse α-mannosidic glycoepitopes in important intra- and intercellular glycoimmunological processes is gaining recognition. With a focus on the mannose-rich asparagine (N-linked) glycosylation type, this review summarises the increasing volume of literature covering human and non-human protein mannosylation, including their structures, biosynthesis and spatiotemporal expression. The review also covers their known interactions with specialised host and microbial mannose-recognising C-type lectin receptors (mrCLRs) and antibodies (mrAbs) during inflammation and pathogen infection. Advances in molecular mapping technologies have recently revealed novel immuno-centric mannose-terminating truncated N-glycans, termed paucimannosylation, on human proteins. The cellular presentation of α-mannosidic glycoepitopes on N-glycoproteins appears tightly regulated; α-mannose determinants are relative rare glycoepitopes in physiological extracellular environments, but may be actively secreted or leaked from cells to transmit potent signals when required. Simultaneously, our understanding of the molecular basis on the recognition of mannosidic epitopes by mrCLRs including DC-SIGN, mannose receptor, mannose binding lectin and mrAb is rapidly advancing, together with the functional implications of these interactions in facilitating an effective immune response during physiological and pathophysiological conditions. Ultimately, deciphering these complex mannose-based receptor-ligand interactions at the detailed molecular level will significantly advance our understanding of immunological disorders and infectious diseases, promoting the development of future therapeutics to improve patient clinical outcomes.