Polyacrylamide-based glycoconjugates as tools in glycobiology

A protocol to isolate, identify, and verify glucose- or carbohydrate-binding receptors

Sensing, transport, and utilization of glucose is pivotal to the maintenance of energy homeostasis in animals. Although transporters involved in mobilizing glucose across different cellular compartments are fairly well known, the receptors that bind glucose to mediate its effects independently of glucose metabolism remain largely unrecognized. Establishing precise and reproducible methods to identify glucose receptors in the brain or other peripheral organs will pave the way for comprehending the role of glucose signaling pathways in maintaining, regulating, and reprogramming cellular metabolic needs. Identification of such potential glucose receptors will also likely lead to development of effective therapeutics for treatment of diabetes and related metabolic disorders. Commercially available biotin or radiolabeled glucose conjugates have low molecular weight; therefore, they do not provide enough sensitivity and density to isolate glucose receptors. Here, we describe a protocol to isolate, identify, and verify glucose-binding receptor/s using high molecular weight glucose (or other carbohydrate) conjugates. We have produced 30 kDa glucose- (or other carbohydrate-) biotin-polyacrylamide (PAA) conjugates with mole fractions of 80:5:15% respectively. These conjugates are used with biotin-streptavidin biochemistry, In-cell ELISA, and surface plasmon resonance (SPR) methods to isolate, identify, and verify glucose- or carbohydrate-binding receptors. We first demonstrate how streptavidin-coated magnetic beads are employed to immobilize glucose-biotin-PAA conjugates. Then, these beads are used to enrich and isolate glucose-binding proteins from tissue homogenates or from single-cell suspensions. The enriched or isolated proteins are subjected to mass spectrometry/proteomics to reveal the identity of top candidate proteins as potential glucose receptors. We then describe how the In-cell ELISA method is used to verify the interaction of glucose with its potential receptor through stable expression of the receptor in-vitro. We further demonstrate how a highly sensitive SPR method can be used to measure the binding kinetics of glucose with its receptor. In summary, we describe a protocol to isolate, identify, and verify glucose- or carbohydrate-binding receptors using magnetic beads, In-cell ELISA, and SPR. This protocol will form the future basis of studying glucose or carbohydrate receptor signaling pathways in health and in disease.

ADGRL1 is a glucose receptor involved in mediating energy and glucose homeostasis

AIMS/HYPOTHESIS

The brain is a major consumer of glucose as an energy source and regulates systemic glucose as well as energy balance. Although glucose transporters such as GLUT2 and sodium-glucose cotransporter 2 (SGLT2) are known to regulate glucose homeostasis and metabolism, the identity of a receptor that binds glucose to activate glucose signalling pathways in the brain is unknown. In this study, we aimed to discover a glucose receptor in the mouse hypothalamus.

METHODS

Here we used a high molecular mass glucose-biotin polymer to enrich glucose-bound mouse hypothalamic neurons through cell-based affinity chromatography. We then subjected the enriched neurons to proteomic analyses and identified adhesion G-protein coupled receptor 1 (ADGRL1) as a top candidate for a glucose receptor. We validated glucose-ADGRL1 interactions using CHO cells stably expressing human ADGRL1 and ligand-receptor binding assays. We generated and determined the phenotype of global Adgrl1-knockout mice and hypothalamus-specific Adgrl1-deficient mice. We measured the variables related to glucose and energy homeostasis in these mice. We also generated an Adgrl1Cre mouse model to investigate the role of ADGRL1 in sensing glucose using electrophysiology.

RESULTS

Adgrl1 is highly expressed in the ventromedial nucleus of the hypothalamus (VMH) in mice. Lack of Adgrl1 in the VMH in mice caused fasting hyperinsulinaemia, enhanced glucose-stimulated insulin secretion and insulin resistance. In addition, the Adgrl1-deficient mice had impaired feeding responses to glucose and fasting coupled with abnormal glucose sensing and decreased physical activity before development of obesity and hyperglycaemia. In female mice, ovariectomy was necessary to reveal the contribution of ADGRL1 to energy and glucose homeostasis.

CONCLUSIONS/INTERPRETATION

Altogether, our findings demonstrate that ADGRL1 binds glucose and is involved in energy as well as glucose homeostasis in a sex-dependent manner. Targeting ADGRL1 may introduce a new class of drugs for the treatment of type 2 diabetes and obesity.

Missing the sweet spot: one of the two N-glycans on human Gb3/CD77 synthase is expendable

N-glycosylation is a ubiquitous posttranslational modification that may influence folding, subcellular localization, secretion, solubility and oligomerization of proteins. In this study, we examined the effects of N-glycans on the activity of human Gb3/CD77 synthase, which catalyzes the synthesis of glycosphingolipids with terminal Galα1 → 4Gal (Gb3 and the P1 antigen) and Galα1 → 4GalNAc disaccharides (the NOR antigen). The human Gb3/CD77 synthase contains two occupied N-glycosylation sites at positions N121 and N203. Intriguingly, we found that while the N-glycan at N203 is essential for activity and correct subcellular localization, the N-glycan at N121 is dispensable and its absence did not reduce, but, surprisingly, even increased the activity of the enzyme. The fully N-glycosylated human Gb3/CD77 synthase and its glycoform missing the N121 glycan correctly localized in the Golgi, whereas a glycoform without the N203 site partially mislocalized in the endoplasmic reticulum. A double mutein missing both N-glycans was inactive and accumulated in the endoplasmic reticulum. Our results suggest that the decreased specific activity of human Gb3/CD77 synthase glycovariants results from their improper subcellular localization and, to a smaller degree, a decrease in enzyme solubility. Taken together, our findings show that the two N-glycans of human Gb3/CD77 synthase have opposing effects on its properties, revealing a dual nature of N-glycosylation and potentially a novel regulatory mechanism controlling the biological activity of proteins.

Recombinant SARS-CoV-2 S Protein Binds to Glycans of the Lactosamine Family in vitro

Many viruses, beside binding to their main cell target, interact with other molecules that promote virus adhesion to the cell; often, these additional targets are glycans. The main receptor for SARS-CoV-2 is a peptide motif in the ACE2 protein. We studied interaction of the recombinant SARS-CoV-2 spike (S) protein with an array of glycoconjugates, including various sialylated, sulfated, and other glycans, and found that the S protein binds some (but not all) glycans of the lactosamine family. We suggest that parallel influenza infection will promote SARS-CoV-2 adhesion to the respiratory epithelial cells due to the unmasking of lactosamine chains by the influenza virus neuraminidase.

40 years of glyco-polyacrylamide in glycobiology

Polyacrylamide conjugates of glycans have long been widely used in many research areas of glycobiology, mainly for immobilizing glycans in solid-phase assays and as multivalent inhibitors. Pending biotin tag allows immobilizing Glyc-PAA quantitatively on any surface, and acts as a tracer for detection of carbohydrate-binding proteins. However, the scope of already realized capabilities of these probes is immeasurably richer than those listed above. This review is not so much about routine as about less common, but not less significant applications. Also, the data on the glycopolymers themselves, their molecular weight, size and polymer chain flexibility are presented, as well as the methods of synthesis, clusterisation and entropy factor in their interaction with proteins.

Profiling of Naturally Occurring Antibodies to the Thomsen-Friedenreich Antigen in Health and Cancer: The Diversity and Clinical Potential

The Thomsen-Friedenreich (TF) antigen is expressed in a majority of human tumors due to aberrant glycosylation in cancer cells. There is strong evidence that humoral immune response to TF represents an effective mechanism for the elimination of cancer cells that express TF-positive glycoconjugates. The presence of naturally occurring antibodies to tumor-associated TF and cancer-specific changes in their levels, isotype distribution and interrelation, avidity, and glycosylation profile make these Abs a convenient and ubiquitous marker for cancer diagnostics and prognostics. In this review, we attempt to summarize the latest data on the potential of TF-specific Abs for cancer diagnostics and prognostics.

Preparation and in vivo evaluation of glyco-gold nanoparticles carrying synthetic mycobacterial hexaarabinofuranoside

A number of bacterial glycans are specific markers for the detection and the serological identification of microorganisms and are also widely used as antigenic components of vaccines. The use of gold nanoparticles as carriers for glyco-epitopes is becoming an important alternative to the traditional conjugation with proteins and synthetic polymers. In this study, we aimed to prepare and evaluate in vivo glyco-gold nanoparticles (glyco-GNPs) bearing the terminal-branched hexaarabinofuranoside fragment (Ara₆) of arabinan domains of lipoarabinomannan and arabinogalactan, which are principal polysaccharides of the cell wall of Mycobacterium tuberculosis, the causative agent of tuberculosis. In particular, we were interested whether the antibodies generated against Ara₆-GNPs would recognize the natural saccharides on the cell surface of different mycobacterial strains. Two synthetic Ara₆ glycosides with amino-functionalized spacer aglycons differing in length and hydrophilicity were directly conjugated with spherical gold nanoparticles (d = 15 nm) to give two sets of glyco-GNPs, which were used for the immunization of rabbits. Dot assays revealed cross-reactions between the two obtained antisera with the hexaarabinofuranoside and the 2-aminoethyl aglycon used for the preparation of glyco-GNPs. Both antisera contained high titers of antibodies specific for Mycobacteria as shown by enzyme-linked immunosorbent assay using M. bovis and M. smegmatis cells as antigens while there was only a weak response to M. phlei cells and no interaction with E. coli cells. The results obtained suggest that glyco-GNPs are promising agents for the generation of anti-mycobacterial antibodies.

Label-free sensitive detection of influenza virus using PZT discs with a synthetic sialylglycopolymer receptor layer

We describe rapid, label-free detection of Influenza A viruses using the first radial mode of oscillations of lead zirconate titanate (PZT) piezoelectric discs with a 2 mm radius and 100 µm thickness fabricated from a piezoelectric membrane. The discs are modified with a synthetic sialylglycopolymer receptor layer, and the coated discs are inserted in a flowing virus suspension. Label-free detection of the virus is achieved by monitoring the disc radial mode resonance frequency shift. Piezo transducers with sialylglycopolymer sensor layers exhibited a long lifetime, a high sensitivity and the possibility of regeneration. We demonstrate positive, label-free detection of Influenza A viruses at concentrations below 105 virus particles per millilitre. We show that label-free, selective, sensitive detection of influenza viruses by home appliances is possible in principle.

Label-free sensitive detection of influenza virus using PZT discs with a synthetic sialylglycopolymer receptor layer

We describe rapid, label-free detection of Influenza A viruses using the first radial mode of oscillations of lead zirconate titanate (PZT) piezoelectric discs with a 2 mm radius and 100 µm thickness fabricated from a piezoelectric membrane. The discs are modified with a synthetic sialylglycopolymer receptor layer, and the coated discs are inserted in a flowing virus suspension. Label-free detection of the virus is achieved by monitoring the disc radial mode resonance frequency shift. Piezo transducers with sialylglycopolymer sensor layers exhibited a long lifetime, a high sensitivity and the possibility of regeneration. We demonstrate positive, label-free detection of Influenza A viruses at concentrations below 10⁵ virus particles per millilitre. We show that label-free, selective, sensitive detection of influenza viruses by home appliances is possible in principle.

Cluster binding studies with two anti-Thomsen-Friedenreich (anti-core-1, CD176, TF) antibodies: Evidence for a multiple TF epitope

Antibodies to carbohydrate epitopes are often of the IgM isotype and require multiple binding for sufficient avidity. Therefore clusters of epitopes are preferred antigenic sites in these cases. We have examined the type of clusters recognized by two anti-Thomsen-Friedenreich (TF, core-1, CD176) IgM antibodies, NM-TF1 and NM-TF2, using several different sets of TF-carrying synthetic glycoconjugates in ELISA experiments. To our surprise, the single most important factor determining binding strength was a close vicinity of several TF glycans at distances of ≤1 nm. Considering the known dimensions of IgM antibodies, our data strongly suggest that a cluster of up to four TF moieties, presenting as a "multiple epitope", is required to attach to a single combining site in order to result in adequate binding strength. This effect can also be achieved by "surrogate-multiple epitopes" consisting of separate TF-carrying molecules in close vicinity. In addition, it was found that serine-linked TFs are stronger bound than threonine-linked TFs by both antibodies. This peculiar type of cluster recognition may contribute to improved avidity and explicit tumor specificity.

Synthetic glycoscapes: addressing the structural and functional complexity of the glycocalyx

The glycocalyx is an information-dense network of biomacromolecules extensively modified through glycosylation that populates the cellular boundary. The glycocalyx regulates biological events ranging from cellular protection and adhesion to signalling and differentiation. Owing to the characteristically weak interactions between individual glycans and their protein binding partners, multivalency of glycan presentation is required for the high-avidity interactions needed to trigger cellular responses. As such, biological recognition at the glycocalyx interface is determined by both the structure of glycans that are present as well as their spatial distribution. While genetic and biochemical approaches have proven powerful in controlling glycan composition, modulating the three-dimensional complexity of the cell-surface 'glycoscape' at the sub-micrometre scale remains a considerable challenge in the field. This focused review highlights recent advances in glycocalyx engineering using synthetic nanoscale glycomaterials, which allows for controlled de novo assembly of complexity with precision not accessible with traditional molecular biology tools. We discuss several exciting new studies in the field that demonstrate the power of precision glycocalyx editing in living cells in revealing and controlling the complex mechanisms by which the glycocalyx regulates biological processes.

Weak Multivalent Binding of Influenza Hemagglutinin Nanoparticles at a Sialoglycan-Functionalized Supported Lipid Bilayer

Quantification of the multivalent interactions of influenza viruses binding at interfaces may provide ways to tackle key biological questions regarding influenza virulence and zoonoses. Yet, the deconvolution of the contributions of molecular and interfacial parameters, such as valency, interaction area, and receptor density, to the binding of whole viruses is hindered by difficulties in the direct determination of these parameters. We report here a chemical platform technology to study the binding of multivalent recombinant hemagglutinin (rHA) nanoparticles at artificial sialoglycan cell receptor-presenting interfaces in which all these parameters can be derived, thus allowing the desired full and quantitative binding analysis. SiO2 substrates were functionalized with supported lipid bilayers containing a targeted and tunable fraction of a biotinylated lipid, followed by the adsorption of streptavidin and biotinylated polyvalent 2,3- or 2,6-sialyl lactosamine (SLN). rHA nanoparticles were used as a virus mimic to provide a good prediction of the number of interactions involved in binding. Low nanomolar affinities and selectivities for binding at the 2,6-SLN platforms were observed for rHA particles from three different virus variants. When fitting the data to a multivalency model, the nanomolar overall affinity appears to be achieved by 6-9 HA-sugar molecular interaction pairs, which individually present a rapid association/dissociation behavior. This dynamic behavior may be an essential biological attribute in the functioning of the influenza virus.

IgG Antibodies to GlcNAcβ and Asialo-GM2 (GA2) Glycans as Potential Markers of Liver Damage in Chronic Hepatitis C and the Efficacy of Antiviral Treatment

Total serum IgG level is a surrogate marker of hepatitis C (HC) severity. Antibodies (Abs) to microbial glycans could be markers of HC severity caused by the translocation of microbial products. The level of anti-glycan (AG) Abs was analysed in serum samples of patients (n = 128) with chronic HC in ELISA using fourteen synthetic glycans present in microbes and adhesins to evaluate the association of Abs with clinical parameters and the efficacy of antiviral treatment. The anti-GlcNAcβ IgG level was significantly higher in patients with fibrosis (P = 0.021) and severe portal inflammation (P < 0.001) regardless of other clinical parameters. The ROC curve analysis showed sensitivity of 0.59, specificity of 0.84, and AUC of 0.71 in discriminating F0 from F1–4 (HCV genotype-1b-infected patients). The level of anti-GA2 Abs before Peg-IFN/RBV treatment was significantly higher in nonsustained viral response (non-SVR) to treatment than in SVR (P = 0.033). ROC analysis showed sensitivity of 0.62, specificity of 0.70, and AUC of 64. Correlations of AG Abs to clinical parameters were found. The quantification of anti-GlcNAcβ Abs deserves attention in assessment of the hepatic damage while anti-GA2 Abs may be a sign of immune response related to the antiviral treatment.

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.

Reduced background autofluorescence for cell imaging using nanodiamonds and lanthanide chelates

Bio-imaging is a key technique in tracking and monitoring important biological processes and fundamental biomolecular interactions, however the interference of background autofluorescence with targeted fluorophores is problematic for many bio-imaging applications. This study reports on two novel methods for reducing interference with cellular autofluorescence for bio-imaging. The first method uses fluorescent nanodiamonds (FNDs), containing nitrogen vacancy centers. FNDs emit at near-infrared wavelengths typically higher than most cellular autofluorescence; and when appropriately functionalized, can be used for background-free imaging of targeted biomolecules. The second method uses europium-chelating tags with long fluorescence lifetimes. These europium-chelating tags enhance background-free imaging due to the short fluorescent lifetimes of cellular autofluorescence. In this study, we used both methods to target E-selectin, a transmembrane glycoprotein that is activated by inflammation, to demonstrate background-free fluorescent staining in fixed endothelial cells. Our findings indicate that both FND and Europium based staining can improve fluorescent bio-imaging capabilities by reducing competition with cellular autofluorescence. 30 nm nanodiamonds coated with the E-selectin antibody was found to enable the most sensitive detective of E-selectin in inflamed cells, with a 40-fold increase in intensity detected.

Improved spot morphology for printed glycan arrays

Despite considerable success studying glycan-binding proteins using printed glycan arrays (PGAs), unambiguous quantitation of spot intensities by fluorescent readers remains a challenge. The main obstacles are the varying spot shape and size and in-spot fluorescence distribution caused by uneven drying of the printed drops. Two methods have been suggested for solving this problem: using polymeric glycoconjugates, which makes it possible to equalize the physicochemical properties (hydrophobicity, charge, and size) of different glycans, and applying a glycan solution on a slide coated with a thin oil mask, which hinders evaporation of the drop. Both approaches yield spots with similar sizes and an even distribution of the signal across the spot and are likely to be useful for improving the prints of other classes of molecules.

Synthetic Mucus Nanobarriers for Identification of Glycan-Dependent Primary Influenza A Infection Inhibitors

Current drugs against the influenza A virus (IAV) act by inhibiting viral neuraminidase (NA) enzymes responsible for the release of budding virions from sialoglycans on infected cells. Here, we describe an approach focused on a search for inhibitors that reinforce the protective functions of mucosal barriers that trap viruses en route to the target cells. We have generated mimetics of sialo-glycoproteins that insert into the viral envelope to provide a well-defined mucus-like environment encapsulating the virus. By introducing this barrier, which the virus must breach using its NA enzymes to infect a host cell, into a screening platform, we have been able to identify compounds that provide significant protection against IAV infection. This approach may facilitate the discovery of potent new IAV prophylactics among compounds with NA activities too weak to emerge from traditional drug screens.

Human Gb3/CD77 synthase reveals specificity toward two or four different acceptors depending on amino acid at position 211, creating P(k), P1 and NOR blood group antigens

Human Gb3/CD77 synthase (α1,4-galactosyltransferase, P(k) synthase), encoded by A4GALT gene, is known for synthesis of Gal(α1-4)Gal moiety in globotriaosylceramide (Gb3Cer, CD77, P(k) blood group antigen), a glycosphingolipid of the globo series. Recently, it was shown that c.631C > G mutation in A4GALT, which causes p.Q211E substitution in the open reading frame of the enzyme, broadens the enzyme specificity, making it able also to synthesize Gal(α1-4)GalNAc moiety, which constitutes the defining terminal disaccharide of the NOR antigen (carried by two glycosphingolipids: NOR1 and NOR2). Terminal Gal(α1-4)Gal disaccharide is also present in another glycosphingolipid blood group antigen, called P1, which together with P(k) and NOR comprises the P1PK blood group system. Despite several attempts, it was never clearly shown that P1 antigen is synthesized by Gb3/CD77 synthase, leaving open an alternative hypothesis that there are two homologous α1,4-galactosyltransferases in humans. In this study, using recombinant Gb3/CD77 synthase produced in insect cells, we show that the consensus enzyme synthesizes both the P(k) and P1 antigens, while its p.Q211E variant additionally synthesizes the NOR antigen. This is the first direct biochemical evidence that Gb3/CD77 synthase is able to synthesize two different glycosphingolipid antigens: P(k) and P1, and when p.Q211E substitution is present, the NOR antigen is also synthesized.

Determination of receptor specificities for whole influenza viruses using multivalent glycan arrays

Influenza viruses bind to mucosal glycans to gain entry into a host organism and initiate infection. The target glycans are often displayed in multivalent arrangements on proteins; however, how glycan presentation influences viral specificity is poorly understood. Here, we report a microarray platform approximating native glycan display to facilitate such studies.

Synthetic sialylglycopolymer receptor for virus detection using cantilever-based sensors

We describe the rapid, label-free detection of Influenza A viruses using a cantilever transducer modified with a synthetic sialylglycopolymer receptor layer.

Functionalized halloysite multivalent glycocluster as a new drug delivery system

A new design for halloysite nanotube materials was obtained by grafting chemically modified cyclodextrin units onto the nanotube surface. In particular, grafted cyclodextrins were decorated with thiosaccharide pendants, in order to mimic the well-known binding of sugars to proteins and the glyco-cluster effect occurring during cellular recognition events. The obtained materials were characterized by using a combination of varied techniques (FT-IR spectroscopy, thermogravimetric analysis, scanning electron microscopy, dynamic light scattering, turbidimetry), and their potential drug-delivery abilities were tested by studying their interactions with the common naturally occurring anticancer agent curcumin. A suitable model describing the interaction between our materials and curcumin is proposed.

RAFT-based tri-component fluorescent glycopolymers: synthesis, characterization and application in lectin-mediated bacterial binding study

A group of fluorescent statistical glycopolymers, prepared via reversible addition–fragmentation chain-transfer (RAFT)-based polymerizations, were successfully employed in lectin-mediated bacterial binding studies. The resultant glycopolymers contained three different monomers: N-(2-hydroxyethyl) acrylamide (HEAA), N-(2-aminoethyl) methacrylamide (AEMA) and N-(2-glyconamidoethyl)-methacrylamides possessing different pendant sugars. Low dispersities (≤1.32) and predictable degrees of polymerization were observed among the products. After the polymerization, the glycopolymers were further modified by different succinimidyl ester fluorophores targeting the primary amine groups on AEMA. With their binding specificities being confirmed by testing with lectin coated agarose beads, the glycopolymers were employed in bacterial binding studies, where polymers containing α-galactose or β-galactose as the pendant sugar were specifically bound by two clinically important pathogens Pseudomonas aeruginosa and Staphylococcus aureus, respectively. This is the first report of using RAFT-based glycopolymers in bacterial binding studies, and the ready access to tri-component statistical glycopolymers also warrants further exploration of their utility in other glycobiological applications.

Tailored glycopolymers as anticoagulant heparin mimetics

Heparin and its low molecular weight derivatives are clinical therapeutics used to treat and prevent blood clots, but are prone to side effects and contamination. Here we describe the design and expedient synthesis of heparin-based glycopolymers that are potent and potentially safer mimetics of heparin. The mimetics exhibited strong activity against proteases in the coagulation cascade and prolonged blood clot times in human plasma with efficacies similar to those of clinical anticoagulants.

A new generation of carbohydrate-based therapeutics: recombinant mucin-type fusion proteins as versatile inhibitors of protein-carbohydrate interactions

Cell surface carbohydrates are essential for a multitude of biomedically important interactions that take place at the cell surface. Carbohydrate-binding proteins are, therefore, significant targets for the development of carbohydrate-based inhibitors. Due to their multivalent character, monovalent low-molecular-weight sugar homologues or analogues are usually poor inhibitors of these interactions. Recent advances in organic and chemoenzymatic synthesis of carbohydrates will undoubtedly increase the pace by which new multivalent carbohydrate-based drugs are developed. Knowledge gained on the glycosyltransferases that are involved in glycan biosynthesis can be used to engineer host cells for recombinant production of proteins with tailored glycan substitution. In particular, recombinant mucin-type proteins can serve as natural scaffolds for multivalent presentation of therapeutic carbohydrate determinants.

Glycodendrimers prevent HIV transmission via DC-SIGN on dendritic cells

Dendritic cells (DCs) are antigen-presenting cells efficient in capturing pathogens, and processing their antigenic determinants for presentation to antigen-specific T cells to induce robust immune responses. Their location at peripheral tissues and the expression of pattern-recognition receptors, among them DC-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN), facilitates the capture of pathogens before spreading. However, some pathogens have developed strategies to escape the immune system. One of the most successful is HIV-1, which targets DC-SIGN for transport to the lymph node where the virus infects CD4(+) T cells. Contact of HIV-1 with DC-SIGN is thus the first event in the pathogenic cascade and, therefore, it is the primary target point for therapies aimed at HIV infection prevention. DC-SIGN recognizes specific glycans on HIV-1 and this interaction can be blocked by competitive inhibition through glycans. Although the affinity of glycans is relatively low, multivalency may increase avidity and the strength to compete with HIV-1 virions. We have designed multivalent dendrimeric compounds based on Lewis-type antigens that bind DC-SIGN with high selectivity and avidity and that effectively block gp120 binding to DC-SIGN and, consequently, HIV transmission to CD4(+) T cells. Binding to DC-SIGN and gp120 inhibition was higher on glycodendrimers with larger molecular diameter, indicating that the geometry of the compounds is an important factor determining their functionality. Our compounds elicited DC-SIGN internalization, a property of the receptor upon triggering, but did not affect the maturation status of DCs. Thus, Le(X) glycodendrimers could be incorporated into topic prophylactic approaches for the prevention of HIV-1 transmission.

The relation of the level of serum anti-TF, -Tn and -alpha-Gal IgG to survival in gastrointestinal cancer patients

OBJECTIVE

To evaluate the relation of the level of serum anti-TF, -Tn and -αGal carbohydrate antibodies to survival in gastrointestinal cancer patients.

METHODS

The level of anti-TF (Thomsen-Friedenreich antigen), -Tn and -αGal IgG was analysed in the serum of patients with gastric (n = 83) and colorectal (n = 51) cancers in the long-term follow-up, using ELISA with polyacrylamide glycoconjugates. To evaluate overall survival and the risk of death, the Kaplan-Meier method and the Cox proportional hazards model were used in the univariate analysis of patients groups.

RESULTS

A significantly better survival was observed: (1) in patients with an increased level of anti-TF antibodies (all, stage III, T2-4, N1-2 and G3; P = 0.004-0.038, HR = 0.16-0.46); and (2) in patients with an increased level of anti-Tn antibodies (G1-2 tumors; P = 0.034-0.042, HR = 0.34-0.47). A significantly worse survival was observed in gastrointestinal, gastric and colorectal groups with an increased level of serum anti-αGal antibodies. This association depended on the patho-morphology of tumors (all, stages I-II, III, T2-4, N0, N1-2 and G1-2; P = 0.006-0.048, HR = 1.99-2.33). In the combined assessment of the anti-TF and -αGal antibodies level of the whole gastrointestinal group (n = 53), P = 0.002, HR = 0.25, 95% CI 0.094-0.655. In the follow-up, the survival time was shorter in patients whose level of anti-αGal antibodies rose (P = 0.009-0.040, HR = 2.18-4.27). The level of anti-TF antibodies inversely correlated with neutrophil/lymphocyte ratio (NLR, r = - 0.401, P = 0.004, n = 49). Patients with a higher level of anti-αGal antibodies and NLR values demonstrated a significantly worse survival (P = 0.009, HR = 2.98, n = 48).

CONCLUSIONS

The preoperative levels of anti-TF, -Tn and -αGal antibodies and their dynamics are of prognostic significance. The method for the determination of circulating anti-carbohydrate antibodies may be a useful supplement in clinical outcome assessment.

Mannosylated mucin-type immunoglobulin fusion proteins enhance antigen-specific antibody and T lymphocyte responses

Targeting antigens to antigen-presenting cells (APC) improve their immunogenicity and capacity to induce Th1 responses and cytotoxic T lymphocytes (CTL). We have generated a mucin-type immunoglobulin fusion protein (PSGL-1/mIgG(2b)), which upon expression in the yeast Pichia pastoris became multivalently substituted with O-linked oligomannose structures and bound the macrophage mannose receptor (MMR) and dendritic cell-specific intercellular adhesion molecule-3 grabbing non-integrin (DC-SIGN) with high affinity in vitro. Here, its effects on the humoral and cellular anti-ovalbumin (OVA) responses in C57BL/6 mice are presented.OVA antibody class and subclass responses were determined by ELISA, the generation of anti-OVA CTLs was assessed in (51)Cr release assays using in vitro-stimulated immune spleen cells from the different groups of mice as effector cells and OVA peptide-fed RMA-S cells as targets, and evaluation of the type of Th cell response was done by IFN-γ, IL-2, IL-4 and IL-5 ELISpot assays.Immunizations with the OVA - mannosylated PSGL-1/mIgG(2b) conjugate, especially when combined with the AbISCO®-100 adjuvant, lead to faster, stronger and broader (with regard to IgG subclass) OVA IgG responses, a stronger OVA-specific CTL response and stronger Th1 and Th2 responses than if OVA was used alone or together with AbISCO®-100. Also non-covalent mixing of mannosylated PSGL-1/mIgG(2b), OVA and AbISCO®-100 lead to relatively stronger humoral and cellular responses. The O-glycan oligomannoses were necessary because PSGL-1/mIgG(2b) with mono- and disialyl core 1 structures did not have this effect.Mannosylated mucin-type fusion proteins can be used as versatile APC-targeting molecules for vaccines and as such enhance both humoral and cellular immune responses.

Density variant glycan microarray for evaluating cross-linking of mucin-like glycoconjugates by lectins

Interactions of mucin glycoproteins with cognate receptors are dictated by the structures and spatial organization of glycans that decorate the mucin polypeptide backbone. The glycan-binding proteins, or lectins, that interact with mucins are often oligomeric receptors with multiple ligand binding domains. In this work, we employed a microarray platform comprising synthetic glycopolymers that emulate natural mucins arrayed at different surface densities to evaluate how glycan valency and spatial separation affect the preferential binding mode of a particular lectin. We evaluated a panel of four lectins (Soybean agglutinin (SBA), Wisteria floribunda lectin (WFL), Vicia villosa-B-4 agglutinin (VVA), and Helix pomatia agglutin (HPA)) with specificity for α-N-acetylgalactosamine (α-GalNAc), an epitope displayed on mucins overexpressed in many adenocarcinomas. While these lectins possess the ability to agglutinate A₁-blood cells carrying the α-GalNAc epitope and cross-link low valency glycoconjugates, only SBA showed a tendency to form intermolecular cross-links among the arrayed polyvalent mucin mimetics. These results suggest that glycopolymer microarrays can reveal discrete higher-order binding preferences beyond the recognition of individual glycan epitopes. Our findings indicate that glycan valency can set thresholds for cross-linking by lectins. More broadly, well-defined synthetic glycopolymers enable the integration of glycoconjugate structural and spatial diversity in a single microarray screening platform.

Receptor-binding profiles of H7 subtype influenza viruses in different host species

Influenza viruses of gallinaceous poultry and wild aquatic birds usually have distinguishable receptor-binding properties. Here we used a panel of synthetic sialylglycopolymers and solid-phase receptor-binding assays to characterize receptor-binding profiles of about 70 H7 influenza viruses isolated from aquatic birds, land-based poultry, and horses in Eurasia and America. Unlike typical duck influenza viruses with non-H7 hemagglutinin (HA), all avian H7 influenza viruses, irrespective of the host species, displayed a poultry-virus-like binding specificity, i.e., preferential binding to sulfated oligosaccharides Neu5Acα2-3Galβ1-4(6-O-HSO(3))GlcNAc and Neu5Acα2-3Galβ1-4(Fucα1-3)(6-O-HSO(3))GlcNAc. This phenotype correlated with the unique amino acid sequence of the amino acid 185 to 189 loop of H7 HA and seemed to be dependent on ionic interactions between the sulfate group of the receptor and Lys193 and on the lack of sterical clashes between the fucose residue and Gln222. Many North American and Eurasian H7 influenza viruses displayed weak but detectable binding to the human-type receptor moiety Neu5Acα2-6Galβ1-4GlcNAc, highlighting the potential of H7 influenza viruses for avian-to-human transmission. Equine H7 influenza viruses differed from other viruses by preferential binding to the N-glycolyl form of sialic acid. Our data suggest that the receptor-binding site of contemporary H7 influenza viruses in aquatic and terrestrial birds was formed after the introduction of their common precursor from ducks to a new host, presumably, gallinaceous poultry. The uniformity of the receptor-binding profile of H7 influenza viruses in various wild and domestic birds indicates that there is no strong receptor-mediated host range restriction in birds on viruses with this HA subtype. This notion agrees with repeated interspecies transmission of H7 influenza viruses from aquatic birds to poultry.

Specificity of serum anti-A(di) IgG antibodies from patients with gastrointestinal cancer

Changes in the glycosylation in cancer may lead to an aberrant expression of A, B incompatible or xenogeneic blood group related antigens. To characterize the specificity of IgG antibodies to A, B, and related glycans in sera of gastrointestinal cancer patients, serum probes and affinity-isolated antibodies were analyzed in the indirect and competitive ELISA using a set of homogenous polyacrylamide (PAA) glycoconjugates. Monoreactive antibodies recognizing A(di) (I) and cross-reactive antibodies to A(di)/B(di)/B(tri) (II) or A(di)/A(tri)/Fs(di)/Core5 (III) were affinity-isolated on A(di)-PAA-Sepharose. The population I showed a higher affinity to A(di)-PAA than cross-reactive antibodies. The antibodies II were more specific to B(di) and may belong to the core alpha-Gal reactive antibodies but are also capable of recognizing A(di). The antibodies III were more specific to A(tri); they agglutinated A-erythrocytes and belong to anti-A isoantibodies reactive to xenogeneic oligosaccharides. The purified antibody samples were non- or faintly reactive to Tn. The IC(50) values of PAA glycoconjugates ranged from 6 × 10(-8) to 7 × 10(-6) M. No or weak binding of antibodies to the unrelated antigens used in the detection of polyreactivity (ferritin, casein, and DNA) was observed.

Immobilization of polyacrylamide-based glycoconjugates on solid phase in immunosorbent assays

Our experience in coating of solid surfaces with glycans, mainly for obtaining routine glycoarrays based on immunological plates, is summarized. Three polystyrene coating techniques are described: direct physical adsorption, covalent binding, and immobilization using the biotin tag. Protocols for studies on anticarbohydrate antibodies are considered, with special emphasis on the application niches of different immobilization techniques as related to the specificity of each method of glycan-binding protein assay, as well as the problems of background binding and quantitative estimation of the results.

Solid-phase assays of receptor-binding specificity

Influenza virus attachment to sialic acid-containing molecules on the cell surface initiates the infection. The spectrum of functional receptors on target cells and decoy receptors on cells and epithelial mucus varies substantially between animal species leading to variations in the receptor-binding specificity of viruses circulating in these species. Analysis of the receptor specificity of different animal and human influenza viruses can give insight into factors and mechanisms that determine viral host range, tissue and cell tropism, replication efficiency, and pathogenesis. Knowledge of viral receptor specificity may also be useful for the development of more efficient influenza vaccines and anti-influenza drugs.A majority of known receptor specificity assays measure influenza virus binding to sialic acid-containing natural and synthetic compounds (receptor analogues). Here, we describe protocols of two solid-phase enzyme-linked receptor-binding assays which are technically similar to standard ELISA. Each assay determines binding of the virus immobilized in the wells of 96-well plate to receptor analogues in solution. In the direct binding assay, the virus binds to either synthetic biotinylated sialylglycopolymers or to peroxidase-labeled sialylglycoprotein fetuin (Fet-HRP); the apparent association constants of the virus-receptor complexes are calculated from the Scatchard plots of the binding data. In the fetuin-binding inhibition assay, the virus is incubated with a mixture of unlabeled receptor analogue and standard preparation of Fet-HRP; the association constant for analogue is calculated based on the level of its competition with Fet-HRP.

Specific antibody filter (SAF) binding capacity enhancement to remove anti-A antibodies

Removal of Anti-A/B antibodies prior to ABO-incompatible transplantation can prevent hyperacute organ rejection. We are developing a specific antibody filter (SAF) device to selectively remove ABO blood group antibodies from the whole blood by utilizing immunoaffinity adsorption. The device consists of ultrafiltration hollow fiber membranes with synthetic antigens specific to bind blood group antibodies immobilized on the inner lumenal walls of the fibers. The aim of this study was to evaluate the effect of antigen molecular weight and surface activation process to increase the antibody binding capacity of the fiber membrane surface. A new higher molecular weight antigen Atri-pNSA-1000 compared with Atri-pNPA-30 (A-trisaccharide (Atri) conjugated to activated polymers of Mol. wt. 1000 kDa and 30 kDa, respectively) was employed to improve accessibility of the antigen to bind antibodies. Also, a cyanogen bromide (CNBr) based surface activation method mediated by TEA in neutral pH medium was used to enhance the number of active sites for antigen binding compared to a strong basic medium of NaOH. Using a CNBr/TEA activation method and by immobilizing Atri-pNSA-1000 antigen, an antibody binding capacity (∼0.01 monoclonal anti-A IgM nmol/cm(2)) was achieved on the fiber surface. This binding capacity was sufficient to reduce monoclonal antibody titer from 1:128 to final titer below 1:4 with a surface area to volume ratio that is similar to commercial dialysis device (∼1.1 m(2) surface area for an average body blood volume of 5 L).

Synthesis of glycopolymers for microarray applications via ligation of reducing sugars to a poly(acryloyl hydrazide) scaffold

In this paper, we report on a general synthetic strategy for the assembly of glycopolymers that capitalizes on the intrinsic reactivity of reducing glycans toward hydrazides to form stable cyclic N-glycosides. We developed a poly(acryloyl hydrazide) (PAH) scaffold to which we conjugated a variety of reducing glycans ranging in structure from simple mono- and disaccharides to considerably more complex human milk and blood oligosaccharides. The conjugation proceeds under mild conditions with excellent ligation efficiencies and in a stereoselective manner, providing glycopolymers with pendant glycans accommodated mostly in their cyclic β-glycosidic form. Utilizing a biotin-terminated PAH scaffold prepared via RAFT polymerization, we quickly assembled a panel of glycopolymers that we microarrayed on streptavidin-coated glass. We then demonstrated that in these microarrays, the glycopolymer ligands bind lectins according to the structures of their pendant glycans. Importantly, glycopolymers containing biologically relevant branched oligosaccharides, such as sialyl Lewis^x, as well as sulfated glycosaminoglycan-like epitopes can be readily prepared using our methodology.

Molecular adaptation of an H7N3 wild duck influenza virus following experimental multiple passages in quail and turkey

To investigate the molecular adaptation of influenza viruses during natural interspecies transmission, we performed a phenotypic and genotypic analysis of a low-pathogenic duck H7N3 influenza virus after experimental passages in turkey and quail. Results obtained showed differences in the HA receptor-binding and in NA enzyme activities in viruses recovered after passages in quail, compared to those obtained from passages in turkey. Sequencing of the HA, NA and genes of internal proteins of the viruses obtained from quail and turkey, identified several amino acid substitutions in comparison with the progenitor virus. Of note, in the quail-adapted viruses the emergence of a 23-amino acid deletion in the stalk of the NA and the introduction of a glycosylation site in the HA were a reminiscence of changes typically observed in nature confirming a potential role of the quail in the adaptation of wild birds viruses to domestic poultry.

Is there a clinical need for a diagnostic test allowing detection of chain type-specific anti-A and anti-B?

BACKGROUND

Hemagglutination for detection and semiquantification of ABO antibodies is associated with large center-to-center variations and poor reproducibility. Because acceptance for transplantation and diagnosis of rejection in ABO-incompatible transplantation rely on the levels and specificity of ABO antibodies, reproducible tests that allow their detection and specificity determination are required.

STUDY DESIGN AND METHODS

The level of chain type-specific anti-A and anti-B were analyzed in the sera of 44 healthy individuals of known ABO blood group using an enzyme-linked immunosorbent assay (ELISA) with polyacrylamide (PAA) conjugates of blood group A and B trisaccharides or Type 2 chain A and B tetrasaccharides. Selected sera were further analyzed by hemagglutination and in an ELISA with Types 1 to 4 chain A or B neoglycolipids (NGL) as antigens.

RESULTS

Immunoglobulin (Ig)G anti-A and anti-B levels were higher (p ≤ 0.05) in blood group O than in B and A individuals. More IgM anti-A and anti-B cross-reactivity was detected in AB serum on PAA-conjugated A and B trisaccharides than on the tetrasaccharides. One of 11 blood group B and two of 12 A individuals had IgG antibodies binding the tetrasaccharide despite lack of, or very low reactivity with, the trisaccharides. IgG antibodies preferring the A and B Type 2 tetrasaccharides were of the IgG2 subclass. The NGL ELISA further supported the presence of chain type-specific anti-A and -B antibodies among nonsensitized, healthy individuals.

CONCLUSION

An ELISA with structurally defined ABH antigens will allow the antibody class and fine specificity of ABO antibodies to be determined, which may improve risk assessment in ABO-incompatible transplantation.

A polymorphism in the hemagglutinin of the human isolate of a highly pathogenic H5N1 influenza virus determines organ tropism in mice

We characterized a human H5N1 virus isolate (KAN-1) encoding a hemagglutinin (HA) with a K-to-E substitution at amino acid position 222 that was previously described to be selected in the lung of the infected patient. In mice, the growth of the HA(222E)-encoding virus was mainly confined to the lung, but reversion to 222K allowed virus to spread to the brain. The HA(222E) variant showed an overall reduced binding affinity compared to that of HA(222K) for synthetic Neu5Ac2-3Gal-terminated receptor analogues, except for one analogue [Neu5Acalpha2-3Galbeta1-4(Fucalpha1-3)(6-HSO(3))GlcNAcbeta, Su-SLe(x)]. Our results suggest that human-derived mutations in HA of H5N1 viruses can affect viral replication efficiency and organ tropism.

Distribution and function of macrophage galactose-type C-type lectin 2 (MGL2/CD301b): efficient uptake and presentation of glycosylated antigens by dendritic cells

Dendritic cells (DCs) express cell surface lectins that are potentially involved in the recognition, uptake, and presentation of glycosylated foreign substances. A unique calcium-type (C-type) lectin, the macrophage galactose (Gal)-type C-type lectin (MGL/CD301) expressed on DCs, is thought to participate in the recognition of molecules from both altered self and pathogens due to its monosaccharide specificity for Gal and N-acetylgalactosamine (GalNAc). Although mice have two MGL genes, Mgl1 and Mgl2, their distinct roles have not been previously explored. The present report characterizes the properties of MGL2 by examining its distribution and its role in antigen presentation by DCs. We generated an MGL2-specific monoclonal antibody and examined MGL2 expression in tissues by immunohistochemistry and in isolated cells by flow cytometry. The cells reactive with this antibody were shown to be a portion of MGL1-expressing cells, mostly conventional DCs. Internalization of soluble polyacrylamide polymers (PAA) with alpha-GalNAc residues (GalNAc-PAA) by bone marrow-derived DCs (BM-DCs) was mediated by MGL2, as revealed by a comparison of Mgl1(-/-) and Mgl2(-/-) BM-DCs with wild-type BM-DCs. Biotinylated GalNAc-PAA conjugated to streptavidin (SAv) was more efficiently presented to SAv-primed T cells by BM-DCs than beta-N-acetylglucosamine-PAA conjugated to SAv or SAv alone as shown by thymidine uptake and cytokine production. This is the first report that demonstrates the involvement of GalNAc residues in antigen uptake and presentation by DCs that lead to CD4(+) T cell activation.

Glycoprobes as a tool for the study of lectins expressed on tumor cells

Polyacrylamide glycoconjugates, Glyc-PAA, having various tags or labels are convenient tools for analysis of cellular lectins. Adaptation of such glycoprobes for flow cytometry allows us to reveal lectins expressed on cell surface and analyze their carbohydrate specificity as well as functionality. Localization of lectins is visualized by labeling of cells with fluorescein-tagged glycoprobes, Glyc-PAA-fluo, in combination with fluorescent microscopy techniques. Additionally, biotinylated glycoprobes can be immobilized on magnetic particles making it possible to separate a cell population according to its carbohydrate-binding profile. Here, we exemplify application of glycoprobes in the study of cellular siglecs and galectins, as well as lectin patterning of tumor cells. The specificity of sialic acid binding membrane-anchored lectins, siglecs-1, -5, -7, -8 and -9 was determined using this methodology. To study the carbohydrate-binding profile of soluble galactoside-binding lectins, galectins-1 or -3, these were loaded on (initially galectin free) Raji cells and probed using Glyc-PAA-fluo. Lessons learned from this model system allowed us to study the galectin distribution pattern of tumors: cells obtained from mice carrying mammary adenocarcinoma or lymphoma were probed with Glyc-PAA-fluo using flow cytometry. Disaccharide 6OSuLacdiNAc was shown to be the most potent probe for adenocarcinoma cells, demonstrating that 6OSuLacdiNAc-binding molecules accumulate on cell surface in a patch-wise distribution.