Oligolysine-based saccharide clusters: synthesis and specificity

Synthesis of glycopeptides and glycopeptide conjugates

Protein glycosylation is a key post-translational modification important to many facets of biology. Glycosylation can have critical effects on protein conformation, uptake and intracellular routing. In immunology, glycosylation of antigens has been shown to play a role in self/non-self distinction and the effective uptake of antigens. Improperly glycosylated proteins and peptide fragments, for instance those produced by cancerous cells, are also prime candidates for vaccine design. To study these processes, access to peptides bearing well-defined glycans is of critical importance. In this review, the key approaches towards synthetic, well-defined glycopeptides, are described, with a focus on peptides useful for and used in immunological studies. Special attention is given to the glycoconjugation approaches that have been developed in recent years, as these enable rapid synthesis of various (unnatural) glycopeptides, enabling powerful carbohydrate structure/activity studies. These techniques, combined with more traditional total synthesis and chemoenzymatic methods for the production of glycopeptides, should help unravel some of the complexities of glycobiology in the near future.

Assembly, postsynthetic modification and hepatocyte targeting by multiantennary, galactosylated soft structures

Enzyme modifiable, hollow self-assembled structures offer an excellent scope for multiantennary delivery vectors.

Carbohydrate-lectin interactions assayed by SPR

Surface plasmon resonance is a valuable tool to determine the affinity between glycoconjugates and sugar-binding proteins such as plant and animal lectins. The main interest of using such an approach is that neither the lectins - which are proteins - nor their ligands - natural compounds such as glycoproteins, oligosaccharides, polysaccharides, or synthetic glycoconjugates such as glycoclusters or neoglycoproteins - require any tag. Because lectins bear several binding sites, they behave like immunoglobulin eliciting avidity phenomena. This peculiarity may lead to erroneous results if special conditions are not applied. We obtained best and reproducible results when the lectin was immobilized and its ligands were used as soluble analytes. With heterogeneous glycoconjugates such as neoglycoproteins (which are heterogeneous in terms of nature, number, and position of sugar residues) or a mixture of oligosaccharides, the data may be more accurately gathered by using the Sips approach, which has been used to determine mean binding constants of polyclonal antibodies. With small analytes such as oligosaccharides, we found it convenient to determine binding constants by using an inhibitory approach: a neoglycoprotein (M (r) = approximately 80,000) was allowed to bind to the immobilized lectin and small oligosaccharides were used as inhibitors. With larger glycoconjugates such as peptides substituted with glycoclusters, direct binding measurements gave accurate results. Because of the availability of low-cost simple sugars (mono- or disaccharides) it is very convenient to use large concentrations of such carbohydrates to clean the sensor chips instead of more drastic cleaning solutions such as acids or alkali, in such a way that the immobilized lectin is stable for many experiments.

Gold manno-glyconanoparticles: multivalent systems to block HIV-1 gp120 binding to the lectin DC-SIGN

The HIV envelope glycoprotein gp120 takes advantage of the high-mannose clusters on its surface to target the C-type lectin dendritic cell-specific intracellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) on dendritic cells. Mimicking the cluster presentation of oligomannosides on the virus surface is a strategy for designing carbohydrate-based antiviral agents. Bio-inspired by the cluster presentation of gp120, we have designed and prepared a small library of multivalent water-soluble gold glyconanoparticles (manno-GNPs) presenting truncated (oligo)mannosides of the high-mannose undecasaccharide Man(9)GlcNAc(2) and have tested them as inhibitors of DC-SIGN binding to gp120. These glyconanoparticles are ligands for DC-SIGN, which also interacts in the early steps of infection with a large number of pathogens through specific recognition of associated glycans. (Oligo)mannosides endowed with different spacers ending in thiol groups, which enable attachment of the glycoconjugates to the gold surface, have been prepared. manno-GNPs with different spacers and variable density of mannose (oligo)saccharides have been obtained and characterized. Surface plasmon resonance (SPR) experiments with selected manno-GNPs have been performed to study their inhibition potency towards DC-SIGN binding to gp120. The tested manno-GNPs completely inhibit the binding from the micro- to the nanomolar range, while the corresponding monovalent mannosides require millimolar concentrations. manno-GNPs containing the disaccharide Manalpha1-2Manalpha are the best inhibitors, showing more than 20 000-fold increased activity (100 % inhibition at 115 nM) compared to the corresponding monomeric disaccharide (100 % inhibition at 2.2 mM). Furthermore, increasing the density of dimannoside on the gold platform from 50 to 100 % does not improve the level of inhibition.

Surface plasmon resonance study of protein-carbohydrate interactions using biotinylated sialosides

Lectins are carbohydrate binding proteins found in plants, animals, and microorganisms. They serve as important models for understanding protein-carbohydrate interactions at the molecular level. We report here the fabrication of a novel sensing interface of biotinylated sialosides to probe lectin-carbohydrate interactions using surface plasmon resonance spectroscopy (SPR). The attachment of carbohydrates to the surface using biotin-NeutrAvidin interactions and the implementation of an inert hydrophilic hexaethylene glycol spacer (HEG) between the biotin and the carbohydrate result in a well-defined interface, enabling desired orientational flexibility and enhanced access of binding partners. The specificity and sensitivity of lectin binding were characterized using Sambucus nigra agglutinin (SNA) and other lectins including Maackia amurensis lectin (MAL), concanavalin A (Con A), and wheat germ agglutinin (WGA). The results indicate that alpha2,6-linked sialosides exhibit high binding affinity to SNA, while alteration in sialyl linkage and terminal sialic acid structure compromises the affinity by a varied degree. Quantitative analysis yields an equilibrium dissociation constant (KD) of 777 +/- 93 nM for SNA binding to Neu5Ac alpha2,6-LHEB. Transient SPR kinetics confirms the K D value from the equilibrium binding studies. A linear relationship was obtained in the 10-100 microg/mL range with limit of detection of approximately 50 nM. Weak interactions with MAL, Con A, and WGA were also quantified. The control experiment with bovine serum albumin indicates that nonspecific interaction on this surface is insignificant over the concentration range studied. Multiple experiments can be performed on the same substrate using a glycine stripping buffer, which selectively regenerates the surface without damaging the sialoside or the biotin-NeutrAvidin interface. This surface design retains a high degree of native affinity for the carbohydrate motifs, allowing distinction of sialyl linkages and investigation pertaining to the effect of functional group on binding efficiency. It could be easily modified to identify and quantify binding patterns of any low-affinity biologically relevant systems, opening new avenues for probing carbohydrate-protein interactions in real time.

Fucosylated Pentaerythrityl Phosphodiester Oligomers (PePOs): Automated Synthesis of DNA-Based Glycoclusters and Binding to Pseudomonas aeruginosa Lectin (PA-IIL)

The synthesis of propargylated pentaerythrityl phosphodiester oligomers (PePOs) was achieved using a DNA synthesizer with a bis-propargylated pentaerythritol-based phosphoramidite. An azido fucose derivative was reacted under "click" chemistry conditions activated by microwaves to construct a series of glycosylated PePOs bearing 4, 6, 8, and 10 L-fucose residues. Binding to the fucose-specific bacterial lectin (PA-IIL) was determined for the fucosylated PePOs through an enzyme-linked lectin amplification competition assay. The IC50 values measured are 10-20 times better than for monovalent l-fucose and denotate for a "macromolecular" effect rather than a "cluster" effect.

Synthesis of glycocluster-tumor antigenic peptide conjugates for dendritic cell targeting

The use of dendritic cells (DC) for the development of therapeutic cancer vaccines is attractive because of their unique ability to present tumor epitopes via the MHC class I pathway to induce cytotoxic CD8+ T lymphocyte responses. C-Type membrane lectins, DC-SIGN and the mannose receptor (MR), present on the DC surface, recognize oligosaccharides containing mannose and/or fucose and mediate sugar-specific endocytosis of synthetic oligolysine-based glycoclusters. We therefore asked whether a glycotargeting approach could be used to induce uptake and presentation of tumor antigens by DC. To this end, we designed and synthesized glycocluster conjugates containing a CD8+ epitope of the Melan-A/Mart-1 melanoma antigen. These glycocluster-Melan-A conjugates were obtained by coupling glycosynthons: oligosaccharyl-pyroglutamyl-beta-alanine derivatives containing either disaccharides, a dimannoside (Manalpha-6Man) or lactoside, or a Lewis oligosaccharide, to Melan-A 16-40 peptide comprising the 26-35 HLA-A2 restricted T cell epitope, extended with an oligolysine stretch at the C-terminal end. We showed by confocal microscopy and flow cytometry that fluorescent-labeled Melan-A glycoclusters containing either dimannoside or Lewis oligosaccharide were taken up by DC and concentrated in acidic vesicles; conversely lactoside glycopeptides were not at all taken up. Furthermore, using surface plasmon resonance, we showed that dimannoside and Lewis-Melan-A conjugates bind MR and DC-SIGN with high affinity. DC loaded with these conjugates, but not with the lactose-Melan-A conjugate, led to an efficient presentation of the Melan-A epitope eliciting a CD8+ T-lymphocyte response. These data suggest that synthetically designed glycocluster-tumor antigen conjugates may induce antigen cross-presentation by DC and represent a promising tool for the development of tumor vaccines.

Influence of a neoglycolipid and its PEO-lipid moiety on the organization of phospholipid monolayers

The surface properties of the neoglycolipid (GlcNAcE(3)G(28)) and of its PEO-lipid (E(3)G(28)) moiety mixed with phospholipids (dipalmitoylphosphatidylcholine, DPPC; distearoylphosphatidylcholine, DSPC; diarachidoylphosphatidylcholine, DAPC; and dibehenoylphosphatidylcholine, DBPC) were studied in Langmuir monolayers at various mixture compositions and surface pressures. The pi-A isotherms of the pure compounds revealed that because of the presence of the sugar group in its molecule, GlcNAcE(3)G(28) collapsed at a higher surface pressure and occupied a larger molecular area than the PEO-lipid moiety. It was also observed that the presence of the PEO-lipid (E(3)G(28)) in the mixtures triggered a strong alteration of both phospholipid pi-A isotherm profiles and surface diffraction spectra, an indication that the disordering of the initially structured phospholipid monolayers took place. Unlike E(3)G(28), GlcNAcE(3)G(28) did not disorganize phospholipid monolayers but generated a partial segregation of the film-forming components. The calculated excess free energies of mixing (DeltaG(exc)) for GlcNAcE(3)G(28)-phospholipid mixtures enabled us to predict the stability of such systems.

SPR Studies of Carbohydrate-Protein Interactions: Signal Enhancement of Low-Molecular-Mass Analytes by Organoplatinum(II)-Labeling

The relatively insensitive surface plasmon resonance (SPR) signal detection of low-molecular-mass analytes that bind with weak affinity to a protein--for example, carbohydrate-lectin binding--is hampering the use of biosensors in interaction studies. In this investigation, low-molecular-mass carbohydrates have been labeled with an organoplatinum(II) complex of the type [PtCl(NCN-R)]. The attachment of this complex increased the SPR response tremendously and allowed the detection of binding events between monosaccharides and lectins at very low analyte concentrations. The platinum atom inside the organoplatinum(II) complex was shown to be essential for the SPR-signal enhancement. The organoplatinum(II) complex did not influence the specificity of the biological interaction, but both the signal enhancement and the different binding character of labeled compounds when compared with unlabeled ones makes the method unsuitable for the direct calculation of biologically relevant kinetic parameters. However, the labeling procedure is expected to be of high relevance for qualitative binding studies and relative affinity ranking of small molecules (not restricted only to carbohydrates) to receptors, a process of immense interest in pharmaceutical research.

Photoswitchable cluster glycosides as tools to probe carbohydrate-protein interactions: synthesis and lectin-binding studies of azobenzene containing multivalent sugar ligands

Synthetic cluster glycosides have often been used to unravel mechanisms of carbohydrate-protein interactions. Although synthetic cluster glycosides are constituted on scaffolds to achieve high avidities in lectin binding, there have been no known attempts to modulate the orientations of the sugar clusters with the aid of a functional scaffold onto which the sugar units are linked. Herein, we describe synthesis, physical, and lectin-binding studies of a series of alpha-D-mannopyranoside and beta-D-galactopyranosyl-(1-->4)-beta-D-glucopyranoside glycoclusters that are attached to a photoswitchable azobenzenoid core. These glycoclusters were synthesized by the amidation of amine-tethered glycopyranosides with azobenzene carbonyl chlorides. From kinetic studies, the cis forms of the azobenzene-glycopyranoside derivative were found to be more stable in aqueous solutions than in organic solvents. Molecular modeling studies were performed to estimate the relative geometries of the photoswitchable glycoclusters in the trans- and cis-isomeric forms. Isothermal titration calorimetry (ITC) was employed to assess the binding of these glycoclusters to lectins peanut agglutinin (PNA) and concanavalin A (Con A). Although binding affinities were enhanced several orders higher as the valency of the sugar was increased, a biphasic-binding profile in ITC plots was observed during few glycoclusters lectin-binding processes. The biphasic-binding profile indicates a "cooperativity" in the binding process. An important outcome of this study is that in addition to inherent clustering of the sugar units as a molecular feature, an induced clustering emanates because of the isomerization of the trans form of the azobenzene scaffold to the cis-isomeric form.

A survey of the year 2002 commercial optical biosensor literature

We have compiled 819 articles published in the year 2002 that involved commercial optical biosensor technology. The literature demonstrates that the technology's application continues to increase as biosensors are contributing to diverse scientific fields and are used to examine interactions ranging in size from small molecules to whole cells. Also, the variety of available commercial biosensor platforms is increasing and the expertise of users is improving. In this review, we use the literature to focus on the basic types of biosensor experiments, including kinetics, equilibrium analysis, solution competition, active concentration determination and screening. In addition, using examples of particularly well-performed analyses, we illustrate the high information content available in the primary response data and emphasize the impact of including figures in publications to support the results of biosensor analyses.

Oligolysine-based oligosaccharide clusters: selective recognition and endocytosis by the mannose receptor and dendritic cell-specific intercellular adhesion molecule 3 (ICAM-3)-grabbing nonintegrin

Dendritic cells are potent antigen-presenting cells that express several membrane lectins, including the mannose receptor and DC-SIGN (dendritic cell-specific ICAM-3-grabbing nonintegrin). To identify highly specific ligands for these dendritic cell receptors, oligosaccharides were converted into glycosynthons (Os1) and were used to prepare oligolysine-based glycoclusters, Os-[Lys(Os)]n-Ala-Cys-NH2. Clusters containing two to six dimannosides as well as clusters containing four or five pentasaccharides (Lewisa or Lewisx) or hexasaccharides (Lewisb) were synthesized. The thiol group of the appended cysteine residue allows easy tagging by a fluorescent probe or convenient substitution with an antigen. Surface plasmon resonance was used to determine the affinity of the different glycoclusters for purified mannose receptor and DC-SIGN, whereas flow cytometry and confocal microscopy analysis allowed assessment of cell uptake of fluoresceinyl-labeled glycoclusters. Dimannoside clusters are recognized by the mannose receptor with an affinity constant close to 106 liter.mol-1 but have a very low affinity for DC-SIGN (less than 104 liter x mol-1). Conversely, Lewis clusters have a higher affinity toward DC-SIGN than toward the mannose receptor. Dimannoside clusters are efficiently taken up by human dendritic cells as well as by rat fibroblasts expressing the mannose receptor but not by HeLa cells or rat fibroblasts expressing DC-SIGN; DC-SIGN-expressing cells take up Lewis clusters. The results suggest that ligands containing dimannoside clusters can be used specifically to target the mannose receptor, whereas ligands containing Lewis clusters will be targeted to DC-SIGN.