Polyisobutylene-based glycopolymers as potent inhibitors for in vitro insulin aggregation

A family of amphiphilic diblock copolymers containing a hydrophobic polyisobutylene (PIB, M_n = 1000 g mol^-1) segment and a hydrophilic block with sugar pendants has been synthesized by combining living cationic and reversible addition-fragmentation chain transfer (RAFT) polymerization techniques; to explore their potential in insulin fibrillation inhibition. The glucose content in the hydrophilic segment has been tailor-made from 20 to 57 units to prepare block copolymers. The removal of the acetates from the pendent glucose units resulted in amphiphilic block copolymers that generated micellar aggregates in aqueous media. The treatment of insulin with these block copolymers affected the fibril formation process which was demonstrated using an array of biophysical techniques, namely, thioflavin T (ThT) fluorescence, tyrosine (Tyr) fluorescence, Nile red (NR) fluorescence, isothermal titration calorimetry (ITC), etc. The Tyr fluorescence assay and NR fluorescence study revealed the crucial role of hydrophobic interaction in the inhibition process, whereas ITC measurements confirmed the importance of polar interaction. Thus, the block copolymers exhibit potent inhibition of insulin fibrillation owing to hydrophobic (from PIB segment) and glycosidic cluster effect (from sugar pendant block).

High-throughput chemical and chemoenzymatic approaches to saccharide-coated magnetic nanoparticles for MRI

There is a need for biofunctionalised magnetic nanoparticles for many biomedical applications, including MRI contrast agents that have a range of surface properties and functional groups. A library of eleven adducts, each formed by condensing a reducing sugar with a catechol hydrazide, for nanoparticle functionalisation has been created using a high-throughput chemical synthesis methodology. The enzymatic transformation of an N-acetylglucosamine (GlcNAc) adduct into an N-acetyllactosamine adduct by β-1,4-galactosyltransferase illustrates how chemoenzymatic methods could provide adducts bearing complex and expensive glycans. Superparamagnetic iron oxide nanoparticles (8 nm diameter, characterised by TEM, DLS and SQUID) were coated with these adducts and the magnetic resonance imaging (MRI) properties of GlcNAc-labelled nanoparticles were determined. This straightforward approach can produce a range of MRI contrast agents with a variety of biofunctionalised surfaces.

‘One-pot’ sequential enzymatic modification of synthetic glycolipids in vesicle membranes

To create vesicles with cell-targeting coatings, two soluble enzymes were used to directly glycosylate vesicle surfaces in a ‘one-pot’ procedure.

Thermally-induced lateral assembly of a PEG-containing amphiphile triggering vesicle budding

A macrocyclic amphiphile consisting of a thermo-responsive octaethylene glycol chain with hydrophobic aromatic and aliphatic units undergoes lateral self-assembly in a liquid-disordered-state phospholipid bilayer membrane upon heating, which further leads to vesicle budding.

A Bifunctional Spin Label for Ligand Recognition on Surfaces

In situ monitoring of biomolecular recognition, especially at surfaces, still presents a significant technical challenge. Electron paramagnetic resonance (EPR) of biomolecules spin-labeled with nitroxides can offer uniquely sensitive and selective insights into these processes, but new spin-labeling strategies are needed. The synthesis and study of a bromoacrylaldehyde spin label (BASL), which features two attachment points with orthogonal reactivity is reported. The first examples of mannose and biotin ligands coupled to aqueous carboxy-functionalized gold nanoparticles through a spin label are presented. EPR spectra were obtained for the spin-labeled ligands both free in solution and attached to nanoparticles. The labels were recognized by the mannose-binding lectin, Con A, and the biotin-binding protein avidin-peroxidase. Binding gave quantifiable changes in the EPR spectra from which binding profiles could be obtained that reflect the strength of binding in each case.

A versatile approach towards multivalent saccharide displays on magnetic nanoparticles and phospholipid vesicles

A simple synthetic route has been devised for the production of coating agents that can give multivalent displays of saccharides on the surface of magnetite nanoparticles and phospholipid vesicles. A versatile and potentially high-throughput condensation reaction allowed the rapid synthesis of a variety of glycosylhydrazide conjugates with lipid, resorcinol or catechol termini, each in good yield and high anomeric purity. The hydrolytic stability of these adducts was assessed in D2O at different pD values using (1)H-NMR spectroscopy, whilst quartz crystal microbalance with dissipation monitoring (QCM-D) confirmed that the saccharide functionality on bilayers and on nanoparticles was still available to lectins. These multivalent saccharide displays promoted nanoparticle interactions with cells, for example N-acetylglucosamine-coated nanoparticles interacted much more effectively with 3T3 fibroblasts than uncoated nanoparticles with these cells. Despite potential sensitivity to oxidation, catechol coatings on magnetite nanoparticles were found to be more stable and generate better nanoparticle interactions with fibroblasts than resorcinol coatings.

The sweet quartet: Binding of fucose to the norovirus capsid

Human noroviruses bind histo-blood group antigens (HBGAs) and this interaction is thought to be important for an infection. We identified two additional fucose-binding pockets (termed fucose-3/4 sites) on a genogroup II human (GII.10) norovirus-protruding (P) dimer using X-ray crystallography. Fucose-3/4 sites were located between two previously determined HBGA binding pockets (termed fucose-1/2 sites). We found that four fucose molecules were capable of binding altogether at fucose-1/2/3/4 sites on the P dimer, though the fucose molecules bound in a dose-dependent and step-wise manner. We also showed that HBGA B-trisaccharide molecules bound in a similar way at the fucose-1/2 sites. Interestingly, we discovered that the monomers of the P dimer were asymmetrical in an unliganded state and when a single B-trisaccharide molecule bound, but were symmetrical when two B-trisaccharide molecules bound. We postulate that the symmetrical dimers might favor HBGA binding interactions at fucose-1/2 sites.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

Chondroitin sulfate cluster of epiphycan from salmon nasal cartilage defines binding specificity to collagens

Epiphycan (EPY) from salmon nasal cartilage has a glycosaminoglycan (GAG) domain that is heavily modified by chondroitin 4-sulfate and chondroitin 6-sulfate. The functional role of the GAG domain has not been investigated. The interaction of EPY with collagen was examined in vitro using surface plasmon resonance analysis. EPY was found to bind to type I collagen via clustered chondroitin sulfate (CS), while a single chain of CS was unable to bind. Types I, III, VII, VIII and X collagen showed high binding affinity with EPY, whereas types II, IV, V, VI and IX showed low binding affinities. Chemical modification of lysine residues in collagen decreased the affinity with the clustered CS. These results suggest that lysine residues of collagen are involved in the interaction with the clustered CS, and the difference in lysine modification defines the binding affinity to EPY. The clustered CS was also involved in an inter-saccharide interaction, and formed self-associated EPY. CS of EPY promoted fibril formation of type I collagen.

Sialylation of lactosyl lipids in membrane microdomains byT. cruzi trans-sialidase

SolubleT. cruzi trans-sialidase transformed a synthetic lactosyl glycolipid in microdomains more slowly than the same substrate dispersed across the bilayer surface, producing phospholipid vesicles with a Neu5Ac(α2-3)Gal(β1-4)Glc “glycocalyx”.

Modulation of in-membrane receptor clustering upon binding of multivalent ligands

In living cells and biomimetic systems alike, multivalent ligands in solution can induce clustering of membrane receptors. The link between the receptor clustering and the ligand binding remains, however, poorly defined. Using minimalist divalent ligands, we develop a model that allows quantifying the modulation of receptor clustering by binding of ligands with any number of binding sites. The ligands, with weak binding affinity for the receptor and with binding sites held together by flexible linkers, lead to nearly quantitative clustering upon binding in a wide range of experimental conditions, showing that efficient modulation of receptor clustering does not require pre-organization or large binding affinities per binding site. Simulations show that, in the presence of ligands with five or more binding sites, an on/off clustering response follows a very small change in receptor density in the membrane, which is consistent with the highly cooperative behavior of multivalent biomolecular systems.

Mannose-decorated cyclodextrin vesicles: The interplay of multivalency and surface density in lectin-carbohydrate recognition

Cyclodextrin vesicles are versatile models for biological cell membranes since they provide a bilayer membrane that can easily be modified by host–guest interactions with functional guest molecules. In this article, we investigate the multivalent interaction of the lectin concanavalin A (ConA) with cyclodextrin vesicles decorated with mannose–adamantane conjugates with one, two or three adamantane units as well as one or two mannose units. The carbohydrate–lectin interaction in this artificial, self-assembled glycocalyx was monitored in an agglutination assay by the increase of optical density at 400 nm. It was found that there is a close relation between the carbohydrate density at the cyclodextrin vesicle surface and the multivalent interaction with ConA, and the most efficient interaction (i.e., fastest agglutination at lowest concentration) was observed for mannose–adamantane conjugates, in which both the cyclodextrin–adamantane and the lectin–mannose interaction is inherently multivalent.

Multivalent interaction of cyclodextrin vesicles, carbohydrate guests, and lectins: a kinetic investigation

An artificial glycocalix self-assembles when unilamellar bilayer vesicles of amphiphilic β-cyclodextrins are decorated with maltose- and lactose-adamantane conjugates by host-guest interactions. The maltose-decorated vesicles aggregate in the presence of lectin concanavalin A whereas the lactose-decorated vesicles aggregate in the presence of lectin peanut agglutinin. The kinetics of the orthogonal multivalent interfacial interactions present in this ternary system of vesicles, carbohydrates, and lectins were studied by time-dependent measurements of the optical density at 400 nm. The average vesicle and vesicle aggregate sizes were monitored by dynamic light scattering. The aggregation process was evaluated as a function of lectin concentration, vesicle concentration, and surface coverage of the vesicles by the carbohydrate-adamantane conjugates. The initial rate of vesicle aggregation scales linearly with the lectin as well as the cyclodextrin vesicle concentration. Furthermore, each lectin requires a characteristic critical density of carbohydrates at the vesicle surface. These observations allow a prediction of the response of the ternary supramolecular system at different concentrations of its components. Also, the effective binding site separation in a multivalent receptor such as a multiple binding site protein can be accurately determined. This methodology can be extended to multivalent noncovalent interactions in other ligand-receptor systems at interfaces.

Incorporation of fluorous glycosides to cell membrane and saccharide chain elongation by cellular enzymes

A series of fluorous-tagged glycosides with different number of fluorine atoms are incorporated into the cells, transported to Golgi, elongated by cellular enzymes, and then released to the culture medium. Fluorine content strongly affects on the affinity for cell membrane and glycosylation. Essentially, the fluorocarbon chain in fluorous compound and the hydrocarbon chain are not miscible. However, the fluorous-tagged glycosides have affinity for cell membrane because of its amphiphilicity. The affinity of fluoro-amphiphilic compound for cell membrane is discussed using critical micelle concentration. The separation of glycosylated products by solvent extraction or fluorous solid phase extraction cartridges is also discussed.

Preparation of aminoethyl glycosides for glycoconjugation

The synthesis of a number of aminoethyl glycosides of cell-surface carbohydrates, which are important intermediates for glycoarray synthesis, is described. A set of protocols was developed which provide these intermediates, in a short number of steps, from commercially available starting materials.

Nanodomain formation in lipid membranes probed by time-resolved fluorescence

Time-resolved fluorescence measurements on liposomes prepared with 1 mol % pyrene-labeled lipids (PLLs) with a headgroup bearing either an alcohol (PSOH) or an imido diacetic acid (PSIDA) and 99 mol % 1-palmitoyl-2-oleyl-3-sn-phosphatidylcholines (POPC) or 99 mol % distearylphosphatidylcholine (DSPC) were performed to investigate how lipids phase separate within the membrane bilayer. Global analysis of the fluorescence decays with the fluorescence blob model (FBM) led to the conclusion that the PLLs were homogeneously distributed on the surface of POPC vesicles while the PLLs phase-separated in the DSPC vesicles. The analysis yielded the fraction of aggregated pyrenes, f(agg). The large f(agg) values found for PSIDA suggest that the imido diacetic acid headgroup of PSIDA induces self-aggregation and phase separation in both membranes. The addition of external cations such as Cu(2+) and La(3+) was shown to hinder diffusional encounters between PSIDAs. The cations seem to target preferentially unassociated PSIDAs rather than aggregated PSIDA clusters. Accounting for the quenching of pyrene by Cu(2+) enables one to use PSIDA to probe the microviscosity of the lipid membrane. Using this effect, the environment of PSIDA in the DSPC membrane was found to be about 6 times more viscous than that in the POPC membrane. This difference is attributed to the difference in viscosity of the fluid POPC membrane and the gel-like DSPC membranes.