Synthesis of Tailor-Made Glycoconjugate Mimetics of Heparan Sulfate That Bind IFN-γ in the Nanomolar Range

Rational synthesis of a heparan sulfate saccharide that promotes the activity of BMP2

Heparan sulfate (HS) is a glycosaminoglycan (GAG) found throughout nature and is involved in a wide range of functions including modulation of cell signalling via sequestration of growth factors. Current consensus is that the specificity of HS motifs for protein binding are individual for each protein. Given the structural complexity of HS the synthesis of libraries of these compounds to probe this is not trivial. Herein we present the synthesis of an HS decamer, the design of which was undertaken rationally from previously published data for HS binding to the growth factor BMP-2. The biological activity of this HS decamer was assessed in vitro, showing that it had the ability to both bind BMP-2 and increase its thermal stability as well as enhancing the bioactivity of BMP-2 in vitro in C2C12 cells. At the same time no undesired anticoagulant effect was observed. This decamer was then analysed in vivo in a rabbit model where higher bone formation, bone mineral density (BMD) and trabecular thickness were observed over an empty defect or collagen implant alone. This indicated that the HS decamer was effective in promoting bone regeneration in vivo.

Synthesis of Anticoagulant Pentasaccharide Fondaparinux via 3,5-Dimethyl-4-(2'-phenylethynylphenyl)phenyl Glycosides

Here, we disclosed a convenient procedure for the preparation of EPP [3,5-dimethyl-4-(2'-phenylethynylphenyl)phenyl] glycosides and their application to an effective synthesis of fondaparinux, the clinically approved anticoagulant heparin pentasaccharide. The use of EPP glycosides in the one-pot orthogonal glycosylation for the synthesis of heparin-like tetrasaccharides has also been achieved.

Expedient Synthesis of a Library of Heparan Sulfate-Like "Head-to-Tail" Linked Multimers for Structure and Activity Relationship Studies

Heparan sulfate (HS) plays important roles in many biological processes. The inherent complexity of naturally existing HS has severely hindered the thorough understanding of their structure-activity relationship. To facilitate biological studies, a new strategy has been developed to synthesize a HS-like pseudo-hexasaccharide library, where HS disaccharides were linked in a "head-to-tail" fashion from the reducing end of a disaccharide module to the non-reducing end of a neighboring module. Combinatorial syntheses of 27 HS-like pseudo-hexasaccharides were achieved. This new class of compounds bound with fibroblast growth factor 2 (FGF-2) with similar structure-activity trends as HS oligosaccharides bearing native glycosyl linkages. The ease of synthesis and the ability to mirror natural HS activity trends suggest that the new head-to-tail linked pseudo-oligosaccharides could be an exciting tool to facilitate the understanding of HS biology.

Chemoenzymatic Synthesis of Heparan Sulfate Oligosaccharides having a Domain Structure

Heparan sulfate (HS) has a domain structure in which regions that are modified by epimerization and sulfonation (NS domains) are interspersed by unmodified fragments (NA domains). There is data to support that domain organization of HS can regulate binding of proteins, however, such model has been difficult to probe. Here, we report a chemoenzymatic methodology that can provide HS oligosaccharides composed of two or more NS domains separated by NA domains of different length. It is based on the chemical synthesis of a HS oligosaccharide that enzymatically was extended by various GlcA-GlcNAc units and terminated in GlcNAc having an azido moiety at C-6 position. HS oligosaccharides having an azide and alkyne moiety could be assembled by copper catalyzed alkyne-azide cycloaddition to give compounds having various NS domains separated by unsulfonated regions. Competition binding studies showed that the length of an NA domain modulates the binding of the chemokines CCL5 and CXCL8.

Pentosan Polysulphate (PPS), a Semi-Synthetic Heparinoid DMOAD With Roles in Intervertebral Disc Repair Biology emulating The Stem Cell Instructive and Tissue Reparative Properties of Heparan Sulphate

This review highlights the attributes of pentosan polysulphate (PPS) in the promotion of intervertebral disc (IVD) repair processes. PPS has been classified as a disease modifying osteoarthritic drug (DMOAD) and many studies have demonstrated its positive attributes in the countering of degenerative changes occurring in cartilaginous tissues during the development of osteoarthritis (OA). Degenerative changes in the IVD also involve inflammatory cytokines, degradative proteases and cell signalling pathways similar to those operative in the development of OA in articular cartilage. PPS acts as a heparan sulphate (HS) mimetic to effect its beneficial effects in cartilage. The IVD contains small cell membrane HS-proteoglycans (HSPGs) such as syndecan, and glypican and a large multifunctional HS/chondroitin sulphate (CS) hybrid proteoglycan (HSPG2/perlecan) that have important matrix stabilising properties and sequester, control and present growth factors from the FGF, VEGF, PDGF and BMP families to cellular receptors to promote cell proliferation, differentiation and matrix synthesis. HSPG2 also has chondrogenic properties and stimulates the synthesis of extracellular matrix (ECM) components, expansion of cartilaginous rudiments and has roles in matrix stabilisation and repair. Perlecan is a perinuclear and nuclear proteoglycan in IVD cells with roles in chromatin organisation and control of transcription factor activity, immunolocalises to stem cell niches in cartilage, promotes escape of stem cells from quiescent recycling, differentiation and attainment of pluripotency and migratory properties. These participate in tissue development and morphogenesis, ECM remodelling and repair. PPS also localises in the nucleus of stromal stem cells, promotes development of chondroprogenitor cell lineages, ECM synthesis and repair and discal repair by resident disc cells. The availability of recombinant perlecan and PPS offer new opportunities in repair biology. These multifunctional agents offer welcome new developments in repair strategies for the IVD.

Strategies in Synthesis of Heparin/Heparan Sulfate Oligosaccharides: 2000-Present

Heparin and heparan sulfate are members of the glycosaminoglycan family that are involved in a multitude of biological processes. The great interests in the anticoagulant properties of heparin have stimulated major advances in synthetic strategies toward clinically effective analogues, as demonstrated importantly by the approval of the fully synthetic pentasaccharide fragment, termed fondaparinux (Arixtra®), of the heparin macromolecule for treatment of deep-vein thrombosis. Given the highly complex nature of heparin and heparan sulfate, the chemical synthesis of their components is a challenging endeavor. In the past decade, multiple approaches have been developed to improve the overall synthetic efficiency. New strategies have emerged that can generate libraries of oligosaccharide components of heparin and heparan sulfate. This article discusses recent developments in the assembly of heparin and heparan sulfate oligosaccharides and the associated challenges in their synthesis.

Heparanized chitosans: towards the third generation of chitinous biomaterials

The functionalization of chitosans is an emerging research area in the design of solutions for a wide range of biomedical applications. In particular, the modification of chitosans to incorporate sulfate groups has generated great interest since they show structural similarity to heparin and heparan sulfates. Most of the biomedical applications of heparan sulfates are derived from their ability to bind different growth factors and other proteins, as through these interactions they can modulate the cellular response. This review aims to summarize the most recent advances in the synthesis, and structural and physicochemical characterization of heparanized chitosan, a remarkably interesting family of polysaccharides that have demonstrated the ability to mimic heparan sulfates as ligands for different proteins, thereby exerting their biological activity by mimicking the function of these glycosaminoglycans.

Discrimination of deletion to point cytokine mutants based on an array of cross-reactive receptors mimicking protein recognition by heparan sulfate

Differential sensing of proteins based on cross-reactive arrays and pattern recognition is a promising technique for the detection and identification of proteins. In this study, a rational biomimetic strategy has been used to prepare sensing materials capable of discriminating structurally similar proteins, such as deletion and point mutants of a cytokine, by mimicking the biological properties of heparan sulfate (HS). Using the self-assembly of two disaccharides, lactose and sulfated lactose at various ratios on the surface of a chip, an array of combinatorial cross-reactive receptors has been prepared. Coupling with surface plasmon resonance imaging (SPRi), the obtained cross-reactive array is very efficient for protein sensing. It is able to detect HS binding proteins (HSbps) such as IFNγ at nanomolar concentrations. Moreover, such a system is capable of discriminating between IFNγ and its mutants with good selectivity.

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

Comparison of disaccharide donors for heparan sulfate synthesis: uronic acids vs. their pyranose equivalents

Late oxidation of hexose based building blocks or the use of uronic acid containing building blocks are two complementary strategies in the synthesis of glycosaminoglycans, the latter simplifiying the later stages of the process. Here we report the synthesis and evaluation of various disaccharide donors-uronic acids and their pyranose equivalents-for the synthesis of heparan sulfate, using an established protective group strategy. Hexose based "imidate" type donors perform well in the studied glycosylations, while their corresponding uronate esters fall short; a uronate ester thioglycoside performs equal to, if not better than, a hexose thioglycoside equivalent.

Synthesis of Heparan Sulfate- and Dermatan Sulfate-Related Oligosaccharides via Iterative Chemoselective Glycosylation Exploiting Conformationally Disarmed [2.2.2] l-Iduronic Lactone Thioglycosides

Heparan sulfate (HS) and dermatan sulfate (DS) are l-iduronic acid containing glycosaminoglycans (GAGs) which are implicated in a number of biological processes and conditions including cancer and viral infection. Chemical synthesis of HS and DS is required to generate structurally defined oligosaccharides for a biological study. Herein, we present a new synthetic approach to HS and DS oligosaccharides using chemoselective glycosylation which relies on a disarmed [2.2.2] l-ido lactone motif. The strategy provides a general approach for iterative-reducing end chain extension, using only shelf-stable thioglycoside building blocks, exploiting a conformational switch to control reactivity, and thus requires no anomeric manipulation steps between glycosylations.

The role of heparin/heparan sulphate in the IFN-γ-led Arena

IFN-γ (Interferon-gamma) is a pleiotropic cytokine. It is often involved in a variety of physiological processes by binding to the cell surface transmembrane receptor (IFN-γR) to initiate a series of signalling pathways that transmit external signals from cell surface receptors to the cell nucleus. Heparan sulphate (HS), a highly sulphated linear polysaccharide, is ubiquitous on the mammalian cell surface and extracellular matrix. Electrostatic interactions can be generated between the highly sulphated HS region and specific basic amino acid residues in the IFN-γ structure, thereby detaining IFN-γ on the cell surface, and the concentration of IFN-γ on the cell surface is thus, changed. IFN-γ retained on the cell surface will optimize the binding of IFN-γ to the transmembrane receptor resulting in high efficiency signalling. Heparin is a glycosaminoglycan with a structure similar to HS. The structural similarity provides a basis for modelling exogenous heparin dependence for interference with IFN-γ function. This model can be summarized as follows: First, the competitive binding effect; heparin bound to cytokines by competing with membrane-associated HS, causes a decrease in cytokine concentration on the cell surface. Second, the principle of priority occupancy; heparin can occupy the receptor binding site on cytokines, partially preventing the IFN-γ-IFN-γR interaction. These two models interfere with IFN-γ signal transmission. To decipher the mechanism by which heparin influences IFN-γ activity, studies of the structure-activity relationship are in progress. This paper summarizes research progress on the IFN-γ signalling pathway, heparin interference with IFN-γ activity and the structure-activity relationship between heparin and IFN-γ.

Substrate binding mode and catalytic mechanism of human heparan sulfate d-glucuronyl C5 epimerase

Heparan sulfate (HS) is a linear, complex polysaccharide that modulates the biological activities of proteins through binding sites made by a series of Golgi-localized enzymes. Of these, glucuronyl C5-epimerase (Glce) catalyzes C5-epimerization of the HS component, d-glucuronic acid (GlcA), into l-iduronic acid (IdoA), which provides internal flexibility to the polymer and forges protein-binding sites to ensure polymer function. Here we report crystal structures of human Glce in the unbound state and of an inactive mutant, as assessed by real-time NMR spectroscopy, bound with a (GlcA-GlcNS)_n substrate or a (IdoA-GlcNS)_n product. Deep infiltration of the oligosaccharides into the active site cleft imposes a sharp kink within the central GlcNS-GlcA/IdoA-GlcNS trisaccharide motif. An extensive network of specific interactions illustrates the absolute requirement of N-sulfate groups vicinal to the epimerization site for substrate binding. At the epimerization site, the GlcA/IdoA rings are highly constrained in two closely related boat conformations, highlighting ring-puckering signatures during catalysis. The structure-based mechanism involves the two invariant acid/base residues, Glu499 and Tyr578, poised on each side of the target uronic acid residue, thus allowing reversible abstraction and readdition of a proton at the C5 position through a neutral enol intermediate, reminiscent of mandelate racemase. These structures also shed light on a convergent mechanism of action between HS epimerases and lyases and provide molecular frameworks for the chemoenzymatic synthesis of heparin or HS analogs.

Microwave-assisted simultaneous O,N-sulfonation in the synthesis of heparin-like oligosaccharides

Simultaneous O,N-sulfonation of heparin-like saccharides was achieved in short reaction times and excellent yields (>90%) under microwave irradiation.

Divergent Synthesis of Heparan Sulfate Oligosaccharides

Heparan sulfates are implicated in a wide range of biological processes. A major challenge in deciphering their structure and activity relationship is the synthetic difficulties to access diverse heparan sulfate oligosaccharides with well-defined sulfation patterns. In order to expedite the synthesis, a divergent synthetic strategy was developed. By integrating chemical synthesis and two types of O-sulfo transferases, seven different hexasaccharides were obtained from a single hexasaccharide precursor. This approach combined the flexibility of chemical synthesis with the selectivity of enzyme-catalyzed sulfations, thus simplifying the overall synthetic operations. In an attempt to establish structure activity relationships of heparan sulfate binding with its receptor, the synthesized oligosaccharides were incorporated onto a glycan microarray, and their bindings with a growth factor FGF-2 were examined. The unique combination of chemical and enzymatic approaches expanded the capability of oligosaccharide synthesis. In addition, the well-defined heparan sulfate structures helped shine light on the fine substrate specificities of biosynthetic enzymes and confirm the potential sequence of enzymatic reactions in biosynthesis.

Making the longest sugars: a chemical synthesis of heparin-related [4] n oligosaccharides from 16-mer to 40-mer

The chemical synthesis of long oligosaccharides remains a major challenge. In particular, the synthesis of glycosaminoglycan (GAG) oligosaccharides belonging to the heparin and heparan sulfate (H/HS) family has been a high profile target, particularly with respect to the longer heparanome. Herein we describe a synthesis of the longest heparin-related oligosaccharide to date and concurrently provide an entry to the longest synthetic oligosaccharides of any type yet reported. Specifically, the iterative construction of a series of [4] _n -mer heparin-backbone oligosaccharides ranging from 16-mer through to the 40-mer in length is described. This demonstrates for the first time the viability of generating long sequence heparanoids by chemical synthesis, via practical solution-phase synthesis. Pure-Shift HSQC NMR provides a dramatic improvement in anomeric signal resolution, allowing full resolution of all 12 anomeric protons and extrapolation to support anomeric integrity of the longer species. A chemically pure 6-O-desfulfated GlcNS-IdoAS icosasaccharide (20-mer) represents the longest pure synthetic heparin-like oligosaccharide.

Heparin/Heparan sulfate proteoglycans glycomic interactome in angiogenesis: biological implications and therapeutical use

Angiogenesis, the process of formation of new blood vessel from pre-existing ones, is involved in various intertwined pathological processes including virus infection, inflammation and oncogenesis, making it a promising target for the development of novel strategies for various interventions. To induce angiogenesis, angiogenic growth factors (AGFs) must interact with pro-angiogenic receptors to induce proliferation, protease production and migration of endothelial cells (ECs). The action of AGFs is counteracted by antiangiogenic modulators whose main mechanism of action is to bind (thus sequestering or masking) AGFs or their receptors. Many sugars, either free or associated to proteins, are involved in these interactions, thus exerting a tight regulation of the neovascularization process. Heparin and heparan sulfate proteoglycans undoubtedly play a pivotal role in this context since they bind to almost all the known AGFs, to several pro-angiogenic receptors and even to angiogenic inhibitors, originating an intricate network of interaction, the so called "angiogenesis glycomic interactome". The decoding of the angiogenesis glycomic interactome, achievable by a systematic study of the interactions occurring among angiogenic modulators and sugars, may help to design novel antiangiogenic therapies with implications in the cure of angiogenesis-dependent diseases.

Can we produce heparin/heparan sulfate biomimetics using "mother-nature" as the gold standard?

Heparan sulfate (HS) and heparin are glycosaminoglycans (GAGs) that are heterogeneous in nature, not only due to differing disaccharide combinations, but also their sulfate modifications. HS is well known for its interactions with various growth factors and cytokines; and heparin for its clinical use as an anticoagulant. Due to their potential use in tissue regeneration; and the recent adverse events due to contamination of heparin; there is an increased surge to produce these GAGs on a commercial scale. The production of HS from natural sources is limited so strategies are being explored to be biomimetically produced via chemical; chemoenzymatic synthesis methods and through the recombinant expression of proteoglycans. This review details the most recent advances in the field of HS/heparin synthesis for the production of low molecular weight heparin (LMWH) and as a tool further our understanding of the interactions that occur between GAGs and growth factors and cytokines involved in tissue development and repair.

Synthesis of glycosaminoglycan mimetics through sulfation of polyphenols

In nearly all cases of biological activity of sulfated GAGs, the sulfate group(s) are critical for interacting with target proteins. A growing paradigm is that appropriate small, sulfated, nonsaccharide GAG mimetics can be designed to either mimic or interfere with the biological functions of natural GAG sequences resulting in the discovery of either antagonist or agonist agents. A number of times these sulfated NSGMs can be computationally designed based on the parent GAG-protein interaction. The small sulfated NSGMs may possess considerable aromatic character so as to engineer hydrophobic, hydrogen-bonding, Coulombic or cation-pi forces in their interactions with target protein(s) resulting in higher specificity of action relative to parent GAGs. The sulfated NSGMs can be easily synthesized in one step from appropriate natural polyphenols through chemical sulfation under microwave-based conditions. We describe step-by-step procedures to perform microwave-based sulfation of several small polyphenol scaffolds so as to prepare homogenous NSGMs containing one to more than 10 sulfate groups per molecule in high yields.

Synthesis and biological evaluation of a unique heparin mimetic hexasaccharide for structure-activity relationship studies

To date, the structure-activity relationship studies of heparin/heparan sulfate with their diverse binding partners such as growth factors, cytokines, chemokines, and extracellular matrix proteins have been limited yet provide early insight that specific sequences contribute to this manifold biological role. This has led to an impetus for the chemical synthesis of oligosaccharide fragments of these complex polysaccharides, which can provide an effective tool for this goal. The synthesis of three heparin mimetic hexasaccharides with distinct structural patterns is described herein, and the influence of the targeted substitution on their bioactivity profiles is studied using in vitro affinity and/or inhibition toward different growth factors and proteins. Additionally, the particularly challenging synthesis of an irregular hexasaccharide is reported, which, interestingly, in spite of being considerably structurally similar with its two counterparts, displayed a unique and remarkably distinct profile in the test assays.

New glucuronic acid donors for the modular synthesis of heparan sulfate oligosaccharides

Although hundreds of heparan sulfate (HS) binding proteins have been implicated in a myriad of physiological and pathological processes, very little information is known about ligand requirements for binding and mediating biological activities by these proteins. We report here a streamlined approach for the preparation of modular disaccharide building blocks that will facilitate the assembly of libraries of HS oligosaccharides for structure-activity relationship studies. In particular, we have found that glucuronic acid donors, which usually perform poorly in glycosylations, can give high yields of coupling products when the C-2 hydroxyl is protected with a permanent 4-acetoxy-2,2-dimethyl butanoyl- (PivOAc) or temporary levulinoyl (Lev) ester and the C-4 hydroxyl modified with a selectively removable 2-methylnaphthyl (Nap) ether. It has been shown that the PivOAc ester can be removed without affecting sulfate esters making it an ideal protecting group for HS oligosaccharide assembly. Iduronic acid donors exhibit more favorable glycosyl donating properties and a compound protected with a Lev ester at C-2 and an Fmoc function at the C-4 hydroxyl gave coupling products in high yield. The new donors avoid post-glycosylation oxidation and therefore allow the facile preparation of modular disaccharide building blocks.

Efficient synthesis of a library of heparin tri- and tetrasaccharides relevant to the substrate of heparanase

A robust glycosylation protocol was fixed to construct the GlcN–(1α→4)-GlcA/IdoA linkagesen routeto heparin oligosaccharides.

Tetrasaccharide iteration synthesis of a heparin-like dodecasaccharide and radiolabelling for in vivo tissue distribution studies

Heparin-like oligosaccharides mediate numerous important biological interactions, of which many are implicated in various diseases. Synthetic improvements are central to the development of such oligosaccharides as therapeutics and, in addition, there are no methods to elucidate the pharmacokinetics of structurally defined heparin-like oligosaccharides. Here we report an efficient two-cycle [4+4+4] tetrasaccharide-iteration-based approach for rapid chemical synthesis of a structurally defined heparin-related dodecasaccharide, combined with the incorporation of a latent aldehyde tag, unmasked in the final step of chemical synthesis, providing a generic end group for labelling/conjugation. We exploit this latent aldehyde tag for ³H radiolabelling to provide the first example of this kind of agent for monitoring in vivo tissue distribution and in vivo stability of a biologically active, structurally defined heparin related dodecasaccharide. Such studies are critical for the development of related saccharide therapeutics, and the data here establish that a biologically active, synthetic, heparin-like dodecasaccharide provides good organ distribution, and serum lifetimes relevant to developing future oligosaccharide therapeutics.

Heparin-like oligosaccharides are implicated in various diseases. Hansen et al. report an efficient two-cycle [4+4+4] tetrasaccharide-iteration-based approach to synthesize a structurally defined heparin dodecasaccharide with a latent aldehyde tag for labelling and conjugation.

Synthesis of a targeted library of heparan sulfate hexa- to dodecasaccharides as inhibitors of β-secretase: potential therapeutics for Alzheimer's disease

Heparan sulfates (HS) are a class of sulfated polysaccharides that function as dynamic biological regulators of the functions of diverse proteins. The structural basis of these interactions, however, remains elusive, and chemical synthesis of defined structures represents a challenging but powerful approach for unravelling the structure-activity relationships of their complex sulfation patterns. HS has been shown to function as an inhibitor of the β-site cleaving enzyme β-secretase (BACE1), a protease responsible for generating the toxic Aβ peptides that accumulate in Alzheimer's disease (AD), with 6-O-sulfation identified as a key requirement. Here, we demonstrate a novel generic synthetic approach to HS oligosaccharides applied to production of a library of 16 hexa- to dodecasaccharides targeted at BACE1 inhibition. Screening of this library provided new insights into structure-activity relationships for optimal BACE1 inhibition, and yielded a number of potent non-anticoagulant BACE1 inhibitors with potential for development as leads for treatment of AD through lowering of Aβ peptide levels.

Discovery of allosteric modulators of factor XIa by targeting hydrophobic domains adjacent to its heparin-binding site

To discover promising sulfated allosteric modulators (SAMs) of glycosaminoglycan-binding proteins (GBPs), such as human factor XIa (FXIa), we screened a library of 26 synthetic, sulfated quinazolin-4(3H)-ones (QAOs) resulting in the identification of six molecules that reduced the Vmax of substrate hydrolysis without influencing the KM. Mutagenesis of residues of the heparin-binding site (HBS) of FXIa introduced a nearly 5-fold loss in inhibition potency supporting recognition of an allosteric site. Fluorescence studies showed a sigmoidal binding profile indicating highly cooperative binding. Competition with a positively charged, heparin-binding polymer did not fully nullify inhibition suggesting importance of hydrophobic forces to binding. This discovery suggests the operation of a dual-element recognition process, which relies on an initial Coulombic attraction of anionic SAMs to the cationic HBS of FXIa that forms a locked complex through tight interaction with an adjacent hydrophobic patch. The dual-element strategy may be widely applicable for discovering SAMs of other GBPs.

First gram-scale synthesis of a heparin-related dodecasaccharide

The first example of a gram-scale synthesis of a structurally defined, heparin-related dodecasaccharide is reported. An iterative 14-step process using an iduronate donor disaccharide delivers >1g quantities of the dodecasaccharide sequence [GlcNS-IdoA2S](6)-OMe in 15% overall yield from the reducing terminal disaccharide, a 2 orders of magnitude increase in scale for access to synthetic heparanoid dodecasaccharide mimetics. The synthesis also delivers multigram amounts of the protected oligosaccharides from tetra- through to dodecasaccharide.

Convergent stereoselective synthesis of multiple sulfated GlcNα(1,4)GlcAβ(1,4) dodecasaccharides

In this paper, we describe an effective method for the elongation of a GlcNα(1,4)GlcAβ(1,4) sequence using a GlcNTrocα(1,4)GlcA disaccharide unit and the synthesis of the N- and/or O-sulfated GlcNα(1,4)GlcAβ(1,4) oligosaccharides. N-Troc protection of GlcNα(1,4)GlcA units was effective for the synthesis of the GlcNα(1,4)GlcAβ(1,4) oligosaccharides in comparison with the azido substituent. The GlcNα(1,4)GlcAβ(1,4) dodecasaccharide was successfully prepared by the direct β-selective glycosidation of glucuronate in the GlcNα(1,4)GlcAβ(1,4)GlcNα(1,4)GlcAβ(1,4) tetrasaccharide. In addition, the synthesis of the N- and/or O-sulfated GlcNα(1,4)GlcAβ(1,4) oligosaccharides was accomplished by fluorous-assisted deprotection and sulfation. The fluorous-assisted synthetic technology applied to the highly polar sulfated oligosaccharide permits it to be more easily separated from the highly polar reagents, such as SO(3)·NEt(3).

Synthesis of amine-functionalized heparin oligosaccharides for the investigation of carbohydrate-protein interactions in microtiter plates

The synthesis of well-defined oligosaccharides is crucial for the establishment of structure-activity relationships for specific sequences of heparin, contributing to the understanding of the biological role of this polysaccharide. It is highly convenient that the synthetic oligosaccharides contain an orthogonal functional group that allows selective conjugation of the probes and expands their use as chemical tools in glycobiology. We present here the synthesis of a series of amine-functionalized heparin oligosaccharides using an n+2 modular approach. The conditions of the glycosylation reactions were carefully optimized to produce efficiently the desired synthetic intermediates with an N-benzyloxycarbonyl-protected aminoethyl spacer at the reducing end. The use of microwave heating greatly facilitates O- and N-sulfation steps, avoiding experimental problems associated with these reactions. The synthesized oligosaccharides were immobilized in 384-well microtiter plates and successfully probed with a heparin-binding protein, the basic fibroblast growth factor FGF-2. The use of hexadecyltrimethylammonium bromide minimized the amount of sugar required for attachment to the solid support. Using this approach we quantified heparin-protein interactions, and surface dissociation constants for the synthetic heparin derivatives were determined.

Toward automated oligosaccharide synthesis

Carbohydrates have been shown to play important roles in biological processes. The pace of development in carbohydrate research is, however, relatively slow due to the problems associated with the complexity of carbohydrate structures and the lack of general synthetic methods and tools available for the study of this class of biomolecules. Recent advances in synthesis have demonstrated that many of these problems can be circumvented. In this Review, we describe the methods developed to tackle the problems of carbohydrate-mediated biological processes, with particular focus on the issue related to the development of the automated synthesis of oligosaccharides. Further applications of carbohydrate microarrays and vaccines to human diseases are also highlighted.

Heparan sulfate glycomics: towards systems biology strategies

HSs (heparan sulfates) are a complex family of cell-surface and matrix polysaccharides that have diverse biological functions, underpinned by structurally diverse patterns of backbone chain modification, especially by sulfate groups. These variant structures represent a molecular code, the 'heparanome', that confers the ability to interact selectively with a wide interactome of proteins, the 'heparactome', and thereby influence a network of cellular events. It is becoming increasingly apparent that understanding the structure-activity relationships of these enigmatic molecules requires the development of a holistic systems biology view of their structure and interactions. In the present paper, I describe some of the new tools available to realize this strategy, and discuss the future potential for the combined application of glycomics and other '-omics' approaches to define the molecular code of the heparanome.

Hijacking of the pleiotropic cytokine interferon-γ by the type III secretion system of Yersinia pestis

Yersinia pestis, the causative agent of bubonic plague, employs its type III secretion system to inject toxins into target cells, a crucial step in infection establishment. LcrV is an essential component of the T3SS of Yersinia spp, and is able to associate at the tip of the secretion needle and take part in the translocation of anti-host effector proteins into the eukaryotic cell cytoplasm. Upon cell contact, LcrV is also released into the surrounding medium where it has been shown to block the normal inflammatory response, although details of this mechanism have remained elusive. In this work, we reveal a key aspect of the immunomodulatory function of LcrV by showing that it interacts directly and with nanomolar affinity with the inflammatory cytokine IFNγ. In addition, we generate specific IFNγ mutants that show decreased interaction capabilities towards LcrV, enabling us to map the interaction region to two basic C-terminal clusters of IFNγ. Lastly, we show that the LcrV-IFNγ interaction can be disrupted by a number of inhibitors, some of which display nanomolar affinity. This study thus not only identifies novel potential inhibitors that could be developed for the control of Yersinia-induced infection, but also highlights the diversity of the strategies used by Y. pestis to evade the immune system, with the hijacking of pleiotropic cytokines being a long-range mechanism that potentially plays a key role in the severity of plague.

Preactivation-based, one-pot combinatorial synthesis of heparin-like hexasaccharides for the analysis of heparin-protein interactions

Heparin (HP) and heparan sulfate (HS) play important roles in many biological events. Increasing evidence has shown that the biological functions of HP and HS can be critically dependent upon their precise structures, including the position of the iduronic acids and sulfation patterns. However, unraveling the HP code has been extremely challenging due to the enormous structural variations. To overcome this hurdle, we investigated the possibility of assembling a library of HP/HS oligosaccharides using a preactivation-based, one-pot glycosylation method. A major challenge in HP/HS oligosaccharide synthesis is stereoselectivity in the formation of the cis-1,4-linkages between glucosamine and the uronic acid. Through screening, suitable protective groups were identified on the matching glycosyl donor and acceptor, leading to stereospecific formation of both the cis-1,4- and trans-1,4-linkages present in HP. The protective group chemistry designed was also very flexible. From two advanced thioglycosyl disaccharide intermediates, all of the required disaccharide modules for library preparation could be generated in a divergent manner, which greatly simplified building-block preparation. Furthermore, the reactivity-independent nature of the preactivation-based, one-pot approach enabled us to mix the building blocks. This allowed rapid assembly of twelve HP/HS hexasaccharides with systematically varied and precisely controlled backbone structures in a combinatorial fashion. The speed and the high yields achieved in glycoassembly without the need to use a large excess of building blocks highlighted the advantages of our approach, which can be of general use to facilitate the study of HP/HS biology. As a proof of principle, this panel of hexasaccharides was used to probe the effect of backbone sequence on binding with the fibroblast growth factor-2 (FGF-2). A trisaccharide sequence of 2-O-sulfated iduronic acid flanked by N-sulfated glucosamines was identified to be the minimum binding motif and N-sulfation was found to be critical. This provides useful information for further development of more potent compounds towards FGF-2 binding, which can have potential applications in wound healing and anticancer therapy.

Modular synthesis of heparan sulfate oligosaccharides for structure-activity relationship studies

Although hundreds of heparan sulfate binding proteins have been identified and implicated in a myriad of physiological and pathological processes, very little information is known about the ligand requirements for binding and mediating biological activities by these proteins. This difficulty results from a lack of technology for establishing structure-activity relationships, which in turn is due to the structural complexity of natural heparan sulfate (HS) and difficulties of preparing well-defined HS oligosaccharides. To address this deficiency, we developed a modular approach for the parallel combinatorial synthesis of HS oligosaccharides that utilizes a relatively small number of selectively protected disaccharide building blocks, which can easily be converted into glycosyl donors and acceptors. The utility of the modular building blocks has been demonstrated by the preparation of a library of 12 oligosaccharides, which has been employed to probe the structural features of HS for inhibiting the protease, BACE-1. The complex variations in activity with structural changes support the view that important functional information is embedded in HS sequences. Furthermore, the most active derivative provides an attractive lead compound for the preparation of more potent compounds, which may find use as a therapeutic agent for Alzheimer's disease.