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.

Enzymatic synthesis of heparan sulfate and heparin

Heparan sulfate (HS) polysaccharide chains have been shown to orchestrate distinct biological functions in several systems. Study of HS structure-function relations is, however, hampered due to the lack of availability of HS in sufficient quantities as well as the molecular heterogeneity of naturally occurring HS. Enzymatic synthesis of HS is an attractive alternative to the use of naturally occurring HS, as it reduces molecular heterogeneity, or a long and daunting chemical synthesis of HS. Heparosan, produced by E. coli K5 bacteria, has a structure similar to the unmodified HS backbone structure and can be used as a precursor in the enzymatic synthesis of HS-like polysaccharides. Here, we describe an enzymatic approach to synthesize several specifically sulfated HS polysaccharides for biological studies using the heparosan backbone and a combination of recombinant biosynthetic enzymes such as C5-epimerase and sulfotransferases.

Production of size-defined heparosan, heparan sulfate, and heparin oligosaccharides by enzymatic depolymerization

Glycosaminoglycans (GAG) are most commonly isolated as large polymers from various animal origins, the functional units of which are oligosaccharides, which bind their target proteins to induce conformational changes, compete with other ligands, or facilitate the formation of signaling complexes. One example, the extensively studied heparin pentasaccharide sequence-which binds antithrombin-III, inducing a conformational change that increases its serpin protease activity by 1,000-fold-is unique in that no other specific GAG-protein structure-function relations have been described to the same degree. Thus, production of heparan sulfate (HS) oligosaccharides is critical for obtaining specific structural information regarding the binding interactions of GAG and their ligands (typically proteins). Purely synthetic methods of oligosaccharide synthesis are possible, but the cost, time requirement, and difficulty of their preparation prohibit library synthesis in significant amounts. Herein, the use of bacterial heparin lyases for the production of HS oligosaccharides via enzymatic depolymerization of HS polymers is discussed. The separation and purification of these oligosaccharides by liquid chromatography are also described.

Chemoenzymatic synthesis of heparan sulfate and heparin

Heparan sulfate is a polysaccharide that plays essential physiological functions in the animal kingdom. Heparin, a highly sulfated form of heparan sulfate, is a widely prescribed anticoagulant drug worldwide. The heparan sulfate and heparin isolated from natural sources are highly heterogeneous mixtures differing in their polysaccharide chain lengths and sulfation patterns. The access to structurally defined heparan sulfate and heparin is critical to probe the contribution of specific sulfated saccharide structures to the biological functions as well as for the development of the next generation of heparin-based anticoagulant drugs. The synthesis of heparan sulfate and heparin, using a purely chemical approach, has proven extremely difficult, especially for targets larger than octasaccharides having a high degree of site-specific sulfation. A new chemoenzymatic method has emerged as an effective alternative approach. This method uses recombinant heparan sulfate biosynthetic enzymes combined with unnatural uridine diphosphate-monosaccharide donors. Recent examples demonstrate the successful synthesis of ultra-low molecular weight heparin, low-molecular weight heparin and bioengineered heparin with unprecedented efficiency. The new method provides an opportunity to develop improved heparin-based therapeutics.

Preparation of heparan sulfate-like polysaccharide and application in stem cell chondrogenic differentiation

Heparan sulfate is a component of the extracellular matrix (ECM) that modulates individual development and cell growth through its interaction with growth factors. Structurally, heparan sulfate consists of repeating linear sulfated poly-anionic disaccharide structures. The K5 polysaccharide has the same structure as heparosan, and is the capsular polysaccharide of Escherichia coli K5 strain which serves as a precursor in heparin and heparan sulfate biosynthesis. Here, we prepared sulfated K5 polysaccharides that are structurally similar to heparan sulfate and investigated their biocompatibility and bioactivity in stem cell chondrogenic differentiation. Briefly, sulfation groups were added to -NH- and/or -OH of a precursor heparosan and the modified heparosan was qualitatively analyzed by FT-IR, (1)H NMR, and (13)C NMR techniques. Cell viability was not significantly affected by the sulfated K5 capsular polysaccharide. Relative mRNA expression of the chondrogenic differentiation marker COL2A1 was significantly upregulated in cells treated with the N,O-sulfated K5 polysaccharide confirming that the sulfated K5 capsular polysaccharide is able to stimulate chondrogenic differentiation. The main sulfation pattern for chondrogenic activity is N,6-O sulfation and the activity was not proportional to the sulfation level. This type of mimic was prepared in nearly a gram scale, supporting further structural study and 3 dimension stem cell culture. Together, the results of this study show that sulfated K5 capsular polysaccharides are able to stimulate chondrogenic differentiation without affecting cell viability.

Fluorous-assisted chemoenzymatic synthesis of heparan sulfate oligosaccharides

The chemoenzymatic synthesis of heparan sulfate tetrasaccharide (1) and hexasaccharide (2) with a fluorous tag attached at the reducing end is reported. The fluorous tert-butyl dicarbonate ((F)Boc) tag did not interfere with enzymatic recognition for both elongation and specific sulfation, and flash purification was performed by standard fluorous solid-phase extraction (FSPE). Based on an (F)Boc attached disaccharide as acceptor, a series of partial N-sulfated, 6-O-sulfated heparan sulfate oligosaccharides were successfully synthesized employing fluorous techniques.

Isolation of low-molecular-weight heparin/heparan sulfate from marine sources

The glycosaminoglycan (heparin and heparan sulfate) are polyanionic sulfated polysaccharides mostly recognized for its anticoagulant activity. In many countries, low-molecular-weight heparins have replaced the unfractionated heparin, owing to its high bioavailability, half-life, and less adverse effect. The low-molecular-weight heparins differ in mode of preparation (chemical or enzymatic synthesis and chromatography fractionations) and as a consequence in molecular weight distribution, chemical structure, and pharmacological activities. Bovine and porcine body parts are at present used for manufacturing of commercial heparins, and the appearance of mad cow disease and Creutzfeldt-Jakob disease in humans has limited the use of bovine heparin. Consequently, marine organisms come across the new resource for the production of low-molecular-weight heparin and heparan sulfate. The importance of this chapter suggests that the low-molecular-weight heparin and heparan sulfate from marine species could be alternative sources for commercial heparin.

Fluorous supported modular synthesis of heparan sulfate oligosaccharides

The modular synthesis of heparan sulfate fragments is greatly facilitated by employing an anomeric aminopentyl linker protected by a benzyloxycarbonyl group modified by a perfluorodecyl tag, which made it possible to purify highly polar intermediates by fluorous solid phase extraction. This tagging methodology made it also possible to perform repeated glycosylations to drive reactions to completion.

Synthetic heparan sulfate oligosaccharides inhibit endothelial cell functions essential for angiogenesis

Background

Heparan sulfate (HS) is an important regulator of the assembly and activity of various angiogenic signalling complexes. However, the significance of precisely defined HS structures in regulating cytokine-dependent angiogenic cellular functions and signalling through receptors regulating angiogenic responses remains unclear. Understanding such structure-activity relationships is important for the rational design of HS fragments that inhibit HS-dependent angiogenic signalling complexes.

Methodology/Principal Findings

We synthesized a series of HS oligosaccharides ranging from 7 to 12 saccharide residues that contained a repeating disaccharide unit consisting of iduronate 2-O-sulfate linked to glucosamine with or without N-sulfate. The ability of oligosaccharides to compete with HS for FGF2 and VEGF₁₆₅ binding significantly increased with oligosaccharide length and sulfation. Correspondingly, the inhibitory potential of oligosaccharides against FGF2- and VEGF₁₆₅-induced endothelial cell responses was greater in longer oligosaccharide species that were comprised of disaccharides bearing both 2-O- and N-sulfation (2SNS). FGF2- and VEGF₁₆₅-induced endothelial cell migration were inhibited by longer 2SNS oligosaccharide species with 2SNS dodecasaccharide activity being comparable to that of receptor tyrosine kinase inhibitors targeting FGFR or VEGFR-2. Moreover, the 2SNS dodecasaccharide ablated FGF2- or VEGF₁₆₅-induced phosphorylation of FAK and assembly of F-actin in peripheral lamellipodia-like structures. In contrast, FGF2-induced endothelial cell proliferation was only moderately inhibited by longer 2SNS oligosaccharides. Inhibition of FGF2- and VEGF₁₆₅-dependent endothelial tube formation strongly correlated with oligosaccharide length and sulfation with 10-mer and 12-mer 2SNS oligosaccharides being the most potent species. FGF2- and VEGF₁₆₅-induced activation of MAPK pathway was inhibited by biologically active oligosaccharides correlating with the specific phosphorylation events in FRS2 and VEGFR-2, respectively.

Conclusion/Significance

These results demonstrate structure-function relationships for synthetic HS saccharides that suppress endothelial cell migration, tube formation and signalling induced by key angiogenic cytokines.

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.

Using an enzymatic combinatorial approach to identify anticoagulant heparan sulfate structures

Heparan sulfate (HS) represents a major class of glycans that perform central physiological functions. Emerging HS and glycosaminoglycan microarray techniques are used to interrogate the structure and function relationship to develop novel therapeutic agents. Availability of HS with specific sulfation patterns has been a limiting factor and impedes the accuracy of HS glycomics studies. Although organic synthesis provides oligosaccharides, these may not fully represent the biological functions of polysaccharides. Here, we present a study for developing an enzyme-based approach to synthesize a polysaccharide library with different sulfation patterns. Using different combinations of biosynthetic enzymes, we synthesized eight unique polysaccharides. We discovered that polysaccharides without the iduronic acid residue displayed strong binding affinity to antithrombin and high anti-Xa and anti-IIa activities. The enzyme-based synthetic approach could become a general method for discovering new HS structures with unique biological functions.

Combinatorial enzymatic synthesis of heparan sulfate

Escherichia coli K5 heparosan was enzymatically modified by Chen and colleagues to construct a library of heparan sulfate polysaccharides for evaluation, leading to the discovery that a 2-O-sulfoiduronic acid residue is not essential for antithrombin-mediated anticoagulant activity in larger oligosaccharide and polysaccharide structures.

Orthogonal sulfation strategy for synthetic heparan sulfate ligands

[reaction: see text] An orthogonal sulfation strategy involving six different protecting groups has been developed for generating sulfated carbohydrate libraries based on heparan. Chemoselective cleavage conditions (optimized for a heparan disaccharide) can be performed in the presence of sulfate esters as well as the remaining protecting groups.

Heparin Derivatives as Inhibitors of BACE-1, the Alzheimer's β-Secretase, with Reduced Activity against Factor Xa and Other Proteases

Heparan sulfate (HS) regulates processing of the amyloid precursor protein by the Alzheimer's beta-secretase (BACE-1). An HS analogue, porcine intestinal mucosal heparin, was systematically modified at the principal positions of O-sulfation and N-sulfation/acetylation and tested for BACE-1 inhibitory and anti factor Xa activities. The derivative with the highest anti-BACE-1 to anti-Xa activity ratio contained N-acetyl and 2-O- and 6-O-sulfates and also exhibited attenuated activities against cathepsin-D and renin, two other structurally related aspartyl proteases.

From D-glucose to biologically potent L-hexose derivatives: synthesis of alpha-L-iduronidase fluorogenic detector and the disaccharide moieties of bleomycin A2 and heparan sulfate

A novel and convenient route for the synthesis of biologically potent and rare L-hexose derivatives from D-glucose is described. Conversion of diacetone-alpha-D-glucose (14) into 1,2:3,5-di-O-isopropylidene-beta-L-idofuranose (19) was efficiently carried out in two steps. Orthogonal isopropylidene rearrangement of compound 19 led to 1,2:5,6-di-O-isopropylidene-beta-L-idofuranose (27), which underwent regioselective epimerization at the C3 position to give the L-talo- and 3-functionalized L-idofuranosyl derivatives. Hydrolysis of compound 19 under acidic conditions furnished 1,6-anhydro-beta-L-idopyranose (35) in excellent yield, which was successfully transformed into the corresponding L-allo, L-altro, L-gulo, and L-ido derivatives via regioselective benzylation, benzoylation, triflation and nucleophilic substitution as the key steps. Applications of these 1,6-anhydro-beta-L-hexopyranoses as valuable building blocks to the syntheses of 4-methylcoumarin-7-yl-alpha-L-iduronic acid and the disaccharide moieties of bleomycin A(2) as well as heparan sulfate are highlighted.

New set of orthogonal protecting groups for the modular synthesis of heparan sulfate fragments

Six strategically chosen monosaccharide building blocks, which are protected by a novel set of four orthogonal protecting groups (Lev, Fmoc, TBDPS, and All), can be employed for the efficient synthesis of the 20 disaccharide moieties found in heparan sulfate. The properly protected disaccharide building blocks can be converted into glycosyl donors and acceptors, which can be used for the modular synthesis of a wide range of well-defined oligosaccharides that differ in sulfation pattern. [structure: see text]

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

We have recently described the preparation of three building blocks for the combinatorial synthesis of heparan sulfate (HS) fragments. Herein we show that one of these building blocks (disaccharide 4) allows the preparation, in high yields and with total alpha stereoselectivity, of tetra-, hexa- and octasaccharides from the heparin (HP) regular region, by using 2+2, 2+4 and 4+4 glycosylation strategies, respectively. These oligosaccharides were processed into sulfated derivatives bearing an allyl moiety in the anomeric position. The UV-promoted conjugation of these compounds with alpha,omega-bis(thio)poly(ethylene glycol) spacers of three different lengths allowed us to prepare nine benzylated glycoconjugates. After final deprotection, the glycoconjugates 1 a-c, 2 a-c and 3 a-c were obtained and their ability to inhibit the interaction between IFN-gamma and HP was tested by using surface plasmon resonance detection. Compound 3 b, containing two HP octasaccharides linked by a 50-A linker was able to inhibit the IFN-gamma/HP interaction with an IC(50) value of approximately 35 nM. In addition, the nine glycoconjugates were perfect tools in the study to ascertain the topology of the IFN-gamma binding site on HS. Compounds 1 a-c, 2 a-c and 3 a-c, by mimicking the alternating sulfated and nonsulfated regions found in HS, thus comprise the first example of a library of synthetic HS mimetics giving access to the "second level of molecular diversity" found in HS.

Highly Diverse Heparan Sulfate Analogue Libraries: Providing Access to Expanded Areas of Sequence Space for Bioactivity Screening

Structurally diverse heparan sulfate analogue libraries were produced chemicoenzymatically from heparin. They possess vastly more heterogeneity than tissue heparan sulfates, expand the sequence space available for screening, and can help identify minimal structural features associated with activity. Library components are likely to exhibit fewer nonspecific interactions and side-effects than heparin or simple chemically modified heparin. A strategy for their use is illustrated for the fibroblast growth factor-receptor tyrosine kinase signaling system.

Use of Cerny Epoxides for the Accelerated Synthesis of Glycosaminoglycans

[reaction: see text]. 1,6:2,3-Dianhydrohexopyranoses (Cerny epoxides) are versatile intermediates for the synthesis of glycosaminoglycans. Complex heparan and chondroitin sulfate disaccharide synthons can be assembled from a single common precursor in a short sequence of steps.

Recent chemical and enzymatic approaches to the synthesis of glycosaminoglycan oligosaccharides

Glycosaminoglycans, highly charged polycarboxylated, polysulfated polysaccharides, are an important class of therapeutic agents and investigational drug candidates. Heparin has been widely used as a clinical anticoagulant for over 60 years. Low molecular weight heparins have begun to displace heparin and recently a synthetic heparin pentasaccharide was approved for clinical use in Europe. In addition to heparin (and the related heparan sulfate glycosaminoglycan), dermatan sulfate, chondroitin sulfate, hyaluronan and their derivatives are all in various stages of clinical evaluation. This review focuses on the chemical and chemoenzymatic synthesis of glycosaminoglycan oligosaccharides. Recent advances in functional group protection chemistry, conversion of D-gluco to L-ido or D-galacto configurations, glycosylation reactions and the preparation and use of novel starting materials in acidic oligosaccharide synthesis are discussed.

A novel generation of heparan sulfate mimetics for the treatment of prion diseases

The accumulation of PrP(res), the protease-resistant abnormal form of the host-encoded cellular prion protein, PrP(C), plays a central role in transmissible spongiform encephalopathies. Human contamination by bovine spongiform encephalopathy (BSE) has propelled many scientific teams on a highway for anti-prion drug development. This study reports that heparan sulfate mimetics (HMs), developed originally for their effect on tissue regeneration, abolish prion propagation in scrapie-infected GT1 cells. PrP(res) does not reappear for up to 50 days post-treatment. When tested in vivo, one of these compounds, HM2602, hampered PrP(res) accumulation in scrapie- and BSE-infected mice and prolonged significantly the survival time of 263K scrapie-infected hamsters. Interestingly, HM2602 is an apparently less toxic and more potent inhibitor of PrP(res) accumulation than dextran sulfate 500, a molecule known to exhibit anti-prion properties in vivo. Kinetics of PrP(res) disappearance in vitro and unaffected PrP(C) levels during treatment suggest that HMs are able to block the conversion of PrP(C) into PrP(res). It is speculated that HMs act as competitors of endogenous heparan sulfates known to act as co-receptors for the prion protein. Since these molecules are particularly amenable to drug design, their anti-prion potential could be developed further and optimized for the treatment of prion diseases.

A synthetic heparan sulfate pentasaccharide, exclusively containing L-iduronic acid, displays higher affinity for FGF-2 than its D-glucuronic acid-containing isomers

It has been suggested that the FGF-2 binding site on heparan sulfate chains is a trisulfated pentasaccharide containing three hexuronic acid units. The configuration at C-5 of two of them being undetermined, we have synthesized the four possible pentasaccharides, and have evaluated their FGF-2 binding affinity through in vitro biological assays. The pentasaccharide containing L-iduronic acid as the sole hexuronic acid showed higher affinity for FGF-2 than the other pentasaccharides, where one hexuronic acid unit at least is D-glucuronic acid.

Approaches to the synthesis of heparin

The biosynthesis of heparin is initiated by formation of [GlcA-GlcNAc]n polysaccharide chains linked to the core protein of a proteoglycan structure. The polymer is transformed into the mature polysaccharide by a series of modification reactions which involve N-deacetylation and N-sulfation of GlcNAc units, C5 epimerization of GlcA to IdoA residues, and O-sulfation at different positions. Incomplete modification, controlled in part by the substrate specificities of the corresponding enzymes, provides the complex saccharide sequences that are typical for heparin and, in particular, for heparan sulfate. One such structure is the antithrombin-binding region which is comprised by a specific pentasaccharide sequence with a 3-O-sulfated GlcN marker group. Aspects of regulation of polymer modification are discussed.