Strategies in synthesis of heparin/heparan sulfate oligosaccharides: 2000-present

Glycosaminoglycan microarrays for studying glycosaminoglycan-protein systems

More than 3000 proteins are now known to bind to glycosaminoglycans (GAGs). Yet, GAG-protein systems are rather poorly understood in terms of selectivity of recognition, molecular mechanism of action, and translational promise. High-throughput screening (HTS) technologies are critically needed for studying GAG biology and developing GAG-based therapeutics. Microarrays, developed within the past two decades, have now improved to the point of being the preferred tool in the HTS of biomolecules. GAG microarrays, in which GAG sequences are immobilized on slides, while similar to other microarrays, have their own sets of challenges and considerations. GAG microarrays are rapidly becoming the first choice in studying GAG-protein systems. Here, we review different modalities and applications of GAG microarrays presented to date. We discuss advantages and disadvantages of this technology, explain covalent and non-covalent immobilization strategies using different chemically reactive groups, and present various assay formats for qualitative and quantitative interpretations, including selectivity screening, binding affinity studies, competitive binding studies etc. We also highlight recent advances in implementing this technology, cataloging of data, and project its future promise. Overall, the technology of GAG microarray exhibits enormous potential of evolving into more than a mere screening tool for studying GAG - protein systems.

Recent advances in the synthesis of extensive libraries of heparan sulfate oligosaccharides for structure-activity relationship studies

Heparan sulfate (HS) is a linear, sulfated and highly negatively-charged polysaccharide that plays important roles in many biological events. As a member of the glycosaminoglycan (GAG) family, HS is commonly found on mammalian cell surfaces and within the extracellular matrix. The structural complexities of natural HS polysaccharides have hampered the comprehension of their biological functions and structure-activity relationships (SARs). Although the sulfation patterns and backbone structures of HS can be major determinants of their biological activities, obtaining significant amounts of pure HS from natural sources for comprehensive SAR studies is challenging. Chemical and enzyme-based synthesis can aid in the production of structurally well-defined HS oligosaccharides. In this review, we discuss recent innovations enabling the syntheses of large libraries of HS and how these libraries can provide insights into the structural preferences of various HS binding proteins.

Chemical Synthesis of Δ-4,5 Unsaturated Heparan Sulfate Oligosaccharides for Biomarker Discovery

A methodology is described that can provide heparan sulfate oligosaccharides having a Δ4,5-double bond, which are needed as analytical standards and biomarkers for mucopolysaccharidoses. It is based on chemical oligosaccharide synthesis followed by modification of the C-4 hydroxyl of the terminal uronic acid moiety as methanesulfonate. This leaving group is stable under conditions used to remove temporary protecting groups, O-sulfation, and hydrogenolysis. Treatment with NaOH results in elimination of the methanesulfonate and formation of a Δ4,5-double bond.

Sulfation of Heparan and Chondroitin Sulfate Ligands Enables Cell-Specific Homing of Nanoprobes

Demystifying the sulfation code of glycosaminoglycans (GAGs) to induce precise homing of nanoparticles in tumor cells or neurons influences the development of a potential drug- or gene-delivery system. However, GAGs, particularly heparan sulfate (HS) and chondroitin sulfate (CS), are structurally highly heterogeneous, and synthesizing well-defined HS/CS composed nanoparticles is challenging. Here, we decipher how specific sulfation patterns on HS and CS regulate receptor-mediated homing of nanoprobes in primary and secondary cells. We discovered that aggressive cancer cells such as MDA-MB-231 displayed a strong uptake of GAG-nanoprobes compared to mild or moderately aggressive cancer cells. However, there was no selectivity towards the GAG sequences, thus indicating the presence of more than one form of receptor-mediated uptake. However, U87 cells, olfactory bulb, and hippocampal primary neurons showed selective or preferential uptake of CS-E-coated nanoprobes compared to other GAG-nanoprobes. Furthermore, mechanistic studies revealed that the 4,6-O-disulfated-CS nanoprobe used the CD44 and caveolin-dependent endocytosis pathway for uptake. These results could lead to new opportunities to use GAG nanoprobes in nanomedicine.

Synthesis of a Systematic 64-Membered Heparan Sulfate Tetrasaccharide Library

Heparan sulfate (HS) has multifaceted biological activities. To date, no libraries of HS oligosaccharides bearing systematically varied sulfation structures are available owing to the challenges in synthesizing a large number of HS oligosaccharides. To overcome the obstacles and expedite the synthesis, a divergent approach was designed, where 64 HS tetrasaccharides covering all possible structures of 2-O-, 6-O- and N-sulfation with the glucosamine-glucuronic acid-glucosamine-iduronic acid backbone were successfully produced from a single strategically protected tetrasaccharide intermediate. This extensive library helped identify the structural requirements for HS sequences to have strong fibroblast growth factor-2 binding but a weak affinity for platelet factor-4. Such a strategy to separate out these two interactions could lead to new HS-based potential therapeutics without the dangerous adverse effect of heparin-induced thrombocytopenia.

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.

State-of-the-art glycosaminoglycan characterization

Glycosaminoglycans (GAGs) are heterogeneous acidic polysaccharides involved in a range of biological functions. They have a significant influence on the regulation of cellular processes and the development of various diseases and infections. To fully understand the functional roles that GAGs play in mammalian systems, including disease processes, it is essential to understand their structural features. Despite having a linear structure and a repetitive disaccharide backbone, their structural analysis is challenging and requires elaborate preparative and analytical techniques. In particular, the extent to which GAGs are sulfated, as well as variation in sulfate position across the entire oligosaccharide or on individual monosaccharides, represents a major obstacle. Here, we summarize the current state-of-the-art methodologies used for GAG sample preparation and analysis, discussing in detail liquid chromatograpy and mass spectrometry-based approaches, including advanced ion activation methods, ion mobility separations and infrared action spectroscopy of mass-selected species.

Surface charge changes in spike RBD mutations of SARS-CoV-2 and its variant strains alter the virus evasiveness via HSPGs: A review and mechanistic hypothesis

With the COVID-19 pandemic continuing, more contagious SARS-CoV-2 variants, including Omicron, have been emerging. The mutations, especially those that occurred on the spike (S) protein receptor-binding domain (RBD), are of significant concern due to their potential capacity to increase viral infectivity, virulence, and breakthrough antibodies' protection. However, the molecular mechanism involved in the pathophysiological change of SARS-CoV-2 mutations remains poorly understood. Here, we summarized 21 RBD mutations and their human angiotensin-converting enzyme 2 (hACE2) and/or neutralizing antibodies' binding characteristics. We found that most RBD mutations, which could increase surface positive charge or polarity, enhanced their hACE2 binding affinity and immune evasion. Based on the dependence of electrostatic interaction of the epitope residue of virus and docking protein (like virus receptors or antibodies) for its invasion, we postulated that the charge and/or polarity changes of novel mutations on the RBD domain of S protein could affect its affinity for the hACE2 and antibodies. Thus, we modeled mutant S trimers and RBD-hACE2 complexes and calculated their electrotactic distribution to study surface charge changes. Meanwhile, we emphasized that heparan sulfate proteoglycans (HSPGs) might play an important role in the hACE2-mediated entry of SARS-CoV-2 into cells. Those hypotheses provide some hints on how SARS-CoV-2 mutations enhance viral fitness and immune evasion, which may indicate potential ways for drug design, next-generation vaccine development, and antibody therapies.

Excavating proteoglycan structure-function relationships: Modern approaches to capture the interactions of ancient biomolecules

Proteoglycans are now well regarded as key facilitators of cell biology. While a majority of their interactions and functions are attributed to the decorating glycosaminoglycan chains, there is a growing appreciation for the roles of the proteoglycan core protein and for considering proteoglycans as replete protein-glycan conjugates. This appreciation, seeded by early work in proteoglycan biology, is now being advanced and exalted by modern approaches in chemical glycobiology. In this review, we discuss up-and-coming methods to unearth the fine-scale architecture of proteoglycans that modulate their functions and interactions. Crucial to these efforts is the production of chemically defined materials, including semi-synthetic proteoglycans and the in situ capture of interacting proteins. Together, the integration of chemical biology approaches promises to expedite the dissection of the structural heterogeneity of proteoglycans and deliver refined insight into their functions.

Automated Solid Phase Assisted Synthesis of a Heparan Sulfate Disaccharide Library

Heparan sulfate (HS) regulates a wide range of biological events, including blood coagulation, cancer development, cell differentiation, and viral infections. It is generally recognized that structures of HS can critically...

Discovery of rare sulfated N-unsubstituted glucosamine based heparan sulfate analogs selectively activating chemokines

Achieving selective inhibition of chemokines with structurally well-defined heparan sulfate (HS) oligosaccharides can provide important insights into cancer cell migration and metastasis. However, HS is highly heterogeneous in chemical composition, which limits its therapeutic use. Here, we report the rational design and synthesis of N-unsubstituted (NU) and N-acetylated (NA) heparan sulfate tetrasaccharides that selectively inhibit structurally homologous chemokines. HS analogs were produced by divergent synthesis, where fully protected HS tetrasaccharide precursor was subjected to selective deprotection and regioselectively O-sulfated, and O-phosphorylated to obtain 13 novel HS tetrasaccharides. HS microarray and SPR analysis with a wide range of chemokines revealed the structural significance of sulfation patterns and NU domain in chemokine activities for the first time. Particularly, HT-3,6S-NH revealed selective recognition by CCL2 chemokine. Further systematic interrogation of the role of HT-3,6S-NH in cancer demonstrated an effective blockade of CCL2 and its receptor CCR2 interactions, thereby impairing cancer cell proliferation, migration and invasion, a step towards designing novel drug molecules.

Glycosaminoglycans: Carriers and Targets for Tailored Anti-Cancer Therapy

The tumor microenvironment (TME) is composed of cancerous, non-cancerous, stromal, and immune cells that are surrounded by the components of the extracellular matrix (ECM). Glycosaminoglycans (GAGs), natural biomacromolecules, essential ECM, and cell membrane components are extensively altered in cancer tissues. During disease progression, the GAG fine structure changes in a manner associated with disease evolution. Thus, changes in the GAG sulfation pattern are immediately correlated to malignant transformation. Their molecular weight, distribution, composition, and fine modifications, including sulfation, exhibit distinct alterations during cancer development. GAGs and GAG-based molecules, due to their unique properties, are suggested as promising effectors for anticancer therapy. Considering their participation in tumorigenesis, their utilization in drug development has been the focus of both industry and academic research efforts. These efforts have been developing in two main directions; (i) utilizing GAGs as targets of therapeutic strategies and (ii) employing GAGs specificity and excellent physicochemical properties for targeted delivery of cancer therapeutics. This review will comprehensively discuss recent developments and the broad potential of GAG utilization for cancer therapy.

Chemical synthesis of human syndecan-4 glycopeptide bearing O-, N-sulfation and multiple aspartic acids for probing impacts of the glycan chain and the core peptide on biological functions

Proteoglycans are a family of complex glycoproteins with glycosaminoglycan chains such as heparan sulfate (HS) attached to the core protein backbone. Due to the high structural heterogeneity of HS in nature, it is challenging to decipher the respective roles of the HS chain and the core protein on proteoglycan functions. While the sulfation patterns of HS dictate many activities, the core protein can potentially impact HS functions. In order to decipher this, homogeneous proteoglycan glycopeptides are needed. Herein, we report the first successful synthesis of proteoglycan glycopeptides bearing multiple aspartic acids in the core peptide and O- and N-sulfations in the glycan chain, as exemplified by the syndecan-4 glycopeptides. To overcome the high acid sensitivities of sulfates and base sensitivities of the glycopeptide during synthesis, a new synthetic approach has been developed to produce a sulfated glycan chain on a peptide sequence prone to the formation of aspartimide side products. The availability of the structurally well-defined synthetic glycopeptide enabled the investigation of their biological functions including cytokine, growth factor binding and heparanase inhibition. Interestingly, the glycopeptide exhibited context dependent enhancement or decrease of biological activities compared to the peptide or the glycan alone. The results presented herein suggest that besides varying the sulfation patterns of HS, linking the HS chain to core proteins as in proteoglycans may be an additional approach to modulate biological functions of HS in nature.

Novel Insights into the Existence of the Putative UDP-Glucuronate 5-Epimerase Specificity

C5-epimerases are promising tools for the production of rare l-hexoses from their more common d-counterparts. On that account, UDP-glucuronate 5-epimerase (UGA5E) attracts attention as this enzyme could prove to be useful for the synthesis of UDP-l-iduronate. Interestingly, l-iduronate is known as a precursor for the production of heparin, an effective anticoagulant. To date, the UGA5E specificity has only been detected in rabbit skin extract, and the respective enzyme has not been characterized in detail or even identified at the molecular level. Accordingly, the current work aimed to shed more light on the properties of UGA5E. Therefore, the pool of putative UGA5Es present in the UniProt database was scrutinized and their sequences were clustered in a phylogenetic tree. However, the examination of two of these enzymes revealed that they actually epimerize UDP-glucuronate at the 4- rather than 5-position. Furthermore, in silico analysis indicated that this should be the case for all sequences that are currently annotated as UGA5E and, hence, that such activity has not yet been discovered in nature. The detected l-iduronate synthesis in rabbit skin extract can probably be assigned to the enzyme chondroitin-glucuronate C5-epimerase, which catalyzes the conversion of d-glucuronate to l-iduronate on a polysaccharide level.

Chemoenzymatic Synthesis of Heparan Sulfate Mimetic Glycopolymers and Their Interactions with the Receptor for Advanced Glycation End-Product

Heparan sulfate (HS) is a sulfated polysaccharide presenting on the animal cell surface and in the extracellular matrix that plays a critical role in a range of different biological processes. The receptor for advanced glycation end-products (RAGE), as a major receptor linked to numerous diseases, can interact with HS to activate signaling of RAGE ligands in the disease process. We herein report a concise chemoenzymatic approach for the synthesis of HS-mimetic glycopolymers as RAGE antagonists. Glycopolymer with GlcA units was chemically synthesized and subsequently modified by enzymatic elongation and chemical sulfation. Six HS-mimetic glycopolymers with pendant sulfated di-, tri-, or tetrasaccharides were successfully prepared for evaluation of their interaction with RAGE. Upon evaluation, the HS-mimetic glycopolymer decorated with overall sulfated trisaccharides was determined to have the highest binding affinity with RAGE. These findings would promote the further exploration of HS-RAGE interactions and the development of RAGE antagonists.

Automated solution-phase syntheses of alpha 1 → 2, 1 → 3 type rhamnans and rhamnan sulfate fragments

Rhamnan and rhamnan sulfate are naturally occurring carbohydrates that have important biological functions and possible therapeutic applications, but studies are limited to the microheterogeneous mixtures from natural sources. This work reports the first synthesis of any sulfated rhamnan fragments and successful automation of the process with a recently developed automated solution-phase approach using N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf) promotor and levulinoyl ester deprotection conditions. The automated solution-phase activation/deprotection approach was initially able to create alpha 1 → 2, 1 → 3 type rhamnan di- and trisaccharide in moderate yields. Once these targets were achieved, a process to use SO₃•pyridine complex in DMF for sulfation compatible with an automated solution-phase liquid handling system was developed and successfully applied to carbohydrate sulfation to create two rhamnan sulfate fragments with differing monosulfation patterns.

Concise chemoenzymatic synthesis of heparan sulfate analogues as potent BACE-1 inhibitors

Heparan sulfate (HS) and heparin, representative members of the glycosaminoglycans, possess distinct biological functions in terms of their specific interactions with hundreds of binding proteins. However, the structural properties of HS and heparin are complex due to their variable repeating motifs, different chain lengths and sulfation patterns. A concise chemoenzymatic approach has been developed to obtain well-defined low molecular weight (LMW) HS analogues. Pasteurella multocida heparosan synthase-2 (PmHS2) was utilized to fabricate the HS backbones with controllable chain lengths ranging from 14mer to 26mer. Moreover, regioselective and overall sulfation were conducted by chemical approach. The persulfated HS analogues exhibited more potent beta-site amyloid precursor protein (APP)-cleaving enzyme-1 (BACE-1) inhibitory activity than heparin and enoxaparin, and enhanced BACE-1 inhibitions were also found with the increasing molecular size of the HS analogues. This approach supplies the promising LMW HS analogues for the potential development of novel anti-Alzheimer's drugs.

Design, synthesis, and evaluation of heparan sulfate mimicking glycopolymers for inhibiting heparanase activity

Heparanase is an enzyme which cleaves heparan sulfate (HS) polysaccharides of the extracellular matrix. It is a regulator of tumor behavior, plays a key role in kidney related diseases and autoimmune diabetes. We report herein the use of computational studies to extract the natural HS-heparanase interactions as a template for the design of HS mimicking glycopolymers. Upon evaluation, a glycopolymer with 12 repeating units was determined to be the most potent inhibitor and to have tight-binding characteristics. This glycopolymer also lacks anticoagulant activity.

Replacement of the L-iduronic acid unit of the anticoagulant pentasaccharide idraparinux by a 6-deoxy-L-talopyranose - Synthesis and conformational analysis

One critical part of the synthesis of heparinoid anticoagulants is the creation of the L-iduronic acid building block featured with unique conformational plasticity which is crucial for the anticoagulant activity. Herein, we studied whether a much more easily synthesizable sugar, the 6-deoxy-L-talose, built in a heparinoid oligosaccharide, could show a similar conformational plasticity, thereby can be a potential substituent of the L-idose. Three pentasaccharides related to the synthetic anticoagulant pentasaccharide idraparinux were prepared, in which the L-iduronate was replaced by a 6-deoxy-L-talopyranoside unit. The talo-configured building block was formed by C4 epimerisation of the commercially available L-rhamnose with high efficacy at both the monosaccharide and the disaccharide level. The detailed conformational analysis of these new derivatives, differing only in their methylation pattern, was performed and the conformationally relevant NMR parameters, such as proton-proton coupling constants and interproton distances were compared to the corresponding ones measured in idraparinux. The lack of anticoagulant activity of these novel heparin analogues could be explained by the biologically not favorable 1C4 chair conformation of their 6-deoxy-L-talopyranoside residues.

Binding and neurotoxicity mitigation of toxic tau oligomers by synthetic heparin like oligosaccharides

Well-defined heparin like oligosaccharides up to decasaccharides were synthesized. It was discovered for the first time that heparin oligosaccharides, as short as tetrasaccharides, can bind with the most toxic tau species, i.e., tau oligomers with nM KD. The binding significantly reduced the cellular uptake of toxic tau oligomers and protected the cells from tau oligomer induced cytotoxicity.

Chemically Precise Glycoengineering Improves Human Insulin

Diabetes is a leading cause of death worldwide and results in over 3 million annual deaths. While insulin manages the disease well, many patients fail to comply with injection schedules, and despite significant investment, a more convenient oral formulation of insulin is still unavailable. Studies suggest that glycosylation may stabilize peptides for oral delivery, but the demanding production of homogeneously glycosylated peptides has hampered transition into the clinic. We report here the first total synthesis of homogeneously glycosylated insulin. After characterizing a series of insulin glycoforms with systematically varied O-glycosylation sites and structures, we demonstrate that O-mannosylation of insulin B-chain Thr27 reduces the peptide's susceptibility to proteases and self-association, both critical properties for oral dosing, while maintaining full activity. This work illustrates the promise of glycosylation as a general mechanism for regulating peptide activity and expanding its therapeutic use.

Glycosidase Inhibition by Multivalent Presentation of Heparan Sulfate Saccharides on Bottlebrush Polymers

We report herein the first-time exploration of the attachment of well-defined saccharide units onto a synthetic polymer backbone for the inhibition of a glycosidase. More specifically, glycopolymers endowed with heparan sulfate (HS) disaccharides were established to inhibit the glycosidase, heparanase, with an IC50 value in the low nanomolar range (1.05 ± 0.02 nm), a thousand-fold amplification over its monovalent counterpart. The monomeric moieties of these glycopolymers were designed in silico to manipulate the well-established glycotope of heparanase into an inhitope. Studies concluded that (1) the glycopolymers are hydrolytic stable toward heparanase, (2) longer polymer length provides greater inhibition, and (3) increased local saccharide density (monoantennary vs diantennary) is negligible due to hindered active site of heparanase. Furthermore, HS oligosaccharide and polysaccharide controls illustrate the enhanced potency of a multivalent scaffold. Overall, the results on these studies of the multivalent presentation of saccharides on bottlebrush polymers serve as the platform for the design of potent glycosidase inhibitors and have potential to be applied to other HS-degrading proteins.

Heparin Mimetics: Their Therapeutic Potential

Heparin mimetics are synthetic and semi-synthetic compounds that are highly sulfated, structurally distinct analogues of glycosaminoglycans. These mimetics are often rationally designed to increase potency and binding selectivity towards specific proteins involved in disease manifestations. Some of the major therapeutic arenas towards which heparin mimetics are targeted include: coagulation and thrombosis, cancers, and inflammatory diseases. Although Fondaparinux, a rationally designed heparin mimetic, is now approved for prophylaxis and treatment of venous thromboembolism, the search for novel anticoagulant heparin mimetics with increased affinity and fewer side effects remains a subject of research. However, increasingly, research is focusing on the non-anticoagulant activities of these molecules. Heparin mimetics have potential as anti-cancer agents due to their ability to: (1) inhibit heparanase, an endoglycosidase which facilitates the spread of tumor cells; and (2) inhibit angiogenesis by binding to growth factors. The heparin mimetic, PI-88 is in clinical trials for post-surgical hepatocellular carcinoma and advanced melanoma. The anti-inflammatory properties of heparin mimetics have primarily been attributed to their ability to interact with: complement system proteins, selectins and chemokines; each of which function differently to facilitate inflammation. The efficacy of low/non-anticoagulant heparin mimetics in animal models of different inflammatory diseases has been demonstrated. These findings, plus clinical data that indicates heparin has anti-inflammatory activity, will raise the momentum for developing heparin mimetics as a new class of therapeutic agent for inflammatory diseases.

Heparan Sulfate Microarray Reveals That Heparan Sulfate-Protein Binding Exhibits Different Ligand Requirements

Heparan sulfates (HS) are linear sulfated polysaccharides that modulate a wide range of physiological and disease-processes. Variations in HS epimerization and sulfation provide enormous structural diversity, which is believed to underpin protein binding and regulatory properties. The ligand requirements of HS-binding proteins have, however, been defined in only a few cases. We describe here a synthetic methodology that can rapidly provide a library of well-defined HS oligosaccharides. It is based on the use of modular disaccharides to assemble several selectively protected tetrasaccharides that were subjected to selective chemical modifications such as regioselective O- and N-sulfation and selective de-sulfation. A number of the resulting compounds were subjected to enzymatic modifications by 3-O-sulfotransferases-1 (3-OST1) to provide 3-O-sulfated derivatives. The various approaches for diversification allowed one tetrasaccharide to be converted into 12 differently sulfated derivatives. By employing tetrasaccharides with different backbone compositions, a library of 47 HS-oligosaccharides was prepared and the resulting compounds were used to construct a HS microarray. The ligand requirements of a number of HS-binding proteins including fibroblast growth factor 2 (FGF-2), and the chemokines CCL2, CCL5, CCL7, CCL13, CXCL8, and CXCL10 were examined using the array. Although all proteins recognized multiple compounds, they exhibited clear differences in structure-binding characteristics. The HS microarray data guided the selection of compounds that could interfere in biological processes such as cell proliferation. Although the library does not cover the entire chemical space of HS-tetrasaccharides, the binding data support a notion that changes in cell surface HS composition can modulate protein function.

Endogenous regeneration: Engineering growth factors for stroke

Despite the efforts in developing therapeutics for stroke, recombinant tissue plasminogen activator (rtPA) remains the only FDA approved drug for ischemic stroke. Regenerative medicine targeting endogenous growth factors has drawn much interest in the clinical field as it provides potential restoration for the damaged brain tissue without being limited by a narrow therapeutic window. To date, most of the translational studies using regenerative medicines have encountered problems and failures. In this review, we discuss the effects of some trophic factors which include of erythropoietin (EPO), brain derived neurotrophic factor (BDNF), granulocyte-colony stimulating factor (G-CSF), vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), epidermal growth factor (EGF) and heparin binding epidermal growth factor (HB-EGF) in experimental ischemic stroke models and elaborate the lost in translation of the candidate growth factors from bench to bedside. Several new methodologies have been developed to overcome the caveats in translational studies. This review highlights the latest bioengineering approaches including the controlled release and delivery of growth factors by hydrogel-based scaffolds and the enhancement of half-life and selectivity of growth factors by a novel approach facilitated by glycosaminoglycans.

Introduction: General Aspects of the Chemical Biology of Glycoproteins

This chapter is meant to serve as an introduction to the remainder of the book by providing general background on the chemical biology of glycoproteins as well as a brief review of the chapters that follow. The purpose here is to introduce some basic concepts common to many forms of glycosylation for those readers who may be unfamiliar with the field. We begin with a discussion of the strategies and methods used to study protein glycosylation. During the overview, an effort is made to highlight a few relevant aspects of chemical glycobiology, including glycoprotein biosynthesis and a brief description of the synthesis and function of glycoproteins. Finally, we have a summary of the contributions from chemical biology over the years. It is our hope that, after reading this introductory chapter, the reader will have a broad view of the chemical glycobiology field as it currently stands and a deeper appreciation for some of the unique ideas that chemical biology brings to the field.

Obstacles and solutions for chemical synthesis of syndecan-3 (53-62) glycopeptides with two heparan sulfate chains

Proteoglycans play critical roles in many biological events. Due to their structural complexities, strategies towards synthesis of this class of glycopeptides bearing well-defined glycan chains are urgently needed. In this work, we give the full account of the synthesis of syndecan-3 glycopeptide (53-62) containing two different heparan sulfate chains. For assembly of glycans, a convergent 3+2+3 approach was developed producing two different octasaccharide amino acid cassettes, which were utilized towards syndecan-3 glycopeptides. The glycopeptides presented many obstacles for post-glycosylation manipulation, peptide elongation, and deprotection. Following screening of multiple synthetic sequences, a successful strategy was finally established by constructing partially deprotected single glycan chain containing glycopeptides first, followed by coupling of the glycan-bearing fragments and cleavage of the acyl protecting groups.

Heparin-Mimicking Polymers: Synthesis and Biological Applications

Heparin is a naturally occurring, highly sulfated polysaccharide that plays a critical role in a range of different biological processes. Therapeutically, it is mostly commonly used as an injectable solution as an anticoagulant for a variety of indications, although it has also been employed in other forms such as coatings on various biomedical devices. Due to the diverse functions of this polysaccharide in the body, including anticoagulation, tissue regeneration, anti-inflammation, and protein stabilization, and drawbacks of its use, analogous heparin-mimicking materials are also widely studied for therapeutic applications. This review focuses on one type of these materials, namely, synthetic heparin-mimicking polymers. Utilization of these polymers provides significant benefits compared to heparin, including enhancing therapeutic efficacy and reducing side effects as a result of fine-tuning heparin-binding motifs and other molecular characteristics. The major types of the various polymers are summarized, as well as their applications. Because development of a broader range of heparin-mimicking materials would further expand the impact of these polymers in the treatment of various diseases, future directions are also discussed.

Integrated Approach to Identify Heparan Sulfate Ligand Requirements of Robo1

An integrated methodology is described to establish ligand requirements for heparan sulfate (HS) binding proteins based on a workflow in which HS octasaccharides are produced by partial enzymatic degradation of natural HS followed by size exclusion purification, affinity enrichment using an immobilized HS-binding protein of interest, putative structure determination of isolated compounds by a hydrophilic interaction chromatography-high-resolution mass spectrometry platform, and chemical synthesis of well-defined HS oligosaccharides for structure-activity relationship studies. The methodology was used to establish the ligand requirements of human Roundabout receptor 1 (Robo1), which is involved in a number of developmental processes. Mass spectrometric analysis of the starting octasaccharide mixture and the Robo1-bound fraction indicated that Robo1 has a preference for a specific set of structures. Further analysis was performed by sequential permethylation, desulfation, and pertrideuteroacetylation followed by online separation and structural analysis by MS/MS. Sequences of tetrasaccharides could be deduced from the data, and by combining the compositional and sequence data, a putative octasaccharide ligand could be proposed (GlA-GlcNS6S-IdoA-GlcNS-IdoA2S-GlcNS6S-IdoA-GlcNAc6S). A modular synthetic approach was employed to prepare the target compound, and binding studies by surface plasmon resonance (SPR) confirmed it to be a high affinity ligand for Robo1. Further studies with a number of tetrasaccharides confirmed that sulfate esters at C-6 are critical for binding, whereas such functionalities at C-2 substantially reduce binding. High affinity ligands were able to reverse a reduction in endothelial cell migration induced by Slit2-Robo1 signaling.

Endoglycosidases for the Synthesis of Polysaccharides and Glycoconjugates

Recent advances in glycobiology have implicated essential roles of oligosaccharides and glycoconjugates in many important biological recognition processes, including intracellular signaling, cell adhesion, cell differentiation, cancer progression, host-pathogen interactions, and immune responses. A detailed understanding of the biological functions, as well as the development of carbohydrate-based therapeutics, often requires structurally well-defined oligosaccharides and glycoconjugates, which are usually difficult to isolate in pure form from natural sources. To meet with this urgent need, chemical and chemoenzymatic synthesis has become increasingly important as the major means to provide homogeneous compounds for functional glycocomics studies and for drug/vaccine development. Chemoenzymatic synthesis, an approach that combines chemical synthesis and enzymatic manipulations, is often the method of choice for constructing complex oligosaccharides and glycoconjugates that are otherwise difficult to achieve by purely chemical synthesis. Among these, endoglycosidases, a class of glycosidases that hydrolyze internal glycosidic bonds in glycoconjugates and polysaccharides, are emerging as a very attractive class of enzymes for synthetic purposes, due to their transglycosylation activity and their capability of transferring oligosaccharide units en bloc in a single step, in contrast to the limitation of monosaccharide transfers by common glycosyltransferases. In this chapter, we provide an overview on the application of endoglycosidases for the synthesis of complex carbohydrates, including oligosaccharides, polysaccharides, glycoproteins, glycolipids, proteoglycans, and other biologically relevant polysaccharides. The scope, limitation, and future directions of endoglycosidase-catalyzed synthesis are discussed.

The stereoselectivities of tributyltin hydride-mediated reductions of 5-bromo-D-glucuronides to L-iduronides are dependent on the anomeric substituent: syntheses and DFT calculations

One of the shortest synthetic routes to L-iduronic acid derivatives is via free radical reduction of the C-5 bromide of the corresponding protected D-glucuronic acid derivative. The epimerization of such C-5 bromides to the L-ido derivatives via reaction with tributyltin hydride was investigated. It was found that the stereoselectivity of the reaction was dependent on the anomeric substituent. If the substituent was fluoride the L-ido product was obtained exclusively in 65-72% yield whereas the O-methyl or O-acetyl derivatives led to isomeric mixtures of both the L-ido and D-gluco products in different ratios depending on the reaction conditions. DFT calculations were performed to determine the stereoelectronic factors that favour formation of the L-ido isomer from the fluoride and suggest the selectivity is due to a transition state gauche effect and an Sn-F interaction.

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.

Synthetic Approaches to L-Iduronic Acid and L-Idose: Key Building Blocks for the Preparation of Glycosaminoglycan Oligosaccharides

L-Iduronic acid (IdoA) is an important monosaccharide component of glycosaminoglycans (GAGs) such as heparin, heparan sulfate and dermatan sulfate. GAGs are complex, highly sulfated polysaccharides that mediate a multitude of physiological and pathological processes via their interactions with a range of diverse proteins. The main challenge in the synthesis of GAG oligosaccharides is the efficient gram-scale preparation of IdoA building blocks since neither IdoA nor L-idose is commercially available or readily accessible from natural sources. In this review, the different synthetic approaches for the preparation of IdoA and its derivatives, including L-idose, are presented and discussed. Derivatives of the latter are often used in GAG synthesis and are elaborated to IdoA via selective oxidation at C-6 after incorporation into a GAG chain. Particular focus will be given to the preparation of IdoA synthons most commonly used for GAG oligosaccharide synthesis, and on the progress made since the last systematic review in this area.

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.