Kinetic and Structural Studies of Interactions between Glycosaminoglycans and Langerin

Surface plasmon resonance based-optical biosensor: Emerging diagnostic tool for early detection of diseases

The development of diagnostic tools remains at the center of the health care system. In recent times optical biosensors have been widely applied in the scientific community, especially for monitoring protein-protein or nucleic acid hybridization interactions. Optical biosensors-derived surface plasmon resonance (SPR) technology has appeared as a revolutionary technology at the current times. This review focuses on the research work in molecular biomarker evaluation using the technique based on SPR for translational clinical diagnosis. The review has covered both communicable and noncommunicable diseases by using different bio-fluids of the patient's sample for diagnosis of the diseases. An increasing number of SPR approaches have been developed in healthcare research and fundamental biological studies. The utility of SPR in the area of biosensing basically lies in its noninvasive diagnostic and prognostic feature due to its label-free high sensitivity and specificity properties. This makes SPR an invaluable tool with precise application in the recognition of different stages of the disease.

Interactions between heparin and SARS-CoV-2 spike glycoprotein RBD from omicron and other variants

Heparan sulfate (HS) acts as a co-receptor of angiotensin-converting enzyme 2 (ACE2) by interacting with severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) spike glycoprotein (SGP) facilitating host cell entry of SARS-CoV-2 virus. Heparin, a highly sulfated version of heparan sulfate (HS), interacts with a variety of proteins playing key roles in many physiological and pathological processes. In this study, SARS-CoV-2 SGP receptor binding domain (RBD) wild type (WT), Delta and Omicron variants were expressed in Expi293F cells and used in the kinetic and structural analysis on their interactions with heparin. Surface plasmon resonance (SPR) analysis showed the binding kinetics of SGP RBD from WT and Delta variants were very similar while Omicron variant SGP showed a much higher association rate. The SGP from Delta and Omicron showed higher affinity (K D ) to heparin than the WT SGP. Competition SPR studies using heparin oligosaccharides indicated that binding of SGP RBDs to heparin requires chain length greater than 18. Chemically modified heparin derivatives all showed reduced interactions in competition assays suggesting that all the sulfo groups in the heparin polysaccharide were critical for binding SGP RBDs with heparin. These interactions with heparin are pH sensitive. Acidic pH (pH 6.5, 5.5, 4.5) greatly increased the binding of WT and Delta SGP RBDs to heparin, while acidic pH slightly reduced the binding of Omicron SGP RBD to heparin compared to binding at pH 7.3. In contrast, basic pH (pH 8.5) greatly reduced the binding of Omicron SGP RBDs to heparin, with much less effects on WT or Delta. The pH dependence indicates different charged residues were present at the Omicron SGP-heparin interface. Detailed kinetic and structural analysis of the interactions of SARS-CoV-2 SGP RBDs with heparin provides important information for designing anti-SARS-CoV-2 molecules.

Intermolecular Interactions between a Membrane Protein and a Glycolipid Essential for Membrane Protein Integration

Inducing newly synthesized proteins to appropriate locations is an indispensable biological function in every organism. Integration of proteins into biomembranes in Escherichia coli is mediated by proteinaceous factors, such as Sec translocons and an insertase YidC. Additionally, a glycolipid named MPIase (membrane protein integrase), composed of a long sugar chain and pyrophospholipid, was proven essential for membrane protein integration. We reported that a synthesized minimal unit of MPIase possessing only one trisaccharide, mini-MPIase-3, involves an essential structure for the integration activity. Here, to elucidate integration mechanisms using MPIase, we analyzed intermolecular interactions of MPIase or its synthetic analogs with a model substrate, the Pf3 coat protein, using physicochemical methods. Surface plasmon resonance (SPR) analyses revealed the importance of a pyrophosphate for affinity to the Pf3 coat protein. Compared with mini-MPIase-3, natural MPIase showed faster association and dissociation due to its long sugar chain despite the slight difference in affinity. To focus on more detailed MPIase substructures, we performed docking simulations and saturation transfer difference-nuclear magnetic resonance. These experiments yielded that the 6-O-acetyl group on glucosamine and the phosphate of MPIase play important roles leading to interactions with the Pf3 coat protein. The high affinity of MPIase to the hydrophobic region and the basic amino acid residues of the protein was suggested by docking simulations and proven experimentally by SPR using protein mutants devoid of target regions. These results demonstrated the direct interactions of MPIase with a substrate protein and revealed detailed mechanisms of membrane protein integration.

GAG Multivalent Systems to interact with Langerin

Langerin is a C-type Lectin expressed at the surface of Langerhans cells, which play a pivotal role in protecting organisms against pathogen infections. To address this aim, Langerin presents at least two recognition sites, one Ca2+-dependent and another one independent, capable of recognizing a variety of carbohydrate ligands. In contrast to other lectins, Langerin recognizes sulfated glycosaminoglycans (GAGs), a family of complex and heterogeneous polysaccharides present in the cell membrane and the extracellular matrix at the interphase generated in the trimeric form of Langerin but absent in the monomeric form. The complexity of these oligosaccharides has impeded the development of well-defined monodisperse structures to study these interaction processes. However, in the last few decades, an improvement of synthetic developments to achieve the preparation of carbohydrate multivalent systems mimicking the GAGs has been described. Despite all these contributions, very few examples are reported where the GAG multivalent structures are used to evaluate the interaction with Langerin. These molecules should pave the way to explore these GAG-Langerin interactions.

Investigation of the Differences in Antithrombin to Heparin Binding among Antithrombin Budapest 3, Basel, and Padua Mutations by Biochemical and In Silico Methods

Antithrombin (AT) is a serine protease inhibitor, its activity is highly accelerated by heparin. Mutations at the heparin-binding region lead to functional defect, type II heparin-binding site (IIHBS) AT deficiency. The aim of this study was to investigate and compare the molecular background of AT Budapest 3 (p.Leu131Phe, ATBp3), AT Basel (p.Pro73Leu), and AT Padua (p.Arg79His) mutations. Advanced in silico methods and heparin-binding studies of recombinant AT proteins using surface plasmon resonance method were used. Crossed immunoelectrophoresis and Differential Scanning Fluorimetry (NanoDSF) were performed in plasma samples. Heparin affinity of AT Padua was the lowest (KD = 1.08 × 10^-6 M) and had the most severe consequences affecting the allosteric pathways of activation, moreover significant destabilizing effects on AT were also observed. KD values for AT Basel, ATBp3 and wild-type AT were 7.64 × 10^-7 M, 2.15 × 10^-8 M and 6.4 × 10^-10 M, respectively. Heparin-binding of AT Basel was slower, however once the complex was formed the mutation had only minor effect on the secondary and tertiary structures. Allosteric activation of ATBp3 was altered, moreover decreased thermostability in ATBp3 homozygous plasma and increased fluctuations in multiple regions of ATBp3 were observed by in silico methods suggesting the presence of a quantitative component in the pathogenicity of this mutation due to molecular instability.

Keratan sulfate-based glycomimetics using Langerin as a target for COPD: lessons from studies on Fut8 and core fucose

Glycosylation represents one of the most abundant posttranslational modification of proteins. Glycosylation products are diverse and are regulated by the cooperative action of various glycosyltransferases, glycosidases, substrates thereof: nucleoside sugars and their transporters, and chaperons. In this article, we focus on a glycosyltransferase, α1,6-fucosyltransferase (Fut8) and its product, the core fucose structure on N-glycans, and summarize the potential protective functions of this structure against emphysema and chronic obstructive pulmonary disease (COPD). Studies of FUT8 and its enzymatic product, core fucose, are becoming an emerging area of interest in various fields of research including inflammation, cancer and therapeutics. This article discusses what we can learn from studies of Fut8 and core fucose by using knockout mice or in vitro studies that were conducted by our group as well as other groups. We also include a discussion of the potential protective functions of the keratan sulfate (KS) disaccharide, namely L4, against emphysema and COPD as a glycomimetic. Glycomimetics using glycan analogs is one of the more promising therapeutics that compensate for the usual therapeutic strategy that involves targeting the genome and the proteome. These typical glycans using KS derivatives as glycomimetics, will likely become a clue to the development of novel and effective therapeutic strategies.

The structure-activity relationship of the interactions of SARS-CoV-2 spike glycoproteins with glucuronomannan and sulfated galactofucan from Saccharina japonica

The SARS-CoV-2 spike glycoproteins (SGPs) and human angiotensin converting enzyme 2 (ACE2) are the two key targets for the prevention and treatment of COVID-19. Host cell surface heparan sulfate (HS) is believed to interact with SARS-CoV-2 SGPs to facilitate host cell entry. In the current study, a series of polysaccharides from Saccharina japonica were prepared to investigate the structure-activity relationship on the binding abilities of polysaccharides (oligosaccharides) to pseudotype particles, including SARS-CoV-2 SGPs, and ACE2 using surface plasmon resonance. Sulfated galactofucan (SJ-D-S-H) and glucuronomannan (Gn) displayed strongly inhibited interaction between SARS-CoV-2 SGPs and heparin while showing negligible inhibition of the interaction between SARS-CoV-2 SGPs and ACE2. The IC50 values of SJ-D-S-H and Gn in blocking heparin SGP binding were 27 and 231 nM, respectively. NMR analysis showed that the structure of SJ-D-S-H featured with a backbone of 1, 3-linked α-L-Fucp residues sulfated at C4 and C2/C4 and 1, 3-linked α-L-Fucp residues sulfated at C4 and branched with 1, 6-linked β-D-galacto-biose; Gn had a backbone of alternating 1, 4-linked β-D-GlcAp residues and 1, 2-linked α-D-Manp residues. The sulfated galactofucan and glucuronomannan showed strong binding ability to SARS-CoV-2 SGPs, suggesting that these polysaccharides might be good candidates for preventing and/or treating SARS-CoV-2.

Interactions of fibroblast growth factors with sulfated galactofucan from Saccharina japonica

A total 68 types of marine algae oligosaccharides and polysaccharides were prepared and used to study the structure-activity relationship of oligosaccharides and polysaccharides in their interactions with fibroblast growth factors (FGF) 1 and 2. Factors considered include different types of algae, extraction methods, molecular weight, sulfate content and fractions. In the case of low molecular weight polysaccharide (SJ-D) from Saccharina japonica and its fractions eluting from anion exchange column, both 1.0 M NaCl fraction (SJ-D-I) and 2.0 M NaCl fraction (SJ-D-S) had stronger binding affinity than the parent SJ-D, suggesting that sulfated galactofucans represented the major tight binding component. Nuclear magnetic resonance showed that SJ-D-I was a typical sulfated galactofucan, composed of four units: 1, 3-linked 4-sulfated α-L-fucose (Fuc); 1, 3-linked 2, 4-disulfated α-L-Fuc; 1, 6-linked 4-sulfated β-D-Gal and/or 1, 6-linked 3, 4-sulfated β-D-Gal. Modification by autohydrolysis to oligosaccharides and desulfation decreased the FGF binding affinity while oversulfation increased the affinity. The solution-based affinities of SJ-D-I to FGF1 and FGF2 were 69 nM and 3.9 nM, suggesting that SJ-D-I showed better preferentially binding to FGF1 than a natural ligand, heparin, suggesting that sulfated galactofucan might represent a good regulator of FGF1.

Fucosylated Chondroitin Sulfate 9-18 Oligomers Exhibit Molecular Size-Independent Antithrombotic Activity while Circulating in the Blood

Fucosylated chondroitin sulfate (FCS) oligosaccharides extracted from sea cucumber and depolymerized exhibit potent anticoagulant activity. Knowledge of the antithrombotic activity of different size oligosaccharides and their fucose (Fuc) branch sulfation pattern should promote their development for clinical applications. We prepared highly purified FCS trisaccharide repeating units from hexasaccharide (6-mer) to octadecasaccharide (18-mer), including those with 2,4-disulfated and 3,4-disulfated Fuc branches. All 10 oligosaccharides were identified by their nuclear magnetic resonance structures and ESI-FTMS spectroscopy. In vitro anticoagulant activities and surface plasmon resonance binding tests indicated those of larger molecular sizes and 2,4-disulfated Fuc branches showed stronger anticoagulant effects with respect to anti-FXase activity, as well as stronger binding to FIXa among various clotting proteins. However, both types of FCS 9-mer to 18-mer exhibited molecular size-independent potent antithrombotic activity in vivo at the same dose. In addition, both types of the FCS 6-mer exhibited favorable antithrombotic activity in vivo, although they showed weak anticoagulant activity in vitro. Combining absorption and metabolism studies, we conclude that FCS 9-18 oligomers could remain in the circulation to interact with various clotting proteins to prevent thrombus formation, and appreciable quantities of these oligomers could be excreted through the kidneys. All FCS 9-18 oligomers also resulted in no bleeding, hypotension, or platelet aggregation risk during blood circulation. Thus, FCS 9-18 oligomers with 2,4-disulfated or 3,4-disulfated Fuc branches exhibit potent and safe antithrombotic activity needed for clinical applications.

Photoaffinity Probes for the Identification of Sequence-Specific Glycosaminoglycan-Binding Proteins

Glycosaminoglycan (GAG)-protein interactions mediate critical physiological and pathological processes, such as neuronal plasticity, development, and viral invasion. However, mapping GAG-protein interaction networks is challenging as these interactions often require specific GAG sulfation patterns and involve transmembrane receptors or extracellular matrix-associated proteins. Here, we report the first GAG polysaccharide-based photoaffinity probes for the system-wide identification of GAG-binding proteins in living cells. A general platform for the modular, efficient assembly of various chondroitin sulfate (CS)-based photoaffinity probes was developed. Systematic evaluations led to benzophenone-containing probes that efficiently and selectively captured known CS-E-binding proteins in vitro and in cells. Importantly, the probes also enabled the identification of >50 new proteins from living neurons that interact with the neuroplasticity-relevant CS-E sulfation motif. Several candidates were independently validated and included membrane receptors important for axon guidance, innate immunity, synapse development, and synaptic plasticity. Overall, our studies provide a powerful approach for mapping GAG-protein interaction networks, revealing new potential functions for these polysaccharides and linking them to diseases such as Alzheimer's and autism.

Molecular Recognition in C-Type Lectins: The Cases of DC-SIGN, Langerin, MGL, and L-Sectin

Carbohydrates play a pivotal role in intercellular communication processes. In particular, glycan antigens are key for sustaining homeostasis, helping leukocytes to distinguish damaged tissues and invading pathogens from healthy tissues. From a structural perspective, this cross‐talk is fairly complex, and multiple membrane proteins guide these recognition processes, including lectins and Toll‐like receptors. Since the beginning of this century, lectins have become potential targets for therapeutics for controlling and/or avoiding the progression of pathologies derived from an incorrect immune outcome, including infectious processes, cancer, or autoimmune diseases. Therefore, a detailed knowledge of these receptors is mandatory for the development of specific treatments. In this review, we summarize the current knowledge about four key C‐type lectins whose importance has been steadily growing in recent years, focusing in particular on how glycan recognition takes place at the molecular level, but also looking at recent progresses in the quest for therapeutics.

Interactions between Sclerostin and Glycosaminoglycans

Sclerostin (SOST) is a glycoprotein having many important functions in the regulation of bone formation as a key negative regulator of Wnt signaling in bone. Surface plasmon resonance (SPR), which allows for a direct quantitative analysis of the label-free molecular interactions in real-time, has been widely used for the biophysical characterization of glycosaminoglycan (GAG)-protein interactions. In the present study, we report kinetics, structural analysis and the effects of physiological conditions (e.g., salt concentrations, Ca²⁺ and Zn²⁺concentrations) on the interactions between GAGs and recombinant human (rh) and recombinant mouse (rm) SOST using SPR. SPR results revealed that both SOSTs bind heparin with high affinity (rhSOST-heparin, K_D~36 nM and rmSOST-heparin, K_D~77 nM) and the shortest oligosaccharide of heparin that effectively competes with full size heparin for SOST binding is octadecasaccharide (18mer). This heparin binding protein also interacts with other highly sulfated GAGs including, disulfated-dermatan sulfate and chondroitin sulfate E. In addition, liquid chromatography-mass spectrometry was used to characterize the structure of sulfated GAGs that bound to SOST.

Major Differences between the Self-Assembly and Seeding Behavior of Heparin-Induced and in Vitro Phosphorylated Tau and Their Modulation by Potential Inhibitors

Self-assembly of the microtubule-associated protein tau into neurotoxic oligomers, fibrils, and paired helical filaments, and cell-to-cell spreading of these pathological tau species are critical processes underlying the pathogenesis of Alzheimer’s disease and other tauopathies. Modulating the self-assembly process and inhibiting formation and spreading of such toxic species are promising strategies for therapy development. A challenge in investigating tau self-assembly in vitro is that, unlike most amyloidogenic proteins, tau does not aggregate in the absence of posttranslational modifications (PTM), aggregation inducers, or preformed seeds. The most common induction method is addition of polyanions, such as heparin; yet, this artificial system may not represent adequately tau self-assembly in vivo, which is driven by aberrant phosphorylation and other PTMs, potentially leading to in vitro data that do not reflect the behavior of tau and its interaction with modulators in vivo. To tackle these challenges, methods for in vitro phosphorylation of tau to produce aggregation-competent forms recently have been introduced (Despres et al. (2017) Proc. Natl. Acad. Sci. U.S.A., 114, 9080−908528784767). However, the oligomerization, seeding, and interaction with assembly modulators of the different forms of tau have not been studied to date. To address these knowledge gaps, we compared here side-by-side the self-assembly and seeding activity of heparin-induced tau with two forms of in vitro phosphorylated tau and tested how the molecular tweezer CLR01, a negatively charged compound, affected these processes. Tau was phosphorylated by incubation either with activated extracellular signal-regulated kinase 2 or with a whole rat brain extract. Seeding activity was measured using a fluorescence-resonance energy transfer-based biosensor-cell method. We also used solution-state NMR to investigate the binding sites of CLR01 on tau and how they were impacted by phosphorylation. Our systematic structure–activity relationship study demonstrates that heparin-induced tau behaves differently from in vitro phosphorylated tau. The aggregation rates of the different forms are distinct as is the intracellular localization of the induced aggregates, which resemble brain-derived tau strains suggesting that heparin-induced tau and in vitro phosphorylated tau have different conformations, properties, and activities. CLR01 inhibits aggregation and seeding of both heparin-induced and in vitro phosphorylated tau dose-dependently, although heparin induction interferes with the interaction between CLR01 and tau.

A Specific, Glycomimetic Langerin Ligand for Human Langerhans Cell Targeting

Langerhans cells are a subset of dendritic cells residing in the epidermis of the human skin. As such, they are key mediators of immune regulation and have emerged as prime targets for novel transcutaneous cancer vaccines. Importantly, the induction of protective T cell immunity by these vaccines requires the efficient and specific delivery of both tumor-associated antigens and adjuvants. Langerhans cells uniquely express Langerin (CD207), an endocytic C-type lectin receptor. Here, we report the discovery of a specific, glycomimetic Langerin ligand employing a heparin-inspired design strategy and structural characterization by NMR spectroscopy and molecular docking. The conjugation of this glycomimetic to liposomes enabled the specific and efficient targeting of Langerhans cells in the human skin. We further demonstrate the doxorubicin-mediated killing of a Langerin+ monocyte cell line, highlighting its therapeutic and diagnostic potential in Langerhans cell histiocytosis, caused by the abnormal proliferation of Langerin+ myeloid progenitor cells. Overall, our delivery platform provides superior versatility over antibody-based approaches and novel modalities to overcome current limitations of dendritic cell-targeted immuno- and chemotherapy.

Characterization of structural motifs for interactions between glycosaminoglycans and proteins

Glycosaminoglycans (GAGs) are a family of linear and anionic polysaccharides that play essential roles in many biological and physiological processes. Interactions between GAGs and proteins regulate function in many proteins and are related to many human diseases and disorders. The structural motifs and mechanisms for interactions between GAGs and proteins are not fully understood. Specific bindings, including minor but unique sequences sporadically distributed along the GAG chains or variably sulfated domains interspersed by undersulfated regions, may be specifically recognized by defined domains of a variety of proteins. Understanding the molecular basis of these interactions will provide a template for developing novel glycotherapeutic agents. The present article reviews recent methodologies and progress on the characterization of structural motifs in both GAGs and proteins involved in GAG-protein interactions. The analytical approaches are categorized into three groups: affinity-based methods; molecular docking, nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography; and mass spectrometry (MS) techniques. The advantages and limitations of each category of methods are discussed and are based on examples of using these techniques to investigate binding between GAGs and proteins.

Glycan Determinants of Heparin-Tau Interaction

Tau aggregates into paired helical filaments within neurons, a pathological hallmark of Alzheimer's disease. Heparin promotes tau aggregation and recently has been shown to be involved in the cellular uptake of tau aggregates. Although the tau-heparin interaction has been extensively studied, little is known about the glycan determinants of this interaction. Here, we used surface plasmon resonance (SPR) and NMR spectroscopy to characterize the interaction between two tau fragments, K18 and K19, and several polysaccharides, including heparin, heparin oligosaccharides, chemically modified heparin, and related glycans. Using a heparin-immobilized chip, SPR revealed that tau K18 and K19 bind heparin with a K_D of 0.2 and 70 μM, respectively. In SPR competition experiments, N-desulfation and 2-O-desulfation had no effect on heparin binding to K18, whereas 6-O-desulfation severely reduced binding, suggesting a critical role for 6-O-sulfation in the tau-heparin interaction. The tau-heparin interaction became stronger with longer-chain heparin oligosaccharides. As expected for an electrostatics-driven interaction, a moderate amount of salt (0.3 M NaCl) abolished binding. NMR showed the largest chemical-shift perturbation (CSP) in R2 in tau K18, which was absent in K19, revealing differential binding sites in K18 and K19 to heparin. Dermatan sulfate binding produced minimal CSP, whereas dermatan disulfate, with the additional 6-O-sulfo group, induced much larger CSP. 2-O-desulfated heparin induced much larger CSP in K18 than 6-O-desulfated heparin. Our data demonstrate a crucial role for the 6-O-sulfo group in the tau-heparin interaction, which to our knowledge has not been reported before.

Novel method for measurement of heparin anticoagulant activity using SPR

A novel method has been developed for the easy measurement of heparin's anticoagulant activity using surface plasmon resonance. The anticoagulant activity of target heparin was evaluated by measuring the competitive antithrombin III binding of analyte heparin in the solution phase and USP heparin immobilized on chip surface. Heparins, obtained from different animal sources, and low molecular weight heparins were analyzed. The results were reproducible and correlated well with the results of chromogenic assays (correlation coefficient r = 0.98 for anti-Xa and r = 0.94 for anti-IIa). This protocol provides many advantages, significantly minimizing time, cost and the complications of chromogenic assay methods.

Calcium-Independent Activation of an Allosteric Network in Langerin by Heparin Oligosaccharides

The C-type lectin receptor Langerin is a glycan-binding protein that serves as an uptake receptor on Langerhans cells and is essential for the formation of Birbeck granules. Whereas most Langerin ligands are recognized by a canonical Ca²⁺ -dependent binding site, heparins have been proposed to make additional contacts to a secondary, Ca²⁺ -independent site. Glycan array screening and biomolecular NMR spectroscopy were employed to investigate the molecular mechanism of these interactions. We observed that binding of heparin hexasaccharides to a secondary site did not require the presence of Ca²⁺ and activated a previously identified intradomain allosteric network of Langerin (thus far only associated with Ca²⁺ affinity and release). We propose a communication hub between these two binding sites, which sheds new light on modulatory functions of Langerin-heparin interactions.

Surface Plasmon Resonance: A Boon for Viral Diagnostics

Rapidly evolving viral strains leading to epidemics and pandemics necessitates quick diagnostics and treatment to halt the progressive march of the disease. Optical biosensors like surface plasmon resonance (SPR) have emerged in recent times as a most reliable diagnostic device owing to their portability, reproducibility, sensitivity and specificity. SPR analyzes the kinetics of biomolecular interactions in a label-free manner. It has surpassed the conventional virus detection methods in its utility, particularly in medical diagnostics and healthcare. However, the requirement of high-end infrastructure setup and trained manpower are some of the roadblocks in realizing the true potential of SPR. This platform needs further improvisation in terms of simplicity, affordability and portability before it could be utilized in need-based remote areas of under-developed and developing countries with limited infrastructure.

pH-Sensitive N-doped carbon dots–heparin and doxorubicin drug delivery system: preparation and anticancer research

In this study, doxorubicin (DOX) hydrochloride as a model drug, N-doped carbon dots as a drug carrier, and heparin as an auxiliary medicine were selected to design and prepare a multi-functional drug delivery system with pH-triggered drug release.