Characterisation of the Dynamic Interactions between Complex N ‐Glycans and Human CD22

Sequence variety in the CC' loop of Siglec-8/9/3 determines the recognitions to sulfated oligosaccharides

Siglecs are important lectins found in different types of immune cells and function as regulatory molecules by recognizing self-associated glycans and converting extracellular interactions into signals for inhibiting immune cell functions. Although many Siglecs have been found to show broad specificities and recognize different types of sulfated oligosaccharides, Siglec-8 and Siglec-9 displayed a high degree of specificity for sialyl N-acetyllactosamine (sLacNAc) with sulfations at O6-positions of the galactose (6'-sulfation) and N-acetylglucosamine (6-sulfation), respectively. Siglec-3 was recently discovered to bind sLacNAc both sulfations. In addition to a conserved arginine residue for binding to sialic acid residue, the sequence variety in the CC' loop may provide binding specificities to sulfated oligosaccharides in Siglecs. Thus, the present study employed molecular models to study the impact of different residues in the CC' loops of Siglec-8/9/3 to the recognitions of 6-sulfations in Gal and/or GlcNAc of sLacNAc. The negatively charged residues in the CC' loop of Siglec-9 formed unfavorable electrostatic repulsions with the 6-sulfate in Gal and resulted no recognitions, in contrast to the favorable interactions formed between the positively charged residues in the CC' loop of Siglec-8 and the 6-sulfate in Gal resulting strong specificity. A two-state binding model was proposed for Siglec-3 recognizing 6-sulfations in Gal and GlcNAc of sLacNAc, as the neutral residues in the CC' loop of Siglec-3 could not form strong favorable interactions to lock the 6-sulfate in Gal within a single binding pose or strong unfavorable interactions to repel the 6-sulfate in Gal. The oligosaccharide adopted two distinctive binding poses and oriented the sulfate groups to form interactions with residues in the CC' loop and G-strand. The present study provided a structural mechanism for the sequence variety in the CC' loop of Siglec-8/9/3 determining the recognitions to the sulfated oligosaccharides and offered insights into the binding specificities for Siglecs.

Conformationally Constrained Sialyl Analogues as New Potential Binders of h-CD22

Here, two conformationally constrained sialyl analogues were synthesized and characterized in their interaction with the inhibitory Siglec, human CD22 (h-CD22). An orthogonal approach, including biophysical assays (SPR and fluorescence), ligand-based NMR techniques, and molecular modelling, was employed to disentangle the interaction mechanisms at a molecular level. The results showed that the Sialyl-TnThr antigen analogue represents a promising scaffold for the design of novel h-CD22 inhibitors. Our findings also suggest that the introduction of a biphenyl moiety at position 9 of the sialic acid hampers canonical accommodation of the ligand in the protein binding pocket, even though the affinity with respect to the natural ligand is increased. Our results address the search for novel modifications of the Neu5Ac-α(2-6)-Gal epitope, outline new insights for the design and synthesis of high-affinity h-CD22 ligands, and offer novel prospects for therapeutic intervention to prevent autoimmune diseases and B-cell malignancies.

Molecular Recognition Insights of Sialic Acid Glycans by Distinct Receptors Unveiled by NMR and Molecular Modeling

All cells are decorated with a highly dense and complex structure of glycan chains, which are mostly attached to proteins and lipids. In this context, sialic acids are a family of nine-carbon acidic monosaccharides typically found at the terminal position of glycan chains, modulating several physiological and pathological processes. Sialic acids have many structural and modulatory roles due to their negative charge and hydrophilicity. In addition, the recognition of sialic acid glycans by mammalian cell lectins, such as siglecs, has been described as an important immunological checkpoint. Furthermore, sialic acid glycans also play a pivotal role in host-pathogen interactions. Various pathogen receptors exposed on the surface of viruses and bacteria are responsible for the binding to sialic acid sugars located on the surface of host cells, becoming a critical point of contact in the infection process. Understanding the molecular mechanism of sialic acid glycans recognition by sialic acid-binding proteins, present on the surface of pathogens or human cells, is essential to realize the biological mechanism of these events and paves the way for the rational development of strategies to modulate sialic acid-protein interactions in diseases. In this perspective, nuclear magnetic resonance (NMR) spectroscopy, assisted with molecular modeling protocols, is a versatile and powerful technique to investigate the structural and dynamic aspects of glycoconjugates and their interactions in solution at the atomic level. NMR provides the corresponding ligand and protein epitopes, essential for designing and developing potential glycan-based therapies. In this review, we critically discuss the current state of knowledge about the structural features behind the molecular recognition of sialic acid glycans by different receptors, naturally present on human cells or pathogens, disclosed by NMR spectroscopy and molecular modeling protocols.

Molecular recognition of sialoglycans by streptococcal Siglec-like adhesins: toward the shape of specific inhibitors

Streptococcus gordonii and Streptococcus sanguinis, commensal bacteria present in the oral cavity of healthy individuals, upon entry into the bloodstream can become pathogenic, causing infective endocarditis (IE). Sialic acid-binding serine-rich repeat adhesins on the microbial surface represent an important factor of successful infection to cause IE. They contain Siglec-like binding regions (SLBRs) that variously recognize different repertoires of O-glycans, with some strains displaying high selectivity and others broader specificity. We here dissect at an atomic level the mechanism of interaction of SLBR-B and SLBR-H from S. gordonii with a multivarious approach that combines NMR spectroscopy and computational and biophysical studies. The binding pockets of both SLBRs are broad enough to accommodate extensive interactions with sialoglycans although with key differences related to strain specificity. Furthermore, and significantly, the pattern of interactions established by the SLBRs are mechanistically very different from those reported for mammalian Siglecs despite them having a similar fold. Thus, our detailed description of the binding modes of streptococcal Siglec-like adhesins sparks the development of tailored synthetic inhibitors and therapeutics specific for Streptococcal adhesins to counteract IE, without impairing the interplay between Siglecs and glycans.

We here present a detailed molecular description of sialoglycans recognition by Siglec-like adhesins from S. gordonii opening the venue for the design of specific inhibitors to influence the propensity of streptococci to cause infective endocarditis.

Investigation of protein-ligand complexes by ligand-based NMR methods

Molecular recognition is at the base of all biological events and its knowledge at atomic level is pivotal in the development of new drug design approaches. NMR spectroscopy is one of the most widely used technique to detect and characterize transient ligand-receptor interactions in solution. In particular, ligand-based NMR approaches, including NOE-based NMR techniques, diffusion experiments and relaxation methods, are excellent tools to investigate how ligands interact with their receptors. Here we describe the key structural information that can be achieved on binding processes thanks to the combined used of advanced NMR and computational methods. Saturation Transfer Difference NMR (STD-NMR), WaterLOGSY, diffusion- and relaxation-based experiments, together with tr-NOE techniques allow, indeed, to investigate the ligand behavior when bound to a receptor, determining, among others, the epitope map of the ligand and its bioactive conformation. The combination of these NMR techniques with computational methods, including docking, molecular dynamics and CORCEMA-ST analysis, permits to define and validate an accurate 3D model of protein-ligand complexes.

Recent Advances in the Chemical Biology of N-Glycans

Asparagine-linked N-glycans on proteins have diverse structures, and their functions vary according to their structures. In recent years, it has become possible to obtain high quantities of N-glycans via isolation and chemical/enzymatic/chemoenzymatic synthesis. This has allowed for progress in the elucidation of N-glycan functions at the molecular level. Interaction analyses with lectins by glycan arrays or nuclear magnetic resonance (NMR) using various N-glycans have revealed the molecular basis for the recognition of complex structures of N-glycans. Preparation of proteins modified with homogeneous N-glycans revealed the influence of N-glycan modifications on protein functions. Furthermore, N-glycans have potential applications in drug development. This review discusses recent advances in the chemical biology of N-glycans.

Behavior of glycolylated sialoglycans in the binding pockets of murine and human CD22

Siglecs (sialic acid binding immunoglobulin (Ig)-like lectins) constitute a group of 15 human and 9 murine cell-surface transmembrane receptors belonging to the I-type lectin family, mostly expressed on innate immune cells and characterized by broadly similar structural features. Here, the prominent inhibitory CD22 (Siglec-2), well known in maintaining tolerance and preventing autoimmune responses on B cells, is studied in its human and murine forms in complex with sialoglycans. In detail, the role of the N-glycolyl neuraminic acid (Neu5Gc) moiety in the interaction with both orthologues was explored. The analysis of the binding mode was carried out by the combination of NMR spectroscopy, computational approaches, and CORCEMA-ST calculations. Our findings provide a first model of Neu5Gc recognition by h-CD22 and show a comparable molecular recognition profile by h- and m-CD22. These data open the way to innovative diagnostic and/or therapeutic methodologies to be used in the modulation of the immune responses.

Mass Spectrometry-Based Shotgun Glycomics for Discovery of Natural Ligands of Glycan-Binding Proteins

Glycans attached to lipids and membrane-bound and secreted proteins and peptides mediate many important physiological and pathophysiological processes through interactions with glycan-binding proteins (GBPs). However, uncovering functional glycan ligands is challenging due to the large number of naturally occurring glycan structures, the limited availability of glycans in their purified form, the low affinities of GBP-glycan interactions, and limitations in existing binding assays. This work explores the application of catch-and-release electrospray ionization mass spectrometry (CaR-ESI-MS) for screening libraries of N-glycans derived from natural sources. The assay was tested by screening a small-defined library of complex N-glycans at equimolar concentrations against plant and human GBPs with known specificities for either α2-3- or α2-6-linked sialosides, with affinities in the millimolar to micromolar range. Validation experiments, performed in negative ion mode, revealed that bound N-glycan ligands are readily released, as intact deprotonated ions, from GBPs in the gas phase using collision-induced dissociation. Moreover, the relative abundances of the released ligands closely match their solution affinities. The results obtained for a natural N-glycan library produced from cultured immune cells serve to highlight the ease with which CaR-ESI-MS can screen complex mixtures of N-glycans for interactions. Additionally, scaling the relative abundances of released glycan ligands according to their relative abundances in solution, as determined by hydrophilic interaction-ultrahigh-performance liquid chromatography of the fluorescently labeled library, allows the relative affinities of glycan ligands to be ranked.

Structural advances of Siglecs: insight into synthetic glycan ligands for immunomodulation

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are transmembrane proteins of the immunoglobulin (Ig) superfamily predominantly expressed on the cells of our immune system. Siglecs recognize sialic acid via their terminal V-set domain. In mammals, sialic acid-terminated glycolipids and glycoproteins are the ligands of Siglecs, and the monomeric affinity of Siglecs for their sialic acid-containing ligands is weak. Significant efforts have been devoted toward the development of chemically modified sialoside ligands to target Siglecs with higher affinity and selectivity. In this review we discuss natural and synthetic sialoside ligands for each human Siglec, emphasizing the ligand binding determinants uncovered from recent advances in protein structural information. Potential therapeutic applications of these ligands are also discussed.

Unveiling Molecular Recognition of Sialoglycans by Human Siglec-10

Siglec-10 is an inhibitory I-type lectin selectively recognizing sialoglycans exposed on cell surfaces, involved in several patho-physiological processes. The key role Siglec-10 plays in the regulation of immune cell functions has made it a potential target for the development of immunotherapeutics against a broad range of diseases. However, the crystal structure of the protein has not been resolved for the time being and the atomic description of Siglec-10 interactions with complex glycans has not been previously unraveled. We present here the first insights of the molecular mechanisms regulating the interaction between Siglec-10 and naturally occurring sialoglycans. We used combined spectroscopic, computational and biophysical approaches to dissect glycans' epitope mapping and conformation upon binding in order to afford a description of the 3D complexes. Our outcomes provide a structural perspective for the rational design and development of high-affinity ligands to control the receptor functionality.

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We unveiled the molecular basis of sialoglycans recognition by Siglec-10

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The conformation of sialoglycans drives the interaction with the protein

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Siglec-10 is able to recognize and bind complex N-glycans

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Our outcomes may open the venue for the design and development of novel glycomimetics

Glycan structures and their interactions with proteins. A NMR view

Carbohydrate molecules are essential actors in key biological events, being involved as recognition points for cell-cell and cell-matrix interactions related to health and disease. Despite outstanding advances in cryoEM, X-ray crystallography and NMR still remain the most employed techniques to unravel their conformational features and to describe the structural details of their interactions with biomolecular receptors. Given the intrinsic flexibility of saccharides, NMR methods are of paramount importance to deduce the extent of motion around their glycosidic linkages and to explore their receptor-bound conformations. We herein present our particular view on the latest advances in NMR methodologies that are permitting to magnify their applications for deducing glycan conformation and dynamics and understanding the recognition events in which there are involved.