Structure-guided design of sialic acid-based Siglec inhibitors and crystallographic analysis in complex with sialoadhesin

Unraveling Molecular Recognition of Glycan Ligands by Siglec-9 via NMR Spectroscopy and Molecular Dynamics Modeling

Human sialic-acid-binding immunoglobulin-like lectin-9 (Siglec-9) is a glycoimmune checkpoint receptor expressed on several immune cells. Binding of Siglec-9 to sialic acid containing glycans (sialoglycans) is well documented to modulate its functions as an inhibitory receptor. Here, we first assigned the amino acid backbone of the Siglec-9 V-set domain (Siglec-9d1), using well-established triple resonance three-dimensional nuclear magnetic resonance (NMR) methods. Then, we combined solution NMR and molecular dynamic simulation methods to decipher the molecular details of the interaction of Siglec-9 with the natural ligands α2,3 and α2,6 sialyl lactosamines (SLN), sialyl Lewis X (sLeX), and 6-O sulfated sLeX and with two synthetically modified sialoglycans that bind with high affinity. As expected, Neu5Ac is accommodated between the F and G β-strands at the canonical sialic acid binding site. Addition of a heteroaromatic scaffold 9N-5-(2-methylthiazol-4-yl)thiophene sulfonamide (MTTS) at the C9 position of Neu5Ac generates new interactions with the hydrophobic residues located at the G-G' loop and the N-terminal region of Siglec-9. Similarly, the addition of the aromatic substituent (5-N-(1-benzhydryl-1H-1,2,3-triazol-4-yl)methyl (BTC)) at the C5 position of Neu5Ac stabilizes the conformation of the long and flexible B'-C loop present in Siglec-9. These results expose the underlying mechanism responsible for the enhanced affinity and specificity for Siglec-9 for these two modified sialoglycans and sheds light on the rational design of the next generation of modified sialoglycans targeting Siglec-9.

Synthesis and Binding Mode Predictions of Novel Siglec-7 Ligands

Siglec-7 regulates immune cell activity and is a promising target for immunomodulation. Here, we report the discovery of novel sialic acid derivatives binding to Siglec-7. Synthesis and affinity measurements are complemented by high-quality models of sialoside-Siglec-7 complexes based on molecular dynamics (MD) simulations on the microsecond time scale. We provide details for the predicted binding modes for the new ligands, e.g., that an extension of the carbon backbone leads to a different molecular interaction pattern with the receptor and the nearby water structure than found for known Siglec-7 ligands. Further on, we uncover some shortcomings of the GLYCAM06 and GAFF2 force fields when used for the simulation of sialoside-based glycomimetics. Our results open new opportunities for the rational design of Siglec-7 inhibitors. In addition, we provide strategies on how to use and visualize MD simulations to describe and investigate sialoside-Siglec complexes in general.

Glycomimetics for the inhibition and modulation of lectins

Carbohydrates are essential mediators of many processes in health and disease. They regulate self-/non-self- discrimination, are key elements of cellular communication, cancer, infection and inflammation, and determine protein folding, function and life-times. Moreover, they are integral to the cellular envelope for microorganisms and participate in biofilm formation. These diverse functions of carbohydrates are mediated by carbohydrate-binding proteins, lectins, and the more the knowledge about the biology of these proteins is advancing, the more interfering with carbohydrate recognition becomes a viable option for the development of novel therapeutics. In this respect, small molecules mimicking this recognition process become more and more available either as tools for fostering our basic understanding of glycobiology or as therapeutics. In this review, we outline the general design principles of glycomimetic inhibitors (Section 2). This section is then followed by highlighting three approaches to interfere with lectin function, i.e. with carbohydrate-derived glycomimetics (Section 3.1), novel glycomimetic scaffolds (Section 3.2) and allosteric modulators (Section 3.3). We summarize recent advances in design and application of glycomimetics for various classes of lectins of mammalian, viral and bacterial origin. Besides highlighting design principles in general, we showcase defined cases in which glycomimetics have been advanced to clinical trials or marketed. Additionally, emerging applications of glycomimetics for targeted protein degradation and targeted delivery purposes are reviewed in Section 4.

Improved synthesis of CD22-binding sialosides and its application for further development of potent CD22 inhibitors

CD22, one of the sialic acid-binding immunoglobulin-like lectins (Siglecs), regulates B lymphocyte signaling via its interaction with glycan ligands bearing the sequence Neu5Ac/Gcα(2→6)Gal. We have developed the synthetic sialoside GSC-718 as a ligand mimic for CD22 and identified it as a potent CD22 inhibitor. Although the synthesis of CD22-binding sialosides including GSC-718 has been reported by our group, the synthetic route was unfortunately not suitable for large-scale synthesis. In this study, we developed an improved scalable synthetic procedure for sialosides which utilized 1,5-lactam formation as a key step. The improved procedure yielded sialosides incorporating a series of aglycones at the C2 position. Several derivatives with substituted benzyl residues as aglycones were found to bind to mouse CD22 with affinity comparable to that of GSC-718. The new procedure developed in this study affords sialosides in sufficient quantities for cell-based assays, and will facilitate the search for promising CD22 inhibitors that have therapeutic potential.

The role of sialic acid-binding immunoglobulin-like-lectin-1 (siglec-1) in immunology and infectious disease

Siglec-1, also known as Sialoadhesin (Sn) and CD169 is highly conserved among vertebrates and with 17 immunoglobulin-like domains is Siglec-1 the largest member of the Siglec family. Expression of Siglec-1 is found primarily on dendritic cells (DCs), macrophages and interferon induced monocyte. The structure of Siglec-1 is unique among siglecs and its function as a receptor is also different compared to other receptors in this class as it contains the most extracellular domains out of all the siglecs. However, the ability of Siglec-1 to internalize antigens and to pass them on to lymphocytes by allowing dendritic cells and macrophages to act as antigen presenting cells, is the main reason that has granted Siglec-1's key role in multiple human disease states including atherosclerosis, coronary artery disease, autoimmune diseases, cell-cell signaling, immunology, and more importantly bacterial and viral infections. Enveloped viruses for example have been shown to manipulate Siglec-1 to increase their virulence by binding to sialic acids present on the virus glycoproteins allowing them to spread or evade immune response. Siglec-1 mediates dissemination of HIV-1 in activated tissues enhancing viral spread via infection of DC/T-cell synapses. Overall, the ability of Siglec-1 to bind a variety of target cells within the immune system such as erythrocytes, B-cells, CD8+ granulocytes and NK cells, highlights that Siglec-1 is a unique player in these essential processes.

Targeting Human CD22/Siglec-2 with Dimeric Sialosides as Novel Oligosaccharide Mimetics

Significant interest in the development of high-affinity ligands for Siglecs exists due to the various therapeutically relevant functions of these proteins. Here, we report a new strategy to develop and design Siglec ligands as disialyl-oligosaccharide mimetics exemplified on Siglec-2 (CD22). We report insights into development of dimeric ligands with high affinity and avidity to cell surface-expressed CD22, assay development, tool compounds, structure activity relationships, and biological data on calcium flux regulation in B-cells. The binding modes of selected ligands have been modeled based on state-of-the-art molecular dynamics simulations on the microsecond timescale, providing detailed views on ligand binding and opening a new perspective on drug design efforts for Siglecs. High-avidity dimeric ligands containing a linker opening the way towards bispecifics are presented as well.

Cooperativity of catalytic and lectin-like domain of Trypanosoma congolense trans-sialidase modulates its catalytic activity

Trans-sialidases (TS) represent a multi-gene family of unusual enzymes, which catalyse the transfer of terminal sialic acids (Sia) from sialoglycoconjugates to terminal galactose or N-acetylgalactosamine residues of oligosaccharides without the requirement of CMP-Neu5Ac, the activated Sia used by typical sialyltransferases. Enzymes comprise a N-terminal catalytic domain (CD) followed by a lectin-like domain (LD). Most work on trypanosomal TS has been done on enzymatic activities focusing on the CD of TS from Trypanosoma cruzi (causing Chagas disease in Latin America), subspecies of Trypanosoma brucei, (causing human sleeping sickness in Africa) and Trypanosoma congolense (causing African Animal Trypanosomosis in livestock). Previously, we demonstrated that T. congolense TS (TconTS)-LD binds to several carbohydrates, such as 1,4-β-mannotriose. In this study we investigated the influence of TconTS3-LD on Sia transfer efficiency of TconTS1a-CD by swapping domains. in silico analysis on structure models of TconTS enzymes revealed the potential of domain swaps between TconTS1a and TconTS3 without structural disruptions of the enzymes overall topologies. Recombinant domain swapped TconTS1a/TS3 showed clear Sia transfer activity, when using fetuin and lactose as Sia donor and acceptor substrates, respectively. While Sia transfer activity remained unchanged from the level of TconTS1a, hydrolytic release of free Neu5Ac as a side product was suppressed resulting in increased transfer efficiency. Presence of 1,4-β-mannotriose during TS reactions modulates enzyme activities enhancing transfer efficiency possibly due to occupation of the binding site in TconTS1a-LD. Interestingly this effect was in the same range as that observed when swapping TconTS1a-CD and TconTS3-LD. In summary, this study demonstrate the proof-of-principle for swapping CDs and LDs of TconTS and that TconTS3-LD influences enzymatic activity of TconTS1a-CD providing evidence that LDs play pivotal roles in modulating activities and biological functions of TconTS and possibly other TS.

Siglecs as Therapeutic Targets in Cancer

Hypersialylation is a common post-translational modification of protein and lipids found on cancer cell surfaces, which participate in cell-cell interactions and in the regulation of immune responses. Sialic acids are a family of nine-carbon α-keto acids found at the outermost ends of glycans attached to cell surfaces. Given their locations on cell surfaces, tumor cells aberrantly overexpress sialic acids, which are recognized by Siglec receptors found on immune cells to mediate broad immunomodulatory signaling. Enhanced sialylation exposed on cancer cell surfaces is exemplified as "self-associated molecular pattern" (SAMP), which tricks Siglec receptors found on leukocytes to greatly down-regulate immune responsiveness, leading to tumor growth. In this review, we focused on all 15 human Siglecs (including Siglec XII), many of which still remain understudied. We also highlighted strategies that disrupt the course of Siglec-sialic acid interactions, such as antibody-based therapies and sialic acid mimetics leading to tumor cell depletion. Herein, we introduced the central roles of Siglecs in mediating pro-tumor immunity and discussed strategies that target these receptors, which could benefit improved cancer immunotherapy.

Prevention of vascular-allograft rejection by protecting the endothelial glycocalyx with immunosuppressive polymers

Systemic immunosuppression for the mitigation of immune rejection after organ transplantation causes adverse side effects and constrains the long-term benefits of the transplanted graft. Here we show that protecting the endothelial glycocalyx in vascular allografts via the enzymatic ligation of immunosuppressive glycopolymers under cold-storage conditions attenuates the acute and chronic rejection of the grafts after transplantation in the absence of systemic immunosuppression. In syngeneic and allogeneic mice that received kidney transplants, the steric and immunosuppressive properties of the ligated polymers largely protected the transplanted grafts from ischaemic reperfusion injury, and from immune-cell adhesion and thereby immunocytotoxicity. Polymer-mediated shielding of the endothelial glycocalyx following organ procurement should be compatible with clinical procedures for transplant preservation and perfusion, and may reduce the damage and rejection of transplanted organs after surgery.

Cell Properties of Lung Tissue-Resident Macrophages Propagated by Co-Culture with Lung Fibroblastic Cells from C57BL/6 and BALB/c Mice

Tissue-resident macrophages (Mø) originating from foetal precursors are maintained by self-renewal under tissue/organ-specific microenvironments (niches). We recently developed a simple propagation method applicable to tissue-resident Mø by co-culturing. Here, we examined the properties of lung tissue-resident Mø propagated by co-culturing with lung interstitial cells. The intracardially and intratracheally perfused lung from BALB/c and C57BL/6 mice could minimise the contamination of alveolar Mø and lung monocytes. Lung tissue-resident Mø could be largely propagated under standard culture media along with the propagation of lung interstitial cells demonstrating a fibroblastic morphology. Propagated lung Mø showed characteristic expression properties for Mø/monocyte markers: high expressions of CD11b, CD64 and CD206; substantial expressions of Mertk; and negative expressions of Ly6C, MHC II and Siglec-F. These properties fit with those of lung interstitial Mø of a certain population that can undergo self-renewal. Propagated fibroblastic cells by co-culturing with lung Mø possessed niche properties such as Csf1 and Tgfb1 expression. Propagated lung Mø from both the mouse types were polarised to an M2 phenotype highly expressing arginase 1 without M2 inducer treatment, whereas the M1 inducers significantly increased the iNOS-positive cell percentages in C57BL/6 mice relative to those in BALB/c mice. This is the first study to demonstrate fundamental properties of lung tissue-resident Mø propagated by co-culturing. Propagated lung Mø showing features of lung interstitial Mø can serve as an indispensable tool for investigating SARS-CoV-2 diseases, although lung interstitial Mø have gained little attention in terms of their involvement in SARS-CoV-2 disease pathology, in contrast to alveolar and recruited Mø.

Novel approaches to glycomimetic design: development of small molecular weight lectin antagonists

Introduction: The direct binding of carbohydrates or those presented on glycoproteins or glycolipids to proteins is the primary effector of many biological responses. One class of carbohydrate-binding proteins, lectins are important in all forms of life. Their functions in animals include regulating cell adhesion, glycoprotein synthesis, metabolism, and mediating immune system response while in bacteria and viruses a lectin-mediated carbohydrate-protein interaction between host cells and the pathogen initiates pathogenesis of the infection.Areas covered: In this review, the authors outline the structural and functional pathogenesis of lectins from bacteria, amoeba, and humans. Mimics of a carbohydrate are referred to as glycomimetics, which are much smaller in molecular weight and are devised to mimic the key binding interactions of the carbohydrate while also allowing additional contacts with the lectin. This article emphasizes the various approaches used over the past 10-15 years in the rational design of glycomimetic ligands.Expert opinion: Medicinal chemistry efforts enabled by X-ray structural biology have identified small-molecule glycomimetic lectin antagonists that have entered or are nearing clinical trials. A common theme in these strategies is the use of biaryl ring systems to emulate the carbohydrate interactions with the lectin.

Surface chemistry-mediated modulation of adsorbed albumin folding state specifies nanocarrier clearance by distinct macrophage subsets

Controlling nanocarrier interactions with the immune system requires a thorough understanding of the surface properties that modulate protein adsorption in biological fluids, since the resulting protein corona redefines cellular interactions with nanocarrier surfaces. Albumin is initially one of the dominant proteins to adsorb to nanocarrier surfaces, a process that is considered benign or beneficial by minimizing opsonization or inflammation. Here, we demonstrate the surface chemistry of a model nanocarrier can be engineered to stabilize or denature the three-dimensional conformation of adsorbed albumin, which respectively promotes evasion or non-specific clearance in vivo. Interestingly, certain common chemistries that have long been considered to convey stealth properties denature albumin to promote nanocarrier recognition by macrophage class A1 scavenger receptors, providing a means for their eventual removal from systemic circulation. We establish that the surface chemistry of nanocarriers can be specified to modulate adsorbed albumin structure and thereby tune clearance by macrophage scavenger receptors.

Current Status on Therapeutic Molecules Targeting Siglec Receptors

The sialic acid-binding immunoglobulin-type of lectins (Siglecs) are receptors that recognize sialic acid-containing glycans. In the majority of the cases, Siglecs are expressed on immune cells and play a critical role in regulating immune cell signaling. Over the years, it has been shown that the sialic acid-Siglec axis participates in immunological homeostasis, and that any imbalance can trigger different pathologies, such as autoimmune diseases or cancer. For all this, different therapeutics have been developed that bind to Siglecs, either based on antibodies or being smaller molecules. In this review, we briefly introduce the Siglec family and we compile a description of glycan-based molecules and antibody-based therapies (including CAR-T and bispecific antibodies) that have been designed to therapeutically targeting Siglecs.

Design, synthesis and molecular docking study of α-triazolylsialosides as non-hydrolyzable and potent CD22 ligands

Ligand 1 was the first reported example of monomeric high-affinity synthetic CD22 ligand that regulated B cell activation in vitro, augmented antibody production and regulated immune responses in mice. Replacing O-glycoside linkage of 1 by nitrogen of triazole by click reaction afforded compounds which are as potent as the parent compound. The synthesis of the new compounds is straightforward with fewer synthetic steps and higher yield. Such a strategy provided stable ligand that can bind avidly and can be conjugated to drugs for B-cell targeting or multimeric formation. The new compounds were screened for their affinity to CD22, using surface plasmon resonance (SPR). Compound 12 was obtained as a bioisosteric analogue and an anomerically stable imitation of 1. It was, also, screened for MAG to test for selectivity and analyzed by molecular docking and dynamic simulation to explore the potential binding modes and source of selectivity within CD22. Our results could enable the development of small molecule drug capable of modulating the activity of CD22 in autoimmune diseases and malignancies derived from B-cells.

A Potent Mimetic of the Siglec-8 Ligand 6'-Sulfo-Sialyl Lewisx

Siglecs are members of the immunoglobulin gene family containing sialic acid binding N-terminal domains. Among them, Siglec-8 is expressed on various cell types of the immune system such as eosinophils, mast cells and weakly on basophils. Cross-linking of Siglec-8 with monoclonal antibodies triggers apoptosis in eosinophils and inhibits degranulation of mast cells, making Siglec-8 a promising target for the treatment of eosinophil- and mast cell-associated diseases such as asthma. The tetrasaccharide 6'-sulfo-sialyl Lewis^x has been identified as a specific Siglec-8 ligand in glycan array screening. Here, we describe an extended study enlightening the pharmacophores of 6'-sulfo-sialyl Lewis^x and the successful development of a high-affinity mimetic. Retaining the neuraminic acid core, the introduction of a carbocyclic mimetic of the Gal moiety and a sulfonamide substituent in the 9-position gave a 20-fold improved binding affinity. Finally, the residence time, which usually is the Achilles tendon of carbohydrate/lectin interactions, could be improved.

A Novel Siglec-4 Derived Spacer Improves the Functionality of CAR T Cells Against Membrane-Proximal Epitopes

A domain that is often neglected in the assessment of chimeric antigen receptor (CAR) functionality is the extracellular spacer module. However, several studies have elucidated that membrane proximal epitopes are best targeted through CARs comprising long spacers, while short spacer CARs exhibit highest activity on distal epitopes. This finding can be explained by the requirement to have an optimal distance between the effector T cell and target cell. Commonly used long spacer domains are the CH2-CH3 domains of IgG molecules. However, CARs containing these spacers generally show inferior in vivo efficacy in mouse models compared to their observed in vitro activity, which is linked to unspecific Fcγ-Receptor binding and can be abolished by mutating the respective regions. Here, we first assessed a CAR therapy targeting membrane proximal CD20 using such a modified long IgG1 spacer. However, despite these mutations, this construct failed to unfold its observed in vitro cytotoxic potential in an in vivo model, while a shorter but less structured CD8α spacer CAR showed complete tumor clearance. Given the shortage of well-described long spacer domains with a favorable functionality profile, we designed a novel class of CAR spacers with similar attributes to IgG spacers but without unspecific off-target binding, derived from the Sialic acid-binding immunoglobulin-type lectins (Siglecs). Of five constructs tested, a Siglec-4 derived spacer showed highest cytotoxic potential and similar performance to a CD8α spacer in a CD20 specific CAR setting. In a pancreatic ductal adenocarcinoma model, a Siglec-4 spacer CAR targeting a membrane proximal (TSPAN8) epitope was efficiently engaged in vitro, while a membrane distal (CD66c) epitope did not activate the T cell. Transfer of the TSPAN8 specific Siglec-4 spacer CAR to an in vivo setting maintained the excellent tumor killing characteristics being indistinguishable from a TSPAN8 CD8α spacer CAR while outperforming an IgG4 long spacer CAR and, at the same time, showing an advantageous central memory CAR T cell phenotype with lower release of inflammatory cytokines. In summary, we developed a novel spacer that combines cytotoxic potential with an advantageous T cell and cytokine release phenotype, which make this an interesting candidate for future clinical applications.

Nano-immunoimaging

Immunoimaging is a rapidly growing field stoked in large part by the intriguing triumphs of immunotherapy. On the heels of immunotherapy's successes, there exists a growing need to evaluate tumor response to therapy particularly immunotherapy, stratify patients into responders vs. non-responders, identify inflammation, and better understand the fundamental roles of immune system components to improve both immunoimaging and immunotherapy. Innovative nanomaterials have begun to provide novel opportunities for immunoimaging, in part due to their sensitivity, modularity, capacity for many potentially varied ligands (high avidity), and potential for multifunctionality/multimodality imaging. This review strives to comprehensively summarize the integration of nanotechnology and immunoimaging, and the field's potential for clinical applications.

Siglecs as Immune Cell Checkpoints in Disease

Sialic acid-binding immunoglobulin-type lectins (Siglecs) are expressed on the majority of white blood cells of the immune system and play critical roles in immune cell signaling. Through recognition of sialic acid-containing glycans as ligands, they help the immune system distinguish between self and nonself. Because of their restricted cell type expression and roles as checkpoints in immune cell responses in human diseases such as cancer, asthma, allergy, neurodegeneration, and autoimmune diseases they have gained attention as targets for therapeutic interventions. In this review we describe the Siglec family, its roles in regulation of immune cell signaling, current efforts to define its roles in disease processes, and approaches to target Siglecs for treatment of human disease.

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.

A Sulfonamide Sialoside Analogue for Targeting Siglec-8 and -F on Immune Cells

The Siglec family of cell surface receptors have emerged as attractive targets for cell-directed therapies due to their restricted expression on immune cells, endocytic properties, and ability to modulate receptor signaling. Human Siglec-8, for instance, has been identified as a therapeutic target for the treatment of eosinophil and mast cell disorders. A promising strategy to target Siglecs involves the use of liposomal nanoparticles with a multivalent display of Siglec ligands. A key challenge for this approach is the identification of a high affinity ligand for the target Siglec. Here, we report the development of a ligand of Siglec-8 and its closest murine functional orthologue Siglec-F that is capable of targeting liposomes to cells expressing Siglec-8 or -F. A glycan microarray library of synthetic 9-N-sulfonyl sialoside analogues was screened to identify potential lead compounds. The best ligand, 9-N-(2-naphthyl-sulfonyl)-Neu5Acα2-3-[6-O-sulfo]-Galβ1-4GlcNAc (6'-O-sulfo ^NSANeu5Ac) combined the lead 2-naphthyl sulfonyl C-9 substituent with the preferred sulfated scaffold. The ligand 6'-O-sulfo ^NSANeu5Ac was conjugated to lipids for display on liposomes to evaluate targeted delivery to cells. Targeted liposomes showed strong in vitro binding/uptake and selectivity to cells expressing Siglec-8 or -F and, when administered to mice, exhibit in vivo targeting to Siglec-F⁺ eosinophils.

Characterization of Glycoproteins with the Immunoglobulin Fold by X-Ray Crystallography and Biophysical Techniques

Glycoproteins on the surface of cells play critical roles in cellular function, including signalling, adhesion and transport. On leukocytes, several of these glycoproteins possess immunoglobulin (Ig) folds and are central to immune recognition and regulation. Here, we present a platform for the design, expression and biophysical characterization of the extracellular domain of human B cell receptor CD22. We propose that these approaches are broadly applicable to the characterization of mammalian glycoprotein ectodomains containing Ig domains. Two suspension human embryonic kidney (HEK) cell lines, HEK293F and HEK293S, are used to express glycoproteins harbouring complex and high-mannose glycans, respectively. These recombinant glycoproteins with different glycoforms allow investigating the effect of glycan size and composition on ligand binding. We discuss protocols for studying the kinetics and thermodynamics of glycoprotein binding to biologically relevant ligands and therapeutic antibody candidates. Recombinant glycoproteins produced in HEK293S cells are amenable to crystallization due to glycan homogeneity, reduced flexibility and susceptibility to endoglycosidase H treatment. We present methods for soaking glycoprotein crystals with heavy atoms and small molecules for phase determination and analysis of ligand binding, respectively. The experimental protocols discussed here hold promise for the characterization of mammalian glycoproteins to give insight into their function and investigate the mechanism of action of therapeutics.

CD22-Binding Synthetic Sialosides Regulate B Lymphocyte Proliferation Through CD22 Ligand-Dependent and Independent Pathways, and Enhance Antibody Production in Mice

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are expressed in various immune cells and most of them carry signaling functions. High-affinity synthetic sialoside ligands have been developed for various Siglecs. Therapeutic potentials of the nanoparticles and compounds that contain multiple numbers of these sialosides and other reagents such as toxins and antigens have been demonstrated. However, whether immune responses can be regulated by monomeric sialoside ligands has not yet been known. CD22 (also known as Siglec-2) is an inhibitory molecule preferentially expressed in B lymphocytes (B cells) and is constitutively bound and functionally regulated by α2,6 sialic acids expressed on the same cell (cis-ligands). Here, we developed synthetic sialosides GSC718 and GSC839 that bind to CD22 with high affinity (IC₅₀ ~100 nM), and inhibit ligand binding of CD22. When B cells are activated by B cell antigen receptor (BCR) ligation, both GSC718 and GSC839 downregulate proliferation of B cells, and this regulation requires both CD22 and α2,6 sialic acids. This result suggests that these sialosides regulate BCR ligation-induced B cell activation by reversing endogenous ligand-mediated regulation of CD22. By contrast, GSC718 and GSC839 augment B cell proliferation induced by TLR ligands or CD40 ligation, and this augmentation requires CD22 but not α2,6 sialic acids. Thus, these sialosides appear to enhance B cell activation by directly suppressing the inhibitory function of CD22 independently of endogenous ligand-mediated regulation. Moreover, GSC839 augments B cell proliferation that depends on both BCR ligation and CD40 ligation as is the case for in vivo B cell responses to antigens, and enhanced antibody production to the extent comparable to CpG oligonuleotides or a small amount of alum. Although these known adjuvants induce production of the inflammatory cytokines or accumulation of inflammatory cells, CD22-binding sialosides do not. Thus, synthetic sialosides that bind to CD22 with high-affinity modulate B cell activation through endogenous ligand-dependent and independent pathways, and carry an adjuvant activity without inducing inflammation.

New Human CD22/Siglec-2 Ligands with a Triazole Glycoside

CD22 is a member of the Siglec family. Considerable attention has been drawn to the design and synthesis of new Siglec ligands to explore target biology and innovative therapies. In particular, CD22-ligand-targeted nanoparticles with therapeutic functions have proved successful in preclinical settings for blood cancers, autoimmune diseases, and tolerance induction. Here we report the design, synthesis and affinity evaluation of a new class of Siglec ligands: namely sialic acid derivatives with a triazole moiety replacing the natural glycoside oxygen atom. In addition, we describe important and surprising differences in binding to CD22 expressed at the cell surface for compounds with distinct valences. The new class of compounds might serve as a template for the design of ligands for other members of the Siglec family and next-generation CD22-ligand-based targeted therapies.

Glycolipids and Lectins in Endocytic Uptake Processes

A host of endocytic processes has been described at the plasma membrane of eukaryotic cells. Their categorization has most commonly referenced cytosolic machinery, of which the clathrin coat has occupied a preponderant position. In what concerns intra-membrane constituents, the focus of interest has been on phosphatidylinositol lipids and their capacity to orchestrate endocytic events on the cytosolic leaflet of the membrane. The contribution of extracellular determinants to the construction of endocytic pits has received much less attention, depite the fact that (glyco)sphingolipids are exoplasmic leaflet fabric of membrane domains, termed rafts, whose contributions to predominantly clathrin-independent internalization processes is well recognized. Furthermore, sugar modifications on extracellular domains of proteins, and sugar-binding proteins, termed lectins, have also been linked to the uptake of endocytic cargoes at the plasma membrane. In this review, we first summarize these contributions by extracellular determinants to the endocytic process. We thus propose a molecular hypothesis - termed the GL-Lect hypothesis - on how GlycoLipids and Lectins drive the formation of compositional nanoenvrionments from which the endocytic uptake of glycosylated cargo proteins is operated via clathrin-independent carriers. Finally, we position this hypothesis within the global context of endocytic pathway proposals that have emerged in recent years.

Structures of the Streptococcus sanguinis SrpA Binding Region with Human Sialoglycans Suggest Features of the Physiological Ligand

Streptococcus sanguinis is a leading cause of bacterial infective endocarditis, a life-threatening infection of heart valves. S. sanguinis binds to human platelets with high avidity, and this adherence is likely to enhance virulence. Previous studies suggest that a serine-rich repeat adhesin termed SrpA mediates the binding of S. sanguinis to human platelets via its interaction with sialoglycans on the receptor GPIbα. However, in vitro binding assays with SrpA and defined sialoglycans failed to identify specific high-affinity ligands. To improve our understanding of the interaction between SrpA and human platelets, we determined cocrystal structures of the SrpA sialoglycan binding region (SrpA_BR) with five low-affinity ligands: three sialylated trisaccharides (sialyl-T antigen, 3'-sialyllactose, and 3'-sialyl-N-acetyllactosamine), a sialylated tetrasaccharide (sialyl-Lewis^X), and a sialyl galactose disaccharide component common to these sialoglyans. We then combined structural analysis with mutagenesis to further determine whether our observed interactions between SrpA_BR and glycans are important for binding to platelets and to better map the binding site for the physiological receptor. We found that the sialoglycan binding site of SrpA_BR is significantly larger than the sialoglycans cocrystallized in this study, which suggests that binding of SrpA to platelets either is multivalent or occurs via a larger, disialylated glycan.

TIM-1 acts a dual-attachment receptor for Ebolavirus by interacting directly with viral GP and the PS on the viral envelope

Ebolavirus can cause hemorrhagic fever in humans with a mortality rate of 50%–90%. Currently, no approved vaccines and antiviral therapies are available. Human TIM1 is considered as an attachment factor for EBOV, enhancing viral infection through interaction with PS located on the viral envelope. However, reasons underlying the preferable usage of hTIM-1, but not other PS binding receptors by filovirus, remain unknown. We firstly demonstrated a direct interaction between hTIM-1 and EBOV GP in vitro and determined the crystal structures of the Ig V domains of hTIM-1 and hTIM-4. The binding region in hTIM-1 to EBOV GP was mapped by chimeras and mutation assays, which were designed based on structural analysis. Pseudovirion infection assays performed using hTIM-1 and its homologs as well as point mutants verified the location of the GP binding site and the importance of EBOV GP-hTIM-1 interaction in EBOV cellular entry.

Synthesis and biological evaluation of sialyl-oligonucleotide conjugates targeting leukocyte B trans-membranal receptor CD22 as delivery agents for nucleic acid drugs

Antisense oligonucleotides (ASOs) modified with ligands which target cell surface receptors have the potential to significantly improve potency in the target tissue. This has recently been demonstrated using triantennary N-acetyl d-galactosamine conjugated ASOs. CD22 is a cell surface receptor expressed exclusively on B cells thus presenting an attractive target for B cell specific delivery of drugs. Herein, we reported the synthesis of monovalent and trivalent ASO conjugates with biphenylcarbonyl (BPC) modified sialic acids and their study as ASO delivery agents into B cells. CD22 positive cells exhibited reduced potency when treated with ligand modified ASOs and mechanistic examination suggested reduced uptake into cells potentially as a result of sequestration of ASO by other cell-surface proteins.

Sialic Acid Mimetics to Target the Sialic Acid-Siglec Axis

Sialic acid sugars are vital regulators of the immune system through binding to immunosuppressive sialic acid-binding immunoglobulin-like lectin (Siglec) receptors on immune cells. Aberrant sialic acid-Siglec interactions are associated with an increasing number of pathologies including infection, autoimmunity, and cancer. Therefore, the sialic acid-Siglec axis is an emerging target to prevent or affect the course of several diseases. Chemical modifications of the natural sialic acid ligands have led to sialic acid mimetics (SAMs) with improved binding affinity and selectivity towards Siglecs. Recent progress in glycobiotechnology allows the presentation of these SAMs on nanoparticles, polymers, and living cells via bioorthogonal synthesis. These developments now enable the detailed study of the sialic acid-Siglec axis including its therapeutic potential as an immune modulator.

Controlled Multi-functionalization Facilitates Targeted Delivery of Nanoparticles to Cancer Cells

A major objective of nanomedicine is to combine in a controlled manner multiple functional entities into a single nanoscale device to target particles with great spatial precision, thereby increasing the selectivity and potency of therapeutic drugs. A multifunctional nanoparticle is described for controlled conjugation of a cytotoxic drug, a cancer cell targeting ligand, and an imaging moiety. The approach is based on the chemical synthesis of polyethylene glycol that at one end is modified by a thioctic acid for controlled attachment to a gold core. The other end of the PEG polymers is modified by a hydrazine, amine, or dibenzocyclooctynol moiety for conjugation with functional entities having a ketone, activated ester, or azide moiety, respectively. The conjugation approach allowed the controlled attachment of doxorubicin through an acid-labile hydrazone linkage, an Alexa Fluor dye through an amide bond, and a glycan-based ligand for the cell surface receptor CD22 of B-cells using strain promoted azide-alkyne cycloaddition. The incorporation of the ligand for CD22 led to rapid entry of the nanoparticle by receptor-mediated endocytosis. Covalent attachment of doxorubicin via hydrazone linkage caused pH-responsive intracellular release of doxorubicin and significantly enhanced the cytotoxicity of nanoparticles. A remarkable 60-fold enhancement in cytotoxicity of CD22 (+) lymphoma cells was observed compared to non- targeted nanoparticles.

Therapeutic Targeting of Siglecs using Antibody- and Glycan-Based Approaches

The sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of immunomodulatory receptors whose functions are regulated by their glycan ligands. Siglecs are attractive therapeutic targets because of their cell type-specific expression pattern, endocytic properties, high expression on certain lymphomas/leukemias, and ability to modulate receptor signaling. Siglec-targeting approaches with therapeutic potential encompass antibody- and glycan-based strategies. Several antibody-based therapies are in clinical trials and continue to be developed for the treatment of lymphoma/leukemia and autoimmune disease, while the therapeutic potential of glycan-based strategies for cargo delivery and immunomodulation is a promising new approach. Here we review these strategies with special emphasis on emerging approaches and disease areas that may benefit from targeting the Siglec family.

Chemical Lectinology: Tools for Probing the Ligands and Dynamics of Mammalian Lectins In Vivo

The importance and complexity associated with the totality of glycan structures, i.e. the glycome, has garnered significant attention from chemists and biologists alike. However, what is lacking from this biochemical picture is how cells, tissues, and organisms interpret glycan patterns and translate this information into appropriate responses. Lectins, glycan-binding proteins, are thought to bridge this gap by decoding the glycome and dictating cell fate based on the underlying chemical identities and properties of the glycome. Yet, our understanding of the in vivo ligands and function for most lectins is still incomplete. This review focuses on recent advances in chemical tools to study the specificity and dynamics of mammalian lectins in live cells. A picture emerges of lectin function that is highly sensitive to its organization, which in turn drastically shapes immunity and cancer progression. We hope this review will inspire biologists to make use of these new techniques and stimulate chemists to continue developing innovative approaches to probe lectin biology in vivo.

Sialic Acids on Varicella-Zoster Virus Glycoprotein B Are Required for Cell-Cell Fusion

Varicella-zoster virus (VZV) is a member of the human Herpesvirus family that causes varicella (chicken pox) and zoster (shingles). VZV latently infects sensory ganglia and is also responsible for encephalomyelitis. Myelin-associated glycoprotein (MAG), a member of the sialic acid (SA)-binding immunoglobulin-like lectin family, is mainly expressed in neural tissues. VZV glycoprotein B (gB) associates with MAG and mediates membrane fusion during VZV entry into host cells. The SA requirements of MAG when associating with its ligands vary depending on the specific ligand, but it is unclear whether the SAs on gB are involved in the association with MAG. In this study, we found that SAs on gB are essential for the association with MAG as well as for membrane fusion during VZV infection. MAG with a point mutation in the SA-binding site did not bind to gB and did not mediate cell-cell fusion or VZV entry. Cell-cell fusion and VZV entry mediated by the gB-MAG interaction were blocked by sialidase treatment. N-glycosylation or O-glycosylation inhibitors also inhibited the fusion and entry mediated by gB-MAG interaction. Furthermore, gB with mutations in N-glycosylation sites, i.e. asparagine residues 557 and 686, did not associate with MAG, and the cell-cell fusion efficiency was low. Fusion between the viral envelope and cellular membrane is essential for host cell entry by herpesviruses. Therefore, these results suggest that SAs on gB play important roles in MAG-mediated VZV infection.

Discovery of multifold modified sialosides as human CD22/Siglec-2 ligands with nanomolar activity on B-cells

Sialic acids are abundant in higher domains of life and lectins recognizing sialosaccharides are heavily involved in the regulation of the human immune system. Modified sialosides are useful tools to explore the functions of those lectins, especially members of the Siglec (sialic acid binding immunoglobulin like lectin) family. Here we report design, synthesis, and affinity evaluation of novel sialoside classes with combined modification at positions 2, 4, and 9 or 2, 3, 4, and 9 of the sialic acid scaffold as human CD22 (human Siglec-2) ligands. They display up to 7.5 × 10(5)-fold increased affinity over αMe Neu5Ac (the minimal Siglec ligand). CD22 is a negative regulating coreceptor of the B-cell receptor (BCR). In vitro experiments with a human B-lymphocyte cell line showed functional blocking of CD22 upon B-cell receptor (BCR) stimulation in the presence of nanomolar concentrations of the novel ligands. The observed increased Ca(2+) response corresponds to enhanced cell activation, providing an opportunity to therapeutically modulate B-lymphocyte responses, e.g., in immune deficiencies and infections.

Recent advances toward the development of inhibitors to attenuate tumor metastasis via the interruption of lectin-ligand interactions

Aberrant glycosylation is a well-recognized phenomenon that occurs on the surface of tumor cells, and the overexpression of a number of ligands (such as TF, sialyl Tn, and sialyl Lewis X) has been correlated to a worse prognosis for the patient. These unique carbohydrate structures play an integral role in cell-cell communication and have also been associated with more metastatic cancer phenotypes, which can result from binding to lectins present on cell surfaces. The most well studied metastasis-associated lectins are the galectins and selectins, which have been correlated to adhesion, neoangiogenesis, and immune-cell evasion processes. In order to slow the rate of metastatic lesion formation, a number of approaches have been successfully developed which involve interfering with the tumor lectin-substrate binding event. Through the generation of inhibitors, or by attenuating lectin and/or carbohydrate expression, promising results have been observed both in vitro and in vivo. This article briefly summarizes the involvement of lectins in the metastatic process and also describes different approaches used to prevent these undesirable carbohydrate-lectin binding events, which should ultimately lead to improvement in current cancer therapies.

PILRα and PILRβ have a siglec fold and provide the basis of binding to sialic acid

Paired immunoglobulin-like type 2 receptor α (PILRα) and β (PILRβ) belong to the PILR family and are related to innate immune regulation in various species. Despite their high sequence identity, PILRα and PILRβ are shown to have variant sialic acid (SA) binding avidities. To explore the molecular basis of this interaction, we solved the crystal structures of PILRα and PILRβ at resolutions of 1.6 Å and 2.2 Å, respectively. Both molecules adopt a typical siglec fold but use a hydrophobic bond to substitute the siglec-specific disulfide linkage for protein stabilization. We further used HSV-1 glycoprotein B (gB) as a representative molecule to study the PILR-SA interaction. Deploying site-directed mutagenesis, we demonstrated that three residues (Y2, R95, and W108) presented on the surface of PILRα form the SA binding site equivalent to those in siglecs but are arranged in a unique linear mode. PILRβ differs from PILRα in one of these three residues (L108), explaining its inability to engage gB. Mutation of L108 to tryptophan in PILRβ restored the gB-binding capacity. We further solved the structure of this PILRβ mutant complexed with SA, which reveals the atomic details mediating PILR/SA recognition. In comparison with the free PILR structures, amino acid Y2 oriented variantly in the complex structure, thereby disrupting the linear arrangement of PILR residues Y2, R95, and W108. In conclusion, our study provides significant implications for the PILR-SA interaction and paves the way for understanding PILR-related ligand binding.

Analysis of the binding sites of porcine sialoadhesin receptor with PRRSV

Porcine reproductive and respiratory syndrome virus (PRRSV) can infect pigs and cause enormous economic losses to the pig industry worldwide. Porcine sialoadhesin (pSN) and CD163 have been identified as key viral receptors on porcine alveolar macrophages (PAM), a main target cell infected by PRRSV. In this study, the protein structures of amino acids 1-119 from the pSN and cSN (cattle sialoadhesin) N-termini (excluding the 19-amino acid signal peptide) were modeled via homology modeling based on mSN (mouse sialoadhesin) template structures using bioinformatics tools. Subsequently, pSN and cSN homology structures were superposed onto the mSN protein structure to predict the binding sites of pSN. As a validation experiment, the SN N-terminus (including the wild-type and site-directed-mutant-types of pSN and cSN) was cloned and expressed as a SN-GFP chimera protein. The binding activity between SN and PRRSV was confirmed by WB (Western blotting), FAR-WB (far Western blotting), ELISA (enzyme-linked immunosorbent assay) and immunofluorescence assay. We found that the S107 amino acid residue in the pSN N-terminal played a crucial role in forming a special cavity, as well as a hydrogen bond for enhancing PRRSV binding during PRRSV infection. S107 may be glycosylated during PRRSV infection and may also be involved in forming the cavity for binding PRRSV along with other sites, including W2, Y44, S45, R97, R105, W106 and V109. Additionally, S107 might also be important for pSN binding with PRRSV. However, the function of these binding sites must be confirmed by further studies.

CH-π "T-shape" interaction with histidine explains binding of aromatic galactosides to Pseudomonas aeruginosa lectin LecA

The galactose specific lectin LecA mediates biofilm formation in the opportunistic pathogen P. aeruginosa . The interaction between LecA and aromatic β-galactoside biofilm inhibitors involves an intermolecular CH-π T-shape interaction between C(ε1)-H of residue His50 in LecA and the aromatic ring of the galactoside aglycone. The generality of this interaction was tested in a diverse family of β-galactosides. LecA binding to aromatic β-galactosides (KD ∼ 8 μM) was consistently stronger than to aliphatic β-galactosides (KD ∼ 36 μM). The CH-π interaction was observed in the X-ray crystal structures of six different LecA complexes, with shorter than the van der Waals distances indicating productive binding. Related XH/cation/π-π interactions involving other residues were identified in complexes of aromatic glycosides with a variety of carbohydrate binding proteins such as concanavalin A. Exploiting such interactions might be generally useful in drug design against these targets.

On-chip synthesis and screening of a sialoside library yields a high affinity ligand for Siglec-7

The Siglec family of sialic acid-binding proteins are differentially expressed on white blood cells of the immune system and represent an attractive class of targets for cell-directed therapy. Nanoparticles decorated with high-affinity Siglec ligands show promise for delivering cargo to Siglec-bearing cells, but this approach has been limited by a lack of ligands with suitable affinity and selectivity. Building on previous work employing solution-phase sialoside library synthesis and subsequent microarray screening, we herein report a more streamlined 'on-chip' synthetic approach. By printing a small library of alkyne sialosides and subjecting these to 'on-chip' click reactions, the largest sialoside analogue library to date was generated. Siglec-screening identified a selective Siglec-7 ligand, which when displayed on liposomal nanoparticles, allows for targeting of Siglec-7(+) cells in peripheral human blood. In silico docking to the crystal structure of Siglec-7 provides a rationale for the affinity gains observed for this novel sialic acid analogue.

Biochemical characterization and crystal structure of a GH10 xylanase from termite gut bacteria reveal a novel structural feature and significance of its bacterial Ig-like domain

Bacterial Ig-like (Big) domains are commonly distributed in glycoside hydrolases (GH), but their structure and function remains undefined. Xylanase is a GH, and catalyzes the hydrolysis of the internal β-xylosidic linkages of xylan. In this study, we report the molecular cloning, biochemical and biophysical characterization, and crystal structure of a termite gut bacterial xylanase, Xyl-ORF19, which was derived from gut bacteria of a wood-feeding termite (Globitermes brachycerastes). The protein architecture of Xyl-ORF19 reveals that it has two domains, a C-terminal GH10 catalytic domain and an N-terminal Big_2 non-catalytic domain. The catalytic domain folds in an (α/β)8 barrel as most GH10 xylanases do, but it has two extra β-strands. The non-catalytic domain is structurally similar to an immunoglobulin-like domain of intimins. The recombinant enzyme without the non-catalytic domain has fairly low catalytic activity, and is different from the full-length enzyme in kinetic parameters, pH and temperature profiles, which suggests the non-catalytic domain could affect the enzyme biochemical and biophysical properties as well as the role for enzyme localization. This study provides a molecular basis for future efforts in xylanase bioengineering.

Click and pick: identification of sialoside analogues for siglec-based cell targeting

Click 'n' chips: Azide and alkyne-bearing sialic acids (purple diamond; see picture) were subjected to high-throughput click chemistry to generate a library of sialic acid analogues. Microarray printing of the library and screening with the siglec family of sialic-acid-binding proteins, led to the identification of high-affinity ligands for siglec-9 and siglec-10.

Targeting of CD22-positive B-cell lymphoma cells by synthetic divalent sialic acid analogues

CD22 is an inhibitory co-receptor of the B-cell receptor (BCR) on B cells. Since CD22 is ubiquitously expressed in the B-cell lineage and CD22 endocytosis can be triggered efficiently, antibodies and antibody-based immunotoxins against CD22 are used to target B cells both in B-cell lymphomas and leukemias, as well as in autoimmune diseases. CD22 recognizes α2,6-linked sialic acids as endogenous ligands. We have developed new synthetic sialosides as ligands for human CD22. These sialosides bind CD22 on human B cells with high affinity and can efficiently enhance IgM-triggered Ca(2+) signaling. We coupled these sialosides to Pseudomonas exotoxin A to generate a novel CD22 ligand-based immunotoxin. This sialoside-exotoxin-A construct can specifically kill CD22-positive B-cell lymphoma cells. It binds specifically to CD22-positive B-cell lymphoma cells and is dominant over endogenous cis-ligands on the B-cell surface. The sialoside-exotoxin-A construct is efficiently internalized by endocytosis into B-cell lymphoma cell lines. Thus we show the development of a new therapeutic compound for targeting CD22 on human B cells, both for B-cell lymphoma, as well as for B-cell-mediated autoimmune diseases.