Natural ligands for CD33-related Siglecs?

CD33 as a leukocyte-associated marker expressed on human spermatozoa

OBJECTIVE

Sialic acid-binding immunoglobulin-type lectins (Siglecs) are commonly present on immune cells and often mediate cell-to-cell interactions and signaling. Studies have shown the presence of Siglecs 1, 2, 5, 6, 10 and 14 on human spermatozoa. To the best of our knowledge, the expression of CD33 on spermatozoa has not yet been studied. Semen samples were collected from 25 healthy men with normal semen status. CD33 expression on purified spermatozoa was evaluated by flow cytometry methods.

RESULTS

The results demonstrate the expression of CD33 on the surface of purified spermatozoa. The mean (± SD) of MFI (mean fluorescence intensity) was 12.85 (± 1.33) and the mean percentage of spermatozoa that express CD33 was 73.75 (± 3.75). Results were obtained showing that spermatozoa express CD33 (or Siglec-3) on their surface. The physiological role of these molecules on spermatozoa remains to be determined. It is recommended that further research be undertaken regarding the role of Siglecs (such as CD33) on spermatozoa apoptosis.

Siglecs in allergy and asthma

The term "allergic diseases" encompasses several common, IgE-mediated conditions that range from being annoying to those that are life-threatening. Available treatments include active avoidance of the instigating allergen and the use of a variety of oral, inhaled, intranasal, intraocular and injected agents. While most individuals with allergies do well with existing therapies, there are still unmet therapeutic needs. Siglecs (sialic acid-binding, immunoglobulin-like lectins) are a family of single-pass transmembrane I-type lectins found on various subsets of cells, especially those of the immune system. All Siglecs have extracellular domains recognizing sialoside ligands, and most contain cytoplasmic domains with inhibitory signaling activity. This review focuses on Siglecs that likely play a role in regulating allergic and asthmatic responses, and how specific Siglecs, expressed on cells such as eosinophils and mast cells, are being targeted for therapeutic benefit.

Non-canonical roles of Siglecs: Beyond sialic acid-binding and immune cell modulation

Siglecs (Sialic acid-binding immunoglobulin-type lectins) are I-type lectins that bind with sialic acid ligands (Sia). Most are expressed on the surface of leukocytes and are involved in immune regulation and possess immune tyrosine-based inhibitory motif (ITIM) in the intracellular domain, thus leading to inhibition of the immune response. This signaling is instrumental in maintaining quiescence under physiological conditions and acts as a brake for inflammatory cascades. By contrast, activating Siglecs carry positively charged residues in the transmembrane domain and interact with immune tyrosine-based activating motif (ITAM)-containing proteins, a DNAX-activating protein of 10-12 kDa (DAP10/12), to activate immune cells. There are various characteristics of Siglecs that do not fit within the classification of Siglec receptors as being either inhibitory or activating in nature. This review focuses on elucidating the non-canonical functions and interactions of Siglec receptors, which include Sia-independent interactions such as protein-protein interactions and interactions with lipids or other sugars. This review also summarizes Siglec expression and function on non-immune cells, and non-classical signaling of the receptor. Thus, this review will be beneficial to researchers interested in the field of Siglecs and sialic acid biology.

Phagocytosis: Phenotypically Simple Yet a Mechanistically Complex Process

Phagocytosis is a pivotal immunological process, and its discovery by Elia Metchnikoff in 1882 was a step toward the establishment of the innate immune system as a separate branch of immunology. Elia Metchnikoff received the Nobel Prize in physiology and medicine for this discovery in 1908. Since its discovery almost 140 years before, phagocytosis remains the hot topic of research in immunology. The phagocytosis research has seen a great advancement since its first discovery. Functionally, phagocytosis is a simple immunological process required to engulf and remove pathogens, dead cells and tumor cells to maintain the immune homeostasis. However, mechanistically, it is a very complex process involving different mechanisms, induced and regulated by several pattern recognition receptors, soluble pattern recognition molecules, scavenger receptors (SRs) and opsonins. These mechanisms involve the formation of phagosomes, their maturation into phagolysosomes causing pathogen destruction or antigen synthesis to present them to major histocompatibility complex molecules for activating an adaptive immune response. Any defect in this mechanism may predispose the host to certain infections and inflammatory diseases (autoinflammatory and autoimmune diseases) along with immunodeficiency. The article is designed to discuss its mechanistic complexity at each level, varying from phagocytosis induction to phagolysosome resolution.

Microglial Drug Targets in AD: Opportunities and Challenges in Drug Discovery and Development

Alzheimer’s disease (AD) is a large and increasing unmet medical need with no disease-modifying treatment currently available. Genetic evidence from genome-wide association studies (GWASs) and gene network analysis has clearly revealed a key role of the innate immune system in the brain, of which microglia are the most important element. Single-nucleotide polymorphisms (SNPs) in genes predominantly expressed in microglia have been associated with altered risk of developing AD. Furthermore, microglia-specific pathways are affected on the messenger RNA (mRNA) expression level in post-mortem AD tissue and in mouse models of AD. Together these findings have increased the interest in microglia biology, and numerous scientific reports have proposed microglial molecules and pathways as drug targets for AD. Target identification and validation are generally the first steps in drug discovery. Both target validation and drug lead identification for central nervous system (CNS) targets and diseases entail additional significant obstacles compared to peripheral targets and diseases. This makes CNS drug discovery, even with well-validated targets, challenging. In this article, we will illustrate the special challenges of AD drug discovery by discussing the viability/practicality of possible microglia drug targets including cluster of differentiation 33 (CD33), K_Ca3.1, kynurenines, ionotropic P2 receptor 7 (P2X7), programmed death-1 (PD-1), Toll-like receptors (TLRs), and triggering receptor expressed in myeloid cells 2 (TREM2).

Targeting sialic acid-Siglec interactions to reverse immune suppression in cancer

Changes in sialic acids in cancer have been observed for many years. In particular, the increase of sialoglycan density or hypersialylation in tumors has been described. Recent studies have identified mechanisms for immune evasion based on sialoglycan interactions with immunoregulatory Siglec receptors that are exploited by tumor cells and microorganisms alike. Siglecs are mostly inhibitory receptors similar to known immune checkpoints including PD-1 or CTLA-4 that are successfully targeted with blocking antibodies for cancer immunotherapy. Here, we summarize the known changes of sialic acids in cancer and the role Siglec receptors play in cancer immunity. We also focus on potential ways to target these Siglec receptors or sialoglycans in order to improve anti-cancer immunity.

The Neuro-Immune-Regulators (NIREGs) Promote Tissue Resilience; a Vital Component of the Host's Defense Strategy against Neuroinflammation

An effective protective inflammatory response in the brain is crucial for the clearance of pathogens (e.g. microbes, amyloid fibrils, prion^SC) and should be closely regulated. However, the CNS seems to have limited tissue resilience to withstand the detrimental effects of uncontrolled inflammation compromising functional recovery and tissue repair. Newly described neuro-immune-regulators (NIREGs) are functionally related proteins regulating the severity and duration of the host inflammatory response. NIREGs such as CD200, CD47 and CX3CL1 are vital for increasing tissue resilience and are constitutively expressed by neurons. The interaction with co-receptors (CD200R, CD172a, CX3CR1) will maintain microglia in the resting phenotype, directing aggressive microglia phenotype and limiting bystander injuries. Neurons can also express many of the complement NIREGs (CD55, CD46, CD59 and factor H). Neurons and glia also express suppressor of cytokine signaling proteins (SOCS) down regulating janus kinase-signal transducer and activator of transcription (JAK/STAT) pathway and to lead to the polarization of microglia towards anti-inflammatory phenotype. Other NIREGs such as serine protease inhibitors (serpins) and thrombomodulin (CD141) inhibit neurotoxic systemic coagulation proteins such as thrombin. The unfolded protein response (UPR) detects misfolded proteins and other stressors to prevent irreversible cell injury. Microglial pattern recognition receptors (PRR) (TREM-2, CR3, FcγR) are important to clear apoptotic cells and cellular debris but in non-phlogystic manner through inhibitory signaling pathways. The TYRO3, Axl, Mer (TAM) tyrosine receptor kinases activated by Gas 6 and PROS1 regulate inflammation by inhibiting Toll like receptors (TLR) /JAK-STAT activation and contribute to NIREG's functions.

Microglial Lectins in Health and Neurological Diseases

Microglia are the innate sentinels of the central nervous system (CNS) and are responsible for the homeostasis and immune defense of the CNS. Under the influence of the local environment and cell-cell interaction, microglia exhibit a multidimensional and context-dependent phenotypes that can be cytotoxic and neuroprotective. Recent studies suggest that microglia express multitudinous types of lectins, including galectins, Siglecs, mannose-binding lectins (MBLs) and other glycan binding proteins. Because most studies that examine lectins focus on the peripheral system, the functions of lectins have not been critically investigated in the CNS. In addition, the types of brain cells that contribute to the altered levels of lectins present in diseases are often unclear. In this review, we will discuss how galectins, Siglecs, selectins and MBLs contribute to the dynamic functions of microglia. The interacting ligands of these lectins are complex glycoconjugates, which consist of glycoproteins and glycolipids that are expressed on microglia or surrounding cells. The current understanding of the heterogeneity and functions of glycans in the brain is limited. Galectins are a group of pleotropic proteins that recognize both β-galactoside-containing glycans and non- β-galactoside-containing proteins. The function and regulation of galectins have been implicated in immunomodulation, neuroinflammation, apoptosis, phagocytosis and oxidative bursts. Most Siglecs are expressed at a low level on the plasma membrane and bind to sialic acid residues for immunosurveillance and cell-cell communication. Siglecs are classified based on their inhibitory and activatory downstream signaling properties. Inhibitory Siglecs negatively regulate microglia activation upon recognizing the intact sialic acid patterns and vice versa. MBLs are expressed upon infection in cytoplasm and can be secreted in order to recognize molecules containing terminal mannose as an innate immune defense machinery. Most importantly, multiple studies have reported dysregulation of lectins in neurological disorders. Here, we reviewed recent studies on microglial lectins and their functions in CNS health and disease, and suggest that these lectin families are novel, potent therapeutic targets for neurological diseases.

Pathogen-Derived Carbohydrate Recognition in Molluscs Immune Defense

Self-nonself discrimination is a common theme for all of the organisms in different evolutionary branches, which is also the most fundamental step for host immune protection. Plenty of pattern recognition receptors (PRRs) with great diversity have been identified from different organisms to recognize various pathogen-associated molecular patterns (PAMPs) in the last two decades, depicting a complicated scene of host-pathogen interaction. However, the detailed mechanism of the complicate PAMPs–PRRs interactions at the contacting interface between pathogens and hosts is still not well understood. All of the cells are coated by glycosylation complex and thick carbohydrates layer. The different polysaccharides in extracellular matrix of pathogen-host are important for nonself recognition of most organisms. Coincidentally, massive expansion of PRRs, majority of which contain recognition domains of Ig, leucine-rich repeat (LRR), C-type lectin (CTL), C1q and scavenger receptor (SR), have been annotated and identified in invertebrates by screening the available genomic sequence. The phylum Mollusca is one of the largest groups in the animal kingdom with abundant biodiversity providing plenty of solutions about pathogen recognition and immune protection, which might offer a suitable model to figure out the common rules of immune recognition mechanism. The present review summarizes the diverse PRRs and common elements of various PAMPs, especially focusing on the structural and functional characteristics of canonical carbohydrate recognition proteins and some novel proteins functioning in molluscan immune defense system, with the objective to provide new ideas about the immune recognition mechanisms.

Evidence for C1q-mediated crosslinking of CD33/LAIR-1 inhibitory immunoreceptors and biological control of CD33/LAIR-1 expression

C1q collagen-like region (CLR) engaging and activating the LAIR-1 inhibitory immunoreceptor represents a non-complement mechanism for maintaining immune quiescence. Given the binding promiscuity of C1q’s globular region (gC1q), we hypothesized that C1q concurrently associates with distinct inhibitory immunoreceptors to produce C1q-mediated modulatory networking. Like LAIR-1, CD33 inhibitory immunoreceptors are highly expressed on monocytes. Binding CD33 restricts cell activation/differentiation; however, natural ligands for CD33 remain elusive. CD33 has IgC2-like domains potentially recognized by gC1q. Thus, we asked whether C1q binds to CD33 and if C1q mediates CD33/LAIR-1 crosslinking. Our findings demonstrate that C1q and gC1q interact with CD33 to activate its inhibitory motifs, while CLR does not. Whole C1q is required to crosslink CD33 and LAIR-1 and concurrently activate CD33/LAIR-1 inhibitory motifs. While C1q binds CD33C2 domains, decreased C1q-CD33 interactions resulting from sialic acid masking of CD33C2 domains suggests a process for regulating C1q-CD33 activity. Consistent with defective self-tolerance, CD33/LAIR-1 expression is reduced in systemic lupus erythematosus (SLE) myelomonocytes. The anti-inflammatory cytokine M-CSF, but not DC growth factors, sustains CD33/LAIR-1 expression on both healthy and SLE cells suggesting further biological control of C1q-CD33/LAIR-1 processes.

Soluble Siglec-5 associates to PSGL-1 and displays anti-inflammatory activity

Interactions between endothelial selectins and the leukocyte counter-receptor PSGL1 mediates leukocyte recruitment to inflammation sites. PSGL1 is highly sialylated, making it a potential ligand for Siglec-5, a leukocyte-receptor that recognizes sialic acid structures. Binding assays using soluble Siglec-5 variants (sSiglec-5/C4BP and sSiglec-5/Fc) revealed a dose- and calcium-dependent binding to PSGL1. Pre-treatment of PSGL1 with sialidase reduced Siglec-5 binding by 79 ± 4%. In confocal immune-fluorescence assays, we observed that 50% of Peripheral Blood Mononuclear Cells (PBMCs) simultaneously express PSGL1 and Siglec-5. Duolink-proximity ligation analysis demonstrated that PSGL1 and Siglec-5 are in close proximity (<40 nm) in 31 ± 4% of PBMCs. In vitro perfusion assays revealed that leukocyte-rolling over E- and P-selectin was inhibited by sSiglec-5/Fc or sSiglec-5/C4BP, while adhesion onto VCAM1 was unaffected. When applied to healthy mice (0.8 mg/kg), sSiglec-5/C4BP significantly reduced the number of rolling leukocytes under basal conditions (10.9 ± 3.7 versus 23.5 ± 9.3 leukocytes/field/min for sSiglec-5/C4BP-treated and control mice, respectively; p = 0.0093). Moreover, leukocyte recruitment was inhibited over a 5-h observation period in an in vivo model of TNFalpha-induced inflammation following injection sSiglec-5/C4BP (0.8 mg/kg). Our data identify PSGL1 as a ligand for Siglec-5, and soluble Siglec-5 variants appear efficient in blocking PSGL1-mediated leukocyte rolling and the inflammatory response in general.

Galectin-1 and Galectin-3 Constitute Novel-Binding Partners for Factor VIII

OBJECTIVE

Recent studies have demonstrated that galectin-1 (Gal-1) and galectin-3 (Gal-3) can bind von Willebrand factor and directly modulate von Willebrand factor-dependent early thrombus formation in vivo. Because the glycans expressed on human factor VIII (FVIII) are similar to those of von Willebrand factor, we investigated whether galectins might also bind and modulate the activity of FVIII.

APPROACH AND RESULTS

Immunosorbant assays and surface plasmon resonance analysis confirmed that Gal-1 and Gal-3 bound purified FVIII with high affinity. Exoglycosidase removal of FVIII N-linked glycans significantly reduced binding to both Gal-1 and Gal-3. Moreover, combined removal of both the N- and O-glycans of FVIII further attenuated Gal-3 binding. Notably, specific digestion of FVIII high-mannose glycans at N239 and N2118 significantly impaired FVIII affinity for Gal-1. Importantly Gal-1, but not Gal-3, bound to free FVIII in the plasma milieu, and significantly inhibited FVIII functional activity. Interestingly, commercial recombinant FVIII (rFVIII) concentrates are manufactured in different cell lines and differ in their glycosylation profiles. Although the biological mechanism has not been defined, recent studies in previously untreated patients with severe hemophilia A reported significant differences in inhibitor development associated with different rFVIII products. Interestingly, Gal-1 and Gal-3 both displayed enhanced affinity for BHK-rFVIII compared with CHO-rFVIII. Furthermore, binding of Gal-1 and Gal-3 to BDD-FVIII was markedly reduced compared with full-length rFVIII.

CONCLUSIONS

We have identified Gal-1 and Gal-3 as novel-binding partners for human FVIII and demonstrated that Gal-1 binding can influence the procoagulant activity of FVIII.

Gene conversions are frequent but not under positive selection in the Siglec gene families of primates

Siglecs are cell surface proteins that belong to the immunoglobulin superfamily and which bind sialic acids. They are composed of two groups, the conserved Siglecs and the CD33-related Siglecs. Previous studies have reported the occurrence of gene conversions between human CD33-related Siglecs and suggested that these conversions are adaptive because they increase the diversity of these immunoglobulin-related genes. Here, we analyze the Siglec genes of five primate species and show that gene conversions are not observed between conserved Siglec genes but that they are frequent between primate CD33-related Siglecs. The gene conversions between CD33-related Siglec genes only occur between similar genes and equally frequently in sialic acid binding and nonbinding domains. Furthermore, dN/dS ratio tests show that most of the Ig-like V-type 1 and the Ig-like C2-type 1 domains of Siglec genes evolve either neutrally or under purifying selection and that gene conversions were not responsible for the positively selected regions detected in the Ig-like V-type1 domain of the human SIGLEC7 and SIGLEC9 genes. Our results suggest that the frequent gene conversions between CD33-related Siglec genes are simply a consequence of the high degree of sequence similarity of these genes and that they are not adaptive.

Exploration of sialic acid diversity and biology using sialoglycan microarrays

Sialic acids (Sias) are a group of α-keto acids with a nine-carbon backbone, which display many types of modifications in nature. The diversity of natural Sia presentations is magnified by a variety of glycosidic linkages to underlying glycans, the sequences and classes of such glycans, as well as the spatial organization of Sias with their surroundings. This diversity is closely linked to the numerous and varied biological functions of Sias. Relatively large libraries of natural and unnatural Sias have recently been chemically/chemoenzymatically synthesized and/or isolated from natural sources. The resulting sialoglycan microarrays have proved to be valuable tools for the exploration of diversity and biology of Sias. Here we provide an overview of Sia diversity in nature, the approaches used to generate sialoglycan microarrays, and the achievements and challenges arising.

T cell regulation mediated by interaction of soluble CD52 with the inhibitory receptor Siglec-10

Functionally diverse T cell populations interact to maintain homeostasis of the immune system. We found that human and mouse antigen-activated T cells with high expression of the lymphocyte surface marker CD52 suppressed other T cells. CD52(hi)CD4(+) T cells were distinct from CD4(+)CD25(+)Foxp3(+) regulatory T cells. Their suppression was mediated by soluble CD52 released by phospholipase C. Soluble CD52 bound to the inhibitory receptor Siglec-10 and impaired phosphorylation of the T cell receptor-associated kinases Lck and Zap70 and T cell activation. Humans with type 1 diabetes had a lower frequency and diminished function of CD52(hi)CD4(+) T cells responsive to the autoantigen GAD65. In diabetes-prone mice of the nonobese diabetic (NOD) strain, transfer of lymphocyte populations depleted of CD52(hi) cells resulted in a substantially accelerated onset of diabetes. Our studies identify a ligand-receptor mechanism of T cell regulation that may protect humans and mice from autoimmune disease.

Siglec-8 as a drugable target to treat eosinophil and mast cell-associated conditions

Siglecs (sialic acid immunoglobulin-like lectins) are members of the immunoglobulin gene family that contain sialoside binding N-terminal domains. They are cell surface proteins found predominantly on cells of the immune system. Among them, Siglec-8 is uniquely expressed by human eosinophils and mast cells, as well as basophils. Engaging this structure with antibodies or glycan ligands results in apoptosis in human eosinophils and inhibition of release of preformed and newly generated mediators from human mast cells without affecting their survival. Pro-apoptotic effects are also seen when its closest functional paralog, Siglec-F, on mouse eosinophils is similarly engaged in vitro, and beneficial effects are observed after administration of Siglec-F antibody using models of eosinophilic pulmonary and gastrointestinal inflammation in vivo. Siglec-8 targeting may thus provide a means to specifically inhibit or deplete these cell types. Cell-directed therapies are increasingly sought after by the pharmaceutical industry for their potential to reduce side effects and increase safety. The challenge is to identify suitable targets on the cell type of interest, and selectively deliver a therapeutic agent. By targeting Siglec-8, monoclonal antibodies and glycan ligand-conjugated nanoparticles may be ideally suited for treatment of eosinophil and mast cell-related diseases, such as asthma, chronic rhinosinusitis, chronic urticaria, hypereosinophilic syndromes, mast cell and eosinophil malignancies and eosinophilic gastrointestinal disorders.

Multifarious roles of sialic acids in immunity

Sialic acids are a diverse family of monosaccharides widely expressed on all cell surfaces of vertebrates and so-called "higher" invertebrates, and on certain bacteria that interact with vertebrates. This overview surveys examples of biological roles of sialic acids in immunity, with emphasis on an evolutionary perspective. Given the breadth of the subject, the treatment of individual topics is brief. Subjects discussed include biophysical effects regulation of factor H; modulation of leukocyte trafficking via selectins; Siglecs in immune cell activation; sialic acids as ligands for microbes; impact of microbial and endogenous sialidases on immune cell responses; pathogen molecular mimicry of host sialic acids; Siglec recognition of sialylated pathogens; bacteriophage recognition of microbial sialic acids; polysialic acid modulation of immune cells; sialic acids as pathogen decoys or biological masks; modulation of immunity by sialic acid O-acetylation; sialic acids as antigens and xeno-autoantigens; antisialoglycan antibodies in reproductive incompatibility; and sialic-acid-based blood groups.

Factor VIII and von Willebrand factor are ligands for the carbohydrate-receptor Siglec-5

BACKGROUND

Factor VIII (FVIII) and von Willebrand factor (VWF) circulate in plasma in a tight non-covalent complex, being critical to hemostasis. Although structurally unrelated, both share the presence of sialylated glycan-structures, making them potential ligands for sialic-acid-binding-immunoglobulin-like-lectins (Siglecs).

DESIGN AND METHODS

We explored the potential interaction between FVIII/VWF and Siglec-5, a receptor expressed in macrophages using various experimental approaches, including binding experiments with purified proteins and cell-binding studies with Siglec-5 expressing cells. Finally, Siglec-5 was overexpressed in mice via hydrodynamic gene transfer.

RESULTS

In different systems using purified proteins, saturable, dose-dependent and reversible interactions between a soluble Siglec-5 fragment and both hemostatic proteins were found. Sialidase treatment of VWF resulted in a complete lack of Siglec-5 binding. In contrast, sialidase treatment left interactions between FVIII and Siglec-5 unaffected. FVIII and VWF also bound to cellsurface exposed Siglec-5, as was visualized by classical immunostaining as well as by Duolinkproximity ligation assays. Co-localization of FVIII and VWF with early endosomal markers further suggested that binding to Siglec-5 is followed by endocytosis of the proteins. Finally, overexpression of human Siglec-5 in murine hepatocytes following hydrodynamic gene transfer resulted in a significant decrease in plasma levels of FVIII and VWF in these mice.

CONCLUSIONS

Our data indicate that FVIII and VWF may act as a ligand for Siglec-5, and that Siglec-5 may contribute to the regulation of plasma levels of the FVIII/VWF complex.

Microglial activatory (immunoreceptor tyrosine-based activation motif)- and inhibitory (immunoreceptor tyrosine-based inhibition motif)-signaling receptors for recognition of the neuronal glycocalyx

Microglia sense intact or lesioned cells of the central nervous system (CNS) and respond accordingly. To fulfill this task, microglia express a whole set of recognition receptors. Fc receptors and DAP12 (TYROBP)-associated receptors such as microglial triggering receptor expressed on myeloid cells-2 (TREM2) and the complement receptor-3 (CR3, CD11b/CD18) trigger the immunoreceptor tyrosine-based activation motif (ITAM)-signaling cascade, resulting in microglial activation, migration, and phagocytosis. Those receptors are counter-regulated by immunoreceptor tyrosine-based inhibition motif (ITIM)-signaling receptors, such as sialic acid-binding immunoglobulin superfamily lectins (Siglecs). Siglecs recognize the sialic acid cap of healthy neurons thus leading to an ITIM signaling that turns down microglial immune responses and phagocytosis. In contrast, desialylated neuronal processes are phagocytosed by microglial CR3 signaling via an adaptor protein containing an ITAM. Thus, the aberrant terminal glycosylation of neuronal surface glycoproteins and glycolipids could serve as a flag for microglia, which display a multitude of diverse carbohydrate-binding receptors that monitor the neuronal physical condition and respond via their ITIM- or ITAM-signaling cascade accordingly.

Siglecs as sensors of self in innate and adaptive immune responses

Siglecs are expressed on most white blood cells of the immune system and are known to modulate the activity of cell signaling receptors via regulatory motifs in their cytoplasmic domains. This immunoglobulin subfamily of coreceptors recognize sialic acid containing glycans as ligands, which are found on glycoproteins and glycolipids of all mammalian cells. By virtue of their ability to recognize this common structural element, siglecs are increasingly recognized for their ability to help immune cells distinguish between self and nonself, and dampen autoimmune responses.

Identification of galectin-1 and galectin-3 as novel partners for von Willebrand factor

OBJECTIVE

Although von Willebrand factor (VWF) is a heavily glycosylated protein, its potential to associate with glycan-binding proteins is poorly investigated. Here, we explored its interaction with the glycan-binding proteins galectin-1 and galectin-3.

METHODS AND RESULTS

Immunofluorescence analysis using Duolink proximity ligation assays revealed that VWF colocalizes with galectin-1 and galectin-3 in endothelial cells, both before and after stimulation of endothelial cells. Moreover, galectin-1 was found along the typical VWF bundles that are released by endothelial cells. Galectin-1 and galectin-3 could be coprecipitated with VWF from plasma in immunoprecipitation assays, whereas plasma levels of galectin-1 and galectin-3 were significantly reduced in VWF-deficient mice. Binding studies using purified proteins confirmed that VWF could directly interact with both galectins, predominantly via its N-linked glycans. In search of the physiological relevance of the VWF-galectin interaction, we found that inhibition of galectins in in vitro perfusion assays was associated with increased VWF-platelet string formation, a phenomenon that was reproduced in galectin-1/galectin-3 double-deficient mice. These mice were also characterized by a more rapid formation of initial thrombi following ferric chloride-induced injury.

CONCLUSIONS

We have identified galectin-1 and galectin-3 as novel partners for VWF, and these proteins may modulate VWF-mediated thrombus formation.

Microglial immunoreceptor tyrosine-based activation and inhibition motif signaling in neuroinflammation

Elimination of extracellular aggregates and apoptotic neural membranes without inflammation is crucial for brain tissue homeostasis. In the mammalian central nervous system, essential molecules in this process are the Fc receptors and the DAP12-associated receptors which both trigger the microglial immunoreceptor tyrosine-based activation motif- (ITAM-) Syk-signaling cascade. Microglial triggering receptor expressed on myeloid cells-2 (TREM2), signal regulatory protein-β1, and complement receptor-3 (CD11b/CD18) signal via the adaptor protein DAP12 and activate phagocytic activity of microglia. Microglial ITAM-signaling receptors are counter-regulated by immunoreceptor tyrosine-based inhibition motif- (ITIM-) signaling molecules such as sialic acid-binding immunoglobulin superfamily lectins (Siglecs). Siglecs can suppress the proinflammatory and phagocytic activity of microglia via ITIM signaling. Moreover, microglial neurotoxicity is alleviated via interaction of Siglec-11 with sialic acids on the neuronal glycocalyx. Thus, ITAM- and ITIM-signaling receptors modulate microglial phagocytosis and cytokine expression during neuroinflammatory processes. Their dysfunction could lead to impaired phagocytic clearance and neurodegeneration triggered by chronic inflammation.

Alleviation of neurotoxicity by microglial human Siglec-11

Sialic acid-binding Ig superfamily lectins (Siglecs) are members of the Ig superfamily that recognize sialic acid residues of glycoproteins. Siglec-11 is a recently identified human-specific CD33-related Siglec that binds to alpha2,8-linked polysialic acids and is expressed on microglia, the brain resident innate immune cells. Polysialylated neuronal cell adhesion molecule (PSA-NCAM) is a putative ligand of Siglec-11. We observed gene transcription and protein expression of Siglec-11 splice variant 2 in human brain tissue samples by RT-PCR and Western blot analysis. Siglec-11 was detected on microglia in human brain tissue by immunohistochemistry. Human Siglec-11 splice variant 2 was ectopically expressed by a lentiviral vector system in cultured murine microglial cells. Stimulation of Siglec-11 by cross-linking suppressed the lipopolysaccharides (LPS)-induced gene transcription of the proinflammatory mediators interleukin-1beta and nitric oxide synthase-2 in microglia. Furthermore, phagocytosis of apoptotic neuronal material was reduced in Siglec-11 transduced microglia. Expression of PSA-NCAM was detected on microglia and neurons by immunohistochemistry and RT-PCR. Coculture of microglia transduced with Siglec-11 and neurons demonstrated neuroprotective function of Siglec-11. The neuroprotective effect of Siglec-11 was dependent on polysialic acid (PSA) residues on neurons, but independent on PSA on microglia. Thus, data demonstrate that human Siglec-11 ectopically expressed on murine microglia interacts with PSA on neurons, reduces LPS-induced gene transcription of proinflammatory mediators, impairs phagocytosis and alleviates microglial neurotoxicity.

Metabolic labeling of glycoconjugates with photocrosslinking sugars

Protein-carbohydrate interactions play essential roles in a variety of biological processes. This class of interactions is particularly important in development, immunology, infection, and carcinogenesis. However, the transient nature of glycan-dependent interactions hampers efforts to detect and characterize these complexes. Photocrosslinking is emerging as a powerful tool to discover and study glycan-dependent complexes. Through the use of photocrosslinking groups, UV irradiation can be employed to introduce a covalent bond between two transiently interacting molecules. Here we describe the use of metabolic oligosaccharide engineering to incorporate a photocrosslinkable sugar into cellular glycoconjugates and the use of this photocrosslinker to covalently capture glycan-mediated interactions.

Glycosynaptic microdomains controlling tumor cell phenotype through alteration of cell growth, adhesion, and motility

Glycosphingolipids (GSLs) GM3 (NeuAcalpha3Galbeta4Glcbeta1Cer) and GM2 (GalNAcbeta4[NeuAcalpha3]Galbeta4Glcbeta1Cer) inhibit (i) cell growth through inhibition of tyrosine kinase associated with growth factor receptor (GFR), (ii) cell adhesion/motility through inhibition of integrin-dependent signaling via Src kinases, or (iii) both cell growth and motility by blocking "cross-talk" between integrins and GFRs. These inhibitory effects are enhanced when GM3 or GM2 are in complex with specific tetraspanins (TSPs) (CD9, CD81, CD82). Processes (i)-(iii) occur through specific organization of GSLs with key molecules (TSPs, caveolins, GFRs, integrins) in the glycosynaptic microdomain. Some of these processes are shared with epithelial-mesenchymal transition induced by TGFbeta or under hypoxia, particularly that associated with cancer progression.