Siglec-E Negatively Regulates the Activation of TLR4 by Controlling Its Endocytosis

Roles for Siglec-glycan interactions in regulating immune cells

Cell surface complex carbohydrates, known as glycans, are positioned to be the first point of contact between two cells. Indeed, interactions between glycans with glycan-binding can modulate cell-cell interactions. This concept is particularly relevant for immune cells, which use an array of glycan-binding proteins to help in the process of differentiating 'self' from 'non-self'. This is exemplified by the sialic acid-binding immunoglobulin-type lectins (Siglecs), which recognize sialic acid. Given that sialic acid is relatively unique to vertebrates, immune cells leverage Siglecs to recognize sialic acid as a marker of 'self'. Siglecs serve many biological roles, with most of these functions regulated through interactions with their sialoglycan ligands. In this review, we provide a comprehensive update on the ligands of Siglecs and how Siglec-sialoglycan interactions help regulate immune cells in the adaptive and innate immune system.

Identification of a potential sialylation-related pattern for the prediction of prognosis and immunotherapy response in small cell lung cancer

Our study aimed to establish a novel system for quantifying sialylation patterns and comprehensively analyze their relationship with immune cell infiltration (ICI) characterization, prognosis, and therapeutic sensitivity in small cell lung cancer (SCLC). We conducted a thorough assessment of the sialylation patterns in 100 patients diagnosed with SCLC. Our primary focus was on analyzing the expression levels of 7 prognostic sialylation-related genes. To evaluate and quantify these sialylation patterns, we devised a sialylation score (SS) using principal component analysis algorithms. Prognostic value and therapeutic sensitivities were then evaluated using multiple methods. The GSE176307 was used to verify the predictive ability of SS for immunotherapy. Our study identified 2 distinct clusters based on sialylation patterns. Sialylation cluster B exhibited a lower level of induced ICI therapy and immune-related signaling enrichment, which was associated with a poorer prognosis. Furthermore, there were significant differences in prognosis, response to targeted inhibitors, and immunotherapy between the high and low SS groups. Patients with high SS were characterized by decreased immune cell infiltration, chemokine and immune checkpoint expression, and poorer response to immunotherapy, while the low SS group was more likely to benefit from immunotherapy. This work showed that the evaluation of sialylation subtypes will help to gain insight into the heterogeneity of SCLC. The quantification of sialylation patterns played a non-negligible role in the prediction of ICI characterization, prognosis and individualized therapy strategies.

Sialic acid in the regulation of blood cell production, differentiation and turnover

Sialic acid is a unique sugar moiety that resides in the distal and most accessible position of the glycans on mammalian cell surface and extracellular glycoproteins and glycolipids. The potential for sialic acid to obscure underlying structures has long been postulated, but the means by which such structural changes directly affect biological processes continues to be elucidated. Here, we appraise the growing body of literature detailing the importance of sialic acid for the generation, differentiation, function and death of haematopoietic cells. We conclude that sialylation is a critical post-translational modification utilized in haematopoiesis to meet the dynamic needs of the organism by enforcing rapid changes in availability of lineage-specific cell types. Though long thought to be generated only cell-autonomously within the intracellular ER-Golgi secretory apparatus, emerging data also demonstrate previously unexpected diversity in the mechanisms of sialylation. Emphasis is afforded to the mechanism of extrinsic sialylation, whereby extracellular enzymes remodel cell surface and extracellular glycans, supported by charged sugar donor molecules from activated platelets.

RNA Sequencing of Whole Blood in Premature Coronary Artery Disease: Identification of Novel Biomarkers and Involvement of T Cell Imbalance

Serum biomarkers were explored based on the peripheral blood gene expression profiles of premature coronary artery disease (PCAD). RNA sequencing (RNA-Seq) was used to detect PCAD-specific differentially expressed genes (DEGs). Quantitative real-time polymerase chain reaction (RT-PCR) was used to validate the most significant DEGs, and enzyme-linked immunosorbent assay (ELISA) was utilized to quantify the effect on corresponding serum proteins. Fifty-nine PCAD-specific DEGs were identified. Functional analysis showed positive regulation of T cell-mediated cytotoxicity, regulation of T cell-mediated immunity, and the regulation of alpha-beta T cell proliferation which were enriched in PCAD. RT-PCR validated the significant difference in the expression of BAG6, MUC5B, and APOA2 between PCAD and late-onset coronary artery disease (LCAD) patients. ELISA validation showed serum MUC5B increased dramatically in PCAD when compared to LCAD. Our study found T cells contribute to the occurrence of PCAD, and the inflammatory factor MUC5B may be a novel serum marker in PCAD patients.

Simultaneous control of infection and inflammation with keratin-derived antibacterial peptides targeting TLRs and co-receptors

Controlling infection-driven inflammation is a major clinical dilemma because of limited therapeutic options and possible adverse effects on microbial clearance. Compounding this difficulty is the continued emergence of drug-resistant bacteria, where experimental strategies aiming to augment inflammatory responses for enhanced microbial killing are not applicable treatment options for infections of vulnerable organs. As with corneal infections, severe or prolonged inflammation jeopardizes corneal transparency, leading to devastating vision loss. We hypothesized that keratin 6a-derived antimicrobial peptides (KAMPs) may be a two-pronged remedy capable of tackling bacterial infection and inflammation at once. We used murine peritoneal neutrophils and macrophages, together with an in vivo model of sterile corneal inflammation, to find that nontoxic and prohealing KAMPs with natural 10- and 18-amino acid sequences suppressed lipoteichoic acid (LTA)- and lipopolysaccharide (LPS)-induced NFκB and IRF3 activation, proinflammatory cytokine production, and phagocyte recruitment independently of their bactericidal function. Mechanistically, KAMPs not only competed with bacterial ligands for cell surface Toll-like receptor (TLR) and co-receptors (MD2, CD14, and TLR2) but also reduced cell surface availability of TLR2 and TLR4 through promotion of receptor endocytosis. Topical KAMP treatment effectively alleviated experimental bacterial keratitis, as evidenced by substantial reductions of corneal opacification, inflammatory cell infiltration, and bacterial burden. These findings reveal the TLR-targeting activities of KAMPs and demonstrate their therapeutic potential as a multifunctional drug for managing infectious inflammatory disease.

Multivalent sialic acid materials for biomedical applications

Sialic acid is a kind of monosaccharide expressed on the non-reducing end of glycoproteins or glycolipids. It acts as a signal molecule combining with its natural receptors such as selectins and siglecs (sialic acid-binding immunoglobulin-like lectins) in intercellular interactions like immunological surveillance and leukocyte infiltration. The last few decades have witnessed the exploration of the roles that sialic acid plays in different physiological and pathological processes and the use of sialic acid-modified materials as therapeutics for related diseases like immune dysregulation and virus infection. In this review, we will briefly introduce the biomedical function of sialic acids in organisms and the utilization of multivalent sialic acid materials for targeted drug delivery as well as therapeutic applications including anti-inflammation and anti-virus.

Ablation of Siglec-E augments brain inflammation and ischemic injury

Sialic acid immunoglobulin-like lectin E (Siglec-E) is a subtype of pattern recognition receptors found on the surface of myeloid cells and functions as a key immunosuppressive checkpoint molecule. The engagement between Siglec-E and the ligand α_2,8-linked disialyl glycans activates the immunoreceptor tyrosine-based inhibitory motif (ITIM) in its intracellular domain, mitigating the potential risk of autoimmunity amid innate immune attacks on parasites, bacteria, and carcinoma. Recent studies suggest that Siglec-E is also expressed in the CNS, particularly microglia, the brain-resident immune cells. However, the functions of Siglec-E in brain inflammation and injuries under many neurological conditions largely remain elusive. In this study, we first revealed an anti-inflammatory role for Siglec-E in lipopolysaccharide (LPS)-triggered microglial activation. We then found that Siglec-E was induced within the brain by systemic treatment with LPS in mice in a dose-dependent manner, while its ablation exacerbated hippocampal reactive microgliosis in LPS-treated animals. The genetic deficiency of Siglec-E also aggravated oxygen-glucose deprivation (OGD)-induced neuronal death in mouse primary cortical cultures containing both neurons and glial cells. Moreover, Siglec-E expression in ipsilateral brain tissues was substantially induced following middle cerebral artery occlusion (MCAO). Lastly, the neurological deficits and brain infarcts were augmented in Siglec-E knockout mice after moderate MCAO when compared to wild-type animals. Collectively, our findings suggest that the endogenous inducible Siglec-E plays crucial anti-inflammatory and neuroprotective roles following ischemic stroke, and thus might underlie an intrinsic mechanism of resolution of inflammation and self-repair in the brain.

Transgenic mouse models to study the physiological and pathophysiological roles of human Siglecs

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are important immunomodulatory receptors. Due to differences between human and mouse Siglecs, defining the in vivo roles for human Siglecs (hSiglecs) can be challenging. One solution is the development and use of hSiglec transgenic mice to assess the physiological roles of hSiglecs in health and disease. These transgenic mice can also serve as important models for the pre-clinical testing of immunomodulatory approaches that are based on targeting hSiglecs. Four general methods have been used to create hSiglec-expressing transgenic mice, each with associated advantages and disadvantages. To date, transgenic mouse models expressing hSiglec-2 (CD22), -3 (CD33), -7, -8, -9, -11, and -16 have been created. This review focuses on both the generation of these hSiglec transgenic mice, along with the important findings that have been made through their study. Cumulatively, hSiglec transgenic mouse models are providing a deeper understanding of the differences between human and mice orthologs/paralogs, mechanisms by which Siglecs regulate immune cell signaling, physiological roles of Siglecs in disease, and different paradigms where targeting Siglecs may be therapeutically advantageous.

Sialylation of TLR2 initiates osteoclast fusion

The molecular control of osteoclast formation is still not clearly elucidated. Here, we show that a process of cell recognition mediated by Siglec15-TLR2 binding is indispensable and occurs prior to cell fusion in RANKL-mediated osteoclastogenesis. Siglec15 has been shown to regulate osteoclastic bone resorption. However, the receptor for Siglec15 has not been identified, and the signaling mechanism involving Siglec15 in osteoclast function remains unclear. We found that Siglec15 bound sialylated TLR2 as its receptor and that the binding of sialylated TLR2 to Siglec15 in macrophages committed to the osteoclast-lineage initiated cell fusion for osteoclast formation, in which sialic acid was transferred by the sialyltransferase ST3Gal1. Interestingly, the expression of Siglec15 in macrophages was activated by M-CSF, whereas ST3Gal1 expression was induced by RANKL. Both Siglec15-specific deletion in macrophages and intrafemoral injection of sialidase abrogated cell recognition and reduced subsequent cell fusion for the formation of osteoclasts, resulting in increased bone formation in mice. Thus, our results reveal that cell recognition mediated by the binding of sialylated TLR2 to Siglec15 initiates cell fusion for osteoclast formation.

The role of the Siglec-G ITIM domain during bacterial infection

Siglecs, membrane-bound lectins of the sialic acid-binding immunoglobulin superfamily, inhibit immune responses by recruiting tyrosine phosphatases (e.g., SHP-1 and SHP-2) through their cytoplasmic immunoreceptor tyrosine-based inhibition motif (ITIM) domain. The role of Siglecs in infection has been extensively studied, but downstream signaling through the ITIM domain remains unclear. Here, we used a GST pull-down assay to identify additional proteins associated with the ITIM domain during bacterial infection. Gdi2 bound to ITIM under normal homeostasis, but Rab1a was recruited to ITIM during bacterial infection. Western blot analysis confirmed the presence of SHP-1 and SHP-2 in eluted ITIM-associated proteins under normal homeostasis. We confirmed the association of ITIM with Gdi2 or Rab1a by transfection of corresponding expression vectors in 293T cells followed by immunoprecipitation-western blot assay. Thus, ITIM's role in the inhibition of the immune response during bacterial infection may be regulated by interaction with Gdi2 and Rab1a in addition to SHP-1 and SHP-2.

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.

Interplay Between Sialic Acids, Siglec-E, and Neu1 Regulates MyD88- and TRIF-Dependent Pathways for TLR4-Activation During Leishmania donovani Infection

TLR4 activates two distinct signaling pathways involving adaptors MyD88 and TRIF to produce proinflammatory cytokines and type-I interferon respectively. How Leishmania donovani suppresses these pathways is not well studied. We earlier reported, TLR4 is hypersialylated due to reduced membrane-bound neuraminidase (Neu1) on infected-macrophages. We hypothesized that such enhanced sialoglycoconjugates on host cells may modulate the interactions with siglecs- which are the inhibitory receptors. Here, we examined the impact of such sialylation on overall TLR4 activation both in murine cell line J774A.1 and primary bone marrow derived macrophages (BMDM). Supporting this hypothesis, we demonstrated siglec-E engages hypersialylated TLR4 during infection. Such sialic acids-siglec-E interaction enhanced siglec-E phosphorylation that mediated its strong association with SHP1/SHP2 and also upregulated their phosphorylation in both types of macrophages. Pre-treatment of parasites and host cells with neuraminidase reduced SHP1/SHP2 phosphorylation and triggered TLR4 activation respectively through enhanced nuclear translocation of p-65. Moreover, a reciprocal interplay between Neu1 and siglec-E differentially regulates MyD88- and TRIF-pathways through sialic acids on TLR4 as their common substrate during infection. Correspondingly, Neu1 overexpression enhanced MyD88-signaling while still suppressing TRIF-activation. However, silencing siglec-E specifically activated TRIF-signaling. Pro-inflammatory cytokines corresponding to MyD88 and TRIF pathways were also upregulated respectively. Additionally, Neu1 overexpression or siglec-E silencing prevented TLR4 ubiquitination and subsequent degradation by Triad3A. Neu1-overexpression and siglec-E-silencing together followed by infection activated both MyD88 and TRIF-signaling through their enhanced TLR4-association. This elevated the MyD88-specific cytokines and TRIF-mediated IRF3 and IFN-β genes, thus upregulating the pro-inflammatory cytokines and nitric oxide levels and reduced anti-inflammatory cytokines. All these significantly inhibited parasite survival in macrophages thus demonstrating a previously unidentified dualistic regulation of TLR4signaling pathways activation through sialic acids by interplay of Neu1 and siglec-E during Leishmania infection.

Selective Response to Bacterial Infection by Regulating Siglec-E Expression

Interactions between microbes and hosts can be a benign, deleterious, or even fatal, resulting in death of the host, the microbe, or both. Sialic acid-binding immunoglobulin-like lectins (Siglecs) suppress infection responses to sialylated pathogens. However, most pathogens are nonsialylated. Here we determined Siglecs respond to nonsialylated Gram-negative bacteria (Escherichia coli 25922 and DH5α) and Gram-positive bacteria (Staphylococcus aureus and Listeria monocytogenes). We found that Siglece-/- mice had higher mortality than wild-type mice following Gram-negative but not Gram-positive bacterial infection. Better survival in wild-type mice depended on more efficient clearance of Gram-negative than Gram-positive bacteria. Gram-negative bacteria upregulated Siglec-E, thus increasing reactive oxygen species (ROS); Tyr432 in the ITIM domain of Siglec-E was required to increase ROS. Moreover, Gram-negative bacteria upregulated Siglec-E via TLR4/MyD88/JNK/NF-κB/AP-1, whereas Gram-positive bacteria downregulated Siglec-E via TLR2/RANKL/TRAF6/Syk. Thus, our study describes a fundamentally new role for Siglec-E during infection.

Polysialic acid and Siglec-E orchestrate negative feedback regulation of microglia activation

Polysialic acid (polySia) emerges as a novel regulator of microglia activity. We recently identified polysialylated proteins in the Golgi compartment of murine microglia that are released in response to inflammatory stimulation. Since exogenously added polySia is able to attenuate the inflammatory response, we proposed that the release of polysialylated proteins constitutes a mechanism for negative feedback regulation of microglia activation. Here, we demonstrate that translocation of polySia from the Golgi to the cell surface can be induced by calcium depletion of the Golgi compartment and that polysialylated proteins are continuously released for at least 24 h after the onset of inflammatory stimulation. The latter was unexpected, because polySia signals detected by immunocytochemistry are rapidly depleted. However, it indicates that the amount of released polySia is much higher than anticipated based on immunostaining. This may be crucial for microglial responses during traumatic brain injury (TBI), as we detected polySia signals in activated microglia around a stab wound in the adult mouse brain. In BV2 microglia, the putative polySia receptor Siglec-E is internalized during lipopolysaccharide (LPS)-induced activation and in response to polySia exposure, indicating interaction. Correspondingly, CRISPR/Cas9-mediated Siglec-E knockout prevents inhibition of pro inflammatory activation by exogenously added polySia and leads to a strong increase of the LPS response. A comparable increase of LPS-induced activation has been observed in microglia with abolished polySia synthesis. Together, these results indicate that the release of the microglia-intrinsic polySia pool, as implicated in TBI, inhibits the inflammatory response by acting as a trans-activating ligand of Siglec-E.

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.

Immunoregulatory Siglec ligands are abundant in human and mouse aorta and are up-regulated by high glucose

AIM

Inflammation is a driving force in development of atherosclerosis, and hyperglycemia is a significant risk factor for angiopathy. Siglec-9, expressed on human neutrophils and macrophages, engages specific glycan ligands on tissues to diminish ongoing inflammation.

MATERIALS AND METHOD

Siglec-9 ligands on human aorta were characterized and the effects of high glucose exposure on the expression of ligands for Siglec-9 on human umbilical vein endothelial cells (HUV-EC-C) in vitro and ligands for the comparable siglec (Siglec-E) on mouse aorta in vivo were studied.

KEY FINDINGS

Siglec-9 ligands were expressed broadly on human aorta, as well as on HUV-EC-C. Siglec-9 ligands on HUV-EC-C were sharply up-regulated under high glucose exposure in vitro, as were Siglec-E ligands on the aortas of hyperglycemic mice. Exposure of HUV-EC-C to high-glucose resulted in consistent inhibitory changes in co-cultured macrophages including increased apoptosis and decreased phagocytosis. Control of Siglec-9 ligand expression on HUV-EC-C was downstream of changes in an enzyme involved in their biosynthesis, UDP-galactose-4-epimerase (GALE) and increased cellular N-acetylgalactosamine. The alteration of GALE was associated with the regulatory microRNA hsa-let-7f.

SIGNIFICANCE

We conclude that exposure to high-glucose results in up-regulation of immune inhibitory Siglec-9 sialoglycan ligands on aorta and HUV-EC-C cells downstream of altered GALE and GalNAc expression, resulting in up-regulation of apoptosis and decrease of phagocytic activity of macrophages. Changes in Siglec-9 sialoglycan ligand expression on vascular endothelial cells may be a natural response to the initial steps of atherosclerosis and might be a potential target to regulate inflammation in diabetic angiopathy.

Overexpression of Toll-like Receptor 4-linked Mitogen-activated Protein Kinase Signaling Contributes to Internalization of Escherichia coli in Sheep

Escherichia coli is one of the most common causal pathogens of mastitis in milk-producing mammals. Toll-like receptor 4 (TLR4) is important for host recognition of this bacteria. Increased activation of TLR4 can markedly enhance the internalization of E. coli. In this study, the relationship between TLR4 and mitogen-activated protein kinase (MAPK) signaling pathways in mediating E. coli internalization was evaluated in sheep monocytes. Using a TLR4-overexpressing transgenic (Tg) sheep model, we explored the bacterial internalization mechanism in sheep. We found that monocytes of Tg sheep could phagocytize more bacteria and exhibited higher adhesive capacity. The specific inhibition of p38 MAPK or c-Jun N-terminal kinase (JNK) or extracellular signal-regulated kinases (ERKs) reduced TLR4-dependent internalization of bacteria into sheep monocytes. Furthermore, the inhibition of MAPK signaling down-regulated the adhesive capacity of monocytes and the expression of scavenger receptors and adhesion molecules. Taken together, the overexpression of TLR4 in transgenic sheep enhanced the internalization of E. coli via MAPK signaling.

Expression of Siglec-E Alters the Proteome of Lipopolysaccharide (LPS)-Activated Macrophages but Does Not Affect LPS-Driven Cytokine Production or Toll-Like Receptor 4 Endocytosis

Siglec-E is a murine CD33-related siglec that functions as an inhibitory receptor and is expressed mainly on neutrophils and macrophage populations. Recent studies have suggested that siglec-E is an important negative regulator of lipopolysaccharide (LPS)-toll-like receptor 4 (TLR4) signaling and one report (1) claimed that siglec-E is required for TLR4 endocytosis following uptake of Escherichia coli by macrophages and dendritic cells (DCs). Our attempts to reproduce these observations using cells from wild-type (WT) and siglec-E-deficient mice were unsuccessful. We used a variety of assays to determine if siglec-E expressed by different macrophage populations can regulate TLR4 signaling in response to LPS, but found no consistent differences in cytokine secretion in vitro and in vivo, comparing three different strains of siglec-E-deficient mice with matched WT controls. No evidence was found that the siglec-E deficiency was compensated by expression of siglecs-F and -G, the other murine inhibitory CD33-related siglecs. Quantitative proteomics was used as an unbiased approach and provided additional evidence that siglec-E does not suppress inflammatory TLR4 signaling. Interestingly, proteomics revealed a siglec-E-dependent alteration in macrophage protein composition that could be relevant to functional responses in host defense. In support of this, siglec-E-deficient mice exhibited enhanced growth of Salmonella enterica serovar Typhimurium in the liver following intravenous infection, but macrophages lacking siglec-E did not show altered uptake or killing of bacteria in vitro. Using various cell types including bone marrow-derived DCs (BMDCs), splenic DCs, and macrophages from WT and siglec-E-deficient mice, we showed that siglec-E is not required for TLR4 endocytosis following E. coli uptake or LPS challenge. We failed to see expression of siglec-E by BMDC even after LPS-induced maturation, but confirmed previous studies that splenic DCs express low levels of siglec-E. Taken together, our findings do not support a major role of siglec-E in regulation of TLR4 signaling functions or TLR4 endocytosis in macrophages or DCs. Instead, they reveal that induction of siglec-E by LPS can modulate the phenotype of macrophages, the functional significance of which is currently unclear.

Sialic Acids in the Immune Response during Sepsis

Sialic acid-binding immunoglobulin-type lectins (Siglecs) are a group of cell surface transmembrane receptors expressed on immune cells, and regulate immune balance in inflammatory diseases. Sepsis is a life-threatened inflammatory syndrome induced by infection, and the pathogenesis of sepsis includes immune dysregulation, inflammation, and coagulation disorder. Here, we reviewed the various roles acted by Siglecs family in the pathogenesis of sepsis. Siglec-1, Siglec-5, and Siglec-14 play bidirectional roles through modulation of inflammation and immunity. Siglec-2 regulates the immune balance during infection by modulating B cell and T cell response. Siglec-9 helps endocytosis of toll-like receptor 4, regulates macrophages polarization, and inhibits the function of neutrophils during infection. Siglec-10 inhibits danger-associated molecular patterns induced inflammation, helps the initiation of antigen response by T cells, and decreases B-1a cell population to weaken inflammation. Regulating the Siglecs function in the different stages of sepsis holds great potential in the therapy of sepsis.