The human sialic acid-binding immunoglobulin-like lectin Siglec-9 and its murine homolog Siglec-E control osteoclast activity and bone resorption

Sialylated glycoproteins bind to Siglec-9 in a cis manner on platelets to suppress platelet activation

BACKGROUND

The endogenous negative regulation of platelets is important in preventing spontaneous thrombosis, while the mechanism of homeostasis is incompletely understood.

OBJECTIVES

In this study, we aimed to explore whether Siglec-9 plays a negative regulative role and identify the ligand of Siglec-9 on platelets.

METHODS

To determine the role of Siglec-9 on platelets, platelet factor 4-cre:Siglec-Eflox/flox knockout mouse model and human platelet in vitro culture system were used. Furthermore, recombinant glycoprotein (GP) of Siglec-9 ligand on platelets was expressed and used.

RESULTS

We found that Siglec-E conditional knockout can lead to significant increase in platelet coagulation activities both in vivo and in vitro, which strongly suggests that Siglec-9/E plays an inhibitory physiological role in platelet activation. Siglec-9 ligand is an O-link GP with an α2,3-linked sialic acid terminal structure, and the protein carrier of the ligand is mucin-like region of GPIbα. Our data further showed that the ligands on platelets could not engage Siglec-9 on other cells via trans-binding, which indicates that the ligands on platelets play a self-modulation role. Furthermore, we provided evidence that the activation of Siglec-9 pathway with exogenous specific ligands could inhibit the activity of platelets. These data demonstrate a previously unanticipated role for GPIbα in inhibiting platelet activation and provide a novel mechanism for the homeostasis of platelets.

CONCLUSIONS

We conclude that the cis-binding between mucin-like region of GPIbα and Siglec-9 acts as a "parking brake" on platelet activation. This finding provides a potential druggable target for novel antiplatelet medicine.

Understanding the Glycosylation Pathways Involved in the Biosynthesis of the Sulfated Glycan Ligands for Siglecs

Carbohydrate sulfation plays a pivotal role in modulating the strength of Siglec-glycan interactions. Recently, new aspects of Siglec binding to sulfated cell surface carbohydrates have been discovered, but the class of glycan presenting these sulfated Siglec ligands has not been fully elucidated. In this study, the contribution of different classes of glycans to cis and trans Siglec ligands was investigated within cells expressing the carbohydrate sulfotransferase 1 (CHST1) or CHST2. For some Siglecs, the glycan class mediating binding was clear, such as O-glycans for Siglec-7 and N-glycans for Siglec-2 and Siglec-9. Both N-glycans and mucin-type O-glycans contributed to ligands for Siglec-3, -5, -8, and -15. However, significant levels of Siglec-3 and -8 ligands remained in CHST1-expressing cells lacking complex N-glycans and mucin-type O-glycans. A combination of genetic, pharmacological, and enzymatic treatment strategies ruled out heparan sulfates and glycoRNA as contributors, although Siglec-8 did exhibit some binding to glycolipids. Genetic disruption of O-mannose glycans within CHST1-expressing cells had a small but significant impact on Siglec-3 and -8 binding, demonstrating that this class of glycans can present sulfated Siglec ligands. We also investigated the ability of sulfated cis ligands to mask Siglec-3 and Siglec-7. For Siglec-7, cis ligands were again found to be mucin-type O-glycans. While N-glycans were the major sulfated trans ligands for Siglec-3, disruption of complex mucin-type O-glycans had the largest impact on Siglec-3 masking. Overall, this study enhances our knowledge of the types of sulfated glycans that can serve as Siglec ligands.

Siglecs-mediated immune regulation in neurological disorders

The surfaces of various immune cells are rich in glycan chains, including the sialic-acid-binding immunoglobulin-like lectins (Siglecs) family. As an emerging glyco-immune checkpoint, Siglecs have the ability to bind and interact with various glycoproteins, thereby eliciting a series of downstream reactions to modulate the immune response. The impact of Siglecs has been extensively studied in tumor immunotherapy. However, research in neurological disorders and neurological diseases is very limited, and therapeutic options involving Siglecs need further exploration. Siglecs play a crucial role in the development, homeostasis, and repair processes of the nervous system, especially in degenerative diseases. This review summarizes studies on the immunomodulatory role mediated by Siglecs expressed on different immune cells in various neurological disorders, elucidates how dysregulated sialic acid contributes to several psychiatric disorders, and discusses the progress and limitations of research on the treatment of neurological disorders.

Comprehensive Analysis of Sialylation-Related Gene Profiles and Their Impact on the Immune Microenvironment in Periodontitis

Periodontitis is a chronic inflammatory disease strongly influenced by host's immune response. Aberrant sialylation on cell surface plays a key role in inflammation and immunity. This study aims to identify sialylation-related genes associated with periodontitis and explore their impact on periodontal immune microenvironment. Differential expression analysis and machine learning were employed to determine core sialylation-related genes after datasets were retrieved and integrated. A diagnostic model incorporating these genes was constructed, following the immune cell infiltration analysis. Consensus clustering and weighted gene co-expression network analysis stratified periodontitis patients into subgroups and identified associated module genes. Single-cell sequencing data was further utilized to investigate the impact of sialylation on the periodontal immune microenvironment with pseudo-time series analysis and cell communication analysis. Periodontitis had a higher sialylation score with six key sialylation genes (CHST2, SELP, ST6GAL1, ST3GAL1, NEU1, FCN1) identified. The multi-gene diagnostic model demonstrated high accuracy and efficacy. Significant associations were observed between the key genes and immune cell populations, such as monocytes and B cells, in the periodontal immune microenvironment. Clustering analysis revealed two distinct sialylation-related subgroups with differential immune profiles. Single-cell data showed a significantly higher expression of sialylation-related genes in monocytes, which was found to significantly impact their developmental processes as well as their intercellular communication with B cells. The six identified sialylation-related genes hold potential as periodontitis biomarkers. High sialylation expression can impact the differentiation and cell-cell communication of monocytes. Sialylation-related genes are closely associated with alterations in the periodontal immune microenvironment.

Siglec-E Ligand Downregulation on Hippocampus Neurons Induced Inflammation in Sevoflurane-Associated Perioperative Neurocognitive Disorders in Aged Mice

Activated microglia-induced inflammation in the hippocampus plays an important role in perioperative neurocognitive disorders. Previous studies have shown that sialic acid-binding immunoglobulin-like lectin 3 (hSiglec-3, ortholog of mouse Siglec-E) engagement in microglia and its glycan ligands on neurons contributes to inflammatory homeostasis through an endogenous negative regulation pathway. This study aimed to explore whether the glycan ligand alteration on neurons plays a role in sevoflurane-induced perioperative neurocognitive disorders. This study's data has shown that a slight Siglec-E ligands' expression decrease does not induce inflammation homeostasis disruption. We also demonstrated that the ligand level on neurons was decreased with age, and the reduced Siglec-E ligand expression on neurons caused via sevoflurane was induced by neuraminidase 1. Furthermore, this study has shown that the Siglec-E ligand expression decline caused by age and sevoflurane treatment could decrease the ligands' level, thus leading to inflammatory homeostasis disruption. This research provided a novel mechanism for perioperative neurocognitive disorder susceptibility in the elderly.

Functional and analytical recapitulation of osteoclast biology on demineralized bone paper

Osteoclasts are the primary target for osteoporosis drug development. Recent animal studies revealed the crucial roles of osteoblasts in regulating osteoclastogenesis and the longer lifespans of osteoclasts than previously thought with fission and recycling. However, existing culture platforms are limited to replicating these newly identified cellular processes. We report a demineralized bone paper (DBP)-based osteoblast culture and osteoclast assay platform that replicates osteoclast fusion, fission, resorption, and apoptosis with high fidelity and analytical power. An osteoid-inspired DBP supports rapid and structural mineral deposition by osteoblasts. Coculture osteoblasts and bone marrow monocytes under biochemical stimulation recapitulate osteoclast differentiation and function. The DBP-based bone model allows longitudinal quantitative fluorescent monitoring of osteoclast responses to bisphosphonate drug, substantiating significantly reducing their number and lifespan. Finally, we demonstrate the feasibility of humanizing the bone model. The DBP-based osteo assay platforms are expected to advance bone remodeling-targeting drug development with improved prediction of clinical outcomes.

Glycobiology in osteoclast differentiation and function

Glycans, either alone or in complex with glycan-binding proteins, are essential structures that can regulate cell biology by mediating protein stability or receptor dimerization under physiological and pathological conditions. Certain glycans are ligands for lectins, which are carbohydrate-specific receptors. Bone is a complex tissue that provides mechanical support for muscles and joints, and the regulation of bone mass in mammals is governed by complex interplay between bone-forming cells, called osteoblasts, and bone-resorbing cells, called osteoclasts. Bone erosion occurs when bone resorption notably exceeds bone formation. Osteoclasts may be activated during cancer, leading to a range of symptoms, including bone pain, fracture, and spinal cord compression. Our understanding of the role of protein glycosylation in cells and tissues involved in osteoclastogenesis suggests that glycosylation-based treatments can be used in the management of diseases. The aims of this review are to clarify the process of bone resorption and investigate the signaling pathways mediated by glycosylation and their roles in osteoclast biology. Moreover, we aim to outline how the lessons learned about these approaches are paving the way for future glycobiology-focused therapeutics.

Roles of the Siglec family in bone and bone homeostasis

Tremendous progress has been seen in the study of the role of sialic acid binding im-munoglobulin type lectins (Siglecs) in osteoimmunology in the past two decades. Interest in Siglecs as immune checkpoints has grown from the recognition that Siglecs have relevance to human disease. Siglecs play important roles in inflammation and cancer, and play key roles in immune cell signaling. By recognizing common sialic acid containing glycans on glycoproteins and glycolipids as regulatory receptors for immune cell signals, Siglecs are expressed on most immune cells and play important roles in normal homeostasis and self-tolerance. In this review, we describe the role that the siglec family plays in bone and bone homeostasis, including the regulation of osteoclast differentiation as well as recent advances in inflammation, cancer and osteoporosis. Particular emphasis is placed on the relevant functions of Siglecs in self-tolerance and as pattern recognition receptors in immune responses, thereby potentially providing emerging strategies for the treatment of bone related diseases.

Polysialic acid promotes remyelination in cerebellar slice cultures by Siglec-E-dependent modulation of microglia polarization

Multiple sclerosis is an inflammatory demyelinating disease of the central nervous system. Spontaneous restoration of myelin after demyelination occurs, but its efficiency declines during disease progression. Efficient myelin repair requires fine-tuning inflammatory responses by brain-resident microglia and infiltrating macrophages. Accordingly, promising therapeutic strategies aim at controlling inflammation to promote remyelination. Polysialic acid (polySia) is a polymeric glycan with variable chain lengths, presented as a posttranslational modification on select protein carriers. PolySia emerges as a negative regulator of inflammatory microglia and macrophage activation and has been detected on oligodendrocyte precursors and reactive astrocytes in multiple sclerosis lesions. As shown recently, polySia-modified proteins can also be released by activated microglia, and the intrinsically released protein-bound and exogenously applied free polySia were equally able to attenuate proinflammatory microglia activation via the inhibitory immune receptor Siglec-E. In this study, we explore polySia as a candidate substance for promoting myelin regeneration by immunomodulation. Lysophosphatidylcholine-induced demyelination of organotypic cerebellar slice cultures was used as an experimental model to analyze the impact of polySia with different degrees of polymerization (DP) on remyelination and inflammation. In lysophosphatidylcholine-treated cerebellar slice cultures, polySia-positive cells were abundant during demyelination but largely reduced during remyelination. Based on the determination of DP24 as the minimal polySia chain length required for the inhibition of inflammatory BV2 microglia activation, pools with short and long polySia chains (DP8-14 and DP24-30) were generated and applied to slice cultures during remyelination. Unlike DP8-14, treatment with DP24-30 significantly improved remyelination, increased arginase-1-positive microglia ratios, and reduced the production of nitric oxide in wildtype, but not in Siglec-E-deficient slice cultures. In vitro differentiation of oligodendrocytes was not affected by DP24-30. Collectively, these results suggest a beneficial effect of exogenously applied polySia DP24-30 on remyelination by Siglec-E-dependent microglia regulation.

Sialic acid and food allergies: The link between nutrition and immunology

Food allergies (FA), a major public health problem recognized by the World Health Organization, affect an estimated 3%-10% of adults and 8% of children worldwide. However, effective treatments for FA are still lacking. Recent advances in glycoimmunology have demonstrated the great potential of sialic acids (SAs) in the treatment of FA. SAs are a group of nine-carbon α-ketoacids usually linked to glycoproteins and glycolipids as terminal glycans. They play an essential role in modulating immune responses and may be an effective target for FA intervention. As exogenous food components, sialylated polysaccharides have anti-FA effects. In contrast, as endogenous components, SAs on immunoglobulin E and immune cell surfaces contribute to the pathogenesis of FA. Given the lack of comprehensive information on the effects of SAs on FA, we reviewed the roles of endogenous and exogenous SAs in the pathogenesis and treatment of FA. In addition, we considered the structure-function relationship of SAs to provide a theoretical basis for the development of SA-based FA treatments.

Genome-wide studies reveal factors associated with circulating uromodulin and its relationships to complex diseases

Uromodulin (UMOD) is a major risk gene for monogenic and complex forms of kidney disease. The encoded kidney-specific protein uromodulin is highly abundant in urine and related to chronic kidney disease, hypertension, and pathogen defense. To gain insights into potential systemic roles, we performed genome-wide screens of circulating uromodulin using complementary antibody-based and aptamer-based assays. We detected 3 and 10 distinct significant loci, respectively. Integration of antibody-based results at the UMOD locus with functional genomics data (RNA-Seq, ATAC-Seq, Hi-C) of primary human kidney tissue highlighted an upstream variant with differential accessibility and transcription in uromodulin-synthesizing kidney cells as underlying the observed cis effect. Shared association patterns with complex traits, including chronic kidney disease and blood pressure, placed the PRKAG2 locus in the same pathway as UMOD. Experimental validation of the third antibody-based locus, B4GALNT2, showed that the p.Cys466Arg variant of the encoded N-acetylgalactosaminyltransferase had a loss-of-function effect leading to higher serum uromodulin levels. Aptamer-based results pointed to enzymes writing glycan marks present on uromodulin and to their receptors in the circulation, suggesting that this assay permits investigating uromodulin's complex glycosylation rather than its quantitative levels. Overall, our study provides insights into circulating uromodulin and its emerging functions.

Polysialic Acid in the Immune System

Polysialic acid (polySia) is a highly regulated polymer of sialic acid (Sia) with such potent biophysical characteristics that when expressed drastically influences the interaction properties of cells. Although much of what is known of polySia in mammals has been elucidated from the study of its role in the central nervous system (CNS), polySia is also expressed in other tissues, including the immune system where it presents dynamic changes during differentiation, maturation, and activation of different types of immune cells of the innate and adaptive response, being involved in key regulatory mechanisms. At least six polySia protein carriers (CCR7, ESL-1, NCAM, NRP2, ST8Sia 2, and ST8Sia 4) are expressed in different types of immune cells, but there is still much to be explored in regard not only to the regulatory mechanisms that determine their expression and the structure of polySia chains but also to the identification of the cis- and trans- ligands of polySia that establish signaling networks. This review summarizes the current knowledge on polySia in the immune system, addressing its biosynthesis, its tools for identification and structural characterization, and its functional roles and therapeutic implications.

Metabolomic biomarkers of low BMD: a systematic review

Due to the metabolic nature of osteoporosis, this study was conducted to identify metabolomic studies investigating the metabolic profile of low bone mineral density (BMD) and osteoporosis. A comprehensive systematic literature search was conducted through PubMed, Web of Science, Scopus, and Embase databases up to April 08, 2020, to identify observational studies with cross-sectional or case-control designs investigating the metabolic profile of low BMD in adults using biofluid specimen via metabolomic platform. The quality assessment panel specified for the "omics"-based diagnostic research (QUADOMICS) tool was used to estimate the methodologic quality of the included studies. Ten untargeted and one targeted approach metabolomic studies investigating biomarkers in different biofluids through mass spectrometry or nuclear magnetic resonance platforms were included in the systematic review. Some metabolite panels, rather than individual metabolites, showed promising results in differentiating low BMD from normal. Candidate metabolites were of different categories including amino acids, followed by lipids and carbohydrates. Besides, certain pathways were suggested by some of the studies to be involved. This systematic review suggested that metabolic profiling could improve the diagnosis of low BMD. Despite valuable findings attained from each of these studies, there was great heterogeneity regarding the ethnicity and age of participants, samples, and the metabolomic platform. Further longitudinal studies are needed to validate the results and confirm the predictive role of metabolic profile on low BMD and fracture. It is also mandatory to address and minimize the heterogeneity in future studies by using reliable quantitative methods. Summary: Due to the metabolic nature of osteoporosis, researchers have considered metabolomic studies recently. This systematic review showed that metabolic profiling including different categories of metabolites could improve the diagnosis of low BMD. However, great heterogeneity was observed and it is mandatory to address and minimize the heterogeneity in future studies.

Gene Expression Profiles Analyzed Using Integrating RNA Sequencing, and Microarray Reveals Increased Inflammatory Response, Proliferation, and Osteoclastogenesis in Pigmented Villonodular Synovitis

Background

Pigmented villonodular synovitis (PVNS) is a rare condition that involves benign proliferation of the synovial tissue and is characterized by severe joint destruction and high recurrence even after surgical resection. However, poor understanding of the pathogenesis limits its effective therapy.

Method

In this study, gene expression profiles of six patients with PVNS, 11 patients with osteoarthritis (OA), nine patients with rheumatoid arthritis (RA) (E-MTAB-6141), and three healthy subjects (GSE143514) were analyzed using integrating RNA sequencing (RNA-seq) and microarray to investigate the PVNS transcriptome. Gene ontology, string, and cytoscape were used to determine the gene functional enrichment. Cell functional molecules were detected using flow cytometry or immunohistochemical test to identify the cell subset and function. CD14⁺ cells were isolated and induced to osteoclast to evaluate the monocyte/macrophage function.

Results

The most obvious local manifestations of PVNS were inflammation, including increased immune cells infiltration and cytokine secretion, and tumor phenotypes. High proportion of inflammatory cells, including T cells, natural killer (NK) cells, NKT cells, and B cells were recruited from the blood. Th17 and monocytes, especially classical monocytes but not nonclassical monocytes, increased in PVNS synovium. An obvious increase in osteoclastogenesis and macrophage activation was observed locally. Elevated expression of MMP9, SIGLEC 15, and RANK were observed in myeloid cell of PVNS than OA. When compared with RA, osteoclast differentiation and myeloid cell activation are PVNS-specific characters, whereas T cell activation is shared by PVNS and RA.

Conclusion

The transcriptional expression characteristics of PVNS showed increased immune response, cell migration, and osteoclastogenesis. Osteoclast differentiation is only observed in PVNS but not RA, whereas T-cell activation is common in inflammatory arthritis.