Functional and structural investigation of a broadly neutralizing SARS-CoV-2 antibody.

Since its emergence, SARS-CoV-2 has been continuously evolving, hampering the effectiveness of current vaccines against COVID-19. mAbs can be used to treat patients at risk of severe COVID-19. Thus, the development of broadly protective mAbs and an understanding of the underlying protective mechanisms are of great importance. Here, we isolated mAbs from donors with breakthrough infection with Omicron subvariants using a single-B cell screening platform. We identified a mAb, O5C2, which possesses broad-spectrum neutralization and antibody-dependent cell-mediated cytotoxic activities against SARS-CoV-2 variants, including EG.5.1. Single-particle analysis by cryo-electron microscopy revealed that O5C2 targeted an unusually large epitope within the receptor-binding domain of spike protein that overlapped with the angiotensin-converting enzyme 2 binding interface. Furthermore, O5C2 effectively protected against BA.5 Omicron infection in vivo by mediating changes in transcriptomes enriched in genes involved in apoptosis and interferon responses. Our findings provide insights into the development of pan-protective mAbs against SARS-CoV-2.

Quantitative Analysis of Siglec Ligands by Flow Cytometry.

Siglecs (sialic acid-binding, immunoglobulin superfamily, lectins) are a family of transmembrane receptor-type glycan recognition proteins in vertebrates that are primarily expressed on leukocytes and regulate immune responses. Siglecs are involved in several diseases, such as cancer and neurodegenerative diseases. Most Siglecs suppress the activation of leukocytes by recognizing ligands containing sialic acid, a group of acidic sugars commonly found in vertebrate glycans, but rare among microbes. Siglec ligands are critical in the interaction between leukocytes and target cells. The abundance of the Siglec ligand is influenced by both the abundance of the glycoconjugate carrier (glycoprotein or glycolipid) and that of the terminal glycan epitope directly recognized by the Siglec. Therefore, a direct approach to evaluate the expression level of a Siglec ligand on cells of interest is to analyze the binding of recombinant Siglec protein to these cells. In this article, we describe a protocol for semi-quantitatively analyzing the expression level of Siglec ligands via flow cytometry using recombinant Siglec-Fc fusion protein. Support protocols describe how to remove sialic acids from the cell surface with sialidase under mild conditions to demonstrate the sialic acid dependence of Siglec binding, and the preparation of recombinant Siglec-Fc fusion proteins by transient transfection of mammalian cells. © 2023 Wiley Periodicals LLC. Basic Protocol: Quantitative analysis of Siglec ligands on mammalian cells via flow cytometry with recombinant Siglec-Fc fusion protein Support Protocol 1: Sialidase treatment of mammalian cells Support Protocol 2: Preparation of recombinant Siglec-Fc fusion protein via transient transfection of mammalian cells.

Molecular Basis and Role of Siglec-7 Ligand Expression on Chronic Lymphocytic Leukemia B Cells.

Siglec-7 (sialic acid-binding immunoglobulin-like lectin 7) is an immune checkpoint-like glycan recognition protein on natural killer (NK) cells. Cancer cells often upregulate Siglec ligands to subvert immunosurveillance, but the molecular basis of Siglec ligands has been elusive. In this study, we investigated Siglec-7 ligands on chronic lymphocytic leukemia (CLL) B cells. CLL B cells express higher levels of Siglec-7 ligands compared with healthy donor B cells, and enzymatic removal of sialic acids or sialomucins makes them more sensitive to NK cell cytotoxicity. Gene knockout experiments have revealed that the sialyltransferase ST6GalNAc-IV is responsible for the biosynthesis of disialyl-T (Neu5Acα2-3Galß1-3[Neu5Acα2-6]GalNAcα1-), which is the glycotope recognized by Siglec-7, and that CD162 and CD45 are the major carriers of this glycotope on CLL B cells. Analysis of public transcriptomic datasets indicated that the low expression of GCNT1 (encoding core 2 GlcNAc transferase, an enzyme that competes against ST6GalNAc-IV) and high expression of ST6GALNAC4 (encoding ST6GalNAc-IV) in CLL B cells, together enhancing the expression of the disialyl-T glycotope, are associated with poor patient prognosis. Taken together, our results determined the molecular basis of Siglec-7 ligand overexpression that protects CLL B cells from NK cell cytotoxicity and identified disialyl-T as a potential prognostic marker of CLL.

Recent Progress in the Methodologies to Identify Physiological Ligands of Siglecs.

Siglecs, a family of receptor-like lectins, recognize glycoproteins and/or glycolipids containing sialic acid in the extracellular space and transduce intracellular signaling. Recently, researchers uncovered significant contributions of Siglecs in cancer immunity, renewing interest in this family of proteins. Previous extensive studies have defined how Siglecs recognize glycan epitopes (glycotopes). Nevertheless, the biological role of these glycotopes has not been fully evaluated. Recent studies using live cells have begun unraveling the constituents of Siglec ligands. These studies demonstrated that glycoprotein scaffolds (counter-receptors) displaying glycotopes are sometimes just as important as the glycotope itself. These new insights may guide future efforts to develop therapeutic agents to target the Siglec - ligand axis.

Identification and functional characterization of a Siglec-7 counter-receptor on K562 cells.

Sialic acid (Sia)-binding immunoglobulin-like lectin 7 (Siglec-7) is an inhibitory receptor primarily expressed on natural killer (NK) cells and monocytes. Siglec-7 is known to negatively regulate the innate immune system through Sia binding to distinguish self and nonself; however, a counter-receptor bearing its natural ligand remains largely unclear. Here, we identified a counter-receptor of Siglec-7 using K562 hematopoietic carcinoma cells presenting cell surface ligands for Siglec-7. We affinity-purified the ligands using Fc-ligated recombinant Siglec-7 and diSia-dextran polymer, a strong inhibitor for Siglec-7. We then confirmed the counter-receptor for Siglec-7 as leukosialin (CD43) through mass spectrometry, immunoprecipitation, and proximity labeling. Additionally, we demonstrated that the cytotoxicity of NK cells toward K562 cells was suppressed by overexpression of leukosialin in a Siglec-7-dependent manner. Taken together, our data suggest that leukosialin on K562 is a counter-receptor for Siglec-7 on NK cells and that a cluster of the Sia-containing glycan epitope on leukosialin is key as trans-ligand for unmasking the cis-ligand.

Preparation of Recombinant Siglecs and Identification of Their Ligands.

Siglecs are transmembrane receptor-like vertebrate lectins that recognize glycans containing sialic acid. Most Siglecs also interact with intracellular signal transduction molecules, and modulate immune responses. Recombinant soluble Siglecs fused with the fragment crystallizable (Fc) region of immunoglobulin G (Siglec-Fc) are a versatile tool for the investigation of Siglec functions. We describe protocols for the production of recombinant Siglec-Fc, the analysis of expression of Siglec ligands by flow cytometry, and the identification of the Siglec ligand candidates based on proximity labeling.

Novel Zebrafish Mono-α2,8-sialyltransferase (ST8Sia VIII): An Evolutionary Perspective of α2,8-Sialylation.

The mammalian mono-α2,8-sialyltransferase ST8Sia VI has been shown to catalyze the transfer of a unique sialic acid residues onto core 1 O-glycans leading to the formation of di-sialylated O-glycosylproteins and to a lesser extent to diSia motifs onto glycolipids like GD1a. Previous studies also reported the identification of an orthologue of the ST8SIA6 gene in the zebrafish genome. Trying to get insights into the biosynthesis and function of the oligo-sialylated glycoproteins during zebrafish development, we cloned and studied this fish α2,8-sialyltransferase homologue. In situ hybridization experiments demonstrate that expression of this gene is always detectable during zebrafish development both in the central nervous system and in non-neuronal tissues. Intriguingly, using biochemical approaches and the newly developed in vitro MicroPlate Sialyltransferase Assay (MPSA), we found that the zebrafish recombinant enzyme does not synthetize diSia motifs on glycoproteins or glycolipids as the human homologue does. Using comparative genomics and molecular phylogeny approaches, we show in this work that the human ST8Sia VI orthologue has disappeared in the ray-finned fish and that the homologue described in fish correspond to a new subfamily of α2,8-sialyltransferase named ST8Sia VIII that was not maintained in Chondrichtyes and Sarcopterygii.

Systems glycomics of adult zebrafish identifies organ-specific sialylation and glycosylation patterns.

The emergence of zebrafish Danio rerio as a versatile model organism provides the unique opportunity to monitor the functions of glycosylation throughout vertebrate embryogenesis, providing insights into human diseases caused by glycosylation defects. Using a combination of chemical modifications, enzymatic digestion and mass spectrometry analyses, we establish here the precise glycomic profiles of eight individual zebrafish organs and demonstrate that the protein glycosylation and glycosphingolipid expression patterns exhibits exquisite specificity. Concomitant expression screening of a wide array of enzymes involved in the synthesis and transfer of sialic acids shows that the presence of organ-specific sialylation motifs correlates with the localized activity of the corresponding glycan biosynthesis pathways. These findings provide a basis for the rational design of zebrafish lines expressing desired glycosylation profiles.

Priming mass spectrometry-based sulfoglycomic mapping for identification of terminal sulfated lacdiNAc glycotope.

In an effort to prime our mass spectrometry (MS)-based sulfoglycomic mapping platform technology for facile identification of sulfated lacdiNAc (GalNAcß1-4GlcNAcß1-), we have re-examined the N-glycans of bovine thyroid stimulating hormone. We showed that MALDI-MS mapping of permethylated glycans in negative ion mode can give an accurate representation of the sulfated glycans and, through MS/MS, diagnostic ions can be derived that we can collectively define the presence of a terminal sulfated lacdiNAc moiety at high sensitivity. Based on these ions, which can also be produced by nanoESI-MS(n), we demonstrated that the glycome of an ovarian carcinoma cell line, RMG-1, comprises a high abundance of sulfated lacdiNAc epitopes carried on multiantennary complex type N-glycans alongside fucosylated, sialylated and/or sulfated lacNAc antennae. This represents the first report of a natural glycomic occurrence of sulfated lacdiNAc on a cell line, as opposed to other better-characterized presence on secreted glycoproteins from a handful of sources. It is anticipated that with improved methods of detection such as that developed in this work, we are likely to identify a wider occurrence of sulfated lacdiNAc and be able to more accurately delineate the regulatory mechanism dictating the choice of a cell type in synthesizing sulfated, sialylated, fucosylated and/or non-substituted lacdiNAc.

Mapping the expressed glycome and glycosyltransferases of zebrafish liver cells as a relevant model system for glycosylation studies.

The emergence of zebrafish as a model organism for human diseases was accompanied by the development of cellular model systems that extended the possibilities for in vitro manipulation and in vivo studies after cell implantation. The exploitation of zebrafish cell systems is, however, still hampered by the lack of genomic and biochemical data. Here, we lay a path toward the efficient use of ZFL, a zebrafish liver-derived cell system, as a platform for studying glycosylation. To achieve this, we established the glycomic profile of ZFL by a combination of mass spectrometry and NMR. We demonstrated that glycoproteins were substituted by highly sialylated multiantennary N-glycans, some of them comprising the unusual zebrafish epitope Galß1-4[Neu5Ac(α2,3)]Galß1-4[Fuc(α1,3)]GlcNAc, and core 1 multisialylated O-glycans. Similarly, these analyses established that glycolipids were dominated by sialylated gangliosides. In parallel, analyzing the expression patterns of all putative sialyl- and fucosyltransferases, we directly correlated the identified structures to the set of enzymes involved in ZFL glycome. Finally, we demonstrated that this cell system was amenable to metabolic labeling using functionalized monosaccharides that permit in vivo imaging of glycosylation processes. Altogether, glycomics, genomics, and functional studies established ZFL as a relevant cellular model for the study of glycosylation.

Sialome analysis of the cephalochordate Branchiostoma belcheri, a key organism for vertebrate evolution.

Sialic acid, a common terminal substitution of glycoconjugates, has been so far consistently identified in all vertebrates as well as in a growing number of bacterial species. It is assumed to be widely distributed among animal species of the deuterostome phylum, based on its identification in few echinoderm and all vertebrate species. However, whole sections of deuterostome, especially those intermediate species between invertebrates and vertebrates including cephalochordates, urochordates and hemichordates, are still unexplored in term of sialylation capacities. The discovery of functional sialic acid machinery in some of these species may shed new light onto the evolution of glycosylation capacities in deuterostome lineage. In a first approach, we investigated the sialylation pattern of a cephalocordate species, Branchiostoma belcheri, which occupies a strategic phylogenetic position to understand the transition of invertebrates toward vertebrates. Structural analysis of B. belcheri glycoconjugates established that this organism synthesizes large quantities of various sialic acids, some of which present rare or novel structures such as methylated sialic acids. These sialic acids were shown to be mainly associated with mono- and disialylated core 1-type O-glycans. Moreover, screening of the animal organs revealed the existence of exquisite tissue specificity in the distribution of sialic acids. Description of sialylation profiles was then correlated with the expression patterns of key enzymes involved in the biosynthesis of major forms of sialic acids, which provides the first complete overview of the sialylation patterns in cephalochordates.

Human sperm binding is mediated by the sialyl-Lewis(x) oligosaccharide on the zona pellucida.

Human fertilization begins when spermatozoa bind to the extracellular matrix coating of the oocyte, known as the zona pellucida (ZP). One spermatozoan then penetrates this matrix and fuses with the egg cell, generating a zygote. Although carbohydrate sequences on the ZP have been implicated in sperm binding, the nature of the ligand was unknown. Here, ultrasensitive mass spectrometric analyses revealed that the sialyl-Lewis(x) sequence [NeuAcα2-3Galß1-4(Fucα1-3)GlcNAc], a well-known selectin ligand, is the most abundant terminal sequence on the N- and O-glycans of human ZP. Sperm-ZP binding was largely inhibited by glycoconjugates terminated with sialyl-Lewis(x) sequences or by antibodies directed against this sequence. Thus, the sialyl-Lewis(x) sequence represents the major carbohydrate ligand for human sperm-egg binding.

Molecular cloning and characterization of the expression pattern of the zebrafish alpha2, 8-sialyltransferases (ST8Sia) in the developing nervous system.

Sialyltransferases are Golgi type II transmembrane glycoproteins involved in the biosynthesis of sialylated glycolipids and glycoproteins. These sialylated compounds play fundamental roles in the development of a variety of tissues including the nervous system. In this study, we have molecularly cloned from zebrafish sources, the orthologues of the six human alpha2,8-sialyltransferases (ST8Sia), a family of sialyltransferases implicated in the alpha2-8-mono-, oligo-, and poly-sialylation of glycoproteins and gangliosides and we have analysed their expression pattern in the embryonic zebrafish nervous system, using in situ hybridization. Our results show that all six ST8Sia exhibit distinct and overlapping patterns of expression in the developing zebrafish central nervous system with spatial and temporal regulation of the expression of these genes, which suggests a role for the alpha2-8-sialylated compounds in the developing nervous system.

Developmental regulation of oligosialylation in zebrafish.

Zebrafish appears as a relevant model for the functional study of glycoconjugates along vertebrate's development. Indeed, as a prelude to such studies, we have previously identified a vast array of potentially stage-specific glycoconjugates, which structures are reminiscent of glycosylation pathways common to all vertebrates. In the present study, we have focused on the identification and regulation of major protein and lipids associated alpha2-8-linked oligosialic acids motifs in the early development of zebrafish. By a combination of partial hydrolysis, anion exchange HPLC-FD and mass spectrometry, we demonstrated that glycoproteins and glycolipids differed by the extent and the nature of their substituting oligosialylated sequences. Furthermore, relative quantifications showed that alpha2-8-linked sialylation was differentially regulated in both families of glycoconjugates along development. Accordingly, we established that alpha2,8-sialyltransferase mRNA levels was directly correlated with changes of alpha2,8-sialylation status of glycolipids, but independent of those observed on major glycoproteins that appear to originate from the mother.

Glycomic survey mapping of zebrafish identifies unique sialylation pattern.

Functional genomics and proteomics studies of the developmental glycobiology of zebrafish are greatly hampered by the current lack of knowledge on its glycosylation profile. To furnish the requisite structural basis for a more insightful functional delineation and genetic manipulation, we have initiated a survey mapping of the possible expression of stage-specific glycoconjugates in zebrafish. High-sensitivity mass spectrometry (MS) analysis in conjunction with the usual array of enzymatic and chemical derivatization was employed as the principal method for rapid differential mapping of the glycolipids and sequentially liberated N- and O-glycans from the total extracts. We demonstrated that all developmental stages of the zebrafish under investigation, from fertilized eggs to hatched embryos, synthesize oligomannosyl types of N-glycans, as well as complex types with additionally beta4-galactosylated, Neu5Ac/Neu5Gc monosialylated Lewis x termini. A combination of collision-induced dissociation (CID)-MS/MS and nuclear magnetic resonance (NMR) analyses led to the identification of an abundant and unusual mucin-type O-glycosylation, based on a novel sequence Fucalpha1-3GalNAcbeta1-4(Neu5Ac/Neu5Gcalpha2-3)Galbeta1-3GalNAc. This core structure may be further oligosialylated, but exclusively in the earlier development stages. Similarly, MS and MS/MS analyses of the extracted glycolipid fraction revealed the presence of a heterogeneous family of oligosialylated lactosylceramide compounds. In contrast to the O-glycans, these glycolipids only appear in the later development stages, suggesting a complex pattern of regulation for sialyltransferase activities during zebrafish embryogenesis.