ABH blood group antigens in N-glycan of human glycophorin A

Potentially causal association between immunoglobulin G N-glycans and cardiometabolic diseases: Bidirectional two-sample Mendelian randomization study

BACKGROUND

Observational studies support that altered immunoglobulin G (IgG) N-glycosylation and inflammatory factors are associated with cardiometabolic diseases (CMDs); nevertheless, the causality between them remains unclear.

METHODS

Two-sample Mendelian randomization (MR) analyses were conducted to systematically investigate the bidirectional causality between IgG N-glycans and nine CMDs in both East Asians and Europeans.

RESULTS

In the forward MR analysis, the univariable MR analysis presented suggestive causality of 14 and eight genetically instrumented IgG N-glycans with CMDs in East Asians and Europeans, respectively; the multivariable MR analysis showed that ten and 11 pairs of glycan-CMD associations were identified in East Asian and European populations, respectively. In the reverse MR analysis, based on East Asians and Europeans, the univariable MR analysis presented suggestive causality of seven and 12 genetically instrumented CMDs with IgG N-glycans, respectively; the multivariable MR analysis presented that six and five CMD-glycan causality were found in East Asian and Europeans, respectively.

CONCLUSIONS

The comprehensive MR analyses provide suggestive evidence of bidirectional causality between IgG N-glycans and CMDs. This work helps to understand the molecular mechanism of the occurrence/progression of CMDs, optimize existing and develop new strategies to prevent CMDs, and contribute to the early identification of high-risk groups of CMDs.

Differential enzymatic deglycosylation reveals attachment of red cell B antigen onto the carbohydrate moiety of glycophorin A and glycophorin B

BACKGROUND AND OBJECTIVES

Early studies indicate that red cell A and B antigens are attached primarily onto band 3 and GLUT1 on the erythrocyte membrane and little onto glycophorin A (GPA) and glycophorin B (GPB). But as GPA and band 3 form stable protein complexes and GPA is much more heavily glycosylated than band 3, this study re-examined the association between ABO antigens and GPA/GPB.

MATERIALS AND METHODS

Band 3/GPA-associated protein complexes were first immunoprecipitated, followed by differential enzymatic deglycosylation that removed sialic acids, N-glycans and O-glycans. Serological anti-A (BIRMA 1) and anti-B IgM (GAMA 110) could be used for western blot (WB); however, only the anti-B IgM showed significant reactivity for the immunoprecipitates isolated by anti-band 3. The expression of the B antigen in un-deglycosylated and differentially deglycosylated band 3 immunoprecipitates was thus compared.

RESULTS

Besides attachment to band 3, red cell B antigen expressed substantially on GPA monomer and homodimer, GPA*GPB heterodimer, and GPB monomer and dimer via attachments through the N- and O-glycans.

CONCLUSION

Immunoprecipitation (IP), as a means of protein separation and concentration, was used in combination with a WB to differentiate glycosylation on different proteins and oligomers. This study implemented differential enzymatic deglycosylation during IP of the band 3 complexes. This combined approach allowed separate identification of the B antigen on GPA/GPB monomer and dimer and GPA*GPB heterodimer, and band 3 on the WB and verified non-trivial expression of the B antigen on GPA and GPB on the erythrocyte surface.

N-Glycomics of Human Erythrocytes

Glycosylation is a complex post-translational modification that conveys functional diversity to glycoconjugates. Cell surface glycosylation mediates several biological activities such as induction of the intracellular signaling pathway and pathogen recognition. Red blood cell (RBC) membrane N-glycans determine blood type and influence cell lifespan. Although several proteomic studies have been carried out, the glycosylation of RBC membrane proteins has not been systematically investigated. This work aims at exploring the human RBC N-glycome by high-sensitivity MALDI-MS techniques to outline a fingerprint of RBC N-glycans. To this purpose, the MALDI-TOF spectra of healthy subjects harboring different blood groups were acquired. Results showed the predominant occurrence of neutral and sialylated complex N-glycans with bisected N-acetylglucosamine and core- and/or antennary fucosylation. In the higher mass region, these species presented with multiple N-acetyllactosamine repeating units. Amongst the detected glycoforms, the presence of glycans bearing ABO(H) antigens allowed us to define a distinctive spectrum for each blood group. For the first time, advanced glycomic techniques have been applied to a comprehensive exploration of human RBC N-glycosylation, providing a new tool for the early detection of distinct glycome changes associated with disease conditions as well as for understanding the molecular recognition of pathogens.

Bisecting GlcNAc Is a General Suppressor of Terminal Modification of N-glycan

Glycoproteins are decorated with complex glycans for protein functions. However, regulation mechanisms of complex glycan biosynthesis are largely unclear. Here we found that bisecting GlcNAc, a branching sugar residue in N-glycan, suppresses the biosynthesis of various types of terminal epitopes in N-glycans, including fucose, sialic acid and human natural killer-1. Expression of these epitopes in N-glycan was elevated in mice lacking the biosynthetic enzyme of bisecting GlcNAc, GnT-III, and was conversely suppressed by GnT-III overexpression in cells. Many glycosyltransferases for N-glycan terminals were revealed to prefer a nonbisected N-glycan as a substrate to its bisected counterpart, whereas no up-regulation of their mRNAs was found. This indicates that the elevated expression of the terminal N-glycan epitopes in GnT-III-deficient mice is attributed to the substrate specificity of the biosynthetic enzymes. Molecular dynamics simulations further confirmed that nonbisected glycans were preferentially accepted by those glycosyltransferases. These findings unveil a new regulation mechanism of protein N-glycosylation.

The structures of glycophorin C N-glycans, a putative component of the GPC receptor site for Plasmodium falciparum EBA-140 ligand

Glycophorins C and D are highly glycosylated integral sialoglycoproteins of human red blood cell membranes carrying the Gerbich blood group antigens. The O- and N-glycosidic chains of the major erythrocyte glycoprotein (Lisowska E. 2001, Antigenic properties of human glycophorins - an update. Adv Exp Med Biol, 491:155-169; Tomita M and Marchesi VT. 1975, Amino-acid sequence and oligosaccharide attachment sites of human erythrocyte glycophorin. Proc Natl Acad Sci USA, 72:2964-2968.) are well characterized but the structure of GPC N-glycans has remained unknown. This problem became important since it was reported that GPC N-glycans play an essential role in the interaction with Plasmodium falciparum EBA-140 merozoite ligand. The elucidation of these structures seems essential for full characterization of the GPC binding site for the EBA-140 ligand. We have employed detailed structural analysis using sequential mass spectrometry to show that many GPC N-glycans contain H2 antigen structures and several contain polylactosamine structures capped with fucose. The results obtained indicate structural heterogeneity of the GPC N-glycans and show the existence of structural elements not found in glycophorin A N-glycans. Our results also open a possibility of new interpretation of the data concerning the binding of P. falciparum EBA-140 ligand to GPC. We hypothesize that preferable terminal fucosylation of N-glycosidic chains containing repeating lactosamine units of the GPC Gerbich variant could be an explanation for why the EBA-140 ligand does not react with GPC Gerbich and an indication that the EBA-140 interaction with GPC is distinctly dependent on the GPC N-glycan structure.

Molecular deciphering of the ABO system as a basis for novel diagnostics and therapeutics in ABO incompatible transplantation

In recent years ABO incompatible kidney transplantation (KTx) has become a more or less clinical routine procedure with graft and patient survival similar to those of ABO compatible transplants. Antigen-specific immunoadsorption (IA) for anti-A and anti-B antibody removal constitutes in many centers an important part of the treatment protocol. ABO antibody titration by hemagglutination is guiding the treatment; both if the recipient can be transplanted as well as in cases of suspected rejections if antibody removal should be performed. Despite the overall success of ABO incompatible KTx, there is still room for improvements and an extension of the technology to include other solid organs. Based on an increased understanding of the structural complexity and tissue distribution of ABH antigens and the fine epitope specificity of the ABO antibody repertoire, improved IA matrices and ABO antibody diagnostics should be developed. Furthermore, understanding the molecular mechanisms behind accommodation of ABO incompatible renal allografts could make it possible to induce long-term allograft acceptance also in human leukocyte antigen (HLA) sensitized recipients and, perhaps, also make clinical xenotransplantation possible.

Typing of blood-group antigens on neutral oligosaccharides by negative-ion electrospray ionization tandem mass spectrometry

Blood-group antigens, such as those containing fucose and bearing the ABO(H)- and Lewis-type determinants expressed on the carbohydrate chains of glycoproteins and glycolipids, and also on unconjugated free oligosaccharides in human milk and other secretions, are associated with various biological functions. We have previously shown the utility of negative-ion electrospay ionization tandem mass spectrometry with collision-induced dissociation (ESI-CID-MS/MS) for typing of Lewis (Le) determinants, for example, Le(a), Le(x), Le(b), and Le(y) on neutral and sialylated oligosaccharide chains. In the present report, we extended the strategy to characterization of blood-group A-, B-, and H-determinants on type 1 and type 2 and also on type 4 globoside chains to provide a high sensitivity method for typing of all the major blood-group antigens, including the A, B, H, Le(a), Le(x), Le(b), and Le(y) determinants, present in oligosaccharides. Using the principles established, we identified two minor unknown oligosaccharide components present in the products of enzymatic synthesis by bacterial fermentation. We also demonstrated that the unique fragmentations derived from the D- and (0,2)A-type cleavages observed in ESI-CID-MS/MS, which are important for assigning blood-group and chain types, only occur under the negative-ion conditions for reducing sugars but not for reduced alditols or under positive-ion conditions.

One-step immunopurification and lectinochemical characterization of the Duffy atypical chemokine receptor from human erythrocytes

Duffy antigen/receptor for chemokines (DARC) is a glycosylated seven-transmembrane protein acting as a blood group antigen, a chemokine binding protein and a receptor for Plasmodium vivax malaria parasite. It is present on erythrocytes and endothelial cells of postcapillary venules. The N-terminal extracellular domain of the Duffy glycoprotein carries Fy(a)/Fy(b) blood group antigens and Fy6 linear epitope recognized by monoclonal antibodies. Previously, we have shown that recombinant Duffy protein expressed in K562 cells has three N-linked oligosaccharide chains, which are mainly of complex-type. Here we report a one-step purification method of Duffy protein from human erythrocytes. DARC was extracted from erythrocyte membranes in the presence of 1% n-dodecyl-β-D-maltoside (DDM) and 0.05% cholesteryl hemisuccinate (CHS) and purified by affinity chromatography using immobilized anti-Fy6 2C3 mouse monoclonal antibody. Duffy glycoprotein was eluted from the column with synthetic DFEDVWN peptide containing epitope for 2C3 monoclonal antibody. In this single-step immunoaffinity purification method we obtained highly purified DARC, which migrates in SDS-polyacrylamide gel as a major diffuse band corresponding to a molecular mass of 40-47 kDa. In ELISA purified Duffy glycoprotein binds anti-Duffy antibodies recognizing epitopes located on distinct regions of the molecule. Results of circular dichroism measurement indicate that purified DARC has a high content of α-helical secondary structure typical for chemokine receptors. Analysis of DARC glycans performed by means of lectin blotting and glycosidase digestion suggests that native Duffy N-glycans are mostly triantennary complex-type, terminated with α2-3- and α2-6-linked sialic acid residues with bisecting GlcNAc and α1-6-linked fucose at the core.

A novel series of anti-human glycophorin A (CD235a) antibodies defining five extra- and intracellular epitopes

Glycophorin A (GPA, CD235a) is a major membrane glycoprotein and marker of cells of the erythroid lineage. It is also the target of Plasmodium falciparum and of influenza virus. We describe a novel series of 10 antibodies towards GPA, recognizing four extra- and intracellular peptide epitopes of this molecule (defined by epitope mapping) and one mixed peptide/carbohydrate epitope. All antibodies bind better to the desialylated than to the fully sialylated molecule, including those specific for the intracellular epitope. For some of the antibodies (representing all five epitopes) functional binding constants were determined by Surface Plasmon Resonance. The new panel complements the already known anti-glycophorin antibodies and offers several potential applications for, e.g., differential diagnosis of erythroleukemias, lineage analyses of erythroid cells, isolation of senescent erythrocytes, or a highly sensitive neuraminidase assay.