The nature and abundance of human red cell surface glycoproteins

Desipramine induces eryptosis in human erythrocytes, an effect blunted by nitric oxide donor sodium nitroprusside and N-acetyl-L-cysteine but enhanced by Calcium depletion

Background: Desipramine a representative of tricyclic antidepressants (TCAs) promotes recovery of depressed patients by inhibition of reuptake of neurotransmitters serotonin (SER) and norepinephrine (NE) in the presynaptic membrane by directly blocking their respective transporters SERT and NET.Aims: To study the effect of desipramine on programmed erythrocyte death (eryptosis) and explore the underlying mechanisms.Methods: Phosphatidylserine (PS) exposure on the cell surface as marker of cell death was estimated from annexin-V-binding, cell volume from forward scatter in flow cytometry. Hemolysis was determined photometrically, and intracellular glutathione [GSH]i from high performance liquid chromatography.Results: Desipramine dose-dependently significantly enhanced the percentage of annexin-V-binding cells and didn´t impact glutathione (GSH) synthesis. Desipramine-induced eryptosis was significantly reversed by pre-treatment of erythrocytes with either nitric oxide (NO) donor sodium nitroprusside (SNP) or N-acetyl-L-cysteine (NAC). The highest inhibitory effect was obtained by using both inhibitors together. Calcium (Ca2+) depletion aggravated desipramine-induced eryptosis. Changing the order of treatment, i.e. desipramine first followed by inhibitors, could not influence the inhibitory effect of SNP or NAC.Conclusion: Antidepressants-caused intoxication can be treated by SNP and NAC, respectively. B) Patients with chronic hypocalcemia should not be treated with tricyclic anti-depressants or their dose should be noticeably reduced.

Frequencies of MNS Blood Group Antigens and Phenotypes in Southwestern Saudi Arabia

PURPOSE

Knowledge of the prevalence of blood group antigens in a given population is important for the prevention of hemolytic reactions. The MNS blood group system (002) has four polymorphic antigens-M, N, S, and s. Anti-S and anti-s antibodies may result in immediate and delayed hemolytic transfusion reactions, and hemolytic disease of the fetus and newborn may occur. The present study investigated the frequencies of the main antigens and phenotypes of the MNS blood group system.

SUBJECTS AND METHODS

We randomly obtained 149 samples from anonymous Saudi blood donors living in Jazan Province. Serotyping was conducted using a gel card to investigate (M, N, S, and s) antigens and phenotypes.

RESULTS

The frequencies of MNS antigens were as follows: M = 89.26%, N = 51.67%, S = 61.07%, and s = 82.55%. Regarding the MNS phenotypes, nine phenotypes were observed in the study population. The most common phenotype was M+N-S+s+ (n = 36, 24.16%), in contrast to the least common phenotype M+N-S-s- (n = 1, 0.67%). The prevalence of the MNS phenotypes in the current study population was highly and significantly different from that in Europeans (P = 0.044) and African Americans (P = 0.000).

CONCLUSION

In summary, this study reports the frequencies of the MNS antigens and phenotypes in Jazan Province, Saudi Arabia. The most common phenotype was M+N-S+s+, whereas the least observed phenotype was M+N-S-s-. The outcomes of this study may assist the blood banks in Jazan Province to establish an extended phenotyping protocol including the MNS antigens, in particular S and s antigens, to preclude any alloimmunization events.

Nanoarchitecture and dynamics of the mouse enteric glycocalyx examined by freeze-etching electron tomography and intravital microscopy

The glycocalyx is a highly hydrated, glycoprotein-rich coat shrouding many eukaryotic and prokaryotic cells. The intestinal epithelial glycocalyx, comprising glycosylated transmembrane mucins, is part of the primary host-microbe interface and is essential for nutrient absorption. Its disruption has been implicated in numerous gastrointestinal diseases. Yet, due to challenges in preserving and visualizing its native organization, glycocalyx structure-function relationships remain unclear. Here, we characterize the nanoarchitecture of the murine enteric glycocalyx using freeze-etching and electron tomography. Micrometer-long mucin filaments emerge from microvillar-tips and, through zigzagged lateral interactions form a three-dimensional columnar network with a 30 nm mesh. Filament-termini converge into globular structures ~30 nm apart that are liquid-crystalline packed within a single plane. Finally, we assess glycocalyx deformability and porosity using intravital microscopy. We argue that the columnar network architecture and the liquid-crystalline packing of the filament termini allow the glycocalyx to function as a deformable size-exclusion filter of luminal contents.

Sun, Krystofiak et al. show the nanoarchitecture of the murine enteric glycocalyx, glycoprotein-rich coat covering cells and assess its porosity and deformability in mice, providing a comprehensive structural framework. This study suggests that the glycocalyx may function as a deformable size-exclusion filter of luminal contents.

Genetic Evidence for Erythrocyte Receptor Glycophorin B Expression Levels Defining a Dominant Plasmodium falciparum Invasion Pathway into Human Erythrocytes

Plasmodium falciparum, the parasite that causes the deadliest form of malaria, has evolved multiple proteins known as invasion ligands that bind to specific erythrocyte receptors to facilitate invasion of human erythrocytes. The EBA-175/glycophorin A (GPA) and Rh5/basigin ligand-receptor interactions, referred to as invasion pathways, have been the subject of intense study. In this study, we focused on the less-characterized sialic acid-containing receptors glycophorin B (GPB) and glycophorin C (GPC). Through bioinformatic analysis, we identified extensive variation in glycophorin B (GYPB) transcript levels in individuals from Benin, suggesting selection from malaria pressure. To elucidate the importance of the GPB and GPC receptors relative to the well-described EBA-175/GPA invasion pathway, we used an ex vivo erythrocyte culture system to decrease expression of GPA, GPB, or GPC via lentiviral short hairpin RNA transduction of erythroid progenitor cells, with global surface proteomic profiling. We assessed the efficiency of parasite invasion into knockdown cells using a panel of wild-type P. falciparum laboratory strains and invasion ligand knockout lines, as well as P. falciparum Senegalese clinical isolates and a short-term-culture-adapted strain. For this, we optimized an invasion assay suitable for use with small numbers of erythrocytes. We found that all laboratory strains and the majority of field strains tested were dependent on GPB expression level for invasion. The collective data suggest that the GPA and GPB receptors are of greater importance than the GPC receptor, supporting a hierarchy of erythrocyte receptor usage in P. falciparum.

Evaluating the diffusion coefficient of dopamine at the cell surface during amperometric detection: disk vs ring microelectrodes

During exocytosis, small quantities of neurotransmitters are released by the cell. These neurotransmitters can be detected quantitatively using electrochemical methods, principally with disk carbon fiber microelectrode amperometry. An exocytotic event then results in the recording of a current peak whose characteristic features are directly related to the mechanisms of exocytosis. We have compared two exocytotic peak populations obtained from PC12 cells with a disk carbon fiber microelectrode and with a pyrolyzed carbon ring microelectrode array, with a 500 nm ring thickness. The specific shape of the ring electrode allows for precise analysis of diffusion processes at the vicinity of the cell membrane. Peaks obtained with a ring microelectrode array show a distorted average shape, owing to increased diffusion pathways. This result has been used to evaluate the diffusion coefficient of dopamine at the surface of a cell, which is up to an order of magnitude smaller than that measured in free buffer. The lower rate of diffusion is discussed as resulting from interactions with the glycocalyx.

Identifying novel Plasmodium falciparum erythrocyte invasion receptors using systematic extracellular protein interaction screens

The invasion of host erythrocytes by the parasite Plasmodium falciparum initiates the blood stage of infection responsible for the symptoms of malaria. Invasion involves extracellular protein interactions between host erythrocyte receptors and ligands on the merozoite, the invasive form of the parasite. Despite significant research effort, many merozoite surface ligands have no known erythrocyte binding partner, most likely due to the intractable biochemical nature of membrane-tethered receptor proteins and their interactions. The few receptor–ligand pairs that have been described have largely relied on sourcing erythrocytes from patients with rare blood groups, a serendipitous approach that is unsatisfactory for systematically identifying novel receptors. We have recently developed a scalable assay called AVEXIS (for AVidity-based EXtracellular Interaction Screen), designed to circumvent the technical difficulties associated with the identification of extracellular protein interactions, and applied it to identify erythrocyte receptors for orphan P. falciparum merozoite ligands. Using this approach, we have recently identified Basigin (CD147) and Semaphorin-7A (CD108) as receptors for RH5 and MTRAP respectively. In this essay, we review techniques used to identify Plasmodium receptors and discuss how they could beapplied in the future to identify novel receptors both for Plasmodium parasites but also other pathogens.

Structural basis for recognition of the pore-forming toxin intermedilysin by human complement receptor CD59

Pore-forming proteins containing the structurally conserved membrane attack complex/perforin fold play an important role in immunity and host-pathogen interactions. Intermedilysin (ILY) is an archetypal member of a cholesterol-dependent cytolysin subclass that hijacks the complement receptor CD59 to make cytotoxic pores in human cells. ILY directly competes for the membrane attack complex binding site on CD59, rendering cells susceptible to complement lysis. To understand how these bacterial pores form in lipid bilayers and the role CD59 plays in complement regulation, we determined the crystal structure of human CD59 bound to ILY. Here, we show the ILY-CD59 complex at 3.5 Å resolution and identify two interfaces mediating this host-pathogen interaction. An ILY-derived peptide based on the binding site inhibits pore formation in a CD59-containing liposome model system. These data provide insight into how CD59 coordinates ILY monomers, nucleating an early prepore state, and suggest a potential mechanism of inhibition for the complement terminal pathway.

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Crystal structure of the ILY-CD59 complex defines two interfaces

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Our two binding interfaces are supported by previous mutagenesis studies

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An ILY-derived peptide competes for binding in a liposome model system

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Our model provides a structural basis for CD59 nucleation of an ILY early prepore

Single cell amperometry reveals glycocalyx hinders the release of neurotransmitters during exocytosis

The diffusional hindrance of the glycocalyx along the cell surface on exocytotic peaks, observed with single cell amperometry, was investigated. Partial digestion of the glycocalyx with neuraminidase led to the observation of faster peaks, as shown by varied peak parameters. This result indicates that diffusion of small molecules in the partially digested glycocalyx is 2.2 faster than in the intact glycocalyx. Similarly, neutralization of the negative charges present in the cell microenvironment led to faster peak kinetics. The analysis of the vesicular efflux indicates that the diffusion coefficient of dopamine at the cell surface is at most 45% of the diffusion coefficient in free solution. This study shows that the glycocalyx plays an important role in the diffusion kinetics of processes along the cell surface, including exocytotic events.

ABO blood groups influence macrophage-mediated phagocytosis of Plasmodium falciparum-infected erythrocytes

Erythrocyte polymorphisms associated with a survival advantage to Plasmodium falciparum infection have undergone positive selection. There is a predominance of blood group O in malaria-endemic regions, and several lines of evidence suggest that ABO blood groups may influence the outcome of P. falciparum infection. Based on the hypothesis that enhanced innate clearance of infected polymorphic erythrocytes is associated with protection from severe malaria, we investigated whether P. falciparum-infected O erythrocytes are more efficiently cleared by macrophages than infected A and B erythrocytes. We show that human macrophages in vitro and mouse monocytes in vivo phagocytose P. falciparum-infected O erythrocytes more avidly than infected A and B erythrocytes and that uptake is associated with increased hemichrome deposition and high molecular weight band 3 aggregates in infected O erythrocytes. Using infected A₁, A₂, and O erythrocytes, we demonstrate an inverse association of phagocytic capacity with the amount of A antigen on the surface of infected erythrocytes. Finally, we report that enzymatic conversion of B erythrocytes to type as O before infection significantly enhances their uptake by macrophages to observed level comparable to that with infected O wild-type erythrocytes. These data provide the first evidence that ABO blood group antigens influence macrophage clearance of P. falciparum-infected erythrocytes and suggest an additional mechanism by which blood group O may confer resistance to severe malaria.

Plasmodium falciparum malaria is considered to be one of the strongest forces for evolutionary selection pressure on the human genome. Different red blood cell variants associated with a survival advantage to P. falciparum infection have undergone positive selection. Blood group O is found more frequently in malaria-endemic regions and has been associated with protection against severe malaria and death. However the biological basis of protection remains unclear. In this study, we investigated innate immune clearance of P. falciparum-infected erythrocytes by macrophages as a possible mode of protection. We show that macrophages clear P. falciparum-infected O erythrocytes more avidly than infected A and B erythrocytes. We also report that enzymatic conversion of infected blood group B red cells to type as “O” like erythrocytes significantly enhances their uptake by macrophages to a level comparable to that observed with infected O wild type erythrocytes. These data provide the first evidence that clearance of P. falciparum-infected erythrocytes is influenced by human ABO blood groups and suggest a new mechanism by which blood group O may contribute to protection against severe malaria.

Targeting recombinant thrombomodulin fusion protein to red blood cells provides multifaceted thromboprophylaxis

Thrombin generates fibrin and activates platelets and endothelium, causing thrombosis and inflammation. Endothelial thrombomodulin (TM) changes thrombin's substrate specificity toward cleavage of plasma protein C into activated protein C (APC), which opposes its thrombotic and inflammatory activities. Endogenous TM activity is suppressed in pathologic conditions, and antithrombotic interventions involving soluble TM are limited by rapid blood clearance. To overcome this problem, we fused TM with a single chain fragment (scFv) of an antibody targeted to red blood cells. scFv/TM catalyzes thrombin-mediated generation of activated protein C and binds to circulating RBCs without apparent damage, thereby prolonging its circulation time and bioavailability orders of magnitude compared with soluble TM. In animal models, a single dose of scFv/TM, but not soluble TM, prevents platelet activation and vascular occlusion by clots. Thus, scFv/TM serves as a prodrug and provides thromboprophylaxis at low doses (0.15 mg/kg) via multifaceted mechanisms inhibiting platelets and coagulation.

Basigin is a receptor essential for erythrocyte invasion by Plasmodium falciparum

Erythrocyte invasion by Plasmodium falciparum is central to the pathogenesis of malaria. Invasion requires a series of extracellular recognition events between erythrocyte receptors and ligands on the merozoite, the invasive form of the parasite. None of the few known receptor-ligand interactions involved are required in all parasite strains, indicating that the parasite is able to access multiple redundant invasion pathways. Here, we show that we have identified a receptor-ligand pair that is essential for erythrocyte invasion in all tested P. falciparum strains. By systematically screening a library of erythrocyte proteins, we have found that the Ok blood group antigen, basigin, is a receptor for PfRh5, a parasite ligand that is essential for blood stage growth. Erythrocyte invasion was potently inhibited by soluble basigin or by basigin knockdown, and invasion could be completely blocked using low concentrations of anti-basigin antibodies; importantly, these effects were observed across all laboratory-adapted and field strains tested. Furthermore, Ok(a-) erythrocytes, which express a basigin variant that has a weaker binding affinity for PfRh5, had reduced invasion efficiencies. Our discovery of a cross-strain dependency on a single extracellular receptor-ligand pair for erythrocyte invasion by P. falciparum provides a focus for new anti-malarial therapies.

Effects of natural selection and gene conversion on the evolution of human glycophorins coding for MNS blood polymorphisms in malaria-endemic African populations

Malaria has been a very strong selection pressure in recent human evolution, particularly in Africa. Of the one million deaths per year due to malaria, more than 90% are in sub-Saharan Africa, a region with high levels of genetic variation and population substructure. However, there have been few studies of nucleotide variation at genetic loci that are relevant to malaria susceptibility across geographically and genetically diverse ethnic groups in Africa. Invasion of erythrocytes by Plasmodium falciparum parasites is central to the pathology of malaria. Glycophorin A (GYPA) and B (GYPB), which determine MN and Ss blood types, are two major receptors that are expressed on erythrocyte surfaces and interact with parasite ligands. We analyzed nucleotide diversity of the glycophorin gene family in 15 African populations with different levels of malaria exposure. High levels of nucleotide diversity and gene conversion were found at these genes. We observed divergent patterns of genetic variation between these duplicated genes and between different extracellular domains of GYPA. Specifically, we identified fixed adaptive changes at exons 3-4 of GYPA. By contrast, we observed an allele frequency spectrum skewed toward a significant excess of intermediate-frequency alleles at GYPA exon 2 in many populations; the degree of spectrum distortion is correlated with malaria exposure, possibly because of the joint effects of gene conversion and balancing selection. We also identified a haplotype causing three amino acid changes in the extracellular domain of glycophorin B. This haplotype might have evolved adaptively in five populations with high exposure to malaria.

The functional importance of blood group-active molecules in human red blood cells

Antigens of 23 of the 30 human blood group systems are defined by the amino acid sequence of red cell membrane proteins. The antigens of DI, RH, RHAG, MNS, GE and CO systems are carried on blood group-active proteins (Band 3, D and CE polypeptides, RhAG, Glycophorins A and B, Glycophorins C and D and Aquaporin 1, respectively) which are expressed at high levels (>200,000 copies/red cell). These major proteins contribute to essential red cell functions either directly as membrane transporters and by providing linkage to the underlying red cell skeleton or by facilitating the membrane assembly of the protein complexes involved in these processes. The proteins expressing antigens of the remaining 17 blood group systems are much less abundant (<20,000 copies/red cell) and their functional importance for the circulating red cell is largely unknown. Human gene knock-outs (null phenotypes) have been described for many of these minor blood group-active proteins, but only absence of Kx glycoprotein has been clearly linked with pathology directly related to the function of circulating red cells. Recent evidence suggesting the normal quality control system for glycoprotein synthesis is altered during the latter stages of red cell production raises the possibility that many of these low abundance blood group-active proteins are vestigial. In sickle cell disease and polycythaemia vera, elevated Lutheran glycoprotein expression may contribute to pathology. Dyserythropoiesis with reduced antigen expression can result from mutations in the erythroid transcription factors GATA-1 and EKLF.

Shotgun glycomics: a microarray strategy for functional glycomics

Major challenges of glycomics are to characterize a glycome and identify functional glycans as ligands for glycan-binding proteins (GBPs). To address these issues we have developed a general strategy termed shotgun glycomics. We focus on glycosphingolipids (GSLs), a challenging class of glycoconjugates recognized by toxins, antibodies, and GBPs. We derivatized GSLs extracted from cells with a heterobifunctional fluorescent tag suitable for covalent immobilization. Fluorescent GSLs were separated by multidimensional chromatography, quantified, and coupled to glass slides to create GSL shotgun microarrays. The microarrays were interrogated with cholera toxin, antibodies, and sera from patients with Lyme disease to identify biologically relevant GSLs that were subsequently characterized by mass spectrometry. Shotgun glycomics incorporating GSLs and potentially glycoprotein-derived glycans provides an approach to accessing the complex glycomes of animal cells and offers a strategy for focusing structural analyses on functionally significant glycans.

The sialome--far more than the sum of its parts

The glycome is defined as the glycan repertoire of cells, tissues, and organisms, as found under specified conditions. The vastly diverse glycome is generated by a nontemplate driven biosynthesis, which is indirectly encoded in the genome, and very dynamic. Due to this overwhelming diversity, glycomic analysis must be approached at different hierarchical levels of complexity. In this review five such levels of complexity and the experimental approaches used for analysis at each level are discussed for a subclass of the glycome: the sialome. The sialome, in analogy to the canopy of a forest, covers the cell membrane with diverse array of complex sialylated structures. Sialome complexity includes modification of sialic acid core structure (the leaves and flowers), the linkage to the underlying sugar (the stems), the identity, and arrangement of the underlying glycans (the branches), the structural attributes of the underlying glycans (the trees), and finally, the spatial organization of the sialoglycans in relation to components of the intact cell surface (the forest). Understanding the full complexity of the sialome thus requires combined analyses at multiple levels, that is, the sialome is far more than the sum of its parts.

Detection of hereditary pyropoikilocytosis by the eosin-5-maleimide (EMA)-binding test is attributable to a marked reduction in EMA-reactive transmembrane proteins

INTRODUCTION

Hereditary spherocytosis (HS) and hereditary pyropoikilocytosis (HPP, severe form of hereditary elliptocytosis) are unrelated red cell disorders caused by defects in distinct regions of the red cell cytoskeleton. The high predictive value of the eosin-5-maleimide (EMA)-binding test for the diagnosis of HS is because of its interaction with transmembrane proteins band 3, Rh protein, Rh glycoprotein and CD47, which are reduced on HS red cells. Our study was undertaken to determine why EMA-labelled HPP red cells were previously found to give much lower fluorescence readings than HS.

METHODS

Flow cytometry was used to determine the relative amounts of monoclonal antibodies bound to red cells from normal adults, HS and HPP groups. Confocal microscopy was used to visualise the overall staining pattern of the red cells with selected antibodies.

RESULTS

In flow cytometry, HPP red cells gave lower antibody binding to the four EMA-reactive membrane proteins than HS red cells and bound less antibody to glycophorins A and C, and CD59. Confocal images of Rh protein and band 3 immunostaining revealed a greater number of HPP red cells having partial or no fluorescence than in HS and normal controls.

CONCLUSION

Lesser amounts of EMA-reactive membrane proteins were detected in HPP than HS red cells, thus confirming their lower fluorescence readings in the EMA-binding test. The concomitant reduction in glycophorins A and C, and CD59 in HPP could have caused cellular contraction, resulting in poikilocytosis.

The relationship between blood groups and disease

The relative contribution of founder effects and natural selection to the observed distribution of human blood groups has been debated since blood group frequencies were shown to differ between populations almost a century ago. Advances in our understanding of the migration patterns of early humans from Africa to populate the rest of the world obtained through the use of Y chromosome and mtDNA markers do much to inform this debate. There are clear examples of protection against infectious diseases from inheritance of polymorphisms in genes encoding and regulating the expression of ABH and Lewis antigens in bodily secretions particularly in respect of Helicobacter pylori, norovirus, and cholera infections. However, available evidence suggests surviving malaria is the most significant selective force affecting the expression of blood groups. Red cells lacking or having altered forms of blood group-active molecules are commonly found in regions of the world in which malaria is endemic, notably the Fy(a-b-) phenotype and the S-s- phenotype in Africa and the Ge- and SAO phenotypes in South East Asia. Founder effects provide a more convincing explanation for the distribution of the D- phenotype and the occurrence of hemolytic disease of the fetus and newborn in Europe and Central Asia.

ABO blood group glycans modulate sialic acid recognition on erythrocytes

ABH(O) blood group polymorphisms are based on well-known intraspecies variations in structures of neutral blood cell surface glycans in humans and other primates. Whereas natural antibodies against these glycans can act as barriers to blood transfusion and transplantation, the normal functions of this long-standing evolutionary polymorphism remain largely unknown. Although microbial interactions have been suggested as a selective force, direct binding of lethal pathogens to ABH antigens has not been reported. We show in this study that ABH antigens found on human erythrocytes modulate the specific interactions of 3 sialic acid-recognizing proteins (human Siglec-2, 1918SC influenza hemagglutinin, and Sambucus nigra agglutinin) with sialylated glycans on the same cell surface. Using specific glycosidases that convert A and B glycans to the underlying H(O) structure, we show ABH antigens stabilize sialylated glycan clusters on erythrocyte membranes uniquely for each blood type, generating differential interactions of the 3 sialic acid-binding proteins with erythrocytes from each blood type. We further show that by stabilizing such structures ABH antigens can also modulate sialic acid-mediated interaction of pathogens such as Plasmodium falciparum malarial parasite. Thus, ABH antigens can noncovalently alter the presentation of other cell surface glycans to cognate-binding proteins, without themselves being a direct ligand.

Invasion by P. falciparum merozoites suggests a hierarchy of molecular interactions

Central to the pathology of malaria disease are the repeated cycles of parasite invasion and destruction of human erythrocytes. In Plasmodium falciparum, the most virulent species causing malaria, erythrocyte invasion involves several specific receptor-ligand interactions that direct the pathway used to invade the host cell, with parasites varying in their dependency on these different pathways. Gene disruption of a key invasion ligand in the 3D7 parasite strain, the P. falciparum reticulocyte binding-like homolog 2b (PfRh2b), resulted in the parasite invading via a novel pathway. Here, we show results that suggest the molecular basis for this novel pathway is not due to a molecular switch but is instead mediated by the redeployment of machinery already present in the parent parasite but masked by the dominant role of PfRh2b. This would suggest that interactions directing invasion are organized hierarchically, where silencing of dominant invasion ligands reveal underlying alternative pathways. This provides wild parasites with the ability to adapt to immune-mediated selection or polymorphism in erythrocyte receptors and has implications for the use of invasion-related molecules in candidate vaccines.

Identification of a novel hybrid glycophorin gene encoding GP.Hop

BACKGROUND

The GP.Hop (Mi.IV) phenotype expresses the MNS low-incidence antigens Mur, Hop, TSEN, MINY, and MUT. Because serologically similar MNS phenotypes expressing some or all of these antigens were shown to be carried by hybrid GP(B-A-B) proteins, it was proposed that a similar protein would be found for GP.Hop. The identification of a second GP.Hop propositus (ES) initiated a study to determine the molecular basis of this phenotype.

STUDY DESIGN AND METHODS

Serologic tests and immunoblotting analysis with glycophorin-specific antibodies were performed. GYPB, the gene encoding the GPB protein, was cloned and sequenced after reverse transcription PCR amplification of total RNA isolated from ES. GYPB-specific primers encompassing GYPB pseudoexon 3, intron 3, and exon 4 were also used to clone and sequence genomic DNA from ES and MH, the original GP.Hop proband.

RESULTS

Serologic and immunochemical data confirmed that ES's RBCs carried antigens associated with the GP.Hop phenotype. Sequencing of ES's cDNA demonstrated the presence of genes predicted to encode s-specific GPB and an S-specific GP(B-A-B) hybrid in which the 3' end of GYPB pseudoexon 3 had been replaced by a short nucleotide sequence from exon 3 of the GPA gene (GYPA). The hybrid nucleotide sequence contained sequence motifs previously shown to be required for the expression of the Mur, Hop, TSEN, MINY, and MUT, which is consistent with their presence as detected serologically. Genomic DNA analysis found that the crossover point in GYPB pseudoexon 3 was identical in ES and MH.

CONCLUSIONS

The GP.Hop phenotype is produced by a hybrid GP(B-A-B) protein caused by a DNA insertion of GYPA into GYPB. The composition of the hybrid protein is GPB(1-26)-GPpsiB(27-50)-GPA(51-58)-GPB(S)(59-103).

Immune-mediated agglutination of cytoskeleton-free RBC microvesicles

BACKGROUND

The factors contributing to RBC agglutination are complex. The RBC cytoskeleton's participation in and contribution to this phenomenon are difficult to separate from those of the plasma membrane. Immunoreactive, cytoskeleton-free, band 3-enriched microvesicles can be generated from normal RBCs. Band 3 has been defined as an important antigen in autoimmune hemolytic anemia (AIHA).

STUDY DESIGN AND METHODS

RBC microvesicles devoid of major cytoskeletal proteins were generated and sensitized with eluates obtained from AIHA patients, DAT-positive blood donors, and antisera to common RBC antigens. Monoclonal anti-human IgG was added and agglutination was investigated. Autoantibody-specific binding was evaluated by employing (125)I protein A.

RESULTS

RBC vesicle agglutination with a 4+ anti-human globulin score was obtained with 10 autoantibody eluates from AIHA patients and anti-D (3+), but not with eluates from 20 DAT-positive blood donors or antisera directed to eight other common RBC antigens. Microvesicles sensitized with AIHA eluates bound 67 to 167 times as much (125)I protein A radioactivity as did those incubated with buffered normal saline and 18 to 45 times more than vesicles incubated with normal serum.

CONCLUSION

The major proteins of the RBC cytoskeleton are not required for supporting IgG immune-mediated agglutination of RBC microvesicles.

Red cell antigens on band 3 and glycophorin A

Band 3 and glycophorin A (GPA) are the two most abundant integral proteins of the red cell membrane, being present in approximately 10(6) copies per cell. The main functions of band 3 are membrane anion transport and maintenance of red cell membrane stability through interaction with the cytoskeleton. GPA plays an important role in prevention of red cell aggregation in the circulation and contribution to the glycocalyx. The extracellular domains of both proteins are highly polymorphic. Band 3 carries the antigens (currently 19) of the Diego blood group system and GPA and glycophorin B the antigens (currently 43) of the MNS system. There is substantial evidence that band 3 and GPA associate in the red cell membrane and the Wr(b) antigen, although a product of the band 3 gene, is known to require a complex of GPA and band 3 for normal expression. The discovery of a novel GPA mutation (Ala65-->Pro) giving rise to aberrant Wr(b) expression has been informative with regard to the site of interaction of the two proteins. The extensive array of GPA-related antigens is largely due to genetic events between two closely linked genes and different genetic mechanisms can give rise to the same antigen. This is in contrast to the antigens on band 3 which are exclusively due to single nucleotide mutations in the band 3 gene.

Fine specificities of murine anti-Mg monoclonal antibodies

The specificities of two murine anti-Mg monoclonal IgG1 antibodies, 3B10 and 2D5, were determined by pepscan analysis. The peptides which correspond to various fragments of amino-terminal portions of glycophorin A of group M (GPA-M), N (GPA-N) and Mg (GPA-Mg), and replacement analogues of some of these peptides, were synthesized on plastic pins and tested for binding of the antibodies. Both antibodies bound strongly to the N-terminal Mg octapeptide 1LSTNEVAM8, but they showed different subspecificities. The essential fragment of the epitope 2D5 are amino acid residues 2STNEV6. Replacement of any of these amino acid residues by Ala, and replacement of Glu5 residue by Gly, abolished or strongly reduced the antibody binding, but replacement of Asn4 by Thr gave only a moderate decrease of peptide activity. In contrast, the Leu1 and Asn4 residues were most essential components of the epitope 3B10, while Ser2, Thr3 and Glu5 seemed to be less important. Our present results and earlier ones on the specificity of human anti-Mg alloantibodies and monoclonal anti-M/Mg antibodies showed that antibodies reacting with Mg antigen recognize different fragments and/or different amino acid residues of the amino- terminal nonglycosylated domain of GPA-Mg. The knowledge of fine specificities of antibodies reacting with Mg antigen is interesting in view of the presence of anti-Mg alloantibodies in 1-2% of human sera.

[A molecular approach to the structure, polymorphism and function of blood groups]

Biochemical and molecular genetic studies have contributed to our molecular knowledge of blood group-associated molecules in the past few years. Among the 23 blood group systems presently identified, almost all have a molecular basis and present investigations are oriented towards the analysis of genetic polymorphisms, tissue-specific expression and structure-function relationships. Antigens defined by carbohydrate structures, among which ABO, Hh, Lewis and Secretor are the main representative species, are indirect gene products. They are synthesized by Golgi-resident glycosyltransferases, which are the direct products of the blood group genes. Many of these enzymes have been cloned and the molecular basis of the silent phenotypes, for instance 0, Bombay/paraBombay, Le(a-b-) and non-secretor, has been elucidated. However, the glycosyltransferases involved in the biosynthesis of Pk, P and P1 antigens are not yet characterized. A large number of blood group antigens carried by red cell polypeptides expressed at the cell surface are not related to a carbohydrate structure, and these proteins are direct blood group gene products. Most have been cloned and characterized recently, for instance MN antigens (glycophorin A), Ss antigens (glycophorin B), Gerbich antigens (glycophorins C and D) and antigens encoded by the RH, LW, KEL, FY, JK, XG, LU and XK loci. Other antigens have been located on proteins already identified, for instance the Cromer antigens on DAF, Knops antigens on CR1, Indian and AnWj antigens on CD44, Yt antigens on AChE, Diego, Wr, Rga and Warr on Band 3, Colton antigens on AQP-1 (water channel). The SC (Scianna) et DO (Dombrock) systems, however, still resist to molecular cloning. On the basis of this information, a tentative classification of blood group antigens into five functional categories is emerging: - Transporters and channels, - Receptors and ligands, - Adhesion molecules, - Enzymes, - Structural proteins. This review will focus on these recent findings and will illustrate how these studies may bring new information for analysis of normal and abnormal phenotypes and for understanding both the mechanisms of tissue specific expression and the potential function of these antigens, particularly those expressed in non-erythroid lineage. In addition, since our knowledge of the molecular basis of blood group polymorphisms has significantly increased, new genotyping techniques potentially useful in clinical applications will become available.

Detailed physical mapping of the genes encoding glycophorins A, B and E, as revealed by P1 plasmids containing human genomic DNA

Human glycophorins A, B and E (GPA, GPB and GPE) are members of the glycophorin gene family encompassing a 330-kb genomic segment located on chromosome 4, band q31 [Onda et al., J. Biol. Chem., 269 (1994) 13013-13020]. This gene family was apparently generated by two successive duplications of an ancestral gene. One of the progenitor genes, resulting from the first duplication, directly evolved into the GPA gene. The other progenitor gene acquired a unique 3'-region sequence and was then duplicated to yield GPB and GPE. In order to obtain a more detailed understanding of how these different members of the gene family evolved, we isolated several P1 plasmid clones encoding GPA, GPB or GPE. The precise locations of exon 1 and the exon encoding the transmembrane (TM) domain in GPA, GPB and GPE were then determined by hybridization with specific probes after restricted DNA fragments were separated by pulsed-field gel electrophoresis. The results obtained showed that the distances between exon 1 and exon 2 are almost equal for GPA and GPB, whereas this distance is larger in GPE. In contrast, the distance between exon 2 and the exon encoding the TM domain was shown to be the same among GPA, GPB and GPE. These results suggest that the gene divergence, i.e., insertions or deletions, took place after two successive duplications and supports the hypothesis that GPE acquired a portion of the GPA sequence surrounding exon 2 by gene conversion.

Blood group antigens defined by the amino acid sequences of red cell surface proteins

The antigens of 18 blood group systems are expressed on proteins that are intrinsic to the red cell. The proteins which carry the antigens of these systems have been identified and primary sequence information is available for all but two (SC, DO). Several different functional groups are evident. Antigens of the DI, CO, RH, XK and JK systems are located on proteins which have the structure of membrane transport proteins. The FY antigens mark a cytokine receptor. The IN, LW, XG antigens are associated with molecules which have adhesion functions and the LU glycoprotein also has a structure which suggests a role in adhesion. YT and KEL antigens are located on cell surface enzymes and the CR and KN antigen on molecules involved in complement regulation. Finally, the MN and GE antigens are located on sialic acid-rich glycoproteins (glycophorins A, B and C/D respectively), a group of molecules which do not, as yet, have a clearly defined function. The molecular basis of antigens in several blood group systems have been defined and shown to depend upon the amino acid sequence.

The major integral proteins of the human red cell

The structures and functions of the major human red cell integral membrane proteins are summarized in this review. The proteins that are discussed are the anion transporter (band 3), the sialic acid-rich glycophorins and the glucose transporter. Band 3 (AE1) is a member of a family of anion transporters which carry out Cl-/HCO3- exchange. AE1 is largely restricted to red cells and functions in CO2 transport between the tissues and lungs. In addition to its transport function band 3 acts as an anchor site to the membrane of the red cell skeleton, and also binds a number of cytoplasmic red cell proteins. Variant forms of band 3 are known and some of these have an effect on red cell function and viability. The glycophorins comprise three major proteins, glycophorin A (GPA), glycophorin B (GPB) and glycophorin C (GPC). GPA and GPB (together with another putative gene product, GPE) are closely related products of highly homologous genes located in tandem on the human chromosome. The similarity between the genes gives rise to a number of genetic variants as a result of unequal crossover events. The gene products are erythroid specific. The function of the proteins is not clearly established, but GPA appears to have a role in facilitating the movement of band 3 to the cell surface during the biosynthesis of the latter. The GPC gene is not related to the GPA, GPB and GPE gene family. This gene gives rise to GPC and a form of GPC which is truncated at the N-terminus and is designated GPD. GPC functions in anchoring the red cell skeleton to the membrane, and absence of the protein is associated with red cell abnormalities. GPC transcripts are found in many other tissues, where they probably also have a role in cytoskeletal interactions. The red cell glucose transporter (GLUT1) is a member of the gene family of passive glucose transporters. GLUT1 is not erythroid specific but is also present in several other tissues.

An immunoblotting procedure for screening glycophorins and band 3 protein in the same blots. Identification of glycophorin and band 3 variant forms

An immunoblotting procedure is described which makes it possible to screen multiple blood samples for the presence of glycophorin and band 3 variant forms with altered electrophoretic mobility. The procedure can be simplified by using whole red blood cell hemolysates instead of membranes for SDS-polyacrylamide gel electrophoresis. The use of hemolysates also has the advantage that antigens sensitive to proteolysis are not degraded in vitro. The same nitrocellulose blots were used for immunoenzymatic detection of glycophorins with a set of anti-glycophorin monoclonal antibodies, and for autoradiographic detection of band 3-derived bands with 125I-labeled anti-band 3 monoclonal antibody. The screening of 157 Caucasian blood samples revealed the presence of a slower-migrating form of band 3 in seven cases and variant glycophorin in one case. The variant glycophorin exhibited the features of hybrid glycophorin of B-A type.

Human erythrocyte glycophorins: protein and gene structure analyses

Human RBCs glycophorins are integral membrane proteins rich in sialic acids that carry blood group antigenic determinants and serve as ligands for viruses, bacteria, and parasites. These molecules have long been used as a general model of membrane proteins and as markers to study normal and pathological differentiation of the erythroid tissue. The RBC glycophorins known as GPA, GPB, GPC, GPD, and GPE have recently been fully characterized at both the protein and the DNA levels, and these studies have demonstrated conclusively that these molecules can be subdivided into two groups that are distinguished by distinct properties. The first group includes the major proteins GPA and GPB, which carry the MN and Ss blood group antigens, respectively, and a recently characterized protein, GPE, presumably expressed at a low level on RBCs. All three proteins are structurally homologous and are essentially erythroid specific. The respective genes are also strikingly homologous up to a transition site defined by an Alu repeat sequence located about 1 Kb downstream from the exon encoding the transmembrane regions. Downstream of the transition site, the GPB and GPE sequences are still homologous, but diverge completely from those of GPA. The three glycophorin genes are organized in tandem on chromosome 4q28-q31, and define a small gene cluster that presumably evolved by duplication from a common ancestral gene. Most likely two sequential duplications occurred, the first, about 9 to 35 million years ago, generated a direct precursor of the GPA gene, and the second, about 5 to 21 million years ago, generated the GPB and GPE genes and that involved a gene that acquired its specific 3' end by homologous recombination through Alu repeats. Numerous variants of GPA and GPB usually detected by abnormal expression of the blood group MNSs antigens are known. An increasing number of these variants have been structurally defined by protein and molecular genetic analyses, and have been shown to result from point mutations, gene deletions, hybrid gene fusion products generated by unequal crossing-over (not at Alu repeats), and microconversion events. The second group of RBC membrane glycophorins includes the minor proteins GPC and GPD both of which carry blood group Gerbich antigens. Protein and nucleic acid analysis indicated that GPD is a truncated form of GPC in its N-terminal region, and that both proteins are produced by a unique gene called GE (Gerbich), which is present as a single copy per haploid genome and is located on chromosome 2q14-q21.(ABSTRACT TRUNCATED AT 400 WORDS)