Lack of the human choline transporter-like protein SLC44A2 causes hearing impairment and a rare red blood phenotype

Blood phenotypes are defined by the presence or absence of specific blood group antigens at the red blood cell (RBC) surface, due to genetic polymorphisms among individuals. The recent development of genomic and proteomic approaches enabled the characterization of several enigmatic antigens. The choline transporter-like protein CTL2 encoded by the SLC44A2 gene plays an important role in platelet aggregation and neutrophil activation. By investigating alloantibodies to a high-prevalence antigen of unknown specificity, found in patients with a rare blood type, we showed that SLC44A2 is also expressed in RBCs and carries a new blood group system. Furthermore, we identified three siblings homozygous for a large deletion in SLC44A2, resulting in complete SLC44A2 deficiency. Interestingly, the first-ever reported SLC44A2-deficient individuals suffer from progressive hearing impairment, recurrent arterial aneurysms, and epilepsy. Furthermore, SLC44A2_null individuals showed no significant platelet aggregation changes and do not suffer from any apparent hematological disorders. Overall, our findings confirm the function of SLC44A2 in hearing preservation and provide new insights into the possible role of this protein in maintaining cerebrovascular homeostasis.

Genetic ablation of Rhbg in the mouse does not impair renal ammonium excretion

NH(4)(+) transport by the distal nephron and NH(4)(+) detoxification by the liver are critical for achieving regulation of acid-base balance and to avoid hyperammonemic hepatic encephalopathy, respectively. Therefore, it has been proposed that rhesus type B glycoprotein (Rhbg), a member of the Mep/Amt/Rh NH(3) channel superfamily, may be involved in some forms of distal tubular acidosis and congenital hyperammonemia. We have tested this hypothesis by inactivating the RHbg gene in the mouse by insertional mutagenesis. Histochemical studies analyses confirmed that RHbg knockout (KO) mice did not express Rhbg protein. Under basal conditions, the KO mice did not exhibit encephalopathy and survived well. They did not exhibit hallmarks of distal tubular acidosis because neither acid-base status, serum potassium concentration, nor bone mineral density was altered by RHbg disruption. They did not have hyperammonemia or disturbed hepatic NH(3) metabolism. Moreover, the KO mice adapted to a chronic acid-loading challenge by increasing urinary NH(4)(+) excretion as well as their wild-type controls. Finally, transepithelial NH(3) diffusive permeability, or NH(3) and NH(4)(+) entry across the basolateral membrane of cortical collecting duct cells, measured by in vitro microperfusion of collecting duct from KO and wild-type mice, was identical with no apparent effect of the absence of Rhbg protein. We conclude that Rhbg is not a critical determinant of NH(4)(+) excretion by the kidney and of NH(4)(+) detoxification by the liver in vivo.

Protein kinase A-dependent phosphorylation of Lutheran/basal cell adhesion molecule glycoprotein regulates cell adhesion to laminin alpha5

Lutheran (Lu) blood group and basal cell adhesion molecule (B-CAM) antigens reside on two glycoprotein (gp) isoforms Lu and Lu(v13) that belong to the Ig superfamily and differ only by the size of their cytoplasmic tail. Lu/B-CAM gps have been recognized as laminin alpha5 receptors on red blood cells and epithelial cells in multiple tissues. It has been shown that sickle red cells exhibit enhanced adhesion to laminin alpha5 when intracellular cAMP is up-regulated by physiological stimuli such as epinephrine and that this signaling pathway is protein kinase A- and Lu/B-CAM-dependent. In this study, we analyzed the relationship between the phosphorylation status of Lu/B-CAM gps and their adhesion function to laminin alpha5. We showed that Lu isoform was phosphorylated in sickle red cells as well as in erythroleukemic K562 and epithelial Madin-Darby canine kidney cells and that this phosphorylation is enhanced by different stimuli of the PKA pathway. Lu gp is phosphorylated by glycogen synthase kinase 3 beta, casein kinase II, and PKA at serines 596, 598, and 621, respectively. Alanine substitutions of serines 596 and 598 abolished phosphorylation by glycogen synthase kinase 3 beta and casein kinase II, respectively, but had no effect on adhesion of K562 cells to laminin under flow conditions. Conversely, mutation of serine 621 prevented phosphorylation by PKA and dramatically reduced cell adhesion. Furthermore, stimulation of K562 cells by epinephrine increased Lu gp phosphorylation by PKA and enhanced adhesion to laminin. It is postulated that modulation of the phosphorylation state of Lu gp might be a critical factor for the sickle red cells adhesiveness to laminin alpha5 in sickle cell disease.

Direct interaction between the Lu/B-CAM adhesion glycoproteins and erythroid spectrin†

Lutheran (Lu) and Lu(v13), two glycoprotein (gp) isoforms belonging to the immunoglobulin superfamily, represent adhesion molecules that act as erythrocyte receptors for laminin 10/11. These two gps, which differ only by the length of their cytoplasmic tail, carry both Lu blood group and Basal Cell Adhesion Molecule (B-CAM) antigens. Here, analysis of the Triton extractability of recombinant Lu and Lu(v13) gps in K562 transfected cells showed that both gps were mainly associated with the detergent-insoluble material. Patching experiments using Cholera Toxin subunit B indicated that Lu gps were not localized in lipid rafts. Glutathione-S-transferase capture assays showed that the cytoplasmic domain of Lu and Lu(v13) bound to erythroid spectrin, present in a low ionic strength extract from red cell ghosts. Direct interaction with spectrin was confirmed by plasmon resonance assays. Site-directed mutagenesis mapped a major interaction site with spectrin to the RK573-574 motif, located on the cytoplasmic tail of Lu gp, in close vicinity to the inner leaflet of the membrane lipid bilayer. The two Lu adhesion gps represent the first example of a direct link between transmembrane proteins and spectrin in red blood cells. Since Lu gps are low abundant proteins, we speculate that their interaction with spectrin might be critical for signalling and receptor function rather than for participating in the linkage of the lipid bilayer to the red cell skeleton.

The molecular basis of the Rhesus antigen Ew

BACKGROUND

The Rhesus antigen Ew (ISBT designation 004 011) was first described in 1955. It is defined by a specific antibody, but its molecular genetic basis has not yet been resolved.

STUDY DESIGN AND METHODS

Two individuals serologically characterized to express the rare Rhesus antigen Ew were analyzed by sequencing of all 10 exons of the RHCE gene.

RESULTS

A nucleotide exchange at position 500 (T500A) resulting in a Met167Lys amino acid substitution was found in both individuals. Moreover, we show that an individual carrying the Ew antigen is capable to produce an alloantibody against the wild-type E antigen.

CONCLUSION

The single-point mutation T500A in exon 4 of the RHCE gene is a molecular basis of the rare Rhesus antigen Ew.

Sequence, evolution and ligand binding properties of mammalian Duffy antigen/receptor for chemokines

The Duffy antigen/ receptor for chemokine, DARC, acts as a widely expressed promiscuous chemokine receptor and as the erythrocyte receptor for Plasmodium vivax. To gain insight into the evolution and structure/function relations of DARC, we analyzed the binding of anti-human Fy monoclonal antibodies (mAbs) and human chemokines to red blood cells (RBCs) from 11 nonhuman primates and two nonprimate mammals, and we elucidated the structures of the DARC genes from gorilla, gibbon, baboon, marmoset, tamarin, night monkey and cattle. CXCL-8 and CCL-5 chemokine binding analysis indicated that the promiscuous binding profile characteristic of DARC is conserved across species. Among three mAbs that detected the Fy6 epitope by flow cytometric analysis of human and chimpanzee RBCs, only one reacted with night monkey and squirrel monkey. Only chimpanzee RBCs bound a significant amount of the anti-Fy3 mAb. Fy3 was also poorly detected on RBCs from gorilla, baboon and rhesus monkey, but not from new world monkeys. Alignment of DARC homologous sequences allowed us to construct a phylogenetic tree in which all branchings were in accordance with current knowledge of primate phylogeny. Although DARC was expected to be under strong internal and external selection pressure, in order to maintain chemokine binding and avoid Plasmodium vivax binding, respectively, our present study did not provide arguments in favor of a selection pressure on the extracellular domains involved in ligand specificity. The amino acid variability of DARC-like polypeptides was found to be well correlated with the hydrophilicity indexes, with the highest divergence on the amino-terminal extracellular domain. Analysis of the deduced amino acid sequences highlighted the conservation of some amino acid residues, which should prove to be critical for the structural and functional properties of DARC.

Evidence that the red cell skeleton protein 4.2 interacts with the Rh membrane complex member CD47

Rh(null) red cells are characteristically stomato-spherocytic. This and other evidence suggest that the Rh complex represents a major attachment site between the membrane lipid bilayer and the erythroid skeleton. As an attempt to identify the linking protein(s) between the red cell skeleton and the Rh complex, we analyzed the expression of Rh, RhAG, CD47, LW, and glycophorin B proteins in red cells from patients with hereditary spherocytosis associated with complete protein 4.2 deficiency but normal band 3 (4.2(-)HS). Flow cytometric and immunoblotting analysis revealed a severe reduction of CD47 (up to 80%) and a slower mobility of RhAG on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, possibly reflecting an overglycosylation state. Unexpectedly, 4.2(-/-) mice, which are anemic, displayed a normal red cell expression of CD47 and RhAG. These results suggest that human protein 4.2, through interaction with CD47, is involved in the skeleton linkage and/or membrane translocation of the Rh complex. However, these potential role(s) of protein 4.2 might be not conserved across species. Finally, the absence or low expression of red cell CD47 in CD47(-/-) mice and in some humans carrying RHCE gene variants (D--, D., and R(N)), respectively, had no detectable effect on protein 4.2 and RhAG expression. Since these cells are morphologically normal with no sign of hemolysis, it is assumed that CD47 deficiency per se is not responsible for the cell shape abnormalities and for the compensated hemolytic anemia typical of 4.2(-) and Rh(null) red cells.

Structural characterization of the epitope recognized by the new anti-Fy6 monoclonal antibody NaM 185-2C3

The epitope recognized by a new anti-Fy6 monoclonal antibody (MoAb) (clone name: NaM185-2C3) was characterized using peptides synthesized on pins (Epitope scanning kit). The clone was obtained from splenocytes of mice immunized with CHO cells expressing the recombinant Duffy glycoprotein. NaM185-2C3 recognized a linear epitope, the essential portion of which was pentapeptide Phe-Glu-Asp-Val-Trp comprising amino acid residues 22-26 of the main (336aa) isoform of the Duffy antigen. All the amino acid residues of the epitope, except Asp, were essential for the antibody-binding, because they could not be replaced by any or most other amino acid residues. The Asp residue could be replaced by most other amino acid residues and its replacement by some amino acid residues gave a distinct increase in the antibody-binding. The MoAb NaM185-2C3, similarly as other anti-Fy6 antibodies, inhibits interleukin (IL)-8-binding to the Duffy antigen. A part of the results was presented at ISBT meeting (Blanchard et al., 1998, Vox Sanguinis, 74, S1, Abstract no. 71).

[Value of RHD fetal genotyping in the prevention of anti-D immunization]

Anti-D prophylaxis is currently applied in France after birth of an RhD positive infant, after interruption of pregnancy and after some antenatal immunizing events (amniocentesis...). However this program does not cover all the prenatal exposures to fetal RhD antigen, and maternal Rh immunization continues to occur. DNA RhD genotyping of the fetus is now reliably performed on amniotic fluid, and pre diagnostic studies on fetal DNA extracted from maternal plasma are promising. The widespread use of fetal RhD genotyping on maternal blood would allow the antenatal administration of Rh immunoglobuline in all Rh negative patients bearing an Rh positive fetus, insofar as it would preclude exposing the other Rh negative patients to the above plasma derived and rather expensive blood product.

The nonexpression of CD36 on reticulocytes and mature red blood cells does not modify the clinical course of patients with sickle cell anemia

It is thought that an increase in the adhesion of circulating reticulocytes to the vascular endothelium may initiate the vascular occlusion underlying the painful crises and organ failures typical of sickle cell disease (SCD). At least 2 receptors, usually present on reticulocytes, seem to be involved in this adhesion process: glycoprotein CD36 (glycoprotein IV) and integrin alpha(4)beta(1) (very late activation antigen--4). Recently, a high frequency of the platelet CD36--deficient phenotype was reported in black Africans. The frequency of this deficiency was similar in subjects with and without SCD. The role of CD36 in vaso-occlusion was then investigated by comparing the clinical course in 2 groups of black Africans homozygous for hemoglobin S, with and without CD36 deficiency, but similar in age, sex, geographical origin, number of alpha-globin genes, and beta-globin gene haplotype. Flow cytometry showed that CD36 was absent from the circulating red blood cells and reticulocytes of platelet CD36--deficient individuals but present on those from patients with normal platelet CD36 expression, and that alpha(4)beta(1) integrin levels were similar on the reticulocytes of the 2 groups. Neither clinical severity, as evaluated by the frequency and characteristics of vaso-occlusive events, nor biological data differed significantly in the 2 groups of patients. Finally, although CD36 has been suggested to play a critical role in the pathogenesis of vaso-occlusion, this study, despite including only a small number of patients, supports the idea that the modulation of expression of a single type of adhesion molecule is insufficient to counteract the pathological process leading to vaso-occlusion in SCD patients. (Blood. 2001;98:966-971)

Characterization of the laminin binding domains of the Lutheran blood group glycoprotein

Lutheran (Lu) blood group antigens and the basal cell adhesion molecule antigen reside on two glycoproteins that belong to the Ig superfamily (IgSF) and carry five Ig-like extracellular domains. These glycoproteins act as widely expressed adhesion molecules and represent the unique receptors for laminin-10/11 in erythroid cells. Here, we report the mapping of IgSF domains responsible for binding to laminin. In plasmonic resonance surface experiments, only recombinant Lu proteins containing the N-terminal IgSF domains 1-3 were able to bind laminin-10/11 and to inhibit binding of laminin to Lu-expressing K562 cells. Mutant recombinant proteins containing only IgSF domain 1, domains 1 + 2, domains 1 + 3, domains 2 + 3, domain 3, domain 4, domain 5, and domains 4 + 5 failed to bind laminin as well as a construct containing all of the extracellular domains except domain 3. Altogether, these results indicate that IgSF domains 1-3 are involved in laminin binding and that a specific spatial arrangement of these three first domains is most probably necessary for interaction. Neither the RGD nor the N-glycosylation motifs present in IgSF domain 3 were involved in laminin binding.

Two new alleles of the RHCE gene in Black individuals: the RHce allele ceMO and the RHcE allele cEMI

Six unrelated individuals of Afro-Caribbean origin, whose red cells have a marked reduction of the Rhe antigen expression, have been identified. All exhibited the same serological profile with anti-e monoclonal antibodies and lacked expression of the high frequency e-related antigen hrS. Transcripts and genomic analysis showed that these phenotypes resulted from the presence of two new RHCE alleles, ceMO and cEMI. The ceMO allele corresponded to a RHce gene carrying a G667T mutation (exon 5) and was detected at the homozygous state in sample 1 and at the heterozygous state in samples 2-6. The G667T mutation resulted in a Val223Phe substitution on the Rhce polypeptide, in close proximity to Ala226 (e-antigen polymorphism), which might account for the altered expression of e. The ceMO allele is also associated with the lack of expression of the hrS antigen. The absence of the hrS antigen expression may have implications in transfusion as hrS-negative individuals may develop clinically significant antibodies. The cEMI allele corresponded to a silent RHE allele carrying a nine nucleotide deletion within exon 3 and was detected at the heterozygous state in sample 2. This deletion resulted in a shortened polypeptide of 414 residues (instead of 417) that was absent (or severely reduced) at the red cell surface, as the E antigen was undetectable using serology and Western blot analysis with anti-E reagents. In DNA-based polymerase chain reaction genotyping for RHE determination, the cEMI allele provided a false positive result as the cells carrying this allele are serologically phenotyped as E-negative. The incidence of this allele in the Black population is unknown but, as shown already for D genotyping, one must exercise caution when genotyping is performed to detect the e/E polymorphism.

Flow cytometric analysis of the association between blood group-related proteins and the detergent-insoluble material of K562 cells and erythroid precursors

The linkage between blood group-related cell surface proteins and the detergent-insoluble material (DIM) was estimated by flow cytometry using a panel of specific monoclonal antibodies (mAbs) as a comparison of the antibody-binding capacity of intact and Triton-X100-treated cells. Studies were performed with K562 cells expressing endogenous or recombinant proteins and with human erythroid progenitors during their proliferation and differentiation in vitro. Glycophorin C (GPC) was found to be Triton-insoluble in both cellular models. When expressed (erythroid progenitors), Band 3 remained Triton-insoluble. Glycophorin A (GPA), however, behaved as Triton-soluble or insoluble according to the absence (K562) or the presence (erythroid progenitors) of Band 3 respectively. Comparison of the cellular models regarding the proteins that compose the Rh complex also indicated that Rh(D), RhAG and CD47 were resistant to Triton extraction in cells lacking Band 3. Similarly, RhAG and CD47 remained predominantly Triton-insoluble in K562 cells and early progenitors before Rh and Band 3 expression. Further analysis showed that the Kell protein was DIM-associated. In contrast, CD99 and DARC (Fy) proteins were not, or were very poorly, DIM-associated. Additionally, the adhesion molecules CD44 and Lu were completely or partially resistant to detergent extraction respectively. Deletion of the Lu cytoplasmic tail or its replacement by the cytoplasmic domain of GPC resulted in significant increase or decrease of the Triton solubility of the transfected proteins respectively. These data suggest that Triton insolubility of Lu results in part from direct attachment of its cytoplasmic tail with the cytoskeleton. We assume that this method should provide a useful tool to map interaction sites localized in the cytoplasmic domain of recombinant transmembrane proteins.

Structural and functional diversity of blood group antigens

Biochemical and molecular genetic studies have revealed that blood group antigens are present on cell surface molecules of wide structural diversity, including carbohydrate epitopes on glycoproteins and/or glycolipids, and peptide antigens on proteins inserted within the membrane via single or multi-pass transmembrane domains, or via glycosylphosphatidylinositol linkages. These studies have also shown that some blood group antigens are carried by complexes consisting of several membrane components which may be lacking or severely deficient in rare blood group 'null' phenotypes. In addition, although all blood group antigens are serologically detectable on red blood cells (RBCs), most of them are also expressed in non-erythroid tissues, raising further questions on their physiological function under normal and pathological conditions. In addition to their structural diversity, blood group antigens also possess wide functional diversity, and can be schematically subdivided into five classes: i) transporters and channels; ii) receptors for ligands, viruses, bacteria and parasites; iii) adhesion molecules; iv) enzymes; and v) structural proteins. The purpose of this review is to summarize recent findings on these molecules, and in particular to illustrate the existing structure-function relationships.

Transcriptional regulation of the KEL gene and Kell protein expression in erythroid and non-erythroid cells

The Kell blood-group antigen was originally reported to be a protein expressed in erythroid tissue only. Transcriptional analysis of the KEL promoter activity in human erythroleukaemia K562 and epithelial HeLa cells by electrophoretic mobility-shift and supershift assays, chloramphenicol acetyltransferase assays, co-transfection studies and site-directed mutagenesis provided the following results: (i) the KEL promoter exhibits a strong transcriptional activity in K562 cells and, unexpectedly, a basal non-erythroid activity in HeLa cells, (ii) up-regulation of the 5' distal promoter activity occurs only in the erythroid context, and (iii) two motifs localized in the exon 1 region, which bind the Sp1/Sp3 and the human GATA-1/Ku70/80 factors, were required for down-regulation of the promoter activity, but inhibition of the promoter activity by the repressing factors in HeLa cells was incomplete. KEL expression in HeLa cells was performed further by primer-extension analysis, which revealed the presence of a low amount of Kell transcript correlating with basal expression of the Kell protein in these cells, as shown by immunopurification and Western-blot analysis. DNA sequencing of the transcript revealed a sequence identical to that obtained from erythroid tissue. In human tissues, KEL expression was investigated by dot-blot analysis and revealed high levels of Kell mRNAs, particularly in brain tissues, testis and lymphoid tissues. Moreover, most tissues analysed exhibited low levels of Kell transcripts. The Kell protein was also detected by immunohistochemistry in the Sertoli cells of the testis and in lymphoid tissues like spleen and tonsil, specifically localized in the follicular dendritic cells. Altogether, the results indicated that KEL expression is not restricted to erythroid tissue.

Expression of chemokines and chemokine receptors during human renal transplant rejection

Infiltration of renal allografts by leukocytes is a hallmark of acute transplant rejection. Chemokines attract leukocytes bearing specific chemokine receptors, and the specific leukocyte chemokine receptor phenotype is associated with types of immune responses, ie, T helper subtype 1 (Th1; CXC chemokine receptor 3 [CXCR3], CC chemokine receptor 5 [CCR5]) versus Th2 (CCR3, CCR4, CCR8). We studied the expression of the chemokine monocyte chemoattractant protein-1 and the chemokine receptors CCR2B and CXCR4 messenger RNA (mRNA) by in situ hybridization, as well as the chemokine receptors Duffy antigen receptor for chemokines (DARC) and CCR5 protein by immunohistochemistry in renal biopsy specimens with acute cellular rejection (n = 12) and acute vascular rejection (n = 8), transplant nephrectomy specimens (n = 6), and normal areas of tumor nephrectomy specimens (n = 5). CC chemokines and CC chemokine receptor mRNA expression were evaluated by ribonuclease protection assay in specimens from four transplant nephrectomies and one tumor nephrectomy. Upregulation of mRNAs for the chemokines, interferon-inducible protein-10 (IP-10); regulated on activation normal T-cell expressed and secreted; macrophage inflammatory protein-1alpha (MIP-1alpha); MIP-1beta; and lymphotactin, as well as the chemokine receptors, CCR2 and CCR5, were documented during allograft rejection. CCR1 mRNA was detectable in both allografts and controls, but CCR3 and CCR8 were absent. The number of CXCR4, CCR5, and CCR2B mRNAs expressing leukocytes and DARC-positive vessels increased during rejection episodes. CXCR4 mRNA was the most widely expressed. Leukocytes in diffuse interstitial infiltrates were mainly CCR5 positive, but in areas in which leukocytes formed nodular aggregates of infiltrating cells, the number of CCR5-positive cells was low. Instead, leukocytes in these nodular aggregates mainly expressed CXCR4. DARC was expressed on peritubular capillaries, where it was upregulated in areas of interstitial infiltration. Induction of chemokines during renal allograft rejection is accompanied by infiltration of leukocytes bearing the respective chemokine receptors. The upregulation of the CXCR3 ligand IP-10, as well as CCR5 and its ligands, in the absence of CCR3 and CCR8 is indicative that renal allograft rejection is primarily the result of a Th1-type immune response.

Molecular basis of red cell protein antigen deficiencies

Blood group antigens reside on cell surface molecules of wide structural and functional diversity. Although all are serologically detectable on red blood cells, most are also expressed in non erythroid tissues, but with a few exceptions their biological role on erythrocytes and tissues often remains elusive. Deficiencies of these molecules seen either in rare blood group "null" phenotypes and/or associated with red cell membrane disorders, offer the opportunity to clarify their role in the cell membrane and to correlate their molecular abnormalities with cell dysfunctions. This review will summarize some of the present knowledge on these defects and on gene targeting studies developed to analyze the function of these molecules in animal models.

Macrothrombocytopenia with abnormal demarcation membranes in megakaryocytes and neutropenia with a complete lack of sialyl-Lewis-X antigen in leukocytes--a new syndrome?

A new megathrombocytopenic syndrome with giant platelets in peripheral blood and severe thrombocytopenia was diagnosed in a 4-month-old boy. His clinical course included repeated hemorrhagic incidents leading to death at age 37 months. Bone marrow ultrastructural analysis revealed numerous dystrophic megakaryocytes with giant membrane complexes. Although these features were similar to those described for megakaryocytes in mice lacking the gene for transcription factor p45-NF-E2, no abnormalities in the p45-NF-E2 gene could be documented. Platelet membrane analysis showed a reduction in glycoprotein (GP) Ib, but normal content of GPIIb and GPIIIa. Analysis of genes encoding for GPIb alpha and beta, GPV, and GPIX ruled out the possibility that the observed platelet abnormality is a variant of Bernard-Soulier syndrome. A moderate neutropenia was associated with a complete lack of expression of sialyl-Lewis-X on the surface of polymorphonuclear neutrophils. A common defect in posttranslational modification of glycoproteins could account for the diverse cellular abnormalities.

New procedures for glycophorin A purification with high yield and high purity

Glycophorin A (GPA) is the major glycoprotein of the human erythrocyte membrane. It is known to form, in SDS gels as well as in a membrane environment, homodimers, and also heterodimers with the homologous molecule Glycophorin B (GPB). It is shown in this report that the propensity of GPA to dimerize with GPB precludes satisfactory preparation with high yield of pure GPA using classical techniques including SEC and RPLC. It was demonstrated using multiple angle light scattering that GPA is eluted from RPLC columns as dimers. A convenient procedure was devised which allowed us to get pure GPA with high yield. This procedure consists of selectively blocking GPA-GPB heterodimer formation by selective modification of Cysteine 50 of GPB before RPLC.

The human Rhesus-associated RhAG protein and a kidney homologue promote ammonium transport in yeast

The Rhesus blood-group antigens are defined by a complex association of membrane polypeptides that includes the non-glycosylated Rh proteins (RhD and RhCE) and the RHag glycoprotein, which is strictly required for cell surface expression of these antigens. RhAG and the Rh polypeptides are erythroid-specific transmembrane proteins belonging to the same family (36% identity). Despite their importance in transfusion medicine, the function of RhAG and Rh proteins remains unknown, except that their absence in Rh(null) individuals leads to morphological and functional abnormalities of erythrocytes, known as the Rh-deficiency syndrome. We recently found significant sequence similarity between the Rh family proteins, especially RhAG, and Mep/Amt ammonium transporters. We show here that RhAG and also RhGK, a new human homologue expressed in kidney cells only, function as ammonium transport proteins when expressed in yeast. Both specifically complement the growth defect of a yeast mutant deficient in ammonium uptake. Moreover, ammonium efflux assays and growth tests in the presence of toxic concentrations of the analogue methylammonium indicate that RhAG and RhGK also promote ammonium export. Our results provide the first experimental evidence for a direct role of RhAG and RhGK in ammonium transport. These findings are of high interest, because no specific ammonium transport system has been characterized so far in human.

The Duffy antigen receptor for chemokines is up-regulated during acute renal transplant rejection and crescentic glomerulonephritis

BACKGROUND

Recruitment of leukocytes during immune responses requires the coordinate expression of adhesion molecules in concert with chemokines and their receptors. The Duffy antigen receptor for chemokines (DARC) binds multiple chemokines and is expressed on postcapillary venules in the normal kidney. The chemokine receptor CCR5, which shares the ligand regulated upon activation, normal T-cell expressed and secreted (RANTES) with DARC, is expressed by infiltrating T cells in the renal interstitium. As DARC might be involved in the attraction of CCR5-positive cells, we studied the distribution of DARC and CCR5 in two forms of cell-mediated renal injury: renal allograft rejection and crescentic glomerulonephritis (cGN).

METHODS

A total of 87 renal specimens, including 12 pretransplant biopsies, 47 transplant biopsies (Banff 1, N = 10; Banff 2, N = 19; and various other lesions N = 18), and 28 biopsies from patients with cGN, was analyzed. Immunohistochemistry for CCR5 and DARC was performed on serial sections of formalin-fixed and paraffin-embedded tissue.

RESULTS

Compared with pretransplant biopsies, the mean number of DARC-positive interstitial venules was significantly increased during both transplant rejection and cGN. This was accompanied by an infiltration of CCR5-positive leukocytes. During transplant rejection, the number and distribution of CCR5-positive cells correlated with DARC-positive venules. Infiltrating CCR5-positive leukocytes were found mainly in the interstitium, often clustering around Bowman's capsules in biopsies from cGN. The number of glomerular CCR5 positive cells is low, but they are common in a subset of crescents.

CONCLUSIONS

We hypothesize that the increased number of DARC-positive venules in areas of interstitial injury and the colocalization with CCR5-positive infiltrating leukocytes may indicate a role for endothelial DARC expression during leukocyte adhesion and interstitial infiltration.

Binding sites of leukocyte beta 2 integrins (LFA-1, Mac-1) on the human ICAM-4/LW blood group protein

The red cell ICAM-4/LW blood group glycoprotein, which belongs to the family of intercellular adhesion molecules (ICAMs), has been reported to interact with CD11a/CD18 (LFA-1) and CD11b/CD18 (Mac-1) beta(2) integrins. To better define the basis of the ICAM-4/beta(2) integrin interaction, we have generated wild-type, domain-deleted and mutated recombinant chimeric ICAM-4-Fc proteins and analyzed their interaction in a cellular adhesion assay with LFA-1 and Mac-1 L-cell stable transfectants. We found that monoclonal antibodies against CD11a, CD11b, CD18, or LW(ab) block adhesion of transfectant L-cells to immobilized ICAM-4-Fc protein and that the ICAM-4/beta(2) integrin interaction was highly sensitive to the presence of the divalent cations Ca(2+) and Mg(2+). Deletion of individual Ig-domains D1 or D2 of the extracellular part of ICAM-4 showed that LFA-1 binds to the first Ig-like domain, whereas the Mac-1 binding site encompassed both the first and the second Ig-like domains. Based on the crystal structure of ICAM-2, we propose a model for the Ig-like domains D1 and D2 of ICAM-4. Accordingly, by site-directed mutagenesis of 22 amino acid positions spread out on all faces of the ICAM-4 molecule, we identified four exposed residues, Leu(80), Trp(93), and Arg(97) on the CFG face and Trp(77) on the E-F loop of domain D1 that may contact LFA-1 as part of the binding site. However, the single and double mutants R52E and T91Q on the CFG face of domain D1, which correspond to the key residues Glu(34) and Gln(73) for ICAM-1 binding to LFA-1, had no effect on LFA-1 binding. In contrast, all mutants on the CFG face of domain D1 and residues Glu(151) and Thr(154) in the C'-E loop of the domain D2 seem to play a dominant role in Mac-1 binding. These data suggest that the binding site for LFA-1 on ICAM-4 overlaps but is distinct from the Mac-1 binding site.

Molecular heterogeneity of the Jk(null) phenotype: expression analysis of the Jk(S291P) mutation found in Finns

Polymerase chain reaction genotyping of 32 unrelated Jk(null) individuals originating predominantly from Polynesia and Finland indicated that all were homozygous for the JK*B polymorphism and that 17 of 32, including the 14 Polynesians, carried a 3'-acceptor splice site mutation of intron 5 that resulted in the skipping of exon 6 (called mutation Jk delta 6). The remaining 15 Jk(null) donors from Finland were homozygous for a new T871C transition resulting in a S291P amino acid substitution at a consensus N-glycosylation site of the Jk polypeptide. Transcription-translation assays revealed that the Jk(S291P) mutant was translated into a glycosylated component as efficiently as the wild-type Jk polypeptide (wt Jk)] in the presence of microsomes, thus indicating that the S291P mutation has no effect on the N-glycosylation pattern of the Jk protein. Expression studies in Xenopus oocytes revealed that the Jk(S291P) polypeptide functions as a urea transporter, but the transport activity and the membrane expression level of the mutant protein was reduced to a similar extent. A substantial fraction of the mutant protein was retained intracellularly suggesting that the transit to the plasma membrane was reduced, presumably because of the S-->P mutation. After transfection in erythroleukemia K562 cells the wild-type, but not the mutant, protein was efficiently expressed at the cell surface. Because the Jk(S291P) mutant polypeptide was not present in human red cells from Jk(null) individuals, expression data in the erythroid context clearly indicates that the S-->P mutation is the molecular basis of the Finnish Jk(null) phenotype. (Blood. 2000;96:1566-1573)

Quantitative analysis of XG blood group and CD99 antigens on human red cells

CD99, the product of the MIC2 gene, exhibits an erythroid-specific quantitative polymorphism co-regulated with the Xga blood group polymorphism. The co-expression of X-linked MIC2 and XG genes is presumably controlled at the transcriptional level by a single XGR locus in the pseudoautosomal region of sexual chromosomes. This locus is composed of two alleles, XGR(low) and XGR(high), which determine low or high CD99 levels (CD99-L, CD99-H) and the Xg(a-)/ Xg(a+) status. To test this hypothesis, the phenotypic relationship between Xga and CD99 antigens on human RBCs was investigated by quantitative flow cytometry using NBL-1 (anti-Xga) and 12E7 (anti-CD99) monoclonal antibodies and semi-quantitative estimate of membrane proteins and RNA by Western blot and Northern blot, respectively. The antibody binding capacity of RBCs, which is an estimation of the antigen density, was determined for 118 blood donors including 60 males and 58 females. Xg(a+) RBCs, which all belong to the group of CD99-H expressors, carry 159+/-13 and 960+/-50 copies of Xga and CD99 molecules/cell, respectively. Xg(a-) RBCs have no Xga antigen, but are subdivided into CD99-H (all male) and CD99-L expressors carrying 747+/-28 and 200+/-22 CD99 copies/cell, respectively, with identical CD99 levels between CD99-L males and females. However, among males, the CD99 expression was higher in Xg(a+) than in Xg(a-)/CD99-H individuals (P<0.01). In addition, CD99-H expressors in Xg(a+) males could be clearly subdivided into two categories, high and super high expressors, which are presumably heterozygous and homozygous for the XGR(high) allele, which fits the above hypothesis. This was not the case for Xg(a+) females where CD99-H subcategories were not found. Quantitative differences were confirmed by Western blot analysis of red cell membrane preparations from individuals of different Xga and CD99 phenotypes and by Northern blot analysis showing that the reticulocytes from CD99-L individuals expressed a reduced level of MIC2 transcripts compared to CD99-H donors. These findings further support the hypothesis of a single genetic control of CD99 and Xga expression by the XGR locus.

A murine monoclonal antibody against Kx protein which reacts also with beta-spectrin

Kx is a polytopic membrane protein of human erythrocytes carrying the Kx blood group antigen, which is deficient in rare patients with McLeod syndrome. Kx is disulphide bond linked to the Kell glycoprotein, which is a bitopic type II membrane protein carrying the Kell blood group antigen. Mice immunized with a synthetic peptide predicted to be located on the second external loop of Kx produced a monoclonal antibody called 3E12 which does not recognize red cells with common Kell phenotype by agglutination and flow cytometry. 3E12 recognizes the Kx protein and the spectrin beta-chain on western blots, the affinity for these two proteins being lowered with increasing ionic strength. Linear epitopes recognized by 3E12 are E116EIEKE121 and L484AQELEKE491 on the Kx protein and spectrin beta-chain, respectively. To quantify the relative amount of Kx in Empigen BB extracts of red cell membranes, an ELISA for Kx was set up which showed conclusively that (i) there is less Kx in membranes of K0 individuals (lacking the Kell glycoprotein) than in membranes of common individuals, and (ii) that all common individuals, typed as K+k-, K-k+ and K+k+, have the same amount of Kx on their red cell membranes. When an erythrocyte membrane detergent extract from one K0 individual was chromatographed on an immobilized 3E12 column, a minute amount of authentic Kell glycoprotein was recovered in acid eluted fractions, indicating that at least the K0 individual under study may still produce some Kell protein.

A study of the coregulation and tissue specificity of XG and MIC2 gene expression in eukaryotic cells

CD99, the product of the MIC2 gene, exhibits an erythroid-specific quantitative polymorphism coregulated with the polymorphism of the XG blood group gene. As a preliminary study of this phenomenon, human XG and CD99 recombinant proteins were expressed in murine RAG cells and analyzed by flow cytometry. Both proteins were expressed independently and at a similar level in single and double transfectants. Immunoprecipitation and Western blot analysis, using the murine monoclonal antibodies NBL-1 and 12E7, revealed species of 26 kd (XG) and 32 kd (CD99), respectively. A putative 28-kd intracellular precursor of CD99 was also detected, as was a 26-kd species after neuraminidase treatment of CD99-expressing cells. No evidence of association or complex formation between XG and CD99 proteins could be proven, either on transfected RAG cells or on human erythrocytes. These results were confirmed using somatic hybrids between single transfectants. These findings suggest that the phenotypic relationship between XG and CD99 is mostly regulated at the transcriptional level, but they do not formally exclude some posttranscriptional effect. Studies on the tissue specificity of XG expression showed that surface expression of the XG protein could not be restored in somatic hybrids between B-lymphoblastoid cell lines from Xg(a+) persons and fibroblasts (RAG) or erythroid (MEL) cells. RT-PCR analysis of the transcripts revealed the existence of an XG mRNA in each cell line, suggesting that the tissue-specific regulation of cell surface XG expression occurs either at a quantitative transcriptional level or is a posttranscriptional event. By Northern blot analysis, XG transcripts were detected in erythroid tissues and several nonerythroid tissues. (Blood. 2000;95:1819-1826)

Analysis of deletions in three McLeod patients: exclusion of the XS locus from the Xp21.1-Xp21.2 region

The McLeod syndrome is a rare X-linked recessive disorder characterized by blood group, neuromuscular and haematopoietic abnormalities. It is caused by XK gene defects and may include large deletions in the Xp21 region. Analysis of three unrelated McLeod patients for the presence of the XK, DMD, CYBB, ETX1, RPGR and OTC loci, as well as for the DXS709 marker, revealed deletions from the 39th exon of DMD to the ETX1 locus (patient Be), from the XK to RPGR loci (patient Bi) and from the XK to CYBB loci (patient Lh). All three patients normally expressed the Lutheran (Lu) red cell antigens, thus excluding the interval between the RPGR and DMD genes as site of the XS locus, previously mapped to the Xp21.2-Xq21.1 region and thought to regulate the expression of the LU blood group gene on chromosome 19.

Immunopurification of the blood group RhD protein from human erythrocyte membranes

Rh proteins are membrane proteins encoded by genes at the blood group RH locus. They are of paramount importance in transfusion medicine, but their function is still unknown. Biochemical and biophysical studies of these proteins are scarce since only minute amounts of the very hydrophobic Rh proteins, can be purified from human erythrocytes. Recently, a human monoclonal antibody (LOR-15C9) was described as having the unique property to recognize the Rh30 protein carrying the major blood group D specificity (RhD protein), either in a membrane detergent extract or when blotted on a membrane. In this report, we describe one-step purification of the RhD protein from detergent extracts of red cell membranes, based on immunoaffinity chromatography carried out with immobilized LOR-15C9 IgG. The technique yielded RhD protein with high purity which was devoid of other associated proteins (RhAG, CD47, LW and GPB) that comprise the Rh complex in the erythrocyte membrane. By contrast immunoprecipitation performed with the same antibody led to co-isolation of both RhD and RhAG.

RH blood group system and molecular basis of Rh-deficiency

Rhesus (Rh) antigens are defined by a complex association of membrane polypeptides that are missing or severely deficient from the red cells of rare Rhnull individuals who suffer a clinical syndrome of varying severity characterized by abnormalities of the red cell shape, cation transport and membrane phospholipid organization. The Rhnull phenotype is an inherited condition that may arise from homozygosity either for a 'suppressor' gene unrelated to the RH locus ('regulator type') or for a silent allele at the RH locus itself ('amorph type'). A current model suggests that the proteins of the Rh complex (Rh, RhAG, CD47, LW, GPB) are assembled by non-covalent bonds and that it is not assembled or transported to the cell surface when one subunit is missing. Rh and RhAG proteins belong to the same protein family and are quantitatively the major components that form the core of the complex, which is firmly linked to the membrane skeleton. Molecular analysis of Rhnull individuals has revealed that abnormalities occur only at the RHAG and RH loci, without alteration of the genes encoding the accessory chains. Mutations of the RHAG gene, but not of RH, occur in all Rhnull individuals of the regulator type (including Rhmod) investigated so far (13 cases), strongly suggesting that RHAG mutants act as 'suppressors' and not as transcriptional regulators of the RH genes and that variable expression of the RHAG alleles may account for the Rhmod phenotypes (exhibiting weak expression of Rh antigens). Conversely, mutations of the RHCE gene, but not of RHAG, occur in two unrelated Rhnull individuals of the amorph type, supporting the view that RH mutants result from a 'silent' allele at the RH locus. These findings strongly support the Rh complex model since when either the Rh or RhAG protein is missing, the assembly and/or transport of the Rh complex is defective. Transcriptional as well as post-transcriptional mechanisms may account for the molecular abnormalities, but experimental evidence based on expression models is required to test these hypotheses, in the hope that they may help to clarify the biological role of the Rh proteins in the red cell membrane.

Characterization of a mouse liminin receptor gene homologous to the human blood group Lutheran gene

The human Lutheran (Lu) blood group antigens are carried by two glycoproteins (gps) that belong to the immunoglobulin (Ig) superfamily. These gps represent adhesion molecules that function as the unique erythroid receptors for laminin. We report here the cloning and functional expression of the orthologous mouse Lu mRNA as well as the genomic organization of the mouse Lu gene. The deduced human and mouse Lu gps share 72.5% identity and similar organization of the Ig-like domains. As in the human, the mouse Lu gene is organized in 15 exons. The proximal promoter showed consensus CACC-binding sites whereas the distal promoter exhibits a GATA-1-binding site and multiple E boxes. Like the human gene, the mouse Lu gene is also widely expressed among tissues but is transcribed as a unique 2.4-kb mRNA species. Expression of the mouse Lu mRNA is upregulated upon dimethyl sulfoxide-induced erythroid differentiation of murine erythroleukemia cells (MEL). During mouse embryonic development, the Lu transcript is detected as early as day 7 of gestation. Analysis of transfected human erythroleukemia K562 cells indicated that the adhesive properties of the Lu gps to laminin are conserved between human and mouse.

Isoforms of the Lutheran/basal cell adhesion molecule glycoprotein are differentially delivered in polarized epithelial cells. Mapping of the basolateral sorting signal to a cytoplasmic di-leucine motif

Lu and Lu(v13) are two glycoprotein (gp) isoforms that belong to the immunoglobulin superfamily and carry both the Lutheran (Lu) blood group antigens and the basal cell adhesion molecule epithelial cancer antigen. Lu (85 kDa) and Lu(v13) (78 kDa) gps, which differ only in the length of their cytoplasmic domain, are adhesion molecules that bind laminin. In nonerythroid tissues, the Lu/basal cell adhesion molecule antigens are predominantly expressed in the endothelium of blood vessel walls and in the basement membrane region of normal epithelial cells, whereas they exhibit a nonpolarized expression in some epithelial cancers. Here, we analyzed the polarization of Lu and Lu(v13) gps in epithelial cells by confocal microscopy and domain-selective biotinylation assays. Differentiated human colon carcinoma Caco-2 cells exhibited a polarized expression of endogenous Lu antigens associated with a predominant expression of the Lu isoform at the basolateral domain of the plasma membrane and a very low expression of the Lu(v13) isoform at both the apical and basolateral domains. Analysis of transfected Madin-Darby canine kidney cells revealed a basolateral expression of Lu gp and a nonpolarized expression of Lu(v13) gp. Delivery of Lu(v13) to both apical and basolateral surfaces showed similar kinetics, indicating that this isoform is directly transported to each surface domain. A dileucine motif at position 608-609, specific to the Lu isoform, was characterized as a dominant basolateral sorting signal that prevents Lu gp from taking the apical delivery pathway.

The E-box of the human glycophorin B promoter is involved in the erythroid-specific expression of the GPB gene

Previous studies performed on the glycophorin B (GPB) expression demonstrated that this gene is expressed in erythroid cells only and that the ubiquitous factor Ku70 is involved in the process. Here, we investigated the contribution of the -70 E-box sequence toward the GPB promoter expression. We found that the E-box bound two factors, the USF1/USF2 protein and an unidentified ubiquitous protein which was named factor U. Site-directed mutagenesis performed on the -70 E-box showed that the USF factor had an activating effect in CAT assays. Conversely, mutation of the -70 E-box that impaired the binding of factor U led to a positive CAT activity in nonerythroid cells and thus to the loss of the erythroid-specific expression of the GPB gene. This indicates that, in addition to the Ku70 factor, the extinction of the GPB promoter expression in nonerythroid cells depends also on the repressing effect of the factor U.

Time-course expression of polypeptides carrying blood group antigens during human erythroid differentiation

The time course expression of blood group antigens was examined by flow cytometry using a two-phase liquid culture system that supports the proliferation and maturation of human erythroid progenitors from adult peripheral blood. The progression towards erythroid differentiation was followed by the expression changes of the transferrin receptor (CD71++) and glycophorin A (GPA+). Four main categories of blood group markers were identified: (i) those characterized by an early expression like ABO (A), Kell (K:2) and Rh50 which were detected in the Epo-independent phase 1, (ii) those including GPC (Gerbich, Ge antigens) and Fy6 which were expressed in the late phase 1, (iii) GPA (MN antigens), Wrb (Band 3/GPA interaction), Rh(D, Cc/Ee) and LW which appeared during the Epo-dependent phase 2 and (iv) those like Jk3 and Lub which were expressed in late phase 2. Regarding blood group molecules exhibiting adhesive properties (LW/ICAM-4, Oka and Lu) the most significant event was a sharp decrease of Oka (neurothelin) expression with the concomitant loss of ICAMs expression during the later stage of differentiation. These studies suggest that Oka, ICAMs and LW might contribute to the adhesive interactions involved in the formation of erythroblastic islands and attachment to stroma cells and the extracellular matrix. We also noted an asynchronous expression of the proteins that compose the core of the Rh complex, since Rh50 glycoprotein was expressed earlier than Rh(D, CE) proteins.

At physiological expression levels the Kidd blood group/urea transporter protein is not a water channel

The Kidd (JK) blood group locus encodes a urea transporter that is expressed on human red cells and on endothelial cells of the vasa recta in the kidney. Here, we report the identification in human erythroblasts of a novel cDNA, designated HUT11A, which encodes a protein identical to the previously reported erythroid HUT11 urea transporter, except for a Lys(44) --> Glu substitution and a Val-Gly dipeptide deletion after proline 227, which leads to a polypeptide of 389 residues versus 391 in HUT11. Genomic typing by polymerase chain reaction and transcript analysis by ribonuclease protection assay demonstrated that HUT11A encodes the true Kidd blood group/urea transporter protein, which carries only 2 Val-Gly motifs. Upon expression at high levels in Xenopus oocytes, the physiological Kidd/urea transporter HUT11A conferred a rapid transfer of urea (which was insensitive to p-chloromercuribenzene sulfonate or phloretin), a high water permeability, and a selective uptake of small solutes including amides and diols, but not glycerol and meso-erythritol. However, at plasma membrane expression levels close to the level observed in the red cell membrane, HUT11A-mediated water transport and small solutes uptake were absent and the urea transport was poorly inhibited by p-chloromercuribenzene sulfonate, but strongly inhibited by phloretin. These findings show that, at physiological expression levels, the HUT11A transporter confers urea permeability but not water permeability, and that the observed water permeability is a feature of the red cell urea transporter when expressed at unphysiological high levels.

Structure and expression of the mouse homologue of the XK gene

The human Kx blood group antigen is carried by a 37,000 M(r) apparent molecular mass membrane polypeptide which is deficient in rare individuals with the McLeod syndrome. The X-linked human XK gene is transcribed in many tissues including adult skeletal muscle and brain, sieges of disorders observed in McLeod syndrome. We report here the cloning of the orthologous mouse XK mRNA. Comparison of XK from human and mouse revealed 80% sequence similarity at the amino acid level. The mouse XK gene is organized in two exons and is expressed in many tissues, but its expression pattern is slightly different from that of the human gene. The presence in mouse erythrocyte membrane of a 43,000 M(r) Kx-related protein was demonstrated by immunoblotting with a rabbit antiserum directed against the human protein. With non-reduced samples, a 140,000 M(r) species was detected instead of the 43,000 M(r) protein, suggesting that, as demonstrated in the Kx polypeptide might be complexed with another protein in mouse red cells, presumably the homologue of the human Kell protein of 93,000 M(r).

Normal human serum contains natural antibodies reactive with autologous ABO blood group antigens

It is widely accepted that the serum of healthy individuals contains natural antibodies only against those blood group A or B antigens that are not expressed on the individual's red blood cells. The mechanisms involved in tolerance to autologous blood group antigens remain unclear. In the present study, we show that IgM and IgG antibodies reactive with autologous blood group antigens are present in the immunoglobulin fraction of normal human serum. Natural IgG anti-A antibodies purified by affinity chromatography from IgG of individuals of blood group A exhibited an affinity for A trisaccharide antigen in the micromolar range and agglutinated A red cells at sixfold higher concentrations than those required for agglutination with affinity-purified anti-A IgG of individuals of blood group B. Whereas autoantibodies reactive with self A and B antigens are readily detected in purified IgG and IgM fractions, their expression is restricted in whole serum as a result of complementary interactions between variable regions of antibodies. These observations suggest that tolerance to autologous ABO blood group antigens is dependent on peripheral control of antibody autoreactivity.

The extracellular domain of the human erythropoietin receptor: expression as a fusion protein in Escherichia coli, purification, and biological properties

We developed an efficient production system of the soluble extracellular domain of the human erythropoietin receptor (sEPO-R) and characterized the binding of erythropoietin (EPO) with the purified recombinant protein. The sEPO-R, fused to the maltose binding protein (MBP), was expressed as a soluble protein in the periplasm of Escherichia coli (E. coli) and did not accumulate in inclusion bodies. After lysis of the bacteria by an osmotic shock, the fusion protein was purified by affinity chromatography on amylose followed by size exclusion chromatography (SEC). Specific binding of 125I-labelled EPO to the sEPO-R was demonstrated by competitive and saturation binding assays. A single affinity class (Kd = 0.25 nM) of the binding site was evident by Scatchard analysis. This value is similar to the Kd observed between EPO and the EPO-R of high affinity present on human erythroid progenitors. The complex has a molecular size corresponding to a 1:1 complex of EPO and the fusion protein.

Cloning, expression, and chromosomal mapping of a human ATPase II gene, member of the third subfamily of P-type ATPases and orthologous to the presumed bovine and murine aminophospholipid translocase

Recently, a P-type ATPase was cloned from bovine chromaffin granules (b-ATPase II) and a mouse teratocarcinoma cell line (m-ATPase II) and was shown to be homologous to the Saccharomyces cerevisiae DRS2 gene, the inactivation of which resulted in defective transport of phosphatidylserine. Here, we report the cloning from a human skeletal muscle cDNA library of a human ATPase II (h-ATPase II), orthologous to the presumed bovine and mouse aminophospholipid translocase (95.3 and 95.9% amino acid identity, respectively). Compared with the bovine and mouse counterparts, the cloned h-ATPase II polypeptide exhibits a similar membrane topology, but contains 15 additional amino acids (1163 vs 1148) located in the second intracytoplasmic loop, near the DKTGTLT-phosphorylation site. However, RT-PCR analysis performed with RNA from different human tissues and cell lines revealed that the coding sequence for these 15 residues is sometimes present and sometimes absent, most likely as a result of a tissue-specific alternative splicing event. The h-ATPase II gene, which was mapped to chromosome 4p14-p12, is expressed as a 9.5-kb RNA species in a large variety of tissues, but was not detected in liver, testis, and placenta, nor in the erythroleukemic cell line K562.

The repressor which binds the -75 GATA motif of the GPB promoter contains Ku70 as the DNA binding subunit

Glycophorin B (GPB) is an abundant cell surface glycoprotein which is only expressed in human erythroid cells. Previous functional analysis demonstrated that the repression of the GPB promoter is determined by the binding of a ubiquitous factor which recognizes a GATA motif centered at position -75. In erythroid cells this ubiquitous factor is displaced by the binding of the erythroid-specific factor hGATA1. Here, we have identified the Ku70 protein as a candidate GPB repressor DNA binding subunit through the screening of a human HeLa expression library using the -75 GATA sequence as bait (one-hybrid method). Electrophoretic mobility shift assays demonstrated that the ubiquitous factor that binds the -75 GATA sequence was the Ku70-Ku80 (Ku) heterodimer. Co-transfection experiments demonstrated that overexpression of Ku70 in the K562 erythroleukeamic cell line resulted in transcriptional repression of the chloramphenicol acetyltransferase reporter gene when placed under the control of the wild-type GPB promoter. Conversely, no repression was observed when a mutation that abolished the binding of Ku was introduced in the GPB promoter construct. Altogether, these results indicate that Ku binds in vivo to the -75 WGATAR motif and is involved in negative regulation of the GPB promoter. These findings suggest that, besides its role in many functions, Ku is also involved in transcriptional regulation of erythroid genes.

The members of the RH gene family (RH50 and RH30) followed different evolutionary pathways

The evolution of the RH gene family is characterized by two major duplication events, the first one originating the RH50 and RH30 genes and the second one giving rise to RHCE and RHD, the two paralogous RH30 genes which encode the Rh blood group antigens in human. The new sequence data obtained here for mouse RH50 and RH30 and for macaque RH50 allowed us to compare the evolutionary rates of the two genes and to show that RH50 evolved about 2.6 times more slowly than RH30 at nonsynonymous positions. This result implies that Rh50 proteins were evolutionarily more conserved compared to Rh30 polypeptides, thus being indicative of the functional significance of the former protein in species as distantly related as sponge and human. The duplication event leading to RH50 and RH30 genes was estimated to have occurred between 250 and 346 million years ago. Moreover, we could also estimate that the duplication event producing the RHCE and RHD genes occurred some 8.5 +/- 3.4 million years ago, in the common ancestor of human, chimpanzee, and gorilla. Interestingly, this event seems to coincide with the appearance in these species of a G-to-T mutation in the RH50 gene which created a stop codon in the corresponding transcript. This led to an Rh50 C-terminal cytoplasmic domain shorter than that found in orangutan and early primates.

Characterization of a UDP-Gal:Galbeta1-3GalNAc alpha1, 4-galactosyltransferase activity in a Mamestra brassicae cell line

The binding of Bandeiraea simplicifolia lectin-I isolectin B4 on the endogenous glycoproteins of different insect cell lines led us to characterize for the first time a UDP-Gal:Galbeta1-3GalNAc alpha1, 4-galactosyltransferase in a Mamestra brassicae cell line (Mb). The study of the acceptor specificity indicated that the Mb alpha-galactosyltransferase prefers Galbeta1-3-R as acceptor, and among such glycans, the relative substrate activity Vmax/Km was equal to 20 microliters.mg-1.h-1 for Galbetal-3GlcNAcbeta1-O-octyl and to 330 microliters.mg-1.h-1 for Galbeta1-3GalNAcalpha-1-O-benzyl, showing clearly that Galbeta1-3GalNAc disaccharide was the more suitable acceptor substrate for Mb alpha-galactosyltransferase activity. Nuclear magnetic resonance and mass spectrometry data allowed us to establish that the Mb alpha-galactosyltransferase synthesizes one unique product, Galalpha1-4Galbeta1-3GalNAcalpha1-O-benzyl. The Galbeta1-3GalNAc disaccharide is usually present on O-glycosylation sites of numerous asialoglycoproteins and at the nonreducing end of some glycolipids. We observed that Mb alpha1,4-galactosyltransferase catalyzed the transfer of galactose onto both natural acceptors. Finally, we demonstrated that the trisaccharide Galalpha1-4Galbeta1-3GalNAcalpha1-O-benzyl was able to inhibit anti-PK monoclonal antibody-mediated hemagglutination of human blood group PK1 and PK2 erythrocytes.

High adenosine deaminase level among healthy probands of Diamond Blackfan anemia (DBA) cosegregates with the DBA gene region on chromosome 19q13. The DBA Working Group of Société d'Immunologie Pédiatrique (SHIP)

Phenotypic characterization of Diamond Blackfan Anemia (DBA) patients and their relatives was performed in 54 families. Complete blood count, fetal hemoglobin level, erythrocyte i antigen expression, and erythrocyte adenosine deaminase (eADA) activities were quantitated in patients and relatives. eADA was elevated in 28 of 34 transfusion-independent DBA patients, whereas persistence of erythrocyte i antigen was noticed in only 10 of 20 DBA patients. High eADA activities were also found in 14 of 149 healthy family members, allowing us to identify an isolated high eADA phenotype in these families. In contrast, increase in erythrocyte i antigen expression, elevated fetal hemoglobin levels, and macrocytosis were much less frequently noted in nonaffected members of the DBA families studied. Importantly, isolated high eADA phenotype was found to be significantly associated with genetic markers on chromosome 19 that segregate with the DBA phenotype. Isolated high eADA phenotype thus seems to reflect a silent phenotype of DBA in affected families. These findings suggest that elevated eADA activity in unaffected individuals needs to be taken into account during genetic assessment of DBA families and could be used for accurate assessment of mode of inheritance.