The red blood cell band 3 variant (band 3Biceêtrel:R490C) associated with dominant hereditary spherocytosis causes defective membrane targeting of the molecule and a dominant negative effect

Molecular analysis of sequence and splice variants of the human SLC13A4 sulfate transporter

The solute linked carrier 13A4 gene (SLC13A4) is abundantly expressed in the human and mouse placenta where it is proposed to transport nutrient sulfate to the fetus. In mice, targeted disruption of placental Slc13a4 leads to severe and lethal fetal phenotypes, however the involvement of SLC13A4 in human development is unknown. A search of the NCBI and Ensembl gene databases identified two alternatively spliced SLC13A4 mRNA transcripts and 98 SLC13A4 gene variants, including 85 missense, 4 splice site, 5 frameshift and 2 nonsense variants, as well as 2 in-frame deletions. We examined the relative abundance of the two SLC13A4 mRNA transcripts and then compared the sulfate transport function and plasma membrane expression of both isoforms as well as 6 sequence variants that predict disrupted SLC13A4 protein structure and function. SLC13A4 mRNA variant 1 has three additional nucleotides CAG compared to SLC13A4 mRNA variant 2 as a result of alternative splicing at the 5'-end of exon 6. Using qRT-PCR, we show a 4-fold higher abundance of SLC13A4 mRNA variant 1 compared to variant 2 in term human placentas and cultured BeWo and JEG-3 cell lines. The corresponding SLC13A4 protein isoforms 1 and 2 were found to have similar sulfate uptake activity and apical membrane expression in cultured MDCK cells. In addition, sulfate uptake into MDCK cells was similar between SLC13A4 isoform 1 and four missense variants N300S, F310C, E360Q and I570V, whereas V513M and frameshift variant L72Sfs led to partial (≈75% decrease) and complete loss-of-function, respectively. Localisation of these variants in MDCK cells showed N300S, E360Q, V513M and I570V expression on the apical plasma membrane, L72Sfs intracellularly and F310C on both apical and basolateral membranes. Our finding of partial and complete loss-of-function variants warrants further studies of the potential involvement of SLC13A4 in fetal pathophysiology.

The cytoskeletal binding domain of band 3 is required for multiprotein complex formation and retention during erythropoiesis

Band 3 is the most abundant protein in the erythrocyte membrane and forms the core of a major multiprotein complex. The absence of band 3 in human erythrocytes has only been reported once, in the homozygous band 3 Coimbra patient. We used in vitro culture of erythroblasts derived from this patient, and separately short hairpin RNA-mediated depletion of band 3, to investigate the development of a band 3-deficient erythrocyte membrane and to specifically assess the stability and retention of band 3 dependent proteins in the absence of this core protein during terminal erythroid differentiation. Further, using lentiviral transduction of N-terminally green fluorescent protein-tagged band 3, we demonstrated the ability to restore expression of band 3 to normal levels and to rescue secondary deficiencies of key proteins including glycophorin A, protein 4.2, CD47 and Rh proteins arising from the absence of band 3 in this patient. By transducing band 3-deficient erythroblasts from this patient with band 3 mutants with absent or impaired ability to associate with the cytoskeleton we also demonstrated the importance of cytoskeletal connectivity for retention both of band 3 and of its associated dependent proteins within the reticulocyte membrane during the process of erythroblast enucleation.

Restoration of proper trafficking to the cell surface for membrane proteins harboring cysteine mutations

A common phenotype for many genetic diseases is that the cell is unable to deliver full-length membrane proteins to the cell surface. For some forms of autism, hereditary spherocytosis and color blindness, the culprits are single point mutations to cysteine. We have studied two inheritable cysteine mutants of cyclic nucleotide-gated channels that produce achromatopsia, a common form of severe color blindness. By taking advantage of the reactivity of cysteine’s sulfhydryl group, we modified these mutants with chemical reagents that attach moieties with similar chemistries to the wild-type amino acids’ side chains. We show that these modifications restored proper delivery to the cell membrane. Once there, the channels exhibited normal functional properties. This strategy might provide a unique opportunity to assess the chemical nature of membrane protein traffic problems.

An intramolecular transport metabolon: fusion of carbonic anhydrase II to the COOH terminus of the Cl(-)/HCO(3)(-)exchanger, AE1

Anion exchanger 1 (AE1) is the plasma membrane Cl(-)/HCO(3)(-) exchanger of erythrocytes. Carbonic anhydrases (CA) provide substrate for AE1 by catalyzing the reaction, H(2)O + CO(2) ↔ HCO(3)(-) + H(+). The physical complex of CAII with AE1 has been proposed to maximize anion exchange activity. To examine the effect of CAII catalysis on AE1 transport rate, we fused either CAII-wild type or catalytically inactive CAII-V143Y to the cytoplasmic COOH terminus of AE1 to form AE1.CAII and AE1.CAII-V143Y, respectively. When expressed in transfected human embryonic kidney 293 cells, AE1.CAII had a similar Cl(-)/HCO(3)(-) exchange activity to AE1 alone, as assessed by the flux of H(+) equivalents (87 ± 4% vs. AE1) or rate of change of intracellular Cl(-) concentration (93 ± 4% vs. AE1), suggesting that CAII does not activate AE1. In contrast, AE1.CAII-V143Y displayed transport rates for H(+) equivalents and Cl(-) of 55 ± 2% and of 40 ± 2%, versus AE1. Fusion of CAII to AE1 therefore reduces anion transport activity, but this reduction is compensated for during Cl(-)/HCO(3)(-) exchange by the presence of catalytically active CAII. Overexpression of free CAII-V143Y acts in a dominant negative manner to reduce AE1-mediated HCO(3)(-) transport by displacement of endogenous CAII-wild type from its binding site on AE1. To examine whether AE1.CAII bound endogenous CAII, we coexpressed CAII-V143Y along with AE1 or AE1.CAII. The bicarbonate transport activity of AE1 was inhibited by CAII-V143Y, whereas the activity of AE1.CAII was unaffected by CAII-V143Y, suggesting impaired transport activity upon displacement of functional CAII from AE1 but not AE1.CAII. Taken together, these data suggest that association of functional CAII with AE1 increases Cl(-)/HCO(3)(-) exchange activity, consistent with the HCO(3)(-) transport metabolon model.

A prospective study to assess the predictive value for hereditary spherocytosis using five laboratory tests (cryohemolysis test, eosin-5'-maleimide flow cytometry, osmotic fragility test, autohemolysis test, and SDS-PAGE) on 50 hereditary spherocytosis families in Argentina

This prospective study was carried out to assess the usefulness of five laboratory tests in the diagnosis of hereditary spherocytosis (HS), based on the correlation of erythrocyte membrane protein defects with clinical and laboratory features, and also to determine the membrane protein deficiencies detected in Argentina. Of 116 patients and their family members tested, 62 of them were diagnosed to have HS. The specificity of cryohemolysis (CH) test was 95.2%, and its cut-off value to distinguish HS from normal was 2.8%. For flow cytometry, cut-off points of 17% for mean channel fluorescence (MCF) decrease and 14% coefficient of variation (CV) increase showed 95.9% and 92.2% specificity, respectively. Both tests showed the highest percentages of positive results for diagnosis. Either CH or flow cytometry was positive in 93.5% of patients. In eight patients, flow cytometry was positive only through CV increase. Protein defects were detected in 72.3% of patients; ankyrin and spectrin were the most frequently found deficiencies. The CV of the fluorescence showed significantly higher increases in moderate and severe anemia than in mild anemia (p = 0.003). Severity of anemia showed no other correlation with tests results, type of deficient protein, inheritance pattern, or neonatal jaundice. CH and flow cytometry are easy methods with the highest diagnostic accuracy. Simultaneous reading of mean channel fluorescence (MCF) decrease and CV increase improve diagnostic usefulness of flow cytometry. This test seems to be a reliable predictor of severity. The type of detected protein deficiency has no predictive value for outcome. Predominant ankyrin and spectrin deficiencies agree with reports from other Latin American countries.

Band 3 Edmonton I, a novel mutant of the anion exchanger 1 causing spherocytosis and distal renal tubular acidosis

dRTA (distal renal tubular acidosis) and HS (hereditary spherocytosis) are two diseases that can be caused by mutations in the gene encoding the AE1 (anion exchanger 1; Band 3). dRTA is characterized by defective urinary acidification, leading to metabolic acidosis, renal stones and failure to thrive. HS results in anaemia, which may require regular blood transfusions and splenectomy. Mutations in the gene encoding AE1 rarely cause both HS and dRTA. In the present paper, we describe a novel AE1 mutation, Band 3 Edmonton I, which causes dominant HS and recessive dRTA. The patient is a compound heterozygote with the new mutation C479W and the previously described mutation G701D. Red blood cells from the patient presented a reduced amount of AE1. Expression in a kidney cell line showed that kAE1 (kidney AE1) C479W is retained intracellularly. As kAE1 is a dimer, we performed co-expression studies and found that, in kidney cells, kAE1 C479W and G701D proteins traffic independently from each other despite their ability to form heterodimers. Therefore the patient carries one kAE1 mutant that is retained in the Golgi (G701D) and another kAE1 mutant (C479W) located in the endoplasmic reticulum of kidney cells, and is thus probably unable to reabsorb bicarbonate into the blood. We conclude that the C479W mutant is a novel trafficking mutant of AE1, which causes HS due to a decreased cell-surface AE1 protein and results in dRTA due to its intracellular retention in kidney.

Dominant-negative effect of Southeast Asian ovalocytosis anion exchanger 1 in compound heterozygous distal renal tubular acidosis

The human chloride/bicarbonate AE1 (anion exchanger) is a dimeric glycoprotein expressed in the red blood cell membrane,and expressed as an N-terminal (Delta1-65) truncated form, kAE1(kidney AE1), in the basolateral membrane of alpha-intercalated cells in the distal nephron. Mutations in AE1 can cause SAO (Southeast Asian ovalocytosis) or dRTA (distal renal tubular acidosis), an inherited kidney disease resulting in impaired acid secretion. The dominant SAO mutation (Delta400-408) that results in an inactive transporter and altered erythrocyte shape occurs in manydRTA families, but does not itself result in dRTA. Compound heterozygotes of four dRTA mutations (R602H, G701D, DeltaV850 and A858D) with SAO exhibit dRTA and abnormal red blood cell properties. Co-expression of kAE1 and kAE1 SAO with the dRTAmutantswas studied in polarized epithelial MDCK(Madin-Darbycanine kidney) cells. Like SAO, the G701D and DeltaV850 mutants were predominantly retained intracellularly, whereas the R602H and A858D mutants could traffic to the basolateral membrane. When co-expressed in transfected cells, kAE1 WT (wild-type)and kAE1 SAO could interact with the dRTA mutants. MDCK cells co-expressing kAE1 SAO with kAE1 WT, kAE1 R602Hor kAE1 A858D showed a decrease in cell-surface expression of the co-expressed proteins. When co-expressed, kAE1 WT colocalized with the kAE1 R602H, kAE1 G701D, kAE1 DeltaV850 and kAE1 A858D mutants at the basolateral membrane, whereaskAE1 SAO co-localized with kAE1 WT, kAE1 R602H, kAE1 G701D, kAE1 DeltaV850 and kAE1 A858D in MDCK cells. The decrease in cell-surface expression of the dRTAmutants as a result of the interaction with kAE1 SAO would account for the impaired expression of functional kAE1 at the basolateral membrane of alpha-intercalated cells, resulting in dRTA in compound heterozygous patients.

Glycophorin A: Band 3 aid

Band 3 (B3) is a major site of cytoskeletal attachment to the erythrocyte membrane and is important for gas exchange. A truncated isoform of B3 (kB3) is expressed in the alpha-intercalated cells of the kidney and its functional activity and basolateral localization are essential for acid secretion. B3 mutations generally lead to red blood cell (RBC) specific disease (hereditary spherocytosis (HS), Southeast Asian Ovalocytosis or hereditary stomatocytosis) or kidney disease (distal Renal Tubular Acidosis--dRTA). It is rare for both the RBC and kidney disease phenotypes to co-exist, but this does occur in knockout mice, and also in humans (B3 Coimbra and B3 Courcouronne) or cattle with homozygous HS mutations. This is because RBCs express a B3 chaperone-like molecule in the form of Glycophorin A that can rescue the majority of B3 mutations that cause dRTA but probably not the majority of HS mutations. The study of naturally occurring B3 variant blood and expression of B3 or kB3 mutants in heterologous expression systems has provided valuable information concerning B3 trafficking and interactions in the RBC and kidney. This article will review these studies and comment on our current understanding of the interaction between GPA with B3 and also on the proposed B3 centred macrocomplex.

Ubiquitylation-independent ER-associated degradation of an AE1 mutant associated with dominant hereditary spherocytosis in cattle

Various mutations in the AE1 (anion exchanger 1, band 3) gene cause dominant hereditary spherocytosis, a common congenital hemolytic anemia associated with deficiencies of AE1 of different degrees and loss of mutant protein from red blood cell membranes. To determine the mechanisms underlying decreases in AE1 protein levels, we employed K562 and HEK293 cell lines and Xenopus oocytes together with bovine wild-type AE1 and an R664X nonsense mutant responsible for dominant hereditary spherocytosis to analyze protein expression, turnover, and intracellular localization. R664X-mutant protein underwent rapid degradation and caused specifically increased turnover and impaired trafficking to the plasma membrane of the wild-type protein through hetero-oligomer formation in K562 cells. Consistent with those observations, co-expression of mutant and wild-type AE1 reduced anion transport by the wild-type protein in oocytes. Transfection studies in K562 and HEK293 cells revealed that the major pathway mediating degradation of both R664X and wild-type AE1 employed endoplasmic reticulum (ER)-associated degradation through the proteasomal pathway. Proteasomal degradation of R664X protein appeared to be independent of both ubiquitylation and N-glycosylation, and aggresome formation was not observed following proteasome inhibition. These findings indicate that AE1 R664X protein, which is associated with dominant hereditary spherocytosis, has a dominant-negative effect on the expression of wild-type AE1.

Molecular mechanisms of autosomal dominant and recessive distal renal tubular acidosis caused by SLC4A1 (AE1) mutations

Mutations of SLC4A1 (AE1) encoding the kidney anion (Cl⁻/HCO₃⁻) exchanger 1 (kAE1 or band 3) can result in either autosomal dominant (AD) or autosomal recessive (AR) distal renal tubular acidosis (dRTA). The molecular mechanisms associated with SLC4A1 mutations resulting in these different modes of inheritance are now being unveiled using transfected cell systems. The dominant mutants kAE1 R589H, R901X and S613F, which have normal or insignificant changes in anion transport function, exhibit intracellular retention with endoplasmic reticulum (ER) localization in cultured non-polarized and polarized cells, while the dominant mutants kAE1 R901X and G609R are mis-targeted to apical membrane in addition to the basolateral membrane in cultured polarized cells. A dominant-negative effect is likely responsible for the dominant disease because heterodimers of kAE1 mutants and the wild-type protein are intracellularly retained. The recessive mutants kAE1 G701D and S773P however exhibit distinct trafficking defects. The kAE1 G701D mutant is retained in the Golgi apparatus, while the misfolded kAE1 S773P, which is impaired in ER exit and is degraded by proteosome, can only partially be delivered to the basolateral membrane of the polarized cells. In contrast to the dominant mutant kAE1, heterodimers of the recessive mutant kAE1 and wild-type kAE1 are able to traffic to the plasma membrane. The wild-type kAE1 thus exhibits a ‘dominant-positive effect’ relative to the recessive mutant kAE1 because it can rescue the mutant proteins from intracellular retention to be expressed at the cell surface. Consequently, homozygous or compound heterozygous recessive mutations are required for presentation of the disease phenotype. Future work using animal models of dRTA will provide additional insight into the pathophysiology of this disease.

Band 3 anion exchanger and its involvement in erythrocyte and kidney disorders

Recent developments in the structure of erythrocyte band 3 and its role in hereditary spherocytosis and distal renal tubular acidosis are described. The crystal structure of the N-terminal cytoplasmic domain provides a basis for understanding the organization of ankyrin and other peripheral membrane proteins around band 3. Band 3 also binds integral membrane proteins, including the Rh protein complex and CD47. Band 4.2 is important in these associations, which link the Rh complex to the skeleton. It is suggested that band 3 forms the scaffold for a protein assembly that could transduce signals from the cell exterior and modulate the transport and mechanical properties of the erythrocyte. The involvement of band 3 in distal renal tubular acidosis is reviewed. The article discusses a likely mechanism for dominant distal renal tubular acidosis in which associations between the normal and mutant protein alter the plasma membrane targeting of the normal protein in the kidney.

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.

Coexpression of band 3 mutants and Rh polypeptides: differential effects of band 3 on the expression of the Rh complex containing D polypeptide and the Rh complex containing CcEe polypeptide

K562 cells were stably transfected with cDNAs encoding the band 3 found in Southeast Asian ovalocytosis (B3SAO, deletion of residues 400-408), band 3 with a transport-inactivating E681Q point mutation (B3EQ), or normal band 3 (B3). Flow cytometric analysis and quantitative immunoblotting revealed that B3SAO expressed alone was translocated to the plasma membrane, at levels similar to B3 or B3EQ. Nine monoclonal antibodies that reacted with extracellular loops of B3 also reacted with B3SAO, although the affinity of most antibodies for the mutant protein was reduced. Both known Wr(b) epitopes were expressed on K562/B3SAO cells, demonstrating that B3SAO interacts with glycophorin A. The growth rates of K562 clones expressing equivalent amounts of B3 and B3EQ were the same, suggesting that the potentially toxic transport function of band 3 may be regulated in K562 cells. The band 3-mediated enhancement of Rh antigen reactivity and the depression of Rh epitopes on SAO erythrocytes were investigated by comparing the coexpression of B3, B3SAO, or B3EQ in K562 clones expressing exogenous RhcE or RhD polypeptides. The results are consistent with an interaction between band 3 and the Rh polypeptide-Rh glycoprotein (RhAG) complex, which may enhance translocation of the complex or affect its conformation in the plasma membrane. The data suggest that the interaction between band 3 and the RhD-RhAG complex is weaker than it is between band 3 and the RhCcEe-RhAG complex.

Trafficking and folding defects in hereditary spherocytosis mutants of the human red cell anion exchanger

Hereditary spherocytosis (HS) is a common inherited hemolytic anemia caused by mutations in erythrocyte proteins including the anion exchanger, AE1 (band 3). This study examined seven missense mutations (L707P, R760Q, R760W, R808C, H834P, T837M, and R870W) located in the membrane domain of the human AE1 that are associated with this disease. The HS mutants, constructed in full-length AE1 cDNA, could be transiently expressed to similar levels in HEK 293 cells. Immunofluorescence, cell surface biotinylation, and pulse chase labeling showed that the HS mutants all exhibited defective cellular trafficking from the endoplasmic reticulum to the plasma membrane. Impaired binding to an inhibitor affinity matrix indicated that the mutant proteins had non-native structures and may be misfolded. Further characterization of the HS R760Q mutant showed no change in its oligomeric structure or turnover (half-life = 15 h) compared to wild-type AE1, suggesting the mutant was not aggregated or targeted for rapid degradation via the proteasome. Intracellular retention of HS mutant AE1 would lead to destruction of the protein during erythroid development and would account for the lack of HS mutant AE1 in the plasma membrane of the mature red cell.