Glycophorin A: Band 3 aid

Red blood cells-derived components as biomimetic functional materials: Matching versatile delivery strategies based on structure and function

Red blood cells (RBCs), often referred to as "intelligent delivery systems", can serve as biological or hybrid drug carriers due to their inherent advantages and characteristics. This innovative approach has the potential to enhance biocompatibility, pharmacokinetics, and provide targeting properties for drugs. By leveraging the unique structure and contents of RBCs, drug-loading pathways can be meticulously designed to align with these distinctive features. This review article primarily discusses the drug delivery strategies and their applications that are informed by the structural and functional properties of the main components of RBCs, including living RBCs, membranes, hollow RBCs, and hemoglobin. Overall, this review article would assist efforts to make better decisions on optimization and rational utilization of RBCs derivatives-based drug delivery strategies for the future direction in clinical translation.

Increased Anion Exchanger-1 (Band 3) on the Red Blood Cell Membrane Accelerates Scavenging of Nitric Oxide Metabolites and Predisposes Hypertension Risks

The erythrocyte membrane is highly specialized with ∼1 million anion exchanger-1 (AE1) per cell for rapid membrane permeation of HCO3-(aq), as most blood CO2(g) is carried in this hydrated anionic form. People with the GP.Mur blood type have more AE1 on their erythrocyte membrane, and they excrete CO2(g) more efficiently. Unexpectedly, GP.Mur/increased AE1 is also associated with higher blood pressure (BP). To solve this, we knocked the human GYP.Mur gene into C57BL/6J mice at 3'-UTR of GYPA to generate GPMur knock-in (KI) mice. KI of human GYP.Mur increased murine AE1 expression on the red blood cells (RBC). GPMur KI mice were naturally hypertensive, with normal kidney functions and lipid profiles. Blood NO3- [the stable nitric oxide (NO) reservoir] was significantly lower in the GPMur mice. GPMur KI also accelerated AE1-mediated NO2- influx into the RBCs and intraerythrocytic NO2-/NO processing. From tests with different categories of antihypertensives, hypertension in GPMur mice responded best to direct arterial vasodilator hydralazine, suggesting that vasodilator deficiency is the leading cause of "GPMur/AE1-triggered hypertension." In conclusion, we showed that GPMur/increased AE1 predisposed hypertension risks. Mechanistically, higher AE1 expression increased RBC membrane permeability for NO2- and consequently accelerated erythroid NO2-/NO metabolism; this is associated with lower NO bioavailability and higher BP. As hypertension affects a quarter of the world population and GP.Mur is a common Southeast Asian (SEA) blood type, this work may serve as a primer for "GPMur (biomarker)-based" therapeutic development for hypertension.

A Balance between Transmembrane-Mediated ER/Golgi Retention and Forward Trafficking Signals in Glycophorin-Anion Exchanger-1 Interaction

Anion exchanger-1 (AE1) is the main erythroid Cl-/HCO3- transporter that supports CO2 transport. Glycophorin A (GPA), a component of the AE1 complexes, facilitates AE1 expression and anion transport, but Glycophorin B (GPB) does not. Here, we dissected the structural components of GPA/GPB involved in glycophorin-AE1 trafficking by comparing them with three GPB variants-GPBhead (lacking the transmembrane domain [TMD]), GPBtail (mainly the TMD), and GP.Mur (glycophorin B-A-B hybrid). GPB-derived GP.Mur bears an O-glycopeptide that encompasses the R18 epitope, which is present in GPA but not GPB. By flow cytometry, AE1 expression in the control erythrocytes increased with the GPA-R18 expression; GYP.Mur+/+ erythrocytes bearing both GP.Mur and GPA expressed more R18 epitopes and more AE1 proteins. In contrast, heterologously expressed GPBtail and GPB were predominantly localized in the Golgi apparatus of HEK-293 cells, whereas GBhead was diffuse throughout the cytosol, suggesting that glycophorin transmembrane encoded an ER/Golgi retention signal. AE1 coexpression could reduce the ER/Golgi retention of GPB, but not of GPBtail or GPBhead. Thus, there are forward-trafficking and transmembrane-driven ER/Golgi retention signals encoded in the glycophorin sequences. How the balance between these opposite trafficking signals could affect glycophorin sorting into AE1 complexes and influence erythroid anion transport remains to be explored.

Architecture of the human erythrocyte ankyrin-1 complex

The stability and shape of the erythrocyte membrane is provided by the ankyrin-1 complex, but how it tethers the spectrin-actin cytoskeleton to the lipid bilayer and the nature of its association with the band 3 anion exchanger and the Rhesus glycoproteins remains unknown. Here we present structures of ankyrin-1 complexes purified from human erythrocytes. We reveal the architecture of a core complex of ankyrin-1, the Rhesus proteins RhAG and RhCE, the band 3 anion exchanger, protein 4.2, glycophorin A and glycophorin B. The distinct T-shaped conformation of membrane-bound ankyrin-1 facilitates recognition of RhCE and, unexpectedly, the water channel aquaporin-1. Together, our results uncover the molecular details of ankyrin-1 association with the erythrocyte membrane, and illustrate the mechanism of ankyrin-mediated membrane protein clustering.

Cell Physiology and Molecular Mechanism of Anion Transport by Erythrocyte Band 3/AE1

The major transmembrane protein of the red blood cell, known as band 3, AE1, and SLC4A1, has two main functions: 1) catalysis of Cl^-/HCO₃^- exchange, one of the steps in CO₂ excretion; 2) anchoring the membrane skeleton. This review summarizes the 150 year history of research on red cell anion transport and band 3 as an experimental system for studying membrane protein structure and ion transport mechanisms. Important early findings were that red cell Cl^- transport is a tightly coupled 1:1 exchange and band 3 is labeled by stilbenesulfonate derivatives that inhibit anion transport. Biochemical studies showed that the protein is dimeric or tetrameric (paired dimers) and that there is one stilbenedisulfonate binding site per subunit of the dimer. Transport kinetics and inhibitor characteristics supported the idea that the transporter acts by an alternating access mechanism with intrinsic asymmetry. The sequence of band 3 cDNA provided a framework for detailed study of protein topology and amino acid residues important for transport. The identification of genetic variants produced insights into the roles of band 3 in red cell abnormalities and distal renal tubular acidosis. The publication of the membrane domain crystal structure made it possible to propose concrete molecular models of transport. Future research directions include improving our understanding of the transport mechanism at the molecular level and of the integrative relationships among band 3, hemoglobin, carbonic anhydrase, and gradients (both transmembrane and subcellular) of HCO₃^-, Cl^-, O₂, CO₂, pH, and NO metabolites during pulmonary and systemic capillary gas exchange.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Red Blood Cell AE1/Band 3 Transports in Dominant Distal Renal Tubular Acidosis Patients

Introduction

Anion exchanger 1 (AE1) (SLC4A1 gene product) is a membrane protein expressed in both kidney and red blood cells (RBCs): it exchanges extracellular bicarbonate (HCO₃^–) for intracellular chloride (Cl^–) and participates in acid−base homeostasis. AE1 mutations in kidney α-intercalated cells can lead to distal renal tubular acidosis (dRTA). In RBC, AE1 (known as band 3) is also implicated in membrane stability: deletions can cause South Asian ovalocytosis (SAO).

Methods

We retrospectively collected clinical and biological data from patients harboring dRTA due to a SLC4A1 mutation and analyzed HCO₃^– and Cl^– transports (by stopped-flow spectrophotometry) and expression (by flow cytometry, fluorescence activated cell sorting, and Coomassie blue staining) in RBCs, as well as RBC membrane stability (ektacytometry).

Results

Fifteen patients were included. All experience nephrolithiasis and/or nephrocalcinosis, 2 had SAO and dRTA (dRTA SAO+), 13 dominant dRTA (dRTA SAO−). The latter did not exert specific RBC membrane anomalies. Both HCO₃^– and Cl^– transports were lower in patients with dRTA SAO+ than in those with dRTA SAO− or controls. Using 3 different extracellular probes, we report a decreased expression (by 52%, P < 0.05) in dRTA SAO+ patients by fluorescence activated cell sorting, whereas total amount of protein was not affected.

Conclusion

Band 3 transport function and expression in RBCs from dRTA SAO− patients is normal. However, in SAO RBCs, impaired conformation of AE1/band 3 corresponds to an impaired function. Thus, the driver of acid−base defect during dominant dRTA is probably an impaired membrane expression.

Genetic conflicts with Plasmodium parasites and functional constraints shape the evolution of erythrocyte cytoskeletal proteins

Plasmodium parasites exerted a strong selective pressure on primate genomes and mutations in genes encoding erythrocyte cytoskeleton proteins (ECP) determine protective effects against Plasmodium infection/pathogenesis. We thus hypothesized that ECP-encoding genes have evolved in response to Plasmodium-driven selection. We analyzed the evolutionary history of 15 ECP-encoding genes in primates, as well as of their Plasmodium-encoded ligands (KAHRP, MESA and EMP3). Results indicated that EPB42, SLC4A1, and SPTA1 evolved under pervasive positive selection and that episodes of positive selection tended to occur more frequently in primate species that host a larger number of Plasmodium parasites. Conversely, several genes, including ANK1 and SPTB, displayed extensive signatures of purifying selection in primate phylogenies, Homininae lineages, and human populations, suggesting strong functional constraints. Analysis of Plasmodium genes indicated adaptive evolution in MESA and KAHRP; in the latter, different positively selected sites were located in the spectrin-binding domains. Because most of the positively selected sites in alpha-spectrin localized to the domains involved in the interaction with KAHRP, we suggest that the two proteins are engaged in an arms-race scenario. This observation is relevant because KAHRP is essential for the formation of “knobs”, which represent a major virulence determinant for P. falciparum.

Interaction of the human erythrocyte Band 3 anion exchanger 1 (AE1, SLC4A1) with lipids and glycophorin A: Molecular organization of the Wright (Wr) blood group antigen

The Band 3 (AE1, SLC4A1) membrane protein is found in red blood cells and in kidney where it functions as an electro-neutral chloride/bicarbonate exchanger. In this study, we have used molecular dynamics simulations to provide the first realistic model of the dimeric membrane domain of human Band 3 in an asymmetric lipid bilayer containing a full complement of phospholipids, including phosphatidylinositol 4,5–bisphosphate (PIP₂) and cholesterol, and its partner membrane protein Glycophorin A (GPA). The simulations show that the annular layer in the inner leaflet surrounding Band 3 was enriched in phosphatidylserine and PIP₂ molecules. Cholesterol was also enriched around Band 3 but also at the dimer interface. The interaction of these lipids with specific sites on Band 3 may play a role in the folding and function of this anion transport membrane protein. GPA associates with Band 3 to form the Wright (Wr) blood group antigen, an interaction that involves an ionic bond between Glu658 in Band 3 and Arg61 in GPA. We were able to recreate this complex by performing simulations to allow the dimeric transmembrane portion of GPA to interact with Band 3 in a model membrane. Large-scale simulations showed that the GPA dimer can bridge Band 3 dimers resulting in the dynamic formation of long strands of alternating Band 3 and GPA dimers.

Human Band 3 (AE1, SLC4A1), an abundant 911 amino acid glycoprotein, catalyzes the exchange of bicarbonate and chloride across the red blood cell membrane, a process necessary for efficient respiration. Malfunction of Band 3 leads to inherited diseases such as Southeast Asian Ovalocytosis, hereditary spherocytosis and distal renal tubular acidosis. Despite much available structural and functional data about Band 3, key questions about the conformational changes associated with transport and the molecular details of its interaction with lipids and other proteins remain unanswered. In this study, we have used computer simulations to investigate the dynamics of Band 3 in lipid bilayers that resemble the red blood cell plasma membrane. Our results suggest that negatively charged phospholipids and cholesterol interact strongly with Band 3 forming an annulus around the protein. Glycophorin A (GPA) interacts with Band 3 to form the Wright (Wr) blood group antigen. We were able to recreate this complex and show that GPA promotes the clustering of Band 3 in red blood cell membranes. Understanding the molecular details of the interaction of Band 3 with GPA has provided new insights into the nature of the Wright blood group antigen.

Hypokalemic Distal Renal Tubular Acidosis

Distal renal tubular acidosis (DRTA) is defined as hyperchloremic, non-anion gap metabolic acidosis with impaired urinary acid excretion in the presence of a normal or moderately reduced glomerular filtration rate. Failure in urinary acid excretion results from reduced H⁺ secretion by intercalated cells in the distal nephron. This results in decreased excretion of NH₄⁺ and other acids collectively referred as titratable acids while urine pH is typically above 5.5 in the face of systemic acidosis. The clinical phenotype in patients with DRTA is characterized by stunted growth with bone abnormalities in children as well as nephrocalcinosis and nephrolithiasis that develop as the consequence of hypercalciuria, hypocitraturia, and relatively alkaline urine. Hypokalemia is a striking finding that accounts for muscle weakness and requires continued treatment together with alkali-based therapies. This review will focus on the mechanisms responsible for impaired acid excretion and urinary potassium wastage, the clinical features, and diagnostic approaches of hypokalemic DRTA, both inherited and acquired.

Novel messenger RNAs for body fluid identification

In forensic investigations, the identification of the cellular or body fluid source of biological evidence can provide crucial probative information for the court. Messenger RNA (mRNA) profiling has become a valuable tool for body fluid and cell type identification due to its high sensitivity and compatibility with DNA analysis. However, using a single marker to determine the somatic origin of a sample can lead to misinterpretation as a result of cross-reactions. While false positives may be avoided through the simultaneous detection of multiple markers per body fluid, this approach is currently limited by the small number of known differentially expressed mRNAs. Here we characterise six novel mRNAs, partly identified from RNA-Seq, which can supplement existing markers for the detection of circulatory blood, semen (with and without spermatozoa), and menstrual fluid: HBD and SLC4A1 for blood, TNP1 for spermatozoa, KLK2 for seminal fluid, and MMP3 and STC1 for menstrual fluid. Respective expression profiles were evaluated by singleplex endpoint reverse transcription polymerase chain reaction (RT-PCR). HBD, SLC4A1, and KLK2 were specific to their target body fluids. TNP1, MMP3, and STC1 each cross-reacted with two non-target samples; however, these signals were below 350RFU, not reproducible, and likely resulted from large body fluid inputs. All candidates were more sensitive for the detection of their target body fluids than corresponding well-known mRNAs, in particular those for menstrual fluid. The increased sensitivities were statistically significant, except for KLK2. Thus, the new mRNAs introduced here are promising new targets for improved body fluid profiling.

Involvement of Band3 in the efflux of sphingosine 1-phosphate from erythrocytes

Sphingosine 1-phosphate (S1P) is a bioactive lipid mediator that is thought to be involved in various diseases. Although the main source of S1P in the plasma is erythrocytes, how S1P is exported from erythrocytes has not been elucidated. When we differentiated K562 cells into erythroblast-like cells with sodium butyrate, we observed that the efflux of S1P was increased without increased expression of previously proposed S1P transporters, while the expression levels of Band3 were increased. Therefore, in this study, we investigated the involvement of Band 3, the most characteristic membranous transporter for erythrocytes, in S1P efflux, using 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid, disodium salt (H2DIDS), which is an inhibitor of Band3. First, we treated human washed erythrocytes with H₂DIDS and found that H₂DIDS decreased the S1P levels in the supernatant, while it increased the cellular S1P contents. Next, when we injected H₂DIDS into mice, the plasma S1P level was significantly decreased. Finally, when we overexpressed or suppressed Band3 in K562 cells, S1P efflux was enhanced or decreased, respectively, while the overexpression of Band3 in HEK293 cells did not modulate S1P efflux. These results suggested the possible involvement of Band3 in the transport of S1P, a multi-functional bioactive phospholipid, from erythrocytes.

PDLIM5 links kidney anion exchanger 1 (kAE1) to ILK and is required for membrane targeting of kAE1

Anion exchanger 1 (AE1) mediates Cl⁻/HCO₃⁻ exchange in erythrocytes and kidney intercalated cells where it functions to maintain normal bodily acid-base homeostasis. AE1’s C-terminal tail (AE1C) contains multiple potential membrane targeting/retention determinants, including a predicted PDZ binding motif, which are critical for its normal membrane residency. Here we identify PDLIM5 as a direct binding partner for AE1 in human kidney, via PDLIM5’s PDZ domain and the PDZ binding motif in AE1C. Kidney AE1 (kAE1), PDLIM5 and integrin-linked kinase (ILK) form a multiprotein complex in which PDLIM5 provides a bridge between ILK and AE1C. Depletion of PDLIM5 resulted in significant reduction in kAE1 at the cell membrane, whereas over-expression of kAE1 was accompanied by increased PDLIM5 levels, underscoring the functional importance of PDLIM5 for proper kAE1 membrane residency, as a crucial linker between kAE1 and actin cytoskeleton-associated proteins in polarized cells.

The NF-κB p65/miR-23a-27a-24 cluster is a target for leukemia treatment

p65 is a transcription factor that is involved in many physiological and pathologic processes. Here we report that p65 strongly binds to the miR-23a-27a-24 cluster promoter to up-regulate its expression. As bone marrow-derived cells differentiate into red blood cells in vitro, p65/miR-23a-27a-24 cluster expression increases sharply and then declines before the appearance of red blood cells, suggesting that this cluster is negatively related to erythroid terminal differentiation. Bioinformatic and molecular biology experiments confirmed that the miR-23a-27a-24 cluster inhibited the expression of the erythroid proteome and contributed to erythroleukemia progression. In addition, high level of the p65/miR-23a-27a-24 cluster was found in APL and AML cell lines and in nucleated peripheral blood cells from leukemia patients. Furthermore, anti-leukemia drugs significantly inhibited the expression of the p65/miR-23a-27a-24 cluster in leukemia cells. Administration of the p65 inhibitor parthenolide significantly improved hematology and myelogram indices while prolonging the life span of erythroleukemia mice. Meanwhile, stable overexpression of these three miRNAs in mouse erythroleukemia cells enhanced cell malignancy. Our findings thus connect a novel regulation pathway of the p65/miR-23a-27a-24 cluster with the erythroid proteome and provide an applicable approach for treating leukemia.

Advances in understanding the pathogenesis of the red cell volume disorders

Genetic defects of erythrocyte transport proteins cause disorders of red blood cell volume that are characterized by abnormal permeability to the cations Na(+) and K(+) and, consequently, by changes in red cell hydration. Clinically, these disorders are associated with chronic haemolytic anaemia of variable severity and significant co-morbidities, such as iron overload. This review provides an overview of recent insights into the molecular basis of this group of rare anaemias involving cation channels and transporters dysfunction. To date, a total of 5 different membrane proteins have been reported to be responsible for volume homeostasis alteration when mutated, 3 of them leading to overhydrated cells (AE1 [also termed SLC4A1], RHAG and GLUT1 [also termed SCL2A1) and 2 others to dehydrated cells (PIEZO1 and the Gardos Channel). These findings are not only of basic scientific interest, but also of direct clinical significance for improving diagnostic procedures and identify potential approaches for novel therapeutic strategies.

Band 3, the human red cell chloride/bicarbonate anion exchanger (AE1, SLC4A1), in a structural context

The crystal structure of the dimeric membrane domain of human Band 3(1), the red cell chloride/bicarbonate anion exchanger 1 (AE1, SLC4A1), provides a structural context for over four decades of studies into this historic and important membrane glycoprotein. In this review, we highlight the key structural features responsible for anion binding and translocation and have integrated the following topological markers within the Band 3 structure: blood group antigens, N-glycosylation site, protease cleavage sites, inhibitor and chemical labeling sites, and the results of scanning cysteine and N-glycosylation mutagenesis. Locations of mutations linked to human disease, including those responsible for Southeast Asian ovalocytosis, hereditary stomatocytosis, hereditary spherocytosis, and distal renal tubular acidosis, provide molecular insights into their effect on Band 3 folding. Finally, molecular dynamics simulations of phosphatidylcholine self-assembled around Band 3 provide a view of this membrane protein within a lipid bilayer.

Merozoite surface protein 1 recognition of host glycophorin A mediates malaria parasite invasion of red blood cells

Plasmodium falciparum invasion of human red blood cells (RBCs) is an intricate process requiring a number of distinct ligand-receptor interactions at the merozoite-erythrocyte interface. Merozoite surface protein 1 (MSP1), a highly abundant ligand coating the merozoite surface in all species of malaria parasites, is essential for RBC invasion and considered a leading candidate for inclusion in a multiple-subunit vaccine against malaria. Our previous studies identified an interaction between the carboxyl-terminus of MSP1 and RBC band 3. Here, by employing phage display technology, we report a novel interaction between the amino-terminus of MSP1 and RBC glycophorin A (GPA). Mapping of the binding domains established a direct interaction between malaria MSP1 and human GPA within a region of MSP1 known to potently inhibit P falciparum invasion of human RBCs. Furthermore, a genetically modified mouse model lacking the GPA- band 3 complex in RBCs is completely resistant to malaria infection in vivo. These findings suggest an essential role of the MSP1-GPA-band 3 complex during the initial adhesion phase of malaria parasite invasion of RBCs.

Physical and functional links between anion exchanger-1 and sodium pump

Anion exchanger-1 (AE1) mediates chloride-bicarbonate exchange across the plasma membranes of erythrocytes and, via a slightly shorter transcript, kidney epithelial cells. On an omnivorous human diet, kidney AE1 is mainly active basolaterally in α-intercalated cells of the collecting duct, where it is functionally coupled with apical proton pumps to maintain normal acid-base homeostasis. The C-terminal tail of AE1 has an important role in its polarized membrane residency. We have identified the β1 subunit of Na(+),K(+)-ATPase (sodium pump) as a binding partner for AE1 in the human kidney. Kidney AE1 and β1 colocalized in renal α-intercalated cells and coimmunoprecipitated (together with the catalytic α1 subunit of the sodium pump) from human kidney membrane fractions. ELISA and fluorescence titration assays confirmed that AE1 and β1 interact directly, with a Kd value of 0.81 μM. GST-AE1 pull-down assays using human kidney membrane proteins showed that the last 11 residues of AE1 are important for β1 binding. siRNA-induced knockdown of β1 in cell culture resulted in a significant reduction in kidney AE1 levels at the cell membrane, whereas overexpression of kidney AE1 increased cell surface sodium pump levels. Notably, membrane staining of β1 was reduced throughout collecting ducts of AE1-null mouse kidney, where increased fractional excretion of sodium has been reported. These data suggest a requirement of β1 for proper kidney AE1 membrane residency, and that activities of AE1 and the sodium pump are coregulated in kidney.

The SLC4 family of bicarbonate (HCO₃⁻) transporters

The SLC4 family consists of 10 genes (SLC4A1-5; SLC4A7-11). All encode integral membrane proteins with very similar hydropathy plots-consistent with 10-14 transmembrane segments. Nine SLC4 members encode proteins that transport HCO3(-) (or a related species, such as CO3(2-)) across the plasma membrane. Functionally, eight of these proteins fall into two major groups: three Cl-HCO3 exchangers (AE1-3) and five Na(+)-coupled HCO3(-) transporters (NBCe1, NBCe2, NBCn1, NBCn2, NDCBE). Two of the Na(+)-coupled transporters (NBCe1, NBCe2) are electrogenic; the other three Na(+)-coupled HCO3(-) transporters and all three AEs are electroneutral. In addition, two other SLC4 members (AE4, SLC4A9 and BTR1, SLC4A11) do not yet have a firmly established function. Most, though not all, SLC4 members are functionally inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS). SLC4 proteins play important roles many modes of acid-base homeostasis: the carriage of CO2 by erythrocytes, the transport of H(+) or HCO3(-) by several epithelia, as well as the regulation of cell volume and intracellular pH.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Genetic causes and mechanisms of distal renal tubular acidosis

The primary or hereditary forms of distal renal tubular acidosis (dRTA) have received increased attention because of advances in the understanding of the molecular mechanism, whereby mutations in the main proteins involved in acid-base transport result in impaired acid excretion. Dysfunction of intercalated cells in the collecting tubules accounts for all the known genetic causes of dRTA. These cells secrete protons into the tubular lumen through H(+)-ATPases functionally coupled to the basolateral anion exchanger 1 (AE1). The substrate for both transporters is provided by the catalytic activity of the cytosolic carbonic anhydrase II (CA II), an enzyme which is also present in the proximal tubular cells and osteoclasts. Mutations in ATP6V1B1, encoding the B-subtype unit of the apical H(+) ATPase, and ATP6V0A4, encoding the a-subtype unit, lead to the loss of function of the apical H(+) ATPase and are usually responsible for patients with autosomal recessive dRTA often associated with early or late sensorineural deafness. Mutations in the gene encoding the cytosolic CA II are associated with the autosomal recessive syndrome of osteopetrosis, mixed distal and proximal RTA and cerebral calcification. Mutations in the AE1, the gene that encodes the Cl(-)/HCO(3)(-) exchanger, usually present as dominant dRTA, but a recessive pattern has been recently described. Several studies have shown trafficking defects in the mutant protein rather than the lack of function as the major mechanism underlying the pathogenesis of dRTA from AE1 mutations.

Mature erythrocyte membrane homeostasis is compromised by loss of the GATA1-FOG1 interaction

GATA1 plays essential roles in erythroid gene expression. The N-terminal finger of GATA1 (GATA1-Nf) is important for association with FOG1. Substitution mutations in GATA1-Nf, such as GATA1V205M that diminish the GATA1-FOG1 association, have been identified in human thrombocytopenia and anemia cases. A mouse model of human thrombocytopenia has been established using a transgenic complementation rescue approach; GATA1-deficient mice were successfully rescued from embryonic lethality by excess expression of GATA1V205G, but rescued adult mice suffered from severe thrombocytopenia. In this study, we examined GATA1-deficient mice rescued with GATA1V205G at a comparable level to endogenous GATA1. Mice rescued with this level of GATA1V205G rarely survive to adulthood. Rescued newborns suffered from severe anemia and jaundice accompanied with anisocytosis and spherocytosis. Expression of Slc4a1, Spna1, and Aqp1 genes (encoding the membrane proteins band-3, α-spectrin, and aquaporin-1, respectively) were strikingly diminished, whereas expression of other canonical GATA1-target genes, such as Alas2, were little affected. Lack of these membrane proteins provoked perturbation of membrane skeleton. Importantly, the red cells exhibited increased reactive oxygen species accumulation. These results thus demonstrate that the loss of the GATA1-FOG1 interaction causes a unique combination of membrane protein deficiency and disturbs the function of GATA1 in maintaining erythroid homeostasis.

The SLC4A1 gene is under differential selective pressure in primates infected by Plasmodium falciparum and related parasites

Malaria is a disease caused by Plasmodium parasites and is responsible for high mortality in humans. This disease is caused by four different species of Plasmodium though the main source of mortality is Plasmodium falciparum. Humans have a number of genetic adaptations that act to combat Plasmodium. One adaptation is a deletion in the SLC4A1 gene that leads to Southeast Asian ovalocytosis (SAO). There is evidence that SAO erythrocytes are resistant to multiple Plasmodium species. Here we analyze SLC4A1 in 23 primates and mammals to test for differential selective pressures among different primate lineages. Because primates are infected with both human Plasmodium parasites and their relatives, this analysis can be used to test which human Plasmodium parasite is the likely target of SAO. A significantly different pattern of molecular evolution was found in humans and African apes, species that are infected by P. falciparum and its relatives. This effect was restricted to the cytosolic domain of the SLC4A1 gene. The evidence is consistent with a different selective regime operating on this gene domain in humans and African apes, when compared to other primates and mammals. Alternatively, this pattern is consistent with a relaxation of selection or weak adaptive evolution operating on a small number of amino acids. The adaptive interpretation of the results is consistent with the SAO allele of the SLC4A1 gene interacting with P. falciparum in humans, rather than other Plasmodium parasites. However, additional investigation of the relationship between SLC4A1 variants and Plasmodium in humans and African apes is required to test whether the different selective regime in humans and African apes is due to natural selection or relaxed constraint.

Clathrin-mediated endocytosis of mammalian erythroid AE1 anion exchanger facilitated by a YXXΦ or a noncanonical YXXXΦ motif in the N-terminal stretch

To explore the roles of the conserved YXXΦ-type motif in the erythroid-specific N-terminal stretch of anion exchanger 1 (AE1), cell surface expression and internalization of various mutants derived from murine erythroid AE1 tagged with an N-terminal enhanced green fluorescent protein and an extracellular FLAG (EGFP-mAE1Flag) were explored in K562 and HEK293 cells. EGFP-mAE1Flag showed rapid internalization, in association with the internalizations of transferrin and the endogenous AE1 chaperone-like protein glycophorin A in K562 cells. Disruption of the conserved Y72VEL sequence markedly reduced the internalization and increased the relative abundance of cell-surface AE1, whereas substitution of the N-terminal region from bovine AE1 that lacks the relevant motif for the corresponding region had less of an effect on internalization. Deletion or substitution mutations of the Y7EDQL sequence in the bovine N-terminal stretch resulted in the decreased internalization of the AE1 proteins. Cell surface biotinylation and deglycosylation studies showed that approximately 30% of the cell-surface EGFP-mAE1Flag and several other mutants was sorted to the plasma membrane without N-glycan maturation in the Golgi apparatus. These findings indicate that the conserved YXXΦ sequence or a noncanonical YXXXΦ sequence in the N-terminal region facilitates the endocytic recycling of erythroid AE1 through a clathrin-mediated pathway.

Band 3 missense mutations and stomatocytosis: insight into the molecular mechanism responsible for monovalent cation leak

Missense mutations in the erythroid band 3 protein (Anion Exchanger 1) have been associated with hereditary stomatocytosis. Features of cation leaky red cells combined with functional expression of the mutated protein led to the conclusion that the AE1 point mutations were responsible for Na(+) and K(+) leak through a conductive mechanism. A molecular mechanism explaining mutated AE1-linked stomatocytosis involves changes in AE1 transport properties that become leaky to Na(+) and K(+). However, another explanation suggests that point-mutated AE1 could regulate a cation leak through other transporters. This short paper intends to discuss these two alternatives.

Anion exchanger 1 in red blood cells and kidney: Band 3's in a pod

The bicarbonate/chloride exchanger 1 (AE1, Band 3) is abundantly expressed in the red blood cell membrane, where it is involved in gas exchange and functions as a major site of cytoskeletal attachment to the erythrocyte membrane. A truncated kidney isoform (kAE1) is highly expressed in type A intercalated cells of the distal tubules, where it is vital for urinary acidification. Recently, kAE1 has emerged as a novel physiologically significant protein in the kidney glomerulus. This minireview will discuss the known interactions of kAE1 in the podocytes and the possible mechanisms whereby this important multispanning membrane protein may contribute to the function of the glomerular filtration barrier and prevent proteinuria.

Interactions of mouse glycophorin A with the dRTA-related mutant G719D of the mouse Cl-/HCO3- exchanger Ae1

The AE1 mutation G701D, associated with recessive distal renal tubular acidosis (dRTA), produces only minimal erythroid phenotype, reflecting erythroid-specific expression of stimulatory AE1 subunit glycophorin A (GPA). GPA transgene expression could theoretically treat recessive dRTA in patients and in mice expressing cognate Ae1 mutation G719D. However, human (h) GPA and mouse (m) Gpa amino acid sequences are widely divergent, and mGpa function in vitro has not been investigated. We therefore studied in Xenopus oocytes the effects of coexpressed mGpa and hGPA on anion transport by erythroid (e) and kidney (k) isoforms of wild-type mAe1 (meAe1, mkAe1) and of mAe1 mutant G719D. Coexpression of hGPA or mGpa enhanced the function of meAe1 and mkAe1 and rescued the nonfunctional meAe1 and mkAe1 G719D mutants through increased surface expression. Progressive N-terminal truncation studies revealed a role for meAe1 amino acids 22-28 in GPA-responsiveness of meAe1 G719D. MouseN-cyto/humanTMD and humanN-cyto/mouseTMD kAE1 chimeras were active and GPA-responsive. In contrast, whereas chimera mkAe1N-cyto/hkAE1 G701DTMD was GPA-responsive, chimera hkAE1N-cyto/mkAe1 G719DTMD was GPA-insensitive. Moreover, whereas the isolated transmembrane domain (TMD) of hAE1 G701D was GPA-responsive, that of mAe1 G719D was GPA-insensitive. Thus, mGpa increases surface expression and activity of meAe1 and mkAe1. However, the G719D mutation renders certain mAe1 mutant constructs GPA-unresponsive and highlights a role for erythroid-specific meAe1 amino acids 22-28 in GPA-responsiveness.

Mis-trafficking of bicarbonate transporters: implications to human diseasesThis paper is one of a selection of papers published in a Special Issue entitled CSBMCB 53rd Annual Meeting — Membrane Proteins in Health and Disease, and has undergone the Journal’s usual peer review process

Bicarbonate is a waste product of mitochondrial respiration and one of the main buffers in the human body. Thus, bicarbonate transporters play an essential role in maintaining acid-base balance but also during fetal development as they ensure tight regulation of cytosolic and extracellular environments. Bicarbonate transporters belong to two gene families, SLC4A and SLC26A. Proteins from these two families are widely expressed, and thus mutations in their genes result in various diseases that affect bones, pancreas, reproduction, brain, kidneys, eyes, heart, thyroid, red blood cells, and lungs. In this minireview, we discuss the current state of knowledge regarding the effect of SLC4A and SLC26A mutants, with a special emphasis on mutants that have been studied in mammalian cell lines and how they correlate with phenotypes observed in mice models.

Glyceraldehyde 3-phosphate dehydrogenase is required for band 3 (anion exchanger 1) membrane residency in the mammalian kidney

The mammalian kidney isoform of the essential chloride-bicarbonate exchanger AE1 differs from its erythrocyte counterpart, being shorter at its N terminus. It has previously been reported that the glycolytic enzyme GAPDH interacts only with erythrocyte AE1, by binding to the portion not found in the kidney isoform. (Chu H, Low PS. Biochem J 400:143–151, 2006). We have identified GAPDH as a candidate binding partner for the C terminus of both AE1 and AE2. We show that full-length AE1 and GAPDH coimmunoprecipitated from both human and rat kidney as well as from Madin-Darby canine kidney (MDCK) cells stably expressing kidney AE1, while in human liver, AE2 coprecipitated with GAPDH. ELISA and glutathione S-transferase (GST) pull-down assays using GST-tagged C-terminal AE1 fusion protein confirmed that the interaction is direct; fluorescence titration revealed saturable binding kinetics with K_d 2.3 ± 0.2 μM. Further GST precipitation assays demonstrated that the D⁹⁰²EY residues in the D⁹⁰²EYDE motif located within the C terminus of AE1 are important for GAPDH binding. In vitro GAPDH activity was unaffected by C-terminal AE1 binding, unlike in erythrocytes. Also, differently from red cell N-terminal binding, GAPDH-AE1 C-terminal binding was not disrupted by phosphorylation of AE1 in kidney AE1-expressing MDCK cells. Importantly, small interfering RNA knockdown of GAPDH in these cells resulted in significant intracellular retention of AE1, with a concomitant reduction in AE1 at the cell membrane. These results indicate differences between kidney and erythrocyte AE1/GAPDH behavior and show that in the kidney, GAPDH is required for kidney AE1 to achieve stable basolateral residency.

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

Anion exchanger 1 interacts with nephrin in podocytes

The central role of the multifunctional protein nephrin within the macromolecular complex forming the glomerular slit diaphragm is well established, but the mechanisms linking the slit diaphragm to the cytoskeleton and to the signaling pathways involved in maintaining the integrity of the glomerular filter remain incompletely understood. Here, we report that nephrin interacts with the bicarbonate/chloride transporter kidney anion exchanger 1 (kAE1), detected by yeast two-hybrid assay and confirmed by immunoprecipitation and co-localization studies. We confirmed low-level glomerular expression of kAE1 in human and mouse kidneys by immunoblotting and immunofluorescence microscopy. We observed less kAE1 in human glomeruli homozygous for the NPHS1(FinMaj) nephrin mutation, whereas kAE1 expression remained unchanged in the collecting duct. We could not detect endogenous kAE1 expression in NPHS1(FinMaj) podocytes in primary culture, but heterologous re-introduction of wild-type nephrin into these podocytes rescued kAE1 expression. In kidneys of Ae1(-/-) mice, nephrin abundance was normal but its distribution was altered along with the reported kAE1-binding protein integrin-linked kinase (ILK). Ae1(-/-) mice had increased albuminuria with glomerular enlargement, mesangial expansion, mesangiosclerosis, and expansion of the glomerular basement membrane. Glomeruli with ILK-deficient podocytes also demonstrated altered AE1 and nephrin expression, further supporting the functional interdependence of these proteins. These data suggest that the podocyte protein kAE1 interacts with nephrin and ILK to maintain the structure and function of the glomerular basement membrane.

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.

Molecular physiology and genetics of Na+-independent SLC4 anion exchangers

Plasmalemmal Cl(-)/HCO(3)(-) exchangers are encoded by the SLC4 and SLC26 gene superfamilies, and function to regulate intracellular pH, [Cl(-)] and cell volume. The Cl(-)/HCO(3)(-) exchangers of polarized epithelial cells also contribute to transepithelial secretion and reabsorption of acid-base equivalents and Cl(-). This review focuses on Na(+)-independent electroneutral Cl(-)/HCO(3)(-) exchangers of the SLC4 family. Human SLC4A1/AE1 mutations cause the familial erythroid disorders of spherocytic anemia, stomatocytic anemia and ovalocytosis. A largely discrete set of AE1 mutations causes familial distal renal tubular acidosis. The Slc4a2/Ae2(-/-) mouse dies before weaning with achlorhydria and osteopetrosis. A hypomorphic Ae2(-/-) mouse survives to exhibit male infertility with defective spermatogenesis and a syndrome resembling primary biliary cirrhosis. A human SLC4A3/AE3 polymorphism is associated with seizure disorder, and the Ae3(-/-) mouse has increased seizure susceptibility. The transport mechanism of mammalian SLC4/AE polypeptides is that of electroneutral Cl(-)/anion exchange, but trout erythroid Ae1 also mediates Cl(-) conductance. Erythroid Ae1 may mediate the DIDS-sensitive Cl(-) conductance of mammalian erythrocytes, and, with a single missense mutation, can mediate electrogenic SO(4)(2-)/Cl(-) exchange. AE1 trafficking in polarized cells is regulated by phosphorylation and by interaction with other proteins. AE2 exhibits isoform-specific patterns of acute inhibition by acidic intracellular pH and independently by acidic extracellular pH. In contrast, AE2 is activated by hypertonicity and, in a pH-independent manner, by ammonium and by hypertonicity. A growing body of structure-function and interaction data, together with emerging information about physiological function and structure, is advancing our understanding of SLC4 anion exchangers.

Kinetic evidence for modulation by glycophorin A of a conformational equilibrium between two states of band 3 (SLC4A1) bound reversibly by the competitive inhibitor DIDS

Recent evidence has suggested that erythrocytes naturally deficient in glycophorin A (GPA) have a reduced V(max) for monovalent anion exchange. Unanswered is whether miss-folding of band 3 during biosynthesis, or the absence of GPA modulation of properly folded band 3 is responsible. Here, I determine the effect of selective depletion of GPA on the kinetics of reversible binding of the competitive transport inhibitor DIDS (4,4'-diisothiocyanato-2,2'-stilbenedisulfonate) to properly folded band 3. Reversible binding of DIDS follows biphasic kinetics: a fast phase {DIDS+band 3<-->(DIDS-band 3), k(1), k(-1)} and a slower phase {(DIDS-band 3)<-->(DIDS-band 3), k(2), k(-2)}. Selective depletion of GPA was accomplished by pretreating membranes with Triton X-100, over a range where erythrocyte hemolysis is inhibited by the detergent (0% to 0.03%, v/v). Pretreatment with sublytic Triton X-100: (a) virtually completely depleted GPA, (b) did not deplete membrane-bound band 3, and (c) slowed the overall rate of reversible binding of DIDS to band 3. Data analysis and model simulation studies indicated that the decrease in the rate of binding of DIDS was due exclusively to a decrease in k(-2), with no change in the initial rate of binding. Thus, depletion of GPA does not alter the native conformation of band 3 at the DIDS binding site, but rather modulates a conformational equilibrium between two states of the binary complex formed by the competitive inhibitor DIDS, reversibly bound to properly folded band 3.

Human kidney anion exchanger 1 localisation in MDCK cells is controlled by the phosphorylation status of two critical tyrosines

An important question in renal physiology is how the alpha-intercalated cells of the kidney regulate the distribution of the basolateral kidney anion exchanger 1 (kAE1) according to systemic acid-base status. Previous work using a MDCKI model system demonstrated that kAE1 basolateral targeting requires an N-terminal determinant and a critical C-terminal tyrosine (Y904). Here, we show that the N-terminal determinant is residue Y359, because a Y359A substitution mutant was mistargeted to the apical membrane. Further determinants might exist because a range of N-terminal kAE1 truncations that contained Y359 were incorrectly targeted to the TGN. Y359 and Y904 in kAE1 are phosphorylated upon pervanadate treatment and this phosphorylation is sensitive to specific Src kinase family inhibitors. We tested a range of stimuli on this model system and only the application of high nonphysiological concentrations of extracellular bicarbonate, and to a lesser extent hypertonicity or hyperosmolarity, induced tyrosine phosphorylation of kAE1. Treatment with pervanadate caused internalisation of kAE1 from the plasma membrane, but treatment with high concentrations of bicarbonate did not, because of the hypertonicity of the solution. We propose that alpha-intercalated cells control the distribution of kAE1 by reversible phosphorylation of tyrosine residues Y359 and Y904.