Transport activity of chimaeric AE2-AE3 chloride/bicarbonate anion exchange proteins

Altered gene expression in slc4a11-/- mouse cornea highlights SLC4A11 roles

SLC4A11 is a H⁺/NH₃/water transport protein, of corneal endothelial cells. SLC4A11 mutations cause congenital hereditary endothelial dystrophy and some cases of Fuchs endothelial corneal dystrophy. To probe SLC4A11’s roles, we compared gene expression in RNA from corneas of 17-week-old slc4a11^−/− (n = 3) and slc4a11^+/+ mice (n = 3) and subjected to RNA sequencing. mRNA levels for a subset of genes were also assessed by quantitative real-time reverse transcription PCR (qRT RT-PCR). Cornea expressed 13,173 genes, which were rank-ordered for their abundance. In slc4a11^−/− corneas, 100 genes had significantly altered expression. Abundant slc14a1 expression, encoding the urea transporter UT-A, suggests a significant role in the cornea. The set of genes with altered expression was subjected to Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses, revealing that alterations clustered into extracellular region, cytoskeleton, cell adhesion and plasma membrane functions. Gene expression changes further clustered into classes (with decreasing numbers of genes): cell fate and development, extracellular matrix and cell adhesion, cytoskeleton, ion homeostasis and energy metabolism. Together these gene changes confirm earlier suggestions of a role of SLC4A11 in ion homeostasis, energy metabolism, cell adhesion, and reveal an unrecognized SLC4A11 role in cytoskeletal organization.

pH and male fertility: making sense on pH homeodynamics throughout the male reproductive tract

In the male reproductive tract, ionic equilibrium is essential to maintain normal spermatozoa production and, hence, the reproductive potential. Among the several ions, HCO3- and H+ have a central role, mainly due to their role on pH homeostasis. In the male reproductive tract, the major players in pH regulation and homeodynamics are carbonic anhydrases (CAs), HCO3- membrane transporters (solute carrier 4-SLC4 and solute carrier 26-SLC26 family transporters), Na+-H+ exchangers (NHEs), monocarboxylate transporters (MCTs) and voltage-gated proton channels (Hv1). CAs and these membrane transporters are widely distributed throughout the male reproductive tract, where they play essential roles in the ionic balance of tubular fluids. CAs are the enzymes responsible for the production of HCO3- which is then transported by membrane transporters to ensure the maturation, storage, and capacitation of the spermatozoa. The transport of H+ is carried out by NHEs, Hv1, and MCTs and is essential for the electrochemical balance and for the maintenance of the pH within the physiological limits along the male reproductive tract. Alterations in HCO3- production and transport of ions have been associated with some male reproductive dysfunctions. Herein, we present an up-to-date review on the distribution and role of the main intervenient on pH homeodynamics in the fluids throughout the male reproductive tract. In addition, we discuss their relevance for the establishment of the male reproductive potential.

Molecular cloning and functional characterization of zebrafish Slc4a3/Ae3 anion exchanger

The zebrafish genome encodes two slc4a1 genes, one expressed in erythroid tissues and the other in the HR (H(+)-ATPase-rich) type of embryonic skin ionocytes, and two slc4a2 genes, one in proximal pronephric duct and the other in several extrarenal tissues of the embryo. We now report cDNA cloning and functional characterization of zebrafish slc4a3/ae3 gene products. The single ae3 gene on chromosome 9 generates at least two low-abundance ae3 transcripts differing only in their 5'-untranslated regions and encoding a single definitive Ae3 polypeptide of 1170 amino acids. The 7 kb upstream of the apparent initiator Met in ae3 exon 3 comprises multiple diverse, mobile repeat elements which disrupt and appear to truncate the Ae3 N-terminal amino acid sequence that would otherwise align with brain Ae3 of other species. Embryonic ae3 mRNA expression was detected by whole mount in situ hybridization only in fin buds at 24-72 hpf, but was detectable by RT-PCR across a range of embryonic and adult tissues. Epitope-tagged Ae3 polypeptide was expressed at or near the surface of Xenopus oocytes, and mediated low rates of DIDS-sensitive (36)Cl(-)/Cl(-) exchange in influx and efflux assays. As previously reported for Ae2 polypeptides, (36)Cl(-) transport by Ae3 was inhibited by both extracellular and intracellular acidic pH, and stimulated by alkaline pH. However, zebrafish Ae3 differed from Ae2 polypeptides in its insensitivity to NH4Cl and to hypertonicity. We conclude that multiple repeat elements have disrupted the 5'-end of the zebrafish ae3 gene, associated with N-terminal truncation of the protein and reduced anion transport activity.

Structure, function, and regulation of the SLC4 NBCe1 transporter and its role in causing proximal renal tubular acidosis

PURPOSE OF REVIEW

There has been significant progress in our understanding of the structural and functional properties and regulation of the electrogenic sodium bicarbonate cotansporter NBCe1, a membrane transporter that plays a key role in renal acid-base physiology. The NBCe1 variant NBCe1-A mediates basolateral electrogenic sodium-base transport in the proximal tubule and is critically required for transepithelial bicarbonate absorption. Mutations in NBCe1 cause autosomal recessive proximal renal tubular acidosis (pRTA). The review summarizes recent advances in this area.

RECENT FINDINGS

A topological model of NBCe1 has been established that provides a foundation for future structure-functional studies of the transporter. Critical residues and regions have been identified in NBCe1 that play key roles in its structure, function (substrate transport, electrogenicity) and regulation. The mechanisms of how NBCe1 mutations cause pRTA have also recently been elucidated.

SUMMARY

Given the important role of proximal tubule transepithelial bicarbonate absorption in systemic acid-base balance, a clear understanding of the structure-functional properties of NBCe1 is a prerequisite for elucidating the mechanisms of defective transepithelial bicarbonate transport in pRTA.

NBCe1 as a model carrier for understanding the structure-function properties of Na⁺ -coupled SLC4 transporters in health and disease

SLC4 transporters are membrane proteins that in general mediate the coupled transport of bicarbonate (carbonate) and share amino acid sequence homology. These proteins differ as to whether they also transport Na(+) and/or Cl(-), in addition to their charge transport stoichiometry, membrane targeting, substrate affinities, developmental expression, regulatory motifs, and protein-protein interactions. These differences account in part for the fact that functionally, SLC4 transporters have various physiological roles in mammals including transepithelial bicarbonate transport, intracellular pH regulation, transport of Na(+) and/or Cl(-), and possibly water. Bicarbonate transport is not unique to the SLC4 family since the structurally unrelated SLC26 family has at least three proteins that mediate anion exchange. The present review focuses on the first of the sodium-dependent SLC4 transporters that was identified whose structure has been most extensively studied: the electrogenic Na(+)-base cotransporter NBCe1. Mutations in NBCe1 cause proximal renal tubular acidosis (pRTA) with neurologic and ophthalmologic extrarenal manifestations. Recent studies have characterized the important structure-function properties of the transporter and how they are perturbed as a result of mutations that cause pRTA. It has become increasingly apparent that the structure of NBCe1 differs in several key features from the SLC4 Cl(-)-HCO3 (-) exchanger AE1 whose structural properties have been well-studied. In this review, the structure-function properties and regulation of NBCe1 will be highlighted, and its role in health and disease will be reviewed in detail.

Proximal renal tubular acidosis mediated by mutations in NBCe1-A: unraveling the transporter's structure-functional properties

NBCe1 belongs to the SLC4 family of base transporting membrane proteins that plays a significant role in renal, extrarenal, and systemic acid-base homeostasis. Recent progress has been made in characterizing the structure-function properties of NBCe1 (encoded by the SLC4A4 gene), and those factors that regulate its function. In the kidney, the NBCe1-A variant that is expressed on the basolateral membrane of proximal tubule is the key transporter responsible for overall transepithelial bicarbonate absorption in this nephron segment. NBCe1 mutations impair transepithelial bicarbonate absorption causing the syndrome of proximal renal tubular acidosis (pRTA). Studies of naturally occurring NBCe1 mutant proteins in heterologous expression systems have been very helpful in elucidation the structure-functional properties of the transporter. NBCe1 mutations are now known to cause pRTA by various mechanisms including the alteration of the transporter function (substrate ion interaction, electrogenicity), abnormal processing to the plasma membrane, and a perturbation in its structural properties. The elucidation of how NBCe1 mutations cause pRTA in addition to the recent studies which have provided further insight into the topology of the transporter have played an important role in uncovering its critically important structural-function properties.

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.

Boric acid increases the expression levels of human anion exchanger genes SLC4A2 and SLC4A3

Boron is an important micronutrient in plants and animals. The role of boron in living systems includes coordinated regulation of gene expression, growth and proliferation of higher plants and animals. There are several well-defined genes associated with boron transportation and tolerance in plants and these genes show close homology with human anion exchanger genes. Mutation of these genes also characterizes some genetic disorders. We investigated the toxic effects of boric acid on HEK293 cells and mRNA expression of anion exchanger (SLC4A1, SLC4A2 and SLC4A3) genes. Cytotoxicity of boric acid at different concentrations was tested by using the methylthiazolyldiphenyl-tetrazolium bromide assay. Gene expression profiles were examined using quantitative real-time PCR. In the HEK293 cells, the nontoxic upper concentration of boric acid was 250 μM; more than 500 μM caused cytotoxicity. The 250 μM boric acid concentration increased gene expression level of SLC4A2 up to 8.6-fold and SLC4A3 up to 2.6-fold, after 36-h incubation. There was no significant effect of boric acid on SLC4A1 mRNA expression levels.

SLC4A transporters

SLC4A gene family proteins include bicarbonate transporters that move HCO(3)(-) across the plasma membrane and regulate intracellular pH and transepithelial movement of acid-base equivalents. These transporters are Cl/HCO(3) exchangers, electrogenic Na/HCO(3) cotransporters, electroneutral Na/HCO(3) cotransporters, and Na(+)-driven Cl/HCO(3) exchanger. Studies of the bicarbonate transporters in vitro and in vivo have demonstrated their physiological importance for acid-base homeostasis at the cellular and systemic levels. Recent advances in structure/function analysis have also provided valuable information on domains or motifs critical for regulation, ion translocation, and protein topology. This chapter focuses on the molecular mechanisms of ion transport along with associated structural aspects from mutagenesis of particular residues and from chimeric constructs. Structure/function studies have helped to understand the mechanism by which ion substrates are moved via the transporters. This chapter also describes some insights into the structure of SLC4A1 (AE1) and SLC4A4 (NBCe1) transporters. Finally, as some SLC4A transporters exist in concert with other proteins in the cells, the structural features associated with protein-protein interactions are briefly discussed.

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.

SLC4A11 mutations in Fuchs endothelial corneal dystrophy

The endothelial (posterior) corneal dystrophies, which result from primary endothelial dysfunction, include Fuchs endothelial corneal dystrophy (FECD), posterior polymorphous corneal dystrophy (PPCD) and congenital hereditary endothelial dystrophy (CHED). Mutations in SLC4A11 gene have been recently identified in patients with recessive CHED (CHED2). In this study, we show that heterozygous mutations in the SLC4A11 gene also cause late-onset FECD. Four heterozygous mutations [three missense mutations (E399K, G709E and T754M) and one deletion mutation (c.99-100delTC)] absent in ethnically matched controls were identified in a screen of 89 FECD patients. Missense mutations involved amino acid residues showing high interspecies conservation, indicating that mutations at these sites would be deleterious. Accordingly, immunoblot analysis, biochemical assay of cell surface localization and confocal immunolocalization showed that missense proteins encoded by the mutants were defective in localization to the cell surface. Our data suggests that SLC4A11 haploinsufficiency and gradual accumulation of the aberrant misfolded protein may play a role in FECD pathology and that reduced levels of SLC4A11 influence the long-term viability of the neural crest derived corneal endothelial cells.

Acute regulation of mouse AE2 anion exchanger requires isoform-specific amino acid residues from most of the transmembrane domain

The widely expressed anion exchanger polypeptide AE2/SLC4A2 is acutely inhibited by acidic intracellular (pH(i)), by acidic extracellular pH (pH(o)), and by the calmodulin inhibitor, calmidazolium, whereas it is acutely activated by NH(4)(+). The homologous erythroid/kidney AE1/SLC4A1 polypeptide is insensitive to these regulators. Each of these AE2 regulatory responses requires the presence of AE2's C-terminal transmembrane domain (TMD). We have now measured (36)Cl(-) efflux from Xenopus oocytes expressing bi- or tripartite AE2-AE1 chimeras to define TMD subregions in which AE2-specific sequences contribute to acute regulation. The chimeric AE polypeptides were all functional at pH(o) 7.4, with the sole exception of AE2((1-920))/AE1((613-811))/AE2((1120-1237)). Reciprocal exchanges of the large third extracellular loops were without effect. AE2 regulation by pH(i), pH(o) and NH(4)(+) was retained after substitution of C-terminal AE2 amino acids 1120-1237 (including the putative second re-entrant loop, two TM spans and the cytoplasmic tail) with the corresponding AE1 sequence. In contrast, the presence of this AE2 C-terminal sequence was both necessary and sufficient for inhibition by calmidazolium. All other tested TMD substitutions abolished AE2 pH(i) sensitivity, abolished or severely attenuated sensitivity to pH(o) and removed sensitivity to NH(4)(+). Loss of AE2 pH(i) sensitivity was not rescued by co-expression of a complementary AE2 sequence within separate full-length chimeras or AE2 subdomains. Thus, normal regulation of AE2 by pH and other ligands requires AE2-specific sequence from most regions of the AE2 TMD, with the exceptions of the third extracellular loop and a short C-terminal sequence. We conclude that the individual TMD amino acid residues previously identified as influencing acute regulation of AE2 exert that influence within a regulatory structure requiring essential contributions from multiple regions of the AE2 TMD.

Interactions of transmembrane carbonic anhydrase, CAIX, with bicarbonate transporters

Association of some plasma membrane bicarbonate transporters with carbonic anhydrase enzymes forms a bicarbonate transport metabolon to facilitate metabolic CO(2)-HCO(3)(-) conversions and coupled HCO(3)(-) transport. The transmembrane carbonic anhydrase, CAIX, with its extracellular catalytic site, is highly expressed in parietal and other cells of gastric mucosa, suggesting a role in acid secretion. We examined in transfected HEK293 cells the functional and physical interactions between CAIX and the parietal cell Cl(-)/HCO(3)(-) exchanger AE2 or the putative Cl(-)/HCO(3)(-) exchanger SLC26A7. Coexpression of CAIX increased AE2 transport activity by 28 +/- 7% and also activated transport mediated by AE1 and AE3 (32 +/- 10 and 37 +/- 9%, respectively). In contrast, despite a transport rate comparable to that of AE3, coexpressed CAIX did not alter transport associated with SLC26A7. The CAIX-associated increase of AE2 activity did not result from altered AE2 expression or cell surface processing. CAIX was coimmunoprecipitated with the coexpressed SLC4 polypeptides AE1, AE2, and AE3, but not with SLC26A7. GST pull-down assays with a series of domain-deleted forms of CAIX revealed that the catalytic domain of CAIX mediated interaction with AE2. AE2 and CAIX colocalized in human gastric mucosa, as indicated by coimmunofluorescence. This is the first example of a functional and physical interaction between a bicarbonate transporter and a transmembrane carbonic anhydrase. We conclude that CAIX can bind to some Cl(-)/HCO(3)(-) exchangers to form a bicarbonate transport metabolon.

Interaction of integrin-linked kinase with the kidney chloride/bicarbonate exchanger, kAE1

Kidney anion exchanger 1 (kAE1) mediates chloride/bicarbonate exchange at the basolateral membrane of kidney alpha-intercalated cells, thereby facilitating bicarbonate reabsorption into the blood. Human kAE1 lacks the N-terminal 65 residues of the erythroid form (AE1, band 3), which are essential for binding of cytoskeletal and cytosolic proteins. Yeast two-hybrid screening identified integrin-linked kinase (ILK), a serine/threonine kinase, and an actin-binding protein as an interacting partner with the N-terminal domain of kAE1. Interaction between kAE1 and ILK was confirmed in co-expression experiments in HEK 293 cells and is mediated by a previously unidentified calponin homology domain in the kAE1 N-terminal region. The calponin homology domain of kAE1 binds the C-terminal catalytic domain of ILK to enhance association of kAE1 with the actin cytoskeleton. Overexpression of ILK increased kAE1 levels at the cell surface as shown by flow cytometry, cell surface biotinylation, and anion transport activity assays. Pulse-chase experiments revealed that ILK associates with kAE1 early in biosynthesis, likely in the endoplasmic reticulum. ILK co-localized with kAE1 at the basolateral membrane of polarized Madin-Darby canine kidney cells and in alpha-intercalated cells of human kidneys. Taken together these results suggest that ILK and kAE1 traffic together from the endoplasmic reticulum to the basolateral membrane. ILK may provide a linkage between kAE1 and the underlying actin cytoskeleton to stabilize kAE1 at the basolateral membrane, resulting in higher levels of cell surface expression.

Carbonic anhydrase inhibition prevents and reverts cardiomyocyte hypertrophy

Hypertrophic cardiomyocyte growth contributes substantially to the progression of heart failure. Activation of the plasma membrane Na+-H+ exchanger (NHE1) and Cl- -HCO3- exchanger (AE3) has emerged as a central point in the hypertrophic cascade. Both NHE1 and AE3 bind carbonic anhydrase (CA), which activates their transport flux, by providing H+ and HCO3-, their respective transport substrates. We examined the contribution of CA activity to the hypertrophic response of cultured neonatal and adult rodent cardiomyocytes. Phenylephrine (PE) increased cell size by 37 +/- 2% and increased expression of the hypertrophic marker, atrial natriuretic factor mRNA, twofold in cultured neonatal rat cardiomyocytes. Cell size was also increased in adult cardiomyocytes subjected to angiotensin II or PE treatment. These effects were associated with increased expression of cytosolic CAII protein and the membrane-anchored isoform, CAIV. The membrane-permeant CA inhibitor, 6-ethoxyzolamide (ETZ), both prevented and reversed PE-induced hypertrophy in a concentration-dependent manner in neonate cardiomyocytes (IC50=18 microm). ETZ and the related CA inhibitor methazolamide prevented hypertrophy in adult cardiomyocytes. In addition, ETZ inhibited transport activity of NHE1 and the AE isoform, AE3, with respective EC50 values of 1.2 +/- 0.3 microm and 2.7 +/- 0.3 microm. PE significantly increased neonatal cardiomyocyte Ca2+ transient frequency from 0.33 +/- 0.4 Hz to 0.77 +/- 0.04 Hz following 24 h treatment; these Ca2+ -handling abnormalities were completely prevented by ETZ (0.28 +/- 0.07 Hz). Our study demonstrates a novel role for CA in mediating the hypertrophic response of cardiac myocytes to PE and suggests that CA inhibition represents an effective therapeutic approach towards mitigation of the hypertrophic phenotype.

The electrogenicity of the rat sodium-bicarbonate cotransporter NBCe1 requires interactions among transmembrane segments of the transporter

The electrogenic Na+-HCO3- cotransporter (NBCe1) plays a central role in intracellular pH (pHi) regulation as well as HCO3- secretion by pancreatic ducts and HCO3- reabsorption by renal proximal tubules. To understand the structural requirements for the electrogenicity of NBCe1, we constructed chimeras of NBCe1-A and the electroneutral NBCn1-B, and used two-electrode voltage clamp to measure electrogenic transporter current in Xenopus oocytes exposed to 5% CO2-26 mm HCO3- (pH 7.40). The chimera consisting of NBCe1-A (i.e. NBCe1-A 'background') with the cytoplasmic N-terminal domain (Nt) of NBCn1-B had a reversal potential of -156.3 mV (compared with a membrane potential Vm of -43.1 mV in a HCO3(-)-free solution) and a slope conductance of 3.0 microS (compared with 12.5 microS for NBCe1-A). Also electrogenic were chimeras with an NBCe1-A background but with NBCn1-B contributing the extracellular loop (L) between transmembrane segment (TM) 5 and 6 (-140.9 mV/11.1 microS), the cytoplasmic C-terminal domain (Ct; -123.8 mV/9.7 microS) or Nt + L + Ct (-120.9 mV/3.7 microS). Reciprocal chimeras (with an NBCn1 background but with NBCe1 contributing Nt, L, Ct or Nt + L + Ct) produced no measurable electrogenic transporter currents in the presence of CO2-HCO3-. pHi recovered from an acid load, but without the negative shift of Vm that is characteristic of electrogenic Na+-HCO3- cotransporters. Thus, these chimeras were electroneutral, as were two others consisting of NBCe1(Nt-L)/NBCn1(TM6-Ct) and NBCn1(Nt-L)/NBCe1(TM6-Ct). We propose that the electrogenicity of NBCe1 requires interactions between TM1-5 and TM6-13.

SLC4 base (HCO3 -, CO3 2-) transporters: classification, function, structure, genetic diseases, and knockout models

In prokaryotic and eukaryotic organisms, biochemical and physiological processes are sensitive to changes in H(+) activity. For these processes to function optimally, a variety of proteins have evolved that transport H(+)/base equivalents across cell and organelle membranes, thereby maintaining the pH of various intracellular and extracellular compartments within specific limits. The SLC4 family of base (HCO(3)(-), CO(3)(2(-))) transport proteins plays an essential role in mediating Na(+)- and/or Cl(-)-dependent base transport in various tissues and cell types in mammals. In addition to pH regulation, specific members of this family also contribute to vectorial transepithelial base transport in several organ systems including the kidney, pancreas, and eye. The importance of these transporters in mammalian cell biology is highlighted by the phenotypic abnormalities resulting from spontaneous SLC4 mutations in humans and targeted deletions in murine knockout models. This review focuses on recent advances in our understanding of the molecular organization and functional properties of SLC4 transporters and their role in disease.

Involvement of AE3 isoform of Na(+)-independent Cl(-)/HCO(3)(-) exchanger in myocardial pH(i) recovery from intracellular alkalization

Myocardial pH(i) recovery from intracellular alkalization results in part from the acid load (-J(H+)) carried by Cl(-)/HCO(3)(-) anion-exchangers (AE). Three AE isoforms, AE1, AE2 and AE3, have been identified in cardiac membranes, but the function of each isoform on pH(i) homeostasis is still under investigation. This work explored, by means of specific antibodies, the role of AE3 isoform in myocardial pH(i) regulation. We developed rabbit polyclonal antibodies against the extracellular "loops": one connecting the fifth to sixth and the other one the seventh to eighth transmembrane domains (loops 3 and 4, respectively) of AE3, and their effect on pH(i) regulation was studied in rat papillary muscles. The anti-AE3 loop 3 antibody decreased -J(H+) in response to myocardial alkalization (from a mean control value of 1.06+/-0.26 to 0.32+/-0.13 mmol/L/min, n=7, P<0.05) without affecting the baseline pH(i) (7.22+/-0.03 vs. 7.21+/-0.04). The anti-AE3 loop 4 antibody did not modify either pH(i) recovery or baseline pH(i). Under control conditions, endothelin-1 (ET-1) increased -J(H+) in response to myocardial alkalization from 1.30+/-0.18 to 2.01+/-0.33 mmol/L /min (n=5, P<0.05). This effect of ET-1 on -J(H+) was abolished by anti-AE3 loop 3 antibody. In addition, the MgATP-induced stimulation of AE activity was reduced by the anti-AE3 loop 3 antibody. These data support the key role of the AE3 isoform in myocardial pH(i) recovery from alkaline loads and also in the stimulatory effect of ET-1 on AE activity. To a lesser extent, it may also contribute to the effect of MgATP on pH(i).

Cisplatin Carbonato Complexes. Implications for Uptake, Antitumor Properties, and Toxicity

The reaction of aquated cisplatin with carbonate which is present in culture media and blood is described. The first formed complex is a monochloro monocarbonato species, which upon continued exposure to carbonate slowly forms a biscarbonato complex. The formation of carbonato species under conditions that simulate therapy may have important implications for uptake, antitumor properties, and toxicity of cisplatin.

Shared apical sorting of anion exchanger isoforms AE2a, AE2b1, and AE2b2 in primary hepatocytes

AE2 (SLC4A2) is the member of the Na(+)-independent anion exchanger (AE) family putatively involved in the secretion of bicarbonate to bile. In humans, three variants of AE2 mRNA have been described: the full-length transcript AE2a (expressed from the upstream promoter in most tissues), and alternative transcripts AE2b(1) and AE2b(2) (driven from alternate promoter sequences in a tissue-restricted manner, mainly in liver and kidney). These transcripts would result in AE protein isoforms with short N-terminal differences. To ascertain their translation, functionality, and membrane sorting, we constructed expression vectors encoding each AE2 isoform fused to GFP at the C-terminus. Transfected HEK293 cells showed expression of functional GFP-tagged AE2 proteins, all three isoforms displaying comparable AE activities. Primary rat hepatocytes transfected with expression vectors and repolarized in a collagen-sandwich configuration showed a microtubule-dependent apical sorting of each AE2 isoform. This shared apical sorting is liver-cell specific, as sorting of AE2 isoforms was basolateral in control experiments on polarized kidney MDCK cells. Hepatocytic apical targeting of AE2 isoforms suggests that they all may participate in the canalicular secretion of bicarbonate to bile.

Regulation of the human NBC3 Na+/HCO3- cotransporter by carbonic anhydrase II and PKA

Human NBC3 is an electroneutral Na(+)/HCO(3)(-) cotransporter expressed in heart, skeletal muscle, and kidney in which it plays an important role in HCO(3)(-) metabolism. Cytosolic enzyme carbonic anhydrase II (CAII) catalyzes the reaction CO(2) + H(2)O left arrow over right arrow HCO(3)(-) + H(+) in many tissues. We investigated whether NBC3, like some Cl(-)/HCO(3)(-) exchange proteins, could bind CAII and whether PKA could regulate NBC3 activity through modulation of CAII binding. CAII bound the COOH-terminal domain of NBC3 (NBC3Ct) with K(d) = 101 nM; the interaction was stronger at acid pH. Cotransfection of HEK-293 cells with NBC3 and CAII recruited CAII to the plasma membrane. Mutagenesis of consensus CAII binding sites revealed that the D1135-D1136 region of NBC3 is essential for CAII/NBC3 interaction and for optimal function, because the NBC3 D1135N/D1136N retained only 29 +/- 22% of wild-type activity. Coexpression of the functionally dominant-negative CAII mutant V143Y with NBC3 or addition of 100 microM 8-bromoadenosine to NBC3 transfected cells reduced intracellular pH (pH(i)) recovery rate by 31 +/- 3, or 38 +/- 7%, respectively, relative to untreated NBC3 transfected cells. The effects were additive, together decreasing the pH(i) recovery rate by 69 +/- 12%, suggesting that PKA reduces transport activity by a mechanism independently of CAII. Measurements of PKA-dependent phosphorylation by mass spectroscopy and labeling with [gamma-(32)P]ATP showed that NBC3Ct was not a PKA substrate. These results demonstrate that NBC3 and CAII interact to maximize the HCO(3)(-) transport rate. Although PKA decreased NBC3 transport activity, it did so independently of the NBC3/CAII interaction and did not involve phosphorylation of NBC3Ct.

Direct extracellular interaction between carbonic anhydrase IV and the human NBC1 sodium/bicarbonate co-transporter

Sodium/bicarbonate co-transporters (NBC) are crucial in the regulation of intracellular pH (pH(i)) and HCO(3)(-) metabolism. Electrogenic NBC1 catalyzes HCO(3)(-) fluxes in mammalian kidney, pancreas, and heart cells. Carbonic anhydrase IV (CAIV), which is also present in these tissues, is glycosylphosphatidyl inositol-anchored to the outer surface of the plasma membrane where it catalyzes the hydration-dehydration of CO(2)/HCO(3)(-). The physical and functional interactions of CAIV and NBC1 were investigated. NBC1 activity was measured by changes of pH(i) in NBC1-transfected HEK293 cells subjected to acid loads. Cotransfection of CAIV with NBC1 increased the rate of pH(i) recovery by 44 +/- 3%, as compared to NBC1-alone. In contrast, CAIV did not increase the functional activity of G767T-NBC1 (mutated on the fourth extracellular loop (EC4) of NBC1), and G767T-NBC1, unlike wild-type NBC1, did not interact with CAIV in glutathione-S-transferase pull-down assays. This indicates that G767 of NBC1 is directly involved in CAIV interaction. NBC1-mediated pH(i) recovery rate after acid load was inhibited by 40 +/- 7% when coexpressed with the inactive human CAII mutant, V143Y. V143Y CAII competes with endogenous CAII for interaction with NBC1 at the inner surface of the plasma membrane, which indicates that NBC1/CAII interaction is needed for full pH(i) recovery activity. We conclude that CAIV binds EC4 of NBC1, and this interaction is essential for full NBC1 activity. The tethering of CAII and CAIV close to the NBC1 HCO(3)(-) transport site maximizes the transmembrane HCO(3)(-) gradient local to NBC1 and thereby activates the transport rate.