Functional characterization and regulation by pH of murine AE2 anion exchanger expressed in Xenopus oocytes

Structural and functional insights into the lipid regulation of human anion exchanger 2

Anion exchanger 2 (AE2) is an electroneutral Na+-independent Cl-/HCO3- exchanger belongs to the SLC4 transporter family. The widely expressed AE2 participates in a variety of physiological processes, including transepithelial acid-base secretion and osteoclastogenesis. Both the transmembrane domains (TMDs) and the N-terminal cytoplasmic domain (NTD) are involved in regulation of AE2 activity. However, the regulatory mechanism remains unclear. Here, we report a 3.2 Å cryo-EM structure of the AE2 TMDs in complex with PIP2 and a 3.3 Å full-length mutant AE2 structure in the resting state without PIP2. We demonstrate that PIP2 at the TMD dimer interface is involved in the substrate exchange process. Mutation in the PIP2 binding site leads to the displacement of TM7 and further stabilizes the interaction between the TMD and the NTD. Reduced substrate transport activity and conformation similar to AE2 in acidic pH indicating the central contribution of PIP2 to the function of AE2.

The structural basis of the pH-homeostasis mediated by the Cl-/HCO3- exchanger, AE2

The cell maintains its intracellular pH in a narrow physiological range and disrupting the pH-homeostasis could cause dysfunctional metabolic states. Anion exchanger 2 (AE2) works at high cellular pH to catalyze the exchange between the intracellular HCO₃^- and extracellular Cl^-, thereby maintaining the pH-homeostasis. Here, we determine the cryo-EM structures of human AE2 in five major operating states and one transitional hybrid state. Among those states, the AE2 shows the inward-facing, outward-facing, and intermediate conformations, as well as the substrate-binding pockets at two sides of the cell membrane. Furthermore, critical structural features were identified showing an interlock mechanism for interactions among the cytoplasmic N-terminal domain and the transmembrane domain and the self-inhibitory effect of the C-terminal loop. The structural and cell-based functional assay collectively demonstrate the dynamic process of the anion exchange across membranes and provide the structural basis for the pH-sensitive pH-rebalancing activity of AE2.

Oxalate Flux Across the Intestine: Contributions from Membrane Transporters

Epithelial oxalate transport is fundamental to the role occupied by the gastrointestinal (GI) tract in oxalate homeostasis. The absorption of dietary oxalate, together with its secretion into the intestine, and degradation by the gut microbiota, can all influence the excretion of this nonfunctional terminal metabolite in the urine. Knowledge of the transport mechanisms is relevant to understanding the pathophysiology of hyperoxaluria, a risk factor in kidney stone formation, for which the intestine also offers a potential means of treatment. The following discussion presents an expansive review of intestinal oxalate transport. We begin with an overview of the fate of oxalate, focusing on the sources, rates, and locations of absorption and secretion along the GI tract. We then consider the mechanisms and pathways of transport across the epithelial barrier, discussing the transcellular, and paracellular components. There is an emphasis on the membrane-bound anion transporters, in particular, those belonging to the large multifunctional Slc26 gene family, many of which are expressed throughout the GI tract, and we summarize what is currently known about their participation in oxalate transport. In the final section, we examine the physiological stimuli proposed to be involved in regulating some of these pathways, encompassing intestinal adaptations in response to chronic kidney disease, metabolic acid-base disorders, obesity, and following gastric bypass surgery. There is also an update on research into the probiotic, Oxalobacter formigenes, and the basis of its unique interaction with the gut epithelium. © 2021 American Physiological Society. Compr Physiol 11:1-41, 2021.

OEsophageal Ion Transport Mechanisms and Significance Under Pathological Conditions

Ion transporters play an important role in several physiological functions, such as cell volume regulation, pH homeostasis and secretion. In the oesophagus, ion transport proteins are part of the epithelial resistance, a mechanism which protects the oesophagus against reflux-induced damage. A change in the function or expression of ion transporters has significance in the development or neoplastic progression of Barrett’s oesophagus (BO). In this review, we discuss the physiological and pathophysiological roles of ion transporters in the oesophagus, highlighting transport proteins which serve as therapeutic targets or prognostic markers in eosinophilic oesophagitis, BO and esophageal cancer. We believe that this review highlights important relationships which might contribute to a better understanding of the pathomechanisms of esophageal diseases.

Carbonic anhydrase IX and acid transport in cancer

Alterations in tumour metabolism and acid/base regulation result in the formation of a hostile environment, which fosters tumour growth and metastasis. Acid/base homoeostasis in cancer cells is governed by the concerted interplay between carbonic anhydrases (CAs) and various transport proteins, which either mediate proton extrusion or the shuttling of acid/base equivalents, such as bicarbonate and lactate, across the cell membrane. Accumulating evidence suggests that some of these transporters interact both directly and functionally with CAIX to form a protein complex coined the 'transport metabolon'. Transport metabolons formed between bicarbonate transporters and CAIX require CA catalytic activity and have a function in cancer cell migration and invasion. Another type of transport metabolon is formed by CAIX and monocarboxylate transporters. In this complex, CAIX functions as a proton antenna for the transporter, which drives the export of lactate and protons from the cell. Since CAIX is almost exclusively expressed in cancer cells, these transport metabolons might serve as promising targets to interfere with tumour pH regulation and energy metabolism. This review provides an overview of the current state of research on the function of CAIX in tumour acid/base transport and discusses how CAIX transport metabolons could be exploited in modern cancer therapy.

An extracellular acidic cleft confers profound H+-sensitivity to epithelial sodium channels containing the δ-subunit in Xenopus laevis

The limited sodium availability of freshwater and terrestrial environments was a major physiological challenge during vertebrate evolution. The epithelial sodium channel (ENaC) is present in the apical membrane of sodium-absorbing vertebrate epithelia and evolved as part of a machinery for efficient sodium conservation. ENaC belongs to the degenerin/ENaC protein family and is the only member that opens without an external stimulus. We hypothesized that ENaC evolved from a proton-activated sodium channel present in ionocytes of freshwater vertebrates and therefore investigated whether such ancestral traits are present in ENaC isoforms of the aquatic pipid frog Xenopus laevis Using whole-cell and single-channel electrophysiology of Xenopus oocytes expressing ENaC isoforms assembled from αβγ- or δβγ-subunit combinations, we demonstrate that Xenopus δβγ-ENaC is profoundly activated by extracellular acidification within biologically relevant ranges (pH 8.0-6.0). This effect was not observed in Xenopus αβγ-ENaC or human ENaC orthologs. We show that protons interfere with allosteric ENaC inhibition by extracellular sodium ions, thereby increasing the probability of channel opening. Using homology modeling of ENaC structure and site-directed mutagenesis, we identified a cleft region within the extracellular loop of the δ-subunit that contains several acidic amino acid residues that confer proton-sensitivity and enable allosteric inhibition by extracellular sodium ions. We propose that Xenopus δβγ-ENaC can serve as a model for investigating ENaC transformation from a proton-activated toward a constitutively-active ion channel. Such transformation might have occurred during the evolution of tetrapod vertebrates to enable bulk sodium absorption during the water-to-land transition.

CK2 is a key regulator of SLC4A2-mediated Cl-/HCO3- exchange in human airway epithelia

Transepithelial bicarbonate secretion by human airway submucosal glands and surface epithelial cells is crucial to maintain the pH-sensitive innate defence mechanisms of the lung. cAMP agonists stimulate HCO₃^- secretion via coordinated increases in basolateral HCO₃^- influx and accumulation, as well as CFTR-dependent HCO₃^- efflux at the luminal membrane of airway epithelial cells. Here, we investigated the regulation of a basolateral located, DIDS-sensitive, Cl^-/HCO₃^- exchanger, anion exchanger 2 (AE2; SLC4A2) which is postulated to act as an acid loader, and therefore potential regulator of HCO₃^- secretion, in human airway epithelial cells. Using intracellular pH measurements performed on Calu-3 cells, we demonstrate that the activity of the basolateral Cl^-/HCO₃^- exchanger was significantly downregulated by cAMP agonists, via a PKA-independent mechanism and also required Ca²⁺ and calmodulin under resting conditions. AE2 contains potential phosphorylation sites by a calmodulin substrate, protein kinase CK2, and we demonstrated that AE2 activity was reduced in the presence of CK2 inhibition. Moreover, CK2 inhibition abolished the activity of AE2 in primary human nasal epithelia. Studies performed on mouse AE2 transfected into HEK-293T cells confirmed almost identical Ca²⁺/calmodulin and CK2 regulation to that observed in Calu-3 and primary human nasal cells. Furthermore, mouse AE2 activity was reduced by genetic knockout of CK2, an effect which was rescued by exogenous CK2 expression. Together, these findings are the first to demonstrate that CK2 is a key regulator of Cl^--dependent HCO₃^- export at the serosal membrane of human airway epithelial cells.

A mathematical model of osteoclast acidification during bone resorption

Bone resorption by osteoclasts occurs through the creation of a sealed extracellular compartment (ECC), or pit, adjacent to the bone that is subsequently acidified through a complex biological process. The low pH of the pit dissolves the bone mineral and activates acid proteases that further break down the bone matrix. There are many ion channels, transporters, and soluble proteins involved in osteoclast mediated resorption, and in the past few years, there has been an increased understanding of the identity and properties of some key proteins such as the ClC-7 Cl^-/H⁺ antiporter and the H_V1 proton channel. Here we present a detailed mathematical model of osteoclast acidification that includes the influence of many of the key regulatory proteins. The primary enzyme responsible for acidification is the vacuolar H⁺-ATPase (V-ATPase), which pumps protons from the cytoplasm into the pit. Unlike the acidification of small lysosomes, the pit is so large that protons become depleted from the cytoplasm. Hence, proton buffering and production in the cytoplasm by carbonic anhydrase II (CAII) is potentially important for proper acidification. We employ an ordinary differential equations (ODE)-based model that accounts for the changes in ionic species in the cytoplasm and the resorptive pit. Additionally, our model tracks ionic flow between the cytoplasm and the extracellular solution surrounding the cell. Whenever possible, the properties of individual channels and transporters are calibrated based on electrophysiological measurements, and physical properties of the cell, such as buffering capacity, surface areas, and volumes, are estimated based on available data. Our model reproduces many of the experimental findings regarding the role of key proteins in the acidification process, and it allows us to estimate, among other things, number of active pumps, protons moved, and the influence of particular mutations implicated in disease.

Stromal uptake and transmission of acid is a pathway for venting cancer cell-generated acid

Proliferation and invasion of cancer cells require favorable pH, yet potentially toxic quantities of acid are produced metabolically. Membrane-bound transporters extrude acid from cancer cells, but little is known about the mechanisms that handle acid once it is released into the poorly perfused extracellular space. Here, we studied acid handling by myofibroblasts (colon cancer-derived Hs675.T, intestinal InMyoFib, embryonic colon-derived CCD-112-CoN), skin fibroblasts (NHDF-Ad), and colorectal cancer (CRC) cells (HCT116, HT29) grown in monoculture or coculture. Expression of the acid-loading transporter anion exchanger 2 (AE2) (SLC4A2 product) was detected in myofibroblasts and fibroblasts, but not in CRC cells. Compared with CRC cells, Hs675.T and InMyoFib myofibroblasts had very high capacity to absorb extracellular acid. Acid uptake into CCD-112-CoN and NHDF-Ad cells was slower and comparable to levels in CRC cells, but increased alongside SLC4A2 expression under stimulation with transforming growth factor β1 (TGFβ1), a cytokine involved in cancer-stroma interplay. Myofibroblasts and fibroblasts are connected by gap junctions formed by proteins such as connexin-43, which allows the absorbed acid load to be transmitted across the stromal syncytium. To match the stimulatory effect on acid uptake, cell-to-cell coupling in NHDF-Ad and CCD-112-CoN cells was strengthened with TGFβ1. In contrast, acid transmission was absent between CRC cells, even after treatment with TGFβ1. Thus, stromal cells have the necessary molecular apparatus for assembling an acid-venting route that can improve the flow of metabolic acid through tumors. Importantly, the activities of stromal AE2 and connexin-43 do not place an energetic burden on cancer cells, allowing resources to be diverted for other activities.

Amelogenins as potential buffers during secretory-stage amelogenesis

Amelogenins are the most abundant protein species in forming dental enamel, taken to regulate crystal shape and crystal growth. Unprotonated amelogenins can bind protons, suggesting that amelogenins could regulate the pH in enamel in situ. We hypothesized that without amelogenins the enamel would acidify unless ameloblasts were buffered by alternative ways. To investigate this, we measured the mineral and chloride content in incisor enamel of amelogenin-knockout (AmelX(-/-)) mice and determined the pH of enamel by staining with methyl-red. Ameloblasts were immunostained for anion exchanger-2 (Ae2), a transmembrane pH regulator sensitive for acid that secretes bicarbonate in exchange for chloride. The enamel of AmelX(-/-) mice was 10-fold thinner, mineralized in the secretory stage 1.8-fold more than wild-type enamel and containing less chloride (suggesting more bicarbonate secretion). Enamel of AmelX(-/-) mice stained with methyl-red contained no acidic bands in the maturation stage as seen in wild-type enamel. Secretory ameloblasts of AmelX(-/-) mice, but not wild-type mice, were immunopositive for Ae2, and stained more intensely in the maturation stage compared with wild-type mice. Exposure of AmelX(-/-) mice to fluoride enhanced the mineral content in the secretory stage, lowered chloride, and intensified Ae2 immunostaining in the enamel organ in comparison with non-fluorotic mutant teeth. The results suggest that unprotonated amelogenins may regulate the pH of forming enamel in situ. Without amelogenins, Ae2 could compensate for the pH drop associated with crystal formation.

Sodium is not required for chloride efflux via chloride/bicarbonate exchanger from rat thymic lymphocytes

Sodium-dependent Cl⁻/HCO₃⁻ exchanger acts as a chloride (Cl⁻) efflux in lymphocytes. Its functional characterization had been described when Cl⁻ efflux was measured upon substituting extracellular sodium (Na⁺) by N-methyl-D-glucamine (NMDG). For Na⁺ and Cl⁻ substitution, we have used D-mannitol or NMDG. Thymocytes of male Wistar rats aged 7–9 weeks were used and intracellular Cl⁻ was measured by spectrofluorimetry using MQAE dye in bicarbonate buffers. Chloride efflux was measured in a Cl⁻-free buffer (Cl⁻ substituted with isethionate acid) and in Na⁺ and Cl⁻-free buffer with D-mannitol or with NMDG. The data have shown that Cl⁻ efflux is mediated in the absence of Na⁺ in a solution containing D-mannitol and is inhibited by H₂DIDS. Mathematical modelling has shown that Cl⁻ efflux mathematical model parameters (relative membrane permeability, relative rate of exchanger transition, and exchanger efficacy) were the same in control and in the medium in which Na⁺ had been substituted by D-mannitol. The net Cl⁻ efflux was completely blocked in the NMDG buffer. The same blockage of Cl⁻ efflux was caused by H₂DIDS. The study results allow concluding that Na⁺ is not required for Cl⁻ efflux via Cl⁻/HCO₃⁻ exchanger. NMDG in buffers cannot be used for substituting Na⁺ because NMDG inhibits the exchanger.

A substrate access tunnel in the cytosolic domain is not an essential feature of the solute carrier 4 (SLC4) family of bicarbonate transporters

Anion exchanger 1 (AE1; Band 3; SLC4A1) is the founding member of the solute carrier 4 (SLC4) family of bicarbonate transporters that includes chloride/bicarbonate AEs and Na(+)-bicarbonate co-transporters (NBCs). These membrane proteins consist of an amino-terminal cytosolic domain involved in protein interactions and a carboxyl-terminal membrane domain that carries out the transport function. Mutation of a conserved arginine residue (R298S) in the cytosolic domain of NBCe1 (SLC4A4) is linked to proximal renal tubular acidosis and results in impaired transport function, suggesting that the cytosolic domain plays a role in substrate permeation. Introduction of single and double mutations at the equivalent arginine (Arg(283)) and at an interacting glutamate (Glu(85)) in the cytosolic domain of human AE1 (cdAE1) had no effect on the cell surface expression or the transport activity of AE1 expressed in HEK-293 cells. In addition, the membrane domain of AE1 (mdAE1) efficiently mediated anion transport. A 2.1-Å resolution crystal structure of cdΔ54AE1 (residues 55-356 of cdAE1) lacking the amino-terminal and carboxyl-terminal disordered regions, produced at physiological pH, revealed an extensive hydrogen-bonded network involving Arg(283) and Glu(85). Mutations at these residues affected the pH-dependent conformational changes and stability of cdΔ54AE1. As these structural alterations did not impair functional expression of AE1, the cytosolic and membrane domains operate independently. A substrate access tunnel within the cytosolic domain is not present in AE1 and therefore is not an essential feature of the SLC4 family of bicarbonate transporters.

Substrate specificity of the electrogenic sodium/bicarbonate cotransporter NBCe1-A (SLC4A4, variant A) from humans and rabbits

In the basolateral membrane of proximal-tubule cells, NBCe1-A (SLC4A4, variant A), operating with an apparent Na(+):HCO(3)(-) stoichiometry of 1:3, contributes to the reclamation of HCO(3)(-) from the glomerular filtrate, thereby preventing whole body acidosis. Others have reported that NBCe1-like activity in human, rabbit, and rat renal preparations is substantially influenced by lithium, sulfite, oxalate, and harmaline. These data were taken as evidence for the presence of distinct Na(+) and CO(3)(2-) binding sites in NBCe1-A, favoring a model of 1 Na(+):1 HCO(3)(-):1 CO(3)(2-). Here, we reexamine these findings by expressing human or rabbit NBCe1-A clones in Xenopus oocytes. In oocytes, NBCe1-A exhibits a 1:2 stoichiometry and could operate in one of five thermodynamically equivalent transport modes: 1) cotransport of Na(+) + 2 HCO(3)(-), 2) cotransport of Na(+) + CO(3)(2-), 3) transport of NaCO(3)(-), 4) exchange of Na(+) + HCO(3)(-) for H(+), or 5) HCO(3)(-)-activated exchange of Na(+) for 2 H(+). In contrast to the behavior of NBCe1-like activity in renal preparations, we find that cloned NBCe1-A is only slightly stimulated by Li(+), not at all influenced by sulfite or oxalate, and only weakly inhibited by harmaline. These negative data do not uniquely support any of the five models above. In addition, we find that NBCe1-A mediates a small amount of Na(+)-independent NO(3)(-) transport and that NBCe1-A is somewhat inhibited by extracellular benzamil. We suggest that the features of NBCe1-like activity in renal preparations are influenced by yet-to-be-identified renal factors. Thus the actual ionic substrates of NBCe1 remain to be identified.

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.

Substitution of transmembrane domain Cys residues alters pH(o)-sensitive anion transport by AE2/SLC4A2 anion exchanger

AE2/SLC4A2 is the most widely expressed of the Na(+)-independent SLC4 Cl(-)/HCO3 (-) exchangers and is essential for postnatal survival, but its structure remains unknown. We have generated and expressed a mouse AE2 construct devoid of transmembrane domain cysteine (Cys) residues, mAE2Cys-less, to enhance the utility of Cys-substitution mutagenesis for structural and structure-function studies of mAE2. mAE2Cys-less expressed in Xenopus oocytes exhibited partial reduction of stilbene disulfonate-sensitive anion exchange activity. This activity was independent of the mAE2 N-terminal cytosolic domain and was accompanied by near-normal surface expression, without change in K 1/2 for extracellular Cl(-). mAE2Cys-less exhibited wildtype activation of anion exchange by hypertonicity and by NH4Cl, and wildtype inhibition of anion exchange by acidic intracellular pH (pHi) in the absence of NH4 (+). However, inhibition of anion exchange by extracellular pH (pHo) exhibited an alkaline shifted pHo(50) value of at least 0.6-0.7 pH units. Although SO4 (2-) transport by mAE2Cys-less resembled wildtype mAE2 in its stimulation by acidic pHo, the absence of transmembrane domain Cys residues abrogated activation of oxalate transport by acidic pHo. The contrasting enhancement of SO4 (2-) transport by alkaline pHo in the mAE1 anion translocation pathway mutant E699Q (Am J Physiol Cell Physiol 295: C302) was phenocopied by the corresponding mutant E1007Q in both AE2 and AE2Cys-less. However, the absence of transmembrane domain Cys residues exacerbated the reduced basal anion transport function exhibited by this and other missense substitutions at AE2 residue E1007. AE2Cys-less will be a valuable experimental tool for structure-function studies of the SLC4 gene family, but its utility for studies of AE2 regulation by extracellular pH must be evaluated in the context of its alkaline-shifted pHo sensitivity, resembling that of AE2 gastric parietal cell variant AE2c1.

Bicarbonate-dependent chloride transport drives fluid secretion by the human airway epithelial cell line Calu-3

Anion and fluid secretion are both defective in cystic fibrosis (CF); however, the transport mechanisms are not well understood. In this study, Cl(-) and HCO(3)(-) secretion was measured using genetically matched CF transmembrane conductance regulator (CFTR)-deficient and CFTR-expressing cell lines derived from the human airway epithelial cell line Calu-3. Forskolin stimulated the short-circuit current (I(sc)) across voltage-clamped monolayers, and also increased the equivalent short-circuit current (I(eq)) calculated under open-circuit conditions. I(sc) was equivalent to the HCO(3)(-) net flux measured using the pH-stat technique, whereas I(eq) was the sum of the Cl(-) and HCO(3)(-) net fluxes. I(eq) and HCO(3)(-) fluxes were increased by bafilomycin and ZnCl(2), suggesting that some secreted HCO(3)(-) is neutralized by parallel electrogenic H(+) secretion. I(eq) and fluid secretion were dependent on the presence of both Na(+) and HCO(3)(-). The carbonic anhydrase inhibitor acetazolamide abolished forskolin stimulation of I(eq) and HCO(3)(-) secretion, suggesting that HCO(3)(-) transport under these conditions requires catalysed synthesis of carbonic acid. Cl(-) was the predominant anion in secretions under all conditions studied and thus drives most of the fluid transport. Nevertheless, 50-70% of Cl(-) and fluid transport was bumetanide-insensitive, suggesting basolateral Cl(-) loading by a sodium-potassium-chloride cotransporter 1 (NKCC1)-independent mechanism. Imposing a transepithelial HCO(3)(-) gradient across basolaterally permeabilized Calu-3 cells sustained a forskolin-stimulated current, which was sensitive to CFTR inhibitors and drastically reduced in CFTR-deficient cells. Net HCO(3)(-) secretion was increased by bilateral Cl(-) removal and therefore did not require apical Cl(-)/HCO(3)(-) exchange. The results suggest a model in which most HCO(3)(-) is recycled basolaterally by exchange with Cl(-), and the resulting HCO(3)(-)-dependent Cl(-) transport provides an osmotic driving force for fluid secretion.

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.

Pendrin function and regulation in Xenopus oocytes

SLC26A4/PDS mutations cause Pendred Syndrome and non-syndromic deafness. but some aspects of function and regulation of the SLC26A4 polypeptide gene product, pendrin, remain controversial or incompletely understood. We have therefore extended the functional analysis of wildtype and mutant pendrin in Xenopus oocytes, with studies of isotopic flux, electrophysiology, and protein localization. Pendrin mediated electroneutral, pH-insensitive, DIDS-insensitive anion exchange, with extracellular K((1/2)) (in mM) of 1.9 (Cl(-)), 1.8 (I(-)), and 0.9 (Br(-)). The unusual phenotype of Pendred Syndrome mutation E303Q (loss-of-function with normal surface expression) prompted systematic mutagenesis at position 303. Only mutant E303K exhibited loss-of-function unrescued by forced overexpression. Mutant E303C was insensitive to charge modification by methanethiosulfonates. The corresponding mutants SLC26A2 E336Q, SLC26A3 E293Q, and SLC26A6 E298Q exhibited similar loss-of-function phenotypes, with wildtype surface expression also documented for SLC26A2 E336Q. The strong inhibition of wildtype SLC26A2, SLC26A3, and SLC26A6 by phorbol ester contrasts with its modest inhibition of pendrin. Phorbol ester inhibition of SLC26A2, SLC26A3, and SLC26A6 was blocked by coexpressed kinase-dead PKCδ but was without effect on pendrin. Mutation of SLC26A2 serine residues conserved in PKCδ -sensitive SLC26 proteins but absent from pendrin failed to reduce PKCδ sensitivity of SLC26A2 (190).

Anion secretion by a model epithelium: more lessons from Calu-3

Anion transport drives fluid into the airways and is essential for humidifying inspired air and supplying surface liquid for mucociliary transport. Despite the importance of airway secretion in diseases such as cystic fibrosis, the cellular mechanisms remain poorly understood, in part due to the small size and complicated structure of the submucosal glands that produce most of the fluid. The Calu-3 human lung adenocarcinoma cell line has become a popular model for studying airway secretion because it can be cultured as a flat sheet, expresses the cystic fibrosis transmembrane conductance regulator and several acinar cell markers, forms polarized monolayers with tight junctions, has robust cAMP-stimulated anion transport, and responds to secretagogues that regulate the glands in vivo. However, some properties of Calu-3 cells are less consistent with those of native tissue. In particular, Calu-3 monolayers do not secrete chloride when stimulated by forskolin under short-circuit conditions. Bicarbonate ions are thought to carry the short-circuit current (I(sc)) and the drive secretion of alkaline fluid, in contrast to the neutral pH secretions that are produced by submucosal glands. Calu-3 cells also have abnormal chromosomes and characteristics of both serous and mucus cells. In this article, we discuss Calu-3 as a model in light of our ongoing studies, which suggest that Calu-3 monolayers resemble submucosal glands more closely than was previously thought. For example, we find that net HCO(3)(-) flux fully accounts for I(sc) as previously suggested but Cl(-) is the main anion transported under physiological conditions. A novel, HCO(3)(-) -dependent mechanism of Cl(-) transport is emerging which may explain secretion by Calu-3 and perhaps other epithelial cells.

Effects of chronic acetazolamide administration on gas exchange and acid-base control in pulmonary circulation in exercising horses

REASONS FOR PERFORMING STUDY

Carbonic anhydrase (CA) catalyses the hydration/dehydration reaction of CO(2) and increases the rate of Cl(-) and HCO(3)(-) exchange between the erythrocytes and plasma. Therefore, chronic inhibition of CA has a potential to attenuate CO(2) output and induce greater metabolic and respiratory acidosis in exercising horses.

OBJECTIVES

To determine the effects of Carbonic anhydrase inhibition on CO(2) output and ionic exchange between erythrocytes and plasma and their influence on acid-base balance in the pulmonary circulation (across the lung) in exercising horses with and without CA inhibition.

METHODS

Six horses were exercised to exhaustion on a treadmill without (Con) and with CA inhibition (AczTr). CA inhibition was achieved with administration of acetazolamide (10 mg/kg bwt t.i.d. for 3 days and 30 mg/kg bwt before exercise). Arterial, mixed venous blood and CO(2) output were sampled at rest and during exercise. An integrated physicochemical systems approach was used to describe acid base changes.

RESULTS

AczTr decreased the duration of exercise by 45% (P < 0.0001). During the transition from rest to exercise CO(2) output was lower in AczTr (P < 0.0001). Arterial PCO(2) (P < 0.0001; mean ± s.e. 71 ± 2 mmHg AczTr, 46 ± 2 mmHg Con) was higher, whereas hydrogen ion (P = 0.01; 12.8 ± 0.6 nEq/l AczTr, 15.5 ± 0.6 nEq/l Con) and bicarbonate (P = 0.007; 5.5 ± 0.7 mEq/l AczTr, 10.1 ± 1.3 mEq/l Con) differences across the lung were lower in AczTr compared to Con. No difference was observed in weak electrolytes across the lung. Strong ion difference across the lung was lower in AczTr (P = 0.0003; 4.9 ± 0.8 mEq AczTr, 7.5 ± 1.2 mEq Con), which was affected by strong ion changes across the lung with exception of lactate.

CONCLUSIONS

CO(2) and chloride changes in erythrocytes across the lung seem to be the major contributors to acid-base and ions balance in pulmonary circulation in exercising horses.

Regulation of pendrin by pH: dependence on glycosylation

Mutations in the anion exchanger pendrin are responsible for Pendred syndrome, an autosomal recessive disease characterized by deafness and goitre. Pendrin is highly expressed in kidney collecting ducts, where it acts as a chloride/bicarbonate exchanger and thereby contributes to the regulation of acid–base homoeostasis and blood pressure. The present study aimed to characterize the intrinsic properties of pendrin. Mouse pendrin was transfected in HEK (human embryonic kidney) 293 and OKP (opossum kidney proximal tubule) cells and its activity was determined by monitoring changes in the intracellular pH induced by variations of transmembrane anion gradients. Combining measurements of pendrin activity with mathematical modelling we found that its affinity for Cl−, HCO3− and OH− varies with intracellular pH, with increased activity at low intracellular pH. Maximal pendrin activity was also stimulated at low extracellular pH, suggesting the presence of both intracellular and extracellular proton regulatory sites. We identified five putative pendrin glycosylation sites, only two of which are used. Mutagenesis-induced disruption of pendrin glycosylation did not alter its cell-surface expression or polarized targeting to the apical membrane and basal activity, but fully abrogated its sensitivity to extracellular pH. The hither to unknown regulation of pendrin by external pH may constitute a key mechanism in controlling ionic exchanges across the collecting duct and inner ear.

Hydrogen ion dynamics in human red blood cells

Our understanding of pH regulation within red blood cells (RBCs) has been inferred mainly from indirect experiments rather than from in situ measurements of intracellular pH (pH(i)). The present work shows that carboxy-SNARF-1, a pH fluorophore, when used with confocal imaging or flow cytometry, reliably reports pH(i) in individual, human RBCs, provided intracellular fluorescence is calibrated using a 'null-point' procedure. Mean pH(i) was 7.25 in CO(2)/HCO(3)(-)-buffered medium and 7.15 in Hepes-buffered medium, and varied linearly with extracellular pH (slope of 0.77). Intrinsic (non-CO(2)/HCO(3)(-)-dependent) buffering power, estimated in the intact cell (85 mmol (l cell)(-1) (pH unit)(-1) at resting pH(i)), was somewhat higher than previous estimates from cell lysates (50-70 mmol (l cell)(-1) (pH unit)(-1)). Acute displacement of pH(i) (superfusion of weak acids/bases) triggered rapid pH(i) recovery. This was mediated via membrane Cl(-)/HCO(3)(-) exchange (the AE1 gene product), irrespective of whether recovery was from an intracellular acid or base load, and with no evident contribution from other transporters such as Na(+)/H(+) exchange. H(+)-equivalent flux through AE1 was a linear function of [H(+)](i) and reversed at resting pH(i), indicating that its activity is not allosterically regulated by pH(i), in contrast to other AE isoforms. By simultaneously monitoring pH(i) and markers of cell volume, a functional link between membrane ion transport, volume and pH(i) was demonstrated. RBC pH(i) is therefore tightly regulated via AE1 activity, but modulated during changes of cell volume. A comparable volume-pH(i) link may also be important in other cell types expressing anion exchangers. Direct measurement of pH(i) should be useful in future investigations of RBC physiology and pathology.

A biophysical model for integration of electrical, osmotic, and pH regulation in the human bronchial epithelium

A dynamical biophysical model for the functioning of an epithelium is presented. This model integrates the electrical and osmotic behaviors of the epithelium, taking into account intracellular conditions. The specific tissue modeled is the human bronchial epithelium, which is of particular interest, as it is the location of the most common lethal symptoms of cystic fibrosis. The model is implemented in a modular form to facilitate future application of the code to other epithelial tissue by inputting different transporters, channels, and geometric parameters. The model includes pH regulation as an integral component of overall regulation of epithelial function, through the interdependence of pH, bicarbonate concentration, and current. The procedures for specification, the validation of the model, and parametric studies are presented using available experimental data of cultured human bronchial epithelium. Parametric studies are performed to elucidate a), the contribution of basolateral chloride channels to the short-circuit current functional form, and b), the role that regulation of basolateral potassium conductance plays in epithelial function.

A SLC4-like anion exchanger from renal tubules of the mosquito (Aedes aegypti): evidence for a novel role of stellate cells in diuretic fluid secretion

Transepithelial fluid secretion across the renal (Malpighian) tubule epithelium of the mosquito (Aedes aegypti) is energized by the vacuolar-type (V-type) H(+)-ATPase and not the Na(+)-K(+)-ATPase. Located at the apical membrane of principal cells, the V-type H(+)-ATPase translocates protons from the cytoplasm to the tubule lumen. Secreted protons are likely to derive from metabolic H(2)CO(3), which raises questions about the handling of HCO(3)(-) by principal cells. Accordingly, we tested the hypothesis that a Cl/HCO(3) anion exchanger (AE) related to the solute-linked carrier 4 (SLC4) superfamily mediates the extrusion of HCO(3)(-) across the basal membrane of principal cells. We began by cloning from Aedes Malpighian tubules a full-length cDNA encoding an SLC4-like AE, termed AeAE. When expressed heterologously in Xenopus oocytes, AeAE is both N- and O-glycosylated and mediates Na(+)-independent intracellular pH changes that are sensitive to extracellular Cl(-) concentration and to DIDS. In Aedes Malpighian tubules, AeAE is expressed as two distinct forms: one is O-glycosylated, and the other is N-glycosylated. Significantly, AeAE immunoreactivity localizes to the basal regions of stellate cells but not principal cells. Concentrations of DIDS that inhibit AeAE activity in Xenopus oocytes have no effects on the unstimulated rates of fluid secretion mediated by Malpighian tubules as measured by the Ramsay assay. However, in Malpighian tubules stimulated with kinin or calcitonin-like diuretic peptides, DIDS reduces the diuretic rates of fluid secretion to basal levels. In conclusion, Aedes Malpighian tubules express AeAE in the basal region of stellate cells, where this transporter may participate in producing diuretic rates of transepithelial fluid secretion.

Sensors and regulators of intracellular pH

Protons dictate the charge and structure of macromolecules and are used as energy currency by eukaryotic cells. The unique function of individual organelles therefore depends on the establishment and stringent maintenance of a distinct pH. This, in turn, requires a means to sense the prevailing pH and to respond to deviations from the norm with effective mechanisms to transport, produce or consume proton equivalents. A dynamic, finely tuned balance between proton-extruding and proton-importing processes underlies pH homeostasis not only in the cytosol, but in other cellular compartments as well.

High rates of HCO3- secretion and Cl- absorption against adverse gradients in the marine teleost intestine: the involvement of an electrogenic anion exchanger and H+-pump metabolon?

Anion exchange contributes significantly to intestinal Cl(-) absorption in marine teleost fish and is thus vital for successful osmoregulation. This anion exchange process leads to high luminal HCO(3)(-) concentrations (up to approximately 100 mmol l(-1)) and high pH and results in the formation of CaCO(3) precipitates in the intestinal lumen. Recent advances in our understanding of the transport processes involved in intestinal anion exchange in marine teleost fish include the demonstration of a role for the H(+)-pump (V-ATPase) in apical H(+) extrusion and the presence of an electrogenic (nHCO(3)(-)/Cl(-)) exchange protein (SLC26a6). The H(+)-V-ATPase defends against cellular acidification, which might otherwise occur as a consequence of the high rates of base secretion. In addition, apical H(+) extrusion probably maintains lower HCO(3)(-) concentrations in the unstirred layer at the apical surface than in the bulk luminal fluids and thus facilitates continued anion exchange. Furthermore, H(+)-V-ATPase activity hyperpolarizes the apical membrane potential that provides the driving force for apical electrogenic nHCO(3)(-)/Cl(-) exchange, which appears to occur against both Cl(-) and HCO(3)(-) electrochemical gradients. We propose that a similar coupling between apical H(+) extrusion and nHCO(3)(-)/Cl(-) exchange accounts for Cl(-) uptake in freshwater fish and amphibians against very steep Cl(-) gradients.

Characteristics of Cl- uptake in rat alveolar type I cells

Although Cl- transport in fetal lung is important for fluid secretion and normal lung development, the role of Cl- transport in adult lung is not well understood. In physiological studies, the cystic fibrosis transmembrane regulator (CFTR) plays a role in fluid absorption in the distal air spaces of adult lung, and alveolar type II cells cultured for 5 days have the capacity to transport Cl-. Although both alveolar type I and type II cells express CFTR, it has previously not been known whether type I cells transport Cl-. We studied Cl- uptake in isolated type I cells directly, using either radioisotopic tracers or halide-sensitive fluorescent indicators. By both methods, type I cells take up Cl-. In the presence of beta-adrenergic agonist stimulation, Cl- uptake can be inhibited by CFTR antagonists. Type I cells express both the Cl-/HCO3- anion exchanger AE2 and the voltage-gated Cl- channels CLC5 and CLC2. Inhibitors of AE2 also block Cl- uptake in type I cells. Together, these results demonstrate that type I cells are capable of Cl- uptake and suggest that the effects seen in whole lung studies establishing the importance of Cl- movement in alveolar fluid clearance may be, in part, the result of Cl- transport across type I cells.

Studies on bicarbonate transporters and carbonic anhydrase in porcine nonpigmented ciliary epithelium

PURPOSE

Bicarbonate transport plays a role in aqueous humor (AH) secretion. The authors examined bicarbonate transport mechanisms and carbonic anhydrase (CA) in porcine nonpigmented ciliary epithelium (NPE).

METHODS

Cytoplasmic pH (pH(i)) was measured in cultured porcine NPE loaded with BCECF. Anion exchanger (AE), sodium bicarbonate cotransporter (NBC), and CA were examined by RT-PCR and immunolocalization. AH secretion was measured in the intact porcine eye using a fluorescein dilution technique.

RESULTS

Anion exchanger AE2, CAII, and CAIV were abundant in the NPE layer. In cultured NPE superfused with a CO(2)/HCO(3)(-)-free HEPES buffer, exposure to a CO(2)/HCO(3)(-)-containing buffer caused rapid acidification followed by a gradual increase in pH(i). Subsequent removal of CO(2)/HCO(3)(-) with HEPES buffer caused rapid alkalinization followed by a gradual decrease in pH(i). The rate of gradual alkalinization after the addition of HCO(3)(-)/CO(2) was inhibited by sodium-free conditions, DIDS, and the CA inhibitors acetazolamide and methazolamide but not by the Na-H exchange inhibitor dimethylamiloride or low-chloride buffer. The phase of gradual acidification after removal of HCO(3)(-)/CO(2) was inhibited by DIDS, acetazolamide, methazolamide, and low-chloride buffer. DIDS reduced baseline pH(i). In the intact eye, DIDS and acetazolamide reduced AH secretion by 25% and 44%, respectively.

CONCLUSIONS

The results suggest the NPE uses a Na(+)-HCO(3)(-) cotransporter to import bicarbonate and a Cl(-)/HCO(3)(-) exchanger to export bicarbonate. CA influences the rate of bicarbonate transport. AE2, CAII, and CAIV are enriched in the NPE layer of the ciliary body, and their coordinated function may contribute to AH secretion by effecting bicarbonate transport into the eye.

Mouse Ae1 E699Q mediates SO42-i/anion-o exchange with [SO42-]i-dependent reversal of wild-type pHo sensitivity

The SLC4A1/AE1 gene encodes the electroneutral Cl(-)/HCO(3)(-) exchanger of erythrocytes and renal type A intercalated cells. AE1 mutations cause familial spherocytic and stomatocytic anemias, ovalocytosis, and distal renal tubular acidosis. The mutant mouse Ae1 polypeptide E699Q expressed in Xenopus oocytes cannot mediate Cl(-)/HCO(3)(-) exchange or (36)Cl(-) efflux but exhibits enhanced dual sulfate efflux mechanisms: electroneutral exchange of intracellular sulfate for extracellular sulfate (SO(4)(2-)(i)/SO(4)(2-)(o) exchange), and electrogenic exchange of intracellular sulfate for extracellular chloride (SO(4)(2-)(i)/Cl(-)(o) exchange). Whereas wild-type AE1 mediates 1:1 H(+)/SO(4)(2-) cotransport in exchange for either Cl(-) or for the H(+)/SO(4)(2-) ion pair, mutant Ae1 E699Q transports sulfate without cotransport of protons, similar to human erythrocyte AE1 in which the corresponding E681 carboxylate has been chemically converted to the alcohol (hAE1 E681OH). We now show that in contrast to the normal cis-stimulation by protons of wild-type AE1-mediated SO(4)(2-) transport, both SO(4)(2-)(i)/Cl(-)(o) exchange and SO(4)(2-)(i)/SO(4)(2-)(o) exchange mediated by mutant Ae1 E699Q are inhibited by acidic pH(o) and activated by alkaline pH(o). hAE1 E681OH displays a similarly altered pH(o) dependence of SO(4)(2-)(i)/Cl(-)(o) exchange. Elevated [SO(4)(2-)](i) increases the K(1/2) of Ae1 E699Q for both extracellular Cl(-) and SO(4)(2-), while reducing inhibition of both exchange mechanisms by acid pH(o). The E699Q mutation also leads to increased potency of self-inhibition by extracellular SO(4)(2-). Study of the Ae1 E699Q mutation has revealed the existence of a novel pH-regulatory site of the Ae1 polypeptide and should continue to provide valuable paths toward understanding substrate selectivity and self-inhibition in SLC4 anion transporters.

Characterization of human SLC4A10 as an electroneutral Na/HCO3 cotransporter (NBCn2) with Cl- self-exchange activity

The SLC4A10 gene product, commonly known as NCBE, is highly expressed in rodent brain and has been characterized by others as a Na(+)-driven Cl-HCO(3) exchanger. However, some of the earlier data are not consistent with Na(+)-driven Cl-HCO(3) exchange activity. In the present study, northern blot analysis showed that, also in humans, NCBE transcripts are predominantly expressed in brain. In some human NCBE transcripts, splice cassettes A and/or B, originally reported in rats and mice, are spliced out. In brain cDNA, we found evidence of a unique partial splice of cassette B that is predicted to produce an NCBE protein with a novel C terminus containing a protein kinase C phosphorylation site. We used pH-sensitive microelectrodes to study the molecular physiology of human NCBE expressed in Xenopus oocytes. In agreement with others we found that NCBE mediates the 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid-sensitive, Na(+)-dependent transport of HCO(3)(-). For the first time, we demonstrated that this transport process is electroneutral. Using Cl(-)-sensitive microelectrodes positioned at the oocyte surface, we found that, unlike both human and squid Na(+)-driven Cl-HCO(3) exchangers, human NCBE does not normally couple the net influx of HCO(3)(-) to a net efflux of Cl(-). Moreover we found that that the (36)Cl efflux from NCBE-expressing oocytes, interpreted by others to be coupled to the influx of Na(+) and HCO(3)(-), actually represents a CO(2)/HCO(3)(-)-stimulated Cl(-) self-exchange not coupled to either Na(+) or net HCO(3)(-) transport. We propose to rename NCBE as the second electroneutral Na/HCO(3) cotransporter, NBCn2.

Enhanced formation of a HCO3- transport metabolon in exocrine cells of Nhe1-/- mice

Cl(-) influx across the basolateral membrane is a limiting step in fluid production in exocrine cells and often involves functionally linked Cl(-)/HCO(3)(-) (Ae) and Na(+)/H(+) (Nhe) exchange mechanisms. The dependence of this major Cl(-) uptake pathway on Na(+)/H(+) exchanger expression was examined in the parotid acinar cells of Nhe1(-/-) and Nhe2(-/-) mice, both of which exhibited impaired fluid secretion. No change in Cl(-)/HCO(3)(-) exchanger activity was detected in Nhe2-deficient mice. Conversely, Cl(-)/HCO(3)(-) exchanger activity increased nearly 4-fold in Nhe1-deficient mice, despite only minimal or any change in mRNA and protein levels of the anion exchanger Ae2. Acetazolamide completely blocked the increase in Cl(-)/HCO(3)(-) exchanger activity in Nhe1-null mice suggesting that increased anion exchange required carbonic anhydrase activity. Indeed, the parotid glands of Nhe1(-/-) mice expressed higher levels of carbonic anhydrase 2 (Car2) polypeptide. Moreover, the enhanced Cl(-)/HCO(3)(-) exchange activity was accompanied by an increased abundance of Car2.Ae2 complexes in the parotid plasma membranes of Nhe1(-/-) mice. Anion exchanger activity was also significantly reduced in Car2-deficient mice, consistent with an important role of a putative Car2.Ae2 HCO(3)(-) transport metabolon in parotid exocrine cell function. Increased abundance of this HCO(3)(-) transport metabolon is likely one of the multiple compensatory changes in the exocrine parotid gland of Nhe1(-/-) mice that together attenuate the severity of in vivo electrolyte and acid-base balance perturbations.

Functional characterization of Cl-/HCO3- exchange in villous cells of the mouse ileum

At least three kinds of Cl(-)/HCO(3)(-) exchangers, SLC26A3, SLC26A6 and AE2, have been demonstrated to be expressed in the intestinal epithelial cell. To examine the functional expression of these exchangers in the native enterocyte, we studied the Cl(-)/HCO(3)(-)- exchange activity in isolated villi from the mouse ileum by microfluorometric intracellular pH (pH(i)) measurement. The pH(i) value increased upon Cl(-) removal when the villus was superfused with an HCO(3)(-)/CO(2)-buffered solution, while the response was blunted when superfused with an HCO(3)(-)/CO(2)-free, Hepes-buffered solution. The recovery of pH(i) value induced by Cl(-) re-addition (after initial Cl(-) removal) was totally or partially mimicked by the addition of Br(-), I(-), F(-), NO(3)(-), or SO(4)(2-) (in the absence of Cl(-)). The increase in pH(i) value induced by Cl(-) removal was partially inhibited in the presence of DIDS (30 muM), tenidap (10 muM), niflumic acid (30 muM) or NPPB (30 muM). Increasing the K(+) concentration from 5 mM to 60 mM in the superfusion solution induced a reversible increase in pH(i) value under the HCO(3)(-)/CO(2)-buffered condition, while it had hardly any effect on pH(i) under the Hepesbuffered condition. The K(+)-induced pH(i) changes were partially suppressed by removing Cl(-) from the superfusion solution. These results, together with the reported findings of mouse slc26a3, slc26a6 and AE2 in heterologously expressed systems, suggest the possibility that these three exchangers may all be functionally expressed in mouse ileal villous cells.

Role of nonconserved charged residues of the AE2 transmembrane domain in regulation of anion exchange by pH

The ubiquitous AE2/SLC4A2 anion exchanger is acutely and independently regulated by intracellular (pH(i)) and extracellular pH (pH(o)), whereas the closely related AE1/SLC4A1 of the red cell and renal intercalated cell is relatively pH-insensitive. We have investigated the contribution of nonconserved charged residues within the C-terminal transmembrane domain (TMD) of AE2 to regulation by pH through mutation to the corresponding AE1 residues. AE2-mediated Cl(-)/Cl(-) exchange was measured as 4,4'-di-isothiocyanatostilbene-2,2'-disulfonic acid-sensitive (36)Cl(-) efflux from Xenopus oocytes by varying pH(i) at constant pH(o), and by varying pH(o) at near-constant pH(i). All mutations of nonconserved charged residues of the AE2 TMD yielded functional protein, but mutations of some conserved charged residues (R789E, R1056A, R1134C) reduced or abolished function. Individual mutation of AE2 TMD residues R921, F922, P1077, and R1107 exhibited reduced pH(i) sensitivity compared to wt AE2, whereas TMD mutants K1153R, R1155K, R1202L displayed enhanced sensitivity to acidic pH(i). In addition, pH(o) sensitivity was significantly acid- shifted when nonconserved AE2 TMD residues E981, K982, and D1075 were individually converted to the corresponding AE1 residues. These results demonstrate that multiple conserved charged residues are important for basal transport function of AE2 and that certain nonconserved charged residues of the AE2 TMD are essential for wild-type regulation of anion exchange by pH(i) and pH(o).

Bicarbonate-rich choleresis induced by secretin in normal rat is taurocholate-dependent and involves AE2 anion exchanger

Canalicular bile is modified along bile ducts through reabsorptive and secretory processes regulated by nerves, bile salts, and hormones such as secretin. Secretin stimulates ductular cystic fibrosis transmembrane conductance regulator (CFTR)-dependent Cl- efflux and subsequent biliary HCO3- secretion, possibly via Cl-/HCO3- anion exchange (AE). However, the contribution of secretin to bile regulation in the normal rat, the significance of choleretic bile salts in secretin effects, and the role of Cl-/HCO3- exchange in secretin-stimulated HCO3- secretion all remain unclear. Here, secretin was administered to normal rats with maintained bile acid pool via continuous taurocholate infusion. Bile flow and biliary HCO3- and Cl- excretion were monitored following intrabiliary retrograde fluxes of saline solutions with and without the Cl- channel inhibitor 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) or the Cl-/HCO3- exchange inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). Secretin increased bile flow and biliary excretion of HCO3- and Cl-. Interestingly, secretin effects were not observed in the absence of taurocholate. Whereas secretin effects were all blocked by intrabiliary NPPB, DIDS only inhibited secretin-induced increases in bile flow and HCO3- excretion but not the increased Cl- excretion, revealing a role of biliary Cl-/HCO3- exchange in secretin-induced, bicarbonate-rich choleresis in normal rats. Finally, small hairpin RNA adenoviral constructs were used to demonstrate the involvement of the Na+-independent anion exchanger 2 (AE2) through gene silencing in normal rat cholangiocytes. AE2 gene silencing caused a marked inhibition of unstimulated and secretin-stimulated Cl-/HCO3- exchange. In conclusion, maintenance of the bile acid pool is crucial for secretin to induce bicarbonate-rich choleresis in the normal rat and that this occurs via a chloride-bicarbonate exchange process consistent with AE2 function.

Alkaline-shifted pH Sensitivity of AE2c1-mediated Anion Exchange Reveals Novel Regulatory Determinants in the AE2 N-terminal Cytoplasmic Domain

The mouse anion exchanger AE2/SLC4A2 Cl(-)/HCO(-)(3) exchanger is essential to post-weaning life. AE2 polypeptides regulate pH(i), chloride concentration, cell volume, and transepithelial ion transport in many tissues. Although the AE2a isoform has been extensively studied, the function and regulation of the other AE2 N-terminal variant mRNAs of mouse (AE2b1, AE2b2, AE2c1, and AE2c2) have not been examined. We now present an extended analysis of AE2 variant mRNA tissue distribution and function. We show in Xenopus oocytes that all AE2 variant polypeptides except AE2c2 mediated Cl(-) transport are subject to inhibition by acidic pH(i) and to activation by hypertonicity and NH(+)(4). However, AE2c1 differs from AE2a, AE2b1, and AE2b2 in its alkaline-shifted pH(o)((50)) (7.70 +/- 0.11 versus 6.80 +/- 0.05), suggesting the presence of a novel AE2a pH-sensitive regulatory site between amino acids 99 and 198. Initial N-terminal deletion mutagenesis restricted this site to the region between amino acids 120 and 150. Further analysis identified AE2a residues 127-129, 130-134, and 145-149 as jointly responsible for the difference in pH(o)((50)) between AE2c1 and the longer AE2a, AE2b1, and AE2b2 polypeptides. Thus, AE2c1 exhibits a unique pH(o) sensitivity among the murine AE2 variant polypeptides, in addition to a unique tissue distribution. Physiological coexpression of AE2c1 with other AE2 variant polypeptides in the same cell should extend the range over which changing pH(o) can regulate AE2 transport activity.

Functional and Molecular Adaptation of Cl-/HCO3- Exchanger to Chronic Alkaline Media in RenalCells

The Cl(-)/HCO3- exchanger (AE) is one of the mechanisms that cells have developed to adjust pH Despite its importance, the role of AE isoforms in controlling steady-state pH during alkalosis has not been widely investigated. In the present study, we have evaluated whether conditions simulating acute and chronic metabolic alkalosis affected the transport activity and protein levels of Cl-/HCO3- exchangers in a rat cortical collecting duct cell line (RCCD1). pH(i) was monitored using the fluorescent dye BCECF in monolayers grown on permeable supports. Anion exchanger function was assessed by the response of pH(i) to acute chloride removal. RT-PCR and immunoblot assays were also performed. Our results showed that RCCD1 cells express two members of the anion exchanger gene family: AE2 and AE4. Functional studies demonstrated that while in acute alkalosis pH(i) became alkaline and was not regulated, after 48 h adaptation; steady-state pH(i) reached a value similar to the physiological one. Chronic treated cells also resulted in a 3-fold rise in Cl(-)/HCO3- exchange activity together with a 2.2-fold increase in AE2, but not AE4, protein abundance. We conclude that RCCD1 cells can adapt to chronic extracellular alkalosis reestablishing its steady-state pH(i) and that AE2 would play a key role in cell homeostasis.

Bicarbonate exporting transporters in the ovine ruminal epithelium

In order to stabilize the intraruminal pH, bicarbonate secretion by the ruminal epithelium seems to be an important prerequisite. The present study therefore focussed on the characterization of bicarbonate exporting systems in ruminal epithelial cells. Intracellular pH (pH(i)) was measured spectrofluorometrically in primary cultured ruminal epithelial cells loaded with the pH-sensitive fluorescent dye, 2,7-bis(carboxyethyl)-5(6')-carboxyfluorescein acetomethyl ester. Switching from CO2/HCO3- -buffered to HEPES-buffered solution caused a rapid intracellular alkalinization followed by a counter-regulation towards initial pH(i). The recovery of pH(i) was dependent upon extracellular chloride, but independent of extracellular sodium. Adding 500 microM H2DIDS significantly reduced the increase of pH(i). For further characterization of the bicarbonate exporting systems, we tested the ability to reverse the direction from HCO3- export to import in the absence of sodium and chloride. Under sodium and chloride-free conditions, counter-regulation after CO2-induced pH(i) decrease did not differ from pH(i) recovery in the presence of sodium and chloride. Existence of bicarbonate exporting systems in cultured ruminal epithelial cells and intact ruminal epithelium was verified by reverse transcription polymerase chain reaction (RT-PCR). Using RT-PCR and subsequent sequencing, expression of mRNA encoding for AE2, DRA and PAT1 could be found. Bicarbonate exporting systems could therefore be detected both on the functional and structural level.

Acute pH-dependent Regulation of AE2-mediated Anion Exchange Involves Discrete Local Surfaces of the NH2-terminal Cytoplasmic Domain

We have previously defined in the NH2-terminal cytoplasmic domain of the mouse AE2/SLC4A2 anion exchanger a critical role for the highly conserved amino acids (aa) 336-347 in determining wild-type pH sensitivity of anion transport. We have now engineered hexa-Ala ((A)6) and individual amino acid substitutions to investigate the importance to pH-dependent regulation of AE2 activity of the larger surrounding region of aa 312-578. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive 36Cl- efflux from AE2-expressing Xenopus oocytes was monitored during changes in pHi or pHo in HEPES-buffered and in 5% CO2/HCO3- -buffered conditions. Wild-type AE2-mediated 36Cl- efflux was profoundly inhibited at low pHo, with a pHo(50) value = 6.75 +/- 0.05 and was stimulated up to 10-fold by intracellular alkalinization. Individual mutation of several amino acid residues at non-contiguous sites preceding or following the conserved sequence aa 336-347 attenuated pHi and/or pHo sensitivity of 36Cl- efflux. The largest attenuation of pH sensitivity occurred with the AE2 mutant (A)6357-362. This effect was phenocopied by AE2 H360E, suggesting a crucial role for His360. Homology modeling of the three-dimensional structure of the AE2 NH2-terminal cytoplasmic domain (based on the structure of the corresponding region of human AE1) predicts that those residues shown by mutagenesis to be functionally important define at least one localized surface region necessary for regulation of AE2 activity by pH.

Identification of a basolateral Cl-/HCO3- exchanger specific to gastric parietal cells

The basolateral Cl(-)/HCO(3)(-) exchanger in parietal cells plays an essential role in gastric acid secretion mediated via the apical gastric H(+)-K(+)-ATPase. Here, we report the identification of a new Cl(-)/HCO(3)(-) exchanger, which shows exclusive expression in mouse stomach and kidney, with expression in the stomach limited to the basolateral membrane of gastric parietal cells. Tissue distribution studies by RT-PCR and Northern hybridizations demonstrated the exclusive expression of this transporter, also known as SLC26A7, to stomach and kidney, with the stomach expression significantly more abundant. No expression was detected in the intestine. Cellular distribution studies by RT-PCR and Northern hybridizations demonstrated predominant localization of SLC26A7 in gastric parietal cells. Immunofluorescence labeling localized this exchanger exclusively to the basolateral membrane of gastric parietal cells, and functional studies in oocytes indicated that SLC26A7 is a DIDS-sensitive Cl(-)/HCO(3)(-) exchanger that is active in both acidic and alkaline pH(i). On the basis of its unique expression pattern and function, we propose that SLC26A7 is a basolateral Cl(-)/HCO(3)(-) exchanger in gastric parietal cells and plays a major role in gastric acid secretion.

Evidence for the presence of three different anion exchangers in a red cell. Functional expression studies in Xenopus oocytes

Anion exchangers (AE) are transmembrane proteins catalyzing electroneutral exchange of Cl(-) for HCO3-. To date, three different genes coding for this protein, AE1, AE2 and AE3, have been identified in many species. AE1 is considered to be the unique anion exchanger expressed in erythrocytes. In this paper we propose the presence of three different AEs in skate erythrocytes, a skAE1, a skAE2 and a skAE3, cloned by RT-PCR (reverse-transcriptase polymerase chain reaction). These three skAE have a similar predicted secondary structure. All three skAE are divided in two main domains: a hydrophilic cytoplasmic N-terminal domain and a C-terminal domain crossing the lipid bilayer at least 12 times. The greatest similarity is found in the membrane-spanning domain of the three skAE. The size as well as the amino-acid sequence of the cytoplasmic domain differ significantly among three anion exchangers. Functional expression studies in Xenopus oocytes led to the conclusion that skAE-1 and -2 share some functional features (Cl-dependence and DIDS sensitivity). The skAE3 could not be expressed in Xenopus oocytes. These data are in agreement with expression data obtained with AEs of different species utilizing the oocyte system. It is highly probable that these three new AE sequences come from three different genes, thus suggesting for the first time the presence of the three AE genes in Chondrichthyes.

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

Chloride/bicarbonate anion exchangers (AEs), found in the plasma membrane of most mammalian cells, are involved in pH regulation and bicarbonate metabolism. Although AE2 and AE3 are highly similar in sequence, AE2-transport activity was 10-fold higher than AE3 (41 versus 4 mM x min(-1) respectively), when expressed by transient transfection of HEK-293 cells. AE2-AE3 chimaeras were constructed to define the region responsible for differences in transport activity. The level of AE2 expression was approx. 30% higher than that of AE3. Processing to the cell surface, studied by chemical labelling and confocal microscopy, showed that AE2 is processed to the cell surface approx. 8-fold more efficiently than AE3. The efficiency of cell-surface processing was dependent on the cytoplasmic domain, since the AE2 domain conferred efficient processing upon the AE3 membrane domain, with a predominant role for amino acids 322-677 of AE2. AE2 that was expressed in HEK-293 cells was glycosylated, but little of AE3 was. However, AE2 expressed in the presence of the glycosylation inhibitor, tunicamycin, was not glycosylated, yet retained 85 +/- 8% of anion-transport activity. Therefore glycosylation has little, if any, role in the cell-surface processing or activity of AE2 or AE3. We conclude that the low anion-transport activity of AE3 in HEK-293 cells is due to low level processing to the plasma membrane, possibly owing to protein interactions with the AE3 cytoplasmic domain.

Bicarbonate transport proteins

Bicarbonate is not freely permeable to membranes. Yet, bicarbonate must be moved across membranes, as part of CO2 metabolism and to regulate cell pH. Mammalian cells ubiquitously express bicarbonate transport proteins to facilitate the transmembrane bicarbonate flux. These bicarbonate transporters, which function by different transport mechanisms, together catalyse transmembrane bicarbonate movement. Recent advances have allowed the identification of several new bicarbonate transporter genes. Bicarbonate transporters cluster into two separate families: (i) the anion exachanger (AE) family of Cl-/HCO3- exchangers is related in sequence to the NBC family of Na+/HCO3- cotransporters and the Na(+)-dependent Cl/HCO3- exchangers and (ii) some members of the SLC26a family of sulfate transporters will also transport bicarbonate but are not related in sequence to the AE/NBC family of transporters. This review summarizes our understanding of the mammalian bicarbonate transporter superfamily.

Role of bicarbonate in the regulation of intracellular pH in the mammalian ventricular myocyte

Bicarbonate is important for pHi control in cardiac cells. It is a major part of the intracellular buffer apparatus, it is a substrate for sarcolemmal acid-equivalent transporters that regulate intracellular pH, and it contributes to the pHo sensitivity of steady-state pHi, a phenomenon that may form part of a whole-body response to acid/base disturbances. Both bicarbonate and H+/OH- transporters participate in the sarcolemmal regulation of pHi, namely Na(+)-HCO3-cotransport (NBC), Cl(-)-HCO3- exchange (i.e., anion exchange, AE), Na(+)-H+ exchange (NHE), and Cl(-)-OH- exchange (CHE). These transporters are coupled functionally through changes of pHi, while pHi is linked to [Ca2+]i through secondary changes in [Na+] mediated by NBC and NHE. Via such coupling, decreases of pHo and pHi can ultimately lead to an elevation of [Ca2+]i, thereby influencing cardiac contractility and electrical rhythm. Bicarbonate is also an essential component of an intracellular carbonic buffer shuttle that diffusively couples cytoplasmic pH to the sarcolemma and minimises the formation of intracellular pH microdomains. The importance of bicarbonate is closely linked to the activity of the enzyme carbonic anhydrase (CA). Without CA activity, intracellular bicarbonate-dependent buffering, membrane bicarbonate transport, and the carbonic shuttle are severely compromised. There is a functional partnership between CA and HCO3- transport. Based on our observations on intracellular acid mobility, we propose that one physiological role for CA is to act as a pH-coupling protein, linking bulk pH to the allosteric H+ control sites on sarcolemmal acid/base transporters.

Inhibition of apical Cl-/OH- exchange activity in Caco-2 cells by phorbol esters is mediated by PKCepsilon

The present studies were undertaken to examine the possible regulation of apical membrane Cl-/OH- exchanger in Caco-2 cells by protein kinase C (PKC). The effect of the phorbol ester phorbol 12-myristate 13-acetate (PMA), an in vitro PKC agonist, on OH- gradient-driven 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive 36Cl uptake in Caco-2 cells was assessed. The results demonstrated that PMA decreased apical Cl-/OH- exchanger activity via phosphatidylinositol 3-kinase (PI3-kinase)-mediated activation of PKCepsilon. The data consistent with these conclusions are as follows: 1) short-term treatment of cells for 1-2 h with PMA (100 nM) significantly decreased Cl-/OH- exchange activity compared with control (4alpha-PMA); 2) pretreatment of cells with specific PKC inhibitors chelerythrine chloride, calphostin C, and GF-109203X completely blocked the inhibition of Cl-/OH- exchange activity by PMA; 3) specific inhibitors for PKCepsilon (Ro-318220) but not PKCalpha (Go-6976) significantly blocked the PMA-mediated inhibition; 4) specific PI3-kinase inhibitors wortmannin and LY-294002 significantly attenuated the inhibitory effect of PMA; and 5) PI3-kinase activators IRS-1 peptide and phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P(3)] mimicked the effects of PMA. These findings provide the first evidence for PKCepsilon-mediated inhibition of Cl-/OH- exchange activity in Caco-2 cells and indicate the involvement of the PI3-kinase-mediated pathways in the regulation of Cl- absorption in intestinal epithelial cells.

Regulation of AE2-mediated Cl- transport by intracellular or by extracellular pH requires highly conserved amino acid residues of the AE2 NH2-terminal cytoplasmic domain

We reported recently that regulation by intracellular pH (pH(i)) of the murine Cl-/HCO(3)(-) exchanger AE2 requires amino acid residues 310-347 of the polypeptide's NH(2)-terminal cytoplasmic domain. We have now identified individual amino acid residues within this region whose integrity is required for regulation of AE2 by pH. 36Cl- efflux from AE2-expressing Xenopus oocytes was monitored during variation of extracellular pH (pH(o)) with unclamped or clamped pH(i), or during variation of pH(i) at constant pH(o). Wild-type AE2-mediated 36Cl- efflux was profoundly inhibited by acid pH(o), with a value of pH(o50) = 6.87 +/- 0.05, and was stimulated up to 10-fold by the intracellular alkalinization produced by bath removal of the preequilibrated weak acid, butyrate. Systematic hexa-alanine [(A)6]bloc substitutions between aa 312-347 identified the greatest acid shift in pH(o(50)) value, approximately 0.8 pH units in the mutant (A)6 342-347, but only a modest acid-shift in the mutant (A)6 336-341. Two of the six (A)6 mutants retained normal pH(i) sensitivity of 36Cl- efflux, whereas the (A)6 mutants 318-323, 336-341, and 342-347 were not stimulated by intracellular alkalinization. We further evaluated the highly conserved region between aa 336-347 by alanine scan and other mutagenesis of single residues. Significant changes in AE2 sensitivity to pH(o) and to pH(i) were found independently and in concert. The E346A mutation acid-shifted the pH(o(0) value to the same extent whether pH(i) was unclamped or held constant during variation of pH(o). Alanine substitution of the corresponding glutamate residues in the cytoplasmic domains of related AE anion exchanger polypeptides confirmed the general importance of these residues in regulation of anion exchange by pH. Conserved, individual amino acid residues of the AE2 cytoplasmic domain contribute to independent regulation of anion exchange activity by pH(o) as well as pH(i).

An alternate pathway of cAMP-stimulated Cl secretion across the NKCC1-null murine duodenum

BACKGROUND & AIMS

Adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated anion secretion across the duodenal epithelium requires the cystic fibrosis transmembrane conductance regulator (CFTR) in the apical membrane and anion uptake proteins in the basolateral membrane. NKCC1, the epithelial Na(+)/K(+)/2Cl(-) cotransporter, is the major protein responsible for Cl(-) uptake. In this study, we evaluate the role of NKCC1 in determining the relative rates of transepithelial Cl(-) and HCO(3)(-) secretion during cAMP stimulation of the duodenum.

METHODS

Bicarbonate and chloride secretion across duodenal mucosa was measured in Ussing chambers by pH stat and (36)Cl flux methods using mice with either gene-targeted deletion of NKCC1 (NKCC1-/-) or bumetanide blockade of NKCC1.

RESULTS

Total anion secretion stimulated by forskolin treatment of NKCC1-null duodenum resulted from approximately equivalent rates of electrogenic chloride, electrogenic bicarbonate, and electroneutral bicarbonate secretion. Evaluation of the alternate chloride secretory pathway indicated chloride uptake by a basolateral membrane anion exchange process with characteristics consistent with the anion exchanger isoform AE2.

CONCLUSIONS

Chloride uptake by basolateral anion exchanger activity (AE2) supports intracellular cAMP-stimulated chloride secretion in the NKCC1-null duodenum. A model for the alternate chloride secretion pathway is proposed whereby chloride uptake via AE2 is coupled to basolateral NaHCO(3) cotransport to support CFTR-mediated chloride and bicarbonate secretion.

Role of JNK in hypertonic activation of Cl(-)-dependent Na(+)/H(+) exchange in Xenopus oocytes

In the course of studying the hypertonicity-activated ion transporters in Xenopus oocytes, we found that activation of endogenous oocyte Na(+)/H(+) exchange activity (xoNHE) by hypertonic shrinkage required Cl(-), with an EC(50) for bath [Cl(-)] of approximately 3mM. This requirement for chloride was not supported by several nonhalide anions and was not shared by xoNHE activated by acid loading. Hypertonicity-activated xoNHE exhibited an unusual rank order of inhibitory potency among amiloride derivatives and was blocked by Cl(-) transport inhibitors. Chelation of intracellular Ca(2+) by injection of EGTA blocked hypertonic activation of xoNHE, although many inhibitors of Ca(2+)-related signaling pathways were without inhibitory effect. Hypertonicity activated oocyte extracellular signal-regulated kinase 1/2 (ERK1/2), but inhibitors of neither ERK1/2 nor p38 prevented hypertonic activation of xoNHE. However, hypertonicity also stimulated a Cl(-)-dependent increase in c-Jun NH(2)-terminal kinase (JNK) activity. Inhibition of JNK activity prevented hypertonic activation of xoNHE but not activation by acid loading. We conclude that hypertonic activation of Na(+)/H(+) exchange in Xenopus oocytes requires Cl(-) and is mediated by activation of JNK.

Regulation of AE2 anion exchanger by intracellular pH: critical regions of the NH(2)-terminal cytoplasmic domain

The role of intracellular pH (pH(i)) in regulation of AE2 function in Xenopus oocytes remains unclear. We therefore compared AE2-mediated (36)Cl(-) efflux from Xenopus oocytes during imposed variation of extracellular pH (pH(o)) or variation of pH(i) at constant pH(o). Wild-type AE2-mediated (36)Cl(-) efflux displayed a steep pH(o) vs. activity curve, with pH(o(50)) = 6.91 +/- 0.04. Sequential NH(2)-terminal deletion of amino acid residues in two regions, between amino acids 328 and 347 or between amino acids 391 and 510, shifted pH(o(50)) to more acidic values by nearly 0.6 units. Permeant weak acids were then used to alter oocyte pH(i) at constant pH(o) and were shown to be neither substrates nor inhibitors of AE2-mediated Cl(-) transport. At constant pH(o), AE2 was inhibited by intracellular acidification and activated by intracellular alkalinization. Our data define structure-function relationships within the AE2 NH(2)-terminal cytoplasmic domain, which demonstrates distinct structural requirements for AE2 regulation by intracellular and extracellular protons.

Differential expression and regulation of AE2 anion exchanger subtypes in rabbit parietal and mucous cells

1. The anion exchanger isoform 2 (AE2) gene encodes three subtypes (AE2a, b and c), which have different N-termini and tissue distributions. AE2 is expressed at high levels in the stomach, where it is thought to mediate basolateral base exit during acid production. The present study investigated if the three AE2 subtypes are differentially expressed and regulated in different cell types within the gastric mucosa. 2. The cloning strategy to obtain rabbit AE2a, b and c cDNAs combined genomic PCR and RT-PCR based on primers deduced from the rat sequences. Semiquantitative RT-PCR using homologous primers revealed much higher AE2 mRNA expression in rabbit parietal cells (PCs) than in mucous cells (MCs). The subtype expression pattern was AE2b >> AE2c > or = AE2a in PCs and AE2a >AE2b >> AE2c in MCs. Sequence analysis revealed the presence of a highly conserved protein kinase C (PKC) consensus sequence in the AE2a alternative N-terminus. 3. Maximal Cl(-)-HCO(3)(-) exchange rates, measured fluorometrically in BCECF-loaded cultured gastric cells, were much higher in PCs than MCs. PKC activation by phorbol ester stimulated maximal Cl(-)-HCO(3)(-) exchange rates in MCs but not in PCs, whereas forskolin had no effect in each cell type. 4. In summary, rabbit PCs and MCs, which originate from the same gastric stem cell population, display a completely different AE2 subtype expression pattern. Therefore, AE2 subtype expression is not organ specific but cell type specific. The different regulation of anion exchange in parietal and mucous cells suggests that AE2 subtypes may be differentially regulated.