Localization of the Cl-/HCO3- anion exchanger binding site to the amino-terminal region of carbonic anhydrase II

Carbonic anhydrase, its inhibitors and vascular function

It has been known for some time that Carbonic Anhydrase (CA, EC 4.2.1.1) plays a complex role in vascular function, and in the regulation of vascular tone. Clinically employed CA inhibitors (CAIs) are used primarily to lower intraocular pressure in glaucoma, and also to affect retinal blood flow and oxygen saturation. CAIs have been shown to dilate vessels and increase blood flow in both the cerebral and ocular vasculature. Similar effects of CAIs on vascular function have been observed in the liver, brain and kidney, while vessels in abdominal muscle and the stomach are unaffected. Most of the studies on the vascular effects of CAIs have been focused on the cerebral and ocular vasculatures, and in particular the retinal vasculature, where vasodilation of its vessels, after intravenous infusion of sulfonamide-based CAIs can be easily observed and measured from the fundus of the eye. The mechanism by which CAIs exert their effects on the vasculature is still unclear, but the classic sulfonamide-based inhibitors have been found to directly dilate isolated vessel segments when applied to the extracellular fluid. Modification of the structure of CAI compounds affects their efficacy and potency as vasodilators. CAIs of the coumarin type, which generally are less effective in inhibiting the catalytically dominant isoform hCA II and unable to accept NO, have comparable vasodilatory effects as the primary sulfonamides on pre-contracted retinal arteriolar vessel segments, providing insights into which CA isoforms are involved. Alterations of the lipophilicity of CAI compounds affect their potency as vasodilators, and CAIs that are membrane impermeant do not act as vasodilators of isolated vessel segments. Experiments with CAIs, that shed light on the role of CA in the regulation of vascular tone of vessels, will be discussed in this review. The role of CA in vascular function will be discussed, with specific emphasis on findings with the effects of CA inhibitors (CAI).

Potential Novel Role of Membrane-Associated Carbonic Anhydrases in the Kidney

Carbonic anhydrases (CAs), because they catalyze the interconversion of carbon dioxide (CO₂) and water into bicarbonate (HCO₃^-) and protons (H⁺), thereby influencing pH, are near the core of virtually all physiological processes in the body. In the kidneys, soluble and membrane-associated CAs and their synergy with acid-base transporters play important roles in urinary acid secretion, the largest component of which is the reabsorption of HCO₃^- in specific nephron segments. Among these transporters are the Na⁺-coupled HCO₃^- transporters (NCBTs) and the Cl^--HCO₃^- exchangers (AEs)-members of the "solute-linked carrier" 4 (SLC4) family. All of these transporters have traditionally been regarded as "HCO₃^-" transporters. However, recently our group has demonstrated that two of the NCBTs carry CO₃^2- rather than HCO₃^- and has hypothesized that all NCBTs follow suit. In this review, we examine current knowledge on the role of CAs and "HCO₃^-" transporters of the SLC4 family in renal acid-base physiology and discuss how our recent findings impact renal acid secretion, including HCO₃^- reabsorption. Traditionally, investigators have associated CAs with producing or consuming solutes (CO₂, HCO₃^-, and H⁺) and thus ensuring their efficient transport across cell membranes. In the case of CO₃^2- transport by NCBTs, however, we hypothesize that the role of membrane-associated CAs is not the appreciable production or consumption of substrates but the minimization of pH changes in nanodomains near the membrane.

Local Attraction of Substrates and Co-Substrates Enhances Weak Acid and Base Transmembrane Transport

The transmembrane transport of weak acid and base metabolites depends on the local pH conditions that affect the protonation status of the substrates and the availability of co-substrates, typically protons. Different protein designs ensure the attraction of substrates and co-substrates to the transporter entry sites. These include electrostatic surface charges on the transport proteins and complexation with seemingly transport-unrelated proteins that provide substrate and/or proton antenna, or enzymatically generate substrates in place. Such protein assemblies affect transport rates and directionality. The lipid membrane surface also collects and transfers protons. The complexity in the various systems enables adjustability and regulation in a given physiological or pathophysiological situation. This review describes experimentally shown principles in the attraction and facilitation of weak acid and base transport substrates, including monocarboxylates, ammonium, bicarbonate, and arsenite, plus protons as a co-substrate.

Proton Transport in Cancer Cells: The Role of Carbonic Anhydrases

Intra- and extracellular pH regulation is a pivotal function of all cells and tissues. Net outward transport of H⁺ is a prerequisite for normal physiological function, since a number of intracellular processes, such as metabolism and energy supply, produce acid. In tumor tissues, distorted pH regulation results in extracellular acidification and the formation of a hostile environment in which cancer cells can outcompete healthy local host cells. Cancer cells employ a variety of H⁺/HCO₃⁻-coupled transporters in combination with intra- and extracellular carbonic anhydrase (CA) isoforms, to alter intra- and extracellular pH to values that promote tumor progression. Many of the transporters could closely associate to CAs, to form a protein complex coined “transport metabolon”. While transport metabolons built with HCO₃⁻-coupled transporters require CA catalytic activity, transport metabolons with monocarboxylate transporters (MCTs) operate independently from CA catalytic function. In this article, we assess some of the processes and functions of CAs for tumor pH regulation and discuss the role of intra- and extracellular pH regulation for cancer pathogenesis and therapeutic intervention.

Carbonic anhydrases enhance activity of endogenous Na-H exchangers and not the electrogenic Na/HCO3 cotransporter NBCe1-A, expressed in Xenopus oocytes

KEY POINTS

According to the HCO 3 - metabolon hypothesis, direct association of cytosolic carbonic anhydrases (CAs) with the electrogenic Na/HCO3 cotransporter NBCe1-A speeds transport by regenerating/consuming HCO 3 - . The present work addresses published discrepancies as to whether cytosolic CAs stimulate NBCe1-A, heterologously expressed in Xenopus oocytes. We confirm the essential elements of the previous experimental observations, taken as support for the HCO 3 - metabolon hypothesis. However, using our own experimental protocols or those of others, we find that NBCe1-A function is unaffected by cytosolic CAs. Previous conclusions that cytosolic CAs do stimulate NBCe1-A can be explained by an unanticipated stimulatory effect of the CAs on an endogenous Na-H exchanger. Theoretical analyses show that, although CAs could stimulate non- HCO 3 - transporters (e.g. Na-H exchangers) by accelerating CO2 / HCO 3 - -mediated buffering of acid-base equivalents, they could not appreciably affect transport rates of NBCe1 or other transporters carrying HCO 3 - , CO 3 = , or NaCO 3 - ion pairs.

ABSTRACT

The HCO 3 - metabolon hypothesis predicts that cytosolic carbonic anhydrase (CA) binds to NBCe1-A, promotes HCO 3 - replenishment/consumption, and enhances transport. Using a short step-duration current-voltage (I-V) protocol with Xenopus oocytes expressing eGFP-tagged NBCe1-A, our group reported that neither injecting human CA II (hCA II) nor fusing hCA II to the NBCe1-A carboxy terminus affects background-subtracted NBCe1 slope conductance (GNBC ), which is a direct measure of NBCe1-A activity. Others - using bovine CA (bCA), untagged NBCe1-A, and protocols keeping holding potential (Vh ) far from NBCe1-A's reversal potential (Erev ) for prolonged periods - found that bCA increases total membrane current (ΔIm ), which apparently supports the metabolon hypothesis. We systematically investigated differences in the two protocols. In oocytes expressing untagged NBCe1-A, injected with bCA and clamped to -40 mV, CO2 / HCO 3 - exposures markedly decrease Erev , producing large transient outward currents persisting for >10 min and rapid increases in [Na+ ]i . Although the CA inhibitor ethoxzolamide (EZA) reduces both ΔIm and d[Na+ ]i /dt, it does not reduce GNBC . In oocytes not expressing NBCe1-A, CO2 / HCO 3 - triggers rapid increases in [Na+ ]i that both hCA II and bCA enhance in concentration-dependent manners. These d[Na+ ]i /dt increases are inhibited by EZA and blocked by EIPA, a Na-H exchanger (NHE) inhibitor. In oocytes expressing untagged NBCe1-A and injected with bCA, EIPA abolishes the EZA-dependent decreases in ΔIm and d[Na+ ]i /dt. Thus, CAs/EZA produce their ΔIm and d[Na+ ]i /dt effects not through NBCe1-A, but endogenous NHEs. Theoretical considerations argue against a CA stimulation of HCO 3 - transport, supporting the conclusion that an NBCe1-A- HCO 3 - metabolon does not exist in oocytes.

Transport Metabolons and Acid/Base Balance in Tumor Cells

Solid tumors are metabolically highly active tissues, which produce large amounts of acid. The acid/base balance in tumor cells is regulated by the concerted interplay between a variety of membrane transporters and carbonic anhydrases (CAs), which cooperate to produce an alkaline intracellular, and an acidic extracellular, environment, in which cancer cells can outcompete their adjacent host cells. Many acid/base transporters form a structural and functional complex with CAs, coined "transport metabolon". Transport metabolons with bicarbonate transporters require the binding of CA to the transporter and CA enzymatic activity. In cancer cells, these bicarbonate transport metabolons have been attributed a role in pH regulation and cell migration. Another type of transport metabolon is formed between CAs and monocarboxylate transporters, which mediate proton-coupled lactate transport across the cell membrane. In this complex, CAs function as "proton antenna" for the transporter, which mediate the rapid exchange of protons between the transporter and the surroundings. These transport metabolons do not require CA catalytic activity, and support the rapid efflux of lactate and protons from hypoxic cancer cells to allow sustained glycolytic activity and cell proliferation. Due to their prominent role in tumor acid/base regulation and metabolism, transport metabolons might be promising drug targets for new approaches in cancer therapy.

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.

Uncoupling of Carbonic Anhydrase from Na-H exchanger-1 in Experimental Colitis: A Possible Mechanistic Link with Na-H Exchanger

In this study, we investigated a mechanistic link between Na–H exchanger-1 (NHE-1) and carbonic anhydrase (CA) in experimental colitis induced in the rats by intrarectal administration of trinitrobenzenesulphonic acid (TNBS). Western blot analysis showed CA-I and CA-II as the major isoforms and CA-IV as a minor one in the colon, and they all are expressed as minor isoforms in the ileum. Co-immunoprecipitation and confocal immunofluorescence microscopy showed colocalization of NHE-1 with CA-I and CA-II, but not with CA-IV. TNBS significantly reduced the levels of NHE-1 and CA protein isoforms in the colon, but not in the uninflamed ileum. A similar reduction profile of the expression of CA isozymes was also obtained in ex vivo treatment of normal colon strips with TNF-α. The level of uncoupling as detected by co-immunoprecipitation was significantly more pronounced. A peptide (83 aa) from the NHE-1 C-terminus demonstrated binding of CA-II only, but not of the CA-I or CA-IV isoform. Furthermore, the profile of inflammatory test markers confirmed inflammation in the tissue used. These findings taken together suggest an inflammation-induced uncoupling of CA and NHE-1, which might be a putative mechanism for reducing the activity of NHE-1 in experimental colitis. This uncoupling might lead to an intracellular accumulation of H^+, resulting in acidosis and necrosis in the inflamed colon.

Exhaustive-exercise-induced oxidative stress alteration of erythrocyte oxygen release capacity

The aim of the present study was to explore the effect of exhaustive running exercise in the oxygen release capacity of rat erythrocytes. Rats were divided into sedentary control, moderate running exercise, and exhaustive running exercise groups. The thermodynamic and kinetic properties of the erythrocyte oxygen release process of the different groups were tested. We also determined the degree of band-3 oxidation and phosphorylation, anion transport activity, and carbonic anhydrase isoform II activity. Biochemical studies suggested that exhaustive running significantly increased oxidative injury parameters in thiobarbituric acid reactive substances and methaemoglobin levels. Furthermore, exhaustive running significantly decreased anion transport activity and carbonic anhydrase isoform II activity. Thermodynamic analysis indicated that erythrocytes oxygen release ability also significantly increased due to elevated 2,3-DPG level after exhaustive running. Kinetic analysis indicated that exhaustive running resulted in significantly decreased T50 value. We presented evidence that exhaustive running remarkably impacted thermodynamic and kinetic properties of RBC oxygen release. In addition, changes in 2,3-DPG levels and band-3 oxidation and phosphorylation could be the driving force for exhaustive-running-induced alterations in erythrocyte oxygen release thermodynamic and kinetic properties.

A surface proton antenna in carbonic anhydrase II supports lactate transport in cancer cells

Many tumor cells produce vast amounts of lactate and acid, which have to be removed from the cell to prevent intracellular lactacidosis and suffocation of metabolism. In the present study, we show that proton-driven lactate flux is enhanced by the intracellular carbonic anhydrase CAII, which is colocalized with the monocarboxylate transporter MCT1 in MCF-7 breast cancer cells. Co-expression of MCTs with various CAII mutants in Xenopus oocytes demonstrated that CAII facilitates MCT transport activity in a process involving CAII-Glu69 and CAII-Asp72, which could function as surface proton antennae for the enzyme. CAII-Glu69 and CAII-Asp72 seem to mediate proton transfer between enzyme and transporter, but CAII-His64, the central residue of the enzyme's intramolecular proton shuttle, is not involved in proton shuttling between the two proteins. Instead, this residue mediates binding between MCT and CAII. Taken together, the results suggest that CAII features a moiety that exclusively mediates proton exchange with the MCT to facilitate transport activity.

Adaptor protein 1 B mu subunit does not contribute to the recycling of kAE1 protein in polarized renal epithelial cells

Mutations in the gene encoding the kidney anion exchanger 1 (kAE1) can lead to distal renal tubular acidosis (dRTA). dRTA mutations reported within the carboxyl (C)-terminal tail of kAE1 result in apical mis-targeting of the exchanger in polarized renal epithelial cells. As kAE1 physically interacts with the μ subunit of epithelial adaptor protein 1 B (AP-1B), we investigated the role of heterologously expressed μ1B subunit of the AP-1B complex for kAE1 retention to the basolateral membrane in polarized porcine LLC-PK1 renal epithelial cells that are devoid of endogenous AP-1B. We confirmed the interaction and close proximity between kAE1 and μ1B using immunoprecipitation and proximity ligation assay, respectively. Expressing the human μ1B subunit in these cells decreased significantly the amount of cell surface kAE1 at the steady state, but had no significant effect on kAE1 recycling and endocytosis. We show that (i) heterologous expression of μ1B displaces the physical interaction of endogenous GAPDH with kAE1 WT supporting that both AP-1B and GAPDH proteins bind to an overlapping site on kAE1 and (ii) phosphorylation of tyrosine 904 within the potential YDEV interaction motif does not alter the kAE1/AP-1B interaction. We conclude that μ1B subunit is not involved in recycling of kAE1.

A magnificent enzyme superfamily: carbonic anhydrases, their purification and characterization

In this paper, we reviewed the purification and characterization methods of the α-carbonic anhydrase (CA, EC 4.2.1.1) class. Six genetic families (α-, β-, γ-, δ-, ζ- and η-CAs) all know to date, all encoding such enzymes in organisms widely distributed over the phylogenetic tree. Starting from the manuscripts published in the 1930s on the isolation and purification of α-CAs from blood and other tissues, and ending with the recent discovery of the last genetic family in protozoa, the η-CAs, considered for long time an α-CA, we present historically the numerous and different procedures which were employed for obtaining these catalysts in pure form. α-CAs possess important application in medicine (as many human α-CA isoforms are drug targets) as well as biotechnological processes, in which the enzymes are ultimately used for CO2 capture in order to mitigate the global warming effects due to greenhouse gases. Recently, it was discovered an involvement of CAs in cancerogenesis as well as infection caused by pathogenic agents such as bacteria, fungi and protozoa. Inhibition studies of CAs identified in the genome of the aforementioned organisms might lead to the discovery of innovative drugs with a novel mechanism of action.

Carbonic anhydrase II binds to and increases the activity of the epithelial sodium-proton exchanger, NHE3

Two-thirds of sodium filtered by the renal glomerulus is reabsorbed from the proximal tubule via a sodium/proton exchanger isoform 3 (NHE3)-dependent mechanism. Since sodium and bicarbonate reabsorption are coupled, we postulated that the molecules involved in their reabsorption [NHE3 and carbonic anhydrase II (CAII)] might physically and functionally interact. Consistent with this, CAII and NHE3 were closely associated in a renal proximal tubular cell culture model as revealed by a proximity ligation assay. Direct physical interaction was confirmed in solid-phase binding assays with immobilized CAII and C-terminal NHE3 glutathione-S-transferase fusion constructs. To assess the effect of CAII on NHE3 function, we expressed NHE3 in a proximal tubule cell line and measured NHE3 activity as the rate of intracellular pH recovery, following an acid load. NHE3-expressing cells had a significantly greater rate of intracellular pH recovery than controls. Inhibition of endogenous CAII activity with acetazolamide significantly decreased NHE3 activity, indicating that CAII activates NHE3. To ascertain whether CAII binding per se activates NHE3, we expressed NHE3 with wild-type CAII, a catalytically inactive CAII mutant (CAII-V143Y), or a mutant unable to bind other transporters (CAII-HEX). NHE3 activity increased upon wild-type CAII coexpression, but not in the presence of the CAII V143Y or HEX mutant. Together these studies support an association between CAII and NHE3 that alters the transporter's activity.

Analysis of the binding moiety mediating the interaction between monocarboxylate transporters and carbonic anhydrase II

Proton-coupled monocarboxylate transporters (MCTs) mediate the exchange of high energy metabolites like lactate between different cells and tissues. We have reported previously that carbonic anhydrase II augments transport activity of MCT1 and MCT4 by a noncatalytic mechanism, while leaving transport activity of MCT2 unaltered. In the present study, we combined electrophysiological measurements in Xenopus oocytes and pulldown experiments to analyze the direct interaction between carbonic anhydrase II (CAII) and MCT1, MCT2, and MCT4, respectively. Transport activity of MCT2-WT, which lacks a putative CAII-binding site, is not augmented by CAII. However, introduction of a CAII-binding site into the C terminus of MCT2 resulted in CAII-mediated facilitation of MCT2 transport activity. Interestingly, introduction of three glutamic acid residues alone was not sufficient to establish a direct interaction between MCT2 and CAII, but the cluster had to be arranged in a fashion that allowed access to the binding moiety in CAII. We further demonstrate that functional interaction between MCT4 and CAII requires direct binding of the enzyme to the acidic cluster (431)EEE in the C terminus of MCT4 in a similar fashion as previously shown for binding of CAII to the cluster (489)EEE in the C terminus of MCT1. In CAII, binding to MCT1 and MCT4 is mediated by a histidine residue at position 64. Taken together, our results suggest that facilitation of MCT transport activity by CAII requires direct binding between histidine 64 in CAII and a cluster of glutamic acid residues in the C terminus of the transporter that has to be positioned in surroundings that allow access to CAII.

Bicarbonate transport in health and disease

Bicarbonate (HCO3(-)) has a central place in human physiology as the waste product of mitochondrial energy production and for its role in pH buffering throughout the body. Because bicarbonate is impermeable to membranes, bicarbonate transport proteins are necessary to enable control of bicarbonate levels across membranes. In humans, 14 bicarbonate transport proteins, members of the SLC4 and SLC26 families, function by differing transport mechanisms. In addition, some anion channels and ZIP metal transporters contribute to bicarbonate movement across membranes. Defective bicarbonate transport leads to diseases, including systemic acidosis, brain dysfunction, kidney stones, and hypertension. Altered expression levels of bicarbonate transporters in patients with breast, colon, and lung cancer suggest an important role of these transporters in cancer.

Degradation mechanism of a Golgi-retained distal renal tubular acidosis mutant of the kidney anion exchanger 1 in renal cells

Distal renal tubular acidosis (dRTA) can be caused by mutations in the SLC4A1 gene encoding the anion exchanger 1 (AE1). Both recessive and dominant mutations result in mistrafficking of proteins, preventing them from reaching the basolateral membrane of renal epithelial cells, where their function is needed. In this study, we show that two dRTA mutants are prematurely degraded. Therefore, we investigated the degradation pathway of the kidney AE1 G701D mutant that is retained in the Golgi. Little is known about degradation of nonnative membrane proteins from the Golgi compartments in mammalian cells. We show that the kidney AE1 G701D mutant is polyubiquitylated and degraded by the lysosome and the proteosome. This mutant reaches the plasma membrane, where it is endocytosed and degraded by the lysosome via a mechanism dependent on the peripheral quality control machinery. Furthermore, we show that the function of the mutant is rescued at the cell surface upon inhibition of the lysosome and incubation with a chemical chaperone. We conclude that modulating the peripheral quality control machinery may provide a novel therapeutic option for treatment of patients with dRTA due to a Golgi-retained mutant.

Contrasting Cu, Fe, and Zn isotopic patterns in organs and body fluids of mice and sheep, with emphasis on cellular fractionation

We report Cu, Fe, and Zn natural isotope compositions in organs, body fluids, diets and feces of mice and sheep. Large and systematic isotope variability is observed, notably in the δ(66)Zn in liver and δ(65)Cu in kidneys, but significant differences exist between mice, sheep and humans, especially in the δ(66)Zn value of blood. The results are interpreted with reference to current knowledge of metal trafficking and redox conditions in cells. In general, the light isotopes preferentially fractionate into 'softer' bonds involving sulfur such as cysteine and glutathione, whereas heavy isotopes fractionate into 'harder' bonds involving nitrogen (histidine) and even more oxygen, notably hydroxides, phosphates, and carbonates. Bonds involving the reduced forms Cu(+) and Fe(2+) are enriched in the light isotopes relative to bonds involving the oxidized Cu(2+) and Fe(3+) forms. Differences in blood Zn isotope abundances between mice, sheep and humans may reflect a different prevalence of Zn ZIP transporters. The isotopically heavy Cu in the kidneys may reflect isotope fractionation during redox processes and may be relevant to ascorbate degradation into oxalate.

Physiological functions of the alpha class of carbonic anhydrases

Carbonic anhydrases are ubiquitous enzymes that catalyze the reversible hydration of carbon dioxide. These enzymes are of ancient origin as they are found in the deepest of branches of the evolutionary tree. Of the five different classes of carbonic anhydrases, the alpha class has perhaps received the most attention because of its role in human pathology. This review focuses on the physiological function of this class of carbonic anhydrases organized by their cellular location.

Carbonic anhydrases and their interplay with acid/base-coupled membrane transporters

Carbonic anhydrases (CAs) have not only been identified as ubiquitous enzymes catalyzing the fast reversible hydration of carbon dioxide to generate or consume protons and bicarbonate, but also as intra- and extracellular proteins, which facilitate transport function of many acid/base transporting membrane proteins, coined 'transport metabolon'. Functional interaction between CAs and acid/base transporters, such as chloride/bicarbonate exchanger (AE), sodium-bicarbonate cotransporter (NBC) and sodium/hydrogen exchanger (NHE) has been shown to require both catalytic CA activity as well as direct binding of the enzyme to specific sites on the transporter. In contrast, functional interaction between different CA isoforms and lactate-proton-cotransporting monocarboxylate transporters (MCT) has been found to be isoform-specific and independent of CA catalytic activity, but seems to require an intramolecular proton shuttle within the enzyme. In this chapter, we review the various types of interactions between acid/base-coupled membrane carriers and different CA isoforms, as studied in vitro, in intact Xenopus oocytes, and in various mammalian cell types. Furthermore, we discuss recent findings that indicate the significance of these 'transport metabolons' for normal cell functions.

Activity and distribution of intracellular carbonic anhydrase II and their effects on the transport activity of anion exchanger AE1/SLC4A1

We have investigated the previously published 'metabolon hypothesis' postulating that a close association of the anion exchanger 1 (AE1) and cytosolic carbonic anhydrase II (CAII) exists that greatly increases the transport activity of AE1. We study whether there is a physical association of and direct functional interaction between CAII and AE1 in the native human red cell and in tsA201 cells coexpressing heterologous fluorescent fusion proteins CAII-CyPet and YPet-AE1. In these doubly transfected tsA201 cells, YPet-AE1 is clearly associated with the cell membrane, whereas CAII-CyPet is homogeneously distributed throughout the cell in a cytoplasmic pattern. Förster resonance energy transfer measurements fail to detect close proximity of YPet-AE1 and CAII-CyPet. The absence of an association of AE1 and CAII is supported by immunoprecipitation experiments using Flag-antibody against Flag-tagged AE1 expressed in tsA201 cells, which does not co-precipitate native CAII but co-precipitates coexpressed ankyrin. Both the CAII and the AE1 fusion proteins are fully functional in tsA201 cells as judged by CA activity and by cellular HCO3(-) permeability (P(HCO3(-))) sensitive to inhibition by 4,4-Diisothiocyano-2,2-stilbenedisulfonic acid. Expression of the non-catalytic CAII mutant V143Y leads to a drastic reduction of endogenous CAII and to a corresponding reduction of total intracellular CA activity. Overexpression of an N-terminally truncated CAII lacking the proposed site of interaction with the C-terminal cytoplasmic tail of AE1 substantially increases intracellular CA activity, as does overexpression of wild-type CAII. These variously co-transfected tsA201 cells exhibit a positive correlation between cellular P(HCO3(-)) and intracellular CA activity. The relationship reflects that expected from changes in cytoplasmic CA activity improving substrate supply to or removal from AE1, without requirement for a CAII-AE1 metabolon involving physical interaction. A functional contribution of the hypothesized CAII-AE1 metabolon to erythroid AE1-mediated HCO3(-) transport was further tested in normal red cells and red cells from CAII-deficient patients that retain substantial CA activity associated with the erythroid CAI protein lacking the proposed AE1-binding sequence. Erythroid P(HCO3(-)) was indistinguishable in these two cell types, providing no support for the proposed functional importance of the physical interaction of CAII and AE1. A theoretical model predicts that homogeneous cytoplasmic distribution of CAII is more favourable for cellular transport of HCO3(-) and CO2 than is association of CAII with the cytoplasmic surface of the plasma membrane. This is due to the fact that the relatively slow intracellular transport of H(+) makes it most efficient to place the CA in the vicinity of the haemoglobin molecules, which are homogeneously distributed over the cytoplasm.

Membrane transport metabolons

In this review evidence from a wide variety of biological systems is presented for the genetic, functional, and likely physical association of membrane transporters and the enzymes that metabolize the transported substrates. This evidence supports the hypothesis that the dynamic association of transporters and enzymes creates functional membrane transport metabolons that channel substrates typically obtained from the extracellular compartment directly into their cellular metabolism. The immediate modification of substrates on the inner surface of the membrane prevents back-flux through facilitated transporters, increasing the efficiency of transport. In some cases products of the enzymes are themselves substrates for the transporters that efflux the products in an exchange or antiport mechanism. Regulation of the binding of enzymes to transporters and their mutual activities may play a role in modulating flux through transporters and entry of substrates into metabolic pathways. Examples showing the physical association of transporters and enzymes are provided, but available structural data is sparse. Genetic and functional linkages between membrane transporters and enzymes were revealed by an analysis of Escherichia coli operons encoding polycistronic mRNAs and provide a list of predicted interactions ripe for further structural studies. This article supports the view that membrane transport metabolons are important throughout Nature in organisms ranging from bacteria to humans.

Transport activity of the sodium bicarbonate cotransporter NBCe1 is enhanced by different isoforms of carbonic anhydrase

Transport metabolons have been discussed between carbonic anhydrase II (CAII) and several membrane transporters. We have now studied different CA isoforms, expressed in Xenopus oocytes alone and together with the electrogenic sodium bicarbonate cotransporter 1 (NBCe1), to determine their catalytic activity and their ability to enhance NBCe1 transport activity. pH measurements in intact oocytes indicated similar activity of CAI, CAII and CAIII, while in vitro CAIII had no measurable activity and CAI only 30% of the activity of CAII. All three CA isoforms increased transport activity of NBCe1, as measured by the transport current and the rate of intracellular sodium rise in oocytes. Two CAII mutants, altered in their intramolecular proton pathway, CAII-H64A and CAII-Y7F, showed significant catalytic activity and also enhanced NBCe1 transport activity. The effect of CAI, CAII, and CAII mutants on NBCe1 activity could be reversed by blocking CA activity with ethoxyzolamide (EZA, 10 µM), while the effect of the less EZA-sensitive CAIII was not reversed. Our results indicate that different CA isoforms and mutants, even if they show little enzymatic activity in vitro, may display significant catalytic activity in intact cells, and that the ability of CA to enhance NBCe1 transport appears to depend primarily on its catalytic activity.

Semisynthesis of fluorescent metabolite sensors on cell surfaces

Progress in understanding signal transduction and metabolic pathways is hampered by a shortage of suitable sensors for tracking metabolites, second messengers, and neurotransmitters in living cells. Here we introduce a class of rationally designed semisynthetic fluorescent sensor proteins, called Snifits, for measuring metabolite concentrations on the cell surface of mammalian cells. Functional Snifits are assembled on living cells through two selective chemical labeling reactions of a genetically encoded protein scaffold. Our best Snifit displayed fluorescence intensity ratio changes on living cells significantly higher than any previously reported cell-surface-targeted fluorescent sensor protein. This work establishes a generally applicable and rational strategy for the generation of cell-surface-targeted fluorescent sensor proteins for metabolites of interest.

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

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

Transport activity of the high-affinity monocarboxylate transporter MCT2 is enhanced by extracellular carbonic anhydrase IV but not by intracellular carbonic anhydrase II

The ubiquitous enzyme carbonic anhydrase isoform II (CAII) has been shown to enhance transport activity of the proton-coupled monocarboxylate transporters MCT1 and MCT4 in a non-catalytic manner. In this study, we investigated the role of cytosolic CAII and of the extracellular, membrane-bound CA isoform IV (CAIV) on the lactate transport activity of the high-affinity monocarboxylate transporter MCT2, heterologously expressed in Xenopus oocytes. In contrast to MCT1 and MCT4, transport activity of MCT2 was not altered by CAII. However, coexpression of CAIV with MCT2 resulted in a significant increase in MCT2 transport activity when the transporter was coexpressed with its associated ancillary protein GP70 (embigin). The CAIV-mediated augmentation of MCT2 activity was independent of the catalytic activity of the enzyme, as application of the CA-inhibitor ethoxyzolamide or coexpressing the catalytically inactive mutant CAIV-V165Y did not suppress CAIV-mediated augmentation of MCT2 transport activity. Furthermore, exchange of His-88, mediating an intramolecular H(+)-shuttle in CAIV, to alanine resulted only in a slight decrease in CAIV-mediated augmentation of MCT2 activity. The data suggest that extracellular membrane-bound CAIV, but not cytosolic CAII, augments transport activity of MCT2 in a non-catalytic manner, possibly by facilitating a proton pathway other than His-88.

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

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

Topology models of anion exchanger 1 that incorporate the anti-parallel V-shaped motifs found in the EM structureThis paper is one of a selection of papers published in a Special Issue entitled CSBMCB 53rd Annual Meeting — Membrane Proteins in Health and Disease, and has undergone the Journal’s usual peer review process

We recently published the three-dimensional structure of the membrane domain of human erythrocyte anion exchanger 1 (AE1) at 7.5 Å resolution, solved by electron crystallography. The structure exhibited distinctive anti-parallel V-shaped motifs, which protrude from the membrane bilayer on both sides. Similar motifs exist in the previously reported structure of a bacterial chloride channel (ClC)-type protein. Here, we propose two topology models of AE1 that reflect the anti-parallel V-shaped structural motifs. One is assumed to have structural similarity with the ClC protein and the other is only assumed to have internal repeats, as is often the case with transporters. Both models are consistent with most topological results reported thus far for AE1, each having advantages and disadvantages.

Cytosolic H+ microdomain developed around AE1 during AE1-mediated Cl-/HCO3- exchange

Microdomains, regions of discontinuous cytosolic solute concentration enhanced by rapid solute transport and slow diffusion rates, have many cellular roles. pH-regulatory membrane transporters, like the Cl−/HCO3− exchanger AE1, could develop H+ microdomains since AE1 has a rapid transport rate and cytosolic H+ diffusion is slow. We examined whether the pH environment surrounding AE1 differs from other cellular locations. As AE1 drives Cl−/HCO3− exchange, differences in pH, near and remote from AE1, were monitored by confocal microscopy using two pH-sensitive fluorescent proteins: deGFP4 (GFP) and mNectarine (mNect). Plasma membrane (PM) pH (defined as ∼1 μm region around the cell periphery) was monitored by GFP fused to AE1 (GFP.AE1), and mNect fused to an inactive mutant of the Na+-coupled nucleoside co-transporter, hCNT3 (mNect.hCNT3). GFP.AE1 to mNect.hCNT3 distance was varied by co-expression of different amounts of the two proteins in HEK293 cells. As the GFP.AE1–mNect.hCNT3 distance increased, mNect.hCNT3 detected the Cl−/HCO3− exchange-associated cytosolic pH change with a time delay and reduced rate of pH change compared to GFP.AE1. We found that a H+ microdomain 0.3 μm in diameter forms around GFP.AE1 during physiological HCO3− transport. Carbonic anhydrase isoform II inhibition prevented H+ microdomain formation. We also measured the rate of H+ movement from PM GFP.AE1 to endoplasmic reticulum (ER), using mNect fused to the cytosolic face of ER-resident calnexin (CNX.mNect). The rate of H+ diffusion through cytosol was 60-fold faster than along the cytosolic surface of the plasma membrane. The pH environment surrounding pH regulatory transport proteins may differ as a result of H+ microdomain formation, which will affect nearby pH-sensitive processes.

Intramolecular proton shuttle supports not only catalytic but also noncatalytic function of carbonic anhydrase II

Carbonic anhydrases (CAs) catalyze the reversible hydration of CO(2) to HCO(3)(-) and H(+). The rate-limiting step in this reaction is the shuttle of protons between the catalytic center of the enzyme and the bulk solution. In carbonic anhydrase II (CAII), the fastest and most wide-spread isoform, this H(+) shuttle is facilitated by the side chain of His64, whereas CA isoforms such as carbonic anhydrase III (CAIII), which lack such a shuttle, have only low catalytic activity in vitro. By using heterologous protein expression in Xenopus oocytes, we tested the role of this intramolecular H(+) shuttle on CA activity in an intact cell. The data revealed that CAIII, shown in vitro to have ∼1,000-fold reduced activity as compared with CAII, displays significant catalytic activity in the intact cell. Furthermore, we tested the hypothesis that the H(+) shuttle in CAII itself can facilitate transport activity of the monocarboxylate transporters 1 and 4 (MCT1/4) independent of catalytic activity. Our results show that His64 is essential for the enhancement of lactate transport via MCT1/4, because a mutation of this residue to alanine (CAII-H64A) abolishes the CAII-induced increase in MCT1/4 activity. However, injection of 4-methylimidazole, which acts as an exogenous H(+) donor/acceptor, can restore the ability of CAII-H64A to enhance transport activity of MCT1/4. These findings support the hypothesis that the H(+) shuttle in CAII not only facilitates CAII catalytic activity but also can enhance activity of acid-/base-transporting proteins such as MCT1/4 in a direct, noncatalytic manner, possibly by acting as an "H(+)-collecting antenna."

Kinetic and crystallographic studies of the role of tyrosine 7 in the active site of human carbonic anhydrase II

The rate limiting step in catalysis of bicarbonate dehydration by human carbonic anhydrase II (HCA II) is an intramolecular proton transfer from His64 to the zinc-bound hydroxide. We have examined the role of Tyr7 using site-specific mutagenesis and measuring catalysis by the ¹⁸O exchange method using membrane inlet mass spectrometry. The side chain of Tyr7 in HCA II extends into the active-site cavity about 7 Å from the catalytic zinc atom. Replacement of Tyr7 with eight other amino acids had no effect on the interconversion of bicarbonate and CO₂, but in some cases caused enhancements in the rate constant of proton transfer by nearly 10-fold. The variant Y7I HCA II enhanced intramolecular proton transfer approximately twofold; its structure was determined by X-ray crystallography at 1.5 Å resolution. No changes were observed in the ordered solvent structure in the active-site cavity or in the conformation of the side chain of the proton shuttle His64. However, the first 11 residues of the amino-terminal chain in Y7I HCA II assumed an alternate conformation compared with the wild type. Differential scanning calorimetry showed variants at position 7 had a melting temperature approximately 8 °C lower than that of the wild type.

Nonenzymatic augmentation of lactate transport via monocarboxylate transporter isoform 4 by carbonic anhydrase II

Monocarboxylate transporters (MCTs) are carriers of high-energy metabolites like lactate and pyruvate, and different MCT isoforms are expressed in a wide range of cells and tissues. Transport activity of MCT isoform 1 (MCT1), heterologously expressed in Xenopus oocytes, has previously been shown to be supported by carbonic anhydrase II (CAII) in a noncatalytic manner. In the present study, we investigated possible interactions of CAII with MCT4, expressed in Xenopus oocytes. MCT4 transport activity is enhanced both by injected and by coexpressed CAII, similar to MCT1, with the highest augmentation at low extracellular pH and low lactate concentrations. CAII-induced augmentation in MCT4 transport activity is independent from the enzyme's catalytic function, as shown by application of the CA inhibitor ethoxyzolamide and by coexpression of MCT4 with the catalytically inactive mutant CAII-V143Y.

Tumor metabolism of lactate: the influence and therapeutic potential for MCT and CD147 regulation

Tumor metabolism consists of complex interactions between oxygenation states, metabolites, ions, the vascular network and signaling cascades. Accumulation of lactate within tumors has been correlated with poor clinical outcomes. While its production has negative implications, potentially contributing to tumor progression, the implications of the ability of tumors to utilize lactate can offer new therapeutic targets for the future. Monocarboxylate transporters (MCTs) of the SLC16A gene family influence substrate availability, the metabolic path of lactate and pH balance within the tumor. CD147, a chaperone to some MCT subtypes, contributes to tumor progression and metastasis. The implications and consequences of lactate utilization by tumors are currently unknown; therefore future research is needed on the intricacies of tumor metabolism. The possibility of metabolic modification of the tumor microenvironment via regulation or manipulation of MCT1 and CD147 may prove to be promising avenues of therapeutic options.

Evaluating the role of carbonic anhydrases in the transport of HCO3--related species

The soluble enzyme carbonic anhydrase II (CAII) plays an important role in CO(2) influx and efflux by red blood cells (RBCs), a process initiated by changes in the extracellular [CO(2)] (CO(2)-initiated CO(2) transport). Evidence suggests that CAII may be part of a macromolecular complex at the inner surface of the RBC membrane. Some have suggested CAII specifically binds to a motif on the cytoplasmic C terminus (Ct) of the Cl-HCO(3) exchanger AE1 and some other members of the SLC4 family of HCO(3)(-) transporters, a transport metabolon. Moreover, others have suggested that this bound CAII enhances the transport of HCO(3)(-)-related species-HCO(3)(-), CO(3)(), or CO(3)() ion pairs-when the process is initiated by altering the activity of the transporter (HCO(3)(-)-initiated HCO(3)(-) transport). In this review, I assess the theoretical roles of CAs in the transport of CO(2) and HCO(3)(-)-related species, concluding that although the effect of bound CAII on CO(2)-initiated CO(2) transport is expected to be substantial, the effect of bound CAs on HCO(3)(-)-initiated HCO(3)(-) transport is expected to be modest at best. I also assess the experimental evidence for CAII binding to AE1 and other transporters, and the effects of this binding on HCO(3)(-)-initiated HCO(3)(-) transport. The early conclusion that CAII binds to the Ct of AE1 appears to be the result of unpredictable effects of GST in the GST fusion proteins used in the studies. The early conclusion that bound CAII speeds HCO(3)(-)-initiated HCO(3)(-) transport appears to be the result of CAII accelerating the pH changes used as a read-out of transport. Thus, it appears that CAII does not bind directly to AE1 or other SLC4 proteins, and that bound CAII does not substantially accelerate HCO(3)(-)-initiated HCO(3)(-) transport.

Bicarbonate homeostasis in excitable tissues: role of AE3 Cl-/HCO3- exchanger and carbonic anhydrase XIV interaction

Bicarbonate transport and metabolism are key elements of normal cellular function. Two alternate transcripts of anion exchanger 3 (AE3), full-length (AE3fl) and cardiac (AE3c), are expressed in central nervous system (CNS), where AE3 catalyzes electroneutral Cl(-)/HCO(3)(-) exchange across the plasma membrane of neuronal and glial cells of CNS. Anion exchanger isoforms, AE3fl and AE3c, associate with the carbonic anhydrases (CA) CAII and CAIV, forming a HCO(3)(-) transport metabolon, to maximize HCO(3)(-) flux across the plasma membrane. CAXIV, with catalytic domain anchored to the extracellular surface, is also expressed in CNS. Here physical association of AE3 and CAXIV was examined by coimmunoprecipitation experiments, using mouse brain and retinal lysates. CAXIV immunoprecipitated with anti-AE3 antibody, and both AE3 isoforms were immunoprecipitated using anti-CAXIV antibody, indicating CAXIV and AE3 interaction in the CNS. Confocal images revealed colocalization of CAXIV and AE3 in Müller and horizontal cells, in the mouse retina. Cl(-)/HCO(3)(-) exchange activity of AE3fl was investigated in transiently transfected human embryonic kidney 293 cells, using intracellular fluorescence measurements of BCECF, to monitor intracellular pH. CAXIV increased the rate of AE3fl-mediated HCO(3)(-) transport by up to 120%, which was suppressed by the CA inhibitor acetazolamide. Association of AE3 and CAXIV may represent a mechanism to enhance disposal of waste CO(2) and to balance pH in excitable tissues.

Cloning and characterization of novel human SLC4A8 gene products encoding Na+-driven Cl-/HCO3(-) exchanger variants NDCBE-A, -C, and -D

The reported sequences of the human and mouse Na+-driven Cl-/HCO3(-) exchangers (NDCBEs) differ greatly in their extreme cytosolic COOH termini (Ct). In human NDCBE (NDCBE-B), a 17-amino acid (aa) sequence replaces 66 aa at the equivalent position in mouse NDCBE (NDCBE-A). We performed 5'- and 3'-rapid amplification of cDNA ends (RACE) on human brain cDNA, followed by PCR of full-length cDNAs to determine whether the human SLC4A8 gene was capable of producing the mouselike Ct sequence. Our study confirmed the presence in human cDNA of mouse NDCBE-like transcripts (human NDCBE-A) and also disclosed the existence of three further novel NDCBE transcripts that we have called NDCBE-C, NDCBE-D, and NDCBE-D'. The novel NDCBE-C/D/D' transcripts initiate at a novel "exon 0" positioned approximately 35 kb upstream of the first exon of NDCBE-A/B. NDCBE-C/D/D' protein products are predicted to be truncated by 54 aa in the cytosolic NH(2) terminus (Nt) compared with NDCBE-A/B. Our data, combined with a new in silico analysis of partial transcripts reported by others in the region of the human SLC4A8 gene, increase the known extent of the SLC4A8 gene by 49 kb, to 124 kb. A functional comparison of NDCBE-A/B/C/D expressed in Xenopus oocytes demonstrates that the Nt variation does not affect the basal functional expression of NDCBE, but those with the shorter Ct have a 25-50% reduced functional expression compared with those with the longer Ct. By comparison with an artificially truncated NDCBE that contains neither 17-aa nor 66-aa Ct cassette, we determined that the functional difference is unrelated to the 66-aa cassette of NDCBE-A/C, but is instead due to an inhibitory effect of the 17-aa cassette of NDCBE-B/D.

Glycophorin A: Band 3 aid

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

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.

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.

Membrane-bound carbonic anhydrases in osteoclasts

Osteoclasts are multinucleated bone-resorbing cells that use multiple pH regulation mechanisms to create an acidic pH in the resorption lacuna. Carbonic anhydrase II and vacuolar H(+)-ATPases produce and transport protons, while chloride channels provide a Cl(-) flux into the resorption site. These activities are required for inorganic matrix dissolution that precedes enzymatic removal of organic bone matrix. In other cell types it has become evident that carbonic anhydrase isoenzymes interact with AE proteins to form transport metabolons that regulate intracellular pH. Membrane-bound carbonic anhydrase isoenzymes may also compensate for the lack of cytoplasmic carbonic anhydrase II. Therefore, our goal was to explore the expression of membrane-bound carbonic anhydrase (CA) isoenzymes CA IV, CA IX, CA XII and CA XIV in bone-resorbing osteoclasts. Immunohistochemistry and confocal microscopy showed expression of CA IV, CA XII and CA XIV in cultured rat and human osteoclasts. To confirm these results, RT-PCR was used. Immunohistochemistry revealed distinct staining patterns for CA IV, CA XII and CA XIV in rat trabecular bone specimens. A plasma membrane staining was observed in bone lining cells with the CA XII antibody while osteoclast plasma membranes were stained with CA IV and CA XIV antibodies. Confocal microscopy of cultured human osteoclasts showed a punctated intracellular CA IV staining and a perinuclear CA XIV staining while no CA IX or CA XII staining was observed. To evaluate the physiological role of membrane-bound CAs in osteoclasts, we used PCS, a novel membrane-impermeable CA inhibitor. Increased osteoclast number and bone resorption activity was observed in rat osteoclast cultures exposed to a low concentration of PCS while higher concentrations affected cell survival. PCS treatment also disturbed intracellular acidification in osteoclasts, as determined by live cell microscopy. In conclusion, our data shows that membrane-bound carbonic anhydrase isoenzymes CA IV and CA XIV are expressed both at mRNA and protein levels in osteoclasts in vivo and in vitro. In addition, the inhibitor experiments provide novel evidence to support the hypothesis that intracellular pH regulation in osteoclasts may indeed involve transport metabolons.

Why bicarbonate?

Bicarbonate is a simple single carbon molecule that plays surprisingly important roles in diverse biological processes. Among these are photosynthesis, the Krebs cycle, whole-body and cellular pH regulation, and volume regulation. Since bicarbonate is charged it is not permeable to lipid bilayers. Mammalian membranes thus contain bicarbonate transport proteins to facilitate the specific transmembrane movement of HCO3(-). This review provides a wide-ranging view of the biochemistry of bicarbonate and its membrane transporters, revealing what makes the study of bicarbonate transport such a rewarding activity.

The role of carbonic anhydrases in renal physiology

Carbonic anhydrase (CA) catalyzes the reversible hydration of CO(2). CA is expressed in most segments of the kidney. CAII and CAIV predominate in human and rabbit kidneys; in rodent kidneys, CAXII, and CAXIV are also present. CAIX is expressed by renal cell carcinoma (RCC). Most of these isoforms, except for rodent CAIV, have high turnover rates. CAII is a cytoplasmic enzyme, whereas the others are membrane-associated; CAIV is anchored by glycosylphosphatidylinositol linkage. Membrane polarity is apical for CAXIV, basolateral for CAXII, and apical and basolateral for CAIV. Luminal membrane CAs facilitate the dehydration of carbonic acid (H(2)CO(3)) that is formed when secreted protons combine with filtered bicarbonate. Basolateral CA enhances the efflux of bicarbonate via dehydration of H(2)CO(3). CAII and CAIV can associate with bicarbonate transporters (e.g., AE1, kNBC1, NBC3, and SCL26A6), and proton antiporter, NHE1 in a membrane protein complex called a transport metabolon. CAXII and CAXIV may also be associated with transporters in normal kidney and CAIX in RCCs. The multiplicity of CAs implicates their importance in acid-base and other solute transport along the nephron. For example, CAII on the cytoplasmic face and CAIV on the extracellular surface provide the 'push' and 'pull' for bicarbonate transport by supplying and dissipating substrate respectively.

Evidence against a direct interaction between intracellular carbonic anhydrase II and pure C-terminal domains of SLC4 bicarbonate transporters

Based on solid-phase binding assays with enzyme-linked immunosorbent assay detection, previous investigators suggested that intracellular carbonic anhydrase II (CA II) interacts at high affinity with the C-terminal (Ct) domains of SLC4 bicarbonate-transport proteins, expressed as glutathione S-transferase (GST) fusion proteins, to form functional HCO3- metabolons. Here we re-evaluated this protein-protein interaction using two solid-phase binding assays. We first compared the ability of the Ct domain of three SLC4 transporters, SLC4-A1 (AE1), SLC4-A4 (NBCe1), and SLC4-A8 (NDCBE), to bind immobilized CA II, using enzyme-linked immunosorbent assay detection. We found that when expressed as GST fusion proteins, all three bind to CA II (Kd 300-600 nM) better than does pure GST. However, we detected no binding of pure SLC4-Ct peptides to immobilized CA II. Second, we reversed assay orientation by immobilizing the SLC4-Ct fusion proteins or peptides. We found that more CA II binds to GST than to any of the three GST-SLC4-Ct fusion proteins. Furthermore, we detected no binding of CA II to any of the immobilized pure SLC4-Ct peptides. Finally, we used surface plasmon resonance to detect possible rapid interactions between CA II and the pure peptides. Although we detected acetazolamide binding to immobilized CA II and specific antibodies binding to immobilized SLC4-Ct peptides, we detected no binding of CA II to immobilized SLC4-Ct or vice versa. Thus, although an HCO3 metabolon may exist, CA II cannot bind directly to pure SLC4-Ct peptides and can bind to GST-SLC4-Ct fusion proteins only when the CA II is immobilized and the fusion protein is soluble, and not vice versa.

Chloride Uptake and Base Secretion in Freshwater Fish: A Transepithelial Ion‐Transport Metabolon?

Despite all the efforts and technological advances during the last few decades, the cellular mechanisms for branchial chloride uptake in freshwater (FW) fish are still unclear. Although a tight 1 : 1 link with HCO-3 secretion has been established, not much is known about the identity of the ion-transporting proteins involved or the energizing steps that allow for the inward transport of Cl- against the concentration gradient. We propose a new model for Cl- uptake in FW fish whereby the combined action of an apical anion exchanger, cytoplasmic carbonic anhydrase, and basolateral V-type H+ -ATPase creates a local [HCO-3] high enough to energize Cl- uptake. Our model is based on analyses of structure-function relationships, reinterpretation of previous results, and novel observations about gill cell subtypes and immunolocalization of the V-H+ -ATPase.

Effect of Human Carbonic Anhydrase II on the Activity of the Human Electrogenic Na/HCO3 Cotransporter NBCe1-A in Xenopus Oocytes

Others report that carbonic anhydrase II (CA II) binds to the C termini of the anion exchanger AE1 and the electrogenic Na/HCO3 cotransporter NBCe1-A, enhancing transport. After injecting oocytes with NBCe1-A cRNA (Day 0), we measured NBC current (I(NBC)) by two-electrode voltage clamp (Day 3), injected CA II protein + Tris or just Tris (Day 3), measured I(NBC) or the initial rate at which the intracellular pH fell (dpH(i)/dt) upon applying 5% CO2 (Day 4), exposed oocytes to the permeant CA inhibitor ethoxzolamide (EZA), and measured I(NBC) or dpH(i)/dt (Day 4). Because dpH(i)/dt was greater in CA II than Tris oocytes, and EZA eliminated the difference, injected CA II was functional. I(NBC) slope conductance was unaffected by injecting CA II. Moreover, EZA had identical effects in CA II versus Tris oocytes. Thus, injected CA II does not enhance NBC activity. In a second protocol, we made a fusion protein with enhanced green fluorescent protein (EGFP) at the 5' end of NBCe1-A and CA II at the 3' end (EGFP-e1-CAII). We measured I(NBC) or dpH(i)/dt (days 3-4), exposed oocytes to EZA, and measured I(NBC) or dpH(i)/dt (Day 3-4). dpH(i)/dt was greater in oocytes expressing EGFP-e1-CA II versus EGFP-e1, and EZA eliminated the difference. Thus, fused CA II was functional. Slope conductances of EGFP-e1-CAII versus EGFP-e1 oocytes were indistinguishable, and EZA had no effect. Thus, even when fused to NBCe1-A, CA II does not enhance NBCe1-A activity.

The structure and function of carbonic anhydrase isozymes in the respiratory system of vertebrates

Carbonic anhydrase is a ubiquitous metalloenzyme that catalyzes the reversible hydration/dehydration of carbon dioxide. To date, 16 different CA isozymes have been identified in mammals, and several novel isozymes have also been identified in non-mammalian vertebrates. These isozymes are involved in many physiological processes; however, one of the most important roles is facilitating the transport and subsequent excretion of carbon dioxide. As such, CA isozymes are found at virtually every step of the process, including the metabolic site of CO(2) production (muscle), the circulating red blood cells, and the primary respiratory surface (gills/lungs). This review will examine the structural characteristics that are integral to CAs participation in respiration, as well as highlight the specific roles and tissues that the different CA isozymes are involved in.

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.