Conservation of an AE3 Cl-/HCO3- exchanger cardiac-specific exon and promoter region and AE3 mRNA expression patterns in murine and human hearts

Mouse models of SLC4-linked disorders of HCO3--transporter dysfunction

The SLC4 family Cl^-/[Formula: see text] cotransporters (NBCe1, NBCe2, NBCn1, and NBCn2) contribute to a variety of vital physiological processes including pH regulation and epithelial fluid secretion. Accordingly, their dysfunction can have devastating effects. Disorders such as epilepsy, hemolytic anemia, glaucoma, hearing loss, osteopetrosis, and renal tubular acidosis are all genetically linked to SLC4-family gene loci. This review summarizes how studies of Slc4-modified mice have enhanced our understanding of the etiology of SLC4-linked pathologies and the interpretation of genetic linkage studies. The review also surveys the novel disease signs exhibited by Slc4-modified mice which could either be considered to presage their description in humans, or to highlight interspecific differences. Finally, novel Slc4-modified mouse models are proposed, the study of which may further our understanding of the basis and treatment of SLC4-linked disorders of [Formula: see text]-transporter dysfunction.

Critical role of bicarbonate and bicarbonate transporters in cardiac function

Bicarbonate is one of the major anions in mammalian tissues and extracellular fluids. Along with accompanying H⁺, HCO₃^- is generated from CO₂ and H₂O, either spontaneously or via the catalytic activity of carbonic anhydrase. It serves as a component of the major buffer system, thereby playing a critical role in pH homeostasis. Bicarbonate can also be utilized by a variety of ion transporters, often working in coupled systems, to transport other ions and organic substrates across cell membranes. The functions of HCO₃^- and HCO₃^--transporters in epithelial tissues have been studied extensively, but their functions in heart are less well understood. Here we review studies of the identities and physiological functions of Cl^-/HCO₃^- exchangers and Na⁺/HCO₃^- cotransporters of the SLC4A and SLC26A families in heart. We also present RNA Seq analysis of their cardiac mRNA expression levels. These studies indicate that slc4a3 (AE3) is the major Cl^-/HCO₃^- exchanger and plays a protective role in heart failure, and that Slc4a4 (NBCe1) is the major Na⁺/HCO₃^- cotransporter and affects action potential duration. In addition, previous studies show that HCO₃^- has a positive inotropic effect in the perfused heart that is largely independent of effects on intracellular Ca²⁺. The importance of HCO₃^- in the regulation of contractility is supported by experiments showing that isolated cardiomyocytes exhibit sharply enhanced contractility, with no change in Ca²⁺ transients, when switched from Hepes-buffered to HCO₃^-- buffered solutions. These studies demonstrate that HCO₃^- and HCO₃^--handling proteins play important roles in the regulation of cardiac function.

Inhibition of peripheral anion exchanger 3 decreases formalin-induced pain

We determined the role of chloride-bicarbonate anion exchanger 3 in formalin-induced acute and chronic rat nociception. Formalin (1%) produced acute (first phase) and tonic (second phase) nociceptive behaviors (flinching and licking/lifting) followed by long-lasting evoked secondary mechanical allodynia and hyperalgesia in both paws. Local peripheral pre-treatment with the chloride-bicarbonate anion exchanger inhibitors 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid prevented formalin-induced nociception mainly during phase 2. These drugs also prevented in a dose-dependent fashion long-lasting evoked secondary mechanical allodynia and hyperalgesia in both paws. Furthermore, post-treatment (on day 1 or 6) with 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid reversed established hypersensitivity. Anion exchanger 3 was expressed in dorsal root ganglion neurons and it co-localized with neuronal nuclei protein (NeuN), substance P and purinergic P2X3 receptors. Furthermore, Western blot analysis revealed a band of about 85 kDa indicative of anion exchanger 3 protein expression in dorsal root ganglia of naïve rats, which was enhanced at 1 and 6 days after 1% formalin injection. On the other hand, this rise failed to occur during 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid exposure. These results suggest that anion exchanger 3 is present in dorsal root ganglia and participates in the development and maintenance of short and long-lasting formalin-induced nociception.

Loss of the AE3 anion exchanger in a hypertrophic cardiomyopathy model causes rapid decompensation and heart failure

The AE3 Cl(-)/HCO(3)(-) exchanger is abundantly expressed in the sarcolemma of cardiomyocytes, where it mediates Cl(-)-uptake and HCO(3)(-)-extrusion. Inhibition of AE3-mediated Cl(-)/HCO(3)(-) exchange has been suggested to protect against cardiac hypertrophy; however, other studies indicate that AE3 might be necessary for optimal cardiac function. To test these hypotheses we crossed AE3-null mice, which appear phenotypically normal, with a hypertrophic cardiomyopathy mouse model carrying a Glu180Gly mutation in α-tropomyosin (TM180). Loss of AE3 had no effect on hypertrophy; however, survival of TM180/AE3 double mutants was sharply reduced compared with TM180 single mutants. Analysis of cardiac performance revealed impaired cardiac function in TM180 and TM180/AE3 mutants. TM180/AE3 double mutants were more severely affected and exhibited little response to β-adrenergic stimulation, a likely consequence of their more rapid progression to heart failure. Increased expression of calmodulin-dependent kinase II and protein phosphatase 1 and differences in methylation and localization of protein phosphatase 2A were observed, but were similar in single and double mutants. Phosphorylation of phospholamban on Ser16 was sharply increased in both single and double mutants relative to wild-type hearts under basal conditions, leading to reduced reserve capacity for β-adrenergic stimulation of phospholamban phosphorylation. Imaging analysis of isolated myocytes revealed reductions in amplitude and decay of Ca(2+) transients in both mutants, with greater reductions in TM180/AE3 mutants, consistent with the greater severity of their heart failure phenotype. Thus, in the TM180 cardiomyopathy model, loss of AE3 had no apparent anti-hypertrophic effect and led to more rapid decompensation and heart failure.

The murine AE4 promoter predominantly drives type B intercalated cell specific transcription

AE4 is an anion exchanger almost exclusively expressed in the collecting ducts of the kidney. This very restricted expression prompted us to analyze its transcription in more detail. 5' RACE yielded alternative transcriptional start sites that are predicted to code for N-terminal protein variants. Comparison of the 5' genomic sequence between species identified a transcriptionally active region with three conserved spans. In transgenic mice beta-galactosidase expression driven by this fragment resembled endogenous AE4 expression and was predominantly restricted to type B intercalated cells. Hence this promoter could prove useful to target type B intercalated cells by genetic approaches.

Impaired cardiac contractility in mice lacking both the AE3 Cl-/HCO3- exchanger and the NKCC1 Na+-K+-2Cl- cotransporter: effects on Ca2+ handling and protein phosphatases

To analyze the cardiac functions of AE3, we disrupted its gene (Slc4a3) in mice. Cl(-)/HCO3(-) exchange coupled with Na+-dependent acid extrusion can mediate pH-neutral Na+ uptake, potentially affecting Ca2+ handling via effects on Na+/Ca2+ exchange. AE3 null mice appeared normal, however, and AE3 ablation had no effect on ischemia-reperfusion injury in isolated hearts or cardiac performance in vivo. The NKCC1 Na+-K+-2Cl(-) cotransporter also mediates Na+ uptake, and loss of NKCC1 alone does not impair contractility. To further stress the AE3-deficient myocardium, we combined the AE3 and NKCC1 knock-outs. Double knock-outs had impaired contraction and relaxation both in vivo and in isolated ventricular myocytes. Ca2+ transients revealed an apparent increase in Ca2+ clearance in double null cells. This was unlikely to result from increased Ca2+ sequestration, since the ratio of phosphorylated phospholamban to total phospholamban was sharply reduced in all three mutant hearts. Instead, Na+/Ca2+ exchanger activity was found to be enhanced in double null cells. Systolic Ca2+ was unaltered, however, suggesting more direct effects on the contractile apparatus of double null myocytes. Expression of the catalytic subunit of protein phosphatase 1 was increased in all mutant hearts. There was also a dramatic reversal, between single null and double null hearts, in the carboxymethylation and localization to the myofibrillar fraction, of the catalytic subunit of protein phosphatase 2A, which corresponded to the loss of normal contractility in double null hearts. These data show that AE3 and NKCC1 affect Ca2+ handling, PLN regulation, and expression and localization of major cardiac phosphatases and that their combined loss impairs cardiac function.

Cardiac hypertrophy in anion exchanger 1-null mutant mice with severe hemolytic anemia

Anion exchanger 1 (AE1; SLC4A1), the plasma membrane Cl(-)/HCO(3)(-) exchanger of erythrocytes, is also expressed in heart. The aim of this study was to assess the role of AE1 in heart function through study of AE1-null (AE1(-/-)) mice, which manifest severe hemolytic anemia resulting from erythrocyte fragility. Heart weight-to-body weight ratios were significantly higher in the AE1(-/-) mice than in wild-type (AE1(+/+)) littermates at both 1-3 days postnatal (3.01 +/- 0.38 vs. 1.45 +/- 0.04) and at 7 days postnatal (9.45 +/- 0.53 vs. 4.13 +/- 0.41), indicating that loss of AE1 led to cardiac hypertrophy. Heterozygous (AE1(+/-)) mice had no signs of cardiac hypertrophy. Morphology of the adult AE1(-/-) mutant heart revealed an increased left ventricular mass, accompanied by increased collagen deposition and fibrosis. M-mode echocardiography revealed dysfunction of the AE1(-/-) hearts, including dilated left ventricle end diastole and systole and expanded left ventricular mass compared with AE1(+/+) hearts. Expression of intracellular pH-regulatory mechanisms in the hypertrophic myocardium of neonate AE1(-/-) mutant mice was indistinguishable from AE1(+/-) and AE1(+/+) mice, as assessed by quantitative real-time RT-PCR. Confocal immunofluorescence revealed that, in normal mouse myocardium, AE1 is sarcolemmal, whereas AE3 and slc26a6 are found both at the sarcolemma and in internal membranes (T tubules and sarcoplasmic reticulum). These results indicate that AE1(-/-) mice, which suffer from severe hemolytic anemia and spherocytosis, display cardiac hypertrophy and impaired cardiac function, reminiscent of findings in patients with hereditary abnormalities of red blood cells. No essential role for AE1 in heart function was found.

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

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

Involvement of the Na+-independent Cl-/HCO3- exchange (AE) isoform in the compensation of myocardial Na+/H+ isoform 1 hyperactivity in spontaneously hypertensive rats

Enhanced activity of Na+/H+ isoform 1 (NHE-1) and the Na+-independent Cl-/HCO3- exchange (AE) is a feature of the hypertrophied myocardium in spontaneously hypertensive rats (SHR). The present study explored the possibility that sustained intracellular acidosis due to increased myocardial acid loading through AE causes NHE-1 enhancement. To this aim, SHR were treated for 2 weeks with a rabbit polyclonal antibody against an AE3 isoform that was recently developed and proven to have inhibitory effects on myocardial AE activity. We then compared the AE activity in the left ventricle papillary muscles isolated from untreated SHR with antiAE3-treated SHR; AE activity was measured in terms of the rate of intracellular pH recovery after an intracellular alkali load was introduced. AE activity was diminished by approximately 70% in SHR treated with the antiAE3 antibody, suggesting that the AE3 isoform is a major carrier of acid-equivalent influx in the hypertrophied myocardium. However, the antibody treatment failed to normalize NHE-1 activity that remained elevated in the myocardium of normotensive rats. The data therefore rule out the possibility that NHE-1 hyperactivity in hypertensive myocardium was due to sustained intracellular acidosis induced by increased AE activity that characterizes SHR myocardial tissue.

HCO(3)(-)-independent rescue from apoptosis by stilbene derivatives in rat cardiomyocytes

Apoptosis of rat cardiomyocytes induced by staurosporine is prevented by a stilbene derivative (DIDS), which is a known blocker of both Cl(-)/HCO(3)(-) exchangers and Cl(-) channels. To clarify its target, staurosporine-induced activation of caspase-3, DNA laddering and cell death were examined in cultured rat cardiomyocytes. Removal of ambient HCO(3)(-), which minimizes the function of Cl(-)/HCO(3)(-) exchangers, failed to affect the preventive effect of DIDS on apoptosis. A carboxylate analog Cl(-) channel blocker, which does not block Cl(-)/HCO(3)(-) exchangers, also inhibited apoptotic events. Thus, rescue by DIDS of cardiomyocytes from apoptosis is mediated by blockage of Cl(-) channels.

Cardiac muscle cell cytoskeletal protein 4.1: Analysis of transcripts and subcellular location?relevance to membrane integrity, microstructure, and possible role in heart failure

The spectrin-based cytoskeleton assembly has emerged as a major player in heart functioning; however, cardiac protein 4.1, a key constituent, is uncharacterized. Protein 4.1 evolved to protect cell membranes against mechanical stresses and to organize membrane microstructure. 4.1 Proteins are multifunctional and, among other activities, link integral/signaling proteins on the plasma and internal membranes with the spectrin-based cytoskeleton. Four genes, EPB41, EPB41L1, EPB41L2, and EPB41L3 encode proteins 4.1R, 4.1N, 4.1G, and 4.1B, respectively. All are extensively spliced. Different isoforms are expressed according to tissue and developmental state, individual function being controlled through inclusion/exclusion of interactive domains. We have defined mouse and human cardiac 4.1 transcripts; other than 4. 1B in humans, all genes show activity. Cardiac transcripts constitutively include conserved FERM and C-terminal domains; both interact with membrane-bound signaling/transport/cell adhesion molecules. Variable splicing within and adjacent to the central spectrin/actin-binding domain enables regulation of cytoskeleton-binding activity. A novel heart-specific exon occurs in human 4.1G, but not in mouse. Immunofluorescence reveals 4.1 staining within mouse cardiomyocytes; thus, both at the plasma membrane and, interdigitated with sarcomeric myosin, across myofibrils in regions close to the sarcoplasmic reticulum. These are all regions to which spectrin locates. 4.1R in human heart shows similar distribution; however, there is limited plasma membrane staining. We conclude that cardiac 4.1s are highly regulated in their ability to crosslink plasma/integral cell membranes with the spectrin-actin cytoskeleton. We speculate that over the repetitive cycles of heart muscle contraction and relaxation, 4.1s are likely to locate, support, and coordinate functioning of key membrane-bound macromolecular assemblies.

Slc26a6: a cardiac chloride-hydroxyl exchanger and predominant chloride-bicarbonate exchanger of the mouse heart

Bicarbonate facilitate more than 50% of pH recovery in the acidotic myocardium, and have roles in cardiac hypertrophy and steady-state pH regulation. To determine which bicarbonate transporters are responsible for this activity, we measured the expression levels of all known HCO3(-)-anion exchange proteins in mouse heart, by quantitative real time RT-PCR. Bicarbonate-anion exchangers are members of either the SLC4A or the SLC26A gene families. In neonatal and adult myocardium, AE1 (Slc4a1), AE2 (Slc4a2), AE3 (Slc4a3) (AE3fl and AE3c variants), Slc26a3 and Slc26a6 were expressed. Adult hearts expressed Slc26a3 and Slc4a1-3 mRNAs at similar levels, while Slc26a6 mRNA was about seven-fold higher than AE3, which was more abundant than any other. Immunohistochemistry revealed that Slc26a6 and AE3 are present in the plasma membrane of ventricular myocytes. Slc26a6 expression levels were higher in ventricle than atrium, whereas AE3 was detected only in ventricle. Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange activity of SLC26A6 and AE3 were investigated in transfected HEK293 cells, using intracellular fluorescence measurements of 2',7'-bis (2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), to monitor intracellular pH (pH(i)). Rates of pH(i) change were measured under HCO3(-)-containing (Cl(-)-HCO(3)(-)) or nominally HCO3(-)-free (Cl(-)-OH(-)) conditions. HCO3(-) fluxes were similar for cells expressing AE3fl, SLC26A6 or Slc26a3, suggesting that they have similar transport activity. However, only SLC26A6 and Slc26a3 functioned as Cl(-)-OH(-) exchangers. Activation of alpha-adrenergic receptors, which stimulates protein kinase C, inhibited SLC26A6 Cl(-)-HCO(3)(-) exchange activity. We conclude that Slc26a6 is the predominant Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchanger of the myocardium and that Slc26a6 is negatively regulated upon alpha-adrenergic stimulation.

Molecular mapping of the conductance activity linked to tAE1 expressed in Xenopus oocyte

It was previously shown that expressed in Xenopus oocyte the trout (tAE1) and the mouse (mAE1) anion exchangers behave differently: both elicit anion exchange activity but only tAE1 induces a transport of organic solutes correlated with an anion conductance. In order to identify the structural domains involved in the induction of tAE1 channel activity, chimeras have been prepared between mouse and trout AE1. As some constructs were not expressed at the plasma membrane, skate exchanger (skAE1) was used instead of mouse exchanger to complete the structure-function analysis. The present paper shows that skAE1, highly similar to mAE1, does not induce a chloride conductance when expressed in Xenopus oocyte. Construct expression analysis showed that only tAE1 transmembrane domain is linked to the anion conductance. More precisely, we identified two regions composed of helices 6, 7 and 8 and putative helices 12 and 13 which are required for this function.

HNF1alpha upregulates the human AE2 anion exchanger gene (SLC4A2) from an alternate promoter

The human AE2 gene (SLC4A2) is transcribed in a widespread fashion from the upstream promoter, the resultant full-length transcript AE2a being encountered in most tissues. Moreover, alternate promoter sequences within intron 2 may drive tissue-restricted expression of variants AE2b(1) and AE2b(2), mainly in liver and kidney. AE2b(2) proximal promoter sequences are highly active in transfected liver-derived HepG2 cells and contain an HNF1 motif. Mutation-disruption of this motif dramatically decreased alternate promoter activity in HepG2 cells but not in prostate-derived PC-3 cells. Electromobility shift and supershift assays indicated that HNF1alpha from HepG2 nuclear extracts binds the HNF1 sequence. Transactivation studies in PC-3 cells showed enhanced activity of the wild-type construct upon cotransfection with an HNF1alpha expression plasmid, while activity of the HNF1-mutated construct remained unaffected. Since liver AE2 is putatively involved in the biliary secretion of bicarbonate, HNF1alpha may have a role in increasing bicarbonate secretion in response to certain stimuli.

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.

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.

[Ion transporters and cardiovascular diseases: pH control or modulation of intracellular calcium concentration]

The regulation of the intracellular pH is under tight control by several ion transport systems including the sodium-proton exchanger, the sodium-bicarbonate cotransporter and the chlore-bicarbonate anion exchanger. While the activation of the anion exchange induces a cellular acidification, both the sodium-proton exchanger and the sodium-bicarbonate cotransporter are responsible for a protection against acidosis by extruding protons or importing bicarbonate. These transporters are transmembrane proteins whose activity is regulated by several mechanisms including phosphorylation, calcium binding and which are involved in several pathophysiologic processes such as ischemia, hypertrophy and arrhythmias. Recent studies suggest that the activation of these transporters during various diseases induces an increase in intracellular calcium concentration. Therefore, inhibiting these transporters could represent novel therapeutic strategies for the treatment of cardiovascular diseases.

The extracellular component of a transport metabolon. Extracellular loop 4 of the human AE1 Cl-/HCO3- exchanger binds carbonic anhydrase IV

Cytosolic carbonic anhydrase II (CAII) and the cytoplasmic C-terminal tails of chloride/bicarbonate anion exchange (AE) proteins associate to form a bicarbonate transport metabolon, which maximizes the bicarbonate transport rate. To determine whether cell surface-anchored carbonic anhydrase IV (CAIV) interacts with AE proteins to accelerate the bicarbonate transport rate, AE1-mediated bicarbonate transport was monitored in transfected HEK293 cells. Expression of the inactive CAII V143Y mutant blocked the interaction between endogenous cytosolic CAII and AE1, AE2, and AE3 and inhibited their transport activity (53 +/- 3, 49 +/- 10, and 35 +/- 1% inhibition, respectively). However, in the presence of V143Y CAII, expression of CAIV restored full functional activity to AE1, AE2, and AE3 (AE1, 101 +/- 3; AE2, 85 +/- 5; AE3, 108 +/- 1%). In Triton X-100 extracts of transfected HEK293 cells, resolved by sucrose gradient ultracentrifugation, CAIV recruitment to the position of AE1 suggested a physical interaction between CAIV and AE1. Gel overlay assays showed a specific interaction between CAIV and AE1, AE2, and AE3. Glutathione S-transferase pull-down assays revealed that the interaction between CAIV and AE1 occurs on the large fourth extracellular loop of AE1. We conclude that AE1 and CAIV interact on extracellular loop 4 of AE1, forming the extracellular component of a bicarbonate transport metabolon, which accelerates the rate of AE-mediated bicarbonate transport.

AE anion exchangers in atrial tumor cells

Intracellular pH homeostasis and intracellular Cl(-) concentration in cardiac myocytes are regulated by anion exchange mechanisms. In physiological extracellular Cl(-) concentrations, Cl(-)/HCO(3)(-) exchange promotes intracellular acidification and Cl(-) loading sensitive to inhibition by stilbene disulfonates. We investigated the expression of AE anion exchangers in the AT-1 mouse atrial tumor cell line. Cultured AT-1 cells exhibited a substantial basal Na(+)-independent Cl(-)/HCO(3)(-) (but not Cl(-)/OH(-)) exchange activity that was inhibited by DIDS but not by dibenzamidostilbene disulfonic acid (DBDS). AT-1 cell Cl(-)/HCO(3)(-) activity was stimulated two- to threefold by extracellular ATP and ANG II. AE mRNAs detected by RT-PCR in AT-1 cells included brain AE3 (bAE3), cardiac AE3 (cAE3), AE2a, AE2b, AE2c1, AE2c2, and erythroid AE1 (eAE1), but not kidney AE1 (kAE1). Cultured AT-1 cells expressed AE2, cAE3, and bAE3 polypeptides, which were detected by immunoblot and immunocytochemistry. An AE1-like epitope was detected by immunocytochemistry but not by immunoblot. Both bAE3 and cAE3 were present in intact AT-1 tumors. Cultured AT-1 cells provide a useful system for the study of mediators and regulators of Cl(-)/HCO(3)(-) exchange activity in an atrial cell type.

Molecular cloning and genomic organization of the mouse AE2 anion exchanger gene

The molecular organization of the AE2 (SLC4A2) gene, a member of the multigene family encoding sodium-independent chloride/bicarbonate anion exchangers, has previously been described in both humans and rats. In these two species, AE2 shows alternate promoter usages and tissue-specific expression of isoforms in a similar, but not identical, fashion. Here we report the molecular cloning and organization of the entire mouse AE2 gene. The gene consists of 23 exons and 22 introns and spans about 17 kb. Moreover, it drives transcription of N-terminal truncated isoforms from alternate promoter sequences in a way analogous to that described for rat and/or human orthologs. Thus, sequences within intron 2 function as overlapping alternate promoters for truncated isoforms AE2b(1) and AE2b(2), and sequences of intron 5 drive transcription of isoforms AE2c(1) and AE2c(2). Each of these variants has a specific alternative first exon, while remaining exons are common to the complete form of the message AE2a, the diversity at 5' leading to different N-termini in corresponding encoded proteins. As expected, mouse AE2 promoter sequences and the patterns of tissue expression of AE2 isoforms resemble rat counterparts more closely than human ones.

Tissue-specific N-terminal isoforms from overlapping alternate promoters of the human AE2 anion exchanger gene

Previously, we isolated the human AE2 (SLC4A2) gene, a member of the sodium-independent anion exchanger family. Rat ortholog of this gene was reported to drive alternative transcription yielding N-terminal variants of the AE2a message. We thus analyzed the human AE2 gene in this regard. Using HepG2 cells, two alternative first exons, each splicing to exon 3 in alternative transcripts, were found to be transcribed from overlapping sequences of intron 2. Exon 1b(1) corresponds to the rat variant "b" and encodes three initial residues (MTQ) in AE2b(1) isoform that replace the first 17 amino acids of AE2a protein, while the novel exon 1b(2) encodes eight initial residues (MDFLLRPQ) in AE2b(2) isoform. The relative abundance of AE2b(1) and AE2b(2) mRNAs was about 10% of AE2a mRNA each. Alternate promoter sequences have multiple potential binding motifs for liver-enriched factors, and dual-luciferase assays indicated that they possess the ability for driving transcription in transiently transfected HepG2 cells. Tissue survey showed that expression of human AE2b(1) and AE2b(2) transcripts is restricted to liver and kidney, while AE2a mRNA was encountered in all examined tissues. Our findings reveal a characteristic tissue-specific expression of two N-terminal variants of human AE2 from overlapping sequences within intron 2, one of which is a novel isoform.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Flanking sequences modulate the cell specificity of M-CAT elements

M-CAT elements mediate both muscle-specific and non-muscle-specific transcription. We used artificial promoters to dissect M-CAT elements derived from the cardiac troponin T promoter, whose regulation is highly striated muscle specific. We show that muscle-specific M-CAT-dependent expression requires two distinct components: the core heptameric M-CAT motif (5'-CATTCCT-3'), which constitutes the canonical binding site for TEF-1-related proteins, and specific sequences immediately flanking the core motif that bind an additional factor(s). These factors are found in higher-order M-CAT DNA-protein complexes with TEF-1 proteins. Non-muscle-specific promoters are produced when the sequences flanking the M-CAT motif are removed or modified to match those of non-muscle-specific promoters such as the simian virus 40 promoter. Moreover, a mutation of the 5'-flanking region of the cardiac troponin T M-CAT-1 element upregulated expression in nonmuscle cells. That mutation also disrupts a potential E box that apparently does not bind myogenic basic helix-loop-helix proteins. We propose a model in which M-CAT motifs are potentially active in many cell types but are modulated through protein binding to specific flanking sequences. In nonmuscle cells, these flanking sequences bind a factor(s) that represses M-CAT-dependent activity. In muscle cells, on the other hand, the factor(s) binding to these flanking sequences contributes to both the cell specificity and the overall transcriptional strength of M-CAT-dependent promoters.

Three N-terminal variants of the AE2 Cl-/HCO3- exchanger are encoded by mRNAs transcribed from alternative promoters

Multiple AE2 Cl-/HCO3- exchanger mRNAs have been identified in rat. To determine the genetic basis for these mRNAs and whether they encode different variants of the exchanger, we used both rapid amplification of cDNA ends and S1 nuclease protection protocols and examined the organization of the gene. mRNAs encoding three N-terminal variants of AE2 (AE2a, AE2b, and AE2c) were identified and shown to be transcribed from alternative promoters. The AE2a transcription unit consists of 23 exons, with exons 1 and 2 containing 5'-untranslated sequence and the first 17 codons. The first exon of AE2b is located in intron 2; it contains 5'-untranslated sequence and an alternative 3-amino acid N-terminal coding sequence and is spliced to exon 3. The first exon of AE2c is located in intron 5; it consists of 5'-untranslated sequence and is spliced to exon 6, which contains the translation initiation codon corresponding to Met-200 of AE2a. Northern analysis shows that AE2a is expressed in all tissues, AE2b exhibits a more restricted distribution with highest levels in stomach, and AE2c is expressed only in stomach. Thus, the use of alternative promoters leads to the production of three N-terminal variants of AE2 that exhibit tissue-specific patterns of expression.