Pendrin: an apical Cl-/OH-/HCO3- exchanger in the kidney cortex

Voltage-dependent electrogenic chloride/proton exchange by endosomal CLC proteins

Eukaryotic members of the CLC gene family function as plasma membrane chloride channels, or may provide neutralizing anion currents for V-type H(+)-ATPases that acidify compartments of the endosomal/lysosomal pathway. Loss-of-function mutations in the endosomal protein ClC-5 impair renal endocytosis and lead to kidney stones, whereas loss of function of the endosomal/lysosomal protein ClC-7 entails osteopetrosis and lysosomal storage disease. Vesicular CLCs have been thought to be Cl- channels, in particular because ClC-4 and ClC-5 mediate plasma membrane Cl- currents upon heterologous expression. Here we show that these two mainly endosomal CLC proteins instead function as electrogenic Cl-/H+ exchangers (also called antiporters), resembling the transport activity of the bacterial protein ClC-e1, the crystal structure of which has already been determined. Neutralization of a critical glutamate residue not only abolished the steep voltage-dependence of transport, but also eliminated the coupling of anion flux to proton counter-transport. ClC-4 and ClC-5 may still compensate the charge accumulation by endosomal proton pumps, but are expected to couple directly vesicular pH gradients to Cl- gradients.

Role of hensin in mediating the adaptation of the cortical collecting duct to metabolic acidosis

PURPOSE OF REVIEW

The cortical collecting duct is able to secrete HCO3-, a state that can be converted to acid secretion during metabolic acidosis. Bicarbonate secretion in this segment is mediated by beta-intercalated cells whereas alpha-intercalated cells perform acid secretion. During metabolic acidosis, the number of beta-intercalated cells is reduced while that of alpha-intercalated cells increases without a change in the total number of intercalated cells, suggesting conversion of one cell type to another. Using an immortalized intercalated cell line we found that this adaptation is mediated by an extracellular protein named hensin. Hensin is secreted as a monomer which is then polymerized in the extracellular environment by a complex process requiring at least three other proteins.

RECENT FINDINGS

We describe that a cyclophilin, via its cis/trans prolyl isomerase activity, is required for this polymerization. This may explain the distal renal tubular acidosis observed with cyclosporin A therapy. In addition, galectin-3 is needed to aggregate the protein. Finally, we recently found that activation of integrins is also necessary for the development of the hensin fiber. Hensin is expressed in all epithelia and deletion of its gene is embryonic lethal at an early stage when the first columnar epithelia develop.

SUMMARY

These studies suggest that the response of intercalated cells to metabolic acidosis uses a pathway that is involved in terminal differentiation of columnar epithelia.

Molecular analysis of the PDS gene in a nonconsanguineous Sicilian family with Pendred's syndrome

OBJECTIVE

The autosomal recessive Pendred's syndrome is defined by congenital sensorineural deafness, goiter, and impaired iodide organification. It is caused by mutations in the Pendred's syndrome (PDS) gene that encodes pendrin, a chloride/iodide transporter expressed in the thyroid, the inner ear, and the kidney. In this study we performed clinical and molecular analyses in three siblings from a nonconsanguineous Sicilian family who presented with the clinical features of Pendred's syndrome. PATIENTS AND MOLECULAR ANALYSES: In two sisters and one brother, the clinical diagnosis of Pendred's syndrome was established based on the findings of sensorineural hearing loss and large goiters. Thyroid function tests, perchlorate discharge tests, thyroid ultrasound, and scintigraphy were performed in all affected individuals. Exons 2 to 21 of the PDS gene were amplified by polymerase chain reaction (PCR) and both strands were submitted to direct sequence analysis.

RESULTS

The clinical diagnosis of Pendred's syndrome was supported by a positive perchlorate discharge test in the three afflicted siblings. Direct sequence analysis of the PDS gene revealed that all three harbored one allele with a novel mutation 890delC leading to a frameshift mutation and premature stop codon at position 302 (FS297 > 302X). On the other allele, two of the siblings had a previously described transition 1226G > A, which results in the substitution of arginine by histidine at position 409 (R409H). In the index patient, no mutation could be identified on the other allele. In functional studies, these mutants lose the ability of pendrin to mediate iodide efflux.

CONCLUSIONS

All three patients included in this study presented with the classic Pendred syndrome triad. Two siblings were compound heterozygous for mutations in the coding region of the PDS gene. The third individual could have an unidentified mutation in a regulatory or intronic region of the PDS gene, or an identical phenotype caused by distinct pathogenic mechanisms.

SLC26A6 and SLC26A7 Anion Exchangers Have a Distinct Distribution in Human Kidney

BACKGROUND

The anion transporters SLC26A6 (PAT1) and SLC26A7, transporting at least chloride, oxalate, sulfate and bicarbonate, show a distinct expression and function in different mammalian species. They are expressed in kidney, but their exact localization in human kidney has not been studied. We therefore examined SLC26A6 and A7 expression in human kidneys.

METHODS

The localization of SLC26A6 and A7 in different segments of human nephrons was studied by RT-PCR and immunohistochemistry by comparing to the tubular markers PNRA, CD10, Tamm-Horsfall antigen, high molecular weight cytokeratin, CK7, AQP2 and H(+)V-ATPase.

RESULTS

In human kidney, SLC26A6 is expressed in distal segments of proximal tubules, parts of the thin and thick ascending limbs of Henle's loops, macula densa, distal convoluted tubules and a subpopulation of intercalated cells of collecting ducts. SLC26A7 is expressed in extraglomerular mesangial cells and a subpopulation of intercalated cells of collecting ducts.

CONCLUSION

Our results show that in human kidney SLC26A6 and A7 have a distinct, partially overlapping expression in distal segments of nephrons. The distribution partly differs from that found previously in rodent kidneys.

Inhibition of N-methyl-N-nitrosourea-induced mammary carcinogenesis by molecular iodine (I2) but not by iodide (I-) treatment Evidence that I2 prevents cancer promotion

We analyzed the effect of molecular iodine (I2), potassium iodide (KI) and a subclinical concentration of thyroxine (T4) on the induction and promotion of mammary cancer induced by N-methyl-N-nitrosourea. Virgin Sprague-Dawley rats received short or continuous treatment. Continuous I2 treated rats exhibited a strong and persistent reduction in mammary cancer incidence (30%) compared to controls (72.7%). Interruption of short or long term treatments resulted in a higher incidence in mammary cancer compared to the control groups. The protective effect of I2 was correlated with the highest expression of the I-/Cl- transporter pendrin and with the lowest levels of lipoperoxidation expression in mammary glands. Triiodothyronine serum levels and Na+/I- symporter, lactoperoxidase, or p53 expression did not show any changes. In conclusion continuous I2 treatment has a potent antineoplastic effect on the progression of mammary cancer and its effect may be related to a decrease in the oxidative cell environment.

Hearing loss associated with enlarged vestibular aqueduct and Mondini dysplasia is caused by splice-site mutation in the PDS gene

Recessive mutations of PDS gene are the common causes of Pendred syndrome and non-syndromic hearing loss associated with temporal bone abnormalities ranging from isolated enlargement of the vestibular aqueduct (EVA) to Mondini dysplasia. In this study we evaluate the relationship between EVA and Mondini dysplasia in 10 prelingual deaf patients and PDS gene mutation. One of three mutations, IVS7-2A-->G, IVS16-6G-->A or IVS15+5G-->A, was identified in the PDS gene in each patient. In family studies of four probands with the IVS7-2A-->G mutation, we found that this mutation was inherited from the same mutant alleles of parental origin. The effect of IVS7-2A-->G mutation on PDS gene expression was determined by reverse transcription and polymerase chain reaction (RT-PCR). Sequencing of the RT-PCR products revealed that the PDS transcripts from the allele with IVS7-2A-->G mutation lose the entire exon 8, resulting in a joining of exons 7 and 9. Deletion of the exon 8 results in frameshift and premature termination of translation. Haplotype analysis showed a significant haplotype shared among the family members carrying IVS7-2A-->G mutation, suggesting that they may be derived from a common ancestor. Our results provide evidence that hearing loss with EVA and Mondini dysplasia may be caused by splice-site mutation in the PDS gene.

SLC26A9 is expressed in gastric surface epithelial cells, mediates Cl-/HCO3- exchange, and is inhibited by NH4+

HCO3- secretion by gastric mucous cells is essential for protection against acidic injury and peptic ulcer. Herein we report the identification of an apical HCO3- transporter in gastric surface epithelial cells. Northern hybridization and RT-PCR demonstrate the expression of this transporter, also known as SLC26A9, in mouse and rat stomach and trachea (but not kidney). In situ hybridization in mouse stomach showed abundant expression of SLC26A9 in surface epithelial cells with apical localization on immunofluorescence labeling. Functional studies in HEK-293 cells demonstrated that SLC26A9 mediates Cl-/HCO3- exchange and is also capable of Cl--independent HCO3- extrusion. Unlike other anion exchangers or transport proteins reported to date, SLC26A9 activity is inhibited by ammonium (NH4+). The inhibitory effect of NH4+ on gastric HCO3- secretion was also indicated by reduced gastric juxtamucosal pH (pHjm) in rat stomach in vivo. This report is the first to describe the inhibition of HCO3- transport in vitro and the reduction of pHjm in stomach in vivo by NH4+. Given its critical localization on the apical membrane of surface epithelial cells, its ability to transport HCO3-, and its inhibition by NH4+, we propose that SLC26A9 mediates HCO3- secretion in surface epithelial cells and is essential for protection against acidic injury in the stomach. Disease states that are associated with increased ammonia (NH3)/NH4+ generation (e.g., Helicobacter pylori) may impair gastric HCO3- secretion and therefore predispose patients to peptic ulcer by inhibiting SLC26A9.

The immunohistochemical analysis of pendrin in the mouse inner ear

Pendred's syndrome (PS) is an autosomal recessive disorder characterized by deafness and goiter, which are caused by mutations in the Pendred's syndrome gene (PDS). PDS encodes a membrane protein named pendrin that is considered to act as an anion transporter. An expression pattern of the PDS ortholog (Pds) mRNA in the auditory and vestibular systems has been reported in mice, and the localization of pendrin has been reported recently. We generated antipeptide antibodies against human pendrin, and performed immunohistochemical analysis of mouse inner ears. We detected pendrin in the endolymphatic duct and sac, and the utricle, saccule, and external sulcus. In the endolymphatic duct and sac, the expression of pendrin was apparent at the apical membrane. In addition, we detected pendrin in the spiral ligament, Claudius cells, Deiter's cells, and the spiral ganglion of the cochlea. Our results are key to defining the role of pendrin in inner ear development and elucidating the pathogenic mechanisms underlying deafness in PS.

Genetic basis of hearing loss associated with enlarged vestibular aqueducts in Koreans

Sensorineural hearing loss associated with enlargement of the vestibular aqueduct (EVA) can be associated with mutations of the SLC26A4 gene. In western populations, less than one-half of the affected individuals with EVA have two mutant SLC26A4 alleles, and EVA is frequently caused by unknown genetic or environmental factors alone or in combination with a single SLC26A4 mutation as part of a complex trait. In this study, we ascertained 26 Korean probands with EVA and performed nucleotide sequence analysis to detect SLC26A4 mutations. All subjects had bilateral EVA, and 20 of 26 were sporadic (simplex) cases. Fourteen different mutations were identified, including nine novel mutations. Five mutations were recurrent and accounted for 80% of all mutant alleles, providing a basis for the design and interpretation of cost-efficient mutation detection algorithms. Two mutant alleles were identified in 21 (81%), one mutant allele was detected in three (11%), and zero mutant allele was detected in two (8%) of 26 probands. The high proportion of Korean probands with two SLC26A4 mutations may reflect a reduced frequency of other genetic or environmental factors causing EVA in comparison to western populations.

Renal and intestinal transport defects in Slc26a6-null mice

SLC26A6 (PAT1, CFEX) is an anion exchanger that is expressed on the apical membrane of the kidney proximal tubule and the small intestine. Modes of transport mediated by SLC26A6 include Cl-/formate exchange, Cl-/HCO3- exchange, and Cl-/oxalate exchange. To study its role in kidney and intestinal physiology, gene targeting was used to prepare mice lacking Slc26a6. Homozygous mutant Slc26a6-/- mice appeared healthy and exhibited a normal blood pressure, kidney function, and plasma electrolyte profile. In proximal tubules microperfused with a low-HCO3-/high-Cl- solution, the baseline rate of fluid absorption (Jv), an index of NaCl transport under these conditions, was the same in wild-type and null mice. However, the stimulation of Jv by oxalate observed in wild-type mice was completely abolished in Slc26a6-null mice (P<0.05). Formate stimulation of Jv was partially reduced in null mice, but the difference from the response in wild-type mice did not reach statistical significance. Apical membrane Cl-/base exchange activity, assayed with the pH-sensitive dye BCPCF in microperfused proximal tubules, was decreased by 58% in Slc26a6-/- animals (P<0.001 vs. wild types). In the duodenum, the baseline rate of HCO3- secretion measured in mucosal tissue mounted in Ussing chambers was decreased by approximately 30% (P<0.03), whereas the forskolin-stimulated component of HCO3- secretion was the same in wild-type and Slc26a6-/- mice. We conclude that Slc26a6 mediates oxalate-stimulated NaCl absorption, contributes to apical membrane Cl-/base exchange in the kidney proximal tubule, and also plays an important role in HCO3- secretion in the duodenum.

The Cl−/HCO3−exchanger pendrin in the rat kidney is regulated in response to chronic alterations in chloride balance

Pendrin (Pds; Slc26A4) is a new anion exchanger that is believed to mediate apical Cl−/HCO3−exchange in type B and non-A-non-B intercalated cells of the connecting tubule and cortical collecting duct. Recently, it has been proposed that this transporter may be involved in NaCl balance and blood pressure regulation in addition to its participation in the regulation of acid-base status. The purpose of our study was to determine the regulation of Pds protein abundance during chronic changes in chloride balance. Rats were subjected to either NaCl, NH4Cl, NaHCO3, KCl, or KHCO3loading for 6 days or to a low-NaCl diet or chronic furosemide administration. Pds protein abundance was estimated by semiquantitative immunoblotting in renal membrane fractions isolated from the cortex of treated and control rats. We observed a consistent inverse relationship between Pds expression and diet-induced changes in chloride excretion independent of the administered cation. Conversely, NaCl depletion induced by furosemide was associated with increased Pds expression. We conclude that Pds expression is specifically regulated in response to changes in chloride balance.

The bicarbonate transport metabolon

To allow cells to control their pH and bicarbonate levels, cells express bicarbonate transport proteins that rapidly and selectively move bicarbonate across the plasma membrane. Physical interactions have been identified between the carbonic anhydrase isoform, CAII, and the erythrocyte membrane Cl- /HCO3(-) anion exchanger, AE1, mediated by an acidic motif in the AE1 C-terminus. We have found that the presence of CAII attached to AE1 accelerates AE1 HCO3(-) transport activity, as AE1 moves bicarbonate either into or out of the cell. In efflux mode the presence of CAII attached to AE1 will increase the local concentration of bicarbonate at the AE1 transport site. As bicarbonate is transported into the cell by AE1, the presence of CAII on the cytosolic surface accelerates transport by consumption of bicarbonate, thereby maximizing the transmembrane bicarbonate concentration gradient experienced by the AE1 molecule. Functional and physical interactions also occur between CAII and Na+/HCO3(-) co-transporter isoforms NBC1 and NBC3. All examined bicarbonate transport proteins, except the DRA (SLC26A3) Cl-/HCO3(-) exchange protein, have a consensus CAII binding site in their cytoplasmic C-terminus. Interestingly, CAII does not bind DRA. CAIV is anchored to the extracellular surface of cells via a glycosylphosphatidyl inositol linkage. We have identified extracellular regions of AE1 and NBC1 that directly interact with CAIV, to form a physical complex between the proteins. In summary, bicarbonate transporters directly interact with the CAII and CAIV carbonic anhydrases to increase the transmembrane bicarbonate flux. The complex of a bicarbonate transporter with carbonic anhydrase forms a "Bicarbonate Transport Metabolon."

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

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.

Ear and kidney syndromes: molecular versus clinical approach

The association between ear and kidney anomalies is not usually due to an insult to the embryo. In recent years, many essential development control genes that coordinate the assembly and function of kidney and ear have been discovered through the generation of animal mutants and have increased our understanding of the mechanisms of human oto-renal diseases. Here, we describe ear and kidney clinical syndromes and their molecular expression.

Mechanism of iodide/chloride exchange by pendrin

We performed an electrophysiological study to investigate ion transport of pendrin and thereby understand the pathogenesis of Pendred syndrome. Using pendrin-transfected COS-7 cells, we could show that pendrin transports both iodide and chloride measured as voltage-dependent inward and outward membrane currents. Chloride in the culture medium, [Cl-]o, was efficiently exchanged with cytoplasmic iodide, [I-]i, under physiological concentrations, indicating that pendrin is important for chloride uptake and iodide efflux. Although exchange of iodide in the medium, [I-]o, with cytoplasmic chloride, [Cl-]i, was observed, a significantly high concentration of iodide (10 mm) was required. In addition, either iodide or chloride was required on both sides of the cell membrane for the anion exchange activity of pendrin, indicating that iodide and chloride activate the exchange activity of pendrin while they are transported. The present study further supports that pendrin is responsible for the iodide efflux in thyroid cells where intracellular iodide concentration is high and that the general function of pendrin in other tissues is to transport chloride through exchange with other anions.

Activation of the apical Na+/H+ exchanger NHE3 by formate: a basis of enhanced fluid and electrolyte reabsorption by formate in the kidney

Formate stimulates sodium chloride and fluid reabsorption in kidney proximal tubule; however, the exact cellular mechanism of this effect remains unknown. We hypothesized that the primary target of formate is the apical Na(+)/H(+) exchanger. Here, we demonstrate that formate directly enhances the apical Na(+)/H(+) exchanger (NHE3) activity in mouse kidney proximal tubule. In the absence of CO(2)/HCO(3)(-), addition of formate (500 microM) to the bath and lumen of microperfused mouse kidney proximal tubule caused significant intracellular alkalinization, with intracellular pH (pH(i)) increasing from baseline levels 7.17 +/- 0.01 to 7.55 +/- 0.01 (P < 0.001, n = 14), with a Delta pH of 0.38 +/- 0.02. Removal of luminal chloride did not block cell pH alkalinization by formate (baseline pH of 7.26 +/- 0.01 to 7.53 +/- 0.01 with formate, P < 0.001, n = 10), indicating that the apical Cl(-)/OH(-) exchanger was not the primary mediator of the effect of formate on cell pH. However, removal of sodium from the lumen or addition of EIPA completely prevented cell pH alkalinization. Addition of formate to the lumen and bath in the outer medullary collecting duct, which does not express any apical Na(+)/H(+) exchanger, did not cause any cell pH alkalinization. At lower concentrations (50 microM), formate caused significant pH(i) alkalinization in proximal tubule cells, with pH(i) increasing from baseline levels 7.15 +/- 0.02 to 7.36 +/- 0.02 (P < 0.02, n = 11). Acetate, at 50 microM, had no effect on pH(i). Formate's effect was observed both in the absence and presence of CO(2)/HCO(3)(-) in the media. We conclude that formate stimulates the apical Na(+)/H(+) exchanger NHE3 in the kidney proximal tubule. We propose that formate stimulation of chloride reabsorption in the proximal tubule is indirect and is secondary to the activation of apical Na(+)/H(+) exchanger NHE3, which then leads to the stimulation of the apical chloride/base exchanger.

Gating of CFTR by the STAS domain of SLC26 transporters

Chloride absorption and bicarbonate secretion are vital functions of epithelia, as highlighted by cystic fibrosis and diseases associated with mutations in members of the SLC26 chloride-bicarbonate exchangers. Many SLC26 transporters (SLC26T) are expressed in the luminal membrane together with CFTR, which activates electrogenic chloride-bicarbonate exchange by SLC26T. However, the ability of SLC26T to regulate CFTR and the molecular mechanism of their interaction are not known. We report here a reciprocal regulatory interaction between the SLC26T DRA, SLC26A6 and CFTR. DRA markedly activates CFTR by increasing its overall open probablity (NP(o)) sixfold. Activation of CFTR by DRA was facilitated by their PDZ ligands and binding of the SLC26T STAS domain to the CFTR R domain. Binding of the STAS and R domains is regulated by PKA-mediated phosphorylation of the R domain. Notably, CFTR and SLC26T co-localize in the luminal membrane and recombinant STAS domain activates CFTR in native duct cells. These findings provide a new understanding of epithelial chloride and bicarbonate transport and may have important implications for both cystic fibrosis and diseases associated with SLC26T.

The SLC26 gene family of multifunctional anion exchangers

The ten-member SLC26 gene family encodes anion exchangers capable of transporting a wide variety of monovalent and divalent anions. The physiological role(s) of individual paralogs is evidently due to variation in both anion specificity and expression pattern. Three members of the gene family are involved in genetic disease; SLC26A2 in chondrodysplasias, SLC26A3 in chloride-losing diarrhea, and SLC26A4 in Pendred syndrome and hereditary deafness (DFNB4). The analysis of Slc26a4-null mice has significantly enhanced the understanding of the roles of this gene in both health and disease. Targeted deletion of Slc26a5 has in turn revealed that this paralog is essential for electromotor activity of cochlear outer hair cells and thus for cochlear amplification. Anions transported by the SLC26 family, with variable specificity, include the chloride, sulfate, bicarbonate, formate, oxalate and hydroxyl ions. The functional versatility of SLC26A6 identifies it as the primary candidate for the apical Cl(-)-formate/oxalate and Cl(-)-base exchanger of brush border membranes in the renal proximal tubule, with a central role in the reabsorption of Na(+)-Cl(-) from the glomerular ultrafiltrate. At least three of the SLC26 exchangers mediate electrogenic Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange; the stoichiometry of Cl(-)-HCO(3)(-) exchange appears to differ between SLC26 paralogs, such that SLC26A3 transports >/=2 Cl(-) ions per HCO(3)(-) ion, whereas SLC26A6 transports >/=2 HCO(3)(-) ions per Cl(-) ion. SLC26 Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange is activated by the cystic fibrosis transmembrane regulator (CFTR), implicating defective regulation of these exchangers in the reduced HCO(3)(-) transport seen in cystic fibrosis and related disorders; CFTR-independent activation of these exchangers is thus an important and novel goal for the future therapy of cystic fibrosis.

Cl(-)/HCO(3)(-) exchange is acetazolamide sensitive and activated by a muscarinic receptor-induced [Ca(2+)](i) increase in salivary acinar cells

Large volumes of saliva are generated by transepithelial Cl(-) movement during parasympathetic muscarinic receptor stimulation. To gain further insight into a major Cl(-) uptake mechanism involved in this process, we have characterized the anion exchanger (AE) activity in mouse serous parotid and mucous sublingual salivary gland acinar cells. The AE activity in acinar cells was Na(+) independent, electroneutral, and sensitive to the anion exchange inhibitor DIDS, properties consistent with the AE members of the SLC4A gene family. Localization studies using a specific antibody to the ubiquitously expressed AE2 isoform labeled acini in both parotid and sublingual glands. Western blot analysis detected an approximately 170-kDa protein that was more highly expressed in the plasma membranes of sublingual than in parotid glands. Correspondingly, the DIDS-sensitive Cl(-)/HCO(3)(-) exchanger activity was significantly greater in sublingual acinar cells. The carbonic anhydrase antagonist acetazolamide markedly inhibited, whereas muscarinic receptor stimulation enhanced, the Cl(-)/HCO(3)(-) exchanger activity in acinar cells from both glands. Intracellular Ca(2+) chelation prevented muscarinic receptor-induced upregulation of the AE, whereas raising the intracellular Ca(2+) concentration with the Ca(2+)-ATPase inhibitor thapsigargin mimicked the effects of muscarinic receptor stimulation. In summary, carbonic anhydrase activity was essential for regulating Cl(-)/HCO(3)(-) exchange in salivary gland acinar cells. Moreover, muscarinic receptor stimulation enhanced AE activity through a Ca(2+)-dependent mechanism. Such forms of regulation may play important roles in modulating fluid and electrolyte secretion by salivary gland acinar cells.

SLC26A7: a basolateral Cl-/HCO3- exchanger specific to intercalated cells of the outer medullary collecting duct

The outer medullary collecting duct (OMCD) plays an important role in bicarbonate reabsorption and acid-base regulation. An apical V-type H+-ATPase and a basolateral Cl-/HCO3- exchanger, located in intercalated cells of OMCD, mediate the bicarbonate reabsorption. Here we report the identification of a new basolateral Cl-/HCO3- exchanger in OMCD intercalated cells in rat kidney. Northern hybridizations demonstrated the predominant expression of this transporter, also known as SLC26A7, in the outer medulla, with lower expression levels in the inner medulla. SLC26A7 was recognized as a approximately 90-kDa band in the outer medulla by immunoblot analysis and was localized on the basolateral membrane of a subset of OMCD cells by immunocytochemical staining. No labeling was detected in the cortex. Double-immunofluorescence labeling with the aquaporin-2 and SLC26A7 antibodies or anion exchanger-1 and SLC26A7 antibodies identified the SLC26A7-expressing cells as alpha-intercalated cells. Functional studies in oocytes demonstrated that increasing the osmolality of the media (to simulate the physiological milieu in the medulla) increased the Cl-/HCO3- exchanger activity mediated via SLC26A7 by about threefold (P < 0.02 vs. normal condition). We propose that SLC26A7 is a basolateral Cl-/HCO3- exchanger in intercalated cells of the OMCD and may play an important role in bicarbonate reabsorption in medullary collecting duct.

Unique regulation of anion/HCO3- exchangers by constitutive nitric oxide in rabbit small intestine

In the rabbit small intestine, there are three functionally different brush-border membrane (BBM) anion/HCO3- exchangers: 1) Cl/HCO3- exchange on the BBM of villus cells responsible for coupled NaCl absorption; 2) Cl/HCO3- exchange on the BBM of crypt cells possibly involved in HCO3- secretion; and 3) short-chain fatty acid (SCFA)/HCO3- exchange on the BBM of villus cells, which facilitates SCFA absorption. Although constitutive nitric oxide (cNO) has been postulated to alter many gastrointestinal tract functions, how cNO may specifically alter these three transporters is unknown. Inhibition of cNO synthase with NG-nitro-L-arginine methyl ester (L-NAME) 1) did not affect villus cell BBM Cl/HCO3 change, 2) stimulated crypt cell BBM Cl/HCO3- exchange, and 3) inhibited villus cell BBM SCFA/HCO3- exchange. D-NAME, an inactive analog of L-NAME, and L-N6-(1-iminoethyl)lysine, a more selective inhibitor of inducible NO, did not affect these transport processes. Kinetic studies demonstrated that 1) the mechanism of inhibition of crypt cell BBM Cl/HCO3- exchange is secondary to a decrease in the maximal rate of uptake of Cl, without an alteration in the affinity of the transporter for Cl, and 2) the mechanism of stimulation of villus cell BBM SCFA/HCO3- exchange is secondary to an increase in the affinity of the transporter for SCFA without an alteration in the maximal rate of uptake of SCFA. These results indicate that cNO uniquely regulates the three BBM anion/HCO3- transporters in the rabbit small intestine.

Identification of an apical Cl-/HCO-3 exchanger in rat kidney proximal tubule

SLC26A6 (or putative anion transporter 1, PAT1) is located on the apical membrane of mouse kidney proximal tubule and mediates Cl-/HCO3- exchange in in vitro expression systems. We hypothesized that PAT1 along with a Cl-/HCO3- exchange is present in apical membranes of rat kidney proximal tubules. Northern hybridizations indicated the exclusive expression of SLC26A6 (PAT1 or CFEX) in rat kidney cortex, and immunocytochemical staining localized SLC26A6 on the apical membrane of proximal tubules, with complete prevention of the labeling with the preadsorbed serum. To examine the functional presence of apical Cl-/HCO3- exchanger, proximal tubules were isolated, microperfused, loaded with the pH-sensitive dye BCPCF-AM, and examined by digital ratiometric imaging. The pH of the perfusate and bath was kept at 7.4. Buffering capacity was measured, and transport rates were calculated as equivalent base flux. The results showed that in the presence of basolateral DIDS (to inhibit Na+-HCO3- cotransporter 1) and apical EIPA (to inhibit Na+/H+ exchanger 3), the magnitude of cell acidification in response to addition of luminal Cl- was approximately 5.0-fold higher in the presence than in the absence of CO2/HCO3-. The Cl--dependent base transport was inhibited by approximately 61% in the presence of 0.5 mM luminal DIDS. The presence of physiological concentrations of oxalate in the lumen (200 microM) did not affect the Cl-/HCO3- exchange activity. These results are consistent with the presence of SLC26A6 (PAT1) and Cl-/HCO3- exchanger activity in the apical membrane of rat kidney proximal tubule. We propose that SLC26A6 is likely responsible for the apical Cl-/HCO3- (and Cl-/OH-) exchanger activities in kidney proximal tubule.

HUMANNONSYNDROMICSENSORINEURALDEAFNESS

Given the unique biological requirements of sound transduction and the selective advantage conferred upon a species capable of sensitive sound detection, it is not surprising that up to 1% of the approximately 30,000 or more human genes are necessary for hearing. There are hundreds of monogenic disorders for which hearing loss is one manifestation of a syndrome or the only disorder and therefore is nonsyndromic. Herein we review the supporting evidence for identifying over 30 genes for dominantly and recessively inherited, nonsyndromic, sensorineural deafness. The state of knowledge concerning their biological roles is discussed in the context of the controversies within an evolving understanding of the intricate molecular machinery of the inner ear.

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

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

Expression of SSAT, a novel biomarker of tubular cell damage, increases in kidney ischemia-reperfusion injury

Ischemia-reperfusion injury (IRI) is the major cause of acute renal failure in native and allograft kidneys. Identifying the molecules and pathways involved in the pathophysiology of renal IRI will yield valuable new diagnostic and therapeutic information. To identify differentially regulated genes in renal IRI, RNA from rat kidneys subjected to an established renal IRI protocol (bilateral occlusion of renal pedicles for 30 min followed by reperfusion) and time-matched kidneys from sham-operated animals was subjected to suppression subtractive hybridization. The level of spermidine/spermine N(1)-acetyltransferase (SSAT) mRNA, an essential enzyme for the catabolism of polyamines, increased in renal IRI. SSAT expression was found throughout normal kidney tubules, as detected by nephron segment RT-PCR. Northern blots demonstrated that the mRNA levels of SSAT are increased by greater than threefold in the renal cortex and by fivefold in the renal medulla at 12 h and returned to baseline at 48 h after ischemia. The increase in SSAT mRNA was paralleled by an increase in SSAT protein levels as determined by Western blot analysis. The concentration of putrescine in the kidney increased by approximately 4- and approximately 7.5-fold at 12 and 24 h of reperfusion, respectively, consistent with increased functional activity of SSAT. To assess the specificity of SSAT for tubular injury, a model of acute renal failure from Na(+) depletion (without tubular injury) was studied; SSAT mRNA levels remained unchanged in rats subjected to Na(+) depletion. To distinguish SSAT increases from the effects of tubular injury vs. uremic toxins, SSAT was increased in cis-platinum-treated animals before the onset of renal failure. The expression of SSAT mRNA and protein increased by approximately 3.5- and >10-fold, respectively, in renal tubule epithelial cells subjected to ATP depletion and metabolic poisoning (an in vitro model of kidney IRI). Our results suggest that SSAT is likely a new marker of tubular cell injury that distinguishes acute prerenal from intrarenal failure.

Origins and frequencies of SLC26A4 (PDS) mutations in east and south Asians: global implications for the epidemiology of deafness

Recessive mutations of SLC26A4 (PDS) are a common cause of Pendred syndrome and non-syndromic deafness in western populations. Although south and east Asia contain nearly one half of the global population, the origins and frequencies of SLC26A4 mutations in these regions are unknown. We PCR amplified and sequenced seven exons of SLC26A4 to detect selected mutations in 274 deaf probands from Korea, China, and Mongolia. A total of nine different mutations of SLC26A4 were detected among 15 (5.5%) of the 274 probands. Five mutations were novel and the other four had seldom, if ever, been identified outside east Asia. To identify mutations in south Asians, 212 Pakistani and 106 Indian families with three or more affected offspring of consanguineous matings were analysed for cosegregation of recessive deafness with short tandem repeat markers linked to SLC26A4. All 21 SLC26A4 exons were PCR amplified and sequenced in families segregating SLC26A4 linked deafness. Eleven mutant alleles of SLC26A4 were identified among 17 (5.4%) of the 318 families, and all 11 alleles were novel. SLC26A4 linked haplotypes on chromosomes with recurrent mutations were consistent with founder effects. Our observation of a diverse allelic series unique to each ethnic group indicates that mutational events at SLC26A4 are common and account for approximately 5% of recessive deafness in south Asians and other populations.

Isoforms of SLC26A6 mediate anion transport and have functional PDZ interaction domains

The solute carrier gene family SLC26 consists of tissue-specific anion exchanger genes, three of them associated with distinct human recessive disorders. By a genome-driven approach, several new SLC26 family members have been identified, including a kidney- and pancreas-specific gene, SLC26A6. We report the functional characterization of SLC26A6 and two new alternatively spliced variants, named SLC26A6c and SLC26A6d. Immunofluorescence studies on transiently transfected cells indicated membrane localization and indicated that both NH(2)- and COOH-terminal tails of the SLC26A6 variants are located intracellularly, suggesting a topology with an even number of transmembrane domains. Functional expression of the three proteins in Xenopus oocytes demonstrated Cl(-) and SO(4)(2-) transport activity. In addition, the transport of SO(4)(2-) and Cl(-) was inhibited by DIDS and HCO(3)(-). We demonstrated also that the COOH terminus of SLC26A6 binds to the first and second PDZ domains of the Na(+)/H(+) exchanger (NHE)3 kinase A regulatory protein (E3KARP) and NHE3 regulatory factor (NHERF) proteins in vitro. Truncation of the last three amino acids (TRL) of SLC26A6 abrogated the interaction but did not affect transport function. These results demonstrate that SLC26A6 and its two splice variants can function as anion transporters linked to PDZ-interaction pathways. Our results support the general concept of microdomain organization for ion transport and suggest a mechanism for cystic fibrosis transmembrane regulator (CFTR)-mediated SLC26A6 upregulation in pancreatic duct cells.

Role of luminal anion and pH in distal tubule potassium secretion

Potassium secretory flux (J(K)) by the distal nephron is regulated by systemic and luminal factors. In the present investigation, J(K) was measured with a double-barreled K(+) electrode during paired microperfusion of superficial segments of the rat distal nephron. We used control solutions (100 mM NaCl, pH 7.0) and experimental solutions in which Cl(-) had been replaced with a less permeant anion and/or pH had been increased to 8.0. J(K) increased when Cl(-) was replaced by either acetate ( approximately 37%), sulfate ( approximately 32%), or bicarbonate ( approximately 62%), and also when the pH of the control perfusate was increased ( approximately 26%). The majority (80%) of acetate-stimulated J(K) was Ba(2+) sensitive, but furosemide (1 mM) further reduced secretion ( approximately 10% of total), suggesting that K(+)-Cl(-) cotransport was operative. Progressive reduction in luminal Cl(-) concentration from 100 to 20 to 2 mM caused increments in J(K) that were abolished by inhibitors of K(+)-Cl(-) cortransport, i.e., furosemide and [(dihydroindenyl)oxy]alkanoic acid. Increasing the pH of the luminal perfusion fluid also increased J(K) even in the presence of Ba(2+), suggesting that this effect cannot be accounted for only by K(+) channel modulation of K(+) secretion in the distal nephron of the rat. Collectively, these data suggest a role for K(+)-Cl(-) cotransport in distal nephron K(+) secretion.

Localization of pendrin in mouse kidney

Pendrin is an anion exchanger expressed in type B intercalated cells of the cortical collecting duct (CCD). Whether pendrin localizes to other nephron segments with intercalated cells is unknown. Moreover, whether pendrin is expressed in proximal tubule is debated. Thus the distribution of pendrin mRNA and protein expression in mouse kidney was investigated by using light and electron microscopic immunohistochemistry and quantitative real-time PCR. We observed that pendrin mRNA is expressed mainly in cortex. Within cortex, pendrin mRNA is at least fivefold higher in CCD and the connecting tubule (CNT) than in the other segments. Pendrin protein was observed in a subset of cells within the distal convoluted tubule as well as in type B and in non-A-non-B intercalated cells of the CNT and CCD. In type B intercalated cells, pendrin immunoreactivity was highest in apical cytoplasmic vesicles with little immunolabel along the apical plasma membrane. In non-A-non-B intercalated cells, intense pendrin immunoreactivity was detected along the apical plasma membrane. These differences in the subcellular distribution of pendrin immunolabel were confirmed by morphometric analysis. In conclusion, pendrin is expressed in the mouse distal convoluted tubule, CCD, and CNT along the apical plasma membrane of non-A-non-B intercalated cells and in subapical cytoplasmic vesicles of type B intercalated cells.

Regulation of the apical Cl-/HCO-3 exchanger pendrin in rat cortical collecting duct in metabolic acidosis

Pendrin is an apical Cl(-)/OH(-)/HCO(3)(-) exchanger in beta-intercalated cells (beta-ICs) of rat and mouse cortical collecting duct (CCD). However, little is known about its regulation in acid-base disorders. Here, we examined the regulation of pendrin in metabolic acidosis, a condition known to decrease HCO(3)(-) secretion in CCD. Rats were subjected to NH(4)Cl loading for 4 days, which resulted in metabolic acidosis. Apical Cl(-)/HCO(3)(-) exchanger activity in beta-ICs was determined as amplitude and rate of intracellular pH change when Cl was removed in isolated, microperfused CCDs. Intracellular pH was measured by single-cell digital ratiometric imaging using fluorescent pH-sensitive dye 2',7'-bis-(3-carboxypropyl)-5-(and-6)-carboxyfluorescein-AM. Pendrin mRNA expression in kidney cortex was examined by Northern blot hybridizations. Expression of pendrin protein was assessed by indirect immunofluorescence. Microperfused CCDs isolated from acidotic rats demonstrated approximately 60% reduction in apical Cl(-)/HCO(3)(-) exchanger activity in beta-ICs (P < 0.001 vs. control). Northern blot hybridizations indicated that the mRNA expression of pendrin in kidney cortex decreased by 68% in acidotic animals (P < 0.02 vs. control). Immunofluorescence labeling demonstrated significant reduction in pendrin expression in CCDs of acidotic rats. We conclude that metabolic acidosis decreases the activity of the apical Cl(-)/HCO(3)(-) exchanger in beta-ICs of the rat CCD by reducing the expression of pendrin. Adaptive downregulation of pendrin in metabolic acidosis indicates the important role of this exchanger in acid-base regulation in the CCD.

Regulation of the expression of the Cl-/anion exchanger pendrin in mouse kidney by acid-base status

BACKGROUND

Pendrin belongs to a superfamily of Cl-/anion exchangers and is expressed in the inner ear, the thyroid gland, and the kidney. In humans, mutations in pendrin cause Pendred syndrome characterized by sensorineural deafness and goiter. Recently pendrin has been localized to the apical side of non-type A intercalated cells of the cortical collecting duct, and reduced bicarbonate secretion was demonstrated in a pendrin knockout mouse model. To investigate a possible role of pendrin in modulating acid-base transport in the cortical collecting duct, we examined the regulation of expression of pendrin by acid-base status in mouse kidney.

METHODS

Mice were treated orally either with an acid or bicarbonate load (0.28 mol/L NH4Cl or NaHCO3) or received a K+-deficient diet for one week. Immunohistochemistry and Western blotting was performed.

RESULTS

Acid-loading caused a reduction in pendrin protein expression levels within one day and decreased expression to 23% of control levels after one week. Concomitantly, pendrin protein was shifted from the apical membrane to the cytosol, and the relative abundance of pendrin positive cells declined. Similarly, in chronic K+-depletion, known to elicit a metabolic alkalosis, pendrin protein levels decreased and pendrin expression was shifted to an intracellular pool with the relative number of pendrin positive cells reduced. In contrast, following oral bicarbonate loading pendrin was found exclusively in the apical membrane and the relative number of pendrin positive cells increased.

CONCLUSIONS

These results are in agreement with a potential role of pendrin in bicarbonate secretion and regulation of acid-base transport in the cortical collecting duct.

Colocalization of the apical Cl-/HCO3- exchanger PAT1 and gastric H-K-ATPase in stomach parietal cells

The apical Cl-/HCO exchanger called the putative anion transporter (PAT1; SLC26A6) is expressed on apical membranes of villus cells in the duodenum, but its location in the stomach remains unknown. Here we examined the cell distribution and membrane location of PAT1 in mouse stomach. Immunofluorescence labeling studies with anti-PAT1 antibodies and Dolichos biflorus agglutinin indicated the exclusive expression of PAT1 in gastric parietal cells. Double immunocytochemical staining revealed colocalization of PAT1 with the gastric H-K-ATPase, consistent with expression in tubulovesicles and/or the secretory canaliculus. Radiolabeled 36Cl flux studies demonstrated the functional presence of Cl-/HCO exchange in purified tubulovesicles of parietal cells. The expression of PAT1 was significantly decreased in parietal cells of gastric H-K-ATPase-null mice, which exhibit a sharp reduction in tubulovesicle membranes. These data indicate that the Cl-/HCO exchanger PAT1 is localized on tubulovesicular membranes, and they are consistent with the hypothesis that it functions in the maintenance of intravesicular ion concentrations in the resting state and dehydration of vesicles derived from the secretory membranes following the transition from the stimulated to the resting state.

Formate-stimulated NaCl absorption in the proximal tubule is independent of the pendrin protein

A significant fraction of active chloride reabsorption across the apical membrane of the proximal tubule is mediated by a chloride/formate exchange process, whereby intracellular formate drives the transport of chloride into the cell. When chloride/formate exchange operates in parallel with Na(+)/H(+) exchange and H(+)-coupled recycling of formate, the net result is electroneutral NaCl reabsorption. Pendrin is the protein product of the PDS gene (SLC26A4) and functions in several different anion exchange modes, including chloride/formate exchange. Pendrin is expressed in the kidney and may serve as the transporter responsible for formate-dependent NaCl reabsorption. In the present study, Pds-knockout mice were used to determine the role of pendrin in proximal tubule chloride reabsorption. We show that formate-dependent NaCl absorption in microperfused proximal tubules is similar between wild-type and pendrin-deficient mice. In addition, there is no difference in the rate of formate-mediated chloride transport in brush-border membrane vesicles isolated from wild-type and pendrin-deficient mice. These studies demonstrate that pendrin is not responsible for formate-dependent NaCl reabsorption in the proximal tubule.

The functional and physical relationship between the DRA bicarbonate transporter and carbonic anhydrase II

COOH-terminal cytoplasmic tails of chloride/bicarbonate anion exchangers (AE) bind cytosolic carbonic anhydrase II (CAII) to form a bicarbonate transport metabolon, a membrane protein complex that accelerates transmembrane bicarbonate flux. To determine whether interaction with CAII affects the downregulated in adenoma (DRA) chloride/bicarbonate exchanger, anion exchange activity of DRA-transfected HEK-293 cells was monitored by following changes in intracellular pH associated with bicarbonate transport. DRA-mediated bicarbonate transport activity of 18 +/- 1 mM H+ equivalents/min was inhibited 53 +/- 2% by 100 mM of the CAII inhibitor, acetazolamide, but was unaffected by the membrane-impermeant carbonic anhydrase inhibitor, 1-[5-sulfamoyl-1,3,4-thiadiazol-2-yl-(aminosulfonyl-4-phenyl)]-2,6-dimethyl-4-phenyl-pyridinium perchlorate. Compared with AE1, the COOH-terminal tail of DRA interacted weakly with CAII. Overexpression of a functionally inactive CAII mutant, V143Y, reduced AE1 transport activity by 61 +/- 4% without effect on DRA transport activity (105 +/- 7% transport activity relative to DRA alone). We conclude that cytosolic CAII is required for full DRA-mediated bicarbonate transport. However, DRA differs from other bicarbonate transport proteins because its transport activity is not stimulated by direct interaction with CAII.

A molecular mechanism for aberrant CFTR-dependent HCO(3)(-) transport in cystic fibrosis

Aberrant HCO(3)(-) transport is a hallmark of cystic fibrosis (CF) and is associated with aberrant Cl(-)-dependent HCO(3)(-) transport by the cystic fibrosis transmembrane conductance regulator (CFTR). We show here that HCO(3)(-) current by CFTR cannot account for CFTR-activated HCO(3)(-) transport and that CFTR does not activate AE1-AE4. In contrast, CFTR markedly activates Cl(-) and OH(-)/HCO(3)(-) transport by members of the SLC26 family DRA, SLC26A6 and pendrin. Most notably, the SLC26s are electrogenic transporters with isoform-specific stoichiometries. DRA activity occurred at a Cl(-)/HCO(3)(-) ratio > or =2. SLC26A6 activity is voltage regulated and occurred at HCO(3)(-)/Cl(-) > or =2. The physiological significance of these findings is demonstrated by interaction of CFTR and DRA in the mouse pancreas and an altered activation of DRA by the R117H and G551D mutants of CFTR. These findings provide a molecular mechanism for epithelial HCO(3)(-) transport (one SLC26 transporter-electrogenic transport; two SLC26 transporters with opposite stoichiometry in the same membrane domain-electroneutral transport), the CF-associated aberrant HCO(3)(-) transport, and reveal a new function of CFTR with clinical implications for CF and congenital chloride diarrhea.

Immunocytochemical localization of pendrin in intercalated cell subtypes in rat and mouse kidney

Recent studies have demonstrated that a novel anion exchanger, pendrin, is expressed in the apical domain of type B intercalated cells in the mammalian collecting duct. The purpose of this study was 1) to determine the expression and distribution of pendrin along the collecting duct and connecting tubule of mouse and rat kidney and establish whether pendrin is expressed in the non-A-non-B intercalated cells and 2) to determine the intracellular localization of pendrin in the different populations of intercalated cells by immunoelectron microscopy. A peptide-derived affinity-purified antibody was generated that specifically recognized pendrin in immunoblots of rat and mouse kidney. Immunohistochemistry and confocal laser scanning microscopy demonstrated the presence of pendrin in apical domains of all type B intercalated cells in mouse and rat connecting tubule and collecting duct. In addition, strong pendrin immunostaining was observed in non-A-non-B intercalated cells. There was no labeling of type A intercalated cells. Immunoelectron microscopy demonstrated that pendrin was located in the apical plasma membrane and intracellular vesicles of both type B intercalated cells and non-A-non-B cells; the latter was identified by the presence of H(+)-ATPase in the apical plasma membrane. The results of this study demonstrate that both pendrin and H(+)-ATPase are expressed in the apical plasma membrane of non-A-non-B intercalated cells, suggesting that these cells are capable of both HCO and proton secretion. Furthermore, the presence of pendrin in both the apical plasma membrane and the apical intracellular vesicles of type B and non-A-non-B intercalated cells suggests that HCO secretion may be regulated by trafficking of pendrin between the two membrane compartments.

Molecular characterization of the murine Slc26a6 anion exchanger: functional comparison with Slc26a1

We report the molecular and functional characterization of murine Slc26a6, the putative apical chloride-formate exchanger of the proximal tubule. The Slc26a6 transcript is expressed in several tissues, including kidney. Alternative splicing of the second exon generates two distinct isoforms, denoted Slc26a6a and Slc26a6b, which differ in the inclusion of a 23-residue NH(2)-terminal extension. Functional comparison with murine Slc26a1, the basolateral oxalate exchanger of the proximal tubule, reveals a number of intriguing differences. Whereas Slc26a6 is capable of Cl(-), SO, formate, and oxalate uptake when expressed in Xenopus laevis oocytes, Slc26a1 transports only SO and oxalate. Measurement of intracellular pH during the removal of extracellular Cl(-) in the presence and absence of HCO indicates that Slc26a6 functions as both a Cl(-)/HCO and a Cl(-)/OH(-) exchanger; simultaneous membrane hyperpolarization during these experimental maneuvers reveals that HCO and OH(-) transport mediated by Slc26a6 is electrogenic. Cis-inhibition and efflux experiments indicate that Slc26a6 can mediate the exchange of both Cl(-) and SOwith a number of substrates, including formate and oxalate. In contrast, SO and oxalate transport by Slc26a1 are mutually cis-inhibited but activated significantly by extracellular halides, lactate, and formate. The data indicate that Slc26a6 encodes an apical Cl(-)/formate/oxalate and Cl(-)/base exchanger and reveal significant mechanistic differences between apical and basolateral oxalate exchangers of the proximal tubule.

Pendrin immunoreactivity in the gill epithelium of a euryhaline elasmobranch

Pendrin is an anion exchanger in the cortical collecting duct of the mammalian nephron that appears to mediate apical Cl(-)/HCO3(-) exchange in bicarbonate-secreting intercalated cells. The goals of this study were to determine 1) if pendrin immunoreactivity was present in the gills of a euryhaline elasmobranch (Atlantic stingray, Dasyatis sabina), and 2) if branchial pendrin immunoreactivity was influenced by environmental salinity. Immunoblots detected pendrin immunoreactivity in Atlantic stingray gills; pendrin immunoreactivity was greatest in freshwater stingrays compared with freshwater stingrays acclimated to seawater (seawater acclimated) and marine stingrays. Using immunohistochemistry, pendrin-positive cells were detected on both gill lamellae and interlamellar regions of freshwater stingrays but were more restricted to interlamellar regions in seawater-acclimated and marine stingray gills. Pendrin immunolabeling in freshwater stingray gills was more apical, discrete, and intense compared with seawater-acclimated and marine stingrays. Regardless of salinity, pendrin immunoreactivity occurred on the apical region of cells rich with basolateral vacuolar-proton-ATPase, and not in Na(+)-K(+)-ATPase-rich cells. We suggest that a pendrin-like transporter may contribute to apical Cl(-)/HCO3(-) exchange in gills of Atlantic stingrays from both freshwater and marine environments.

AE4 is a DIDS-sensitive Cl(-)/HCO(-)(3) exchanger in the basolateral membrane of the renal CCD and the SMG duct

The renal cortical collecting duct (CCD) plays an important role in systemic acid-base homeostasis. The beta-intercalated cells secrete most of the HCO(-)(3), which is mediated by a luminal, DIDS-insensitive, Cl(-)/HCO(-)(3) exchange. The identity of the luminal exchanger is a matter of debate. Anion exchanger isoform 4 (AE4) cloned from the rabbit kidney was proposed to perform this function (Tsuganezawa H et al. J Biol Chem 276: 8180-8189, 2001). By contrast, it was proposed (Royaux IE et al. Proc Natl Acad Sci USA 98: 4221-4226, 2001) that pendrin accomplishes this function in the mouse CCD. In the present work, we cloned, localized, and characterized the function of the rat AE4. Northern blot and RT-PCR showed high levels of AE4 mRNA in the CCD. Expression in HEK-293 and LLC-PK(1) cells showed that AE4 is targeted to the plasma membrane. Measurement of intracellular pH (pH(i)) revealed that AE4 indeed functions as a Cl(-)/HCO(-)(3) exchanger. However, AE4 activity was inhibited by DIDS. Immunolocalization revealed species-specific expression of AE4. In the rat and mouse CCD and the mouse SMG duct AE4 was in the basolateral membrane. By contrast, in the rabbit, AE4 was in the luminal and lateral membranes. In both, the rat and rabbit CCD AE4 was in alpha-intercalated cells. Importantly, localization of AE4 was not affected by the systemic acid-base status of the rats. Therefore, we conclude that expression and possibly function of AE4 is species specific. In the rat and mouse AE4 functions as a Cl(-)/HCO(-)(3) exchanger in the basolateral membrane of alpha-intercalated cells and may participate in HCO(-)(3) absorption. In the rabbit AE4 may contribute to HCO(-)(3) secretion.

Characterization and semiquantitative analyses of pendrin expressed in normal and tumoral human thyroid tissues

The gene mutated in Pendred syndrome (PDS), the PDS gene, is expressed in the inner ear, kidney, and thyroid. It encodes a membrane protein named pendrin that is endowed with the function of anion transporter or exchanger. It has been postulated that in the thyroid pendrin could participate in the transport of iodide from the cell to the lumen of follicles. We generated antipeptide antibodies directed against the C- terminal sequence of human pendrin 1) to characterize the protein expressed in the human thyroid, and 2) to analyze its expression level in relation to the functional activity of thyroid tissue. In denaturing conditions, a single molecular species of 110-115 kDa was identified in human thyroid membrane fractions. After treatment of thyroid membranes with N-glycosidase F, pendrin had an apparent molecular mass of 85 kDa. Analyzed by ultracentrifugation on sucrose gradient in nondenaturing conditions, pendrin sedimented as a main 120- to 140-kDa component. Pendrin was assayed by semiquantitative Western blot in thyroid membrane fractions from 25 hyper- or hypofunctioning tumors and paired normal tissue samples. Pendrin was increased 2-fold in toxic adenomas, was not significantly altered in follicular adenoma, and was decreased, on the average, by 35% in papillary carcinomas compared with levels in paired normal tissue. The variations in the pendrin tissue content and PDS transcript levels, assayed by RT-PCR on duplicate samples of the same tumors, were similar. In conclusion, we show that pendrin expressed by the human thyroid gland is a mainly monomeric glycoprotein and that the level of expression of pendrin, although somewhat related, only moderately varied with the functional status of the thyroid tissue.

Mutations of the PDS gene, encoding pendrin, are associated with protein mislocalization and loss of iodide efflux: implications for thyroid dysfunction in Pendred syndrome

Pendred syndrome (PDS) is an autosomal recessive disorder characterized by deafness and goiter. Phenotypic heterogeneity is observed in affected individuals, and thyroid dysfunction is particularly variable. The syndrome is caused by mutations in the PDS (SLC26A4) gene, encoding an anion transporter pendrin, which localizes to the apical membrane of thyroid follicular cells. PDS is thought to enable efflux iodide into the follicle lumen. More than 50 diseases causing mutations of PDS have been reported. Here we have investigated the effect of nine PDS missense mutations on pendrin localization and iodide transport with the view to understanding their functional impact. As demonstrated by transient expression of green fluorescent protein-tagged pendrin mutant constructs in mammalian cell lines, appropriate trafficking to the plasma membrane was observed for only two mutants. The remaining PDS mutants appear to be retained within the endoplasmic reticulum following transfection. Iodide efflux assays were performed using human embryonic kidney 293 cells transfected with mutant pendrin and cotransfected with sodium iodide transporter to provide a mechanism of iodide uptake. The results indicated loss of pendrin iodide transport for all mislocalizing mutations. However, PDS mutants are associated with variable thyroid dysfunction in affected subjects. We concluded that additional genetic and/or environmental factors influence the thyroid activity in Pendred syndrome.

Identification of an apical Cl(-)/HCO3(-) exchanger in the small intestine

HCO3(-) secretion is the most important defense mechanism against acid injury in the duodenum. However, the identity of the transporter(s) mediating apical HCO3(-) secretion in the duodenum remains unknown. A family of anion exchangers, which include downregulated in adenoma (DRA or SLC26A3), pendrin (PDS or SLC26A4), and the putative anion transporter (PAT1 or SLC26A6) has recently been identified. DRA and pendrin mediate Cl(-)/base exchange; however, the functional identity and distribution of PAT1 (SLC26A6) is not known. In these studies, we investigated the functional identity, tissue distribution, and membrane localization of PAT1. Expression studies in Xenopus oocytes demonstrated that PAT1 functions in Cl(-)/HCO3(-) exchange mode. Tissue distribution studies indicated that the expression of PAT1 is highly abundant in the small intestine but is low in the colon, a pattern opposite that of DRA. PAT1 was also abundantly detected in stomach and heart. Immunoblot analysis studies identified PAT1 as a approximately 90 kDa protein in the duodenum. Immunohistochemical studies localized PAT1 to the brush border membranes of the villus cells of the duodenum. We propose that PAT1 is an apical Cl(-)/HCO3(-) exchanger in the small intestine.

Prestin, a new type of motor protein

Prestin, a transmembrane protein found in the outer hair cells of the cochlea, represents a new type of molecular motor, which is likely to be of great interest to molecular cell biologists. In contrast to enzymatic-activity-based motors, prestin is a direct voltage-to-force converter, which uses cytoplasmic anions as extrinsic voltage sensors and can operate at microsecond rates. As prestin mediates changes in outer hair cell length in response to membrane potential variations, it might be responsible for sound amplification in the mammalian hearing organ.

Acid incubation reverses the polarity of intercalated cell transporters, an effect mediated by hensin

Metabolic acidosis causes a reversal of polarity of HCO(3)(-) flux in the cortical collecting duct (CCD). In CCDs incubated in vitro in acid media, beta-intercalated (HCO(3)(-)-secreting) cells are remodeled to functionally resemble alpha-intercalated (H(+)-secreting) cells. A similar remodeling of beta-intercalated cells, in which the polarity of H(+) pumps and Cl(-)/HCO(3)(-) exchangers is reversed, occurs in cell culture and requires the deposition of polymerized hensin in the ECM. CCDs maintained 3 h at low pH ex vivo display a reversal of HCO(3)(-) flux that is quantitatively similar to an effect previously observed in acid-treated rabbits in vivo. We followed intracellular pH in the same beta-intercalated cells before and after acid incubation and found that apical Cl/HCO(3) exchange was abolished following acid incubation. Some cells also developed basolateral Cl(-)/HCO(3)(-) exchange, indicating a reversal of intercalated cell polarity. This adaptation required intact microtubules and microfilaments, as well as new protein synthesis, and was associated with decreased size of the apical surface of beta-intercalated cells. Addition of anti-hensin antibodies prevented the acid-induced changes in apical and basolateral Cl(-)/HCO(3)(-) exchange observed in the same cells and the corresponding suppression of HCO(3)(-) secretion. Acid loading also promoted hensin deposition in the ECM underneath adapting beta-intercalated cells. Hence, the adaptive conversion of beta-intercalated cells to alpha-intercalated cells during acid incubation depends upon ECM-associated hensin.

A transport metabolon. Functional interaction of carbonic anhydrase II and chloride/bicarbonate exchangers

The cytoplasmic carboxyl-terminal domain of AE1, the plasma membrane chloride/bicarbonate exchanger of erythrocytes, contains a binding site for carbonic anhydrase II (CAII). To examine the physiological role of the AE1/CAII interaction, anion exchange activity of transfected HEK293 cells was monitored by following the changes in intracellular pH associated with AE1-mediated bicarbonate transport. AE1-mediated chloride/bicarbonate exchange was reduced 50-60% by inhibition of endogenous carbonic anhydrase with acetazolamide, which indicates that CAII activity is required for full anion transport activity. AE1 mutants, unable to bind CAII, had significantly lower transport activity than wild-type AE1 (10% of wild-type activity), suggesting that a direct interaction was required. To determine the effect of displacement of endogenous wild-type CAII from its binding site on AE1, AE1-transfected HEK293 cells were co-transfected with cDNA for a functionally inactive CAII mutant, V143Y. AE1 activity was maximally inhibited 61 +/- 4% in the presence of V143Y CAII. A similar effect of V143Y CAII was found for AE2 and AE3cardiac anion exchanger isoforms. We conclude that the binding of CAII to the AE1 carboxyl-terminus potentiates anion transport activity and allows for maximal transport. The interaction of CAII with AE1 forms a transport metabolon, a membrane protein complex involved in regulation of bicarbonate metabolism and transport.

Downregulated in adenoma and putative anion transporter are regulated by CFTR in cultured pancreatic duct cells

The mechanism of the pancreatic ductal HCO secretion defect in cystic fibrosis (CF) is not well defined. However, a lack of apical Cl(-)/HCO exchange may exist in CF. To test this hypothesis, we examined the expression of Cl(-)/HCO exchangers in cultured pancreatic duct epithelial cells with physiological features prototypical of CF [CFPAC-1 cells lacking a functional CF transmembrane conductance regulator (CFTR)] or normal duct cells (CFPAC-1 cells transfected with functional wild-type CFTR, CFPAC-WT). Cl(-)/HCO exchange activity, assayed with the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein in cells grown on coverslips, increased about twofold in cells transfected with functional CFTR. This correlated with increased apical (36)Cl influx in cells expressing functional CFTR and grown on permeable support. Northern hybridizations indicated the induction of downregulated in adenoma (DRA) in cells expressing functional CFTR. The expression of putative anion transporter PAT1 also increased significantly in cells expressing functional CFTR. DRA was detected at high levels in native mouse pancreas by Northern hybridization and localized to the apical domain of the duct cells by immunohistochemical studies. In conclusion, CFTR upregulates DRA and PAT1 expression in cultured pancreatic duct cells. We propose that the pancreatic HCO secretion defect in CF patients is partly due to the downregulation of apical Cl(-)/HCO exchange activity mediated by DRA (and possibly PAT1).

Genetic defects in thyroid hormone synthesis

Thyroid hormone synthesis requires a normally developed thyroid gland, a properly functioning hypothalamic-pituitary-thyroid axis, and sufficient iodine intake. This article focuses on genetic defects in this axis. Defects that are primarily of developmental origin are discussed in our associated article in this issue. Defects in hormone synthesis usually are associated with the development of a goiter, provided that the bioactivity and action of thyrotropin (TSH) are not impaired. In contrast, hypoplasia of the gland may be caused by developmental defects, bioinactive TSH, or resistance to TSH at the level of the receptor or its signaling pathway. At the other end of the spectrum, hyperthyroidism may result from gain of function mutations in genes regulating growth and function.

Identification of a chloride-formate exchanger expressed on the brush border membrane of renal proximal tubule cells

A key function of the proximal tubule is retrieval of most of the vast quantities of NaCl and water filtered by the kidney. Physiological studies using brush border vesicles and perfused tubules have indicated that a major fraction of Cl(-) reabsorption across the apical membrane of proximal tubule cells occurs via Cl(-)-formate exchange. The molecular identity of the transporter responsible for renal brush border Cl(-)-formate exchange has yet to be elucidated. As a strategy to identify one or more anion exchangers responsible for mediating Cl(-) reabsorption in the proximal tubule, we screened the expressed sequence tag database for homologs of pendrin, a transporter previously shown to mediate Cl(-)-formate exchange. We now report the cDNA cloning of CFEX, a mouse pendrin homolog with expression in the kidney by Northern analysis. Sequence analysis indicated that CFEX very likely represents the mouse ortholog of human SLC26A6. Immunolocalization studies detected expression of CFEX, but not pendrin, on the brush border membrane of proximal tubule cells. Functional expression studies in Xenopus oocytes demonstrated that CFEX mediates Cl(-)-formate exchange. Taken together, these observations identify CFEX as a prime candidate to mediate Cl(-)-formate exchange in the proximal tubule and thereby to contribute importantly to renal NaCl reabsorption. Given its wide tissue distribution, CFEX also may contribute to transcellular Cl(-) transport in additional epithelia such as the pancreas and contribute to transmembrane Cl(-) transport in nonepithelial tissues such as the heart.