Putting bicarbonate on the spot: pharmacological insights for CFTR correction in the airway epithelium

Introduction: Cystic fibrosis (CF) is caused by defective Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) proteins. CFTR controls chloride (Cl-) and bicarbonate (HCO3 -) transport into the Airway Surface Liquid (ASL). We investigated the impact of F508del-CFTR correction on HCO3 - secretion by studying transepithelial HCO3 - fluxes. Methods: HCO3 - secretion was measured by pH-stat technique in primary human respiratory epithelial cells from healthy subjects (WT) and people with CF (pwCF) carrying at least one F508del variant. Its changes after CFTR modulation by the triple combination VX445/661/770 and in the context of TNF-α+IL-17 induced inflammation were correlated to ASL pH and transcriptional levels of CFTR and other HCO3 - transporters of airway epithelia such as SLC26A4 (Pendrin), SLC26A9 and NBCe1. Results: CFTR-mediated HCO3 - secretion was not detected in F508del primary human respiratory epithelial cells. It was rescued up to ∼ 80% of the WT level by VX-445/661/770. In contrast, TNF-α+IL-17 normalized transepithelial HCO3 - transport and increased ASL pH. This was related to an increase in SLC26A4 and CFTR transcript levels. VX-445/661/770 induced an increase in pH only in the context of inflammation. Effects on HCO3 - transport were not different between F508del homozygous and F508del compound heterozygous CF airway epithelia. Conclusion: Our studies show that correction of F508del-CFTR HCO3 - is not sufficient to buffer acidic ASL and inflammation is a key regulator of HCO3 - secretion in CF airways. Prediction of the response to CFTR modulators by theratyping should take into account airway inflammation.

pH-dependence of the Plasmodium falciparum chloroquine resistance transporter is linked to the transport cycle

The chloroquine resistance transporter, PfCRT, of the human malaria parasite Plasmodium falciparum is sensitive to acidic pH. Consequently, PfCRT operates at 60% of its maximal drug transport activity at the pH of 5.2 of the digestive vacuole, a proteolytic organelle from which PfCRT expels drugs interfering with heme detoxification. Here we show by alanine-scanning mutagenesis that E207 is critical for pH sensing. The E207A mutation abrogates pH-sensitivity, while preserving drug substrate specificity. Substituting E207 with Asp or His, but not other amino acids, restores pH-sensitivity. Molecular dynamics simulations and kinetics analyses suggest an allosteric binding model in which PfCRT can accept both protons and chloroquine in a partial noncompetitive manner, with increased proton concentrations decreasing drug transport. Further simulations reveal that E207 relocates from a peripheral to an engaged location during the transport cycle, forming a salt bridge with residue K80. We propose that the ionized carboxyl group of E207 acts as a hydrogen acceptor, facilitating transport cycle progression, with pH sensing as a by-product.

Airway surface hyperviscosity and defective mucociliary transport by IL-17/TNF-α are corrected by β-adrenergic stimulus

The fluid covering the surface of airway epithelia represents a first barrier against pathogens. The chemical and physical properties of the airway surface fluid are controlled by the activity of ion channels and transporters. In cystic fibrosis (CF), loss of CFTR chloride channel function causes airway surface dehydration, bacterial infection, and inflammation. We investigated the effects of IL-17A plus TNF-α, 2 cytokines with relevant roles in CF and other chronic lung diseases. Transcriptome analysis revealed a profound change with upregulation of several genes involved in ion transport, antibacterial defense, and neutrophil recruitment. At the functional level, bronchial epithelia treated in vitro with the cytokine combination showed upregulation of ENaC channel, ATP12A proton pump, ADRB2 β-adrenergic receptor, and SLC26A4 anion exchanger. The overall result of IL-17A/TNF-α treatment was hyperviscosity of the airway surface, as demonstrated by fluorescence recovery after photobleaching (FRAP) experiments. Importantly, stimulation with a β-adrenergic agonist switched airway surface to a low-viscosity state in non-CF but not in CF epithelia. Our study suggests that CF lung disease is sustained by a vicious cycle in which epithelia cannot exit from the hyperviscous state, thus perpetuating the proinflammatory airway surface condition.

A variant of ASIC2 mediates sodium retention in nephrotic syndrome

Idiopathic nephrotic syndrome (INS) is characterized by proteinuria and renal sodium retention leading to edema. This sodium retention is usually attributed to epithelial sodium channel (ENaC) activation after plasma aldosterone increase. However, most nephrotic patients show normal aldosterone levels. Using a corticosteroid-clamped (CC) rat model of INS (CC-PAN), we showed that the observed electrogenic and amiloride-sensitive Na retention could not be attributed to ENaC. We then identified a truncated variant of acid-sensing ion channel 2b (ASIC2b) that induced sustained acid-stimulated sodium currents when coexpressed with ASIC2a. Interestingly, CC-PAN nephrotic ASIC2b-null rats did not develop sodium retention. We finally showed that the expression of the truncated ASIC2b in the kidney was dependent on the presence of albumin in the tubule lumen and activation of ERK in renal cells. Finally, the presence of ASIC2 mRNA was also detected in kidney biopsies from patients with INS but not in any of the patients with other renal diseases. We have therefore identified a variant of ASIC2b responsible for the renal Na retention in the pathological context of INS.

Airway Surface Liquid pH Regulation in Airway Epithelium Current Understandings and Gaps in Knowledge

Knowledge on the mechanisms of acid and base secretion in airways has progressed recently. The aim of this review is to summarize the known mechanisms of airway surface liquid (ASL) pH regulation and their implication in lung diseases. Normal ASL is slightly acidic relative to the interstitium, and defects in ASL pH regulation are associated with various respiratory diseases, such as cystic fibrosis. Basolateral bicarbonate (HCO3-) entry occurs via the electrogenic, coupled transport of sodium (Na+) and HCO3-, and, together with carbonic anhydrase enzymatic activity, provides HCO3- for apical secretion. The latter mainly involves CFTR, the apical chloride/bicarbonate exchanger pendrin and paracellular transport. Proton (H+) secretion into ASL is crucial to maintain its relative acidity compared to the blood. This is enabled by H+ apical secretion, mainly involving H+/K+ ATPase and vacuolar H+-ATPase that carry H+ against the electrochemical potential gradient. Paracellular HCO3- transport, the direction of which depends on the ASL pH value, acts as an ASL protective buffering mechanism. How the transepithelial transport of H+ and HCO3- is coordinated to tightly regulate ASL pH remains poorly understood, and should be the focus of new studies.

Analysis of CLCNKB mutations at dimer-interface, calcium-binding site, and pore reveals a variety of functional alterations in ClC-Kb channel leading to Bartter syndrome

Pathological missense mutations in CLCNKB gene give a wide spectrum of clinical phenotypes in Bartter syndrome type III patients. Molecular analysis of the mutated ClC-Kb channels can be helpful to classify the mutations according to their functional alteration. We investigated the functional consequences of nine mutations in the CLCNKB gene causing Bartter syndrome. We first established that all tested mutations lead to decreased ClC-Kb currents. Combining electrophysiological and biochemical methods in Xenopus laevis oocytes and in MDCKII cells, we identified three classes of mutations. One class is characterized by altered channel trafficking. p.A210V, p.P216L, p.G424R, and p.G437R are totally or partially retained in the endoplasmic reticulum. p.S218N is characterized by reduced channel insertion at the plasma membrane and altered pH-sensitivity; thus, it falls in the second class of mutations. Finally, we found a novel class of functionally inactivated mutants normally present at the plasma membrane. Indeed, we found that p.A204T alters the pH-sensitivity, p.A254V abolishes the calcium-sensitivity. p.G219C and p.G465R are probably partially inactive at the plasma membrane. In conclusion, most pathogenic mutants accumulate partly or totally in intracellular compartments, but some mutants are normally present at the membrane surface and simultaneously show a large range of altered channel gating properties.

Phosphomimetic substitution at Ser-33 of the chloroquine resistance transporter PfCRT reconstitutes drug responses in Plasmodium falciparum

The chloroquine resistance transporter PfCRT of the human malaria parasite Plasmodium falciparum confers resistance to the former first-line antimalarial drug chloroquine, and it modulates the responsiveness to a wide range of quinoline and quinoline-like compounds. PfCRT is post-translationally modified by phosphorylation, palmitoylation, and, possibly, ubiquitination. However, the impact of these post-translational modifications on P. falciparum biology and, in particular, the drug resistance-conferring activity of PfCRT has remained elusive. Here, we confirm phosphorylation at Ser-33 and Ser-411 of PfCRT of the chloroquine-resistant P. falciparum strain Dd2 and show that kinase inhibitors can sensitize drug responsiveness. Using CRISPR/Cas9 genome editing to generate genetically engineered PfCRT variants in the parasite, we further show that substituting Ser-33 with alanine reduced chloroquine and quinine resistance by ∼50% compared with the parental P. falciparum strain Dd2, whereas the phosphomimetic amino acid aspartic acid could fully and glutamic acid could partially reconstitute the level of chloroquine/quinine resistance. Transport studies conducted in the parasite and in PfCRT-expressing Xenopus laevis oocytes linked phosphomimetic substitution at Ser-33 to increased transport velocity. Our data are consistent with phosphorylation of Ser-33 relieving an autoinhibitory intramolecular interaction within PfCRT, leading to a stimulated drug transport activity. Our findings shed additional light on the function of PfCRT and suggest that chloroquine could be reevaluated as an antimalarial drug by targeting the kinase in P. falciparum that phosphorylates Ser-33 of PfCRT.

ANP-stimulated Na+ secretion in the collecting duct prevents Na+ retention in the renal adaptation to acid load

We have recently reported that type A intercalated cells of the collecting duct secrete Na⁺ by a mechanism coupling the basolateral type 1 Na⁺-K⁺-2Cl^- cotransporter with apical type 2 H⁺-K⁺-ATPase (HKA2) functioning under its Na⁺/K⁺ exchange mode. The first aim of the present study was to evaluate whether this secretory pathway is a target of atrial natriuretic peptide (ANP). Despite hyperaldosteronemia, metabolic acidosis is not associated with Na⁺ retention. The second aim of the present study was to evaluate whether ANP-induced stimulation of Na⁺ secretion by type A intercalated cells might account for mineralocorticoid escape during metabolic acidosis. In Xenopus oocytes expressing HKA2, cGMP, the second messenger of ANP, increased the membrane expression, activity, and Na⁺-transporting rate of HKA2. Feeding mice with a NH₄Cl-enriched diet increased urinary excretion of aldosterone and induced a transient Na⁺ retention that reversed within 3 days. At that time, expression of ANP mRNA in the collecting duct and urinary excretion of cGMP were increased. Reversion of Na⁺ retention was prevented by treatment with an inhibitor of ANP receptors and was absent in HKA2-null mice. In conclusion, paracrine stimulation of HKA2 by ANP is responsible for the escape of the Na⁺-retaining effect of aldosterone during metabolic acidosis.

Increased expression of ATP12A proton pump in cystic fibrosis airways

Proton secretion mediated by ATP12A protein on the surface of the airway epithelium may contribute to cystic fibrosis (CF) lung disease by favoring bacterial infection and airway obstruction. We studied ATP12A in fresh bronchial samples and in cultured epithelial cells. In vivo, ATP12A expression was found almost exclusively at the apical side of nonciliated cells of airway epithelium and in submucosal glands, with much higher expression in CF samples. This could be due to bacterial infection and inflammation, since treating cultured cells with bacterial supernatants or with IL-4 (a cytokine that induces goblet cell hyperplasia) increased the expression of ATP12A in nonciliated cells. This observation was associated with upregulation and translocation of ATP1B1 protein from the basal to apical epithelial side, where it colocalizes with ATP12A. ATP12A function was evaluated by measuring the pH of the apical fluid in cultured epithelia. Under resting conditions, CF epithelia showed more acidic values. This abnormality was minimized by inhibiting ATP12A with ouabain. Following treatment with IL-4, ATP12A function was markedly increased, as indicated by strong acidification occurring under bicarbonate-free conditions. Our study reveals potentially novel aspects of ATP12A and remarks its importance as a possible therapeutic target in CF and other respiratory diseases.

In silico model of the human ClC-Kb chloride channel: pore mapping, biostructural pathology and drug screening

The human ClC-Kb channel plays a key role in exporting chloride ions from the cytosol and is known to be involved in Bartter syndrome type 3 when its permeation capacity is decreased. The ClC-Kb channel has been recently proposed as a potential therapeutic target to treat hypertension. In order to gain new insights into the sequence-structure-function relationships of this channel, to investigate possible impacts of amino-acid substitutions, and to design novel inhibitors, we first built a structural model of the human ClC-Kb channel using comparative modeling strategies. We combined in silico and in vitro techniques to analyze amino acids involved in the chloride ion pathway as well as to rationalize the possible role of several clinically observed mutations leading to the Bartter syndrome type 3. Virtual screening and drug repositioning computations were then carried out. We identified six novel molecules, including 2 approved drugs, diflusinal and loperamide, with Kd values in the low micromolar range, that block the human ClC-Kb channel and that could be used as starting point to design novel chemical probes for this potential therapeutic target.

Iron is a substrate of the Plasmodium falciparum chloroquine resistance transporter PfCRT in Xenopus oocytes

The chloroquine resistance transporter of the human malaria parasite Plasmodium falciparum, PfCRT, is an important determinant of resistance to several quinoline and quinoline-like antimalarial drugs. PfCRT also plays an essential role in the physiology of the parasite during development inside erythrocytes. However, the function of this transporter besides its role in drug resistance is still unclear. Using electrophysiological and flux experiments conducted on PfCRT-expressing Xenopus laevis oocytes, we show here that both wild-type PfCRT and a PfCRT variant associated with chloroquine resistance transport both ferrous and ferric iron, albeit with different kinetics. In particular, we found that the ability to transport ferrous iron is reduced by the specific polymorphisms acquired by the PfCRT variant as a result of chloroquine selection. We further show that iron and chloroquine transport via PfCRT is electrogenic. If these findings in the Xenopus model extend to P. falciparum in vivo, our data suggest that PfCRT might play a role in iron homeostasis, which is essential for the parasite's development in erythrocytes.

Rattlesnake Phospholipase A2 Increases CFTR-Chloride Channel Current and Corrects ∆F508CFTR Dysfunction: Impact in Cystic Fibrosis

Deletion of Phe508 in the nucleotide binding domain (∆F508-NBD1) of the cystic fibrosis transmembrane regulator (CFTR; a cyclic AMP-regulated chloride channel) is the most frequent mutation associated with cystic fibrosis. This mutation affects the maturation and gating of CFTR protein. The search for new high-affinity ligands of CFTR acting as dual modulators (correctors/activators) presents a major challenge in the pharmacology of cystic fibrosis. Snake venoms are a rich source of natural multifunctional proteins, potential binders of ion channels. In this study, we identified the CB subunit of crotoxin from Crotalus durissus terrificus as a new ligand and allosteric modulator of CFTR. We showed that CB interacts with NBD1 of both wild type and ∆F508CFTR and increases their chloride channel currents. The potentiating effect of CB on CFTR activity was demonstrated using electrophysiological techniques in Xenopus laevis oocytes, in CFTR-HeLa cells, and ex vivo in mouse colon tissue. The correcting effect of CB was shown by functional rescue of CFTR activity after 24-h ΔF508CFTR treatments with CB. Moreover, the presence of fully glycosylated CFTR was observed. Molecular docking allowed us to propose a model of the complex involving of the ABCβ and F1-like ATP-binding subdomains of ΔF508-NBD1. Hydrogen-deuterium exchange analysis confirmed stabilization in these regions, also showing allosteric stabilization in two other distal regions. Surface plasmon resonance competition studies showed that CB disrupts the ∆F508CFTR-cytokeratin 8 complex, allowing for the escape of ∆F508CFTR from degradation. Therefore CB, as a dual modulator of ΔF508CFTR, constitutes a template for the development of new anti-CF agents.

Characterization of SLC26A9 in patients with CF-like lung disease

Diffuse bronchiectasis is a common problem in respiratory clinics. We hypothesized that mutations in the solute carrier 26A9 (SLC26A9) gene, encoding for a chloride (Cl(-)) transporter mainly expressed in lungs, may lead to defects in mucociliary clearance. We describe two missense variants in the SLC26A9 gene in heterozygote patients presenting with diffuse idiopathic bronchiectasis : p.Arg575Trp, identified in a patient also heterozygote for p.Phe508del in the CFTR gene; and p.Val486Ile. Expression of both mutants in Xenopus laevis oocytes abolished SLC26A9-mediated Cl(-) conductance without decreasing protein membrane expression. Coexpression of CFTR with SLC26A9-p.Val486Ile resulted in a significant increase in the Cl(-) current induced by PKA stimulation, similar to that obtained in oocytes expressing CFTR and SLC26A9-WT. In contrast, coexpression of CFTR with SLC26A9-p.Arg575Trp inhibited SLC26A9-enhanced CFTR activation upon PKA. Further structure-function analyses led us to propose a site encompassing Arg575 in the SLC26A9-STAS domain for CFTR-SLC26A9 interaction. We hypothesize that SLC26A9-p.Arg575Trp prevented SLC26A9-mediated functional activation of CFTR by altering SLC26A9-CFTR interaction. Although we cannot confirm that these mutations by themselves are deleterious, we propose that they trigger the pathogenic role of a single CFTR mutation and provide insight into a novel mechanism of Cl(-) transport alteration across the respiratory mucosa, based on functional inhibition of CFTR.

Functional and electrophysiological characterization of four non-truncating mutations responsible for creatine transporter (SLC6A8) deficiency syndrome

Intellectual disability coupled with epilepsy are clinical hallmarks of the creatine (Cr) transporter deficiency syndrome resulting from mutations in the SLC6A8 gene. So far characterization of pathogenic mutations of SLC6A8 has been limited to Cr uptake. The aim of our study was to characterize the electrogenic and pharmacological properties of non truncating SLC6A8 mutations identified in patients presenting variable clinical severity. Electrophysiological and pharmacological properties of four mutants (including two novel ones) were studied in X. laevis oocyte expression system. Creatine uptake was assessed with [(14)C]-Cr in X. laevis and patients' fibroblasts. Subcellular localization was determined by immunofluorescence and western blot. All mutants were properly targeted to the plasma membrane in both systems. Mutations led to the complete loss of both electrogenic and transport activities in X. laevis and Cr uptake in patients' fibroblasts. Among the Cr analogs tested, guanidinopropionate induced an electrogenic activity with the normal SLC6A8 transporter similar to creatine whereas a phosphocreatine derivative, PCr-Mg-CPLX, resulted in partial activity. SLC6A8 mutants displayed no electrogenic activity with all Cr analogs tested in X. laevis oocytes. Although the mutations altered various domains of SLC6A8 Cr uptake and electrogenic properties were completely inhibited and could not be dissociated. Besides the metabolic functions of Cr, the loss of SLC6A8 electrogenic activity, demonstrated here for the first time, may also play a role in the altered brain functions of the patients.

Functional interaction between CFTR and the sodium-phosphate co-transport type 2a in Xenopus laevis oocytes

Background

A growing number of proteins, including ion transporters, have been shown to interact with Cystic Fibrosis Transmembrane conductance Regulator (CFTR). CFTR is an epithelial chloride channel that is involved in Cystic Fibrosis (CF) when mutated; thus a better knowledge of its functional interactome may help to understand the pathophysiology of this complex disease. In the present study, we investigated if CFTR and the sodium-phosphate co-transporter type 2a (NPT2a) functionally interact after heterologous expression of both proteins in Xenopus laevis oocytes.

Methodology/Findings

NPT2a was expressed alone or in combination with CFTR in X. laevis oocytes. Using the two-electrode voltage-clamp technique, the inorganic phosphate-induced current (IPi) was measured and taken as an index of NPT2a activity. The maximal IPi for NPT2a substrates was reduced when CFTR was co-expressed with NPT2a, suggesting a decrease in its expression at the oolemna. This was consistent with Western blot analysis showing reduced NPT2a plasma membrane expression in oocytes co-expressing both proteins, whereas NPT2a protein level in total cell lysate was the same in NPT2a- and NPT2a+CFTR-oocytes. In NPT2a+CFTR- but not in NPT2a-oocytes, IPi and NPT2a surface expression were increased upon PKA stimulation, whereas stimulation of Exchange Protein directly Activated by cAMP (EPAC) had no effect. When NPT2a-oocytes were injected with NEG2, a short amino-acid sequence from the CFTR regulatory domain that regulates PKA-dependent CFTR trafficking to the plasma membrane, IPi values and NPT2a membrane expression were diminished, and could be enhanced by PKA stimulation, thereby mimicking the effects of CFTR co-expression.

Conclusion/Perspectives

We conclude that when both CFTR and NPT2a are expressed in X. laevis oocytes, CFTR confers to NPT2a a cAMPi-dependent trafficking to the membrane. This functional interaction raises the hypothesis that CFTR may play a role in phosphate homeostasis.

A new human NHERF1 mutation decreases renal phosphate transporter NPT2a expression by a PTH-independent mechanism

BACKGROUND

The sodium-hydrogen exchanger regulatory factor 1 (NHERF1) binds to the main renal phosphate transporter NPT2a and to the parathyroid hormone (PTH) receptor. We have recently identified mutations in NHERF1 that decrease renal phosphate reabsorption by increasing PTH-induced cAMP production in the renal proximal tubule.

METHODS

We compared relevant parameters of phosphate homeostasis in a patient with a previously undescribed mutation in NHERF1 and in control subjects. We expressed the mutant NHERF1 protein in Xenopus Oocytes and in cultured cells to study its effects on phosphate transport and PTH-induced cAMP production.

RESULTS

We identified in a patient with inappropriate renal phosphate reabsorption a previously unidentified mutation (E68A) located in the PDZ1 domain of NHERF1.We report the consequences of this mutation on NHERF1 function. E68A mutation did not modify cAMP production in the patient. PTH-induced cAMP synthesis and PKC activity were not altered by E68A mutation in renal cells in culture. In contrast to wild-type NHERF1, expression of the E68A mutant in Xenopus oocytes and in human cells failed to increase phosphate transport. Pull down experiments showed that E68A mutant did not interact with NPT2a, which robustly interacted with wild type NHERF1 and previously identified mutants. Biotinylation studies revealed that E68A mutant was unable to increase cell surface expression of NPT2a.

CONCLUSIONS

Our results indicate that the PDZ1 domain is critical for NHERF1-NPT2a interaction in humans and for the control of NPT2a expression at the plasma membrane. Thus we have identified a new mechanism of renal phosphate loss and shown that different mutations in NHERF1 can alter renal phosphate reabsorption via distinct mechanisms.

The testis anion transporter TAT1 (SLC26A8) physically and functionally interacts with the cystic fibrosis transmembrane conductance regulator channel: a potential role during sperm capacitation

The Slc26 gene family encodes several conserved anion transporters implicated in human genetic disorders, including Pendred syndrome, diastrophic dysplasia and congenital chloride diarrhea. We previously characterized the TAT1 (testis anion transporter 1; SLC26A8) protein specifically expressed in male germ cells and mature sperm and showed that in the mouse, deletion of Tat1 caused male sterility due to a lack of sperm motility, impaired sperm capacitation and structural defects of the flagella. Ca(2+), Cl(-) and HCO(3)(-) influxes trigger sperm capacitation events required for oocyte fertilization; these events include the intracellular rise of cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA)-dependent protein phosphorylation. The cystic fibrosis transmembrane conductance regulator (CFTR) is expressed in mature sperm and has been shown to contribute to Cl(-) and HCO(3)(-) movements during capacitation. Furthermore, several members of the SLC26 family have been described to form complexes with CFTR, resulting in the reciprocal regulation of their activities. We show here that TAT1 and CFTR physically interact and that in Xenopus laevis oocytes and in CHO-K1 cells, TAT1 expression strongly stimulates CFTR activity. Consistent with this, we show that Tat1 inactivation in mouse sperm results in deregulation of the intracellular cAMP content, preventing the activation of PKA-dependent downstream phosphorylation cascades essential for sperm activation. These various results suggest that TAT1 and CFTR may form a molecular complex involved in the regulation of Cl(-) and HCO(3)(-) fluxes during sperm capacitation. In humans, mutations in CFTR and/or TAT1 may therefore be causes of asthenozoospermia and low fertilizing capacity of sperm.

Resveratrol rescues cAMP-dependent anionic transport in the cystic fibrosis pancreatic cell line CFPAC1

BACKGROUND AND PURPOSE

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-dependent chloride channel in the plasma membrane of epithelia whose mutation is the cause of the genetic disease cystic fibrosis (CF). The most frequent CFTR mutation is deletion of Phe(508) and this mutant protein (delF508CFTR) does not readily translocate to the plasma membrane and is rapidly degraded within the cell. We hypothesized that treating epithelial cells with resveratrol, a natural polyphenolic, phyto-ooestrogenic compound from grapes, could modulate both the expression and localization of CFTR.

EXPERIMENTAL APPROACH

Cells endogenously expressing CFTR (MDCK1 and CAPAN1 cells) or delF508CFTR (CFPAC1 and airway epithelial cells, deriving from human bronchial biopsies) were treated with resveratrol for 2 or 18 h. The effect of this treatment on CFTR and delF508CFTR expression and localization was evaluated using RT-PCR, Western blot and immunocytochemistry. Halide efflux was measured with a fluorescent dye and with halide-sensitive electrodes. Production of interleukin-8 by these cells was assayed by ELISA.

KEY RESULTS

Resveratrol treatment increased CFTR expression or maturation in immunoblotting experiments in MDCK1 cells or in CFPAC1 cells. Indirect immunofluorescence experiments showed a shift of delF508CFTR localization towards the (peri)-membrane area in CFPAC1 cells and in human airway epithelial cells. A cAMP-dependent increase in membrane permeability to halide was detected in resveratrol-treated CFPAC1 cells, and was inhibited by a selective inhibitor of CFTR.

CONCLUSION AND IMPLICATIONS

These results show that resveratrol modulated CFTR expression and localization and could rescue cAMP-dependent chloride transport in delF508CFTR cells.

PfCHA is a mitochondrial divalent cation/H+ antiporter in Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is capable of adapting to vastly different extracellular Ca(2+) environments while maintaining tight control of its intracellular Ca(2+) concentration. The mechanisms underpinning Ca(2+) homeostasis in this important pathogen are only partly understood. Here we have functionally expressed the putative Ca(2+)/H(+) antiporter PfCHA in Xenopus laevis oocytes. Our data suggest that PfCHA mediates H(+)-coupled Ca(2+) and Mn(2+) exchange. The apparent dissociation constant K(M) for Ca(2+) of 2.2 +/- 0.7 mM and the maximal velocity V(max) of 0.6 +/- 0.1 nmol per oocyte per hour are consistent with PfCHA being a low-affinity high-capacity Ca(2+) carrier. In the parasite, PfCHA was found to localize to the mitochondrion. Physiological studies conducted with live parasitized erythrocytes, and using Fluo-4 and Rhod-2 to monitor cytoplasmic and mitochondrial Ca(2+) dynamics, suggest that the mitochondrion serves as a dynamic Ca(2+) store and that PfCHA functions as a Ca(2+) efflux system expelling excess Ca(2+) from the mitochondrion. PfCHA lacks appreciable homologies to the human mitochondrial Ca(2+)/H(+) exchanger and might represent an evolutionary divergent class of mitochondrial cation antiporter, which, in turn, might provide novel opportunities for intervention.

NHERF1 mutations and responsiveness of renal parathyroid hormone

Impaired renal phosphate reabsorption, as measured by dividing the tubular maximal reabsorption of phosphate by the glomerular filtration rate (TmP/GFR), increases the risks of nephrolithiasis and bone demineralization. Data from animal models suggest that sodium-hydrogen exchanger regulatory factor 1 (NHERF1) controls renal phosphate transport. We sequenced the NHERF1 gene in 158 patients, 94 of whom had either nephrolithiasis or bone demineralization. We identified three distinct mutations in seven patients with a low TmP/GFR value. No patients with normal TmP/GFR values had mutations. The mutants expressed in cultured renal cells increased the generation of cyclic AMP (cAMP) by parathyroid hormone (PTH) and inhibited phosphate transport. These NHERF1 mutations suggest a previously unrecognized cause of renal phosphate loss in humans.

Control of basal CFTR gene expression by bicarbonate-sensitive adenylyl cyclase in human pulmonary cells

The CFTR protein, encoded by the gene whose mutations induce Cystic Fibrosis, is an anion channel devoted mainly to chloride and bicarbonate transmembrane transport, but which also regulates transport of several other ions. Moreover, it is implicated in the cell response to inflammation, and, reciprocally, cftr gene expression is modulated by inflammatory stimuli and transduction pathways. Looking for a control of CFTR expression by ionic conditions, we investigated the effect of altered extracellular bicarbonate ion concentration on CFTR expression in human pulmonary Calu-3 cells. We found that basal cftr gene transcription is enhanced when extracellular HCO(3)(-) concentration increases from 0 to 25 mmol/l. The transduction pathway controlled by these extracellular [HCO(3)(-)] variations includes cAMP production linked to the stimulation of soluble adenylyl cyclase (sAC), and nuclear accumulation of the transcription factor, CREB. Basal membrane content in CFTR protein exhibits the same variations as cftr mRNA in cells incubated in the presence of extracellular [HCO(3)(-)] between 0 and 25 mmol/l, and is also decreased by inhibiting sAC in the presence of HCO(3)(-). These results show that bicarbonate-controlled sAC stimulation must be taken into account in cell physiology and that basal CFTR expression depends on an ionic parameter.

Ammonium Homeostasis and Human Rhesus Glycoproteins

The brain ammonium production is detoxified by astrocytes, the gut ammonium production is detoxified by hepatic cells, and the renal ammonium production plays a major role in renal acid excretion. As a result of ammonium handling in these organs, the ammonium and pH values are strictly regulated in plasma. Up until recently, it was accepted that mammalian cell transmembrane ammonium transport was due to NH(4)(+) transport by non-specific transporting systems, and to non-ionic NH(3) diffusion, whereas lower organisms (such as bacteria, yeasts and plants) were endowed with specific ammonium transporters (Amts). Sequence homologies between Amts and human Rhesus (Rh) glycoproteins (RhAG, from erythroid cells, and RhBG and RhCG from epithelial cells) raised the hypothesis that Rh glycoproteins act as specific ammonium transporters, further sustained by the polarized distribution of RhBG and RhCG in gut, kidney and liver. Results from functional studies agree that Rh glycoproteins are the first ammonium transporters reported in mammals. However, the nature of the transported specie(s) is much debated: in particular, it is proposed that Rh glycoproteins mediate a direct NH(3) transport, or that they mediate an indirect NH(3) transport (resulting from NH(4)(+) for H(+) exchange). Direct NH(3) transport (associated or not with NH(4)(+) transport) raises the exciting hypothesis that Rh glycoproteins may also transport other gases than NH(3) (namely, CO(2)).

The challenge of understanding ammonium homeostasis and the role of the Rh glycoproteins

Rh glycoproteins belong to the superfamily of ammonium transporters, but until recent functional studies their functional role was unknown. This review focuses on the functional results obtained in our laboratory after the heterologous expression of RhAG (the erythroid Rh glycoprotein) and RhCG (an epithelial Rh glycoprotein). RhAG and RhCG were expressed in two different expression systems (HeLa cells and Xenopus laevis oocytes) that differed in their endogenous membrane permeabilities for NH3 and NH4+. To check if RhAG and RhCG are ammonium transporters, we measured intracellular pH changes in cells exposed to an ammonium-containing solution, and analyzed the ammonium-induced NH3 and NH4+ transmembrane fluxes in control versus transfected cells. We observed that RhAG and RhCG expression induced an enhancement of the ammonium-induced initial alkalinization (related to NH3 influx into the cell) and secondary acidification (related to NH4+ influx into the cell). Moreover, sub-millimolar ammonium concentrations induced inward currents in voltage-clamped RhAG- and in RhCG-expressing oocytes. Taken together, these results show not only that RhAG and RhCG are ammonium transporters, but also that they are promoting the transmembrane transport of NH3 and of NH4+. Data from our laboratory and from other groups raise several questions that are discussed.

Rescue of DeltaF508-CFTR (cystic fibrosis transmembrane conductance regulator) by curcumin: involvement of the keratin 18 network

The most common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, DeltaF508, causes retention of DeltaF508-CFTR in the endoplasmic reticulum and leads to the absence of CFTR Cl(-) channels in the plasma membrane. DeltaF508-CFTR retains some Cl(-) channel activity so increased expression of DeltaF508-CFTR in the plasma membrane can restore Cl(-) secretion deficiency. Recently, curcumin was shown to rescue DeltaF508-CFTR localization and function. In our previous work, the keratin 18 (K18) network was implicated in DeltaF508-CFTR trafficking. Here, we hypothesized that curcumin could restore a functional DeltaF508-CFTR to the plasma membrane acting via the K18 network. First, we analyzed the effects of curcumin on the localization of DeltaF508-CFTR in different cell lines (HeLa cells stably transfected with wild-type CFTR or DeltaF508-CFTR, CALU-3 cells, or cystic fibrosis pancreatic epithelial cells CFPAC-1) and found that it was significantly delocalized toward the plasma membrane in DeltaF508-CFTR-expressing cells. We also performed a functional assay for the CFTR chloride channel in CFPAC-1 cells treated or not with curcumin and detected an increase in a cAMP-dependent chloride efflux in treated DeltaF508-CFTR-expressing cells. The K18 network then was analyzed by immunocytochemistry and immunoblot exclusively in curcumin-treated or untreated CFPAC-1 cells because of their endogenic DeltaF508-CFTR expression. After curcumin treatment, we observed a remodeling of the K18 network and a significant increase in K18 Ser52 phosphorylation, a site directly implicated in the reorganization of intermediate filaments. With these results, we propose that K18 as a new therapeutic target and curcumin, and/or its analogs, might be considered as potential therapeutic agents for cystic fibrosis.

Expression of the human erythroid Rh glycoprotein (RhAG) enhances both NH3 and NH4+ transport in HeLa cells

The erythroid Rh-associated glycoprotein (RhAG) is strictly required for the expression of the Rh blood group antigens carried by Rh (D,CE) proteins. A biological function for RhAG in ammonium transport has been suggested by its ability to improve survival of an ammonium-uptake-deficient yeast. We investigated the function of RhAG by studying the entry of NH3/NH4+ in HeLa cells transiently expressing the green fluorescent protein (GFP)-RhAG fusion protein and using a fluorescent proton probe to measure intracellular pH (pHi). Under experimental conditions that reduce the intrinsic Na/H exchanger activity, exposure of control cells to a 10 mM NH4Cl- containing solution induces the classic pHi response profile of cells having a high permeability to NH3 (PNH3) but relatively low permeability to NH4+ (PNH4). In contrast, under the same conditions, the pHi profile of cells expressing RhAG clearly indicated an increased PNH4, as evidenced by secondary reacidification during NH4Cl exposure and a pHi undershoot below the initial resting value upon its removal. Measurements of pHi during methylammonium exposure showed that RhAG expression enhances the influx of both the unprotonated and ionic forms of methylammonium. Using a mathematical model to adjust passive permeabilities for a fit to the pHi profiles, we found that RhAG expression resulted in a threefold increase of PNH4 and a twofold increase of PNH3. Our results are the first evidence that the human erythroid RhAG increases the transport of both NH3 and NH4+.

Evidence for Activation of Endogenous Transporters in Xenopus laevis Oocytes Expressing the Plasmodium falciparum Chloroquine Resistance Transporter, PfCRT

A large body of genetic, reverse genetic, and epidemiological data has linked chloroquine-resistant malaria to polymorphisms within a gene termed pfcrt in the human malarial parasite Plasmodium falciparum. To investigate the biological function of the chloroquine resistance transporter, PfCRT, as well as its role in chloroquine resistance, we functionally expressed this protein in Xenopus laevis oocytes. Our data show that PfCRT-expressing oocytes exhibit a depolarized resting membrane potential and a higher intracellular pH compared with control oocytes. Pharmacological and electrophysiological studies link the higher intracellular pH to an enhanced amiloride-sensitive H(+) extrusion and the low membrane potential to an activated nonselective cation conductance. The finding that both properties are independent of each other, together with the fact that they are endogenously present in X. laevis oocytes, supports a model in which PfCRT activates transport systems. Our data suggest that PfCRT plays a role as a direct or indirect activator or modulator of other transporters.

Chloride transport in the renal proximal tubule

The renal proximal tubule is responsible for most of the renal sodium, chloride, and bicarbonate reabsorption. Micropuncture studies and electrophysiological techniques have furnished the bulk of our knowledge about the physiology of this tubular segment. As a consequence of the leakiness of this epithelium, paracellular ionic transport--in particular that of Cl(-)--is of considerable importance in this first part of the nephron. It was long accepted that proximal Cl(-) reabsorption proceeds solely paracellularly, but it is now known that transcellular Cl(-) transport also exists. Cl(-) channels and Cl(-)-coupled transporters are involved in transcellular Cl(-) transport. In the apical membrane, Cl(-)/anion (formate, oxalate and bicarbonate) exchangers represent the first step in transcellular Cl(-) reabsorption. A basolateral Cl(-)/HCO(3)(-) exchanger, involved in HCO(3)(-) reclamation, participates in the rise of intracellular Cl(-) activity above its equilibrium value, and thus also contributes to the creation of an outwardly directed electrochemical Cl(-) gradient across the cell membranes. This driving force favours Cl(-) diffusion from the cell to the lumen and to the interstitium. In the basolateral membrane, the main mechanism for transcellular Cl(-) reabsorption is a Cl(-) conductance, but a Na(+)-driven Cl(-)/HCO(3)(-) exchanger may also participate in Cl(-) reabsorption.

NH3 is involved in the NH4+ transport induced by the functional expression of the human Rh C glycoprotein

Renal ammonium (NH3 + NH4+) transport is a key process for body acid-base balance. It is well known that several ionic transport systems allow NH4+ transmembrane translocation without high specificity NH4+, but it is still debated whether NH3, and more generally, gas, may be transported by transmembrane proteins. The human Rh glycoproteins have been proposed to mediate ammonium transport. Transport of NH4+ and/or NH3 by the epithelial Rh C glycoprotein (RhCG) may be of physiological importance in renal ammonium excretion because RhCG is mainly expressed in the distal nephron. However, RhCG function is not yet established. In the present study, we search for ammonium transport by RhCG. RhCG function was investigated by electrophysiological approaches in RhCG-expressing Xenopus laevis oocytes. In the submillimolar concentration range, NH4Cl exposure induced inward currents (IAM) in voltage-clamped RhCG-expressing cells, but not in control cells. At physiological extracellular pH (pHo) = 7.5, the amplitude of IAM increased with NH4Cl concentration and membrane hyperpolarization. The amplitude of IAM was independent of external Na+ or K+ concentrations but was enhanced by alkaline pHo and decreased by acid pHo. The apparent affinity of RhCG for NH4+ was affected by NH3 concentration and by changing pHo, whereas the apparent affinity for NH3 was unchanged by pHo, consistent with direct NH3 involvement in RhCG function. The enhancement of methylammonium-induced current by NH3 further supported this conclusion. Exposure to 500 microm NH4Cl induced a biphasic intracellular pH change in RhCG-expressing oocytes, consistent with both NH3 and NH4+ enhanced influx. Our results support the hypothesis of a specific role for RhCG in NH3 and NH4+ transport.

Nephrolithiasis and osteoporosis associated with hypophosphatemia caused by mutations in the type 2a sodium-phosphate cotransporter

BACKGROUND

Epidemiologic studies suggest that genetic factors confer a predisposition to the formation of renal calcium stones or bone demineralization. Low serum phosphate concentrations due to a decrease in renal phosphate reabsorption have been reported in some patients with these conditions, suggesting that genetic factors leading to a decrease in renal phosphate reabsorption may contribute to them. We hypothesized that mutations in the gene coding for the main renal sodium-phosphate cotransporter (NPT2a) may be present in patients with these disorders.

METHODS

We studied 20 patients with urolithiasis or bone demineralization and persistent idiopathic hypophosphatemia associated with a decrease in maximal renal phosphate reabsorption. The coding region of the gene for NPT2a was sequenced in all patients. The functional consequences of the mutations identified were analyzed by expressing the mutated RNA in Xenopus laevis oocytes.

RESULTS

Two patients, one with recurrent urolithiasis and one with bone demineralization, were heterozygous for two distinct mutations. One mutation resulted in the substitution of phenylalanine for alanine at position 48, and the other in a substitution of methionine for valine at position 147. Phosphate-induced current and sodium-dependent phosphate uptake were impaired in oocytes expressing the mutant NPT2a. Coinjection of oocytes with wild-type and mutant RNA indicated that the mutant protein had altered function.

CONCLUSIONS

Heterozygous mutations in the NPT2a gene may be responsible for hypophosphatemia and urinary phosphate loss in persons with urolithiasis or bone demineralization.

Functional characterization of a calcium-sensing receptor mutation in severe autosomal dominant hypocalcemia with a Bartter-like syndrome

The extracellular Ca(2+)-sensing receptor (CaSR) plays an essential role in extracellular Ca(2+) homeostasis by regulating the rate of parathyroid hormone (PTH) secretion and the rate of calcium reabsorption by the kidney. Activation of the renal CaSR is thought to inhibit paracellular divalent cation reabsorption in the cortical ascending limb (cTAL) both directly and indirectly via a decrease in NaCl transport. However, in patients with autosomal dominant hypocalcemia (ADH), caused by CaSR gain-of-function mutations, a defect in tubular NaCl reabsorption with renal loss of NaCl has not been described so far. This article describes a patient with ADH due to a gain-of-function mutation in the CaSR, L125P, associated with a Bartter-like syndrome that is characterized by a decrease in distal tubular fractional chloride reabsorption rate and negative NaCl balance with secondary hyperaldosteronism and hypokalemia. The kinetics of activation of the L125P mutant receptor expressed in HEK-293 cells, assessed by measuring CaSR-stimulated changes in intracellular Ca(2+) and ERK activity, showed a dramatic reduction in the EC(50) for extracellular Ca(2+) compared with the wild-type and a loss-of-function mutant CaSR (I40F). This study describes the first case of ADH associated with a Bartter-like syndrome. It is herein proposed that the L125P mutation of the CaSR, which represents the most potent gain-of-function mutation reported so far, may reduce NaCl reabsorption in the cTAL sufficiently to result in renal loss of NaCl with secondary hyperaldosteronism and hypokalemia.

Effect of reactive oxygen species on NH4+ permeation in Xenopus laevis oocytes

To investigate the effects of reactive oxygen species (ROS) on NH4+ permeation in Xenopus laevis oocytes, we used intracellular double-barreled microelectrodes to monitor the changes in membrane potential (V(m)) and intracellular pH (pH(i)) induced by a 20 mM NH4Cl-containing solution. Under control conditions, NH4Cl exposure induced a large membrane depolarization (to V(m) = 4.0 +/- 1.5 mV; n = 21) and intracellular acidification [reaching a change in pH(i) (DeltapH(i)) of 0.59 +/- 0.06 pH units in 12 min]; the initial rate of cell acidification (dpH(i)/dt) was 0.06 +/- 0.01 pH units/min. Incubation of the oocytes in the presence of H2O2 or beta-amyloid protein had no marked effect on the NH4Cl-induced DeltapH(i). By contrast, in the presence of photoactivated rose bengal (RB), tert-butyl-hydroxyperoxide (t-BHP), or xanthine/xanthine oxidase (X/XO), the same experimental maneuver induced significantly greater DeltapH(i) and dpH(i)/dt. These increases in DeltapH(i) and dpH(i)/dt were prevented by the ROS scavengers histidine and desferrioxamine, suggesting involvement of the reactive species (1)DeltagO2 and.OH. Using the voltage-clamp technique to identify the mechanism underlying the ROS-measured effects, we found that RB induced a large increase in the oocyte membrane conductance (G(m)). This RB-induced G(m) increase was prevented by 1 mM diphenylamine-2-carboxylate (DPC) and by a low Na+ concentration in the bath. We conclude that RB, t-BHP, and X/XO enhance NH4+ influx into the oocyte via activation of a DPC-sensitive nonselective cation conductance pathway.

Effect of locally applied drugs on the pH of luminal fluid in the endolymphatic sac of guinea pig

The aim of the present work was to assess the effect of various drugs applied locally on the pH of the luminal fluid (pH(lum)) in guinea pig endolymphatic sac. pH(lum) and transepithelial potential, when measured in vivo by means of double-barrelled pH-sensitive microelectrodes, were 7.06 +/- 0.08 and +6.1 +/- 0.34 mV (mean +/- SE; n = 84), respectively, which is consistent with a net acid secretion in the luminal fluid of the endolymphatic sac. Bafilomycin and acetazolamide increased and decreased, respectively, pH(lum). Amiloride, ethylisopropylamiloride, ouabain, and Schering 28080 had no effect on pH(lum). Results obtained with inhibitors of anionic transport systems were inconclusive; e.g., DIDS reduced pH(lum), whereas neither SITS nor triflocin had any effect. We conclude that bafilomycin-sensitive H(+)-ATPase activity accounts for the transepithelial acid gradient measured in the endolymphatic sac and that intracellular and membrane-bound carbonic anhydrase probably participates in regulating endolymphatic sac pH(lum). The relationship between acid pH, endolymph volume, and Ménière's disease remains to be further investigated.

Ammonium transport by the colonic H(+)-K(+)-ATPase expressed in Xenopus oocytes

Functional expression of the rat colonic H(+)-K(+)-ATPase was obtained by coexpressing its catalytic alpha-subunit and the beta(1)-subunit of the Na(+)-K(+)-ATPase in Xenopus laevis oocytes. We observed that, in oocytes expressing the rat colonic H(+)-K(+)-ATPase but not in control oocytes (expressing beta(1) alone), NH(4)Cl induced a decrease in (86)Rb uptake and the initial rate of intracellular acidification induced by extracellular NH(4)Cl was enhanced, consistent with NH(+)(4) influx via the colonic H(+)-K(+)-ATPase. In the absence of extracellular K(+), only oocytes expressing the colonic H(+)-K(+)-ATPase were able to acidify an extracellular medium supplemented with NH(4)Cl. In the absence of extracellular K(+) and in the presence of extracellular NH(+)(4), intracellular Na(+) activity in oocytes expressing the colonic H(+)-K(+)-ATPase was lower than that in control oocytes. A kinetic analysis of (86)Rb uptake suggests that NH(+)(4) acts as a competitive inhibitor of the pump. Taken together, these results are consistent with NH(+)(4) competition for K(+) on the external site of the colonic H(+)-K(+)-ATPase and with NH(+)(4) transport mediated by this pump.

Evidence for apical K conductance and Na-K-2Cl cotransport in the endolymphatic sac of guinea pig

The transepithelial potential in the endolymphatic sac (ESP) was recorded up to 60 min after apical injection of ouabain, bumetanide, quinine, barium, tetraethylammonium, and 4-aminopyridine. After control injection, ESP decreased by 74% and completely recovered at 30 min. After ouabain, barium, or quinine injection, the ESP time course was similar to that in the control group. After bumetanide, tetraethylammonium, or 4-aminopyridine injection, complete recovery was only observed at 60 min. These results suggest that apical K+ conductance and Na-K-2Cl cotransporter could be involved in the genesis of ESP.

Extracellular ATP raises cytosolic calcium and activates basolateral chloride conductance in Necturus proximal tubule

1. Extracellular nucleotides modulate ionic transport mechanisms in various epithelia. In the present study, we investigated the effects of extracellular ATP on the intracellular free Ca+2 concentration ([Ca2+]i) and electrophysiological properties of Necturus maculosus proximal convoluted tubule (PCT). 2. ATP raised [Ca2+]i in microdissected fura-2-loaded PCTs (half-maximal effect, approximately mumol 1(-1) ATP). The initial ATP-induced changes in [Ca2+]i were not blunted by the removal of external Ca2+ nor by the presence of Ca2+ channel blockers, but were abolished by thapsigargin and suramin. The sequence for the potency of various agonists on [Ca2+]i was 2-methylthioATP (2MeSATP) = ADP = ATP >> UTP, 2',3',-O-(4-benzoilbenzoil) ATP (BzATP), alpha, beta-methylene ATP (AMPCPP), adenosine. 3. In vivo electrophysiological measurements showed that 100 mumol 1(-1) peritubular ATP added to a Ringer solution reduced the basolateral cell membrane potential (Vm) and increased the cell membrane input conductance. In a low Cl- solution, this ATP-induced depolarization was enhanced. These effects were inhibited by 1 mmol l-1 SITS, consistent with the activation of a basolateral Cl- conductance. 4. The ATP-induced change in Vm was reproduced by ADP but not by UTP or adenosine, and was prevented by suramin. 5. The ATP-induced membrane depolarization was not influenced by thapsigargin, BAPTA AM, or staurosporine and was not reproduced by manoeuvres increasing [Ca2+]i or intracellular cAMP content. 6. We conclude that, in Necturus PCT, a P2y receptor mobilizes Ca2+ mainly from intracellular pools and increases a basolateral Cl- conductance, GCl. The activation of GCl occurs by a mechanism which is not related either to an increase in [Ca2+]i or cAMP content, or to PKC activation.

Does the colonic H,K-ATPase also act as an Na,K-ATPase?

We previously have demonstrated that the colonic P-ATPase alpha subunit cDNA encodes an H,K-ATPase when expressed in Xenopus laevis oocytes. Besides its high level of amino acid homology (75%) with the Na,K-ATPase, the colonic H,K-ATPase also shares a common pharmacological profile with Na,K-ATPase, because both are ouabain-sensitive and Sch 28080-insensitive. These features raise the possibility that an unrecognized property of the colonic H, K-ATPase would be Na+ translocation. To test this hypothesis, ion-selective microelectrodes were used to measure the intracellular Na+ activity of X. laevis oocytes expressing various combinations of P-ATPase subunits. The results show that expression in oocytes of the colonic H,K-ATPase affects intracellular Na+ homeostasis in a way similar to the expression of the Bufo marinus Na,K-ATPase; intracellular Na+ activity is lower in oocytes expressing the colonic H,K-ATPase or the B. marinus Na,K-ATPase than in oocytes expressing the gastric H,K-ATPase or a beta subunit alone. In oocytes expressing the colonic H,K-ATPase, the decrease in intracellular Na+ activity persists when diffusive Na+ influx is enhanced by functional expression of the amiloride-sensitive epithelial Na+ channel, suggesting that the decrease is related to increased active Na+ efflux. The Na+ decrease depends on the presence of K+ in the external medium and is inhibited by 2 mM ouabain, a concentration that inhibits the colonic H,K-ATPase. These data are consistent with the hypothesis that the colonic H,K-ATPase may transport Na+, acting as an (Na,H),K-ATPase. Despite its molecular and functional characterization, the physiological role of the colonic (Na,H),K-ATPase in colonic and renal ion homeostasis remains to be elucidated.

Symmetric pH dependence of buffering power in giant fused cells from frog kidney proximal tubule

This study measures the intrinsic buffering power (beta(i)) of giant fused cells from the proximal kidney tubule of the frog (Rana ridibunda) as a function of intracellular pH (pHi). We monitored pHi and transmembrane potential difference during acid or alkaline cell loading, achieved by removal of NH4Cl-containing solutions or CO2-HCO3(-)-equilibrated solutions, respectively, in the absence of extracellular Na+. Data were well fit by the equation for a single, monoprotic buffer with a maximum beta(i) at a pHi of 7.39 +/- 0.06 and a total buffer concentration of 30.7 +/- 1.6 mM (means +/- SD). From pHi measurements obtained during CO2-HCO3- exposure, we also calculated the buffering power afforded by the CO2-HCO3- pair, and we show its increasing contribution to total buffering power at increasing PCO2 and pHi. To our knowledge, this is the first report of a cell type in which intrinsic cell buffers can be adequately approximated as a single monoprotic buffer with a negative logarithm of apparent dissociation constant in the normal physiological range and essentially symmetric dependence on pHi in both acid and alkaline ranges.

The rat distal colon P-ATPase alpha subunit encodes a ouabain-sensitive H+, K+-ATPase

The functional properties and the pharmacological profile of the recently cloned cDNA colonic P-ATPase alpha subunit (Crowson, M.S., and Shull, G.E. (1992) J. Biol. Chem. 267, 13740-13748) were investigated by using the Xenopus oocyte expression system. Xenopus oocytes were injected with alpha subunit cRNAs from Bufo marinus bladder or rat distal colon and/or with beta subunit cRNA from B. marinus bladder. Two days after injection, K+ uptake was measured by using 86 Rb+ as a K+ surrogate, and pH measurements were performed by means of ion-selective microelectrodes. Co-injection of alpha and beta subunit cRNAs led to a large increase in 86Rb+ uptake, an intracellular alkalinization, and an extracellular medium acidification, as compared to alpha or beta injection alone. These results indicate that the colonic P-ATPase alpha subunit, like the bladder alpha subunit, acts as a functional H+,K+-ATPase, and that co-expression of alpha and beta subunits is required for the function. External K+ activation of the 86Rb+ uptake had a K1/2 of approximately 440 microM for the bladder isoform (consistent with the previously reported value (Jaisser, F., Horisberger, J.D., Geering, K., and Rossier, B.C. (1993) J. Cell. Biol. 123, 1421-1431) and a K1/2 of approximately 730 microM for the colonic isoform. Sch28080 was ineffective to reduce 86Rb+ uptake whereas ouabain inhibited the activity expressed from rat colon alpha subunit with a Ki of 970 microM when measured at the Vmax of the enzyme. We conclude that, when expressed in Xenopus oocytes, the rat colon P-ATPase alpha subunit encodes a ouabain-sensitive H+,K+-ATPase.

Further investigation of ionic diffusive properties and of NH4+ pathways in Xenopus laevis oocyte cell membrane

To study the ionic diffusive properties and the NH4+ pathways in the Xenopus laevis oocyte cell membrane, we recorded the effects of various inhibitors on membrane potential (Vm) and membrane resistance (Rm); intracellular acidification was taken as an index of NH4+ influx from the bath to the cytoplasm. The following results were obtained: in the control state, barium and quinine (Q) ions depolarized Vm and raised Rm, consistent with inhibition of K+ conductance(s). Diphenylamine-2-carboxylic acid (DPC), 3',5'-dichlorodiphenylamine-2-carboxylic acid (DCDPC) and gadolinium ions hyperpolarized Vm and increased Rm, suggesting the inhibition of nonselective cationic conductance(s). In the presence of 20 mmol/l NH4Cl, Vm depolarized, Rm fell, and intracellular pH (pHi) decreased, consistent with an NH4+ influx. In the presence of DPC, the same manoeuvre induced a biphasic Vm change (i.e. a spike depolarization followed by a membrane hyperpolarization) and a fall of Rm; in most oocytes, intracellular acidification persisted and was reversible upon adding ouabain (Oua). These results indicate that a DPC-sensitive conductance is not the unique NH4+ pathway and that Na, K-ATPase may also mediate NH4+ influx. However, Oua did not prevent the Rm decrease, suggesting that ouabain-insensitive rheogenic pathway(s) are activated. Thus, we investigated the Vm change induced by NH4Cl addition in the presence of DPC: the spike depolarization followed by secondary hyperpolarization became a plateau depolarization when Q was added, suggesting involvement of Q-sensitive pathway(s) in the above described biphasic Vm change. In the presence of DPC, Q and Oua, intracellular acidification upon adding NH4Cl persisted consistent with further NH4+ influx through quinine-, DPC- and Oua-insensitive pathway(s).

Differential regulation of membrane potential and conductance via intra- and extracellular pH in fused proximal tubular cells of frog kidney

Intracellular pH (pHi), membrane potential (Vm) and membrane conductance (Gm) in fused proximal tubular cells of the frog kidney, were determined at three extracellular pH (pHo) values, 7.5, 8.5 and 6.5. Imposed changes of pHo by +/- 1 pH unit induced parallel but smaller shifts of pHi. The alkaline milieu hyperpolarized the cells and increased Gm, whereas the acid milieu depolarized and lowered Gm. We subsequently introduced a weak acid and its conjugate base (acetic acid/acetate), or a weak base and its conjugate acid (NH3/NH4+), at pHo 7.5, 8.5 and 6.5 to shift pHi without altering pHo, or to shift pHi against imposed changes of pHo. From these experiments, we observed that under some circumstances Vm varied with pHo but without Gm or pHi changes, whereas under other circumstances changes of Gm occurred during alterations of pHi while pHo and Vm remained unaltered. At pHi approximately 6.5 associated with Vm approximately -10 mV, Gm dramatically increased to quasi-infinite values. This increase was not an artifact since Gm returned to its control value following recovery to the control solution or in the presence of hyperosmotic solution. In conclusion, we demonstrate a differential regulation whereby Vm and Gm are controlled by pHo and pHi: pHo modulates mainly Vm and pHi modulates chiefly Gm. Furthermore, at pHi approximately 6.5 and Vm approximately -10 mV, our data reveal a large Gm that tends towards infinite values in a reversible fashion.

Triflocin, a novel inhibitor for the Na-HCO3 symport in the proximal tubule

1. Triflocin, applied at millimolar concentration hyperpolarizes the basolateral membrane of Necturus proximal convoluted tubular cells, in vivo. 2. Barium, 2.5 x 10(-3) M, ouabain, 10(-3) M, or amiloride 10(-4) M, fail to prevent this hyperpolarization. 3. Triflocin has no effect on the intracellular chloride activity. 4. In physiological acid base conditions, Triflocin increases intracellular pH. 5. Upon an acute isohydric hypercapnia, Triflocin depolarizes the basolateral membrane potential. 6. It is concluded that, Triflocin inhibits the basolateral electrogenic Na-(HCO3)n > 1 cotransport in proximal tubules.

Change of apparent stoichiometry of proximal-tubule Na(+)-HCO3- cotransport upon experimental reversal of its orientation

Electrogenic cotransport of Na+ with HCO3- has been reported in numerous tissues. It has always been shown with a net transfer of negative charge, but in some situations achieves net outward transport of both species with a stoichiometry of at least three HCO3- ions per Na+ ion (3:1), and in other situations achieves net inward transport of both species and has a stoichiometry of at most two HCO3- ions per Na+ ion (2:1). This suggests either that there may be more than one protein responsible for Na(+)-HCO3- cotransport in different tissues or that if there is a single protein, its stoichiometry may differ depending on the orientation of net transport. The present study, using conventional or double-barreled ion-selective microelectrodes to follow basolateral membrane potential and intracellular pH or Na+ activity in Necturus proximal convoluted tubule in vivo, shows that the orientation of the basolateral Na(+)-HCO3- cotransporter can be reversed upon switching from a perfusate simulating normal acid-base conditions to one that imposes peritubular isohydric hypercapnia. Moreover, accompanying the reversal of orientation is a change of apparent stoichiometry from 3:1 to 2:1. Given that the observed change of orientation and accompanying change of apparent stoichiometry occur within seconds and in the same preparation, these results suggest that a single transport protein is responsible for both types of behavior.

Luminal pH in the amphibian distal tubule: effects of carbonic anhydrase and carbonic anhydrase inhibitors

To better delineate acid-base transport properties in the distal tubule (DT) of Necturus in vivo, we 1) studied the effects of peritubular (pt) isohydric increase of PCO2 and [HCO3-]pt on luminal pH (pHlu), and 2) measured the steady-state pHlu under various experimental conditions. The experiments were carried out on initial (DTi) or distal (DTd) loops of the DT in control state and then during intravenous infusion of carbonic anhydrase (CA) or CA inhibitors (CAI). In control state, isohydric increase of PCO2 and [HCO3-]pt results in transient acidification of the DTi lumen, whereas in DTd lumen the same maneuver yields sustained (plateau) acidification. Under systemic infusion of CAI, isohydric increase of PCO2 and [HCO3-]pt lowers pHlu (sustained fall of pHlu) in DTi and DTd, whereas under CA infusion both segments exhibit only transient acidification. During intravenous infusion with benzolamide DTi steady-state pHlu falls, suggesting that this maneuver inhibits a functional luminal CA, in contrast to the DTd, whose pHlu remains unaltered. Intravenous infusion of CA significantly increases steady-state DTd pHlu; by contrast, steady-state pHlu in DTi does not change. These data are consistent with the presence of functional luminal CA in the DTi, whereas the DTd segment lacks the luminal enzyme.