Anoctamin-1/TMEM16A is the major apical iodide channel of the thyrocyte.

Iodide is captured by thyrocytes through the Na(+)/I(-) symporter (NIS) before being released into the follicular lumen, where it is oxidized and incorporated into thyroglobulin for the production of thyroid hormones. Several reports point to pendrin as a candidate protein for iodide export from thyroid cells into the follicular lumen. Here, we show that a recently discovered Ca(2+)-activated anion channel, TMEM16A or anoctamin-1 (ANO1), also exports iodide from rat thyroid cell lines and from HEK 293T cells expressing human NIS and ANO1. The Ano1 mRNA is expressed in PCCl3 and FRTL-5 rat thyroid cell lines, and this expression is stimulated by thyrotropin (TSH) in rat in vivo, leading to the accumulation of the ANO1 protein at the apical membrane of thyroid follicles. Moreover, ANO1 properties, i.e., activation by intracellular calcium (i.e., by ionomycin or by ATP), low but positive affinity for pertechnetate, and nonrequirement for chloride, better fit with the iodide release characteristics of PCCl3 and FRTL-5 rat thyroid cell lines than the dissimilar properties of pendrin. Most importantly, iodide release by PCCl3 and FRTL-5 cells is efficiently blocked by T16Ainh-A01, an ANO1-specific inhibitor, and upon ANO1 knockdown by RNA interference. Finally, we show that the T16Ainh-A01 inhibitor efficiently blocks ATP-induced iodide efflux from in vitro-cultured human thyrocytes. In conclusion, our data strongly suggest that ANO1 is responsible for most of the iodide efflux across the apical membrane of thyroid cells.

Ionic selectivity of volume-sensitive currents in human epithelial cells.

The ion selectivity of swelling-activated Cl- currents has been investigated in three different human epithelial cell lines, two derived from the airway epithelium (9HTEo- and CFNPE9o-) and one from a colon carcinoma (T84). The relative permeability of volume-sensitive currents with respect to Cl- is: I- (1.19) greater than NO3- (1.07) approximately Br-(1.05) greater than Cl-(1.0) greater than F-(0.5) approximately HCO3-(0.48) greater than isethionate(0.28) greater than aspartate (0.14) approximately gluconate(0.13) approximately SO4(2-)(0.12). This type of ion selectivity is similar to that described for depolarization-activated outwardly rectifying Cl- channels found in epithelial cells.

A volume-sensitive chloride conductance revealed in cultured human keratinocytes by 36Cl- efflux and whole-cell patch clamp recording.

The Cl- transport mechanism responsible for the stimulation of 36Cl- efflux after exposure to hypotonic medium (210 mosmol/kg) was investigated in human keratinocytes. The involvement of the anion exchanger and of the Cl-/cation cotransporters was ruled out by the finding that replacement of extracellular Cl- by the poorly permeant anion gluconate, and the addition of bumetanide and furosemide, inhibitors of the Na+/K+/Cl- and K+/Cl- cotransporters, respectively, failed to significantly reduce the activation of Cl- efflux by hypotonic medium. 'Whole cell' configuration of the patch clamp technique directly revealed the presence of a macroscopic Cl- current, which was evoked by incubation with hypotonic medium and was reversed by elevation of the extracellular osmolality. Volume-sensitive current showed outward rectification of the current-voltage relationship and time-dependent inactivation at depolarizing voltages. This current was Cl- selective, because the zero-current reversal potential approached the Cl- equilibrium potential, when extracellular Cl- was replaced by gluconate. 0.1 mM 1,9-dideoxyforskolin significantly reduced either 36Cl- efflux and the Cl- current, suggesting that the Cl- efflux and the macroscopic current activated after exposure to hypotonic medium are mediated by the same pathway. Electronic cell sizing showed that in keratinocytes hypotonic swelling was not followed by a significant regulatory volume decrease response.

Characterization of a murine gene homologous to the bovine CaCC chloride channel.

The bovine CaCC protein is a putative Ca2+-dependent Cl- channel of airway epithelial cells. Therefore, CaCC proteins could contribute to transepithelial Cl- transport and accordingly modify the phenotype of cystic fibrosis (CF) patients. We have identified a murine EST containing a full-length cDNA coding for a 902-amino-acid protein highly homologous to bovine CaCC. The murine gene (mCaCC) maps to chromosome 3 at the H2-H3 band and is expressed, as indicated by Northern blot analysis, in mouse skin and kidney but not in brain, heart, lung or testis. RT-PCR indicates a low expression in tracheal epithelial cells. Heterologous expression of mCaCC in Xenopus oocytes elicits membrane currents that are anion-selective and inhibited by DIDS and by niflumic acid, a blocker of the endogenous chloride current in oocytes. The identification of genes belonging to the CaCC family will help to evaluate their role as ion channels or channel regulators and their actual contribution to epithelial chloride transport.

Characterization of the human gene coding for the swelling-dependent chloride channel ICln at position 11q13.5-14.1 (CLNS1A) and further characterization of the chromosome 6 (CLNS1B) localization.

Expression cloning revealed a chloride channel (ICln) that we found to be fundamental for the regulatory volume decrease in a variety of cells. The chromosomal localization of the human ICln-gene showed two loci, one at chromosome 11 in position q13.5-q14.1, termed CLNS1A, and a second one at chromosome 6 at position p12.1-q13, termed CLNS1B. In this study, we offer a detailed characterization of the CLNS1A gene and provide the exact position (6p12) and sequence data of CLNS1B, an intronless gene 91.3% homologous to the coding region of CLNS1A.

A class of non-selective cation channels in human fibroblasts.

Non-selective cation channels were detected in membrane patches of cultured human fibroblasts. The channels had a unitary conductance which ranged from 14 to 25 pS in symmetrical 130 mM NaCl and were permeable to both sodium and potassium ions. Open channel probability was dependent either on the membrane potential and the Ca2+ concentration on the intracellular side of the membrane. High Ca2+ concentrations in the millimolar range were needed to keep the channel active.

Molecular cloning and functional characterization of a GABA/betaine transporter from human kidney.

The human homologue of the canine GABA/betaine transporter (BGT-1) was isolated from a kidney inner medulla cDNA library. The coding sequence predicts a 614 amino acids protein with the typical features of neurotransmitter transporter family. The gene maps to chromosome 12p13 and, in addition to kidney, is also expressed in brain, liver, heart, skeletal muscle, and placenta. Functional studies reveal a Km = 20 microM for GABA transport and a coupling to Na+ and Cl- with a stoichiometry 3 Na+:2 Cl-:1 GABA. At 500 microM the GABA transport was inhibited by various compounds with the following potency order: quinidine > verapamil > phloretin > betaine.

Green fluorescent protein-based halide indicators with improved chloride and iodide affinities.

The green fluorescent protein YFP-H148Q is sensitive to halides by a mechanism involving halide binding and a shift in pK(a). However, a limitation of YFP-H148Q is its low halide sensitivity, with K(d)>100 mM for Cl(-). Indicators with improved sensitivities are needed for cell transport studies, particularly in drug discovery by high-throughput screening, and for measurement of Cl(-) concentration in subcellular organelles. YFP-H148Q libraries were generated in which pairs of residues in the vicinity of the halide binding site were randomly mutated. An automated procedure was developed to screen bacterial colonies for improved halide sensitivity. Analysis of 1536 clones revealed improved anion sensitivities with K(d) down to 2 mM for I(-) (I152L), 40 mM for Cl(-) (V163S), and 10 mM for NO(3)(-) (I152L). The anion-sensitive mechanism of these indicators was established and their utility in cells was demonstrated using transfected cells expressing the cystic fibrosis transmembrane conductance regulator chloride channel.

Extracellular 2-chloroadenosine and ATP stimulate volume-sensitive Cl- current and calcium mobilization in human tracheal 9HTEo- cells.

The perforated-patch whole-cell technique was used to record membrane currents in epithelial cells (9HTEo-) obtained from the human tracheal epithelium. Extracellular application of 2-chloroadenosine and ATP (0.01-100 microM) caused activation of Cl- currents similar to those regulated by cell volume in airway and intestinal cells. This response was inhibited by increasing extracellular osmolality, by omission of extracellular Ca2+, or by the addition of the A2 adenosine receptor antagonist 3,7-dimethyl-1-propargylxanthine (DMPX). Fluorimetric measurements with fura-2 reveal that 2-chloroadenosine and ATP elicited both a Ca2+ influx through the plasma membrane and a release from intracellular stores.

Regulation of transepithelial ion transport by two different purinoceptors in the apical membrane of canine kidney (MDCK) cells.

1. The effect of extracellular nucleotides on the transepithelial ion transport of Madin Darby canine kidney cells (MDCK) was investigated. Cells were grown up to confluency on permeable supports and the short circuit current (ISC) was measured with an Ussing chamber-like mini-perfusion system. 2. Apical ATP stimulated a biphasic ISC increase consisting of a first rapid and transient peak followed by a broader one. 3. The first peak evoked by ATP was reversibly blocked by basilen blue (BB) in a concentration-dependent fashion, with an EC50 of 7.5 microM. 4. The P2 gamma receptor agonist, 2-methylthioATP (2-MeSATP) caused a single transient ISC increase that was completely blocked by pretreatment with BB. On the contrary, the P2x agonist, alpha, beta-methylene ATP (alpha, beta-meATP) was almost completely ineffective on ISC. UTP essentially induced a monophasic response the time-course of which resembled that of the second peak stimulated by ATP. The agonist potency order was 2-MeSATP > or = ATP >> UTP, alpha, beta-meATP for the first peak and UTP > or = ATP > 2-MeSATP > alpha, beta-meATP for the second peak. 5. Monolayer incubation with the membrane permeable calcium chelator [bis-o-aminophenoxy)-ethane-N,N,N',N',-tetraacetic acid, tetra(acetoximethyl)-ester] (BAPTA/AM) inhibited the ATP-evoked first peak. 6. The non-hydrolyzable ATP analogue, adenosine-5'-O-(3-thio)-trisphosphate (ATP-gamma-S) elicited a biphasic response similar to that of ATP. The P1 receptor agonist, 2-chloroadenosine and CGS-21680, were almost unable to induce an ISC increase.2+ increase. The second induces prostaglandin synthesis probably through a P2U receptor activation.

Insensitivity of volume-sensitive chloride currents to chromones in human airway epithelial cells.

Chromones (sodium cromoglycate and sodium nedocromil) block cell swelling-activated Cl- channels in NIH-3T3 fibroblasts and endothelial cells. This has led to hypothesize that cell volume regulation might be involved in asthma pathogenesis. Using whole-cell patch-clamp experiments, we studied the effect of chromones on volume-sensitive Cl- currents in transformed human tracheal epithelial cells (9HTEo-) and in primary cultures of human bronchial epithelial cells (BE). Cl- currents activated by hypotonic shock were poorly blocked by extracellular nedocromil or cromoglycate. The block was voltage-dependent since it was observed only at positive membrane potentials. At the concentration of 5 mM, the current inhibition by both chromones at +80 mV was about 40% for 9HTEo- and only 20% for BE. Intracellular application of chromones elicited a voltage-independent inhibition in 9HTEo- cells. Under this condition, volume-sensitive Cl- currents were reduced at all membrane potentials (60 and 45% inhibition by 2 mM nedocromil and cromoglycate respectively). In contrast intracellular chromones were ineffective in BE cells. The relative refractoriness to chromones, in contrast with the high sensitivity shown by other Cl- channels, suggests that the epithelial volume-sensitive Cl- channel is not involved in asthma.

An improved method to obtain highly differentiated monolayers of human bronchial epithelial cells.

Electrophysiological studies of human bronchial epithelial cells in vitro are limited by the scarcity of biological material available for primary culture. To overcome this problem, we set up a protocol in which the cell number is first enlarged in LHC9/RPMI 1640 serum-free medium for up to six passages, each passage giving a four- to eightfold amplification. The cells are then plated at high density on permeable supports. Cell differentiation, monitored by measuring transepithelial potential difference (PD) and electrical resistance (R), is induced with a medium containing serum and a cocktail of different supplements and hormones. Maximal values of PD and R, obtained after 4-7 d of culture on permeable supports, are around -50 mV and 3000-4000 omega/cm2, respectively. Ussing chamber experiments show that basal short-circuit current (Isc) is partially inhibited by the epithelial Na+ channel blocker amiloride. Stimulation with a cAMP-elevating agent induces a Isc increase that is inhibited by the cystic fibrosis transmembrane conductance regulator (CFTR) blocker glibenclamide. Our culture protocol provides a large number of differentiated bronchial epithelial cell monolayers starting from a low amount of material. This characteristic is useful for in vitro studies of ion transport in airway epithelium.

Binding site of activators of the cystic fibrosis transmembrane conductance regulator in the nucleotide binding domains.

The use of substances that could activate the defective chloride channels of the mutant cystic fibrosis transmembrane conductance regulator (CFTR) has been suggested as possible therapy for cystic fibrosis. Using epithelia formed by cells stably transfected with wildtype or mutant (G551D, G1349D) CFTR, we estimated the apparent dissociation constant, K(D), of a series of CFTR activators by measuring the increase in the apical membrane current. Modification of apparent K(D) of CFTR activators by mutations of the nucleotide-binding domains (NBDs) suggests that the binding site might be in these regions. The human NBD structure was predicted by homology with murine NBD1. An NBD1-NBD2 complex was constructed by overlying monomers to a bacterial ABC transporter NBD dimer in the "head-to-tail" conformation. Binding sites for CFTR activators were predicted by molecular docking. Comparison of theoretical binding free energy estimated in the model to free energy estimated from the apparent dissociation constants, K(D), resulted in a remarkably good correlation coefficient for one of the putative binding sites, located in the interface between NBD1 and NBD2.

Biophysical characteristics of swelling-activated Cl- channels in human tracheal 9HTEo-cells.

The question of whether a single molecule can account for every observed swelling-activated Cl- current deserves to be addressed and biophysical description seems to be an adequate criterion to classify these channels. We studied the biophysical properties of swelling-activated Cl- currents in 9HTEo-cells using whole-cell and outside-out patch clamp recordings. Hypotonic shock activated outwardly rectifying currents that inactivated at potentials higher than 20 mV. The decay phase of the current was well fitted by two exponential functions and both time constants were voltage-dependent. Two voltage-dependent time constants were also necessary to describe reactivation. The midpoint of current inactivation was 54 mV. The voltage dependence of kinetics did not significantly change by modifying the extracellular NaCl concentration while the inactivation midpoint slightly shifted. In conclusion, our results indicate that the voltage-dependent properties of the swelling-activated Cl- currents in 9HTEo- cells are largely independent from the extracellular ionic strength and the extracellular Cl- concentration. Excised patches from cells exposed to hypotonic shock showed single channel currents that inactivated at positive membrane potentials and displayed chord conductance of approximately 60 pS at 100 mV and of approximately 20 pS at -80 mV. The permeability sequence for the single channel was I- > Br- > Cl- > gluconate and currents were blocked by Reactive blue 2. These properties indicate that intermediate conductance outwardly rectifying channels are responsible for the macroscopic swelling-activated current.

A large conductance Cl- channel revealed by patch-recordings in human fibroblasts.

A Cl- channel with large single-unit conductance and characteristic voltage-dependent inactivation was studied on cultured human fibroblasts. The channel was activated only after excision and lasting depolarization of the membrane patch. In inside-out configuration and in symmetrical 135 mM NaCl, the conductance was 300 pS. The channel was usually open at the membrane potentials between -20 to +20 mV, while more negative or positive voltages closed the channel. The time course of this apparent inactivation process was dependent on increasing potential. Recovery from inactivation was made possible by returning the membrane potential to 0 mV. The channel was selective to Cl- over Na+ with a PCl/PNa of 6. The order of permeability among anions was: I greater than Br = Cl greater than isethionate greater than F greater than glutamate. The channel was blocked by internal application of a derivative of the diphenylamine-2-carboxilate (Blocker 144) but not by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid.

Volume regulatory taurine release in human tracheal 9HTEo- and multidrug resistant 9HTEo-/Dx cells.

The intracellular taurine release evoked by hypotonic shock is accomplished by volume-activated Cl- channels whose activity has been related to the expression of the multidrug resistance protein (MDR-1). We studied taurine transport in 9HTEo- cells and in the derived cell line 9HTEo-/Dx expressing MDR-1. [3H]taurine release from preloaded cells increased upon reduction of extracellular osmolality. This process was not inhibited by preincubation with phorbol 12-myristate 13-acetate but was reduced by inhibitors of volume-sensitive Cl- channels such as 1,9-dideoxiforskolin, La3+, and arachidonate. Verapamil, a substrate of MDR-1, increased the osmotically evoked taurine efflux. Replacement of extracellular Cl- with I- or gluconate or of extracellular Na+ with Li+ significantly reduced the taurine efflux, whereas substitution of N-methyl-D-glucamine for Na+ increased it. Application of ATP and 2-chloroadenosine stimulated the efflux in isotonic medium. No differences were seen between 9HTEo- and 9HTEo-/Dx cells with respect to hypotonically induced taurine efflux and the response to phorbol ester, channel blockers, ion replacement, and purinergic agents. Our results reveal novel properties of the osmotically induced taurine release and demonstrate its independence from MDR-1 gene expression.

Low Ca2+-sensitive maxi-K+ channels in human cultured fibroblasts.

The patch clamp technique was used to reveal single channel activity in the membrane of human cultured fibroblasts. The most frequently detected ion channel type was a Ca2+-dependent K+ channel with a conductance of 287 +/- 38 pS in symmetrical 130 mM KCl. The channel showed a peculiar low Ca2+-sensitivity compared to that of similar channels in other preparations. In fact micromolar values of internal Ca2+ were not effective in the channel activation, except at high depolarizing membrane potentials. The activity was highly increased only when the channel was exposed to relatively high internal Ca2+ concentrations (0.2-2.0 mM).

Why is the cystic fibrosis gene so frequent?

The high incidence of cystic fibrosis (CF) in most European populations (and populations of European descent) can be explained by different hypotheses that can be tested using the available data concerning this disorder. Among the five hypotheses discussed (genetic heterogeneity, high rate of mutation, meiotic drive, drift and heterozygote advantage), only the last is supported by experimental data. The following conclusions can be drawn from the evidence that we have reviewed: (1) CF is a single gene disorder (genetically homogeneous). (2) Haplotypes associated with the CF gene suggest that only a few mutations (the same gene located in 7q13 is always affected) are responsible for the disorder. (3) CF with pancreatic insufficiency is mainly associated with a single haplotype, whereas CF with pancreatic sufficiency is more frequently associated with different haplotypes. (4) A selective advantage consisting of higher resistance to Cl- -secreting diarrhoeas might have favored, in the past, survival of infants heterozygous for the CF gene.

A forskolin and verapamil sensitive K+ current in human tracheal cells.

A voltage-dependent K+ current has been revealed in whole-cell recordings carried out on immortalized cells obtained from the human tracheal epithelium. At positive membrane potentials the current shows a time dependent inactivation which is accelerated by increasing the depolarizing step. Forskolin, a direct activator of adenylyl cyclase, and verapamil, a Ca2+ channel blocker, induce the K+ current to inactivate more rapidly. Control experiments show that the action of these two compounds is not mediated by cyclic AMP and Ca2+. The application of 1,9-dideoxyforskolin, an analogue which does not stimulate adenylate cyclase, inhibits the current in the same way as forskolin; on the contrary, the dibutyryl analogue of cyclic AMP is ineffective. Furthermore, eliminating extracellular Ca2+ does not affect K+ current kinetics. Tetraethylammonium is an effective blocker of this current with an IC50 of 0.3 mM.