CLC-3 knockout hints at swelling-activated chloride channel complexity.
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Functional identification of a sarcolemmal chloride channel from bovine tracheal smooth muscle.
The biophysical and pharmacological characteristics of unitary Cl- currents from bovine tracheal smooth muscle cells were studied after reconstitution of microsomal vesicles into planar lipid bilayers. Two types of currents were recorded simultaneously in KCl buffer: the well-defined Ca(2+)-dependent K+ conductance [GK(Ca)] and a much smaller Cl- current, indicating that the Cl- channels under scrutiny originate from the same membrane as the GK(Ca)-type channels, the plasma membrane of airway smooth muscle (ASM) cells. The GK(Ca) activities were eliminated by the use of CsCl buffer. The average unitary Cl- conductance measured in 50 mM trans-250 mM cis CsCl was 77 +/- 6 pS (n = 21), and the reversal potential measured in various CsCl gradients followed the Cl- equilibrium potential as determined from the Nernst equation. In contrast with the previous reports describing the Ca2+ sensitivity of macroscopic ASM Cl- currents, this channel was found to be insensitive to cytoplasmic and extracellular Ca2+ levels. Phosphorylation cocktails, including protein kinases A, G, or C, did not alter the activity of the channel nor did changes in pH. Among a series of Cl- channel inhibitors, 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid [50% effective concentration (EC50) = 30 microM] and 5-nitro-2-(3-phenylpropylamino) benzoic acid (EC50 = 130 microM) were the most potent blockers of the current examined. The exact role of this surface Cl- conductance remains unclear, and its involvement in cellular activity needs further investigation.
4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
Expression and targeting to the plasma membrane of xClC-K, a chloride channel specifically expressed in distinct tubule segments of Xenopus laevis kidney.
ClC-K channels are Cl- channels specifically expressed in vertebrate kidneys. Although their heterologous functional expression is still controversial, indirect evidence points to them as major factors involved in Cl- reabsorption in the nephron. We cloned xClC-K, an amphibian (Xenopus) homologue of mammalian ClC-K. The cDNA encodes a 77 kDa protein presenting 62% similarity with human ClC-Kb. The protein is monoglycosylated and is expressed primarily in the Xenopus kidney. It is localized in the basolateral membranes of proximal convoluted tubules of the nephron and in the apical region of the diluting segments. Heterologous expression of xClC-K in HEK-293 cells showed that the full-length protein is glycosylated and targeted to the cell membrane, but no associated Cl- current could be observed with the patch-clamp recording technique. N-glycosylation of both the native kidney channel and the recombinant protein expressed in HEK-293 conferred on them anomalous behaviour in denaturing PAGE, which is indicative of strong interactions at the extracellular side of the plasma membrane. The expression of ClC-K channels in both mesonephric and metanephric kidneys will permit further comparative physiological studies of Cl- permeabilities at the molecular level.
Amidohydrolases↗
Glucose-inhibition of glucagon secretion involves activation of GABAA-receptor chloride channels.
The endocrine part of the pancreas plays a central role in blood-glucose regulation. It is well established that an elevation of glucose concentration reduces secretion of the hyperglycaemia-associated hormone glucagon from pancreatic alpha 2 cells. The mechanisms involved, however, remain unknown. Electrophysiological studies have demonstrated that alpha 2 cells generate Ca2+-dependent action potentials. The frequency of these action potentials, which increases under conditions that stimulate glucagon release, is not affected by glucose or insulin. The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is present in the endocrine part of the pancreas at concentrations comparable to those encountered in the central nervous system, and co-localizes with insulin in pancreatic beta cells. We now describe a mechanism whereby GABA, co-secreted with insulin from beta cells, may mediate part of the inhibitory action of glucose on glucagon secretion by activating GABAA-receptor Cl- channels in alpha 2 cells. These observations provide a model for feedback regulation of glucagon release, which may be of significance for the understanding of the hypersecretion of glucagon frequently associated with diabetes.
Animals↗
ClC-2 chloride channels contribute to HTC cell volume homeostasis.
Membrane Cl(-) channels play an important role in cell volume homeostasis and regulation of volume-sensitive cell transport and metabolism. Heterologous expression of ClC-2 channel cDNA leads to the appearance of swelling-activated Cl(-) currents, consistent with a role in cell volume regulation. Since channel properties in heterologous models are potentially modified by cellular background, we evaluated whether endogenous ClC-2 proteins are functionally important in cell volume regulation. As shown by whole cell patch clamp techniques in rat HTC hepatoma cells, cell volume increases stimulated inwardly rectifying Cl(-) currents when non-ClC-2 currents were blocked by DIDS (100 microM). A cDNA closely homologous with rat brain ClC-2 was isolated from HTC cells; identical sequence was demonstrated for ClC-2 cDNAs in primary rat hepatocytes and cholangiocytes. ClC-2 mRNA and membrane protein expression was demonstrated by in situ hybridization, immunocytochemistry, and Western blot. Intracellular delivery of antibodies to an essential regulatory domain of ClC-2 decreased ClC-2-dependent currents expressed in HEK-293 cells. In HTC cells, the same antibodies prevented activation of endogenous Cl(-) currents by cell volume increases or exposure to the purinergic receptor agonist ATP and delayed HTC cell volume recovery from swelling. These studies provide further evidence that mammalian ClC-2 channel proteins are functional and suggest that in HTC cells they contribute to physiological changes in membrane Cl(-) permeability and cell volume homeostasis.
4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
Side-chain charge effects and conductance determinants in the pore of ClC-0 chloride channels.
The charge on the side chain of the internal pore residue lysine 519 (K519) of the Torpedo ClC-0 chloride (Cl-) channel affects channel conductance. Experiments that replace wild-type (WT) lysine with neutral or negatively charged residues or that modify the K519C mutant with various methane thiosulfonate (MTS) reagents show that the conductance of the channel decreases when the charge at position 519 is made more negative. This charge effect on the channel conductance diminishes in the presence of a high intracellular Cl- concentration ([Cl-]i). However, the application of high concentrations of nonpermeant ions, such as glutamate or sulfate (SO42-), does not change the conductance, suggesting that the electrostatic effects created by the charge at position 519 are unlikely due to a surface charge mechanism. Another pore residue, glutamate 127 (E127), plays an even more critical role in controlling channel conductance. This negatively charged residue, based on the structures of the homologous bacterial ClC channels, lies 4-5 A from K519. Altering the charge of this residue can influence the apparent Cl- affinity as well as the saturated pore conductance in the conductance-Cl- activity curve. Amino acid residues at the selectivity filter also control the pore conductance but mutating these residues mainly affects the maximal pore conductance. These results suggest at least two different conductance determinants in the pore of ClC-0, consistent with the most recent crystal structure of the bacterial ClC channel solved to 2.5 A, in which multiple Cl--binding sites were identified in the pore. Thus, we suggest that the occupancy of the internal Cl--binding site is directly controlled by the charged residues located at the inner pore mouth. On the other hand, the Cl--binding site at the selectivity filter controls the exit rate of Cl- and therefore determines the maximal channel conductance.
Amino Acid Substitution↗
Exterior site occupancy infers chloride-induced proton gating in a prokaryotic homolog of the ClC chloride channel.
The ClC family of anion channels mediates the efficient, selective permeation of Cl(-) across the biological membranes of living cells under the driving force of an electrochemical gradient. In some eukaryotes, these channels are known to exhibit a unique gating mechanism, which appears to be triggered by the permeant Cl(-) anion. We infer details of this gating mechanism by studying the free energetics of Cl(-) occupancy in the pore of a prokaryotic ClC homolog. These free energetics were gleaned from 30 ns of molecular dynamics simulation on an approximately 133,000-atom system consisting of a hydrated membrane embedded StClC transporter. The binding sites for Cl(-) in the transporter were determined for the cases where the putative gating residue, Glu(148), was protonated and unprotonated. When the glutamate gate is protonated, Cl(-) favorably occupies an exterior site, S(ext), to form a queue of anions in the pore. However, when the glutamate gate is unprotonated, Cl(-) cannot occupy this site nor, consequently, pass through the pore. An additional, previously undetected, site was found in the pore near the outer membrane that exists regardless of the protonation state of Glu(148). Although this suggests that, for the prokaryotic homolog, protonation of Glu(148) may be the first step in transporting Cl(-) at the expense of H(+) transport in the opposite direction, an evolutionary argument might suggest that Cl(-) opens the ClC gate in eukaryotic channels by inducing the conserved glutamate's protonation. During an additional 20 ns free dynamics simulation, the newly discovered outermost site, S(out), and the innermost site, S(int), were seen to allow spontaneous exchange of Cl(-) ions with the bulk electrolyte while under depolarization conditions.
Algorithms↗
Expression of cystic fibrosis transmembrane conductance regulator corrects defective chloride channel regulation in cystic fibrosis airway epithelial cells.
The cystic fibrosis transmembrane conductance regulator (CFTR) was expressed in cultured cystic fibrosis airway epithelial cells and Cl- channel activation assessed in single cells using a fluorescence microscopic assay and the patch-clamp technique. Expression of CFTR, but not of a mutant form of CFTR (delta F508), corrected the Cl- channel defect. Correction of the phenotypic defect demonstrates a causal relationship between mutations in the CFTR gene and defective Cl- transport which is the hallmark of the disease.
Cell Membrane Permeability↗
CFTR and outward rectifying chloride channels are distinct proteins with a regulatory relationship.
In cystic fibrosis (CF), numerous epithelial cell functions are abnormal, including Cl- conductance, sodium absorption, mucin sulphation and enzyme secretion. Although the CF gene product, the cystic fibrosis transmembrane conductance regulator (CFTR), functions as a small linear Cl- channel, it is difficult to attribute such pleiotropic disease manifestations solely to a defect in Cl- conductance. This has led to speculation that CFTR regulates the activity of other proteins. One possible example is the protein kinase A activation of outward rectifying Cl- channels (ORCC), which is defective in membrane patches excised from CF cells. Whether CFTR regulates the activity of an independent anion channel is debatable, because ORCC occur exclusively in excised membrane patches and could be an excision-induced molecular derivative of CFTR. 'Knockout' mice that lack CFTR provide a means to define the relationship between CFTR and ORCC. Here we report that ORCC are present in CFTR(-/-) mouse nasal epithelial cells and thus cannot be a derivative of the CFTR molecule. Also ORCC were regulated by protein kinase A in membrane patches from normal but not CFTR(-/-) cells. These observations are the first, to our knowledge definitive demonstration that CFTR regulates the activity of another protein.
3T3 Cells↗
Propofol produces differences in behavior but not chloride channel function between selected lines of mice.
We report differential central nervous system (CNS) sensitivity to propofol between Long Sleep (LS) and Short Sleep (SS) mice, selectively bred for their differential CNS sensitivity to ethanol. Intravenous propofol requirements for loss of righting reflex, or sleep time, were measured to define the extent of this sensitivity. LS mice slept approximately two times longer than SS mice at equal doses. Awakening plasma and brain levels of the SS line were, respectively, two and three times that of the LS line (P < 0.0001). This suggests that the LS and SS sleep time difference is CNS mediated, and that propofol and ethanol may share common genes that determine anesthetic sensitivities. The ethanol effect may be at least partially mediated by gamma-aminobutyric acid-A (GABAA) receptor function. Propofol had no differential effect on GABAA receptor function, as measured by chloride flux in LS and SS brain microsac preparations. Either the GABAA receptor does not mediate propofol sleep time, or qualitative differences cannot be demonstrated using 36Cl- uptake in brain membranes.
Anesthetics, Intravenous↗
Swelling-activated chloride channels in cardiac physiology and pathophysiology.
Characteristics and functions of the cardiac swelling-activated Cl current (I(Cl,swell)) are considered in physiologic and pathophysiologic settings. I(Cl,swell) is broadly distributed throughout the heart and is stimulated not only by osmotic and hydrostatic increases in cell volume, but also by agents that alter membrane tension and direct mechanical stretch. The current is outwardly rectifying, reverses between the plateau and resting potentials (E(m)), and is time-independent over the physiologic voltage range. Consequently, I(Cl,swell) shortens action potential duration, depolarizes E(m), and acts to decrease cell volume. Because it is activated by stimuli that also activate cation stretch-activated channels, I(Cl,swell) should be considered as a potential effector of mechanoelectrical feedback. I(Cl,swell) is activated in ischemic and non-ischemic dilated cardiomyopathies and perhaps during ischemia and reperfusion. I(Cl,swell) plays a role in arrhythmogenesis, myocardial injury, preconditioning, and apoptosis of myocytes. As a result, I(Cl,swell) potentially is a novel therapeutic target.
Action Potentials↗
Identification of intracellular and extracellular domains mediating signal transduction in the inhibitory glycine receptor chloride channel.
Fast synaptic neurotransmission is mediated by transmitter-activated conformational changes in ligand-gated ion channel receptors, culminating in opening of the integral ion channel pore. Human hereditary hyperekplexia, or startle disease, is caused by mutations in both the intracellular or extracellular loops flanking the pore-lining M2 domain of the glycine receptor alpha1 subunit. These flanking domains are designated the M1-M2 loop and the M2-M3 loop respectively. We show that four startle disease mutations and six additional alanine substitution mutations distributed throughout both loops result in uncoupling of the ligand binding sites from the channel activation gate. We therefore conclude that the M1-M2 and M2-M3 loops act in parallel to activate the channel. Their locations strongly suggest that they act as hinges governing allosteric control of the M2 domain. As the members of the ligand-gated ion channel superfamily share a common structure, this signal transduction model may apply to all members of this superfamily.
Allosteric Regulation↗
Potassium and chloride channels in freshly isolated rat odontoblasts.
It has been suggested that understanding the physiological properties of odontoblasts may be important in understanding the mechanisms underlying both metabolic and transductive processes in dental pulp. Because ion flux(es) may play a critical role in these events, it is of particular interest to understand ionic mechanisms in odontoblast cells. Thus, the aim of this study was to use patch-clamp recording techniques to examine the properties of resident ion channels in freshly dissociated odontoblasts. In recordings made in potassium-rich solutions, cells displayed at least three distinct channel amplitudes, with conductances of 130 +/- 18 pS, 52 +/- 4 pS, and 25 +/- 2 pS, respectively. Channel activity persisted in the presence of potassium salts of impermeant anions, and could be abolished by barium, a non-specific potassium channel blocker. In addition to the potassium conductances, we saw two separate anion channels in the odontoblast membrane. These channels were predominantly chloride-selective, weakly permeable to both acetate and aspartate, and had conductances of 391 +/- 64 pS and 24 +/- 3 pS. While questions remain regarding the functional role of these and other ion channels that presumably reside in the odontoblast membrane, our results demonstrate that it is possible to study ionic mechanisms of the odontoblast at the level of the single cell.
Animals↗
Intracellular calcium ions activate a low-conductance chloride channel in smooth-muscle cells isolated from human mesenteric artery.
Calcium-activated chloride currents were studied by the patch-clamp technique in vascular smooth muscle cells (VSMC) isolated from human mesenteric arteries. Bath application of 20 mM caffeine caused the cell membrane to depolarize by a calcium-activated inward current that peaked to -654 +/- 230 pA (holding potential -50 mV). Cell-attached, at the same time inwardly directed single-channel currents were detected with an amplitude of -0.22 pA. In open-cell-attached patches channel activity was triggered by elevating [Ca2+]i to 10 microM. At -60 mV the mean amplitude of the current was -0.24 pA and the mean open time of the channels was 28 ms. Plotting the amplitude of the current versus the test potential yielded a single-channel conductance of 2.8 +/- 0.5 pS. The currents disappeared when [Cl-] was reduced from 150 mM to 5 mM at the cytosolic side of the inside-out patch at a holding potential of -60 mV (calculated reversal potential -58 mV) suggesting that the calcium-activated current was a chloride current. This suggests that, in human mesenteric VSMC, elevation of [Ca2+]i activates a low-conductance chloride channel, which may mediate the agonist-induced depolarization of the cell membrane.
Action Potentials↗
Severely impaired urine-concentrating ability in mice lacking the CLC-K1 chloride channel.
To analyze the physiological functions of CLC-K1 in vivo, we generated mice lacking CLC-K1 by targeted gene disruption. Homozygous mutant Clcnk1-/- mice produced approximately 5 times more urine than Clcnk1+/- and Clcnk1+/+ mice. After 24-hour water deprivation, Clcnk1-/- mice became severely dehydrated and lethargic. Intraperitoneal injection of the V2 agonist, deamino-Cys(1), D-Arg(8) vasopressin, induced an increase in urine osmolarity in Clcnk1+/- and Clcnk1+/+ mice from approximately 1,000 to approximately 3,000 mosm/kg H(2)O, whereas the increase in Clcnk1-/- mice was only from approximately 600 to approximately 840 mosm/kg H(2)O, indicating nephrogenic diabetes insipidus in Clcnk1-/- mice. These results clearly established that CLC-K1 plays a major role in the urinary-concentrating mechanisms.
Animals↗
Inhibition of prostaglandin E2 synthesis by a blocker of epithelial chloride channels.
Arginine-vasopressin (AVP) elicits a variety of responses in cultured rat mesangial cells, among them stimulation of prostaglandin biosynthesis and activation of Cl- channels. AVP produced an 11-fold increase over basal levels in prostaglandin E2 release from cultured mesangial cells. This response was completely inhibited by 25 microM indomethacin and 82 +/- 5% inhibited by 25 microM 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) which is a potent blocker of epithelial Cl- channels. The IC50 for NPPB inhibition of prostaglandin E2 release was 8 microM. Indomethacin and NPPB at 25 microM also inhibited AVP-stimulated cellular accumulation of prostaglandin E2 by 98% and 79 +/- 7% respectively. The inhibitory effect of NPPB was not due to interference with the cellular response to AVP since at 50 microM it did not block AVP-stimulated release of arachidonate metabolites from cells metabolically labeled with [3H]-arachidonic acid. It is suggested that NPPB inhibition of prostaglandin E2 synthesis is at the cyclooxygenase level on the basis of its structural similarity to the fenamic acid type of cyclooxygenase inhibitors.
Animals↗
CLIC6, a member of the intracellular chloride channel family, interacts with dopamine D(2)-like receptors.
To identify proteins interacting with the C-terminal cytoplasmic tail of the dopamine D(3) receptor (D(3)R), we used the two-hybrid system to screen a rat brain cDNA library. We isolated three partial cDNAs encoding, respectively, for the MUPP1 multi-PDZ protein, for the N-terminal region of radixin, for GIPC and for a 160-amino acid open reading frame sharing high homology with the human CLIC6, also identified as parchorin in rabbit. In the two-hybrid system, CLIC6 was also able to interact with the D(2)R and D(4)R. The interaction between D(3)R and CLIC6 was confirmed by the use of a GST-D(3)R C-terminus fusion protein and COS cell extracts transiently expressing epitope-tagged CLIC6. In adult rat brain, CLIC6 mRNA expression was restricted to the choroid plexus, the striatal proliferative subventricular zone and the cerebellum where it is co-expressed with the D(3)R in the Purkinje cells of the lobules IX and X. CLIC6 mRNA was also detected in the pituitary in the posterior lobe and in cells co-expressing the D(2)R at the border between the intermediate and anterior lobes. In transfected HEK293 cells, D(3)R and CLIC6 co-localized at the plasma-membrane. No effect of CLIC6 transfection was observed on either intracellular chloride concentration or D(3)R/D(2)R-mediated response. In two-hybrid system, CLIC6 also interacted with MUPP1 and radixin but not GIPC, suggesting it could take part in a complex with D(2)-like receptors, not only by direct interaction with their C-termini, but also through interactions with scaffolding proteins.
Amino Acid Sequence↗
Characterization of CFTR expression and chloride channel activity in human endothelia.
The cystic fibrosis transmembrane conductance regulator (CFTR) functions as a low-conductance, cAMP-regulated chloride (Cl-) channel in a variety of cell types, such as exocrine epithelial cells. Our results demonstrate that human primary endothelial cells isolated from umbilical vein (HUVEC) and lung microvasculature (HLMVEC) also express CFTR as determined via RT-PCR and immunohistochemical and immunoprecipitation analyses. Moreover, Cl- efflux and whole cell patch-clamp analyses reveal that HUVEC (n = 6 samples, P < 0.05) and HLMVEC (n = 5 samples, P < 0.05) display cyclic nucleotide-stimulated Cl- transport that is inhibited by the CFTR selective Cl- channel blocker glibenclamide but not by the blocker DIDS, indicative of CFTR Cl- channel activity. Taken together, these findings demonstrate that human endothelial cells derived from multiple organ systems express CFTR and that CFTR functions as a cyclic nucleotide-regulated Cl- channel in human endothelia.
Base Sequence↗