Characterization and regulation of a chloride channel from bovine tracheal epithelium

Exogenous transforming growth factor-β1 prevents the inflow of fluoride to ameleoblasts through regulation of voltage-gated chloride channels 5 and 7

Dental fluorosis is a global issue. Although there are multiple causes of dental fluorosis, the precise mechanism remains controversial. Previous studies have demonstrated that extracellular fluoride may promote an accumulation of fluoride ions in ameloblasts, which may induce oxidative and endoplasmic reticulum stresses, leading to dental fluorosis. However, the exact process by which fluoride ions enter cells has not been determined. In the present study, intracellular fluoride concentration was determined using a newly developed specific fluorescent probe called probe 1. Under high extracellular fluoride concentrations, the fluorescence intensity of the ameloblasts increased, however, exogenous transforming growth factor-β1 (TGF-β1) was able to inhibit the increase. Furthermore, changes in the expression of the voltage-gated chloride channels 5 and 7 (ClC5 and ClC-7), which are responsible for the transport of fluoride were investigated. The results indicated that fluoride reduced the expression of endogenous TGF-β1 and increased the expression of ClC-5 and ClC-7. Additionally, exogenous TGF-β1 reduced the expression of ClC-5 and ClC-7. The results of the present study indicate that exogenous TGF-β1 may prevent accumulation of fluoride in ameloblasts through the regulation of ClC-5 and ClC-7 under high extracellular fluoride concentrations.

Functional and pharmacological characterization of volume-regulated anion channels in human normal and cystic fibrosis bronchial and nasal epithelial cells

Volume-regulated anion channels (VRACs) are widely present in various cell types and have important functions ranging from regulatory volume decrease to control of cell proliferation and apoptosis. Here we aimed to compare the biophysical features and pharmacological profiles of VRAC currents in healthy and cystic fibrosis (CF) respiratory epithelial cells in order to characterize these currents both functionally and pharmacologically. Whole-cell electrophysiology was used to characterize the VRAC current in normal (16HBE14o-; HBE) and CF cell lines (CFBE14o-; CFBE), as well as in native human nasal epithelial cells. Application of hypotonic solution produced current responses of similar sizes in both HBE and CFBE cells. Biophysical properties of VRACs, such as instantaneous activation and deactivation upon voltage step, some inactivation at potentials positive to 40 mV and outwardly-rectifying I-V curves, were indistinguishable in both cell types. Extensive pharmacological analysis of the currents revealed a similar pharmacological profile in response to three blockers--NPPB, DCPIB and DIDS. Native primary human nasal epithelial cells from both healthy and CF volunteers also showed typical VRAC responses of comparable sizes. VRACs in these cells were more sensitive to external solution hypotonicity compared to HBE and CFBE cells. In all cell types studied robust VRAC currents could be induced at constant cell volume by G-protein activation with GTPγS infusion. This study provides the first extensive comparative functional and pharmacological analysis of VRAC currents in normal and CF airway epithelial cells and shows that VRACs are unimpaired molecularly or functionally in CF.

Inhibition of matrix metalloproteinase MMP-2 activates chloride current in human airway epithelial cells

Matrix metalloproteinases (MMPs) are involved in the remodeling and degradation of the extracellular matrix. Recently, it has been found that MMPs also contribute to processes not directly related to tissue remodeling, such as platelet aggregation or degranulation of airway gland cells. Since mucus secretion is closely related to ion channel function, we investigated whether MMPs could also be involved in the regulation of ion channels. We used human airway submucosal cell line Calu-3 to study the effects of MMPs on whole-cell current and transepithelial short-circuit current (Isc). Phenanthroline, a specific inhibitor of MMPs, increased whole-cell current with the half-maximally effective dose of 5.2 µM, and reversibly activated Isc in transepithelial measurements. Current stimulated by phenanthroline displayed linear current-voltage relationships and had inhibitor pharmacology and ion selectivity consistent with cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel activity. Zymography and Western blot showed significant expression of MMP-2 in Calu-3 cells. Moreover, anti-MMP-2 antibodies (1 µg/mL) increased whole-cell current and Isc, whereas human recombinant MMP-2 (10 ng/mL) reduced it. We also studied the expression of MMPs and the effects of phenanthroline on whole-cell current in A549 cells, which are derived from airway surface epithelium and do not express CFTR Cl- channels. While these cells also showed significant expression of MMP-2, inhibition of this enzyme with phenanthroline exerted no significant effect on whole-cell current. It is concluded that MMP-2 is involved in the regulation of CFTR Cl- channels in human airways.Key words: matrix metalloproteinases, Cl- current, Calu-3 cells, zymography, phenanthroline.

Activation of bovine tracheal chloride channels by amino group-specific reagents

1. The predominant Cl- channel in bovine tracheal epithelial cells has a conductance of approximately 71 pS and accounts for more than 80 % of the total chloride conductance. We examined the effects of protein-modifying reagents on channel function and found that amino groups are critically involved in gating. 2. Patch clamp studies showed that lysine-specific reagents, such as dimethyl adipimidate (DMA), significantly increased the channel open probability, but not its conductance. This suggests that modified residues are involved in the gating mechanism, but are distant from the channel permeation pathway. 3. Kinetic analysis of channel activity showed that histograms of open and closed durations could be well fitted by double exponential distributions, suggesting that the channel has at least two open and two closed states. DMA did not change the number of open or closed states, but increased channel mean open time. 4. Since membrane impermeant reagents were effective only from the extracellular side, we conclude that lysine residues in the extracellular domain of the channel are critically involved in gating. These residues may present an important target for site-directed mutagenesis and pharmacological activation of Cl- channels in epithelial cells.

Structural requirements and ionic mechanism of the Mytilus inhibitory peptides (MIPs) on Helix central neurons

1. The structural and ionic requirements for potent FVamide-induced inhibition were investigated using Helix aspersa central neurons under current or voltage clamp. 2. For potent FVamide inhibition the full hexapeptide sequence, GSPYFVamide, was required. The rank order of decreasing potency was GSPYFVamide >> SPYFVamide> PYFVamide > YFVamide. 3. GSPYFVamide inhibition involved an increase in conductance to both potassium and chloride ions as demonstrated by ion substitution experiments and addition of 4-aminopyridine, tetraethylammonium, 4,4-di-isothiocyanatostilbene-2,2'-disulfonic acid, 4,4-dinitrostilbene-2,2'-disulfonic acid disodium salt and furosemide to the solution bathing the preparation.

A large conductance, Ca2+-activated K+ channel in a human lung epithelial cell line (A549)

A large conductance, Ca2+-activated K+ channel in a human lung epithelial cell line (A549) was identified using the single channel patch clamp technique. Channel conductance was 242 +/- 33 pS (n = 67) in symmetrical KCl (140 mM). The channel was activated by membrane depolarization and increased cytosolic Ca2+. High selectivity was observed for K+ over Rb+(0.49) > Cs+(0.14) > Na+(0.09). Open probability was significantly decreased by Ba2+ (5 mM) and quinidine (5 mM) to either surface, but TEA (5 mM) was only effective when added to the external surface. All effects were reversible. Increasing cytosolic Ca2+ concentration from 10(-7) to 10(-6) M caused an increase in open probability from near zero to fully activated. ATP decreased open probability at approximately 2 mM, but the effect was variable. The channel was almost always observed together with a smaller conductance channel, although they could both be seen individually. We conclude that A549 cells contain large conductance Ca2+-activated K+ channels which could explain a major fraction of the K+ conductance in human alveolar epithelial membranes.

Heterogeneity of chloride channels in the apical membrane of isolated mitochondria-rich cells from toad skin

The isolated epithelium of toad skin was disintegrated into single cells by treatment with collagenase and trypsine. Chloride channels of cell-attached and excised inside-out apical membrane-patches of mitochondria-rich cells were studied by the patch-clamp technique. The major population of Cl- channels constituted small 7-pS linear channels in symmetrical solutions (125 mM Cl-). In cell-attached and inside-out patches the single channel i/V-relationship could be described by electrodiffusion of Cl- with a Goldmann-Hodgkin-Katz permeability of, PCl = 1.2 x 10(-14) - 2.6 x 10(-14) cm3. s-1. The channel exhibited voltage-independent activity and could be activated by cAMP. This channel is a likely candidate for mediating the well known cAMP-induced transepithelial Cl- conductance of the amphibian skin epithelium. Another population of Cl- channels exhibited large, highly variable conductances (upper limit conductances, 150-550 pS) and could be activated by membrane depolarization. A group of intermediate-sized Cl(-)-channels included: (a) channels (mean conductance, 30 pS) with linear or slightly outwardly rectifying i/V-relationships and activity occurring in distinct "bursts," (b) channels (conductance-range, 10-27 pS) with marked depolarization-induced activity, and (c) channels with unresolvable kinetics. The variance of current fluctuations of such "noisy" patches exhibited a minimum close to the equilibrium-potential for Cl-. With channels occurring in only 38% of sealed patches and an even lower frequency of voltage-activated channels, the chloride conductance of the apical membrane of mitochondria-rich cells did not match quantitatively that previously estimated from macroscopic Ussing-chamber experiments. From a qualitative point of view, however, we have succeeded in demonstrating the existence of Cl-channels in the apical membrane with features comparable to macroscopic predictions, i.e., activation of channel gating by cAMP and, in a few patches, also by membrane depolarization.