Biophysical properties of a chloride channel in the apical membrane of a secretory epithelial cell

Effect of apical chloride concentration on the measurement of responses to CFTR modulation in airway epithelia cultured from nasal brushings

INTRODUCTION

One method for assessing the in vitro response to CFTR-modulating compounds is by analysis of epithelial monolayers in an Ussing chamber, where the apical and basolateral surfaces are isolated and the potential difference, short-circuit current, and transepithelial resistance can be monitored. The effect of a chloride ion gradient across airway epithelia on transepithelial chloride transport and the magnitude of CFTR modulator efficacy were examined.

METHODS

CFTR-mediated changes in the potential difference and transepithelial currents of primary human nasal epithelial cell cultures were quantified in Ussing chambers with either symmetrical solutions or reduced chloride solutions in the apical chamber. CFTR activity in homozygous F508del CFTR epithelia was rescued by treatment with VX-661, C4/C18, 4-phenylbutyrate (4-PBA) for 24 hr at 37°C or by incubation at 29°C for 48 hr.

RESULTS

Imposing a chloride gradient increased CFTR-mediated and CaCC-mediated ion transport. Treatment of F508del CFTR homozygous cells with CFTR modulating compounds increased CFTR activity, which was significantly more evident in the presence of a chloride gradient. This observation was recapitulated with temperature-mediated F508del CFTR correction.

CONCLUSIONS

Imposing a chloride gradient during Ussing chamber measurements resulted in increased CFTR-mediated ion transport in expanded non-CF and F508del CFTR homozygous epithelia. In F508del CFTR homozygous epithelia, the magnitude of response to CFTR modulating compounds or low temperature was greater when assayed with a chloride gradient compared to symmetrical chloride, resulting in an apparent increase in measured efficacy. Future work may direct which methodologies utilized to quantify CFTR modulator response in vitro are most appropriate for the estimation of in vivo efficacy.

Regulation of human placental chloride channel by arachidonic acid and other cis unsaturated fatty acids

OBJECTIVE

Arachidonic acid has been implicated in the modulation of various transport processes, including conductive chloride transport in brush border membranes in the human placenta. The purpose of this work was to explore the effects of some cis unsaturated fatty acids on the electrophysiologic properties of the maxi chloride channels present in apical membranes from human placenta.

STUDY DESIGN

Apical membrane chloride channels from human term placentas were reconstituted in giant liposomes. These cell-sized liposomes, generated by a cycle of dehydration and rehydration, are suitable for electrophysiologic studies by the patch-clamp method.

RESULTS

Low micromolar concentrations of arachidonic acid reversibly inhibit maxi chloride channels in excised patches. Other cis unsaturated fatty acids, such as oleic and linoleic acids, show similar blockade. The inhibition was dose dependent. The maxi chloride channel can also be inhibited by 4,4 -diisothiocyanatostilbene-2,2 -disulfonic acid, a known chloride channel inhibitor.

CONCLUSIONS

Our results identify the apical membrane maxi chloride channel as a possible electrophysi ologic counterpart of 4,4 -diisothiocyanatostilbene-2, 2 -disulfonic acid and cis unsaturated fatty acid-inhibited conductance previously described in brush border membranes of the human placenta. From a functional point of view the control of these channels by arachidonic acid may be of great importance in placental physiologic characteristics. Regulation of chloride channels could be important in the control of electrolyte and fluid transfer across the placenta. In addition, if these channels contribute to setting the membrane potential their regulation could have consequences for nutrient transport and delivery to the fetus. The electrophysiologic identification of these channels and their regulation might help to unravel their possible role in transplacental transport in normal and pathologic placental tissue.

Cytoskeletal actin gates a Cl- channel in neocortical astrocytes

Increases in astroglial Cl- conductance accompany changes in cell morphology and disassembly of cytoskeletal actin, but Cl- channels underlying these conductance increases have not been described. We characterize an outwardly rectifying Cl- channel in rodent neocortical cultured astrocytes and describe how cell shape and cytoskeletal actin modulate channel gating. In inside-out patch-clamp recordings from cultured astrocytes, outwardly rectifying Cl- channels either were spontaneously active or inducible in quiescent patches by depolarizing voltage steps. Average single-channel conductance was 36 pS between -60 and -80 mV and was 75 pS between 60 and 80 mV in symmetrical (150 mM NaCl) solutions. The permeability ratio (PNa/PCl) was 0.14 at lower ionic strength but increased at higher salt concentrations. Both ATP and 4, 4-diisothiocyanostilbene-2,2'-disulfonic acid produced a flicker block, whereas Zn2+ produced complete inhibition of channel activity. The frequency of observing both spontaneous and inducible Cl- channel activity was markedly higher in stellate than in flat, polygonally shaped astrocytes. In addition, cytoskeletal actin modulated channel open-state probability (PO) and conductance at negative membrane potentials, controlling the degree of outward rectification. Direct application of phalloidin, which stabilizes actin, preserved low PO and promoted lower conductance levels at negative potentials. Lower PO also was induced by direct application of polymerized actin. The actions of phalloidin and actin were reversed by coapplication of gelsolin and cytochalasin D, respectively. These results provide the first report of an outwardly rectifying Cl- channel in neocortical astrocytes and demonstrate how changes in cell shape and cytoskeletal actin may control Cl- conductance in these cells.

The effect of phospholipase A2 on chloride transport by pancreatic secretory granules

Secretory granules from the rat pancreas contain electrolyte transport pathways that may contribute to exocrine fluid production. The Cl- selective transport pathway was measured indirectly in isolated granules by ionophore-induced lysis after suspension in isotonic KCl. This Cl- transport was shown to respond to alterations in the granule membrane lipid environment. Exogenously added phospholipase A2 (PLA2) caused an increase of up to 193% in the Cl- specific transport by the isolated granules. In addition, the products of PLA2 hydrolysis, lysophospholipids and unesterified fatty acids, directly increased the rate of Cl- transport when incubated with granules in vitro. Lysophospholipids (2.0 micrograms/ml) increased the Cl- transport between 280-450% (depending on the lysophospholipid species). Similarly, free fatty acids (10 microM) increased the granule Cl- transport from 25% with capric acid (10:0) to 255% with arachidonic acid (20:4). The relative extent of stimulation by fatty acids was dependent on their carbon chain length and to a lesser extent, the degree of unsaturation. The inhibition (68%) of PLA2 promoted granule lysis by 4-acetamido-4'-isothiocyanatostilbene 2,2'-disulfonic acid (0.5 mM) also suggests that the effect is specific for the granule Cl- channel. Thus, the data show that zymogen granule Cl- transport is influenced by membrane lipids and supports a role for PLA2 in controlling electrolyte transport during stimulus-secretion coupling.

Regulation of Cl- channels in normal and cystic fibrosis airway epithelial cells by extracellular ATP

The rate of Cl- secretion by human airway epithelium is determined, in part, by apical cell membrane Cl- conductance. In cystic fibrosis airway epithelia, defective regulation of Cl- conductance decreases the capability to secrete Cl-. Here we report that extracytosolic ATP in the luminal bath of cultured human airway epithelia increased transepithelial Cl- secretion and apical membrane Cl- permeability. Single-channel studies in excised membrane patches revealed that ATP increased the open probability of outward rectifying Cl- channels. The latter effect occurs through a receptor mechanism that requires no identified soluble second messengers and is insensitive to probes of G protein function. These results demonstrate a mode of regulation of anion channels by binding ATP at the extracellular surface. Regulation of Cl- conductance by external ATP is preserved in cystic fibrosis airway epithelia.

Swelling-induced and depolarization-induced C1-channels in normal and cystic fibrosis epithelial cells

Cl- currents induced by cell swelling were characterized at the whole cell and single-channel levels in primary cultures of normal and cystic fibrosis (CF) epithelial cells and in the T84 cell line. Currents recorded in normal and CF cells were indistinguishable. At 22-24 degrees C with isotonic CsCl in the pipette, initial whole cell outward current density at 100 mV in unswollen cells was 2-4 pA/pF. The current density increased with time during whole cell recording up to 100 pA/pF in isotonic solutions and up to 200 pA/pF in a hypotonic bath, though values typically ranged between 10 and 70 pA/pF. Currents were outwardly rectifying, active at negative voltages, started to inactivate above approximately 40 mV, and were blocked by 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS). Single Cl- channels (approximately 50 pS near 0 mV) with an outwardly rectifying current-voltage relation were recorded in cell-attached and outside-out patches from swollen cells. The channels were mostly open at negative voltages and inactivated at positive voltages with a voltage dependence similar to the whole cell currents. Channel activity decreased rapidly (channel rundown) after seal formation. After swelling-induced channel activity had ceased, outwardly rectifying, depolarization-induced Cl- channels (ORDIC channels) were activated in some patches. The swelling-induced and ORDIC single-channel currents were similar, but some consistent differences were observed. ORDIC channels were often closed at resting voltages (-70 to -50 mV), while swelling-induced channels were always open in this voltage range. In addition, ORDIC channels started to inactivate at more positive voltages (approximately 90 vs. approximately 50 mV), rectified more, and had smaller conductances (approximately 25 pS near 0 mV), shorter mean open durations (approximately 70 vs. approximately 350 ms), and more open-channel noise than swelling-induced channels. The two types of currents might arise from separate channel proteins or from a single channel molecule in different states.

Chloride transport across the membrane of parotid secretory granules

The Cl- transport pathways in secretory granules isolated from the parotid glands of rats were characterized by the technique of ionophore-induced lysis in defined salt solutions. The granules were shown to possess a Cl- conductance that exhibited a distinct anion selectivity with a sequence I- greater than Br- greater than Cl- greater than F- greater than SO4(2-) much greater than gluconate-. This conductance could be reduced approximately 40% by the stilbene 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) from the cytoplasmic side; the half-maximal concentration for inhibition was 50 microM. Furthermore, the apparent Cl- conductance was reduced by outwardly directed granule H+ gradients and stimulated by inwardly directed gradients. An outwardly directed H+ gradient mimics the in vivo environment and may serve in a regulatory capacity, providing for a tonic inhibition of transport until the granule fuses with the luminal membrane. The granules also possessed a Cl(-)-HCO3- exchange based on electroneutrality of Cl- uptake and stimulation of this uptake by HCO3-. This pathway displayed a different anion selectivity, I- greater than Br- greater than F- greater than Cl- much greater than SO4(2-) much greater than gluconate-, and was not inhibited by SITS on the cytoplasmic side. The presence of these electrolyte transport pathways in the granule membrane is consistent with the production of primary fluid by parotid acinar cells after fusion of granules with the luminal plasma membrane.