Role of the cytoskeleton in the regulation of Cl- channels in human embryonic skeletal muscle cells

Hypotonic activation of volume-sensitive outwardly rectifying anion channels (VSOACs) requires coordinated remodeling of subcortical and perinuclear actin filaments

Cell volume regulation requires activation of volume-sensitive outwardly rectifying anion channels (VSOACs). The actin cytoskeleton may participate in the activation of VSOACs but the roles of the two major actin pools remain undefined. We hypothesized that structural reorganization of both subcortical and perinuclear actin filaments (F-actin) contributes to the hypotonic activation of VSOACs. Hypotonic stress of pulmonary artery smooth muscle cells (PASMCs) was associated with reorganization of both peripheral and perinuclear F-actin, and with activation of VSOACs. Preincubation with cytochalasin D caused prominent dissociation of perinuclear, but not of subcortical F-actin. Cytochalasin D failed to induce isotonic activation and delayed the hypotonic activation of VSOACs. F-actin stabilization by phalloidin delayed both the hypotonic stress-induced dissociation of membrane-associated actin filaments and the activation kinetics of VSOACs. PKCepsilon, which was proposed to phosphorylate and inhibit VSOACs, colocalized primarily with F-actin and the net kinase activity remained unchanged during hypotonic cell swelling. In conclusion, normal hypotonic activation of VSOACs requires disruption of peripheral F-actin but intact perinuclear F-actin; interference with this pattern of actin reorganization delays the activation kinetics of VSOACs. The cell swelling-induced peripheral actin dissociation may underlie the observed translocation of PKCepsilon, which leads to a net decrease of PKCepsilon inhibitory activity in submembranous sites. Thus, reorganization of actin and PKCepsilon may establish conditions for mechano- and/or signal transduction-mediated activation of VSOACs.

High-resolution scanning patch-clamp: new insights into cell function

Cell specialization is often governed by the spatial distribution of ion channels and receptors on the cell surface. So far, little is known about functional ion channel localization. This is due to a lack of satisfactory methods for investigating ion channels in an intact cell and simultaneously determining the channels' positions accurately. We have developed a novel high-resolution scanning patch-clamp technique that enables the study of ion channels, not only in small cells, such as sperm, but in submicrometer cellular structures, such as epithelial microvilli, fine neuronal dendrites, and, particularly, T-tubule openings of cardiac myocytes. In cardiac myocytes, as in most excitable cells, action potential propagation depends essentially on the properties of ion channels that are functionally and spatially coupled. We found that the L-type calcium and chloride channels are distributed and colocalized in the region of T-tubule openings, but not in other regions of the myocyte. In addition, chloride channels were found in narrowly defined regions of Z-grooves. This finding suggests a new synergism between these types of channels that may be relevant for action potential propagation along the T-tubule system and excitation-contraction coupling.

Functional characterization of recombinant human ClC-4 chloride channels in cultured mammalian cells

Members of the ClC chloride channel family participate in several physiological processes and are linked to human genetic diseases. The physiological role of ClC-4 is unknown and previous detailed characterizations of recombinant human ClC-4 (hClC-4) have provided conflicting results. To re-examine the hClC-4 phenotype, recombinant hClC-4 was expressed in three distinct mammalian cell lines and characterized using patch-clamp techniques. In all cells, the expression of hClC-4 generated strongly outward-rectifying Cl(-) currents with the conductance sequence: SCN(-) >> NO(3)(-) >> Cl(-) > Br(-) approximate I(-) >> aspartate. Continuous activity of hClC-4 was sustained to different degrees by internal nucleotides: ATP approximately ATPgammaS >> AMP-PNP approximate GTP > ADP. Although non-hydrolysable nucleotides are sufficient for channel function, ATP hydrolysis is required for full activity. Changing the extracellular (2 mM or nominal Ca(2+)-free) or intracellular Ca(2+) (25 or 250 nM) concentration did not alter hClC-4 currents. Acidification of external pH (pH(o)) inhibited hClC-4 currents (half-maximal inhibition approximate 6.19), whereas neither external alkalinization to pH 8.4 nor internal acidification to pH 6.0 reduced current levels. Single-channel recordings demonstrated a Cl(-) channel active only at depolarizing potentials with a slope conductance of approximately 3 pS. Acidic pH(o) did not alter single-channel conductance. We conclude that recombinant hClC-4 encodes a small-conductance, nucleotide-dependent, Ca(2+)-independent outward-rectifying chloride channel that is inhibited by external acidification. This detailed characterization will be highly valuable in comparisons of hClC-4 function with native chloride channel activities and for future structure-function correlations.

Ca2+ modulation of volume-regulated anion channels: evidence for colocalization with store-operated channels

Ca2+ regulation of Cl- current induced by cell swelling (I(CI,swell)) in response to hypotonicity was studied in human prostate cancer epithelial cells (LNCaP) by using the patch-clamp technique. Increase of global intracellular Ca2+ ([Ca2+]in) to 1 mM as well as variations of the extracellular Ca2+ ([Ca2+]out) in the 0 to 10 mM range did not affect time course of the development, maximal amplitude, rectification properties, and kinetics of I(CI,swell). However, the presence of 0.1 mM thapsigargin (TG), an inhibitor of endoplasmic reticulum (ER) Ca2+ pump, resulted in a more than 50% inhibition of ICI,swell. The blockade of plasma membrane store-operated channels (SOCs), activated in the presence of TG, by 2 mM Ni2+ prevented TG-conferred I(CI,swell) inhibition by extracellular Ca2+. In the presence of TG and Ca2+, the cells failed to exhibit regulatory volume decrease. We conclude that interaction between volume-regulated anion channels (VRACs) carrying I(CI,swell) and Ca2+ occurs in the microdomains from the inner surface of the membrane that are not accessible to the changes in [Ca2+]in, but can be readily reached by Ca2+ entering the cell via plasma membrane, especially through SOCs. Preferred access of SOC-transported Ca2+ to VRAC suggests colocalization of these channels in the cell membrane.

Ca2+-sensing receptor-mediated regulation of volume-sensitive Cl- channels in human epithelial cells

Since extracellular Ca2+ or Mg2+ has been reported to modulate swelling-activated Cl- currents, we examined the expression of the G protein-coupled Ca2+-sensing receptor (CaR) and its involvement in the regulation of volume-sensitive Cl- channels in a human epithelial cell line (Intestine 407). Reverse transcriptase-polymerase chain reaction and immunoblotting analysis showed that Intestine 407 cells express CaR mRNA and protein. The swelling-activated whole-cell Cl- current was voltage-independently augmented by extracellular Ca2+ or Mg2+. In addition, Ca2+ or Mg2+ voltage-dependently accelerated the inactivation kinetics of the Cl- current. Neomycin, spermine and La3+ augmented volume-sensitive Cl- currents. However, these CaR agonists failed to affect depolarization-induced inactivation. Intracellular application of GTPgammaS, but not GDPbeta]S, increased the amplitude of the swelling-induced Cl- current without affecting the basal current. The upregulating effect of Ca2+ on the Cl- current amplitude was abolished by either GTPgammaS or GDPbetaS. In contrast, GTPgammaS and GDPbetaS failed to affect the inactivation kinetics of the Cl- current and the accelerating effect of Ca2+ thereon. The Cl- current amplitude was enlarged by stimulation with forskolin, dibutyryl cAMP and IBMX. During the cAMP stimulation, extracellular Ca2+ failed to increase the Cl- current but did accelerate depolarization-induced inactivation. It is concluded that stimulation of the CaR induces upregulation of volume-sensitive Cl- channels via a G protein-mediated increase in intracellular cAMP in the human epithelial cell. However, the accelerating effect of extracellular divalent cations on the inactivation kinetics of the Cl- current is induced by a mechanism independent of the CaR and cAMP.

Volume-regulated chloride conductance in the LNCaP human prostate cancer cell line

Patch-clamp recordings were used to study ion currents induced by cell swelling caused by hypotonicity in human prostate cancer epithelial cells, LNCaP. The reversal potential of the swelling-evoked current suggested that Cl(-) was the primary charge carrier (termed I(Cl,swell)). The selectivity sequence of the underlying volume-regulated anion channels (VRACs) for different anions was Br(-) approximately I(-) > Cl(-) > F(-) > methanesulfonate >> glutamate, with relative permeability numbers of 1.26, 1.20, 1.0, 0.77, 0.49, and 0.036, respectively. The current-voltage patterns of the whole cell currents as well as single-channel currents showed moderate outward rectification. Unitary VRAC conductance was determined at 9.6 +/- 1.8 pS. Conventional Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 microM) and DIDS (100 microM) inhibited whole cell I(Cl,swell) in a voltage-dependent manner, with the block decreasing from 39.6 +/- 9.7% and 71.0 +/- 11. 0% at +50 mV to 26.2 +/- 7.2% and 14.5 +/- 6.6% at -100 mV, respectively. Verapamil (50 microM), a standard Ca(2+) antagonist and P-glycoprotein function inhibitor, depressed the current by a maximum of 15%. Protein tyrosine kinase inhibitors downregulated I(Cl,swell) (genistein with an IC(50) of 2.6 microM and lavendustin A by 60 +/- 14% at 1 microM). The protein tyrosine phosphatase inhibitor sodium orthovanadate (500 microM) stimulated I(Cl,swell) by 54 +/- 11%. We conclude that VRACs in human prostate cancer epithelial cells are modulated via protein tyrosine phosphorylation.

Role of the F-actin cytoskeleton in the RVD and RVI processes in Ehrlich ascites tumor cells

The role of the F-actin cytoskeleton in cell volume regulation was studied in Ehrlich ascites tumor cells, using a quantitative rhodamine-phalloidin assay, confocal laser scanning microscopy, and electronic cell sizing. A hypotonic challenge (160 mOsm) was associated with a decrease in cellular F-actin content at 1 and 3 min and a hypertonic challenge (600 mOsm) with an increase in cellular F-actin content at 1, 3, and 5 min, respectively, compared to isotonic (310 mOsm) control cells. Confocal visualization of F-actin in fixed, intact Ehrlich cells demonstrated that osmotic challenges mainly affect the F-actin in the cortical region of the cells, with no visible changes in F-actin in other cell regions. The possible role of the F-actin cytoskeleton in RVD was studied using 0. 5 microM cytochalasin B (CB), cytochalasin D (CD), or chaetoglobosin C (ChtC), a cytochalasin analog with little or no affinity for F-actin. Recovery of cell volume after hypotonic swelling was slower in cells pretreated for 3 min with 0.5 microM CB, but not in CD- and ChtC-treated cells, compared to osmotically swollen control cells. Moreover, the maximal cell volume after swelling was decreased in CB-treated, but not in CD- or Chtc-treated cells. Following a hypertonic challenge imposed using the RVD/RVI protocol, recovery from cell shrinkage was slower in CB-treated, but not in CD- or Chtc-treated cells, whereas the minimal cell volume after shrinkage was unaltered by either of these treatments. It is concluded that osmotic cell swelling and shrinkage elicit a decrease and an increase in the F-actin content in Ehrlich cells, respectively. The RVD and RVI processes are inhibited by 0.5 microM CB, but not by 0.5 microM CD, which is more specific for actin.

Modulation of glioma cell migration and invasion using Cl(-) and K(+) ion channel blockers

Human malignant gliomas are highly invasive tumors. Mechanisms that allow glioma cells to disseminate, migrating through the narrow extracellular brain spaces are poorly understood. We recently demonstrated expression of large voltage-dependent chloride (Cl(-)) currents, selectively expressed by human glioma cells in vitro and in situ (Ullrich et al., 1998). Currents are sensitive to several Cl(-) channel blockers, including chlorotoxin (Ctx), (Ullrich and Sontheimer; 1996; Ullrich et al; 1996), tetraethylammonium chloride (TEA), and tamoxifen (Ransom and Sontheimer, 1998). Using Transwell migration assays, we show that blockade of glioma Cl(-) channels specifically inhibits tumor cell migration in a dose-dependent manner. Ctx (5 microM), tamoxifen (10 microM), and TEA (1 mM) also prevented invasion of human glioma cells into fetal rat brain aggregates, used as an in vitro model to assess tumor invasiveness. Anion replacement studies suggest that permeation of chloride ions through glioma chloride channel is obligatory for cell migration. Osmotically induced cell swelling and subsequent regulatory volume decrease (RVD) in cultured glioma cells were reversibly prevented by 1 mM TEA, 10 microM tamoxifen, and irreversibly blocked by 5 microM Ctx added to the hypotonic media. Cl(-) fluxes associated with adaptive shape changes elicited by cell swelling and RVD in glioma cells were inhibited by 5 microM Ctx, 10 microM tamoxifen, and 1 mM TEA, as determined using the Cl(-)-sensitive fluorescent dye 6-methoxy-N-ethylquinolinium iodide. Collectively, these data suggest that chloride channels in glioma cells may enable tumor invasiveness, presumably by facilitating cell shape and cell volume changes that are more conducive to migration and invasion.

Lysophosphatidic acid stimulates neurotransmitter-like conductance changes that precede GABA and L-glutamate in early, presumptive cortical neuroblasts

During neurogenesis in the embryonic cerebral cortex, the classical neurotransmitters GABA and L-glutamate stimulate ionic conductance changes in ventricular zone (VZ) neuroblasts. Lysophosphatidic acid (LPA) is a bioactive phospholipid producing myriad effects on cells including alterations in membrane conductances (for review, see Moolenaar et al., 1995). Developmental expression patterns of its first cloned receptor gene, lpA1/vzg-1 (Hecht et al., 1996; Fukushima et al., 1998) in the VZ suggested that functional LPA receptors were synthesized at these early times, and thus, LPA could be an earlier stimulus to VZ cells than the neurotransmitters GABA and L-glutamate. To address this possibility, primary cultures of electrically coupled, presumptive cortical neuroblast clusters were identified by age, morphology, electrophysiological profile, BrdU incorporation, and nestin immunostaining. Single cells from cortical neuroblast cell lines were also examined. Whole-cell variation of the patch-clamp technique was used to record from nestin-immunoreactive cells after stimulation by local administration of ligands. After initial plating at embryonic day 11 (E11), cells responded only to LPA but not to GABA or L-glutamate. Continued growth in culture for up to 12 hr produced more LPA-responsive cells, but also a growing population of GABA- or L-glutamate-responsive cells. Cultures from E12 embryos showed LPA as well as GABA and L-glutamate responses, with LPA-responsive cells still representing a majority. Overall, >50% of cells responded to LPA with depolarization mediated by either chloride or nonselective cation conductances. These data implicate LPA as the earliest reported extracellular stimulus of ionic conductance changes for cortical neuroblasts and provide evidence for LPA as a novel, physiological component in CNS development.

The cytoskeleton and cell signaling: component localization and mechanical coupling

The three-dimensional intracellular network formed by the filamentous polymers comprising the cytoskeletal affects the way cells sense their extracellular environment and respond to stimuli. Because the cytoskeleton is viscoelastic, it provides a continuous mechanical coupling throughout the cell that changes as the cytoskeleton remodels. Such mechanical effects, based on network formation, can influence ion channel activity at the plasma membrane of cells and may conduct mechanical stresses from the cell membrane to internal organelles. As a result, both rapid responses such as changes in intracellular Ca2+ and slower responses such as gene transcription or the onset of apoptosis can be elicited or modulated by mechanical perturbations. In addition to mechanical features, the cytoskeleton also provides a large negatively charged surface on which many signaling molecules including protein and lipid kinases, phospholipases, and GTPases localize in response to activation of specific transmembrane receptors. The resulting spatial localization and concomitant change in enzymatic activity can alter the magnitude and limit the range of intracellular signaling events.

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.

Downregulation of volume-activated Cl- currents during muscle differentiation

We have used the whole cell configuration of the patch-clamp technique to investigate volume-activated Cl- currents in BC3H1 and C2C12 cells, two mouse muscle cell lines that can be switched from a proliferating to a differentiated musclelike state. Reducing the extracellular osmolality by 40% evoked large Cl- currents in proliferating BC3H1 and C2C12 cells. These currents were outwardly rectifying and had an anion permeability sequence as follows: I- > Br- > Cl- >> gluconate. They were inhibited by >50% by flufenamic acid (500 microM), niflumic acid (500 microM), and 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 microM) but were relatively insensitive to tamoxifen (100 microM). A reduction in the serum concentration in the culture medium induced growth arrest in both cell lines, and the cells started to differentiate into spindle-shaped nonfusing muscle cells (BC3H1) or myotubes (C2C12). This differentiation was accompanied by a drastic decrease in the magnitude of the volume-activated Cl- currents. The close correlation between volume-activated Cl- currents and cell proliferation suggests that these currents may be involved in cell proliferation.

Hypo-osmotic cell swelling activates the p38 MAP kinase signalling cascade

Hypo-osmotic swelling of human Intestine 407 cells leads to a significant increase of intracellular MAPKAP-kinase 2 activity and Hsp27 phosphorylation. Pre-treatment of the cells with the p38 MAP kinase inhibitor SB-203580 blocks this activation, indicating that the hypotonicity-induced activation of MAPKAP kinase 2 is, similarly to that described for hyperosmotic treatment, the result of an activated p38 MAP kinase cascade. The activation of MAPKAP kinase 2 proceeds with kinetics similar to that of one of the first physiological responses of hypo-osmotic treatment, the opening of compensatory Cl- channels. However, inhibition of the p38 MAP kinase cascade does not block the osmo-sensitive anion efflux and, vice versa, activation of p38 MAP kinase by cytokines and anisomycin does not increase the efflux. These results indicate that the p38 MAP kinase cascade is not directly involved in Cl- channel activation but instead may play a role in subsequent cellular repair processes.

Whole-cell chloride currents in rat astrocytes accompany changes in cell morphology

Astrocytes can change shape dramatically in response to increased physiological and pathological demands, yet the functional consequences of morphological change are unknown. We report the expression of Cl- currents after manipulations that alter astrocyte morphology. Whole-cell Cl- currents were elicited after (1) rounding up cells by brief exposure to trypsin; (2) converting cells from a flat polygonal to a process-bearing (stellate) morphology by exposure to serum-free Ringer's solution; and (3) swelling cells by exposure to hypo-osmotic solution. Zero-current potentials approximated the Nernst for Cl-, and rectification usually followed that predicted by the constant-field equation. We observed heterogeneity in the activation and inactivation kinetics, as well as in the relative degree of outward versus inward rectification. Cl- conductances were inhibited by 4, 4-diisothiocyanostilbene-2,2'-disulfonic acid (200 microM) and by Zn2+ (1 mM). Whole-cell Cl- currents were not expressed in cells without structural change. We investigated whether changes in cytoskeletal actin accompanying changes in astrocytic morphology play a role in the induction of shape-dependent Cl- currents. Cytochalasins, which disrupt actin polymers by enhancing actin-ATP hydrolysis, elicited whole-cell Cl- conductances in flat, polygonal astrocytes. In stellate cells, elevated intracellular Ca2+ (2 microM), which can depolymerize actin, enhanced Cl- currents, and high intracellular ATP (5 mM), required for repolymerization, reduced Cl- currents. Modulation of Cl- current by Ca2+ and ATP was blocked by concurrent whole-cell dialysis with phalloidin and DNase, respectively. Phalloidin stabilizes actin polymers and DNase inhibits actin polymerization. Dialysis with phalloidin also prevented hypo-osmotically activated Cl- currents. These results demonstrate how the expression of astrocyte Cl- currents can be dependent on cell morphology, the structure of actin, Ca2+ homeostasis, and metabolism.

Activation of the osmo-sensitive chloride conductance involves P21rho and is accompanied by a transient reorganization of the F-actin cytoskeleton

Hypo-osmotic stimulation of human Intestine 407 cells rapidly activated compensatory CL- and K+ conductances that limited excessive cell swelling and, finally, restored the original cell volume. Osmotic cell swelling was accompanied by a rapid and transient reorganization of the F-actin cytoskeleton, affecting both stress fibers as well as apical ruffles. In addition, an increase in total cellular F-actin was observed. Pretreatment of the cells with recombinant Clostridium botulinum C3 exoenzyme, but not with mutant enzyme (C3-E173Q) devoid of ADP-ribosyltransferase activity, greatly reduced the activation of the osmo-sensitive anion efflux, suggesting a role for the ras-related GTPase p21rho. In contrast, introducing dominant negative N17-p21rac into the cells did not affect the volume-sensitive efflux. Cell swelling-induced reorganization of F-actin coincided with a transient, C3 exoenzyme-sensitive tyrosine phosphorylation of p125 focal adhesion kinase (p125FAK) as well as with an increase in phosphatidylinositol-3-kinase (PtdIns-3-kinase) activity. Pretreatment of the cells with wortmannin, a specific inhibitor of PtdIns-3-kinase, largely inhibited the volume-sensitive ion efflux. Taken together, our results indicate the involvement of a p21rho signaling cascade and actin filaments in the activation of volume-sensitive chloride channels.

Modulation of a volume-regulated chloride current by F-actin

We have examined whether F-actin integrity is involved in activation of a volume-regulated Cl- current (VRChlC) in B-lymphocytes. VRChlC activation was initiated in response to establishing a whole cell recording in the presence of a hyposmotic gradient. Parallel confocal microscopy experiments using Rhodamine-Phalloidin (R-P) as a specific marker of F-actin showed that the submembrane actin ring is reversibly disrupted in response to an hyposmotic gradient. Disruptions of cortical F-actin integrity by 50 microM cytochalasin B (CB) does not trigger activation of VRChlC under isosmotic conditions or potentiate the rate of activation when the osmolarity of the extracellular solution was decreased by 75%. However, incubation with CB increased the rate of VRChlC activation in response to a 90% hyposmotic gradient. Phalloidin, a stabilizer of F-actin, decreases the rate of VRChlC activation in response to a 90% gradient, but has no effect in response to a 75% gradient. These observations suggest that disassembly of cortical F-actin is not critical for VRChlC activation in B-lymphocytes. The integrity of cortical F-actin, however, can exert a modulatory effect on the rate of VRChlC activation in the presence of a hyposmotic gradient.