The volume-activated chloride current in endothelial cells from bovine pulmonary artery is not modulated by phosphorylation

Enhancement of an outwardly rectifying chloride channel in hippocampal pyramidal neurons after cerebral ischemia

Cerebral ischemia induces delayed, selective neuronal death in the CA1 region of the hippocampus. The underlying molecular mechanisms remain unclear, but it is known that apoptosis is involved in this process. Chloride efflux has been implicated in the progression of apoptosis in various cell types. Using both the inside-out and whole-cell configurations of the patch-clamp technique, the present study characterized an outwardly rectifying chloride channel (ORCC) in acutely dissociated pyramid neurons in the hippocampus of adult rats. The channel had a nonlinear current-voltage relationship with a conductance of 42.26±1.2pS in the positive voltage range and 18.23±0.96pS in the negative voltage range, indicating an outward rectification pattern. The channel is Cl(-) selective, and the open probability is voltage-dependent. It can be blocked by the classical Cl(-) channel blockers DIDS, SITS, NPPB and glibenclamide. We examined the different changes in ORCC activity in CA1 and CA3 pyramidal neurons at 6, 24 and 48h after transient forebrain ischemia. In the vulnerable CA1 neurons, ORCC activity was persistently enhanced after ischemic insult, whereas in the invulnerable CA3 neurons, no significant changes occurred. Further analysis of channel kinetics suggested that multiple openings are a major contributor to the increase in channel activity after ischemia. Pharmacological blockade of the ORCC partly attenuated cell death in the hippocampal neurons. We propose that the enhanced activity of ORCC might contribute to selective neuronal damage in the CA1 region after cerebral ischemia, and that ORCC may be a therapeutic target against ischemia-induced cell death.

Identification of key signaling molecules involved in the activation of the swelling-activated chloride current in human glioblastoma cells

The swelling-activated chloride current (I Cl,Vol) is abundantly expressed in glioblastoma (GBM) cells, where it controls cell volume and invasive migration. The transduction pathway mediating I Cl,Vol activation in GBM cells is, however, poorly understood. By means of pharmacological and electrophysiological approaches, on GL-15 human GBM cells we found that I Cl,Vol activation by hypotonic swelling required the activity of a U73122-sensitive phospholipase C (PLC). I Cl,Vol activation could also be induced by the membrane-permeable diacylglycerol (DAG) analog OAG. In contrast, neither calcium (Ca(2+)) chelation by BAPTA-AM nor changes in PKC activity were able to affect I Cl,Vol activation by hypotonic swelling. We further found that R59022, an inhibitor of diacylglycerol kinase (DGK), reverted I Cl,Vol activation, suggesting the involvement of phosphatidic acid. In addition, I Cl,Vol activation required the activity of a EHT1864-sensitive Rac1 small GTPase and the resulting actin polymerization, as I Cl,Vol activation was prevented by cytochalasin B. We finally show that I Cl,Vol can be activated by the promigratory fetal calf serum in a PLC- and DGK-dependent manner. This observation is potentially relevant because blood serum can likely come in contact with glioblastoma cells in vivo as a result of the tumor-related partial breakdown of the blood-brain barrier. Given the relevance of I Cl,Vol in GBM cell volume regulation and invasiveness, the several key signaling molecules found in this study to be involved in the activation of the I Cl,Vol may represent potential therapeutic targets against this lethal cancer.

Tamoxifen inhibits migration of estrogen receptor-negative hepatocellular carcinoma cells by blocking the swelling-activated chloride current

Tamoxifen is a triphenylethylene non-steroidal antiestrogen anticancer agent. It also shows inhibitory effects on metastasis of estrogen receptor (EsR)-independent tumors, but the underlying mechanism is unclear. It was demonstrated in this study that, in EsR-negative and highly metastatic human hepatocellular carcinoma MHCC97H cells, tamoxifen-inhibited cell migration, volume-activated Cl(-) currents (I(Cl,vol)) and regulatory volume decrease (RVD) in a concentration-dependent manner with a similar IC(50). Analysis of the relationships between migration, I(Cl,vol) and RVD showed that cell migration was positively correlated with I(Cl,vol) and RVD. Knockdown of the expression of ClC-3 Cl(-) channel proteins by ClC-3 shRNA or siRNA inhibited I(Cl,vol), and cell migration, and these inhibitory effects could not be increased further by addition of tamoxifen in the medium. The results suggest that knockdown of ClC-3 expression may deplete the effects of tamoxifen; tamoxifen may inhibit cell migration by modulating I(Cl,vol) and cell volume. Moreover, tamoxifen decreased the activity of protein kinase C (PKC) and the effects were reversed by the PKC activator PMA. Activation of PKC by PMA could competitively downregulate the inhibitory effects of tamoxifen on I(Cl,vol). PMA promoted cell migration, and knockdown of ClC-3 expression by ClC-3 siRNA abolished the PMA effect on cell migration. The results suggest that tamoxifen may inhibit I(Cl,vol) by suppressing PKC activation; I(Cl,vol) may be an EsR-independent target for tamoxifen in the anti-metastatic action on cancers, especially on EsR-negative cancers. The finding may have an implication in the clinical use of tamoxifen in the treatments of both EsR-positive and EsR-negative cancers.

Regulatory role of tyrosine phosphorylation in the swelling-activated chloride current in isolated rabbit articular chondrocytes

Articular chondrocytes are exposed in vivo to the continually changing osmotic environment and thus require volume regulatory mechanisms. The present study was designed to investigate (i) the functional role of the swelling-activated Cl(-) current (I(Cl,swell)) in the regulatory volume decrease (RVD) and (ii) the regulatory role of tyrosine phosphorylation in I(Cl,swell), in isolated rabbit articular chondrocytes. Whole-cell membrane currents were recorded from chondrocytes in isosmotic, hyposmotic and hyperosmotic external solutions under conditions where Na(+), K(+) and Ca(2+) currents were minimized. The cell surface area was also measured using microscope images from a separate set of chondrocytes and was used as an index of cell volume. The isolated chondrocytes exhibited a RVD during sustained exposure to hyposmotic solution, which was mostly inhibited by the I(Cl,swell) blocker 4-(2-butyl-6,7-dichloro-2-cyclopentyl-indan-1-on-5-yl)oxobutyric acid (DCPIB) at 20 microM. Exposure to a hyposmotic solution activated I(Cl,swell), which was also largely inhibited by 20 microM DCPIB. I(Cl,swell) in rabbit articular chondrocytes had a relative taurine permeability (P(tau)/P(Cl)) of 0.21. Activation of I(Cl,swell) was significantly reduced by the protein tyrosine kinase (PTK) inhibitor genistein (30 microM) but was only weakly affected by its inactive analogue daidzein (30 microM). Intracellular application of protein tyrosine phosphatase (PTP) inhibitor sodium orthovanadate (250 and 500 microM) resulted in a gradual activation of a Cl(-) current even in isosmotic solutions. This Cl(-) current was almost completely inhibited by 4,4-diisothiocyanatostilbene-2,2-disulfonate (DIDS, 500 microM) and was also largely suppressed by exposure to hyperosmotic solution, thus indicating a close similarity to I(Cl,swell). Pretreatment of chondrocytes with genistein significantly prevented the activation of the Cl(-) current by sodium orthovanadate, suggesting that the basal activity of endogenous PTK is required for the activation of this Cl(-) current. Our results provide evidence to indicate that activation of I(Cl,swell) is involved in RVD in isolated rabbit articular chondrocytes and is facilitated by tyrosine phosphorylation.

Regulation of cardiac volume-sensitive chloride channel by focal adhesion kinase and Src kinase

The volume-sensitive chloride current ( ICl,swell) is found in the mammalian myocardium and is activated by osmotic swelling. The goal of this study was to examine the importance of the tyrosine kinases focal adhesion kinase (FAK) and Src kinase in cardiac ICl,swellregulation. Neonatal rat ventricular myocytes were cultured on collagen membranes and infected with adenovirus expressing β-galactosidase (AdLacZ), FAK, or FAK-related nonkinase. FAK-related nonkinase (FRNK) is an endogenous cardiac protein, which functions as an inhibitor of FAK. Whole cell patch-clamp recordings demonstrated that osmotic swelling was associated with the activation of an outward rectifying current in uninfected and AdLacZ-infected cells. Consistent with the properties of ICl,swell, this current displayed a reversal potential close to the equilibrium potential for Cl−; was inhibited by the Cl−channel blockers 4,4′-dinitrostilbene-2,2′-disulfonic acid, 5-nitro-2-(3-phenylpropylamino)-benzoic acid, and tamoxifen; and was eliminated in hypertonic solution. In addition to activating ICl,swell, hypotonic swelling enhanced the tyrosine phosphorylation of multiple cardiac proteins including those in the range of 68–70 and 120–130 kDa. Pretreatment of the cells with the drug 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolo[3,4-d]pyrimidine, an inhibitor of FAK and Src, diminished swelling-induced phosphorylation of these proteins but paradoxically increased ICl,swell. Furthermore, overexpression of FRNK but not FAK caused a twofold augmentation in ICl,swelland increased the rate of current activation. Thus the tyrosine kinases FAK and Src contribute to the regulation of ICl,swell.

Propofol inhibits volume-sensitive chloride channels in human coronary artery smooth muscle cells

Volume-sensitive chloride channels (VSCC) play an important role in regulation of cell volume and electrical activity. Activation of vascular smooth muscle VSCC causes smooth muscle depolarization and contraction. We investigated the effects of propofol on VSCC in cultured human coronary artery smooth muscle cells by using the chloride-sensitive dye 6-methoxy-N-ethylquinolinium (MEQ). To activate VSCC, cells were superfused for 2 min with hypotonic gluconate solutions and then potassium thiocyanate solution. The percentage reduction in MEQ fluorescence during 60 s in the presence of potassium thiocyanate was measured and used as an index of VSCC activity. 5-Nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), a well characterized chloride channel blocker, and propofol were dissolved in hypotonic gluconate solution to test their effect on VSCC activity. The reduction in fluorescence was inversely related to osmolality, indicating that activation of VSCC is osmolality dependent. Hypotonic gluconate solution (210 mOsm/kg H(2)O) reduced fluorescence by 38.9% +/- 2.6% of the baseline value. The reduction in fluorescence was dose-dependently inhibited by NPPB. Propofol at 0.3, 1, 3, 10, 30, and 100 micro g/mL significantly inhibited the reduction in fluorescence to 23.6% +/- 4.8%, 19.7% +/- 7.4%, 18.2% +/- 3.5%, 17.6% +/- 5.0%, 15.8% +/- 3.1%, and 10.3% +/- 3.9% of the baseline value, respectively. Our results indicate that propofol inhibits VSCC in a dose-dependent manner in human coronary artery smooth muscle cells.

Potentiation of the osmosensitive taurine release and cell volume regulation by cytosolic Ca2+rise in cultured cerebellar astrocytes

Hyposmolarity (-30%) in cultured cerebellar astrocytes raised cytosolic Ca2+ concentration ([Ca2+]i) from 160 to 400 nM and activated the osmosensitive taurine release (OTR) pathway. Although OTR is essentially [Ca2+]i-independent, further increase in [Ca2+]i by ionomycin strongly enhanced OTR, with a more robust effect at low and mild osmolarity reductions. Ionomycin did not affect isosmotic taurine efflux. OTR was decreased by tyrphostin A25 and increased by ortho-vanadate, suggesting a modulation by tyrosine kinase or phosphorylation state. Inhibition of phosphatidylinositol-3-kinase activity by wortmannin markedly decreased OTR and the ionomycin increase. Conversely, OTR and the ionomycin effect were independent of ERK1/ERK2 activation. OTR and its potentiation by ionomycin differed in their sensitivity to CaM and CaMK blockers and in the requirement of an intact cytoskeleton for the ionomycin effect, but not for normal OTR. Changes in the actin cytoskeleton organization elicited by hyposmolarity were not observed in ionomycin-treated cells, which may permit the operation of CaM/CaMK pathways involved in the OTR potentiation by [Ca2+]i rise. OTR potentiation by [Ca2+]i requires the previous or simultaneous activation/operation of the taurine release mechanism and is not modifying its set point, but rather increasing the effectiveness of the pathway, resulting in a more efficient volume regulation. This may have a beneficial effect in pathological situations with concurrent swelling and [Ca2+]i elevation in astrocytes.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Human cervical cancer cells use Ca2+ signalling, protein tyrosine phosphorylation and MAP kinase in regulatory volume decrease

1. This study was aimed at identifying the signalling pathways involved in the activation of volume-regulatory mechanisms of human cervical cancer cells. 2. Osmotic swelling of human cervical cancer cells induced a substantial increase in intracellular Ca2+ ([Ca2+]i) by the activation of Ca2+ entry across the cell membrane, as well as Ca2+ release from intracellular stores. This Ca2+ signalling was critical for the normal regulatory volume decrease (RVD) response. 3. The activation of swelling-activated ion and taurine transport was significantly inhibited by tyrosine kinase inhibitors (genistein and tyrphostin AG 1478) and potentiated by the tyrosine phosphatase inhibitor Na3VO4. However, the Src family of tyrosine kinases was not involved in regulation of the swelling-activated Cl- channel. 4. Cell swelling triggered mitogen-activated protein (MAP) kinase cascades leading to the activation of extracellular signal-regulated kinase 1 and 2 (ERK1/ERK2) and p38 kinase. The volume-responsive ERK1/ERK2 signalling pathway linked with the activation of K+ and Cl- channels, and taurine transport. However, the volume-regulatory mechanism was independent of the activation of p38 MAP kinase. 5. The phosphorylated ERK1/ERK2 expression following a hypotonic shock was up-regulated by protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and down-regulated by PKC inhibitor staurosporine. The response of ERK activation to hypotonicity also required Ca2+ entry and depended on tyrosine kinase and mitogen-activated/ERK-activating kinase (MEK) activity. 6. Considering the results overall, osmotic swelling promotes the activation of tyrosine kinase and ERK1/ERK2 and raises intracellular Ca2+, all of which play a crucial role in the volume-regulatory mechanism of human cervical cancer cells.

Regulation of ion channels by protein tyrosine phosphorylation

Ion channels are regulated by protein phosphorylation and dephosphorylation of serine, threonine, and tyrosine residues. Evidence for the latter process, tyrosine phosphorylation, has increased substantially since this topic was last reviewed. In this review, we present a comprehensive summary and synthesis of the literature regarding the mechanism and function of ion channel regulation by protein tyrosine kinases and phosphatases. Coverage includes the majority of voltage-gated, ligand-gated, and second messenger-gated channels as well as several types of channels that have not yet been cloned, including store-operated Ca2+ channels, nonselective cation channels, and epithelial Na+ and Cl- channels. Additionally, we discuss the critical roles that channel-associated scaffolding proteins may play in localizing protein tyrosine kinases and phosphatases to the vicinity of ion channels.

Regulation of I(Cl,swell) in neuroblastoma cells by G protein signaling pathways

Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) activated the I(Cl,swell) anion channel in N1E115 neuroblastoma cells in a swelling-independent manner. GTPgammaS-induced current was unaffected by ATP removal and broadly selective tyrosine kinase inhibitors, demonstrating that phosphorylation events do not regulate G protein-dependent channel activation. Pertussis toxin had no effect on GTPgammaS-induced current. However, cholera toxin inhibited the current approximately 70%. Exposure of cells to 8-bromoadenosine 3',5'-cyclic monophosphate did not mimic the effect of cholera toxin, and its inhibitory action was not prevented by treatment of cells with an inhibitor of adenylyl cyclase. These results demonstrate that GTPgammaS does not act through Galpha(i/o) GTPases and that Galpha(s)/Gbetagamma G proteins inhibit the channel and/or channel regulatory mechanisms through cAMP-independent mechanisms. Swelling-induced activation of I(Cl,swell) was stimulated two- to threefold by GTPgammaS and inhibited by 10 mM guanosine 5'-O-(2-thiodiphosphate). The Rho GTPase inhibitor Clostridium difficile toxin B inhibited both GTPgammaS- and swelling-induced activation of I(Cl,swell). Taken together, these findings indicate that Rho GTPase signaling pathways regulate the I(Cl,swell) channel via phosphorylation-independent mechanisms.

Effects of anti-oestrogens and beta-estradiol on calcium uptake by cardiac sarcoplasmic reticulum

1. Tamoxifen and a group of structurally similar non-steroidal, triphenolic compounds inhibit the oestrogen receptor. In addition to this action, these anti-oestrogens are known to inhibit some types of plasma membrane ion channels and other proteins through mechanisms that do not appear to involve their interactions with the estrogen receptor but could be the result of their effect on membrane lipid structure or fluidity. 2. We studied the effects of beta-estradiol and three anti-oestrogens (tamoxifen, 4-hydroxytamoxifen and clomiphene) on Ca(2+) uptake into sarcoplasmic reticulum (SR) vesicles isolated from canine cardiac ventricular tissue. 3. The antiestrogens all inhibit SR Ca(2+) uptake in a concentration-dependent manner (order of potency: tamoxifen > 4-hydroxytamoxifen > or = clomiphene). Although these compounds rapidly inhibit net Ca(2+) uptake they do not have a similar rapid effect on the ATPase activity of the SR Ca pump. beta-estradiol has no effect on Ca(2+) uptake nor does it alter the inhibitory action of tamoxifen on the SR. 4. The differences in the effects of beta-estradiol and the anti-oestrogens on cardiac SR Ca(2+) uptake do not correlate with differences in the ways in which these compounds have been reported to interact with membrane lipids. Our results are consistent, however, with direct effects on a membrane protein (possibly an SR Cl(-) or K(+) channel).

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.

Signaling events during swelling and regulatory volume decrease

Brain cell swelling compromises neuronal function and survival by the risk of generation of ischemia episodes as compression of small vessels occurs due to the limits to expansion imposed by the rigid skull. External osmolarity reductions or intracellular accumulation of osmotically active solutes result in cell swelling which can be counteracted by extrusion of osmolytes through specific efflux pathways. Characterization of these pathways has received considerable attention, and there is now interest in the understanding of the intracellular signaling events involved in their activation and regulation. Calcium and calmodulin, phosphoinositides and cAMP may act as second messengers, carrying the information about a cell volume change into signaling enzymes. Small GTPases, protein tyrosine kinases and phospholipases, also appear to be part of the signaling cascades ultimately modulating the osmolyte efflux pathways. This review focus on i) the influence of hyposmotic and isosmotic swelling on these signaling events and molecules and ii) the effects of manipulating their function on the osmolyte fluxes, particularly K+, CI- and amino acids, and on the consequent efficiency of cell volume adjustment.

Role of Rho and Rho kinase in the activation of volume-regulated anion channels in bovine endothelial cells

1. We have studied the modulation of volume-regulated anion channels (VRACs) by the small GTPase Rho and by one of its targets, Rho kinase, in calf pulmonary artery endothelial (CPAE) cells. 2. RT-PCR and immunoblot analysis showed that both RhoA and Rho kinase are expressed in CPAE cells. 3. ICl,swell, the chloride current through VRACs, was activated by challenging CPAE cells with a 25 % hypotonic extracellular solution (HTS) or by intracellular perfusion with a pipette solution containing 100 microM GTPgammaS. 4. Pretreatment of CPAE cells with the Clostridium C2IN-C3 fusion toxin, which inactivates Rho by ADP ribosylation, significantly impaired the activation of ICl,swell in response to the HTS. The current density at +100 mV was 49 +/- 13 pA pF-1 (n = 17) in pretreated cells compared with 172 +/- 17 pA pF-1 (n = 21) in control cells. 5. The volume-independent activation of ICl,swell by intracellular perfusion with GTPgammaS was also impaired in C2IN-C3-pretreated cells (31 +/- 7 pA pF-1, n = 11) compared with non-treated cells (132 +/- 21 pA pF-1, n = 15). 6. Activation of ICl,swell was pertussis toxin (PTX) insensitive. 7. Y-27632, a blocker of Rho kinase, inhibited ICl,swell and delayed its activation. 8. Inhibition of Rho and of Rho kinase by the above-described treatments did not affect the extent of cell swelling in response to HTS. 9. These experiments provide strong evidence that the Rho-Rho kinase pathway is involved in the VRAC activation cascade.

Mechanical stimulation regulates voltage-gated potassium currents in cardiac microvascular endothelial cells

Vascular endothelial cells are constantly exposed to mechanical forces resulting from blood flow and transmural pressure. The goal of this study was to determine whether mechanical stimulation alters the properties of endothelial voltage-gated K+ channels. Cardiac microvascular endothelial cells (CMECs) were isolated from rat ventricular muscle and cultured on thin sheets of silastic membranes. Membrane currents were measured with the use of the whole-cell arrangement of the patch-clamp technique in endothelial cells subjected to static stretch for 24 hours and compared with measurements from control, nonstretched cells. Voltage steps positive to -30 mV resulted in the activation of a time-dependent, delayed rectifier K+current (IK) in the endothelial cells. Mechanically induced increases of 97%, 355%, and 106% at +30 mV were measured in the peak amplitude of IK in cells stretched for 24 hours by 5%, 10%, and 15%, respectively. In addition, the half-maximal voltage required for IK activation was shifted from +34 mV in the nonstretched cells to -5 mV in the stretched cells. Although IK in both groups of CMECs was blocked to a similar extent by tetraethylammonium, currents in the stretched endothelial cells displayed an enhanced sensitivity to inhibition by charybdotoxin. Preincubation of the CMECs with either pertussis toxin or phorbol 12-myristate 13-acetate during the 24 hours of cell stretch did not prevent the increase in IK. The application of phorbol 12-myristate 13-acetate and static stretch stimulated the proliferation of CMECs. Stretch-induced regulation of K+ channels may be important to control the resting potential of the endothelium and may contribute to capillary growth during periods of mechanical perturbation.

A serine residue in ClC-3 links phosphorylation-dephosphorylation to chloride channel regulation by cell volume

In many mammalian cells, ClC-3 volume-regulated chloride channels maintain a variety of normal cellular functions during osmotic perturbation. The molecular mechanisms of channel regulation by cell volume, however, are unknown. Since a number of recent studies point to the involvement of protein phosphorylation/dephosphorylation in the control of volume-regulated ionic transport systems, we studied the relationship between channel phosphorylation and volume regulation of ClC-3 channels using site-directed mutagenesis and patch-clamp techniques. In native cardiac cells and when overexpressed in NIH/3T3 cells, ClC-3 channels were opened by cell swelling or inhibition of endogenous PKC, but closed by PKC activation, phosphatase inhibition, or elevation of intracellular Ca2+. Site-specific mutational studies indicate that a serine residue (serine51) within a consensus PKC-phosphorylation site in the intracellular amino terminus of the ClC-3 channel protein represents an important volume sensor of the channel. These results provide direct molecular and pharmacological evidence indicating that channel phosphorylation/dephosphorylation plays a crucial role in the regulation of volume sensitivity of recombinant ClC-3 channels and their native counterpart, ICl.vol.

Hypotonicity activates transcription through ERK-dependent and -independent pathways in renal cells

Acute hypotonic shock (50% dilution of medium with sterile water, but not with isotonic NaCl) activated the extracellular signal response kinase (ERK) mitogen-activated protein (MAP) kinases in renal medullary cells, as measured by Western analysis with a phospho-ERK-specific antibody and by in vitro kinase assay of epitope-tagged ERKs immunoprecipitated from stable HA-ERK transfectants. Hypotonicity also activated the transcription factor and ERK substrate Elk-1 in a partially PD-98059-sensitive fashion, as assessed by chimeric reporter gene assay. Consistent with these data, hypotonic stress activated transcription of the immediate-early gene transcription factor Egr-1 in a partially PD-98059-sensitive fashion. Hypotonicity-inducible Egr-1 transcription was mediated in part through 5'-flanking regions containing serum response elements and in part through the minimal Egr-1 promoter. Elimination of the Ets motifs adjacent to key regulatory serum response elements in the Egr-1 promoter diminished the effect of hypotonicity but failed to abolish it. Interestingly, hypotonicity also transiently activated p38 and c-Jun NH2-terminal kinase 1, as determined by immunoblotting with anti-phospho-MAP kinase antibodies. Taken together, these data strongly suggest that hypotonicity activates immediate-early gene transcription in renal medullary cells via MAP kinase kinase-dependent and -independent mechanisms.

Effect of cell swelling on membrane and cytoplasmic distribution of pICln

pICln is found ubiquitously in mammalian cells and is postulated to play a critical role in cell volume regulation. Mutagenesis studies led to the proposal that pICln is a swelling-activated anion channel. However, recent studies in Madin-Darby canine kidney cells and endothelial cells have shown that the protein is localized primarily to the cytoplasm. It has therefore been postulated that activation involves reversible translocation of pICln from the cytoplasm and insertion into the plasma membrane. We tested this hypothesis using several different approaches. Fractionation of C6 glioma cells into plasma membrane- and cytoplasm-containing fractions demonstrated that approximately 90% of the recovered pICln was confined to the cytosol. Swelling had no effect on the relative amount of protein present in the plasma membrane fraction. Immunofluorescence microscopy revealed that pICln is localized primarily, if not exclusively, to the cytoplasm of swollen and nonswollen cells. Similarly, transfection of cells with a green fluorescent protein-labeled pICln construct failed to reveal any membrane localization of the protein. These findings do not support the hypothesis that pICln is a volume regulatory anion channel activated by swelling-induced membrane insertion.

Mitogen-activated protein and tyrosine kinases in the activation of astrocyte volume-activated chloride current

Astrocytes swell during neuronal activity as they accumulate K+ to buffer the increase in external K+ released from neurons. This swelling activates volume-sensitive Cl- channels, which are thought to be important in regulatory volume decrease and in the response of the CNS to trauma and excitotoxicity. Mitogen-activated protein (MAP) kinases also are activated by cell volume changes, but their roles in volume regulation are unknown. We have investigated the role of tyrosine and MAP kinases in the activation of volume-activated Cl- channels in cultured astrocytes, using whole-cell patch-clamp recording and Western immunoblots. As previously described, hypo-osmotic solution induced an outwardly rectifying Cl- current, which was blocked by NPPB and SITS. This Cl- current did not depend on [Ca2+ ]i because it was still observed when 20 mM BAPTA was included in the pipette, but it did exhibit rundown when ATP was omitted. Inhibition of tyrosine kinases with genistein or tyrphostin A23 (but not the inactive agents daidzein and tyrphostin A1) blocked the Cl- current. The MAP kinase kinase (MEK) inhibitor PD 98059 reversibly inhibited activation of the Cl- current by hypo-osmotic solution. Western immunoblots showed that genistein or PD 98059 blocked activation of Erk-1 and Erk-2 by hypo-osmotic solution in astrocytes. Therefore, activation of tyrosine and MAP kinases by swelling is a critical step in the opening of volume-sensitive Cl- channels.

Regulation of a swelling-activated chloride current in bovine endothelium by protein tyrosine phosphorylation and G proteins

1. The role of protein tyrosine phosphorylation and of G proteins in the activation of a swelling-activated Cl- current (ICl,swell) in calf pulmonary artery endothelial (CPAE) cells was studied using the whole-cell patch clamp technique. ICl,swell was activated by reducing the extracellular osmolality by either 12.5% (mild hypotonicity) or 25% (strong hypotonicity). 2. The protein tyrosine kinase (PTK) inhibitors tyrphostin B46, tyrphostin A25 and genistein inhibited ICl,swell with IC50 values of, respectively, 9.2 +/- 0.2, 61.4 +/- 1.7 and 62.9 +/- 1.3 microM. Tyrphostin A1, a tyrphostin analogue with little effect on PTK activity, and daidzein, an inactive genistein analogue, were without effect on ICl,swell. 3. The protein tyrosine phosphatase (PTP) inhibitors Na3VO4 (200 microM) and dephostatin (20 microM) potentiated ICl,swell activated by mild hypotonicity by 47 +/- 9 and 69 +/- 15%, respectively. 4. Intracellular perfusion with GTP gamma S (100 microM) transiently activated a Cl- current with an identical biophysical and pharmacological profile to ICl,swell. This current was inhibited by the tested PTK inhibitors and potentiated by the PTP inhibitors. Hypertonicity-induced cell shrinkage completely inhibited the GTP gamma S-activated Cl- current. 5. Intracellular perfusion with GDP beta S (1 mM) caused a time-dependent inhibition of ICl,swell, which was more pronounced when the current was activated by mild hypotonicity. 6. Our results demonstrate that the activity of endothelial swelling-activated Cl- channels is dependent on tyrosine phosphorylation and suggest that G proteins regulate the sensitivity to cell swelling.

Cl- and K+ conductances activated by cell swelling in primary cultures of rabbit distal bright convoluted tubules

Ionic currents induced by cell swelling were characterized in primary cultures of rabbit distal bright convoluted tubule (DCTb) by the whole cell patch-clamp technique. Cl- currents were produced spontaneously by whole cell recording with an isotonic pipette solution or by exposure to a hypotonic stress. Initial Cl- currents exhibited outwardly rectifying current-voltage relationship, whereas steady-state currents showed strong decay with depolarizing pulses. The ion selectivity sequence was I- = Br- > Cl- >> glutamate. Currents were inhibited by 0.1 mM 5-nitro-2-(3-phenylpropylamino) benzoic acid and 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and strongly blocked by 1 mM diphenylamine-2-carboxylate. Currents were insensitive to intracellular Ca2+ but required the presence of extracellular Ca2+. They were not activated in cells pretreated with 200 nM staurosporine, 50 microM LaCl3, 10 microM nifedipine, 100 microM verapamil, 5 microM tamoxifen, and 50 microM dideoxyforskolin. Staurosporine, tamoxifen, verapamil, or the absence of external Ca2+ was without effect on the fully developed Cl- currents. Osmotic shock also activated K+ currents in Cl- free conditions. These currents were time independent, activated at depolarized potentials, and inhibited by 5 mM BaCl2. The activation of Cl- and K+ currents by an osmotic shock may be implicated in regulatory volume decrease in DCTb cells.

Role for anions in pulmonary endothelial permeability

beta-Adrenergic stimulation reduces albumin permeation across pulmonary artery endothelial monolayers and induces changes in cell morphology that are mediated by Cl- flux. We tested the hypothesis that anion-mediated changes in endothelial cells result in changes in endothelial permeability. We measured permeation of radiolabeled albumin across bovine pulmonary arterial endothelial monolayers when the extracellular anion was Cl-, Br-, I-, F-, acetate (Ac-), gluconate (G-), and propionate (Pr-). Permeability to albumin (Palbumin) was calculated before and after addition of 0.2 mM of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), which reduces permeability. In Cl-, the Palbumin was 3.05 +/- 0.86 x 10(-6) cm/s and fell by 70% with the addition of IBMX. The initial Palbumin was lowest for Pr- and Ac-. Initial Palbumin was higher in Br-, I-, G-, and F- than in Cl-. A permeability ratio was calculated to examine the IBMX effect. The greatest IBMX effect was seen when Cl- was the extracellular anion, and the order among halide anions was Cl- > Br- > I- > F-. Although the level of extracellular Ca2+ concentration ([Ca2+]o) varied over a wide range in the anion solutions, [Ca2+]o did not systematically affect endothelial permeability in this system. When Cl- was the extracellular anion, varying [Ca2+]o from 0.2 to 2.8 mM caused a change in initial Palbumin but no change in the IBMX effect. The anion channel blockers 4-acetamido-4'-isothiocyanotostilbene-2, 2'-disulfonic acid (0.25 mM) and anthracene-9-carboxylic acid (0.5 mM) significantly altered initial Palbumin and the IBMX effect. The anion transport blockers bumetanide (0.2 mM) and furosemide (1 mM) had no such effects. We conclude that extracellular anions influence bovine pulmonary arterial endothelial permeability and that the pharmacological profile fits better with the activity of anion channels than with other anion transport processes.

Swelling-activated efflux of taurine and other organic osmolytes in endothelial cells

We used a combined biochemical, pharmacological, and electrophysiological approach to study the effects of hyposmotic swelling on organic osmolyte efflux in endothelial cells (EC). In [3H]taurine-loaded monolayers of calf pulmonary artery EC (CPAEC), hyposmolality activated time- and dose-dependent effluxes of [3H]taurine. Swelling-activated [3H]taurine efflux (Jtau swell)in CPAEC was inhibited by the anion channel blockers tamoxifen, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), fenamates, and also quinine (in a pH-dependent manner), ATP, and the phospholipase A2 inhibitor 4-bromophenacyl bromide. In contrast, Jtau swell was partly or totally insensitive to bumetanide, forskolin, phorbol 12-myristate 13-acetate, and staurosporine. Swelling also activated myo-[3H]inositol efflux that was blocked by tamoxifen, NPPB, DIDS, and niflumic acid. Moreover, the cellular content of taurine and other amino acids was significantly reduced in osmotically activated CPAEC. Finally, in whole cell patch-clamp experiments, taurine, glycine, aspartate, and glutamate exhibited significant permeability for swelling-activated anion channels. In conclusion, hyposmotic swelling activates efflux of taurine and other organic osmolytes in EC. In addition, our results suggest that anion channels may provide a pathway for swelling-activated efflux of organic osmolytes in EC.

Expression of human pICln and ClC-6 in Xenopus oocytes induces an identical endogenous chloride conductance

pICln is a protein that induces an outwardly rectifying, nucleotide-sensitive chloride current (ICln) when expressed in Xenopus oocytes, but its precise function (plasma-membrane anion channel versus cytosolic regulator of a channel) remains controversial. We now report that a chloride current identical to ICln is induced when Xenopus oocytes are injected with human ClC-6 RNA. Indeed, both the pICln and the ClC-6 induced current are outwardly rectifying, they inactivate slowly at positive potentials and have an anion permeability sequence NO3- > I- > Br- > Cl- > gluconate. Cyclamate, NPPB, and extracellular cAMP block the induced currents. The success rate of current expression is significantly increased when the injected Xenopus oocytes are incubated at a higher temperature (24 or 37 degrees C) prior to the analysis. In addition, the ICln current was detected in 6.2% of noninjected control Xenopus oocytes. We therefore conclude that the ICln current in Xenopus oocytes corresponds to an endogenous conductance that can be activated by expression of structurally unrelated proteins. Furthermore, functional, biochemical, and morphological observations did not support the notion that pICln resides in the plasma membrane either permanently or transiently after cell swelling. Thus, it is unlikely that pICln forms the channel that is responsible for the ICln current in Xenopus oocytes.

Calcium-activated chloride channels in bovine pulmonary artery endothelial cells

1. We characterized Ca(2+)-activated Cl- currents in calf pulmonary artery endothelial (CPAE) cells by using a combined patch clamp and fura-2 microfluorescence technique to simultaneously measure ionic currents and the intracellular Ca2+ concentration, [Ca2+]i. 2. Various procedures that increased [Ca2+]i, such as stimulation with ATP or ionomycin, or loading the cells with Ca2+ via the patch pipette, activated a strongly outwardly rectifying current with a reversal potential close to the Cl- equilibrium potential. Changing the extracellular Cl- concentration shifted this reversal potential as predicted for a Cl- current. Buffering Ca2+ rises with BAPTA prevented ATP from activating the current. 3. Ca(2+)-activated Cl- currents could be distinguished from volume-activated Cl- currents, which were sometimes coactivated in the same cell. The latter showed much less outward rectification, their activation was voltage independent, and they could be inhibited by exposing the cells to hypertonic solutions. 4. The permeability ratio for the Ca(2+)-activated conductance of the anions iodide:chloride: gluconate was 1.71 +/- 0.06:1:0.39 +/- 0.03 (n = 12). 5. This Ca(2+)-activated Cl- current, ICl, Ca, inactivated rapidly at negative potentials and activated slowly at positive potentials. Outward tail currents were slowly decaying, while inward tail currents decayed much faster. 6. 4,4'-Diisothiocyanatostilbene-2,2'-disulphonic-acid (DIDS) and niflumic acid inhibited Icl,Ca in a voltage-dependent manner, i.e. they exerted a more potent block at positive potentials. The block by N-phenylanthracilic acid (NPA), 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) and tamoxifen was voltage independent. Niflumic acid and tamoxifen were the most potent blockers. 7. The single-channel conductance was 7.9 +/- 0.7 pS (n = 15) at 300 mM extracellular Cl-. The channel open probability was high at positive potentials, but very small at negative potentials. 8. It is concluded that [Ca2+]i activates small-conductance Cl- channels in endothelial cells, which coexist with the volume-activated Cl- channels described previously.

Anion, cation, and zwitterion selectivity of phospholemman channel molecules

Phospholemman (PLM), a 72-amino acid membrane protein with a single transmembrane domain, forms taurine-selective ion channels in lipid bilayers. Because taurine forms zwitterions, a taurine-selective channel might have binding sites for both anions and cations. Here we show that PLM channels indeed allow fluxes of both cations and anions, making instantaneous and voltage-dependent transitions among conformations with drastically different ion selectivity characteristics. This surprising and novel ion channel behavior offers a molecular explanation for selective taurine flux across cell membranes and may explain why molecules in the phospholemman family can induce cation- or anion-selective conductances when expressed in Xenopus oocytes.

Annexin II modulates volume-activated chloride currents in vascular endothelial cells

The membrane-associated, microfilament-binding protein annexin II is abundantly expressed in endothelial cells from calf pulmonary artery (CPAE cells). We have analyzed its role in the regulation of volume-activated chloride currents (ICl, vol) by loading the cells via the patch pipette with a peptide comprising the N-terminal 14 residues of annexin II. This sequence harbors the binding site for the intracellular annexin II ligand, p11, and the peptide interferes with the annexin II-p11 complex formation. Loading of a CPAE cell with this peptide caused a gradual decrease in the amplitude of ICl, vol during repetitive stimulations with a 28% hypotonic extracellular solution. This run down of the current was virtually absent in untreated cells and in cells that were loaded with a mutated 14-amino acid peptide, which has a single amino acid replacement known to result in a more than 1000 times reduced affinity for binding to p11. We conclude that annexin II-p11 complex formation is either directly or indirectly involved in the activation of ICl, vol in endothelial cells.

Volume-activated Cl- channels

1. An increase in cell volume activates, in most mammalian cells, a Cl- current, ICl,vol. This current is involved in a variety of cellular functions, such as the maintenance of a constant cell volume, pH regulation, and control of membrane potential. It might also play a role in the regulation of cell proliferation and in the processes that control transition from proliferation to differentiation. This review focuses on various aspects of this current, including its biophysical characterisation and its functional role for various cell processes. 2. Volume-activated Cl- channels show all outward rectification. Iodide is more permeable than chloride. In some cell types, ICl,vol inactivates at positive potentials. Single channel conductance can be divided mainly into two groups: small (< 5 pS) and medium conductance channels (around 50 pS). 3. The pharmacology and modulation of these channels are reviewed in detail, and suggest the existence of an heterogeneous family of multiple volume-activated Cl- channels. 4. Molecular candidates for this channel (i.e. ClC-2, a member of the ClC-family of voltage-dependent Cl- channels, the mdr-1 encoded P-glycoprotein, the nucleotide-sensitive pICln protein and phospholemman) will be discussed.