Hypotonically activated chloride current in HSG cells

TRPM7 is an essential regulator for volume-sensitive outwardly rectifying anion channel

Animal cells can regulate their volume after swelling by the regulatory volume decrease (RVD) mechanism. In epithelial cells, RVD is attained through KCl release mediated via volume-sensitive outwardly rectifying Cl^- channels (VSOR) and Ca²⁺-activated K⁺ channels. Swelling-induced activation of TRPM7 cation channels leads to Ca²⁺ influx, thereby stimulating the K⁺ channels. Here, we examined whether TRPM7 plays any role in VSOR activation. When TRPM7 was knocked down in human HeLa cells or knocked out in chicken DT40 cells, not only TRPM7 activity and RVD efficacy but also VSOR activity were suppressed. Heterologous expression of TRPM7 in TRPM7-deficient DT40 cells rescued both VSOR activity and RVD, accompanied by an increase in the expression of LRRC8A, a core molecule of VSOR. TRPM7 exerts the facilitating action on VSOR activity first by enhancing molecular expression of LRRC8A mRNA through the mediation of steady-state Ca²⁺ influx and second by stabilizing the plasmalemmal expression of LRRC8A protein through the interaction between LRRC8A and the C-terminal domain of TRPM7. Therefore, TRPM7 functions as an essential regulator of VSOR activity and LRRC8A expression.

Cell Death Induction and Protection by Activation of Ubiquitously Expressed Anion/Cation Channels. Part 1: Roles of VSOR/VRAC in Cell Volume Regulation, Release of Double-Edged Signals and Apoptotic/Necrotic Cell Death

Cell volume regulation (CVR) is essential for survival and functions of animal cells. Actually, normotonic cell shrinkage and swelling are coupled to apoptotic and necrotic cell death and thus called the apoptotic volume decrease (AVD) and the necrotic volume increase (NVI), respectively. A number of ubiquitously expressed anion and cation channels are involved not only in CVD but also in cell death induction. This series of review articles address the question how cell death is induced or protected with using ubiquitously expressed ion channels such as swelling-activated anion channels, acid-activated anion channels and several types of TRP cation channels including TRPM2 and TRPM7. The Part 1 focuses on the roles of the volume-sensitive outwardly rectifying anion channels (VSOR), also called the volume-regulated anion channel (VRAC), which is activated by cell swelling or reactive oxygen species (ROS) in a manner dependent on intracellular ATP. First we describe phenotypical properties, the molecular identity, and physical pore dimensions of VSOR/VRAC. Second, we highlight the roles of VSOR/VRAC in the release of organic signaling molecules, such as glutamate, glutathione, ATP and cGAMP, that play roles as double-edged swords in cell survival. Third, we discuss how VSOR/VRAC is involved in CVR and cell volume dysregulation as well as in the induction of or protection from apoptosis, necrosis and regulated necrosis under pathophysiological conditions.

The Relationship Between Actin Cytoskeleton and Membrane Transporters in Cisplatin Resistance of Cancer Cells

Cisplatin [cis-diamminedichloroplatinum (II)] is a platinum-based anticancer drug widely used for the treatment of various cancers. It forms interstrand and intrastrand cross-linking with DNA and block DNA replication, resulting in apoptosis. On the other hand, intrinsic and acquired cisplatin resistance restricts its therapeutic effects. Although some studies suggest that dramatic epigenetic alternations are involved in the resistance triggered by cisplatin, the mechanism is complicated and remains poorly understood. Recent studies reported that cytoskeletal structures regulate cisplatin sensitivity and that activities of membrane transporters contribute to the development of resistance to cisplatin. Therefore, we focus on the roles of actin filaments and membrane transporters in cisplatin-induced apoptosis. In this review, we summarize the relationship between actin cytoskeleton and membrane transporters in the cisplatin resistance of cancer cells.

Impaired actin filaments decrease cisplatin sensitivity via dysfunction of volume-sensitive Cl- channels in human epidermoid carcinoma cells

Cisplatin is a widely used platinum-based anticancer drug in the chemotherapy of numerous human cancers. However, cancer cells acquire resistance to cisplatin. So far, functional loss of volume-sensitive outwardly rectifying (VSOR) Cl^- channels has been reported to contribute to cisplatin resistance of cancer cells. Here, we analyzed protein expression patterns of human epidermoid carcinoma KB cells and its cisplatin-resistant KCP-4 cells. Intriguingly, KB cells exhibited higher β-actin expression and clearer actin filaments than KCP-4 cells. The β-actin knockdown in KB cells decreased VSOR Cl^- currents and inhibited the regulatory volume decrease (RVD) process after cell swelling. Consistently, KB cells treated with cytochalasin D, which depolymerizes actin filaments, showed smaller VSOR Cl^- currents and slower RVD. Cytochalasin D also inhibited cisplatin-triggered apoptosis in KB cells. These results suggest that the disruption of actin filaments cause the dysfunction of VSOR Cl^- channels, which elicits resistance to cisplatin in human epidermoid carcinoma cells.

Properties of Taurine Release in Glucose-Free Media in Hippocampal Slices from Developing and Adult Mice

The release of preloaded [(3)H]taurine from hippocampal slices from developing 7-day-old and young adult 3-month-old mice was studied in a superfusion system in the absence of glucose. These hypoglycemic conditions enhanced the release at both ages, the effect being markedly greater in developing mice. A depolarizing K(+) concentration accentuated the release, which indicates that it was partially mediated by exocytosis. The anion channel blockers were inhibitory, witnessing the contribution of ion channels. NO-generating agents fomented the release as a sign of the participation of excitatory amino acid receptors. The other second messenger systems were apparently less efficient. The much greater taurine release could be a reason for the well-known greater tolerance of developing nervous tissue to lack of glucose.

Activation of ATP secretion via volume-regulated anion channels by sphingosine-1-phosphate in RAW macrophages

We report the activation of outwardly rectifying anion currents by sphingosine-1-phosphate (S1P) in the murine macrophage cell line RAW 264.7. The S1P-induced current is mainly carried by anions, because the reversal potential of the current was shifted by replacement of extracellular Cl(-) by glutamate(-) but not when extracellular Na(+) was substituted by Tris(+). The inhibition of the current by hypertonic extracellular or hypotonic intracellular solution as well as the inhibitory effects of NPPB, tamoxifen, and glibenclamide indicates that the anion current is mediated by volume-regulated anion channels (VRAC). The S1P effect was blocked by intracellular GDPβS and W123, which points to signaling via the S1P receptor 1 (S1PR1) and G proteins. As cytochalasin D diminished the action of S1P, we conclude that the actin cytoskeleton is involved in the stimulation of VRAC. S1P and hypotonic extracellular solution induced secretion of ATP from the macrophages, which in both cases was blocked in a similar way by typical VRAC blockers. We suppose that the S1P-induced ATP secretion in macrophages via activation of VRAC constitutes a functional link between sphingolipid and purinergic signaling in essential processes such as inflammation and migration of leukocytes as well as phagocytosis and the killing of intracellular bacteria.

Autoantibodies against muscarinic type 3 receptor in Sjögren's syndrome inhibit aquaporin 5 trafficking

Sjögren's syndrome (SjS) is a chronic autoimmune disease that mainly targets the salivary and lacrimal glands. It has been controversial whether anti-muscarinic type 3 receptor (α-M3R) autoantibodies in patients with SjS inhibit intracellular trafficking of aquaporin-5 (AQP5), water transport protein, leading to secretory dysfunction. To address this issue, GFP-tagged human AQP5 was overexpressed in human salivary gland cells (HSG-hAQP5) and monitored AQP5 trafficking to the plasma membrane following carbachol (CCh, M3R agonist) stimulation. AQP5 trafficking was indeed mediated by M3R stimulation, shown in partial blockage of trafficking by M3R-antagonist 4-DAMP. HSG-hAQP5 pre-incubated with SjS plasma for 24 hours significantly reduced AQP5 trafficking with CCh, compared with HSG-hAQP5 pre-incubated with healthy control (HC) plasma. This inhibition was confirmed by monoclonal α-M3R antibody and pre-absorbed plasma. Interestingly, HSG-hAQP5 pre-incubated with SjS plasma showed no change in cell volume, compared to the cells incubated with HC plasma showing shrinkage by twenty percent after CCh-stimulation. Our findings clearly indicate that binding of anti-M3R autoantibodies to the receptor, which was verified by immunoprecipitation, suppresses AQP5 trafficking to the membrane and contribute to impaired fluid secretion in SjS. Our current study urges further investigations of clinical associations between SjS symptoms, such as degree of secretory dysfunction, cognitive impairment, and/or bladder irritation, and different profiles (titers, isotypes, and/or specificity) of anti-M3R autoantibodies in individuals with SjS.

Influence of the extracellular matrix and integrins on volume-sensitive osmolyte anion channels in C2C12 myoblasts

The purpose of this study was to determine whether extracellular matrix (ECM) composition through integrin receptors modulated the volume-sensitive osmolyte anion channels (VSOACs) in skeletal muscle-derived C2C12 cells. Cl(-) currents were recorded in whole cell voltage-clamped cells grown on laminin (LM), fibronectin (FN), or in the absence of a defined ECM (NM). Basal membrane currents recorded in isotonic media (300 mosmol/kg) were larger in cells grown on FN (3.8-fold at +100 mV) or LM (8.8-fold at +100 mV) when compared with NM. VSOAC currents activated by cell exposure to hypotonic solution were larger in cells grown on LM (1.72-fold at +100 mV) or FN (1.75-fold at +100 mV) compared with NM. Additionally, the kinetics of VSOAC activation was approximately 27% quicker on FN and LM. These currents were tamoxifen sensitive, displayed outward rectification, reversed at the equilibrium potential of Cl(-) and inactivated at potentials >+60 mV. Specific knockdown of beta(1)-integrin by short hairpin RNA interference strongly inhibited the VSOAC Cl(-) currents in cells plated on FN. In conclusion, ECM composition and integrins profoundly influence the biophysical properties and mechanisms of onset of VSOACs.

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.

Reduced volume-regulated outwardly rectifying anion channel activity in ventricular myocyte of type 1 diabetic mice

The currents through the volume-regulated outwardly rectifying anion channel (VRAC) were measured in single ventricular myocytes obtained from streptozotocin (STZ)-induced diabetic mice, using whole-cell voltage-clamp method. In myocytes from STZ-diabetic mice, the density of VRAC current induced by hypotonic perfusion was markedly reduced, compared with that in the cells form normal control mice. Video-image analysis showed that the regulatory volume decrease (RVD), which was seen in normal cells after osmotic swelling, was almost lost in myocytes from STZ-diabetic mice. Some mice were pretreated with 3-O-methylglucose before STZ injection, to prevent the STZ's beta cell toxicity. In the myocytes obtained from such mice, the magnitude of VRAC current and the degree of RVD seen during hypotonic challenge were almost normal. Incubation of the myocytes from STZ-diabetic mice with insulin reversed the attenuation of VRAC current. These findings suggested that the STZ-induced chronic insulin-deficiency was an important causal factor for the attenuation of VRAC current. Intracellular loading of the STZ-diabetic myocytes with phosphatidylinositol 3,4,5-trisphosphate (PIP3), but not phosphatidylinositol 4,5-bisphosphate (PIP2), also reversed the attenuation of VRAC current. Furthermore, treatment of the normal cells with wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, suppressed the development of VRAC current. We postulate that an impairment PI3K-PIP3 pathway, which may be insulin-dependent, is responsible for the attenuation of VRAC currents in STZ-diabetic myocytes.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Role of Calcium in Volume-Activated Chloride Currents in a Mouse Cholangiocyte Cell Line

Volume-activated Cl(-) channels (VACCs) play vital roles in many cells including cholangiocytes. Previously, we characterized the VACCs in mouse cholangiocytes. Since calcium plays an important role in VACC regulation in many cells, we have studied the effect of calcium modulation on the regulatory volume decrease (RVD) and VACC currents in mouse bile duct cells (MBDCs). Cell volume measurements were assessed by a Coulter counter with cell sizer, and conventional whole-cell patch-clamp techniques were used to study the role of calcium on RVD and VACC currents. Cell volume study indicated that MBDCs exhibited RVD, which was inhibited by 5-nitro-2'-(3-phenylpropylamino)-benzoate (NPPB), 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) and 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra-acetoxymethyl ester (BAPTA-AM) but not by removal of extracellular calcium. During hypotonic challenge, MBDCs exhibited an outwardly rectified current, which was significantly inhibited by administration of classical chloride channel inhibitors such as NPPB and tamoxifen. Chelation of the intracellular calcium with BAPTA-AM or removal of extracellular calcium and calcium channel blocker had no significant effect on VACC currents during hypotonic challenge. In addition to VACC, MBDC had a calcium-activated chloride channel, which was inhibited by NPPB. The present study is the first to systemically study the role of calcium on the VACC and RVD in mouse cholangiocytes and demonstrates that a certain level of intracellular calcium is necessary for RVD but the activation of VACC during RVD does not require calcium. These findings suggest that calcium does not have a direct regulatory role on VACC but has a permissive role on RVD in cholangiocytes.

Selective block of swelling-activated Cl- channels over cAMP-dependent Cl- channels in ventricular myocytes

The objective of this study on guinea-pig and rabbit ventricular myocytes was to evaluate the sensitivities of swelling-activated Cl- current (ICl(swell)) and cAMP-dependent cystic fibrosis transmembrane regulator (CFTR) Cl- current (ICl(CFTR)) to block by dideoxyforskolin and verapamil. The currents were recorded from whole-cell configured myocytes that were dialysed with a Cs+-rich pipette solution and superfused with either isosmotic Na+-, K+-, Ca2+-free solution that contained 140 mM sucrose or hyposmotic sucrose-free solution. Forskolin-activated ICl(CFTR) was inhibited by reference blocker anthracene-9-carboxylic acid but unaffected by < or = 200 microM dideoxyforskolin and verapamil. However, dideoxyforskolin and verapamil had strong inhibitory effects on outwardly-rectifying, inactivating, distilbene-sensitive ICl(swell); IC50 values were approximately 30 microM, and blocks were voltage-independent and reversible. The results establish that dideoxyforskolin and verapamil can be used to distinguish between ICl(CFTR) and ICl(swell) in heart cells, and expand the pharmacological characterization of cardiac ICl(swell).

Calcium receptor message, expression and function decrease in differentiating keratinocytes

Calcium-sensing receptor (CaSR) expression and function were studied in proliferating and differentiating cultured human gingival keratinocytes (HGKs). CaSR mRNA and protein were present in proliferating HGKs cultured in 0.03 mM [Ca(2+)] and decreased in cells induced to differentiate by culturing in 1.2 mM [Ca(2+)] for 2 days. CaSR protein was also detected in gingival tissue. Exposure to 10 mM extracellular [Ca(2+)] activated two sequential whole-cell currents. The first was a small, transient calcium release activated calcium current I(CRAC)-like current with an inwardly rectifying I-V curve. The second current was larger with a linear I-V curve. Both currents were significantly decreased in differentiating cells. Neither neomycin nor gadolinium induced changes in whole cell currents nor in intracellular [Ca(2+)], but neomycin inhibited the late large current. Extracellular Ca(2+) increased intracellular [Ca(2+)] of proliferating HGKs in a dose-dependent fashion. Comparison of the time-courses of the whole-cell currents and the intracellular [Ca(2+)] responses indicated both induced currents supported a Ca(2+) influx. Extracellular [Mg(2+)] changes did not affect intracellular [Ca(2+)]. La(3+) and 2-APB inhibited the whole cell current and intracellular [Ca(2+)] changes. The results indicate that the CaSR signaling response likely plays a major role in initiating Ca(2+) induced differentiation responses in HGKs.

CFTR null mutation altered cAMP-sensitive and swelling-activated Cl- currents in primary cultures of mouse nephron

The role of cystic fibrosis transmembrane conductance regulator (CFTR) in the control of Cl(-) currents was studied in mouse kidney. Whole cell clamp was used to analyze Cl(-) currents in primary cultures of proximal and distal convoluted and cortical collecting tubules from wild-type (WT) and cftr knockout (KO) mice. In WT mice, forskolin activated a linear Cl(-) current only in distal convoluted and cortical collecting tubule cells. This current was not recorded in KO mice. In both mice, Ca(2+)-dependent Cl(-) currents were recorded in all segments. In WT mice, volume-sensitive Cl(-) currents were implicated in regulatory volume decrease during hypotonicity. In KO mice, regulatory volume decrease and swelling-activated Cl(-) current were impaired but were restored by adenosine perfusion. Extracellular ATP also restored swelling-activated Cl(-) currents. The effect of ATP or adenosine was blocked by 8-cyclopentyl-1,3-diproxylxanthine. The ecto-ATPase inhibitor ARL-67156 inhibited the effect of hypotonicity and ATP. Finally, in KO mice, volume-sensitive Cl(-) currents are potentially functional, but the absence of CFTR precludes their activation by extracellular nucleosides. This observation strengthens the hypothesis that CFTR is a modulator of ATP release in epithelia.

Phloretin differentially inhibits volume-sensitive and cyclic AMP-activated, but not Ca-activated, Cl(-) channels

Some phenol derivatives are known to block volume-sensitive Cl(-) channels. However, effects on the channel of the bisphenol phloretin, which is a known blocker of glucose uniport and anion antiport, have not been examined. In the present study, we investigated the effects of phloretin on volume-sensitive Cl(-) channels in comparison with cyclic AMP-activated CFTR Cl(-) channels and Ca(2+)-activated Cl(-) channels using the whole-cell patch-clamp technique. Extracellular application of phloretin (over 10 microM) voltage-independently, and in a concentration-dependent manner (IC(50) approximately 30 microM), inhibited the Cl(-) current activated by a hypotonic challenge in human epithelial T84, Intestine 407 cells and mouse mammary C127/CFTR cells. In contrast, at 30 microM phloretin failed to inhibit cyclic AMP-activated Cl(-) currents in T84 and C127/CFTR cells. Higher concentrations (over 100 microM) of phloretin, however, partially inhibited the CFTR Cl(-) currents in a voltage-dependent manner. At 30 and 300 microM, phloretin showed no inhibitory effect on Ca(2+)-dependent Cl(-) currents induced by ionomycin in T84 cells. It is concluded that phloretin preferentially blocks volume-sensitive Cl(-) channels at low concentrations (below 100 microM) and also inhibits cyclic AMP-activated Cl(-) channels at higher concentrations, whereas phloretin does not inhibit Ca(2+)-activated Cl(-) channels in epithelial cells.

Chloride channels regulate HIT cell volume but cannot fully account for swelling-induced insulin secretion

Insulin-secreting pancreatic islet beta-cells possess anion-permeable Cl- channels (I(Cl,islet)) that are swelling-activated, but the role of these channels in the cells is unclear. The Cl- channel blockers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and niflumic acid were evaluated for their ability to inhibit I(Cl,islet) in clonal beta-cells (HIT cells). Both drugs blocked the channel, but the blockade due to niflumic acid was less voltage-dependent than the blockade due to DIDS. HIT cell volume initially increased in hypotonic solution and was followed by a regulatory volume decrease (RVD). The addition of niflumic acid and, to a lesser extent, DIDS to the hypotonic solution potentiated swelling and blocked the RVD. In isotonic solution, niflumic acid produced swelling, suggesting that islet Cl- channels are activated under basal conditions. The channel blockers glyburide, gadolinium, or tetraethylammonium-Cl did not alter hypotonic-induced swelling or volume regulation. The Na/K/2Cl transport blocker furosemide produced cell shrinkage in isotonic solution and blocked cell swelling normally induced by hypotonic solution. Perifused HIT cells secreted insulin when challenged with hypotonic solutions. However, this could not be completely attributed to I(Cl,islet)-mediated depolarization, because secretion persisted even when Cl- channels were fully blocked. To test whether blocker-resistant secretion occurred via a distal pathway, distal secretion was isolated using 50 mmol/l potassium and diazoxide. Under these conditions, glucose-dependent secretion was blunted, but hypotonically induced secretion persisted, even with Cl- channel blockers present. These results suggest that beta-cell swelling stimulates insulin secretion primarily via a distal I(Cl,islet)-independent mechanism, as has been proposed for K(ATP)-independent glucose- and sulfonylurea-stimulated insulin secretion. Reverse transcriptase-polymerase chain reaction of HIT cell mRNA identified a CLC-3 transcript in HIT cells. In other systems, CLC-3 is believed to mediate swelling-induced outwardly rectifying Cl- channels. This suggests that the proximal effects of swelling to regulate cell volume may be mediated by CLC-3 or a closely related Cl- channel.

ClC-2 chloride channels contribute to HTC cell volume homeostasis

Membrane Cl(-) channels play an important role in cell volume homeostasis and regulation of volume-sensitive cell transport and metabolism. Heterologous expression of ClC-2 channel cDNA leads to the appearance of swelling-activated Cl(-) currents, consistent with a role in cell volume regulation. Since channel properties in heterologous models are potentially modified by cellular background, we evaluated whether endogenous ClC-2 proteins are functionally important in cell volume regulation. As shown by whole cell patch clamp techniques in rat HTC hepatoma cells, cell volume increases stimulated inwardly rectifying Cl(-) currents when non-ClC-2 currents were blocked by DIDS (100 microM). A cDNA closely homologous with rat brain ClC-2 was isolated from HTC cells; identical sequence was demonstrated for ClC-2 cDNAs in primary rat hepatocytes and cholangiocytes. ClC-2 mRNA and membrane protein expression was demonstrated by in situ hybridization, immunocytochemistry, and Western blot. Intracellular delivery of antibodies to an essential regulatory domain of ClC-2 decreased ClC-2-dependent currents expressed in HEK-293 cells. In HTC cells, the same antibodies prevented activation of endogenous Cl(-) currents by cell volume increases or exposure to the purinergic receptor agonist ATP and delayed HTC cell volume recovery from swelling. These studies provide further evidence that mammalian ClC-2 channel proteins are functional and suggest that in HTC cells they contribute to physiological changes in membrane Cl(-) permeability and cell volume homeostasis.

Extracellular adenosine modulates a volume-sensitive-like chloride conductance in immortalized rabbit DC1 cells

Cl(-) currents induced by cell swelling were characterized in an immortalized cell line (DC1) derived from rabbit distal bright convoluted tubule by the whole cell patch-clamp techniques and by (125)I(-) efflux experiments. Exposure of cells to a hypotonic shock induced outwardly rectifying Cl(-) currents that could be blocked by 0.1 mM 5-nitro-2-(3-phenylpropyl-amino)benzoic acid, 1 mM DIDS, and by 1 mM diphenylamine-2-carboxylate. (125)I(-) efflux experiments showed that exposure of the monolayer to a hypotonic medium increased (125)I(-) loss. Preincubation of cells with LaCl(3) or GdCl(3) prevented the development of the response. The addition of 10 microM adenosine to the bath medium activated outwardly rectifying whole cell currents similar to those recorded after hypotonic shock. This conductance was inhibited by the A(1)-receptor antagonist 8-cyclopentyl-1,3-diproxylxanthine (DPCPX), LaCl(3), or GdCl(3) and was activated by GTPgammaS. The selective A(1)-receptor agonist N(6)-cyclopentyladenosine (CPA) mimicked the effect of hypotonicity on (125)I(-) efflux. The CPA-induced increase of (125)I(-) efflux was inhibited by DPCPX and external application of LaCl(3) or GdCl(3). Adenosine also enhanced Mn(2+) influx across the apical membrane. Overall, the data show that DC1 cells possess swelling- and adenosine-activated Cl(-) conductances that share identical characteristics. The activation of both conductances involved Ca(2+) entry into the cell, probably via mechanosensitive Ca(2+) channels. The effects of adenosine are mediated via A(1) receptors that could mediate the purinergic regulation of the volume-sensitive Cl(-) conductance.

Microvillar ion channels: cytoskeletal modulation of ion fluxes

The recently presented theory of microvillar Ca(2+)signaling [Lange, K. (1999) J. Cell. Physiol.180, 19-35], combined with Manning's theory of "condensed counterions" in linear polyelectrolytes [Manning, G. S. (1969). J. Chem. Phys.51, 924-931] and the finding of cable-like ion conductance in actin filaments [Lin, E. C. & Cantiello, H. F. (1993). Biophys. J.65, 1371-1378], allows a systematic interpretation of the role of the actin cytoskeleton in ion channel regulation. Ion conduction through actin filament bundles of microvilli exhibits unique nonlinear transmission properties some of which closely resemble that of electronic semiconductors: (1) bundles of microfilaments display significant resistance to cation conduction and (2) this resistance is decreased by supply of additional energy either as thermal, mechanical or electromagnetic field energy. Other transmission properties, however, are unique for ionic conduction in polyelectrolytes. (1) Current pulses injected into the filaments were transformed into oscillating currents or even into several discrete charge pulses closely resembling that of single-channel recordings. Discontinuous transmission is due to the existence of counterion clouds along the fixed anionic charge centers of the polymer, each acting as an "ionic capacitor". (2) The conductivity of linear polyelectrolytes strongly decreases with the charge number of the counterions; thus, Ca(2+)and Mg(2+)are effective modulator of charge transfer through linear polyelectrolytes. Field-dependent formation of divalent cation plugs on either side of the microvillar conduction line may generate the characteristic gating behavior of cation channels. (3) Mechanical movement of actin filament bundles, e.g. bending of hair cell microvilli, generates charge translocations along the filament structure (mechano-electrical coupling). (4) Energy of external fields, by inducing molecular dipoles within the polyelectrolyte matrix, can be transformed into mechanical movement of the system (electro-mechanical coupling). Because ionic transmission through linear polyelectrolytes is very slow compared with electronic conduction, only low-frequency electromagnetic fields can interact with the condensed counterion systems of linear polyelectrolytes. The delineated characteristics of microvillar ion conduction are strongly supported by the phenomenon of electro-mechanical coupling (reverse transduction) in microvilli of the audioreceptor (hair) cells and the recently reported dynamics of Ca(2+)signaling in microvilli of audio- and photoreceptor cells. Due to the cell-specific expression of different types and combinations of ion channels and transporters in the microvillar tip membrane of differentiated cells, the functional properties of this cell surface organelle are highly variable serving a multitude of different cellular functions including receptor-mediated effects such as Ca(2+)signaling, regulation of glucose and amino acid transport, as well as modulation of membrane potential. Even mechanical channel activation involved in cell volume regulation can be deduced from the systematic properties of the microvillar channel concept. In addition, the specific ion conduction properties of microfilaments combined with their proposed role in Ca(2+)signaling make microvilli the most likely cellular site for the interaction with external electric and magnetic fields.

Modulation of volume-sensitive Cl - channels and cell volume by actin filaments and microtubules in human cervical cancer HT-3 cells

Hypotonicity activates volume-sensitive Cl- currents, which are implicated in the regulatory volume decrease (RVD) responses and transport of taurine in human cervical cancer HT-3 cells. In this study, the role of cytoskeleton in the regulation of volume-sensitive Cl- channels and RVD responses in HT-3 cells was studied. Cells were incubated with various compounds, which depolymerized or polymerized cytoskeletal elements, i.e. actin filaments and microtubules. The hypotonicity-induced changes in Cl- conductance and in cell volume were measured by whole-cell voltage clamping and cell size monitoring, respectively. Our results show that in HT-3 cells hypotonicity activated an outward rectified Cl- current that was abrogated by Cl- channel blockers. Cytochalasin B, an actin-depolymerizing compound, induced a substantial increase in Cl- conductance under isotonic condition and potentiated the expression of Cl- currents in hypotonic stress. Phorbol 12-myristate 13-acetate (PMA) significantly inhibited the cytochalasin B-induced activation of Cl- conductance under isotonic condition. On the other hand, treatment with cytochalasin B significantly prolonged the RVD responses. Phalloidin, a stabilizer of actin polymerization, did not change the basal currents under isotonic condition, but completely abolished the increase in whole-cell Cl- conductance elicited by hypotonicity and retarded the cell volume recovery. Colchicine, a microtubule-assembly inhibitor, had no effect on either basal Cl- conductance or volume-sensitive Cl- current and was unable to inhibit the RVD responses. Taxol, a microtubule-stabilizing compound, did not alter the basal Cl- conductance, but inhibited the activation of volume-sensitive Cl- channels as well as the process of RVD in a dose-dependent manner. These data support the notion that functional integrity of actin filaments and microtubules plays critical roles in maintaining the RVD responses and activation of Cl- channels in human cervical cancer HT-3 cells.

Synergetic activation of outwardly rectifying Cl- currents by hypotonic stress and external Ca2+ in murine osteoclasts

1. An outwardly rectifying Cl- (ORCl) current of murine osteoclasts was activated by hypotonic stimulation. The current was characterized by rapid activation, little inactivation, strong outward rectification, blockage by DIDS and permeability to organic acids (pyruvate and glutamate). 2. The hypotonically activated ORCl current was inhibited by intracellular dialysis with an ATP-free pipette solution, but not by replacement of ATP with a poorly hydrolysable ATP analogue adenosine 5'-O-(3-thiotriphosphate). The current amplitude was reduced when intracellular alkalinity increased over the pH range 6.6-8.0. 3. Intracellular application of cytochalasin D occasionally activated the ORCl current without hypotonic stress, but inhibited activation of the ORCl current by hypotonic stimulation. The hypotonically activated ORCl current was unaffected by a non-actin-depolymerizing cytochalasin, chaetoglobosin C, but partially inhibited by deoxyribonuclease I. 4. Removal of extracellular Ca2+ inhibited activation of the ORCl current by hypotonic shock, but did not reduce the current once activated. The hypotonically activated ORCl current was partially decreased by intracellular dialysis with 20 mM EGTA. 5. With 10 mM Ca2+ in the extracellular medium, the ORCl current was activated in response to more minor decreases in osmolarity than with 1 mM Ca2+. The increased sensitivity to hypotonicity was mimicked by increasing the intracellular Ca2+ level (pCa 6.5). 6. These results suggest that hypotonic stimulation and a rise in the extracellular Ca2+ level synergistically activate the ORCl channel of murine osteoclasts, and that the activating process is modified by multiple intracellular factors (pH, ATP and actin cytoskeletal organization).

Regulation of KCa current by store-operated Ca2+ influx depends on internal Ca2+ release in HSG cells

This study examines the Ca2+ influx-dependent regulation of the Ca2+-activated K+ channel (KCa) in human submandibular gland (HSG) cells. Carbachol (CCh) induced sustained increases in the KCa current and cytosolic Ca2+ concentration ([Ca2+]i), which were prevented by loading cells with 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid (BAPTA). Removal of extracellular Ca2+ and addition of La3+ or Gd3+, but not Zn2+, inhibited the increases in KCa current and [Ca2+]i. Ca2+ influx during refill (i.e., addition of Ca2+ to cells treated with CCh and then atropine in Ca2+-free medium) failed to evoke increases in the KCa current but achieved internal Ca2+ store refill. When refill was prevented by thapsigargin, Ca2+ readdition induced rapid activation of KCa. These data provide further evidence that intracellular Ca2+ accumulation provides tight buffering of [Ca2+]i at the site of Ca2+ influx (H. Mogami, K. Nakano, A. V. Tepikin, and O. H. Petersen. Cell 88: 49-55, 1997). We suggest that the Ca2+ influx-dependent regulation of the sustained KCa current in CCh-stimulated HSG cells is mediated by the uptake of Ca2+ into the internal Ca2+ store and release via the inositol 1,4,5-trisphosphate-sensitive channel.

Swelling-induced arachidonic acid release via the 85-kDa cPLA2 in human neuroblastoma cells

Arachidonic acid or its metabolites have been implicated in the regulatory volume decrease (RVD) response after hypotonic cell swelling in some mammalian cells. The present study investigated the role of arachidonic acid (AA) during RVD in the human neuroblastoma cell line CHP-100. During the first nine minutes of hypo-osmotic exposure the rate of 3H-arachidonic acid (3H-AA) release increased to 250 +/- 19% (mean +/- SE, n = 22) as compared with cells under iso-osmotic conditions. This release was significantly inhibited after preincubation with AACOCF3, an inhibitor of the 85-kDa cytosolic phospholipase A2 (cPLA2). This indicates that a PLA2, most likely the 85-kDa cPLA2 is activated during cell swelling. In contrast, preincubation with U73122, an inhibitor of phospholipase C, did not affect the swelling-induced release of 3H-AA. Swelling-activated efflux of 36Cl and 3H-taurine were inhibited after preincubation with AACOCF3. Thus the swelling-induced activation of cPLA2 may be essential for stimulation of both 36Cl and 3H-taurine efflux during RVD. As the above observation could result from a direct effect of AA or its metabolite leukotriene D4 (LTD4), the effects of these agents were investigated on swelling-induced 36Cl and 3H-taurine effluxes. In the presence of high concentrations of extracellular AA, the swelling-induced efflux of 36Cl and 3H-taurine were inhibited significantly. In contrast, addition of exogenous LTD4 had no significant effect on the swelling-activated 36Cl efflux. Furthermore, exogenous AA increased cytosolic calcium levels as measured in single cells loaded with the calcium sensitive dye Fura-2. On the basis of these results we propose that cell swelling activates phospholipase A2 and that this activation via an increased production of AA or some AA metabolite(s) other than LTD4 is essential for RVD.

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.

Interferon-gamma-induced JAK2 and STAT1 signalling in a human salivary gland cell line

We have used a human salivary gland cell line (HSG) as a possible in vitro model to evaluate the effects of IFN-gamma on human salivary gland epithelium (Wu et al., 1994, 1996, 1997). In the present study, we examined the JAK-STAT signal-transduction pathway in IFN-gamma-treated HSG cells. We demonstrate that JAK2 and Stat1 are phosphorylated at tyrosine residues in a time- and concentration-dependent manner following exposure to IFN-gamma. In addition, we show that activation of this signalling pathway is decreased by the addition of a blocking antibody to the IFN-gamma receptor. The same maneuver is also able to reduce by approximately 50-70% the surface expression of two IFN-gamma-induced immunoregulatory molecules: HLA-DR and ICAM-1. These results demonstrate that the JAK2 and Stat1 signalling pathway is active in salivary-derived epithelial cells and may contribute to their immunopathologic destruction.

Synergism between hypotonically induced calcium release and fatty acyl-CoA esters induced calcium release from intracellular stores

The non-mitochondrial Ca2+ stores in permeabilized A7r5 cells responded to a decrease in Mg-ATP concentration with a pronounced Ca2+ release if 20 microM CoA was present. This release was rather specific for the preincubation or removal of ATP. ATP gamma S was much less effective and AMP-PNP, GTP, ITP, CTP, UTP, ADP, AMP, adenosine and adenine had no effect. CoA activated with an EC50 of 6 microM. Dephospho-CoA was a less effective cofactor and desulfo-CoA was ineffective. The release induced by Mg-ATP removal did not occur in the presence of 2% fatty acid-free bovine serum albumin and did not develop at 4 degrees C. All these findings suggest that CoA had to be acylated by endogenous fatty-acyl-CoA synthetase to become effective. Myristoyl- and palmitoyl-CoA esters were identified as the most effective cofactors for the release. Ca2+ release induced by removing Mg-ATP did not occur if the osmolality of the medium was kept constant by addition of mannitol, sucrose, KCl, MgCl2 or Mg-GTP, indicating that the decrease in tonicity was the trigger for the release. Mg-ATP plus CoA also synergized with Ca2+ release induced by a hypotonic shock imposed by diluting the medium with H2O. Osmolality changes induced by decreasing the Mg-ATP concentration were more effective in releasing Ca2+ than equal decreases in concentration of all solutes. We conclude that fatty acyl-CoA esters sensitize the hypotonically induced Ca2+ release from the non-mitochondrial Ca2+ stores.

A swelling-activated chloride current in rat sympathetic neurones

1. We have tested whether neurones show a swelling-induced Cl- current following hypotonic shock, by recording membrane current responses and cell volume changes in voltage clamped isolated rat sympathetic neurones during application of hypotonic solutions. 2. Using both whole-cell and perforated patch recording methods, hypotonic solution caused cell swelling and the activation of an inward Cl- current at -60 mV. This current showed weak outward rectification with no obvious time dependence. It was inhibited by SITS (0.3-1 mM), NPPB (30-300 microM) and niflumic acid (50-200 microM), but not by tamoxifen (10 microM). 3. Hypotonic solution did not cause a rise in intracellular Ca2+ concentration as measured by simultaneous indo-1 fluorescence. Also, neither the volume change nor Cl- current were affected by the removal of external Ca2+ or internal Ca2+ buffering to < or = 1 nM with EGTA. 4. The Cl- current was unaffected by an inhibitor of protein kinase C (PKC; GF109203X, 3 microM) or by omission of ATP from the pipette solution. 5. Cells exhibited a regulatory volume decrease during sustained exposure to hypotonic solution. This was completely inhibited by 0.5 mM niflumic acid. 6. It is concluded that osmotic swelling induces an outwardly rectifying, Ca2(+)- and PKC-independent Cl- current in these nerve cells. It is suggested that this current may be involved in volume regulatory mechanisms.

Excitatory actions of GABA after neuronal trauma

GABA is the dominant inhibitory neurotransmitter in the CNS. By opening Cl- channels, GABA generally hyperpolarizes the membrane potential, decreases neuronal activity, and reduces intracellular Ca2+ of mature neurons. In the present experiment, we show that after neuronal trauma, GABA, both synaptically released and exogenously applied, exerted a novel and opposite effect, depolarizing neurons and increasing intracellular Ca2+. Different types of trauma that were effective included neurite transection, replating, osmotic imbalance, and excess heat. The depolarizing actions of GABA after trauma increased Ca2+ levels up to fourfold in some neurons, occurred in more than half of the severely injured neurons, and was long lasting (>1 week). The mechanism for the reversed action of GABA appears to be a depolarized Cl- reversal potential that results in outward rather than inward movement of Cl-, as revealed by gramicidin-perforated whole-cell patch-clamp recording. The consequent depolarization and resultant activation of the nimodipine sensitive L- and conotoxin-sensitive N-type voltage-activated Ca2+ channel allows extracellular Ca2+ to enter the neuron. The long-lasting capacity to raise Ca2+ may give GABA a greater role during recovery from trauma in modulating gene expression, and directing and enhancing outgrowth of regenerating neurites. On the negative side, by its depolarizing actions, GABA could increase neuronal damage by raising cytosolic Ca2+ levels in injured cells. Furthermore, the excitatory actions of GABA after neuronal injury may contribute to maladaptive signal transmission in affected GABAergic brain circuits.

Autocrine signaling through ATP release represents a novel mechanism for cell volume regulation

Recovery of cell volume in response to osmotic stress is mediated in part by increases in the Cl- permeability of the plasma membrane. These studies evaluate the hypothesis that ATP release and autocrine stimulation of purinergic (P2) receptors couple increases in cell volume to opening of Cl- channels. In HTC rat hepatoma cells, swelling induced by hypotonic exposure increased membrane Cl- current density to 44.8 +/- 7.1 pA/pF at -80 mV. Both the rate of volume recovery and the increase in Cl- permeability were inhibited in the presence of the ATP hydrolase apyrase (3 units/ml) or by exposure to the P2 receptor blockers suramin and Reactive Blue 2 (10-100 microM). Cell swelling also stimulated release of ATP. Hypotonic exposure increased the concentration of ATP in the effluent of perfused cells by 170 +/- 36 nM in the presence of a nucleotidase inhibitor (P < 0.01). In whole-cell recordings with ATP as the charge carrier, cell swelling increased membrane current density approximately 30-fold to 16.5 +/- 10.4 pA/pF. These findings indicate that increases in cell volume lead to efflux of ATP through opening of a conductive pathway consistent with a channel, and that extracellular ATP is required for recovery from swelling. ATP may function as an autocrine factor that couples increases in cell volume to opening of Cl- channels through stimulation of P2 receptors.

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.

Mycoplasma orale infection affects K+ and Cl- currents in the HSG salivary gland cell line

The relations between K+ channel and Cl- channel currents and mycoplasma infection status were studied longitudinally in HSG cells, a human submandibular gland cell line. The K+ channel currents were disrupted by the occurrence of mycoplasma infection: muscarinic activation of K+ channels and K+ channel expression as estimated by ionomycin- or hypotonically induced K+ current responses were all decreased. Similar decreases in ionomycin- and hypotonically induced responses were observed for Cl- channels, but only the latter decrease was statistically significant. Also, Cl- currents could be elicited more frequently than K+ currents (63% of cases versus 0%) in infected cells when tested by exposure to hypotonic media, indicating that mycoplasma infection affects K+ channels relatively more than Cl- channels. These changes occurred in the originally infected cells, were ameliorated when the infection was cleared with sparfloxacin, and recurred when the cells were reinfected. Such changes would be expected to result in hyposecretion of salivary fluid if they occurred in vivo.

Hypotonically induced calcium release from intracellular calcium stores

Osmotic cell swelling induced by hypotonic stress is associated with a rise in intracellular Ca2+ concentration, which is at least partly due to a release of Ca2+ from internal stores. Since osmotic influx of water dilutes the cytoplasmic milieu, we have investigated how nonmitochondrial Ca2+ stores in permeabilized A7r5 cells respond to a reduction in cytoplasmic tonicity. We now present experimental evidence for a direct Ca2+ release from the stores when exposed to a hypotonic medium. The release is graded, but does not occur through the inositol trisphosphate or the ryanodine receptor. Ca2+ seems to be released through the passive leak pathway, and this phenomenon can be partially inhibited by divalent cations in the following order of potency: Ni2+ = Co2+ > Mn2+ > Mg2+ > Ba2+. This release also occurs in intact A7r5 cells. This novel mechanism of hypotonically induced Ca2+ release is therefore an inherent property of the stores, which can occur in the absence of second messengers. Intracellular stores can therefore act as osmosensors.