Hyposmotic activation of ICl,swell in rabbit nonpigmented ciliary epithelial cells involves increased ClC-3 trafficking to the plasma membrane

Endocytosis and exocytosis protect cells against severe membrane tension variations

The ability of cells to regulate their shape and volume is critical for many cell functions. How endocytosis and exocytosis, as important ways of membrane trafficking, affect cellular volume regulation is still unclear. Here, we develop a theoretical framework to study the dynamics of cell volume, endocytosis, and exocytosis in response to osmotic shocks and mechanical loadings. This model can not only explain observed dynamics of endocytosis and exocytosis during osmotic shocks but also predict the dynamics of endocytosis and exocytosis during cell compressions. We find that a hypotonic shock stimulates exocytosis, while a hypertonic shock stimulates endocytosis; and exocytosis in turn allows cells to have a dramatic change in cell volume but a small change in membrane tension during hyposmotic swelling, protecting cells from rupture under high tension. In addition, we find that cell compressions with various loading speeds induce three distinct dynamic modes of endocytosis and exocytosis. Finally, we show that increasing endocytosis and exocytosis rates reduce the changes in cell volume and membrane tension under fast cell compression, whereas they enhance the changes in cell volume and membrane tension under slow cell compression. Together, our findings reveal critical roles of endocytosis and exocytosis in regulating cell volume and membrane tension.

Search for Upstream Cell Volume Sensors: The Role of Plasma Membrane and Cytoplasmic Hydrogel

The plasma membrane plays a prominent role in the regulation of cell volume by mediating selective transport of extra- and intracellular osmolytes. Recent studies show that upstream sensors of cell volume changes are mainly located within the cytoplasm that displays properties of a hydrogel and not in the plasma membrane. Cell volume changes occurring in anisosmotic medium as well as in isosmotic environment affect properties of cytoplasmic hydrogel that, in turn, trigger rapid regulatory volume increase and decrease (RVI and RVD). The downstream signaling pathways include reorganization of 2D cytoskeleton and altered composition of polyphosphoinositides located on the inner surface of the plasma membrane. In addition to its action on physico-chemical properties of cytoplasmic hydrogel, cell volume changes in anisosmotic conditions affect the ionic strength of the cytoplasm and the [Na⁺]_i/[K⁺]_i ratio. Elevated intracellular ionic strength evoked by long term exposure of cells to hypertonic environment resulted in the activation of TonEBP and augmented expression of genes controlling intracellular organic osmolyte levels. The role of Na⁺_i/K⁺_i -sensitive, Ca²⁺_i -mediated and Ca²⁺_i-independent mechanisms of excitation-transcription coupling in cell volume-adjustment remains unknown.

Cell volume changes regulate slick (Slo2.1), but not slack (Slo2.2) K+ channels

Slick (Slo2.1) and Slack (Slo2.2) channels belong to the family of high-conductance K⁺ channels and have been found widely distributed in the CNS. Both channels are activated by Na⁺ and Cl⁻ and, in addition, Slick channels are regulated by ATP. Therefore, the roles of these channels in regulation of cell excitability as well as ion transport processes, like regulation of cell volume, have been hypothesized. It is the aim of this work to evaluate the sensitivity of Slick and Slack channels to small, fast changes in cell volume and to explore mechanisms, which may explain this type of regulation. For this purpose Slick and Slack channels were co-expressed with aquaporin 1 in Xenopus laevis oocytes and cell volume changes of around 5% were induced by exposure to hypotonic or hypertonic media. Whole-cell currents were measured by two electrode voltage clamp. Our results show that Slick channels are dramatically stimulated (196% of control) by cell swelling and inhibited (57% of control) by a decrease in cell volume. In contrast, Slack channels are totally insensitive to similar cell volume changes. The mechanism underlining the strong volume sensitivity of Slick channels needs to be further explored, however we were able to show that it does not depend on an intact actin cytoskeleton, ATP release or vesicle fusion. In conclusion, Slick channels, in contrast to the similar Slack channels, are the only high-conductance K⁺ channels strongly sensitive to small changes in cell volume.

The ClC-3 chloride channel and osmoregulation in the European sea bass, Dicentrarchus labrax

Dicentrarchus labrax migrates between sea (SW), brackish and fresh water (FW) where chloride concentrations and requirements for chloride handling change: in FW, fish absorb chloride and restrict renal losses; in SW, they excrete chloride. In this study, the expression and localization of ClC-3 and Na(+)/K(+)-ATPase (NKA) were studied in fish adapted to SW, or exposed to FW from 10 min to 30 days. In gills, NKA-α1 subunit expression transiently increased from 10 min and reached a stabilized intermediate expression level after 24 h in FW. ClC-3 co-localized with NKA in the basolateral membrane of mitochondria-rich cells (MRCs) at all conditions. The intensity of MRC ClC-3 immunostaining was significantly higher (by 50 %) 1 h after the transfer to FW, whereas the branchial ClC-3 protein expression was 30 % higher 7 days after the transfer as compared to SW. This is consistent with the increased number of immunopositive MRCs (immunostained for NKA and ClC-3). However, the ClC-3 mRNA expression was significantly lower in FW gills. In the kidney, after FW transfer, a transient decrease in NKA-α1 subunit expression was followed by significantly higher stable levels from 24 h. The low ClC-3 protein expression detected at both salinities was not observed by immunocytochemistry in the SW kidney; ClC-3 was localized in the basal membrane of the collecting ducts and tubules 7 and 30 days after transfer to FW. Renal ClC-3 mRNA expression, however, seemed higher in SW than in FW. The potential role of this chloride channel ClC-3 in osmoregulatory and osmosensing mechanisms is discussed.

Mechanisms of ATP release, the enabling step in purinergic dynamics

The only effective intervention to slow onset and progression of glaucomatous blindness is to lower intraocular pressure (IOP). Among other modulators, adenosine receptors (ARs) exert complex regulation of IOP. Agonists of A(3)ARs in the ciliary epithelium activate Cl(-) channels, favoring increased formation of aqueous humor and elevated IOP. In contrast, stimulating A(1)ARs in the trabecular outflow pathway enhances release of matrix metalloproteinases (MMPs) from trabecular meshwork (TM) cells, reducing resistance to outflow of aqueous humor to lower IOP. These opposing actions are thought to be initiated by cellular release of ATP and its ectoenzymatic conversion to adenosine. This view is now supported by our identification of six ectoATPases in trabecular meshwork (TM) cells and by our observation that external ATP enhances TM-cell secretion of MMPs through ectoenzymatic formation of adenosine. ATP release is enhanced by cell swelling and stretch. Also, enhanced ATP release and downstream MMP secretion is one mediator of the action of actin depolymerization to reduce outflow resistance. Inflow and outflow cells share pannexin-1 and connexin hemichannel pathways for ATP release. However, vesicular release and P2X(7) release pathways were functionally limited to inflow and outflow cells, respectively, suggesting that blocking exocytosis might selectively inhibit inflow, lowering IOP.

Immunolocalization of chloride transporters to gill epithelia of euryhaline teleosts with opposite salinity-induced Na+/K+-ATPase responses

Opposite patterns of branchial Na(+)/K(+)-ATPase (NKA) responses were found in euryhaline milkfish (Chanos chanos) and pufferfish (Tetraodon nigroviridis) upon salinity challenge. Because the electrochemical gradient established by NKA is thought to be the driving force for transcellular Cl(-) transport in fish gills, the aim of this study was to explore whether the differential patterns of NKA responses found in milkfish and pufferfish would lead to distinct distribution of Cl(-) transporters in their gill epithelial cells indicating different Cl(-) transport mechanisms. In this study, immunolocalization of various Cl(-) transport proteins, including Na(+)/K(+)/2Cl(-) cotransporter (NKCC), cystic fibrosis transmembrane conductance regulator (CFTR), anion exchanger 1 (AE1), and chloride channel 3 (ClC-3), were double stained with NKA, the basolateral marker of branchial mitochondrion-rich cells (MRCs), to reveal the localization of these transporter proteins in gill MRC of FW- or SW-acclimated milkfish and pufferfish. Confocal microscopic observations showed that the localization of these transport proteins in the gill MRCs of the two studied species were similar. However, the number of gill NKA-immunoreactive (IR) cells in milkfish and pufferfish exhibited to vary with environmental salinities. An increase in the number of NKA-IR cells should lead to the elevation of NKA activity in FW milkfish and SW pufferfish. Taken together, the opposite branchial NKA responses observed in milkfish and pufferfish upon salinity challenge could be attributed to alterations in the number of NKA-IR cells. Furthermore, the localization of these Cl(-) transporters in gill MRCs of the two studied species was identical. It depicted the two studied euryhaline species possess the similar Cl(-) transport mechanisms in gills.

Ion channels in pediatric CNS Atypical Teratoid/Rhabdoid Tumor (AT/RT) cells: potential targets for novel therapeutic agents

The central nervous system Atypical Teratoid/Rhabdoid Tumor (CNS AT/RT) is a highly malignant neoplasm that commonly affects infants and young children, and has an extremely poor prognosis. Recently, a small subset of ion channels have been found to be over-expressed in a variety of malignant cells, thus emerging as potential therapeutic targets for difficult to treat tumors. We have studied the electrophysiological properties of AT/RT cell lines with particular attention to cell volume sensitive ion channels (VSC). This class of membrane proteins can play a fundamental role in cellular processes relevant to tumor development. We have found that chloride selective VSCs are particularly active in AT/RT cell lines, compared to non-tumor cells. We evaluated specific inhibitors for activity against chloride selective VSCs and consequently for their ability to inhibit the growth and survival of AT/RT cells in vitro. The results demonstrated that the extent of volume sensitive membrane current inhibition by these agents was correlated with their potency in AT/RT cell growth inhibition in vitro. In addition, we showed that ion channel inhibition enhanced the activity of certain anti-neoplastic agents, suggesting its value in effective drug combination protocols. Results presented provide preliminary in vitro data for possible evaluation of distinct ion channels as plausible therapeutic targets in the treatment of AT/RT.

The ClC-3 chloride channels in cardiovascular disease

ClC-3 is a member of the ClC voltage-gated chloride (Cl(-)) channel superfamily. Recent studies have demonstrated the abundant expression and pleiotropy of ClC-3 in cardiac atrial and ventricular myocytes, vascular smooth muscle cells, and endothelial cells. ClC-3 Cl(-) channels can be activated by increase in cell volume, direct stretch of β1-integrin through focal adhesion kinase and many active molecules or growth factors including angiotensin II and endothelin-1-mediated signaling pathways, Ca(2+)/calmodulin-dependent protein kinase II and reactive oxygen species. ClC-3 may function as a key component of the volume-regulated Cl(-) channels, a superoxide anion transport and/or NADPH oxidase interaction partner, and a regulator of many other transporters. ClC-3 has been implicated in the regulation of electrical activity, cell volume, proliferation, differentiation, migration, apoptosis and intracellular pH. This review will highlight the major findings and recent advances in the study of ClC-3 Cl(-) channels in the cardiovascular system and discuss their important roles in cardiac and vascular remodeling during hypertension, myocardial hypertrophy, ischemia/reperfusion, and heart failure.

alpha-Adrenoceptor-mediated depletion of phosphatidylinositol 4, 5-bisphosphate inhibits activation of volume-regulated anion channels in mouse ventricular myocytes

BACKGROUND AND PURPOSE

Volume-regulated anion channels (VRACs) play an important role in cell-volume regulation. alpha(1)-Adrenoceptor stimulation by phenylephrine (PE) suppressed the hypotonic activation of VRAC current in mouse ventricular cells and regulatory volume decrease (RVD) was also absent in PE-treated cells. We examined whether the effects of alpha(1)-adrenoceptor stimuli on VRAC current were modulated by phosphatidylinositol signalling.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp method was used to record the hypotonicity-induced VRAC current in mouse ventricular cells. RVD was analyzed by videomicroscopic measurement of cell images.

KEY RESULTS

The attenuation of VRAC current by PE was suppressed by alpha(1A)-adrenoceptor antagonists (prazosin and WB-4101), anti-G(q) protein antibody and a specific phosphoinositide-specific phospholipase C (PLC) inhibitor (U-73122), but not by antagonists for alpha(1B)-, alpha(1D)- or beta-adrenoceptor, or protein kinase C inhibitors. The inhibition of VRAC by PE was antagonized by intracellular excess phosphatidylinositol 4,5-bisphosphate (PIP(2)), while intracellular anti-PIP(2) antibody (PIP(2) Ab) inhibited the activation of VRAC currents. When cells were loaded with phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) with or without PIP(2) Ab, PE little affected the VRAC current. Extracellular m-3M3FBS (an activator of PLC) suppressed VRAC in the absence of PE, and this effect was reversed by intracellular excess PIP(2).

CONCLUSIONS AND IMPLICATIONS

Our results indicate that the stimulation of alpha(1A)-adrenoceptors by PE inhibited the activation of cardiac VRAC current via PIP(3) depletion brought about by PLC-dependent reduction of membrane PIP(2) level.

Chloride channel ClC-3 in gills of the euryhaline teleost, Tetraodon nigroviridis: expression, localization and the possible role of chloride absorption

Previous studies have reported the mechanisms of ion absorption and secretion by diverse membrane transport proteins in gills of various teleostean species. To date, however, the chloride channel expressed in the basolateral membrane of mitochondrion-rich (MR) cells for Cl(-) uptake in freshwater (FW) fish is still unknown. In this study, the combination of bioinformatics tools [i.e. National Center for Biotechnology Information (NCBI) database, Tetraodon nigroviridis (spotted green pufferfish) genome database (Genoscope), BLAT and BLASTn] were used to identify the gene of ClC-3 (TnClC-3), a member of the CLC chloride channel family in the T. nigroviridis genome. RT-PCR analysis revealed that the gene encoding for the ClC-3 protein was widely expressed in diverse tissues (i.e. gill, kidney, intestine, liver and brain) of FW- and seawater (SW)-acclimated pufferfish. In whole-mount double immunofluorescent staining, branchial ClC-3-like immunoreactive protein was localized to the basolateral membrane of Na(+)/K(+)-ATPase (NKA) immunoreactive cells in both the FW- and SW-acclimated pufferfish. In response to salinity, the levels of transcript of branchial TnClC-3 were similar between FW and SW fish. Moreover, the membrane fraction of ClC-3-like protein in gills was 2.7-fold higher in FW compared with SW pufferfish. To identify whether the expression of branchial ClC-3-like protein specifically responded to lower environmental [Cl(-)], the pufferfish were acclimated to artificial waters either with a normal (control) or lower Cl(-) concentration (low-Cl). Immunoblotting of membrane fractions of gill ClC-3-like protein showed the expression was about 4.3-fold higher in pufferfish acclimated to the low-Cl environment than in the control group. Furthermore, branchial ClC-3-like protein was rapidly elevated in response to acute changes of environmental salinity or [Cl(-)]. Taken together, pufferfish ClC-3-like protein was expressed in the basolateral membrane of gill MR cells, and the protein amounts were stimulated by hyposmotic and low-Cl environments. The enhancement of ClC-3-like protein may trigger the step of basolateral Cl(-) absorption of the epithelium to carry out iono- and osmoregulatory functions of euryhaline pufferfish gills.

CLC-3 chloride channels in the pulmonary vasculature

Volume-sensitive outwardly rectifying anion channels (VSOACs) are expressed in pulmonary artery smooth muscle cells (PASMCs) and have been implicated in cell proliferation, growth, apoptosis and protection against oxidative stress. In this chapter, we review the properties of native VSOACs in PASMCs, and consider the evidence that ClC-3, a member of the ClC superfamily of voltage dependent Cl- channels, may be responsible for native VSOACs in PASMCs. Finally, we examine whether or not native VSOACs and heterologously expressed ClC-3 channels function as bona fide chloride channels or as chloride/proton antiporters.

Molecular interaction and functional regulation of ClC-3 by Ca2+/calmodulin-dependent protein kinase II (CaMKII) in human malignant glioma

Glioblastoma multiforme is the most common and lethal primary brain cancer in adults. Tumor cells diffusely infiltrate the brain making focal surgical and radiation treatment challenging. The invasion of glioma cells into normal brain is facilitated by the activity of ion channels aiding dynamic regulation of cell volume. Recent studies have specifically implicated ClC-3, a voltage-gated chloride channel, in this process. However, the interaction between ClC-3 activity and cell movement is poorly understood. Here, we demonstrate that ClC-3 is highly expressed on the plasma membrane of human glioma cells where its activity is regulated through phosphorylation via Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Intracellular infusion of autoactivated CaMKII via patch pipette enhanced chloride currents 3-fold, and this regulation was inhibited by autocamtide-2 related inhibitory peptide, a CaMKII-specific inhibitor. CaMKII modulation of chloride currents was also lost upon stable small hairpin RNA knockdown of ClC-3 channels indicating a specific interaction of ClC-3 and CaMKII. In ClC-3-expressing cells, inhibition of CaMKII reduced glioma invasion to the same extent as direct inhibition of ClC-3. The importance of the molecular interaction of ClC-3 and CaMKII is further supported by our finding that CaMKII co-localizes and co-immunoprecipitates with ClC-3. ClC-3 and CaMKII also co-immunoprecipitate in tissue biopsies from patients diagnosed with grade IV glioblastoma. These tumor samples show 10-fold higher ClC-3 protein expression than nonmalignant brain. These data suggest that CaMKII is a molecular link translating intracellular calcium changes, which are intrinsically associated with glioma migration, to changes in ClC-3 conductance required for cell movement.

Cardiac-specific, inducible ClC-3 gene deletion eliminates native volume-sensitive chloride channels and produces myocardial hypertrophy in adult mice

Native volume-sensitive outwardly rectifying anion channels (VSOACs) play a significant role in cell volume homeostasis in mammalian cells. However, the molecular correlate of VSOACs has been elusive to identify. The short isoform of ClC-3 (sClC-3) is a member of the mammalian ClC gene family and has been proposed to be a molecular candidate for VSOACs in cardiac myocytes and vascular smooth muscle cells. To directly test this hypothesis, and assess the physiological role of ClC-3 in cardiac function, we generated a novel line of cardiac-specific inducible ClC-3 knock-out mice. These transgenic mice were maintained on a doxycycline diet to preserve ClC-3 expression; removal of doxycycline activates Cre recombinase to inactivate the Clcn3 gene. Echocardiography revealed dramatically reduced ejection fraction and fractional shortening, and severe signs of myocardial hypertrophy and heart failure in the knock-out mice at both 1.5 and 3 weeks off doxycycline. In mice off doxycycline, time-dependent inactivation of ClC-3 gene expression was confirmed in atrial and ventricular cells by qRT-PCR and Western blot analysis. Electrophysiological examination of native VSOACs in isolated atrial and ventricular myocytes 3 weeks off doxycycline revealed a complete elimination of the currents, whereas at 1.5 weeks, VSOAC current densities were significantly reduced, compared to age-matched control mice maintained on doxycycline. These results indicate that ClC-3 is a key component of native VSOACs in mammalian heart and plays a significant cardioprotective role against cardiac hypertrophy and failure.

CARDIAC-SPECIFIC OVEREXPRESSION OF THE HUMAN SHORT CLC-3 CHLORIDE CHANNEL ISOFORM IN MICE

1. ClC-3 has been proposed as a molecular candidate responsible for volume-sensitive outwardly rectifying anion channels (VSOAC) in cardiac and smooth muscle cells. To further test this hypothesis, we produced a novel line of transgenic mice with cardiac-specific overexpression of the human short ClC-3 isoform (hsClC-3). 2. Northern and western blot analyses demonstrated that mRNA and protein levels of the short isoform (sClC-3) in the heart were significantly increased in hsClC-3-overexpressing (OE) mice compared with wild-type (WT) mice. Heart weight : bodyweight ratios for OE mice were significantly smaller compared with age-matched WT mice. 3. Electrocardiogram recordings indicated no difference at rest, whereas echocardiographic recordings revealed consistent reductions in left ventricular diastolic diameter, left ventricular posterior wall thickness at end of diastole and interventricular septum thickness in diastole in OE mice. 4. The VSOAC current densities in atrial cardiomyocytes were significantly increased by ClC-3 overexpression compared with WT cells. No differences in VSOAC current properties in OE and WT atrial myocytes were observed in terms of outward rectification, anion permeability (I(-) > Cl(-) > Asp(-)) and inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid and glibenclamide. The VSOAC in atrial myocytes from both groups were totally abolished by phorbol-12,13-dibutyrate (a protein kinase C activator) and by intracellular dialysis of an N-terminal anti-ClC-3 antibody. 5. Cardiac cell volume measurements revealed a significant acceleration of the rate of regulatory volume decrease (RVD) in OE myocytes compared with WT. 6. In conclusion, enhanced VSOAC currents and acceleration of the time-course of RVD in atrial myocytes of OE mice is strong evidence supporting an essential role of sClC-3 in native VSOAC function in mouse atrial 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.

Species variation in biology and physiology of the ciliary epithelium: similarities and differences

Glaucoma is a leading cause of irreversible blindness worldwide. Lowering intraocular pressure (IOP) is the only strategy documented to delay the appearance and retard the progression of vision loss. One major approach for lowering IOP is to slow the rate of aqueous humor formation by the ciliary epithelium. As discussed in the present review, the transport basis for this secretion is largely understood. However, several substantive issues are yet to be resolved, including the integrated regulation of secretion, the functional topography of the ciliary epithelium, and the degree and significance of species variation in aqueous humor inflow. This review discusses species differences in net secretion, particularly of Cl(-) and HCO(3)(-) secretion. Identifying animal models most accurately mimicking aqueous humor formation in the human will facilitate development of future novel initiatives to lower IOP.

ClC-3 is required for LPA-activated Cl− current activity and fibroblast-to-myofibroblast differentiation

To determine the effects of chloride channel 3 (ClC-3) knockdown and overexpression on lysophosphatidic acid (LPA)- and volume-regulated anion channel Cl− currents ( ICl,LPA and ICl,VRAC, respectively), cell differentiation, and cell volume regulation, a short hairpin RNA (shRNA) expression system based on a mouse U6 promoter was used to knock down ClC-3 in human corneal keratocytes and human fetal lung fibroblasts. ClC-3 overexpression was achieved by electroporating full-length ClC-3, within a pcDNA3.1 vector, into these two cell lines. RT-PCR and Western blot analysis were used to detect ClC-3 mRNA and protein levels. Whole cell perforated patch-clamp recording was used to measure ICl,LPA and ICl,VRAC currents, and fluorescence-activated cell sorting analysis was used to measure cell volume regulation. ClC-3 knockdown significantly decreased ICl,LPA and ICl,VRAC activity in the presence of transforming growth factor-β1 (TGF-β1) compared with controls, whereas ClC-3 overexpression resulted in increased ICl,LPA activity in the absence of TGF-β1. ClC-3 knockdown also resulted in a reduction of α-smooth muscle actin (α-SMA) protein levels in the presence of TGF-β1, whereas ClC-3 overexpression increased α-SMA protein expression in the absence of TGF-β1. In addition, keratocytes transfected with ClC-3 shRNA had a significantly blunted regulatory volume decrease response following hyposmotic stimulation compared with controls. These data confirm that ClC-3 is important in VRAC function and cell volume regulation, is associated with the ICl,LPA current activity, and participates in the fibroblast-to-myofibroblast transition.

Hypotonic activation of short ClC3 isoform is modulated by direct interaction between its cytosolic C-terminal tail and subcortical actin filaments

Short ClC3 isoform (sClC3) functions as a volume-sensitive outwardly rectifying anion channel (VSOAC) in some cell types. In previous studies, we have shown that the hypotonic activation of sClC3 is linked to cell swelling-mediated remodeling of the actin cytoskeleton. In the present study, we have tested the hypothesis that the cytosolic tails of sClC3 bind to actin directly and that binding modulates the hypotonic activation of the channel. Co-sedimentation assays in vitro demonstrated a strong binding between the glutathione S-transferase-fused cytosolic C terminus of sClC3 (GST-sClC3-CT) to filamentous actin (F-actin) but not to globular monomeric actin (G-actin). The GST-fused N terminus (GST-sClC3-NT) exhibited low binding affinity to both G- and F-actin. Co-sedimentation experiments with progressively truncated GST-sClC3-CT indicated that the F-actin binding region is located between amino acids 690 and 760 of sClC3. Two synthetic peptides mapping basic clusters of the cytosolic sClC3-CT (CTP2, isoleucine 716 to leucine 734; and CTP3, proline 688 to proline 709) prevented binding of GST-sClC3-CT to F-actin in vitro. Dialysis into NIH/3T3 cells of these two peptides (but not of synthetic peptide CTP1 (isoleucine 737 to glutamine 748)) reduced the maximal current density by 60 and 38%, respectively. Based on these results, we have concluded that, by direct interaction with subcortical actin filaments, sClC3 contributes to the hypotonic stress-induced VSOACs in NIH/3T3 cells.

The ClC-3 Cl− channel in cell volume regulation, proliferation and apoptosis in vascular smooth muscle cells

The volume-regulated Cl(-) current (I(Cl.vol)) is responsible for the transmembrane Cl(-) transport that is involved in cell volume regulatory mechanisms. Although the regulation of cell volume is a fundamental function of healthy cells for maintaining constant size, the molecular genetic identification of I(Cl.vol) is still being debated. Recent studies in vascular smooth muscle support the idea that ClC-3, a member of the voltage-gated ClC Cl(-) channel family, is the molecular component involved in the activation or regulation of I(Cl.vol). Moreover, gene-targeting studies in vascular smooth muscle cells (VSMCs) and other cell types indicate emerging roles of ClC-3 in cell proliferation and apoptosis. These findings indicate that ClC-3 might be involved in modulating vascular remodeling in hypertension and arteriosclerosis.

Effects of hyposmotic stress on exocytosis in isolated turbot, Scophthalmus maximus, hepatocytes

The effect of hyposmotic shock on exocytosis was examined in isolated hepatocytes of turbot, a marine flatfish, using the molecular probe FM1-43. Sudden exposure to a reduced osmolality caused an increase in cell exocytic activity related to the osmotic gradient between intra- and extracellular fluids. Cytoskeletal microtubules could contribute to this hyposmotic-induced exocytosis since colchicine inhibited the process. Protein kinase C, phosphatidylinositol-3 kinase, phospholipases A2, C and D could constitute key enzymes in the mechanism since their inhibition by specific agents altered the hyposmotic-induced exocytic activity. Moreover, arachidonic acid and derivates from the 5-lipoxygenase pathway as well as calcium could participate in the process. As regulatory volume decrease (RVD) exhibited by turbot hepatocytes following hyposmotic stimulation involves similar features, a potential role of exocytosis in volume regulation is suggested. In particular, exocytosis could serve RVD by contributing to ATP release since this latter process similarly appeared to be phospholipase D-dependent and related to the osmotic gradient. This study provides the first evidence of a volume-sensitive exocytosis that could aim at volume constancy in a marine teleost fish cell type.

Expression of the chloride channel CLC-K in human airway epithelial cells

Airway submucosal gland function is severely disrupted in cystic fibrosis (CF), as a result of genetic mutation of the cystic fibrosis transmembrane conductance regulator (CFTR), an apical membrane Cl(-) channel. To identify other Cl(-) channel types that could potentially substitute for lost CFTR function in these cells, we investigated the functional and molecular expression of Cl(-) channels in Calu-3 cells, a human cell line model of the submucosal gland serous cell. Whole cell patch clamp recording from these cells identified outwardly rectified, pH- and calcium-sensitive Cl(-) currents that resemble those previously ascribed to ClC-K type chloride channels. Using reverse transcription polymerase chain reaction, we identified expression of mRNA for ClC-2, ClC-3, ClC-4, ClC-5, ClC-6, ClC-7, ClC-Ka, and ClC-Kb, as well as the common ClC-K channel beta subunit barttin. Western blotting confirmed that Calu-3 cells express both ClC-K and barttin protein. Thus, Calu-3 cells express multiple members of the ClC family of Cl(-) channels that, if also expressed in native submucosal gland serous cells within the CF lung, could perhaps act to partially substitute lost CFTR function. Furthermore, this work represents the first evidence for functional ClC-K chloride channel expression within the lung.

Functional role of amino terminus in ClC-3 chloride channel regulation by phosphorylation and cell volume

AIM

This study investigated the functional role of the ClC-3 amino-terminus in channel regulation in response to changes in cell volume.

METHODS

Wild-type sClC-3 tagged with a green fluorescence protein (GFP) at the C-terminus was used as a template to construct a number of deletion mutants which were functionally expressed in NIH-3T3 cells. Whole cell and single channel patch-clamp electrophysiology was used to determine the functional properties of heterologously expressed channels.

RESULTS

The first 100 amino acids of the ClC-3 N-terminus were removed and the truncated channel (sClC-3DeltaNT) was functionally expressed. Immunocytochemistry confirmed membrane expression of both wtsClC-3 and sClC-3DeltaNT channels in NIH/3T3 cells. sClC-3DeltaNT yielded constitutively active functional channels, which showed no response to protein kinase C or changes in cell volume. Deletion of a cluster of negatively charged amino acids 16-21 (sClC-3Delta16-21) within the N-terminus also yielded a constitutively active open channel phenotype, indicating these amino acids are involved in the N-type regulation. Intracellular delivery of a thiol-phosphorylated peptide corresponding to N-terminal residues 12-61 (NT peptide) markedly inhibited sClC-3DeltaNT whole-cell and single-channel currents, further confirming the essential role of the N-terminus in volume regulation of channel activity.

CONCLUSIONS

These data strongly suggest the N-terminus of sClC-3 channels acts as a blocking particle inhibiting the flow of anions through the channel pore. This 'N-type' regulation of sClC-3 channels may be an important transducing mechanism linking changes in cell volume and channel protein phosphorylation to channel gating.

Swelling-activated chloride channels in aqueous humour formation: on the one side and the other

Aqueous humour is secreted by the ciliary epithelium comprising pigmented and non-pigmented cell layers facing the stroma and aqueous humour respectively. Net chloride secretion likely limits the rate of aqueous humour formation and proceeds in three steps: stromal chloride entry into pigmented cells, diffusion through gap junctions and final non-pigmented cell secretion. Swelling-activated chloride channels function on both epithelial surfaces. At the stromal surface, swelling- and cyclic adenosine monophosphate-activated maxi-chloride channels can recycle chloride, reducing net chloride secretion. At the aqueous-humour surface, swelling- and A3 adenosine receptor-activated chloride channels subserve chloride release into the aqueous humour. The similar macroscopic properties of the two non-pigmented cell chloride currents suggest that both flow through a common conduit. In addition, measurements of intraocular pressure (IOP) in living wild-type and mutant mice have confirmed that A3 adenosine receptor-activated agonists and antagonists increase and lower IOP respectively. Isolated ciliary epithelial cells are commonly perfused with hypotonic solution to probe and characterize chloride channels, but the physiological role of swelling-activated channels has been unclear without knowing their epithelial distribution. Recently, hypotonic challenge has been found to stimulate the chloride-sensitive short-circuit current across the intact bovine ciliary epithelium, suggesting that the net effect of the swelling-activated chloride currents is oriented to enhance aqueous humour formation. Taken together, the results suggest that swelling-activated chloride channels are predominantly oriented to enhance aqueous humour secretion, and these chloride channels at the aqueous surface may be identical with adenosine receptor-activated chloride channels which likely modulate aqueous inflow and IOP in the living mouse.

Diversity of Cl(-) channels

Cl(-) channels are widely found anion pores that are regulated by a variety of signals and that play various roles. On the basis of molecular biologic findings, ligand-gated Cl(-) channels in synapses, cystic fibrosis transmembrane conductors (CFTRs) and ClC channel types have been established, followed by bestrophin and possibly by tweety, which encode Ca(2+)-activated Cl(-) channels. The ClC family has been shown to possess a variety of functions, including stabilization of membrane potential, excitation, cell-volume regulation, fluid transport, protein degradation in endosomal vesicles and possibly cell growth. The molecular structure of Cl(-) channel types varies from 1 to 12 transmembrane segments. By means of computer-based prediction, functional Cl(-) channels have been synthesized artificially, revealing that many possible ion pores are hidden in channel, transporter or unidentified hydrophobic membrane proteins. Thus, novel Cl(-)-conducting pores may be occasionally discovered, and evidence from molecular biologic studies will clarify their physiologic and pathophysiologic roles.