A voltage-gated chloride channel in ascidian embryos modulated by both the cell cycle clock and cell volume

Identification and validation of a signature involving voltage-gated chloride ion channel genes for prediction of prostate cancer recurrence

Voltage-gated chloride ion channels (CLCs) are transmembrane proteins that maintain chloride ion homeostasis in various cells. Accumulating studies indicated CLCs were related to cell growth, proliferation, and cell cycle. Nevertheless, the role of CLCs in prostate cancer (PCa) has not been systematically profiled. The purpose of this study was to investigate the expression profiles and biofunctions of CLCs genes, and construct a novel risk signature to predict biochemical recurrence (BCR) of PCa patients. We identified five differentially expressed CLCs genes in our cohort and then constructed a signature composed of CLCN2 and CLCN6 through Lasso-Cox regression analysis in the training cohort from the Cancer Genome Atlas (TCGA). The testing and entire cohorts from TCGA and the GSE21034 from the Gene Expression Omnibus (GEO) were used as internal and independent external validation datasets. This signature could divide PCa patients into the high and low risk groups with different prognoses, was apparently correlated with clinical features, and was an independent excellent prognostic indicator. Enrichment analysis indicated our signature was primarily concentrated in cellular process and metabolic process. The expression patterns of CLCN2 and CLCN6 were detected in our own cohort based immunohistochemistry staining, and we found CLCN2 and CLCN6 were highly expressed in PCa tissues compared with benign tissues and positively associated with higher Gleason score and shorter BCR-free time. Functional experiments revealed that CLCN2 and CLCN6 downregulation inhibited cell proliferation, colony formation, invasion, and migration, but prolonged cell cycle and promoted apoptosis. Furthermore, Seahorse assay showed that silencing CLCN2 or CLCN6 exerted potential inhibitory effects on energy metabolism in PCa. Collectively, our signature could provide a novel and robust strategy for the prognostic evaluation and improve treatment decision making for PCa patients.

Leucine-rich repeat containing protein LRRC8A is essential for swelling-activated Cl- currents and embryonic development in zebrafish

A volume‐regulated anion channel (VRAC) has been electrophysiologically characterized in innumerable mammalian cell types. VRAC is activated by cell swelling and mediates the volume regulatory efflux of Cl⁻ and small organic solutes from cells. Two groups recently identified the mammalian leucine‐rich repeat containing protein LRRC8A as an essential VRAC component. LRRC8A must be coexpressed with at least one of the other four members of this gene family, LRRC8B‐E, to reconstitute VRAC activity in LRRC8^−/− cells. LRRC8 genes likely arose with the origin of chordates. We identified LRRC8A and LRRC8C‐E orthologs in the zebrafish genome and demonstrate that zebrafish embryo cells and differentiated adult cell types express a swelling‐activated Cl⁻ current indistinguishable from mammalian VRAC currents. Embryo cell VRAC currents are virtually eliminated by morpholino knockdown of the zebrafish LRRC8A ortholog lrrc8aa. VRAC activity is fully reconstituted in LRRC8^−/− human cells by coexpression of zebrafish lrrc8aa and human LRRC8C cDNAs. lrrc8aa expression varies during zebrafish embryogenesis and lrrc8aa knockdown causes pericardial edema and defects in trunk elongation and somatogenesis. Our studies provide confirmation of the importance of LRRC8A in VRAC activity and establish the zebrafish as a model system for characterizing the molecular regulation and physiological roles of VRAC and LRRC8 proteins.

Research and progress on ClC‑2 (Review)

Chloride channel 2 (ClC-2) is one of the nine mammalian members of the ClC family. The present review discusses the molecular properties of ClC‑2, including CLCN2, ClC‑2 promoter and the structural properties of ClC‑2 protein; physiological properties; functional properties, including the regulation of cell volume. The effects of ClC‑2 on the digestive, respiratory, circulatory, nervous and optical systems are also discussed, in addition to the mechanisms involved in the regulation of ClC‑2. The review then discusses the diseases associated with ClC‑2, including degeneration of the retina, Sjögren's syndrome, age‑related cataracts, degeneration of the testes, azoospermia, lung cancer, constipation, repair of impaired intestinal mucosa barrier, leukemia, cystic fibrosis, leukoencephalopathy, epilepsy and diabetes mellitus. It was concluded that future investigations of ClC‑2 are likely to be focused on developing specific drugs, activators and inhibitors regulating the expression of ClC‑2 to treat diseases associated with ClC‑2. The determination of CLCN2 is required to prevent and treat several diseases associated with ClC‑2.

The Role of Ion Channels in Cell Proliferation

Significant progress has been made in identifying physiologically important growth fac tors, receptors, and signal transduction pathways involved in the control of normal and malignant cell proliferation. More recent studies suggest that changes in the proliferative state of a cell are coupled to specific changes in membrane properties, which suggests that there are links between ion channel activity and cell proliferation in both neuronal and non-neuronal cells. Changes in ion channel expression may be necessary to permit cell cycle progression and, ultimately, cell proliferation. The potential mechanisms in volved in the translation of ion channel activity into changes in gene expression are discussed. NEUROSCIENTIST 5:70-73, 1999

Characterization of Human Dermal Fibroblasts in Fabry Disease

Fabry disease (FD) is a hereditary X-linked metabolic lysosomal storage disorder due to insufficient amounts or a complete lack of the lysosomal enzyme α-galactosidase A (α-GalA). The loss of α-GalA activity leads to an abnormal accumulation of globotriaosylcerami (Gb3) in lysosomes and other cellular components of different tissues and cell types, affecting the cell function. However, whether these biochemical alterations also modify functional processes associated to the cell mitotic ability is still unknown. The goal of the present study was to characterize lineages of human dermal fibroblasts (HDFs) of FD patients and healthy controls focusing on Gb3 accumulation, expression of chloride channels that regulate proliferation, and proliferative activity. The biochemical and functional analyses indicate the existence of quantitative differences in some but not all the parameters of cytoskeletal organization, proliferation, and differentiation processes.

Regulation of cell volume and water transport--an old fundamental role of the renin angiotensin aldosterone system components at the cellular level

The expression and the role of renin angiotensin aldosterone system (RAAS) components on regulation of cell volume and water transport on vertebrates and invertebrates were reviewed. The presence of these components even in simple organisms like leeches and their relevance for the control of cellular volume and water transport supports the view that the expression of these components, at cellular level, is an acquisition which was preserved throughout evolution.

Ion channels in volume regulation of clonal kidney cells

OBJECTIVES

Clonal kidney cells (Vero cells) are extensively utilized in the manufacture of biological preparations for disease diagnostics and therapeutics and also in preparation of vaccines. In all cells, regulation of volume is an essential function coupled to a variety of physiological processes and is a topic of interest. The objective here was to investigate involvement of ion channels in the process of volume regulation of Vero cells.

METHODS

Involvement of ion channels in cell volume regulation was studied using video-microscopy and flow cytometry. Pharmacologically unaltered cells of different sizes, which are presumably at different phases of the cell cycle, were used.

RESULTS

Ion transport inhibitors altered all phases of regulatory volume decrease (RVD) of Vero cells, rate of initial cell swelling, V(max) and volume recovery. Effects were dependent on type of inhibitor and on cell size (cell cycle phase). Participation of aquaporins in RVD was suggested. Inhibitors decelerated growth, arresting Vero cells at the G(0) /G(1) phase boundary. Electrophysiological study confirmed presence of volume-activated Cl(-) channels and K(+) channels in plasmatic membranes of the cells.

CONCLUSION

Vero cells of all sizes maintained the ability to recover from osmotic swelling. Activity of ion channels was one of the key factors that controlled volume regulation and proliferation of the cells.

Chloride channels: often enigmatic, rarely predictable

Until recently, anion (Cl(-)) channels have received considerably less attention than cation channels. One reason for this may be that many Cl(-) channels perform functions that might be considered cell-biological, like fluid secretion and cell volume regulation, whereas cation channels have historically been associated with cellular excitability, which typically happens more rapidly. In this review, we discuss the recent explosion of interest in Cl(-) channels, with special emphasis on new and often surprising developments over the past five years. This is exemplified by the findings that more than half of the ClC family members are antiporters, and not channels, as was previously thought, and that bestrophins, previously prime candidates for Ca(2+)-activated Cl(-) channels, have been supplanted by the newly discovered anoctamins and now hold a tenuous position in the Cl(-) channel world.

Anoctamin/TMEM16 family members are Ca2+-activated Cl- channels

Ca(2+)-activated Cl- channels (CaCCs) perform many important functions in cell physiology including secretion of fluids from acinar cells of secretory glands, amplification of olfactory transduction, regulation of cardiac and neuronal excitability, mediation of the fast block to polyspermy in amphibian oocytes, and regulation of vascular tone. Although a number of proteins have been proposed to be responsible for CaCC currents, the anoctamin family (ANO, also known as TMEM16) exhibits characteristics most similar to those expected for the classical CaCC. Interestingly, this family of proteins has previously attracted the interest of both developmental and cancer biologists. Some members of this family are up-regulated in a number of tumours and functional deficiency in others is linked to developmental defects.

Proteomic characterization of the cytotoxic mechanism of gold (III) porphyrin 1a, a potential anticancer drug

There has been increasing interest in the potential applications of gold (III) complexes as anticancer drugs with higher cytotoxicity and fewer side effects than existing metal anticancer drugs. Our previous findings demonstrated that gold (III) porphyrin 1a preferentially induced apoptosis in a cancer cell line (SUNE1). In this study, we identified differentially expressed proteins related to the drug's cytotoxic action by comparing the protein alterations induced by gold (III) porphyrin 1a and cisplatin treatments. Several clusters of altered proteins were identified, including cellular structure and stress-related chaperone proteins, proteins involved in reactive oxygen species and enzyme proteins, translation factors, proteins that mediate cell proliferation or differentiation, and proteins participating in the internal degradation systems. Our results indicated that multiple factors leading to apoptosis were involved in drug cytotoxicity in SUNE1 cells. The balance between pro-apoptotic and anti-apoptotic signals determined the final fate of cancer cells.

Cell cycle-dependent activity of the volume- and Ca2+-activated anion currents in Ehrlich lettre ascites cells

Recent evidence implicates the volume-regulated anion current (VRAC) and other anion currents in control or modulation of cell cycle progression; however, the precise involvement of anion channels in this process is unclear. Here, Cl- currents in Ehrlich Lettre Ascites (ELA) cells were monitored during cell cycle progression, under three conditions: (i) after osmotic swelling (i.e., VRAC), (ii) after an increase in the free intracellular Ca2+ concentration (i.e., the Ca2+-activated Cl- current, CaCC), and (iii) under steady-state isotonic conditions. The maximal swelling-activated VRAC current decreased in G1 and increased in early S phase, compared to that in G0. The isotonic steady-state current, which seems to be predominantly VRAC, also decreased in G1, and increased again in early S phase, to a level similar to that in G0. In contrast, the maximal CaCC current (500 nM free Ca2+ in the pipette), was unaltered from G0 to G1, but decreased in early S phase. A novel high-affinity anion channel inhibitor, the acidic di-aryl-urea NS3728, which inhibited both VRAC and CaCC, attenuated ELA cell growth, suggesting a possible mechanistic link between cell cycle progression and cell cycle-dependent changes in the capacity for conductive Cl- transport. It is suggested that in ELA cells, entrance into the S phase requires an increase in VRAC activity and/or an increased potential for regulatory volume decrease (RVD), and at the same time a decrease in CaCC magnitude.

Structure and function of CLCA proteins

CLCA proteins were discovered in bovine trachea and named for a calcium-dependent chloride conductance found in trachea and in other secretory epithelial tissues. At least four closely located gene loci in the mouse and the human code for independent isoforms of CLCA proteins. Full-length CLCA proteins have an unprocessed mass ratio of approximately 100 kDa. Three of the four human loci code for the synthesis of membrane-associated proteins. CLCA proteins affect chloride conductance, epithelial secretion, cell-cell adhesion, apoptosis, cell cycle control, mucus production in asthma, and blood pressure. There is a structural and probable functional divergence between CLCA isoforms containing or not containing beta4-integrin binding domains. Cell cycle control and tumor metastasis are affected by isoforms with the binding domains. These isoforms are expressed prominently in smooth muscle, in some endothelial cells, in the central nervous system, and also in secretory epithelial cells. The isoform with disrupted beta4-integrin binding (hCLCA1, pCLCA1, mCLCA3) alters epithelial mucus secretion and ion transport processes. It is preferentially expressed in secretory epithelial tissues including trachea and small intestine. Chloride conductance is affected by the expression of several CLCA proteins. However, the dependence of the resulting electrical signature on the expression system rather than the CLCA protein suggests that these proteins are not independent Ca2+-dependent chloride channels, but may contribute to the activity of chloride channels formed by, or in conjunction with, other proteins.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Ion Channels and Cancer

Membrane ion channels are essential for cell proliferation and appear to have a role in the development of cancer. This has initially been demonstrated for potassium channels and is meanwhile also suggested for other cation channels and Cl- channels. For some of these channels, like voltage-gated ether à go-go and Ca2+-dependent potassium channels as well as calcium and chloride channels, a cell cycle-dependent function has been demonstrated. Along with other membrane conductances, these channels control the membrane voltage and Ca2+ signaling in proliferating cells. Homeostatic parameters, such as the intracellular ion concentration, cytosolic pH and cell volume, are also governed by the activity of ion channels. Thus it will be an essential task for future studies to unravel cell cycle-specific effects of ion channels and non-specific homeostatic functions. When studying the role of ion channels in cancer cells, it is indispensable to choose experimental conditions that come close to the in vivo situation. Thus, environmental parameters, such as low oxygen pressure, acidosis and exposure to serum proteins, have to be taken into account. In order to achieve clinical application, more studies on the original cancer tissue are required, and improved animal models. Finally, it will be essential to generate more potent and specific inhibitors of ion channels to overcome the shortcomings of some of the current approaches.

Re-expression of detachment-inducible chloride channel mCLCA5 suppresses growth of metastatic breast cancer cells

The calcium-activated chloride channel hCLCA2 has been identified as a candidate tumor suppressor in human breast cancer. It is greatly down-regulated in breast cancer, and its re-expression suppresses tumorigenesis by an unknown mechanism. To establish a mouse model, we identified the mouse ortholog of hCLCA2, termed mCLCA5, and investigated its behavior in mammary epithelial cell lines and tissues. Expression in the immortalized cell line HC11 correlated with slow or arrested growth. Although rapidly dividing, sparsely plated cells had low levels of expression, mCLCA5 was induced by 10-fold when cells became confluent and 30-fold when cells were deprived of growth factors or anchorage. The apoptosis effector Bax was induced in parallel. Like hCLCA2, mCLCA5 was down-regulated in metastatic mammary tumor cell lines such as 4T1 and CSML-100. Ectopic re-expression in 4T1 cells caused a 20-fold reduction in colony survival relative to vector control. High mCLCA5 expression in stable clones inhibited proliferation and enhanced sensitivity to detachment. Moreover, mCLCA5 was induced in lactating and involuting mammary gland, correlating with differentiation and onset of apoptosis. Together, these results establish mCLCA5 as the mouse ortholog of hCLCA2, demonstrate that mCLCA5 is a detachment-sensitive growth inhibitor, and suggest a mechanism whereby these channels may antagonize mammary tumor progression.

From genes to integrative physiology: ion channel and transporter biology in Caenorhabditis elegans

The stunning progress in molecular biology that has occurred over the last 50 years drove a powerful reductionist approach to the study of physiology. That same progress now forms the foundation for the next revolution in physiological research. This revolution will be focused on integrative physiology, which seeks to understand multicomponent processes and the underlying pathways of information flow from an organism's "parts" to increasingly complex levels of organization. Genetically tractable and genomically defined nonmammalian model organisms such as the nematode Caenorhabditis elegans provide powerful experimental advantages for elucidating gene function and the molecular workings of complex systems. This review has two main goals. The first goal is to describe the experimental utility of C. elegans for investigating basic physiological problems. A detailed overview of C. elegans biology and the experimental tools, resources, and strategies available for its study is provided. The second goal of this review is to describe how forward and reverse genetic approaches and direct behavioral and physiological measurements in C. elegans have generated novel insights into the integrative physiology of ion channels and transporters. Where appropriate, I describe how insights from C. elegans have provided new understanding of the physiology of membrane transport processes in mammals.

Cl- channel blockers inhibit transition of quiescent (G0) fibroblasts into the cell cycle

Modulation of ion permeability during the cell cycle is one of the key events in cell cycle progression. We have compared the effects of K+ and Cl- channel blockers on the cell cycle in synchronous and asynchronous NIH3T3 cells. The Cl- channel blocker 5-N-2-(3-phenylpropylamino) benzoic acid (NPPB; 0.2 mM) inhibited entry into S phase in synchronous cells but not in asynchronous cells, while the K+ channel blocker 4-aminopyridine (4-AP) showed similar inhibitory effects in both conditions. In NIH3T3 cells synchronized by serum deprivation/replenishment, G0-to-G1 transition occurred within 8 h after serum addition, and the G1/S checkpoint at 10-14 h. NPPB applied only at 0-8 or 8-14 h after serum addition inhibited entry into S phase. Cl- permeability measured as 125I efflux increased at 4 and 10 h after serum addition. Ki-67-negative cells, which represent quiescent G0 phase cells, progressively decreased in number until 8 h after serum addition. The Cl- channel blockers (NPPB and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid [DIDS]) but not the K+ channel blocker (4-AP) significantly decreased the rate of reduction in number of Ki-67-negative cells. These data indicate that an increase in Cl- permeability plays an important role in reentry of quiescent cells into the proliferating phase, in addition to the known effects on passage through the G1/S checkpoint.

Cell cycle-dependent expression of volume-activated chloride currents in nasopharyngeal carcinoma cells

Patch-clamping and cell image analysis techniques were used to study the expression of the volume-activated Cl(-) current, I(Cl(vol)), and regulatory volume decrease (RVD) capacity in the cell cycle in nasopharyngeal carcinoma cells (CNE-2Z). Hypotonic challenge caused CNE-2Z cells to swell and activated a Cl(-) current with a linear conductance, negligible time-dependent inactivation, and a reversal potential close to the Cl(-) equilibrium potential. The sequence of anion permeability was I(-) > Br(-) > Cl(-) > gluconate. The Cl(-) channel blockers tamoxifen, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), and ATP inhibited I(Cl(vol)). Synchronous cultures of cells were obtained by the mitotic shake-off technique and by a double chemical-block (thymidine and hydroxyurea) technique. The expression of I(Cl(vol)) was cell cycle dependent, being high in G(1) phase, downregulated in S phase, but increasing again in M phase. Hypotonic solution activated RVD, which was cell cycle dependent and inhibited by the Cl(-) channel blockers NPPB, tamoxifen, and ATP. The expression of I(Cl(vol)) was closely correlated with the RVD capacity in the cell cycle, suggesting a functional relationship. Inhibition of I(Cl(vol)) by NPPB (100 microM) arrested cells in G(0)/G(1). The data also suggest that expression of I(Cl(vol)) and RVD capacity are actively modulated during the cell cycle. The volume-activated Cl(-) current associated with RVD may therefore play an important role during the cell cycle progress.

Regulatory volume decrease is actively modulated during the cell cycle

Nasopharyngeal carcinoma cells, CNE-2Z, when swollen by 47% hypotonic solution, exhibited a regulatory volume decrease (RVD). The RVD was inhibited by extracellular applications of the chloride channel blockers tamoxifen (30 microM; 61% inhibition), 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 100 microM; 60% inhibition), and ATP (10 mM; 91% inhibition). The level and time constant of RVD varied greatly between cells. Most cells conducted an incomplete RVD, but a few had the ability to recover their volume completely. There was no obvious correlation between cell volume and RVD capacity. Flow cytometric analysis showed that highly synchronous cells were obtained by the mitotic shake-off technique and that the cells progressed through the cell cycle synchronously when incubated in culture medium. Combined application of DNA synthesis inhibitors, thymidine and hydroxyurea arrested cells at the G1/S boundary and 87% of the cells reached S phase 4 h after being released. RVD capacity changed significantly during the cell cycle progression in cells synchronized by shake-off technique. RVD capacity being at its highest in G1 phase and lowest in S phase. The RVD capacity in G1 (shake-off cells sampled after 4 h of incubation), S (obtained by chemical arrest), and M cells (selected under microscope) was 73, 33, and 58%, respectively, and the time constants were 435, 769, and 2,000 sec, respectively. We conclude that RVD capacity is actively modulated in the cell cycle and RVD may play an important role in cell cycle progress.

M phase-specific expression and phosphorylation-dependent ubiquitination of the ClC-2 channel

Cl(-) channel activities vary during the cell cycle and are thought to play various roles including regulation of cell volume. We have shown previously that ClC-2 channels are directly phosphorylated and functionally regulated by the M phase-specific cyclin-dependent kinase p34(cdc2)/cyclin B. We investigate here to determine whether the expression levels of ClC-2 channel protein vary during the cell cycle. Immunoblot and immunocytochemical analyses of cells cycle-synchronized by serum depletion/replenishment reveal that ClC-2 channel protein is expressed predominantly at M phase in cells with two nuclei and a clear constriction ring, whereas RNA blot analysis shows that ClC-2 mRNA expression does not change during the cell cycle. Ubiquitin assays reveal that the ClC-2 channels are ubiquitinated at M phase, whereas the magnitude of ubiquitination is suppressed by incubation with olomoucine, an inhibitor of p34(cdc2)/cyclin B, and it is almost completely abolished in ClC-2 channels having an S632A mutation, which cannot be phosphorylated by p34(cdc2)/cyclin B, indicating that ubiquitination of ClC-2 channels requires phosphorylation by M phase-specific p34(cdc2)/cyclin B. Regulation at the post-transcriptional level, including phosphorylation-dependent ubiquitination, may contribute to M phase-specific expression of ClC-2 channels. Cell cycle-dependent regulation of expression at the protein level in addition to the regulation of function suggests that the ClC-2 channel plays a physiological role in the cell cycle.

Cell cycle- and swelling-induced activation of a Caenorhabditis elegans ClC channel is mediated by CeGLC-7alpha/beta phosphatases

ClC voltage-gated anion channels have been identified in bacteria, yeast, plants, and animals. The biophysical and structural properties of ClCs have been studied extensively, but relatively little is known about their precise physiological functions. Furthermore, virtually nothing is known about the signaling pathways and molecular mechanisms that regulate channel activity. The nematode Caenorhabditis elegans provides significant experimental advantages for characterizing ion channel function and regulation. We have shown previously that the ClC Cl- channel homologue CLH-3 is expressed in C. elegans oocytes, and that it is activated during meiotic maturation and by cell swelling. We demonstrate here that depletion of intracellular ATP or removal of Mg2+, experimental maneuvers that inhibit kinase function, constitutively activate CLH-3. Maturation- and swelling-induced channel activation are inhibited by type 1 serine/threonine phosphatase inhibitors. RNA interference studies demonstrated that the type 1 protein phosphatases CeGLC-7alpha and beta, both of which play essential regulatory roles in mitotic and meiotic cell cycle events, mediate CLH-3 activation. We have suggested previously that CLH-3 and mammalian ClC-2 are orthologues that play important roles in heterologous cell-cell interactions, intercellular communication, and regulation of cell cycle-dependent physiological processes. Consistent with this hypothesis, we show that heterologously expressed rat ClC-2 is also activated by serine/threonine dephosphorylation, suggesting that the two channels have common regulatory mechanisms.

Phosphorylation and functional regulation of ClC-2 chloride channels expressed in Xenopus oocytes by M cyclin-dependent protein kinase

Many dramatic alterations in various cellular processes during the cell cycle are known to involve ion channels. In ascidian embryos and Caenorhabditis elegans oocytes, for example, the activity of inwardly rectifying Cl(-) channels is enhanced during the M phase of the cell cycle, but the mechanism underlying this change remains to be established. We show here that the volume-sensitive Cl(-) channel, ClC-2 is regulated by the M-phase-specific cyclin-dependent kinase, p34(cdc2)/cyclin B. ClC-2 channels were phosphorylated by p34(cdc2)/cyclin B in both in vitro and cell-free phosphorylation assays. ClC-2 phosphorylation was inhibited by olomoucine and abolished by a (632)Ser-to-Ala (S632A) mutation in the C-terminus, indicating that (632)Ser is a target of phosphorylation by p34(cdc2)/cyclin B. Injection of activated p34(cdc2)/cyclin B attenuated the ClC-2 currents but not the S632A mutant channel currents expressed in Xenopus oocytes. ClC-2 currents attenuated by p34(cdc2)/cyclin B were increased by application of the cyclin-dependent kinase inhibitor, olomoucine (100 microM), an effect that was inhibited by calyculin A (5 nM) but not by okadaic acid (5 nM). A yeast two-hybrid system revealed a direct interaction between the ClC-2 C-terminus and protein phosphatase 1. These data suggest that the ClC-2 channel is also counter-regulated by protein phosphatase 1. In addition, p34(cdc2)/cyclin B decreased the magnitude of ClC-2 channel activation caused by cell swelling. As the activities of both p34(cdc2)/cyclin B and protein phosphatase 1 vary during the cell cycle, as does cell volume, the ClC-2 channel could be regulated physiologically by these factors.

Molecular structure and physiological function of chloride channels

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

Tumor suppression by a proapoptotic calcium-activated chloride channel in mammary epithelium

Little is known of the roles played by ion channels in cancer. Here we describe a pair of closely related calcium-activated chloride channels whose differential regulation in normal, apoptotic, and transformed mouse cells suggests that channel function is proapoptotic and antineoplastic. While mCLCA1 predominates over mCLCA2 under normal physiological conditions, this relationship is reversed by apoptotic stress both in developing mammary gland and in cultured HC11 mammary epithelial cells. Consistent with an apoptosis-promoting role, splicing of mCLCA2 is disrupted in apoptosis-resistant tumor cell lines and in HC11 cells selected for resistance to detachment-induced apoptosis (anoikis). Unexpectedly, mCLCA1 message is also down-regulated in these cells by at least 30-fold. These results suggest that both genes antagonize survival of mammary tumor cells by sensitizing them to anoikis. When MCF7 or HEK293 tumor cells were transfected with plasmids encoding either mCLCA1 or mCLCA2, colony formation was greatly reduced relative to a vector-transfected control, demonstrating that calcium-sensitive chloride channel (CLCA) expression is deleterious to tumor cell survival. Furthermore, mammary epithelial cells overexpressing mCLCA2 had twice the rate of apoptosis of normal cells when subjected to serum starvation and formed multinuclear giants at a high frequency in normal culture, suggesting that mCLCA2 can promote either apoptosis or senescence.

A slow outward current and a hypoosmolality induced anion conductance in embryonic chicken osteoclasts

In this paper we report on a hypoosmolality induced current, I(osmo), in embryonic chicken osteoclasts, which could only be studied when blocking a simultaneously active, unidentified slow outward current, I(slo). I(slo) was observed in all of the examined cells when both the intracellular and extracellular solutions contained sodium as the major cation and no potassium. The current was outwardly rectifying and activated at membrane potentials more positive than -44 +/- 12 mV (n = 31). The time to half activation of the current was also voltage dependent and was 350 ms at Vm = +80 mV, and 78 ms at Vm = +120 mV. The current did not inactivate during periods up to 5 s. Extracellular 4-AP (5 mM), TEA (5 mM) and Ba2+ (1 mM), blockers of K+ conductances in chicken osteoclasts, did not influence I(slo). However, I(slo) was inhibited by 50 microM extracellular verapamil, which allowed us to study I(osmo) in isolation. Exposure of the osteoclasts to hypotonic solution resulted in the development of a depolarization activated I(osmo). It developed after a 1-min delay and reached its maximum within 10 minutes. Half-maximal activation occurred after 4.4 +/- 0.9 min (n = 9). The current activated within a few ms upon depolarization and did not inactivate during at least 5 sec. I(osmo) reversed around the calculated Nernst potential for Cl- (E(Cl) = +7.3 mV and V(rev) = +5.4 +/- 3.6 mV, n = 9). The underlying conductance, G(osmo) exhibited moderate outward rectification around 0 mV in symmetrical Cl- solutions. Ion substitution experiments showed that G(osmo) is an anion conductance with P(Cl) approximately = P(F) > P(gluc) >> P(Na). I(osmo) was blocked by 0.5 mM SITS but 50 microM verapamil, 5 mM TEA, 5 mM 4-AP, 1 mM Ba2+, 50 microM cytochalasin D and 0.5 mM alendronate did not have any effect on the current. Cl- currents have been implicated in charge neutralization during osteoclastic acid secretion for bone resorption. The present results imply that osmolality may be a factor controlling this charge neutralization.

Developmentally regulated cell cycle dependence of swelling-activated anion channel activity in the mouse embryo

Anion channels activated by increased cell volume are a nearly ubiquitous mechanism of cell volume regulation, including in early preimplantation mouse embryos. Here, we show that the swelling-activated anion current (ICl,swell) in early mouse embryos is cell-cycle dependent, and also that this dependence is developmentally regulated. ICl,swell is present both in first meiotic prophase (germinal vesicle stage) mouse oocytes and in unfertilized mature oocytes in second meiotic metaphase, and it persists after fertilization though the 1-cell and 2-cell stages. ICl,swell was found to remain unchanged during metaphase at the end of the 1-cell stage. However, ICl,swell decreased during prophase and became nearly undetectable upon entry into metaphase at the end of the 2-cell stage. Entry into prophase/metaphase was required for the decrease in ICl,swell at the end of the 2-cell stage, since it persisted indefinitely in 2-cell embryos arrested in late G2. There is considerable evidence that the channel underlying ICl,swell is not only permeable to inorganic anions, but to organic osmolytes as well. We found a similar pattern of cell cycle and developmental dependence in the 1-cell and 2-cell stages for the swelling-induced increase in permeability to the organic osmolyte glycine. Thus, entry into metaphase deactivates ICl,swell in embryos, but only after developmental progression through the 2-cell stage.

Regulation of ClC-2 chloride channels in T84 cells by TGF-α

The almost ubiquitously expressed ClC-2 chloride channel is activated by hyperpolarization and osmotic cell swelling. Osmotic swelling also activates a different class of outwardly rectifying chloride channels, and several reports point to a link between protein tyrosine phosphorylation and activation of these channels. This study examines the possibility that transforming growth factor-α (TGF-α) modulates ClC-2 activity in human colonic epithelial (T84) cells. TGF-α (0.17 nM) irreversibly inhibited ClC-2 current in nystatin-perforated whole cell patch-clamp experiments, whereas a superimposed reversible activation of the current was observed at 8.3 nM TGF-α. Both effects required activation of the intrinsic epidermal growth factor receptor (EGFR) tyrosine kinase activity, of phosphoinositide 3-kinase, and of protein kinase C. With microspectrofluorimetry of the pH-sensitive fluorescent dye 2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, TGF-α was shown to reversibly alkalinize T84 cells at 8.3 nM but not at 0.17 nM, suggesting that 8.3 nM TGF-α-induced alkalinization activates ClC-2 current. This study indicates that ClC-2 channels are targets for EGFR signaling in epithelial cells.

Cell cycle-related changes in regulatory volume decrease and volume-sensitive chloride conductance in mouse fibroblasts

Cell cycle-related changes in the ability to regulate cell volume following hyposmotic swelling were studied in mouse fibroblasts using videomicroscopy and the whole-cell patch clamp technique. Regulatory volume decrease (RVD) and volume-sensitive Cl- conductance (G(Cl,vol)) were measured: (1) in proliferating cells of different sizes; (2) in cells arrested in defined phases of the cell cycle (G1, G1/S, S, and M phases) using mevastatin, mimosine, hydroxyurea, aphidicolin, cytosine beta-D-arabinofuranoside, and taxol; and (3) in serum-starved cells (G(0) state). Cells in all groups were able to undergo RVD, although the cells approaching mitosis (i.e., the largest cells in proliferating cultures and the taxol-treated cells) had the lowest rates of shrinkage during RVD. In agreement with this finding, the density of G(Cl,vol) was stable in proliferating and cell cycle-arrested cells for most of the cell cycle, with the exception of the cells approaching mitosis and the new daughter cells where the density was decreased to half. The impairment of RVD was greatest in serum-starved cells which also had the lowest density of G(Cl,vol). We conclude that proliferating cells maintain an ability to recover from osmotic swelling as they progress through the cell cycle, although this ability may be compromised during mitosis.

Osmosensitivity of an inwardly rectifying chloride current revealed by whole-cell and perforated-patch recordings in cultured rat cortical astrocytes

The osmosensitivity of the inwardly rectifying Cl(-) current (I(Clh)), expressed by primary cultured rat neocortical astrocytes long-term treated with dibutyryl cyclic AMP, was investigated in the whole-cell and perforated-patch modes. In whole-cell experiments, whereas hypotonic extracellular solution (Delta=100 mOsmol) did not cause any change in I(Clh), hypertonicity produced a slowly developing, approximately 40% reversible decrease in current magnitude. By contrast, in perforated-patch experiments, exposure to a less hypertonic saline (Delta=50 mOsmol) depressed the current to approximately 50%, and hypotonicity induced a approximately 50% slow increase in I(Clh). These differences in osmosensitivity between the two experimental modes suggest that the osmoregulation of I(Clh) may be mediated by complex intracellular mechanism(s), which appear(s) to be partly compromised by the dialysis of the astrocytic cytoplasm.

The nuclear chloride ion channel NCC27 is involved in regulation of the cell cycle

NCC27 is a nuclear chloride ion channel, identified in the PMA-activated U937 human monocyte cell line. NCC27 mRNA is expressed in virtually all cells and tissues and the gene encoding NCC27 is also highly conserved. Because of these factors, we have examined the hypothesis that NCC27 is involved in cell cycle regulation. Electrophysiological studies in Chinese hamster ovary (CHO-K1) cells indicated that NCC27 chloride conductance varied according to the stage of the cell cycle, being expressed only on the plasma membrane of cells in G2/M phase. We also demonstrate that Cl- ion channel blockers known to block NCC27 led to arrest of CHO-K1 cells in the G2/M stage of the cell cycle, the same stage at which this ion channel is selectively expressed on the plasma membrane. These data strongly support the hypothesis that NCC27 is involved, in some as yet undetermined manner, in regulation of the cell cycle.

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

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

Characterization of pICln phosphorylation state and a pICln-associated protein kinase

pICln is a ubiquitous cellular protein that has been proposed to be a volume-sensitive Cl- channel or a channel regulator. Detailed biochemical, cellular and molecular characterization of pICln is required to understand its function. Our goal in the present investigation was to define further the biochemical properties of pICln and the proteins that associate with it. Immunoprecipitation of pICln from 32P-orthophosphoric acid-labeled C6 glioma cells revealed that the protein is phosphorylated constitutively, primarily on serine residues. Protein kinase activity was detected in pICln immunoprecipitates, revealing that a constitutively active protein kinase co-precipitates with pICln. A specific association between pICln and a protein kinase was also observed in affinity assays using a recombinant GST-pICln fusion protein. The pICln-associated kinase displayed broad substrate specificity and was inhibited in a concentration-dependent manner by heparin, zinc and 5,6-dichloro-1-beta-D-ribofuranosylbenose (DRB). These characteristics resembled those of casein kinase I and II. The pICln-associated kinase was not recognized, however, by antibodies against these two enzymes. Association of the kinase with pICln was disrupted by increasing concentrations of NaCl in the washing buffer, suggesting that electrostatic interactions are involved in kinase binding. Mutagenesis experiments corroborated this observation. Truncation of pICln demonstrated that two highly charged clusters of acidic amino acid residues are both necessary and sufficient for kinase binding. Phosphopeptide mapping demonstrated that pICln contains at least two phosphorylated serine residues that are located on trypsin cleavage fragments rich in acidic amino acid residues. We propose that the kinase or a kinase binding protein binds to acidic amino acids located between D101 and Y156 and phosphorylates nearby serine residues.

A cytoplasmic cell cycle controls the activity of a K+ channel in pre-implantation mouse embryos

We previously have reported that the activity of a 240 pS K+ channel varies during the cell cycle in pre-implantation mouse embryos. In the present study, we show that: (i) the cycling of channel activity is not prevented by inhibiting protein synthesis and hence does not involve cyclin-dependent kinase 1 (cdk1)-cyclin B; and (ii) the cycling of channel activity continues in anucleate zygote fragments with a time course similar to that observed in nucleate fragments. We further demonstrate that: (i) persistent activation of the K+ channel in one-cell embryos arrested in metaphase requires the maintenance of an active cdk1-cyclin B complex; and (ii) both DNA synthesis inhibition with aphidicolin and DNA damage produced by mitomycin C prevent the down-regulation of the channel at the start of S phase by a mechanism that requires tyrosine kinase activation. Thus, the 240 pS K+ channel in these cells is controlled by a previously unsuspected cytoplasmic clock that functions independently of the well-known clock controlling the chromosomal cell cycle, but can interact with it.

The development of voltage-gated ion channels and its relation to activity-dependent development events

Spontaneous activity is an essential feature in the development of the nervous system. The patterns of activity and the waveform and ionic dependence of the action potentials that occur during such activity are fine-tuned to carry out certain developmental functions, and are therefore generally not compatible with the mature physiological function of the cell. For this reason, the patterns of ion channel development that create spontaneous activity early in the development of a given cell type are complex and not easily predicted from the mature properties of that same cell. Ion channels are often found that are specific to early stages of development, and that either are not retained in the mature cell or whose properties are greatly changed during later differentiation. The exact significance of such patterns of channel development is just now becoming clear, as we understand more about the mechanisms linking spontaneous activity to later developmental events.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Cell cycle-dependent expression of a glioma-specific chloride current: proposed link to cytoskeletal changes

We recently demonstrated expression of a novel, glioma-specific Cl- current in glial-derived tumor cells (gliomas), including stable cell lines such as STTG1, derived from a human anaplastic astrocytoma. We used STTG1 cells to study whether glioma Cl- channel (GCC) activity is regulated during cell cycle progression. Cells were arrested in defined stages of cell cycle (G0, G1, G1/S, S, and M phases) using serum starvation, mevastatin, hydroxyurea, demecolcine, and cytosine beta-D-arabinofuranoside. Cell cycle arrest was confirmed by measuring [3H]thymidine incorporation and by DNA flow cytometry. Using whole cell patch-clamp recordings, we demonstrate differential changes in GCC activity after cell proliferation and cell cycle progression was selectively altered; specifically, channel expression was low in serum-starved, G0-arrested cells, increased significantly in early G1, decreased during S phase, and increased after arrest in M phase. Although the link between the cell cycle and GCC activity is not yet clear, we speculate that GCCs are linked to the cytoskeleton and that cytoskeletal rearrangements associated with cell division lead to the observed changes in channel activity. Consistent with this hypothesis, we demonstrate the activation of GCC by disruption of F-actin using cytochalasin D or osmotic cell swelling.

Hypotonicity and thrombin activate taurine efflux in BC3H1 and C2C12 myoblasts that is down regulated during differentiation

The efflux of organic osmolytes such as taurine is an important mechanism by which cells regulate their volume. The effects of hypotonicity and thrombin on taurine efflux were studied in BC3H1 and C2C12 cells, two mouse myoblastic cell lines that can be induced to differentiate with serum deprivation. In proliferating cultures of both cell types preloaded with [3H]taurine, exposure to 27% hypotonicity activated a 10- to 20-fold increase in [3H]taurine efflux (Jtau). This effect was blocked by the C1- channel inhibitors NPPB and flufenamic acid. Thrombin and the thrombin receptor agonist SFLLRN also activated Jtau that was abolished by NPPB and flufenamic acid. Together, hypotonicity and thrombin synergistically activated Jtau. In differentiated myocytes, the effect of thrombin was abolished, while that of hypotonicity was significantly reduced. These results suggest that (i) hypotonicity and thrombin activate taurine-permeable anion channels in BC3H1 and C2C12 cells, and (ii) these anion channels may be involved in cell proliferation.

Osmosensitivity of the hyperpolarization-activated chloride current in human intestinal T84 cells

The osmosensitivity of the hyperpolarization-activated chloride current (I(Clhyp)) in T84 cells was studied using the whole cell patch-clamp recording configuration. Hypotonicity is known to activate an outwardly rectifying chloride current (HIORC) distinct from I(Clhyp) in these cells. The differing sensitivities of HIORC and I(Clhyp) toward inhibitors (1,9-dideoxyforskolin blocked HIORC but not I(Clhyp), and Cd2+ inhibited I(Clhyp) but not HIORC) allowed us to investigate the osmoregulation of I(Clhyp). Hypotonicity induced an increase in I(Clhyp) amplitude. Protein phosphatase inhibitors prevented this effect, and hypotonic solutions became slightly inhibitory. Hypertonicity resulted in a transient increase in I(Clhyp) amplitude followed by a large decrease. The complex responses of I(Clhyp) to osmotic changes indicate that these signals affect the same channel via multiple transduction pathways. The responses of I(Clhyp) to hypotonicity have features in common with the responses of ClC-2 channels expressed in Xenopus oocytes (activation) and with hyperpolarization-activated chloride currents in other cell types, such as osteoblasts and mandibular duct cells (inhibition).

Downregulation of volume-activated Cl- currents during muscle differentiation

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