Mechanisms of hypoosmotic volume regulation in glioma cells

Expression and functional role of potassium-chloride cotransporters (KCC) in astrocytes and C6 glioma cells

Brain edema formation following brain injury is a serious but still poorly treatable medical condition. The understanding of volume regulation in astrocytes, the main cells involved in the formation of cytotoxic brain edema, is key for the development of novel treatment strategies. This study investigates the role of potassium-chloride cotransporters (KCC) for cell volume regulation in glial cells. PCR revealed the expression of KCC isoforms in a glial cell line (C6) and primary cultured astrocytes. Specific inhibition of KCCs caused glial cell swelling and resulted in a complete inhibition of regulatory volume decrease upon hypotonic medium-induced cell swelling. Therefore, our results show that KCCs play an important role in the maintenance and regulation of cell volume in astrocytes.

Inhibition of ERK and JNK Decreases Both Osmosensitive Taurine Release and Cell Proliferation in Glioma Cells

Cell swelling is associated with the activation of an increase in the osmosensitive taurine release (OTR) rate, which serves to decrease cell volume as part of a process known as regulatory volume decrease. OTR, which is sensitive to many pharmacological agents including anion channel blockers and signalling pathway modulators, has also been suggested to play a role in cell cycle progression. At non-cytotoxic concentrations, the anion channel blocker NPPB (25 microM), the extra-cellular signal-regulated kinase inhibitor PD98059 (50 microM), and the c-Jun NH2-terminal kinase inhibitor SP 600125 (5 microM) each decreased the OTR rate by > or =50%, decreased cell proliferation, and increased G0/G1 cell cycle arrest.

Brain amino acids during hyponatremia in vivo: clinical observations and experimental studies

Hyponatremia is a highly morbid condition, present in a wide range of human pathologies, that exposes patients to encephalopathic complication and the risk of permanent brain damage and death. Treating hyponatremia has proved to be difficult and still awaits safe management, avoiding the morbid sequelae of demyelinizing and necrotic lesions associated with the use of hypertonic solutions. During acute and chronic hyponatremia in vivo, the brain extrudes the excessive water by decreasing its content of electrolytes and organic osmolytes. At the cellular level, a similar response occurs upon cell swelling. Among the organic osmolytes involved in both responses, free amino acids play a prominent role because of the large intracellular pools often found in nerve cells. An overview of the changes in brain amino acid content during hyponatremia in vivo is presented and the contribution of these changes to the adaptive cell responses involved in volume regulation discussed. Additionally, new data are provided concerning changes in amino acid levels in different regions of the central nervous system after chronic hyponatremia. Results favor the role of taurine, glutamine, glutamate, and aspartate as the main amino acid osmolytes involved in the brain adaptive response to hyponatremia in vivo. Deeper knowledge of the adaptive overall and cellular brain mechanisms activated during hyponatremia would lead to the design of safer therapies for the hyponatremic patient.

Regulation of a Ca2+-activated K+ channel by calcium-sensing receptor involves p38 MAP kinase

By using pharmacological and molecular approaches, we previously showed that the G-protein-coupled, extracellular calcium (Ca2+(o))-sensing receptor (CaR) regulates a large-conductance (approximately 140 pS), Ca(2+)-activated K+ channel [IK(Ca); CAKC] in U87 astrocytoma cells. Here we show that elevated Ca2+(o) stimulates extracellular-signal-regulated kinase (ERK1/2) and p38 MAP kinase (MAPK). The effect of high Ca2+(o) on p38 MAPK but not ERK1/2 is CaR mediated, insofar as transduction with a dominant-negative CaR (R185Q) using recombinant adeno-associated virus (rAAV) attenuated the activation of p38 MAPK but not of ERK1/2. p38 MAPK activation by the CaR is likely to be protein kinase C (PKC) independent, in that the pan-PKC inhibitor GF109203X failed to abolish the high-Ca2+(o)-induced phosphorylation of p38 MAPK. Consistently with our data on the activation of this kinase, we observed that inhibiting p38 MAPK blocked the activation of the CAKC induced by the specific pharmacological CaR activator NPS R-467. In contrast, inhibiting MEK1 only transiently inhibited the activation of this K+ channel by NPS R-467, despite the continued presence of the antagonist. Similarly to the lack of any effect of the PKC inhibitor on the activation of ERK1/2 and p38 MAPK, inhibiting PKC had no effect on NPS R-467-induced activation of this channel. Therefore, our data show that the CaR, acting via p38 MAPK, regulates a large-conductance CAKC in U87 cells, a process that is PKC independent. Large-conductance CAKCs play an important role in the regulation of cellular volume, so our results have important implications for glioma cell volume regulation.

Impaired regulatory volume decrease in freshly isolated cholangiocytes from cystic fibrosis mice: implications for cystic fibrosis transmembrane conductance regulator effect on potassium conductance

Various K(+) and Cl(-) channels are important in cell volume regulation and biliary secretion, but the specific role of cystic fibrosis transmembrane conductance regulator in cholangiocyte cell volume regulation is not known. The goal of this research was to study regulatory volume decrease (RVD) in bile duct cell clusters (BDCCs) from normal and cystic fibrosis (CF) mouse livers. Mouse BDCCs without an enclosed lumen were prepared as described (Cho, W. K. (2002) Am. J. Physiol. 283, G1320-G1327). The isotonic solution consisted of HEPES buffer with 40% of the NaCl replaced with isomolar amounts of sucrose, whereas hypotonic solution was the same as isotonic solution without sucrose. The cell volume changes were indirectly assessed by measuring cross-sectional area (CSA) changes of the BDCCs using quantitative videomicroscopy. Exposure to hypotonic solutions increased relative CSAs of normal BDCCs to 1.20 +/- 0.01 (mean +/- S.E., n = 50) in 10 min, followed by RVD to 1.07 +/- 0.01 by 40 min. Hypotonic challenge in CF mouse BDCCs also increased relative CSA to 1.20 +/- 0.01 (n = 53) in 10 min but without significant recovery. Coadministration of the K(+)-selective ionophore valinomycin restored RVD in CF mouse BDCCs, suggesting that the impaired RVD was likely from a defect in K(+) conductance. Moreover, this valinomycin-induced RVD in CF mice was inhibited by 5-nitro-2'-(3-phenylpropylamino)-benzoate, indicating that it is not from nonspecific effects. Neither cAMP nor calcium agonists could reverse the impaired RVD seen in CF cholangiocytes. Our conclusion is that CF mouse cholangiocytes have defective RVD from an impaired K(+) efflux pathway, which could not be reversed by cAMP nor calcium agonists.

Recovery from lactacidosis-induced glial cell swelling with the aid of exogenous anion channels

Hypotonic challenge induces transient swelling in glial cells, which is typically followed by a regulatory volume decrease (RVD). In contrast, lactic acidosis (lactacidosis) induces persistent cell swelling in astrocytes without an accompanying RVD. In the present study, we studied the mechanisms by which lactacidosis interferes with normal volume regulation in rat astrocytic glioma C6 cells. Following exposure of C6 cells to a hypotonic challenge, a current was detected that exhibited properties consistent with those of volume-sensitive outwardly rectifying (VSOR) anion channels. When exposed to in vitro conditions designed to simulate lactacidosis, C6 cells failed to respond to hypotonic stress with an RVD, and VSOR anion currents were not activated. When added to C6 cells, an anion channel-forming protein purified from Helicobacter pylori, VacA, was found to form anion-selective channels in the plasma membrane, and the activity of the VacA channel was not affected by lactacidosis (pH 6.2). Cells preincubated with VacA and then exposed to lactacidotic conditions underwent transient swelling followed by RVD. In contrast, application of a cation ionophore, gramicidin, failed to inhibit lactacidosis-induced persistent cell swelling. From these results, we conclude that inhibition of a volume-sensitive anion channel contributes to persistent swelling induced by lactacidosis in glial cells. Introduction of anion channel activity into glial cells might provide a novel approach for treating cerebral edema, which is associated with lactacidosis in cerebral ischemia or head injury.

Influence of calcium on regulatory volume decrease: role of potassium channels

In most cell types, hyposmotic swelling consistently elicits an increase in the concentration of cytosolic Ca2+ - [Ca2+]i - with contributions of extracellular and intracellular sources. The mechanisms of Ca2+ entry and release from endogenous sources are not fully clarified and may be cell specific. The ubiquity of the swelling-evoked [Ca2+]i rise makes Ca2+ a likely candidate for a role as osmotransducing signal. However, the regulatory volume decrease (RVD) which follows swelling and the osmolyte fluxes involved in this process are not always Ca2+ dependent. It was found that, with a few exceptions, in most cell types the osmosensitive Cl- efflux pathway and the swelling-activated organic osmolyte fluxes are Ca2+ independent. In contrast, Ca2+-dependent or Ca2+-independent K+ fluxes activated by swelling are detected, depending on the cell type. The close correlation found in this review between the Ca2+ dependence of RVD and that of the K+ channels activated by swelling led to the conclusion that it is the type of osmosensitive K+ pathway which largely confers the Ca2+ dependence to RVD. Interestingly, this coincidence of Ca2+-dependent K+ efflux and RVD is found predominantly in epithelial cells, whereas in nonepithelial cells both processes are largely Ca2+ independent. In these cells, the [Ca2+]i rise elicited by swelling may be an epiphenomenon.

Isovolumetric regulation of rat glial cells during development and correction of hypo-osmolality

Rat C6 glioma cells undergo regulatory volume decrease (RVD) following sudden exposure to hypo-osmolality, but little or no regulatory volume increase (RVI) is observed when cells cultured in hypo-osmotic media are suddenly returned to isoosmolality. Because C6 glioma cells would rarely be exposed to sudden large changes in osmolality in vivo, we examined the ability of these cells to maintain their volume, termed 'isovolumetric regulation', when exposed to gradual changes in osmolality. When osmolality was gradually reduced by reduction of NaCl concentration from 300 to 250 mOsmol/kg at a rate of 0.4 mOsmol/kg/min or less cells were able to maintain their volume, while at higher rates, the cells swelled. Cells which were cultured in hypo-osmotic (200 mOsmol/kg) media for 3 days exhibited isovolumetric regulation at rates of osmolality increase of 0.5 mOsmol/kg/min or less over the range of 200-250 mOsmol/kg. We conclude that rat C6 glioma cells can sensitively regulate their volume over the osmolality range of pathophysiologic interest at rates of osmolality change which are faster than those generally seen in clinical conditions.

Relevance of calcium homeostasis in glial cell swelling from acidosis

Tissue acidosis from trauma or ischemia induces cytotoxic brain edema, mainly affecting astrocytes. In vitro, lactacidosis induces a dose-dependent swelling of glial cells. Activation of membrane transporters and channels, also involved in regulation of intracellular pH (pHi), has been identified as underlying mechanism, although details are poorly understood. We have currently studied whether Ca(2+)-ions play a role in acidosis-induced glial swelling and the associated intracellular acidification. The medium pH of a cell suspension (C6 glioma) was lowered from control (7.4) to 6.2 by lactic acid. Cell volume (CV) and pHi were assessed by flow cytometry. During acidosis in normal medium (2.2 mM Ca2+) CV reached a maximum of 125.1%. In a calcium-free medium swelling from acidosis was inhibited by 74%, while additional buffering of intracellular calcium (Ca2+i) by BAPTA-AM had no further effect. Buffering of Ca2+i alone did not affect the CV increase from acidosis at all. pHi which is decreasing during acidosis was not influenced by the above modifications. The present experiments indicate that lactacidosis-induced glial swelling depends on the presence of extracellular Ca(2+)-ions, while alterations of Ca2+i do not seem to be involved.

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.

Role of calcium ions in acidosis-induced glial swelling

Tissue acidosis occurring in cerebral ischemia and traumatic brain injury is a mediator of cytotoxic brain edema. In vitro, extracellular lactacidosis induces swelling of glial cells in a dose dependent manner. pH-regulatory membrane transporters and channels have been identified which are involved in the increase of the glial cell volume. Underlying mechanisms of their activation are poorly understood, however. We have, therefore, addressed the question, whether and how Ca(2+)-ions play a role in acidosis-induced glial swelling and intracellular acidification. For that purpose C6 glioma cells were suspended and the pH in the medium was lowered from 7.4 (baseline) to 6.2 by isotonic lactic acid. Cell volume and intracellular pH (pHi) were assessed by flow cytometry. In the presence of Ca(2+)-ions the cell volume reached a maximum of 125.1% from acidosis. In experiments using a calcium-free suspension medium, cell swelling from acidosis was inhibited by 74%. Additional buffering of intracellular calcium (Ca2+i) had no further inhibitory effect on acidosis-induced cell swelling, while buffering of Ca2+i by BAPTA-AM alone did not affect the glial volume increase secondary to administration of lactic acid. pHi which was decreasing from acidosis was not affected by the experimental modifications of the Ca(2+)-concentration in the medium or cytosol. The present data indicate that lactacidosis-induced glial swelling depends on the presence of extracellular Ca(2+)-ions, while release of Ca(2+)-ions from intracellular stores does not seem to be involved.

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.

Mechanotransducing ion channels in C6 glioma cells

Mechanotransducing (MS) ion channels and images of the patch membrane were studied in cell-attached patches in C6 glioma cells. MS channel density was approximately 0.08 to 0.5 channels/microns2, channel conductance was approximately 40 pS (at -40 mV), and the reversal potential was +15 mV. Replacement of NaCl with KCl, CsCl, or Na gluconate in the pipette solution was without substantial effect on the current-voltage relationship. Replacement of NaCl with NMDG (N-Methyl-D-Glucamine) Cl or reducing NaCl decreased the amplitude of inward currents at negative membrane potentials and caused the reversal potential to shift in the negative direction. Rapid application of suction to the back of the pipette usually elicited a fast (< 0.1 s) appearance of channel activity. The peak (phasic) in channel activity was followed by a decrease to a constant (tonic) level of activity. The reduction in channel activity--called adaptation--was reduced at depolarizing membrane potentials and disappeared if too much pressure was applied. Positive pressure caused the patch membrane to curve toward the pipette tip, move in the direction of the tip, and evoke MS channel activity. Removal of the positive pressure caused the patch to move back to the original position. Conversely, negative pressure caused the patch membrane to curve away from the pipette tip, move away from the tip, and elicit MS channel activity. Gigohm seal resistances were always maintained during translational movement of the patch membrane. Tonic MS channel activity was not associated with translational movements of the patch membrane. Phasic and tonic channel activity were independent of the sign of curvature of the patch membrane. C6 glioma cells have rapidly adapting voltage-dependent MS ion channels, which are non-selective for monovalent cations, and belong to the stretch-activating class of mechanosensory ion channels. Adaptation in MS channels may allow the cell to limit the influx of cations in response to mechanical input. The selective loss of adaptation suggests that the MS channel's gate receives input from two sources. A minimal viscoelastic mechanical model of adaptation and two alternative models for translational movement of the patch are presented.

Volume regulation in rat brain glial cells: lack of a substantial contribution of free amino acids

Volume regulation of C6 glioma cells was studied while the bath osmolality was reduced from 300 to 150 mosmol/kg. Exposure to a hyposmotic challenge elicited a typical regulatory volume decrease (RVD). No regulatory volume increase was observed upon restoration of isosmotic conditions. During a second subsequent hyposmotic challenge, the cells did not respond with RVD. High extracellular K+ concentration and the K+ channel blockers Ba2+ and quinine inhibited the RVD. RVD was abolished after Cl- was replaced by gluconate and by the Cl- channel blocker 5-nitro-2(3-phenylpropylamino)benzoic acid. Amino acid (AA) concentration in cell and perfusate was determined. In control, cell content was only 26 mmol/l. Hypotonicity increased the efflux of AA from 0.14 to 0.60 mmol/min. During the second hyposmotic challenge, the release was 0.32 mmol/min. The data show that C6 cells adjust their volume under hyposmotic conditions but lose the ability to restore their volume during a subsequent hyposmotic treatment. K+ and Cl- are the main osmolytes involved in volume adjustment through conductive pathways. AA do not contribute substantially to cell volume regulation.