Rho family GTP binding proteins are involved in the regulatory volume decrease process in NIH3T3 mouse fibroblasts

Hydrogen peroxide potentiates volume-sensitive excitatory amino acid release via a mechanism involving Ca2+/calmodulin-dependent protein kinase II

Excessive excitatory amino acid (EAA) release in cerebral ischemia is a major mechanism responsible for neuronal damage and death. A substantial fraction of ischemic EAA release occurs via volume-regulated anion channels (VRACs). Hydrogen peroxide (H2O2), which is abundantly produced during ischemia and reperfusion, activates a number of protein kinases critical for VRAC functioning and has recently been reported to activate VRACs. In the present study, we explored the effects of H2O2 on volume-dependent EAA release in cultured astrocytes, measured as the release of preloaded D-[3H]aspartate. 100-1,000 microm H2O2 enhanced swelling-induced EAA release by approximately 2.5-3-fold (EC50 approximately 10 microM). The VRAC blockers ATP, phloretin, and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) potently inhibited both control swelling-induced and the H2O2-potentiated release, suggesting a role for VRACs. The H2O2-induced component of EAA release was attenuated by the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM) and completely eliminated by the calmodulin antagonists trifluoperazine and W-7 and the Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93. Inhibitors of tyrosine kinases, protein kinase C, and the myosin light chain kinase were ineffective in blocking the H2O2 response. H2O2 treatment of swollen astrocytes, but not swelling alone, resulted in CaMKII activation that was inhibited by KN-93, as determined by a phospho-Thr286 CaMKII antibody. These data demonstrate that H2O2 strongly up-regulates astrocytic volume-sensitive EAA release via a CaMKII-dependent mechanism and in this way may potently promote pathological EAA release and brain damage in ischemia.

ATP regulates anion channel-mediated organic osmolyte release from cultured rat astrocytes via multiple Ca2+-sensitive mechanisms

Ubiquitously expressed volume-regulated anion channels (VRACs) are activated in response to cell swelling but may also show limited activity in nonswollen cells. VRACs are permeable to inorganic anions and small organic osmolytes, including the amino acids aspartate, glutamate, and taurine. Several recent reports have demonstrated that neurotransmitters or hormones, such as ATP and vasopressin, induce or strongly potentiate astrocytic whole cell Cl(-) currents and amino acid release, which are inhibited by VRAC blockers. In the present study, we explored the intracellular signaling mechanisms mediating the effects of ATP on d-[(3)H]aspartate release via the putative VRAC pathway in rat primary astrocyte cultures. Cells were exposed to moderate (5%) or substantial (30%) reductions in medium osmolarity. ATP strongly potentiated d-[(3)H]aspartate release in both moderately swollen and substantially swollen cells. These ATP effects were blocked (>or=80% inhibition) by intracellular Ca(2+) chelation with BAPTA-AM, calmodulin inhibitors, or a combination of the inhibitors of protein kinase C (PKC) and calmodulin-dependent kinase II (CaMK II). In contrast, control d-[(3)H]aspartate release activated by the substantial hyposmotic swelling showed little (<or=25% inhibition) sensitivity to the same pharmacological agents. These data indicate that ATP regulates VRAC activity via two separate Ca(2+)-sensitive signaling cascades involving PKC and CaMK II and that cell swelling per se activates VRACs via a separate Ca(2+)/calmodulin-independent signaling mechanism. Ca(2+)-dependent organic osmolyte release via VRACs may contribute to the physiological functions of these channels in the brain, including astrocyte-to-neuron intercellular communication.

Ca2+-mediated Potentiation of the Swelling-induced Taurine Efflux from HeLa Cells: On the Role of Calmodulin and Novel Protein Kinase C Isoforms

The present work sets out to investigate how Ca(2+) regulates the volume-sensitive taurine-release pathway in HeLa cells. Addition of Ca(2+)-mobilizing agonists at the time of exposure to hypotonic NaCl medium augments the swelling-induced taurine release and subsequently accelerates the inactivation of the release pathway. The accelerated inactivation is not observed in hypotonic Ca(2+)-free or high-K(+) media. Addition of Ca(2+)-mobilizing agonists also accelerates the regulatory volume decrease, which probably reflects activation of Ca(2+)-activated K(+) channels. The taurine release from control cells and cells exposed to Ca(2+) agonists is equally affected by changes in cell volume, application of DIDS and arachidonic acid, indicating that the volume-sensitive taurine leak pathway mediates the Ca(2+)-augmented taurine release. Exposure to Ca(2+)-mobilizing agonists prior to a hypotonic challenge also augments a subsequent swelling-induced taurine release even though the intracellular Ca(2+)-concentration has returned to the unstimulated level. The Ca(2+)-induced augmentation of the swelling-induced taurine release is abolished by inhibition of calmodulin, but unaffected by inhibition of calmodulin-dependent kinase II, myosin light chain kinase and calcineurin. The effect of Ca(2+)-mobilizing agonists is mimicked by protein kinase C (PKC) activation and abolished in the presence of the PKC inhibitor Gö6850 and following downregulation of phorbol ester-sensitive PKC isoforms. It is suggested that Ca(2+) regulates the volume-sensitive taurine-release pathway through activation of calmodulin and PKC isoforms belonging to the novel subclass (nPKC).

Mechanisms of the ATP potentiation of hyposmotic taurine release in Swiss 3T3 fibroblasts

Reducing osmolarity by 35% increased (3)H-taurine efflux from Swiss 3T3 fibroblasts from 0.5% to a peak of 5.7%. The presence of ATP (10-100 microM; EC(50) 1.5 microM) increased taurine efflux up to 10%, and decreased the set point for hyposmotically stimulated taurine release (HTR). ATP potentiation was mimicked by UTP, reduced by addition of suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and unaffected by ADP, beta,gamma-methylene-ATP (beta,gamma-ATP) or 2-methylthio-ATP (Me-ATP), suggesting its mediation by purinergic P2Y(2) and P2Y(4) metabotropic receptors. Under isosmotic conditions ATP increased the cytosolic [Ca(2+)] ([Ca(2+)](i)) markedly, but did not increase taurine release. HTR was independent of external Ca(2+) but was reduced (by 56-59%) by BAPTA-AM, thapsigargin-induced depletion of intracellular Ca(2+) stores, or phospholipase C (PLC) inhibition. Blockade of calmodulin (CaM) or calmodulin kinase II (CaMKII) reduced HTR by 54% and 76%, respectively. The ATP-mediated potentiation was prevented fully by all these treatments. HTR was reduced by 30-50% by blockers of protein tyrosine kinases (AG18), phosphoinositide 3-kinase (PI3K) (wortmannin), p21rho (toxin B), p21rho-kinase (Y27632) and the stress-activated kinase p38 (PD169316). ATP-mediated potentiation was reduced similarly by these blockers. Simultaneous inhibition of PI3K and CaMKII abolished HTR. Altogether, these results suggest a modulatory effect of ATP, probably exerted by a potentiation of the Ca(2+)-dependent fraction of HTR. This fraction has as signalling elements a PLC-dependent [Ca(2+)](i) increase, resulting from Ca(2+) released from thapsigargin-sensitive internal stores, followed by activation of CaM/CaMKII reactions. The Ca(2+)/ATP effect operates only when the Ca(2+)-independent, tyrosine kinase-mediated pathway is already activated. Suggested elements of cross-talk between the two pathways are PLC, PI3K and CaMKII.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

Hypotonic activation of volume-sensitive outwardly rectifying chloride channels in cultured PASMCs is modulated by SGK

The serum- and glucocorticoid-inducible kinase (SGK) is a serine/threonine protein kinase (PK) transcriptionally regulated by corticoids, serum, and cell volume. SGK regulates cell volume of various cells by effects on Na(+) and K(+) transport through membrane channels. We hypothesized a role for SGK in the activation of volume-sensitive osmolyte and anion channels (VSOACs) in cultured canine pulmonary artery smooth muscle cells (PASMCs). Intracellular dialysis through the patch electrode of recombinant active SGK, but not kinase-dead Delta60-SGK-K127M, heat-inactivated SGK, or active Akt1, partially activated VSOACs under isotonic conditions. Dialysis of active SGK before cell exposure to hypotonic medium significantly accelerated the activation kinetics and increased the maximal density of VSOAC current. Exposure of PASMCs to hypotonic medium (230 mosM) activated phosphatidylinositol 3-kinases (PI3Ks) and their downstream targets Akt/PKB and SGK but not PKC-epsilon. Inhibition of PI3Ks with wortmannin reduced the activation rate and maximal amplitude of VSOACs. Immunoprecipitated ClC-3 channels were phosphorylated by PKC-epsilon but not by SGK in vitro, suggesting that SGK may activate VSOACs indirectly. These data indicate that the PI3K-SGK cascade is activated on hypotonic swelling of PASMCs and, in turn, affects downstream signaling molecules linked to activation of VSOACs.

Volume changes and whole cell membrane currents activated during gradual osmolarity decrease in C6 glioma cells: contribution of two types of K+ channels

Volume changes and whole cell ionic currents activated by gradual osmolarity reductions (GOR) of 1.8 mosM/min were characterized in C6 glioma cells. Cells swell less in GOR than after sudden osmolarity reductions (SOR), the extent of swelling being partly Ca(2+) dependent. In nominally Ca(2+)-free conditions, GOR activated predominantly whole cell outward currents. Cells depolarized from the initial -79 mV to a steady state of -54 mV reached at 18% osmolarity reduction [hyposmolarity of -18% (H-18%)]. Recordings of Cl(-) and K(+) currents showed activation at H-3% of an outwardly rectifying Cl(-) current, with conductance of 1.6 nS, sensitive to niflumic acid and 5-nitro-2-(3-phenylpropylamino)benzoic acid, followed at H-18% by an outwardly rectifying K(+) current with conductance of 4.1 nS, inhibited by clofilium but insensitive to the typical K(+) channel blockers. With 200 nM Ca(2+) in the patch pipette, whole cell currents activated at H-3% and at H-13% cells depolarized from -77 to -63 mV. A K(+) current activated at H-1%, showing a rapid increase in conductance, suppressed by charybdotoxin and insensitive to clofilium. These results show the operation of two different K(+) channels in response to GOR in the same cell type, activated by Ca(2+) and osmolarity and with different osmolarity activation thresholds. Taurine and glutamate efflux, monitored by labeled tracers, showed delayed osmolarity thresholds of H-39 and H-33%, respectively. This observation clearly separates the Cl(-) and amino acid osmosensitive pathways. The delayed amino acid efflux may contribute to counteract swelling at more stringent osmolarity reductions.

Hypotonicity induces membrane protrusions and actin remodeling via activation of small GTPases Rac and Cdc42 in Rat-1 fibroblasts

An important consequence of cell swelling is the reorganization of the F-actin cytoskeleton in different cell types. We demonstrate in this study by means of rhodamine-phalloidin labeling and fluorescence microscopy that a drastic reorganization of F-actin occurs in swollen Rat-1 fibroblasts: stress fibers disappear and F-actin patches are formed in peripheral extensions at the cell border. Moreover, we demonstrate that activation of both Rac and Cdc42, members of the family of small Rho GTPases, forms the link between the hypotonic stimulation and F-actin reorganization. Indeed, inhibition of the small GTPases RhoA, Rac, and Cdc42 (by Clostridium difficile toxin B) prevents the hypotonicity-induced reorganization of the actin cytoskeleton, whereas inhibition of RhoA alone (by C. limosum C3 exoenzyme) does not preclude this rearrangement. Second, a direct activation and translocation toward the actin patches underneath the plasma membrane is observed for endogenous Rac and Cdc42 (but not for RhoA) during cell swelling. Finally, transfection of Rat-1 fibroblasts with constitutively active RhoA, dominant negative Rac, or dominant negative Cdc42 abolishes the swelling-induced actin reorganization. Interestingly, application of cRGD, a competitor peptide for fibronectin-integrin association, induces identical membrane protrusions and changes in the F-actin cytoskeleton that are also inhibited by C. difficile toxin B and dominant negative Rac or Cdc42. Moreover, cRGD also induces a redistribution of endogenous Rac and Cdc42 to the newly formed submembranous F-actin patches. We therefore conclude that hypotonicity and cRGD remodel the F-actin cytoskeleton in Rat-1 fibroblasts in a Rac/Cdc42-dependent way.

Hyperosmotic stress activates Rho: differential involvement in Rho kinase-dependent MLC phosphorylation and NKCC activation

Hyperosmotic stress initiates adaptive responses, including phosphorylation of myosin light chain (MLC) and concomitant activation of Na+-K+-Cl- cotransporter (NKCC). Because the small GTPase Rho is a key regulator of MLC phosphorylation, we investigated 1) whether Rho is activated by hyperosmotic stress, and if so, what the triggering factors are, and 2) whether the Rho/Rho kinase (ROK) pathway is involved in MLC phosphorylation and NKCC activation. Rho activity was measured in tubular epithelial cells by affinity pulldown assay. Hyperosmolarity induced rapid (<1 min) and sustained (>20 min) Rho activation that was proportional to the osmotic concentration and reversed within minutes upon restoration of isotonicity. Both decreased cell volume at constant ionic strength and elevated total ionic strength at constant cell volume were capable of activating Rho. Changes in [Na+] and [K+] at normal total salinity failed to activate Rho, and Cl- depletion did not affect the hyperosmotic response. Thus alterations in cellular volume and ionic strength but not individual ion concentrations seem to be the critical triggering factors. Hyperosmolarity induced mono- and diphosphorylation of MLC, which was abrogated by the Rho-family blocker Clostridium toxin B. ROK inhibitor Y-27632 suppressed MLC phosphorylation under isotonic conditions and prevented its rise over isotonic levels in hypertonically stimulated cells. ML-7 had a smaller inhibitory effect. In contrast, it abolished the hypertonic activation of NKCC, whereas Y-27632 failed to inhibit this response. Thus hyperosmolarity activates Rho, and Rho/ROK pathway contributes to basal and hyperosmotic MLC phosphorylation. However, the hypertonic activation of NKCC is ROK independent, implying that the ROK-dependent component of MLC phosphorylation can be uncoupled from NKCC activation.

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

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