Inhibition by Cl- channel blockers of the volume-activated, diffusional mechanism of inositol transport in primary astrocytes in culture

Peroxynitrite enhances astrocytic volume-sensitive excitatory amino acid release via a src tyrosine kinase-dependent mechanism

Volume-regulated anion channels (VRACs) are critically important for cell volume homeostasis, and under pathological conditions contribute to neuronal damage via excitatory amino (EAA) release. The precise mechanisms by which brain VRACs are activated and/or modulated remain elusive. In the present work we explored the possible involvement of nitric oxide (NO) and NO-related reactive species in the regulation of VRAC activity and EAA release, using primary astrocyte cultures. The NO donors sodium nitroprusside and spermine NONOate did not affect volume-activated d-[3H]aspartate release. In contrast, the peroxynitrite (ONOO-) donor 3-morpholinosydnomine hydrochloride (SIN-1) increased volume-dependent EAA release by approx. 80-110% under identical conditions. Inhibition of ONOO- formation with superoxide dismutase completely abolished the effects of SIN-1. Both the volume- and SIN-1-induced EAA release were sensitive to the VRAC blockers NPPB and ATP. Further pharmacological analysis ruled out the involvement of cGMP-dependent reactions and modification of sulfhydryl groups in the SIN-1-inducedmodulation of EAA release. The src family tyrosine kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo [3,4-d]pyrimidine (PP2), but not its inactive analog PP3, abolished the effects of SIN-1. A broader spectrum tyrosine kinase inhibitor tyrphostin A51, also completely eliminated the SIN-1-induced EAA release. Our data suggest that ONOO- up-regulates VRAC activity via a src tyrosine kinase-dependent mechanism. This modulation may contribute to EAA-mediated neuronal damage in ischemia and other pathological conditions favoring cell swelling and ONOO- production.

Inositol and higher inositol phosphates in neural tissues: homeostasis, metabolism and functional significance

Inositol phospholipids and inositol phosphates mediate well-established functions in signal transduction and in Ca2+ homeostasis in the CNS and non-neural tissues. More recently, there has been renewed interest in other roles that both myo-inositol and its highly phosphorylated forms may play in neural function. We review evidence that myo-inositol serves as a clinically relevant osmolyte in the CNS, and that its hexakisphosphate and pyrophosphorylated derivatives may play roles in such diverse cellular functions as DNA repair, nuclear RNA export and synaptic membrane trafficking.

Regulation of Myo-inositol homeostasis in differentiated human NT2-N neurons

We have investigated the possible role of second messengers on inositol homeostasis in NT2-N cells, human central nervous system neurons obtained by terminal differentiation of teratocarcinoma precursors. Uptake of inositol into NT2-N neurons was inhibited approximately 10% by protein kinase C (PKC) activation but was unaffected by either the presence of cyclic nucleotide analogs or changes in the intracellular concentration of Ca2+. Efflux of inositol from NT2-N neurons was enhanced in hypotonic buffer but virtually eliminated by inclusion of the Cl- channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, a result which indicates the involvement of a volume-sensitive organic osmolyte-anion channel. Volume-sensitive inositol efflux was stimulated approximately 30% following activation of PKC or elevation of the cytosolic Ca2+ concentration but was unaffected by protein kinase A activation. These results suggest that whereas inositol uptake into NT2-N neurons is relatively refractory to regulation, volume-sensitive inositol efflux may be significantly affected by intracellular signaling events.

Glutamate release through volume-activated channels during spreading depression

Volume-sensitive organic anion channels (VSOACs) in astrocytes are activated by cell swelling and are permeable to organic anions, such as glutamate and taurine. We have examined the release of glutamate through VSOACs during the propagation of spreading depression (SD). SD was induced by bath application of ouabain in hippocampal brain slices and was monitored by imaging intrinsic optical signals, a technique that provides a measure of cellular swelling. The onset of SD was associated with increased light transmittance, confirming previous studies that cellular swelling occurs during SD. NMDA receptor antagonists, either noncompetitive (MK-801, 10-50 microM) or competitive (CGS-17355, 100 microM), reduced the rate of propagation of SD, indicating that glutamate release contributes to SD onset. SD still occurred in zero Ca(2+)-EGTA (0-Ca(2+)-EGTA) solution, a manipulation that depresses synaptic transmission. HPLC measurements indicated that, even in this solution, there was significant glutamate release. Two lines of experiments indicated that glutamate was released through VSOACs during SD. First, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), a blocker of VSOACs, depressed the rate of propagation of SD in a manner similar to NMDA antagonists. Second, NPPB inhibited the release of glutamate during SD in 0-Ca(2+)-EGTA external solution. These results indicate that cellular swelling during SD causes the activation of VSOACs and the release of glutamate by permeation through this channel. Cellular swelling is a result of neuronal activity and is observed during excitotoxicity. Therefore, glutamate release from VSOAC activation could occur under conditions of cell swelling and contribute to excitotoxic damage.

Differentiated human NT2-N neurons possess a high intracellular content of myo-inositol

myo-Inositol plays a key role in signal transduction and osmotic regulation events in the CNS. Despite the known high concentrations of inositol in the human CNS, relatively little is known about its distribution within the different cell types. In this report, inositol homeostasis was studied in NT2-N cells, a unique cell culture model of human CNS neurons. Differentiation of precursor NT2 teratocarcinoma cells into NT2-N neurons by means of retinoic acid treatment resulted in an increase in inositol concentration from 24 to 195 nmol/mg of protein. After measurement of intracellular water spaces, inositol concentrations of 1.6 and 17.4 mM were calculated for NT2 and NT2-N cells, respectively. The high concentrations of inositol in NT2-N neurons could be explained by (1) an increased uptake of inositol (3.7 vs. 1.6 nmol/mg of protein/h, for NT2-N and NT2 cells, respectively) and (2) a decreased efflux of inositol (1.7%/h for NT2-N neurons vs. 9.0%/h for NT2 cells). Activity of inositol synthase, which mediates de novo synthesis of inositol, was not detected in either cell type. The observation that CNS neurons maintain a high intracellular concentration of inositol may be relevant to the regulation of both phosphoinositide signaling and osmotic stress events in the CNS.

Pharmacology of CFTR chloride channel activity

Pharmacology of CFTR Chloride Channel Activity. Physiol. Rev. 79, Suppl.: S109-S144, 1999. - The pharmacology of cystic fibrosis transmembrane conductance regulator (CFTR) is at an early stage of development. Here we attempt to review the status of those compounds that modulate the Cl- channel activity of CFTR. Three classes of compounds, the sulfonylureas, the disulfonic stilbenes, and the arylaminobenzoates, have been shown to directly interact with CFTR to cause channel blockade. Kinetic analysis has revealed the sulfonylureas and arylaminobenzoates interact with the open state of CFTR to cause blockade. Suggestive evidence indicates the disulfonic stilbenes act by a similar mechanism but only from the intracellular side of CFTR. Site-directed mutagenesis studies indicate the involvement of specific amino acid residues in the proposed transmembrane segment 6 for disulfonic stilbene blockade and segments 6 and 12 for arylaminobenzoate blockade. Unfortunately, these compounds (sulfonylureas, disulfonic stilbenes, arylaminobenzoate) also act at a number of other cellular sites that can indirectly alter the activity of CFTR or the transepithelial secretion of Cl-. The nonspecificity of these compounds has complicated the interpretation of results from cellular-based experiments. Compounds that increase the activity of CFTR include the alkylxanthines, phosphodiesterase inhibitors, phosphatase inhibitors, isoflavones and flavones, benzimidazolones, and psoralens. Channel activation can arise from the stimulation of the cAMP signal transduction cascade, the inhibition of inactivating enzymes (phosphodiesterases, phosphatases), as well as the direct binding to CFTR. However, in contrast to the compounds that block CFTR, a detailed understanding of how the above compounds increase the activity of CFTR has not yet emerged.

Expression of pI(Cln) mRNA in cultured bovine lens epithelial cells: response to changes in cell volume

PURPOSE

The authors recently established a link between swelling-activated myo-inositol efflux and chloride movement via anion channels in cultured bovine lens epithelial cells (BLECs). To further define this pathway, the relationship between cell volume, myo-inositol movement and mRNA expression of pI(Cln), a proposed chloride channel regulatory protein was investigated.

METHODS

To demonstrate the effect of cell volume changes on pIcln transcription, BLECs were exposed to either hypertonic or hypotonic medium conditions. For rapid cellular shrinkage, BLECs were maintained at confluence in physiologic medium (257+/-2 mosm) then transferred to sodium hypertonic medium (473+/-6 mosm) or raffinose hypertonic medium (452+/-2 mosm). For rapid cellular swelling, cells were switched from sodium hypertonic medium to physiologic medium+/-tamoxifen. The expression of pI(Cln) mRNA was determined by Northern blot analysis.

RESULTS

Upon cell volume reduction (increasing intracellular osmolality), BLECs upregulate the expression of pI(Cln) mRNA. Contrastly, when cell volume rapidly increases (decreasing intracellular osmolality), BLECs moderately downregulate pIcln mRNA, with expression levels reaching near physiologic control by 24 h. Blockage of swelling-activated chloride movement and osmolyte efflux with either tamoxifen or niflumic acid enhances the downregulation of pIcln mRNA expression.

CONCLUSIONS

In cultured BLECs, pI(Cln) transcriptional regulation appears to be responsive to cell volume fluctuations. These data suggest a converse relationship exists between pIcln mRNA expression and changes in cell volume.

Amino acid release during volume regulation by cardiac cells: cellular mechanisms

Mechanisms of amino acid efflux during volume regulation in hypoosmotically treated isolated rat hearts were studied by collecting the coronary artery perfusate and analysis by high pressure liquid chromatography. Hypoosmotic stress resulted in marked percentage increases in perfusate taurine, aspartate and glutamate levels, smaller increases in phosphoethanolamine, glycine and alanine and non-significant increases in serine and glutamine. Amino acid levels declined during reperfusion with isosmotic perfusate. The anion channel blocker 4-acetamido-4-isothiocyanostilbene-2:2'-disulfonic acid (SITS, 500 microM) significantly reduced hypoosmotic release of taurine, aspartate, glutamate and glycine. Furosemide reduced hypoosmotically-evoked releases of taurine, glycine, alanine and phosphoethanolamine. The polyunsaturated amino acids, arachidonic and linoleic also reduced amino acid efflux. Phospholipase A2 inhibition with 7,7-dimethyleicosadienoic acid (DEDA, 2 microM) reduced osmotically-evoked releases of taurine, aspartate and glutamate. 4-Bromophenacyl bromide (1 microM) inhibited osmotically-evoked release of glutamate and glycine. Combined applications of SITS + DEDA markedly reduced osmotically evoked release of all eight amino acids. Glutamate and aspartate effluxes were not inhibited by the glutamate transport inhibitor dihydrokainic acid (1 mM). These results indicate that the hypoosmotic stress, by inducing cell swelling, can initiate an amino acid efflux as part of a regulatory volume decrease. An opening of anion-permeant channels and phospholipase activation appear to be involved in the regulatory volume decrease phenomenon.

Volume regulation in NIH/3T3 cells not expressing P-glycoprotein. I. Regulatory volume decrease

Exposure of NIH/3T3 fibroblasts not expressing P-glycoprotein to 50, 30, 20, and 10% hyposmotic solutions led to cell volume increases of 70, 32, 21, and 12%, respectively. After swelling, NIH/3T3 cells exhibited regulatory volume decrease (RVD), attaining complete volume recovery after 30 min except in 50% hyposmotic solution, in which volume recovery was 76%. RVD was accelerated by gramicidin and inhibited by the Cl channel blockers 5-nitro-2-(3-phenylpropylamino)-benzoic acid, 1,9-dideoxyforskolin, dipyridamole, and niflumic acid and by the K channel, blocker quinidine. RVD was reduced 15% by removal of extracellular Ca. The pathway opened by hypotonicity was highly permeable to K and Rb and only partly permeable to other cations. Most anions were able to permeate, with a permeability ranking of nitrate > benzoate = iodide > thiocyanate > chloride > > gluconate. The pathway was permeable to neutral amino acids, with a permeability ranking of glycine > alanine > glutamate > taurine > gamma-aminobutyric acid > glutamine. The pathway was not permeable to basic amino acids. These results show that, despite the absence of P-glycoprotein, NIH/3T3 cells exhibit RVD with properties similar to those expressed in most cell types.

Inhibition by anion channel blockers of ischemia-evoked release of excitotoxic and other amino acids from rat cerebral cortex

Neuronal and glial cell swelling occurs rapidly in ischemia as part of the cytotoxic response. Astrocytic swelling is known to result in large extracellular increases in certain amino acids, including glutamate, aspartate and taurine, as part of the regulatory volume decrease (RVD) response inherent to these and other cells. RVD in astrocytic cultures is inhibited by anion channel blockers. In this study, we compared the effects of three anion channel blockers on the ischemia/reperfusion-evoked release of amino acids from the in vivo rat cerebral cortex. Twenty minutes of four vessel cerebral ischemia caused significant increases in cortical superfusate levels of aspartate, glutamate, GABA, taurine and phosphoethanolamine. During reperfusion there were delayed increases in the level of glycine, alanine and serine. Glutamine levels were not affected. Cl- channel blockers, 4-acetamido-4'-isothiocyanostrilbene-2,2'-disulfonic acid (SITS, 2 mM), 5-nitro-2-(3-phenyl-propylamino)benzoic acid (NPPB, 350 microM) and dipyridamole (200 microM) depressed basal releases of glutamate and taurine and the ischemia/reperfusion-evoked releases of aspartate, glutamate, taurine and phosphoethanolamine. The results suggest that diffusion of amino acids through an anion channel may be partially responsible for the elevated extracellular levels of excitotoxic and other amino acids that occur during cerebral ischemia/reperfusion.

The role of swelling-induced anion channels during neuronal volume regulation

Regulation of cell volume is an essential function of most mammalian cells. In the cells of the central nervous system, maintenance of cell osmolarity and, hence, volume, is particularly crucial because of the restrictive nature of the skull. Cell volume regulation involves a variety of pathways, with considerable differences between cell types. One common pathway activated during hypo-osmotic stress involves chloride (Cl-) channels. However, hypo-osmotically stimulated anion permeability can be regulated by a diverse array of second messengers. Although neuronal swelling can occur in a number of pathological and nonpathological conditions, our understanding of neuronal volume regulation is limited. This article summarizes our current understanding of the role of anion channels during neuronal volume regulation.

Volume regulatory taurine release in human tracheal 9HTEo- and multidrug resistant 9HTEo-/Dx cells

The intracellular taurine release evoked by hypotonic shock is accomplished by volume-activated Cl- channels whose activity has been related to the expression of the multidrug resistance protein (MDR-1). We studied taurine transport in 9HTEo- cells and in the derived cell line 9HTEo-/Dx expressing MDR-1. [3H]taurine release from preloaded cells increased upon reduction of extracellular osmolality. This process was not inhibited by preincubation with phorbol 12-myristate 13-acetate but was reduced by inhibitors of volume-sensitive Cl- channels such as 1,9-dideoxiforskolin, La3+, and arachidonate. Verapamil, a substrate of MDR-1, increased the osmotically evoked taurine efflux. Replacement of extracellular Cl- with I- or gluconate or of extracellular Na+ with Li+ significantly reduced the taurine efflux, whereas substitution of N-methyl-D-glucamine for Na+ increased it. Application of ATP and 2-chloroadenosine stimulated the efflux in isotonic medium. No differences were seen between 9HTEo- and 9HTEo-/Dx cells with respect to hypotonically induced taurine efflux and the response to phorbol ester, channel blockers, ion replacement, and purinergic agents. Our results reveal novel properties of the osmotically induced taurine release and demonstrate its independence from MDR-1 gene expression.