Swelling-activated efflux of taurine and other organic osmolytes in endothelial cells

Metabolomic Disparities in Intraocular Fluid Across Varied Stages of Cataract Progression: Implications for the Analysis of Cataract Development

Introduction: The lens's metabolic demands are met through a continuous circulation of aqueous humor, encompassing a spectrum of components such as organic and inorganic ions, carbohydrates, glutathione, urea, amino acids, proteins, oxygen, carbon dioxide, and water. Metabolomics is a pivotal tool, offering an initial insight into the complexities of integrated metabolism. In this investigative study, we systematically scrutinize the composition of intraocular fluid in individuals afflicted with cataracts. Methods: The investigation involved a comprehensive analysis of aqueous humor samples from a cohort comprising 192 patients. These individuals were stratified by utilizing the SPONCS classification system, delineating distinct groups characterized by the hardness of cataracts. The analytical approach employed targeted quantitative metabolite analysis using HILIC-based liquid chromatography coupled with high-resolution mass spectrometric detection. The metabolomics data analysis was performed with MetaboAnalyst 5.0. Results: The results of the enrichment analysis have facilitated the inference that the discerned disparities among groups arise from disruptions in taurine and hypotaurine metabolism, variations in tryptophan metabolism, and modifications in mitochondrial beta-oxidation of short-chain saturated fatty acids and pyrimidine metabolism. Conclusion: A decline in taurine concentration precipitates diminished glutathione activity, prompting an elevated requirement for NAD+ and instigating tryptophan metabolism along the kynurenine pathway. Activation of this pathway is additionally prompted by interferon-gamma and UV radiation, leading to the induction of IDO. Concurrently, heightened mitochondrial beta-oxidation signifies a distinctive scenario in translocating fatty acids into the mitochondria, enhancing energy production.

Physiological Functions of the Volume-Regulated Anion Channel VRAC/LRRC8 and the Proton-Activated Chloride Channel ASOR/TMEM206

Volume-regulated anion channels (VRACs) and the acid-sensitive outwardly rectifying anion channel (ASOR) mediate flux of chloride and small organic anions. Although known for a long time, they were only recently identified at the molecular level. VRACs are heteromers consisting of LRRC8 proteins A to E. Combining the essential LRRC8A with different LRRC8 paralogues changes key properties of VRAC such as conductance or substrate selectivity, which is how VRACs are involved in multiple physiological functions including regulatory volume decrease, cell proliferation and migration, cell death, purinergic signalling, fat and glucose metabolism, insulin signalling, and spermiogenesis. VRACs are also involved in pathological conditions, such as the neurotoxic release of glutamate and aspartate. Certain VRACs are also permeable to larger, organic anions, including antibiotics and anti-cancer drugs, making them an interesting therapeutic target. ASOR, also named proton-activated chloride channel (PAC), is formed by TMEM206 homotrimers on the plasma membrane and on endosomal compartments where it mediates chloride flux in response to extracytosolic acidification and plays a role in the shrinking and maturation of macropinosomes. ASOR has been shown to underlie neuronal swelling which causes cell death after stroke as well as promoting the metastasis of certain cancers, making them intriguing therapeutic targets as well.

Leucine-rich repeat containing 8A contributes to the expansion of The potential role of leucine-rich repeat-containing protein 8A in central nervous system: current situation and prospect

Cell swelling usually initiates the regulatory volume decrease (RVD) process mediated mainly by volume-regulated anion channels (VRACs), which are formed by multiple different leucine-rich repeat-containing protein 8 (LRRC8) family members. VRAC currents have been widely recorded in astrocytes, neurons and microglia in the brain, and VRACs have been suggested to be involved in the important pathogenesis of cell swelling-related central nervous system (CNS) diseases, such as ischemic stroke, epilepsy and epileptogenesis, glioblastoma (GBM), and so on. Recently, the increasing studies started to focus on LRRC8A (SWELL1), an obligatory subunit of VRAC indentified in 2014, which may be the key target to regulate the VRAC functions. After cerebral ischemia, the swollen astrocytes, neurons and microglia can activate LRRC8A-dependent VRACs, which may respectively promote the release of excitatory amino acids (EAA), interaction with ionotropic glutamate receptors, and regulating inflammation, suggesting the pleiotropic roles of LRRC8A in swollen brain cells. For the treatment of cell swelling-related CNS diseases, specific targeting LRRC8A may be a superior strategy to inhibit swollen-induced VRAC hyperactivity without blocking the normal VRAC function.

Ion Channel Involvement in Tumor Drug Resistance

Over 90% of deaths in cancer patients are attributed to tumor drug resistance. Resistance to therapeutic agents can be due to an innate property of cancer cells or can be acquired during chemotherapy. In recent years, it has become increasingly clear that regulation of membrane ion channels is an important mechanism in the development of chemoresistance. Here, we review the contribution of ion channels in drug resistance of various types of cancers, evaluating their potential in clinical management. Several molecular mechanisms have been proposed, including evasion of apoptosis, cell cycle arrest, decreased drug accumulation in cancer cells, and activation of alternative escape pathways such as autophagy. Each of these mechanisms leads to a reduction of the therapeutic efficacy of administered drugs, causing more difficulty in cancer treatment. Thus, targeting ion channels might represent a good option for adjuvant therapies in order to counteract chemoresistance development.

Atherosclerosis Linked to Aberrant Amino Acid Metabolism and Immunosuppressive Amino Acid Catabolizing Enzymes

Cardiovascular disease is the leading global health concern and responsible for more deaths worldwide than any other type of disorder. Atherosclerosis is a chronic inflammatory disease in the arterial wall, which underpins several types of cardiovascular disease. It has emerged that a strong relationship exists between alterations in amino acid (AA) metabolism and the development of atherosclerosis. Recent studies have reported positive correlations between levels of branched-chain amino acids (BCAAs) such as leucine, valine, and isoleucine in plasma and the occurrence of metabolic disturbances. Elevated serum levels of BCAAs indicate a high cardiometabolic risk. Thus, BCAAs may also impact atherosclerosis prevention and offer a novel therapeutic strategy for specific individuals at risk of coronary events. The metabolism of AAs, such as L-arginine, homoarginine, and L-tryptophan, is recognized as a critical regulator of vascular homeostasis. Dietary intake of homoarginine, taurine, and glycine can improve atherosclerosis by endothelium remodeling. Available data also suggest that the regulation of AA metabolism by indoleamine 2,3-dioxygenase (IDO) and arginases 1 and 2 are mediated through various immunological signals and that immunosuppressive AA metabolizing enzymes are promising therapeutic targets against atherosclerosis. Further clinical studies and basic studies that make use of animal models are required. Here we review recent data examining links between AA metabolism and the development of atherosclerosis.

A 30-year journey from volume-regulated anion currents to molecular structure of the LRRC8 channel

Strange et al. review recent advances in our understanding of the molecular and structural basis of volume-regulated anion channel function within the framework of classical biophysical and physiological studies.

The swelling-activated anion channel VRAC has fascinated and frustrated physiologists since it was first described in 1988. Multiple laboratories have defined VRAC’s biophysical properties and have shown that it plays a central role in cell volume regulation and possibly other fundamental physiological processes. However, confusion and intense controversy surrounding the channel’s molecular identity greatly hindered progress in the field for >15 yr. A major breakthrough came in 2014 with the demonstration that VRAC is a heteromeric channel encoded by five members of the Lrrc8 gene family, Lrrc8A–E. A mere 4 yr later, four laboratories described cryo-EM structures of LRRC8A homomeric channels. As the melee of structure/function and physiology studies begins, it is critical that this work be framed by a clear understanding of VRAC biophysics, regulation, and cellular physiology as well as by the field’s past confusion and controversies. That understanding is essential for the design and interpretation of structure/function studies, studies of VRAC physiology, and studies aimed at addressing the vexing problem of how the channel detects cell volume changes. In this review we discuss key aspects of VRAC biophysics, regulation, and function and integrate these into our emerging understanding of LRRC8 protein structure/function.

Cell Volume Control in Healthy Brain and Neuropathologies

Regulation of cellular volume is a critical homeostatic process that is intimately linked to ionic and osmotic balance in the brain tissue. Because the brain is encased in the rigid skull and has a very complex cellular architecture, even minute changes in the volume of extracellular and intracellular compartments have a very strong impact on tissue excitability and function. The failure of cell volume control is a major feature of several neuropathologies, such as hyponatremia, stroke, epilepsy, hyperammonemia, and others. There is strong evidence that such dysregulation, especially uncontrolled cell swelling, plays a major role in adverse pathological outcomes. To protect themselves, brain cells utilize a variety of mechanisms to maintain their optimal volume, primarily by releasing or taking in ions and small organic molecules through diverse volume-sensitive ion channels and transporters. In principle, the mechanisms of cell volume regulation are not unique to the brain and share many commonalities with other tissues. However, because ions and some organic osmolytes (e.g., major amino acid neurotransmitters) have a strong impact on neuronal excitability, cell volume regulation in the brain is a surprisingly treacherous process, which may cause more harm than good. This topical review covers the established and emerging information in this rapidly developing area of physiology.

Expression of LRRC8/VRAC Currents in Xenopus Oocytes: Advantages and Caveats

Volume-regulated anion channels (VRACs) play a role in controlling cell volume by opening upon cell swelling. Apart from controlling cell volume, their function is important in many other physiological processes, such as transport of metabolites or drugs, and extracellular signal transduction. VRACs are formed by heteromers of the pannexin homologous protein LRRC8A (also named Swell1) with other LRRC8 members (B, C, D, and E). LRRC8 proteins are difficult to study, since they are expressed in all cells of our body, and the channel stoichiometry can be changed by overexpression, resulting in non-functional heteromers. Two different strategies have been developed to overcome this issue: complementation by transient transfection of LRRC8 genome-edited cell lines, and reconstitution in lipid bilayers. Alternatively, we have used Xenopus oocytes as a simple system to study LRRC8 proteins. Here, we have reviewed all previous experiments that have been performed with VRAC and LRRC8 proteins in Xenopus oocytes. We also discuss future strategies that may be used to perform structure-function analysis of the VRAC in oocytes and other systems, in order to understand its role in controlling multiple physiological functions.

Investigation of LRRC8-Mediated Volume-Regulated Anion Currents in Xenopus Oocytes

Volume-regulated anion channels (VRACs) play an important role in controlling cell volume by opening upon cell swelling. Recent work has shown that heteromers of LRRC8A with other LRRC8 members (B, C, D, and E) form the VRAC. Here, we used Xenopus oocytes as a simple system to study LRRC8 proteins. We discovered that adding fluorescent proteins to the C-terminus resulted in constitutive anion channel activity. Using these constructs, we reproduced previous findings indicating that LRRC8 heteromers mediate anion and osmolyte flux with subunit-dependent kinetics and selectivity. Additionally, we found that LRRC8 heteromers mediate glutamate and ATP flux and that the inhibitor carbenoxolone acts from the extracellular side, binding to probably more than one site. Our results also suggest that the stoichiometry of LRRC8 heteromers is variable, with a number of subunits ≥6, and that the heteromer composition depends on the relative expression of different subunits. The system described here enables easy structure-function analysis of LRRC8 proteins.

Selective transport of neurotransmitters and modulators by distinct volume-regulated LRRC8 anion channels

In response to swelling, mammalian cells release chloride and organic osmolytes through volume-regulated anion channels (VRACs). VRACs are heteromers of LRRC8A and other LRRC8 isoforms (LRRC8B to LRRC8E), which are co-expressed in HEK293 and most other cells. The spectrum of VRAC substrates and its dependence on particular LRRC8 isoforms remains largely unknown. We show that, besides the osmolytes taurine and myo-inositol, LRRC8 channels transport the neurotransmitters glutamate, aspartate and γ-aminobutyric acid (GABA) and the co-activator D-serine. HEK293 cells engineered to express defined subsets of LRRC8 isoforms were used to elucidate the subunit-dependence of transport. Whereas LRRC8D was crucial for the translocation of overall neutral compounds like myo-inositol, taurine and GABA, and sustained the transport of positively charged lysine, flux of negatively charged aspartate was equally well supported by LRRC8E. Disruption of LRRC8B or LRRC8C failed to decrease the transport rates of all investigated substrates, but their inclusion into LRRC8 heteromers influenced the substrate preference of VRAC. This suggested that individual VRACs can contain three or more different LRRC8 subunits, a conclusion confirmed by sequential co-immunoprecipitations. Our work suggests a composition-dependent role of VRACs in extracellular signal transduction.

Volume-regulated anion channel--a frenemy within the brain

The volume-regulated anion channel (VRAC) is a ubiquitously expressed yet highly enigmatic member of the superfamily of chloride/anion channels. It is activated by cellular swelling and mediates regulatory cell volume decrease in a majority of vertebrate cells, including those in the central nervous system (CNS). In the brain, besides its crucial role in cellular volume regulation, VRAC is thought to play a part in cell proliferation, apoptosis, migration, and release of physiologically active molecules. Although these roles are not exclusive to the CNS, the relative significance of VRAC in the brain is amplified by several unique aspects of its physiology. One important example is the contribution of VRAC to the release of the excitatory amino acid neurotransmitters glutamate and aspartate. This latter process is thought to have impact on both normal brain functioning (such as astrocyte-neuron signaling) and neuropathology (via promoting the excitotoxic death of neuronal cells in stroke and traumatic brain injury). In spite of much work in the field, the molecular nature of VRAC remained unknown until less than 2 years ago. Two pioneer publications identified VRAC as the heterohexamer formed by the leucine-rich repeat-containing 8 (LRRC8) proteins. These findings galvanized the field and are likely to result in dramatic revisions to our understanding of the place and role of VRAC in the brain, as well as other organs and tissues. The present review briefly recapitulates critical findings in the CNS and focuses on anticipated impact on the LRRC8 discovery on further progress in neuroscience research.

Intracellular levels of glutamate in swollen astrocytes are preserved via neurotransmitter reuptake and de novo synthesis: implications for hyponatremia

Hyponatremia and several other CNS pathologies are associated with substantial astrocytic swelling. To counteract cell swelling, astrocytes lose intracellular osmolytes, including l-glutamate and taurine, through volume-regulated anion channel. In vitro, when swollen by exposure to hypo-osmotic medium, astrocytes lose endogenous taurine faster, paradoxically, than l-glutamate or l-aspartate. Here, we explored the mechanisms responsible for differences between the rates of osmolyte release in primary rat astrocyte cultures. In radiotracer assays, hypo-osmotic efflux of preloaded [(14) C]taurine was indistinguishable from d-[(3) H]aspartate and only 30-40% faster than l-[(3) H]glutamate. However, when we used HPLC to measure the endogenous intracellular amino acid content, hypo-osmotic loss of taurine was approximately fivefold greater than l-glutamate, and no loss of l-aspartate was detected. The dramatic difference between loss of endogenous taurine and glutamate was eliminated after inhibition of both glutamate reuptake [with 300 μM dl-threo-β-benzyloxyaspartic acid (TBOA)] and glutamate synthesis by aminotransferases [with 1 mM aminooxyacetic acid (AOA)]. Treatment with TBOA+AOA made reductions in the intracellular taurine and l-glutamate levels approximately equal. Taken together, these data suggest that swollen astrocytes actively conserve intracellular glutamate via reuptake and de novo synthesis. Our findings likely also explain why in animal models of acute hyponatremia, extracellular levels of taurine are dramatically elevated with minimal impact on extracellular l-glutamate. We identified mechanisms that allow astrocytes to conserve intracellular l-glutamate (Glu) upon exposure to hypo-osmotic environment. Cell swelling activates volume-regulated anion channel (VRAC) and triggers loss of Glu, taurine (Tau), and other cytosolic amino acids. Glu is conserved via reuptake by Na(+) -dependent transporters and de novo synthesis in the reactions of mitochondrial transamination (TA). These findings explain why, in acute hyponatremia, extracellular levels of Tau can be dramatically elevated with minimal changes in extracellular Glu.

LRRC8A protein is indispensable for swelling-activated and ATP-induced release of excitatory amino acids in rat astrocytes

In mammals, cellular swelling activates release of small organic osmolytes, including the excitatory amino acids (EAA) glutamate and aspartate, via a ubiquitously expressed volume-regulated chloride/anion channel (VRAC). Pharmacological evidence suggests that VRAC plays plural physiological and pathological roles, including excitotoxic release of glutamate in stroke. However, the molecular identity of this pathway was unknown. Two recent studies discovered that LRRC8 gene family members encode heteromeric VRAC composed of LRRC8A plus LRRC8B-E, which mediate swelling-activated Cl(-) currents and taurine release in human non-neural cells (Z. Qiu et al. Cell 157: 447, 2014; F.K. Voss et al. Science 344: 634, 2014). Here, we tested the contribution of LRRC8A to the EAA release in brain glia. We detected and quantified expression levels of LRRC8A-E in primary rat astrocytes with quantitative RT-PCR and then downregulated LRRC8A with gene-specific siRNAs. In astrocytes exposed to hypo-osmotic media, LRRC8A knockdown dramatically reduced swelling-activated release of the EAA tracer D-[(3)H]aspartate. In parallel HPLC assays, LRRC8A siRNA prevented hypo-osmotic media-induced loss of the endogenous intracellular L-glutamate and taurine. Furthermore, downregulation of LRRC8A completely ablated the ATP-stimulated release of D-[(3)H]aspartate and [(14)C]taurine from non-swollen astrocytes. Overall, these data indicate that LRRC8A is an indispensable component of a permeability pathway that mediates both swelling-activated and agonist-induced amino acid release in brain glial cells.

SWELL1, a plasma membrane protein, is an essential component of volume-regulated anion channel

Maintenance of a constant cell volume in response to extracellular or intracellular osmotic changes is critical for cellular homeostasis. Activation of a ubiquitous volume-regulated anion channel (VRAC) plays a key role in this process; however, its molecular identity in vertebrates remains unknown. Here, we used a cell-based fluorescence assay and performed a genome-wide RNAi screen to find components of VRAC. We identified SWELL1 (LRRC8A), a member of a four-transmembrane protein family with unknown function, as essential for hypotonicity-induced iodide influx. SWELL1 is localized to the plasma membrane, and its knockdown dramatically reduces endogenous VRAC currents and regulatory cell volume decrease in various cell types. Furthermore, point mutations in SWELL1 cause a significant change in VRAC anion selectivity, demonstrating that SWELL1 is an essential VRAC component. These findings enable further molecular characterization of the VRAC channel complex and genetic studies for understanding the function of VRAC in normal physiology and disease.

Taurine and atherosclerosis

Taurine is abundantly present in most mammalian tissues and plays a role in many important physiological functions. Atherosclerosis is the underlying mechanism of cardiovascular disease including myocardial infarctions, strokes and peripheral artery disease and remains a major cause of morbidity and mortality worldwide. Studies conducted in laboratory animal models using both genetic and dietary models of hyperlipidemia have demonstrated that taurine supplementation retards the initiation and progression of atherosclerosis. Epidemiological studies have also suggested that taurine exerts preventive effects on cardiovascular diseases. The present review focuses on the effects of taurine on the pathogenesis of atherosclerosis. In addition, the potential mechanisms by which taurine suppress the development of atherosclerosis will be discussed.

Identification of an orally active opioid receptor-like 1 (ORL1) receptor antagonist 4-{3-[(2R)-2,3-dihydroxypropyl]-2-oxo-2,3-dihydro-1H-benzimidazol-1-yl}-1-[(1S,3S,4R)-spiro[bicyclo[2.2.1]heptane-2,1'-cyclopropan]-3-ylmethyl]piperidine as clinical candidate

Our efforts to optimize prototype opioid receptor-like 1 (ORL1) antagonist 1 led to the discovery of 4-{3-[(2R)-2,3-dihydroxypropyl]-2-oxo-2,3-dihydro-1H-benzimidazol-1-yl}-1-[(1S,3S,4R)-spiro[bicyclo[2.2.1]heptane-2,1'-cyclopropan]-3-ylmethyl]piperidine 10. 10 showed potent ORL1 antagonistic activity, excellent selectivity over other opioid receptors, and in vivo efficacy after oral dosing. Currently clinical trials of 10 are underway.

Swelling-induced taurine transport: relationship with chloride channels, anion-exchangers and other swelling-activated transport pathways

Cells have to regulate their volume in order to survive. Moreover, it is now evident that cell volume per se and the membrane transport processes which regulate it, comprise an important signalling unit. For example, macromolecular synthesis, apoptosis, cell growth and hormone secretion are all influenced by the cellular hydration state. Therefore, a thorough understanding of volume-activated transport processes could lead to new strategies being developed to control the function and growth of both normal and cancerous cells. Cell swelling stimulates the release of ions such as K(+) and Cl(-) together with organic osmolytes, especially the beta-amino acid taurine. Despite being the subject of intense research interest, the nature of the volume-activated taurine efflux pathway is still a matter of controversy. On the one hand it has been suggested that osmosensitive taurine efflux utilizes volume-sensitive anion channels whereas on the other it has been proposed that the band 3 anion-exchanger is a swelling-induced taurine efflux pathway. This article reviews the evidence for and against a role of anion channels and exchangers in osmosensitive taurine transport. Furthermore, the distinct possibility that neither pathway is involved in taurine transport is highlighted. The putative relationship between swelling-induced taurine transport and volume-activated anionic amino acid, alpha-neutral amino acid and K(+) transport is also examined.

The orphanin FQ/nociceptin (OFQ/N) system

Orphanin FQ/nociceptin (OFQ/N) was the first novel neuropeptide discovered as the natural ligand of an orphan G protein-coupled receptor (GPCR). Orphan GPCRs are proteins classified as receptors on the basis of their sequence similarities to known GPCRs but that lack the ligands that activate them in vivo. One such orphan GPCR exhibited sequence similarities with the opioid receptors. OFQ/N was isolated as its natural ligand and shown to also share sequence similarities to the opioid peptides. This led to numerous studies attempting to find functional similarities and differences between the OFQ/N and opioid systems. This chapter will summarize our knowledge of the OFQ/N system and of its roles in the organism.

Prostanoid receptors regulate the volume-sensitive efflux of osmolytes from murine fibroblasts via a cyclic AMP-dependent mechanism

The ability of prostanoid receptors to regulate the volume-dependent efflux of the organic osmolyte taurine from murine fibroblasts (L cells) via a cAMP-dependent mechanism has been examined. Incubation of L cells under hypoosmotic conditions resulted in a time-dependent efflux of taurine, the threshold of release occurring at 250 mOsM. Addition of prostaglandin E(1) (PGE(1)) potently (EC(50) = 2.5 nM) enhanced the volume-dependent efflux of taurine at all time points examined and increased the threshold for osmolyte release to 290 mOsM. Maximal PGE(1) stimulation (250-300% of basal) of taurine release was observed at 250 mOsM. Of the PGE analogs tested, only the EP(2)-selective agonist butaprost (9-oxo-11alpha,16S-dihydroxy-17-cyclobutyl-prost-13E-en-1-oic acid) was able to enhance taurine efflux. Inclusion of 1,9-dideoxyfoskolin, 5-nitro-2-(3-phenylpropylamino) benzoic acid, or 4-[(2-butyl-6,7-dicloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]-butanoic acid blocked the ability of PGE(1) to enhance taurine release, indicating the mediation of a volume-sensitive organic osmolyte and anion channel. The ability of PGE(1) to increase osmolyte release from L cells was mimicked by the addition of agents that inhibit cAMP breakdown, directly activate adenylyl cyclase, or are cell-permeant analogs of cAMP. Taurine release elicited by either PGE(1) or 8-(4-chlorophenylthio)-cAMP was attenuated by >70% in L cells that had been stably transfected with a mutant regulatory subunit of cAMP-dependent protein kinase (PKA). PGE(1) stimulation of taurine efflux was not attenuated by either depletion of intracellular calcium or inhibition of protein kinase C. The results indicate that activation of prostanoid receptors on murine fibroblasts enhances osmolyte release via a cAMP and PKA-dependent mechanism.

Differential effects of pyrethroids on volume-sensitive anion and organic osmolyte pathways

1. There are no effective ways of screening for potential modulators of volume-regulated anion channels in their native cell type. Generally, cell lines are used for this purpose. Using HeLa and C6 glioma cells, we identified the pyrethroids as a novel class of compounds that inhibit taurine efflux through volume-regulated anion transport pathways in these cells. Subsequently, we examined their effects on volume-regulated anion channels in guinea-pig ventricular myocytes to determine whether results obtained using cell lines could be extrapolated to other tissues. 2. Tetramethrin inhibited taurine efflux in both HeLa and C6 glioma cells with Ki values of approximately 26 and 16 micro mol/L, respectively. Bioallethrin and fenpropathrin inhibited volume-sensitive taurine efflux from C6 glioma cells, but not from HeLa cells. The Ki values for bioallethrin and fenpropathrin were 70 and 59 micro mol/L, respectively. 3. Volume-sensitive I- efflux was observed in HeLa cells but not in C6 glioma cells, suggesting that the taurine efflux pathway in C6 glioma cells may be different to that of the I- efflux pathway. Cyfluthrin, tetramethrin, fenpropathrin, tefluthrin and bioallethrin all significantly inhibited volume-sensitive I- efflux from HeLa cells at 100 micro mol/L. 4. Patch-clamp experiments have shown inhibition of ICl,vol in guinea-pig ventricular myocytes by fenpropathrin, but not tetramethrin or cypermethrin, at 100 micro mol/L. This revealed that further differences exist between ICl,vol in guinea-pig ventricular myocytes and the anion transport pathways in C6 glioma and HeLa cells. 5. In conclusion, we have shown that pyrethroids differentially inhibit volume-regulated anion and taurine efflux in a number of cell types. Because these compounds have different effects in different cells, it is likely that: (i) more than one pathway is involved in the volume-sensitive transport of anions and organic osmolytes; and (ii) the molecular identities of the channels underlying anion transport are different. Finally, for the reasons given above, care should be taken when extrapolating data from one cell type to another. However, in the absence of an existing high-throughput screen, taurine efflux still represents a viable route for the identification of potential modulators of volume-regulated ion channels.

[ATP release pathways in vascular endothelial cells]

Vascular endothelial cells regulate vascular tonus, growth, and angiogenesis in response to mechanical stresses. ATP release is one of well-known mechanosensitive responses in endothelial cells. Released ATP induces Ca(2+) responses and nitric oxide production in neighboring cells in an auto/paracrine manner. Mechanosensitive and agonist-induced ATP releases are also observed in other cell types, but the cellular mechanisms and pathways of ATP release are largely unknown. Reported candidates for ATP release pathways are ABC proteins including P-glycoprotein and CFTR, exocytosis of ATP-containing vesicles, and ATP-permeable anion channels. In vascular endothelium, vesicular exocytosis, volume-regulated anion channels (VRAC), and connexin hemichannels have been reported as candidates for ATP release pathways. We found that VRAC inhibitors suppressed hypotonic stress-induced ATP release in bovine aortic endothelial cells. Furthermore, extracellular ATP suppressed VRAC current in a voltage dependent manner, which could be fitted to the permeation-blocker model with a Kd(0) of 1 mM and delta value of 0.41. However, it should be noted that VRAC is probably not the only pathway for ATP release in the endothelium, because basal ATP release was not inhibited by VRAC inhibitors. Further investigations are definitely warranted to clarify the details and therapeutic significance of mechanosensitive ATP release in the endothelium.

Osmotic swelling-provoked release of organic osmolytes in human intestinal epithelial cells

Human Intestine 407 cells respond to osmotic cell swelling by the activation of Cl−- and K+-selective ionic channels, as well as by stimulating an organic osmolyte release pathway readily permeable to taurine and phosphocholine. Unlike the activation of volume-regulated anion channels (VRAC), activation of the organic osmolyte release pathway shows a lag time of ∼30–60 s, and its activity persists for at least 8–12 min. In contrast to VRAC activation, stimulation of organic osmolyte release did not require protein tyrosine phosphorylation, active p21rho, or phosphatidylinositol 3-kinase activity and was insensitive to Cl−channel blockers. Treatment of the cells with putative organic anion transporter inhibitors reduced the release of taurine only partially or was found to be ineffective. The efflux was blocked by a subclass of organic cation transporter (OCT) inhibitors (cyanine-863 and decynium-22) but not by other OCT inhibitors (cimetidine, quinine, and verapamil). Brief treatment of the cells with phorbol esters potentiated the cell swelling-induced taurine efflux, whereas addition of the protein kinase C (PKC) inhibitor GF109203X largely inhibited the response, suggesting that PKC is involved. Increasing the level of intracellular Ca2+by using A-23187- or Ca2+-mobilizing hormones, however, did not affect the magnitude of the response. Taken together, the results indicate that the hypotonicity-induced efflux of organic osmolytes is independent of VRAC and involves a PKC-dependent step.

Regulation of swelling-activated chloride channels in embryonic chick heart cells

Swelling-activated Cl- currents, I(Cl,swell) were measured during hyposmotic shock in white Leghorn embryonic chick heart cells using the whole-cell recording of patch-clamp technique. Genistein, an inhibitor of protein tyrosine kinase (PTK), suppressed I(Cl,swell). Under isosmotic condition phorbol 12-myristate 13-acetate (PMA), an activator of PKC, elicited the Cl- current similar to that in hyposmotic solution, whereas hyposmotic shock did not elicit I(Cl,swell) in chelerythrine chloride(an inhibitor of PKC)-treated cells. Confocal microscopy experiments using FITC-phalloidin as a fluorescent label of F-actin showed that the actin network was moved from cortical region of the cell to the center after hyposmotic shock as compared with the image under isosmotic condition. When the cells were treated with cytochalasin B (CB) or cytochalasin D (CD) under isosmotic condition the disruption of the F-actin integrity was observed, and I(Cl,swell) was not elicited. With combination treatment of CB with PMA, hyposmotic solution could not elicited I(Cl,swell). The results suggested that the role of PTK, probably receptor tyrosine kinase, for regulation of I(Cl,swell) appeared to be at upstream site related to the role of F-actin. Then PKC signal pathway was activated somehow and finally change in the polymerization state of cytoskeleton led to activate the swelling-activated Cl- channels. These results demonstrate clearly that PTK, PKC and F-actin are important factors for regulation of I(Cl,swell), in embryonic chick heart cells as compared with often controversial results reported in different cell types.

Taurine: evidence of physiological function in the retina

Taurine is a free amino acid found in high millimolar concentrations in mammalian tissue and is particularly abundant in the retina. Mammals synthesize taurine endogenously with varying abilities, with some species more dependent on dietary sources of taurine than others. Human children appear to be more dependent on dietary taurine than adults. Specifically, it has been established that visual dysfunction in both human and animal subjects results from taurine deficiency. Moreover, the deficiency is reversed with simple nutritional supplementation with taurine. The data suggest that taurine is an important neurochemical factor in the visual system. However, the exact function or functions of taurine in the retina are still unresolved despite continuing scientific study. Nevertheless, the importance of taurine in the retina is implied in the following experimental findings: (1) Taurine exhibits significant effects on biochemical systems in vitro. (2) The distribution of taurine is tightly regulated in the different retinal cell types through the development of the retina. (3) Taurine depletion results in significant retinal lesions. (4) Taurine release and uptake has been found to employ distinct regulatory mechanisms in the retina.

Osmoregulation and cell volume regulation in the preimplantation embryo

The early preimplantation mammalian embryo possesses mechanisms that regulate intracellular osmolarity and cell volume. While transport of osmotically active inorganic ions might play a role in this process in embryos, the major mechanisms that have been identified and studied are those that employ organic osmolytes. Organic osmolytes provide a substantial portion of intracellular osmotic support in embryos and are required for their development under in vivo conditions. The main osmolytes that have been identified in cleavage stage embryos are accumulated via two transport systems of the neurotransmitter transporter family active in early preimplantation embryos--the glycine transport system (GLY) and the beta-amino acid transport system (system beta). While system beta has been established to have a similar role in many other cells, this is a novel function for the GLY transport system. The intracellular concentration of organic osmolytes such as glycine in early preimplantation embryos is regulated by tonicity, allowing the embryo to regulate its volume against shrinkage and to control its internal osmolarity. In addition, the cells of the embryo can regulate against an increase in volume via controlled release of osmolytes from the cytoplasm. This is mediated by a swelling-activated anion channel that is also highly permeable to a range of organic osmolytes, and which closely resembles similar channels found in many other cell types (VSOAC channels). Together, these mechanisms appear to regulate cell volume in the egg through the early cleavage stages of embryogenesis, after which there are indications that the mechanisms of osmoregulation change.

Molecular characterization of taurine transport in bovine aortic endothelial cells

Cultured bovine aortic endothelial (BAE) cells expressed a Na(+)/Cl(-)-dependent taurine uptake activity that saturated with an apparent K(0.5) of approximately 4.9 microM for taurine and was inhibited by beta-alanine, guanidinoethane sulfonate, and homotaurine. We isolated a taurine transporter clone from a BAE cell cDNA library that revealed >91% sequence identity at the amino acid level to the previously cloned high-affinity mammalian taurine transporters. The biochemical and pharmacological properties of the bovine taurine transporter cDNA expressed in Xenopus oocyte was similar to those of the high-affinity taurine transporter. Surprisingly, F(-) blocked taurine uptake in BAE cells with an IC(50) of approximately 17.5 mM. The endogenous taurine uptake was also inhibited by the protein kinase C activator phorbol 12-myristate 13-acetate, but not by its inactive analog, 4 alpha-phorbol 12,13-didecanoate. The endogenous uptake was stimulated, however, by hypertonic stress and the increase was due to an increase in the V(max) of taurine uptake. Our results provide the first description of a molecular mechanism that may be responsible for maintaining the intracellular taurine content in the endothelial cells.

Cardiac chloride channels: physiology, pharmacology and approaches for identifying novel modulators of activity

Drugs that block cardiac cation channels have been marketed as the therapeutic answer to cardiac arrhythmia. However, such molecules have been only moderately successful at improving the survival of cardiac patients, and so new targets have been needed for future antiarrhythmic agents. This article outlines the properties and roles of Cl(-) channels, which are one of these new targets, and describes an approach for identifying novel CI(2) channel modulators.

Pharmacological characterization of volume-sensitive, taurine permeable anion channels in rat supraoptic glial cells

To characterize the volume-sensitive, osmolyte permeable anion channels responsible for the osmodependent release of taurine from supraoptic nucleus (SON) astrocytes, we investigated the pharmacological properties of the [(3)H]-taurine efflux from acutely isolated SON. Taurine release induced by hypotonic stimulus (250 mosmol l(-1)) was not antagonized by the taurine transporter blocker guanidinoethyl sulphonate, confirming the lack of implication of the transporter. The osmodependent release of taurine was blocked by a variety of Cl(-) channel inhibitors with the order of potency: NPPB>niflumic acid>DPC>DIDS>ATP. On the other hand, release of taurine was only weakly affected by other compounds (dideoxyforskolin, 4-bromophenacyl bromide, mibefradil) known to block volume-activated anion channels in other cell preparations, and was completely insensitive to tamoxifen, a broad inhibitor of these channels. Although the molecular identity of volume-sensitive anion channels is not firmly established, a few genes have been postulated as potential candidates to encode such channels. We checked the expression in the SON of three of them, ClC(3), phospholemman and VDAC(1), and found that the transcripts of these genes are found in SON neurons, but not in astrocytes. Similar observation was previously reported for ClC(2). In conclusion, the osmodependent taurine permeable channels of SON astrocytes display a particular pharmacological profile, suggesting the expression of a particular type or subtype of volume-sensitive anion channel, which is likely to be formed by yet unidentified proteins.

Swelling-induced taurine release without chloride channel activity in Xenopus laevis oocytes expressing anion channels and transporters

Taurine is an important osmolyte involved in cell volume regulation. During regulatory volume decrease it is released via a volume-sensitive organic osmolyte/anion channel. Several molecules have been suggested as candidates for osmolyte release. In this study, we chose three of these, namely ClC-2, ClC-3 and ICln, because of their expression in rat astrocytes, a cell type which is known to release taurine under hypotonic stress, and their activation by hypotonic shock. As all three candidates were also suggested to be chloride channels, we investigated their permeability for both chloride and taurine under isotonic and hypotonic conditions using the Xenopus laevis oocyte expression system. We found a volume-sensitive increase of chloride permeability in ClC-2-expressing oocytes only. Yet, the taurine permeability was significantly increased under hypotonic conditions in oocytes expressing any of the tested candidates. Further experiments confirmed that the detected taurine efflux does not represent unspecific leakage. These results suggest that ClC-2, ClC-3 and ICln either participate in taurine transport themselves or upregulate an endogenous oocyte osmolyte channel. In either case, the taurine efflux of oocytes not being accompanied by an increased chloride flux suggests that taurine and chloride can be released via two separate pathways.

Tyrosine phosphorylation modulates the osmosensitivity of volume-dependent taurine efflux from glial cells in the rat supraoptic nucleus

1. In the supraoptic nucleus, taurine, selectively released in an osmodependent manner by glial cells through volume-sensitive anion channels, is likely to inhibit neuronal activity as part of the osmoregulation of vasopressin release. We investigated the involvement of various kinases in the activation of taurine efflux by measuring [3H]taurine release from rat acutely isolated supraoptic nuclei. 2. The protein tyrosine kinase inhibitors genistein and tyrphostin B44 specifically reduced, but did not suppress, both the basal release of taurine and that evoked by a hypotonic stimulus. Inhibition of tyrosine phosphatase by orthovanadate had the opposite effect. 3. The tyrosine kinase and phosphatase inhibitors shifted the relationship between taurine release and medium osmolarity in opposite directions, suggesting that tyrosine phosphorylation modulates the osmosensitivity of taurine release, but is not necessary for its activation. 4. Genistein also increased the amplitude of the decay of the release observed during prolonged hypotonic stimulation. Potentiation of taurine release by tyrosine kinases could serve to maintain a high level of taurine release in spite of cell volume regulation. 5. Taurine release was unaffected by inhibitors and/or activators of PKA, PKC, MEK and Rho kinase. 6. Our results demonstrate a unique regulation by protein tyrosine kinase of the osmosensitivity of taurine efflux in supraoptic astrocytes. This points to the presence of specific volume-dependent anion channels in these cells, or to a specific activation mechanism or regulatory properties. This may relate to the particular role of the osmodependent release of taurine in this structure in the osmoregulation of neuronal activity.

Inhibition of angiogenesis by blockers of volume-regulated anion channels

Osmotic cell swelling activates an outwardly rectifying Cl(-) current in endothelial cells that is mediated by volume-regulated anion channels (VRACs). In the past, we have shown that serum-induced proliferation of endothelial cells is arrested in the presence of compounds that potently block the endothelial VRACs. Here we report on the effects of four chemically distinct VRAC blockers [5-nitro-2-(3-phenylpropylamino)benzoic acid] (NPPB), mibefradil, tamoxifen, and clomiphene-on several models of experimental angiogenesis. Mibefradil (20 microM), NPPB (100 microM), tamoxifen (20 microM), and clomiphene (20 microM) inhibited tube formation by rat microvascular endothelial cells plated on matrigel by 42.9 +/- 8.8%, 25.3 +/- 10.4%, 32.2 +/- 4.5%, and 20 +/- 5.8%, respectively (p < 0.05). Additionally, NPPB (50-100 microM) and mibefradil (10-30 microM) significantly inhibited bFGF (10 ng/ml) + TNFalpha (2.5 ng/ml)-stimulated microvessel formation by human microvascular endothelial cells plated on fibrin by 30-70%. Furthermore, NPPB, mibefradil, and clomiphene concentration dependently inhibited spontaneous microvessel formation in the rat aorta-ring assay and vessel development in the chick chorioallantoic membrane assay. These results suggest that VRAC blockers are potent inhibitors of angiogenesis and thus might serve as therapeutic tools in tumor growth and other angiogenesis-dependent diseases.

Reduced intracellular ionic strength as the initial trigger for activation of endothelial volume-regulated anion channels

Most mammalian cell types, including endothelial cells, respond to cell swelling by activating a Cl- current termed ICl,swell, but it is not known how the physical stimulus of cell swelling is transferred to the channels underlying ICl,swell. We have investigated the precise relation between cell volume and ICl,swell in endothelial cells by performing whole-cell current recordings while continuously monitoring cell thickness (Tc) as a measure for cell volume. The time course of Tc was accurately predicted by a theoretical model that describes volume changes of patch-clamped cells in response to changes in the extracellular osmolality (OSMo). This model also predicts significant changes in intracellular ionic strength (Gammai) when OSMo is altered. Under all experimental conditions ICl,swell closely followed the changes in Gammai, whereas ICl,swell and cell volume were often found to change independently. These results do not support the hypothesis that Gammai regulates the volume set point for activation of ICl,swell. Instead, they are in complete agreement with a model in which a decrease of Gammai rather than an increase in cell volume is the initial trigger for activation of ICl,swell.

Amino acids are compatible osmolytes for volume recovery after hypertonic shrinkage in vascular endothelial cells

The response to chronic hypertonic stress has been studied in human endothelial cells derived from saphenous veins. In complete growth medium the full recovery of cell volume requires several hours and is neither associated with an increase in cell K+ nor hindered by bumetanide but depends on an increased intracellular pool of amino acids. The highest increase is exhibited by neutral amino acid substrates of transport system A, such as glutamine and proline, and by the anionic amino acid glutamate. Transport system A is markedly stimulated on hypertonic stress, with an increase in activity roughly proportional to the extent and the duration of the osmotic shrinkage. Cycloheximide prevents the increase in transport activity of system A and the recovery of cell volume. It is concluded that human endothelial cells counteract hypertonic stress through the stimulation of transport system A and the consequent expansion of the intracellular amino acid pool.

Functional genomics and the discovery of new drug targets

Functional genomics can be defined as the search for the physiological role of a gene for which only its primary sequence is known. One example of a successful functional genomics adventure is the search for the natural ligands of orphan G protein-coupled receptors (GPCRs). GPCRs are proteins containing 7 hydrophobic domains that are the recognition sites of neurotransmitters and neuropeptides. Although many of these have been shown to interact with known natural ligands, several bind ligands that have not been thus far isolated. These are the so-called "orphan" GPCRs. As an example of functional genomics, an "orphan receptor strategy" has been developed to identify the natural ligands of orphan GPCRs. We describe that the application of this strategy has already led to the identification of 4 new neuropeptides and report on what has been learned about these neuropeptides. We finally discuss the importance of the application of the orphan receptor strategy to the development of novel drugs.

Cyclic AMP analogs induce synthesis, processing, and secretion of prepro nociceptin/orphanin FQ-derived peptides by NS20Y neuroblastoma cells

Recent studies have shown that cAMP analogs can induce expression of prepro (pp) orphanin FA (OFQ)/nociceptin-related gene products in NS20Y mouse neuroblastoma cells (Saito et al. [1996]. J Biol Chem 271, 15615-15622). Additionally, exposure of NS20Y cells to cAMP analogs promoted neurite outgrowth and large dense-core vesicle formation. Even though an OFQ-like precursor (called 27K) was identified in NS20Y cell extracts, no secretion of OFQ-related peptides was detected. We have used reversed-phase high-performance liquid chromatography combined with a specific radioimmunoassay for OFQ(1-17) to determine if NS20Y cells secrete ppOFQ-derived peptides when stimulated by the cAMP analog ctp-cAMP. We found that NS20Y cells secreted abundant amounts of OFQ-derived products when stimulated by cAMP analogs. We also have determined that secretion of OFQ peptides was both time and concentration dependent and reversible on removal of cAMP analogs from the culture medium. In addition, the opioid agonist D-Pen2-D-Pen5-enkephalin inhibited forskolin-stimulated OFQ peptide secretion. Further, the synthetic glucocorticoid dexamethasone virtually abolished ctp-cAMP-stimulated OFQ peptide secretion. These results suggest that the biosynthesis, processing, and secretion of the OFQ neuropeptide transmitter system can be modulated through intracellular cAMP levels and that these functions are regulated by opioids and molecules involved in mediating the stress response. The NS20Y cell system will be extremely valuable for studying the regulation of OFQ-derived peptides by a variety of intra-cellular and extracellular signaling pathways.

Orphan receptors and the concept of reverse physiology: discovery of the novel neuropeptide orphanin FQ/nociceptin

The cloning of numerous orphan members from the supergene family of G protein-coupled receptors implies the existence of many as yet undiscovered neurotransmitters and neuropeptides. Recently, new technologies were developed to isolate natural ligands for orphan receptors, using the receptor as a biological sensor during the purification process. This manuscript will present the concept and technology of an approach which starts from a cloned receptor to ultimately describe the physiological functions of the transmitter system. This strategy inverts the classical order of biomedical research and was thus termed "reverse physiology". The first natural ligand isolated by this strategy is a peptide with significant similarity to the opioid peptides and has been named orphanin FQ or nociceptin (OFQ/NOC). Evidence for characterizing OFQ/NOC as a genuine neuropeptide will be reviewed. OFQ/NOC is biosynthetically derived from a larger precursor protein which may encode additional bioactive peptides. Since its discovery, a large number of studies have described numerous physiological functions of OFQ/NOC. Because of its relation to the opioid system, much attention has been focused on the involvement of OFQ/NOC in nociception, sometimes with controversial results. However, the pharmacological profile of the OFQ/NOC system suggests a clear separation from the opioids. The discovery of OFQ/NOC and the subsequent analyses of its physiological functions is an example which has already been followed by the identification of two other novel neuropeptides. The orphan receptor strategy holds a lot of promises for the postgenomic era, helping to fill the vast amount of sequence data with life.

Effects of sulphonylureas on the volume-sensitive anion channel in rat pancreatic beta-cells

1. The volume-sensitive anion conductance in rat pancreatic beta-cells was studied directly using the conventional whole-cell and perforated patch recording techniques, and indirectly by measuring 3H-taurine efflux from pre-loaded, perifused islets. 2. Using the conventional whole-cell recording configuration, activation of the outwardly-rectifying, DIDS-sensitive conductance was induced by glibenclamide (10 microM) but not by tolbutamide (100 microM) nor by meglitinide (20 microM). A high concentration of glibenclamide (100 microM) caused a voltage-and time-dependent inhibition of the conductance. Tolbutamide had a modest inhibitory effect on swelling-induced inward currents. 3. In perforated patch recordings, glibenclamide, tolbutamide and meglitinide were all without effect on the conductance, although activation could be induced under these conditions by exposure to a hypotonic bath solution. 4. The rate of efflux of 3H-taurine, a marker for activity of the volume-sensitive anion channel, from preloaded, perifused islets was markedly stimulated by exposure to a hypotonic solution. However, glibenclamide and tolbutamide were both without effect. 5. Electrical activity of beta-cells in response to glibenclamide or tolbutamide was not inhibited by 4,4'-dithiocyanatostilbene-2,2'-disulphonic acid (DIDS), an inhibitor of the volume-sensitive anion channel. 6. It is concluded that activity of the volume-sensitive anion conductance in rat pancreatic beta-cells is not modulated by the sulphonylurea receptor. The activation of the conductance by glibenclamide in whole-cell recordings could be the result of a non-specific interaction of the drug with plasma membrane lipids.

Conductive pathways for chloride and oxalate in rabbit ileal brush-border membrane vesicles

To evaluate the possibility that an apical membrane conductive pathway for oxalate is present in the rabbit distal ileum, we studied oxalate ([14C]oxalate) and chloride (36Cl) uptake into brush-border membrane vesicles enriched 15- to 18-fold in sucrase activity. Voltage-sensitive pathways for oxalate and chloride were identified by the stimulation of uptake provided by an inwardly directed potassium diffusion potential in the presence of valinomycin. Additionally, outwardly directed oxalate (or chloride) gradients stimulated [14C]oxalate (or 36Cl) uptake to a greater degree in the absence of valinomycin (when intracellular and extracellular potassium are equal) than in the presence of valinomycin. Voltage-dependent anion uptake was poorly saturable: apparent affinity constants were 141 +/- 17 and 126 +/- 8 mM for chloride and oxalate, respectively. Activation energies for the voltage-dependent uptake processes were low: 4.7 and 6.3 kcal/mol for chloride and oxalate, respectively. Sensitivity profiles of voltage-dependent chloride and oxalate uptake to anion transport inhibitors were similar. We conclude that an anion conductance is present in the apical membranes of ileal enterocytes and that this conductance is a candidate pathway for oxalate efflux from the enterocyte during transepithelial oxalate secretion.

Functional genomics: the search for novel neurotransmitters and neuropeptides

Functional genomics can be defined as the search for the physiological role of a gene for which only its primary sequence is known. Most of the genes encoding proteins containing seven hydrophobic stretches code for G protein-coupled receptors (GPCRs). Although many of these have been shown to interact with known natural ligands, several bind ligands which have not been thus far isolated. These are the so-called orphan GPCRs. As an example of functional genomics, an 'orphan receptor strategy' has been developed to identify the natural ligands of orphan GPCRs. The application of this strategy is bound to revolutionize our understanding of the diversity of the primary messengers which modulate synaptic transmission. This review discusses the basic concepts and some of the particular problems associated with the orphan receptor strategy. The strategy's potential is exemplified by its successes which culminated in the discovery of the neuropeptides 'orphanin FQ/nociceptin' and 'orexins/hypocretins'. The steps that led to the characterization of these neuropeptides are discussed as are some of the further studies that have addressed the roles of these neuropeptides. To conclude, some of the implications of the application of the orphan receptor strategy are discussed.

Disruption of mitochondrial respiration inhibits volume-regulated anion channels and provokes neuronal cell swelling

Hypoxia and inhibitors of mitochondrial respiration impair the regulatory volume decrease (RVD) of cerebellar granule neurons after hypotonic swelling. RVD is linked to the opening of volume-regulated anion channels (VRACs). VRACs are outwardly rectifying, inactivate slowly during maintained depolarization, and are permeable to the cellular organic osmolyte taurine. Channel activation requires nonhydrolytic ATP binding and is not modulated by intracellular ADP. VRAC opening is reversibly depressed by hypoxia and by mitochondrial inhibitors such as oligomycin, rotenone, and antimycin A. These results demonstrate that neuronal VRAC activation and swelling are both tightly linked to cellular energy. Moreover, the findings reported in this work may have a particular significance for inherited mitochondrial human diseases, such as mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), which cause brain swelling and edema.

Regulation of a swelling-activated chloride current in bovine endothelium by protein tyrosine phosphorylation and G proteins

1. The role of protein tyrosine phosphorylation and of G proteins in the activation of a swelling-activated Cl- current (ICl,swell) in calf pulmonary artery endothelial (CPAE) cells was studied using the whole-cell patch clamp technique. ICl,swell was activated by reducing the extracellular osmolality by either 12.5% (mild hypotonicity) or 25% (strong hypotonicity). 2. The protein tyrosine kinase (PTK) inhibitors tyrphostin B46, tyrphostin A25 and genistein inhibited ICl,swell with IC50 values of, respectively, 9.2 +/- 0.2, 61.4 +/- 1.7 and 62.9 +/- 1.3 microM. Tyrphostin A1, a tyrphostin analogue with little effect on PTK activity, and daidzein, an inactive genistein analogue, were without effect on ICl,swell. 3. The protein tyrosine phosphatase (PTP) inhibitors Na3VO4 (200 microM) and dephostatin (20 microM) potentiated ICl,swell activated by mild hypotonicity by 47 +/- 9 and 69 +/- 15%, respectively. 4. Intracellular perfusion with GTP gamma S (100 microM) transiently activated a Cl- current with an identical biophysical and pharmacological profile to ICl,swell. This current was inhibited by the tested PTK inhibitors and potentiated by the PTP inhibitors. Hypertonicity-induced cell shrinkage completely inhibited the GTP gamma S-activated Cl- current. 5. Intracellular perfusion with GDP beta S (1 mM) caused a time-dependent inhibition of ICl,swell, which was more pronounced when the current was activated by mild hypotonicity. 6. Our results demonstrate that the activity of endothelial swelling-activated Cl- channels is dependent on tyrosine phosphorylation and suggest that G proteins regulate the sensitivity to cell swelling.