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

Do Amino Acid Antiporters Have Asymmetric Substrate Specificity?

Amino acid antiporters mediate the 1:1 exchange of groups of amino acids. Whether substrate specificity can be different for the inward and outward facing conformation has not been investigated systematically, although examples of asymmetric transport have been reported. Here we used LC-MS to detect the movement of ¹²C- and ¹³C-labelled amino acid mixtures across the plasma membrane of Xenopus laevis oocytes expressing a variety of amino acid antiporters. Differences of substrate specificity between transporter paralogs were readily observed using this method. Our results suggest that antiporters are largely symmetric, equalizing the pools of their substrate amino acids. Exceptions are the antiporters y⁺LAT1 and y⁺LAT2 where neutral amino acids are co-transported with Na⁺ ions, favouring their import. For the antiporters ASCT1 and ASCT2 glycine acted as a selective influx substrate, while proline was a selective influx substrate of ASCT1. These data show that antiporters can display non-canonical modes of transport.

A GC-MS/Single-Cell Method to Evaluate Membrane Transporter Substrate Specificity and Signaling

Amino acid transporters play a vital role in metabolism and nutrient signaling pathways. Typically, transport activity is investigated using single substrates and competing amounts of other amino acids. We used GC-MS and LC-MS for metabolic screening of Xenopus laevis oocytes expressing various human amino acid transporters incubated in complex media to establish their comprehensive substrate profiles. For most transporters, amino acid selectivity matched reported substrate profiles. However, we could not detect substantial accumulation of cationic amino acids by SNAT4 and ATB0,+ in contrast to previous reports. In addition, comparative substrate profiles of two related sodium neutral amino acid transporters known as SNAT1 and SNAT2, revealed the latter as a significant leucine accumulator. As a consequence, SNAT2, but not SNAT1, was shown to be an effective activator of the eukaryotic cellular growth regulator mTORC1. We propose, that metabolomic profiling of membrane transporters in Xe nopus laevis oocytes can be used to test their substrate specificity and role in intracellular signaling pathways.

Use of Xenopus laevis Oocytes to Study Auxin Transport

Xenopus laevis oocytes are an expression system that is particularly well suited for the characterization of membrane transporters. Oocytes possess only very little endogenous transport systems and therefore transporters can be studied with a high signal-to-noise ratio. This book chapter provides the basic methods to use Xenopus oocytes for the characterization of transporters by radiotracer experiments. While the methods described here were established to study auxin transport they can easily be adapted to study other hormone transporters and their substrates.

Downregulation of chloride channel ClC-2 by Janus kinase 3

Janus kinase-3 (JAK3) fosters proliferation and counteracts apoptosis of lymphocytes and tumor cells. The gain of function mutation (A572V)JAK3 has been discovered in acute megakaryoplastic leukemia. JAK3 is inactivated by replacement of lysine by alanine in the catalytic subunit ((K855A)JAK3). Regulation of cell proliferation and apoptosis involves altered activity of Cl(-) channels. The present study, thus, explored whether JAK3 modifies the function of the small conductance Cl(-) channel ClC-2. To this end, ClC-2 was expressed in Xenopus oocytes with or without wild-type JAK3, (A568V)JAK3 or (K851A)JAK3, and the Cl(-) channel activity determined by dual-electrode voltage clamp. Channel protein abundance in the cell membrane was determined utilizing chemiluminescence. As a result, expression of ClC-2 was followed by a marked increase of cell membrane conductance. The conductance was significantly decreased following coexpression of JAK3 or (A568V)JAK3, but not by coexpression of (K851A)JAK3. Exposure of the oocytes expressing ClC-2 together with (A568V)JAK3 to the JAK3 inhibitor WHI-P154 (4-[(3'-bromo-4'-hydroxyphenyl)amino]-6,7-dimethoxyquinazoline, 22 μM) increased the conductance. Coexpression of (A568V)JAK3 decreased the ClC-2 protein abundance in the cell membrane of ClC-2 expressing oocytes. The decline of conductance in ClC-2 and (A568V)JAK3 coexpressing oocytes following inhibition of channel protein insertion by brefeldin A (5 μM) was similar in oocytes expressing ClC-2 with (A568V)JAK3 and oocytes expressing ClC-2 alone, indicating that (A568V)JAK3 might slow channel protein insertion into rather than accelerating channel protein retrieval from the cell membrane. In conclusion, JAK3 downregulates ClC-2 activity and thus counteracts Cl(-) exit-an effect possibly influencing cell proliferation and apoptosis.

Xenopus laevis Oocytes

Xenopus oocytes are a versatile expression system particularly suited for membrane transporters and channels. Oocytes have little background activity and therefore offer a very high signal-to-noise ratio for transporter and channel characterization. This chapter provides an overview of the basic methods used for the analysis of membrane transporters in this system, including preparation of oocytes, assays of transport activity, protocols for immunostaining and fluorescence microscopy, and other assays to study surface expression.

Channel-like characteristics of the low-affinity barley phosphate transporter PHT1;6 when expressed in Xenopus oocytes

Remobilization of inorganic phosphate (P(i)) within a plant is critical for sustaining growth and seed production under external P(i) fluctuation. The barley (Hordeum vulgare) transporter HvPHT1;6 has been implicated in P(i) remobilization. In this report, we expressed HvPHT1;6 in Xenopus laevis oocytes, allowing detailed characterization of voltage-dependent fluxes and currents induced by HvPHT1;6. HvPHT1;6 increased efflux of P(i) near oocyte resting membrane potentials, dependent on external P(i) concentration. Time-dependent inward currents were observed when membrane potentials were more negative than -160 mV, which was consistent with nH(+):HPO(4)(2-) (n > 2) cotransport, based on simultaneous radiotracer and oocyte voltage clamping, dependent upon P(i) concentration gradient and pH. Time- and voltage-dependent inward currents through HvPHT1;6 were also observed for SO(4)(2-)and to a lesser degree for NO(3)(-)Cl(-)but not for malate. Inward and outward currents showed linear dependence on the concentration of external HPO(4)(2-)similar to low-affinity P(i) transport in plant studies. The electrophysiological properties of HvPHT1;6, which locates to the plasma membrane when expressed in onion (Allium cepa) epidermal cells, are consistent with its suggested role in the remobilization of P(i) in barley plants.

AR-C155858 is a potent inhibitor of monocarboxylate transporters MCT1 and MCT2 that binds to an intracellular site involving transmembrane helices 7-10

In the present study we characterize the properties of the potent MCT1 (monocarboxylate transporter 1) inhibitor AR-C155858. Inhibitor titrations of L-lactate transport by MCT1 in rat erythrocytes were used to determine the K_i value and number of AR-C155858-binding sites (Et) on MCT1 and the turnover number of the transporter (k_cat). Derived values were 2.3±1.4 nM, 1.29±0.09 nmol per ml of packed cells and 12.2±1.1 s⁻¹ respectively. When expressed in Xenopus laevis oocytes, MCT1 and MCT2 were potently inhibited by AR-C155858, whereas MCT4 was not. Inhibition of MCT1 was shown to be time-dependent, and the compound was also active when microinjected, suggesting that AR-C155858 probably enters the cell before binding to an intracellular site on MCT1. Measurement of the inhibitor sensitivity of several chimaeric transporters combining different domains of MCT1 and MCT4 revealed that the binding site for AR-C155858 is contained within the C-terminal half of MCT1, and involves TM (transmembrane) domains 7–10. This is consistent with previous data identifying Phe³⁶⁰ (in TM10) and Asp³⁰² plus Arg³⁰⁶ (TM8) as key residues in substrate binding and translocation by MCT1. Measurement of the K_m values of the chimaeras for L-lactate and pyruvate demonstrate that both the C- and N-terminal halves of the molecule influence transport kinetics consistent with our proposed molecular model of MCT1 and its translocation mechanism that requires Lys³⁸ in TM1 in addition to Asp³⁰² and Arg³⁰⁶ in TM8 [Wilson, Meredith, Bunnun, Sessions and Halestrap (2009) J. Biol. Chem. 284, 20011–20021].

Anionic leak currents through the Na+/monocarboxylate cotransporter SMCT1

SMCT1 is a Na-coupled cotransporter of short chain monocarboxylates, which is expressed in the apical membrane of diverse epithelia such as colon, renal cortex, and thyroid. We previously reported that SMCT1 cotransport was reduced by extracellular Cl− replacement with cyclamate− and that the protein exhibited an ostensible anionic leak current. In this paper, we have revisited the interaction between small monovalent anions and SMCT cotransport and leak currents. We found that the apparent Cl− dependence of cotransport was due to inhibition of this protein by the replacement anion cyclamate, whereas several other replacement anions function as substrates for SMCT1; a suitable replacement anion (MES−) was identified. The observed outward leak currents represented anionic influx and favored larger anions (NO3−>I−>Br−>Cl−); currents in excess of 1 μA (at +50 mV) could be observed and exhibited a quasilinear relationship with anion concentrations up to 100 mM. Application of 25 mM bicarbonate did not produce measurable leak currents. The leak current displayed outward rectification, which disappeared when external Na+ was replaced by N-methyl-d-glucamine+. More precisely, external Na+ blocked the leak current in both directions, but its Ki value rose rapidly when membrane potential became positive. Thus SMCT1 possesses a anionic leak current that becomes significant whenever external Na+ concentration is reduced. The presence of this leak current may represent a second function for SMCT1 in addition to cotransporting short chain fatty acids, and future experiments will determine whether this function serves a physiological role in tissues where SMCT1 is expressed.

Placental chloride channels: a review

The human placental syncytiotrophoblast (hSTB) is a polarized epithelial structure, that forms the main barrier to materno-fetal exchange. The chloride (Cl(-)) channels in other epithelial tissues contribute to several functions, such as maintenance of the membrane potential, volume regulation, absorption and secretion. Additionally, the contributions of Cl(-) channels to these functions are demonstrated by certain diseases and knock-out animal models. There are multiple lines of evidence for the presence of Cl(-) channels in the hSTB, which could contribute to different placental functions. However, both the mechanism by which these channels are involved in the physiology of the placenta, and their molecular identities are still unclear. Furthermore, a correlation between altered Cl(-) channels functions and pathological pregnancies is beginning to emerge. This review summarizes recent developments on conductive placental chloride transport, and discusses its potential implications for placental physiology.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

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 maxi-chloride channel in human syncytiotrophoblast: a pathway for taurine efflux in placental volume regulation?

Taurine (Tau), the most abundant amino acid in fetal blood, is highly concentrated in human placenta. During pregnancy, Tau is involved in the neurological development of the fetus, and in volume regulation of the placenta. The placenta may release taurine in parallel with K(+) and Cl(-) in response to an increase in cell volume. However, the pathway for the volume-activated taurine efflux is unknown. One candidate is a voltage-dependent Maxi-chloride channel from apical syncytiotrophoblast membrane (MVM), with a conductance over 200pS and multiple subconductance states. Our aim was to study whether this channel could be a Tau conductive pathway in the MVM. Purified human placental MVM were reconstituted into giant liposomes suitable for patch clamp recordings. Typical Maxi-chloride channel activity was detected in symmetrical chloride (Cl(-)) solutions, and then taurine (Tau), Aspartate (Asp), and glutamate (Glu) solutions were used in the bath of excised patches to detect single channel currents carried by these anions. The relative permeabilities (P), estimated from the shift in reversal potential of current-voltage curves after anion replacement, were as follows: Chloride>Taurine=Glutamate=Aspartate. In Tau symmetric conditions using equivalent Cl(-) concentrations, the slope conductance was 62.4+/-7.3pS. The data shows that Tau and other amino acids diffuse through the Maxi-chloride channel, which could be of great importance as part of the mechanism involved in the volume regulation process in human placenta.

Heterologous expression of the glutamine transporter SNAT3 in Xenopus oocytes is associated with four modes of uncoupled transport

The glutamine transporter SNAT3 (SLC38A3, former SN1) plays a major role in glutamine release from brain astrocytes and in glutamine uptake into hepatocytes and kidney epithelial cells. Here we expressed rat SNAT3 in oocytes of Xenopus laevis and reinvestigated its transport modes using two-electrode voltage clamp and pH-sensitive microelectrodes. In addition to the established coupled Na+-glutamine-cotransport/H+ antiport, we found that there are three conductances associated with SNAT3, two dependent and one independent of the amino acid substrate. The glutamine-dependent conductance is carried by cations at pH 7.4, whereas at pH 8.4 the inward currents are still dependent on the presence of external Na+ but are carried by H+. Mutation of threonine 380 to alanine abolishes the cation conductance but leaves the proton conductance intact. Under Na+-free conditions, where the substrate-dependent conductance is suppressed, a substrate-independent, outwardly rectifying current becomes apparent at pH 8.4 that is carried by K+ and H+. In addition, we identified a glutamine-dependent uncoupled Na+/H+ exchange activity that becomes apparent upon removal of Na+ in the presence of glutamine. In conclusion, our results suggest that, in addition to coupled transport, SNAT3 mediates four modes of uncoupled ion movement across the membrane.

Transepithelial Taurine Transport in Caco-2 Cell Monolayers

Here we characterized transepithelial taurine transport in monolayers of cultured human intestinal Caco-2 cells by analyzing kinetic apical and basolateral uptake and efflux parameters. Basolateral uptake was Na(+)- and Cl(-)- dependent and was inhibited by beta-amino acids. Uptake by this membrane showed properties similar to those of the apical TauT system. In both membranes, taurine uptake fitted a model consisting of a non-saturable plus a saturable component, with a higher half-saturation constant and transport capacity at the apical membrane (K(m), 17.1 micromol/L; V(max), 28.4 pmol.cm(-2).5 min(-1)) than in the basolateral domain (K(m), 9.46 micromol/L; V(max), 5.59 pmol.cm(-2).5 min(-1)). The non-saturable influx component, estimated in the absence of Na(+) and Cl(-), showed no significant differences between apical and basolateral membranes (K(D), 89.2 and 114.7 nL.cm(-2) . 5 min(-1), respectively). Taurine efflux from the cells is a diffusive process, as shown in experiments using preloaded cells and in trans-stimulation studies (apical K(D),72.7 and basolateral K(D), 50.1 nL.cm(-2).5 min(-1)). Basolateral efflux rates were significantly lower than passive influx rates. We conclude that basolateral taurine uptake in Caco-2 cells is mediated by a transport mechanism that shares some properties with the apical system TauT. Moreover, calculation of unidirectional and transepithelial taurine fluxes reveals that apical influx of this amino acid is higher than basolateral efflux rates, thereby enabling epithelial cells to accumulate taurine against a concentration gradient.

Tyrosine kinase inhibition affects skate anion exchanger isoform I alterations after volume expansion

Upon exposure to hypotonic medium, skate red blood cells swell and then reduce their volume by releasing organic osmolytes and associated water. The regulatory volume decrease is inhibited by stilbenes and anion exchange inhibitors, suggesting involvement of the red blood cell anion exchanger skAE1. To determine the role of tyrosine phosphorylation, red blood cells were volume expanded with and without prior treatment with the tyrosine kinase inhibitor piceatannol. At the concentration used, 130 microM, piceatannol nearly completely inhibits p72(syk), a tyrosine kinase previously shown to phosphorylate skAE1 (M. W. Musch, E. H. Hubert, and L. Goldstein. J Biol Chem 274: 7923-7928, 1999). Hyposmotic-induced volume expansion stimulated association of p72(syk) with a light membrane fraction of skate red blood cells. Piceatannol did not inhibit this association but decreased hyposmotically stimulated increased skAE1 tyrosine phophorylation. Movement of skAE1 from an intracellular to a surface detergent-resistant membrane domain and tetramer formation were not inhibited by piceatannol treatment. Two effects of hyposmotic-induced volume expansion, decreased band 4.1 binding and increased ankyrin, were both inhibited by piceatannol. These results suggest that at least one event requiring p72(syk) activation is pivotal for hyposmotic-induced increased transport; however, steps that do not require tyrosine phosphorylation may also play a role.

Serum and glucocorticoid inducible kinases functionally regulate ClC-2 channels

ClC-2 participates in the regulation of neuronal excitability, chloride secretion, and cell volume. The ClC-2 sequence contains a consensus site (Ser82) for phosphorylation by the serum and glucocorticoid inducible kinase isoforms SGK1-3. Thus, the present study explored whether ClC-2 is regulated by those kinases. ClC-2 expression in Xenopus oocytes induced inwardly rectifying currents that increased upon coexpression of SGK1-3 and the related kinase PKB. The stimulatory effect was still present upon disruption of the SGK phosphorylation site. SGKs can phosphorylate the ubiquitin ligase Nedd4-2 and prevent Nedd4-2 from binding to its target. Therefore, the role of Nedd4-2 in ClC-2 modulation was investigated. ClC-2 activity decreased upon Nedd4-2 coexpression, an effect reversed by the kinases. According to chemiluminescence ClC-2 membrane abundance was enhanced by SGKs and diminished by Nedd4-2. These observations suggest that SGK1-3 and Nedd4-2 regulate ClC-2 at least in part by modulating ClC-2 abundance at the plasma membrane.

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.

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.

Biophysical and pharmacological characterization of hypotonically activated chloride currents in cortical astrocytes

Rat cortical astrocytes regulate their cell volume in response to hypotonic challenge. This regulation is believed to depend largely on the release of chloride or organic osmolytes through anion channels. Using whole-cell recordings, we identified weakly outwardly rectifying chloride currents that could be activated in response to hypotonic challenge. These currents exhibited the following permeability sequence upon replacement of chloride in the bathing solution with various anions: I- > NO3- > Cl- > Gluc- > or = MeS- > Ise-. Interestingly, extracellular I-, albeit showing the greatest permeability, blocked the currents with an IC50 of approximately 50 mM. Currents were almost completely inhibited by 123 microM NPPB and partially inhibited by 200 microM niflumic acid or 200 microM DIDS. Additionally, the total number of Cl- ions effluxed through the hypotonically activated channels was markedly similar to the total solute efflux during volume regulation. We therefore propose the hypotonically activated chloride channel as a major contributor to volume regulation of astrocytes. To examine potential candidate chloride channel genes expressed by astrocytes, we employed RT-PCR to demonstrate the presence of transcripts for ClC-2, 3, 4, 5, and 7, as well as for VDAC and CFTR in cultured astrocytes. Moreover, we performed immunostaining with antibodies against each of these channels and showed the strongest expression of ClC-2 and ClC-3, strong expression of ClC-5 and VDAC, weak expression of ClC-7 and very weak expression of ClC-4 and CFTR. Intriguingly, although we found at least seven Cl- channel proteins from three different gene families in astrocytes, none appeared to be active in resting cells.

Astroglial glutamine transport by system N is upregulated by glutamate

Release of glutamine from astrocytes is an essential step of the glutamate-glutamine cycle, and hence for the maintenance of neuronal glutamate and gamma-aminobutyric acid (GABA) pools. The glutamine transporter SNAT3 (SN1) has recently been identified as one of the major mediators of glutamine efflux from astrocytes. We investigated the regulation of SNAT3 mediated glutamine transport in cultured astrocytes. Incubation of primary astrocyte cultures with physiological concentrations of glutamate resulted in a rapid, about twofold, upregulation of SNAT3-mediated transport activity. The effect was not mediated by glutamate receptors but required uptake of glutamate into astrocytes. Both net uptake and net efflux increased after treatment of cells with glutamate, excluding an acceleration of the transport by way of an exchange mechanism. Elevated intracellular glutamate most likely reduces the K(m) of SNAT3 for its substrate glutamine. The results suggest that astrocytes respond actively to the release of glutamate by increasing glutamine release and thereby may modulate glutamatergic neurotransmission.

Cystinuria-specific rBAT(R365W) mutation reveals two translocation pathways in the amino acid transporter rBAT-b0,+AT

Apical reabsorption of dibasic amino acids and cystine in kidney is mediated by the heteromeric amino acid antiporter rBAT/b(0,+)AT (system b(0,+)). Mutations in rBAT cause cystinuria type A, whereas mutations in b(0,+)AT cause cystinuria type B. b(0,+)AT is the catalytic subunit, whereas it is believed that rBAT helps the routing of the rBAT/b(0,+)AT heterodimeric complex to the plasma membrane. In the present study, we have functionally characterized the cystinuria-specific R365W (Arg(365)-->Trp) mutation of human rBAT, which in addition to a trafficking defect, alters functional properties of the b(0,+) transporter. In oocytes, where human rBAT interacts with the endogenous b(0,+)AT subunit to form an active transporter, the rBAT(R365W) mutation caused a defect of arginine efflux without altering arginine influx or apparent affinities for intracellular or extracellular arginine. Transport of lysine or leucine remained unaffected. In HeLa cells, functional expression of rBAT(R365W)/b(0,+)AT was observed only at the permissive temperature of 33 degrees C. Under these conditions, the mutated transporter showed 50% reduction of arginine influx and a similar decreased accumulation of dibasic amino acids. Efflux of arginine through the rBAT(R365W)/b(0,+)AT holotransporter was completely abolished. This supports a two-translocation-pathway model for antiporter b(0,+), in which the efflux pathway in the rBAT(R365W)/b(0,+)AT holotransporter is defective for arginine translocation or dissociation. This is the first direct evidence that mutations in rBAT may modify transport properties of system b(0,+).

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Hypotonic-induced transport pathways in Xenopus laevis erythrocytes: taurine fluxes

Taurine fluxes in Xenopus laevis red cells were studied in vitro in media of different tonicities. Both influx and efflux increased 3-10 times reversibly when dilution of the medium exceeded 30%. The absolute values of uptake ranged between 5 and 30 micromol/l cells.h at extracellular taurine concentration of 1 mmol/l, but is poorly selective as almost the same uptake was measured for choline and sucrose. Q(10) of 2.77 and an activation energy of 71.90+/-7.37 kJ/mol were calculated for the uptake process. Taurine uptake was reduced 50% in the absence of Cl(-), whereas the alkali cations (Na(+), K(+), Li(+) and Rb(+)) supported it similarly. Taurine uptake was greatly increased in Ca(2+)-free solution, and was inhibited by alkaline pH. The inhibitor of anion exchange protein, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (IC(50)=25 microM) and the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino) benzoic acid and [(dihydro-indenyl) oxy] alkanoic acid (IC(50)<20 microM) inhibited taurine uptake effectively. Isoproterenol did not affect taurine uptake in isotonic, nor in hypotonic solution. The uptake was reduced slowly to near the original, control level within 15-30 min in hypotonic solutions, indicating deactivation of the hypotonic-induced taurine pathway.

Secretion and cell volume regulation by salivary acinar cells from mice lacking expression of the Clcn3 Cl- channel gene

Salivary gland acinar cells shrink when Cl(-) currents are activated following cell swelling induced by exposure to a hypotonic solution or in response to calcium-mobilizing agonists. The molecular identity of the Cl(-) channel(s) in salivary cells involved in these processes is unknown, although ClC-3 has been implicated in several tissues as a cell-volume-sensitive Cl(-) channel. We found that cells isolated from mice with targeted disruption of the Clcn3 gene undergo regulatory volume decrease in a fashion similar to cells from wild-type littermates. Consistent with a normal regulatory volume decrease response, the magnitude and the kinetics of the swell-activated Cl(-) currents in cells from ClC-3-deficient mice were equivalent to those from wild-type mice. It has also been suggested that ClC-3 is activated by Ca(2+)-calmodulin-dependent protein kinase II; however, the magnitude of the Ca(2+)-dependent Cl(-) current was unchanged in the Clcn3(-/-) animals. In addition, we observed that ClC-3 appeared to be highly expressed in the smooth muscle cells of glandular blood vessels, suggesting a potential role for this channel in saliva production by regulating blood flow, yet the volume and ionic compositions of in vivo stimulated saliva from wild-type and null mutant animals were comparable. Finally, in some cells ClC-3 is an intracellular channel that is thought to be involved in vesicular acidification and secretion. Nevertheless, the protein content of saliva was unchanged in Clcn3(-/-) mice. Our results demonstrate that the ClC-3 Cl(-) channel is not a major regulator of acinar cell volume, nor is it essential for determining the secretion rate and composition of saliva.

Chloride/anion channels in glial cell membranes

At least seven different chloride/anion currents have now been identified in astrocytes, oligodendrocytes/Schwann cells, and microglia. Only for two of these currents is the corresponding gene known. One of these genes is not encoding for a chloride channel, but for a class of mitochondria-like pores also found in cell membranes. Astrocytes and oligodendrocytes differ in their resting properties: astrocytes accumulate chloride but do not have a significant permeability. Oligodendrocytes have a close to passive distribution and a significant permeability. Under certain circumstances, astrocytes can express a resting chloride conductance. Reactive and neoplastic astrocytes as well as astrocytes with an altered shape exhibit a resting conductance. The function of these channels certainly involves volume regulation. Other possible functions are potassium homeostasis, migration, proliferation (in microglia), and involvement in spreading depression waves. Of greatest interest are two phenomena discovered in situ: The ClC-2 channel is only found in astrocytic endfeet near blood capillaries adjacent to neuronal GABA(A) receptors. In the supraoptic nucleus of the hypothalamus, there is an osmosensitive astrocytic taurine release. This released taurine interacts with glycine receptors in neighboring neurons, causing inhibition. It is assumed that with the future availability of more in situ, rather than in vitro, studies, an increased number of such complex interactions between glial cells, neurons, and blood vessels will be discovered.

Regulation of the glutamine transporter SN1 by extracellular pH and intracellular sodium ions

The glutamine transporter SN1 has recently been identified as one of the major glutamine transporters in hepatocytes and brain astrocytes. It appears to be the molecular correlate of system N amino acid transport. Two different transport mechanisms have been proposed for this transporter. These are an electroneutral mechanism, in which glutamine uptake is coupled to an exchange of 1Na+ and 1H+, or an electrogenic mechanism coupled to the exchange of 2Na+ against 1H+. This study was performed to solve these discrepancies and to investigate the reversibility of the transporter. When SN1 was expressed in Xenopus laevis oocytes, glutamine uptake was accompanied by a cotransport of 2-3 Na+ ions as determined by 22Na+ fluxes. However, at the same time a rapid release of intracellular Na+ was observed indicating an active exchange of Na+ ions. The driving force of the proton electrochemical gradient was equivalent to that of the sodium electrochemical gradient. Acidification of the extracellular medium caused the transporter to run in reverse and to release glutamine. Determination of accumulation ratios at different driving forces were in agreement with an electroneutral 1Na+-glutamine cotransport-1H+ antiport. Inward currents that were observed during glutamine uptake were much smaller than expected for a stoichiometric cotransport of charges. A slippage mode in the transporter mechanism and pH-regulated endogenous oocyte cation channels are likely to contribute to the observed currents.

Phloretin differentially inhibits volume-sensitive and cyclic AMP-activated, but not Ca-activated, Cl(-) channels

Some phenol derivatives are known to block volume-sensitive Cl(-) channels. However, effects on the channel of the bisphenol phloretin, which is a known blocker of glucose uniport and anion antiport, have not been examined. In the present study, we investigated the effects of phloretin on volume-sensitive Cl(-) channels in comparison with cyclic AMP-activated CFTR Cl(-) channels and Ca(2+)-activated Cl(-) channels using the whole-cell patch-clamp technique. Extracellular application of phloretin (over 10 microM) voltage-independently, and in a concentration-dependent manner (IC(50) approximately 30 microM), inhibited the Cl(-) current activated by a hypotonic challenge in human epithelial T84, Intestine 407 cells and mouse mammary C127/CFTR cells. In contrast, at 30 microM phloretin failed to inhibit cyclic AMP-activated Cl(-) currents in T84 and C127/CFTR cells. Higher concentrations (over 100 microM) of phloretin, however, partially inhibited the CFTR Cl(-) currents in a voltage-dependent manner. At 30 and 300 microM, phloretin showed no inhibitory effect on Ca(2+)-dependent Cl(-) currents induced by ionomycin in T84 cells. It is concluded that phloretin preferentially blocks volume-sensitive Cl(-) channels at low concentrations (below 100 microM) and also inhibits cyclic AMP-activated Cl(-) channels at higher concentrations, whereas phloretin does not inhibit Ca(2+)-activated Cl(-) channels in epithelial cells.

Reduction of phospholemman expression decreases osmosensitive taurine efflux in astrocytes

The role of phospholemman (PLM) in taurine and Cl(-) efflux elicited by 30% hyposmotic solution was studied in cultured cerebellar astrocytes with reduced PLM expression by antisense oligonucleotide (AO) treatment. PLM, a substrate for protein kinases (PK) C and A, is a protein that increases an anion current in Xenopus oocytes and forms taurine-selective channels in lipid bilayers. Taurine contributes as an osmolyte to regulatory volume decrease (RVD) and is highly permeable through PLM channels in bilayers. Two antisense oligonucleotides (AO1 and AO2) effectively decreased the expression of the PLM protein by 40% and 30%, respectively, and markedly reduced [(3)H]taurine efflux by 67% and 62%. AO treatment also decreased the osmosensitive release of Cl(-), followed as (125)I. The inhibition of Cl(-) efflux (23% for AO1 and 13% for AO2) was notably lower than for [(3)H]taurine. The contribution of PKC and PKA in the function of PLM was also evaluated in astrocytes. Pharmacological activation or inhibition of PKC and PKA revealed that the osmosensitive taurine efflux is essentially PKC-independent while (125)I efflux is reduced by the PKC blockers H-7 (21%) and Gö6983 (41%). The PKA activator forskolin and dbcAMP increased taurine efflux by 66-70% and (125)I efflux by 21-45%. Norepinephrine increased the osmosensitive taurine efflux at about the same extent as dbcAMP and forskolin, and this was reduced by PKA blockers. These results suggest that PLM plays a role in RVD in astrocytes by predominantly influencing taurine fluxes, which are modulated by PKA but not PKC.