A large, multiple-conductance chloride channel in normal human T lymphocytes

Unique Properties of Nutrient Channels on Plasmodium-Infected Erythrocytes

Intracellular malaria parasites activate an ion and organic solute channel on their host erythrocyte membrane to acquire a broad range of essential nutrients. This plasmodial surface anion channel (PSAC) facilitates the uptake of sugars, amino acids, purines, some vitamins, and organic cations, but remarkably, it must exclude the small Na+ ion to preserve infected erythrocyte osmotic stability in plasma. Although molecular, biochemical, and structural studies have provided fundamental mechanistic insights about PSAC and advanced potent inhibitors as exciting antimalarial leads, important questions remain about how nutrients and ions are transported. Here, I review PSAC's unusual selectivity and conductance properties, which should guide future research into this important microbial ion channel.

Physiologic roles of P2 receptors in leukocytes

Since their discovery in the 1970s, purinergic receptors have been shown to play key roles in a wide variety of biologic systems and cell types. In the immune system, purinergic receptors participate in innate immunity and in the modulation of the adaptive immune response. In particular, P2 receptors, which respond to extracellular nucleotides, are widely expressed on leukocytes, causing the release of cytokines and chemokines and the formation of inflammatory mediators, and inducing phagocytosis, degranulation, and cell death. The activity of these receptors is regulated by ectonucleotidases-expressed in these same cell types-which regulate the availability of nucleotides in the extracellular environment. In this article, we review the characteristics of the main purinergic receptor subtypes present in the immune system, focusing on the P2 family. In addition, we describe the physiologic roles of the P2 receptors already identified in leukocytes and how they can positively or negatively modulate the development of infectious diseases, inflammation, and pain.

The ATP-Releasing Maxi-Cl Channel: Its Identity, Molecular Partners and Physiological/Pathophysiological Implications

The Maxi-Cl phenotype accounts for the majority (app. 60%) of reports on the large-conductance maxi-anion channels (MACs) and has been detected in almost every type of cell, including placenta, endothelium, lymphocyte, cardiac myocyte, neuron, and glial cells, and in cells originating from humans to frogs. A unitary conductance of 300-400 pS, linear current-to-voltage relationship, relatively high anion-to-cation selectivity, bell-shaped voltage dependency, and sensitivity to extracellular gadolinium are biophysical and pharmacological hallmarks of the Maxi-Cl channel. Its identification as a complex with SLCO2A1 as a core pore-forming component and two auxiliary regulatory proteins, annexin A2 and S100A10 (p11), explains the activation mechanism as Tyr23 dephosphorylation at ANXA2 in parallel with calcium binding at S100A10. In the resting state, SLCO2A1 functions as a prostaglandin transporter whereas upon activation it turns to an anion channel. As an efficient pathway for chloride, Maxi-Cl is implicated in a number of physiologically and pathophysiologically important processes, such as cell volume regulation, fluid secretion, apoptosis, and charge transfer. Maxi-Cl is permeable for ATP and other small signaling molecules serving as an electrogenic pathway in cell-to-cell signal transduction. Mutations at the SLCO2A1 gene cause inherited bone and gut pathologies and malignancies, signifying the Maxi-Cl channel as a perspective pharmacological target.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Molecular Identities and ATP Release Activities of Two Types of Volume-Regulatory Anion Channels, VSOR and Maxi-Cl

An elaborate volume regulation system based on interplay of ion channels and transporters was evolved to cope with constant osmotic challenges caused by intensive metabolism, transport and other physiological/pathophysiological events. In animal cells, two types of anion channels are directly activated by cell swelling and involved in the regulatory volume decrease (RVD): volume-sensitive outwardly rectifying anion channel (VSOR), also called volume-regulated anion channel (VRAC), and Maxi-Cl which is the most major type of maxi-anion channel (MAC). These two channels have very different biophysical profiles and exhibit opposite dependence on intracellular ATP. After several decades of verifying many false-positive candidates for VSOR and Maxi-Cl, LRRC8 family proteins emerged as major VSOR components, and SLCO2A1 protein as a core of Maxi-Cl. Still, neither of these proteins alone can fully reproduce the native channel phenotypes suggesting existence of missing components. Although both VSOR and Maxi-Cl have pores wide enough to accommodate bulky ATP^4- and MgATP^2- anions, evidence accumulated hitherto, based on pharmacological and gene silencing experiments, suggests that Maxi-Cl, but not VSOR, serves as one of the major pathways for the release of ATP from swollen and ischemic/hypoxic cells. Relations of VSOR and Maxi-Cl with diseases and their selective pharmacology are the topics promoted by recent advance in molecular identification of the two volume-activated, volume-regulatory anion channels.

The organic anion transporter SLCO2A1 constitutes the core component of the Maxi-Cl channel

The maxi-anion channels (MACs) are expressed in cells from mammals to amphibians with ~60% exhibiting a phenotype called Maxi-Cl. Maxi-Cl serves as the most efficient pathway for regulated fluxes of inorganic and organic anions including ATP However, its molecular entity has long been elusive. By subjecting proteins isolated from bleb membranes rich in Maxi-Cl activity to LC-MS/MS combined with targeted siRNA screening, CRISPR/Cas9-mediated knockout, and heterologous overexpression, we identified the organic anion transporter SLCO2A1, known as a prostaglandin transporter (PGT), as a key component of Maxi-Cl. Recombinant SLCO2A1 exhibited Maxi-Cl activity in reconstituted proteoliposomes. When SLCO2A1, but not its two disease-causing mutants, was heterologously expressed in cells which lack endogenous SLCO2A1 expression and Maxi-Cl activity, Maxi-Cl currents became activated. The charge-neutralized mutant became weakly cation-selective with exhibiting a smaller single-channel conductance. Slco2a1 silencing in vitro and in vivo, respectively, suppressed the release of ATP from swollen C127 cells and from Langendorff-perfused mouse hearts subjected to ischemia-reperfusion. These findings indicate that SLCO2A1 is an essential core component of the ATP-conductive Maxi-Cl channel.

Extracellular ATP and adenosine: The Yin and Yang in immune responses?

Extracellular adenosine 5'-triphosphate (ATP) and adenosine molecules are intimately involved in immune responses. ATP is mostly a pro-inflammatory molecule and is released during hypoxic condition and by necrotic cells, as well as by activated immune cells and endothelial cells. However, under certain conditions, for instance at low concentrations or at prolonged exposure, ATP may also have anti-inflammatory properties. Extracellular ATP can activate both P2X and P2Y purinergic receptors. Extracellular ATP can be hydrolyzed into adenosine in a two-step enzymatic process involving the ectonucleotidases CD39 (ecto-apyrase) and CD73. These enzymes are expressed by many cell types, including endothelial cells and immune cells. The counterpart of ATP is adenosine, which is produced by breakdown of intra- or extracellular ATP. Adenosine has mainly anti-inflammatory effects by binding to the adenosine, or P1, receptors (A1, A2A, A2B, and A3). These receptors are also expressed in many cells, including immune cells. The final effect of ATP and adenosine in immune responses depends on the fine regulatory balance between the 2 molecules. In the present review, we will discuss the current knowledge on the role of these 2 molecules in the immune responses.

Plasmalemmal VDAC controversies and maxi-anion channel puzzle

The maxi-anion channel has been observed in many cell types from the very beginning of the patch-clamp era. The channel is highly conductive for chloride and thus can modulate the resting membrane potential and play a role in fluid secretion/absorption and cell volume regulation. A wide nanoscopic pore of the maxi-anion channel permits passage of excitatory amino acids and nucleotides. The channel-mediated release of these signaling molecules is associated with kidney tubuloglomerular feedback, cardiac ischemia/hypoxia, as well as brain ischemia/hypoxia and excitotoxic neurodegeneration. Despite the ubiquitous expression and physiological/pathophysiological significance, the molecular identity of the maxi-anion channel is still obscure. VDAC is primarily a mitochondrial protein; however several groups detected it on the cellular surface. VDAC in lipid bilayers reproduced the most important biophysical properties of the maxi-anion channel, such as a wide nano-sized pore, closure in response to moderately high voltages, ATP-block and ATP-permeability. However, these similarities turned out to be superficial, and the hypothesis of plasmalemmal VDAC as the maxi-anion channel did not withstand the test by genetic manipulations of VDAC protein expression. VDAC on the cellular surface could also function as a ferricyanide reductase or a receptor for plasminogen kringle 5 and for neuroactive steroids. These ideas, as well as the very presence of VDAC on plasmalemma, remain to be scrutinized by genetic manipulations of the VDAC protein expression. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Activation of maxi-anion channel by protein tyrosine dephosphorylation

The maxi-anion channel with a large single-channel conductance of >300 pS, and unknown molecular identity, is functionally expressed in a large variety of cell types. The channel is activated by a number of experimental maneuvers such as exposing cells to hypotonic or ischemic stress. The most effective and consistent method of activating it is patch membrane excision. However, the activation mechanism of the maxi-anion channel remains poorly understood at present. In the present study, involvement of phosphorylation/dephosphorylation in excision-induced activation was examined. In mouse mammary fibroblastic C127 cells, activity of the channel was suppressed by intracellular application of Mg-ATP, but not Mg-5'-adenylylimidodiphosphate (AMP-PNP), in a concentration-dependent manner. When a cocktail of broad-spectrum tyrosine phosphatase inhibitors was applied, channel activation was completely abolished, whereas inhibitors of serine/threonine protein phosphatases had no effect. On the other hand, protein tyrosine kinase inhibitors brought the channel out of an inactivated state. In mouse adult skin fibroblasts (MAFs) in primary culture, similar maxi-anion channels were found to be activated on membrane excision, in a manner sensitive to tyrosine phosphatase inhibitors. In MAFs isolated from animals deficient in receptor protein tyrosine phosphatase (RPTP)zeta, activation of the maxi-anion channel was significantly slower and less prominent compared with that observed in wild-type MAFs; however, channel activation was restored by transfection of the RPTPzeta gene. Thus it is concluded that activation of the maxi-anion channel involves protein dephosphorylation mediated by protein tyrosine phosphatases that include RPTPzeta in mouse fibroblasts, but not in C127 cells.

The maxi-anion channel: a classical channel playing novel roles through an unidentified molecular entity

The maxi-anion channel is widely expressed and found in almost every part of the body. The channel is activated in response to osmotic cell swelling, to excision of the membrane patch, and also to some other physiologically and pathophysiologically relevant stimuli, such as salt stress in kidney macula densa as well as ischemia/hypoxia in heart and brain. Biophysically, the maxi-anion channel is characterized by a large single-channel conductance of 300-400 pS, which saturates at 580-640 pS with increasing the Cl(-) concentration. The channel discriminates well between Na(+) and Cl(-), but is poorly selective to other halides exhibiting weak electric-field selectivity with an Eisenman's selectivity sequence I. The maxi-anion channel has a wide pore with an effective radius of approximately 1.3 nm and permits passage not only of Cl(-) but also of some intracellular large organic anions, thereby releasing major extracellular signals and gliotransmitters such as glutamate(-) and ATP(4-). The channel-mediated efflux of these signaling molecules is associated with kidney tubuloglomerular feedback, cardiac ischemia/hypoxia, as well as brain ischemia/hypoxia and excitotoxic neurodegeneration. Despite the ubiquitous expression, well-defined properties and physiological/pathophysiological significance of this classical channel, the molecular entity has not been identified. Molecular identification of the maxi-anion channel is an urgent task that would greatly promote investigation in the fields not only of anion channel but also of physiological/pathophysiological signaling in the brain, heart and kidney.

Purinergic regulation of neutrophil chemotaxis

Chemotaxis allows polymorphonuclear neutrophils (PMN) to rapidly reach infected and inflamed sites. However, excessive influx of PMN damages host tissues. Better knowledge of the mechanisms that control PMN chemotaxis may lead to improved treatments of inflammatory diseases. Recent findings suggest that ATP and adenosine are involved in PMN chemotaxis. Therefore, these purinergic signaling processes may be suitable targets for novel therapeutic approaches to ameliorate host tissue damage.

Inhibition of a cardiac sarcoplasmic reticulum chloride channel by tamoxifen

Anion and cation channels present in the sarcoplasmic reticulum (SR) are believed to be necessary to maintain the electroneutrality of SR membrane during Ca(2+) uptake by the SR Ca(2+) pump (SERCA). Here we incorporated canine cardiac SR ion channels into lipid bilayers and studied the effects of tamoxifen and other antiestrogens on these channels. A Cl(-) channel was identified exhibiting multiple subconductance levels which could be divided into two primary conductance bands. Tamoxifen decreases the time the channel spends in its higher, voltage-sensitive band and the mean channel current. The lower, voltage-insensitive, conductance band is not affected by tamoxifen, nor is a K(+) channel present in the cardiac SR preparation. By examining SR Ca(2+) uptake, SERCA ATPase activity, and SR ion channels in the same preparation, we also estimated SERCA transport current, SR Cl(-) and K(+) currents, and the density of SERCA, Cl(-), and K(+) channels in cardiac SR membranes.

ATP release via anion channels

ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.

Inwardly rectifying chloride channel activity in intestinal pacemaker cells

Cl(-) channels are proposed to play a role in gut pacemaker activity, but little is known about the characteristics of Cl(-) channels in interstitial cells of Cajal (ICC), the intestinal pacemaker cells. The objective of the present study was to identify whole cell Cl(-) currents in ICC associated with previously observed single-channel activity and to characterize its inward rectification. Whole cell patch-clamp studies showed that ICC express an inwardly rectifying Cl(-) current that was not sensitive to changes in cation composition of the extracellular solutions. Currents were not affected by replacing all cations with N-methyl-d-glucamine (NMDG(+)). Whole cell currents followed the Cl(-) equilibrium potential and were inhibited by DIDS and 9-anthracene carboxylic acid. Ramp protocols of single-channel activity showed that inward rectification was due to reduction in single-channel open probability, not a reduction in single-channel conductance. Single-channel data led to the hypothesis that strong cooperation exists between 30-pS channels that show less cooperation at potentials positive to the reversal potential. Hence, an inwardly rectifying Cl(-) channel plays a prominent role in determining pacemaker activity in the gut.

High-conductance chloride channels generate pacemaker currents in interstitial cells of Cajal

BACKGROUND & AIMS

Interstitial cells of Cajal (ICCs) are responsible for slow, wave-driven, rhythmic, peristaltic motor patterns in the gastrointestinal tract. The aim was to identify and characterize the ion channels that generate the underlying pacemaker activity.

METHODS

Single ion channel recordings were obtained from nonenzymatically isolated ICCs and studied by using the cell attached and inside-out configurations of the patch clamp technique.

RESULTS

A high-conductance chloride channel was observed in ICCs that was spontaneously and rhythmically active at the same frequency as the rhythmic inward currents defining ICC pacemaker activity, 20-30 cycles/min at room temperature. Main conductance levels occurred between 122-144 pS and between 185-216 pS. Periodicity in the channel opening coincided with periodicity in membrane potential change, hence, at the single channel level, chloride channels were seen to be associated with the generation of rhythmic changes in membrane potential.

CONCLUSIONS

ICCs harbor high-conductance chloride channels that participate in the generation of pacemaker activity and may become a target for pharmacologic treatment of gut motor disorders.

Extramitochondrial porin: facts and hypotheses

Mitochondrial porin, or VDAC, is a pore-forming protein abundant in the outer mitochondrial membrane. Several publications have reported extramitochondrial localizations as well, but the evidence was considered insufficient by many, and the presence of porin in nonmitochondrial cellular compartments has remained in doubt for a long time. We have now obtained new data indicating that the plasma membrane of hematopoietic cells contains porin, probably located mostly in caveolae or caveolae-like domains. Porin was purified from the plasma membrane of intact cells by a procedure utilizing the membrane-impermeable labeling reagent NH-SS-biotin and streptavidin affinity chromatography, and shown to have the same properties as mitochondrial porin. A channel with properties similar to that of isolated VDAC was observed by patch-clamping intact cells. This review discusses the evidence supporting extramitochondrial localization, the putative identification of the plasma membrane porin with the "maxi" chloride channel, the hypothetical mechanisms of sorting porin to various cellular membrane structures, and its possible functions.

Studies on human porin XIX: zinc ions reduce the voltage-dependence of human type-1 porin integrated in artificial planar lipid bilayers; barium ions do not

We report on the first data concerning the effects of ionic zinc or barium on human type-1 porin. The channel enriched as mature protein from B-lymphocyte crude membranes was integrated into artificial planar lipid bilayers and both agonists were applied as their chloride salts at 100 microM. Relative conductance was measured from -80 to 80 mV. The presence of Zn2+ resulted in a distinct reduction of the voltage dependence of the channel, the effect appearing to be symmetric. The addition of Ba2+ did not produce any measurable effect on human porin. According to our knowledge we present the first results on the interaction of bivalent metal ions and eukaryotic type-1 porin. The putative physiological relevance of the data is discussed.

Large conductance channel in plasma membranes of astrocytic cells is functionally related to mitochondrial VDAC-channels

Large conductance anion channels with similar electrophysiological characteristics were found in plasma membranes and in outer mitochondrial membranes of various cell types. Although their large conductance and their peculiar voltage dependence point to a close relation, it was questioned whether they belong to the same family. We therefore compared some biochemical features of a plasmalemmal channel with those known from the mitochondrial channel. Current events were recorded from excised patches of plasma membranes of a rat astrocytic cell line (RGCN). The underlying channels exhibited a conductance of 401 +/- 50 pS. Open probability was highest between +/- 10 mV and gradually approached zero beyond +/- 25 mV. Activity as induced by voltage ramps between +/- 40 mV appeared after a delay of up to several min. The delay could be reduced by bathing either side of the patch in an acidic Ringer solution (pH 6.2). 1 mM Al3+ increased the open time at potentials more positive than 20 mV. 10 mM dextran sulfate (MW 8000) caused reversible flickering, increasing the closed probability. 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid (DIDS) also caused a reversible flickering into the closed state, reducing the apparent single channel amplitude by up to 70% at 0.5 mM DIDS. Application of 5 mM ATP resulted in reversible blockade; ATP was more effective from the outside than from the inside (blocking activity 65% vs. 16% of the patches). We conclude that the large conductance anion channel from astrocytic cells displays electrophysiological and pharmacological characteristics that resemble those of VDAC (Voltage Dependent Anion Channel) from the outer mitochondrial membrane.

Characterization and relative abundance of maxi-chloride channels in Epstein-Barr virus (EBV) producer: B95-8 cells

Several Epstein-Barr virus (EBV)-transformed cell lines were used to investigate the pathogenesis of lymphoproliferative diseases and nasopharyngeal carcinoma. The studies focus on the events occurring inside the membrane. On only one occasion, the cell membrane of EBV-transformed B lymphocytes from a cystic fibrosis patient was found to express defective Cl channels (CFTR; Cystic Fibrosis Transmembrane conductance Regulator), as in the airway epithelial cell. No other type of channel in EBV-transformed cells has so far been investigated. In this study, the cell membrane of the B95-8 cell was examined by the patch-clamp technique and compared to the non-EBV-infected BJAB cell. The high conductance (approximately 300 pS) maxi-chloride (Cl) channel activity was the most frequently observed event in inside-out configurations. Under similar experimental conditions, we have found a significantly higher probability of detecting maxi-Cl channel activity on the cell membrane of B95-8 cells (69%) than on BJAB cells (27%), or as previously reported on resting murine B lymphocytes (38%) or intact human T lymphocytes (37%). The relative abundance of the maxi-Cl channel on B95-8 cells may be linked to EBV infection and/or secretory ability.

Voltage-dependent chloride channels: invertebrates to man

Chloride channels are ubiquitous proteins found in invetebrates to man. Cl- is one of the most abundant biological anions and accounts for a measurable fraction of the electrical conductance of many biological membranes. Physiologically this contributes to cellular processes, including pH regulation, volume regulation, generation of the resting membrane potential, and regulation of membrane excitability. The unitary conductance of voltage-dependent Cl- channels is as diverse as the number of different types of Cl- channels described ranging from 5-450 pS. Cl- channels are highly anion selective passing at least ten anionic species, including all of the halides. Cl- channels are blocked by various agents, including aromatic acids, inorganic cations, and protons. Maintaining high resting conductance and normal excitability, regulating cell volume, and modulating hormone action are some examples of the functions of Cl- channels. Despite the large amount of data accumulated on voltage-dependent Cl- channels, identifying subsets within this class of channels with coherent biophysical features that subserve each specific function is still not possible. At present, the molecular structure for every type of functional Cl- channels has not been determined, but future identification of cloned Cl- channel structures should provide a clearer understanding of the functional properties of background Cl- channels.

Properties of K+ and Cl- channels and their involvement in proliferation of rat microglial cells

Essentially pure (>95%) cultures of microglia were established from neopallia of newborn rats and used for whole-cell patch-clamp recording of electrophysiological properties and for proliferation studies. Two types of cultures were examined: 1) "Primary" cultures were grown in culture medium with serum and used within 3 weeks of isolation; 2) and "Colony-stimulating factor (CSF)-1-stimulated" cultures were derived from 3-week-old "primary" cultures by passaging and culturing them for several weeks longer in the presence of conditioned medium enriched in CSF-1. Microglia in the "primary" cultures expressed: 1) an inwardly rectifying K+ current (Kir) that was inhibited by Ba2+; 2) an outwardly rectifying K+ current (Kv) with many similarities to the cloned Kv1.3 channel of lymphocytes, including block by nanomolar concentrations of charybdotoxin (ChTX) and margatoxin (MgTX); and 3) an outwardly rectifying anion current with time- and voltage-independent gating. The anion current is activated reversibly under cell swelling conditions, i.e., after exposure to a hypo-osmotic bathing medium. The anion channels are highly permeable to Cl-, measurably permeable to gluconate (P(gluconate)/ PCl = 0.34), and blocked by flufenamic acid, 4-nitro-2-(3-phenylpropylamino)- benzoic acid (NPPB), and 6, 7-dichloro-2-cyclopentyl-2, 3-dihydro-2-methyl-1-oxo-1H-inden-5-yl (oxy) acetic acid (IAA-94). Microglia in the "CSF-1-stimulated" cultures expressed Kir and Cl- current, but not Kv current. Proliferation in the latter type of cultures could be slowed by omission of the CSF-1 enriched supernatant for 2 days and stimulated by adding back the conditioned medium. This "CSF-1-stimulated" proliferation was inhibited by Ba2+ (Kir blocker), and the Cl(-)-channel blockers flufenamic acid, NPPB, and IAA-94, whereas the Kv blockers ChTX and MgTX had no effect. Thus, Kir and Cl- channels appear to be necessary for "CSF-1-stimulated" proliferation of rat microglia, and there is no evidence that even a transient activation of Kv is necessary.

Chloride-channel block inhibits T lymphocyte activation and signalling

Both large- and small-conductance chloride (Cl-) channels have been found in human T lymphocytes; however, apart from possible roles in mediating regulatory volume decrease, their functions are not understood. We have used patch-clamp electrophysiology, Ca2+ spectrofluorometry, and Western blot assay for phosphotyrosine to investigate the effects of blocking Cl- channels on proliferation and on specific events in the activation of normal human T cells. Four chemically distinct Cl- channel blockers inhibited both the small-conductance Cl- channels and phytohemagglutinin (PHA)-induced lymphocyte proliferation in a similar dose-dependent manner; their order of potency was 5-nitro-2(3-phenylpropylamino)-benzoic acid (NPPB) > 4,4'-diisothiocyano-2,2'-disulfonic acid (DIDS) > flufenamic acid >> IAA-94. The Cl- channel blockers inhibited both the PHA-induced mobilization of Ca2+ and the rapid tyrosine phosphorylation of several polypeptides. Cell proliferation was not rescued by the Ca+ ionophore ionomycin or by addition of exogenous interleukin-2 (IL-2). Moreover, the blockers also inhibited phosphotyrosine expression in IL-2-treated, activated lymphoblasts. Thus, our results support a role for Cl- channels in early, PHA-evoked signalling and in later, II-2-dependent stages of lymphocyte activation and proliferation.

Chloride permeability of rat brain membrane vesicles correlates with thiamine triphosphate content

Incubation of rat brain homogenates with thiamine or thiamine diphosphate (TDP) leads to a synthesis of thiamine triphosphate (TTP). In membrane vesicles subsequently prepared from the homogenates, increased TTP content correlates with increased 36Cl- uptake. A hyperbolic relationship was obtained with a K0.5 of 0.27 nmol TTP/mg protein. In crude mitochondrial fractions from the brains of animals previously treated with thiamine or sulbutiamine, a positive correlation between 36Cl- uptake and TTP content was found. These results, together with other results previously obtained with the patch-clamp technique, suggest that TTP is an activator of chloride channels having a large unit conductance.

Calcium-dependent chloride current induced by axotomy in rat sympathetic neurons

1. Seven to ten days after sectioning their axons, rat sympathetic neurons were studied using intracellular recording techniques in an in vitro preparation of the superior cervical ganglion. 2. In 75% of axotomized cells, an after-depolarization (ADP) was observed following spike firing or depolarization with intracellular current pulses. Discontinuous single-electrode voltage-clamp techniques were employed to study the ADP. When the membrane potential was clamped at the resting level just after an action potential, a slow inward current was recorded in cells that showed an ADP. 3. In the presence of TTX and TEA, inward peaks and outward currents were recorded during depolarizing voltage jumps, followed by slowly decaying inward tail currents accompanied by large increases in membrane conductance. The inward peak and tail currents activated between -10 and -20 mV and reached maximum amplitudes around 0 mV. With depolarizing jumps to between +40 and +50 mV, net outward currents were recorded during the depolarizing jumps but inward tail currents were still activated. 4. In the presence of the Ca2+ channel blocker cadmium, or when Ca2+ was substituted by Mg2+, the ADP disappeared. In voltage-clamped cells, cadmium blocked the inward tail currents. The reversal potential for the inward tail current was approximately -15 mV. Substitution of the extracellular NaCl by sucrose or sodium isethionate increased the amplitude of the inward tail current, and displaced its equilibrium potential to more positive values. Changes in extracellular [K+] did not appreciably affect the inward tail current amplitude or equilibrium potential. Niflumic acid, a blocker of chloride channels activated by Ca2+, almost completely blocked the tail current. 5. No ADPs were observed in non-axotomized neurons, and when depolarizing pulses were applied while in voltage clamp no inward tail currents were evoked in these normal cells. 6. It is concluded that axotomy of sympathetic ganglion cells produces the appearance of a Ca(2+)-dependent chloride current responsible for the ADP observed following spike firing.

Thiamine triphosphate activates an anion channel of large unit conductance in neuroblastoma cells

In neuroblastoma cells, the intracellular thiamine triphosphate (TTP) concentration was found to be about 0.5 microM, which is several times above the amount of cultured neurons or glial cells. In inside-out patches, addition of TTP (1 or 10 microM) to the bath activated an anion channel of large unit conductance (350-400 pS) in symmetrical 150 mM NaCl solution. The activation occurred after a delay of about 4 min and was not reversed when TTP was washed out. A possible explanation is that the channel has been irreversibly phosphorylated by TTP. The channel open probability (Po) shows a bell-shaped behavior as a function of pipette potential (Vp). Po is maximal for -25 mV < Vp < 10 mV and steeply decreases outside this potential range. From reversal potentials, permeability ratios of PCl/PNa = 20 and PCl/Pgluconate = 3 were estimated. ATP (5 mM) at the cytoplasmic side of the channel decreased the mean single channel conductance by about 50%, but thiamine derivatives did not affect unit conductance; 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (0.1 mM) increased the flickering of the channel between the open and closed state, finally leading to its closure. Addition of oxythiamine (1 mM), a thiamine antimetabolite, to the pipette filling solution potentiates the time-dependent inactivation of the channel at Vp = -20 mV but had the opposite effect at +30 mV. This finding corresponds to a shift of Po towards more negative resting membrane potentials. These observations agree with our previous results showing a modulation of chloride permeability by thiamine derivatives in membrane vesicles from rat brain.

Calcium ionophore plus excision induce a large conductance chloride channel in membrane patches of human colon carcinoma cells HT-29cl.19A

In excised inside-out membrane patches of the human colon carcinoma HT-29cl.19A cells a large conductance (373 +/- 10 pS) chloride channel was found. Channel activity could only be observed after excision of patches from cells incubated with calcium ionophore. The channel was never observed in cell-attached patches. The channel was strongly voltage dependent, being open only between +30 and -30 mV clamp potentials. The selectivity sequence among anions, deduced from reversal potentials, was I > Br > Cl > F > gluconate. The PNa/PCl was 0.09. Although a similar type of channel has been described earlier, this is the first report stating its appearance in patches of intestinal epithelial cells requiring the combined action of Ca2+ ionophore and excision, suggesting its control by an intracellular compound.

Correlated ion flux through parallel pores: application to channel subconductance states

Many ion channels that normally gate fully open or shut have recently been observed occasionally to display well-defined subconductance states with conductances much less than those of the fully open channel. One model of this behavior is a channel consisting of several parallel pores with a strong correlation between the flux in each pore such that, normally, they all conduct together but, under special circumstances, the pores may transfer to a state in which only some of them conduct. This paper introduces a general technique for modeling correlated pores, and explores in detail by computer simulation a particular model based upon electric interaction between the pores. Correlation is obtained when the transient electric field of ions passing through the pores acts upon a common set of ionizable residues of the channel protein, causing transient changes in their effective pK and hence in their charged state. The computed properties of such a correlated parallel pore channel with single occupation of each pore are derived and compared to those predicted for a single pore that can contain more than one ion at a time and also to those predicted for a model pore with fluctuating barriers. Experiments that could distinguish between the present and previous models are listed.

Chloride channels of high conductance in the microvillous membrane of term human placenta

The patch clamp technique has been used to study ion channels in the undisturbed microvillous membrane of the human placental syncytiotrophoblast. In villi from 55 placentae delivered by caesarean section, high resistance seals were achieved in approximately 30 percent of attempts. Of these, a large conductance chloride channel was identified in seven inside-out and two 'cell' attached patches. The channel had the following properties: (a) a slope conductance of 313 +/- 9 pS, (b) the presence of sub-conductance states, (c) voltage dependency, being open predominantly between +/- 20 mV and inactivating at more extreme potentials and (d) inhibition by DIDS (4-acetamido-4'-diisothiocyanostilbene 2,2-disulphonic acid). These are characteristic features of 'maxi' chloride channels which have been identified in a variety of cell types (Gogelein, 1988). The role of the chloride channel in ion transport by or homeostasis of the syncytiotrophoblast has yet to be determined.

Two types of chloride channel in the apical membrane of rat choroid plexus epithelial cells

The patch clamp technique has been used to study ion channel activity in the apical (ventricular) membrane of epithelial cells from the rat choroid plexus. Two different classes of Cl(-)-selective channel were identified. A low conductance (26 pS) channel which was the predominant feature in cell-attached and inside-out patches. The occurrence of this channel appeared to increase in tissue bathed in forskolin. It was activated in inside-out patches by increasing the Ca2+ concentration at the intracellular face of the membrane and by depolarising potentials. The second class of channel was observed infrequently (2% of patches) and appeared to be similar to 'maxi'-Cl- channels which have been described in many other cell types. It had a conductance of 320 pS, opened to sub-conductance levels and displayed a marked voltage dependence in inside-out patches. The possible contribution of these channels to Cl- transport during the production of cerebrospinal fluid (CSF) is discussed.

Voltage-sensitive chloride channels of large conductance in the membrane of pig aortic endothelial cells

Single, large-conductance chloride-selective channels were studied in the membrane of pig aortic endothelial cells. These channels were usually inactive in cell-attached recordings and activated spontaneously upon formation of inside-out patches or amphotericin B-perforated vesicles. Channel activity was voltage dependent, with a maximum open probability within the range of -20 mV to + 20 mV. Addition of 1 mM Zn2+ to either the cytoplasmic or extracellular side blocked channel activity reversibly. Extracellular 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) blocked the channels; the concentration necessary for half-maximum blockade was 100 mumol/l. The frequency of observing channels in cell-attached patches increased from less than 5% to 27% when cells were treated for several minutes with 1 mumol/l bradykinin and to 80% in the presence of the calcium ionophore A23187 (1 mumol/l). Both agents increase the cytoplasmic Ca2+ concentration, thereby stimulating nitric oxide (NO) synthesis and cGMP formation in endothelial cells. Sodium nitroprusside (100 mumol/l), which spontaneously releases NO, did not increase Cl- channel activity in intact cells. Polymyxin B (100 mumol/l), an inhibitor of protein kinase C, clearly enhanced Cl- channel activity in intact cells, resulting in the observation of Cl- channels in 70% of cell-attached patches. Our results demonstrate the existence of a large-conductance (LC-type) Cl- channel in vascular endothelium which is subject to a complex cellular regulation, possibly involving inhibition via phosphorylation by protein kinase C, and activation by a Ca2(+)-dependent process which is different from the NO/cGMP pathway.

Characterization of large-conductance chloride channels in rabbit colonic smooth muscle

1. A large-conductance Cl- channel was characterized in cell-free membrane patches from the rabbit longitudinal colonic smooth muscle using the patch clamp technique. In addition, the regulation of these channels by neurokinin-1 (NK-1) receptor agonists and G proteins was studied. 2. No spontaneous channel activity was observed in cell-attached patches at the cell resting potential, or in excised patches at pipette potentials (Vp) between -20 and 20 mV. In excised patches, channel activity could be induced in thirty-six out of ninety-six patches by holding the patch at Vp values more negative than -60 mV or more positive than 60 mV. Once induced, the channel showed a bell-shaped voltage activation curve in high symmetric [Cl-], with maximal open probability between 20 and -5 mV. Varying cytosolic calcium concentration ([Ca2+]) between 5 x 10(-8) M and 1.0 mM had no effect on the voltage activation of the channel. 3. In inside-out and outside-out patches, when pipette and bath solutions contained equal [Cl-] (130 mM), the anion channel showed a linear current-voltage (I-V) relationship between -60 and 60 mV with a slope conductance of 309 +/- 20 pS (n = 13). Reversal potential measurements indicated that the channel was selective for Cl- over Na+ and K+ (PCl/PNa = 6:1). 4. Channel openings from the closed state to the full open state as well as transitions through smaller conductance states were observed. The smallest detectable substate had a conductance of 15.6 pS. Based on the similarities in selectivity and linearity of the I-V curve of the smaller conductances with the full open state, and kinetic analysis of channel activity, it is concluded that the large conductance channel is composed of multiple substates which can either open and close independently, or simultaneously via a main gate. 5. The stilbene derivative diiso-thiocyanato-stilbene-disulphonic acid (DIDS) and the diphenylamine-2-carboxylate analogue 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) caused a dose-dependent, reversible flicker block of the small conductance and significantly reduced the macroscopic current flow through the channel. 6. In quiescent outside-out patches, when the pipette contained a 140 mM-CsCl solution with 10(-6) M-CaCl2, 1.2 mM-MgCl2 and 1 mM-GTP, and the bath contained Ringer solution, addition of the NK-1 receptor antagonists substance P methylester resulted in activation of the full conductance state and of smaller substates.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for extra-mitochondrial localization of the VDAC/porin channel in eucaryotic cells

The expression of bacterial porin in outer membranes of gram-negative bacteria and of mitochondrial porin or voltage-dependent anion channel (VDAC) in outer mitochondrial membranes (OMM) of eucaryotic cells was demonstrated about 15 years ago. However, the expression of VDAC in the plasmalemma (PLM) of transformed human B lymphoblasts has recently been indicated by cytotoxicity and indirect immunofluorescence studies. New data suggest that the expression of VDAC may be even more widespread. Different cell types express porin channels in their PLM and in intracellular membranes other than OMM. The functional expression of these channels may differ in the various compartments since recent experiments have demonstrated that the voltage dependence and ion selectivity of mitochondrial VDAC may be altered by their interaction with modulators. The present paper proposes a unifying concept for the ion-selective channels of cell membranes, in particular, those whose regulation is affected in cystic fibrosis.

High conductance anion channel in Schwann cell vesicles from rat spinal roots

Potassium uptake, possibly together with chloride, is one of the presumed functions of Schwann cells in the peripheral nervous system. However, the presence of chloride channels has not been demonstrated in adult Schwann cells. We present here a new method which allows single channel recordings to be made from Schwann cells in situ without enzymatic treatment. Isolated rat spinal roots were split mechanically into several bundles. Within about 30 min after this procedure small bleb-like vesicles (approximately 20-30 microns in diameter) with a clean surface appeared at the edges of the fibre bundles. Immunofluorescence microscopy with a surface marker for Schwann cell membranes (monoclonal antibody O4) revealed that the vesicles originate from Schwann cells. In standard patch clamp recordings with symmetrical bath and pipette solutions (excised inside-out configuration) an anion channel with the following characteristics was mainly observed: 1) single channel slope conductance of 337 +/- 5 pS in 125 mM KCl and 209 +/- 6 pS in 125 mM K+ methylsulphate; 2) ion permeability ratio: PCl/PK/Pgluconate = 1/0.12/0.06; 3) linear current-voltage relationship (range +/- 60 mV); and 4) voltage- and time-dependent inactivation (the channel was most active at potentials +/- 20 mV). Pharmacologically, the channel was completely blocked with zinc (1 mM) and barium (10 mM). A similar anion channel, showing characteristics 1-4), has been described in cultured Schwann cells of newborn rats (Gray et al., 1984). We now demonstrate that this channel is also present in adult Schwann cells in situ.

Modulation of potassium channels in intact human T lymphocytes

1. A voltage-dependent K+ channel called the 'n' type (for 'normal') is the most prevalent ion channel found in whole-cell recordings from T lymphocytes. In whole-cell patch-clamp recordings activity of the n-type channel is affected by mitogenic agents, pH, Ca2+ and temperature but not by cyclic nucleotides. Because channel properties and regulation can depend on cytoplasmic components we sought to reassess the properties of K+ channels in intact, normal human T lymphocytes using cell-attached, patch-clamp recordings. In the present study, we show that the predominant K+ channel in resting, intact cells is the n type and is affected by voltage, temperature and Ca2+ in ways similar to the disrupted cell. Moreover, K+ channels are activated by agents that raise cyclic AMP in intact cells. 2. In cell-attached recordings, we found voltage-activated K+ channels in about 60% of patches at room temperature. The channel was K+ selective as judged from the reversal potential under different Ka(+)-K+ gradients and at different resting membrane potentials. Some patches were subsequently excised and the selectivity further confirmed. The current-voltage relation was inwardly rectifying under symmetrical K+ concentrations and had a slope conductance of 9.4 pS at 50 mV depolarized and 23.8 pS at 50 mV hyperpolarized from the resting potential. From the reversal potentials under various conditions the cell resting potential was -51 +/- 1 mV in normal NaCl saline and about 0 mV when the bath contained 150 mM-KCl saline. Two other types of K+ channel were seen in resting, intact cells, but were much less common (less than 5% and 11% of patches). A large-conductance K+ channel was seen in less than 1% of inside-out patches. 3. The predominant K+ channel in intact, resting T lymphocytes was confirmed as the n type underlying the whole-cell K+ current evoked by voltage steps. In cell-attached patches there was a low, steady-state level of activity at the resting potential but activity was greatly increased by depolarizing voltage jumps. Steady-state inactivation could be removed by a hyperpolarizing pre-pulse. Ensemble currents constructed by summing channel openings during repeated voltage jumps showed sigmoid kinetics of current activation and a monoexponential decay phase. These kinetics were well fitted by a Hodgkin-Huxley-type n4j kinetic model with time constants very similar to the whole-cell current of disrupted cells. Moreover, the kinetics depended on the external K+ concentration as previous research has shown.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of stilbene-type anion channel blockers on the immune response during experimental allergic neuritis (EAN)

We have studied the role of anion channel gating for the autoimmune response in experimental allergic neuritis (EAN) induced by bovine peripheral myelin (BPM). The influence of the stilbene-type anion channel blockers SITS and DIDS on T cell function was assessed by measurement of proliferation and by counting of interferon-gamma (IFN-gamma) secreting cells (IFN-gamma-sc) in response to BPM and phytohemagglutinin (PHA). SITS caused a dose-dependent increase of spontaneous proliferative activity as well as of proliferation in response to the antigenic stimulus BPM. In contrast, the drug caused a decrease of proliferation of cells stimulated with PHA. The number of cells induced to IFN-gamma secretion was reduced by SITS. The suppressive effect was dependent on the degree of activity of cells without drugs. Cultures showing high numbers of BPM reactive T cells were more easily suppressed than cultures with low numbers of BPM reactive T cells. Our results suggest that anion channel gating is involved in the triggering of T cells to IFN-gamma secretion. The anion channel signal pathway in lymphocytes could be a target for pharmacological intervention in inflammatory disorders. In the presently used autoimmune model, EAN, the net effect of in vivo treatment with SITS resulted in worsening of clinical signs and increased inflammatory cell infiltration in sciatic nerve, whereas the in vitro conductivity of sciatic nerve was not significantly affected by the drug. Thus anion channel gating seems to regulate activities of immune cells, and drugs with anion channel blocking properties may have effects that enhance autoimmune disease.

Role of ion channels in lymphocytes

Ion channels, and ion fluxes in general, appear to regulate a wide variety of processes important to lymphocyte function in normal and disease states. These include resting ionic homeostasis and the more complex signaling events involved in activation, proliferation, cytotoxic function, and volume regulation. The wider application of patch-clamp and microfluorimetry techniques to lymphocytes has helped to clarify some issues and raised many more. It seems likely that rapid progress will be made in our understanding of these areas through a combination of immunological, biochemical, and electrophysiological approaches.

[A flip-flop model of the chloride channel complex explains the dysregulation of the chloride flow in the plasmalemma of cells in cystic fibrosis]

The basic defect in cystic fibrosis is the chloride impermeability of the plasmalemma in different cells. A candidate for the chloride channel, thought to be affected in the syndrome, is "Porin 31HL" recently described by us. The molecule is i) expressed in the plasmalemma of different cells, it has ii) a molecular mass of 31,000 Daltons, it shows iii) high conductance in artificial membranes and it can be iv) modified by 4,4'-Diisothiocyanatostilbene-2,2'-disulfonate. A porin in the outer membrane of cells should furthermore v) be regulated by modulators. All these characters of "Porin 31HL" correspond to those given in literature for chloride channels. The regulation of the channels can be explained by a two component flip flop model.

Regulation of Cl- channels by multifunctional CaM kinase

cAMP kinase has been shown to mediate the cAMP pathway for regulation of Cl- channels in lymphocytes, but the mediator of an alternative, Ca2+ pathway has not been identified. We show here that Ca2+ ionophore activates Cl- currents in cell-attached and whole-cell patch-clamp recordings of Jurkat T lymphocytes, but this activation is not direct. The effect of Ca2+ ionophore on whole-cell Cl- currents is inhibited by a specific peptide inhibitor of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase). Furthermore, Cl- channels are activated in excised patches by purified CaM kinase in a fashion that mimics the effect of Ca2+ ionophore in cell-attached recordings. These results suggest that CaM kinase mediates the Ca2+ pathway of Cl- channel activation.

High-conductance anion channels in embryonic chick osteogenic cells

Patch-clamp measurements done on excised membrane patches obtained from 1-5 day cultured embryonic chick osteoblasts, osteocytes, and periosteal fibroblasts revealed the existence of a high-conductance anion channel: 371 +/- 63 pS when measured under symmetrical 158 mM Cl- conditions. The channel frequently displayed subconductance levels. The ion selectivity of the channel expressed as the (an)ion to chloride permeability ratio was as follows: Cl- (1.0) greater than methylsulfate- (0.71) greater than gluconate- (0.25) greater than glutamate- (0.17) greater than Na+ = K+ (0.10). In addition, the channel had a significant permeability for inorganic phosphate ions. The channel was found in about 1% of the cell-attached patches, which indicates that the channel is under the control of as yet unknown intracellular factors. Once activated by patch excision, the channel was voltage dependent and active at potentials close to 0 mV. At potentials outside the range of +/- 10 mV channel activity decreased. This process proceeded faster at increasing membrane potentials of either polarity. Returning to potentials close to 0 mV caused reopening of the channels within seconds if the preceding voltage step led to complete closure of the channels. Channel activity did not depend noticeably on intracellular and extracellular CA2+ ions. The channel is not unique to (chick) osteogenic cells but has been demonstrated in excised patches obtained from excitable and other nonexcitable cells. Although its presence in a wide variety of cell types suggests that the channel plays a general role in as yet unknown cell physiologic processes, the channel may also have specific functions in osteogenic cells, for example providing a pathway for phosphate ions during mineralization.