Wide nanoscopic pore of maxi-anion channel suits its function as an ATP-conductive pathway

Ferula sesquiterpenes, ferutinin, galbanic acid and karatavic acid, suppress thymocyte volume regulation and proliferation by blocking the volume-sensitive anion channel

BACKGROUND

T cell development and maturation requires efficient cell volume regulation (CVR) system. Although the molecular basis of CVR is being rapidly elucidated, pharmacology of its key components remains poorly developed. Biopharmaceuticals specifically targeting CVR and its central player, the volume-sensitive outwardly rectifying anion channel (VSOR/VRAC), are necessary to uncover relationships between the channel, CVR and cell proliferation and survival.

METHODS

The effects of three Ferula sesquiterpenes, ferutinin, galbanic acid and karatavic acid on the regulatory volume decrease (RVD) of freshly isolated thymocytes by light transmittance, on proliferation of primary cultured thymocytes by cell counting and on the VSOR/VRAC by patch-clamp were evaluated.

RESULTS

Ferutinin, galbanic acid and karatavic acid exerted a profound inhibitory effect on RVD of thymocytes, leading to proliferation arrest. All three sesquiterpenes blocked VSOR/VRAC in a voltage-independent "cork-in-bottle" manner with half-maximal efficiencies comparable to those for RVD. Hill coefficients of 2.0-3.3 imply that positively cooperated binding of 2-3 molecules of the Ferula sesquiterpenes to VSOR/VRAC is required to suppress cell proliferation via inhibition of CVR. The Ferula sesquiterpenes were not apoptogenic, but induced necrotic cell death, which was pronounced for ferutinin and less manifested for galbanic and karatavic acids. VSOR/VRAC and RVD inhibition did not correlate with necrotic cell death induction.

CONCLUSION

The VSOR/VRAC channel blockage by Ferula sesquiterpenes was found to impair the CVR machinery of thymocytes, resulting in suppression of cell proliferation. The necrotic cell death is not a direct consequence of VSOR/VRAC and RVD inhibition, likely involving other cellular pathways.

Cryo-EM structure of the human Pannexin-3 channel

Pannexin-3 (PANX3) is a member of the pannexin family of large-pore, ATP-permeable channels conserved across vertebrates. PANX3 contributes to various developmental and pathophysiological processes by permeating ATP and Ca2+ ions; however, the structural basis of PANX3 channel function remains unclear. Here, we present the cryo-EM structure of human PANX3 at 2.9-3.2 Å. The PANX3 channel is heptameric and forms a transmembrane pore along the central symmetric axis. The narrowest constriction of the pore is composed of an isoleucine ring located in the extracellular region, and its size is comparable to that of other pannexins. A structural variability analysis revealed prominent structural dynamics in intracellular regions. Our structural studies provide a foundation for understanding the detailed properties of pannexin channels.

Physiology of the volume-sensitive/regulatory anion channel VSOR/VRAC: part 2: its activation mechanisms and essential roles in organic signal release

The volume-sensitive outwardly rectifying or volume-regulated anion channel, VSOR/VRAC, which was discovered in 1988, is expressed in most vertebrate cell types, and is essentially involved in cell volume regulation after swelling and in the induction of cell death. This series of review articles describes what is already known and what remains to be uncovered about the functional and molecular properties as well as the physiological and pathophysiological roles of VSOR/VRAC. This Part 2 review article describes, from the physiological and pathophysiological standpoints, first the pivotal roles of VSOR/VRAC in the release of autocrine/paracrine organic signal molecules, such as glutamate, ATP, glutathione, cGAMP, and itaconate, as well as second the swelling-independent and -dependent activation mechanisms of VSOR/VRAC. Since the pore size of VSOR/VRAC has now well been evaluated by electrophysiological and 3D-structural methods, the signal-releasing activity of VSOR/VRAC is here discussed by comparing the molecular sizes of these organic signals to the channel pore size. Swelling-independent activation mechanisms include a physicochemical one caused by the reduction of intracellular ionic strength and a biochemical one caused by oxidation due to stimulation by receptor agonists or apoptosis inducers. Because some organic substances released via VSOR/VRAC upon cell swelling can trigger or augment VSOR/VRAC activation in an autocrine fashion, swelling-dependent activation mechanisms are to be divided into two phases: the first phase induced by cell swelling per se and the second phase caused by receptor stimulation by released organic signals.

Electrophysiological characterization of a schistosome transient receptor potential channel activated by praziquantel

Ion channels have proved to be productive targets for anthelmintic chemotherapy. One example is the recent discovery of a parasitic flatworm ion channel targeted by praziquantel (PZQ), the main clinical therapy used for treatment of schistosomiasis. The ion channel activated by PZQ - a transient receptor potential ion channel of the melastatin subfamily, named TRPMPZQ - is a Ca2+-permeable ion channel expressed in all parasitic flatworms that are PZQ-sensitive. However, little is currently known about the electrophysiological properties of this target that mediates the deleterious action of PZQ on many trematodes and cestodes. Here, we provide a detailed biophysical characterization of the properties of Schistosoma mansoni TRPMPZQ channel (Sm.TRPMPZQ) in response to PZQ. Single channel electrophysiological analysis demonstrated that Sm.TRPMPZQ when activated by PZQ is a non-selective, large conductance, voltage-insensitive cation channel that displays distinct properties from human TRPM paralogs. Sm.TRPMPZQ is Ca2+-permeable but does not require Ca2+ for channel gating in response to PZQ. TRPMPZQ from Schistosoma japonicum (Sj.TRPMPZQ) and Schistosoma haematobium (Sh.TRPMPZQ) displayed similar characteristics. Profiling Sm.TRPMPZQ responsiveness to PZQ has established a biophysical signature for this channel that will aid future investigation of endogenous TRPMPZQ activity, as well as analyses of endogenous and exogenous regulators of this novel, druggable antiparasitic target.

Mechanisms of Pannexin 1 (PANX1) Channel Mechanosensitivity and Its Pathological Roles

Pannexins (PANX) were cloned based on their sequence homology to innexins (Inx), invertebrate gap junction proteins. Although there is no sequence homology between PANX and connexins (Cx), these proteins exhibit similar configurations. The PANX family has three members, PANX1, PANX2 and PANX3. Among them, PANX1 has been the most extensively studied. The PANX1 channels are activated by many factors, including high extracellular K⁺ ([K⁺]_e), high intracellular Ca²⁺ ([Ca²⁺]_i), Src family kinase (SFK)-mediated phosphorylation, caspase cleavage and mechanical stimuli. However, the mechanisms mediating this mechanosensitivity of PANX1 remain unknown. Both force-from-lipids and force-from-filaments models are proposed to explain the gating mechanisms of PANX1 channel mechanosensitivity. Finally, both the physiological and pathological roles of mechanosensitive PANX1 are discussed.

Probing Protein Nanopores with Poly(ethylene glycol)s

Neutral water-soluble poly(ethylene glycol)s (PEGs) have been extensively explored in protein nanopore research for the past several decades. The principal use of PEGs is to investigate the membrane protein ion channel physical characteristics and transport properties. In addition, protein nanopores are used to study polymer-protein interactions and polymer physicochemical properties. In this review, we focus on the biophysical studies on probing protein ion channels with PEGs, specifically on nanopore sizing by PEG partitioning. We discuss the fluctuation analysis of ion channel currents in response to the PEGs moving within their confined geometries. The advantages, limitations, and recent developments of the approach are also addressed. This article is protected by copyright. All rights reserved.

Properties, Structures, and Physiological Roles of Three Types of Anion Channels Molecularly Identified in the 2010's

Molecular identification was, at last, successfully accomplished for three types of anion channels that are all implicated in cell volume regulation/dysregulation. LRRC8A plus LRRC8C/D/E, SLCO2A1, and TMEM206 were shown to be the core or pore-forming molecules of the volume-sensitive outwardly rectifying anion channel (VSOR) also called the volume-regulated anion channel (VRAC), the large-conductance maxi-anion channel (Maxi-Cl), and the acid-sensitive outwardly rectifying anion channel (ASOR) also called the proton-activated anion channel (PAC) in 2014, 2017, and 2019, respectively. More recently in 2020 and 2021, we have identified the S100A10-annexin A2 complex and TRPM7 as the regulatory proteins for Maxi-Cl and VSOR/VRAC, respectively. In this review article, we summarize their biophysical and structural properties as well as their physiological roles by comparing with each other on the basis of their molecular insights. We also point out unsolved important issues to be elucidated soon in the future.

Effects of Weak Nonspecific Interactions with ATP on Proteins

Adenosine triphosphate (ATP) is an immensely well-studied metabolite serving multiple key biochemical roles as the major chemical energy currency in living systems, a building block of ribonucleic acids, and a phosphoryl group donor in kinase-mediated signaling. Intriguingly, ATP has been recently proposed to act as a hydrotrope that inhibits aggregation of amyloidogenic proteins; however, the underlying mechanism and the general physicochemical effect that coexistence with ATP exerts on proteins remain unclear. By combining NMR spectroscopy and MD simulations, here we observed weak but unambiguously measurable and concentration-dependent noncovalent interactions between ATP and various proteins. The interactions were most pronounced for an intrinsically disordered protein (α-synuclein) and for residues in flexible regions (e.g., loops or termini) of two representative folded proteins (ubiquitin and the dimeric ubiquitin-binding domain of p62). As shown by solution NMR, a consequence of the ATP-protein interaction was altered hydration of solvent-exposed residues in the protein. The observation that ATP interacted with all three proteins suggests that ATP is a general nonspecific binder of proteins. Several complementary biophysical methods further confirmed that, at physiological concentrations of ∼5-10 mM, ATP starts to form oligomeric states via magnesium-chelating and chelation-independent mechanisms, in agreement with previous studies. Although the observed ATP-protein interaction was relatively weak overall, the high ratio of ATP (monomeric free ATP, mono- and divalent ion-bound ATP, oligomeric and chelated ATP) to proteins in cells suggests that most proteins are likely to encounter transient interactions with ATP (and chemically similar metabolites) that confer metabolite-mediated protein surface protection.

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.

Structure versus function: Are new conformations of pannexin 1 yet to be resolved?

Pannexin 1 (Panx1) plays a decisive role in multiple physiological and pathological settings, including oxygen delivery to tissues, mucociliary clearance in airways, sepsis, neuropathic pain, and epilepsy. It is widely accepted that Panx1 exerts its role in the context of purinergic signaling by providing a transmembrane pathway for ATP. However, under certain conditions, Panx1 can also act as a highly selective membrane channel for chloride ions without ATP permeability. A recent flurry of publications has provided structural information about the Panx1 channel. However, while these structures are consistent with a chloride selective channel, none show a conformation with strong support for the ATP release function of Panx1. In this Viewpoint, we critically assess the existing evidence for the function and structure of the Panx1 channel and conclude that the structure corresponding to the ATP permeation pathway is yet to be determined. We also list a set of additional topics needing attention and propose ways to attain the large-pore, ATP-permeable conformation of the Panx1 channel.

Lytic and sublytic effects of gossypol on red blood cells and thymocytes

Gossypol is a natural polyphenol presently considered as a promising biological phytochemical with a range of activities including anticancer. We examined volume regulation-dependent effects of gossypol using erythrocytes and thymic lymphocytes. Gossypol effectively lysed human red blood cells (RBC) with a half-maximal concentration of 67.4 ± 1.6 μmol/L and in a non-colloid osmotic manner. Sublytic gossypol doses of 1-10 μmol/L significantly protected RBC from osmotic hemolysis, but potentiated their sensitivity to the colloid-osmotic lysis induced by a pore-former nystatin. When added to the thymocytes suspension, gossypol caused a strong depression of the ability of cells to restore their volume under hypoosmotic stress with a half-maximal activity at 2.1 ± 0.3 μmol/L. Gossypol suppressed regulatory volume decrease under experimental conditions, when cationic permeability was controlled by gramicidin D, and volume recovery depended mainly on anionic conductance, suggesting that the polyphenol inhibits the swelling-induced anion permeability. In direct patch-clamp experiments, gossypol inhibited the volume-sensitive outwardly rectifying (VSOR) chloride channel in thymocytes and in human HCT116 and HeLa cells, possibly by a mechanism when gossypol molecule with a radius close to the size of channel pore plugs into the narrowest portion of the native VSOR chloride channel. Micromolar gossypol suppressed proliferation of thymocytes, HCT116 and HeLa cells. VSOR blockage may represent new mechanism of anticancer activity of gossypol in addition to its action as a BH3-mimetic.

SARS-CoV-2 may hijack GPCR signaling pathways to dysregulate lung ion and fluid transport

The tropism of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a virus responsible for the ongoing coronavirus disease 2019 (COVID-19) pandemic, toward the host cells is determined, at least in part, by the expression and distribution of its cell surface receptor, angiotensin-converting enzyme 2 (ACE2). The virus further exploits the host cellular machinery to gain access into the cells; its spike protein is cleaved by a host cell surface transmembrane serine protease 2 (TMPRSS2) shortly after binding ACE2, followed by its proteolytic activation at a furin cleavage site. The virus primarily targets the epithelium of the respiratory tract, which is covered by a tightly regulated airway surface liquid (ASL) layer that serves as a primary defense mechanism against respiratory pathogens. The volume and viscosity of this fluid layer is regulated and maintained by a coordinated function of different transport pathways in the respiratory epithelium. We argue that SARS-CoV-2 may potentially alter evolutionary conserved second-messenger signaling cascades via activation of G protein-coupled receptors (GPCRs) or by directly modulating G protein signaling. Such signaling may in turn adversely modulate transepithelial transport processes, especially those involving cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial Na⁺ channel (ENaC), thereby shifting the delicate balance between anion secretion and sodium absorption, which controls homeostasis of this fluid layer. As a result, activation of the secretory pathways including CFTR-mediated Cl⁻ transport may overwhelm the absorptive pathways, such as ENaC-dependent Na⁺ uptake, and initiate a pathophysiological cascade leading to lung edema, one of the most serious and potentially deadly clinical manifestations of COVID-19.

P2 Receptors in Cardiac Myocyte Pathophysiology and Mechanotransduction

ATP is a major energy source in the mammalian cells, but it is an extracellular chemical messenger acting on P2 purinergic receptors. A line of evidence has shown that ATP is released from many different types of cells including neurons, endothelial cells, and muscle cells. In this review, we described the distribution of P2 receptor subtypes in the cardiac cells and their physiological and pathological roles in the heart. So far, the effects of external application of ATP or its analogues, and those of UTP on cardiac contractility and rhythm have been reported. In addition, specific genetic alterations and pharmacological agonists and antagonists have been adopted to discover specific roles of P2 receptor subtypes including P2X4-, P2X7-, P2Y2- and P2Y6-receptors in cardiac cells under physiological and pathological conditions. Accumulated data suggest that P2X4 receptors may play a beneficial role in cardiac muscle function, and that P2Y2- and P2Y6-receptors can induce cardiac fibrosis. Recent evidence further demonstrates P2Y1 receptor and P2X4 receptor as important mechanical signaling molecules to alter membrane potential and Ca²⁺ signaling in atrial myocytes and their uneven expression profile between right and left atrium.

Extracellular ATP: A Feasible Target for Cancer Therapy

Adenosine triphosphate (ATP) is one of the main biochemical components of the tumor microenvironment (TME), where it can promote tumor progression or tumor suppression depending on its concentration and on the specific ecto-nucleotidases and receptors expressed by immune and cancer cells. ATP can be released from cells via both specific and nonspecific pathways. A non-regulated release occurs from dying and damaged cells, whereas active release involves exocytotic granules, plasma membrane-derived microvesicles, specific ATP-binding cassette (ABC) transporters and membrane channels (connexin hemichannels, pannexin 1 (PANX1), calcium homeostasis modulator 1 (CALHM1), volume-regulated anion channels (VRACs) and maxi-anion channels (MACs)). Extracellular ATP acts at P2 purinergic receptors, among which P2X7R is a key mediator of the final ATP-dependent biological effects. Over the years, P2 receptor- or ecto-nucleotidase-targeting for cancer therapy has been proposed and actively investigated, while comparatively fewer studies have explored the suitability of TME ATP as a target. In this review, we briefly summarize the available evidence suggesting that TME ATP has a central role in determining tumor fate and is, therefore, a suitable target for cancer therapy.

Cryo-EM structure of human Cx31.3/GJC3 connexin hemichannel

Connexin family proteins assemble into hexameric channels called hemichannels/connexons, which function as transmembrane channels or dock together to form gap junction intercellular channels (GJIChs). We determined the cryo-electron microscopy structures of human connexin 31.3 (Cx31.3)/GJC3 hemichannels in the presence and absence of calcium ions and with a hearing-loss mutation R15G at 2.3-, 2.5-, and 2.6-Å resolutions, respectively. Compared with available structures of GJICh in open conformation, Cx31.3 hemichannel shows substantial structural changes of highly conserved regions in the connexin family, including opening of calcium ion-binding tunnels, reorganization of salt-bridge networks, exposure of lipid-binding sites, and collocation of amino-terminal helices at the cytoplasmic entrance. We also found that the hemichannel has a pore with a diameter of ~8 Å and selectively transports chloride ions. Our study provides structural insights into the permeant selectivity of Cx31.3 hemichannel.

Structural Basis of Enhanced Facilitated Diffusion of DNA-Binding Protein in Crowded Cellular Milieu

Although the fast association between DNA-binding proteins (DBPs) and DNA is explained by a facilitated diffusion mechanism, in which DBPs adopt a weighted combination of three-dimensional diffusion and one-dimensional (1D) sliding and hopping modes of transportation, the role of cellular environment that contains many nonspecifically interacting proteins and other biomolecules is mostly overlooked. By performing large-scale computational simulations with an appropriately tuned model of protein and DNA in the presence of nonspecifically interacting bulk and DNA-bound crowders (genomic crowders), we demonstrate the structural basis of the enhanced facilitated diffusion of DBPs inside a crowded cellular milieu through, to our knowledge, novel 1D scanning mechanisms. In this one-dimensional scanning mode, the protein can float along the DNA under the influence of nonspecific interactions of bulk crowder molecules. The search mode is distinctly different compared to usual 1D sliding and hopping dynamics in which protein diffusion is regulated by the DNA electrostatics. In contrast, the presence of genomic crowders expedites the target search process by transporting the protein over DNA segments through the formation of a transient protein-crowder bridged complex. By analyzing the ruggedness of the associated potential energy landscape, we underpin the molecular origin of the kinetic advantages of these search modes and show that they successfully explain the experimentally observed acceleration of facilitated diffusion of DBPs by molecular crowding agents and crowder-concentration-dependent enzymatic activity of transcription factors. Our findings provide crucial insights into gene regulation kinetics inside the crowded cellular milieu.

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.

ATP Release Channels

Adenosine triphosphate (ATP) has been well established as an important extracellular ligand of autocrine signaling, intercellular communication, and neurotransmission with numerous physiological and pathophysiological roles. In addition to the classical exocytosis, non-vesicular mechanisms of cellular ATP release have been demonstrated in many cell types. Although large and negatively charged ATP molecules cannot diffuse across the lipid bilayer of the plasma membrane, conductive ATP release from the cytosol into the extracellular space is possible through ATP-permeable channels. Such channels must possess two minimum qualifications for ATP permeation: anion permeability and a large ion-conducting pore. Currently, five groups of channels are acknowledged as ATP-release channels: connexin hemichannels, pannexin 1, calcium homeostasis modulator 1 (CALHM1), volume-regulated anion channels (VRACs, also known as volume-sensitive outwardly rectifying (VSOR) anion channels), and maxi-anion channels (MACs). Recently, major breakthroughs have been made in the field by molecular identification of CALHM1 as the action potential-dependent ATP-release channel in taste bud cells, LRRC8s as components of VRACs, and SLCO2A1 as a core subunit of MACs. Here, the function and physiological roles of these five groups of ATP-release channels are summarized, along with a discussion on the future implications of understanding these channels.

Maxi-anion channels play a key role in glutamate-induced ATP release from mouse astrocytes in primary culture

Astrocytes are an abundant source of ATP, which might be released from the cytosol into extracellular spaces under various conditions and even affect cell fate under some circumstances. In the present study, we aimed to evaluate the pathway(s) contributing toward glutamate-induced ATP release from mouse astrocytes. Firstly, our study of cultured astrocytes showed marked ATP release in response to stimuli of glutamate at different concentrations (0.1-1 mM), with an interesting bimodal distribution in time course. Inhibitors or blockers of potential pathways for ATP release such as exocytotic vesicular release, gap junction hemichannels, P2X7 receptors, and volume-sensitive outwardly rectifying chloride channels had no significant effects on the observed ATP release. In contrast, glutamate-induced ATP release from astrocytes was significantly inhibited by gadolinium (50 µM), an inhibitor of a maxi-anion channel; meanwhile, the application of gadolinium can allay glutamate-induced cell injury significantly. Thus, we propose that the maxi-anion channel might play an important role in glutamate-induced ATP release from mouse astrocytes and inhibition of maxi-anion channel activities to reduce ATP release can produce protective effects in the case of glutamate stimuli.

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.

Nanometer-Scale Permeabilization and Osmotic Swelling Induced by 5-ns Pulsed Electric Fields

High-intensity nanosecond pulsed electric fields (nsPEFs) permeabilize cell membranes. Although progress has been made toward an understanding of the mechanism of nsPEF-induced membrane poration, the dependence of pore size and distribution on pulse duration, strength, number, and repetition rate remains poorly defined experimentally. In this paper, we characterize the size of nsPEF-induced pores in living cell membranes by isosmotically replacing the solutes in pulsing media with polyethylene glycols and sugars before exposing Jurkat T lymphoblasts to 5 ns, 10 MV/m electric pulses. Pore size was evaluated by analyzing cell volume changes resulting from the permeation of osmolytes through the plasma membrane. We find that pores created by 5 ns pulses have a diameter between 0.7 and 0.9 nm at pulse counts up to 100 with a repetition rate of 1 kHz. For larger number of pulses, either the pore diameter or the number of pores created, or both, increase with increasing pulse counts. But the prevention of cell swelling by PEG 1000 even after 2000 pulses suggests that 5 ns, 10 MV/m pulses cannot produce pores with a diameter larger than 1.9 nm.

Purinergic control of lysenin's transport and voltage-gating properties

Lysenin, a pore-forming protein extracted from the coelomic fluid of the earthworm Eisenia foetida, manifests cytolytic activity by inserting large conductance pores in host membranes containing sphingomyelin. In the present study, we found that adenosine phosphates control the biological activity of lysenin channels inserted into planar lipid membranes with respect to their macroscopic conductance and voltage-induced gating. Addition of ATP, ADP, or AMP decreased the macroscopic conductance of lysenin channels in a concentration-dependent manner, with ATP being the most potent inhibitor and AMP the least. ATP removal from the bulk solutions by buffer exchange quickly reinstated the macroscopic conductance and demonstrated reversibility. Single-channel experiments pointed to an inhibition mechanism that most probably relies on electrostatic binding and partial occlusion of the channel-conducting pathway, rather than ligand gating induced by the highly charged phosphates. The Hill analysis of the changes in macroscopic conduction as a function of the inhibitor concentration suggested cooperative binding as descriptive of the inhibition process. Ionic screening significantly reduced the ATP inhibitory efficacy, in support of the electrostatic binding hypothesis. In addition to conductance modulation, purinergic control over the biological activity of lysenin channels has also been observed to manifest as changes of the voltage-induced gating profile. Our analysis strongly suggests that not only the inhibitor's charge but also its ability to adopt a folded conformation may explain the differences in the observed influence of ATP, ADP, and AMP on lysenin's biological activity.

Epithelia of the ovine and bovine forestomach express basolateral maxi-anion channels permeable to the anions of short-chain fatty acids

It has long been established that the absorption of short-chain fatty acids (SCFA) across epithelia stimulates sodium proton exchange. The apically released protons are not available as countercations for the basolateral efflux of SCFA anions and a suitable transport model is lacking. Patch clamp and microelectrode techniques were used to characterize an anion conductance expressed by cultured cells of the sheep and bovine rumen and the sheep omasum and to localize the conductance in the intact tissue. Cells were filled with a Na-gluconate solution and superfused with sodium salts of acetate, propionate, butyrate, or lactate. Reversal potential rose and whole cell current at +100 mV decreased with the size of the anion. Anion-induced currents could be blocked by diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS), NPPB (200 μmol l(-1)), or pCMB (1 mmol l(-1)). In patches of bovine ruminal cells, single channels were observed with a conductance for chloride (327 ± 11 pS), acetate (115 ± 8 pS), propionate (102 ± 10 pS), butyrate (81 ± 2 pS), and gluconate (44 ± 3 pS). Channels expressed by sheep rumen and omasum were similar. Microelectrode experiments suggest basolateral localization. In conclusion, forestomach epithelia express basolateral maxi-anion channels with a permeability sequence of chloride > acetate > propionate > butyrate. SCFA absorption may resemble functionally coupled transport of NaCl, with the Na(+)/K(+)-ATPase driving the basolateral efflux of the anion through a channel. Since protons are apically extruded, the model accurately predicts that influx of buffers with saliva is essential for the pH homeostasis of the ruminant forestomach.

Claudins and the modulation of tight junction permeability

Claudins are tight junction membrane proteins that are expressed in epithelia and endothelia and form paracellular barriers and pores that determine tight junction permeability. This review summarizes our current knowledge of this large protein family and discusses recent advances in our understanding of their structure and physiological functions.

Shear stress-induced Ca²⁺ mobilization in MDCK cells is ATP dependent, no matter the primary cilium

Primary cilium has emerged as mechanosensor to subtle flow variations in epithelial cells, but its role in shear stress detection remains controversial. To probe the function of this non-motile organelle in shear stress detection by cells, we compared calcium signalling responses induced by shear stress in ciliated and unciliated MDCK cells. Cytosolic free Ca²⁺ ([Ca²⁺]i) was measured using Fura-PE3 video imaging fluorescence microscopy in response to shear stress due to laminar flow (385 μl s⁻¹). Our results show that both unciliated and ciliated MDCK cells are shear stress sensitive via ATP release and autocrine feedback through purinergic receptors. However, purinergic calcium signals differed in response intensity and receptor subtypes. In unciliated cells, shear stress-induced elevation in [Ca²⁺]i was predominantly mediated through P2X receptors (P2XR). In contrast, calcium mobilization in ciliated MDCK cells resulted from P2YRs and store-operated Ca²⁺-permeable channels besides P2XRs. These findings lend support to the hypothesis that ATP release in response to shear stress is independent of the primary cilium and that transduction of mechanical strain into a specific biochemical responses stems on the mobilization of different sets of purinergic receptors.

Volume-sensitive anion channels mediate osmosensitive glutathione release from rat thymocytes

Glutathione (GSH) is a negatively charged tripeptide, which is a major determinant of the cellular redox state and defense against oxidative stress. It is assembled inside and degraded outside the cells and is released under various physiological and pathophysiological conditions. The GSH release mechanism is poorly understood at present. In our experiments, freshly isolated rat thymocytes were found to release GSH under normal isotonic conditions at a low rate of 0.82±0.07 attomol/cell/min and that was greatly enhanced under hypoosomotic stimulation to reach a level of 6.1±0.4 attomol/cell/min. The swelling-induced GSH release was proportional to the cell density in the suspension and was temperature-dependent with relatively low activation energy of 5.4±0.6 kcal/mol indicating a predominant diffusion mechanism of GSH translocation. The osmosensitive release of GSH was significantly inhibited by blockers of volume-sensitive outwardly rectifying (VSOR) anion channel, DCPIB and phloretin. In patch-clamp experiments, osmotic swelling activated large anionic conductance with the VSOR channel phenotype. Anion replacement studies suggested that the thymic VSOR anion channel is permeable to GSH(-) with the permeability ratio P(GSH)/P(Cl) of 0.32 for influx and 0.10 for efflux of GSH. The osmosensitive GSH release was trans-stimulated by SLCO/OATP substrates, probenecid, taurocholic acid and estrone sulfate, and inhibited by an SLC22A/OAT blocker, p-aminohippuric acid (PAH). The inhibition by PAH was additive to the effect of DCPIB or phloretin implying that PAH and DCPIB/phloretin affected separate pathways. We suggest that the VSOR anion channel constitutes a major part of the γ-glutamyl cycle in thymocytes and, in cooperation with OATP-like and OAT-like transporters, provides a pathway for the GSH efflux from osmotically swollen cells.

Maxi-anion channel and pannexin 1 hemichannel constitute separate pathways for swelling-induced ATP release in murine L929 fibrosarcoma cells

The maxi-anion channel plays a classically recognized role in controlling the membrane potential through the chloride conductance. It also has novel functions as a regulated pathway for the release of the anionic signaling molecules ATP and excitatory amino acids from cells subjected to osmotic perturbation, ischemia, or hypoxia. Because hemichannels formed by pannexins and connexins have been reported to mediate ATP release from a number of cell types, these hemichannels may represent the molecular correlate of the maxi-anion channel. Here, we found that L929 fibrosarcoma cells express functional maxi-anion channels which mediate a major portion of swelling-induced ATP release, and that ATP released via maxi-anion channels facilitates the regulatory volume decrease after osmotic swelling. Also, it was found that the cells express the mRNA for pannexin 1, pannexin 2, and connexin 43. Hypotonicity-induced ATP release was partially suppressed not only by known blockers of the maxi-anion channel but also by several blockers of pannexins including the pannexin 1-specific blocking peptide (10)Panx1 and small interfering (si)RNA against pannexin 1 but not pannexin 2. The inhibitory effects of maxi-anion channel blockers and pannexin 1 antagonists were additive. In contrast, maxi-anion channel activity was not affected by pannexin 1 antagonists and siRNAs against pannexins 1 and 2. Although a connexin 43-specific blocking peptide, Gap27, slightly suppressed hypotonicity-induced ATP release, maxi-anion channel activity was not affected by Gap27 or connexin 43-specific siRNA. Thus, it is concluded that the maxi-anion channel is a molecular entity distinct from pannexin 1, pannexin 2, and connexin 43, and that the maxi-anion channel and the hemichannels constitute separate pathways for swelling-induced ATP release in L929 cells.

Purinoceptor signaling in malaria-infected erythrocytes

Human erythrocytes are endowed with ATP release pathways and metabotropic and ionotropic purinoceptors. This review summarizes the pivotal function of purinergic signaling in erythrocyte control of vascular tone, in hemolytic septicemia, and in malaria. In malaria, the intraerythrocytic parasite exploits the purinergic signaling of its host to adapt the erythrocyte to its requirements.

Vesicular and conductive mechanisms of nucleotide release

Extracellular nucleotides and nucleosides promote a vast range of physiological responses, via activation of cell surface purinergic receptors. Virtually all tissues and cell types exhibit regulated release of ATP, which, in many cases, is accompanied by the release of uridine nucleotides. Given the relevance of extracellular nucleotide/nucleoside-evoked responses, understanding how ATP and other nucleotides are released from cells is an important physiological question. By facilitating the entry of cytosolic nucleotides into the secretory pathway, recently identified vesicular nucleotide and nucleotide-sugar transporters contribute to the exocytotic release of ATP and UDP-sugars not only from endocrine/exocrine tissues, but also from cell types in which secretory granules have not been biochemically characterized. In addition, plasma membrane connexin hemichannels, pannexin channels, and less-well molecularly defined ATP conducting anion channels have been shown to contribute to the release of ATP (and UTP) under a variety of conditions.

Visualization of flow-induced ATP release and triggering of Ca2+ waves at caveolae in vascular endothelial cells

Endothelial cells (ECs) release ATP in response to shear stress, a fluid mechanical force generated by flowing blood but, although its release has a crucial role in controlling a variety of vascular functions by activating purinergic receptors, the mechanism of ATP release has never been established. To analyze the dynamics of ATP release, we developed a novel chemiluminescence imaging method by using cell-surface-attached firefly luciferase and a CCD camera. Upon stimulation of shear stress, cultured human pulmonary artery ECs simultaneously released ATP in two different manners, a highly concentrated, localized manner and a less concentrated, diffuse manner. The localized ATP release occurred at caveolin-1-rich regions of the cell membrane, and was blocked by caveolin-1 knockdown with siRNA and the depletion of plasma membrane cholesterol with methyl-β-cyclodexrin, indicating involvement of caveolae in localized ATP release. Ca(2+) imaging with Fluo-4 combined with ATP imaging revealed that shear stress evoked an increase in intracellular Ca(2+) concentration and the subsequent Ca(2+) wave that originated from the same sites as the localized ATP release. These findings suggest that localized ATP release at caveolae triggers shear-stress-dependent Ca(2+) signaling in ECs.

Swelling-activated anion channels are essential for volume regulation of mouse thymocytes

Channel-mediated trans-membrane chloride movement is a key process in the active cell volume regulation under osmotic stress in most cells. However, thymocytes were hypothesized to regulate their volume by activating a coupled K-Cl cotransport mechanism. Under the patch-clamp, we found that osmotic swelling activates two types of macroscopic anion conductance with different voltage-dependence and pharmacology. At the single-channel level, we identified two types of events: one corresponded to the maxi-anion channel, and the other one had characteristics of the volume-sensitive outwardly rectifying (VSOR) chloride channel of intermediate conductance. A VSOR inhibitor, phloretin, significantly suppressed both macroscopic VSOR-type conductance and single-channel activity of intermediate amplitude. The maxi-anion channel activity was largely suppressed by Gd(3+) ions but not by phloretin. Surprisingly, [(dihydroindenyl)oxy] alkanoic acid (DIOA), a known antagonist of K-Cl cotransporter, was found to significantly suppress the activity of the VSOR-type single-channel events with no effect on the maxi-anion channels at 10 μM. The regulatory volume decrease (RVD) phase of cellular response to hypotonicity was mildly suppressed by Gd(3+) ions and was completely abolished by phloretin suggesting a major impact of the VSOR chloride channel and modulatory role of the maxi-anion channel. The inhibitory effect of DIOA was also strong, and, most likely, it occurred via blocking the VSOR Cl(-) channels.

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.

Molecular mechanisms of purine and pyrimidine nucleotide release

Given the widespread importance of purinergic receptor-evoked signaling, understanding how ATP and other nucleotides are released from cells in a regulated manner is an essential physiological question. Nonlytic release of ATP, UTP, UDP-glucose, and other nucleotides occurs in all cell types and tissues via both constitutive mechanisms, that is, in the absence of external stimuli, and to a greater extent in response to biochemical or mechanical/physical stimuli. However, a molecular understanding of the processes regulating nucleotide release has only recently begun to emerge. It is generally accepted that nucleotide release occurs in two different scenarios, exocytotic release from the secretory pathway or via conductive/transport mechanisms, and a critical review of our current understanding of these mechanisms is presented in this chapter.

ATP hydrolysis-dependent asymmetry of the conformation of CFTR channel pore

Despite substantial efforts, the entire cystic fibrosis transmembrane conductance regulator (CFTR) protein proved to be difficult for structural analysis at high resolution, and little is still known about the actual dimensions of the anion-transporting pathway of CFTR channel. In the present study, we therefore gauged geometrical features of the CFTR Cl(-) channel pore by a nonelectrolyte exclusion technique. Polyethylene glycols with a hydrodynamic radius (R (h)) smaller than 0.95 nm (PEG 300-1,000) added from the intracellular side greatly suppressed the inward unitary anionic conductance, whereas only molecules with R (h) ≤ 0.62 nm (PEG 200-400) applied extracellularly were able to affect the outward unitary anionic currents. Larger molecules with R (h) = 1.16-1.84 nm (PEG 1,540-3,400) added from either side were completely excluded from the pore and had no significant effect on the single-channel conductance. The cut-off radius of the inner entrance of CFTR channel pore was assessed to be 1.19 ± 0.02 nm. The outer entrance was narrower with its cut-off radius of 0.70 ± 0.16 nm and was dilated to 0.93 ± 0.23 nm when a non-hydrolyzable ATP analog, 5'-adenylylimidodiphosphate (AMP-PNP), was added to the intracellular solution. Thus, it is concluded that the structure of CFTR channel pore is highly asymmetric with a narrower extracellular entrance and that a dilating conformational change of the extracellular entrance is associated with the channel transition to a non-hydrolytic, locked-open state.

Manipulation of cell volume and membrane pore comparison following single cell permeabilization with 60- and 600-ns electric pulses

Intense nanosecond-duration electric pulses (nsEP) open stable nanopores in the cell membrane, followed by cell volume changes due to water uptake or expulsion, as regulated by the osmolality balance of pore-impermeable solutes inside and outside the cell. The size of pores opened by either fifty 60-ns EP (~13 kV/cm) or five, 600-ns EP (~6 kV/cm) in GH3 cells was estimated by isoosmotic replacement of bath NaCl with polyethylene glycols and sugars. Such replacement reduced cell swelling or resulted in transient or sustained cell shrinking in response to EP. depending on the availability of pores permeable to the test solute. Unexpectedly, solute substitutions showed that for the same integral area of pores opened by 60- and 600-ns treatments (as estimated by cell volume changes), the pore sizes were similar. However, the 600-ns exposure triggered significantly higher cell uptake of propidium. We concluded that 600-ns EP opened a greater number of larger (propidium-permeable pores), but the fraction of the larger pores in the entire pore population was insufficient to contribute to cell volume changes. For both the 60- and 600-ns exposures, cell volume changes were determined by pores smaller than 0.9 nm in diameter; however, the diameter increased with increasing the nsEP intensity.

Pannexin 1 is the conduit for low oxygen tension-induced ATP release from human erythrocytes

Erythrocytes release ATP in response to exposure to the physiological stimulus of lowered oxygen (O(2)) tension as well as pharmacological activation of the prostacyclin receptor (IPR). ATP release in response to these stimuli requires activation of adenylyl cyclase, accumulation of cAMP, and activation of protein kinase A. The mechanism by which ATP, a highly charged anion, exits the erythrocyte in response to lowered O(2) tension or receptor-mediated IPR activation by iloprost is unknown. It was demonstrated previously that inhibiting pannexin 1 with carbenoxolone inhibits hypotonically induced ATP release from human erythrocytes. Here we demonstrate that three structurally dissimilar compounds known to inhibit pannexin 1 prevent ATP release in response to lowered O(2) tension but not to iloprost-induced ATP release. These results suggest that pannexin 1 is the conduit for ATP release from erythrocytes in response to lowered O(2) tension. However, the identity of the conduit for iloprost-induced ATP release remains unknown.

Extracellular ATP and P2Y2 receptors mediate intercellular Ca(2+) waves induced by mechanical stimulation in submandibular gland cells: Role of mitochondrial regulation of store operated Ca(2+) entry

Coordination of Ca(2+) signaling among cells contributes to synchronization of salivary gland cell function. However, mechanisms that underlie this signaling remain elusive. Here, intercellular Ca(2+) waves (ICW) in submandibular gland cells were investigated using Fura-2 fluorescence imaging. Mechanical stimulation of single cells induced ICW propagation from the stimulated cells through approximately 7 layers of cells or approximately 120microm. Our findings indicate that an extracellular ATP-dependent pathway is involved because the purinergic receptor antagonist suramin and the ATP hydrolyzing enzyme apyrase blocked ICW propagation. However, the gap junction uncoupler oleamide had no effect. ATP is released from mechanically stimulated cells possibly through opening of mechanosensitive maxi-anion channels, and does not appear to be directly linked to cytosolic Ca(2+). The ICW is propagated by diffusing ATP, which activates purinergic receptors in neighboring cells. This purinergic signaling induces a Ca(2+) transient that is dependent on Ca(2+) release via IP(3) receptors in the ER and store operated Ca(2+) entry (SOCE). Finally, inhibition of mitochondrial Ca(2+) uptake modified ICW indicating an important role of these organelles in this phenomenon. These studies increase our understanding of purinergic receptor signaling in salivary gland cells, and its role as a coordination mechanism of Ca(2+) signals induced by mechanical stimulation.

Pannexin 1 contributes to ATP release in airway epithelia

ATP is a paracrine regulator of critical airway epithelial cell functions, but the mechanism of its release is poorly understood. Pannexin (Panx) proteins, related to invertebrate innexins, form channels (called pannexons) that are able to release ATP from several cell types. Thus, ATP release via pannexons was examined in airway epithelial cells. Quantitative RT-PCR showed Panx1 expression in normal human airway epithelial cells during redifferentiation at the air-liquid interface (ALI), at a level comparable to that of alveolar macrophages; Panx3 was not expressed. Immunohistochemistry showed Panx1 expression at the apical pole of airway epithelia. ALI cultures exposed to hypotonic stress released ATP to an estimated maximum of 255 (+/-64) nM within 1 minute after challenge (n = 6 cultures from three different lungs) or to approximately 1.5 (+/-0.4) microM, recalculated to a normal airway surface liquid volume. Using date- and culture-matched cells (each n > or = 16 from 4 different lungs), the pannexon inhibitors carbenoxolone (10 microM) and probenecid (1 mM), but not the connexon inhibitor flufenamic acid (100 microM), inhibited ATP release by approximately 60%. The drugs affected Panx1 currents in Xenopus oocytes expressing exogenous Panx1 correspondingly. In addition, suppression of Panx1 expression using lentivirus-mediated production of shRNA in differentiated airway epithelial cells inhibited ATP release upon hypotonic stress by approximately 60% as well. These data not only show that Panx1 is expressed apically in differentiated airway epithelial cells but also that it contributes to ATP release in these cells.

Surface accessibility and conformational changes in the N-terminal domain of type I inositol trisphosphate receptors: studies using cysteine substitution mutagenesis

To identify surface-accessible residues and monitor conformational changes of the type I inositol 1,4,5-trisphosphate receptor protein in membranes, we have introduced 10 cysteine substitutions into the N-terminal ligand-binding domain. The reactivity of these mutants with progressively larger maleimide-polyethylene glycol derivatives (MPEG) was measured using a gel shift assay of tryptic fragments. The results indicate that the mutations fall into four categories as follows: sites that are highly accessible based on reactivity with the largest 20-kDa MPEG (S2C); sites that are moderately accessible based on reactivity only with 5-kDa MPEG (S6C, S7C, A189C, and S277C); sites whose accessibility is markedly enhanced by Ca(2+) (S171C, S277C, and A575C); and sites that are inaccessible irrespective of incubation conditions (S217C, A245C, and S436C). The stimulation of accessibility induced by Ca(2+) at the S277C site occurred with an EC(50) of 0.8 mum and was mimicked by Sr(2+) but not Ba(2+). Inositol 1,4,5-trisphosphate alone did not affect reactivity of any of the mutants in the presence or absence of Ca(2+). The data are interpreted using crystal structures and EM reconstructions of the receptor. Our data identify N-terminal regions of the protein that become exposed upon Ca(2+) binding and suggest possible orientations of the suppressor and ligand-binding domains that have implications for the mechanism of gating of the channel.

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.