Effect of Glycyrrhetic Acid Derivatives on Regulation of Thymocyte Volume

We studied the effects of glycyrrhetinic acid (bioactive aglycone of glycyrrhizin) and its ester derivatives at positions C-3 and C-30 on the cell volume regulation in rat thymocytes under conditions of hypoosmotic stress. Native glycyrrhetinic acid completely suppressed this process with half-maximal concentration of 12.7±1.4 μM and Hill coefficient of 3.1±0.6. Formation of esters at C-3 (esters with the acetic, cinnamic and methoxi-cinnamic acid) and at C-30 (methyl ester) drastically decreased the inhibitory activity of the molecule, suggesting that intact hydroxyl group at C-3 and carboxyl group at C-30 are structurally important determinants of biological activity of glycyrrhetinic acid towards volume regulation of thymic lymphocytes.

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.

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.

Annexin A2-S100A10 Represents the Regulatory Component of Maxi-Cl Channel Dependent on Protein Tyrosine Dephosphorylation and Intracellular Ca²⁺

BACKGROUND/AIMS

Maxi-anion channel (Maxi-Cl) is ubiquitously expressed and involved in a number of important cell functions especially by serving as an ATP release pathway. We recently identified SLCO2A1 as its essential core component. However, the regulatory component required for the channel activation/inactivation remains unidentified.

METHODS

In the present study, to identify the regulatory component, we made genome-wide analysis combined with siRNA screening and performed patch-clamp studies and ATP release assay after gene silencing and overexpression.

RESULTS

Comparative microarray analysis between Maxi-Cl-rich C127 and -deficient C1300 cells revealed highly differential expression not only of SLCO2A1 but also of four annexin family members. Gene silencing study showed that Anxa2 is involved in Maxi-Cl activity. The Maxi-Cl events appeared in C1300 cells by overexpression of Slco2a1 and more efficiently by that of Slco2a1 plus Anxa2. Immunoprecipitation assay supported the interaction between ANXA2 and SLCO2A1. Suppressive effects of overexpression of a phospho-mimicking mutant of Anxa2, Anxa2-Y23E, indicated that protein tyrosine dephosphorylation dependence of Maxi-Cl is conferred by ANXA2. Maxi-Cl activity was suppressed by gene silencing of S100A10, a binding partner of ANXA2, and by applying a synthetic ANXA2 peptide, Ac-(1-14), which interferes with the ANXA2-S100A10 complex formation. Intracellular Ca²⁺ dependence of Maxi-Cl activity was abolished by S100a10 knockdown.

CONCLUSION

The ANXA2-S100A10 complex represents the regulatory component of Maxi-Cl conferring protein tyrosine dephosphorylation dependence and intracellular Ca²⁺ sensitivity on this 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.

Effect of plant flavonoids on the volume regulation of rat thymocytes under hypoosmotic stress

BACKGROUND

Cell volume regulation and volume-regulated anion channels are critical for cell survival in non-isosmotic conditions, and dysregulation of this system is detrimental. Although genes and proteins underlying this basic cellular machinery were recently identified, the pharmacology remains poorly explored.

METHODS

We examined effects of 16 flavonoids on the regulatory volume decrease (RVD) of thymocytes under hypoosmotic stress assessed by light transmittance and on the activity of volume-sensitive chloride channel by patch-clamp technique.

RESULTS

Comparison of effects of flavonoids on RVD revealed a group of four active substances with lehmannin being the strongest inhibitor (IC50 = 8.8 μM). Structure-functional comparison suggested that hydrophobicity brought about by methoxy, prenyl or lavandulyl groups as well as by the absence of glucosyl fragment together with localization of the phenyl ring B at the position C2 (which is at C3 in totally inactive isoflavones) are important structural determinants for the flavonoids activity as volume regulation inhibitors. All active flavonoids suppressed RVD under Gramicidin D-NMDG hypotonic stress conditions when cationic permeability was increased by an ionophore, gramicidin D, with all extracellular monovalent cations replaced with bulky NMDG+ suggesting that they target volume-sensitive anionic permeability. While effects of hispidulin and pulicarin were only partial, lehmannin and pinocembrin completely abolished RVD under Gramicidin D-NMDG conditions. In direct patch-clamp experiments, lehmannin and pinocembrin produced a strong inhibiting effect on the swelling-induced whole-cell chloride conductance in a voltage-independent manner.

CONCLUSION

Lehmannin, pinocembrin, and possibly hispidulin and pulicarin may serve as leads for developing effective low-toxic immunomodulators.

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.

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.

Pore formation by Vibrio cholerae cytolysin requires cholesterol in both monolayers of the target membrane

Vibrio cholerae cytolysin (VCC) forms oligomeric transmembrane pores in cholesterol-rich membranes. To better understand this process, we used planar bilayer membranes. In symmetric membranes, the rate of the channel formation by VCC has a superlinear dependency on the cholesterol membrane fraction. Thus, more than one cholesterol molecule can facilitate VCC-pore formation. In asymmetric membranes, the rate of pore formation is limited by the leaflet with the lower cholesterol content. Methyl-beta-cyclodextrin, which removes cholesterol from membranes, rapidly inhibits VCC pore formation, even when it is added to the side opposite that of VCC addition. The results suggest that cholesterol in both membrane leaflets aid VCC-pore formation and that either leaflet can function as a kinetic bottleneck with respect to the rate of pore-formation.

Conductance and ion selectivity of a mesoscopic protein nanopore probed with cysteine scanning mutagenesis

Nanometer-scale proteinaceous pores are the basis of ion and macromolecular transport in cells and organelles. Recent studies suggest that ion channels and synthetic nanopores may prove useful in biotechnological applications. To better understand the structure-function relationship of nanopores, we are studying the ion-conducting properties of channels formed by wild-type and genetically engineered versions of Staphylococcus aureus alpha-hemolysin (alphaHL) reconstituted into planar lipid bilayer membranes. Specifically, we measured the ion selectivities and current-voltage relationships of channels formed with 24 different alphaHL point cysteine mutants before and after derivatizing the cysteines with positively and negatively charged sulfhydryl-specific reagents. Novel negative charges convert the selectivity of the channel from weakly anionic to strongly cationic, and new positive charges increase the anionic selectivity. However, the extent of these changes depends on the channel radius at the position of the novel charge (predominantly affects ion selectivity) or on the location of these charges along the longitudinal axis of the channel (mainly alters the conductance-voltage curve). The results suggest that the net charge of the pore wall is responsible for cation-anion selectivity of the alphaHL channel and that the charge at the pore entrances is the main factor that determines the shape of the conductance-voltage curves.

Protein electrostriction: a possibility of elastic deformation of the α-hemolysin channel by the applied field

While conformational flexibility of proteins is widely recognized as one of their functionally crucial features and enjoys proper attention for this reason, their elastic properties are rarely discussed. In ion channel studies, where the voltage-induced or ligand-induced conformational transitions, gating, are the leading topic of research, the elastic structural deformation by the applied electric field has never been addressed at all. Here we examine elasticity using a model channel of known crystal structure-Staphylococcus aureus alpha-hemolysin. Working with single channels reconstituted into planar lipid bilayers, we first show that their ionic conductance is asymmetric with voltage even at the highest salt concentration used where the static charges in the channel interior are maximally shielded. Second, choosing 18-crown-6 as a molecular probe whose size is close to the size of the narrowest part of the alpha-hemolysin pore, we analyze the blockage of the channel by the crown/K(+) complex. Analysis of the blockage within the framework of the Woodhull model in its generalized form demonstrates that the model is able to correctly describe the crown effect only if the parameters of the model are considered to be voltage-dependent. Specifically, one has to include either a voltage-dependent barrier for crown release to the cis side of the channel or voltage-dependent interactions between the binding site and the crown. We suggest that the voltage sensitivity of both the ionic conductance of the channel seen at the highest salt concentration and its blockage by the crown reflects a field-induced deformation of the pore.

Probing the volume changes during voltage gating of Porin 31BM channel with nonelectrolyte polymers

To probe the volume changes of the voltage-dependent anion-selective channel (VDAC), the nonelectrolyte exclusion technique was taken because it is one of the few existing methods that may define quite accurately the rough geometry of lumen of ion channels (in membranes) for which there is no structural data.Here, we corroborate the data from our previous study [FEBS Lett. 416 (1997) 187] that the gross structural features of VDAC in its highest conductance state are asymmetric with respect to the plane of the membrane, and state that this asymmetry is not dependent on sign of voltage applied. Hence, the plasticity of VDAC does not play a role in the determination of lumen geometry at this state and the asymmetry is an internal property of the channel. We also show that the apparent diameter of the cis segment of the pore decreases slightly from 2 to 1.8 nm when the channel's conductance decreases from its high to low state. However, the trans funnel segment undergoes a more marked change in polymer accessible volume. Specifically, its larger diameter decreases from approximately 4 to 2.4 nm. Supposing the channel's total length is 4.6 nm, the apparent change in channel volume during this transition is estimated to be about 10 nm(3), i.e. about 40% of the channel's volume in the high conductance state.

Lumen geometry of ion channels formed by Vibrio cholerae EL Tor cytolysin elucidated by nonelectrolyte exclusion

Vibrio cholerae EL Tor cytolysin, a water-soluble protein with a molecular mass of 63 kDa, forms small pores in target cell membranes. In this communication, planar lipid bilayers under voltage clamp conditions were used to investigate the geometric properties of the pores. It was established that all cytolysin channels were inserted into membranes with the same orientation. Sharp asymmetry in the I-V curve of fully open cytolysin channels persisting at high electrolyte concentrations indicated asymmetry in the geometry of the channel lumen. Using the nonelectrolyte exclusion method, evidence was obtained that the cis opening of the channel had a larger diameter (< or = 1.9 nm) than the trans opening (< or = 1.6 nm). The channel lumen appeared constricted, with a diameter of < or = 1.2 nm. Cup-shaped lumen geometry was deduced for both channel openings, which appeared to be connected to each other via a central narrow part. The latter contributed significantly to the total electrical resistance and determined the discontinuous character of channel filling with nonelectrolytes. Comparisons of the properties of pores formed by cytolysins of two V. cholerae biotypes (EL Tor and non-O1) indicated that the two ion channels possessed a similar geometry.

Electrophysiological evidence for heptameric stoichiometry of ion channels formed by Staphylococcus aureus alpha-toxin in planar lipid bilayers

Staphylococcal alpha-toxin forms homo-oligomeric channels in lipid bilayers and cell membranes. Here, we report that electrophysiological monitoring of single-channel function using a derivatized cysteine substitution mutant allows accurate determination of the subunit stoichiometry of the oligomer in situ. The electrophysiological phenotype of channels formed in planar lipid bilayers with the cysteine replacement mutant I7C is equal to that of the wild type. When pores were formed with I7C, alterations of several channel properties were observed upon modification with SH reagents. Decreases in conductance then occurred that were seen only as negative voltage was applied. At the level of single channels, these were manifest as stepwise changes in conductance, each step most probably reflecting modification of a single SH group within the oligomer. Because seven steps were observed, the functional channel formed by alpha-toxin in planar lipid membranes is a heptamer.

Polymeric nonelectrolytes to probe pore geometry: application to the alpha-toxin transmembrane channel

Asymmetrical (one-sided) application of penetrating water-soluble polymers, polyethylene glycols (PEGs), to a well-defined channel formed by Staphylococcus aureus alpha-toxin is shown to probe channel pore geometry in more detail than their symmetrical (two-sided) application. Polymers added to the cis side of the planar lipid membrane (the side of protein addition) affect channel conductance differently than polymers added to the trans side. Because a satisfactory theory quantitatively describing PEG partitioning into a channel pore does not exist, we apply the simple empirical rules proposed previously (, J. Membr. Biol. 161:83-92) to gauge the size of pore openings as well as the size and position of constrictions along the pore axis. We estimate the radii of the two openings of the channel to be practically identical and equal to 1. 2-1.3 nm. Two apparent constrictions with radii of approximately 0. 9 nm and approximately 0.6-0.7 nm are inferred to be present in the channel lumen, the larger one being closer to the cis side. These structural findings agree well with crystallographic data on the channel structure (, Science. 274:1859-1866) and verify the practicality of polymer probing. The general features of PEG partitioning are examined using available theoretical considerations, assuming there is no attraction between PEG and the channel lumen. It is shown that the sharp dependence of the partition coefficient on polymer molecular weight found under both symmetrical and asymmetrical polymer application can be rationalized within a "hard sphere nonideal solution model." This finding is rather surprising because PEG forms highly flexible coils in water with a Kuhn length of only several Angstroms.

Heparin influence on alpha-staphylotoxin formed channel

The effects of heparin on ion channels formed by Staphylococcus aureus alpha-toxin (ST channel) in lipid bilayers were studied under voltage clamp conditions. Heparin concentrations as small as 100 pM induced a sharp dose-dependent increase in channel voltage sensitivity. This was only observed when heparin was added to the negative-potential side of lipid bilayers in the presence of divalent cations. Divalent cations differ in their efficiency: Zn2+>Ca2+>Mg2+. The apparent positive gating charge increased 2-3-fold with heparin addition as well as with acidification of the bathing solution. 'Free' carboxyl groups and carboxyl groups in ion pairs of the protein moiety are hypothesized to interact with sulfated groups of heparin through divalent cation bridges. The cis mouth of the channel (that protrudes beyond the membrane plane on the side of ST addition and to which voltage was applied) is less sensitive to heparin than the trans-mouth. It is suggested that charged residues which interact with heparin at the cis mouth of ST channels and which contribute to the effective gating charge at negative voltage may be physically different from those at the trans mouth and at positive voltage.

Channel-sizing experiments in multichannel bilayers

The possibility of obtaining information about the radius of high and low conductance states of channels in multichannel membranes was tested experimentally. In spite of the interference of non-electrolytes on the numbers of channels that appeared in the membrane, the non-electrolyte-exclusion method was successfully adapted to multichannel bilayers to estimate the radius of the larger opening of the low conductance state of the channel induced by Staphylococcus aureus alpha-toxin. At the pH used, the channel transition to a low conductance state was accompanied by a decrease of the opening radius from 1.3 +/- 0.2 nm to 0.9 +/- 0.1 nm. The determination criteria for maximum size of a channel opening when using the non-electrolyte exclusion method is discussed.

The hinge portion of the S. aureus alpha-toxin crosses the lipid bilayer and is part of the trans-mouth of the channel

This paper compares the functional properties of ion channels formed in planar lipid membranes by the wild and mutant Staphylococcus aureus alpha-toxin. It was shown that replacement of the amino acid Gly at position 130 by Cys in the primary structure of the toxin decreases the single-channel conductance with a concomitant decrease in the pH at which the channel becomes unable to discriminate between Cl- and K+ ions. The mutation also induced an increase in the asymmetry in the current-voltage relationship of the channel. The results of our experiments suggest that the trans-mouth of the channel is responsible for all the observed changes in channel properties. It was assumed that this entrance is built by the glycine-rich hinge portion of the toxin and is situated close to the surface of monolayer facing the trans-compartment.

Ionophoretic properties of ferutinin

The influence of the natural terpenoid ferutinin (4-oxy-6-(4-oxybenzoyloxy) dauc-8,9-en), isolated from the plant Ferula tenuisecta, on ion permeability of biological and artificial membranes was investigated. It was shown that ferutinin, in the concentration range 1-50 microM, increases the permeability of thymocytes, mitochondria, sarcoplasmic reticulum, liposomes and bilayer lipid membranes (BLM) for Ca2+. Ferutinin establishes a transmembrane potential in BLM equal to the Nernst's potential. The permeability ratio for Na+/Ca2+ is 0.41. The dependence of BLM conductivity on ferutinin concentration is linear. The stoichiometry of the ferutinin:Ca2+ complex is 2, assuming the formation of a structure with participation of two terpenoid molecules and one Ca2+ ion.

Pore-forming properties of proteolytically nicked staphylococcal alpha-toxin: the ion channel in planar lipid bilayer membranes

Staphylococcal alpha-toxin is a single-chain protein with a molecular mass of 33.2 kDa, which can form large water-filled pores both in lipid bilayers and in erythrocyte membranes. Limited proteolysis of the purified toxin with proteinase K led to time-dependent changes of all the functional features of the channels formed by the toxin. Single-channel conductance in planar bilayers was decreased about threefold. The anion selectivity of the channel was replaced with cation selectivity and the asymmetry in the current-voltage relationship of the channel became more pronounced. At the same time the nicked toxin kept its full ability to form ion channels in lipid bilayers, although it lost a considerable part of its hemolytic activity. In planar bilayers and in erythrocyte membranes, the proteolytically nicked toxin actually formed channels with a slightly smaller diameter (approximately 1.2 times) than that formed by the native toxin. This decrease was not marked enough to explain changes in the biological effects of the nicked toxin. The change in channel selectivity induced by the cleavage is considered to be the major determinant of the changes in the biological effects of the nicked toxin.