A consensus segment in the M2 domain of the hP2X7 receptor shows ion channel activity in planar lipid bilayers and in biological membranes

The P2X₇ receptor (P2X₇R) is an ATP-gated, cation-selective channel permeable to Na⁺, K⁺ and Ca²⁺. This channel has also been associated with the opening of a non-selective pore that allows the flow of large organic ions. However, the biophysical properties of the P2X₇R have yet to be characterized unequivocally. We investigated a region named ADSEG, which is conserved among all subtypes of P2X receptors (P2XRs). It is located in the M2 domain of hP2X₇R, which aligns with the H5 signature sequence of potassium channels. We investigated the channel forming ability of ADSEG in artificial planar lipid bilayers and in biological membranes using the cell-attached patch-clamp techniques. ADSEG forms channels, which exhibit a preference for cations. They are voltage independent and show long-term stability in planar lipid bilayers as well as under patch-clamping conditions. The open probability of the ADSEG was similar to that of native P2X₇R. The conserved part of the M2 domain of P2X₇R forms ionic channels in planar lipid bilayers and in biological membranes. Its electrophysiological characteristics are similar to those of the whole receptor. Conserved and hydrophobic part of the M2 domain forms ion channels.     

Pannexin1 channels act downstream of P2X7 receptors in ATP-induced murine T-cell death

Death of murine T cells induced by extracellular ATP is mainly triggered by activation of purinergic P2X₇ receptors (P2X₇Rs). However, a link between P2X₇Rs and pannexin1 (Panx1) channels, which are non-selective, has been recently demonstrated in other cell types. In this work, we characterized the expression and cellular distribution of pannexin family members (Panxs 1, 2 and 3) in isolated T cells. Panx1 was the main pannexin family member clearly detected in both helper (CD4⁺) and cytotoxic (CD8⁺) T cells, whereas low levels of Panx2 were found in both T-cell subsets. Using pharmacological and genetic approaches, Panx1 channels were found to mediate most ATP-induced ethidium uptake since this was drastically reduced by Panx1 channel blockers (¹⁰Panx1, Probenecid and low carbenoxolone concentration) and absent in T cells derived from Panx1^−/− mice. Moreover, electrophysiological measurements in wild-type CD4⁺ cells treated with ATP unitary current events and pharmacological sensitivity compatible with Panx1 channels were found. In addition, ATP release from T cells treated with 4Br-{"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187, a calcium ionophore, was completely blocked with inhibitors of both connexin hemichannels and Panx1 channels. Panx1 channel blockers drastically reduced the ATP-induced T-cell mortality, indicating that Panx1 channels mediate the ATP-induced T-cell death. However, mortality was not reduced in T cells of Panx1^−/− mice, in which levels of P2X₇Rs and ATP-induced intracellular free Ca²⁺ responses were enhanced suggesting that P2X₇Rs take over Panx1 channels lose-function in mediating the onset of cell death induced by extracellular ATP.     

Monovalent and divalent cation permeability and block of neuronal nicotinic receptor channels in rat parasympathetic ganglia

Acetylcholine-evoked currents mediated by activation of nicotinic receptors in rat parasympathetic neurons were examined using whole-cell voltage clamp. The relative permeability of the neuronal nicotinic acetylcholine (nACh) receptor channel to monovalent and divalent inorganic and organic cations was determined from reversal potential measurements. The channel exhibited weak selectivity among the alkali metals with a selectivity sequence of Cs+ > K+ > Rb+ > Na+ > Li+, and permeability ratios relative to Na+ (Px/PNa) ranging from 1.27 to 0.75. The selectivity of the alkaline earths was also weak, with the sequence of Mg2+ > Sr2+ > Ba2+ > Ca2+, and relative permeabilities of 1.10 to 0.65. The relative Ca2+ permeability (PCa/PNa) of the neuronal nACh receptor channel is approximately fivefold higher than that of the motor endplate channel (Adams, D. J., T. M. Dwyer, and B. Hille. 1980. Journal of General Physiology. 75:493-510). The transition metal cation, Mn2+ was permeant (Px/PNa = 0.67), whereas Ni2+, Zn2+, and Cd2+ blocked ACh-evoked currents with half-maximal inhibition (IC50) occurring at approximately 500 microM, 5 microM and 1 mM, respectively. In contrast to the muscle endplate AChR channel, that at least 56 organic cations which are permeable to (Dwyer et al., 1980), the majority of organic cations tested were found to completely inhibit ACh- evoked currents in rat parasympathetic neurons. Concentration-response curves for guanidinium, ethylammonium, diethanolammonium and arginine inhibition of ACh-evoked currents yielded IC50's of approximately 2.5- 6.0 mM. The organic cations, hydrazinium, methylammonium, ethanolammonium and Tris, were measureably permeant, and permeability ratios varied inversely with the molecular size of the cation. Modeling suggests that the pore has a minimum diameter of 7.6 A. Thus, there are substantial differences in ion permeation and block between the nACh receptor channels of mammalian parasympathetic neurons and amphibian skeletal muscle which represent functional consequences of differences in the primary structure of the subunits of the ACh receptor channel.     

Pannexin1 channels act downstream of P2X7 receptors in ATP-induced murine T-cell death

Death of murine T cells induced by extracellular ATP is mainly triggered by activation of purinergic P2X₇ receptors (P2X₇Rs). However, a link between P2X₇Rs and pannexin1 (Panx1) channels, which are non-selective, has been recently demonstrated in other cell types. In this work, we characterized the expression and cellular distribution of pannexin family members (Panxs 1, 2 and 3) in isolated T cells. Panx1 was the main pannexin family member clearly detected in both helper (CD4⁺) and cytotoxic (CD8⁺) T cells, whereas low levels of Panx2 were found in both T-cell subsets. Using pharmacological and genetic approaches, Panx1 channels were found to mediate most ATP-induced ethidium uptake since this was drastically reduced by Panx1 channel blockers (¹⁰Panx1, Probenecid and low carbenoxolone concentration) and absent in T cells derived from Panx1^?/? mice. Moreover, electrophysiological measurements in wild-type CD4⁺ cells treated with ATP unitary current events and pharmacological sensitivity compatible with Panx1 channels were found. In addition, ATP release from T cells treated with 4Br-A23187, a calcium ionophore, was completely blocked with inhibitors of both connexin hemichannels and Panx1 channels. Panx1 channel blockers drastically reduced the ATP-induced T-cell mortality, indicating that Panx1 channels mediate the ATP-induced T-cell death. However, mortality was not reduced in T cells of Panx1^?/? mice, in which levels of P2X₇Rs and ATP-induced intracellular free Ca²⁺ responses were enhanced suggesting that P2X₇Rs take over Panx1 channels lose-function in mediating the onset of cell death induced by extracellular ATP.     

Carbenoxolone Blocks Endotoxin-Induced Protein Kinase R (PKR) Activation and High Mobility Group Box 1 (HMGB1) Release

The pathogen- and damage-associated molecular patterns (for example, bacterial endotoxin and adenosine 5′-triphosphate [ATP]) activate the double-stranded RNA-activated protein kinase R (PKR) to trigger the inflammasome-dependent high mobility group box 1 (HMGB1) release. Extracellular ATP contributes to the inflammasome activation through binding to the plasma membrane purinergic P2X₇ receptor (P2X₇R), triggering the opening of P2X₇R channels and the pannexin-1 (panx-1) hemichannels permeable for larger molecules up to 900 daltons. It was previously unknown whether panx-1 channel blockers can abrogate lipopolysaccharide (LPS)-induced PKR activation and HMGB1 release in innate immune cells. Here we demonstrated that a major gancao (licorice) component (glycyrrhizin, or glycyrrhizic acid) derivative, carbenoxolone (CBX), dose dependently abrogated LPS-induced HMGB1 release in macrophage cultures with an estimated IC₅₀ ≈ 5 μmol/L. In an animal model of polymicrobial sepsis (induced by cecal ligation and puncture [CLP]), repetitive CBX administration beginning 24 h after CLP led to a significant reduction of circulating and peritoneal HMGB1 levels, and promoted a significant increase in animal survival rates. As did P2X₇R antagonists (for example, oxidized ATP, oATP), CBX also effectively attenuated LPS-induced P2X₇R/panx-1 channel activation (as judged by Lucifer Yellow dye uptake) and PKR phosphorylation in primary peritoneal macrophages. Collectively, these results suggested that CBX blocks LPS-induced HMGB1 release possibly through impairing PKR activation, supporting the involvement of PKR in the regulation of HMGB1 release.     

P-Loop Residues Critical for Selectivity in K+ Channels Fail to Confer Selectivity to Rabbit HCN4 Channels

HCN channels are thought to be structurally similar to K_v channels, but show much lower selectivity for K⁺. The ∼3.3 Å selectivity filter of K⁺ channels is formed by the pore-lining sequence XT(V/I)GYG, with X usually T, and is held stable by key residues in the P-loop. Differences in the P-loop sequence of HCN channels (eg. the pore-lining sequence L₄₇₈C₄₇₉IGYG) suggest these residues could account for differences in selectivity between these channel families. Despite being expressed, L478T/C479T HCN4 channels did not produce current. Since threonine in the second position is highly conserved in K⁺ channels, we also studied C479T channels. Based on permeability ratios (P_X/P_K), C479T HCN4 channels (K⁺(1)>Rb⁺(0.85)>Cs⁺(0.59)>Li⁺(0.50)≥Na⁺(0.49)) were less selective than WT rabbit HCN4 (K⁺(1)>Rb⁺(0.48)>Cs⁺(0.31)≥Na⁺(0.29)>Li⁺(0.03)), indicating that the TIGYG sequence is insufficient to confer K⁺ selectivity to HCN channels. C479T HCN4 channels had an increased permeability to large organic cations than WT HCN4 channels, as well as increased unitary K⁺ conductance, and altered channel gating. Collectively, these results suggest that HCN4 channels have larger pores than K⁺ channels and replacement of the cysteine at position 479 with threonine further increases pore size. Furthermore, selected mutations in other regions linked previously to pore stability in K⁺ channels (ie. S475D, S475E and F471W/K472W) were also unable to confer K⁺ selectivity to C479T HCN4 channels. Our findings establish the presence of the TIGYG pore-lining sequence does not confer K⁺ selectivity to rabbit HCN4 channels, and suggests that differences in selectivity of HCN4 versus K⁺ channels originate from differences outside the P-loop region.     

The permeation of organic cations through cAMP-gated channels in mammalian olfactory receptor neurons

The permeation of monovalent organic cations through adenosine 3,5-cyclic monophosphate-(cAMP) activated channels was studied by recording macroscopic currents in excised inside-out membrane patches from the dendritic knobs of isolated mammalian olfactory receptor neurons (ORNs). Current-voltage relations were measured when bathing solution Na⁺ was replaced by monovalent organic cations. Permeability ratios relative to Na⁺ ions were calculated from changes in reversal potentials. Some of the small organic cations tested included ammonium (NH ₄ ⁺ ), hydroxylammonium and formamidinium, with relative permeability ratios of 1.41, 2.3 and 1.01 respectively. The larger methylated and ethylated ammonium ions studied included: DMA (dimethylammonium), TMA (tetramethylammonium) and TEA (tetraethylammonium) and they all had permeability ratios larger than 0.09. Even large cations such as choline, arginine and tris(hydroxymethyl)aminomethane (Tris) were appreciably permeant through the cAMP-activated channel with permeability ratios ranging from 0.19 to 0.7. The size of the permeating cations, as assessed by molecular weight, was a good predictor of the permeability. The permeability sequence of the cAMP-activated channel in our study was P_NH4 > P_Na > p_DMA > p_TMA > P_Choline > P_TEA. Higher permeability ratios of hydroxylammonium, arginine and tris(hydroxymethyl)aminomethane cannot be explained by ionic size alone. Our results indicate that: (i) cAMP-activated channels poorly select between monovalent cations; (ii) the pore dimension must be at least 6.5 × 6.5 Å, in order to allow TEA and Tris to permeate and (iii) molecular sieving must be an important mechanism for the permeation of large organic ions through the channels with specific ion binding playing a smaller role than in other structurally similar channels. In addition, the results clearly indicate that cyclic nucleotide-gated (CNG) channels in different cells are not the same, the olfactory CNG channel being different from that of the photoreceptors, particularly with respect to the permeation of large organic cations, which the ORN channels allow to permeate readily.This work was supported by the Australian Research Council of Australia.     

Monovalent Ion Selectivity Sequences of the Rat Connexin43 Gap Junction Channel

The relative permeability sequences of the rat connexin 43 (rCx43) gap junction channel to seven cations and chloride were examined by double whole cell patch clamp recording of single gap junction channel currents in rCx43 transfected neuroblastoma 2A (N2A) cell pairs. The measured maximal single channel slope conductances (γ_j, in pS) of the junctional current-voltage relationships in 115 mM XCl were RbCl (103) ≥ CsCl (102) > KCl (97) > NaCl (79) ≥ LiCl (78) > TMACl (65) > TEACl (53) and for 115 mM KY were KBr (105) > KCl (97) > Kacetate (77) > Kglutamate (61). The single channel conductance-aqueous mobility relationships for the test cations and anions were linear. However, the predicted minimum anionic and cationic conductances of these plots did not accurately predict the rCx43 channel conductance in 115 mM KCl. Instead, the conductance of the rCx43 channel in 115 mM KCl was accurately predicted from cationic and anionic conductance-mobility plots by applying a mobility scaling factor D_x/D_o, which depends upon the relative radii of the permeant ions to an estimated pore radius. Relative permeabilities were determined for all of the monovalent cations and anions tested from asymmetric salt reversal potential measurements and the Goldman-Hodgkin-Katz voltage equation. These experiments estimate the relative chloride to potassium permeability to be 0.13. The relationship between the relative cation permeability and hydrated radius was modeled using the hydrodynamic equation assuming a pore radius of 6.3 ± 0.4 Å. Our data quantitatively demonstrate that the rCx43 gap junction channel is permeable to monovalent atomic and organic cations and anions and the relative permeability sequences are consistent with an Eisenman sequence II or I, respectively. These predictions about the rCx43 channel pore provide a useful basis for future investigations into the structural determinants of the conductance and permeability properties of the connexin channel pore.     

Activation,Permeability, and Inhibition of Astrocytic and Neuronal Large Pore (Hemi)channels

Astrocytes and neurons express several large pore (hemi)channels that may open in response to various stimuli, allowing fluorescent dyes, ions, and cytoplasmic molecules such as ATP and glutamate to permeate. Several of these large pore (hemi)channels have similar characteristics with regard to activation, permeability, and inhibitor sensitivity. Consequently, their behaviors and roles in astrocytic and neuronal (patho)physiology remain undefined. We took advantage of the Xenopus laevis expression system to determine the individual characteristics of several large pore channels in isolation. Expression of connexins Cx26, Cx30, Cx36, or Cx43, the pannexins Px1 or Px2, or the purinergic receptor P2X₇ yielded functional (hemi)channels with isoform-specific characteristics. Connexin hemichannels had distinct sensitivity to alterations of extracellular Ca²⁺ and their permeability to dyes and small atomic ions (conductance) were not proportional. Px1 and Px2 exhibited conductance at positive membrane potentials, but only Px1 displayed detectable fluorescent dye uptake. P2X₇, in the absence of Px1, was permeable to fluorescent dyes in an agonist-dependent manner. The large pore channels displayed overlapping sensitivity to the inhibitors Brilliant Blue, gadolinium, and carbenoxolone. These results demonstrated isoform-specific characteristics among the large pore membrane channels; an open (hemi)channel is not a nonselective channel. With these isoform-specific properties in mind, we characterized the divalent cation-sensitive permeation pathway in primary cultured astrocytes. We observed no activation of membrane conductance or Cx43-mediated dye uptake in astrocytes nor in Cx43-expressing C6 cells. Our data underscore that although Cx43-mediated transport is observed in overexpressing cell systems, such transport may not be detectable in native cells under comparable experimental conditions.     

High Ca2+ permeability of a peptide-gated DEG/ENaC from Hydra

Degenerin/epithelial Na⁺ channels (DEG/ENaCs) are Na⁺ channels that are blocked by the diuretic amiloride. In general, they are impermeable for Ca²⁺ or have a very low permeability for Ca²⁺. We describe here, however, that a DEG/ENaC from the cnidarian Hydra magnipapillata, the Hydra Na⁺ channel (HyNaC), is highly permeable for Ca²⁺ (P_Ca/P_Na = 3.8). HyNaC is directly gated by Hydra neuropeptides, and in Xenopus laevis oocytes expressing HyNaCs, RFamides elicit currents with biphasic kinetics, with a fast transient component and a slower sustained component. Although it was previously reported that the sustained component is unselective for monovalent cations, the selectivity of the transient component had remained unknown. Here, we show that the transient current component arises from secondary activation of the Ca²⁺-activated Cl⁻ channel (CaCC) of Xenopus oocytes. Inhibiting the activation of the CaCC leads to a simple on–off response of peptide-activated currents with no apparent desensitization. In addition, we identify a conserved ring of negative charges at the outer entrance of the HyNaC pore that is crucial for the high Ca²⁺ permeability, presumably by attracting divalent cations to the pore. At more positive membrane potentials, the binding of Ca²⁺ to the ring of negative charges increasingly blocks HyNaC currents. Thus, HyNaC is the first member of the DEG/ENaC gene family with a high Ca²⁺ permeability.     

Connexin hemichannel and pannexin channel electrophysiology: How do they differ?

Connexin hemichannels are postulated to form a cell permeabilization pore for the uptake of fluorescent dyes and release of cellular ATP. Connexin hemichannel activity is enhanced by low external [Ca²⁺]_o, membrane depolarization, metabolic inhibition, and some disease-causing gain-of-function connexin mutations. This paper briefly reviews the electrophysiological channel conductance, permeability, and pharmacology properties of connexin hemichannels, pannexin 1 channels, and purinergic P2X₇ receptor channels as studied in exogenous expression systems including Xenopus oocytes and mammalian cell lines such as HEK293 cells. Overlapping pharmacological inhibitory and channel conductance and permeability profiles makes distinguishing between these channel types sometimes difficult. Selective pharmacology for Cx43 hemichannels (Gap19 peptide), probenecid or FD&C Blue #1 (Brilliant Blue FCF, BB FCF) for Panx1, and A740003, A438079, or oxidized ATP (oATP) for P2X7 channels may be the best way to distinguish between these three cell permeabilizing channel types. Endogenous connexin, pannexin, and P2X₇ expression should be considered when performing exogenous cellular expression channel studies. Cell pair electrophysiological assays permit the relative assessment of the connexin hemichannel/gap junction channel ratio not often considered when performing isolated cell hemichannel studies.     

Developmental changes in the cochlear hair cell mechanotransducer channel and their regulation by transmembrane channel–like proteins

Vibration of the stereociliary bundles activates calcium-permeable mechanotransducer (MT) channels to initiate sound detection in cochlear hair cells. Different regions of the cochlea respond preferentially to different acoustic frequencies, with variation in the unitary conductance of the MT channels contributing to this tonotopic organization. Although the molecular identity of the MT channel remains uncertain, two members of the transmembrane channel–like family, Tmc1 and Tmc2, are crucial to hair cell mechanotransduction. We measured MT channel current amplitude and Ca²⁺ permeability along the cochlea’s longitudinal (tonotopic) axis during postnatal development of wild-type mice and mice lacking Tmc1 (Tmc1−/−) or Tmc2 (Tmc2−/−). In wild-type mice older than postnatal day (P) 4, MT current amplitude increased ∼1.5-fold from cochlear apex to base in outer hair cells (OHCs) but showed little change in inner hair cells (IHCs), a pattern apparent in mutant mice during the first postnatal week. After P7, the OHC MT current in Tmc1−/− (dn) mice declined to zero, consistent with their deafness phenotype. In wild-type mice before P6, the relative Ca²⁺ permeability, P_Ca, of the OHC MT channel decreased from cochlear apex to base. This gradient in P_Ca was not apparent in IHCs and disappeared after P7 in OHCs. In Tmc1−/− mice, P_Ca in basal OHCs was larger than that in wild-type mice (to equal that of apical OHCs), whereas in Tmc2−/−, P_Ca in apical and basal OHCs and IHCs was decreased compared with that in wild-type mice. We postulate that differences in Ca²⁺ permeability reflect different subunit compositions of the MT channel determined by expression of Tmc1 and Tmc2, with the latter conferring higher P_Ca in IHCs and immature apical OHCs. Changes in P_Ca with maturation are consistent with a developmental decrease in abundance of Tmc2 in OHCs but not in IHCs.     

The New Permeability Pathways Induced by the Malaria Parasite in the Membrane of the Infected Erythrocyte: Comparison of Results Using Different Experimental Techniques

The membrane of erythrocytes infected with malaria parasites is highly permeable to a large variety of solutes, including anions, carbohydrates, amino acids, nucleosides, organic and inorganic cations and small peptides. The altered permeability is presumed to be due to the activation of endogenous dormant channels, the new permeability pathways. The latter have been studied by different techniques—isosmotic lysis and tracer fluxes—and recently by patch-clamping. Here we analyze all available published data and we show that there is generally a good agreement between the two first methods. From the fluxes we calculate the number of channels per cell using reasonable assumptions as to the radius of the channel, and assuming that penetration through the channel is by diffusion through a water-filled space. The number of channels so calculated is <10 for most solutes, but ~400 for anions and the nucleosides thymidine and adenosine. This latter number is not far from that calculated from patch-clamp experiments. However, the anion flux measured directly by tracer is an order of magnitude larger than expected from conductance measurements. We conclude that the new permeability pathways consist of two types of channels; one is present in small number, and is charge- and size-selective. The other type is about 100-fold more abundant and is anion-selective, but does not admit non-electrolytes other than perhaps nucleosides.     

The pore properties of human nociceptor channel TRPA1 evaluated in single channel recordings

TRPA channels detect stimuli of different sensory modalities, including a broad spectrum of chemosensory stimuli, noxious stimuli associated with tissue damage and inflammation, mechanical stimuli, and thermal stimuli. Despite a growing understanding of potential modulators, agonists, and antagonists for these channels, the exact mechanisms of channel regulation and activation remain mostly unknown or controversial and widely debated. Relatively little is also known about the basic biophysical parameters of both native and heterologously expressed TRPA channels. Here we use conventional single channel inside-out and outside-out patch recording from the human TRPA1 channel transiently expressed in human embryonic kidney 293T cells to characterize the selectivity of the channel for inorganic mono-/divalent and organic monovalent cations in the presence of allylisothiocyanate (AITC). We show the relative permeability of the hTRPA1 channel to inorganic cations to be:and to organic cations:Na(+)(1.0)≥ dimethylamine (0.99)>trimethylamine (0.7)>tetramethylammonium (0.4)>N-methyl-d-glucamine (0.1). Activation of the hTRPA1 channels by AITC appears to recruit the channels to a conformational state with an increased permeability to large organic cations. The pore of the channels in this state can be characterized as dilated by approximately 1-2.5 ?. These findings provide important insight into the basic fundamental properties and function of TRPA1 channels in general and human TRPA1 channel in particular.     

Plasma Membrane Cholesterol as a Regulator of Human and Rodent P2X7 Receptor Activation and Sensitization

P2X7 receptors are nonselective cation channels gated by high extracellular ATP, but with sustained activation, receptor sensitization occurs, whereby the intrinsic pore dilates, making the cell permeable to large organic cations, which eventually leads to cell death. P2X7 receptors associate with cholesterol-rich lipid rafts, but it is unclear how this affects the properties of the receptor channel. Here we show that pore-forming properties of human and rodent P2X7 receptors are sensitive to perturbations of cholesterol levels. Acute depletion of cholesterol with 5 mm methyl-β-cyclodextrin (MCD) caused a substantial increase in the rate of agonist-evoked pore formation, as measured by the uptake of ethidium dye, whereas cholesterol loading inhibited this process. Patch clamp analysis of P2X7 receptor currents carried by Na⁺ and N-methyl-d-glucamine (NMDG⁺) showed enhanced activation and current facilitation following cholesterol depletion. This contrasts with the inhibitory effect of methyl-β-cyclodextrin reported for other P2X subtypes. Mutational analysis suggests the involvement of an N-terminal region and a proximal C-terminal region that comprises multiple cholesterol recognition amino acid consensus (CRAC) motifs, in the cholesterol sensitivity of channel gating. These results reveal cholesterol as a negative regulator of P2X7 receptor pore formation, protecting cells from P2X7-mediated cell death.     

N-methyl-D-glucamine and propidium dyes utilize different permeation pathways at rat P2X(7) receptors

Activation of membrane P2X₇ receptors by extracellular ATP [or its analog 2',3'-O-(4-benzoylbenzoyl)-ATP] results in the opening within several milliseconds of an integral ion channel that is permeable to small cations. If the ATP application is maintained for several seconds, two further sequelae occur: there is a gradual increase in permeability to the larger cation N-methyl-D-glucamine and the cationic propidium dye quinolinium, 4-[(3-methyl-2(3H)-benzoxazolylidene)methyl]-1-[3-(triethylammonio)propyl]diiodide (YO-PRO-1) enters the cell. The similarity in the time course of these two events has led to the widespread view that N-methyl-D-glucamine and YO-PRO-1 enter through a common permeation pathway, the "dilating" P2X₇ receptor pore. Here we provide two independent lines of evidence against this view. We studied single human embryonic kidney cells expressing rat P2X₇ receptors with patch-clamp recordings of membrane current and with fluorescence measurements of YO-PRO-1 uptake. First, we found that maintained application of the ATP analog did not cause any increase in N-methyl-D-glucamine permeability when the extracellular solution contained its normal sodium concentration, although YO-PRO-1 uptake was readily observed. Second, we deleted a cysteine-rich 18-amino acid segment in the intracellular juxtamembrane region of the P2X₇ receptor. This mutated receptor showed normal YO-PRO-1 uptake but had no permeability to N-methyl-D-glucamine. Together, the clear differential effects of extracellular sodium ions or of mutation of the receptor strongly suggest that N-methyl-D-glucamine and YO-PRO-1 do not enter the cell by the same permeation pathway. ATP; cation channel; permeability; quinolinium, 4-[(3-methyl-2(3H)-benzoxazolylidene)methyl]-1-[3-(triethylammonio)propyl]diiodide     

Selectivity of cations and nonelectrolytes for acetylcholine-activated channels in cultured muscle cells

The selectivity of acetylcholine (A-Ch)-activated channels for alkali cations, organic cations, and nonelectrolytes in cultured muscle cells has been studied. To test the effect of size, charge, and hydrogen- binding capacity of permeant molecules on their permeability, we have obtained the selectivity sequences of alkali cations, compared the permeability of pairs of permeant molecules with similar size and shape but differing in charge, and studied the permeability of amines of different hydrogen bonding capacity. ACh-activated channels transport alkali cations of small hydration radii and high mobility. The molecules with positive charge and (or) a hydrogen-bond donating moiety are more permeable than the ones without. On the other hand, several nonelectrolytes, i.e., ethylene glycol, formamide, and urea, do have a small, but measurable, permeability through the channels. These results are consistent with a model that ACh-activated channel is a water- filled pore containing dipoles or hydrogen bond accepting groups and a negative charged site with a pK of 4.8.     

Cation permeability and cation-anion interactions in a mutant GABA-gated chloride channel from Drosophila.

To investigate the structural basis of anion selectivity of Drosophila GABA-gated Cl(-) channels, the permeation properties of wild-type and mutant channels were studied in Xenopus oocytes. This work focused on asparagine 319, which by homology is one amino acid away from a putative extracellular ring of charge that regulates cation permeation in nicotinic receptors. Mutation of this residue to aspartate reduced channel conductance, and mutation to lysine or arginine increased channel conductance. These results are consistent with an electrostatic interaction between this site and permeating anions. The lysine mutant, but not the arginine mutant, formed a channel that is permeable to cations, and this cannot be explained in terms of electrostatics. The lysine mutant had a 25-mV reversal potential in solutions with symmetrical Cl(-) and asymmetrical cations. The permeability ratio of K(+) to Cl(-) was determined as 0. 33 from reversal potential measurements in KCl gradients. Experiments with large organic cations and anions showed that cation permeation can only be seen in the presence of Cl(-), but Cl(-) permeation can be seen in the absence of permeant cations. Measurements of permeability ratios of organic anions indicated that the lysine mutant has an increased pore size. The cation permeability of the lysine-containing mutant channel cannot be accounted for by a simple electrostatic interaction with permeating ions. It is likely that lysine substitution causes a structural change that extends beyond this one residue to influence the positions of other channel-forming residues. Thus protein conformation plays an important role in enabling ion channels to distinguish between anions and cations.     

Activation and Inhibition of TMEM16A Calcium-Activated Chloride Channels

Calcium-activated chloride channels (CaCC) encoded by family members of transmembrane proteins of unknown function 16 (TMEM16) have recently been intensely studied for functional properties as well as their physiological roles as chloride channels in various tissues. One technical hurdle in studying these channels is the well-known channel rundown that frequently impairs the precision of electrophysiological measurements for the channels. Using experimental protocols that employ fast-solution exchange, we circumvented the problem of channel rundown by normalizing the Ca²⁺-induced current to the maximally-activated current obtained within a time period in which the channel rundown was negligible. We characterized the activation of the TMEM16A-encoded CaCC (also called ANO1) by Ca²⁺, Sr²⁺, and Ba²⁺, and discovered that Mg²⁺ competes with Ca²⁺ in binding to the divalent-cation binding site without activating the channel. We also studied the permeability of the ANO1 pore for various anions and found that the anion occupancy in the pore–as revealed by the permeability ratios of these anions–appeared to be inversely correlated with the apparent affinity of the ANO1 inhibition by niflumic acid (NFA). On the other hand, the NFA inhibition was neither affected by the degree of the channel activation nor influenced by the types of divalent cations used for the channel activation. These results suggest that the NFA inhibition of ANO1 is likely mediated by altering the pore function but not through changing the channel gating. Our study provides a precise characterization of ANO1 and documents factors that can affect divalent cation activation and NFA inhibition of ANO1.