Activation and block of mouse muscle-type nicotinic receptors by tetraethylammonium

Null method to estimate the maximal PA at subsaturating concentrations of agonist

The maximal probability of being in an active state (PA,max) is a measure of gating efficacy for a given agonist acting on a given receptor channel. In macroscopic electrophysiological recordings, PA,max is typically estimated by comparing the amplitude of the current response to a saturating concentration of a test agonist to that of a reference agonist with known PA. Here, we describe an approach to estimate the PA,max for low-efficacy agonists at subsaturating concentrations. In this approach, the amplitude of the response to a high-efficacy control agonist applied alone is compared with the amplitude of the response to a control agonist coapplied with the low-efficacy test agonist that binds to the same site(s). If the response to the combination is larger than the response to the control agonist alone, then the PA,max of the test agonist is greater than the PA of the control response. Conversely, if the response to the control agonist is reduced upon exposure to the test agonist, then the PA,max of the test agonist is smaller than the PA of the control response. The exact PA,max of the test agonist can be determined by testing its effect at different concentrations of the control agonist to estimate the PA at which the effect changes direction. The main advantage of this approach lies in the ability to use low, subsaturating concentrations of the test agonist. The model-based predictions are supported by observations from activation of heteromeric and homomeric GABAA receptors by combinations of high- and low-efficacy orthosteric agonists.

Crystal structure of the tetra-ethyl-ammonium salt of the non-steroidal anti-inflammatory drug nimesulide (polymorph II)

The crystal structure of the tetra-ethyl-ammonium salt of the non-steroidal anti-inflammatory drug nimesulide (polymorph II) (systematic name: tetra-ethyl-ammonium N-methane-sulfonyl-4-nitro-2-phen-oxy-anilinide), C8H20N+·C13H11N2O5S-, was determined using single-crystal X-ray diffraction. The title compound crystallizes in the monoclinic space group P21/c with one tetra-ethyl-ammonium cation and one nimesulide anion in the asymmetric unit. In the crystal, the ions are linked by C-H⋯N and C-H⋯O hydrogen bonds and C-H⋯π inter-actions. There are differences in the geometry of both the nimesulide anion and the tetra-ethyl-ammonium cation in polymorphs I [Rybczyńska & Sikorski (2023 ▸). Sci. Rep. 13, 17268] and II of the title compound.

The synthesis, thermal behaviour, spectral and structural characterization, and in silico prediction of pharmacokinetic parameters of tetraalkylammonium salts of non-steroidal anti-inflammatory drug nimesulide

The synthesis, spectral properties, thermal analysis, structural characterization and in silico prediction of pharmacokinetic parameters of tetramethylammonium (compound 1) and tetraethylammonium (compound 2) salt of nimesulide were described in this article. Both compounds crystallize in the monoclinic P21/n space group, with one tetraalkylammonium cation and one nimesulide anion in the asymmetric unit and their crystal structures are stabilized by C-H···O hydrogen bonds between ions. Additionally, structures of title compounds are stabilized by π-π interactions (compound 1), or C-H···π interactions (compound 2) between nimesulide anions. The TG and DSC measurements show that compound 1 melts at a temperature higher than nimesulide, whereas the compound 2 melts at a temperature lower than nimesulide. The MALDI-TOF, 1H NMR, 13C NMR and ATR-FTIR analyses confirm the SCXRD study, that in compounds 1 and 2 nimesulide exists in an ionized form. Studies performed by SWISS ADME and ProTOX II tools, predict to be oral bioavailability of both salts obtained, and one of them (compound 1) is predicted to be well-absorbed by digestive system, while both compounds obtained are classified into toxicity class 4.

Activation of Caenorhabditis elegans Levamisole-Sensitive and Mammalian Nicotinic Receptors by the Antiparasitic Bephenium

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels involved in neuromuscular transmission. In nematodes, muscle nAChRs are targets of antiparasitic drugs. Bephenium is an anthelmintic compound whose molecular action in the free-living nematode Caenorhabditis elegans, which is a model for anthelmintic drug discovery, is poorly known. We explored the effect of bephenium on C. elegans locomotion and applied single-channel recordings to identify its molecular target, mechanism of action, and selectivity between mammalian and C. elegans nAChRs. As in parasites, bephenium paralyzes C. elegans A mutant strain lacking the muscle levamisole-sensitive nAChR (L-AChR) shows full resistance to bephenium, indicating that this receptor is the target site. Bephenium activates L-AChR channels from larvae muscle cells in the micromolar range. Channel activity is similar to that elicited by levamisole, appearing mainly as isolated brief openings. Our analysis revealed that bephenium is an agonist of L-AChR and an open-channel blocker at higher concentrations. It also activates mammalian muscle nAChRs. Opening events are significantly briefer than those elicited by ACh and do not appear in activation episodes at a range of concentrations, indicating that it is a very weak agonist of mammalian nAChRs. Recordings in the presence of ACh showed that bephenium acts as a voltage-dependent channel blocker and a low-affinity agonist. Molecular docking into homology-modeled binding-site interfaces represent the binding mode of bephenium that explains its partial agonism. Given the great diversity of helminth nAChRs and the overlap of their pharmacological profiles, unraveling the basis of drug receptor-selectivity will be required for rational design of anthelmintic drugs.

Cre-expressing neurons in visual cortex of Ntsr1-Cre GN220 mice are corticothalamic and are depolarized by acetylcholine

The Ntsr1-Cre GN220 mouse expresses Cre-recombinase in corticothalamic (CT) neurons in neocortical layer 6. It is not known if the other major types of pyramidal neurons in this layer also express this enzyme. By electrophysiological recordings in slices and histological analysis of the uptake of retrogradely transported beads we show that Cre-positive neurons are CT and not corticocortical or corticoclaustral types. Furthermore, we show that Ntsr1-Cre-positive cells are immuno-positive for the nuclear transcription factor Forkhead box protein P2 (FoxP2). We conclude that Cre-expression is limited to a specific type of pyramidal neuron: CT. However, it appears as not all CT neurons are Cre-expressing; there are indications that the penetrance of the gene is about 90%. We demonstrate the utility of assigning a specific identity to individual neurons by determining that the CT neurons are potently modulated by acetylcholine acting on both nicotinic and muscarinic acetylcholine receptors. These results corroborate the suggested function of these neurons in regulating the gain of thalamocortical transfer of sensory information depending on attentional demand and state of arousal.

Isoform-specific mechanisms of α3β4*-nicotinic acetylcholine receptor modulation by the prototoxin lynx1

This study investigates-for the first time to our knowledge-the existence and mechanisms of functional interactions between the endogenous mammalian prototoxin, lynx1, and α3- and β4-subunit-containing human nicotinic acetylcholine receptors (α3β4*-nAChRs). Concatenated gene constructs were used to express precisely defined α3β4*-nAChR isoforms (α3β4)₂β4-, (α3β4)₂α3-, (α3β4)₂α5(398D)-, and (α3β4)₂α5(398N)-nAChR in Xenopus oocytes. In the presence or absence of lynx1, α3β4*-nAChR agonist responses were recorded by using 2-electrode voltage clamp and single-channel electrophysiology, whereas radioimmunolabeling measured cell-surface expression. Lynx1 reduced (α3β4)₂β4-nAChR function principally by lowering cell-surface expression, whereas single-channel effects were primarily responsible for reducing (α3β4)₂α3-nAChR function [decreased unitary conductance (≥50%), altered burst proportions (3-fold reduction in the proportion of long bursts), and enhanced closed dwell times (3- to 6-fold increase)]. Alterations in both cell-surface expression and single-channel properties accounted for the reduction in (α3β4)₂α5-nAChR function that was mediated by lynx1. No effects were observed when α3β4*-nAChRs were coexpressed with mutated lynx1 (control). Lynx1 is expressed in the habenulopeduncular tract, where α3β4*-α5*-nAChR subtypes are critical contributors to the balance between nicotine aversion and reward. This gives our findings a high likelihood of physiologic significance. The exquisite isoform selectivity of lynx1 interactions provides new insights into the mechanisms and allosteric sites [α(-)-interface containing] by which prototoxins can modulate nAChR function.-George, A. A., Bloy, A., Miwa, J. M., Lindstrom, J. M., Lukas, R. J., Whiteaker, P. Isoform-specific mechanisms of α3β4*-nicotinic acetylcholine receptor modulation by the prototoxin lynx1.

Muscle-Type Nicotinic Receptor Modulation by 2,6-Dimethylaniline, a Molecule Resembling the Hydrophobic Moiety of Lidocaine

To identify the molecular determinants responsible for lidocaine blockade of muscle-type nAChRs, we have studied the effects on this receptor of 2,6-dimethylaniline (DMA), which resembles lidocaine’s hydrophobic moiety. Torpedo marmorata nAChRs were microtransplanted to Xenopus oocytes and currents elicited by ACh (I_ACh), either alone or co-applied with DMA, were recorded. DMA reversibly blocked I_ACh and, similarly to lidocaine, exerted a closed-channel blockade, as evidenced by the enhancement of I_ACh blockade when DMA was pre-applied before its co-application with ACh, and hastened I_ACh decay. However, there were marked differences among its mechanisms of nAChR inhibition and those mediated by either the entire lidocaine molecule or diethylamine (DEA), a small amine resembling lidocaine’s hydrophilic moiety. Thereby, the IC₅₀ for DMA, estimated from the dose-inhibition curve, was in the millimolar range, which is one order of magnitude higher than that for either DEA or lidocaine. Besides, nAChR blockade by DMA was voltage-independent in contrast to the increase of I_ACh inhibition at negative potentials caused by the more polar lidocaine or DEA molecules. Accordingly, virtual docking assays of DMA on nAChRs showed that this molecule binds predominantly at intersubunit crevices of the transmembrane-spanning domain, but also at the extracellular domain. Furthermore, DMA interacted with residues inside the channel pore, although only in the open-channel conformation. Interestingly, co-application of ACh with DEA and DMA, at their IC₅₀s, had additive inhibitory effects on I_ACh and the extent of blockade was similar to that predicted by the allotopic model of interaction, suggesting that DEA and DMA bind to nAChRs at different loci. These results indicate that DMA mainly mimics the low potency and non-competitive actions of lidocaine on nAChRs, as opposed to the high potency and voltage-dependent block by lidocaine, which is emulated by the hydrophilic DEA. Furthermore, it is pointed out that the hydrophobic (DMA) and hydrophilic (DEA) moieties of the lidocaine molecule act differently on nAChRs and that their separate actions taken together account for most of the inhibitory effects of the whole lidocaine molecule on nAChRs.

The atypical cation-conduction and gating properties of ELIC underscore the marked functional versatility of the pentameric ligand-gated ion-channel fold

The superfamily of pentameric ligand-gated ion channels (pLGICs) is unique among ionotropic receptors in that the same overall structure has evolved to generate multiple members with different combinations of agonist specificities and permeant-ion charge selectivities. However, aside from these differences, pLGICs have been typically regarded as having several invariant functional properties. These include pore blockade by extracellular quaternary-ammonium cations in the micromolar-to-millimolar concentration range (in the case of the cation-selective members), and a gain-of-function phenotype, which manifests as a slower deactivation time course, as a result of mutations that reduce the hydrophobicity of the transmembrane pore lining. Here, we tested this notion on three distantly related cation-selective members of the pLGIC superfamily: the mouse muscle nicotinic acetylcholine receptor (nAChR), and the bacterial GLIC and ELIC channels. Remarkably, we found that, whereas low millimolar concentrations of TMA(+) and TEA(+) block the nAChR and GLIC, neither of these two quaternary-ammonium cations blocks ELIC at such concentrations; instead, both carry measurable inward currents when present as the only cations on the extracellular side. Also, we found that, whereas lidocaine binding speeds up the current-decay time courses of the nAChR and GLIC in the presence of saturating concentrations of agonists, the binding of lidocaine to ELIC slows this time course down. Furthermore, whereas mutations that reduce the hydrophobicity of the side chains at position 9' of the M2 α-helices greatly slowed the deactivation time course of the nAChR and GLIC, these mutations had little effect--or even sped up deactivation--when engineered in ELIC. Our data indicate that caution should be exercised when generalizing results obtained with ELIC to the rest of the pLGICs, but more intriguingly, they hint at the possibility that ELIC is a representative of a novel branch of the superfamily with markedly divergent pore properties despite a well-conserved three-dimensional architecture.

Toxicity of tetramethylammonium hydroxide to aquatic organisms and its synergistic action with potassium iodide

The aquatic ecotoxicity of chemicals involved in the manufacturing process of thin film transistor liquid crystal displays was assessed with a battery of four selected acute toxicity bioassays. We focused on tetramethylammonium hydroxide (TMAH, CAS No. 75-59-2), a widely utilized etchant. The toxicity of TMAH was low when tested in the 72 h-algal growth inhibition test (Pseudokirchneriellia subcapitata, EC50=360 mg L(-1)) and the Microtox® test (Vibrio fischeri, IC50=6.4 g L(-1)). In contrast, the 24h-microcrustacean immobilization and the 96 h-fish mortality tests showed relatively higher toxicity (Daphnia magna, EC50=32 mg L(-1) and Oryzias latipes, LC50=154 mg L(-1)). Isobologram and mixture toxicity index analyses revealed apparent synergism of the mixture of TMAH and potassium iodide when examined with the D. magna immobilization test. The synergistic action was unique to iodide over other halide salts i.e. fluoride, chloride and bromide. Quaternary ammonium ions with longer alkyl chains such as tetraethylammonium and tetrabutylammonium were more toxic than TMAH in the D. magna immobilization test.

Poly(ethylene glycol) as a scaffold for high-affinity open-channel blockers of the mouse nicotinic acetylcholine receptor

High-affinity blockers for an ion channel often have complex molecular structures that are synthetically challenging and/or laborious. Here we show that high-affinity blockers for the mouse nicotinic acetylcholine receptor (AChR) can be prepared from a structurally simple material, poly(ethylene glycol) (PEG). The PEG-based blockers (PQ1–5), comprised of a flexible octa(ethylene glycol) scaffold and two terminal quaternary ammonium groups, exert low- to sub-micromolar affinities for the open AChR pore (measured via single-channel analysis of AChRs expressed in human embryonic kidney cells). PQ1–5 are comparable in pore-binding affinity to the strongest AChR open-channel blockers previously reported, which have complex molecular structures. These results suggest a general approach for designing potent open-channel blockers from a structurally flexible polymer. This design strategy involves simple synthetic procedures and does not require detailed information about the structure of an ion-channel pore.

Endothelium-derived hyperpolarizing factor mediates bradykinin-stimulated tissue plasminogen activator release in humans

AIMS

Bradykinin (BK) stimulates tissue plasminogen activator (t-PA) release from human endothelium. Although BK stimulates both nitric oxide and endothelium-derived hyperpolarizing factor (EDHF) release, the role of EDHF in t-PA release remains unexplored. This study sought to determine the mechanisms of BK-stimulated t-PA release in the forearm vasculature of healthy human subjects.

METHODS

In 33 healthy subjects (age 40.3 ± 1.9 years), forearm blood flow (FBF) and t-PA release were measured at rest and after intra-arterial infusions of BK (400 ng/min) and sodium nitroprusside (3.2 mg/min). Measurements were repeated after intra-arterial infusion of tetraethylammonium chloride (TEA; 1 µmol/min), fluconazole (0.4 µmol·min(-1)·l(-1)), and N(G)-monomethyl-L-arginine (L-NMMA, 8 µmol/min) to block nitric oxide, and their combination in separate studies.

RESULTS

BK significantly increased net t-PA release across the forearm (p < 0.0001). Fluconazole attenuated both BK-mediated vasodilation (-23.3 ± 2.7% FBF, p < 0.0001) and t-PA release (from 50.9 ± 9.0 to 21.3 ± 8.9 ng/min/100 ml, p = 0.02). TEA attenuated FBF (-14.7 ± 3.2%, p = 0.002) and abolished BK-stimulated t-PA release (from 22.9 ± 5.7 to -0.8 ± 3.6 ng/min/100 ml, p = 0.0002). L-NMMA attenuated FBF (p < 0.0001), but did not inhibit BK-induced t-PA release (nonsignificant).

CONCLUSION

BK-stimulated t-PA release is partly due to cytochrome P450-derived epoxides and is inhibited by K(+)Ca channel blockade. Thus, BK stimulates both EDHF-dependent vasodilation and t-PA release.

Energy for wild-type acetylcholine receptor channel gating from different choline derivatives

Agonists, including the neurotransmitter acetylcholine (ACh), bind at two sites in the neuromuscular ACh receptor channel (AChR) to promote a reversible, global change in protein conformation that regulates the flow of ions across the muscle cell membrane. In the synaptic cleft, ACh is hydrolyzed to acetate and choline. Replacement of the transmitter's ester acetyl group with a hydroxyl (ACh→choline) results in a + 1.8 kcal/mol reduction in the energy for gating generated by each agonist molecule from a low- to high-affinity change of the transmitter binding site (ΔG(B)). To understand the distinct actions of structurally related agonist molecules, we measured ΔG(B) for 10 related choline derivatives. Replacing the hydroxyl group of choline with different substituents, such as hydrogen, chloride, methyl, or amine, increased the energy for gating (i.e., it made ΔG(B) more negative relative to choline). Extending the ethyl hydroxide tail of choline to propyl and butyl hydroxide also increased this energy. Our findings reveal the amount of energy that is available for the AChR conformational change provided by different, structurally related agonists. We speculate that a hydrogen bond between the choline hydroxyl and the backbone carbonyl of αW149 positions this agonist's quaternary ammonium group so as to reduce the cation-π interaction between this moiety and the aromatic groups at the binding site.

Ca2+-activated K+ channels of small and intermediate conductance control eNOS activation through NAD(P)H oxidase

Ca(2+)-activated K(+) channels (K(Ca)) and NO play a central role in the endothelium-dependent control of vasomotor tone. We evaluated the interaction of K(Ca) with NO production in isolated arterial mesenteric beds of the rat. In phenylephrine-contracted mesenteries, acetylcholine (ACh)-induced vasodilation was reduced by NO synthase (NOS) inhibition with N(ω)-nitro-L-arginine (L-NA), but in the presence of tetraethylammonium, L-NA did not further affect the response. In KCl-contracted mesenteries, the relaxation elicited by 100 nM ACh or 1 μM ionomycin was abolished by L-NA, tetraethylammonium, or simultaneous blockade of small-conductance K(Ca) (SK(Ca)) channels with apamin and intermediate-conductance K(Ca) (IK(Ca)) channels with triarylmethane-34 (TRAM-34). Apamin-TRAM-34 treatment also abolished 100 nM ACh-activated NO production, which was associated with an increase in superoxide formation. Endothelial cell Ca(2+) buffering with BAPTA elicited a similar increment in superoxide. Apamin-TRAM-34 treatment increased endothelial NOS phosphorylation at threonine 495 (P-eNOS(Thr495)). Blockade of NAD(P)H oxidase with apocynin or superoxide dismutation with PEG-SOD prevented the increment in superoxide and changes in P-eNOS(Thr495) observed during apamin and TRAM-34 application. Our results indicate that blockade of SK(Ca) and IK(Ca) activates NAD(P)H oxidase-dependent superoxide formation, which leads to inhibition of NO release through P-eNOS(Thr495). These findings disclose a novel mechanism involved in the control of NO production.

Functional characterization of the α5(Asn398) variant associated with risk for nicotine dependence in the α3β4α5 nicotinic receptor

Smoking is a major cause for premature death. Work aimed at identifying genetic factors that contribute to nicotine addiction has revealed several single nucleotide polymorphisms (SNPs) that are linked to smoking-related behaviors such as nicotine dependence and level of smoking. One of these SNPs leads to an aspartic acid-to-asparagine substitution in the nicotinic receptor α5 subunit at amino acid position 398 [rs16969968; α5(Asn398)]. The α5 subunit is expressed both in the brain and in the periphery. In the brain, it associates with the α4 and β2 subunits to form α4β2α5 receptors. In the periphery, the α5 subunit combines with the α3 and β4 subunits to form the major ganglionic postsynaptic nicotinic receptor subtype. The α3β4α5 receptor regulates a variety of autonomic responses such as control of cardiac rate, blood pressure, and perfusion. In this paradigm, the α5(Asn398) variant may act by regulating autonomic responses that may affect nicotine intake by humans. Here, we have investigated the effect of the α5(Asn398) variant on the function of the α3β4α5 receptor. The wild-type or variant α5 subunits were coexpressed with the α3 and β4 subunits in human embryonic kidney 293 cells. The properties of the receptors were studied using whole-cell and single-channel electrophysiology. The data indicate that the introduction of the α5(Asn398) mutation has little effect on the pharmacology of receptor activation, receptor desensitization, or single-channel properties. We propose that the effect of the α5(Asn398) variant on nicotine use is not mediated by an action on the physiological or pharmacological properties of the α3β4α5 subtype.

The activation mechanism of alpha1beta2gamma2S and alpha3beta3gamma2S GABAA receptors

The α₁β₂γ₂ and α₃β₃γ₂ are two isoforms of γ-aminobutyric acid type A (GABA_A) receptor that are widely distributed in the brain. Both are found at synapses, for example in the thalamus, where they mediate distinctly different inhibitory postsynaptic current profiles, particularly with respect to decay time. The two isoforms were expressed in HEK293 cells, and single-channel activity was recorded from outside-out patches. The kinetic characteristics of both isoforms were investigated by analyzing single-channel currents over a wide range of GABA concentrations. α₁β₂γ₂ channels exhibited briefer active periods than α₃β₃γ₂ channels over the entire range of agonist concentrations and had lower intraburst open probabilities at subsaturating concentrations. Activation mechanisms were constructed by fitting postulated schemes to data recorded at saturating and subsaturating GABA concentrations simultaneously. Reaction mechanisms were ranked according to log-likelihood values and how accurately they simulated ensemble currents. The highest ranked mechanism for both channels consisted of two sequential binding steps, followed by three conducting and three nonconducting configurations. The equilibrium dissociation constant for GABA at α₃β₃γ₂ channels was ∼2.6 µM compared with ∼19 µM for α₁β₂γ₂ channels, suggesting that GABA binds to the α₃β₃γ₂ channels with higher affinity. A notable feature of the mechanism was that two consecutive doubly liganded shut states preceded all three open configurations. The lifetime of the third shut state was briefer for the α₃β₃γ₂ channels. The longer active periods, higher affinity, and preference for conducting states are consistent with the slower decay of inhibitory currents at synapses that contain α₃β₃γ₂ channels. The reaction mechanism we describe here may also be appropriate for the analysis of other types of GABA_A receptors and provides a framework for rational investigation of the kinetic effects of a variety of therapeutic agents that activate or modulate GABA_A receptors and hence influence synaptic and extrasynaptic inhibition in the central nervous system.

Agonist and blocking actions of choline and tetramethylammonium on human muscle acetylcholine receptors

Choline has been used widely as an agonist for the investigation of gain-of-function mutants of the nicotinic acetylcholine receptor. It is useful because it behaves like a partial agonist. The efficacy of choline is difficult to measure because choline blocks the channel at concentrations about four times lower than those that activate it. We have fitted activation mechanisms to single-channel activity elicited from HEK-expressed human recombinant muscle nicotinic receptors by choline and by tetramethylammonium (TMA). Channel block by the agonist was incorporated into the mechanisms that were fitted, and block was found not to be selective for the open state. The results also suggest that channel block is very fast and that the channel can shut almost as fast as normal when the blocker was bound. Single-channel data are compatible with a mechanism in which choline is actually a full agonist, its maximum response being limited only by channel block. However, they are also compatible with a mechanism incorporating a pre-opening conformation change ('flip') in which choline is a genuine partial agonist. The latter explanation is favoured by concentration jump experiments, and by the fact that only this mechanism fits the TMA data. We propose that choline, like TMA, is a partial agonist because it is very ineffective (approximately 600-fold less than acetylcholine) at eliciting the initial, pre-opening conformation change. Once flipping has occurred, all agonists, even choline, open the channel with similar efficiency.

A novel signaling pathway: fibroblast nicotinic receptor alpha1 binds urokinase and promotes renal fibrosis

The nicotinic acetylcholine receptor alpha1 (nAChRalpha1) was investigated as a potential fibrogenic molecule in the kidney, given reports that it may be an alternative urokinase (urokinase plasminogen activator; uPA) receptor in addition to the classical receptor uPAR. In a mouse obstructive uropathy model of chronic kidney disease, interstitial fibroblasts were identified as the primary cell type that bears nAChRalpha1 during fibrogenesis. Silencing of the nAChRalpha1 gene led to significantly fewer interstitial alphaSMA(+) myofibroblasts (2.8 times decreased), reduced interstitial cell proliferation (2.6 times decreased), better tubular cell preservation (E-cadherin 14 times increased), and reduced fibrosis severity (24% decrease in total collagen). The myofibroblast-inhibiting effect of nAChRalpha1 silencing in uPA-sufficient mice disappeared in uPA-null mice, suggesting that a uPA-dependent fibroblastic nAChRalpha1 pathway promotes renal fibrosis. To further establish this possible ligand-receptor relationship and to identify downstream signaling pathways, in vitro studies were performed using primary cultures of renal fibroblasts. (35)S-Labeled uPA bound to nAChRalpha1 with a K(d) of 1.6 x 10(-8) m, which was displaced by the specific nAChRalpha1 inhibitor d-tubocurarine in a dose-dependent manner. Pre-exposure of uPA to the fibroblasts inhibited [(3)H]nicotine binding. The uPA binding induced a cellular calcium influx and an inward membrane current that was entirely prevented by d-tubocurarine preincubation or nAChRalpha1 silencing. By mass spectrometry phosphoproteome analyses, uPA stimulation phosphorylated nAChRalpha1 and a complex of signaling proteins, including calcium-binding proteins, cytoskeletal proteins, and a nucleoprotein. This signaling pathway appears to regulate the expression of a group of genes that transform renal fibroblasts into more active myofibroblasts characterized by enhanced proliferation and contractility. This new fibrosis-promoting pathway may also be relevant to disorders that extend beyond chronic kidney disease.

MaxiK potassium channels in the function of chemoreceptor cells of the rat carotid body

Hypoxia activates chemoreceptor cells of the carotid body (CB) promoting an increase in their normoxic release of neurotransmitters. Catecholamine (CA) release rate parallels the intensity of hypoxia. Coupling of hypoxia to CA release requires cell depolarization, produced by inhibition of O(2)-regulated K(+) channels, and Ca(2+) entering the cells via voltage-operated channels. In rat chemoreceptor cells hypoxia inhibits large-conductance, calcium-sensitive K channels (maxiK) and a two-pore domain weakly inward rectifying K(+) channel (TWIK)-like acid-sensitive K(+) channel (TASK)-like channel, but the significance of maxiK is controversial. A proposal envisions maxiK contributing to set the membrane potential (E(m)) and the hypoxic response, but the proposal is denied by authors finding that maxiK inhibition does not depolarize chemoreceptor cells or alters intracellular Ca(2+) concentration or CA release in normoxia or hypoxia. We found that maxiK channel blockers (tetraethylammonium and iberiotoxin) did not modify CA release in rat chemoreceptor cells, in either normoxia or hypoxia, and iberiotoxin did not alter the Ca(2+) transients elicited by hypoxia. On the contrary, both maxiK blockers increased the responses elicited by dinitrophenol, a stimulus we demonstrate does not affect maxiK channels in isolated patches of rat chemoreceptor cells. We conclude that in rat chemoreceptor cells maxiK channels do not contribute to the genesis of the E(m), and that their full inhibition by hypoxia, preclude further inhibition by maxiK channel blockers. We suggest that full inhibition of this channel is required to generate the spiking behavior of the cells in acute hypoxia.

An activity-based protein profiling probe for the nicotinic acetylcholine receptor

Activity-based protein profiling (ABPP) has been used extensively to characterize the physiological functions of enzymes but has not yet been extended to ion channels. We have synthesized a state-dependent photoaffinity probe for the nicotinic acetylcholine receptor (nAChR) as a proof of concept for the development of ion channel directed ABPP probes. The candidate probe BPyneTEA comprises an nAChR binding moiety, a benzophenone moiety for photolabeling, and an alkyne moiety for biotinylation via "click chemistry". Single-molecule current measurements show that BPyneTEA blocks both the closed and open (i.e., nondesensitized) conformations of the nAChR with similar kinetics. In living cells, BPyneTEA photolabels the closed state selectively over the inactive desensitized state. BPyneTEA thus shows promise as a probe for nondesensitized nAChRs and may be useful in studying the molecular physiology of desensitization. The structure and reactivity of ion channel pores in general suggest that they will be a broadly useful target for ABPP probes.

Probing protein packing surrounding the residues in and flanking the nicotinic acetylcholine receptor M2M3 loop

Nicotinic acetylcholine receptors (nAChR) are cation-selective, ligand-gated ion channels of the cysteine (Cys)-loop gene superfamily. The recent crystal structure of a bacterial homolog from Erwinia chrysanthemi (ELIC) agrees with previous structures of the N-terminal domain of AChBP (acetylcholine-binding protein) and of the electron-microscopy-derived Torpedo nAChR structure. However, the ELIC transmembrane domain is significantly more tightly packed than the corresponding region of the Torpedo nAChR. We investigated the tightness of protein packing surrounding the extracellular end of the M2 transmembrane segment and around the loop connecting the M2 and M3 segments using the substituted cysteine accessibility method. The M2 20' to 27' residues were highly water accessible and the variation in reaction rates were consistent with this region being alpha-helical. At all positions tested, the presence of ACh changed methanethiosulfonate ethylammonium (MTSEA) modification rates by <10-fold. In the presence of ACh, reaction rates for residues in the last extracellular alpha-helical turn of M2 and in the M2M3 loop increased, whereas rates in the penultimate alpha-helical turn of M2 decreased. Only three of eight M2M3 loop residues were accessible to MTSEA in both the presence and absence of ACh. We infer that the protein packing around the M2M3 loop is tight, consistent with its location at the interdomain interface where it is involved in the transduction of ligand binding in the extracellular domain to gating in the transmembrane domain. Our data indicate that the Torpedo nAChR transmembrane domain structure is a better model than the ELIC structure for eukaryotic Cys-loop receptors.

Activation and block of the adult muscle-type nicotinic receptor by physostigmine: single-channel studies

The plant-derived acetylcholinesterase inhibitor physostigmine has previously been shown to act on the nicotinic acetylcholine receptor (nAChR) causing either direct activation or potentiation of currents elicited by low concentrations of nicotinic agonists, or, at higher concentrations, channel block. We examined mouse adult-type muscle nAChR activation by physostigmine and found that channel activation by physostigmine exhibits many characteristics common with channel activity elicited by nicotinic agonists. Single-channel conductance was indistinguishable, and mutants known to slow channel closing in the presence of nicotinic agonists had a similar effect in the presence of physostigmine. However, physostigmine is a very inefficacious agonist. The presence of physostigmine did not alter the effective opening rate for a subsaturating dosage of carbachol, suggesting that physostigmine does not interact with the nicotinic agonist binding site. Mutations to a residue (alphaLys125) previously identified as part of the putative binding site for physostigmine reduced the duration of openings elicited by physostigmine, but the effects were generally small and, in most cases, nonsignificant. At higher concentrations, physostigmine blocked channel activity. Block manifested as a reduction in the mean open time and the emergence of a closed state, with a mean duration of 3 to 7 ms. The properties of block were consistent with two equivalent blocking sites per receptor with microscopic binding and unbinding rate constants for physostigmine of 20 microM(-1) s(-1) and 450 s(-1) (K(D) = 23 microM). These observations indicate that physostigmine is able to activate muscle nAChR by interacting with a site other than the nicotinic ligand binding site.

Diverse inhibitory actions of quaternary ammonium cholinesterase inhibitors on Torpedo nicotinic ACh receptors transplanted to Xenopus oocytes

BACKGROUND AND PURPOSE

This work was aimed at comparing and analysing the effects and mechanisms of action of the quaternary ammonium cholinesterase inhibitors (QChEIs) BW284c51, decamethonium and edrophonium, on nicotinic ACh receptor (nAChR) function.

EXPERIMENTAL APPROACH

nAChRs purified from Torpedo electroplax were transplanted to oocytes and currents elicited by ACh (I(ACh)) either alone or in presence of these QChEIs were recorded.

KEY RESULTS

None of the QChEIs, by itself, elicited changes in membrane conductance; however, when co-applied with ACh, all of them decreased I(ACh) in a concentration-dependent way. The mechanisms of nAChR inhibition were different for these QChEIs. BW284c51 blockade was non-competitive and voltage-dependent, although it also affected the n(H) of the dose-response curve. By contrast, decamethonium and edrophonium inhibition, at -60 mV, was apparently competitive and did not modify either desensitisation or n(H). Decamethonium effects were voltage-independent and washed out slowly after its removal; by contrast, edrophonium blockade had strong voltage dependence and its effects disappeared quickly after its withdrawal. Analysis of the voltage-dependent blockade indicated that BW284c51 bound to a shallow site into the channel pore, whereas edrophonium bound to a deeper locus. Accordingly, additive inhibitory effects on I(ACh) were found among any pairs of these QChEIs.

CONCLUSIONS AND IMPLICATIONS

The tested QChEIs bound to the nAChR at several and different loci, which might account for their complex inhibitory behaviour, acting both as allosteric effectors and, in the case of BW284c51 and edrophonium, as open channel blockers.

Block of muscle nicotinic receptors by choline suggests that the activation and desensitization gates act as distinct molecular entities

Ion channel block in muscle acetylcholine nicotinic receptors (AChRs) is an extensively reported phenomenon. Yet, the mechanisms underlying the interruption of ion flow or the interaction of the blocker with the channel's gates remain incompletely characterized. In this paper, we studied fast channel block by choline, a quaternary-ammonium cation that is also an endogenous weak agonist of this receptor, and a valuable tool in structure-function studies. Analysis of the single-channel current amplitude as a function of both choline concentration and voltage revealed that extracellular choline binds to the open-channel pore with millimolar apparent affinity (K(B) congruent with 12 mM in the presence of approximately 155 mM monovalent and 3.5 mM divalent, inorganic cations), and that it permeates the channel faster than acetylcholine. This, together with its relatively small size ( approximately 5.5 A along its longest axis), suggests that the pore-blocking choline binding site is the selectivity filter itself, and that current blockages simply reflect the longer-lived sojourns of choline at this site. Kinetic analysis of single-channel traces indicated that increasing occupancy of the pore-blocking site by choline (as judged from the reduction of the single-channel current amplitude) is accompanied by the lengthening of (apparent) open interval durations. Consideration of a number of possible mechanisms firmly suggests that this prolongation results from the local effect of choline interfering with the operation of the activation gate (closure of blocked receptors is slower than that of unblocked receptors by a factor of approximately 13), whereas closure of the desensitization gate remains unaffected. Thus, we suggest that these two gates act as distinct molecular entities. Also, the detailed understanding gained here on how choline distorts the observed open-time durations can be used to compensate for this artifact during activation assays. This correction is necessary if we are to understand how choline binds to and gates the AChR.

Galantamine activates muscle-type nicotinic acetylcholine receptors without binding to the acetylcholine-binding site

Galantamine (Reminyl; Janssen Pharmaceutica, Titusville, NJ) belongs to a class of acetylcholinesterase inhibitors approved for symptomatic treatment of Alzheimer's disease. The drug presumably acts by raising and prolonging the profile of acetylcholine (ACh) via an inhibitory effect on the esterase. However, there is also evidence demonstrating that galantamine can activate the nicotinic ACh receptor or modulate its activation by ACh. In this study, we have examined the ability of galantamine to directly activate the muscle-type nicotinic ACh receptor or to modulate receptor activation by selected nicotinic agonists. Studies of direct activation by galantamine demonstrated that this ligand is a low-efficacy agonist of the muscle-type ACh receptor. Point mutations in the M2-M3 linker (alphaS269I) and the M2 transmembrane domain (epsilonT264P) had similar effects on receptor activation by galantamine and nicotinic agonists, suggesting that the general features of receptor activation by galantamine are similar to that in the presence of ACh. Experiments performed in the simultaneous presence of galantamine and various nicotinic ligands showed that channel activation by the nicotinic ligands studied (ACh, carbachol, and choline) was not affected by the presence of galantamine at concentrations up to 100 microm. In addition, galantamine did not reduce the initial rate of binding for 125I-alpha-bungarotoxin. These results demonstrate that galantamine does not interfere with the occupation of the nicotinic agonist binding site by ACh, carbachol, or choline. We conclude that galantamine activates the muscle-type ACh receptor by interacting with a binding site that is distinct from the site for nicotinic agonists.

Activation of heteroliganded mouse muscle nicotinic receptors

The activation of the mouse muscle-type nicotinic acetylcholine receptor was studied in the presence of carbachol, and in the simultaneous presence of carbachol and choline. The channel currents were recorded under steady-state conditions using cell-attached single-channel patch clamp, and during transient exposures to the agonists using a piezo-driven fast application system. The presence of choline resulted in inhibition of currents elicited by carbachol. The inhibitory effect of choline manifested as a reduction in the effective opening rate (increase in the mean intracluster closed time duration) in single-channel recordings. In the fast application experiments, the peak current amplitude was reduced and the current rise time increased when choline was co-applied with carbachol. The data were analysed according to a model in which receptor interactions with carbachol and choline resulted in three types of ligation: receptors occupied by two carbachol molecules, receptors occupied by two choline molecules, and receptors in which one agonist binding site was occupied by carbachol and the other by choline, i.e. heteroliganded receptors. All three agonist-bound receptor populations could open albeit with different efficacies. The affinity of the resting receptor to choline was estimated to be 1-2 mm, and heteroliganded receptors opened with an opening rate constant of approximately 3000 s(-1). The results of the analysis suggest that the presence of choline in the neuromuscular junction in vivo has little effect on the time course of synaptic currents. Nevertheless, the contribution of heteroliganded receptors should be taken into consideration when the receptor is exposed simultaneously to two or more agonists.