Spontaneous mobility of GABAA receptor M2 extracellular half relative to noncompetitive antagonist action

Structures of the ISWI-nucleosome complex reveal a conserved mechanism of chromatin remodeling

Chromatin remodelers are diverse enzymes, and different models have been proposed to explain how these proteins work. Here we report the 3.3 Å-resolution cryogenic electron microscopy (cryo-EM) structures of Saccharomyces cerevisiae ISWI (ISW1) in complex with the nucleosome in adenosine diphosphate (ADP)-bound and ADP-BeF_x-bound states. The data show that after nucleosome binding, ISW1 is activated by substantial rearrangement of the catalytic domains, with the regulatory AutoN domain packing the first RecA-like core and the NegC domain being disordered. The high-resolution structure reveals local DNA distortion and translocation induced by ISW1 in the ADP-bound state, which is essentially identical to that induced by the Snf2 chromatin remodeler, suggesting a common mechanism of DNA translocation. The histone core remains largely unperturbed, and prevention of histone distortion by crosslinking did not inhibit the activity of yeast ISW1 or its human homolog. Together, our findings suggest a general mechanism of chromatin remodeling involving local DNA distortion without notable histone deformation.

Identification of a possible secondary picrotoxin-binding site on the GABA(A) receptor

The type A GABA receptors (GABARs) are ligand-gated ion channels (LGICs) found in the brain and are the major inhibitory neurotransmitter receptors. Upon binding of an agonist, the GABAR opens and increases the intraneuronal concentration of chloride ions, thus hyperpolarizing the cell and inhibiting the transmission of the nerve action potential. GABARs also contain many other modulatory binding pockets that differ from the agonist-binding site. The composition of the GABAR subunits can alter the properties of these modulatory sites. Picrotoxin is a noncompetitive antagonist for LGICs, and by inhibiting GABAR, picrotoxin can cause overstimulation and induce convulsions. We use addition of picrotoxin to probe the characteristics and possible mechanism of an additional modulatory pocket located at the interface between the ligand-binding domain and the transmembrane domain of the GABAR. Picrotoxin is widely regarded as a pore-blocking agent that acts at the cytoplasmic end of the channel. However, there are also data to suggest that there may be an additional, secondary binding site for picrotoxin. Through homology modeling, molecular docking, and molecular dynamics simulations, we show that binding of picrotoxin to this interface pocket correlates with these data, and negative modulation occurs at the pocket via a kinking of the pore-lining helices into a more closed orientation.

Molecular dynamics investigation of Cl(-) transport through the closed and open states of the 2α12β2γ2 GABA(A) receptor

The α1β2γ2 gamma-aminobutyric type A receptor (GABA(A)R) is one of the most widely expressed GABA(A)R subtypes in the mammalian brain. GABA(A)Rsbelonging to the Cys-loop superfamily of ligand-gated ion channels have been identified as key targets for many clinical drugs, and the motions that govern the gating mechanism are still not well understood. In this study, an open-state GABA(A)R was constructed using the structure of the glutamate-gated chloride channel (GluCl), which has a high sequence identity to GABA(A)R. A closed-state model was constructed using the structure of the nicotinic acetylcholine receptor (nAChR). Molecular dynamics simulations of the open-state and closed-state GABA(A)R were performed. We calculated the electrostatic potential of the two conformations, the pore radius of the two ion channels and the root-mean-square fluctuation. We observed the presence of two positively charged girdles around the ion channel and found flexible regions in the GABA(A)R. Then, the free-energy of chloride ion permeations through the closed-state and open-state G GABA(A)R has been estimated using adaptive biasing force (ABF) simulation. For the closed-state G GABA(A)R, we observed two major energy barriers for chloride ion translocation in the transmembrane domain (TMD). For the open-state GABA(A)R, there was only one energy barrier formed by two Thr261 (α1), two Thr255 (β2) and one Thr271 (γ2). By using ABF simulation, the overall free-energy profile is obtained for Cl(-) transporting through GABA(A)R, which gives a complete map of the ion channel of Cl(-) permeation.

Experimental determination of the vertical alignment between the second and third transmembrane segments of muscle nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChR) are members of the Cys-loop ligand-gated ion channel superfamily. Muscle nAChR are heteropentamers that assemble from two α, and one each of β, γ, and δ subunits. Each subunit is composed of three domains, extracellular, transmembrane and intracellular. The transmembrane domain consists of four α-helical segments (M1-M4). Pioneering structural information was obtained using electronmicroscopy of Torpedo nAChR. The recently solved X-ray structure of the first eukaryotic Cys-loop receptor, a truncated (intracellular domain missing) glutamate-gated chloride channel α (GluClα) showed the same overall architecture. However, a significant difference with regard to the vertical alignment between the channel-lining segment M2 and segment M3 was observed. Here, we used functional studies utilizing disulfide trapping experiments in muscle nAChR to determine the spatial orientation between M2 and M3. Our results are in agreement with the vertical alignment as obtained when using the GluClα structure as a template to homology model muscle nAChR, however, they cannot be reconciled with the current Torpedo nAChR model. The vertical M2-M3 alignments as observed in X-ray structures of prokaryotic Gloeobacter violaceus ligand-gated ion channel and GluClα are in agreement. Our results further confirm that this alignment in Cys-loop receptors is conserved between prokaryotes and eukaryotes.

Neuroactive insecticides: targets, selectivity, resistance, and secondary effects

Neuroactive insecticides are the principal means of protecting crops, people, livestock, and pets from pest insect attack and disease transmission. Currently, the four major nerve targets are acetylcholinesterase for organophosphates and methylcarbamates, the nicotinic acetylcholine receptor for neonicotinoids, the γ-aminobutyric acid receptor/chloride channel for polychlorocyclohexanes and fiproles, and the voltage-gated sodium channel for pyrethroids and dichlorodiphenyltrichloroethane. Species selectivity and acquired resistance are attributable in part to structural differences in binding subsites, receptor subunit interfaces, or transmembrane regions. Additional targets are sites in the sodium channel (indoxacarb and metaflumizone), the glutamate-gated chloride channel (avermectins), the octopamine receptor (amitraz metabolite), and the calcium-activated calcium channel (diamides). Secondary toxic effects in mammals from off-target serine hydrolase inhibition include organophosphate-induced delayed neuropathy and disruption of the cannabinoid system. Possible associations between pesticides and Parkinson's and Alzheimer's diseases are proposed but not established based on epidemiological observations and mechanistic considerations.

Influences on blockade by t-butylbicyclo-phosphoro-thionate of GABA(A) receptor spontaneous gating, agonist activation and desensitization

Picrotoxin and t-butylbicyclophosphorothionate (TBPS) are GABA(A) receptor (GABA(A)R) open channel blockers. However, picrotoxin displaceable [(35)S]TBPS binding to α1β2γ2 GABA(A)Rs occurs in the absence of GABA, suggesting that access to the binding site is independent of activation. Alternatively, spontaneous gating may provide access to the channel. In the absence of episodic GABA application, picrotoxin and TBPS blocked (by 91 ± 3% and 85 ± 5%, respectively) GABA-evoked currents mediated by α1β2γ2 receptors. We used two approaches to inhibit spontaneous GABA(A)R gating, bicuculline, which inhibits spontaneous current in the absence of exogenous agonist and the α1(K278M) mutant subunit. Whole-cell patch-clamp recordings demonstrated that α1(K278M)β2γ2 receptors have negligible spontaneous gating. Application of bicuculline to α1β2γ2 receptors in the absence of exogenous GABA caused a 35% reduction of current blockade by TBPS and reduced [(35)S]TBPS binding by 25%. Consistent with this, in the absence of exogenous GABA, α1(K278M)β2γ2 receptors exhibited reduced blockade by TBPS current compared to wild-type receptors. These data suggest that a decrease in spontaneous gating reduces accessibility of TBPS to its binding site. GABA application during picrotoxin or TBPS administration enhanced α1β2γ2 receptor blockade (to 98% in both cases). The GABA-dependent component of TBPS blockade accounts for the stimulation of [(35)S]TBPS binding to α1β2γ2 receptors seen with GABA (1 μm) application. Moreover, application of GABA at concentrations that cause significant steady-state desensitization reduced [(35)S]TBPS binding. The α1(K278M) subunit slowed desensitization kinetics and increased the rate of deactivation of GABA-evoked currents. Furthermore, there was a marked increase in the GABA EC(50) for desensitization of α1(K278M)β2γ2 receptors associated with a large increase in the GABA-dependent stimulation of [(35)S]TBPS binding. These data establish a relationship between GABA(A)R function and the three phases of [(35)S]TBPS binding seen in the absence and the presence of GABA.

Binding modes of noncompetitive GABA-channel blockers revisited using engineered affinity-labeling reactions combined with new docking studies

The binding modes of noncompetitive GABA(A)-channel blockers were re-examined taking into account the recent description of the 3D structure of prokaryotic pentameric ligand-gated ion channels, which provided access to new mammalian or insect GABA receptor models, emphasizing their transmembrane portion. Two putative binding modes were deciphered for this class of compounds, including the insecticide fipronil, located nearby either the intra- or the extracellular part of the membrane, respectively. These results are in full agreement with previously described affinity-labeling reactions performed with GABA(A) noncompetitive blockers (Perret et al. J. Biol. Chem.1999, 274, 25350-25354).

GABAA receptor open-state conformation determines non-competitive antagonist binding

The γ-aminobutyric acid (GABA) type A receptor (GABA(A)R) is one of the most important targets for insecticide action. The human recombinant β3 homomer is the best available model for this binding site and 4-n-[(3)H]propyl-4'-ethynylbicycloorthobenzoate ([(3)H]EBOB) is the preferred non-competitive antagonist (NCA) radioligand. The uniquely high sensitivity of the β3 homomer relative to the much-less-active but structurally very-similar β1 homomer provides an ideal comparison to elucidate structural and functional features important for NCA binding. The β1 and β3 subunits were compared using chimeragenesis and mutagenesis and various combinations with the α1 subunit and modulators. Chimera β3/β1 with the β3 subunit extracellular domain and the β1 subunit transmembrane helices retained the high [(3)H]EBOB binding level of the β3 homomer while chimera β1/β3 with the β1 subunit extracellular domain and the β3 subunit transmembrane helices had low binding activity similar to the β1 homomer. GABA at 3μM stimulated heteromers α1β1 and α1β3 binding levels more than 2-fold by increasing the open probability of the channel. Addition of the α1 subunit rescued the inactive β1/β3 chimera close to wildtype α1β1 activity. EBOB binding was significantly altered by mutations β1S15'N and β3N15'S compared with wildtype β1 and β3, respectively. However, the binding activity of α1β1S15'N was insensitive to GABA and α1β3N15'S was stimulated much less than wildtype α1β3 by GABA. The inhibitory effect of etomidate on NCA binding was reduced more than 5-fold by the mutation β3N15'S. Therefore, the NCA binding site is tightly regulated by the open-state conformation that largely determines GABA(A) receptor sensitivity.

Genetic polymorphisms in organic cation transporter 1 (OCT1) in Chinese and Japanese populations exhibit altered function

Organic cation transporter 1 (OCT1; SLC22A1) seems to play a role in the efficacy and disposition of the widely used antidiabetic drug metformin. Genetic variants in OCT1 have been identified largely in European populations. Metformin is increasingly being used in Asian populations where the incidence of type 2 diabetes (T2D) is on the rise. The goal of this study is to identify genetic variants of OCT1 in Chinese and Japanese populations, which may potentially modulate response to metformin. We used recent data from the 1000 Genomes Project (Chinese and Japanese) and direct sequencing of selected amplicons of OCT1 in 66 DNA samples from Japanese patients with T2D. A total of six nonsynonymous variants were identified. Three of them (Q97K, P117L, and R206C) had not been functionally characterized previously and had allele frequencies of 0.017, 0.023 and 0.008, respectively. The uptake of metformin in cells expressing Q97K, P117L, and R206C was significantly reduced relative to the OCT1 reference (62 ± 4.3, 55 ± 6.8, and 22 ± 1.5% for Q97K, P117L, and R206C, respectively). Kinetic studies indicated that P117L and R206C exhibited a reduced V(max), whereas Q97K showed an increased K(m). The green fluorescent protein (GFP)-tagged Q97K and P117L variants localized to the plasma membrane, whereas the GFP-tagged R206C was retained mainly in the endoplasmic reticulum. Replacement of the highly conserved R206 with different amino acids modulated the subcellular localization and function of the transporter. This study suggests that nonsynonymous variants of OCT1 in Chinese and Japanese populations may affect the differential response to metformin.

Ginkgolide X is a potent antagonist of anionic Cys-loop receptors with a unique selectivity profile at glycine receptors

The novel ginkgolide analog ginkgolide X was characterized functionally at human glycine and gamma-aminobutyric acid type A receptors (GlyRs and GABA(A)Rs, respectively) in the fluorescence-based FLIPR(TM) Membrane Potential assay. The compound inhibited the signaling of all GABA(A)R subtypes included in the study with high nanomolar/low micromolar IC(50) values, except the rho 1 receptor at which it was a significantly weaker antagonist. Ginkgolide X also displayed high nanomolar/low micromolar IC(50) values at the homomeric alpha1 and alpha2 GlyRs, whereas it was inactive at the heteromeric alpha 1 beta and alpha 2 beta subtypes at concentrations up to 300 microm. Thus, the functional properties of the compound were significantly different from those of the naturally occurring ginkgolides A, B, C, J, and M but similar to those of picrotoxin. In a mutagenesis study the 6' M2 residues in the GlyR ion channel were identified as the primary molecular determinant of the selectivity profile of ginkgolide X, and a 6' M2 ring consisting of five Thr residues was found to be of key importance for its activity at the GABA(A)R. Conformational analysis and docking of low-energy conformations of the native ginkgolide A and ginkgolide X into a alpha1 GlyR homology model revealed two distinct putative binding sites formed by the 6' M2 residues together with the 2' residues and the 10' and 13' residues, respectively. Thus, we propose that the distinct functionalities of ginkgolide X compared with the other ginkgolides could arise from different flexibility and thus different binding modes to the ion channel of the anionic Cys-loop receptor.

The insecticide fipronil and its metabolite fipronil sulphone inhibit the rat alpha1beta2gamma2L GABA(A) receptor

BACKGROUND AND PURPOSE

Fipronil is the active ingredient in a number of widely used insecticides. Human exposure to fipronil leads to symptoms (headache, nausea and seizures) typically associated with the antagonism of GABA(A) receptors in the brain. In this study, we have examined the modulation of the common brain GABA(A) receptor subtype by fipronil and its major metabolite, fipronil sulphone.

EXPERIMENTAL APPROACH

Whole-cell and single-channel recordings were made from HEK 293 cells transiently expressing rat alpha1beta2gamma2L GABA(A) receptors.

KEY RESULTS

The major effect of fipronil was to increase the rate of current decay in macroscopic recordings. In single-channel recordings, the presence of fipronil resulted in shorter cluster durations without affecting the intracluster open and closed time distributions or the single-channel conductance. The alpha1V256S mutation, previously shown alleviate channel inhibition by inhibitory steroids and several insecticides, had a relatively small effect on channel block by fipronil. The mode of action of fipronil sulphone was similar to that of its parent compound but the metabolite was less potent at inhibiting the alpha1beta2gamma2L receptor.

CONCLUSIONS AND IMPLICATIONS

We conclude that exposure to fipronil induces accumulation of receptors in a novel, long-lived blocked state. This process proceeds in parallel with and independently of, channel desensitization. The lower potency of fipronil sulphone indicates that the conversion serves as a detoxifying process in mammalian brain.