Identification of a GABAA receptor anesthetic binding site at subunit interfaces by photolabeling with an etomidate analog

Molecular mechanisms of the GABA type A receptor function

The GABA type A receptor (GABAAR) belongs to the family of pentameric ligand-gated ion channels and plays a key role in inhibition in adult mammalian brains. Dysfunction of this macromolecule may lead to epilepsy, anxiety disorders, autism, depression, and schizophrenia. GABAAR is also a target for multiple physiologically and clinically relevant modulators, such as benzodiazepines (BDZs), general anesthetics, and neurosteroids. The first GABAAR structure appeared in 2014, but the past years have brought a particularly abundant surge in structural data for these receptors with various ligands and modulators. Although the open conformation remains elusive, this novel information has pushed the structure-function studies to an unprecedented level. Electrophysiology, mutagenesis, photolabeling, and in silico simulations, guided by novel structural information, shed new light on the molecular mechanisms of receptor functioning. The main goal of this review is to present the current knowledge of GABAAR functional and structural properties. The review begins with an outline of the functional and structural studies of GABAAR, accompanied by some methodological considerations, especially biophysical methods, enabling the reader to follow how major breakthroughs in characterizing GABAAR features have been achieved. The main section provides a comprehensive analysis of the functional significance of specific structural elements in GABAARs. We additionally summarize the current knowledge on the binding sites for major GABAAR modulators, referring to the molecular underpinnings of their action. The final chapter of the review moves beyond examining GABAAR as an isolated macromolecule and describes the interactions of the receptor with other proteins in a broader context of inhibitory plasticity. In the final section, we propose a general conclusion that agonist binding to the orthosteric binding sites appears to rely on local interactions, whereas conformational transitions of bound macromolecule (gating) and allosteric modulation seem to reflect more global phenomena involving vast portions of the macromolecule.

The Role of GABA Receptors in Anesthesia and Sedation: An Updated Review

GABA (γ-aminobutyric acid) receptors are constituents of many inhibitory synapses within the central nervous system. They are formed by 5 subunits out of 19 various subunits: α1-6, β1-3, γ1-3, δ, ε, θ, π, and ρ1-3. Two main subtypes of GABA receptors have been identified, namely GABAA and GABAB. The GABAA receptor (GABAAR) is formed by a variety of combinations of five subunits, although both α and β subunits must be included to produce a GABA-gated ion channel. Other subunits are γ, δ, ε, π, and ϴ. GABAAR has many isoforms, that dictate, among other properties, their differing affinities and conductance. Drugs acting on GABAAR form the cornerstone of anesthesia and sedation practice. Some such GABAAR agonists used in anesthesia practice are propofol, etomidate, methohexital, thiopental, isoflurane, sevoflurane, and desflurane. Ketamine, nitrous oxide, and xenon are not GABAR agonists and instead inhibit glutamate receptors-mainly NMDA receptors. Inspite of its many drawbacks such as pain in injection, quick and uncontrolled conversion from sedation to general anesthesia and dose-related cardiovascular depression, propofol remains the most popular GABAR agonist employed by anesthesia providers. In addition, being formulated in a lipid emulsion, contamination and bacterial growth is possible. Literature is rife with newer propofol formulations, aiming to address many of these drawbacks, and with some degree of success. A nonemulsion propofol formulation has been developed with cyclodextrins, which form inclusion complexes with drugs having lipophilic properties while maintaining aqueous solubility. Inhalational anesthetics are also GABA agonists. The binding sites are primarily located within α+/β- and β+/α- subunit interfaces, with residues in the α+/γ- interface. Isoflurane and sevoflurane might have slightly different binding sites providing unexpected degree of selectivity. Methoxyflurane has made a comeback in Europe for rapid provision of analgesia in the emergency departments. Penthrox (Galen, UK) is the special device designed for its administration. With better understanding of pharmacology of GABAAR agonists, newer sedative agents have been developed, which utilize "soft pharmacology," a term pertaining to agents that are rapidly metabolized into inactive metabolites after producing desired therapeutic effect(s). These newer "soft" GABAAR agonists have many properties of ideal sedative agents, as they can offer well-controlled, titratable activity and ultrashort action. Remimazolam, a modified midazolam and methoxycarbonyl-etomidate (MOC-etomidate), an ultrashort-acting etomidate analog are two such examples. Cyclopropyl methoxycarbonyl metomidate is another second-generation soft etomidate analog that has a greater potency and longer half-life than MOC-etomidate. Additionally, it might not cause adrenal axis suppression. Carboetomidate is another soft analog of etomidate with low affinity for 11β-hydroxylase and is, therefore, unlikely to have clinically significant adrenocortical suppressant effects. Alphaxalone, a GABAAR agonist, is recently formulated in combination with 7-sulfobutylether-β-cyclodextrin (SBECD), which has a low hypersensitivity profile.

Structural insights into GABAA receptor potentiation by Quaalude

Methaqualone, a quinazolinone marketed commercially as Quaalude, is a central nervous system depressant that was used clinically as a sedative-hypnotic, then became a notorious recreational drug in the 1960s-80s. Due to its high abuse potential, medical use of methaqualone was eventually prohibited, yet it persists as a globally abused substance. Methaqualone principally targets GABAA receptors, which are the major inhibitory neurotransmitter-gated ion channels in the brain. The restricted status and limited accessibility of methaqualone have contributed to its pharmacology being understudied. Here, we use cryo-EM to localize the GABAA receptor binding sites of methaqualone and its more potent derivative, PPTQ, to the same intersubunit transmembrane sites targeted by the general anesthetics propofol and etomidate. Both methaqualone and PPTQ insert more deeply into subunit interfaces than the previously-characterized modulators. Binding of quinazolinones to this site results in widening of the extracellular half of the ion-conducting pore, following a trend among positive allosteric modulators in destabilizing the hydrophobic activation gate in the pore as a mechanism for receptor potentiation. These insights shed light on the underexplored pharmacology of quinazolinones and further elucidate the molecular mechanisms of allosteric GABAA receptor modulation through transmembrane binding sites.

Forty Years Searching for Neurosteroid Binding Sites on GABAA Receptors

Brain inhibition is a vital process for controlling and sculpting the excitability of the central nervous system in healthy individuals. This level of control is provided over several timescales and involves the neurotransmitter GABA acting at inhibitory synapses to: rapidly inhibit neurons by activating the GABAA receptor; over a slower timescale, to tonically activate extrasynaptic GABAA receptors to provide a low level of background inhibition; and finally, to activate G-protein coupled GABAB receptors to control transmitter release by inhibiting presynaptic Ca2+ channels whilst providing postsynaptic inhibition via K+ channel activation. From this plethora of roles for GABA and its receptors, the GABAA receptor isoform is of major interest due to its dynamic functional plasticity, which in part, is due to being targeted by modulatory brain neurosteroids derived from sex and stress hormones. This family of neurosteroids can, depending on their structure, potentiate, activate and also inhibit the activity of GABAA receptors to affect brain inhibition. This review tracks the methods that have been deployed in probing GABAA receptors, and charts the sterling efforts made by several groups to locate the key neurosteroid binding sites that affect these important receptors. Increasing our knowledge of these binding sites will greatly facilitate our understanding of the physiological roles of neurosteroids and will help to advance their use as novel therapeutics to combat debilitating brain diseases.

Substituted Cysteine Modification and Protection with n-Alkyl-MTS Reagents Quantifies Steric Changes Induced by a Mutation in Anesthetic Binding Sites on GABA Type A Receptors

Multiple approaches, including cryogenic electron microscopy (cryo-EM), indicate that the anesthetics etomidate and propofol modulate α1β2/3γ2 GABAA receptors by binding in overlapping transmembrane inter-subunit sites near βM286 and αL232 sidechains. High-precision approaches in functional receptors are needed for comparisons with cryo-EM. We previously used substituted cysteine modification and protection (SCAMP) with n-alkyl-methanethiosulfonate (MTS) reagents and electrophysiology in α1β3M286Cγ2L receptors to estimate the distance from etomidate to β3M286 with precision near 1.3 Å. Here, we address three more aims using this approach: (i) SCAMP with etomidate was tested in α1L232Cβ3γ2L receptors; (ii) studies in α1L232Wβ3M286Cγ2L receptors assessed whether α1L232W displaces etomidate relative to β3M286C; and (iii) results with propofol were compared with those with etomidate. Voltage-clamp electrophysiology in Xenopus oocytes was used to assess persistent functional changes after exposing cysteine-substituted receptors to methyl-MTS through n-decyl-MTS. Overlap of modified cysteine sidechains with bound anesthetic was inferred when anesthetic co-application with alkyl-MTS reagent blocked the development of persistent effects. In α1L232Cβ3γ2L receptors, only pentyl-MTS and hexyl-MTS induced persistent effects that were unaltered by etomidate co-application, precluding a direct estimate of intermolecular distance. In α1L232Wβ3M286Cγ2L receptors, sidechain overlap with bound etomidate was inferred for modifications with ethyl-MTS through n-pentyl-MTS, with unambiguous cut-on and cut-off. Comparison with results in α1β3M286Cγ2L reveals that α1L232W, which increases maximal sidechain length by 2.1 Å, displaces etomidate closer to β3M286C by about 1.3 Å. Propofol results largely mirrored those with etomidate. These findings indicate that both etomidate and propofol bind within 1 Å of α1L232, consistent with cryo-EM structures. SIGNIFICANCE STATEMENT: We combined electrophysiology, cysteine substitutions, and n-alkyl-methanethiosulfonate modifiers in functional GABAA receptors to enable precise estimates of the distance between β3M286C sidechains and anesthetics (etomidate and propofol) bound in transmembrane β+/α- inter-subunit pockets. Comparing results in α1β3M286Cγ2L and α1L232Wβ3M286Cγ2L receptors reveals that α1L232W mutations displace both anesthetics toward β3M286C, indicating that these anesthetics bind within 1 Å of the α1L232 sidechain in functional receptors, consistent with cryogenic electron microscopy structures derived under nonphysiologic conditions.

Chemical, Pharmacological, and Structural Characterization of Novel Acrylamide-Derived Modulators of the GABAA Receptor

Acrylamide-derived compounds have been previously shown to act as modulators of members of the Cys-loop transmitter-gated ion channel family, including the mammalian GABAA receptor. Here we have synthesized and functionally characterized the GABAergic effects of a series of novel compounds (termed "DM compounds") derived from the previously characterized GABAA and the nicotinic α7 receptor modulator (E)-3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2). Fluorescence imaging studies indicated that the DM compounds increase apparent affinity to the transmitter by up to 80-fold in the ternary αβγ GABAA receptor. Using electrophysiology, we show that the DM compounds, and the structurally related (E)-3-furan-2-yl-N-phenylacrylamide (PAM-4), have concurrent potentiating and inhibitory effects that can be isolated and observed under appropriate recording conditions. The potentiating efficacies of the DM compounds are similar to those of neurosteroids and benzodiazepines (ΔG ∼ -1.5 kcal/mol). Molecular docking, functionally confirmed by site-directed mutagenesis experiments, indicate that receptor potentiation is mediated by interactions with the classic anesthetic binding sites located in the transmembrane domain of the intersubunit interfaces. Inhibition by the DM compounds and PAM-4 was abolished in the receptor containing the α1(V256S) mutation, suggestive of similarities in the mechanism of action with that of inhibitory neurosteroids. Functional competition and mutagenesis experiments, however, indicate that the sites mediating inhibition by the DM compounds and PAM-4 differ from those mediating the action of the inhibitory steroid pregnenolone sulfate. SIGNIFICANCE STATEMENT: We have synthesized and characterized the actions of novel acrylamide-derived compounds on the mammalian GABAA receptor. We show that the compounds have concurrent potentiating effects mediated by the classic anesthetic binding sites, and inhibitory actions that bear mechanistic resemblance to but do not share binding sites with, the inhibitory steroid pregnenolone sulfate.

Mutational Analysis of Anesthetic Binding Sites and Their Effects on GABAA Receptor Activation and Modulation by Positive Allosteric Modulators of the α7 Nicotinic Receptor

The positive allosteric modulators (PAMs) of the α7 nicotinic receptor N-(5-Cl-2-hydroxyphenyl)-N'-[2-Cl-5-(trifluoromethyl)phenyl]-urea (NS-1738) and (E)-3-(furan-2-yl)-N-(p-tolyl)-acrylamide (PAM-2) potentiate the α1β2γ2L GABAA receptor through interactions with the classic anesthetic binding sites located at intersubunit interfaces in the transmembrane domain of the receptor. In the present study, we employed mutational analysis to investigate in detail the involvement and contributions made by the individual intersubunit interfaces to receptor modulation by NS-1738 and PAM-2. We show that mutations to each of the anesthetic-binding intersubunit interfaces (β+/α-, α+/β-, and γ+/β-), as well as the orphan α+/γ- interface, modify receptor potentiation by NS-1738 and PAM-2. Furthermore, mutations to any single interface can fully abolish potentiation by the α7-PAMs. The findings are discussed in the context of energetic additivity and interactions between the individual binding sites.

In Vivo Photoadduction of Anesthetic Ligands in Mouse Brain Markedly Extends Sedation and Hypnosis

Photoaffinity ligands are best known as tools used to identify the specific binding sites of drugs to their molecular targets. However, photoaffinity ligands have the potential to further define critical neuroanatomic targets of drug action. In the brains of WT male mice, we demonstrate the feasibility of using photoaffinity ligands in vivo to prolong anesthesia via targeted yet spatially restricted photoadduction of azi-m-propofol (aziPm), a photoreactive analog of the general anesthetic propofol. Systemic administration of aziPm with bilateral near-ultraviolet photoadduction in the rostral pons, at the border of the parabrachial nucleus and locus coeruleus, produced a 20-fold increase in the duration of sedative and hypnotic effects compared with control mice without UV illumination. Photoadduction that missed the parabrachial-coerulean complex also failed to extend the sedative or hypnotic actions of aziPm and was indistinguishable from nonadducted controls. Paralleling the prolonged behavioral and EEG consequences of on target in vivo photoadduction, we conducted electrophysiologic recordings in rostral pontine brain slices. Using neurons within the locus coeruleus to further highlight the cellular consequences of irreversible aziPm binding, we demonstrate transient slowing of spontaneous action potentials with a brief bath application of aziPm that becomes irreversible on photoadduction. Together, these findings suggest that photochemistry-based strategies are a viable new approach for probing CNS physiology and pathophysiology.SIGNIFICANCE STATEMENT Photoaffinity ligands are drugs capable of light-induced irreversible binding, which have unexploited potential to identify the neuroanatomic sites of drug action. We systemically administer a centrally acting anesthetic photoaffinity ligand in mice, conduct localized photoillumination within the brain to covalently adduct the drug at its in vivo sites of action, and successfully enrich irreversible drug binding within a restricted 250 µm radius. When photoadduction encompassed the pontine parabrachial-coerulean complex, anesthetic sedation and hypnosis was prolonged 20-fold, thus illustrating the power of in vivo photochemistry to help unravel neuronal mechanisms of drug action.

Modulation of the mammalian GABAA receptor by type I and type II positive allosteric modulators of the α7 nicotinic acetylcholine receptor

BACKGROUND AND PURPOSE

Positive allosteric modulators of the α7 nicotinic acetylcholine (nACh) receptor (α7-PAMs) possess promnesic and procognitive properties and have potential in the treatment of cognitive and psychiatric disorders including Alzheimer's disease and schizophrenia. Behavioural studies in rodents have indicated that α7-PAMs can also produce antinociceptive and anxiolytic effects that may be associated with positive modulation of the GABAA receptor. The overall goal of this study was to investigate the modulatory actions of selected α7-PAMs on the GABAA receptor.

EXPERIMENTAL APPROACH

We employed a combination of cell fluorescence imaging, electrophysiology, functional competition and site-directed mutagenesis to investigate the functional and structural mechanisms of modulation of the GABAA receptor by three representative α7-PAMs.

KEY RESULTS

We show that the α7-PAMs at micromolar concentrations enhance the apparent affinity of the GABAA receptor for the transmitter and potentiate current responses from the receptor. The compounds were equi-effective at binary αβ and ternary αβγ GABAA receptors. Functional competition and site-directed mutagenesis indicate that the α7-PAMs bind to the classic anaesthetic binding sites in the transmembrane region in the intersubunit interfaces, which results in stabilization of the active state of the receptor.

CONCLUSION AND IMPLICATIONS

We conclude that the tested α7-PAMs are micromolar-affinity, intermediate- to low-efficacy allosteric potentiators of the mammalian αβγ GABAA receptor. Given the similarities in the in vitro sensitivities of the α7 nACh and α1β2γ2L GABAA receptors to α7-PAMs, we propose that doses used to produce nACh receptor-mediated behavioural effects in vivo are likely to modulate GABAA receptor function.

General anesthesia bullies the gut: a toxic relationship with dysbiosis and cognitive dysfunction

Perioperative neurocognitive disorder (PND) is a common surgery outcome affecting up to a third of the elderly patients, and it is associated with high morbidity and increased risk for Alzheimer's disease development. PND is characterized by cognitive impairment that can manifest acutely in the form of postoperative delirium (POD) or after hospital discharge as postoperative cognitive dysfunction (POCD). Although POD and POCD are clinically distinct, their development seems to be mediated by a systemic inflammatory reaction triggered by surgical trauma that leads to dysfunction of the blood-brain barrier and facilitates the occurrence of neuroinflammation. Recent studies have suggested that the gut microbiota composition may play a pivotal role in the PND development by modulating the risk of neuroinflammation establishment. In fact, modulation of gut microbiome composition with pre- and probiotics seems to be effective for the prevention and treatment of PND in animals. Interestingly, general anesthetics seem to have major responsibility on the gut microbiota composition changes following surgery and, consequently, can be an important element in the process of PND initiation. This concept represents an important milestone for the understanding of PND pathogenesis and may unveil new opportunities for the development of preventive or mitigatory strategies against the development of these conditions. The aim of this review is to discuss how anesthetics used in general anesthesia can interact and alter the gut microbiome composition and contribute to PND development by favoring the emergence of neuroinflammation.

State-dependent inhibition of GABA receptor channels by the ectoparasiticide fluralaner

γ-Aminobutyric acid (GABA) receptors (GABARs) are ligand-gated Cl^- channels, which cause an influx of Cl^- that inhibits excitation in postsynaptic cells upon activation. GABARs are important targets for drugs and pest control chemicals. We previously reported that the isoxazoline ectoparasiticide fluralaner inhibits GABA-induced currents in housefly (Musca domestica) GABARs by binding to the putative binding site in the transmembrane subunit interface. In the present study, we investigated whether fluralaner inhibits the GABA response in the GABAR activated state, the resting state, or both, using two-electrode voltage clamp electrophysiology protocols. We found that inhibition progresses over time to steady-state levels by repeated short applications of GABA during fluralaner perfusion. The GABA response was not impaired by fluralaner treatment in the GABAR resting state. However, once inhibited, the GABA response was not restored by repeated applications of GABA. These findings suggest that fluralaner might reach the binding site of the activated conformation of GABARs in a stepwise fashion and tightly bind to it.

Druggable Lipid Binding Sites in Pentameric Ligand-Gated Ion Channels and Transient Receptor Potential Channels

Lipids modulate the function of many ion channels, possibly through direct lipid-protein interactions. The recent outpouring of ion channel structures by cryo-EM has revealed many lipid binding sites. Whether these sites mediate lipid modulation of ion channel function is not firmly established in most cases. However, it is intriguing that many of these lipid binding sites are also known sites for other allosteric modulators or drugs, supporting the notion that lipids act as endogenous allosteric modulators through these sites. Here, we review such lipid-drug binding sites, focusing on pentameric ligand-gated ion channels and transient receptor potential channels. Notable examples include sites for phospholipids and sterols that are shared by anesthetics and vanilloids. We discuss some implications of lipid binding at these sites including the possibility that lipids can alter drug potency or that understanding protein-lipid interactions can guide drug design. Structures are only the first step toward understanding the mechanism of lipid modulation at these sites. Looking forward, we identify knowledge gaps in the field and approaches to address them. These include defining the effects of lipids on channel function in reconstituted systems using asymmetric membranes and measuring lipid binding affinities at specific sites using native mass spectrometry, fluorescence binding assays, and computational approaches.

Discovery and functional characterization of N-(thiazol-2-yl)-benzamide analogs as the first class of selective antagonists of the Zinc-Activated Channel (ZAC)

The Zinc-Activated Channel (ZAC) is an atypical member of the Cys-loop receptor (CLR) superfamily of pentameric ligand-gated ion channels, with its very different endogenous agonists and signalling properties. In this study, a compound library screening at ZAC resulted in the identification of 2-(5-bromo-2-chlorobenzamido)-4-methylthiazole-5-methyl ester (1) as a novel ZAC antagonist. The structural determinants for ZAC activity in 1 were investigated by functional characterization of 61 analogs at ZAC expressed in Xenopus oocytes by two-electrode voltage clamp electrophysiology, and couple of analogs exerting more potent ZAC inhibition than 1 were identified (IC₅₀ values: 1-3 μM). 1 and N-(4-(tert-butyl)thiazol-2-yl)-3-fluorobenzamide (5a, TTFB) were next applied in studies of the functional properties and the mode of action of this novel class of ZAC antagonists. TTFB was a roughly equipotent antagonist of Zn⁺- and H⁺-evoked ZAC signaling and of spontaneous ZAC activity, and the slow on-set of its channel block suggested that its ZAC inhibition is state-dependent. TTFB was found to be a selective ZAC antagonist, exhibiting no significant agonist, antagonist or modulatory activity at 5-HT₃A, α₃β₄ nicotinic acetylcholine, α₁β₂γ_2S GABA_A or α₁ glycine receptors at 30 μM. 1 displayed largely non-competitive antagonism of Zn²⁺-induced ZAC signalling, and TTFB was demonstrated to target the transmembrane and/or intracellular domains of the receptor, which collectively suggests that the N-(thiazol-2-yl)-benzamide analog acts a negative allosteric modulator of ZAC. We propose that this first class of selective ZAC antagonists could constitute useful pharmacological tools in future explorations of the presently poorly elucidated physiological functions governed by this CLR.

Etomidate and its Analogs: A Review of Pharmacokinetics and Pharmacodynamics

Etomidate is a hypnotic agent that is used for the induction of anesthesia. It produces its effect by acting as a positive allosteric modulator on the γ-aminobutyric acid type A receptor and thus enhancing the effect of the inhibitory neurotransmitter γ-aminobutyric acid. Etomidate stands out among other anesthetic agents by having a remarkably stable cardiorespiratory profile, producing no cardiovascular or respiratory depression. However, etomidate suppresses the adrenocortical axis by the inhibition of the enzyme 11β-hydroxylase. This makes the drug unsuitable for administration by a prolonged infusion. It also makes the drug unsuitable for administration to critically ill patients. Etomidate has relatively large volumes of distributions and is rapidly metabolized by hepatic esterases into an inactive carboxylic acid through hydrolyzation. Because of the decrease in popularity of etomidate, few modern extensive pharmacokinetic or pharmacodynamic studies exist. Over the last decade, several analogs of etomidate have been developed, with the aim of retaining its stable cardiorespiratory profile, whilst eliminating its suppressive effect on the adrenocortical axis. One of these molecules, ABP-700, was studied in extensive phase I clinical trials. These found that ABP-700 is characterized by small volumes of distribution and rapid clearance. ABP-700 is metabolized similarly to etomidate, by hydrolyzation into an inactive carboxylic acid. Furthermore, ABP-700 showed a rapid onset and offset of clinical effect. One side effect observed with both etomidate and ABP-700 is the occurrence of involuntary muscle movements. The origin of these movements is unclear and warrants further research.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Mechanisms underlying the selectivity of meta-diamides between insect resistance to dieldrin (RDL) and human γ-aminobutyric acid (GABA) and glycine receptors

BACKGROUND

Meta-diamides [3-benzamido-N-(4-(perfluoropropan-2-yl)phenyl)benzamides] show high insecticide activity by acting as antagonists to the insect resistance to dieldrin (RDL) γ-aminobutyric acid (GABA) receptors. In contrast, low-level antagonist activities of meta-diamides have been demonstrated against the human GABA type A receptor (GABA_A R) α1β2γ2S, mammalian GABA_A R α1β3γ2S, and the human glycine receptor (GlyR) α1β. Glycine residue 336 in the membrane-spanning region M3 of the Drosophila RDL GABA receptor is essential for its high sensitivity to meta-diamide 7, [3-benzamido-N-(2-bromo-4-(perfluoropropan-2-yl)-6-(trifluoromethyl)phenyl)-2-fluorobenzamide].

RESULTS

We examined the effects of an equivalent mutation (M288G) in spontaneously opened human GABA_A R β3 homomers using membrane potential assay. Picrotoxin and fipronil blocked spontaneously opened human GABA_A Rs β3 and β3-M286G in a concentration-dependent manner. In contrast, meta-diamide 7 did not block spontaneously opened GABA_A R β3 homomers, although meta-diamide 7 blocked spontaneously opened GABA_A R β3-M286G homomers. In addition, inhibitory potency of meta-diamide 7 for GABA-induced membrane potential change in cells expressing GABA_A R α1β3-M286G was much higher than that in cells expressing GABA_A R α1β3. In the same way, the equivalent mutation (A288G) in GlyR α1 increased the inhibitory potency of meta-diamide 7 for GlyRs α1 and α1β.

CONCLUSION

Studies substituting an equivalent mutation (M288G) in spontaneously opening human GABA_A R β3 homomers and human GABA_A Rs α1β3 heteromers suggest that M286 in human GABA_A R β3 is important for the low sensitivity to meta-diamide 7. In this study, we summarize the mechanisms underlying the selectivity of meta-diamides between insect RDL and human GABA and glycine receptors. © 2020 Society of Chemical Industry.

Important amino acids for function of the insect Rdl GABA receptor

BACKGROUND

Insect Rdl GABA receptor is an important insecticide target. To design a novel insecticide, studies on the structures of homologous pentameric ligand-gated ion channels provide information about important amino acids that are necessary for the function of insect Rdl GABA receptors.

RESULTS

L9'A, T12'A, T13'A, T13'S, M15'S, and M15'N mutations in the Drosophila Rdl GABA receptor subunit caused the protein to spontaneously adopt the open state conformation. In contrast, the S16'A, S16'T, S17'A, and S17'H mutant homomers showed the same levels of agonist and antagonist sensitivity as the wild-type receptor. The G336M mutation in the Drosophila Rdl GABA receptor abolished the agonist activities of ivermectin and milbemectin, but the F339M mutation did not. Additionally, the F339M mutation caused spontaneous opening of the receptor. In the Drosophila Rdl model, the hydrophobic girdle plays an important role in stabilization of the closed state. Mutations which decrease hydrophobic interactions resulted in spontaneous opening, supporting the importance of the hydrophobic girdle for keeping the channel closed. Through a mutational study of transmembrane 3 (TM3) cytoplasmic domain and Rdl GABA receptor modeling, hydrophobic interactions between TM3 and TM4 and intersubunit interaction were demonstrated to be important for channel gating. Alternatively, the intrasubunit interaction between TM2 and TM3 domains were less important for channel gating in case of Drosophila Rdl GABA receptor.

CONCLUSION

This study demonstrates important amino acids critical to the function of the Drosophila Rdl GABA receptor based on the mutational studies and Drosophila Rdl GABA receptor modeling approach. © 2020 Society of Chemical Industry.

Molecular Determinants Underlying Delta Selective Compound 2 Activity at δ-Containing GABAA Receptors

Delta selective compound 2 (DS2; 4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridin-3-yl]benzamide) is one of the most widely used tools to study selective actions mediated by δ-subunit-containing GABAA receptors. DS2 was discovered over 10 years ago, but despite great efforts, the precise molecular site of action has remained elusive. Using a combination of computational modeling, site-directed mutagenesis, and cell-based pharmacological assays, we probed three potential binding sites for DS2 and analogs at α 4 β 1 δ receptors: an α 4 (+) δ (-) interface site in the extracellular domain (ECD), equivalent to the diazepam binding site in αβγ 2 receptors, and two sites in the transmembrane domain (TMD) - one in the α 4 (+) β 1 (-) and one in the α 4 (-) β 1 (+) interface, with the α 4 (-) β 1 (+) site corresponding to the binding site for etomidate and a recently disclosed low-affinity binding site for diazepam. We show that mutations in the ECD site did not abrogate DS2 modulation. However, mutations in the TMD α 4 (+) β 1 (-) interface, either α 4(S303L) of the α 4 (+) side or β 1(I289Q) of the β 1 (-) side, convincingly disrupted the positive allosteric modulation by DS2. This was consistently demonstrated both in an assay measuring membrane potential changes and by whole-cell patch-clamp electrophysiology and rationalized by docking studies. Importantly, general sensitivity to modulators was not compromised in the mutated receptors. This study sheds important light on the long-sought molecular recognition site for DS2, refutes the misconception that the selectivity of DS2 for δ-containing receptors is caused by a direct interaction with the δ-subunit, and instead points toward a functional selectivity of DS2 and its analogs via a surprisingly well conserved binding pocket in the TMD. SIGNIFICANCE STATEMENT: δ-Containing GABAA receptors represent potential drug targets for the treatment of several neurological conditions with aberrant tonic inhibition, yet no drugs are currently in clinical use. With the identification of the molecular determinants responsible for positive modulation by the known compound delta selective compound 2, the ground is laid for design of ligands that selectively target δ-containing GABAA receptor subtypes, for better understanding of tonic inhibition, and ultimately, for rational development of novel drugs.

Substituted Cysteine Modification and Protection with n-Alkyl- Methanethiosulfonate Reagents Yields a Precise Estimate of the Distance between Etomidate and a Residue in Activated GABA Type A Receptors

The anesthetic etomidate modulates synaptic α1β2/3γ2 GABAA receptors via binding sites located in transmembrane β+/α- interfaces. Various approaches indicate that etomidate binds near β2/3M286 side chains, including recent cryogenic electron microscopy images in α1β2γ2L receptors under nonphysiologic conditions with ∼3.5-Å resolution. We hypothesized that substituted cysteine modification and protection experiments using variably sized n-alkyl-methanethiosulfonate (MTS) reagents could precisely estimate the distance between bound etomidate and β3M286 side chains in activated functional receptors. Using voltage-clamp electrophysiology in Xenopus oocytes expressing α1β3M286Cγ2L GABAA receptors, we measured functional changes after exposing GABA-activated receptors to n-alkyl-MTS reagents, from methyl-MTS to n-decyl-MTS. Based on previous studies using a large sulfhydryl reagent, we anticipated that cysteine modifications large enough to overlap etomidate sites would cause persistently increased GABA sensitivity and decreased etomidate modulation and that etomidate would hinder these modifications, reducing effects. Based on altered GABA or etomidate sensitivity, ethyl-MTS and larger n-alkyl-MTS reagents modified GABA-activated α1β3M286Cγ2L GABAA receptors. Receptor modification by n-propyl-MTS or larger reagents caused persistently increased GABA sensitivity and decreased etomidate modulation. Receptor-bound etomidate blocked β3M286C modification by n-propyl-MTS, n-butyl-MTS, and n-hexyl-MTS. In contrast, GABA sensitivity was unaltered by receptor exposure to methyl-MTS or ethyl-MTS, and ethyl-MTS modification uniquely increased etomidate modulation. These results reveal a "cut-on" between ethyl-MTS and n-propyl-MTS, from which we infer that -S-(n-propyl) is the smallest β3M286C appendage that overlaps with etomidate sites. Molecular models of the native methionine and -S-ethyl and -S-(n-propyl) modified cysteines suggest that etomidate is located between 1.7 and 3.0 Å from the β3M286 side chain. SIGNIFICANCE STATEMENT: Precise spatial relationships between drugs and their receptor sites are essential for mechanistic understanding and drug development. This study combined electrophysiology, a cysteine substitution, and n-alkyl-methanethiosulfonate modifiers, creating a precise molecular ruler to estimate the distance between a α1β3γ2L GABA type A receptor residue and etomidate bound in the transmembrane β+/α- interface.

Structure, Function and Physiology of 5-Hydroxytryptamine Receptors Subtype 3

5-hydroxytryptamine receptor subtype 3 (5-HT₃R) is a pentameric ligand-gated ion channel (pLGIC) involved in neuronal signaling. It is best known for its prominent role in gut-CNS signaling though there is growing interest in its other functions, particularly in modulating non-serotonergic synaptic activity. Recent advances in structural biology have provided mechanistic understanding of 5-HT₃R function and present new opportunities for the field. This chapter gives a broad overview of 5-HT₃R from a physiological and structural perspective and then discusses the specific details of ion permeation, ligand binding and allosteric coupling between these two events. Biochemical evidence is summarized and placed within a physiological context. This perspective underscores the progress that has been made as well as outstanding challenges and opportunities for future 5-HT₃R research.

Discovery of the EL-0052 as a potential anesthetic drug

As a γ-aminobutyric acid A receptor (GABA_AR) inhibitor, etomidate fulfills several characteristics of an ideal anesthetic agent, such as rapid onset with rapid clearance and high potency, along with cardiovascular stability. Unfortunately, etomidate has been reported to inhibit CYP11B1 at hypnotic doses, which is associated with a marked increase in patient deaths due to this unexpected off-target effect. In this study, molecular docking was used to simulate the binding mode of etomidate with GABA_AR and CYP11B1. Based on the in-depth analysis of the binding mode, strong electron-withdrawing group on the C4 position of the imidazole ring was introduced to reduce the charge density of the nitrogen, which is beneficial in reducing the coordination bond between the imidazole nitrogen and heme iron in CYP11B1, as well as in reducing the adrenocortical suppression. Based on the results of ADMET property prediction, MEP analysis, and molecular docking simulation, 4-fluoroetomidate (EL-0052) was designed and synthesized. In vivo studies in rats and mice confirmed that EL-0052 had the efficacy similar to etomidate, but without adrenocortical suppression. These findings suggested that EL-0052 was superior to etomidate and support the continued development of EL-0052 as a preclinical candidate as an anesthetic.

Drug Development in Channelopathies: Allosteric Modulation of Ligand-Gated and Voltage-Gated Ion Channels

Ion channels have been characterized as promising drug targets for treatment of numerous human diseases. Functions of ion channels can be fine-tuned by allosteric modulators, which interact with channels and modulate their activities by binding to sites spatially discrete from those of orthosteric ligands. Positive and negative allosteric modulators have presented a plethora of potential therapeutic advantages over traditionally orthosteric agonists and antagonists in terms of selectivity and safety. This thematic review highlights the discovery of representative allosteric modulators for ligand-gated and voltage-gated ion channels, discussing in particular their identifications, locations, and therapeutic uses in the treatment of a range of channelopathies. Additionally, structures and functions of selected ion channels are briefly described to aid in the rational design of channel modulators. Overall, allosteric modulation represents an innovative targeting approach, and the corresponding modulators provide an abundant but challenging landscape for novel therapeutics targeting ligand-gated and voltage-gated ion channels.

From Methaqualone and Beyond: Structure-Activity Relationship of 6-, 7-, and 8-Substituted 2,3-Diphenyl-quinazolin-4(3H)-ones and in Silico Prediction of Putative Binding Modes of Quinazolin-4(3H)-ones as Positive Allosteric Modulators of GABAA Receptors

Methaqualone (2-methyl-3-(o-tolyl)-quinazolin-4(3H)-one, MTQ) is a moderately potent positive allosteric modulator (PAM) of GABA_A receptors (GABA_ARs). In a previous structure-activity relationship (SAR) study probing the importance of 2- and 3-substituents in the quinazolin-4(3H)-one scaffold, several potent GABA_AR PAMs were identified, including 2,3-diphenylquinazolin-4(3H)-one (PPQ) and 3-(2-chlorophenyl)-2-phenylquinazolin-4(3H)-one (Cl-PPQ). Here, PPQ was applied as lead in a SAR study of 6-, 7-, and 8-substituents in the quinazolin-4(3H)-one by synthesis and functional characterization of 36 PPQ analogs at various GABA_AR subtypes. While none of the new analogs were significantly more potent than PPQ or displayed pronounced subtype selectivity across the GABA_ARs tested, several interesting SAR observations were extracted from the study. In an in silico study, the putative binding modes of MTQ, PPQ, and Cl-PPQ in the transmembrane β₂⁽⁺⁾/α₁^(-) interface of the α₁β₂γ_2S GABA_AR were predicted. Several plausible binding modes were identified for the three PAMs, and rationalization of the molecular basis for their different modulatory potencies was attempted.

Cryo-EM: The Resolution Revolution and Drug Discovery

Single-particle cryogenic electron microscopy (cryo-EM) has been elevated to the mainstream of structural biology propelled by technological advancements in numerous fronts, including imaging analysis and the development of direct electron detectors. The drug discovery field has watched with (initial) skepticism and wonder at the progression of the technique and how it revolutionized the molecular understanding of previously intractable targets. This article critically assesses how cryo-EM has impacted drug discovery in diverse therapeutic areas. Targets that have been brought into the realm of structure-based drug design by cryo-EM and are thus reviewed here include membrane proteins like the GABA_A receptor, several TRP channels, and G protein-coupled receptors, and multiprotein complexes like the ribosomes, the proteasome, and eIF2B. We will describe these studies highlighting the achievements, challenges, and caveats.

Competitive Antagonism of Etomidate Action by Diazepam: In Vitro GABAA Receptor and In Vivo Zebrafish Studies

BACKGROUND

Recent cryo-electron microscopic imaging studies have shown that in addition to binding to the classical extracellular benzodiazepine binding site of the α1β3γ2L γ-aminobutyric acid type A (GABAA) receptor, diazepam also binds to etomidate binding sites located in the transmembrane receptor domain. Because such binding is characterized by low modulatory efficacy, the authors hypothesized that diazepam would act in vitro and in vivo as a competitive etomidate antagonist.

METHODS

The concentration-dependent actions of diazepam on 20 µM etomidate-activated and 6 µM GABA-activated currents were defined (in the absence and presence of flumazenil) in oocyte-expressed α1β3γ2L GABAA receptors using voltage clamp electrophysiology. The ability of diazepam to inhibit receptor labeling of purified α1β3γ2L GABAA receptors by [H]azietomidate was assessed in photoaffinity labeling protection studies. The impact of diazepam (in the absence and presence of flumazenil) on the anesthetic potencies of etomidate and ketamine was compared in a zebrafish model.

RESULTS

At nanomolar concentrations, diazepam comparably potentiated etomidate-activated and GABA-activated GABAA receptor peak current amplitudes in a flumazenil-reversible manner. The half-maximal potentiating concentrations were 39 nM (95% CI, 27 to 55 nM) and 26 nM (95% CI, 16 to 41 nM), respectively. However, at micromolar concentrations, diazepam reduced etomidate-activated, but not GABA-activated, GABAA receptor peak current amplitudes in a concentration-dependent manner with a half-maximal inhibitory concentration of 9.6 µM (95% CI, 7.6 to 12 µM). Diazepam (12.5 to 50 µM) also right-shifted the etomidate-concentration response curve for direct activation without reducing the maximal response and inhibited receptor photoaffinity labeling by [H]azietomidate. When administered with flumazenil, 50 µM diazepam shifted the etomidate (but not the ketamine) concentration-response curve for anesthesia rightward, increasing the etomidate EC50 by 18-fold.

CONCLUSIONS

At micromolar concentrations and in the presence of flumazenil to inhibit allosteric modulation via the classical benzodiazepine binding site of the GABAA receptor, diazepam acts as an in vitro and in vivo competitive etomidate antagonist.

Site-specific effects of neurosteroids on GABAA receptor activation and desensitization

This study examines how site-specific binding to three identified neurosteroid-binding sites in the α₁β₃ GABA_A receptor (GABA_AR) contributes to neurosteroid allosteric modulation. We found that the potentiating neurosteroid, allopregnanolone, but not its inhibitory 3β-epimer epi-allopregnanolone, binds to the canonical β₃(+)–α₁(-) intersubunit site that mediates receptor activation by neurosteroids. In contrast, both allopregnanolone and epi-allopregnanolone bind to intrasubunit sites in the β₃ subunit, promoting receptor desensitization and the α₁ subunit promoting effects that vary between neurosteroids. Two neurosteroid analogues with diazirine moieties replacing the 3-hydroxyl (KK148 and KK150) bind to all three sites, but do not potentiate GABA_AR currents. KK148 is a desensitizing agent, whereas KK150 is devoid of allosteric activity. These compounds provide potential chemical scaffolds for neurosteroid antagonists. Collectively, these data show that differential occupancy and efficacy at three discrete neurosteroid-binding sites determine whether a neurosteroid has potentiating, inhibitory, or competitive antagonist activity on GABA_ARs.

The general anesthetic etomidate and fenamate mefenamic acid oppositely affect GABAAR and GlyR: a structural explanation

GABA and glycine act as inhibitory neurotransmitters in the CNS. Inhibitory neurotransmission is mediated via activation of ionotropic GABA_A and glycine receptors. We used a modeling approach to explain the opposite effects of the general anesthetic etomidate (ETM) and fenamate mefenamic acid (MFA) on GABA- and glycine-activated currents recorded in isolated cerebellar Purkinje cells and hippocampal pyramidal neurons, respectively. These drugs potentiated GABA_ARs but blocked GlyRs. We built a homology model of α1β GlyR based on the cryo-EM structure of open α1 GlyR, used the α1β3γ2 GABA_AR structure from the PDB, and applied Monte-Carlo energy minimization to optimize models of receptors and ligand-receptor complexes. In silico docking suggests that ETM/MFA bind at the transmembrane β( +)/α( -) intersubunit interface in GABA_AR. Our models predict that the bulky side chain of the highly conserved Arg^19' residue at the plus interface side wedges the interface and maintains the conducting receptor state. We hypothesized that MFA/ETM binding at the β( +)/α( -) interface leads to prolongation of receptor life-time in the open state. Having analyzed different GABA_AR and GlyR structures available in the PDB, we found that mutual arrangement of the Arg^19' and Gln^-26' side chains at the plus and minus interface sides, respectively, plays an important role when the receptor switches from the open to closed state. We show that this process is accompanied by narrowing of the intersubunit interfaces, leading to extrusion of the Arg^19' side chain from the interface. Our models allow us to explain the lack of GlyR potentiation in our electrophysiological experiments.

Shared structural mechanisms of general anaesthetics and benzodiazepines

Most general anaesthetics and classical benzodiazepine drugs act through positive modulation of γ-aminobutyric acid type A (GABAA) receptors to dampen neuronal activity in the brain1-5. However, direct structural information on the mechanisms of general anaesthetics at their physiological receptor sites is lacking. Here we present cryo-electron microscopy structures of GABAA receptors bound to intravenous anaesthetics, benzodiazepines and inhibitory modulators. These structures were solved in a lipidic environment and are complemented by electrophysiology and molecular dynamics simulations. Structures of GABAA receptors in complex with the anaesthetics phenobarbital, etomidate and propofol reveal both distinct and common transmembrane binding sites, which are shared in part by the benzodiazepine drug diazepam. Structures in which GABAA receptors are bound by benzodiazepine-site ligands identify an additional membrane binding site for diazepam and suggest an allosteric mechanism for anaesthetic reversal by flumazenil. This study provides a foundation for understanding how pharmacologically diverse and clinically essential drugs act through overlapping and distinct mechanisms to potentiate inhibitory signalling in the brain.

Optical control of neuronal ion channels and receptors

Light-controllable tools provide powerful means to manipulate and interrogate brain function with relatively low invasiveness and high spatiotemporal precision. Although optogenetic approaches permit neuronal excitation or inhibition at the network level, other technologies, such as optopharmacology (also known as photopharmacology) have emerged that provide molecular-level control by endowing light sensitivity to endogenous biomolecules. In this Review, we discuss the challenges and opportunities of photocontrolling native neuronal signalling pathways, focusing on ion channels and neurotransmitter receptors. We describe existing strategies for rendering receptors and channels light sensitive and provide an overview of the neuroscientific insights gained from such approaches. At the crossroads of chemistry, protein engineering and neuroscience, optopharmacology offers great potential for understanding the molecular basis of brain function and behaviour, with promises for future therapeutics.

The mechanisms of potentiation and inhibition of GABAA receptors by non-steroidal anti-inflammatory drugs, mefenamic and niflumic acids

Fenamates mefanamic and niflumic acids (MFA and NFA) induced dual potentiating and inhibitory effects on GABA currents recorded in isolated cerebellar Purkinje cells using the whole-cell patch-clamp and fast-application techniques. Regardless of the concentration, both drugs induced a pronounced prolongation of the current response. We demonstrated that the same concentration of drugs can produce both potentiating and inhibitory effects, depending on the GABA concentration, which indicates that both processes take place simultaneously and the net effect depends on the concentrations of both the agonist and fenamate. We found that the NFA-induced block is strongly voltage-dependent. The Woodhull analysis of the block suggests that NFA has two binding sites in the pore - shallow and deep. We built a homology model of the open GABA_AR based on the cryo-EM structure of the open α1 GlyR and applied Monte-Carlo energy minimization to optimize the ligand-receptor complexes. A systematic search for MFA/NFA binding sites in the GABA_AR pore revealed the existence of two sites, the location of which coincides well with predictions of the Woodhull model. In silico docking suggests that two fenamate molecules are necessary to occlude the pore. We showed that MFA, acting as a PAM, competes with an intravenous anesthetic etomidate for a common binding site. We built structural models of MFA and NFA binding at the transmembrane β(+)/α(-) intersubunit interface. We suggested a hypothesis on the molecular mechanism underlying the prolongation of the receptor lifetime in open state after MFA/NFA binding and β subunit specificity of the fenamate potentiation.

Etomidate and Etomidate Analog Binding and Positive Modulation of γ-Aminobutyric Acid Type A Receptors: Evidence for a State-dependent Cutoff Effect

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Naphthalene-etomidate, an etomidate analog containing a bulky phenyl ring substituent group, possesses very low γ-aminobutyric acid type A (GABAA) receptor efficacy and acts as an anesthetic-selective competitive antagonist. Using etomidate analogs containing phenyl ring substituents groups that range in volume, we tested the hypothesis that this unusual pharmacology is caused by steric hindrance that reduces binding to the receptor's open state.

METHODS

The positive modulatory potencies and efficacies of etomidate and phenyl ring-substituted etomidate analogs were electrophysiology defined in oocyte-expressed α1β3γ2L GABAA receptors. Their binding affinities to the GABAA receptor's two classes of transmembrane anesthetic binding sites were assessed from their abilities to inhibit receptor labeling by the site-selective photolabels [H]azi-etomidate and tritiated R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid.

RESULTS

The positive modulatory activities of etomidate and phenyl ring-substituted etomidate analogs progressively decreased with substituent group volume, reflecting significant decreases in both potency (P = 0.005) and efficacy (P < 0.0001). Affinity for the GABAA receptor's two β - α anesthetic binding sites similarly decreased with substituent group volume (P = 0.003), whereas affinity for the receptor's α - β/γ - β sites did not (P = 0.804). Introduction of the N265M mutation, which is located at the β - α binding sites and renders GABAA receptors etomidate-insensitive, completely abolished positive modulation by naphthalene-etomidate.

CONCLUSIONS

Steric hindrance selectively reduces phenyl ring-substituted etomidate analog binding affinity to the two β - α anesthetic binding sites on the GABAA receptor's open state, suggesting that the binding pocket where etomidate's phenyl ring lies becomes smaller as the receptor isomerizes from closed to open.

Identifying Drugs that Bind Selectively to Intersubunit General Anesthetic Sites in the α1β3γ2 GABAAR Transmembrane Domain

GABAA receptors (GABAARs) are targets for important classes of clinical agents (e.g., anxiolytics, anticonvulsants, and general anesthetics) that act as positive allosteric modulators (PAMs). Previously, using photoreactive analogs of etomidate ([3H]azietomidate) and mephobarbital [[3H]1-methyl-5-allyl-5-(m-trifluoromethyl-diazirynylphenyl)barbituric acid ([3H]R-mTFD-MPAB)], we identified two homologous but pharmacologically distinct classes of general anesthetic binding sites in the α1β3γ2 GABAAR transmembrane domain at β +-α - (β + sites) and α +-β -/γ +-β - (β - sites) subunit interfaces. We now use competition photolabeling with [3H]azietomidate and [3H]R-mTFD-MPAB to identify para-substituted propofol analogs and other drugs that bind selectively to intersubunit anesthetic sites. Propofol and 4-chloro-propofol bind with 5-fold selectivity to β +, while derivatives with bulkier lipophilic substitutions [4-(tert-butyl)-propofol and 4-(hydroxyl(phenyl)methyl)-propofol] bind with ∼10-fold higher affinity to β - sites. Similar to R-mTFD-MPAB and propofol, these drugs bind in the presence of GABA with similar affinity to the α +-β - and γ +-β - sites. However, we discovered four compounds that bind with different affinities to the two β - interface sites. Two of these bind with higher affinity to one of the β - sites than to the β + sites. We deduce that 4-benzoyl-propofol binds with >100-fold higher affinity to the γ +-β - site than to the α +-β - or β +-α - sites, whereas loreclezole, an anticonvulsant, binds with 5- and 100-fold higher affinity to the α +-β - site than to the β + and γ +-β - sites. These studies provide a first identification of PAMs that bind selectively to a single intersubunit site in the GABAAR transmembrane domain, a property that may facilitate the development of subtype selective GABAAR PAMs.

Monod-Wyman-Changeux Allosteric Shift Analysis in Mutant α1β3γ2L GABAA Receptors Indicates Selectivity and Crosstalk among Intersubunit Transmembrane Anesthetic Sites

Propofol, etomidate, and barbiturate anesthetics are allosteric coagonists at pentameric α1β3γ2 GABAA receptors, modulating channel activation via four biochemically established intersubunit transmembrane pockets. Etomidate selectively occupies the two β +/α - pockets, the barbiturate photolabel R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid (R-mTFD-MPAB) occupies homologous α +/β - and γ +/β - pockets, and propofol occupies all four. Functional studies of mutations at M2-15' or M3-36' loci abutting these pockets provide conflicting results regarding their relative contributions to propofol modulation. We electrophysiologically measured GABA-dependent channel activation in α1β3γ2L or receptors with single M2-15' (α1S270I, β3N265M, and γ2S280W) or M3-36' (α1A291W, β3M286W, and γ2S301W) mutations, in the absence and presence of equipotent clinical range concentrations of etomidate, R-mTFD-MPAB, and propofol. Estimated open probabilities were calculated and analyzed using global two-state Monod-Wyman-Changeux models to derive log(d) parameters proportional to anesthetic-induced channel modulating energies (where d is the allosteric anesthetic shift factor). All mutations reduced the log(d) values for anesthetics occupying both abutting and nonabutting pockets. The Δlog(d) values [log(d, mutant) - log(d, wild type)] for M2-15' mutations abutting an anesthetic's biochemically established binding sites were consistently larger than the Δlog(d) values for nonabutting mutations, although this was not true for the M3-36' mutant Δlog(d) values. The sums of the anesthetic-associated Δlog(d) values for sets of M2-15' or M3-36' mutations were all much larger than the wild-type log(d) values. Mutant Δlog(d) values qualitatively reflect anesthetic site occupancy patterns. However, the lack of Δlog(d) additivity undermines quantitative comparisons of distinct site contributions to anesthetic modulation because the mutations impaired both abutting anesthetic binding effects and positive cooperativity between anesthetic binding sites.

Multiple functional neurosteroid binding sites on GABAA receptors

Neurosteroids are endogenous modulators of neuronal excitability and nervous system development and are being developed as anesthetic agents and treatments for psychiatric diseases. While gamma amino-butyric acid Type A (GABA_A) receptors are the primary molecular targets of neurosteroid action, the structural details of neurosteroid binding to these proteins remain ill defined. We synthesized neurosteroid analogue photolabeling reagents in which the photolabeling groups were placed at three positions around the neurosteroid ring structure, enabling identification of binding sites and mapping of neurosteroid orientation within these sites. Using middle-down mass spectrometry (MS), we identified three clusters of photolabeled residues representing three distinct neurosteroid binding sites in the human α₁β₃ GABA_A receptor. Novel intrasubunit binding sites were identified within the transmembrane helical bundles of both the α₁ (labeled residues α₁-N⁴⁰⁸, Y⁴¹⁵) and β₃ (labeled residue β₃-Y⁴⁴²) subunits, adjacent to the extracellular domains (ECDs). An intersubunit site (labeled residues β₃-L²⁹⁴ and G³⁰⁸) in the interface between the β₃(+) and α₁(−) subunits of the GABA_A receptor pentamer was also identified. Computational docking studies of neurosteroid to the three sites predicted critical residues contributing to neurosteroid interaction with the GABA_A receptors. Electrophysiological studies of receptors with mutations based on these predictions (α₁-V²²⁷W, N⁴⁰⁸A/Y⁴¹¹F, and Q²⁴²L) indicate that both the α₁ intrasubunit and β₃-α₁ intersubunit sites are critical for neurosteroid action.

Novel neurosteroid analogue photolabeling reagents identify three specific neurosteroid binding sites on α1β3 GABAA receptors, showing that a site between the α and β subunits, as well as a site within the α-subunit, contribute to neurosteroid-mediated enhancement of GABAA currents.

Neurosteroids are cholesterol metabolites produced by neurons and glial cells that participate in central nervous system (CNS) development, regulate neuronal excitability, and modulate complex behaviors such as mood. Exogenously administered neurosteroid analogues are effective sedative hypnotics and are being developed as antidepressants and anticonvulsants. Gamma amino-butyric acid Type A (GABA_A) receptors, the principal ionotropic inhibitory neurotransmitter receptors in the brain, are the primary functional target of neurosteroids. Understanding the molecular details of neurosteroid interactions with GABA_A receptors is critical to understanding their mechanism of action and developing specific and effective therapeutic agents. In the current study, we developed a suite of neurosteroid analogue affinity labeling reagents, which we used to identify three distinct binding sites on GABA_A receptors and to determine the orientation of neurosteroid binding in each site. Electrophysiological studies performed on receptors with mutations designed to disrupt the identified binding sites showed that two of the three sites contribute to neurosteroid modulation of GABA_A currents. The distinct patterns of neurosteroid affinity, binding orientation, and effect provide the potential for the development of isoform-specific agonists, partial agonists, and antagonists with targeted therapeutic effects.

A structural look at GABAA receptor mutations linked to epilepsy syndromes

Understanding the genetic variation in GABA_A receptor subunit genes (GABRs), GABRA1-6, GABRB1-3, GABRG1-3 and GABRD, in individuals affected by epilepsy may improve the diagnosis and treatment of epilepsy syndromes through identification of disease-associated variants. However, the lack of functional analysis and validation of many novel and previously reported familial and de novo mutations have made it challenging to address meaningful gene associations with epilepsy syndromes. GABA_A receptors belong to the Cys-loop receptor family. Even though GABA_A receptor mutant residues are widespread among different GABRs, their frequent occurrence in important structural domains that share common functional features suggests associations between structure and function.

Cryo-EM structure of the human α1β3γ2 GABAA receptor in a lipid bilayer

Type A γ-aminobutyric acid (GABAA) receptors are pentameric ligand-gated ion channels and the main drivers of fast inhibitory neurotransmission in the vertebrate nervous system1,2. Their dysfunction is implicated in a range of neurological disorders, including depression, epilepsy and schizophrenia3,4. Among the numerous assemblies that are theoretically possible, the most prevalent in the brain are the α1β2/3γ2 GABAA receptors5. The β3 subunit has an important role in maintaining inhibitory tone, and the expression of this subunit alone is sufficient to rescue inhibitory synaptic transmission in β1-β3 triple knockout neurons6. So far, efforts to generate accurate structural models for heteromeric GABAA receptors have been hampered by the use of engineered receptors and the presence of detergents7-9. Notably, some recent cryo-electron microscopy reconstructions have reported 'collapsed' conformations8,9; however, these disagree with the structure of the prototypical pentameric ligand-gated ion channel the Torpedo nicotinic acetylcholine receptor10,11, the large body of structural work on homologous homopentameric receptor variants12 and the logic of an ion-channel architecture. Here we present a high-resolution cryo-electron microscopy structure of the full-length human α1β3γ2L-a major synaptic GABAA receptor isoform-that is functionally reconstituted in lipid nanodiscs. The receptor is bound to a positive allosteric modulator 'megabody' and is in a desensitized conformation. Each GABAA receptor pentamer contains two phosphatidylinositol-4,5-bisphosphate molecules, the head groups of which occupy positively charged pockets in the intracellular juxtamembrane regions of α1 subunits. Beyond this level, the intracellular M3-M4 loops are largely disordered, possibly because interacting post-synaptic proteins are not present. This structure illustrates the molecular principles of heteromeric GABAA receptor organization and provides a reference framework for future mechanistic investigations of GABAergic signalling and pharmacology.

A half century of γ-aminobutyric acid

γ-aminobutyric acid has become one of the most widely known neurotransmitter molecules in the brain over the last 50 years, recognised for its pivotal role in inhibiting neural excitability. It emerged from studies of crustacean muscle and neurons before its significance to the mammalian nervous system was appreciated. Now, after five decades of investigation, we know that most neurons are γ-aminobutyric-acid-sensitive, it is a cornerstone of neural physiology and dysfunction to γ-aminobutyric acid signalling is increasingly documented in a range of neurological diseases. In this review, we briefly chart the neurodevelopment of γ-aminobutyric acid and its two major receptor subtypes: the γ-aminobutyric acidA and γ-aminobutyric acidB receptors, starting from the humble invertebrate origins of being an 'interesting molecule' acting at a single γ-aminobutyric acid receptor type, to one of the brain's most important neurochemical components and vital drug targets for major therapeutic classes of drugs. We document the period of molecular cloning and the explosive influence this had on the field of neuroscience and pharmacology up to the present day and the production of atomic γ-aminobutyric acidA and γ-aminobutyric acidB receptor structures. γ-Aminobutyric acid is no longer a humble molecule but the instigator of rich and powerful signalling processes that are absolutely vital for healthy brain function.

In Search of GABAA Receptor's Neurosteroid Binding Sites

Neurosteroids (NS) are the main modulators of γ-aminobutyric acid type A receptors (GABA_ARs), which are the ligand-gated channels target of the major inhibitory neurotransmitter in vertebrates. As a consequence of their ability to modify inhibitory functions in the brain, NS have high physiological and clinical relevance. Accumulated evidence has strongly suggested that NS binding sites were located in the GABA_AR transmembrane domain; however the specific localization of these sites has remained an enigma for decades. Fortunately, recent resolution of GABA_ARs crystal structures, together with computational strategies applied to investigate the NS binding, has paved the way to rationalizing the molecular basis of NS modulation. This work reviews from a historical perspective the road followed for establishing the GABA_AR/NS binding mode, from their initial molecular modeling to the latest findings. Furthermore, a comparative analysis describing the NS binding is provided, plus a preliminary analysis of putative NS sites in other assemblies.

Design, Synthesis, and Pharmacological Evaluation of Novel β2/3 Subunit-Selective γ-Aminobutyric Acid Type A (GABAA) Receptor Modulators

Subunit-selective modulation of γ-aminobutyric acid type A receptors (GABA_AR) is considered to exert fewer side effects compared to unselective clinically used drugs. Here, the β2/3 subunit-selective GABA_AR modulators valerenic acid (VA) and loreclezole (LOR) guided the synthesis of novel subunit-selective ligands with simplified structures. We studied their effects on GABA_ARs expressed in Xenopus laevis oocytes using two-microelectrode voltage clamp technique. Five compounds showed significantly more efficacious modulation of GABA-evoked currents than VA and LOR with retained potency and selectivity. Compound 18 [( E)-2-Cyano-3-(2,4-dichlorophenyl)but-2-enamide] induced the highest maximal modulation of GABA-induced chloride currents ( E_max: 3114 ± 242%), while 12 [( Z)-3-(2,4-dichlorophenyl)but-2-enenitrile] displayed the highest potency (EC₅₀: 13 ± 2 μM). Furthermore, in hippocampal neurons 12 facilitated phasic and tonic GABAergic inhibition, and in vivo studies revealed significantly more potent protection against pentylenetetrazole (PTZ)-induced seizures compared to VA and LOR. Collectively, compound 12 constitutes a novel, simplified, and subunit-selective GABA_AR modulator with low-dose anticonvulsant activity.

Structural basis of neurosteroid anesthetic action on GABAA receptors

Type A γ-aminobutyric acid receptors (GABA_ARs) are inhibitory pentameric ligand-gated ion channels in the brain. Many anesthetics and neurosteroids act through binding to the GABA_AR transmembrane domain (TMD), but the structural basis of their actions is not well understood and no resting-state GABA_AR structure has been determined. Here, we report crystal structures of apo and the neurosteroid anesthetic alphaxalone-bound desensitized chimeric α1GABA_AR (ELIC-α1GABA_AR). The chimera retains the functional and pharmacological properties of GABA_ARs, including potentiation, activation and desensitization by alphaxalone. The apo-state structure reveals an unconventional activation gate at the intracellular end of the pore. The desensitized structure illustrates molecular determinants for alphaxalone binding to an inter-subunit TMD site. These structures suggest a plausible signaling pathway from alphaxalone binding at the bottom of the TMD to the channel gate in the pore-lining TM2 through the TM1–TM2 linker. The study provides a framework to discover new GABA_AR modulators with therapeutic potential.

The anesthetic alphaxalone binds γ-aminobutyric acid type A receptors (GABA_ARs) that play an important role in regulating sensory processes. Here the authors present the structures of a α1GABA_AR chimera in the resting state and in an alphaxalone-bound desensitized state, which might facilitate the development of new GABA_AR modulators.

Current advances of carbene-mediated photoaffinity labeling in medicinal chemistry

Photoaffinity labeling (PAL) in combination with a chemical probe to covalently bind its target upon UV irradiation has demonstrated considerable promise in drug discovery for identifying new drug targets and binding sites. In particular, carbene-mediated photoaffinity labeling (cmPAL) has been widely used in drug target identification owing to its excellent photolabeling efficiency, minimal steric interference and longer excitation wavelength. Specifically, diazirines, which are among the precursors of carbenes and have higher carbene yields and greater chemical stability than diazo compounds, have proved to be valuable photolabile reagents in a diverse range of biological systems. This review highlights current advances of cmPAL in medicinal chemistry, with a focus on structures and applications for identifying small molecule-protein and macromolecule-protein interactions and ligand-gated ion channels, coupled with advances in the discovery of targets and inhibitors using carbene precursor-based biological probes developed in recent decades.

Alphaxalone Binds in Inner Transmembrane β+-α- Interfaces of α1β3γ2 γ-Aminobutyric Acid Type A Receptors

BACKGROUND

Neurosteroids like alphaxalone are potent anxiolytics, anticonvulsants, amnestics, and sedative-hypnotics, with effects linked to enhancement of γ-aminobutyric acid type A (GABAA) receptor gating in the central nervous system. Data locating neurosteroid binding sites on synaptic αβγ GABAA receptors are sparse and inconsistent. Some evidence points to outer transmembrane β-α interfacial pockets, near sites that bind the anesthetics etomidate and propofol. Other evidence suggests that steroids bind more intracellularly in β-α interfaces.

METHODS

The authors created 12 single-residue β3 cysteine mutations: β3T262C and β3T266C in β3-M2; and β3M283C, β3Y284C, β3M286C, β3G287C, β3F289C, β3V290C, β3F293C, β3L297C, β3E298C, and β3F301C in β3-M3 helices. The authors coexpressed α1 and γ2L with each mutant β3 subunit in Xenopus oocytes and electrophysiologically tested each mutant for covalent sulfhydryl modification by the water-soluble reagent para-chloromercuribenzenesulfonate. Then, the authors assessed whether receptor-bound alphaxalone, etomidate, or propofol blocked cysteine modification, implying steric hindrance.

RESULTS

Eleven mutant β3 subunits, when coexpressed with α1 and γ2L, formed functional channels that displayed varied sensitivities to the three anesthetics. Exposure to para-chloromercuribenzenesulfonate produced irreversible functional changes in ten mutant receptors. Protection by alphaxalone was observed in receptors with β3V290C, β3F293C, β3L297C, or β3F301C mutations. Both etomidate and propofol protected receptors with β3M286C or β3V290C mutations. Etomidate also protected β3F289C. In α1β3γ2L structural homology models, all these protected residues are located in transmembrane β-α interfaces.

CONCLUSIONS

Alphaxalone binds in transmembrane β-α pockets of synaptic GABAA receptors that are adjacent and intracellular to sites for the potent anesthetics etomidate and propofol.

GABAA receptor: Positive and negative allosteric modulators

gamma-Aminobutyric acid (GABA)-mediated inhibitory neurotransmission and the gene products involved were discovered during the mid-twentieth century. Historically, myriad existing nervous system drugs act as positive and negative allosteric modulators of these proteins, making GABA a major component of modern neuropharmacology, and suggesting that many potential drugs will be found that share these targets. Although some of these drugs act on proteins involved in synthesis, degradation, and membrane transport of GABA, the GABA receptors Type A (GABAAR) and Type B (GABABR) are the targets of the great majority of GABAergic drugs. This discovery is due in no small part to Professor Norman Bowery. Whereas the topic of GABABR is appropriately emphasized in this special issue, Norman Bowery also made many insights into GABAAR pharmacology, the topic of this article. GABAAR are members of the ligand-gated ion channel receptor superfamily, a chloride channel family of a dozen or more heteropentameric subtypes containing 19 possible different subunits. These subtypes show different brain regional and subcellular localization, age-dependent expression, and potential for plastic changes with experience including drug exposure. Not only are GABAAR the targets of agonist depressants and antagonist convulsants, but most GABAAR drugs act at other (allosteric) binding sites on the GABAAR proteins. Some anxiolytic and sedative drugs, like benzodiazepine and related drugs, act on GABAAR subtype-dependent extracellular domain sites. General anesthetics including alcohols and neurosteroids act at GABAAR subunit-interface trans-membrane sites. Ethanol at high anesthetic doses acts on GABAAR subtype-dependent trans-membrane domain sites. Ethanol at low intoxicating doses acts at GABAAR subtype-dependent extracellular domain sites. Thus GABAAR subtypes possess pharmacologically specific receptor binding sites for a large group of different chemical classes of clinically important neuropharmacological agents. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Optoanesthesia: Use of Anesthetic Photolabels In Vivo

Investigation of how anesthetics produce hypnosis requires knowledge of their effects at the molecular, neuronal, circuit, and whole-brain network level. Anesthetic photolabels have long been used to explore how anesthetics bind and affect known protein targets, but they could potentially assist in investigation of anesthetic effects at higher organizational levels of the central nervous system. Here, we advocate the use and provide detailed methods for the application of anesthetic photolabels in slice electrophysiology and in intact animals as a means of investigating anesthetic effects on distinct circuits and brain centers.

Native System and Cultured Cell Electrophysiology for Investigating Anesthetic Mechanisms

Anesthetic agents interact with a variety of ion channels and membrane-bound receptors, often at agent-specific binding sites of a single protein. These molecular-level interactions are ultimately responsible for producing the clinically anesthetized state. Between these two scales of effect, anesthetic agents can be studied in terms of how they impact the physiology of neuronal circuits, individual neurons, and cells expressing individual receptor types. The acutely dissected hippocampal slice is one of the most extensively studied and characterized preparations of intact neural tissue and serves as a highly useful experimental model system to test hypotheses of anesthetic mechanisms. Specific agent-receptor interactions and their effect on excitable membranes can further be defined with molecular precision in cell-based expression systems. We highlight several approaches in these respective systems that we have used and that also have been used by many investigators worldwide. We emphasize economy and quality control, to allow an experimenter to carry out these types of studies in a rigorous and efficient manner.

Combining Mutations and Electrophysiology to Map Anesthetic Sites on Ligand-Gated Ion Channels

General anesthetics are known to act in part by binding to and altering the function of pentameric ligand-gated ion channels such as nicotinic acetylcholine and γ-aminobutyric acid type A receptors. Combining heterologous expression of the subunits that assemble to form these ion channels, mutagenesis techniques and voltage-clamp electrophysiology have enabled a variety of "structure-function" approaches to questions of where anesthetic binds to these ion channels and how they enhance or inhibit channel function. Here, we review the evolution of concepts and experimental strategies during the last three decades, since molecular biological and electrophysiological tools became widely used. Topics covered include: (1) structural models as interpretive frameworks, (2) various electrophysiological approaches and their limitations, (3) Monod-Wyman-Changeux allosteric models as functional frameworks, (4) structural strategies including chimeras and point mutations, and (5) methods based on cysteine substitution and covalent modification. We discuss in particular depth the experimental design considerations for substituted cysteine modification-protection studies.