Identification of propofol binding sites in a nicotinic acetylcholine receptor with a photoreactive propofol analog

Current State of Potential Mechanisms Supporting Low Intensity Focused Ultrasound for Neuromodulation

Low intensity focused ultrasound (LIFU) has been gaining traction as a non-invasive neuromodulation technology due to its superior spatial specificity relative to transcranial electrical/magnetic stimulation. Despite a growing literature of LIFU-induced behavioral modifications, the mechanisms of action supporting LIFU's parameter-dependent excitatory and suppressive effects are not fully understood. This review provides a comprehensive introduction to the underlying mechanics of both acoustic energy and neuronal membranes, defining the primary variables for a subsequent review of the field's proposed mechanisms supporting LIFU's neuromodulatory effects. An exhaustive review of the empirical literature was also conducted and studies were grouped based on the sonication parameters used and behavioral effects observed, with the goal of linking empirical findings to the proposed theoretical mechanisms and evaluating which model best fits the existing data. A neuronal intramembrane cavitation excitation model, which accounts for differential effects as a function of cell-type, emerged as a possible explanation for the range of excitatory effects found in the literature. The suppressive and other findings need additional theoretical mechanisms and these theoretical mechanisms need to have established relationships to sonication parameters.

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.

Identifying c-fos Expression as a Strategy to Investigate the Actions of General Anesthetics on the Central Nervous System

Although general anesthetics have been used in the clinic for more than 170 years, the ways in which they induce amnesia, unconsciousness, analgesia, and immobility remain elusive. Modulations of various neural nuclei and circuits are involved in the actions of general anesthetics. The expression of the immediate-early gene c-fos and its nuclear product, c-fos protein, can be induced by neuronal depolarization; therefore, c-fos staining is commonly used to identify the activated neurons during sleep and/or wakefulness, as well as in various physiological conditions in the central nervous system. Identifying c-fos expression is also a direct and convenient method to explore the effects of general anesthetics on the activity of neural nuclei and circuits. Using c-fos staining, general anesthetics have been found to interact with sleep- and wakefulness-promoting systems throughout the brain, which may explain their ability to induce unconsciousness and emergence from general anesthesia. This review summarizes the actions of general anesthetics on neural nuclei and circuits based on a c-fos expression.

Menthol Binding to the Human α4β2 Nicotinic Acetylcholine Receptor Facilitated by Its Strong Partitioning in the Membrane

We utilize various computational methodologies to study menthol's interaction with multiple organic phases, a lipid bilayer, and the human α4β2 nicotinic acetylcholine receptor (nAChR), the most abundant nAChR in the brain. First, force field parameters developed for menthol are validated in alchemical free energy perturbation simulations to calculate solvation free energies of menthol in water, dodecane, and octanol and compare the results against experimental data. Next, umbrella sampling is used to construct the free energy profile of menthol permeation across a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. The results from a flooding simulation designed to study the water-membrane partitioning of menthol in a POPC lipid bilayer are used to determine the penetration depth and the preferred orientation of menthol in the bilayer. Finally, employing both docking and flooding simulations, menthol is shown to bind to different sites on the human α4β2 nAChR. The most likely binding mode of menthol to a desensitized membrane-embedded α4β2 nAChR is identified to be via a membrane-mediated pathway in which menthol binds to the sites at the lipid-protein interface after partitioning in the membrane. A rare but distinct binding mode in which menthol binds to the extracellular opening of receptor's ion permeation pore is also reported.

Structural Basis for a Bimodal Allosteric Mechanism of General Anesthetic Modulation in Pentameric Ligand-Gated Ion Channels

Ion channel modulation by general anesthetics is a vital pharmacological process with implications for receptor biophysics and drug development. Functional studies have implicated conserved sites of both potentiation and inhibition in pentameric ligand-gated ion channels, but a detailed structural mechanism for these bimodal effects is lacking. The prokaryotic model protein GLIC recapitulates anesthetic modulation of human ion channels, and it is accessible to structure determination in both apparent open and closed states. Here, we report ten X-ray structures and electrophysiological characterization of GLIC variants in the presence and absence of general anesthetics, including the surgical agent propofol. We show that general anesthetics can allosterically favor closed channels by binding in the pore or favor open channels via various subsites in the transmembrane domain. Our results support an integrated, multi-site mechanism for allosteric modulation, and they provide atomic details of both potentiation and inhibition by one of the most common general anesthetics.

Electrophysiological-mechanical coupling in the neuronal membrane and its role in ultrasound neuromodulation and general anaesthesia

The current understanding of the role of the cell membrane is in a state of flux. Recent experiments show that conventional models, considering only electrophysiological properties of a passive membrane, are incomplete. The neuronal membrane is an active structure with mechanical properties that modulate electrophysiology. Protein transport, lipid bilayer phase, membrane pressure and stiffness can all influence membrane capacitance and action potential propagation. A mounting body of evidence indicates that neuronal mechanics and electrophysiology are coupled, and together shape the membrane potential in tight coordination with other physical properties. In this review, we summarise recent updates concerning electrophysiological-mechanical coupling in neuronal function. In particular, we aim at making the link with two relevant yet often disconnected fields with strong clinical potential: the use of mechanical vibrations-ultrasound-to alter the electrophysiogical state of neurons, e.g., in neuromodulation, and the theories attempting to explain the action of general anaesthetics. STATEMENT OF SIGNIFICANCE: General anaesthetics revolutionised medical practice; now an apparently unrelated technique, ultrasound neuromodulation-aimed at controlling neuronal activity by means of ultrasound-is poised to achieve a similar level of impact. While both technologies are known to alter the electrophysiology of neurons, the way they achieve it is still largely unknown. In this review, we argue that in order to explain their mechanisms/effects, the neuronal membrane must be considered as a coupled mechano-electrophysiological system that consists of multiple physical processes occurring concurrently and collaboratively, as opposed to sequentially and independently. In this framework the behaviour of the cell membrane is not the result of stereotypical mechanisms in isolation but instead emerges from the integrative behaviour of a complexly coupled multiphysics system.

Towards a Comprehensive Understanding of Anesthetic Mechanisms of Action: A Decade of Discovery

Significant progress has been made in the 21st century towards a comprehensive understanding of the mechanisms of action of general anesthetics, coincident with progress in structural biology and molecular, cellular, and systems neuroscience. This review summarizes important new findings that include target identification through structural determination of anesthetic binding sites, details of receptors and ion channels involved in neurotransmission, and the critical roles of neuronal networks in anesthetic effects on memory and consciousness. These recent developments provide a comprehensive basis for conceptualizing pharmacological control of amnesia, unconsciousness, and immobility.

A photoreactive analog of allopregnanolone enables identification of steroid-binding sites in a nicotinic acetylcholine receptor

Many neuroactive steroids potently and allosterically modulate pentameric ligand-gated ion channels, including GABA_A receptors (GABA_AR) and nicotinic acetylcholine receptors (nAChRs). Allopregnanolone and its synthetic analog alphaxalone are GABA_AR-positive allosteric modulators (PAMs), whereas alphaxalone and most neuroactive steroids are nAChR inhibitors. In this report, we used 11β-(p-azidotetrafluorobenzoyloxy)allopregnanolone (F₄N₃Bzoxy-AP), a general anesthetic and photoreactive allopregnanolone analog that is a potent GABA_AR PAM, to characterize steroid-binding sites in the Torpedo α₂βγδ nAChR in its native membrane environment. We found that F₄N₃Bzoxy-AP (IC₅₀ = 31 μm) is 7-fold more potent than alphaxalone in inhibiting binding of the channel blocker [³H]tenocyclidine to nAChRs in the desensitized state. At 300 μm, neither steroid inhibited binding of [³H]tetracaine, a closed-state selective channel blocker, or of [³H]acetylcholine. Photolabeling identified three distinct [³H]F₄N₃Bzoxy-AP-binding sites in the nAChR transmembrane domain: 1) in the ion channel, identified by photolabeling in the M2 helices of βVal-261 and δVal-269 (position M2-13'); 2) at the interface between the αM1 and αM4 helices, identified by photolabeling in αM1 (αCys-222/αLeu-223); and 3) at the lipid-protein interface involving γTrp-453 (M4), a residue photolabeled by small lipophilic probes and promegestone, a steroid nAChR antagonist. Photolabeling in the ion channel and αM1 was higher in the nAChR-desensitized state than in the resting state and inhibitable by promegestone. These results directly indicate a steroid-binding site in the nAChR ion channel and identify additional steroid-binding sites also occupied by other lipophilic nAChR antagonists.

Binding of the general anesthetic sevoflurane to ion channels

The direct-site hypothesis assumes general anesthetics bind ion channels to impact protein equilibrium and function, inducing anesthesia. Despite advancements in the field, a first principle all-atom demonstration of this structure-function premise is still missing. We focus on the clinically used sevoflurane interaction to anesthetic-sensitive Kv1.2 mammalian channel to resolve if sevoflurane binds protein’s well-characterized open and closed structures in a conformation-dependent manner to shift channel equilibrium. We employ an innovative approach relying on extensive docking calculations and free-energy perturbation of all potential binding sites revealed by the latter, and find sevoflurane binds open and closed structures at multiple sites under complex saturation and concentration effects. Results point to a non-trivial interplay of site and conformation-dependent modes of action involving distinct binding sites that increase channel open-probability at diluted ligand concentrations. Given the challenge in exploring more complex processes potentially impacting channel-anesthetic interaction, the result is revealing as it demonstrates the process of multiple anesthetic binding events alone may account for open-probability shifts recorded in measurements.

General anesthetics are central to modern medicine, yet their microscopic mechanism of action is still unknown. Here, we demonstrate that a clinically used anesthetic, sevoflurane, binds the mammalian voltage-gated potassium channel Kv1.2 effecting a shift in its open probability, even at low concentrations. The results, supported by recent experimental measurements, are promising as they demonstrate that the molecular process of direct binding of anesthetic to ion channels play a relevant role in anesthesia.

Intravenous Anesthetic Protects Hepatocyte from Reactive Oxygen Species-Induced Cellular Apoptosis during Liver Transplantation In Vivo

Background

Liver transplantation leads to liver ischemia/reperfusion (I/R) injury, resulting in early graft dysfunction and failure. Exacerbations of oxidative stress and inflammatory response are key processes in the development of liver I/R injury. Intravenous anesthetic propofol potent effects on free radical scavenging and protects livers against I/R injury. However, the role and mechanism of propofol-mediated hepatic protection in liver transplantation is poorly understood. The aim of this study was to evaluate the role of propofol postconditioning in the liver I/R injury after liver transplantation.

Methods

Forty-eight rats were randomly divided into six groups: rats receiving either sham operation or orthotopic autologous liver transplantation (OALT) in the absence or presence of propofol (high dose and low dose) postconditioning or intralipid control or VAS2870 (Nox2 special inhibitor). Eight hours after OALT or sham operation, parameters of organ injury, oxidative stress, inflammation, and NADPH-associated proteins were assessed.

Results

After OALT, severe liver pathological injury was observed that was associated with increases of serum AST and ALT, which were attenuated by propofol postconditioning. In addition, especially high dose of propofol postconditioning reduced TNF-α, IL-1β, IL-6, TLR4, and NF-κB inflammatory pathway, accompanied with decrease of neutrophil elastase activity, MPO activity, 8-isoprotane, p47^phox and gp91^phox protein expressions, and increase of SOD activity. Inhibition of Nox2 by VAS2870 conferred similar protective effects in liver transplantation.

Conclusion

Liver transplantation leads to severe inflammation and oxidative stress with NADPH oxidase activation. Propofol postconditioning reduces liver I/R injury after liver transplantation partly via inhibiting NADPH oxidase Nox2 and the subsequent inflammation and oxidative stress.

Allosteric potentiation of a ligand-gated ion channel is mediated by access to a deep membrane-facing cavity

Theories of general anesthesia have shifted in focus from bulk lipid effects to specific interactions with membrane proteins. Target receptors include several subtypes of pentameric ligand-gated ion channels; however, structures of physiologically relevant proteins in this family have yet to define anesthetic binding at high resolution. Recent cocrystal structures of the bacterial protein GLIC provide snapshots of state-dependent binding sites for the common surgical agent propofol (PFL), offering a detailed model system for anesthetic modulation. Here, we combine molecular dynamics and oocyte electrophysiology to reveal differential motion and modulation upon modification of a transmembrane binding site within each GLIC subunit. WT channels exhibited net inhibition by PFL, and a contraction of the cavity away from the pore-lining M2 helix in the absence of drug. Conversely, in GLIC variants exhibiting net PFL potentiation, the cavity was persistently expanded and proximal to M2. Mutations designed to favor this deepened site enabled sensitivity even to subclinical concentrations of PFL, and a uniquely prolonged mode of potentiation evident up to ∼30 min after washout. Dependence of these prolonged effects on exposure time implicated the membrane as a reservoir for a lipid-accessible binding site. However, at the highest measured concentrations, potentiation appeared to be masked by an acute inhibitory effect, consistent with the presence of a discrete, water-accessible site of inhibition. These results support a multisite model of transmembrane allosteric modulation, including a possible link between lipid- and receptor-based theories that could inform the development of new anesthetics.

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.

Propofol inhibits prokaryotic voltage-gated Na+ channels by promoting activation-coupled inactivation

Propofol is widely used in the clinic for the induction and maintenance of general anesthesia. As with most general anesthetics, however, our understanding of its mechanism of action remains incomplete. Local and general anesthetics largely inhibit voltage-gated Na⁺ channels (Navs) by inducing an apparent stabilization of the inactivated state, associated in some instances with pore block. To determine the biophysical and molecular basis of propofol action in Navs, we investigated NaChBac and NavMs, two prokaryotic Navs with distinct voltage dependencies and gating kinetics, by whole-cell patch clamp electrophysiology in the absence and presence of propofol at clinically relevant concentrations (2-10 µM). In both Navs, propofol induced a hyperpolarizing shift of the pre-pulse inactivation curve without any significant effects on recovery from inactivation at strongly hyperpolarized voltages, demonstrating that propofol does not stabilize the inactivated state. Moreover, there was no evidence of fast or slow pore block by propofol in a non-inactivating NaChBac mutant (T220A). Propofol also induced hyperpolarizing shifts of the conductance-voltage relationships with negligible effects on the time constants of deactivation at hyperpolarized voltages, indicating that propofol does not stabilize the open state. Instead, propofol decreases the time constants of macroscopic activation and inactivation. Adopting a kinetic scheme of Nav gating that assumes preferential closed-state recovery from inactivation, a 1.7-fold acceleration of the rate constant of activation and a 1.4-fold acceleration of the rate constant of inactivation were sufficient to reproduce experimental observations with computer simulations. In addition, molecular dynamics simulations and molecular docking suggest that propofol binding involves interactions with gating machinery in the S4-S5 linker and external pore regions. Our findings show that propofol is primarily a positive gating modulator of prokaryotic Navs, which ultimately inhibits the channels by promoting activation-coupled inactivation.

Nicotinic acetylcholine receptors at the single-channel level

Over the past four decades, the patch clamp technique and nicotinic ACh (nACh) receptors have established an enduring partnership. Like all good partnerships, each partner has proven significant in its own right, while their union has spurred innumerable advances in life science research. A member and prototype of the superfamily of pentameric ligand-gated ion channels, the nACh receptor is a chemo-electric transducer, binding ACh released from nerves and rapidly opening its channel to cation flow to elicit cellular excitation. A subject of a Nobel Prize in Physiology or Medicine, the patch clamp technique provides unprecedented resolution of currents through single ion channels in their native cellular environments. Here, focusing on muscle and α7 nACh receptors, we describe the extraordinary contribution of the patch clamp technique towards understanding how they activate in response to neurotransmitter, how subtle structural and mechanistic differences among nACh receptor subtypes translate into significant physiological differences, and how nACh receptors are being exploited as therapeutic drug targets.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc/.

X-Ray Crystallographic Studies for Revealing Binding Sites of General Anesthetics in Pentameric Ligand-Gated Ion Channels

X-ray crystallography is a powerful tool in structural biology and can offer insight into structured-based understanding of general anesthetic action on various relevant molecular targets, including pentameric ligand-gated ion channels (pLGICs). In this chapter, we outline the procedures for expression and purification of pLGICs. Optimization of crystallization conditions, especially to achieve high-resolution structures of pLGICs bound with general anesthetics, is also presented. Case studies of pLGICs bound with the volatile general anesthetic isoflurane, 2-bromoethanol, and the intravenous general anesthetic ketamine are revisited.

Recent progress on the molecular pharmacology of propofol

The precise mechanism by which propofol enhances GABAergic transmission remains unclear, but much progress has been made regarding the underlying structural and dynamic mechanisms. Furthermore, it is now clear that propofol has additional molecular targets, many of which are functionally influenced at concentrations achieved clinically. Focusing primarily on molecular targets, this brief review attempts to summarize some of this recent progress while pointing out knowledge gaps and controversies. It is not intended to be comprehensive but rather to stimulate further thought, discussion, and study on the mechanisms by which propofol produces its pleiotropic effects.

Permeating disciplines: Overcoming barriers between molecular simulations and classical structure-function approaches in biological ion transport

Ion translocation across biological barriers is a fundamental requirement for life. In many cases, controlling this process-for example with neuroactive drugs-demands an understanding of rapid and reversible structural changes in membrane-embedded proteins, including ion channels and transporters. Classical approaches to electrophysiology and structural biology have provided valuable insights into several such proteins over macroscopic, often discontinuous scales of space and time. Integrating these observations into meaningful mechanistic models now relies increasingly on computational methods, particularly molecular dynamics simulations, while surfacing important challenges in data management and conceptual alignment. Here, we seek to provide contemporary context, concrete examples, and a look to the future for bridging disciplinary gaps in biological ion transport. This article is part of a Special Issue entitled: Beyond the Structure-Function Horizon of Membrane Proteins edited by Ute Hellmich, Rupak Doshi and Benjamin McIlwain.

Enantiomeric barbiturates bind distinct inter- and intrasubunit binding sites in a nicotinic acetylcholine receptor (nAChR)

Nicotinic acetylcholine receptors (nAChRs) and γ-aminobutyric acid type A receptors (GABA_ARs) are members of the pentameric ligand-gated ion channel superfamily. Drugs acting as positive allosteric modulators of muscle-type α₂βγδ nAChRs, of use in treatment of neuromuscular disorders, have been hard to identify. However, identification of nAChR allosteric modulator binding sites has been facilitated by using drugs developed as photoreactive GABA_AR modulators. Recently, R-1-methyl-5-allyl-5-(m-trifluoromethyl-diazirinylphenyl) barbituric acid (R-mTFD-MPAB), an anesthetic and GABA_AR potentiator, has been shown to inhibit Torpedo α₂βγδ nAChRs, binding in the ion channel and to a γ⁺-α^- subunit interface site similar to its GABA_AR intersubunit binding site. In contrast, S-1-methyl-5-propyl-5-(m-trifluoromethyl-diazirinylphenyl) barbituric acid (S-mTFD-MPPB) acts as a convulsant and GABA_AR inhibitor. Photolabeling studies established that S-mTFD-MPPB binds to the same GABA_AR intersubunit binding site as R-mTFD-MPAB, but with negative rather than positive energetic coupling to GABA binding. We now show that S-mTFD-MPPB binds with the same state (agonist) dependence as R-mTFD-MPAB within the nAChR ion channel, but it does not bind to the intersubunit binding site. Rather, S-mTFD-MPPB binds to intrasubunit sites within the α and δ subunits, photolabeling αVal-218 (αM1), δPhe-232 (δM1), δThr-274 (δM2), and δIle-288 (δM3). Propofol, a general anesthetic that binds to GABA_AR intersubunit sites, inhibited [³H]S-mTFD-MPPB photolabeling of these nAChR intrasubunit binding sites. These results demonstrate that in an nAChR, the subtle difference in structure between S-mTFD-MPPB and R-mTFD-MPAB (chirality; 5-propyl versus 5-allyl) determines selectivity for intra- versus intersubunit sites, in contrast to GABA_ARs, where this difference affects state dependence of binding to a common site.

Emerging Molecular Mechanisms of Signal Transduction in Pentameric Ligand-Gated Ion Channels

Nicotinic acetylcholine, serotonin type 3, γ-amminobutyric acid type A, and glycine receptors are major players of human neuronal communication. They belong to the family of pentameric ligand-gated ion channels, sharing a highly conserved modular 3D structure. Recently, high-resolution structures of both open- and closed-pore conformations have been solved for a bacterial, an invertebrate, and a vertebrate receptor in this family. These data suggest that a common gating mechanism occurs, coupling neurotransmitter binding to pore opening, but they also pinpoint significant differences among subtypes. In this Review, we summarize the structural and functional data in light of these gating models and speculate about their mechanistic consequences on ion permeation, pathological mutations, as well as functional regulation by orthosteric and allosteric effectors.

Concentration-Dependent Binding of Small Ligands to Multiple Saturable Sites in Membrane Proteins

Membrane proteins are primary targets for most therapeutic indications in cancer and neurological diseases, binding over 50% of all known small molecule drugs. Understanding how such ligands impact membrane proteins requires knowledge on the molecular structure of ligand binding, a reasoning that has driven relentless efforts in drug discovery and translational research. Binding of small ligands appears however highly complex involving interaction to multiple transmembrane protein sites featuring single or multiple occupancy states. Within this scenario, looking for new developments in the field, we investigate the concentration-dependent binding of ligands to multiple saturable sites in membrane proteins. The study relying on docking and free-energy perturbation provides us with an extensive description of the probability density of protein-ligand states that allows for computation of thermodynamic properties of interest. It also provides one- and three-dimensional spatial descriptions for the ligand density across the protein-membrane system which can be of interest for structural purposes. Illustration and discussion of the results are shown for binding of the general anesthetic sevoflurane against Kv1.2, a mammalian ion channel for which experimental data are available.

Shedding Light on Anesthetic Mechanisms: Application of Photoaffinity Ligands

Anesthetic photoaffinity ligands have had an increasing presence within anesthesiology research. These ligands mimic parent general anesthetics and allow investigators to study anesthetic interactions with receptors and enzymes; identify novel targets; and determine distribution within biological systems. To date, nearly all general anesthetics used in medicine have a corresponding photoaffinity ligand represented in the literature. In this review, we examine all aspects of the current methodologies, including ligand design, characterization, and deployment. Finally we offer points of consideration and highlight the future outlook as more photoaffinity ligands emerge within the field.

Mapping General Anesthetic Sites in Heteromeric γ-Aminobutyric Acid Type A Receptors Reveals a Potential For Targeting Receptor Subtypes

IV general anesthetics, including propofol, etomidate, alphaxalone, and barbiturates, produce important actions by enhancing γ-aminobutyric acid type A (GABAA) receptor activation. In this article, we review scientific studies that have located and mapped IV anesthetic sites using photoaffinity labeling and substituted cysteine modification protection. These anesthetics bind in transmembrane pockets between subunits of typical synaptic GABAA receptors, and drugs that display stereoselectivity also show remarkably selective interactions with distinct interfacial sites. These results suggest strategies for developing new drugs that selectively modulate distinct GABAA receptor subtypes.

The Role of the Hydroxyl Group in Propofol-Protein Target Recognition: Insights from ONIOM Studies

Propofol (PFL, 1-hydroxyl-2,6-diisopropylbenzene) is currently used widely as one of the most well-known intravenous anesthetics to relieve surgical suffering, but its mechanism of action is not yet clear. Previous experimental studies have demonstrated that the hydroxyl group of PFL plays a dominant role in the molecular recognition of PFL with receptors that lead to hypnosis. To further explore the mechanism of anesthesia induced by PFL in the present work, the exact binding features and interaction details of PFL with three important proteins, human serum albumin (HSA), the pH-gated ion channel from Gloeobacter violaceus (GLIC), and horse spleen apoferritin (HSAF), were investigated systematically by using a rigorous three-layer ONIOM (M06-2X/6-31+G*:PM6:AMBER) method. Additionally, to further characterize the possible importance of such hydroxyl interactions, a similar set of calculations was carried out on the anesthetically inactive fropofol (FFL, 1-fluoro-2,6-diisopropylbenzene) in which the fluorine was substituted for the hydroxyl. According to the ONIOM calculations, atoms in molecules (AIM) analyses, and electrostatic potential (ESP) analyses, the significance of hydrogen bond, halogen bond, and hydrophobic interactions in promoting proper molecular recognition was revealed. The binding interaction energies of PFL with different proteins were generally larger than FFL and are a significant determinant of their differential anesthetic efficacies. Interestingly, although the hydrogen-bonding effect of the hydroxyl moiety was prominent in propofol, the substitution of the 1-hydroxyl by a fluorine atom did not prevent FFL from binding to the protein via a halogen-bonding interaction. It therefore became clear that multiple specific interactions rather than just hydrogen or halogen bonds must be taken into account to explain the different anesthesia endpoints caused by PFL and FFL. The contributions of key residues in ligand-receptor binding were also quantified, and the calculated results agreed with many available experimental observations. This work will provide complementary insights into the molecular mechanisms of anesthetic action for PFL from a robust theoretical point of view. This will not only assist in interpreting experimental observations but will also help to develop working hypotheses for further experiments and future drug design.

Synthesis and pharmacological evaluation of neurosteroid photoaffinity ligands

Neuroactive steroids are potent positive allosteric modulators of GABA_A receptors (GABA_AR), but the locations of their GABA_AR binding sites remain poorly defined. To discover these sites, we synthesized two photoreactive analogs of alphaxalone, an anesthetic neurosteroid targeting GABA_AR, 11β-(4-azido-2,3,5,6-tetrafluorobenzoyloxy)allopregnanolone, (F4N3Bzoxy-AP) and 11-aziallopregnanolone (11-AziAP). Both photoprobes acted with equal or higher potency than alphaxalone as general anesthetics and potentiators of GABA_AR responses, left-shifting the GABA concentration - response curve for human α1β3γ2 GABA_ARs expressed in Xenopus oocytes, and enhancing [³H]muscimol binding to α1β3γ2 GABA_ARs expressed in HEK293 cells. With EC₅₀ of 110 nM, 11-AziAP is one the most potent general anesthetics reported. [³H]F4N3Bzoxy-AP and [³H]11-AziAP, at anesthetic concentrations, photoincorporated into α- and β-subunits of purified α1β3γ2 GABA_ARs, but labeling at the subunit level was not inhibited by alphaxalone (30 μM). The enhancement of photolabeling by ³H-azietomidate and ³H-mTFD-MPAB in the presence of either of the two steroid photoprobes indicates the neurosteroid binding site is different from, but allosterically related to, the etomidate and barbiturate sites. Our observations are consistent with two hypotheses. First, F4N3Bzoxy-AP and 11-aziAP bind to a high affinity site in such a pose that the 11-photoactivatable moiety, that is rigidly attached to the steroid backbone, points away from the protein. Second, F4N3Bzoxy-AP, 11-aziAP and other steroid anesthetics, which are present at very high concentration at the lipid-protein interface due to their high lipophilicity, act via low affinity sites, as proposed by Akk et al. (Psychoneuroendocrinology2009, 34S1, S59-S66).

Photoaffinity Labeling of Pentameric Ligand-Gated Ion Channels: A Proteomic Approach to Identify Allosteric Modulator Binding Sites

Photoaffinity labeling techniques have been used for decades to identify drug binding sites and to study the structural biology of allosteric transitions in transmembrane proteins including pentameric ligand-gated ion channels (pLGIC). In a typical photoaffinity labeling experiment, to identify drug binding sites, UV light is used to introduce a covalent bond between a photoreactive ligand (which upon irradiation at the appropriate wavelength converts to a reactive intermediate) and amino acid residues that lie within its binding site. Then protein chemistry and peptide microsequencing techniques are used to identify these amino acids within the protein primary sequence. These amino acid residues are located within homology models of the receptor to identify the binding site of the photoreactive probe. Molecular modeling techniques are then used to model the binding of the photoreactive probe within the binding site using docking protocols. Photoaffinity labeling directly identifies amino acids that contribute to drug binding sites regardless of their location within the protein structure and distinguishes them from amino acids that are only involved in the transduction of the conformational changes mediated by the drug, but may not be part of its binding site (such as those identified by mutational studies). Major limitations of photoaffinity labeling include the availability of photoreactive ligands that faithfully mimic the properties of the parent molecule and protein preparations that supply large enough quantities suitable for photoaffinity labeling experiments. When the ligand of interest is not intrinsically photoreactive, chemical modifications to add a photoreactive group to the parent drug, and pharmacological evaluation of these chemical modifications become necessary. With few exceptions, expression and affinity-purification of proteins are required prior to photolabeling. Methods to isolate milligram quantities of highly enriched pLGIC suitable for photoaffinity labeling experiments have been developed. In this chapter, we discuss practical aspects of experimental strategies to identify allosteric modulator binding sites in pLGIC using photoaffinity labeling.

Structural Basis of Alcohol Inhibition of the Pentameric Ligand-Gated Ion Channel ELIC

The structural basis for alcohol modulation of neuronal pentameric ligand-gated ion channels (pLGICs) remains elusive. We determined an inhibitory mechanism of alcohol on the pLGIC Erwinia chrysanthemi (ELIC) through direct binding to the pore. X-ray structures of ELIC co-crystallized with 2-bromoethanol, in both the absence and presence of agonist, reveal 2-bromoethanol binding in the pore near T237(6') and the extracellular domain (ECD) of each subunit at three different locations. Binding to the ECD does not appear to contribute to the inhibitory action of 2-bromoethanol and ethanol as indicated by the same functional responses of wild-type ELIC and mutants. In contrast, the ELIC-α1β3GABA_AR chimera, replacing the ELIC transmembrane domain (TMD) with the TMD of α1β3GABA_AR, is potentiated by 2-bromoethanol and ethanol. The results suggest a dominant role of the TMD in modulating alcohol effects. The X-ray structures and functional measurements support a pore-blocking mechanism for inhibitory action of short-chain alcohols.

Role of the Cys Loop and Transmembrane Domain in the Allosteric Modulation of α4β2 Nicotinic Acetylcholine Receptors

Allosteric modulators of pentameric ligand-gated ion channels are thought to act on elements of the pathways that couple agonist binding to channel gating. Using α4β2 nicotinic acetylcholine receptors and the α4β2-selective positive modulators 17β-estradiol (βEST) and desformylflustrabromine (dFBr), we have identified pathways that link the binding sites for these modulators to the Cys loop, a region that is critical for channel gating in all pentameric ligand-gated ion channels. Previous studies have shown that the binding site for potentiating βEST is in the C-terminal (post-M4) region of the α4 subunit. Here, using homology modeling in combination with mutagenesis and electrophysiology, we identified the binding site for potentiating dFBr on the top half of a cavity between the third (M3) and fourth transmembrane (M4) α-helices of the α4 subunit. We found that the binding sites for βEST and dFBr communicate with the Cys loop, through interactions between the last residue of post-M4 and Phe¹⁷⁰ of the conserved FPF sequence of the Cys loop, and that these interactions affect potentiating efficacy. In addition, interactions between a residue in M3 (Tyr³⁰⁹) and Phe¹⁶⁷, a residue adjacent to the Cys loop FPF motif, also affect dFBr potentiating efficacy. Thus, the Cys loop acts as a key control element in the allosteric transduction pathway for potentiating βEST and dFBr. Overall, we propose that positive allosteric modulators that bind the M3-M4 cavity or post-M4 region increase the efficacy of channel gating through interactions with the Cys loop.

Understanding the Bases of Function and Modulation of α7 Nicotinic Receptors: Implications for Drug Discovery

The nicotinic acetylcholine receptor (nAChR) belongs to a superfamily of pentameric ligand-gated ion channels involved in many physiologic and pathologic processes. Among nAChRs, receptors comprising the α7 subunit are unique because of their high Ca(2+) permeability and fast desensitization. nAChR agonists elicit a transient ion flux response that is further sustained by the release of calcium from intracellular sources. Owing to the dual ionotropic/metabotropic nature of α7 receptors, signaling pathways are activated. The α7 subunit is highly expressed in the nervous system, mostly in regions implicated in cognition and memory and has therefore attracted attention as a novel drug target. Additionally, its dysfunction is associated with several neuropsychiatric and neurologic disorders, such as schizophrenia and Alzheimer's disease. α7 is also expressed in non-neuronal cells, particularly immune cells, where it plays a role in immunity, inflammation, and neuroprotection. Thus, α7 potentiation has emerged as a therapeutic strategy for several neurologic and inflammatory disorders. With unique activation properties, the receptor is a sensitive drug target carrying different potential binding sites for chemical modulators, particularly agonists and positive allosteric modulators. Although macroscopic and single-channel recordings have provided significant information about the underlying molecular mechanisms and binding sites of modulatory compounds, we know just the tip of the iceberg. Further concerted efforts are necessary to effectively exploit α7 as a drug target for each pathologic situation. In this article, we focus mainly on the molecular basis of activation and drug modulation of α7, key pillars for rational drug design.

Common Anesthetic-binding Site for Inhibition of Pentameric Ligand-gated Ion Channels

BACKGROUND

Identifying functionally relevant anesthetic-binding sites in pentameric ligand-gated ion channels (pLGICs) is an important step toward understanding the molecular mechanisms underlying anesthetic action. The anesthetic propofol is known to inhibit cation-conducting pLGICs, including a prokaryotic pLGIC from Erwinia chrysanthemi (ELIC), but the sites responsible for functional inhibition remain undetermined.

METHODS

We photolabeled ELIC with a light-activated derivative of propofol (AziPm) and performed fluorine-19 nuclear magnetic resonance experiments to support propofol binding to a transmembrane domain (TMD) intrasubunit pocket. To differentiate sites responsible for propofol inhibition from those that are functionally irrelevant, we made an ELIC-γ-aminobutyric acid receptor (GABAAR) chimera that replaced the ELIC-TMD with the α1β3GABAAR-TMD and compared functional responses of ELIC-GABAAR and ELIC with propofol modulations.

RESULTS

Photolabeling showed multiple AziPm-binding sites in the extracellular domain (ECD) but only one site in the TMD with labeled residues M265 and F308 in the resting state of ELIC. Notably, this TMD site is an intrasubunit pocket that overlaps with binding sites for anesthetics, including propofol, found previously in other pLGICs. Fluorine-19 nuclear magnetic resonance experiments supported propofol binding to this TMD intrasubunit pocket only in the absence of agonist. Functional measurements of ELIC-GABAAR showed propofol potentiation of the agonist-elicited current instead of inhibition observed on ELIC.

CONCLUSIONS

The distinctly different responses of ELIC and ELIC-GABAAR to propofol support the functional relevance of propofol binding to the TMD. Combining the newly identified TMD intrasubunit pocket in ELIC with equivalent TMD anesthetic sites found previously in other cationic pLGICs, we propose this TMD pocket as a common site for anesthetic inhibition of pLGICs.

Noncompetitive Inhibition of 5-HT3 Receptors by Citral, Linalool, and Eucalyptol Revealed by Nonlinear Mixed-Effects Modeling

Citral, eucalyptol, and linalool are widely used as flavorings, fragrances, and cosmetics. Here, we examined their effects on electrophysiological and binding properties of human 5-HT3 receptors expressed in Xenopus oocytes and human embryonic kidney 293 cells, respectively. Data were analyzed using nonlinear mixed-effects modeling to account for random variance in the peak current response between oocytes. The oils caused an insurmountable inhibition of 5-HT-evoked currents (citral IC50 = 120 µM; eucalyptol = 258 µM; linalool = 141 µM) and did not compete with fluorescently labeled granisetron, suggesting a noncompetitive mechanism of action. Inhibition was not use-dependent but required a 30-second preapplication. Compound washout caused a slow (∼180 seconds) but complete recovery. Coapplication of the oils with bilobalide or diltiazem indicated they did not bind at the same locations as these channel blockers. Homology modeling and ligand docking predicted binding to a transmembrane cavity at the interface of adjacent subunits. Liquid chromatography coupled to mass spectrometry showed that an essential oil extracted from Lippia alba contained 75.9% citral. This inhibited expressed 5-HT3 receptors (IC50 = 45 µg ml(-1)) and smooth muscle contractions in rat trachea (IC50 = 200 µg ml(-1)) and guinea pig ileum (IC50 = 20 µg ml(-1)), providing a possible mechanistic explanation for why this oil has been used to treat gastrointestinal and respiratory ailments. These results demonstrate that citral, eucalyptol, and linalool inhibit 5-HT3 receptors, and their binding to a conserved cavity suggests a valuable target for novel allosteric modulators.

Structural Basis for Xenon Inhibition in a Cationic Pentameric Ligand-Gated Ion Channel

GLIC receptor is a bacterial pentameric ligand-gated ion channel whose action is inhibited by xenon. Xenon has been used in clinical practice as a potent gaseous anaesthetic for decades, but the molecular mechanism of interactions with its integral membrane receptor targets remains poorly understood. Here we characterize by X-ray crystallography the xenon-binding sites within both the open and "locally-closed" (inactive) conformations of GLIC. Major binding sites of xenon, which differ between the two conformations, were identified in three distinct regions that all belong to the trans-membrane domain of GLIC: 1) in an intra-subunit cavity, 2) at the interface between adjacent subunits, and 3) in the pore. The pore site is unique to the locally-closed form where the binding of xenon effectively seals the channel. A putative mechanism of the inhibition of GLIC by xenon is proposed, which might be extended to other pentameric cationic ligand-gated ion channels.

Propofol and AZD3043 Inhibit Adult Muscle and Neuronal Nicotinic Acetylcholine Receptors Expressed in Xenopus Oocytes

Propofol is a widely used general anaesthetic with muscle relaxant properties. Similarly as propofol, the new general anaesthetic AZD3043 targets the GABA_A receptor for its anaesthetic effects, but the interaction with nicotinic acetylcholine receptors (nAChRs) has not been investigated. Notably, there is a gap of knowledge about the interaction between propofol and the nAChRs found in the adult neuromuscular junction. The objective was to evaluate whether propofol or AZD3043 interact with the α1β1δε, α3β2, or α7 nAChR subtypes that can be found in the neuromuscular junction and if there are any differences in affinity for those subtypes between propofol and AZD3043. Human nAChR subtypes α1β1δε, α3β2, and α7 were expressed into Xenopus oocytes and studied with an automated voltage-clamp. Propofol and AZD3043 inhibited ACh-induced currents in all of the nAChRs studied with inhibitory concentrations higher than those needed for general anaesthesia. AZD3043 was a more potent inhibitor at the adult muscle nAChR subtype compared to propofol. Propofol and AZD3043 inhibit nAChR subtypes that can be found in the adult NMJ in concentrations higher than needed for general anaesthesia. This finding needs to be evaluated in an in vitro nerve-muscle preparation and suggests one possible explanation for the muscle relaxant effect of propofol seen during higher doses.

Insights into the Nature of Anesthetic-Protein Interactions: An ONIOM Study

Anesthetics have been employed widely to relieve surgical suffering, but their mechanism of action is not yet clear. For over a century, the mechanism of anesthesia was previously thought to be via lipid bilayer interactions. In the present work, a rigorous three-layer ONIOM(M06-2X/6-31+G*:PM6:AMBER) method was utilized to investigate the nature of interactions between several anesthetics and actual protein binding sites. According to the calculated structural features, interaction energies, atomic charges, and electrostatic potential surfaces, the amphiphilic nature of anesthetic-protein interactions was demonstrated for both inhalational and injectable anesthetics. The existence of hydrogen and halogen bonding interactions between anesthetics and proteins was clearly identified, and these interactions served to assist ligand recognition and binding by the protein. Within all complexes of inhalational or injectable anesthetics, the polarization effects play a dominant role over the steric effects and induce a significant asymmetry in the otherwise symmetric atomic charge distributions of the free ligands in vacuo. This study provides new insight into the mechanism of action of general anesthetics in a more rigorous way than previously described. Future rational design of safer anesthetics for an aging and more physiologically vulnerable population will be predicated on this greater understanding of such specific interactions.

Direct Pore Binding as a Mechanism for Isoflurane Inhibition of the Pentameric Ligand-gated Ion Channel ELIC

Pentameric ligand-gated ion channels (pLGICs) are targets of general anesthetics, but molecular mechanisms underlying anesthetic action remain debatable. We found that ELIC, a pLGIC from Erwinia chrysanthemi, can be functionally inhibited by isoflurane and other anesthetics. Structures of ELIC co-crystallized with isoflurane in the absence or presence of an agonist revealed double isoflurane occupancies inside the pore near T237(6′) and A244(13′). A pore-radius contraction near the extracellular entrance was observed upon isoflurane binding. Electrophysiology measurements with a single-point mutation at position 6′ or 13′ support the notion that binding at these sites renders isoflurane inhibition. Molecular dynamics simulations suggested that isoflurane binding was more stable in the resting than in a desensitized pore conformation. This study presents compelling evidence for a direct pore-binding mechanism of isoflurane inhibition, which has a general implication for inhibitory action of general anesthetics on pLGICs.

Effects of single-shot and steady-state propofol anaesthesia on rocuronium dose-response relationship: a randomised trial

BACKGROUND

Similar to volatile anaesthetics, propofol may influence neuromuscular transmission. We hypothesised that the administration of propofol influenced the potency of rocuronium depending on the duration of the administration.

METHODS

After consent, patients scheduled for elective surgery randomly received rocuronium either after induction of anaesthesia with propofol (2 min of propofol, n = 36) or after 30 min of propofol infusion (30 min of propofol, n = 36). Remifentanil was given in both groups. Neuromuscular monitoring was performed by calibrated electromyography. The dose-response relationship of rocuronium was determined with a single-bolus technique (0.07, 0.1, 0.15, 0.2, 0.3 and 0.45 mg/kg rocuronium). The primary endpoints were the ED50 and ED95 of rocuronium after 2 and 30 min propofol. Data are presented as means with (95% confidence interval). The trial is registered with the Eudra-CT: 2009-012815-16.

RESULTS

A total of 72 patients were included. Time to maximal neuromuscular blockade was significantly shorter in patients after 30 min of propofol [3.3 min (2.9-3.7)] compared with patients anaesthetised with 2 min of propofol [4.6 min (4.0-5.2)]. After 30 min of propofol, the slope of the dose-response curve was significantly steeper (30 min of propofol: 4.34 [3.62-5.05]; 2 min of propofol: [3.34 (2.72-3.96)], resulting in lower ED95 values of rocuronium (30 min of propofol: 0.287 mg/kg [0.221-0.368]; 2 min of propofol [0.391 mg/kg (0.296-0.520)]. The ED50 were not different between groups.

CONCLUSION

The potency of rocuronium was significantly enhanced after propofol infusion for 30 min. Estimates of potency those are usually determined during steady-state anaesthesia might underestimate rocuronium requirements for endotracheal intubation at the time of induction.

Allosteric modulation of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are receptors for the neurotransmitter acetylcholine and are members of the 'Cys-loop' family of pentameric ligand-gated ion channels (LGICs). Acetylcholine binds in the receptor extracellular domain at the interface between two subunits and research has identified a large number of nAChR-selective ligands, including agonists and competitive antagonists, that bind at the same site as acetylcholine (commonly referred to as the orthosteric binding site). In addition, more recent research has identified ligands that are able to modulate nAChR function by binding to sites that are distinct from the binding site for acetylcholine, including sites located in the transmembrane domain. These include positive allosteric modulators (PAMs), negative allosteric modulators (NAMs), silent allosteric modulators (SAMs) and compounds that are able to activate nAChRs via an allosteric binding site (allosteric agonists). Our aim in this article is to review important aspects of the pharmacological diversity of nAChR allosteric modulators and to describe recent evidence aimed at identifying binding sites for allosteric modulators on nAChRs.

Desformylflustrabromine (dFBr) and [3H]dFBr-Labeled Binding Sites in a Nicotinic Acetylcholine Receptor

Desformylflustrabromine (dFBr) is a positive allosteric modulator (PAM) of α4β2 and α2β2 nAChRs that, at concentrations >1 µM, also inhibits these receptors and α7 nAChRs. However, its interactions with muscle-type nAChRs have not been characterized, and the locations of its binding site(s) in any nAChR are not known. We report here that dFBr inhibits human muscle (αβεδ) and Torpedo (αβγδ) nAChR expressed in Xenopus oocytes with IC50 values of ∼ 1 μM. dFBr also inhibited the equilibrium binding of ion channel blockers to Torpedo nAChRs with higher affinity in the nAChR desensitized state ([(3)H]phencyclidine; IC50 = 4 μM) than in the resting state ([(3)H]tetracaine; IC50 = 60 μM), whereas it bound with only very low affinity to the ACh binding sites ([(3)H]ACh, IC50 = 1 mM). Upon irradiation at 312 nm, [(3)H]dFBr photoincorporated into amino acids within the Torpedo nAChR ion channel with the efficiency of photoincorporation enhanced in the presence of agonist and the agonist-enhanced photolabeling inhibitable by phencyclidine. In the presence of agonist, [(3)H]dFBr also photolabeled amino acids in the nAChR extracellular domain within binding pockets identified previously for the nonselective nAChR PAMs galantamine and physostigmine at the canonical α-γ interface containing the transmitter binding sites and at the noncanonical δ-β subunit interface. These results establish that dFBr inhibits muscle-type nAChR by binding in the ion channel and that [(3)H]dFBr is a photoaffinity probe with broad amino acid side chain reactivity.

Role for the propofol hydroxyl in anesthetic protein target molecular recognition

Propofol is a widely used intravenous general anesthetic. We synthesized 2-fluoro-1,3-diisopropylbenzene, a compound that we call "fropofol", to directly assess the significance of the propofol 1-hydroxyl for pharmacologically relevant molecular recognition in vitro and for anesthetic efficacy in vivo. Compared to propofol, fropofol had a similar molecular volume and only a small increase in hydrophobicity. Isothermal titration calorimetry and competition assays revealed that fropofol had higher affinity for a protein site governed largely by van der Waals interactions. Within another protein model containing hydrogen bond interactions, propofol demonstrated higher affinity. In vivo, fropofol demonstrated no anesthetic efficacy, but at high concentrations produced excitatory activity in tadpoles and mice; fropofol also antagonized propofol-induced hypnosis. In a propofol protein target that contributes to hypnosis, α1β2γ2L GABAA receptors, fropofol demonstrated no significant effect alone or on propofol positive allosteric modulation of the ion channel, suggesting an additional requirement for the 1-hydroxyl within synaptic GABAA receptor site(s). However, fropofol caused similar adverse cardiovascular effects as propofol by a dose-dependent depression of myocardial contractility. Our results directly implicate the propofol 1-hydroxyl as contributing to molecular recognition within protein targets leading to hypnosis, but not necessarily within protein targets leading to side effects of the drug.

Ethanol Modulation is Quantitatively Determined by the Transmembrane Domain of Human α1 Glycine Receptors

BACKGROUND

Mutagenesis and labeling studies have identified amino acids from the human α1 glycine receptor (GlyR) extracellular, transmembrane (TM), and intracellular domains in mediating ethanol (EtOH) potentiation. However, limited high-resolution structural data for physiologically relevant receptors in this Cys-loop receptor superfamily have made pinpointing the critical amino acids difficult. Homologous ion channels from lower organisms provide conserved models for structural and functional properties of Cys-loop receptors. We previously demonstrated that a single amino acid variant of the Gloeobacter violaceus ligand-gated ion channel (GLIC) produced EtOH and anesthetic sensitivity similar to that of GlyRs and provided crystallographic evidence for EtOH binding to GLIC.

METHODS

We directly compared EtOH modulation of the α1 GlyR and GLIC to a chimera containing the TM domain from human α1 GlyRs and the ligand-binding domain of GLIC using 2-electrode voltage-clamp electrophysiology of receptors expressed in Xenopus laevis oocytes.

RESULTS

EtOH potentiated α1 GlyRs in a concentration-dependent manner in the presence of zinc-chelating agents, but did not potentiate GLIC at pharmacologically relevant concentrations. The GLIC/GlyR chimera recapitulated the EtOH potentiation of GlyRs, without apparent sensitivity to zinc chelation. For chimera expression in oocytes, it was essential to suppress leakage current by adding 50 μM picrotoxin to the media, a technique that may have applications in expression of other ion channels.

CONCLUSIONS

Our results are consistent with a TM mechanism of EtOH modulation in Cys-loop receptors. This work highlights the relevance of bacterial homologs as valuable model systems for studying ion channel function of human receptors and demonstrates the modularity of these channels across species.

Macroscopic and macromolecular specificity of alkylphenol anesthetics for neuronal substrates

We used a photoactive general anesthetic called meta-azi-propofol (AziPm) to test the selectivity and specificity of alkylphenol anesthetic binding in mammalian brain. Photolabeling of rat brain sections with [³H]AziPm revealed widespread but heterogeneous ligand distribution, with [³H]AziPm preferentially binding to synapse-dense areas compared to areas composed largely of cell bodies or myelin. With [³H]AziPm and propofol, we determined that alkylphenol general anesthetics bind selectively and specifically to multiple synaptic protein targets. In contrast, the alkylphenol anesthetics do not bind to specific sites on abundant phospholipids or cholesterol, although [³H]AziPm shows selectivity for photolabeling phosphatidylethanolamines. Together, our experiments suggest that alkylphenol anesthetic substrates are widespread in number and distribution, similar to those of volatile general anesthetics, and that multi-target mechanisms likely underlie their pharmacology.

Photoaffinity labeling in target- and binding-site identification

Photoaffinity labeling (PAL) using a chemical probe to covalently bind its target in response to activation by light has become a frequently used tool in drug discovery for identifying new drug targets and molecular interactions, and for probing the location and structure of binding sites. Methods to identify the specific target proteins of hit molecules from phenotypic screens are highly valuable in early drug discovery. In this review, we summarize the principles of PAL including probe design and experimental techniques for in vitro and live cell investigations. We emphasize the need to optimize and validate probes and highlight examples of the successful application of PAL across multiple disease areas.

Activation of human 5-hydroxytryptamine type 3 receptors via an allosteric transmembrane site

In common with other members of the Cys-loop family of pentameric ligand-gated ion channels, 5-hydroxytryptamine type 3 receptors (5-HT3Rs) are activated by the binding of a neurotransmitter to an extracellular orthosteric site, located at the interface of two adjacent receptor subunits. In addition, a variety of compounds have been identified that modulate agonist-evoked responses of 5-HT3Rs, and other Cys-loop receptors, by binding to distinct allosteric sites. In this study, we examined the pharmacological effects of a group of monoterpene compounds on recombinant 5-HT3Rs expressed in Xenopus oocytes. Two phenolic monoterpenes (carvacrol and thymol) display allosteric agonist activity on human homomeric 5-HT3ARs (64 ± 7% and 80 ± 4% of the maximum response evoked by the endogenous orthosteric agonist 5-HT, respectively). In addition, at lower concentrations, where agonist effects are less apparent, carvacrol and thymol act as potentiators of responses evoked by submaximal concentrations of 5-HT. By contrast, carvacrol and thymol have no agonist or potentiating activity on the closely related mouse 5-HT3ARs. Using subunit chimeras containing regions of the human and mouse 5-HT3A subunits, and by use of site-directed mutagenesis, we have identified transmembrane amino acids that either abolish the agonist activity of carvacrol and thymol on human 5-HT3ARs or are able to confer this property on mouse 5-HT3ARs. By contrast, these mutations have no significant effect on orthosteric activation of 5-HT3ARs by 5-HT. We conclude that 5-HT3ARs can be activated by the binding of ligands to an allosteric transmembrane site, a conclusion that is supported by computer docking studies.

Anesthetics target interfacial transmembrane sites in nicotinic acetylcholine receptors

General anesthetics are a heterogeneous group of small amphiphilic ligands that interact weakly at multiple allosteric sites on many pentameric ligand gated ion channels (pLGICs), resulting in either inhibition, potentiation of channel activity, or both. Allosteric principles imply that modulator sites must change configuration and ligand affinity during receptor state transitions. Thus, general anesthetics and related compounds are useful both as state-dependent probes of receptor structure and as potentially selective modulators of pLGIC functions. This review focuses on general anesthetic sites in nicotinic acetylcholine receptors, which were among the first anesthetic-sensitive pLGIC experimental models studied, with particular focus on sites formed by transmembrane domain elements. Structural models place many of these sites at interfaces between two or more pLGIC transmembrane helices both within subunits and between adjacent subunits, and between transmembrane helices and either lipids (the lipid-protein interface) or water (i.e. the ion channel). A single general anesthetic may bind at multiple allosteric sites in pLGICs, producing a net effect of either inhibition (e.g. blocking the ion channel) or enhanced channel gating (e.g. inter-subunit sites). Other general anesthetic sites identified by photolabeling or crystallography are tentatively linked to functional effects, including intra-subunit helix bundle sites and the lipid-protein interface. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Phenylalanine in the pore of the Erwinia ligand-gated ion channel modulates picrotoxinin potency but not receptor function

The Erwinia ligand-gated ion channel (ELIC) is a bacterial homologue of eukaryotic Cys-loop ligand-gated ion channels. This protein has the potential to be a useful model for Cys-loop receptors but is unusual in that it has an aromatic residue (Phe) facing into the pore, leading to some predictions that this protein is incapable of ion flux. Subsequent studies have shown this is not the case, so here we probe the role of this residue by examining the function of the ELIC in cases in which the Phe has been substituted with a range of alternative amino acids, expressed in Xenopus oocytes and functionally examined. Most of the mutations have little effect on the GABA EC₅₀, but the potency of the weak pore-blocking antagonist picrotoxinin at F16′A-, F16′D-, F16′S-, and F16′T-containing receptors was increased to levels comparable with those of Cys-loop receptors, suggesting that this antagonist can enter the pore only when residue 16′ is small. T6′S has no effect on picrotoxinin potency when expressed alone but abolishes the increased potency when combined with F16′S, indicating that the inhibitor binds at position 6′, as in Cys-loop receptors, if it can enter the pore. Overall, the data support the proposal that the ELIC pore is a good model for Cys-loop receptor pores if the role of F16′ is taken into consideration.

Multiple propofol-binding sites in a γ-aminobutyric acid type A receptor (GABAAR) identified using a photoreactive propofol analog

Propofol acts as a positive allosteric modulator of γ-aminobutyric acid type A receptors (GABAARs), an interaction necessary for its anesthetic potency in vivo as a general anesthetic. Identifying the location of propofol-binding sites is necessary to understand its mechanism of GABAAR modulation. [(3)H]2-(3-Methyl-3H-diaziren-3-yl)ethyl 1-(phenylethyl)-1H-imidazole-5-carboxylate (azietomidate) and R-[(3)H]5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl)barbituric acid (mTFD-MPAB), photoreactive analogs of 2-ethyl 1-(phenylethyl)-1H-imidazole-5-carboxylate (etomidate) and mephobarbital, respectively, have identified two homologous but pharmacologically distinct classes of intersubunit-binding sites for general anesthetics in the GABAAR transmembrane domain. Here, we use a photoreactive analog of propofol (2-isopropyl-5-[3-(trifluoromethyl)-3H-diazirin-3-yl]phenol ([(3)H]AziPm)) to identify propofol-binding sites in heterologously expressed human α1β3 GABAARs. Propofol, AziPm, etomidate, and R-mTFD-MPAB each inhibited [(3)H]AziPm photoincorporation into GABAAR subunits maximally by ∼ 50%. When the amino acids photolabeled by [(3)H]AziPm were identified by protein microsequencing, we found propofol-inhibitable photolabeling of amino acids in the β3-α1 subunit interface (β3Met-286 in β3M3 and α1Met-236 in α1M1), previously photolabeled by [(3)H]azietomidate, and α1Ile-239, located one helical turn below α1Met-236. There was also propofol-inhibitable [(3)H]AziPm photolabeling of β3Met-227 in βM1, the amino acid in the α1-β3 subunit interface photolabeled by R-[(3)H]mTFD-MPAB. The propofol-inhibitable [(3)H]AziPm photolabeling in the GABAAR β3 subunit in conjunction with the concentration dependence of inhibition of that photolabeling by etomidate or R-mTFD-MPAB also establish that each anesthetic binds to the homologous site at the β3-β3 subunit interface. These results establish that AziPm as well as propofol bind to the homologous intersubunit sites in the GABAAR transmembrane domain that binds etomidate or R-mTFD-MPAB with high affinity.