Positive Allosteric Modulation of Kv Channels by Sevoflurane: Insights into the Structural Basis of Inhaled Anesthetic Action

A randomised, double-blind, placebo-controlled study to determine the analgesic efficacy, safety and tolerability of VPX638 administered topically to painful wounds

New analgesics are needed for painful wounds. Multiple reports suggest that topical sevoflurane may have analgesic effects. This placebo-controlled randomised trial evaluated the analgesic efficacy and safety of VPX638 (topical sevoflurane). Seventy-eight participants with painful wounds, were enrolled at eight Australian centres and randomly allocated to receive 2 × 5 mL of VPX638 (N = 39) or placebo (N = 40) during one wound dressing change. Numerical pain rating scores and use of opioids were recorded for 24 h. The primary endpoint was pain during wound cleaning, secondary endpoints evaluated pain for 24 h after drug application and opioids use. There was no significant difference in mean pain scores during wound cleaning between VPX638 and placebo (0.854; p = 0.23). The mean differences in summed pain intensity difference from baseline suggested VPX638 provided greater analgesia compared to placebo over 8 h (p < 0.02), 12 h (p < 0.01) and 24 h (p < 0.05) and significantly longer duration of analgesia, 24.3 h for VPX638 versus 7.1 h for placebo (p < 0.01). In the 24 h after drug administration, participants receiving VPX638 had a 50% decrease in opioid use over 24 h compared with placebo. VPX638 appeared safe and well-tolerated. In conclusion, this small placebo-controlled randomised trial suggested that VPX638 provides analgesia and is opioid-sparing for up to 24 h after wound cleaning. It supports the need for further evaluation of the benefit of VPX638 as a topical analgesic for painful wounds.

Regulating the activity of GABAergic neurons in the ventral pallidum alters the general anesthesia effect of propofol

The full mechanism of action of propofol, a commonly administered intravenous anesthetic drug in clinical practice, remains elusive. The focus of this study was the role of GABAergic neurons which are the main neuron group in the ventral pallidum (VP) closely associated with anesthetic effects in propofol anesthesia. The activity of VP GABAergic neurons following propofol anesthesia in Vgat-Cre mice was observed via detecting c-Fos immunoreactivity by immunofluorescence and western blotting. Subsequently, chemogenetic techniques were employed in Vgat-Cre mice to regulate the activity of VP GABAergic neurons. The role of VP GABAergic neurons in generating the effects of general anesthesia induced by intravenous propofol was further explored through behavioral tests of the righting reflex. The results revealed that c-Fos expression in VP GABAergic neurons in Vgat-Cre mice dramatically decreased after propofol injection. Further studies demonstrated that chemogenetic activation of VP GABAergic neurons during propofol anesthesia shortened the duration of anesthesia and promoted wakefulness. Conversely, the inhibition of VP GABAergic neurons extended the duration of anesthesia and facilitated the effects of anesthesia. The results obtained in this study suggested that regulating the activity of GABAergic neurons in the ventral pallidum altered the effect of propofol on general anesthesia.

Allosteric Modulation of Membrane Proteins by Small Low-Affinity Ligands

Membrane proteins may respond to a variety of ligands under an applied external stimulus. These ligands include small low-affinity molecules that account for functional effects in the mM range. Understanding the modulation of protein function by low-affinity ligands requires characterizing their atomic-level interactions under dilution, challenging the current resolution of theoretical and experimental routines. Part of the problem derives from the fact that small low-affinity ligands may interact with multiple sites of a membrane protein in a highly degenerate manner to a degree that it is better conceived as a partition phenomenon, hard to track at the molecular interface of the protein. Looking for new developments in the field, we rely on the classic two-state Boltzmann model to devise a novel theoretical description of the allosteric modulation mechanism of membrane proteins in the presence of small low-affinity ligands and external stimuli. Free energy stability of the partition process and its energetic influence on the protein coupling with the external stimulus are quantified. The outcome is a simple formulation that allows the description of the equilibrium shifts of the protein in terms of the grand-canonical partition function of the ligand at dilute concentrations. The model's predictions of the spatial distribution and response probability shift across a variety of ligand concentrations, and thermodynamic conjugates can be directly compared to macroscopic measurements, making it especially useful to interpret experimental data at the atomic level. Illustration and discussion of the theory is shown in the context of general anesthetics and voltage-gated channels for which structural data are available.

Concentration-Dependent Thermodynamic Analysis of the Partition Process of Small Ligands into Proteins

In the category of functional low-affinity interactions, small ligands may interact with multiple protein sites in a highly degenerate manner. Better conceived as a partition phenomenon at the molecular interface of proteins, such low-affinity interactions appear to be hidden to our current experimental resolution making their structural and functional characterization difficult in the low concentration regime of physiological processes. Characterization of the partition phenomenon under higher chemical forces could be a relevant strategy to tackle the problem provided the results can be scaled back to the low concentration range. Far from being trivial, such scaling demands a concentration-dependent understanding of self-interactions of the ligands, structural perturbations of the protein, among other molecular effects. Accordingly, we elaborate a novel and detailed concentration-dependent thermodynamic analysis of the partition process of small ligands aiming at characterizing the stability and structure of the dilute phenomenon from high concentrations. In analogy to an “aggregate” binding constant of a small molecule over multiple sites of a protein receptor, the model defines the stability of the process as a macroscopic equilibrium constant for the partition number of ligands that can be used to analyze biochemical and functional data of two-component systems driven by low-affinity interactions. Acquisition of such modeling-based structural information is expected to be highly welcome by revealing more traceable protein-binding spots for non-specific ligands.

How Do Modulators Affect the Orthosteric and Allosteric Binding Pockets?

Allosteric modulators (AMs) that bind allosteric sites can exhibit greater selectivity than the orthosteric ligands and can either enhance agonist-induced receptor activity (termed positive allosteric modulator or PAM), inhibit agonist-induced activity (negative AM or NAM), or have no effect on activity (silent AM or SAM). Until now, it is not clear what the exact effects of AMs are on the orthosteric active site or the allosteric binding pocket(s). In the present work, we collected both the three-dimensional (3D) structures of receptor-orthosteric ligand and receptor-orthosteric ligand-AM complexes of a specific target protein. Using our novel algorithm toolset, molecular complex characterizing system (MCCS), we were able to quantify the key residues in both the orthosteric and allosteric binding sites along with potential changes of the binding pockets. After analyzing 21 pairs of 3D crystal or cryo-electron microscopy (cryo-EM) complexes, including 4 pairs of GPCRs, 5 pairs of ion channels, 11 pairs of enzymes, and 1 pair of transcription factors, we found that the binding of AMs had little impact on both the orthosteric and allosteric binding pockets. In return, given the accurately predicted allosteric binding pocket(s) of a drug target of medicinal interest, we can confidently conduct the virtual screening or lead optimization without concern that the huge conformational change of the pocket could lead to the low accuracy of virtual screening.

lncRNA KCNQ1OT1 reverses the effect of sevoflurane on hepatocellular carcinoma progression via regulating the miR-29a-3p/CBX3 axis

Sevoflurane (SEVO) is widely applied as an anesthetic, which exerts antitumor capacity in various cancers, including hepatocellular carcinoma (HCC). Previous studies indicated that long non-coding RNA KCNQ1 opposite strand/antisense transcript 1 (KCNQ1OT1) was upregulated, while microRNA-29a-3p (miR-29a-3p) was downregulated in HCC. Thus, we aimed to explore the roles of KCNQ1OT1 and miR-29a-3p in HCC cells exposed to SEVO. Cell proliferation, apoptosis, migration, and invasion were assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, flow cytometry, and transwell assays, respectively. The levels of genes were determined by quantitative real-time polymerase chain reaction (qRT-PCR) or western blot. Furthermore, the interaction between miR-29a-3p and KCNQ1OT1 or chromebox protein homolog 3 (CBX3) was predicted by Starbase or Targetscan, and then confirmed by dual-luciferase reporter assay. We found that the levels of KCNQ1OT1 and CBX3 were decreased, while miR-29a-3p was increased in SEVO-treated HCC cells. KCNQ1OT1 overexpression weakened the inhibitory effects of SEVO on HCC cell proliferation, apoptosis, migration, and invasion. Interestingly, KCNQ1OT1 bound to miR-29a-3p, and miR-29a-3p targeted CBX3. KCNQ1OT1 upregulated CBX3 level by repressing miR-29a-3p expression. Furthermore, KCNQ1OT1 exerted tumor promotion in HCC cells via suppressing miR-29a-3p to regulate CBX3 expression. Collectively, our findings demonstrated that KCNQ1OT1 regulated the antitumor effects of SEVO on HCC cells through modulating the miR-29a-3p/CBX3 axis, providing a theoretical basis for the treatment of HCC.

Sevoflurane excites nociceptive sensory neurons by inhibiting K+ conductances in rats

Sevoflurane, which is preferentially used as a day-case anesthetic based on its low blood solubility, acts on the central nervous system and exerts analgesic effects. However, it still remains unknown whether sevoflurane affects the excitability of nociceptive sensory neurons. Therefore, we conducted this study to examine the effects of sevoflurane on the excitability of small-sized dorsal root ganglion (DRG) neurons of rats using the whole-cell patch-clamp technique. In a voltage-clamp condition, sevoflurane elicited the membrane current in a concentration-dependent manner, in which the reversal potential was similar to the equilibrium potential of K⁺. In a current-clamp condition, sevoflurane directly depolarized the membrane potentials in a concentration-dependent manner. Moreover, at a clinically relevant concentration, sevoflurane decreased the threshold for action potential generation. These findings suggest that sevoflurane acts on the leak K⁺ channels to increase the excitability of DRG neurons. Sevoflurane increased the half-width of single action potentials, which resulted from the inhibition of voltage-gated K⁺ currents, including the fast inactivating A-type and non-inactivating delayed rectifier K⁺ currents. Our study indicates that sevoflurane could exhibit pronociceptive effects on nociceptive sensory neurons by inhibiting K⁺ conductances.

Electromechanical coupling of the Kv1.1 voltage-gated K+ channel is fine-tuned by the simplest amino acid residue in the S4-S5 linker

Investigating the Shaker-related K⁺ channel Kv1.1, the dysfunction of which is responsible for episodic ataxia 1 (EA1), at the functional and molecular level provides valuable understandings on normal channel dynamics, structural correlates underlying voltage-gating, and disease-causing mechanisms. Most studies focused on apparently functional amino acid residues composing voltage-gated K⁺ channels, neglecting the simplest ones. Glycine at position 311 of Kv1.1 is highly conserved both evolutionarily and within the Kv channel superfamily, is located in a region functionally relevant (the S4-S5 linker), and results in overt disease when mutated (p.G311D). By mutating the G311 residue to aspartate, we show here that the channel voltage-gating, activation, deactivation, inactivation, and window currents are markedly affected. In silico, modeling shows this glycine residue is strategically placed at one end of the linker helix which must be free to both bend and move past other portions of the protein during the channel's opening and closing. This is befitting of a glycine residue as its small neutral side chain allows for movement unhindered by interaction with any other amino acid. Results presented reveal the crucial importance of a distinct glycine residue, within the S4-S5 linker, in the voltage-dependent electromechanical coupling that control channel gating.

Hydrophobic Drug/Toxin Binding Sites in Voltage-Dependent K+ and Na+ Channels

In the Na_v channel family the lipophilic drugs/toxins binding sites and the presence of fenestrations in the channel pore wall are well defined and categorized. No such classification exists in the much larger K_v channel family, although certain lipophilic compounds seem to deviate from binding to well-known hydrophilic binding sites. By mapping different compound binding sites onto 3D structures of Kv channels, there appear to be three distinct lipid-exposed binding sites preserved in K_v channels: the front and back side of the pore domain, and S2-S3/S3-S4 clefts. One or a combination of these sites is most likely the orthologous equivalent of neurotoxin site 5 in Na_v channels. This review describes the different lipophilic binding sites and location of pore wall fenestrations within the K_v channel family and compares it to the knowledge of Na_v channels.

Donepezil Inhibits Acetylcholinesterase via Multiple Binding Modes at Room Temperature

Donepezil is a second generation acetylcholinesterase (AChE) inhibitor for treatment of Alzheimer's disease (AD). AChE is important for neurotransmission at neuromuscular junctions and cholinergic brain synapses by hydrolyzing acetylcholine into acetate and choline. In vitro data support that donepezil is a reversible, mixed competitive and noncompetitive inhibitor of AChE. The experimental fact then suggests a more complex binding mechanism beyond the molecular view in X-ray models resolved at cryogenic temperatures that show a unique binding mode of donepezil in the active site of the enzyme. Aiming at clarifying the mechanism behind that mixed competitive and noncompetitive nature of the inhibitor, we have applied molecular dynamics (MD) simulations and docking and free-energy calculations to investigate microscopic details and energetics of donepezil association for conditions of substrate-free and -bound states of the enzyme. Liquid-phase MD simulation at room temperature shows AChE transits between "open" and "closed" conformations to control accessibility to the active site and ligand binding. As shown by docking and free-energy calculations, association of donepezil involves its reversible axial displacement and reorientation in the active site of the enzyme, assisted by water molecules. Donepezil binds equally well the main-door anionic binding site PAS, the acyl pocket, and the catalytic site CAS by respectively adopting outward-inward-inward orientations regardless of substrate occupancy-the overall stability of that reaction process depends however on co-occupancy of the enzyme being preferential for its substrate-free state. All together, our findings support a physiologically relevant mechanism of AChE inhibition by donepezil involving multistable interactions modes at the molecular origin of the inhibitor's activity.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

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.

Sevoflurane Impairs Insulin Secretion and Tissue-Specific Glucose Uptake In Vivo

The use of anaesthetics severely influences substrate metabolism. This poses challenges for patients in clinical settings and for the use of animals in diabetes research. Sevoflurane can affect regulation of glucose homoeostasis at several steps, but the tissue-specific response remains to be determined. The aim of the study was to investigate the pharmacological effect of sevoflurane anaesthesia on glucose homoeostasis during hyperinsulinaemic clamp conditions, the gold standard method for assessment of whole-body insulin sensitivity. Conscious mice (n = 6) and mice under sevoflurane anaesthesia (n = 8) underwent a hyperinsulinaemic clamp where constant infusion of insulin and donor blood was administered during variable glucose infusion to maintain isoglycaemia. 2-[1-¹⁴ C]-deoxy-D-glucose was infused to determine tissue-specific uptake of glucose in adipose tissue, heart, brain and skeletal muscle. Sevoflurane anaesthesia severely impaired insulin-stimulated whole-body glucose uptake demonstrated by a 50% lower glucose infusion rate (GIR). This was associated with decreased glucose uptake in brain, soleus, triceps and gastrocnemius muscles in sevoflurane-anaesthetized mice compared to conscious mice. Plasma-free fatty acids (FFA), a potent inducer of insulin resistance, increased by 42% in mice during sevoflurane anaesthesia. In addition, insulin secretion from pancreatic β-cell was lower in fasted, anaesthetized mice. Sevoflurane anaesthesia impairs insulin secretion, induces insulin resistance in mice and reduces glucose uptake in non-insulin-sensitive tissue like the brain. The underlying mechanisms may involve sevoflurane-induced mobilization of FFA.

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.

Regulatory Effect of General Anesthetics on Activity of Potassium Channels

General anesthesia is an unconscious state induced by anesthetics for surgery. The molecular targets and cellular mechanisms of general anesthetics in the mammalian nervous system have been investigated during past decades. In recent years, K⁺ channels have been identified as important targets of both volatile and intravenous anesthetics. This review covers achievements that have been made both on the regulatory effect of general anesthetics on the activity of K⁺ channels and their underlying mechanisms. Advances in research on the modulation of K⁺ channels by general anesthetics are summarized and categorized according to four large K⁺ channel families based on their amino-acid sequence homology. In addition, research achievements on the roles of K⁺ channels in general anesthesia in vivo, especially with regard to studies using mice with K⁺ channel knockout, are particularly emphasized.

Effects of neoadjuvant chemotherapy on minimum alveolar concentration values of sevoflurane and desflurane in patients with hepatocellular carcinoma complicated with jaundice

The effects of neoadjuvant chemotherapy on the minimum alveolar concentration (MAC) values of sevoflurane and desflurane in patients with hepatocellular carcinoma (HCC) complicated with jaundice were investigated. Eighty patients with HCC complicated with jaundice were selected. Forty patients underwent the neoadjuvant chemotherapy and were grouped into the desflurane group (Group D) and the sevoflurane group (Group S). Patients in all chemotherapy groups received 2 cycles of chemotherapy prior to surgery and underwent surgical treatment 3 weeks after chemotherapy. The remaining 40 patients in the control group were divided into the desflurane group (Group C1) and the sevoflurane group (Group C2). Changes in MAP, HR and BIS at different time points before and after anesthesia induction and skin incision were compared among the groups. Results showed that there were no significant differences in MAP, HR and BIS before anesthesia induction (T0) (P>0.05); at each time point from T1 to T6, MAP, HR and BIS of Group D were significantly lower than those of Group C1 (P>0.05). Furthermore, MAP, HR and BIS of Group S were significantly lower than those of Group C2 (P>0.05). The MAC^Mean of sevoflurane and desflurane were compared among all patient groups using the mean method. MAC^Mean values of Group D were significantly lower than those of Group C1 (P<0.05). Notably, MAC^Dixon values of sevoflurane and desflurane were compared among all patient groups using the Dixon method and the differences were statistically significant (P<0.05). Logistic regression analyses were conducted, respectively, which revealed that the MAC of sevoflurane and desflurane were associated with whether patients received the neoadjuvant chemotherapy. MAC^Log of sevoflurane and desflurane were decreased in patients receiving the neoadjuvant chemotherapy. The results suggested that neoadjuvant chemotherapy can reduce MAC values of sevoflurane and desflurane in HCC patients complicated with jaundice and may improve these patients' sensitivity to sevoflurane and desflurane.

Time-Resolved Neutron Interferometry and the Mechanism of Electromechanical Coupling in Voltage-Gated Ion Channels

The mechanism of electromechanical coupling for voltage-gated ion channels (VGICs) involved in neurological signal transmission, primarily Nav- and Kv-channels, remains unresolved. Anesthetics have been shown to directly impact this mechanism, at least for Kv-channels. Molecular dynamics computer simulations can now predict the structures of VGICs embedded within a hydrated phospholipid bilayer membrane as a function of the applied transmembrane voltage, but significant assumptions are still necessary. Nevertheless, these simulations are providing new insights into the mechanism of electromechanical coupling at the atomic level in 3-D. We show that time-resolved neutron interferometry can be used to investigate directly the profile structure of a VGIC, vectorially oriented within a single hydrated phospholipid bilayer membrane at the solid-liquid interface, as a function of the applied transmembrane voltage in the absence of any assumptions or potentially perturbing modifications of the VGIC protein and/or the host membrane. The profile structure is a projection of the membrane's 3-D structure onto the membrane normal and, in the absence of site-directed deuterium labeling, is provided at substantially lower spatial resolution than the atomic level. Nevertheless, this novel approach can be used to directly test the validity of the predictions from molecular dynamics simulations. We describe the key elements of our novel experimental approach, including why each is necessary and important to providing the essential information required for this critical comparison of "simulation" vs "experiment." In principle, the approach could be extended to higher spatial resolution and to include the effects of anesthetics on the electromechanical coupling mechanism in VGICs.

Electrophysiological Analysis of Voltage-Gated Ion Channel Modulation by General Anesthetics

Voltage-gated ion channels (VGICs) of excitable tissues are emerging as targets likely involved in both the therapeutic and toxic effects of inhaled and intravenous general anesthetics. Whereas sevoflurane and propofol inhibit voltage-gated Na⁺ channels (Navs), sevoflurane potentiates certain voltage-gated K⁺ channels (Kvs). The combination of these effects would dampen neural excitability and, therefore, might contribute to the clinical endpoints of general anesthesia. As the body of work regarding the interaction of general anesthetics with VGICs continues to grow, a multidisciplinary approach involving functional, biochemical, structural, and computational techniques, many of which are detailed in other chapters, has increasingly become necessary to solve the molecular mechanism of general anesthetic action on VGICs. Here, we focus on electrophysiological and modeling approaches and methodologies to describe how our work has elucidated the biophysical basis of the inhibition Navs by propofol and the potentiation of Kvs by sevoflurane.

Identification of binding sites contributing to volatile anesthetic effects on GABA type A receptors

Most general anesthetics enhance GABA type A (GABA_A) receptor activity at clinically relevant concentrations. Sites of action of volatile anesthetics on the GABA_A receptor remain unknown, whereas sites of action of many intravenous anesthetics have been identified in GABA_A receptors by using photolabeling. Here, we used photoactivatable analogs of isoflurane (AziISO) and sevoflurane (AziSEVO) to locate their sites on α₁β₃γ_2L and α₁β₃ GABA_A receptors. As with isoflurane and sevoflurane, AziISO and AziSEVO enhanced the currents elicited by GABA. AziISO and AziSEVO each labeled 10 residues in α₁β₃ receptors and 9 and 8 residues, respectively, in α₁β₃γ_2L receptors. Photolabeled residues were concentrated in transmembrane domains and located in either subunit interfaces or in the interface between the extracellular domain and the transmembrane domain. The majority of these transmembrane residues were protected from photolabeling with the addition of excess parent anesthetic, which indicated specificity. Binding sites were primarily located within α+/β- and β+/α- subunit interfaces, but residues in the α+/γ- interface were also identified, which provided a basis for differential receptor subtype sensitivity. Isoflurane and sevoflurane did not always share binding sites, which suggests an unexpected degree of selectivity.-Woll, K. A., Zhou, X., Bhanu, N. V., Garcia, B. A., Covarrubias, M., Miller, K. W., Eckenhoff, R. G. Identification of binding sites contributing to volatile anesthetic effects on GABA type A receptors.

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.

Photoaffinity Ligand for the Inhalational Anesthetic Sevoflurane Allows Mechanistic Insight into Potassium Channel Modulation

Sevoflurane is a commonly used inhaled general anesthetic. Despite this, its mechanism of action remains largely elusive. Compared to other anesthetics, sevoflurane exhibits distinct functional activity. In particular, sevoflurane is a positive modulator of voltage-gated Shaker-related potassium channels (K_v1.x), which are key regulators of action potentials. Here, we report the synthesis and validation of azisevoflurane, a photoaffinity ligand for the direct identification of sevoflurane binding sites in the K_v1.2 channel. Azisevoflurane retains major sevoflurane protein binding interactions and pharmacological properties within in vivo models. Photoactivation of azisevoflurane induces adduction to amino acid residues that accurately reported sevoflurane protein binding sites in model proteins. Pharmacologically relevant concentrations of azisevoflurane analogously potentiated wild-type K_v1.2 and the established mutant K_v1.2 G329T. In wild-type K_v1.2 channels, azisevoflurane photolabeled Leu317 within the internal S4-S5 linker, a vital helix that couples the voltage sensor to the pore region. A residue lining the same binding cavity was photolabeled by azisevoflurane and protected by sevoflurane in the K_v1.2 G329T. Mutagenesis of Leu317 in WT K_v1.2 abolished sevoflurane voltage-dependent positive modulation. Azisevoflurane additionally photolabeled a second distinct site at Thr384 near the external selectivity filter in the K_v1.2 G329T mutant. The identified sevoflurane binding sites are located in critical regions involved in gating of K_v channels and related ion channels. Azisevoflurane has thus emerged as a new tool to discover inhaled anesthetic targets and binding sites and investigate contributions of these targets to general anesthesia.

Isoflurane modulates activation and inactivation gating of the prokaryotic Na+ channel NaChBac

The pharmacological effects of inhaled anesthetics on ion channel function are poorly understood. Sand et al. analyze macroscopic gating of the prokaryotic voltage-gated sodium channel, NaChBac, using a six-state kinetic scheme and demonstrate that isoflurane modulates microscopic gating properties.

Voltage-gated Na⁺ channels (Na_v) have emerged as important presynaptic targets for volatile anesthetic (VA) effects on synaptic transmission. However, the detailed biophysical mechanisms by which VAs modulate Na_v function remain unclear. VAs alter macroscopic activation and inactivation of the prokaryotic Na⁺ channel, NaChBac, which provides a useful structural and functional model of mammalian Na_v. Here, we study the effects of the common general anesthetic isoflurane on NaChBac function by analyzing macroscopic Na⁺ currents (I_Na) in wild-type (WT) channels and mutants with impaired (G229A) or enhanced (G219A) inactivation. We use a previously described six-state Markov model to analyze empirical WT and mutant NaChBac channel gating data. The model reproduces the mean empirical gating manifest in I_Na time courses and optimally estimates microscopic rate constants, valences (z), and fractional electrical distances (x) of forward and backward transitions. The model also reproduces gating observed for all three channels in the absence or presence of isoflurane, providing further validation. We show using this model that isoflurane increases forward activation and inactivation rate constants at 0 mV, which are associated with estimated chemical free energy changes of approximately −0.2 and −0.7 kcal/mol, respectively. Activation is voltage dependent (z ≈ 2e₀, x ≈ 0.3), inactivation shows little voltage dependence, and isoflurane has no significant effect on either. Forward inactivation rate constants are more than 20-fold greater than backward rate constants in the absence or presence of isoflurane. These results indicate that isoflurane modulates NaChBac gating primarily by increasing forward activation and inactivation rate constants. These findings support accumulating evidence for multiple sites of anesthetic interaction with the channel.

Sites and Functional Consequence of Alkylphenol Anesthetic Binding to Kv1.2 Channels

Inhalational general anesthetics, such as sevoflurane and isoflurane, modulate a subset of brain Kv1 potassium channels. However, the Kv1.2 channel is resistant to propofol, a commonly used intravenous alkylphenol anesthetic. We hypothesize that propofol binds to a presumed pocket involving the channel's S4-S5 linker, but functional transduction is poor and, therefore, propofol efficacy is low. To test this hypothesis, we used a photoactive propofol analog (meta-aziPropofol = AziPm) to directly probe binding and electrophysiological and mutational analyses in Xenopus oocytes to probe function. We find that AziPm photolabels L321 in the S4-S5 linker of both the wild-type Kv1.2 and a mutant Kv1.2 (G329 T) with a novel gating phenotype. Furthermore, whereas propofol does not significantly modulate Kv1.2 WT but robustly potentiates Kv1.2 G329T, AziPm inhibits Kv1.2 WT and also potentiates Kv1.2 G329T. Kv1.2 modulation by AziPm was abolished by two mutations that decreased hydrophobicity at L321 (L321A and L321F), confirming the specific significance of the S4-S5 linker in the mechanism of general anesthetic modulation. Since AziPm binds to Kv1.2 G329T and shares the propofol ability to potentiate this mutant, the parent propofol likely also binds to the Kv1.2 channel. However, binding and alkylphenol-induced transduction are seemingly sensitive to the conformation of the S4-S5 linker site (altered by G329T) and subtle differences in the chemical structures of propofol and AziPm. Overall, the results are consistent with a mechanism of general anesthetic modulation that depends on the complementarity of necessary ligand binding and permissive ion channel conformations that dictate modulation and efficacy.