Molecular mapping of general anesthetic sites in a voltage-gated ion channel

HC. Relevance of Cortical and Hippocampal Interneuron Functional Diversity to General Anesthetic Mechanisms: A Narrative Review

General anesthetics disrupt brain processes involved in consciousness by altering synaptic patterns of excitation and inhibition. In the cerebral cortex and hippocampus, GABAergic inhibition is largely mediated by inhibitory interneurons, a heterogeneous group of specialized neuronal subtypes that form characteristic microcircuits with excitatory neurons. Distinct interneuron subtypes regulate specific excitatory neuron networks during normal behavior, but how these interneuron subtypes are affected by general anesthetics is unclear. This narrative review summarizes current principles of the synaptic architecture of cortical and interneuron subtypes, their contributions to different forms of inhibition, and their roles in distinct neuronal microcircuits. The molecular and cellular targets in these circuits that are sensitive to anesthetics are reviewed in the context of how anesthetics impact interneuron function in a subtype-specific manner. The implications of this functional interneuron diversity for mechanisms of anesthesia are discussed, as are their implications for anesthetic-induced changes in neural plasticity and overall brain function.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

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.

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.

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.

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.

Protonation state of inhibitors determines interaction sites within voltage-gated sodium channels

Voltage-gated sodium channels are essential for carrying electrical signals throughout the body, and mutations in these proteins are responsible for a variety of disorders, including epilepsy and pain syndromes. As such, they are the target of a number of drugs used for reducing pain or combatting arrhythmias and seizures. However, these drugs affect all sodium channel subtypes found in the body. Designing compounds to target select sodium channel subtypes will provide a new therapeutic pathway and would maximize treatment efficacy while minimizing side effects. Here, we examine the binding preferences of nine compounds known to be sodium channel pore blockers in molecular dynamics simulations. We use the approach of replica exchange solute tempering (REST) to gain a more complete understanding of the inhibitors' behavior inside the pore of NavMs, a bacterial sodium channel, and NavPas, a eukaryotic sodium channel. Using these simulations, we are able to show that both charged and neutral compounds partition into the bilayer, but neutral forms more readily cross it. We show that there are two possible binding sites for the compounds: (i) a site on helix 6, which has been previously determined by many experimental and computational studies, and (ii) an additional site, occupied by protonated compounds in which the positively charged part of the drug is attracted into the selectivity filter. Distinguishing distinct binding poses for neutral and charged compounds is essential for understanding the nature of pore block and will aid the design of subtype-selective sodium channel inhibitors.

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.

A critical test of Drosophila anaesthetics: Isoflurane and sevoflurane are benign alternatives to cold and CO2

Anaesthesia is often a necessary step when studying insects like the model organism Drosophila melanogaster. Most studies of Drosophila and other insects that require anaesthesia use either cold exposure or carbon dioxide exposure to induce a narcotic state. These anaesthetic methods are known to disrupt physiology and behavior with increasing exposure, and thus ample recovery time is required prior to experimentation. Here, we examine whether two halogenated ethers commonly used in vertebrate anaesthesia, isoflurane and sevoflurane, may serve as alternative means of insect anaesthesia. Using D. melanogaster, we generated dose-response curves to identify exposure times for each anaesthetic (cold, CO₂, isoflurane and sevoflurane) that allow for five-minutes of experimental manipulation of the animals after the anaesthetic was removed (i.e. 5min recovery doses). We then compared the effects of this practical dose on high temperature, low temperature, starvation, and desiccation tolerance, as well as locomotor activity and fecundity of female flies following recovery from anaesthesia. Cold, CO₂ and isoflurane each had significant or near significant effects on the traits measured, but the specific effects of each anaesthetic differed, and effects on stress tolerance generally did not persist if the flies were given 48h to recover from anaesthesia. Sevoflurane had no measureable effect on any of the traits examined. Care must be taken when choosing an anaesthetic in Drosophila research, as the impacts of specific anaesthetics on stress tolerance, behavior and reproduction can widely differ. Sevoflurane may be a practical alternative to cold and CO₂ anaesthesia in insects - particularly if flies are to be used for experiments shortly after anesthesia.

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.

Biophysical studies of cholesterol effects on chromatin

Changes in chromatin structure regulate gene expression and genome maintenance. Molecules that bind to the nucleosome, the complex of DNA and histone proteins, are key modulators of chromatin structure. Previous work indicated that cholesterol, a ubiquitous cellular lipid, may bind to chromatin in vivo, suggesting a potential function for lipids in modulating chromatin architecture. However, the molecular mechanisms of cholesterol’s action on chromatin structure have remained unclear. Here, we explored the biophysical impact of cholesterol on nucleosome and chromatin fibers reconstituted in vitro and characterized in silico the cholesterol binding to the nucleosome. Our findings support that cholesterol assists 10 and 30 nm chromatin formation and induces folding of long chromatin fibers as a result of direct interaction of the cholesterol to six nucleosomal binding sites.

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.

Mechanistic Insights into the Modulation of Voltage-Gated Ion Channels by Inhalational Anesthetics

General anesthesia is a relatively safe medical procedure, which for nearly 170 years has allowed life saving surgical interventions in animals and people. However, the molecular mechanism of general anesthesia continues to be a matter of importance and debate. A favored hypothesis proposes that general anesthesia results from direct multisite interactions with multiple and diverse ion channels in the brain. Neurotransmitter-gated ion channels and two-pore K+ channels are key players in the mechanism of anesthesia; however, new studies have also implicated voltage-gated ion channels. Recent biophysical and structural studies of Na+ and K+ channels strongly suggest that halogenated inhalational general anesthetics interact with gates and pore regions of these ion channels to modulate function. Here, we review these studies and provide a perspective to stimulate further advances.

Gambierol and n-alkanols inhibit Shaker Kv channel via distinct binding sites outside the K(+) pore

The marine polycyclic-ether toxin gambierol and 1-butanol (n-alkanol) inhibit Shaker-type Kv channels by interfering with the gating machinery. Competition experiments indicated that both compounds do not share an overlapping binding site but gambierol is able to affect 1-butanol affinity for Shaker through an allosteric effect. Furthermore, the Shaker-P475A mutant, which inverses 1-butanol effect, is inhibited by gambierol with nM affinity. Thus, gambierol and 1-butanol inhibit Shaker-type Kv channels via distinct parts of the gating machinery.

Voltage-sensor conformation shapes the intra-membrane drug binding site that determines gambierol affinity in Kv channels

Marine ladder-shaped polyether toxins are implicated in neurological symptoms of fish-borne food poisonings. The toxin gambierol, produced by the marine dinoflagellate Gambierdiscus toxicus, belongs to the group of ladder-shaped polyether toxins and inhibits Kv3.1 channels with nanomolar affinity through a mechanism of gating modification. Binding determinants for gambierol localize at the lipid-exposed interface of the pore forming S5 and S6 segments, suggesting that gambierol binds outside of the permeation pathway. To explore a possible involvement of the voltage-sensing domain (VSD), we made different chimeric channels between Kv3.1 and Kv2.1, exchanging distinct parts of the gating machinery. Our results showed that neither the electro-mechanical coupling nor the S1-S3a region of the VSD affect gambierol sensitivity. In contrast, the S3b-S4 part of the VSD (paddle motif) decreased gambierol sensitivity in Kv3.1 more than 100-fold. Structure determination by homology modeling indicated that the position of the S3b-S4 paddle and its primary structure defines the shape and∖or the accessibility of the binding site for gambierol, explaining the observed differences in gambierol affinity between the channel chimeras. Furthermore, these findings explain the observed difference in gambierol affinity for the closed and open channel configurations of Kv3.1, opening new possibilities for exploring the VSDs as selectivity determinants in drug design.

Alkanols inhibit voltage-gated K(+) channels via a distinct gating modifying mechanism that prevents gate opening

Alkanols are small aliphatic compounds that inhibit voltage-gated K⁺ (K_v) channels through a yet unresolved gating mechanism. K_v channels detect changes in the membrane potential with their voltage-sensing domains (VSDs) that reorient and generate a transient gating current. Both 1-Butanol (1-BuOH) and 1-Hexanol (1-HeOH) inhibited the ionic currents of the Shaker K_v channel in a concentration dependent manner with an IC₅₀ value of approximately 50 mM and 3 mM, respectively. Using the non-conducting Shaker-W434F mutant, we found that both alkanols immobilized approximately 10% of the gating charge and accelerated the deactivating gating currents simultaneously with ionic current inhibition. Thus, alkanols prevent the final VSD movement(s) that is associated with channel gate opening. Applying 1-BuOH and 1-HeOH to the Shaker-P475A mutant, in which the final gating transition is isolated from earlier VSD movements, strengthened that neither alkanol affected the early VSD movements. Drug competition experiments showed that alkanols do not share the binding site of 4-aminopyridine, a drug that exerts a similar effect at the gating current level. Thus, alkanols inhibit Shaker-type K_v channels via a unique gating modifying mechanism that stabilizes the channel in its non-conducting activated state.

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

Inhalational general anesthesia results from the poorly understood interactions of haloethers with multiple protein targets, which prominently includes ion channels in the nervous system. Previously, we reported that the commonly used inhaled anesthetic sevoflurane potentiates the activity of voltage-gated K⁺ (Kv) channels, specifically, several mammalian Kv1 channels and the Drosophila K-Shaw2 channel. Also, previous work suggested that the S4-S5 linker of K-Shaw2 plays a role in the inhibition of this Kv channel by n-alcohols and inhaled anesthetics. Here, we hypothesized that the S4-S5 linker is also a determinant of the potentiation of Kv1.2 and K-Shaw2 by sevoflurane. Following functional expression of these Kv channels in Xenopus oocytes, we found that converse mutations in Kv1.2 (G329T) and K-Shaw2 (T330G) dramatically enhance and inhibit the potentiation of the corresponding conductances by sevoflurane, respectively. Additionally, Kv1.2-G329T impairs voltage-dependent gating, which suggests that Kv1.2 modulation by sevoflurane is tied to gating in a state-dependent manner. Toward creating a minimal Kv1.2 structural model displaying the putative sevoflurane binding sites, we also found that the positive modulations of Kv1.2 and Kv1.2-G329T by sevoflurane and other general anesthetics are T1-independent. In contrast, the positive sevoflurane modulation of K-Shaw2 is T1-dependent. In silico docking and molecular dynamics-based free-energy calculations suggest that sevoflurane occupies distinct sites near the S4-S5 linker, the pore domain and around the external selectivity filter. We conclude that the positive allosteric modulation of the Kv channels by sevoflurane involves separable processes and multiple sites within regions intimately involved in channel gating.

Volatile anesthetics inhibit sodium channels without altering bulk lipid bilayer properties

Although general anesthetics are clinically important and widely used, their molecular mechanisms of action remain poorly understood. Volatile anesthetics such as isoflurane (ISO) are thought to alter neuronal function by depressing excitatory and facilitating inhibitory neurotransmission through direct interactions with specific protein targets, including voltage-gated sodium channels (Na(v)). Many anesthetics alter lipid bilayer properties, suggesting that ion channel function might also be altered indirectly through effects on the lipid bilayer. We compared the effects of ISO and of a series of fluorobenzene (FB) model volatile anesthetics on Na(v) function and lipid bilayer properties. We examined the effects of these agents on Na(v) in neuronal cells using whole-cell electrophysiology, and on lipid bilayer properties using a gramicidin-based fluorescence assay, which is a functional assay for detecting changes in lipid bilayer properties sensed by a bilayer-spanning ion channel. At clinically relevant concentrations (defined by the minimum alveolar concentration), both the FBs and ISO produced prepulse-dependent inhibition of Na(v) and shifted the voltage dependence of inactivation toward more hyperpolarized potentials without affecting lipid bilayer properties, as sensed by gramicidin channels. Only at supra-anesthetic (toxic) concentrations did ISO alter lipid bilayer properties. These results suggest that clinically relevant concentrations of volatile anesthetics alter Na(v) function through direct interactions with the channel protein with little, if any, contribution from changes in bulk lipid bilayer properties. Our findings further suggest that changes in lipid bilayer properties are not involved in clinical anesthesia.

Butanol isomers exert distinct effects on voltage-gated calcium channel currents and thus catecholamine secretion in adrenal chromaffin cells

Butanol (C₄H₁₀OH) has been used both to dissect the molecular targets of alcohols/general anesthetics and to implicate phospholipase D (PLD) signaling in a variety of cellular functions including neurotransmitter and hormone exocytosis. Like other primary alcohols, 1-butanol is a substrate for PLD and thereby disrupts formation of the intracellular signaling lipid phosphatidic acid. Because secondary and tertiary butanols do not undergo this transphosphatidylation, they have been used as controls for 1-butanol to implicate PLD signaling. Recently, selective pharmacological inhibitors of PLD have been developed and, in some cases, fail to block cellular functions previously ascribed to PLD using primary alcohols. For example, exocytosis of insulin and degranulation of mast cells are blocked by primary alcohols, but not by the PLD inhibitor FIPI. In this study we show that 1-butanol reduces catecholamine secretion from adrenal chromaffin cells to a much greater extent than tert-butanol, and that the PLD inhibitor VU0155056 has no effect. Using fluorescent imaging we show the effect of these drugs on depolarization-evoked calcium entry parallel those on secretion. Patch-clamp electrophysiology confirmed the peak amplitude of voltage-gated calcium channel currents (I_Ca) is inhibited by 1-butanol, with little or no block by secondary or tert-butanol. Detailed comparison shows for the first time that the different butanol isomers exert distinct, and sometimes opposing, effects on the voltage-dependence and gating kinetics of I_Ca. We discuss these data with regard to PLD signaling in cellular physiology and the molecular targets of general anesthetics.

Modulation of a voltage-gated Na+ channel by sevoflurane involves multiple sites and distinct mechanisms

Halogenated inhaled general anesthetic agents modulate voltage-gated ion channels, but the underlying molecular mechanisms are not understood. Many general anesthetic agents regulate voltage-gated Na(+) (NaV) channels, including the commonly used drug sevoflurane. Here, we investigated the putative binding sites and molecular mechanisms of sevoflurane action on the bacterial NaV channel NaChBac by using a combination of molecular dynamics simulation, electrophysiology, and kinetic analysis. Structural modeling revealed multiple sevoflurane interaction sites possibly associated with NaChBac modulation. Electrophysiologically, sevoflurane favors activation and inactivation at low concentrations (0.2 mM), and additionally accelerates current decay at high concentrations (2 mM). Explaining these observations, kinetic modeling suggests concurrent destabilization of closed states and low-affinity open channel block. We propose that the multiple effects of sevoflurane on NaChBac result from simultaneous interactions at multiple sites with distinct affinities. This multiple-site, multiple-mode hypothesis offers a framework to study the structural basis of general anesthetic action.

Novel activation of voltage-gated K(+) channels by sevoflurane

BACKGROUND

Halogenated inhaled anesthetics modulate voltage-gated ion channels by unknown mechanisms.

RESULTS

Biophysical analyses revealed novel activation of K(v) channels by the inhaled anesthetic sevoflurane.

CONCLUSION

K(v) channel activation by sevoflurane results from the positive allosteric modulation of activation gating.

SIGNIFICANCE

The unique activation of K(v) channels by sevoflurane demonstrates novel anesthetic specificity and offers new insights into allosteric modulation of channel gating. Voltage-gated ion channels are modulated by halogenated inhaled general anesthetics, but the underlying molecular mechanisms are not understood. Alkanols and halogenated inhaled anesthetics such as halothane and isoflurane inhibit the archetypical voltage-gated Kv3 channel homolog K-Shaw2 by stabilizing the resting/closed states. By contrast, sevoflurane, a more heavily fluorinated ether commonly used in general anesthesia, specifically activates K-Shaw2 currents at relevant concentrations (0.05-1 mM) in a rapid and reversible manner. The concentration dependence of this modulation is consistent with the presence of high and low affinity interactions (K(D) = 0.06 and 4 mM, respectively). Sevoflurane (<1 mM) induces a negative shift in the conductance-voltage relation and increases the maximum conductance. Furthermore, suggesting possible roles in general anesthesia, mammalian Kv1.2 and Kv1.5 channels display similar changes. Quantitative description of the observations by an economical allosteric model indicates that sevoflurane binding favors activation gating and eliminates an unstable inactivated state outside the activation pathway. This study casts light on the mechanism of the novel sevoflurane-dependent activation of Kv channels, which helps explain how closely related inhaled anesthetics achieve specific actions and suggests strategies to develop novel Kv channel activators.

Insight into the modulation of Shaw2 Kv channels by general anesthetics: structural and functional studies of S4-S5 linker and S6 C-terminal peptides in micelles by NMR

The modulation of the Drosophila Shaw2 Kv channel by 1-alkanols and inhaled anesthetics is correlated with the involvement of the S4-S5 linker and C-terminus of S6, and consistent with stabilization of the channel's closed state. Structural analysis of peptides from S4-S5 (L45) and S6 (S6c), by nuclear magnetic resonance and circular dichroism spectroscopy supports that an α-helical conformation was adopted by L45, while S6c was only in an unstable/dynamic partially folded α-helix in dodecylphosphocholine micelles. Solvent accessibility and paramagnetic probing of L45 revealed that L45 lies parallel to the surface of micelles with charged and polar residues pointing towards the solution while hydrophobic residues are buried inside the micelles. Chemical shift perturbation introduced by 1-butanol on residues Gln320, Thr321, Phe322 and Arg323 of L45, as well as Thr423 and Gln424 of S6c indicates possible anesthetic binding sites on these two important components in the channel activation apparatus. Diffusion measurements confirmed the association of L45, S6c and 1-butanol with micelles which suggests the capability of 1-butanol to influence a possible interaction of L45 and S6c in the micelle environment.

Perturbed voltage-gated channel activity in perturbed bilayers: implications for ectopic arrhythmias arising from damaged membrane

The ceaseless opening and closing of the voltage-gated channels (VGCs) underlying cardiac rhythmicity is controlled, in each VGC, by four mobile voltage sensors embedded in bilayer. Every action potential necessitates extensive packing/repacking of voltage sensor domains with adjacent interacting lipid molecules. This renders VGC activity mechanosensitive (MS), i.e., energetically sensitive to the bilayer's mechanical state. Irreversible perturbations of sarcolemmal bilayer such as those associated with ischemia, reperfusion, inflammation, cortical-cytoskeleton abnormalities, bilayer-disrupting toxins, diet aberrations, etc, should therefore perturb VGC activity. Disordered/fluidized bilayer states that facilitate voltage sensor repacking, and thus make VGC opening too easy could, therefore, explain VGC-leakiness in these conditions. To study this in membrane patches we impose mechanical blebbing injury during pipette aspiration-induced membrane stretch, a process that modulates VGC activity irreversibly (plastic regime) and then, eventually, reversibly (elastic regime). Because of differences in sensor-to-gate coupling among different VGCs, their responses to stretch fall into two major categories, MS-Speed, MS-Number, exemplified by Nav and Cav channels. For particular VGCs in perturbed bilayers, leak mechanisms depend on whether or not the rate-limiting voltage-dependent step is MS. Mode-switch transitions might also be mechanosensitive and thus play a role. Incorporated mathematically in axon models, plastic-regime Nav responses elicit ectopic firing behaviors typical of peripheral neuropathies. In cardiomyocytes with mild bleb damage, Nav and/or Cav leaks from irreversible MS modulation (MS-Speed, MS-Number, respectively) could, similarly, foster ectopic arrhythmias. Where pathologically leaky VGCs reside in damaged bilayer, peri-channel bilayer disorder/fluidity conditions could be an important "target feature" for anti-arrhythmic VGC drugs.

Computer Simulations of Voltage-Gated Cation Channels

The relentless growth in computational power has seen increasing applications of molecular dynamics (MD) simulation to the study of membrane proteins in realistic membrane environments, which include explicit membrane lipids, water and ions. The concomitant increasing availability of membrane protein structures for ion channels, and transporters -- to name just two examples -- has stimulated many of these MD studies. In the case of voltage-gated cation channels (VGCCs) recent computational works have focused on ion-conduction and gating mechanisms, along with their regulation by agonist/antagonist ligands. The information garnered from these computational studies is largely inaccessible to experiment and is crucial for understanding the interplay between the structure and function as well as providing new directions for experiments. This article highlights recent advances in probing the structure and function of potassium channels and offers a perspective on the challenges likely to arise in making analogous progress in characterizing sodium channels.