Molecular determinants of voltage-dependent gating and binding of pore-blocking drugs in transmembrane segment IIIS6 of the Na(+) channel alpha subunit

Structural mapping of Nav1.7 antagonists

Voltage-gated sodium (Na_v) channels are targeted by a number of widely used and investigational drugs for the treatment of epilepsy, arrhythmia, pain, and other disorders. Despite recent advances in structural elucidation of Na_v channels, the binding mode of most Na_v-targeting drugs remains unknown. Here we report high-resolution cryo-EM structures of human Na_v1.7 treated with drugs and lead compounds with representative chemical backbones at resolutions of 2.6-3.2 Å. A binding site beneath the intracellular gate (site BIG) accommodates carbamazepine, bupivacaine, and lacosamide. Unexpectedly, a second molecule of lacosamide plugs into the selectivity filter from the central cavity. Fenestrations are popular sites for various state-dependent drugs. We show that vinpocetine, a synthetic derivative of a vinca alkaloid, and hardwickiic acid, a natural product with antinociceptive effect, bind to the III-IV fenestration, while vixotrigine, an analgesic candidate, penetrates the IV-I fenestration of the pore domain. Our results permit building a 3D structural map for known drug-binding sites on Na_v channels summarized from the present and previous structures.

Calculations of the binding free energies of the Comprehensive in vitro Proarrhythmia Assay (CiPA) reference drugs to cardiac ion channels

The evaluation of the inhibitory activities of drugs on multiple cardiac ion channels is required for the accurate assessment of proarrhythmic risks. Moreover, the in silico prediction of such inhibitory activities of drugs on cardiac channels can improve the efficiency of the drug-development process. Here, we performed molecular docking simulations to predict the complex structures of 25 reference drugs that were proposed by the Comprehensive in vitro Proarrhythmia Assay consortium using two cardiac ion channels, the human ether-a-go-go-related gene (hERG) potassium channel and human NaV1.5 (hNaV1.5) sodium channel, with experimentally available structures. The absolute binding free energy (ΔGbind) values of the predicted structures were calculated by a molecular dynamics-based method and compared with the experimental half-maximal inhibitory concentration (IC50) data. Furthermore, the regression analysis between the calculated values and negative of the common logarithm of the experimental IC50 values (pIC50) revealed that the calculated values of four and ten drugs deviated significantly from the regression lines of the hERG and hNaV1.5 channels, respectively. We reconsidered the docking poses and protonation states of the drugs based on the experimental data and recalculated their ΔGbind values. Finally, the calculated ΔGbind values of 24 and 19 drugs correlated with their experimental pIC50 values (coefficients of determination=0.791 and 0.613 for the hERG and hNaV1.5 channels, respectively). Thus, the regression analysis between the calculated ΔGbind and experimental IC50 data ensured the realization of an increased number of reliable complex structures.

Molecular determinants of μ-conotoxin KIIIA interaction with the human voltage-gated sodium channel NaV1.7

The voltage-gated sodium (NaV) channel subtype NaV1.7 plays a critical role in pain signaling, making it an important drug target. Here we studied the molecular interactions between μ-Conotoxin KIIIA (KIIIA) and the human NaV1.7 channel (hNaV1.7). We developed a structural model of hNaV1.7 using Rosetta computational modeling and performed in silico docking of KIIIA using RosettaDock to predict residues forming specific pairwise contacts between KIIIA and hNaV1.7. We experimentally validated these contacts using mutant cycle analysis. Comparison between our KIIIA-hNaV1.7 model and the cryo-EM structure of KIIIA-hNaV1.2 revealed key similarities and differences between NaV channel subtypes with potential implications for the molecular mechanism of toxin block. The accuracy of our integrative approach, combining structural data with computational modeling, experimental validation, and molecular dynamics simulations, suggests that Rosetta structural predictions will be useful for rational design of novel biologics targeting specific NaV channels.

Molecular basis of the different effects of procainamide and N-acetylprocainamide on the maximum upstroke velocity and half-decay time of the cardiac action potential in guinea pig papillary muscle

Procainamide (PA) and its in vivo metabolite, N-acetylprocainamide (NAPA), display some pharmacological differences. Although it is agreed that PA is a class IA antiarrhythmic, it has been reported that NAPA is a pure class III antiarrhythmic that affects only the repolarizing phase of the cardiac action potential. This last concept, observed exclusively in dogs, gained wide acceptance, appearing in classic pharmacology textbooks. However, evidence in species such as mice and rats indicates that NAPA can affect cardiac Na+ channels, which is unexpected for a pure class III antiarrhythmic drug. To further clarify this issue, the effects of PA (used as a reference drug) and NAPA on the maximum upstroke velocity (Vmax) and half-decay time (HDT) of the cardiac action potential were examined in the isolated right papillaris magnus of the guinea pig heart. Both PA and NAPA affected Vmax at lower concentrations than required to affect HDT, and NAPA had weaker effects on both variables. Thus, NAPA displayed typical class IA antiarrhythmic behavior. Therefore, the concept that NAPA is a pure class III antiarrhythmic drug is more species-dependent than previously envisioned. In addition, we demonstrated that the differential pharmacology of PA and NAPA is explainable, in molecular terms, by steric hindrance of the effects of NAPA and the greater number of potent aromatic-aromatic and cation π interactions with Na+ or K+ cardiac channels for PA.

Blockers of Skeletal Muscle Nav1.4 Channels: From Therapy of Myotonic Syndrome to Molecular Determinants of Pharmacological Action and Back

The voltage-gated sodium channels represent an important target for drug discovery since a large number of physiological processes are regulated by these channels. In several excitability disorders, including epilepsy, cardiac arrhythmias, chronic pain, and non-dystrophic myotonia, blockers of voltage-gated sodium channels are clinically used. Myotonia is a skeletal muscle condition characterized by the over-excitability of the sarcolemma, resulting in delayed relaxation after contraction and muscle stiffness. The therapeutic management of this disorder relies on mexiletine and other sodium channel blockers, which are not selective for the Na_v1.4 skeletal muscle sodium channel isoform. Hence, the importance of deepening the knowledge of molecular requirements for developing more potent and use-dependent drugs acting on Na_v1.4. Here, we review the available treatment options for non-dystrophic myotonia and the structure-activity relationship studies performed in our laboratory with a focus on new compounds with potential antimyotonic activity.

Molecular Modeling of Cardiac Sodium Channel with Mexiletine

A sodium channel blocker mexiletine (MEX) is used to treat chronic pain, myotonia and some arrhythmias. Mutations in the pore domain (PD) of voltage-gated sodium channels differently affect tonic block (TB) and use-dependent block (UDB) by MEX. Previous studies identified several MEX-sensing residues in the hNav1.5 channel and demonstrated that the channel block by MEX increases with activation of the voltage-sensing domain III (VSDIII), whereas MEX stabilizes the activated state of VSDIII. Structural rationales for these observations are unclear. Here, Monte Carlo (MC) energy minimizations were used to dock MEX and its more potent analog, Thio-Me2, into the hNav1.5 cryo-EM structure with activated VSDs and presumably inactivated PD. Computations yielded two ensembles of ligand binding poses in close contacts with known MEX-sensing residues in helices S6III, S6IV and P1IV. In both ensembles, the ligand NH3 group approached the cation-attractive site between backbone carbonyls at the outer-pore bottom, while the aromatic ring protruded ether into the inner pore (putative UDB pose) or into the III/IV fenestration (putative TB pose). In silico deactivation of VSDIII shifted helices S4-S5III, S5III, S6III and S6IV and tightened the TB site. In a model with activated VSDIII and three resting VSDs, MC-minimized energy profile of MEX pulled from the TB site towards lipids shows a deep local minimum due to interactions with 11 residues in S5III, P1III, S6III and S6IV. The minimum may correspond to an interim binding site for MEX in the hydrophobic path to the TB site along the lipid-exposed sides of repeats III and IV where 15 polar and aromatic residues would attract cationic blockers. The study explains numerous experimental data and suggests the mechanism of allosteric modification of the MEX binding site by VSDIII.

Epilepsy-Induced High Affinity Blockade of the Cardiac Sodium Current INa by Lamotrigine; A Potential for Acquired Arrythmias

Lamotrigine is widely prescribed to treat bipolar neurological disorder and epilepsy. It exerts its antiepileptic action by blocking voltage-gated sodium channels in neurons. Recently, the US Food and Drug Administration issued a warning on the use of Lamotrigine after observations of conduction anomalies and Brugada syndrome patterns on the electrocardiograms of epileptic patients treated with the drug. Brugada syndrome and conduction disturbance are both associated with alterations of the cardiac sodium current (I_Na) kinetics and amplitude. In this study, we used the patch clamp technique on cardiomyocytes from epileptic rats to test the hypothesis that Lamotrigine also blocks I_Na in the heart. We found that Lamotrigine inhibited 60% of I_Na peak amplitude and reduced cardiac excitability in epileptic rats but had little effect in sham animals. Moreover, Lamotrigine inhibited 67% of I_NaL and, more importantly, prolonged the action potential refractory period in epileptic animals. Our results suggest that enhanced affinity of Lamotrigine for I_Na may in part explain the clinical phenotypes observed in epileptic patients.

Computational design of peptides to target NaV1.7 channel with high potency and selectivity for the treatment of pain

The voltage-gated sodium Na_V1.7 channel plays a key role as a mediator of action potential propagation in C-fiber nociceptors and is an established molecular target for pain therapy. ProTx-II is a potent and moderately selective peptide toxin from tarantula venom that inhibits human Na_V1.7 activation. Here we used available structural and experimental data to guide Rosetta design of potent and selective ProTx-II-based peptide inhibitors of human Na_V1.7 channels. Functional testing of designed peptides using electrophysiology identified the PTx2-3127 and PTx2-3258 peptides with IC₅₀s of 7 nM and 4 nM for hNa_V1.7 and more than 1000-fold selectivity over human Na_V1.1, Na_V1.3, Na_V1.4, Na_V1.5, Na_V1.8, and Na_V1.9 channels. PTx2-3127 inhibits Na_V1.7 currents in mouse and human sensory neurons and shows efficacy in rat models of chronic and thermal pain when administered intrathecally. Rationally designed peptide inhibitors of human Na_V1.7 channels have transformative potential to define a new class of biologics to treat pain.

Differences in local anaesthetic and antiepileptic binding in the inactivated state of human sodium channel Nav1.4

Voltage-gated sodium channels play a vital role in nerve and muscle cells, enabling them to encode and transmit electrical signals. Currently, there exist several classes of drugs that aim to inhibit these channels for therapeutic purposes, including local anesthetics, antiepileptics and antiarrhythmics. However, sodium-channel-inhibiting drugs lack subtype specificity; instead, they inhibit all sodium channels in the human body. Improving understanding of the mechanisms of binding of existing nonselective drugs is important in providing insight into how subtype-selective drugs could be developed. This study used molecular dynamics simulations to investigate the binding of the antiepileptics carbamazepine and lamotrigine and the local anesthetic lidocaine in neutral and charged states to the recently resolved human Nav1.4 channel. Replica exchange solute tempering was used to enable greater sampling of each compound within the pore. It was found that all four compounds show similarities in their binding sites within the pore. However, the positions of the carbamazepine and lamotrigine did not occlude the center of the pore but preferentially bound to homologous domain DII and DIII. The charged and neutral forms of lidocaine positioned themselves more centrally in the pore, with more common interactions with DIV. The best localized binding site was for charged lidocaine, whose aromatic moiety interacted with Y1593, whereas the amine projected toward the selectivity filter. Comparisons with our previous simulations and published structures highlight potential differences between tonic and use-dependent block related to conformational changes occurring in the pore.

Bioisosteric Modification of To042: Synthesis and Evaluation of Promising Use-Dependent Inhibitors of Voltage-Gated Sodium Channels

Three analogues of To042, a tocainide-related lead compound recently reported for the treatment of myotonia, were synthesized and evaluated in vitro as skeletal muscle sodium channel blockers possibly endowed with enhanced use-dependent behavior. Patch-clamp experiments on hNav1.4 expressed in HEK293 cells showed that N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine, the aryloxyalkyl bioisostere of To042, exerted a higher use-dependent block than To042 thus being able to preferentially block the channels in over-excited membranes while preserving healthy tissue function. It also showed the lowest active transport across BBB according to the results of P-glycoprotein (P-gp) interacting activity evaluation and the highest cytoprotective effect on HeLa cells. Quantum mechanical calculations and dockings gave insights on the most probable conformation of the aryloxyalkyl bioisostere of To042 in solution and the target residues involved in the binding, respectively. Both approaches indicated the conformations that might be adopted in both the unbound and bound state of the ligand. Overall, N-[(naphthalen-1-yl)methyl]-4-[(2,6-dimethyl)phenoxy]butan-2-amine exhibits an interesting toxico-pharmacological profile and deserves further investigation.

Trigeminal neuralgia: An overview from pathophysiology to pharmacological treatments

The trigeminal nerve (V) is the fifth and largest of all cranial nerves, and it is responsible for detecting sensory stimuli that arise from the craniofacial area. The nerve is divided into three branches: ophthalmic (V1), maxillary (V2), and mandibular (V3); their cell bodies are located in the trigeminal ganglia and they make connections with second-order neurons in the trigeminal brainstem sensory nuclear complex. Ascending projections via the trigeminothalamic tract transmit information to the thalamus and other brain regions responsible for interpreting sensory information. One of the most common forms of craniofacial pain is trigeminal neuralgia. Trigeminal neuralgia is characterized by sudden, brief, and excruciating facial pain attacks in one or more of the V branches, leading to a severe reduction in the quality of life of affected patients. Trigeminal neuralgia etiology can be classified into idiopathic, classic, and secondary. Classic trigeminal neuralgia is associated with neurovascular compression in the trigeminal root entry zone, which can lead to demyelination and a dysregulation of voltage-gated sodium channel expression in the membrane. These alterations may be responsible for pain attacks in trigeminal neuralgia patients. The antiepileptic drugs carbamazepine and oxcarbazepine are the first-line pharmacological treatment for trigeminal neuralgia. Their mechanism of action is a modulation of voltage-gated sodium channels, leading to a decrease in neuronal activity. Although carbamazepine and oxcarbazepine are the first-line treatment, other drugs may be useful for pain control in trigeminal neuralgia. Among them, the anticonvulsants gabapentin, pregabalin, lamotrigine and phenytoin, baclofen, and botulinum toxin type A can be coadministered with carbamazepine or oxcarbazepine for a synergistic approach. New pharmacological alternatives are being explored such as the active metabolite of oxcarbazepine, eslicarbazepine, and the new Nav1.7 blocker vixotrigine. The pharmacological profiles of these drugs are addressed in this review.

Differential effect of lacosamide on Nav1.7 variants from responsive and non-responsive patients with small fibre neuropathy

Small fibre neuropathy is a common pain disorder, which in many cases fails to respond to treatment with existing medications. Gain-of-function mutations of voltage-gated sodium channel Na_v1.7 underlie dorsal root ganglion neuronal hyperexcitability and pain in a subset of patients with small fibre neuropathy. Recent clinical studies have demonstrated that lacosamide, which blocks sodium channels in a use-dependent manner, attenuates pain in some patients with Na_v1.7 mutations; however, only a subgroup of these patients responded to the drug. Here, we used voltage-clamp recordings to evaluate the effects of lacosamide on five Na_v1.7 variants from patients who were responsive or non-responsive to treatment. We show that, at the clinically achievable concentration of 30 μM, lacosamide acts as a potent sodium channel inhibitor of Na_v1.7 variants carried by responsive patients, via a hyperpolarizing shift of voltage-dependence of both fast and slow inactivation and enhancement of use-dependent inhibition. By contrast, the effects of lacosamide on slow inactivation and use-dependence in Na_v1.7 variants from non-responsive patients were less robust. Importantly, we found that lacosamide selectively enhances fast inactivation only in variants from responders. Taken together, these findings begin to unravel biophysical underpinnings that contribute to responsiveness to lacosamide in patients with small fibre neuropathy carrying select Na_v1.7 variants.

Effects of Deltamethrin Acute Exposure on Nav1.6 Channels and Medium Spiny Neurons of the Nucleus Accumbens

Exposure to pyrethroids, a popular insecticide class that targets voltage-gated Na+ (Nav) channels, has been correlated to an increase in diagnosis of neurodevelopmental disorders, such as attention deficit hyperactive disorder (ADHD), in children. Dysregulation of medium spiny neurons (MSNs) firing in the nucleus accumbens (NAc) is thought to play a critical role in the pathophysiology of ADHD and other neurodevelopmental disorders. The Nav1.6 channel is the primary molecular determinant of MSN firing and is sensitive to modification by pyrethroids. Building on previous studies demonstrating that deltamethrin (DM), a commonly used pyrethroid, leads to use-dependent enhancement of sodium currents, we characterized the effect of the toxin on long-term inactivation (LTI) of the Nav1.6 channel, a parameter known to affect neuronal firing, and characterized changes in MSN intrinsic excitability. We employed whole-cell patch-clamp electrophysiology to measure sodium currents in HEK-293 cells stably expressing Nav1.6 channels and intrinsic excitability of MSNs in the brain slice preparation. We found that in response to repetitive stimulation acute exposure to 10 μM DM potentiated a build-up of residual sodium currents and modified availability of Nav1.6 by inducing LTI. In the NAc, DM modified MSN intrinsic excitability increasing evoked action potential firing frequency and inducing aberrant action potentials with low amplitude and depolarized voltage threshold, phenotypes that could be explained by DM induced changes on the Nav1.6 channel. These results provide a potential initial mechanism of toxicity of DM that could lead to disruption of the NAc circuitry overtime, increasing the risk of ADHD and other neurodevelopmental disorders.

Structures Illuminate Cardiac Ion Channel Functions in Health and in Long QT Syndrome

The cardiac action potential is critical to the production of a synchronized heartbeat. This electrical impulse is governed by the intricate activity of cardiac ion channels, among them the cardiac voltage-gated potassium (Kv) channels KCNQ1 and hERG as well as the voltage-gated sodium (Nav) channel encoded by SCN5A. Each channel performs a highly distinct function, despite sharing a common topology and structural components. These three channels are also the primary proteins mutated in congenital long QT syndrome (LQTS), a genetic condition that predisposes to cardiac arrhythmia and sudden cardiac death due to impaired repolarization of the action potential and has a particular proclivity for reentrant ventricular arrhythmias. Recent cryo-electron microscopy structures of human KCNQ1 and hERG, along with the rat homolog of SCN5A and other mammalian sodium channels, provide atomic-level insight into the structure and function of these proteins that advance our understanding of their distinct functions in the cardiac action potential, as well as the molecular basis of LQTS. In this review, the gating, regulation, LQTS mechanisms, and pharmacological properties of KCNQ1, hERG, and SCN5A are discussed in light of these recent structural findings.

Lidocaine Binding Enhances Inhibition of Nav1.7 Channels by the Sulfonamide PF-05089771

PF-05089771 is an aryl sulfonamide Nav1.7 channel blocker that binds to the inactivated state of Nav1.7 channels with high affinity but binds only weakly to channels in the resting state. Such aryl sulfonamide Nav1.7 channel blockers bind to the extracellular surface of the S1-S4 voltage-sensor segment of homologous Domain 4, whose movement is associated with inactivation. This binding site is different from that of classic sodium channel inhibitors like lidocaine, which also bind with higher affinity to the inactivated state than the resting state but bind at a site within the pore of the channel. The common dependence on gating state with distinct binding sites raises the possibility that inhibition by aryl sulfonamides and by classic local anesthetics might show an interaction mediated by their mutual state dependence. We tested this possibility by examining the state-dependent inhibition by PF-05089771 and lidocaine of human Nav1.7 channels expressed in human embryonic kidney 293 cells. At -80 mV, where a small fraction of channels are in an inactivated state under drug-free conditions, inhibition by PF-05089771 was both enhanced and speeded in the presence of lidocaine. The results suggest that lidocaine binding to the channel enhances PF-05089771 inhibition by altering the equilibrium between resting states (with D4S4 in the inner position) and inactivated states (with D4S4 in the outer position). The gating state-mediated interaction between the compounds illustrates a principle applicable to many state-dependent agents. SIGNIFICANCE STATEMENT: The results show that lidocaine enhances the degree and rate of inhibition of Nav1.7 channels by the aryl sulfonamide compound PF-05089771, consistent with state-dependent binding by lidocaine increasing the fraction of channels presenting a high-affinity binding site for PF-05089771 and suggesting that combinations of agents targeted to the pore-region binding site of lidocaine and the external binding site of aryl sulfonamides may have synergistic actions.

Veratridine: A Janus-Faced Modulator of Voltage-Gated Sodium Ion Channels

Voltage-gated sodium ion channels (Na_Vs) are integral to both neuronal and muscular signaling and are a primary target for a number of proteinaceous and small molecule toxins. Included among these neurotoxins is veratridine (VTD), a C-nor-D homosteroidal alkaloid from the seeds of members of the Veratrum genus. VTD binds to Na_V within the pore region, causing a hyperpolarizing shift in the activation threshold in addition to reducing peak current. We have characterized the activity of VTD against heterologously expressed rat Na_V1.4 and have demonstrated that VTD acts on the channel as either an agonist or antagonist depending on the nature of the electrophysiological stimulation protocol. Structure-activity studies with VTD and VTD derivatives against Na_V mutants show that the functional duality of VTD can be decoupled. These findings suggest that the dichotomous activity of VTD may derive from two distinct, use-dependent binding orientations of the toxin.

Structure and Pharmacology of Voltage-Gated Sodium and Calcium Channels

Voltage-gated sodium and calcium channels are evolutionarily related transmembrane signaling proteins that initiate action potentials, neurotransmission, excitation-contraction coupling, and other physiological processes. Genetic or acquired dysfunction of these proteins causes numerous diseases, termed channelopathies, and sodium and calcium channels are the molecular targets for several major classes of drugs. Recent advances in the structural biology of these proteins using X-ray crystallography and cryo-electron microscopy have given new insights into the molecular basis for their function and pharmacology. Here we review this recent literature and integrate findings on sodium and calcium channels to reveal the structural basis for their voltage-dependent activation, fast and slow inactivation, ion conductance and selectivity, and complex pharmacology at the atomic level. We conclude with the theme that new understanding of the diseases and therapeutics of these channels will be derived from application of the emerging structural principles from these recent structural analyses.

Noncanonical Ion Channel Behaviour in Pain

Ion channels contribute fundamental properties to cell membranes. Although highly diverse in conductivity, structure, location, and function, many of them can be regulated by common mechanisms, such as voltage or (de-)phosphorylation. Primarily considering ion channels involved in the nociceptive system, this review covers more novel and less known features. Accordingly, we outline noncanonical operation of voltage-gated sodium, potassium, transient receptor potential (TRP), and hyperpolarization-activated cyclic nucleotide (HCN)-gated channels. Noncanonical features discussed include properties as a memory for prior voltage and chemical exposure, alternative ion conduction pathways, cluster formation, and silent subunits. Complementary to this main focus, the intention is also to transfer knowledge between fields, which become inevitably more separate due to their size.

The molecular determinants of R-roscovitine block of hERG channels

Human ether-à-go-go-related gene (Kv11.1, or hERG) is a potassium channel that conducts the delayed rectifier potassium current (I_Kr) during the repolarization phase of cardiac action potentials. hERG channels have a larger pore than other K⁺channels and can trap many unintended drugs, often resulting in acquired LQTS (aLQTS). R-roscovitine is a cyclin-dependent kinase (CDK) inhibitor that induces apoptosis in colorectal, breast, prostate, multiple myeloma, other cancer cell lines, and tumor xenografts, in micromolar concentrations. It is well tolerated in phase II clinical trials. R-roscovitine inhibits open hERG channels but does not become trapped in the pore. Two-electrode voltage clamp recordings from Xenopus oocytes expressing wild-type (WT) or hERG pore mutant channels (T623A, S624A, Y652A, F656A) demonstrated that compared to WT hERG, T623A, Y652A, and F656A inhibition by 200 μM R-roscovitine was ~ 48%, 29%, and 73% weaker, respectively. In contrast, S624A hERG was inhibited more potently than WT hERG, with a ~ 34% stronger inhibition. These findings were further supported by the IC₅₀ values, which were increased for T623A, Y652A and F656A (by ~5.5, 2.75, and 42 fold respectively) and reduced 1.3 fold for the S624A mutant. Our data suggest that while T623, Y652, and F656 are critical for R-roscovitine-mediated inhibition, S624 may not be. Docking studies further support our findings. Thus, R-roscovitine’s relatively unique features, coupled with its tolerance in clinical trials, could guide future drug screens.

Evaluation of the aggregation process in a mixture of propofol and benzocaine

We report on a mass-resolved IR spectrosopic study on propofol-benzocaine aggregates. This is a complex system due to the several conformational isomers that both monomers may adopt and to the combination of functional groups they present, which allow the molecules to interact in many possible ways. However, our results demonstrate that a single conformation is favored for each stoichiometry. In the heterodimer, propofol acts as a proton donor to the ester group of benzocaine, while the whole cluster is stabilized by dispersive forces. These dispersive forces account for an important part of the system's stabilization energy as the calculations suggest. Propofol does not show any affinity for the amino group of benzocaine, even when a second molecule of propofol is introduced. These results demonstrate the difficulty in anticipating the aggregation preferences of even small organic molecules.

Structural basis for antiarrhythmic drug interactions with the human cardiac sodium channel

Voltage-gated sodium channels play a central role in cellular excitability and are key targets for drug development. Recent breakthroughs in high-resolution cryo-electron microscopy protein structure determination, Rosetta computational protein structure modeling, and multimicrosecond molecular dynamics simulations are empowering advances in structural biology to study the atomistic details of channel−drug interactions. We used Rosetta structural computational modeling and molecular dynamics simulations to study the interactions of antiarrhythmic and local anesthetic drugs with cardiac sodium channel. Our results provide crucial atomic-scale mechanistic insights into the channel–drug interactions, necessary for the rational design of novel modulators of the human cardiac sodium channel to be used for the treatment of cardiac arrhythmias.

The human voltage-gated sodium channel, hNa_V1.5, is responsible for the rapid upstroke of the cardiac action potential and is target for antiarrhythmic therapy. Despite the clinical relevance of hNa_V1.5-targeting drugs, structure-based molecular mechanisms of promising or problematic drugs have not been investigated at atomic scale to inform drug design. Here, we used Rosetta structural modeling and docking as well as molecular dynamics simulations to study the interactions of antiarrhythmic and local anesthetic drugs with hNa_V1.5. These calculations revealed several key drug binding sites formed within the pore lumen that can simultaneously accommodate up to two drug molecules. Molecular dynamics simulations identified a hydrophilic access pathway through the intracellular gate and a hydrophobic access pathway through a fenestration between DIII and DIV. Our results advance the understanding of molecular mechanisms of antiarrhythmic and local anesthetic drug interactions with hNa_V1.5 and will be useful for rational design of novel therapeutics.

Mexiletine Block of Voltage-Gated Sodium Channels: Isoform- and State-Dependent Drug-Pore Interactions

Mexiletine is a class Ib antiarrhythmic drug and is also used clinically to reduce or prevent myotonia. In addition, mexiletine has neuroprotective effects in models of brain ischemia. We compared state-dependent affinities of mexiletine for Na_v1.2, Na_v1.4, and Na_v1.5 and examined the effects of mutations of transmembrane segment S6 residues on mexiletine block of Na_v1.5. Three channel isoforms had similar affinities of mexiletine for the rested state, and Na_v1.4 and Na_v1.5 had similar affinities for the open and inactivated states, while Na_v1.2 had lower affinity for these states than Na_v1.4 and Na_v1.5. Mutational studies revealed that the largest affinity change was observed for an Ala substitution of Phe in domain IV S6. In our homology modeling based on the bacterial Na⁺ channel, mexiletine changed its location and orientation in the pore depending on the state of the channel, irrespective of the channel isoform. Mexiletine occurred in the upper part in the pore in the open state and lower in the closed state. High-affinity binding of mexiletine in the open states of Na_v1.4 and Na_v1.5 was caused by a π-π interaction with Phe, whereas mexiletine was located away from Phe in the open state of Na_v1.2. These results provide crucial information on the mechanism of isoform differences in state-dependent block by local anesthetics and related drugs. Mexiletine at upper locations in the open state may effectively cause an electrostatic mechanism of block.

Fenestrations control resting-state block of a voltage-gated sodium channel

Potency of drug action is usually determined by binding to a specific receptor site on target proteins. In contrast to this conventional paradigm, we show here that potency of local anesthetics (LAs) and antiarrhythmic drugs (AADs) that block sodium channels is controlled by fenestrations that allow drug access to the receptor site directly from the membrane phase. Voltage-gated sodium channels initiate action potentials in nerve and cardiac muscle, where their hyperactivity causes pain and cardiac arrhythmia, respectively. LAs and AADs selectively block sodium channels in rapidly firing nerve and muscle cells to relieve these conditions. The structure of the ancestral bacterial sodium channel Na_VAb, which is also blocked by LAs and AADs, revealed fenestrations connecting the lipid phase of the membrane to the central cavity of the pore. We cocrystallized lidocaine and flecainide with Na_vAb, which revealed strong drug-dependent electron density in the central cavity of the pore. Mutation of the contact residue T206 greatly reduced drug potency, confirming this site as the receptor for LAs and AADs. Strikingly, mutations of the fenestration cap residue F203 changed fenestration size and had graded effects on resting-state block by flecainide, lidocaine, and benzocaine, the potencies of which were altered from 51- to 2.6-fold in order of their molecular size. These results show that conserved fenestrations in the pores of sodium channels are crucial pharmacologically and determine the level of resting-state block by widely used drugs. Fine-tuning drug access through fenestrations provides an unexpected avenue for structure-based design of ion-channel-blocking drugs.

Blocking ion channels induced by antifungal lipopeptide syringomycin E with amide-linked local anesthetics

The effects of the amide-linked (lidocaine (LDC), mepivacaine (MPV), prilocaine (PLC)) and ester-bound local anesthetics (benzocaine (BZC), procaine (PRC), and tetracaine (TTC)) on the pore-forming activity of the antifungal lipopeptide syringomycin E (SRE) in lipid bilayers were studied. Independently on electrolyte concentration in the membrane bathing solution the observed changes in conductance of SRE channels agreed with the altered membrane dipole potential under the action of ester-bound local anesthetics. Effects of aminoamides in diluted and concentrated solutions were completely different. At 0.1 M KCl (pH 7.4) the effects of amide-linked anesthetics were in accordance with changes in the membrane surface potential, while at 2 M KCl aminoamides blocked ion passage through the SRE channels, leading to sharp reductions in pore conductance at negative voltages and 100-fold decreases in the channel lifetimes. The effects were not practically influenced by the membrane lipid composition. The interaction cooperativity implied the existence of specific binding sites for amide-bound anesthetics in SRE channels.

Polyamine Modulation of Anticonvulsant Drug Response: A Potential Mechanism Contributing to Pharmacoresistance in Chronic Epilepsy

Despite the development of numerous novel anticonvulsant drugs, ∼30% of epilepsy patients remain refractory to antiepileptic drugs (AEDs). Many established and novel AEDs reduce hyperexcitability via voltage- and use-dependent inhibition of voltage-gated Na⁺ channels. For the widely used anticonvulsant carbamazepine (CBZ), use-dependent block of Na⁺ channels is significantly reduced both in experimental and human epilepsy. However, the molecular underpinnings of this potential cellular mechanism for pharmacoresistance have remained enigmatic.Here, we describe the mechanism that leads to the emergence of CBZ-resistant Na⁺ channels. We focused on the endogenous polyamine system, which powerfully modulates Na⁺ channels in a use-dependent manner. We had shown previously that the intracellular polyamine spermine is reduced in chronic epilepsy, resulting in increased persistent Na⁺ currents. Because spermine and CBZ both bind use-dependently in spatial proximity within the Na⁺ channel pore, we hypothesized that spermine loss might also be related to diminished CBZ response. Using the pilocarpine model of refractory epilepsy in male rats and whole-cell patch-clamp recordings, we first replicated the reduction of use-dependent block by CBZ in chronically epileptic animals. We then substituted intracellular spermine via the patch pipette in different concentrations. Under these conditions, we found that exogenous spermine significantly rescues use-dependent block of Na⁺ channels by CBZ. These findings indicate that an unexpected modulatory mechanism, depletion of intracellular polyamines, leads both to increased persistent Na⁺ currents and to diminished CBZ sensitivity of Na⁺ channels. These findings could lead to novel strategies for overcoming pharmacoresistant epilepsy that target the polyamine system.SIGNIFICANCE STATEMENT Pharmacoresistant epilepsy affects ∼18 million people worldwide, and intense efforts have therefore been undertaken to uncover the underlying molecular and cellular mechanisms. One of the key known candidate mechanisms of pharmacoresistance has been a loss of use-dependent Na⁺ channel block by the anticonvulsant carbamazepine (CBZ), both in human and experimental epilepsies. Despite intense scrutiny, the molecular mechanisms underlying this phenomenon have not been elucidated. We now show that a loss of intracellular spermine in chronic epilepsy is a major causative factor leading to the development of CBZ-resistant Na⁺ currents. This finding can be exploited both for the screening of anticonvulsants in expression systems, and for novel strategies to overcome pharmacoresistance that target the polyamine system.

Insight into tetrodotoxin blockade and resistance mechanisms of Nav 1.2 sodium channel by theoretical approaches

Na_v 1.2, a member of voltage-gated sodium channels (Na_v s) that are responsible for the generation and propagation of action potentials along the cell membrane, and play a vital role in the process of information transmission within the nervous system and muscle contraction, is preferentially expressed in the central nervous system. As a potent and selective blocker of Na_v s, tetrodotoxin (TTX) has been extensively studied in biological and chemical sciences, whereas the detailed mechanism by which it blocks nine Na_v 1 channel subtypes remain elusive. Despite the high structural similarity, the TTX metabolite 4,9-anhydro-TTX is 161 times less effective toward the mammalian Na_v 1.2, which puzzled us to ask a question why such a subtle structural variation results in the largely binding affinity difference. In the current work, an integrated computational strategy, including homology modeling, induced fit docking, explicit-solvent MD simulations, and free energy calculations, was employed to investigate the binding mechanism and conformational determinants of TTX analogs. Based on the computational results, the H-bond interactions between C4-OH and C9-OH of TTX and the outer ring carboxylates of the selectivity-filter residues, and the cation-π interaction between the primary amine of guanidinium of TTX and Phe385 determine the difference of their binding affinities. Moreover, the computationally simulations were carried out for the D384N and E945K mutants of hNa_v 1.2-TTX, and the rank of the predicted binding free energies is in accordance with the experimental data. These observations provide a valuable model to design potent and selective neurotoxins of Na_v 1.2 and shed light on the blocking mechanism of TTX to sodium channels.

Mechanisms of Drug Binding to Voltage-Gated Sodium Channels

Voltage-gated sodium (Na⁺) channels are expressed in virtually all electrically excitable tissues and are essential for muscle contraction and the conduction of impulses within the peripheral and central nervous systems. Genetic disorders that disrupt the function of these channels produce an array of Na⁺ channelopathies resulting in neuronal impairment, chronic pain, neuromuscular pathologies, and cardiac arrhythmias. Because of their importance to the conduction of electrical signals, Na⁺ channels are the target of a wide variety of local anesthetic, antiarrhythmic, anticonvulsant, and antidepressant drugs. The voltage-gated family of Na⁺ channels is composed of α-subunits that encode for the voltage sensor domains and the Na⁺-selective permeation pore. In vivo, Na⁺ channel α-subunits are associated with one or more accessory β-subunits (β₁-β₄) that regulate gating properties, trafficking, and cell-surface expression of the channels. The permeation pore of Na⁺ channels is divided in two parts: the outer mouth of the pore is the site of the ion selectivity filter, while the inner cytoplasmic pore serves as the channel activation gate. The cytoplasmic lining of the permeation pore is formed by the S6 segments that include highly conserved aromatic amino acids important for drug binding. These residues are believed to undergo voltage-dependent conformational changes that alter drug binding as the channels cycle through the closed, open, and inactivated states. The purpose of this chapter is to broadly review the mechanisms of Na⁺ channel gating and the models used to describe drug binding and Na⁺ channel inhibition.

Mechanisms and models of cardiac sodium channel inactivation

Shortly after cardiac Na+ channels activate and initiate the action potential, inactivation ensues within milliseconds, attenuating the peak Na+ current, INa, and allowing the cell membrane to repolarize. A very limited number of Na+ channels that do not inactivate carry a persistent INa, or late INa. While late INa is only a small fraction of peak magnitude, it significantly prolongs ventricular action potential duration, which predisposes patients to arrhythmia. Here, we review our current understanding of inactivation mechanisms, their regulation, and how they have been modeled computationally. Based on this body of work, we conclude that inactivation and its connection to late INa would be best modeled with a "feet-on-the-door" approach where multiple channel components participate in determining inactivation and late INa. This model reflects experimental findings showing that perturbation of many channel locations can destabilize inactivation and cause pathological late INa.

Single rat muscle Na+ channel mutation confers batrachotoxin autoresistance found in poison-dart frog Phyllobates terribilis

Poison-dart Phyllobates terribilis frogs sequester lethal amounts of steroidal alkaloid batrachotoxin (BTX) in their skin as a defense mechanism against predators. BTX targets voltage-gated Na⁺ channels and enables them to open persistently. How BTX autoresistance arises in such frogs remains a mystery. The BTX receptor has been delineated along the Na⁺ channel inner cavity, which is formed jointly by four S6 transmembrane segments from domains D1 to D4. Within the P. terribilis muscle Na⁺ channel, five amino acid (AA) substitutions have been identified at D1/S6 and D4/S6. We therefore investigated the role of these naturally occurring substitutions in BTX autoresistance by introducing them into rat Nav1.4 muscle Na⁺ channel, both individually and in combination. Our results showed that combination mutants containing an N1584T substitution all conferred a complete BTX-resistant phenotype when expressed in mammalian HEK293t cells. The single N1584T mutant also retained its functional integrity and became exceptionally resistant to 5 µM BTX, aside from a small residual BTX effect. Single and combination mutants with the other four S6 residues (S429A, I433V, A445D, and V1583I) all remained highly BTX sensitive. These findings, along with diverse BTX phenotypes of N1584K/A/D/T mutant channels, led us to conclude that the conserved N1584 residue is indispensable for BTX actions, probably functioning as an integral part of the BTX receptor. Thus, complete BTX autoresistance found in P. terribilis muscle Na⁺ channels could emerge primarily from a single AA substitution (asparagine→threonine) via a single nucleotide mutation (AAC→ACC).

Forty Years of Sodium Channels: Structure, Function, Pharmacology, and Epilepsy

Voltage-gated sodium channels initiate action potentials in brain neurons. In the 1970s, much was known about the function of sodium channels from measurements of ionic currents using the voltage clamp method, but there was no information about the sodium channel molecules themselves. As a postdoctoral fellow and staff scientist at the National Institutes of Health, I developed neurotoxins as molecular probes of sodium channels in cultured neuroblastoma cells. During those years, Bruce Ransom and I crossed paths as members of the laboratories of Marshall Nirenberg and Philip Nelson and shared insights about sodium channels in neuroblastoma cells from my work and electrical excitability and synaptic transmission in cultured spinal cord neurons from Bruce's pioneering electrophysiological studies. When I established my laboratory at the University of Washington in 1977, my colleagues and I used those neurotoxins to identify the protein subunits of sodium channels, purify them, and reconstitute their ion conductance activity in pure form. Subsequent studies identified the molecular basis for the main functions of sodium channels-voltage-dependent activation, rapid and selective ion conductance, and fast inactivation. Bruce Ransom and I re-connected in the 1990s, as ski buddies at the Winter Conference on Brain Research and as faculty colleagues at the University of Washington when Bruce became our founding Chair of Neurology and provided visionary leadership of that department. In the past decade my work on sodium channels has evolved into structural biology. Molecular modeling and X-ray crystallographic studies have given new views of sodium channel function at atomic resolution. Sodium channels are also the molecular targets for genetic diseases, including Dravet Syndrome, an intractable pediatric epilepsy disorder with major co-morbidities of cognitive deficit, autistic-like behaviors, and premature death that is caused by loss-of-function mutations in the brain sodium channel NaV1.1. Our work on a mouse genetic model of this disease has shown that its multi-faceted pathophysiology and co-morbidities derive from selective loss of electrical excitability and action potential firing in GABAergic inhibitory neurons, which disinhibits neural circuits throughout the brain and leads directly to the epilepsy, premature death and complex co-morbidities of this disease. It has been rewarding for me to use our developing knowledge of sodium channels to help understand the pathophysiology and to suggest potential therapeutic approaches for this devastating childhood disease.

Mutagenesis of the NaChBac sodium channel discloses a functional role for a conserved S6 asparagine

Asparagine is conserved in the S6 transmembrane segments of all voltage-gated sodium, calcium, and TRP channels identified to date. A broad spectrum of channelopathies including cardiac arrhythmias, epilepsy, muscle diseases, and pain disorders is associated with its mutation. To investigate its effects on sodium channel functional properties, we mutated the simple prokaryotic sodium channel NaChBac. Electrophysiological characterization of the N225D mutant reveals that this conservative substitution shifts the voltage-dependence of inactivation by 25 mV to more hyperpolarized potentials. The mutant also displays greater thermostability, as determined by synchrotron radiation circular dichroism spectroscopy studies of purified channels. Based on our analyses of high-resolution structures of NaChBac homologues, we suggest that the side-chain amine group of asparagine 225 forms one or more hydrogen bonds with different channel elements and that these interactions are important for normal channel function. The N225D mutation eliminates these hydrogen bonds and the structural consequences involve an enhanced channel inactivation.

Consequences of acute Nav1.1 exposure to deltamethrin

BACKGROUND

Pyrethroid insecticides are the most popular class of insecticides in the world, despite their near-ubiquity, their effects of delaying the onset of inactivation of voltage-gated sodium (Nav) channels have not been well-evaluated in all the mammalian Nav isoforms.

OBJECTIVE

Here we compare the well-studied Nav1.6 isoforms to the less-understood Nav1.1 in their responses to acute deltamethrin exposure.

METHODS

We used patch-clamp electrophysiology to record sodium currents encoded by either Nav1.1 or Nav1.6 channels stably expressed in HEK293 cells. Protocols evaluating both resting and use-dependent modification were employed.

RESULTS

We found that exposure of both isoforms to 10μM deltamethrin significantly potentiated persistent and tail current densities without affecting peak transient current densities, and only Nav1.1 maintained these significant effects at 1μM deltamethrin. Window currents increased for both as well, and while only Nav1.6 displayed changes in activation slope and V1/2 of steady-state inactivation for peak currents, V1/2 of persistent current activation was hyperpolarized of ∼10mV by deltamethrin in Nav1.1 cells. Evaluating use-dependence, we found that deltamethrin again potentiated persistent and tail current densities in both isoforms, but only Nav1.6 demonstrated use-dependent enhancement, indicating the primary deltamethrin-induced effects on Nav1.1 channels are not use-dependent.

CONCLUSION

Collectively, these data provide evidence that Nav1.1 is indeed vulnerable to deltamethrin modification at lower concentrations than Nav1.6, and this effect is primarily mediated during the resting state.

GENERAL SIGNIFICANCE

These findings identify Nav1.1 as a novel target of pyrethroid exposure, which has major implications for the etiology of neuropsychiatric disorders associated with loss of Nav1.1-expressing inhibitory neurons.

Mechanism of sodium channel block by local anesthetics, antiarrhythmics, and anticonvulsants

Local anesthetics, antiarrhythmics, and anticonvulsants include both charged and electroneutral compounds that block voltage-gated sodium channels. Prior studies have revealed a common drug-binding region within the pore, but details about the binding sites and mechanism of block remain unclear. Here, we use the x-ray structure of a prokaryotic sodium channel, NavMs, to model a eukaryotic channel and dock representative ligands. These include lidocaine, QX-314, cocaine, quinidine, lamotrigine, carbamazepine (CMZ), phenytoin, lacosamide, sipatrigine, and bisphenol A. Preliminary calculations demonstrated that a sodium ion near the selectivity filter attracts electroneutral CMZ but repels cationic lidocaine. Therefore, we further docked electroneutral and cationic drugs with and without a sodium ion, respectively. In our models, all the drugs interact with a phenylalanine in helix IVS6. Electroneutral drugs trap a sodium ion in the proximity of the selectivity filter, and this same site attracts the charged group of cationic ligands. At this position, even small drugs can block the permeation pathway by an electrostatic or steric mechanism. Our study proposes a common pharmacophore for these diverse drugs. It includes a cationic moiety and an aromatic moiety, which are usually linked by four bonds.

Lacosamide Inhibition of Nav1.7 Voltage-Gated Sodium Channels: Slow Binding to Fast-Inactivated States

Lacosamide is an antiseizure agent that targets voltage-dependent sodium channels. Previous experiments have suggested that lacosamide is unusual in binding selectively to the slow-inactivated state of sodium channels, in contrast to drugs like carbamazepine and phenytoin, which bind tightly to fast-inactivated states. Using heterologously expressed human Nav1.7 sodium channels, we examined the state-dependent effects of lacosamide. Lacosamide induced a reversible shift in the voltage dependence of fast inactivation studied with 100-millisecond prepulses, suggesting binding to fast-inactivated states. Using steady holding potentials, lacosamide block was very weak at -120 mV (3% inhibition by 100 µM lacosamide) but greatly enhanced at -80 mV (43% inhibition by 100 µM lacosamide), where there is partial fast inactivation but little or no slow inactivation. During long depolarizations, lacosamide slowly (over seconds) put channels into states that recovered availability slowly (hundreds of milliseconds) at -120 mV. This resembles enhancement of slow inactivation, but the effect was much more pronounced at -40 mV, where fast inactivation is complete, but slow inactivation is not, than at 0 mV, where slow inactivation is maximal, more consistent with slow binding to fast-inactivated states than selective binding to slow-inactivated states. Furthermore, inhibition by lacosamide was greatly reduced by pretreatment with 300 µM lidocaine or 300 µM carbamazepine, suggesting that lacosamide, lidocaine, and carbamazepine all bind to the same site. The results suggest that lacosamide binds to fast-inactivated states in a manner similar to other antiseizure agents but with slower kinetics of binding and unbinding.

Molecular Mechanism of Action and Selectivity of Sodium Ch annel Blocker Insecticides

Sodium channel blocker insecticides (SCBIs) are a relatively new class of insecticides that are represented by two commercially registered compounds, indoxacarb and metaflumizone. SCBIs, like pyrethroids and DDT, target voltage-gated sodium channels (VGSCs) to intoxicate insects. In contrast to pyrethroids, however, SCBIs inhibit VGSCs at a distinct receptor site that overlaps those of therapeutic inhibitors of sodium channels, such as local anesthetics, anticonvulsants and antiarrhythmics. This review will recount the development of the SCBI insecticide class from its roots as chitin synthesis inhibitors, discuss the symptoms of poisoning and evidence supporting inhibition of VGSCs as their mechanism of action, describe the current model for SCBI-induced inhibition of VGSCs, present a model for the receptor for SCBIs on VGSCs, and highlight differences between data collected from mammalian and insect experimental models.

Isolated pores dissected from human two-pore channel 2 are functional

Multi-domain voltage-gated ion channels appear to have evolved through sequential rounds of intragenic duplication from a primordial one-domain precursor. Whereas modularity within one-domain symmetrical channels is established, little is known about the roles of individual regions within more complex asymmetrical channels where the domains have undergone substantial divergence. Here we isolated and characterised both of the divergent pore regions from human TPC2, a two-domain channel that holds a key intermediate position in the evolution of voltage-gated ion channels. In HeLa cells, each pore localised to the ER and caused Ca²⁺ depletion, whereas an ER-targeted pore mutated at a residue that inactivates full-length TPC2 did not. Additionally, one of the pores expressed at high levels in E. coli. When purified, it formed a stable, folded tetramer. Liposomes reconstituted with the pore supported Ca²⁺ and Na⁺ uptake that was inhibited by known blockers of full-length channels. Computational modelling of the pore corroborated cationic permeability and drug interaction. Therefore, despite divergence, both pores are constitutively active in the absence of their partners and retain several properties of the wild-type pore. Such symmetrical ‘pore-only’ proteins derived from divergent channel domains may therefore provide tractable tools for probing the functional architecture of complex ion channels.

The hitchhiker's guide to the voltage-gated sodium channel galaxy

Eukaryotic voltage-gated sodium (Na_v) channels contribute to the rising phase of action potentials and served as an early muse for biophysicists laying the foundation for our current understanding of electrical signaling. Given their central role in electrical excitability, it is not surprising that (a) inherited mutations in genes encoding for Na_v channels and their accessory subunits have been linked to excitability disorders in brain, muscle, and heart; and (b) Na_v channels are targeted by various drugs and naturally occurring toxins. Although the overall architecture and behavior of these channels are likely to be similar to the more well-studied voltage-gated potassium channels, eukaryotic Na_v channels lack structural and functional symmetry, a notable difference that has implications for gating and selectivity. Activation of voltage-sensing modules of the first three domains in Na_v channels is sufficient to open the channel pore, whereas movement of the domain IV voltage sensor is correlated with inactivation. Also, structure–function studies of eukaryotic Na_v channels show that a set of amino acids in the selectivity filter, referred to as DEKA locus, is essential for Na⁺ selectivity. Structures of prokaryotic Na_v channels have also shed new light on mechanisms of drug block. These structures exhibit lateral fenestrations that are large enough to allow drugs or lipophilic molecules to gain access into the inner vestibule, suggesting that this might be the passage for drug entry into a closed channel. In this Review, we will synthesize our current understanding of Na_v channel gating mechanisms, ion selectivity and permeation, and modulation by therapeutics and toxins in light of the new structures of the prokaryotic Na_v channels that, for the time being, serve as structural models of their eukaryotic counterparts.

Important Role of Asparagines in Coupling the Pore and Votage-Sensor Domain in Voltage-Gated Sodium Channels

Voltage-gated sodium (NaV) channels contain an α-subunit incorporating the channel's pore and gating machinery composed of four homologous domains (DI-DIV), with a pore domain formed by the S5 and S6 segments and a voltage-sensor domain formed by the S1-S4 segments. During a membrane depolarization movement, the S4s in the voltage-sensor domains exert downstream effects on the S6 segments to control ionic conductance through the pore domain. We used lidocaine, a local anesthetic and antiarrhythmic drug, to probe the role of conserved Asn residues in the S6s of DIII and DIV in NaV1.5 and NaV1.4. Previous studies have shown that lidocaine binding to the pore domain causes a decrease in the maximum gating (Qmax) charge of ∼38%, and three-fourths of this decrease results from the complete stabilization of DIII-S4 (contributing a 30% reduction in Qmax) and one-fourth is due to partial stabilization of DIV-S4 (a reduction of 8-10%). Even though substitutions for the Asn in DIV-S6 in NaV1.5, N1764A and N1764C, produce little ionic current in transfected mammalian cells, they both express robust gating currents. Anthopleurin-A toxin, which inhibits movement of DIV-S4, still reduced Qmax by nearly 30%, a value similar to that observed in wild-type channels, in both N1764A and N1764C. By applying lidocaine and measuring the gating currents, we demonstrated that Asn residues in the S6s of DIII and DIV are important for coupling their pore domains to their voltage-sensor domains, and that Ala and Cys substitutions for Asn in both S6s result in uncoupling of the pore domains from their voltage-sensor domains. Similar observations were made for NaV1.4, although substitutions for Asn in DIII-S6 showed somewhat less uncoupling.

The Selective Nav1.7 Inhibitor, PF-05089771, Interacts Equivalently with Fast and Slow Inactivated Nav1.7 Channels

Voltage-gated sodium (Na_v) channel inhibitors are used clinically as analgesics and local anesthetics. However, the absence of Na_v channel isoform selectivity of current treatment options can result in adverse cardiac and central nervous system side effects, limiting their therapeutic utility. Human hereditary gain- or loss-of-pain disorders have demonstrated an essential role of Na_v1.7 sodium channels in the sensation of pain, thus making this channel an attractive target for new pain therapies. We previously identified a novel, state-dependent human Na_v1.7 selective inhibitor (PF-05089771, IC₅₀ = 11 nM) that interacts with the voltage-sensor domain (VSD) of domain IV. We further characterized the state-dependent interaction of PF-05089771 by systematically varying the voltage, frequency, and duration of conditioning prepulses to provide access to closed, open, and fast- or slow-inactivated states. The current study demonstrates that PF-05089771 exhibits a slow onset of block that is depolarization and concentration dependent, with a similarly slow recovery from block. Furthermore, the onset of block by PF-05089771 develops with similar rates using protocols that bias channels into predominantly fast- or slow-inactivated states, suggesting that channel inhibition is less dependent on the availability of a particular inactivated state than the relative time that the channel is depolarized. Taken together, the inhibitory profile of PF-05089771 suggests that a conformational change in the domain IV VSD after depolarization is necessary and sufficient to reveal a high-affinity binding site with which PF-05089771 interacts, stabilizing the channel in a nonconducting conformation from which recovery is slow.

New (arylalkyl)azole derivatives showing anticonvulsant effects could have VGSC and/or GABAAR affinity according to molecular modeling studies

(Arylalkyl)azoles (AAAs) emerged as a novel class of antiepileptic agents with the invention of nafimidone and denzimol. Several AAA derivatives with potent anticonvulsant activities have been reported so far, however neurotoxicity was usually an issue. We prepared a set of ester derivatives of 1-(2-naphthyl)-2-(1H-1,2,4-triazol-1-yl)ethanone oxime and evaluated their anticonvulsant and neurotoxic effects in mice. Most of our compounds were protective against maximal electroshock (MES)- and/or subcutaneous metrazol (s.c. MET)-induced seizures whereas none of them showed neurotoxicity. Nafimidone and denzimol have an activity profile similar to that of phenytoin or carbamazepine, both of which are known to inhibit voltage-gated sodium channels (VGSCs) as well as to enhance γ-aminobutiric acid (GABA)-mediated response. In order to get insights into the effects of our compounds on VGSCs and A-type GABA receptors (GABA_ARs) we performed docking studies using homology model of Na⁺ channel inner pore and GABA_AR as docking scaffolds. We found that our compounds bind VGSCs in similar ways as phenytoin, carbamazepine, and lamotrigine. They showed strong affinity to benzodiazepine (BZD) binding site and their binding interactions were mainly complied with the experimental data and the reported BZD binding model.

A case of non-dystrophic myotonia with concomitant mutations in the SCN4A and CLCN1 genes

Non-dystrophic myotonias are caused by mutations of either the skeletal muscle chloride (CLCN1) or sodium channel (SCN4A) gene. They exhibit several distinct phenotypes, including myotonia congenita, paramyotonia congenita and sodium channel myotonia, and a genotype-phenotype correlation has been established. However, there are atypical cases that do not fit with the standard classification. We report a case of 27-year-old male who had non-dystrophic myotonia with periodic paralysis and two heterozygous mutations, E950K in CLCN1 and F1290L in SCN4A. His mother, who exhibited myotonia without paralytic attack, only harbored E950K, and no mutations were identified in his asymptomatic father. Therefore, the E950K mutation was presumed to be pathogenic, although it was reported as an extremely rare genetic variant. The proband experienced paralytic attacks that lasted for weeks and were less likely to be caused by CLCN1 mutation alone. Functional analysis of the F1290L mutant channel heterologously expressed in cultured cells revealed enhanced activation inducing membrane hyperexcitability. We therefore propose that the two mutations had additive effects on membrane excitability that resulted in more prominent myotonia in the proband. Our case stresses the value of performing genetic analysis of both CLCN1 and SCN4A genes for myotonic patients with an atypical phenotype.

Understanding Sodium Channel Function and Modulation Using Atomistic Simulations of Bacterial Channel Structures

Sodium channels are chief proteins involved in electrical signaling in the nervous system, enabling critical functions like heartbeat and brain activity. New high-resolution X-ray structures for bacterial sodium channels have created an opportunity to see how these proteins operate at the molecular level. An important challenge to overcome is establishing relationships between the structures and functions of mammalian and bacterial channels. Bacterial sodium channels are known to exhibit the main structural features of their mammalian counterparts, as well as several key functional characteristics, including selective ion conduction, voltage-dependent gating, pore-based inactivation and modulation by local anesthetic, antiarrhythmic and antiepileptic drugs. Simulations have begun to shed light on each of these features in the past few years. Despite deviations in selectivity signatures for bacterial and mammalian channels, simulations have uncovered the nature of the multiion conduction mechanism associated with Na(+) binding to a high-field strength site established by charged glutamate side chains. Simulations demonstrated a surprising level of flexibility of the protein, showing that these side chains are active participants in the permeation process. They have also uncovered changes in protein structure, leading to asymmetrical collapses of the activation gate that have been proposed to correspond to inactivated structures. These observations offer the potential to examine the mechanisms of state-dependent drug activity, focusing on pore-blocking and pore-based slow inactivation in bacterial channels, without the complexities of inactivation on multiple timescales seen in eukaryotic channels. Simulations have provided molecular views of the interactions of drugs, consistent with sites predicted in mammalian channels, as well as a wealth of other sites as potential new drug targets. In this chapter, we survey the new insights into sodium channel function that have emerged from studies of simpler bacterial channels, which provide an excellent learning platform, and promising avenues for mechanistic discovery and pharmacological development.

Na+ channel function, regulation, structure, trafficking and sequestration

This paper is the second of a series of three reviews published in this issue resulting from the University of California Davis Cardiovascular Symposium 2014: Systems approach to understanding cardiac excitation-contraction coupling and arrhythmias: Na(+) channel and Na(+) transport. The goal of the symposium was to bring together experts in the field to discuss points of consensus and controversy on the topic of sodium in the heart. The present review focuses on Na(+) channel function and regulation, Na(+) channel structure and function, and Na(+) channel trafficking, sequestration and complexing.