Structure of the Cardiac Sodium Channel

How could simulations elucidate Nav1.5 channel blockers mechanism?

Tao and Corry used metadynamics, an enhanced sampling method to identify and classify Nav channel blockers.

Drugs exhibit diverse binding modes and access routes in the Nav1.5 cardiac sodium channel pore

Small molecule inhibitors of the sodium channel are common pharmacological agents used to treat a variety of cardiac and nervous system pathologies. They act on the channel via binding within the pore to directly block the sodium conduction pathway and/or modulate the channel to favor a non-conductive state. Despite their abundant clinical use, we lack specific knowledge of their protein-drug interactions and the subtle variations between different compound structures. This study investigates the binding and accessibility of nine different compounds in the pore cavity of the Nav1.5 sodium channel using enhanced sampling simulations. We find that most compounds share a common location of pore binding-near the mouth of the DII-III fenestration-associated with the high number of aromatic residues in this region. In contrast, some other compounds prefer binding within the lateral fenestrations where they compete with lipids, rather than binding in the central cavity. Overall, our simulation results suggest that the drug binding within the pore is highly promiscuous, with most drugs having multiple low-affinity binding sites. Access to the pore interior via two out of four of the hydrophobic fenestrations is favorable for the majority of compounds. Our results indicate that the polyspecific and diffuse binding of inhibitors in the pore contributes to the varied nature of their inhibitory effects and can be exploited for future drug discovery and optimization.

The differential impacts of equivalent gating-charge mutations in voltage-gated sodium channels

Voltage-gated sodium (Nav) channels are pivotal for cellular signaling, and mutations in Nav channels can lead to excitability disorders in cardiac, muscular, and neural tissues. A major cluster of pathological mutations localizes in the voltage-sensing domains (VSDs), resulting in either gain-of-function, loss-of-function effects, or both. However, the mechanism behind this functional diversity of mutations at equivalent positions remains elusive. Through hotspot analysis, we identified three gating charges (R1, R2, and R3) as major mutational hotspots in VSDs. The same amino acid substitutions at equivalent gating-charge positions in VSDI and VSDII of the cardiac sodium channel Nav1.5 show differential gating property impacts in electrophysiology measurements. We conducted molecular dynamics (MD) simulations on wild-type channels and six mutants to elucidate the structural basis of their differential impacts. Our 120-µs MD simulations with applied external electric fields captured VSD state transitions and revealed the differential structural dynamics between equivalent R-to-Q mutants. Notably, we observed transient leaky conformations in some mutants during structural transitions, offering a detailed structural explanation for gating-pore currents. Our salt-bridge network analysis uncovered VSD-specific and state-dependent interactions among gating charges, countercharges, and lipids. This detailed analysis revealed how mutations disrupt critical electrostatic interactions, thereby altering VSD permeability and modulating gating properties. By demonstrating the crucial importance of considering the specific structural context of each mutation, our study advances our understanding of structure-function relationships in Nav channels. Our work establishes a robust framework for future investigations into the molecular basis of ion channel-related disorders.

Understanding the role of mutations in voltage-gated sodium ion channels for cardiovascular disorders

Elhanafy et al. used Molecular Dynamics simulations and electrophysiology to show how identical mutations in the volgage sending domain of sodium channels can yield differential functional effects.

α-Adrenoreceptor blocker phentolamine inhibits voltage-gated sodium channels via the local anaesthetic binding site

BACKGROUND AND PURPOSE

Phentolamine is a non-selective α-adrenoreceptor antagonist used to reverse local anaesthesia, for example, during dental procedures when a vasoconstrictor is co-applied. Phentolamine-mediated vasodilation leads to faster clearance of injected drugs. Previous electrophysiological studies hypothesized that phentolamine acts as a modulator of voltage-gated sodium channels, which could conflict with its indication as local anaesthetic reversal agent.

EXPERIMENTAL APPROACH

We performed manual and high throughput patch-clamp recordings on HEK and CHO cells expressing NaV1.7 and NaV1.5. We investigated the effects of phentolamine on sodium channel biophysics and the additive impact of phentolamine on cells preconditioned with the local anaesthetic mexiletine. We used site-directed mutagenesis, homology modelling and drug docking to identify phentolamine's binding site. We compared the effect on sodium channels with other clinically established α-adrenoreceptor antagonists.

KEY RESULTS

Phentolamine inhibits NaV1.7 in HEK and CHO cells with an IC50 value of 72 and 57 μM and NaV1.5 in CHO cells with an IC50 of 27 μM. Phentolamine enhances the tonic block induced by the local anaesthetic mexiletine. Phentolamine binds to sodium channels at the local anaesthetic receptor site. The α-adrenoreceptor antagonists alfuzosin, urapidil and phenoxybenzamine show lower potency on NaV1.5 and NaV1.7 in patch-clamp recordings.

CONCLUSIONS AND IMPLICATIONS

Phentolamine blocks voltage-gated sodium channels via the local anaesthetic receptor site. This may conflict with its current indication as an antidote for local anaesthetics. We propose alternative α-adrenoreceptor antagonists as possible candidates for local anaesthetic reversal because these are less potent inhibitors of both cardiac and neuronal voltage-gated sodium channels.

Molecular Regulation of Cardiomyocyte Maturation

PURPOSE OF THE REVIEW

This review aims to discuss the process of cardiomyocyte maturation, with a focus on the underlying molecular mechanisms required to form a fully functional heart. We examine both long-standing concepts associated with cardiac maturation and recent developments, and the overall complexity of molecularly integrating all the processes that lead to a mature heart.

RECENT FINDINGS

Cardiac maturation, defined here as the sequential changes that occurring before the heart reaches full maturity, has been a subject of investigation for decades. Recently, there has been a renewed, highly focused interest in this process, driven by clinically motivated research areas where enhancing maturation may lead to improved therapeutic opportunities. These include using pluripotent stem cell models for cell therapy and disease modeling, as well as recent advancements in adult cardiac regeneration approaches. We highlight key processes underlying maturation of the heart, including cellular and organ growth, and electrophysiological, metabolic, and contractile maturation. We further discuss how these processes integrate and interact to contribute to the overall complexity of the developing heart. Finally, we emphasize the transformative potential for translating relevant maturation concepts to emerging models of heart disease and regeneration.

Elucidating the roles of voltage sensors in NaV1.9 activation and inactivation through a spider toxin

The gating process of voltage-gated sodium (NaV) channels is extraordinary intrinsic and involves numerous factors, such as voltage-sensing domain (VSD), the N-terminus and C-terminus, and the auxiliary subunits. To date, the gating mechanism of NaV channel has not been clearly elucidated. NaV1.9 has garnered significant attention due to its slow gating kinetics. Due to the challenges of NaV1.9 heterologous expression, research on its gating mechanism is relatively limited. Whether there are any differences in the functions of the four VSDs in NaV1.9 compared to those in other subtypes remains an open question. Here, we employed the established chimera method to transplant the S3b-S4 motif from the VSDIV of the toxin-sensitive donor channel (NaV1.9) into the receptor channel (NaV1.9/1.8 DIV S3b-S4 chimera). This modification imparted animal toxin sensitivity to the other three VSDs. Our results demonstrate that all four VSDs of NaV1.9 are involved in channel opening, VSDIII and VSDIV are primarily involved in regulating fast inactivation, and VSDII also regulates the steady-state inactivation of channels. These findings provide a new insight into the gating mechanism of NaV1.9.

Ion channel traffic jams: the significance of trafficking deficiency in long QT syndrome

A well-balanced ion channel trafficking machinery is paramount for the normal electromechanical function of the heart. Ion channel variants and many drugs can alter the cardiac action potential and lead to arrhythmias by interfering with mechanisms like ion channel synthesis, trafficking, gating, permeation, and recycling. A case in point is the Long QT syndrome (LQTS), a highly arrhythmogenic disease characterized by an abnormally prolonged QT interval on ECG produced by variants and drugs that interfere with the action potential. Disruption of ion channel trafficking is one of the main sources of LQTS. We review some molecular pathways and mechanisms involved in cardiac ion channel trafficking. We highlight the importance of channelosomes and other macromolecular complexes in helping to maintain normal cardiac electrical function, and the defects that prolong the QT interval as a consequence of variants or the effect of drugs. We examine the concept of "interactome mapping" and illustrate by example the multiple protein-protein interactions an ion channel may undergo throughout its lifetime. We also comment on how mapping the interactomes of the different cardiac ion channels may help advance research into LQTS and other cardiac diseases. Finally, we discuss how using human induced pluripotent stem cell technology to model ion channel trafficking and its defects may help accelerate drug discovery toward preventing life-threatening arrhythmias. Advancements in understanding ion channel trafficking and channelosome complexities are needed to find novel therapeutic targets, predict drug interactions, and enhance the overall management and treatment of LQTS patients.

Nav1.2 channel mutations preventing fast inactivation lead to SCN2A encephalopathy

SCN2A gene-related early-infantile developmental and epileptic encephalopathy (EI-DEE) is a rare and severe disorder that manifests in early infancy. SCN2A mutations affecting the fast inactivation gating mechanism can result in altered voltage dependence and incomplete inactivation of the encoded neuronal Nav1.2 channel and lead to abnormal neuronal excitability. In this study, we evaluated clinical data of seven missense Nav1.2 variants associated with DEE and performed molecular dynamics simulations, patch-clamp electrophysiology and dynamic clamp real-time neuronal modelling to elucidate the molecular and neuron-scale phenotypic consequences of the mutations. The N1662D mutation almost completely prevented fast inactivation without affecting activation. The comparison of wild-type and N1662D channel structures suggested that the ambifunctional hydrogen bond formation between residues N1662 and Q1494 is essential for fast inactivation. Fast inactivation could also be prevented with engineered Q1494A or Q1494L Nav1.2 channel variants, whereas Q1494E or Q149K variants resulted in incomplete inactivation and persistent current. Molecular dynamics simulations revealed a reduced affinity of the hydrophobic IFM-motif to its receptor site with N1662D and Q1494L variants relative to wild-type. These results demonstrate that the interactions between N1662 and Q1494 underpin the stability and the orientation of the inactivation gate and are essential for the development of fast inactivation. Six DEE-associated Nav1.2 variants, with mutations mapped to channel segments known to be implicated in fast inactivation were also evaluated. Remarkably, the L1657P variant also prevented fast inactivation and produced biophysical characteristics that were similar to those of N1662D, whereas the M1501V, M1501T, F1651C, P1658S and A1659V variants resulted in biophysical properties that were consistent with gain-of-function and enhanced action potential firing of hybrid neurons in dynamic action potential clamp experiments. Paradoxically, low densities of N1662D or L1657P currents potentiated action potential firing, whereas increased densities resulted in sustained depolarization. Our results provide novel structural insights into the molecular mechanism of Nav1.2 channel fast inactivation and inform treatment strategies for SCN2A-related EI-DEE. The contribution of non-inactivating Nav1.2 channels to neuronal excitability may constitute a distinct cellular mechanism in the pathogenesis of SCN2A-related DEE.

A misstep in the multistep process of fast inactivation

JGP study (Liu and Bezanilla. https://doi.org/10.1085/jgp.202413667) reveals that a sodium channel mutant blocks fast inactivation downstream of inactivation particle binding, diverting the channel into an alternative open state.

A sodium channel mutant removes fast inactivation with the inactivation particle bound

Fast inactivation is a key feature of voltage-gated sodium channels and is pivotal for countless physiological functions. Despite the prevalence of the canonical ball-and-chain model, more recent structural results suggest that fast inactivation requires multiple conformational changes beyond the binding of the inactivation particle, the IFM motif. Combining ionic current, gating current, and fluorescent measurements, here we showed that a double mutant at the bottom of the pore domain (CW) removes fast inactivation by interrupting the communication of the IFM motif and the pore. Instead of triggering fast inactivation, the IFM motif binding in CW allows the channel to enter an alternative open state. This alternative open state severely influenced the voltage sensor movements and was not accessible to wild type or other fast inactivation-deficient channels. Our results highlight the multistep nature of the fast inactivation process in mammalian voltage-gated sodium channels and demonstrate that CW modifies the channel behaviors more profoundly than simple removal of fast inactivation.

Differential Inhibition by Cenobamate of Canonical Human Nav1.5 Ion Channels and Several Point Mutants

Cenobamate is a new and highly effective antiseizure compound used for the treatment of adults with focal onset seizures and particularly for epilepsy resistant to other antiepileptic drugs. It acts on multiple targets, as it is a positive allosteric activator of γ-aminobutyric acid type A (GABAA) receptors and an inhibitor of neuronal sodium channels, particularly of the late or persistent Na+ current. We recently evidenced the inhibitory effects of cenobamate on the peak and late current component of the human cardiac isoform hNav1.5. The determined apparent IC50 values of 87.6 µM (peak) and 46.5 µM (late current) are within a clinically relevant range of concentrations (the maximal plasma therapeutic effective concentration for a daily dose of 400 mg in humans is 170 µM). In this study, we built a 3D model of the canonical hNav1.5 channel (UniProt Q14524-1) in open conformation using AlphaFold2, embedded it in a DPPC lipid bilayer, corrected the residue protonation state (pH 7.2) with H++, and added 2 Na+ ions in the selectivity filter. By molecular docking, we found the cenobamate binding site in the central cavity. We identified 10-point mutant variants in the binding site region and explored them via docking and MD. Mutants N1462K/Y (rs1064795922, rs199473614) and M1765R (rs752476527) (by docking) and N932S (rs2061582195) (by MD) featured higher predicted affinity than wild-type.

Harnessing Deep Learning Methods for Voltage-Gated Ion Channel Drug Discovery

Voltage-gated ion channels (VGICs) are pivotal in regulating electrical activity in excitable cells and are critical pharmaceutical targets for treating many diseases including cardiac arrhythmia and neuropathic pain. Despite their significance, challenges such as achieving target selectivity persist in VGIC drug development. Recent progress in deep learning, particularly diffusion models, has enabled the computational design of protein binders for any clinically relevant protein based solely on its structure. These developments coincide with a surge in experimental structural data for VGICs, providing a rich foundation for computational design efforts. This review explores the recent advancements in computational protein design using deep learning and diffusion methods, focusing on their application in designing protein binders to modulate VGIC activity. We discuss the potential use of these methods to computationally design protein binders targeting different regions of VGICs, including the pore domain, voltage-sensing domains, and interface with auxiliary subunits. We provide a comprehensive overview of the different design scenarios, discuss key structural considerations, and address the practical challenges in developing VGIC-targeting protein binders. By exploring these innovative computational methods, we aim to provide a framework for developing novel strategies that could significantly advance VGIC pharmacology and lead to the discovery of effective and safe therapeutics.

Multiple gating processes associated with the distal end of the S6 segment of domain II in the Nav channels

Voltage-gated sodium (Nav) channels are transmembrane proteins that play crucial roles in the initiation and propagation of action potentials (APs) in excitable tissues such as the heart, muscles, and nerves. The distal ends of the four domain S6 segments of Nav channels contain hydrophobic residues, which form an intracellular gate. This gate allows Nav channels to control ion flux in excitable cells by opening and closing. However, the mechanism of the distal end of Domain II (DII) S6 segment in channel gating remains unclear. In this study, using whole-cell patch clamp recording, we systematically investigated the biophysical characteristics of various mutants L811 site (located at the distal end of the DII S6 segment) of Nav1.9 and the corresponding L796P mutant of Nav1.4. We found that the mutations significantly shifted the activation and inactivation curves, slowed the fast inactivation, accelerated the slow inactivation and use-dependent slow inactivation, and L811P altered the ion selectivity of the channel. Therefore, our findings suggest that the distal end of the DII S6 segment in Nav channels plays a pivotal role in regulating multiple gating processes.

Genetic background and clinical phenotype in a Vietnamese cohort with Brugada syndrome: A whole exome sequencing study

Objectives

The aim of this study was to report the spectrum of genetic variations and clinical phenotype in a Vietnamese cohort with confirmed Brugada syndrome (BrS) using the whole exome sequencing (WES).

Methods

Fifty patients with confirmed BrS were included in this study. Genomic DNA samples were extracted from peripheral blood and conducted for WES. The variants were annotated using ANNOVAR. The variants in the 13 reported genes associated with BrS were filtered, predicted the functional impact using eight computational tools, and classified according to the 2015 ACMG guidelines.

Results

Arrhythmic events were documented in one-fifth of the participants. Twenty-four probands were identified to carry 36 variants in 13 genes. Majority of the variants in our study was SCN5A variants (9/36 variants, 25%), followed by KCNH2 variants (5/36 variants, 14%). The prevalence of SCN5A carriers was 16%; while the prevalence of minor gene carriers was less than 10%. Nine novel missense variants were identified, including four missense SCN5A variants (p.E901D, p.F853L, p.L377F, and p.H184R), two missense ANK2 variants (p.S2845L and V1497L), one missense CACNA1C variant (M1126V), one missense DSP variant (p.K478N), and one intron splicing JUP variant (c.1498-5G>C).

Conclusion

Our study underscores the primary significance of the SCN5A gene in BrS, as indicated by variant prevalence, carrier rates, pathogenicity per ACMG classification, in silico predictions, and its correlation with clinical phenotypes. Longitudinal study with larger sample size, pedigree, Sanger sequence confirmation, and functional analysis is recommended.

Pediatric flecainide pharmacogenomics: a roadmap to delivering precision-based care to pediatrics arrhythmias

Flecainide acetate is a Class 1c anti-arrhythmic with a potent sodium voltage gated channel blockade which is utilized for the second-line treatment of tachyarrhythmias in children and adults. Given its narrow therapeutic index, the individualization of drug therapy is of utmost importance for clinicians. Despite efforts to improve anti-arrhythmic drug therapy, there remain knowledge gaps regarding the impact of variation in the genes relevant to flecainide's disposition and response. This variability is compounded in developing children whose drug disposition and response pathways may remain immature. The purpose of this comprehensive review is to outline flecainide's disposition and response pathways while simultaneously highlighting opportunities for prospective investigation in the pediatric population.

Structural modeling of ion channels using AlphaFold2, RoseTTAFold2, and ESMFold

Ion channels play key roles in human physiology and are important targets in drug discovery. The atomic-scale structures of ion channels provide invaluable insights into a fundamental understanding of the molecular mechanisms of channel gating and modulation. Recent breakthroughs in deep learning-based computational methods, such as AlphaFold, RoseTTAFold, and ESMFold have transformed research in protein structure prediction and design. We review the application of AlphaFold, RoseTTAFold, and ESMFold to structural modeling of ion channels using representative voltage-gated ion channels, including human voltage-gated sodium (NaV) channel - NaV1.8, human voltage-gated calcium (CaV) channel - CaV1.1, and human voltage-gated potassium (KV) channel - KV1.3. We compared AlphaFold, RoseTTAFold, and ESMFold structural models of NaV1.8, CaV1.1, and KV1.3 with corresponding cryo-EM structures to assess details of their similarities and differences. Our findings shed light on the strengths and limitations of the current state-of-the-art deep learning-based computational methods for modeling ion channel structures, offering valuable insights to guide their future applications for ion channel research.

A structural atlas of druggable sites on Nav channels

Voltage-gated sodium (Nav) channels govern membrane excitability by initiating and propagating action potentials. Consistent with their physiological significance, dysfunction, or mutations in these channels are associated with various channelopathies. Nav channels are thereby major targets for various clinical and investigational drugs. In addition, a large number of natural toxins, both small molecules and peptides, can bind to Nav channels and modulate their functions. Technological breakthrough in cryo-electron microscopy (cryo-EM) has enabled the determination of high-resolution structures of eukaryotic and eventually human Nav channels, alone or in complex with auxiliary subunits, toxins, and drugs. These studies have not only advanced our comprehension of channel architecture and working mechanisms but also afforded unprecedented clarity to the molecular basis for the binding and mechanism of action (MOA) of prototypical drugs and toxins. In this review, we will provide an overview of the recent advances in structural pharmacology of Nav channels, encompassing the structural map for ligand binding on Nav channels. These findings have established a vital groundwork for future drug development.

Recent Advances in Expression Screening and Sample Evaluation for Structural Studies of Membrane Proteins

Membrane proteins are involved in numerous biological processes and represent more than half of all drug targets; thus, structural information on these proteins is invaluable. However, the low expression level of membrane proteins, as well as their poor stability in solution and tendency to precipitate and aggregate, are major bottlenecks in the preparation of purified membrane proteins for structural studies. Traditionally, the evaluation of membrane protein constructs for structural studies has been quite time consuming and expensive since it is necessary to express and purify the proteins on a large scale, particularly for X-ray crystallography. The emergence of fluorescence detection size exclusion chromatography (FSEC) has drastically changed this situation, as this method can be used to rapidly evaluate the expression and behavior of membrane proteins on a small scale without the need for purification. FSEC has become the most widely used method for the screening of expression conditions and sample evaluation for membrane proteins, leading to the successful determination of numerous structures. Even in the era of cryo-EM, FSEC and the new generation of FSEC derivative methods are being widely used in various manners to facilitate structural analysis. In addition, the application of FSEC is not limited to structural analysis; this method is also widely used for functional analysis of membrane proteins, including for analysis of oligomerization state, screening of antibodies and ligands, and affinity profiling. This review presents the latest advances and applications in membrane protein expression screening and sample evaluation, with a particular focus on FSEC methods.

An all-atom model of the human cardiac sodium channel in a lipid bilayer

Voltage-gated sodium channels (NaV) are complex macromolecular proteins that are responsible for the initial upstroke of an action potential in excitable cells. Appropriate function is necessary for many physiological processes such as heartbeat, voluntary muscle contraction, nerve conduction, and neurological function. Dysfunction can have life-threatening consequences. During the past decade, there have been significant advancements with ion channel structural characterization by CryoEM, yet descriptions of cytosolic components are often lacking. Many investigations have biophysically characterized reconstituted cytosolic components and their interactions. However, extrapolating the structural alterations and allosteric communication within an intact ion channel can be challenging. To address this, we have developed an all-atom model of the human cardiac sodium channel (NaV1.5) in a lipid bilayer with explicit salt and water. Our simulations contain descriptions of cytosolic components that are poorly predicted by AlphaFold and lacking in many CryoEM structures. Leveraging the latest advancements of the Amber force fields (ff19sb and Lipid21) and water model (OPC), our simulations improved protein backbone torsion angles and generated structural information across time (four independent one-microsecond simulations). Our analysis provided descriptions of lipid and solvent contacts and insight into the C-Terminal Domain - inactivation gate and inactivation gate - latch receptor interactions.

Structural biology and molecular pharmacology of voltage-gated ion channels

Voltage-gated ion channels (VGICs), including those for Na+, Ca2+ and K+, selectively permeate ions across the cell membrane in response to changes in membrane potential, thus participating in physiological processes involving electrical signalling, such as neurotransmission, muscle contraction and hormone secretion. Aberrant function or dysregulation of VGICs is associated with a diversity of neurological, psychiatric, cardiovascular and muscular disorders, and approximately 10% of FDA-approved drugs directly target VGICs. Understanding the structure-function relationship of VGICs is crucial for our comprehension of their working mechanisms and role in diseases. In this Review, we discuss how advances in single-particle cryo-electron microscopy have afforded unprecedented structural insights into VGICs, especially on their interactions with clinical and investigational drugs. We present a comprehensive overview of the recent advances in the structural biology of VGICs, with a focus on how prototypical drugs and toxins modulate VGIC activities. We explore how these structures elucidate the molecular basis for drug actions, reveal novel pharmacological sites, and provide critical clues to future drug discovery.

Scorpion α-toxin LqhαIT specifically interacts with a glycan at the pore domain of voltage-gated sodium channels

Voltage-gated sodium (Nav) channels sense membrane potential and drive cellular electrical activity. The deathstalker scorpion α-toxin LqhαIT exerts a strong action potential prolonging effect on Nav channels. To elucidate the mechanism of action of LqhαIT, we determined a 3.9 Å cryoelectron microscopy (cryo-EM) structure of LqhαIT in complex with the Nav channel from Periplaneta americana (NavPas). We found that LqhαIT binds to voltage sensor domain 4 and traps it in an "S4 down" conformation. The functionally essential C-terminal epitope of LqhαIT forms an extensive interface with the glycan scaffold linked to Asn330 of NavPas that augments a small protein-protein interface between NavPas and LqhαIT. A combination of molecular dynamics simulations, structural comparisons, and prior mutagenesis experiments demonstrates the functional importance of this toxin-glycan interaction. These findings establish a structural basis for the specificity achieved by scorpion α-toxins and reveal the conserved glycan as an essential component of the toxin-binding epitope.

Role of protein domains in trafficking and localization of the voltage-gated sodium channel β2 subunit

The voltage-gated sodium (NaV) channel is critical for cardiomyocyte function since it is responsible for action potential initiation and its propagation throughout the cell. It consists of a protein complex made of a pore forming α subunit and associated β subunits, which regulate α subunit function and subcellular localization. We previously showed the implication of N-linked glycosylation and S-acylation of β2 in its polarized trafficking. Here, we present evidence of β2 dimerization. Moreover, we demonstrate the implication of the cytoplasmic tail, extracellular loop, and transmembrane domain on proper β2 folding and export to the cell surface of polarized Madin-Darby canine kidney cells. Substantial alteration, or lack of any of these domains, leads to accumulation of β2 in the endoplasmic reticulum, along with impaired complex N-glycosylation, which is needed for its efficient surface delivery. We also show that these alterations to β2 affected to a certain extent NaV1.5 surface localization. Conversely, however, NaV1.5 had little or no influence on β2 trafficking, its localization to the surface, or homodimer formation. Altogether, our data link the architecture of the β2 domains to the establishment of its proper subcellular localization. These findings could provide valuable insights to gain a deeper comprehension of the elusive biology of β subunits in excitable cells, such as neurons and cardiomyocytes.

ELUCIDATING THE DIFFERENTIAL IMPACTS OF EQUIVALENT GATING-CHARGE MUTATIONS IN VOLTAGE-GATED SODIUM CHANNELS

Voltage-gated sodium (Nav) channels are pivotal for cellular signaling and mutations in Nav channels can lead to excitability disorders in cardiac, muscular, and neural tissues. A major cluster of pathological mutations localizes in the voltage-sensing domains (VSDs), resulting in either gain-of-function (GoF), loss-of-function (LoF) effects, or both. However, the mechanism behind this functional divergence of mutations at equivalent positions remains elusive. Through hotspot analysis, we identified three gating charges (R1, R2, and R3) as major mutational hotspots in VSDs. The same amino-acid substitutions at equivalent gating-charge positions in VSDI and VSDII of the cardiac sodium channel Nav1.5 show differential gating-property impacts in electrophysiology measurements. We conducted 120 μs molecular dynamics (MD) simulations on wild-type and six mutants to elucidate the structural basis of their differential impacts. Our μs-scale MD simulations with applied external electric fields captured VSD state transitions and revealed the differential structural dynamics between equivalent R-to-Q mutants. Notably, we observed transient leaky conformations in some mutants during structural transitions, offering a detailed structural explanation for gating-pore currents. Our salt-bridge network analysis uncovered VSD-specific and state-dependent interactions among gating charges, countercharges, and lipids. This detailed analysis elucidated how mutations disrupt critical electrostatic interactions, thereby altering VSD permeability and modulating gating properties. By demonstrating the crucial importance of considering the specific structural context of each mutation, our study represents a significant leap forward in understanding structure-function relationships in Nav channels. Our work establishes a robust framework for future investigations into the molecular basis of ion channel-related disorders.

Modulating voltage-gated sodium channels to enhance differentiation and sensitize glioblastoma cells to chemotherapy

BACKGROUND

Glioblastoma (GBM) stands as the most prevalent and aggressive form of adult gliomas. Despite the implementation of intensive therapeutic approaches involving surgery, radiation, and chemotherapy, Glioblastoma Stem Cells contribute to tumor recurrence and poor prognosis. The induction of Glioblastoma Stem Cells differentiation by manipulating the transcriptional machinery has emerged as a promising strategy for GBM treatment. Here, we explored an innovative approach by investigating the role of the depolarized resting membrane potential (RMP) observed in patient-derived GBM sphereforming cell (GSCs), which allows them to maintain a stemness profile when they reside in the G0 phase of the cell cycle.

METHODS

We conducted molecular biology and electrophysiological experiments, both in vitro and in vivo, to examine the functional expression of the voltage-gated sodium channel (Nav) in GSCs, particularly focusing on its cell cycle-dependent functional expression. Nav activity was pharmacologically manipulated, and its effects on GSCs behavior were assessed by live imaging cell cycle analysis, self-renewal assays, and chemosensitivity assays. Mechanistic insights into the role of Nav in regulating GBM stemness were investigated through pathway analysis in vitro and through tumor proliferation assay in vivo.

RESULTS

We demonstrated that Nav is functionally expressed by GSCs mainly during the G0 phase of the cell cycle, suggesting its pivotal role in modulating the RMP. The pharmacological blockade of Nav made GBM cells more susceptible to temozolomide (TMZ), a standard drug for this type of tumor, by inducing cell cycle re-entry from G0 phase to G1/S transition. Additionally, inhibition of Nav substantially influenced the self-renewal and multipotency features of GSCs, concomitantly enhancing their degree of differentiation. Finally, our data suggested that Nav positively regulates GBM stemness by depolarizing the RMP and suppressing the ERK signaling pathway. Of note, in vivo proliferation assessment confirmed the increased susceptibility to TMZ following pharmacological blockade of Nav.

CONCLUSIONS

This insight positions Nav as a promising prognostic biomarker and therapeutic target for GBM patients, particularly in conjunction with temozolomide treatment.

Effects of donors' and recipients' preoperative serum sodium on the prognosis of liver transplantation

Dysnatremia is common in donors and recipients of liver transplantation (LT). However, the influence of dysnatremia on LT prognosis remains controversial. This study aimed to investigate effects of donors' and recipients' serum sodium on LT prognosis. We retrospectively reviewed 248 recipients who underwent orthotopic LT at our center between January 2016 and December 2018. Donors and recipients perioperative and 3-year postoperative clinical data were included. Delta serum sodium was defined as the donors' serum sodium minus the paired recipients' serum sodium. Donors with serum sodium > 145 mmol/L had significantly higher preoperative blood urea nitrogen (BUN) (P < 0.01) and creatinine (Cr) (P < 0.01) than others. Preoperative total bilirubin (TBIL) (P < 0.01), direct bilirubin (DBIL) (P < 0.01), BUN (P < 0.01), Cr (P < 0.01) were significantly higher in the hyponatremia group of recipients than the other groups, but both of donors' and recipients' serum sodium had no effect on the LT prognosis. In the delta serum sodium < 0 mmol/L group, TBIL (P < 0.01) and DBIL (P < 0.01) were significantly higher in postoperative 1 week than the other groups, but delta serum sodium had no effect on the postoperative survival rates. Dysnatremia in donors and recipients of LT have no effect on postoperative survival rates, hepatic and renal function, but recipients with higher serum sodium than donors have significantly higher TBIL and DBIL at 1 week postoperatively.

Shensong yangxin, a multi-functional traditional Chinese medicine for arrhythmia: A review of components, pharmacological mechanisms, and clinical applications

As a common cardiovascular disease (CVD), Arrhythmia refers to any abnormality in the origin, frequency, rhythm, conduction velocity, timing, pathway, sequence, or other aspect of cardiac impulses, and it is one of the common cardiovascular diseases in clinical practice. At present, various ion channel blockers are used for treatment of arrhythmia that include Na+ ion channel blockers, K+ ion channel blockers and Ca2+ ion channel blockers. While these drugs offer benefits, they have led to a gradual increase in drug-related adverse reactions across various systems. As a result, the quest for safe and effective antiarrhythmic drugs is pressing. Recent years have seen some advancements in the treatment of ventricular arrhythmias using traditional Chinese medicine(TCM). The theory of Luobing in TCM has proposed a new drug intervention strategy of "fast and slow treatment, integrated regulation" leading to a shift in mindset from "antiarrhythmic" to "rhythm-regulating". Guided by this theory, the development of Shen Song Yang Xin Capsules (SSYX) has involved various Chinese medicinal ingredients that comprehensively regulate the myocardial electrophysiological mechanism, exerting antiarrhythmic effects on multiple ion channels and non-ion channels. Similarly, in clinical studies, evidence-based research has confirmed that SSYX combined with conventional antiarrhythmic drugs can more effectively reduce the occurrence of arrhythmias. Therefore, this article provides a comprehensive review of the composition and mechanisms of action, pharmacological components, network pharmacology analysis, and clinical applications of SSYX guided by the theory of Luobing, aiming to offer valuable insights for improved clinical management of arrhythmias and related research.

Structural basis of inhibition of human NaV1.8 by the tarantula venom peptide Protoxin-I

Voltage-gated sodium channels (NaVs) selectively permit diffusion of sodium ions across the cell membrane and, in excitable cells, are responsible for propagating action potentials. One of the nine human NaV isoforms, NaV1.8, is a promising target for analgesics, and selective inhibitors are of interest as therapeutics. One such inhibitor, the gating-modifier peptide Protoxin-I derived from tarantula venom, blocks channel opening by shifting the activation voltage threshold to more depolarised potentials, but the structural basis for this inhibition has not previously been determined. Using monolayer graphene grids, we report the cryogenic electron microscopy structures of full-length human apo-NaV1.8 and the Protoxin-I-bound complex at 3.1 Å and 2.8 Å resolution, respectively. The apo structure shows an unexpected movement of the Domain I S4-S5 helix, and VSDI was unresolvable. We find that Protoxin-I binds to and displaces the VSDII S3-S4 linker, hindering translocation of the S4II helix during activation.

Inhibition of TTX-S Na+ currents by a novel blocker QLS-278 for antinociception

Genetic loss-of-function mutations of Nav1.7 channel, abundantly expressed in peripheral nociceptive neurons, cause congenital insensitivity to pain (CIP) in humans, indicating that selective inhibition of the channel may lead to potential therapy of pain disorders. In this study, we investigated a novel compound, 5-chloro-N-(cyclopropylsulfonyl)-2-fluoro-4-(2-(8-(furan-2-ylmethyl)-8-azaspiro [4.5] decan-2-yl) ethoxy) benzamide (QLS-278) that inhibits Nav1.7 channel and exhibits anti-nociceptive activity. Compound QLS-278 exhibits inactivation- and concentration-dependent inhibition of macroscopic currents of Nav1.7 channels stably expressed in HEK293 cells with an IC50 of 1.2 {plus minus} 0.2 μM. QLS-278 causes a hyperpolarization shift of the channel inactivation and delays recovery from inactivation, without an obvious effect on voltage-dependent activation. In mouse DRG neurons, QLS-278 suppresses native TTX-sensitive Nav currents and also reduces neuronal firing. Moreover, QLS-278 dose-dependently relieves neuropathic pain induced by spared nerve injury and inflammatory pain induced by formalin without significant alteration of spontaneous locomotor activity in mice. Altogether, our identification of the novel compound QLS-278 may hold developmental potential for the treatment of chronic pain. Significance Statement QLS-278, a novel voltage-gated sodium Nav1.7 channel blocker, inhibits native TTX-S Na+ current and reduces action potential firings in DRG sensory neurons. QLS-278 also exhibits antinociceptive activity in mouse models of pain, thus demonstrating potential for the development of a treatment for chronic pain.

The chemistry of electrical signaling in sodium channels from bacteria and beyond

Electrical signaling is essential for all fast processes in biology, but its molecular mechanisms have been uncertain. This review article focuses on studies of bacterial sodium channels in order to home in on the essential molecular and chemical mechanisms underlying transmembrane ion conductance and voltage-dependent gating without the overlay of complex protein interactions and regulatory mechanisms in mammalian sodium channels. This minimalist approach has yielded a nearly complete picture of sodium channel function at the atomic level that are mostly conserved in mammalian sodium channels, including sodium selectivity and conductance, voltage sensing and activation, electromechanical coupling to pore opening and closing, slow inactivation, and pathogenic dysfunction in a debilitating channelopathy. Future studies of nature's simplest sodium channels may continue to yield key insights into the fundamental molecular and chemical principles of their function and further elucidate the chemical basis of electrical signaling.

1,4-Disubstituted Piperazin-2-Ones as Selective Late Sodium Current Inhibitors with QT Interval Shortening Properties in Isolated Rabbit Hearts

Late sodium current (INa) inhibitors are a new subclass of antiarrhythmic agents. To overcome the drawbacks, e.g., low efficacy and inhibition effect on K+ current, of the FDA-approved late INa inhibitor ranolazine, chain amide 6a-6q, 1,4-disubstituted piperazin-2-ones 7a-7s, and their derivatives 8a-8n were successively designed, synthesized, and evaluated in vitro on the NaV1.5-transfected HEK293T cells by the whole-cell patch clamp recording assay at the concentration of 40 μM. Among the new skeleton compounds, 7d showed the highest efficacy (IC50 = 2.7 μM) and good selectivity (peak/late ratio >30 folds), as well as excellent pharmacokinetics properties in mice (T1/2 of 3.5 h, F = 90%, 3 mg/kg, po). It exhibited low hERG inhibition and was able to reverse the ATX-II-induced augmentation of late INa phenotype of LQT3 model in isolated rabbit hearts. These results suggest the application potentials of 7d in the treatments of arrhythmias related to the enhancement of late INa.

Genomic analysis of an Ecuadorian individual carrying an SCN5A rare variant

BACKGROUND

Ion channels, vital transmembrane protein complexes, regulate ion movement within cells. Germline variants in channel-encoding genes lead to channelopathies. The sodium channels in cardiac cells exhibit a structure of an alpha subunit and one to two beta subunits. The alpha subunit, encoded by the SCN5A gene, comprises four domains.

CASE PRESENTATION

A fifteen-year-old Ecuadorian female with atrial flutter and abnormal sinus rhythm with no familial history of cardiovascular disease underwent NGS with the TruSight Cardio kit (Illumina). A likely pathogenic SCN5A gene variant (NM_188056.2:c.2677 C > Tp. Arg893Cys) was identified, associated with arrhythmias, long QT, atrial fibrillation, and Brugada syndrome. Ancestral analysis revealed a predominant European component (43.9%), followed by Native American (35.7%) and African (20.4%) components.

CONCLUSIONS

The participant presents atrial flutter and conduction disorders, despite lacking typical cardiovascular risk factors. The proband carries a SCN5A variant that has not been previously reported in Latin America and may be associated to her phenotype. The documented arginine-to-cysteine substitution at position 893 in the protein is crucial for various cellular functions. The subject's mixed genetic composition highlights potential genetic contributors to atrial flutter, emphasizing the need for comprehensive genetic studies, particularly in mixed populations like Ecuadorians. This case underscores the importance of genetic analysis for personalized treatment and the significance of studying diverse genetic backgrounds in understanding cardiovascular diseases.

Evolution of ion channels in cetaceans: a natural experiment in the tree of life

Cetaceans represent a natural experiment within the tree of life in which a lineage changed from terrestrial to aquatic habitats. This shift involved phenotypic modifications, representing an opportunity to explore the genetic bases of phenotypic diversity. Among the different molecular systems that maintain cellular homeostasis, ion channels are crucial for the proper physiological functioning of all living species. This study aims to explore the evolution of ion channels during the evolutionary history of cetaceans. To do so, we created a bioinformatic pipeline to annotate the repertoire of ion channels in the genome of the species included in our sampling. Our main results show that cetaceans have, on average, fewer protein-coding genes and a higher percentage of annotated ion channels than non-cetacean mammals. Signals of positive selection were detected in ion channels related to the heart, locomotion, visual and neurological phenotypes. Interestingly, we predict that the NaV1.5 ion channel of most toothed whales (odontocetes) is sensitive to tetrodotoxin, similar to NaV1.7, given the presence of tyrosine instead of cysteine, in a specific position of the ion channel. Finally, the gene turnover rate of the cetacean crown group is more than three times faster than that of non-cetacean mammals.

GOLEM: Automated and Robust Cryo-EM-Guided Ligand Docking with Explicit Water Molecules

A detailed understanding of ligand-protein interaction is essential for developing rational drug-design strategies. In recent years, technological advances in cryo-electron microscopy (cryo-EM) brought a new era to the structural determination of biological macromolecules and assemblies at high resolution, marking cryo-EM as a promising tool for studying ligand-protein interactions. However, even in high-resolution cryo-EM results, the densities for the bound small-molecule ligands are often of lower quality due to their relatively dynamic and flexible nature, frustrating their accurate coordinate assignment. To address the challenge of ligand modeling in cryo-EM maps, here we report the development of GOLEM (Genetic Optimization of Ligands in Experimental Maps), an automated and robust ligand docking method that predicts a ligand's pose and conformation in cryo-EM maps. GOLEM employs a Lamarckian genetic algorithm to perform a hybrid global/local search for exploring the ligand's conformational, orientational, and positional space. As an important feature, GOLEM explicitly considers water molecules and places them at optimal positions and orientations. GOLEM takes into account both molecular energetics and the correlation with the cryo-EM maps in its scoring function to optimally place the ligand. We have validated GOLEM against multiple cryo-EM structures with a wide range of map resolutions and ligand types, returning ligand poses in excellent agreement with the densities. As a VMD plugin, GOLEM is free of charge and accessible to the community. With these features, GOLEM will provide a valuable tool for ligand modeling in cryo-EM efforts toward drug discovery.

Conformational photo-trapping in NaV1.5: Inferring local motions at the "inactivation gate"

Rapid and effectual inactivation in voltage-gated sodium channels is required for canonical action-potential firing. This "fast" inactivation arises from swift and reversible protein conformational changes that utilize transmembrane segments and the cytoplasmic linker between channel domains III and IV. Until recently, fast inactivation had been accepted to rely on a "ball-and-chain" mechanism whereby a hydrophobic triplet of DIII-IV amino acids (IFM) impairs conductance by binding to a site in central pore of the channel made available by channel opening. New structures of sodium channels have upended this model. Specifically, cryo-electron microscopic structures of eukaryotic sodium channels depict a peripheral binding site for the IFM motif, outside of the pore, opening the possibility of a yet unidentified allosteric mechanism of fast-inactivation gating. We set out to study fast inactivation by photo-trapping human sodium channels in various functional states under voltage control. This was achieved by genetically encoding the crosslinking unnatural amino acid benzophenone phenylalanine at various sites within the DIII-IV linker in the cardiac sodium channel NaV1.5. These data show dynamic state- and positional-dependent trapping of the transient conformations associated with fast inactivation, each yielding different phenotypes and rates of trapping. These data reveal distinct conformational changes that underlie fast inactivation and point to a dynamic environment around the IFM locus.

Resurgent current in context: Insights from the structure and function of Na and K channels

Discovered just over 25 years ago in cerebellar Purkinje neurons, resurgent Na current was originally described operationally as a component of voltage-gated Na current that flows upon repolarization from relatively depolarized potentials and speeds recovery from inactivation, increasing excitability. Its presence in many excitable cells and absence from others has raised questions regarding its biophysical and molecular mechanisms. Early studies proposed that Na channels capable of generating resurgent current are subject to a rapid open-channel block by an endogenous blocking protein, which binds upon depolarization and unblocks upon repolarization. Since the time that this mechanism was suggested, many physiological and structural studies of both Na and K channels have revealed aspects of gating and conformational states that provide insights into resurgent current. These include descriptions of domain movements for activation and inactivation, solution of cryo-EM structures with pore-blocking compounds, and identification of native blocking domains, proteins, and modulatory subunits. Such results not only allow the open-channel block hypothesis to be refined but also link it more clearly to research that preceded it. This review considers possible mechanisms for resurgent Na current in the context of earlier and later studies of ion channels and suggests a framework for future research.

Shining a Light on Venom-Peptide Receptors: Venom Peptides as Targeted Agents for In Vivo Molecular Imaging

Molecular imaging has revolutionised the field of biomedical research by providing a non-invasive means to visualise and understand biochemical processes within living organisms. Optical fluorescent imaging in particular allows researchers to gain valuable insights into the dynamic behaviour of a target of interest in real time. Ion channels play a fundamental role in cellular signalling, and they are implicated in diverse pathological conditions, making them an attractive target in the field of molecular imaging. Many venom peptides exhibit exquisite selectivity and potency towards ion channels, rendering them ideal agents for molecular imaging applications. In this review, we illustrate the use of fluorescently-labelled venom peptides for disease diagnostics and intraoperative imaging of brain tumours and peripheral nerves. Finally, we address challenges for the development and clinical translation of venom peptides as nerve-targeted imaging agents.

Anesthetic Mechanisms: Synergistic Interactions With Lipid Rafts and Voltage-Gated Sodium Channels

Despite successfully utilizing anesthetics for over 150 years, the mechanism of action remains relatively unknown. Recent studies have shown promising results, but due to the complex interactions between anesthetics and their targets, there remains a clear need for further mechanistic research. We know that lipophilicity is directly connected to anesthetic potency since lipid solubility relates to anesthetic partition into the membrane. However, clinically relevant concentrations of anesthetics do not significantly affect lipid bilayers but continue to influence various molecular targets. Lipid rafts are derived from liquid-ordered phases of the plasma membrane that contain increased concentrations of cholesterol and sphingomyelin and act as staging platforms for membrane proteins, including ion channels. Although anesthetics do not perturb membranes at clinically relevant concentrations, they have recently been shown to target lipid rafts. In this review, we summarize current research on how different types of anesthetics-local, inhalational, and intravenous-bind and affect both lipid rafts and voltage-gated sodium channels, one of their major targets, and how those effects synergize to cause anesthesia and analgesia. Local anesthetics block voltage-gated sodium channel pores while also disrupting lipid packing in ordered membranes. Inhalational anesthetics bind to the channel pore and the voltage-sensing domain while causing an increase in the number, size, and diameter of lipid rafts. Intravenous anesthetics bind to the channel primarily at the voltage-sensing domain and the selectivity filter, while causing lipid raft perturbation. These changes in lipid nanodomain structure possibly give proteins access to substrates that have translocated as a result of these structural alterations, resulting in lipid-driven anesthesia. Overall, anesthetics can impact channel activity either through direct interaction with the channel, indirectly through the lipid raft, or both. Together, these result in decreased sodium ion flux into the cell, disrupting action potentials and producing anesthetic effects. However, more research is needed to elucidate the indirect mechanisms associated with channel disruption through the lipid raft, as not much is known about anionic lipid products and their influence over voltage-gated sodium channels. Anesthetics' effect on S-palmitoylation, a promising mechanism for direct and indirect influence over voltage-gated sodium channels, is another auspicious avenue of research. Understanding the mechanisms of different types of anesthetics will allow anesthesiologists greater flexibility and more specificity when treating patients.

Dynamic Changes in Ion Channels during Myocardial Infarction and Therapeutic Challenges

In different areas of the heart, action potential waveforms differ due to differences in the expressions of sodium, calcium, and potassium channels. One of the characteristics of myocardial infarction (MI) is an imbalance in oxygen supply and demand, leading to ion imbalance. After MI, the regulation and expression levels of K+, Ca2+, and Na+ ion channels in cardiomyocytes are altered, which affects the regularity of cardiac rhythm and leads to myocardial injury. Myocardial fibroblasts are the main effector cells in the process of MI repair. The ion channels of myocardial fibroblasts play an important role in the process of MI. At the same time, a large number of ion channels are expressed in immune cells, which play an important role by regulating the in- and outflow of ions to complete intracellular signal transduction. Ion channels are widely distributed in a variety of cells and are attractive targets for drug development. This article reviews the changes in different ion channels after MI and the therapeutic drugs for these channels. We analyze the complex molecular mechanisms behind myocardial ion channel regulation and the challenges in ion channel drug therapy.

Toward high-resolution modeling of small molecule-ion channel interactions

Ion channels are critical drug targets for a range of pathologies, such as epilepsy, pain, itch, autoimmunity, and cardiac arrhythmias. To develop effective and safe therapeutics, it is necessary to design small molecules with high potency and selectivity for specific ion channel subtypes. There has been increasing implementation of structure-guided drug design for the development of small molecules targeting ion channels. We evaluated the performance of two RosettaLigand docking methods, RosettaLigand and GALigandDock, on the structures of known ligand-cation channel complexes. Ligands were docked to voltage-gated sodium (NaV), voltage-gated calcium (CaV), and transient receptor potential vanilloid (TRPV) channel families. For each test case, RosettaLigand and GALigandDock methods frequently sampled a ligand-binding pose within a root mean square deviation (RMSD) of 1-2 Å relative to the experimental ligand coordinates. However, RosettaLigand and GALigandDock scoring functions cannot consistently identify experimental ligand coordinates as top-scoring models. Our study reveals that the proper scoring criteria for RosettaLigand and GALigandDock modeling of ligand-ion channel complexes should be assessed on a case-by-case basis using sufficient ligand and receptor interface sampling, knowledge about state-specific interactions of the ion channel, and inherent receptor site flexibility that could influence ligand binding.

Unveiling Tst3, a Multi-Target Gating Modifier Scorpion α Toxin from Tityus stigmurus Venom of Northeast Brazil: Evaluation and Comparison with Well-Studied Ts3 Toxin of Tityus serrulatus

Studies on the interaction sites of peptide toxins and ion channels typically involve site-directed mutations in toxins. However, natural mutant toxins exist among them, offering insights into how the evolutionary process has conserved crucial sequences for activities and molecular target selection. In this study, we present a comparative investigation using electrophysiological approaches and computational analysis between two alpha toxins from evolutionarily close scorpion species of the genus Tityus, namely, Tst3 and Ts3 from T. stigmurus and T. serrulatus, respectively. These toxins exhibit three natural substitutions near the C-terminal region, which is directly involved in the interaction between alpha toxins and Nav channels. Additionally, we characterized the activity of the Tst3 toxin on Nav1.1-Nav1.7 channels. The three natural changes between the toxins did not alter sensitivity to Nav1.4, maintaining similar intensities regarding their ability to alter opening probabilities, delay fast inactivation, and induce persistent currents. Computational analysis demonstrated a preference for the down conformation of VSD4 and a shift in the conformational equilibrium towards this state. This illustrates that the sequence of these toxins retained the necessary information, even with alterations in the interaction site region. Through electrophysiological and computational analyses, screening of the Tst3 toxin on sodium isoform revealed its classification as a classic α-NaTx with a broad spectrum of activity. It effectively delays fast inactivation across all tested isoforms. Structural analysis of molecular energetics at the interface of the VSD4-Tst3 complex further confirmed this effect.

Unplugging lateral fenestrations of NALCN reveals a hidden drug binding site within the pore region

The sodium (Na+) leak channel (NALCN) is a member of the four-domain voltage-gated cation channel family that includes the prototypical voltage-gated sodium and calcium channels (NaVs and CaVs, respectively). Unlike NaVs and CaVs, which have four lateral fenestrations that serve as routes for lipophilic compounds to enter the central cavity to modulate channel function, NALCN has bulky residues (W311, L588, M1145, and Y1436) that block these openings. Structural data suggest that occluded fenestrations underlie the pharmacological resistance of NALCN, but functional evidence is lacking. To test this hypothesis, we unplugged the fenestrations of NALCN by substituting the four aforementioned residues with alanine (AAAA) and compared the effects of NaV, CaV, and NALCN blockers on both wild-type (WT) and AAAA channels. Most compounds behaved in a similar manner on both channels, but phenytoin and 2-aminoethoxydiphenyl borate (2-APB) elicited additional, distinct responses on AAAA channels. Further experiments using single alanine mutants revealed that phenytoin and 2-APB enter the inner cavity through distinct fenestrations, implying structural specificity to their modes of access. Using a combination of computational and functional approaches, we identified amino acid residues critical for 2-APB activity, supporting the existence of drug binding site(s) within the pore region. Intrigued by the activity of 2-APB and its analogues, we tested compounds containing the diphenylmethane/amine moiety on WT channels. We identified clinically used drugs that exhibited diverse activity, thus expanding the pharmacological toolbox for NALCN. While the low potencies of active compounds reiterate the pharmacological resistance of NALCN, our findings lay the foundation for rational drug design to develop NALCN modulators with refined properties.

A Tissue Distribution Study of Propafenone in an Intentional Fatal Poisoning Case

Propafenone (PPF) belongs to the class 1C antiarrhythmics and can cause electrocardiogram-associated adverse/toxic effects. Cases of PPF intoxication are rarely investigated. We developed a novel and selective GC-MS/MS method for the determination of PPF and its tissue distribution in an intentional fatal poisoning case, which is applicable to PPF quantification in the range of therapeutic to lethal concentrations in complex post-mortem samples. A simple and effective sample pretreatment was applied to all analyzed samples. PPF was determined without the need for dilution, even in highly complex samples containing a wide range of analyte concentrations. Quantification was performed using the standard addition method, developed and validated according to the ICH M10 guidelines. The obtained results indicated that the PPF concentration in the serum from blood taken while alive, before therapy, was the highest ever reported in the literature. Despite the intensive therapy after the patients' admission, the PPF concentrations in the lungs, spleen, femoral blood and cardiac blood were fatal or abnormally high. On the other hand, the concentrations in the liver and skeletal muscle were lower or approximately the same as observed in cases with therapeutic doses. To the best of our knowledge, the distribution of PPF has not been investigated in fatal intoxication cases and can be helpful in clinical or forensic toxicology.

Honeybee CaV4 has distinct permeation, inactivation, and pharmacology from homologous NaV channels

DSC1, a Drosophila channel with sequence similarity to the voltage-gated sodium channel (NaV), was identified over 20 years ago. This channel was suspected to function as a non-specific cation channel with the ability to facilitate the permeation of calcium ions (Ca2+). A honeybee channel homologous to DSC1 was recently cloned and shown to exhibit strict selectivity for Ca2+, while excluding sodium ions (Na+), thus defining a new family of Ca2+ channels, known as CaV4. In this study, we characterize CaV4, showing that it exhibits an unprecedented type of inactivation, which depends on both an IFM motif and on the permeating divalent cation, like NaV and CaV1 channels, respectively. CaV4 displays a specific pharmacology with an unusual response to the alkaloid veratrine. It also possesses an inactivation mechanism that uses the same structural domains as NaV but permeates Ca2+ ions instead. This distinctive feature may provide valuable insights into how voltage- and calcium-dependent modulation of voltage-gated Ca2+ and Na+ channels occur under conditions involving local changes in intracellular calcium concentrations. Our study underscores the unique profile of CaV4 and defines this channel as a novel class of voltage-gated Ca2+ channels.

Biophysical mechanisms of myocardium sodium channelopathies

Genetic variants of gene SCN5A encoding the alpha-subunit of cardiac voltage-gated sodium channel Nav1.5 are associated with various diseases, including long QT syndrome (LQT3), Brugada syndrome (BrS1), and progressive cardiac conduction disease (PCCD). In the last decades, the great progress in understanding molecular and biophysical mechanisms of these diseases has been achieved. The LQT3 syndrome is associated with gain-of-function of sodium channels Nav1.5 due to impaired inactivation, enhanced activation, accelerated recovery from inactivation or the late current appearance. In contrast, BrS1 and PCCD are associated with the Nav1.5 loss-of-function, which in electrophysiological experiments can be manifested as reduced current density, enhanced fast or slow inactivation, impaired activation, or decelerated recovery from inactivation. Genetic variants associated with congenital arrhythmias can also disturb interactions of the Nav1.5 channel with different proteins or drugs and cause unexpected reactions to drug administration. Furthermore, mutations can affect post-translational modifications of the channels and their sensitivity to pH and temperature. Here we briefly review the current knowledge on biophysical mechanisms of LQT3, BrS1 and PCCD. We focus on limitations of studies that use heterologous expression systems and induced pluripotent stem cells (iPSC) derived cardiac myocytes and summarize our understanding of genotype-phenotype relations of SCN5A mutations.

Dexmedetomidine and Perioperative Arrhythmias

The highly selective α2-adrenoceptor agonist dexmedetomidine is a commonly used sedative drug for patients undergoing anesthesia and intensive care treatment. Several studies have indicated that dexmedetomidine may have a potential role in preventing and treating perioperative tachyarrhythmias. However, the specific effect and mechanism of action of dexmedetomidine in this context remain unclear. Dexmedetomidine is known to regulate the electrophysiologic function of the myocardium by inhibiting the function of the sinus node and atrioventricular node, as well as affecting myocardial repolarization. This paper aims to provide a theoretical basis for the prevention and treatment of perioperative arrhythmias by summarizing the effects of dexmedetomidine on myocardial electrophysiologic function and its impact on different types of arrhythmias.

Structural mechanism of voltage-gated sodium channel slow inactivation

Voltage-gated sodium (NaV) channels mediate a plethora of electrical activities. NaV channels govern cellular excitability in response to depolarizing stimuli. Inactivation is an intrinsic property of NaV channels that regulates cellular excitability by controlling the channel availability. The fast inactivation, mediated by the Ile-Phe-Met (IFM) motif and the N-terminal helix (N-helix), has been well-characterized. However, the molecular mechanism underlying NaV channel slow inactivation remains elusive. Here, we demonstrate that the removal of the N-helix of NaVEh (NaVEhΔN) results in a slow-inactivated channel, and present cryo-EM structure of NaVEhΔN in a potential slow-inactivated state. The structure features a closed activation gate and a dilated selectivity filter (SF), indicating that the upper SF and the inner gate could serve as a gate for slow inactivation. In comparison to the NaVEh structure, NaVEhΔN undergoes marked conformational shifts on the intracellular side. Together, our results provide important mechanistic insights into NaV channel slow inactivation.

A review of the direct targets of the cannabinoids cannabidiol, Δ9-tetrahydrocannabinol, N-arachidonoylethanolamine and 2-arachidonoylglycerol

Marijuana has been used by humans for thousands of years for both medicinal and recreational purposes. This included the treatment of pain, inflammation, seizures, and nausea. In the 1960s, the structure of the principal psychoactive ingredient Δ9-tetrahydrocannabinol was determined, and over the next few decades, two cannabinoid receptors were characterized along with the human endocannabinoid system and what it affects. This includes metabolism, the cardiovascular and reproductive systems, and it is involved in such conditions as inflammation, cancer, glaucoma, and liver and musculoskeletal disorders. In the central nervous system, the endocannabinoid system has been linked to appetite, learning, memory, and conditions such as depression, anxiety, schizophrenia, stroke, multiple sclerosis, neurodegeneration, addiction, and epilepsy. It was the profound effectiveness of cannabidiol, a non-psychoactive ingredient of marijuana, to relieve the symptoms of Dravet syndrome, a severe form of childhood epilepsy, that recently helped spur marijuana research. This has helped substantially to change society's attitude towards this potential source of useful drugs. However, research has also revealed that the actions of endocannabinoids, such as anandamide and 2-arachidonoylglycerol, and the phytocannabinoids, tetrahydrocannabinol and cannabidiol, were not just due to interactions with the two cannabinoid receptors but by acting directly on many other targets including various G-protein receptors and cation channels, such as the transient receptor potential channels for example. This mini-review attempts to survey the effects of these 4 important cannabinoids on these currently identified targets.

Variable Penetrance and Expressivity of a Rare Pore Loss-of-Function Mutation (p.L889V) of Nav1.5 Channels in Three Spanish Families

A novel rare mutation in the pore region of Nav1.5 channels (p.L889V) has been found in three unrelated Spanish families that produces quite diverse phenotypic manifestations (Brugada syndrome, conduction disease, dilated cardiomyopathy, sinus node dysfunction, etc.) with variable penetrance among families. We clinically characterized the carriers and recorded the Na+ current (INa) generated by p.L889V and native (WT) Nav1.5 channels, alone or in combination, to obtain further insight into the genotypic-phenotypic relationships in patients carrying SCN5A mutations and in the molecular determinants of the Nav1.5 channel function. The variant produced a strong dominant negative effect (DNE) since the peak INa generated by p.L889V channels expressed in Chinese hamster ovary cells, either alone (-69.4 ± 9.0 pA/pF) or in combination with WT (-62.2 ± 14.6 pA/pF), was significantly (n ≥ 17, p < 0.05) reduced compared to that generated by WT channels alone (-199.1 ± 44.1 pA/pF). The mutation shifted the voltage dependence of channel activation and inactivation to depolarized potentials, did not modify the density of the late component of INa, slightly decreased the peak window current, accelerated the recovery from fast and slow inactivation, and slowed the induction kinetics of slow inactivation, decreasing the fraction of channels entering this inactivated state. The membrane expression of p.L889V channels was low, and in silico molecular experiments demonstrated profound alterations in the disposition of the pore region of the mutated channels. Despite the mutation producing a marked DNE and reduction in the INa and being located in a critical domain of the channel, its penetrance and expressivity are quite variable among the carriers. Our results reinforce the argument that the incomplete penetrance and phenotypic variability of SCN5A loss-of-function mutations are the result of a combination of multiple factors, making it difficult to predict their expressivity in the carriers despite the combination of clinical, genetic, and functional studies.