Structure of the human voltage-gated sodium channel Nav1.4 in complex with β1

Sensory neuron sodium channels as pain targets; from cocaine to Journavx (VX-548, suzetrigine)

Voltage-gated sodium channels underpin electrical signaling in sensory neurons. Their activity is an essential element in the vast majority of pain conditions, making them significant drug targets. Sensory neuron sodium channels play roles not only in afferent signaling but also in a range of efferent regulatory mechanisms. Side effects through actions on other cell types and efferent signaling are thus important issues to address during analgesic drug development. As an example, the human genetic evidence for NaV1.7 as an ideal pain target contrasts with the side effects of NaV1.7 antagonists. In this review, we describe the history and progress toward the development of useful analgesic drugs and the renewed focus on NaV1.8 as a key target in pain treatment. NaV1.8 antagonists alone or in combination with other analgesics are likely to provide new opportunities for pain relief for the vast number of people (about 33% of the population) impacted by chronic pain, particularly present in aging populations.

A robust expression system reveals distinct gating mechanisms and calmodulin regulation of NaV1.9 channels

NaV1.9 is a voltage-gated Na+ channel subtype with unique gating properties that are poorly understood, partly due to the lack of reliable heterologous expression systems. Here, we present a transient expression protocol that produces robust mouse NaV1.9 currents, enabling direct electrophysiological comparisons with native dorsal root ganglion neurons. To further understand the low current density observed in human NaV1.9, we created chimeras with NaV1.5 and identified a role for the C-tail-specifically the IQ motif and EF-hand-in regulating current densities, likely due to a weak affinity for calmodulin. Isothermal titration calorimetry experiments indicated that, unlike other NaV channel subtypes, calmodulin binding to the C-tail is likely too weak to occur under physiological conditions. Markedly, the pre-IQ region did not influence channel expression but was responsible for conferring the characteristic depolarized voltage dependency of inactivation of NaV1.9. Our findings provide insights into the unique gating mechanisms of NaV1.9 and demonstrate the robustness of this platform for structure-function studies.

Preorganized Electric Fields in Voltage-Gated Sodium Channels

Enzymes are reported to catalyze reactions by generating electric fields that promote the evolution of the reaction in the active site. Although seldom used outside enzymatic catalysis, electrostatic preorganization theory and language of electric fields can be generalized to other biological macromolecules. Herein, we performed molecular dynamics simulations of human Nav1.5, Nav1.6, and Nav1.7 with the atomic multipole optmimized energetics for biomolecular applications  polarizable force field. We show that in the absence of an external potential, charged and uncharged residues generate strong electric fields that assist in Na+ motion in the pore. This work emphasizes the importance of charge-dipole interactions in modulating Na+ dynamics, in addition to charge-charge interactions, the focus of a majority of previous studies. Finally, we find that residues share a high level of mutual information through electric fields that can enable the optimization of allosteric pathways.

The α/β3 complex of human voltage-gated sodium channel hNav1.7 to study mechanistic differences in presence and absence of auxiliary subunit β3

CONTEXT

In the context of structural interactomics, we generated a 3D model between α and β3 subunits for the hitherto unknown human voltage-gated sodium channel complex (hNa 1.7α/β3). We embedded our 3D model in a membrane lipid bilayer for molecular dynamics (MD) simulations of the sodium cation passage from the outer vestibule through the inner pore segment of our hNa 1.7 complex in presence and absence of auxiliary subunit β3 with remarkable changes close to electrophysiological study results. A complete passage could not be expected due to because the inactivated state of the underlying 3D template. A complete sodium ion passage would require an open state of the channel. The computed observations concerning side chain rearrangements for favorable cooperativity under evolutionary neighborhood conditions, favorable and unfavorable amino acid interactions, proline kink, loop, and helix displacements were all found in excellent keeping with the extant literature without any exception nor contradiction. Complex-stabilizing pairs of interacting amino acids with evolutionary neighborhood complementary were identified.

METHODS

The following tools were used: sequence search and alignment by FASTA and Clustal Omega; 3D model visualization and homology modeling by Vega ZZ, SPDBV, Chimera and Modeller, respectively; missing sections (loops) by Alphafold; geometry optimization prior to MD runs by GROMACS 2021.4 under the CHARMM 36 force field; local healing of bad contacts by SPDBV based on its Ramachandran plots; protein-protein docking by HDOCK 2.4; membrane insertion assisted by OPM; Berendsen V-rescaling for NVT; Parrinello-Rahman and Nose-Hoover for MPT; MD analyses by VMD and XMGRACE.

Structural basis of human Nav1.5 gating mechanisms

Voltage-gated Nav1.5 channels are central to the generation and propagation of cardiac action potentials. Aberrations in their function are associated with a wide spectrum of cardiac diseases including arrhythmias and heart failure. Despite decades of progress in Nav1.5 biology, the lack of structural insights into intracellular regions has hampered our understanding of its gating mechanisms. Here, we present two cryo-EM structures of human Nav1.5 in open states, revealing sequential conformational changes in gating charges of the voltage-sensing domains (VSDs) and several intracellular regions. Despite the channel being in the open state, these structures show repositioning, but no dislodging of the IFM motif in the receptor site. Molecular dynamics analyses show our structures with CTD conduct Na+ ions. Notably, our structural findings highlight a dynamic C-terminal domain (CTD) and III-IV linker interaction, which regulates the conformation of VSDs and pore opening. Electrophysiological studies confirm that disrupting this interaction alters fast inactivation of Nav1.5. Together, our structure-function studies establish a foundation for understanding the gating mechanisms of Nav1.5 and the mechanisms underlying CTD-related channelopathies.

Epilepsy and autophagy modulators: a therapeutic split

Epilepsy is a neurological disease characterized by repeated unprovoked seizure. Epilepsy is controlled by anti-epileptic drugs (AEDs); however, one third of epileptic patients have symptoms that are not controlled by AEDs in a condition called refractory epilepsy. Dysregulation of macroautophagy/autophagy is involved in the pathogenesis of epilepsy. Autophagy prevents the development and progression of epilepsy through regulating the balance between inhibitory and excitatory neurotransmitters. Induction of autophagy and autophagy-related proteins could be a novel therapeutic strategy in the management of epilepsy. Despite the protective role of autophagy against epileptogenesis and epilepsy, its role in status epilepticus is perplexing and might reflect its nature as a double-edged sword. Autophagy inducers play a critical role in reducing seizure frequency and severity, and could be an adjuvant treatment in the management of epilepsy. However, autophagy inhibitors also have an anticonvulsant effect. Therefore, the aim of the present mini-review is to discuss the potential role of autophagy in the pathogenesis of epileptogenesis and epilepsy, and how autophagy modulators affect epileptogenesis and epilepsy.

Missense variants pathogenicity annotation from homologous proteins

MOTIVATION

High-throughput DNA sequencing has revealed millions of single nucleotide variants (SNVs) in the human genome, with a small fraction linked to disease. The effect of missense variants, which alter the protein sequence, is particularly challenging to interpret due to the scarcity of clinical annotations and experimental information. While using conservation and structural information, current prediction tools still struggle to predict variant pathogenicity. In this study, we explored the pathogenicity of homologous missense variants-variants in equivalent positions across homologous proteins-focusing on proteins involved in autosomal dominant diseases.

RESULTS

Our analysis of 2976 pathogenic and 17 555 non-pathogenic homologous variants demonstrated that pathogenicity can be extrapolated with 95% accuracy within a family, or up to 98% for closer homologs. Remarkably, the evaluation of 27 commonly used mutation predictor methods revealed that they were not fully capturing this biological feature. To facilitate the exploration of homologous variants, we created HomolVar, a web server that computationally predicts the pathogenesis of missense variants using annotations from homologous variants, freely available at https://rarevariants.org/HomolVar. Overall, these findings and the accompanying tool offer a robust method for predicting the pathogenicity of unannotated variants, enhancing genotype-phenotype correlations, and contributing to diagnosing rare genetic disorders.

AVAILABILITY AND IMPLEMENTATION

HomolVar is freely available at https://rarevariants.org/HomolVar.

Inhibition of NMDA receptors and other ion channel types by membrane-associated drugs

N-methyl-D-aspartate receptors (NMDARs) are ligand-gated ion channels present at most excitatory synapses in the brain that play essential roles in cognitive functions including learning and memory consolidation. However, NMDAR dysregulation is implicated in many nervous system disorders. Diseases that involve pathological hyperactivity of NMDARs can be treated clinically through inhibition by channel blocking drugs. NMDAR channel block can occur via two known mechanisms. First, in traditional block, charged drug molecules can enter the channel directly from the extracellular solution after NMDAR activation and channel opening. Second, uncharged molecules of channel blocking drug can enter the hydrophobic plasma membrane, and upon NMDAR activation the membrane-associated drug can transit into the channel through a fenestration within the NMDAR. This membrane-associated mechanism of action is called membrane to channel inhibition (MCI) and is not well understood despite the clinical importance of NMDAR channel blocking drugs. Intriguingly, a hydrophobic route of access for drugs is not unique to NMDARs. Our review will address inhibition of NMDARs and other ion channels by membrane-associated drugs and consider how the path of access may affect a drug's therapeutic potential.

Hyperpolarization Modulation of the T-Type hCav3.2 Channel by Human Synenkephalin [1-53], a Shrew Neurotoxin Analogue without Paralytic Effects

Mammalian secreted venoms mainly consist of peptides and proteases used for defense or predation. Blarina paralytic peptides (BPPs), mealworm-targeting neurotoxins from shrew, are very similar to human synenkephalin. This peptide is released from proenkephalin in the brain along with opioid peptides that mediate analgesic and antidepressant effects, though its physiological function is unclear. Here, we synthesized and characterized human synenkephalin [1-53] (hSYN) and reveal its disulfide bond connectivity. Similar to BPP2, hSYN caused a hyperpolarizing shift in the human T-type voltage-gated calcium channel (hCav3.2) at 0.74 µM, but did not paralyze mealworms. Molecular docking and molecular dynamics simulations showed that hSYN and BPP2 interact with hCav3.2 channel differently, due to differences in polar residues. Since Cav3.2 channel regulates neuronal excitability and is implicated in conditions like autism and epilepsy, our findings on hSYN could provide insight into the channel gating and agonistic mechanisms, along with potential pathways for developing treatments for neurological disorders.

Inherent fast inactivation particle of Nav channels as a new binding site for a neurotoxin

Neurotoxins derived from animal venoms are indispensable tools for probing the structure and function of voltage-gated sodium (Nav) channels. Utilizing a novel centipede peptide toxin called rpTx1, we show that the "inherent inactivation particle" of Nav channels represents a binding site for a neurotoxin. The toxin comprises two functional domains: one for cell penetration and one for modulating Nav channel activity. After crossing the cell membrane, rpTx1 preferentially binds to and stabilizes the IFMT motif (the conserved core region of the fast inactivation particle in mammalian Nav channels) in the unbound state, preventing this motif from associating with its receptor site and thereby inhibiting the fast inactivation of Nav channels. This competition between rpTx1 and the receptor site for interacting with the IFMT motif may account for the higher activity of rpTx1 on Nav1.8 than on other Nav subtypes, given the weaker relative affinity between the receptor site and the IFMT motif of Nav1.8. Overall, this study should promote the investigation of the intracellular modulation of Nav channels by neurotoxins.

Brief analysis of Nav1.7 inhibitors: Mechanism of action and new research trends

Nav1.7 has been the most studied ion channel among the 9 subtypes of sodium ion, and it is also one of the popular analgesic targets in recent years. Compared with opioid receptors, because of its advantages of targeting a variety of pain types and being unrelated to addiction, many related inhibitors have been excavated for it, including old drugs and new uses, peptides, and new skeleton small molecules. Some of these inhibitors have reached the second phase of clinical research, and some are still in the laboratory research stage. So far, no exclusive Nav1.7 inhibitor has successfully passed the third phase of clinical research and entered the field of vision of patients. This article reviews the action sites and mechanisms of different Nav1.7 inhibitors in terms of historical background and related analgesic activities, and also summarizes the related inhibitors that are currently under active development, hoping to provide useful information for the research of new Nav1.7 inhibitors.

Normal locomotion in zebrafish lacking the sodium channel NaV1.4 suggests that the need for muscle action potentials is not universal

Extensive studies over decades have firmly established the concept that action potentials (APs) in muscles are indispensable for muscle contraction. To re-examine the significance of APs, we generated zebrafish lacking APs by editing the scn4aa and scn4ab genes, which together encode NaV1.4 (NaVDKO), using the CRISPR-Cas9 system. Surprisingly, the escape response of NaVDKOs to tactile stimuli, both in the embryonic and adult stages, was indistinguishable from that of wild-type (WT) fish. Ca2+ imaging using the calcium indicator protein GCaMP revealed that myofibers isolated from WT fish could be excited by the application of acetylcholine (ACh), even in the presence of tetrodotoxin (TTX) indicating that NaVs are dispensable for skeletal muscle contraction in zebrafish. Mathematical simulations showed that the end-plate potential was able to elicit a change in membrane potential large enough to activate the dihydropyridine receptors of the entire muscle fiber owing to the small fiber size and the disseminated distribution of neuromuscular synapses in both adults and embryos. Our data demonstrate that NaVs are not essential for muscle contraction in zebrafish and that the physiological significance of NaV1.4 in muscle is not uniform across vertebrates.

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.

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.

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 depolarized 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.

Voltage sensors

Widely distributed in all kingdoms of life, voltage sensors in the membrane serve important functions via their movements driven by changes in voltage across the membrane (membrane potential). A voltage sensor domain contains 4 transmembrane segments (S1-S4). The S1-S3 helices form a hydrophobic constriction site (HCS, also known as the gating charge transfer center) that spans roughly one-third of the membrane thickness. Flanked by aqueous vestibules connected to the extracellular solution above the HCS or cytoplasmic solution below the HCS, the HCS forms a gating pore for the S4 segment bearing multiple basic residues. Membrane potential changes cause S4 to move through the HCS in a 310 helical conformation. This S4 translocation generates a gating current as the positively charged S4 basic residues traverse the membrane electric field, transferring these gating charges from one aqueous vestibule to the other. For voltage-gated ion channels with their voltage sensor domains connected to pore domains, the HCS in the voltage sensor domain allows S4 but not ions to go through, while the channel pore formed by the pore domains mediates ion permeation. Voltage sensor mutations could result in ω currents that are conducted through the gating pore of mutant voltage-gated ion channels. These ω currents may cause pathological consequences in patients with periodic paralysis. Besides voltage-gated ion channels, the sperm-specific Na+/H+ exchanger and voltage-sensing phosphatases contain voltage sensors for membrane potential regulation. Notably, voltage-gated proton channels that are important for pH homeostasis are formed solely by the voltage sensor domain, which mediates proton permeation. SIGNIFICANCE STATEMENT: Voltage sensors mediate voltage regulation of ion channels, transporters, and phosphatases. The voltage sensor domain composed of 4 transmembrane segments (S1-S4) focuses the membrane electric field to the hydrophobic constriction site. To mediate voltage regulation, S4 basic residues within a 310 helix move across the hydrophobic constriction site without concurrent ion flow through this gating pore. As a counterexample, voltage-gated proton channels are formed by the voltage sensor to mediate proton permeation. These ingeniously engineered voltage sensors are conserved throughout evolution.

High-performance solid-state proton gating membranes based on two-dimensional hydrogen-bonded organic framework composites

Biological ion channels exhibit strong gating effects due to their zero-current closed states. However, the gating capabilities of artificial nanochannels have typically fallen short of biological channels, primarily owing to the larger nanopores that fail to completely block ion transport in the off-states. Here, we demonstrate solid-state hydrogen-bonded organic frameworks-based membranes to achieve high-performance ambient humidity-controlled proton gating, accomplished by switching the proton transport pathway instead of relying on conventional ion blockage/activation effects. Density functional theory calculations reveal that the reversible formation and disruption of humidity-induced water bridges within the frameworks facilitates the switching of proton transport mode from the adsorption site hopping to the Grotthuss mechanism. This transition, coupled with the introduction of bacterial cellulose to enhance desorption/adsorption of water clusters, enables us to achieve a superior proton gating ratio of up to 5740, surpassing state-of-the-art solid-state gating devices. Moreover, the developed membrane operates entirely on solid-state principles, rendering it highly versatile for a myriad of applications from environmental detection to human health monitoring. This study offers perspectives for the design of efficient proton gating systems.

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.

Isoform-specific N-linked glycosylation of NaV channel α-subunits alters β-subunit binding sites

Voltage-gated sodium channel α-subunits (NaV1.1-1.9) initiate and propagate action potentials in neurons and myocytes. The NaV β-subunits (β1-4) have been shown to modulate α-subunit properties. Homo-oligomerization of β-subunits on neighboring or opposing plasma membranes has been suggested to facilitate cis or trans interactions, respectively. The interactions between several NaV channel isoforms and β-subunits have been determined using cryogenic electron microscopy (cryo-EM). Interestingly, the NaV cryo-EM structures reveal the presence of N-linked glycosylation sites. However, only the first glycan moieties are typically resolved at each site due to the flexibility of mature glycan trees. Thus, existing cryo-EM structures may risk de-emphasizing the structural implications of glycans on the NaV channels. Herein, molecular modeling and all-atom molecular dynamics simulations were applied to investigate the conformational landscape of N-linked glycans on NaV channel surfaces. The simulations revealed that negatively charged sialic acid residues of two glycan sites may interact with voltage-sensing domains. Notably, two NaV1.5 isoform-specific glycans extensively cover the α-subunit region that, in other NaV channel α-subunit isoforms, corresponds to the binding site for the β1- (and likely β3-) subunit immunoglobulin (Ig) domain. NaV1.8 contains a unique N-linked glycosylation site that likely prevents its interaction with the β2 and β4-subunit Ig-domain. These isoform-specific glycans may have evolved to facilitate specific functional interactions, for example, by redirecting β-subunit Ig-domains outward to permit cis or trans supraclustering within specialized cellular compartments such as the cardiomyocyte perinexal space. Further experimental work is necessary to validate these predictions.

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.

Mapping O- and N-Glycosylation in Transmembrane and Interface Regions of Proteins: Insights from a Database Search Study

Glycosylation is a critical post-translational modification that influences protein folding, stability and function. While extensively studied in extracellular and intracellular regions, glycosylation within transmembrane (TM) regions and at membrane interfaces remains poorly understood. This study aimed to map O- and N-glycosylation sites in these regions using a comprehensive database search and structural validation where possible. Extensive database searches revealed glycosylation sites in a range of membrane proteins. Only the sites falling in the TM regions and at the membrane interface (according to Uniprot annotations) were retained. The location of these sites was confirmed based on available 3D structures. We identified 32 O-glycosylation sites and 7 N-glycosylation sites in the TM domains of 29 proteins. O-GlcNAc sites validated as located within TM regions presented side chains either oriented toward the lipid bilayer or buried within the protein. N-glycosylation sites predicted in protein TM regions were largely confined to interface or extracellular domains. The results obtained here highlight the occurrence of glycosylation in TM regions of proteins and at membrane interfaces. This dataset provides a valuable foundation for the further exploration of structural and functional roles of glycosylation in membrane-associated regions.

Differential Regulation of Hyaluronan Synthesis by Three Isoforms of Hyaluronan Synthases in Mammalian Cells

Hyaluronan (HA) is one of the crucial components of the extracellular matrix in vertebrates and is synthesized by three hyaluronan synthases (HASs), namely HAS1, HAS2, and HAS3. The low expression level of HASs in normal keratinocytes and other various types of cells presents a recognized challenge, impeding biological and pathological research on their localization. In this study, the human proteins HAS1, HAS2, and HAS3 with fused maltose-binding protein (MBP) tags were successfully expressed at high levels and purified for the first time in HEK293F cells. The enzymatic properties of the three HAS proteins were further characterized and compared. A pulse-field gel electrophoresis analysis of the size distribution of the hyaluronan generated in vitro by the membrane proteins demonstrated that the three HAS isoforms generate HA polymer chains at different molecular masses. Kinetic studies demonstrated that the three HAS proteins differed in their catalytic efficiency and apparent Km values for the two substrates, UDP-GlcA and UDP-GlcNAc. Furthermore, the cellular hyaluronan secretion by the three isoenzymes was evaluated and quantified in the HEK 293T cells transfected with GFP-tagged HAS1-GFP, HAS2-GFP, and HAS3-GFP using an ELISA assay. These findings enhance our understanding of the membrane protein HASs in mammalian cells.

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 unique mechanism of transfluthrin action revealed by mapping its binding sites in the mosquito sodium channel

Pyrethroid insecticides exert their toxic action by prolonging the opening of insect voltage-gated sodium channels, resulting in the characteristic tail current during membrane repolarization in voltage clamp experiments. Permethrin (PMT) and deltamethrin (DMT), representative type I and type II pyrethroids, respectively, are predicted to bind to two lipid-exposed pyrethroid receptor sites, PyR1 and PyR2, at the lipid-exposed interfaces of repeats II/III and I/II, respectively. Transfluthrin (TF), a volatile type I pyrethroid and mosquito repellent, has received increased attention in the global combat of vector-borne human diseases. However, the electrophysiological and molecular bases of TF action on insect sodium channels remain unexplored. In this study we discovered that, unlike DMT and PMT, TF barely induces the characteristic tail current of the Aedes aegypti mosquito sodium channel (AaNav1-1) expressed in Xenopus oocytes. Instead, TF induces a unique persistent current. We docked TF into the AlphaFold2 model of AaNav1-1 and found that the tetrafluorophenyl ring of TF binds to alpha helices S5, P1, and S6, but not to the linker helices S4-S5 within either PyR1 or PyR2. In agreement with the model, functional examination of 15 AaNav1-1 mutants demonstrated that substitutions of DMT/PMT-sensing residues in helices S5, P1, and S6, but not in the linker-helices S4-S5, altered channel sensitivity to TF. These results revealed the unique action of TF on channel gating and suggest a distinct subtype of type I pyrethroids with a previously uncharacterized pattern of interactions with residues at the dual pyrethroid receptor sites.

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.

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.

ω-Grammotoxin-SIA inhibits voltage-gated Na+ channel currents

ω-Grammotoxin-SIA (GrTX-SIA) was originally isolated from the venom of the Chilean rose tarantula and demonstrated to function as a gating modifier of voltage-gated Ca2+ (CaV) channels. Later experiments revealed that GrTX-SIA could also inhibit voltage-gated K+ (KV) channel currents via a similar mechanism of action that involved binding to a conserved S3-S4 region in the voltage-sensing domains (VSDs). Since voltage-gated Na+ (NaV) channels contain homologous structural motifs, we hypothesized that GrTX-SIA could inhibit members of this ion channel family as well. Here, we show that GrTX-SIA can indeed impede the gating process of multiple NaV channel subtypes with NaV1.6 being the most susceptible target. Moreover, molecular docking of GrTX-SIA onto NaV1.6, supported by a p.E1607K mutation, revealed the voltage sensor in domain IV (VSDIV) as being a primary site of action. The biphasic manner in which current inhibition appeared to occur suggested a second, possibly lower-sensitivity binding locus, which was identified as VSDII by using KV2.1/NaV1.6 chimeric voltage-sensor constructs. Subsequently, the NaV1.6p.E782K/p.E838K (VSDII), NaV1.6p.E1607K (VSDIV), and particularly the combined VSDII/VSDIV mutant lost virtually all susceptibility to GrTX-SIA. Together with existing literature, our data suggest that GrTX-SIA recognizes modules in NaV channel VSDs that are conserved among ion channel families, thereby allowing it to act as a comprehensive ion channel gating modifier peptide.

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.

Structural basis for the rescue of hyperexcitable cells by the amyotrophic lateral sclerosis drug Riluzole

Neuronal hyperexcitability is a key element of many neurodegenerative disorders including the motor neuron disease Amyotrophic Lateral Sclerosis (ALS), where it occurs associated with elevated late sodium current (INaL). INaL results from incomplete inactivation of voltage-gated sodium channels (VGSCs) after their opening and shapes physiological membrane excitability. However, dysfunctional increases can cause hyperexcitability-associated diseases. Here we reveal the atypical binding mechanism which explains how the neuroprotective ALS-treatment drug riluzole stabilises VGSCs in their inactivated state to cause the suppression of INaL that leads to reversed cellular overexcitability. Riluzole accumulates in the membrane and enters VGSCs through openings to their membrane-accessible fenestrations. Riluzole binds within these fenestrations to stabilise the inactivated channel state, allowing for the selective allosteric inhibition of INaL without the physical block of Na+ conduction associated with traditional channel pore binding VGSC drugs. We further demonstrate that riluzole can reproduce these effects on a disease variant of the non-neuronal VGSC isoform Nav1.4, where pathologically increased INaL is caused directly by mutation. Overall, we identify a model for VGSC inhibition that produces effects consistent with the inhibitory action of riluzole observed in models of ALS. Our findings will aid future drug design and supports research directed towards riluzole repurposing.

Mapping structural distribution and gating-property impacts of disease-associated mutations in voltage-gated sodium channels

Thousands of voltage-gated sodium (Nav) channel variants contribute to a variety of disorders, including epilepsy, cardiac arrhythmia, and pain disorders. Yet, the effects of more variants remain unclear. The conventional gain-of-function (GoF) or loss-of-function (LoF) classifications are frequently employed to interpret mutations' effects and guide therapy for sodium channelopathies. Our study challenges this binary classification by analyzing 525 mutations associated with 34 diseases across 366 electrophysiology studies, revealing that diseases with similar GoF/LoF effects can stem from unique molecular mechanisms. Utilizing UniProt data, we mapped over 2,400 disease-associated missense mutations across Nav channels. This analysis pinpoints key mutation hotspots and maps patterns of gating-property impacts for the mutations, respectively, located around the selectivity filter, activation gate, fast inactivation region, and voltage-sensing domains. This study shows great potential to enhance prediction accuracy for mutational effects based on the structural context, paving the way for targeted drug design in precision medicine.

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.

Diverse biophysical mechanisms in voltage-gated sodium channel Nav1.4 variants associated with myotonia

Mutations in SCN4A gene encoding Nav1.4 channel α-subunit, are known to cause neuromuscular disorders such as myotonia or paralysis. Here, we study the effect of two amino acid replacements, K1302Q and G1306E, in the DIII-IV loop of the channel, corresponding to mutations found in patients with myotonia. We combine clinical, electrophysiological, and molecular modeling data to provide a holistic picture of the molecular mechanisms operating in mutant channels and eventually leading to pathology. We analyze the existing clinical data for patients with the K1302Q substitution, which was reported for adults with or without myotonia phenotypes, and report two new unrelated patients with the G1306E substitution, who presented with severe neonatal episodic laryngospasm and childhood-onset myotonia. We provide a functional analysis of the mutant channels by expressing Nav1.4 α-subunit in Xenopus oocytes in combination with β1 subunit and recording sodium currents using two-electrode voltage clamp. The K1302Q variant exhibits abnormal voltage dependence of steady-state fast inactivation, being the likely cause of pathology. K1302Q does not lead to decelerated fast inactivation, unlike several other myotonic mutations such as G1306E. For both mutants, we observe increased window currents corresponding to a larger population of channels available for activation. To elaborate the structural rationale for our experimental data, we explore the contacts involving K/Q1302 and E1306 in the AlphaFold2 model of wild-type Nav1.4 and Monte Carlo-minimized models of mutant channels. Our data provide the missing evidence to support the classification of K1302Q variant as likely pathogenic and may be used by clinicians.

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.

Voltage-Gated Ion Channels: Structure, Pharmacology and Photopharmacology

Voltage-gated ion channels are transmembrane proteins responsible for the generation and propagation of action potentials in excitable cells. Over the last decade, advancements have enabled the elucidation of crystal structures of ion channels. This progress in structural understanding, particularly in identifying the binding sites of local anesthetics, opens avenues for the design of novel compounds capable of modulating ion conduction. However, many traditional drugs lack selectivity and come with adverse side effects. The emergence of photopharmacology has provided an orthogonal way of controlling the activity of compounds, enabling the regulation of ion conduction with light. In this review, we explore the central pore region of voltage-gated sodium and potassium channels, providing insights from both structural and pharmacological perspectives. We discuss the different binding modes of synthetic compounds that can physically occlude the pore and, therefore, block ion conduction. Moreover, we examine recent advances in the photopharmacology of voltage-gated ion channels, introducing molecular approaches aimed at controlling their activity by using photosensitive drugs.

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.

A versatile residue numbering scheme for Nav and Cav channels

Voltage-gated sodium (Nav) and calcium (Cav) channels are responsible for the initiation of electrical signals. They have long been targeted for the treatment of various diseases. The mounting number of cryoelectron microscopy (cryo-EM) structures for diverse subtypes of Nav and Cav channels from multiple organisms necessitates a generic residue numbering system to establish the structure-function relationship and to aid rational drug design or optimization. Here we suggest a structure-based residue numbering scheme, centering around the most conserved residues on each of the functional segments. We elaborate the generic numbers through illustrative examples, focusing on representative drug-binding sites of eukaryotic Nav and Cav channels. We also extend the numbering scheme to compare common disease mutations among different Nav subtypes. Application of the generic residue numbering scheme affords immediate insights into hotspots for pathogenic mutations and critical loci for drug binding and will facilitate drug discovery targeting Nav and Cav channels.

Conotoxins Targeting Voltage-Gated Sodium Ion Channels

Voltage-gated sodium (NaV) channels are intimately involved in the generation and transmission of action potentials, and dysfunction of these channels may contribute to nervous system diseases, such as epilepsy, neuropathic pain, psychosis, autism, and cardiac arrhythmia. Many venom peptides selectively act on NaV channels. These include conotoxins, which are neurotoxins secreted by cone snails for prey capture or self-defense but which are also valuable pharmacological tools for the identification and/or treatment of human diseases. Typically, conotoxins contain two or three disulfide bonds, and these internal crossbraces contribute to conotoxins having compact, well defined structures and high stability. Of the conotoxins containing three disulfide bonds, some selectively target mammalian NaV channels and can block, stimulate, or modulate these channels. Such conotoxins have great potential to serve as pharmacological tools for studying the functions and characteristics of NaV channels or as drug leads for neurologic diseases related to NaV channels. Accordingly, discovering or designing conotoxins targeting NaV channels with high potency and selectivity is important. The amino acid sequences, disulfide bond connectivity, and three-dimensional structures are key factors that affect the biological activity of conotoxins, and targeted synthetic modifications of conotoxins can greatly improve their activity and selectivity. This review examines NaV channel-targeted conotoxins, focusing on their structures, activities, and designed modifications, with a view toward expanding their applications. SIGNIFICANCE STATEMENT: NaV channels are crucial in various neurologic diseases. Some conotoxins selectively target NaV channels, causing either blockade or activation, thus enabling their use as pharmacological tools for studying the channels' characteristics and functions. Conotoxins also have promising potential to be developed as drug leads. The disulfide bonds in these peptides are important for stabilizing their structures, thus leading to enhanced specificity and potency. Together, conotoxins targeting NaV channels have both immediate research value and promising future application prospects.

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.

Eugenol and lidocaine inhibit voltage-gated Na+ channels from dorsal root ganglion neurons with different mechanisms

Eugenol (EUG) is a bioactive monoterpenoid used as an analgesic, preservative, and flavoring agent. Our new data show EUG as a voltage-gated Na+ channel (VGSC) inhibitor, comparable but not identical to lidocaine (LID). EUG inhibits both total and only TTX-R voltage-activated Na+ currents (INa) recorded from VGSCs naturally expressed on dorsal root ganglion sensory neurons in rats. Inhibition is quick, fully reversible, and dose-dependent. Our biophysical and pharmacological analyses showed that EUG and LID inhibit VGSCs with different mechanisms. EUG inhibits VGSCs with a dose-response relationship characterized by a Hill coefficient of 2, while this parameter for the inhibition by LID is 1. Furthermore, in a different way from LID, EUG modified the voltage dependence of both the VGSC activation and inactivation processes and the recovery from fast inactivated states and the entry to slow inactivated states. In addition, we suggest that EUG, but not LID, interacts with VGSC pre-open-closed states, according to our data.

Electric Eel Biomimetics for Energy Storage and Conversion

The electric eel is known as the most powerful creature to generate electricity with a discharge voltage up to 860 V and peak current up to 1 A. These surprising properties are the results of billions of years of evolution on the electrical biological structure and bulk, and now have triggered great research interest in electric eel biomimetics for designing innovated configurations and components of energy storage and conversion devices. In this review, first, the bioelectrical behavior of electric eels is surveyed, followed by the physiological structure to reveal the discharge characteristics and principles of electric organs and electrocytes. Additionally, underlying electrochemical mechanisms and models for calculating the potential and current of electrocytes are presented. Central to this review is the recent progress of electric-eel-inspired innovations and applications for energy storage and conversion, particularly including novel power sources, triboelectric nanogenerators, and nanochannel ion-selective membranes for salinity gradient energy harvesting. Finally, insights on the challenges at the moment and the perspectives on the future research prospects are critically compiled. It is suggested that energy-related electric eel biomimetics will greatly boost the development of next-generation high performance, green, and functional electronics.

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.

Dilation of ion selectivity filters in cation channels

Ion channels establish the voltage gradient across cellular membranes by providing aqueous pathways for ions to selectively diffuse down their concentration gradients. The selectivity of any given channel for its favored ions has conventionally been viewed as a stable property, and in many cation channels, it is determined by an ion-selectivity filter within the external end of the ion-permeation pathway. In several instances, including voltage-activated K+ (Kv) channels, ATP-activated P2X receptor channels, and transient receptor potential (TRP) channels, the ion-permeation pathways have been proposed to dilate in response to persistent activation, dynamically altering ion permeation. Here, we discuss evidence for dynamic ion selectivity, examples where ion selectivity filters exhibit structural plasticity, and opportunities to fill gaps in our current understanding.

Multiple mutations in the Nav1.4 sodium channel of New Guinean toxic birds provide autoresistance to deadly batrachotoxin

Toxicity has evolved multiple times across the tree of life and serves important functions related to hunting, defence and parasite deterrence. Toxins are produced either in situ by the toxic organism itself or associated symbionts, or acquired through diet. The ability to exploit toxins from external sources requires adaptations that prevent toxic effects on the consumer (autoresistance). Here, we examine genomic adaptations that could facilitate autoresistance to the diet-acquired potent neurotoxic alkaloid batrachotoxin (BTX) in New Guinean toxic birds. Our work documents two new toxic bird species and shows that toxic birds carry multiple mutations in the SCN4A gene that are under positive selection. This gene encodes the most common vertebrate muscle Nav channel (Nav1.4). Molecular docking results indicate that some of the mutations that are present in the pore-forming segment of the Nav channel, where BTX binds, could reduce its binding affinity. These mutations should therefore prevent the continuous opening of the sodium channels that BTX binding elicits, thereby preventing muscle paralysis and ultimately death. Although these mutations are different from those present in Neotropical Phyllobates poison dart frogs, they occur in the same segments of the Nav1.4 channel. Consequently, in addition to uncovering a greater diversity of toxic bird species than previously known, our work provides an intriguing example of molecular-level convergent adaptations allowing frogs and birds to ingest and use the same neurotoxin. This suggests that genetically modified Nav1.4 channels represent a key adaptation to BTX tolerance and exploitation across vertebrates.

Functional effects of drugs and toxins interacting with NaV1.4

NaV1.4 is a voltage-gated sodium channel subtype that is predominantly expressed in skeletal muscle cells. It is essential for producing action potentials and stimulating muscle contraction, and mutations in NaV1.4 can cause various muscle disorders. The discovery of the cryo-EM structure of NaV1.4 in complex with β1 has opened new possibilities for designing drugs and toxins that target NaV1.4. In this review, we summarize the current understanding of channelopathies, the binding sites and functions of chemicals including medicine and toxins that interact with NaV1.4. These substances could be considered novel candidate compounds or tools to develop more potent and selective drugs targeting NaV1.4. Therefore, studying NaV1.4 pharmacology is both theoretically and practically meaningful.

Structural basis of human Nav1.5 gating mechanisms

Voltage-gated Nav1.5 channels are central to the generation and propagation of cardiac action potentials1. Aberrations in their function are associated with a wide spectrum of cardiac diseases including arrhythmias and heart failure2-5. Despite decades of progress in Nav1.5 biology6-8, the lack of structural insights into intracellular regions has hampered our understanding of its gating mechanisms. Here we present three cryo-EM structures of human Nav1.5 in previously unanticipated open states, revealing sequential conformational changes in gating charges of the voltage-sensing domains (VSDs) and several intracellular regions. Despite the channel being in the open state, these structures show the IFM motif repositioned in the receptor site but not dislodged. In particular, our structural findings highlight a dynamic C-terminal domain (CTD) and III-IV linker interaction, which regulates the conformation of VSDs and pore opening. Electrophysiological studies confirm that disrupting this interaction results in the fast inactivation of Nav1.5. Together, our structure-function studies establish a foundation for understanding the gating mechanisms of Nav1.5 and the mechanisms underlying CTD-related channelopathies.