The molecular basis of neuronal excitability

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.

Voltage gated sodium and calcium channels: Discovery, structure, function, and Pharmacology

Voltage-gated sodium channels initiate action potentials in nerve and muscle, and voltage-gated calcium channels couple depolarization of the plasma membrane to intracellular events such as secretion, contraction, synaptic transmission, and gene expression. In this Review and Perspective article, I summarize early work that led to identification, purification, functional reconstitution, and determination of the amino acid sequence of the protein subunits of sodium and calcium channels and showed that their pore-forming subunits are closely related. Decades of study by antibody mapping, site-directed mutagenesis, and electrophysiological recording led to detailed two-dimensional structure-function maps of the amino acid residues involved in voltage-dependent activation and inactivation, ion permeation and selectivity, and pharmacological modulation. Most recently, high-resolution three-dimensional structure determination by X-ray crystallography and cryogenic electron microscopy has revealed the structural basis for sodium and calcium channel function and pharmacological modulation at the atomic level. These studies now define the chemical basis for electrical signaling and provide templates for future development of new therapeutic agents for a range of neurological and cardiovascular diseases.

Ionic channels in nerve membranes, 50 years on

This retrospective traces the hypothesis of ion channels from an early statement in a 1970 essay in this journal (Hille, B., 1970, Prog. Biophys. Mol. Biol. 21, 1-32) to its realization today in biophysical, molecular, biochemical, and structural terms. The Na⁺ and K⁺ channels of the action potential have been isolated, reconstituted, cloned, mutated, and expressed. They are conformationally flexible, multi-pass glycosylated membrane proteins. Refined atomic structures of several conformational states are known. The discoveries over this half century history illustrate the growth of a field from initial ideas to a mature discipline of biology, physiology, and biomedical science.

Extreme synchronization events in a Kuramoto model: The interplay between resource constraints and explosive transitions

Many living and artificial systems possess structural and dynamical properties of complex networks. One of the most exciting living networked systems is the brain, in which synchronization is an essential mechanism of its normal functioning. On the other hand, excessive synchronization in neural networks reflects undesired pathological activity, including various forms of epilepsy. In this context, network-theoretical approach and dynamical modeling may uncover deep insight into the origins of synchronization-related brain disorders. However, many models do not account for the resource consumption needed for the neural networks to synchronize. To fill this gap, we introduce a phenomenological Kuramoto model evolving under the excitability resource constraints. We demonstrate that the interplay between increased excitability and explosive synchronization induced by the hierarchical organization of the network forces the system to generate short-living extreme synchronization events, which are well-known signs of epileptic brain activity. Finally, we establish that the network units occupying the medium levels of hierarchy most strongly contribute to the birth of extreme events emphasizing the focal nature of their origin.

Chemicals with increasingly complex modes of action result in greater variation in sensitivity between earthworm species

The scale of variation in species sensitivity to toxicants has been theoretically linked to mode of action. Specifically, it has been proposed there will be greater variations for chemicals with a putative specific biological target than for toxicants with a non-specific narcotic mechanism. Here we test the hypothesis that mode of action is related to variation in sensitivity in a specifically designed experiment for species from a single ecologically important terrestrial taxa, namely earthworms. Earthworm toxicity tests were conducted with five species for four chemicals, providing a series of increasingly complex modes of action: a putative narcotic polycyclic aromatic hydrocarbon (fluoranthene), and three insecticides (chlorpyrifos, cypermethrin, imidacloprid) with known neuronal receptor targets. Across all the chemicals, the standard epigeic test species Eisenia fetida and Lumbricus rubellus, were generally among the two least sensitive, while the endogenic Aporrectodea caliginosa and Megascolecidae Amynthas gracilis were generally more sensitive (never being among the two least sensitive species). This indicates a potential for bias in the earthworm ecotoxicology literature, which is dominated by studies in epigeic Lumbricidae, but contains few endogeic or Megascolecidae data. Results confirmed the lowest range of variation in sensitivities for effects on reproduction was for fluoranthene (2.5 fold). All insecticides showed greater variation for species sensitivity (cypermethrin: 7.5 fold, chlorpyrifos: 10.3 fold, imidacloprid: 31.5 fold) consistent with the specific mechanisms of the pesticides. Difference in toxicodynamics, based on mode of action specificity and receptor complexity was reflected in the magnitude of sensitivity variation. However, measurements of tissue concentrations also indicated the potential importance of toxicokinetics in explaining species sensitivity variations for chlorpyrifos and cypermethrin.

Structural Basis for High-Affinity Trapping of the NaV1.7 Channel in Its Resting State by Tarantula Toxin

Voltage-gated sodium channels initiate electrical signals and are frequently targeted by deadly gating-modifier neurotoxins, including tarantula toxins, which trap the voltage sensor in its resting state. The structural basis for tarantula-toxin action remains elusive because of the difficulty of capturing the functionally relevant form of the toxin-channel complex. Here, we engineered the model sodium channel NaVAb with voltage-shifting mutations and the toxin-binding site of human NaV1.7, an attractive pain target. This mutant chimera enabled us to determine the cryoelectron microscopy (cryo-EM) structure of the channel functionally arrested by tarantula toxin. Our structure reveals a high-affinity resting-state-specific toxin-channel interaction between a key lysine residue that serves as a "stinger" and penetrates a triad of carboxyl groups in the S3-S4 linker of the voltage sensor. By unveiling this high-affinity binding mode, our studies establish a high-resolution channel-docking and resting-state locking mechanism for huwentoxin-IV and provide guidance for developing future resting-state-targeted analgesic drugs.

Highlighting membrane protein structure and function: A celebration of the Protein Data Bank

Biological membranes define the boundaries of cells and compartmentalize the chemical and physical processes required for life. Many biological processes are carried out by proteins embedded in or associated with such membranes. Determination of membrane protein (MP) structures at atomic or near-atomic resolution plays a vital role in elucidating their structural and functional impact in biology. This endeavor has determined 1198 unique MP structures as of early 2021. The value of these structures is expanded greatly by deposition of their three-dimensional (3D) coordinates into the Protein Data Bank (PDB) after the first atomic MP structure was elucidated in 1985. Since then, free access to MP structures facilitates broader and deeper understanding of MPs, which provides crucial new insights into their biological functions. Here we highlight the structural and functional biology of representative MPs and landmarks in the evolution of new technologies, with insights into key developments influenced by the PDB in magnifying their impact.

The conformational cycle of a prototypical voltage-gated sodium channel

Electrical signaling was a dramatic development in evolution, allowing complex single-cell organisms like Paramecium to coordinate movement and early metazoans like worms and jellyfish to send regulatory signals rapidly over increasing distances. But how are electrical signals generated in biology? In fact, voltage-gated sodium channels conduct sodium currents that initiate electrical signals in all kingdoms of life, from bacteria to man. They are responsible for generating the action potential in vertebrate nerve and muscle, neuroendocrine cells, and other cell types1,2. Because of the high level of conservation of their core structure, it is likely that their fundamental mechanisms of action are conserved as well. Here we describe the complete cycle of conformational changes that a bacterial sodium channel undergoes as it transitions from resting to activated/open and inactivated/closed states, based on high-resolution structural studies of a single sodium channel. We further relate this conformational cycle of the ancestral sodium channel to the function of its vertebrate orthologs. The strong conservation of amino acid sequence and three-dimensional structure suggests that this model, at a fundamental level, is relevant for both prokaryotic and eukaryotic sodium channels, as well as voltage-gated calcium and potassium channels.

Structure of the human sodium leak channel NALCN in complex with FAM155A

NALCN, a sodium leak channel expressed mainly in the central nervous system, is responsible for the resting Na+ permeability that controls neuronal excitability. Dysfunctions of the NALCN channelosome, NALCN with several auxiliary subunits, are associated with a variety of human diseases. Here, we report the cryo-EM structure of human NALCN in complex with FAM155A at an overall resolution of 3.1 angstroms. FAM155A forms extensive interactions with the extracellular loops of NALCN that may help stabilize NALCN in the membrane. A Na+ ion-binding site, reminiscent of a Ca2+ binding site in Cav channels, is identified in the unique EEKE selectivity filter. Despite its 'leaky' nature, the channel is closed and the intracellular gate is sealed by S6I, II-III linker and III-IV linker. Our study establishes the molecular basis of Na+ permeation and voltage sensitivity, and provides important clues to the mechanistic understanding of NALCN regulation and NALCN channelosome-related diseases.

Structure and Pharmacology of Voltage-Gated Sodium and Calcium Channels

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

Structure of the Cardiac Sodium Channel

Voltage-gated sodium channel Na_v1.5 generates cardiac action potentials and initiates the heartbeat. Here, we report structures of Na_V1.5 at 3.2-3.5 Å resolution. Na_V1.5 is distinguished from other sodium channels by a unique glycosyl moiety and loss of disulfide-bonding capability at the Na_Vβ subunit-interaction sites. The antiarrhythmic drug flecainide specifically targets the central cavity of the pore. The voltage sensors are partially activated, and the fast-inactivation gate is partially closed. Activation of the voltage sensor of Domain III allows binding of the isoleucine-phenylalanine-methionine (IFM) motif to the inactivation-gate receptor. Asp and Ala, in the selectivity motif DEKA, line the walls of the ion-selectivity filter, whereas Glu and Lys are in positions to accept and release Na⁺ ions via a charge-delocalization network. Arrhythmia mutation sites undergo large translocations during gating, providing a potential mechanism for pathogenic effects. Our results provide detailed insights into Na_v1.5 structure, pharmacology, activation, inactivation, ion selectivity, and arrhythmias.

Resting-State Structure and Gating Mechanism of a Voltage-Gated Sodium Channel

Voltage-gated sodium (NaV) channels initiate action potentials in nerve, muscle, and other electrically excitable cells. The structural basis of voltage gating is uncertain because the resting state exists only at deeply negative membrane potentials. To stabilize the resting conformation, we inserted voltage-shifting mutations and introduced a disulfide crosslink in the VS of the ancestral bacterial sodium channel NaVAb. Here, we present a cryo-EM structure of the resting state and a complete voltage-dependent gating mechanism. The S4 segment of the VS is drawn intracellularly, with three gating charges passing through the transmembrane electric field. This movement forms an elbow connecting S4 to the S4-S5 linker, tightens the collar around the S6 activation gate, and prevents its opening. Our structure supports the classical "sliding helix" mechanism of voltage sensing and provides a complete gating mechanism for voltage sensor function, pore opening, and activation-gate closure based on high-resolution structures of a single sodium channel protein.

Memory Allocation: Mechanisms and Function

Memories for events are thought to be represented in sparse, distributed neuronal ensembles (or engrams). In this article, we review how neurons are chosen to become part of a particular engram, via a process of neuronal allocation. Experiments in rodents indicate that eligible neurons compete for allocation to a given engram, with more excitable neurons winning this competition. Moreover, fluctuations in neuronal excitability determine how engrams interact, promoting either memory integration (via coallocation to overlapping engrams) or separation (via disallocation to nonoverlapping engrams). In parallel with rodent studies, recent findings in humans verify the importance of this memory integration process for linking memories that occur close in time or share related content. A deeper understanding of allocation promises to provide insights into the logic underlying how knowledge is normally organized in the brain and the disorders in which this process has gone awry.

Distribution and function of voltage-gated sodium channels in the nervous system

Voltage-gated sodium channels (VGSCs) are the basic ion channels for neuronal excitability, which are crucial for the resting potential and the generation and propagation of action potentials in neurons. To date, at least nine distinct sodium channel isoforms have been detected in the nervous system. Recent studies have identified that voltage-gated sodium channels not only play an essential role in the normal electrophysiological activities of neurons but also have a close relationship with neurological diseases. In this study, the latest research findings regarding the structure, type, distribution, and function of VGSCs in the nervous system and their relationship to neurological diseases, such as epilepsy, neuropathic pain, brain tumors, neural trauma, and multiple sclerosis, are reviewed in detail.

Sodium channel subtypes are differentially localized to pre- and post-synaptic sites in rat hippocampus

Voltage-gated Na⁺ channels (Na_v ) modulate neuronal excitability, but the roles of the various Na_v subtypes in specific neuronal functions such as synaptic transmission are unclear. We investigated expression of the three major brain Na_v subtypes (Na_v 1.1, Na_v 1.2, Na_v 1.6) in area CA1 and dentate gyrus of rat hippocampus. Using light and electron microscopy, we found labeling for all three Na_v subtypes on dendrites, dendritic spines, and axon terminals, but the proportion of pre- and post-synaptic labeling for each subtype varied within and between subregions of CA1 and dentate gyrus. In the central hilus (CH) of the dentate gyrus, Na_v 1.1 immunoreactivity was selectively expressed in presynaptic profiles, while Na_v 1.2 and Na_v 1.6 were expressed both pre- and post-synaptically. In contrast, in the stratum radiatum (SR) of CA1, Na_v 1.1, Na_v 1.2, and Na_v 1.6 were selectively expressed in postsynaptic profiles. We next compared differences in Na_v subtype expression between CH and SR axon terminals and between CH and SR dendrites and spines. Na_v 1.1 and Na_v 1.2 immunoreactivity was preferentially localized to CH axon terminals compared to SR, and in SR dendrites and spines compared to CH. No differences in Na_v 1.6 immunoreactivity were found between axon terminals of CH and SR or between dendrites and spines of CH and SR. All Na_v subtypes in both CH and SR were preferentially associated with asymmetric synapses rather than symmetric synapses. These findings indicate selective presynaptic and postsynaptic Na_v expression in glutamatergic synapses of CH and SR supporting neurotransmitter release and synaptic plasticity.

Structure of the Nav1.4-β1 Complex from Electric Eel

Voltage-gated sodium (Na_v) channels initiate and propagate action potentials. Here, we present the cryo-EM structure of EeNa_v1.4, the Na_v channel from electric eel, in complex with the β1 subunit at 4.0 Å resolution. The immunoglobulin domain of β1 docks onto the extracellular L5_I and L6_IV loops of EeNa_v1.4 via extensive polar interactions, and the single transmembrane helix interacts with the third voltage-sensing domain (VSD_III). The VSDs exhibit "up" conformations, while the intracellular gate of the pore domain is kept open by a digitonin-like molecule. Structural comparison with closed Na_vPaS shows that the outward transfer of gating charges is coupled to the iris-like pore domain dilation through intricate force transmissions involving multiple channel segments. The IFM fast inactivation motif on the III-IV linker is plugged into the corner enclosed by the outer S4-S5 and inner S6 segments in repeats III and IV, suggesting a potential allosteric blocking mechanism for fast inactivation.

The chemical basis for electrical signaling

Electrical signals generated by minute currents of ions moving across cell membranes are central to all rapid processes in biology. Initiation and propagation of electrical signals requires voltage-gated sodium (Na_V) and calcium (Ca_V) channels. These channels contain a tetramer of membrane-bound subunits or domains comprising a voltage sensor and a pore module. Voltage-dependent activation occurs as membrane depolarization drives outward movements of positive gating changes in the voltage sensor via a sliding-helix mechanism, which leads to a conformational change in the pore module that opens its intracellular activation gate. A unique negatively charged site in the selectivity filter conducts hydrated Na⁺ or Ca²⁺ rapidly and selectively. Ion conductance is terminated by voltage-dependent inactivation, which causes asymmetric pore collapse. This Review focuses on recent advances in structure and function of Na_V and Ca_V channels that expand our current understanding of the chemical basis for electrical signaling mechanisms conserved from bacteria to humans.

Risks for public health related to the presence of tetrodotoxin (TTX) and TTX analogues in marine bivalves and gastropods

Tetrodotoxin (TTX) and its analogues are produced by marine bacteria and have been detected in marine bivalves and gastropods from European waters. The European Commission asked EFSA for a scientific opinion on the risks to public health related to the presence of TTX and TTX analogues in marine bivalves and gastropods. The Panel on Contaminants in the Food Chain reviewed the available literature but did not find support for the minimum lethal dose for humans of 2 mg, mentioned in various reviews. Some human case reports describe serious effects at a dose of 0.2 mg, corresponding to 4 μg/kg body weight (bw). However, the uncertainties on the actual exposure in the studies preclude their use for derivation of an acute reference dose (ARfD). Instead, a group ARfD of 0.25 μg/kg bw, applying to TTX and its analogues, was derived based on a TTX dose of 25 μg/kg bw at which no apathy was observed in an acute oral study with mice, applying a standard uncertainty factor of 100. Estimated relative potencies for analogues are lower than that of TTX but are associated with a high degree of uncertainty. Based on the occurrence data submitted to EFSA and reported consumption days only, average and P95 exposures of 0.00-0.09 and 0.00-0.03 μg/kg bw, respectively, were calculated. Using a large portion size of 400 g bivalves and P95 occurrence levels of TTX, with exception of oysters, the exposure was below the group ARfD in all consumer groups. A concentration below 44 μg TTX equivalents/kg shellfish meat, based on a large portion size of 400 g, was considered not to result in adverse effects in humans. Liquid chromatography with tandem mass spectroscopy (LC-MS/MS) methods are the most suitable for identification and quantification of TTX and its analogues, with LOQs between 1 and 25 μg/kg.

Structure of a eukaryotic voltage-gated sodium channel at near-atomic resolution

Voltage-gated sodium (Na_v) channels are responsible for the initiation and propagation of action potentials. They are associated with a variety of channelopathies and are targeted by multiple pharmaceutical drugs and natural toxins. Here, we report the cryogenic electron microscopy structure of a putative Na_v channel from American cockroach (designated Na_vPaS) at 3.8 angstrom resolution. The voltage-sensing domains (VSDs) of the four repeats exhibit distinct conformations. The entrance to the asymmetric selectivity filter vestibule is guarded by heavily glycosylated and disulfide bond-stabilized extracellular loops. On the cytoplasmic side, a conserved amino-terminal domain is placed below VSD_I, and a carboxy-terminal domain binds to the III-IV linker. The structure of Na_vPaS establishes an important foundation for understanding function and disease mechanism of Na_v and related voltage-gated calcium channels.

FGF13 modulates the gating properties of the cardiac sodium channel Nav1.5 in an isoform-specific manner

FGF13 (FHF2), the major fibroblast growth factor homologous factor (FHF) in rodent heart, directly binds to the C-terminus of the main cardiac sodium channel, Na_V1.5. Knockdown of FGF13 in cardiomyocytes induces slowed ventricular conduction by altering Na_V1.5 function. FGF13 has five splice variants, each of which possess the same core region and C terminus but differing in their respective N termini. Whether and how these alternatively spliced N termini impart isoform-specific regulation of Na_V1.5, however, has not been reported. Here, we exploited a heterologous expression to explore the specific modulatory effects of FGF13 splice variants FGF13S, FGF13U and FGF13YV on Na_V1.5 function. We found these three splice variants differentially modulated Na_V1.5 current density. Although steady-state activation was unaltered by any of the FGF13 isoforms (compared to control cells expressing Nav1.5 but not expressing FGF13), open-state fast inactivation and closed-state fast inactivation were markedly slowed, steady-state availability was significantly shifted toward the depolarizing direction, and the window current was increased by each of FGF13 isoforms. Most strikingly, FGF13S hastened the rate of Na_V1.5 entry into the slow inactivation state and induced a dramatic slowing of recovery from inactivation, which caused a large decrease in current after either low or high frequency stimulation. Overall, these data showed the diversity of the roles of the FGF13 N-termini in Na_V1.5 channel modulation and suggested the importance of isoform-specific regulation.

Deciphering voltage-gated Na(+) and Ca(2+) channels by studying prokaryotic ancestors

Voltage-gated sodium channels (NaVs) and calcium channels (CaVs) are involved in electrical signaling, contraction, secretion, synaptic transmission, and other physiological processes activated in response to depolarization. Despite their physiological importance, the structures of these closely related proteins have remained elusive because of their size and complexity. Bacterial NaVs have structures analogous to a single domain of eukaryotic NaVs and CaVs and are their likely evolutionary ancestor. Here we review recent work that has led to new understanding of NaVs and CaVs through high-resolution structural studies of their prokaryotic ancestors. New insights into their voltage-dependent activation and inactivation, ion conductance, and ion selectivity provide realistic structural models for the function of these complex membrane proteins at the atomic level.

The KCNE2 K⁺ channel regulatory subunit: Ubiquitous influence, complex pathobiology

The KCNE single-span transmembrane subunits are encoded by five-member gene families in the human and mouse genomes. Primarily recognized for co-assembling with and functionally regulating the voltage-gated potassium channels, the broad influence of KCNE subunits in mammalian physiology belies their small size. KCNE2 has been widely studied since we first discovered one of its roles in the heart and its association with inherited and acquired human Long QT syndrome. Since then, physiological analyses together with human and mouse genetics studies have uncovered a startling array of functions for KCNE2, in the heart, stomach, thyroid and choroid plexus. The other side of this coin is the variety of interconnected disease manifestations caused by KCNE2 disruption, involving both excitable cells such as cardiomyocytes, and non-excitable, polarized epithelia. Kcne2 deletion in mice has been particularly instrumental in illustrating the potential ramifications within a monogenic arrhythmia syndrome, with removal of one piece revealing the unexpected complexity of the puzzle. Here, we review current knowledge of the function and pathobiology of KCNE2.

Protein assemblies of sodium and inward rectifier potassium channels control cardiac excitability and arrhythmogenesis

The understanding of how cardiac ion channels function in the normal and the diseased heart has greatly increased over the last four decades thanks to the advent of patch-clamp technology and, more recently, the emergence of genetics, as well as cellular and molecular cardiology. However, our knowledge of how these membrane-embedded proteins physically interact with each other within macromolecular complexes remains incomplete. This review focuses on how the main cardiac inward sodium channel (NaV1.5) and the strong inward rectifier potassium channel (Kir2.1) function within macromolecular complexes to control cardiac excitability. It has become increasingly clear that these two important ion channel proteins physically interact with multiple other protein partners and with each other from early stages of protein trafficking and targeting through membrane anchoring, recycling, and degradation. Recent findings include compartmentalized regulation of NaV1.5 channel expression and function through a PDZ (postsynaptic density protein, Drosophila disc large tumor suppressor, and zonula occludens-1 protein) domain-binding motif, and interaction of caveolin-3 with Kir2.1 and ankyrin-G as a molecular platform for NaV1.5 signaling. At the cardiomyocyte membrane, NaV1.5 and Kir2.1 interact through at least two distinct PDZ domain-scaffolding proteins (synapse-associated protein-97 and α1-syntrophin), thus modulating reciprocally their cell-surface expression at two different microdomains. Emerging evidence also shows that inheritable mutations in plakophilin-2, ankyrin-G, dystrophin, syntrophin, synapse-associated protein-97, and caveolin-3, among others, modify functional expression and/or localization in the cardiac cell of NaV1.5, Kir2.1 or both to give rise to arrhythmogenic diseases. Unveiling the mechanistic underpinnings of macromolecular interactions should increase our understanding of inherited and acquired arrhythmogenic cardiac diseases and may lead to advances in therapy.

Molecular regulators of nerve conduction - Lessons from inherited neuropathies and rodent genetic models

Myelinated nerve fibers are highly compartmentalized. Helically wrapped lipoprotein membranes of myelin are integrated with subsets of proteins specifically in each compartment to shape the physiological behavior of these nerve fibers. With the advance of molecular biology and genetics, many functions of these proteins have been revealed over the past decade. In this review, we will first discuss how action potential propagation has been understood by classical electrophysiological studies. In particular, the discussion will be concentrated on how the geometric dimensions of myelinated nerve fibers (such as internodal length and myelin thickness) may affect nerve conduction velocity. This discussion will then extend into how specific myelin proteins may shape these geometric parameters, thereby regulating action potential propagation. For instance, periaxin may specifically affect the internodal length, but not other parameters. In contrast, neuregulin-1 may affect myelin thickness, but not axon diameter or internodal length. Finally, we will discuss how these basic neurobiological observations can be applied to inherited peripheral nerve diseases.

Giant ankyrin-G: a critical innovation in vertebrate evolution of fast and integrated neuronal signaling

Axon initial segments (AISs) and nodes of Ranvier are sites of clustering of voltage-gated sodium channels (VGSCs) in nervous systems of jawed vertebrates that facilitate fast long-distance electrical signaling. We demonstrate that proximal axonal polarity as well as assembly of the AIS and normal morphogenesis of nodes of Ranvier all require a heretofore uncharacterized alternatively spliced giant exon of ankyrin-G (AnkG). This exon has sequence similarity to I-connectin/Titin and was acquired after the first round of whole-genome duplication by the ancestral ANK2/ANK3 gene in early vertebrates before development of myelin. The giant exon resulted in a new nervous system-specific 480-kDa polypeptide combining previously known features of ANK repeats and β-spectrin-binding activity with a fibrous domain nearly 150 nm in length. We elucidate previously undescribed functions for giant AnkG, including recruitment of β4 spectrin to the AIS that likely is regulated by phosphorylation, and demonstrate that 480-kDa AnkG is a major component of the AIS membrane "undercoat' imaged by platinum replica electron microscopy. Surprisingly, giant AnkG-knockout neurons completely lacking known AIS components still retain distal axonal polarity and generate action potentials (APs), although with abnormal frequency. Giant AnkG-deficient mice live to weaning and provide a rationale for survival of humans with severe cognitive dysfunction bearing a truncating mutation in the giant exon. The giant exon of AnkG is required for assembly of the AIS and nodes of Ranvier and was a transformative innovation in evolution of the vertebrate nervous system that now is a potential target in neurodevelopmental disorders.

Voltage-gated sodium channels were differentially expressed in human normal prostate, benign prostatic hyperplasia and prostate cancer cells

Voltage-gated sodium channels (VGSCs) are expressed not only in excitable cells but also in numerous metastatic cells, particularly in certain types of cancer cells. In some types of cancer, including prostate cancer, the expression of VGSCs is associated with cancer migration, invasion and metastasis in vivo. However, the detailed expression profiles of VGSC α subunits in normal human prostate, in prostatic hyperplasia and prostatic cancer remain controversial. In the present study, quantitative polymerase chain reaction was used to systematically detect all subtypes of VGSC α subunits in normal human prostate, benign prostatic hyperplasia (BPH) and prostate cancer cells. The expression profile of VGSC α subunits was observed to differ between these cell types. Nav1.5 was the major isoform expressed in normal human prostate tissue, while Nav1.5 and Nav1.2 were the predominant isoforms in BPH tissue. However, in PC-3 and LNCaP cells, two typical prostate cancer cell lines, Nav1.6 and Nav1.7 were abundantly expressed. By comparing the relative expression levels of Nav1.5, Nav1.6 and Nav1.7 in these cells, the mRNA levels of Nav1.6 and Nav1.7 were identified to be 6- to 27-fold higher in PC-3 and LNCaP cells than in either normal or BPH samples (P<0.05); however, Nav1.5 mRNA levels were relatively lower compared with those of Nav1.6 or Nav1.7 in all cells analyzed. To confirm whether Nav1.6 and Nav1.7 expression in cancer cells was functional, a patch-clamp technique was used to record whole-cell currents. A tetrodotoxin-sensitive sodium current was successfully recorded in PC-3 cells, but not in LNCaP cells. It was concluded that although all types of VGSC α subunits exhibited low expression levels in normal prostate and BPH cells, both Nav1.6 and Nav1.7 were significantly upregulated in the prostate cancer cell lines, suggesting these subtypes may be potential diagnostic markers and therapeutic targets for certain types of prostate cancer in humans.

Biology of the KCNQ1 Potassium Channel

Ion channels are essential for basic cellular function and for processes including sensory perception and intercellular communication in multicellular organisms. Voltage-gated potassium (Kv) channels facilitate dynamic cellular repolarization during an action potential, opening in response to membrane depolarization to facilitate K+ efflux. In both excitable and nonexcitable cells other, constitutively active, K+ channels provide a relatively constant repolarizing force to control membrane potential, ion homeostasis, and secretory processes. Of the forty known human Kv channel pore-forming α subunits that coassemble in various combinations to form the fundamental tetrameric channel pore and voltage sensor module, KCNQ1 is unique. KCNQ1 stands alone in having the capacity to form either channels that are voltage-dependent and require membrane depolarization for activation, or constitutively active channels. In mammals, KCNQ1 regulates processes including gastric acid secretion, thyroid hormone biosynthesis, salt and glucose homeostasis, and cell volume and in some species is required for rhythmic beating of the heart. In this review, the author discusses the unique functional properties, regulation, cell biology, diverse physiological roles, and involvement in human disease states of this chameleonic K+ channel.

Vascular endothelial growth factor modulates voltage-gated Na(+) channel properties and depresses action potential firing in cultured rat hippocampal neurons

Vascular endothelial growth factor (VEGF), an angiogenic factor, was found to modulate synaptic plasticity by affecting K(+) and Ca(2+) channels and protect neuron from death by depressing glutamatergic transmission. However, whether VEGF also modulates neuronal activity through modulating voltage-gated Na(+) channels (VGSCs), a main determinant of neuronal excitability, we observed the effects of VEGF on Na(+) channel properties and function on cultured rat hippocampal neurons through whole-cell patch-clamp recording. We found that VEGF decreased the Na(+) channel excitability by shifting the voltage-dependence of steady-state inactivation to more hyperpolarized direction, and increasing the time constants of recovery from inactivation without significantly affecting the activation process. The effect of VEGF on Na(+) channel steady-state inactivation was inhibited by the specific VEGF Flk-1 receptor antagonist SU1498, but was not affected by protein kinase C (PKC)-activator 1-oleoyl-2-acetyl-sn-glycerol (OAG). Furthermore, the inhibition of Na(+) currents by VEGF was frequency-dependent. In addition, the frequency of neuron firing evoked by current injection was reversibly depressed by VEGF. Therefore, our results suggest a potential role of VGSCs in the modulation of VEGF on neuronal excitability.

The Hodgkin-Huxley heritage: from channels to circuits

The Hodgkin-Huxley studies of the action potential, published 60 years ago, are a central pillar of modern neuroscience research, ranging from molecular investigations of the structural basis of ion channel function to the computational implications at circuit level. In this Symposium Review, we aim to demonstrate the ongoing impact of Hodgkin's and Huxley's ideas. The Hodgkin-Huxley model established a framework in which to describe the structural and functional properties of ion channels, including the mechanisms of ion permeation, selectivity, and gating. At a cellular level, the model is used to understand the conditions that control both the rate and timing of action potentials, essential for neural encoding of information. Finally, the Hodgkin-Huxley formalism is central to computational neuroscience to understand both neuronal integration and circuit level information processing, and how these mechanisms might have evolved to minimize energy cost.

Capsaicin Blocks the Hyperpolarization-Activated Inward Currents via TRPV1 in the Rat Dorsal Root Ganglion Neurons

Capsaicin, the pungent ingredient in hot pepper, activates nociceptors to produce pain and inflammation. However, prolonged exposures of capsaicin will cause desensitization to nociceptive stimuli. Hyperpolarization-activated cation currents (I(h)) contribute to the maintenance of the resting membrane potential and excitability of neurons. In the cultured dorsal root ganglion (DRG) neurons, we investigated mechanisms underlying capsaicin-mediated modulation of I(h) using patch clamp recordings. Capsaicin (1 µM) inhibited I(h) only in the capsaicin-sensitive neurons. The capsaicin-induced inhibition of I(h) was prevented by preexposing the TRPV1 antagonist, capsazepine (CPZ). Capsaicin-induced inhibition of I(h) was dose dependent (IC(50)= 0.68 µM) and partially abolished by intracellular BAPTA and cyclosporin A, specific calcineurin inhibitor. In summary, the inhibitory effects of capsaicin on I(h) are mediated by activation of TRPV1 and Ca(2+)-triggered cellular responses. Analgesic effects of capsaicin have been thought to be related to desensitization of nociceptive neurons due to depletion of pain-related substances. In addition, capsaicin-induced inhibition of I(h) is likely to be important in understanding the analgesic mechanism of capsaicin.

Crystal structure of an orthologue of the NaChBac voltage-gated sodium channel

Voltage-gated sodium (Na(v)) channels are essential for the rapid depolarization of nerve and muscle, and are important drug targets. Determination of the structures of Na(v) channels will shed light on ion channel mechanisms and facilitate potential clinical applications. A family of bacterial Na(v) channels, exemplified by the Na(+)-selective channel of bacteria (NaChBac), provides a useful model system for structure-function analysis. Here we report the crystal structure of Na(v)Rh, a NaChBac orthologue from the marine alphaproteobacterium HIMB114 (Rickettsiales sp. HIMB114; denoted Rh), at 3.05 Å resolution. The channel comprises an asymmetric tetramer. The carbonyl oxygen atoms of Thr 178 and Leu 179 constitute an inner site within the selectivity filter where a hydrated Ca(2+) resides in the crystal structure. The outer mouth of the Na(+) selectivity filter, defined by Ser 181 and Glu 183, is closed, as is the activation gate at the intracellular side of the pore. The voltage sensors adopt a depolarized conformation in which all the gating charges are exposed to the extracellular environment. We propose that Na(v)Rh is in an 'inactivated' conformation. Comparison of Na(v)Rh with Na(v)Ab reveals considerable conformational rearrangements that may underlie the electromechanical coupling mechanism of voltage-gated channels.

Structural basis for functional diversity of animal toxins

The diversity of biological functions that are exerted by toxins from snake and scorpion venoms is associated with a limited number of structural frameworks. At present, one predominant basic fold has been observed among scorpion toxins whereas six folds have been found among snake toxins. Most toxin folds have the capacity to accept multiple insertions, deletions and mutations and to exert various recognition functions. We suggest that such folds may serve as guides to engineer new protein functions.

Engineering biosynthetic excitable tissues from unexcitable cells for electrophysiological and cell therapy studies

Patch-clamp recordings in single-cell expression systems have been traditionally used to study the function of ion channels. However, this experimental setting does not enable assessment of tissue-level function such as action potential (AP) conduction. Here we introduce a biosynthetic system that permits studies of both channel activity in single cells and electrical conduction in multicellular networks. We convert unexcitable somatic cells into an autonomous source of electrically excitable and conducting cells by stably expressing only three membrane channels. The specific roles that these expressed channels have on AP shape and conduction are revealed by different pharmacological and pacing protocols. Furthermore, we demonstrate that biosynthetic excitable cells and tissues can repair large conduction defects within primary 2- and 3-dimensional cardiac cell cultures. This approach enables novel studies of ion channel function in a reproducible tissue-level setting and may stimulate the development of new cell-based therapies for excitable tissue repair.

A null mutation of the neuronal sodium channel NaV1.6 disrupts action potential propagation and excitation-contraction coupling in the mouse heart

Evidence supports the expression of brain-type sodium channels in the heart. Their functional role, however, remains controversial. We used global Na(V)1.6-null mice to test the hypothesis that Na(V)1.6 contributes to the maintenance of propagation in the myocardium and to excitation-contraction (EC) coupling. We demonstrated expression of transcripts encoding full-length Na(V)1.6 in isolated ventricular myocytes and confirmed the striated pattern of Na(V)1.6 fluorescence in myocytes. On the ECG, the PR and QRS intervals were prolonged in the null mice, and the Ca(2+) transients were longer in the null cells. Under patch clamping, at holding potential (HP) = -120 mV, the peak I(Na) was similar in both phenotypes. However, at HP = -70 mV, the peak I(Na) was smaller in the nulls. In optical mapping, at 4 mM [K(+)](o), 17 null hearts showed slight (7%) reduction of ventricular conduction velocity (CV) compared to 16 wild-type hearts. At 12 mM [K(+)](o), CV was 25% slower in a subset of 9 null vs. 9 wild-type hearts. These results highlight the importance of neuronal sodium channels in the heart, whereby Na(V)1.6 participates in EC coupling, and represents an intrinsic depolarizing reserve that contributes to excitation.

KCNE1 and KCNE2 inhibit forward trafficking of homomeric N-type voltage-gated potassium channels

Potassium currents generated by voltage-gated potassium (Kv) channels comprising α-subunits from the Kv1, 2, and 3 subfamilies facilitate high-frequency firing of mammalian neurons. Within these subfamilies, only three α-subunits (Kv1.4, Kv3.3, and Kv3.4) generate currents that decay rapidly in the open state because an N-terminal ball domain blocks the channel pore after activation-a process termed N-type inactivation. Despite its importance to shaping cellular excitability, little is known of the processes regulating surface expression of N-type α-subunits, versus their slowly inactivating (delayed rectifier) counterparts. Here we found that currents generated by homomeric Kv1.4, Kv3.3, and Kv3.4 channels are all strongly suppressed by the single transmembrane domain ancillary (β) subunits KCNE1 and KCNE2. A combination of electrophysiological, biochemical, and immunofluorescence analyses revealed this suppression is due to KCNE1 and KCNE2 retaining Kv1.4 and Kv3.4 intracellularly, early in the secretory pathway. The retention is specific, requires α-β coassembly, and does not involve the dynamin-dependent endocytosis pathway. However, the small fraction of Kv3.4 that escapes KCNE-dependent retention is regulated by dynamin-dependent endocytosis. The findings illustrate two contrasting mechanisms controlling surface expression of N-type Kv α-subunits and therefore, potentially, cellular excitability and refractory periods.

Ion channel voltage sensors: structure, function, and pathophysiology

Voltage-gated ion channels generate electrical signals in species from bacteria to man. Their voltage-sensing modules are responsible for initiation of action potentials and graded membrane potential changes in response to synaptic input and other physiological stimuli. Extensive structure-function studies, structure determination, and molecular modeling are now converging on a sliding-helix mechanism for electromechanical coupling in which outward movement of gating charges in the S4 transmembrane segments catalyzed by sequential formation of ion pairs pulls the S4-S5 linker, bends the S6 segment, and opens the pore. Impairment of voltage-sensor function by mutations in Na+ channels contributes to several ion channelopathies, and gating pore current conducted by mutant voltage sensors in Na(V)1.4 channels is the primary pathophysiological mechanism in hypokalemic periodic paralysis. The emerging structural model for voltage sensor function opens the way to development of a new generation of ion-channel drugs that act on voltage sensors rather than blocking the pore.

The M-superfamily of conotoxins: a review

The focus of this review is the M-superfamily of Conus venom peptides. Disulfide rich peptides belonging to the M-superfamily have three loop regions and the cysteine arrangement: CC-C-C-CC, where the dashes represent loops one, two, and three, respectively. Characterization of M-superfamily peptides has demonstrated that diversity in cystine connectivity occurs between different branches of peptides even though the cysteine pattern remains consistent. This superfamily is subdivided into five branches, M-1 through M-5, based on the number of residues in the third loop region, between the fourth and fifth cysteine residues. M-superfamily peptides appear to be ubiquitous in Conus venom. They are largely unexplained in indigenous biological function, and they represent an active area of research within the scientific community.

Lactate production and neurotransmitters; evidence from microdialysis studies

Recent studies have found that lactate metabolism plays a significant role in energy supply during acute neural activation in the brain. We will review evidence from microdialysis studies for a relationship between neurotransmitters and lactate production, as revealed in studies of the effects of psychotropic drugs on stress-induced enhancement of extracellular lactate concentrations. Glutamate enhances stress-induced lactate production via activation of N-methyl-D-asparate receptors, and is affected by uptake of glutamate through glutamate transporters. Findings from microdialysis studies suggest that major neurotransmitters, including norepinephrine, dopamine, serotonin, and GABA (via benzodiazepine-receptors) affect lactate production, depending on brain areas, especially during stress. Among these neurotransmitters, glutamate may principally contribute to the regulation of lactate production, with other neurotransmitter systems affecting the extracellular lactate levels in a glutamate-mediated manner. The role for anaerobic metabolism in the supply of energy, as represented by lactate dynamics, deserves further clarification. Monitoring with intracerebral microdialysis is a reliable method for this purpose. Research into this area is likely to provide a novel insight into the mode of action of psychotropic drugs, and the pathophysiology of some of the stress-related mental disorders as well.

Voltage-gated ion channels and gating modifier toxins

Voltage-gated sodium, calcium, and potassium channels generate electrical signals required for action potential generation and conduction and are the molecular targets for a broad range of potent neurotoxins. These channels are built on a common structural motif containing six transmembrane segments and a pore loop. Their pores are formed by the S5/S6 segments and the pore loop between them, and they are gated by bending of the S6 segments at a hinge glycine or proline residue. The voltage sensor domain consists of the S1-S4 segments, with positively charged residues in the S4 segment serving as gating charges. The diversity of toxin action on these channels is illustrated by sodium channels, which are the molecular targets for toxins that act at six or more distinct receptor sites on the channel protein. Both hydrophilic low molecular weight toxins and larger polypeptide toxins physically block the pore and prevent sodium conductance. Hydrophobic alkaloid toxins and related lipid-soluble toxins act at intramembrane sites and alter voltage-dependent gating of sodium channels via an allosteric mechanism. In contrast, polypeptide toxins alter channel gating by voltage-sensor trapping through binding to extracellular receptor sites, and this toxin interaction has now been modeled at the atomic level for a beta-scorpion toxin. The voltage-sensor trapping mechanism may be a common mode of action for polypeptide gating modifier toxins acting on all of the voltage-gated ion channels.

The signaling pathway in photoresponses that may be mediated by visual pigments in erythrophores of Nile tilapia

The ability to increase the synthesis or vary the distribution of pigment in response to light is an important feature of many pigment cells. Unlike other light-sensitive pigment cells, erythrophores of Nile tilapia change the direction of pigment migration depending on the peak wavelength of incident light: light near 365, 400 or 600 nm induces pigment aggregation, while dispersion occurs in response to light at 500 nm. How these phenomena are achieved is currently unknown. In the present study, the phototransduction involved in the pigment dispersion caused by light at 500 nm or the aggregation by light at 600 nm was examined, using pertussis toxin, cholera toxin, blockers of ion channels, various chemicals affecting serial steps of signaling pathways and membrane-permeable cAMP analog. The results show that light-induced bidirectional movements in tilapia erythrophores may be controlled by cytosolic cAMP levels via Gi- or Gs-type G proteins. In addition, RT-PCR demonstrated for the first time the expression of mRNAs encoding red and green opsins in tilapia fins, only where erythrophores exist. Here, we suggest that multiple cone-type visual pigments may be present in the erythrophores, and that unique cascades in which such opsins couple to Gi or Gs-type G proteins are involved in the photoresponses in these pigment cells. Thus, tilapia erythrophore system seems to be a nice model for understanding the photoresponses of cells other than visual cells.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.