beta-Scorpion toxin modifies gating transitions in all four voltage sensors of the sodium channel

Fifty years of gating currents and channel gating

We celebrate this year the 50th anniversary of the first electrophysiological recordings of the gating currents from voltage-dependent ion channels done in 1973. This retrospective tries to illustrate the context knowledge on channel gating and the impact gating-current recording had then, and how it continued to clarify concepts, elaborate new ideas, and steer the scientific debate in these 50 years. The notion of gating particles and gating currents was first put forward by Hodgkin and Huxley in 1952 as a necessary assumption for interpreting the voltage dependence of the Na and K conductances of the action potential. 20 years later, gating currents were actually recorded, and over the following decades have represented the most direct means of tracing the movement of the gating charges and gaining insights into the mechanisms of channel gating. Most work in the early years was focused on the gating currents from the Na and K channels as found in the squid giant axon. With channel cloning and expression on heterologous systems, other channels as well as voltage-dependent enzymes were investigated. Other approaches were also introduced (cysteine mutagenesis and labeling, site-directed fluorometry, cryo-EM crystallography, and molecular dynamics [MD] modeling) to provide an integrated and coherent view of voltage-dependent gating in biological macromolecules. The layout of this retrospective reflects the past 50 years of investigations on gating currents, first addressing studies done on Na and K channels and then on other voltage-gated channels and non-channel structures. The review closes with a brief overview of how the gating-charge/voltage-sensor movements are translated into pore opening and the pathologies associated with mutations targeting the structures involved with the gating currents.

Scorpion Peptides and Ion Channels: An Insightful Review of Mechanisms and Drug Development

The Buthidae family of scorpions consists of arthropods with significant medical relevance, as their venom contains a diverse range of biomolecules, including neurotoxins that selectively target ion channels in cell membranes. These ion channels play a crucial role in regulating physiological processes, and any disturbance in their activity can result in channelopathies, which can lead to various diseases such as autoimmune, cardiovascular, immunological, neurological, and neoplastic conditions. Given the importance of ion channels, scorpion peptides represent a valuable resource for developing drugs with targeted specificity for these channels. This review provides a comprehensive overview of the structure and classification of ion channels, the action of scorpion toxins on these channels, and potential avenues for future research. Overall, this review highlights the significance of scorpion venom as a promising source for discovering novel drugs with therapeutic potential for treating channelopathies.

Characterising ion channel structure and dynamics using fluorescence spectroscopy techniques

Ion channels undergo major conformational changes that lead to channel opening and ion conductance. Deciphering these structure-function relationships is paramount to understanding channel physiology and pathophysiology. Cryo-electron microscopy, crystallography and computer modelling provide atomic-scale snapshots of channel conformations in non-cellular environments but lack dynamic information that can be linked to functional results. Biophysical techniques such as electrophysiology, on the other hand, provide functional data with no structural information of the processes involved. Fluorescence spectroscopy techniques help bridge this gap in simultaneously obtaining structure-function correlates. These include voltage-clamp fluorometry, Förster resonance energy transfer, ligand binding assays, single molecule fluorescence and their variations. These techniques can be employed to unearth several features of ion channel behaviour. For instance, they provide real time information on local and global rearrangements that are inherent to channel properties. They also lend insights in trafficking, expression, and assembly of ion channels on the membrane surface. These methods have the advantage that they can be carried out in either native or heterologous systems. In this review, we briefly explain the principles of fluorescence and how these have been translated to study ion channel function. We also report several recent advances in fluorescence spectroscopy that has helped address and improve our understanding of the biophysical behaviours of different ion channel families.

The 70-year search for the voltage sensing mechanism of ion channels

This retrospective on the voltage‐sensing mechanisms and gating models of ion channels begins in 1952 with the charged gating particles postulated by Hodgkin and Huxley, viewed as charges moving across the membrane and controlling its permeability to Na⁺ and K⁺ ions. Hodgkin and Huxley postulated that their movement should generate small and fast capacitive currents, which were recorded 20 years later as gating currents. In the early 1980s, several voltage‐dependent channels were cloned and found to share a common architecture: four homologous domains or subunits, each displaying six transmembrane α‐helical segments, with the fourth segment (S4) displaying four to seven positive charges invariably separated by two non‐charged residues. This immediately suggested that this segment was serving as the voltage sensor of the channel (the molecular counterpart of the charged gating particle postulated by Hodgkin and Huxley) and led to the development of the sliding helix model. Twenty years later, the X‐ray crystallographic structures of many voltage‐dependent channels allowed investigation of their gating by molecular dynamics. Further understanding of how channels gate will benefit greatly from the acquisition of high‐resolution structures of each of their relevant functional or structural states. This will allow the application of molecular dynamics and other approaches. It will also be key to investigate the energetics of channel gating, permitting an understanding of the physical and molecular determinants of gating. The use of multiscale hierarchical approaches might finally prove to be a rewarding strategy to overcome the limits of the various single approaches to the study of channel gating.

Neurotoxicity of Tityus bahiensis (brown scorpion) venom in sympathetic vas deferens preparations and neuronal cells

Systemic scorpion envenomation is characterized by massive neurotransmitter release from peripheral nerves mediated primarily by scorpion venoms neurotoxins. Tityus bahiensis is one of the medically most important species in Brazil, but its venom pharmacology, especially regarding to peripheral nervous system, is poorly understood. Here, we evaluated the T. bahiensis venom activity on autonomic (sympathetic) neurotransmission by using a variety of approaches, including vas deferens twitch-tension recordings, electrophysiological measurements (resting membrane potentials, spontaneous excitatory junctional potentials and whole-cell patch-clamp), calcium imaging and histomorphological analysis. Low concentrations of venom (≤ 3 μg/mL) facilitated the electrically stimulated vas deferens contractions without affecting postsynaptic receptors or damaging the smooth muscle cells. Transient TTX-sensitive sustained contractions and resting membrane depolarization were mediated mainly by massive spontaneous ATP release. High venom concentrations (≥ 10 μg/mL) blocked the muscle contractions and induced membrane depolarization. In neuronal cells (ND7-23wt), the venom increased the peak sodium current, modified the current-voltage relationship by left-shifting the Nav-channel activation curve, thereby facilitating the opening of these channels. The venom also caused a time-dependent increase in neuronal calcium influx. These results indicate that the sympathetic hyperstimulation observed in systemic envenomation is presynaptically driven, probably through the interaction of α- and β-toxins with neuronal sodium channels.

Centipede KCNQ Inhibitor SsTx Also Targets KV1.3

It was recently discovered that Ssm Spooky Toxin (SsTx) with 53 residues serves as a key killer factor in red-headed centipede’s venom arsenal, due to its potent blockage of the widely expressed KCNQ channels to simultaneously and efficiently disrupt cardiovascular, respiratory, muscular, and nervous systems, suggesting that SsTx is a basic compound for centipedes’ defense and predation. Here, we show that SsTx also inhibits K_V1.3 channel, which would amplify the broad-spectrum disruptive effect of blocking K_V7 channels. Interestingly, residue R12 in SsTx extends into the selectivity filter to block K_V7.4, however, residue K11 in SsTx replaces this ploy when toxin binds on K_V1.3. Both SsTx and its mutant SsTx_R12A inhibit cytokines production in T cells without affecting the level of K_V1.3 expression. The results further suggest that SsTx is a key molecule for defense and predation in the centipedes’ venoms and it evolves efficient strategy to disturb multiple physiological targets.

Closed- and open-state models of human skeletal muscle sodium channel

Voltage-gated sodium channels play important roles in human physiology. However, their complexity hinders the understanding of their physiology and pathology at atomic level. We took advantage of the structural reports of similar channels obtained by cryo-EM (EeNav1.4, and NavPaS), and constructed models of human Nav1.4 channels at closed and open states. The open-state model is very similar to the recently published cryo-EM structure of hNav1.4. The comparison of both models shows shifts of the voltage sensors (VS) of DIII and DIV. The activated position of VS-DII in the closed model was demonstrated by Ts1 docking, thereby confirming the requirement that VS-DI, VS-DII and VS-DIII must be activated for the channel to open. The interactions observed with VS-DIII suggest a stepwise, yet fast, transition from resting to activated state. These models provide structural insights on the closed-open transition of the channel.

Subtype Specificity of β-Toxin Tf1a from Tityus fasciolatus in Voltage Gated Sodium Channels

Scorpion venoms are a complex mixture of components. Among them the most important are peptides, which presents the capacity to interact and modulate several ion channel subtypes, including voltage-gated sodium channels (Na_V). Screening the activity of scorpion toxins on different subtypes of Na_V reveals the scope of modulatory activity and, in most cases, low channel selectivity. Until now there are approximately 60 scorpion toxins experimentally assayed on Na_V channels. However, the molecular bases of interaction between scorpion toxins and Na_V channels are not fully elucidated. The activity description of new scorpion toxins is crucial to enhance the predictive strength of the structural–function correlations of these Na_V modulatory molecules. In the present work a new scorpion toxin (Tf1a) was purified from Tityus fasciolatus venom by RP-HPLC, and characterized using electrophysiological experiments on different types of voltage-gated sodium channels. Tf1a was able to modify the normal function of Na_V tested, showing to be a typical β-NaScTx. Tf1a also demonstrated an unusual capability to alter the kinetics of Na_V1.5.

Electrophysiological characterization of Tityus obscurus β toxin 1 (To1) on Na+-channel isoforms

To1, previously named Tc49b, is a peptide neurotoxin isolated from venom of the scorpion Tityus obscurus that is responsible for lethal human poisoning cases in the Brazilian Amazonian region. Previously, To1 was shown to be lethal to mice and to change Na⁺ permeation in cerebellum granular neurons from rat brain. In addition, To1 did not affect Shaker B K⁺ channels. Based on sequence similarities, To1 was described as a β-toxin. In the present work, To1 was purified from T. obscurus venom and submitted to an electrophysiological characterization in human and invertebrate Na_V channels. The analysis of the electrophysiological experiments reveal that To1 enhances the open probability at more negative potentials of human Na_V 1.3 and 1.6, of the insect channel BgNa_V1 and of arachnid VdNa_V1 channel. In addition, To1 reduces the peak of Na⁺ currents in some of the Na_Vs tested. These results support the classification of the To1 as a β-toxin. A structure and functional comparison to other β-toxins that share sequence similarity to To1 is also presented.

Ts1 from the Brazilian scorpion Tityus serrulatus: A half-century of studies on a multifunctional beta like-toxin

The Tityus serrulatus scorpion species represents a serious human health threat to in Brazil because it is among the animals that produces the most dangerous venoms for mammals in South America. Its venom has provided several highly selective ligands that specifically interact with sodium and potassium channels. During the past decades, several international groups published an increasing amount of data on the isolation and the chemical, pharmacological and immunological characterisation of its main β-toxin, Ts1. In this review, we compiled the best available past and recent knowledge on Ts1. Aside from its intricate purification, the state-of-the-art understanding concerning its pharmacological activities is presented. Its solved three-dimensional structure is shown, as well as the possible surface areas of contact between Ts1 and its diverse voltage-gated Na⁺ channel targets. Organisations of the gene and the precursor encoding Ts1 are also tackled based on available cDNA clones or on information obtained from polymerase chain reactions of stretches of scorpion DNA. At last, the immunological studies complete with Ts1 to set up an efficient immunotherapy against the Tityus serrulatus venom are summarized.

Using voltage-sensor toxins and their molecular targets to investigate NaV 1.8 gating

Voltage-gated sodium (Na_V ) channel gating is a complex phenomenon which involves a distinct contribution of four integral voltage-sensing domains (VSDI, VSDII, VSDIII and VSDIV). Utilizing accrued pharmacological and structural insights, we build on an established chimera approach to introduce animal toxin sensitivity in each VSD of an acceptor channel by transferring in portable S3b-S4 motifs from the four VSDs of a toxin-susceptible donor channel (Na_V 1.2). By doing so, we observe that in Na_V 1.8, a relatively unexplored channel subtype with distinctly slow gating kinetics, VSDI-III participate in channel opening whereas VSDIV can regulate opening as well as fast inactivation. These results illustrate the effectiveness of a pharmacological approach to investigate the mechanism underlying gating of a mammalian Na_V channel complex.

Nav channel binder containing a specific conjugation-site based on a low toxicity β-scorpion toxin

Voltage-gated sodium (Nav) channels play a key role in generating action potentials which leads to physiological signaling in excitable cells. The availability of probes for functional studies of mammalian Nav is limited. Here, by introducing two amino acid substitutions into the beta scorpion toxin Ts1, we have chemically synthesized a novel binder [S14R, W50Pra]Ts1 for Nav with high affinity, low dissociation rate and reduced toxicity while retaining the capability of conjugating Ts1 with molecules of interests for different applications. Using the fluorescent-dye conjugate, [S14R, W50Pra(Bodipy)]Ts1, we confirmed its binding to Nav1.4 through Lanthanide-based Resonance Energy Transfer. Moreover, using the gold nanoparticle conjugate, [S14R, W50Pra(AuNP)]Ts1, we were able to optically stimulate dorsal root ganglia neurons and generate action potentials with visible light via the optocapacitive effect as previously reported. [S14R, W50Pra]Ts1 is a novel probe with great potential for wider applications in Nav-related neuroscience research.

The envenomation of general physiology throughout the last century

Sack discusses the evolution of toxin research in JGP over the last 100 years.

Toxins are the poisonous products of organisms. Toxins serve vital defensive and offensive functions for those that harbor them: stinging scorpions, pesticidal plants, sanguinary snakes, fearless frogs, sliming snails, noxious newts, and smarting spiders. For physiologists, toxins are integral chemical tools that hijack life’s fundamental processes with remarkable molecular specificity. Our understanding of electrophysiological phenomena has been transformed time and time again with the help of some terrifying toxins. For this reason, studies of toxin mechanism are an important and enduring facet of The Journal of General Physiology (JGP). This Milestone in Physiology reflects on toxins studied in JGP over its first 100 years, what they have taught us, and what they have yet to reveal.

Mechanisms of noncovalent β subunit regulation of NaV channel gating

Voltage-gated Na_V channels are modulated by two different noncovalent accessory subunits: β1 and β3. Zhu et al. present data showing that β1 and β3 cause distinct effects on channel gating because they interact with Na_V channels at different locations. β3 regulates the voltage sensor in domain III, whereas β1 regulates the one in domain IV.

Voltage-gated Na⁺ (Na_V) channels comprise a macromolecular complex whose components tailor channel function. Key components are the non-covalently bound β1 and β3 subunits that regulate channel gating, expression, and pharmacology. Here, we probe the molecular basis of this regulation by applying voltage clamp fluorometry to measure how the β subunits affect the conformational dynamics of the cardiac Na_V channel (Na_V1.5) voltage-sensing domains (VSDs). The pore-forming Na_V1.5 α subunit contains four domains (DI–DIV), each with a VSD. Our results show that β1 regulates Na_V1.5 by modulating the DIV-VSD, whereas β3 alters channel kinetics mainly through DIII-VSD interaction. Introduction of a quenching tryptophan into the extracellular region of the β3 transmembrane segment inverted the DIII-VSD fluorescence. Additionally, a fluorophore tethered to β3 at the same position produced voltage-dependent fluorescence dynamics strongly resembling those of the DIII-VSD. Together, these results provide compelling evidence that β3 binds proximally to the DIII-VSD. Molecular-level differences in β1 and β3 interaction with the α subunit lead to distinct activation and inactivation recovery kinetics, significantly affecting Na_V channel regulation of cell excitability.

Mapping of voltage sensor positions in resting and inactivated mammalian sodium channels by LRET

Voltage-gated sodium channels (Navs) play crucial roles in excitable cells. Although vertebrate Nav function has been extensively studied, the detailed structural basis for voltage-dependent gating mechanisms remain obscure. We have assessed the structural changes of the Nav voltage sensor domain using lanthanide-based resonance energy transfer (LRET) between the rat skeletal muscle voltage-gated sodium channel (Nav1.4) and fluorescently labeled Nav1.4-targeting toxins. We generated donor constructs with genetically encoded lanthanide-binding tags (LBTs) inserted at the extracellular end of the S4 segment of each domain (with a single LBT per construct). Three different Bodipy-labeled, Nav1.4-targeting toxins were synthesized as acceptors: β-scorpion toxin (Ts1)-Bodipy, KIIIA-Bodipy, and GIIIA-Bodipy analogs. Functional Nav-LBT channels expressed in Xenopus oocytes were voltage-clamped, and distinct LRET signals were obtained in the resting and slow inactivated states. Intramolecular distances computed from the LRET signals define a geometrical map of Nav1.4 with the bound toxins, and reveal voltage-dependent structural changes related to channel gating.

Sodium Channels and Venom Peptide Pharmacology

Venomous animals including cone snails, spiders, scorpions, anemones, and snakes have evolved a myriad of components in their venoms that target the opening and/or closing of voltage-gated sodium channels to cause devastating effects on the neuromuscular systems of predators and prey. These venom peptides, through design and serendipity, have not only contributed significantly to our understanding of sodium channel pharmacology and structure, but they also represent some of the most phyla- and isoform-selective molecules that are useful as valuable tool compounds and drug leads. Here, we review our understanding of the basic function of mammalian voltage-gated sodium channel isoforms as well as the pharmacology of venom peptides that act at these key transmembrane proteins.

Voltage-gated sodium channels viewed through a structural biology lens

Voltage-gated sodium (Nav) channels initiate and propagate action potentials in excitable cells, and are frequently dysregulated or mutated in human disease. Despite decades of intense physiological and biophysical research, eukaryotic Nav channels have so far eluded high-resolution structure determination because of their biochemical complexity. Recently, simpler bacterial voltage-gated sodium (BacNav) channels have provided templates to understand the structural basis of voltage-dependent activation, inactivation, ion selectivity, and drug block in eukaryotic Nav and related voltage-gated calcium (Cav) channels. Further breakthroughs employing BacNav channels have also enabled visualization of bound small molecule modulators that can guide the rational design of next generation therapeutics. This review will highlight the emerging structural biology of BacNav channels and its contribution to our understanding of the gating, ion selectivity, and pharmacological regulation of eukaryotic Nav (and Cav) channels.

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

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

Molecular Interactions between Tarantula Toxins and Low-Voltage-Activated Calcium Channels

Few gating-modifier toxins have been reported to target low-voltage-activated (LVA) calcium channels, and the structural basis of toxin sensitivity remains incompletely understood. Studies of voltage-gated potassium (Kv) channels have identified the S3b–S4 “paddle motif,” which moves at the protein-lipid interface to drive channel opening, as the target for these amphipathic neurotoxins. Voltage-gated calcium (Cav) channels contain four homologous voltage sensor domains, suggesting multiple toxin binding sites. We show here that the S3–S4 segments within Cav3.1 can be transplanted into Kv2.1 to examine their individual contributions to voltage sensing and pharmacology. With these results, we now have a more complete picture of the conserved nature of the paddle motif in all three major voltage-gated ion channel types (Kv, Nav, and Cav). When screened with tarantula toxins, the four paddle sequences display distinct toxin binding properties, demonstrating that gating-modifier toxins can bind to Cav channels in a domain specific fashion. Domain III was the most commonly and strongly targeted, and mutagenesis revealed an acidic residue that is important for toxin binding. We also measured the lipid partitioning strength of all toxins tested and observed a positive correlation with their inhibition of Cav3.1, suggesting a key role for membrane partitioning.

Functional Site-Directed Fluorometry

Initially developed in the mid-1990s to examine the conformational changes of the canonical Shaker voltage-gated potassium channel, functional site-directed fluorometry has since been expanded to numerous other voltage-gated and ligand-gated ion channels as well as transporters, pumps, and other integral membrane proteins. The power of functional site-directed fluorometry, also known as voltage-clamp fluorometry, lies in its ability to provide information on the conformational changes in a protein in response to changes in its environment with high temporal resolution while simultaneously monitoring the function of that protein. Over time, applications of site-directed fluorometry have expanded to examine the interactions of ion channels with modulators ranging from membrane potential to ligands to accessory protein subunits to lipids. In the future, the range of questions answerable by functional site-directed fluorometry and its interpretive power should continue to improve, making it an even more powerful technique for dissecting the conformational dynamics of ion channels and other membrane proteins.

Molecular motions that shape the cardiac action potential: Insights from voltage clamp fluorometry

Very recently, voltage-clamp fluorometry (VCF) protocols have been developed to observe the membrane proteins responsible for carrying the ventricular ionic currents that form the action potential (AP), including those carried by the cardiac Na(+) channel, NaV1.5, the L-type Ca(2+) channel, CaV1.2, the Na(+)/K(+) ATPase, and the rapid and slow components of the delayed rectifier, KV11.1 and KV7.1. This development is significant, because VCF enables simultaneous observation of ionic current kinetics with conformational changes occurring within specific channel domains. The ability gained from VCF, to connect nanoscale molecular movement to ion channel function has revealed how the voltage-sensing domains (VSDs) control ion flux through channel pores, mechanisms of post-translational regulation and the molecular pathology of inherited mutations. In the future, we expect that this data will be of great use for the creation of multi-scale computational AP models that explicitly represent ion channel conformations, connecting molecular, cell and tissue electrophysiology. Here, we review the VCF protocol, recent results, and discuss potential future developments, including potential use of these experimental findings to create novel computational models.

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

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

Allosteric interactions between receptor site 3 and 4 of voltage-gated sodium channels: a novel perspective for the underlying mechanism of scorpion sting-induced pain

BACKGROUND

BmK I, a site-3-specific modulator of voltage-gated sodium channels (VGSCs), causes pain and hyperalgesia in rats, while BmK IT2, a site-4-specific modulator of VGSCs, suppresses pain-related responses. A stronger pain-related effect has been previously attributed to Buthus martensi Karsch (BmK) venom, which points out the joint pharmacological effect in the crude venom.

METHODS

In order to detect the joint effect of BmK I and BmK IT2 on ND7-23 cells, the membrane current was measured by whole cell recording. BmK I and BmK IT2 were applied successively and jointly, and the synergistic modulations of VGSCs on ND7-23 cells were detected.

RESULTS

Larger peak INa and more negative half-activation voltage were elicited by joint application of BmK I and BmK IT2 than by application of BmK I or BmK IT2 alone. Compared to the control, co-applied BmK I and BmK IT2 also significantly prolonged the time constant of inactivation.

CONCLUSIONS

Our results indicated that site-4 toxin (BmK IT2) could enhance the pharmacological effect induced by site-3 toxin (BmK I), suggesting a stronger effect elicited by both toxins that alone usually exhibit opposite pharmacological effects, which is related to the allosteric interaction between receptor site 3 and site 4. Meanwhile, these results may bring a novel perspective for exploring the underlying mechanisms of scorpion sting-induced pain.

Direct Measurement of Cardiac Na+ Channel Conformations Reveals Molecular Pathologies of Inherited Mutations

BACKGROUND

Dysregulation of voltage-gated cardiac Na(+) channels (NaV1.5) by inherited mutations, disease-linked remodeling, and drugs causes arrhythmias. The molecular mechanisms whereby the NaV1.5 voltage-sensing domains (VSDs) are perturbed to pathologically or therapeutically modulate Na(+) current (INa) have not been specified. Our aim was to correlate INa kinetics with conformational changes within the 4 (DI-DIV) VSDs to define molecular mechanisms of NaV1.5 modulation.

METHOD AND RESULTS

Four NaV1.5 constructs were created to track the voltage-dependent kinetics of conformational changes within each VSD, using voltage-clamp fluorometry. Each VSD displayed unique kinetics, consistent with distinct roles in determining INa. In particular, DIII-VSD deactivation kinetics were modulated by depolarizing pulses with durations in the intermediate time domain that modulates late INa. We then used the DII-VSD construct to probe the molecular pathology of 2 Brugada syndrome mutations (A735V and G752R). A735V shifted DII-VSD voltage dependence to depolarized potentials, whereas G752R significantly slowed DII-VSD kinetics. Both mutations slowed INa activation, although DII-VSD activation occurred at higher potentials (A735V) or at later times (G752R) than ionic current activation, indicating that the DII-VSD allosterically regulates the rate of INa activation and myocyte excitability.

CONCLUSIONS

Our results reveal novel mechanisms whereby the NaV1.5 VSDs regulate channel activation and inactivation. The ability to distinguish distinct molecular mechanisms of proximal Brugada syndrome mutations demonstrates the potential of these methods to reveal how inherited mutations, post-translational modifications, and antiarrhythmic drugs alter NaV1.5 at the molecular level.

The Scorpion Toxin Tf2 from Tityus fasciolatus Promotes Nav1.3 Opening

We identified Tf2, the first β-scorpion toxin from the venom of the Brazilian scorpion Tityus fasciolatus. Tf2 is identical to Tb2-II found in Tityus bahiensis. We found that Tf2 selectively activates human (h)Na_v1.3, a neuronal voltage-gated sodium (Na_v) subtype implicated in epilepsy and nociception. Tf2 shifts hNa_v1.3 activation voltage to more negative values, thereby opening the channel at resting membrane potentials. Seven other tested mammalian Na_v channels (Na_v1.1-1.2; Na_v1.4-1.8) expressed in Xenopus oocytes are insensitive upon application of 1 μM Tf2. Therefore, the identification of Tf2 represents a unique addition to the repertoire of animal toxins that can be used to investigate Na_v channel function.

Tarantula toxins use common surfaces for interacting with Kv and ASIC ion channels

Tarantula toxins that bind to voltage-sensing domains of voltage-activated ion channels are thought to partition into the membrane and bind to the channel within the bilayer. While no structures of a voltage-sensor toxin bound to a channel have been solved, a structural homolog, psalmotoxin (PcTx1), was recently crystalized in complex with the extracellular domain of an acid sensing ion channel (ASIC). In the present study we use spectroscopic, biophysical and computational approaches to compare membrane interaction properties and channel binding surfaces of PcTx1 with the voltage-sensor toxin guangxitoxin (GxTx-1E). Our results show that both types of tarantula toxins interact with membranes, but that voltage-sensor toxins partition deeper into the bilayer. In addition, our results suggest that tarantula toxins have evolved a similar concave surface for clamping onto α-helices that is effective in aqueous or lipidic physical environments.

Photosensitivity of neurons enabled by cell-targeted gold nanoparticles

Unmodified neurons can be directly stimulated with light to produce action potentials, but such techniques have lacked localization of the delivered light energy. Here we show that gold nanoparticles can be conjugated to high-avidity ligands for a variety of cellular targets. Once bound to a neuron, these particles transduce millisecond pulses of light into heat, which changes membrane capacitance, depolarizing the cell and eliciting action potentials. Compared to non-functionalized nanoparticles, ligand-conjugated nanoparticles highly resist convective washout and enable photothermal stimulation with lower delivered energy and resulting temperature increase. Ligands targeting three different membrane proteins were tested; all showed similar activity and washout resistance. This suggests that many types of ligands can be bound to nanoparticles, preserving ligand and nanoparticle function, and that many different cell phenotypes can be targeted by appropriate choice of ligand. The findings have applications as an alternative to optogenetics and potentially for therapies involving neuronal photostimulation.

Total chemical synthesis of biologically active fluorescent dye-labeled Ts1 toxin

Ts1 toxin is a protein found in the venom of the Brazilian scorpion Tityus serrulatus. Ts1 binds to the domain II voltage sensor in the voltage-gated sodium channel Nav and modifies its voltage dependence. In the work reported here, we established an efficient total chemical synthesis of the Ts1 protein using modern chemical ligation methods and demonstrated that it was fully active in modifying the voltage dependence of the rat skeletal muscle voltage-gated sodium channel rNav1.4 expressed in oocytes. Total synthesis combined with click chemistry was used to label the Ts1 protein molecule with the fluorescent dyes Alexa-Fluor 488 and Bodipy. Dye-labeled Ts1 proteins retained their optical properties and bound to and modified the voltage dependence of the sodium channel Nav. Because of the highly specific binding of Ts1 toxin to Nav, successful chemical synthesis and labeling of Ts1 toxin provides an important tool for biophysical studies, histochemical studies, and opto-pharmacological studies of the Nav protein.

PKA phosphorylation reshapes the pharmacological kinetics of BmK AS, a unique site-4 sodium channel-specific modulator

Although modulation of the activity of voltage-gated sodium channels (VGSCs) by protein kinase A (PKA) phosphorylation has been investigated in multiple preparations, the pharmacological sensitivity of VGSCs to scorpion toxins after PKA phosphorylation has rarely been approached. In this study, the effects of BmK AS, a sodium channel-specific modulator from Chinese scorpion Buthus martensi Karsch, on the voltage-dependent activation and inactivation of Nav1.2 were examined before and after PKA activation. After PKA phosphorylation, the pattern of dose-dependent modulation of BmK AS, on both Nav1.2α and Nav1.2 (α + β1) was reshaped. Meanwhile, the shifts in voltage-dependency of activation and inactivation induced by BmK AS were attenuated. The results suggested that PKA might play a role in different patterns how β-like toxins such as BmK AS modulate gating properties and peak currents of VGSCs.

The voltage sensor module in sodium channels

The mechanism by which voltage-gated ion channels respond to changes in membrane polarization during action potential signaling in excitable cells has been the subject of research attention since the original description of voltage-dependent sodium and potassium flux in the squid giant axon. The cloning of ion channel genes and the identification of point mutations associated with channelopathy diseases in muscle and brain has facilitated an electrophysiological approach to the study of ion channels. Experimental approaches to the study of voltage gating have incorporated the use of thiosulfonate reagents to test accessibility, fluorescent probes, and toxins to define domain-specific roles of voltage-sensing S4 segments. Crystallography, structural and homology modeling, and molecular dynamics simulations have added computational approaches to study the relationship of channel structure to function. These approaches have tested models of voltage sensor translocation in response to membrane depolarization and incorporate the role of negative countercharges in the S1 to S3 segments to define our present understanding of the mechanism by which the voltage sensor module dictates gating particle permissiveness in excitable cells.

Animal toxins influence voltage-gated sodium channel function

Voltage-gated sodium (Nav) channels are essential contributors to neuronal excitability, making them the most commonly targeted ion channel family by toxins found in animal venoms. These molecules can be used to probe the functional aspects of Nav channels on a molecular level and to explore their physiological role in normal and diseased tissues. This chapter summarizes our existing knowledge of the mechanisms by which animal toxins influence Nav channels as well as their potential application in designing therapeutic drugs.

Molecular bases for the asynchronous activation of sodium and potassium channels required for nerve impulse generation

Most action potentials are produced by the sequential activation of voltage-gated sodium (Nav) and potassium (Kv) channels. This is mainly achieved by the rapid conformational rearrangement of voltage-sensor (VS) modules in Nav channels, with activation kinetics up to 6-fold faster than Shaker-type Kv channels. Here, using mutagenesis and gating current measurements, we show that a 3-fold acceleration of the VS kinetics in Nav versus Shaker Kv channels is produced by the hydrophilicity of two "speed-control" residues located in the S2 and S4 segments in Nav domains I-III. An additional 2-fold acceleration of the Nav VS kinetics is provided by the coexpression of the β1 subunit, ubiquitously found in mammal tissues. This study uncovers the molecular bases responsible for the differential activation of Nav versus Kv channels, a fundamental prerequisite for the genesis of action potentials.

Outward stabilization of the voltage sensor in domain II but not domain I speeds inactivation of voltage-gated sodium channels

To determine the roles of the individual S4 segments in domains I and II to activation and inactivation kinetics of sodium current ( INa) in NaV1.5, we used a tethered biotin and avidin approach after a site-directed cysteine substitution was made in the second outermost Arg in each S4 (DI-R2C and DII-R2C). We first determined the fraction of gating charge contributed by the individual S4's to maximal gating current (Qmax), and found that the outermost Arg residue in each S4 contributed ∼19% to Qmax with minimal contributions by other arginines. Stabilization of the S4's in DI-R2C and DII-R2C was confirmed by measuring the expected reduction in Qmax. In DI-R2C, stabilization resulted in a decrease in peak INa of ∼45%, while its peak current-voltage ( I-V) and voltage-dependent Na channel availability (SSI) curves were nearly unchanged from wild type (WT). In contrast, stabilization of the DII-R2C enhanced activation with a negative shift in the peak I-V relationship by −7 mV and a larger −17 mV shift in the voltage-dependent SSI curve. Furthermore, its INa decay time constants and time-to-peak INa became more rapid than WT. An explanation for these results is that the depolarized conformation of DII-S4, but not DI-S4, affects the receptor for the inactivation particle formed by the interdomain linker between DIII and IV. In addition, the leftward shifts of both activation and inactivation and the decrease in Gmax after stabilization of the DII-S4 support previous studies that showed β-scorpion toxins trap the voltage sensor of DII in an activated conformation.

Opening the shaker K+ channel with hanatoxin

Voltage-activated ion channels open and close in response to changes in membrane voltage, a property that is fundamental to the roles of these channels in electrical signaling. Protein toxins from venomous organisms commonly target the S1–S4 voltage-sensing domains in these channels and modify their gating properties. Studies on the interaction of hanatoxin with the Kv2.1 channel show that this tarantula toxin interacts with the S1–S4 domain and inhibits opening by stabilizing a closed state. Here we investigated the interaction of hanatoxin with the Shaker Kv channel, a voltage-activated channel that has been extensively studied with biophysical approaches. In contrast to what is observed in the Kv2.1 channel, we find that hanatoxin shifts the conductance–voltage relation to negative voltages, making it easier to open the channel with membrane depolarization. Although these actions of the toxin are subtle in the wild-type channel, strengthening the toxin–channel interaction with mutations in the S3b helix of the S1-S4 domain enhances toxin affinity and causes large shifts in the conductance–voltage relationship. Using a range of previously characterized mutants of the Shaker Kv channel, we find that hanatoxin stabilizes an activated conformation of the voltage sensors, in addition to promoting opening through an effect on the final opening transition. Chimeras in which S3b–S4 paddle motifs are transferred between Kv2.1 and Shaker Kv channels, as well as experiments with the related tarantula toxin GxTx-1E, lead us to conclude that the actions of tarantula toxins are not simply a product of where they bind to the channel, but that fine structural details of the toxin–channel interface determine whether a toxin is an inhibitor or opener.

S1-S3 counter charges in the voltage sensor module of a mammalian sodium channel regulate fast inactivation

The movement of positively charged S4 segments through the electric field drives the voltage-dependent gating of ion channels. Studies of prokaryotic sodium channels provide a mechanistic view of activation facilitated by electrostatic interactions of negatively charged residues in S1 and S2 segments, with positive counterparts in the S4 segment. In mammalian sodium channels, S4 segments promote domain-specific functions that include activation and several forms of inactivation. We tested the idea that S1-S3 countercharges regulate eukaryotic sodium channel functions, including fast inactivation. Using structural data provided by bacterial channels, we constructed homology models of the S1-S4 voltage sensor module (VSM) for each domain of the mammalian skeletal muscle sodium channel hNaV1.4. These show that side chains of putative countercharges in hNaV1.4 are oriented toward the positive charge complement of S4. We used mutagenesis to define the roles of conserved residues in the extracellular negative charge cluster (ENC), hydrophobic charge region (HCR), and intracellular negative charge cluster (INC). Activation was inhibited with charge-reversing VSM mutations in domains I-III. Charge reversal of ENC residues in domains III (E1051R, D1069K) and IV (E1373K, N1389K) destabilized fast inactivation by decreasing its probability, slowing entry, and accelerating recovery. Several INC mutations increased inactivation from closed states and slowed recovery. Our results extend the functional characterization of VSM countercharges to fast inactivation, and support the premise that these residues play a critical role in domain-specific gating transitions for a mammalian sodium channel.

Inhibitory effect of the recombinant Phoneutria nigriventer Tx1 toxin on voltage-gated sodium channels

Phoneutria nigriventer toxin Tx1 (PnTx1, also referred to in the literature as Tx1) exerts inhibitory effect on neuronal (Na(V)1.2) sodium channels in a way dependent on the holding potential, and competes with μ-conotoxins but not with tetrodotoxin for their binding sites. In the present study we investigated the electrophysiological properties of the recombinant toxin (rPnTx1), which has the complete amino acid sequence of the natural toxin with 3 additional residues: AM on the N-terminal and G on the C-terminal. At the concentration of 1.5 μM, the recombinant toxin inhibits Na(+) currents of dorsal root ganglia neurons (38.4 ± 6.1% inhibition at -80 mV holding potential) and tetrodotoxin-resistant Na(+) currents (26.2 ± 4.9% at the same holding potential). At -50 mV holding potential the inhibition of the total current reached 71.3 ± 2.3% with 1.5 μM rPnTx1. The selectivity of rPnTx1 was investigated on ten different isoforms of voltage-gated sodium channels expressed in Xenopus oocytes. The order of potency for rPnTx1 was: rNa(V)1.2 > rNa(V)1.7 ≈ rNa(V)1.4 ≥ rNa(V)1.3 > mNa(V)1.6 ≥ hNa(V)1.8. No effect was seen on hNa(V)1.5 and on the arthropods isoforms (DmNa(V)1, BGNa(V)1.1a and VdNa(V)1). The IC(50) for Na(V)1.2 was 33.7 ± 2.9 nM with a maximum inhibition of 83.3 ± 1.9%. The toxin did not alter the voltage-dependence of channel gating and was effective on Na(V)1.2 channels devoid of inactivation. It was ineffective on neuronal calcium channels. We conclude that rPnTx1 has a promising selectivity, and that it may be a valuable model to achieve pharmacological activities of interest for the treatment of channelopathies and neuropathic pain.

Scorpion β-toxin interference with NaV channel voltage sensor gives rise to excitatory and depressant modes

Scorpion β toxins, peptides of ∼70 residues, specifically target voltage-gated sodium (Na_V) channels to cause use-dependent subthreshold channel openings via a voltage–sensor trapping mechanism. This excitatory action is often overlaid by a not yet understood depressant mode in which Na_V channel activity is inhibited. Here, we analyzed these two modes of gating modification by β-toxin Tz1 from Tityus zulianus on heterologously expressed Na_V1.4 and Na_V1.5 channels using the whole cell patch-clamp method. Tz1 facilitated the opening of Na_V1.4 in a use-dependent manner and inhibited channel opening with a reversed use dependence. In contrast, the opening of Na_V1.5 was exclusively inhibited without noticeable use dependence. Using chimeras of Na_V1.4 and Na_V1.5 channels, we demonstrated that gating modification by Tz1 depends on the specific structure of the voltage sensor in domain 2. Although residue G658 in Na_V1.4 promotes the use-dependent transitions between Tz1 modification phenotypes, the equivalent residue in Na_V1.5, N803, abolishes them. Gating charge neutralizations in the Na_V1.4 domain 2 voltage sensor identified arginine residues at positions 663 and 669 as crucial for the outward and inward movement of this sensor, respectively. Our data support a model in which Tz1 can stabilize two conformations of the domain 2 voltage sensor: a preactivated outward position leading to Na_V channels that open at subthreshold potentials, and a deactivated inward position preventing channels from opening. The results are best explained by a two-state voltage–sensor trapping model in that bound scorpion β toxin slows the activation as well as the deactivation kinetics of the voltage sensor in domain 2.

Structure-function map of the receptor site for β-scorpion toxins in domain II of voltage-gated sodium channels

Voltage-gated sodium (Na(v)) channels are the molecular targets of β-scorpion toxins, which shift the voltage dependence of activation to more negative membrane potentials by a voltage sensor-trapping mechanism. Molecular determinants of β-scorpion toxin (CssIV) binding and action on rat brain sodium channels are located in the S1-S2 (IIS1-S2) and S3-S4 (IIS3-S4) extracellular linkers of the voltage-sensing module in domain II. In IIS1-S2, mutations of two amino acid residues (Glu(779) and Pro(782)) significantly altered the toxin effect by reducing binding affinity. In IIS3-S4, six positions surrounding the key binding determinant, Gly(845), define a hot spot of high-impact residues. Two of these substitutions (A841N and L846A) reduced voltage sensor trapping. The other three substitutions (N842R, V843A, and E844N) increased voltage sensor trapping. These bidirectional effects suggest that the IIS3-S4 loop plays a primary role in determining both toxin affinity and efficacy. A high resolution molecular model constructed with the Rosetta-Membrane modeling system reveals interactions of amino acid residues in sodium channels that are crucial for toxin action with residues in CssIV that are required for its effects. In this model, the wedge-shaped CssIV inserts between the IIS1-S2 and IIS3-S4 loops of the voltage sensor, placing key amino acid residues in position to interact with binding partners in these extracellular loops. These results provide new molecular insights into the voltage sensor-trapping model of toxin action and further define the molecular requirements for the development of antagonists that can prevent or reverse toxicity of scorpion toxins.

Functional properties and toxin pharmacology of a dorsal root ganglion sodium channel viewed through its voltage sensors

The voltage-activated sodium (Nav) channel Nav1.9 is expressed in dorsal root ganglion (DRG) neurons where it is believed to play an important role in nociception. Progress in revealing the functional properties and pharmacological sensitivities of this non-canonical Nav channel has been slow because attempts to express this channel in a heterologous expression system have been unsuccessful. Here, we use a protein engineering approach to dissect the contributions of the four Nav1.9 voltage sensors to channel function and pharmacology. We define individual S3b–S4 paddle motifs within each voltage sensor, and show that they can sense changes in membrane voltage and drive voltage sensor activation when transplanted into voltage-activated potassium channels. We also find that the paddle motifs in Nav1.9 are targeted by animal toxins, and that these toxins alter Nav1.9-mediated currents in DRG neurons. Our results demonstrate that slowly activating and inactivating Nav1.9 channels have functional and pharmacological properties in common with canonical Nav channels, but also show distinctive pharmacological sensitivities that can potentially be exploited for developing novel treatments for pain.

Molecular basis of the tarantula toxin jingzhaotoxin-III (β-TRTX-Cj1α) interacting with voltage sensors in sodium channel subtype Nav1.5

With conserved structural scaffold and divergent electrophysiological functions, animal toxins are considered powerful tools for investigating the basic structure-function relationship of voltage-gated sodium channels. Jingzhaotoxin-III (β-TRTX-Cj1α) is a unique sodium channel gating modifier from the tarantula Chilobrachys jingzhao, because the toxin can selectively inhibit the activation of cardiac sodium channel but not neuronal subtypes. However, the molecular basis of JZTX-III interaction with sodium channels remains unknown. In this study, we showed that JZTX-III was efficiently expressed by the secretory pathway in yeast. Alanine-scanning analysis indicated that 2 acidic residues (Asp1, Glu3) and an exposed hydrophobic patch, formed by 4 Trp residues (residues 8, 9, 28 and 30), play important roles in the binding of JZTX-III to Nav1.5. JZTX-III docked to the Nav1.5 DIIS3-S4 linker. Mutations S799A, R800A, and L804A could additively reduce toxin sensitivity of Nav1.5. We also demonstrated that the unique Arg800, not emerging in other sodium channel subtypes, is responsible for JZTX-III selectively interacting with Nav1.5. The reverse mutation D816R in Nav1.7 greatly increased the sensitivity of the neuronal subtype to JZTX-III. Conversely, the mutation R800D in Nav1.5 decreased JZTX-III's IC₅₀ by 72-fold. Therefore, our results indicated that JZTX-III is a site 4 toxin, but does not possess the same critical residues on sodium channels as other site 4 toxins. Our data also revealed the underlying mechanism for JZTX-III to be highly specific for the cardiac sodium channel.

Substitutions in the domain III voltage-sensing module enhance the sensitivity of an insect sodium channel to a scorpion beta-toxin

Scorpion β-toxins bind to the extracellular regions of the voltage-sensing module of domain II and to the pore module of domain III in voltage-gated sodium channels and enhance channel activation by trapping and stabilizing the voltage sensor of domain II in its activated state. We investigated the interaction of a highly potent insect-selective scorpion depressant β-toxin, Lqh-dprIT(3), from Leiurus quinquestriatus hebraeus with insect sodium channels from Blattella germanica (BgNa(v)). Like other scorpion β-toxins, Lqh-dprIT(3) shifts the voltage dependence of activation of BgNa(v) channels expressed in Xenopus oocytes to more negative membrane potentials but only after strong depolarizing prepulses. Notably, among 10 BgNa(v) splice variants tested for their sensitivity to the toxin, only BgNa(v)1-1 was hypersensitive due to an L1285P substitution in IIIS1 resulting from a U-to-C RNA-editing event. Furthermore, charge reversal of a negatively charged residue (E1290K) at the extracellular end of IIIS1 and the two innermost positively charged residues (R4E and R5E) in IIIS4 also increased the channel sensitivity to Lqh-dprIT(3). Besides enhancement of toxin sensitivity, the R4E substitution caused an additional 20-mV negative shift in the voltage dependence of activation of toxin-modified channels, inducing a unique toxin-modified state. Our findings provide the first direct evidence for the involvement of the domain III voltage-sensing module in the action of scorpion β-toxins. This hypersensitivity most likely reflects an increase in IIS4 trapping via allosteric mechanisms, suggesting coupling between the voltage sensors in neighboring domains during channel activation.

Open- and closed-state fast inactivation in sodium channels: differential effects of a site-3 anemone toxin

The role of sodium channel closed-state fast inactivation in membrane excitability is not well understood. We compared open- and closed-state fast inactivation, and the gating charge immobilized during these transitions, in skeletal muscle channel hNa(V)1.4. A significant fraction of total charge movement and its immobilization occurred in the absence of channel opening. Simulated action potentials in skeletal muscle fibers were attenuated when pre-conditioned by sub-threshold depolarization. Anthopleurin A, a site-3 toxin that inhibits gating charge associated with the movement of DIVS4, was used to assess the role of this voltage sensor in closed-state fast inactivation. Anthopleurin elicited opposing effects on the gating mode, kinetics and charge immobilized during open- versus closed-state fast inactivation. This same toxin produced identical effects on recovery of channel availability and remobilization of gating charge, irrespective of route of entry into fast inactivation. Our findings suggest that depolarization promoting entry into fast inactivation from open versus closed states provides access to the IFMT receptor via different rate-limiting conformational translocations of DIVS4.

Inhibition of the activation pathway of the T-type calcium channel Ca(V)3.1 by ProTxII

Toxins have been used extensively to probe the gating mechanisms of voltage-gated ion channels. Relatively few such tools are available to study the low-voltage activated T-type Ca channels, which underlie thalamic neuron firing and affect sleep, resistance to seizures, and weight gain. Here we show that ProTxII, a peptide toxin recently isolated from the venom of the tarantula spider Thrixopelma pruriens, dose-dependently inhibited Ca(V)3.1 causing a decrease in current (81.6% +/- 3.1% at -30 mV in 5 microM toxin) and a positive shift in the voltage range of activation (+34.5 mV +/- 4.4 mV). Toxin-modified currents were slower to activate and faster to deactivate and they displayed a longer lag in the onset of current, i.e. the Cole-Moore shift, consistent with the inhibition of gating transitions along the activation pathway, particularly the final opening transition. Single-channel current amplitude and total gating charge were unaffected by toxin, ruling out a change in ion flux or channel dropout as mechanisms for the decrease in macroscopic conductance. A positive shift in the voltage range of gating charge movement (+30.6 mV +/- 2.6 mV shift in the voltage of half maximal charge movement in the presence of 5 microM toxin) confirmed that ProTxII-induced gating perturbations in this channel occur at the level of the voltage sensors, and kinetic modeling based on these findings suggested that reductions in current magnitude could be largely accounted for by kinetic perturbations of activation.

Use of voltage clamp fluorimetry in understanding potassium channel gating: a review of Shaker fluorescence data

Voltage clamp fluorimetry (VCF) utilizes fluorescent probes that covalently bind to cysteine residues introduced into proteins and emit light as a function of their environment. Measurement of this emitted light during membrane depolarization reveals changes in the emission level as the environment of the labelled residue changes. This allows for the correlation of channel gating events with movement of specific protein moieties, at nanosecond time resolution. Since the pioneering use of this technique to investigate Shaker potassium channel activation movements, VCF has become an invaluable technique used to understand ion channel gating. This review summarizes the theory and some of the data on the application of the VCF technique. Although its usage has expanded beyond voltage-gated potassium channels and VCF is now used in a number of other voltage- and ligand-gated channels, we will focus on studies conducted in Shaker potassium channels, and what they have told us about channel activation and inactivation gating.

Targeting voltage sensors in sodium channels with spider toxins

Voltage-activated sodium (Nav) channels are essential in generating and propagating nerve impulses, placing them amongst the most widely targeted ion channels by toxins from venomous organisms. An increasing number of spider toxins have been shown to interfere with the voltage-driven activation process of mammalian Nav channels, possibly by interacting with one or more of their voltage sensors. This review focuses on our existing knowledge of the mechanism by which spider toxins affect Nav channel gating and the possible applications of these toxins in the drug discovery process.

Differential effects of Tityus bahiensis scorpion venom on tetrodotoxin-sensitive and tetrodotoxin-resistant sodium currents

We examined modification of sodium channel gating by Tityus bahiensis scorpion venom (TbScV), and compared effects on native tetrodotoxin-sensitive and tetrodotoxin-resistant sodium currents from rat dorsal root ganglion neurons and cardiac myocytes. In neurons, TbScV dramatically reduced the rate of sodium current inactivation, increased current amplitude, and caused a negative shift in the voltage-dependence of activation and inactivation of tetrodotoxin-sensitive channels. Enhanced activation of modified sodium channels was independent of a depolarizing prepulse. We identified two components of neuronal tetrodotoxin-resistant current with biophysical properties similar to those described for NaV1.8 and NaV1.9. In contrast to its effects on neuronal tetrodotoxin-sensitive current, TbScV caused a small decrease in neuronal tetrodotoxin-resistant sodium current amplitude and the gating modifications described above were absent. A third tetrodotoxin-resistant current, NaV1.5 recorded in rat cardiac ventricular myocytes, was inhibited approximately 50% by TbScV, and the remaining current exhibited markedly slowed activation and inactivation. In conclusion, TbScV has very different effects on different sodium channel isoforms. Among the neuronal types, currents resistant to tetrodotoxin are also resistant to gating modification by TbScV. The cardiac tetrodotoxin-resistant current has complex sensitivity that includes both inhibition of current amplitude and slowing of activation and inactivation.