Site-3 toxins and cardiac sodium channels

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

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

Differential regulation of cardiac sodium channels by intracellular fibroblast growth factors

Voltage-gated sodium (NaV) channels are responsible for the initiation and propagation of action potentials. In the heart, the predominant NaV1.5 α subunit is composed of four homologous repeats (I-IV) and forms a macromolecular complex with multiple accessory proteins, including intracellular fibroblast growth factors (iFGF). In spite of high homology, each of the iFGFs, iFGF11-iFGF14, as well as the individual iFGF splice variants, differentially regulates NaV channel gating, and the mechanisms underlying these differential effects remain elusive. Much of the work exploring iFGF regulation of NaV1.5 has been performed in mouse and rat ventricular myocytes in which iFGF13VY is the predominant iFGF expressed, whereas investigation into NaV1.5 regulation by the human heart-dominant iFGF12B is lacking. In this study, we used a mouse model with cardiac-specific Fgf13 deletion to study the consequences of iFGF13VY and iFGF12B expression. We observed distinct effects on the voltage-dependences of activation and inactivation of the sodium currents (INa), as well as on the kinetics of peak INa decay. Results in native myocytes were recapitulated with human NaV1.5 heterologously expressed in Xenopus oocytes, and additional experiments using voltage-clamp fluorometry (VCF) revealed iFGF-specific effects on the activation of the NaV1.5 voltage sensor domain in repeat IV (VSD-IV). iFGF chimeras further unveiled roles for all three iFGF domains (i.e., the N-terminus, core, and C-terminus) on the regulation of VSD-IV, and a slower time domain of inactivation. We present here a novel mechanism of iFGF regulation that is specific to individual iFGF isoforms and that leads to distinct functional effects on NaV channel/current kinetics.

Historical Perspective of the Characterization of Conotoxins Targeting Voltage-Gated Sodium Channels

Marine toxins have potent actions on diverse sodium ion channels regulated by transmembrane voltage (voltage-gated ion channels) or by neurotransmitters (nicotinic acetylcholine receptor channels). Studies of these toxins have focused on varied aspects of venom peptides ranging from evolutionary relationships of predator and prey, biological actions on excitable tissues, potential application as pharmacological intervention in disease therapy, and as part of multiple experimental approaches towards an understanding of the atomistic characterization of ion channel structure. This review examines the historical perspective of the study of conotoxin peptides active on sodium channels gated by transmembrane voltage, which has led to recent advances in ion channel research made possible with the exploitation of the diversity of these marine toxins.

Characterization of the First Animal Toxin Acting as an Antagonist on AT1 Receptor

The renin-angiotensin system (RAS) is one of the main regulatory systems of cardiovascular homeostasis. It is mainly composed of angiotensin-converting enzyme (ACE) and angiotensin II receptors AT1 and AT2. ACE and AT1 are targets of choice for the treatment of hypertension, whereas the AT2 receptor is still not exploited due to the lack of knowledge of its physiological properties. Peptide toxins from venoms display multiple biological functions associated with varied chemical and structural properties. If Brazilian viper toxins have been described to inhibit ACE, no animal toxin is known to act on AT1/AT2 receptors. We screened a library of toxins on angiotensin II receptors with a radioligand competition binding assay. Functional characterization of the selected toxin was conducted by measuring second messenger production, G-protein activation and β-arrestin 2 recruitment using bioluminescence resonance energy transfer (BRET) based biosensors. We identified one original toxin, A-CTX-cMila, which is a 7-residues cyclic peptide from Conus miliaris with no homology sequence with known angiotensin peptides nor identified toxins, displaying a 100-fold selectivity for AT1 over AT2. This toxin shows a competitive antagonism mode of action on AT1, blocking Gαq, Gαi3, GαoA, β-arrestin 2 pathways and ERK_1/2 activation. These results describe the first animal toxin active on angiotensin II receptors.

The Sea Anemone Neurotoxins Modulating Sodium Channels: An Insight at Structure and Functional Activity after Four Decades of Investigation

Many human cardiovascular and neurological disorders (such as ischemia, epileptic seizures, traumatic brain injury, neuropathic pain, etc.) are associated with the abnormal functional activity of voltage-gated sodium channels (VGSCs/NaVs). Many natural toxins, including the sea anemone toxins (called neurotoxins), are an indispensable and promising tool in pharmacological researches. They have widely been carried out over the past three decades, in particular, in establishing different NaV subtypes functional properties and a specific role in various pathologies. Therefore, a large number of publications are currently dedicated to the search and study of the structure-functional relationships of new sea anemone natural neurotoxins-potential pharmacologically active compounds that specifically interact with various subtypes of voltage gated sodium channels as drug discovery targets. This review presents and summarizes some updated data on the structure-functional relationships of known sea anemone neurotoxins belonging to four structural types. The review also emphasizes the study of type 2 neurotoxins, produced by the tropical sea anemone Heteractis crispa, five structurally homologous and one unique double-stranded peptide that, due to the absence of a functionally significant Arg14 residue, loses toxicity but retains the ability to modulate several VGSCs subtypes.

µ-Theraphotoxin Pn3a inhibition of CaV3.3 channels reveals a novel isoform-selective drug binding site

Low voltage-activated calcium currents are mediated by T-type calcium channels CaV3.1, CaV3.2, and CaV3.3, which modulate a variety of physiological processes including sleep, cardiac pace-making, pain, and epilepsy. CaV3 isoforms' biophysical properties, overlapping expression, and lack of subtype-selective pharmacology hinder the determination of their specific physiological roles in health and disease. We have identified μ-theraphotoxin Pn3a as the first subtype-selective spider venom peptide inhibitor of CaV3.3, with >100-fold lower potency against the other T-type isoforms. Pn3a modifies CaV3.3 gating through a depolarizing shift in the voltage dependence of activation thus decreasing CaV3.3-mediated currents in the normal range of activation potentials. Paddle chimeras of KV1.7 channels bearing voltage sensor sequences from all four CaV3.3 domains revealed preferential binding of Pn3a to the S3-S4 region of domain II (CaV3.3DII). This novel T-type channel pharmacological site was explored through computational docking simulations of Pn3a, site-directed mutagenesis, and full domain II swaps between CaV3 channels highlighting it as a subtype-specific pharmacophore. This research expands our understanding of T-type calcium channel pharmacology and supports the suitability of Pn3a as a molecular tool in the study of the physiological roles of CaV3.3 channels.

The Pharmacological and Structural Basis of the AahII–NaV1.5 Interaction and Modulation by the Anti-AahII Nb10 Nanobody

Scorpion α-toxins are neurotoxins that target the fast inactivation mechanism of voltage-gated sodium (Na_V) channels leading to several neuro- and cardiotoxic effects in mammals. The toxin AahII is the most active α-toxin from the North African scorpion Androctonus australis Hector that slows the fast inactivation of Na_V channels. To fight scorpion envenomation, an anti-AahII nanobody named NbAahII10 (Nb10) was developed. The efficiency of this nanobody has been evaluated in vivo on mice, but its mechanism of action at the cellular level remains unknown. Here we have shown that AahII toxin slows the fast inactivation of the adult cardiac Na_V1.5 channels, expressed in HEK293 cells, in a dose-dependent manner, while current amplitude was not affected. The inactivation of Na_V1.5 is slower by a factor of 4, 7, and 35 in the presence of [AahII] at 75, 150, and 300 nM, respectively. The washout partially reversed the toxin effect on inactivation from 8.3 ± 0.9 ms to 5.2 ± 1.2 ms at 75 nM. We have also demonstrated that the highly neutralizing Nb10 can fully reverse the effect of AahII toxin on the channel inactivation kinetics even at the 1:1 M ratio. However, the 1:0.5 M ratio is not able to neutralize completely the AahII effect. Therefore, the application of Nb10 promotes a partial abolishment of AahII action. Bioinformatic analysis and prediction of Na_V1.5-driven docking with AahII show that Ala39 and Arg62 of AahII play a crucial role to establish a stable interaction through H-bound interactions with Gln1615 and Lys1616 (S3–S4 extracellular loop) and Asp1553 (S1–S2 loop) from the voltage-sensing domain IV (VSD4) of Na_V1.5, respectively. From this, we notice that AahII shares the same contact surface with Nb10. This strongly suggests that Nb10 dynamically replaces AahII toxin from its binding site on the Na_V1.5 channel. At the physiopathological level, Nb10 completely neutralized the enhancement of breast cancer cell invasion induced by AahII. In summary, for the first time, we made an electrophysiological and structural characterization of the neutralization potent of Nb10 against the α-scorpion toxin AahII in a cellular model overexpressing Na_V1.5 channels.

Structural Pharmacology of Voltage-Gated Sodium Channels

Voltage-gated sodium (Na_V) channels initiate and propagate action potentials in excitable tissues to mediate key physiological processes including heart contraction and nervous system function. Accordingly, Na_V channels are major targets for drugs, toxins and disease-causing mutations. Recent breakthroughs in cryo-electron microscopy have led to the visualization of human Na_V1.1, Na_V1.2, Na_V1.4, Na_V1.5 and Na_V1.7 channel subtypes at high-resolution. These landmark studies have greatly advanced our structural understanding of channel architecture, ion selectivity, voltage-sensing, electromechanical coupling, fast inactivation, and the molecular basis underlying Na_V channelopathies. Na_V channel structures have also been increasingly determined in complex with toxin and small molecule modulators that target either the pore module or voltage sensor domains. These structural studies have provided new insights into the mechanisms of pharmacological action and opportunities for subtype-selective Na_V channel drug design. This review will highlight the structural pharmacology of human Na_V channels as well as the potential use of engineered and chimeric channels in future drug discovery efforts.

Effects of FGF14 and NaVβ4 deletion on transient and resurgent Na current in cerebellar Purkinje neurons

Voltage-gated Na channels of Purkinje cells are specialized to maintain high availability during high-frequency repetitive firing. They enter fast-inactivated states relatively slowly and undergo a voltage-dependent open-channel block by an intracellular protein (or proteins) that prevents stable fast inactivation and generates resurgent Na current. These properties depend on the pore-forming α subunits, as well as modulatory subunits within the Na channel complex. The identity of the factors responsible for open-channel block remains a question. Here we investigate the effects of genetic mutation of two Na channel auxiliary subunits highly expressed in Purkinje cells, Na_Vβ4 and FGF14, on modulating Na channel blocked as well as inactivated states. We find that although both Na_Vβ4 and the FGF14 splice variant FGF14-1a contain sequences that can generate resurgent-like currents when applied to Na channels in peptide form, deletion of either protein, or both proteins simultaneously, does not eliminate resurgent current in acutely dissociated Purkinje cell bodies. Loss of FGF14 expression does, however, reduce resurgent current amplitude and leads to an acceleration and stabilization of inactivation that is not reversed by application of the site-3 toxin, anemone toxin II (ATX). Tetrodotoxin (TTX) sensitivity is higher for resurgent than transient components of Na current, and loss of FGF14 preferentially affects a highly TTX-sensitive subset of Purkinje α subunits. The data suggest that Na_V1.6 channels, which are known to generate the majority of Purkinje cell resurgent current, bind TTX with high affinity and are modulated by FGF14 to facilitate open-channel block.

Structural basis of α-scorpion toxin action on Nav channels

Fast inactivation of voltage-gated sodium (Na_v) channels is essential for electrical signaling, but its mechanism remains poorly understood. Here we determined the structures of a eukaryotic Na_v channel alone and in complex with a lethal α-scorpion toxin, AaH2, by electron microscopy, both at 3.5-angstrom resolution. AaH2 wedges into voltage-sensing domain IV (VSD4) to impede fast activation by trapping a deactivated state in which gating charge interactions bridge to the acidic intracellular carboxyl-terminal domain. In the absence of AaH2, the S4 helix of VSD4 undergoes a ~13-angstrom translation to unlatch the intracellular fast-inactivation gating machinery. Highlighting the polypharmacology of α-scorpion toxins, AaH2 also targets an unanticipated receptor site on VSD1 and a pore glycan adjacent to VSD4. Overall, this work provides key insights into fast inactivation, electromechanical coupling, and pathogenic mutations in Na_v channels.

A perspective on Na and K channel inactivation

Armstrong and Hollingworth discuss inactivation in the light of modern structural data from K and Na channels.

We are wired with conducting cables called axons that rapidly transmit electrical signals (e.g., “Ouch!”) from, for example, the toe to the spinal cord. Because of the high internal resistance of axons (salt water rather than copper), a signal must be reinforced after traveling a short distance. Reinforcement is accomplished by ion channels, Na channels for detecting the signal and reinforcing it by driving it further positive (to near 50 mV) and K channels for then restoring it to the resting level (near −70 mV). The signal is called an action potential and has a duration of roughly a millisecond. The return of membrane voltage (V_m) to the resting level after an action potential is facilitated by “inactivation” of the Na channels: i.e., an internal particle diffuses into the mouth of any open Na channel and temporarily blocks it. Some types of K channels also show inactivation after being open for a time. N-type inactivation of K channels has a relatively fast time course and involves diffusion of the N-terminal of one of the channel’s four identical subunits into the channel’s inner mouth, if it is open. This mechanism is similar to Na channel inactivation. Both Na and K channels also display slower inactivation processes. C inactivation in K channels involves changes in the channel’s outer mouth, the “selectivity filter,” whose normal function is to prevent Na⁺ ions from entering the K channel. C inactivation deforms the filter so that neither K⁺ nor Na⁺ can pass.

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

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

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

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

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

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

Cardiac Na Channels: Structure to Function

Heart rhythms arise from electrical activity generated by precisely timed opening and closing of ion channels in individual cardiac myocytes. Opening of the primary cardiac voltage-gated sodium (NaV1.5) channel initiates cellular depolarization and the propagation of an electrical action potential that promotes coordinated contraction of the heart. The regularity of these contractile waves is critically important since it drives the primary function of the heart: to act as a pump that delivers blood to the brain and vital organs. When electrical activity goes awry during a cardiac arrhythmia, the pump does not function, the brain does not receive oxygenated blood, and death ensues. Perturbations to NaV1.5 may alter the structure, and hence the function, of the ion channel and are associated downstream with a wide variety of cardiac conduction pathologies, such as arrhythmias.

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.

Molecular determinant for the tarantula toxin Jingzhaotoxin-I slowing the fast inactivation of voltage-gated sodium channels

Peptide toxins often have divergent pharmacological functions and are powerful tools for a deep review on the current understanding of the structure-function relationships of voltage-gated sodium channels (VGSCs). However, knowing about the interaction of site 3 toxins from tarantula venoms with VGSCs is not sufficient. In the present study, using whole-cell patch clamp technique, we determined the effects of Jingzhaotoxin-I (JZTX-I) on five VGSC subtypes expressed in HEK293 cells. The results showed that JZTX-I could inhibit the inactivation of rNav1.2, rNav1.3, rNav1.4, hNav1.5 and hNav1.7 channels with the IC50 of 870 ± 8 nM, 845 ± 4 nM, 339 ± 5 nM, 335 ± 9 nM, and 348 ± 6 nM, respectively. The affinity of the toxin interaction with subtypes (rNav1.4, hNav1.5, and hNav1.7) was only 2-fold higher than that for subtypes (rNav1.2 and rNav1.3). The toxin delayed the inactivation of VGSCs without affecting the activation and steady-state inactivation kinetics in the physiological range of voltages. Site-directed mutagenesis indicated that the toxin interacted with site 3 located at the extracellular S3-S4 linker of domain IV, and the acidic residue Asp at the position1609 in hNav1.5 was crucial for JZTX-I activity. Our results provide new insights in single key residue that allows toxins to recognize distinct ion channels with similar potency and enhance our understanding of the structure-function relationships of toxin-channel interactions.

Target-Driven Evolution of Scorpion Toxins

It is long known that peptide neurotoxins derived from a diversity of venomous animals evolve by positive selection following gene duplication, yet a force that drives their adaptive evolution remains a mystery. By using maximum-likelihood models of codon substitution, we analyzed molecular adaptation in scorpion sodium channel toxins from a specific species and found ten positively selected sites, six of which are located at the core-domain of scorpion α-toxins, a region known to interact with two adjacent loops in the voltage-sensor domain (DIV) of sodium channels, as validated by our newly constructed computational model of toxin-channel complex. Despite the lack of positive selection signals in these two loops, they accumulated extensive sequence variations by relaxed purifying selection in prey and predators of scorpions. The evolutionary variability in the toxin-bound regions of sodium channels indicates that accelerated substitutions in the multigene family of scorpion toxins is a consequence of dealing with the target diversity. This work presents an example of atypical co-evolution between animal toxins and their molecular targets, in which toxins suffered from more prominent selective pressure from the channels of their competitors. Our discovery helps explain the evolutionary rationality of gene duplication of toxins in a specific venomous species.

Molecular basis of the inhibition of the fast inactivation of voltage-gated sodium channel Nav1.5 by tarantula toxin Jingzhaotoxin-II

Jingzhaotoxin-II (JZTX-II) is a 32-residue peptide from the Chinese tarantula Chilobrachys jingzhao venom, and preferentially inhibits the fast inactivation of the voltage-gated sodium channels (VGSCs) in rat cardiac myocytes. In the present study, we elucidated the action mechanism of JZTX-II inhibiting hNav1.5, a VGSC subtype mainly distributed in human cardiac myocytes. Among the four VGSC subtypes tested, hNav1.5 was the most sensitive to JZTX-II (EC50=125±4nM). Although JZTX-II had little or no effect on steady-state inactivation of the residual currents conducted by hNav1.5, it caused a 10mV hyperpolarized shift of activation. Moreover, JZTX-II increased the recovery rate of hNav1.5 channels, which should lead to a shorter transition from the inactivation to closed state. JZTX-II dissociated from toxin-channel complex via extreme depolarization and subsequently rebound to the channel upon repolarization. Mutagenesis analyses showed that the domain IV (DIV) voltage-sensor domain (VSD) was critical for JZTX-II binding to hNav1.5 and some mutations located in S1-S2 and S3-S4 extracellular loops of hNav1.5 DIV additively reduced the toxin sensitivity of hNav1.5. Our data identified the mechanism underlying JZTX-II inhibiting hNav1.5, similar to scorpion α-toxins, involving binding to neurotoxin receptor site 3.

Resurgent current of voltage-gated Na(+) channels

Resurgent Na(+) current results from a distinctive form of Na(+) channel gating, originally identified in cerebellar Purkinje neurons. In these neurons, the tetrodotoxin-sensitive voltage-gated Na(+) channels responsible for action potential firing have specialized mechanisms that reduce the likelihood that they accumulate in fast inactivated states, thereby shortening refractory periods and permitting rapid, repetitive, and/or burst firing. Under voltage clamp, step depolarizations evoke transient Na(+) currents that rapidly activate and quickly decay, and step repolarizations elicit slower channel reopening, or a 'resurgent' current. The generation of resurgent current depends on a factor in the Na(+) channel complex, probably a subunit such as NaVβ4 (Scn4b), which blocks open Na(+) channels at positive voltages, competing with the fast inactivation gate, and unblocks at negative voltages, permitting recovery from an open channel block along with a flow of current. Following its initial discovery, resurgent Na(+) current has been found in nearly 20 types of neurons. Emerging research suggests that resurgent current is preferentially increased in a variety of clinical conditions associated with altered cellular excitability. Here we review the biophysical, molecular and structural mechanisms of resurgent current and their relation to the normal functions of excitable cells as well as pathophysiology.

Sequence variations at I260 and A1731 contribute to persistent currents in Drosophila sodium channels

Tetrodotoxin-sensitive persistent sodium currents, INaP, that activate at subthreshold voltages, have been detected in numerous vertebrate and invertebrate neurons. These currents are believed to be critical for regulating neuronal excitability. However, the molecular mechanism underlying INaP is controversial. In this study, we identified an INaP with a broad range of voltage dependence, from -60mV to 20mV, in a Drosophila sodium channel variant expressed in Xenopus oocytes. Mutational analysis revealed that two variant-specific amino acid changes, I260T in the S4-S5 linker of domain I (ILS4-S5) and A1731V in the voltage sensor S4 of domain IV (IVS4), contribute to the INaP. I260T is critical for the portion of INaP at hyperpolarized potentials. The T260-mediated INaP is likely the result of window currents flowing in the voltage range where the activation and inactivation curves overlap. A1731V is responsible for impaired inactivation and contributes to the portion of INaP at depolarized potentials. Furthermore, A1731V causes enhanced activity of two site-3 toxins which induce persistent currents by inhibiting the outward movement of IVS4, suggesting that A1731V inhibits the outward movement of IVS4. These results provided molecular evidence for the involvement of distinct mechanisms in the generation of INaP: T260 contributes to INaP via enhancement of the window current, whereas V1731 impairs fast inactivation probably by inhibiting the outward movement of IVS4.

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.

Electrophysiological characteristics of a SCN5A voltage sensors mutation R1629Q associated with Brugada syndrome

Brugada syndrome (BrS) is an inherited arrhythmogenic syndrome leading to sudden cardiac death, partially associated with autosomal dominant mutations in SCN5A, which encodes the cardiac sodium channel alpha-subunit (Nav1.5). To date some SCN5A mutations related with BrS have been identified in voltage sensor of Nav1.5. Here, we describe a dominant missense mutation (R1629Q) localized in the fourth segment of domain IV region (DIV-S4) in a Chinese Han family. The mutation was identified by direct sequencing of SCN5A from the proband's DNA. Co-expression of Wild-type (WT) or R1629Q Nav1.5 channel and hβ1 subunit were achieved in human embryonic kidney cells by transient transfection. Sodium currents were recorded using whole cell patch-clamp protocols. No significant changes between WT and R1629Q currents were observed in current density or steady-state activation. However, hyperpolarized shift of steady-state inactivation curve was identified in cells expressing R1629Q channel (WT: V1/2 = -81.1 ± 1.3 mV, n = 13; R1629Q: V1/2 = -101.7 ± 1.2 mV, n = 18). Moreover, R1629Q channel showed enhanced intermediate inactivation and prolonged recovery time from inactivation. In summary, this study reveals that R1629Q mutation causes a distinct loss-of-function of the channel due to alter its electrophysiological characteristics, and facilitates our understanding of biophysical mechanisms of BrS.

Domain IV voltage-sensor movement is both sufficient and rate limiting for fast inactivation in sodium channels

Voltage-gated sodium channels are critical for the generation and propagation of electrical signals in most excitable cells. Activation of Na(+) channels initiates an action potential, and fast inactivation facilitates repolarization of the membrane by the outward K(+) current. Fast inactivation is also the main determinant of the refractory period between successive electrical impulses. Although the voltage sensor of domain IV (DIV) has been implicated in fast inactivation, it remains unclear whether the activation of DIV alone is sufficient for fast inactivation to occur. Here, we functionally neutralize each specific voltage sensor by mutating several critical arginines in the S4 segment to glutamines. We assess the individual role of each voltage-sensing domain in the voltage dependence and kinetics of fast inactivation upon its specific inhibition. We show that movement of the DIV voltage sensor is the rate-limiting step for both development and recovery from fast inactivation. Our data suggest that activation of the DIV voltage sensor alone is sufficient for fast inactivation to occur, and that activation of DIV before channel opening is the molecular mechanism for closed-state inactivation. We propose a kinetic model of sodium channel gating that can account for our major findings over a wide voltage range by postulating that DIV movement is both necessary and sufficient for fast inactivation.

Antagonism of lidocaine inhibition by open-channel blockers that generate resurgent Na current

Na channels that generate resurgent current express an intracellular endogenous open-channel blocking protein, whose rapid binding upon depolarization and unbinding upon repolarization minimizes fast and slow inactivation. Na channels also bind exogenous compounds, such as lidocaine, which functionally stabilize inactivation. Like the endogenous blocking protein, these use-dependent inhibitors bind most effectively at depolarized potentials, raising the question of how lidocaine-like compounds affect neurons with resurgent Na current. We therefore recorded lidocaine inhibition of voltage-clamped, tetrodotoxin-sensitive Na currents in mouse Purkinje neurons, which express a native blocking protein, and in mouse hippocampal CA3 pyramidal neurons with and without a peptide from the cytoplasmic tail of NaVβ4 (the β4 peptide), which mimics endogenous open-channel block. To control channel states during drug exposure, lidocaine was applied with rapid-solution exchange techniques during steps to specific voltages. Inhibition of Na currents by lidocaine was diminished by either the β4 peptide or the native blocking protein. In peptide-free CA3 cells, prolonging channel opening with a site-3 toxin, anemone toxin II, reduced lidocaine inhibition; this effect was largely occluded by open-channel blockers, suggesting that lidocaine binding is favored by inactivation but prevented by open-channel block. In constant 100 μm lidocaine, current-clamped Purkinje cells continued to fire spontaneously. Similarly, the β4 peptide reduced lidocaine-dependent suppression of spiking in CA3 neurons in slices. Thus, the open-channel blocking protein responsible for resurgent current acts as a natural antagonist of lidocaine. Neurons with resurgent current may therefore be less susceptible to use-dependent Na channel inhibitors used as local anesthetic, antiarrhythmic, and anticonvulsant drugs.

Purification and cDNA cloning of a novel neurotoxic peptide (Acra3) from the scorpion Androctonus crassicauda

Androctonus crassicauda is one of the Southeastern Anatolian scorpions of Turkey with ethno-medical and toxicological importance. Two toxic peptides (Acra1 and Acra2) were isolated and characterized from the venom of this scorpion. In this communication, the isolation of an additional toxin (Acra3) by chromatographic separations (HPLC and TSK-gel sulfopropyl) and its chemical and functional characterization is reported. Acra3 is a 7620Da molecular weight peptide, with 66 amino acid residues crosslinked by four disulfide bridges. The gene coding for this peptide was cloned and sequenced. Acra3 is anticipated to undergo post-translational modifications at the C-terminal region, having an amidated serine as last residue. Injection of Acra3 induces severe neurotoxic events in mice, such as: excitability and convulsions, leading to the death of the animals within a few minutes after injection. Electrophysiological assays conducted with pure Acra3, using cells that specifically expressed sodium channels (Nav1.1-Nav1.6) showed no clear effect. The exact molecular target of Acra3 remained undiscovered, similar to three other scorpion peptides that clustered very closely in the phylogenetic tree included here. The exact target of these four peptides is not very clear.

Modulation of neuronal sodium channels by the sea anemone peptide BDS-I

Blood-depressing substance I (BDS-I), a 43 amino-acid peptide from sea anemone venom, is used as a specific inhibitor of Kv3-family potassium channels. We found that BDS-I acts with even higher potency to modulate specific types of voltage-dependent sodium channels. In rat dorsal root ganglion (DRG) neurons, 3 μM BDS-I strongly enhanced tetrodotoxin (TTX)-sensitive sodium current but weakly inhibited TTX-resistant sodium current. In rat superior cervical ganglion (SCG) neurons, which express only TTX-sensitive sodium current, BDS-I enhanced current elicited by small depolarizations and slowed decay of currents at all voltages (EC(50) ∼ 300 nM). BDS-I acted with exceptionally high potency and efficacy on cloned human Nav1.7 channels, slowing inactivation by 6-fold, with an EC(50) of approximately 3 nM. BDS-I also slowed inactivation of sodium currents in N1E-115 neuroblastoma cells (mainly from Nav1.3 channels), with an EC(50) ∼ 600 nM. In hippocampal CA3 pyramidal neurons (mouse) and cerebellar Purkinje neurons (mouse and rat), BDS-I had only small effects on current decay (slowing inactivation by 20-50%), suggesting relatively weak sensitivity of Nav1.1 and Nav1.6 channels. The biggest effect of BDS-I in central neurons was to enhance resurgent current in Purkinje neurons, an effect reflected in enhancement of sodium current during the repolarization phase of Purkinje neuron action potentials. Overall, these results show that BDS-I acts to modulate sodium channel gating in a manner similar to previously known neurotoxin receptor site 3 anemone toxins but with different isoform sensitivity. Most notably, BDS-I acts with very high potency on human Nav1.7 channels.

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.

Assessing use-dependent inhibition of the cardiac Na(+/-) current (I(Na)) in the PatchXpress automated patch clamp

INTRODUCTION

The cardiac Na+ current (I(Na)) underlies the rapid depolarization of the cardiac myocyte, and block of the current slows cardiac conduction and increases the risk of ventricular arrhythmia. A feature of Na+ channel block termed use-dependence is important to the assessment of blocking potency. We developed a robust automated patch clamp assay to rapidly and routinely assess the use-dependent block of I(Na) by drug candidates. The assay clarifies whether drug candidates block more potently at increased heart rates and provides a quantitative score of use-dependence.

METHODS

A use-dependent cardiac I(Na) assay was implemented on the PatchXpress 7000A, an automated whole-cell patch clamp device, using a HEK cell line stably expressing the human cardiac Na+ channel, Na(V)1.5. Stable recordings lasting up to 30 minutes were achieved by selection of holding potential (-100 mV) as well as an appropriate osmotic gradient to prevent time-dependent loss of cell capacitance and current. The final protocol allows evaluation of I(Na) inhibition at three pulsing rates at three test concentrations for each recorded cell.

RESULTS

IC(50) values obtained for three standard I(Na) blockers lidocaine, mexiletine, and flecainide, at pulsing frequencies of 0.2 Hz, 1 Hz, and 3 Hz, were compared to IC(50) values obtained with conventional pipette patch clamp of the Na(V)1.5 cell line and of guinea pig cardiac myocytes using matched voltage protocols and pulsing rates. Absolute potencies were well correlated only under conditions of matched holding potential and fell within an approximately three-fold window. While absolute potencies could vary widely with holding potential, the fold increases in potency with increases in pulsing rates were less prone to variation of the holding potential.

DISCUSSION

Use-dependence of cardiac Na+ channel block can be rapidly assessed in the PatchXpress platform and quantified at early stages of drug development to guide lead optimization.

Effects of recombinant neurotoxins on single Na(+) channels in isolated rat hippocampal neurons

Four recombinant neurotoxins Hk2a, Hk7a, Hk8a, Hk16a, originally from a sea anemone species Anthopleura sp., were obtained by fusion expression of their genes in Escherichia coli. These neurotoxins were composed of 47 amino acid residues, among which the differences were found at positions 14, 22, 25, and 37, respectively. The effects of the four neurotoxins on single-channel current of sodium in rat hippocampal neurons were studied by cell-attached patch clamp. Each neurotoxin 2 microM could modulate the sodium channel by prolonging the opening dwell time and increasing the open probability, but did not change the amplitude of sodium channel currents. Based on the studies of the structure-function relationship, we found that Hk7a displayed the biggest increase of the open probability because His14 (from Arg14) makes its structure seem more compact in comparison with the other three toxins and Ap-A. Phe25 (Hk8a, Hk16a), which varied from Ala25 (Hk2a, Hk7a), showed that phenyl group might interfere with other key amino acid residue to decrease the activity of toxins. Arg37 (from His37) in Hk8a contributed to decrease of open probability. In our work, it was shown that these important amino acid sites might provide a reliable proof for the future pharmaceutical design.

Marine natural products

This review covers the literature published in 2007 for marine natural products, with 948 citations(627 for the period January to December 2007) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, cnidarians,bryozoans, molluscs, tunicates, echinoderms and true mangrove plants. The emphasis is on new compounds (961 for 2007), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.1 Introduction, 2 Reviews, 3 Marine microorganisms and phytoplankton, 4 Green algae, 5 Brown algae, 6 Red algae, 7 Sponges, 8 Cnidarians, 9 Bryozoans, 10 Molluscs, 11 Tunicates (ascidians),12 Echinoderms, 13 Miscellaneous, 14 Conclusion, 15 References.

Evidence for multiple effects of ProTxII on activation gating in Na(V)1.5

The peptide toxin ProTxII, recently isolated from the venom of the tarantula spider Thrixopelma pruriens, modifies gating in voltage-gated Na+ and Ca2+ channels. ProTxII is distinct from other known Na+ channel gating modifier toxins in that it affects activation, but not inactivation. It shifts activation gating positively and decreases current magnitude such that the dose-dependence of toxin action measured at a single potential reflects both effects. To test the extent to which these effects were independent, we tracked several different measures of current amplitude, voltage-dependent activation, and current kinetics in Na(V)1.5 in a range of toxin concentrations. Changes in voltage dependence and a decrease in G(max) appeared at relatively low concentrations (40-100 nM) while a positive shift in the voltage range of activation was apparent at higher toxin concentrations (> or =500 nM). Because ProTxII carries a net +4 charge we tested whether electrostatic interactions contributed to toxin action. We examined the effects of ProTxII in the presence of high extracellular Ba2+, known to screen and/or bind to surface charge. Some, but not all aspects of ProTxII modification were sensitive to the presence of Ba2+ indicating the contribution of an electrostatic, surface charge-like mechanism and supporting the idea of a multi-faceted toxin-channel interaction.

Potentiation of E-4031-induced torsade de pointes by HMR1556 or ATX-II is not predicted by action potential short-term variability or triangulation

BACKGROUND AND PURPOSE

Torsade de pointes (TdP) can be induced by a reduction in cardiac repolarizing capacity. The aim of this study was to assess whether IKs blockade or enhancement of INa could potentiate TdP induced by IKr blockade and to investigate whether short-term variability (STV) or triangulation of action potentials preceded TdP.

EXPERIMENTAL APPROACH

Experiments were performed in open-chest, pentobarbital-anaesthetized, alpha 1-adrenoceptor-stimulated, male New Zealand White rabbits, which received three consecutive i.v. infusions of either the IKr blocker E-4031 (1, 3 and 10 nmol kg(-1) min(-1)), the IKs blocker HMR1556 (25, 75 and 250 nmol kg(-1) min(-1)) or E-4031 and HMR1556 combined. In a second study rabbits received either the same doses of E-4031, the INa enhancer, ATX-II (0.4, 1.2 and 4.0 nmol kg(-1)) or both of these drugs. ECGs and epicardial monophasic action potentials were recorded.

KEY RESULTS

HMR1556 alone did not cause TdP but increased E-4031-induced TdP from 25 to 80%. ATX-II alone caused TdP in 38% of rabbits, as did E-4031; 75% of rabbits receiving both drugs had TdP. QT intervals were prolonged by all drugs but the extent of QT prolongation was not related to the occurrence of TdP. No changes in STV were detected and triangulation was only increased after TdP occurred.

CONCLUSIONS AND IMPLICATIONS

Giving modulators of ion channels in combination substantially increased TdP but, in this model, neither STV nor triangulation of action potentials could predict TdP.

Outward stabilization of the S4 segments in domains III and IV enhances lidocaine block of sodium channels

The anti-arrhythmic drug lidocaine has been shown to have a lower affinity for block of voltage-gated sodium channels at hyperpolarized potentials compared to depolarized potentials. Concomitantly, lidocaine reduces maximum gating charge (Qmax) by 40% resulting from the complete stabilization of the S4 in domain III in an outward, depolarized position and partial stabilization of the S4 in domain IV in wild-type Na+ channels (Na(V)1.5). To investigate whether the pre-positioning of the S4 segments in these two domains in a depolarized conformation increases affinity for lidocaine block, a cysteine residue was substituted for the 3rd outermost charged residue in the S4 of domain III (R3C-DIII) and for the 2nd outermost Arg in S4 of domain IV (R2C-DIV) in Na(V)1.5. After biotinylation by exposure to extracellular MTSEA-biotin the mutated S4s became stabilized in an outward, depolarized position. For Na+ channels containing both mutations (R3C-DIII + R2C-DIV) the IC50 for rested-state lidocaine block decreased from 194 +/- 15 microM in control to 28 +/- 2 microM after MTSEA-biotin modification. To determine whether an intact inactivation gate (formed by the linker between domains III and IV) was required for local anaesthetic drugs to modify Na+ channel gating currents, a Cys was substituted for the Phe in the IFM motif of the inactivation gate (ICM) and then modified by intracellular MTSET (WT-ICM(MTSET)) before exposure to intracellular QX-222, a quarternary amine. Although WT-ICM(MTSET) required higher concentrations of drug to block I(Na) compared to WT, Qmax decreased by 35% and the V1/2 shifted leftward as previously demonstrated for WT. The effect of stabilization of the S4s in domains III and IV in the absence of an intact inactivation gate on lidocaine block was determined for R3C-DIII + ICM, R2C-DIV + ICM and R3C-DIII + R2C-DIV + ICM, and compared to WT-ICM. IC50 values were 1360 +/- 430 microM, 890 +/- 70 microM, 670 +/- 30 microM and 1920 +/- 60 microM, respectively. Thermodynamic mutant-cycle analysis was consistent with additive (i.e. independent) contributions from stabilization of the individual S4s in R3C-DIII + ICM and R2C-DIV + ICM. We conclude that the positions of the S4s in domains III and IV are major determinants of the voltage dependence of lidocaine affinity.