Tarantula toxins interacting with voltage sensors in potassium channels

A bivalent tarantula toxin activates the capsaicin receptor, TRPV1, by targeting the outer pore domain

Toxins have evolved to target regions of membrane ion channels that underlie ligand binding, gating, or ion permeation, and have thus served as invaluable tools for probing channel structure and function. Here, we describe a peptide toxin from the Earth Tiger tarantula that selectively and irreversibly activates the capsaicin- and heat-sensitive channel, TRPV1. This high-avidity interaction derives from a unique tandem repeat structure of the toxin that endows it with an antibody-like bivalency. The "double-knot" toxin traps TRPV1 in the open state by interacting with residues in the presumptive pore-forming region of the channel, highlighting the importance of conformational changes in the outer pore region of TRP channels during activation.

Molecular diversity of spider venom

Spider venom, a factor that has played a decisive role in the evolution of one of the most successful groups of living organisms, is reviewed. Unique molecular diversity of venom components including substances of variable structure (from simple low molecular weight compounds to large multidomain proteins) with different functions is considered. Special attention is given to the structure, properties, and biosynthesis of toxins of polypeptide nature.

Electrostatic tuning of cellular excitability

Voltage-gated ion channels regulate the electric activity of excitable tissues, such as the heart and brain. Therefore, treatment for conditions of disturbed excitability is often based on drugs that target ion channels. In this study of a voltage-gated K channel, we propose what we believe to be a novel pharmacological mechanism for how to regulate channel activity. Charged lipophilic substances can tune channel opening, and consequently excitability, by an electrostatic interaction with the channel's voltage sensors. The direction of the effect depends on the charge of the substance. This was shown by three compounds sharing an arachidonyl backbone but bearing different charge: arachidonic acid, methyl arachidonate, and arachidonyl amine. Computer simulations of membrane excitability showed that small changes in the voltage dependence of Na and K channels have prominent impact on excitability and the tendency for repetitive firing. For instance, a shift in the voltage dependence of a K channel with -5 or +5 mV corresponds to a threefold increase or decrease in K channel density, respectively. We suggest that electrostatic tuning of ion channel activity constitutes a novel and powerful pharmacological approach with which to affect cellular excitability.

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.

Voltage-gated potassium channels as therapeutic targets

The human genome encodes 40 voltage-gated K(+) channels (K(V)), which are involved in diverse physiological processes ranging from repolarization of neuronal and cardiac action potentials, to regulating Ca(2+) signalling and cell volume, to driving cellular proliferation and migration. K(V) channels offer tremendous opportunities for the development of new drugs to treat cancer, autoimmune diseases and metabolic, neurological and cardiovascular disorders. This Review discusses pharmacological strategies for targeting K(V) channels with venom peptides, antibodies and small molecules, and highlights recent progress in the preclinical and clinical development of drugs targeting the K(V)1 subfamily, the K(V)7 subfamily (also known as KCNQ), K(V)10.1 (also known as EAG1 and KCNH1) and K(V)11.1 (also known as HERG and KCNH2) channels.

Target promiscuity and heterogeneous effects of tarantula venom peptides affecting Na+ and K+ ion channels

Venom-derived peptide modulators of ion channel gating are regarded as essential tools for understanding the molecular motions that occur during the opening and closing of ion channels. In this study, we present the characterization of five spider toxins on 12 human voltage-gated ion channels, following observations about the target promiscuity of some spider toxins and the ongoing revision of their "canonical" gating-modifying mode of action. The peptides were purified de novo from the venom of Grammostola rosea tarantulas, and their sequences were confirmed by Edman degradation and mass spectrometry analysis. Their effects on seven tetrodotoxin-sensitive Na(+) channels, the three human ether-à-go-go (hERG)-related K(+) channels, and two human Shaker-related K(+) channels were extensively characterized by electrophysiological techniques. All the peptides inhibited ion conduction through all the Na(+) channels tested, although with distinctive patterns. The peptides also affected the three pharmaceutically relevant hERG isoforms differently. At higher concentrations, all peptides also modified the gating of the Na(+) channels by shifting the activation to more positive potentials, whereas more complex effects were recorded on hERG channels. No effects were evident on the two Shaker-related K(+) channels at concentrations well above the IC(50) value for the affected channels. Given the sequence diversity of the tested peptides, we propose that tarantula toxins should be considered both as multimode and target-promiscuous ion channel modulators; both features should not be ignored when extracting mechanistic interpretations about ion channel gating. Our observations could also aid in future structure-function studies and might help the development of novel ion channel-specific drugs.

Symptom in search of a toxin: muscle spasms following bites by Old World tarantula spiders (Lampropelma nigerrimum, Pterinochilus murinus, Poecilotheria regalis) with review

BACKGROUND

Tarantula spiders are widely kept and bred in captivity by both adults and children. Their bites are generally considered harmless.

AIM

To explore the effects of envenoming by Old World tarantulas.

DESIGN AND METHODS

Clinical studies and review of conventional literature and hobbyist web sites.

RESULTS

Two men bitten on their index fingers by pet Old World tarantula spiders, Lampropelma nigerrimum (Ornithoctoninae) and Pterinochilus murinus (Harpactirinae) in England, developed intense local pain, swelling and episodic, agonising, generalised muscle cramps. In one of them, cramps persisted for 7 days and serum creatine kinase concentration was mildly elevated. A third man bitten on a finger by Poecilotheria regalis (Poecilotheriinae), suffered persistent local cramps in the affected hand. Reports since 1803, including recent ones on hobbyist web-sites, have been largely overlooked. They mentioned muscle spasms after bites by these and other genera of Old World tarantulas, including Eumenophorus, Selenocosmia and Stromatopelma. The severe muscle spasms seen in two of our patients were a challenge to medical treatment and might, under some circumstances, have been life threatening. They demand a toxinological explanation.

CONCLUSION

Bites by several genera of African, Asian and Australasian tarantulas can cause systemic neurotoxic envenoming. In the absence of available antivenom, severe persistent muscle spasms, reminiscent of latrodectism, pose a serious therapeutic challenge. Discovery of the toxin responsible would be of scientific and potential clinical benefit. Tarantula keepers should be warned of the danger of handling these animals incautiously.

Structural determinants of the high affinity extracellular zinc binding site on Cav3.2 T-type calcium channels

Ca(v)3.2 T-type channels contain a high affinity metal binding site for trace metals such as copper and zinc. This site is occupied at physiologically relevant concentrations of these metals, leading to decreased channel activity and pain transmission. A histidine at position 191 was recently identified as a critical determinant for both trace metal block of Ca(v)3.2 and modulation by redox agents. His(191) is found on the extracellular face of the Ca(v)3.2 channel on the IS3-S4 linker and is not conserved in other Ca(v)3 channels. Mutation of the corresponding residue in Ca(v)3.1 to histidine, Gln(172), significantly enhances trace metal inhibition, but not to the level observed in wild-type Ca(v)3.2, implying that other residues also contribute to the metal binding site. The goal of the present study is to identify these other residues using a series of chimeric channels. The key findings of the study are that the metal binding site is composed of a Asp-Gly-His motif in IS3-S4 and a second aspartate residue in IS2. These results suggest that metal binding stabilizes the closed conformation of the voltage-sensor paddle in repeat I, and thereby inhibits channel opening. These studies provide insight into the structure of T-type channels, and identify an extracellular motif that could be targeted for drug development.

Searching for interesting channels: pairing selection and molecular evolution methods to study ion channel structure and function

The pairing of selection and screening methods with randomly mutated libraries can be an exceptionally powerful means for probing the functions of biological molecules and for developing novel regents from random libraries of peptides and oligonucleotides. The use of such approaches is beginning to permeate the ion channel field where they are being deployed to uncover fundamental aspects about ion channel structure and gating, small molecule-channel interactions, and the development of novel agents to control channel activity.

A polyether biotoxin binding site on the lipid-exposed face of the pore domain of Kv channels revealed by the marine toxin gambierol

Gambierol is a marine polycyclic ether toxin belonging to the group of ciguatera toxins. It does not activate voltage-gated sodium channels (VGSCs) but inhibits Kv1 potassium channels by an unknown mechanism. While testing whether Kv2, Kv3, and Kv4 channels also serve as targets, we found that Kv3.1 was inhibited with an IC(50) of 1.2 +/- 0.2 nM, whereas Kv2 and Kv4 channels were insensitive to 1 microM gambierol. Onset of block was similar from either side of the membrane, and gambierol did not compete with internal cavity blockers. The inhibition did not require channel opening and could not be reversed by strong depolarization. Using chimeric Kv3.1-Kv2.1 constructs, the toxin sensitivity was traced to S6, in which T427 was identified as a key determinant. In Kv3.1 homology models, T427 and other molecular determinants (L348, F351) reside in a space between S5 and S6 outside the permeation pathway. In conclusion, we propose that gambierol acts as a gating modifier that binds to the lipid-exposed surface of the pore domain, thereby stabilizing the closed state. This site may be the topological equivalent of the neurotoxin site 5 of VGSCs. Further elucidation of this previously undescribed binding site may explain why most ciguatoxins activate VGSCs, whereas others inhibit voltage-dependent potassium (Kv) channels. This previously undescribed Kv neurotoxin site may have wide implications not only for our understanding of channel function at the molecular level but for future development of drugs to alleviate ciguatera poisoning or to modulate electrical excitability in general.

S3b amino acid substitutions and ancillary subunits alter the affinity of Heteropoda venatoria toxin 2 for Kv4.3

Heteropoda venatoria toxin 2 (HpTx2) is an inhibitor cystine knot (ICK)-gating modifier toxin that selectively inhibits Kv4 channels. To characterize the molecular determinants of interaction, we performed alanine scanning of the Kv4.3 S3b region. HpTx2-Kv4.3 interaction had an apparent K(d) value of 2.3 microM. Two alanine mutants in Kv4.3 increased K(d) values to 6.4 microM for V276A and 25 microM for L275A. Simultaneous mutation of both amino acids to alanine nearly eliminated toxin interaction. Unlike Hanatoxin and other well characterized ICK toxins, HpTx2 binding does not require a charged amino acid for interaction. To determine whether the identity of the S3b binding site amino acids altered HpTx2 specificity, we constructed Kv4.3 [LV275IF]. This mutation decreased the K(d) value to 0.54 microM, suggesting that the hydrophobic character of the putative binding site is the most important property for interaction with HpTx2. One mutant, N280A, caused stronger interaction of HpTx2 with Kv4.3; the K(d) value for Kv4.3 [N280A] was 0.26 microM. To understand Kv4.3-based transient outward currents in native tissues, we tested the affinity of HpTx2 for Kv4.3 coexpressed with KChIP2b. The toxin's K(d) value for Kv4.3 + KChIP2b was 0.95 microM. KChIP2b stabilizes the closed state of Kv4.3, suggesting that the increased toxin affinity is due to increased stabilization of the closed state. These data show that HpTx2 binding to Kv4.3 has aspects in common with other ICK gating modifier toxins but that the interventions that increase toxin affinity suggest flexibility toward channel binding that belies its unusual specificity for Kv4 channels.

Structure, function, and modification of the voltage sensor in voltage-gated ion channels

Voltage-gated ion channels are crucial for both neuronal and cardiac excitability. Decades of research have begun to unravel the intriguing machinery behind voltage sensitivity. Although the details regarding the arrangement and movement in the voltage-sensor domain are still debated, consensus is slowly emerging. There are three competing conceptual models: the helical-screw, the transporter, and the paddle model. In this review we explore the structure of the activated voltage-sensor domain based on the recent X-ray structure of a chimera between Kv1.2 and Kv2.1. We also present a model for the closed state. From this we conclude that upon depolarization the voltage sensor S4 moves approximately 13 A outwards and rotates approximately 180 degrees, thus consistent with the helical-screw model. S4 also moves relative to S3b which is not consistent with the paddle model. One interesting feature of the voltage sensor is that it partially faces the lipid bilayer and therefore can interact both with the membrane itself and with physiological and pharmacological molecules reaching the channel from the membrane. This type of channel modulation is discussed together with other mechanisms for how voltage-sensitivity is modified. Small effects on voltage-sensitivity can have profound effects on excitability. Therefore, medical drugs designed to alter the voltage dependence offer an interesting way to regulate excitability.

Large-scale movement within the voltage-sensor paddle of a potassium channel-support for a helical-screw motion

The size of the movement and the molecular identity of the moving parts of the voltage sensor of a voltage-gated ion channel are debated. In the helical-screw model, the positively charged fourth transmembrane segment S4 slides and rotates along negative counter charges in S2 and S3, while in the paddle model, S4 carries the extracellular part of S3 (S3b) as a cargo. Here, we show that S4 slides 16-26 A along S3b. We introduced pairs of cysteines in S4 and S3b of the Shaker K channel to make disulfide bonds. Residue 325 in S3b makes close and state-dependent contacts with a long stretch of residues in S4. A disulfide bond between 325 and 360 was formed in the closed state, while a bond between 325 and 366 was formed in the open state. These data are not compatible with the voltage-sensor paddle model, but support the helical-screw model.

Extent of voltage sensor movement during gating of shaker K+ channels

Voltage-driven activation of Kv channels results from conformational changes of four voltage sensor domains (VSDs) that surround the K(+) selective pore domain. How the VSD helices rearrange during gating is an area of active research. Luminescence resonance energy transfer (LRET) is a powerful spectroscopic ruler uniquely suitable for addressing the conformational trajectory of these helices. Using a geometric analysis of numerous LRET measurements, we were able to estimate LRET probe positions relative to existing structural models. The experimental movement of helix S4 does not support a large 15-20 A transmembrane "paddle-type" movement or a near-zero A vertical "transporter-type" model. Rather, our measurements demonstrate a moderate S4 displacement of 10 +/- 5 A, with a vertical component of 5 +/- 2 A. The S3 segment moves 2 +/- 1 A in the opposite direction and is therefore not moving as an S3-S4 rigid body.

Tarantula huwentoxin-IV inhibits neuronal sodium channels by binding to receptor site 4 and trapping the domain ii voltage sensor in the closed configuration

Peptide toxins with high affinity, divergent pharmacological functions, and isoform-specific selectivity are powerful tools for investigating the structure-function relationships of voltage-gated sodium channels (VGSCs). Although a number of interesting inhibitors have been reported from tarantula venoms, little is known about the mechanism for their interaction with VGSCs. We show that huwentoxin-IV (HWTX-IV), a 35-residue peptide from tarantula Ornithoctonus huwena venom, preferentially inhibits neuronal VGSC subtypes rNav1.2, rNav1.3, and hNav1.7 compared with muscle subtypes rNav1.4 and hNav1.5. Of the five VGSCs examined, hNav1.7 was most sensitive to HWTX-IV (IC(50) approximately 26 nM). Following application of 1 microm HWTX-IV, hNav1.7 currents could only be elicited with extreme depolarizations (>+100 mV). Recovery of hNav1.7 channels from HWTX-IV inhibition could be induced by extreme depolarizations or moderate depolarizations lasting several minutes. Site-directed mutagenesis analysis indicated that the toxin docked at neurotoxin receptor site 4 located at the extracellular S3-S4 linker of domain II. Mutations E818Q and D816N in hNav1.7 decreased toxin affinity for hNav1.7 by approximately 300-fold, whereas the reverse mutations in rNav1.4 (N655D/Q657E) and the corresponding mutations in hNav1.5 (R812D/S814E) greatly increased the sensitivity of the muscle VGSCs to HWTX-IV. Our data identify a novel mechanism for sodium channel inhibition by tarantula toxins involving binding to neurotoxin receptor site 4. In contrast to scorpion beta-toxins that trap the IIS4 voltage sensor in an outward configuration, we propose that HWTX-IV traps the voltage sensor of domain II in the inward, closed configuration.

Lipid bilayer deformation and the free energy of interaction of a Kv channel gating-modifier toxin

A number of membrane proteins act via binding at the water/lipid bilayer interface. An important example of such proteins is provided by the gating-modifier toxins that act on voltage-gated potassium (Kv) channels. They are thought to partition to the headgroup region of lipid bilayers, and so provide a good system for probing the nature of interactions of a protein with the water/bilayer interface. We used coarse-grained molecular dynamics simulations to compute the one-dimensional potential of mean force (i.e., free energy) profile that governs the interaction between a Kv channel gating-modifier toxin (VSTx1) and model phospholipid bilayers. The reaction coordinate sampled corresponds to the position of the toxin along the bilayer normal. The course-grained representation of the protein and lipids enabled us to explore extended time periods, revealing aspects of toxin/bilayer dynamics and energetics that would be difficult to observe on the timescales currently afforded by atomistic molecular dynamics simulations. In particular, we show for this model system that the bilayer deforms as it interacts with the toxin, and that such deformations perturb the free energy profile. Bilayer deformation therefore adds an additional layer of complexity to be addressed in investigations of membrane/protein systems. In particular, one should allow for local deformations that may arise due to the spatial array of charged and hydrophobic elements of an interfacially located membrane protein.

The Janus-faced atracotoxins are specific blockers of invertebrate K(Ca) channels

The Janus-faced atracotoxins are a unique family of excitatory peptide toxins that contain a rare vicinal disulfide bridge. Although lethal to a wide range of invertebrates, their molecular target has remained enigmatic for almost a decade. We demonstrate here that these toxins are selective, high-affinity blockers of invertebrate Ca(2+)-activated K(+) (K(Ca)) channels. Janus-faced atracotoxin (J-ACTX)-Hv1c, the prototypic member of this toxin family, selectively blocked K(Ca) channels in cockroach unpaired dorsal median neurons with an IC(50) of 2 nm, but it did not significantly affect a wide range of other voltage-activated K(+), Ca(2+) or Na(+) channel subtypes. J-ACTX-Hv1c blocked heterologously expressed cockroach large-conductance Ca(2+)-activated K(+) (pSlo) channels without a significant shift in the voltage dependence of activation. However, the block was voltage-dependent, indicating that the toxin probably acts as a pore blocker rather than a gating modifier. The molecular basis of the insect selectivity of J-ACTX-Hv1c was established by its failure to significantly inhibit mouse mSlo currents (IC(50) approximately 10 mum) and its lack of activity on rat dorsal root ganglion neuron K(Ca) channel currents. This study establishes the Janus-faced atracotoxins as valuable tools for the study of invertebrate K(Ca) channels and suggests that K(Ca) channels might be potential insecticide targets.

Portability of paddle motif function and pharmacology in voltage sensors

Voltage-sensing domains enable membrane proteins to sense and react to changes in membrane voltage. Although identifiable S1-S4 voltage-sensing domains are found in an array of conventional ion channels and in other membrane proteins that lack pore domains, the extent to which their voltage-sensing mechanisms are conserved is unknown. Here we show that the voltage-sensor paddle, a motif composed of S3b and S4 helices, can drive channel opening with membrane depolarization when transplanted from an archaebacterial voltage-activated potassium channel (KvAP) or voltage-sensing domain proteins (Hv1 and Ci-VSP) into eukaryotic voltage-activated potassium channels. Tarantula toxins that partition into membranes can interact with these paddle motifs at the protein-lipid interface and similarly perturb voltage-sensor activation in both ion channels and proteins with a voltage-sensing domain. Our results show that paddle motifs are modular, that their functions are conserved in voltage sensors, and that they move in the relatively unconstrained environment of the lipid membrane. The widespread targeting of voltage-sensor paddles by toxins demonstrates that this modular structural motif is an important pharmacological target.

Painful toxins acting at TRPV1

Many plant and animal toxins cause aversive behaviors in animals due to their pungent or unpleasant taste or because they cause other unpleasant senstations like pain. This article reviews the current state of knowledge of toxins that act at the TRPV1 ion channel, which is expressed in primary sensory neurons, is activated by multiple painful stimuli and is thought to be a key pain sensor and integrator. The recent finding that painful peptide "vanillotoxin" components of tarantula toxin activate the TRPV1 ion channel to cause pain led us to survey what is known about toxins that act at this receptor. Toxins from plants, spiders and jellyfish are considered. Where possible, structural information about sites of interaction is considered in relation to toxin-binding sites on the Kv ion channel, for which more structural information exists. We discuss a developing model where toxin agonists such as resiniferatoxin and vanillotoxins are proposed to interact with a region of TRPV1 that is homologous to the "voltage sensor" in the Kv1.2 ion channel, to open the channel and activate primary sensory nerves, causing pain.

Tarantula toxins interact with voltage sensors within lipid membranes

Voltage-activated ion channels are essential for electrical signaling, yet the mechanism of voltage sensing remains under intense investigation. The voltage-sensor paddle is a crucial structural motif in voltage-activated potassium (K_v) channels that has been proposed to move at the protein–lipid interface in response to changes in membrane voltage. Here we explore whether tarantula toxins like hanatoxin and SGTx1 inhibit K_v channels by interacting with paddle motifs within the membrane. We find that these toxins can partition into membranes under physiologically relevant conditions, but that the toxin–membrane interaction is not sufficient to inhibit K_v channels. From mutagenesis studies we identify regions of the toxin involved in binding to the paddle motif, and those important for interacting with membranes. Modification of membranes with sphingomyelinase D dramatically alters the stability of the toxin–channel complex, suggesting that tarantula toxins interact with paddle motifs within the membrane and that they are sensitive detectors of lipid–channel interactions.

Gating currents from a Kv3 subfamily potassium channel: charge movement and modification by BDS-II toxin

Kv3 channels have a major role in determining neuronal excitability, and are characterized by ultra-rapid kinetics of gating and a high activation threshold. However, the gating currents, which occur as a result of positional changes of the charged elements in the channel structure during activation, are not well understood. Here we report a study of gating currents from wild-type Kv3.2b channels, expressed in human embryonic kidney (HEK) cells to facilitate high time-resolution recording. On-gating currents (I(g,on)) had extremely rapid kinetics such that at +80 mV, the time constant for the decay of I(g,on) was only approximately 0.3 ms. Decay of I(g,on) appeared mono-exponential at all potentials studied, and in support of this, the charge-voltage (Q-V) relationship was fitted with a single Boltzmann function, supporting the idea that only one charge system is required to account for the time course of I(g,on) and the voltage dependence of Q(on). The voltage (V((1/2))) for half movement of gating charge was -8.4 +/- 4.0 mV (n = 6), which closely matches the voltage dependence of activation of Kv3.2b ionic currents reported before. Depolarizations to more positive potentials than 0 mV decreased the amplitude and slowed the decay of the off-gating currents (I(g,off)), suggesting that a rate-limiting step in opening was present in Kv3 channels as in Shaker and other Kv channels. Return of charge was negatively shifted along the potential axis with a V((1/2)) of Q(off) of -80.9 +/- 0.8 mV (n = 3), which allowed approximately 90% charge return upon repolarization to -100 mV. BDS-II toxin apparently reduced I(g,on), and greatly slowed the kinetics of I(g,on), while shifting the Q-V relationship in the depolarizing direction. However, the Q-V relationship remained well fitted by a single Boltzmann function. These data provide the first description of Kv3 gating currents and give further insight into the interaction of BDS toxins and Kv3 channels.

APETx1 from Sea Anemone Anthopleura elegantissima Is a Gating Modifier Peptide Toxin of the Human Ether-a-go-go- Related Potassium Channel

We studied the mechanism of action and the binding site of APETx1, a peptide toxin purified from sea anemone, on the human ether-a-go-go-related gene (hERG) channel. Similar to the effects of gating modifier toxins (hanatoxin and SGTx) on the voltage-gated potassium (Kv) 2.1 channel, APETx1 shifts the voltage-dependence of hERG activation in the positive direction and suppresses its current amplitudes elicited by strong depolarizing pulses that maximally activate the channels. The APETx1 binding site is distinctly different from that of a pore-blocking peptide toxin, BeKm-1. Mutations in the S3b region of hERG have dramatic impact on the responsiveness to APETx1: G514C potentiates whereas E518C abolishes the APETx1 effect. Restoring the negative charge at position 518 (methanethiosulfonate ethylsulfonate modification of 518C) partially restores APETx1 responsiveness, supporting an electrostatic interaction between E518 and APETx1. Among the three hERG isoforms, hERG1 and hERG3 are equally responsive to APETx1, whereas hERG2 is insensitive. The key feature seems to be an arginine residue uniquely present at the 514-equivalent position in hERG2, where the other two isoforms possess a glycine. Our data show that APETx1 is a gating modifier toxin of the hERG channel, and its binding site shares characteristics with those of gating modifier toxin binding sites on other Kv channels.

Isolation and characterization of a novel toxin from the venom of the spider Grammostola rosea that blocks sodium channels

This communication reports the chemical and physiological characterization of a novel peptide (GrTx1) isolated from the venom of the "rosean-tarantula"Grammostola rosea. This component was one among more than 15 distinct components separated from the soluble venom by high-performance liquid chromatography (HPLC). GrTx1 has 29 amino-acid residues, compactly folded by three disulfide bridges with a molecular weight of 3697 Da. Here we show that this peptide blocks Na(+) currents of neuroblastoma F-11 cells with an IC(50) of 2.8+/-0.1 microM, up to a maximum of about 85% at 10 microM. Moreover, the right-shift (+20.1+/-0.4 mV) of the fractional voltage-dependent conductance could be also compatible with a putative "gating-modifier" mechanism. No effects were seen on common K(+) channels, such as K(v)1.1 and 1.4, using concentrations of toxin up to 10 microM. Sequence analysis reveals that GrTx1 is closely related to other spider toxins reported to affect various distinct ion channel functions. A critical analysis of this study suggests the necessity to search for other potential receptor sites in order to establish the preferred specificity of these kind of peptides.

Gating modifier peptides as probes of pancreatic β-cell physiology

Pancreatic beta-cells depolarize in response to glucose and fire calcium-dependent actions potentials that trigger insulin secretion. The major current responsible for action potential repolarization in these cells is a delayed rectifier and Kv2.1 subunits are thought be a major contributor of the delayed rectifier channels. Hence, blockers of Kv2.1 channels might prolong action potentials and enhance calcium influx and insulin secretion. However, the lack of specific small molecule Kv2.1 inhibitors has hindered the testing of this mechanism. Importantly, several gating modifier peptides inhibit Kv2.1 channels in a relatively specific fashion. Hanatoxin (HaTX) and guangxitoxin-1 (GxTX-1) are examples that have been used to probe the role of Kv2.1 channels in beta-cell physiology. Both HaTX and GxTX-1 strongly inhibit the Kv current of beta-cells from various species, arguing that Kv2.1 subunits contribute significantly to the beta-cell delayed rectifier. GxTX-1 prolongs glucose-triggered action potentials, enhances glucose-dependent intracellular calcium elevations and augments glucose-dependent insulin secretion. Taken together, these data suggest that blockers of Kv2.1 channels may be a useful approach to the design of novel therapeutic agents for the treatment of type 2 diabetes. These studies highlight the utility of gating modifier peptides in the study of physiological systems.