Novel interactions between K+ channels and scorpion toxins

The binding and mechanism of a positive allosteric modulator of Kv3 channels

Small-molecule modulators of diverse voltage-gated K+ (Kv) channels may help treat a wide range of neurological disorders. However, developing effective modulators requires understanding of their mechanism of action. We apply an orthogonal approach to elucidate the mechanism of action of an imidazolidinedione derivative (AUT5), a highly selective positive allosteric modulator of Kv3.1 and Kv3.2 channels. AUT5 modulation involves positive cooperativity and preferential stabilization of the open state. The cryo-EM structure of the Kv3.1/AUT5 complex at a resolution of 2.5 Å reveals four equivalent AUT5 binding sites at the extracellular inter-subunit interface between the voltage-sensing and pore domains of the channel's tetrameric assembly. Furthermore, we show that the unique extracellular turret regions of Kv3.1 and Kv3.2 essentially govern the selective positive modulation by AUT5. High-resolution apo and bound structures of Kv3.1 demonstrate how AUT5 binding promotes turret rearrangements and interactions with the voltage-sensing domain to favor the open conformation.

Methionine-isoleucine dichotomy at a key position in scorpion toxins inhibiting voltage-gated potassium channels

Previous studies have identified some key amino acid residues in scorpion toxins blocking potassium channels. In particular, the most numerous toxins belonging to the α-KTx family and affecting voltage-gated potassium channels (KV) present a conserved K-C-X-N motif in the C-terminal half of their sequence. Here, we show that the X position of this motif is almost always occupied by either methionine or isoleucine. We compare the activity of three pairs of peptides that differ just by this residue on a panel of KV1 channels and find that toxins bearing methionine affect preferentially KV1.1 and 1.6 isoforms. The refined K-C-M/I-N motif stands out as the principal structural element of α-KTx conferring high affinity and selectivity to KV channels.

Cm28, a scorpion toxin having a unique primary structure, inhibits KV1.2 and KV1.3 with high affinity

The Cm28 in the venom of Centruroides margaritatus is a short peptide consisting of 27 amino acid residues with a mol wt of 2,820 D. Cm28 has <40% similarity with other known α-KTx from scorpions and lacks the typical functional dyad (lysine-tyrosine) required to block KV channels. However, its unique sequence contains the three disulfide-bond traits of the α-KTx scorpion toxin family. We propose that Cm28 is the first example of a new subfamily of α-KTxs, registered with the systematic number α-KTx32.1. Cm28 inhibited voltage-gated K+ channels KV1.2 and KV1.3 with Kd values of 0.96 and 1.3 nM, respectively. There was no significant shift in the conductance-voltage (G-V) relationship for any of the channels in the presence of toxin. Toxin binding kinetics showed that the association and dissociation rates are consistent with a bimolecular interaction between the peptide and the channel. Based on these, we conclude that Cm28 is not a gating modifier but rather a pore blocker. In a selectivity assay, Cm28 at 150 nM concentration (>100× Kd value for KV1.3) did not inhibit KV1.5, KV11.1, KCa1.1, and KCa3.1 K+ channels; NaV1.5 and NaV1.4 Na+ channels; or the hHV1 H+ channel but blocked ∼27% of the KV1.1 current. In a biological functional assay, Cm28 strongly inhibited the expression of the activation markers interleukin-2 receptor and CD40 ligand in anti-CD3-activated human CD4+ effector memory T lymphocytes. Cm28, due to its unique structure, may serve as a template for the generation of novel peptides targeting KV1.3 in autoimmune diseases.

Optimization of Pichia pastoris Expression System for High-Level Production of Margatoxin

Margatoxin (MgTx) is a high-affinity blocker of voltage-gated potassium (Kv) channels. It inhibits Kv1.1–Kv1.3 ion channels in picomolar concentrations. This toxin is widely used to study physiological function of Kv ion channels in various cell types, including immune cells. Isolation of native MgTx in large quantities from scorpion venom is not affordable. Chemical synthesis and recombinant production in Escherichia coli need in vitro oxidative refolding for proper disulfide bond formation, resulting in a very low yield of peptide production. The Pichia pastoris expression system offers an economical approach to overcome all these limitations and gives a higher yield of correctly refolded recombinant peptides. In this study, improved heterologous expression of recombinant MgTx (rMgTx) in P. pastoris was obtained by using preferential codons, selecting the hyper-resistant clone against Zeocin, and optimizing the culturing conditions. About 36 ± 4 mg/L of >98% pure His-tagged rMgTx (TrMgTx) was produced, which is a threefold higher yield than has been previously reported. Proteolytic digestion of TrMgTx with factor Xa generated untagged rMgTx (UrMgTx). Both TrMgTx and UrMgTx blocked the Kv1.2 and Kv1.3 currents (patch-clamp) (K_d for Kv1.2 were 64 and 14 pM, and for Kv1.3, 86 and 50 pM, respectively) with comparable potency to the native MgTx. The analysis of the binding kinetics showed that TrMgTx had a lower association rate than UrMgTx for both Kv1.2 and Kv1.3. The dissociation rate of both the analogues was the same for Kv1.3. However, in the case of Kv1.2, TrMgTx showed a much higher dissociation rate with full recovery of the block than UrMgTx. Moreover, in a biological functional assay, both peptides significantly downregulated the expression of early activation markers IL2R and CD40L in activated CD4⁺ T_EM lymphocytes whose activation was Kv1.3 dependent. In conclusion, the authors report that the Pichia expression system is a powerful method to produce disulfide-rich peptides, the overexpression of which could be enhanced noticeably through optimization strategies, making it more cost-effective. Since the presence of the His-tag on rMgTx only mildly altered the block equilibrium and binding kinetics, recombinant toxins could be used in ion channel research without removing the tag and could thus reduce the cost and time demand for toxin production.

Colombian Scorpion Centruroides margaritatus: Purification and Characterization of a Gamma Potassium Toxin with Full-Block Activity on the hERG1 Channel

The Colombian scorpion Centruroides margaritatus produces a venom considered of low toxicity. Nevertheless, there are known cases of envenomation resulting in cardiovascular disorders, probably due to venom components that target ion channels. Among them, the humanether-à-go-go-Related gene (hERG1) potassium channels are critical for cardiac action potential repolarization and alteration in its functionality are associated with cardiac disorders. This work describes the purification and electrophysiological characterization of a Centruroides margaritatus venom component acting on hERG1 channels, the CmERG1 toxin. This novel peptide is composed of 42 amino acids with a MW of 4792.88 Da, folded by four disulfide bonds and it is classified as member number 10 of the γ-KTx1 toxin family. CmERG1 inhibits hERG1 currents with an IC₅₀ of 3.4 ± 0.2 nM. Despite its 90.5% identity with toxin ɣ-KTx1.1, isolated from Centruroides noxius, CmERG1 completely blocks hERG1 current, suggesting a more stable plug of the hERG channel, compared to that formed by other ɣ-KTx.

A Molecular Lid Mechanism of K+ Channel Blocker Action Revealed by a Cone Peptide

Many venomous organisms carry in their arsenal short polypeptides that block K⁺ channels in a highly selective manner. These toxins may compete with the permeating ions directly via a "plug" mechanism or indirectly via a "pore-collapse" mechanism. An alternative "lid" mechanism was proposed but remained poorly defined. Here we study the Drosophila Shaker channel block by Conkunitzin-S1 and Conkunitzin-C3, two highly similar toxins derived from cone venom. Despite their similarity, the two peptides exhibited differences in their binding poses and biophysical assays, implying discrete action modes. We show that while Conkunitzin-S1 binds tightly to the channel turret and acts via a "pore-collapse" mechanism, Conkunitzin-C3 does not contact this region. Instead, Conk-C3 uses a non-conserved Arg to divert the permeant ions and trap them in off-axis cryptic sites above the SF, a mechanism we term a "molecular-lid". Our study provides an atomic description of the "lid" K⁺ blocking mode and offers valuable insights for the design of therapeutics based on venom peptides.

Pore-modulating toxins exploit inherent slow inactivation to block K+ channels

Voltage-dependent potassium channels (K_vs) gate in response to changes in electrical membrane potential by coupling a voltage-sensing module with a K⁺-selective pore. Animal toxins targeting K_vs are classified as pore blockers, which physically plug the ion conduction pathway, or as gating modifiers, which disrupt voltage sensor movements. A third group of toxins blocks K⁺ conduction by an unknown mechanism via binding to the channel turrets. Here, we show that Conkunitzin-S1 (Cs1), a peptide toxin isolated from cone snail venom, binds at the turrets of K_v1.2 and targets a network of hydrogen bonds that govern water access to the peripheral cavities that surround the central pore. The resulting ectopic water flow triggers an asymmetric collapse of the pore by a process resembling that of inherent slow inactivation. Pore modulation by animal toxins exposes the peripheral cavity of K⁺ channels as a novel pharmacological target and provides a rational framework for drug design.

AbeTx1 Is a Novel Sea Anemone Toxin with a Dual Mechanism of Action on Shaker-Type K⁺ Channels Activation

Voltage-gated potassium (K_V) channels regulate diverse physiological processes and are an important target for developing novel therapeutic approaches. Sea anemone (Cnidaria, Anthozoa) venoms comprise a highly complex mixture of peptide toxins with diverse and selective pharmacology on K_V channels. From the nematocysts of the sea anemone Actinia bermudensis, a peptide that we named AbeTx1 was purified and functionally characterized on 12 different subtypes of K_V channels (K_V1.1⁻K_V1.6; K_V2.1; K_V3.1; K_V4.2; K_V4.3; K_V11.1; and, Shaker IR), and three voltage-gated sodium channel isoforms (Na_V1.2, Na_V1.4, and BgNa_V). AbeTx1 was selective for Shaker-related K⁺ channels and is capable of inhibiting K⁺ currents, not only by blocking the K⁺ current of K_V1.2 subtype, but by altering the energetics of activation of K_V1.1 and K_V1.6. Moreover, experiments using six synthetic alanine point-mutated analogs further showed that a ring of basic amino acids acts as a multipoint interaction for the binding of the toxin to the channel. The AbeTx1 primary sequence is composed of 17 amino acids with a high proportion of lysines and arginines, including two disulfide bridges (Cys1⁻Cys4 and Cys2⁻Cys3), and it is devoid of aromatic or aliphatic amino acids. Secondary structure analysis reveals that AbeTx1 has a highly flexible, random-coil-like conformation, but with a tendency of structuring in the beta sheet. Its overall structure is similar to open-ended cyclic peptides found on the scorpion κ-KTx toxins family, cone snail venoms, and antimicrobial peptides.

Loop Replacement Enhances the Ancestral Antibacterial Function of a Bifunctional Scorpion Toxin

On the basis of the evolutionary relationship between scorpion toxins targeting K⁺ channels (KTxs) and antibacterial defensins (Zhu S., Peigneur S., Gao B., Umetsu Y., Ohki S., Tytgat J. Experimental conversion of a defensin into a neurotoxin: Implications for origin of toxic function. Mol. Biol. Evol. 2014, 31, 546–559), we performed protein engineering experiments to modify a bifunctional KTx (i.e., weak inhibitory activities on both K⁺ channels and bacteria) via substituting its carboxyl loop with the structurally equivalent loop of contemporary defensins. As expected, the engineered peptide (named MeuTXKα3-KFGGI) remarkably improved the antibacterial activity, particularly on some Gram-positive bacteria, including several antibiotic-resistant opportunistic pathogens. Compared with the unmodified toxin, its antibacterial spectrum also enlarged. Our work provides a new method to enhance the antibacterial activity of bifunctional scorpion venom peptides, which might be useful in engineering other proteins with an ancestral activity.

Venom-based peptide therapy: insights into anti-cancer mechanism

The 5-year relative survival rate of all types of cancer has increased significantly over the past three decades partly due to the targeted therapy. However, still there are many targeted therapy drugs could play a role only in a portion of cancer patients with specific molecular alternation. It is necessary to continue to develop new biological agents which could be used alone and/or in combination with current FDA approved drugs to treat complex cancer diseases. Venom-based drugs have been used for hundreds of years in human history. Nevertheless, the venom-origin of the anti-cancer drug do rarely appear in the pharmaceutical market; and this is due to the fact that the mechanism of action for a large number of the venom drug such as venom-based peptide is not clearly understood. In this review, we focus on discussing some identified venom-based peptides and their anti-cancer mechanisms including the blockade of cancer cell proliferation, invasion, angiogenesis, and metastasis (hallmarks of cancer) to fulfill the gap which is hindering their use in cancer therapy. Furthermore, it also highlights the importance of immunotherapy based on venom peptide. Overall, this review provides readers for further understanding the mechanism of venom peptide and elaborates on the need to explore peptide-based therapeutic strategies.

Correlation between ontogenetic dietary shifts and venom variation in Australian brown snakes (Pseudonaja)

Venom is a key evolutionary trait, as evidenced by its widespread convergent evolution across the animal kingdom. In an escalating prey-predator arms race, venoms evolve rapidly to guarantee predatory or defensive success. Variation in venom composition is ubiquitous among snakes. Here, we tested variation in venom activity on substrates relevant to blood coagulation among Pseudonaja (brown snake) species, Australian elapids responsible for the majority of medically important human envenomations in Australia. A functional approach was employed to elucidate interspecific variation in venom activity in all nine currently recognised species of Pseudonaja. Fluorometric enzymatic activity assays were performed to test variation in whole venom procoagulant activity among species. Analyses confirmed the previously documented ontogenetic shift from non-coagulopathic venom in juveniles to coagulopathic venom as adults, except for the case of P. modesta, which retains non-coagulopathic venom as an adult. These shifts in venom activity correlate with documented ontogenetic shifts in diet among brown snakes from specialisation on reptilian prey as juveniles (and throughout the life cycle of P. modesta), to a more generalised diet in adults that includes mammals. The results of this study bring to light findings relevant to both clinical and evolutionary toxinology.

BmP02 Atypically Delays Kv4.2 Inactivation: Implication for a Unique Interaction between Scorpion Toxin and Potassium Channel

BmP02, a short-chain peptide with 28 residues from the venom of Chinese scorpion Buthus martensi Karsch, has been reported to inhibit the transient outward potassium currents (I_to) in rat ventricular muscle cells. However, it remains unclear whether BmP02 modulates the Kv4.2 channel, one of the main contributors to I_to. The present study investigated the effects of BmP02 on Kv4.2 kinetics and its underlying molecular mechanism. The electrophysiological recordings showed that the inactivation of Kv4.2 expressed in HEK293T cells was significantly delayed by BmP02 in a dose-response manner with EC₅₀ of ~850 nM while the peak current, activation and voltage-dependent inactivation of Kv4.2 were not affected. Meanwhile, the recovery from inactivation of Kv4.2 was accelerated and the deactivation was slowed after the application of BmP02. The site-directed mutagenesis combined with computational modelling identified that K347 and K353, located in the turret motif of the Kv4.2, and E4/E5, D20/D21 in BmP02 are key residues to interact with BmP02 through electrostatic force. These findings not only reveal a novel interaction between Kv4.2 channel and its peptidyl modulator, but also provide valuable information for design of highly-selective Kv4.2 modulators.

Molecular basis for the toxin insensitivity of scorpion voltage-gated potassium channel MmKv1

Our work is the first to uncover the mechanisms by which scorpions resist their own venoms at the ion channel receptor level and enriched our knowledge of the co-evolution of venomous animals and their venoms for hundreds of million years.

Kalium: a database of potassium channel toxins from scorpion venom

Kalium (http://kaliumdb.org/) is a manually curated database that accumulates data on potassium channel toxins purified from scorpion venom (KTx). This database is an open-access resource, and provides easy access to pages of other databases of interest, such as UniProt, PDB, NCBI Taxonomy Browser, and PubMed. General achievements of Kalium are a strict and easy regulation of KTx classification based on the unified nomenclature supported by researchers in the field, removal of peptides with partial sequence and entries supported by transcriptomic information only, classification of β-family toxins, and addition of a novel λ-family. Molecules presented in the database can be processed by the Clustal Omega server using a one-click option. Molecular masses of mature peptides are calculated and available activity data are compiled for all KTx. We believe that Kalium is not only of high interest to professional toxinologists, but also of general utility to the scientific community.

Database URL: http://kaliumdb.org/

A single-point mutation enhances dual functionality of a scorpion toxin

Scorpion venom represents a tremendous, hitherto partially explored peptide library that has been proven to be useful not only for understanding ion channels but also for drug design. MeuTXKα3 is a functionally unknown scorpion toxin-like peptide. Here we describe new transcripts of this gene arising from alternative polyadenylation and its biological function as well as a mutant with a single-point substitution at site 30. Native-like MeuTXKα3 and its mutant were produced in Escherichia coli and their toxic function against Drosophila Shaker K(+) channel and its mammalian counterparts (rKv1.1-rKv1.3) were assayed by two-electrode voltage clamp technique. The results show that MeuTXKα3 is a weak toxin with a wide-spectrum of activity on both Drosophila and mammalian K(+) channels. The substitution of a proline at site 30 by an asparagine, an evolutionarily conserved functional residue in the scorpion α-KTx family, led to an increased activity on rKv1.2 and rKv1.3 but a decreased activity on the Shaker channel without changing the potency on rKv1.1, suggesting a key role of this site in species selectivity of scorpion toxins. MeuTXKα3 was also active on a variety of bacteria with lethal concentrations ranging from 4.66 to 52.01μM and the mutant even had stronger activity on some of these bacterial species. To the best of our knowledge, this is the first report on a bi-functional short-chain peptide in the lesser Asian scorpion venom. Further extensive mutations of MeuTXKα3 at site 30 could help improve its K(+) channel-blocking and antibacterial functions.

Characterization of Kbot21 Reveals Novel Side Chain Interactions of Scorpion Toxins Inhibiting Voltage-Gated Potassium Channels

Scorpion toxins are important pharmacological tools for probing the physiological roles of ion channels which are involved in many physiological processes and as such have significant therapeutic potential. The discovery of new scorpion toxins with different specificities and affinities is needed to further characterize the physiology of ion channels. In this regard, a new short polypeptide called Kbot21 has been purified to homogeneity from the venom of Buthus occitanus tunetanus scorpion. Kbot21 is structurally related to BmBKTx1 from the venom of the Asian scorpion Buthus martensii Karsch. These two toxins differ by only two residues at position 13 (R /V) and 24 (D/N).Despite their very similar sequences, Kbot21 and BmBKTx1 differ in their electrophysiological activities. Kbot21 targets K_V channel subtypes whereas BmBKTx1 is active on both big conductance (BK) and small conductance (SK) Ca²⁺-activated K⁺ channel subtypes, but has no effects on Kv channel subtypes. The docking model of Kbot21 with the Kv1.2 channel shows that the D24 and R13 side-chain of Kbot21 are critical for its interaction with K_V channels.

Engineering a peptide inhibitor towards the KCNQ1/KCNE1 potassium channel (IKs)

The KCNQ1/KCNE1 channel (IKs) plays important roles in the physiological and pathological process of heart, but no potent peptide acting on this channel has been reported. In this work, we found that the natural scorpion venom hardly inhibited KCNQ1/KCNE1 channel currents. Based on this observation, we attempted to use three natural scorpion toxins ChTX, BmKTX and OmTx2 with two different structural folds as templates to engineer potent peptide inhibitors towards the KCNQ1/KCNE1 channel. Pharmacological experiments showed that when we screen with 1μM MT2 peptide, an analog derived from BmKTX toxin, KCNQ1/KCNE1 channel currents could be effectively inhibited. Concentration-dependent experiments showed that MT2 inhibited the KCNQ1/KCNE1 channel with an IC50 value of 4.6±1.9μM. The mutagenesis experiments indicated that MT2 peptide likely used Lys26 residue to interact with the KCNQ1/KCNE1 channel. With MT2 as a new template, we further designed a more potent MT2-2 peptide, which selectively inhibited the KCNQ1/KCNE1 channel with an IC50 of 1.51±0.62μM. Together, this work provided a much potent KCNQ1/KCNE1 channel peptide inhibitor so far, and highlighted the role of molecular strategy in developing potent peptide inhibitors for the natural toxin-insensitive orphan receptors.

A RNA-seq approach to identify putative toxins from acrorhagi in aggressive and non-aggressive Anthopleura elegantissima polyps

BACKGROUND

The use of venom in intraspecific aggression is uncommon and venom-transmitting structures specifically used for intraspecific competition are found in few lineages of venomous taxa. Next-generation transcriptome sequencing allows robust characterization of venom diversity and exploration of functionally unique tissues. Using a tissue-specific RNA-seq approach, we investigate the venom composition and gene ontology diversity of acrorhagi, specialized structures used in intraspecific competition, in aggressive and non-aggressive polyps of the aggregating sea anemone Anthopleura elegantissima (Cnidaria: Anthozoa: Hexacorallia: Actiniaria: Actiniidae).

RESULTS

Collectively, we generated approximately 450,000 transcripts from acrorhagi of aggressive and non-aggressive polyps. For both transcriptomes we identified 65 candidate sea anemone toxin genes, representing phospholipase A2s, cytolysins, neurotoxins, and acrorhagins. When compared to previously characterized sea anemone toxin assemblages, each transcriptome revealed greater within-species sequence divergence across all toxin types. The transcriptome of the aggressive polyp had a higher abundance of type II voltage gated potassium channel toxins/Kunitz-type protease inhibitors and type II acrorhagins. Using toxin-like proteins from other venomous taxa, we also identified 612 candidate toxin-like transcripts with signaling regions, potentially unidentified secretory toxin-like proteins. Among these, metallopeptidases and cysteine rich (CRISP) candidate transcripts were in high abundance. Furthermore, our gene ontology analyses identified a high prevalence of genes associated with "blood coagulation" and "positive regulation of apoptosis", as well as "nucleoside: sodium symporter activity" and "ion channel binding". The resulting assemblage of expressed genes may represent synergistic proteins associated with toxins or proteins related to the morphology and behavior exhibited by the aggressive polyp.

CONCLUSION

We implement a multifaceted approach to investigate the assemblage of expressed genes specifically within acrorhagi, specialized structures used only for intraspecific competition. By combining differential expression, phylogenetic, and gene ontology analyses, we identify several candidate toxins and other potentially important proteins in acrorhagi of A. elegantissima. Although not all of the toxins identified are used in intraspecific competition, our analysis highlights some candidates that may play a vital role in intraspecific competition. Our findings provide a framework for further investigation into components of venom used exclusively for intraspecific competition in acrorhagi-bearing sea anemones and potentially other venomous animals.

Variability of Potassium Channel Blockers in Mesobuthus eupeus Scorpion Venom with Focus on Kv1.1: AN INTEGRATED TRANSCRIPTOMIC AND PROTEOMIC STUDY

The lesser Asian scorpion Mesobuthus eupeus (Buthidae) is one of the most widely spread and dispersed species of the Mesobuthus genus, and its venom is actively studied. Nevertheless, a considerable amount of active compounds is still under-investigated due to the high complexity of this venom. Here, we report a comprehensive analysis of putative potassium channel toxins (KTxs) from the cDNA library of M. eupeus venom glands, and we compare the deduced KTx structures with peptides purified from the venom. For the transcriptome analysis, we used conventional tools as well as a search for structural motifs characteristic of scorpion venom components in the form of regular expressions. We found 59 candidate KTxs distributed in 30 subfamilies and presenting the cysteine-stabilized α/β and inhibitor cystine knot types of fold. M. eupeus venom was then separated to individual components by multistage chromatography. A facile fluorescent system based on the expression of the KcsA-Kv1.1 hybrid channels in Escherichia coli and utilization of a labeled scorpion toxin was elaborated and applied to follow Kv1.1 pore binding activity during venom separation. As a result, eight high affinity Kv1.1 channel blockers were identified, including five novel peptides, which extend the panel of potential pharmacologically important Kv1 ligands. Activity of the new peptides against rat Kv1.1 channel was confirmed (IC50 in the range of 1-780 nm) by the two-electrode voltage clamp technique using a standard Xenopus oocyte system. Our integrated approach is of general utility and efficiency to mine natural venoms for KTxs.

AaTX1, from Androctonus australis scorpion venom: purification, synthesis and characterization in dopaminergic neurons

We have purified the AaTX1 peptide from the Androctonus australis (Aa) scorpion venom, previously cloned and sequenced by Legros and collaborators in a venom gland cDNA library from Aa scorpion. AaTX1 belongs to the α-Ktx15 scorpion toxins family (αKTx15-4). Characterized members of this family share high sequence similarity and were found to block preferentially IA-type voltage-dependent K(+) currents in rat cerebellum granular cells in an irreversible way. In the current work, we studied the effects of native AaTX1 (nAaTX1) using whole-cell patch-clamp recordings of IA current in substantia nigra pars compacta dopaminergic neurons. At 250 nM, AaTX1 induces 90% decrease in IA current amplitude. Its activity was found to be comparable to that of rAmmTX3 (αKTx15-3), which differs by only one conserved (R/K) amino acid in the 19th position suggesting that the difference between R19 and K19 in AaTX1 and AmmTX3, respectively, may not be critical for the toxins' effects. Molecular docking of both toxins with Kv4.3 channel is in agreement with experimental data and suggests the implication of the functional dyade K27-Y36 in toxin-channel interactions. Since AaTX1 is not highly abundant in Aa venom, it was synthesized as well as AmmTX3. Synthetic peptides, native AaTX1 and rAmmTX3 peptides showed qualitatively the same pharmacological activity. Overall, these data identify a new biologically active toxin that belongs to a family of peptides active on Kv4.3 channel.

Cytotoxicity of recombinant tamapin and related toxin-like peptides on model cell lines

The scorpion toxin tamapin displays the most potent and selective blockage against KCa2.2 channels known to date. In this work, we report the biosynthesis, three-dimensional structure, and cytotoxicity on cancer cell lines (Jurkat E6-1 and human mammary breast cancer MDA-MB-231) of recombinant tamapin and five related peptides bearing mutations on residues (R6A,R7A, R13A, R6A-R7A, and GS-tamapin) that were previously suggested to be important for tamapin's activity. The indicated cell lines were used as they constitutively express KCa2.2 channels. The studied toxin-like peptides displayed lethal responses on Jurkat T cells and breast cancer cells; their effect is dose- and time-dependent with IC50 values in the nanomolar range. The order of potency is r-tamapin>GS-tamapin>R6A>R13A>R6A-R7A>R7A for Jurkat T cells and r-tamapin>R7A for MDA-MB-231 breast cancer cells. Our structural determination by NMR demonstrated that r-tamapin preserves the folding of the αKTx5 subfamily and that neither single nor double alanine mutations affect the three-dimensional structure of the wild-type peptide. In contrast, our activity assays show that changes in cytotoxicity are related to the chemical nature of certain residues. Our results suggest that the toxic activity of r-tamapin on Jurkat and breast cancer cells could be mediated by the interaction of charged residues in tamapin with KCa2.2 channels via the apoptotic cell death pathway.

Modulation of TRP ion channels by venomous toxins

Venoms are evolutionarily fine-tuned mixtures of small molecules, peptides, and proteins-referred to as toxins-that have evolved to specifically modulate and interfere with the function of diverse molecular targets within the envenomated animal. Many of the identified toxin targets are membrane receptors and ion channels. Due to their high specificity, toxins have emerged as an invaluable tool set for the molecular characterization of ion channels, and a selected group of toxins even have been developed into therapeutics. More recently, TRP ion channels have been included as targets for venomous toxins. In particular, a number of apparently unrelated peptide toxins target the capsaicin receptor TRPV1 to produce inflammatory pain. These toxins have turned out to be invaluable for structural and functional characterizations of the capsaicin receptor. If toxins will serve similar roles for other TRP ion channels, only future will tell.

Functional evolution of scorpion venom peptides with an inhibitor cystine knot fold

The ICK (inhibitor cystine knot) defines a large superfamily of polypeptides with high structural stability and functional diversity. Here, we describe a new scorpion venom-derived K⁺ channel toxin (named λ-MeuKTx-1) with an ICK fold through gene cloning, chemical synthesis, nuclear magnetic resonance spectroscopy, Ca²⁺ release measurements and electrophysiological recordings. λ-MeuKTx-1 was found to adopt an ICK fold that contains a three-strand anti-parallel β-sheet and a 3₁₀-helix. Functionally, this peptide selectively inhibits the Drosophila Shaker K⁺ channel but is not capable of activating skeletal-type Ca²⁺ release channels/ryanodine receptors, which is remarkably different from the previously known scorpion venom ICK peptides. The removal of two C-terminal residues of λ-MeuKTx-1 led to the loss of the inhibitory activity on the channel, whereas the C-terminal amidation resulted in the emergence of activity on four mammalian K⁺ channels accompanied by the loss of activity on the Shaker channel. A combination of structural and pharmacological data allows the recognition of three putative functional sites involved in channel blockade of λ-MeuKTx-1. The presence of a functional dyad in λ-MeuKTx-1 supports functional convergence among scorpion venom peptides with different folds. Furthermore, similarities in precursor organization, exon–intron structure, 3D-fold and function suggest that scorpion venom ICK-type K⁺ channel inhibitors and Ca²⁺ release channel activators share a common ancestor and their divergence occurs after speciation between buthidae and non-buthids. The structural and functional characterizations of the first scorpion venom ICK toxin with K⁺ channel-blocking activity sheds light on functionally divergent and convergent evolution of this conserved scaffold of ancient origin.

Protease inhibitors from marine venomous animals and their counterparts in terrestrial venomous animals

The Kunitz-type protease inhibitors are the best-characterized family of serine protease inhibitors, probably due to their abundance in several organisms. These inhibitors consist of a chain of ~60 amino acid residues stabilized by three disulfide bridges, and was first observed in the bovine pancreatic trypsin inhibitor (BPTI)-like protease inhibitors, which strongly inhibit trypsin and chymotrypsin. In this review we present the protease inhibitors (PIs) described to date from marine venomous animals, such as from sea anemone extracts and Conus venom, as well as their counterparts in terrestrial venomous animals, such as snakes, scorpions, spiders, Anurans, and Hymenopterans. More emphasis was given to the Kunitz-type inhibitors, once they are found in all these organisms. Their biological sources, specificity against different proteases, and other molecular blanks (being also K⁺ channel blockers) are presented, followed by their molecular diversity. Whereas sea anemone, snakes and other venomous animals present mainly Kunitz-type inhibitors, PIs from Anurans present the major variety in structure length and number of Cys residues, with at least six distinguishable classes. A representative alignment of PIs from these venomous animals shows that, despite eventual differences in Cys assignment, the key-residues for the protease inhibitory activity in all of them occupy similar positions in primary sequence. The key-residues for the K⁺ channel blocking activity was also compared.

Two conserved arginine residues from the SK3 potassium channel outer vestibule control selectivity of recognition by scorpion toxins

Potassium channel functions are often deciphered by using selective and potent scorpion toxins. Among these toxins, only a limited subset is capable of selectively blocking small conductance Ca(2+)-activated K(+) (SK) channels. The structural bases of this selective SK channel recognition remain unclear. In this work, we demonstrate the key role of the electric charges of two conserved arginine residues (Arg-485 and Arg-489) from the SK3 channel outer vestibule in the selective recognition by the SK3-blocking BmP05 toxin. Indeed, individually substituting these residues with histidyl or lysyl (maintaining the positive electric charge partially or fully), although decreasing BmP05 affinity, still preserved the toxin sensitivity profile of the SK3 channel (as evidenced by the lack of recognition by many other types of potassium channel-sensitive charybdotoxin). In contrast, when Arg-485 or Arg-489 of the SK3 channel was mutated to an acidic (Glu) or alcoholic (Ser) amino acid residue, the channel lost its sensitivity to BmP05 and became susceptible to the "new" blocking activity by charybdotoxin. In addition to these SK3 channel basic residues important for sensitivity, two acidic residues, Asp-492 and Asp-518, also located in the SK3 channel outer vestibule, were identified as being critical for toxin affinity. Furthermore, molecular modeling data indicate the existence of a compact SK3 channel turret conformation (like a peptide screener), where the basic rings of Arg-485 and Arg-489 are stabilized by strong ionic interactions with Asp-492 and Asp-518. In conclusion, the unique properties of Arg-485 and Arg-489 (spatial orientations and molecular interactions) in the SK3 channel account for its toxin sensitivity profile.

Recombinant expression of margatoxin and agitoxin-2 in Pichia pastoris: an efficient method for production of KV1.3 channel blockers

The K(v)1.3 voltage-gated potassium channel regulates membrane potential and calcium signaling in human effector memory T cells that are key mediators of autoimmune diseases such as multiple sclerosis, type 1 diabetes, and rheumatoid arthritis. Thus, subtype-specific K(v)1.3 blockers have potential for treatment of autoimmune diseases. Several K(v)1.3 channel blockers have been characterized from scorpion venom, all of which have an α/β scaffold stabilized by 3-4 intramolecular disulfide bridges. Chemical synthesis is commonly used for producing these disulfide-rich peptides but this approach is time consuming and not cost effective for production of mutants, fusion proteins, fluorescently tagged toxins, or isotopically labelled peptides for NMR studies. Recombinant production of K(v)1.3 blockers in the cytoplasm of E. coli generally necessitates oxidative refolding of the peptides in order to form their native disulfide architecture. An alternative approach that avoids the need for refolding is expression of peptides in the periplasm of E. coli but this often produces low yields. Thus, we developed an efficient Pichia pastoris expression system for production of K(v)1.3 blockers using margatoxin (MgTx) and agitoxin-2 (AgTx2) as prototypic examples. The Pichia system enabled these toxins to be obtained in high yield (12-18 mg/L). NMR experiments revealed that the recombinant toxins adopt their native fold without the need for refolding, and electrophysiological recordings demonstrated that they are almost equipotent with the native toxins in blocking K(V)1.3 (IC(50) values of 201±39 pM and 97 ± 3 pM for recombinant AgTx2 and MgTx, respectively). Furthermore, both recombinant toxins inhibited T-lymphocyte proliferation. A MgTx mutant in which the key pharmacophore residue K28 was mutated to alanine was ineffective at blocking K(V)1.3 and it failed to inhibit T-lymphocyte proliferation. Thus, the approach described here provides an efficient method of producing toxin mutants with a view to engineering K(v)1.3 blockers with therapeutic potential.

Expression and characterization of a novel scorpine-like peptide Ev37, from the scorpion Euscorpiops validus

Scorpion venom contains a group of two-domain peptides that function to block potassium channels or have cytolytic activities. These peptides, whose functions are poorly studied, are named β-KTx or scorpine-like peptides. Ev37, the first identified gene in the Euscorpiidae family, which encoded a novel scorpine-like peptide, was cloned from the venom cDNA library of scorpion Euscorpiops validus. Sequence analysis showed that the mature Ev37 peptide contained 78 amino acid residues, which formed two structural domains: a putative α-helical N-terminus and a C-terminus with the cysteine-stabilized α/β motif. The peptide rEv37 and two truncated peptides representing the individual structural domains (Ev37-N and Ev37-C) were expressed in Escherichia coli and purified for functional study. Unlike classic scorpine-like peptides, rEv37 and truncated peptides showed no cytolytic activity against bacteria or eukaryotic cells. Interestingly, rEv37 selectively inhibited Kv1.3 channel without effectively blocking Kv1.1 and Kv1.2 channels. Neither Ev37-N nor Ev37-C blocked Kv1.3 channel, suggesting that both the N-terminal and C-terminal domain of Ev37 are likely involved in the interaction with Kv1.3 channel. These results not only enrich our knowledge of scorpion toxins from scorpine-like subfamily but also provide a novel template with unique structure for designing new types of selective Kv1.3 blockers.

A novel class of antimicrobial peptides from the scorpion Heterometrus spinifer

The venom peptides from the scorpion Heterometrus spinifer have been poorly characterized so far. Here, we identified a novel class of antimicrobial peptides from the venom gland of H. spinifer, which were referred to as HsAp, HsAp2, HsAp3 and HsAp4, respectively. Each of the four peptides consists of 29 amino acid residues, and is cationic and weakly amphipathic. They display no significant homology to any other known peptides, and thus represent a new family of venom peptides from scorpions. Antimicrobial assay showed that HsAp is able to inhibit the growth of both Gram-negative and Gram-positive bacteria with the MIC values of 11.8-51.2 μM. HsAp is also able to inhibit the growth of the tested fungus. Genomic analysis indicated that the genes of all the four peptides are intronless. Our studies expand the families of antimicrobial peptides from scorpions.

Identification of a new specific Kv1.3 channel blocker, Ctri9577, from the scorpion Chaerilus tricostatus

Scorpion toxins are valuable resources for discovering new ion channel modulators and drug candidates. Potassium channel Kv1.3 is an important pharmacological target of T cell-mediated autoimmune diseases, which are encouraging the screening and design of the specific peptide blockers for Kv1.3 channel. Ctri9577, the first neurotoxin gene of Chaerilidae family was cloned from the venom of the scorpion Chaerilus tricostatus through the constructing its cDNA library. The sequence analysis showed that the mature peptide of Ctri9577 contained 39 amino acid residues including six conserved cysteines, whose low sequence similarity indicated that it was a new member of α-KTx15 subfamily. By using expression and purification technology, the recombinant peptide was obtained. Subsequently, the electrophysiological experiments indicated that the Ctri9577 peptide selectively inhibited Kv1.3 channel current with an IC(50) of 0.49±0.45 nM without effectively blocking potassium channels Kv1.1, Kv1.2, hERG and SK3. All these findings not only enrich the knowledge of toxins from the Chaerilidae family, but also present a novel potential drug candidate targeting Kv1.3 channels for the therapy of autoimmune diseases.

Potassium Channels Blockers from the Venom of Androctonus mauretanicus mauretanicus

K⁺ channels selectively transport K⁺ ions across cell membranes and play a key role in regulating the physiology of excitable and nonexcitable cells. Their activation allows the cell to repolarize after action potential firing and reduces excitability, whereas channel inhibition increases excitability. In eukaryotes, the pharmacology and pore topology of several structural classes of K⁺ channels have been well characterized in the past two decades. This information has come about through the extensive use of scorpion toxins. We have participated in the isolation and in the characterization of several structurally distinct families of scorpion toxin peptides exhibiting different K⁺ channel blocking functions. In particular, the venom from the Moroccan scorpion Androctonus mauretanicus mauretanicus provided several high-affinity blockers selective for diverse K⁺ channels  (SK_Ca,  K_v4.x, and  K_v1.x K⁺ channel families). In this paper, we summarize our work on these toxin/channel interactions.

Structure, function, and chemical synthesis of Vaejovis mexicanus peptide 24: a novel potent blocker of Kv1.3 potassium channels of human T lymphocytes

Animal venoms are rich sources of ligands for studying ion channels and other pharmacological targets. Proteomic analyses of the soluble venom from the Mexican scorpion Vaejovis mexicanus smithi showed that it contains more than 200 different components. Among them, a 36-residue peptide with a molecular mass of 3864 Da (named Vm24) was shown to be a potent blocker of Kv1.3 of human lymphocytes (K(d) ∼ 3 pM). The three-dimensional solution structure of Vm24 was determined by nuclear magnetic resonance, showing the peptide folds into a distorted cystine-stabilized α/β motif consisting of a single-turn α-helix and a three-stranded antiparallel β-sheet, stabilized by four disulfide bridges. The disulfide pairs are formed between Cys6 and Cys26, Cys12 and Cys31, Cys16 and Cys33, and Cys21 and Cys36. Sequence analyses identified Vm24 as the first example of a new subfamily of α-type K(+) channel blockers (systematic number α-KTx 23.1). Comparison with other Kv1.3 blockers isolated from scorpions suggests a number of structural features that could explain the remarkable affinity and specificity of Vm24 toward Kv1.3 channels of lymphocytes.

Toxin modulators and blockers of hERG K(+) channels

The K(+) channel encoded by the Ether-á-go-go-Related Gene (ERG) is expressed in different tissues of different animal species. There are at least three subtypes of this channel, being the sub-type 1 (ERG1) crucial in the repolarization phase of the cardiac action potential. Mutations in this gene can affect the properties of the channel producing the type II long QT syndrome (LQTS2) and many drugs are also known to affect this channel with a similar side effect. Various scorpion, spider and sea anemone toxins affect the ERG currents by blocking the ion-conducting pore from the external side or by modulating channel gating through binding to the voltage-sensor domain. By doing so, these toxins become very useful tools for better understanding the structural and functional characteristics of these ion channels. This review discusses the interaction between the ERG channels and the peptides isolated from venoms of these animals. Special emphasis is placed on scorpion toxins, although the effects of several spider venom toxins and anemone toxins will be also revised.

Looking over toxin-K(+) channel interactions. Clues from the structural and functional characterization of α-KTx toxin Tc32, a Kv1.3 channel blocker

α-KTx toxin Tc32, from the Amazonian scorpion Tityus cambridgei, lacks the dyad motif, including Lys27, characteristic of the family and generally associated with channel blockage. The toxin has been cloned and expressed for the first time. Electrophysiological experiments, by showing that the recombinant form blocks Kv1.3 channels of olfactory bulb periglomerular cells like the natural Tc32 toxin, when tested on the Kv1.3 channel of human T lymphocytes, confirmed it is in an active fold. The nuclear magnetic resonance-derived structure revealed it exhibits an α/β scaffold typical of the members of the α-KTx family. TdK2 and TdK3, all belonging to the same α-KTx 18 subfamily, share significant sequence identity with Tc32 but diverse selectivity and affinity for Kv1.3 and Kv1.1 channels. To gain insight into the structural features that may justify those differences, we used the recombinant Tc32 nuclear magnetic resonance-derived structure to model the other two toxins, for which no experimental structure is available. Their interaction with Kv1.3 and Kv1.1 has been investigated by means of docking simulations. The results suggest that differences in the electrostatic features of the toxins and channels, in their contact surfaces, and in their total dipole moment orientations govern the affinity and selectivity of toxins. In addition, we found that, regardless of whether the dyad motif is present, it is always a Lys side chain that physically blocks the channels, irrespective of its position in the toxin sequence.

Hg1, novel peptide inhibitor specific for Kv1.3 channels from first scorpion Kunitz-type potassium channel toxin family

The potassium channel Kv1.3 is an attractive pharmacological target for autoimmune diseases. Specific peptide inhibitors are key prospects for diagnosing and treating these diseases. Here, we identified the first scorpion Kunitz-type potassium channel toxin family with three groups and seven members. In addition to their function as trypsin inhibitors with dissociation constants of 140 nM for recombinant LmKTT-1a, 160 nM for LmKTT-1b, 124 nM for LmKTT-1c, 136 nM for BmKTT-1, 420 nM for BmKTT-2, 760 nM for BmKTT-3, and 107 nM for Hg1, all seven recombinant scorpion Kunitz-type toxins could block the Kv1.3 channel. Electrophysiological experiments showed that six of seven scorpion toxins inhibited ~50-80% of Kv1.3 channel currents at a concentration of 1 μM. The exception was rBmKTT-3, which had weak activity. The IC(50) values of rBmKTT-1, rBmKTT-2, and rHg1 for Kv1.3 channels were ~129.7, 371.3, and 6.2 nM, respectively. Further pharmacological experiments indicated that rHg1 was a highly selective Kv1.3 channel inhibitor with weak affinity for other potassium channels. Different from classical Kunitz-type potassium channel toxins with N-terminal regions as the channel-interacting interfaces, the channel-interacting interface of Hg1 was in the C-terminal region. In conclusion, these findings describe the first scorpion Kunitz-type potassium channel toxin family, of which a novel inhibitor, Hg1, is specific for Kv1.3 channels. Their structural and functional diversity strongly suggest that Kunitz-type toxins are a new source to screen and design potential peptides for diagnosing and treating Kv1.3-mediated autoimmune diseases.

Interacting sites of scorpion toxin ErgTx1 with hERG1 K+ channels

Peptides purified from scorpion venoms were shown to interact with specific amino acid residues present in the outer vestibule of various sub-types of potassium channels, occluding the pore and causing a decrement of K(+) permeability through the membrane of excitable and non excitable cells. This communication describes the identification of several interacting sites of toxin ErgTx1, a toxin purified from the venom of the scorpion Centruroides noxius, with the human ERG1 K(+) channels, by means of site-directed mutagenesis of specific residues of the toxin. Recombinant mutants of the gene coding for ErgTx1 were expressed heterologously in Escherichia coli, properly folded and their affinities and interactions with hERG1 channels were determined by patch-clamp techniques. Residues in position Y14, Y17 and F37 of the solvent exposed hydrophobic surface, and charged residues at the position K13 and K38 of ErgTx1 were shown to cause a decrement of the affinity from 20 folds to 3 orders of magnitude, thus suggesting that they are certainly participating on the binding surface of this toxin towards the hERG1 channels. Double mutants at positions K13 and F37, Y14 and F37, Y17 and F37 and K13 and K38 were also prepared and assayed, but the results obtained are not much different from the single point mutants of ErgTx1. The results of the present work indicate the most probable surface area of ErgTx1 that makes contact with the hERG channels.

Molding the business end of neurotoxins by diversifying evolution

A diverse range of organisms utilize neurotoxins that target specific ion channels and modulate their activity. Typically, toxins are clustered into several multigene families, providing an organism with the upper hand in the never-ending predator-prey arms race. Several gene families, including those encoding certain neurotoxins, have been subject to diversifying selection forces, resulting in rapid gene evolution. Here we sought a spatial pattern in the distribution of both diversifying and purifying selection forces common to neurotoxin gene families. Utilizing the mechanistic empirical combination model, we analyzed various toxin families from different phyla affecting various receptors and relying on diverse modes of action. Through this approach, we were able to detect clear correlations between the pharmacological surface of a toxin and rapidly evolving domains, rich in positively selected residues. On the other hand, patches of negatively selected residues were restricted to the nontoxic face of the molecule and most likely help in stabilizing the tertiary structure of the toxin. We thus propose a mutual evolutionary strategy of venomous animals in which adaptive molecular evolution is directed toward the toxin active surface. Furthermore, we propose that the binding domains of unstudied toxins could be readily predicted using evolutionary considerations.

The new kappa-KTx 2.5 from the scorpion Opisthacanthus cayaporum

The kappa-KTx family of peptides, which is the newest K⁺-channel blocker family from scorpion venom, is present in scorpions from the families Scorpionidae and Liochelidae. Differently from the other scorpion KTx families, the three-dimensional structure of the known kappa-KTxs toxins is formed by two parallel α-helices linked by two disulfide bridges. Here, the characterization of a new kappa-KTx peptide, designated kappa-KTx 2.5, derived from the Liochelidae scorpion Opisthacanthus cayaporum, is described. This peptide was purified by HPLC and found to be identical to OcyC8, a predicted mature sequence precursor (UniProtKB C5J89) previously described by our group. The peptide was chemically synthesized and the circular dichroism (CD) spectra of both, native and synthetic, conducted at different temperatures in water and water/trifluoroethanol (TFE), showed a predominance of α-helices. The kappa-KTx 2.5 is heat stable and was shown to be a blocker of K⁺-currents on hKv1.1, and hKv1.4, with higher affinity for Kv1.4 channels (IC₅₀= 71 μM). Similarly to the other kappa-KTxs, the blockade of K⁺-channels occurred at micromolar concentrations, leading to uncertainness about their proper molecular target, and consequently their pharmacologic effect. In order to test other targets, kappa-KTx2.5 was tested on other K⁺-channels, on Na⁺-channels, on bacterial growth and on smooth muscle tissue, a known assay to identify possible bradykinin-potentiating peptides, due to the presence of two contiguous prolines at the C-terminal sequence. It has no effect on the targets used except on hKv1.1, and hKv1.4 expressed in Chinese hamster ovary cells. Since the only plausible function found for kappa-KTx2.5 seems to be the blockade of K⁺-channels, a discussion regarding the analysis of structure-function relationships is included in this communication, based on sequence alignments of members of the kappa-KTx toxin family, and on computational simulation of a structural model of the kappa-KTx2.5-Kv1.2 complex.