OsK2, a new selective inhibitor of Kv1.2 potassium channels purified from the venom of the scorpion Orthochirus scrobiculosus

Fifty Years of Animal Toxin Research at the Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry RAS

This review covers briefly the work carried out at our institute (IBCh), in many cases in collaboration with other Russian and foreign laboratories, for the last 50 years. It discusses the discoveries and studies of various animal toxins, including protein and peptide neurotoxins acting on the nicotinic acetylcholine receptors (nAChRs) and on other ion channels. Among the achievements are the determination of the primary structures of the α-bungarotoxin-like three-finger toxins (TFTs), covalently bound dimeric TFTs, glycosylated cytotoxin, inhibitory cystine knot toxins (ICK), modular ICKs, and such giant molecules as latrotoxins and peptide neurotoxins from the snake, as well as from other animal venoms. For a number of toxins, spatial structures were determined, mostly by 1H-NMR spectroscopy. Using this method in combination with molecular modeling, the molecular mechanisms of the interactions of several toxins with lipid membranes were established. In more detail are presented the results of recent years, among which are the discovery of α-bungarotoxin analogs distinguishing the two binding sites in the muscle-type nAChR, long-chain α-neurotoxins interacting with α9α10 nAChRs and with GABA-A receptors, and the strong antiviral effects of dimeric phospholipases A2. A summary of the toxins obtained from arthropod venoms includes only highly cited works describing the molecules' success story, which is associated with IBCh. In marine animals, versatile toxins in terms of structure and molecular targets were discovered, and careful work on α-conotoxins differing in specificity for individual nAChR subtypes gave information about their binding sites.

KV1.2 channel-specific blocker from Mesobuthus eupeus scorpion venom: Structural basis of selectivity

Scorpion venom is an unmatched source of selective high-affinity ligands of potassium channels. There is a high demand for such compounds to identify and manipulate the activity of particular channel isoforms. The objective of this study was to obtain and characterize a specific ligand of voltage-gated potassium channel K_V1.2. As a result, we report the remarkable selectivity of the peptide MeKTx11-1 (α-KTx 1.16) from Mesobuthus eupeus scorpion venom to this channel isoform. MeKTx11-1 is a high-affinity blocker of K_V1.2 (IC₅₀ ∼0.2 nM), while its activity against K_V1.1, K_V1.3, and K_V1.6 is 10 000, 330 and 45 000 fold lower, respectively, as measured using the voltage-clamp technique on mammalian channels expressed in Xenopus oocytes. Two substitutions, G9V and P37S, convert MeKTx11-1 to its natural analog MeKTx11-3 (α-KTx 1.17) having 15 times lower activity and reduced selectivity to K_V1.2. We produced MeKTx11-1 and MeKTx11-3 as well as their mutants MeKTx11-1(G9V) and MeKTx11-1(P37S) recombinantly and demonstrated that point mutations provide an intermediate effect on selectivity. Key structural elements that explain MeKTx11-1 specificity were identified by molecular modeling of the toxin-channel complexes. Confirming our molecular modeling predictions, site-directed transfer of these elements from the pore region of K_V1.2 to K_V1.3 resulted in the enhanced sensitivity of mutant K_V1.3 channels to MeKTx11-1. We conclude that MeKTx11-1 may be used as a selective tool in neurobiology.

Animal toxins for channelopathy treatment

Ion channels are transmembrane proteins that allow passive flow of ions inside and/or outside of cells or cell organelles. Except mutations lead to nonfunctional protein production or abolished receptor entrance on the membrane surface an altered channel may have two principal conditions that can be corrected. The channel may conduct fewer ions through (loss-of-function mutations) or too many ions (gain-of-function mutations) compared to a normal channel. Toxins from animal venoms are specialised molecules that are generally oriented toward interactions with ion channels. This is a result of long coevolution between predators and their prey. On the molecular level, toxins activate or inhibit ion channels, so they are ideal molecules for restoring conductance in mutated channels. Another aspect of this long coevolution is that a broad variety of toxins have been fine tuned to recognize the channels of different species, keeping many amino acids substitution among sequences. Many peptide ligands with high selectivity to specific receptor subtypes have been isolated from animal venoms, some of which are absolutely non-toxic to humans and mammalians. It is expected that molecules that are selective to each known receptor can be found in animal venoms, but the pool of toxins currently does not override all receptors described as being involved in channelopathies. Modern investigating methods have enhanced the search process for selective ligands. One prominent method is a site-directed mutagenesis of existing toxins to change the selectivity or/and affinity to the selected receptor, which has shown positive results. This article is part of the Special Issue entitled 'Channelopathies.'

Arthropod toxins acting on neuronal potassium channels

Arthropod venoms are a rich mixture of biologically active compounds exerting different physiological actions across diverse phyla and affecting multiple organ systems including the central nervous system. Venom compounds can inhibit or activate ion channels, receptors and transporters with high specificity and affinity providing essential insights into ion channel function. In this review, we focus on arthropod toxins (scorpions, spiders, bees and centipedes) acting on neuronal potassium channels. A brief description of the K⁺ channels classification and structure is included and a compendium of neuronal K⁺ channels and the arthropod toxins that modify them have been listed. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

C-Terminal residues in small potassium channel blockers OdK1 and OSK3 from scorpion venom fine-tune the selectivity

We report isolation, sequencing, and electrophysiological characterization of OSK3 (α-KTx 8.8 in Kalium and Uniprot databases), a potassium channel blocker from the scorpion Orthochirus scrobiculosus venom. Using the voltage clamp technique, OSK3 was tested on a wide panel of 11 voltage-gated potassium channels expressed in Xenopus oocytes, and was found to potently inhibit Kv1.2 and Kv1.3 with IC₅₀ values of ~331nM and ~503nM, respectively. OdK1 produced by the scorpion Odontobuthus doriae differs by just two C-terminal residues from OSK3, but shows marked preference to Kv1.2. Based on the charybdotoxin-potassium channel complex crystal structure, a model was built to explain the role of the variable residues in OdK1 and OSK3 selectivity.

Scorpion toxin peptide action at the ion channel subunit level

This review categorizes functionally validated actions of defined scorpion toxin (SCTX) neuropeptides across ion channel subclasses, highlighting key trends in this rapidly evolving field. Scorpion envenomation is a common event in many tropical and subtropical countries, with neuropharmacological actions, particularly autonomic nervous system modulation, causing significant mortality. The primary active agents within scorpion venoms are a diverse group of small neuropeptides that elicit specific potent actions across a wide range of ion channel classes. The identification and functional characterisation of these SCTX peptides has tremendous potential for development of novel pharmaceuticals that advance knowledge of ion channels and establish lead compounds for treatment of excitable tissue disorders. This review delineates the unique specificities of 320 individual SCTX peptides that collectively act on 41 ion channel subclasses. Thus the SCTX research field has significant translational implications for pathophysiology spanning neurotransmission, neurohumoral signalling, sensori-motor systems and excitation-contraction coupling. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Purification, molecular cloning and functional characterization of HelaTx1 (Heterometrus laoticus): the first member of a new κ-KTX subfamily

Given their medical importance, most attention has been paid toward the venom composition of scorpions of the Buthidae family. Nevertheless, research has shown that the venom of scorpions of other families is also a remarkable source of unique peptidyl toxins. The κ-KTx family of voltage-gated potassium channel (VGPC) scorpion toxins is hereof an example. From the telson of the scorpion Heterometrus laoticus (Scorpionidae), a peptide, HelaTx1, with unique primary sequence was purified through HPLC and sequenced by Edman degradation. Based on the amino acid sequence, the peptide could be cloned and the cDNA sequence revealed. HelaTx1 was chemically synthesized and functionally characterized on VGPCs of the Shaker-related, Shab-related, Shaw-related and Shal-related subfamilies. Furthermore, the toxin was also tested on small- and intermediate conductance Ca(2+)-activated K(+) channels. From the channels studied, K(v)1.1 and K(v)1.6 were found to be the most sensitive (K(v)1.1 EC(50)=9.9±1.6 μM). The toxin did not alter the activation of the channels. Competition experiments with TEA showed that the toxin is a pore blocker. Mutational studies showed that the residues E353 and Y379 in the pore of K(v)1.1 act as major interaction points for binding of the toxin. Given the amino acid sequence, the predicted secondary structure and the biological activity on VGPCs, HelaTx1 should be included in the κ-KTX family. Based on a phylogenetic study, we rearranged this family of VGPC toxins into five subfamilies and suggest that HelaTx1 is the first member of the new κ-KTx5 subfamily.

Proteomic analysis of the venom from the scorpion Tityus stigmurus: biochemical and physiological comparison with other Tityus species

The venom from the Brazilian scorpion Tityus stigmurus was fractionated by high performance liquid chromatography (HPLC) and the corresponding components were used for molecular mass determination using electrospray ion trap mass spectrometry. One hundred distinct components were clearly assigned showing molecular masses from 216.5 to 44,800.0 Da. Fifteen new components were isolated and sequenced, four of them to completion: Tst-3 (similar to Na(+) channel specific scorpion toxins), Tst-17 (a K(+) channel blocking peptide similar to Tc1), Tst beta KTx (a peptide with identical sequence as that of TsTX-K beta toxin earlier described to exist in T. serrulatus venom) and finally a novel proline-rich peptide of unknown function. Among the eleven components partially sequenced were two enzymes: hyaluronidase and lysozyme. The first enzyme has a molecular mass of 44,800.0 Da. This enzyme showed high activity against the substrate hyaluronan in vitro. Amino acid sequence of the second enzyme showed that it is similar to other known lysozymes, with similar molecular mass and sequence to that of bona fide lysozymes reported in public protein data banks. Finally, this communication reports a correlation among HPLC retention times and molecular masses of folded scorpion toxins as well as a comparative structural and physiological analysis of components from the venom of several species of the genus Tityus.

The first potassium channel toxin from the venom of the Iranian scorpion Odonthobuthus doriae

The very first member of K(+) channels toxins from the venom of the Iranian scorpion Odonthobuthus doriae (OdK1) was purified, sequenced and characterized physiologically. OdK1 has 29 amino acids, six conserved cysteines and a pI value of 4.95. Based on multiple sequence alignments, OdK1 was classified as alpha-KTx 8.5. The pharmacological effects of OdK1 were studied on six different cloned K(+) channels (vertebrate Kv1.1-Kv1.5 and Shaker IR) expressed in Xenopus laevis oocytes. Interestingly, OdK1 selectively inhibited the currents through Kv1.2 channels with an IC50 value of 183+/-3 nM but did not affect any of the other channels.

Mass spectrometry strategies for venom mapping and peptide sequencing from crude venoms: case applications with single arthropod specimen

Due to their complexity and diversity, animal venoms represent an extensive source of bioactive compounds such as peptides and proteins. Conventional approaches for their characterization often require large quantities of biological material. Current mass spectrometry (MS) techniques now give access to a wealth of information in a short working time frame with minute amounts of sample. Such MS approaches may now be used for the discovery of novel compounds, and once their structure has been determined they may be synthesized and tested for functional activity. Molecular mass fingerprints of venoms allow the rapid identification of known toxins as well as preliminary structural characterization of new compounds. De novo peptide sequencing by tandem mass spectrometry (MS/MS) offers rapid access to partial or total primary peptide structures. This article, written as a tutorial, also contains new material: molecular mass fingerprint analysis of Orthochirus innesi scorpion venom, and identification of components from bumblebee Bombus lapidarius venom, both collected from one single specimen. The structure of the three major peptides detected in the Bombus venom was fully characterized in one working day by de novo sequencing using an electrospray ionization hybrid quadrupole time-of-flight instrument (ESI-QqTOF) and a matrix-assisted laser desorption ionization time-of-flight instrument (MALDI-LIFT-TOF-TOF). After presenting the MS-based sequence elucidation, perspectives in using MS and MS/MS techniques in toxinology are discussed.

Proteomic analysis of the venom and characterization of toxins specific for Na+- and K+-channels from the Colombian scorpion Tityus pachyurus

The Colombian scorpion Tityus pachyurus is toxic to humans and is capable of producing fatal accidents, but nothing is known about its venom components. This communication reports the separation of at least 57 fractions from the venom by high performance liquid chromatography. From these, at least 104 distinct molecular weight compounds were identified by mass spectrometry analysis. The complete amino acid sequences of three peptides were determined and the partial sequences of three others were also identified. Electrophysiological experiments conducted with ion-channels expressed heterologously on Sf9 cells showed the presence of a potent Shaker B K(+)-channel blocker. This peptide (trivial name Tpa1) contains 23 amino acid residues closely packed by three disulfide bridges with a molecular mass of 2,457 atomic mass units. It is the third member of the sub-family 13, for which the systematic name is proposed to be alpha-KTx13.3. The mice assay showed clearly the presence of toxic peptides to mammals. One of them named Tpa2, containing 65 amino acid residues with molecular mass of 7,522.5 atomic mass units, is stabilized by four disulfide bridges. It was shown to modify the Na(+)-currents of F-11 and TE671 cells in culture, similar to the beta scorpion toxins. These results demonstrate the presence of toxic peptides in the venom of T. pachyurus and confirm that accidents with this species of scorpion should be considered an important human hazard in Colombia.

Accurate prediction of scorpion toxin functional properties from primary structures

Scorpion toxins are common experimental tools for studies of biochemical and pharmacological properties of ion channels. The number of functionally annotated scorpion toxins is steadily growing, but the number of identified toxin sequences is increasing at much faster pace. With an estimated 100,000 different variants, bioinformatic analysis of scorpion toxins is becoming a necessary tool for their systematic functional analysis. Here, we report a bioinformatics-driven system involving scorpion toxin structural classification, functional annotation, database technology, sequence comparison, nearest neighbour analysis, and decision rules which produces highly accurate predictions of scorpion toxin functional properties.

A novel scorpion toxin blocking small conductance Ca2+ activated K+ channel

Small conductance calcium activated potassium channels (SK) are crucial in the regulation of cell firing frequency in the nervous system and other tissues. In the present work, a novel SK channel blocker, designated BmSKTx1, was purified from the scorpion Buthus martensi Karsh venom. The sequence of the N-terminal 22 amino acid residues was determined by Edman degradation. Using this sequence information, the full-length cDNA and genomic gene of BmSKTx1 were cloned and sequenced. By these analyses, BmSKTx1 was found to be a peptide composed of 31 amino acid residues with three disulfide bonds. It shared little sequence homology with other known scorpion alpha-KTxs but showed close relationship with SK channel blockers in the phylogenetic tree. According to the previous nomenclature, BmSKTx1 was classified as alpha-KTx14.1. We examined the effects of BmSKTx1 on different ion channels of rat adrenal chromaffin cells (RACC) and locust dorsal unpaired median (DUM) neurons. BmSKTx1 selectively inhibited apamin-sensitive SK currents in RACC with Kd of 0.72 microM and Hill coefficient of 2.2. And it had no effect on Na+, Ca2+, Kv, and BK currents in DUM neuron, indicating that BmSKTx1 was a selective SK toxin.

Synthesis and characterization of Pi4, a scorpion toxin from Pandinus imperator that acts on K+ channels

Pi4 is a 38-residue toxin cross-linked by four disulfide bridges that has been isolated from the venom of the Chactidae scorpion Pandinus imperator. Together with maurotoxin, Pi1, Pi7 and HsTx1, Pi4 belongs to the alpha KTX6 subfamily of short four-disulfide-bridged scorpion toxins acting on K+ channels. Due to its very low abundance in venom, Pi4 was chemically synthesized in order to better characterize its pharmacology and structural properties. An enzyme-based cleavage of synthetic Pi4 (sPi4) indicated half-cystine pairings between Cys6-Cys27, Cys12-32, Cys16-34 and Cys22-37, which denotes a conventional pattern of scorpion toxin reticulation (Pi1/HsTx1 type). In vivo, sPi4 was lethal after intracerebroventricular injection to mice (LD50 of 0.2 microg per mouse). In vitro, addition of sPi4 onto Xenopus laevis oocytes heterologously expressing various voltage-gated K+ channel subtypes showed potent inhibition of currents from rat Kv1.2 (IC50 of 8 pm) and Shaker B (IC50 of 3 nm) channels, whereas no effect was observed on rat Kv1.1 and Kv1.3 channels. The sPi4 was also found to compete with 125I-labeled apamin for binding to small-conductance Ca(2+)-activated K+ (SK) channels from rat brain synaptosomes (IC50 value of 0.5 microm). sPi4 is a high affinity blocker of the Kv1.2 channel. The toxin was docked (BIGGER program) on the Kv channel using the solution structure of sPi4 and a molecular model of the Kv1.2 channel pore region. The model suggests a key role for residues Arg10, Arg19, Lys26 (dyad), Ile28, Lys30, Lys33 and Tyr35 (dyad) in the interaction and the associated blockage of the Kv1.2 channel.

Novel interactions between K+ channels and scorpion toxins

K(+) channels are macromolecules embedded in biological membranes, where they play a key role in cellular excitability and signal transduction pathways. Knowledge of their structure should help improve our understanding of their function and lead to the design of therapeutic compounds. Most pharmacological and structural characteristics of these channels have been elucidated by using high-affinity channel blockers isolated from scorpion venoms. Recent data on the three-dimensional structures of K(+) channels and novel scorpion toxins suggest a variety of novel interacting modes of these channels and toxins, which should help increase our understanding of the K(+) channel structure-function relationship.

An overview of toxins and genes from the venom of the Asian scorpion Buthus martensi Karsch

Among the different scorpion species, Buthus martensi Karsch (BmK), a widely distributed scorpion species in Asia, has received a lot of attention. Indeed, over the past decade, more than 70 different peptides, toxins or homologues have been isolated and more peptides are probably still to be revealed. This review is focusing on the many peptides isolated from the venom of this scorpion, their targets, their genes and their structures. The aim is to give both a 'state of the art' view of the research on BmK venom and an illustration of the complexity of this scorpion venom. In the present manuscript, we have listed the different ion channel toxins and homologues isolated from the venom of BmK, either from the literature or from databases. We have described here 51 long-chain peptides related to the Na(+) channel toxins family: 34 related to the alpha-toxin family, four related to the excitatory insect toxin family, 10 related to the depressant insect toxin, one beta-like toxin plus two peptides, BmK AS and AS1, that act on ryanodine receptors. We also listed 18 peptides related to the K(+) channel toxin family: 14 short chain toxins or homologues, two long chain K(+) toxin homologues and two putative K(+) toxin precursors. Additionally, two chlorotoxin like peptides (Bm-12 and 12 b) have been isolated in the venom of BmK. Besides these ion channels toxins, two peptides without disulfide bridges (the bradykinin-potentiating peptide BmK bpp and BmK n1) and three peptides with no known functions have also been discovered in this venom. We have also taken the opportunity of this review to update the classification of scorpion K(+) toxins () which now presents 17 subfamilies instead of the 12 described earlier. The work on the venom of BmK led to the discovery of two new subfamilies, alpha-KT x 14 and alpha-KT x 17.