Functional site of bukatoxin, an alpha-type sodium channel neurotoxin from the Chinese scorpion (Buthus martensi Karsch) venom: probable role of the (52)PDKVP(56) loop

Biochemical and Proteomic Characterization, and Pharmacological Insights of Indian Red Scorpion Venom Toxins

The Indian red scorpion (Mesobuthus tamulus) is one of the world's deadliest scorpions, with stings representing a life-threatening medical emergency. This species is distributed throughout the Indian sub-continent, including eastern Pakistan, eastern Nepal, and Sri Lanka. In India, Indian red scorpions are broadly distributed in western Maharashtra, Saurashtra, Kerala, Andhra Pradesh, Tamil Nadu, and Karnataka; however, fatal envenomations have been recorded primarily in the Konkan region of Maharashtra. The Indian red scorpion venom proteome comprises 110 proteins belonging to 13 venom protein families. The significant pharmacological activity is predominantly caused by the low molecular mass non-enzymatic Na+ and K+ ion channel toxins. Other minor toxins comprise 15.6% of the total venom proteome. Indian red scorpion stings induce the release of catecholamine, which leads to pathophysiological abnormalities in the victim. A strong correlation has been observed between venom proteome composition and local (swelling, redness, heat, and regional lymph node involvement) and systemic (tachycardia, mydriasis, hyperglycemia, hypertension, toxic myocarditis, cardiac failure, and pulmonary edema) manifestations. Immediate administration of antivenom is the preferred treatment for Indian red scorpion stings. However, scorpion-specific antivenoms have exhibited poor immunorecognition and neutralization of the low molecular mass toxins. The proteomic analysis also suggests that Indian red scorpion venom is a rich source of pharmacologically active molecules that may be envisaged as drug prototypes. The following review summarizes the progress made towards understanding the venom proteome of the Indian red scorpion and addresses the current understanding of the pathophysiology associated with its sting.

Toxin Ct1a, from venom of Centruroides tecomanus, modifies the spontaneous firing frequency of neurons in the suprachiasmatic nucleus

The peptide, denominated Ct1a, is a β-toxin of 66 amino acids, isolated from venom of the scorpion, Centruroides tecomanus, collected in Colima, Mexico. This toxin was purified using size exclusion, cationic exchange, and reverse phase chromatography. It is the most abundant toxin, representing 1.7% of the soluble venom. Its molecular mass of 7588.9 Da was determined by mass spectrometry. The amino acid sequence was determined by Edman degradation and confirmed by transcriptomic analysis. Since neurons of the suprachiasmatic nucleus (SCN) maintain a spontaneous firing rate (SFR), we evaluated the physiological effects of toxin Ct1a on these neurons. The SFR exhibited a bimodal concentration-dependent response: 100 nM of Ct1a increased the SFR by 223%, whereas 500 nM and 1000 nM reduced it to 42% and 7%, respectively. Control experiments, consisting of recordings of the SFR during a time similar to that used in Ct1a testing, showed stability throughout the trials. Experiments carried out with denatured Ct1a toxin (500 nM) caused no variation in SFR recordings. Action potentials of SCN neurons, before and after Ct1a (100 nM) showed changes in the time constants of depolarization and repolarization phases, amplitude, and half-time. Finally, recordings of hNav1.6 sodium currents indicated that Ct1a shifts the channel activation to a more negative potential and reduces the amplitude of the peak current. These results all demonstrate that toxin Ct1a affects the SFR of SCN neurons by acting upon sodium channels of sub-type 1.6, implicating them in regulation of the SFR of SCN neurons.

Toxinology provides multidirectional and multidimensional opportunities: A personal perspective

In nature, toxins have evolved as weapons to capture and subdue the prey or to counter predators or competitors. When they are inadvertently injected into humans, they cause symptoms ranging from mild discomfort to debilitation and death. Toxinology is the science of studying venoms and toxins that are produced by a wide variety of organisms. In the past, the structure, function and mechanisms of most abundant and/or most toxic components were characterized to understand and to develop strategies to neutralize their toxicity. With recent technical advances, we are able to evaluate and determine the toxin profiles using transcriptomes of venom glands and proteomes of tiny amounts of venom. Enormous amounts of data from these studies have opened tremendous opportunities in many directions of basic and applied research. The lower costs for profiling venoms will further fuel the expansion of toxin database, which in turn will provide greater exciting and bright opportunities in toxin research.

Scorpion Venom Heat-Resistant Peptide is Neuroprotective against Cerebral Ischemia-Reperfusion Injury in Association with the NMDA-MAPK Pathway

Scorpion venom heat-resistant peptide (SVHRP) is a component purified from Buthus martensii Karsch scorpion venom. Our previous studies have shown that SVHRP is neuroprotective in models of Alzheimer's disease and Parkinson's disease. The present study aimed to explore the potential neuroprotective effects of SVHRP on cerebral ischemia/reperfusion (I/R) injury, using a mouse model of middle cerebral artery occlusion/reperfusion (MCAO/R) and a cellular model of oxygen-glucose deprivation/reoxygenation (OGD/R). Our results showed that SVHRP treatment decreased the neurological deficit scores, edema formation, infarct volume and neuronal loss in the MCAO/R mice, and protected primary neurons against OGD/R insult. SVHRP pretreatment suppressed the alterations in protein levels of N-methyl-D-aspartate receptors (NMDARs) and phosphorylated p38 MAPK as well as some proinflammatory factors in both the animal and cellular models. These results suggest that SVHRP has neuroprotective effects against cerebral I/R injury, which might be associated with inhibition of the NMDA-MAPK-mediated excitotoxicity.

The role of Ser54 in the antinociceptive activity of BmK9, a neurotoxin from the scorpion Buthus martensii Karsch

Residue 54 has been shown to be important for bioactivity in several toxins. However, its role in the antinociceptive activity of toxins has not been evaluated yet. In this study, site-directed mutagenesis and mouse acetic acid writhing test were used to investigate the role of Ser54 in the antinociceptive activity of BmK9 neurotoxin from the Buthus martensii Karsch scorpion. Detailed mutagenesis analysis revealed that substitution of Ser54 by various polar amino acids produced no significant change in the antinociceptive activity, while all substitutions of nonpolar amino acid for Ser54 led to a significant loss of antinociceptive activity. Following the conformational analysis, it was suggested that Ser54 in BmK9 plays a functional role in the antinociceptive activity, the residue exerts its effect by means of a side-chain hydrogen bond.

Voltage-gated sodium channel modulation by scorpion alpha-toxins

Voltage-gated Na(+) channels are integral membrane proteins that function as a gateway for a selective permeation of sodium ions across biological membranes. In this way, they are crucial players for the generation of action potentials in excitable cells. Voltage-gated Na(+) channels are encoded by at least nine genes in mammals. The different isoforms have remarkably similar functional properties, but small changes in function and pharmacology are biologically well-defined, as underscored by mutations that cause several diseases and by modulation of a myriad of compounds, respectively. This review will stress on the modulation of voltage-gated Na(+) channels by scorpion alpha-toxins. Nature has designed these two classes of molecules as if they were predestined to each other: an inevitable 'encounter' between a voltage-gated Na(+) channel isoform and an alpha-toxin from scorpion venom indeed results in a dramatically changed Na(+) current phenotype with clear-cut consequences on electrical excitability and sometimes life or death. This fascinating aspect justifies an overview on scorpion venoms, their alpha-toxins and the Na(+) channel targets they are built for, as well as on the molecular determinants that govern the selectivity and affinity of this 'inseparable duo'.

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.

Assignment of voltage-gated potassium channel blocking activity to kappa-KTx1.3, a non-toxic homologue of kappa-hefutoxin-1, from Heterometrus spinifer venom

A new family of weak K(+) channel toxins (designated kappa-KTx) with a novel "bi-helical" scaffold has recently been characterized from Heterometrus fulvipes (Scorpionidae) venom. Based on the presence of the minimum functional dyad (Y5 and K19), kappa-hefutoxin-1 (kappa-KTx1.1) was investigated and found to block Kv 1.2 (IC(50) approximately 40 microM) and Kv 1.3 (IC(50) approximately 150 microM) channels. In the present study, kappa-KTx1.3, that shares approximately 60% identity with kappa-hefutoxin 1, has been isolated from Heterometrus spinifer venom. Interestingly, despite the presence of the functional dyad (Y5 and K19), kappa-KTx1.3 failed to reproduce the K(+) channel blocking activity of kappa-hefutoxin-1. Since the dyad lysine in kappa-KTx1.3 was flanked by another lysine (K20), it was hypothesized that this additional positive charge could hinder the critical electrostatic interactions known to occur between the dyad lysine and the Kv 1 channel selectivity filter. Hence, mutants of kappa-KTx1.3, substituting K20 with a neutral (K20A) or a negatively (K20E) or another positively (K20R) charged amino acid were synthesized. kappa-KTx1.3 K20E, in congruence with kappa-hefutoxin 1 with respect to subtype selectivity and affinity, produced blockade of Kv 1.2 (IC(50) = 36.8+/-4.9 microM) and Kv 1.3 (IC(50)=53.7+/-6.7 microM) but not Kv 1.1 channels. kappa-KTx1.3 K20A produced blockade of both Kv 1.2 (IC(50) = 36.9+/-4.9 microM) and Kv 1.3 (IC(50)=115.7+/-7.3 microM) and in addition, acquired affinity for Kv 1.1 channels (IC(50) =1 10.7+/-7.7 microM). kappa-KTx1.3 K20R failed to produce any blockade on the channel subtypes tested. These data suggest that the presence of an additional charged residue in a position adjacent to the dyad lysine impedes the functional block of Kv 1 channels produced by kappa-KTx1.3.

Excitement ahead: structure, function and mechanism of snake venom phospholipase A2 enzymes

Venom phospholipase A2 (PLA2) enzymes share similarity in structure and catalytic function with mammalian enzymes. However, in contrast to mammalian enzymes, many are toxic and induce a wide spectrum of pharmacological effects. Thus structure-function relationship of this group of small proteins is subtle, but complex puzzle to protein biochemists, molecular biologists, toxinologists, pharmacologists and physiologists. This review describes the present status of our understanding of their structure, function and mechanism. It was proposed that their unique ability to 'target' themselves to a specific organ or tissue is due to their high affinity binding to specific proteins which act as receptors (more precisely, acceptors). This specific binding of PLA2 is conferred by the presence of a 'pharmacological site' on its surface which is independent of the catalytic site. The high affinity interaction of PLA2 with its acceptor (or target protein) is probably due to the complementarity, in terms of charges, hydrophobicity and van der Waal's contact surfaces, between the pharmacological site and the binding site on the surface of the acceptor protein. Upon binding to the target, the PLA2 can induce its pharmacological effects by mechanisms either dependent on or independent of its catalytic activity. Because of the unprecedented wide spectrum of specific targeting to various tissues and organs, identification of the pharmacological sites has potential for exploitation in development of novel systems useful for 'delivering' specific proteins to a particular target tissue or organ. Thus research in this field will provide a lot of exciting opportunities.

Purification and primary structure determination of Tf4, the first bioactive peptide isolated from the venom of the Brazilian scorpion Tityus fasciolatus

In the present study Tityus fasciolatus crude venom toxicity was evaluated and we also report the purification and characterization of a 6.6 kDa neurotoxin isolated from T. fasciolatus venom. This new toxin, named Tf4, has a molecular mass of 6614Da and its primary structure is homologous to TbIT-I from T. bahiensis and TsTX-VI and TsNTxP from T. serrulatus. Tf4 delays frog sodium channel inactivation reversibly, but it is non-toxic to mammals or crustaceans. An attempt to identify the residues responsible for the partial loss of toxicity in Tf4 was carried out based on homology modeling and sequence comparison.

Sequence and structure-activity relationship of a scorpion venom toxin with nitrergic activity in rabbit corpus cavernosum

An alpha-toxin responsible for nitric oxide (NO) release in rabbit corpus cavernosum (RbCC) was isolated from Tityus serrulatus venom (TSV). The isolated peptide (molecular mass of 7427.66+/-0.15 Da) was identified as Ts3 after determination of Cys residues, N-terminal amino acid analysis, and proteolytic peptide mapping. Ts3 (30 nM) markedly relaxed the RbCC; this response was blocked by the NO synthesis inhibitor N(omega)-nitro-L-arginine methyl ester (100 microM) and the Na+ channel blocker tetrodotoxin (100 nM). Synthetic peptides based on either Ts3 (P1-16, P17-32, P33-48, P49-64, P9-24, P25-40, P41-56, YGLPDKVPTKT) or Bukatoxin (isolated from Buthus martensi Karsch scorpion venom) sequence (Buka11, Buka11-B, PDKVP, PDSEP) were assayed. These peptides slightly relaxed the RbCC, and such an effect was independent of Na+ channel activation or NO release. Our results indicate that Ts3 exerts nitrergic actions and contributes to the relaxing activity of TSV in RbCC, thus providing a valuable tool to investigate the mechanisms underlying nerve activation in erectile tissues, because NO released from nitrergic fibers plays a key role in the erectile process. Our findings revealed the key importance of the Ts3 structure three-dimensional conformation maintenance for biological activity, because linear peptide sequences neither presented substantial relaxations nor was this effect related to nitrergic activity.

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.

Autonomic effects of some scorpion venoms and toxins

1. The autonomic effects of venoms and toxins from several species of scorpions, including the Indian red scorpion Mesobuthus tamulus, the Chinese scorpion Buthus martensi Karsch and the Israeli scorpion Leiurus quinquestriatus quinquestriatus, all belonging to Buthidae, and the Asian black scorpions Heterometrus longimanus and Heterometrus spinifer, belonging to Scorpionidae, are reviewed. 2. The effects of the venoms of M. tamulus and L. q. quinquestriatus on noradrenergic and nitrergic transmission in the rat isolated anococcygeus muscle revealed that both venoms mediated their pharmacological effects via a prejunctional mechanism involving the activation of voltage-sensitive sodium channels with consequent release of neurotransmitters that mediate target organ responses, similar to the effects mediated by other alpha-scorpion toxins. 3. Two new toxins, Makatoxin I and Bukatoxin, were purified to homogeneity from the venom of B. martensi Karsch. Determination of their complete amino acid sequences confirmed that both toxins belonged to the class of alpha-scorpion toxins. The effects of both toxins on noradrenergic and nitrergic transmission in the rat anococcygeus muscle provided firm evidence that their pharmacological actions also closely resembled those mediated by other alpha-scorpion toxins on neuronal voltage-sensitive sodium channels. 4. The venoms of H. longimanus and H. spinifer were found to have high concentrations of noradrenaline (1.8 +/- 0.3 mmol/L) and relatively high concentrations of acetylcholine (79.8 +/- 1.7 micromol/L) together with noradrenaline (146.7 +/- 19.8 micromol/L), respectively, which can account for their potent direct cholinergic and noradrenergic agonist actions in the rat anococcygeus muscle. 5. Our studies confirmed that the rat anococcygeus muscle is an excellent nerve-smooth muscle preparation for investigating the effects of bioactive agents on noradrenergic and nitrergic transmission, as well as the direct agonist actions of these agents on post-synaptic alpha-adrenoceptors and M3 muscarinic cholinoceptors. Although many studies, including our own, have documented that scorpion venoms and toxins mediate their primary effects via a prejunctional mechanism that leads to the marked release of various autonomic neurotransmitters, our studies have shown that there are exceptions to this generally accepted phenomenon. In particular, we have provided firm evidence to show that the venoms from H. longimanus and H. spinifer do not have such a prejunctional site of action but, instead, the venoms mediate their autonomic effects through direct agonist actions on post-junctional muscarinic M3 cholinoceptors and alpha-adrenoceptors.

Biochemical and pharmacological characterization of the venom of the black scorpion Heterometrus spinifer

The sting of the black scorpion Heterometrus spinifer, which can cause intense localized pain, has not been reported to produce lethal cardiovascular complications, which are well known to result from scorpion envenomation as a consequence of a massive release of catecholamines. Therefore, we have undertaken a biochemical and pharmacological characterization of the venom of H. spinifer. Pharmacologically, the venom (0.125 microL/mL) produced a marked, reversible contracture in the chick biventer cervicis muscle that was blocked by d-tubocurarine (2 microM) but not by tetrodotoxin (5 microM) and omega-conotoxin GVIA (3 microM). The anticholinesterase neostigmine (1 microM) potentiated the contracture by 5.3-fold. An ultra-filtrate fraction of MW < 3000 (F3K) of the venom produced a similar contracture in the biventer muscle, whereas the retentate of MW > 3000 did not. In the rat anococcygeus muscle, the venom produced a contractile response that was partially (37.4 +/- 1.6%) blocked by atropine (5 microM); phentolamine (5 microM) blocked the remaining response. Tetrodotoxin (5 microM) did not block the contractile response of the venom on the anococcygeus muscle. Electrospray ionization-mass spectrometry/mass spectrometry confirmed the presence of high concentrations of acetylcholine (79.8 +/- 1.7 microM) and norepinephrine (146.7 +/- 19.8 microM) in H. spinifer venom, which can fully account for the observed cholinergic and adrenergic effects. In contrast to scorpion venoms that selectively target neuronal ion channels in mediating transmitter release, our data show that H. spinifer venom does not possess such activity, which likely explains the apparent lack of lethality of black scorpion envenomation.