Purification, structure and activity of three insect toxins from Buthus occitanus tunetanus venom

One contractive and two depressant toxins active on insect were purified by high-performance liquid chromatography from the venom of Buthus occitanus tunetanus (Bot). The two depressant toxins, BotIT4 and BotIT5, differ only at position 6 (Arg for Lys) and are equally toxic to insects (LD50 to Blatella germanica = 110 ng/100 mg body weight). They show a strong antigenic cross-reaction with a depressive toxin from Leiurus quinquestriatus quinquestriatus (LqqIT2). The two toxins are able to inhibit with high affinity (K0.5 between 2 and 3 nM) the specific binding of the radioiodinated excitatory insect toxin (125I-AaHIT) on its receptor site on Periplaneta americana synaptosomal membranes. These toxins depolarize the cockroach axon, irreversibly block the action potential, and slow down and very progressively block the transmembrane transient Na+ current. The contracturant toxin BotIT1 is highly toxic to B. germanica (LD50 = 60 ng/ 100 mg body weight) and barely toxic to mice (LD50 = 1 microgram/20 g body weight) when injected intracerebroventricularly. It does not compete with 125I-AaHIT for its receptor site on P. americana synaptosomal membranes. On cockroach axon, BotIT1 develops plateau potentials and slows down the inactivation mechanism of the Na+ channels. Thus, BotIT1 belongs to the group of alpha insect-selective toxins and shows a strong sequence identity (> 90%) with Lqh alpha IT and LqqIII, two insect alpha-toxins previously purified from the venom of L. q. hebraeus and L. q. quinquestriatus. respectively.

In vivo protection against Androctonus australis hector scorpion toxin and venom by immunization with a synthetic analog of toxin II

A synthetic peptide mimicking the North African scorpion Androctonus australis hector toxin II was designed and produced by chemical solid-phase synthesis. It contains the entire sequence of toxin II (64 amino acid residues), with each half-cystine being replaced by the isosteric residue a-aminobutyric acid, and was thus devoid of disulfide bridges. This construct was totally nontoxic in mice even if large amounts, equivalent to 1000 times the LD50 of the original toxin, were injected by the intracerebroventricular route. The synthetic peptide, either as a monomer or polymerized by means of glutaraldehyde, induced the production of antitoxin neutralizing antibodies in immunized mice and rabbits. After three injections with either the monomeric or polymerized synthetic peptide, the immunized mice were protected against several lethal doses of the corresponding native toxin or scorpion venom. Six months after immunization, the mice were completely protected against challenge with eight LD50 of the original toxin. The protection was better when the polymerized synthetic peptide was used. One month after the start of the immunization program, it showed a good correlation between antibody titer and protection. However, antibody titer decreased with time but protection remained high. This suggests that additional factors other than circulating antibodies play a role in protective activity.

Biochemical and pharmacological characterization of a depressant insect toxin from the venom of the scorpion Buthacus arenicola

A depressant toxin active on insects, Buthacus arenicola IT2, was isolated from the venom of the North African scorpion B. arenicola and its structural and pharmacological properties were investigated. B. arenicola IT2 is a single polypeptide of 61 amino acid residues, including 8 half-cystines but no methionine and histidine, with a molecular mass of 6835 Da. Its amino acid sequence is 79-95% identical to other depressant toxins from scorpions. When injected into the cockroach Blatella germanica, B. arenicola IT2 induced a slow depressant flaccid paralysis with a LD50 of 175 ng. B. arenicola IT2 has two non-interacting binding sites in cockroach neuronal membranes: one of high affinity (Kd1 = 0.11 +/- 0.04 nM) and low capacity (Bmax1 = 2.2 +/- 0.6 pmol/mg), and one of low affinity (Kd2 = 24 +/- 7 nM) and high capacity (Bmax2 = 226 +/- 92 pmol/mg). Its binding to these two sites was completely inhibited by Leiurus quinquestriatus quinquestriatus IT2, a depressant toxin from L. quinquestriatus quinquestriatus. Reciprocal-binding experiments between B. arenicola IT2 and the excitatory insect-toxin A. australis Hector IT revealed competition between the two toxins for the high-affinity sites of B. arenicola IT2. B. arenicola IT2 has a higher affinity than L. quinquestriatus hebraeus IT2, a depressant toxin from L. quinquestriatus hebraeus. Thus, B. arenicola IT2 represents an interesting tool to study the receptor site for depressant toxins on insect sodium channels.

Chemical synthesis and characterization of maurotoxin, a short scorpion toxin with four disulfide bridges that acts on K+ channels

Maurotoxin is a toxin isolated from the venom of the Tunisian chactoid scorpion Scorpio maurus. It is a 34-amino-acid peptide cross-linked by four disulfide bridges. Maurotoxin competes with radiolabeled apamin and kaliotoxin for binding to rat-brain synaptosomes. Due to its very low concentration in venom (0.6% of the proteins), maurotoxin was chemically synthesized by means of an optimized solid-phase technique. The synthetic maurotoxin was characterized. It was lethal to mice following intracerebroventricular injection (LD50, 80 ng/mouse). The synthetic maurotoxin competed with 125I-apamin and 125I-kaliotoxin for binding to rat-brain synaptosomes with half-maximal effects at concentrations of 5 nM and 0.2 nM, respectively. Synthetic maurotoxin was tested on K+ channels and was found to block the Kv1.1, Kv1.2, and Kv1.3 currents with half-maximal blockage (IC50) at 37, 0.8 and 150 nM, respectively. Thus, maurotoxin is a scorpion toxin with four disulfide bridges that acts on K+ channels. The half-cystine pairings of synthetic maurotoxin were identified by enzymatic cleavage. The pairings were Cys3-Cys24, Cys9-Cys29, Cys13-Cys19 and Cys31-Cys34. This disulfide organization is unique among known scorpion toxins. The physicochemical and pharmacological properties of synthetic maurotoxin were indistinguishable from those of natural maurotoxin, which suggests that natural maurotoxin adopts the same half-cystine pairing pattern. The conformation of synthetic maurotoxin was investigated by means of circular dichroism spectroscopy and molecular modeling. In spite of its unusual half-cystine pairings, the synthetic-maurotoxin conformation appears to be similar to that of other short scorpion toxins.

Characterization of PO1, a new peptide ligand of the apamin-sensitive Ca2+ activated K+ channel

A new peptide ligand of the small conductance Ca2+ activated K+ channels has been purified from the venom (obtained by manual rather than electrical stimulation of the scorpion Androctonus mauretanicus mauretanicus), by following the inhibition of the 125I-apamin binding to its receptor on rat brain synaptosomes. Only one step on a C18 reversed-phase high-performance liquid chromatography column was necessary to obtain PO1. Its K0.5 for the apamin binding site was 100 nM. The amino acid sequence of PO1 is different from those of leiurotoxin and PO5. For the first time the same peptide was also purified from the venoms of two other species of North African scorpions, Androctonus australis and Buthus occitanus tunetanus. PO1 was chemically synthesized by the solid-phase technique and fully characterized. A model of PO1 was constructed by amino acid replacement using PO5 nuclear magnetic resonance studies as the starting model. Structure-activity relationships between these toxins and their receptor are discussed.

Adenosine and neuropathic pain

Recent studies have reported the possibilities of relieving neuropathic pain by administering adenosine or its analogs. In order to determine if there exists a metabolic anomaly of this nucleoside in patients with neuropathic pain, circulating adenosine levels were compared in three patient groups. The first was composed of individuals suffering from neuropathic pain, the second of patients with nervous system lesions in the absence of pain, and the third was composed of patients suffering from pain resulting from excessive nociception. The adenosine blood levels of these patients were compared to those of a control group. Finally, adenosine in the cerebrospinal fluid (CSF) of some patients was also assayed. The results show that there are reduced levels of blood and CSF adenosine in patients with neuropathic pain. This adenosine deficiency could explain the potential therapeutic effects of administering adenosine or its analogs.

Adenosine and the nervous system: clinical implications

After a review of the metabolism and pharmacology of adenosine, this work will examine the various therapeutic possibilities involving the use of agonists or antagonists of adenosine A1 or A2 receptors in neurological disorders. Promising preclinical results have been obtained with epilepsy, cerebral ischemia, alcoholism, and pain.

A new scorpion venom toxin paralytic to insects that affects Na+ channel activation. Purification, structure, antigenicity and mode of action

A new toxin, BotIT2, with a unique mode of action on the isolated giant axon of the cockroach Periplaneta americana and DUM (dorsal unpaired median) neurons, has been purified from the venom of the scorpion Buthus occitanus tunetanus. Its structural, antigenic and pharmacological properties are compared to those of three other groups of neurotoxins found in Buthidae scorpion venoms. Like excitatory, depressant and alpha-type insect-selective neurotoxins, BotIT2 is toxic to insects, but shows the following common and distinctive characteristics. (a) As alpha-type toxins, BotIT2 lack strict selectivity to insects; they have measurable but low toxicity to mice. (b) As depressant toxins and unlike alpha-type toxins, BotIT2 is able to displace iodinated AaHIT from its binding sites in insect neuronal membranes. This indicates that the binding site for BotIT2 is identical, contiguous or in allosteric interaction with that of AaHIT and depressant toxins. (c) The BotIT2 amino acid sequence shows strong similarity to depressant toxins. However, unexpectedly, despite this high sequence similarity, BotIT2 shares moderate cross-antigenic reactivity with depressant toxins. (d) Voltage and current-clamp studies show that BotIT2 induces limited depolarization concomitantly with the development of depolarizing after potential, repetitive activity and later plateau potentials terminated by bursts. Under voltage-clamp conditions, BotIT2 specifically acts on Na+ channels by decreasing the peak Na+ current and by simultaneously inducing a new current with very slow activation/deactivation kinetics. The voltage dependence of this slow current is not significantly different from that of the control current. These observations indicate that BotIT2 chiefly modifies the kinetics of axonal and DUM neuronal membrane Na(+)-channel activation.

Anti-HIV activity of multibranched peptide constructs derived either from the cleavage sequence or from the transmembrane domain (gp41) of the human immunodeficiency virus type 1 envelope

Multibranched peptides (SPCs) derived either from the fusion protein (gp41) sequence or from the cleavage sequence of the human immunodeficiency virus type 1 envelope were chemically synthesized and tested for their ability to inhibit both syncytium formation and HIV production in CD4+ cells. The gp41-derived SPCs had no effect. In contrast, an SPC encompassing the envelope cleavage sites strongly inhibited both HIV Env-induced syncytium formation and viral production.

Two neurotoxins (BmK I and BmK II) from the venom of the scorpion Buthus martensi Karsch: purification, amino acid sequences and assessment of specific activity

Two neurotoxins, BmK I and BmK II, were purified from the venom of the Chinese scorpion Buthus martensi Karsch. The complete amino acid sequences of both toxins, each containing 64 amino acid residues, were determined by the automatic sequencing of reduced and S-carboxymethylated toxins and their peptides, obtained after cleavage with TPCK-treated trypsin and Staphylococcus aureus V8 protease, respectively. Toxicity as minimum lethal dose tested by i.c.v. injection in mice showed that BmK I was six times more potent than BmK II. Only two amino acid replacements were found: at position 59 Val in BmK I was replaced by Ile in BmK II, and at position 62 a basic Lys residue in BmK I was substituted by a neutral Asn residue in BmK II. These features suggest that the positively charged residue (Lys or Arg) in the C-terminal position 62 (or 61 or 63) may also play an important role in facilitating the interaction between scorpion neurotoxins and the receptor on sodium channels. The effects of BmK I on nerve excitability were examined with the crayfish axon using intracellular recording and voltage-clamp conditions. The results indicate that BmK I preferentially blocks the sodium channel inactivation process. Thus, functional and structural similarities suggest that BmK I and BmK II belong to group 3 of scorpion alpha-type toxins.

Synthesis and characterization of leiurotoxin I analogs lacking one disulfide bridge: evidence that disulfide pairing 3-21 is not required for full toxin activity

Leiurotoxin I (Lei-NH2), a toxin isolated from the venom of the scorpion Leiurus quinquestriatus hebraeus, is a blocker of the apamin-sensitive Ca(2+)-activated K+ channels. It is a 31-residue polypeptide cross-linked by three disulfide bridges which are presumably between Cys3-Cys21, Cys8-Cys26, and Cys12-Cys28. To investigate the role of these disulfides, analogs of Lei-NH2 lacking one disulfide bridge (i.e., [Abu3,21]Lei-NH2, [Abu8,26]Lei-NH2, and [Abu12,28]Lei-NH2) were chemically synthesized by selective replacement of each pair of half-cystines forming a bridge by two alpha-aminobutyrate (Abu) residues. The two disulfide pairings of the main folded form of the synthetic analogs were established by enzymatic proteolysis. They were as expected between Cys8-Cys26 and Cys12-Cys28 for [Abu3,21]Lei-NH2 but were unexpectedly between Cys3-Cys12 and Cys21-Cys28 for [Abu8,26]Lei-NH2 and between Cys3-Cys8 and Cys21-Cys26 for [Abu12,28]Lei-NH2. The synthetic peptides were tested in vitro for their capacity to compete with the binding of [125I]apamin to rat brain synaptosomes and in vivo for their neurotoxicity in mice. In both assays, [Abu3,21]Lei-NH2 exhibited full Lei-NH2-like activity whereas [Abu8,26]Lei-NH2 and [Abu12,28]-Lei-NH2 possessed only residual activities (< 2% native toxin activity). This suggests that disulfide bridge Cys3-Cys21 is not essential per se for high toxin activity. Circular dichroism (CD) spectroscopy of the three analogs showed that only [Abu3,21]Lei-NH2 exhibited a CD spectrum similar to that of Lei-NH2, suggesting they both adopt closely related conformations, in agreement with the pharmacological data. Structural models of the analogs were constructed on the basis of the disulfide pairing assignment and compared with that of Lei-NH2.

Novel anti-platelet aggregation polypeptides from Vipera lebetina venom: isolation and characterization

Lebetins 1 and Lebetins 2, two polypeptide groups that inhibit platelet aggregation, were isolated from Vipera lebetina venom by gel filtration and reverse phase chromatography. Amino acid sequencing indicated that the first group contains two major polypeptides of 13 and 12 residues; their molecular weight was determined by electrospray mass spectrometry. The second was composed of two peptides of 38 and 37 residues, each with one disulfide bond. Sequence analysis revealed neither RGD sequence nor homology with other proteins including known snake or tick polypeptides. Lebetins 1 were Pro and Lys rich peptides and their sequences were identical to the N-terminus of Lebetins 2. Lebetins inhibited platelet aggregation induced by thrombin, collagen and PAF-acether. The 50% concentration that inhibited human and rabbit platelet aggregation induced by thrombin was 590 nM and 125 nM for Lebetins 1 and 100 nM and 8 nM for Lebetins 2, respectively. Lebetins 1 and Lebetins 2 also inhibited fibrinogen-induced aggregation of alpha-chymotrypsin-treated platelets as well as in vivo collagen-induced thrombocytopenia in rats with half effective doses of 2 nmol/kg and 4.2 nmol/kg, respectively. Lebetins were not toxic after intravenous injection into mice and rats. These polypeptides form novel platelet inhibitors that could be used to delineate further the mechanisms of platelet aggregation and serve as a model for developing antithrombotic agents.

Tetrodotoxin reverses brevetoxin allosteric inhibition of scorpion alpha-toxin binding on rat brain sodium channels

Voltage-sensitive sodium channels are responsible for the initiation of action potentials in many excitable cells. Several neurotoxins bind to distinct receptor sites on sodium channels and reveal strong allosteric interactions among them. Scorpion alpha toxins, which inhibit sodium channel inactivation by binding to receptor site 3, have been very important tools to study sodium channel structure and function. Recently, we have shown that brevetoxin induce a strong negative allosteric modulation on scorpion alpha-toxin binding on rat brain sodium channels, in contrast to previously published studies. In this report we have examined the reasons for this discrepancy and found new, unexpected allosteric interactions between the tetrodotoxin and brevetoxin receptor sites, using scorpion alpha-toxin as sensitive probe for subtle conformational changes on sodium channels. Tetrodotoxin reverses the negative modulation induced by brevetoxin on scorpion alpha-toxin binding, revealing new dynamic interactions in sodium channel structure.

Characterization of a new peptide from Tityus serrulatus scorpion venom which is a ligand of the apamin-binding site

A new ligand (Ts kappa) of the apamin binding site on rat brain synaptosomes (K0.5 = 300 pM) was purified and characterized from the venom of Tityus serrulatus. It is a polypeptide toxin of 35 amino acid residues, with three disulfide bridges. Its cDNA was amplified from a venom gland cDNA library and the nucleotide sequence determined. A model of Ts kappa was constructed by amino acid replacement using charybdotoxin structure as determined by 1H nuclear magnetic resonance as starting model.

Activity-induced internalization and rapid degradation of sodium channels in cultured fetal neurons

A regulatory mechanism for neuronal excitability consists in controlling sodium channel density at the plasma membrane. In cultured fetal neurons, activation of sodium channels by neurotoxins, e.g., veratridine and alpha-scorpion toxin (alpha-ScTx) that enhance the channel open state probability induced a rapid down-regulation of surface channels. Evidence that the initial step of activity-induced sodium channel down-regulation is mediated by internalization was provided by using 125I-alpha-ScTx as both a channel probe and activator. After its binding to surface channels, the distribution of 125I-alpha-ScTx into five subcellular compartments was quantitatively analyzed by EM autoradiography. 125I-alpha-ScTx was found to accumulate in tubulovesicular endosomes and disappear from the cell surface in a time-dependent manner. This specific distribution was prevented by addition of tetrodotoxin (TTX), a channel blocker. By using a photoreactive derivative to covalently label sodium channels at the surface of cultured neurons, we further demonstrated that they are degraded after veratridine-induced internalization. A time-dependent decrease in the amount of labeled sodium channel alpha subunit was observed after veratridine treatment. After 120 min of incubation, half of the alpha subunits were cleaved. This degradation was prevented totally by TTX addition and was accompanied by the appearance of an increasing amount of a 90-kD major proteolytic fragment that was already detected after 45-60 min of veratridine treatment. Exposure of the photoaffinity-labeled cells to amphotericin B, a sodium ionophore, gave similar results. In this case, degradation was prevented when Na+ ions were substituted by choline ions and not blocked by TTX. After veratridine- or amphotericin B-induced internalization of sodium channels, breakdown of the labeled alpha subunit was inhibited by leupeptin, while internalization was almost unaffected. Thus, cultured fetal neurons are capable of adjusting sodium channel density by an activity-dependent endocytotic process that is triggered by Na+ influx.

Characterization of alpha-neurotoxin and phospholipase A2 activities from Micrurus venoms. Determination of the amino acid sequence and receptor-binding ability of the major alpha-neurotoxin from Micrurus nigrocinctus nigrocinctus

New World elapids are coral snakes that belong to the genus Micrurus, and for which the venom biochemistry is mostly unknown. Analysis has been difficult because the coral snakes produce small quantities of venom. Clinical observations following bites show mainly neurotoxic effects. Experimentally, cardiotoxic, haemolytic and myotoxic activities are also reported. An experimental approach, using reverse-phase high-performance liquid chromatography and specific assays for alpha-neurotoxin and phospholipase A2 activities, was conducted on milligram quantities of venoms from three Micrurus species from Costa Rica; M. nigrocinctus nigrocinctus, M. alleni yatesi and M. multifasciatus. Neurotoxicity was determined by competition binding experiments with the Torpedo marmorata acetylcholine receptor. Phospholipase A2 activity was measured by fluorimetry using a pyrene lipid substrate. In this way, we purified and characterized seven alpha-neurotoxins, five phospholipases A2 and four toxin homologs. The amino acid sequence of the major alpha-neurotoxin from M. nigrocinctus nigrocinctus venom was fully determined and compared to Old Word representatives. Distance matrix data were generated to set up phylogeny relationships among elapid short-chain alpha-neurotoxins, which proved to be in accordance with the taxonomic classification and geographical distribution of snake species.

Scorpion toxins affecting sodium current inactivation bind to distinct homologous receptor sites on rat brain and insect sodium channels

Sodium channels posses receptor sites for many neurotoxins, of which several groups were shown to inhibit sodium current inactivation. Receptor sites that bind alpha- and alpha-like scorpion toxins are of particular interest since neurotoxin binding at these extracellular regions can affect the inactivation process at intramembranal segments of the channel. We examined, for the first time, the interaction of different scorpion neurotoxins, all affecting sodium current inactivation and toxic to mammals, with alpha-scorpion toxin receptor sites on both mammalian and insect sodium channels. As specific probes for rat and insect sodium channels, we used the radiolabeled alpha-scorpion toxins AaH II and LqhalphaIT, the most active alpha-toxins on mammals and insect, respectively. We demonstrate that the different scorpion toxins may be classified to several groups, according to their in vivo and in vitro activity on mammalian and insect sodium channels. Analysis of competitive binding interaction reveal that each group may occupy a distinct receptor site on sodium channels. The alpha-mammal scorpion toxins and the anti-insect Lqh alphaIT bind to homologous but not identical receptor sites on both rat brain and insect sodium channels. Sea anemone toxin ATX II, previously considered to share receptor site 3 with alpha-scorpion toxins, is suggested to bind to a partially overlapping receptor site with both AaH II and Lqh alphaIT. Competitive binding interactions with other scorpion toxins suggest the presence of a putative additional receptor site on sodium channels, which may bind a unique group of these scorpion toxins (Bom III and IV), active on both mammals and insects. We suggest the presence of a cluster of receptor sites for scorpion toxins that inhibit sodium current inactivation, which is very similar on insect and rat brain sodium channels, in spite of the structural and pharmacological differences between them. The sea anemone toxin ATX II is also suggested to bind within this cluster.

1H-NMR-derived secondary structure and the overall fold of the potent anti-mammal and anti-insect toxin III from the scorpion Leiurus quinquestriatus quinquestriatus

We describe the secondary structure and the overall fold of toxin III from the venom of the scorpion Leiurus quinquestriatus quinquestriatus determined using two-dimensional-1H-NMR spectroscopy. This protein, which contains 64 amino acids and 4 disulfide bridges, belongs to the long-chain toxin category and is highly toxic to both mammals and insects. The overall fold was determined on the basis of 1208 inter-proton-distance restraints derived from NOE measurements and 90 psi, phi dihedral-angle restraints derived from NOE connectivities and 3JNH-alphaH coupling constants using the HABAS program. This fold, which mainly consists of an alpha-helix packed against a small antiparallel three-stranded beta-sheet, and of several turns and loops, is similar to that of other long-chain scorpion toxins. Aromatic and non-polar residues form several patches on the surface of the protein which alternate with patches of charged and polar residues. Such a topology should be important in the interactions of toxin III with sodium channels in membranes. Two weakly constrained loops introduce some flexibility to the structure which could be related to the activity of this toxin. The central core of toxin III is compared with the cysteine-stabilized alpha beta motif (an alpha-helix connected to a beta-sheet through two disulfide bridges) found in insect defensins and plant thionins. Defensins and thionins are small proteins (approximately 40--50 amino acid residues) containing three or four disulfide bridges, respectively. This comparison confirms that the cysteine-stabilized alpha beta motif is a common core to a number of small proteins from different origins and having different activities.