Genes and peptides from the scorpion Centruroides sculpturatus Ewing, that recognize Na(+)-channels

Structural and Functional Characterization of a Novel Scorpion Toxin that Inhibits NaV1.8 via Interactions With the DI Voltage Sensor and DII Pore Module

Voltage-gated sodium channel Na_V1.8 regulates transmission of pain signals to the brain. While Na_V1.8 has the potential to serve as a drug target, the molecular mechanisms that shape Na_V1.8 gating are not completely understood, particularly mechanisms that couple activation to inactivation. Interactions between toxin producing animals and their predators provide a novel approach for investigating Na_V structure-function relationships. Arizona bark scorpions produce Na⁺ channel toxins that initiate pain signaling. However, in predatory grasshopper mice, toxins inhibit Na_V1.8 currents and block pain signals. A screen of synthetic peptide toxins predicted from bark scorpion venom showed that peptide NaTx36 inhibited Na⁺ current recorded from a recombinant grasshopper mouse Na_V1.8 channel (OtNa_V1.8). Toxin NaTx36 hyperpolarized OtNa_V1.8 activation, steady-state fast inactivation, and slow inactivation. Mutagenesis revealed that the first gating charge in the domain I (DI) S4 voltage sensor and an acidic amino acid (E) in the DII SS2 – S6 pore loop are critical for the inhibitory effects of NaTx36. Computational modeling showed that a DI S1 – S2 asparagine (N) stabilizes the NaTx36 – OtNa_V1.8 complex while residues in the DI S3 – S4 linker and S4 voltage sensor form electrostatic interactions that allow a toxin glutamine (Q) to contact the first S4 gating charge. Surprisingly, the models predicted that NaTx36 contacts amino acids in the DII S5 – SS1 pore loop instead of the SS2 – S6 loop; the DII SS2 – S6 loop motif (QVSE) alters the conformation of the DII S5 – SS1 pore loop, enhancing allosteric interactions between toxin and the DII S5 – SS1 pore loop. Few toxins have been identified that modify Na_V1.8 gating. Moreover, few toxins have been described that modify sodium channel gating via the DI S4 voltage sensor. Thus, NaTx36 and OtNa_V1.8 provide tools for investigating the structure-activity relationship between channel activation and inactivation gating, and the connection to alternative pain phenotypes.

Reduced Toxicity of Centruroides vittatus (Say, 1821) May Result from Lowered Sodium β Toxin Gene Expression and Toxin Protein Production

Body tissue and venom glands from an eastern population of the scorpion Centruroides vittatus (Say, 1821) were homogenized and molecular constituents removed to characterize putative sodium β toxin gene diversity, RT-qPCR, transcriptomic, and proteomic variation. We cloned sodium β toxins from genomic DNA, conducted RT-qPCR experiments with seven sodium β toxin variants, performed venom gland tissue RNA-seq, and isolated venom proteins for mass spectrophotometry. We identified >70 putative novel sodium β toxin genes, 111 toxin gene transcripts, 24 different toxin proteins, and quantified sodium β toxin gene expression variation among individuals and between sexes. Our analyses contribute to the growing evidence that venom toxicity among scorpion taxa and their populations may be associated with toxin gene diversity, specific toxin transcripts variation, and subsequent protein production. Here, slight transcript variation among toxin gene variants may contribute to the major toxin protein variation in individual scorpion venom composition.

Identification and Characterization of Novel Proteins from Arizona Bark Scorpion Venom That Inhibit Nav1.8, a Voltage-Gated Sodium Channel Regulator of Pain Signaling

The voltage-gated sodium channel Nav1.8 is linked to neuropathic and inflammatory pain, highlighting the potential to serve as a drug target. However, the biophysical mechanisms that regulate Nav1.8 activation and inactivation gating are not completely understood. Progress has been hindered by a lack of biochemical tools for examining Nav1.8 gating mechanisms. Arizona bark scorpion (Centruroides sculpturatus) venom proteins inhibit Nav1.8 and block pain in grasshopper mice (Onychomys torridus). These proteins provide tools for examining Nav1.8 structure–activity relationships. To identify proteins that inhibit Nav1.8 activity, venom samples were fractioned using liquid chromatography (reversed-phase and ion exchange). A recombinant Nav1.8 clone expressed in ND7/23 cells was used to identify subfractions that inhibited Nav1.8 Na⁺ current. Mass-spectrometry-based bottom-up proteomic analyses identified unique peptides from inhibitory subfractions. A search of the peptides against the AZ bark scorpion venom gland transcriptome revealed four novel proteins between 40 and 60% conserved with venom proteins from scorpions in four genera (Centruroides, Parabuthus, Androctonus, and Tityus). Ranging from 63 to 82 amino acids, each primary structure includes eight cysteines and a “CXCE” motif, where X = an aromatic residue (tryptophan, tyrosine, or phenylalanine). Electrophysiology data demonstrated that the inhibitory effects of bioactive subfractions can be removed by hyperpolarizing the channels, suggesting that proteins may function as gating modifiers as opposed to pore blockers.

Centruroides margaritatus scorpion complete venom exerts cardiovascular effects through alpha-1 adrenergic receptors

Centruroides margaritatus scorpion stings are common in Colombia. However, the cardiovascular toxicity of the venom has not been clarified.

AIM

To study the effect and mechanisms of action of the complete venom of C. margaritatus (CmV) on the murine cardiovascular system.

METHODS

We evaluated the in vivo effect of CmV LD50 on the mean arterial pressure (MABP), heart rate, and surface electrocardiogram in male adult normotensive Wistar rats. Ex vivo, we evaluated the vascular reactivity of rat aortic rings to increasing concentrations (1 to 60 μg/mL) of CmV using the blockers L-NAME, indomethacin, seratrodast, and prazosin.

RESULTS

In the first hour of poisoning, CmV increased the MABP. In the second hour after poisoning, the heart rate decreased as the normalized PR interval and QT corrected increased. After that, cardiovascular shock was demonstrated by a drastic fall in the MABP and signs of cardiac conduction system block. In aortic rings, CmV caused a direct vasoconstrictor effect mediated by alpha-1 adrenergic receptors and counteracted by nitric oxide.

CONCLUSION

The direct vascular and probably the cardiac alpha-1 effects likely explain the transient hypertension and the maintenance of cardiac function, while interval lengthening may be due to K+ channel blockage. Afterwards, the effects of both the alpha-1 pathway and the K+ channel pathway converged, resulting in fatal cardiovascular shock. This knowledge could aid in understanding the dynamics of the effects of the venom and in designing treatments to address its cardiovascular effects.

Defensive Venoms: Is Pain Sufficient for Predator Deterrence?

Pain, though unpleasant, is adaptive in calling an animal’s attention to potential tissue damage. A long list of animals representing diverse taxa possess venom-mediated, pain-inducing bites or stings that work by co-opting the pain-sensing pathways of potential enemies. Typically, such venoms include toxins that cause tissue damage or disrupt neuronal activity, rendering painful stings honest indicators of harm. But could pain alone be sufficient for deterring a hungry predator? Some venomologists have argued “no”; predators, in the absence of injury, would “see through” the bluff of a painful but otherwise benign sting or bite. Because most algogenic venoms are also toxic (although not vice versa), it has been difficult to disentangle the relative contributions of each component to predator deterrence. Southern grasshopper mice (Onychomys torridus) are voracious predators of arthropods, feeding on a diversity of scorpion species whose stings vary in painfulness, including painful Arizona bark scorpions (Centruroides sculpturatus) and essentially painless stripe-tailed scorpions (Paravaejovis spinigerus). Moreover, southern grasshopper mice have evolved resistance to the lethal toxins in bark scorpion venom, rendering a sting from these scorpions painful but harmless. Results from a series of laboratory experiments demonstrate that painful stings matter. Grasshopper mice preferred to prey on stripe-tailed scorpions rather than bark scorpions when both species could sting; the preference disappeared when each species had their stingers blocked. A painful sting therefore appears necessary for a scorpion to deter a hungry grasshopper mouse, but it may not always be sufficient: after first attacking and consuming a painless stripe-tailed scorpion, many grasshopper mice went on to attack, kill, and eat a bark scorpion even when the scorpion was capable of stinging. Defensive venoms that result in tissue damage or neurological dysfunction may, thus, be required to condition greater aversion than venoms causing pain alone.

Scorpions from Mexico: From Species Diversity to Venom Complexity

Scorpions are among the oldest terrestrial arthropods, which are distributed worldwide, except for Antarctica and some Pacific islands. Scorpion envenomation represents a public health problem in several parts of the world. Mexico harbors the highest diversity of scorpions in the world, including some of the world's medically important scorpion species. The systematics and diversity of Mexican scorpion fauna has not been revised in the past decade; and due to recent and exhaustive collection efforts as part of different ongoing major revisionary systematic projects, our understanding of this diversity has changed compared with previous assessments. Given the presence of several medically important scorpion species, the study of their venom in the country is also important. In the present contribution, the diversity of scorpion species in Mexico is revised and updated based on several new systematic contributions; 281 different species are recorded. Commentaries on recent venomic, ecological and behavioral studies of Mexican scorpions are also provided. A list containing the most important peptides identified from 16 different species is included. A graphical representation of the different types of components found in these venoms is also revised. A map with hotspots showing the current knowledge on scorpion distribution and areas explored in Mexico is also provided.

Rescuable folding defective NaV1.1 (SCN1A) mutants in epilepsy: properties, occurrence, and novel rescuing strategy with peptides targeted to the endoplasmic reticulum

Mutations of the voltage gated Na(+) channel Na(V)1.1 (SCN1A) are important causes of different genetic epilepsies and can also cause familial hemiplegic migraine (FHM-III). In previous studies, some rescuable epileptogenic folding defective mutants located in domain IV of Na(V)1.1 have been identified, showing partial loss of function also with maximal rescue. Variable rescue may be one of the causes of phenotypic variability, and rescue might be exploited for therapeutic approaches. Recently, we have identified a folding defective FHM-III Na(V)1.1 mutant that showed overall gain of function when rescued, consistent with a differential pathomechanism. Here, we have evaluated functional properties and cell surface expression of six Na(V)1.1 epileptogenic missense mutations in different rescuing conditions, including a novel one that we have developed expressing a selective sodium channel toxin (CsEI) targeted to the endoplasmic reticulum (ER). All the mutants showed loss of function and reduced cell surface expression, consistently with possibility of rescue. Four of them were rescuable by incubation at low temperature and interactions with different co-expressed proteins or a pharmacological chaperone (phenytoin). Notably, CsEI was able to rescue four mutants. Thus, Na(V)1.1 folding defective mutants can be relatively common and mutations inducing rescuable folding defects are spread in all Na(V)1.1 domains. Importantly, epileptogenic mutants showed overall loss of function even upon rescue, differently than FHM-III ones. The effectiveness of CsEI demonstrates that interactions in the ER are sufficient for inducing rescue, and provides a proof of concept for developing possible therapeutic approaches that may overcome some limitations of pharmacological chaperones.

Scorpion and spider venom peptides: gene cloning and peptide expression

This communication reviews most of the important findings related to venom components isolated from scorpions and spiders, mainly by means of gene cloning and expression. Rather than revising results obtained by classical biochemical studies that report structure and function of venom components, here the emphasis is placed on cloning and identification of genes present in the venomous glands of these arachnids. Aspects related to cDNA library construction, specific or random ESTs cloning, transcriptome analysis, high-throughput screening, heterologous expression and folding are briefly discussed, showing some numbers of species and components already identified, but also shortly mentioning limitations and perspectives of research for the future in this field.

Isolation and characterization of CvIV4: a pain inducing α-scorpion toxin

Background

Among scorpion species, the Buthidae produce the most deadly and painful venoms. However, little is known regarding the venom components that cause pain and their mechanism of action. Using a paw-licking assay (Mus musculus), this study compared the pain-inducing capabilities of venoms from two species of New World scorpion (Centruroides vittatus, C. exilicauda) belonging to the neurotoxin-producing family Buthidae with one species of non-neurotoxin producing scorpion (Vaejovis spinigerus) in the family Vaejovidae. A pain-inducing α-toxin (CvIV4) was isolated from the venom of C. vittatus and tested on five Na⁺ channel isoforms.

Principal Findings

C. vittatus and C. exilicauda venoms produced significantly more paw licking in Mus than V. spinigerus venom. CvIV4 produced paw licking in Mus equivalent to the effects of whole venom. CvIV4 slowed the fast inactivation of Na_v1.7, a Na⁺ channel expressed in peripheral pain-pathway neurons (nociceptors), but did not affect the Na_v1.8-based sodium currents of these neurons. CvIV4 also slowed the fast inactivation of Na_v1.2, Na_v1.3 and Na_v1.4. The effects of CvIV4 are similar to Old World α-toxins that target Na_v1.7 (AahII, BmK MI, LqhIII, OD1), however the primary structure of CvIV4 is not similar to these toxins. Mutant Na_v1.7 channels (D1586A and E1589Q, DIV S3–S4 linker) reduced but did not abolish the effects of CvIV4.

Conclusions

This study: 1) agrees with anecdotal evidence suggesting that buthid venom is significantly more painful than non-neurotoxic venom; 2) demonstrates that New World buthids inflict painful stings via toxins that modulate Na⁺ channels expressed in nociceptors; 3) reveals that Old and New World buthids employ similar mechanisms to produce pain. Old and New World α-toxins that target Na_v1.7 have diverged in sequence, but the activity of these toxins is similar. Pain-inducing toxins may have evolved in a common ancestor. Alternatively, these toxins may be the product of convergent evolution.

Scorpion envenomation and antivenom therapy

Scorpion envenomation is a dangerous and common global event that can result in a variety of toxic clinical effects. These are typically managed with supportive care or antivenom. Antivenom use is controversial because of conflicting evidence of effectiveness for adrenergic toxicity. However, both controlled and uncontrolled studies have shown that antivenom is effective in resolving neuromotor toxicity associated with envenomations by the scorpions of genus Centruroides.

Antivenom for critically ill children with neurotoxicity from scorpion stings

BACKGROUND

Clinically significant scorpion envenomation by Centruroides sculpturatus produces a dramatic neuromotor syndrome and respiratory insufficiency that often necessitate intensive supportive care. We hypothesized that a scorpion-specific F(ab')(2) antivenom would promptly resolve clinical symptoms in children with this syndrome.

METHODS

In a randomized, double-blind study, the efficacy of scorpion-specific F(ab')(2) antivenom, as compared with placebo, was assessed in 15 children 6 months to 18 years of age who were admitted to a pediatric intensive care unit with clinically significant signs of scorpion envenomation. The primary end point was the resolution of the clinical syndrome within 4 hours after administration of the study drug. Secondary end points included the total dose of concomitant midazolam for sedation and quantitative plasma venom levels, before and after treatment.

RESULTS

The clinical syndrome resolved more rapidly among recipients of the antivenom than among recipients of placebo, with a resolution of symptoms in all eight antivenom recipients versus one of seven placebo recipients within 4 hours after treatment (P=0.001). More midazolam was administered in the placebo recipients than in the antivenom recipients (mean cumulative dose, 4.61 vs. 0.07 mg per kilogram of body weight; P=0.01). Plasma venom concentrations were undetectable in all eight antivenom recipients but in only one placebo recipient 1 hour after treatment (P=0.001).

CONCLUSIONS

Among critically ill children with neurotoxic effects of scorpion envenomation, intravenous administration of scorpion-specific F(ab')(2) antivenom resolved the clinical syndrome within 4 hours, reduced the need for concomitant sedation with midazolam, and reduced the levels of circulating unbound venom. (ClinicalTrials.gov number, NCT00685230.)

Serum levels and urine detection of Centruroides sculpturatus venom in significantly envenomated patients

INTRODUCTION

Envenomation by Centruroides sculpturatus can cause systemic signs and symptoms requiring treatment. The toxicokinetics of C. sculpturatus venom has not been described.

METHODS

Venom components were separated for cross-reactivity testing. Serum and urine collected from three patients envenomated by C. sculpturatus had venom levels determined by sandwich enzyme-linked immunosorbent assay (ELISA).

RESULTS

Western blot analysis indicated recognition of C. sculpturatus venom by Alacramyn, an equine F(ab')(2) antivenom developed against Centruroides scorpion venoms, including C. sculpturatus. Serum venom levels in ng/mL with post-envenomation times in minutes (min) were as follows: 85-year-old woman = 8.2 (approximately 150), 2.8 (515), 1.6 (1,200); 14-month-old girl = 29.7 (approximately 50), 5.0 (729); 3-year-old girl = 11.1 (approximately 313), urine venom level of 9.0 (approximately 490).

CONCLUSION

There is sufficient venom cross-antigenicity among different Centruroides species to allow this ELISA technique with Alacramyn to determine serum and urine C. sculpturatus venom concentrations in envenomated patients.

Sequence analysis and phylogenetic relationship of genes encoding heterodimeric phospholipases A2 from the venom of the scorpion Anuroctonus phaiodactylus

Some scorpion venom contain heterodimeric phospholipases A2. They were shown to be toxic to insects and to cause edema and/or hemolysis of mammalian erythrocytes. This manuscript describes the results of cDNA cloning of five different heterodimeric phospholipases from the venomous glands of the Mexican scorpion Anuroctonus phaiodactylus. The amino acid sequence deduced from the heterodimeric phospholipases open reading frames corresponds in each case to a different isoform. The nucleotide sequences corresponding to two of these genes were also obtained by directly sequencing genomic DNA. The cDNA isoforms show high similarity with the heterodimeric phospholipase Phaiodactylipin purified from the same scorpion. However, similar phospholipases were also found in scorpions from other species and the sequences available were used to construct a phylogenetic tree. In order to understand better the gene structure and phylogeny of these enzymes we analyzed their sequences and compared them with secretory phospholipases of other sources from groups I, II and III. The genomic DNA sequence of a similar phospholipase from bee venomous glands was also cloned. The information available on a Drosophila phospholipase was included in this analysis. The phospholipases of groups I and II contain a conserved exon-intron structure (four or five exons of the mature segment of the enzyme are separated by three or four introns). Also, the gene structure of the phospholipases from A. phaiodactylus and that of the bee venom, belonging to group III phospholipases, are interrupted by three introns. The mature peptide of the bee enzyme is a single polypeptide chain, coded by four exons, whereas those from the scorpion studied here although having four exons, showed the presence of two different polypeptides in its native state. The mature protein is processed after synthesis, producing the heterodimeric structure: a long and a short-peptide chain, linked by a disulfide bridge. The small subunit is the one coded by the fourth exon. The human phospholipase A2 and that of Drosophila, also classified into the group III phospholipases, have a considerably different exon-intron organization.

Identification of a Novel Class of Nicotinic Receptor Antagonists

The venoms of predatory marine snails (Conus spp.) contain diverse mixtures of peptide toxins with high potency and selectivity for a variety of voltage-gated and ligand-gated ion channels. Here we describe the chemical and functional characterization of three novel conotoxins, alphaD-VxXIIA, alphaD-VxXIIB, and alphaD-VxXIIC, purified from the venom of Conus vexillum. Each toxin was observed as an approximately 11-kDa protein by LC/MS, size exclusion chromatography, and SDS-PAGE. After reduction, the peptide sequences were determined by Edman degradation chemistry and tandem MS. Combining the sequence data together with LC/MS and NMR data revealed that in solution these toxins are pseudo-homodimers of paired 47-50-residue peptides. The toxin subunits exhibited a novel arrangement of 10 conserved cystine residues, and additional post-translational modifications contributed heterogeneity to the proteins. Binding assays and two-electrode voltage clamp analyses showed that alphaD-VxXIIA, alphaD-VxXIIB, and alphaD-VxXIIC are potent inhibitors of nicotinic acetylcholine receptors (nAChRs) with selectivity for alpha7 and beta2 containing neuronal nAChR subtypes. These dimeric conotoxins represent a fifth and highly divergent structural class of conotoxins targeting nAChRs.

Diversity of long-chain toxins in Tityus zulianus and Tityus discrepans venoms (Scorpiones, Buthidae): molecular, immunological, and mass spectral analyses

In Venezuela, stings by Tityus zulianus scorpions produce cardiorespiratory arrest, whereas envenoming by Tityus discrepans involves gastrointestinal/pancreatic complications, suggesting structural and/or functional differences. We sought to compare their toxin repertoires through immunological, molecular, and mass spectral analyses. First, in vivo tests showed that neutralization of T. zulianus venom toxicity by the anti-T. discrepans antivenom was not complete. To compare T. discrepans and T. zulianus long-chain (sodium channel-active) toxins, their most toxic Sephadex G-50 fractions, TdII and TzII, were subjected to acid-urea PAGE, which showed differences in composition. Amplification of toxin-encoding mRNAs using a leader peptide-based oligonucleotide rendered cDNAs representing twelve T. discrepans and two T. zulianus distinct toxin transcripts, including only one shared component, indicating divergence between T. zulianus and T. discrepans 5' region-encoded, toxin signal peptides. A 3'-UTR polymorphism was also noticed among the transcripts encoding shared components Tz1 and Td4. MALDI-TOF MS profiling of TdII and TzII produced species-specific spectra, with seven of the individual masses matching those predicted by cDNA sequencing. Phylogenetic analysis showed that the unique T. zulianus transcript-encoded sequence, Tz2, is structurally related to Tityus serrulatus and Centruroides toxins. Together with previous reports, this work indicates that T. zulianus and T. discrepans toxin repertoires differ structurally and functionally.

BmP09, a “Long Chain” Scorpion Peptide Blocker of BK Channels

A novel "long chain" toxin BmP09 has been purified and characterized from the venom of the Chinese scorpion Buthus martensi Karsch. The toxin BmP09 is composed of 66 amino acid residues, including eight cysteines, with a mass of 7721.0 Da. Compared with the B. martensi Karsch AS-1 as a Na(+) channel blocker (7704.8 Da), the BmP09 has an exclusive difference in sequence by an oxidative modification at the C terminus. The sulfoxide Met-66 at the C terminus brought the peptide a dramatic switch from a Na(+) channel blocker toaK(+) channel blocker. Upon probing the targets of the toxin BmP09 on the isolated mouse adrenal medulla chromaffin cells, where a variety of ion channels coexists, we found that the toxin BmP09 specifically blocked large conductance Ca(2+)- and voltage-dependent K(+) channels (BK) but not Na(+) channels at a range of 100 nm concentration. This was further confirmed by blocking directly the BK channels encoded with mSlo1 alpha-subunits in Xenopus oocytes. The half-maximum concentration EC(50) of BmP09 was 27 nm, and the Hill coefficient was 1.8. In outside-out patches, the 100 nm BmP09 reduced approximately 70% currents of BK channels without affecting the single-channel conductance. In comparison with the "short chain" scorpion peptide toxins such as Charybdotoxin, the toxin BmP09 behaves much better in specificity and reversibility, and thus it will be a more efficient tool for studying BK channels. A three-dimensional simulation between a BmP09 toxin and an mSlo channel shows that the Lys-41 in BmP09 lies at the center of the interface and plugs into the entrance of the channel pore. The stable binding between the toxin BmP09 and the BK channel is favored by aromatic pi -pi interactions around the center.

Biochemical, genetic and physiological characterization of venom components from two species of scorpions: Centruroides exilicauda Wood and Centruroides sculpturatus Ewing

Current literature concerning the taxonomic names of two possibly distinct species of scorpions from the genus Centruroides (sculpturatus and/or exilicauda) is controversial. This communication reports the results of biochemical, genetic and electrophysiological experiments conducted with C. exilicauda Wood of Baja California (Mexico) and C. sculpturatus Ewing of Arizona (USA). The chromatographic profile fractionation of the soluble venom from both species of scorpions is different. The N-terminal amino acid sequence for nine toxins of C. exilicauda was determined and compared with those from C. sculpturatus. Lethality tests conducted in mice support the idea that C. exilicauda venom should be expected to be medically less important than C. sculpturatus. Thirteen genes from the venomous glands of the scorpion C. exilicauda were obtained and compared with previously published sequences from genes of the species C. sculpturatus. Genes coding for cytochrome oxidase I and II of both species were also sequenced. A phylogenetic tree was generated with this information showing important differences between them. Additionally, the results of electrophysiological assays conducted with the venom from both species on the Ca(2+)-dependent K(+)-channels, showed significant differences. These results strongly support the conclusion that C. exilicauda and C. sculpturatus are in fact two distinct species of scorpions.

Phaiodotoxin, a novel structural class of insect-toxin isolated from the venom of the Mexican scorpion Anuroctonus phaiodactylus

A peptide called phaiodotoxin was isolated from the venom of the scorpion Anuroctonus phaiodactylus. It is lethal to crickets, but non toxic to mice at the doses assayed. It has 72 amino acid residues, with a molecular mass of 7971 atomic mass units. Its covalent structure was determined by Edman degradation and mass spectrometry; it contains four disulfide-bridges, of which one of the pairs is formed between cysteine-7 and cysteine-8 (positions Cys63-Cys71). The other three pairs are formed between Cys13-Cys38, Cys23-Cys50 and Cys27-Cys52. Comparative sequence analysis shows that phaiodotoxin belongs to the long-chain subfamily of scorpion peptides. Several genes coding for this peptide and similar ones were cloned by PCR, using cDNA prepared from the RNA of venomous glands of this scorpion. Electrophysiological assays conducted with this toxin in several mammalian cell lines (TE671, COS7, rat GH3 and cerebellum granular cells), showed no effect on Na+ currents. However, it shifts the voltage dependence of activation and inactivation of insect Na+ channels (para/tipE) to more negative and positive potentials, respectively. Therefore, the 'window' current is increased by 225%, which is thought to be the cause of its toxicity toward insects. Phaiodotoxin is the first toxic peptide ever purified from a scorpion of the family Iuridae.

Phaiodactylipin, a glycosylated heterodimeric phospholipase A2 from the venom of the scorpion Anuroctonus phaiodactylus

Phaiodactylipin was purified from the venom of the scorpion Anuroctonus phaiodactylus. It is the first protein to be purified from a scorpion of the family Iuridae and has a molecular mass of 19 172 atomic mass units. The mature protein is composed of two subunits, the large one consisting of 108 amino acid residues, whereas the small subunit has only 18 residues, and the structure is stabilized by five disulfide bridges. The heterodimer is expressed from a single message containing 769 base pairs and a signal peptide with 16 and/or 25 amino acid residues. During maturation an internal hexapeptide is excised. There are three putative sites of N-glycosylation, one of which is situated in the small subunit region. The carbohydrate composition of this site was determined by mass spectrometry analysis and was found to contain three hexoses, two N-acetyl-hexoses and two deoxyhexoses. The protein has a calcium dependent phospholipase A(2) type of activity. It is lethal to arthropods (insects and isopods), but not toxic to mammals, using doses up to 20 microg per 20 g mouse body weight. For crickets, a dose of 5 microg per animal is lethal; however, when injected into mice it is capable of causing only muscular inflammation, without rupture of the basal membrane of cells. It has a direct hemolytic effect in human erythrocytes and retards the coagulation time of blood. It is an unusual phospholipase A(2), with only 36% and 50% amino acid sequence identities to the closest known phospholipases, imperatoxin I and phospholipin, respectively. Identities with bee and Heloderma venom phospholipase are only in the order of 28%.

A Subfamily of Acidic α-K+ Toxins

Three homologous acidic peptides have been isolated from the venom of three different Parabuthus scorpion species, P. transvaalicus, P. villosus, and P. granulatus. Analysis of the primary sequences reveals that they structurally belong to subfamily 11 of short chain alpha-K(+)-blocking peptides (Tytgat, J., Chandy, K. G., Garcia, M. L., Gutman, G. A., Martin-Eauclaire, M. F., van der Walt, J. J., and Possani, L. D. (1999) Trends Pharmacol. Sci. 20, 444-447). These toxins are 36-37 amino acids in length and have six aligned cysteine residues, but they differ substantially from the other alpha-K(+) toxins because of the absence of the critical Lys(27) and their total overall negative charge. Parabutoxin 1 (PBTx1), which has been expressed by recombinant methods, has been submitted to functional characterization. Despite the lack of the Lys(27), this toxin blocks several Kv1-type channels heterologously expressed in Xenopus oocytes but with low affinities (micromolar range). Because a relationship between the biological activity and the acidic residue substitutions may exist, we set out to elucidate the relative impact of the acidic character of the toxin and the lack of the critical Lys(27) on the weak activity of PBTx1 toward Kv1 channels. To achieve this, a specific mutant named rPBTx1 T24F/V26K was made recombinantly and fully characterized on Kv1-type channels heterologously expressed in Xenopus oocytes. Analysis of rPBTx1 T24F/V26K displaying an affinity toward Kv1.2 and Kv1.3 channels in the nanomolar range shows the importance of the functional dyad above the acidic character of this toxin.

A novel class of peptide found in scorpion venom with neurodepressant effects in peripheral and central nervous system of the rat

A novel toxin, named Cll9, was isolated from the venom of the scorpion Centruroides limpidus limpidus Karsch. It is composed of 63 amino acid residues closely packed by four disulfide bridges. It showed no apparent effect when injected to insects, crustaceans and i.p. to mice. However, when i.c.v. injected in the rat it immediately induced sleep, suggesting that it has a neurodepressant effect. We confirmed this by showing that it has a strong antiepileptic action, as assessed with the penicillin focus model. Its effectiveness in inhibiting Na(+) permeability in (cultured) rat peripheral ganglia further supports its neurodepressant actions. However, this peptide did not affect other Na(+) channels such as those from cerebellum granular cells in culture or the rSkM1 Na(+) channels expressed in HEK293. The cDNA and genomic regions encoding this peptide were cloned and sequenced. This peptide is synthesized as a precursor of 84 amino acid residues and processed by removing 19 amino acids (signal peptide) from the amino terminal region and a couple of lysine residues from the carboxyl end. The presence of an intron of 777 bases interrupting the region encoding the signal peptide was also revealed. A comparison of its primary sequence, with more than 100 scorpion toxins known, showed that together with toxin CsE9 they constitute a new subfamily of peptides considered to be one of the most divergent groups of scorpion toxin-like peptides discovered.

Two new scorpion toxins that target voltage-gated Ca2+ and Na+ channels

This report describes the isolation, primary structure determination, and functional characterization of two similar toxins from the scorpion Parabuthus granulatus named kurtoxin-like I and II (KLI and KLII, respectively). KLII from P. granulatus is identical to kurtoxin from Parabuthus transvaalicus (a 63 amino-acid long toxin) whereas KLI is a new peptide containing 62 amino acid residues closely packed by four disulfide bridges with a molecular mass of 7244. Functional assays showed that both toxins, KLI and kurtoxin from P. granulatus, potently inhibit native voltage-gated T-type Ca(2+) channel activity in mouse male germ cells. In addition, KLI was shown to significantly affect the gating mechanisms of recombinant Na(+) channels and weakly block alpha(1)3.3Ca(V) channels expressed in Xenopus oocytes. KLI and kurtoxin from P. granulatus represent new probes to study the role of ion channels in germ cells, as well as in cardiac and neural tissue.

A large number of novel Ergtoxin-like genes and ERG K+-channels blocking peptides from scorpions of the genus Centruroides

Twenty-three novel sequences similar to Ergtoxin (ErgTx) were obtained by direct sequencing of peptides or deduced from gene cloned using cDNAs of venomous glands of Centruroides (C.) elegans, C. exilicauda, C. gracilis, C. limpidus limpidus, C. noxius and C. sculpturatus. These peptides have from 42 to 47 amino acid residues cross-linked by four disulfide bridges. They share sequence similarities (60-98% compared with ErgTx1) and were shown to block ERG K(+)-channels of F-11 clone (N18TG-2xrat DRG) cultured cells. An unrooted phylogenetic tree analysis of these peptides showed that they conform at least five different subfamilies, of which three are novel subfamilies.

A short-chain peptide toxin isolated from Centruroides sculpturatus scorpion venom inhibits ether-à-go-go-related gene K(+) channels

From the venom of the American scorpion Centruroides sculpturatus Ewing we have isolated a minute peptide fraction (named CsEKerg1) which reversibly inhibits the current through ERG (ether-à-go-go-related gene) K(+) channels. Isolation was done by CM-cellulose column chromatography and reversed phase high-performance liquid chromatography. To test for an effect on ERG channels we used NG108-15 neuroblastomaxglioma hybrid cells voltage-clamped in the whole-cell mode. CsEKerg1 contains 43 amino acids and has a molecular weight of 4833. Its amino acid sequence is similar but not identical to that of ergtoxin, a peptide isolated recently from the venom of the Mexican scorpion Centruroides noxius [FASEB J. 13 (1999) 953]. Half inhibition of ERG current occurs at a peptide concentration of 1.12microg/ml.