Activation of the ryanodine receptor Ca2+ release channel of sarcoplasmic reticulum by a novel scorpion venom

Activation of voltage-gated sodium channels by BmK NT1 augments NMDA receptor function through Src family kinase signaling pathway in primary cerebellar granule cell cultures

Voltage-gated sodium channels (VGSCs) are responsible for the generation and propagation of action potentials in excitable cells and are the molecular targets of an array of neurotoxins. BmK NT1, an α-scorpion toxin obtained from the scorpion Buthus martensii Karsch (BmK), produces neurotoxicity that is associated with extracellular Ca²⁺ influx through Na⁺-Ca²⁺ exchangers, N-methyl-d-aspartic acid (NMDA) receptors, and L-type Ca²⁺ channels in cultured cerebellar granule cells (CGCs). In the present study, we demonstrated that BmK NT1 triggered concentration-dependent release of excitatory neurotransmitters, glutamate and aspartate; both effects were eliminated by VGSC blocker, tetrodotoxin. More importantly, we demonstrated that a threshold concentration of BmK NT1 that produced marginal Ca²⁺ influx and neuronal death augmented glutamate-induced Ca²⁺ elevation and neuronal death in CGCs. BmK NT1-augmented glutamate-induced Ca²⁺ influx and neuronal death were suppressed by tetrodotoxin and MK-801 suggesting that the augmentation was through activation of VGSCs and NMDA receptors. Consistently, BmK NT1 also enhanced NMDA-induced Ca²⁺ influx. Further mechanistic investigations demonstrated that BmK NT1 increased the expression level of NMDA receptors on the plasma membrane and increased the phosphorylation level of NR2B at Tyr1472. Src family kinase inhibitor, 1-tert-butyl-3-(4-chlorophenyl)pyrazolo[3,4-d]pyrimidin-4-yl]amine (PP2), but not the inactive analogue, 4-amino-1-phenylpyrazolo[3,4-d]pyrimidine (PP3), eliminated BmK NT1-triggered NR2B phosphorylation, NMDA receptor trafficking, as well as BmK NT1-augmented NMDA Ca²⁺ response and neuronal death. Considered together, these data demonstrated that both presynaptic (excitatory amino acid release) and postsynaptic mechanisms (augmentation of NMDA receptor function) are critical for VGSC activation-induced neurotoxicity in primary CGC cultures.

Tityus stigmurus venom causes genetic damage in blood and testicular cells and affects the number and morphology of gametogenic lineage cells in mice

Scorpion envenomation represents an important health problem in many parts of the world, due to the high number and severity of accidents. Recent studies demonstrated that some species can produce venoms with genetic damage potential. Here, we evaluated whether Tityus stigmurus venom causes genetic damage in blood and testicular cells of Swiss mice. We also analyzed the effect of the venom on the number of spermatogenic lineage cells. Five groups of mice received 0.387 mg/kg of the venom, intraperitoneally; one group received saline solution (control group). Blood and testicular cells were collected for comet assay and histological analysis at different times after treatment (1, 2, 6, 12, and 48 h). Blood was also collected 48 h after treatment for the micronucleus test in erythrocytes. Histological analysis was performed by counting cells of the spermatogenic lineages; the nuclear area of elongated spermatocytes was also evaluated. Treatment with the venom induced DNA damage that endured from 1 h to 48 h, as confirmed by the comet assay. The micronucleus test demonstrated that the venom induced mutations in erythrocytes. The number of spermatogonia and rounded spermatids decreased in some groups; the number of elongated spermatids increased, and their nuclear size decreased 1 h after treatment. Genetic damage can be caused directly by the venom, but we suggested that reactive oxygen species that result from inflammatory process caused by the envenomation may have an important role in the DNA damage. Genetic damage and apoptosis may explain the changes in the number of spermatogenic cells. Furthermore, the decrease in nuclear area may result from chromatin loss. Genetic damage in testicular cells, associated with alterations in the number and morphology of spermatogenic cells, can result in reproduction disorders in animals, or humans, stung by T. stigmurus.

Atrial Infarction-Induced Spontaneous Focal Discharges and Atrial Fibrillation in Sheep: Role of Dantrolene-Sensitive Aberrant Ryanodine Receptor Calcium Release

BACKGROUND

The mechanisms underlying spontaneous atrial fibrillation (AF) associated with atrial ischemia/infarction are incompletely elucidated. Here, we investigate the mechanisms underlying spontaneous AF in an ovine model of left atrial myocardial infarction (LAMI).

METHODS AND RESULTS

LAMI was created by ligating the atrial branch of the left anterior descending coronary artery. ECG loop recorders were implanted to monitor AF episodes. In 7 sheep, dantrolene-a ryanodine receptor blocker-was administered in vivo during the 8-day observation period (LAMI-D, 2.5 mg/kg, IV, BID). LAMI animals experienced numerous spontaneous AF episodes during the 8-day monitoring period that were suppressed by dantrolene (LAMI, 26.1±5.1; sham, 4.3±1.1; LAMI-D, 2.8±0.8; mean±SEM episodes per sheep, P<0.01). Optical mapping showed spontaneous focal discharges (SFDs) originating from the ischemic/normal-zone border. SFDs were calcium driven, rate dependent, and enhanced by isoproterenol (0.03 µmol/L, from 210±87 to 3816±1450, SFDs per sheep) but suppressed by dantrolene (to 55.8±32.8, SFDs per sheep, mean±SEM). SFDs initiated AF-maintaining reentrant rotors anchored by marked conduction delays at the ischemic/normal-zone border. NOS1 (NO synthase-1) protein expression decreased in ischemic zone myocytes, whereas NADPH (nicotinamide adenine dinucleotide phosphate, reduced form) oxidase and xanthine oxidase enzyme activities and reactive oxygen species (DCF [6-carboxy-2',7'-dichlorodihydrofluorescein diacetate]-fluorescence) increased. CaM (calmodulin) aberrantly increased [³H]ryanodine binding to cardiac RyR2 (ryanodine receptors) in the ischemic zone. Dantrolene restored the physiological binding of CaM to RyR2.

CONCLUSIONS

Atrial ischemia causes spontaneous AF episodes in sheep, caused by SFDs that initiate reentry. Nitroso-redox imbalance in the ischemic zone is associated with intense reactive oxygen species production and altered RyR2 responses to CaM. Dantrolene administration normalizes the CaM response, prevents LAMI-related SFDs, and AF initiation. These findings provide novel insights into the mechanisms underlying ischemia-related atrial arrhythmias.

Venom Gland Transcriptomic and Proteomic Analyses of the Enigmatic Scorpion Superstitionia donensis (Scorpiones: Superstitioniidae), with Insights on the Evolution of Its Venom Components

Venom gland transcriptomic and proteomic analyses have improved our knowledge on the diversity of the heterogeneous components present in scorpion venoms. However, most of these studies have focused on species from the family Buthidae. To gain insights into the molecular diversity of the venom components of scorpions belonging to the family Superstitioniidae, one of the neglected scorpion families, we performed a transcriptomic and proteomic analyses for the species Superstitionia donensis. The total mRNA extracted from the venom glands of two specimens was subjected to massive sequencing by the Illumina protocol, and a total of 219,073 transcripts were generated. We annotated 135 transcripts putatively coding for peptides with identity to known venom components available from different protein databases. Fresh venom collected by electrostimulation was analyzed by LC-MS/MS allowing the identification of 26 distinct components with sequences matching counterparts from the transcriptomic analysis. In addition, the phylogenetic affinities of the found putative calcins, scorpines, La1-like peptides and potassium channel κ toxins were analyzed. The first three components are often reported as ubiquitous in the venom of different families of scorpions. Our results suggest that, at least calcins and scorpines, could be used as molecular markers in phylogenetic studies of scorpion venoms.

Transcriptome analysis of scorpion species belonging to the Vaejovis genus

Scorpions belonging to the Buthidae family have traditionally drawn much of the biochemist's attention due to the strong toxicity of their venoms. Scorpions not toxic to mammals, however, also have complex venoms. They have been shown to be an important source of bioactive peptides, some of them identified as potential drug candidates for the treatment of several emerging diseases and conditions. It is therefore important to characterize the large diversity of components found in the non-Buthidae venoms. As a contribution to this goal, this manuscript reports the construction and characterization of cDNA libraries from four scorpion species belonging to the Vaejovis genus of the Vaejovidae family: Vaejovis mexicanus, V. intrepidus, V. subcristatus and V. punctatus. Some sequences coding for channel-acting toxins were found, as expected, but the main transcribed genes in the glands actively producing venom were those coding for non disulfide-bridged peptides. The ESTs coding for putative channel-acting toxins, corresponded to sodium channel β toxins, to members of the potassium channel-acting α or κ families, and to calcium channel-acting toxins of the calcin family. Transcripts for scorpine-like peptides of two different lengths were found, with some of the species coding for the two kinds. One sequence coding for La1-like peptides, of yet unknown function, was found for each species. Finally, the most abundant transcripts corresponded to peptides belonging to the long chain multifunctional NDBP-2 family and to the short antimicrobials of the NDBP-4 family. This apparent venom composition is in correspondence with the data obtained to date for other non-Buthidae species. Our study constitutes the first approach to the characterization of the venom gland transcriptome for scorpion species belonging to the Vaejovidae family.

Scorpion venom components that affect ion-channels function

The number and types of venom components that affect ion-channel function are reviewed. These are the most important venom components responsible for human intoxication, deserving medical attention, often requiring the use of specific anti-venoms. Special emphasis is given to peptides that recognize Na(+)-, K(+)- and Ca(++)-channels of excitable cells. Knowledge generated by direct isolation of peptides from venom and components deduced from cloned genes, whose amino acid sequences are deposited into databanks are nowadays in the order of 1.5 thousands, out of an estimate biodiversity closed to 300,000. Here the diversity of components is briefly reviewed with mention to specific references. Structural characteristic are discussed with examples taken from published work. The principal mechanisms of action of the three different types of peptides are also reviewed. Na(+)-channel specific venom components usually are modifier of the open and closing kinetic mechanisms of the ion-channels, whereas peptides affecting K(+)-channels are normally pore blocking agents. The Ryanodine Ca(++)-channel specific peptides are known for causing sub-conducting stages of the channels conductance and some were shown to be able to internalize penetrating inside the muscle cells.

Comparative study between the protective effects of Saudi and Egyptian antivenoms, alone or in combination with ion channel modulators, against deleterious actions of Leiurus quinquestriatus scorpion venom

This study compared efficacy of two polyvalent antivenoms (Saudi Arabian and Egyptian), against lethality and pathophysiological changes of Leiurus quinquestriatus quinquestriatus (LQQ) scorpion venom in mice. Additionally, the study examined whether treatment with selected ion channel modulators, lidocaine, nimodipine or amiodarone would be effective, alone or combined with the antivenoms. The protein concentration of the Saudi antivenom was 1/3 of Egyptian, indicating lesser immunogenicity, while both preservative contents were within limits. In immunodiffusion experiments, both exhibited prominent precipitin bands indicating high concentrations of specific antibodies. Neutralizing capacities (60-70 LD(50)) stated on labels were confirmed. Both antivenoms significantly (P < 0.001) prolonged survival time (from 26.9 +/- 1.18 min, 100% dead with venom to 224-300 min, 0-30% dead) of envenomed mice, whether injected iv before or 5 min after venom. Injection of either antivenom plus ion channel modulators, gave comparable results to that observed in mice treated with antivenoms alone. The Na(+) channel blocker lidocaine and the Ca(2+) channel blocker nimodipine on their own significantly protected the animals (P < 0.05), but to a lesser extent. The two antivenoms, significantly ameliorated the venom-evoked changes in serum LDH (P < 0.001) and CKMB (P < 0.01) plus cardiac TNFalpha and nitrate/nitrite levels (P < 0.001). When combined with lidocaine or nimodipine, the effects were not greater than antivenom alone. Moreover, the antivenoms ameliorated characteristic venom-evoked changes in the isolated perfused Langendorff hearts. Lidocaine and amiodarone were more effective than nimodipine. In Conclusion both Saudi and Egyptian antivenoms protected mice from the pathological and lethal effects of LQQ scorpion. Sodium and calcium channel blockers, lidocaine and nimodipine, may be useful when antivenoms are not available.

Polymorphic ventricular tachycardia and abnormal Ca2+ handling in very-long-chain acyl-CoA dehydrogenase null mice

Patients with mutations in the mitochondrial very-long-chain acyl-CoA dehydrogenase (VLCAD) gene are at risk for cardiomyopathy, myocardial dysfunction, ventricular tachycardia (VT), and sudden cardiac death. The mechanism is not known. Here we report a novel mechanism of VT in mice lacking VLCAD (VLCAD(-/-)). These mice exhibited polymorphic VT and increased incidence of VT after isoproterenol infusion. Polymorphic VT was induced in 10 out of 12 VLCAD(-/-) mice (83%) when isoproterenol was used. One out of 10 VLCAD(-/-) mice with polymorphic VT had VT with the typical bidirectional morphology. At the molecular level, VLCAD(-/-) cardiomyocytes showed increased levels of cardiac ryanodine receptor 2, phospholamban, and calsequestrin with increased [(3)H]ryanodine binding in heart microsomes. At the single cardiomyocyte level, VLCAD(-/-) cardiomyocytes showed significant increase in diastolic indo 1 and fura 2 fluorescence, with increased Ca(2+) transient amplitude. These changes were associated with altered Ca(2+) dynamics, to include: faster sarcomere contraction, larger time derivative of the upstroke, and shorter time-to-minimum sarcomere length compared with VLCAD(+/+) control cells. The L-type Ca(2+) current characteristics were not different under voltage-clamp conditions in the two VLCAD genotypes. Sarcoplasmic reticulum Ca(2+) load measured as normalized integrated Na(+)/Ca(2+) exchange current after rapid caffeine application was increased by 48% in VLCAD(-/-) cells. We conclude that intracellular Ca(2+) handling represents a possible molecular mechanism of arrhythmias in mice and perhaps in VLCAD-deficient humans.

Altered Ca2+ sparks and gating properties of ryanodine receptors in aging cardiomyocytes

To investigate the cellular mechanisms for altered cardiac function in senescence, we measured Ca(2+) transients and Ca(2+) sparks in ventricular cardiomyocytes from 6- to 24-month-old Fisher 344 (F344) rat hearts. The single channel properties of ryanodine receptors from adult and senescent hearts were also studied. In senescent myocytes, we observed a decreased peak [Ca(2+)](i) amplitude and an increased time constant for decay (tau), both of which correlated with a reduced Ca(2+) content of the sarcoplasmic reticulum (SR). Our studies also revealed that senescent cardiomyocytes had an increased frequency of Ca(2+) sparks and a slight but statistically significant decrease in average amplitude, full-width-at-half-maximum (FWHM) and full-duration-at-half-maximum (FDHM). Single channel recordings of ryanodine receptors (RyR2) demonstrated that in aging hearts, the open probability (P(o)) of RyR2 was increased but the mean open time was shorter, providing a molecular correlate for the increased frequency of Ca(2+) sparks and decreased size of sparks, respectively. Thus, modifications of normal RyR2 gating properties may play a role in the altered Ca(2+) homeostasis observed in senescent myocytes.

Increased leakage of sarcoplasmic reticulum Ca2+ contributes to abnormal myocyte Ca2+ handling and shortening in sepsis*

OBJECTIVE

Changes in cardiac function due to sepsis have been widely reported. However, the underlying mechanisms remain poorly understood. In the mammalian heart, myocyte function and intracellular calcium homeostasis are closely coupled. In this study we tested the hypothesis that alterations in cardiac calcium homeostasis due to sepsis underlie the observed myocyte dysfunction.

DESIGN

Randomized prospective animal study.

SETTING

Research laboratory.

SUBJECTS

Male Sprague-Dawley rats weighing 250-275 g.

INTERVENTIONS

We induced sepsis by cecal ligation and puncture in the rat, which mimics the type of infection caused by perforation of the intestine in humans.

MEASUREMENTS AND RESULTS

Forty-eight hours after cecal ligation and puncture, isolated cardiac ventricular cardiomyocytes demonstrated a 57% decreased peak systolic [Ca]. The time constant of the Ca transient increased 71% and 57% in myocytes obtained 24 hrs and 48 hrs after cecal ligation and puncture, respectively. The average shortening of cardiomyocytes 48 hrs after cecal ligation and puncture was significantly decreased. To investigate the cellular mechanisms of altered Ca transients and myocyte shortening, we measured Ca sparks, the spontaneous local Ca release events in cardiomyocytes at resting states. The Ca spark frequency progressively increased in myocytes 24 hrs and 48 hrs after cecal ligation and puncture. The total activity of sparks also increased compared with sham-operated animals. The overall leakage of sarcoplasmic reticulum Ca in resting states was increased in sepsis and resulted in reduced sarcoplasmic reticulum Ca content.

CONCLUSIONS

Abnormal Ca leakage from the sarcoplasmic reticulum contributes significantly to the depressed myocyte shortening in sepsis. In the future, modalities that prevent this Ca leakage may prove beneficial in the treatment of sepsis-induced myocyte shortening.

Epitope mapping of the antigenic protein TsNTxP from Tityus serrulatus scorpion venom using mouse, rabbit and sheep antibodies

In the present investigation we used native and recombinant TsNTxP to elicit antibodies in three different animal models (mouse, rabbit and sheep). Differences among anti-TsNTxP antibodies were analyzed using sets of overlapping pentadecapeptides of the TsNTxP amino acid sequence and also modified peptides to reveal key residues in antibody-peptide binding. Despite the identification of similar peptides by the antibodies in the C and N-terminal, peculiarities of each system were observed including the level of reactivity and also the number and type of key residues in the continuous epitopes of TsNTxP. In addition, in vitro neutralization assays indicated that sheep are an alternative and efficient model for the production of anti-Tityus serrulatus venom.

Activation of Skeletal Ryanodine Receptors by Two Novel Scorpion Toxins from Buthotus judaicus

Buthotus judaicus toxin 1 (BjTx-1) and toxin 2 (BjTx-2), two novel peptide activators of ryanodine receptors (RyR), were purified from the venom of the scorpion B. judaicus. Their amino acid sequences differ only in 1 residue out of 28 (residue 16 corresponds to Lys in BjTx-1 and Ile in BjTx-2). Despite a slight difference in EC(50), both toxins increased binding of [(3)H]ryanodine to skeletal sarcoplasmic reticulum at micromolar concentrations but had no effect on cardiac or liver microsomes. Their activating effect was Ca(2+)-dependent and was synergized by caffeine. B. judaicus toxins also increased binding of [(3)H]ryanodine to the purified RyR1, suggesting that a direct protein-protein interaction mediates the effect of the peptides. BjTx-1 and BjTx-2 induced Ca(2+) release from Ca(2+)-loaded sarcoplasmic reticulum vesicles in a dose-dependent manner and induced the appearance of long lived subconductance states in skeletal RyRs reconstituted into lipid bilayers. Three-dimensional structural modeling reveals that a cluster of positively charged residues (Lys(11) to Lys(16)) is a prominent structural motif of both toxins. A similar structural motif is believed to be important for activation of RyRs by imperatoxin A (IpTx(a)), another RyR-activating peptide (Gurrola, G. B., Arevalo, C., Sreekumar, R., Lokuta, A. J., Walker, J. W., and Valdivia, H. H. (1999) J. Biol. Chem. 274, 7879-7886). Thus, it is likely that B. judaicus toxins and imperatoxin A bind to RyRs by means of electrostatic interactions that lead to massive conformational changes in the channel protein. The different affinity and structural diversity of this family of scorpion peptides makes them excellent peptide probes to identify RyR domains that trigger the channel to open.

The calmodulin binding region of the skeletal ryanodine receptor acts as a self-modulatory domain

A synthetic peptide (CaMBP) matching amino acids 3614-3643 of the skeletal ryanodine receptor (RyR1) binds to both Ca2+-free calmodulin (CaM) and Ca2+-bound CaM with nanomolar affinity [J. Biol. Chem. 276 (2001) 2069]. We report here that CaMBP increases [3H]ryanodine binding to RyR1 in a dose- and Ca2+-dependent manner; it also induces Ca2+ release from SR vesicles, and increases open probability (P(o)) of single RyR channels reconstituted in planar lipid bilayers. Further, CaMBP removes CaM associated with SR vesicles and increases [3H]ryanodine binding to purified RyR1, suggesting that its mechanism of action is two-fold: it removes endogenous inhibitors and also interacts directly with complementary regions in RyR1. Remarkably, the N-terminus of CaMBP activates RyRs while the C-terminus of CaMBP inhibits RyR activity, suggesting the presence of two discrete functional subdomains within this region. A ryr1 mutant lacking this region, RyR1-Delta3614-3643, was constructed and expressed in dyspedic myoblasts (RyR1-knockout). The depolarization-, caffeine- and 4-chloro-m-cresol (4-CmC)-induced Ca2+ transients in these cells were dramatically reduced compared with cells expressing wild type RyR1. Deletion of the 3614-3643 region also resulted in profound changes in unitary conductance and channel gating. We thus propose that the RyR1 3614-3643 region acts not only as the CaM binding site, but also as an important modulatory domain for RyR1 function.

Lobe-dependent regulation of ryanodine receptor type 1 by calmodulin

Calmodulin activates the skeletal muscle Ca(2+) release channel RYR1 at nm Ca(2+) concentrations and inhibits the channel at microm Ca(2+) concentrations. Using a deletion mutant of calmodulin, we demonstrate that amino acids 2-8 are required for high affinity binding of calmodulin to RYR1 at both nm and microm Ca(2+) concentrations and are required for maximum inhibition of the channel at microm Ca(2+) concentrations. In contrast, the addition of three amino acids to the N terminus of calmodulin increased the affinity for RYR1 at both nm and microm Ca(2+) concentrations, but destroyed its functional effects on RYR1 at nm Ca(2+). Using both full-length RYR1 and synthetic peptides, we demonstrate that the calmodulin-binding site on RYR1 is likely to be noncontiguous, with the C-terminal lobe of both apocalmodulin and Ca(2+)-calmodulin binding to amino acids between positions 3614 and 3643 and the N-terminal lobe binding at sites that are not proximal in the primary sequence. Ca(2+) binding to the C-terminal lobe of calmodulin converted it from an activator to an inhibitor, but an interaction with the N-terminal lobe was required for a maximum effect on RYR1. This interaction apparently depends on the native sequence or structure of the first few amino acids at the N terminus of calmodulin.

The effects of lignocaine on actions of the venom from the yellow scorpion "Leiurus quinquestriatus" in vivo and in vitro

Many toxins from scorpion venoms activate sodium channels, thereby enhancing neurotransmitter release. The aim of the present work was to determine if the in vivo and in vitro effects of Leiurus quinquestriatus venom (LQQ) could be ameliorated by lignocaine, a sodium channel blocker. In urethane anaesthetised rabbits, LQQ venom (0.5 mg kg(-1), i.v.) caused initial hypotension and bradycardia followed by hypertension, pulmonary oedema, electrocardiographic changes indicating conduction defects, ischaemia, infarction, and then hypotension and death. Lignocaine (1 mg kg(-1) i.v. bolus initially, followed by i.v. infusion of 50 microg kg(-1) min(-1)) significantly attenuated the majority of the venom-evoked effects and reduced mortality. Addition of LQQ venom (1, 3 and 10 microg ml(-1)) to chick biventer cervicis, guinea pig ileum, and rat vas deferens preparations, increased the height of electrically-induced twitches, elevated resting tension, and caused autorhythmic oscillations. Lignocaine (3 x 10(-4)-1.2 x 10(-3) M) greatly attenuated these venom-evoked actions in the three preparations. Antagonists of appropriate neurotransmitters were also tested to determine the contribution of released transmitters to LQQ effects. Atropine significantly decreased the venom-elicited effects on guinea pig ileum preparations, while prazosin and guanethidine significantly reduced the venom's actions on rat vas deferens. In chick biventer cervicis preparations, tubocurarine and hexamethonium significantly attenuated the venom-induced effects. This study supports the hypothesis that many effects of LQQ venom involve the release of neurotransmitters and may be ameliorated by treatment with lignocaine.

Characterization of four distinct monoclonal antibodies specific to BmK AS-1, a novel scorpion bioactive polypeptide

Four monoclonal antibodies designed as 2#, 3#, 4# and 5# have been raised against a novel bioactive polypeptide BmK AS-1 purified from the Chinese scorpion Buthus martensi Karsch. All of these antibodies exhibited specific affinity with antigen by ELISA and Biosensor assay. Western blot analysis showed that 3# and 4# were able to recognize the denatured antigen, but not 2# and 5#. These antibodies could cross-react with BmK AS, but not with other types of BmK neurotoxins such as BmK I (an alpha-like toxin) and BmK IT (an excitatory insect-selective toxin), and in which only 5# can partially react with BmK IT2 (a depressant insect-selective toxin). Immunocytochemical staining demonstrated that 3#, 4# and 5# antibodies can visualize the antigen bound to the membrane of SK-N-SH neuroblast cells, with the exception of 2#. This suggests that either conformation alteration of receptor binding might be prone to nonvisualization or the epitope recognized by antibody 2# might be overlapped with receptor binding sites of antigen. The antibodies developed in the study should provide powerful new tools for investigating the structure/function relationship and pharmacological mechanism of scorpion neurotoxins.

Gene cloning and sequencing of BmK AS and BmK AS-1, two novel neurotoxins from the scorpion Buthus martensi Karsch

Based on the known amino acid sequences of BmK AS and BmK AS-1, the gene specific primers were designed and synthesized for 3' and 5' RACE (Rapid Amplification of cDNA Ends). Their partial cDNA fragments obtained by 3' and 5' RACE were cloned and sequenced, and the full length cDNA sequences of BmK AS and BmK AS-1 were then completed by overlapping their two partial cDNA sequences, respectively. The predicted amino acid sequences both consist of 85 amino acid residues including a putative signal peptide of 19 residues and a mature toxin of 66 residues. They are different in 17 amino acid residues, among them 11 residues in the mature toxin. The predicted amino acid sequences of BmK AS and BmK AS-1 were almost consistent with those determined and revised (personal communication), only different in one and two residues at their COO-terminal parts, respectively. Based on the determined cDNA sequences, and using the total DNAs isolated from the scorpion venom glands as a template, the genomic DNAs of BmK AS and BmK AS-1 were also amplified by PCR and sequenced. It showed that no intron was inserted in their open reading frames, while in the exon of signal peptide sequences of other Na+, K+ and Cl- channel toxins from the same scorpion, an intron is usually found. However, the Northern blot hybridization results indicated that the sizes of their mRNA should be around 800 bp. Their extra sequences around 400 bp which might function as an intron should be located at their 5' untranslated regions.

Molecular cloning and genomic analysis of TsNTxp: an immunogenic protein from Tityus serrulatus scorpion venom

A non-toxic protein (TsNTxP) from Tityus serrulatus scorpion venom has been shown to be an efficient immunogen and anti-TsNTxP antibodies recognize and neutralize the effect of Tityus serrulatus venom [Chávez-Olórtegui et al., 1997. Toxicon 35, 213-221]. With the purpose of studying the organization of the gene that code for this protein, we have isolated a full length cDNA clone for TsNTxP from a cDNA expression library using anti-TsNTxP antibodies. The nucleotide sequence of the gene that encodes TsNTxP was also obtained and it reveals the presence of an intron within the signal peptide sequence. The TsNTxP gene showed high degree of similarity with genes encoding toxins from scorpions of the genus Tityrus.

Novel mechanisms involved in superoxide anion radical-triggered Ca2+ release from cardiac sarcoplasmic reticulum linked to cyclic ADP-ribose stimulation

It has been suggested that cyclic adenosine 5'-diphosphoribose (cADPR) directly activates the cardiac isoform of the ryanodine receptor (RyR)/Ca2+ release channel. We have previously shown that selective activation of RyR/Ca2+ release channel by superoxide anion radical (O2.-) is dependent of the presence of calmodulin and identified calmodulin as a functional mediator of O2.- -triggered Ca2+ release through the RyR/Ca2+ release channel of cardiac sarcoplasmic reticulum (SR). We now demonstrate that although the effect of O2.- on Ca2+ efflux from RyR/Ca2+ release channel at higher concentrations ( >5 microM) is due to its ability to produce a loss in function of calmodulin thereby decreasing calmodulin inhibition, O2.- radicals at lower concentrations (<5 microM) may be able to stimulate Ca2+ release only in the presence of calmodulin from the SR via increased cADPR synthesis; it is also shown that cADPR is a modulator that can activate the Ca2+-release mechanism when it is in a sensitized state by the presence of calmodulin, possibly, at physiological concentration. In addition, the SR vesicles immediately upon addition of cADPR, but not NAD+, did exhibit Ca2+ efflux stimulation. When heart homogenate was incubated with O2.-, conversion of NAD+ into cADPR was stimulated; the reduction of homogenate Ca2+ uptake (by increasing Ca2+ efflux through RyR/Ca2+ release channel) occurred. Thus O2.- radical is responsible for cADPR formation from NAD+ in the cellular environment outside of the SR of heart muscle. The results presented here provide the first evidence of a messenger role for O2.- radical in cADPR-mediated Ca2+ mobilization in myocardium.

Superoxide anion radical-triggered Ca2+ release from cardiac sarcoplasmic reticulum through ryanodine receptor Ca2+ channel

The ryanodine receptor Ca2+ channel (RyRC) constitutes the Ca2+-release pathway in sarcoplasmic reticulum (SR) of cardiac muscle. A direct mechanical and a Ca2+-triggered mechanism (Ca2+-induced Ca2+ release) have been proposed to explain the in situ activation of Ca2+ release in cardiac muscle. A variety of chemical oxidants have been shown to activate RyRC; however, the role of modification induced by oxygen-derived free radicals in pathological states of the muscle remains to be elucidated. It has been hypothesized that oxygen-derived free radicals initiate Ca2+-mediated functional changes in or damage to cardiac muscle by acting on the SR and promoting an increase in Ca2+ release. We confirmed that superoxide anion radical (O2-) generated from hypoxanthine-xanthine oxidase reaction decreases calmodulin content and increases 45Ca2+ efflux from the heavy fraction of canine cardiac SR vesicles; hypoxanthine-xanthine oxidase also decreases Ca2+ free within the intravesicular space of the SR with no effect on Ca2+-ATPase activity. Current fluctuations through single Ca2+-release channels have been monitored after incorporation into planar phospholipid bilayers. We demonstrate that activation of the channel by O2- is dependent of the presence of calmodulin and identified calmodulin as a functional mediator of O2--triggered Ca2+ release through the RyRC. For the first time, we show that O2- stimulates Ca2+ release from heavy SR vesicles and suggest the importance of accessory proteins such as calmodulin in modulating the effect of O2-. The decreased calmodulin content induced by oxygen-derived free radicals, especially O2-, is a likely mechanism of accumulation of cytosolic Ca2+ (due to increased Ca2+ release from SR) after reperfusion of the ischemic heart.

Pandinus imperator scorpion venom blocks voltage-gated K+ channels in human lymphocytes

Using the patch-clamp technique, we determined that Pandinus imperator scorpion venom blocked whole-cell n-type K+ currents in human peripheral blood lymphocytes in a dose-dependent manner with Kd = 0.02 microgram/ml. K+ channel block was instantaneous and removable by washing with venom-free extracellular solution. The venom-induced block was independent of membrane potential. The venom did not influence activation and inactivation kinetics of the K+ channels, however, accelerated recovery from inactivation. Purified peptides Pi1, Pi2, and Pi3 from the P. imperator venom powerfully blocked Kv1.3 channels in human lymphocytes with Kd values of 9.7 nM, 50 pM, and 0.5 nM, respectively. Flow cytometric membrane potential measurements with the oxonol dye showed that Pi2, the most effective peptide toxin of the P. imperator venom, depolarizes human lymphocytes in accordance with its K+ channel blocking effect.

Action of babycurus-toxin 1 from the east African scorpion Babycurus centrurimorphus on the isolated cockroach giant axon

A toxin named babycurus-toxin 1 (mol. wt 8191), from telson extracts of the scorpion Babycurus centrurimorphus, was found to depolarize the cockroach giant axon. It progressively blocked the evoked action potentials after a short period of limited repetitive activity and after 30 min of toxin action it became impossible to evoke responses to current stimulations. Voltage-clamp experiments on the sodium current indicated that the toxin in micromolar concentrations progressively decreased the transient inward peak sodium current, but also slowed the activation phase of this sodium current and maintained an inward current during the voltage pulses, which deactivated slowly. The toxin also induced in the insect axon a slowly activating-deactivating component of the sodium current. This suggests that the toxin modifies both activation and inactivation mechanisms of sodium channels. Thus there is some similarity in the electrophysiological effects between BcTx1 and the beta-toxins active on mammals.

Effect of Leiurus quinquestriatus hebreus venom on calcium and deoxyglucose uptake in cultured cardiac cells

The effects of scorpion venom Leiurus quinquestriatus hebreus were studied on cardiac cells grown in culture. The venom (30 micrograms/ml) increased significantly (P < 0.05) Ca2+ uptake into intact cardiocytes and to sarcoplasmic reticulum of skinned cells. [3H]Deoxyglucose uptake was also increased significantly (P < 0.05) in venom treated cardiocytes. It was found that fractions I and III of the venom, separated by gel filtration and ion exchange chromatography, are responsible for the increased Ca2+ uptake by the sarcoplasmic reticulum, whereas fraction IIb, III and IV are responsible for the accelerated rate of uptake of 45Ca and [3H]deoxyglucose by intact cells. Ca channel blockers prevented these effects and similar results were obtained by propranolol. Thus, it is concluded that the venom exerts its effect through activation of beta-adrenoceptors which causes the opening of L-type Ca channels.

Calcium-induced calcium release in neurones

Neurones express several subtypes of intracellular Ca2+ channels, which are regulated by cytoplasmic calcium concentration ([Ca2+]c) and provide the pathway for Ca(2+)-induced Ca2+ release (CICR) from endoplasmic reticulum Ca2+ stores. The initial studies of CICR which employed several pharmacological tools (and in particular caffeine and ryanodine) demonstrated that: (i) caffeine induces intracellular calcium release in various peripheral and central neurones; and (ii) inhibition of CICR affects the parameters of depolarization-triggered [Ca2+]c responses. Experiments with caffeine demonstrated also that Ca2+ release from internal pools was incremental, suggesting the coexistence of several subpopulations of Ca2+ release channels with different sensitivity to caffeine. The CICR availability in neurones is controlled by both the Ca2+ content of the internal stores and the basal [Ca2+]c. Direct comparison of transmembrane Ca2+ influx with plasmalemmal Ca2+ current and [Ca2+]c elevation performed on sympathetic, sensory and cerebellar Purkinje neurones revealed the gradual activation of CICR. The efficacy of CICR may be regulated by the newly discovered second messenger cADP ribose (cADPR), although the mechanism of signal transduction involving cADPR is still unknown. CICR in neurones may be important in creation of local [Ca2+]c signals and could be involved in a regulation of numerous neuronal functions.

Peptide probe of ryanodine receptor function. Imperatoxin A, a peptide from the venom of the scorpion Pandinus imperator, selectively activates skeletal-type ryanodine receptor isoforms

We have used [3H]ryanodine binding experiments and single channel recordings to provide convergent descriptions of the effect of imperatoxin A (IpTxa), a approximately 5-kDa peptide from the venom of the scorpion Pandinus imperator (Valdivia, H. H., Kirby, M. S., Lederer, W. J., and Coronado, R. (1992) Proc. Ntl. Acad. Sc. U.S.A. 89, 12185-12189) on Ca2+ release channels/ryanodine receptors (RyR) of sarcoplasmic reticulum (SR). At nanomolar concentrations, IpTxa increased the binding of [3H]ryanodine to skeletal SR and, to a lesser extent, to cerebellum microsomes. The activating effect of IpTxa on skeletal SR was Ca(2+)-dependent, synergized by caffeine, and independent of other modulators of RyRs. However, IpTxa had negligible effects on tissues where the expression of skeletal-type RyR isoforms (RyR1) is small or altogether absent, i.e. cardiac, cerebrum, and liver microsomes. Thus, IpTxa may be used as a ligand capable of discriminating between RyR isoforms with nanomolar affinity. IpTxa increased the open probability (Po) of rabbit skeletal muscle RyRs by increasing the frequency of open events and decreasing the duration of the closed lifetimes. This activating effect was dose-dependent (ED50 = 10 nM), had a fast onset, and was fully reversible. Purified RyR from solubilized skeletal SR displayed high affinity for [3H]ryanodine with a KD of 6.1 nM and Bmax of approximately 30 pmol/mg of protein. IpTxa increased [3H]ryanodine binding noncompetitively by increasing Bmax to approximately 60 pmol/mg of protein. These results suggested the presence of an IpTxa-binding site on the RyR or a closely associated regulatory protein. This site appears to be distinct from the caffeine- and adenine nucleotide-regulatory sites. IpTxa may prove a useful tool to identify regulatory domains critical for channel gating and to dissect the contribution of skeletal-type RyRs to intracellular Ca2+ waveforms generated by stimulation of different RyR isoforms.

Rapid adaptation of cardiac ryanodine receptors: modulation by Mg2+ and phosphorylation

Channel adaptation is a fundamental feature of sarcoplasmic reticulum calcium release channels (called ryanodine receptors, RyRs). It permits successive increases in the intracellular concentration of calcium (Ca2+) to repeatedly but transiently activate channels. Adaptation of RyRs in the absence of magnesium (Mg2+) and adenosine triphosphate is an extremely slow process (taking seconds). Photorelease of Ca2+ from nitrophenyl-EGTA, a photolabile Ca2+ chelator, demonstrated that RyR adaptation is rapid (milliseconds) in canine heart muscle when physiological Mg2+ concentrations are present. Phosphorylation of the RyR by protein kinase A increased the responsiveness of the channel to Ca2+ and accelerated the kinetics of adaptation. These properties of the RyR from heart may also be relevant to other cells in which multiple agonist-dependent triggering events regulate cellular functions.

Ryanodine and theophylline-induced depletion of energy stores in amphibian muscle

The effects of high and low levels of ryanodine on theophylline-induced energy depletion were studied in isolated frog sartorius muscle. Whereas low concentrations of ryanodine (1-10 microM) did not change high energy phosphate contents (PE) after 60 min, high levels (100 microM) reduced resting energy contents by 60% after 60 min. Subcontracture levels of theophylline (2 mM), in the presence of high ryanodine, produced an 80% PE depletion, suggesting possible additive or synergistic effects of these two agents. In contrast to theophylline-induced depletion, neither the ryanodine-induced depletion nor the theophylline-plus-ryanodine-induced depletion of PE seemed sensitive to inhibition by 1 mM procaine. This suggests that there may be differences in the mechanisms whereby methylxanthines and ryanodine deplete energy stores and evoke contractures in amphibian skeletal muscle.

Calcium-dependent block of ryanodine receptor channel of swine skeletal muscle by direct binding of calmodulin

The interaction of the Ca2+ binding protein calmodulin (CaM) with the ryanodine receptor of the sarcoplasmic reticulum (SR) of pig skeletal muscle was investigated by [3H]-ryanodine binding, planar bilayer recordings, and rapid filtration of 45Ca(2+)-loaded SR. Inhibition of [3H]-ryanodine binding by CAM was phosphorylation-independent, had an IC50 of approximately 0.1 microM and was optimal at 10 microM Ca(2+). CaM also inhibited [3H]-ryanodine binding to CHAPS-solubilized and purified ryanodine receptors, suggesting a direct CaM-ryanodine receptor interaction. In single channel recordings, CaM blocked Ca2+ release channels in a Ca(2+)-dependent manner by decreasing the number of open events per unit time without affecting the mean open time or unitary channel conductance. Rapid filtration of 45Ca2+ passively loaded into SR vesicles showed that CaM blocked Ca2+ release within milliseconds of exposure of SR to a Ca2+ release medium containing 10 microM CaM. In controls, an increase in extravesicular Ca2+ from 7 nM to 10 microM resulted in a release of 47 +/- 10% of the 45Ca2+ in 20 ms. CaM reduced the release to 23 +/- 12% in the same period. These results are compatible with a direct mechanism of Ca2+ release channel blockade by CaM and suggest that CaM could play a significant role in the inactivation of SR Ca2+ release during excitation-contraction coupling.

Myotoxin a reduces the threshold for calcium-induced calcium release in skeletal muscle

Myotoxin a, isolated from the venom of the prairie rattlesnake Crotalus viridis viridis, induces necrosis in skeletal muscle. In isolated organelles, it has been reported that myotoxin a reduces Ca2+ uptake into the sarcoplasmic reticulum. The effects of the toxin on Ca2+ regulation were examined in heavy sarcoplasmic reticulum fractions from human and equine skeletal muscle. Ca2+ uptake and release (the threshold of Ca(2+)-induced Ca2+ release) were examined by dual wavelength spectrophotometry. The toxin lowered the threshold of Ca(2+)-induced Ca2+ release in a dose-dependent manner (1-10 microM) and this effect was antagonized by ruthenium red, a Ca2+ release channel blocker. Ca2+ uptake into equine heavy sarcoplasmic reticulum was not decreased by myotoxin a (10 microM) when Ca2+ release was blocked by ruthenium red. [3H]Ryanodine binding to equine heavy sarcoplasmic reticulum was converted from a relatively low affinity state to a higher affinity state by myotoxin a. These results suggest that the dominant effect of myotoxin a is to increase the Ca2+ sensitivity for the opening of the calcium release channel (ryanodine receptor). Myotoxin a may prove to be a useful tool to probe the modulation of calcium release in sarcoplasmic reticulum fractions.

Cardiovascular effects of Buthus martensii (Karsch) scorpion venom

Buthus martensii (Karsch) (BMK) scorpion envenomation is a common medical problem in China and BMK scorpion has been widely used in traditional Chinese medicine. However, the cardiovascular effects of this venom have not been systematically investigated. In the present study, i.v. BMK venom injection significantly increased the blood pressure in conscious rats in a concentration-dependent manner (ED50 = 59 +/- 12 micrograms/kg). The increase in blood pressure occurred within 1 min of injection of the venom and was sustained for more than 50 min. Heart rate was not changed by the venom in conscious rats. In vitro studies with BMK venom revealed the increase in the force of contraction, without modification of the contraction frequency (within 20 min) of isolated atrial strips. Contractions of isolated arterial strips from aorta, renal and vertebral arteries were also enhanced by BMK venom with a time lag of 8 min between the application of the venom and the initiation of the contraction. Furthermore, BMK venom-induced rises in blood pressure in vivo and increased contraction of isolated vessel strips were inhibited by prazosin and tolazoline, respectively, two alpha 1-adrenergic antagonists. BMK venom alone did not alter intracellular calcium concentrations, [Ca2+]i, in cultured vascular smooth muscle cells and endothelial cells. However, BMK venom significantly increased the metabolism of InsP3 in dispersed cardiac myocytes, indicating a direct effect on cardiac myocytes. These results demonstrate the significant cardiovascular effects of BMK venom, which may be mediated by an alteration in InsP3 in cardiac myocytes but not by [Ca2+]i in vascular smooth muscle cells.

Protein kinase A-activated chloride channel is inhibited by the Ca(2+)-calmodulin complex in cardiac sarcoplasmic reticulum

Cardiac sarcoplasmic reticulum (SR) has several chloride (Cl-) channels, which may neutralize the charge across the SR membrane generated by Ca2+ movement. We recently reported a novel 116-picosiemen Cl- channel that is activated by protein kinase A-dependent phosphorylation in cardiac SR. This Cl- channel may serve as a target protein in the receptor-dependent regulation of cardiac excitation-contraction coupling. To understand further regulatory mechanisms, the effects of Ca2+ on the Cl- channel were studied using the planar lipid bilayer-vesicle fusion technique. In the presence of calmodulin (CaM, 0.1 mumol/L per microgram SR vesicles), Ca2+ (3 mumol/L to 1 mmol/L) added to the cis solution reduced the channel openings in a concentration-dependent fashion, whereas Ca2+ (1 nmol/L to 1 mmol/L) alone or CaM (0.1 to 1 mumol/L per microgram SR vesicles) with 1 nmol/L Ca2+ did not affect the channel activity. This inhibitory effect of Ca2+ in the presence of CaM was prevented by CaM inhibitors N-(6 aminohexyl)-5-chloro-1-naphthalenesulfonamide and calmidazolium but not by CaM kinase II inhibitor KN62. These results suggest that the Ca(2+)-CaM complex itself, but not CaM kinase II, is involved in this channel inhibition. Thus, the cardiac SR 116-picosiemen Cl- channel is regulated not only by protein kinase A-dependent phosphorylation but also by the cytosolic Ca(2+)-CaM complex. This is a novel second messenger-mediated regulation of Cl- channels in cardiac SR membrane.

Cloning and characterization of cDNAs that code for Na(+)-channel-blocking toxins of the scorpion Centruroides noxius Hoffmann

With the purpose of studying the organization and characteristics of the genes that code for toxins present in the venom of the Mexican scorpion, Centruroides noxius Hoffmann (CnH), we prepared a lambda gt11 cDNA library from the venom glands. Using specific oligodeoxyribonucleotides (oligos) designed according to known amino acid (aa) sequences of CnH toxins (STox), we detected several positive clones, determined their nucleotide (nt) sequences and deduced their aa sequences. A comparative analysis of these sequences with previously reported STox revealed that CnH cDNAs code for a family of very similar STox. The cDNA coding for a known STox, II-10, was cloned. Additionally, three other complete (new) nt sequences were obtained for cDNAs encoding peptides similar to STox 1 from CnH or variants 2 and 3 from Centruroides sculpturatus Ewing. Southern blot genomic DNA analysis showed a minimum size of approximately 600 bp as EcoRI fragments for elements of this family. PCR amplifications of CnH genomic DNA and hybridization of PCR products with specific probes indicated that the genomic structural regions that code for these genes do not contain introns, or at least not large introns.

Dantrolene and azumolene inhibit [3H]PN200-110 binding to porcine skeletal muscle dihydropyridine receptors

We tested whether the hydantoin muscle relaxants dantrolene, azumolene, or aminodantrolene could alter the binding of [3H]PN200-110 to transverse tubule dihydropyridine receptors or the binding of [3H]ryanodine to junctional sarcoplasmic reticulum Ca2+ release channels. All three drugs inhibited [3H]PN200-110 binding with azumolene (IC50 approximately 20 microM) 3-5 times more potent than dantrolene or aminodantrolene. In contrast, 100 microM azumolene and dantrolene produced a small inhibition of [3H]ryanodine binding (less than 25%) while aminodantrolene was essentially inert. Hence there was a preferential interaction of hydantoins with dihydropyridine receptors instead of ryanodine receptors. Skeletal muscle dihydropyridine receptors may participate in the mechanism of action of dantrolene and azumolene.

Cardiac sarcoplasmic reticulum chloride channels regulated by protein kinase A

In heart cells, several plasma membrane ion channels are targets for phosphorylation. However, it is not known whether sarcoplasmic reticulum (SR) ion channels, which are also essential in the regulation of cardiac function, are regulated by second-messenger systems. Here, we show that a Cl- channel in the cardiac SR membrane is activated by the catalytic subunit of protein kinase A (PKA). Purified cardiac heavy SR vesicles were incorporated into planar lipid bilayers. This channel spontaneously inactivated within a few minutes after the channel was incorporated into the bilayer. Mg-ATP (2-5 mM), but not the nonhydrolyzable ATP analogue 5'-adenylylimidodiphosphate, added to the cis solution prevented this spontaneous channel inactivation. After the inactivation process occurred, the catalytic subunit of PKA (with 0.05 mM Mg-ATP) reactivated this channel. These effects of Mg-ATP and PKA on the Cl- channel were prevented by an inhibitor of PKA. Thus, these results suggest that this SR Cl- channel is a novel target of PKA-dependent phosphorylation in cardiac muscle regulation.

Bee venom melittin is a potent toxin for reducing the threshold for calcium-induced calcium release in human and equine skeletal muscle

The modulation of Ca2+ release by synthetic bee venom melittin was examined in equine and human terminal cisternae-containing fractions. Melittin (0.1 microM) decreased the threshold of Ca(2+)-induced Ca2+ release by 20% in equine muscle and by 36% in human muscle. If terminal cisternae fractions were first preloaded with Ca2+ to greater than about 75% of the threshold of Ca(2+)-induced Ca2+ release and then melittin added, an immediate and sustained release of Ca2+ occurred in preparations from both species. Addition of melittin after a Ca2+ preload of < 50% of the threshold of Ca(2+)-induced Ca2+ release did not elicit sustained Ca2+ release. Ruthenium red (10 microM) antagonized all effects of melittin on Ca2+ release. Melittin (0.1-10 microM) did not affect [3H]ryanodine binding. Melittin (0.1 microM) slightly (10%) inhibited the Ca2+ pump and this action was not antagonized by ruthenium red. These findings suggest that melittin may be an important new probe of the Ca(2+)-modulated Ca2+ release process that does not act at the ryanodine binding site.