Effects of Sr2+ and Mg2+ on the phospholipase A and the presynaptic neuromuscular blocking actions of beta-bungarotoxin, crotoxin and taipoxin

Biophysical studies suggest a new structural arrangement of crotoxin and provide insights into its toxic mechanism

Crotoxin (CTX) is the main neurotoxin found in Crotalus durissus rattlesnake venoms being composed by a nontoxic and non-enzymatic component (CA) and a toxic phospholipase A2 (CB). Previous crystallographic structures of CTX and CB provided relevant insights: (i) CTX structure showed a 1:1 molecular ratio between CA and CB, presenting three tryptophan residues in the CA/CB interface and one exposed to solvent; (ii) CB structure displayed a tetrameric conformation. This study aims to provide further information on the CTX mechanism of action by several biophysical methods. Our data show that isolated CB can in fact form tetramers in solution; however, these tetramers can be dissociated by CA titration. Furthermore, CTX exhibits a strong reduction in fluorescence intensity and lifetime compared with isolated CA and CB, suggesting that all tryptophan residues in CTX may be hidden by the CA/CB interface. By companying spectroscopy fluorescence and SAXS data, we obtained a new structural model for the CTX heterodimer in which all tryptophans are located in the interface, and the N-terminal region of CB is largely exposed to the solvent. Based on this model, we propose a toxic mechanism of action for CTX, involving the interaction of N-terminal region of CB with the target before CA dissociation.

Biodistribution and Lymphatic Tracking of the Main Neurotoxin of Micrurus fulvius Venom by Molecular Imaging

The venom of the Eastern coral snake Micrurus fulvius can cause respiratory paralysis in the bitten patient, which is attributable to β-neurotoxins (β-NTx). The aim of this work was to study the biodistribution and lymphatic tracking by molecular imaging of the main β-NTx of M. fulvius venom. β-NTx was bioconjugated with the chelator diethylenetriaminepenta-acetic acid (DTPA) and radiolabeled with the radionuclide Gallium-67. Radiolabeling efficiency was 60%–78%; radiochemical purity ≥92%; and stability at 48 h ≥ 85%. The median lethal dose (LD₅₀) and PLA₂ activity of bioconjugated β-NTx decreased 3 and 2.5 times, respectively, in comparison with native β-NTx. The immune recognition by polyclonal antibodies decreased 10 times. Biodistribution of β-NTx-DTPA-⁶⁷Ga in rats showed increased uptake in popliteal, lumbar nodes and kidneys that was not observed with ⁶⁷Ga-free. Accumulation in organs at 24 h was less than 1%, except for kidneys, where the average was 3.7%. The inoculation site works as a depot, since 10% of the initial dose of β-NTx-DTPA-⁶⁷Ga remains there for up to 48 h. This work clearly demonstrates the lymphatic system participation in the biodistribution of β-NTx-DTPA-⁶⁷Ga. Our approach could be applied to analyze the role of the lymphatic system in snakebite for a better understanding of envenoming.

Neuromuscular activity of the venoms of the Colombian coral snakes Micrurus dissoleucus and Micrurus mipartitus: an evolutionary perspective

The venoms of coral snakes (genus Micrurus) are known to induce a broad spectrum of pharmacological activities. While some studies have investigated their potential human effects, little is known about their mechanism of action in terms of the ecological diversity and evolutionary relationships among the group. In the current study we investigated the neuromuscular blockade of the venom of two sister species Micrurus mipartitus and Micrurus dissoleucus, which exhibit divergent ecological characteristics in Colombia, by using the chick biventer cervicis nerve-muscle preparation. We also undertook a phylogenetic analysis of these species and their congeners, in order to provide an evolutionary framework for the American coral snakes. The venom of M. mipartitus caused a concentration-dependant inhibition (3-10 μg/ml) of nerve-mediated twitches and significantly inhibited contractile responses to exogenous ACh (1 mM), but not KCl (40 mM), indicating a postsynaptic mechanism of action. The inhibition of indirect twitches at the lower venom dose (3 μg/ml) showed to be triphasic and the effect was further attenuated when PLA2 was inhibited. M. dissoleucus venom (10-50 μg/ml) failed to produce a complete blockade of nerve-mediated twitches within a 3 h time period and significantly inhibited contractile responses to exogenous ACh (1 mM) and KCl (40 mM), indicating both postsynaptic and myotoxic mechanisms of action. Myotoxic activity was confirmed by morphological studies of the envenomed tissues. Our results demonstrate a hitherto unsuspected diversity of pharmacological actions in closely related species which exhibit divergent ecological characteristics; these results have important implications for both the clinical management of Coral snake envenomings and the design of Micrurus antivenom.

Bothrops snake myotoxins induce a large efflux of ATP and potassium with spreading of cell damage and pain

Myotoxins play a major role in the pathogenesis of the envenomations caused by snake bites in large parts of the world where this is a very relevant public health problem. We show here that two myotoxins that are major constituents of the venom of Bothrops asper, a deadly snake present in Latin America, induce the release of large amounts of K(+) and ATP from skeletal muscle. We also show that the released ATP amplifies the effect of the myotoxins, acting as a "danger signal," which spreads and causes further damage by acting on purinergic receptors. In addition, the release of ATP and K(+) well accounts for the pain reaction characteristic of these envenomations. As Bothrops asper myotoxins are representative of a large family of snake myotoxins with phospholipase A(2) structure, these findings are expected to be of general significance for snake bite envenomation. Moreover, they suggest potential therapeutic approaches for limiting the extent of muscle tissue damage based on antipurinergic drugs.

The structural events associated with the binding of divalent cations to β-bungarotoxin

In order to address the mechanism why the Ca2+ was crucial for the manifestation of the phospholipase A2 (PLA2) activity of beta-bungarotoxin (beta-BuTx), four divalent cations were used to assess their influences on the catalytic activity and the fine structures of beta-BuTx. Substitution Mg2+ or Sr2+ for Ca2+ in the substrate solution was found to cause a decrease in the PLA2 activity to approximately 15 or 6% of that in the presence of Ca2+. However, only marginally detectable PLA2 activity was observed with the addition of Ba2+. The nonpolarity of 8-anilinonaphthalene-1-sulfonate (ANS)-binding site of beta-BuTx markedly increased with the binding of cations to beta-BuTx. The negative ellipticity noted with the CD spectra of beta-BuTx increased upon the binding of cations too. With the exception of Ba2+, the order of the ability of cations to enhance the intensity of ANS fluorescence or increase the increment of negative ellipticity was Sr2+ > Ca2+ > Mg2+, which was the same order as the increase in their atomic radii. However, the energy transfer from Trp fluorescence emission to ANS was most effective upon the addition of Ca2+. Moreover, the extent of glutaraldehyde crosslinking between A chain and B chain decreased in the presence of cations. Nevertheless, the binding affinities of beta-BuTx for the four cations were similar. These results, together with the findings that the ANS molecule binds at the active site of the A chain in beta-BuTx, suggest that the binding of Ca2+ to beta-BuTx induces subtly conformational changes occurred at the active site for exerting the activity of beta-BuTx. Moreover, the change in the gross conformation induced by the binding of Ca2+ may affect the interaction between A chain and B chain, and consequently the activity of beta-BuTx as well.

Taipoxin induces F-actin fragmentation and enhances release of catecholamines in bovine chromaffin cells

Adrenomedullary bovine chromaffin cells were used to study the uptake and cellular effects of the phospholipase type A2 (PLA2) neurotoxin taipoxin in a neuroendocrine model. This toxin entered rapidly inside cultured cells. Within 1 h, taipoxin accumulated on the plasma membrane, independently of calcium presence, and caused fragmentation of the F-actin cytoskeleton. Toxin-induced cell death occurred after 24 h of incubation with the appearance of toxin containing large vesicles. Secretory experiments performed in cell populations showed an increased exocytosis in taipoxin-treated cells stimulated by depolarization or by incubation with the calcium-ionophore A23187. Like F-actin fragmentation, this effect is abolished by replacement of Ca2+ with Sr2+ during toxin incubation. The effect of taipoxin on exocytosis is not enhanced by latrunculin A, a F-actin disassembling drug altering secretion. Secretory studies in single taipoxin-treated cells using amperometry, showed an increase in the number of released vesicles without modification of the kinetic parameters of individual vesicle fusions. Taken together, these results suggest that taipoxin causes F-actin fragmentation and enhances secretion by redistribution of vesicles among secretory pools.

What does beta-bungarotoxin do at the neuromuscular junction?

beta-Bungarotoxin from the Taiwan banded krait, Bungarus multicinctus is a basic protein (pI=9.5), with a molecular weight of 21,800 consisting of two different polypeptide subunits. A phospholipase A(2) subunit named the A-chain and a non-phospholipase A(2) subunit named the B-chain, which is homologous to Kunitz protease inhibitors. The A-chain and the B-chain are covalently linked by one disulphide bridge. On mouse hemi-diaphragm nerve-muscle preparations, partially paralysed by lowering the external Ca(2+) concentration, beta-bungarotoxin classically produces triphasic changes in the contraction responses to indirect nerve stimulation. The initial transient inhibition of twitches (phase 1) is followed by a prolonged facilitatory phase (phase 2) and finally a blocking phase (phase 3). These changes in twitch tension are mimicked, to some extent, by similar changes to end plate potential amplitude and miniature end plate potential frequency. The first and second phases are phospholipase-independent and are thought to be due to the B-chain (a dendrotoxin mimetic) binding to or near to voltage-dependent potassium channels. The last phase (phase 3) is phospholipase dependent and is probably due to phospholipase A(2)-mediated destruction of membrane phospholipids in motor nerve terminals.

Mechanism of action of beta-bungarotoxin, a presynaptically acting phospholipase A2 neurotoxin: its effect on protein phosphorylation in rat brain synaptosomes

The snake venom phospholipase A2 neurotoxin, beta-bungarotoxin, acts presynaptically to alter acetylcholine release in both the peripheral and central nervous systems. In investigating the mechanism of this action, we found that beta-bungarotoxin inhibited phosphorylation of synapsin I, GAP-43 and MARCKS in rat brain synaptosomes. This inhibition was not due to the inhibition of ATP synthesis, action of arachidonic acid metabolites, or stimulation of phosphatase activities. Furthermore, the activities of Ca2+/calmodulin-kinase II, cAMP-kinase and protein kinase C were not altered by beta-bungarotoxin in either synaptic plasma membranes or cytosol. When synaptic plasma membranes were treated with beta-bungarotoxin, MARCKS phosphorylation was inhibited, and this inhibition was overcome by the addition of exogenous protein kinase C. These results suggest that the interaction between MARCKS and endogenous protein kinase C is altered by beta-bungarotoxin. In contrast, Naja naja atra phospholipase A2, a typical phospholipase A2 enzyme, had effects on phosphorylation which were different from those of beta-bungarotoxin: (1) inhibition of phosphorylation of synapsin I in intact synaptosomes was less potent than that by beta-bungarotoxin; (2) it stimulated basal phosphorylation of GAP-43 and MARCKS; and (3) it increased the activity of protein kinase C. The inhibition of synapsin I phosphorylation by N. n. atra phospholipase A2 in intact synaptosomes may be due to the inhibition of ATP synthesis. The stimulation of GAP-43 and MARCKS by N. n. atra phospholipase A2 can be explained by the production of arachidonic acid, which stimulated protein kinase C activity to a similar extent as that caused by N. n. atra phospholipase A2. Thus, the mechanism of action of beta-bungarotoxin appears to be quite different from that of a phospholipase A2 enzyme, suggesting that phospholipase A2 activity of beta-bungarotoxin may not be essential for its action. beta-Bungarotoxin may be a useful tool to study the physiological role of phosphorylation of synaptosomal proteins in neurotransmitter release.

Differential effects of snake venom phospholipase A2 neurotoxin (beta-bungarotoxin) and enzyme (Naja naja atra) on protein kinases

The phospholipase A2 (PLA2) neurotoxin, beta-bungarotoxin (beta-BuTX), presynaptically alters acetylcholine release. We previously found that beta-BuTX inhibits protein phosphorylation in rat brain synaptosomes. This inhibition was not due to the inhibition of ATP synthesis, the action of arachidonic acid (AA) metabolites, or the stimulation of phosphatase activities. A typical PLA2 enzyme from Naja naja atra (N. n. atra) venom also inhibited phosphorylation but with lesser potency than that of beta-BuTX. We now report the effects of beta-BuTX and N. n. atra PLA2 on the activities of protein kinases. Treatments of synaptic plasma membrane or cytosol with N. n. atra PLA2 stimulated the activities of cAMP-dependent kinase, Ca2+/calmodulin-dependent kinase II, and protein kinase C (PKC), whereas beta-BuTX had no effect on these kinases. Calyculin A, a phosphatase-1 and -2A inhibitor, increased the stimulation of phosphorylation by N. n. atra PLA2, indicating that the stimulation is not due to an inhibition of phosphatase activities. The stimulation of PKC by N. n. atra PLA2 appears to be mediated by free fatty acids (FFAs) resulting from phospholipid hydrolysis by PLA2, since (1) treatment of either synaptic plasma membrane or cytosol with N. n. atra PLA2 produced large amounts of FFAs, and (2) AA, an exogenous FFA, stimulated PKC activity to an extent similar to that caused by N. n. atra PLA2. Thus, the mechanisms of action of beta-BuTX and N. n. atra PLA2 appear quite different from each other although both agents inhibit phosphorylation in intact synaptosomes.

beta-Bungarotoxin blocks phorbol ester-stimulated phosphorylation of MARCKS, GAP-43 and synapsin I in rat brain synaptosomes

The phospholipase A2 neurotoxin, beta-bungarotoxin, presynaptically blocks acetylcholine release. Its mechanism of action is unknown; however, our previous studies suggest that it inhibits phosphorylation of synaptosomal proteins, which might be expected to decrease neurotransmitter release. In our present study, we found that 1 nM beta-BuTX blocked phorbol ester-stimulated phosphorylation of GAP-43, MARCKS and synapsin I without affecting their basal phosphorylation. In contrast, a 1 nM concentration of the non-neurotoxic enzyme. Naja naja atra phospholipase A2 did not affect the phorbol ester-stimulated phosphorylation of these proteins but increased the basal phosphorylation of GAP-43 and MARCKS. Although it has been suggested that cytosolic calmodulin is increased by phosphorylation of the protein kinase C substrates, GAP-43 and MARCKS, we found no change in calmodulin levels by phorbol ester or beta-bungarotoxin. The stimulation of phosphorylation by Naja naja atra phospholipase A2 may be due to products liberated as a result of its phospholipase A2 activity. In contrast, the inhibition of phosphorylation by beta-bungarotoxin appears to be due to an action which may be unrelated its relatively weak phospholipase A2 activity. Inhibition of phosphorylation by beta-bungarotoxin is a possible mechanism by which it could block acetylcholine release. Furthermore, beta-bungarotoxin may be a useful tool to study the physiological role of phosphorylation of synaptosomal proteins in neurotransmitter release.

Ammodytoxin A acceptor in bovine brain synaptic membranes

Ammodytoxin A, the presynaptic neurotoxin from Vipera ammodytes ammodytes venom, was found to bind specifically and with high affinity to bovine cortex synaptic membrane preparation. The detected ammodytoxin A high-affinity binding was characterized by equilibrium binding analysis which revealed a single high-affinity binding site with Kd 4.13 nM and Bmax 6.67 pmoles/mg of membrane protein. 125I-ammodytoxin A was covalently cross-linked to its neuronal acceptor using a chemical cross-linking technique. As revealed by subsequent SDS-PAGE analysis and autoradiography, 125I-ammodytoxin A specifically attached to membrane components with apparent mol. wts 53,000-56,000. Besides by the native ammodytoxin A, the binding of radioiodinated ammodytoxin A to the neuronal acceptor was highly attenuated, also by other two iso-neurotoxins from V. a. ammodytes venom, ammodytoxins B and C, and neurotoxin crotoxin B from the venom of the South American rattlesnake (Crotalus durissus terrificus). Vipera berus berus phospholipase A2 was a weaker inhibitor, whereas nontoxic phospholipase A2, ammodytoxin I2 and myotoxic phospholipase A2 homologue, ammodytin L, both from V. a. ammodytes venom as well, were very weak inhibitors. No inhibitory effect on 125I-ammodytoxin A specific binding at all was, however, obtained with alpha-dendrotoxin, beta-bungarotoxin and crotoxin A, respectively. Treatment of synaptic membranes with proteinase K and Staphylococcus aureus V-8 proteinase, a combination of PNGase F and neuroaminidase, heat or acid lowered the 125I-ammodytoxin A specific binding to various extents but never completely abolished it. The ammodytoxin A binding site in bovine synaptic membranes is thus most likely a combination of membrane glycoprotein acceptor and membrane phospholipids. As ammodytoxin A reduced the second negative component of the perineural waveform, measured on mouse triangularis sterni preparation, which is very likely a result of an inhibition of a fraction of the terminal K+ currents, the ammodytoxin A acceptor could well be connected with K+ channels.

Inhibition of phosphorylation of synapsin I and other synaptosomal proteins by beta-bungarotoxin, a phospholipase A2 neurotoxin

Some snake venom neurotoxins, such as beta-bungarotoxin (beta-BuTX), which possess relatively low phospholipase A2 (PLA2) activity, act presynaptically to alter acetylcholine (ACh) release both in the periphery and in the CNS. In investigating the mechanism of this action, we found that beta-BuTX (5 and 15 nM) inhibited phosphorylation, in both resting and depolarized synaptosomes, of a wide range of proteins, including synapsin I. Naja naja atra PLA2, which has higher PLA2 activity, also inhibited phosphorylation but was less potent than beta-BuTX. At 1 nM, beta-BuTX and N. n. atra PLA2 inhibited phosphorylation of synapsin I only in depolarized synaptosomes. Synaptosomal ATP levels were not affected by 5 or 15 nM beta-BuTX or by 5 nM N. n. atra PLA2. Limited proteolysis, using Staphylococcus aureus V-8 protease, indicated that beta-BuTX inhibited phosphorylation of synapsin I in both the head and the tail regions. The inhibition of phosphorylation was not antagonized by nordihydroguaiaretic acid or indomethacin, suggesting that arachidonic acid derivatives do not mediate this inhibition. Furthermore, inhibition of phosphorylation by beta-BuTX and N. n. atra PLA2 was not altered in the presence of the phosphatase inhibitor okadaic acid, suggesting that stimulation of phosphatase activity is not responsible for this inhibition. Inhibition of protein phosphorylation by PLA2 neurotoxins and enzymes may be associated with an inhibition of ACh release.

Differential effects of presynaptic phospholipase A2 neurotoxins on Torpedo synaptosomes

The effects of several phospholipase A2 neurotoxins from snake venoms were examined on purely cholinergic synaptosomes from Torpedo electric organ. The noncatalytic component A of crotoxin had no effect, whereas its phospholipase component B, used alone or complexed to component A, elicited a rapid and dose-dependent acetylcholine (ACh) release and a depolarization of the preparation. Subsequent ACh release evoked by high K+ levels or calcium ionophore was identical to the control after the action of component A but reduced after the action of crotoxin or of component B. These effects were not observed when the phospholipase A2 activity of the toxin was blocked either by replacing Ca2+ by Ba2+ (respectively, activator and inhibitor of phospholipase A2) or by alkylation of component B with p-bromophenacyl bromide. beta-Bungarotoxin, another very potent phospholipase A2 neurotoxin, induced release of little ACh, did not affect ionophore-evoked ACh release, but significantly reduced depolarization-induced ACh release. The single-chain phospholipase A2 neurotoxin agkistrodotoxin behaved like crotoxin component B. A nonneurotoxic phospholipase A2 from mammalian pancrease induced release of an amount of ACh similar to that released by crotoxin but did not affect the evoked responses. The obvious differences in effect of the various neurotoxins suggest that they exert their specific actions on the excitation-secretion coupling process at different sites or by different mechanisms.

Inhibition of phosphorylation of rat synaptosomal proteins by snake venom phospholipase A2 neurotoxins (beta-bungarotoxin, notexin) and enzymes (Naja naja atra, Naja nigricollis)

Some snake venom neurotoxins, such as beta-bungarotoxin (beta-BuTX) and notexin, which inhibit the release of neurotransmitter at both peripheral and central presynaptic terminals possess phospholipase A2 activity. In contrast, most snake venom phospholipase A2 enzymes such as those isolated from Naja naja atra and Naja nigricollis are structurally homologous to these neutrotoxins but do not have any specific or potent presynaptic action although they have higher enzymatic activities than the neurotoxins. In order to investigate the mechanisms of presynaptic action of the snake venom neurotoxins, we studied their effects on phosphorylation of rat brain synaptosomal proteins. It is known that phosphorylation of synapsin I, a neuron specific and synaptic vesicle associated phosphoprotein, increases neurotransmitter release. Incubation of cerebral cortical synaptosomes with 32P-orthophosphate at 37 degrees C for 30 min, caused significant phosphorylation of a wide mol. wt range of proteins including most markedly those proteins in the mol. wt range (81,000-86,000) of synapsin I. Both snake venom phospholipase A2 neurotoxins and enzymes (5, 15 and 50 nM) inhibited phosphorylation in a Ca2(+)-dependent manner with the following order of potencies: beta-BuTX greater than N.n. atra phospholipase A2 greater than or equal to notexin greater than N. nigricollis phospholipase A2. Five nanomoles of beta-BuTX, which has the lowest phospholipase A2 activity, inhibited phosphorylation of a wide range of mol. wt proteins (51,000-188,000) by 42-58%. At the same concentration, N.n. atra phospholipase A2 (which possesses the highest enzymatic activity), notexin and N. nigricollis phospholipase A2 caused less inhibition than beta-BuTX, ranging from 0-40% depending on the agent used. These results indicate that there is no correlation between their potencies in inhibiting phosphorylation and the levels of their phospholipase A2 activities. An inhibitory activity on phosphorylation may be at least partially responsible for a presynaptically-induced block of neurotransmitter release.

Binding of divalent and trivalent cations with crotoxin and with its phospholipase and its non-catalytic subunits: effects on enzymatic activity and on the interaction of phospholipase component with phospholipids

We have studied the interaction of divalent and trivalent with a potent phospholipase A(2) neurotoxin, crotoxin, from Crotalus durissus terrificus venom. The pharmacological action of crotoxin requires dissociation of its catalytic subunit (component B) and of its non-enzymatic chaperone subunit (component A), then the binding of the phospholipase subunit to target sites on cellular membranes and finally phospholipid hydrolysis. In this report, we show that the phospholipase A(2) activity of crotoxin and of component B required Ca2+ and that other divalent cations (Sr2+, Cd2+ and Ba2+) and trivalent lanthanide ions are inhibitors. The lowest phospholipase A(2) activity was observed in the presence of Ba2+, which proved to be a competitive inhibitor of Ca2+. The binding of divalent cations and trivalent lanthanide ions to crotoxin and to its subunits has been examined by equilibrium dialysis and by spectrofluorimetric methods. We found that crotoxin binds two divalent cations per mole with different affinities; the site presenting the highest affinity (K(d) in the mM range) in involved in the activation (or inhibition) of the phospholipase A(2) activity and must therefore be located on component B, the other site (K(d) higher than 10 mM) is probably localized on component A and does not play any role in the catalytic activity of crotoxin. We also observed that crotoxin component B binds to vesicular and micellar phospholipids, even in the absence of divalent cations. The affinity of this interaction either does not change or else increases by an order of magnitude in the presence of divalent cations.

Tryptophan 110, a residue involved in the toxic activity but not in the enzymatic activity of notexin

We prepared two derivatives of notexin, a phospholipase A2 from Notechis scutatus scutatus venom, by modifying the protein with 2-nitrophenylsulfenylchloride, a tryptophan-specific reagent. One derivative was modified at both tryptophans 20 and 110 whereas the other was modified at tryptophan 20. Evidence based on circular dichroic analysis and antigenicity towards a notexin-specific monoclonal antibody indicated that derivatization at both tryptophans did not affect the tertiary structure of notexin. Concomitant modification of tryptophans 20 and 110 induced a marked decrease in the capacity of notexin to kill mice and to block neuromuscular transmission in the chick biventer cervicis preparation, whereas selective modification at tryptophan 20 had no effect on the lethal properties of notexin. This implies that the decrease in the lethal properties of notexin after derivatization was due to modification at tryptophan 110. However, the diderivatized notexin retained full enzymatic activity, implying that neither tryptophan 20 and tryptophan 110 are involved in the catalytic function of the molecule. We conclude that notexin harbours two functional sites. One of them corresponds to the enzymatic site, whereas the other, which includes tryptophan 110, provides specific toxic characteristics to notexin. By reference to previous crystallographic studies, the relative spatial positions of elements involved in toxicity and the catalytic site, we propose a possible orientation of notexin with respect to its putative membrane-bound target.

The importance of phospholipase A2 in the early induction by crotoxin of biphasic changes in endplate potentials at the frog neuromuscular junction

In Ca2+-Ringer, crotoxin induced a rapid fall in amplitude of endplate potentials followed by a marked secondary rise but, in Ca2+-free, Sr2+-EGTA Ringer, crotoxin failed to elicit either response. In Ca2+-washout experiments, the onset of giant miniature endplate potentials indicated that binding of crotoxin occurred in Sr2+-EGTA Ringer. Facilitation of endplate potential amplitude due to a closely spaced twin impulse was increased to 162% of control near the peak of the crotoxin-induced secondary phase. These findings are consistent with the phospholipase A2 subunit acting specifically at the active zone.

Do chemical modifications dissociate between the enzymatic and pharmacological activities of beta bungarotoxin and notexin?

We have measured enzymatic, hemolytic and anticoagulant activities, lethal potencies and effects on contractions of the phrenic nerve-diaphragm preparation, by chemically modified derivatives of beta bungarotoxin (beta BuTX) and notexin, two presynaptically acting toxins which have PLA2 activity. The following chemical modifications of beta BuTX were tested: alkylation and methylation of histidine 48, alkylation of tryptophan 19, sulfonylation of tyrosine 68, oxidation of methionines 6 and 8, semicarbazide addition under varied conditions to carboxyl groups, varied extents of carbamylation or trinitrophenylation of lysines and guanidination of all lysines with or without trinitrophenylation of the N-terminal asparagine. Only the histidine, tryptophan and tyrosine residues were modified in notexin. The results obtained were compared with those previously obtained using chemically modified derivatives of Naja nigricollis and Naja naja atra PLA2 enzymes which do not have a specific presynaptic site of action. The results with oxidized methionine and lysine-modified derivatives of beta BuTX are supportive of the suggestions of others that the N-terminal region and basic residues away from the enzymatic active region contribute towards the beta type presynaptic neurotoxicity of the PLA2 toxins. Using modified derivatives of beta BuTX and notexin, the dissociations between enzymatic activities and pharmacological properties were not as marked as previously observed with N. nigricollis and N. n. atra PLA2; nevertheless, several dissociations were noted. We conclude that, just as with non-presynaptically acting PLA2 enzymes, some pharmacological actions of presynaptically acting PLA2 toxins may occur independently of phospholipid hydrolysis.

Early induction by crotoxin of biphasic frequency changes and giant miniature endplate potentials in frog muscle

1. Following the addition of crotoxin (250 nM) at the frog neuromuscular junction, there was an initial fall in frequency of miniature endplate potentials (m.e.p.ps), followed by a secondary rise which was characterized by the appearance of large spontaneous potentials (giants, g.m.e.p.ps) and an occasional large potential of the burst type. 2. In the presence of 2-(4-phenylpiperidino)cyclohexanol (AH5183, vesicamol), an inhibitor of vesicular acetylcholine uptake, the frequency of g.m.e.p.ps induced by crotoxin was reduced. 3. The characteristic changes in m.e.p.p. frequency and amplitude distribution were absent with crotoxin in Sr-EGTA Ringer. In the presence of high concentrations of Mn (3.6 or 5.4 mM with 0.9 mM Ca), the crotoxin-induced initial fall and the onset of the secondary rise in m.e.p.p. and g.m.e.p.p. frequencies were slower. The timing of these phases was unaffected by Ca concentrations ranging from 6.3 to 0.9 mM. 4. High concentrations of Mn ions partially inhibited the phospholipase A2 activity of crotoxin on artificial phospholipid membranes. This also supports the involvement of the Ca-dependent phospholipase A2 subunit in both phases of the physiological action of the toxin. 5. G.m.e.p.ps were associated with a moderate increase in m.e.p.p. frequency (2-3 s-1) and were of a time-course similar to that of m.e.p.ps. They persisted after washing with medium lacking Ca ions and in the presence of Ca-Mn Ringer that blocked evoked responses. 6. It is concluded that crotoxin, acting through its phospholipase A2 subunit, produces specific disturbances of synaptic exocytosis and vesicle formation in the axolemma of the motor nerve terminal which lead to biphasic changes in m.e.p.p. frequency and the onset of large spontaneous potentials.

beta-Bungarotoxin-induced phospholipid hydrolysis in rat brain synaptosomes: effect of replacement of calcium by strontium

We tested whether, upon substitution of Ca2+ by Sr2+ in a medium containing beta-bungarotoxin, sufficient Ca2+ remained bound to the tissue to support phospholipid hydrolysis in rat brain synaptosomes. The phrenic nerve--diaphragm preparation could not be used, since replacement of Ca2+ by Sr2+ prolonged time to block of indirectly evoked contractions; however, no phospholipid hydrolysis could be detected (either in the presence of Ca2+ or Sr2+), due to the small amounts of presynaptic terminals. Following initial exposure of synaptosomes to a Ca2+ containing medium and then removal of Ca2+, incubation with beta-bungarotoxin (1 or 10 micrograms/ml) caused significant phospholipid hydrolysis whether or not Sr2+ was present. Therefore, conclusions as to whether phospholipase A2 activity is required for presynaptic actions of beta-bungarotoxin cannot be derived from studies in which Sr2+ is used to inhibit enzymatic activity.

Comparison of enzymatic and pharmacological activities of lysine-49 and aspartate-49 phospholipases A2 from Agkistrodon piscivorus piscivorus snake venom

The basic Lys-49 phospholipase A2 (PLA2) from Agkistrodon piscivorus piscivorus venom is homologous to the basic Asp-49 PLA2 from the same venom as well as other snake venom PLA2 enzymes. It differs, however, in several respects, most important being replacement of the previously invariant Asp-49 at the calcium binding site by Lys, resulting in a reversed order of addition of calcium and phospholipid, phospholipid binding first. Although the preferences for phospholipid substrates of the two enzymes are identical, the apparent Vmax of the Lys-49 PLA2 was only 1.4 to 3% that of the Asp-49 enzyme. Similarly, the Lys-49 PLA2, compared to the Asp-49 PLA2 had less than 3% of the intraventricular lethal potency and 4% of the anticoagulant activity. The intravenous lethal potency of the Lys-49 enzyme was 20% that of the Asp-49 PLA2 and both had little direct hemolytic activity. In contrast, both enzymes were approximately equipotent on the phrenic nerve-diaphragm preparation and on the isolated ventricle strip of the heart. On the cardiac and neuromuscular preparations, the effects of the Asp-49 PLA2 were accompanied by hydrolysis of phosphatidylcholine and phosphatidylethanolamine, whereas no phospholipid hydrolysis was observed with the Lys-49 PLA2. Evaluation of the present results, along with earlier findings using Asp-49 PLA2 enzymes from Naja nigricollis, Hemachatus haemachatus and Naja naja atra venoms, allows us to conclude that: The A. p. piscivorus Asp-49 PLA2 enzyme resembles the Asp-49 enzymes from N. n. atra and H. haemachatus. In contrast, the A. p. piscivorus Lys-49 PLA2 has much lower enzymatic and anticoagulant activities than the Asp-49 enzymes, but equal cardiotoxic and junctional effects. In contrast to some previous suggestions, basic PLA2 enzymes are not necessarily more toxic than neutral or acidic enzymes. Pharmacological effects upon the heart and phrenic nerve-diaphragm preparation correlate neither with in vitro measurements of PLA2 activity nor with actual levels of phospholipid hydrolysis in the heart or diaphragm. This suggests that PLA2 enzymes exert effects independent of phospholipid hydrolysis.

The mechanism of action of beta-bungarotoxin at the presynaptic plasma membrane

The beta-bungarotoxin-induced depolarization of the synaptosomal plasma membrane monitored by the efflux of 86Rb+ is potentiated by raising the albumin in the incubation, is Ca2+-dependent and is due neither to inhibition of the (Na+ + K+)-dependent ATPase nor to activation of the voltage-dependent Na+ channel. Occupancy of the beta-bungarotoxin-binding site by dendrotoxin inhibits partially the action of beta-bungarotoxin. The efflux of 86Rb+ is parallelled by a release of lactate dehydrogenase from the synaptosome, and the two processes are maximal with 2 nM-toxin. Digitonin induces a release of 86Rb+ and lactate dehydrogenase closely similar to that seen with beta-bungarotoxin. It is concluded that the toxicity of beta-bungarotoxin for mammalian nerve terminals can be largely accounted for by specific site-directed phospholipase A2-induced permeabilization of the plasma membrane.

The biphasic effect of phospholipase A2 inhibitors on axon elongation

We have analyzed changes in axon elongation rate in vitro induced by three different phospholipase A2 inhibitors, namely, bromophenacyl bromide (BPB), quinacrine and Sr2+. Spinal ganglia were obtained from 19.5-day-old rat fetuses and explanted onto polyornithine substrata. Axon length was measured at 1, 2 and 3 days in vitro (d.i.v.). We have previously shown that BPB added at 5 hr in vitro (h.i.v.) modifies the structure of growth cones and inhibits or promotes axon elongation according to its concentration. Now, we have observed that 0.5 x 10(-6) M BPB also stimulates axon elongation when added at 1 d.i.v. Culture media with different Ca2+ concentrations were used to test Sr2+ added at 1 d.i.v. In 2.3 mM Ca2+ only an inhibitory effect was observed with 8 mM Sr2+. On the other hand, in 0.3 mM Ca2+, axon growth was stimulated by 0.6-1.2 mM Sr2+ but inhibited by 6 mM Sr2+. Quinacrine, added at 5 h.i.v. was inhibitory at 10(-5) M and showed no effects at 10(-6) M. However, after washing quinacrine, normal elongation rate was recovered by those previously treated with 10(-5) M and axon growth was enhanced in those treated with 10(-6) M. Since three different phospholipase A2 inhibitors, tested in different situations, produce a similar biphasic effect on axon elongation rate, we postulate that this enzymatic activity is involved in the motility of axon growth cones.

Antagonistic action of uranyl nitrate on presynaptic neurotoxins from snake venoms

Uranyl ion (UO2+2) antagonized the neuromuscular blocking action and phospholipase A2 activity of neurotoxins which act presynaptically [beta-bungarotoxin (beta-BuTX) and crotoxin] but did not affect the action of alpha-bungarotoxin and tetrodotoxin. On the basis of the kinetic analysis of the UO2+2 and strontium ion (Sr2+) antagonism of muscle paralysis induced by beta-bungarotoxin, it was found that they inhibited both the binding of the toxin and the steps following binding that brought about the neuromuscular blocking action of beta-bungarotoxin. Uranyl ion was about 50 times more potent than Sr2+ in antagonizing beta-bungarotoxin. High Ca2+ (10 mM) abolished but low Ca2+ (0.25-1.25 mM) medium enhanced the antagonizing action of UO2+2 and Sr2+. In low Ca2+ medium, UO2+2 markedly potentiated the amplitude of the twitch, subsequent addition of beta-bungarotoxin produced three phases of effects on the twitches, e.g. an initial depression, followed by the second facilitation and finally a rapid depression of twitches; however, approx. 70 min after beta-bungarotoxin the small twitches reached a steady state which persisted for more than 350 min. Therefore, it is evident that UO2+2 is the most potent antagonist of beta-bungarotoxin so far tested.

Characterization of monoclonal antibodies against beta-bungarotoxin and their use as structural probes for related phospholipase A2 enzymes and presynaptic phospholipase neurotoxins

Hybridoma lines secreting monoclonal antibodies against a phospholipase-inactive derivative of the presynaptic neurotoxin, beta-bungarotoxin, have been established. These antibodies, either of the IgG1 or IgG2b isotype with affinities in the range 1-2 X 10(8) 1/mol, recognized a single immunodominant region of native beta-bungarotoxin, most probably located on the A (phospholipase homologue) chain of the toxin. Using plate-adsorbed radioimmunoassay procedures, antibodies reacted with native beta-bungarotoxin and other beta-bungarotoxin isotoxins as well as with the non-toxic phospholipase A also present in Bungarus multicinctus venom. Other phospholipase A enzymes and presynaptic phospholipase neurotoxins did not show any competition with beta-bungarotoxin in the radioimmunoassay. Globulin fractions of monoclonal antibodies partially inhibited the phospholipase activity of beta-bungarotoxin.

beta-Bungarotoxin antagonizes the effect of alpha-latrotoxin from black widow spider venom on the neuromuscular junction

Sustained contraction of the chick biventer cervicis nerve-muscle preparations evoked by alpha-latrotoxin was antagonized quickly by beta-bungarotoxin. This effect of beta-bungarotoxin was dependent on its phospholipase A2 activity. In contrast, pancreatic phospholipase A2 was ineffective even at a much higher dose. It is concluded that alpha-latrotoxin needs intact presynaptic membrane to exert its effect.

Cellular and mitochondrial changes induced in the structure of murine skeletal muscle by crotoxin, a neurotoxic phospholipase A2 complex

Following a single i.m. injection of a sublethal dose of crotoxin into the hindlimb of mice, degenerative changes were observed in the soleus muscle in the ensuing 72 hr using light and electron microscopy. A similar degree of myonecrosis resulted from the injection of the phospholipase A2 subunit alone, whereas crotoxin reconstituted from a chemically modified phospholipase A2 subunit with low catalytic activity showed weak myonecrosis and the non-enzymic subunit, crotapotin, was inactive. Visible defects of the sarcolemma were associated with underlying necrotic areas and attraction of macrophages. Internally, hypercontraction bands were often present and mitochondria in the rounded, orthodox configurational state contained linear dark bodies as part of their crystal organization. In fibre segments with slow onset of degeneration, many mitochondria additionally contained clusters of electron opaque deposit and in mitochondria, heavily laden with such deposits, energy-dispersive microprobe analysis showed the presence of high concentration of calcium. It was concluded that the primary site of action of crotoxin was hydrolysis of the sarcolemmal membrane and that mitochondrial changes were associated with calcium accumulation in response to an altered internal environment. Regeneration of fibres was rapid.

Dissociation of pharmacological and enzymatic activities of snake venom phospholipases A2 by modification of carboxylate groups

The carboxylate groups in an acidic and in a basic phospholipase A2 (PLA2) enzyme, purified, respectively, from Naja naja atra and Naja nigricollis snake venoms, were modified with carbodiimide and semicarbazide. The derivatives modified at pH 3.5 and pH 5.5 had less than 1% (N. nigricollis) or 2% (N. n. atra) residual enzymatic activity, whereas 12-16% enzymatic activity remained following modification at pH 5.5 in the presence of Ca2+. In marked contrast, these derivatives retained variable, but significantly greater, levels of lethal potency, hemolytic and anticoagulant activities, and abilities to block indirectly and directly induced contractions of the diaphragm. By this modification of aspartic and glutamic acid residues we have, for the first time, obtained derivatives of PLA2 which selectively retain greater pharmacological activity relative to enzymatic activity. Previously, we had found that modification of lysine and arginine residues produced derivatives which retain enzymatic activity but show a loss of pharmacological properties. These findings suggest that some pharmacological effects of snake venom PLA2 enzymes are due to a non-enzymatic action, suggesting two distinct but perhaps overlapping active sites.

Neurotransmitter release and nerve terminal morphology at the frog neuromuscular junction affected by the dye Erythrosin B

1. The quantal release of neurotransmitter and the fine structure of frog neuromuscular junctions has been examined in the presence of the xanthene dye Erythrosin B.2. At concentrations of 10 muM or greater, Erythrosin B produced time- and dose-dependent increases in transmitter release from presynaptic nerve terminals.3. Miniature end-plate potential (m.e.p.p.) frequency increased in an exponential manner during continuous exposure to the dye. The rate constant for this exponential was dose-dependent, increasing with concentrations from 10 muM to 1 mM.4. The amplitude of evoked end-plate potentials (e.p.p.s) also increased exponentially during dye treatment, primarily due to an increase in quantal content. Rate constants for this effect were also dose-dependent, and were approximately 1/5 as large as those for m.e.p.p.s.5. While the frequency of m.e.p.p.s was increasing, their amplitude distribution did not qualitatively change. Thus the dye has little effect on the size of individual quanta.6. The presynaptic effects of Erythrosin B were irreversible under these experimental conditions. Brief exposure to the dye caused increases in m.e.p.p. frequency and e.p.p. amplitude which were maintained at steady levels during extensive rinsing with dye-free Ringer solution.7. Prolonged exposure to the dye caused an eventual decrease in m.e.p.p. frequency and abolition of e.p.p.s. Coincident with this decline ;giant' m.e.p.p.s as large as 40 mV were observed.8. At dye concentrations greater than approximately 200 muM, Erythrosin B rapidly and reversibly increased the membrane potential and input resistance of muscle fibres. This post-synaptic effect was small and variable in normal saline, but was pronounced in low potassium solutions.9. During the period that release was enhanced by Erythrosin B, presynaptic nerve terminals contained the normal complement of synaptic vesicles and other organelles. Mitochondria were swollen in this condition.10. After m.e.p.p. frequency declined below normal levels and ;giant' m.e.p.p.s appeared, the number of synaptic vesicles within nerve terminals declined and dilated cisternae were present. Mitochondria were swollen further.11. These results do not reveal any mechanism to explain the ability of Erythrosin B to increase transmitter release, but the decline in release may be caused by partial depletion of synaptic vesicles. The ;giant' m.e.p.p.s could be due to the discharge of acetylcholine from cisternae.

Ceruleotoxin: identification in the venom of Bungarus fasciatus, molecular properties and importance of phospholipase A2 activity for neurotoxicity

Ceruleotoxin is a potent neurotoxin which was originally purified from a batch of venom labelled Bungarus caeruleus, from the Pasteur Institute. Since NOBLE et al. have shown that this batch differs in its protein composition from that of B. caeruleus provided by Miami Serpentarium, we decided to clarify this point by comparing the composition of venoms from various Bungarus species of several origins. Although individual variations exist between samples of the same species, the venom from B. multicinctus, B. caeruleus and B. fasciatus possess characteristic protein compositions which allowed us to identify the batch used to purify ceruleotoxin as a B. fasciatus venom. We identified and purified ceruleotoxin from each of the five samples of B. fasciatus venoms tested. We failed to find this neurotoxin in either B. multicinctus or B. caeruleus venoms. Purified ceruleotoxin is a slightly basic protein with an isoelectric point of 7.4 which possesses a significant phospholipase A2 activity (200 mumoles lecithin hydrolyzed per min per mg) and a high lethal potency (i.v. LD50 in mice 0.03-0.07 mg/kg). It is composed of two identical subunits of 13,000 mol. wt. which resemble pancreas and snake venom phospholipases in their amino acid composition. Like crotoxin, ceruleotoxin irreversibly blocks the postsynaptic response of Torpedo and Electrophorus electroplaques to cholinergic agonists without preventing the binding of acetylcholine to its receptor. By hydrolyzing critical lipids of the postsynaptic membrane, it stabilizes the acetylcholine receptor - ionophore assembly in a desensitized state.

Alterations in spontaneous transmitter release by divalent cations after treatment of the neuromuscular junction with beta-bungarotoxin

1. Spontaneous transmitter release was studied at the frog sartorius neuromuscular junction in the presence of a variety of cations before and after treatment with the specific presynaptic neurotoxin, beta-bungarotoxin (beta-BuTX). 2. Treatment with beta-BuTX produced a maintained increase in spontaneous release, as indicated by the miniature end-plate potential (m.e.p.p.) frequency. It was demonstrated that the m.e.p.p. frequency remained dependent on the extracellular calcium concentration. 3. A 30 mM increase in extracellular sodium chloride produced a reversible increase in frequency only after beta-BuTX treatment, indicating that beta-BuTX had increased the permeability of the presynaptic terminal. 4. Furthermore, several divalent cations other than calcium were shown to either maintain or greatly increase the m.e.p.p. frequency after beta-BuTX treatment (before toxin treatment replacement of calcium by these divalent cations produced only small changes in frequency). The relative effectiveness of the divalent cations tested in increasing spontaneous transmitter release after toxin treatment was Co2+ congruent to Ni2+ greater than Mg2+ greater than Ca2+ congruent to Sr2+ greater than Mn2+. The effect of cobalt, which increased the m.e.p.p. frequency 6.5 times after toxin treatment, was studied in detail.

Protease inhibitor homologues from mamba venoms: facilitation of acetylcholine release and interactions with prejunctional blocking toxins

1 Five polypeptides, which were isolated from elapid snake venoms and which are structurally related to protease inhibitors, were tested for action on isolated biventer cervicis nerve-muscle preparations of the chick. 2 Dendrotoxin from the Eastern green mamba (Dendroaspis angusticeps) and toxins K and I from the black mamba (Dendroaspis polylepis polylepis) increased to indirect stimulation without affecting responses to exogenous acetylcholine, carbachol of KCl. 3 The two other protease inhibitor homologues, HHV-II from Ringhals cobra (Hemachatus haemachatus) and NNV-II from Cape cobra (Naja nivea) did not increase responses to nerve stimulation. Trypsin inhibitor from bovine pancreas also had no facilitatory effects on neuromuscular transmission. 4 The facilitatory toxins from mamba venoms interacted with the prejunctional blocking toxins, beta-bungarotoxin, crotoxin and notexin, but not with taipoxin. The blocking effects of beta-bungarotoxin were reduced by pretreatment with the mamba toxins, whereas the blocking actions of crotoxin and notexin were enhanced. 5 The results indicate that protease inhibitor homologues from mamba venoms form a new class of neurotoxin, which acts to increase the release of acetylcholine in response to motor nerve stimulation. 6 From the interaction studies it is concluded that the facilitatory toxins bind to motor nerve terminals at sites related to those occupied by the prejunctional blocking toxins. However, differences in interactions with individual toxins suggest that there must be several related binding sites on the nerve terminals.

Antibodies to beta-bungarotoxin and its phospholipase inactive derivative

Antisera were raised against the presynaptic neurotoxin beta-bungarotoxin and against its phospholipase-inactive derivative, modified by reaction with p-bromophenacyl bromide. The cross-reactivity of the antisera to other phospholipase A2 enzymes and polypeptide neurotoxins was examined. The antisera inhibited both the neurotoxic effects of beta-bungarotoxin at the frog motor endplate and the enzymatic activity of the toxin on model phospholipid membranes, although it is unlikely that the catalytic active centre is the locus of any major determinant.

Site of action of caudoxin, a neurotoxic phospholipase A2 from the horned puff adder (Bitis caudalis) venom

Caudoxin, a single-chain phospholipase A2 isolated from the venom of Bitis caudalis is a toxic polypeptide with a formula weight of 13,332 dalton. The LD50 in mice (i.p.) was 0.18 (0.15-0.22) mg/kg. In the chick biventer cervicis muscle preparation the toxin (1-10 micrograms per ml) caused complete neuromuscular blockade without affecting the response of the muscle to acetylcholine. In the mouse phrenic nerve-diaphragm preparation, the toxin abolished the indirectly elicited contraction without inhibiting that evoked directly. When this preparation was bathed in a low calcium (0.6 mM) medium, the toxin induced a triphasic change in the indirectly evoked contractions: an immediate initial inhibition followed by augmentation and then a second phase of inhibition leading to irreversible neuromuscular blockade. Electrophysiological studies in the same preparation showed a similar triphasic change in the quantal content of endplate potentials. The frequency of miniature endplate potentials first increased and then decreased, while the resting membrane potential was not significantly decreased by the toxin. Histological study showed that the toxin caused local myonecrosis only at a higher dose (2 mg/kg mouse). It is concluded that caudoxin produced a neuromuscular block by acting selectively on a presynaptic site. However, the site of binding appears to be different from that of beta-bungarotoxin since combination of the toxin with beta-bungarotoxin caused potentiation of its neuromuscular blocking action rather than addition.

Presynaptic toxicity of the histidine-modified, phospholipase A2-inactive, beta-bungarotoxin, crotoxin and notexin

Beta-Bungarotoxin, crotoxin and notexin were treated extensively with p-bromophenacyl bromide in order to modify the histidine group of the active site and to greatly decrease the phospholipase A2(PLA) catalytic activity. They were studied for their residual presynaptic effects on the mouse isolated phrenic nerve-diaphragm and chick biventer cervicis muscle preparations. The modified toxins still had 1.7-5.2% PLA activity, which was inhibited by Sr2+, as is the enzyme activity of the native toxins. The neuromuscular blocking activity of these modified toxins, which was reduced 30-60 fold in the mouse diaphragm, was due to a presynaptic effect, as judged from the unchanged amplitude of m.e.p.p.s in the blocked preparations. In contrast to the native toxins, however, the presynaptic effect of modified beta-bungarotoxin and notexin was neither antagonized by Sr2+ nor accelerated by increasing the rate of nerve stimulation, indicating that the presynaptic effects of the modified beta-bungarotoxin and notexin are not likely to be due to their PLA enzyme activity. The modified toxins retained a much greater fraction of their early presynaptic effects in comparison to their enzymatic and neuromuscular blocking activities, although the time-course of the early effects was delayed. The results indicate that the modified neurotoxins per se have their own presynaptic effects, which are unrelated to the PLA enzyme activities.

Beta-bungarotoxin stimulates the synthesis and accumulation of acetylcholine in rat phrenic nerve diaphragm preparations

1. The effects of beta-bungarotoxin on acetylcholine (ACh) synthesis, tissue content and release have been studied in the rat diaphragm. A gas chromatographic mass spectrometric assay was used to measure ACh and choline. 2. Within 30 min, beta-bungarotoxin (0.14 or 1.4 micrograms/ml.) caused a significant increase in tissue ACh content. This increase was apparent prior to the final inhibition by beta-bungarotoxin of evoked (10 Hz) ACh release. 3. The toxin enhanced the incorporation of [2H4]Ch into [2H4]ACh in both resting and stimulated preparations. 4. Hemicholinium-3 blocked the rise in diaphragm ACh normally produced by beta-bungarotoxin. 5. Beta-Bungarotoxin did not directly activate choline acetyltransferase in muscle homogenates. 6. The toxin-induced rise in tissue ACh was largely absent in Ca2+-free solutions which contained either EGTA (1 mM) or SrCl2 (2 or 10 mM). 7. Non-neurotoxic phospholipases A2, fatty acids and the neurotoxic phospholipase A2, notexin, did not cause ACh accumulation in the diaphragm. 8. Beta-Bungarotoxin did not stimulate ACh synthesis in denervated muscle. 9. The extra ACh which accumulated after beta-bungarotoxin did not contribute to enhanced release by nerve impulses even when 4-aminopyridine was added to the medium. High K+ solution and black widow spider venom were also ineffective in increasing output from toxin-treated diaphragms relative to controls that had not been treated with beta-bungarotoxin. 10. Prior injection of a rat with botulinum toxin prevented the accumulation of ACh due to beta-bungarotoxin. Tubocurarine, however, did not antagonize beta-bungarotoxin. 11. These data indicate that beta-bungarotoxin has a unique capacity to inhibit ACh release and stimulate ACh synthesis in diaphragm nerve endings. The results are discussed in terms of a possible action of beta-bungarotoxin to raise the level of ionized Ca in the nerve terminal cytosol.

A further study of the phospholipase-independent action of beta-bungarotoxin at frog end-plates

1. The effect of beta-bungarotoxin (beta-BuTx) at the frog neuromuscular junction has been investigated further in order to distinguish more clearly between phospholipase- independent and phospholipase-dependent actions on transmitter release. 2. Inhibition of the enzymatic activity, by substitution of strontium for calcium, allowed determination of the dose-response curve of the early rapid decrease in transmitter release caused by the toxin. In the presence of strontium ions there was, however, still about 7% residual enzymatic activity, and electrophysiological evidence of it could be seen in room-temperature experiments at high concentrations of beta-BuTx. This residual enzymatic activity could be suppressed by lowering the temperature to 5 degrees C. 3. In normal calcium-Ringer solution beta-BuTx produced the typical triphasic effect on the amplitude of end-plate potentials (e.p.p.s). Lowering the temperature markedly delayed an then diminished the secondary transient increase. There was, however, comparatively little temperature influence on the first rapid decrease in e.p.p. amplitude. Enzymatic assays confirmed the temperature dependence of the toxin's phospholipase activity on model phospholipid substrates. 4. The kinetics of the phospholipase-independent action of beta-BuTx were examined in strontium-Ringer compared to calcium-Ringer solution, as well as in calcium-Ringer at different temperatures. Both the time to onset of inhibition and the time to 50% inhibition of the e.p.p., during the first phase of toxin action, are temperature-dependent and briefer in calcium than in strontium-Ringer solution. It is suggested that calcium is more effective than strontium in promoting this phospholipase- independent interaction of beta-BuTx with the nerve terminal membrane.