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

Electrospray ionization mass spectrometry as a critical tool for revealing new properties of snake venom phospholipase A2

Results from high-performance liquid chromatography/nano-electrospray ionization tandem mass spectrometry (HPLC/nESI-MS/MS) coupled to two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (2D SDS-PAGE) indicated that the monomer and dimer of phospholipase A(2) (PLA(2)) coexisted in crude Chinese Agkistrodon blomhoffii Ussurensis snake venom (ABUSV). Then, an acidic PLA(2) with the accurate molecular mass of 13979.6 Da was purified from ABUSV (mo-ABUSV-aPLA(2)). MS/MS-derived peptides from ABUSV-aPLA(2) were compared with other homologous snake venom PLA(2)s, which in turn showed that ABUSV-aPLA(2) is a novel snake venom PLA(2). Meanwhile, the ABUSV-aPLA(2) dimer (di-ABUSV-aPLA(2)) was also obtained. MS/MS analysis identified the same peptides from di-ABUSV-aPLA(2) as from mo-ABUSV-aPLA(2), which indicates that di-ABUSV-aPLA(2) is a homodimer. One Ca(2+) ion is contained per ABUSV-aPLA(2). The Ca(2+) ion is critical for both the hydrolytic activity and the structure of ABUSV-aPLA(2). Pro-Q Emerald and Pro-Q Diamond specific glycoprotein and phosphoprotein staining combined with MS/MS analysis indicated that the ABUSV-aPLA(2) is both a glycoprotein and a phosphoprotein, which to our knowledge is the first such report for a snake venom PLA(2) and thus provides new threads for the study of the functions and structures of snake venom PLA(2)s. One phosphorylation site and the size of the glycan chain are determined by using HPLC/nESI-MS/MS and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS. The delicate utilization of ESI-MS can exert tremendous impact on protein sciences.

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.

Presynaptically acting snake venom phospholipase A2 enzymes attack unique substrates

Synaptosomes were incubated with bovine serum albumin (BSA) to examine whether the presynaptic action of snake venom phospholipase A2 (PLA2) toxins is due either to the release of fatty acids resistant to extraction by BSA or to the liberation of a specific fatty acid type. In the presence of BSA (0.5% or 1.0%) two PLA2 enzymes from Naja naja atra and Naja naja kaouthia snake venoms that do not have a predominant presynaptic action at the neuromuscular junction (PS-) did not stimulate acetylcholine (ACh) release from synaptosomes. In contrast, two PLA2 enzymes (beta-bungarotoxin, scutoxin) that do have a predominant presynaptic action at the neuromuscular junction (PS+) did stimulate ACh release. BSA did not antagonize PS- enzymes by more efficiently extracting the fatty acids produced by these enzymes relative to PS+ enzymes. While absolute amounts of total and unsaturated fatty acid produced overlapped for the PS- and PS+ enzymes, the two PS+ enzymes produced a significantly greater absolute amount and relative percentage of palmitic acid (16:0) than did either of the PS- enzymes. However, the levels of free palmitic acid remaining in the synaptosomes where they would exert effects on ACh release were similar for the N. n. kaouthia PLA2 (PS-) and beta-bungarotoxin (PS+). Therefore, the total (supernatant plus synaptosomal) amount of palmitic acid produced per se did not account for stimulation of ACh release, since the greater amounts produced by the PS+ enzymes were removed from the synaptosomes by BSA. The production of higher levels of palmitic acid suggests either that PS+ enzymes gain access to sites containing phospholipid substrates unavailable to the PS- enzymes, or that they have a different substrate preference. These findings suggest new possibilities for the mechanism of PS+PLA2 action, including site-directed enzymatic activity and protein acylation.

Inhibition of ACh release at an Aplysia synapse by neurotoxic phospholipases A2: specific receptors and mechanisms of action

1. Monochain (OS2) and multichain (taipoxin) neurotoxic phospholipases A2 (PLA2), purified from taipan snake venom, both inhibited ACh release at a concentration of 20 nM (90% inhibition in 2 h) at an identified synapse from buccal ganglion of Aplysia californica. 2. The Na+ current was unchanged upon application of either OS2 or taipoxin. Conversely, presynaptic K+ currents (IA and IK) were increased by taipoxin but not by OS2. In addition, OS2 induced a significant decrease of the presynaptic Ca2+ current (30%) while taipoxin increased this latter current by 20-30%. 3. Bee venom PLA2, another monochain neurotoxic PLA2, also inhibited ACh release while non-toxic enzymatically active PLA2s like OS1 (also purified from taipan snake venom) or porcine pancreatic PLA2 elicited a much weaker inhibition of ACh release, suggesting a specific action of neurotoxic PLA2s versus non-toxic PLA2s on ACh release. 4. Using iodinated OS2, specific high affinity binding sites with molecular masses of 140 and 18 kDa have been identified on Aplysia ganglia. The maximal binding capacities were 55 and 300-400 fmol (mg protein)-1 for membrane preparations from whole and buccal ganglia, respectively. These binding sites are of high affinity for neurotoxic PLA2s (Kd values, 100-800 pM) and of very low affinity for non-toxic PLA2s (Kd values in the micromolar range), thus indicating that these binding sites are presumably involved in the blockade of ACh release by neurotoxic PLA2s.

Bovine serum albumin does not completely block synaptosomal cholinergic activities of presynaptically acting snake venom phospholipase A2 enzymes

Bovine serum albumin (BSA), which binds fatty acids, was used to test the contribution of free fatty acid to the presynaptic toxicity of phospholipase A2 (PLA2) enzymes. The effects of BSA on inhibition of [14C]choline uptake and stimulation of [14C]acetylcholine (ACh) release in synaptosomes by PLA2 enzymes that do not have a predominant presynaptic action at the neuromuscular junction (PS-) were compared with those on the cholinergic actions of PLA2 enzymes that do have a predominant presynaptic action at the neuromuscular junction (PS+). The inhibition of choline uptake by the Naja naja atra PLA2, a PS- PLA2, was completely antagonized by BSA (0.5%); whereas that by beta-bungarotoxin, a PS+ PLA2, was unaffected by BSA. The inhibition of choline uptake by two other PS+ PLA2 toxins (scutoxin and pseudexin) was partially antagonized by BSA. The effects of the PLA2 enzymes were antagonized in the same manner by BSA whether on Na(+)-dependent or on Na(+)-independent choline uptake. Likewise, the stimulation of ACh release by two PS- PLA2 enzymes (from Naja naja atra and Naja naja kaouthia snake venoms) was completely blocked by BSA; whereas that by beta-bungarotoxin was unaffected and that by scutoxin and pseudexin was only partially antagonized by BSA. The results suggest that the PS- PLA2 enzymes are completely dependent on fatty acid production for their cholinergic toxicity and that BSA can be used to investigate further the neurotoxic mechanisms of PS+ PLA2 enzymes in 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.