Snake presynaptic neurotoxins with phospholipase A2 activity induce punctate swellings of neurites and exocytosis of synaptic vesicles

New Insights into Immunopathology Associated to Bothrops lanceolatus Snake Envenomation: Focus on PLA2 Toxin

The systemic increase in inflammatory mediator levels can induce diverse pathological disorders, including potentially thrombus formation, which may be lethal. Among the clinical conditions in which the formation of thrombi dictates the patient's prognosis, envenomation by Bothrops lanceolatus should be emphasized, as it can evolve to stroke, myocardial infarction and pulmonary embolism. Despite their life-threatening potential, the immunopathological events and toxins involved in these reactions remain poorly explored. Therefore, in the present study, we examined the immunopathological events triggered by a PLA₂ purified from B. lanceolatus venom, using an ex vivo human blood model of inflammation. Our results showed that the purified PLA₂ from the venom of B. lanceolatus damages human erythrocytes in a dose dependent way. The cell injury was associated with a decrease in the levels of CD55 and CD59 complement regulators on the cell surface. Moreover, the generation of anaphylatoxins (C3a and C5a) and the soluble terminal complement complex (sTCC) indicates that human blood exposure to the toxin activates the complement system. Increased production of TNF-α, CXCL8, CCL2 and CCL5 followed complement activation. The venom PLA₂ also triggered the generation of lipid mediators, as evidenced by the detected high levels of LTB₄, PGE₂ and TXB₂. The scenario here observed of red blood cell damage, dysfunctions of the complement regulatory proteins, accompanied by an inflammatory mediator storm, suggests that B. lanceolatus venom PLA₂ contributes to the thrombotic disorders present in the envenomed individuals.

The Phospholipase Activity of Ammodytoxin, a Prototype Snake Venom β-Neurotoxin, Is Not Obligatory for Cell Internalisation and Translocation to Mitochondria

β-Neurotoxins are secreted phospholipase A2 molecules that inhibit transmission in neuromuscular synapses by poisoning the motor neurons. These toxins specifically and rapidly internalise into the nerve endings of motor neurons. Ammodytoxin (Atx) is a prototype β-neurotoxin from the venom of the nose-horned viper (Vipera ammodytes ammodytes). Here, we studied the relevance of the enzymatic activity of Atx in cell internalisation and subsequent intracellular movement using transmission electron microscopy (TEM). We prepared a recombinant, enzymatically inactive mutant of Atx, Atx(D49S), labelled with gold nanoparticles (GNP), and incubated this with PC12 cells, to analyse its localisation by TEM. Atx(D49S)-GNP internalised into the cells. Inside the cells, Atx(D49S)-GNP was detected in different vesicle-like structures, cytosol, endoplasmic reticulum and mitochondria, where it was spotted in the intermembrane space and matrix. Co-localization of fluorescently labelled Atx(D49S) with mitochondria in PC12 cells by confocal fluorescence microscopy confirmed the reliability of results generated using Atx(D49S)-GNP and TEM and allowed us to conclude that the phospholipase activity of Atx is not obligatory for its cell internalisation and translocation into the mitochondrial intermembrane space and matrix.

Gyroxin, a toxin from Crotalus durissus terrificus snake venom, induces a calcium dependent increase in glutamate release in mice brain cortical synaptosomes

Gyroxin is a thrombin-like toxin obtained from the venom of the South American rattlesnake, Crotalus durissus terrificus. Literature has reported "gyroxin syndrome" characterized, in mice, as series of aberrant motor behavior, known as barrel rotation, mainly after intraperitoneal administration. Despites several studies, a physiological mechanism of "gyroxin syndrome" are still not completely understood. In this context, alterations on the central nervous system (CNS), especially causing neurotoxic events, are pointed out as likely candidates. Then, we decided to investigate whether gyroxin induces alterations in glutamate release, one of the most important neurotransmitter involved in neurotoxicity. For that, we performed all experiments, in vitro, using a model of mice brain cortical synaptosomes. Notably, our results indicate that the administration of gyroxin on purified presynaptic brain cortical terminals resulted in an extracellular Ca²⁺- dependent raise in glutamate release. Indeed, our results also showed that gyroxin increases intrasynaptosomal calcium (Ca²⁺) levels through acting on voltage gated calcium channels (VGCC), specifically N and P/Q subtypes. Moreover, our data show that gyroxin increases exocytosis rate. Interestingly, these data suggest that gyroxin might induce neurotoxicity by increasing glutamate levels. However, future investigations are needed in order to elucidate the nature of the following events.

Anhydroecgonine methyl ester (AEME), a cocaine pyrolysis product, impairs glutathione-related enzymes response and increases lipid peroxidation in the hippocampal cell culture

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AEME and cocaine decreased GPx, GR and GST activities after 3 and 6 h of exposure.

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AEME and cocaine increased MDA after 48 h of exposure.

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AEME-cocaine combination decreased GPx, GR and GST activities after 3 and 6 h.

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AEME-cocaine combination showed an additive effect on MDA after 48 h of exposure.

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A higher neurotoxic effect after crack cocaine use is suggested.

Crack cocaine smokers inhale, alongside with cocaine, its pyrolysis product, anhydroecgonine methyl ester (AEME). We have previously described AEME neurotoxic effect and its additive effect when co-incubated with cocaine. Our aim was to evaluate, the effect of AEME, cocaine and AEME-cocaine combination on glutathione peroxidase (GPx), glutathione reductase (GR) and glutathione S-transferase (GST) activities after 3 and 6 h of exposure, periods previous to neuronal death. Lipid peroxidation was evaluated through malonaldehyde (MDA) levels at 3, 6, 24 and 48 h of exposure. All treated groups reduced neuronal viability after 24 h of exposure. AEME and cocaine decreased GPx, GR and GST activities after 3 and 6 h, with an increase in MDA levels after 48 h. AEME-cocaine combination decreased the enzymes activities after 3 and 6 h, showing an additive effect in MDA levels after 48 h. These data show that the glutathione-related enzymes imbalance caused by AEME, cocaine or AEME-cocaine combination exposure preceded neuronal death and lipid peroxidation. Moreover, the additive effect on lipid peroxidation observed with AEME-cocaine exposure after 48 h, suggest a higher neurotoxic effect after crack cocaine use when compared to cocaine alone.

An Agonist of the CXCR4 Receptor Strongly Promotes Regeneration of Degenerated Motor Axon Terminals

The activation of the G-protein coupled receptor CXCR4 by its ligand CXCL12α is involved in a large variety of physiological and pathological processes, including the growth of B cells precursors and of motor axons, autoimmune diseases, stem cell migration, inflammation, and several neurodegenerative conditions. Recently, we demonstrated that CXCL12α potently stimulates the functional recovery of damaged neuromuscular junctions via interaction with CXCR4. This result prompted us to test the neuroregeneration activity of small molecules acting as CXCR4 agonists, endowed with better pharmacokinetics with respect to the natural ligand. We focused on NUCC-390, recently shown to activate CXCR4 in a cellular system. We designed a novel and convenient chemical synthesis of NUCC-390, which is reported here. NUCC-390 was tested for its capability to induce the regeneration of motor axon terminals completely degenerated by the presynaptic neurotoxin α-Latrotoxin. NUCC-390 was found to strongly promote the functional recovery of the neuromuscular junction, as assayed by electrophysiology and imaging. This action is CXCR4 dependent, as it is completely prevented by AMD3100, a well-characterized CXCR4 antagonist. These data make NUCC-390 a strong candidate to be tested in human therapy to promote nerve recovery of function after different forms of neurodegeneration.

Emerging evidence for the modulation of exocytosis by signalling lipids

Membrane fusion is a key event in exocytosis of neurotransmitters and hormones stored in intracellular vesicles. In this process, soluble N‐ethylmaleimide sensitive factor attachment protein receptor (SNARE) proteins are essential components of the exocytotic molecular machinery, while lipids have been seen traditionally as structural elements. However, the so‐called signalling lipids, such as sphingosine and arachidonic acid, interact with SNAREs and directly modulate the frequency and mode of fusion events. Interestingly, recent work has proved that the sphingosine analogue FTY‐720, used in the treatment of multiple sclerosis, mimics the effects of signalling lipids. In the present Review, we discuss recent investigations suggesting that endogenous signalling lipids and synthetic analogues can modulate important physiological aspects of secretion, such as quantal release, vesicle recruitment into active sites, vesicle transport and even organelle fusion in the cytosol. Therefore, these compounds are far from being merely structural components of cellular membranes.

Variability in venom composition of European viper subspecies limits the cross-effectiveness of antivenoms

Medically relevant cases of snakebite in Europe are predominately caused by European vipers of the genus Vipera. Systemic envenoming by European vipers can cause severe pathology in humans and different clinical manifestations are associated with different members of this genus. The most representative vipers in Europe are V. aspis and V. berus and neurological symptoms have been reported in humans envenomed by the former but not by the latter species. In this study we determined the toxicological profile of V. aspis and V. berus venoms in vivo in mice and we tested the effectiveness of two antivenoms, commonly used as antidotes, in counteracting the specific activities of the two venoms. We found that V. aspis, but not V. berus, is neurotoxic and that this effect is due to the degeneration of peripheral nerve terminals at the NMJ and is not neutralized by the two tested antisera. Differently, V. berus causes a haemorrhagic effect, which is efficiently contrasted by the same antivenoms. These results indicate that the effectiveness of different antisera is strongly influenced by the variable composition of the venoms and reinforce the arguments supporting the use polyvalent antivenoms.

CXCL12α/SDF-1 from perisynaptic Schwann cells promotes regeneration of injured motor axon terminals

The neuromuscular junction has retained through evolution the capacity to regenerate after damage, but little is known on the inter-cellular signals involved in its functional recovery from trauma, autoimmune attacks, or neurotoxins. We report here that CXCL12α, also abbreviated as stromal-derived factor-1 (SDF-1), is produced specifically by perisynaptic Schwann cells following motor axon terminal degeneration induced by α-latrotoxin. CXCL12α acts via binding to the neuronal CXCR4 receptor. A CXCL12α-neutralizing antibody or a specific CXCR4 inhibitor strongly delays recovery from motor neuron degeneration in vivo Recombinant CXCL12α in vivo accelerates neurotransmission rescue upon damage and very effectively stimulates the axon growth of spinal cord motor neurons in vitro These findings indicate that the CXCL12α-CXCR4 axis plays an important role in the regeneration of the neuromuscular junction after motor axon injury. The present results have important implications in the effort to find therapeutics and protocols to improve recovery of function after different forms of motor axon terminal damage.

Schwann cells are activated by ATP released from neurons in an in vitro cellular model of Miller Fisher syndrome

The neuromuscular junction is exposed to different types of insult, including mechanical trauma, toxins and autoimmune antibodies and, accordingly, has retained through evolution a remarkable ability to regenerate. Regeneration is driven by multiple signals that are exchanged among the cellular components of the junction. These signals are largely unknown. Miller Fisher syndrome is a variant of Guillain-Barré syndrome caused by autoimmune antibodies specific for epitopes of peripheral axon terminals. Using an animal model of Miller Fisher syndrome, we recently reported that a monoclonal anti-polysialoganglioside GQ1b antibody plus complement damages nerve terminals with production of mitochondrial hydrogen peroxide, which activates Schwann cells. Several additional signaling molecules are likely to be involved in the activation of the regeneration program in these cells. Using an in vitro cellular model consisting of co-cultured primary neurons and Schwann cells, we found that ATP is released by neurons injured by the anti-GQ1b antibody plus complement. Neuron-derived ATP acts as an alarm messenger for Schwann cells, where it induces the activation of intracellular pathways, including calcium signaling, cAMP and CREB, which, in turn, produce signals that promote nerve regeneration. These results contribute to defining the cross-talk taking place at the neuromuscular junction when it is attacked by anti-gangliosides autoantibodies plus complement, which is crucial for nerve regeneration and is also likely to be important in other peripheral neuropathies.

Antivenom for Neuromuscular Paralysis Resulting From Snake Envenoming

Antivenom therapy is currently the standard practice for treating neuromuscular dysfunction in snake envenoming. We reviewed the clinical and experimental evidence-base for the efficacy and effectiveness of antivenom in snakebite neurotoxicity. The main site of snake neurotoxins is the neuromuscular junction, and the majority are either: (1) pre-synaptic neurotoxins irreversibly damaging the presynaptic terminal; or (2) post-synaptic neurotoxins that bind to the nicotinic acetylcholine receptor. Pre-clinical tests of antivenom efficacy for neurotoxicity include rodent lethality tests, which are problematic, and in vitro pharmacological tests such as nerve-muscle preparation studies, that appear to provide more clinically meaningful information. We searched MEDLINE (from 1946) and EMBASE (from 1947) until March 2017 for clinical studies. The search yielded no randomised placebo-controlled trials of antivenom for neuromuscular dysfunction. There were several randomised and non-randomised comparative trials that compared two or more doses of the same or different antivenom, and numerous cohort studies and case reports. The majority of studies available had deficiencies including poor case definition, poor study design, small sample size or no objective measures of paralysis. A number of studies demonstrated the efficacy of antivenom in human envenoming by clearing circulating venom. Studies of snakes with primarily pre-synaptic neurotoxins, such as kraits (Bungarus spp.) and taipans (Oxyuranus spp.) suggest that antivenom does not reverse established neurotoxicity, but early administration may be associated with decreased severity or prevent neurotoxicity. Small studies of snakes with mainly post-synaptic neurotoxins, including some cobra species (Naja spp.), provide preliminary evidence that neurotoxicity may be reversed with antivenom, but placebo controlled studies with objective outcome measures are required to confirm this.

ATP Released by Injured Neurons Activates Schwann Cells

Injured nerve terminals of neuromuscular junctions (NMJs) can regenerate. This remarkable and complex response is governed by molecular signals that are exchanged among the cellular components of this synapse: motor axon nerve terminal (MAT), perisynaptic Schwann cells (PSCs), and muscle fiber. The nature of signals that govern MAT regeneration is ill-known. In the present study the spider toxin α-latrotoxin has been used as tool to investigate the mechanisms underlying peripheral neuroregeneration. Indeed this neurotoxin induces an acute, specific, localized and fully reversible damage of the presynaptic nerve terminal, and its action mimics the cascade of events that leads to nerve terminal degeneration in injured patients and in many neurodegenerative conditions. Here we provide evidence of an early release by degenerating neurons of adenosine triphosphate as alarm messenger, that contributes to the activation of a series of intracellular pathways within Schwann cells that are crucial for nerve regeneration: Ca(2+), cAMP, ERK1/2, and CREB. These results contribute to define the cross-talk taking place among degenerating nerve terminals and PSCs, involved in the functional recovery of the NMJ.

M1 and M3 muscarinic receptors may play a role in the neurotoxicity of anhydroecgonine methyl ester, a cocaine pyrolysis product

The smoke of crack cocaine contains cocaine and its pyrolysis product, anhydroecgonine methyl ester (AEME). AEME possesses greater neurotoxic potential than cocaine and an additive effect when they are combined. Since atropine prevented AEME-induced neurotoxicity, it has been suggested that its toxic effects may involve the muscarinic cholinergic receptors (mAChRs). Our aim is to understand the interaction between AEME and mAChRs and how it can lead to neuronal death. Using a rat primary hippocampal cell culture, AEME was shown to cause a concentration-dependent increase on both total [³H]inositol phosphate and intracellular calcium, and to induce DNA fragmentation after 24 hours of exposure, in line with the activation of caspase-3 previously shown. Additionally, we assessed AEME activity at rat mAChR subtypes 1–5 heterologously expressed in Chinese Hamster Ovary cells. l-[N-methyl-³H]scopolamine competition binding showed a preference of AEME for the M₂ subtype; calcium mobilization tests revealed partial agonist effects at M₁ and M₃ and antagonist activity at the remaining subtypes. The selective M₁ and M₃ antagonists and the phospholipase C inhibitor, were able to prevent AEME-induced neurotoxicity, suggesting that the toxicity is due to the partial agonist effect at M₁ and M₃ mAChRs, leading to DNA fragmentation and neuronal death by apoptosis.

Inhibition of botulinum neurotoxins interchain disulfide bond reduction prevents the peripheral neuroparalysis of botulism

Botulinum neurotoxins (BoNTs) form a growing family of metalloproteases with a unique specificity either for VAMP, SNAP25 or syntaxin. The BoNTs are grouped in seven different serotypes indicated by letters from A to G. These neurotoxins enter the cytosol of nerve terminals via a 100 kDa chain which binds to the presynaptic membrane and assists the translocation of a 50 kDa metalloprotease chain. These two chains are linked by a single disulfide bridge which plays an essential role during the entry of the metalloprotease chain in the cytosol, but thereafter it has to be reduced to free the proteolytic activity. Its reduction is mediated by thioredoxin which is continuously regenerated by its reductase. Here we show that inhibitors of thioredoxin reductase or of thioredoxin prevent the specific proteolysis of VAMP by the four VAMP-specific BoNTs: type B, D, F and G. These compounds are effective not only in primary cultures of neurons, but also in preventing the in vivo mouse limb neuroparalysis. In addition, one of these inhibitors, Ebselen, largely protects mice from the death caused by a systemic injection. Together with recent results obtained with BoNTs specific for SNAP25 and syntaxin, the present data demonstrate the essential role of the thioredoxin-thioredoxin reductase system in reducing the interchain disulfide during the nerve intoxication mechanism of all serotypes. Therefore its inhibitors should be considered for a possible use to prevent botulism and for treating infant botulism.

Mitochondrial alarmins released by degenerating motor axon terminals activate perisynaptic Schwann cells

An acute and highly reproducible motor axon terminal degeneration followed by complete regeneration is induced by some animal presynaptic neurotoxins, representing an appropriate and controlled system to dissect the molecular mechanisms underlying degeneration and regeneration of peripheral nerve terminals. We have previously shown that nerve terminals exposed to spider or snake presynaptic neurotoxins degenerate as a result of calcium overload and mitochondrial failure. Here we show that toxin-treated primary neurons release signaling molecules derived from mitochondria: hydrogen peroxide, mitochondrial DNA, and cytochrome c. These molecules activate isolated primary Schwann cells, Schwann cells cocultured with neurons and at neuromuscular junction in vivo through the MAPK pathway. We propose that this inter- and intracellular signaling is involved in triggering the regeneration of peripheral nerve terminals affected by other forms of neurodegenerative diseases.

Neurotoxicity of coral snake phospholipases A2 in cultured rat hippocampal neurons

The neurotoxicity of two secreted Phospholipases A2 from Brazilian coral snake venom in rat primary hippocampal cell culture was investigated. Following exposure to Mlx-8 or Mlx-9 toxins, an increase in free cytosolic Ca(2+) and a reduction in mitochondrial transmembrane potential (ΔΨm) became evident and occurred prior to the morphological changes and cytotoxicity. Exposure of hippocampal neurons to Mlx-8 or Mlx-9 caused a decrease in the cell viability as assessed by MTT and LDH assays. Inspection using fluorescent images and ultrastructural analysis by scanning and transmission electron microscopy showed that multiphase injury is characterized by overlapping cell death phenotypes. Shrinkage, membrane blebbing, chromatin condensation, nucleosomal DNA fragmentation and the formation of apoptotic bodies were observed. The most striking alteration observed in the electron microscopy was the fragmentation and rarefaction of the neuron processes network. Degenerated terminal synapses, cell debris and apoptotic bodies were observed among the fragmented fibers. Numerous large vacuoles as well as swollen mitochondria and dilated Golgi were noted. Necrotic signs such as a large amount of cellular debris and membrane fragmentation were observed mainly when the cells were exposed to highest concentration of the PLA2-neurotoxins. PLA2s exposed cultures showed cytoplasmic vacuoles filled with cell debris, clusters of mitochondria presented mitophagy-like structures that are in accordance to patterns of programmed cell death by autophagy. Finally, we demonstrated that the sPLA2s, Mlx-8 and Mlx-9, isolated from the Micrurus lemniscatus snake venom induce a hybrid cell death with apoptotic, autophagic and necrotic features. Furthermore, this study suggests that the augment in free cytosolic Ca(2+) and mitochondrial dysfunction are involved in the neurotoxicity of Elapid coral snake venom sPLA2s.

Neutralisation of specific surface carboxylates speeds up translocation of botulinum neurotoxin type B enzymatic domain

Botulinum neurotoxins translocate their enzymatic domain across vesicular membranes. The molecular triggers of this process are unknown. Here, we tested the possibility that this is elicited by protonation of conserved surface carboxylates. Glutamate-48, glutamate-653 and aspartate-877 were identified as possible candidates and changed into amide. This triple mutant showed increased neurotoxicity due to faster cytosolic delivery of the enzymatic domain; membrane translocation could take place at less acidic pH. Thus, neutralisation of specific negative surface charges facilitates membrane contact permitting a faster initiation of the toxin membrane insertion.

Neurotoxicity in snakebite--the limits of our knowledge

Snakebite is classified by the WHO as a neglected tropical disease. Envenoming is a significant public health problem in tropical and subtropical regions. Neurotoxicity is a key feature of some envenomings, and there are many unanswered questions regarding this manifestation. Acute neuromuscular weakness with respiratory involvement is the most clinically important neurotoxic effect. Data is limited on the many other acute neurotoxic manifestations, and especially delayed neurotoxicity. Symptom evolution and recovery, patterns of weakness, respiratory involvement, and response to antivenom and acetyl cholinesterase inhibitors are variable, and seem to depend on the snake species, type of neurotoxicity, and geographical variations. Recent data have challenged the traditional concepts of neurotoxicity in snake envenoming, and highlight the rich diversity of snake neurotoxins. A uniform system of classification of the pattern of neuromuscular weakness and models for predicting type of toxicity and development of respiratory weakness are still lacking, and would greatly aid clinical decision making and future research. This review attempts to update the reader on the current state of knowledge regarding this important issue.

Calpains participate in nerve terminal degeneration induced by spider and snake presynaptic neurotoxins

α-latrotoxin and snake presynaptic phospholipases A2 neurotoxins target the presynaptic membrane of axon terminals of the neuromuscular junction causing paralysis. These neurotoxins display different biochemical activities, but similarly alter the presynaptic membrane permeability causing Ca(2+) overload within the nerve terminals, which in turn induces nerve degeneration. Using different methods, here we show that the calcium-activated proteases calpains are involved in the cytoskeletal rearrangements that we have previously documented in neurons exposed to α-latrotoxin or to snake presynaptic phospholipases A2 neurotoxins. These results indicate that calpains, activated by the massive calcium influx from the extracellular medium, target fundamental components of neuronal cytoskeleton such as spectrin and neurofilaments, whose cleavage is functional to the ensuing nerve terminal fragmentation.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Biochemistry of envenomation

Venoms and toxins are of significant interest due to their ability to cause a wide range of pathophysiological conditions that can potentially result in death. Despite their wide distribution among plants and animals, the biochemical pathways associated with these pathogenic agents remain largely unexplored. Impoverished and underdeveloped regions appear especially susceptible to increased incidence and severity due to poor socioeconomic conditions and lack of appropriate medical treatment infrastructure. To facilitate better management and treatment of envenomation victims, it is essential that the biochemical mechanisms of their action be elucidated. This review aims to characterize downstream envenomation mechanisms by addressing the major neuro-, cardio-, and hemotoxins as well as ion-channel toxins. Because of their use in folk and traditional medicine, the biochemistry behind venom therapy and possible implications on conventional medicine will also be addressed.

Structural analysis of trimeric phospholipase A2 neurotoxin from the Australian taipan snake venom

Snake pre-synaptic neurotoxins endowed with phospholipase A(2) activity are potent inducers of paralysis through the specific disruption of the neuromuscular junction pre-synaptic membrane and represent a valuable tool for investigating neuronal degeneration and recovery. They have different structural complexity and a wide range of lethal potency and enzymatic activity, although they share a similar mechanism of action. Although no correlation has been reported between neurotoxicity and enzymatic activity, toxicity increases with structural complexity and phospholipase A(2) oligomers show 10-fold lower LD(50) values compared to their monomeric counterparts. To date, no structural study has been performed on multimeric SPANs with the aim of shedding light on the correlation between structural complexity and neurotoxicity. In the present study, we investigated the structure of taipoxin, a trimeric phospholipase A(2) neurotoxin, as well as that of its subunits, by X-ray crystallography and small angle X-ray scattering analysis. We present the high-resolution structure of two isoforms of the taipoxin β subunit, which show no neurotoxic activity but enhance the activity of the other subunits in the complex. One isoform shows no structural change that could justify the lack of activity. The other displays three point mutations in critical positions for the catalytic activity. Moreover, we designed a model for the quaternary structure of taipoxin under physiological conditions, in which the three subunits are organized into a flat holotoxin with the substrate binding sockets exposed on the same side of the complex, which suggests a role for this interface in the toxin-membrane interaction.

Guanidination of notexin promotes its phospholipase A(2) activity-independent fusogenicity on vesicles with lipid-supplied negative curvature

To address the requirement of phospholipase A(2) (PLA(2)) activity in membrane fusion events and membrane perturbation activity of notexin and guanidinated notexin (Gu-notexin), the present study was conducted. Notexin and Gu-notexin did not show PLA(2) activity after the removal of Ca(2+) with EDTA. Metal-free notexin and Gu-notexin were found to induce membrane leakage and fusion of phospholipid vesicles. Fusogenic activity of native and modified notexin correlated positively with their membrane-damaging activity underlying the deprivation of PLA(2) activity. Compared with Ca(2+)-bound Gu-notexin, fusogenicity of metal-free Gu-notexin was notably increased by incorporation of cholesterol, cholesterol sulfate, phosphatidylethanolamine, α-tocopherol and phosphatidic acid that supplied negative curvature into phospholipid bilayer. The ability of Gu-notexin to induce membrane fusion of vesicles with lipid-supplied negative curvature was higher than that of notexin regardless of the absence or presence of Ca(2+). Consistently, metal-free Gu-notexin markedly induced membrane fusion of red blood cells (RBCs) compared with metal-free notexin, and fusion activity of metal-free Gu-notexin on cholesterol-depleted RBCs notably reduced. Compared with notexin, Gu-notexin highly induced uptake of calcein-loaded phosphatidylcholine (PC)/cholesterol and PC/cholesterol sulfate vesicles by K562 cells in the presence of EDTA. Taken together, our data suggest that notexin and Gu-notexin could induce vesicle leakage and fusion via a PLA(2) activity-independent mechanism, and guanidination promotes PLA(2) activity-independent fusogenicity of notexin on vesicles with lipid-supplied negative curvature.

Double anchorage to the membrane and intact inter-chain disulfide bond are required for the low pH induced entry of tetanus and botulinum neurotoxins into neurons

Tetanus and botulinum neurotoxins are di-chain proteins that cause paralysis by inhibiting neuroexocytosis. These neurotoxins enter into nerve terminals via endocytosis inside synaptic vesicles, whose acidic pH induces a structural change of the neurotoxin molecule that becomes capable of translocating its L chain into the cytosol, via a transmembrane protein-conducting channel made by the H chain. This is the least understood step of the intoxication process primarily because it takes place inside vesicles within the cytosol. In the present study, we describe how this passage was made accessible to investigation by making it to occur at the surface of neurons. The neurotoxin, bound to the plasma membrane in the cold, was exposed to a warm low pH extracellular medium and the entry of the L chain was monitored by measuring its specific metalloprotease activity with a ratiometric method. We found that the neurotoxin has to be bound to the membrane via at least two anchorage sites in order for a productive low-pH induced structural change to take place. In addition, this process can only occur if the single inter-chain disulfide bond is intact. The pH dependence of the conformational change of tetanus neurotoxin and botulinum neurotoxin B, C and D is similar and take places in the same slightly acidic range, which comprises that present inside synaptic vesicles. Based on these and previous findings, we propose a stepwise sequence of molecular events that lead from toxin binding to membrane insertion.

Mass spectrometry analysis of the phospholipase A(2) activity of snake pre-synaptic neurotoxins in cultured neurons

Snake pre-synaptic phospholipase A(2) neurotoxins paralyse the neuromuscular junction by releasing phospholipid hydrolysis products that alter curvature and permeability of the pre-synaptic membrane. Here, we report results deriving from the first chemical analysis of the action of these neurotoxic phospholipases in neurons, made possible by the use of high sensitivity mass spectrometry. The time-course of the phospholipase A(2) activity (PLA(2)) hydrolysis of notexin, beta-bungarotoxin, taipoxin and textilotoxin acting in cultured neurons was determined. At variance from their enzymatic activities in vitro, these neurotoxins display comparable kinetics of lysophospholipid release in neurons, reconciling the large discrepancy between their in vivo toxicities and their in vitro enzymatic activities. The ratios of the lyso derivatives of phosphatidyl choline, ethanolamine and serine obtained here together with the known distribution of these phospholipids among cell membranes, suggest that most PLA(2) hydrolysis takes place on the cell surface. Although these toxins were recently shown to enter neurons, their intracellular hydrolytic action and the activation of intracellular PLA(2)s appear to contribute little, if any, to the phospholipid hydrolysis measured here.

Snake phospholipase A2 neurotoxins enter neurons, bind specifically to mitochondria, and open their transition pores

Snake presynaptic neurotoxins with phospholipase A(2) activity are potent inducers of paralysis through inhibition of the neuromuscular junction. These neurotoxins were recently shown to induce exocytosis of synaptic vesicles following the production of lysophospholipids and fatty acids and a sustained influx of Ca(2+) from the medium. Here, we show that these toxins are able to penetrate spinal cord motor neurons and cerebellar granule neurons and selectively bind to mitochondria. As a result of this interaction, mitochondria depolarize and undergo a profound shape change from elongated and spaghetti-like to round and swollen. We show that snake presynaptic phospholipase A(2) neurotoxins facilitate opening of the mitochondrial permeability transition pore, an inner membrane high-conductance channel. The relative potency of the snake neurotoxins was similar for the permeability transition pore opening and for the phospholipid hydrolysis activities, suggesting a causal relationship, which is also supported by the effect of phospholipid hydrolysis products, lysophospholipids and fatty acids, on mitochondrial pore opening. These findings contribute to define the cellular events that lead to intoxication of nerve terminals by these snake neurotoxins and suggest that mitochondrial impairment is an important determinant of their toxicity.

Presynaptic neurotoxins with enzymatic activities

Toxins that alter neurotransmitter release from nerve terminals are of considerable scientific and clinical importance. Many advances were recently made in the understanding of their molecular mechanisms of action and use in human therapy. Here, we focus on presynaptic neurotoxins, which are very potent inhibitors of the neurotransmitter release because they are endowed with specific enzymatic activities: (1) clostridial neurotoxins with a metallo-proteolytic activity and (2) snake presynaptic neurotoxins with a phospholipase A2 activity.

Phospholipase A2 activity-independent membrane-damaging effect of notexin

To elucidate whether the phospholipase A(2) (PLA(2)) activity of notexin was exclusively associated with the manifestation of its pharmacological activities, the interaction of notexin with phospholipid liposomes was explored by fluorescence and CD measurement underlying the conditions of depriving its PLA(2) activity. Although a higher membrane-damaging activity was noted with Ca(2+)-bound notexin, abolishment of PLA(2) activity by EDTA and Sr(2+) could not diminish the membrane-damaging activity of notexin. Fluorescence-quenching studies and CD measurement indicated that Ca(2+)-bound, Sr(2+)-bound or metal-free notexin did not adopt the same conformation upon binding with phospholipids. Regardless of the presence of Ca(2+), Sr(2+) or EDTA, self-quenching assay with rhodamine-labeled notexin revealed that the toxin pertained to form oligomer when it bound with liposomes. Although Lys-modified notexin retained full PLA(2) activity, a notable decrease in membrane-damaging activity was observed. These results indicate that notexin could directly cause a leakage of membrane via a PLA(2) activity-independent manner, and implicate that another biological event contributes to the activity of notexin in vivo.

Understanding the molecular mechanism underlying the presynaptic toxicity of secreted phospholipases A2

An important group of toxins, whose action at the molecular level is still a matter of debate, is secreted phospholipases A(2) (sPLA(2)s) endowed with presynaptic or beta-neurotoxicity. The current belief is that these beta-neurotoxins (beta-ntxs) exert their toxicity primarily due to their extracellular enzymatic action on the plasma membrane of motoneurons at the neuromuscular junction. However, the discovery of several extra- and intracellular proteins, with high binding affinity for snake venom beta-ntxs, has raised the question as to whether this explanation is adequate to account for all the observed phenomena in the process of presynaptic toxicity. The purpose of this review is to critically examine the various published studies, including the most recent results on internalization of a beta-ntx into motor nerve terminals, in order to contribute to a better understanding of the molecular mechanism of beta-neurotoxicity. As a result, we propose that presynaptic neurotoxicity of sPLA(2)s is a result of both extra- and intracellular actions of beta-ntxs, involving enzymatic activity as well as interaction of the toxins with intracellular proteins affecting the cycling of synaptic vesicles in the axon terminals of vertebrate motoneurons.

Glycogen synthase kinase 3 activation is essential for the snake phospholipase A2 neurotoxin-induced secretion in chromaffin cells

Neuroendocrine chromaffin cells were used to study the mechanism of the snake phospholipase A2 (PLA2) neurotoxin enhancement of exocytosis. Notexin, beta-bungarotoxin, taipoxin or textilotoxin enhanced the fast release of catecholamines elicited by flash photolysis of cytosolic caged calcium. Such an increase correlates with the capacity of these neurotoxins to cause fragmentation of the F-actin cortical barrier with subsequent accumulation of vesicles in the proximity of the plasma membrane. These PLA2 neurotoxins do not act via protein kinase C activation, which is known to promote F-actin fragmentation. Lithium, RO31-8220 and SB216763, three inhibitors of the glycogen synthase kinase 3, prevent both the alteration of the F-actin peripheral cortex and the enhancement of fast release elicited by these neurotoxins. In addition, glycogen synthase kinase 3 has been detected by immunolocalization in a membranous compartment of the chromaffin cell endoplasmic reticulum (ER). These results suggest that the activation of this enzyme plays a major role in the enhancement of exocytosis of the readily releasable granules caused by PLA2 neurotoxins in neuroendocrine chromaffin cells.

The roles of NADPH oxidase and phospholipases A2 in oxidative and inflammatory responses in neurodegenerative diseases

Reactive oxygen species (ROS) are produced in mammalian cells through enzymic and non-enzymic mechanisms. Although some ROS production pathways are needed for specific physiological functions, excessive production is detrimental and is regarded as the basis of numerous neurodegenerative diseases. Among enzymes producing superoxide anions, NADPH oxidase is widespread in mammalian cells and is an important source of ROS in mediating physiological and pathological processes in the cardiovascular and the CNS. ROS production is linked to the alteration of intracellular calcium homeostasis, activation of Ca(2+)-dependent enzymes, alteration of cytoskeletal proteins, and degradation of membrane glycerophospholipids. There is evolving evidence that ROS produced by NADPH oxidase regulate neuronal functions and degrade membrane phospholipids through activation of phospholipases A(2) (PLA(2)). This review is intended to cover recent studies describing ROS generation from NADPH oxidase in the CNS and its downstream activation of PLA(2), namely, the group IV cytosolic cPLA(2) and the group II secretory sPLA(2). A major focus is to elaborate the dual role of NADPH oxidase and PLA(2) in mediating the oxidative and inflammatory responses in neurodegenerative diseases, including cerebral ischemia and Alzheimer's disease. Elucidation of the signaling pathways linking NADPH oxidase with the multiple forms of PLA(2) will be important in understanding the oxidative and degradative mechanisms that underline neuronal damage and glial activation and will facilitate development of therapeutic intervention for prevention and treatment of these and other neurodegenerative diseases.

Calcium Influx and Mitochondrial Alterations at Synapses Exposed to Snake Neurotoxins or Their Phospholipid Hydrolysis Products

Snake presynaptic phospholipase A2 neurotoxins (SPANs) bind to the presynaptic membrane and hydrolyze phosphatidylcholine with generation of lysophosphatidylcholine (LysoPC) and fatty acid (FA). The LysoPC+FA mixture promotes membrane fusion, inducing the exocytosis of the ready-to-release synaptic vesicles. However, also the reserve pool of synaptic vesicles disappears from nerve terminals intoxicated with SPAN or LysoPC+FA. Here, we show that LysoPC+FA and SPANs cause a large influx of extracellular calcium into swollen nerve terminals, which accounts for the extensive synaptic vesicle release. This is paralleled by the change of morphology and the collapse of membrane potential of mitochondria within nerve bulges. These results complete the picture of events occurring at nerve terminals intoxicated by SPANs and define the LysoPC+FA lipid mixture as a novel and effective agonist of synaptic vesicle release.

An Envelope-determined Endocytic Route of Viral Entry Allows HIV-1 to Escape from Secreted Phospholipase A2 Entry Blockade

Secreted phospholipases A(2) (sPLA(2)s) represent a new class of human immunodeficiency virus (HIV) inhibitors that block the early steps of virus entry into cells. Here, we applied an in vitro evolution/selection procedure to select, from primary HIV isolates, an emerging variant (HIV(RBV-3)) able to actively infect cells in the presence of sPLA(2)s. HIV(RBV-3) represents a very atypical HIV-1 isolate because, in contrast to others, this virus requires a functional endocytic machinery to infect cells. Indeed, endocytosis inhibitors that affect endosome acidification (bafilomycin A(1), monensin) and/or endosomal trafficking (nocodazole, latrunculin A) drastically reduced HIV(RBV-3) replication. Using a standardized PCR-assay, we showed that endocytosis inhibitors block HIV(RBV-3) entry just before the reverse transcription step. Concurrently, to identify the viral proteins responsible for the HIV(RBV-3) atypical behaviour, we constructed a HIV-1 molecular chimera bearing different HIV(RBV-3) proteins. We demonstrated that the sole presence of the HIV(RBV-3) envelope glycoprotein is enough, not only to confer the resistance to sPLA(2)s, but also to direct HIV(RBV-3) to the endosomal-dependent entry pathway. Interestingly, HIV(RBV-3) envelope glycoprotein sequencing revealed an unusual structural pattern with the presence of rare mutations in the N-terminal region and V1-V2 envelope loop sequence extensions. Taken together, we conclude that HIV-1 may escape from entry inhibitors, such as sPLA2s, through the selection of a particular HIV-1 envelope glycoprotein that allows HIV to infect cells via an alternative entry route that relies on endosome trafficking.

A lysolecithin/fatty acid mixture promotes and then blocks neurotransmitter release at the Drosophila melanogaster larval neuromuscular junction

The study of the effect of snake presynaptic neurotoxins with phospholipase A2 activity on nerve terminals has recently unveiled the inhibitory action of a lysophosphatidylcholine (LysoPC)/fatty acid mixture. We report here that these neurotoxins have no activity on Drosophila melanogaster nerve terminals. However, a 1:1 mixture of LysoPC and oleic acid induces an early increase, followed by an inhibition of both evoked and spontaneous neurotransmitter release. This effect is also induced by LysoPC alone. The present findings provide an indirect evidence that the lipid hemifusion-to-pore transition is a key event in neuroexocytosis in Drosophila. Moreover, these findings substantiate the use of LysoPC as a general agonist of membrane fusion at nerve terminals. This novel tool could contribute to the unraveling of the molecular steps involved in neuroexocytosis, particularly in Drosophila, where it is straightforward to combine it with electrophysiology and genetics.

Reversible skeletal neuromuscular paralysis induced by different lysophospholipids

Lysophosphatidylcholine rapidly paralyses the neuromuscular junction (NMJ), similarly to snake phospholipase A2 neurotoxins, implicating a lipid hemifusion-pore transition in neuroexocytosis. The mode and kinetics of NMJ paralysis of different lysophospholipids (lysoPLs) in high or low [Mg2+] was investigated. The following order of potency was found: lysophosphatidylcholine>lysophosphatidylethanolamine>lysophosphatidic acid>lysophosphatidylserine>lysophosphatidylglycerol. The latter two lysoPLs closely mimic the profile of paralysis caused by the toxins in high [Mg2+]. This paralysis is fully reversed by albumin washing. These findings provide novel insights on the mode of action of snake neurotoxins and qualify lysoPLs as novel agents to study neuroexocytosis.