Effect of the venom of Glycera convoluta on the spontaneous quantal release of transmitter

An Exploration of Novel Bioactives from the Venomous Marine Annelid Glycera alba

The immense biodiversity of marine invertebrates makes them high-value targets for the prospecting of novel bioactives. The present study investigated proteinaceous toxins secreted by the skin and proboscis of Glycera alba (Annelida: Polychaeta), whose congenerics G. tridactyla and G. dibranchiata are known to be venomous. Proteomics and bioinformatics enabled the detection of bioactive proteins that hold potential for biotechnological applications, including toxins like glycerotoxins (GLTx), which can interfere with neuromuscular calcium channels and therefore have value for the development of painkillers, for instance. We also identified proteins involved in the biosynthesis of toxins. Other proteins of interest include venom and toxin-related bioactives like cysteine-rich venom proteins, many of which are known to interfere with the nervous system. Ex vivo toxicity assays with mussel gills exposed to fractionated protein extracts from the skin and proboscis revealed that fractions potentially containing higher-molecular-mass venom proteins can exert negative effects on invertebrate prey. Histopathology, DNA damage and caspase-3 activity suggest significant cytotoxic effects that can be coadjuvated by permeabilizing enzymes such as venom metalloproteinases M12B. Altogether, these encouraging findings show that venomous annelids are important sources of novel bioactives, albeit illustrating the challenges of surveying organisms whose genomes and metabolisms are poorly understood.

Comparative analyses of glycerotoxin expression unveil a novel structural organization of the bloodworm venom system

BACKGROUND

We present the first molecular characterization of glycerotoxin (GLTx), a potent neurotoxin found in the venom of the bloodworm Glycera tridactyla (Glyceridae, Annelida). Within the animal kingdom, GLTx shows a unique mode of action as it can specifically up-regulate the activity of Cav2.2 channels (N-type) in a reversible manner. The lack of sequence information has so far hampered a detailed understanding of its mode of action.

RESULTS

Our analyses reveal three ~3.8 kb GLTx full-length transcripts, show that GLTx represents a multigene family, and suggest it functions as a dimer. An integrative approach using transcriptomics, quantitative real-time PCR, in situ hybridization, and immunocytochemistry shows that GLTx is highly expressed exclusively in four pharyngeal lobes, a previously unrecognized part of the venom apparatus.

CONCLUSIONS

Our results overturn a century old textbook view on the glycerid venom system, suggesting that it is anatomically and functionally much more complex than previously thought. The herein presented GLTx sequence information constitutes an important step towards the establishment of GLTx as a versatile tool to understand the mechanism of synaptic function, as well as the mode of action of this novel neurotoxin.

The Utility of Genome Skimming for Phylogenomic Analyses as Demonstrated for Glycerid Relationships (Annelida, Glyceridae)

Glyceridae (Annelida) are a group of venomous annelids distributed worldwide from intertidal to abyssal depths. To trace the evolutionary history and complexity of glycerid venom cocktails, a solid backbone phylogeny of this group is essential. We therefore aimed to reconstruct the phylogenetic relationships of these annelids using Illumina sequencing technology. We constructed whole-genome shotgun libraries for 19 glycerid specimens and 1 outgroup species (Glycinde armigera). The chosen target genes comprise 13 mitochondrial proteins, 2 ribosomal mitochondrial genes, and 4 nuclear loci (18SrRNA, 28SrRNA, ITS1, and ITS2). Based on partitioned maximum likelihood as well as Bayesian analyses of the resulting supermatrix, we were finally able to resolve a robust glycerid phylogeny and identified three clades comprising the majority of taxa. Furthermore, we detected group II introns inside the cox1 gene of two analyzed glycerid specimens, with two different insertions in one of these species. Moreover, we generated reduced data sets comprising 10 million, 4 million, and 1 million reads from the original data sets to test the influence of the sequencing depth on assembling complete mitochondrial genomes from low coverage genome data. We estimated the coverage of mitochondrial genome sequences in each data set size by mapping the filtered Illumina reads against the respective mitochondrial contigs. By comparing the contig coverage calculated in all data set sizes, we got a hint for the scalability of our genome skimming approach. This allows estimating more precisely the number of reads that are at least necessary to reconstruct complete mitochondrial genomes in Glyceridae and probably non-model organisms in general.

A Polychaete's powerful punch: venom gland transcriptomics of Glycera reveals a complex cocktail of toxin homologs

Glycerids are marine annelids commonly known as bloodworms. Bloodworms have an eversible proboscis adorned with jaws connected to venom glands. Bloodworms prey on invertebrates, and it is known that the venom glands produce compounds that can induce toxic effects in animals. Yet, none of these putative toxins has been characterized on a molecular basis. Here we present the transcriptomic profiles of the venom glands of three species of bloodworm, Glycera dibranchiata, Glycera fallax and Glycera tridactyla, as well as the body tissue of G. tridactyla. The venom glands express a complex mixture of transcripts coding for putative toxin precursors. These transcripts represent 20 known toxin classes that have been convergently recruited into animal venoms, as well as transcripts potentially coding for Glycera-specific toxins. The toxins represent five functional categories: Pore-forming and membrane-disrupting toxins, neurotoxins, protease inhibitors, other enzymes, and CAP domain toxins. Many of the transcripts coding for putative Glycera toxins belong to classes that have been widely recruited into venoms, but some are homologs of toxins previously only known from the venoms of scorpaeniform fish and monotremes (stonustoxin-like toxin), turrid gastropods (turripeptide-like peptides), and sea anemones (gigantoxin I-like neurotoxin). This complex mixture of toxin homologs suggests that bloodworms employ venom while predating on macroscopic prey, casting doubt on the previously widespread opinion that G. dibranchiata is a detritivore. Our results further show that researchers should be aware that different assembly methods, as well as different methods of homology prediction, can influence the transcriptomic profiling of venom glands.

Partial purification of ?-glycerotoxin, a presynaptic neurotoxin from the venom glands of the polychaete annelid glycera convoluta

The venom secreted from glands appended to the jaws of Glycera convoluta, a Polychaete Annelid, increases the spontaneous quantal release of transmitter from nerve terminals. The component that is biologically active on vertebrate cholinergic nerve terminals has recently been shown to be a high molecular weight protein. In the present work, the crude extract from the venom apparatus was shown to be toxic for mammals and crustaceans. It was fractionated by gel filtrations and ion exchange chromatographies. The biologically active component at frog neuromuscular junctions, ?-glycerotoxin, was purified more than 1,000-fold. It is distinct from the components that are toxic for crustaceans. Purified ?-glycerotoxin is a globular protein of 300,000 +/- 20,000 mol wt. It has a Stokes radius of 65 A and a sedimentation coefficient of 11 S. By its molecular properties, ?-glycerotoxin appears distinct from other neurotoxins such as ?-latrotoxin, which also trigger transmitter release.

Glycerotoxin from Glycera convoluta stimulates neurosecretion by up-regulating N-type Ca2+ channel activity

We report here the purification of glycerotoxin from the venom of Glycera convoluta, a novel 320 kDa protein capable of reversibly stimulating spontaneous and evoked neurotransmitter release at the frog neuromuscular junction. However, glycerotoxin is ineffective at the murine neuromuscular junction, which displays a different subtype of voltage- dependent Ca(2+) channels. By sequential and selective inhibition of various types of Ca(2+) channels, we found that glycerotoxin was acting via Ca(v)2.2 (N-type). In neuroendocrine cells, it elicits a robust, albeit transient, influx of Ca(2+) sensitive to the Ca(v)2.2 blockers omega-conotoxin GVIA and MVIIA. Moreover, glycerotoxin triggers a Ca(2+) transient in human embryonic kidney (HEK) cells over-expressing Ca(v)2.2 but not Ca(v)2.1 (P/Q-type). Whole-cell patch-clamp analysis of Ca(v)2.2 expressing HEK cells revealed an up-regulation of Ca(2+) currents due to a leftward shift of the activation peak upon glycerotoxin addition. A direct interaction between Ca(v)2.2 and this neurotoxin was revealed by co-immunoprecipitation experiments. Therefore, glycerotoxin is a unique addition to the arsenal of tools available to unravel the mechanism controlling Ca(2+)-regulated exocytosis via the specific activation of Ca(v)2.2.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

A unifying hypothesis for acetylcholine release

Mediatophore is the only nerve terminal membrane protein known to translocate acetylcholine upon calcium action. It is localized at the active zone. In this review we attempted to describe its role in relation to the vesicular and membrane protein complexes that are formed at the active zone. The model pictures a possible set of sequential steps that lead to exocytosis. The smallest quantal events are attributed to mediatophore opening momentarily, while synaptic vesicles synchronize release by controlling the calcium microdomain. A clear distinction is made between sub-quantal ACh release preserved after Botulinum toxin action, and exocytosis of vesicular contents. A cybernetic model for release and exocytosis related to protein interactions is presented for future works.

Large-scale purification of presynaptic plasma membranes from Torpedo marmorata electric organ

The presynaptic plasma membrane (PSPM) of cholinergic nerve terminals was purified from Torpedo electric organ using a large-scale procedure. Up to 500 g of frozen electric organ were fractioned in a single run, leading to the isolation of greater than 100 mg of PSPM proteins. The purity of the fraction is similar to that of the synaptosomal plasma membrane obtained after subfractionation of Torpedo synaptosomes as judged by its membrane-bound acetylcholinesterase activity, the number of Glycera convoluta neurotoxin binding sites, and the binding of two monoclonal antibodies directed against PSPM. The specificity of these antibodies for the PSPM is demonstrated by immunofluorescence microscopy.

Inactivation of acetylcholine release from Torpedo synaptosomes in response to prolonged depolarizations

The release of acetylcholine (ACh) from purely cholinergic Torpedo synaptosomes was monitored continuously using a chemiluminescent assay (Israël & Lesbats, 1981 a, b). Upon prolonged K+ depolarization in the presence of Ca2+, the release of ACh was transient and returned to a steady low level in about 3 min. Addition of the Ca2+ ionophore A23187 triggered the release again, suggesting that neither depletion of the transmitter store nor an inhibition of the release mechanism itself were involved in this phasic response, but rather an inactivation of the Ca2+ entry. The release response evoked by adding Ca2+ back after exposure of the synaptosomes to high K+ (70 mM) and low Ca2+ (0.57 mM) solution inactivates as a function of the duration of the pre-depolarization with a two-component time course with rapid (tau = 5.5 s) and slow phases (tau = 143 s). This response to Ca2+ addition was more strikingly reduced as the level of depolarization during pre-treatment was increased. The inactivation was found to be dose dependent with respect to the amount of Ca2+ present during the pre-depolarization period (conditioning Ca2+). Moreover, the presence of EGTA during pre-treatment with high-K+ solutions increased the response to applied Ca2+. These observations suggest that Ca2+ entry itself was responsible for this inactivation. No inactivation was found when ACh release was induced by the depolarizing agent Gramicidin D, except when external Na+ was replaced by Li+. This result indicates that part of the Ca2+ influx promoted by Gramicidin D depends on a Na+ entry, and may be mediated by the Na-Ca exchange mechanism.

Relationship between presynaptic membrane potential and acetylcholine release in synaptosomes from Torpedo electric organ

The membrane potential of purely cholinergic synaptosomes isolated from Torpedo electric organ was monitored with fluorescent carbocyanine dyes. An increased fluorescence was associated with depolarization and a quenching with hyperpolarization. Fluorescence data provided evidence that Torpedo synaptosomes have a membrane potential mainly driven by a K+ diffusion potential and a membrane potential of about -50 mV could be estimated after calibration of fluorescence signals with ionophore antibiotics. The release of acetylcholine (ACh) from Torpedo synaptosomes was monitored continuously by measuring the light emitted by a chemiluminescent method (Israël & Lesbats, 1981 a). Using fluorescence data, the release of ACh was expressed as a function of membrane potential. The relationship between presynaptic potential and transmitter release as determined by biochemical methods at cholinergic nerve endings showed striking similarities to that observed at the squid giant synapse. Several substances were also tested with regard to their depolarizing and releasing properties and it was found that the toxin isolated from the venom of the annelid Glycera convoluta, which induced a large increase in quantal release of transmitter (Manaranche, Thieffry, & Israël, 1980) promoted a depolarization of Torpedo synaptosomes in addition to ACh release.

Alpha-latrotoxin and glycerotoxin differ in target specificity and in the mechanism of their neurotransmitter releasing action

alpha-Latrotoxin, a high molecular weight protein (130,000) purified from the venom of the black widow spider, and a partially purified neurotoxin, glycerotoxin, prepared from extracts of the jaw glands of the polichaete annelid Glycera convoluta, were previously found to induce similar effects (stimulation of quantal acetylcholine release) at the frog neuromuscular junction. In the present study parallel experiments performed with these two toxins revealed that only glycerotoxin was able to release acetylcholine from Torpedo electric organ synaptosomes, while alpha-latrotoxin did not affect release in this system. In contrast, alpha-latrotoxin stimulated release of dopamine from PC12 cells (a cloned neurosecretory cell line), whereas glycerotoxin was almost inactive. In rat brain synaptosomes both toxins were active. Preincubation of synaptosomal membranes with glycerotoxin was without effect on the subsequent binding of alpha-latrotoxin. Glycerotoxin application induced depolarization of synaptosomal plasma membrane, massive Ca2+ influx, marked increase of the cytosolic Ca2+ concentration, and stimulation of catecholamine release. The latter effect occurred to the same extent when glycerotoxin was applied either in complete medium (containing both Ca2+ and Mg2+), Ca2+-free medium or divalent cation-free medium. Some of these effects of glycerotoxin in rat brain synaptosomes (depolarization, increased Ca2+ influx and increased cytosolic Ca2+ concentration) resemble effects previously reported for alpha-latrotoxin. However, the secretory response induced by the latter was reduced in Ca2+-free, and abolished in divalent cation-free media. The different target specificity and the lack of binding competition of the two toxins could be due to their ability to recognize different receptors whose distribution overlap only in part in the cellular systems we have studied. The differences in action, on the other hand, could depend on postreceptor events, possibly related to the transmembrane insertion of toxin molecules demonstrated by others in artificial lipid membranes.

Reconstitution of a functional synaptosomal membrane possessing the protein constituents involved in acetylcholine translocation

Reconstitution of a functional presynaptic membrane possessing calcium-dependent acetylcholine release properties has been achieved. The proteoliposomal membrane obtained gains its acetylcholine-releasing capabilities from presynaptic membrane proteins. At the peak of acetylcholine release, intramembrane particles became more numerous in one of the proteoliposomal membrane faces. This phenomenon resembles the intramembrane particle rearrangements found in stimulated synaptosomes. No visible structures capable of releasing acetylcholine as a result of the calcium influx were found inside the proteoliposomes. This supports the view that the release of free cytosolic acetylcholine from stimulated nerve terminals can be directly attributed to presynaptic membrane proteins. These proteins were extracted in a functional form from the synaptosomal membrane.

Binding of a Glycera convoluta neurotoxin to cholinergic nerve terminal plasma membranes

The crude extract of venom glands of the polychaete annelid Glycera convoluta triggers a large Ca2+-dependent acetylcholine release from both frog motor nerve terminals and Torpedo electric organ synaptosomes. This extract was partially purified by Concanavalin A affinity chromatography. The biological activity was correlated in both preparations to a 300,000-dalton band, as shown by gel electrophoresis. This confirmed previous determinations obtained with chromatographic methods. This glycoprotein binds to presynaptic but not postsynaptic plasma membranes isolated from Torpedo electric organ. Pretreatment of intact synaptosomes by pronase abolished both the binding and the venom-induced acetylcholine release without impairing the high K+-induced acetylcholine release. Pretreatment of nerve terminal membranes by Concanavalin A similarly prevented the binding and the biological response. Binding to Torpedo membranes was still observed in the presence of EGTA. An antiserum directed to venom glycoproteins inhibited the neurotoxin so we could directly follow its binding to the presynaptic membrane. Glycera convoluta neurotoxin has to bind to a ectocellularly oriented protein of the presynaptic terminal to induce transmitter release.

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.

Induction of ion-permeable channels by the venom of the fanged bloodworm Glycera dibranchiata

Venom from the poison glands of the polychaete annelid Glycera convoluta has been reported to dramatically increase the frequency of miniature end-plate potentials at the frog and crayfish neuromuscular junctions, without causing detectable ultrastructural changes. We report here that addition of venom from the related annelid Glycera dibranchiata to one side of a lipid bilayer results in the formation of ion-permeable channels in the membrane. The channel forming activity was found in the void volume of a Sephadex G-25 column (mol. wt. greater than 5000). The conductance of a single channel is about 350 pmho in 0.1 M NaCl and is ohmic. The channels exhibit moderate (but not ideal) cation selectivity in NaCl or KCl gradients. Other selectivity measurements suggest that Ca2+ and Mg2+ are also permeable. The channels show a slight voltage sensitivity. The steady state conductance at--70 mV (side opposite venom) is about 5 times the conductance at + 70 mV. We suggest that these channels in the venom may evoke transmitter release at neuromuscular junctions either by (1) depolarizing the pre-synaptic terminal and thus opening voltage-dependent Ca2+ channels, or (2) directly allowing Ca2+ to enter the terminal. Black widow spider venom is known to produce similar effects on neuromuscular junctions and lipid bilayers. The single channel conductances and ionic selectivities of the channels found in the venoms of Glycera and Latrodectus are strikingly similar. Taken together, these results suggest that channel formation can explain the electrophysiologic effects of these two different venoms.

Continuous determination by a chemiluminescent method of acetylcholine release and compartmentation in Torpedo electric organ synaptosomes

The detection of acetylcholine (ACh) with a chemiluminescent procedure enables one to follow continuously the release of transmitter from stimulated synaptosomes and to study the compartmentation of ACh in resting and active nerve terminals. A compartment of ACh liberated almost entirely by a single freezing and thawing could be directly measured and compared with a compartment of ACh resistant to several cycles of freezing and thawing but liberated by a detergent (60-70% of the total). It is the compartment liberated by freezing and thawing that is reduced when synaptosomes are stimulated. Up to half the total synaptosomal ACh content is readily releasable provided the calcium entry is maintained, or if a strong releasing agent such as the venom of Glycera convoluta is used. In addition, it is shown that synaptosomes contain only negligible amounts of choline, and that the proportion of the two ACh compartments is not influenced by changing extracellular calcium just before their determination.

Simultaneous release of acetylcholine and ATP from stimulated cholinergic synaptosomes

The release of acetylcholine (ACh) and ATP from pure cholinergic synaptosomes isolated from the electric organ of Torpedo was studied in the same perfused sample. A presynaptic ATP release was demonstrated either by depolarization with KCl or after the action of a venom extracted from the annelid Glycera convoluta (GV). The release of ATP exhibited similar kinetics to that of ACh release and was therefore probably closely related to the latter. The ACh/ATP ratio in perfusates after KCl depolarization was 45; this was much higher than the ACh/ATP ratio in cholinergic synaptic vesicles, which was 5. The ACh/ATP ratio released after the action of GV was also higher than that of synaptic vesicles. These differences are discussed. The stoichiometry of that of synaptic vesicles. These differences are discussed. The stoichiometry of ACh and ATP release is not consistent with the view that the whole synaptic vesicle content is released by exocytosis after KCl depolarization, as is the case for chromaffin cells in the adrenal medulla.