Isolation of pure cholinergic nerve endings from Torpedo electric organ. Evaluation of their metabolic properties

Synaptotagmin II and gangliosides bind independently with botulinum neurotoxin B but each restrains the other

Botulinum neurotoxin type B (BoNT/B) initiates its toxicity by binding to synaptotagmin II (SytII) and gangliosides GD1a and GT1b on the neural membrane. We synthesized two 27-residue peptides that carry the BoNT/B binding sites on mouse SytII (mSytII 37-63) or human SytII (hSytII 34-60). BoNT/B bound to these peptides, but showed substantially higher binding to mSytII peptide than to hSytII peptide. The mSytII peptide inhibited almost completely BoNT/B binding to synaptosomes (snps) and displayed a high affinity. BoNT/B bound strongly to mSytII peptide and binding was inhibited by the peptide. Binding of BoNT/B to snps was also inhibited (~80 %) by a larger excess of gangliosides GD1a or GT1b. The mSytII peptide inhibited very strongly (at least 80 %) the toxin binding to snps, while the two gangliosides were much less efficient inhibitors requiring much larger excess to achieve similar inhibition levels. Furthermore, gangliosides GD1a or GT1b inhibited BoNT/B binding to mSytII peptide at a much larger excess than the inhibition by mSytII peptide. Conversely, BoNT/B bound well to each ganglioside and binding could be inhibited by the correlate ganglioside and much less efficiently by the mSytII peptide. There was no apparent collaboration between mSytII peptide and either ganglioside. mSytII peptide displayed some protective activity in vivo in mice against a lethal BoNT/B dose. We concluded that SytII peptide and gangliosides bind independently but, with their binding sites on BoNT/B being spatially close, each can influence BoNT/B binding to the other due to regional conformational perturbations or steric interference or both. Ganglioside involvement in BoNT/B binding might help in toxin translocation and endocytosis.

Location of the synaptosome-binding regions on botulinum neurotoxin B

The regions of botulinum neurotoxin B (BoNT/B) involved in binding to mouse brain synaptosomes (snps) were localized. Sixty 19-residue overlapping peptides (peptide C31 consisted of 24 residues) encompassing BoNT/B H chain (residues 442-1291) were synthesized and used to inhibit binding of (125)I-labeled BoNT/B to snps. Synaptosome-binding regions were noncompeting and existed on both H(N) and H(C) domains of neurotoxin. At 37 °C, inhibitory activities on H(N) resided, in decreasing order, in peptides 638-656 (26.7%), 596-614 (18.2%), 512-530 (13.9%), 778-796 (13.8%), and 526-544 (11.6%). On H(C), activity resided in decreasing order in peptides 1170-1188 (44.6%), 1128-1146 (21.6%), 1184-1202 (18.6%), 1156-1174 (13.0%), 946-964 (11.8%), 1114-1132 (11.2%), 1100-1118 (6.2%), 876-894 (6.1%), 1268-1291 (4.6%), and 1226-1244 (4.3%). The 45 remaining H(N) and H(C) peptides had no activity. At 4 °C, peptide C24 (1170-1188) remained quite active (inhibiting, 31.2%), while activities of peptides N15, C21, and C25 were little under 10%. The snp-binding regions contained sites that bind synaptotagmin II and gangliosides. Despite the low degree of sequence homology, BoNT/B and BoNT/A display significant structural homology and appeared to bind in part to the same snp-binding regions. Binding of each labeled toxin to snps was inhibited ~50% by the other toxin, 70-72% by its correlate H(C), and by the H(C) of the other toxin [29% (BoNT/A by H(C) of B) or 32% (BoNT/B by H(C) of A)]. In the three-dimensional structure of BoNT/B, the greater part of H(C), one H(N) face, and part of the belt on the same side interact with snps. Thus, BoNT/B binds to snps through the H(C) head and employs regions on one H(N) face and the belt, reserving flexibility for the belt's unbound part to release the light chain. Most snp-binding regions coincide or overlap with blocking antibody (Ab)-binding regions explaining how such Abs prevent BoNT/B toxicity.

Synaptic vesicles control the time course of neurotransmitter secretion via a Ca²+/H+ antiport

We investigated the physiological role of the vesicular Ca2+/H+ antiport in rapid synaptic transmission using the Torpedo electric organ (a modified neuromuscular system). By inhibiting V-type H+-transporting ATPase (V-ATPase), bafilomycin A1 dissipates the H+ gradient of synaptic vesicles, thereby abolishing the Ca2+/H+ antiport driving force. In electrophysiology experiments, bafilomycin A1 significantly prolonged the duration of the evoked electroplaque potential. A biochemical assay for acetylcholine (ACh) release showed that the effect of bafilomycin A1 was presynaptic. Indeed, bafilomycin A1 increased the amount of radio-labelled ACh released in response to paired-pulse stimulation. Bafilomycin A1 also enhanced Ca2+-dependent ACh release from isolated nerve terminals (synaptosomes). The bafilomycin-induced electroplaque potential lengthening did not arise from cholinesterase inhibition, since eserine (which also prolonged the electroplaque potential) strongly decreased evoked ACh release. Bafilomycin A1 augmented the amount of calcium accumulating in nerve terminals following a short tetanic stimulation and delayed subsequent calcium extrusion. By reducing stimulation-dependent calcium accumulation in synaptic vesicles, bafilomycin A1 diminished the corresponding depletion of vesicular ACh, as tested using both intact tissue and isolated synaptic vesicles. Strontium ions inhibit the vesicular Ca2+/H+ antiport, while activating transmitter release at concentrations one order of magnitude higher than Ca2+ does. In the presence of Sr2+ the time course of the electroplaque potential was also prolonged but, unlike bafilomycin A1, Sr2+ enhanced facilitation in paired-pulse experiments. It is therefore proposed that the vesicular Ca2+/H+ antiport function is to shorten 'phasic' transmitter release, allowing the synapse to transmit briefer impulses and so to work at higher frequencies.

Physical and functional interactions of cyclophilin B with neuronal actin and peroxiredoxin-1 are modified by oxidative stress

Presynaptic actin was identified as a new Torpedo cyclophilin B partner captured in pull-down experiments and by coimmunoprecipitation. The cyclophilin B-actin pull-down interaction was insensitive to the blockade of peptidyl cis/trans prolyl isomerase and calcineurin activities and to the latrunculin A- and jasplakinolide-mediated perturbation of F-actin polymerization. Conversely, it was reduced by ATP and stimulated by a low Cu(2+) treatment of synaptosomes and by acrolydan-conjugated cyclophilin B. This Cu(2+)-induced stress, in parallel, stimulates the formation of GSH adducts with cysteines of synaptosomal actin followed by its deglutathionylation and its dimerization in the presence of higher Cu(2+) concentrations. The reversibility of the thiol processing of actin occurred in the same range of Cu(2+) concentrations that mediated a stronger cyclophilin B-actin interaction, suggesting cyclophilin B participation in antioxidant processes. Among 2-Cys-peroxiredoxin isoforms, mainly peroxiredoxin-1 was found in cell bodies and nerve endings. Functionally, both Torpedo and human peroxiredoxin-1 were activated in vitro by Torpedo cyclophilin B. Moreover, cyclophilin B, like thioredoxins, maintained an H(2)O(2)-dependent peroxidase activity of peroxiredoxin-1 in the presence of dithiothreitol. Thus, the monocysteinic Torpedo cyclophilin B is able to sustain peroxiredoxin-1 activity and might be involved in the presynaptic defense against oxidative stress affecting G-actin posttranslational changes and its redox signaling in nerve ending compartments.

Ionic dependence of the velocity of release of ATP from permeabilized cholinergic synaptic vesicles

Evidence is provided to show that synaptic vesicles have an internal matrix. Suspensions of cholinergic synaptic vesicles isolated from the electric organ of Torpedo marmorata fish were permeabilized in solutions containing low concentrations of Na(+) or Ca(2+). The release of ATP from the vesicular matrix was 10 times more effective with Ca(2+) than with Na(+). We ascertained whether these two cations induced a different velocity of release of ATP from the matrix. The release of ATP was monitored with the chemiluminescent reaction of luciferin-luciferase. The light signal generated was the result of the kinetics of ATP release of the enzymatic reaction. To overcome the kinetics of the enzymatic reaction, the light records were deconvoluted. The actual kinetics of ATP release of vesicles containing Na(+) or Ca(2+) were coincident. To validate this result, comparison was made with ATP release from intact nerve terminals which were already deconvoluted. The results show that the real time course of release is longer than that obtained from synaptic vesicles. This was as expected given that the release of neurotransmitters is due to successive molecular steps of synaptic vesicle exocytosis.

Comparative effects of aluminum and ouabain on synaptosomal choline uptake, acetylcholine release and (Na+/K+)ATPase

Closing the gap between adverse health effects of aluminum and its mechanisms of action still represents a huge challenge. Cholinergic dysfunction has been implicated in neuronal injury induced by aluminum. Previously reported data also indicate that in vivo and in vitro exposure to aluminum inhibits the mammalian (Na(+)/K(+))ATPase, an ubiquitous plasma membrane pump. This study was undertaken with the specific aim of determining whether in vitro exposure to AlCl(3) and ouabain, the foremost utilized selective inhibitor of (Na(+)/K(+))ATPase, induce similar functional modifications of cholinergic presynaptic nerve terminals, by comparing their effects on choline uptake, acetylcholine release and (Na(+)/K(+))ATPase activity, on subcellular fractions enriched in synaptic nerve endings isolated from rat brain, cuttlefish optic lobe and torpedo electric organ. Results obtained show that choline uptake by rat synaptosomes was inhibited by submillimolar AlCl(3), whereas the amount of choline taken up by synaptosomes isolated from cuttlefish and torpedo remained unchanged. Conversely, choline uptake was reduced by ouabain to a large extent in all synaptosomal preparations analyzed. In contrast to ouabain, which modified the K(+) depolarization evoked release of acetylcholine by rat, cuttlefish and torpedo synaptosomal fractions, AlCl(3) induced reduction of stimulated acetylcholine release was only observed when rat synaptosomes were challenged. Finally, it was observed that the aluminum effect on cuttlefish and torpedo synaptosomal (Na(+)/K(+))ATPase activity was slight when compared to its inhibitory action on mammalian (Na(+)/K(+))ATPase. In conclusion, inhibition of (Na(+)/K(+))ATPase by AlCl(3) and ouabain jeopardized the high-affinity (Na(+)-dependent, hemicholinium-3 sensitive) uptake of choline and the Ca(2+)-dependent, K(+) depolarization evoked release of acetylcholine by rat, cuttlefish and torpedo synaptosomal fractions. The effects of submillimolar AlCl(3) on choline uptake and acetylcholine release only resembled those of ouabain when rat synaptosomes were assayed. Therefore, important differences were found between the species regarding the cholinotoxic action of aluminum. The variability of (Na(+)/K(+))ATPase sensitivity to aluminum of cholinergic neurons might contribute to their differential susceptibility to this neurotoxic agent.

Inhibition by human sera of botulinum neurotoxin-A binding to synaptosomes: A new assay for blocking and non-blocking antibodies

The mouse protection assay (MPA), which is an in vivo assay, is currently the most widely used method for monitoring blocking antibodies (Abs) in botulinum neurotoxin (BoNT)-treated patients. In recent studies we found that a number of the regions on the heavy (H) subunit of BoNT/A that bind blocking mouse Abs coincided, or overlapped, with the regions that bind to mouse synaptosomes (snps). This suggested that blocking anti-BoNT/A Abs would be expected to inhibit BoNT/A binding to snps. In the present work, we analyzed sera from 58 cervical dystonia (CD) patients who had been treated with BOTOX (a preparation of BoNT/A serotype) for blocking Abs by MPA and by their abilities to inhibit in vitro the binding of 125I-labeled active BoNT/A or inactive toxin (toxoid) to mouse brain snps. With active 125I-labeled BoNT/A-snps binding, the MPA-positive sera (n = 30) displayed inhibition levels that were distinctly higher (mean = 21.1 +/- 5.8) than those obtained with MPA-negative sera (n = 28) (mean = -1.3 +/- 3.9; p < 0.0001) or control sera (n = 19) (mean = -3.4 +/- 2.8; p < 0.0001). Similarly, inhibition levels by MPA-positive sera of 125I-labeled toxoid snp-binding (mean = 48.6 +/- 8.7) were distinctly higher than inhibition by MPA-negative sera (mean=10.0+/-7.6; p < 0.0001) or control sera (mean = 1.8 +/- 6.9; p < 0.0001). Thus, using labeled active toxin or toxoid, the inhibition assay correlated very well with the MPA. The inhibitory activity of the non-protective sera generally correlated with the duration of survival after toxin challenge (correlation coefficients of inhibition: active toxin = 0.445; p = 0.0167; inactive toxoid = 0.774; p < 0.0001). It is concluded that the snp-inhibition assay reported here is reliable, reproducible and correlates very well with the MPA. It requires much less serum (0.75% of the amount needed for the MPA) and is considerably less costly than the MPA. With either 125I-labeled active toxin or toxoid, it is possible to distinguish CD sera that have blocking Abs from those that lack such Abs. Since the results with the toxoid were as discriminating as those of the active toxin, it would not even be necessary to use active toxin in these assays.

Synapsin associates with cyclophilin B in an ATP- and cyclosporin A-dependent manner

Immunophilins are ubiquitous enzymes responsible for proline isomerisation during protein synthesis and for the chaperoning of several membrane proteins. These activities can be blocked by the immunosuppressants cyclosporin A, FK506 and rapamycin. It has been shown that all three immunosuppressants have neurotrophic activity and can modulate neurotransmitter release, but the molecular basis of these effects is currently unknown. Here, we show that synapsin I, a synaptic vesicle-associated protein, can be purified from Torpedo cholinergic synaptosomes through its affinity to cyclophilin B, an immunophilin that is particularly abundant in brain. The interaction is direct and conserved in mammals, and shows a dissociation constant of about 0.5 microM in vitro. The binding between the two proteins can be disrupted by cyclosporin A and inhibited by physiological concentrations of ATP. Furthermore, cyclophilin B co-localizes with synapsin I in rat synaptic vesicle fractions and its levels in synaptic vesicle-containing fractions are decreased in synapsin knockout mice. These results suggest that immunophilins are involved in the complex protein networks operating at the presynaptic level and implicate the interaction between cyclophilin B and synapsins in presynaptic function.

Specific sorting of the a1 isoform of the V-H+ATPase a subunit to nerve terminals where it associates with both synaptic vesicles and the presynaptic plasma membrane

Vacuolar H+ATPase (V-ATPase) accumulates protons inside various intracellular organelles, generating the electrochemical proton gradient required for many vital cellular processes. V-ATPase is a complex enzyme with many subunits that are organized into two domains. The membrane domain that translocates protons contains a proteolipid oligomer of several c subunits and a 100 kDa a subunit. Several a-subunit isoforms have been described that are important for tissue specificity and targeting to different membrane compartments, and could also result in the generation of V-ATPases with different functional properties. In the present report, we have cloned the Torpedo marmorata a1 isoform. This isoform was found to be addressed specifically to nerve endings, whereas VATPases in the neuron cell bodies contain a different a-subunit isoform. In nerve terminals, the V-ATPase membrane domain is present not only in synaptic vesicles but also in the presynaptic plasma membrane, where its density could reach 200 molecules microm(-2). This V-ATPase interacts with VAMP-2 and with the SNARE complexes involved in synaptic vesicle docking and exocytosis.

Selective prion protein binding to synaptic components is modulated by oxidative and nitrosative changes induced by copper(II) and peroxynitrite in cholinergic synaptosomes, unveiling a role for calcineurin B and thioredoxin

Choline acetyltransferase (ChAT) and choline transport are decreased after nitrosative stress. ChAT activity is altered in scrapie-infected neurons, where oxidative stress develops. Cellular prion protein (PrPc) may play a neuroprotective function in participating in the redox control of neuronal environment and regulation of copper metabolism, a role impaired when PrPc is transformed into PrPSc in prion pathologies. The complex cross-talk between PrPc and cholinergic neurons was analyzed in vitro using peroxynitrite and Cu2+ treatments on nerve endings isolated from Torpedo marmorata, a model of the motoneuron pre-synaptic element. Specific interactions between solubilized synaptic components and recombinant ovine prion protein (PrPrec) could be demonstrated by Biacore technology. Peroxynitrite abolished this interaction in a concentration-dependent way and induced significant alterations of neuronal targets. Interaction was restored by prior addition of peroxynitrite trapping agents. Cu2+ (in the form of CuSO4) treatment of synaptosomes triggered a milder oxidative effect leading to a bell-shaped increase of PrPrec binding to synaptosomal components, counteracted by the natural thiol agents, glutathione and thioredoxin. Copper(II)-induced modifications of thiols in several neuronal proteins. A positive correlation was observed between PrPrec binding and immunoreactive changes for calcineurin B and its partners, suggesting a synergy between calcineurin complex and PrP for copper regulation.

The neuronal choline transporter CHT1 is regulated by immunosuppressor-sensitive pathways

The immunosuppressor cyclosporin A inhibits the peptidyl-prolyl-cis/trans-isomerase activity of cyclophilins and the resulting complex inhibits the phosphatase activity of calcineurin. Both enzymes were detected in peripheral nerve endings isolated from the electric organ of Torpedo and shown to be affected by 10 micro m cyclosporin A. Among the cholinergic properties studied, choline uptake was specifically inhibited by cyclosporin A to a maximum of 40%. Cyclosporin A decreased the rate of choline transport but not the binding of the non-transportable choline analogue hemicholinium-3, indicating that the number of membrane transporters was not affected. Through the use of two other immunosuppressors, FK506, which also inhibits calcineurin, and rapamycin, which does not, two different mechanisms of choline uptake inhibition were uncovered. FK506 inhibited the rate of choline transport, whereas rapamycin diminished the affinity for choline. The Torpedo homologue of the high affinity choline transporter CHT1 was cloned and its activity was reconstituted in Xenopus oocytes. Choline uptake by oocytes expressing tCHT1 was inhibited by all three immunosuppressors and also by microinjection of the specific calcineurin autoinhibitory domain A457-481, indicating that the phosphatase calcineurin regulates CHT1 activity and could be the common target of cyclosporin and FK506. Rapamycin, which changed the affinity of the transporter, may have acted through an immunophilin on the isomerization of critical prolines that are found in the tCHT1 sequence.

Role of mediatophore in connection with proteins of the active zone in synaptic transmission

Mediatophore is a protein purified from Torpedo electric organ synaptosomes, which translocates acetylcholine (ACh) upon calcium action after reconstitution in artificial membranes. After expression in transfected cells, it endows these cells with a calcium-dependent release mechanism displaying clear quantal properties. The role of mediatophore in synaptic transmission is discussed in relation to the ultrastructural organization of the active zone and the cytosolic high calcium microdomains that transiently appear after presynaptic membrane depolarization.

Neurotransmitter release at rapid synapses

The classical concept of the vesicular hypothesis for acetylcholine (ACh) release, one quantum resulting from exocytosis of one vesicle, is becoming more complicated than initially thought. 1) synaptic vesicles do contain ACh, but the cytoplasmic pool of ACh is the first to be used and renewed on stimulation. 2) The vesicles store not only ACh, but also ATP and Ca(2+) and they are critically involved in determining the local Ca(2+) microdomains which trigger and control release. 3) The number of exocytosis pits does increase in the membrane upon nerve stimulation, but in most cases exocytosis happens after the precise time of release, while it is a change affecting intramembrane particles which reflects more faithfully the release kinetics. 4) The SNARE proteins, which dock vesicles close to Ca(2+) channels, are essential for the excitation-release coupling, but quantal release persists when the SNAREs are inactivated or absent. 5) The quantum size is identical at the neuromuscular and nerve-electroplaque junctions, but the volume of a synaptic vesicle is eight times larger in electric organ; at this synapse there is enough ACh in a single vesicle to generate 15-25 large quanta, or 150-200 subquanta. These contradictions may be only apparent and can be resolved if one takes into account that an integral plasmalemmal protein can support the formation of ACh quanta. Such a protein has been isolated, characterised and called mediatophore. Mediatophore has been localised at the active zones of presynaptic nerve terminals. It is able to release ACh with the expected Ca(2+)-dependency and quantal character, as demonstrated using mediatophore-transfected cells and other reconstituted systems. Mediatophore is believed to work like a pore protein, the regulation of which is in turn likely to depend on the SNARE-vesicle docking apparatus.

The presynaptic calcium channel is part of a transmembrane complex linking a synaptic laminin (alpha4beta2gamma1) with non-erythroid spectrin

Nerve regeneration studies at the neuromuscular junction (NMJ) suggest that synaptic basal lamina components tell the returning axon where to locate neurotransmitter release machinery, including synaptic vesicle clusters and active zones. Good candidates for these components are the synaptic laminins (LNs) containing alpha4, alpha5, or beta2 chains. Results from a beta2 laminin knockout mouse have suggested a linkage of this extracellular laminin to cytosolic synaptic vesicle clusters. Here we report such a transmembrane link at the electric organ synapse, which is homologous to the NMJ. We immunopurified electric organ synaptosomes and found on their surface two laminins of 740 and 900 kDa. The 740 kDa laminin has a composition of alpha4beta2gamma1 (laminin-9). Immunostaining reveals that as in the NMJ, alpha4 and beta2 chains are concentrated at the electric organ synapse. Using detergent-solubilized synaptosomes, we immunoprecipitated a complex containing alpha4beta2gamma1 laminin, the voltage-gated calcium channel, and the cytoskeletal protein spectrin. Other presynaptic proteins such as 900 kDa laminin are not found in this complex. We hypothesize that alpha4beta2gamma1 laminin in the synaptic basal lamina attaches to calcium channel, which in turn is attached to cytosolic spectrin. Spectrin could then organize synaptic vesicle clusters by binding vesicle-associated proteins.

The presynaptic calcium channel is part of a transmembrane complex linking a synaptic laminin (alpha4beta2gamma1) with non-erythroid spectrin

Nerve regeneration studies at the neuromuscular junction (NMJ) suggest that synaptic basal lamina components tell the returning axon where to locate neurotransmitter release machinery, including synaptic vesicle clusters and active zones. Good candidates for these components are the synaptic laminins (LNs) containing alpha4, alpha5, or beta2 chains. Results from a beta2 laminin knockout mouse have suggested a linkage of this extracellular laminin to cytosolic synaptic vesicle clusters. Here we report such a transmembrane link at the electric organ synapse, which is homologous to the NMJ. We immunopurified electric organ synaptosomes and found on their surface two laminins of 740 and 900 kDa. The 740 kDa laminin has a composition of alpha4beta2gamma1 (laminin-9). Immunostaining reveals that as in the NMJ, alpha4 and beta2 chains are concentrated at the electric organ synapse. Using detergent-solubilized synaptosomes, we immunoprecipitated a complex containing alpha4beta2gamma1 laminin, the voltage-gated calcium channel, and the cytoskeletal protein spectrin. Other presynaptic proteins such as 900 kDa laminin are not found in this complex. We hypothesize that alpha4beta2gamma1 laminin in the synaptic basal lamina attaches to calcium channel, which in turn is attached to cytosolic spectrin. Spectrin could then organize synaptic vesicle clusters by binding vesicle-associated proteins.

VAMP (synaptobrevin) is present in the plasma membrane of nerve terminals

Synaptic vesicle docking and exocytosis require the specific interaction of synaptic vesicle proteins (such as VAMP/synaptobrevin) with presynaptic plasma membrane proteins (such as syntaxin and SNAP 25). These proteins form a stable, SDS-resistant, multimolecular complex, the SNARE complex. The subcellular distribution of VAMP and syntaxin within Torpedo electric organ nerve endings was studied by immunogoldlabeling of SDS-digested freeze-fracture replicas (Fujimoto, 1995). This technique allowed us to visualize large surface areas of the presynaptic plasma membrane and numerous synaptic vesicles from rapidly frozen nerve endings and synaptosomes. VAMP was found associated with synaptic vesicles, as also shown by conventional electron microscopy immunolabeling, and to the presynaptic plasma membrane (P leaflet). Syntaxin was also detected in the nerve ending plasma membrane, without gold labeling of synaptic vesicles. Comparison of gold particle densities suggests that the presynaptic plasma membrane contains 3 VAMP molecules per molecule of syntaxin. After biotinylation of intact synaptosomes, the synaptosomal plasma membrane was isolated on Streptavidin coated magnetic beads. Its antigenic content was compared to that of purified synaptic vesicles. VAMP was present in both membranes whereas syntaxin and SNAP 25 were highly enriched in the synaptosomal plasma membrane. This membrane has a low content of classical synaptic vesicle proteins (synaptophysin, SV2 and the vesicular acetylcholine transporter). The VAMP to syntaxin stoichiometry in the isolated synaptosomal membrane was estimated by comparison with purified antigens and close to 2, in accordance with morphological data. SDS-resistant SNARE complexes were detected in the isolated presynaptic membrane but absent in purified synaptic vesicles. Taken together, these results show that the presence of VAMP in the plasma membrane of nerve endings cannot result from exocytosis of synaptic vesicles, a process which could, as far as SNAREs are concerned, very much resemble homotypic fusion.

A syntaxin-SNAP 25-VAMP complex is formed without docking of synaptic vesicles

We show herein that syntaxin is already associated with SNAP 25 and VAMP during fast axonal transport, and in isolated synaptic vesicles, before docking of these secretory organelles at the active zones.

Control of the calcium concentration involved in acetylcholine release and its facilitation: an additional role for synaptic vesicles?

2,5-Diterbutyl-1,4-benzohydroquinone, a specific blocker of Ca2+-ATPase pumps, increased acetylcholine release from an identified synapse of Aplysia, as well as from Torpedo and mouse caudate nucleus synaptosomes. Because 2,5-diterbutyl-1,4-benzohydroquinone does not change the presynaptic Ca2+ influx, the enhancement of acetylcholine release could be due to an accumulation of Ca2+ in the terminal. This possibility was further checked by studying the effects of 2,5-diterbutyl-1,4-benzohydroquinone on twin pulse facilitation, classically attributed to residual Ca2+. While preventing the fast sequestration of Ca2+ by presynaptic organelles, 2,5-diterbutyl-1,4-benzohydroquinone magnified both twin pulse facilitation observed under low extracellular Ca2+ concentration and twin pulse dysfacilitation observed under high extracellular Ca2+ concentration. Thus, it is concluded that 2,5-diterbutyl-1,4-benzohydroquinone, by preventing Ca2+ buffering near transmitter release sites, modulates acetylcholine release. As 2,5-diterbutyl-1,4-benzohydroquinone was also shown to decrease by 50% the uptake of 45Ca2+ by isolated synaptic vesicles, we propose that synaptic vesicles can control the presynaptic Ca2+ concentration triggering the release of neurotransmitter.

In vitro reconstitution of neurotransmitter release

The vesicular hypothesis has stimulated fruitful investigations on many secreting systems. In the case of rapid synaptic transmission, however, the hypothesis has been found difficult to reconcile with a number of well established observations. Brief impulses of transmitter molecules (quanta) are emitted from nerve terminals at the arrival of an action potential by a mechanism which is under the control of multiple regulations. It is therefore not surprising that quantal release could be disrupted by experimental manipulation of a variety of cellular processes, such as a) transmitter uptake, synthesis, or transport, b) energy supply, c) calcium entry, sequestration and extrusion, d) exo- or endocytosis, e) expression of vesicular and plasmalemmal proteins, f) modulatory systems and second messengers, g) cytoskeleton integrity, etc. Hence, the approaches by "ablation strategy" do not provide unequivocal information on the final step of the release process since there are so many ways to stop the release. We propose an alternate approach: the "reconstitution strategy". To this end, we developed several preparations for determining the minimal system supporting Ca2+-dependent transmitter release. Release was reconstituted in proteoliposomes, Xenopus oocytes and transfected cell lines. Using these systems, it appears that a presynaptic plasmalemmal proteolipid, that we called mediatophore should be considered as a key molecule for the generation of transmitter quanta in natural synapses.

Polarized sorting of nicotinic acetylcholine receptors to the postsynaptic membrane in Torpedo electrocyte

Several regulatory mechanisms contribute to the accumulation and maintenance of high concentrations of acetylcholine receptors (AChR) at the postsynaptic membrane of the neuromuscular junction, including compartmentalized gene transcription, targeting, clustering and anchoring to the cytoskeleton. The targeting of the AChR to the postsynaptic membrane is likely to involve a polarized sorting in the exocytic pathway. In this work, we used the electrocyte of Torpedo marmorata electric organ to study the intracellular trafficking of neosynthesized AChR and its delivery to the postsynaptic membrane. Gradient centrifugation and immunoisolation techniques have led to the isolation of two populations of post-Golgi transport vesicles (PGVs) enriched in proteins of either the innervated (AChR) or non-innervated (Na,K-ATPase) membrane domains of the cell. Immunolabelling of these vesicles at the EM level disclosed that very few PGVs contained both proteins. In AChR-enriched vesicles, high sialylation of AchR molecules, an expected post-translational modification of proteins exiting the trans-Golgi network, and the presence of a marker of the exocytic pathway (Rab6p), indicate that these vesicles are carriers engaged in the Golgi-to-plasma membrane transport. These data suggest that AChR and Na,K-ATPase are sorted intracellularly most likely within the trans-Golgi network. Furthermore, EM analysis and immunogold-labelling experiments provided in situ evidence that the AChR-containing PGVs are conveyed to the postsynaptic membrane, possibly by a microtubule-dependent transport mechanism. Our data therefore provide the first evidence that the targeting of receptors for neurotransmitters to synaptic sites could be contributed by intracellular sorting and polarized delivery in the exocytic pathway.

Acetylcholine release and the cholinergic genomic locus

Choline acetyltransferase and vesicular acetylcholine-transporter genes are adjacent and coregulated. They define a cholinergic locus that can be turned on under the control of several factors, including the neurotrophins and the cytokines. Hirschprung's disease, or congenital megacolon, is characterized by agenesis of intramural cholinergic ganglia in the colorectal region. It results from mutations of the RET (GDNF-activated) and the endothelin-receptor genes, causing a disregulation in the cholinergic locus. Using cultured cells, it was shown that the cholinergic locus and the proteins involved in acetylcholine (ACh) release can be expressed separately ACh release could be demonstrated by means of biochemical and electrophysiological assays even in noncholinergic cells following preloading with the transmitter. Some noncholinergic or even nonneuronal cell types were found to be capable of releasing ACh quanta. In contrast, other cells were incompetent for ACh release. Among them, neuroblastoma N18TG-2 cells were rendered release-competent by transfection with the mediatophore gene. Mediatophore is an ACh-translocating protein that has been purified from plasma membranes of Torpedo nerve terminal; it confers a specificity for ACh to the release process. The mediatophores are activated by Ca2+; but with a slower time course, they can be desensitized by Ca2+. A strictly regulated calcium microdomain controls the synchronized release of ACh quanta at the active zone. In addition to ACh and ATP, synaptic vesicles have an ATP-dependent Ca2+ uptake system; they transiently accumulate Ca2+ after a brief period of stimulation. Those vesicles that are docked close to Ca2+ channels are therefore in the best position to control the profile and dynamics of the Ca2+ microdomains. Thus, vesicles and their whole set of associated proteins (SNAREs and others) are essential for the regulation of the release mechanism in which the mediatophore seems to play a key role.

Nitric oxide and peroxynitrite affect differently acetylcholine release, choline acetyltransferase activity, synthesis, and compartmentation of newly formed acetylcholine in Torpedo marmorata synaptosomes

Recent reports proposed that nitric oxide was a modulator of cholinergic transmission. Here, we examined the role of NO on cholinergic metabolism in a model of the peripheral cholinergic nervous synapse: synaptosomes from Torpedo electric organ. The presence of NO synthase was immunodetected in the cell bodies, in the nerve ending area of nerve-electroplate tissue and in the electroplates. Exogenous source of NO was provided from SIN1, a donor of NO and O2-., and an end-derivative peroxynitrite (ONOO-). SIN1 increased calcium-dependent acetylcholine (ACh) release induced by KCl depolarization or a calcium ionophore A23187. The formation of ONOO- was continuously followed by a new chemiluminescent assay. The addition of superoxide dismutase, that decreases the formation of ONOO-, did not impair the stimulation of ACh release, suggesting that NO itself was the main stimulating agent. When the endogenous source of NO was blocked by proadifen, an inhibitor of cytochrome P450 activity of NO synthase, both KCl- and A23187-induced ACh release were abolished; nevertheless, the inhibitor Ng-monomethyl-L-arginine did not modify ACh release when applied in a short time duration of action. Both NO synthase inhibitors reduced the synthesis of ACh from the radioactive precursor acetate and its incorporation into synaptic vesicles as did ONOO- chemically synthesized or formed from SIN1. In addition, choline acetyltransferase activity was strongly inhibited by ONOO- and SIN1 but not by the NO donors SNAP and SNP or, by NO synthase inhibitors. Altogether these results indicate that NO and ONOO modulate presynaptic cholinergic metabolism in the micromolar range, NO (up to 100 microM) being a stimulating agent of ACh release and ONOO- being an inhibitor of ACh synthesis and choline acetyltransferase activity.

Characterization of presynaptic proteins involved in synaptic vesicle exocytosis in the nervous system of Torpedo marmorata

Synaptobrevin, SNAP-25 and syntaxin (SNAP receptor proteins) are molecular components that play a key role in the exocytotic machinery of synaptic vesicles. Their presence, distribution and interactions are reported in central and peripheral nervous systems of the electric fish Torpedo marmorata. These three proteins form a protein complex in all the nervous system regions tested, including the electric lobe and the electric organ which is innervated by pure cholinergic nerve terminals. Immunoblot analysis revealed a double protein pattern of SNAP-25 in the anterior brain and cerebellum, although a single protein band corresponding to SNAP-25 was observed in the electromotor system. Moreover, SNAP-25 showed a differential distribution in the electromotor system. It was present along nerve fibres and terminals that innervated the electric organ but it was not detected in nerve terminals at the electric lobe. Immunoisolation experiments using anti-synaptobrevin antibodies showed a tissue-specific co-existence of SNAP-25 and syntaxin with synaptobrevin in the immunoisolated organelles. In conclusion, the molecular components of the exocytotic machinery are shown to be conserved in Torpedo, although some differences mainly on SNAP-25, suggest a potential diversity in the regulation of neurosecretion.

Evidence to suggest that extracellular acetate is accumulated by rat hippocampal cholinergic nerve terminals for acetylcholine formation and release

It is well established that extracellular choline is transported into central cholinergic nerve terminals by 'high' and 'low' affinity processes to form the neurotransmitter acetylcholine (ACh). The intent of the present investigation was to ascertain whether extracellular acetate might also be transported into central cholinergic nerve terminals to form ACh. To test this possibility, rat hippocampal tissue was incubated with varying concentrations of extracellular [1-(14)C]acetate (0.1-100 microM) and the uptake of [1-(14)C]acetate and the amount of [14C]ACh formed by the tissue determined. The results indicated that the uptake of extracellular [1-(14)C]acetate was temperature-dependent and saturable having an apparent Michaelis constant (Km) of 22 microM. The formation of [14C]ACh in the tissue as a function of extracellular [1-(14)C]acetate appeared to occur by both 'high' and 'low' affinity processes with apparent Km values of 0.5 and 19.6 microM, respectively. In other experiments, three inhibitors (lithium, allicin and sodium) of acetyl CoA synthetase (EC 6.2.1.1 acetate: CoA ligase), the enzyme which converts acetate to acetyl CoA when ATP and CoA are present, inhibited [1-(14)C]acetate uptake and the amount of [14C]ACh formed from that [1-(14)C]acetate. Additionally, vesamicol, an inhibitor of ACh transport into synaptic vesicles, blocked the filling of a synaptic vesicle-enriched fraction of hippocampal tissue with newly synthesized [14C]ACh formed from extracellular [1-(14)C]acetate. High K+ depolarization of hippocampal tissue loaded with extracellular [1-(14)C]acetate not only increased the synthesis but also the release of [14C]ACh. These results suggest that extracellular acetate is recycled by rat hippocampal cholinergic nerve terminals for the formation and release of ACh. They also suggest that the enzyme acetyl CoA synthetase mediates extracellular acetate uptake into hippocampal cholinergic nerve terminals by metabolizing it to acetyl CoA and thereby creating a diffusion gradient for it to follow.

Association of syntaxin with SNAP 25 and VAMP (synaptobrevin) in Torpedo synaptosomes

Two proteins of the presynaptic plasma membrane, syntaxin and SNAP 25, and VAMP/ synaptobrevin, a synaptic vesicle membrane protein, form stable protein complexes which are involved in the docking and fusion of synaptic vesicles at the mammalian brain presynaptic membrane. Similar protein complexes were revealed in an homogeneous population of cholinergic synaptosomes purified from Torpedo electric organ by combining velocity sedimentation and immunoprecipitation experiments. After CHAPS solubilization, virtually all the nerve terminal syntaxin was found in the form of large 16 S complexes, in association with 65% of SNAP 25 and 15% of VAMP. Upon Triton X100 solubilization, syntaxin was still recovered in association with SNAP 25 and VAMP but in smaller 8 S complexes. A small (2-5%) percentage of the nerve terminal 15 kDa proteolipid subunit of the v-H+ATPase and of mediatophore was copurified with syntaxin, using two different antisyntaxin monoclonal antibodies. The use of an homogeneous population of peripheral cholinergic nerve terminals allowed us to extend results on the composition of the brain presynaptic protein complexes to the Torpedo electric organ synapse, a model of the rapid neuromuscular synapses.

Ca(2+)-dependent changes of acetylcholine release and IP3 mass in Torpedo cholinergic synaptosomes

The aim of the present study was to investigate possible changes of inositol 1,4,5-trisphosphate (IP3) mass in Torpedo cholinergic synaptosomes in conditions promoting stimulated acetylcholine (ACh) release. For this purpose, we used a radioreceptor IP3 mass assay and a chemiluminescent method for ACh detection. Torpedo cholinergic synaptosomes have consistent IP3 mass levels under resting conditions. The IP3 mass was neither modified by changes in external Ca2+ nor by a Ca(2+)-free medium containing EGTA. IP3 mass and ACh release, measured in the same conditions and in parallel, were increased by depolarization with high K+ and by the ionophores A-23/87 and gramicidin-D in a manner dependent on external Ca2+ emphasizing that Ca2+ entry, independently of the influx mechanism involved, leads to an IP3 increase. The phospholipase C beta inhibitors U-73122 and U-73343 reduced K(+)-stimulated IP3 levels while K(+)-evoked ACh release was almost completely blocked suggesting an additional effect of these drugs on depolarization-neurotransmitter secretion coupling. The effect reported showing an increase of IP3 by agents that stimulate ACh release may suggest a possible link between IP3 metabolism and the neurotransmitter release mechanism. However, such a link is probably not a direct one as implied by the results obtained with the inhibitors of phospholipase C.

Activation and desensitisation of acetylcholine release by zinc at Torpedo nerve terminals

Treatment with 100 or 250 microM ZnCl2 irreversibly blocked neurotransmission in the Torpedo electric organ by inhibiting acetylcholine (ACh) release. In Zn2+-treated tissue, release failure did not result from impairment of Ca2+ entry since stimulation still provoked an accumulation of Ca2+. Also pretreatment of isolated synaptosomes with Zn2+ inhibited to the same extent the release elicited by KCl-evoked depolarisation and the release elicited by using the Ca2+ ionophore A23187. On the other hand, after application of A23187, Zn2+ by itself efficiently triggered ACh release from synaptosomes. This dual effect of Zn2+ was also observed to occur in proteoliposomes equipped with mediatophore (a protein of the presynaptic membrane characterised by its capability to support Ca2+-dependent transmitter release). Hence, Zn2+ mimicked two fundamental actions of Ca2+ on nerve terminals, which are: (1) the immediate activation of release, and (2) a more slowly developing desensitisation of release. Zn2+ was more powerful than Ca2+ for both actions. It is concluded that the dual action of Zn2+ on the mediatophore protein accounts at least in part for its complex effects on neurotransmission.

Characterization of alpha-neurotoxin and phospholipase A2 activities from Micrurus venoms. Determination of the amino acid sequence and receptor-binding ability of the major alpha-neurotoxin from Micrurus nigrocinctus nigrocinctus

New World elapids are coral snakes that belong to the genus Micrurus, and for which the venom biochemistry is mostly unknown. Analysis has been difficult because the coral snakes produce small quantities of venom. Clinical observations following bites show mainly neurotoxic effects. Experimentally, cardiotoxic, haemolytic and myotoxic activities are also reported. An experimental approach, using reverse-phase high-performance liquid chromatography and specific assays for alpha-neurotoxin and phospholipase A2 activities, was conducted on milligram quantities of venoms from three Micrurus species from Costa Rica; M. nigrocinctus nigrocinctus, M. alleni yatesi and M. multifasciatus. Neurotoxicity was determined by competition binding experiments with the Torpedo marmorata acetylcholine receptor. Phospholipase A2 activity was measured by fluorimetry using a pyrene lipid substrate. In this way, we purified and characterized seven alpha-neurotoxins, five phospholipases A2 and four toxin homologs. The amino acid sequence of the major alpha-neurotoxin from M. nigrocinctus nigrocinctus venom was fully determined and compared to Old Word representatives. Distance matrix data were generated to set up phylogeny relationships among elapid short-chain alpha-neurotoxins, which proved to be in accordance with the taxonomic classification and geographical distribution of snake species.

Functional reconstitution of KCl-evoked, Ca(2+)-dependent acetylcholine release system in Xenopus oocytes microinjected with presynaptic plasma membranes and synaptic vesicles

We have developed a new method for the generation of functionally active presynaptic chimeras in Xenopus laevis oocytes. Frog oocytes injected with presynaptic subcellular fractions extracted from the electric organ of Torpedo marmorata release acetylcholine in a calcium-dependent manner upon chemical stimulation. Neither oocytes injected without presynaptic plasma membranes nor oocytes injected with ghost erythrocyte plasma membrane instead of presynaptic plasma membrane release acetylcholine. This suggests that specific presynaptic components necessary for KCl-evoked, Ca(2+)-dependent acetylcholine release become functionally integrated in the Xenopus laevis oocytes. Moreover, rhodaminated presynaptic plasma membranes and the synaptic vesicle protein synaptophysin are detected on the oocyte surface by fluorescence or immunofluorescence, respectively, showing that the injected presynaptic components are incorporated into the membrane of the frog oocyte. Furthermore, Botulinum neurotoxin type A, a specific blocker of acetylcholine release in the neuromuscular junction, inhibits the neurotransmitter release from the chimerical oocytes. This suggests that targets for toxin action are also functionally incorporated in the oocyte upon injection of membranous presynaptic components. Our results show that oocytes injected with presynaptic components behave as cholinergic nerve ending chimeras, at least in terms of neurotransmitter release and toxin targets. The system bypasses some problems associated with messenger RNA expression because not only proteins, but native presynaptic components are incorporated. This new technique may provide a useful approach for electrophysiological and pharmacological studies in order to characterize the synaptic transmission.

Cyclosporin A affects functions concerning acetylcholine release of cholinergic Torpedo synaptosomes

The effect of cyclosporin A was investigated on Torpedo synaptosomes. Cyclosporin A inhibits KCl-evoked acetylcholine release (up to 50% at 1 mu M) and was inactive on acetylcholine release induced by a Ca2+ ionophore, A23187. Interestingly, when the synaptosomes were pretreated with cyclosporin A, this immunosuppressor did abolish the modulation of A23187-induced acetylcholine release produced by two other drugs, cetiedil (alpha-cyclohexyl-3-thienyl acetic acid 2-(hexahydro-1H-azepin-1-yl) ethyl ester, citrate salt) and MR16728 (N-(N'-hexamethylene imino)-propyl-phenyl-cyclohexyl-methyl acetamide, chlorhydrate), which were previously shown to be inhibitory and stimulatory, respectively. Moreover, cyclosporin A and MR16728 are competitive inhibitors of [3H]cetiedil binding to purified synaptosomal presynaptic membranes (dissociation constant of 181.9 nM). These results suggest that presynaptic proteins involved in acetylcholine release (directly or indirectly through cyclophilin) are potential targets of cyclosporin A in Torpedo synaptosomes.

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.

Tetanus toxin inhibits spontaneous quantal release and cleaves VAMP/synaptobrevin

Tetanus toxin decreased the frequency of spontaneous events at the electric organ of Torpedo marmorata. This reduction was up to 70% in poisoned electric organ. According to distribution analysis of miniature end plate currents, only a subpopulation of events which have small amplitudes were recorded after poisoning. Furthermore, isolated cholinergic nerve terminals showed a decrease in VAMP/synaptobrevin when poisoned with tetanus toxin under similar conditions. The relationship between the two effects of the toxin, i.e. inhibition of vesicle exocytosis and peptidase activity on synaptobrevin, is discussed.

Evaluation of the evidence for ion channels in synaptic vesicles

Synaptic vesicles (SVs) have been the focus of much research for many years, however only recently have ion channels from SV membranes been reported. There is now convincing evidence that SVs contain ion channels. This conclusion is based on direct experimental results from several different laboratories using the patch clamp or planar lipid bilayer technique on SVs and neurosecretory granules (NSG). Some limitations of patch clamping and of fusing synamptic vesicles to a bilayer are described and the advantages of the nystatin/ergosterol fusion method are presented. Six different channels appear to exist in SV (or NSG) membranes. Two large channels (250 and 154 pS) have been observed in SVs isolated from mammalian brain, two channels (180 and 13 pS) from Torpedo electric organ, and two channels (130 and 30-40 pS) from NSG. The three larger channels from each set (250, 180 and 130 pS7) are novel in that they have a subconductance state. The 154 pS channel has been identified as synaptophysin but the identity and function of the other channels is unknown. Although some of the channels are gated by voltage, only the 130 pS channel is modulated by Ca2+. Further knowledge of what regulates these channels is mandatory if we are to determine the physiological significance of these channels.

Tetanus toxin mechanism of action in Torpedo electromotor system: a study on different steps in the intoxication process

The mechanism of action of tetanus toxin was characterized in the electromotor system of Torpedo marmorata either at peripheral and central nervous system. The consecutive steps of the intoxication pathway were observed: (i) [125I]tetanus toxin specifically bound to neuronal plasma membranes isolated both from electric organ and electric lobe of Torpedo, exhibiting one and two binding sites respectively; (ii) [125I]tetanus toxin was internalized into nerve terminals and retrogradely transported to the electric lobe after its injection in the electric organ; (iii) finally, intracellular effect of tetanus toxin was studied either at electric organ and electric lobe membrane fractions. In both preparations tetanus toxin cleaved synaptobrevin, as detected by immunoblotting methods. In conclusion, our findings exhibit the presence of two different populations of acceptors for tetanus toxin in central and peripheral nervous system and show that synaptobrevin cleavage may account for intracellular toxicity in Torpedo.

Tacrine-induced increase in the release of spontaneous high quantal content events in Torpedo electric organ

1. The anticholinesterases, tacrine (100 microM) and physostigmine (60 microM) had different effects on the amplitude distribution and kinetics of miniature endplate currents (m.e.p.cs) recorded extracellularly from the electric organ of Torpedo marmorata. 2. Tacrine increased the ratio of giant miniatures (larger than 4 mV of amplitude) to more than 20% of recorded spontaneous events. In the presence of physostigmine such events represented only 4%. 3. Both tacrine and physostigmine increased the rise time and the decay phase of normal-sized m.e.p.cs when compared to control conditions. Both effects were significantly greater for tacrine. 4. We have tested the specificity of the tacrine effect on ectoenzyme activities associated with plasma membranes of these pure cholinergic nerve endings. Tacrine does not act unspecifically on every ectoenzyme, because it is not able to block the ectoapyrase activity even at a concentration 100 fold greater than that required to inhibit 94% of AChE. 5. We conclude that the differential effects of tacrine and physostigmine can be explained in terms of undetermined presynaptic actions of tacrine, while comparable effects of the two compounds can be explained through a shared anticholinesterase activity.

Spontaneous release of acetylcholine from Torpedo synaptosomes: effect of cetiedil and its analogue MR 16728

The effects of cetiedil and its analogue MR 16728 were examined on spontaneous acetylcholine release measured with a chemiluminescent assay using choline oxidase in a synaptosomal suspension obtained from Torpedo marmorata electric organ. Evoked acetylcholine release is inhibited by cetiedil, whereas this drug enhances spontaneous extracellular Ca(2+)-independent acetylcholine release (up to 340%). This effect was examined as a function of cetiedil concentration and incubation time. On the other hand, the analogue MR 16728, which enhances A23187-evoked acetylcholine release, also enhances spontaneous Ca(2+)-independent acetylcholine release. Cetiedil and MR 16728 effects on spontaneous acetylcholine release were also examined in the presence of Ca2+. Addition of Ca2+ enhanced spontaneous acetylcholine release by 75%, and cetiedil and MR 16728 stimulation was maintained but with different levels of enhancement. Thus, these results show that the processes responsible for evoked and spontaneous acetylcholine release are sensitive but in different ways to drugs of the cetiedil family.

High resolution labeling of cholinergic nerve terminals using a specific fully active biotinylated botulinum neurotoxin type A

We report here on the synthesis and characterization of a fully active biotinylated derivative of the botulinum neurotoxin type A. Different ratios of biotin: botulinum toxin were tested to optimize derivatizing conditions and a ratio of 35:1 was selected for further experiments. The average number of biotin groups per toxin molecule was estimated to be 7.8, occurring at both heavy and light chains, and almost all externally located and easily accessible to recognition by streptavidin. The modified toxin retained its toxicity and its ability to interact with biological membranes. Apart from its suitability for detection in Western blots and in microtiter well plates, biotinylated botulinum toxin proved to be adequate for morphological labeling studies at both light and electron microscopy. Peroxidase histochemistry in cryostat sections of intoxicated rat hemidiaphragm muscles showed a distinct labeling of end-plates. Electron microscopy studies were performed on the electric organ of Torpedo marmorata using colloidal gold-conjugated streptavidin for detection. After intoxication of electric organ fragments with the modified toxin, gold labels were found associated with the presynaptic plasma membrane of nerve terminals and with the membrane of synaptic vesicles. Moreover, the distribution of biotinylated botulinum toxin binding sites over the membrane of synaptosomes isolated from the electric organ of Torpedo and their relationship with intramembrane particles were analyzed using the replica-staining label-fracture technique. It was found that the toxin is never associated with intramembrane particles.

The same 15 kDa proteolipid subunit is a constituent of two different proteins in Torpedo, the acetylcholine releasing protein mediatophore and the vacuolar H+ ATPase

Using the monoclonal antibody 15K1, we have studied, at the cellular and subcellular levels, the distribution of a 15 kDa proteolipid, identified as the subunit of mediatophore, a presynaptic membrane protein able to release acetylcholine when activated by calcium. Aside from the electric lobe, the antigen distribution in the brain of Torpedo paralleled that of the synaptic vesicle antigen SV2 and did not appear to be related to that of acetylcholine and choline acetyltransferase. The 15 kDa proteolipid antigen was therefore present in all nerve endings and not restricted to cholinergic ones. At the ultrastructural level, on cholinergic nerve endings, the antigen was detected associated to synaptic vesicles and, to a lesser extent, to the presynaptic plasma membrane. Indeed, considering the high sequence homology between the mediatophore subunit (Birman et al., 1990) and the proteolipid subunit of the vacuolar type H+ ATPase, a major enzyme constituent of synaptic vesicles, this distribution was not surprising. To determine whether antibody 15K1 recognizes the vacuolar type H+ ATPase, we chose a non neuronal cell type which possesses a high content of this enzyme, the kidney proton secreting epithelial cells. Indeed, antibody 15K1 intensely labelled the apical plasma membrane of mitochondria rich epithelial cells in kidney tubules. A high density of the antigen was also found associated to intracellular membrane structures such as lysosomal multivesicular bodies, both in kidney epithelial cells and in electromotoneurons. The 15 kDa proteolipid antigen was associated with other vacuolar H+ ATPase subunits in kidney membranes which was not the case in presynaptic plasma membranes. This illustrates that the 15 kDa proteolipid antigen is a constituent of two different protein complexes, which exhibit very different functional properties.

Ganglioside composition of subcellular fractions, including pre- and postsynaptic membranes, from Torpedo electric organ

Gangliosides were isolated from four subcellular fractions of the electric organ of Torpedo marmorata: synaptosomes, presynaptic membranes, postsynaptic membranes, and synaptic vesicle membranes. This exploited a principal advantage offered by this tissue: facile separation of pre-and postsynaptic elements. Total ganglioside concentration in presynaptic membranes was approximately twice that of synaptosomes and 15 times that of postsynaptic membranes (47.7, 24.4, and 3.21 micrograms of lipid sialic acid per mg protein, respectively). Synaptic vesicle membranes had the highest overall concentration (78.9) relative to protein, but a concentration approximately comparable to that of presynaptic membranes when expressed relative to phospholipid. The thin-layer patterns of these two fractions were similar, both in terms of total pattern and the specific pattern of gangliotetraose structures as revealed by overlay with cholera toxin B subunit; these were notable for the paucity of monosialo structures and the virtual absence of GM1. Postsynaptic membranes, on the other hand, had a significantly higher content of monosialogangliosides including the presence of GM1. The synaptosomal pattern resembled that of the presynaptic membranes and synaptic vesicles. Thus, a clear difference in ganglioside pattern could be discerned between the pre- and postsynaptic elements of the electric organ.

Effect of N,N'-dicyclohexylcarbodiimide on compartmentation and release of newly synthesized and preformed acetylcholine in Torpedo synaptosomes

Using isolated cholinergic synaptosomes prepared from Torpedo electric organ, we studied the effects of N,N'-dicyclohexylcarbodiimide (DCCD) on acetylcholine (ACh) synthesis, compartmentation, and release after stimulation. Whereas ACh synthesis was unchanged, ACh compartmentation inside synaptosomes was affected by the presence of DCCD. In resting conditions, the uptake into the synaptic vesicle pool of newly synthesized ACh (i.e., [14C]ACh synthesized in the presence of the drug) was progressively and markedly inhibited as the duration of DCCD preincubation was increased, whereas compartmentation of endogenous ACh was unchanged in the presence of DCCD. After stimulation, the release of endogenous ACh from DCCD-treated synaptosomes was similar to that of control, in contrast to the release of [14C]ACh, which was markedly inhibited. This inhibition was observed whatever the conditions of stimulation used (gramicidin D, calcium ionophore A23187, or KCl depolarization). The study of the compartmentation of [14C]ACh during stimulation revealed a transfer of highly labeled ACh from the free to the bound ACh compartment in the presence of DCCD, suggesting the existence of several ACh subcompartments within the free and bound ACh pools. The present results are discussed in comparison with the previously reported effects of vesamicol (AH5183) on ACh compartmentation and release.

Thirty years of synaptosome research

Detached synapses (synaptosomes), first isolated by the author in 1958 and identified as such in 1960, are sealed presynaptic nerve terminals often with a portion of the target cell--sometimes amounting to a complete dendritic spine--adhering to their external surface. They can be prepared in high yield from brain tissue and also in decreasing yield from spinal cord, retina, sympathetic ganglia, myenteric plexus and electric organs. They are sealed structures which, under metabolizing conditions, respire, take up oxygen and glucose, extrude Na+, accumulate K+, maintain a normal membrane potential and, on depolarization, release transmitter in a Ca(2+)-dependent manner. They thus provide an excellent preparation with which to investigate synaptic function without the complications encountered with synapses in situ. They also serve as the parent fraction for preparations of synaptic vesicles and other synaptic components.

Agelenopsis aperta venom and FTX, a purified toxin, inhibit acetylcholine release in Torpedo synaptosomes

The presence of P-type calcium channels in synaptosomes prepared from electric organ of Torpedo marmorata was investigated by using the venom of Agelenopsis aperta, a toxin purified from it, FTX, and its synthetic analog. We analysed the action of these agents on acetylcholine release which was continuously followed using a chemiluminescent assay. Agelenopsis aperta venom, FTX and synthetic FTX inhibit acetylcholine release from synaptosomes induced by a presynaptic membrane depolarization with 60 mM KCl. A stronger inhibition of acetylcholine release was observed with the venom than with FTX (70 and 50%, respectively). Another way of triggering acetylcholine release from Torpedo synaptosomes is to insert in the presynaptic membrane a calcium ionophore A23187 which allows the bypass of the natural calcium channels. The venom of Agelenopsis aperta inhibits A23187-evoked acetylcholine release. Purified and synthetic FTX does not possess this property, suggesting that this inhibition of acetylcholine release was due to other toxins of the venom. Another type of pharmacological sensitivity of Torpedo calcium channels was also demonstrated using omega-conotoxin GVIA. At a concentration of 20 microM, this toxin was able to inhibit about 35% of KCl-evoked acetylcholine release. When FTX + omega-conotoxin GVIA were applied together, the inhibitory effect on KCl-evoked acetylcholine release was not significantly increased in comparison with the one observed with FTX alone. In conclusion, we examined the effect of different agents on acetylcholine release from Torpedo marmorata electric organ synaptosomes; acetylcholine release was elicited with KCl depolarization and followed continuously with a chemiluminescent assay.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel antagonist omega-conotoxin binds to intramembrane particles of isolated nerve terminals

Voltage-sensitive calcium channels play a key role in evoked neurotransmitter release and their distribution in presynaptic membranes can be critical for fast signalling at chemical synapses. Using a biotinylated derivative of the neuronal calcium channel antagonist, omega-conotoxin, and a combination of colloidal gold labeling and freeze-fracture techniques, we have labeled calcium channels present at the membrane of nerve terminals isolated from the electric organ of Torpedo marmorata. The biotinylated blocker exerts an inhibitory action on the high potassium-evoked release of adenosine triphosphate as the native toxin does and its interaction with biological membranes is specific as shown in displacement experiments. This study shows that an antagonist specific for voltage-activated calcium channels binds to intramembrane particles in presynaptic membranes, reinforcing the idea that these particles, concentrated at neurotransmitter release sites, effectively represent calcium channels.

The proportion of amphiphilic choline acetyltransferase in Drosophila melanogaster is higher than in rat or Torpedo and is developmentally regulated

We show that in the central nervous system of the fly, Drosophila melanogaster, choline acetyltransferase (ChAT) activity exists under two molecular forms, a soluble, hydrophilic form and a membrane-bound, amphiphilic form. This is based on the following demonstrations of differential solubilization and interaction with non-denaturing detergents: sequential extraction of Drosophila heads produced low-salt-soluble (83-87%) and detergent-soluble (6-7%) ChAT activity. Sedimentation in sucrose gradients of detergent-soluble ChAT was found to be influenced by the type of detergent present in the gradient (Triton X-100 and Brij 96). This was not the case for low-salt-soluble ChAT. To further confirm these findings, we subjected Drosophila heads to Triton X-114 fractionation. This method, which yielded 12% of amphiphilic ChAT activity, separates hydrophilic from amphiphilic proteins. Compared to central nervous tissue of rat and Torpedo electric lobes, Drosophila head contained the highest proportion of amphiphilic ChAT activity. Synaptosomes isolated from Torpedo electric organ exhibited higher levels of amphiphilic ChAT than did electric lobes. Of the three animal species analyzed here, the Torpedo amphiphilic enzyme was the most hydrophobic and the rat enzyme the least hydrophobic. The proportion of amphiphilic ChAT was analyzed during Drosophila development. The percentage of this activity increased about 7 times from embryo to larva and then remained constant until the adult fly age.

The effect of MR16728, a cetiedil analogue, on acetylcholine release in Torpedo synaptosomes

MR16728, a cetiedil analogue, enhanced acetylcholine (ACh) release (up to 145% of control) from Torpedo synaptosomes when the release was triggered by a Ca2+ ionophore, A23187 or ionomycin, in the presence of 4 mM Ca2+ in the release medium, but inhibited ACh release induced by KCl depolarization of the presynaptic membrane. MR16728 also inhibited Ca(2+)-ATPase activity measured in purified synaptosomal presynaptic membranes. We studied the stimulation by MR16728 as a function of its concentration; the half-maximal effect was reached at the concentration of 13.5 microM. Moreover the stimulation was more pronounced (up to 300%) when a low concentration of Ca2+ (in the 10-micromolar range) was added to the medium. The enhancement of ACh release was also observed in proteoliposomes having incorporated mediatophore, a presynaptic membrane protein, able to release ACh in a Ca(2+)-dependent manner. Thus, mediatophore is a potential presynaptic target for MR16728.

Calcium-dependent and -independent acetylcholine release from electric organ synaptosomes by pardaxin: evidence of a biphasic action of an excitatory neurotoxin

The effect of pardaxin, a new excitatory neurotoxin, on neurotransmitter release was tested using purely cholinergic synaptosomes of Torpedo marmorata electric organ. Pardaxin elicited the release of acetylcholine with a biphasic dose dependency. At low concentrations (up to 3 x 10(-7) M), the release was calcium-dependent and synaptosomal structure was well preserved as revealed by electron microscopy and measurements of occluded lactate dehydrogenase activity. At concentrations from 3 x 10(-7) M to 10(-5) M, the pardaxin-induced release of acetylcholine was independent of extracellular calcium, and occluded synaptosomal lactate dehydrogenase activity was lowered, indicating a synaptosomal membrane perturbation. Electron microscopy of 10(-6) M pardaxin-treated synaptosomes revealed nerve terminals depleted of synaptic vesicles and containing cisternae. At higher toxin concentrations (> or = 10(-5) M), there were striking effects on synaptosomal morphology and occluded lactate dehydrogenase activity, suggesting a membrane lytic effect. We conclude that, at low concentrations, this neurotoxin is a promising tool to investigate calcium-dependent mechanisms of neurotransmitter release in the nervous system.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)