Acetylcholine release from proteoliposomes equipped with synaptosomal membrane constituents

Mediatophore, a protein supporting quantal acetylcholine release

After having reconstituted in artificial membranes the calcium-dependent acetylcholine release step, and shown that essential properties of the mechanism were preserved, we purified from Torpedo electric organ nerve terminals a protein, the mediatophore, able to release acetylcholine upon calcium action. A plasmid encoding for Torpedo mediatophore was introduced into cells deficient for acetylcholine release and for the expression of the cholinergic genomic locus defined by the co-regulated choline acetyltransferase and vesicular transporter genes. The transfected cells became able to release acetylcholine in response to a calcium influx in the form of quanta. The cells had to be loaded with acetylcholine since they did not synthesize it, and without transporter they could not concentrate it in vesicles. We may then attribute the observed quanta to mediatophores. We know from previous works that like the release mechanism, mediatophore is activated at high calcium concentrations and desensitized at low calcium concentrations. Therefore only the mediatophores localized within the calcium microdomain would be activated synchronously. Synaptic vesicles have been shown to take up calcium and those of the active zone are well situated to control the diffusion of the calcium microdomain and consequently the synchronization of mediatophores. If this was the case, synchronization of mediatophores would depend on vesicular docking and on proteins ensuring this process.Key words: acetylcholine release, presynaptic proteins, quantal release, mediatophore, transfection.

Acetylcholine translocating protein: mediatophore at rat neuromuscular synapses

The neuromuscular synapses of the rat sternomastoid muscles contain a membrane protein, mediatophore, that endows artificial membranes with a calcium-dependent acetylcholine release mechanism. Mediatophore and choline acetylase had similar distributions along the muscle. Sciatic nerve membranes contain mediatophore, and a purified preparation was obtained from the nerve.

Transmitter release: prepackaging and random mechanism or dynamic and deterministic process

Stepwise variations in end-plate potential amplitudes that are also multiples of spontaneous miniature end-plate potentials (MEPPs) demonstrate a quantal nature of evoked transmitter release at the vertebrate neuromuscular junction. Both the number of quanta which form relatively small end-plate potentials (EPPs) and the time intervals between MEPPs were found to fit Poisson statistics. These observations suggested that the release process randomly liberates uniform quantities of transmitter. Initial studies showed that quantal size remained stable after seemingly high rates of release which was interpreted to indicate that a large store of equally sized, equally available, and independently releasable quanta are present in the nerve terminals. The observation of numerous presynaptic vesicles that contain transmitter provided a morphological basis for prepacked transmitter (i.e., quanta). However, physiological studies over the last 15 years have yielded data that are difficult to incorporate into the quantum-vesicle hypothesis. With normal conditions and during most treatments which increase the rate of release, two classes of MEPPs have been found and both show a substructure. The bell-MEPP class was characterized by Fatt and Katz and the smaller skew-MEPP class has been studied by Kriebel. The ratio of the two classes and substructure compositions of both classes are variable. Short series of MEPPs and unitary EPPs (U-EPPs) show preferred amplitudes and longer series of MEPPs and U-EPPs show stepwise variations in amplitude. Slow-MEPPs and giant MEPPs belong to the skew class and represent nearly synchronous bursts of smaller MEPPs. Transmitter packet formation, preferred amplitudes, stepwise variations in amplitudes, random-like distributions and organized bursts can be simulated by a simple deterministic system, the drop formation process, that is known for its periodic and chaotic behaviors which are determined by the single parameter of flow rate. MEPP intervals, sizes and classes, are also dependent on rates of release which demonstrate that the release process(es) is highly organized and sensitive to different conditions. We demonstrate that the processes of drop formation and release of a packet of transmitter have similar properties and that deterministic characteristics describe MEPP and U-EPP time dependencies and amplitude substructures. The data and model presented here suggest that packet size of acetylcholine may be determined at the moment of release.

Calcium-induced desensitization of acetylcholine release from synaptosomes or proteoliposomes equipped with mediatophore, a presynaptic membrane protein

A "fatigue" of acetylcholine (ACh) release is described in cholinergic synaptosomes stimulated with the calcium ionophore A23187 or gramicidin. A small conditioning calcium entry, which did not trigger a large ACh release, led to a decrease of transmitter release elicited by a second large calcium influx. This fatigue was half-maximal at approximately 30 microM external calcium and developed in a few minutes. In contrast, activation of release by calcium was very rapid and was half-maximal at approximately 0.5 mM external calcium. Activation and desensitization of release could be attributed to the recently identified presynaptic membrane protein, the "mediatophore." Proteoliposomes equipped with purified mediatophore showed a calcium-dependent activation and "fatigue" of ACh release similar to that of synaptosomes. It was found that the ionophore A23187 rapidly equilibrated internal and external calcium concentrations in proteoliposomes. Thus, the external calcium concentration gave the internal concentration required for activation or desensitization of proteoliposomal ACh release. The mediatophore showed remarkable calcium binding properties (20 sites/molecule) with a KD of 25 microM. The physiological implications of desensitization on the organization of release sites are discussed.

Cetiedil, a drug that inhibits acetylcholine release in Torpedo electric organ

The effects of cetiedil, a vasodilatator substance with reported anticholinergic properties, were examined on cholinergic presynaptic functions at the nerve electroplaque junction of Torpedo marmorata using either synaptosomes or slices of intact tissue. Cetiedil abolished the calcium-dependent release of acetylcholine (ACh) triggered by depolarization or by addition of A23187 ionophore, a finding localizing the site of action downstream from the calcium entry step. In addition, a direct effect on the release process itself was indicated by the observation that cetiedil blocks the release of ACh mediated by a recently isolated presynaptic membrane protein, the mediatophore, reconstituted into ACh-containing proteoliposomes. In all three preparations, ACh release was inhibited by cetiedil with a Ki of 5-8 microM. Under the conditions used in these release experiments, the synthesis of ACh and its compartmentation within the nerve terminals were not modified. However, the drug was able to reduce high-affinity choline uptake and vesicular ACh incorporation when it was given together with the radioactive precursor, a result showing that cetiedil has a broad inhibitory action on cholinergic uptake processes.

Surplus acetylcholine and acetylcholine release in the rat diaphragm

1. Skeletal muscles from rat, mouse and frog were incubated under different conditions and the amounts of acetylcholine (ACh) extractable from the tissue and released into the medium were determined by mass fragmentography. In some experiments measurements were made of the amounts of ACh ('bound' ACh) surviving in a muscle homogenate to which an excess of acetylcholinesterase had been added. In other experiments the membrane potentials, end-plate potentials (e.p.p.s), and miniature end-plate potentials (m.e.p.p.s) were studied. 2. During incubation in Ringer medium the ACh content of the rat hemidiaphragm usually did not change, but after inhibition of cholinesterase by soman the ACh content rose gradually from about 100 to 150 pmol to a plateau of about 400 pmol after 4 h. A similar formation of 'surplus ACh' after cholinesterase inhibition was found in the mouse diaphragm, but not in the frog sartorius muscle. 3. Surplus ACh accumulated predominantly in the end-plate region of the rat diaphragm. In muscles, 16-18 h after in vivo denervation, the capacity to form surplus ACh was decreased by more than 80%. 4. The amount of ACh diffusing from the resting hemidiaphragm into the incubation medium ('resting release') varied between 0.5 and 0.9 pmol min-1 in different experiments; it remained at the same level during accumulation of surplus ACh. It was reduced by more than 80% 16-18 h after denervation. 5. The amplitude of m.e.p.p.s and e.p.p.s did not increase while surplus ACh was accumulating. 6. Incubation of hemidiaphragms in Ringer solution containing [3H]choline caused the formation of [3H]ACh. Additional amounts of [3H]choline were incorporated into ACh when the nerve was stimulated for 60 min. However, incubation in the presence of soman (3,3-dimethyl-2-butylmethylphosphonofluoridate), in the absence of stimulation, did not cause an increase of the [3H]ACh content of the muscles. 7. From hemidiaphragms with active cholinesterase about 120 pmol ACh was lost after prolonged nerve stimulation or incubation with 50 mM-KCl in the presence of hemicholinium-3, and about 35 pmol remained in the tissue. In soman-treated muscles, containing surplus ACh, about as much ACh was released by nervous stimulation as from untreated hemidiaphragms, and much more ACh remained unreleased. 8. Transection of the muscle at both sides of the end-plate or incubation of intact muscles in the presence of 50 mM-KCl depolarized the muscle fibres to -35 and -31 mV, respectively. Surplus ACh was partially released by 50 mM-KCl, but not by muscle transection.(ABSTRACT TRUNCATED AT 400 WORDS)

Solubilization and partial purification of a presynaptic membrane protein ensuring calcium-dependent acetylcholine release from proteoliposomes

In previous work, it was shown that cytoplasmic acetylcholine decreased on stimulation of Torpedo electric organ or synaptosomes in a strictly calcium-dependent manner. This led to the hypothesis that the presynaptic membrane contained an element translocating acetylcholine when activated by calcium. To test this hypothesis, the presynaptic membrane constituents were incorporated into the membranes of liposomes filled with acetylcholine. The proteoliposomes thus obtained released the transmitter in response to a calcium influx. The kinetics and calcium dependency of acetylcholine release were comparable for proteoliposomes and synaptosomes. The presynaptic membrane element ensuring calcium-dependent acetylcholine release is most probably a protein, since it was susceptible to Pronase, but only when the protease had access to the intracellular face of the presynaptic membrane. Postsynaptic membrane fractions contained very low amounts of this protein. It was extracted from the presynaptic membrane under alkaline conditions in the form of a protein-lipid complex of large size and low density which was partially purified. The specificity of the calcium-dependent release for acetylcholine was tested with proteoliposomes filled with equal amounts of acetylcholine and choline or acetylcholine and ATP. In both cases, acetylcholine was released preferentially. After cholate solubilization and gel filtration, the protein ensuring the calcium-dependent acetylcholine release was recovered at a high apparent molecular weight (between 600,000 and 200,000 daltons), its apparent sedimentation coefficient being 17S after cholate elimination. This protein is probably an essential coin of the transmitter release mechanism. We propose to name it mediatophore.

Reconstitution of carrier-mediated choline transport in proteoliposomes prepared from presynaptic membranes of Torpedo electric organ, and its internal and external ionic requirements

Proteoliposomes made by a butanol-sonication technique from electric organ presynaptic membranes showed choline transport activity. In contrast to intact nerve terminals, the uptake of choline was dissociated from its conversion to acetylcholine in this preparation. The kinetics of choline uptake by proteoliposomes was best described by two Michaelis-Menten components. At a low concentration of choline, uptake was inhibited by hemicholinium-3 and required external Na+ and, thus, closely resembled high-affinity choline uptake by intact cholinergic nerve terminals. Choline transport could be driven by the Na+ gradient and by the transmembrane potential (inside negative) but did not directly require ATP. External Cl-, but not a Cl- gradient, was needed for choline transport activity. It is suggested that internal K+ plays a role in the retention of choline inside the proteoliposome. Proteoliposomes should prove a useful tool for both biochemical and functional studies of the high-affinity choline carrier.

High-affinity, sodium-gradient-dependent transport of choline into vesiculated presynaptic plasma membrane fragments from the electric organ of Torpedo marmorata and reconstitution of the solubilized transporter into liposomes

Vesiculated fragments of presynaptic plasma membranes have been isolated from the purely cholinergic electromotor nerve terminals of Torpedo marmorata. Synaptosomes, generated from the terminals by homogenization, were separated on a discontinuous Ficoll gradient and then lysed by osmotic shock at 2 degrees C, pH 8.5 in the presence of 0.1 mM MgCl2. These conditions for lysis were optimal for choline transport. Electron micrographs of lysed synaptosomes showed vesiculated membranes with diameters smaller than those of synaptosomes; occasionally, synaptic vesicles were observed attached to them. Intact mitochondria or synaptosomes and basal laminae were not present. High-affinity (KT = 1.7 microM) uptake of choline into these vesiculated membrane fragments showed: an absolute dependence on the Na+ gradient (outside greater than inside), a transient Na+-gradient-dependent accumulation of choline over the equilibrium concentration (over-shoot), electrogenicity and rheogenicity, since the uptake was further stimulated in the presence of a Na+ gradient by valinomycin, dependence on the presence of external Cl-, and partial dependence on a Cl- gradient (outside greater than inside), high-affinity (Ki = 25 nM) inhibition by hemicholinium-3 and temperature sensitivity. The plasma membranes were further purified by centrifugal density gradient fractionation on a 4-12% Ficoll gradient. Several enzymes and polypeptides copurified with the specific binding sites for choline present in the membranes. The fraction with the most binding sites was one denser than 12% Ficoll. This was also the fraction richest in acetylcholinesterase, 5'-nucleotidase and polypeptides of relative molecular mass, Mr (X 10(-3)) of greater than 200, 140, 68 (doublet), 57, 54 and 28. Acetylcholinesterase was positively identified as a Mr 68 000 component by immune blot. By contrast the ouabain-sensitive ATPase showed a negative correlation with choline binding sites. When the solubilized proteins of the vesiculated membranes were transferred to liposomes, they conferred on the latter the capacity to take up choline in a manner closely resembling its transport in natural membranes but with an initial (one minute) rate of uptake approximately 10-times greater per mg of protein. Several proteins were selectively transferred to the liposomes including ones of Mr (X 10(-3)) 34, 42, 47, 54, 60, 68, 92, 160 and greater than 200. The polypeptides of Mr (X 10(-3)) 140, 57 and 28 were lost in the transfer.(ABSTRACT TRUNCATED AT 400 WORDS)

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.