The structure and stoichiometry of electric ray synaptic vesicles

The non-specific ion channel in Torpedo ocellata fused synaptic vesicles

1. Synaptic vesicles were isolated and fused into large structures with a diameter of more than 20 microns to characterize their ionic channels. The 'cell'-attached and inside-out configurations of the patch clamp technique were used. 2. Two types of ion channels were most frequently observed: a low conductance chloride channel and a high conductance non-specific channel. 3. The non-specific channel has a main conducting state and a substate. The main conducting state has a slope conductance of 246 +/- 15 pS (+/- S.E.M., n = 15), in the presence of different combinations of KCl and potassium glutamate. 4. From the reversal potentials of the current-voltage (I-V) relation, it was concluded that this channel conducts both Cl- and K+. 5. The non-specific channel is highly voltage dependent: under steady-state voltages it has a high open probability near 0 mV and does not inactivate; when the membrane is hyperpolarized (pipette side more positive), the open probability decreases dramatically. 6. Voltage pulses showed that upon hyperpolarization (from holding potentials between -20 and + 20 mV), the channels deactivated; when the membrane was stepped back to the holding potential, the channels reactivated rapidly. 7. In a number of experiments, when the pipette side was made more negative than the bath, the open probability also decreased. 8. Frequently, a substate with a conductance of about 44 +/- 4% (+/- S.E.M., n = 3) of the main state was detected. 9. We speculate that this non-specific ion channel may have different roles at the various stages of the life cycle of the synaptic vesicle. When the synaptic vesicle is an intracellular structure, it might help its transmitter-concentrating capacity by dissipating the polarization. After fusion with the surface membrane, it might constitute an additional conductance pathway, taking part in frequency modulation of synaptic transmission.

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)

Regulation of the vesamicol receptor in cholinergic synaptic vesicles by acetylcholine and an endogenous factor

Cholinergic synaptic vesicles obtained from Torpedo electric organ have an active transport system for acetylcholine (ACh). Linked to ACh transport is a cytoplasmically oriented receptor for the inhibitory drug (-)-trans-2-(4-phenylpiperidino)cyclohexanol (vesamicol, formerly AH5183). Storage of freshly isolated vesicles for several days leads to more vesamicol binding. This can be induced immediately by hyposmotic lysis of the vesicles, which reseal to form right-side-out ghosts. The increased drug binding was due to a twofold increase in the affinity and a 20% increase in the amount of the receptor expressed, probably as a result of the release of an endogenous factor. Binding of vesamicol to ghosts was specifically inhibited by exogenous ACh acting with a dissociation constant of 18 mM. This suggests that the vesamicol binding site probably is linked to a low-affinity ACh binding site that is different from the higher affinity transport binding site. Equilibrium and kinetic attempts to determine whether exogenous ACh acts on the outside or the inside of the ghost membrane to inhibit vesamicol binding failed because of rapid equilibration of exogenous ACh across the ghost membrane. It is argued that the endogenous factor released by hyposmotic lysis might be ACh. Potential roles for such a transmembrane signal regulating the vesamicol receptor are discussed.

Ganglioside composition of synaptic vesicles from Torpedo electric organ

We have studied the ganglioside content and pattern of synaptic vesicles isolated from the electric organs of two species of Torpedinidae, Torpedo californica and Torpedo marmorata. The ganglioside concentrations were high relative to protein content (77 and 58 micrograms of N-acetylneuraminic acid/mg of protein, respectively), owing to the low protein-to-lipid ratio; however, they were also appreciable in relation to phospholipid (15.6 and 10.0 micrograms of N-acetylneuraminic acid/mg of phospholipid). The fact that a membrane fraction that separated from synaptic vesicles of T. californica on a controlled-pore glass-bead column and constituted the main potential source of contamination in this preparation had a lower ganglioside content and a different TLC pattern than synaptic vesicles indicated the relatively high purity of the latter. Most of the gangliosides from synaptic vesicles of both species migrated on TLC in the vicinity of standards with three or more sialic acids. Synaptosomes from T. marmorata had a higher lipid N-acetylneuraminic acid/phospholipid ratio and a different TLC pattern than synaptic vesicles. Considering these results and other data appearing recently in the literature, we suggest that reexamination of synaptic vesicles from mammalian brain for the possible presence of gangliosides is warranted.

Selectivity and regulation in the phospholipase A2-mediated attack on cholinergic synaptic vesicles by beta-bungarotoxin

The total fatty acid composition of purified Torpedo californica electric organ synaptic vesicles was determined by GLC analysis of methyl esters. Limit amounts of fatty acids released by high concentrations of either beta-bungarotoxin (beta-BuTx) or Naja naja venom phospholipase A2 (PLA2) acting in deoxycholate are reported. The time and enzyme concentration dependence for beta-BuTx- and PLA2-induced release of fatty acids from intact synaptic vesicles indicate that PLA2 is 100- to 1,000-fold more active. The Ca2+ dependence for beta-BuTx-induced release of fatty acids also was determined. ATP inhibits beta-BuTx- but not PLA2-induced release of fatty acids from vesicles in a manner that can not be ascribed only to chelation of the required Ca2+. ATP, other nucleotides, and adenosine have complex effects on beta-BuTx-induced release of fatty acids from egg yolk phosphatidylcholine dispersed in deoxycholate. The results suggest that beta-BuTx-mediated hydrolysis of the cholinergic synaptic vesicle membrane is approximately 10- to 100-fold more effective at causing uncoupling of vesicles than is PLA2 and that the enzymatic activity of beta-BuTx is subject to regulation by nucleotide-like factors.

Stoichiometries of acetylcholine uptake, release, and drug inhibition in Torpedo synaptic vesicles: heterogeneity in acetylcholine transport and storage

Highly purified Torpedo electric organ synaptic vesicles form a 49 nM suspension at 1 mg protein/ml. Under active transport conditions hundreds of molecules of [3H]acetylcholine ([3H]ACh) can be accumulated per vesicle, which requires the ACh transporter to undergo multiple turnovers. The transport blocker trans-2-(4-phenylpiperidino)cyclohexanol (AH5183) has no effect on storage of endogenous ACh by vesicles. In contrast, AH5183, other blocking drugs, and nonradioactive ACh caused a rapid release of at least 30-63 molecules of newly transported [3H]ACh per vesicle. Thus AH5183 distinguishes recently transported "new" vesicular ACh from "old" endogenous ACh. l-AH5183 inhibits transport of ACh with a half-inhibitory concentration of 16 +/- 7 nM at 12 nM vesicles and 115 +/- 34 nM at 120 nM vesicles. With the assumption that AH5183 acts on a receptor in an unamplified manner about 2.7 or fewer receptors per vesicle need to be occupied to cause inhibition of ACh transport. The apparent amplification in the number of [3H]ACh molecules per vesicle that are released by AH5183 suggests that AH5183 inhibits ACh storage by an indirect mechanism that distinguishes new from old ACh.

Biochemical evidence that vesicles are the source of the acetylcholine released from stimulated PC12 cells

Treatment of PC12 cells with AH5183 at concentrations of 40 nM to 40 microM inhibited the loading of newly synthesized acetylcholine into storage vesicles, but it had little effect on choline uptake, acetylcholine synthesis, or the vesicular content of previously loaded acetylcholine. AH5183 at 4 microM inhibited the Ba2+-evoked release of newly synthesized acetylcholine but not of older stores of acetylcholine. These data indicate that the vesicles are the source of the acetylcholine released from stimulated cells. AH5183 had little effect on the vesicular loading of dopamine and on the evoked release of dopamine, but at concentrations of 4-40 microM it did inhibit dopamine uptake by the cells.

Specificity and properties of the nucleotide carrier in chromaffin granules from bovine adrenal medulla

1. The influence of various substances on the uptake of [3H]ATP and [14C]-noradrenaline into isolated bovine chromaffin granules was investigated. The carrier-mediated [3H]ATP uptake is specifically inhibited by SO42-, PO43- and phosphoenolpyruvate. Compounds with carboxylic acid or sulphonic acid groups had no significant inhibitory effects on either uptake. 2. 35SO42-, 32PO43- and phosphoenol[14C]pyruvate are taken up into chromaffin granules by a temperature-dependent process that is inhibited by atractyloside, uncouplers of oxidative phosphorylation and lipid-permeant anions. The apparent Km of 35SO42- uptake is 0.4 mM. 3. These results indicate that the nucleotide carrier in chromaffin granules has a broad specificity, transporting compounds with two strong negative charges. 4. Amino acid probes influence the uptake of ATP and catecholamines differently. Pyridoxal phosphate inhibits both uptake processes, 4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid preferentially blocks ATP uptake, whereas phenylglyoxal blocks only ATP transport. It is suggested that the nucleotide carrier possesses arginine residues in a functionally important position. 5. The significance of these results obtained on isolated granules for the function of chromaffin granules within the cell is discussed.

Calmodulin stimulation of 45Ca2+ transport and protein phosphorylation in cholinergic synaptic vesicles

Cholinergic synaptic vesicles isolated from the electric organ of Torpedo californica exhibit ATP-dependent uptake of 45Ca2+ that is stimulated by exogenous calmodulin. ATP-independent uptake also occurs, but it is only weakly stimulated by calmodulin. Saturating calmodulin decreased the Michaelis constant for ATP-dependent 45Ca2+ uptake from 52 +/- 0.4 to 12 +/- 0.2 microM and increased the maximal velocity from 3.4 +/- 0.3 to 5.2 +/- 0.5 nmol/mg of protein per min. The dose-response curve for calmodulin-dependent stimulation showed a maximal increase of 3.5-fold in the uptake rate; 0.2 microM calmodulin gave half-maximal stimulation. The activity of the vesicle-associated ATPase was unaffected. Incubation of vesicles with [gamma-32P]ATP and Ca2+ resulted in phosphorylation of four polypeptides of molecular weights about 64,000, 58,000, 54,000, and 41,000 when calmodulin was added. Vesicles that were previously phosphorylated and purified exhibited 2-fold enhanced ATP-independent uptake of 45Ca2+. Cyclic AMP could not substitute for calmodulin. The calcium transport system of the cholinergic synaptic vesicle is regulated by a calcicalmodulin-dependent protein kinase that is vesicle-associated.