The involvement of lysophosphoglycerides in neurotransmitter release; the composition and turnover of phospholipids of synaptic vesicles of guinea-pig cerebral cortex and Torpedo electric organ and the effect of stimulation

RIBEYE(B)-domain binds to lipid components of synaptic vesicles in an NAD(H)-dependent, redox-sensitive manner

Synaptic ribbons are needed for fast and continuous exocytosis in ribbon synapses. RIBEYE is a main protein component of synaptic ribbons and is necessary to build the synaptic ribbon. RIBEYE consists of a unique A-domain and a carboxyterminal B-domain, which binds NAD(H). Within the presynaptic terminal, the synaptic ribbons are in physical contact with large numbers of synaptic vesicle (SV)s. How this physical contact between ribbons and synaptic vesicles is established at a molecular level is not well understood. In the present study, we demonstrate that the RIBEYE(B)-domain can directly interact with lipid components of SVs using two different sedimentation assays with liposomes of defined chemical composition. Similar binding results were obtained with a SV-containing membrane fraction. The binding of liposomes to RIBEYE(B) depends upon the presence of a small amount of lysophospholipids present in the liposomes. Interestingly, binding of liposomes to RIBEYE(B) depends on NAD(H) in a redox-sensitive manner. The binding is enhanced by NADH, the reduced form, and is inhibited by NAD⁺, the oxidized form. Lipid-mediated attachment of vesicles is probably part of a multi-step process that also involves additional, protein-dependent processes.

Cholesterol facilitates the native mechanism of Ca2+-triggered membrane fusion

The process of regulated exocytosis is defined by the Ca2+-triggered fusion of two apposed membranes, enabling the release of vesicular contents. This fusion step involves a number of energetically complex steps and requires both protein and lipid membrane components. The role of cholesterol has been investigated using isolated release-ready native cortical secretory vesicles to analyze the Ca2+-triggered fusion step of exocytosis. Cholesterol is a major component of vesicle membranes and we show here that selective removal from membranes, selective sequestering within membranes, or enzymatic modification causes a significant inhibition of the extent, Ca2+ sensitivity and kinetics of fusion. Depending upon the amount incorporated, addition of exogenous cholesterol to cholesterol-depleted membranes consistently recovers the extent, but not the Ca2+ sensitivity or kinetics of fusion. Membrane components of comparable negative curvature selectively recover the ability to fuse, but are unable to recover the kinetics and Ca2+ sensitivity of vesicle fusion. This indicates at least two specific positive roles for cholesterol in the process of membrane fusion: as a local membrane organizer contributing to the efficiency of fusion, and, by virtue of its intrinsic negative curvature, as a specific molecule working in concert with protein factors to facilitate the minimal molecular machinery for fast Ca2+-triggered fusion.

Reduced palmitate turnover in brain phospholipids of pentobarbital-anesthetized rats

Our laboratory has reported that pentobarbital-induced anesthesia reduced the incorporation of intravenously injected radiolabeled palmitic acid into brain phospholipids. To determine if this decrease reflected a pentobarbital-induced decrease in palmitate turnover in phospholipids, we applied our method and model to study net flux and turnover of palmitate in brain phospholipids (1). Awake, light and deep pentobarbital (25-70 mg/kg, iv) anesthetized rats were infused with [9,10-3H]palmitate over a 5 min period. Brain electrical activity was monitored by electroencephalography. An isoelectric electroencephalogram characterized deep pentobarbital anesthesia. Net incorporation rates (J(FA,i)) and turnover rates (Fi) of palmitate were calculated. J(FA,i) for palmitate incorporated into phospholipids was dramatically reduced by pentobarbital treatment in a dose-dependent manner, by 70% and 90% respectively for lightly and deeply anesthetized animals, compared with awake controls. Turnover rates for palmitate in total phospholipid and individual phospholipid classes were decreased by nearly 70% and 90% for lightly and deeply anesthetized animals, respectively. Thus, pentobarbital decreases, in a dose-dependent manner, the turnover of palmitate in brain phospholipids. This suggests that palmitate turnover is closely coupled to brain functional activity.

Activities of enzymes in platelet activating factor biosynthetic pathways in the gerbil model of cerebral ischemia

The activities of enzymes in platelet activating factor (PAF) biosynthetic pathways were analyzed in hippocampal and cerebral cortical regions of normal and ischemic gerbil brain to assess changes in enzyme activities and potential modulators that could explain the accentuated production of PAF seen in ischemia. Global forebrain ischemia was produced by bilateral carotid artery ligation, and the effectiveness of the ligation was shown by free fatty acid release and ATP depletion. Specific activities of 1-alkyl-2-acetyl-sn-glycerol (AAG) choline phosphotransferase, 1-alkyl-sn-glycero-3-phosphate (AGP) acetyl transferase, and 1-alkyl-sn-glycero-3-phosphocholine (lyso PAF) acetyl transferase in tissue homogenates were in the ratio 4:1:0.1, respectively. Sham-operated and ischemic or ischemic-reperfused tissues showed similar activities for individual enzymes, indicating that enzyme levels or activation states did not change in ischemic or reperfused tissues. However, small metabolites (relevant to ischemia) added to the in vitro assays did modify enzyme activities. Physiological concentrations of MgATP severely inhibited AGP acetyl transferase activity, and this resulted in the ratio of AGP acyl transferase to AGP acetyl transferase activities changing from 48:1 in the presence of 2.5 mM MgATP to 6:1 in the absence of MgATP. This suggests that falling ATP levels in cerebral ischemia may promote the de novo pathway of PAF biosynthesis by releasing inhibition of AGP acetyl transferase. Lyso PAF acetyl transferase was much less active than AGP acetyl transferase and was also inhibited by MgATP. AAG choline phosphotransferase was not inhibited by MgATP but was inhibited by calcium. However the superior specific activity of the choline phosphotransferase in comparison with the AGP acetyl transferase suggested that the lowered choline phosphotransferase activity in the presence of rising intracellular calcium would not seriously compromise the synthesis of PAF by the de novo route. Both acetyl transferase enzymes were also inhibited by oleoyl CoA.

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.

Stimulatory effect of veratridine on lysophosphatidylethanolamine formation in rat brain synaptosomes

[3H]-Arachidonic acid incorporation into phospholipids of synaptosomal lysates prepared from veratridine-treated synaptosomes was examined. Synaptosomal lysates were shown to acylate exogenously added lysophosphatidylcholine, lysophosphatidylinositol, and lysophosphatidylethanolamine, when incubated with [3H]-arachidonic acid, ATP, CoA and MgCl2, yielding the respective phospholipids. Preincubation of synaptosomes with veratridine for 30 sec gave rise to an increase in [3H]-arachidonic acid incorporation into phosphatidylethanolamine, but not phosphatidylcholine nor phosphatidylinositol, indicating that lysophosphatidylethanolamine might be produced by veratridine. This increase of radioactivity in phosphatidylethanolamine caused by veratridine was completely inhibited by 1 microM tetrodotoxin or in calcium-free condition. These observations show that lysophosphatidylethanolamine was formed in a calcium-dependent manner and accumulated in synaptosomes treated with veratridine, which may relate to its action on the sodium channel and enhanced calcium influx.

The rapid incorporation of radioactive fatty acid into triacylglycerols during the in vitro acylation of native lipids of neuronal nuclei

Using neuronal nuclei (N1) and microsomes (P3) isolated from cerebral cortices of 15-day-old rabbits, the incorporation of [14C]oleate was followed in vitro, making use of fatty acid activation factors and endogenous membrane acyl acceptors. Of the lipids of N1, it was triacylglycerol which showed the highest rates of labelling and which represented 71-80% of the total incorporated radioactivity in this fraction. Specific rates of N1 triacylglycerol formation were 63-166 times those of P3 triacylglycerols (based upon membrane phospholipid content). In P3, phospholipids made up 85% of the total microsomal lipid labelling. The incorporation of oleate was dependent upon ATP and coenzyme A, and acyl-CoA synthetase activities were demonstrated in N1 and P3 (specific activity ratio, N1:P3 = 4.5). Using exogenous [14C]oleoyl-CoA, high rates of N1 triacylglycerol labelling were still seen relative to P3, but rates of diacylglycerol and phospholipid labelling were substantially elevated in both fractions in contrast to rates found using [14C]oleate. By increasing levels of endogenous diacylglycerol using preincubations with phospholipase C, a 3-fold increase was seen in specific rates of triacylglycerol formation in both fractions in subsequent assays with [14C]oleate. A 4.5-fold increase in N1 diacylglycerol concentrations was found when N1 was incubated for 10 min in the absence of fatty acid, ATP and coenzyme A. It is concluded that neuronal nuclei have a very active diacylglycerol acyltransferase as well as the ability to generate diacylglycerol substrates.

Lipids of synaptic vesicles: relevance to the mechanism of membrane fusion

Synaptic vesicles from the electric organ of the marine ray Narcine brasiliensis, purified to at least 90% homogeneity, were analyzed for the lipid and fatty acid content of their membranes. The major lipids (mol %) were phosphatidylcholine (32.3%), phosphatidylethanolamine (20.5%), phosphatidylserine (6.1%), sphingomyelin (3.0%), and cholesterol (33.3%), a composition which did not differ greatly from that of the parent electric organ. While the number of double bonds per fatty acid molecule was similar for both synaptic vesicle and whole electric organ phospholipids, the vesicles were highly enriched in docosahexenoic acid (22:6). Reaction with the amine labeling reagents isethionylacetimidate and trinitrobenzenesulfonic acid indicated that 40% of the phosphatidylserine and 60% of the phosphatidylethanolamine are present on the external (cytoplasmic) surface of the synaptic vesicle. These data on a natural fusing membrane have relevance to models of membrane fusion, which have been based largely on studies of in vitro fusion using synthetic membranes.

The isolation and lipid composition of subcellular fractions derived from neuronal perikarya isolated in bulk from rabbit cerebral cortex

(1) Neuronal perikarya were isolated from rabbit cerebral cortex and were homogenized and separated into a number of subcellular membrane fractions using differential and discontinuous density gradient centrifugation. (2) The efficiency of homogenizing the nerve cell bodies could be greatly increased by the preliminary passage of the cell body fraction through a micropipet tip. (3) Of a number of density media, a discontinuous gradient of metrizamide gave the best resolution of the mitochondrial and lysosomal marker enzymes found in the crude mitochondrial fraction. This yielded a purified mitochondrial fraction and several lighter membrane fractions. (4) The lighter membrane fractions in metrizamide contained a degree of mitochondrial contamination which could be removed by a second gradient spin on 1.2 M sucrose, producing a second mitochondrial fraction and two light membrane fractions. (5) Based on marker analyses, fractions enriched in nuclei, mitochondria, microsomes and plasma membrane/lysosomes were produced. (6) The two mitochondrial fractions showed the highest, and the nuclear fraction the lowest specific phospholipid content. Cholesterol: phospholipid molar ratios showed a gradient of values from a low (0.2) for the mitochondria and nuclei to an intermediate value (0.4) for the microsomes to a high (0.6) for the light membrane fractions. (7) Phospholipid distributions indicated that for the nuclear and mitochondrial fractions phosphatidylinositol was more abundant than sphingomyelin or phosphatidylserine, while for the microsomes and the two light membrane fractions these three phospholipids were present in almost equal amounts.