Neural phospholipases hydrolysing phosphatidylinositol and their possible role in stimulated turnover of this phospholipid

K+-dependent stimulation of dopamine synthesis in striatal synaptosomes is mediated by protein kinase C

Dopamine synthesis rate was measured in striatal synaptosomes. Removal of Na+ increased synthesis rate; this was blocked in Ca2+-free medium and by addition of the Ca2+/calmodulin inhibitor N-6-aminohexyl-5-chloro-1-naphthalenesulfonamide (W7). The increase in dopamine synthesis rate caused by the addition of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) was blocked by the protein kinase C inhibitor polymyxin B. K+-stimulated synthesis was unchanged in Ca2+-free medium or by addition of W7; it was blocked by polymyxin B. The effect of 50 mM K+ was additive with that of 8-Br cyclic AMP and of Na+ removal; the combined effect of 50 mM K+ and TPA was no greater than that of either alone. These results suggest that stimulation of dopamine synthesis in striatal synaptosomes by 50 mM K+ is mediated by protein kinase C.

Formation of diacylglycerol and degradation of phosphatidylinositol induced in rat lymphocytes by non-esterified oleic or linoleic acid

Rat spleen lymphocytes were incubated for 3 h with [14C]arachidonic acid in foetal calf serum. It was found that arachidonic acid distributed into phospholipids in the order phosphatidylcholine greater than phosphatidylethanolamine greater than phosphatidylinositol. After labelling with arachidonic acid the lymphocytes were washed, and incubated for up to 2 h with non-radioactive palmitic, oleic or linoleic acid dissolved in ethanol. The presence of ethanol or palmitic acid during a 2 h post-incubation had little effect on the amount of radioactivity found in different lipid fractions. Both oleic acid and linoleic acid, however, brought about an accumulation (up to 8-fold) of radioactivity in the diacylglycerol fraction. These fatty acids also brought about a change of radioactivity in several phospholipids, notably in phosphatidylinositol, which lost more than 50% of its counts during the 2 h incubation. Although maximum effects were seen at 2 h, diacylglycerol radioactivity was increased by 100% within 5 min after adding the fatty acids. The minimum concentration of fatty acids used (50 microM) gave an almost maximum response. The results indicate that unsaturated fatty acids may activate phosphatidylinositol phosphodiesterase in lymphocytes, as they do in brain. The possibility that a phospholipase A is activated is discussed. Possible implications for any experiments in which cells are incubated with fatty acids are pointed out.

Receptor-mediated increases in phosphatidylinositol turnover in neuron-like cell lines

Muscarinic receptors found in the N1E-115 mouse neuroblastoma cell line were tested for their ability to mediate stimulation of phosphatidylinositol (PI) turnover. This study was facilitated by the development of a new solvent system (acetone:butanol:acetic acid:water, 3:5:1:1) for the rapid and consistent separation of PI by one-dimensional thin-layer chromatography. Cholinergic stimulation caused as much as a 680% increase in the incorporation of 32P into PI. Enhanced incorporation of 32P into PI could be measured as early as 4 min after stimulation began. By 20 min, the rate of incorporation by stimulated cells had decreased to that of unstimulated cells, indicating desensitization. The magnitude of the response was dependent on the extent of receptor occupancy, and the response elicited by a saturating dose of carbamylcholine was blocked completely by 10(-7) M atropine, a specific muscarinic antagonist. Chronic stimulation, known to cause a loss of receptor binding sites, led to a 90% decrease in the maximum response even after a 40-min withdrawal period. Replacement of Na+ ions in the medium with choline or K+ severely impaired the ability of the cells to incorporate added 32P into PI (90 and 50%, respectively). Removal of the putative second messenger Ca2+ for short periods of time by the addition of excess EGTA did not alter either basal or muscarinic-stimulated PI turnover.

Biosynthesis of phosphatidylinositol in Crithidia fasciculata

Microsomal preparations from the protozoan (Crithidia fasciculata were shown to incorporate myo-[2-3H]inositol into phosphatidylinositol by both the CDPdiacylglycerol:myo-inositol phosphatidyltransferase reaction and by a myo-inositol exchange reaction. Non-ionic detergent and Mg2+ were necessary for the measurement of transferase activity. Untreated preparations could not be saturated with Mg2+, even at very high concentrations (50-75 mM). However, low concentrations of EGTA (75 micro M) both stimulated the activity 3-fold and reduced the Mg2+ required for saturation to 15-20 mM. EGTA also increased the apparent Km for CDPdiacylglycerol while increasing the sensitivity to substrate inhibition above 1 mM. The transferase activity was inhibited by relatively low concentrations of Ca2+ (50 micro M). This and the EGTA effect suggest a possible role for Ca2+ in the modulation of phosphatidylinositol synthesis. The myo-inositol exchange activity required Mn2+, was insensitive to Ca2+ inhibition and was only slightly stimulated by detergents and EGTA. This activity was preferentially inactivated by heating at 50 degrees C in the presence of Triton X-100. In a detergent solubilized preparation the exchange activity but not the transferase exhibited a non-specific requirement for phospholipid. The differences in properties of the two activities suggest the presence of a separate exchange enzyme.

The calcium-dependent phosphatidylinositol-phosphodiesterase of rat brain. Mechanisms of suppression and stimulation

1. The activity of the soluble, calcium-dependent phosphatidylinositol-specific phosphodiesterase (EC 3.1.4.10) against [32P]phosphatidylinositol has been investigated. 2. KC1 (only at neutral pH), Mg2+, positively-charged proteins such as histone, and phospholipids containing a choline headgroup are all inhibitory to the enzyme. Choline-phospholipids cause a 90% inhibition at an equimolar ratio to phosphatidylinositol. 3. Other phospholipids (phosphatidylglycerol, phosphatidylserine, phosphatidylethanolamine and phosphatidic acid) are all potent stimulators of the enzyme: maximum stimulation being observed at a ratio of 1 mol activator/5--10 mol phosphatidylinositol. 4. Unsaturated amphiphiles such as oleic and oleoyl alcohol also stimulate the activity, maximum stimulation being observed at about an equimolar ratio to phosphatidylinositol. Saturated amphiphiles (such as stearic acid and stearoyl alcohol) are less effective. 5. The activation by acidic phospholipids and unsaturated amphiphiles appear to be independent as they are additive and, under certain conditions, synergistic. 6. Both types of stimulator (independently or together) can reverse the inhibition caused by histone or phosphatidylcholine. 7. Possible mechanisms of the suppression of the phosphatidylinositol phosphodiesterase in vivo, of its activation, and of the amplification of phosphatidylinositol breakdown are discussed.