Phosphatidic acid and polyphosphoinositide formation in a broken cell preparation from rat brain: Effects of different incubation conditions

Effects of ethanol on phosphorylation of lipids in rat synaptic plasma membranes

Synaptic plasma membranes (SPM) isolated from rat cerebral cortex contain lipid kinases for conversion of phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), and diacylglycerol (DG) to PIP, phosphatidylinositol 4,5-bisphosphate (PIP2), and phosphatidic acid (PA), respectively. These anionic phospholipids are important in signal transduction mechanisms and are required for synaptic function. The effect of ethanol and other aliphatic alcohols on phosphorylation of these lipids in SPM has not been established. Incubation of SPM with [gamma-32P]ATP resulted in labeling of PIP, lyso-PIP, PIP2, and PA. Ethanol (50-200 mM) added to the incubation system showed a dose-dependent decrease in labeling of PIP2, but not PIP or PA. To a lesser extent, labeling of PIP2 was also inhibited by 1-propanol, but neither isopropanol nor 1-butanol could alter the PIP2 labeling pattern. Under similar incubation conditions, labeling of PIP and PA in SPM was not altered by ethanol, 1-propanol, iso-propanol, but 1-butanol stimulated PIP labeling with a peak at 25 mM. Addition of exogenous PIP to the incubation mixture led to an increase in labeling of PIP2, suggesting that the endogenous PIP pool in SPM is limiting for the synthesis of PIP2 in SPM. Interestingly, when SPM were incubated with exogenous PIP, addition of ethanol (50-100 mM) to this incubation mixture resulted in an increase in PIP2 labeling. Taken together, these results suggest a specific effect of ethanol on PIP kinase in SPM, and this effect seems to be dependent on the location and/or amount of PIP in the membrane.

Phosphoinositide kinase activities in synaptosomes prepared from brains of patients with Alzheimer's disease and controls

Previously, phosphatidylinositol (PI) kinase activity in cytosolic fractions prepared from postmortem tissue of the cerebral cortex from patients with Alzheimer's disease (AD) appeared to be lower than that of age-matched controls [Jolles et al., J. Neurochem., 58 (1992) 2326-2329]. In the study presented here, PI and PIP (phosphatidylinositol phosphate) kinase activities were studied in synaptosomes prepared from postmortem brain tissue of AD patients and age-matched controls. Firstly, PI kinase activity in synaptosomes prepared from the frontal superior gyrus of AD brain was 30% lower than in synaptosomes prepared from postmortem tissue of control brain. PIP kinase activity was the same in AD and control synaptosomes. Secondly, the yield of synaptosomal protein (micrograms protein per mg tissue wet weight) was lower in preparations from AD brain than in preparations from control brain, which could be a manifestation of a loss of presynaptic terminals in the frontal cortex. These results suggest that the difference in PI kinase activity between AD and control brain tissue may originate from differences in intact neurons in view of the fact that synaptosomes can originate only from intact neurons.

Phosphorylation of lipids in rat primary glial cells and immortalized astrocytes (DITNC)

Phosphatidylinositol 4,5-bisphosphate, a substrate in the signal transduction pathway involving phospholipase C, is synthesized in cell membranes by phosphorylation of phosphatidylinositol (PI) and phosphatidylinositol 4-phosphate (PI-4-P). Incubation with [gamma-32P]adenosine triphosphate of membranes isolated from primary glial cells in culture and from an immortalized astrocyte cell line (DITNC) and subsequent separation of the lipids by high-performance thin-layer chromatography revealed a number of labeled lipid bands. Further analysis of the deacylated products by high-performance liquid chromatography indicated the presence of two PI-4-P bands and a band corresponding to lysoPIP (PIP, PI phosphate). The two PI-4-P bands were more prominent in the samples from astrocyte cell membranes than from synaptic plasma membranes and plasma membranes. Analysis of the fatty acid composition of PI by gas-liquid chromatography revealed that both 20:3n-9 and 20:4n-6 were present in PI of cultured cell membranes, whereas the brain membranes contained exclusively 20:4n-6. The two PI-4-P bands in these membranes can be attributed to the presence of different molecular species of PI. Assignment of the fraction corresponding to lysoPIP was consistent with the observation that an increase in label in this lipid band occurred upon incubation of DITNC cell supernatant with lysoPI (1-stearoyl PI). This suggests that endogenous lysoPI present in the cell membranes can be phosphorylated to form lysoPIP.

Platelet phosphatidylinositol kinase activity is not altered in Alzheimer disease

We previously reported a specific decline in phosphatidylinositol (PI) kinase activity in the neocortex of patients with Alzheimer disease (AD) as compared to controls, whereas phosphatidylinositol phosphate (PIP) kinase activity appeared not to be affected (Jolles et al., 1992). In search of a possible systemic effect of AD, in the present study we investigated phosphoinositide kinase activity in platelets from patients with AD and from control subjects. The study was based on the notion that disease-specific abnormalities in the brain could be reflected in blood platelets. PI kinase activity was studied in platelet homogenates and in a salt-solubilized protein fraction of platelets, because of the difference in subcellular localization of the different types of PI kinases. In addition, NADH cytochrome-C reductase was measured in platelet homogenates as a marker for the endoplasmic reticulum, to detect a possible proliferation of the endoplasmic reticulum. AD patients and normal elderly controls showed no difference in PI kinase activity in either enzyme fraction. Furthermore, NADH cytochrome-C reductase activity and the protein/phospholipid ratio per 10(6) platelets were the same for AD patients and controls. This was taken as an indication that platelets in AD patients do not show proliferation of intracellular membranes.

Behavioural and biochemical effects of acute central metabolic inhibition: effects of acetyl-l-carnitine

In the present study we evaluated a new method to assess the behavioural and biochemical effects of a brief period of acute hypoxia in the brain. In this method, cyanide is injected into the lateral ventricles. Spatial navigation performance in a Morris task was found to be impaired 1 and 5 min after an i.c.v. injection of 5.0 micrograms cyanide but not after 2.5 micrograms cyanide. Increased rate of phosphatidic acid formation, reflecting increased phospholipase C activity, were observed after injection of 5.0 micrograms cyanide, indicating that energy-dependent phosphoinositide metabolism was affected. Chronic treatment with acetyl-l-carnitine attenuated the cyanide-induced behavioural deficit, but had no effect on energy-dependent phosphoinositide metabolism. The results suggest that, in this model, acetyl-l-carnitine may act via free fatty acid metabolism, by increasing the reservoir of activated acyl groups which are involved in the reacylation of membrane phospholipids.

Evidence for a new inositol phospholipid in rat brain mitochondria

Phosphorylation of phosphatidylinositol (PI), phosphatidylinositol monophosphate (PIP) and diacylglycerol (DAG) was studied in rat brain cortex myelin, synaptosomal and mitochondrial fractions, with ATP as phosphate donor and endogenous phospholipids as substrate. All fractions had PI, PIP and DAG phosphorylating activity with their own characteristic subcellular distribution. However, in the mitochondrial fraction an unidentified lipid was phosphorylated, which had a slower Rf value than PIP2 on TLC. After hydrolysis of the polar head group of the lipid and separation on anion exchange columns, it appeared to be a phosphoinositide. The elution profile showed that it was not phosphatidylinositol trisphosphate, or a lyso-compound. The available evidence suggests that the unknown inositol phospholipid in rat brain mitochondria is a phosphatidylinositol 4,5-bisphosphate isomer, although the possibility of it being a glycosyl-phosphoinositide cannot be excluded.

Brain phosphatidic acid and polyphosphoinositide formation in a broken cell preparation: regional distribution and the effect of age

The effect of age on phosphate incorporation into phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid (PA) was studied. Lysed crude synaptosomal fractions of different brain regions of 3-month-old and 32-month-old Brown Norway rats were used. The brain regions tested were the hippocampus, frontal cortex, occipital/parietal cortex, entorhinal/pyriformal cortex, striatum/septum, thalamus and hypothalamus. The individual specific phosphorylating activities were unevenly distributed within the brain of Brown Norway rats. Strikingly, the distribution of phosphate incorporation into PIP2 was opposite from that of phosphate incorporation into PA. Phosphate incorporation into PA decreased (-15%) with age in almost all brain regions tested, whereas phosphate incorporation into PIP2 decreased with age only in the frontal cortex (-20%) and in the hypothalamus (-8%). The effects of age may reflect a deterioration of phosphoinositide metabolism, with its function in signal transduction coupled to receptors via G-proteins, in the brain regions involved. In addition, there was an age related decrease in protein content and total phospholipid phosphorus content of lysed crude synaptosomal preparations of all brain regions. The high correlation between the changes in these parameters may be indicative of a decrease in the number or size of synaptosomes with age in the brain regions involved.

Phosphatidylinositol kinase is reduced in Alzheimer's disease

Phosphatidylinositol (PI) kinase and PI phosphate (PIP) kinase activities were measured in postmortem samples of brain tissue from patients with Alzheimer's disease and nondemented control subjects. A membrane-free cytosolic fraction from four neocortical locations, with exogenous inositol lipids as the substrate, was used. Tissue from patients with Alzheimer's disease was characterized by reduced PIP formation; the reduction was 50% in prefrontal cortex, temporal cortex, and parietal cortex and 40% in precentral gyrus. In contrast, no alterations were found in PI bisphosphate formation in these four neocortical locations. The specific changes in PI kinase but not PIP kinase activity suggest that the findings may have functional relevance to the involvement of brain membrane processes in Alzheimer's disease.